Novel effectors of insulin action in the liver

Project Summary/Abstract: Type 2 diabetes is an epidemic health problem, but in part due to the incomplete understanding on the underlying mechanisms, the current treatments or preventive options are limited. A central paradox in the pathogenesis of type 2 diabetes is the selective mode of hepatic insulin resistance, in which insulin fails to suppress hepatic gluconeogenesis but continues to stimulate lipogenesis, resulting in hyperglycemia and hypertriglyceridemia. Although the acute regulation of glucose and lipid metabolism is largely through changes in metabolite flux and allosteric modulation of key enzyme activities, the chronic regulation of metabolism requires gene transcription. As a cofactor that links multiple transcription factors to RNA polymerase II, the Mediator complex has merged as an important regulator of metabolism. The mammalian Mediator complex is composed of up to 30 subunits. Our central hypothesis is that the Mediator complex integrates hormonal and/or nutritional signals with metabolic gene expression by connecting relevant transcription factors to RNA polymerase II through specific binding domains within particular Mediator subunits. This proposal is focused on a novel interaction between the Mediator subunit MED15 and GATA4 transcription factor. In addition to our previous work showing that MED15 stimulates lipogenesis by co- activating SREBP-1c transcription factor, our preliminary studies support a role of hepatic GATA4/MED15 complex in activating gluconeogenesis and in the development of insulin resistance. Interestingly, although the molecular mechanisms are unknown, human genetic studies indicate that mutations in Gata4 gene (likely gain of function) among all reported genetic variations display the strongest correlation with hypertriglyceridemia. Our hypothesis will be tested in two Specific Aims: Aim 1 will study insulin regulation of hepatic GATA4 in glucose metabolism, and Aim 2 will study the role of hepatic MED15 coactivating SREBP-1c and GATA4 in insulin resistance. A combined genetic and gene delivery approaches together with metabolic, biochemical and molecular analyses will be used to carry out these Aims. All key animal models, reagents and techniques have been established, and supportive preliminary results have been obtained. Overall, successful completion of the proposed studies will yield a novel insight into the mechanisms underlying selective hepatic insulin resistance, and may also aid the development of novel interventional strategies against type 2 diabetes.