Insulin signaling in primary adipocytes in insulin sensitive and insulin resistant states

Sammanfattning: Increasing numbers of people world-wide develops the disease type 2 diabetes. Development of type 2 diabetes is characterized by a shift from an insulin sensitive state to an insulin resistant state in peripheral insulin responding organs, which originates from the development of insulin resistance in the adipose tissue. Insulin resistance in combination with reduced pancreatic insulin secretion lead to overt type 2 diabetes.In this thesis, the insulin signaling network in primary adipocytes was analyzed. Key proteins and mechanisms were studied to gain deeper knowledge of signaling both in the insulin sensitive state and in the insulin resistant state produced by rapid weight gain as well as in type 2 diabetes.The surface of the adipocyte is dotted with invaginations in the cell membrane called caveolae that act as important metabolic and signaling platforms in adipocytes, and also harbor the insulin receptor. In paper I we show that insulin stimulation of primary adipocytes results in a rapid phosphorylation of the insulin receptor and caveolin-1, and that internalization of the proteins is mediated by endocytosis of caveolae.Weight gain due to overfeeding and obesity has been associated with the development of insulin resistance in insulin sensitive tissues such as the adipose tissue. In paper II we show that short-term overfeeding for one month of lean subjects results in an insulin resistant state. At the end of the study, the subjects had developed a mild systemic insulin resistance. Moreover, in isolated subcutaneous adipocytes we found several alterations of the insulin signaling pathway that mimicked alterations found in isolated subcutaneous adipocytes from subjects with type 2 diabetes.In paper III we present a first dynamic mathematical model of the insulin signaling network in human adipocytes that are based on experimental data acquired in a consistent fashion. The model takes account of insulin signaling in both the healthy, insulin sensitive state and in the insulin resistant state of type 2 diabetes. We show that attenuated mTORC1-mediated positive feedback to control of phosphorylation of IRS1 at Ser307 is an essential component of the insulin resistant state of type 2 diabetes. A future application of the model is the identification and evaluation of drug targets for the treatment of insulin resistance and type 2 diabetes.In paper IV we examine the protein kinase that catalyzes the insulin stimulated mTORC1- mediated feedback to IRS1. We find that the phosphorylation of IRS1 at Ser307 is not likely to be catalyzed by the kinases S6K1, mTOR or PKB. However, a catalyzing protein kinase for the in vitro phosphorylation of IRS1 at Ser307 was found to be associated with the complex mTORC1.In conclusion, this thesis provide new insights and characterize mechanisms of the intrinsically complex insulin signaling network of primary adipocytes, both in insulin sensitive and insulin resistant states.