The role of interferon regulatory factor-1 in cytokine-induced mRNA expression and cell death in murine pancreatic beta-cells.

Combinations of cytokines, including interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma), induce nitric oxide (NO) production and cell death in pancreatic islet cells. We have previously shown that these events are preceded by increased expression of the transcription factor interferon regulatory factor-1 (IRF-1). We utilized an IRF-1 knockout mouse (IRF-1-/-) to investigate the role of IRF-1 in cytokine-induced islet- and beta-cell gene expression and cell death. For this purpose, pancreatic islets or FACS-purified beta-cells were isolated from wild type (wt) or IRF-1-/- mice. These cells were exposed for different time points to IL-1beta (50 U/mI), IFN-gamma (1,000 U/ml) and/or TNF-alpha (1,000 U/ml) before being harvested for determination of viability (by nuclear dyes) and mRNA expression (by RT-PCR with specific primers). Following a 24 hours exposure to IL-1beta or IL-1beta + IFN-gamma, pancreatic islets isolated from IRF-1-/- mice presented a 30-50% reduction in medium nitrite accumulation and inducible NO-synthase (iNOS) expression. Interestingly, both wt and IRF-1-/- purified beta-cells failed to produce NO in response to IL-1beta alone, but presented a similar increase in nitrite accumulation and iNOS expression following exposure to IL-1beta + IFN-gamma. The basal expression of MHC class I mRNA was lower in IRF-1-/- islet cells (30% reduction), but there was a similar 2-4 fold-increase in MHC expression in islet cells from both strains following cytokine exposure. IL-1beta induced serine protease inhibitor-3 (SPI-3; a putative cellular "defense" protein) mRNA expression in both wt and IRF-1-/- islets or beta-cells. IFN-gamma decreased the IL-1beta-induced SPI-3 expression in wt islets or beta-cells, but induced a 5-fold increase in the expression of this mRNA in IRF-1-/- islets cells, suggesting that IRF-1 mediates an inhibitory effect of IFN-gamma on SPI-3 expression. Treatment of whole islets for 3 days with IL-1beta + IFN-gamma induced significantly more islet cell death in wt than in IRF-1-/- mice (respectively 85 +/- 3% versus 31 +/- 4% dead cells). On the other hand, prolonged exposure (3-9 days) of FACS-purified beta-cells to the same cytokines, or a combination of 3 cytokines, led to a similar increase in cell death in both IRF-1-/- and wt islets. In conclusion, IRF-1 contributes to cytokine-induced islet iNOS expression and cell death. These effects are absent in purified beta-cells, suggesting that IRF-1 may mediate its effects on whole islets via activation of non-endocrine cells (e.g. macrophages and ductal cells) present in these preparations.