Insulin receptor substrate 1 regulation of sarco-endoplasmic reticulum calcium ATPase 3 in insulin-secreting β-cells

Abstract

We have previously characterized an insulin receptor substrate 1 (IRS-1)-overexpressing β-cell line. These β-cells demonstrated elevated fractional insulin secretion and elevated cytosolic Ca2+ levels compared with wild-type and vector controls. This effect of IRS-1 may be mediated via an interaction with the sarco-endoplasmic reticulum calcium ATPase (SERCA). Here we demonstrate that IRS-1 and IRS-2 localize to an endoplasmic reticulum (ER)-enriched fraction in β-cells using subcellular fractionation. We also observe co-localization of both IRS-1 and IRS-2 with ER marker proteins using immunofluorescent confocal microscopy. Furthermore, immuno-electron microscopy studies confirm that IRS-1 and SERCA3b localize to vesicles derived from the ER. In Chinese hamster ovary-T (CHO-T) cells transiently transfected with SERCA3b alone or together with IRS-1, SERCA3b co-immunoprecipitates with IRS-1. This interaction is enhanced with insulin treatment. SERCA3b also co-immunoprecipitates with IRS-1 in wild-type and IRS-1-overexpressing β-cell lines. Ca2+ uptake in ER-enriched fractions prepared from wild-type and IRS-1-overexpressing cell lines shows no significant difference, indicating that the previously observed decrease in Ca2+ uptake by IRS-1-overexpressing cells is not the result of a defect in SERCA. Treatment of wild-type β-cells with thapsigargin, an inhibitor of SERCA, resulted in an increase in glucose-stimulated fractional insulin secretion similar to that observed in IRS-1-overexpressing cells. The colocalization of IRS proteins and SERCA in the ER of β-cells increases the likelihood that these proteins can interact with one another. Co-immunoprecipitation of IRS-1 and SERCA in CHO-T cells and β-cells confirms that these proteins do indeed interact directly. Pharmacological inhibition of SERCA in β-cells results in enhanced secretion of insulin. Taken together, our data suggest that interaction between IRS proteins and SERCA is an important regulatory step in insulin secretion.