Previous studies in our laboratory have demonstrated that depletion of either cdk2 or cdk1 alone in cancer cell lines does not prevent the G1/S or G2/M transition, so that cells are still able to proliferate (Cai et al. Cancer Research 66: 9270, 2006). Cdks 1 and 2 are able to compensate for one another; for example, after cdk1 depletion, novel cyclin B-cdk2 complexes facilitate G2/M progression. However, dual depletion of both cdk2 and 1 resulted in G2/M arrest and/or cell death. We hypothesized that depletion of either cdk1 or cdk2 alone would be more likely to sensitize cells to DNA damaging agents than dual depletion because of continued proliferation. We have demonstrated that shRNA- mediated depletion of cdk1 results in sensitization of the non-small cell lung cancer cell lines NCI-H1299 and A549 to a range of DNA damaging treatments, including cisplatin and ionizing radiation. In contrast, cdk2 depletion did not affect sensitivity to DNA damage. In non-transformed retinal pigment epithelial cells, cdk1 depletion afforded potent G2/M arrest, which protected from S phase specific DNA damaging agents, so that sensitization appears restricted to transformed cell types. In the absence of cdk1, several proteins that are substrates of ATR/ATM kinases are not phosphorylated in response to DNA damage, including Chk1 and Chk2. BRCA1 is a key mediator of ATR/ATM responses to DNA damage and facilitates ATR/ATM phosphorylation of a number of their non-chromatin-bound substrates. We have shown that after DNA damage, in the absence of cdk1, BRCA1 is not recruited to sites of damaged DNA. Consequently, there is a failure to properly activate the checkpoint response pathway, resulting in abrogation of DNA damage-induced S phase arrest and increased apoptosis. Furthermore, cdk1 directly phosphorylates BRCA1 at S1189/S1191 and S1497, and mutation of these sites reduced the ability of HA-tagged BRCA1 to form foci. Cdk1 knockdown results were confirmed using the selective small molecule cdk1 inhibitor RO-3306. We therefore propose cdk1 inhibition as a novel strategy to sensitize BRCA1 proficient cancer cells to DNA damaging treatments by mimicking the BRCA1 loss-of-function phenotype.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3316.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO