Altered expression and activity of G₁/S cyclins and cyclin-dependent kinases characterize squamous cell carcinomas of the head and neck

Progressive deregulation of the cell-division cycle is thought to contribute to the establishment and progression of neoplasia. Previously, we have documented the in vivo inactivation of p161(INK4A), an inhibitor of G₁ cyclin-dependent kinases, in squamous cell carcinomas of the head and neck region. In the present study, we extend these findings by examining the expression and functional activity of cyclin-dependent kinases (CDKs) and their regulatory subunits using a model system of cell lines derived from squamous cell carcinomas. Increased activity of CDK4 and 6 was universal in tumor cells compared with normal keratinocytes, reflecting over-expression of either or both kinases. In contrast to other studies, overexpression of cyclin D1, a regulatory subunit of CDK4 and 6, was not observed. Increased activity of CDK2 was less frequent and was related to over-expression of cyclin A and/or E. All tumor cell lines showed increased expression of proliferating cell nuclear antigen compared to normal keratinocytes. Four SCC cell lines, including one tumor-metastasis pair derived from a single patient, failed to express the p15(INK4B) transcript. Western blot analysis of cell lysates revealed normal or reduced levels of p27(KIP1) in tumor cells compared to normal keratinocytes. However, failure to express wild-type p53 was not reflected by lower levels of p21(WAF1). Our data suggest that cell- cycle deregulation is likely to occur by multiple mechanisms during the genesis of head and neck squamous cell carcinomas. Furthermore, p161(INK4A) is likely to be the primary target for inactivation on chromosome 9p21 in these tumors as p15(INK4B) loss occurs less frequently.