Overexpression of G(11 alpha) and isoforms of phospholipase C in islet beta-cells reveals a lack of correlation between inositol phosphate accumulation and insulin secretion

It has been suggested that insulin secretion from pancreatic islets may be mediated in part by activation of phospholipases C (PLCs) and phosphoinosit ide hydrolysis. The purpose of this study was to determine whether the rela tively modest fuel-stimulated insulin secretion responses of rodent beta-ce ll lines might be explained by inadequate expression or activation of PLC i soforms. We have found that two insulinoma cell lines, INS-1 and beta G 40/ 110, completely lack PLC-delta 1 expression but have levels of expression o f PLC-beta 1, -beta 2, -beta 3, -delta 2, and -gamma 1 that are similar to or slightly reduced from those found in fresh rat islets. Adenovirus-mediat ed overexpression of PLC-delta 1, -delta 1, or -beta 3 in INS-1 or beta G 4 0/110 cells results in Little or no enhancement in inositol phosphate (IP) accumulation and no improvement in insulin secretion when the cells are sti mulated with glucose or carbachol, despite the fact that the overexpressed proteins are fully active in cell extracts. Overexpression of PLC-beta 1 or -beta 3 in normal rat islets elicits a larger increase in IP accumulation but, again, has no effect on insulin secretion. Because the effect of carba chol on insulin secretion is thought to be mediated through muscarinic rece ptors that link to the G(q/11) class of heterotrimeric G proteins, we also overexpressed G(11 alpha) in INS-1 cells, either alone or in concert with o verexpression of PLC-beta 1 or -beta 3. Overexpression of G(11 alpha) enhan ces TP accumulation, an effect slightly potentiated by co-overexpression of PLC-beta 1 or -beta 3, but these maneuvers do not affect glucose or carbac hol-stimulated insulin secretion. In sum, our studies show a lack of correl ation between IP accumulation and insulin secretion in INS-1 cells, beta G 40/110 cells, or cultured rat islets. We conclude that overexpression of PL C isoforms and/or G(11 alpha) is not an effective means of enhancing fuel r esponsiveness in the insulinoma cell lines studied.