In obese conditions, the hypertrophic expansion of adipose tissue has many common features with the growth of solid tumors. Hypoxia in obese adipose tissue as well as tumorous tissues induces the expression of the transcription factor HIF-1α to stimulate the high expression of proinflammatory cytokines, including IL-6, tumor necrosis factor-α (TNF-α) and CCL2[
78,
79]. IL-6 is involved in various biological activities, including immune regulation, hematopoiesis, and tumorigenesis[
80]. In the mammary adipose tissue of humans and mice with obesity, IL-6 can be secreted by macrophages with a proinflammatory phenotype in an NADPH oxidase 2-dependent manner. Furthermore, IL-6 stimulates macrophage polarization toward the M2 phenotype and excessive proliferation
via the upregulation of IL-4 receptor α (IL-4Rα)[
81]. IL-6 facilitates the stem-like properties of breast cancer cells by interacting with glycoprotein 130[
82]. Second, TNF-α released from tumor and stromal cells acts as a critical inflammatory factor in the obesity-induced microenvironment. A recent study showed that macrophages derived from bone marrow from IL-1β
−/− mice underwent M1 polarization, while TNF-α deletion inhibited M1 macrophage polarization[
83]. In mice with knockout of TNF-α receptors (TNFR1 and TNFR2) (RKO), adipose tissue in high-fat/sucrose-diet-fed RKO mice exhibited elevated infiltration of macrophages; however, compared with those in wild-type mice, the macrophage phenotype markers in RKO mice were characterized by anti-inflammatory M2 over proinflammatory M1 markers[
84]. Thus, TNF-α has a vital effect on the classically polarized phenotype of macrophages to promote inflammatory reactions. The CCL2/C-C chemokine receptor 2 (CCR2) pathway derived from hypertrophic adipocytes is enabled to recruit more macrophages in obese adipose tissue[
61]. Subsequently, CCL2 upregulates the mannose receptor (CD206) in stimulating CD11b
+ cells, and CD206 is a classical marker of M2-polarized macrophages[
85]. Likewise, CCL2 promotes the generation of IL-10 induced by LPS in macrophages, while the blockade of CCL2 leads to an increased level of M1-associated genes and reduces the production of M2-associated markers in human macrophages[
86]. Therefore, CCL2 not only recruits macrophages to the microenvironment but also shapes the M2-like polarization of macrophages. Moreover, IL-33 and IL-4 promote the alternative polarization of macrophages through metabolic reprogramming in a transcription factor GATA3-dependent manner[
87]. Specifically, IL-4 induces de novo adipogenesis by activating the transcription of sterol regulatory element-binding protein 1. During the process of lipogenesis, a large amount of NADPH is consumed, and the levels of reactive oxygen species are elevated, which acts as a secondary messenger to signal adequate de novo lipogenesis and promote alternative polarization of macrophages[
88].