Interleukin-lβ (IL-1β) has been implicated as an effector molecule of β-cell destruction in autoimmune diabetes. IL-1β inhibits insulin secretion from pancreatic β cells by stimulating the expression of inducible nitric oxide synthase (iNOS) that generates the free radical nitric oxide. IL-1β also induces the coexpression of the inducible isoform of cyclooxygenase (COX-2) that results in the overproduction of proinflammatory prostaglandins. The current studies were designed to characterize the involvement of protease(s) in the signaling pathway of IL-1β-induced iNOS and COX-2 expression by rat islets and transformed rat pancreatic β-cells. Because of the limitations of cell numbers of purified primary β-cells obtained from rat islets, biochemical and molecular studies were performed using the rat insulinoma β-cell line RINm5F. A serine protease inhibitor, Nα-P-tosyl-L- lysine chloromethyl ketone (TLCK), and a proteasome complex (26S) inhibitor, MG 132, inhibited IL-1β-induced nitrite formation, an oxidation product of nitric oxide produced by iNOS, in a concentration-dependent manner, with complete inhibition observed at 100 μmol/l and 10 μmol/l, respectively. Both TLCK and MG 132 also inhibited iNOS gene expression at the level of mRNA and protein. In an analogous manner, TLCK (100 μmol/l) and MG 132 (10 μmol/l) inhibited IL-1β-induced COX-2 enzyme activity (PGE2 formation) and COX-2 gene expression at the level of mRNA and protein. In human islets, the proteasome inhibitor MG 132 also inhibited the formation of the products of iNOS and COX-2 enzyme activity, nitrite, and PGE2, respectively. These findings suggest that the inhibitory action of TLCK and MG 132 on iNOS and COX-2 expression precedes transcription. The transcription factor NFκB is essential for activation of a number of cytokine-inducible enzymes and was evaluated as a possible sire of protease action necessary for IL-1β-induced coexpression of iNOS and COX-2. TLCK and MG 132 inhibited both IL-1β- induced activation of NFκB and degradation of IκBα by islets and RINm5F cells. These results implicate protease activation as an early signaling event in IL-1β-induced inhibition of β-cell function. This study also suggests that IL-1β-induced iNOS and COX-2 coexpression by pancreatic β- cells share a common signaling pathway in utilizing the proteasome complex (26S) and the transcription factor NFκB, and it identifies sites of intervention to prevent the overproduction of their inflammatory products.