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

In order to elucidate the mechanisms by which estrogens and antiestrogens modulate the growth of breast cancer cells, we have characterized the changes induced by estradiol that occur during the G1 phase of the cell cycle of MCF-7 human mammary carcinoma cells. Addition of estradiol relieves the cell cycle block created by tamoxifen treatment, leading to marked activation of cyclin E-cdk2 complexes and phosphorylation of the retinoblastoma protein within 6 h. Cyclin D1 levels increase significantly while the levels of cyclin E, cdk2, and the p21 and p27 cdk inhibitors are relatively constant. However, the p21 cdk inhibitor shifts from its association with cyclin E-cdk2 to cyclin D1-cdk4, providing an explanation for the observed activation of the cyclin E-cdk2 complexes. These results support the notion that cyclin D1 has an important role in steroid-dependent cell proliferation and that estrogen, by regulating the activities of G1 cyclin-dependent kinases, can control the proliferation of breast cancer cells.
Mol Cell Biol 1997 Jul
PMID:Estrogen-dependent cyclin E-cdk2 activation through p21 redistribution. 919 41

Cdc25A, a phosphatase essential for G1-S transition, associates with, dephosphorylates, and activates the cell cycle kinase cyclin E-cdk2. p21CIP1 and p27 are cyclin-dependent kinase (cdk) inhibitors induced by growth-suppressive signals such as p53 and transforming growth factor beta (TGF-beta). We have identified a cyclin binding motif near the N terminus of Cdc25A that is similar to the cyclin binding Cy (or RR LFG) motif of the p21CIP1 family of cdk inhibitors and separate from the catalytic domain. Mutations in this motif disrupt the association of Cdc25A with cyclin E- or cyclin A-cdk2 in vitro and in vivo and selectively interfere with the dephosphorylation of cyclin E-cdk2. A peptide based on the Cy motif of p21 competitively disrupts the association of Cdc25A with cyclin-cdks and inhibits the dephosphorylation of the kinase. p21 inhibits Cdc25A-cyclin-cdk2 association and the dephosphorylation of cdk2. Conversely, Cdc25A, which is itself an oncogene up-regulated by the Myc oncogene, associates with cyclin-cdk and protects it from inhibition by p21. Cdc25A also protects DNA replication in Xenopus egg extracts from inhibition by p21. These results describe a mechanism by which the Myc- or Cdc25A-induced oncogenic and p53- or TGF-beta-induced growth-suppressive pathways counterbalance each other by competing for cyclin-cdks.
Mol Cell Biol 1997 Aug
PMID:p21CIP1 and Cdc25A: competition between an inhibitor and an activator of cyclin-dependent kinases. 923 91

Retinoic acid (RA) inhibition of breast cancer cell growth is associated with an accumulation of cells in G1 phase of the cell cycle. We have investigated the effects of RA on the expression and activity of cell cycle-regulatory proteins in MCF-7 human breast cancer cells. Flow cytometry analysis of MCF-7 cells treated with RA revealed a decrease in the percentage of cells in S phase by 48 h, which was maximal by 72 h. Phosphorylation of the retinoblastoma protein (pRb) was partially reduced in RA-treated cells accompanied by a decrease in the level of retinoblastoma protein. Expression of the cyclin D1 transcript was reduced by 48 h and cyclin-dependent kinase 2 (cdk2) mRNA levels declined within 8 h posttreatment followed by a decrease in cyclin D1 and cdk2 protein levels. Message and protein levels of cdk4 and cdc2 were not affected by RA. While cdk4 activity was similar in control and RA-treated cells, cdk2 activity began to decrease within 48 h of exposure to RA and was profoundly reduced after 72 h. This reduced activity was associated with decreased phosphorylation of cdk2. The decrease in cdk2 activity occurred in the absence of RA-mediated increases in the levels of the cdk inhibitors p21 and p27. However, assays of cdk2 from pooled lysates from RA-treated and control cells showed that RA-treated cells contain a cdk2-inhibitory activity. Our results show that RA inhibits cell cycle progression of MCF-7 cells by inhibiting cdk2 mRNA and protein production and by decreasing cdk2 activity.
Mol Endocrinol 1997 Aug
PMID:CDK2 is a target for retinoic acid-mediated growth inhibition in MCF-7 human breast cancer cells. 925 11

Depending on the tissue, progesterone is classified as a proliferative or a differentiative hormone. To explain this paradox, and to simplify analysis of its effects, we used a breast cancer cell line (T47D-YB) that constitutively expresses the B isoform of progesterone receptors. These cells are resistant to the proliferative effects of epidermal growth factor (EGF). Progesterone treatment accelerates T47D-YB cells through the first mitotic cell cycle, but arrests them in late G1 of the second cycle. This arrest is accompanied by decreased levels of cyclins D1, D3, and E, disappearance of cyclins A and B, and sequential induction of the cyclin-dependent kinase (cdk) inhibitors p21 and p27(Kip1). The retinoblastoma protein is hypophosphorylated and extensively down-regulated. The activity of the cell cycle-dependent protein kinase, cdk2, is regulated biphasically by progesterone: it increases initially, then decreases. This is consistent with the biphasic proliferative increase followed by arrest produced by one pulse of progesterone. A second treatment with progesterone cannot restart proliferation despite adequate levels of transcriptionally competent PR. Instead, a second progesterone dose delays the fall of p21 and enhances the rise of p27(Kip1), thereby intensifying the progesterone resistance in an autoinhibitory loop. However, during the progesterone-induced arrest, the cell cycling machinery is poised to restart. The first dose of progesterone increases the levels of EGF receptors and transiently sensitizes the cells to the proliferative effects of EGF. We conclude that progesterone is neither inherently proliferative nor antiproliferative, but that it is capable of stimulating or inhibiting cell growth depending on whether treatment is transient or continuous. We also suggest that the G1 arrest after progesterone treatment is accompanied by cellular changes that permit other, possibly tissue-specific, factors to influence the final proliferative or differentiative state.
Mol Endocrinol 1997 Oct
PMID:Biphasic regulation of breast cancer cell growth by progesterone: role of the cyclin-dependent kinase inhibitors, p21 and p27(Kip1). 932 42

The minute virus of mice, an autonomous parvovirus, requires entry of host cells into the S phase of the cell cycle for its DNA to be amplified and its genes expressed. This work focuses on the P4 promoter of this parvovirus, which directs expression of the transcription unit encoding the parvoviral nonstructural polypeptides. These notably include protein NS1, necessary for the S-phase-dependent burst of parvoviral DNA amplification and gene expression. The activity of the P4 promoter is shown to be regulated in a cell cycle-dependent manner. At the G1/S-phase transition, the promoter is activated via a cis-acting DNA element which interacts with phase-specific complexes containing the cellular transcription factor E2F. It is inhibited, on the other hand, in cells arrested in G1 due to contact inhibition. This inhibitory effect is not observed in serum-starved cells. It is mediated in cis by cyclic AMP response elements (CREs). Unlike serum-starved cells, confluent cells accumulate the cyclin-dependent kinase inhibitor p27, suggesting that the switch from CRE-mediated activation to CRE-mediated repression involves the p27 protein. Accordingly, plasmid-driven overexpression of p27 causes down-modulation of promoter P4 in growing cells, depending on the presence of at least two functional CREs. No such effect is observed with two other cyclin-dependent kinase inhibitors, p16 and p21. Given the importance of P4-driven synthesis of protein NS1 in parvoviral DNA amplification and gene expression, the stringent S-phase dependency of promoter P4 is likely a major determinant of the absolute requirement of the minute virus of mice for host cell proliferation.
Mol Cell Biol 1998 Jan
PMID:Opposite transcriptional effects of cyclic AMP-responsive elements in confluent or p27KIP-overexpressing cells versus serum-starved or growing cells. 941 88

It has been proposed that the functions of the cyclin-dependent kinase inhibitors p21(Cip1/Waf1) and p27Kip1 are limited to cell cycle control at the G1/S-phase transition and in the maintenance of cellular quiescence. To test the validity of this hypothesis, p21 was expressed in a diverse panel of cell lines, thus isolating the effects of p21 activity from the pleiotropic effects of upstream signaling pathways that normally induce p21 expression. The data show that at physiological levels of accumulation, p21, in addition to its role in negatively regulating the G1/S transition, contributes to regulation of the G2/M transition. Both G1- and G2-arrested cells were observed in all cell types, with different preponderances. Preponderant G1 arrest in response to p21 expression correlated with the presence of functional pRb. G2 arrest was more prominent in pRb-negative cells. The arrest distribution did not correlate with the p53 status, and proliferating-cell nuclear antigen (PCNA) binding activity of p21 did not appear to be involved, since p27, which lacks a PCNA binding domain, produced similar arrest distributions [corrected], DNA endoreduplication occurred in pRb-negative but not in pRb-positive cells, suggesting that functional pRb is necessary to prevent DNA replication in p21 G2-arrested cells. These results suggest that the primary target of the Cip/Kip family of inhibitors leading to efficient G1 arrest as well as to blockade of DNA replication from either G1 or G2 phase is the pRb regulatory system. Finally, the tendency of Rb-negative cells to undergo endoreduplication cycles when p21 is expressed may have negative implications in the therapy of Rb-negative cancers with genotoxic agents that activate the p53/p21 pathway.
Mol Cell Biol 1998 Jan
PMID:Effects of p21(Cip1/Waf1) at both the G1/S and the G2/M cell cycle transitions: pRb is a critical determinant in blocking DNA replication and in preventing endoreduplication. 941 9

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

The steroid hormone progesterone regulates proliferation and differentiation in the mammary gland and uterus by cell cycle phase-specific actions. In breast cancer cells the predominant effect of synthetic progestins is long-term growth inhibition and arrest in G1 phase. Progestin-mediated growth arrest of T-47D breast cancer cells was preceded by inhibition of cyclin D1-Cdk4, cyclin D3-Cdk4, and cyclin E-Cdk2 kinase activities in vitro and reduced phosphorylation of pRB and p107. This was accompanied by decreases in the expression of cyclins D1, D3, and E, decreased abundance of cyclin D1- and cyclin D3-Cdk4 complexes, increased association of the cyclin-dependent kinase (CDK) inhibitor p27 with the remaining Cdk4 complexes, and changes in the molecular masses and compositions of cyclin E complexes. In control cells cyclin E eluted from Superdex 200 as two peaks of approximately 120 and approximately 200 kDa, with the 120-kDa peak displaying greater cyclin E-associated kinase activity. Following progestin treatment, almost all of the cyclin E was in the 200-kDa, low-activity form, which was associated with the CDK inhibitors p21 and p27; this change preceded the inhibition of cell cycle progression. These data suggest preferential formation of this higher-molecular-weight, CDK inhibitor-bound form and a reduced number of cyclin E-Cdk2 complexes as mechanisms for the decreased cyclin E-associated kinase activity following progestin treatment. Ectopic expression of cyclin D1 in progestin-inhibited cells led to the reappearance of the 120-kDa active form of cyclin E-Cdk2 preceding the resumption of cell cycle progression. Thus, decreased cyclin expression and consequent increased CDK inhibitor association are likely to mediate the decreases in CDK activity accompanying progestin-mediated growth inhibition.
Mol Cell Biol 1998 Apr
PMID:Mechanisms of cyclin-dependent kinase inactivation by progestins. 952 53

We report that cyclin D3/cdk4 kinase activity is regulated by p27(kip1) in BALB/c 3T3 cells. The association of p27(kip1) was found to result in inhibition of cyclin D3 activity as measured by immune complex kinase assays utilizing cyclin D3-specific antibodies. The ternary p27(kip1)/cyclin D3/cdk4 complexes do exhibit kinase activity when measured in immune complex kinase assays utilizing p27(kip1)-specific antibodies. The association of p27(kip1) with cyclin D3 was highest in quiescent cells and declined upon mitogenic stimulation, concomitantly with declines in the total level of p27(kip1) protein. The decline in this association could be elicited by PDGF treatment alone; this was not sufficient, however, for activation of cyclin D3 activity, which also required the presence of factors in platelet-poor plasma in the culturing medium. Unlike cyclin D3 activity, which was detected only in growing cells, p27(kip1) kinase activity was present throughout the cell cycle. Since we found that the p27(kip1) activity was dependent on cyclin D3 and cdk4, we compared the substrate specificity of the active ternary complex containing p27(kip1) and the active cyclin D3 lacking p27(kip1) by tryptic phosphopeptide mapping of GST-Rb phosphorylated in vitro and also by comparing the relative phosphorylation activity toward a panel of peptide substrates. We found that ternary p27(kip1)/cyclin D3/cdk4 complexes exhibited a different specificity than the active binary cyclin D3/cdk4 complexes, suggesting that p27(kip1) has the capacity to both inhibit cyclin D/cdk4 activity as well as to modulate cyclin D3/cdk4 activity by altering its substrate preference.
Mol Biol Cell 1998 Aug
PMID:Cyclin D3-associated kinase activity is regulated by p27kip1 in BALB/c 3T3 cells. 969 68

Loss-of-function mutations of p16(INK4a) have been identified in a large number of human tumors. An established biochemical function of p16 is its ability to specifically inhibit cyclin D-dependent kinases in vitro, and this inhibition is believed to be the cause of the p16-mediated G1 cell cycle arrest after reintroduction of p16 into p16-deficient tumor cells. However, a mutant of Cdk4, Cdk4(N158), designed to specifically inhibit cyclin D-dependent kinases through dominant negative interference, was unable to arrest the cell cycle of the same cells (S. van den Heuvel and E. Harlow, Science 262:2050-2054, 1993). In this study, we determined functional differences between p16 and Cdk4(N158). We show that p16 and Cdk4(N158) inhibit the kinase activity of cellular cyclin D1 complexes through different mechanisms. p16 dissociated cyclin D1-Cdk4 complexes with the release of bound p27(KIP1), while Cdk4(N158) formed complexes with cyclin D1 and p27. In cells induced to overexpress p16, a higher portion of cellular p27 formed complexes with cyclin E-Cdk2, and Cdk2-associated kinase activities were correspondingly inhibited. Cells engineered to express moderately elevated levels of cyclin E became resistant to p16-mediated growth suppression. These results demonstrate that inhibition of cyclin D-dependent kinase activity may not be sufficient to cause G1 arrest in actively proliferating tumor cells. Inhibition of cyclin E-dependent kinases is required in p16-mediated growth suppression.
Mol Cell Biol 1998 Sep
PMID:Requirement of cyclin E-Cdk2 inhibition in p16(INK4a)-mediated growth suppression. 971 Jun 13


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