Osteopontin is an activator of human adipose tissue macrophages and directly affects adipocyte function

Osteopontin (OPN) is highly up-regulated in adipose tissue in human and murine obesity and has been recently shown to be functionally involved in the pathogenesis of obesity-induced adipose tissue inflammation and associated insulin resistance in mice. OPN is a protein with multiple functions and acts as a chemokine and an inflammatory cytokine through a variety of different receptors (CD44, integrins). It is expressed in many cell types including adipose tissue macrophages (ATM). However, the target cells of OPN action in obese adipose tissue are still elusive. Here, we investigated expression of OPN receptors and the impact of OPN on ATM, adipocytes, and other cells of human adipose tissue. We found broad expression of OPN receptors in different adipose tissue cell types including adipocytes. OPN stimulated inflammatory signaling pathways and secretion of cytokines in model macrophages as well as isolated human ATM. Moreover, OPN impaired differentiation and insulin sensitivity of primary adipocytes as determined by peroxisomal proliferator-activated receptor-γ and adiponectin gene expression and insulin-stimulated glucose uptake. Furthermore, OPN induced inflammatory signaling in human adipocytes. In conclusion, OPN activates ATM and interferes with adipocyte function. Thus these data underline the potential of OPN as a therapeutic target for obesity-induced complications.