Insulin protects islets from apoptosis via Pdx1 and specific changes in the human islet proteome

Abstract

Insulin is both a hormone regulating energy metabolism and a growth factor. We and others have shown that physiological doses of insulin initiate complex signals in primary human and mouse ␤-cells, but the functional significance of insulin's effects on this cell type remains unclear. In the present study, the role of insulin in ␤-cell apoptosis was examined. Exogenous insulin completely prevented apoptosis induced by serum withdrawal when given at picomolar or low nanomolar concentrations but not at higher concentrations, indicating that physiological concentrations of insulin are antiapoptotic and that insulin signaling is self-limiting in islets. Insulin treatment was associated with the nuclear localization of Pdx1 and the prosurvival effects of insulin were largely absent in islets 50% deficient in Pdx1, providing direct evidence that Pdx1 is a signaling target of insulin. Physiological levels of insulin did not increase Akt phosphorylation, and the protective effects of insulin were only partially altered in islets lacking 80% of normal Akt activity, suggesting the presence of additional insulin-regulated antiapoptotic pathways. Proteomic analysis of insulin-treated human islets revealed significant changes in multiple proteins. Bridge-1, a Pdx1-binding partner and regulator of ␤-cell survival, was increased significantly at low insulin doses. Together, these data suggest that insulin can act as a master regulator of islet survival by regulating Pdx1.diabetes ͉ pancreatic ␤-cells ͉ programmed cell death ͉ autocrine insulin feedback signaling ͉ maturity onset diabetes of the young

Sustained production of spliced X-box binding protein 1 (XBP1) induces pancreatic beta cell dysfunction and apoptosis

“…In addition, viruses encoding truncated forms of XBP1 induced little (Ad-tXBP1s) or no apoptosis (Ad-XBP1u), and there was a close correlation between XBP1s activity and apoptosis, indicating that the increased apoptosis is specifically triggered by XBP1s. It is likely that the discrepancy between fibroblasts and beta cells is due to the role, discovered here, of XBP1s in inhibiting the expression of Pdx1 and Mafa, two key beta cell-specific transcription factors that are crucial for maintaining function [34] and, at least for Pdx1, survival [38,39]. Xbp1 knockdown partially reversed Pdx1 downregulation, but failed to prevent cytokine-or CPA-induced apoptosis, suggesting that Pdx1 downregulation is not a major determinant of beta cell apoptosis in response to cytokines or CPA.…”

Reciprocal modulation of adult beta cell maturity by activin A and follistatin

“…Cell culture Human islets were kindly provided by G. Warnock and the Ike Barber Human Islet Transplant Laboratory (Vancouver General Hospital, Vancouver, BC, Canada) and cultured as described [11]. Mouse islets were isolated from 10-to 12-week-old C57BL/6J mice as described [25] and cultured overnight in RPMI 1640 with 10% (vol./vol.)…”

“…Factorial design of experiments [6] was used to compare multiple candidate growth and differentiation factors simultaneously. Based on factors reported to influence the development or differentiation of beta cells, we examined the effects of glucose [7], nicotinamide [8], exendin 4 [9], insulin [10,11], IGF-1 [12], betacellulin [12], laminin-1 [13], epidermal growth factor [14], retinoic acid [15], gastrin17 [14], hepatocyte growth factor [16] and activin A [17]. To date, the effects of these factors and their interactions have not been systematically compared in one study.…”

A feat of metabolic proportions: Pdx1 orchestrates islet development and function in the maintenance of glucose homeostasis

“…Based upon these studies, several mechanisms have been postulated: (a) increased β cell apoptosis via and Bcl-2), with resulting loss of downregulation of anti-apoptotic genes (Bcl XL functional cell mass [56,67], (b) loss of activity of key Pdx1 target genes whose products are involved in glucose-stimulated insulin transcription and secretion (including Glut2, glucokinase, MafA, Nkx6.1, insulin) [57,63,[68][69][70][71][72][73][74][75][76], and (d) loss of new β cell formation/regeneration [64,77]; in this regard, studies suggest that the action of glucagon-like peptide 1 (GLP1) in enhancing β cell growth and formation in the adult may rely upon activation of Pdx1 in β cells and potential precursor cell types, such as those residing within ducts [74,[78][79][80][81][82].…”