The Transcription Factor AP-2beta Causes Cell Enlargement and Insulin Resistance in 3T3-L1 Adipocytes

We have reported the association of variations in the activating protein-2beta (AP-2beta) transcription factor gene with type 2 diabetes. This gene was preferentially expressed in 3T3-L1 adipocytes in a differentiation stage-dependent manner, and preliminary experiments showed that subjects with the disease-susceptible allele showed stronger expression in adipose tissue than those without the susceptible allele. Thus, we overexpressed the AP-2beta gene in 3T3-L1 adipocytes to clarify whether AP-2beta might play a crucial role in the pathogenesis of type 2 diabetes through dysregulation of adipocyte function. In cells overexpressing AP-2beta, cells increased in size by accumulation of triglycerides accompanied by enhanced glucose uptake. On the contrary, suppression of AP-2beta expression by small interfering RNA inhibited glucose uptake. Enhancement of glucose uptake by AP-2beta overexpression was attenuated by inhibitors of phospholipase C (PLC) and atypical protein kinase Czeta/lambda (PKCzeta/lambda), but not by a phosphatidylinositol 3-kinase (PI3-K) inhibitor. Consistently, we found activation of PLC and atypical PKC, but not PI3-K, by AP-2beta expression. Furthermore, overexpression of PLCgamma enhanced glucose uptake, and this activation was inhibited by an atypical PKC inhibitor, suggesting that the enhanced glucose uptake may be mediated through PLC and atypical PKCzeta/lambda, but not PI3-K. Moreover, we observed the increased tyrosine phosphorylation of Grb2-associated binder-1 (Gab1) and its association with PLCgamma, indicating that Gab1 may be involved in AP-2beta-induced PLCgamma activation. Finally, AP-2beta overexpression was found to relate to the impaired insulin signaling. We propose that AP-2beta is a candidate gene for producing adipocyte hypertrophy and may relate to the abnormal characteristics of adipocytes observed in obesity.