The cellular senescence program evolved as a response to diverse forms of damage and stress. Importantly, although senescence was considered a tissue culture phenomenon for many years, recent in vivo studies demonstrated that cellular senescence represents a potent failsafe mechanism against tumorigenesis, by halting aberrant proliferation of cells harboring cancer-promoting mutations in benign lesions. Moreover, senescence can be restored in a number of cancer cells. It is clear now, as with apoptosis, the senescence response also contributes to the efficacy of certain anticancer agents.

The atypical cyclin-dependent kinase family member CDK5 is a postmitotic kinase whose activity is mainly associated with proper development of the central nervous system (CNS), but accumulating evidence suggests that CDK5 is also involved in other non-neuronal settings and functions in a diverse range of processes, such as wound healing, migration, apoptosis and senescence.

Recent studies in our lab have shown that CDK5 is activated during cellular senescence in both primary and tumor cells, and its activity is required for the senescent morphology change. In the present study, we show that p35, one of the activators of CDK5 in neurons, is required for CDK5 activation during senescence and the expression of p35 itself is upregulated in senescing cells. In addition to the shape change, CDK5/p35 is also required for the altered expression of senescent secretome. We further investigated whether such changes would confer any biological or physiological consequences, for example, on cell proliferation and tumorigenesis. Here we show that CDK5 is required for the expression of senescence markers in murine embryonic fibroblasts, as well as fibrosarcomas. Although acute loss of CDK5 does not affect the proliferation of pre-senescent cells, CDK5 loss cooperates with Ras and dominant-negative p53 (DNp53) in the transformation of mouse embryonic fibroblasts (MEFs), as shown by an increased anchorage-independent growth of Cdk5-null cells in in vitro soft agar assays. In vitro migration/invasion assays performed with these transformed cells also show an increased motility and invasiveness in the Cdk5-null MEFs. However, CDK5 loss does not accelerate tumor formation in a xenograft model using transformed MEFs. Put together, these results suggest that although CDK5 is not directly involved in the regulation of cell proliferation and tumor growth once the senescence barrier is overcome, it may control cell motility and the metastatic potential of cancerous cells. In addition, CDK5 could have profound effects on the microenvironment and neighboring cells through its regulation of the secretome in senescent cells.