Figure 1.

Obesity-induced macrophage infiltration into adipose tissue causes insulin resistance. (A) In adipose tissue in a lean state, most resident macrophages are M2 macrophages that contribute to insulin sensitivity by secreting IL-10. (B) Hyperphagia and lack of exercise cause hypertrophy of adipocytes, which induces MCP-1 secretion to the circulation, leading to the recruitment of circulating monocytes to adipose tissues. These infiltrated monocytes differentiate into activated M1 macrophages, which robustly secrete proinflammatory cytokines such as TNFα, IL-6, and MCP-1, thus contributing to low-grade inflammation in adipose tissue and a decrease of adiponectin. At the same time, these secreted cytokines cause insulin resistance in liver and skeletal muscle by acting as insulin resistance-inducing adipokines.