Paradoxical increase in retinoblastoma protein in colorectal carcinomas may protect cells from apoptosis

The retinoblastoma (Rb) gene is inactivated in a variety of human cancers, but in colorectal carcinomas there is frequently increased expression of th is gene. This is paradoxical in view of the known role of Rb as a tumor sup pressor gene, In the present study, we compared the levels of expression of the Rb protein (pRb) in normal human colorectal mucosa, adenomatous polyps , and carcinomas by immunohistochemistry, In vitro studies were also done t o examine the phenotypic effects of an antisense oligo-deoxynucleotide (AS- Rb) targeted to Rb mRNA in the HCT116 colon carcinoma cell line that expres ses a relatively high level of pRb, The incidence of pRb-positive cells was increased during multistage colorectal carcinogenesis. In vitro treatment of HCT116 cells with AS-Rb decreased the level of pRb by about 70% and also decreased the levels of the cyclin D1 protein and cyclin D1-associated kin ase activity. AS-Rb inhibited growth of HCT116 cells and induced apoptosis, Reporter assays indicated about a 17-fold increase in E2F activity. These findings suggest that the increased expression of pRb in colorectal carcino ma cells may provide a homeostatic mechanism that protects them from growth inhibition and apoptosis, perhaps by counterbalancing potentially toxic ef fects of excessive E2F activity.