Investigating signaling pathway alterations in colon neoplasms using complementary deoxyribonucleic acid microarrays

Retinoids are attracting interest as potential chemopreventive and chemotherapeutic agents for many human cancers. Although the molecular responses elicited by retinoids are poorly understood, these responses in some instances appear to cause cellular differentiation. Development of normal colon epithelial cells proceeds through a systematic differentiation of cells that emerge from stem cells within the base of colon crypts. Genetic mutations in the adenomatous polyposis coli (APC) gene are thought to create colonocytes that demonstrate enhanced proliferation as well as a decrease in differentiation and, therefore, produce colon adenomas and colon carcinomas. Using microarray expression profiling, we have found molecular evidence supporting lack of differentiation as a mechanism contributing to the development of colon adenomas and carcinomas. Our data revealed a prevalence of down-regulated genes in sporadic colon polyps and colon tumors as compared to normal tissue. Among the genes absent from neoplastic colon tissues were a number of retinoic acid (RA) response genes. The absence of RA response genes was paralleled by the lack of expression of retinol dehydrogenase 5 and a novel retinol dehydrogenase, RDHL. Our data indicate that re-introduction of APC leads to re-expression of RDHL. Additionally, RDHL is highly expressed in normal colon compared to other tissues, and the intestine specific transcription factors, cdx1 and cdx2, can regulate the RDHL promoter. In addition to loss of RA biosynthetic genes, there seems to be at least two other means of inactivating the RA response pathway in colon cancer. These include silencing of retinoic acid receptors and silencing of the intestine-specific transcription factor cdx2 through aberrant DNA methylation. That the RA response pathway appears to be targeted for inactivation by three different mechanisms during colon tumorigenesis emphasizes the importance of this pathway in maintaining normally differentiated colonocytes and preventing the formation of colon neoplasms.