NITRIC-OXIDE PREVENTS IL-1-BETA AND IFN-GAMMA-INDUCING FACTOR (IL-18)RELEASE FROM MACROPHAGES BY INHIBITING CASPASE-1 (IL-1-BETA-CONVERTING ENZYME)

Procytokine processing by caspase-1 is required for the maturation and release of IL-IP and IFN-gamma-inducing factor (IGIF) (or IL-18) from activated macrophages (M phi). Nitric oxide (NO) has emerged as a pot ent inhibitor of cysteine proteases, Here, we tested the hypothesis th at NO regulates cytokine release by inhibiting IL-lp-converting enzyme (ICE) or caspase-1 activity, Activated RAW264.7 cells released four t o five times more IL-1 beta, but not TNF-alpha, in the presence of the NO synthase inhibitor N-G-monomethyI-L-arginine. Stimulated peritonea l Mg from wild-type mice (inducible NO synthase (iNOS)(+/+)) also rele ased more IL-1 beta if exposed to NG-monomethyl-L-arginine, whereas Mg from iNOS knockout mice (iNOS(-/-)) did not, Inhibition of NO synthes is in stimulated RAW264.7 cells also resulted in a threefold increase in intracellular caspase-1 activity. The NO donor S-nitroso-N-acetyl-D L-penicillamine inhibited caspase-1 activity in cells as well as the a ctivity of purified recombinant caspase-1 and also prevented the cleav age of pro-IL-1 beta and pro-IGIF by recombinant caspase-1, The inhibi tion of caspase-1 by NO was reversible by the addition of DTT, which i s consistent with S-nitrosylation as the mechanism of caspase-1 inhibi tion. An in vivo role for the regulation of caspase-1 by NO was establ ished in iNOS knockout animals, which exhibited significantly higher p lasma levels of IL-1 beta and IFN-gamma than their wild-type counterpa rts at 10 h following LPS injection. Taken together, these data indica te that NO suppresses IL-1 beta and IGIF processing by inhibiting casp ase-1 activity, providing evidence for a unique role for induced NO in regulating IL-1 beta and IGIF release.