Stimulation of insulin secretion and associated nuclear accumulation of iPLA2β in INS-1 insulinoma cells

Abstract

Accumulating evidence suggests that the cytosolic calcium-independent phospholipase A2 (iPLA2β) manifests a signaling role in insulin-secreting (INS-1) β-cells. Earlier, we reported that insulin-secretory responses to cAMP-elevating agents are amplified in iPLA2β-overexpressing INS-1 cells (Ma Z, Ramanadham S, Bohrer A, Wohltmann M, Zhang S, and Turk J. J Biol Chem276: 13198–13208, 2001). Here, immunofluorescence, immunoaffinity, and enzymatic activity analyses are used to examine distribution of iPLA2β in stimulated INS-1 cells in greater detail. Overexpression of iPLA2β in INS-1 cells leads to increased accumulation of iPLA2β in the nuclear fraction. Increasing glucose concentrations alone results in modest increases in insulin secretion, relative to parental cells, and in nuclear accumulation of the iPLA2β protein. In contrast, cAMP-elevating agents induce robust increases in insulin secretion and in time-dependent nuclear accumulation of iPLA2β fluorescence, which is reflected by increases in nuclear iPLA2β protein content and specific enzymatic activity. The stimulated effects are significantly attenuated in the presence of cell-permeable inhibitors of protein phosphorylation and glycosylation. These findings suggest that conditions that amplify insulin secretion promote translocation of β-cell iPLA2β to the nuclei, where it may serve a crucial signaling role.