The acid sphingomyelinase inhibitor SR33557 counteracts TNF-alpha-mediatedpotentiation of IL-1 beta-induced NF-kappa B activation in the insulin-producing cell line RINm5F

Cytokines induce nitric oxide (NO) production and cell death in insulin-pro ducing cells in vitro but the signaling pathways mediating the cytokine eff ects are not well characterized. The aim of this study was to determine whe ther sphingomyelinase (SMase) participates in cytokine signaling leading to NF-kappaB activation, iNOS induction and cell death in insulin-producing c ells. Acute exposure to IL-1 beta or TNF-alpha did not affect SMase activit ies in rat insulinoma (RINm5F) cells. TNF-alpha activated NF-kappaB in gel shift experiments without inducing iNOS - as assessed by nitrite formation - whereas IL-1 beta stimulated both NF-kappaB activation and iNOS induction . Natural ceramide did not activate NF-kappaB or iNOS. However, both cerami de and TNF-alpha potentiated IL-1 beta- induced activation of NF-kappaB and iNOS. Moreover, the potentiating effects of TNF-alpha were counteracted by the acid SMase inhibitor SR33557. The combination of IL-1 beta and IFN-gam ma induced apoptosis in RINm5F cells, which was paralleled by a modest incr ease in acid SMase, whereas ceramide mainly induced necrosis. It is conclud ed that cytokine-induced beta -cell signaling is associated with the induct ion of iNOS but not with enhanced SMase activities. However, TNF-alpha -med iated potentiation of the IL-1 beta effect may involve an increased sensiti vity to basal acid SMase activity. An increased acid SMase activity may par ticipate in the execution of cytokine-induced beta -cell apoptosis.