Overexpression of G11alpha and isoforms of phospholipase C in islet beta-cells reveals a lack of correlation between inositol phosphate accumulation and insulin secretion.

Abstract

It has been suggested that insulin secretion from pancreatic islets may be mediated in part by activation of phospholipases C (PLCs) and phosphoinositide hydrolysis. The purpose of this study was to determine whether the relatively modest fuel-stimulated insulin secretion responses of rodent beta-cell lines might be explained by inadequate expression or activation of PLC isoforms. We have found that two insulinoma cell lines, INS-1 and betaG 40/110, completely lack PLC-delta1 expression but have levels of expression of PLC-beta1, -beta2, -beta3, -delta2, and -gamma1 that are similar to or slightly reduced from those found in fresh rat islets. Adenovirus-mediated overexpression of PLC-delta1, -beta1, or -beta3 in INS-1 or betaG 40/110 cells results in little or no enhancement in inositol phosphate (IP) accumulation and no improvement in insulin secretion when the cells are stimulated with glucose or carbachol, despite the fact that the overexpressed proteins are fully active in cell extracts. Overexpression of PLC-beta1 or -beta3 in normal rat islets elicits a larger increase in IP accumulation but, again, has no effect on insulin secretion. Because the effect of carbachol on insulin secretion is thought to be mediated through muscarinic receptors that link to the Gq/11 class of heterotrimeric G proteins, we also overexpressed G11alpha in INS-1 cells, either alone or in concert with overexpression of PLC-beta1 or -beta3. Overexpression of G11alpha enhances IP accumulation, an effect slightly potentiated by co-overexpression of PLC-beta1 or -beta3, but these maneuvers do not affect glucose or carbachol-stimulated insulin secretion. In sum, our studies show a lack of correlation between IP accumulation and insulin secretion in INS-1 cells, betaG 40/110 cells, or cultured rat islets. We conclude that overexpression of PLC isoforms and/or G11alpha is not an effective means of enhancing fuel responsiveness in the insulinoma cell lines studied.