STAT5 activation by human GH protects insulin-producing cells against interleukin-1β, interferon-γ and tumour necrosis factor-α-induced apoptosis independent of nitric oxide production

The proinflammatory cytokines interleukin-1β (IL-1β), interferon-γ (IFN-γ) and tumour necrosis factor-α (TNF-α) are toxic to pancreatic β-cells and are implicated in the pathogenesis of type 1 diabetes. We have previously found that GH and prolactin (PRL) stimulate both proliferation and insulin production in pancreatic β-cells and rat insulin-producing INS-1 cells. Here we report that human (h) GH can prevent the apoptotic effects of IL-1β, IFN-γ and TNF-α in INS-1 and INS-1E cells. Using adenovirus-mediated gene transfer, we found that the anti-apoptotic effect of hGH is abrogated by expression of a dominant negative signal transducer and activator of transcription (STAT5) mutant in INS-1E cells. hGH and the cytotoxic cytokines was found to additively increase suppressor of cytokine signalling-3 mRNA expression after 4 h of exposure. In order to identify possible targets for the STAT5-mediated protection of INS-1E cells, we studied the effect of hGH on activation of the transcription factors STAT1 and nuclear factor-κB (NF-κB) by IFN-γ and IL-1β+TNF-α respectively. Gel retardation experiments showed that hGH affects neither IFN-γ+ TNF-α-induced STAT1 DNA binding nor IL-1β and IFN-γ+TNF-α-induced NFκB DNA binding. The lack of influence of hGH on cytokine-mediated activation of STAT1 and NFκB is in accordance with the finding that hGH had only a minor effect on cytokine-induced inducible nitric oxide synthase (iNOS) gene expression and in fact augmented the IL-1β-stimulated nitric oxide production. As the anti-apoptotic Bcl-xL gene has been shown to harbour a STAT5-binding element we measured the expression of Bcl-xL as well as the pro-apoptotic Bax. We found that hGH increased the Bcl-xL/Bax ratio both in the absence and in the presence of cytotoxic cytokines. In conclusion, these results suggested that GH and PRL protect β-cells against cytotoxic cytokines via STAT5-dependent mechanisms distal to iNOS activation possibly at the level of Bcl-xL.