Nitric oxide is an endogenous thiol-reactive molecule that modulates the functions of many regulatory proteins by a thiol-redox mechanism. NO has now been shown to inhibit the activation of apoptosis signal-regulating kinase 1 (ASK1) in murine fibrosarcoma L929 cells through such a mechanism. Exposure of L929 cells to interferon-γ resulted in the endogenous production of NO and in inhibition of the activation of ASK1 by hydrogen peroxide. The interferon-γ-induced inhibition of ASK1 activity was blocked by N G-nitro-L-arginine, an inhibitor of NO synthase. Furthermore, the NO donor S-nitro-N-acetyl-DL-penicillamine (SNAP) inhibited ASK1 activity in vitro, and this inhibition was reversed by thiol-reducing agents such as dithiothreitol and β-mercaptoethanol. SNAP did not inhibit the kinase activities of MKK3, MKK6, or p38 in vitro. The inhibition of ASK1 by interferon-γ was not changed by 1H- (1,2,4)oxadiazolo[4,3-α ]quinoxalin-1-one, an inhibitor of guanylyl cyclase nor was it mimicked by 8-bromo-cyclic GMP. Site-directed mutagenesis revealed that replacement of cysteine 869 of ASK1 by serine rendered this protein resistant to the inhibitory effects both of interferon-γ in intact cells and of SNAP in vitro. Coimmunoprecipitation data showed that NO production inhibited a binding of ASK1, but not ASK1(C869S), to MKK3 or MKK6. Moreover, interferon-γ induced the S-nitrosylation of endogenous ASK1 in L929 cells. Together, these results suggest that NO mediates the interferon-γ-induced inhibition of ASK1 in L929 cells through a thiol-redox mechanism.
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