Comparative effects of overexpression of p27(Kip1) and p21(Cip1/Waf1) on growth and differentiation in human colon carcinoma cells

Recent studies have shown that decreased expression of p27(Kip1) is associated with high grade tumors and an unfavorable prognosis in several types of human cancer. To clarify the role of p27(Kip1) in colon cancer, we have overexpressed this protein in the HT29 colon cancer cell line. The derivatives displayed an increase in the p27(Kip1) protein in cyclin E/CDK2 immunoprecipitates and a decrease in cyclin E-associated kinase activity when compared to vector control clones, providing evidence that the overexpressed protein was functional. Clones with a high level of p27(Kip1) displayed partial growth inhibition in monolayer culture and a decrease in plating efficiency, even though they expressed increased levels of the cyclin D1 protein. Using alkaline phosphatase expression as a marker, we found that the p27(Kip1) overexpressor clones displayed a 2-3-fold increase in sensitivity to induction of differentiation by 2 mM sodium butyrate. In contrast to these results, derivatives of HT29 cells that stably overexpressed p21(Cip1/Waf1) displayed decreased sensitivity to the induction of differentiation. These findings may explain why decreased levels of p27(Kip1) in certain human cancers is associated with high grade (poorly differentiated) tumors, and suggest that strategies that increase the level of p27(Kip1) may be useful in cancer therapy.