Role of adiponectin and inflammation in insulin resistance of Mc3r and Mc4r knockout mice

Objective: To investigate the involvement of hypoadiponectinemia and inflammation in coupling obesity to insulin resistance in melanocortin-3 receptor and melanocortin-4 receptor knockout (KO) mice (Mc3/4rKO). Research Methods and Procedures: Sera and tissue were collected from 6-month-old Mc3rKO, Mc4rKO, and wildtype C57BL6J litter mates maintained on low-fat diet or exposed to high-fat diet (HFD) for 1 or 3 months. Inflammation was assessed by both real-time polymerase chain reaction analysis of macrophage- specific gene expression and immunohistochemistry. Results: Mc4rKO exhibited hypoadiponectinemia, exacerbated by HFD and obesity, previously reported in murine models of obesity. Mc4r deficiency was also associated with high levels of macrophage infiltration of adipose tissue, again exacerbated by HFD. In contrast, Mc3rKO exhibited normal serum adiponectin levels, irrespective of diet or obesity. and a delayed inflammatory response to HFD relative to Mc4rKO. Discussion: Our findings suggest that severe insulin resis- tance of Mc4rKO fed a HFD, as reported in other models of obesity such as leptin-deficient (Lep (ab)/Lep(ab)) and KK-A(y) mice, is linked to reduced serum adiponectin and high levels of inflammation in adipose tissue. Conversely, maintenance of normal serum adiponectin may be a factor in the relatively mild insulin-resistant phenotype of severely obese Mc3rKO. Mc3rKO are, thus, a unique mouse model where obesity is not associated with reduced serum adiponectin levels. A delay in macrophage infiltration of adipose tissue of Mc3rKO during exposure to HFD may also be a factor contributing to the mild insulin resistance in this model.