MINI REVIEW published: 14 February 2022 doi: 10.3389/fcvm.2022.777822 Lipid-Laden Macrophages and Inflammation in Atherosclerosis and Cancer: An Integrative View Miriam Lee-Rueckert 1 , Jani Lappalainen 1 , Petri T. Kovanen 1*† and Joan Carles Escola-Gil 2*† 1 Wihuri Research Institute, Helsinki, Finland, 2 Institut d’Investigacions Biomèdiques (IIB) Sant Pau and CIBER de Diabetes y Enfermedades Metabólicas Asociadas, Barcelona, Spain Atherosclerotic arterial plaques and malignant solid tumors contain macrophages, which participate in anaerobic metabolism, acidosis, and inflammatory processes inherent in the development of either disease. The tissue-resident macrophage populations originate Edited by: from precursor cells derived from the yolk sac and from circulating bone marrow-derived Wen Dai, monocytes. In the tissues, they differentiate into varying functional phenotypes in Versiti Blood Research Institute, response to local microenvironmental stimulation. Broadly categorized, the macrophages United States are activated to polarize into proinflammatory M1 and anti-inflammatory M2 phenotypes; Reviewed by: Xiaobo Wang, yet, noticeable plasticity allows them to dynamically shift between several distinct Columbia University, United States functional subtypes. In atherosclerosis, low-density lipoprotein (LDL)-derived cholesterol Linzhang Huang, Fudan University, China accumulates within macrophages as cytoplasmic lipid droplets thereby generating Elda Favari, macrophage foam cells, which are involved in all steps of atherosclerosis. The University of Parma, Italy conversion of macrophages into foam cells may suppress the expression of given Scott M. Gordon, University of Kentucky, United States proinflammatory genes and thereby initiate their transcriptional reprogramming toward Michihisa Umetani, an anti-inflammatory phenotype. In this particular sense, foam cell formation can be University of Houston, United States considered anti-atherogenic. The tumor-associated macrophages (TAMs) may become *Correspondence: Petri T. Kovanen polarized into anti-tumoral M1 and pro-tumoral M2 phenotypes. Mechanistically, the petri.kovanen@wri.fi TAMs can regulate the survival and proliferation of the surrounding cancer cells and Joan Carles Escola-Gil participate in various aspects of tumor formation, progression, and metastasis. The TAMs jescola@santpau.cat may accumulate lipids, but their type and their specific roles in tumorigenesis are still † These authors share last authorship poorly understood. Here, we discuss how the phenotypic and functional plasticity of Specialty section: macrophages allows their multifunctional response to the distinct microenvironments in This article was submitted to developing atherosclerotic lesions and in developing malignant tumors. We also discuss Lipids in Cardiovascular Disease, how the inflammatory reactions of the macrophages may influence the development of a section of the journal Frontiers in Cardiovascular Medicine atherosclerotic plaques and malignant tumors, and highlight the potential therapeutic Received: 15 September 2021 effects of targeting lipid-laden macrophages in either disease. Accepted: 18 January 2022 Keywords: atherosclerosis, cancer, inflammation, LDL, macrophages Published: 14 February 2022 Citation: Lee-Rueckert M, Lappalainen J, Kovanen PT and Escola-Gil JC (2022) INTRODUCTION Lipid-Laden Macrophages and Inflammation in Atherosclerosis and Macrophages are a heterogeneous group of effector cells with essential roles in metabolism and Cancer: An Integrative View. host defense in tissues. They are innate immune cells capable of recognizing and clearing dead Front. Cardiovasc. Med. 9:777822. cells and pathogens, orchestrating inflammatory and healing processes that occur in response to doi: 10.3389/fcvm.2022.777822 injury. Through a wide variety of functions, macrophages play a central role in organ development Frontiers in Cardiovascular Medicine | www.frontiersin.org 1 February 2022 | Volume 9 | Article 777822 Lee-Rueckert et al. Macrophages in Atherosclerosis and Cancer and body homeostasis both in health and disease (1). With influence the specific courses of an atherosclerotic lesion and the exception of the brain and the intestine, macrophages a tumor. in other organs originate from 2 sources: the embryonic Tissue-resident macrophages can be functionally polarized in yolk sac and the bone marrow. Moreover, each organ has response to a cocktail of growth factors and cytokines present its own particular composition of embryonically derived and in their microenvironment (9). Two distinct phenotypes: the adult-derived macrophage subsets, and each organ dictates M1 (proinflammatory) classically activated or the M2 (anti- the degree to which circulating monocytes replace resident inflammatory) alternatively activated macrophages, have been macrophages after birth (2). Under homeostatic conditions, the generated and defined in experimental cell cultures in which embryonically derived and the adult-derived macrophages co- the macrophages’ responses were evaluated after incubation with exist but it remains unclear whether macrophages of distinct interferon gamma and lipopolysaccharide or with interleukin origins have overlapping or distinct functional imprints within (IL)-4 and IL-13, respectively (9). M1 macrophages are key the same tissue. Yet, it is known that whereas macrophages players in the defense against bacterial infections and rely in a specific tissue are mainly seeded during embryonic on glycolysis to meet the rapid energy consumption and development, infiltration of monocyte-derived macrophages cope with a hypoxic tissue microenvironment, while M2 progressively occurs after birth and, particularly, in response to macrophages are rather involved in tissue repair and wound local inflammatory cues (3). Such extravasation of circulating healing and use fatty acid oxidation to fuel their longer-term monocytes across the blood vessel wall, as it occurs in functions (10). Despite providing a useful framework, this immunoinflammatory diseases like atherosclerosis and cancer, dichotomous classification of macrophages only represents the is a multistep event facilitated by several families of endothelial opposite ends of a wide spectrum of macrophage phenotypes junctional cell adhesion molecules such as intercellular adhesion and is an oversimplification of the macrophage diversity molecule (ICAM)-1, vascular cell adhesion molecule (VCAM)- occurring in response to different microenvironments in vivo. 1, selectins, and integrins. The complex role of the adhesion The granulocyte-macrophage colony-stimulating factor (GM- molecules facilitating the immune cells trafficking across the CSF) and the macrophage colony-stimulating factor (M-CSF), endothelium is beyond the scope of this work, and the reader is widely used to differentiate cultured human monocytes into referred to excellent reviews revisiting this field in the context of macrophages, also, respectively, polarize the cells toward M1- atherosclerosis and cancer (4, 5). and M2-like phenotypes (11), which reflects the complexity Because macrophages generated from recruited blood of the macrophage nomenclature, also when identifying the monocytes can partially replace embryonically derived heterogeneous subtypes generated in vitro. macrophages with variable kinetics, the ontogeny of tissue- The remarkable functional plasticity of macrophages in resident macrophages is flexible rather than static (6). The response to microenvironmental stimuli triggering diverse lack of specific markers to discriminate and selectively target polarization states also involves global changes in the macrophages originated from different sources and living macrophage transcriptome. Such functional plasticity can in distinct biological microenvironments has represented a also modify their evolutionary role from host protection technical limitation to functional studies related to macrophage (originally against bacterial infections) into host damage. Prime ontogeny. In the following sections, we provide detailed examples of such possible transformation of macrophages from information about the ontogeny of arterial and tumoral a friend into a foe are their roles as pathogenic promoters of macrophages. Because the characterization of the macrophage atherosclerosis and cancer. Importantly, whereas atherosclerosis ontogeny in the various organ tissues is still in its infancy development directly involves the accumulation of lipids in and for simplicity’s sake, in further sections of this review we macrophages, the role of lipid-loaded macrophages in cancer will refer to the entire population of macrophages residing is still poorly understood. Despite intrinsic differences between in an atherosclerotic plaque or a tumor as “tissue-resident atherosclerosis and cancer, monocyte accumulation and chronic macrophages” regardless their origins. Thus, they comprise inflammation are common features. Thereby, early studies on the the ones embryonically derived and seeded during the early role of monocytes in atherosclerosis have provided insights into embryonic stage, those derived from monocytes recruited cancer, and vice versa (12), a concept that can be now extended from circulation at post-natal stages, as well as the multiple to the monocyte-derived macrophages and the lipid-loaded generations of macrophages successively derived from foam cells generated from them. Moreover, macrophages play them. The role of the extracellular environment within a critical physiopathological roles in either disease by inducing given tissue is critical for macrophage differentiation and the formation, growth, and rupture of an atherosclerotic plaque, specialization, thereby, irrespective of their ontogeny, tissue- as well as the initiation, growth, and metastasis of a malignant resident macrophages display a high degree of phenotypic and tumor (Table 1). functional plasticity in different tissues and exert multifaceted Excitingly, the modern single-cell omics technologies have roles in diseases such as atherosclerosis, cancer, Alzheimer’s revealed enormous cellular heterogeneity of macrophages and disease, multiple sclerosis, and diabetes (7). Notably, tissue other immune/inflammatory cells in advanced atherosclerotic macrophages can also become lipid-laden cells which, lesions (30–32). The use of these tools has now enabled to then, exert diverse functions in each disease context (8). A perform more detailed transcriptional analyses of macrophages current topic of research is how accumulation of various in the atherosclerotic plaques and also in certain solid tumors. macrophage subsets and their lipid-laden forms are capable to In this sense, atherosclerosis and cancer also show resemblance Frontiers in Cardiovascular Medicine | www.frontiersin.org 2 February 2022 | Volume 9 | Article 777822 Lee-Rueckert et al. Macrophages in Atherosclerosis and Cancer TABLE 1 | Effects of macrophages during the various developmental stages of atherosclerotic plaques and malignant solid tumors. Disease stage Atherosclerotic plaques Malignant solid tumors Initiation The first inflammatory cells to invade an atherosclerotic lesion (as Accessory cells in the malignant tumor, which are involved in blood monocytes) (13) tumor growth and progression (15) The major cellular component of inflammatory cell population in Secretion of chemokines and cytokines that promote development atherosclerotic lesions. Via uptake of modified-LDL, macrophages of tumors, such as IL-6, IL-8, and IL-10 (16) become cholesterol-loaded macrophage foam cells, the hallmarks Activated TAMs promote local inflammation (15, 16) of atherosclerotic lesions (13, 14) Activated intimal macrophages promote local inflammation (14) Progression Proteolytic remodeling of the extracellular matrix of the plaque Proteolytic remodeling of the extracellular matrix of the tumor (21) (17, 18) Promotion of the proliferation of tumor cells directly by secretion of Apoptosis of macrophage foam cells increases plaque size and growth factors, which contributes to the growth of the malignant growth of the atherosclerotic plaque with formation of a necrotic tumor (21–24) lipid core (19) Immune regulation by inhibition of antitumoral responses of T Thinning of the fibrous cap by expression of MMPs with ensuing cell-mediated cytotoxicity (15, 16) formation of the rupture-prone “vulnerable” plaque (17, 18) Facilitation of the motility and intravasation of tumor cells with Promotion of angiogenesis in hypoxic areas of advanced ensuing promotion of their metastatic spreading (15) atherosclerotic lesions (20) Induction of therapeutic resistance (25) Stimulation of endothelial cell proliferation and promotion of neovascularization of the tumor tissue (21) Regression Lesional macrophage may egress from the plaque during plaque Anti-tumor immunity response by phagocytosing cancer cells regression (26, 27) (16, 28, 29) Note that the phenotype of macrophages residing within an atherosclerotic plaque or a tumor will influence both disease progression or regression depending on how they respond to specific microenvironment signals via polarized and functional programs. These signals include the cocktails of cytokines and growth factors present in the interstitial fluids, as well as the hypoxia and the acidic pH characteristic of the atherosclerotic plaques and malignant tumors. (33) and call for more complex in vitro studies exploiting is extensive literature on the macrophage foam cells generated the modern multicellular-type organoid cultures to meet the during atherosclerosis progression and, accordingly, this term current challenges of the microenvironmental plasticity of has been specifically used over the years as referring to the immune/inflammatory cells, such as the macrophages (34). cholesterol-loaded cells present in the plaques. Lack of specific The recent advances in the knowledge of the complex immunohistochemistry markers for human monocyte-derived macrophage diversity, however, also raise important questions macrophages, smooth muscle cells, and dendritic cells has made about the distinct roles of macrophages of fetal or postnatal it challenging to identify the various subpopulations of cells and origins and their proliferative capacities in the inflammatory their corresponding lipid-laden forms in atherosclerotic lesions microenvironment of an atherosclerotic plaque and a malignant (37). Despite it having been reported that vascular smooth muscle tumor. Here, we will much discuss the roles of M1 and cells and dendritic cells also can take up cholesterol-derived M2 macrophages which represent the two extreme states of from intimal LDL particles and so turn into cholesterol-loaded phenotypic macrophage polarization. This apparent lack of foam cells, macrophages are considered the primary source of sophistication regarding the phenotypic diversity and plasticity foam cells in atherosclerosis (38–40). More recently, also lipid- of macrophages derives from the fact that most of the cell laden TAMs have been detected in tumors, however, their lipid culture studies have been performed by using these in vitro cargo appears to be much smaller than that in the macrophage generated phenotypes, which hardly find their pure counterparts foam cells present in atherosclerotic lesions. Moreover, the in the complex tissue environments in vivo (35, 36). Despite the accumulated lipids are triglycerides, glycerophospholipids, and plasticity of macrophages, which allows a dynamic modulation sphingomyelins, rather than cholesteryl esters. Based on the as a continuous phenotype, the M1/M2 dichotomy still persists above considerations on the conventional terminology, we have and serves the studies in which the role of macrophages in applied the term “foamy” only for the macrophages present in atherosclerosis and cancer are examined (21, 32). atherosclerotic lesions, but not for those present in the malignant Based on the current knowledge about the ontogeny and tumors, which have been termed “lipid-laden macrophages.” functions of the various macrophage subsets recently achieved In this respect, it is also important to highlight that the story by applying novel technologies in mice models and humans, of macrophages and foam cells in atherosclerosis was initiated this review aims to compare the relative significance of more than 100 years ago with the classical studies of the Russian macrophages and their lipid-loading capacity on the progression pathologist Nikolai Anitschkow (41). Therefore, cholesterol- of atherosclerosis and cancer. At this point, we need to denote loaded macrophage foam cells were very early recognized as the that the traditional view of a “foam cell” has been used to describe root cause of atherosclerosis and they have been a persistent and the cholesteryl ester-loaded cells present in atherosclerotic lesions central focus of research in this field. In contrast, the research given that their abundant cytoplasmic lipid droplets result in on macrophages and cancer is much younger, moreover, the a bubbly or “foamy” appearance under the microscope. There potential (perhaps relatively less relevant) role of lipid-laden Frontiers in Cardiovascular Medicine | www.frontiersin.org 3 February 2022 | Volume 9 | Article 777822 Lee-Rueckert et al. Macrophages in Atherosclerosis and Cancer macrophages in malignant tumors has called the attention of the adventitial layer, whereas the majority of the macrophages cancer biologists only recently. By revising the recent discoveries which infiltrate the artery around birth arose from monocyte on macrophage biology in atherosclerosis and cancer, we hope to progenitors, thereafter adopting self-renewing capacity (48). The disseminate a piece of challenging interdisciplinary information embryonically originated macrophages express high levels of the on this particular topic in two diseases, which are the leading hyaluronan receptor LYVE-1, which in the adventitia induces causes of death worldwide. collagen degradation by smooth muscle cells, and thereby prevents collagen deposition and arterial stiffness (47, 49). Whether of bone marrow or embryonic origin, tissue-resident MACROPHAGES IN ATHEROGENESIS: macrophages can closely resemble each other, including the ONTOGENY, CHOLESTEROL LOADING, capacity to renew via proliferation (6). Importantly, regardless POLARIZATION, AND FUNCTIONAL of their origin, the proliferation of resident macrophages PHENOTYPES drives the expansion of the atherosclerotic plaque (14). It was recently found that prolonged hypercholesterolemia in Macrophages are the major immune cell population in the the low-density lipoprotein receptor-deficient (Ldlr-/-) mouse atherosclerotic lesion which contribute to the initiation and model led to a complete loss of the resident-derived lipid- progression of atherosclerosis through their diverse roles in laden foam cells, as they were ultimately replaced entirely by cholesterol metabolism and inflammation (42). They are the recruited blood monocytes, indicating the key role of monocyte first inflammatory cells to invade atherosclerotic lesions and the recruitment to sustain macrophage proliferation and to allow the main component of atherosclerotic plaques (13). Macrophages expansion of multiple generations of macrophages during plaque that accumulate in atherosclerotic plaques appear to have a progression (48). diminished capacity to migrate, which contributes to the failure Plasma lipoproteins also cross the endothelial barrier to resolve inflammation and to the progression of these lesions into the arterial intima where they are involved in the to more advanced plaques (14). For a long period, they were regulation of macrophage cholesterol balance. Formation of considered to mainly originate from bone marrow- or spleen- the atherosclerotic lesion in the arterial intima is characterized derived monocytes, which once in circulation can infiltrate the by accumulation of low-density lipoprotein (LDL)-derived arterial intima. Monocytes recruited to atherosclerotic lesions cholesterol in macrophages with the ensuing generation of foam are technically identified as Ly6Chigh cells that originate from cells, and is associated with chronic inflammatory responses both medullary and extramedullary hematopoiesis. The practical (50). In the intima, LDL particles interact with a dense use of the Ly6Chigh , principal marker of mouse monocytes, is extracellular matrix network rich in proteoglycans, collagen, based on similar features in the expression pattern of certain and elastin. Particularly, the interaction with proteoglycans molecules that the cluster of differentiation (CD)14++ CD16− initiates LDL retention in the intima (51) and facilitates various human classical and most abundant monocyte population shares types of modifications of LDL particles, which, increase their with the Ly6Chi mouse monocytes (43). More recently, with the proatherogenic roles. The non-regulated uptake of modified increased availability of avant-garde technologies, highly diverse LDL particles by the CD36 and the SRA1 scavenger receptors cellular communities of macrophages have been detected in in macrophages enables the intracellular accumulation of the advanced human atherosclerotic plaques including a population cholesterol contained in them (13, 14). Intracellularly, the of tissue macrophages that originates during the embryonic cholesteryl esters are first hydrolyzed in lysosomes, and then re- development. Using genetic fate-mapping approaches, it has esterified in the cytoplasmic compartment of the macrophages been shown that arterial macrophages arise embryonically from where they form microscopically visible cholesteryl ester droplets C-X3-C motif chemokine receptor 1 (Cx3cr1)+ precursors typical of foam cells, which, again, are the hallmarks of and postnatally from bone marrow-derived monocytes that atherosclerotic lesions (52). colonize the tissue immediately after birth (44). Thus, arterial In contrast to LDL, the cardioprotective effects of high- macrophages appeared to be early originated from yolk sac- density lipoproteins (HDL) are attributed to their ability to enter derived erythromyeloid precursors and one additional wave of the arterial intima where they stimulate cholesterol efflux from blood-derived monocytes shortly after birth (45, 46). macrophages, which is mechanistically linked to the HDL anti- Upon atherosclerosis development, the resident macrophages inflammatory functions (53). Abundant experimental evidence embryonically originated are replaced by or accompanied by supports the anti-inflammatory role of cholesterol efflux, which recruited monocyte-derived macrophages that adopt a resident- is exemplified by the increase in inflammatory responses in like macrophage phenotype (3). According to studies in mice, the the ATP-binding cassete transporter (ABC)A1/ABCG1-deficient population of yolk sac erythro-myeloid progenitors substantially macrophages. It is also known that HDL and other compounds contributes to the adventitial macrophage population and (as cyclodextrins) can produce anti-inflammatory actions by a gives rise to a defined cluster of resident immune cells variety of mechanisms, most of them based on their cholesterol with homeostatic functions which then declines in numbers efflux capacities (54). In this context, we have shown that various during aging and is not replenished by bone marrow-derived specific anti-inflammatory mechanisms induced by apoA-I, the macrophages (47). In line with this report, only a minor part main apolipoprotein of HDL, require the intactness of its C- of the aortic resident macrophages in adult mice was found terminal domain, which is critical to bind with high affinity to to be embryonically derived and they were mainly located in human coronary artery endothelial cells (55). Frontiers in Cardiovascular Medicine | www.frontiersin.org 4 February 2022 | Volume 9 | Article 777822 Lee-Rueckert et al. Macrophages in Atherosclerosis and Cancer Cholesterol efflux can significantly compensate for the of the plaques. In early lesions, the apoptotic macrophages can excessive influx of LDL-derived cholesterol to the foam cells be rapidly cleared by adjacent macrophages via a phagocytotic and stimulate the anti-atherosclerotic pathway known as reverse process known as efferocytosis. However, efferocytosis is cholesterol transport (RCT) (56). Thus, disruption of the impaired in advanced atherosclerotic plaques, a defect that is endothelial barrier by vasoactive compounds in naturally high- partly attributed to oxidative stress and cytoplasmic saturation HDL murine models in vivo has enhanced the passage of with indigestible material (68). A deficient efferocytosis HDL into the interstitial fluid of the skin and, thereby, contributes to the formation of the necrotic core, which increased the rate of cholesterol transfer from subcutaneously promotes plaque disruption, particularly by thinning the located macrophage foam cells to feces (macrophage-RCT) (57). fibrous cap separating the core from the arterial lumen (19). The most relevant mechanisms of cholesterol efflux involve Progression of the atherosclerotic lesion is also accelerated by the ABCA1 and G1, which upon interaction with lipid-poor degradation of components of the extracellular matrix, such as and mature HDL, respectively, trigger a cascade of events collagen and elastin in the fibrous cap, by different elastolytic associated with the release of cholesterol and phospholipids and proteolytic enzymes secreted locally by macrophages and from the cell surface (58). Both ABC efflux transporters are other intimal cells. These enzymes include several matrix overexpressed in cholesterol-loaded macrophages as a result of metalloproteinases (MMPs) and cysteine proteases secreted by activation of liver X receptor (LXR)/retinoid X receptor (RXR)– macrophages (17, 18). All these macrophage-dependent effects mediated gene transcription (59, 60). Of note, by disrupting contribute to the progression of the atherosclerotic lesion, specialized cholesterol and sphingomyelin-rich lipid rafts of and also render the plaque more vulnerable to rupture with the macrophage plasma membrane that serve as platforms for ensuing acute atherothrombotic complications, such as acute inflammatory signaling pathways, cholesterol efflux mediated by myocardial infarction. Moreover, in the deep hypoxic areas of the ABC transporters is associated with anti-inflammatory effects atherosclerotic plaques, intraplaque neoangiogenesis takes place. and supports the proper functioning of macrophage immune Indeed, the severity of tissue hypoxia correlates with the presence responses (61). ABCA1 in macrophages also functions as an of macrophages and the expression of the hypoxia-inducible anti-inflammatory signaling receptor through activation of signal factor (HIF) and the vascular endothelial growth factor (VEGF) transducer and activator of transcription factor (STAT)3, an in the advanced human atherosclerotic lesions (20). Ruptures of effect that is independent of the induction of cholesterol efflux the fragile microvessels generate intraplaque microhemorrhages (62). Indeed, a recent transcriptome analysis of advanced human and further weaken the plaque (Table 1). atherosclerotic plaques obtained during carotid endarterectomy In the arterial intima, the macrophages adopt different indicated that macrophages were found in distinct populations functional programs in response to various polarizing signals. with diverse activation patterns and the most anti-inflammatory Among them, the colony-stimulating factors GM-CSF and M- foam cell–like cluster was characterized by the expression of CSF can generate disparate proatherogenic and proinflammatory ABC cholesterol efflux transporters and other lipid-related macrophage phenotypes. Notably, immunostained human genes whose expression was most likely driven by intracellular coronary arteries showed that macrophages with similar antigen cholesterol accumulation (31). expression as induced in vitro by M-CSF, i.e., the M2-like Although normal veins and arteries have been found not macrophage phenotype, were predominant within human to express ABCA1 mRNA, in the setting of atherosclerosis atherosclerotic lesions (69). Interestingly, the distribution of widespread expression was observed in macrophages within the macrophage subtypes is not uniform in human atherosclerotic lesions (63). However, further studies in human endarterectomy plaques, being M1 macrophages located at the rupture-prone specimens revealed that the ABCA1 protein is markedly reduced shoulders of mature plaques while the M2 macrophages are in the advanced atherosclerotic lesions, thereby suggesting that away from the lipid core (70). Because GM-CSF and M-CSF a failure to translate ABCA1 mRNA into ABCA1 protein, are soluble glycoproteins widely expressed in the arterial intima or, alternatively, increased degradation of the ABCA1 protein (71) and the formation of cholesterol-loaded macrophage may occur in the advanced plaques (64, 65). Recently, a novel foam cells is intrinsically related to atherogenesis, we recently transcellular movement of cholesterol from cultured macrophage investigated the effect of cholesterol loading on the expression foam cells to adjacent smooth muscle cells has been described of key atheroinflammatory genes in cultured human monocyte- to occur even in the absence of HDL (66). An intriguing derived macrophages differentiated by either CSF (72). We question remains whether macrophage foam cells within an found that, as compared to M1, the M2 macrophage subtype atherosclerotic plaque could unload at least some of their surplus expressed higher levels of CD36 and SRA1 receptors and cholesterol onto adjacent smooth muscle cells. However, the were particularly prone to foam cell formation. Moreover, transfer of cholesterol to intimal smooth muscle cells could the expression of ABCG1 and C-C Motif Chemokine Ligand enhance their conversion into macrophage-like foam cells, and (CCL)2 in the M2 subtype was markedly lower and higher, thereby contribute to the pathogenesis of an atherosclerotic respectively. Since the cholesterol efflux transporter ABCG1 also plaque (67). displays anti-inflammatory effects (53) and CCL2 is an early All cell types present in atherosclerotic plaques, including component of the proinflammatory response in atherosclerosis endothelial cells, smooth muscle cells, lymphocytes, and (73), these data indicated that polarization with M-CSF macrophages, undergo apoptosis, and, importantly, apoptotic induces proinflammatory traits in macrophages. Interestingly, cell death has been shown to occur in macrophage-rich regions cholesterol loading of the M2 polarized macrophages strongly Frontiers in Cardiovascular Medicine | www.frontiersin.org 5 February 2022 | Volume 9 | Article 777822 Lee-Rueckert et al. Macrophages in Atherosclerosis and Cancer suppressed their proinflammatory features as indicated by murine pancreatic ductal adenocarcinoma models by identifying 60-fold upregulation of ABCG1 and 2-fold downregulation of that both inflammatory Ly6Chigh monocytes and tissue-resident CCL2 (72). The finding that cholesterol loading can override macrophages of embryonic origin are sources of TAMs (81). the inflammatory profile of M2 polarized macrophages by Moreover, the TAMs of different origins demonstrate distinct reprogramming the gene expression levels of both ABCG1 and phenotypes and divergent functionality, i.e., whereas monocyte- CCL2 toward an anti-inflammatory phenotype strongly suggests derived TAMs are more potent at sampling tumor antigens, that, in terms of the local atheroinflammatory component embryonically derived TAMs display higher expression of pro- of atherogenesis, macrophage foam cell formation may be fibrotic factors (81). The dynamic changes in the ontogeny of considered an anti-atherogenic event (72), a concept also TAMs during tumor development were also documented in a supported by other reports (74–77). breast cancer mouse model, in which it was found that mammary Interestingly, a mechanistic link between cholesterol tumor growth induced an overall loss of resident macrophages accumulation and suppression of inflammation in macrophages with a concomitant increase in newly arrived monocyte-derived has been suggested due to a regulated accumulation of TAMs (82). desmosterol in cells, which is the last intermediate in the Neoplastic cells in solid tumors maintain intricate interactions pathway of cholesterol biosynthesis (74). A regulated increase of with their surrounding stroma composed of blood-derived cells desmosterol underlies the activation of LXR target genes, such as including macrophages (up to 50%), T cells, granulocytes, ABCA1 and ABCG1, and suppression of the genes responsible and mast cells, as well as peripheral fibroblasts and epithelial for inflammatory responses, thereby disclosing a mechanistic cells (16, 83). Moreover, the primary tumor cells secrete link between cholesterol accumulation and suppression of a variety of chemokines, cytokines, and other factors that macrophage inflammation. Yet, although cholesterol loading promote the mobilization and recruitment of various types reduces the expression of inflammatory genes, the net result of blood cells, notably, monocytes, which become TAMs could lead to an attenuated inflammatory profile, rather than within the tumor microenvironment (16). TAMs are, overall, to an actual anti-inflammatory phenotype of the foam cells. involved in many activities associated with tumor growth Moreover, the anti-inflammatory mechanisms associated with and progression including inflammation, immune regulation, cholesterol-loading (74) likely appear to operate in macrophages angiogenesis, invasion, and metastasis (15). The important role during the initial stages of the development of atherosclerotic of TAMs in tumorigenesis is supported by the fact that TAM lesions. It is not known whether the pathways for the generation numbers have been identified to be an independent prognostic of desmosterol or its intracellular distribution are impaired in factor in several types of cancer, such as in lung cancer, breast the end-stage lesional macrophages of advanced atherosclerotic cancer, and lymphomas (84). Moreover, a depletion of TAMs lesions. The further acquisition of a proinflammatory phenotype has translated into marked clinical benefit in cancer patients, during advanced atherosclerosis could also depend on extrinsic such as in those affected by a diffuse-type giant cell tumor (85). proinflammatory stimuli within the artery wall. These could There is also some evidence pointing out that TAMs can mediate include inflammatory mediators derived from endothelial cells resistance of tumor cells to chemotherapy or radiotherapy by or other types of cells in the arterial intima, increased free activating STAT3 in the tumor cells, which, again, enhances cholesterol (crystals) deposited within the extracellular matrix, the proliferation and survival of malignant cells even during uptake of extracellular cholesterol crystals, and/or exposure to treatment with various chemotherapeutics (25) (Table 1). the debris generated by dying cells. Tumor cells secrete factors that prime immune cells, once in the tumor microenvironment, to gain a tumor-supportive phenotype (86). Accordingly, TAMs can become polarized upon MACROPHAGES IN TUMOR receiving signals from the particular microenvironment they DEVELOPMENT: ONTOGENY, reside in and thereby create inflammatory conditions that POLARIZATION, PRO- AND facilitate the survival and proliferation of cancer cells (21, 87). ANTI-TUMORAL MACROPHAGES The recruitment and differentiation progress of TAMs are also related to local anoxia, inflammation, and high levels of lactic acid Abundant macrophage populations are found in the stromal (16). TAMs are also thought to affect tumor invasion and stromal compartment of solid tumors in virtually all types of malignancy cell migration through the extracellular matrix, i.e., by deposition (78). Similar to atherosclerosis, circulating monocytes can give of various types of collagen and breakdown of these components rise to tumor-associated macrophages (TAMs), which play via secretion of proteolytic enzymes such as matrix MMPs, critical roles at all stages of tumor progression. Despite the dogma serine proteases, and cathepsins (88). However, the interactions that recruitment of monocytes from the periphery by chemotaxis between TAMs and cancer cells are still poorly defined, is the exclusive source of TAMs, it is now known that embryonic- potentially due to different populations of TAMs analyzed in derived TAMs are also present and that they participate in multiple tumor settings. Thus, there are conflicting pieces of tumor development (28, 79). Moreover, recent evidence shows evidence about the role of TAMs heterogeneity regarding their that at least in certain tumors, tissue-specific embryonically pro- and anti-tumoral activities (22). Given the high plasticity derived resident macrophages infiltrate tumor tissues and, thus, of macrophages, the diverse activities of TAMs may also relate represent a significant input source of TAMs (80). Moreover, to the existence of distinct subpopulations of TAMs, which the heterogeneous origin of TAMs has been demonstrated in are associated with different intratumoral microenvironments Frontiers in Cardiovascular Medicine | www.frontiersin.org 6 February 2022 | Volume 9 | Article 777822 Lee-Rueckert et al. Macrophages in Atherosclerosis and Cancer (21). As such, many unanswered questions and paradoxical phenotype during tumor progression (22). In this context, evidence exist regarding the microenvironmental conditions that depending on the type of tumor and the experimental model, the determine the differentiation of TAMs, resulting in opposed blockade of M-CSF signaling has in most cases attenuated cancer activities. It has been shown that macrophage polarization toward progression, a finding with potentially high clinical relevance for tumor-promoting phenotypes is not exclusively the result of the use of the M-CSF receptor inhibitors as cancer therapeutics dysfunctional tissue homeostasis, but instead of a more active (22). Indeed, inhibition of M-CSF by a specific antibody or process driven by reciprocal interactions with both malignant chemical inhibitors has been shown to significantly suppress and stromal cells in the tumor, including the effect of a tumor angiogenesis and lymphangiogenesis (93). dysregulated tissue architecture in tumors caused by cell deaths In tumors, also the cytokines CCL2, CCL11, CCL16, and (88). Cell-to-cell contacts among TAMs, cancer cells, and other CCL21 are major determinants of macrophage infiltration and activated stromal cells can be also of great significance in angiogenesis (16). TAMs also produce CCL2, which primarily determining the phenotype and functionality of TAMs within a contributes to the recruitment of macrophages in tumors (16, tumor. However, although the TAMs do not become polarized 94). Research in mouse models and humans has shown that by their location per se, the distribution pattern of macrophages high levels of tumor-derived CCL2 correlate with an increased between the tumor nest and the tumor stroma, rather than number of TAMs in the tumor tissue, and also with a poor the total number of TAMs, has been reported to be a better cancer prognosis (93). Downregulation of CCL2 expression independent prognostic factor for the overall survival in gastric could therefore be considered a promising target by preventing cancer patients (89). the cancerous tissue from further collecting TAMs (95). In Tumors can also secrete the key macrophage growth factors this context, it would be interesting to investigate whether the GM-CSF and M-CSF (22–24), and, accordingly, they can act expression of CCL2 in TAMs is upregulated by M-CSF, as in an autocrine manner driving the proliferation of TAMs. observed in human macrophage foam cells in vitro (72). If so, Based on the expression of specific markers, TAMs can be M-CSF inhibitors might be of further value in their clinical use. mainly classified into the anti-tumoral M1 phenotype (classically activated state) and the pro-tumoral M2 phenotype (alternatively activated state). TAMs generally have functional properties EXTRACELLULAR MICROENVIRONMENTS associated with an M2-like polarization caused by tumor-derived WITHIN AN ATHEROMA AND A TUMOR: lactic acid or by secretion of immunosuppressive cytokines SOLUTES DERIVED FROM THE BLOOD from different types of cells in the tumor microenvironment AND THE LOCAL CELLS (90, 91), which also highlights the role of the macrophage distribution pattern within the tumor tissue. Thus, TAM The vascular endothelium is a semipermeable barrier that populations consist of M2 and a small fraction of M1 cells. regulates the composition of interstitial fluids by an ultrafiltration Overall, whereas the M1 phenotype is associated with intrinsic process, which applies particularly to the various lipoproteins as phagocytosis and enhanced antitumor inflammatory reactions, the largest solutes in the blood. Depending on the needs, the M2 exerts a repertoire of tumor-promoting capabilities involving vascular permeability is dynamically regulated to maintain tissue immunosuppression, angiogenesis, and neovascularization, as homeostasis, yet, oxidative stress and hypoxia can challenge the well as stromal activation and remodeling of the extracellular normal endothelial permeability (96). In chronic inflammatory matrix (21). No direct link has been shown between the ontogeny diseases such as atherosclerosis and cancer, a cocktail of of TAMs and their pro- or anti-tumoral profile. However, a inflammatory mediators can disrupt the organization of the depletion of TAMs induced by myeloablative chemotherapy has endothelial junctions leading to their opening and significant loss been followed by a transient and massive wave of bone marrow- of the endothelial barrier function (97), among them also the derived monocytes which contributed to the phagocytosis of mast cell-derived (98) and macrophage-derived histamine (99). cancer cells suggesting that such TAMs can be potent effectors It was reported that the vasoactive substance P may induce gaps of an anti-tumoral response (28). Moreover, macrophages in the endothelial junctions of endothelial cells in vessels of the may also act as effector cells by eliminating tumor cells, rat trachea that range from 100 to 400 nm (vs. 5–8 nm in basal particularly via induction of antibody-dependent phagocytosis by blood vessels) (100). In this regard, both the endothelium lining monoclonal antibodies during cancer therapy (29). Therefore, a of an atherosclerotic plaque and the tumor vasculature possess dynamic balance appears to exist between the negative (25) and leaky capillary gaps (101, 102), which facilitates extravasation of positive (22, 73) effects of TAMs during the course of a given plasma lipids that can influence lipid metabolism in the various cancer therapy. populations of tissue-resident macrophages. Since the M2-like TAMs are considered to exert pro-tumoral An enhanced vascular permeability facilitates the passage of activities and the M1-like TAMs can be regarded as anti-tumoral plasma lipoproteins like LDL (22–29 nm particle size range) macrophages, a higher M1/M2 ratio in cancer tissue usually across such dysfunctional endothelial monolayer. Accordingly, in signifies a favorable outcome, whereas a lower M1/M2 ratio hypercholesterolemia, the transendothelial entry of high levels of often indicates poor prognosis in cancer patients (92). Yet, circulating proatherogenic cholesterol-laden LDL particles into transcriptome data have reported that TAMs actually express the arterial wall will increase. Within the subendothelial intima, overlapping M1 and M2 characteristics, which may be also the LDL particles are retained, become modified, e.g., oxidized, derived from a dynamic switch from M1- to M2-like TAM lipolyzed, or proteolyzed, are recognized by scavenger receptors Frontiers in Cardiovascular Medicine | www.frontiersin.org 7 February 2022 | Volume 9 | Article 777822 Lee-Rueckert et al. Macrophages in Atherosclerosis and Cancer and are ingested by the macrophages (26). A fraction of the pH 5.5) reduced ABCA1 mRNA and protein expression in modified LDL particles form aggregates or cholesterol crystals human macrophage foam cells and, thereby impaired their and are taken up by macrophages by means of fluid-phase cholesterol efflux capacity (117). In line with this finding, ABCA1 pinocytosis processes (103, 104). Whichever is the LDL uptake protein has been reported to be markedly reduced in advanced mechanism by macrophages, foam cell formation typically human carotid atherosclerotic lesions, where the intimal fluid ensues. Finally, inflammatory activation of the endothelial cells in most likely has turned acidic (64). A low extracellular pH atherosclerosis-prone areas of the arteries where hemodynamic also decreases the ACAT-dependent cholesterol esterification forces induce endothelial cell dysfunction, also facilitates the in cultured human macrophages and results in intracellular transendothelial migration of immune cells into the intima and, accumulation of unesterified (or “free”) cholesterol (118), which thereby, accelerates vascular inflammation (105–107). could eventually lead to apoptotic cell death. Cancer develops in a complex tumor microenvironment Also within the core of solid tumors, the pH of extracellular where the malignant cells and its stroma, including endothelial fluid may be as low as 5.2 (119). Tumor acidosis can significantly cells, pericytes, fibroblasts, immune cells, and the extracellular affect the phenotypic characteristics of macrophages. A recent matrix coexist during tumor evolution (108). Regarding cancer report found that clear renal cancer cells secreted parathyroid- progression, disruption of the endothelial barrier by tumor- hormone-related protein that promoted the perinephric adipose derived secreted factors, such as VEGF and CCL2, is a critical tissue browning, which in turn enhanced the release of lactate step in cancer cell extravasation (109). Thus, the weakening and tumor growth (120). The accumulation of lactate released by of the endothelial junctions facilitates cancer cells to cross the cancer cells into the tumor microenvironment can function as an endothelial barrier and colonize other tissues. In this particular intrinsic inflammatory mediator that increases the production of instance, atherosclerotic tissue and tumor tissue are at least the inflammatory cytokine interleukin IL17A by macrophages, so partly mirror images of each other regarding disease progression: promoting chronic inflammation (121). Lactate also stimulates the dysfunctional endothelial barrier of an atherosclerotic lesion the macrophage receptor G protein-coupled receptor (GPR) 132 allows entry of circulating inflammatory cells into the developing in TAMs, promoting the development of an M2-like phenotype lesion, while a disrupted endothelial barrier in tumor tissue may that enables migration and invasion of tumor cells (122). open the gates to the exit of tumor cells. Regarding atherosclerotic Moreover, when the impact of a low pH as an independent lesions, no significant exit of cells has been observed, albeit some entity from lactate was dissected, human macrophages incubated studies have provided evidence of egress of lesional macrophages under polarizing conditions in an acidic medium of pH 6.8 or macrophage foam cells back into the circulation or to the acquired a functional state similar to the pro-tumoral phenotype lymphatic vessels during regression of the lesions (26, 27). often ascribed to TAMs, suggesting that sole acidosis is capable Of interest, macrophage efflux from inflammatory sites was to dictate the pro-tumoral functionality of macrophages (123). shown to occur by proteolytic shedding of the soluble integrin- Furthermore, modulation of the macrophage phenotype by β2 followed by its binding to the ICAM-1, which blocks its acidity also occurred in a model of prostate cancer in vivo and adhesion capacity and enables macrophages to leave the site of acted as a significant driver of tumor progression (123). inflammation and also limits further leukocyte infiltration (110). The acidic microenvironments of an atherosclerotic lesion The proper functionality of key metabolic processes mediated and a tumor also activate secreted lysosomal enzymes with an by cytoplasmic enzymes and cell membrane receptors operate at optimum pH within the acidic range. Therefore, the increased pH optimum near 7.3 (111). Notably, both atherosclerosis and activity of various cysteine cathepsins such as D, F, S, and K cancer are distinguished by anaerobic metabolism and acidosis, secreted by macrophage foam cells in atherosclerotic lesions where the low pH of the interstitial fluid is regarded as an (112), and the cathepsins B and S secreted by TAMs, e.g., in breast important denominator. An acidic extracellular pH is found in cancer tissue (124), may, respectively, modulate the progression various inflammatory sites such as various joint diseases and of atherosclerosis and cancer by local proteolytic degradation of in atherosclerotic lesions, where local hypoxia and acidosis can the extracellular matrix (17, 18). Activated cysteine proteases can affect the ongoing immune response (112, 113). In the intimal also contribute to deplete the small lipid-poor preβ-migrating fluid of advanced carotid plaques, pH values below 6.0 have HDL particles (125), which are abundant in human interstitial been reported, and such low pH values were suggested to be a fluids (126). Such proteolytic loss of preβ-HDL in the acidic sign of plaque vulnerability (114). Indeed, acidity enhances the milieu of the intima, by compromising the cholesterol efflux proteolytic, lipolytic, and oxidative modifications of LDL, and capacity of the intimal fluid, would further favor the generation strongly increases their affinity for extracellular proteoglycans and maintenance of cholesterol-loaded macrophage foam cells in the arterial intima and also for the pericellular proteoglycans (127). Regarding cancer, the significance of a compromised on macrophage surfaces, so favoring both extracellular and cholesterol efflux from TAMs is still unclear (see the following intracellular accumulation of cholesterol in atherogenesis (112). section), however, the cathepsin-rich TAMs were found to be Interestingly, a mechanistic switch in the uptake of modified- potent suppressors of Taxol-induced tumor cell death thereby LDL from scavenger receptor-mediated mechanism to non- blunting the chemotherapeutic response (124). specific fluid-phase pinocytosis is likely to occur at acidic Finally, in contrast to other tissues in which the concentration pH (115, 116). Regarding the effect of the extracellular pH of LDL particles is only one tenth of that in the circulation, on the ability of macrophages to release cholesterol, we have in the arterial intima their concentration equals that in the found that sole incubation in acidic culture media (down to circulation (128). The uniquely high concentration of LDL in Frontiers in Cardiovascular Medicine | www.frontiersin.org 8 February 2022 | Volume 9 | Article 777822 Lee-Rueckert et al. Macrophages in Atherosclerosis and Cancer the intimal extracellular fluid prevails because the intimal space lipid-laden TAMs. Interestingly, the lipid droplet-dependent lacks lymphatic capillaries, and is a closed space separated from FA metabolism was shown to induce the immunosuppressive the medial layer of the arterial wall by a largely impermeable phenotype of TAMs, which suggested that targeting lipid droplets internal elastic lamina (129). Efficient lymphatic lipoprotein by chemical inhibitors could be potentially used as a novel anti- drainage could prevent the continuous cholesterol accumulation tumor strategy (134). and the subsequent lesion development in the arterial intima. Only a few lipidomic analyses have been carried out in However, the arterial intima lacks lymphatic capillaries, and they cancer tissues, and this lack of data particularly applies to appear only in very advanced lesions (130). By promoting an TAMs. Moreover, some conflicting results could be attributed abundant macrophage uptake of LDL-derived cholesterol and to the specific cancer models used in the reported studies. formation of the intimal foam cells, such distinctive extracellular A recent study in vitro found that macrophages from both microenvironment could explain the significant differences in human and murine tumor tissues were enriched with lipids lipid composition of the lipid droplets in these cells relative to due to increased lipid uptake (135). TAMs expressed elevated those reported in TAMs. In addition, because of the uniquely levels of CD36, accumulated lipids, and used FA oxidation high levels of LDL in the intima, acidification of the intimal instead of glycolysis for energy. As compared with macrophages fluid may also have major effects on the retention, modification, isolated from normal tissues, neutral lipids were accumulated and ensuing uptake of LDL by intimal macrophages. Thus, in in lipid droplets in TAMs, as revealed by the fluorescent lipid the intimal fluid of an advanced atherosclerotic lesion, a dual dye BODIPY (135). In a model of gastric cancer, lipidomic effect of a low pH and high concentrations of LDL exists, analysis of TAMs generated in the presence of lipid-containing which would favor the increased formation of cholesterol-loaded tumor explant supernatants revealed high levels of triglycerides macrophages. While the acidity of the intimal fluid favors without changes in the levels of cholesteryl esters (136). the intracellular cholesterol accumulation in macrophages, the Another lipidomic study of TAMs cocultured with human extreme high concentrations of LDL in the extracellular matrix of papillary thyroid carcinoma cells found that tumor cells can the intima is sufficient per se to transform resident macrophages stimulate lipid biosynthesis in the TAMs, which enriched into foam cells. In contrast, mere acidification of the interstitial their levels of intracellular lipids, being glycerophospholipids fluid in the absence of LDL excess, as occurs in the tumor milieu, and sphingomyelins the most highly upregulated lipids (137). does not induce cholesterol accumulation in TAMs, as will be Therefore, it appears, overall, that lipid droplets in TAMs described in the next section. accumulate mainly triglycerides with variable amounts of glycerophospholipids and sphingomyelins. Regarding the similarities and differences between the neutral LIPID-LADEN TAMs ARE NOT lipid content in the cytoplasmic lipid droplets of macrophage CHOLESTEROL-LOADED FOAM CELLS foam cells in human atherosclerotic lesions and tumor tissues, we wish to further denote that a dynamic overlap may exist. In contrast to the vast information on the cholesterol- Thus, it was found that the macrophage foam cells isolated laden macrophage foam cells typical of atherosclerosis, the from human aortic atherosclerotic lesions are able, when exposed type of accumulated lipids, their potential origins, and the to hypoxia, to store in their neutral lipid droplets, in addition physiopathological role of the lipid cargo in TAMs remains to be to cholesteryl esters, also significant amounts of triglycerides fully elucidated. Instead, several studies have rather investigated (138). Because unmodified, i.e., native LDL particles, can be lipid metabolism in cancer cells and the sources of extracellular taken up by macrophages via fluid-phase pinocytosis without lipids available for these cells. Yet, because cancer cells and an involvement of lipoprotein receptors (139) and this process TAMs coexist and are embedded in the same interstitial fluid, appears to be enhanced in acidic microenvironments (115, 116), the extracellular lipids identified in the tumor microenvironment it is plausible to expect that it would facilitate the uptake of would be available also for the neighboring TAMs. native LDL also by TAMs. However, in contrast to the LDL- Cancer cells can activate adipocytes and other stromal cells enriched extracellular fluid of an atherosclerotic lesion, the to lipolyze their triglyceride storage, which results in release of LDL levels must be much lower in a well-vascularized tumor fatty acids (FA) into the extracellular space, from where FA are tissue due to the abundant presence of lymph capillaries. As taken by cancer cells via numerous transporters (131). Moreover, a consequence, the contribution of LDL to the generation of several blood components can cross the leaky endothelium lipid-laden TAMs appears to be minute. Thus, stimulation of the of a tumor, among them albumin-bound FA, and very low fluid-phase pinocytosis in macrophages by the low pH of the density lipoprotein (VLDL) and LDL particles, which can release intratumoral environment would rather stimulate FA uptake and FA by the local actions of lipoprotein lipase and secreted ensuing accumulation of triglycerides and glycerophospholipids phospholipase A2, thereby enhancing the FA pool of the tumor in the lipid droplets, as it has been observed in TAMs (136, 137). microenvironment (132). In this respect, a recent clinical study In analogy to the strong association between the formation of reported that high levels of free FA in serum were associated with cholesteryl ester-filled macrophage foam cells and atherosclerosis an increased risk for six types of cancer (133). A FA-enriched progression, the formation of neutral lipid droplets in TAMs intratumoral milieu would facilitate FA uptake by TAMs via has been found to correlate with various types of cancer phagocytosis mediated by the scavenger receptor CD36, resulting (84), and more recently with the human colorectal cancer in the formation of cytoplasmic lipid droplets and generating progression (134). Frontiers in Cardiovascular Medicine | www.frontiersin.org 9 February 2022 | Volume 9 | Article 777822 Lee-Rueckert et al. Macrophages in Atherosclerosis and Cancer While it is obvious that lipid accumulation in TAMs upregulation of ABC cholesterol efflux transporters in the TAMs. can induce profound effects on their functional polarization Yet, the mechanisms by which the expression and activity of (131), how such lipid-induced macrophage reprogramming the key efflux transporters are coordinately upregulated were can specifically affect tumor development is still unclear. not disclosed in any of the referred studies. These findings Thus, various reports have indicated that the accumulated highlight the notion that modulation of cholesterol metabolism lipids can polarize TAMs to gain either a pro-tumoral in TAMs can change their function, and thereby exert profound activity or an immunosuppressive phenotype. In a model effects on tumor growth. Furthermore, these novel data indicate of gastric tumor-bearing mice it was found that TAMs that the macrophage functions primed by the ABC-mediated accumulating high lipid levels were associated with M2-like cholesterol efflux could lead to opposite pathophysiological profiling, which can display pro-tumoral activity (136). Another effects in different cellular microenvironments, i.e., protecting metabolome study reported a metabolic shift in lipid-laden TAMs from atherosclerosis but promoting cancer. increasing the production of proinflammatory cytokines and reactive oxygen species (ROS), which contributed to their pro- tumoral functions (137). In contrast, another report conducted CONCLUDING REMARKS in colorectal cancer patients found that the accumulation of lipid droplets polarized bone marrow-derived myeloid Tissue-resident macrophages are central drivers in the generation cells into an immunosuppressive phenotype of TAMs (134). of an atherosclerotic lesion and also act as important auxiliary Mechanistically, the immunosuppressive phenotype found in immune cells in tumor development. Because macrophages TAMs was controlled by long-chain FA metabolism, specifically, show a high degree of plasticity, both systemic factors and unsaturated FA added to the culture medium (134). These data the local microenvironment in atherosclerotic lesions and indicated that, in addition to the well-known cytokine signaling tumors may contribute to macrophage polarization by inducing effects in modulating macrophage phenotype, lipid droplets transcriptional and functional reprogramming in these cells. are capable to modulate the immunosuppressive capacity of It is conceivable that the different microenvironments within TAMs in the tumor microenvironment. Thus, lipid droplets may a single atherosclerotic lesion or a malignant tumor are become effective targets for chemical inhibitors to block in vitro capable of inducing phenotypically different macrophage subsets. polarization of TAMs and potentially tumor growth in vivo, Moreover, the microenvironments in atherosclerotic lesions and as well. malignant tumors play primary roles by inducing lipid-loading The activity of ABC transporters can profoundly affect the of the macrophages, which is followed by alterations in their organization and activation states of lipid domains in the intracellular metabolism and phenotypic characteristics. Such plasma membrane, which can affect the functionality of the plasticity allows the macrophages to variously influence the cells in many ways (140). Because actively replicating cancer course of the disease. In this scenario, the dynamic interactions cells have a high demand for cholesterol, it follows that loss between the macrophages and their neighboring cells, be they of the ABCA1-dependent cholesterol efflux in these cells may malignant or non-malignant, are of paramount importance. promote carcinogenesis. Indeed, ABCA1 downregulation has Strikingly, the macrophages are ontogenically heterogeneous been observed in prostate cancers (141), and suppression of and both bone marrow- and embryonic-derived macrophages ABCA1 expression by oncogenic mutations or loss-of-function have been identified in the aortic intima and various tumor mutation has been linked to an accumulation of mitochondrial types. These different subsets of macrophages may have cholesterol and malignant cell transformation (142). Recent different impacts on the progression of both diseases. However, studies indicate that, in contrast to cancer cells, increasing the since tissue-resident macrophage expansion appears to be a content of cholesterol in TAMs may exert an inhibitory effect pathological factor in both atherosclerosis and cancer, blockade on tumor development. Thus, it was reported in a mouse of monocyte recruitment and the subsequent concomitant model of bladder cancer and melanoma that Abcg1–/– deficiency increase of the monocyte-attracting protein CCL2 into the shifts the macrophage phenotype toward an M1-like tumoricidal diseased tissue, would at least partially contribute to mitigating phenotype and also inhibits tumor growth, which was associated the disease progression. A question that remains unanswered with the accumulation of cholesterol in the macrophages is whether the macrophage origin or the tissue environment (143). Another work in an ovarian cancer model showed is the main driver of the functionality of the macrophage, or, that genetic deletion of ABC transporters in TAMs reverts whether the complex interplay between both factors is critical. A their tumor-promoting functions (144). This study further summary of the ontogeny and phenotypic features of the tissue- indicated that cancer cells can scavenge membrane cholesterol residing macrophage populations in an atherosclerotic plaque from TAMs resulting in the reprogramming of TAMs toward and a malignant solid tumor is given in Table 2. an immunosuppressive and tumor-promoting (M2) phenotype Because of the extreme plasticity of the macrophages, their (144). In line with this report, a very recent transcriptome study phenotype in any tissue can be considered as a continuous in human lung tumor tissues found that, although the tumors variable. However, M2-polarized macrophages appear to be the themselves were cholesterol-rich, the TAMs were depleted of predominant subpopulation both in atherosclerotic lesions and cholesterol and overexpressed ABCA1 and ABCG1 (145). Overall, in malignant tumors. Although lipid-laden macrophages are the current evidence points out that cancer cells use TAMs present in both diseases, the lipid droplets in macrophage foam as a source of cholesterol and this is associated with the cells from the atherosclerotic intima are rich in cholesteryl esters, Frontiers in Cardiovascular Medicine | www.frontiersin.org 10 February 2022 | Volume 9 | Article 777822 Lee-Rueckert et al. Macrophages in Atherosclerosis and Cancer TABLE 2 | Ontogeny and phenotypic features of the macrophage populations in atherosclerotic plaques and malignant solid tumors. Macrophages Atherosclerotic plaques Malignant solid tumors Ontogenic origin Myeloid and embryonic (3, 44, 45, 47, 48) Myeloid and embryonic (28, 79–81) Properties of myeloid Critical in driving expansion of the plaque beyond early time points Regulation of tumor immunity (81) populations and exacerbation of the evolving plaques (48) Shaping of the tumor expanding microenvironment (28) Properties of embryonic Homeostatic and metabolic regulation of the arterial wall via high Pro-fibrotic transcriptional profile (81) populations expression of the hyaluronan receptor LYVE-1 in the adventitia (47, 49) Polarization into M1- and M2-like subpopulation predominates (69) M2-like subpopulation predominates (90, 91) M2-like phenotypes* Properties of the M1-like Proinflammatory actions (32) Anti-tumoral actions (21) macrophages Predominate in progressing atherosclerotic lesions where they Express proinflammatory cytokines (IL1, IL6, and TNF alpha) and promote calcium deposition in the necrotic core (microcalcification), MHC molecules implicated in killing tumor cells (15) which may lead to plaque rupture (70) Properties of the M2- like Anti-inflammatory actions (32, 70) Pro-tumoral actions (21) macrophages High foam cell-forming capacity (69, 72) Stimulate angiogenesis and metastasis (15, 21) Dominate in regressing plaques where they promote Suppress immune response as a result of secretion of TGF-beta or macrocalcification, which tends to stabilize the plaques. They also IL-10 (15) scavenge apoptotic cells and cell debris, and thereby improving lesion repair and healing (70) Production of CCL2 Yes (73) Yes (16, 94) Production of cysteine Cathepsins D, F, S, and K (112) Cathepsins B and S (124) cathepsins Expression and function of Increased gene expression of ABCA1 and ABCG1 in macrophage Increased gene expression of ABCA1 and ABCG1 in TAMs (145) the ABC cholesterol efflux foam cells (59, 60) Immunosuppressive and pro-tumoral effect in a model of ovarian transporters Anti-atherosclerotic effect and induction of macrophage-RCT cancer (144) (53, 56, 61) Accumulation of Filled with abundant lipid droplets, which give macrophages a Moderate tendency to accumulate lipid droplets. Macrophage lipids cytoplasmic lipid droplets “foamy” appearance. The lipid droplets contain cholesteryl esters and mainly consist of triglycerides, and also of variable amounts of and lipidomic analysis in hypoxic lesion areas, they may contain also triglycerides (52, 138) glycerophospholipids and sphingomyelins (135–137) The information provided in the right column includes only selected illustrative examples of malignant tumors. Note that each malignant tumor tissue is likely to have its unique character. *Given the high plasticity of macrophages, they dynamically switch their in vivo gene expression in response to the polarizing signals present in their respective microenvironments, rather than forming terminally differentiated phenotypes during the progression of either disease. FIGURE 1 | An integrative view of the generation of lipid-laden macrophages in atherosclerosis and cancer. Tissue macrophages (LEFT) are exposed to characteristic microenvironmental conditions present in the extracellular fluids of an atherosclerotic lesion and a malignant tumor (CENTER), which influence the respective lipid cargo of the macrophage foam cells and lipid-laden tumor-associated macrophages (TAMs) (RIGHT). In the extracellular fluid: LDL, low-density lipoprotein; m-LDL, modified low-density lipoprotein; FA, fatty acid; H+, proton. In the intracellular space: CE, cholesteryl ester; TG, triglyceride; PL, phospholipid; SM, sphingomyelin. Frontiers in Cardiovascular Medicine | www.frontiersin.org 11 February 2022 | Volume 9 | Article 777822 Lee-Rueckert et al. Macrophages in Atherosclerosis and Cancer while those in TAMs appear to accumulate triglycerides, and the physiopathological role of lipid-laden macrophages is needed, variable amounts of glycerophospholipids and sphingomyelins. and this challenge particularly applies to tumor development. Independently of the lipid type, lipid loading is followed by Advances in this emergent field of research will solidify novel changes in cell signaling molecules as well as in gene expression therapeutic strategies of targeting different subsets of tissue- profile, which then modify the metabolic programming of the resident macrophages and their lipid-laden counterparts as lipid-laden macrophages. Figure 1 summarizes the characteristic promising tools in the fight against atherosclerosis and cancer— extracellular conditions and ensuing phenotypic features of -but cautiously recognizing their fundamental differences as lipid-laden macrophages in the atherosclerotic lesion and disease-modifying cellular components in these two diseases. in tumors. Finally, in contrast to the classical paradigm that lipid- AUTHOR CONTRIBUTIONS laden macrophages are culprits of chronic inflammation in atherosclerotic plaques, we and others have reported that ML-R, PK, JE-G, and JL: conceptualization and writing-review foamy macrophages may be less inflammatory than their non- and editing. ML-R: writing-original draft preparation. PK foamy counterparts. More recently, in the context of cancer, and JE-G: funding acquisition. All authors contributed to the it was found that the accumulation of lipids in TAMs can article and approved the submitted version. elicit an immunosuppressive macrophage phenotype. Regarding the effect on disease progression, we can surmise that, at FUNDING least in the initial stages of atherosclerosis, the conversion of proinflammatory macrophages into macrophage foam cells with Wihuri Research Institute was maintained by the Jenny and an anti-inflammatory phenotype would slow down the speed Antti Wihuri Foundation. This work was partly funded by the of atherogenesis, while conversion of TAMs into lipid-laden Instituto de Salud Carlos III and FEDER Una manera de hacer TAMs with an immunosuppressive phenotype would promote Europa grant PI1900136 (JE-G), and Pulsus Foundation (ML-R). the progression of cancer. Overall, more complete knowledge of CIBERDEM is an Instituto de Salud Carlos III project. REFERENCES 13. Gui T, Shimokado A, Sun Y, Akasaka T, Muragaki Y. Diverse roles of macrophages in atherosclerosis: from inflammatory biology to biomarker 1. Sreejit G, Fleetwood AJ, Murphy AJ, Nagareddy PR. Origins and diversity discovery. Media Inflamm. (2012) 2012:693083. doi: 10.1155/2012/ of macrophages in health and disease. Clin Transl Immunol. (2020) 9:e1222. 693083 doi: 10.1002/cti2.1222 14. Moore KJ, Sheedy FJ, Fisher EA. Macrophages in atherosclerosis: a 2. Epelman S, Lavine KJ, Randolph GJ. Origin and functions of tissue dynamic balance. Nat Rev Immunol. 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Dysregulation of cholesterol homeostasis in human lung cancer Copyright © 2022 Lee-Rueckert, Lappalainen, Kovanen and Escola-Gil. This is an tissue and tumour-associated macrophages. EBioMedicine. (2021) 72:103578. open-access article distributed under the terms of the Creative Commons Attribution doi: 10.1016/j.ebiom.2021.103578 License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the Conflict of Interest: The authors declare that the research was conducted in the original publication in this journal is cited, in accordance with accepted academic absence of any commercial or financial relationships that could be construed as a practice. No use, distribution or reproduction is permitted which does not comply potential conflict of interest. with these terms. Frontiers in Cardiovascular Medicine | www.frontiersin.org 16 February 2022 | Volume 9 | Article 777822