Fig. 2.

Functional differences between M1 ATMs and M2 ATMs. In the lean state, the major ATMs are the M2 phenotype, and they play roles in maintaining tissue homeostasis. During cold stimulation and caloric restriction, M2 ATMs are involved in the browning of subQ. As obesity progresses both M1 and M2 ATMs become involved in tissue remodeling. M1 ATMs accumulate around necrotic adipocytes and form crown-like structures (CLSs), which further contribute to polarization into the M1 phenotype by activating HIF-1α. By contrast, M2 ATMs somehow prevent the development of the lipodystrophic phenotype, and they may play a role in suppressing inflammatory responses induced by M1 ATMs. In some situations, M2 ATMs induce fibrosis, thereby aggravating insulin resistance