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

Accumulating evidence suggests that the cytosolic calcium-independent phospholipase A 2 (iPLA 2 β) 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 iPLA 2 β-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 iPLA 2 β in stimulated INS-1 cells in greater detail. Overexpression of iPLA 2 β in INS-1 cells leads to increased accumulation of iPLA 2 β 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 iPLA 2 β protein. In contrast, cAMP-elevating agents induce robust increases in insulin secretion and in time-dependent nuclear accumulation of iPLA 2 β fluorescence, which is reflected by increases in nuclear iPLA 2 β 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 iPLA 2 β to the nuclei, where it may serve a crucial signaling role.