Suppression of growth factor-induced CYL1 cyclin gene expression by antiproliferative agents.

A recently identified novel mammalian cyclin (CYL1), induced by growth factors and apparently functional during the G1 phase of the cell cycle, is of potential significance, given that cell division is primarily controlled in G1. We have measured CYL1 gene expression in murine bone marrow-derived macrophages (BMM), a normal cell type dependent upon colony-stimulating factors (CSFs) for survival and proliferation. The induction of CYL1 mRNA levels correlated strongly with stimulation of DNA synthesis, since elevated CYL1 mRNA levels occurred in response to the mitogenic stimuli, CSF-1, and granulocyte/macrophage CSF, but not to nonmitogenic macrophage-activating agents. BMM are subject to cell cycle arrest by numerous agents, including tumor necrosis factor alpha, interferon gamma, bacterial lipopolysaccharide, and agents that increase cAMP. These antiproliferative agents suppressed CSF-1-stimulated CYL1 gene expression, even when added late in G1. This pattern of CYL1 gene expression was remarkably consistent with the ability of these agents to inhibit progression into S phase. The mechanisms of negative growth regulation are largely unknown, and given the likely importance of G1 cyclins in the control of cell division, we propose that antiproliferative agents may exert their effects by suppressing G1 cyclin gene expression.     

Differential phosphorylation of vertebrate p34cdc2 kinase at the G1/S and G2/M transitions of the cell cycle: identification of major phosphorylation sites.

The cdc2 kinase is a key regulator of the eukaryotic cell cycle. The activity of its catalytic subunit, p34cdc2, is controlled by cell cycle dependent interactions with other proteins as well as by phosphorylation--dephosphorylation reactions. In this paper, we examine the phosphorylation state of chicken p34cdc2 at various stages of the cell cycle. By peptide mapping, we detect four major phosphopeptides in chicken p34cdc2; three phosphorylation sites are identified as threonine (Thr) 14, tyrosine (Tyr) 15 and serine (Ser) 277. Analysis of synchronized cells demonstrates that phosphorylation of all four sites is cell cycle regulated. Thr 14 and Tyr 15 are phosphorylated maximally during G2 phase but dephosphorylated abruptly at the G2/M transition, concomitant with activation of p34cdc2 kinase. This result suggests that phosphorylation of Thr 14 and/or Tyr 15 inhibits p34cdc2 kinase activity, in line with the location of these residues within the putative ATP binding site of the kinase. During M phase, p34cdc2 is also phosphorylated, but phosphorylation occurs on a threonine residue distinct from Thr 14. Finally, phosphorylation of Ser 277 peaks during G1 phase and drops markedly as cells progress through S phase, raising the possibility that this modification may contribute to control the proposed G1/S function of the vertebrate p34cdc2 kinase.     

Activation of the phosphatase activity of human cdc25A by a cdk2-cyclin E dependent phosphorylation at the G1/S transition.

Progression through the cell cycle is monitored at two major points: during the G1/S and the G2/M transitions. In most cells, the G2/M transition is regulated by the timing of p34cdc2 dephosphorylation which results in the activation of the kinase activity of the cdc2-cyclin B complex. The timing of p34cdc2 dephosphorylation is determined by the balance between the activity of the kinase that phosphorylates p34cdc2 (wee1 in human cells) and the opposing phosphatase (cdc25C). Both enzymes are regulated and it has been shown that cdc25C is phosphorylated and activated by the cdc2-cyclin B complex. This creates a positive feed-back loop providing a switch used to control the onset of mitosis. Here, we show that another member of the human cdc25 family, cdc25A, undergoes phosphorylation during S phase, resulting in an increase of its phosphatase activity. The phosphorylation of cdc25A is dependent on the activity of the cdc2-cyclin E kinase. Microinjection of anti-cdc25A antibodies into G1 cells blocks entry into S phase. These results indicate that the cdc25A phosphatase is required to enter S phase in human cells and suggest that this enzyme is part of an auto-amplification loop analogous to that described at the G2/M transition. We discuss the nature of the in vivo substrate of the cdc25A phosphatase in S phase and the possible implications for the regulation of S phase entry.     

Regulation of the G1 phase of the cell cycle by periodic stabilization and degradation of the p25rum1 CDK inhibitor.

In fission yeast, the cyclin-dependent kinase (CDK) inhibitor p25(rum1) is a key regulator of progression through the G1 phase of the cell cycle. We show here that p25(rum1) protein levels are sharply periodic. p25(rum1) begins to accumulate at anaphase, persists in G1 and is destroyed during S phase. p25(rum1 )is stabilized and polyubiquitinated in a mutant defective in the 26S proteasome, suggesting that its degradation normally occurs through the ubiquitin-dependent 26S proteasome pathway. Phosphorylation of p25(rum1 )by cdc2-cyclin complexes at residues T58 and T62 is important to target the protein for degradation. Mutation of one or both of these residues to alanine causes stabilization of p25(rum1) and induces a cell cycle delay in G1 and polyploidization due to occasional re-initiation of DNA replication before mitosis. The CDK-cyclin complex cdc2-cig1, which is insensitive to p25(rum1 )inhibition, seems to be the main kinase that phosphorylates p25(rum1). Phosphorylation of p25(rum1) in S phase and G2 serves as the trigger for p25(rum1) proteolysis. Thus, periodic accumulation and degradation of the CDK inhibitor p25(rum1 )in G1 plays a role in setting a threshold of cyclin levels important in determining the length of the pre-Start G1 phase and in ensuring the correct order of cell cycle events.     

A pre-start checkpoint preventing mitosis in fission yeast acts independently of p34cdc2 tyrosine phosphorylation.

We have monitored the tyrosine (Y15) phosphorylated and dephosphorylated forms of p34cdc2 from Schizosaccharomyces pombe as cells proceed through the cell cycle. Y15 is dephosphorylated in G1 before start and becomes phosphorylated only after cells pass start and enter late G1. This transition is associated with a switch from one checkpoint which restrains mitosis in pre-start G1, by a mechanism independent from Y15 phosphorylation, to a second checkpoint acting post-start during late G1 and S phase operating through Y15 phosphorylation. The pre-start checkpoint may act by preventing formation of the p34cdc2/p56cdc13 complex. The complex between Y15-phosphorylated p34cdc2 and p56cdc13 accumulates during S phase and G2, but the level generated is not solely dependent on the amount of p34cdc2 and p56cdc13 present in the cell. The extent of p56cdc13 breakdown at the end of mitosis may be determined by the amount complexed with p34cdc2. We have also shown that an insoluble form of p34cdc2 is associated with the progression of the cell through late G1 into S phase.     

c-Fos/activator protein-1 transactivates wee1 kinase at G(1)/S to inhibit premature mitosis in antigen-specific Th1 cells

M-phase promoting factor is a complex of cdc2 and cyclin B that is regulated positively by cdc25 phosphatase and negatively by wee1 kinase. We isolated the wee1 gene promoter and found that it contains one AP-1 binding motif and is directly activated by the immediate early gene product c-Fos at cellular G(1)/S phase. In antigen-specific Th1 cells stimulated by antigen, transactivation of the c-fos and wee1 kinase genes occurred sequentially at G(1)/S, and the substrate of wee1 kinase, cdc2-Tyr15, was subsequently phosphorylated at late G(1)/S. Under prolonged expression of the c-fos gene, however, the amount of wee1 kinase was increased and its target cdc2 molecule was constitutively phosphorylated on its tyrosine residue, where Th1 cells went into aberrant mitosis. Thus, an immediate early gene product, c-Fos/AP-1, directly transactivates the wee1 kinase gene at G(1)/S. The transient increase in c-fos and wee1 kinase genes is likely to be responsible for preventing premature mitosis while the cells remain in the G(1)/S phase of the cell cycle.     

Mitogenic response to colony-stimulating factor 1.

The proliferative effects of colony-stimulating factor 1 (CSF-1) on cells of the mononuclear phagocyte lineage are mediated by the CSF-1 receptor tyrosine kinase, which triggers intracellular signal transduction through multiple second messenger pathways. CSF-1 is required throughout G1 to stimulate both immediate- and delayed-early responses that ensure entry of macrophages into S phase. Because CSF-1 induces the expression of different sets of cellular genes during early and late G1, products of the immediate-early response might modulate the effects of ensuing receptor signals to facilitate G1 progression. Targets of the delayed-early response include novel cyclin genes that may play a role in S phase commitment.     

Reovirus-Induced ς1s-Dependent G2/M Phase Cell Cycle Arrest Is Associated with Inhibition of p34cdc2

Serotype 3 reoviruses inhibit cellular proliferation by inducing a G(2)/M phase cell cycle arrest. Reovirus-induced G(2)/M phase arrest requires the viral S1 gene-encoded sigma1s nonstructural protein. The G(2)-to-M transition represents a cell cycle checkpoint that is regulated by the kinase p34(cdc2). We now report that infection with serotype 3 reovirus strain Abney, but not serotype 1 reovirus strain Lang, is associated with inhibition and hyperphosphorylation of p34(cdc2). The sigma1s protein is necessary and sufficient for inhibitory phosphorylation of p34(cdc2), since a viral mutant lacking sigma1s fails to hyperphosphorylate p34(cdc2) and inducible expression of sigma1s is sufficient for p34(cdc2) hyperphosphorylation. These studies establish a mechanism by which reovirus can perturb cell cycle regulation.     

p25rum1 promotes proteolysis of the mitotic B-cyclin p56cdc13 during G1 of the fission yeast cell cycle.

The fission yeast Schizosaccharomyces pombe CDK inhibitor p25rum1 plays a major role in regulating cell cycle progression during G1. Here we show that p25rum1 associates with the CDK p34cdc2/p56cdc13 during G1 in normally cycling cells and is required for the rapid proteolysis of p56cdc13. In vitro binding data indicate that p25rum1 has specificity for the B-cyclin p56cdc13 component of the CDK and can bind the cyclin even in the absence of the cyclin destruction box. At the G1-S-phase transition, p25rum1 levels decrease and p56cd13 levels increase. We also show that on release from a G1 block, the rapid disappearance of p25rum1 requires the activity of the CDK p34cdc2/cig1p and that this same CDK phosphorylates p25rum1 in vitro. We propose that the binding of p25rum1 to p56cdc13 promotes cyclin proteolysis during G1, with p25rum1 possibly acting as an adaptor protein, promoting transfer of p56cdc13 to the proteolytic machinery. At the G1-S-phase transition, p25rum1 becomes targeted for proteolysis by a mechanism which may involve p34cdc2/cig1p phosphorylation. As a consequence, at this point in the cell cycle p56cdc13 proteolysis is inhibited, leading to a rise of p56cdc13 levels in preparation for mitosis.     

B-type cyclins regulate G1 progression in fission yeast in opposition to the p25rum1 cdk inhibitor.

The onset of S phase in fission yeast is regulated at Start, the point of commitment to the mitotic cell cycle. The p34cdc2 kinase is essential for G1 progression past Start, but until now its regulation has been poorly understood. Here we show that the cig2/cyc17 B-type cyclin has an important role in G1 progression, and demonstrate that p34cdc2 kinase activity is periodically associated with cig2 in G1. Cells lacking cig2 are defective in G1 progression, and this is particularly clear in small cells that must regulate Start with respect to cell size. We also find that the cig1 B-type cyclin can promote G1 progression. Whilst p25rum1 can inhibit cig2/cdc2 activity in vitro, and may transiently inhibit this complex in vivo, cig1 is regulated independently of p25rum1. Since cig1/cdc2 kinase activity peaks in mitotic cells, and decreases after mitosis with similar kinetics to cdc13-associated kinase activity, we suggest that cig2 is likely to be the principal fission yeast G1 cyclin. cig2 protein levels accumulate in G1 cells, and we propose that p25rum1 may transiently inhibit cig2-associated p34cdc2 activity until the critical cell size required for Start is reached.     

Changes in p34cdc2 kinase activity and cyclin A during induced differentiation of murine erythroleukemia cells.

Hexamethylene bisacetamide (HMBA)-induced murine erythroleukemia (MELC) differentiation is characterized by a prolongation of the initial G1 which follows passage through S phase in the presence of inducer. Commitment to terminal cell division is first detected in a portion of the cell population during this prolonged G1. HMBA-induced commitment is stochastic. This study has examined changes in two known cell cycle regulators, p34cdc2 and cyclin A, in cycle-synchronized MELC in the absence and presence of HMBA. Histone H1 kinase activity of p34cdc2, and the levels of CDC2Mm mRNA, 1.8-kilobase mRNA of cyclin A, and cyclin A protein changed during cell cycle progression in MELC, and all of them were suppressed during G1. The suppression of the H1 kinase activity and cyclin A expression continued through the prolonged G1 in MELC cultured with HMBA, whereas p34cdc2 protein level did not vary through the cell cycle in MELC cultured without or with inducer. Phosphorylation of p34cdc2 in uninduced MELC gradually increased as cells progressed from G1 to S. In induced MELC, an increase in phosphorylation of p34cdc2 occurred during the prolonged G1, and prior to the exit of the bulk of the cells from G1 to S. These results suggest that in HMBA-induced MELC, p34cdc2 phosphorylation per se is not a limiting factor in determining G1 to S progression. The persistent suppression of cyclin A expression and histone H1 kinase activity may play a role in HMBA-induced commitment to terminal differentiation.     

Cell cycle-dependent phosphorylation of human DNA polymerase alpha

The expression of DNA polymerase alpha, a principal chromosome replication enzyme, is constitutive during the cell cycle. We show in this report that DNA polymerase alpha catalytic polypeptide p180 is phosphorylated throughout the cell cycle and is hyperphosphorylated in G2/M phase. The p70 subunit is phosphorylated only in G2/M phase. This cell cycle-dependent phosphorylation is due to cell cycle-dependent kinase(s) and not to phosphatase(s). In vitro evidence indicates the involvement of p34cdc2 kinase in the mitotic phosphorylation of DNA polymerase alpha. Tryptic phosphopeptide maps demonstrate that peptides phosphorylated in vitro are identical to those phosphorylated in vivo. DNA polymerase alpha from mitotic cells is found to have lower affinity for single-stranded DNA than does polymerase alpha from G1/S phase cells. These results imply that the mitotic phosphorylation of polymerase alpha may affect its physical interaction with other replicative proteins and/or with DNA at the replication fork.     

Regulation of p105wee1 and p34cdc2 during meiosis in Schizosaccharomyces pombe.

Temperature-sensitive pat1 mutants of the fission yeast Schizosaccharomyces pombe can be induced to undergo meiosis at the restrictive temperature, irrespective of the mat1 configuration and the nutritional conditions. Using a combination of exit from stationary phase and thermal inactivation of the 52-kilodalton protein kinase that is encoded by the pat1 (also called ran1) gene, highly synchronous meiotic cultures were obtained. Synthesis and tyrosyl phosphorylation of p34cdc2 was evident during meiotic G1 and S phases. During this period there was increased expression of p105wee1, a protein kinase implicated in the tyrosyl phosphorylation of p34cdc2. Following a relatively brief G2 period, during which a reduction in the steady-state level of p105wee1 occurred, there was an approximately 19-fold increase in the histone H1 phosphotransferase activity of p34cdc2. Only a single peak of histone H1 kinase activation was observed, which implies that unlike meiosis in amphibians and echinoderms, p34cdc2 is functional only during one of the meiotic divisions in S. pombe, presumably meiosis II. Stimulation of the kinase activity of p34cdc2 was associated with its tyrosyl dephosphorylation. This is analogous to mitotic M phase and suggests parallels in the mechanism of activation of p34cdc2 during mitosis and one of the meiotic divisions in S. pombe.     

Casein kinase II phosphorylates p34cdc2 kinase in G1 phase of the HeLa cell division cycle.

The activity of p34cdc2 kinase is regulated in the phases of vertebrate cell cycle by mechanisms of phosphorylation and dephosphorylation. In this paper, we demonstrate that casein kinase II (CKII) phosphorylates p34cdc2 in vivo and in vitro at Ser39 during the G1 phase of HeLa cell division cycle. Human p34cdc2 shows a typical phosphorylation sequence motif site for CKII at Ser39 (ES39EEE). In our experiments, either p34cdc2 expressed and purified from bacteria or p34cdc2 immunoprecipitated from HeLa cells enriched in G1 by elutriation were substrates for in vitro phosphorylation by CKII. Phosphoamino acid analysis, N-chlorosuccinimide mapping, and two-dimensional tryptic mapping of p34cdc2 phosphorylated in vitro were performed to determine the phosphorylation site. A synthetic peptide spanning residues 33-50 of human p34cdc2, including the CKII site, was used to map the site. In addition, phosphorylation at Ser39 also occurs in vivo, since p34cdc2 is phosphorylated during G1 on serine, and its two-dimensional tryptic map shows two phosphopeptides that comigrate exactly with the synthetic peptides used as standard.     

Estrogen can regulate the cell cycle in the early G1 phase of yeast by increasing the amount of adenylate cyclase mRNA

The effects of beta-estradiol (estrogen; a minor component of yeast cells) on S. cerevisiae cells in the G0 and G1 phases were examined. Results showed that estrogen stimulated the recovery of growth from G0 arrest induced by nutrient limitation or ts mutation of cdc35 (adenylate cyclase) in the early G1 phase, and inhibited entry into the resting G0 phase by increasing the intracellular cAMP level. However, estrogen had no effect on late G1 arrest induced by the alpha factor or ts mutation of cdc36. Estrogen was found to lead to higher steady-state levels of adenylate cyclase mRNA but not to affect the expression of the RAS1 and RAS2 genes, although these can also alter the intracellular cAMP level. These results suggest that estrogen influences the cell cycle of yeast in the early G1 phase by controlling the level of cAMP through the increase of adenylate cyclase mRNA.     

Expression of the murine homologue of the cell cycle control protein p34cdc2 in T lymphocytes.

The mammalian homologue of the cdc2 gene of the fission yeast Schizosaccharomyces pombe encodes a p34cdc2 cyclin-dependent kinase that regulates the cell cycle of a wide variety of cell types. Resting murine T lymphocytes contained no detectable p34cdc2 protein, histone kinase activity, or specific mRNA for the cdc2 gene. Activation of the T cells by immobilized anti-CD3 resulted in the expression of specific mRNA late in the G1 phase of the cell cycle, and p34cdc2 protein was detectable at or near G1/S. At this point in the cell cycle, the protein was phosphorylated at tyrosine and displayed no H1 histone kinase activity. As the cells progressed through the cycle, the amount of specific mRNA and p34cdc2 increased, and H1 histone kinase activity was detectable when the cells were blocked at G2/M by nocodazole. The activation of T cells by phorbol dibutyrate induced the expression of IL-2R but failed to induce the synthesis of IL-2 or the expression of cdc2-specific mRNA. Under these conditions, the activated cells failed to enter the S phase of the cell cycle. Because the presence of IL-2 added exogenously during activation by phorbol dibutyrate resulted in the expression of cdc2-specific mRNA and progression through the cell cycle, either IL-2 or the interaction with IL-2R may be involved in the expression of cdc2 and regulation of the G1/S transition.     

Delaying the onset of M phase in NIH 3T3 cells blocked in early S phase occurs via accumulating cyclin B1 and tyrosine-phosphorylated p34cdc2 in the nucleus

An affinity-purified antibody (anti-Cdc2C) raised against the carboxy terminal sequence LDNQIKKM of p34^cdc2 uncovered in NIH 3T3 cells a protein subpopulation, the location and the level of accumulation of which evolve during progression through the cell cycle: it first emerges inside the nucleus in late G1/early S phase and continues to build up principally in this location throughout S phase; a cytoplasmic expression then becomes apparent near the end of S phase, develops during G2 and sometimes prevails over the nuclear expression; it finally relocates to the nucleus in early prophase. We propose that a major part of this subpopulation would represent p34^cdc2 molecules existing inside a complex with cyclin B1. NIH 3T3 cells arrested in early S phase with aphidicolin do not commit prematurely to mitosis which indicates that the regulatory pathway involved in preserving the temporal order of S and M phases is functioning in these conditions. Conjugated Western blot analysis and immunofluorescence microscopy showed that cyclin A, cyclin B1 and tyrosine-phosphorylated p34^cdc2 continue to build up predominantly in the nucleus of the arrested cells. After release from the block, the cells rapidly reenter S and G2 phases and, concomitantly, cyclin B1 and tyrosine-phosphorylated p34^cdc2 relocate to the cytoplasm before redistributing again in the nucleus in early prophase. These data would suggest that delaying the onset of M phase in NIH 3T3 cells in which the rate of DNA replication is reduced, is first ensured by a mechanism that prevents the cytoplasmic relocation of inactive p34^cdc2/cyclin B1 complexes continually forming in the nucleus once the G1 period of mitotic cyclin instability is over.