Colorectal cancer is the third leading cause of cancer death in the United States, Colon carcinogenesis is a complex, multi-step process involving progressive changes in signaling pathways regulating intestinal epithelial cell proliferation, differentiation and programmed death. The peptide hormone, gastrin 1-17 (G-17), and its non-amidated precursor, glycine-extended gastrin (G-GIy), exert potent trophic effects on colon cancer cells. The long-term goal is to understand the role of these peptide hormones in the regulation of epithelial cell biology and colon carcinogenesis. Although the growth-promoting effect of these peptides on colon cancers has been extensively documented, the identity of the receptors and intracellular signaling pathways involved remain controversial. The investigators have identified and isolated the cDNA for a novel splice variant of the human cholecystokinin-B/gastrin receptor (CCK-BR), a member of the G protein-coupled receptor (GPCR) superfamily. The splice variant (designated CCK-BRi4sv for intron 4 containing splice variant) encodes a receptor protein containing 69 additional amino acid residues in its putative third intracellular loop domain. CCK-BRi4sv is expressed in adenomatous polyps and colorectal cancers, but not in nonmalignant colonic mucosa adjacent to the cancer. Mouse Balb3T3 cells expressing the splice variant exhibited spontaneous, ligand-independent, oscillatory, increases in [Ca2+]i whereas, the same cells expressing wild-type CCK-BR (CCK-BRwt) did not. Similarly, primary cultures of human cells isolated from freshly resected colorectal cancers exhibited, ligand-independent, oscillatory increases in [Ca2+]. For both Balb3T3 and primary tumor cells, application of G-17 (10 and 200 nM, respectively) caused an increase in [Ca2+]i. Selective CCK-BR antagonists blocked the G- 17-stimulated Ca2+ responses, but not the spontaneous [Ca2+]i oscillations. In addition to spontaneous intracellular signaling, BaIb3T3 cells expressing CCK-BRi4sv exhibited an increased rate of cell proliferation (approximately 2.5-fold), in the absence of G-17, compared to cells expressing wild-type CCK-BR (CCK-BRwt). Based on these findings, the PI hypothesizes that CCK-BRi4sv may regulate colorectal cancer cell growth through both a gastrin-independent and -dependent mechanism and thus play a significant role in colorectal carcinogenesis. Furthermore, the PI hypothesizes that the function of CCK-BRi4sv in colorectal cancer biology is a direct consequence of the structural changes in the third intracellular loop domain, caused by intron retention, and the impact of those changes on intracellular signal transduction. To examine these hypotheses they plan experiments with the following specific aims: 1) to determine the spatial and temporal expression of the CCK-BR splice variant in adenomatous polyps and colon cancers; 2) to determine the effects of intron retention on receptor-mediated intracellular signal transduction and receptor desensitization/internalization; and 3) to determine the effects of ectopic expression of the CCK-BR splice variant on colonic epithelial cell homeostasis and susceptibility to carcinogen-induced colon cancer using a transgenic mouse model. These studies will provide important and new information regarding the role of the novel receptor splice variant and G-17 and G-Gly in epithelial cell biology and colon carcinogenesis. Furthermore, these studies may, in the future, provide the basis for the development of innovative therapeutic strategies for the treatment of peptide hormone-sensitive cancers.
