Adipogenic role of alternatively activated macrophages in β-adrenergic remodeling of white adipose tissue

De novo brown adipogene- sis involves the proliferation and differentiation of progenitors, yet the mechanisms that guide these events in vivo are poorly understood. We previously demonstrated that treatment with a β₃-adrenergic receptor (ADRB3) agonist triggers brown/beige adipogenesis in gonadal white adipose tissue following adipocyte death and clearance by tissue macrophages. The close physical relationship between adipocyte progenitors and tissue macrophages suggested that the macrophages that clear dying adipocytes might generate proadipogenic factors. Flow cytometric analysis of macrophages from mice treated with CL 316,243 identified a subpopulation that contained elevated lipid and expressed CD44. Lipidomic analysis of fluorescence-activated cell sorting-isolated macrophages demonstrated that CD44+ macrophages contained four- to five-fold higher levels of the endogenous peroxi- some-proliferator activated receptor gamma (PPAR_γ) ligands 9-hy- droxyoctadecadienoic acid (HODE), and 13-HODE compared with CD44− macrophages. Gene expression profiling and immunohisto- chemistry demonstrated that ADRB3 agonist treatment upregulated expression of ALOX15, the lipoxygenase responsible for generating 9-HODE and 13-HODE. Using an in vitro model of adipocyte efferocytosis, we found that IL-4-primed tissue macrophages accumulated lipid from dying fat cells and upregulated expression of Alox15. Furthermore, treatment of differentiating adipocytes with 9-HODE and 13-HODE potentiated brown/beige adipogenesis. Collectively, these data indicate that noninflammatory removal of adipocyte remnants and coordinated generation of PPAR_γ ligands by M2 macrophages provides localized adipogenic signals to support de novo brown/beige adipogenesis.