Coordinate up-regulation of cyclin-dependent kinase 4 and its inhibitor p16(INK4) in human glioma cells following chloroethylnitrosourea-induced DNA damage

Anomalies in the genes of the cell cycle regulators, p16(INK4) and CDK4 are highly frequent in human gliomas and other cancers, however, the extent to which these defects are involved in regulating the response of tumor cells to DNA damaging agents is not clear. In this study, using three human malignant glioma cell lines, MGR1, MGR3, and U87MG, we examined changes in gene expression of p16 and/or of its specific target CDK4 following damage of the cellular genome by the chemotherapeutic bifunctional alkylating agent, 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU). Exposure of the cells to 50 mu M BCNU for 24 h induced a significant level of DNA interstrand cross-links in all cell lines. In MCR1 cells (p16(+), Rb+), over the 24 h period, a steady increase in p16 (mRNA and protein) and CDK4 (protein) was observed. The increase in CDK4 and p16 proteins occurred in parallel, and that the two proteins accumulated in complex with each other, resulting in marked inhibition of CDK4 kinase activity. In MGR3 and U87MG cells, both of which lack functional p16 protein (p16(-), Rb+), BCNU, however increased the CDK4 protein levels. In all three cell lines, despite the differences in p16 gene status, BCNU exposure caused significant blockade of the cells at G(2)/M phase of the cell cycle. The coordinated enhancement of the target (CDK4) and the inhibitor (p16) in cellular response to genomic injury may reflect an attempt of the cells to continue progression through the cell cycle (via CDK4), while triggering the cell cycle arrest (via p16), required for the orderly repair of the damage to the genome.