GH inhibits interferon-γ-induced signal transducer and activator of transcription-1 activation and expression of the inducible isoform of nitric oxide synthase in INS-1 cells

Interferon-gamma and TNF alpha synergistically induce the inducible isoform of nitric oxide synthase and elicit severe cytotoxicity in pancreatic beta -cells. We demonstrate here that GH, the well known beta -cell mitogen, inhibits nitric oxide production by reducing inducible nitric oxide synthase gene induction by the two cytokines and counteracts their cytotoxic effect in insulin-secreting INS-I cells. To elucidate the underlying mechanism, we examined activation of the transcription factors implicated in the induction of inducible nitric oxide synthase, signal transducer and activator of transcription-1, and nuclear factor-kappaB. GH inhibited tyrosine phosphorylation and DNA binding of signal transducer and activator of transcription-1 promoted by interferon-gamma, whereas nuclear factor-kappaB activation by TNF alpha was not affected by GH. GH was found to induce suppressor of cytokine signaling-1 and -3, both of which are able to inhibit interferon-gamma activation of signal transducer and activator of transcription-1, suggesting that they are likely to mediate the inhibitory action of GH. Finally, exposure of INS-I cells to interferon-gamma resulted in the impairment of insulin secretion in response to glucose, which was restored by the addition of GH. These results indicate that GH counteracts the effect of interferon-gamma through the inhibition of signal transducer and activator of transcription-1. This action of GH may be sufficient to suppress the synergistic induction of inducible nitric oxide synthase by interferon-gamma and TNF alpha, thereby preventing the cytotoxicity to beta -cells.