CD36 tango in cancer: signaling pathways and functions

Abstract

CD36, a scavenger receptor expressed in multiple cell types, mediates lipid uptake, immunological recognition, inflammation, molecular adhesion, and apoptosis. CD36 is a transmembrane glycoprotein that contains several posttranslational modification sites and binds to diverse ligands, including apoptotic cells, thrombospondin-1 (TSP-1), and fatty acids (FAs). Beyond fueling tumor metastasis and therapy resistance by enhancing lipid uptake and FA oxidation, CD36 attenuates angiogenesis by binding to TSP-1 and thereby inducing apoptosis or blocking the vascular endothelial growth factor receptor 2 pathway in tumor microvascular endothelial cells. Moreover, CD36-driven lipid metabolic reprogramming and functions in tumor-associated immune cells lead to tumor immune tolerance and cancer development. Notable advances have been made in demonstrating the regulatory networks that govern distinct physiological properties of CD36, and this has identified targeting CD36 as a potential strategy for cancer treatment. Here, we provide an overview on the structure, regulation, ligands, functions, and clinical trials of CD36 in cancer.

Introduction

Lipid metabolism is currently attracting increasing research attention because of its critical role in tumor initiation, development, and metastasis ¹. Adipocytes supply adequate fatty acids (FAs) to meet the high energy requirement of tumor cells and fuel their development, and tumor cells also show increased propensity to migrate toward adipose tissues, where they can uptake sufficient lipids ². CD36, a transmembrane glycoprotein (GP) that is also known as FA translocase (FAT), platelet GPIV, GP88, and scavenger receptor class B type 2 (SR-B2), is expressed on the cell surface in multiple cell types, including dendritic cells (DCs), microvascular endothelial cells (MVECs), retinal epithelial cells, monocytes, adipocytes, platelets, enterocytes, microglial cells, and podocytes ³. In tumor tissues, CD36 is expressed in tumor cells and in stromal and immune cells, but the expression level varies in distinct cell types and tumor stages. CD36 expression is invariably low in the in situ stage but increases during metastasis ⁴, and CD36 plays essential roles in lipid homeostasis, angiogenesis, immune response, adhesion, and metastasis in cancer ^{4, 5}. Therefore, CD36 is now regarded as a potential biomarker and therapeutic target for cancer.

CD36: discovery, structure, and distribution

Discovery of CD36

In 1973, Kobylka and Carraway discovered a membrane protein in breast epithelial cells that could not be hydrolyzed in milk fat globules ⁶, and in 1978, the molecule was identified as platelet GPIV, a protein that mediates thrombospondin-1 (TSP-1) binding in platelets ⁷. Subsequently, Tandon found that GPIV featured an overlapping structure with leukocyte-differentiation antigen CD36, and other investigators later determined the effects of CD36 in platelet activation ⁸ and cell adhesion ⁹. In 1993, Abumrad and colleagues demonstrated the FAT activity of CD36, after which the protein in mice was cloned and identified as the mouse ortholog of human CD36, which indicated its role in FA uptake ¹⁰. CD36 is now widely recognized as a scavenger receptor that can absorb long-chain FAs (LCFAs) and oxidized low-density lipoprotein (ox-LDL) ^{11, 12}.

Structure of CD36

The human CD36 gene is located at chromosome 7q11.2 and is related to the GP gene family. The gene is ~46 kb long and includes 15 exons, and polymorphisms have been identified at >32 bases in the human gene ¹³.

Human CD36 contains ~472 amino acids and its estimated molecular weight is 53 kDa ¹⁰; the protein harbors two transmembrane domains, a large extracellular region containing ligand-binding sites, and a single short cytoplasmic tail at each terminal (N and C) (Figure 1). The extracellular domain of CD36 forms two hydrophobic cavities that mediate the uptake of hydrophobic molecules such as FAs, phospholipids, and cholesterol ¹³. Moreover, in CD36, the CD36, LIMP-2, Emp sequence homologous (CLESH) domain residues are negatively charged and can interact with TSP-1 repeat domain 2 (TSR) to regulate tumor angiogenesis, platelet aggregation, and other reactions, and CD36 also harbors a lysine-cluster region that can bind to negatively charged ligands ¹⁴. Almost all end-stage oxidative products are negatively charged, including ox-LDL, apoptotic cells, and advanced oxidation protein products (AOPPs) ¹⁵, which might be present at adequate levels in areas that exhibit heightened metabolism, such as in tumor tissues.

Figure 1.

Structure of CD36. (Above) CD36 harbors two transmembrane domains, a large extracellular region containing ligand-binding sites, and one short cytoplasmic tail at each terminal (N and C). The extracellular domain of CD36 forms two hydrophobic cavities that bind to ligands such as fatty acids and oxidized LDL (ox-LDL). Moreover, the CLESH domain (CD36, LIMP-2, Emp sequence homologous domain) residues are negatively charged and interact with TSP-1 repeat domain 2 (TSR). CD36 undergoes multiple posttranslational modifications, including palmitoylation, acetylation, glycosylation, phosphorylation, ubiquitylation, and disulfide bonding (at three sites), and these modifications control CD36 maturation and localization in cells. Conversely, ligands bind to CD36 in different regions, and the binding leads to the activation of various downstream pathways. (Below) CD36 primary structure showing the hydrophobic domain and binding sites of general ligands. The numbers in different colors denote amino acid sites that undergo posttranslational modification: red, palmitoylation; green, glycosylation; blue, phosphorylation; dark blue, ubiquitylation; purple, acetylation.

Besides the transmembrane form of CD36, a soluble form of the protein, sCD36, is also generated, and this form was shown to be composed of the exposed extracellular domain of transmembrane CD36 that is shed by plasma proteases or a specific subset of circulating microparticles ¹⁶. The mechanism underlying sCD36 generation warrants further investigation.

CD36 distribution in cancer

In tumor tissues, CD36 is expressed in tumor cells ⁴, MVECs ¹⁷, stromal cells ⁵, and immune cells ¹⁸, and the CD36 level varies in distinct cell types. In malignant epidermal tumor cells, such as in the cells of ovarian cancer ¹⁹, gastric cancer ²⁰, glioblastoma (GBM) ²¹, and oral squamous cell carcinoma (OSCC) ⁴, CD36 expression is markedly upregulated, whereas in tumor microvessels, which support tumor development and metastasis, CD36 expression is generally downregulated. CD36 expression is also deficient in the tumor stroma. For example, CD36 expression level in breast cancer stroma is considerably lower than that in adjacent tissues. Furthermore, a strong risk factor associated with high prevalence of breast cancer is high mammographic density (MD), and high-MD (but cancer-free) tissues were also shown to express reduced levels of CD36 ⁵. These lines of evidence suggest that CD36 deficiency is representative of the tumor stroma and high cancer risk: the lower the CD36 level in the stroma, the more aggressive the tumor.

CD36 is widely recognized as a plasma membrane protein. On the cell membrane, CD36 and caveolin-1 can co-polymerize in plasma membrane domains called caveolae ²², which are specialized cell-membrane microdomains that are rich in cholesterol, sphingomyelin, signaling molecules, and receptor proteins and promote the integration of signaling cascades ²³. CD36 can be transported to organelle and cell membranes by intracellular and extracellular vesicles and can regulate FA uptake and the balance of energy metabolism. CD36 transport to the cell membrane can be facilitated by several physiological stimuli, the most potent of which are insulin and muscle contraction. Muscle contraction induces CD36 localization in the cell membrane by activating adenosine 5ʹ-monophosphate-activated protein kinase (AMPK) ²⁴, whereas insulin produces this effect by activating the phosphatidylinositol 3-kinase (PI3K)/AKT signaling axis ²⁵. Moreover, inflammation promotes CD36 transcription, translation, and translocation to the cell surface ²⁶.

CD36 is also present in endosomes, the endoplasmic reticulum (ER), and mitochondria. In the ER, CD36 undergoes modification and maturation as well as ubiquitylation-mediated inactivation. Furthermore, CD36 plays a critical role in mitochondrial FA oxidation (FAO) in skeletal muscle, and CD36 functions in regulating mitochondrial FAO together with carnitine palmitoyltransferase-1 (CPT1) ²⁷. To elucidate CD36 function(s) in cancer, it is of interest to comprehensively investigate the regulation of CD36 expression and distribution.

Regulation and localization of CD36

Regulation of CD36 gene

Several transcription factors contribute to CD36 transcription (Figure 2). For instance, the promoter of CD36 contains peroxisome proliferator-activated receptor (PPAR)-response elements (PPREs) ²⁸, and thus PPARγ ligands, including 9- and 13-hydroxyoctadecadienoic acid (HODE), ox-LDL, and prostaglandin J2 (PGJ2), upregulate CD36 mRNA expression ^{29, 30}. Moreover, interleukin (IL)-4 induces CD36 expression in macrophages by activating intracellular lipoxygenase and the PPARγ ligand PGJ2 ³¹. In gastric cancer, phosphatidylinositol transfer protein cytoplasmic 1 (PITPNC1) upregulates the RNA level of PPARG, and PPARγ then enhances the expression of CD36 and mitochondria CPT1 and thereby elevates FA absorption and promotes FAO and metastasis ³². Conversely, tamoxifen inhibits CD36 expression and ox-LDL accumulation by inactivating PPARγ signaling ³³, and CD36 downregulation induced by transforming growth factor-β (TGF-β) was found to be associated with the phosphorylation and inactivation of PPARγ ³⁴. In macrophages, ox-LDL and its metabolites 9-/13-HODE absorbed by CD36 activate PPAR through protein kinase C (PKC), protein kinase B (PKB), and p38 mitogen-activated protein kinase (MAPK) pathways ³⁵. PPAR combines with retinoid X receptor (RXR) to form dimers at CD36 promoter and enhances CD36 expression, which, in turn, increases ox-LDL uptake; this signaling contributes to foam-cell formation ^{36, 37}. By contrast, oxidized HDL (ox-HDL) inhibits CD36 expression through a PPARγ-dependent mechanism in macrophages ³⁸. Intriguingly, pregnane X receptor (PXR) reaction components, liver X receptor (LXR) nuclear receptor-binding site, and a CCAAT/enhancer-binding protein (C/EBP)-response element were also found in the promoter of CD36 ³⁷, and, interestingly, C/EBP can bind to PPREs and activate PPARγ ³⁹.

Figure 2.

Regulation of CD36. CD36 transcription can be modulated by several transcription factors and their ligands, including PPARs, C/EBP, STAT3, LXR, PXR, FoxO1, and HIF-1α. Noncoding RNAs, including miR-758-5p, MALAT1, miR-4668, and RNAs372, can also regulate CD36 mRNA levels. CD36 is posttranslationally modified in the ER during maturation, and these posttranslational modifications and the AMPK and PI3K/AKT signaling pathways modulate the translocation of CD36 to the membrane. CD36 trafficked to mitochondria functions in cooperation with CPT1 to promote FAO.

Lysophosphatidic acid (LPA) is a phospholipid that stimulates cell proliferation ⁴⁰. LPA can initiate the assembly of nuclear transcription complexes in the microvascular endothelium through protein kinase D (PKD1), which induces the recruitment of a co-repressor, nuclear receptor co-repressor 1 (NCoR1), and histone deacetylase 7 (HDAC7) to the forkhead box O1 (FoxO1) nuclear complex. FoxO1 inhibits CD36 transcription, which leads to diminished cellular binding of TSP-1 ¹⁷. This process might be related to the promotion of tumor angiogenesis. However, the opposite effect was observed in macrophages. Moreover, LPA and its analogs are agonists of PPARγ, which upregulates CD36 expression and induces lipid accumulation through ox-LDL absorption. LPA can also be excreted from stimulated cancer cells. Thus, LPA derived from activated tumor cells might regulate CD36 in adjacent or distant target cells ⁴¹.

Signal transducer and activator of transcription 3 (STAT3) can bind to the interferon-γ-activated sequence (GAS) element sequence (TTCCATGAA) in the CD36 promoter region. Nobiletin, a flavonoid isolated from citrus peel, was shown to block STAT3 binding of the GAS element and CD36 promoter and thereby inhibit angiogenesis, tumor invasion, and metastasis ⁴². Mwaikambo and colleagues recently reported that hypoxia upregulated CD36 expression on retinal MVECs through the hypoxia inducible factor-1 (HIF-1) and PI3K pathways ⁴³. Moreover, Notch signaling can be affected by plasma glucose and inflammatory lipids and is closely related to the metabolic status of cells; the Notch signal inhibits angiogenesis in vascular system development and in solid tumors, which is of important clinical significance in tumor therapy. Japs and colleagues found that in endothelial cells, CD36 transcription was upregulated by molecules that function downstream of Notch binding to the Rbp-jκ-binding sites (TG[G/A]GAA) of CD36 promoter ⁴⁴. Furthermore, CD36 expression at the transcriptional level is enhanced by macrophage colony-stimulating factor (M-CSF) ⁴⁵, natural and modified LDLs ⁴⁶, cellular cholesterol ⁴⁷, glucose molecules ⁴⁸, and IL-4 ⁴⁶, whereas CD36 transcription is downregulated by interferon ⁴⁹, glucocorticoids ⁵⁰, TGF-β ³⁴, tumor necrosis factor-α (TNF-α) ⁵¹, lipopolysaccharide (LPS) ⁵⁰, and statins ⁵².

Recently, noncoding RNAs were found to regulate CD36. In foam cells derived from THP1 macrophages, miR-758-5p reduces cholesterol uptake and lipid accumulation in cells by downregulating CD36 mRNA and protein levels ⁵³. Moreover, ox-LDLs increase the transcription of the long noncoding RNA MALAT1 through the NF-κB pathway, which enriches the binding site for β-catenin on CD36 promoter and increases the uptake of lipids in foam cells ⁵⁴. MiR-4668 and miR-26a can bind to the 3ʹ noncoding region of CD36 and inhibit CD36 transcription ⁵⁵, whereas highly-conserved RNAs372 promotes CD36 transcription by blocking the maturation of miR-4668 ⁵⁶.

In lung cancer, increased methylation of CD36 promotes cancer progression, whereas decitabine and chidamide synergistically inhibit the growth of lung tumor; these findings indicate that CD36 demethylation might represent a potential strategy for tumor therapy ⁵⁷.

Regulation of CD36 protein

Very few previous studies have investigated the regulation of CD36 protein translation, which thus warrants increased research attention in the future. Notably, however, CD36 is known to contain multiple posttranslational modification sites that can be glycosylated, phosphorylated, palmitoylated, acetylated, or ubiquitylated, and these modifications regulate CD36 stability, protein folding, and translocation.

CD36 is glycosylated in the ER and Golgi, and this promotes CD36 structural stabilization and trafficking to the plasma membrane ⁵⁸. CD36 harbors 10 potential glycosylation sites, all within the extracellular ring, and fully glycosylated CD36 is a transmembrane GP receptor of ~88 kDa. High-level glycosylation of CD36 promotes CD36 expression on the cell membrane without affecting its ligand binding ⁵⁹. In intestinal epithelial cells, enhanced glycosylation of CD36 has been reported to increase FA uptake partially through an elevation of membrane CD36 levels ⁶⁰.

CD36 contains two phosphorylation sites, Thr92 and Ser237, that can regulate extracellular ligand binding. CD36 Thr92 phosphorylation in melanoma cells is induced by phorbol-12-myristate-13-acetate (PMA) during protein synthesis and transport through the Golgi ⁶¹, and CD36 Ser237 phosphorylation inhibits FA uptake by platelets and intestinal cells. Because the two CD36 phosphorylation sites are adjacent to the CLESH domain of CD36, their phosphorylation spatially inhibits the binding of TSP-1 and CLESH domain and thereby regulates collagen adhesion. A previous study showed that endothelial CD36 is constitutively phosphorylated at Thr92, which maintains the adhesion of Plasmodium falciparum-infected erythrocytes to human dermal MVECs under flow conditions ⁶². Furthermore, in melanoma cells, CD36 Thr92 phosphorylation has been shown to reduce the recruitment of Src-family proteins after TSP-1 binding to CD36 and blunt vascular-cell signaling ⁶¹.

Palmitoylation regulates protein subcellular localization, membrane interaction, and subcellular transport, and protein palmitoylation level is positively correlated with the plasma level of palmitic acid (PA). CD36 contains four palmitoylation sites, Cys3, Cys7, Cys464, and Cys466, located in the N- and C-terminal intracellular segments. Under PA stimulation, palmitoyl acyltransferases (PATs) induce CD36 palmitoylation in the ER, which is a reversible process. Recently, two aspartate-histidine-histidine-cysteine (DHHC) motif-containing PATs, DHHC4 and DHHC5, were reported to control CD36 palmitoylation at different subcellular locations and affect CD36 plasma membrane translocation and FA-uptake activity ⁶³. Palmitoylated CD36 is located in lipid rafts in the cell membrane and mediates FA adsorption and transport. Whereas palmitoylation increases the lipid-uptake activity of CD36, which promotes the downstream pathways of inflammation and metabolic reprogramming, depalmitoylation inhibits CD36 maturation. Notably, palmitoylation deficiency was found to reduce the half-life of CD36 ⁶⁴. Therefore, inhibition of CD36 palmitoylation might represent a potential strategy for reducing CD36-mediated lipid accumulation and inflammatory signaling. Patients with non-alcoholic steatohepatitis exhibit elevated palmitoylation of CD36, which aggravates lipid accumulation and immune-cell infiltration in the liver ⁶⁵, and tumor cells exhibiting metastatic potential express increased levels of CD36 and show elevated FA-uptake activity. However, the effect of CD36 palmitoylation in tumor cells remains to be investigated.

Protein acetylation is a major mechanism for regulating various protein functions and properties, including DNA replication and transcriptional activity ⁶⁶, protein stability, FA metabolism ⁶⁷, subcellular localization ⁶⁸, and apoptosis ⁶⁹. CD36 contains four acetylation sites, Lys52, Lys166, Lys231, and Lys403, but little is known regarding their biological functions ⁶⁷.

CD36 undergoes ubiquitylation—both polyubiquitylation and monoubiquitylation—at Lys469 and Lys472. Protein polyubiquitylation leads to protein degradation by directing the target proteins to proteasomes, and in the C2C12 mouse muscle cell line, the degree of polyubiquitylation was found to affect the total CD36 expression level rather than CD36 distribution ⁷⁰. The C-terminal lysines in CD36 can also be monoubiquitylated by parkin, and this is considered to be nondegradative ⁷¹. Parkin-knockout mice show reduced hepatic CD36 levels and an attenuated maladaptive response to high-fat diet ⁷². However, because parkin can also trigger polyubiquitylation, parkin functions vary in different tissues ⁷³.

CD36 can also be degraded through the autophagy-lysosome-dependent pathway. C1q/TNF-related protein 13 (CTRP13) is a soluble adipokine that can reduce inflammatory responses, with a decrease in macrophage content, in lesion areas. In patients with coronary artery diseases, CTRP13 is invariably downregulated, and CTRP13 overexpression reduces CD36 levels by attenuating autophagy-lysosome-dependent degradation at the posttranslational level, which decreases ox-LDL uptake and foam-cell formation ⁷⁴.

In summary, CD36 can be regulated at the gene and protein levels. Elevated lipid deposition and inflammatory factors in tumor tissues might induce and alter CD36 expression and distribution, and this could contribute to a high metastatic rate. Another key factor of interest is the level of the CD36 posttranslational modifications that can regulate the binding of ligands.

Various ligands of CD36

The functional diversity of CD36 depends on the binding of distinct ligands. CD36 ligands can be classified as lipid-related and protein-related ligands. As a lipid transaminase, CD36 can absorb lipid molecules, including LCFAs ⁷⁵, ox-LDL ⁷⁶, anionic phospholipids ⁷⁷, and oxidized phospholipids (ox-PLs) ⁷⁸. Moreover, cell-membrane CD36 can transfer FAs to cytosolic FA-binding protein (FABPc) and transport it to mitochondria to provide energy for cell metabolism. CD36 also binds to protein-related ligands such as TSP-1, TSP-2, amyloid proteins, AOPPs ⁷⁹, and advanced glycation end products (AGEs) ¹⁵. TSP-1 can bind to the CLESH domain of CD36 expressed on MVECs, which then activates the downstream Src-family pathway and mediates the apoptosis of endothelial cells.

As a scavenger receptor, CD36 can bind to other transmembrane proteins on the membrane surface, such as integrins (1, 2, and 5) and four-transmembrane proteins (CD9, CD81), which jointly mediate ligand binding and signal transduction. In GBM stem cells, CD36 is co-expressed with integrin-6 and CD133, and a decrease in CD36 results in the loss of integrin-6 expression, self-renewal, and tumor initiation ²¹. Furthermore, by acting as a regulator of the assembly of toll-like receptor (TLR) 4 and 6 heterodimers, CD36 can trigger inflammatory signal transduction in microglia upon encountering exogenous stimuli and ox-LDL ⁸⁰.

CD36 also functions in innate immunity as a pattern-recognition receptor (PRR) by recognizing exogenous materials such as red blood cells infected by Plasmodium falciparum and the cell-wall components of staphylococcus and mycobacterium ⁸¹. Moreover, CD36 recognizes endogenous ligands: CD36 on DCs can recognize apoptotic cells and cross-present antigens to T cells ⁸², and CD36 on macrophages can bind to β-amyloid peptides ⁸³, apoptotic cells ⁸⁴, AOPPs ⁸⁵, and AGEs ⁸⁶.

In tumor cells, CD36 associates with multiple ligands that influence the development of cancer. FA uptake by CD36 for FAO supports metastasis ⁸⁷, whereas CD36 binding of TSP-1 inhibits tumor progression ⁸⁸, and hepatoma cells absorb ox-LDL through CD36 and SR-BI ⁸⁹. These ligands determine the biofunctions of CD36 in cancer.

Functions of CD36 in cancer

In recent decades, in-depth research on CD36 in cancer has elucidated CD36 functions in tumor-cell metabolism, anti-angiogenesis, metastasis, therapy resistance, and tumor immunoediting.

CD36 accelerates tumor growth

Tumor cells are now recognized to possess the ability for enhanced lipid deposition and metabolism to meet the requirements of rapid proliferation and development ⁹⁰. As a scavenger receptor, CD36 exhibits high affinity to transport LCFAs, which contribute to tumor growth. For instance, in a cervical cancer xenograft model, a high oleic acid diet increases tumor volume and weight, and inhibition of CD36 prevents this effect. Furthermore, oleic acid facilitates tumor development by activating Src kinase and the downstream ERK1/2 pathway in a CD36-dependent manner ⁹¹. These findings suggest that targeting CD36 could serve as a potential strategy against tumor growth.

Because of the vigorous metabolism and high energy requirement of cancers, a high demand for nutrients and oxygen frequently exists in the tumor microenvironment. Previous work has shown that in the absence of the ability to promote endogenous growth of new blood vessels, tumor diameter cannot exceed 1-2 mm ⁹². Therefore, the formation and extension of blood vessels in tumor tissues are crucial for tumor initiation, development, and metastasis. TSP-1, which belongs to a family of structurally related proteins, is secreted by activated platelets, various epithelial cells, and mesenchymal cells. TSP-1 harbors a TSR sequence that can interact with the CLESH domain in CD36, as well as a potential TGF-β activation sequence that plays a critical role in tumor-cell metastasis ⁶¹.

TSP-1, when bound to CD36 on the surface of MVECs, inhibits angiogenesis and promotes vascular apoptosis (Figure 3). On the MVEC surface, the binding of the membrane protein CD36 to TSR leads to the association of Fyn with the CD36 membrane complex, which increases the phosphorylation level of Fyn and then triggers p38 activation by caspase-3. Caspase-3 forms a proapoptotic complex and induces the expression of endogenous proapoptotic receptors, such as TNF receptor (TNFR), FAS, and the TRAIL (TNF-related apoptosis-inducing ligand) receptors death receptor (DR) 4 and DR5, which induce the apoptosis of endothelial cells. TSR treatment can reduce tumor growth and microvascular density in isogenic models of malignant melanoma and also in situ in models of human bladder cancer ⁹³. Accordingly, recombinant TSP-2 fragments can inhibit angiogenesis and tumor growth in non-glioma tumors ⁹⁴.

Figure 3.

TSP-1-CD36 signaling induces apoptosis of tumor-associated endothelial cells. TSP-1 binds to CD36 on microvascular endothelial cells and induces the phosphorylation of the cytoplasmic protein tyrosine kinase P59^fyn. Activated P59^fyn stimulates caspase-3-like proteases, which activate and induce the phosphorylation of p38 mitogen-activated protein kinase (MAPK). Nuclear translocation of MAPK results in increased expression of caspase-3 and proapoptotic receptors, which leads to apoptosis. Furthermore, mitochondrial damage releases cytochrome C and reactive oxygen species (ROS), which also trigger the caspase-3 cascade to induce apoptosis. Moreover, TSP-1 biding to CD36 results in the recruitment of Src homology 2 domain-containing protein tyrosine phosphatase (SHP)-1 to the VEGFR2 complex and SHP-1-mediated dephosphorylation of VEGFR2, which inhibits the VEGF pathway and thus leads to anti-angiogenesis.

TSP-1 was recently found to inhibit vascular endothelial growth factor (VEGF) receptor 2 (VEGFR2) level through the CD36 pathway. In the microvascular endothelium, after CD36-TSP-1 binding, Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP-1) is recruited to the VEGFR2 signaling complex, and SHP-1 dephosphorylates this complex and attenuates VEGF signal transduction ⁹⁵. TSP-1 and VEGF engage in an antagonistic relationship and jointly regulate the formation of microvessels. Thus, the CD36/TSP-1 anti-angiogenic signal switches the growth factor-mediated pro-angiogenic response to an anti-angiogenic and pro-apoptotic response ⁹⁶.

In accordance with the aforementioned finding, TSR binds to CD36 on GBM cells and upregulates DR4/5 expression. Subsequently, signaling by DR4/5 initiates GBM apoptosis through TRAIL-induced expression of caspase-8/3/7 ⁸⁸. Together, these results suggest that TSR plays an antitumor role in tumor cells and tumor-related vascular endothelial cells, which reflects the considerable potential TSR holds for use in cancer therapy.

Histidine-rich glycoprotein (HRG) regulates microvascular CD36-TSR signaling; HRG is a 75-kDa protein synthesized by hepatocytes and circulated in plasma at high concentrations. Platelet α-particles are rich in HRG (109-371 ng/platelets), which can be released into specific microenvironments after platelet activation. HRG is a modular protein that can bind to proteoglycans, matrix proteins, divalent cations, and coagulin, and HRG contains a domain that is similar to the CLESH domain of CD36, which can bind to the TSRs of TSP-1/2 and angiostatin. Therefore, HRG, as a soluble decoy receptor of TSP-1/2, blocks the binding of CD36 to TSP-1 and inhibits the anti-angiogenic activity of TSP-1. In an experimental model in which mice were transplanted with Lewis lung-cancer and B16F1 melanoma cells, tumor volume was found to be markedly lower in the HRG-knockout group than in wild-type and CD36-knockout groups, and vascular density was also substantially lower in the HRG-knockout group than in CD36-knockout and wild-type groups ⁹⁷.

Thus, TSP-1 and TSR associate with CD36 and trigger MVEC apoptosis and block VEGF signaling and thereby inhibit tumor growth. Consequently, targeting this pathway might provide a potential strategy for cancer therapy.

CD36 promotes tumor metastasis

Tumor metastasis-initiating cells (MICs) represent a rare and unique population of cells derived from primary tumors that can seed metastasis colonies in secondary organs ⁹⁸. GBM stem cells expressing high levels of CD36 were found to enhance self-renewal and tumor initiation ²¹. In 2016, Pascual reported that in human OSCC, CD44⁺ cell subsets highly express CD36 and lipid-metabolism genes and exhibit the unique ability to initiate metastasis ⁴. CD36-induced metastasis potential has been reported to be based on the lipid metabolism of cancer cells ⁹⁹. Intriguingly, blockade of CD36 in OSCC cells also causes severe intercellular lipid accumulation in cancer cells, which leads to lipotoxic cell death and impaired metastasis ⁴.

CD36 mediates the uptake of FAs, which are key nutrients for tumor metabolism. PA uptake by CD36 has been shown to cause AKT phosphorylation in gastric cancer cells and inhibit glycogen synthase kinase 3β (GSK-3)/β-catenin degradation and promote gastric cancer metastasis ²⁰. Tumor-associated adipocytes can provide sufficient FAs to tumor cells and thus promote their proliferation and metastasis. For example, gastric cancer commonly metastasizes to the greater omentum, which is rich in fat cells, and adipocytes also induce CD36 expression in metastatic and primary human ovarian tumors ¹⁰⁰. In FAO, the rate-limiting step is FA transport into mitochondria, a process in which CPT1 and CPT2 are the key enzymes. CD36 present on mitochondria in human skeletal muscle cells can bind to CPT1, and CD36 upregulation in mitochondria is correlated with increased FAO. Notably, patients with myeloid leukemia who express elevated levels of CD36 also exhibit increased FAO, enhanced expression of oxidative phosphorylation (OXPHOS) and reactive oxygen species (ROS)-related genes, and poor prognosis ¹⁰¹.

CD36 also regulates FAO through the liver kinase B1 (LKB1)-AMPK pathway. AMPK regulates metabolic balance in cells, and this activates the uptake and metabolism of FAs and glucose. CD36 functions as an energy sensor of extracellular FA levels. When the FA level is low, CD36 forms a complex with Fyn and remains inactive, but when PA binds to CD36, Fyn dissociates from the CD36 intracellular domain and fails to phosphorylate LKB1; subsequently, elevated intracytoplasmic LKB1 phosphorylates and activates AMPK, which activates CPT1 and upregulates FAO by inhibiting acetyl-CoA carboxylase (ACC) and reducing malonyl-CoA content ¹⁰². Moreover, AMPK promotes CD36 transport to and localization at the cell membrane ²². For instance, in acute monocytic leukemia, bone marrow adipocytes can promote the expression of CD36 and AMPK genes in cancer cells, which increases FAO and mediates the survival and development of cancer cells ¹⁰³.

Epithelial-mesenchymal transition (EMT) promotes cancer-cell metastasis ¹⁰⁴. Aritro and colleagues found that CD36 uptake of FA led to EMT through Wnt and TGF-β signaling pathways ¹⁰⁵. In the case of diabetic nephropathy, CD36 knockdown in HK-2 cells inhibited high glucose (HG)-induced ROS generation and TGF-β1 expression, which prevented HG-induced EMT in these cells ¹⁰⁶. An analysis of The Cancer Genome Atlas revealed that increased CD36 expression was consistently associated with high EMT in all cancers ¹⁰⁷, although Pascual and colleagues found that CD36⁺ MICs expressed lower levels of EMT genes as compared with CD36^- MICs, which indicated that EMT might not participate in the metastasis initiation of all tumor types ¹⁰⁸.

Taken together, these findings suggest that MICs and the cancer EMT process are both regulated by lipid metabolism through the CD36-dependent pathway. Therefore, CD36 might represent a potential target for the prevention and therapy of cancer metastasis.

As discussed above, TSP-1 has long been accepted to exert antitumor effects in various cancers; however, glioma cells overexpressing a TSP-1 fragment were found to be more aggressive and to decrease survival in animals as compared with glioma cells not expressing the fragment ¹⁰⁹. These results suggest that although the TSP-1 fragment plays an anti-angiogenic role, it might also perform a cancer-promoting function. TSP-1 has been reported to promote metastatic spread by enhancing tumor-cell embolus formation and facilitating cancer-cell adhesion during metastasis ¹¹⁰. Furthermore, TSP-1 can release activated TGF-β by interacting with its potential precursors through RFK and WXXW sequences ¹¹¹. TGF-β induces matrix production and alters integrin expression and thus promotes tumor growth by stabilizing angiogenesis ¹¹², and activated TGF-β also upregulates urokinase (plasminogen activator), urokinase receptor, and plasminogen activator inhibitor-1 (PAI-1) in the proteolytic system and enhances the invasiveness of malignant tumors ¹¹³. Therefore, the TSP-1 effect on malignant metastasis partially depends on the TGF-β concentration at the tumor site in specific tumors. The pro-angiogenic effect of TGF-β and the anti-angiogenic effect of TSP-1 overexpression counterbalance each other, and this hinders the use of TSP-1 as an anti-angiogenesis drug in tumor treatment, a matter that warrants further investigation.

CD36 regulates chemoresistance and radioresistance

Resistance to radiotherapy and chemotherapy poses a major challenge in cancer treatment. Tumor stem cells have been widely reported to play a crucial role in chemotherapy resistance, and metabolic alterations in these cells might be the key to their ability to maintain stemness and drug resistance. In acute myeloid leukemia cells that are resistant to arabinocytosine (AraC, also known as cytarabine), FAO and OXPHOS are elevated and CD36 is upregulated. Notably, reducing FAO, mitochondrial protein synthesis, and electron transfer maintains OXPHOS at a low level, and this can markedly enhance the anti-leukemia effect of AraC ¹⁰¹. In breast cancer, breast adipocytes provide adequate FAs for breast cancer stem cells (BCSCs). Whereas JAK/STAT3 signaling regulates lipid metabolism to promote FAO in BCSCs, which maintains the stemness and chemotherapy resistance of breast cancer cells, inhibiting FAO sensitizes breast cancer cells to chemotherapy ¹¹⁴. Because STAT3 can increase CD36 expression at the transcriptional level, these results suggest that CD36 could play a critical role in the chemoresistance of these cells. Moreover, as a result of their unique metabolic characteristics, CD36⁺ leukemic stem cells accumulate in adipose tissues; this allows the cells to be protected by the gonadal adipose tissue microenvironment and thereby escape the effects of chemotherapy ¹¹⁵.

In addition to killing tumor cells, radiotherapy inevitably causes damage to normal tissues and thus affects the overall therapeutic effect. A previous study showed that CD47 interaction with TSP-1 on the cell surface attenuated the post-treatment recovery ability of normal cells, and that normal tissues of mice showed enhanced resistance to radiation damage when the CD47-TSP-1 interaction was blocked ¹¹⁶. Because CD47 and CD36 can competitively bind to TSP-1, CD36 might endow tumor-radiotherapy resistance to normal tissues and protect nontumor tissues against radiation damage and enhance the efficacy of radiotherapy.

CD36 modulates tumor immunity

In tumor tissues, immune cells play critical roles in facilitating tumor development and progression (Figure 4). CD36 regulates cytokine production, antigen presentation, phagocytosis, and immune tolerance. In macrophages, CD36 and TLRs function in the innate immune response by recognizing and engulfing pathogens and pathogen-associated molecules, including LPS from various gram-negative and gram-positive bacteria ¹¹⁷. CD36 can activate TLR4 and TLR6 to promote the release of inflammatory cytokines ⁸⁰. Moreover, CD36-mediated internalization of ox-LDL leads to lysosomal membrane rupture and activation of NOD-like receptor protein-3 (NLRP3) inflammasome and thereby triggers sterile inflammation ¹¹⁸. CD36 deletion can alleviate the aseptic hepatitis caused by concanavalin A, suppress the infiltration of CD4+ and CD8+ T cells and natural killer (NK) cells, downregulate the levels of p-JNK, p-IKK, and cleaved caspase-3, and reduce the expression of various inflammatory mediators, including TNF-α, C-X-C motif ligand 10 (CXCL10), IL-1, monocyte chemoattractant protein-1 (MCP-1), and IL-6 ¹¹⁹. Because inflammation triggers the initiation, proliferation, invasion, and metastasis of tumor cells, reducing CD36-mediated sterile inflammation might provide the basis for a new mode of antitumor treatment.

Figure 4.

CD36 functions in tumor microenvironment. CD36 regulates downstream Src-family kinases to promote anti-angiogenesis, FAO, and chemoresistance and radioresistance, which lead to tumor metastasis. CD36 also activates Wnt/TGF-β signaling to facilitate tumor metastasis through EMT. CD36 uptakes multiple lipids, such as ox-LDL, LCFAs, and cholesterol. The lipid deposition in immune cells leads to aseptic inflammation and dysfunction of antigen presentation in DCs, which induce tumor immunosuppression. Moreover, CD36 can bind to apoptotic cells and activate cross-priming, which might lead to immunosuppression and tumor development.

Lipid deposition causes dysfunction of antigen presentation by DCs and T-cell activation, and this promotes tumor immune tolerance and progression. As compared with DCs from tumor-free mice and healthy people, the DCs of tumor-bearing mice and cancer patients have been reported to contain a considerably larger proportion of triglycerides ¹²⁰. CD36 is known to play an essential role in lipid uptake, but further investigation is required to elucidate the function of CD36 in lipid deposition in DCs.

In addition to internalizing oxidized lipids, phagocytes and antigen-presenting cells recognize anionic phosphatidylserine (PS) on the surface of apoptotic cells through CD36 ¹²¹. In conjunction with αvβ5 integrin, CD36 recognizes and binds to specific epitopes on the surface of apoptotic cells, and activates CD8+ T cells through cross-priming and thus mediates the killing of malignant or infected cells ¹²². Therefore, reduced CD36 expression might help tumor cells in evading the immune system. Macrophages do not express αvβ5 integrin on the surface, and their surface CD36, in coordination with the entire αvβ3 integrin protein, phagocytoses apoptotic cells, but does not mediate antigen presentation.

In the tumor microenvironment, rapid proliferation of tumor cells leads to a lack of energy supply in the tumor-tissue core, which results in the formation of necrotic foci; consequently, a large number of apoptotic and necrotic cells can be consumed by tumor-associated macrophages (TAMs) and tumor-associated DCs in a CD36-denpendent manner. Because the phagocytosis of DCs and the presentation of apoptotic bodies can affect the TCR library of Treg and Tconv cells ¹²¹, tumor-associated DC phagocytosis might represent one of the mechanisms leading to tumor immune tolerance. Moreover, incubation of macrophages with apoptotic cells was found to increase the release of anti-inflammatory mediators and decrease the secretion of LPS-induced proinflammatory mediators, and therefore the apoptosis of these cells did not elicit a strong inflammatory response ¹²³. Whether this mechanism mediates the development of the immunosuppressive environment in tumor tissues should be of future research interest.

CD36 is expressed during the late period of differentiation in tissue-resident macrophages and circulating and bone marrow-derived monocytes ¹²⁴. Macrophages can polarize toward both M1 (classical) activation and M2 (alternative) activation ¹²⁵. CD36-mediated uptake of triacylglycerol substrates and their subsequent lipolysis by lysosomal acid lipase (LAL) was found to promote M2 activation of macrophages, which then exhibited elevated OXPHOS and enhanced spare respiratory capacity (SRC) ¹²⁶. Macrophage polarization toward M2 activation can also be induced by ox-LDL ¹²⁷, but ox-LDL uptake through CD36 in eosinophils was found to promote the release of proinflammatory cytokines, which facilitated macrophage M1 polarization ¹²⁸. Notably, CD36 expressed in macrophages plays a critical role in atherosclerosis ¹²⁹. Blood-circulating monocytes transmigrate into the arterial intima, where the cells differentiate into macrophages and internalize ox-LDL through CD36 ⁷⁶. Moreover, internalized ox-LDL activates PPAR-γ, which, in turn, upregulates CD36 expression. Further uptake of excess ox-LDL contributes to cholesterol deposition in macrophages and generates foam cells, which is the initial critical step of atherosclerosis ^{29 130}. The interaction of ox-LDL and CD36 also induces cytokine release and immune-cell infiltration, which accelerate the process of atherosclerosis ¹³¹.

TAMs are a group of tumor-infiltrating immune cells that are notable for their tumor-promoting ability ¹³². In breast cancer, apoptotic tumor cells release miR-375 and bind LDL, and this mediates their uptake by TAMs through CD36. Apoptotic tumor cells also release LCFAs and ox-PLs and thus act as carriers of miR-375 for CD36-mediated uptake, and miR-375, in turn, enhances TAM migration and infiltration into tumor spheroids. Moreover, in tumor cells, miR-375 increases CCL2 expression and thereby enhances the recruitment of macrophages, and knocking down CD36 largely prevents miR-375 uptake into macrophages ¹³³. Thus, CD36 facilitates the migration and recruitment of TAMs in tumor tissues, which promotes tumor progression.

Metastasis-associated macrophages (MAMs) function in tumor metastasis by potentiating extravasation and enhancing the survival of metastasizing cancer cells. MAM precursor cells are found in metastasis sites at an early stage and accumulate during metastatic tumor growth ¹³⁴. Distinct from normal circulating classical monocytes, MAMs harbor specific markers such as CD36 that contribute toward suppressing the cytotoxicity of activated CD8+ T cells ¹⁸. In this regard, CD36 might serve as a marker of a subset of MAMs.

Tumor microvesicles are a unique type of extracellular microvesicles that circulate in the peripheral blood of patients with metastatic cancer and contribute to distant metastasis. CD36 expressed on specific tissue immune cells bind and engulf tumor microvesicles, which leads to the invasion and extravasation of microvesicles from vessel walls and promotes metastasis ¹³⁵. Therefore, targeting CD36 could represent a potential immunotherapeutic strategy for cancer.

Clinical trials targeting CD36 for cancer treatment

As discussed in preceding sections, CD36 is closely related to tumor initiation, development, invasion, and metastasis, and thus numerous preclinical studies and clinical trials focused on CD36 have been and are being performed (Table 1). Because CD36 mediates FA uptake on the surface of tumor cells and is related to the metastatic potential of tumor cells, inhibiting CD36-mediated FA uptake on the tumor-cell surface could serve as a strategy for treating metastatic tumors. For example, FA uptake was found to be enhanced in human prostate cancer, and deleting CD36 in the prostate of cancer-susceptible Pten^-/- mice was reported to markedly reduce FAs uptake, which decreased the abundance of oncogenic signaling lipids and attenuated cancer progression ¹³⁶. These findings support the conclusion that CD36-mediated FA uptake is critical for prostate cancer development, and targeting FA uptake might therefore represent an effective strategy for treating prostate cancer. Moreover, synthetic amphipathic helical peptides (SAHPs) that replicate the apolipoprotein domain were found to be capable of substituting for the binding of apolipoprotein to CD36; ELK-SAHPs effectively inhibited pulmonary inflammation and dysfunction in a sepsis model by inhibiting the FA-transport activity of CD36 ¹³⁷. Although the SAHPs clearly inhibited CD36 in the kidney and lung, their functional effect in cancer and in other tissues remains unclear.

Table 1.

Clinical trials and preclinical drugs targeting CD36 for antitumor treatment.

Abbreviations: TRAIL, TNF-related apoptosis-inducing ligand; TSP, thrombospondin; TSR, TSP-1 repeat domain 2.

CD36 expressed in the tumor microvascular endothelium binds to TSP-1 expressed in tumor tissues and mediates the apoptosis of tumor vascular endothelial cells. Cancer-treatment studies targeting TSP-1 binding to CD36 have yielded promising results ¹³⁸. Three modified TSR peptides, ABT-526, ABT-510, and ABT-898, were all found to function effectively in animal experiments, and ABT-510 even reached phase 2 clinical trials. However, ABT-510 failed in phase 2 clinical trials because of ineffective performance and severe adverse events ¹³⁹. Pfizer developed CVX-22 and CVX-045, which contain TSP-1-derived peptidomimetics and an antibody scaffold: CVX-045 caused potent regression of tumor xenografts by reducing tumor microvasculature and increasing necrotic cores, but it failed in a phase 1 clinical trial because of severe side effects and unsatisfactory efficacy ¹⁴⁰; like CVX-045, CVX-22 was also found to produce beneficial effects in the preclinical phase, and thus this compound might serve as a potential CD36-targeting drug in the future ¹⁴¹.

CD47 acts as a “don't eat me” signal to macrophages. TSP-1 interacts with CD47 on immune cells and thereby inactivates antitumor immunosurveillance, and, concomitantly, attenuates the antitumor effect by interacting with CD36. Recently, a cyclic peptide named TAX2, derived from CD47, was discovered to bind TSP-1 directly and antagonize TSP-1/CD47 interaction, which inhibited angiogenesis through the CD36-dependent pathway. More intriguingly, the effects of TAX2 appeared to be restricted to the tumor-associated environment in which TSP-1 was overexpressed ¹⁴². TAX2 induced a switch in TSP-1 binding from CD47 to CD36, which led to an anti-angiogenesis effect in tumors. TAX2 reduced the tumor volume in allogeneic melanoma and effectively inhibited xenograft growth in pancreatic cancer, as well as caused the destruction of tumor-related vascular networks ¹⁴³. In future work, TAX2 use might offer a new strategy for tumor anti-angiogenesis therapy.

Conclusion and Perspectives

CD36 is a multifunctional molecule involved in several diseases, including cancer, metabolic diseases, Alzheimer's disease, and infection. CD36 plays a critical role in lipid uptake, immunological recognition, inflammation, molecular adhesion, and apoptosis, which influence the initiation, development, and progression of cancer. The regulation and localization of CD36 are controlled by complex signaling pathways, which might provide potential drug targets for tumor therapy. CD36 posttranslational modifications, including palmitoylation, phosphorylation, and glycosylation, modulate the translocation and ligand binding of CD36, and this could also potentially offer promising targets for regulating tumor-cell metabolism. Diverse ligands and pathways lead to tumor anti-angiogenesis, metastasis, immunity, and therapy resistance through CD36. CD36 interacts with TSP-1 through the binding of the CLESH domain and TSR, and this contributes to MVEC apoptosis and inhibits VEGF signaling and thereby leads to anti-angiogenesis in tumor tissues. Although TSP-1 might also promote tumor progression through TGF-β in certain tumor types, drugs targeting the TSP-1-CD36 anti-angiogenic effect perform favorably and have already entered phase 2 clinical trials. Moreover, CD36 was also identified to function as the enzyme FAT in the uptake of LCFAs, ox-LDLs, PS, and cholesterol, which promote tumor metastasis. High-level expression of CD36 in tumor cells not only facilitates FAO to supply adequate energy for tumor progression and chemoresistance, but also promotes EMT to enhance tumor-cell migration. Because ablation of CD36-mediated FA uptake attenuates tumor progression, targeting FA uptake by CD36 could represent an effective strategy for tumor treatment. As a scavenger receptor, CD36 plays a vital role in innate immunity. CD36 promotes sterile inflammation and the protumor ability of tumor-associated immune cells, which could serve as a potential target for tumor immunotherapy.

Several antitumor drugs targeting CD36 have already entered clinical trials, but the majority of these have failed because of severe adverse events and unsatisfactory performance. Thus, it is necessary to further investigate the regulation and downstream signaling pathways of CD36 during the manipulation of this target in precise clinical translation in the future.

Abbreviations

ACC

acetyl-CoA carboxylase

AGEs

advanced glycation end products

AMPK

adenosine 5'-monophosphate-activated protein kinase

AOPPs

advanced oxidation protein products

AraC

arabinocytosine

BCSC

breast cancer stem cells

C/EBP

CCAAT/enhancer-binding protein

CLESH

CD36, LIMP-2, Emp Sequence Homologous domain

CPT-1

carnitine palmitoyltransferase-1

CTRP13

C1q/tumor necrosis factor-related protein 13

CXCL10

C-X-C motif ligand 10

DCs

dendritic cells

DHHC

aspartate-histidine-histidine-cysteine

DR

death receptor

EMT

epithelial-mesenchymal transition

ER

endoplasmic reticulum

FABPc

cytosolic fatty acid binding protein

FAO

fatty acid oxidation

FAs

fatty acids

FAT

fatty acid translocase

FoxO1

forkhead box O1

GAS

interferon-gamma activated sequence

GBM

glioblastoma

GM-CSF

granulocyte-macrophage colony-stimulating factor

GP

glycoprotein

GSK-3

glycogen synthase kinase 3β

HDAC7

histone deacetylase 7

HG

high glucose

HIF-1

hypoxia inducible factor 1

HODE

hydroxyoctadecadienoic acid

HRG

histidine-rich glycoprotein

IL4

interleukin-4

IRBCs

plasmodium falciparum-infected erythrocytes

LAL

lysosomal acid lipase

LCFAs

long chain fatty acids

LPA

lysophosphatidic acid

LPS

lipopolysaccharide

LXR

liver X receptor

MAM

metastasis-associated macrophages

MAPK

mitogen-activated protein kinase

MCP-1

monocyte chemoattractant protein-1

M-CSF

macrophage colony-stimulating factor

MD

mammographic density

MICs

metastasis-initiating cells

MVECs

microvascular endothelial cells

NCoR1

nuclear receptor co-repressor 1

NK cells

natural killer cells

NLRP3

NOD-like receptor protein-3

OSCC

oral squamous cell carcinomas

ox-HDL

oxidized HDL

ox-LDL

oxidized low-density lipoprotein

OXPHOS

oxidative phosphorylation

ox-PLs

oxidized phospholipids

PA

palmitic acid

PAI-1

plasminogen activator inhibitor 1

PATs

palmitoyl-acyl transferases

PGJ2

prostaglandin J2

PI3K

phosphatidylinositol 3-kinase

PITPNC1

phosphatidylinositol transfer protein, cytoplasmic 1

PKB

protein kinase B

PKC

protein kinase C

PKD1

protein kinase D

PMA

phorbol-12-myristate-13-acetate

PPAR

peroxisome proliferator-activated receptor

PPREs

peroxisome proliferator-activated receptor response elements

PRR

pattern recognition receptor

PS

phosphatidylserine

PXR

pregnane X receptor

ROS

reactive oxygen species

RXR

retinoid X receptor

SAHPs

synthetic amphipathic helical peptides

SHP-1

Src homology 2 domain-containing protein tyrosine phosphatase 1

SRC

spare respiratory capacity

STAT3

signal transducer and activator of transcription 3

TAMs

tumor-associated macrophages

TGF-β

transforming growth factor-beta

TLR

toll-like receptor

TNFR

tumor necrosis factor receptor

TNFα

tumor necrosis factor-alpha

TRAIL

TNF-related apoptosis-inducing ligand

TSP-1

thrombospondin-1

TSR

TSP-1 repeat domain 2

VEGF

vascular endothelial growth factor

VEGFR2

vascular endothelial growth factor receptor 2