DNA damage invokes mismatch repair-dependent cyclin D1 attenuation and retinoblastoma signaling pathways to inhibit CDK2.

DNA-damage evokes cell cycle checkpoints, which function to maintain genomic integrity. The retinoblastoma tumor suppressor (RB) and mismatch repair complexes are known to contribute to the appropriate cellular response to specific types of DNA damage. However, the signaling pathways through which these proteins impact the cell cycle machinery have not been explicitly determined. RB-deficient murine embryo fibroblasts continued a high degree of DNA replication following the induction of cisplatin damage, but were inhibited for G(2)/M progression. This damage led to RB dephosphorylation/activation and subsequent RB-dependent attenuation of cyclin A and CDK2 activity. In both Rb+/+ and Rb -/- cells, cyclin D1 expression was attenuated following DNA damage. As cyclin D1 is a critical determinant of RB phosphorylation and cell cycle progression, we probed the pathway through which cyclin D1 degradation occurs in response to DNA damage. We found that attenuation of endogenous cyclin D1 is dependent on multiple mismatch repair proteins. We demonstrate that the mismatch repair-dependent attenuation of endogenous cyclin D1 is critical for attenuation of CDK2 activity and induction of cell cycle checkpoints. Together, these studies couple the activity of the retinoblastoma and mismatch repair tumor suppressor pathways through the degradation of cyclin D1 and dual attenuation of CDK2 activity.     

Differential contribution of inhibitory phosphorylation of CDC2 and CDK2 for unperturbed cell cycle control and DNA integrity checkpoints

Inhibition of cyclin-dependent kinases (CDKs) by Thr14/Tyr15 phosphorylation is critical for normal cell cycle progression and is a converging event for several cell cycle checkpoints. In this study, we compared the relative contribution of inhibitory phosphorylation for cyclin A/B1-CDC2 and cyclin A/E-CDK2 complexes. We found that inhibitory phosphorylation plays a major role in the regulation of CDC2 but only a minor role for CDK2 during the unperturbed cell cycle of HeLa cells. The relative importance of inhibitory phosphorylation of CDC2 and CDK2 may reflect their distinct cellular functions. Despite this, expression of nonphosphorylation mutants of both CDC2 and CDK2 triggered unscheduled histone H3 phosphorylation early in the cell cycle and was cytotoxic. DNA damage by a radiomimetic drug or replication block by hydroxyurea stimulated a buildup of cyclin B1 but was accompanied by an increase of inhibitory phosphorylation of CDC2. After DNA damage and replication block, all cyclin-CDK pairs that control S phase and mitosis were to different degrees inhibited by phosphorylation. Ectopic expression of nonphosphorylated CDC2 stimulated DNA replication, histone H3 phosphorylation, and cell division even after DNA damage. Similarly, a nonphosphorylation mutant of CDK2, but not CDK4, disrupted the G2 DNA damage checkpoint. Finally, CDC25A, CDC25B, a dominant-negative CHK1, but not CDC25C or a dominant-negative WEE1, stimulated histone H3 phosphorylation after DNA damage. These data suggest differential contributions for the various regulators of Thr14/Tyr15 phosphorylation in normal cell cycle and during the DNA damage checkpoint.     

Acute reduction of an origin recognition complex (ORC) subunit in human cells reveals a requirement of ORC for Cdk2 activation

The origin recognition complex (ORC) is involved in formation of prereplicative complexes (pre-RCs) on replication origins in the G1 phase. At the G1/S transition, elevated cyclin E-CDK2 activity triggers 1DNA replication to enter S phase. The CDK cycle works as an engine that drives progression of cell cycle events by successive activation of different types of cyclin-CDK. However, how the CDK cycle is coordinated with replication initiation remains elusive. Here we report that acute depletion of ORC2 by RNA interference (RNAi) arrests cells with low cyclin E-CDK2 activity. This result suggests that loss of a replication initiation protein prevents progression of the CDK cycle in G1. p27 and p21 proteins accumulate following ORC2 RNAi and are required for the CDK2 inhibition. Restoration of CDK activity by co-depletion of p27 and p21 allows many ORC2-depleted cells to enter S phase and go on to mitosis. However, in some cells the release of the CDK2 block caused catastrophic events like apoptosis. Therefore, the CDK2 inhibition observed following ORC2 RNAi seems to protect cells from premature S phase entry and crisis in DNA replication. These results demonstrate an unexpected role of ORC2 in CDK2 activation, a linkage that could be important for maintaining genomic stability.     

Differential susceptibility of yeast S and M phase CDK complexes to inhibitory tyrosine phosphorylation

BACKGROUND: Several checkpoint pathways employ Wee1-mediated inhibitory tyrosine phosphorylation of cyclin-dependent kinases (CDKs) to restrain cell-cycle progression. Whereas in vertebrates this strategy can delay both DNA replication and mitosis, in yeast cells only mitosis is delayed. This is particularly surprising because yeasts, unlike vertebrates, employ a single family of cyclins (B type) and the same CDK to promote both S phase and mitosis. The G2-specific arrest could be explained in two fundamentally different ways: tyrosine phosphorylation of cyclin/CDK complexes could leave sufficient residual activity to promote S phase, or S phase-promoting cyclin/CDK complexes could somehow be protected from checkpoint-induced tyrosine phosphorylation. RESULTS: We demonstrate that in Saccharomyces cerevisiae, several cyclin/CDK complexes are protected from inhibitory tyrosine phosphorylation, allowing Clb5,6p to promote DNA replication and Clb3,4p to promote spindle assembly, even under checkpoint-inducing conditions that block nuclear division. In vivo, S phase-promoting Clb5p/Cdc28p complexes were phosphorylated more slowly and dephosphorylated more effectively than were mitosis-promoting Clb2p/Cdc28p complexes. Moreover, we show that the CDK inhibitor (CKI) Sic1p protects bound Clb5p/Cdc28p complexes from tyrosine phosphorylation, allowing the accumulation of unphosphorylated complexes that are unleashed when Sic1p is degraded to promote S phase. The vertebrate CKI p27(Kip1) similarly protects Cyclin A/Cdk2 complexes from Wee1, suggesting that the antagonism between CKIs and Wee1 is evolutionarily conserved. CONCLUSIONS: In yeast cells, the combination of CKI binding and preferential phosphorylation/dephosphorylation of different B cyclin/CDK complexes renders S phase progression immune from checkpoints acting via CDK tyrosine phosphorylation.     

Cyclin/CDK regulates the nucleocytoplasmic localization of the human papillomavirus E1 DNA helicase

Cyclin-dependent kinases (CDKs) play key roles in eukaryotic DNA replication and cell cycle progression. Phosphorylation of components of the preinitiation complex activates replication and prevents reinitiation. One mechanism is mediated by nuclear export of critical proteins. Human papillomavirus (HPV) DNA replication requires cellular machinery in addition to the viral replicative DNA helicase E1 and origin recognition protein E2. E1 phosphorylation by cyclin/CDK is critical for efficient viral DNA replication. We now show that E1 is phosphorylated by CDKs in vivo and that phosphorylation regulates its nucleocytoplasmic localization. We identified a conserved regulatory region for localization which contains a dominant leucine-rich nuclear export sequence (NES), the previously defined cyclin binding motif, three serine residues that are CDK substrates, and a putative bipartite nuclear localization sequence. We show that E1 is exported from the nucleus by a CRM1-dependent mechanism unless the NES is inactivated by CDK phosphorylation. Replication activities of E1 phosphorylation site mutations are reduced and correlate inversely with their increased cytoplasmic localization. Nuclear localization and replication activities of most of these mutations are enhanced or restored by mutations in the NES. Collectively, our data demonstrate that CDK phosphorylation controls E1 nuclear localization to support viral DNA amplification. Thus, HPV adopts and adapts the cellular regulatory mechanism to complete its reproductive program.     

Cyclin D2 compensates for the loss of cyclin D1 in estrogen-induced mouse uterine epithelial cell proliferation

The cell cycle-regulatory protein, cyclin D1, is the sensor that connects the intracellular cell cycle machinery to external signals. Given this central role in the control of cell proliferation, it was surprising that mice lacking the cyclin D1 gene were viable and fertile. Fertility requires 17beta-estradiol (E2)-induced uterine luminal epithelial cell proliferation. In these cells E2 causes the translocation of cyclin D1/cyclin-dependent kinase 4 (CDK4) from the cytoplasm into the nucleus with the consequent phosphorylation of the retinoblastoma protein. In cyclin D1 null mice, E2 also induces retinoblastoma protein phosphorylation and DNA synthesis in a normal manner. CDK4 activity was slightly reduced in the D1 null mice compared with wild-type mice. This CDK4 activity was due to complexes of cyclin D2/CDK4. Cyclin D2 was translocated into the nucleus in response to E2 in the cyclin D1-/- mice to a much greater degree than in wild-type mice. This cyclin D2/CDK4 complex was also able to bind p27kip1 in cyclin D1-/- uterine luminal epithelial cells, allowing for the activation of CDK2. Our data show that in vivo cyclin D2 can completely compensate for the loss of cyclin D1 and reinforces the conclusions that cyclin Ds are the central regulatory point in the proliferative responses of epithelial cells to estrogens.     

Cyclin-dependent kinases and S phase control in mammalian cells

Mammalian DNA replication is an elegantly choreographed process in which multiple components are assembled at the origins to form the prereplication complex. Formation and activation of the prereplication complex requires coordinate actions of G1and S phase cyclin-dependent kinases. Cyclin E-CDK2 and cyclin A-CDK2, together with DBF4-CDC7, phosphorylate several components of the prereplication complex and replication machinery. In this review, we summarize the current understanding of the mechanism of initiation of DNA replication in mammalian cells. The roles of cyclin A/E-CDK2 complexes in driving replication, their relationship with other regulators of S phase, and their role in keeping replication to only once per cell cycle will be discussed. In addition, an important issue is the checks and balances that prevent inappropriate DNA replication, and how a breakdown in these checkpoints can lead to genomic instability and cancer. A critical mediator of these checkpoints, ATM, signals through a comprehensive network of proteins leading to CDK2 inhibition thus preventing DNA synthesis. This will be reviewed in addition to other mechanisms involved in the intra-S phase DNA damage checkpoint.     

Cyclin-Dependent Kinases and S Phase Control in Mammalian Cells

Mammalian DNA replication is an elegantly choreographed process in which multiple components are assembled at the origins to form the prereplication complex. Formation and activation of the prereplication complex requires coordinate actions of G1 and S phase cyclin-dependent kinases. Cyclin E-CDK2 and cyclin A-CDK2, together with DBF4-CDC7, phosphorylate several components of the prereplication complex and replication machinery. In this review, we summarize the current understanding of the mechanism of initiation of DNA replication in mammalian cells. The roles of cyclin A/E-CDK2 complexes in driving replication, their relationship with other regulators of S phase, and their role in keeping replication to only once per cell cycle will be discussed. In addition, an important issue is the checks and balances that prevent inappropriate DNA replication, and how a breakdown in these checkpoints can lead to genomic instability and cancer. A critical mediator of these checkpoints, ATM, signals through a comprehensive network of proteins leading to CDK2 inhibition thus preventing DNA synthesis. This will be reviewed in addition to other mechanisms involved in the intra-S phase DNA damage checkpoint.     

CDKs promote DNA replication origin licensing in human cells by protecting Cdc6 from APC/C-dependent proteolysis

Cyclin-dependent kinases (CDKs) restrict DNA replication origin firing to once per cell cycle by preventing the assembly of prereplicative complexes (pre-RCs; licensing) outside of G1 phase. Paradoxically, under certain circumstances, CDKs such as cyclin E-cdk2 are also required to promote licensing. Here, we show that CDK phosphorylation of the essential licensing factor Cdc6 stabilizes it by preventing its association with the anaphase promoting complex/cyclosome (APC/C). APC/C-dependent Cdc6 proteolysis prevents pre-RC assembly in quiescent cells and, when cells reenter the cell cycle from quiescence, CDK-dependent Cdc6 stabilization allows Cdc6 to accumulate before the licensing inhibitors geminin and cyclin A which are also APC/C substrates. This novel mechanism for regulating protein stability establishes a window of time prior to S phase when pre-RCs can assemble which we propose represents a critical function of cyclin E.     

Varicella-zoster virus infection of human foreskin fibroblast cells results in atypical cyclin expression and cyclin-dependent kinase activity

In its course of human infection, varicella-zoster virus (VZV) infects rarely dividing cells such as dermal fibroblasts, differentiated keratinocytes, mature T cells, and neurons, none of which are actively synthesizing DNA; however, VZV is able to productively infect them and use their machinery to replicate the viral genome. We hypothesized that VZV alters the intracellular environment to favor viral replication by dysregulating cell cycle proteins and kinases. Cyclin-dependent kinases (CDKs) and cyclins displayed a highly unusual profile in VZV-infected confluent fibroblasts: total amounts of CDK1, CDK2, cyclin B1, cyclin D3, and cyclin A protein increased, and kinase activities of CDK2, CDK4, and cyclin B1 were strongly and simultaneously induced. Cyclins B1 and D3 increased as early as 24 h after infection, concurrent with VZV protein synthesis. Confocal microscopy indicated that cyclin D3 overexpression was limited to areas of IE62 production, whereas cyclin B1 expression was irregular across the VZV plaque. Downstream substrates of CDKs, including pRb, p107, and GM130, did not show phosphorylation by immunoblotting, and p21 and p27 protein levels were increased following infection. Finally, although the complement of cyclin expression and high CDK activity indicated a progression through the S and G(2) phases of the cell cycle, DNA staining and flow cytometry indicated a possible G(1)/S blockade in infected cells. These data support earlier studies showing that pharmacological CDK inhibitors can inhibit VZV replication in cultured cells.     

Cyclic AMP-dependent phosphorylation of cyclin D3-bound CDK4 determines the passage through the cell cycle restriction point in thyroid epithelial cells

According to current concepts, the cell cycle commitment after restriction (R) point passage requires the sustained stimulation by mitogens of the synthesis of labile d-type cyclins, which associate with cyclin-dependent kinase (CDK) 4/6 to phosphorylate pRb family proteins and sequester the CDK inhibitor p27kip1. In primary cultures of dog thyroid epithelial cells, the cAMP-dependent cell cycle induced by a sustained stimulation by thyrotropin or forskolin differs from growth factor mitogenic pathways, as cAMP does not upregulate d-type cyclins but increases p27 levels. Instead, cAMP induces the assembly of required cyclin D3-CDK4 complexes, which associate with nuclear p27. In this study, the arrest of forskolin stimulation rapidly slowed down the entry of dog thyrocytes into S phase and the phosphorylation of pRb family proteins. The pRb kinase activity, but not the formation, of the cyclin D3-CDK4-p27 complex was strongly reduced. Using two-dimensional gel electrophoresis, a phosphorylated form of CDK4 was separated. It appeared in response to forskolin and was bound to both cyclin D3 and p27, presumably reflecting the activating Thr-172 phosphorylation of CDK4. Upon forskolin withdrawal or after cycloheximide addition, this CDK4 phosphoform unexpectedly persisted in p27 complexes devoid of cyclin D3 but it disappeared from the more labile cyclin D3 complexes. These data demonstrate that the assembly of the cyclin D3-CDK4-p27 holoenzyme and the subsequent phosphorylation and activation of CDK4 depend on distinct cAMP actions. This provides a first example of a crucial regulation of CDK4 phosphorylation by a mitogenic cascade and a novel mechanism of cell cycle control at the R point.     

The CDK4/CDK6 inhibitor PD0332991 paradoxically stabilizes activated cyclin D3-CDK4/6 complexes

CDK4 and CDK6 bound to D-type cyclins are master integrators of G1 phase cell cycle regulations by initiating the inactivating phosphorylation of the central oncosuppressor pRb. Because of their frequent deregulation in cancer, cyclin D-CDK4/6 complexes are emerging as especially promising therapeutic targets. The specific CDK4/6 inhibitor PD0332991 is currently tested in a growing number of phase II/III clinical trials against a variety of pRb-proficient chemotherapy-resistant cancers. We have previously shown that PD0332991 inhibits not only CDK4/6 activity but also the activation by phosphorylation of the bulk of cyclin D-CDK4 complexes stabilized by p21 binding. Here we show that PD0332991 has either a positive or a negative impact on the activation of cyclin D-CDK4/6 complexes, depending on their binding to p21. Indeed, whereas PD0332991 inhibits the phosphorylation and activity of p21-bound CDK4/6, it specifically stabilized activated cyclin D3-CDK4/6 complexes devoid of p21 and p27. After elimination of PD0332991, these activated cyclin D3-CDK4/6 complexes persisted for at least 24 h, resulting in paradoxical cell cycle entry in the absence of a mitogenic stimulation. This unsuspected positive effect of PD0332991 on cyclin D3-CDK4/6 activation should be carefully assessed in the clinical evaluation of PD0332991, which until now only involves discontinuous administration protocols.     

HeLa cells are phenotypically limiting in cyclin E/CDK2 for efficient human papillomavirus DNA replication

Human papillomaviral (HPV) origin-containing plasmids replicate efficiently in human 293 cells or cell extracts in the presence of HPV origin-recognition protein E2 and replication initiation protein E1, whereas cervical carcinoma-derived, HPV-18-positive HeLa cells or cell extracts support HPV DNA replication poorly. We recently showed that HPV-11 E1 interacts with cyclin/cyclin-dependent kinase (cdk) complexes through an RXL motif and is a substrate for these kinases. E1 mutations in this motif or in candidate cdk phosphorylation sites are impaired in replication, suggesting a role for cdks in HPV replication. We now demonstrate that one limiting activity in HeLa cells is cyclin E/CDK2. Purified cyclin E/CDK2 or cyclin E/CDK3 complex, but not other cdks, partially complemented HeLa cell extracts. Cyclin E/CDK2 expression vectors also enhanced transient HPV replication in HeLa cells. HeLa cell-derived HPV-18 E1 protein is truncated at the carboxyl terminus but can associate with cyclin E/CDK2. This truncated E1 was replication-incompetent and inhibited cell-free HPV replication. These results indicate that HeLa cells are phenotypically limiting in cyclin E/CDK2 for efficient HPV replication, most likely due to sequestration by the endogenous, defective HPV-18 E1 protein. Further analyses of the regulation of HPV E1 and HPV replication by cyclin E may shed light on the roles of cyclin E/CDK2 in cellular DNA replication.     

CDK4 activity in mouse embryos expressing a single D-type cyclin

D-type cyclins (D1, D2, and D3) are components of the cell cycle machinery. Their association with cyclin-dependent kinase 4 (CDK4) and CDK6 causes activation of these protein kinases and leads to phosphorylation and inactivation of the retinoblastoma protein, pRb. Using embryos expressing single D-type cyclin ('cyclin D1-only', 'cyclin D2-only' and 'cyclin D3-only'), we tested whether each of D-type cyclin plays the same role in CDK activation and phosphorylation of pRb during mouse embryonic development. We found that the level of CDK4 activity was similar in wild-type embryos and those expressing only cyclin D3 or cyclin D2. However, we did not detect CDK4 activity in embryos expressing only cyclin D1, despite the fact that this cyclin was able to form complexes with CDK4 and p27(kip1) in wild-type as well as in mutant embryos. Analysis of the expression pattern of mRNA encoding cyclin D1 revealed that the expression of this RNA is regulated temporally during embryogenesis. These data and results from other laboratories indicate that cyclin D1-dependent CDK4 activity is dispensable for normal development of the mouse embryo.     

Association of cyclin A and cdk2 with SV40 DNA in replication initiation complexes is cell cycle dependent

The cell cycle is driven by the sequential activation of a family of cyclin-dependent kinases (CDK) in association with cyclins. In mammalian cells the timing of activation of cyclin A-associated kinase activity coincides with the onset of DNA synthesis in S-phase. Using in vitro replication of SV40 origin-containing DNA as a model system, we have analyzed the proteins associated with DNA during initiation of DNA replication in S-phase cell extracts. This analysis reveals that, in addition to replication initiation proteins, cyclin A and cdk2 are also specifically associated with DNA. The association of cyclin A and cdk2 with DNA during initiation is cell cycle regulated and occurs specifically in the presence of SV40 origin-containing plasmid and SV40 T antigen (the viral replication initiator protein). The interactions among proteins involved in initiation play an important role in DNA replication. We therefore investigated the ability of cyclin A and cdk2 to associate with replication initiation proteins. Under replication initiation conditions, cyclin A and cdk2 from S-phase extracts specifically associate with SV40 T antigen. Further, the interaction of cyclin A-cdk2 with SV40 T antigen is mediated via cyclin A, and purified recombinant cyclin A associates directly with SV40 T antigen. Taken together, our results suggest that cyclin A and cdk2 are components of the SV40 replication initiation complex, and that protein-protein interactions between cyclin A-cdk2 and T antigen may facilitate the association of cyclin A-cdk2 with the complex. Received: 30 July 1996; in revised form: 25 September 1996 / Accepted: 8 October 1996     

Phosphorylation of Ubc9 by Cdk1 enhances SUMOylation activity

Increasing evidence has pointed to an important role of SUMOylation in cell cycle regulation, especially for M phase. In the current studies, we have obtained evidence through in vitro studies that the master M phase regulator CDK1/cyclin B kinase phosphorylates the SUMOylation machinery component Ubc9, leading to its enhanced SUMOylation activity. First, we show that CDK1/cyclin B, but not many other cell cycle kinases such as CDK2/cyclin E, ERK1, ERK2, PKA and JNK2/SAPK1, specifically enhances SUMOylation activity. Second, CDK1/cyclin B phosphorylates the SUMOylation machinery component Ubc9, but not SAE1/SAE2 or SUMO1. Third, CDK1/cyclin B-phosphorylated Ubc9 exhibits increased SUMOylation activity and elevated accumulation of the Ubc9-SUMO1 thioester conjugate. Fourth, CDK1/cyclin B enhances SUMOylation activity through phosphorylation of Ubc9 at serine 71. These studies demonstrate for the first time that the cell cycle-specific kinase CDK1/cyclin B phosphorylates a SUMOylation machinery component to increase its overall SUMOylation activity, suggesting that SUMOylation is part of the cell cycle program orchestrated by CDK1 through Ubc9.