Requirement for p34cdc2 kinase is restricted to mitosis in the mammalian cdc2 mutant FT210

Electrochemical Genosensing of Overexpressed GAPDH Transcripts in Breast Cancer Exosomes

Exosomes are receiving highlighted attention as new biomarkers for the detection of cancer since they are profusely released by tumor cells in different biological fluids. In this paper, the exosomes are preconcentrated from the serum by immunomagnetic separation (IMS) based on a CD326 receptor as a specific epithelial cancer-related biomarker and detected by glyceraldehyde-3-phosphate dehydrogenase (GAPDH) transcripts. Following the lysis of the captured exosomes, the released GAPDH transcripts are amplified by reverse transcription polymerase chain reaction (RT-PCR) with a double-tagging set of primers on poly(dT)-modified-MPs to increase the sensitivity. The double-tagged amplicon is then quantified by electrochemical genosensing. The IMS/double-tagging RT-PCR/electrochemical genosensing approach is first demonstrated for the sensitive detection of exosomes derived from MCF7 breast cancer cells and compared with CTCs in terms of the analytical performance, showing an LOD of 4 × 102 exosomes μL-1. The genosensor was applied to human samples by immunocapturing the exosomes directly from serum from breast cancer patients and showed a higher electrochemical signal (3.3-fold, p < 0.05), when compared with healthy controls, suggesting an overexpression of GAPDH on serum-derived exosomes from breast cancer patients. The detection of GAPDH transcripts is performed from only 1.0 mL of human serum using specific magnetic particles, improving the analytical simplification and avoiding ultracentrifugation steps, demonstrating to be a promising strategy for minimal invasive liquid biopsy.

Differentiation of trophoblast stem cells into giant cells is triggered by p57/Kip2 inhibition of CDK1 activity

Genome endoreduplication during mammalian development is a rare event for which the mechanism is unknown. It first appears when fibroblast growth factor 4 (FGF4) deprivation induces differentiation of trophoblast stem (TS) cells into the nonproliferating trophoblast giant (TG) cells required for embryo implantation. Here we show that RO3306 inhibition of cyclin-dependent protein kinase 1 (CDK1), the enzyme required to enter mitosis, induced differentiation of TS cells into TG cells. In contrast, RO3306 induced abortive endoreduplication and apoptosis in embryonic stem cells, revealing that inactivation of CDK1 triggers endoreduplication only in cells programmed to differentiate into polyploid cells. Similarly, FGF4 deprivation resulted in CDK1 inhibition by overexpressing two CDK-specific inhibitors, p57/KIP2 and p21/CIP1. TS cell mutants revealed that p57 was required to trigger endoreduplication by inhibiting CDK1, while p21 suppressed expression of the checkpoint protein kinase CHK1, thereby preventing induction of apoptosis. Furthermore, Cdk2(-/-) TS cells revealed that CDK2 is required for endoreduplication when CDK1 is inhibited. Expression of p57 in TG cells was restricted to G-phase nuclei to allow CDK activation of S phase. Thus, endoreduplication in TS cells is triggered by p57 inhibition of CDK1 with concomitant suppression of the DNA damage response by p21.

Preventing DNA over-replication: a Cdk perspective

The cell cycle is tightly controlled to ensure that replication origins fire only once per cycle and that consecutive S-phases are separated by mitosis. When controls fail, DNA over-replication ensues: individual origins fire more than once per S-phase (re-replication) or consecutive S-phases occur without intervening mitoses (endoreduplication). In yeast the cell cycle is controlled by a single cyclin dependent kinase (Cdk) that prevents origin licensing at times when it promotes origin firing, and that is inactivated, via proteolysis of its partner cyclin, as cells undergo mitosis. A quantitative model describes three levels of Cdk activity: low activity allows licensing, intermediate activity allows firing but prevents licensing, and high activity promotes mitosis. In higher eukaryotes the situation is complicated by the existence of additional proteins (geminin, Cul4-Ddb1Cdt2, and Emi1) that control licensing. A current challenge is to understand how these various control mechanisms are co-ordinated and why the degree of redundancy between them is so variable. Here the experimental induction of DNA over-replication is reviewed in the context of the quantitative model of Cdk action. Endoreduplication is viewed as a consequence of procedures that cause Cdk activity to fall below the threshold required to prevent licensing, and re-replication as the result of procedures that increase that threshold value. This may help to explain why over-replication does not necessarily require reduced Cdk activity and how different mechanisms conspire to prevent over-replication. Further work is nevertheless required to determine exactly how losing just one licensing control mechanism often causes over-replication, and why this varies between cell systems.

Role for Cdk1 (Cdc2)/cyclin A in preventing the mammalian origin recognition complex's largest subunit (Orc1) from binding to chromatin during mitosis

The eukaryotic origin recognition complex (ORC) selects the genomic sites where prereplication complexes are assembled and DNA replication begins. In proliferating mammalian cells, ORC activity appears to be regulated by reducing the affinity of the Orc1 subunit for chromatin during S phase and then preventing reformation of a stable ORC-chromatin complex until mitosis is completed and a nuclear membrane is assembled. Here we show that part of the mechanism by which this is accomplished is the selective association of Orc1 with Cdk1 (Cdc2)/cyclin A during the G(2)/M phase of cell division. This association accounted for the appearance in M-phase cells of hyperphosphorylated Orc1 that was subsequently dephosphorylated during the M-to-G(1) transition. Moreover, inhibition of Cdk activity in metaphase cells resulted in rapid binding of Orc1 to chromatin. However, chromatin binding was not mediated through increased affinity of Orc1 for Orc2, suggesting that additional events are involved in the assembly of functional ORC-chromatin sites. These results reveal that the same cyclin-dependent protein kinase that initiates mitosis in mammalian cells also concomitantly inhibits assembly of functional ORC-chromatin sites.

Failure to proliferate and mitotic arrest of CDK11(p110/p58)-null mutant mice at the blastocyst stage of embryonic cell development

The CDK11(p110) protein kinases are part of large-molecular-weight complexes that also contain RNA polymerase II, transcriptional elongation factors, and general pre-mRNA splicing factors. CDK11(p110) isoforms may therefore couple transcription and pre-mRNA splicing by their effect(s) on certain proteins required for these processes. The CDK11(p58) kinase isoform is generated from the CDK11(p110) mRNA through the use of an internal ribosome entry site in a mitosis-specific manner, suggesting that this kinase may regulate the cell cycle during mitosis. The in vivo role and necessity of CDK11(p110/p58) kinase function during mammalian development were examined by generating CDK11(p110/p58)-null mice through targeted disruption of the corresponding gene using homologous recombination. While heterozygous mice develop normally, disruption of both CDK11(p110/p58) alleles results in early embryonic lethality due to apoptosis of the blastocyst cells between 3.5 and 4 days postcoitus. Cells within these embryos exhibit both proliferative defect(s) and a mitotic arrest. These results are consistent with the proposed cellular functions of the CDK11(p110/p58) kinases and confirm that the CDK11(p110/p58) kinases are essential for cellular viability as well as normal early embryonic development.

Cdt1 phosphorylation by cyclin A-dependent kinases negatively regulates its function without affecting geminin binding

The current concept regarding cell cycle regulation of DNA replication is that Cdt1, together with origin recognition complex and CDC6 proteins, constitutes the machinery that loads the minichromosome maintenance complex, a candidate replicative helicase, onto chromatin during the G(1) phase. The actions of origin recognition complex and CDC6 are suppressed through phosphorylation by cyclin-dependent kinases (Cdks) after S phase to prohibit rereplication. It has been suggested in metazoan cells that the function of Cdt1 is blocked through binding to an inhibitor protein, geminin. However, the functional relationship between the Cdt1-geminin system and Cdks remains to be clarified. In this report, we demonstrate that human Cdt1 is phosphorylated by cyclin A-dependent kinases dependent on its cyclin-binding motif. Cdk phosphorylation resulted in the binding of Cdt1 to the F-box protein Skp2 and subsequent degradation. In contrast, in vitro DNA binding activity of Cdt1 was inhibited by the phosphorylation. However, geminin binding to Cdt1 was not affected by the phosphorylation. Finally we provide evidence that inactivation of Cdk1 results in Cdt1 dephosphorylation and rebinding to chromatin in murine FT210 cells synchronized around the G(2)/M phase. Taken together, these findings suggest that Cdt1 function is also negatively regulated by the Cdk phosphorylation independent of geminin binding.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-induced changes in activities of nuclear protein kinases and phosphatases affecting DNA binding activity of c-Myc and AP-1 in the livers of guinea pigs

To study the effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on nuclear protein phosphorylation activities, male guinea pigs were treated in vivo with a single 1 microg/kg i.p. injection of TCDD, and the state of protein kinases and phosphatases in the nuclei of the hepatocytes was examined after 1, 10, and 40 days. TCDD was found to cause a rise in nuclear protein tyrosine kinase on day 1, and to a lesser extent on day 10, but this effect diminished almost completely on day 40. TCDD also caused a reduction in nuclear casein kinase II (CKII) activity at all time points. To study the biochemical events taking place at the early stage of the action of TCDD, a short-term in vitro model system was established using explant liver tissues maintained in tissue culture medium. It was found that TCDD caused a rapid reduction of the activity of nuclear CKII with an accompanying increase in the cytosol. Such changes in protein phosphorylation activities were also accompanied by an increase in the DNA binding activity of activator protein 1 (AP-1). The effect of TCDD on nuclear proteins binding to the c-Myc response element DNA was, on the other hand, biphasic: an initial increase of protein binding to the c-Myc response element was followed by suppression. To test the hypothesis that some of the above changes were caused by TCDD-induced changes in protein kinase activity, nuclear proteins isolated from hepatocytes of in vivo treated guinea pigs were incubated with exogenously added Mg2+ and ATP under cell-free conditions. The results showed that this in vitro phosphorylation treatment exacerbated this tendency of increased AP-1 and decreased c-Myc binding to their respective response element DNAs, indicating that kinases and phosphatases present in the isolated nuclear protein preparation were active and capable of modifying protein binding to DNA. Such effects of Mg2+ and ATP on AP-1 were blocked by heparin, indicating that CKII plays an important role in transducing the signal of TCDD into the nucleus.

Comparison of mitotic cyclins and cyclin-dependent kinase expression in keratoacanthoma and squamous cell carcinoma

Disruption of the cell-cycle regulation through over-expression or mutation of cyclins and cyclin-dependent kinases has been implicated in carcinogenesis. In order to determine whether keratoacanthoma (KA) is unique or a variant of squamous cell carcinoma (SCC) and whether expression of mitosis-related antigens are associated with KAs' tendency to regress, we compared the immunohistochemical expression of mitotic cyclins (cyclins A and B) and their cyclin-dependent kinase p34(cdc2) in 21 KAs, 8 regressing KAs, and 28 conventional squamous cell carcinomas. KAs showed both overlap and significant differences in expression of these mitosis-related antigens compared to SCCs. Basal and parabasal pattern of expression of cyclins A and B significantly predominated in KAs in contrast to SCCs which exhibited diffuse pattern (cyclin A 86%/cyclin B 64% vs. 25%/36%, p < 0.01). However, no differences in the highest mean level of expression in 'hot spot' loci of cyclins A and B were identified comparing KAs to SCCs (19%/12% vs. 25%/13%, p > 0.05). For the cyclin-dependent kinase p34(cdc2), no differences in pattern, distribution or mean levels of expression were found. For cyclins A and B, regressing KA showed significantly more regional tumor labeling (88%/88% vs. 57%/33%, p = 0.03) and a lower mean level of immunoreactivity (5%/4% vs. 19%/12%, p = 0.001) compared to mature KAs. These findings indicate a role for mitotic cyclins in the evolution of both SCC and KA. The overlapping patterns of expression for these mitosis-related antigens suggest that KAs represent a variant of SCC that exhibit an overwhelming but not absolute tendency to involute.

Identification of cytosolic aldehyde dehydrogenase 1 from non-small cell lung carcinomas as a flavopiridol-binding protein

The synthetic flavone flavopiridol can be cytostatic or cytotoxic to mammalian cells, depending on the concentration of the drug and the duration of exposure. It has been shown to inhibit the cyclin-dependent kinase (CDK) family of cell cycle regulatory enzymes. However, the existence of additional potential targets for drug action remains a matter of interest to define. To identify cellular targets, flavopiridol was immobilized. CDKs, particularly CDK 4, bound weakly to immobilized flavopiridol when ATP was absent but not in its presence. Two proteins with molecular weights of 40 kDa and 120 kDa had high affinities to the immobilized flavopiridol independent of the presence of ATP. They were present in all cell lines analyzed: cervical (HeLa), prostate and non-small cell lung carcinoma (NSCLC) cell lines. A 60-kDa protein, which was present only in NSCLC cells and bound similarly well to immobilized flavopiridol, was identified as cytosolic aldehyde dehydrogenase class 1 (ALDH-1). The level of this protein correlated with the resistance of NSCLC cell lines to cytotoxicity caused by 500 nM flavopiridol but not higher flavopiridol concentrations. Despite binding to ALDH-1, there was no inhibition of dehydrogenase activity by flavopiridol concentrations as high as 20 microM and flavopiridol was not metabolized by ALDH-1. The results suggest that high cellular levels of ALDH-1 may reduce cytotoxicity of flavopiridol and contribute to relative resistance to the drug. This is the first report that flavopiridol binds to proteins other than CDKs.

Identification and characterization of individual cyclin-dependent kinase complexes from Saccharomyces cerevisiae

In S. cerevisiae, regulation of cell cycle progression is known to be carried out by a single cyclin-dependent kinase homologue, Cdc28p, acting at different stages of the cell cycle in association with various cyclins and other regulatory subunits. However, a still unsolved problem is the identification of the physiologically relevant substrates of the different Cdc28p kinase complexes which participate in this regulation. Purification and characterization of the subunit composition and enzymological properties of these Cdc28p complexes would therefore contribute substantially to our understanding of the molecular mechanisms controlling the cell cycle. We have used a combination of ammonium sulphate fractionation, nickel nitrilotriacetate affinity purification, ATP Sepharose affinity chromatography and Resource Q ion exchange chromatography to purify two different Cdc28p kinase complexes. Using specific clb deletion mutants and plasmid or genomic HA epitope-tagged CLBs, we show that one of these complexes is composed almost exclusively (93% or greater) of Clb2p-Cdc28p, whereas the other is mainly (75% or greater) Clb3p-Cdc28p. These procedures provide the basis for the analysis of regulatory, enzymatic and functional properties of individual Cdc28p kinase complexes.

p34cdc2 in invasive breast cancer: relationship to DNA content, Ki67 index and c-erbB-2 expression

AIMS

One-hundred and eighty-eight cases of human mammary carcinoma were examined immunohistochemically for their expression of Ki67, p34cdc2 and c-erbB-2. DNA image cytometry was performed to evaluate DNA ploidy, Auer type, S-phase fraction (SPF), 5c exceeding rate (5cER) and 2c deviation index (2cDI).

METHODS AND RESULTS

One-hundred and sixty-eight cases were invasive ductal carcinomas, 20 were of invasive lobular type. Routinely assessed oestrogen and progesterone receptor scores were available. The results were analysed statistically in comparison to tumour type, histopathological grade, lymph node status, menopausal status, patient age and overall survival. Ki67 (P < 0.002) and c-erbB-2 (P < 0.0001) correlated well with overall survival (P < 0.0008) and grade (P < 0.038) but not with lymph node status and tumour type. p34cdc2 showed a trend towards a positive correlation with Ki67 (P < 0.058) and a significant negative correlation with receptor status (P < 0.008) but with none of the other parameters examined.

CONCLUSIONS

No association between the DNA measured parameters (Auer type, SPF, 5cER and 2cDI) and survival was found. Our results suggest that c-erbB-2 and Ki67 are parameters which might, in combination with receptor status, help to define subgroups with different outcomes.

Mitotic cyclins and cyclin-dependent kinases in melanocytic lesions

Recent evidence has implicated cyclins and cyclin-dependent kinases in the evolution and progression of various malignancies. We studied the immunohistochemical expression of cyclin A, cyclin B, and cyclin-dependent kinase p34cdc2 in a broad spectrum of benign and malignant melanocytic lesions. Formalin-embedded, parrafin-fixed tissue sections from 66 malignant melanomas (MM) and 60 benign nevi were examined for the expression of these cell-cycle proteins. The results were compared with the standard proliferative marker Ki-67 and mitotic index. MM showed significantly higher immunoreactivity for cyclin A, cyclin B, p34cdc2, and Ki-67 compared with benign nevi. Cyclin A, p34cdc2, and Ki-67 displayed strong co-expression in MM. Overexpression of cyclin A and p34cdc2 correlated with histological type, mitotic activity, Ki-67 index, tumor thickness, Clark's level, and clinical outcome in MM. In invasive MM, increased immunostaining of cyclin A and Ki-67 were associated with decreased patient survival. These findings indicate potential roles of mitotic cyclins and cyclin-dependent kinases in the pathogenesis and progression of malignant melanoma.

Cell cycle- and chromatin binding state-dependent phosphorylation of human MCM heterohexameric complexes. A role for cdc2 kinase

The mammalian MCM protein family, presently with six members, exists in the nuclei in two forms, chromatin-bound and unbound. The former dissociates from chromatin with progression through the S phase. Recently, we have established a procedure to isolate chromatin-bound and unbound complexes containing all six human MCM (hMCM) proteins by immunoprecipitation. In the present study, we applied this procedure to HeLa cells synchronized in each of the G1, S, and G2/M phases and could detect hMCM heterohexameric complexes in all three. In addition, depending on the cell cycle and the state of chromatin association, hMCM2 and 4 in the complexes were found to variously change their phosphorylation states. Concentrating attention on G2/M phase hyperphosphorylation, we found hMCM2 and 4 in the complexes to be good substrates for cdc2/cyclin B in vitro. Furthermore, when cdc2 kinase was inactivated in temperature-sensitive mutant murine FT210 cells, the G2/M hyperphosphorylation of the murine MCM2 and MCM4 and release of the MCMs from chromatin in the G2 phase were severely impaired. Taken together, the data suggest that the six mammalian MCM proteins function and undergo cell cycle-dependent regulation as heterohexameric complexes and that phosphorylation of the complexes by cdc2 kinase may be one of mechanisms negatively regulating the MCM complex-chromatin association.

Uncoupling of the order of the S and M phases: effects of staurosporine on human cell cycle kinases

The protein kinase inhibitor staurosporine (SSP) was employed to study the involvement of kinases in human cell cycle progression. Thirty to 100 ng/ml SSP blocks entry into S phase and M phase. Lack of entry into S phase is due to impaired activity of the retinoblastoma protein kinase. The requirement for any of the SSP-sensitive kinases for cell cycle progression can be abrogated in tumour cells. Therefore, these kinases act in a checkpoint network negatively controlling the initiation of S phase, M phase and cytokinesis, rather than being inherent parts of a substrate-product chain required for the initiation of the cell cycle phases. As a consequence of the lack of certain checkpoint effectors, tumour cells may endoreduplicate or binucleate in the presence of SSP. The latter processes, as well as meiosis, are naturally occurring in specialized cell types, leading to the idea that this checkpoint network controls the order of the cell cycle phases in normal cells. A model is presented where the cell cycle is envisioned as two independently running cycles, the S and the M cycle, which are controlled by intra and intercycledependent checkpoints in human somatic cells. The model accounts for the dependency of S and M phase initiation on the successful completion of the previous M and S phase, respectively, as well as entry into a resting state.

A measure of the mitotic index: studies of the abundance and half-life of p34cdc2 in cultured cells and normal and neoplastic tissues

BACKGROUND

The cdc2 gene encodes a protein kinase, p34cdc2, that is essential for mitosis, and is present at high levels in dividing cells. Classical studies of the levels of this protein in dividing and resting cells used antibodies that cross-react with other members of the CDK family, in particular with CDK2. We have therefore re-examined the abundance of p34cdc2 in a variety of tissues and cell lines, using a highly specific, epitope-mapped monoclonal antibody that does not react with CDK2.

RESULTS

We observed high levels of p34cdc2 in proliferating cells, especially those in neoplastic tissues. Cells that have withdrawn from the cell cycle have low or undetectable levels. At the end of mitosis, the level of p34cdc2 declines, with simple first-order kinetics, with a half-life which is never less than 6h and is more typically about 18h. The persistence of p34cdc2 after the last cell division is comparable to that of PCNA, a commonly used marker of proliferation.

CONCLUSIONS

The immunochemical detection of p34cdc2 provides an accurate, reliable and meaningful measure of the proliferative activity of cells in tissues. We suggest that p34cdc2 should be considered as the most authentic molecular marker of the mitotic index.

Four distinct cyclin-dependent kinases phosphorylate histone H1 at all of its growth-related phosphorylation sites

In mammalian cells, up to six serines and threonines in histone H1 are phosphorylated in vivo in a cell cycle dependent manner that has long been linked with chromatin condensation. Growth-associated H1 kinases, now known as cyclin-dependent kinases (CDKs), are thought to be the enzymes responsible for this process. This paper describes the phosphorylation of histone H1 by four different purified CDKs. The four CDKs phosphorylate only the cell cycle specific phosphorylation sites of H1, indicating that they belong to the kinase class responsible for growth-related H1 phosphorylation in vivo. All four CDKs phosphorylate all of the interphase and mitotic-specific H1 sites. In addition to the (S/T)PXK consensus phosphorylation sites, these four CDKs also phosphorylate a mitotic-specific in vivo H1 phosphorylation site that lacks this sequence. There is no site selectivity among the growth-related phosphorylation sites by any of the four CDKs because all four CDKs phosphorylate all relevant sites. The results imply that the cell cycle dependent H1 phosphorylations observed in vivo must involve differential accessibility of H1 sites at different stages of the cell cycle.

The prognostic significance of p34cdc2 and cyclin D1 protein expression in prostate adenocarcinoma

BACKGROUND

Cyclin-dependent kinases (CDK) and cyclins constitute the subunits of the maturation-promoting factor that controls the process of cell division. High levels of these proteins have been reported in human malignancies of the stomach, colon, breast, and lung, and have been implicated in aberrant cell division and dysregulated tumor growth.

METHODS

p34cdc2 CDK and cyclin D1 (D1) protein expression were evaluated in 140 radical prostatectomy specimens harboring adenocarcinoma (PAC), using the respective monoclonal antibodies on archival tissue sections. In each case, slides stained with hematoxylin and eosin were examined for evaluation of Gleason's grade and pathologic stage. The DNA content of the tumors was determined by the Feulgen method with the CAS200 Image Analyzer (Cell Analysis Systems, Lombard, IL). Nuclear immunoreactivity for the two proteins was semiquantitatively scored, and results were correlated with Gleason's grade, stage, ploidy, metastatic status, and disease recurrence after radical prostatectomy.

RESULTS

p34cdc2 was expressed in 84 of 140 PACs (60%) and correlated with high Gleason's grade (P = 0.0001), advanced pathologic stage (P = 0.01), nondiploid DNA content (P = 0.0001), and metastases (P = 0.04). On multivariate analysis using the Cox proportional hazards model, p34cdc2 immunoreactivity (P = 0.0001) and high Gleason's grade (P = 0.01) each independently predicted disease recurrence. When tumors were of low Gleason's grade and lacked p34cdc2 expression, 4 of 39 PACs (10%) recurred, as compared with 18 of 47 (38%) that recurred when tumors were of high Gleason's grade and expressed p34cdc2 protein. D1 was positive in 31 of 140 PACs (22%) and showed a trend (P = 0.07) of high Gleason's grade, but it did not reach statistical significance with any of the prognostic variables. In the majority of PACs expressing both p34cdc2 and D1 proteins, the adjacent benign prostate acini showed focal, scattered nuclear positivity of the basal and secretory epithelial cells.

CONCLUSIONS

p34cdc2 is expressed in a majority of PACs and correlates with high Gleason's grade, advanced pathologic stage, nondiploid DNA content, and metastases. On multivariate analyses high Gleason's grade and p34cdc2 immunoreactivity predict disease recurrence independently of the pathologic stage. Thus, p34cdc2 appears to play a critical role in the evolution, proliferation, and spread of PACs and may be of prognostic value when applied to initial prostate tissue samples taken by needle biopsy.

S-phase function of Drosophila cyclin A and its downregulation in G1 phase

BACKGROUND

Cyclin E is the normal inducer of S phase in G1 cells of Drosophila embryos. Stable G1 quiescence requires the downregulation both of cyclin E and of other factors that can bypass the normal regulation of cell cycle progression.

RESULTS

High-level expression of cyclin A triggered the G1/S transition in wild-type embryos and in mutant embryos lacking cyclin E. Three types of control downregulated this activity of cyclin A. First, cyclin destruction limited the accumulation of cyclin A protein in G1. Second, inhibitory phosphorylation of cdc2, the kinase partner of cyclin A, reduced the S-phase promoting activity of cyclin A in G1. Third, rux, a protein with unknown biochemical function, limited cyclin A function in G1. Overexpression of rux blocked S phase induction by coexpressed cyclin A and promoted the degradation of cyclin A. Rux also prevented a stable cyclin A mutant from inducing S phase, indicating that inhibition does not require cyclin destruction, and drove the nuclear localization of cyclin A.

CONCLUSIONS

Cyclin A can drive the G1/S transition, but this function is suppressed by three types of control: cyclin A destruction, inhibitory phosphorylation of cdc2, and inhibition by rux. The partly redundant contributions of these three inhibitory mechanisms safeguard the stability of G1 quiescence until the induction of cyclin E. The action of rux during G1 resembles the action of inhibitors of mitotic kinases present during G1 in yeast, although no obvious sequence similarity exists.

Construction by gene targeting in human cells of a "conditional' CDC2 mutant that rereplicates its DNA

We describe a novel gene targeting strategy for the genetic analysis of essential genes in mammalian cells and its use to study the role of the cell cycle control gene CDC2 in human cells. A cell line (HT2-19) was generated in which endogenous CDC2 gene expression and cell viability depend on the presence of an inducer in the growth medium. In the absence of inducer, HT2-19 cells undergo extensive DNA rereplication and apoptosis. Rereplication is indicative of a role for human CDC2 in a control mechanism, previously undetected in mammalian cells, that prevents premature entry into S-phase.

Screening of cell cycle inhibitors from microbial metabolites by a bioassay using a mouse cdc2 mutant cell line, tsFT210

We have established a bioassay system using a mouse cdc2 mutant cell line, tsFT210, to detect inhibitors of the mammalian cell cycle. When cultured at the high temperature, restrictive temperature at 39.4 degrees C, tsFT210 cells can be arrested at G2 phase and are large in size. Four hours after release from G2 arrest, the cells entered into the G1 phase. At this time, G1 phase cells were easily discriminated from the G2/M-cells by their size under microscopic observation. The cell-morphology-based bioassay utilizing tsFT210 cells is very simple and sensitive for detecting cdc2 kinase inhibitors and also G2/M-phase inhibitors of the mammalian cell cycle. To demonstrate the merits of this bioassay, the effects of protein kinase inhibitors isolated from actinomycetes were investigated. RK-286C and RK-1409, which are structurally related to staurosporine, inhibited cell cycle progression at the G2 phase in both G2-synchronized and nonsynchronized cultures of tsFT210 cells. Another kinase inhibitor, sangivamycin, inhibited cell cycle progression at the G2 phase of cells released from temperature arrest but did not inhibit that of the exponentially growing cells. Using the bioassay system, we carried out screening of the cell cycle inhibitors from the microbial metabolites and have discovered several new inhibitors, including novel compounds such as tryprostatins A, B and acetophthalidin. Thus, this bioassay allowed for the detection of cell cycle inhibitors and provided a convenient and useful method for the screening of new inhibitors from the microbial metabolites.

Repression of p27kip1 synthesis by platelet-derived growth factor in BALB/c 3T3 cells

We have investigated the regulation of p27kip1, a cyclin-dependent kinase inhibitor, in BALB/c 3T3 cells during growth factor-stimulated transition from quiescence (G0) to a proliferative (G1) state. The level of p27kip1 protein falls dramatically after mitogenic stimulation and is accompanied by a decrease in cyclin E associated p27kip1, as well as a transient increase in cyclin D1-associated p27kip1 that later declines concomitantly with the loss of total p27kip1. Analysis of metabolically labelled cells revealed that cyclin D2, cyclin D3, and cdk4 were also partnered with p27kip1 in quiescent BALB/c 3T3 cells and that this association decreased after platelet-derived growth factor (PDGF) treatment. Furthermore, the decline in p27kip1 and reduced association with cyclin D3, initiated by the addition of PDGF but not plasma-derived factors, suggested that these changes are involved in competence, the first step in the exit from G0. Synthesis of p27kip1 as determined by incorporation of [35S]methionine was repressed upon mitogenic stimulation, and PDGF was sufficient to elicit this repression within 2 to 3 h. Pulse-chase experiments demonstrated the reduced rate of synthesis was not the result of an increased rate of degradation. Full repression of p27kip1 synthesis required the continued presence of PDGF and failed to occur in the presence of the RNA polymerase inhibitor 5,6-dichlorobenzimidazole riboside. These characteristics demonstrate that repression was a late effect of PDGF and was consistent with our finding that conditional expression of activated H-ras did not affect synthesis of p27kip1. Northern (RNA) analysis of p27kip1 mRNA revealed that the repression was not accompanied by a corresponding decrease in p27kip1 mRNA, suggesting that the PDGF-regulated decrease in p27kip1 expression occurred through a translational mechanism.

Cytoplasmic accumulation of cdc25B phosphatase in mitosis triggers centrosomal microtubule nucleation in HeLa cells

The formation of the mitotic spindle is an essential prerequisite for successful mitosis. The dramatic changes in the level of microtubule (Mt) nucleation at the centrosomes and Mt dynamics that occur in prophase are presumed to be initiated through the activity of cdc2/cyclin B. Here we present data that the cdc25B isoform functions to activate the cytoplasmic pool of cdc2/cyclin B responsible for these events. In contrast to cdc25C, cdc25B is present at low levels in HeLa cells during interphase, but sharply increases in prophase, when cdc25B accumulation in the cytoplasm correlates with prophase spindle formation. Overexpression of wild type and dominant negative mutants of cdc25B and cdc25C shows that prophase Mt nucleation is a consequence of cytoplasmic cdc25B activity, and that cdc25C regulates nuclear G2/M events. Our data also suggest that the functional status of the centrosome can regulate nuclear mitotic events.

The localization of p34cdc2 in the cells of normal, hyperplastic, and malignant epithelial and lymphoid tissues of the oral cavity

The expression of p34cdc2 has been examined in normal, hyperplastic, and malignant oral epithelia and lymphoid tissues. Four monoclonal antibodies were prepared against Xenopus p34cdc2, three of which react specifically with human p34cdc2 and not with p33cdk2. These produced similar patterns of staining in both fixed and frozen sections of human material. Staining occurred mainly in the proliferative compartments of normal and hyperplastic tissues. In normal oral epithelia, parabasal and basal cells were the most strongly stained, with lighter cytoplasmic staining in lower prickle cells. In tonsillar germinal centres, a high proportion of cells was stained, with fewer positive cells in interfollicular zones, a distribution in keeping with the known pattern of cell proliferation. In normal cells, the intracellular location of p34cdc2 was cytoplasmic until early prophase, but in oral squamous cell carcinomas and lymphomas, it was located in both cytoplasm and nucleus during interphase and a larger fraction of cells was positive than in the equivalent normal tissues. Higher-grade neoplasms showed both a higher intensity of staining and a higher proportion of p34cdc2-positive cells.

Differentiation is induced in three-dimensional cultures of brain cells immortalized by the LAP mammalian regulatory system

Immortalized neuroectodermal precursor cell lines were generated from mouse brain by the SV40 large T antigen expressed under the control of the LAP (lac activating protein) mammalian regulatory system. The LAP system permits the reversible expression of T antigen as a function of the exogenous inducer, isopropyl-beta-D-thiogalactopyranoside. Immortalized cells can be stably maintained in an undifferentiated state in monolayer cultures. Cell lines expressed the early neurofilament-like protein nestin, but not markers characteristic for mature cells such as the neurofilament light protein and glial fibrillary acidic protein. Downregulating the LAP-controlled T antigen with isopropyl-beta-D-thiogalactopyranoside was not sufficient to induce differentiation. However, when cells formed three-dimensional aggregates, differentiation to a neuronal phenotype occurred, indicating that cell-cell interaction plays an important role in their differentiation. Cells in aggregates did not proliferate, even in the presence of T antigen, suggesting that an aggregation-induced signal to cease growth was dominant over the growth signal of T antigen. Further morphological differentiation was induced by basic fibroblast growth factor. These immortalized cells should facilitate molecular and cellular studies concerned with the mechanism of commitment, fate determination, and mitotic arrest of neuronal precursor cells in the developing mammalian CNS.

Inactivation of Cdc2 increases the level of apoptosis induced by DNA damage

A number of lines of evidence have suggested a possible involvement of the mitosis-promoting protein kinase Cdc2 in the process of apoptotic cell death, and one recent study concluded that premature activation of Cdc2 is required for apoptosis. Here we have used a temperature-sensitive murine Cdc2 mutant cell line and Cdc2 inhibitor compounds to study the effect of inhibition of this protein kinase on apoptosis induced by DNA-damaging drugs. Inhibition of Cdc2 activity before or during exposure to DNA strand break-inducing drugs had the effect of increasing the level of subsequent apoptosis, as assessed by electron microscopy and flow cytometry. We conclude that, far from being required for cell death, a form of mammalian Cdc2 suppresses apoptosis induced by DNA damage. This form of Cdc2 appears to be active in G2-arrested cells and is therefore presumably distinct from the mitosis-promoting Cdc2-cyclin B heterodimer.

Phosphorylation of the Oxytricha telomere protein: possible cell cycle regulation

In the macronucleus of the ciliate Oxytricha nova, telomeres end with single-stranded (T4G4)2 DNA bound to a heterodimeric telomere protein (alpha beta). Both the alpha and beta subunits (alpha-TP and beta-TP) were phosphorylated in asynchronously growing Oxytricha; beta-TP was phosphorylated to a much higher degree. In vitro, mouse cyclin-dependent kinases (Cdks) phosphorylated beta-TP in a lysine-rich domain that is not required for specific DNA binding but is implicated in higher order structure formation of telomeres. Therefore, phosphorylation of beta-TP could modulate a function of the telomere protein that is separate from specific DNA binding. Phosphoamino acid analysis revealed that the mouse Cdks modify predominantly threonine residues in beta-TP, consistent with the observation that beta-TP contains two consensus Cdk recognition sequences containing threonine residues. In Xenopus egg extracts that undergo cell cycling, beta-TP was phosphorylated in M phase and dephosphorylated in interphase. This work provides the first direct evidence of phosphorylation at telomeres in any organism, as well as indirect evidence for cell cycle regulation of telomere phosphorylation. The Cdc2/cyclin A and Cdc2/cyclin B kinases are required for major mitotic events. An attractive model is that phosphorylation of beta-TP by these kinases is required for the breakdown of telomere associations with each other and/or with nuclear structures prior to nuclear division.

Apoptosis and the cell cycle

In this review, we consider apoptosis as a process intimately linked to the cell cycle. There are several reasons for thinking of apoptosis as a cell cycle phenomenon. First, within the organism, apoptosis is almost exclusively found in proliferating tissues. Second, artificial manipulation of the cell cycle can either prevent or potentiate apoptosis, depending on the point of arrest. Data from such studies have suggested that molecules acting late in G1 are required for apoptosis. Since passage through late G1 into S phase in mammalian cells is known to be regulated by p53 and by activation of cyclin-dependent kinases, we also examine recent studies linking these molecules to the apoptotic pathway.

Distinct modes of cyclin E/cdc2c kinase regulation and S-phase control in mitotic and endoreduplication cycles of Drosophila embryogenesis

Drosophila cyclin E (DmcycE) is required in embryos for S phase of mitotic and endoreduplication cycles. Here, we describe regulatory differences characteristic for these two cell cycle types. While DmcycE transcript levels decline in DmcycE mutant cells programmed for mitotic proliferation, they are maintained and no longer restricted to transient pulses in DmcycE mutant cells programmed for endoreduplication. Moreover, DmcycE expression in endoreduplicating cells is down-regulated by ectopic expression of a heat-inducible cyclin E transgene. DmcycE expression in endoreduplicating tissues, therefore, is restricted by a negative feedback to the transient pulse triggering entry into S-phase. Conversely, during mitotic cycles, where S phase entry is not only dependent on cyclin E but also on progression through M phase, cyclin E and associated Dmcdc2c kinase activity are present throughout the cell cycle. Reinitiation of DNA replication during the G2 phase of the mitotic cell cycle, therefore, is prevented by cyclin E/Dmcdc2c kinase-independent regulation. Observations in cyclin A mutants implicate G2 cyclins in this regulation. Our results suggest molecular explanations for the different rules governing S phase during mitotic and endoreduplication cycles.

Evidence for a NIMA-like mitotic pathway in vertebrate cells

NIMA is essential for entry into mitosis in Aspergillus nidulans. To examine whether there is a NIMA-like pathway in other eukaryotic cell cycles, we expressed NIMA and its dominant negative mutants in two different eukaryotic systems. In Xenopus oocytes, NIMA induced germinal vesicle breakdown without activating Mos, CDC2, or MAP kinase. In HeLa cells, NIMA induced premature mitotic events without activating CDC2, whereas the mutants caused a specific G2 arrest but did not block mutant CDC2T14AY15F-induced premature entry into mitosis. A sequence essential for both these phenotypes was mapped to a region of approximately 100 amino acids lying just after the catalytic domain of NIMA that shows a significant similarity to protein interaction domains in other proteins. These results provide evidence for the existence of a NIMA-like mitotic pathway in vertebrate cells.

Both cdc2 and cdk2 promote S phase initiation in Xenopus egg extracts

Xenopus egg extracts induce S phase DNA replication in added sperm pronuclei in a highly regulated manner, similar to events in vivo. Removal of cyclin-dependant kinases (cdks) or cdk2 from these extracts using affinity matrices severely inhibits initiation of S phase. We have used p13suc1 beads to remove both cdk2 and cdc2 proteins from egg extracts and developed a method to replace either protein alone to assess their capacity to initiate DNA replication. Re-addition of either cdk2 or cdc2 proteins to depleted extracts, through translation of their respective mRNAs, restimulated replication, judged by both total synthesis and labelling index. An ATP-binding-site mutant cdk2 mRNA (cdk2.R33) failed to stimulate replication and inhibited S phase initiation in mock-depleted extracts. Both human and Xenopus cdc2 mRNAs rescued replication in this system. Human mutant mRNAs have been used to show that the stimulation induced requires cdc2 catalytic activity, though not its mitotically active form. Rescue of replication by p34cdc2 is also observed in extracts depleted of cdks with a cdk2 antibody, which still retain much of their endogenous cdc2 protein. We conclude that newly synthesised p34cdc2, but not the inherited ‘old’ form, can induce S phase and in this form may overlap in function with p33cdk2.

The G1/S boundary-specific enhancer of the rat cdc2 promoter

Multiple species of G1 cyclins and cyclin-dependent kinases are induced sequentially during G1 phase, and the expression of cyclin A and cdc2 genes is subsequently induced at the G1/S boundary. To analyze the mechanism of cdc2 promoter activation, the 5'-flanking region of the rat cdc2 gene was isolated and its structural features were characterized. The highly conserved sequence between human and rat cdc2 genes is present in the basal promoter region from positions -183 to -122, which contains the E box, SpI, and E2F motifs. The expression of 5' sequential deletion derivatives of the promoter fused to luciferase cDNA in rat 3Y1 cells revealed the presence of the enhancer element. The presumed enhancer region was further analyzed by the introduction of base substitutions and by the formation of DNA-protein complexes with cell extracts prepared at various times during the G1-to-S-phase progression. These analyses revealed that the enhancer sequence, AAGTTACAAATA, located from -276 to -265, confers strong inducibility on the basal promoter at the G1/S boundary. The base substitutions introduced into the motifs of transcription factors indicated that the E2F motif is essential for the enhancer-dependent activation of the cdc2 promoter at the G1/S boundary. Electrophoretic mobility shift assays and DNase I footprinting showed that a factor which interacts with the enhancer element is induced late in G1 phase.

Chromosome condensation induced by fostriecin does not require p34cdc2 kinase activity and histone H1 hyperphosphorylation, but is associated with enhanced histone H2A and H3 phosphorylation

Chromosome condensation at mitosis correlates with the activation of p34cdc2 kinase, the hyperphosphorylation of histone H1 and the phosphorylation of histone H3. Chromosome condensation can also be induced by treating interphase cells with the protein phosphatase 1 and 2A inhibitors okadaic acid and fostriecin. Mouse mammary tumour FT210 cells grow normally at 32 degrees C, but at 39 degrees C they lose p34cdc2 kinase activity and arrest in G2 because of a temperature-sensitive lesion in the cdc2 gene. The treatment of these G2-arrested FT210 cells with fostriecin or okadaic acid resulted in full chromosome condensation in the absence of p34cdc2 kinase activity or histone H1 hyperphosphorylation. However, phosphorylation of histones H2A and H3 was strongly stimulated, partly through inhibition of histone H2A and H3 phosphatases, and cyclins A and B were degraded. The cells were unable to complete mitosis and divide. In the presence of the protein kinase inhibitor starosporine, the addition of fostriecin did not induce histone phosphorylation and chromosome condensation. The results show that chromosome condensation can take place without either the histone H1 hyperphosphorylation or the p34cdc2 kinase activity normally associated with mitosis, although it requires a staurosporine-sensitive protein kinase activity. The results further suggest that protein phosphatases 1 and 2A may be important in regulating chromosome condensation by restricting the level of histone phosphorylation during interphase, thereby preventing premature chromosome condensation.

Control of cell proliferation during plant development

Knowledge of the control of cell division in eukaryotes has increased tremendously in recent years. The isolation and characterization of the major players from a number of systems and the study of their interactions have led to a comprehensive understanding of how the different components of the cell cycle apparatus are brought together and assembled in a fine-tuned machinery. Many parts of this machine are highly conserved in organisms as evolutionary distant as yeast and animals. Some key regulators of cell division have also been identified in higher plants and have been shown to be functional homologues of the yeast or animal proteins. Although still in its early days, investigations into the regulation of these molecules have provided some clues on how cell division is coupled to plant development.

The kinetics of H1 histone kinase activation during the cell cycle of wild-type and wee mutants of the fission yeast Schizosaccharomyces pombe

H1 histone kinase activity has been followed in selection-synchronised cultures of fission yeast wild-type and wee1 mutant cells, and in induction-synchronised cells of the mutant cdc2-33. The main conclusions are: (1) in all three cases, the peak of activity is near mitosis. (2) The rise in activity is relatively slow starting in wild type at 0.4 of the cycle before mitosis. It is proposed that the beginning of the rise is the first identified event in the mitotic control. (3) The rise is twice as fast in wee and starts nearer to mitosis. (4) In all cases the beginning of the rise is in G2. (5) The fall in activity is also slow, lasting for 0.25 of the cycle, in wild type. Exit from mitosis happens well before activity has fallen to baseline. (6) In a range of size mutants, activity is roughly proportional to cell size. It is suggested that the kinase may have a cytoplasmic function. (7) Estimates have been made of the timing of mitosis in the mutants. In wee, mitosis is 0.14 of the cycle earlier than in wild type because the cells have a longer septated period at the end of the cycle. (8) A novel method has been developed for eliminating the effects of the partial asynchrony in synchronous cultures, without which the kinetic analysis would have been inaccurate.

Simian virus 40 prevents activation of M-phase-promoting factor during lytic infection

Simian virus 40 (SV40) infection stimulates confluent cultures of monkey kidney cells into successive rounds of cellular DNA synthesis without intervening mitosis. As an initial step in defining the mechanisms responsible for viral inhibition of mitosis, M-phase-promoting factor (MPF) was examined in SV40-infected CV-1 cells passing from G2 phase into a second S phase. MPF is a serine-threonine protein kinase that is essential for mitosis in eukaryotic cells. In SV40-infected cells exiting G2 phase, there was a reduced amount of MPF-associated H1 kinase activity relative to that of uninfected cells passing through mitosis. Both subunits of MPF, cyclin B and the p34cdc2 catalytic subunit, were present and in a complex in infected cells. In uninfected cultures, passage through mitosis was associated with the dephosphorylation of the p34cdc2 subunit, which is characteristic of MPF activation. In contrast, the p34cdc2 subunit remained in the tyrosine-phosphorylated, inactive form in SV40-infected cells passing from G2 phase into a second S phase. These results suggest that although the MPF complex is assembled and modified normally, SV40 interferes with pathways leading to MPF activation.

Activity and expression pattern of cyclin-dependent kinase 5 in the embryonic mouse nervous system

Cyclin-dependent kinase 5 (cdk5) was originally isolated on the basis of its close primary sequence homology to the human cdc2 serine/threonine kinase, the prototype of the cyclin-dependent kinases. While kinase activities of both cdc2 and cdk2 are detected in proliferating cells and are essential for cells to progress through the key transition points of the cell cycle, cdk5 kinase activity has been observed only in lysates of adult brain. In this study, we compared the activity and expression of cdk5 with that of cdc2 and cdk2 in the embryonic mouse forebrain. The expression and activity of cdk5 increased progressively as increasing numbers of cells exited the proliferative cycle. In contrast, the expression and activity of cdc2 and cdk2 were maximum at gestational day 11 (E11) when the majority of cells were proliferating and fell to barely detectable levels at E17 at the end of the cytogenetic period. Immunohistochemical studies showed that cdk5 is expressed in postmitotic neurons but not in glial cells or mitotically active cells. Expression of cdk5 was concentrated in fasciculated axons of postmitotic neurons. In contrast to other cell division cycle kinases to which it is closely related, cdk5 appears not to be expressed in dividing cells in the developing brain. These observations suggest that cdk5 may have a role in neuronal differentiation but not in the cell division cycle in the embryonic nervous system.

Reversal of terminal differentiation and control of DNA replication: cyclin A and Cdk2 specifically localize at subnuclear sites of DNA replication

DNA replication in mammalian cells occurs in discrete nuclear foci. Here we show that terminally differentiated myotubes can be induced to reenter S phase and show the same pattern of replication foci as cycling cells. We used this cellular system to analyze the interaction of cell cycle proteins with these foci in vivo. Cyclin A and cdk2, but not cyclin B1 and cdc2, were specifically localized at nuclear replication foci, just like the replication protein proliferating cell nuclear antigen. A potential target of cyclin A and cdk2 is the 34 kd subunit of replication protein A (RPA34). In contrast with the 70 kd subunit, which localizes to the foci, RPA34 was not detected at these replication sites, which may reflect a transient interaction. The specific localization of cyclin A and cdk2 at nuclear replication foci provides a direct link between cell cycle regulation and DNA replication.

Cell cycle dependent distribution of proliferating cell nuclear antigen/cyclin and cdc2-kinase in mouse T-lymphoma cells

The aim of the present study was to investigate bromodeoxyuridine (BrdU) uptake and coordinated distribution of proliferating cell nuclear antigen (PCNA) and p34-cdc2-kinase, two important proteins involved in cell cycle regulation and progression. Flow cytometric analysis of marker proteins in freshly plated mouse T-lymphoma cells (Yac-1 cells), using fluorescein isothiocyanate (FITC)-labeled specific antibodies, showed PCNA distributed throughout the cell cycle with increased intensity in S-phase. PCNA is essential for cells to cycle through S-phase and its synthesis is initiated during late G1-phase before incorporation of BrdU and remains high during active DNA replication. The intensity of PCNA fluorescence increases with the duration of incubation after plating. The cdc2-kinase was detectable in all phases of the cell cycle and the G2-M-phase appears to have the maximum concentrations. The cell cycle analysis of high dose colcemid (2 micrograms/ml) treated Yac-1 cells showed an aneuploid or hypodiploid population. Although the G2-M-phase seems to be the dominating population in aneuploid cells, the concentrations of cdc2-kinase were variable in this phase of cell cycle. The colcemid treatment at 25 ng/ml arrested 96% of cells in S-phase and G2-M-phase, but PCNA expression was evident in a portion of the cell population in G2-M-phase. Although cells blocked in M-phase seem to have high levels of cdc2-kinase, colcemid renders them inactive. From these data, it appears that the down regulation and/or inactivation of cdc2-kinase could be responsible for the colcemid arrest of cells in M-phase.

In vitro fusion of endocytic vesicles is inhibited by cyclin A-cdc2 kinase

Receptor-mediated endocytosis and recycling are inhibited in mitotic mammalian cells, and previous studies have shown that inhibition of endocytic vesicle fusion in vitro occurs via cyclin B-cdc2 kinase. To test for the ability of cyclin A-cdc2 kinase to inhibit endocytic vesicle fusion, we employed recombinant cyclin A proteins. Addition of cyclin A to interphase extracts activated a histone kinase and markedly reduced the efficiency of endocytic vesicle fusion. By a number of criteria, inhibition of fusion was shown to be due to the action of cyclin A, via the mitosis-specific cdc2 kinase, and not an indirect effect through cyclin B. Two-stage incubations were used to demonstrate that at least one target of cyclin A-cdc2 kinase is a cytosolic component of the fusion apparatus. Reconstitution experiments showed that this component was also modified in mitotic cytosols and was unaffected by N-ethyl maleimide treatment.

Cyclin D1 is a nuclear protein required for cell cycle progression in G1

A cascade of events is triggered upon the addition of growth factor to quiescent mammalian cells, which ultimately restarts proliferation by inducing the transition from G0/G1 to S-phase. We have studied cyclin D1, a putative G1 cyclin, in normal diploid human fibroblasts. Cyclin D1 accumulated and reached a maximum level before S-phase upon the addition of serum to quiescent cells. The protein was localized to the nucleus, and it disappeared from the nucleus as cells proceeded into S-phase. Microinjection of anti-cyclin D1 antibodies or antisense plasmid prevented cells from entering S-phase, and the kinetics of inhibition showed that cyclin D1 is required at a point in the cell cycle earlier than cyclin A. These results demonstrate that cyclin D1 is a critical target of proliferative signals in G1.

Regulation of the cell cycle by the cdk2 protein kinase in cultured human fibroblasts

In mammalian cells inhibition of the cdc2 function results in arrest in the G2-phase of the cell cycle. Several cdc2-related gene products have been identified recently and it has been hypothesized that they control earlier cell cycle events. Here we have studied the relationship between activation of one of these cdc2 homologs, the cdk2 protein kinase, and the progression through the cell cycle in cultured human fibroblasts. We found that cdk2 was activated and specifically localized to the nucleus during S phase and G2. Microinjection of affinity-purified anti-cdk2 antibodies but not of affinity-purified anti-cdc2 antibodies, during G1, inhibited entry into S phase. The specificity of these effects was demonstrated by the fact that a plasmid-driven cdk2 overexpression counteracted the inhibition. These results demonstrate that the cdk2 protein kinase is involved in the activation of DNA synthesis.