Fig. 1
The adipose tissue macrophage (ATM) population is a compound system of embryonic and infiltrating monocyte-derived macrophages with the ability of self-renewal. Under physiological conditions, ATMs with different phenotypes perform a variety of physiological functions. ATMs adopt a metabolically activated (MMe) phenotype to promote dead adipocyte clearance through lysosomal exocytosis. Ly6c ATMs support normal adipose physiology upon adoptive transfer by inducing genes related to cholesterol and lipid biosynthesis. Alternatively activated MFehi ATMs display elevated cellular iron content along with an anti-inflammatory and iron-recycling gene expression profile. M2 macrophages induce increased UCP1 levels in adipocytes and promote browning by mimicking the sympathetic signaling pathway. The interplay between ATMs and adipocytes in energy metabolism, glycolysis, OXPHOS, iron handing and even interactions with the nervous system. In obesity, the microenvironment of adipose tissue changes dramatically, and lipotoxicity, hypoxia, unfolded protein responses (UPRs), oxidative stress and other harmful pathological changes occur in succession. Under these conditions, ATMs undergo changes in numbers, phenotype, and metabolic state