PROSTAGLANDINS AND GASTROINTESTINAL DNA DAMAGE
Colorectal cancer, the second leading cause of cancer death in the western world, arises through the acquisition of multiple, independent genetic mutations and subsequent clonal expansion of mutated crypt epithelial stem cells. Programmed cell death, or apoptosis, is an effective method of removal of these genetically damaged epithelial cells. Azoxymethane (AOM) is a potent DNA damaging agent and carcinogen that induces intestinal and colonic tumors in rodents. NSAIDs inhibit AOM-mediated carcinogenesis. This occurs by inhibition of prostaglandin (PG) production through cyclooygenases (COX) COX-1 or COX-2. COX-2 and not COX-1 however is frequently reported to have a key role in tumor progression. PGE2 is critically important in the maintenance of gastrointestinal integrity and is thought to be a major protective component against environmentally-induced damage. PGs, specifically PGE2 has been shown to regulate intestinal stem cell survival and crypt epithelial apoptosis in response to radiation injury. Furthermore PGE2 mediated radioprotection is COX-1 dependent. In order to determine the effects of AOM on intestinal and colonic injury, we administered AOM (10mg/kg) IP to adult C57/BL6 mice. AOM treatment resulted in marked apoptosis in crypt epithelial cells of the jejunum and distal colon within 8 h. Furthermore, we observed a 3-fold induction in COX-1 mRNA, and a 4-fold increase in intestinal PGE2 levels in WT mice. AOM did not increase PGE2 levels in Cox-1 -/- mice. These data taken together suggest that Cox-1 mediated PGE2 synthesis plays a role in the intestinal response to azoxymethane induced DNA damage. The biologic effects of PGE2 are mediated by binding to membrane bound G-protein coupled receptors, termed EP receptors. The EP receptors are encoded by four separate genes (EP1, EP2, EP3 and EP4). These receptors differ in their ligand binding specificity and in down stream signaling pathways. We have recently demonstrated that the EP2 receptor is expressed in the mouse gastrointestinal tract and is upregulated in the mouse intestine following radiation injury. Moreover, we have demonstrated that the EP2 receptor mediates PGE2's radioprotective effects on apoptosis and crypt stem cell survival. These data taken together support our central hypothesis that the EP2 receptor an important component of an adaptive response in crypt epithelial cells in response to genotoxic injury, in which PGE2 signaling through the EP2 receptor regulates stem cell survival and crypt epithelial cell apoptosis. In this proposal we have chosen to examine the role of PGE2 and EP2 receptors in the regulation of gastrointestinal epithelial apoptosis in response to DNA damage induced by AOM through the following specific aims. 1.To correlate the regional differences in AOM -induced crypt epithelial cell apoptosis along the duodenal to colonic axis with regional differences in EP2 receptor distribution 2.To define the mechanisms of AOM-induced radioprotection in the mouse gastrointestinal tract. 3. To determine the role of COX-1 and PGE2 in AOM-mediated tumorigenesis. We believe that a more complete understanding of the role of PGE2 and EP receptor signaling in the regulation of stem cell fate, will be of great importance in improving our knowledge of the pathogenesis of gastrointestinal malignancy.