Fuel and hormone regulation of phospholipase C beta 1 and delta 1 overexpressed in RINm5F pancreatic beta cells

The mechanism by which glucose and other fuels stimulate phosphoinositide-s pecific phospholipase C (PLC) in pancreatic islet beta cells is not known. Previous studies have suggested that glucose may couple to PLC beta1 and PL C delta1. To determine directly if fuels activate these PLC isozymes, clone s stably overexpressing PLC pi or PLC delta1 were generated in the fuel-sen sitive beta cell line RINm5F, and secretagogue regulation of these PLC isof orms was determined. Overexpression of PLC beta1 or PLC delta1 significantl y increased PLC activity in isolated cell fractions, consistent with overex pression of active PLC isoforms in these clones. In paired experiments, sti mulation of inositol phosphate (IP) accumulation by the fuel glyceraldehyde was enhanced in clones overexpressing PLC beta1, in parallel with the G-pr otein alpha subunit activator, AlF4- suggesting a coupling between glyceral dehyde and this PLC isoform. In contrast, overexpression of PLC delta1 had no effect on glyceraldehyde- or AlF4--stimulated IP accumulation. Similarly , IP accumulation stimulated by ionomycin was enhanced in PLC beta1, but no t PLC delta1 clones, indicating that increases in intracellular free calciu m [Ca2+](i) can regulate PLC beta1 but not PLC delta1 overexpressed in this cell line. Interestingly, [Arg(8)] vasopressin-stimulated, but not carbach ol-stimulated, IP accumulation was significantly increased in clones overex pressing either PLC beta1 or PLC delta1. These studies illustrate unique pa thways coupling diverse secretagogues to specific PLC isoforms in islet bet a cells, and demonstrate that glyceraldehyde can activate PLC beta1 but not PLC delta1; whereas, vasopressin, but not carbachol, can stimulate either isoform. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.