Human cyclin E, a new cyclin that interacts with two members of the CDC2 gene family

The mammalian cell cycle in normal and abnormal growth

Cell division, a complex array of intracellular events, occurs in a highly ordered and carefully coordinated manner. This regulation is achieved by the sequential activation and deactivation of the members of a family of serine-threonine-specific protein kinases that consist of regulatory and enzymatic subunits, the cyclins and cyclin-dependent kinases. These enzymes, in turn, regulate the activity of other proteins involved in the mitogenic pathway. Mutations in the components of the regulatory pathways can lead to aberrant growth, including malignancies.

Turnover of cyclin E by the ubiquitin-proteasome pathway is regulated by cdk2 binding and cyclin phosphorylation

Cyclin E is a mammalian G1 cyclin that is both required and rate limiting for entry into S phase. The expression of cyclin E is periodic, peaking at the G1-S transition and then decaying as S phase progresses. To understand the mechanisms underlying cyclin E periodicity, we have investigated the regulation of cyclin E degradation. We find that cyclin E is degraded by the ubiquitin-proteasome system, and that this degradation is regulated by both cdk2 binding and cdk2 catalytic activity. Free cyclin E is readily ubiquitinated and degraded by the proteasome. Binding to cdk2 protects cyclin E from ubiquitination, and this protection is reversed by cdk2 activity in a process that involves phosphorylation of cyclin E itself. The data are most consistent with a model in which cdk2 activity initiates cyclin E degradation by promoting the disassembly of cyclin E-cdk2 complexes, followed by the ubiquitination and degradation of free cyclin E.

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.

The cyclin-dependent kinase inhibitor p40SIC1 imposes the requirement for Cln G1 cyclin function at Start

In yeast, commitment to cell division (Start) is catalyzed by activation of the Cdc28 protein kinase in late G1 phase by the Cln1, Cln2, and Cln3 G1 cyclins. The Clns are essential, rate-limiting activators of Start because cells lacking Cln function (referred to as cln-) arrest at Start and because CLN dosage modulates the timing of Start. At or shortly after Start, the development of B-type cyclin Clb-Cdc28 kinase activity and initiation of DNA replication requires the destruction of p40SIC1, a specific inhibitor of the Clb-Cdc28 kinases. I report here that cln cells are rendered viable by deletion of SIC1. Conversely, in cln1 cln2 cells, which have low CLN activity, modest increases in SIC1 gene dosage cause inviability. Deletion of SIC1 does not cause a general bypass of Start since (cln-)sic1 cells remain sensitive to mating pheromone-induced arrest. Far1, a pheromone-activated inhibitor of Cln-Cdc28 kinases, is dispensable for arrest of (cln-)sic1 cells by pheromone, implying the existence of an alternate Far1-independent arrest pathway. These observations define a pheromone-sensitive activity able to catalyze Start only in the absence of p40SIC1. The existence of this activity means that the B-type cyclin inhibitor p40SIC1 imposes the requirement for Cln function at Start.

Cell cycle regulation of the murine cyclin E gene depends on an E2F binding site in the promoter

Cyclin E controls progression through the G1 phase of the cell cycle in mammalian fibroblasts and potentially in many other cell types. Cyclin E is a rate-limiting activator of cdk2 kinase in late G1. The abundance of cyclin E is controlled by phase-specific fluctuations in the mRNA level; in mammalian fibroblasts, mRNA is not detected under conditions of serum starvation and is accumulated upon serum stimulation, with expression starting in mid-G1. Here, we report the cloning of the murine cyclin E promoter. We isolated a 3.8-kb genomic fragment that contains several transcriptional start sites and confers cell cycle regulation on a luciferase reporter gene. This fragment also supports transcriptional activation by adenovirus E1A, a known upstream regulator of cyclin E gene expression. An E2F binding site which is required for G1-specific activation of the cyclin E promoter in synchronized NIH 3T3 cells was identified in this fragment.

Increased stringency of the 1,25-dihydroxyvitamin D3-induced G1 to S phase block in polyploid HL60 cells

Treatment of mammalian cells with 1,25-dihydroxyvitamin D3 (1,25D3) produces a G1 to S (G1/S) phase cell cycle block. In addition, it has been noted that a smaller proportion of cells accumulates in the G2/M compartment in 1,25D3-treated cultures. Since cyclins have a major influence on the regulation of cell cycle progression, we determined the expression of cyclins A and B as markers of the G2 phase and of cyclin E as the marker of G1/S transition. No increase in the steady-state levels of cyclin A or cyclin B mRNA was detected in the total cell population or in the cyclin B1 protein in the G2/M cell cycle compartment. In contrast, immunodetectable cyclin E protein was increased in cell cultures as a whole and specifically in the G2/M compartment cells. Determination of BrdU incorporation into DNA by flow cytometry showed marked inhibition of DNA replication in cells with DNA content higher than 4C, and autoradiography of 3H-TdR-pulsed cells showed that polynucleated cells did not replicate DNA after 96 h of treatment with 1,25D3 or analogs. Taken together, these experiments show that at least a portion of the G2/M compartment in 1,25D3-arrested cultures of HL60 cells represents G1 cells at a higher ploidy level, which are blocked from entering the high ploidy S phase.

Immunohistochemical study of Cell Cycle Modulators in G(1)-S Transition in Clinical Breast Cancer Tissue

In the present study, we investigated immunolocalization of the modulators of G(1)-S transition by using monoclonal or polyclonal antibodies for each of the modulators in 65 cases of clinical breast cancer. Two prominent cyclin dependent kinase(cdk)-cyclin complexes, cdk4-cyclin D and cdk2-cyclin E, were proved to have different modes of mutual expression. cdk4-positive lesions were found to equal cyclin D-expressing lesions in 55 cases, while the former were more extensive than the latter in 9 cases. On the other hand, cyclin E expression was detected in all the cases examined and was more dominant than that of cdk2/cdc2 in as many as 40 cases whereas the reverse was seen in only 1 case. Interestingly, cdk4(P<0.01)and cyclin E(P<0.05)expressions showed an inverse relationship with the tumor size and the cancer stage. A similar tendency was also detected for two other positive modulators of G(1)-S transition, indicating that cell cycle progression must be regulated by the cancer itself once it has grown to a certain extent. p21, which has been regarded as a universal inhibitor of the cell cycle, was expressed in 43.1% of the cases examined and its immunoreactivity showed an inverse relationship with lymph node metastasis(P<0.05). It also tended to be absent more frequently in T3 or larger cancers and stage III cases. Moreover, two patients who died as a result of cancer and three patients with recurrence were all p21 negative, suggesting that p21 is prognosticably the most significant of all these modulators.

Cell cycle regulation by the retinoblastoma family of growth inhibitory proteins

The retinoblastoma family of growth-inhibitory proteins act by binding and inhibiting several proteins with growth-stimulatory activity, the most prominent of which is the cellular transcription factor E2F. In higher organisms, progression through the cell division cycle is accompanied by the cyclical activation of a number of protein kinases, the cyclin-dependent kinases. Phosphorylation of retinoblastoma family proteins by these cyclin-dependent kinases leads to release of the associated growth-stimulatory proteins which in turn mediate progression through the cell division cycle.

Xenopus cyclin E, a nuclear phosphoprotein, accumulates when oocytes gain the ability to initiate DNA replication

The capacity to initiate DNA replication appears during oocyte maturation in Xenopus. Initiation of S phase is driven by several components which include active cyclin/cdk complexes. We have identified three Xenopus cyclin E clones showing 59% amino acid identity with human cyclin E. The recruitment of cyclin E mRNA, like cdk2 mRNA, into the polysomal fraction during oocyte maturation, results in the accumulation of the corresponding proteins in unfertilized eggs. Cyclin E mRNA remains polyadenylated during cleavage and anti-cyclin E antibodies detect Xlcyclin E in embryonic nuclei at this time. Cdk2 protein is necessary for the phosphorylation of radiolabelled cyclin E added to egg extracts. Radiolabelled Xlcyclin E enters interphase nuclei and, though stable through interphase and mitosis, is not associated with condensed mitotic chromatin. In egg extracts, endogenous Xlcyclin E rapidly associates with nuclei before S phase and remains nuclear throughout interphase, becoming nucleoplasmic in G2/prophase. Under conditions where initiation of replication is limiting in extracts, Xlcyclin E associates only with those nuclei that undergo S phase. These features are entirely consistent with the view that Xlcyclin E is required for initiation of S phase.

Activity of the retinoblastoma family proteins, pRB, p107, and p130, during cellular proliferation and differentiation

Genetic evidence from retinoblastoma patients and experiments describing the mechanism of cellular transformation by the DNA tumor viruses have defined a central role for the retinoblastoma protein (pRB) family of tumor suppressors in the normal regulation of the eukaryotic cell cycle. These proteins, pRB, p107, and p130, act in a cell cycle-dependent manner to regulate the activity of a number of important cellular transcription factors, such as the E2F-family, which in turn regulate expression of genes whose products are important for cell cycle progression. In addition, inhibition of E2F activity by the pRB family proteins is required for cell cycle exit after terminal differentiation or nutrient depletion. The loss of functional pRB, due to mutation of both RB1 alleles, results in deregulated E2F activity and a predisposition to specific malignancies. Similarly, inactivation of the pRB family by the transforming proteins of the DNA tumor viruses overcomes cellular quiescence and prevents terminal differentiation by blocking the interaction of pRB, p107, and p130 with the E2F proteins, leading to cell cycle progression and, ultimately, cellular transformation. Together these two lines of evidence implicate the pRB family of negative cell cycle regulators and the E2F family of transcription factors as central components in the cell cycle machinery.

Purification and characterization of a p13suc1-bound serine/threonine kinase that is expressed in mature rat brain

We previously reported that the histone-H1 kinase activity bound to p13suc1 increased dramatically during development of the rat brain. In the present work, an in situ kinase assay in an SDS/polyacrylamide gel that contained substrate proteins was employed to characterize the enzyme. Two major proteins of 45 kDa and 100 kDa were found to have p13suc1-bound histone-H1 kinase activity. The former (p45) exhibited strong activity towards histone H1 and had weak autophosphorylation activity, whereas the latter (p100) acted on myelin basic protein or histone H1, and underwent autophosphorylation. p45 was further purified from the nuclear-enriched fraction of rat brain to near homogeneity through sequential column chromatographies. The purified enzyme retained its ability to bind specifically to p13suc1, which suggests that this binding does not require a cofactor. The immunochemical and enzymatic properties of p45 revealed that it differs from Cdk that are known to bind to p13suc1 with high affinity. However, in vitro p45 acted on the peptide motif that is conserved among substrates for cyclin-dependent kinases (Cdk) and mitogen-activated protein kinases, which implies that this protein might belong to the large family of proline-directed kinases. The evidence obtained in this study suggest that p45 is a nuclear p13suc1-bound kinase that has unique functions in the mature brain.

Growth factor dependence of progression through G1 and S phases of adult rat hepatocytes in vitro. Evidence of a mitogen restriction point in mid-late G1

Several hepatocyte mitogens have been identified, but the signals triggering the G0/G1 transition and cell cycle progression of hepatocytes remain unknown. Using hepatocyte primary cultures, we investigated the role of epidermal growth factor/pyruvate during the entry into and progression through the G1 phase and analyzed the expression of cell cycle markers. We show that the G0/G1 transition occurs during hepatocyte isolation as evidenced by the expression of early genes such as c-fos, c-jun, and c-myc. In culture, hepatocytes progress through G1 regardless of growth factor stimulation until a restriction point (R point) in mid-late G1 beyond which they cannot complete the cell cycle without mitogenic stimulation. Changes in cell cycle gene expression were associated with progression in G1; the cyclin E mRNA level is low early in G1 but increases at the G1/S boundary, while the protein is constantly detected during cell cycle but undergoes a change of electrophoretic mobility in mid-late G1 after the R point. In addition, a drastic induction of cyclin D1 mRNA and protein, and to a lesser extent of cyclin D2 mRNA, takes place in mitogen-stimulated cells after the R point. In contrast, cyclin D3 mRNA appears early in G1, remains constant in stimulated cells, but accumulates in unstimulated arrested cells, paralleling the cyclin-dependent kinase 4 mRNA expression. These results characterize the different steps of G1 phase in hepatocytes.

Presence and expression of G2 cyclins in the coelenterate hydra

In hydra all cell-cycle control occurs in the G2/M transition. Cyclins acting at this restriction point in the cell cycle belong to the cyclin A and B families. In agreement with this we isolated cDNAs coding for a cyclin A and a cyclin B from the multiheaded mutant of Chlorohydra viridissima and a cyclin B from Hydra vulgaris. The two B-type cyclins from hydra show 85.6% identity at the amino acid level, and 84.8% at the nucleotide level. The relatedness is less extensive than that found for mammals, e.g. human and mouse, and is evidence that the two hydra species diverged early in evolution. From each hydra species only one B-type cyclin was found, showing equal relatedness to the B1 and B2 subtypes of cyclins, hinting at a role as common ancestor before the split into B1 and B2 cyclins occurred. All three hydra cyclins contain regulation signals typical for G2/M cyclins, such as a ubiquitin destruction box at the amino terminus, needed for rapid degradation of the protein, and translation and polyadenylation elements in the 3′ untranslated region to regulate RNA storage and RNA degradation. In hydra cell-cycle times vary depending on feeding regime and growth conditions. Cyclin B RNA expression was found to precede the daily mitotic rhythm induced by feeding. During head regeneration cyclin B expression showed the expected drop early during regeneration and an increase later. At the cellular level strongest expression of cyclin B RNA and protein was detected in interstitial cells which possess with one day the shortest cell-cycle time in hydra. Epithelial cells with a three-day cell-cycle rhythm showed variable, and differentiated cells no cyclin B expression. Regions of hydra containing high numbers of proliferating cells, such as developing buds exhibited elevated levels of cyclin B expression.

Differential expression of D-type G1 cyclins during mouse development and liver regeneration in vivo

D-type G1 cyclins are the primary cell cycle regulators of G1/S transition in eukaryotic cells, and are differentially expressed in a variety of cell lines in vitro. Little is known, however, about the expression patterns of D-type G1 cyclins in normal mouse in vivo. Thus, in the present study, tissue-specific expressions of cyclin D1 and D3 genes were examined in several tissues derived from adult male mice, and stage-specific expression of cyclin genes was studied in brain, liver, and kidney of developing mice from embryonic day 13 to postnatal day 11. Cell cycle-dependent expression of cyclins was also examined in regenerating livers following partial hepatectomy. Our results indicate that (1) cyclins D1 and D3 are expressed in a tissue-specific manner, with cyclin D1 being highly expressed in kidney and D3 in thymus; (2) cyclin D3 mRNA is abundantly expressed in young proliferating tissues and is gradually reduced during development, whereas cyclin D1 mRNA fluctuates during development; and (3) compensatory regeneration of liver induces cyclin D1 gene expression 12 hr after partial hepatectomy, and cyclin D3 gene expression from 36 to 42 hr (at the time of G1/S transition). In conclusion, this study indicates that cyclin D1 and D3 genes are differentially expressed in vivo in a tissue-specific, developmental stage-dependent, and cell cycle-dependent manner.

Transforming growth factor-beta1 modulates p107 function in myeloid cells: correlation with cell cycle progression

Transforming growth factor-beta1 (TGF-beta1) is a potent inhibitor of hematopoietic cell growth. Here we report that TGF-beta1 signals inhibition of IL-3-dependent 32D-123 murine myeloid cell growth by modulating the activities of cyclin E and cyclin-dependent kinase 2 (cdk2) proteins and their complex formation in the G1 phase of the cell cycle. Whereas the cyclin E protein was hyperphosphorylated in TGF-beta1 treated cells, TGF-beta1 decreased both the phosphorylation of cdk2 and the kinase activity of the cyclin E-cdk2 complex. Decreased cyclin E-cdk2 kinase activity correlated with decreased phosphorylation of the retinoblastoma-related protein p107. In support of these observations, transient overexpression of p107 inhibited the proliferation of the myeloid cells, and expression of antisense oligodeoxynucleotides to p107 mRNA blocked TGF-beta1 inhibition of myeloid cell growth. Furthermore, as reported previously, in 32D-123 TGF-beta1 treated cells, c-Myc protein expression was decreased. TGF-beta1 increased the binding of p107 to the transcription factor E2F, leading to decreased c-Myc protein levels. p107 inhibited E2F transactivation activity and was also found to bind the c-Myc protein, suggesting p107 negative regulation of c-Myc protein function. These studies demonstrate the modulation of p107 function by TGF-beta1 and suggest a novel mechanism by which TGF-beta1 blocks cell cycle progression in myeloid cells.

Cyclin G1 and cyclin G2 comprise a new family of cyclins with contrasting tissue-specific and cell cycle-regulated expression

We describe the isolation and characterization of cDNAs encoding full-length human and murine cyclin G1 and a novel human homologue of this cyclin designated cyclin G2. Cyclin G1 is expressed at high levels in skeletal muscle, ovary, and kidney. Following an initial up-regulation from early G1 to G1/S phase, cyclin G1 mRNA is constitutively expressed throughout the cell cycle in T and B cell lines. In contrast, in stimulated peripheral T cells, cyclin G1 mRNA is maximal in early G1 phase and declines in cell cycle progression. Cyclin G1 levels parallel p53 expression in murine B lymphocytes; however, in several human Burkitt's lymphomas, murine lymphocytes treated with transforming growth factor-beta, early murine embryos, and several tissues of p53 null mice, cyclin G1 levels are either inverse of p53 levels or expressed independent of p53. The cyclin G1 homologue, cyclin G2, exhibits 60% nucleotide sequence identity and 53% amino acid sequence identity with cyclin G1, and like cyclin G1, exhibits closest sequence identity to the cyclin A family. Distinct from cyclin G1, the amino acid sequence for cyclin G2 shows a PEST-rich sequence and a potential Shc PTB binding site. Cyclin G2 mRNA is differentially expressed compared to cyclin G1, the highest transcript levels seen in cerebellum, thymus, spleen, prostate, and kidney. In contrast to the constitutive expression of cyclin G1 in lymphocytes, cyclin G2 mRNA appears to oscillate through the cell cycle with peak expression in late S phase.

Inhibition of G1 cyclin-dependent kinase activity during growth arrest of human breast carcinoma cells by prostaglandin A2

Prostaglandin A2 (PGA2) potently inhibits cell proliferation and suppresses tumor growth in vivo, but little is known regarding the molecular mechanisms mediating these effects. Here we demonstrate that treatment of breast carcinoma MCF-7 cells with PGA2 leads to G1 arrest associated with a dramatic decrease in the levels of cyclin D1 and cyclin-dependent kinase 4 (cdk4) and accompanied by an increase in the expression of p21. We further show that these effects occur independent of cellular p53 status. The decline in cyclin D and cdk4 protein levels is correlated with loss in cdk4 kinase activity, cdk2 activity is also significantly inhibited in PGA2-treated cells, an effect closely associated with the upregulation of p21. Immunoprecipitation experiments verified that p21 was indeed complexed with cdk2 in PGA2-treated cells. Additional experiments with synchronized MCF-7 cultures stimulated with serum revealed that treatment with PGA2 prevents the progression of cells from G1 to S. Accordingly, the kinase activity associated with cdk4, cyclin E, and cdk2 immunocomplexes, which normally increases following serum addition, was unchanged in PGA2-treated cells. Furthermore, the retinoblastoma protein (Rb), a substrate of cdk4 and cdk2 whose phosphorylation is necessary for cell cycle progression, remains underphosphorylated in PGA2-treated serum-stimulated cells. These findings indicate that PGA2 exerts its growth-inhibitory effects through modulation of the expression and/or activity of several key G1 regulatory proteins. Our results highlight the chemotherapeutic potential of PGA2, particularly for suppressing growth of tumors lacking p53 function.

The cell cycle in polyploid megakaryocytes is associated with reduced activity of cyclin B1-dependent cdc2 kinase

The platelet precursor, the megakaryocyte, matures to a polyploid cell as a result of DNA replication in the absence of mitosis (endomitosis). The factors controlling endomitosis are accessible to analysis in our megakaryocytic cell line, MegT, generated by targeted expression of temperature-sensitive simian virus 40 large T antigen to megakaryocytes of transgenic mice. We aimed to define whether endomitosis consists of a continuous phase of DNA synthesis (S) or of S phases interrupted by gaps. Analysis of the cell cycle in MegT cells revealed that, upon inactivation of large T antigen, the cells shifted from a mitotic cell cycle to an endomitotic cell cycle consisting of S/Gap phases. The level of the G1/S cyclin, cyclin A, as well as of the G1 phase cyclin, cyclin D3, were elevated at the onset of DNA synthesis, either in MegT cells undergoing a mitotic cell cycle or during endomitosis. In contrast, the level of the mitotic cyclin, cyclin B1, cycled in cells displaying a mitotic cell cycle while not detectable during endomitosis. Comparable levels of the mitotic kinase protein, Cdc2, were detected during the mitotic cell cycle or during endomitosis; however, cyclin B1-dependent Cdc2 kinase activity was largely abolished in the polyploid cells. Fibroblasts immortalized with the same heat-labile oncogene do not display reduced levels of cyclin B1 upon shifting to high temperature nor do they become polyploid, indicating that reduced levels of cyclin B1 is a property of megakaryocytes and not of the T-antigen mutant. We conclude that cellular programming during endoreduplication in megakaryocytes is associated with reduced levels of cyclin B1.

Different expression patterns of cyclins A, D1 and E in human colorectal cancer

The expressions of cyclins A, D1 and E at the protein level were investigated by Western blotting in human colorectal carcinomas and in adjacent non-neoplastic colorectal mucosas. Cyclin E was higher in the cancer tissue than in the non-neoplastic mucosa in 92% patients (35 out of 38 cases). However, the cyclin A expression of the mucosa was higher than that of the cancer tissue in 63% (25 out of 40 cases) cases, and only 4 (10%) cancers had higher cyclin A expression. Eleven cancers (27%) demonstrated expression equivalent to that in the mucosa. Equal expression of cyclin D1 in cancer and mucosal tissues was found in 51% cases (20/39), lower expression of cyclin D1 by cancer tissues was demonstrated in 41% cases (16/39) and only three cancers showed higher expression than the mucosa. Proliferating-cell nuclear antigen immunohistochemistry revealed that the labeling index of the cancer tissue was 43.5 +/- 8.3% while that of the mucosa was only 14.8 +/- 5.1%. These results proved that colorectal cancers express high levels of cyclin E, consistent with a high rate of cell proliferation, whereas most of such cancer lose control of cyclin A and cyclin D1 expression.

Isolation of a new mitotic-like cyclin from Arabidopsis: complementation of a yeast cyclin mutant with a plant cyclin

Cyclins, a large family of proteins, are the regulatory subunits of cyclin-dependent protein kinase that are essential activators of cell cycle progression in eukaryotes. Here we report isolation of a new cyclin cDNA (cyclbAt) from Arabibopsis cDNA libraries using polymerase chain reaction amplified cyclin-box sequences as probes. The deduced amino acid sequence of the isolated cDNA showed the highest sequence similarity with mitotic cyclins. However, the nucleotide and predicted amino acid sequence of cyclbAt is different from five other mitotic-like cyclins that have recently been isolated from the same system, indicating that it is a new mitotic-like cyclin. These results, together with previous reports, suggest that there are at least six different mitotic-like cyclins in Arabidopsis. Expression of cyclbAt in yeast G1 cyclin-minus mutant (DL1) rescued the cyclin-minus phenotype, demonstrating, that plant mitotic-like cyclin can complement cyclin function in yeast. Analysis of expression of cyclbAt in different tissues by reverse transcription-polymerase chain reaction using primers corresponding to a unique region of the cDNA showed that cyclbAt is differentially expressed in different tissues with highest expression in flowers and no detectable expression in leaves.

Dependence of cyclin E-CDK2 kinase activity on cell anchorage

Most nonmalignant cells are anchorage-dependent; they require substrate attachment for growth and, in some instances, survival. This requirement is lost on oncogenic transformation. The cyclin E-CDK2 complex, which is required for the G1-S transition of the cell cycle, was activated in late G1 phase in attached human fibroblasts, but not in fibroblasts maintained in suspension. In transformed fibroblasts the complex was active regardless of attachment. The lack of cyclin E-CDK2 activity in suspended cells appeared to result from increased expression of CDK2 inhibitors and a concomitant decrease in phosphorylation of CDK2 on threonine-160. Suppression of cyclin E-CDK2 activity may thus underlie the anchorage dependence of cell growth.

Interaction of cyclin-dependent kinase 5 (Cdk5) and neuronal Cdk5 activator in bovine brain

Neuronal cdc2-like kinase (Nclk) purified from bovine brain is a heterodimer of Cdk5 and an essential 25-kDa regulatory subunit (Lew, J., and Wang, J. H. (1995) Trends Biochem. Sci. 20, 33-37). The regulatory subunit is an N-terminal truncated derivative of a 35-kDa protein expressed specifically in brain, hence the name neuronal Cdk5 activator, p25/p35nck5a. In this study, we probe the relationship between the two different forms of Nck5a and their interaction with and activation of Cdk5 in bovine brain extract. Using protein fractionation procedures in combination with Western blot analysis and protein kinase assay, three forms of Cdk5 have been detected in bovine brain: a monomeric Cdk5 that can be activated by bacterially expressed GST-p21nck5a, a heterodimer of Cdk5 and p25nck5a that displays high kinase activity, and a Cdk5.p35nck5a complex that is inactive and refractory to GST-p21nck5a activation. Analysis of the Cdk5.p35nck5a complex by gel filtration chromatography indicated that the complex was part of a macromolecular structure with a molecular mass of approximately 670 kDa. When the macromolecular complex was subjected to gel filtration chromatography in the presence of 10% ethylene glycol, the fractions containing both p35nck5a and Cdk5, although eluting at the same position as control, displayed high kinase activity. The result is compatible with the suggestion that the macromolecular complex contained a kinase inhibitory factor that dissociated from the complex in 10% ethylene glycol.

Cdk2 kinase is required for entry into mitosis as a positive regulator of Cdc2-cyclin B kinase activity

In higher eukaryotes, Cdk2 kinase plays an essential role in regulating the G1-S transition. Here, we use cycling Xenopus egg extracts to examine the requirement of Cdk2 kinase on progression into mitosis. Interestingly, when Cdk2 kinase activity is inhibited by the Cdk-specific inhibitor, p21Cip1, a block to mitosis occurs, and inactive Cdc2-cyclin B accumulates. This block occurs in the absence of nuclei and is not due to direct inhibition of Cdc2 by Cip. Importantly, this block to mitosis is reversible by restoring Cdk2-cyclin E kinase activity to a Cip-treated cycling extract. Moreover, immunodepletion of Cdk2 from interphase extracts prevents activation of Cdc2 upon the addition of exogenous cyclin B. Thus, our data show that Cdk2 kinase is a positive regulator of Cdc2-cyclin B complexes and establish a link between Cdk2 kinase and cell cycle progression into mitosis.

Nuclear import of protein kinases and cyclins

Karyophilic and acidic clusters were found in most nonmembrane serine/threonine protein kinases whose primary structure was examined. These karyophilic clusters might mediate the anchoring of the kinase molecules to transporter proteins for their regulated nuclear import and might constitute the nuclear localization signals (NLS) of the kinase molecules. In contrast to protein transcription factors that are exclusively nuclear possessing strong karyophilic peptides composed of at least four arginines (R) and lysines (K) within an hexapeptide flanked by proline and glycine helix-breakers, protein kinases often contain one histidine and three K+R residues; this is proposed to specify a weak NLS structure resulting in the nuclear import of a fraction of the total cytoplasmic kinase molecules as well as in their weak retention in the different ionic strength nuclear environment. Putative NLS peptides in protein kinases may also contain hydrophobic or bulky aromatic amino acids proposed to further diminish their capacity to act as strong NLS. Most kinases lacking karyophilic clusters (c-Mos, v-Mos, sea star MAP, and yeast KIN28, SRA1, SRA3, TPK1, TPK2) also lack acidic clusters, which is in contrast to most kinases containing both acidic and karyophilic peptides; this and the presence of R/K clusters in the transporter proteins supports a role of acidic clusters on kinases in nuclear import. Cyclins B lack karyophilic signals and are proposed to be imported into nuclei via their association with Cdc2.

Structure and gene expression of avian cyclin D2

Avian cyclin D2 (Cyl D2)-encoding cDNA clones were isolated from a chicken UG9 T-cell lambda gt10 library. Sequence analysis revealed a high degree of sequence conservation with both the mouse and human Cyl D2, and somewhat lower similarity with the mouse and human Cyl D1 and D3. The homology is highest between species in the Cyl-box domain which is well conserved among human, mouse and chicken. A single 6.0-kb CYL2 mRNA is produced in both avian B- and T-cells, as expected.

Regulation of the cyclin E gene by transcription factor E2F1

A variety of results point to the transcription factor E2F as a critical determinant of the G1/S-phase transition during the cell cycle in mammalian cells, serving to activate the transcription of a group of genes that encode proteins necessary for DNA replication. In addition, E2F activity appears to be directly regulated by the action of retinoblastoma protein (RB) and RB-related proteins and indirectly regulated through the action of G1 cyclins and associated kinases. We now show that the accumulation of G1 cyclins is regulated by E2F1. E2F binding sites are found in both the cyclin E and cyclin D1 promoters, both promoters are activated by E2F gene products, and at least for cyclin E, the E2F sites contribute to cell cycle-dependent control. Most important, the endogenous cyclin E gene is activated following expression of the E2F1 product encoded by a recombinant adenovirus vector. These results suggest the involvement of E2F1 and cyclin E in an autoregulatory loop that governs the accumulation of critical activities affecting the progression of cells through G1.

Cdk family genes are expressed not only in dividing but also in terminally differentiated mouse germ cells, suggesting their possible function during both cell division and differentiation

The roles of the cyclin dependent kinase (Cdk) family in murine germ cell development have been examined by studying the expression of five Cdk family genes (Cdc2, Cdk2, Cdk4, Pctaire-1, and Pctaire-3) in mouse reproductive organs. Northern blot and in situ hybridization analyses revealed distinctive expression patterns of these genes with striking cellular, lineage, and developmental stage specificity. We observed Cdk expression in cell types with proliferative activity: Cdc2 and Cdk2 expression in premeiotic spermatocytes in the testis, and Cdc2, Cdk2, and Cdk4 expression in granulosa cells of ovarian follicles. Cdc2 transcripts were most abundant in late pachytene to diplotene spermatocytes, soon to undergo meiosis. Surprisingly, we also observed expression of Cdk family genes in non-proliferating cell types. All five Cdk family genes examined were expressed in Sertoli cells of the adult testis, which are no longer mitotically active. With regard to Pctaire-1 and Pctaire-3, the highest levels of expression were observed in postmeiotic spermatids. Immunoblot analysis also revealed the presence of high levels of Pctaire-1 in postmeiotic germ cells. These results suggest that Cdk family kinases may exhibit various functions in germinal and somatic cells during gametogenesis, not only in the cell cycle but also in other regulatory processes, including differentiation.

Molecular cloning and cell cycle-dependent expression of a novel gene that is homologous to cdc37

A cDNA clone, termed N17, was isolated from a rat fibroblast 3Y1 cDNA library by a differential screening technique. The expression of the N17 gene was significantly increased in a variety of rapidly growing cells, including v-src-, v-Ha-ras-, or v-mos-transformed 3Y1 cells, when compared with parental 3Y1 cells. The N17 gene is present as a single copy in rat genome and is evolutionarily conserved among higher eukaryotes. The predicted open reading frame (ORF) encodes a polypeptide of 379 amino acids that exhibits a significant similarity with those of the cell cycle control protein Cdc37. The amount of N17 mRNA starts to be increased in the late G1 phase and the same level was retained until just before the G2/M phase. Taken together, these results suggest that N17 gene product may play a crucial role in the cell cycle control of 3Y1 cells.

The crystal structure of cyclin A

BACKGROUND

Eukaryotic cell cycle progression is regulated by cyclin dependent protein kinases (CDKs) whose activity is regulated by association with cyclins and by reversible phosphorylation. Cyclins also determine the subcellular location and substrate specificity of CDKs. Cyclins exhibit diverse sequences but all share homology over a region of approximately 100 amino acids, termed the cyclin box. From the determination of the structure of cyclin A, together with results from biochemical and genetic analyses, we can identify which parts of the cyclin molecular may contribute to cyclin A structure and function.

RESULTS

We have solved the crystal structure, at 2.0 A resolution, of an active recombinant fragment of bovine cyclin A, cyclin A-3, corresponding to residues 171-432 of human cyclin A. The cyclin box has an alpha-helical fold comprising five alpha helices. This fold is repeated in the C-terminal region, although this region shares negligible sequence similarity with the cyclin box.

CONCLUSIONS

Analysis of residues that are conserved throughout the A, B, and E cyclins identifies two exposed clusters of residues, one of which has recently been shown to be involved in the association with human CDK2. The second cluster may identify another site of cyclin A-protein interaction. Comparison of the structure of the unbound cyclin with the structure of cyclin A complexed with CDK2 reveals that cyclin A does not undergo any significant conformational changes on complex formation. Threading analysis shows that the cyclin-box fold is consistent with the sequences of the transcription factor TFIIB and other functionally related proteins. The structural results indicate a role for the cyclin-box fold both as a template for the cyclin family and as a generalised adaptor molecule in the regulation of transcription.

Sequential activation of cyclin E and cyclin A gene expression by human papillomavirus type 16 E7 through sequences necessary for transformation

To investigate E7-dependent biochemical changes which are involved in cellular transformation, we analyzed the influence of human papillomavirus type 16 (HPV-16) E7 on the expression of cell cycle regulatory proteins. Expression of E7 in established rodent fibroblasts (NIH 3T3), which was shown to be sufficient for transformation of these cells, leads to constitutive expression of the cyclin E and cyclin A genes in the absence of external growth factors. Surprisingly, expression of the cyclin D1 gene, which encodes a major regulator of G1 progression, is unaltered in E7-transformed cells. In transient transfection experiments, the cyclin A gene promoter is activated by E7 via an E2F binding site. In 14/2 cells, which were used as a model system to analyze the role of HPV-16 E7 in the transformation of primary cells, we observed rapid E7-dependent activation of cyclin E gene expression, which can be uncoupled from activation of the cyclin A gene, since the latter requires additional protein synthesis. E7-driven induction of cyclin E and cyclin A gene expression was accompanied by an increase in the associated kinase activities. Two domains of the E7 oncoprotein, which are designated cd1 and cd2, are essential for transformation of rodent fibroblasts. It is shown here that growth factor-independent expression of the cyclin E gene requires cd2 but not cd1, while activation of cyclin A gene expression requires cd1 function in addition to that of cd2. These data suggest that cyclin A gene expression is controlled by two distinct negative signals, one of which also restricts expression of the cyclin E gene. The ability of E7 to separately override each of these inhibitory signals, via cd1 and cd2, cosegregates with its ability to fully transform rodent fibroblasts. Unlike serum growth factors, E7 induces S-phase entry without activating cyclin D1 gene expression, in keeping with the finding that cyclin D1 function is not required in cells transformed by DNA tumor viruses.

Constitutive overexpression of CDK2 inhibits neuronal differentiation of rat pheochromocytoma PC12 cells

Changes in the levels of cyclins A, D, and E, p21, and cyclin-dependent kinase 2 (CDK2) were examined in rat pheochromocytoma PC12 cells during neuronal differentiation induced by nerve growth factor (NGF). Expression of cyclin A decreased to an undetectable level after 5 days of exposure to NGF, while expression of CDK2 decreased gradually after day 3. In contrast, the levels of cyclins D1 and E increased gradually through day 10, yet the amount of cyclin E associated with CDK2 decreased concomitant with a decrease in the CDK2 protein level. p21 expression increased gradually after day 7, while the level of CDK2-associated p21 remained unchanged. When human cDNAs encoding cyclins and CDK2 were introduced into PC12 cells, only CDK2 overexpression inhibited NGF-induced differentiation. The cell lines overexpressing CDK2 showed stable and high levels of CDK2 kinase activity during differentiation, whereas parental and vector-transfected cell lines displayed a marked decline in CDK2 kinase activity 1 day after NGF treatment. In cell lines overexpressing cyclins A, D, and E, this reduction of the kinase activity was not apparent until day 3. These results suggest that down-regulation of CDK2 activity is a crucial event for the neuronal differentiation of PC12 cells.

CDC25 phosphatases as potential human oncogenes

Cyclin-dependent kinases (CDKs) are activated by CDC25 phosphatases, which remove inhibitory phosphate from tyrosine and threonine residues. In human cells, CDC25 proteins are encoded by a multigene family, consisting of CDC25A, CDC25B, and CDC25C. In rodent cells, human CDC25A or CDC25B but not CDC25C phosphatases cooperate with either Ha-RASG12V or loss of RB1 in oncogenic focus formation. Such transformants were highly aneuploid, grew in soft agar, and formed high-grade tumors in nude mice. Overexpression of CDC25B was detected in 32 percent of human primary breast cancers tested. The CDC25 phosphatases may contribute to the development of human cancer.

Early events in DNA replication require cyclin E and are blocked by p21CIP1

Using immunodepletion of cyclin E and the inhibitor protein p21WAF/CIP1, we demonstrate that the cyclin E protein, in association with Cdk2, is required for the elongation phase of replication on single-stranded substrates. Although cyclin E/Cdk2 is likely to be the major target by which p21 inhibits the initiation of sperm DNA replication, p21 can inhibit single-stranded replication through a mechanism dependent on PCNA. While the cyclin E/Cdk2 complex appears to have a role in the initiation of DNA replication, another Cdk kinase, possibly cyclin A/Cdk, may be involved in a later step controlling the switch from initiation to elongation. The provision of a large maternal pool of cyclin E protein shows that regulators of replication are constitutively present, which explains the lack of a protein synthesis requirement for replication in the early embryonic cell cycle.

Induction of G1 arrest by down-regulation of cyclin D3 in T cell hybridomas

The relationship between activation-induced growth inhibition and regulation of the cell cycle progression was investigated in T cell hybridomas by studying the function of the cell cycle-regulating genes such as G1 cyclins and their associated kinases. Activation of T cell hybridomas by anti-T cell receptor antibody induces growth arrest at G1 phase of the cell cycle and subsequently results in activation-driven cell death. Rapid reduction of both messenger RNA and protein level of the cyclin D3 is accompanied by growth arrest upon activation. Although the residual cyclin D3 protein forms a complex with cdk4 protein, cyclin D3-dependent kinase activity is severely impaired. Stable transfectants engineered to express cyclin D3 override the growth arrest upon activation. These results imply that the activation signal through T cell receptor induces the down-regulation of cyclin D3 expression and cyclin D3-dependent kinase activity, leading to growth arrest in G1 phase of the cell cycle in T cells.

Members of the NAP/SET family of proteins interact specifically with B-type cyclins

Cyclin-dependent kinase complexes that contain the same catalytic subunit are able to induce different events at different times during the cell cycle, but the mechanisms by which they do so remain largely unknown. To address this problem, we have used affinity chromatography to identify proteins that bind specifically to mitotic cyclins, with the goal of finding proteins that interact with mitotic cyclins to carry out the events of mitosis. This approach has led to the identification of a 60-kD protein called NAP1 that interacts specifically with members of the cyclin B family. This interaction has been highly conserved during evolution: NAP1 in the Xenopus embryo interacts with cyclins B1 and B2, but not with cyclin A, and the S. cerevisiae homolog of NAP1 interacts with Clb2 but not with Clb3. Genetic experiments in budding yeast indicate that NAP1 plays an important role in the function of Clb2, while biochemical experiments demonstrate that purified NAP1 can be phosphorylated by cyclin B/p34cdc2 kinase complexes, but not by cyclin A/p34cdc2 kinase complexes. These results suggest that NAP1 is a protein involved in the specific functions of cyclin B/p34cdc2 kinase complexes. In addition to NAP1, we found a 43-kD protein in Xenopus that is homologous to NAP1 and also interacts specifically with B-type cyclins. This protein is the Xenopus homolog of the human SET protein, which was previously identified as part of a putative oncogenic fusion protein (Von Lindern et al., 1992).

Enhanced expression of cyclin D1 in senescent human fibroblasts

When human fibroblast, TIG-1, was growth-stimulated with fetal bovine serum, the induction level of cell cycle-dependent genes was generally much lower in senescent cells than in young counterparts. Exceptionally, the expression level of cyclin D1 in senescent cells was constitutively higher than in young cells and further increased after serum stimulation, which was confirmed by Northern and Western blots and immunoprecipitation. This was also true in other human diploid fibroblast lines, TIG-3 and MRC-5. However, cyclin D1-dependent kinase activity was not detected in senescent cells. When sense- or antisense-cyclin D1 cDNA driven by beta-actin promoter was transfected into young TIG-1 cells, the number of appeared colonies from sense-strand transfected cultures was lower than that from antisense-strand-transfected ones. However, clones expressing cyclin D1 at low or undetectable level which were isolated after transfection with antisense-cyclin D1 proliferated up to the same division limit as untransfected and sense-strand transfected cells. Four clones of SV40-transformed TIG-1 expressed cyclin D1 at moderate levels during their extended proliferative lifespan. It appears that, if the extremely overexpressed cyclin D1 could cause an inhibition of cell proliferation at senescent stage, cellular senescence occurs regardless of overexpression of cyclin D1.

Deregulated c-Fos/AP-1 modulates expression of the cyclin and the cdk gene in splenic B cells stimulated with lipopolysaccharide

Overexpression of c-Fos/AP-1 augments proliferation of splenic B cells stimulated with lipopolysaccharide (LPS). To elucidate mechanisms of the augmentation by c-Fos/AP-1, a cell cycle of the LPS-activated B cells from c-fos transgenic mice was analyzed. Cell cycle progression into the S phase was accelerated in the c-fos B cells. Expression of genes related to the cell cycle progression was examined in these B cells. Amount of cyclin D3 and cdk4 mRNA increased in the c-fos B cells at 6 h earlier than that in the control B cells, indicating that the kinetics of these mRNA expressions correlate with the acceleration of cell cycle progression. Furthermore, cyclin D1 and cyclin E mRNA were detected in the c-fos B cells but not in the control B cells. These results indicate that deregulated c-Fos/AP-1 modulates expression of the cyclin and the cdk gene in splenic B cells stimulated with LPS. These modulations may accelerate cell cycle progression and augment proliferation of the B cells.

Identification of a cell-type-specific and E2F-independent mechanism for repression of cdc2 transcription

Human myeloid leukemia cells, such as HL60, U937, and THP1 cells, undergo macrophage differentiation and growth arrest following treatment with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Surprisingly, we find that growth of a significant percentage of THP1 cells is arrested in the G2 phase of the cell cycle. G2 arrest correlates with cell-specific repression of the gene encoding p34cdc2, a crucial regulator of G2/M progression. Intriguingly, TPA-mediated repression of the cdc2 promoter was independent of the transcription factor E2F, distinguishing this pathway from mechanisms responsible for repression of cdc2 transcription in response to serum starvation. The region of the cdc2 promoter required for repression was located from bp -22 to -2 from the major transcriptional start site. This sequence, which we term the R box, directs the uncoupling of the basal promoter from upstream activators following TPA treatment. Analysis of THP1 nuclear proteins revealed a 55-kDa protein that was induced by TPA and interacted with the cdc2 promoter in an R-box-dependent manner. These observations provide evidence for the existence of cell-type- and promoter-specific pathways for the assembly of stable transcriptional initiation complexes that function to differentially regulate the expression of cell cycle control genes in mammalian cells.

Threshold expression of cyclin E but not D type cyclins characterizes normal and tumour cells entering S phase

Complexes of cyclin-dependent kinases (cdk) and their partner cyclins drive the cell through the cell cycle, each such complex phosphorylating a distinct set of proteins at a particular check-point or phase of the cycle. Immunocytochemical detection of cyclins combined with measurement of cellular DNA content by flow cytometry makes it possible to relate expression of each of these proteins with the actual cell cycle position, without the necessity of cell synchronization. In the present study, we have investigated expression of E and D type cyclins in G1 cells and in cells entering S phase, in eight different human hematopoietic and solid tumour cell lines (two leukaemias, a lymphoma, three breast carcinomas, a colon carcinoma and a bladder transitional cell carcinoma) during their exponential phase of growth, as well as in normal mitogen stimulated lymphocytes. In all the cell types studied, the average level of D type cyclin expression was invariable throughout the cell cycle. A great intercellular variability, in particular of the G1 cell subpopulations, and the presence of a large fraction of G1, S and G2 + M cells that were cyclin D negative (20-40% in tumour cell lines and about 80% among lymphocytes), were other characteristic features of D type cyclin expression. In contrast to D type cyclins, the expression of cyclin E was discontinuous during the cycle, peaking at the time of cell entrance to S. Also, a well defined threshold in expression of cyclin E characterized cells that were entering S phase, and virtually no cyclin E negative cells were seen during the early portion of S phase. The data indicate that while cell entrance to S phase is unrelated to expression of D type cyclins (at the time of entrance), accumulation of cyclin E up to critical level is a prerequisite for initiation of DNA replication. The great intercellular variability in expression of D type cyclins and their invariant average level across the cell cycle suggest that these cyclins, in addition to their acknowledged function in promoting cell progression through mid- to late-G1 may have other role(s), related or unrelated to the cell cycle progression. The presence of a large number of D type cyclin negative cells in all phases of the cycle suggests that during exponential growth the cells may not express this protein and yet may traverse the cycle, including G1 phase.

Inhibitors of mammalian G1 cyclin-dependent kinases

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Identification of a developmental timer regulating the stability of embryonic cyclin A and a new somatic A-type cyclin at gastrulation

We have identified a second Xenopus cyclin A, called cyclin A2. Cyclin A2 is a 46.6-kD protein that shows a greater homology to human cyclin A than to the previously identified Xenopus cyclin A1. It is present throughout embryonic development (up to stage 46 at least) and is found in adult tissues as well as in Xenopus tissue culture cell lines. In contrast, cyclin A1 is present in eggs and early embryos but cannot be detected in late embryos or in tissue culture cells. We have found that the maternally stored pools of mRNAs encoding both of these cyclin A proteins are stable until the onset of gastrulation and then are degraded abruptly. At this time, new transcription replaces cyclin A2 mRNA. Interestingly, we have also observed a dramatic change in the stability of the cyclin A proteins at this time. Prior to the onset of gastrulation, cyclin A1 protein is stable during interphase of the cell cycle. At gastrulation, however, both A1 and A2 proteins turn over rapidly during interphase of the cell cycle. Together, these results indicate that developmental programs controlling cyclin A protein and mRNA stability are activated at gastrulation. We have shown that this program is independent of new transcription beginning at the mid-blastula transition. Furthermore, treatment of early stage embryos with cycloheximide demonstrates that activation of this degradative program is independent of cell division and translation. Collectively, our observations suggest that a previously uncharacterized timing mechanism activates new degradative pathways at the onset of gastrulation, which could play an essential role in releasing cells from maternal programming.

Cyclin D1 is dispensable for G1 control in retinoblastoma gene-deficient cells independently of cdk4 activity

To elucidate the regulator-versus-target relationship in the cyclin D1/cdk4/retinoblastoma protein (pRB) pathway, we examined fibroblasts from RB-1 gene-deficient and RB-1 wild-type littermate mouse embryos (ME) and in human tumor cell lines that differed in the status of the RB-1 gene. The RB+/+ and RB-/- ME fibroblasts expressed similar protein levels of D-type cyclins, cdk4, and cdk6, showed analogous spectra and abundance of cellular proteins complexed with cdk4 and/or cyclins D1 and D2, and exhibited comparable associated kinase activities. Of the two human cell lines established from the same sarcoma biopsy, the RB-positive SKUT1B cells contained cdk4 that was mainly associated with D-type cyclins, contrary to a predominant cdk4-p16INK4 complex in the RB-deficient SKUT1A cells. Antibody-mediated neutralization of cyclin D1 arrested the RB-positive ME and SKUT1B cells in G1, whereas this cyclin appeared dispensable in the RB-deficient ME and SKUT1A cells. Lack of requirement for cyclin D1 therefore correlated with absence of functional pRB, regardless of whether active cyclin D1/cdk4 holoenzyme was present in the cells under study. Consistent with a potential role of cyclin D/cdk4 in phosphorylation of pRB, monoclonal anti-cyclin D1 antibodies supporting the associated kinase activity failed to significantly affect proliferation of RB-positive cells, whereas the antibody DCS-6, unable to coprecipitate cdk4, efficiently inhibited G1 progression and prevented pRB phosphorylation in vivo. These data provide evidence for an upstream control function of cyclin D1/cdk4, and a downstream role for pRB, in the order of events regulating transition through late G1 phase of the mammalian cell division cycle.

Human cyclin E, a nuclear protein essential for the G1-to-S phase transition

Cyclin E was first identified by screening human cDNA libraries for genes that would complement G1 cyclin mutations in Saccharomyces cerevisiae and has subsequently been found to have specific biochemical and physiological properties that are consistent with it performing a G1 function in mammalian cells. Most significantly, the cyclin E-Cdk2 complex is maximally active at the G1/S transition, and overexpression of cyclin E decreases the time it takes the cell to complete G1 and enter S phase. We have now found that mammalian cells express two forms of cyclin E protein which differ from each other by the presence or absence of a 15-amino-acid amino-terminal domain. These proteins are encoded by alternatively spliced mRNAs and are localized to the nucleus during late G1 and early S phase. Fibroblasts engineered to constitutively overexpress either form of cyclin E showed elevated cyclin E-dependent kinase activity and a shortened G1 phase of the cell cycle. The overexpressed cyclin E protein was detected in the nucleus during all cell cycle phases, including G0. Although the cyclin E protein could be overexpressed in quiescent cells, the cyclin E-Cdk2 complex was inactive. It was not activated until 6 to 8 h after readdition of serum, 4 h earlier than the endogenous cyclin E-Cdk2. This premature activation of cyclin E-Cdk2 was consistent with the extent of G1 shortening caused by cyclin E overexpression. Microinjection of affinity-purified anti-cyclin E antibodies during G1 inhibited entry into S phase, whereas microinjection performed near the G1/S transition was ineffective. These results demonstrate that cyclin E is necessary for entry into S phase. Moreover, we found that cyclin E, in contrast to cyclin D1, was required for the G1/S transition even in cells lacking retinoblastoma protein function. Therefore, cyclins E and D1 control two different transitions within the human cell cycle.