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

Selective Occupation by E2F and RB of Loci Expressed by RNA Polymerase III

In all cases tested, TFIIIB is responsible for recruiting pol III to its genetic templates. In mammalian cells, RB binds TFIIIB and prevents its interactions with both promoter DNA and pol III, thereby suppressing transcription. As TFIIIB is not recruited to its target genes when bound by RB, the mechanism predicts that pol III-dependent templates will not be occupied by RB; this contrasts with the situation at most genes controlled by RB, where it can be tethered by promoter-bound sequence-specific DNA-binding factors such as E2F. Contrary to this prediction, however, ChIP-seq data reveal the presence of RB in multiple cell types and the related protein p130 at many loci that rely on pol III for their expression, including RMRP, RN7SL, and a variety of tRNA genes. The sets of genes targeted varies according to cell type and growth state. In such cases, recruitment of RB and p130 can be explained by binding of E2F1, E2F4 and/or E2F5. Genes transcribed by pol III had not previously been identified as common targets of E2F family members. The data provide evidence that E2F may allow for the selective regulation of specific non-coding RNAs by RB, in addition to its influence on overall pol III output through its interaction with TFIIIB.

RBL2 Regulates Cardiac Sensitivity to Anthracycline Chemotherapy

Background

Anthracycline chemotherapies cause heart failure in a subset of cancer patients. We previously reported that the anthracycline doxorubicin (DOX) induces cardiotoxicity through the activation of cyclin-dependent kinase 2 (CDK2).

Objectives

The aim of this study was to determine whether retinoblastoma-like 2 (RBL2/p130), an emerging CDK2 inhibitor, regulates anthracycline sensitivity in the heart.

Methods

Rbl2-/- mice and Rbl2+/+ littermates received DOX (5 mg/kg/wk for 4 weeks intraperitoneally, 20 mg/kg cumulative). Heart function was monitored with echocardiography. The association of RBL2 genetic variants with anthracycline cardiomyopathy was evaluated in the SJLIFE (St. Jude Lifetime Cohort Study) and CPNDS (Canadian Pharmacogenomics Network for Drug Safety) studies.

Results

The loss of endogenous Rbl2 increased basal CDK2 activity in the mouse heart. Mice lacking Rbl2 were more sensitive to DOX-induced cardiotoxicity, as evidenced by rapid deterioration of heart function and loss of heart mass. The disruption of Rbl2 exacerbated DOX-induced mitochondrial damage and cardiomyocyte apoptosis. Mechanistically, Rbl2 deficiency enhanced CDK2-dependent activation of forkhead box O1 (FOXO1), leading to up-regulation of the proapoptotic protein Bim. The inhibition of CDK2 desensitized Rbl2-depleted cardiomyocytes to DOX. In wild-type cardiomyocytes, DOX exposure induced Rbl2 expression in a FOXO1-dependent manner. Importantly, the rs17800727 G allele of the human RBL2 gene was associated with reduced anthracycline cardiotoxicity in childhood cancer survivors.

Conclusions

Rbl2 is an endogenous CDK2 inhibitor in the heart and represses FOXO1-mediated proapoptotic gene expression. The loss of Rbl2 increases sensitivity to DOX-induced cardiotoxicity. Our findings suggest that RBL2 could be used as a biomarker to predict the risk of cardiotoxicity before the initiation of anthracycline-based chemotherapy.

Cyclin E: a potential treatment target to reverse cancer chemoresistance by regulating the cell cycle

The cyclin family plays important roles in regulating the proliferative cycle of mammalian cells. Among the members of this family, cyclin E regulates multiple downstream molecules, such as the retinoblastoma susceptibility gene (RB1) and the transcription factor E2F, by interacting with cyclin-dependent kinases (CDKs) and plays an important role in the cell cycle transition from G1 to S phase. Over the years, studies have shown that cyclin E is closely related to the chemotherapy resistance of tumor cells and that its expression in tumor cells is closely related to prognosis. The dysregulated expression of cyclin E has a definite effect not only on the cell cycle regulation of tumor cells but also on the presence of low-molecular-weight cyclin E (LMW-E) and other cyclins that render tumor cells resistant. In addition, many studies in recent years have confirmed that chemotherapy resistance mediated by cyclin E can be reversed. For example, the combination of a cyclin-dependent kinase inhibitor (CKI) with anticancer drugs or the therapeutic targeting of related genes improves chemotherapy resistance by reducing the level or activity of cyclin E in tumor cells. This review summarizes the specific processes by which cyclin E regulates the cell cycle, its relationship to chemotherapy resistance in cancer, and its potential as a clinical therapeutic target to reverse chemotherapy resistance.

Divide or Commit - Revisiting the Role of Cell Cycle Regulators in Adult Hippocampal Neurogenesis

The adult dentate gyrus continuously generates new neurons that endow the brain with increased plasticity, helping to cope with changing environmental and cognitive demands. The process leading to the birth of new neurons spans several precursor stages and is the result of a coordinated series of fate decisions, which are tightly controlled by extrinsic signals. Many of these signals act through modulation of cell cycle (CC) components, not only to drive proliferation, but also for linage commitment and differentiation. In this review, we provide a comprehensive overview on key CC components and regulators, with emphasis on G₁ phase, and analyze their specific functions in precursor cells of the adult hippocampus. We explore their role for balancing quiescence versus self-renewal, which is essential to maintain a lifelong pool of neural stem cells while producing new neurons “on demand.” Finally, we discuss available evidence and controversies on the impact of CC/G₁ length on proliferation versus differentiation decisions.

Methylation of promoter of RBL1 enhances the radioresistance of three dimensional cultured carcinoma cells

Three dimensional (3D) culture in vitro is a new cell culture model that more closely mimics the physiology features of the in vivo environment and is being used widely in the field of medical and biological research. It has been demonstrated that cancer cells cultured in 3D matrices are more radioresistant compared with cells in monolayer (2D). However, the mechanisms causing this difference remain largely unclear. Here we found that the cell cycle distribution and expression of cell cycle regulation genes in 3D A549 cells are different from the 2D. The higher levels of the promotor methylation of cell cycle regulation genes such as RBL1 were observed in 3D A549 cells compared with cells in 2D. The treatments of irradiation or 5-Aza-CdR activated the demethylation of RBL1 promotor and resulted in the increased expression of RBL1 only in 3D A549 cells. Inhibition of RBL1 enhanced the radioresistance and decreased the G2/M phase arrest induced by irradiation in 2D A549 and MCF7 cells. Overexpression of RBL1 sensitized 3D cultured A549 and MCF7 cells to irradiation. Taken together, to our knowledge, it is the first time to revealthat the low expression of RBL1 due to itself promotor methylation in 3D cells enhances the radioresistance. Our finding sheds a new light on understanding the features of the 3D cultured cell model and its application in basic research into cancer radiotherapy and medcine development.

Cell type-specific effects of p27KIP1 loss on retinal development

BACKGROUND

Cyclin-dependent kinase (CDK) inhibitors play an important role in regulating cell cycle progression, cell cycle exit and cell differentiation. p27^KIP1 (p27), one of the major CDK inhibitors in the retina, has been shown to control the timing of cell cycle exit of retinal progenitors. However, the precise role of this protein in retinal development remains largely unexplored. We thus analyzed p27-deficient mice to characterize the effects of p27 loss on proliferation, differentiation, and survival of retinal cells.

METHODS

Expression of p27 in the developing and mature mouse retina was analyzed by immunohistochemistry using antibodies against p27 and cell type-specific markers. Cell proliferation and differentiation were examined in the wild-type and p27-deficient retinas by immunohistochemistry using various cell cycle and differentiation markers.

RESULTS

All postmitotic retinal cell types expressed p27 in the mouse retinas. p27 loss caused extension of the period of proliferation in the developing retinas. This extra proliferation was mainly due to ectopic cell cycle reentry of differentiating cells including bipolar cells, Müller glial cells and cones, rather than persistent division of progenitors as previously suggested. Aberrant cell cycle activity of cones was followed by cone death resulting in a significant reduction in cone number in the mature p27-deficient retinas.

CONCLUSIONS

Although expressed in all retinal cell types, p27 is required to maintain the quiescence of specific cell types including bipolar cells, Müller glia, and cones while it is dispensable for preventing cell cycle reentry in other cell types.

PP2A: more than a reset switch to activate pRB proteins during the cell cycle and in response to signaling cues

In their active hypophosphorylated state, members of the retinoblastoma family of pocket proteins negatively regulate cell cycle progression at least in part by repressing expression of E2F-dependent genes. Mitogen-dependent activation of G1 and G1/S Cyclin Dependent Kinases (CDKs) results in coordinated hyperphosphorylation and inactivation of these proteins, which no longer bind and repress E2Fs. S and G2/M CDKs maintain pocket protein hyperphosphorylated through the end of mitosis. The inactivating action of inducible CDKs is opposed by the Ser/Thr protein phosphatases PP2A and PP1. Various trimeric PP2A holoenzymes have been implicated in dephosphorylation of pocket proteins in response to specific cellular signals and stresses or as part of an equilibrium with CDKs throughout the cell cycle. PP1 has specifically been implicated in dephosphorylation of pRB in late mitosis and early G1. This review is particularly focused on the emerging role of PP2A as a major hub for integration of growth suppressor signals that require rapid inactivation of pocket proteins. Of note, activation of particular PP2A holoenzymes triggers differential activation of pocket proteins in the presence of active CDKs.

The B55α regulatory subunit of protein phosphatase 2A mediates fibroblast growth factor-induced p107 dephosphorylation and growth arrest in chondrocytes

Fibroblast growth factor (FGF)-induced growth arrest of chondrocytes is a unique cell type-specific response which contrasts with the proliferative response of most cell types and underlies several genetic skeletal disorders caused by activating FGF receptor (FGFR) mutations. We have shown that one of the earliest key events in FGF-induced growth arrest is dephosphorylation of the retinoblastoma protein (Rb) family member p107 by protein phosphatase 2A (PP2A), a ubiquitously expressed multisubunit phosphatase. In this report, we show that the PP2A-B55α holoenzyme (PP2A containing the B55α subunit) is responsible for this phenomenon. Only the B55α (55-kDa regulatory subunit, alpha isoform) regulatory subunit of PP2A was able to bind p107, and this interaction was induced by FGF in chondrocytes but not in other cell types. Small interfering RNA (siRNA)-mediated knockdown of B55α prevented p107 dephosphorylation and FGF-induced growth arrest of RCS (rat chondrosarcoma) chondrocytes. Importantly, the B55α subunit bound with higher affinity to dephosphorylated p107. Since the p107 region interacting with B55α is also the site of cyclin-dependent kinase (CDK) binding, B55α association may also prevent p107 phosphorylation by CDKs. FGF treatment induces dephosphorylation of the B55α subunit itself on several serine residues that drastically increases the affinity of B55α for the PP2A A/C dimer and p107. Together these observations suggest a novel mechanism of p107 dephosphorylation mediated by activation of PP2A through B55α dephosphorylation. This mechanism might be a general signal transduction pathway used by PP2A to initiate cell cycle arrest when required by external signals.

p53 can repress transcription of cell cycle genes through a p21(WAF1/CIP1)-dependent switch from MMB to DREAM protein complex binding at CHR promoter elements

The tumor suppressor p53 plays an important role in cell cycle arrest by downregulating transcription. Many genes repressed by p53 code for proteins with functions in G₂/M. A large portion of these genes is controlled by cell cycle-dependent elements (CDE) and cell cycle genes homology regions (CHR) in their promoters. Cyclin B2 is an example of such a gene, with a function at the transition from G₂ to mitosis. We find that p53-dependent downregulation of cyclin B2 promoter activity is dependent on an intact CHR element. In the presence of high levels of p53 or p21^WAF1/CIP1, protein binding to the CHR switches from MMB to DREAM complex by shifting MuvB core-associated proteins from B-Myb to E2F4/DP1/p130. The results suggest a model for p53-dependent transcriptional repression by which p53 directly activates p21^WAF1/CIP1. The inhibitor then prevents further phosphorylation of p130 by cyclin-dependent kinases. The presence of hypophosphorylated pocket proteins shifts the equilibrium for complex formation from MMB to DREAM. In the case of promoters that do not hold CDE or E2F elements, binding of DREAM and MMB solely relies on a CHR site. Thus, p53 can repress target genes indirectly through CHR elements.

The retinoblastoma family of proteins and their regulatory functions in the mammalian cell division cycle

The retinoblastoma (RB) family of proteins are found in organisms as distantly related as humans, plants, and insects. These proteins play a key role in regulating advancement of the cell division cycle from the G1 to S-phases. This is achieved through negative regulation of two important positive regulators of cell cycle entry, E2F transcription factors and cyclin dependent kinases. In growth arrested cells transcriptional activity by E2Fs is repressed by RB proteins. Stimulation of cell cycle entry by growth factor signaling leads to activation of cyclin dependent kinases. They in turn phosphorylate and inactivate the RB family proteins, leading to E2F activation and additional cyclin dependent kinase activity. This propels the cell cycle irreversibly forward leading to DNA synthesis. This review will focus on the basic biochemistry and cell biology governing the regulation and activity of mammalian RB family proteins in cell cycle control.

PP2A holoenzymes negatively and positively regulate cell cycle progression by dephosphorylating pocket proteins and multiple CDK substrates

Cell cycle progression is negatively regulated by the retinoblastoma family of pocket proteins and CDK inhibitors (CKIs). In contrast, CDKs promote progression through multiple phases of the cell cycle. One prominent way by which CDKs promote cell cycle progression is by inactivation of pocket proteins via hyperphosphorylation. Reactivation of pocket proteins to halt cell cycle progression requires dephosphorylation of multiple CDK-phosphorylated sites and is accomplished by PP2A and PP1 serine/threonine protein phosphatases. The same phosphatases are also implicated in dephosphorylation of multiple CDK substrates as cells exit mitosis and reenter the G1 phase of the cell cycle. This review is primarily focused on the role of PP2A and PP1 in the activation of pocket proteins during the cell cycle and in response to signaling cues that trigger cell cycle exit. Other functions of PP2A during the cell cycle will be discussed in brief, as comprehensive reviews on this topic have been published recently (De Wulf et al., 2009; Wurzenberger and Gerlich, 2011).

p107 in the public eye: an Rb understudy and more

p107 and its related family members Rb and p130 are critical regulators of cellular proliferation and tumorigenesis. Due to the extent of functional overlap within the Rb family, it has been difficult to assess which functions are exclusive to individual members and which are shared. Like its family members, p107 can bind a variety of cellular proteins to affect the expression of many target genes during cell cycle progression. Unlike Rb and p130, p107 is most highly expressed during the G1 to S phase transition of the cell cycle in actively dividing cells and accumulating evidence suggests a role for p107 during DNA replication. The specific roles for p107 during differentiation and development are less clear, although emerging studies suggest that it can cooperate with other Rb family members to control differentiation in multiple cell lineages. As a tumor suppressor, p107 is not as potent as Rb, yet studies in knockout mice have revealed some tumor suppressor functions in mice, depending on the context. In this review, we identify the unique and overlapping functions of p107 during the cell cycle, differentiation, and tumorigenesis.

Cell-context specific role of the E2F/Rb pathway in development and disease

Development of the central nervous system (CNS) requires the generation of neuronal and glial cell subtypes in appropriate numbers, and this demands the careful coordination of cell-cycle exit, survival, and differentiation. The E2F/Rb pathway is critical for cell-cycle regulation and also modulates survival and differentiation of distinct cell types in the developing and adult CNS. In this review, we first present the specific temporal patterns of expression of the E2F and Rb family members during CNS development and then discuss the genetic ablation of single or multiple members of these two families. Overall, the available data suggest a time-dependent and cell-context specific role of E2F and Rb family members in the developing and adult CNS.

E2F4 and ribonucleotide reductase mediate S-phase arrest in colon cancer cells treated with chlorophyllin

Chlorophyllin (CHL) is a water-soluble derivative of chlorophyll that exhibits cancer chemopreventive properties, but which also has been studied for its possible cancer therapeutic effects. We report here that human colon cancer cells treated with CHL accumulate in S-phase of the cell cycle, and this is associated with reduced expression levels of p53, p21, and other G(1)/S checkpoint controls. At the same time, E2F1 and E2F4 transcription factors become elevated and exhibit increased DNA binding activity. In CHL-treated colon cancer cells, bromodeoxyuridine pulse-chase experiments provided evidence for the inhibition of DNA synthesis. Ribonucleotide reductase (RR), a pivotal enzyme for DNA synthesis and repair, was reduced at the mRNA and protein level after CHL treatment, and the enzymatic activity was inhibited in a concentration-dependent manner both in vitro and in vivo. Immunoblotting revealed that expression levels of RR subunits R1, R2, and p53R2 were reduced by CHL treatment in HCT116 (p53(+/+)) and HCT116 (p53(-/-)) cells, supporting a p53-independent mechanism. Prior studies have shown that reduced levels of RR small subunits can increase the sensitivity of colon cancer cells to clinically used DNA-damaging agents and RR inhibitors. We conclude that by inhibiting R1, R2, and p53R2, CHL has the potential to be effective in the clinical setting, when used alone or in combination with currently available cancer therapeutic agents.

Cell cycle kinases as therapeutic targets for cancer

Several families of protein kinases orchestrate the complex events that drive the cell cycle, and their activity is frequently deregulated in hyperproliferative cancer cells. Although several molecules that inhibit cell cycle kinases have been developed and clinically screened as potential anticancer agents, none of these has been approved for commercial use and an effective strategy to specifically control malignant cell proliferation has yet to be established. However, recent genetic and biochemical studies have provided information about the requirement for certain cell cycle kinases by specific tumours and specialized tissue types. Here, we discuss the potential and limitations of established cell cycle kinases as targets in anticancer drug discovery as well as novel strategies for the design of new agents.

G1 to S phase cell cycle transition in somatic and embryonic stem cells

It is well known that G1 to S phase transition is tightly regulated by the expression and phosphorylation of a number of well-characterized cyclins, cyclin-dependent kinases and members of the retinoblastoma gene family. In this review we discuss the role of these components in regulation of G1 to S phase transition in somatic cells and human embryonic stem cells. Most importantly, we discuss some new tenable links between maintenance of pluripotency and cell cycle regulation in embryonic stem cells by describing the role that master transcription factors play in this process. Finally, the differences in cell cycle regulation between murine and human embryonic stem cells are highlighted, raising interesting questions regarding their biology and stages of embryonic development from which they have been derived.

From G0 to S phase: a view of the roles played by the retinoblastoma (Rb) family members in the Rb-E2F pathway

Tumor suppressor pRb/p105, pRb/p107, and pRb2/p130 genes belong to the retinoblastoma (Rb) gene family. The members of the Rb gene family and the transcription factor E2F play an essential role in regulating cell cycle and, consequently, cell proliferation. This mini-review describes the mechanisms by which Rb family members and E2F regulate cell cycle progression.

Subversion of cell cycle regulatory pathways

Human cytomegalovirus (HCMV) has evolved numerous strategies to commandeer the host cell for producing viral progeny. The virus manipulates host cell cycle pathways from the early stages of infection to stimulate viral DNA replication at the expense of cellular DNA synthesis. At the same time, cell cycle checkpoints are by-passed, preventing apoptosis and allowing sufficient time for the assembly of infectious virus.

SCAPER, a novel cyclin A-interacting protein that regulates cell cycle progression

Cyclin A/Cdk2 plays an important role during S and G2/M phases of the eukaryotic cell cycle, but the mechanisms by which it regulates cell cycle events are not fully understood. We have biochemically purified and identified SCAPER, a novel protein that specifically interacts with cyclin A/Cdk2 in vivo. Its expression is cell cycle independent, and it associates with cyclin A/Cdk2 at multiple phases of the cell cycle. SCAPER localizes primarily to the endoplasmic reticulum. Ectopic expression of SCAPER sequesters cyclin A from the nucleus and results specifically in an accumulation of cells in M phase of the cell cycle. RNAi-mediated depletion of SCAPER decreases the cytoplasmic pool of cyclin A and delays the G1/S phase transition upon cell cycle re-entry from quiescence. We propose that SCAPER represents a novel cyclin A/Cdk2 regulatory protein that transiently maintains this kinase in the cytoplasm. SCAPER could play a role in distinguishing S phase- from M phase-specific functions of cyclin A/Cdk2.

A small molecule based on the pRb2/p130 spacer domain leads to inhibition of cdk2 activity, cell cycle arrest and tumor growth reduction in vivo

One strategy in the development of anticancer therapeutics has been to arrest malignant proliferation through inhibition of the enzymatic activity of cyclin-dependent kinases (cdks), which are key regulatory molecules of the cell cycle. Over the past few years, numerous compounds with remarkable cdk inhibitory activity have been studied in cancer therapy, although it is very difficult to point out the best cdk to target. An excellent candidate appears to be cdk2, whose alteration is a pathogenic hallmark of tumorigenesis. The small molecule described in our study showed an inhibitory effect on the kinase activity of cdk2, a significant growth arrest observed in a colony formation assay and a reduction in the size of the tumor in nude mice, thus suggesting its potential role as a promising new type of mechanism-based antitumor drug, also for the treatment of hyperproliferative disorders.

Cell cycle control and beyond: emerging roles for the retinoblastoma gene family

Rb family proteins (pRb/p105, Rb2/p130 and p107) play a key role in cell cycle control and are worthily involved in transcription repression and tumor suppression. The mechanisms of transcriptional activation and repression by the Rb gene family has been extensively investigated: pRb, pRb2/p130 and p107 interact with different E2F family factors and can inhibit E2F responsive promoters, interfering with progression of cell cycle, gene transcription, initiation of apoptotic process and cell differentiation. Recent studies have indicated that Rb and Rb2/p130 may be involved in cellular response to DNA damage events, by influencing the transcription of factors involved in DNA repair pathways. In particular, evidences suggest that Rb loss and target gene deregulation impacts on the repair of UV-induced pyrimidine pyrimidone photoproducts (6-4 PP) by regulating the expression of several DNA damage factors involved in UV DNA damage repair processes, including proliferating cell nuclear antigen. Ongoing studies are focused on the mechanisms by which Rb family genes drive cell cycle exit following DNA damage induction, and how Rb gene family's interaction with chromatin remodeling factors can influence DNA repair dynamics.

Involvement of RB gene family in tumor angiogenesis

Angiogenesis, the development of new blood vessels from pre-existing vessels, represents a fundamental step in tumor progression and metastatization. The induction of vasculature is required for growth of the tumor mass, to ensure an adequate supply of oxygen and metabolites to the tumor beyond a critical size. Tumor angiogenesis is a highly regulated process that is controlled physiologically by the tumor microenvironment and genetically by alteration of several oncogenes or tumor suppressor genes. We will focus on recent demonstrations regarding the involvement of the retinoblastoma family proteins (phosphorylated retinoblastoma (pRb), p107 and pRb2/p130) at different levels of the angiogenic process. pRb and its homologs can regulate the expression of pro- and antiangiogenic factors, such as the vascular endothelial growth factor, through an E2F-dependent mechanism. Moreover, pRb is able to modulate also the transcriptional activity of several angiogenesis-related factors like HIF-1, Id2 and Oct-1. pRb2/p130 is required for both differentiation and mobilization of bone marrow-derived endothelial cell precursors and endothelial sprouting from neighboring vessels. The involvement of the pRb pathway in the angiogenesis process has also been demonstrated by different cellular models expressing viral oncoproteins, like human papilloma virus. Moreover, some natural and synthetic compounds demonstrate their antiangiogenetic activity with a mechanism of action involving pRb. Finally, the possible prognostic value of immunohistochemical evaluation of pRb and/or pRb2/p130 expression can represent a useful tool for the characterization of the angiogenic phenotype of specific tumor histotypes.

Rb family proteins as modulators of gene expression and new aspects regarding the interaction with chromatin remodeling enzymes

The pRb family proteins (pRb1/105, p107, pRb2/p130), collectively referred to as pocket proteins, are believed to function primarily as regulators of the mammalian cell cycle progression, and suppressors of cellular growth and proliferation. In addition, different studies suggest that these pocket proteins are also involved in development and differentiation of various tissues. Several lines of evidence indicate that generally pRb-family proteins function through their effect on the transcription of E2F-regulated genes. In fact, each of Rb family proteins binds to distinct members of the E2F transcription factors, which regulate the expression of genes whose protein products are necessary for cell proliferation and to drive cell-cycle progression. Nevertheless, pocket proteins can affect the G1/S transition through E2F-independent mechanisms. More recently, a broad range of evidences indicate that pRb-family proteins associate with a wide variety of transcription factors and chromatin remodeling enzymes forming transcriptional repressor complexes that control gene expression. This review focuses on the complex regulatory mechanisms by which pRb-family proteins tell genes when to switch on and off.

p27Kip1 and p130 cooperate to regulate hematopoietic cell proliferation in vivo

To investigate the potential functional cooperation between p27Kip1 and p130 in vivo, we generated mice deficient for both p27Kip1 and p130. In p27Kip1-/-; p130-/- mice, the cellularity of the spleens but not the thymi is significantly increased compared with that of their p27Kip1-/- counterparts, affecting the lymphoid, erythroid, and myeloid compartments. In vivo cell proliferation is significantly augmented in the B and T cells, monocytes, macrophages, and erythroid progenitors in the spleens of p27Kip1-/-; p130-/- animals. Immunoprecipitation and immunodepletion studies indicate that p130 can compensate for the absence of p27Kip1 in binding to and repressing CDK2 and is the predominant CDK-inhibitor associated with the inactive CDK2 in the p27Kip1-/- splenocytes. The finding that the p27Kip1-/-; p130-/- splenic B cells are hypersensitive to mitogenic stimulations in vitro lends support to the concept that the hyperproliferation of splenocytes is not a result of the influence of their microenvironment. In summary, our findings provide genetic and molecular evidence to show that p130 is a bona fide cyclin-dependent kinase inhibitor and cooperates with p27Kip1 to regulate hematopoietic cell proliferation in vivo.

Role of the interaction between large T antigen and Rb family members in the oncogenicity of JC virus

Human polyomaviruses (JC virus, BK virus and simian virus 40) are causative agents of some human diseases and, interestingly, are involved in processes of cell transformation and oncogenesis. These viruses need the cell cycle machinery of the host cell to complete their replication; so they evolved mechanisms that can interfere with the growth control of infected cells and force them into DNA replication. The retinoblastoma family of proteins (pRb), which includes pRb/p105, p107 and pRb2/p130, acts as one of the most important regulators of the G1/S transition of the cell cycle. Rb proteins represent an important target for viral oncoproteins. Early viral T antigens can bind all members of the pRb family, promoting the activation of the E2F family of transcription factors, thus inducing the expression of genes required for the entry to the S phase. The interaction between early viral antigens and cell cycle regulators represents an important mechanism through which viruses deregulate cell cycle and lead to cell transformation. In this review, we will discuss the effects of the interaction between large T antigen and Rb proteins in JC virus-mediated oncogenesis.

RB, the conductor that orchestrates life, death and differentiation

The retinoblastoma susceptibility gene was the first tumor suppressor gene identified in humans and the first tumor suppressor gene knocked out by targeted deletion in mice. RB serves as a transducer between the cell cycle machinery and promoter-specific transcription factors, its most documented activity being the repression of the E2F family of transcription factors, which regulate the expression of genes involved in cell proliferation and survival. Recent investigations of RB function suggest that it works as a fundamental regulator to coordinate pathways of cellular growth and differentiation. In this review, we unravel the novel role of an equally important aspect of RB in downregulating the differentiation inhibitor EID-1 during cellular differentiation by teasing apart the signal, which elicit differentiation and limit cell cycle progression, since the molecular mechanisms relating to RB activation of differentiation is much less understood. We review the various roles for RB in differentiation of neurons, muscle, adipose tissue, and the retina. In addition, we provide an update for the current models of the role of RB in cell cycle to entry and exit, extending the view toward chromatin remodeling and expose the dichotomies in the regulation of RB family members. We conclude with a discussion of a novel RB regulatory network, incorporating the dynamic contribution of EID family proteins.

Rb2/p130 and protein phosphatase 2A: key mediators of ovarian carcinoma cell growth suppression by all-trans retinoic acid

Despite a number of attempts to improve treatment of ovarian cancer, it remains the most common cause of death from gynecological cancers. Thus, it is very important to identify more effective drugs for treatment and prevention of ovarian cancer. All-trans-retinoic acid (ATRA) has been shown to arrest the growth of ovarian carcinoma cells in G0/G1 and to significantly elevate levels of Rb2/p130 protein, a member of the retinoblastoma family of tumor suppressors. As ATRA treatment leads to a significant increase in the amount of Rb2/p130 protein but not mRNA, the elevated levels of Rb2/p130 protein is likely the result of increased stability. In studies to elucidate the mechanism by which ATRA alters Rb2/p130 stability in ovarian cancer cells, it was determined that PP2A, a serine/threonine phosphatase, binds and dephosphorylates Rb2/p130. Dephosphorylated Rb2/p130 exhibits decreased ubiquitination and thus is not degraded by the proteasome. The sites at which PP2A catalytic subunit (PP2Ac) interacts with Rb2/p130 have been localized to the NLS in the C-terminus of Rb2/p130. These sites are also involved in the interaction of Rb/p130 with importin beta and importin alpha, members of the nuclear transport machinery. It is known that importin alpha recognizes a NLS on a target protein and importin beta binds the nuclear pore complex. Moreover, it has been shown that the binding of importin alpha to NLS significantly decreases with phosphorylation of NLS. In ATRA-treated ovarian carcinoma cells, PP2A binds to Rb2/p130 and dephosphorylates the NLS of Rb2/p130 leading to the interaction of importin alpha with Rb2/p130. Importin beta then binds to the importin alpha-Rb2/p130 complex, leading to the translocation of the Rb2/p130 to the nucleus where it acts to arrest ovarian cancer cells in G1 and suppress proliferation.

CP110 cooperates with two calcium-binding proteins to regulate cytokinesis and genome stability

The centrosome is an integral component of the eukaryotic cell cycle machinery, yet very few centrosomal proteins have been fully characterized to date. We have undertaken a series of biochemical and RNA interference (RNAi) studies to elucidate a role for CP110 in the centrosome cycle. Using a combination of yeast two-hybrid screens and biochemical analyses, we report that CP110 interacts with two different Ca2+-binding proteins, calmodulin (CaM) and centrin, in vivo. In vitro binding experiments reveal a direct, robust interaction between CP110 and CaM and the existence of multiple high-affinity CaM-binding domains in CP110. Native CP110 exists in large (approximately 300 kDa to 3 MDa) complexes that contain both centrin and CaM. We investigated a role for CP110 in CaM-mediated events using RNAi and show that its depletion leads to a failure at a late stage of cytokinesis and the formation of binucleate cells, mirroring the defects resulting from ablation of either CaM or centrin function. Importantly, expression of a CP110 mutant unable to bind CaM also promotes cytokinesis failure and binucleate cell formation. Taken together, our data demonstrate a functional role for CaM binding to CP110 and suggest that CP110 cooperates with CaM and centrin to regulate progression through cytokinesis.

Pocket protein function in melanocyte homeostasis and neoplasia

Melanoma is the most lethal of human skin cancers and its incidence is increasing worldwide [L.K. Dennis (1999). Arch. Dermatol. 135, 275; C. Garbe et al. (2000). Cancer 89, 1269]. Melanomas often metastasize early during the course of the disease and are then highly intractable to current therapeutic regimens [M.F. Demierre and G. Merlino (2004). Curr. Oncol. Rep. 6, 406]. Consequently, understanding the factors that maintain melanocyte homeostasis and prevent their neoplastic transformation into melanoma is of utmost interest from the perspective of therapeutic interdiction. This review will focus on the role of the pocket proteins (PPs), Rb1 (retinoblastoma protein), retinoblastoma-like 1 (Rbl1 also known as p107) and retinoblastoma-like 2 (Rbl2 also known as p130), in melanocyte homeostasis, with particular emphasis on their functions in the cell cycle and the DNA damage repair response. The potential mechanisms of PP deregulation in melanoma and the possibility of PP-independent pathways to melanoma development will also be considered. Finally, the role of the PP family in ultraviolet radiation (UVR)-induced melanoma and the precise contribution that each PP family member makes to melanocyte homeostasis will be discussed in the context of a number of genetically engineered mouse models.

Role for Brm in Cell Growth Control

Recently, we have shown implication of Brm, the catalytic subunit of the SWI/SNF chromatin remodeling complex, in repression of cyclin A expression in quiescent cells. Here, we have examined the fate of cells lacking Brm throughout the cycle. We find that despite elevated levels of cyclins A and E, these cells can respond to serum starvation, however, without reaching a canonical G(0) phase as they continue to express high levels of c-Myc and have an abnormally large average size. The response to serum starvation can be correlated with increased levels of Rb proteins p130 and p107 as well as increased association of p27 with the cyclin-dependent kinases, possibly compensating for the higher levels of G(1) cyclins by reducing their associated kinase activity. After serum stimulation, reentry into the cycle occurs normally, but the S phase is delayed and shorter. In addition, the M phase has an increased duration, and we observed frequent faulty chromosome segregation events in anaphase. Altogether, our data suggest that cells can partially overcome the absence of Brm by activating several compensatory mechanisms to control the cell cycle. However, they remain profoundly affected, unable to enter a canonical quiescent state, presenting a shorter S phase, and finally unable to perform correct chromosome segregation.

Melanocytes in conditional Rb-/- mice are normal in vivo but exhibit proliferation and pigmentation defects in vitro

The function of the retinoblastoma tumour suppressor (Rb1), and the pocket protein family in general, has been implicated as an important focal point for deregulation in many of the molecular pathways mutated in melanoma. We have focused on the role of Rb1 in mouse melanocyte homeostasis using gene targeting and Cre/loxP mediated tissue-specific deletion. We show that constitutive Cre-mediated ablation of Rb1 exon 2 prevents the production of Rb1 and recapitulates the phenotype encountered in other Rb1 knockout mouse models. Mice with conditional melanocyte-specific ablation of Rb1 manifest overtly normal pigmentation and are bereft of melanocytic hyperproliferative defects or apoptosis-induced depigmentation. Histologically, these mice have melanocyte morphology and distribution comparable with control littermates. In contrast, Rb1-null melanocytes removed from their in vivo micro-environment and cultured in vitro display some of the characteristics associated with a transformed phenotype. They proliferate at a heightened rate when compared with control melanocytes and have a decreased requirement for mitogens. With progressive culture the cells depigment at relatively early passage and display a gross morphology which, whilst reminiscent of early passage melanocytes, is generally different to equivalent passage control cells. These results indicate that Rb1 is dispensable for in vivo melanocyte homeostasis when its ablation is targeted from the melanoblast stage onwards, however, when cultured in vitro, Rb1 loss increases melanocyte growth but the cells are not fully transformed.

Rapamycin Disrupts Cyclin/Cyclin-Dependent Kinase/p21/Proliferating Cell Nuclear Antigen Complexes and Cyclin D1 Reverses Rapamycin Action by Stabilizing These Complexes

Rapamycin and its derivatives are promising anticancer agents, but the exact mechanisms by which these drugs induce cell cycle arrest and inhibit tumor growth are unknown. A biochemical analysis of human mammary tumor cell lines indicated that rapamycin-induced antiproliferative effects correlated with down-regulation of cellular p21 levels and the levels of p21 in cyclin-dependent kinase (Cdk) 2 and 4 complexes. Cyclin D1 overexpression reversed rapamycin action and this reversal correlated with increased levels of cellular p21, higher levels of p21 associated with Cdk2, and stabilization of cyclin D1/Cdk2/p21/proliferating cell nuclear antigen (PCNA) complexes. Experiments using a novel cyclin D1-Cdk2 fusion protein or a kinase-dead mutant of the fusion protein indicated that reversal of rapamycin action required not only the formation of complexes with p21 and PCNA but also complex-associated kinase activity. Similar results were observed in vivo. The rapamycin derivative RAD001 (everolimus) inhibited the growth of mouse mammary tumors, which correlated with the disruption of cyclin D1/Cdk2 complexes. The potential implications of these results with respect to the use of rapamycin derivatives in breast cancer therapy are discussed.

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.

The flexible evolutionary anchorage-dependent Pardee's restriction point of mammalian cells: how its deregulation may lead to cancer

Living cells oscillate between the two states of quiescence and division that stand poles apart in terms of energy requirements, macromolecular composition and structural organization and in which they fulfill dichotomous activities. Division is a highly dynamic and energy-consuming process that needs be carefully orchestrated to ensure the faithful transmission of the mother genotype to daughter cells. Quiescence is a low-energy state in which a cell may still have to struggle hard to maintain its homeostasis in the face of adversity while waiting sometimes for long periods before finding a propitious niche to reproduce. Thus, the perpetuation of single cells rests upon their ability to elaborate robust quiescent and dividing states. This led yeast and mammalian cells to evolve rigorous Start [L.H. Hartwell, J. Culotti, J. Pringle, B.J. Reid, Genetic control of the cell division cycle in yeast, Science 183 (1974) 46-51] and restriction (R) points [A.B. Pardee, A restriction point for control of normal animal cell proliferation, Proc. Natl. Acad. Sci. U. S. A. 71 (1974) 1286-1290], respectively, that reduce deadly interferences between the two states by enforcing their temporal insulation though still enabling a rapid transition from one to the other upon an unpredictable change in their environment. The constitutive cells of multi-celled organisms are extremely sensitive in addition to the nature of their adhering support that fluctuates depending on developmental stage and tissue specificity. Metazoan evolution has entailed, therefore, the need for exceedingly flexible anchorage-dependent R points empowered to assist cells in switching between quiescence and division at various times, places and conditions in the same organism. Programmed cell death may have evolved concurrently in specific contexts unfit for the operation of a stringent R point that increase the risk of deadly interferences between the two states (as it happens notably during development). But, because of their innate flexibility, anchorage-dependent R points have also the ability to readily adjust to a changing structural context so as to give mutated cells a chance to reproduce, thereby encouraging tumor genesis. The Rb and p53 proteins, which are regulated by the two products of the Ink4a-Arf locus [C.J. Sherr, The INK4a/ARF network in tumor suppression, Nat. Rev., Mol. Cell Biol. 2 (2001) 731-737], govern separable though interconnected pathways that cooperate to restrain cyclin D- and cyclin E-dependent kinases from precipitating untimely R point transit. The expression levels of the Ink4a and Arf proteins are especially sensitive to changes in cellular shape and adhesion that entirely remodel at the time when cells shift between quiescence and division. The Arf proteins further display an extremely high translational sensitivity and can activate the p53 pathway to delay R point transit, but, only when released from the nucleolus, 'an organelle formed by the act of building a ribosome' [T. Mélèse, Z. Xue, The nucleolus: an organelle formed by the act of building a ribosome, Curr. Opin. Cell Biol. 7 (1995) 319-324]. In this way, the Ink4a/Rb and Arf/p53 pathways emerge as key regulators of anchorage-dependent R point transit in mammalian cells and their deregulation is, indeed, a rule in human cancers. Thus, by selecting the nucleolus to mitigate cell cycle control by the Arf proteins, mammalian cells succeeded in forging a highly flexible R point enabling them to match cell division with a growth rate imposed by factors controlling nucleolar assembling, such as nutrients and adhesion. It is noteworthy that nutrient control of critical size at Start in budding yeast has been shown recently to be governed by a nucleolar protein interaction network [P. Jorgensen, J.L. Nishikawa, B.-J. Breitkreutz, M. Tyers, Systematic identification of pathways that couple cell growth and division in yeast, Science 297 (2002) 395-400].

Cyclin E

E-type cyclins (cyclin E1 and cyclin E2) are expressed during the late G1 phase of the cell cycle until the end of the S-phase. The activity of cyclin E is limiting for the passage of cells through the restriction point "R" which marks a "point of no return" for cells entering the division cycle from a resting state or passing from G1 into S-phase. Expression of cyclin E is regulated on the level of gene transcription mainly by members of the E2F trrnscription factor family and by its degradation via the proteasome pathway. Cyclin E binds and activates the kinase Cdk2 and by phosphorylating its substrates, the so-called "pocket proteins", the cyclic/Cdk2 complexes initiate a cascade of events that leads to the expression of S-phase specific genes. Aside from this specific function as a regulator of S-phase-entry, cyclin E plays a direct role in the initiation of DNA replication, the control of genomic stability, and the centrosome cycle. Surprisingly, recent studies have shown that the once thought essential cyclin E is dispensable for the development of higher eukaryotes and for the mitotic division of eukaryotic cells. Nevertheless, high level cyclin E expression has been associated with the initiation or progression of different human cancers, in particular breast cancer but also leukemia, lymphoma and others. Transgenic mouse models in which cyclin E is constitutively expressed develop malignant diseases, supporting the notion of cyclin E as a dominant onco-protein.

Frequent loss of pRb2/p130 in human ovarian carcinoma

PURPOSE

RB2/p130, a member of the retinoblastoma gene family, maps to human chromosome 16q12.2, a region in which deletions have been found in several human neoplasms including breast, prostatic, and ovarian carcinoma. We sought to evaluate pRb2/p130 protein expression and function in ovarian carcinoma.

EXPERIMENTAL DESIGN

pRb2/p130 expression was detected by immunohistochemical and Western blot analyses in 45 primary ovarian carcinoma samples.

RESULTS

Immunohistochemical analysis revealed loss or decrease of pRb2/p130 expression in 18 cases (40%). pRb2/p130 expression was mostly nuclear and inversely correlated to the tumor grade (P < 0.05). Western blot analysis correlated with immunohistochemical expression. Reverse transcription-PCR followed by Southern blot analysis was performed on a representative set of 20 ovarian carcinomas. RB2/p130 mRNA levels were consistent with protein expression. We found a significant increase in the percentage of G(1)-phase-arrested cells in CAOV3 and A2780 ovarian carcinoma cell lines after transduction with an adenovirus carrying the RB2/p130 gene (Ad-CMV-RB2/p130).

CONCLUSIONS

These data indicate that loss or decrease of pRb2/p130 expression is a frequent event in ovarian carcinoma and is regulated mostly at the transcriptional level. Moreover, pRb2/p130 overexpression is able to arrest cell growth in ovarian carcinoma cells, suggesting the putative role of pRb2/p130 as a tumor suppressor in this malignancy.

Interactions between human cytomegalovirus IE1-72 and cellular p107: functional domains and mechanisms of up-regulation of cyclin E/cdk2 kinase activity

Previous work has demonstrated that the human cytomegalovirus IE1-72 protein is able to bind to the N terminus of p107, and IE1-72 alone is sufficient for alleviation of p107-mediated cell growth suppression. However, the mechanism of this alleviation is unclear. Here, we show that IE1-72 can alleviate p107 inhibition of cyclin E/cdk2 kinase activity. We cotransfected various IE1-72 and p107 constructs into C33A cells and demonstrated that IE1-72 could activate the kinase activity of cyclin E/cdk2. Conversely, IE2-86 did not activate this activity, suggesting that the interaction between p107 and IE1-72 and the subsequent kinase activation are specific. By the use of a series of deletion and point mutants of IE1-72 and p107, we observed that a mutation of the loop region of helix-loop-helix-turn-helix in exon 3 of IE1-72 as well as a mutation of the leucine zipper-2 region in exon 4 of IE1-72 abolished binding to p107. In addition, these two IE1-72 mutants did not alleviate p107 inhibition of cyclin E/cdk2 kinase activity and also failed to alleviate p107 inhibition of the E2F-responsive promoter. Meanwhile, deletion of the N-terminal aa 1 to 175 of p107 abolished both p107 binding with IE1-72 and p107 inhibition of cyclin E/cdk2 kinase activity. This result confirms that the N-terminus aa 1 to 175 region of p107 is a common region where both IE1-72 protein and cyclin E/cdk2 bind. We propose a mechanism in which binding of IE1-72 to p107 displaces cyclin E/cdk2 from p107. Once released from p107, cyclin E/cdk2 is able to function as an active kinase.

End-stage differentiation of neutrophil granulocytes in vivo is accompanied by up-regulation of p27kip1 and down-regulation of CDK2, CDK4, and CDK6

The in vivo expression profiles of cell-cycle proteins regulating G1-to-S-phase transition were determined in three neutrophil precursor populations from human bone marrow: myeloblasts (MBs) and promyelocytes (PMs); myelocytes (MCs) and metamyelocytes (MMs); and band cells (BCs) and segmented neutrophil cells (SCs) and in mature polymorphonuclear neutrophils (PMNs) from peripheral blood. Complete cell-cycle arrest was observed in BCs/SCs and PMNs. Cyclins D1, D2, and D3 were found to be down-regulated during granulopoiesis, whereas a slight increase of cyclin E was seen. In contrast, cyclin-dependent kinase (CDK)2, -4, and -6 were down-regulated from the MC/MM stages and onward. The transcript levels of CDK2, -4, and -6 were concurrently down-regulated. As the only CDK inhibitor, p27kip1 protein and mRNA expression were up-regulated in MCs/MMs and reached peak levels in PMNs. Protein expression of retinoblastoma protein and the related pocket proteins p107 and p130 was down-regulated from the MC/MM stages and onward. This is the first report to describe expression levels of cell-cycle proteins during granulopoiesis in vivo, and it strongly contrasts the observations made in cell-culture systems in vitro.

Protein phosphatase 2A associates with Rb2/p130 and mediates retinoic acid-induced growth suppression of ovarian carcinoma cells

Levels of Rb2/p130 protein are increased 5-10-fold following all-trans-retinoic acid (ATRA) treatment of the retinoid-sensitive ovarian adenocarcinoma cell line CAOV3, but not the retinoid-resistant adenocarcinoma cell line SKOV3. We found that this increase in Rb2/p130 protein levels in ATRA-treated CAOV3 cells was the result of an increased protein stability. Moreover, Rb2/p130 exhibited a decreased ubiquitination following ATRA treatment. Because phosphorylation frequently mediates ubiquitination of proteins, we examined the serine/threonine phosphatase activity in our CAOV3 cells following ATRA treatment. A significant increase in Ser/Thr phosphatase activity was found, which correlated with a rise in the level of protein phosphatase 2A (PP2A) catalytic subunit-alpha. In addition, co-immunoprecipitation and glutathione S-transferase pull-down studies demonstrated that PP2A and Rb2/p130 associate. We have made use of a battery of Rb2/p130 mutants to determine the sites dephosphorylated in response to ATRA treatment of CAOV3 cells. Obligate CDK4 phosphorylation sites seemed most important to the stability of the protein and are among the candidate sites that are dephosphorylated by PP2A following ATRA treatment. Finally, using both small interfering RNA specific to the catalytic subunit of PP2A and a variant of the SKOV3 cell line that overexpresses PP2A, we have shown that modulation of PP2A protein levels correlates with the ability of ATRA to inhibit growth of ovarian carcinoma cells. Our data suggest that ATRA mediates growth inhibition by stabilizing Rb2/p130 via a mechanism that involves induction of PP2A, an enzyme that can potentially dephosphorylate Rb2/p130, thereby protecting it from degradation by the proteasome.

SKP2 associates with p130 and accelerates p130 ubiquitylation and degradation in human cells

p130 is a member of the retinoblastoma family of pocket proteins, which includes pRB and p107. Unlike pRB and p107, p130 protein levels decrease dramatically following its hyperphosphorylation starting in the mid-G1 phase of the cell cycle. However, the mechanism leading to p130 downregulation is unknown. We have found that the proteasome inhibitor, lactacystin, inhibited p130 downregulation in T98G cells progressing through the G1/S transition and S phase and that p130 is multiubiquitylated in 293 cells. We have previously shown that ectopic expression of both cyclin D and E induces phosphorylation and downregulation of p130. Since the SKP1/Cul1/SKP2 E3 ubiquitin ligase complex mediates ubiquitylation of substrates previously phosphorylated by cyclin-dependent kinases, we investigated the potential role of this ubiquitin ligase in mediating p130 downregulation. We found that p130 interacts with SKP1, Cul-1 and SKP2 in human 293 cells. We also found that ectopic coexpression of SKP2 and p130 leads to dose-dependent downregulation of p130, reduces p130 protein half-life and induces p130 ubiquitylation in these cells. Moreover, adenoviral-mediated expression of SKP2 accelerates downregulation of endogenous hyperphosphorylated p130 in mitogen-stimulated T98G cells and primary WI38 fibroblasts. We conclude that p130 is a substrate of the SCF(SKP2) ubiquitin ligase and this E3 ligase regulates p130 abundance during the cell cycle.

Dissecting the contribution of p16(INK4A) and the Rb family to the Ras transformed phenotype

Although oncogenic Ras commonly contributes to the development of cancer, in normal primary cells it induces cell cycle arrest rather than transformation. Here we analyze the additional genetic changes required for Ras to promote cell cycle progression rather than arrest. We show that loss of p53 is sufficient for oncogenic Ras to stimulate proliferation in the absence of extrinsic mitogens in attached cells. However, surprisingly, we find that p53 loss is not sufficient for Ras to overcome anchorage dependence or contact inhibition. In contrast, expression of simian virus 40 (SV40) large T antigen (LT) allows Ras to overcome these additional cell cycle controls. Mutational analysis of SV40 LT shows that this action of SV40 LT depends on its ability to inactivate the retinoblastoma (Rb) family of proteins, in concert with the loss of p53. Importantly, we show that inactivation of the Rb family of proteins can be mimicked by loss of the cyclin-dependent kinase inhibitor p16(INK4A). p16(INK4A) is commonly lost in human tumors, but its contribution to the transformed phenotype is unknown. We demonstrate here a role for p16(INK4A) in the loss of cell cycle controls required for tumorigenesis and show how accumulating genetic changes cooperate and contribute to the transformed phenotype.

Rapamycin potentiates transforming growth factor beta-induced growth arrest in nontransformed, oncogene-transformed, and human cancer cells

Transforming growth factor beta (TGF-beta) induces cell cycle arrest of most nontransformed epithelial cell lines. In contrast, many human carcinomas are refractory to the growth-inhibitory effect of TGF-beta. TGF-beta overexpression inhibits tumorigenesis, and abolition of TGF-beta signaling accelerates tumorigenesis, suggesting that TGF-beta acts as a tumor suppressor in mouse models of cancer. A screen to identify agents that potentiate TGF-beta-induced growth arrest demonstrated that the potential anticancer agent rapamycin cooperated with TGF-beta to induce growth arrest in multiple cell lines. Rapamycin also augmented the ability of TGF-beta to inhibit the proliferation of E2F1-, c-Myc-, and (V12)H-Ras-transformed cells, even though these cells were insensitive to TGF-beta-mediated growth arrest in the absence of rapamycin. Rapamycin potentiation of TGF-beta-induced growth arrest could not be explained by increases in TGF-beta receptor levels or rapamycin-induced dissociation of FKBP12 from the TGF-beta type I receptor. Significantly, TGF-beta and rapamycin cooperated to induce growth inhibition of human carcinoma cells that are resistant to TGF-beta-induced growth arrest, and arrest correlated with a suppression of Cdk2 kinase activity. Inhibition of Cdk2 activity was associated with increased binding of p21 and p27 to Cdk2 and decreased phosphorylation of Cdk2 on Thr(160). Increased p21 and p27 binding to Cdk2 was accompanied by decreased p130, p107, and E2F4 binding to Cdk2. Together, these results indicate that rapamycin and TGF-beta cooperate to inhibit the proliferation of nontransformed cells and cancer cells by acting in concert to inhibit Cdk2 activity.

E2F4 loss suppresses tumorigenesis in Rb mutant mice

The E2F transcription factors mediate the activation or repression of key cell cycle regulatory genes under the control of the retinoblastoma protein (pRB) tumor suppressor and its relatives, p107 and p130. Here we investigate how E2F4, the major "repressive" E2F, contributes to pRB's tumor-suppressive properties. Remarkably, E2F4 loss suppresses the development of both pituitary and thyroid tumors in Rb(+/-) mice. Importantly, E2F4 loss also suppresses the inappropriate gene expression and proliferation of pRB-deficient cells. Biochemical analyses suggest that this tumor suppression occurs via a novel mechanism: E2F4 loss allows p107 and p130 to regulate the pRB-specific, activator E2Fs. We also detect these novel E2F complexes in pRB-deficient cells, suggesting that they play a significant role in the regulation of tumorigenesis in vivo.

The pRb-related protein p130 is regulated by phosphorylation-dependent proteolysis via the protein-ubiquitin ligase SCF(Skp2)

p130 is a tumor suppressor of the pocket protein family whose expression is posttranscriptionally regulated and largely G0 restricted. The mechanism of down-regulation of p130 expression in proliferating cells was investigated. Our results indicate that the decline of p130 expression as G0 cells reenter the cell cycle is due to a decrease in protein stability. The enhancement of p130 turnover in late G1 and S phase compared with G0 and early G1 phase was dependent on Cdk4/6-specific phosphorylation of p130 on Serine 672, and independent of Cdk2 activity. The activity of the ubiquitin ligase complex Skp1-Cul1/Cdc53-F-box protein Skp2 (SCF(Skp2)) and the proteasome were necessary for p130 degradation. In vitro, recombinant Skp2 was able to bind hyperphosphorylated but not dephosphorylated p130. Furthermore, in vitro polyubiquitination of p130 by SCF(Skp2) was specifically dependent on phosphorylation of p130 on Serine 672. Thus, like the Cdk inhibitor p27(Kip1), p130 turnover is regulated by Cdk-dependent G1 phosphorylation leading to ubiquitin-dependent proteolysis.

B-Myb overcomes a p107-mediated cell proliferation block by interacting with an N-terminal domain of p107

B-Myb is a cell-cycle regulated transcription factor which is implicated in cell proliferation and has an essential role in early embryonic development. In this study we examined the functions of B-Myb required to overcome G1 arrest in Saos-2 cells induced by the retinoblastoma-related p107 protein. Our results demonstrated that this activity was independent of B-Myb transactivation function, but correlated with its capacity to form an in vivo complex with p107. A large proportion of B-Myb formed complexes with p107 in cotransfected cells, however, B-Myb bound weakly to the related p130 protein and not at all to pRb. In contrast to the E2F transcription factors, which bind the p107 C-terminal pocket domain, B-Myb recognizes an N-terminal p107 region which overlaps the larger cyclin-binding domain. B-Myb and cyclin A2 formed mutually exclusive complexes with p107, and B-Myb enhanced the activity of co-transfected cyclin E kinase activity, implying that B-Myb affects the cell cycle by preventing sequestration of active cyclin/cdk2 complexes. This study defines a novel function of B-Myb and further suggests that the p107 N-terminus provides an interaction domain for transcription factors involved in cell cycle control.

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.

CP110, a cell cycle-dependent CDK substrate, regulates centrosome duplication in human cells

Centrosome duplication and separation are linked inextricably to certain cell cycle events, in particular activation of cyclin-dependent kinases (CDKs). However, relatively few CDK targets driving these events have been uncovered. Here, we have performed a screen for CDK substrates and have isolated a target, CP110, which is phosphorylated by CDKs in vitro and in vivo. Human CP110 localizes to centrosomes. Its expression is strongly induced at the G1-to-S phase transition, coincident with the initiation of centrosome duplication. RNAi-mediated depletion of CP110 indicates that this protein plays an essential role in centrosome duplication. Long-term disruption of CP110 phosphorylation leads to unscheduled centrosome separation and overt polyploidy. Our data suggest that CP110 is a physiological centrosomal CDK target that promotes centrosome duplication, and its deregulation may contribute to genomic instability.