Activation of cyclin E-dependent kinase activity in colorectal cancer

Although cyclin E gene amplification is reported to be an important event in various cancers, it is rarely found in human colorectal cancers. As one of the candidate factors of other mechanisms relating to cyclin E, we analyzed cyclin E-dependent kinase activity in colorectal cancer. Protein levels of cyclin E, its catalytic subunit, cyclin-dependent kinase 2 (Cdk2), and p21 and p27 were determined by western blot or immunohistochemistry in 27 colorectal cancers and 10 colorectal adenomas, and compared with adjacent normal colonic mucosa. Enzymatic activity of cyclin E-Cdk2 complex in the colorectal neoplasm was measured using in-gel kinase assay using glutathione S-transferase-retinoblastoma (GST-Rb) fusion protein as substrate, and compared with that of normal mucosa. We clearly showed that although the protein level of cyclin E in colorectal cancer and adenoma was similar to that of adjacent normal mucosa, cylin E-dependent kinase activity was increased in all the cases of colorectal cancers and 90% of colorectal adenomas. The relative kinase activity was significantly higher in colorectal cancer (3.7 +/- 1.7 -fold) than colorectal adenomas (2.0 +/- 0.8-fold) (P < 0.004). The relative expression level of Cdk2 protein in cancer was significantly higher than adenoma (4.4 +/- 2.4 vs 2.7 +/- 1.3, P < 0.04), and p21 and p27 were not detected in colorectal cancer and notably decreased in adenoma. The results of this study strongly suggest that activation of cyclin E-dependent kinase activity may play an important role in colorectal cancer, and its level appears to be related to increased Cdk2 and decreased p21 and p27 amounts rather than cyclin E protein level.