The cell cycle is comprised of a series of highly coordinated events culminating in cell growth and division. Cyclin-dependent kinases (CDK) and their cyclin counterparts strictly regulate and drive cell cycle progression and different CDK/cyclin complexes are responsible for the timely occurrence of each phase transition in order to maintain genetic integrity throughout generations. Cancer cells have been frequently found to have a de-regulated CDK activity allowing them to escape the normal cell cycle and proliferate uncontrollably. For these reasons CDKs have been considered attractive targets for cancer therapy and several CDK-inhibitors have been developed and are under intense investigation.
R-Roscovitine (Seliciclib, CYC202; herein referred to as Roscovitine), one of the most promising members of the CDK-inhibitor family, is an orally available adenosine analogue prominently targeting CDK2 (also affecting CDKs 1, 7 and 9 at a much lower rate) with a low off-target effect on other members of the human kinome, and a nice toxicity profile. In preclinical studies Roscovitine has shown significant in vitro and in vivo antitumor activity on a wide panel of human cancers and is currently in phase II clinical trials. Since preclinical experimentation, it has become evident that, CDK-inhibitors, such as Roscovitine, may actually curb the activity of DNA repair machinery[6, 7], hence becoming an attractive candidate for therapeutic association with either radiation therapy[8, 9] or genotoxic agent-based chemotherapy. However, the mechanism of this inhibition is still elusive.
One of the proposed means for CDK-inhibitors to affect DNA repair is through checkpoint deregulation[11–13], but increasing evidence supports a complex network of direct interactions between individual CDKs and proteins that play a key role in DNA damage repair (DDR). It is known that different DNA repair pathways are preferentially activated at specific stages of the cell cycle possibly suggesting a functional crosstalk between CDK/cyclin complexes and DNA repair mechanisms. In particular, CDK2 has been shown to interact with p53, BRCA1, BRCA2, Ku70 and both, CDK1 and CDK2, can modulate BRCA1-BARD1 activity[13, 19]. Moreover, CDK2 knock-down cells have an attenuated capacity to repair DNA damage suggesting a pivotal role for CDK2 in DDR. Given the ability of CDKs to compensate for each other in vivo, overall CDK activity has been proposed to be influential in DDR regulation however CDK2 function seems to have a specific role in some survival pathways.
Cyclins, similarly to CDKs, have been correlated to DDR. Cyclin E levels are upregulated under genotoxic stress conditions and a post-translational cleavage generates an 18-amino acid peptide, which has been shown to interact with Ku70 promoting the release of the pro-apoptotic factor Bax from the inactivating complex Bax/Ku70. Moreover, an increasing amount of data suggests an important role in DDR for the A-type cyclins, and in particular for cyclin A1. Differing from cyclin A2, ubiquitously expressed during the S and G2/M phases of the cell cycle, cyclin A1 is a testis-specific cyclin, which interacts with CDK2 and is involved in germ cell meiosis and spermatogenesis. Cyclin A1 may have a role in carcinogenesis, as it has been found to be over-expressed in acute myeloid leukemia and various other tumour types[23–25], however, its role in cancer is still particularly obscure. In somatic non-testicular tissues, cyclin A1 is not expressed or is expressed at very low basal levels. After genotoxic insult, cyclin A1 mRNA is upregulated in vitro and in vivo. At a molecular level, human CDK2/cyclin A1 complexes interact with members of the Ku family and phosphorylate Ku70[27, 28], a pivotal player in the non-homologous end-joining (NHEJ) double strand break (DSB) repair pathway. Furthermore, under genotoxic conditions the kinase activity of CDK2/cyclin A1 complex increases, while the relative kinase activity of CDK2/cyclin A2 decreases and the CDK2/cyclin A1 complex out-competes with CDK2/cyclin A2 for Ku70 binding. Moreover, it has recently been found that CDK2 phosphorylation status and structure changes upon the cyclin A family member with which it is bound  suggesting a non-redundant function between CDK2/cyclin A1 and CDK2/cyclin A2 complexes. Finally cyclin A1 knockout mice and Xenopus embryos exhibited a clear defect in DNA repair[27, 30] and are more prone to undergo apoptosis.
Taken together these data support that during genotoxic stress differential transcriptional levels and activity of cyclin A family members may redirect CDK2 toward DNA repair resulting in a modulation of NHEJ. Since one of the most relevant effects of CDK inhibitors is the downregulation of cell cycle related cyclins, we investigated if the inhibition of DNA repair mechanisms by Roscovitine may also occur through the modulation of the expression levels of cyclin A family members. Physiological CDK-inhibition, in fact, results in cyclin downregulation through the inhibition of E2F-family transcription factors, which drive and regulate cell cycle-related cyclin transcription. Given that the promoter of the cyclin A1 gene, CCNA1, is different from the other cell cycle-related cyclins, not being under the regulation of E2Fs, here we analyzed the effects of Roscovitine on cyclin A1 expression and modulation of DNA repair mechanisms. We demonstrated that under DNA damaging conditions cyclin A1 is strongly upregulated and localizes to the nucleus. Although Roscovitine alone was not sufficient to reduce the basal levels of cyclin A1, in contrast to cell cycle related cyclins, Roscovitine treatment could abolish the DNA damage-induced cyclin A1 upregulation, reducing NHEJ and significantly hindering DNA repair over time.