Abstract:

The ability of leptin to improve metabolic abnormalities in models of leptin deficiency, lipodystrophy, and even type 1 diabetes is of significant interest. However, the mechanism by which leptin mediates these effects remains ill-defined. Leptin was recently reported to regulate insulin-like growth factor–binding protein-2 (IGFBP2), and adenoviral overexpression of pharmacological levels of IGFBP2 ameliorates diabetic symptoms in many models of diabetes. We sought to determine the role of physiological levels of IGFBP2 in the glucoregulatory action of leptin. To investigate whether physiological levels of IGFBP2 are sufficient to mimic the action of leptin, we treated male ob/ob mice with low-dose IGFBP2 adenovirus (Ad-IGFBP2) or low-dose leptin. Despite similar levels of circulating IGFBP2, leptin but not Ad-IGFBP2 lowered body weight and plasma insulin and improved glucose and insulin tolerance. To elucidate the role of IGFBP2 in normal glucose homeostasis, we knocked down IGFBP2 in male C57BL/6 mice using small interfering RNA to determine whether this would recapitulate any aspect of the ob/ob phenotype. Despite successful IGFBP2 knockdown, body weight, blood glucose, and plasma insulin were unchanged. Finally, to determine whether IGFBP2 is required for the glucoregulatory actions of leptin, we prevented leptin-mediated increases in IGFBP2 in male ob/ob mice using RNA interference. Even though increases in IGFBP2 were blocked, the ability of leptin to decrease body weight, blood glucose, and plasma insulin levels were unaltered. In conclusion, physiological levels of IGFBP2 are neither sufficient to mimic nor required for the physiological action of leptin.