Evidence for Involvement of the Proteasome Complex (26S) and NF kappa B in IL-1 beta-Induced Nitric Oxide and Prostaglandin Production by Rat Islets and RINm5F Cells

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Abstract

Interleukin-1 beta (IL-1 beta) has been implicated as an effector molecule of beta-cell destruction in autoimmune diabetes. IL-1 beta inhibits insulin secretion from pancreatic beta-cells by stimulating the expression of inducible nitric oxide synthase (iNOS) that generates the free radical nitric oxide. IL-1 beta 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-1 beta-induced iNOS and COX-2 expression by rat islets and transformed rat pancreatic beta-cells. Because of the limitations of cell numbers of purified primary beta-cells obtained from rat islets, biochemical and molecular studies were performed using the rat insulinoma beta-cell line RINm5F. A serine protease inhibitor, N alpha-P-tosyl-L-lysine chloromethyl ketone (TLCK), and a proteasome complex (26S) inhibitor, MG 132, inhibited IL-1 beta-induced nitrite formation, an oxidation product of nitric oxide produced by iNOS, in a concentration-dependent manner, with complete inhibition observed at 100 micro mol/l and 10 micro mol/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 micro mol/l) and MG 132 (10 micro mol/l) inhibited IL-1 beta-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 NF kappa B is essential for activation of a number of cytokine-inducible enzymes and was evaluated as a possible site of protease action necessary for IL-1 beta-induced coexpression of iNOS and COX-2. TLCK and MG 132 inhibited both IL-1 beta-induced activation of NF kappa B and degradation of I kappa B alpha by islets and RINm5F cells. These results implicate protease activation as an early signaling event in IL-1 beta-induced inhibition of beta-cell function. This study also suggests that IL-1 beta-induced iNOS and COX-2 coexpression by pancreatic beta-cells share a common signaling pathway in utilizing the proteasome complex (26S) and the transcription factor NF kappa B, and it identifies sites of intervention to prevent the overproduction of their inflammatory products. Diabetes 47:583-591, 1998

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