The Cks1/Cks2 axis fine-tunes Mll1 expression and is crucial for MLL-rearranged leukaemia cell viability.

The Cdc28 protein kinase subunits, Cks1 and Cks2, play dual roles in Cdk-substrate specificity and Cdk-independent protein degradation, in concert with the E3 ubiquitin ligase complexes SCFSkp2 and APCCdc20. Notable targets controlled by Cks include p27 and Cyclin A. Here, we demonstrate that Cks1 and Cks2 proteins interact with both the MllN and MllC subunits of Mll1 (Mixed-lineage leukaemia 1), and together, the Cks proteins define Mll1 levels throughout the cell cycle. Overexpression of CKS1B and CKS2 is observed in multiple human cancers, including various MLL-rearranged (MLLr) AML subtypes. To explore the importance of MLL-Fusion Protein regulation by CKS1/2, we used small molecule inhibitors (MLN4924 and C1) to modulate their protein degradation functions. These inhibitors specifically reduced the proliferation of MLLr cell lines compared to primary controls. Altogether, this study uncovers a novel regulatory pathway for MLL1, which may open a new therapeutic approach to MLLr leukaemia.

Overexpression of Cks1 increases the radiotherapy resistance of esophageal squamous cell carcinoma.

PURPOSES: The Cks1 protein is a member of the highly conserved family of Cks/Suc1 proteins, which interact with Cdks, and was found to be an essential cofactor for efficient Skp2-dependent ubiquitination of p27. The present study was undertaken to examine the expression status of Cks1 in esophageal squamous cell carcinoma and its significance. MATERIALS AND METHODS: The expression of Cks1 in 140 esophageal squamous cell carcinoma patients was examined by immunohistochemistry. The correlations between Cks1 expression and tumor clinicopathologic features were analyzed. The effects of Cks1 expression on radiotherapy results were also examined. RESULTS: In the present study, we found that Cks1 is overexpressed in esophageal squamous cell carcinoma tissues. Elevated expression of Cks1 correlates significantly with tumor stage and positive lymph node metastasis (p < 0.05). Moreover, a significant negative correlation was found between Cks1 expression and the survival of patients who received radiotherapy (p < 0.05). At the molecular level, forced expression of Cks1 promotes the radio-resistance ability of EC9706 cells. Knockdown of Cks1 expression sensitizes cancer cells to radiation, and a wobble mutant of Cks1 that is resistant to Cks1 siRNA can rescue this effect. CONCLUSIONS: These results demonstrate for the first time that overexpression of Cks1 correlates with the increased radiotherapy resistance of esophageal squamous cell carcinoma.

Pbcrk-1, the Plasmodium berghei orthologue of P. falciparum cdc-2 related kinase-1 (Pfcrk-1), is essential for completion of the intraerythrocytic asexual cycle.

The molecular mechanisms underlying gametocytogenesis in malaria parasites are not understood. Plasmodium falciparum cdc2-related kinase 1 (pfcrk-1), a gene that is expressed predominantly in gametocytes, bears homology to the PITSLRE subfamily of cyclin-dependent kinases and has been hypothesized to function as a negative regulator of the cell cycle. We attempted to knock-out pbcrk-1, the P. berghei orthologue of pfcrk-1, but were unable to recover P. berghei parasites with a disrupted pbcrk-1 locus. In contrast, an integration event at this locus that did not result in a loss-of-function of the pbcrk-1 gene was readily observed. This strongly suggests that a functional pbcrk-1 gene product is essential to intraerythrocytic asexual multiplication.

The Evi1 proto-oncoprotein blocks endomitosis in megakaryocytes by inhibiting sustained cyclin-dependent kinase 2 catalytic activity.

The 3q21q26 syndrome leukaemias are characterised by dystrophic megakaryocytes, elevated platelet counts, ectopic EVI1 protein production and poor prognosis. To investigate the molecular basis of this disease, we developed a model system to examine the biological activity of EVI1 in a megakaryocyte progenitor cell line. For this purpose, Evi1 was conditionally expressed in human erythroleukaemia cells (HEL) that progress along the megakaryocyte lineage in the presence of 12-O-tetradecanoylphorbol 13-acetate (TPA). TPA-stimulated HEL cells normally undergo: (1) growth arrest; (2) altered morphology; (3) endomitosis and (4) characteristic changes in gene expression, including reduction of the erythroid-specific glycophoryn A and elevation of the specific glycoproteins GPIIIa and GPVI. Enforced Evi1 expression alone had no effect upon HEL cell proliferation or differentiation but a phenotype was manifest upon stimulation to differentiate. Evi1-expressing, TPA-treated HEL cells still showed growth arrest, had reduced and enhanced glycophoryn A and GPIIIa mRNA's, respectively, but failed to significantly elevate GPVI mRNA. This was accompanied by inhibition of endomitosis and altered cell morphology. Sustained CDK2 catalytic activity, typically associated with megakaryocyte endomitosis, was dramatically decreased in TPA-stimulated Evi1-expressing HEL cells because of significantly reduced levels of cyclin A. Therefore, enforced Evi1 expression could inhibit megakaryocyte differentiation although retention of some characteristic molecular changes, in combination with a block in endomitosis and altered morphology, suggest a defect in lineage progression. These results suggest that ectopic Evi1 expression contributes to a defective megakaryocyte differentiation programme and is likely to contribute to the phenotype observed in 3q21q26 syndrome leukaemias.

Cdk2 is dispensable for cell cycle inhibition and tumor suppression mediated by p27(Kip1) and p21(Cip1).

p27(Kip1) and p21(Cip1) are thought to suppress tumor growth and prevent cell cycle progression by inhibiting Cdk2-cyclin E/A kinases. Since Cdk2 is dispensable for mitotic cell division, we analyzed the activity of these inhibitors in Cdk2-deficient cells. Ectopic expression of p27(Kip1) or p21(Cip1) efficiently inhibits cell cycle progression of Cdk2(-/-) fibroblasts. Loss of p27(Kip1) or p21(Cip1) confers similar proliferative advantages to Cdk2(+/+) and Cdk2(-/-) cells. Moreover, Cdk2 is dispensable for p21(Cip1)-induced cell cycle arrest after DNA damage. Finally, ablation of Cdk2 in p27(Kip1) null mice does not suppress their phenotypic defects, including development of pituitary tumors. These results indicate that Cdk2 is not an essential target for p27(Kip1) and p21(Cip1) in cell cycle inhibition and tumor suppression.

Novel role for cyclin-dependent kinase 2 in neuregulin-induced acetylcholine receptor epsilon subunit expression in differentiated myotubes.

Cyclin-dependent kinases (CDKs) are a family of evolutionarily conserved serine/threonine kinases. CDK2 acts as a checkpoint for the G(1)/S transition in the cell cycle. Despite a down-regulation of CDK2 activity in postmitotic cells, many cell types, including muscle cells, maintain abundant levels of CDK2 protein. This led us to hypothesize that CDK2 may have a function in postmitotic cells. We show here for the first time that CDK2 can be activated by neuregulin (NRG) in differentiated C2C12 myotubes. In addition, this activity is required for expression of the acetylcholine receptor (AChR) epsilon subunit. The switch from the fetal AChRgamma subunit to the adult-type AChRepsilon is required for synapse maturation and the neuromuscular junction. Inhibition of CDK2 activity with either the specific CDK2 inhibitory peptide Tat-LFG or by RNA interference abolished neuregulin-induced AChRepsilon expression. Neuregulin-induced activation of CDK2 also depended on the ErbB receptor, MAPK, and PI3K, all of which have previously been shown to be required for AChRepsilon expression. Neuregulin regulated CDK2 activity through coordinating phosphorylation of CDK2 on Thr-160, accumulation of CDK2 in the nucleus, and down-regulation of the CDK2 inhibitory protein p27 in the nucleus. In addition, we also observed a novel mechanism of regulation of CDK2 activity by a low molecular weight variant of cyclin E in response to NRG. These findings establish CDK2 as an intermediate molecule that integrates NRG-activated signals from both the MAPK and PI3K pathways to AChRepsilon expression and reveal an undiscovered physiological role for CDK2 in postmitotic cells.

Cdk2-dependent phosphorylation of homeobox transcription factor CDX2 regulates its nuclear translocation and proteasome-mediated degradation in human intestinal epithelial cells.

By having demonstrated previously that p27(Kip1), a potent inhibitor of G(1) cyclin-cyclin-dependent kinases complexes, increases markedly during intestinal epithelial cell differentiation, we examined the effect of p27(Kip1) on the activity of the transcription factor CDX2. The present results revealed the following. 1) p27(Kip1) interacts with the CDX2 transcription factor. 2) In contrast to CDX2 mRNA levels, CDX2 protein expression levels significantly increased as soon as Caco-2/15 cells reached confluence, slowed their proliferation, and began their differentiation. The mechanism of CDX2 regulation is primarily related to protein stability, because inhibition of proteasome activity increased CDX2 levels. The half-life of CDX2 protein was significantly enhanced in differentiated versus undifferentiated proliferative intestinal epithelial cells. 3) Cdk2 interacted with CDX2 and phosphorylated CDX2, as determined by pull-down glutathione S-transferase and immunoprecipitation experiments with proliferating undifferentiated Caco-2/15 cell extracts. 4) Treatment of Caco-2/15 cells with MG132 (a proteasome inhibitor) and (R)-roscovitine (a specific Cdk2 inhibitor) induced an increase in CDX2 protein levels. 5) Conversely, ectopic expression of Cdk2 resulted in decreased expression of CDX2 protein. 6) Of note, treatment of proliferative Caco-2/15 cells with (R)-roscovitine or leptomycin (an inhibitor of nuclear export through CRM1) led to an accumulation of CDX2 into the nucleus. These data suggest that CDX2 undergoes CRM1-dependent nuclear export and cytoplasmic degradation in cells in which Cdk2 is activated, such as in proliferative intestinal epithelial cells. The targeted degradation of CDX2 following its phosphorylation by Cdk2 identifies a new mechanism through which CDX2 activity can be regulated in coordination with the cell cycle machinery.

Cdk2 activity is essential for the first to second meiosis transition in porcine oocytes.

The meiotic progression of Xenopus oocytes has been suggested to depend on the activity of cyclin-dependent kinase 2 (Cdk2). We examined whether Cdk2 is involved in the regulation of mammalian oocyte meiosis by injecting porcine oocytes with anti-Cdk2 antibody. At first, the cross-reactivity of the anti-Cdk2 antibody with Cdc2 kinase was evaluated by immunoprecipitation and immunoblotting experiments using porcine granulosa cell extract, and no cross-reactivity with Cdc2 kinase was observed in the antibody used. In the anti-Cdk2 antibody-injected group, 50.7% of the oocytes were arrested in the second metaphase after 50 h of culture and this rate was significantly lower than those in the non-injected intact oocytes or the oocytes injected with mouse IgG (84.5% and 86.7%, respectively). Most of the other oocytes in the antibody-injected group formed a pronucleus without polar bodies or with only one polar body. The cyclin B1 amount in the antibody-injected and activated oocytes was dramatically decreased compared with that in the intact or mouse IgG-injected oocytes after 50 h of culture. These results suggest that Cdk2 is involved in the meiotic maturation of mammalian oocytes, and that the block of Cdk2 activity results in the failure of cyclin B1 accumulation and second meiosis induction.

Replication-dependent destruction of Cdt1 limits DNA replication to a single round per cell cycle in Xenopus egg extracts.

In eukaryotes, prereplication complexes (pre-RCs) containing ORC, Cdc6, Cdt1, and MCM2-7 are assembled on chromatin in the G1 phase. In S phase, when DNA replication initiates, pre-RCs are disassembled, and new pre-RC assembly is restricted until the following G1 period. As a result, DNA replication is limited to a single round per cell cycle. One inhibitor of pre-RC assembly, geminin, was discovered in Xenopus, and it binds and inactivates Cdt1 in S phase. However, removal of geminin from Xenopus egg extracts is insufficient to cause rereplication, suggesting that other safeguards against rereplication exist. Here, we show that Cdt1 is completely degraded by ubiquitin-mediated proteolysis during the course of the first round of DNA replication in Xenopus egg extracts. Degradation depends on Cdk2/Cyclin E, Cdc45, RPA, and polymerase alpha, demonstrating a requirement for replication initiation. Cdt1 is ubiquitinated on chromatin, and this process also requires replication initiation. Once replication has initiated, Cdk2/Cyclin E is dispensable for Cdt1 degradation. When fresh Cdt1 is supplied after the first round of DNA replication, significant rereplication results, and rereplication is enhanced in the absence of geminin. Our results identify a replication-dependent proteolytic pathway that targets Cdt1 and that acts redundantly with geminin to inactivate Cdt1 in S phase.

Critical role of CDK2 for melanoma growth linked to its melanocyte-specific transcriptional regulation by MITF.

The genomic organization of the CDK2 gene, which overlaps the melanocyte-specific gene SILV/PMEL17, poses an interesting regulatory challenge. We show that, despite its ubiquitous expression, CDK2 exhibits tissue-specific regulation by the essential melanocyte lineage transcription factor MITF. In addition, functional studies revealed this regulation to be critical for maintaining CDK2 kinase activity and growth of melanoma cells. Expression levels of MITF and CDK2 are tightly correlated in primary melanoma specimens and predict susceptibility to the CDK2 inhibitor roscovitine. CDK2 depletion suppressed growth and cell cycle progression in melanoma, but not other cancers, corroborating previous results. Collectively, these data indicate that CDK2 activity in melanoma is largely maintained at the transcriptional level by MITF, and unlike other malignancies, it may be a suitable drug target in melanoma.

Direct cell cycle regulation by the fibroblast growth factor receptor (FGFR) kinase through phosphorylation-dependent release of Cks1 from FGFR substrate 2.

Fibroblast growth factors (FGFs) are upstream activators of the mitogen-activated protein kinase pathway and mitogens in a wide variety of cells. However, whether the mitogen-activated protein kinase pathway solely accounts for the induction of cell cycle or antiapoptotic activity of the FGF receptor (FGFR) tyrosine kinase is not clear. Here we report that cell cycle inducer Cks1, which triggers ubiquitination and degradation of p27(Kip1), associates with the unphosphorylated form of FGFR substrate 2 (FRS2), an adaptor protein that is phosphorylated by FGFR kinases and recruits downstream signaling molecules. FGF-dependent activation of FGFR tyrosine kinases induces FRS2 phosphorylation, causes release of Cks1 from FRS2, and promotes degradation of p27(Kip1) in 3T3 cells. Since degradation of p27(Kip1) is a key regulatory step in activation of the cyclin E/A-Cdk complex during the G(1)/S transition of the cell cycle, the results suggest a novel mitogenic pathway whereby FGF and other growth factors that activate FRS2 directly activate cyclin-dependent kinases.

The centrosome in normal and transformed cells.

The centrosome is a unique organelle that functions as the microtubule organizing center in most animal cells. During cell division, the centrosomes form the poles of the bipolar mitotic spindle. In addition, the centrosomes are also needed for cytokinesis. Each mammalian somatic cell typically contains one centrosome, which is duplicated in coordination with DNA replication. Just like the chromosomes, the centrosome is precisely reproduced once and only once during each cell cycle. However, it remains a mystery how this protein-based structure undergoes accurate duplication in a semiconservative manner. Intriguingly, amplification of the centrosome has been found in numerous forms of cancers. Cells with multiple centrosomes tend to form multipolar spindles, which result in abnormal chromosome segregation during mitosis. It has therefore been postulated that centrosome aberration may compromise the fidelity of cell division and cause chromosome instability. Here we review the current understanding of how the centrosome is assembled and duplicated. We also discuss the possible mechanisms by which centrosome abnormality contributes to the development of malignant phenotype.

Noncatalytic requirement for cyclin A-cdk2 in p27 turnover.

Ubiquitin-dependent proteolysis makes a major contribution to decreasing the levels of p27. Ubiquitin-dependent proteolysis of p27(kip1) is growth and cell cycle regulated in two ways: first, skp2, a component of the E3-ubiquitin ligase, is growth regulated, and second, a kinase must phosphorylate the threonine-187 position on p27 so that it can be recognized by skp2. In vitro, p27 is phosphorylated by cyclin E- and cyclin A-associated cdk2 as well as by cyclin B1-cdk1. Having analyzed the effect of different cyclin-cyclin-dependent kinase complexes on ubiquitination of p27 in a reconstitution assay system, we now report a noncatalytic requirement for cyclin A-cdk2. Multiparameter flow cytometric analysis also indicates that p27 turnover correlates best with the onset of S phase, once the levels of cyclin A become nearly maximal. Finally, increasing the amount of both cyclin E-cdk2 and skp2 was less efficient at promoting p27 ubiquitination than was increasing the amount of cyclin A-cdk2 alone in extracts prepared from cultures of >93%-purified G(1) cells. Together these lines of evidence suggest that cyclin A-cdk2 plays an ancillary noncatalytic role in the ubiquitination of p27 by the SCF(skp2) complex.

Cyclin E.

E-type cyclins (cyclin E1 and cyclin E2) are expressed during the late G1 phase of the cell cycle until the end of the S-phase. The activity of cyclin E is limiting for the passage of cells through the restriction point "R" which marks a "point of no return" for cells entering the division cycle from a resting state or passing from G1 into S-phase. Expression of cyclin E is regulated on the level of gene transcription mainly by members of the E2F trrnscription factor family and by its degradation via the proteasome pathway. Cyclin E binds and activates the kinase Cdk2 and by phosphorylating its substrates, the so-called "pocket proteins", the cyclic/Cdk2 complexes initiate a cascade of events that leads to the expression of S-phase specific genes. Aside from this specific function as a regulator of S-phase-entry, cyclin E plays a direct role in the initiation of DNA replication, the control of genomic stability, and the centrosome cycle. Surprisingly, recent studies have shown that the once thought essential cyclin E is dispensable for the development of higher eukaryotes and for the mitotic division of eukaryotic cells. Nevertheless, high level cyclin E expression has been associated with the initiation or progression of different human cancers, in particular breast cancer but also leukemia, lymphoma and others. Transgenic mouse models in which cyclin E is constitutively expressed develop malignant diseases, supporting the notion of cyclin E as a dominant onco-protein.

Promiscuity rules? The dispensability of cyclin E and Cdk2.

The canonical view of the mammalian cell cycle arose from studies of cultured cells rather than mutant organisms. It depicts the many complexes of cyclin and Cdk (cyclin/Cdk) as fulfilling unique and essential steps that dictate the sequential order of cell cycle events. Recent analyses of knockout mice challenge this view. Cdk2 and cyclin E, long thought to be essential, are largely dispensable. Here, we discuss the phenotypes of these and other cyclin/Cdk mutants in genetically tractable metazoa (mouse, fly, and nematode) and explore possible reasons behind similarities and differences among experimental systems and cell types.

Extracellular matrix glycoprotein biglycan enhances vascular smooth muscle cell proliferation and migration.

Proteoglycans are produced and secreted by vascular smooth muscle cells, but the pathophysiological role of these glycoproteins in the vasculature is an enigma. Because the small leucine-rich proteoglycan (SLRP) biglycan is overexpressed in arteriosclerotic lesions, we produced mice constitutively overexpressing biglycan in the vascular smooth muscle, in order to examine the effects on vascular pathology. In the aorta and renal vasculature, increased vascular proliferation was seen both in the basal state and after infusion of angiotensin II (Ang II) in the transgenic mice compared with wild-type controls. In addition, the combination of biglycan overexpression and Ang II infusion resulted in marked increases in vascular smooth muscle cell proliferation and migration in the coronary arteries, as well as increases in fibrosis surrounding the vessels. In vitro, biglycan caused an increase in thymidine incorporation and migration of vascular smooth muscle cells, whereas these parameters were unchanged or reduced in endothelial cells. Moreover, addition of biglycan resulted in an increase in cdk2 expression and decrease in p27 levels in the vascular smooth muscle cells. These results suggest that this extracellular matrix SLRP may be involved in the regulation of vascular smooth muscle growth and migration through cdk2- and p27-dependent pathways. Furthermore, changes in biglycan expression could be a factor influencing the susceptibility of arteries to vascular injury, and may play a direct role in the pathogenesis of vascular lesions.

Cyclin A/Cdk2 complexes regulate activation of Cdk1 and Cdc25 phosphatases in human cells.

Mitotic entry, a critical decision point for maintaining genetic stability, is governed by the cyclin B/Cyclin dependent kinase 1 (Cdc2) complex. In Xenopus oocytes and early embryos, accumulation of cyclin B activates Cdk1, which then phosphorylates and activates the positive regulator Cdc25 in an autocatalytic feedback loop. However, cyclin B levels do not increase as some human cells approach mitosis, and the key factors regulating Cdk1 activation in human cells are unknown. We report here that reducing cyclin A expression by RNA interference (RNAi) in primary human fibroblasts inhibited activation of Cdc25B and Cdc25C and dephosphorylation of Cdk1 on tyrosine (tyr) 15. These results were reproduced in U2-OS cells by inducing the expression of a dominant-negative (dn) mutant of Cdk2, the principal cyclin A binding partner. Cdk2-dn induction could inhibit Cdc25B activity and foster Cdk1 tyr phosphorylation within the S phase, temporally dissociating these events from Cdk1 activation at mitosis. In contrast, reducing Cdk1 expression delayed mitotic entry without markedly impairing Cdc25B or Cdc25C activity. These results suggest that cyclin A/Cdk2 complexes are key regulators of Cdc25 and Cdk1 activation in human cells. This pathway appears to be commonly deregulated in cancer.

Key role for cyclin-dependent kinases in the first and second meiotic divisions of rat spermatocytes.

In all systems examined so far, the G2/M phase transition is controlled by the M-phase promoting factor (MPF), a complex of cdc2 (CDK1) and cyclin B1. Histone H1 kinase activity and MPF components are present in pachytene spermatocytes (PS). However, it has not been demonstrated yet that direct inhibition of MPF activity prevents the G2/M transition in these cells. When roscovitine, a potent inhibitor of CDK1, CDK2, and CDK5 activities, was added to cocultures of PS with Sertoli cells, the number of both secondary spermatocytes and round spermatids formed were lower than in control cultures, despite similar cell viability. This effect of roscovitine was reversible, did not involve the Sertoli cells, and was dependent on the concentration of the inhibitor. Roscovitine did not modify the amount of MPF in these germ cells but inhibited the CDK1- or CDK2-associated histone H1 kinase activity of PS. Hence a functional relationship between cyclin-dependent kinase activity and the spontaneous processing of the first meiotic division and, for the first time, of the second meiotic division of male germ cells is shown.

Enhanced cell cycle progression and down regulation of p21(Cip1/Waf1) by PRL tyrosine phosphatases.

Human PRL-1, PRL-2, and PRL-3 tyrosine phosphatases induce the malignant transformation of epithelial cells. We tested the hypothesis that the oncogenic effects of PRL occur by increasing cellular proliferation. Cells stably transfected with PRL-1 or PRL-2 exhibited 2.7-3.3-fold increases over control cells in the rate of DNA synthesis and the proportion of cells in S-phase, and they progressed more rapidly from G1 into S. In addition, cells overexpressing either PRL-1 or PRL-2 exhibited enhanced cyclin-dependent kinase 2 (CDK2) activity and significantly lower p21(Cip1/Waf1) protein levels, and PRL-1 overexpressing cells had higher cyclin A protein levels than control cells. We conclude that PRL phosphatases increase cell proliferation by stimulating progression from G1 into S phase, and this process may be dependent on the down regulation of the cyclin dependent kinase inhibitor p21(Cip1/Waf1).

Cdk2 knockout mice are viable.

BACKGROUND: Cyclin-dependent kinases (Cdks) and their cyclin regulatory subunits control cell growth and division. Cdk2/cyclin E complexes are thought to be required because they phosphorylate the retinoblastoma protein and drive cells through the G1/S transition into the S phase of the cell cycle. In addition, Cdk2 associates with cyclin A, which itself is essential for cell proliferation during early embryonic development. RESULTS: In order to study the functions of Cdk2 in vivo, we generated Cdk2 knockout mice. Surprisingly, these mice are viable, and therefore Cdk2 is not an essential gene in the mouse. However, Cdk2 is required for germ cell development; both male and female Cdk2(-/-) mice are sterile. Immunoprecipitates of cyclin E1 complexes from Cdk2(-/-) spleen extracts displayed no activity toward histone H1. Cyclin A2 complexes were active in primary mouse embryonic fibroblasts (MEFs), embryo extracts and in spleen extracts from young animals. In contrast, there was little cyclin A2 kinase activity in immortalized MEFs and spleen extracts from adult animals. Cdk2(-/-) MEFs proliferate but enter delayed into S phase. Ectopic expression of Cdk2 in Cdk2(-/-) MEFs rescued the delayed entry into S phase. CONCLUSIONS: 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.