Insulin receptor substrate 1-induced-inhibition of endoplasmic reticulum Ca2+ uptake in beta-cells - Autocrine regulation of intracellular Ca2+ homeostasis and insulin secretion

To understand the role of the insulin receptor pathway in beta-cell functio n, we have generated stable beta-cells (beta IRS1-A) that overexpress by S- fold the insulin receptor substrate-1 (IRS-1) and compared them to vector-e xpressing controls. IRS-1 overexpression dramatically increased basal cytos olic Ca2+ levels from 81 to 278 nM, but it did not affect Ca2+ response to glucose. Overexpression of the insulin receptor also caused an increase in cytosolic Ca2+. Increased cytosolic Ca2+ was due to inhibition of Ca2+ upta ke by the endoplasmic reticulum, because endoplasmic reticulum Ca2+ uptake and content were reduced in beta IRS1-A cells. Fractional insulin secretion was significantly increased 2-fold, and there was a decrease in beta IRS1- A insulin content and insulin biosynthesis. Steady-state insulin mRNA level s and glucose-stimulated ATP were unchanged. High IFS-1 levels also reduced beta-cell proliferation. These data demonstrate a direct link between the insulin receptor signaling pathway and the Ca2+-dependent pathways regulati ng insulin secretion of beta-cells. We postulate that during regulated insu lin secretion, released insulin binds the beta-cell insulin receptor and ac tivates IRS-1, thus further increasing cytosolic Ca2+ by reducing Ca2+ upta ke. We suggest the existence of a novel pathway of autocrine regulation of intracellular Ca2+ homeostasis and insulin secretion in the beta-cell of th e endocrine pancreas.