Unraveling Mechanisms of Insulin Resistance in Type 2 Diabetes in Human Adipocytes Role of extracellular signal regulated kinase 1/2 (ERK1/2) and forkhead box protein 01 (FOX01)

Abstract: Type 2 Diabetes is characterized by hyperglycemia primarily caused due to insulin resistance in insulin responsive tissues and insufficient production of insulin by the β-cells. Insulin resistance appears to develop first in the expanding adipose tissue during caloric surplus and affects other tissues like liver and muscle by ectopic fat accumulation. In spite of significant research in field of insulin signaling, very little has been known about the mechanisms that lead to insulin resistance and T2D.We aim for network-wide knowledge of insulin signaling in human adipocytes and to identify mechanisms that can induce insulin resistance in diabetic individuals. We have herein focused on the transcriptional control of insulin via ERK and FOXO1, and have used mathematical modelling to gain a systems-level understanding of insulin signaling network.Through the work in this thesis, we present for the first time a dynamic comprehensive model for insulin signaling for the adipocytes, for both metabolic and transcriptional control, and that can simulate data from both normal and diabetic individuals. We described insulin regulation of ERK phosphorylation and showed that both its insulin sensitivity and maxima  response to insulin was curtailed in adipocytes from diabetic individuals (Paper I). Our findings indicate that insulin regulated ERK pathway exerts control on transcription not only through phosphorylation of Elk-1 but also through phosphorylation of FOXO1 and exerts translational control via phosphorylation of ribosomal protein S6 (Paper I, II). Furthermore, we showed that insulin-induced FOXO1 phosphorylation or its insulin sensitivity was not impaired in diabetic individuals, although FOXO1 protein level was reduced by 45% in adipocytes from patients with type 2 diabetes. Comprehensive analysis of the detailed insulin signaling model showed that attenuation of the feedback from mTORC1 to IRS1-Ser307 explained dominant part of the insulin resistance seen in adipocytes from diabetic individuals (Paper II). More interestingly, inhibition of FOXO1 with a dominant negative construct of FOXO1, mimicked the diabetic state in the adipocytes, with the similarity extending to both insulin signaling as well as the reduced protein levels, as seen in the diabetic adipocytes. We also show that mTORC1 and FOXO1 maintain each other’s expression/activity in the human adipocytes (Paper II, III). Our findings thus demonstrate that the interplay between mTORC1 and FOXO1 maintains normal insulin signaling in the human adipocytes.