Cyclin A-kinase regulation of E2F-1 DNA binding function underlies suppression of an S phase checkpoint

SU9516, a cyclin-dependent kinase 2 inhibitor, promotes accumulation of high molecular weight E2F complexes in human colon carcinoma cells

The E2F family plays a critical role in the expression of genes required for entry into and progression through S phase. E2F-mediated transcription is repressed by the tumor suppressor retinoblastoma protein (pRb), which results in sequestration of E2F in a multiprotein complex that includes pRb. Derepression of E2F results from a series of complex phosphorylation events mediated by cyclin D/cdk4 and cyclin E/cdk2. We have employed a novel 3-substituted indolinone compound, 3-[1-(3H-imidazol-4-yl)-meth-(Z)-ylidene]-5-methoxy-1,3-dihydro-indol-2-one (SU9516), which selectively inhibits cdk2 activity (Lane et al., Cancer Res 2001;61:6170-7) to investigate these events. Electrophoretic mobility gel shift assays were performed on SU9516-treated and -untreated HT-29, SW480, and RKO human colon cancer cell extracts. Treatment with 5 microM SU9516 prevented dissociation of pRb from E2F1 in all cell lines (HT-29>RKO>SW480). Treatment effects were time-dependent, demonstrating greater inhibition at 48 hr versus 24hr in HT-29 cells. Furthermore, E2F species were sequestered in complexes with p107, p130, DP-1, and cyclins A and E. After a 24-hr treatment with 5 microM SU9516, cyclin D1 and cdk2 levels decreased by 10-60%. These findings delineate a previously undescribed mechanism for SU9516-mediated cell growth arrest through down-regulation of cyclin D1, inhibition of cdk2 levels and activity, and pan-sequestration of E2F.

Molecular and genetic toxicology of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)

The heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), formed when meat containing food is cooked, induces cancer of the colon, prostate and mammary gland of rats, tumours that are strongly associated with a Western diet. After consumption of a meat meal, PhIP is rapidly absorbed, metabolised and bioactivated to DNA damaging species. Thus, PhIP should be considered as a candidate etiological agent for human cancer. Studies in vitro in model mammalian cell culture systems, and in vivo in transgenic animals, have shown that mutation induced by PhIP is dose dependent and describes a mutational "fingerprint" that is characteristic of the chemical. This genetic toxicity is dependent upon CYP1 family metabolic activation and is detectable in these model systems at micro M concentrations. At early time points, PhIP treated cells show subtle signs of toxicity that lead to altered growth and cycling. Using co-culture systems where one cell line bioactivates PhIP with a second cell line as target, we showed in human lymphoblastoid target cells that PhIP induced a dose- and time-dependent S-phase delay of the cell cycle. With time, the cell population became increasingly apoptotic with remaining survivors carrying a mutated gene set. Transcript profiling of treated cells indicated differential expression of genes involved in cell cycle regulation, stress response, receptors and tumour related genes. Prominent was elevation of p21(cip1/waf1) transcript and Western blot analysis confirmed induction of p21(cip1/waf1) and p53 proteins. The dose dependency and temporal aspects of these changes indicate that manipulation of the cell cycle and growth in response to PhIP is a precursor to mutant selection. Reduction of the PhIP dose allows dissection of a different battery of cellular responses that favour cell growth rather than inhibition. This pro-growth stimulus is oestrogen-like and encompasses altered gene expression, proliferation and cell behaviour. In human breast cell lines, these PhIP-mediated pro-oestrogenic responses are inhibited by the anti-oestrogen ICI 182780. This range of molecular and genetic responses induced in cells by PhIP is quite remarkable. Its ability to activate S-phase cell cycle checkpoint, alter gene expression leading to apoptosis and an increased frequency of mutation are probably direct consequences of its genetic toxicity. In contrast, its pro-oestrogenic activity is likely to be a driver of clonal expansion. We suggest that these PhIP-induced genomic and cellular events contrive to manipulate cell cycle and survival. Understanding these molecular processes as well as the genetic toxicology of the chemical will help to define the involvement of PhIP in carcinogenesis and shed light upon its tissue specificity.

Active nuclear import and export pathways regulate E2F-5 subcellular localization

Epidermal keratinocyte differentiation is accompanied by differential regulation of E2F genes, including up-regulation of E2F-5 and its concomitant association with the retinoblastoma family protein p130. This complex appears to play a role in irreversible withdrawal from the cell cycle in differentiating keratinocytes. We now report that keratinocyte differentiation is also accompanied by changes in E2F-5 subcellular localization, from the cytoplasm to the nucleus. To define the molecular determinants of E2F-5 nuclear import, we tested its ability to enter the nucleus in import assays in vitro using digitonin-permeabilized cells. We found that E2F-5 enters the nucleus through mediated transport processes that involve formation of nuclear pore complexes. It has been proposed that E2F-4 and E2F-5, which lack defined nuclear localization signal (NLS) consensus sequences, enter the nucleus in association with NLS-containing DP-2 or pRB family proteins. However, we show that nuclear import of E2F-5 only requires the first N-terminal 56 amino acid residues and is not dependent on interaction with DP or pRB family proteins. Because E2F-5 is predominantly cytoplasmic in undifferentiated keratinocytes and in other intact cells, we also examined whether this protein is subjected to active nuclear export. Indeed, E2F-5 is exported from the nucleus through leptomycin B-sensitive, CRM1-mediated transport, through a region corresponding to amino acid residues 130-154. This region excludes the DNA- and the p130-binding domains. Thus, the subcellular distribution of E2F-5 is tightly regulated in intact cells, through multiple functional domains that direct nucleocytoplasmic shuttling of this protein.

Differential regulation of Rb family proteins and prohibitin during camptothecin-induced apoptosis

Prohibitin, a potential tumor suppressor, is known to induce growth suppression and repress E2F-mediated transcription. These growth regulatory functions of prohibitin require a physical interaction with the Rb protein. We now find that prohibitin protects cells from apoptosis mediated by camptothecin, a topoisomerase I inhibitor. Camptothecin treatment of Ramos B cells leads to the degradation of Rb protein and phosphorylation of its family members, p107 and p130. This correlates with an increase in the levels of cyclin E as well as the kinase activity associated with it. Inactivation of Rb leads to the dissociation and release of free E2F. We find also that E2F activity is induced upon camptothecin treatment, but this increase is absent in prohibitin overexpressing cells. It thus appears that prohibitin may be inhibiting apoptosis by downregulating E2F activity when Rb family members are inactive.

E2F transcriptional repressor complexes are critical downstream targets of p19(ARF)/p53-induced proliferative arrest

The p16(INK4a)/pRB/E2F and p19(ARF)/p53 tumor suppressor pathways are disrupted in most human cancers. Both p19(ARF) and p53 are required for the induction of senescence in primary mouse embryonic fibroblasts (MEFs), but little is known about their downstream targets. Disruption of E2F-mediated transcriptional repression in MEFs caused a general increase in the expression of E2F target genes, including p19ARF. We detected no contribution of E2F-mediated transactivation in this setting, indicating that a predominant role of endogenous E2F in asynchronously growing primary MEFs is to repress its target genes. Moreover, relief of transcriptional repression by E2F rendered MEFs resistant to senescence induced by either p19(ARF), p53, or RAS(V12). Thus, E2F transcriptional repressor complexes are critical downstream targets of antiproliferative p19(ARF)/p53 signaling.

E2F-3B is a physiological target of cyclin A

The E2F family of transcription factors controls the expression of numerous genes that are required for the G(1)/S transition. Among the mechanisms that modulate the activity of the E2F proteins, cyclin A has been found to be important for the down-regulation of E2F-1, -2, and -3A activity after cells have progressed through G(1)/S. Specifically, phosphorylation of these E2F proteins by cyclin A/Cdk2 ultimately results in their necessary degradation as cells progress through S phase. E2F-3B was recently identified as an alternatively spliced form of E2F-3A that was predicted to lack a functional cyclin A binding domain. In this paper, we present considerable evidence that contradicts this prediction. First, we demonstrate binding of cyclin A to E2F-3B as bacterially expressed proteins in vitro. Second, we demonstrate binding of cyclin A to E2F-3B in mammalian cells in vivo. Third, we show that co-expression of cyclin A with E2F-3B significantly reduces E2F-3B-mediated transcriptional activity. Finally, in synchronized cells, we observe down-regulation of E2F-3B protein expression coincident with the up-regulation of cyclin A. We conclude that E2F-3B is a physiological target of cyclin A.

Paradigms of growth control: relation to Cdk activation

The cyclin-dependent kinases (CDKs) play a key role in cell cycle control, and in this review, we focus on the events that regulate their activities. Emphasis is placed on the CDKs that function during the G(1) phase of the cell cycle and on the CDK inhibitor p27(Kip1). We discuss how CDK activation relates to two basic concepts of cell cycle regulation: (i) the need for multiple mitogens for the proliferation of nontransformed cells and (ii) the inhibitory effect of high culture density on proliferative capacity. We also describe how Cdk2 modulates the expression of the alpha subunit of the interleukin-2 receptor in T cells, and address the question of whether p27(Kip1) functions as an activator or inhibitor of the CDKs associated with the D cyclins.

Oxidized low density lipoprotein induces the cyclin-dependent kinase inhibitor p21(waf1) and the tumor suppressor Rb

Oxidized low density lipoprotein (OxLDL) is known to be cytotoxic towards different cell types of the arterial wall, leading to progression of an atherosclerotic plaque. We previously reported that OxLDL activates the tumor suppressor p53 in human fibroblasts [Biochem. Biophys. Res. Commun. 276 (2000) 718]. In the present work, we demonstrate that OxLDL increased intracellular levels of the kinase inhibitor p21(waf1) (p21) and of the tumor suppressor Rb. Concomitantly, level of the hypophosphorylated active form of Rb (HypoP-Rb) was also enhanced. Cycloheximide prevented the OxLDL-induced increase in p21, Rb, and HypoP-Rb, whereas okadaic acid had no effect. This increase was also prevented by the antioxidant vitamin E. In addition, the lipid extract of OxLDL, which includes the lipid peroxidation products, reproduced the action of the OxLDL particle itself. OxLDL and its lipid extract induced an oxidative stress, as assessed by the intracellular levels of reactive oxygen species and lipid peroxidation products. Finally, OxLDL induced a dose-dependent inhibition of DNA synthesis as assessed by thymidine incorporation. These results demonstrate that OxLDL or its lipid peroxidation products, by generation of an oxidative stress, enhances the expression of p21 and Rb genes, leading to an accumulation of the Hypo-P active form of the tumor suppressor Rb. This phenomenon is in accordance with the fact that p21 is a mediator of p53-dependent cell-cycle arrest in G1 and is most probably involved in the cytotoxicity of OxLDL.

The amino-terminus of the E2F-1 transcription factor inhibits DNA replication of autonomously replicating plasmids in mammalian cells

The E2F1 transcription factor plays a pivotal role in driving cells out of a quiescent state and into the S phase of the cell cycle, in part by transactivating genes needed for DNA replication including DHFR, thymidine kinase, and DNA Polymerase alpha. E2F1 has also been implicated in regulating an S phase checkpoint, however its role in this checkpoint is not well defined. To determine how E2F1 affects such a checkpoint, we utilized an in vivo replication assay employing a plasmid based SV40 origin of replication, transfected into cells expressing SV40 large T antigen. Here we show that expression of full length E2F1, or only its N terminus, represses replication from plasmids containing the SV40 origin, while N terminal deletions of E2F1 do not. E2F1 appears to inhibit the elongation phase of replication and not the initiation phase since it does not affect the replication of other cotransfected plasmids containing only the SV40 origin. Further, inhibition of replication is dependent on both the amino-terminus of the E2F1 protein and on a DNA sequence that is contained within the 3' end of the E2F1 cDNA. Additionally, both full-length E2F1, or just its N-terminus, form protein complexes with two portions of the 3' end of the E2F1 cDNA. These data provide a clue to the mechanism by which E2F1 regulates transit through the S phase checkpoint, by acting on a specific DNA sequence via its amino-terminal region, to inhibit elongation of DNA replication.

Gene expression changes in response to E2F1 activation

The p16/RB/E2F regulatory pathway, which controls transit through the G1 restriction point of the cell cycle, is one of the most frequent targets of genetic alterations in human cancer. Any of these alterations results in the deregulated expression of the transcription factor E2F, one of the key mediators of cell cycle progression. Under these conditions, E2F1 also participates in the induction of apoptosis by a p53-dependent pathway, and independently of p53. Recently, we identified the p53-homolog p73 as a first direct target of p53-independent apoptosis. Here, we used a cDNA microarray to screen an inducible E2F1-expressing Saos-2 cell line for E2F1 target genes. Expression analysis by cDNA microarray and RT-PCR revealed novel E2F1 target genes involved in E2F1-regulated cellular functions such as cell cycle control, DNA replication and apoptosis. In addition, the identification of novel E2F1 target genes participating in the processes of angiogenesis, invasion and metastasis supports the view that E2F1 plays a central role in many aspects of cancer development. These results provide new insight into the role of E2F1 in tumorigenesis as a basis for the development of novel anti-cancer therapeutics.

Classical and novel retinoids: their targets in cancer therapy

Retinoids are important mediators of cellular growth and differentiation. Retinoids modulate the growth of both normal and malignant cells through their binding to retinoid nuclear receptors and their subsequent activation. While retinoids have demonstrated therapeutic efficacy in the treatment of acute promyelocytic leukemia, their spectrum of activity remains limited. Other agents such as histone deacetylase inhibitors may significantly increase retinoid activity in a number of malignant cell types. The novel retinoids N-(4-hydroxyphenyl) retinamide (4-HPR) and 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437; AHPN) induce apoptosis in a wide variety of malignant cells. Their mechanism(s) of action remain unclear, although a number of potential targets have been identified. Whether the retinoid receptors are involved in 4-HPR and CD473/AHPN mediated apoptosis remains unclear. Both 4-HPR and CD437/AHPN display significant potential as therapeutic agents in the treatment of a number of premalignant and malignant conditions.

Regulation of E2F1-dependent gene transcription and apoptosis by the ETS-related transcription factor GABPgamma1

The E2F family of transcription factors comprises six related members which are involved in the control of the coordinated progression through the G(1)/S-phase transition of cell cycle or in cell fate decision. Their activity is regulated by pocket proteins, including pRb, p107, and p130. Here we show that E2F1 directly interacts with the ETS-related transcription factor GABPgamma1 in vitro and in vivo. The binding domain interacting with GABPgamma1 was mapped to the C-terminal amino acids 310 to 437 of E2F1, which include its transactivation and pRb binding domain. Among the E2F family of transcription factors, the interaction with GABPgamma1 is restricted to E2F1. DNA-binding E2F1 complexes containing GABPgamma1 are characterized by enhanced E2F1-dependent transcriptional activity. Moreover, GABPgamma1 suppresses E2F1-dependent apoptosis by mechanisms other than the inhibition of the transactivation capacity of E2F1. In summary, our results provide evidence for a novel pRb-independent mechanism regulating E2F1-dependent transcription and apoptosis.

Mild heat shock induces cyclin D1 synthesis through multiple Ras signal pathways

Hyperthermia such as that occurring during fever may improve cell survival during infection, although its mechanism of action is largely unknown. Here we show that acute exposure to mild, but not severe, heat shock induces the synthesis of cyclin D1 that plays a critical role(s) in G1 progression of the cell cycle. This induction seemed to be regulated through multiple Ras signal pathways involving extracellular signal-regulated kinase, phosphatidylinositol 3-kinase, and Rac1/NADPH oxidase, all of which have well been documented to be responsible for growth factor-induced cyclin D1 expression. In a physiological sense, mild heat shock may regulate cell proliferation through inducing cyclin D1 along with growth factors.

Role of EP(1) and EP(4) PGE(2) subtype receptors in serum-induced 3T6 fibroblast cycle progression and proliferation

Recent studies have suggested that prostaglandin E(2) (PGE(2)) subtype receptors (EP) are involved in cellular proliferation and tumor development. We studied the role of EP(1) and EP(4) PGE(2) subtype receptor antagonists AH-6809 and AH-23848B, respectively, in serum-induced 3T6 fibroblast proliferation. This was significantly reduced in a dose-dependent manner (IC(50) approximately 100 and approximately 30 microM, respectively) to an almost complete inhibition, without any cytotoxic effect. However, the effect of each antagonist on 3T6 cell cycle progression clearly differed. Whereas the EP(1) antagonist increased the G(0)/G(1) population, the EP(4) antagonist brought about an accumulation of cells in early S phase. These effects were associated with a decrease in cyclin D and E levels in AH-6809-treated 3T6 cells and lower cyclin A levels in AH-23848B-treated fibroblasts with respect to control cells. The G(0)/G(1) accumulation caused by AH-6809 seems to be intracellular Ca(2+) concentration ([Ca(2+)](i)) dependent, because a 6-h 1 microM thapsigargin treatment allowed G(0)/G(1)-arrested cells to enter S phase. Similarly, treatment with 20 microM forskolin for 6 h allowed S-phase and G(2)/M progression of AH-23848B-treated cells. This study shows that the inhibitory effect of the EP(1) and EP(4) antagonists on serum-induced 3T6 fibroblast growth is due to their effect at various levels of the cell cycle machinery, suggesting that PGE(2) interaction with its different subtype receptors regulates progression through the cell cycle by modulating cAMP and [Ca(2+)](i).

Structure-based design of cyclin-dependent kinase inhibitors

The eukaryotic cell cycle is tightly regulated by the sequential activation and deactivation of the cyclin-dependent kinases (CDKs). Aberrant CDK activity is a common defect in human tumours, and clinically, it often confers a poor prognosis. The strong genetic link between CDKs and the molecular pathology of cancer has provided the rationale for developing small-molecule inhibitors of these kinases. X-ray crystallography recently has revealed the molecular details of CDK regulation by cyclin binding and phosphorylation, and by complex formation with endogenous inhibitors. Knowledge of the structure of CDK2 has been key in driving the design and development of a large number of ATP competitive inhibitors. Crystallography has revealed that the ATP-binding site of CDK2 can accommodate a number of diverse molecular frameworks, exploiting various sites of interaction. In addition, residues outside the main ATP-binding cleft have been identified that could be targeted to increase specificity and potency. These results suggest that it may be possible to design pharmacologically relevant ligands that act as specific and potent inhibitors of CDK activity. We provide a review of the current state of the field, along with an overview of our current inhibitor design studies.

Infection of cells with replication deficient adenovirus induces cell cycle alterations and leads to downregulation of E2F-1

Gene products of recombinant replication-deficient adenovirus vectors of the first generation (Ad vector) can induce cell cycle dysregulation and apoptosis after infection in eukaryotic cells. The mechanisms underlying this complex process are largely unknown. Therefore, we investigated the regulation of the pRb/E2F-1 complex, which controls transition from G(0)/G(1) to S phase of the cell cycle. As Ad vector infection results in a decrease in the number of cells in G(0)/G(1) phase of the cell cycle, we observed a decline of the pRb protein level and, surprisingly, also a decrease of the E2F-1 protein and mRNA level in infected cell lines. Furthermore, in contrast to the reduction of cells in the G(0)/G(1) phase we observed increased protein levels of p53 and p21 proteins. However, as experiments in p53 deficient cell lines indicated, the decrease of pRb and E2F-1 is independent of p53 and p21 expression. Moreover, results obtained with Rb deficient cell lines indicated that the reduced E2F-1 expression is independent of pRb. These results suggest that Ad vector-induced cell cycle dysregulation is associated with a specific downregulation of E2F-1 independent of Rb and p53 genomic status of cells.

Apoptosis: a link between cancer genetics and chemotherapy

Defects in apoptosis underpin both tumorigenesis and drug resistance, and because of these defects chemotherapy often fails. Understanding the molecular events that contribute to drug-induced apoptosis, and how tumors evade apoptotic death, provides a paradigm to explain the relationship between cancer genetics and treatment sensitivity and should enable a more rational approach to anticancer drug design and therapy.

Novel function of the cyclin A binding site of E2F in regulating p53-induced apoptosis in response to DNA damage

We demonstrate here that the E2F1 induced by DNA damage can bind to and promote the apoptotic function of p53 via the cyclin A binding site of E2F1. This function of E2F1 does not require its DP-1 binding, DNA binding, or transcriptional activity and is independent of mdm2. All the cyclin A binding E2F family members can interact and cooperate with p53 to induce apoptosis. This suggests a novel role for E2F in regulating apoptosis in response to DNA damage. Cyclin A, but not cyclin E, prevents E2F1 from interacting and cooperating with p53 to induce apoptosis. However, in response to DNA damage, cyclin A levels decrease, with a concomitant increase in E2F1-p53 complex formation. These results suggest that the binding of E2F1 to p53 can specifically stimulate the apoptotic function of p53 in response to DNA damage.

Disruption of cell cycle kinetics and cyclin-dependent kinase system by ethanol in cultured cerebellar granule progenitors

An in vitro model of neuronal precursors, primary culture of cerebellar granule progenitors (CGPs), was used to investigate the mechanisms underlying ethanol-induced cell cycle damage. The CGP cultures were generated from 3-day-old rats. Ethanol significantly inhibited the proliferation of the CGPs in culture. Analysis of cell cycle kinetics by a cumulative 5-bromo-2'-deoxyuridine (BrdU) labeling technique demonstrated that ethanol exposure increased the duration of the cell cycle and decreased the growth fraction (the cycling population). The duration of the S-phase and total cell cycle was significantly prolonged by ethanol exposure by 220% and 135%, respectively, while the growth fraction was decreased from 44% in the control groups to 22% in the ethanol-exposed cultures. Cyclin-dependent kinase 2 (Cdk2) is a key protein that regulates both the passage from G1 into S, and the S phase progression. The results from in vitro phosphorylation assay and Western blot demonstrated that ethanol dramatically down-regulated both the activity and the expression of Cdk2. In addition, ethanol significantly decreased the expression of Cyclin A and Cyclin D(2). Further studies using in situ TUNEL assay and DNA fragmentation ELISA showed that ethanol caused a delayed apoptosis, i.e. the ethanol-induced apoptosis was evident only after chronic exposure. On the other hand, ethanol did not affect the necrotic index. In conclusion, ethanol decreases the cycling pool of CGPs by inducing cell cycle delay and promoting apoptosis. Ethanol-mediated disturbance of the cyclin-dependent kinase system may be an important mechanism to account for cell cycle arrest in neuronal precursor cells.

Feedback regulation of the MBF transcription factor by cyclin Cig2

The Mlu1-binding factor (MBF) from the fission yeast Schizosaccharomyces pombe contains the proteins Res1p and Res2p and binds to the Mlu1 cell-cycle box (MCB) element in DNA, activating the transcription of genes required for S phase. We report here that the cell-cycle-regulated expression of the cyclin cig2 gene is dependent on MBF. Deletion of MCB elements in the cig2 promoter perturbed the expression not only of cig2 but also of other MBF-dependent genes, indicating that Cig2p could regulate MBF activity. Cig2p can bind to Res2p, promote the phosphorylation of Res1p and inhibit MBF-dependent gene transcription. Cig2p thus forms an autoregulating feedback-inhibition loop with MBF which is important for normal regulation of the cell cycle.

Effect of naturally occurring E2F-4 alterations on transcriptional activation and proliferation in transfected cells

E2F is a family of transcription factors implicated in the regulation of gene expression required for progression through the G(1)-S transition. We have previously detected tumor-specific mutations at a trinucleotide repeat coding sequence of E2F-4 gene in a subset of human sporadic colorectal cancers. The purpose of this study was to investigate the potential functional consequences of these E2F-4 mutations. We transfected NIH3T3 fibroblasts with expression constructs containing wild-type as well as mutant E2F-4 cDNA, and the effect of the E2F-4 mutations on proliferation was examined. Alteration in transactivation of the E2F consensus promoter sequence was also examined by transient cotransfection of a E2F-4 with a DP-2 construct into cultured human cells. Transfected cell clones overexpressing mutant E2F-4 grew more rapidly and showed higher proliferative activity by increased immunohistochemical staining for proliferating cell nuclear antigen (PCNA). All three mutant forms of E2F-4 showed elevated transactivation of the E2F consensus promoter sequence. Thus, expression of mutant E2F-4s confers a growth advantage in vivo, and this effect may be related to the acquisition of a neoplastic phenotype.

Inhibitor binding to active and inactive CDK2: the crystal structure of CDK2-cyclin A/indirubin-5-sulphonate

BACKGROUND

Cyclin-dependent kinase 2 (CDK2) is an important target for structure-based design of antitumor agents. Monomeric CDK2 is inactive. Activation requires rearrangements to key structural elements of the enzyme's active site, which accompany cyclin binding and phosphorylation. To assess the validity of using monomeric CDK2 as a model for the active kinase in structure-based drug design, we have solved the structure of the inhibitor indirubin-5-sulphonate (E226) complexed with phospho-CDK2-cyclin A and compared it with the structure of E226 bound to inactive, monomeric CDK2.

RESULTS

Activation of monomeric CDK2 leads to a rotation of its N-terminal domain relative to the C-terminal lobe. The accompanying change in position of E226 follows that of the N-terminal domain, and its interactions with residues forming part of the adenine binding pocket are conserved. The environment of the ATP-ribose site, not explored by E226, is significantly different in the binary complex compared to the monomeric complex due to movement of the glycine loop. Conformational changes also result in subtle differences in hydrogen bonding and electrostatic interactions between E226's sulphonate and CDK2's phosphate binding site. Affinities calculated by LUDI for the interaction of E226 with active or inactive CDK2 differ by a factor of approximately ten.

CONCLUSIONS

The accuracy of monomeric CDK2 as an inhibitor design template is restricted to the adenine binding site. The general flexibility observed for the glycine loop and subtle changes to the phosphate binding site suggest a need to study interactions between inhibitors and active CDK2 in structure-based drug design programs.

Cyclin A-CDK phosphorylates Sp1 and enhances Sp1-mediated transcription

Cyclin A-mediated activation of cyclin-dependent kinases (CDKs) is essential for cell cycle transversal. Cyclin A activity is regulated on several levels and cyclin A elevation in a number of cancers suggests a role in tumorigenesis. In the present study, we used a modified DNA binding site selection and PCR amplification procedure to identify DNA binding proteins that are potential substrates of cyclin A-CDK. One of the sequences identified is the Sp1 transcription factor binding site. Co-immunoprecipitation experiments show that cyclin A and Sp1 can interact physically. In vitro and in vivo phosphorylation studies indicate that cyclin A-CDK complexes can phosphorylate Sp1. The phosphorylation site is located in the N-terminal region of the protein. Cells overexpressing cyclin A have elevated levels of Sp1 DNA binding activity, suggesting that cyclin A-CDK-mediated phosphorylation augments Sp1 DNA binding properties. In co-transfection studies, cyclin A expression stimulated transcription from an Sp1-regulated promoter. Mutation of the phosphorylation site abrogated cyclin A-CDK-dependent phosphorylation, augmentation of Sp1 transactivation function and DNA binding activity.

Transcriptional repression by the retinoblastoma protein through the recruitment of a histone methyltransferase

The E2F transcription factor controls the cell cycle-dependent expression of many S-phase-specific genes. Transcriptional repression of these genes in G(0) and at the beginning of G(1) by the retinoblasma protein Rb is crucial for the proper control of cell proliferation. Rb has been proposed to function, at least in part, through the recruitment of histone deacetylases. However, recent results indicate that other chromatin-modifying enzymes are likely to be involved. Here, we show that Rb also interacts with a histone methyltransferase, which specifically methylates K9 of histone H3. The results of coimmunoprecipitation experiments of endogenous or transfected proteins indicate that this histone methyltransferase is the recently described heterochromatin-associated protein Suv39H1. Interestingly, phosphorylation of Rb in vitro as well as in vivo abolished the Rb-Suv39H1 interaction. We also found that Suv39H1 and Rb cooperate to repress E2F activity and that Suv39H1 could be recruited to E2F1 through its interaction with Rb. Taken together, these data indicate that Suv39H1 is involved in transcriptional repression by Rb and suggest an unexpected link between E2F regulation and heterochromatin.

Identification of E2F-1/Cyclin A antagonists

A simple method for the synthesis of a rationally designed (S,S)-[Pro-Leu]-spirolactam scaffold is described. This was expanded to a small biased library of compounds mimicking the 'ZRXL' motif in order to identify E2F-1/Cyclin A antagonists. The synthesized compounds were evaluated in an E2F-1/Cyclin A binding assay and moderately active analogues were identified. In addition, the critical roles of Phe, Leu, Lys, and Arg residues of the identified motif were determined.

E2F1-mediated transcriptional inhibition of the plasminogen activator inhibitor type 1 gene

Gene expression of the plasminogen activation system is cell-cycle dependent. Previously, we showed that ectopic expression of E2F1 repressed the plasminogen activator inhibitor type 1 (PAI-1) promoter in a manner dependent on the presence of DNA-binding and transactivation domains of E2F1 but independent of binding to pocket-binding proteins, suggesting a novel mechanism for E2F-mediated negative gene regulation [Koziczak, M., Krek, W. & Nagamine, Y. (2000) Mol. Cell. Biol. 20, 2014-2022]. However, it remains to be seen whether endogenous E2F can exert a similar effect. We report here that down-regulation of PAI-1 gene expression correlates with an increase in endogenous E2F activity. When cells were treated with a cdk2/4-specific inhibitor, which maintains E2F in an inactive state, the decline of serum-induced PAI-1 mRNA levels was suppressed. In mutant U2OS cells expressing a temperature-sensitive retinoblastoma protein (pRB), a shift to a permissive temperature induced PAI-1 mRNA expression. In U2OS cells stably expressing an E2F1-estrogen receptor chimeric protein that could be activated by tamoxifen, PAI-1 gene transcription was markedly reduced by tamoxifen even in the presence of cycloheximide. These results all indicate that endogenous E2F can directly repress the PAI-1 gene. DNase I hypersensitive-site analysis of the PAI-1 promoter suggested the involvement of conformation changes in chromatin structure of the PAI-1 promoter. 5' deletion analysis of the PAI-1 promoter showed that multiple sites were responsible for the E2F negative regulation, some of which were promoter dependent. Interestingly, one of these sites is a p53-binding element.

Expression of E2F-1 and E2F-4 is reduced in primary and metastatic breast carcinomas

The E2F family of transcription factors can induce both cell proliferation and apoptosis. Whether they function as oncogenes or tumor suppressors appears to be tissue specific. Their role in breast carcinogenesis remains unclear. We found a decreased expression of E2F-1 and E2F-4 in 70% (7/10) of primary breast carcinomas and in all (10/10) metastatic nodal tissues when compared with the corresponding normal breast tissue. No tumor-specific mutation was detected, but polymorphisms were identified in E2F-1 exon 5 and in the polyserine tract of E2F-4. The presence of polymorphisms did not correlate with E2F expression. Among the 12 human breast cancer cell lines, one contained a missense mutation in E2F-1 exon 2. Five (42%) cell lines overexpressed E2F-1, while three (25%) expressed low levels of the protein. Our results suggest that not only are the E2Fs likely to function as tumor suppressors in breast cancer, but also that their down-regulation may be important in the development of metastases.

Cell cycle aberrations in the pathogenesis of squamous cell carcinoma of the uterine cervix

Cancer cells are characterized by limitless proliferative autonomy and immunity to inhibitory and apoptotic signals, thus ensuring growth and metastasis [1]. Epidemiological studies have long implicated human papillomavirus (HPV) as a pathogenic agent in cervical cancer. Progress in cancer research now provides an understanding of how these characteristics are achieved by the interaction of HPV proteins with the cell cycle machinery. Expression of oncoproteins E7 and E6 induces immortalization of cells through their inhibitory effects on tumor suppressor proteins pRb and p53, respectively. Undermining of pRb's growth-inhibitory role with release of E2F transcription factors renders the cells independent of mitogenic stimuli. The abundance of growth transcription factors grants limitless proliferative potential by allowing expression of products such as cyclins A, E, and B, dihydrofolate reductase, and DNA polymerase which fuel the various stages of the cell cycle. There is subsequent disruption of both the G1-S and G2-M cell cycle checkpoints. Overexpression of cyclin E results in chromosomal instability and possible unmasking of genetic mutations, allowing disease progression. Cyclin A grants anchorage-independent growth, facilitating tissue invasion and tumor spread. Apoptotic and growth-inhibitory mechanisms are also evaded. p53 is degraded by E6 and its own downstream protein mdm2. Its other downstream protein, p21 is rendered ineffective against cyclin-cyclin-dependent kinase units by E7, as is p27. The understanding of the molecular pathology of disease will provide us with the ability to prognosticate and treat patients more effectively.

The histone deacetylase HDAC3 targets RbAp48 to the retinoblastoma protein

The product of the retinoblastoma susceptibility gene, the Rb protein, functions partly through transcriptional repression of E2F-regulated genes. Repression by Rb is mediated, at least in part, by a histone deacetylase complex, whose enzymatic activity relies on HDAC1, HDAC2 or HDAC3. Recently, we have shown that the Rb-associated histone deacetylase complex contains RbAp48 protein, which interacts with HDAC1 and HDAC2. RbAp48 could favour the deacetylation of histones since it binds directly to histone H4. In agreement with that, we show that transcriptional repression of E2F activity requires the presence of RbAp48. HDAC3 was thought not to interact with RbAp48. However, we found that it shared with HDAC1 the ability to favour the recruitment of RbAp48 to Rb. This latter effect was unlikely to be due to activation of Rb function, since HDAC3 did not increase Rb-E2F1 interaction. Rather, we found, surprisingly, that HDAC3 could physically interact with RbAp48 both in vitro and in living cells. Taken together, our data suggest a model in which Rb mediates the recruitment to E2F-regulating promoters of a repressive complex containing either HDAC1, HDAC2 or HDAC3 and the histone-binding protein RbAp48.

The long form of CDK2 arises via alternative splicing and forms an active protein kinase with cyclins A and E

We have reinvestigated the long form of cyclin-dependent kinase (CDK)2 that is expressed in many rodent cells. We show that the mRNA encoding CDK2L arises by alternative splicing and that the encoded protein can bind to, and be activated by, cyclins A and E. The complex of CDK2L with cyclin A has about half the specific activity of the equivalent CDK2-cyclin A complex. Also, CDK2L--cyclin A is inhibited to the same extent and by the same concentrations of p21(CIP1) as CDK2--cyclin A. The nucleotide sequences of intron V in the human and murine CDK2 genes, where the sequences encoding the 48-residue insert in CDK2L are located, show very high conservation in the position of the alternatively spliced exon and its surroundings. Despite this, we were not able to detect significant expression of CDK2L in human cell lines, although a low level is expressed in COS-1 cells from monkeys.

Ca2+ and BMP-6 signaling regulate E2F during epidermal keratinocyte differentiation

The epidermis consists of a squamous epithelium continuously replenished by committed stem cells, which can either self-renew or differentiate. We demonstrated previously that E2F genes are differentially expressed in developing epidermis (Dagnino, L., Fry, C. J., Bartley, S. M., Farnham, P., Gallie, B. L., and Phillips, R. A. (1997) Cell Growth Differ. 8, 553-563). Thus, we hypothesized that various E2F proteins likely play distinct growth regulatory roles in the undifferentiated stem cells and in terminally differentiated keratinocytes. To further understand the function of E2F genes in epidermal morphogenesis, we have examined the expression, regulation, and protein-protein interactions of E2F factors in undifferentiated cultured murine primary keratinocytes or in cells induced to differentiate with Ca(2+) or BMP-6 (bone morphogenetic protein 6). We find similar patterns of E2F regulation with both differentiating agents and demonstrate a switch in expression from E2F-1, -2, and -3 in undifferentiated, proliferating cells to E2F-5 in terminally differentiated keratinocytes. Inhibition of keratinocyte proliferation by transforming growth factor-beta1 did not enhance E2F-5 protein levels, suggesting that this response is specific to differentiation rather than reversible cell cycle withdrawal. E2F-5 up-regulation is also accompanied by formation of heteromeric nuclear complexes containing E2F5, p130, and histone deacetylase (HDAC) 1. Overexpression of E2F5 specifically inhibited DNA synthesis in undifferentiated keratinocytes in an HDAC-dependent manner, suggesting that E2F-5.p130.HDAC1 complexes are likely involved in the permanent withdrawal from the cell cycle of keratinocytes responding to differentiation stimuli.

[Molecular mechanisms controlling the cell cycle: fundamental aspects and implications for oncology]

INTRODUCTION

Comprehension of cell cycle regulation mechanisms has progressed very quickly these past few years and regulators of the cell cycle have gained widespread importance in cancer. This review first summarizes major advances in the understanding of the control of cell cycle mechanisms. Examples of how this control is altered in tumoral cells are then described.

CURRENT KNOWLEDGE AND KEY POINTS

The typical mammalian cell cycle consists of four distinct phases occurring in a well-defined order, each of which should be completed successfully before the next begins. Progression of eukaryotic cells through major cell cycle transitions is mediated by sequential assembly and activation of a family of serine-threonine protein kinases, the cyclin dependent kinases (CDK). The timing of their activation is determined by their post-translational modifications (phosphorylations/dephosphorylations), and by the association of a protein called cyclin, which is the regulatory subunit of the kinase complex. The cyclin family is divided into two main classes. The 'G1 cyclins' include cyclins C, D1-3, and E, and their accumulation is rate-limiting for progression from the G1 to S phase. The 'mitotic or G2 cyclins', which include cyclin A and cyclin B, are involved in the control of G2/M transition and mitosis. The cyclins bind to and activate the CDK, which leads to phosphorylation (and then inhibition) of the tumor suppressor protein, pRb. pRb controls commitment to progress from the G1 to S phase, at least in part by repressing the activity of the E2F transcription factors known to promote cell proliferation. Both the D-type cyclins and their partner kinases CDK4/6 have proto-oncogenic properties, and their activity is carefully regulated at multiple levels including negative control by two families of CDK inhibitors. While members of the INK4 family (p16INK4A, p15INK4B, p18INK4C, p19INK4D) interact specifically with CDK4 and CDK6, the CIP/KIP inhibitors p21CIP1/WAF1, p27KIP1 and p57KIP2 inhibit a broader spectrum of CDK. The interplay between p16INK4A, cyclin D/CDK, and pRb/E2F together constitute a functional unit collectively known as the 'pRb pathway'. Each of the major components of this mechanism may become deregulated in cancer, and accumulating evidence points to the 'pRb pathway' as a candidate obligatory target in multistep oncogenesis of possibly all human tumor types.

FUTURE PROSPECTS AND PROJECTS

Major advances in the understanding of cell cycle regulation mechanisms provided a better knowledge of the molecular interactions involved in human cancer. This progress has led to the promotion of new therapeutic agents presently in clinical trials or under development. Moreover, the components of the cell cycle are probably involved in other non-cancerous diseases and their role must be defined.

Retinoblastoma tumor suppressor protein signals through inhibition of cyclin-dependent kinase 2 activity to disrupt PCNA function in S phase

The retinoblastoma tumor suppressor protein (RB) is a negative regulator of the cell cycle that inhibits both G(1) and S-phase progression. While RB-mediated G(1) inhibition has been extensively studied, the mechanism utilized for S-phase inhibition is unknown. To delineate the mechanism through which RB inhibits DNA replication, we generated cells which inducibly express a constitutively active allele of RB (PSM-RB). We show that RB-mediated S-phase inhibition does not inhibit the chromatin binding function of MCM2 or RPA, suggesting that RB does not regulate the prereplication complex or disrupt early initiation events. However, activation of RB in S-phase cells disrupts the chromatin tethering of PCNA, a requisite component of the DNA replication machinery. The action of RB was S phase specific and did not inhibit the DNA damage-mediated association of PCNA with chromatin. We also show that RB-mediated PCNA inhibition was dependent on downregulation of CDK2 activity, which was achieved through the downregulation of cyclin A. Importantly, restoration of cyclin-dependent kinase 2 (CDK2)-cyclin A and thus PCNA activity partially restored S-phase progression in the presence of active RB. Therefore, the data presented identify RB-mediated regulation of PCNA activity via CDK2 attenuation as a mechanism through which RB regulates S-phase progression. Together, these findings identify a novel pathway of RB-mediated replication inhibition.

Efficient chondrogenic differentiation of mesenchymal cells in micromass culture by retroviral gene transfer of BMP-2

The multipotential murine embryonic C3H10T1/2 mesenchymal cell line is able to undergo chondrogenesis in vitro, in a high density micromass environment, following treatment with soluble human bone morphogenetic protein-2 (BMP-2). To enhance this process, the human BMP-2 cDNA was cloned into a retroviral expression vector and a high titer, infectious retrovirus (replication defective) was generated. Infection of C3HIOT1/2 cells with this retroviral construct resulted in an infection efficiency of 90-95% and was highly effective in converting cells in micromass culture to a chondrocyte phenotype, as assessed by positive Alcian blue staining for extracellular matrix proteoglycans, increased sulfate incorporation, increased expression of the cartilage marker genes collagen type II and aggrecan, and decreased expression of collagen type I. Interestingly, BMP-2 expression in the micromass cultures also induced the expression of the cell cycle inhibitory protein/differentiation factor p21/WAF1, suggesting its functional involvement in chondrogenesis. The chondrogenic effect of retrovirally expressed BMP-2 in these high-density cultures was limited to the infected cells, since uninfected cells did not chondrify when co-cultured as a nonoverlapping micromass adjacent to BMP-2 expressing cells. These data indicate that retrovirally expressed BMP-2 is highly effective at inducing a chondrocyte phenotype in a multipotential mesenchymal cell line in vitro, and its action is restricted to the infected cell population. These findings should provide a framework for the optimization of chondrogenesis in culture using mesenchymal stem cells and retroviral gene transfer.

A conserved cyclin-binding domain determines functional interplay between anaphase-promoting complex-Cdh1 and cyclin A-Cdk2 during cell cycle progression

Periodic activity of the anaphase-promoting complex (APC) ubiquitin ligase determines progression through multiple cell cycle transitions by targeting cell cycle regulators for destruction. At the G(1)/S transition, phosphorylation-dependent dissociation of the Cdh1-activating subunit inhibits the APC, allowing stabilization of proteins required for subsequent cell cycle progression. Cyclin-dependent kinases (CDKs) that initiate and maintain Cdh1 phosphorylation have been identified. However, the issue of which cyclin-CDK complexes are involved has been a matter of debate, and the mechanism of how cyclin-CDKs interact with APC subunits remains unresolved. Here we substantiate the evidence that mammalian cyclin A-Cdk2 prevents unscheduled APC reactivation during S phase by demonstrating its periodic interaction with Cdh1 at the level of endogenous proteins. Moreover, we identified a conserved cyclin-binding motif within the Cdh1 WD-40 domain and show that its disruption abolished the Cdh1-cyclin A-Cdk2 interaction, eliminated Cdh1-associated histone H1 kinase activity, and impaired Cdh1 phosphorylation by cyclin A-Cdk2 in vitro and in vivo. Overexpression of cyclin binding-deficient Cdh1 stabilized the APC-Cdh1 interaction and induced prolonged cell cycle arrest at the G(1)/S transition. Conversely, cyclin binding-deficient Cdh1 lost its capability to support APC-dependent proteolysis of cyclin A but not that of other APC substrates such as cyclin B and securin Pds1. Collectively, these data provide a mechanistic explanation for the mutual functional interplay between cyclin A-Cdk2 and APC-Cdh1 and the first evidence that Cdh1 may activate the APC by binding specific substrates.

S and G2 phase roles for Cdk2 revealed by inducible expression of a dominant-negative mutant in human cells

Cyclin-dependent kinase 2 (Cdk2) is essential for initiation of DNA synthesis in higher eukaryotes. Biochemical studies in Xenopus egg extracts and microinjection studies in human cells have suggested an additional function for Cdk2 in activation of Cdk1 and entry into mitosis. To further examine the role of Cdk2 in human cells, we generated stable clones with inducible expression of wild-type and dominant-negative forms of the enzyme (Cdk2-wt and Cdk2-dn, respectively). Both exogenous proteins associated efficiently with endogenous cyclins. Cdk2-wt had no apparent effect on the cell division cycle, whereas Cdk2-dn inhibited progression through several distinct stages. Cdk2-dn induction could arrest cells at the G1/S transition, as previously observed in transient expression studies. However, under normal culture conditions, Cdk2-dn induction primarily arrested cells with S and G2/M DNA contents. Several observations suggested that the latter cells were in G2 phase, prior to the onset of mitosis: these cells contained uncondensed chromosomes, low levels of cyclin B-associated kinase activity, and high levels of tyrosine-phosphorylated Cdk1. Furthermore, Cdk2-dn did not delay progression through mitosis upon release of cells from a nocodazole block. Although the G2 arrest imposed by Cdk2-dn was similar to that imposed by the DNA damage checkpoint, the former was distinguished by its resistance to caffeine. These findings provide evidence for essential functions of Cdk2 during S and G2 phases of the mammalian cell cycle.

Additive effect of adenovirus-mediated E2F-1 gene transfer and topoisomerase II inhibitors on apoptosis in human osteosarcoma cells

Recently, it has been demonstrated that Etoposide, a topoisomerase II inhibitor, can induce apoptosis in MDM2-overexpressing tumor cells by inhibition of MDM2 synthesis. We have previously shown that E2F-1 overexpression induces apoptosis of MDM2-overexpressing sarcoma cells, which is related to the inhibition of MDM2 expression. Therefore, the present study was designed to investigate the in vitro and in vivo effect of combined treatment of adenovirus-mediated E2F-1 and topoisomerase II inhibitors on the growth inhibition and apoptosis in human sarcoma cells. Two human sarcoma cell lines, OsACL and U2OS, were treated with topoisomerase II inhibitors (Etoposide and Adriamycin), alone or in combination with adenoviral vectors expressing beta-galactosidase (Ad-LacZ) or E2F-1 (Ad-E2F-1). E2F-1 expression was confirmed by Western blot analysis. Ad-E2F-1 gene transfer at a low dose (multiplicity of infection, 2) markedly increased the sensitivity of human sarcoma cells to topoisomerase II inhibitor treatment. This cooperative effect of E2F-1 and topoisomerase II inhibitors was less marked in SAOS-2 cells (p53 and pRb null). Topoisomerase II inhibitors also cooperated with E2F-1 overexpression to enhance tumor cell killing in an in vivo model using xenografts in nude mice. When combined with Adriamycin or Etoposide, E2F-1 adenovirus therapy resulted in approximately 95% and 85% decrease in tumor size, respectively, compared to controls (P<.05). These results suggest a new chemosensitization strategy that is effective in MDM2-overexpressing tumors and may have clinical utility.

S and G2 phase roles for Cdk2 revealed by inducible expression of a dominant-negative mutant in human cells

Cyclin-dependent kinase 2 (Cdk2) is essential for initiation of DNA synthesis in higher eukaryotes. Biochemical studies in Xenopus egg extracts and microinjection studies in human cells have suggested an additional function for Cdk2 in activation of Cdk1 and entry into mitosis. To further examine the role of Cdk2 in human cells, we generated stable clones with inducible expression of wild-type and dominant-negative forms of the enzyme (Cdk2-wt and Cdk2-dn, respectively). Both exogenous proteins associated efficiently with endogenous cyclins. Cdk2-wt had no apparent effect on the cell division cycle, whereas Cdk2-dn inhibited progression through several distinct stages. Cdk2-dn induction could arrest cells at the G1/S transition, as previously observed in transient expression studies. However, under normal culture conditions, Cdk2-dn induction primarily arrested cells with S and G2/M DNA contents. Several observations suggested that the latter cells were in G2 phase, prior to the onset of mitosis: these cells contained uncondensed chromosomes, low levels of cyclin B-associated kinase activity, and high levels of tyrosine-phosphorylated Cdk1. Furthermore, Cdk2-dn did not delay progression through mitosis upon release of cells from a nocodazole block. Although the G2 arrest imposed by Cdk2-dn was similar to that imposed by the DNA damage checkpoint, the former was distinguished by its resistance to caffeine. These findings provide evidence for essential functions of Cdk2 during S and G2 phases of the mammalian cell cycle.

NF-kappaB and cell-cycle regulation: the cyclin connection

The cyclins are a family of proteins that are centrally involved in cell cycle regulation and which are structurally identified by conserved "cyclin box" regions. They are regulatory subunits of holoenzyme cyclin-dependent kinase (CDK) complexes controlling progression through cell cycle checkpoints by phosphorylating and inactivating target substrates. CDK activity is controlled by cyclin abundance and subcellular location and by the activity of two families of inhibitors, the cyclin-dependent kinase inhibitors (CKI). Many hormones and growth factors influence cell growth through signal transduction pathways that modify the activity of the cyclins. Dysregulated cyclin activity in transformed cells contributes to accelerated cell cycle progression and may arise because of dysregulated activity in pathways that control the abundance of a cyclin or because of loss-of-function mutations in inhibitory proteins.Analysis of transformed cells and cells undergoing mitogen-stimulated growth implicate proteins of the NF-kappaB family in cell cycle regulation, through actions on the CDK/CKI system. The mammalian members of this family are Rel-A (p65), NF-kappaB(1) (p50; p105), NF-kappaB(2) (p52; p100), c-Rel and Rel-B. These proteins are structurally identified by an amino-terminal region of about 300 amino acids, known as the Rel-homology domain. They exist in cytoplasmic complexes with inhibitory proteins of the IkappaB family, and translocate to the nucleus to act as transcription factors when activated. NF-kappaB pathway activation occurs during transformation induced by a number of classical oncogenes, including Bcr/Abl, Ras and Rac, and is necessary for full transforming potential. The avian viral oncogene, v-Rel is an NF-kappaB protein. The best explored link between NF-kappaB activation and cell cycle progression involves cyclin D(1), a cyclin which is expressed relatively early in the cell cycle and which is crucial to commitment to DNA synthesis. This review examines the interactions between NF-kappaB signaling and the CDK/CKI system in cell cycle progression in normal and transformed cells. The growth-promoting actions of NF-kappaB factors are accompanied, in some instances, by inhibition of cellular differentiation and by inhibition of programmed cell death, which involve related response pathways and which contribute to the overall increase in mass of undifferentiated tissue.

DNA replication control through interaction of E2F-RB and the origin recognition complex

The E2F transcription factor and retinoblastoma protein control cell-cycle progression and DNA replication during S phase. Mutations in the Drosophila dE2F1 and dDP genes affect the origin recognition complex (DmORC) and initiation of replication at the chorion gene replication origin. Here we show that mutants of Rbf (an retinoblastoma protein homologue) fail to limit DNA replication. We also show that the dDP, dE2F1 and Rbf proteins are located in a complex with DmORC, and that dE2F1 and DmORC are bound to the chorion origin of replication in vivo. Our results indicate that dE2F1 and Rbf function together at replication origins to limit DNA replication through interactions with DmORC.

E2F-1 induces the stabilization of p53 but blocks p53-mediated transactivation

E2F-1 induces p53 accumulation and E2F-1 and p53 form a physical complex, which affects the ability of E2F-1 to activate transcription. We mapped the domains on E2F-1 that interact with p53 and found two p53-binding domains. To understand the functional consequences of the E2F-1/p53 association on p53 activities we identified the domains of E2F-1 that were responsible for the accumulation of p53. Unexpectedly, we found that the E2F-1 transactivation domain was dispensable for p53 induction. By contrast, further deletion of the DP-1 interaction/'marked' box domain eliminated p53 accumulation. Radiolabeling pulse/chase analysis demonstrated that E2F-1 caused post-translational stabilization of p53. Although E2F-1 caused the stabilization of p53, E2F-1 expression impaired p53-dependent transactivation. Thus, the E2F-1 : p53 interaction may provide a checkpoint function to inactivate overactive E2F-1, but the association may also inactivate p53 transactivation to allow cell cycle progression.

Investigating the hows and whys of DNA endoreduplication

Endoreduplication is a form of nuclear polyploidization that results in multiple, uniform copies of chromosomes. This process is common in plants and animals, especially in tissues with high metabolic activity, and it generally occurs in cells that are terminally differentiated. In plants, endoreduplication is well documented in the endosperm and cotyledons of developing seeds, but it also occurs in many tissues throughout the plant. It is thought that endoreduplication provides a mechanism to increase the level of gene expression, but the function of this process has not been thoroughly investigated. Numerous observations have been made of endoreduplication, or at least extra cycles of S-phase, as a consequence of mutations in genes controlling several aspects of cell cycle regulation. However, until recently there were few studies directed at the molecular mechanisms responsible for this specialized cell cycle. It is suggested that endoreduplication requires nothing more elaborate than a loss of M-phase cyclin-dependent kinase activity and oscillations in the activity of S-phase cyclin-dependent kinase.

Integration of the pRB and p53 cell cycle control pathways

Two fundamental molecular pathways, the pRB and p53 pathways, regulate cell growth and cell death. The importance of these pathways in cellular growth control is underscored by the observation that members of these pathways are found mutated in all human cancers. These two pathways have typically been studied and described independently. However, as we discuss here, recent data have revealed an intimate molecular and genetic interaction between the p53 and pRB pathways.

Biology of human papillomaviruses

Human papillomaviruses (HPVs) cause squamous cancers of epithelial surfaces, of which genital cancers are the most common. In this article we have attempted to describe the properties and functions of the viral proteins of HPV type 16, a common cause of genital cancers, and have tried to suggest how their expression may lead to a dysregulated cell which may become malignant. These viruses are attempting to replicate in terminally differentiating keratinocytes and must stimulate G1 to S-phase progression for the replication of their genome. As part of the successful completion of replication and assembly of infectious virus particles, the virus needs at least partial differentiation to occur. Therefore, at the same time as differentiation is occurring, the nuclei of infected cells are in S-phase. While the mechanisms of action of the viral proteins are not completely understood, researchers are making progress and this article strives to bring together the conclusions from some of this work.

Overexpression of E2F-1 leads to cytokine-independent proliferation and survival in the hematopoietic cell line BaF-B03

Cytokine receptors activate signals that regulate the transcription factor E2F-1, which then coordinates the expression of genes essential for DNA synthesis and cell cycle progression. Overexpression of E2F-1 most often induces S-phase entry followed by apoptosis, but in some cell types it leads to continuous proliferation and transformation. Here, it is shown that constitutive expression of E2F-1 promotes cytokine-independent proliferation in the murine pro-B cell line BaF-B03. There was no enhancement of apoptosis following cytokine withdrawal in these cells, despite the presence of intact p53-dependent apoptotic pathways. Notwithstanding the continuous presence of E2F-1, the cell cycle-dependent expression of cyclin A, cyclin B1, cyclin D1, cyclin E, and proliferating-cell nuclear antigen was restored with a pattern equivalent to that associated with cytokine stimulation. These findings provide evidence that, in the absence of cytokine, constitutive expression of E2F-1 can promote cell cycle progression and prevent apoptosis.

The F-box protein Skp2 is a ubiquitylation target of a Cul1-based core ubiquitin ligase complex: evidence for a role of Cul1 in the suppression of Skp2 expression in quiescent fibroblasts

The ubiquitin protein ligase SCF(Skp2) is composed of Skp1, Cul1, Roc1/Rbx1 and the F-box protein Skp2, the substrate-recognition subunit. Levels of Skp2 decrease as cells exit the cell cycle and increase as cells re-enter the cycle. Ectopic expression of Skp2 in quiescent fibroblasts causes mitogen-independent S-phase entry. Hence, mechanisms must exist for limiting Skp2 protein expression during the G(0)/G(1) phases. Here we show that Skp2 is degraded by the proteasome in G(0)/G(1) and is stabilized when cells re-enter the cell cycle. Rapid degradation of Skp2 in quiescent cells depends on Skp2 sequences that contribute to Cul1 binding and interference with endogenous Cul1 function in serum-deprived cells induces Skp2 expression. Furthermore, recombinant Cul1-Roc1/Rbx1-Skp1 complexes can catalyse Skp2 ubiquitylation in vitro. These results suggest that degradation of Skp2 in G(0)/G(1) is mediated, at least in part, by an autocatalytic mechanism involving a Skp2-bound Cul1-based core ubiquitin ligase and imply a role for this mechanism in the suppression of SCF(Skp2) ubiquitin protein ligase function during the G(0)/G(1) phases of the cell cycle.

NPDC-1, a regulator of neural cell proliferation and differentiation, interacts with E2F-1, reduces its binding to DNA and modulates its transcriptional activity

We have previously identified NPDC-1, a neural factor involved in the control of proliferation and differentiation, and we have shown that the stable introduction of NPDC-1 into transformed cells down-regulates cell proliferation both by increasing the generation time and by suppressing transformed properties. The data presented here indicate that, in vitro, NPDC-1 is able to interact with the transcription factor E2F-1 and some cell cycle proteins, such as D-cyclins and cdk2. In addition, two-hybrid experiments in mammalian cells show that the interaction between NPDC-1 and E2F-1 can also occur in vivo. This interaction reduces the binding of E2F-1 to DNA and its transcriptional activity. Taken together, the data suggest that NPDC-1 could influence cell cycle progression and neural differentiation through its association with E2F-1.

Nonperiodic activity of the human anaphase-promoting complex-Cdh1 ubiquitin ligase results in continuous DNA synthesis uncoupled from mitosis

Ubiquitin-proteasome-mediated destruction of rate-limiting proteins is required for timely progression through the main cell cycle transitions. The anaphase-promoting complex (APC), periodically activated by the Cdh1 subunit, represents one of the major cellular ubiquitin ligases which, in Saccharomyces cerevisiae and Drosophila spp., triggers exit from mitosis and during G(1) prevents unscheduled DNA replication. In this study we investigated the importance of periodic oscillation of the APC-Cdh1 activity for the cell cycle progression in human cells. We show that conditional interference with the APC-Cdh1 dissociation at the G(1)/S transition resulted in an inability to accumulate a surprisingly broad range of critical mitotic regulators including cyclin B1, cyclin A, Plk1, Pds1, mitosin (CENP-F), Aim1, and Cdc20. Unexpectedly, although constitutively assembled APC-Cdh1 also delayed G(1)/S transition and lowered the rate of DNA synthesis during S phase, some of the activities essential for DNA replication became markedly amplified, mainly due to a progressive increase of E2F-dependent cyclin E transcription and a rapid turnover of the p27(Kip1) cyclin-dependent kinase inhibitor. Consequently, failure to inactivate APC-Cdh1 beyond the G(1)/S transition not only inhibited productive cell division but also supported slow but uninterrupted DNA replication, precluding S-phase exit and causing massive overreplication of the genome. Our data suggest that timely oscillation of the APC-Cdh1 ubiquitin ligase activity represents an essential step in coordinating DNA replication with cell division and that failure of mechanisms regulating association of APC with the Cdh1 activating subunit can undermine genomic stability in mammalian cells.