Possible Protective Effect of Membrane Lipid Rafts against Interleukin-1β-Mediated Anti-Proliferative Effect in INS-1 Cells

We recently reported that pancreatic islets from pre-diabetic rats undergo an inflammatory process in which IL-1 beta takes part and controls beta-cell function. In the present study, using the INS-1 rat pancreatic beta-cell line, we investigated the potential involvement of membrane-associated cholesterol-enriched lipid rafts in IL-1 beta signaling and biological effects on insulin secretion, beta-cell proliferation and apoptosis. We show that, INS-1 cells exposure to increasing concentrations of IL-1 beta leads to a progressive inhibition of insulin release, an increase in the number of apoptotic cells and a dose-dependent decrease in pancreatic beta-cell proliferation. Disruption of membrane lipid rafts markedly reduced glucose-stimulated insulin secretion but did not affect either cell apoptosis or proliferation rate, demonstrating that membrane lipid raft integrity is essential for beta-cell secretory function. In the same conditions, IL-1 beta treatment of INS-1 cells led to a slight further decrease in insulin secretion for low concentrations of the cytokine, and a more marked one, similar to that observed in normal cells for higher concentrations. These effects occurred together with an increase in iNOS expression and surprisingly with an upregulation of tryptophane hydroxylase and protein Kinase C in membrane lipid rafts suggesting that compensatory mechanisms develop to counteract IL-1 beta inhibitory effects. We also demonstrate that disruption of membrane lipid rafts did not prevent cytokine-induced cell death recorded after exposure to high IL-1 beta concentrations. Finally, concerning cell proliferation, we bring strong evidence that membrane lipid rafts exert a protective effect against IL-1 beta anti-proliferative effect, possibly mediated at least partly by modifications in ERK and PKB expression/activities. Our results 1) demonstrate that IL-1 beta deleterious effects do not require a cholesterol-dependent plasma membrane compartmentalization of IL-1R1 signaling and 2) confer to membrane lipid rafts integrity a possible protective function that deserves to be considered in the context of inflammation and especially T2D pathogenesis.