Oncogenic mutations of Ras, present in about 50% of colon cancers, regulate cell proliferation and cell survival and alter the sensitivity of cells to chemopreventive agents through modulation of downstream target genes. Using isogenic cell lines that differ only in the presence of mutant Ras, we demonstrated that activating mutations of k-ras inhibit the expression of STAT1 and STAT2, transcription factors required for signaling by IFNs (Klampfer et al, Journal of Biological Chemistry 2003, in appendix). Accordingly, a panel of colon cancer cell lines with k-ras mutation displayed reduced expression of IFN target genes, and inactivation of the mutant k-ras allele in the HCT116 colon cancer cell line was sufficient to restore the expression of STAT1 and STAT2, as well as their downstream target genes. These data led us to the testable hypothesis that activating Ras mutations perturb the balance between STAT1 dependent and STAT1 independent signaling by IFN( and thereby alter its biological activity. Since STAT1 is an important regulator of apoptosis, this raised the possibility that Ras modulates apoptosis in response to chemopreventive agents in a STAT1-dependent manner. We used isogenic cell lines with genetic inactivation of mutant Ras to demonstrate that activated Ras promotes apoptosis in response to the short chain fatty acid butyrate (Klampfer et al, Journal of Biological Chemistry 2004, in appendix). Here we present evidence that, like Ras mutations, STAT1 deficiency promotes butyrate-induced apoptosis and that STAT1 is required for butyrate-induced p21 expression. Finally, we show that loss of p21 favors apoptosis over cell cycle arrest upon butyrate treatment, suggesting that enhanced apoptosis in STAT1 deficient cells may be due to impaired p21 induction.
The aims of this proposal are: 1) to determine the molecular mechanism whereby Ras mutations interfere with IFN/STAT1 signaling, 2) to examine whether targeted expression of mutant Ras interferes with IFN/STAT signaling also in vivo, 3) to define the effect of Ras mutations, and STAT1 deficiency, on the responsiveness of colonic epithelial cells to interferons, and to the prototypic chemopreventive agents for patients with colon cancer predisposition, sulindac and aspirin, and 4) to test the hypothesis that STAT1 is required for transcriptional activation of p21 in response to butyrate and sulindac.
