The Acid Sphingomyelinase Inhibitor SR33557 Counteracts TNF-α-Mediated Potentiation of IL-1β-Induced NF-κB Activation in the Insulin-Producing Cell Line RINm5F

Abstract

Cytokines induce nitric oxide (NO) production and cell death in insulin-producing cells in vitro but the signaling pathways mediating the cytokine effects are not well characterized. The aim of this study was to determine whether sphingomyelinase (SMase) participates in cytokine signaling leading to NF- B activation, iNOS induction and cell death in insulin-producing cells. Acute exposure to IL-1β or TNF-α did not affect SMase activities in rat insulinoma (RINm5F) cells. TNF-α activated NF-B in gel shift experiments without inducing iNOS - as assessed by nitrite formation - whereas IL-1β stimulated both NF-B activation and iNOS induction. Natural ceramide did not activate NF-B or iNOS. However, both cera-mide and TNF-α potentiated IL-1β- induced activation of NF-B and iNOS. Moreover, the potentiating effects of TNF-α were counteracted by the acid SMase inhibitor SR33557. The combination of IL-1β and IFN-γ induced apoptosis in RINm5F cells, which was paralleled by a modest increase in acid SMase, whereas ceramide mainly induced necrosis. It is concluded that cytokine-induced p-cell signaling is associated with the induction of iNOS but not with enhanced SMase activities. However, TNF-α-mediated potentiation of the IL-1 β effect may involve an increased sensitivity to basal acid SMase activity. An increased acid SMase activity may participate in the execution of cytokine-induced P-cell apoptosis

Key Words:

• Interleukin-1
• Tumor necrosis factor-α
• Sphingomyelinase
• NF-B
• iNOS
• Insulin producing cells