The role of Cdk5 in retinoic acid-induced apoptosis of cervical cancer cell line.

Cdk5 is a small serine/threonine protein kinase which belongs to Cdk family. Unlike other Cdk members, so far Cdk5 is known to be irrelevant in cell cycle. Cdk5 kinase activity is regulated by binding with its activator, p35. Our previous results indicate that CdkS and p35 are involved in drugs-induced apoptosis of prostate cancer cells. Retinoic acid (RA) is one of the vitamin A-related compounds. Because of its potency on biological functions, it has been widely studied in its novel actions including the ability to inhibit cancer cell growth and to induce apoptosis. Here, we report that RA treatment decreased the growth of human cervical cancer cell line, HeLa, and Cdk5 contributed to this effect. The involvement of Cdk5 in RA-reduced cell survival was performed by treatments of Cdk5 inhibitor and siRNA. We further identified that RA-induced growth inhibition was partly correlated to Cdk5 activity-related apoptosis by detecting cell cycle distribution of sub G1 phase and the signals of Annexin V staining. In addition, our results also indicated that Cdk5 activity was involved in RA-induced HeLa apoptosis by detecting cleavages of caspase-3 and its substrate, PARP (poly (ADP-ribose) polymerases) Interestingly, the nuclear localizations of Cdk5 and p35 proteins were increased by RA treatment, which, again, suggests the involvement of Cdk5 and p35 in RA-induced apoptotic effects. In conclusion, we provide evidence to suggest that Cdk5 and p35 might play important roles in RA-induced HeLa apoptosis.

Cdk5 Directly Targets Nuclear p21CIP1 and Promotes Cancer Cell Growth.

The significance of Cdk5 in cell-cycle control and cancer biology has gained increased attention. Here we report the inverse correlation between the protein levels of Cdk5 and p21CIP1 from cell-based and clinical analysis. Mechanistically, we identify that Cdk5 overexpression triggers the proteasome-dependent degradation of p21CIP1 through a S130 phosphorylation in a Cdk2-independent manner. Besides, the evidence from cell-based and clinical analysis shows that Cdk5 primarily regulates nuclear p21CIP1 protein degradation. S130A-p21CIP1 mutant enables to block either its protein degradation or the increase of cancer cell growth caused by Cdk5. Notably, Cdk5-triggered p21CIP1 targeting primarily appears in S-phase, while Cdk5 overexpression increases the activation of Cdk2 and its interaction with DNA polymerase δ. The in vivo results show that Cdk2 might play an important role in the downstream signaling to Cdk5. In summary, these findings suggest that Cdk5 in a high expression status promotes cancer growth by directly and rapidly releasing p21CIP1-dependent cell-cycle inhibition and subsequent Cdk2 activation, which illustrates an oncogenic role of Cdk5 potentially applied for future diagnosis and therapy. Cancer Res; 76(23); 6888-900. ©2016 AACR.