p21 Disrupts the interaction between cdk2 and the E2F-p130 complex

HypoxamiR-210-3p regulates mesenchymal stem cells proliferation via P53 & Akt

Transplanted stem cells (˃95%) into ischemic myocardium die because of unfavourable conditions. Moreover, hypoxia role in the cell cycle regulation has been studied in transformed/immortalized cell lines which may have altered cell cycle regulators and/or mutated and, can't be transplanted in patients. We quest to find out the mechanism of cell cycle regulation in mesenchymal stem cells (MSC) to regulate its survival and proliferation in repair processes. Additionally, critically analysed role of hypoxamiR-210-3p, and cell cycle regulators that can regulate cell proliferation under hypoxic conditions. Bone marrow-derived MSC (BM-MSC) isolated from young male Fischer-344 rats by flushing the cavity of femur and propagated in vitro under 1% hypoxia for 72 h showed an increased in cell proliferation ( > 30%, p < 0.05) compared to normoxia. miR-210-3p, role in cell proliferation under hypoxic condition was confirmed by knockdown. Loss of function studies with transfection of anti-mir-210-3p, we observed decrease in proliferation of BM-MSC under hypoxia. Furthermore, BM-MSC proliferation due to miR-210-3p was confirmed using CFSE assay and flow cytometry, in which more cells were observed in S-phase. Mechanistically, western blot analysis showed miR-210-3p inhibition upregulates p53 and p21 expression and subsequent decrease in pAkt under hypoxia. On contrary, CFSE and Western blot under normoxic conditions showed downregulation of p53 and p21 whilst upregulation of pAkt indicated the key role of miR-210-3p in BM-MSC proliferation. Our results demonstrate the role of miR-210-3p in BM-MSC proliferation under both hypoxic and normoxic conditions and illustrate the potential mechanism via the regulation of pAkt, p53 and p21.

Multifaceted p21 in carcinogenesis, stemness of tumor and tumor therapy

Cancer cells possess metabolic properties that are different from those of benign cells. p21, encoded by CDKN1A gene, also named p21^Cip1/WAF1, was first identified as a cyclin-dependent kinase regulator that suppresses cell cycle G1/S phase and retinoblastoma protein phosphorylation. CDKN1A (p21) acts as the downstream target gene of TP53 (p53), and its expression is induced by wild-type p53 and it is not associated with mutant p53. p21 has been characterized as a vital regulator that involves multiple cell functions, including G1/S cell cycle progression, cell growth, DNA damage, and cell stemness. In 1994, p21 was found as a tumor suppressor in brain, lung and colon cancer by targeting p53 and was associated with tumorigenesis and metastasis. Notably, p21 plays a significant role in tumor development through p53-dependent and p53-independent pathways. In addition, expression of p21 is closely related to the resting state or terminal differentiation of cells. p21 is also associated with cancer stem cells and acts as a biomarker for such cells. In cancer therapy, given the importance of p21 in regulating the G1/S and G2 check points, it is not surprising that p21 is implicated in response to many cancer treatments and p21 promotes the effect of oncolytic virotherapy.

p21cip1/waf1 Coordinate Autophagy, Proliferation and Apoptosis in Response to Metabolic Stress

Cancer cells possess metabolic properties that are different from benign cells. These unique characteristics have become attractive targets that are being actively investigated for cancer therapy. p21cip1/waf1, also known as Cyclin-Dependent Kinase inhibitor 1A, is encoded by the CDKN1A gene. It is a major p53 target gene involved in cell cycle progression that has been extensively evaluated. To date, p21 has been reported to regulate various cell functions, both dependent and independent of p53. Besides regulating the cell cycle, p21 also modulates apoptosis, induces senescence, and maintains cellular quiescence in response to various stimuli. p21 transcription is induced in response to stresses, including those from oxidative and chemotherapeutic treatment. A recent study has shown that in response to metabolic stresses such as nutrient and energy depletion, p21 expression is induced to regulate various cell functions. Despite the biological significance, the mechanism of p21 regulation in cancer adaptation to metabolic stress is underexplored and thus represents an exciting field. This review focuses on the recent development of p21 regulation in response to metabolic stress and its impact in inducing cell cycle arrest and death in cancer cells.

Loss of Cyclin-Dependent Kinase Inhibitor Alters Oncolytic Adenovirus Replication and Promotes More Efficient Virus Production

We elucidate the role of p21/Waf-1, a cyclin-dependent kinase inhibitor, on the oncolytic infection and replication cycle of adenovirus by studying both mRNA and adenoviral proteins expression. We found that infection in the absence of p21 causes a significant increase in adenoviral genomes and late gene expression. Similarly, the oncolytic adenoviral infected p21^−/− cells have earlier formation of replication foci and robust replication kinetics that were not observed in the wild type p21/Waf-1 intact cells. These findings suggest a culmination that the presence of intact p21 in host cells causes defects in the oncolytic viral life cycle which results in the production of immature and noninfectious particles.

Doxorubicin Activates Hepatitis B Virus Replication by Elevation of p21 (Waf1/Cip1) and C/EBPα Expression

Hepatitis B virus reactivation is an important medical issue in cancer patients who undergo systemic chemotherapy. Up to half of CHB carriers receiving chemotherapy develop hepatitis and among these cases a notable proportion are associated with HBV reactivation. However, the molecular mechanism(s) through which various chemotherapeutic agents induce HBV reactivation is not yet fully understood. In this study, we investigated the role of the cell cycle regulator p21 (Waf1/Cip1) in the modulation of HBV replication when a common chemotherapeutic agent, doxorubicin, is present. We showed that p21 expression was increased by doxorubicin treatment. This elevation in p21 expression enhanced the expression of CCAAT/enhancer-binding protein α (C/EBPα); such an increase is likely to promote the binding of C/EBPα to the HBV promoter, which will contribute to the activation of HBV replication. Our current study thus reveals the mechanism underlying doxorubicin modulation of HBV replication and provides an increased understanding of HBV reactivation in CHB patients who are receiving systemic chemotherapy.

The molecular basis for the pharmacokinetics and pharmacodynamics of curcumin and its metabolites in relation to cancer

This review addresses the oncopharmacological properties of curcumin at the molecular level. First, the interactions between curcumin and its molecular targets are addressed on the basis of curcumin's distinct chemical properties, which include H-bond donating and accepting capacity of the β-dicarbonyl moiety and the phenylic hydroxyl groups, H-bond accepting capacity of the methoxy ethers, multivalent metal and nonmetal cation binding properties, high partition coefficient, rotamerization around multiple C-C bonds, and the ability to act as a Michael acceptor. Next, the in vitro chemical stability of curcumin is elaborated in the context of its susceptibility to photochemical and chemical modification and degradation (e.g., alkaline hydrolysis). Specific modification and degradatory pathways are provided, which mainly entail radical-based intermediates, and the in vitro catabolites are identified. The implications of curcumin's (photo)chemical instability are addressed in light of pharmaceutical curcumin preparations, the use of curcumin analogues, and implementation of nanoparticulate drug delivery systems. Furthermore, the pharmacokinetics of curcumin and its most important degradation products are detailed in light of curcumin's poor bioavailability. Particular emphasis is placed on xenobiotic phase I and II metabolism as well as excretion of curcumin in the intestines (first pass), the liver (second pass), and other organs in addition to the pharmacokinetics of curcumin metabolites and their systemic clearance. Lastly, a summary is provided of the clinical pharmacodynamics of curcumin followed by a detailed account of curcumin's direct molecular targets, whereby the phenotypical/biological changes induced in cancer cells upon completion of the curcumin-triggered signaling cascade(s) are addressed in the framework of the hallmarks of cancer. The direct molecular targets include the ErbB family of receptors, protein kinase C, enzymes involved in prostaglandin synthesis, vitamin D receptor, and DNA.

Cyclin-dependent kinase inhibitor p21(Waf1): contemporary view on its role in senescence and oncogenesis

p21(Waf1) was identified as a protein suppressing cyclin E/A-CDK2 activity and was originally considered as a negative regulator of the cell cycle and a tumor suppressor. It is now considered that p21(Waf1) has alternative functions, and the view of its role in cellular processes has begun to change. At present, p21(Waf1) is known to be involved in regulation of fundamental cellular programs: cell proliferation, differentiation, migration, senescence, and apoptosis. In fact, it not only exhibits antioncogenic, but also oncogenic properties. This review provides a contemporary understanding of the functions of p21(Waf1) depending on its intracellular localization. On one hand, when in the nucleus, it serves as a negative cell cycle regulator and tumor suppressor, in particular by participating in the launch of a senescence program. On the other hand, when p21(Waf1) is localized in the cytoplasm, it acts as an oncogene by regulating migration, apoptosis, and proliferation.

p21 in cancer: intricate networks and multiple activities

One of the main engines that drives cellular transformation is the loss of proper control of the mammalian cell cycle. The cyclin-dependent kinase inhibitor p21 (also known as p21WAF1/Cip1) promotes cell cycle arrest in response to many stimuli. It is well positioned to function as both a sensor and an effector of multiple anti-proliferative signals. This Review focuses on recent advances in our understanding of the regulation of p21 and its biological functions with emphasis on its p53-independent tumour suppressor activities and paradoxical tumour-promoting activities, and their implications in cancer.

Cell cycle-related transformation of the E2F4-p130 repressor complex

During G0 phase the p130, member of the pRb tumor suppressor protein family, forms a repressor complex with E2F4 which is inactivated in G1/S by hyperphosphorylation of the p130. The role of p130 after G1/S remains poorly investigated. We found that in nuclear extracts of T98G cells, the p130-E2F4-DNA (pp-E2F4) complex does not dissociate at G1/S transition, but instead reverts to the p130-E2F4-cyclin E/A-cdk2 (cyc/cdk-pp-E2F4) complex, which is detected in S and G2/M phases of the cell cycle. Hyperphosphorylation of the p130 at G1/S transition is associated with a decrease of its total amount; however, this protein is still detected during the rest of the cell cycle, and it is increasingly hyperphosphorylated in the cytosol, but continuously dephosphorylated in the nucleus. Both nuclear and cytosol cell fractions in T98G cells contain a hyperphosphorylated form of p130 in complex with E2F4 at S and G2/M cell cycle phases. In contrast to T98G cells, transformation of the p130 containing cyc/cdk-pp-E2F4 complex into the p130-pp-E2F4 repressor does not occur in HeLa cells under growth restriction conditions.

Construction of a cyclin D1-Cdk2 fusion protein to model the biological functions of cyclin D1-Cdk2 complexes

Cyclin D1 is frequently overexpressed in human breast cancers, and cyclin D1 overexpression correlates with poor prognosis. Cyclin D1-Cdk2 complexes were previously observed in human breast cancer cell lines, but their role in cell cycle regulation and transformation was not investigated. This report demonstrates that Cdk2 in cyclin D1-Cdk2 complexes from mammary epithelial cells is phosphorylated on the activating phosphorylation site, Thr(160). Furthermore, cyclin D1-Cdk2 complexes catalyze Rb phosphorylation on multiple sites in vitro. As a model to investigate the biological and biochemical functions of cyclin D1-Cdk2 complexes, and the mechanisms by which cyclin D1 activates Cdk2, a cyclin D1-Cdk2 fusion gene was constructed. The cyclin D1-Cdk2 fusion protein expressed in epithelial cells was phosphorylated on Thr(160) and catalyzed the phosphorylation of Rb on multiple sites in vitro and in vivo. Kinase activity was not observed if either the cyclin D1 or Cdk2 domain was mutationally inactivated. Mutational inactivation of the cyclin D1 domain prevented activating phosphorylation of the Cdk2 domain on Thr(160). These results indicate that the cyclin D1 domain of the fusion protein activated the Cdk2 domain through an intramolecular mechanism. Cells stably expressing the cyclin D1-Cdk2 fusion protein exhibited several hallmarks of transformation including hyperphosphorylation of Rb, resistance to TGFbeta-induced growth arrest, and anchorage-independent proliferation in soft agar. We propose that cyclin D1-Cdk2 complexes mediate some of the transforming effects of cyclin D1 and demonstrate that the cyclin D1-Cdk2 fusion protein is a useful model to investigate the biological functions of cyclin D1-Cdk2 complexes.

Contribution of the p38MAPK signalling pathway to proliferation in human cultured airway smooth muscle cells is mitogen-specific

We have investigated the role of p38MAPK in human airway smooth muscle (HASM) proliferation in response to thrombin and bFGF. The regulation of cyclin D1 mRNA, cyclin D1, cyclin E and p21Cip1 protein levels, and the extent of retinoblastoma protein (pRb) phosphorylation in response to activation of p38MAPK have also been examined. Two distinct inhibitors of p38MAPK, SB 203580 (10 microm) and SB 202190 (10 microm), prevented bFGF (0.3-3 nm)-stimulated cell proliferation, but had no effect on the response to thrombin (0.3-3 U ml(-1)). In cells incubated with thrombin or bFGF for 20 h, there was an increase in p38MAPK phosphorylation in response to bFGF, but not to thrombin. Thrombin and bFGF-stimulated increases in ERK phosphorylation and cyclin D1 mRNA and protein levels were not influenced by SB 203580 pre-treatment. Similarly, cyclin E and p21Cip1 protein levels, measured after 20 h incubation with mitogen, did not appear to be regulated by SB 203580 (10 microm). Although both thrombin and bFGF significantly increased levels of pRb phosphorylation, SB 203580 (10 microm) inhibited only bFGF-stimulated pRb phosphorylation. In addition, SB 203580 (10 microm) selectively inhibited bFGF-stimulated DNA synthesis, suggesting that the antimitogenic actions of SB 203580 on pRb phosphorylation cause cell cycle arrest at late G1 phase. In conclusion, these results indicate that p38MAPK is involved in bFGF-, but not in thrombin-stimulated HASM proliferation. The activation of the p38MAPK pathway by bFGF, but not by thrombin, regulates the phosphorylation of pRb without influencing cyclin D1 expression.

Genotoxic Stress Induces Expression of E2F4, Leading to Its Association with p130 in Prostate Carcinoma Cells

The retinoblastoma (pRb), p107, and p130 pocket proteins bind to the E2F transcription factors to control gene expression. E2F4 protein levels increased and accumulated in the nuclei of prostate carcinoma cells subjected to ionizing radiation (IR). The IR-induced increase of E2F4 levels led to an increase in E2F4 binding to p130 but had no effect on E2F4/p107 or E2F5/p130 complexes. The increase in E2F4/p130 association after IR was observed in prostate carcinoma cells regardless of their sensitivity to androgens, but not in breast carcinoma cells. E2F4/p130 complex formation was dependent on dissociation of p130 from cyclin-dependent kinase 2 and p130 dephosphorylation. Disruption of E2F4 through small interfering RNA prevented p130/E2F4 complex formation and sensitized cells to IR-induced apoptosis, leading to caspase-3 activation, cleavage of its substrate, poly(ADP-ribose) polymerase, and nuclear condensation. The E2F4/p130 pocket protein complex emerges as a new target of radiation in prostate carcinoma cells.

P130 and its truncated form mediate p53-induced cell cycle arrest in Rb(-/-) Saos2 cells

In the present study, we investigate the mechanism of how p53 induces growth arrest in Rb-defective Saos2 cells that express temperature-sensitive mutant p53 (ts p53). The activation of p53 at a permissive temperature (32.5 degrees C) induces the cell cycle arrest at both the G1 and G2 stages. The induction of several p53-responsive genes as well as a small form of p130 (S-p130) was detected upon p53 activation. S-p130 retained the functions as a pocket protein and was dominant over p130 at the protein level after 36 h at 32.5 degrees C. A canonical p53 binding site was identified in intron 4 of p130. Furthermore, a novel p53-inducible transcript containing a partial intron 4 sequence downstream of the p53 binding site and exon 5 of p130 was detected by RT-PCR, suggesting S-p130 is induced by p53 at transcriptional level. The results from gel shift assay and immunoprecipitation showed that S-p130 as well as p130 formed complexes with both E2F1 and E2F4 at a permissive temperature. Moreover, the transient expression of E1A (12S) and E2F1 effectively abrogated p53-induced cell cycle arrest. These results strongly suggested that p130 and its truncated form might substitute Rb in mediating p53-induced cell cycle arrest in Rb(-/-) Saos2 cells.

Cell cycle regulators and hematopoiesis

The cell cycle behavior of hematopoietic cells varies from extended quiescence to spectacular proliferation. Cell cycle regulators choreograph these transitions through variation in the makeup of cyclin-dependent kinase (cdk)-containing complexes and through alteration in protein expression levels and subcellular localization. The mechanisms through which cell cycle regulators couple proliferation, differentiation and survival is coming into sharper focus. Cdk-inhibitors, once thought of solely in terms of a checkpoint function on cycling, are now known to interact directly with proteins and pathways central to differentiation and apoptosis. By shuttling between binding partners committed to discrete functional pathways, cell cycle regulators may directly coordinate proliferation with differentiation, migration and apoptosis.

C-terminal deletion mutant p21(WAF1/CIP1) enhances E2F-1-mediated apoptosis in colon adenocarcinoma cells

The present study was designed to investigate the efficacy of combination gene therapy using adenoviral vectors expressing gene products shown to possess apoptotic activity: E2F-1 (Ad-E2F-1) and a C-terminal deletion mutant of p21(WAF1/cIP1) (Ad-p21(-PCNA)), on growth inhibition and apoptosis of human colon cancer cells in vitro and in vivo. Marked E2F-1 and p21(-PCNA) overexpression in response to adenovirus infection was evident by Western blot analysis. IC(25) concentrations of each virus were used for each treatment in vitro to detect cooperative effects on cell death. Coexpression of E2F-1 and p21(-PCNA) resulted in an additive effect on cell death compared to infection with either virus alone. Cell cycle analysis, poly(ADP-ribose) polymerase (PARP) cleavage and analysis of cell morphology also revealed that coinfection with both Ad-E2F-1 and Ad-p21(-PCNA) enhanced cellular apoptosis compared to either virus alone. Interestingly, E2F-1 protein expression was markedly enhanced in the E2F-1/p21(-PCNA) adenovirus combination compared to Ad-E2F-1 infection alone. However, these same effects were not evident in cells coinfected with Ad-E2F-1 and an adenovirus expressing wild-type human p21(WAF1/CIP1) (Ad-p21(WT)). The increase in E2F-1 expression with coexpression of E2F-1 and p21(-PCNA) was not a result of increased E2F-1 protein stability, but was related to increased transcriptional activity from the CMV promoter. Cell cycle analysis revealed G1 arrest 72 hours following single-gene therapy with either the wild-type or mutant p21, whereas increased accumulation of cells in G2/M phase was demonstrated in the E2F-1-overexpressing cells. In the combined therapies, E2F-1/p21(-PCNA) treatment still resulted in G1 arrest, but E2F-1 was able to counteract the G1 arrest when coinfected with p21(WT). These results provide further evidence of the importance of the p21:PCNA-binding domain in mediating the complex cell cycle interaction between E2F-1 and p21. Simultaneous intratumoral injection of Ad-E2F-1 and Ad-p21(-PCNA) dramatically reduced tumor burden of SW620 xenografts compared to either treatment alone in our in vivo model but not in HT-29 colon cancer xenografts. When combined with Ad-p21(-PCNA), E2F-1 adenovirus therapy resulted in approximately 95% decrease in tumor volume of SW620 tumor xenografts compared with controls (P<.05). In conclusion, although simultaneous delivery of E2F-1 and p21(-PCNA) transgenes results in increased E2F-1 expression and enhanced apoptosis of both SW620 and HT-29 colon cancer cells in vitro, this combination was only effective in the treatment of SW620 metastatic colon cancer in vivo. This may represent a potentially useful combination gene therapy strategy for metastatic colon cancer.

A low abundance pool of nascent p21WAF1/Cip1 is targeted by estrogen to activate cyclin E*Cdk2

Estrogens regulate cell proliferation in target tissues, including breast cancer by stimulating G(1)-S phase transition. Activation of cyclin E.Cdk2 through abrogation of the ability of p21(WAF1/Cip1) to bind to and inhibit cyclin-CDKs is a pivotal event in this process in MCF-7 breast cancer cells. A proposed mechanism is p21 sequestration into cyclin D1.Cdk4/6 complexes driven by estrogen-induced transcriptional activation of cyclin D1 gene expression. However, we now show that some E(2)-induced cyclin E.Cdk2 activation occurs in the absence of increased cyclin D1 levels and requires decreased p21 protein synthesis. Both mechanisms operate in the absence of major changes in total p21 protein levels and instead target a low abundance subset of newly synthesized p21. E(2)-induced activation of cyclin E.Cdk2 is mimicked by targeted inhibition of nascent p21 expression by antisense p21 oligonucleotides. Cyclin E.Cdk2 activation is completely inhibited by a combination of antisense cyclin D1 oligonucleotide transfection and elimination of the decrease in nascent p21 by infection with adenoviral-p21. These findings strongly support a central role for p21 in the early phase of E(2)-induced mitogenesis and highlight a major functional role for newly synthesized CDK inhibitory proteins.

Insulin/insulin-like growth factor-I and estrogen cooperate to stimulate cyclin E-Cdk2 activation and cell Cycle progression in MCF-7 breast cancer cells through differential regulation of cyclin E and p21(WAF1/Cip1)

Estrogens and insulin/insulin-like growth factor-I (IGF-I) are potent mitogens for breast epithelial cells and, when co-administered, induce synergistic stimulation of cell proliferation. To investigate the molecular basis of this effect, a MCF-7 breast cancer cell model was established where serum deprivation and concurrent treatment with the pure estrogen antagonist, ICI 182780, inhibited growth factor and estrogen action and arrested cells in G(0)/G(1) phase. Subsequent stimulation with insulin or IGF-I alone failed to induce significant S-phase entry. However, these treatments increased cyclin D1, cyclin E, and p21 gene expression and induced the formation of active Cdk4 complexes but resulted in only minor increases in cyclin E-Cdk2 activity, likely due to recruitment of the cyclin-dependent kinase (CDK) inhibitor p21(WAF1/Cip1) into these complexes. Treatment with estradiol alone resulted in a greater increase in cyclin D1 gene expression but markedly decreased p21 expression, with a concurrent increase in Cdk4 and Cdk2 activity and subsequent synchronous entry of cells into S phase. Co-administration of insulin/IGF-I and estrogen induced synergistic stimulation of S-phase entry coincident with synergistic activation of high molecular mass (approximately 350 kDa) cyclin E-Cdk2 complexes lacking p21. To determine if the ability of estrogen to deplete p21 was central to these effects, cells stimulated with insulin and estradiol were infected with an adenovirus expressing p21. Induction of p21 to levels equivalent to those following treatment with insulin alone markedly inhibited the synergism between estradiol and insulin on S-phase entry. Thus the ability of estradiol to antagonize the insulin-induced increase in p21 gene expression, with consequent activation of cyclin E-Cdk2, is a central component of the synergistic stimulation of breast epithelial cell proliferation induced by simultaneous activation of the estrogen and insulin/IGF-I signaling pathways.

P27 in cell cycle control and cancer

In order to survive, cells need tight control of cell cycle progression. The control mechanisms are often lost in human cancer cells. The cell cycle is driven forward by cyclin-dependent kinases (CDKs). The CDK inhibitors (CKIs) are important regulators of the CDKs. As the name implies, CKIs were initially shown to negatively regulate CDK activity. However, recent data indicates that the members of the Kip/Cip family of CKIs, including p27, exert both positive and negative regulation of CDK activity at the G1/S phase transition. Mutations of Kip/Cip genes are rare, but p27 knockout mice are tumor prone when challenged with carcinogenic stimuli. Numerous studies of various human non-hematological tumors have identified low expression of p27 as a predictor of poor prognosis. In non-Hodgkin's lymphoma (NHL), we and others have also shown the independent prognostic value of p27 expression. In distinct NHL entities however, shortened survival seems to correlate with high expression of p27. For definitive assessment of the role played by p27 in lymphomagenesis, and the prognostic value of p27 in these tumors, further studies of distinct NHL entities are needed. This review addresses the function of p27 and the other Kip/Cip proteins in G1/S phase transition and their possible role in tumorigenesis with emphasis on p27 and NHL.

Glucocorticoids inhibit developmental stage-specific osteoblast cell cycle. Dissociation of cyclin A-cyclin-dependent kinase 2 from E2F4-p130 complexes

Unique cell cycle control is instituted in confluent osteoblast cultures, driving growth to high density. The postconfluent dividing cells share features with cells that normally exit the cell cycle; p27(kip1) is increased, p21(waf1/cip1) is decreased, free E2F DNA binding activity is reduced, and E2F4 is primarily nuclear. E2F4-p130 becomes the predominant E2F-pocket complex formed on E2F sites, but, unlike the complex that typifies resting cells, cyclin A and CDK2 are also present. Administration of dexamethasone at this, but not earlier stages, results in reduction of cyclin A and CDK2 levels with a parallel decrease in the associated kinase activity, dissociation of cyclin A-CDK2 from the E2F4-p130 complexes, and inhibition of G(1)/S transition. The glucocorticoid-mediated cell cycle attenuation is also accompanied by, but not attributable to, increased p27(kip1) and decreased p21(waf1/cip1) levels. The attenuation of osteoblast growth to high density by dexamethasone is associated with severe impairment of mineralized extracellular matrix formation, unless treatment commences in cultures that have already grown to high density. Both the antimitotic and the antiphenotypic effects are reversible, and both are antagonized by RU486. Thus, glucocorticoids induce premature attenuation of the osteoblast cell cycle, possibly contributing to the osteoporosis induced by these drugs in vivo.

Loss of anti-mitotic effects of Bcl-2 with retention of anti-apoptotic activity during tumor progression in a mouse model

Bcl-2 is an anti-apoptotic and anti-proliferative protein over-expressed in several different human cancers including breast. Gain of Bcl-2 function in mammary epithelial cells was superimposed on the WAP-TAg transgenic mouse model of breast cancer progression to determine its effect on epithelial cell survival and proliferation at three key stages in oncogenesis: the initial proliferative process, hyperplasia, and cancer. During the initial proliferative process, Bcl-2 strongly inhibited both apoptosis and mitotic activity. However as tumorigenesis progressed to hyperplasia and adenocarcinoma, the inhibitory effects on mitotic activity were lost. In contrast, anti-apoptotic activity persisted in both hyperplasias and adenocarcinomas. These results demonstrate that the inhibitory effect of Bcl-2 on epithelial cell proliferation and apoptosis can separate during cancer progression. In this model, retention of anti-apoptotic activity with loss of anti-proliferative action resulted in earlier tumor presentation.

Functions of the retinoblastoma protein

The retinoblastoma protein (pRB) can both positively and negatively regulate transcription. The former correlates with its ability to promote differentiation and the latter with its ability to regulate entry into S-phase. pRB negatively regulates transcription by forming complexes with members of the E2F transcription factor family. These complexes, when bound to E2F sites within certain target genes, actively repress transcription through a variety of mechanisms including physical interaction with adjacent transcriptional activation domains and recruitment of proteins that directly, or indirectly, lead to histone deacetylation. pRB function is, in turn, modulated by phosphorylation mediated by cyclin-dependent kinases. Emerging data suggest that combinatorial control of pRB function may be achieved through the use of different phosphoacceptor sites, different cyclin/cdk docking sites, and different cyclin/cdk complexes. The untimely activation of E2F responsive genes can induce apoptosis. This comes about at least partly through the induction of ARF, which leads to the stabilization and activation of p53. BioEssays 1999;21:950-958.

Growth inhibition by CDK-cyclin and PCNA binding domains of p21 occurs by distinct mechanisms and is regulated by ubiquitin-proteasome pathway

The CDK inhibitor, p21WAF1/Cip1 blocks cell cycle progression. In vitro, the N-terminus of p21 binds and inhibits CDK-cyclin kinase activity, whereas the C-terminus binds and inhibits PCNA (proliferating cell nuclear antigen) function. PCNA is essential for processivity of both DNA polymerase delta and epsilon. We have performed a detailed analysis of growth inhibition by the N- and C-terminal regions of p21, and determined whether the N- and C-terminal regions mediate this effect by different mechanisms. Expression of either the N- or the C-terminal region of p21 inhibits DNA synthesis and cell growth, but not as efficiently as full length p21. The effectiveness of the two p21 domains is dependent on their stability which is determined by the ubiquitin-proteasome pathway. The stabilization of the N- and C-terminal region of p21 increases their effectiveness as inhibitors of DNA synthesis to levels comparable to full length p21. Inhibition of DNA synthesis by the N-terminal region of p21 involves suppression of E2F activity. In contrast, inhibition by the C-terminal region of p21 is not accompanied by suppression of E2F activity, but is mediated via PCNA binding. The C-terminal region of p21 therefore inhibits cell growth by a mechanism distinct from that of the N-terminal region containing the CDK-cyclin inhibitory domain.

The naturally occurring mutants of DDB are impaired in stimulating nuclear import of the p125 subunit and E2F1-activated transcription

The human UV-damaged-DNA binding protein DDB has been linked to the repair deficiency disease xeroderma pigmentosum group E (XP-E), because a subset of XP-E patients lack the damaged-DNA binding function of DDB. Moreover, the microinjection of purified DDB complements the repair deficiency in XP-E cells lacking DDB. Two naturally occurring XP-E mutations of DDB, 82TO and 2RO, have been characterized. They have single amino acid substitutions (K244E and R273H) within the WD motif of the p48 subunit of DDB, and the mutated proteins lack the damaged-DNA binding activity. In this report, we describe a new function of the p48 subunit of DDB, which reveals additional defects in the function of the XP-E mutants. We show that when the subunits of DDB were expressed individually, p48 localized in the nucleus and p125 localized in the cytoplasm. The coexpression of p125 with p48 resulted in an increased accumulation of p125 in the nucleus, indicating that p48 plays a critical role in the nuclear localization of p125. The mutant forms of p48, 2RO and 82TO, are deficient in stimulating the nuclear accumulation of the p125 subunit of DDB. In addition, the mutant 2RO fails to form a stable complex with the p125 subunit of DDB. Our previous studies indicated that DDB can associate with the transcription factor E2F1 and can function as a transcriptional partner of E2F1. Here we show that the two mutants, while they associate with E2F1 as efficiently as wild-type p48, are severely impaired in stimulating E2F1-activated transcription. This is consistent with our observation that both subunits of DDB are required to stimulate E2F1-activated transcription. The results provide insights into the functions of the subunits of DDB and suggest a possible link between the role of DDB in E2F1-activated transcription and the repair deficiency disease XP-E.

Growth inhibition by CDK-cyclin and PCNA binding domains of p21 occurs by distinct mechanisms and is regulated by ubiquitin-proteasome pathway

The CDK inhibitor, p21(WAF1/Cip1) blocks cell cycle progression. In vitro, the N-terminus of p21 binds and inhibits CDK-cyclin kinase activity, whereas the C-terminus binds and inhibits PCNA (proliferating cell nuclear antigen) function. PCNA is essential for processivity of both DNA polymerase delta and epsilon. We have performed a detailed analysis of growth inhibition by the N- and C-terminal regions of p21, and determined whether the N- and C-terminal regions mediate this effect by different mechanisms. Expression of either the N- or the C-terminal region of p21 inhibits DNA synthesis and cell growth, but not as efficiently as full length p21. The effectiveness of the two p21 domains is dependent on their stability which is determined by the ubiquitin-proteasome pathway. The stabilization of the N- and C-terminal region of p21 increases their effectiveness as inhibitors of DNA synthesis to levels comparable to full length p21. Inhibition of DNA synthesis by the N-terminal region of p21 involves suppression of E2F activity. In contrast, inhibition by the C-terminal region of p21 is not accompanied by suppression of E2F activity, but is mediated via PCNA binding. The C-terminal region of p21 therefore inhibits cell growth by a mechanism distinct from that of the N-terminal region containing the CDK-cyclin inhibitory domain.

Molecular mechanisms underlying interferon-alpha-induced G0/G1 arrest: CKI-mediated regulation of G1 Cdk-complexes and activation of pocket proteins

One prominent effect of IFNs is their cell growth-inhibitory activity. The mechanism behind this inhibition of proliferation is still not fully understood. In this study, the effect of IFN-alpha treatment on cell cycle progression has been analysed in three lymphoid cell lines, Daudi, U-266 and H9. Examination of the growth-arrested cell populations shows that Daudi cells accumulate in a G0-like state, whereas U-266 cells arrest later in G1. H9 cells are completely resistant to IFN-alpha's cell growth-inhibitory effects. The G0/G1-phase arrest is preceded by a rapid induction of the cyclin-dependent kinase inhibitors (CKIs), p21 and p15. In parallel, the activities of the G1 Cdks are significantly reduced. In addition to p21/p15 induction, IFN-alpha regulates the expression of another CKI, p27, presumably by a post-transcriptional mechanism. In the G1 Cdk-complexes, there is first an increased binding of p21 and p15 to their respective kinases. At longer exposure times, when Cdk-bound p15 and p21 decline, p27 starts to accumulate. Furthermore, we found that IFN-alpha not only suppresses the phosphorylation of pRb, but also alters the phosphorylation and expression of the other pocket proteins p130 and p107. These data suggest that induction of p21/p15 is involved in the primary IFN-alpha response inhibiting G1 Cdk activity, whereas increased p27 expression is part of a second set of events which keep these Cdks in their inactive form. Moreover, elevated levels of p27 correlated with a dissociation of cyclin E/Cdk2-p130 or p107 complexes to yield cyclin E/Cdk2-p27 complexes. In resistant H9 cells, which possess a homozygous deletion of the p15/p16 genes and lack p21 protein expression, IFN-alpha causes no detectable changes in p27 expression and, furthermore, no effects are observed on either pocket proteins in this cell line. Taken together, these data suggest that the early decline in G1 Cdk activity, subsequent changes in phosphorylation of pocket proteins, and G1/G0 arrest following IFN-alpha treatment, is not primarily due to loss of the G1 kinase components, but result from the inhibitory action of CKIs on these complexes.

C/EBPalpha regulates formation of S-phase-specific E2F-p107 complexes in livers of newborn mice

We previously showed that the rate of hepatocyte proliferation in livers from newborn C/EBPalpha knockout mice was increased. An examination of cell cycle-related proteins showed that the cyclin-dependent kinase (CDK) inhibitor p21 level was reduced in the knockout animals compared to that in wild-type littermates. Here we show additional cell cycle-associated proteins that are affected by C/EBPalpha. We have observed that C/EBPalpha controls the composition of E2F complexes through interaction with the retinoblastoma (Rb)-like protein, p107, during prenatal liver development. S-phase-specific E2F complexes containing E2F, DP, cdk2, cyclin A, and p107 are observed in the developing liver. In wild-type animals these complexes disappear by day 18 of gestation and are no longer present in the newborn animals. In the C/EBPalpha mutant, the S-phase-specific complexes do not diminish and persist to birth. The elevation of levels of the S-phase-specific E2F-p107 complexes in C/EBPalpha knockout mice correlates with the increased expression of several E2F-dependent genes such as those that encode cyclin A, proliferating cell nuclear antigen, and p107. The C/EBPalpha-mediated regulation of E2F binding is specific, since the deletion of another C/EBP family member, C/EBPbeta, does not change the pattern of E2F binding during prenatal liver development. The addition of bacterially expressed, purified His-C/EBPalpha to the E2F binding reaction resulted in the disruption of E2F complexes containing p107 in nuclear extracts from C/EBPalpha knockout mouse livers. Ectopic expression of C/EBPalpha in cultured cells also leads to a reduction of E2F complexes containing Rb family proteins. Coimmunoprecipitation analyses revealed an interaction of C/EBPalpha with p107 but none with cdk2, E2F1, or cyclin A. A region of C/EBPalpha that has sequence similarity to E2F is sufficient for the disruption of the E2F-p107 complexes. Despite its role as a DNA binding protein, C/EBPalpha brings about a change in E2F complex composition through a protein-protein interaction. The disruption of E2F-p107 complexes correlates with C/EBPalpha-mediated growth arrest of hepatocytes in newborn animals.

E2F target genes and cell-cycle checkpoint control

In this review, we will focus on the role played by transcription factors of the E2F/DP family in controlling the expression of genes that carry out important cell-cycle control functions, thereby ensuring ordered progression through the mammalian cell division cycle. The emerging picture is that cell-cycle progression depends on the execution of a regulatory cascade of gene expression, driven by E2F/DP transcription factors, which are in turn regulated by the products of some of these genes. That E2F factors are potent regulators of cell-cycle checkpoints in mammalian cells is supported by experiments demonstrating that ectopic expression of individual E2F family members is sufficient to modulate cell proliferation and apoptosis. It is also clear that deregulation of E2F activity will result in the loss of particular checkpoint controls, thereby predisposing cells to malignant conversion.

Requirement of cyclin E-Cdk2 inhibition in p16(INK4a)-mediated growth suppression

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.

c-Myc or cyclin D1 mimics estrogen effects on cyclin E-Cdk2 activation and cell cycle reentry

Estrogen-induced progression through G1 phase of the cell cycle is preceded by increased expression of the G1-phase regulatory proteins c-Myc and cyclin D1. To investigate the potential contribution of these proteins to estrogen action, we derived clonal MCF-7 breast cancer cell lines in which c-Myc or cyclin D1 was expressed under the control of the metal-inducible metallothionein promoter. Inducible expression of either c-Myc or cyclin D1 was sufficient for S-phase entry in cells previously arrested in G1 phase by pretreatment with ICI 182780, a potent estrogen antagonist. c-Myc expression was not accompanied by increased cyclin D1 expression or Cdk4 activation, nor was cyclin D1 induction accompanied by increases in c-Myc. Expression of c-Myc or cyclin D1 was sufficient to activate cyclin E-Cdk2 by promoting the formation of high-molecular-weight complexes lacking the cyclin-dependent kinase inhibitor p21, as has been described, following estrogen treatment. Interestingly, this was accompanied by an association between active cyclin E-Cdk2 complexes and hyperphosphorylated p130, identifying a previously undefined role for p130 in estrogen action. These data provide evidence for distinct c-Myc and cyclin D1 pathways in estrogen-induced mitogenesis which converge on or prior to the formation of active cyclin E-Cdk2-p130 complexes and loss of inactive cyclin E-Cdk2-p21 complexes, indicating a physiologically relevant role for the cyclin E binding motifs shared by p130 and p21.

Regulation of p21(WAF1) Expression During Normal Myeloid Differentiation

The G1-phase cell-cycle inhibitor p21 has been proposed to mediate growth arrest during differentiation. Upregulation of p21 has been shown in multiple cell lines induced to differentiate; however, the mechanism of p21 induction during normal differentiation is largely unknown. In this report, we use normal hematopoietic precursor cells obtained from umbilical cord to model p21 regulation during differentiation. Myeloid maturation of CD34+ precursor cells is associated with a marked increase in p21 expression at the RNA and protein level. The upregulation of p21 transcripts during differentiation is associated with decreased binding to a highly conserved 44-bp fragment within the p21 promoter. This 44-bp regulatory element binds a novel modulator of p21 expression. It is of considerable interest that, although the binding activity is expressed in p53-negative as well as in p53-positive cells, the DNA sequence recognized by this protein overlaps a PuPuPuC(A/T)(T/A)GPyPyPy consensus sequence for p53.

Regulation of p21(WAF1) Expression During Normal Myeloid Differentiation

The G1-phase cell-cycle inhibitor p21 has been proposed to mediate growth arrest during differentiation. Upregulation of p21 has been shown in multiple cell lines induced to differentiate; however, the mechanism of p21 induction during normal differentiation is largely unknown. In this report, we use normal hematopoietic precursor cells obtained from umbilical cord to model p21 regulation during differentiation. Myeloid maturation of CD34+ precursor cells is associated with a marked increase in p21 expression at the RNA and protein level. The upregulation of p21 transcripts during differentiation is associated with decreased binding to a highly conserved 44-bp fragment within the p21 promoter. This 44-bp regulatory element binds a novel modulator of p21 expression. It is of considerable interest that, although the binding activity is expressed in p53-negative as well as in p53-positive cells, the DNA sequence recognized by this protein overlaps a PuPuPuC(A/T)(T/A)GPyPyPy consensus sequence for p53.

pRB, p107 and p130 as transcriptional regulators: role in cell growth and differentiation

The mammalian cell cycle engine, which is composed of cyclin/CDK holoenzymes, controls the progression throughout the cell cycle by regulating, at least in part, the transcription of two types of genes: genes whose protein products are required for DNA metabolism and genes whose protein products are involved in cell cycle control. Among the targets of cyclin/CDKs, there is a family of negative growth regulators collectively known as pocket proteins. This family of pocket proteins includes the product of the retinoblastoma tumor suppressor gene, pRB and the functionally and structurally related proteins p107 and p130. In this review, the mechanisms by which pocket proteins are thought to regulate cell growth and differentiation are discussed.

Up-regulation of p27Kip1, p21WAF1/Cip1 and p16Ink4a is associated with, but not sufficient for, induction of squamous differentiation

Irreversible growth arrest is an early and integral part of squamous cell differentiation in normal human epidermal keratinocytes (NHEKs) and is assumed to be linked to the control of expression of differentiation-specific genes. In this study, we examine the link between the molecular events associated with growth arrest and the expression of differentiation genes. NHEKs that have been induced to undergo growth arrest and differentiation by suspension culture contain populations in both G1 and G2/M of the cell cycle. The irreversible growth arrest state in NHEKs is characterized by an accumulation of the hypophosphorylated forms of Rb and p130, with subsequent down-regulation of levels of Rb, up-regulation of p130 and associated down-regulation of E2F-regulated genes such as cyclin A. These events correlate with an inhibition of G1 cdk activity, mediated in part by an increase in the cdk inhibitors p21(WAF1/Cip1), p27(Kip1) and p16(Ink4a). Flow cytometric and immunoblot analysis demonstrated that the timing of the up-regulation of p27, p16 and p130 corresponds closely with the induction of the squamous-specific genes cornifin alpha (SPRR-1) and transglutaminase type I, suggesting a close link between control of growth arrest and differentiation. However, growth arrest induced by over-expression of p27, p21 or p16 by recombinant adenovirus is not sufficient to induce expression of the differentiation genes, or to invoke the pattern of cell cycle regulatory protein expression characteristic of the differentiation-specific irreversible growth arrest. We conclude that growth arrest mediated by activation of the Rb pathway is not sufficient to trigger terminal squamous differentiation and additional signals which can be generated during suspension culture are required to promote the complete differentiation program.

Estrogen regulation of cell cycle progression in breast cancer cells

Estrogens are potent mitogens in a number of target tissues including the mammary gland where they play a pivotal role in the development and progression of mammary carcinoma. The demonstration that estrogen-induced mitogenesis is associated with the recruitment of non-cycling, G0, cells into the cell cycle and an increased rate of progression through G1 phase, has focused attention on the estrogenic regulation of molecules with a known role in the control of G1-S phase progression. These experiments provide compelling evidence that estrogens regulate the expression and function of c-Myc and cyclin D1 and activate cyclin E-Cdk2 complexes, all of which are rate limiting for progression from G1 to S phase. Furthermore, these studies reveal a novel mechanism of activation of cyclin E-Cdk2 complexes whereby estrogens promote the formation of high molecular weight complexes lacking the CDK inhibitor p21. Inducible expression of either c-Myc or cyclin D1 can mimic the effects of estrogen in activating the cyclin E-Cdk2 complexes and promoting S phase entry, providing evidence for distinct c-Myc and cyclin D1 pathways in estrogen-induced mitogenesis which converge on the activation of cyclin E-Cdk2. These data provide new mechanistic insights into the known mitogenic effects of estrogens and identify potential downstream targets that contribute to their role in oncogenesis.

Cyclin-dependent kinases: engines, clocks, and microprocessors

Cyclin-dependent kinases (Cdks) play a well-established role in the regulation of the eukaryotic cell division cycle and have also been implicated in the control of gene transcription and other processes. Cdk activity is governed by a complex network of regulatory subunits and phosphorylation events whose precise effects on Cdk conformation have been revealed by recent crystallographic studies. In the cell, these regulatory mechanisms generate an interlinked series of Cdk oscillators that trigger the events of cell division.

Estrogen and progestin regulation of cell cycle progression

Estrogens and progesterone, acting via their specific nuclear receptors, are essential for normal mammary gland development and differentiated function. The molecular mechanisms through which these effects are mediated are not well defined, although significant recent progress has been made in linking steroid hormone action to cell cycle progression. This review summarizes data identifying c-myc and cyclin D1 as major downstream targets of both estrogen- and progestin-stimulated cell cycle progression in human breast cancer cells. Additionally, estrogen induces the formation of high specific activity forms of the cyclin E-Cdk2 enzyme complex lacking the cyclin-dependent kinase (CDK)3 inhibitor, p21. The delayed growth inhibitory effects of progestins, which are likely to be prerequisites for manifestation of their function in differentiation, also involve decreases in cyclin D1 and E gene expression and recruitment of CDK inhibitors into cyclin D1-Cdk4 and cyclin E-Cdk2 complexes. Thus estrogens and progestins affect CDK function not only by effects on cyclin abundance but also by regulating the recruitment of CDK inhibitors and, as yet undefined, additional components which determine the activity of the CDK complexes. These effects of estrogens and progestins are likely to be major contributors to their regulation of mammary epithelial cell proliferation and differentiation.

Site-directed mutant p21 proteins defective in both inhibition of E2F-regulated transcription and disruption of E2F-p130-cyclin-cdk2 complexes

P21 is a regulatory protein that can contribute to cell cycle arrest by inhibiting the cyclin-dependent-kinases (cdks). However, the mechanism that links the inhibition of the cdk activities and the cell cycle arrest is not well established. To investigate this, we studied a purified endogenous cellular complex which contained E2F (in the form of E2F-4), p130, cyclin, and cdk2. This complex of E2F-p130-cyclin-cdk2 is found mainly in cycling cells and is postulated to be an intermediate that leads to the activation of E2F. We previously showed that p21 could disrupt this complex leading to the accumulation of an E2F-p130 complex and the inhibition of E2F-regulated transcription. We analyzed a group of p21 mutants including those that harbored changes in cyclin- and cdk2-binding motifs. We show that both the cyclin and cdk2 binding motifs of p21 are crucial for the disruption of this endogenous complex of E2F-p130-cyclin-cdk2. This suggests a model where the ability of p21 to inhibit the function of this complex is dependent on interactions with both cyclin and cdk2 molecules. This was substantiated by studies with intact cells. P21 mutants that are impaired in their ability to disrupt the cellular E2F-p130-cyclin-cdk2 complex are also shown to be maximally impaired in the ability to repress E2F-regulated transcription.

Opposite transcriptional effects of cyclic AMP-responsive elements in confluent or p27KIP-overexpressing cells versus serum-starved or growing cells

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.

DDB, a putative DNA repair protein, can function as a transcriptional partner of E2F1

The transcription factor E2F1 is believed to be involved in the regulated expression of the DNA replication genes. To gain insights into the transcriptional activation function of E2F1, we looked for proteins in HeLa nuclear extracts that bind to the activation domain of E2F1. Here we show that DDB, a putative DNA repair protein, associates with the activation domain of E2F1. DDB was identified as a heterodimeric protein (48 and 127 kDa) that binds to UV-damaged DNA. We show that the UV-damaged-DNA binding activity from HeLa nuclear extracts can associate with the activation domain of E2F1. Moreover, the 48-kDa subunit of DDB, synthesized in vitro, binds to a fusion protein of E2F1 depending on the C-terminal activation domain. The interaction between DDB and E2F1 can also be detected by coimmunoprecipitation experiments. Immunoprecipitation of an epitope-tagged DDB from cell extracts resulted in the coprecipitation of E2F1. In a reciprocal experiment, immunoprecipitates of E2F1 were found to contain DDB. Fractionation of HeLa nuclear extracts also revealed a significant overlap in the elution profiles of E2F1 and DDB. For instance, DDB, which does not bind to the E2F sites, was enriched in the high-salt fractions containing E2F1 during chromatography through an E2F-specific DNA affinity column. We also observed evidence for a functional interaction between DDB and E2F1 in living cells. For instance, expression of DDB specifically stimulated E2F1-activated transcription. In addition, the transcriptional activation function of a heterologous transcription factor containing the activation domain of E2F1 was stimulated by coexpression of DDB. Moreover, DDB expression could overcome the retinoblastoma protein (Rb)-mediated inhibition of E2F1-activated transcription. The results suggest that this damaged-DNA binding protein can function as a transcriptional partner of E2F1. We speculate that the damaged-DNA binding function of DDB, besides repair, might serve as a negative regulator of E2F1-activated transcription, as damaged DNA will sequester DDB and make it unavailable for E2F1. Furthermore, the binding of DDB to damaged DNA might be involved in downregulating the replication genes during growth arrest induced by damaged DNA.

Activation of p53-p21waf1 pathway in response to disruption of cell-matrix interactions

The proliferation of most cells is strictly dependent on cell-matrix interactions, a phenomenon called anchorage dependence. Because tumor cells often are independent of this regulation, it is important to characterize the molecular pathways that control cellular proliferation after detachment of cells from their matrix. In this report, we investigated a possible role of p53 and one of its target genes, p21(waf1/cip1), as components of anchorage-dependent cell growth control. We found that p53 protein is rapidly activated upon the disruption of cellular attachment. This led to p21 transcriptional activation via two p53-binding sites in its promoter. Elevated p21 protein levels blocked transcription and activity of the cell cycle-regulator cyclin A, and cells became arrested in G1 of the cell cycle. Under the same conditions, fibroblasts from p53 knock-out mice did not activate p21 and did not down-regulate cyclin A expression but rather induced another cell cycle inhibitor, p27. Thus, our results characterize a chain of events, starting from the activation of p53 and proceeding via p21 to cyclin A, that is activated in response to the loss of cellular adherence. This p53-regulated pathway may constitute one of a few redundant systems to ensure proper cell control in multicellular organisms.

p107 and p130 associated cyclin A has altered substrate specificity

We demonstrate that p107 and p130 immune complexes exhibit kinase activity. We have tested such immune complexes with four substrates commonly utilized to assay Cdk activity, including all three known members of the retinoblastoma family. Immunodepletion revealed this kinase activity could be abolished by removal of either cyclin A or Cdk2 but was unaffected by removal of Cdk4 or any D-type cyclin. The appearance of p107 associated activity followed the accumulation of p107 protein. In contrast, the kinase activity associated with p130 immune complexes became apparent after mid-G1, coincident with p130 hyperphosphorylation. GST-Rb, GST-p107, and GST-p130 (where GST indicates glutathione S-transferase) were equally suitable substrates in p107 and p130 immune complex kinase assays, yielding activity equal to 25% of the cyclin A activity present. The p107 and p130 associated activity was unable to phosphorylate histone H1, suggesting the p107 and p130 associated cyclin A/Cdk2 may represent a distinct pool with a distinct substrate specificity. The p107 and p130 associated activity was released from the immune complexes upon incubation with ATP and Mg2+ and exhibited the same substrate preference observed with the untreated immune complex. Our data suggest that p107 and p130 recognize, or form by association, a distinct pool of cyclin A/Cdk2 that preferentially phosphorylates retinoblastoma family members.

p27Kip1 induces an accumulation of the repressor complexes of E2F and inhibits expression of the E2F-regulated genes

p27Kip1 is an inhibitor of the cyclin-dependent kinases and it plays an inhibitory role in the progression of cell cycle through G1 phase. To investigate the mechanism of cell cycle inhibition by p27Kip1, we constructed a cell line that inducibly expresses p27Kip1 upon addition of isopropyl-1-thio-beta-D-galactopyranoside in the culture medium. Isopropyl-1-thio-beta-D-galactopyranoside-induced expression of p27Kip1 in these cells causes a specific reduction in the expression of the E2F-regulated genes such as cyclin E, cyclin A, and dihydrofolate reductase. The reduction in the expression of these genes correlates with the p27Kip1-induced accumulation of the repressor complexes of the E2F family of factors (E2Fs). Our previous studies indicated that p21WAF1 could disrupt the interaction between cyclin/cyclin-dependent kinase 2 (cdk2) and the E2F repressor complexes E2F-p130 and E2F-p107. We show that p27Kip1, like p21WAF1, disrupts cyclin/cdk2-containing complexes of E2F-p130 leading to the accumulation of the E2F-p130 complexes, which is found in growth-arrested cells. In transient transfection assays, expression of p27Kip1 specifically inhibits transcription of a promoter containing E2F-binding sites. Mutants of p27Kip1 harboring changes in the cyclin- and cdk2-binding motifs are deficient in inhibiting transcription from the E2F sites containing reporter gene. Moreover, these mutants of p27Kip1 are also impaired in disrupting the interaction between cyclin/cdk2 and the repressor complexes of E2Fs. Taken together, these observations suggest that p27Kip1 reduces expression of the E2F-regulated genes by generating repressor complexes of E2Fs. Furthermore, the results also demonstrate that p27Kip1 inhibits expression of cyclin A and cyclin E, which are critical for progression through the G1-S phases.

Infection of primary cells by adeno-associated virus type 2 results in a modulation of cell cycle-regulating proteins

It has been demonstrated that infection of primary human cells with adeno-associated viruses (AAV) leads to a decrease in cellular proliferation and to growth arrest. We analyzed the molecular basis of this phenomenon and observed that infection with AAV type 2 (AAV2) had an effect on several factors engaged in the control of the mammalian cell cycle. In particular, all of the pRB family members, pRB, p107, and p130, which are involved in G1 cell cycle checkpoint control, were affected. After infection, a shift from hyper- to hypophosphorylated forms was observed. Cyclins A and B1, which are required for G1/S transition and progression into mitosis, respectively, were downregulated at the transcriptional level as well as at the protein level, whereas the G1 cyclins D1 and E remained unaffected. In addition, the steady-state levels of cyclin-dependent kinases CDK1 and CDK2 and of transcription factor E2F-1 were diminished. Of all the factors known to be involved in phosphorylation of pRB family proteins, only the CDK inhibitor p21WAF1 exhibited a response to AAV2 infection. p21WAF1 mRNA was quickly and progressively upregulated in a p53-independent manner over at least 72 h. Consistent with the increased p21WAF1 protein levels, cyclin E- and cyclin A-dependent kinase activities declined to low levels and E2F-p130-cyclin-CDK2 complexes were disrupted. From these data, we conclude that the major effect of AAV2 infection on primary human fibroblasts appears to be upregulation of p21WAF1 gene expression and thus cell cycle arrest by the suppression of pRB family protein phosphorylation.

p130 and p107 use a conserved domain to inhibit cellular cyclin-dependent kinase activity

The pRB-related proteins p107 and p130 are thought to suppress growth in part through their associations with two important cell cycle kinases, cyclin A-cdk2 and cyclin E-cdk2, and transcription factor E2F. Although each protein plays a critical role in cell proliferation, the functional consequences of the association among growth suppressor, cyclin-dependent kinase, and transcription factor have remained elusive. In an attempt to understand the biochemical properties of such complexes, we reconstituted each of the p130-cyclin-cdk2 and p107-cyclin-cdk2 complexes found in vivo with purified, recombinant proteins. Strikingly, stoichiometric association of p107 or p130 with either cyclin E-cdk2 or cyclin A-cdk2 negated the activities of these kinases. The results of our experiments suggest that inhibition does not result from substrate competition or loss of cdk2 activation. Kinase inhibitory activity was dependent upon an amino-terminal region of p107 that is highly conserved with p130. Further, a role for this amino-terminal region in growth suppression was uncovered by using p107 mutants unable to bind E2F. To determine whether cellular complexes might display similar regulatory properties, we purified p130-cyclin A-cdk2 complexes from human cells and found that such complexes exist in two forms, one that contains E2F-4-DP-1 and one that lacks the heterodimer. These endogenous complexes behaved like the in vitro-reconstituted complexes, exhibiting low levels of associated kinase activity that could be significantly augmented by dissociation of p130. The results of these experiments suggest a mechanism whereby p130 and p107 suppress growth by inhibiting important cell cycle kinases.

Uncoupling of p21 induction and MyoD activation results in the failure of irreversible cell cycle arrest in doxorubicin-treated myocytes

Doxorubicin (Dox, Adriamicin), a potent broad spectrum anthracycline anticancer drug, selectively inhibits muscle specific gene expression in cardiac cells in vivo and prevents terminal differentiation of skeletal muscle cells in vitro. By inducing the expression of the helix-loop-helix (HLH) transcriptional inhibitor ld2, Dox represses the myogenic function of the MyoD family of muscle regulatory factors (MRFs). In many cell types, terminal differentiation is coupled to an irreversible exit from the cell cycle and MyoD plays a critical role in the permanent cell cycle arrest of differentiating myocytes by upregulating the cyclin dependent kinase inhibitor (cdki) p21. Here, we correlate Dox effects on cell cycle with changes of E2F/DP complexes and activity in differentiating C2C12 myocytes. In Dox-treated quiescent myoblasts, which fail to differentiate into myotubes under permissive culture conditions, serum re-stimulation induces cyclin/cdk re-association on the E2F/DP complexes and this correlates with an evident increase in E2F/DP driven transcription and re-entry of myoblasts into the cell cycle. Despite Dox ability to activate the DNA-damage dependent p53/p21 pathway, when induced in the absence of MyoD or other MRFs, p21 fails to maintain the postmitotic state in Dox-treated myocytes induced to differentiate. Thus, uncoupling p21 induction and MyoD activity results in a serum-reversible cell cycle arrest, indicating that MRF specific activation of cdki(s) is required for permanent cell cycle arrest in differentiating muscle cells.