Metformin modulates insulin receptor signaling in normal and cholesterol-treated human hepatoma cells (HepG2)

The effects of the biguanide anti-hyperglycemic agent, metformin (N,N'-dimethyl-biguanide), on insulin signaling was studied in a human hepatoma cell line (HepG2). Cells were cultured in the absence (control cells) or in the presence of 100 microM of a cholesterol derivative, hemisuccinate of cholesterol. Cholesterol hemisuccinate-treatment alters cholesterol and lipid content of HepG2 and modulates membrane fluidity. Cholesterol hemisuccinate-treatment induces a decrease in insulin responsiveness and creates an 'insulin-resistant' state in these cells. Exposure to 100 microM of metformin resulted in a significant enhancement of insulin-stimulated lipogenesis in control and cholesterol hemisuccinate-treated cells. In control cells, metformin altered glycogenesis in a biphasic manner. In cholesterol hemisuccinate-treated cells, metformin inhibited basal glycogenesis but restored insulin-stimulated glycogenesis. Hence, to understand the mechanism of metformin action, we analyzed early steps in the insulin signaling pathway, including insulin receptor autophosphorylation, mitogen-activated-protein kinase and phosphatidylinositol 3-kinase activities, in both control and cholesterol hemisuccinate-treated cells. Overall, the results suggest that metformin may interact with the insulin receptor and/or a component involved in the early steps of insulin signal transduction.