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Query: UNIPROT:P06889 (Mol)
 630,302 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The irreversible G1 arrest in senescent human diploid fibroblasts is probably caused by inactivation of the G1 cyclin-cyclin-dependent kinase (Cdk) complexes responsible for phosphorylation of the retinoblastoma protein (pRb). We show that the Cdk inhibitor p21(Sdi1,Cip1,Waf1), which accumulates progressively in aging cells, binds to and inactivates all cyclin E-Cdk2 complexes in senescent cells, whereas in young cells only p21-free Cdk2 complexes are active. Furthermore, the senescent-cell-cycle arrest occurs prior to the accumulation of the Cdk4-Cdk6 inhibitor p16(Ink4a), suggesting that p21 may be sufficient for this event. Accordingly, cyclin D1-associated phosphorylation of pRb at Ser-780 is lacking even in newly senescent fibroblasts that have a low amount of p16. Instead, the cyclin D1-Cdk4 and cyclin D1-Cdk6 complexes in these cells are associated with an increased amount of p21, suggesting that p21 may be responsible for inactivation of both cyclin E- and cyclin D1-associated kinase activity at the early stage of senescence. Moreover, even in the late stage of senescence when p16 is high, cyclin D1-Cdk4 complexes are persistent, albeit reduced by </=50% compared to young cells. We also provide new evidence that p21 may play a role in inactivation of the DNA replication factor proliferating cell nuclear antigen during early senescence. Finally, because p16 accumulates in parallel with the increases in senescence-associated beta-Gal activity and cell volume that characterize the senescent phenotype, we suggest that p16 upregulation may be part of a differentiation program that is turned on in senescent cells. Since p21 decreases after senescence is achieved, this upregulation of p16 may be essential for maintenance of the senescent-cell-cycle arrest.
Mol Cell Biol 1999 Mar
PMID:Differential roles for cyclin-dependent kinase inhibitors p21 and p16 in the mechanisms of senescence and differentiation in human fibroblasts. 1002 98

The response of the uterine epithelium to female sex steroid hormones provides an excellent model to study cell proliferation in vivo since both stimulation and inhibition of cell proliferation can be studied. Thus, when administered to ovariectomized adult mice 17beta-estradiol (E2) stimulates a synchronized wave of DNA synthesis and cell division in the epithelial cells, while pretreatment with progesterone (P4) completely inhibits this E2-induced cell proliferation. Using a simple method to isolate the uterine epithelium with high purity, we have shown that E2 treatment induces a relocalization of cyclin D1 and, to a lesser extent, cdk4 from the cytoplasm into the nucleus and results in the orderly activation of cyclin E- and cyclin A-cdk2 kinases and hyperphosphorylation of pRb and p107. P4 pretreatment did not alter overall levels of cyclin D1, cdk4, or cdk6 nor their associated kinase activities but instead inhibited the E2-induced nuclear localization of cyclin D1 to below the control level and, to a lesser extent, nuclear cdk4 levels, with a consequent inhibition of pRb and p107 phosphorylation. In addition, it abrogated E2-induced cyclin E-cdk2 activation by dephosphorylation of cdk2, followed by inhibition of cyclin A expression and consequently of cyclin A-cdk2 kinase activity and further inhibition of phosphorylation of pRb and p107. P4 is used therapeutically to oppose the effect of E2 during hormone replacement therapy and in the treatment of uterine adenocarcinoma. This study showing a novel mechanism of cell cycle inhibition by P4 may provide the basis for the development of new antiestrogens.
Mol Cell Biol 1999 Mar
PMID:Progesterone inhibits estrogen-induced cyclin D1 and cdk4 nuclear translocation, cyclin E- and cyclin A-cdk2 kinase activation, and cell proliferation in uterine epithelial cells in mice. 1002 12

Mast cells are widely distributed in human tissues, including the human uterus. However, the function of mast cells in uterine smooth muscle has not been clearly established. Mast cells possess secretory granules containing such substances as heparin, serotonin, histamine and many cytokines. To help establish the role of mast cells in the human myometrium, the action of heparin was investigated using smooth muscle cells (SMC) from normal myometrium and from leiomyoma. The proliferation of cultured myometrial and leiomyomal SMC was inhibited by heparin treatment. Flow cytometric analysis showed that the population in the G1 phase of the cell cycle increased under heparin treatment. Western blotting analysis showed that markers of SMC differentiation such as alpha-smooth muscle actin (alpha-SMA), calponin h1 and cyclin-dependent kinase inhibitor p27 were induced by heparin, whereas cell-cycle-related gene products from the G1 phase of the cell cycle, such as cyclin E and cdk2, were not changed. Taken together, these results indicate that heparin inhibits the proliferation of myometrial and leiomyomal SMC through the induction of alpha-SMA, calponin h1 and p27. We suggest that heparin from mast cells may induce differentiation in uterine SMC and may influence tissue remodelling and reconstruction during physiological and pathophysiological events.
Mol Hum Reprod 1999 Feb
PMID:Heparin inhibits proliferation of myometrial and leiomyomal smooth muscle cells through the induction of alpha-smooth muscle actin, calponin h1 and p27. 1006 69

Transforming growth factor beta (TGF-beta) potently suppresses Mv1Lu mink epithelial cell growth, whereas hepatocyte growth factor (HGF) counteracts TGF-beta-mediated growth inhibition and induces Mv1Lu cell proliferation (J. Taipale and J. Keski-Oja, J. Biol. Chem. 271:4342-4348, 1996). By addressing the cell cycle regulatory mechanisms involved in HGF-mediated release of Mv1Lu cells from TGF-beta inhibition, we show that increased DNA replication is accompanied by phosphorylation of the retinoblastoma protein and alternative regulation of cyclin-Cdk-inhibitor complexes. While TGF-beta treatment decreased the expression of Cdk6, this effect was counteracted by HGF, followed by partial restoration of cyclin D2-associated kinase activity. Notably, HGF failed to prevent TGF-beta induction of p15 and its association with Cdk6. However, HGF reversed the TGF-beta-mediated decrease in Cdk6-associated p27 and cyclin D2-associated Cdk6, suggesting that HGF modifies the TGF-beta response at the level of G1 cyclin complex formation. Counteraction of TGF-beta regulation of Cdk6 by HGF may in turn affect the association of p27 with Cdk2-cyclin E complexes. Though HGF did not differentially regulate the total levels of p27 in TGF-beta-treated cells, p27 immunodepletion experiments suggested that upon treatment with both growth factors, less p27 is associated with Cdk2-cyclin E complexes, in parallel with restoration of the active form of Cdk2 and the associated kinase activity. The results demonstrate that HGF intercepts TGF-beta cell cycle regulation at multiple points, affecting both G1 and G1-S cyclin kinase activities.
Mol Cell Biol 1999 May
PMID:Hepatocyte growth factor releases mink epithelial cells from transforming growth factor beta1-induced growth arrest by restoring Cdk6 expression and cyclin E-associated Cdk2 activity. 1020 89

The tumor suppressor p16(INK4a) inhibits cyclin-dependent kinases 4 and 6. This activates the retinoblastoma protein (pRB) and, through incompletely understood events, arrests the cell division cycle. To permit biochemical analysis of the arrest, we generated U2-OS osteogenic sarcoma cell clones in which p16 transcription could be induced. In these clones, binding of p16 to cdk4 and cdk6 abrogated binding of cyclin D1, p27(KIP1), and p21(WAF1/CIP1). Concomitantly, the total cellular level of p21 increased severalfold via a posttranscriptional mechanism. Most cyclin E-cdk2 complexes associated with p21 and became inactive, expression of cyclin A was curtailed, and DNA synthesis was strongly inhibited. Induction of p21 alone, in a sibling clone, to the level observed during p16 induction substantially reproduced these effects. Overexpression of either cyclin E or A prevented p16 from mediating arrest. We then extended these studies to HCT 116 colorectal carcinoma cells and a p21-null clone derived by homologous recombination. In the parental cells, p16 expression also augmented total cellular and cdk2-bound p21. Moreover, p16 strongly inhibited DNA synthesis in the parental cells but not in the p21-null derivative. These findings indicate that p21-mediated inhibition of cdk2 contributes to the cell cycle arrest imposed by p16 and is a potential point of cooperation between the p16/pRB and p14(ARF)/p53 tumor suppressor pathways.
Mol Cell Biol 1999 May
PMID:Induction of p21(WAF1/CIP1) and inhibition of Cdk2 mediated by the tumor suppressor p16(INK4a). 1020 15

The p300 and CREB binding protein (CBP) transcriptional coactivators interact with a variety of transcription factors and regulate their activity. Among the interactions that have been described, the COOH-terminal region of p300 binds to cyclin E-cyclin-dependent kinase 2 (cyclin E-Cdk2) and TFIIB, as well as to the E1A gene products of adenovirus. Inhibition of Cdk activity by Cdk inhibitors, such as p21 or p27, potentiates NF-kappaB activity and provides a mechanism to coordinate cell cycle progression with the transcription of genes expressed during growth arrest. In this report, we analyze the specific domains of p300 required for the binding of p300 to cyclin E-Cdk2, TFIIB, and E1A and the ability of these proteins to interact with p300, alone or in combination. 12S E1A, an inhibitor of p300-dependent transcription, reduces the binding of TFIIB, but not that of cyclin E-Cdk2, to p300. In contrast, 13S E1A, a pleiotropic transcriptional activator, does not inhibit TFIIB binding to p300, although it enhances the interaction of cyclin E-Cdk2 with p300. Modification of cyclin E-Cdk2 is most likely required for association with p300 since the interaction is observed only with cyclin E-Cdk2 purified from mammalian cells. Domain swap studies show that the cyclin homology domain of TFIIB is involved in interactions with p300, although the homologous region from cyclin E does not mediate this interaction. These findings suggest that p300 or CBP function is regulated by interactions of various proteins with a common coactivator domain.
Mol Cell Biol 1999 Jun
PMID:Specificity of cyclin E-Cdk2, TFIIB, and E1A interactions with a common domain of the p300 coactivator. 1033 Jan 64

c-myc is a cellular proto-oncogene associated with a variety of human cancers and is strongly implicated in the control of cellular proliferation, programmed cell death, and differentiation. We have previously reported the first isolation of a c-myc-null cell line. Loss of c-Myc causes a profound growth defect manifested by the lengthening of both the G1 and G2 phases of the cell cycle. To gain a clearer understanding of the role of c-Myc in cellular proliferation, we have performed a comprehensive analysis of the components that regulate cell cycle progression. The largest defect observed in c-myc-/- cells is a 12-fold reduction in the activity of cyclin D1-Cdk4 and -Cdk6 complexes during the G0-to-S transition. Downstream events, such as activation of cyclin E-Cdk2 and cyclin A-Cdk2 complexes, are delayed and reduced in magnitude. However, it is clear that c-Myc affects the cell cycle at multiple independent points, because restoration of the Cdk4 and -6 defect does not significantly increase growth rate. In exponentially cycling cells the absence of c-Myc reduces coordinately the activities of all cyclin-cyclin-dependent kinase complexes. An analysis of cyclin-dependent kinase complex regulators revealed increased expression of p27(KIP1) and decreased expression of Cdk7 in c-myc-/- cells. We propose that c-Myc functions as a crucial link in the coordinate adjustment of growth rate to environmental conditions.
Mol Cell Biol 1999 Jul
PMID:c-Myc regulates cyclin D-Cdk4 and -Cdk6 activity but affects cell cycle progression at multiple independent points. 1037 16

Human Cdc25 phosphatases play important roles in cell cycle regulation by removing inhibitory phosphates from tyrosine and threonine residues of cyclin-dependent kinases. Three human Cdc25 isoforms, A, B, and C, have been discovered. Cdc25B and Cdc25C play crucial roles at the G(2)/M transition. In the present study, we have investigated the function of human Cdc25A phosphatase. Cell lines that express human Cdc25A in an inducible manner have been generated. Ectopic expression of Cdc25A accelerates the G(1)/S-phase transition, indicating that Cdc25A controls an event(s) that is rate limiting for entry into S phase. Furthermore, we carried out a detailed analysis of the expression and activation of human Cdc25A. Activation of endogenous Cdc25A occurs during late G(1) phase and increases in S and G(2) phases. We further demonstrate that Cdc25A is activated at the same time as cyclin E- and cyclin A-dependent kinases. In vitro, Cdc25A dephosphorylates and activates the cyclin-Cdk complexes that are active during G(1). Overexpression of Cdc25A in the inducible system, however, leads to a premature activation of both cyclin E-Cdk2 and cyclin A-Cdk2 complexes, while no effect of cyclin D-dependent kinases is observed. Furthermore, Cdc25A overexpression induces a tyrosine dephosphorylation of Cdk2. These results suggest that Cdc25A is an important regulator of the G(1)/S-phase transition and that cyclin E- and cyclin A-dependent kinases act as direct targets.
Mol Cell Biol 1999 Sep
PMID:Ectopic expression of Cdc25A accelerates the G(1)/S transition and leads to premature activation of cyclin E- and cyclin A-dependent kinases. 1045 65

Cyclin-dependent kinase inhibitors such as p27(KIP1) have recently been shown to lead to cellular differentiation by causing cell cycle arrest, but it is unknown whether similar events occur in differentiating promyeloid cells. Hematopoietic progenitor cells undergo lineage-restricted differentiation, which is accompanied by expression of distinct maturation markers. Here we show that the classical growth factor insulin-like growth factor I (IGF-I) potently promotes vitamin D(3)-induced macrophage differentiation of promyeloid cells, as assessed by measurement of a coordinate increase in expression of the integrin alpha subunit CD11b, the CD14 lipopolysaccharide receptor, and the macrophage-specific esterase, alpha-naphthyl acetate esterase, as early as 24 h following initiation of terminal differentiation. Addition of IGF-I to cells undergoing vitamin D(3)-induced differentiation also leads to an early increase in expression of cyclin E, phosphorylation of the retinoblastoma tumor suppressor protein, and a doubling of the cell number. Early expression of CD11b (24 h) is simultaneously accompanied by inhibition in the expression of p27(KIP1). Cell cycle analysis with propidium iodide revealed that CD11b expression at 24 h following initiation of differentiation occurs at all phases of the cell cycle instead of only those cells arrested in G(0)/G(1). Similarly, development of a novel double-labeling intra- and extracellular flow-cytometric technique demonstrated that single cells expressing the mature leukocyte differentiation antigen CD11b can also incorporate the thymidine analog bromodeoxyuridine. Likewise, expression of the intracellular DNA polymerase delta cofactor/proliferating-cell nuclear antigen at 24 h is also simultaneously expressed with the surface marker CD11b, indicating that these cells continue to proliferate early in their differentiation program. Finally, at 24 h following induction of differentiation, IGF-I promoted a fourfold increase in the uptake of [(3)H]thymidine by purified populations of CD11b-expressing cells. Taken together, these data demonstrate that the initial steps associated with terminal macrophage differentiation occur concomitantly with progression through the cell cycle and that these very early differentiation events do not require the accumulation of p27(KIP1).
Mol Cell Biol 1999 Sep
PMID:Elevated cyclin E levels, inactive retinoblastoma protein, and suppression of the p27(KIP1) inhibitor characterize early development of promyeloid cells into macrophages. 1045 69

Functional inactivation of the pRB pathway is a very frequent event in human cancer, resulting in deregulated activity of the E2F transcription factors. To understand the functional role of the E2Fs in cell proliferation, we have developed cell lines expressing E2F-1, E2F-2, and E2F-3 fused to the estrogen receptor ligand binding domain (ER). In this study, we demonstrated that activation of all three E2Fs could relieve the mitogen requirement for entry into S phase in Rat1 fibroblasts and that E2F activity leads to a shortening of the G(0)-G(1) phase of the cell cycle by 6 to 7 h. In contrast to the current assumption that E2F-1 is the only E2F capable of inducing apoptosis, we showed that deregulated E2F-2 and E2F-3 activities also result in apoptosis. Using the ERE2F-expressing cell lines, we demonstrated that several genes containing E2F DNA binding sites are efficiently induced by the E2Fs in the absence of protein synthesis. Furthermore, CDC25A is defined as a novel E2F target whose expression can be directly regulated by E2F-1. Data showing that CDC25A is an essential target for E2F-1, since its activity is required for efficient induction of S phase by E2F-1, are provided. Finally, our results show that expression of two E2F target genes, namely CDC25A and cyclin E, is sufficient to induce entry into S phase in quiescent fibroblasts. Taken together, our results provide an important step in defining how E2F activity leads to deregulated proliferation.
Mol Cell Biol 1999 Sep
PMID:CDC25A phosphatase is a target of E2F and is required for efficient E2F-induced S phase. 1045 84


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