Macrophages are particularly important in adipose tissue, and contribute to link obesity-related inflammation and tumor progression. Likewise, macrophages envelope around phagocytosis of a dead or dying adipocyte, this configuration is termed as crown-like structures (CLS) [40]. Notably, the number and the density of CLS is positively correlated to high body mass index, large adipocyte size, postmenopausal status as well as insulin resistance in obese subjects, suggesting the pathophysiologic role CLS played in adipose tissue inflammation[26, 41]. Hence, macrophages infiltration in adipose tissue around tumor seems to have vital clinical significance, especially in macrophage polarization, yet it needs further investigation. The present study describes that exosomal miRNA-155 from the tumor-adipocyte crosstalk increase the secretion of CCL2 and CCL5 in adipocytes, further the chemokines recruit macrophages around the adipocytes and repolarized macrophages towards M2-like subtype. Not surprisingly, deleting macrophages in this context retard tumor growth.
Macrophages are a complicated kind of cells, which express diverse surface markers and have unique anatomical locations[7, 42]. Macrophages are demonstrated to develop from three distinct origins including embryonic precursors, adult haematopoietic stem cells (HSCs) or bone marrow derived monocytes [43]. Likewise, two main phenotypes of macrophages are the classical polarized M1 macrophages and alternatively polarized M2 macrophages[9]. CD11c is discovered as pre- inflammatory biomarkers especially expressed in tissue-resident macrophages[44] while M2 macrophages have an anti-inflammatory effect via up-regulating CD163, CD206 and CD204 [11, 42]. Meanwhile, it frequently exists macrophages reprogramming from one subset into another, partly explaining this diversity of macrophages[45]. In addition, macrophage phenotype can vary from different cancer types and intratumor districts[11]. Our results have shown that CD163+ macrophages not CD11c+ ATMs highly infiltrating in tumor-adipose regions predicted poor survival in patients undergoing breast cancer. It suggests that the macrophages mainly derived from monocyte-derived cells not tissue-resident macrophages. Importantly, M2 macrophages could be activated into an M2c phenotype, which high expression of CD163 and Mer receptor tyrosine kinase (MerTK), thus giving rise to their efficient phagocytosis of apoptotic cells[46]. Described above, macrophages wrap and engulf the dead or dying adipocytes in CLS, suggesting that M2c phenotype might be potential subset of macrophage in this condition. However, M2a macrophages, another subtype of M2 macrophages, express high levels of CD206 but not CD163. These M2a macrophages enrich genes associated with tissue remodeling and wound healing[47]. Taken together, CD163, one of special surface markers expressed M2c macrophage, may be the optimal biomarker in CLS. Importantly, tissue-resident M2c macrophages in tumor tissues enrich genes associated with tissue remodeling, and have increased expression of genes concerning immunosuppression[38]. By contrast, M2a macrophages mainly exert anti-inflammatory and wound-healing effects[47] while M2b macrophages, functioned as immunoregulation, secrete pro- and anti-inflammatory cytokines meantime and are found in tumors[38, 48, 49]. Finally, M2d macrophages represent a novel M2 subset and constitute the major inflammatory component in neoplastic tissue, contributing to angiogenesis and cancer metastasis[50]. However, adipose tissues and cancer contain macrophages with a mixed subtypes[8, 38]. Thus, comprehending the heterogeneity and plasticity of macrophage phenotypes is pivotal for obesity-related cancer.
The chemokines such as CCL2 and CCL5 have the pivotal functions in the recruit and polarization of macrophages in both tumor tissues and fatty tissues[7, 10, 41]. In the case of obesity, the hypertrophic expansion of adipose tissue has many features in common with the growth of solid tumors. The hypoxia in obese adipose tissue induces expression of the transcription factor HIF-1α, which is consistent with tumor hypoxia, thereby increasing expression of proinflammatory cytokines including CCL2 and CCL5[51, 52]. The present results indicate that CLL2 and CCL5 overexpress around the adipose tissues and are positive correlated with CD163+ macrophages infiltration in breast cancer specimens, further promoting M2-like polarization of macrophages via activating pSTAT3. Hypertrophic adipocyte-derived chemotactic CCL2/CC chemokine receptor 2 (CCR2) pathway recruits more macrophages to accumulate in the obese adipose tissue. Subsequently, CCL2 polarizes macrophages toward the M2 phenotype [53]. Additionally, CCL2 enhances the LPS-induced expression of IL-10 in macrophages, while CCL2 blockade results in increased generation of M1 polarization-related genes and cytokines, and decreased generation of M2-related markers in human macrophages[54]. Likewise, CCL5 is elevated and remains high in adipose microenvironment[55], and promotes tumor recurrence by recruiting CCR5-expressing macrophages, which may contribute to collagen deposition in residual tumors[18]. Moreover, CCL5 activates AKT signaling to recruit and repolarize TAMs via bounding to its receptor, CCR5[56]. The CCR5 inhibitors prevent recruitment of monocytes to the tumor and repolarize macrophages with anti-tumoral effects [56, 57]. Therefore, CCL2 and CCL5 not only recruit macrophages to the tumor or obesity-induced microenvironment but shape M2-skewed polarization of macrophages.
Exosome has been defined as a novel way for cell-to-cell communication that interact with a neighbor or distant target cells[19]. Potentially, the exosome-contained specific contents like miRNAs as potential biomarkers and the amount of released exosomes are altered in obesity and cancer. In current results, exosomal miRNA-155 suppresses SOCS6, a negative regulator of the JAK-STAT signaling pathway, further induces the phosphorylation of STAT3 that contributes to the secretion of CCL2 and CCL5. Inhibition of miRNA-155 suppresses the generation of CCL2 and CCL5 and blocks adipocytes-induced tumor growth. Our previous studies have demonstrated that exosomes from the tumor-adipocyte symbiosis contain several specific miRNAs such as miRNA-126, miRNA-144 and miRNA-155[21–23]. Indeed, these exosomal miRNAs play pivotal roles in mediating the differentiation and functions of macrophages[58–61]. For example, the up-regulation of miRNA-155 in macrophages increases CCL2 secretion through directly inhibiting B-cell lymphoma-6 (BCL6) expression and the decreased expression of Arginase-1 and Chil3[58, 62]. Additionally, cancer cell-secreted miRNA-155 targets PPARγ and downregulates its level[22, 23]. Meanwhile, a PPAR𝛾 ligand rosiglitazone converts high fat-induced M2 polarization of macrophages toward anti-inflammatory subtype[63]. By contrast, propranolol, a non-selective sympatholytic 𝛽-blocker, not only alleviates tumor exosome-stimulated cachexia via activating PPARγ[23] but also inhibits the production of IL-10 in M2 macrophages[64]. The specific effects of PPAR𝛾 activators in inhibiting M2 macrophages polarization and the efficacy of these compounds in immunologic/inflammatory diseases requires to further be demonstrated. Taken together, exosomal miRNAs originated from tumor cells may adequately reshape adipocytes in fatty microenvironment to convert macrophages into a pro-tumor niche.
Autophagy has multiple effects on macrophages. On the one hand, autophagy activity in macrophages enhance IL-1β secretion[65]. On the other hand, autophagy may render macrophage phagocytosis[66]. The basic helix-loop-helix transcription factor TFE3 (transcription factor 3) belongs to the MiT/TFE family[67]. It has evidence that TFE3 controls lysosomal biogenesis and autophagy by regulating the expression of several lysosomal and autophagic genes[68]. Normally, TFE3 is located in the cytoplasm and translocates to the nucleus in response to various stress[67]. Importantly, TFE3 is crucial for macrophages activation via inducing autophagy, increasing lysosomal biogenesis, and upregulating numerous proinflammatory cytokines in transcriptional way including CCL2 and CCL5[69]. Transcription factor EB (TFEB) is similar with TFE3[67, 69], and its expression elevates in IL-4 induced M2 macrophages, but reduces in tumor-conditioned medium-treated macrophages. Likewise, TFEB ablation promotes breast tumor growth[70]. Interesting, recent study shows that TFEB reduction was accompanied in some instances by a concomitant increase in TFE3 levels, which might be indicative of a compensatory mechanism between both transcription factors[69]. Consistent with our results, TFE3 but not TFEB promote autophagic flux in activated macrophages.