REVIEW published: 31 January 2020 doi: 10.3389/fonc.2020.00038 PVT1 Long Non-coding RNA in Gastrointestinal Cancer Águeda Martínez-Barriocanal 1,2*, Diego Arango 1,2* and Higinio Dopeso 1* 1 Group of Biomedical Research in Digestive Tract Tumors, CIBBIM-Nanomedicine, Vall d’Hebron University Hospital, Research Institute (VHIR), Universitat Autònoma de Barcelona, Barcelona, Spain, 2 Group of Molecular Oncology, IRB Lleida, Lleida, Spain Whole genome and transcriptome sequencing technologies have led to the identification of many long non-coding RNAs (lncRNAs) and stimulated the research of their role in health and disease. LncRNAs participate in the regulation of critical signaling pathways including cell growth, motility, apoptosis, and differentiation; and their expression has been found dysregulated in human tumors. Thus, lncRNAs have emerged as new players in the initiation, maintenance and progression of tumorigenesis. PVT1 (plasmacytoma variant translocation 1) lncRNA is located on chromosomal 8q24.21, a large locus frequently amplified in human cancers and predictive of increased cancer risk in genome-wide association studies (GWAS). Combined, colorectal and gastric Edited by: adenocarcinomas are the most frequent tumor malignancies and also the leading cause Ihab Younis, of cancer-related deaths worldwide. PVT1 expression is elevated in gastrointestinal Carnegie Mellon University in Qatar, Qatar tumors and correlates with poor patient prognosis. In this review, we discuss the Reviewed by: mechanisms of action underlying PVT1 oncogenic role in colorectal and gastric cancer Yuchen Liu, such as MYC upregulation, miRNA production, competitive endogenous RNA (ceRNA) Shenzhen University, China Olorunseun O. Ogunwobi, function, protein stabilization, and epigenetic regulation. We also illustrate the potential Hunter College (CUNY), United States role of PVT1 as prognostic biomarker and its relationship with resistance to current *Correspondence: chemotherapeutic treatments. Águeda Martínez-Barriocanal agueda.martinez@vhir.org Keywords: PVT1, lncRNA, siRNA, ceRNA, Myc, colorectal/gastric cancer Diego Arango diego.arango@vhir.org Higinio Dopeso INTRODUCTION dopesogj@mskcc.org According to the latest data released by the International Agency for Research on Cancer (IARC), Specialty section: cancer of the digestive tract is the leading cause of cancer and cancer-related death worldwide This article was submitted to (1–3) (Table 1). Digestive tract tumors include malignancies arising in the oral cavity, esophagus, Cancer Genetics, stomach, small, and large intestines, rectum and anus. Briefly, oral cancer is a subgroup of head and a section of the journal neck cancers, characterized by the growth of tumor cells in the lining of the lips, mouth, and upper Frontiers in Oncology throat. Oral squamous cell carcinoma represents the most frequent oral neoplasm and arises from Received: 18 April 2019 epithelial cells. The greatest risk factor for oral cancer is the tobacco and/or alcohol use. However, Accepted: 09 January 2020 the exposure to chemical carcinogens, ultraviolet/ionizing radiation and viral infections, such as Published: 31 January 2020 Human Papilloma virus (HPV), Epstein-Barr Virus (EBV) or Hepatitis C virus (HCV), are also Citation: known to increase oral cancer incidence rate (4). Esophageal cancer occurs when malignant cells Martínez-Barriocanal Á, Arango D and Dopeso H (2020) PVT1 Long arise in the tissue of the esophagus, the tube that transports the food from the mouth to the stomach. Non-coding RNA in Gastrointestinal There are two major subtypes of esophageal cancer, which are epidemiologically and biologically Cancer. Front. Oncol. 10:38. very distinct. Esophageal squamous cell carcinomas occur in the epithelial cells of the mucosa, are doi: 10.3389/fonc.2020.00038 frequently found in the upper and middle sections of the esophagus and associate with risk factors Frontiers in Oncology | www.frontiersin.org 1 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer TABLE 1 | Incidence and mortality rates of tumor malignancies in the digestive tract worldwide. Incidence Mortality Number Ranking Number Ranking Males Females Both sexes Males Females Both sexes Colorectal 1,849,518 3 2 3 880,792 4 3 2 Stomach 1,033,701 4 7 5 782,685 3 5 3 Esophagus 572,034 7 13 7 508,585 6 9 6 Lips/oral cavity 354,864 11 19 16 177,384 12 16 15 Total digestive tract 3,810,117 1 (21.1%) 2,349,446 1 (24.6%) Incidence and mortality numbers for digestive tract malignancies and position occupied in the ranking when compared to other cancers are shown. Percentages considering all tumor sites are indicated in brackets. Data was extracted from the 2018 Global Cancer Observatory (International Agency for Research on Cancer) (1). leading to recurrent chemical or physical insults to mucosa, due to its high prevalence in Asian countries, whose population i.e., tobacco, alcohol, or hot drinks use. Conversely, esophageal and economic growth rates are increasing. The prognosis of adenocarcinomas arise from the glandular cells in the mucosa, gastrointestinal cancer patients largely depends on tumor stage are more abundant in the lower section of the esophagus, and are at the time of diagnosis (Table 2). Additionally, tumor staging associated with gastro-esophageal reflux and obesity as the main strongly influences the clinical management (12–15). Thus, risk factors (5, 6). In turn, gastric cancer occurs when malignant gastrointestinal cancer represents a major health and social cancer cells grow and colonize the wall of the stomach. The most issue requiring great investment from governments worldwide to common form of stomach cancer is the gastric adenocarcinoma, cover the cost of prevention, diagnosis and treatment. which originates in the epithelial cells from the mucosa. Other Efficient clinical management of cancer largely relies on the less common types of stomach cancer include gastro-intestinal identification and study of key mediators of the tumorigenic stromal tumors, which develop in the connective tissue from the process, both at onset and progression. Classically, most of stomach wall, squamous cell carcinomas and carcinoid tumors. the research efforts in tumor biology have been focused on Traditionally, gastric adenocarcinomas are divided into two protein-coding oncogenes, tumor-suppressor genes and DNA main histological subtypes, namely intestinal and diffuse. These repair genes. But with the irruption of whole-transcriptome subtypes follow very different oncogenic programs and thus sequencing (RNAseq) technologies and computational sciences, display different molecular profiles (7). The etiology of gastric we have gained great insights into non-coding RNAs (ncRNAs). cancer is multifactorial but dietary factors, such as high salt These RNAs cover over 90% of the human genome and and nitrate intake, and Helicobacter pylori infection increase the regulate a great variety of cellular processes including chromatin risk of gastric cancer development and progression (8–10). And architecture and remodeling, transcription, post-transcriptional finally, colorectal cancer occurs when tumor cells grow in the modification, epigenetic regulation, and signal transduction, colon or rectum inner lining. The normal intestinal epithelium both in physiological and pathological processes including cancer is maintained by a tight balance of proliferation, migration (16–18). An arbitrary 200-nucleotide length cut-off allows the and cell death. Tumorigenesis occurs when these mechanisms classification of non-coding transcripts into two categories: short become deregulated resulting in cell hyperproliferation and and long ncRNAs. Short ncRNAs are represented by microRNAs loss of differentiation, evidenced by the formation of aberrant (miRNAs), PIWI-interacting RNAs (piRNAs), transcription crypts evolving into adenomatous polyps and subsequently into initiation RNAs (tiRNAs), small nucleolar RNAs (snoRNAs), adenocarcinomas. Only a small proportion of colorectal tumors promoter-associated small RNAs (PASRs), promoter upstream display a mesenchymal origin. Alcohol consumption, smoking, transcripts (PROMPTs), and TSS (transcriptional start site)- high fat diet as well as obesity are well-known risk factors for associated RNAs (TSSa-RNAs). On the other hand, long ncRNAs colorectal cancer (11). (lncRNAs) group heterogeneous non-coding transcripts such as Among all tumor types in the digestive tract, colorectal, and intergenic non-coding RNAs (lincRNAs), ultraconserved regions gastric cancers exhibit the highest incidence and mortality rates. (T-UCRs) and other ncRNAs collectively named lncRNAs (17, Specifically, colorectal cancer is the third most common cancer 19). miRNAs have been the most widely studied class of type in men after lung and prostate cancer; and the second most ncRNAs in tumor biology. Since its discovery two and a common in women after breast cancer. Stomach cancer, in turn, half decades ago, many miRNAs have been implicated in the ranks fourth and seventh regarding its incidence in men and development of multiple human cancers through a wide range women, respectively. Only in 2018, 1.8 million colorectal and of mechanisms. Among all these mechanism, gene silencing has about 1 million stomach cancer cases were diagnosed worldwide, been predominant (20, 21). Mirroring what we have learned accounting for almost 900,000 and 800,000 deaths, respectively about cancer-associated coding genes, we can classify miRNAs (Table 1). Moreover, it is expected that by 2040, the incidence of into oncogenic or tumor suppressor (22). Moreover, miRNAs colorectal cancer will raise by 30% and mortality by 40%; whereas have become actionable targets for cancer treatment and several statistics for stomach cancer will both worriedly increase by 60% therapeutic agents are under development including some that Frontiers in Oncology | www.frontiersin.org 2 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer TABLE 2 | Survival rates for colorectal and gastric cancer. are expressed in a more tissue-specific manner than protein- coding genes. It has been estimated that 78% of all lncRNAs Stage Colorectal Stage Stomach are tissue-specific, while protein-coding genes barely reach Colon Rectum 19% of specificity (39). Additionally, lncRNAs dysregulation in primary tumors and metastasis is observed in body fluids, 0 – – 0 – i.e., whole blood, plasma, urine, saliva, and gastric juice I 92 87 IA 92 (40). And despite the high abundance of ribonucleases in IIA 87 80 IB 87 most of these fluids, lncRNAs are easily detected due to its IIB 63 49 II 63 high stability. This represents a clear advantage to patients IIIA 89 84 IIIA 89 as it allows cancer diagnosis and follow-up using minimally IIIB 69 71 IIIB 69 invasive methodologies. IIIC 53 58 Cancer cells show a plethora of chromosomal abnormalities, IV 11 12 IV 53 including translocations, amplifications, and deletions. The Five-year survival rates (%) for colorectal and gastric cancer patients are shown according plasmacytoma variant translocation 1 (PVT1) gene encodes to the tumor staging system. Stage 0: Abnormal cells are found but confined in the tissue a lncRNA that was first identified when studying a recurrent of origin; Stage I: Tumor cells are found but confined in the tissue of origin; Stage II: translocation breakpoint in the Igκ locus found in murine Tumor cells have spread deeper into the gastrointestinal wall; Stage III: Tumor cells have plamacytomas (41). One year later, a homologous human spread into nearby lymph nodes; Stage IV: Tumor cells have spread to distant tissues and organs. Data was extracted from ESMO Clinical Practice Guidelines (European Society sequence was identified when studying immunoglobulin for Medical Oncology) (12–15). translocations in Burkitt lymphoma (42). We now know that PVT1 gene fusions occur in additional hematologic malignancies, such as non-Hodgkin lymphoma and advanced have reached clinical trials (23–25). Currently, there is a growing multiple myeloma (10, 43, 44). PVT1 gene fusions are also interest in lncRNAs biology and their role in the tumorigenic found in solid tumors although at much lower rates (10, 43, 44). process. These transcripts are generated by RNA polymerase PVT1 exon 1 and intron 1 are most often involved in these II, can be capped, spliced and polyadenylated, but lack an DNA rearrangements (45). In addition, human PVT1 is a obvious open reading frame. According to the latest release of target of genetic gains and amplifications in a large variety of the GENCODE project (GRCh38.p12), which aims to build an cancers, including those of the digestive tract (46, 47). Moreover, encyclopedia of genes and gene variants, the human genome genome-wide association studies (GWAS) identified single contains 16,066 lncRNA genes encoding for 29,566 different nucleotide polymorphisms (SNPs) in the PVT1 locus (8q24) lncRNA transcripts (26–29). This number represents 27% of all that are associated with increased colorectal cancer risk (48). Of annotated human genes. interest, PVT1 locus leads to the production of a cluster of four Next-generation sequencing of large numbers of tumor annotated miRNAs, namely miR-1204, miR-1205, miR-1206, specimens has revealed thousands of lncRNAs aberrantly and miR-1207 (-5p and -3p), being some of them important in expressed in a broad spectrum of cancers (30, 31). Dysregulation the tumorigenic process of colorectal and gastric cancer (49–52). of certain lncRNAs leads to the hyper- or hypoactivation of It is of note that although PVT1 has been mostly studied in the cellular pathways that promote and/or sustain tumor initiation context of cancer, this lncRNA is related to multiple and diverse and progression (32). LncRNAs are involved in a broad range pathologies (Table 3). of processes such as transcriptional regulation of neighboring In this review, we will discuss the current knowledge of protein-coding genes, interference of miRNAs via sequence PVT1 alteration/dysregulation, as well as its contribution to complementary, protein decoy, protein stability control, post- gastrointestinal cancer. transcriptional processing, epigenetic regulation, high-order chromosomal dynamics, telomere biology, and subcellular structural organization (32–35). LncRNAs mediate all these PVT1 EXPRESSION IN NORMAL AND functions through the interaction with proteins, RNAs and TUMOR TISSUE lipids. It is remarkable that unlike other ncRNAs, the function of lncRNAs cannot be inferred from sequence or structure Most lncRNAs exhibit a great number of isoforms and PVT1 and thus, experimental evaluation needs to be conducted for is not an exception (53). Human PVT1 locus resides in a full and accurate biological annotation (30). Because of their chromosome 8q24.21 and contains 21 exons leading to 25 undeniable role in cancer biology, the therapeutic targeting annotated transcript variants (54). These variants arise as of oncogenic lncRNAs and lncRNAs involved in resistance to a consequence of alternative splicing mechanisms mediating treatment has raised significant attention. The main strategies exon skipping and the use of unconventional donor and to inactivate oncogenic lnRNAs aim their post-transcriptional acceptor splice sites. Thanks to consortia such as The degradation with siRNAs, or the steric blockade of lncRNA- Genotype-Tissue Expression Project (GTEx) and The Cancer protein interactions with small molecules, morpholinos, or Genome Atlas (TCGA), we know that among all these PVT1 antisense oligonucleotides (36, 37). Noteworthy, some intrinsic transcripts, 14 are present in tissues at detectable levels (7, features of lncRNAs make them very attractive as cancer 55–58). Specifically, 11 PVT1 transcripts have been detected diagnostic and prognostic biomarkers (38). First, lncRNAs in the normal gastrointestinal mucosa and adenocarcinomas Frontiers in Oncology | www.frontiersin.org 3 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer TABLE 3 | PVT1 in disease. and STAD (stomach adenocarcinoma) samples compared to the corresponding normal tissue was revealed using transcriptomic Disease Reports expression data available at the TCGA repository (57, 80). Cancer 335 Interestingly, the tumorigenic process does not seem to affect the Diabetic nephropathy 9 relative abundance of PVT1 isoforms (Figure 1), being PVT1 Sv- Arthritis 4 217 the isoform also more abundant in tumors. Unfortunately, Cardiac disease 3 the studies investigating the expression and role of PVT1 in Diabetic neuropathy 2 gastrointestinal malignancies have not addressed directly or Asthma 2 indirectly the expression of this isoform (66, 68–72, 75–78). All Vascular disease 1 these observations have been extended to human cancer cell lines. Precisely, CACO2, SW480, SW620, HT29, and HCT116 cells Chronic obstructive pulmonary disease 1 derived from human colorectal tumors displayed higher levels of Epilepsy 1 total PVT1 compared to NCM460, FHC and HCoEpiC normal Schizophrenia 1 colonic epithelial cells (67, 70, 72, 81). Likewise, AGS, MKN45, Multiple sclerosis 1 SGC7901, and BGC823 gastric cancer cell lines have higher PVT1 Muscle atrophy 1 expression compared to GES1 normal gastric epithelium cell line Diabetic retinopathy 1 (76, 79). Sepsis 1 PVT1 expression in stomach cancer is directly regulated by Immunodeficiencies 1 FOXM1 (Forkhead Box M1) transcription factor (75). FOXM1 Vitiligo 1 controls the expression of genes such as MYC, CCNB1, AURKB, Pulpitis 1 and SKP2, which are essential for cell cycle progression at Human pathologies in which PVT1 has been involved. Number of reports in Pubmed DNA replication and mitosis, and therefore important for tumor database are shown. initiation and progression (82–85). Additionally, FOXM1 plays a key role in DNA damage checkpoint participating in the repair of the colon and stomach (Figure 1). Among them, PVT1- of DNA strand breaks (86). PVT1 transcript levels changes upon 217 (ENST00000523190) containing only four exons, is the manipulation of FOXM1 protein expression in gastric cancer most abundant PVT1 transcript in the digestive tract. This cells. Specifically, PVT1 expression is reduced upon FOXM1 heterogeneity in isoform expression needs to be considered silencing and increases after FOXM1 overexpression (75). This when studying the role of PVT1 in carcinogenesis, as the regulation occurs through a direct interaction of FOXM1 biology of each isoform might have a different impact on tumor with two independent binding sites in the PVT1 promoter initiation/progression and patient survival. Several oncogenic that enhances the transcriptional activation of this lncRNA mechanisms have been attributed to PVT1, i.e., as a ceRNA (75). In addition, STAT3 (Signal Transducer and Activator of (competing endogenous RNA) for several miRNAs and as a Transcription 3) has also been shown to regulate the expression source of miRNAs itself (49, 51, 59–65). As a non-coding of PVT1 (77). Persistent STAT3 activation in tumor cells results gene, all of them rely into PVT1 RNA primary and secondary in increased cell proliferation, survival, and invasion; and at structure. Accordingly, the biological activity of PVT1 depends the same time, STAT3 exerts non-cell autonomous effects in on the sequence of the specific PVT1 transcripts expressed the tumor microenvironment by boosting tumor-promoting in a given tissue at a given time. Unfortunately, very few inflammation and suppressing anti-tumor immunity (87). STAT3 reports have taken into consideration the large heterogeneity overexpression in gastric cancer cell lines leads to increased PVT1 in PVT1 isoform expression and consequently, some of the levels, while STAT3 knockdown results in PVT1 transcriptional results described in this review require further investigation. downregulation. These effects are explained by the presence Nonetheless, multiple studies assessing PVT1 expression by of three canonical STAT3 binding motifs within the PVT1 means of microarray technology (66–68) or RT-PCR using promoter, which were confirmed to control the transcription of oligonucleotides amplifying several PVT1 isoforms, have shown this lncRNA (77) (Figure 2). a general overexpression of PVT1 in colorectal tumors compared In colorectal cancer, PVT1 transcription is influenced by p53 to paired normal tissue samples (66, 68–74). Only He et al. have (50). This tumor suppressor is involved in the cell cycle regulation examined the expression of individual PVT1 splice variants (Sv) by transactivating a plethora of protein-coding but also non- in colorectal cancer (68). They found PVT1 Sv-214, Sv-205, Sv- protein-coding genes, that ultimately prevent cell division by 209, Sv-208, Sv-206, Sv-207, Sv-213, Sv-219, Sv-201, and Sv-215 inducing cell cycle arrest, senescence, or apoptosis (88, 89). upregulated in colorectal cancer vs. normal samples (variants Importantly, p53 is the most frequently mutated gene in human ranked by decreasing overexpression fold) (68). Similarly, cancers (90, 91). The promoter of PVT1 harbors a functional increased PVT1 expression has been described in primary gastric p53 response element that enhances the transcription of this tumors compared to the normal gastric mucosa (75–78). PVT1 lncRNA in colorectal cancer cell lines exposed to the DNA isoform Sv-214 overexpression was found in primary gastric damaging agent daunorubicin, or upon increased p53 protein tumor samples compared to adjacent normal gastric tissue levels achieved by the use of Nutlin-3a (50). It is important (79). In good agreement, increased PVT1 expression in COAD to mention that PVT1 upregulation upon these conditions was (colorectal adenocarcinoma), READ (rectal adenocarcinoma) only monitored in p53 wild-type colorectal cancer cell lines and Frontiers in Oncology | www.frontiersin.org 4 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer FIGURE 1 | Expression of PVT1 isoforms in normal and tumor tissue from the gastrointestinal tract. Eleven of the twenty-five predicted PVT1 isoforms are expressed in the gastrointestinal tract. Transcripts variants are ranked from top to bottom according to their expression abundance (%) both in colon and stomach. Purple histograms illustrate the expression in normal tissue samples from GTEX database (colon n = 204 and stomach n = 204). Blue histograms illustrate the expression in tumor tissue samples from TCGA repository (colon adenocarcinoma n = 723 and stomach adenocarcinoma n = 453). Exon display for each PVT1 transcript variant is shown. Data was extracted using UCSC Xena Browser. thus, it remains unknown the ability of mutant p53 proteins to “oasis” for lncRNAs. Up to 12 lncRNAs have been identified in exert an equivalent role. Although most p53 mutations result this locus, and very importantly for colorectal cancer, many of in expression of dominant-negative forms, certain mutations are them regulate Wnt signaling pathway and in turn Myc activity known to confer oncogenic functions (92) (Figure 2). (35, 69) (Figure 2). Wnt signaling is crucial for the initiation and maintenance of colorectal cancer and consistently, this pathway is altered in more than 90% of these malignancies according to PVT1 AND MYC the TCGA data (58). Wnt dysregulation in gastric tumorigenesis is less frequent but affects 10–30% tumors (7, 97). Accordingly, High-resolution analyses of somatic copy-number alterations PVT1 and MYC coamplification (66, 78, 98) and coexpression indicate that 8q24.21 region is one of the most frequently (71, 98, 99) has been observed in colorectal and gastric primary amplified regions across human cancers, including colorectal tumors and cell lines (Figure 3). and gastric cancer, and are often associated with poor prognosis The first evidence of PVT1 in human solid malignancies, and/or drug resistance (7, 58, 93). Interestingly, although the and also its relationship with MYC, came from the study of amplified region spans almost 2 Mb the only protein coding gene COLO320, a colorectal cancer cell line that was known to be in this locus is the MYC oncogene. This fact led to nickname MYC amplified. Two variants of the cell line were isolated: this region as the gene “dessert” locus, and attribute to MYC COLO320-HSR (homogeneous stained region) with MYC allele the tumor promoting effects of gaining supernumerary copies of intrachromosomically amplified; and COLO320-DM, with a 8q24 (66, 94–96). Conversely, 8q24.21 has been shown to be an rearranged and amplified MYC allele that is carried on small Frontiers in Oncology | www.frontiersin.org 5 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer FIGURE 2 | Chromosome 8q24.21 amplification and PVT1 gene structure and regulation. 8q24.21 chromosomal region is frequently amplified in colorectal and gastric cancers (upper). This region contains MYC as the only protein-coding gene (yellow boxes) and several lncRNAs. Those lncRNA involved in MYC or WNT regulation have been depicted (blue and black boxes) (middle). PVT1 lncRNA expression is regulated by FOXM1, STAT3, and p53 transcription factors. PVT1 promoter exhibits four enhancer regions that in normal conditions (WT) boost its own expression. Due to PVT1 promoter mutations and rearrangements (MUT), which are frequently found in cancers, enhancer elements preferentially interact with MYC promoter enhancing the transcription of the oncogene. PVT1 encodes for four miRNAs. miR-1204 has been show to stabilize p53 protein, while miR-1207 has been proven to modulate TERT protein at the post-transcriptional level and expression of stemness and EMT genes by mechanisms currently unknown (lower). Pictograms in purple and green illustrate evidences obtained in colorectal and gastric cancer, respectively. Dark gray boxes indicate gene promoters. Dark blue boxes indicate enhancer regions. Green and red arrows indicate up- and down-regulation, respectively. fragments of extrachromosomal DNA named double minutes PVT1 promoter is recurrently rearranged in human tumors, (100). By studying MYC amplification in COLO320-DM, the including those affecting the digestive tract (10, 44, 104). authors noticed that intron 1 from MYC contained an ectopic It has been demonstrated experimentally that insertions or DNA fragment which they characterized and described as PVT1 deletions in the PVT1 promoter lead to Myc overexpression and in humans (101). However, the architecture of the MYC-PVT1 consequently, enhanced growth in cancer cells (104). Chromatin locus in COLO320-DM was later realized to be unique, since is organized into self-interacting units called topologically PTV1 locus normally lies 57 kb downstream of MYC gene. associating domains (TADs). Contacts between promoter and This chromosomal organization is conserved in other species enhancer regions take place within the TADs, leading to an such as mouse and rat, with syntenic 15qD1 and 7q33 regions, additional layer of gene expression regulation (105). As a respectively (99). consequence, genomic DNA structural variations, i.e., deletions, The expression of PVT1 and MYC in tumors are correlated duplications, insertions, inversions, and translocations, are not only because of their genetic coamplification. The 8q24 able to modify the three-dimensional chromatin topology and region displays a very strong enhancer activity creating feedback promote or suppress promoter-enhancer interactions within loops controlling MYC expression (102). Precisely, lncRNAs the TADs (104, 106). PVT1 gene harbors four intragenic within 8q24 region are efficiently transcribed due to the enhancer elements that under normal conditions establish strong presence of abundant transcriptional factors and mediator interactions with the PVT1 promoter and sustain expression of proteins, and these lncRNAs in turn, increase the enhancer the non-coding transcript. However, under the PVT1 promoter activity by forming chromatin loops and protein bridges to mutational rearrangements found in tumor cells, and even promote the transcription of MYC (71, 98, 103). Interestingly, epigenetic inactivation, PVT1 intragenic enhancer elements Frontiers in Oncology | www.frontiersin.org 6 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer FIGURE 3 | PVT1 and MYC expression in gastrointestinal tumors and cell lines. PVT1 and MYC copy number variation (A,B) and RNAseq expression (C,D) in tumor samples from colorectal (n = 616) and stomach (n = 441) adenocarcinomas (A,C) and established cell lines from colorectal (n = 58) and gastric tumors (n = 38) (B,D). The name of cell lines used in studies investigating the role of PVT1 in gastrointestinal cancer are shown. Gray boxes indicate no alteration. Light blue boxes indicate gene amplification. Dark blue boxes indicate gene deletion. RNAseq expression units: RPKM (Reads Per Kilobase Million). Pearson and Spearman correlation coefficients and associated p-value (Spearman) are shown. interact preferentially with MYC promoter, thus boosting the All these results have been obtained upon genetic transcription of MYC oncogene and its oncogenic activity of manipulation of PVT1 levels in cell line models, and it is tumor cells (104) (Figure 2). important to mention that these manipulations have not been restricted to a single PVT1 isoform (68). The siRNAs often used to achieve downregulation of PVT1 target multiple transcripts, ONCOGENIC PVT1 FUNCTIONS IN and not always those predominantly expressed in colorectal COLORECTAL AND GASTRIC CANCER and gastric tumors. On the other hand, models of PVT1 overexpression do not adequately report the specific isoforms LncRNAs carry out their biological functions through multiple used. The only exception is the study of He et al., where they and diverse mechanisms. Several research groups have convincingly demonstrated in HCT116 and SW480 colon cancer demonstrated the overall ability of PVT1 to sustain in vitro cell lines, the ability of PVT1 Sv-214 to promote proliferation, and in vivo cell growth, clonogenicity, migration, and invasion, stemness, migration and invasion in vitro; as well as in vivo by both in colorectal (66–68, 71, 72, 107) and gastric cancer establishing tumor xenografts and liver metastasis mouse models epithelial cells (75–77, 79). The capacity of PVT1 to negatively in immunodeficient nude mice (68). Importantly, differences regulate apoptosis through the inhibition of the TGFβ pathway, in the PVT1 isoforms downregulated in different studies might is more controversial, as it has not been observed systematically explain the discrepancies reported regarding the role of PVT1 (66, 70–72, 78, 79, 107, 108). PVT1 lncRNA has been shown to to regulate apoptosis. In this line, the suitability of certain exert its protumorigenic activity also in a non-cell autonomous PVT1 knockdown cellular models exhibiting apoptosis to assess manner. Precisely, PVT1 expression in gastric tumors enhances invasion and/or migration properties should be reconsidered. microvessel formation, both in vitro and in vivo, through a mechanism that involves vascular endothelial growth factor miRNA Production A (VEGFA) expression in a STAT3-dependent manner (77). PVT1 locus leads to the production of a cluster of four annotated Formation of new vessels is a crucial step for tumors to maintain miRNAs, namely miR-1204, miR-1205, miR-1206, and miR-1207 the supply of oxygen and nutrients, and VEGFA is a master (-5p and -3p) (49, 51) (Figure 2). Originally, miR-1208 was regulator of this process (109). Consistently, antiangiogenic also mapped into PVT1 locus, but according to latest version agents are routinely used in the clinical practice to treat cancer of Ensemble Genome browser (v98) this miRNA lies outside patients (110, 111). Antiangiogenics are used in combination the lncRNA sequence (54). All these miRNAs are conserved with chemotherapy in the first line of treatment of colorectal in the mouse Pvt1 gene (51). As depicted in Figure 4, most cancer, and several compounds targeting tumor vasculature of the miRNAs contained within the PVT1 transcript variants have been recently approved for the management of gastric expressed in the colon and stomach are found in intronic cancer (13, 14, 112). In fact, expression of PVT1 and VEGFA regions. Spatiotemporal organization of the RNA splicing has in combination predicts poor survival in gastric cancer, further important implications to fully understand the PVT1 loss-of- supporting the use of antiangiogenic drugs in these patients (77). expression experiments found in the literature (113). Gene Frontiers in Oncology | www.frontiersin.org 7 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer FIGURE 4 | miRNAs encoded by PVT1 transcript variants. PTV1 isoforms are ranked according to expression abundance (%) in gastrointestinal normal tissues and tumors. Gray and blue boxes indicate absence and presence of the indicated miRNA, respectively. Nucleotide sequence, length, and location within PVT1 introns or exons is indicated for all miRNAs. silencing using exogenous siRNA against exonic regions does while having tumor suppressor effects in gastric cancer cells not necessary entail that the miRNAs encoded within introns (contrary to the oncogenic role of PVT1). will also be silenced. Upon splicing in the nucleus, introns are available for microRNA processing while the spliced transcript ceRNA is exported to the cytoplasm where it may be targeted by LncRNAs have the ability to modulate gene expression indirectly exogenous siRNAs. This might explain why the use of siRNAs by impairing miRNA activity through sequestration/sponging, against PVT1 in HCT116 and RKO cells did not have any which leads to an effective de-repression of targets of these effect on the expression of miR-1204, miR-1205, and miR-1207- miRNAs (33, 34). This function, named competitive endogenous 5p/3p (66). Contrary, expression of all PVT1-encoded miRNAs RNA (ceRNA), has encountered certain skepticism due to the was enhanced upon exposure of the cells lines to the DNA fact that the physiological expression levels of an individual damaging agent daunorubicin, or upon increased p53 protein lncRNA may not be sufficient to completely suppress the activity levels achieved through Nutlin-3a stimulation (50). This, effect is of miRNAs (117). However, regulation by modestly expressed likely to be indirect because, as mentioned before, PVT1 increases lncRNAs could be magnified through downstream processes, as consequence of p53 stabilization. Among the four PVT1- particularly through the upregulation of transcription factors encoded miRNAs, only miR-1204 seems to engage a positive that by transactivating multiple targets contribute to outcome feedback loop to sustain stabilization of p53 at protein level (50). amplification. This might be the case for PVT1, which as shown The role of PVT1-encoded miRNAs in the initiation and/or here maintains a close relationship with Myc, p53 and STAT3, all progression of malignancies in the stomach or colon is uncertain of them key transcription factors in the tumorigenic process. due to the lack of literature. Only miR-1207 is reported to have an effect in gastric cancer proliferation and invasion, miR-152 or in the stemness properties of colon cancer cells (52, 81). miR-152 suppresses the proliferation and motility of gastric Specifically, it has been shown how ectopic expression of miR- cancer cells targeting CD151 and FGF2, which are cell 1207-5p in the SGC7901 gastric tumor cell line is able to reduce surface receptors well-known to participate in the spreading, proliferation and invasion in vitro and in vivo by targeting migration and invasion of tumors (118–120) (Figure 5). miR- the catalytic subunit of the telomerase complex hTERT (52). 152 expression is downregulated in gastric cancer tissues, most Aberrant expression of hTERT is associated with the metastatic likely as a result of the interaction with PVT1 which has been ability of gastric tumors (114, 115). In turn, miR-1207-5p shown in vitro to effectively sponge this miRNA (60, 118). overexpression in the HCoEpiC normal colon epithelial cell line In good agreement, PVT1 is negatively correlated with miR- enables the formation of primary and secondary spheres by a 152 expression in tumors from gastric cancer patients, and mechanism involving the upregulation at the mRNA level of its genetic manipulation in SGC7901 and BGC823 gastric cell TGFβ, CTNNB1, MMP2 and several colorectal stem cell markers lines influences miR-152 levels and ultimately, CD151 and FGF2 such as CD44, CD133, and CD166 (81). Consistent with the expression (60). It is noteworthy that three independent binding tight relationship between stemness and epithelial-mesenchymal sites for miR-152 have been identified in PVT1, however not all of transition (EMT), Slug, Snail, and vimentin were also found them are present in the transcripts more frequently expressed in upregulated (81, 116) (Figure 2). Interestingly, miR-1207-5p the gastric tissue (Figure 6). No reports are found in the literature miRNA seems to exhibit oncogenic activity in colorectal cancer regarding the role of PVT1 in sponging miR-152 in colorectal Frontiers in Oncology | www.frontiersin.org 8 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer FIGURE 5 | PVT1 interaction with miRNAs and proteins. PVT1 interacts with multiple miRNAs and proteins in cancer cells, leading to post-transcriptional (1) and post-translational (2) regulation of gene expression. PVT1-miRNA interactions promote efficient de-repression of miRNA targets. Specifically, PVT1 hinders miRNAs binding in the 3′ untranslated region of target transcripts, preventing their degradation by the RISC complex and thus increasing protein levels. PVT1-protein interactions stabilize proteins and prevent their degradation. Pictograms in purple and green illustrate PVT1-miRNAs/proteins interactions identified in colorectal and gastric cancer, respectively. tumors. However, as observed for gastric tumorigenesis, this prognosis and thus, tumors engage sophisticated mechanisms miRNA acts as a tumor suppressor in colorectal cancer cell lines to keep its expression at high levels (126). Gastric cancer cell and its expression is downregulated in primary colorectal tumors lines with PVT1 overexpression show increased HIF-1α mRNA (121, 122). and protein levels, that are reduced with a miR-186 mimic. Conversely, the decreased HIF-1α levels achieved by PVT1 miR-186 downregulation are rescued upon usage of a miR-186 inhibitor This tumor suppressor miRNA negatively regulates the (124) (Figure 5). PVT1 binding to miR-186 occurs through a proliferation, invasion and migration of gastric cancer cells by single interacting site, which according to in silico analysis is impairing the EMT process through TWIST1 targeting and present exclusively in PVT1-Sv203 (Figure 6). miR-186 role in degradation (123). miR-186 regulates the expression of hypoxia colorectal cancer pathogenesis is not so clear. Some reports have inducible factor 1α (HIF-1α) and blocks aerobic glycolysis in identified miR-186 [5p] as a tumor suppressor due to its ability gastric cancer cell lines (124). HIF-1α is a key protein regulating to inhibit proliferation, metastasis and EMT of colorectal cancer the response of cells to low levels of oxygen (125). In cancer, HIF- cells by targeting ZEB1, while others point out in the opposite 1α is significantly associated with metastasis, stemness and poor direction (127–129). Frontiers in Oncology | www.frontiersin.org 9 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer FIGURE 6 | Predicted sponge capacity of miRNAs by PVT1 transcript variants. PTV1 isoforms are ranked according to their relative expression abundance (%) in gastrointestinal normal tissues and tumors. Gray and solid blue boxes indicate absence and presence of the nucleotide sequence complementary to the different miRNA, respectively. Patterned blue boxes indicate complementarity with miR-128 principal binding motif, but lack of the additional contact sites and/or difference in length from the main interacting core. Nucleotide sequence of the miRNA binding site is specified. miR-455 RNA-binding protein that inhibits the processing of let-7 family PVT1 acts as a ceRNA to negatively regulate the expression of of miRNAs. Let-7 regulates the translation of mRNAs important miR-455 in HT29 and SW480 colon cancer cell lines (72). Hsa- for the embryo development, cell pluripotency, metabolism, and miR-455 encodes for two miRNAs: miR-455-5p and miR-455-3p. tumor progression (68, 134). In line with this observation, the PVT1 seems to specifically bind to miR-455-5p (Figure 6). This expression of PTV1 Sv-214 and let7a negatively correlated in miRNA is frequently downregulated in colorectal tumors and colorectal cancer patient samples (Figure 5). Notably, Lin28 acts predominantly as a tumor suppressor element by targeting cooperates with Wnt signaling and Snail to drive growth and phosphoinositide-3-kinase regulatory subunit 1 (PIK3R1) and invasiveness in colorectal and gastric tumors, respectively (135– Runt-related transcription factor 2 (RUNX2), both crucial for the 137). Conversely, in gastric cancer cell line systems, miR-128 development of tumors (72, 130, 131) (Figure 5). An equivalent was shown to target TrkC (79) (Figure 5). TrkC/NTRK3 is a tumor suppressor role has been attributed to miR-455 in gastric member of the NTRK neurotrophin tyrosine kinase receptor tumorigenesis (132, 133). family and functions as a tumor suppressor. TrkC is considered a dependence receptor, which is characterized by its ability to induce opposing biological effects depending on the availability miR-128 of the ligand, NT-3. In the presence of the TrkC ligand, a survival In silico analysis identified PVT1 Sv-214 as ceRNA for miR- signal is transduced, whereas its absence results in cleavage of a 128, and the initial observation was confirmed experimentally death-domain peptide and induction of apoptosis. NT-3 levels in colorectal cancer cell lines through gain and loss-of-function are reduced in CRC and consistently with the tumor suppressor experiments against PVT1 Sv-214 isoform and further extended activity of the receptor, tumors tend to accumulate inactivating to gastric cancer cell line model (68, 79). Moreover, the point mutations or silence protein expression by means of expression of miR-128 showed a significant inverse correlation promoter methylation (138, 139). Interestingly, gastric tumors with the expression of PVT1 Sv-214 in colorectal and gastric seem to achieve TrkC inactivation through miR128-mediated primary tumor samples. As depicted in Figure 6, the central transcript degradation. interacting motif between PVT1 Sv-214 and miR-128 is present in several transcript variants. However, the additional contact sites between the lncRNA and the miRNA pointed out by miR-214 the authors are not always present in these variants or Bioinformatics analysis also allowed the identification of PVT1 alternatively differ in length from the central motif. Therefore, as a sponge for miR-214-3p. This miRNA negatively regulates the capacity of these PVT1 isoforms to sponge miR-128 should be the expression of IRS1 (Insulin receptor substrate 1) in the colon confirmed experimentally. miR-128 targets the Lin28 transcript cancer cell line HCT116 (107). IRS1 is a key signaling mediator for degradation in colorectal cancer cell lines (68). Lin28 is an of the insulin/insulin-like growth factor (IGF) system controlling Frontiers in Oncology | www.frontiersin.org 10 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer cellular proliferation, differentiation, EMT, and apoptosis in colorectal cancer (140, 141). Upon phosphorylation by the insulin receptor, IRS1 binds specifically to cellular proteins containing SH2 domains such as phosphatidylinositol 3-kinase (PI3K) regulatory subunit (p85) or GRB2, leading to activation of PI3K and MAPK signaling pathways, respectively (142, 143) (Figure 5). Accordingly, both PVT1 and miR-214-3p sustained the expression and activation of PI3K catalytic subunit (p110) and Akt in colon cancer cell line models (107). No reports have described so far PVT1 binding to miR-214 in gastric cancer. FIGURE 7 | Epigenetic regulation by PVT1. PVT1 recruits EZH2 polycomb However, this might not contribute for stomach carcinogenesis, group protein leading to promoter methylation and transcriptional repression of CDKN2A (p16) and CDKN2B (p15). as miR-214 displays an oncogenic role rather than tumor suppressor in gastric epithelial tumor cells. Specifically, miR-214 was been found upregulated in gastric tumors and sustains cell proliferation, migration and invasion by targeting GSK3β, PTEN, Epigenetic Regulation PRDM16, and A2AR (144, 145). LncRNAs are important epigenetic regulators and in turn, perturbations of epigenetic regulation are thought to be a key feature of many cancers and even driver events (148). Twenty Protein Stabilization percent of all human lncRNAs have been shown to physically LncRNA can also interact with proteins and regulate their associate with Polycomb Repressive Complex 2 (PRC2 complex), stability by preventing post-translational modifications which operates as a transcriptional repressor (149, 150). PVT1 associated with protein degradation. Specifically, PVT1 has has been shown to recruit Enhancer of Zeste homolog 2 been shown to bind and stabilize some proteins that are relevant (EZH2) to epigenetically negatively regulate the expression of in the gastrointestinal tumorigenesis, such as STAT3 and Lin28. p15 and p16 in gastric cancer cell lines (78). Interestingly, in primary gastric tumors PVT1 correlates positively with EZH2 STAT3 and negatively with p15/p16 protein levels (78) (Figure 7). PVT1 has been shown to interact with transcription factor STAT3 in gastric cancer cell lines. Binding of PVT1 to STAT3 restricted STAT3 ubiquitin-proteasomal degradation leading to sustained PVT1 AS A PROGNOSTIC BIOMARKER phosphorylation and nuclear translocation of the transcription PVT1 is clearly dysregulated in gastric and colorectal cancer, factor (77) (Figure 5). As described before, STAT3 has a strong and experimental mechanistic studies convincingly demonstrate role supporting tumor progression and therefore, significant that elevated levels of this lncRNA promote proliferation, efforts have been aimed at the development of specific and angiogenesis and metastasis in human malignancies. non-toxic inhibitors for cancer treatment (146, 147). Consistently, possible associations between PVT1 levels and patient prognosis have been investigated by several independent LIN28 groups. However, as mentioned before, PVT1 exhibits a RNA immunoprecipitation followed by mass spectrometry significant variability in transcript expression and this has not identified Lin28 as a protein interacting directly with PVT1 been rigorously considered when interrogating PVT1 levels in Sv-214 (68). Lin28-PVT1 complex formation prevented Lin28 primary tumors. Most of the studies have determined PVT1 degradation by the proteasome, increasing the steady-state levels expression by means of qPCR-based techniques, and surprisingly of this RNA-binding protein. Consistently, Lin28 protein levels none of them directly assessed the expression of PVT1 Sv-217, were reduced in vitro and in vivo when PVT1 was knocked which according to datasets available in public repositories such down using siRNAs technology, and increased when PVT1 was as TCGA, is the isoform predominantly expressed in colorectal overexpressed (Figure 5). The interaction between Lin28 and and stomach tumors (Figure 1) (151). Additionally, primers PVT1 is known to take place in the cytoplasm and to involve targeting multiple isoforms have been used in most of the the 3′ region of PVT1 Sv-214. Although the specific binding studies, which might account for the differences observed in the site has not been mapped, it is known to be contained within clinicopathological features correlating with PVT1 expression. nucleotides ranging from position 672 and 922 in this PVT1 In colorectal cancer, the expression of PVT1 isoform Sv-214 isoform and encoded by a small portion of exon 17 and full was found to be positively associated with tumor size, advanced exons 18 and 19. Therefore, besides PVT1 Sv-214, only PVT1 stage (III-IV), distant metastasis, and reduced overall and Sv-217 could potentially interact with Lin28. Altogether, Lin28 disease-free survival (70). PVT1 Sv-203 when amplified with is regulated by PVT1 through two independent mechanisms: qPCR primers that could co-amplify Sv-202 and Sv-211 (66) (two (1) post-transcriptionally, preventing Lin28 transcript targeting isoforms that according to genome-wide RNAseq profiling have by mir-128, as described above, and; (2) post-translationally, no detectable expression in colorectal malignancies) correlates reducing Lin28 protein degradation by the proteasome with advanced stage (III-IV), lymph node metastasis, venous machinery (68). invasion, and decreased overall survival (66, 70). However, the Frontiers in Oncology | www.frontiersin.org 11 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer expression of the same PVT1 isoform Sv-203, when determined (70, 157). Likewise, BGC823 and SGC7901 gastric cancer cell together with the expression of Sv-206 and Sv-212 (two isoforms line models resistant to cisplatin exposure, exhibited increased that are moderately expressed in colorectal tumors), was not levels of PVT1 (108). Of note, all the cell lines used in these associated with the clinicopathological features described above studies were all MYC amplified, and in the case of the colorectal (69). Additional studies assessing PVT1 overall levels by means cancer cell lines, also PVT1 amplified. It might have been very of high-throughput technologies such as microarrays or RNA interesting to investigate to what extent elevated PVT1 levels by sequencing, observed that high expression values were associated copy-number gain, are required for drug resistance. Interference with shorter patient overall survival (67, 152). Similarly, in of PVT1 expression using siRNA technology in cisplatin resistant gastric cancer, elevated PVT1 expression was found to be cellular models resulted in a reduction of cell proliferation and associated with high tumor stage (III–IV) (75–78), lymph node engagement of apoptosis (70, 108, 157). Mechanistically, some of metastasis (76) and overall/disease-free survival (78). PVT1 Sv- these effects could be attributed to MDR1 (Multidrug resistance 214 specific evaluation in gastric cancer patients was associated protein 1B) and MRP1 (Multidrug resistance-associated protein with larger tumor size (5 cm cut off), advanced stage (III-IV) 1), which have been shown in vitro to modify their expression and reduced overall survival (79). Collectively, the available at mRNA and protein level in a PTV1-dependent manner (70, literature supports the contention that elevated expression of 108, 157). Both proteins are plasma membrane efflux pumps PVT1 is associated with poor prognosis of gastrointestinal controlling the intracellular bioavailability of substrates at the cancer patients, but additional isoform-specific studies expense of ATP hydrolysis, and are well-known chemotherapy- are warranted. resistance mediators in cancer (159). PVT1 AND RESISTANCE TO THERAPEUTIC CONCLUSIONS AND FUTURE AGENTS PERSPECTIVES Current treatment for locally advanced colorectal and stomach PVT1 is an oncogenic lncRNA in many cancer types, including cancer involves surgical resection of the primary tumor colorectal and stomach cancer. This lncRNA is involved in followed by adjuvant chemotherapy to lower the risk of multifaceted aspects of cancer biology such tumor growth, tumor recurrence. Certain patients are also eligible for metastasis, and response to therapeutic agents through a complex preoperative or neoadjuvant therapy, being radiation-based signaling network that involves interactions with DNA, RNA, treatment the best option for rectal cancer. Several drugs and proteins. PVT1 complexity starts at the transcriptional are approved for the treatment of gastrointestinal cancer, level. Twenty-five splice variants have been described for PVT1 but nowadays ECF chemotherapy (epirubicin, cisplatin, and according the latest release of Ensembl database, being 11 5-fluorouracil) and FOLFOX/FOLFIRI/FOLFOXIRI regimens of them detectable in colorectal and gastric normal/tumor (5-fluorouracil, leucovorin, oxaliplatin, and/or irinotecan) tissues with the current sequencing platforms. Surprisingly, this constitute the standard of care for stomach and colorectal complexity has been largely neglected when addressing PVT1 cancer, respectively (12–15). Intrinsic and acquired resistance expression different in tissues. This is of special relevance if as to therapeutic agents is the first cause of treatment failure in postulated by some authors, PVT1 is intended to be used as cancer patients (153). Therefore, identification of the molecular a biomarker for diagnosis, personalized therapeutic treatment, pathways driving the resistance to chemotherapeutic drugs or even a new therapeutic target. Currently, there are no and the screening of drug response biomarkers in cancer reports associating PVT1 up-regulation with well-established and patients is of paramount importance to maximize response clinically relevant molecular features of colorectal and gastric rates (154–156). tumors such as microsatellite stability (MSI/MSS), CpG island Several studies have investigated the expression of PVT1 methylator phenotype (CIMP+/CIMP−), consensus molecular in colorectal and gastric cancer patients treated with different subtypes (CMS1, CMS2, CMS3, and CMS4 for colorectal cancer, regimens. PVT1 was found upregulated using quantitative RT- and MSI, EBV, CIN, GS for gastric cancer) or mutations in PCR in colorectal cancer patients refractory to 5-fluorouracil (5- key driver genes (APC, CTNNB1, SMAD4, TGFB1, TP53, KRAS, FU) in neoadyuvancy (primary resistance), and also in colorectal PI3K, CDKN2A, CDH1, ERBB2, or RHOA). and gastric cancer patients receiving but not benefiting from Experimentally, more efforts should be invested in the cisplatin (70, 108, 157). These findings were consistent with study of the primary, secondary, and higher-order structure the results obtained using cellular models of acquired drug of PVT1 transcripts and PVT1 promoter region, which is resistance in vitro. HCT116 colon cancer cells resistant to the crucial for understanding the function of this lncRNA at a combination of 5-FU and radiation displayed increased levels molecular level. The role of PVT1 in colorectal and gastric of PVT1 compared to the parental cell line when evaluated cancer has been addressed using gain-and-loss of function using a human lncRNA Expression Array (158). Assessment of experiments using gene overexpression and RNA interference PVT1 transcript levels by quantitative RT-PCR in LOVO and approaches in cell line systems, respectively. PVT1 encodes RKO colon cancer cell lines with acquired resistance to cisplatin, multiple miRNAs in intronic regions. The nuclear localization or HCT8 and HCT116 with secondary resistance to 5-FU also of spliced intronic sequences prevents optimal targeting by the evidenced the transcriptional upregulation of PVT1 lncRNA RNAi machinery, which is mainly localized to the cytoplasm. Frontiers in Oncology | www.frontiersin.org 12 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer Alternative oligo-mediated RNA knockdown strategies, such AUTHOR CONTRIBUTIONS as modified antisense oligonucleotides or gapmers, should be used to block PVT1 miRNA-mediated effects. Ideally, knock- AM-B, DA, and HD wrote the manuscript. out/knock-in mouse models should be used to better assess the role of PVT1 in cancer biology. However, compared to protein FUNDING coding genes, very few animal models have been generated to assess the function of lncRNAs in vivo. Unfortunately, many This study was partially funded by grants of the Spanish Ministry lncRNAs are not highly conserved hindering the identification for Economy and Competitiveness (PI16/00540, AC15/00066, of mouse orthologs and full function evaluation still represents AC19/00095, and PI19/00993) and the Spanish Association a challenge. Against Cancer (AECC GCA15152966ARAN) to DA. REFERENCES 17. Morris KV, Mattick JS. The rise of regulatory RNA. Nat Rev Genet. (2014) 15:423–37. doi: 10.1038/nrg3722 1. International Agency for Research on Cancer. Global Cancer Observatory. 18. Consortium EP. An integrated encyclopedia of DNA elements in the human (2018). Available online at: https://gco.iarc.fr/ (accessed April 18, 2019). genome. Nature. (2012) 489:57–74. doi: 10.1038/nature11247 2. Bray F, Ferlay J, Soerjomataram I, Siegel RL, Torre LA, Jemal A. Global 19. Esteller M. Non-coding RNAs in human disease. Nat Rev Genet. (2011) cancer statistics 2018: GLOBOCAN estimates of incidence and mortality 12:861–74. doi: 10.1038/nrg3074 worldwide for 36 cancers in 185 countries. Cancer J Clin. (2018) 68:394–424. 20. Huntzinger E, Izaurralde E. Gene silencing by microRNAs: contributions of doi: 10.3322/caac.21492 translational repression and mRNA decay. Nat Rev Genet. (2011) 12:99–110. 3. Ferlay J, Colombet M, Soerjomataram I, Mathers C, Parkin DM, Pineros doi: 10.1038/nrg2936 M, et al. Estimating the global cancer incidence and mortality in 2018: 21. Jonas S, Izaurralde E. Towards a molecular understanding of GLOBOCAN sources and methods. Int J Cancer. (2019) 144:1941–53. microRNA-mediated gene silencing. Nat Rev Genet. (2015) 16:421–33. doi: 10.1002/ijc.31937 doi: 10.1038/nrg3965 4. Rivera C, Venegas B. Histological and molecular aspects of oral squamous 22. Croce CM. Causes and consequences of microRNA dysregulation in cancer. cell carcinoma (Review). Oncol Lett. (2014) 8:7–11. doi: 10.3892/ol.2014.2103 Nat Rev Genet. (2009) 10:704–14. doi: 10.1038/nrg2634 5. Smyth EC, Lagergren J, Fitzgerald RC, Lordick F, Shah MA, Lagergren 23. Rupaimoole R, Slack FJ. MicroRNA therapeutics: towards a new era for P, et al. Oesophageal cancer. Nat Rev Dis Primers. (2017) 3:17048. the management of cancer and other diseases. Nat Rev Drug Discov. (2017) doi: 10.1038/nrdp.2017.48 16:203–22. doi: 10.1038/nrd.2016.246 6. Lagergren J, Smyth E, Cunningham D, Lagergren P. Oesophageal cancer. 24. van Rooij E, Kauppinen S. Development of microRNA Lancet. (2017) 390:2383–96. doi: 10.1016/S0140-6736(17)31462-9 therapeutics is coming of age. EMBO Mol Med. (2014) 6:851–64. 7. Cancer Genome Atlas Research N. Comprehensive molecular doi: 10.15252/emmm.201100899 characterization of gastric adenocarcinoma. Nature. (2014) 513:202–9. 25. Kong YW, Ferland-McCollough D, Jackson TJ, Bushell M. doi: 10.1038/nature13480 microRNAs in cancer management. Lancet Oncol. (2012) 13:e249–58. 8. Nagini S. Carcinoma of the stomach: a review of epidemiology, pathogenesis, doi: 10.1016/S1470-2045(12)70073-6 molecular genetics and chemoprevention. World J Gastrointest Oncol. (2012) 26. Harrow J, Frankish A, Gonzalez JM, Tapanari E, Diekhans M, 4:156–69. doi: 10.4251/wjgo.v4.i7.156 Kokocinski F, et al. GENCODE: the reference human genome 9. Ashktorab H, Kupfer SS, Brim H, Carethers JM. Racial disparity annotation for The ENCODE project. Genome Res. (2012) 22:1760–74. in gastrointestinal cancer risk. Gastroenterology. (2017) 153:910–23. doi: 10.1101/gr.135350.111 doi: 10.1053/j.gastro.2017.08.018 27. Frankish A, Diekhans M, Ferreira AM, Johnson R, Jungreis I, Loveland J, 10. Kim HP, Cho GA, Han SW, Shin JY, Jeong EG, Song SH, et al. Novel fusion et al. GENCODE reference annotation for the human and mouse genomes. transcripts in human gastric cancer revealed by transcriptome analysis. Nucleic Acids Res. (2019) 47:D766–73. doi: 10.1093/nar/gky955 Oncogene. (2014) 33:5434–41. doi: 10.1038/onc.2013.490 28. Uszczynska-Ratajczak B, Lagarde J, Frankish A, Guigo R, 11. Kuipers EJ, Grady WM, Lieberman D, Seufferlein T, Sung JJ, Boelens Johnson R. Towards a complete map of the human long non- PG, et al. Colorectal cancer. Nat Rev Dis Primers. (2015) 1:15065. coding RNA transcriptome. Nat Rev Genet. (2018) 19:535–48. doi: 10.1038/nrdp.2015.65 doi: 10.1038/s41576-018-0017-y 12. Smyth EC, Verheij M, Allum W, Cunningham D, Cervantes A, 29. GENCODE. Available from: https://www.gencodegenes.org. Arnold D, et al. Gastric cancer: ESMO clinical practice guidelines for 30. Huarte M. The emerging role of lncRNAs in cancer. Nat Med. (2015) diagnosis, treatment and follow-up. Ann Oncol. (2016) 27(Suppl 5):v38–49. 21:1253–61. doi: 10.1038/nm.3981 doi: 10.1093/annonc/mdw350 31. Sanchez Calle A, Kawamura Y, Yamamoto Y, Takeshita F, Ochiya T. Emerging 13. Labianca R, Nordlinger B, Beretta GD, Mosconi S, Mandala M, Cervantes roles of long non-coding RNA in cancer. Cancer Sci. (2018) 109:2093–100. A, et al. Early colon cancer: ESMO clinical practice guidelines for doi: 10.1111/cas.13642 diagnosis, treatment and follow-up. Ann Oncol. (2013) 24(Suppl 6):vi64–72. 32. Lin C, Yang L. Long noncoding RNA in cancer: wiring signaling circuitry. doi: 10.1093/annonc/mdt354 Trends Cell Biol. (2018) 28:287–301. doi: 10.1016/j.tcb.2017.11.008 14. Van Cutsem E, Cervantes A, Nordlinger B, Arnold D, Group EGW. 33. Mercer TR, Dinger ME, Mattick JS. Long non-coding RNAs: insights into Metastatic colorectal cancer: ESMO clinical practice guidelines for functions. Nat Rev Genet. (2009) 10:155–9. doi: 10.1038/nrg2521 diagnosis, treatment and follow-up. Ann Oncol. (2014) 25(Suppl 3):iii1–9. 34. Wang KC, Chang HY. Molecular mechanisms of long noncoding RNAs. Mol doi: 10.1093/annonc/mdu260 Cell. (2011) 43:904–14. doi: 10.1016/j.molcel.2011.08.018 15. Glynne-Jones R, Wyrwicz L, Tiret E, Brown G, Rodel C, Cervantes A, 35. Shen P, Pichler M, Chen M, Calin GA, Ling H. To Wnt or lose: the et al. Rectal cancer: ESMO clinical practice guidelines for diagnosis, missing non-coding linc in colorectal cancer. Int J Mol Sci. (2017) 18:E2003. treatment and follow-up. Ann Oncol. (2018) 29(Suppl 4):iv263. doi: 10.3390/ijms18092003 doi: 10.1093/annonc/mdy161 36. Arun G, Diermeier SD, Spector DL. Therapeutic targeting of long 16. Anastasiadou E, Jacob LS, Slack FJ. Non-coding RNA networks in cancer. non-coding RNAs in cancer. Trends Mol Med. (2018) 24:257–77. Nat Rev Cancer. (2018) 18:5–18. doi: 10.1038/nrc.2017.99 doi: 10.1016/j.molmed.2018.01.001 Frontiers in Oncology | www.frontiersin.org 13 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer 37. Qi P, Du X. The long non-coding RNAs, a new cancer diagnostic competitively binding microRNA-152. Oncotarget. (2016) 7:62886–97. and therapeutic gold mine. Mod Pathol. (2013) 26:155–65. doi: 10.18632/oncotarget.11709 doi: 10.1038/modpathol.2012.160 60. Li T, Meng XL, Yang WQ. Long noncoding RNA PVT1 acts as a “sponge” 38. Bhan A, Soleimani M, Mandal SS. Long noncoding RNA to inhibit microRNA-152 in gastric cancer cells. Digest Dis Sci. (2017) and cancer: a new paradigm. Cancer Res. (2017) 77:3965–81. 62:3021–8. doi: 10.1007/s10620-017-4508-z doi: 10.1158/0008-5472.CAN-16-2634 61. Zhou Q, Chen F, Zhao J, Li B, Liang Y, Pan W, et al. Long non- 39. Cabili MN, Trapnell C, Goff L, Koziol M, Tazon-Vega B, Regev A, et al. coding RNA PVT1 promotes osteosarcoma development by acting as a Integrative annotation of human large intergenic noncoding RNAs reveals molecular sponge to regulate miR-195. Oncotarget. (2016) 7:82620–33. global properties and specific subclasses. Genes Dev. (2011) 25:1915–27. doi: 10.18632/oncotarget.13012 doi: 10.1101/gad.17446611 62. Wu D, Li Y, Zhang H, Hu X. Knockdown of Lncrna PVT1 enhances 40. Bolha L, Ravnik-Glavac M, Glavac D. Long noncoding RNAs as biomarkers radiosensitivity in non-small cell lung cancer by sponging Mir-195. Cell in cancer. Dis Markers. (2017) 2017:7243968. doi: 10.1155/2017/7243968 Physiol Biochem. (2017) 42:2453–66. doi: 10.1159/000480209 41. Cory S, Graham M, Webb E, Corcoran L, Adams JM. Variant (6;15) 63. Shen CJ, Cheng YM, Wang CL. LncRNA PVT1 epigenetically silences translocations in murine plasmacytomas involve a chromosome 15 locus miR-195 and modulates EMT and chemoresistance in cervical cancer at least 72 kb from the c-myc oncogene. EMBO J. (1985) 4:675–81. cells. J Drug Target. (2017) 25:637–44. doi: 10.1080/1061186X.2017. doi: 10.1002/j.1460-2075.1985.tb03682.x 1307379 42. Graham M, Adams JM. Chromosome 8 breakpoint far 3’ of the c- 64. Kong F, Ma J, Yang H, Yang D, Wang C, Ma X. Long non-coding RNA myc oncogene in a Burkitt’s lymphoma 2;8 variant translocation is PVT1 promotes malignancy in human endometrial carcinoma cells through equivalent to the murine pvt-1 locus. EMBO J. (1986) 5:2845–51. negative regulation of miR-195-5p. Biochim Biophys Acta Mol Cell Res. (2018) doi: 10.1002/j.1460-2075.1986.tb04578.x 1865:1479–90. doi: 10.1016/j.bbamcr.2018.07.008 43. Boerma EG, Siebert R, Kluin PM, Baudis M. Translocations involving 8q24 65. Li PD, Hu JL, Ma C, Ma H, Yao J, Chen LL, et al. Upregulation in Burkitt lymphoma and other malignant lymphomas: a historical review of of the long non-coding RNA PVT1 promotes esophageal squamous cytogenetics in the light of todays knowledge. Leukemia. (2009) 23:225–34. cell carcinoma progression by acting as a molecular sponge of miR- doi: 10.1038/leu.2008.281 203 and LASP1. Oncotarget. (2017) 8:34164–76. doi: 10.18632/oncotarget. 44. Nagoshi H, Taki T, Hanamura I, Nitta M, Otsuki T, Nishida K, et al. Frequent 15878 PVT1 rearrangement and novel chimeric genes PVT1-NBEA and PVT1- 66. Takahashi Y, Sawada G, Kurashige J, Uchi R, Matsumura T, Ueo WWOX occur in multiple myeloma with 8q24 abnormality. Cancer Res. H, et al. Amplification of PVT-1 is involved in poor prognosis via (2012) 72:4954–62. doi: 10.1158/0008-5472.CAN-12-0213 apoptosis inhibition in colorectal cancers. Br J Cancer. (2014) 110:164–71. 45. Cui M, You L, Ren X, Zhao W, Liao Q, Zhao Y. Long non-coding doi: 10.1038/bjc.2013.698 RNA PVT1 and cancer. Biochem Biophys Res Commun. (2016) 471:10–4. 67. Wang C, Zhu X, Pu C, Song X. Upregulated plasmacytoma doi: 10.1016/j.bbrc.2015.12.101 variant translocation 1 promotes cell proliferation, invasion and 46. Cerami E, Gao J, Dogrusoz U, Gross BE, Sumer SO, Aksoy BA, metastasis in colorectal cancer. Mol Med Rep. (2018) 17:6598–604. et al. The cBio cancer genomics portal: an open platform for exploring doi: 10.3892/mmr.2018.8669 multidimensional cancer genomics data. Cancer Discov. (2012) 2:401–4. 68. He F, Song Z, Chen H, Chen Z, Yang P, Li W, et al. Long noncoding doi: 10.1158/2159-8290.CD-12-0095 RNA PVT1-214 promotes proliferation and invasion of colorectal cancer by 47. cBioPortal for Cancer Genomics. Available from: http://www.cbioportal. stabilizing Lin28 and interacting with miR-128. Oncogene. (2018) 38:164–79. org/. doi: 10.1038/s41388-018-0432-8 48. Yao L, Tak YG, Berman BP, Farnham PJ. Functional annotation of colon 69. Ozawa T, Matsuyama T, Toiyama Y, Takahashi N, Ishikawa T, Uetake H, et al. cancer risk SNPs. Nat Commun. (2014) 5:5114. doi: 10.1038/ncomms6114 CCAT1 and CCAT2 long noncoding RNAs, located within the 8q.24.21 ’gene 49. Huppi K, Volfovsky N, Runfola T, Jones TL, Mackiewicz M, Martin desert’, serve as important prognostic biomarkers in colorectal cancer. Ann SE, et al. The identification of microRNAs in a genomically unstable Oncol. (2017) 28:1882–8. doi: 10.1093/annonc/mdx248 region of human chromosome 8q24. Mol Cancer Res. (2008) 6:212–21. 70. Ping G, Xiong W, Zhang L, Li Y, Zhang Y, Zhao Y. Silencing long noncoding doi: 10.1158/1541-7786.MCR-07-0105 RNA PVT1 inhibits tumorigenesis and cisplatin resistance of colorectal 50. Barsotti AM, Beckerman R, Laptenko O, Huppi K, Caplen NJ, Prives C. cancer. Am J Transl Res. (2018) 10:138–49. p53-Dependent induction of PVT1 and miR-1204. J Biol Chem. (2012) 71. Guo K, Yao J, Yu Q, Li Z, Huang H, Cheng J, et al. The expression pattern 287:2509–19. doi: 10.1074/jbc.M111.322875 of long non-coding RNA PVT1 in tumor tissues and in extracellular vesicles 51. Beck-Engeser GB, Lum AM, Huppi K, Caplen NJ, Wang BB, Wabl of colorectal cancer correlates with cancer progression. Tumour Biol. (2017) M. Pvt1-encoded microRNAs in oncogenesis. Retrovirology. (2008) 5:4. 39:1010428317699122. doi: 10.1177/1010428317699122 doi: 10.1186/1742-4690-5-4 72. Chai J, Guo D, Ma W, Han D, Dong W, Guo H, et al. A feedback 52. Chen L, Lu MH, Zhang D, Hao NB, Fan YH, Wu YY, et al. miR- loop consisting of RUNX2/LncRNA-PVT1/miR-455 is involved in the 1207-5p and miR-1266 suppress gastric cancer growth and invasion by progression of colorectal cancer. Am J Cancer Res. (2018) 8:538–50. targeting telomerase reverse transcriptase. Cell Death Dis. (2014) 5:e1034. 73. Zhang R, Li J, Yan X, Jin K, Li W, Liu X, et al. Long noncoding doi: 10.1038/cddis.2013.553 RNA plasmacytoma variant translocation 1 (PVT1) promotes colon cancer 53. Volders PJ, Anckaert J, Verheggen K, Nuytens J, Martens L, Mestdagh P, progression via endogenous sponging mir-26b. Med Sci Monitor. (2018) et al. LNCipedia 5: towards a reference set of human long non-coding RNAs. 24:8685–92. doi: 10.12659/MSM.910955 Nucleic Acids Res. (2019) 47:D135–9. doi: 10.1093/nar/gky1031 74. Gharib E, Anaraki F, Baghdar K, Ghavidel P, Sadeghi H, Nasrabadi PN, et al. 54. Ensembl Genome Browser. Available from: https://www.ensembl.org/ Investigating the diagnostic performance of HOTTIP, PVT1, and UCA1 long 55. Genotype-Tissue Expression (GTEx) project. Available from: https:// noncoding RNAs as a predictive panel for the screening of colorectal cancer gtexportal.org. patients with lymph node metastasis. J Cell Biochem. (2019) 120:14780–90. 56. Consortium GT. The genotype-tissue expression (GTEx) project. Nat Genet. doi: 10.1002/jcb.28739 (2013) 45:580–5. doi: 10.1038/ng.2653 75. Xu MD, Wang Y, Weng W, Wei P, Qi P, Zhang Q, et al. A 57. The Cancer Genome Atlas (TCGA). Available from: https://cancergenome. positive feedback loop of lncRNA-PVT1 and FOXM1 facilitates gastric nih.gov cancer growth and invasion. Clin Cancer Res. (2017) 23:2071–80. 58. Cancer Genome Atlas N. Comprehensive molecular characterization doi: 10.1158/1078-0432.CCR-16-0742 of human colon and rectal cancer. Nature. (2012) 487:330–7. 76. Huang T, Liu HW, Chen JQ, Wang SH, Hao LQ, Liu M, et al. The long doi: 10.1038/nature11252 noncoding RNA PVT1 functions as a competing endogenous RNA by 59. Zheng J, Yu F, Dong P, Wu L, Zhang Y, Hu Y, et al. Long sponging miR-186 in gastric cancer. Biomed Pharmacother. (2017) 88:302–8. non-coding RNA PVT1 activates hepatic stellate cells through doi: 10.1016/j.biopha.2017.01.049 Frontiers in Oncology | www.frontiersin.org 14 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer 77. Zhao J, Du P, Cui P, Qin Y, Hu C, Wu J, et al. LncRNA PVT1 promotes 99. Colombo T, Farina L, Macino G, Paci P. PVT1: a rising star among angiogenesis via activating the STAT3/VEGFA axis in gastric cancer. oncogenic long noncoding RNAs. BioMed Res Int. (2015) 2015:304208. Oncogene. (2018) 37:4094–109. doi: 10.1038/s41388-018-0250-z doi: 10.1155/2015/304208 78. Kong R, Zhang EB, Yin DD, You LH, Xu TP, Chen WM, et al. Long 100. Schwab M, Klempnauer KH, Alitalo K, Varmus H, Bishop M. Rearrangement noncoding RNA PVT1 indicates a poor prognosis of gastric cancer and at the 5′ end of amplified c-myc in human COLO 320 cells is promotes cell proliferation through epigenetically regulating p15 and p16. associated with abnormal transcription. Mol Cell Biol. (1986) 6:2752–5. Mol Cancer. (2015) 14:82. doi: 10.1186/s12943-015-0355-8 doi: 10.1128/MCB.6.7.2752 79. Zhao S, Fan NF, Chen XH, Zhuo CH, Xu CW, Lin RB. Long noncoding 101. Shtivelman E, Bishop JM. The PVT gene frequently amplifies with MYC in RNA PVT1-214 enhances gastric cancer progression by upregulating TrkC tumor cells. Mol Cell Biol. (1989) 9:1148–54. doi: 10.1128/MCB.9.3.1148 expression in competitively sponging way. Euro Rev Med Pharmacol Sci. 102. Ahmadiyeh N, Pomerantz MM, Grisanzio C, Herman P, Jia L, Almendro (2019) 23:4173–84. doi: 10.26355/eurrev_201905_17920 V, et al. 8q24 prostate, breast, and colon cancer risk loci show tissue- 80. Zeng Y, Wang T, Liu Y, Su Z, Lu P, Chen X, et al. LncRNA PVT1 specific long-range interaction with MYC. Proc Natl Acad Sci USA. (2010) as an effective biomarker for cancer diagnosis and detection based on 107:9742–6. doi: 10.1073/pnas.0910668107 transcriptome data and meta-analysis. Oncotarget. (2017) 8:75455–66. 103. Sotelo J, Esposito D, Duhagon MA, Banfield K, Mehalko J, Liao H, et al. Long- doi: 10.18632/oncotarget.20634 range enhancers on 8q24 regulate c-Myc. Proc Natl Acad Sci USA. (2010) 81. Farhana L, Antaki F, Anees MR, Nangia-Makker P, Judd S, Hadden T, et al. 107:3001–5. doi: 10.1073/pnas.0906067107 Role of cancer stem cells in racial disparity in colorectal cancer. Cancer Med. 104. Cho SW, Xu J, Sun R, Mumbach MR, Carter AC, Chen YG, et al. Promoter (2016) 5:1268–78. doi: 10.1002/cam4.690 of lncRNA gene PVT1 is a tumor-suppressor DNA boundary element. Cell. 82. Wierstra I, Alves J. FOXM1, a typical proliferation-associated transcription (2018) 173:1398–412.e22. doi: 10.1016/j.cell.2018.03.068 factor. Biol Chem. (2007) 388:1257–74. doi: 10.1515/BC.2007.159 105. Pombo A, Dillon N. Three-dimensional genome architecture: players and 83. Katoh M, Igarashi M, Fukuda H, Nakagama H, Katoh M. Cancer genetics mechanisms. Nat Rev Mol Cell Biol. (2015) 16:245–57. doi: 10.1038/nrm3965 and genomics of human FOX family genes. Cancer Lett. (2013) 328:198–206. 106. Parolia A, Cieslik M, Chinnaiyan AM. Competing for enhancers: doi: 10.1016/j.canlet.2012.09.017 PVT1 fine-tunes MYC expression. Cell Res. (2018) 28:785–6. 84. Raychaudhuri P, Park HJ. FoxM1: a master regulator of tumor metastasis. doi: 10.1038/s41422-018-0064-0 Cancer Res. (2011) 71:4329–33. doi: 10.1158/0008-5472.CAN-11-0640 107. Shang AQ, Wang WW, Yang YB, Gu CZ, Ji P, Chen C, et al. 85. Laissue P. The forkhead-box family of transcription factors: key molecular Knockdown of long noncoding RNA PVT1 suppresses cell proliferation players in colorectal cancer pathogenesis. Mol Cancer. (2019) 18:5. and invasion of colorectal cancer via upregulation of microRNA- doi: 10.1186/s12943-019-0938-x 214-3p. Am J Physiol Gastrointest Liver Physiol. (2019) 317:G222–32. 86. Zona S, Bella L, Burton MJ, Nestal de Moraes G, Lam EW. FOXM1: doi: 10.1152/ajpgi.00357.2018 an emerging master regulator of DNA damage response and 108. Zhang XW, Bu P, Liu L, Zhang XZ, Li J. Overexpression of long non- genotoxic agent resistance. Biochim Biophys Acta. (2014) 1839:1316–22. coding RNA PVT1 in gastric cancer cells promotes the development of doi: 10.1016/j.bbagrm.2014.09.016 multidrug resistance. Biochem Biophys Res Commun. (2015) 462:227–32. 87. Yu H, Pardoll D, Jove R. STATs in cancer inflammation and doi: 10.1016/j.bbrc.2015.04.121 immunity: a leading role for STAT3. Nat Rev Cancer. (2009) 9:798–809. 109. Carmeliet P. VEGF as a key mediator of angiogenesis in cancer. Oncology. doi: 10.1038/nrc2734 (2005) 69(Suppl 3):4–10. doi: 10.1159/000088478 88. Pfister NT, Prives C. Transcriptional regulation by wild-type and cancer- 110. Sitohy B, Nagy JA, Dvorak HF. Anti-VEGF/VEGFR therapy for related mutant forms of p53. Cold Spring Harb Perspect Med. (2017) 7:26054. cancer: reassessing the target. Cancer Res. (2012) 72:1909–14. doi: 10.1101/cshperspect.a026054 doi: 10.1158/0008-5472.CAN-11-3406 89. Li XL, Zhou J, Chen ZR, Chng WJ. P53 mutations in colorectal cancer 111. Lupo G, Caporarello N, Olivieri M, Cristaldi M, Motta C, Bramanti V, - molecular pathogenesis and pharmacological reactivation. World J et al. Anti-angiogenic therapy in cancer: downsides and new pivots for Gastroenterol. (2015) 21:84–93. doi: 10.3748/wjg.v21.i1.84 precision medicine. Front Pharmacol. (2016) 7:519. doi: 10.3389/fphar.2016. 90. Martincorena I, Raine KM, Gerstung M, Dawson KJ, Haase K, Van Loo 00519 P, et al. Universal patterns of selection in cancer and somatic tissues. Cell. 112. Nienhuser H, Schmidt T. Angiogenesis and anti-angiogenic therapy in (2017) 171:1029–41.e21. doi: 10.1016/j.cell.2017.09.042 gastric cancer. Int J Mol Sci. (2017) 19:43. doi: 10.3390/ijms19010043 91. Bailey MH, Tokheim C, Porta-Pardo E, Sengupta S, Bertrand D, 113. Han J, Xiong J, Wang D, Fu XD. Pre-mRNA splicing: where and when in the Weerasinghe A, et al. Comprehensive characterization of cancer driver genes nucleus. Trends Cell Biol. (2011) 21:336–43. doi: 10.1016/j.tcb.2011.03.003 and mutations. Cell. (2018) 174:1034–5. doi: 10.1016/j.cell.2018.07.034 114. Nandakumar J, Cech TR. Finding the end: recruitment of telomerase to 92. Muller PA, Vousden KH. p53 mutations in cancer. Nat Cell Biol. (2013) telomeres. Nat Rev Mol Cell Biol. (2013) 14:69–82. doi: 10.1038/nrm3505 15:2–8. doi: 10.1038/ncb2641 115. Lu MH, Deng JQ, Cao YL, Fang DC, Zhang Y, Yang SM. Prognostic role 93. Beroukhim R, Mermel CH, Porter D, Wei G, Raychaudhuri S, Donovan of telomerase activity in gastric adenocarcinoma: a meta-analysis. Exp Ther J, et al. The landscape of somatic copy-number alteration across human Med. (2012) 3:728–34. doi: 10.3892/etm.2012.471 cancers. Nature. (2010) 463:899–905. doi: 10.1038/nature08822 116. Fabregat I, Malfettone A, Soukupova J. New insights into the crossroads 94. Huppi K, Pitt JJ, Wahlberg BM, Caplen NJ. The 8q24 gene desert: an between EMT and stemness in the context of cancer. J Clin Med. (2016) oasis of non-coding transcriptional activity. Front Genet. (2012) 3:69. 5:E37. doi: 10.3390/jcm5030037 doi: 10.3389/fgene.2012.00069 117. Balas MM, Johnson AM. Exploring the mechanisms behind long 95. Jin DH, Park SE, Lee J, Kim KM, Kim S, Kim DH, et al. Copy noncoding RNAs and cancer. Noncoding RNA Res. (2018) 3:108–17. number gains at 8q24 and 20q11-q13 in gastric cancer are more common doi: 10.1016/j.ncrna.2018.03.001 in intestinal-type than diffuse-type. PLoS ONE. (2015) 10:e0137657. 118. Zhai R, Kan X, Wang B, Du H, Long Y, Wu H, et al. miR-152 suppresses doi: 10.1371/journal.pone.0137657 gastric cancer cell proliferation and motility by targeting CD151. Tumour 96. Grisanzio C, Freedman ML. Chromosome 8q24-associated cancers and Biol. (2014) 35:11367–73. doi: 10.1007/s13277-014-2471-2 MYC. Genes Cancer. (2010) 1:555–9. doi: 10.1177/1947601910381380 119. Sadej R, Grudowska A, Turczyk L, Kordek R, Romanska HM. CD151 in 97. Flanagan DJ, Vincan E, Phesse TJ. Winding back Wnt signalling: potential cancer progression and metastasis: a complex scenario. Lab Invest. (2014) therapeutic targets for treating gastric cancers. Br J Pharmacol. (2017) 94:41–51. doi: 10.1038/labinvest.2013.136 174:4666–83. doi: 10.1111/bph.13890 120. Akl MR, Nagpal P, Ayoub NM, Tai B, Prabhu SA, Capac CM, et al. Molecular 98. Tseng YY, Moriarity BS, Gong W, Akiyama R, Tiwari A, Kawakami H, and clinical significance of fibroblast growth factor 2 (FGF2 /bFGF) in et al. PVT1 dependence in cancer with MYC copy-number increase. Nature. malignancies of solid and hematological cancers for personalized therapies. (2014) 512:82–6. doi: 10.1038/nature13311 Oncotarget. (2016) 7:44735–62. doi: 10.18632/oncotarget.8203 Frontiers in Oncology | www.frontiersin.org 15 January 2020 | Volume 10 | Article 38 Martínez-Barriocanal et al. PVT1 in Gastrointestinal Cancer 121. Li B, Xie Z, Li B. miR-152 functions as a tumor suppressor in 141. Li H, Batth IS, Qu X, Xu L, Song N, Wang R, et al. IGF-IR signaling in colorectal cancer by targeting PIK3R3. Tumour Biol. (2016) 37:10075–84. epithelial to mesenchymal transition and targeting IGF-IR therapy: overview doi: 10.1007/s13277-016-4888-2 and new insights. Mol Cancer. (2017) 16:6. doi: 10.1186/s12943-016-0576-5 122. Ghazanchaei A, Mansoori B, Mohammadi A, Biglari A, Baradaran B. 142. Taniguchi CM, Emanuelli B, Kahn CR. Critical nodes in signalling pathways: Restoration of miR-152 expression suppresses cell proliferation, survival, and insights into insulin action. Nat Rev Mol Cell Biol. (2006) 7:85–96. migration through inhibition of AKT-ERK pathway in colorectal cancer. J doi: 10.1038/nrm1837 Cell Physiol. (2018) 234:769–76. doi: 10.1002/jcp.26891 143. Metz HE, Houghton AM. Insulin receptor substrate regulation 123. Cao C, Sun D, Zhang L, Song L. miR-186 affects the proliferation, invasion of phosphoinositide 3-kinase. Clin Cancer Res. (2011) 17:206–11. and migration of human gastric cancer by inhibition of Twist1. Oncotarget. doi: 10.1158/1078-0432.CCR-10-0434 (2016) 7:79956–63. doi: 10.18632/oncotarget.13182 144. Yang TS, Yang XH, Wang XD, Wang YL, Zhou B, Song ZS. MiR-214 regulate 124. Liu L, Wang Y, Bai R, Yang K, Tian Z. MiR-186 inhibited aerobic glycolysis gastric cancer cell proliferation, migration and invasion by targeting PTEN. in gastric cancer via HIF-1alpha regulation. Oncogenesis. (2016) 5:e224. Cancer Cell Int. (2013) 13:68. doi: 10.1186/1475-2867-13-68 doi: 10.1038/oncsis.2016.35 145. Li HL, Liang S, Cui JH, Han GY. Targeting of GSK-3beta by miR- 125. Majmundar AJ, Wong WJ, Simon MC. Hypoxia-inducible factors 214 to facilitate gastric cancer cell proliferation and decrease of and the response to hypoxic stress. Mol Cell. (2010) 40:294–309. cell apoptosis. Euro Rev Med Pharmacol Sci. (2018) 22:127–34. doi: 10.1016/j.molcel.2010.09.022 doi: 10.26355/eurrev_201801_14109 126. Yoshida Y, Takahashi K, Okita K, Ichisaka T, Yamanaka S. Hypoxia enhances 146. Yu H, Lee H, Herrmann A, Buettner R, Jove R. Revisiting STAT3 signalling the generation of induced pluripotent stem cells. Cell Stem Cell. (2009) in cancer: new and unexpected biological functions. Nat Rev Cancer. (2014) 5:237–41. doi: 10.1016/j.stem.2009.08.001 14:736–46. doi: 10.1038/nrc3818 127. Li J, Xia L, Zhou Z, Zuo Z, Xu C, Song H, et al. MiR-186-5p upregulation 147. Furqan M, Akinleye A, Mukhi N, Mittal V, Chen Y, Liu D. STAT inhibitors for inhibits proliferation, metastasis and epithelial-to-mesenchymal transition cancer therapy. J Hematol Oncol. (2013) 6:90. doi: 10.1186/1756-8722-6-90 of colorectal cancer cell by targeting ZEB1. Arch Biochem Biophys. (2018) 148. Morlando M, Fatica A. Alteration of epigenetic regulation by long noncoding 640:53–60. doi: 10.1016/j.abb.2018.01.002 RNAs in cancer. Int J Mol Sci. (2018) 19:E570. doi: 10.3390/ijms19020570 128. Islam F, Gopalan V, Vider J, Wahab R, Ebrahimi F, Lu CT, et al. MicroRNA- 149. Khalil AM, Guttman M, Huarte M, Garber M, Raj A, Rivea Morales D, et al. 186-5p overexpression modulates colon cancer growth by repressing the Many human large intergenic noncoding RNAs associate with chromatin- expression of the FAM134B tumour inhibitor. Exp Cell Res. (2017) 357:260– modifying complexes and affect gene expression. Proc Natl Acad Sci USA. 70. doi: 10.1016/j.yexcr.2017.05.021 (2009) 106:11667–72. doi: 10.1073/pnas.0904715106 129. Mazeh H, Mizrahi I, Ilyayev N, Halle D, Brucher B, Bilchik A, et al. 150. Comet I, Riising EM, Leblanc B, Helin K. Maintaining cell identity: PRC2- The diagnostic and prognostic role of microRNA in colorectal cancer - a mediated regulation of transcription and cancer. Nat Rev Cancer. (2016) comprehensive review. J Cancer. (2013) 4:281–95. doi: 10.7150/jca.5836 16:803–10. doi: 10.1038/nrc.2016.83 130. Wang J, Lu Y, Zeng Y, Zhang L, Ke K, Guo Y. Expression profile and 151. The UCSC Xena Browser. Available from: https://xenabrowser.net/ biological function of miR-455-5p in colorectal carcinoma. Oncol Lett. (2019) transcripts/. 17:2131–40. doi: 10.3892/ol.2018.9862 152. Ansari H, Shahrisa A, Birgani YT, Birgani MT, Hajjari M, Asl JM. Long 131. Zhang J, Roberts TM, Shivdasani RA. Targeting PI3K signaling as a noncoding RNAs in colorectal adenocarcinoma; an in silico analysis. Pathol therapeutic approach for colorectal cancer. Gastroenterology. (2011) 141:50– Oncol Res. (2018) 25:1387–94. doi: 10.1007/s12253-018-0428-2 61. doi: 10.1053/j.gastro.2011.05.010 153. Holohan C, Van Schaeybroeck S, Longley DB, Johnston PG. Cancer 132. Ning T, Peng Z, Li S, Qu Y, Zhang H, Duan J, et al. miR-455 inhibits drug resistance: an evolving paradigm. Nat Rev Cancer. (2013) 13:714–26. cell proliferation and migration via negative regulation of EGFR in doi: 10.1038/nrc3599 human gastric cancer. Oncol Rep. (2017) 38:175–82. doi: 10.3892/or.2017. 154. Hammond WA, Swaika A, Mody K. Pharmacologic resistance in 5657 colorectal cancer: a review. Ther Adv Med Oncol. (2016) 8:57–84. 133. Liu J, Zhang J, Li Y, Wang L, Sui B, Dai D. MiR-455-5p acts as a novel doi: 10.1177/1758834015614530 tumor suppressor in gastric cancer by down-regulating RAB18. Gene. (2016) 155. Van der Jeught K, Xu HC, Li YJ, Lu XB, Ji G. Drug resistance and new 592:308–15. doi: 10.1016/j.gene.2016.07.034 therapies in colorectal cancer. World J Gastroenterol. (2018) 24:3834–48. 134. Balzeau J, Menezes MR, Cao S, Hagan JP. The LIN28/let-7 pathway in cancer. doi: 10.3748/wjg.v24.i34.3834 Front Genet. (2017) 8:31. doi: 10.3389/fgene.2017.00031 156. Shi WJ, Gao JB. Molecular mechanisms of chemoresistance in gastric cancer. 135. Tu HC, Schwitalla S, Qian Z, LaPier GS, Yermalovich A, Ku YC, et al. LIN28 World J Gastrointest Oncol. (2016) 8:673–81. doi: 10.4251/wjgo.v8.i9.673 cooperates with WNT signaling to drive invasive intestinal and colorectal 157. Fan H, Zhu JH, Yao XQ. Knockdown of long noncoding RNA PVT1 adenocarcinoma in mice and humans. Genes Dev. (2015) 29:1074–86. reverses multidrug resistance in colorectal cancer cells. Mol Med Rep. (2018) doi: 10.1101/gad.256693.114 17:8309–15. doi: 10.3892/mmr.2018.8907 136. Zhang H, Zong Y, Qiu G, Jia R, Xu X, Wang F, et al. Silencing 158. Chen Z, Cai X, Chang L, Xia Y, Wang L, Hou Y, et al. LINC00152 is a Lin28 promotes apoptosis in colorectal cancer cells by upregulating let7c potential biomarker involved in the modulation of biological characteristics targeting of antiapoptotic BCL2L1. Mol Med Rep. (2018) 17:5143–9. of residual colorectal cancer cells following chemoradiotherapy. Oncol Lett. doi: 10.3892/mmr.2018.8483 (2018) 15:4177–84. doi: 10.3892/ol.2018.7833 137. Yu WW, Jiang H, Zhang CT, Peng Y. The SNAIL/miR-128 axis 159. Choi CH. ABC transporters as multidrug resistance mechanisms and the regulated growth, invasion, metastasis, and epithelial-to-mesenchymal development of chemosensitizers for their reversal. Cancer Cell Int. (2005) transition of gastric cancer. Oncotarget. (2017) 8:39280–95. 5:30. doi: 10.1186/1475-2867-5-30 doi: 10.18632/oncotarget.16849 138. Bardelli A, Parsons DW, Silliman N, Ptak J, Szabo S, Saha S, et al. Mutational Conflict of Interest: The authors declare that the research was conducted in the analysis of the tyrosine kinome in colorectal cancers. Science. (2003) 300:949. absence of any commercial or financial relationships that could be construed as a doi: 10.1126/science.1082596 potential conflict of interest. 139. Luo Y, Kaz AM, Kanngurn S, Welsch P, Morris SM, Wang J, et al. NTRK3 is a potential tumor suppressor gene commonly inactivated by Copyright © 2020 Martínez-Barriocanal, Arango and Dopeso. This is an open-access epigenetic mechanisms in colorectal cancer. PLoS Genet. (2013) 9:e1003552. article distributed under the terms of the Creative Commons Attribution License (CC doi: 10.1371/journal.pgen.1003552 BY). The use, distribution or reproduction in other forums is permitted, provided 140. Vigneri PG, Tirro E, Pennisi MS, Massimino M, Stella S, Romano C, the original author(s) and the copyright owner(s) are credited and that the original et al. The insulin/IGF system in colorectal cancer development and publication in this journal is cited, in accordance with accepted academic practice. resistance to therapy. Front Oncol. (2015) 5:230. doi: 10.3389/fonc.2015. No use, distribution or reproduction is permitted which does not comply with these 00230 terms. Frontiers in Oncology | www.frontiersin.org 16 January 2020 | Volume 10 | Article 38