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

Glucocorticoids inhibit proliferation of many cell types, but the events leading from the activated glucocorticoid receptor (GR) to growth arrest are not understood. Ectopic expression and activation of GR in human osteosarcoma cell lines U2OS and SAOS2, which lack endogenous receptors, result in a G1 cell cycle arrest. GR activation in U2OS cells represses expression of the cyclin-dependent kinases (CDKs) CDK4 and CDK6 as well as their regulatory partner, cyclin D3, leading to hypophosphorylation of the retinoblastoma protein (Rb). We also demonstrate a ligand-dependent reduction in the expression of E2F-1 and c-Myc, transcription factors involved in the G1-to-S-phase transition. Mitogen-activated protein kinase, CDK2, cyclin E, and the CDK inhibitors (CDIs) p27 and p21 are unaffected by receptor activation in U2OS cells. The receptor's N-terminal transcriptional activation domain is not required for growth arrest in U2OS cells. In Rb-deficient SAOS2 cells, however, the expression of p27 and p21 is induced upon receptor activation. Remarkably, in SAOS2 cells that express a GR deletion derivative lacking the N-terminal transcriptional activation domain, induction of CDI expression is abolished and the cells fail to undergo ligand-dependent cell cycle arrest. Similarly, murine S49 lymphoma cells, which, like SAOS2 cells, lack Rb, require the N-terminal activation domain for growth arrest and induce CDI expression upon GR activation. These cell-type-specific differences in receptor domains and cellular targets linking GR activation to cell cycle machinery suggest two distinct regulatory mechanisms of GR-mediated cell cycle arrest: one involving transcriptional repression of G1 cyclins and CDKs and the other involving enhanced transcription of CDIs by the activated receptor.
Mol Cell Biol 1997 Jun
PMID:Glucocorticoid receptor-mediated cell cycle arrest is achieved through distinct cell-specific transcriptional regulatory mechanisms. 915 17

Activins, members of the transforming growth factor-beta family, have been implicated in the regulation of growth and differentiation of various types of cells. We have recently found that activin A induces apoptotic cell death of plasmacytic cells including B cell hybridoma cells and myeloma cells. In the present study, we demonstrated that activin A caused cell-cycle arrest in the G1 phase before appearance of apoptotic cells in mouse B cell hybridoma cells. Phosphorylation of retinoblastoma protein (Rb) and in vitro Rb kinase activity of cyclin-dependent kinase (CDK)4 was inhibited in activin A-treated cells. Analysis of expression of genes regulating Rb phosphorylation revealed that activin A suppressed cyclin D2, the sole D-type cyclin gene expressed in the hybridoma cells, and activated p21CIP1/WAF1 but had no effect on expression of cyclin-dependent kinases (CDK2, CDK4, CDK6) and other CDK inhibitors (p27KIP1, p16INK4a, p15INK4b). Modulation of cyclin D2 and p21CIP1/WAF1 expression resulted in a decrease in level of cyclin D2-CDK4 complex and an increase in level of CDK4 complexed with p21CIP1/WAF1. Moreover, overexpression of cyclin D2 partially abrogated inhibition of Rb phosphorylation and G1 arrest in the hybridoma cells.
Mol Endocrinol 1997 Jul
PMID:Activin A induction of cell-cycle arrest involves modulation of cyclin D2 and p21CIP1/WAF1 in plasmacytic cells. 921 52

The molecular mechanisms that arrest cardiomyocytes in the cell cycle during the postnatal period remain largely unknown. We have examined changes of the expression of cyclins and CDKs, the activity of each CDK in cardiomyocytes during the postnatal period, and have compared those changes with rate of binucleation formation of cardiomyocytes in rats. The mRNA and protein levels of cyclin D1, A and B in cardiomyocytes were high at day 1, then the levels decreased at different rates during the postnatal period. While the protein levels of cyclin A and B rapidly decreased, the protein level of cyclin D1 was relatively constant. The protein levels of CDK4, CDK2, and cdc2 in cardiomyocytes were high at day 1, then their levels gradually decreased. However, the activity of CDK4, which is responsible for G1 phase of cell cycle, was detectable only at day 1. The activity of CDK2 activity, which is responsible for the S phase of cell cycle, was relatively high at day 1, decreased at day 2, abruptly decreased at day 4, maintained the same low level until day 10, and barely or not detectable thereafter in cardiomyocytes. The activity of cdc2 was high at day 1, increased by 20% at day 2, and then gradually decreased thereafter, although approximately 50% of maximum activity was present at day 6. Most cardiomyocytes were mononucleated during the first 2 days postnatal. The percentage of binucleated cardiomyocytes increased from 2.5% at day 2, 14% at day 4, 50% at day 8, 80% at day 14, and had reached adult levels at day 21 after birth. During active binucleation formation in neonatal (from days 1-14) cardiomyocytes, CDK4 or CDK2 was functionally negligible, while cdc2 was functionally active. These data suggest that there were differential and dramatic decrease of CDK4 and CDK2 activities in cardiomyocytes during neonatal period, and the functionally active cdc2 in neonatal cardiomyocytes may be involved in binucleation formation.
J Mol Cell Cardiol 1997 Jul
PMID:Differential and dramatic changes of cyclin-dependent kinase activities in cardiomyocytes during the neonatal period. 1160 19

The molecular mechanisms that regulate the cardiomyocyte cell cycle and its terminal differentiation remain largely unknown. To determine which cyclins or cyclin dependent kinases (CDKs) are important for cardiomyocyte proliferation, we examined the expression of cyclins and CDKs during normal cardiac development. All cyclins and CDKs were highly expressed during embryonic cardiac development, then they decreased at different rates after birth. The mRNAs and proteins of cyclins A and B (G2 and M phase cyclins) were found in embryonic and neonatal hearts, but were not detected in young or adult hearts. In contrast, while the mRNAs of cyclins D1, D2, D3, and E (G1 and S phase cyclins) were observed during all stages of development, the proteins of cyclins D1, D3, and E were observed in hearts at the young growth stage, although the levels decreased differently. Reverse transcriptase-polymerase chain reaction (RT-PCR) using specific cyclin B and D3 primers revealed that cyclins B and D3 originated from cardiomyocytes and noncardiomyocytes. The CDKs (cdc2, CDK2, and CDK4) were highly expressed during embryonic cardiac development and maintained almost constant levels during neonatal periods. However, they were expressed at very low levels at the young and adult stages. The pattern of proliferating cell nuclear antigen (PCNA) expression during cardiac development was similar to the expression of CDKs. These findings suggest that all cyclins and CDKs are involved in the cardiac cell cycle, and that marked and rapid reduction of mitotic cyclins may be associated with the withdrawal of the cardiac cell cycle after birth.
Mol Cells 1997 Jun 30
PMID:Cyclins and cyclin dependent kinases during cardiac development. 926 23

It is well documented that Ras functions as a molecular switch for reentry into the cell cycle at the border between G0 and G1 by transducing extracellular growth stimuli into early G1 mitogenic signals. In the present study, we investigated the role of Ras during the late stage of the G1 phase by using NIH 3T3 (M17) fibroblasts in which the expression of a dominant negative Ras mutant, p21(Ha-Ras[Asn17]), is induced in response to dexamethasone treatment. We found that delaying the expression of Ras(Asn17) until late in the G1 phase by introducing dexamethasone 3 h after the addition of epidermal growth factor (EGF) abolished the downregulation of the p27kip1 cyclin-dependent kinase (CDK) inhibitor which normally occurred during this period, with resultant suppression of cyclin Ds/CDK4 and cyclin E/CDK2 and G1 arrest. The immunodepletion of p27kip1 completely eliminated the CDK inhibitor activity from EGF-stimulated, dexamethasone-treated cell lysate. The failure of p27kip1 downregulation and G1 arrest was also observed in cells in which Ras(Asn17) was induced after growth stimulation with a phorbol ester or alpha-thrombin and was mimicked by the addition late in the G1 phase of inhibitors for phosphatidylinositol-3-kinase. Ras-mediated downregulation of p27kip1 involved both the suppression of synthesis and the stimulation of the degradation of the protein. Unlike the earlier expression of Ras(Asn17) at the border between G0 and G1, its delayed expression did not compromise the EGF-stimulated transient activation of extracellular signal-regulated kinases or inhibit the stimulated expression of a principal D-type cyclin, cyclin D1, until close to the border between G1 and S. We conclude that Ras plays temporally distinct, phase-specific roles throughout the G1 phase and that Ras function late in G1 is required for p27kip1 downregulation and passage through the restriction point, a prerequisite for entry into the S phase.
Mol Cell Biol 1997 Sep
PMID:Ras activity late in G1 phase required for p27kip1 downregulation, passage through the restriction point, and entry into S phase in growth factor-stimulated NIH 3T3 fibroblasts. 927 12

The molecular mechanisms responsible for the alterations in proliferative capacity of cardiac myocytes during development remain unknown; however, cell cycle dependent molecules may be involved. We have determined the expression of cyclins A, D1-3 and E, and cyclin-dependent kinases (CDKs) 2, 4, 5 and 6 and cdc2 in freshly isolated rat cardiac myocytes from fetal (18 days gestation), neonatal (2 days post-natal) and adult animals by immunoblotting. Our results show a dramatic decrease in expression of these proteins during normal cardiac development, such that levels are highest in fetal myocytes but are significantly down-regulated in adult cells (P<0.05, in each case). We also have determined the in vitro kinase activities of cdc2, CDK2, CDK4, CDK5 and CDK6 immunocomplexes in fetal, neonatal and adult myocytes. There was a consistent and significant loss of cdc2, CDK2, CDK4 and CDK6 kinase activities in adult cardiac cell lysates (5.3-, 10.6-, 1.5- and 1.9-fold decreases, respectively) when compared to neonatal samples (P<0.05); CDK5 activity showed a similar trend but failed to reach significance. In conclusion, our results show that the expression and activities of various positive regulators of the cell cycle are down-regulated significantly during development of the cardiac myocyte, concomitant with the loss of proliferative capacity in adult myocytes. Down-regulation of these proteins may be pivotal in the withdrawal of the cardiac myocyte from the cell cycle.
J Mol Cell Cardiol 1997 Aug
PMID:Expression and activities of cyclins and cyclin-dependent kinases in developing rat ventricular myocytes. 928 57

The tumor suppressor protein p53 acts as a transcriptional activator that can mediate cellular responses to DNA damage by inducing apoptosis and cell cycle arrest. p53 is a nuclear phosphoprotein, and phosphorylation has been proposed to be a means by which the activity of p53 is regulated. The cyclin-dependent kinase (CDK)-activating kinase (CAK) was originally identified as a cellular kinase required for the activation of a CDK-cyclin complex, and CAK is comprised of three subunits: CDK7, cyclin H, and p36MAT1. CAK is part of the transcription factor IIH multiprotein complex, which is required for RNA polymerase II transcription and nucleotide excision repair. Because of the similarities between p53 and CAK in their involvement in the cell cycle, transcription, and repair, we investigated whether p53 could act as a substrate for phosphorylation by CAK. While CDK7-cyclin H is sufficient for phosphorylation of CDK2, we show that p36MAT1 is required for efficient phosphorylation of p53 by CDK7-cyclin H, suggesting that p36MAT1 can act as a substrate specificity-determining factor for CDK7-cyclin H. We have mapped a major site of phosphorylation by CAK to Ser-33 of p53 and have demonstrated as well that p53 is phosphorylated at this site in vivo. Both wild-type and tumor-derived mutant p53 proteins are efficiently phosphorylated by CAK. Furthermore, we show that p36 and p53 can interact both in vitro and in vivo. These studies reveal a potential mechanism for coupling the regulation of p53 with DNA repair and the basal transcriptional machinery.
Mol Cell Biol 1997 Dec
PMID:p53 is phosphorylated by CDK7-cyclin H in a p36MAT1-dependent manner. 937 54

A novel 761-amino-acid transcription factor, DMP1, contains a central DNA binding domain that includes three imperfect myb repeats flanked by acidic transactivating domains at the amino and carboxyl termini. D-type cyclins associate with a region of the DMP1 DNA binding domain immediately adjacent to the myb repeats to form heteromeric complexes which detectably interact neither with cyclin-dependent kinase 4 (CDK4) nor with DNA. The segment of D-type cyclins required for its interaction with DMP1 falls outside the "cyclin box," which contains the residues predicted to contact CDK4. Hence, D-type cyclin point mutants that do not interact with CDK4 can still bind to DMP1. Enforced coexpression of either of three D-type cyclins (D1, D2, or D3) with DMP1 in mammalian cells canceled its ability to activate gene expression. This property was not shared by cyclins A, B, C, or H; did not depend upon CDK4 or CDK2 coexpression; was not subverted by a mutation in cyclin D1 that prevents its interaction with CDK4; and was unaffected by inhibitors of CDK4 catalytic activity. Introduction of DMP1 into mouse NIH 3T3 fibroblasts inhibited entry into S phase. Cell cycle arrest depended upon the ability of DMP1 to bind to DNA and to transactivate gene expression and was specifically antagonized by coexpression of D-type cyclins, including a D1 point mutant that does not bind to CDK4. Taken together, these findings suggest that DMP1 induces genes that inhibit S phase entry and that D-type cyclins can override DMP1-mediated growth arrest in a CDK-independent manner.
Mol Cell Biol 1998 Mar
PMID:Gene expression and cell cycle arrest mediated by transcription factor DMP1 is antagonized by D-type cyclins through a cyclin-dependent-kinase-independent mechanism. 948 76

Cardiomyocyte terminal differentiation was examined by studying the interaction of retinoblastoma protein (pRb) family members with E2F during the developmental transition from 17-day fetal to 2-day neonatal. Additionally, the expression pattern of cyclins, cyclin-dependent kinases (CDKs), and CDK inhibitors responsible for modulating the phosphorylation of pRb were studied. p107, pRb, and p130 are regulators of cellular proliferation, differentiation, and cell cycle exit and entry, respectively. The active, underphosphorylated form of these proteins targets the E2F family of transcriptional factors that play a critical role in the control of genes associated with DNA synthesis. Electromobility shift analyses demonstrated E2F complexed with p107 in proliferating fetal cardiomyocytes, whereas in 2-day neonatal cells, E2F was principally associated with p130 and a low level of pRb. At the 2-day neonatal stage, decreased protein levels were observed for cyclins D2, D3, and E, and CDK2 and CDK4. No changes were observed in the mRNA levels of the D-cyclins in neonatal cells; however, the transcripts for cyclins A and E and CDK4 were diminished. In skeletal myoblasts, differentiation is associated with induction of p21, a CDK inhibitor, by a MyoD-dependent pathway. Although heart cells lack MyoD, CDK assays demonstrated that the activity of CDKs 2, 4, and 6 were downregulated in 2-day neonatal cells, and CDC2 was increased. RT-PCR indicated that p21 mRNA was induced 1.4-, 2.0-, and 3.1-fold in the 2-day neonatal, 7-day neonatal, and adult stages, respectively, compared to the 17-day fetal stage. At the protein level, p21 also increased at the 2-day neonatal stage. Kinase inhibitory immunodepletion assays showed that CDK inhibitory activity was markedly increased in the 2-day neonate. Although mRNA levels of the p27 CDK inhibitor were unchanged, its protein level and inhibitory effect on CDK2 and CDK4 were increased. Thus, cardiomyocytes retain the capacity to proliferate until the early neonatal period when a series of changes occur, including a switch in pRb partners, a decrease in CDK levels and induction of CDK inhibitory activity, which is associated with terminal differentiation.
J Mol Cell Cardiol 1998 Mar
PMID:Changes in E2F complexes containing retinoblastoma protein family members and increased cyclin-dependent kinase inhibitor activities during terminal differentiation of cardiomyocytes. 951 32

Cyclin D2 is a member of the family of D-type cyclins that is implicated in cell cycle regulation, differentiation, and oncogenic transformation. To better understand the role of this cyclin in the control of cell proliferation, cyclin D2 expression was monitored under various growth conditions in primary human and established murine fibroblasts. In different states of cellular growth arrest initiated by contact inhibition, serum starvation, or cellular senescence, marked increases (5- to 20-fold) were seen in the expression levels of cyclin D2 mRNA and protein. Indirect immunofluorescence studies showed that cyclin D2 protein localized to the nucleus in G0, suggesting a nuclear function for cyclin D2 in quiescent cells. Cyclin D2 was also found to be associated with the cyclin-dependent kinases CDK2 and CDK4 but not CDK6 during growth arrest. Cyclin D2-CDK2 complexes increased in amounts but were inactive as histone H1 kinases in quiescent cells. Transient transfection and needle microinjection of cyclin D2 expression constructs demonstrated that overexpression of cyclin D2 protein efficiently inhibited cell cycle progression and DNA synthesis. These data suggest that in addition to a role in promoting cell cycle progression through phosphorylation of retinoblastoma family proteins in some cell systems, cyclin D2 may contribute to the induction and/or maintenance of a nonproliferative state, possibly through sequestration of the CDK2 catalytic subunit.
Mol Cell Biol 1998 Jun
PMID:Increased expression of cyclin D2 during multiple states of growth arrest in primary and established cells. 958 57


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