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

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 Chem 276: 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.