881 5-Aminosalicylic Acid Mediates Expression of Cyclooxygenase-2 and 15-Hydroxyprostaglandin Dehydrogenase to Suppress Tumorigenesis in C-HA-RAS Transgenic Mice

Abstract

Background: Cyclooxygenase-2 (COX-2) is a key enzyme that produces prostaglandin E2 (PGE2) and plays an important role in colorectal tumor growth. Several studies showed that COX-2 inhibiting medications are useful for prevention of colorectal cancer (CRC), however, due to possible adverse events, these medications are not routinely recommended for the prevention. In addition to inhibition of COX-2 activity, recent focus is on 15-hydroxyprostaglandin dehydrogenase (15-PGDH), which degrades PGE2, as another mechanism to regulate PGE2. Some studies reported that 5-aminosalicylic acid (5-ASA), the most widely used antiinflammatory medication for inflammatory bowel diseases with demonstrated long term safety, is associated with reduction in risk of CRC, but the mechanism of action is still unclear. To explore the potential of 5-ASA as a safe preventive agent for CRC, we determined the effect of 5-ASA on COX-2 and 15-PGDH expression. We also investigated the preventive effect of 5-ASA in c-Ha-ras transgenic mice. Materials and Methods: To examine whether 5-ASA affects the expression of COX-2 and 15-PGDH, HT29 cells were cultured in the absence or presence of 5-ASA. To investigate the preventive effect of 5-ASA In Vivo, we used c-Ha-ras transgenic mice, which highly mimics human sporadic colorectal tumor development following injection of 1,2dihydrochloride. The transgenic mice were randomized to Ras or Ras+5ASA group. Mice assigned to the Ras+5-ASA group were treated orally with 5-ASA. The dose of 5-ASA used in the experiments were chosen carefully to reflect efficacy and safety for use in humans. Total protein was extracted from HT-29 cells and from murine colons after excluding sites of tumors and Western blotting analysis was performed for on COX-2 and 15-PGDH. The expression of these proteins were quantified and normalized by beta-actin expression. We examined the number and size of tumors in colon specimens, and performed histopathological analysis and immunohistochemistry. Result: 5-ASA significantly suppressed COX-2 and induced 15-PGDH expression in HT29 cells. In the transgenic mice, we confirmed that tumors in this model were sporadic and there was no inflammation in background mucosa and detected strong expression of COX2 in tumor lesions. The expression of 15-PGDH was remarkably weak regardless of colonic area. Oral 5-ASA intake reduced incidence of colorectal tumor formation from 83% to 40% (P = 0.048) and tumor size from 1.9 ± 1.1mm to 1.1 ± 0.2mm (P = 0.015). Furthermore, we observed down-regulation of COX-2 (P = 0.017) and up-regulation of 15-PGDH (P = 0.048) in the colons. Conclusion: We provide evidence that 5-ASA exerts a chemopreventive effect against colorectal tumor development in c-Ha-ras transgenic mice through mediation of COX-2 and 15-PGDH expression.