Islet Secretory Defect in Insulin Receptor Substrate 1 Null Mice Is Linked With Reduced Calcium Signaling and Expression of Sarco(endo)plasmic Reticulum Ca2+-ATPase (SERCA)-2b and -3

Abstract

Mice with deletion of insulin receptor substrate (IRS)-1 (IRS-1 knockout [KO] mice)show mild insulin resistance and defective glucose-stimulated insulin secretion and reduced insulin synthesis. To further define the role of IRS-1 in islet function, we examined the insulin secretory defect in the knockouts using freshly isolated islets and primary ␤-cells. T he insulin/IGF-1 receptor signaling pathway plays a significant role in the regulation of both insulin secretion and synthesis (1-5). We and other laboratories have shown that the insulin receptor substrate (IRS)-1/phosphatidylinositol (PI) 3-kinase pathway is involved in the growth and function of islets (6 -9). Further evidence for a role for this substrate in islet function is derived from IRS-1 knockout (KO) mice that exhibit hyperplastic islets but show a reduced insulin content, decreased insulin mRNA, and decreased secretion in response to glucose and L-arginine (8,10,11). Overexpression of the arginine-to-glycine polymorphism at position 972 in the IRS-1 gene in ␤-cells leads to apoptosis (12) and a reduced insulin secretory response (13), whereas, conversely, overexpression of IRS-1 in ␤-cells has been shown to increase insulin secretion (14).Cytoplasmic calcium is a ubiquitous messenger that is essential for physiological responses in most mammalian cells including pancreatic ␤-cells. Glucose and several other insulin secretagogues depolarize ␤-cells, leading to activation of voltage-sensitive Ca 2ϩ channels and an influx of Ca 2ϩ ions across the plasma membrane. This increase in intracellular free Ca 2ϩ concentration ([Ca 2ϩ ] i ) triggers exocytosis of insulin (15). The Ca 2ϩ store in the endoplasmic reticulum is an important source that can also trigger insulin secretion (16). In either case, the increase in [Ca 2ϩ ] i , observed just before insulin secretion, is followed by rapid lowering of intracellular Ca 2ϩ concentrations either by extrusion and/or by uptake into the endoplasmic reticulum against a concentration gradient. In islet ␤-cells, similar to other eukaryotic cells, these active movements of Ca 2ϩ ions are mediated by Ca 2ϩ pumps (17). The sarco(endo)plasmic reticulum Ca 2ϩ -ATPase (SERCA) is one such intrinsic membrane protein with a single polypeptide chain of 110 -115 kDa. Among the SERCA isoforms that have been identified so far, SERCA-2a, -2b, and -3 are also expressed in pancreatic ␤-cells (18,19). The SERCA proteins have been shown to lower intracellular Ca 2ϩ levels in ␤-cells by sequestering the ions into the endoplasmic reticulum (20,21).Abnormalities in Ca 2ϩ signaling have been related to the development of some forms of type 2 diabetes in several rodent models and in humans (20,22). In the db/db mouse (20) and in Goto-Kakizaki rats (19), a loss of SERCA activity is associated with defects in the patterns of glucose-stimulated changes in [Ca 2ϩ ] i and potentially in insulin secretion. Furthermore, in humans with type 2 diabetes, alteration in the normal pulsatility of insulin secretion has been recognized as a signif...

Pancreatic deletion of insulin receptor substrate 2 reduces beta and alpha cell mass and impairs glucose homeostasis in mice

“…Defects in glucose-stimulated calcium mobilisation are thought to be a component of this abnormality. For example, Insr-, Igf1r-and Irs1-deficient islets display reduced increases in intracellular calcium flux, which, in the case of Irs1-null islets, are thought to be in part due to decreased expression of sarco(endo)plasmic reticulum Ca 2+ -ATPase expression [7,26,27]. PIrs2KO beta cells also showed impaired calcium mobilisation in response to glucose, suggesting that IRS2 signalling is also a component of this mechanism.…”

Participation of prolactin receptors and phosphatidylinositol 3-kinase and MAP kinase pathways in the increase in pancreatic islet mass and sensitivity to glucose during pregnancy

“…The link between insulin signaling and secretion has been demonstrated in IRS1 knockout (KO) mice, which show marked insulin secretory defects in response to glucose and reduced insulin synthesis (Kulkarni et al 1999), and in IRS2 KO mice, which primarily show a decrease in -cell mass (Withers et al 1998). More recently, the defective glucose sensing in IRS1 KO mice was found to be associated with impaired intracellular Ca 2+ signaling (Kulkarni et al 2004). Based on these observations, it is possible that the higher basal activity of the IRSs-PI3K cascades partly mediates -cell growth and improves insulin secretion in pregnant rats.…”

Signal transducer and activator of transcription 3-regulated sarcoendoplasmic reticulum Ca2+-ATPase 2 expression by prolactin and glucocorticoids is involved in the adaptation of insulin secretory response during the peripartum period

“…It is well known that Ca 2C handling in pancreatic b-cells plays an important role in the regulation of insulin secretion. Diminished SERCA expression and activity has been described as a common dysfunction related to the poor insulin secretion observed in animal models of diabetes (Roe 1994, Varadi et al 1996, Levy et al 1998, Marie et al 2001, Kulkarni et al 2004. Although both SERCA2 and SERCA3 isoforms are co-expressed in human and rat islets and in the RINm5F b-cell line (Varadi et al 1996), several studies have suggested a central role for SERCA3 in b-cell function.…”

“…-ATPase (SERCA) activity and expression, has been suggested as the cause of impaired insulin secretion in several animal models of glucose intolerance (Marie et al 2001, Kulkarni et al 2004, including the db/db mice (Roe et al 1994). db/db Mice display obesity and type 2 diabetes, due to the absence of functional leptin receptors, the main activator of STAT3 in pancreatic b-cells (Shafrir 1992, Morton et al 1999.…”