Raf-1 kinase regulates intestinal epithelial cell survival in response to pro-inflammatory stimuli

Abstract

The maintenance of the intestinal epithelium is dependent upon controlled regulation of cell proliferation and apoptosis, or programmed cell death. Dysregulation of either of these processes can compromise the integrity of the intestinal epithelium leading to the development of inflammatory bowel disease (IBD). Although the cause of IBD remains unknown, increased production of pro-inflammatory cytokines, such as tumor necrosis factor (TNF), exacerbates the inflammatory response contributing to epithelial damage. One known target of TNF signaling is Raf-1 kinase, a serine/threonine kinase that is a key regulator of cell proliferation, differentiation and survival. An intestinal epithelium-specific conditional Raf knockout mouse (Raf KOIE) was generated to address the role of Raf in the intestinal epithelium following inflammation-induced injury. Using this mouse model, we demonstrated that Raf expression protects against dextran sulfate sodium (DSS)-induced colitis by promoting colon epithelial cell survival in response to injury and inflammation. Following DSS treatment of colon epithelial cells, Raf stimulates anti-apoptotic signaling pathways through nuclear factor (NF)-kappa B in a novel MEK-independent manner. Raf has previously been identified as a target of TNF receptor (R) signaling; therefore, studies were conducted to determine the requirement for Raf in colon epithelial cell survival following TNF exposure. Further analysis of TNF signaling pathways showed that Raf promotes NF-kappa B p65 phosphorylation downstream of TNFR1 activation. Similar to our findings in the DSS model, MEK inhibition did not effect anti-apoptotic signaling in response to TNF indicating that Raf-mediated cell survival occurs through a MEK-independent mechanism. In summary, we have demonstrated a protective role for Raf kinase in the colon epithelium following acute colitis or TNF exposure. While we show that Raf promotes anti-apoptotic signaling through NF-kappa B, inhibition of MEK kinase activity has no effect colon epithelial cell survival. These findings indicate a novel mechanism in which Raf contributes to maintenance of the colon epithelium under both non-inflammatory conditions and during acute colitis, which may lead to the development of novel therapeutics for the treatment of IBD.