Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.7.11.22 (cdc2)
 8,319 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

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.
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PMID:Human cyclin E, a nuclear protein essential for the G1-to-S phase transition. 773 42

The D-type cyclins are growth factor-regulated delayed early functions which peak at the G1/S transition, are thought to regulate entry into S phase and have been implicated in tumorigenesis. Here, we show that cyclin D2 can co-operate with Ha-Ras to impose a novel transformed state on rat embryo fibroblasts (REF). While clonal cyclin D2/Ha-Ras REF transformants exhibit a characteristic transformed phenotype in high serum, in low serum they arrest cell proliferation and display profound morphological and cytological changes indicating loss of control of cell mass and deregulation of the G1/S transition. Notably, in low serum, despite re-establishment of actin cables and arrest of proliferation, cell mass continues to increase, creating giant cells up to 10 x normal size. Also, during low-serum culture the cells make a very gradual but progressive entry into S phase, reaching a 2.4N DNA content after 6 days. PCNA is expressed and 2N and 4N cells are largely absent, and thus the cells undergo a novel S phase arrest. While transfer to low serum induced the retinoblastoma protein to enter its dephosphorylated state, and cyclin A, cyclin B and cdc2 levels to decrease, all as normal, cyclin E, cdk4, cdk2 and the exogenous cyclin D2 persisted at high levels. These results indicate that cyclin D2 and Ha-Ras can transform cells when mitogenic signals from growth factors are provided. However, in low serum, co-operation of cyclin D2 and Ha-Ras provides only a subset of the progression signals and these are sufficient for G1-related cell mass increase and S phase entry, but are insufficient for full cell cycling.
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PMID:Cyclin D2 and Ha-Ras transformed rat embryo fibroblasts exhibit a novel deregulation of cell size control and early S phase arrest in low serum. 774 96

It has been postulated that the product (pRB) of the retinoblastoma gene dissociates from the E2F-pRB complex upon phosphorylation by cyclin-dependent kinase(s) (cdk). However, there is no direct evident for the regulation of formation of the E2F-pRB complex via phosphorylation by purified cdk. Therefore, we investigated the regulation of formation of this complex by phosphorylation using pRB and purified cyclin A-cdk2, cyclin E-cdk2 or cyclin D1-cdk4. Purified pRB was incubated with nuclear extracts prepared from pRB-defective cells and then subjected to gel mobility shift assays. We confirmed that unphosphorylated pRB associated with various types of E2F but pRB has been phosphorylated by cyclin A-cdk2 did not. We found that E2F-pRB complexes were disrupted as a consequence of phosphorylation by cyclin A-cdk2, and the levels of the free forms of E2Fs increased. We also found that not only the E2F-pRB complexes but also the E2F-p107 complexes were disrupted upon phosphorylation by cyclin A-cdk2. Furthermore, E2F-pRB complexes were disrupted through phosphorylation by cyclin D1-cdk4 and cyclin E-cdk2, as well as by cyclin A-cdk2. These results clearly demonstrate that the phosphorylation of pRB and p107 by cdks regulates the formation of complexes between E2F and pRB or p107.
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PMID:The interactions of E2F with pRB and with p107 are regulated via the phosphorylation of pRB and p107 by a cyclin-dependent kinase. 775 45

The PLSTIRE protein (cyclin-dependent kinase 6 (cdk6)), which shares extensive sequence homology (approximately 70%) with cdk4, was identified as the earliest inducible member of the cdk family of proteins in human T lymphocytes induced to proliferate in vitro by stimulation either with phorbol 12,13-dibutyrate and ionomycin (PDB/I) or PHA. The p40cdk6 protein was present in resting cells and increased amounts were detected 6 h after stimulation. It increased in amount throughout the first cell cycle but was present in reduced amounts at later times. Activity of the kinase, determined by in vitro phosphorylation of recombinant truncated retinoblastoma tumor suppressor gene (Rb) protein (p60Rb), paralleled p40cdk6 protein amounts. Cyclins D2 and D3 were the major cyclins associated with p40cdk6, with D2 predominating in early G1 phase. Both PDB and ionomycin were required for maximal accumulation of p40cdk6, but either agent alone stimulated some increase in amount and activity of the protein. p40cdk6 also increased in amount in cells activated in the presence of cyclosporin A or FK506, drugs that inhibit production of IL-2 and cell proliferation, suggesting that initial induction occurred independently of IL-2-mediated cell cycle progression. Furthermore, increased accumulation of p40cdk6 protein and activity occurred in cells rendered "competent" (responsive to IL-2) by a brief treatment with PDB/I. Thus, increased accumulation of the protein and its activity begin before IL-2/IL-2 receptor interaction, suggesting that the cdk6-cyclin D2 complex might be involved in acquisition of the competent state in human T lymphocytes.
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PMID:Regulation of synthesis and activity of the PLSTIRE protein (cyclin-dependent kinase 6 (cdk6)), a major cyclin D-associated cdk4 homologue in normal human T lymphocytes. 775 65

The retinoblastoma gene (Rb gene) is a tumour-suppressor gene and its product (pRB) is known to act as a negative regulator of the cell cycle. Although lack of pRB expression resulting from gene alterations is considered to be responsible for the genesis of several human malignancies, increased expression of pRB has been demonstrated in a majority of colorectal cancer cases. In the present study, we investigated the expression of pRB as well as that of its related kinases, cdk2 and cdc2, in colorectal cancer, since these kinases have been reported to phosphorylate and inactivate pRB. Western blot analysis revealed that colorectal cancer expressed higher levels of cdk2 and cdc2 than did normal mucosa and that the ratio of the hyperphosphorylated form of pRB was higher in colorectal cancer. Furthermore, immunohistochemical studies showed that cdk2/cdc2 was expressed exclusively in the cancer cells positive for pRB. These results suggest that an increase in the expression of cdk2/cdc2 in colorectal cancer may have prevented pRB from braking the cell cycle through phosphorylation.
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PMID:Coexpression of cdk2/cdc2 and retinoblastoma gene products in colorectal cancer. 777 16

The p130 protein is a recently cloned member of the retinoblastoma protein family. We show here that transformation of NIH3T3-L1 fibroblasts (L1 cells) by the simian virus 40 large T antigen (LTAg) depends on the disruption of DNA binding complexes between transcription factor E2F and p130. LTAg binds to the pocket region of p130 in vivo and disrupts the E2F-p130 complexes. E2F-p130 complexes are present only in quiescent L1 cells and disappear at the G1/S phase boundary concomitantly to induction of DNA synthesis and expression of the E2F-regulated cdc2 gene. p130 is a substrate of cyclin-dependent kinase 2 (Cdk2) in vitro and associates with a Cdk in vivo which is activated upon serum stimulation in late G1. Overexpression of p130 inhibits cdc2 promoter activity and entry of quiescent L1 cells into S phase. The results demonstrate that p130 is negative regulator of cell cycle progression which is specifically targeted by LTAg during cell transformation.
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PMID:A complex between E2F and the pRb-related protein p130 is specifically targeted by the simian virus 40 large T antigen during cell transformation. 778 51

This brief review examines the strict relationships between cell apoptosis and G1 cyclins. It has been shown that the basic role of G1 cyclins is in regulating G1 progression and G1/S transition (the critical cycle point for cell program decisions, including apoptosis) a fatal program for cells unable to bypass G1/S checkpoint 1. Notably, both of the two giant regulators of checkpoint 1 (i.e., p105RB [retinoblastoma oncosuppressor-encoded protein] and p53 dependent WAF1/CIP1) are influenced by or influence G1 cyclins: cyclin E/cdk2 kinase complexes hyperphosphorylate p105RB, induce E2F release, and free G1 exit. On the other hand, p21-WAF1/CIP1 is an inhibitor of cyclin-dependent kinases blocking cells at G1/S. Thus, G1 cyclin activity appears as a conditio sine qua non for G1 exit and apoptosis escape.
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PMID:Apoptosis and the cell cycle. 778 78

Inactivation of the retinoblastoma gene product (pRb) occurs concomitant with the appearance of its hyperphosphorylated form in mid to late G1. Multiple cyclin/CDK complexes are implicated in the cell cycle phosphorylation of pRb. Using in vivo expression systems, we show that cyclins A, E, D1, D2, and D3 each function to phosphorylate and inactivate pRb. In vivo, G1 cyclin/kinase complexes enhance the phosphorylation of pRb, and these effects of cyclin/kinases on pRb can be overcome by the addition of p21, a wide spectrum inhibitor of G1 kinases. Kinases associated with cyclins A, E, and D1 phosporylate pRb indistinguishably in vivo, according to proteolytic maps. Although cyclin D1 has been reported to bind to pRb directly, requiring the pRb-binding motif LXCXE, a mutant D1 lacking the pRb-binding motif remains able to phosphorylate pRb in vivo and in vitro and is also able to reverse the growth-inhibitory properties of pRb in intact cells. Finally, coexpression of G1 cyclins and kinases represses pRb-mediated growth inhibition in Saos-2 cells. The multiplicity of mechanisms for pRb phosphorylation and inactivation suggests that several pathways exist for the regulation of pRb by phosphorylation.
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PMID:G1 cyclins control the retinoblastoma gene product growth regulation activity via upstream mechanisms. 779 7

The orderly progression through the cell cycle is mediated by the sequential activation of several cyclin/cyclin-dependent kinase (cdk) complexes. These kinases phosphorylate a number of cellular substrates, among which is the product of the retinoblastoma gene, pRb. Phosphorylation of pRb in late G1 causes the release of the transcription factor E2F from pRb, resulting in the transcriptional activation of E2F-responsive genes. We show here that phosphorylation of the pRb-related p107 is also cell cycle regulated. p107 is first phosphorylated at 8 hr following serum stimulation of quiescent fibroblasts, which coincides with an increase in cyclin D1 protein levels. Consistent with this, we show that a cyclin D1/cdk4 complex, but not a cyclin E/cdk2 complex, can phosphorylate p107 in vivo. Furthermore, phosphorylation of p107 can be abolished by the overexpression of a dominant-negative form of cdk4. Phosphorylation of p107 results in the loss of the ability to associate with E2F-4, a transcription factor with growth-promoting and oncogenic activity. A p107-induced cell cycle block can be released by cyclin D1/cdk4 but not by cyclin E/cdk2. These data indicate that the activity of p107 is regulated by phosphorylation through D-type cyclins.
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PMID:Regulation of the retinoblastoma protein-related p107 by G1 cyclin complexes. 779 74

The interaction of the retinoblastoma susceptibility gene product (Rb)-related p107 protein with the E2F transcription factor in S-phase cells facilitates the formation of a multicomponent complex also containing cyclin A and the p33cdk2 kinase. We have created a series of p107 mutants to assess the ability of p107 to inhibit E2F function and the role of the cyclin A/cdk2 complex in this process. We find that p107 mutants that do not bind to E2F also fail to repress E2F-dependent transcription. Moreover, we find that the ability of p107 to suppress E2F-dependent transcription is not dependent on the ability of p107 to associate with cyclin A/cdk2. Finally, an analysis of the ability of the p107 mutant proteins to suppress cell growth suggests that both E2F-dependent and E2F-independent events correlate with this activity.
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PMID:The Rb-related p107 protein can suppress E2F function independently of binding to cyclin A/cdk2. 779 40


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