Although Cdk2 is not an essential gene in the mouse, it is required for germ cell development and meiosis. Loss of Cdk2 affects the timing of S phase, suggesting that Cdk2 is involved in regulating progression through the mitotic cell cycle.

The cyclin-dependent kinase inhibitor p27(Kip1) is known as a negative regulator of cell-cycle progression and as a tumour suppressor. Cdk2 is the main target of p27 (refs 2, 3) and therefore we hypothesized that loss of Cdk2 activity should modify the p27(-/-) mouse phenotype. Here, we show that although p27(-/-) Cdk2(-/-) mice developed ovary tumours and tumours in the anterior lobe of the pituitary, we failed to detect any functional complementation in p27(-/-) Cdk2(-/-) double-knockout mice, indicating a parallel pathway regulated by p27. We observed elevated levels of S phase and mitosis in tissues of p27(-/-) Cdk2(-/-) mice concomitantly with elevated Cdc2 activity in p27(-/-) Cdk2(-/-) extracts. p27 binds to Cdc2, cyclin B1, cyclin A2, or suc1 complexes in wild-type and Cdk2(-/-) extracts. In addition, cyclin E binds to and activates Cdc2. Our in vivo results provide strong evidence that Cdc2 may compensate the loss of Cdk2 function.

Cisplatin is one of the most effective chemotherapeutics, but its usefulness is limited by its toxicity to normal tissues, including cells of the kidney proximal tubule. The purpose of these studies was to determine the mechanism of cisplatin cytotoxicity. It was shown in vivo that cisplatin administration induces upregulation of the gene for the p21 cyclin-dependent kinase (cdk) inhibitor in kidney cells. This protein is a positive effector on the fate of cisplatin-exposed renal tubule cells in vivo and in vitro; adenoviral transduction of p21 completely protected proximal tubule cells from cisplatin toxicity. Herein is reported that cdk2 inhibitory drugs protect kidney cells in vivo and in vitro, that transduction of kidney cells in vitro with dominant-negative cdk2 also protected, and that cdk2 knockout cells were resistant to cisplatin. The cdk2 knockout cells regained cisplatin sensitivity after transduction with wild-type cdk2. It is concluded that cisplatin cytotoxicity depends on cdk2 activation and that the mechanism of p21 protection is by direct inhibition of cdk2. This demonstrated the involvement of a protein that previously was associated with cell-cycle progression with pathways of apoptosis. It also was demonstrated that this pathway of cisplatin-induced cell death can be interceded in vivo to prevent nephrotoxicity.J Am Soc Nephrol 17: 2434 -2442 , 2006 . doi: 10.1681 C isplatin is one of the most effective chemotherapeutic agents against testicular and bladder tumors, head and neck, ovarian, breast, and lung cancers, and refractory non-Hodgkin's lymphomas (1,2). The major adverse effect of cisplatin use is nephrotoxicity, in which kidney proximal tubule cells are especially sensitive (3). It is likely that its anticancer activity depends on formation of DNA intrastrand cross-links (4). Several distinct mechanisms have been proposed for cisplatin cytotoxicity in renal tubule cells, including direct DNA damage (5), caspase activation (6), mitochondrial dysfunction (7), formation of reactive oxygen species (8), effects on the endoplasmic reticulum (9), and activation of TNF-␣ apoptotic pathways (10). However, it is unclear whether cisplatin nephrotoxicity depends on any of these pathways or these apoptotic cascades merely amplify more proximal initiated cell death signals.We have shown in vivo that kidney cells entered the cell cycle after cisplatin administration and that the gene for the p21Cip1/WAF1 cell-cycle inhibitor was induced simultaneously (11). The p21 protein interacts with several members of the cell cycle to regulate cell-cycle progression (12-14), and its induction is a positive effector on the fate of renal tubule cells both in vivo and in vitro (15,16). In addition, we recently reported that cells cultured from mouse proximal tubules were completely protected from cisplatin cytotoxicity by adenoviral transduction of human p21 (17). The activity of cyclin-dependent kinase 2 (cdk2), a serine/threonine kinase whose main function is the phosphorylation of substrates necessary for cel...