Rôle de l’enzyme PAS kinase dans la régulation du facteur de transcription PDX-1 dans la cellule bêta pancréatique
Type 2 diabetes (T2D) is characterized by an impaired insulin secretion from the pancreas and a peripheral insulin resistance. In pancreatic beta cells, glucose stimulates insulin production by inducing its gene transcription, translation and secretion. Chronic and simultaneous exposure of pancreatic beta cells to elevated glucose concentrations in the presence of fatty acids lead to the deterioration of their function and the development T2D. However, the molecular mechanisms underlying this regulation are not completely known. The aim of this thesis is to identify the mechanisms involved in the regulation of insulin gene transcription.    PDX-1 is a major transcription factor that is essential for the development and function of the pancreas. In response to glucose, PDX-1 binds to the insulin gene promoter and triggers its transcription. This is inhibited by the fatty acid palmitate. In the first study, we identified two mechanisms of regulation of insulin gene transcription. An ERK1/2 (extracellular-signal-regulated kinase 1 and 2)- and a PASK (Per-Arnt-Sim kinase)-mediated pathways. We also show evidence of a third mechanism that involves inhibition of the expression of the transcription factor MafA by palmitate.  Our data indicate that PASK-mediated regulation of insulin gene transcription is a major pathway. PASK expression is induced by glucose and inhibited palmitate. Its overexpression in MIN6 cells and isolated rat islet mimics the effect of glucose on both insulin gene transcription and PDX-1 expression and it prevents the inhibitory effects of palmitate.   In the second part of this work, we identified a novel role of PASK in the pancreatic beta cell, by which it promotes PDX-1 protein stability, via phosphorylation and inactivation of GSK3 beta (glycogen synthase kinase 3 beta).  Glucose induces PDX-1 nuclear translocation which is required for its binding to the promoter of its target genes. Nuclear exclusion of PDX-1 has been observed in both ex vivo and in vivo models of beta cell dysfunction. Here, we demonstrate the importance of using isolated islets to study endogenous PDX-1 nuclear translocation as this regulation is absent in the insulin secreting cell lines MIN6 and HIT-T15.  This work has led us to identify and better understand the mechanisms of regulation of insulin gene transcription via PDX-1. The molecular targets herein identified, may contribute to the development of new therapeutic approaches T2D treatment.  Keywords : Diabetes, islets of Langerhans, pancreatic beta cells, insulin gene, PDX-1, PASK, GSK3 beta, ERK1/2, PKB, glucose, palmitate.
Glucose Palmitate PDX-1 PASK GSK3b ERK1/2 PKB Diabetes Glucose Palmitate Pancreatic beta cell Islet of Langerhans