Recent progress in Cox-2 selective inhibitors have shown promise to prevent colon cancer, but untoward cardiovascular side effects have dampened the enthusiasm for these agents. Since selective Cox-2 inhibitors appear problematic, increasing efforts are being made to identify other effective and safer chemopreventive agents. A large, phase III, double blind, placebo-controlled trial of Ursodeoxycholic acid (UDCA) to prevent the recurrence of colorectal adenoma was associated with a statistically significant reduction in recurrence of adenomas with high-grade dysplasia. My long-term research objectives are to elucidate the mechanisms by which UDCA exerts anticarcinogenic effects with respect to colon cancer. We have previously demonstrated that supplemental dietary cholic acid promoted the development of azoxymethane-induced rat colonic tumors. In contrast, dietary UDCA inhibited tumorigenesis and suppressed cholic acid-induced tumor promotion. To examine the mechanisms by which UDCA causes anticarcinogenic effects, studies in the proposal demonstrated that UDCA inhibited wild-type Ras activated tumors and generated RasGTPase activating protein (RasGAP) -N and -C terminal fragments. RasGAP C-terminal fragment sensitizes colon cancer cells towards UDCA-induced cell cycle arrest and suppresses Cox-2 expression in these cells. One of the major goals of this investigation is to provide proof of principle that RasGAP C-fragment protein will be useful in improving the efficacy of UDCA in colon cancer prevention. This will be achieved by investigating the growth of tumor xenografts in nude mice established with RasGAP C-transfectants and the effect of UDCA on regulators of G0/G1 cell cycle arrest in cell culture. Additionally, we will also investigate the effect of Intratumoral delivery of a permeable RasGAP C-fragment protein on tumor xenografts. Another goal of this investigation is to identify and characterize the transcriptional and post-transcriptional mechanisms by which RasGAP C-terminal fragment alters constitutive Cox-2 gene expression in colon cancer cells and UDCA suppresses Cox-2 in Aberrant Crypt Foci (ACF), the putative premalignant precursors of colon cancer. This will be achieved by investigating the stromal and epithelial cells of laser capture microdissected ACF in azoxymethane model of colonic carcinogenesis. We will extend our investigation to human samples by isolating ACFs from patients and study RasGAP fragments and Cox-2 regulators to ensure that regulatory components identified in vitro and in vivo are relevant to human disease. Studies in this proposal are, therefore, important for gaining insights into basic molecular regulation of Cox-2 by RasGAP C-fragments, a novel and unexplored mechanism. It will be useful in the development of better therapeutic interventions, as potential Cox-2 inhibitors are considered toxic for human use.
Significance and Relevance to Veterans Health Care The VA health care system has emphasized the importance of colon cancer screening, diagnosis and treatment. In 2002 the VA Gastroenterology Field Advisory Group estimated that there were 4.7 million veterans over the age of 50 and thus at risk for colon cancer and in need of screening. The 2006 VA directive on colon cancer screening noted that colon cancer screening coupled with the cost of treatment for colon cancer makes the issue of cancer prevention in the VA a high priority. The proposed research has great potential to eventually affect the clinical approach to colon cancer by elucidating the mechanisms of a truly preventive approach to colon cancer.
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