Distinct phosphorylation events regulate p130- and p107-mediated repression of E2F-4

The p21CIP1-CDK4-DREAM axis is a master regulator of genotoxic stress-induced cellular senescence

Cellular senescence, a major driver of aging, can be stimulated by DNA damage, and is counteracted by the DNA repair machinery. Here we show that in p16INK4a-deficient cells, senescence induction by the environmental genotoxin B[a]P or ionizing radiation (IR) completely depends on p21CIP1. Immunoprecipitation-based mass spectrometry interactomics data revealed that during senescence induction and maintenance, p21CIP1 specifically inhibits CDK4 and thereby activates the DREAM complex. Genome-wide transcriptomics revealed striking similarities in the response induced by B[a]P and IR. Among the top 100 repressed genes 78 were identical between B[a]P and IR and 76 were DREAM targets. The DREAM complex transcriptionally silences the main proliferation-associated transcription factors E2F1, FOXM1 and B-Myb as well as multiple DNA repair factors. Knockdown of p21CIP1, E2F4 or E2F5 diminished both, repression of these factors and senescence. The transcriptional profiles evoked by B[a]P and IR largely overlapped with the profile induced by pharmacological CDK4 inhibition, further illustrating the role of CDK4 inhibition in genotoxic stress-induced senescence. Moreover, data obtained by live-cell time-lapse microscopy suggest the inhibition of CDK4 by p21CIP1 is especially important for arresting cells which slip through mitosis. Overall, we identified the p21CIP1/CDK4/DREAM axis as a master regulator of genotoxic stress-induced senescence.

Cytomegalovirus infection lengthens the cell cycle of granule cell precursors during postnatal cerebellar development

Human cytomegalovirus (HCMV) infection in infants infected in utero can lead to a variety of neurodevelopmental disorders. However, mechanisms underlying altered neurodevelopment in infected infants remain poorly understood. We have previously described a murine model of congenital HCMV infection in which murine CMV (MCMV) spreads hematogenously and establishes a focal infection in all regions of the brain of newborn mice, including the cerebellum. Infection resulted in disruption of cerebellar cortical development characterized by reduced cerebellar size and foliation. This disruption was associated with altered cell cycle progression of the granule cell precursors (GCPs), which are the progenitors that give rise to granule cells (GCs), the most abundant neurons in the cerebellum. In the current study, we have demonstrated that MCMV infection leads to prolonged GCP cell cycle, premature exit from the cell cycle, and reduced numbers of GCs resulting in cerebellar hypoplasia. Treatment with TNF-α neutralizing antibody partially normalized the cell cycle alterations of GCPs and altered cerebellar morphogenesis induced by MCMV infection. Collectively, our results argue that virus-induced inflammation altered the cell cycle of GCPs resulting in a reduced numbers of GCs and cerebellar cortical hypoplasia, thus providing a potential mechanism for altered neurodevelopment in fetuses infected with HCMV.

Preclinical evaluation of CDK4 phosphorylation predicts high sensitivity of pleural mesotheliomas to CDK4/6 inhibition

Malignant pleural mesothelioma (MPM) is an aggressive cancer with limited therapeutic options. We evaluated the impact of CDK4/6 inhibition by palbociclib in 28 MPM cell lines including 19 patient-derived ones, using various approaches including RNA-sequencing. Palbociclib strongly and durably inhibited the proliferation of 23 cell lines, indicating a unique sensitivity of MPM to CDK4/6 inhibition. When observed, insensitivity to palbociclib was mostly explained by the lack of active T172-phosphorylated CDK4. This was associated with high p16INK4A (CDKN2A) levels that accompany RB1 defects or inactivation, or (unexpectedly) CCNE1 overexpression in the presence of wild-type RB1. Prolonged palbociclib treatment irreversibly inhibited proliferation despite re-induction of cell cycle genes upon drug washout. A senescence-associated secretory phenotype including various potentially immunogenic components was irreversibly induced. Phosphorylated CDK4 was detected in 80% of 47 MPMs indicating their sensitivity to CDK4/6 inhibitors. Its absence in some highly proliferative MPMs was linked to very high p16 (CDKN2A) expression, which was also observed in public datasets in tumours from short-survival patients. Our study supports the evaluation of CDK4/6 inhibitors for MPM treatment, in monotherapy or combination therapy.

Xanthatin suppresses pancreatic cancer cell growth via the ROS/RBL1 signaling pathway: In vitro and in vivo insights

BACKGROUND

As a malignant digestive system tumor, pancreatic cancer has a high mortality rate. Xanthatin is a sesquiterpene lactone monomer compound purified from the traditional Chinese herb Xanthium strumarium L. It has been reported that Xanthatin exhibits inhibitory effects on various cancer cells in retinoblastoma, glioma, hepatoma, colon cancer, lung cancer, as well as breast cancer. However, in pancreatic cancer cells, only one report exists on the suppression of Prostaglandin E2 synthesis and the induction of caspase 3/7 activation in Xanthatin-treated MIA PaCa-2 cells, while systematic in vitro and in vivo investigations and related mechanisms have yet to be explored.

PURPOSE

This research aims to explore the in vitro and in vivo effects of Xanthatin on pancreatic cancer and its molecular mechanisms.

METHODS

The anticancer effects and mechanisms of Xanthatin on pancreatic cancer cells were assessed through employing cell counting kit-8 (CCK-8) assay, lactate dehydrogenase (LDH) assay, carboxyfluorescein diacetate succinimidyl ester (CFDA SE) cell proliferation assay, colony formation assay, wound healing assay, transwell assay, Annexin V-FITC/propidium iodide (PI) dual staining, Hoechst nuclear staining, Western blot analysis, phosphoproteomics, and reactive oxygen species (ROS) measurement. The in vivo anticancer effects of Xanthatin on pancreatic cancer cells were studied using a nude mouse model.

RESULTS

The present study showed that Xanthatin can prevent the proliferation and metastasis of pancreatic cancer cells and trigger the exposure of phosphatidylserine (PS), chromatin condensation, and caspase activation, thereby inducing apoptosis. Phosphoproteomic analysis indicated that Xanthatin inhibits the phosphorylation of the proliferation-associated protein RBL1, and oxidative stress can lead to RBL1 dephosphorylation. Further investigation revealed that Xanthatin significantly upregulates ROS levels in pancreatic cancer cells, and the antioxidant N-acetylcysteine (NAC) can reverse Xanthatin-induced cell proliferation inhibition and apoptosis. In addition, Xanthatin can suppress pancreatic cancer cell growth in a xenograft nude mouse model with low toxicity to the mice.

CONCLUSION

Xanthatin may inhibit the proliferation of pancreatic cancer cells and trigger apoptosis through the ROS/RBL1 signaling pathway.

Mutations in the acetylation hotspots of Rbl2 are associated with increased risk of breast cancer

Retinoblastoma like protein-2 (Rbl2) is functionally regulated by phosphorylation and acetylation. Previously, we demonstrated that lysine K1083 (K1079 in human Rbl2) is a potential target for acetylation but its functional role remains elusive. We investigated alterations in human Rbl2 gene specifically targeting exons 19–22 harbouring acetylatable residues i.e. K1072, K1083 and K1115 through single stranded conformation polymorphism (SSCP) in breast cancer patients. The K1083 was found altered into arginine (R) in 51% of the cases but K1072 and K1115 remained conserved. The ‘K1083R’ mutation impairs the acetylation potential of this motif that may result in functional inactivation of Rbl2. These patients also showed poor survival outcome that highlights prognostic relevance of this residue. NIH3T3 cells expressing glutamine (K1083Q) mutated Rbl2 could not be arrested in G₁ by serum starvation, whereas cells expressing Rbl2 with K1083R showed prolonged G₁ arrest in fluorescence activated cell sorting (FACS) analysis. This suggests that K1083 acetylation is important for G₁/S transition. Further, we performed molecular dynamic simulations (MDS) to analyse kinetics of residue K1083 with Cyc-D1/CDK4. Mutations at K1083 impaired this binding exposing neighbouring residues S1080, P1081, S1082 and R1084, hence enhancing the possibility of accelerated phosphorylation. S1080 has previously been reported as a promising candidate of cell cycle dependent phosphorylation in Rbl2. This highlights significance of mutations in the pocket domain of Rbl2 gene in breast cancer, and also strengthen the supposition that K1083 acetylation is pre-requisite for its phosphorylation.

Cell cycle regulation: p53-p21-RB signaling

The retinoblastoma protein RB and the transcription factor p53 are central tumor suppressors. They are often found inactivated in various tumor types. Both proteins play central roles in regulating the cell division cycle. RB forms complexes with the E2F family of transcription factors and downregulates numerous genes. Among the RB-E2F target genes, a large number code for key cell cycle regulators. Their transcriptional repression by the RB-E2F complex is released through phosphorylation of RB, leading to expression of the cell cycle regulators. The release from repression can be prevented by the cyclin-dependent kinase inhibitor p21/CDKN1A. The CDKN1A gene is transcriptionally activated by p53. Taken together, these elements constitute the p53-p21-RB signaling pathway. Following activation of p53, for example by viral infection or induction of DNA damage, p21 expression is upregulated. High levels of p21 then result in RB-E2F complex formation and downregulation of a large number of cell cycle genes. Thus, p53-dependent transcriptional repression is indirect. The reduced expression of the many regulators leads to cell cycle arrest. Examination of the p53-p21-RB targets and genes controlled by the related p53-p21-DREAM signaling pathway reveals that there is a large overlap of the two groups. Mechanistically this can be explained by replacing RB-E2F complexes with the DREAM transcriptional repressor complex at E2F sites in target promoters. In contrast to RB-E2F, DREAM can downregulate genes also through CHR transcription factor binding sites. This results in a distinct gene set controlled by p53-p21-DREAM signaling independent of RB-E2F. Furthermore, RB has non-canonical functions without binding to E2F and DNA. Such a role of RB supporting DREAM formation may be exerted by the RB-SKP2-p27-cyclin A/E-CDK2-p130-DREAM link. In the current synopsis, the mechanism of regulation by p53-p21-RB signaling is assessed and the overlap with p53-p21-DREAM signaling is examined.

Cyclin F drives proliferation through SCF-dependent degradation of the retinoblastoma-like tumor suppressor p130/RBL2

Cell cycle gene expression programs fuel proliferation and are universally dysregulated in cancer. The retinoblastoma (RB)-family of proteins, RB1, RBL1/p107, and RBL2/p130, coordinately represses cell cycle gene expression, inhibiting proliferation, and suppressing tumorigenesis. Phosphorylation of RB-family proteins by cyclin-dependent kinases is firmly established. Like phosphorylation, ubiquitination is essential to cell cycle control, and numerous proliferative regulators, tumor suppressors, and oncoproteins are ubiquitinated. However, little is known about the role of ubiquitin signaling in controlling RB-family proteins. A systems genetics analysis of CRISPR/Cas9 screens suggested the potential regulation of the RB-network by cyclin F, a substrate recognition receptor for the SCF family of E3 ligases. We demonstrate that RBL2/p130 is a direct substrate of SCF^{cyclin F}. We map a cyclin F regulatory site to a flexible linker in the p130 pocket domain, and show that this site mediates binding, stability, and ubiquitination. Expression of a mutant version of p130, which cannot be ubiquitinated, severely impaired proliferative capacity and cell cycle progression. Consistently, we observed reduced expression of cell cycle gene transcripts, as well a reduced abundance of cell cycle proteins, analyzed by quantitative, iterative immunofluorescent imaging. These data suggest a key role for SCF^{cyclin F} in the CDK-RB network and raise the possibility that aberrant p130 degradation could dysregulate the cell cycle in human cancers.

PP2A/B55α substrate recruitment as defined by the retinoblastoma-related protein p107

Protein phosphorylation is a reversible post-translation modification essential in cell signaling. This study addresses a long-standing question as to how the most abundant serine/threonine protein phosphatase 2 (PP2A) holoenzyme, PP2A/B55α, specifically recognizes substrates and presents them to the enzyme active site. Here, we show how the PP2A regulatory subunit B55α recruits p107, a pRB-related tumor suppressor and B55α substrate. Using molecular and cellular approaches, we identified a conserved region 1 (R1, residues 615-626) encompassing the strongest p107 binding site. This enabled us to identify an 'HxRVxxV619-625' short linear motif (SLiM) in p107 as necessary for B55α binding and dephosphorylation of the proximal pSer-615 in vitro and in cells. Numerous B55α/PP2A substrates, including TAU, contain a related SLiM C-terminal from a proximal phosphosite, 'p[ST]-P-x(4,10)-[RK]-V-x-x-[VI]-R.' Mutation of conserved SLiM residues in TAU dramatically inhibits dephosphorylation by PP2A/B55α, validating its generality. A data-guided computational model details the interaction of residues from the conserved p107 SLiM, the B55α groove, and phosphosite presentation. Altogether, these data provide key insights into PP2A/B55α's mechanisms of substrate recruitment and active site engagement, and also facilitate identification and validation of new substrates, a key step towards understanding PP2A/B55α's role in multiple cellular processes.

RBL1/p107 Expression Levels Are Modulated by Multiple Signaling Pathways

The members of the retinoblastoma (RB) protein family, RB1/p105, retinoblastoma-like (RBL)1/p107 and RBL2/p130 are critical modulators of the cell cycle and their dysregulation has been associated with tumor initiation and progression. The activity of RB proteins is regulated by numerous pathways including oncogenic signaling, but the molecular mechanisms of these functional interactions are not fully defined. We previously demonstrated that RBL2/p130 is a direct target of AKT and it is a key mediator of the apoptotic process induced by AKT inhibition. Here we demonstrated that RBL1/p107 levels are only minorly modulated by the AKT signaling pathway. In contrast, we discovered that RBL1/p107 levels are regulated by multiple pathways linked directly or indirectly to Ca²⁺-dependent signaling. Inhibition of the multifunctional calcium/calmodulin-dependent kinases (CaMKs) significantly reduced RBL1/p107 expression levels and phosphorylation, increased RBL1/p107 nuclear localization and led to cell cycle arrest in G0/G1. Targeting the Ca²⁺-dependent endopeptidase calpain stabilized RBL1/p107 levels and counteracted the reduction of RBL1/p107 levels associated with CaMKs inhibition. Thus, these novel observations suggest a complex regulation of RBL1/p107 expression involving different components of signaling pathways controlled by Ca²⁺ levels, including CaMKs and calpain, pointing out a significant difference with the mechanisms modulating the close family member RBL2/p130.

PARP14 regulates cyclin D1 expression to promote cell-cycle progression

Cyclin D1 is an essential regulator of the G1-S cell-cycle transition and is overexpressed in many cancers. Expression of cyclin D1 is under tight cellular regulation that is controlled by many signaling pathways. Here we report that PARP14, a member of the poly(ADP-ribose) polymerase (PARP) family, is a regulator of cyclin D1 expression. Depletion of PARP14 leads to decreased cyclin D1 protein levels. In cells with a functional retinoblastoma (RB) protein pathway, this results in G1 cell-cycle arrest and reduced proliferation. Mechanistically, we found that PARP14 controls cyclin D1 mRNA levels. Using luciferase assays, we show that PARP14 specifically regulates cyclin D1 3'UTR mRNA stability. Finally, we also provide evidence that G1 arrest in PARP14-depleted cells is dependent on an intact p53-p21 pathway. Our work uncovers a new role for PARP14 in promoting cell-cycle progression through both cyclin D1 and the p53 pathway.

Role of the Holoenzyme PP1-SPN in the Dephosphorylation of the RB Family of Tumor Suppressors During Cell Cycle

Simple Summary

Cell cycle progression is highly regulated by modulating the phosphorylation status of retinoblastoma (RB) family proteins. This process is controlled by a balance in the action of kinases, such as the complexes formed by cyclin-dependent kinases (CDKs) and cyclins, and phosphatases, mainly the protein phosphatase 1 (PP1). However, while the phosphorylation of the RB family has been largely studied, its dephosphorylation is less known. Recently, the PP1-Spinophilin (SPN) holoenzyme has been described as the main phosphatase responsible for the dephosphorylation of RB proteins during the G0/G1 transition and at the end of G1. Here, we describe the regulation of the phosphorylation status of RB family proteins, giving importance not only to their inactivation by phosphorylation but also to their dephosphorylation to restore the cell cycle.

Abstract

Cell cycle progression is highly regulated by modulating the phosphorylation status of the retinoblastoma protein (pRB) and the other two members of the RB family, p107 and p130. This process is controlled by a balance in the action of kinases, such as the complexes formed by cyclin-dependent kinases (CDKs) and cyclins, and phosphatases, mainly the protein phosphatase 1 (PP1). However, while the phosphorylation of the RB family has been largely studied, its dephosphorylation is less known. Phosphatases are holoenzymes formed by a catalytic subunit and a regulatory protein with substrate specificity. Recently, the PP1-Spinophilin (SPN) holoenzyme has been described as the main phosphatase responsible for the dephosphorylation of RB proteins during the G0/G1 transition and at the end of G1. Moreover, SPN has been described as a tumor suppressor dependent on PP1 in lung and breast tumors, where it promotes tumorigenesis by increasing the cancer stem cell pool. Therefore, a connection between the cell cycle and stem cell biology has also been proposed via SPN/PP1/RB proteins.

Roles of plant retinoblastoma protein: cell cycle and beyond

The human retinoblastoma (RB1) protein is a tumor suppressor that negatively regulates cell cycle progression through its interaction with members of the E2F/DP family of transcription factors. However, RB-related (RBR) proteins are an early acquisition during eukaryote evolution present in plant lineages, including unicellular algae, ancient plants (ferns, lycophytes, liverworts, mosses), gymnosperms, and angiosperms. The main RBR protein domains and interactions with E2Fs are conserved in all eukaryotes and not only regulate the G1/S transition but also the G2/M transition, as part of DREAM complexes. RBR proteins are also important for asymmetric cell division, stem cell maintenance, and the DNA damage response (DDR). RBR proteins play crucial roles at every developmental phase transition, in association with chromatin factors, as well as during the reproductive phase during female and male gametes production and embryo development. Here, we review the processes where plant RBR proteins play a role and discuss possible avenues of research to obtain a full picture of the multifunctional roles of RBR for plant life.

Chemotherapy and CDK4/6 Inhibitors: Unexpected Bedfellows

Cyclin-dependent kinases 4 and 6 (CDK4/6) have emerged as important therapeutic targets. Pharmacologic inhibitors of these kinases function to inhibit cell-cycle progression and exert other important effects on the tumor and host environment. Because of their impact on the cell cycle, CDK4/6 inhibitors (CDK4/6i) have been hypothesized to antagonize the antitumor effects of cytotoxic chemotherapy in tumors that are CDK4/6 dependent. However, there are multiple preclinical studies that illustrate potent cooperation between CDK4/6i and chemotherapy. Furthermore, the combination of CDK4/6i and chemotherapy is being tested in clinical trials to both enhance antitumor efficacy and limit toxicity. Exploitation of the noncanonical effects of CDK4/6i could also provide an impetus for future studies in combination with chemotherapy. Thus, while seemingly mutually exclusive mechanisms are at play, the combination of CDK4/6 inhibition and chemotherapy could exemplify rational medicine.

RB, p130 and p107 differentially repress G1/S and G2/M genes after p53 activation

Cell cycle gene expression occurs in two waves. The G1/S genes encode factors required for DNA synthesis and the G2/M genes contribute to mitosis. The Retinoblastoma protein (RB) and DREAM complex (DP, RB-like, E2F4 and MuvB) cooperate to repress all cell cycle genes during G1 and inhibit entry into the cell cycle. DNA damage activates p53 leading to increased levels of p21 and inhibition of cell cycle progression. Whether the G1/S and G2/M genes are differentially repressed by RB and the RB-like proteins p130 and p107 in response to DNA damage is not known. We performed gene expression profiling of primary human fibroblasts upon DNA damage and assessed the effects on G1/S and G2/M genes. Upon p53 activation, p130 and RB cooperated to repress the G1/S genes. In addition, in the absence of RB and p130, p107 contributed to repression of G1/S genes. In contrast, G2/M genes were repressed by p130 and p107 after p53 activation. Furthermore, repression of G2/M genes by p107 and p130 led to reduced entry into mitosis. Our data demonstrates specific roles for RB, p130-DREAM, and p107-DREAM in p53 and p21 mediated repression of cell cycle genes.

Cyclin D-CDK4 relieves cooperative repression of proliferation and cell cycle gene expression by DREAM and RB

The Retinoblastoma protein (RB) restricts cell cycle gene expression and entry into the cell cycle. The RB-related protein p130 forms the DREAM (DP, RB-like, E2F and MuvB) complex and contributes to repression of cell cycle dependent genes during quiescence. Although both RB and DREAM bind and repress an overlapping set of E2F dependent gene promoters, it remains unclear if they cooperate to restrict cell cycle entry. To test the specific contributions of RB and DREAM, we generated RB and p130 knockout cells in primary human fibroblasts. Knockout of both p130 and RB yielded higher levels of cell cycle gene expression in G0 and G1 cells compared to cells with knockout of RB alone, indicating a role for DREAM and RB in repression of cell cycle genes. We observed that RB played a dominant role in E2F dependent gene repression during mid to late G1 while DREAM activity was more prominant during G0 and early G1. Cyclin D - Cyclin Dependent Kinase 4 (CDK4) dependent phosphorylation of p130 occurred during early G1 and led to the release of p130 and MuvB from E2F4 and decreased p130 and MuvB binding to cell cycle promoters. Specific inhibition of CDK4 activity by palbociclib blocked DREAM complex disassembly during cell cycle entry. In addition, sensitivity to CDK4 inhibition was dependent on RB and an intact DREAM complex in both normal cells as well as in palbociclib-sensitive cancer cell lines. Although RB knockout cells were partially resistant to CDK4 inhibition, RB and p130 double knockout cells were significantly more resistant to palbociclib treatment. These results indicate that DREAM cooperates with RB in repressing E2F dependent gene expression and cell cycle entry and supports a role for DREAM as a therapeutic target in cancer.

Nucleosomes positioning around transcriptional start site of tumor suppressor (Rbl2/p130) gene in breast cancer

Dynamic positioning of nucleosomes is pivotal in determining level of genes expression especially on or around transcription start site (TSS) of a gene. Purpose of the current study was to determine nucleosome position around TSS of Rbl2/p130. We investigated Rbl2/p130 expression in connection to nucleosome positions around its TSS among breast tumors and their adjacent normal control tissues (ANCT) using micrococcal nuclease (MNAse) digestion assay and ChIP-PCR analysis. Three fold reduced Rbl2/p130 expression in these tumor tissues were noticed compared to their control tissues. DNA obtained from MNAse digested chromatin was used as PCR template. Region between - 137 to + 140 around TSS was scanned using 3 primer pairs (P1 = - 137 to + 69; P2 = - 90 to + 69; P3 = - 33 to + 140). ~ 66% breast tumors and ~ 26% ANCT samples were positive for P1. The difference was found statistically significant (p = 0.000) with an odd ratio (OD) of 9.143, suggesting that nucleosome formation in this region is ~ 9 times more probable in tumor samples. ~ 73% of the tumor and 60% ANCT were positive for P2, which although is significant (p = 0.035) with OD = 3.250, but less preferable than P1. However, P3 was not found to be a preferred area for nucleosome occupancy (p = 0.670; OD = 1.2). Negative correlations for nucleosome positions were observed especially for P1. Our results indicate that nucleosome are present slightly downstream of TSS in routine, while in case of breast carcinogenesis nucleosomes slides 55 bases upstream of the TSS, aligning + 1 position at the center of nucleosome, hence hindering access to the transcriptional machinery.

Cell cycle arrest through indirect transcriptional repression by p53: I have a DREAM

Activation of the p53 tumor suppressor can lead to cell cycle arrest. The key mechanism of p53-mediated arrest is transcriptional downregulation of many cell cycle genes. In recent years it has become evident that p53-dependent repression is controlled by the p53–p21–DREAM–E2F/CHR pathway (p53–DREAM pathway). DREAM is a transcriptional repressor that binds to E2F or CHR promoter sites. Gene regulation and deregulation by DREAM shares many mechanistic characteristics with the retinoblastoma pRB tumor suppressor that acts through E2F elements. However, because of its binding to E2F and CHR elements, DREAM regulates a larger set of target genes leading to regulatory functions distinct from pRB/E2F. The p53–DREAM pathway controls more than 250 mostly cell cycle-associated genes. The functional spectrum of these pathway targets spans from the G₁ phase to the end of mitosis. Consequently, through downregulating the expression of gene products which are essential for progression through the cell cycle, the p53–DREAM pathway participates in the control of all checkpoints from DNA synthesis to cytokinesis including G₁/S, G₂/M and spindle assembly checkpoints. Therefore, defects in the p53–DREAM pathway contribute to a general loss of checkpoint control. Furthermore, deregulation of DREAM target genes promotes chromosomal instability and aneuploidy of cancer cells. Also, DREAM regulation is abrogated by the human papilloma virus HPV E7 protein linking the p53–DREAM pathway to carcinogenesis by HPV. Another feature of the pathway is that it downregulates many genes involved in DNA repair and telomere maintenance as well as Fanconi anemia. Importantly, when DREAM function is lost, CDK inhibitor drugs employed in cancer treatment such as Palbociclib, Abemaciclib and Ribociclib can compensate for defects in early steps in the pathway upstream from cyclin/CDK complexes. In summary, the p53–p21–DREAM–E2F/CHR pathway controls a plethora of cell cycle genes, can contribute to cell cycle arrest and is a target for cancer therapy.

Cyclin D3 deficiency inhibits skin tumor development, but does not affect normal keratinocyte proliferation

Rearrangement and amplification of the D-type cyclin genes have been reported in human cancer. Previous studies have demonstrated that Ras-mediated skin tumorigenesis depends on pathways that act through cyclin D1 and D2; however, the role of cyclin D3 remains unknown. The present study demonstrates that cyclin D3 ablation does not affect keratinocyte proliferation, but instead increases apoptosis levels in the bulge region of the hair follicle. Consequently, cyclin D3 ablation reduces skin papilloma development in a Ras-dependent carcinogenesis model. Previous results revealed that cyclin D3 preferentially binds to cyclin-dependent kinase 6 (CDK6) in mouse keratinocytes and transgenic expression of CDK6 (K5CDK6 mice) inhibits skin tumor development. Thus, we hypothesized that the inhibitory effect of CDK6 is dependent on cyclin D3 expression. To test this hypothesis, a mouse model that overexpresses CDK6 and does not express cyclin D3 (K5CDK6/cyclin D3-/− compound mouse) was developed. Biochemical analysis of the epidermis of K5CDK6/cyclin D3-/− mice revealed that cyclin D3 ablation resulted in increased expression of cyclin D1 protein, with a consequent elevation in the level of CDK6/cyclin D1 and CDK4/cyclin D1 complexes. These findings were concurrent with the increase skin papilloma malignant progression observed in K5CDK6/cyclin D3-/− mice. In summary the absence of cyclin D3 led to fewer number of papillomas in cyclin D3-ablated mice than in the wild-type owing to increased apoptosis, suggesting that alterations in cell survival are a crucial mechanism for crippling cellular defense against neoplasia. The results of the current study also suggest that although cyclin D3 expression does not alter the tumor suppressive role of CDK6 in skin carcinogenesis, the compensatory increase in cyclin D1 can shift the balance towards malignant progression.

Structural Conservation and E2F Binding Specificity within the Retinoblastoma Pocket Protein Family

The human pocket proteins retinoblastoma (Rb), p107, and p130 are critical negative regulators of the cell cycle and contribute to tumor suppression. While strong structural conservation within the pocket protein family provides for some functional redundancy, important differences have been observed and may underlie the reason that Rb is a uniquely potent tumor suppressor. It has been proposed that distinct pocket protein activities are mediated by their different E2F transcription factor binding partners. In humans, Rb binds E2F1-E2F5, whereas p107 and p130 almost exclusively associate with E2F4 and E2F5. To identify the molecular determinants of this specificity, we compared the crystal structures of Rb and p107 pocket domains and identified several key residues that contribute to E2F selectivity in the pocket family. Mutation of these residues in p107 to match the analogous residue in Rb results in an increase in affinity for E2F1 and E2F2 and an increase in the ability of p107 to inhibit E2F2 transactivation. Additionally, we investigated how phosphorylation by Cyclin-dependent kinase on distinct residues regulates p107 affinity for the E2F4 transactivation domain. We found that phosphorylation of residues S650 and S975 weakens the E2F4 transactivation domain binding. Our data reveal molecular features of pocket proteins that are responsible for their similarities and differences in function and regulation.

HPV 16E7 and 48E7 proteins use different mechanisms to target p130 to overcome cell cycle block

Background

Retinoblastoma like protein 2 (RBL2) or p130 is a member of the pocket protein family, which is infrequently mutated in human tumours. Its expression is posttranscriptionally regulated and largely G0 restricted. We have previously shown that E6/E7 oncoproteins encoded by human papillomavirus (HPV) type 16, which is a high-risk type for cervical cancer development, must target p130 to promote the host cell to exit from quiescence (G0) state and enter S phase of the cell cycle. P130 is associated with the DREAM (DP, RB-like, E2F and MuvB) complex in G0/G1, which prevents S phase progression by repressing transcription of E2F-regulated genes. E7 proteins could potentially disrupt the p130-DREAM complex through two known mechanisms: direct interaction with p130 or induction of cyclin dependent kinase 2 (CDK2) phosphorylation by interacting with its inhibitor, p21^CIP1.

Methods

In this study we have used p130 mutants deficient in binding the E7 LXCXE domain (p130mE7), unphosphorylatable by CDK2 (p130PM22) or a combination of both (p130PM22/mE7) to investigate these mechanisms used by E7 proteins to disrupt the p130-DREAM complex and promote cell cycle progression.

Results

We found that HPV16 E7 binding to p130 through its LXCXE domain was absolutely required to disrupt p130-DREAM to promote S phase of the cell cycle, as HPV16 E7 was unable to suppress p130mE7 but could suppress p130PM22. In contrast, the E7 protein encoded by a cutaneous HPV type that lacks a functional LXCXE domain, HPV 48 E7, was also able to disrupt p130-DREAM to promote cell cycling, but through the alternative mechanism. Thus, HPV48 E7 could suppress a cell cycle block imposed by p130mE7, but was unable to suppress p130PM22.

Conclusions

Overall, these results indicate that suppression of p130 is required for HPV-induced cell cycling, and that different HPV E7 proteins can use alternative mechanisms to achieve this.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-015-0460-8) contains supplementary material, which is available to authorized users.

Structural mechanisms of DREAM complex assembly and regulation

To understand the biochemical mechanisms underpinning DREAM function and regulation, Guiley et al. investigated the structural basis for DREAM assembly. Together, the data inform a novel target interface for studying MuvB and p130 function and the design of inhibitors that prevent tumor escape in quiescence.

The DREAM complex represses cell cycle genes during quiescence through scaffolding MuvB proteins with E2F4/5 and the Rb tumor suppressor paralog p107 or p130. Upon cell cycle entry, MuvB dissociates from p107/p130 and recruits B-Myb and FoxM1 for up-regulating mitotic gene expression. To understand the biochemical mechanisms underpinning DREAM function and regulation, we investigated the structural basis for DREAM assembly. We identified a sequence in the MuvB component LIN52 that binds directly to the pocket domains of p107 and p130 when phosphorylated on the DYRK1A kinase site S28. A crystal structure of the LIN52–p107 complex reveals that LIN52 uses a suboptimal LxSxExL sequence together with the phosphate at nearby S28 to bind the LxCxE cleft of the pocket domain with high affinity. The structure explains the specificity for p107/p130 over Rb in the DREAM complex and how the complex is disrupted by viral oncoproteins. Based on insights from the structure, we addressed how DREAM is disassembled upon cell cycle entry. We found that p130 and B-Myb can both bind the core MuvB complex simultaneously but that cyclin-dependent kinase phosphorylation of p130 weakens its association. Together, our data inform a novel target interface for studying MuvB and p130 function and the design of inhibitors that prevent tumor escape in quiescence.

Global discovery of high-NaCl-induced changes of protein phosphorylation

High extracellular NaCl, such as in the renal medulla, can perturb and even kill cells, but cells mount protective responses that enable them to survive and function. Many high-NaCl-induced perturbations and protective responses are known, but the signaling pathways involved are less clear. Change in protein phosphorylation is a common mode of cell signaling, but there was no unbiased survey of protein phosphorylation in response to high NaCl. We used stable isotopic labeling of amino acids in cell culture coupled to mass spectrometry to identify changes in protein phosphorylation in human embryonic kidney (HEK 293) cells exposed to high NaCl. We reproducibly identify >8,000 unique phosphopeptides in 4 biological replicate samples with a 1% false discovery rate. High NaCl significantly changed phosphorylation of 253 proteins. Western analysis and targeted ion selection mass spectrometry confirm a representative sample of the phosphorylation events. We analyze the affected proteins by functional category to infer how altered protein phosphorylation might signal cellular responses to high NaCl, including alteration of cell cycle, cyto/nucleoskeletal organization, DNA double-strand breaks, transcription, proteostasis, metabolism of mRNA, and cell death.

Signaling through cyclin D-dependent kinases

Research over the past quarter century has identified cyclin D-dependent kinases, CDK4 and CDK6, as the major oncogenic drivers among members of the CDK superfamily. CDK4/6 are rendered hyperactive in the majority of human cancers through a multitude of genomic alterations. Sustained activation of these protein kinases provides cancer cells with the power to enter the cell cycle continuously by triggering G1-S-phase transitions and dramatically shortening the duration of the G1 phase. It has also become clear, however, that CDK4/6 effectively counter cancer cell-intrinsic tumor suppression mechanisms, senescence and apoptosis, which must be overcome during cell transformation and kept at bay throughout all stages of tumorigenesis. As a central 'node' in cellular signaling networks, cyclin D-dependent kinases sense a plethora of mitogenic signals to orchestrate specific transcriptional programs. As the complexity of the cellular signaling network regulated by these oncogenic kinases unfolds, much remains to be learned about its architecture, its dynamics and the consequences of its perturbation.

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.

PP1 and PP2A phosphatases--cooperating partners in modulating retinoblastoma protein activation

The retinoblastoma/pocket protein family is one of the master regulators of the eukaryotic cell cycle. It includes the retinoblastoma protein (Rb) and the related p107 and p130 proteins. The importance of the Rb pathway for homeostasis and tumour suppression is evident from the fact that inactivating mutations in Rb are frequently associated with many cancers. Rbs regulate the cell cycle by controlling the activity of the E2F family of transcription factors. The activity of Rb proteins themselves is modulated by their phosphorylation status at several Ser/Thr residues: phosphorylation by cyclin-dependent kinases inactivates Rb proteins and positively influences the transcription of genes necessary for cell cycle progression. Although the mechanisms of cyclin-dependent kinase-mediated inactivation of Rb proteins are understood in great detail, our knowledge of the process that counteracts Rb phosphorylation is still quite limited. The present review focuses on the Ser/Thr phosphatases that are responsible for the dephosphorylation and thus activation of Rb proteins. Two major scenarios are considered: (a) when pocket proteins are dephosphorylated during regular cell cycle progression and (b) when rapid dephosphorylation is dictated by external stress or growth inhibitory conditions, such as oxidative stress, UV radiation or other DNA-damaging stimuli, and cell differentiation factors. It transpires that protein phosphatase 1 and protein phosphatase 2A can efficiently modulate pocket protein activity in a highly context-dependent manner and both are tightly regulated by the presence of different regulatory subunits or interacting proteins.

Combined effect of cyclin D3 expression and abrogation of cyclin D1 prevent mouse skin tumor development

We have previously demonstrated that ras-mediated skin tumorigenesis depends on signaling pathways that act preferentially through cyclin D1 and D2. Interestingly, the expression of cyclin D3 inhibits skin tumor development, an observation that conflicts with the oncogenic role of D-type cyclins in the mouse epidermis. Here, we show that simultaneous up and downregulation of particular members of the D-type cyclin family is a valuable approach to reduce skin tumorigenesis. We developed the K5D3/cyclin D1(-/-) compound mouse, which overexpresses cyclin D3 but lacks expression of cyclin D1 in the skin. Similar to K5D3 transgenic mice, keratinocytes from K5D3/cyclin D1(-/-) compound mice show a significant reduction of cyclin D2 levels. Therefore, this model allows us to determine the effect of cyclin D3 expression when combined with reduced or absent expression of the remaining two members of the D-type cyclin family in mouse epidermis. Our data show that induced expression of cyclin D3 compensates for the reduced level of cyclin D1 and D2, resulting in normal keratinocyte proliferation. However, simultaneous ablation of cyclin D1 and downregulation of cyclin D2 via cyclin D3 expression resulted in a robust reduction in ras-mediated skin tumorigenesis. We conclude that modulation of the levels of particular members of the D-type cyclin family could be useful to inhibit tumor development and, in particular, ras-mediated tumorigenesis.

Transient arrest in a quiescent state allows ovarian cancer cells to survive suboptimal growth conditions and is mediated by both Mirk/dyrk1b and p130/RB2

Some ovarian cancer cells in vivo are in a reversible quiescent state where they can contribute to cancer spread under favorable growth conditions. The serine/threonine kinase Mirk/dyrk1B was expressed in each of seven ovarian cancer cell lines and in 21 of 28 resected human ovarian cancers, and upregulated in 60% of the cancers. Some ovarian cancer cells were found in a G0 quiescent state, with the highest fraction in a line with an amplified Mirk gene. Suboptimal culture conditions increased the G0 fraction in SKOV3 and TOV21G, but not OVCAR4 cultures. Less than half as many OVCAR4 cells survived under suboptimal culture conditions as shown by total cell numbers, dye exclusion viability studies, and assay of cleaved apoptotic marker proteins. G0 arrest in TOV21G and SKOV3 cells led to increased levels of Mirk, the CDK inhibitor p27, p130/Rb2, and p130/Rb2 complexed with E2F4. The G0 arrest was transient, and cells exited G0 when fresh nutrients were supplied. Depletion of p130/Rb2 reduced the G0 fraction, increased cell sensitivity to serum-free culture and to cisplatin, and reduced Mirk levels. Mirk contributed to G0 arrest by destabilization of cyclin D1. In TOV21G cells, but not in normal diploid fibroblasts, Mirk depletion led to increased apoptosis and loss of viability. Because Mirk is expressed at low levels in most normal adult tissues, the elevated Mirk protein levels in ovarian cancers may present a novel therapeutic target, in particular for quiescent tumor cells which are difficult to eradicate by conventional therapies targeting dividing cells.

Immunodetection of retinoblastoma-related protein and its phosphorylated form in interphase and mitotic alfalfa cells

Plant retinoblastoma-related (RBR) proteins are primarily considered as key regulators of G(1)/S phase transition, with functional roles in a variety of cellular events during plant growth and organ development. Polyclonal antibody against the C-terminal region of the Arabidopsis RBR1 protein also specifically recognizes the alfalfa 115 kDa MsRBR protein, as shown by the antigen competition assay. The MsRBR protein was detected in all cell cycle phases, with a moderate increase in samples representing G(2)/M cells. Antibody against the human phospho-pRb peptide (Ser807/811) cross-reacted with the same 115 kDa MsRBR protein and with the in vitro phosphorylated MsRBR protein C-terminal fragment. Phospho-MsRBR protein was low in G(1) cells. Its amount increased upon entry into the S phase and remained high during the G(2)/M phases. Roscovitine treatment abolished the activity of alfalfa MsCDKA1;1 and MsCDKB2;1, and the phospho-MsRBR protein level was significantly decreased in the treated cells. Colchicine block increased the detected levels of both forms of MsRBR protein. Reduced levels of the MsRBR protein in cells at stationary phase or grown in hormone-free medium can be a sign of the division-dependent presence of plant RBR proteins. Immunolocalization of the phospho-MsRBR protein indicated spots of variable number and size in the labelled interphase nuclei and high signal intensity of nuclear granules in prophase. Structures similar to phospho-MsRBR proteins cannot be recognized in later mitotic phases. Based on the presented western blot and immunolocalization data, the possible involvement of RBR proteins in G(2)/M phase regulation in plant cells is discussed.

Human parvovirus B19 causes cell cycle arrest of human erythroid progenitors via deregulation of the E2F family of transcription factors

Human parvovirus B19 (B19V) is the only human pathogenic parvovirus. It causes a wide spectrum of human diseases, including fifth disease (erythema infectiosum) in children and pure red cell aplasia in immunocompromised patients. B19V is highly erythrotropic and preferentially replicates in erythroid progenitor cells (EPCs). Current understanding of how B19V interacts with cellular factors to regulate disease progression is limited, due to a lack of permissive cell lines and animal models. Here, we employed a recently developed primary human CD36(+) EPC culture system that is highly permissive for B19V infection to identify cellular factors that lead to cell cycle arrest after B19V infection. We found that B19V exploited the E2F family of transcription factors by downregulating activating E2Fs (E2F1 to E2F3a) and upregulating repressive E2Fs (E2F4 to E2F8) in the primary CD36(+) EPCs. B19V nonstructural protein 1 (NS1) was a key viral factor responsible for altering E2F1-E2F5 expression, but not E2F6-E2F8 expression. Interaction between NS1 and E2F4 or E2F5 enhanced the nuclear import of these repressive E2Fs and induced stable G₂ arrest. NS1-induced G₂ arrest was independent of p53 activation and increased viral replication. Downstream E2F4/E2F5 targets, which are potentially involved in the progression from G₂ into M phase and erythroid differentiation, were identified by microarray analysis. These findings provide new insight into the molecular pathogenesis of B19V in highly permissive erythroid progenitors.

Estrogen regulation of cyclin E2 requires cyclin D1 but not c-Myc

During estrogen-induced proliferation, c-Myc and cyclin D1 initiate independent pathways that activate cyclin E1-Cdk2 by sequestration and/or downregulation of the CDK inhibitor p21(Waf1/Cip1), without significant increases in cyclin E1 protein levels. In contrast, cyclin E2 undergoes a marked increase in expression, which occurs within 9 to 12 h of estrogen treatment of antiestrogen-pretreated MCF-7 breast cancer cells. Both E cyclins are important to estrogen action, as small interfering RNA (siRNA)-mediated knockdown of either cyclin E1 or cyclin E2 attenuated estrogen-mediated proliferation. Inducible expression of cyclin D1 upregulated cyclin E2, while siRNA-mediated knockdown of cyclin D1 attenuated estrogen effects on cyclin E2. However, manipulation of c-Myc levels did not profoundly affect cyclin E2. Cyclin E2 induction by estrogen was accompanied by recruitment of E2F1 to the cyclin E1 and E2 promoters, and cyclin D1 induction was sufficient for E2F1 recruitment. siRNA-mediated knockdown of the chromatin remodelling factor CHD8 prevented cyclin E2 upregulation. Together, these data indicate that cyclin E2-Cdk2 activation by estrogen occurs via E2F- and CHD8-mediated transcription of cyclin E2 downstream of cyclin D1. This contrasts with the predominant regulation of cyclin E1-Cdk2 activity via CDK inhibitor association downstream of both c-Myc and cyclin D1 and indicates that cyclins E1 and E2 are not always coordinately regulated.

Differential regulation of cyclin-dependent kinase 4 (CDK4) and CDK6, evidence that CDK4 might not be activated by CDK7, and design of a CDK6 activating mutation

The homologous cyclin-dependent kinases (CDK) CDK4 and CDK6 integrate mitogenic and oncogenic signaling cascades with the cell cycle. Their activation requires binding to a D-type cyclin and then T-loop phosphorylation at T172 and T177 (respectively) by the only CDK-activating kinase identified in animal cells, cyclin H-CDK7. At odds with the existing data showing the constitutive activity of CDK7, we have recently identified the T172 phosphorylation of cyclin D-bound CDK4 as a crucial cell cycle regulatory target. Here we show that T172 phosphorylation of CDK4 is conditioned by its unique proline 173 residue. In contrast to CDK4, CDK6 does not contain such a proline and, unexpectedly, remained poorly phosphorylated and active in a variety of cells. Mutations of proline 173 did not adversely affect CDK4 activation by CDK7, but in cells they abolished CDK4 T172 phosphorylation and activity. Conversely, substituting a proline for the corresponding residue of CDK6 enforced its complete, apparently cyclin-independent T177 phosphorylation and dramatically increased its activity. These results lead us to propose that CDK4 might not be phosphorylated by CDK7 in intact cells but is more likely phosphorylated by another, presumably proline-directed kinase(s). Moreover, they provide a new model of a potentially oncogenic activating mutation of a CDK.

PP2A-mediated dephosphorylation of p107 plays a critical role in chondrocyte cell cycle arrest by FGF

FGF signaling inhibits chondrocyte proliferation, a cell type-specific response that is the basis for several genetic skeletal disorders caused by activating FGFR mutations. This phenomenon requires the function of the p107 and p130 members of the Rb protein family, and p107 dephosphorylation is one of the earliest distinguishing events in FGF-induced growth arrest. To determine whether p107 dephoshorylation played a critical role in the chondrocyte response to FGF, we sought to counteract this process by overexpressing in RCS chondrocytes the cyclin D1/cdk4 kinase complex. CyclinD/cdk4-expressing RCS cells became resistant to FGF-induced p107 dephosphorylation and growth arrest, and maintained significantly high levels of cyclin E/cdk2 activity and of phosphorylated p130 at later times of FGF treatment. We explored the involvement of a phosphatase in p107 dephosphorylation. Expression of the SV40 small T-Ag, which inhibits the activity of the PP2A phosphatase, or knockdown of the expression of the PP2A catalytic subunit by RNA interference prevented p107 dephosphorylation and FGF-induced growth arrest of RCS cells. Furthermore, an association between p107 and PP2A was induced by FGF treatment. Our data show that p107 dephosphorylation is a key event in FGF-induced cell cycle arrest and indicate that in chondrocytes FGF activates the PP2A phosphatase to promote p107 dephosphorylation.

A comprehensive modular map of molecular interactions in RB/E2F pathway

We present, here, a detailed and curated map of molecular interactions taking place in the regulation of the cell cycle by the retinoblastoma protein (RB/RB1). Deregulations and/or mutations in this pathway are observed in most human cancers. The map was created using Systems Biology Graphical Notation language with the help of CellDesigner 3.5 software and converted into BioPAX 2.0 pathway description format. In the current state the map contains 78 proteins, 176 genes, 99 protein complexes, 208 distinct chemical species and 165 chemical reactions. Overall, the map recapitulates biological facts from approximately 350 publications annotated in the diagram. The network contains more details about RB/E2F interaction network than existing large-scale pathway databases. Structural analysis of the interaction network revealed a modular organization of the network, which was used to elaborate a more summarized, higher-level representation of RB/E2F network. The simplification of complex networks opens the road for creating realistic computational models of this regulatory pathway.

PsRBR1 encodes a pea retinoblastoma-related protein that is phosphorylated in axillary buds during dormancy-to-growth transition

In intact plants, cells in axillary buds are arrested at the G1 phase of the cell cycle during dormancy. In mammalian cells, the cell cycle is suppressed at the G1 phase by the activities of retinoblastoma tumor suppressor gene (RB) family proteins, depending on their phosphorylation state. Here, we report the isolation of a pea cDNA clone encoding an RB-related protein (PsRBR1, Accession No. AB012024) with a high degree of amino acid conservation in comparison with RB family proteins. PsRBR1 protein was detected as two polypeptides using an anti-PsRBR1 antibody in dormant axillary buds, whereas it was detected as three polypeptides, which were the same two polypeptides and another larger polypeptide 2 h after terminal decapitation. Both in vitro-synthesized PsPRB1 protein and lambda protein phosphatase-treated PsRBR1 protein corresponded to the smallest polypeptide detected by anti-PsRBR1 antibody, suggesting that the three polypeptides correspond to non-phosphorylated form of PsRBR1 protein, and lower- and higher-molecular mass forms of phosphorylated PsRBR1 protein. Furthermore, in vivo labeling with [32P]-inorganic phosphate indicated that PsRBR1 protein was more phosphorylated before mRNA accumulation of cell cycle regulatory genes such as PCNA. Together these findings suggest that dormancy-to-growth transition in pea axillary buds is regulated by molecular mechanisms of cell cycle control similar to those in mammals, and that the PsRBR1 protein has an important role in suppressing the cell cycle during dormancy in axillary buds.

The Rb family connects with the Tp53 family in skin carcinogenesis

In contrast with the low frequency of alterations found in the Rb gene, the pRb pathway is inactivated in the vast majority of human tumors. A similar situation takes place in mouse models of cancer, including two-stage skin tumorigenesis. This might be explained if the Rb functions are carried out, in its absence, by other proteins that are also controlled by the same upstream regulators and display similar effectors. The other Rb family members, p107 and or p130, are plausible candidates. The embryonic lethality of pRb-deficient animals, which precludes the analysis of the roles of Rb gene in mouse models, has been avoided using tissue-specific deletion of pRb. In epidermis, pRb deletion leads to altered proliferation and differentiation. However, these deficient mice do not develop spontaneous tumors, and chemical carcinogenesis experiments revealed that the absence of pRb renders fewer and smaller tumors than control animals, but showing increased malignant conversion to squamous cell carcinomas (SCC). Detailed biochemical analyses have indicated that, in the absence of pRb, multiple pathways, including the aberrant p53 activation mediated by E2F/p19(ARF), are activated leading to increased tumor apoptosis. As Rb loss in epidermis is functionally compensated by Rbl1 (p107), this might also suggest that p107 could behave as a tumor suppressor. We summarize here our findings in support of this hypothesis. The pRb-;p107-/- epidermis form spontaneous tumors, and the reduction of p107 levels restores the susceptibility of pRb-mice to chemical skin carcinogenesis experiments. Moreover, Rb-deficient keratinocytes are highly susceptible to Ha-ras-induced transformation, and this susceptibility is enhanced by p107 loss. Further functional studies have indicated that the loss of p107 in the absence of pRb produces the reduction of p53-dependent proapoptotic signals through the modulation of p63 and p73 isoforms. In addition, expression profiling analysis has revealed multiple oncogenic alterations that can contribute to tumor susceptibility in epidermis in the absence of pRb and p107.

E2F4 function in G2: maintaining G2-arrest to prevent mitotic entry with damaged DNA

Mammalian cells undergo cell cycle arrest in response to DNA damage through multiple checkpoint mechanisms. One such checkpoint pathway maintains genomic integrity by delaying mitotic progression in response to genotoxic stress. Transition though the G2 phase and entry into mitosis is considered to be regulated primarily by cyclin B1 and its associated catalytically active partner Cdk1. While not necessary for its initiation, the p130 and Rb-dependent target genes have emerged as being important for stable maintenance of a G2 arrest. It was recently demonstrated that by interacting with p130, E2F4 is present in the nuclei and plays a key role in the maintenance of this stable G2 arrest. Increased E2F4 levels and its translocation to the nucleus following genotoxic stress result in downregulation of many mitotic genes and as a result promote a G0-like state. Irradiation of E2F4-depleted cells leads to enhanced cellular DNA double-strand breaks that may be measured by comet assays. It also results in cell death that is characterized by caspase activation, sub-G1 and sub-G2 DNA content, and decreased clonogenic cell survival. Here we review these recent findings and discuss the mechanisms of G2 phase checkpoint activation and maintenance with a particular focus on E2F4.

Transcription of the MAT2A gene, coding for methionine adenosyltransferase, is up-regulated by E2F and Sp1 at a chromatin level during proliferation of liver cells

Methionine adenosyltransferase (MAT) is an essential enzyme because it catalyzes the formation of S-adenosylmethionine, the main methyl donor. Two MAT-encoding genes (MAT1A, MAT2A) are found in mammals. The latter is expressed in proliferating liver, dedifferentiation and cancer, whereas MAT1A is expressed in adult quiescent hepatocytes. Here, we report studies on the molecular mechanisms controlling the induction of MAT2A in regenerating rat liver and in proliferating hepatocytes. The MAT2A is up-regulated at two discrete moments during liver regeneration, as confirmed by RNApol-ChIP analysis. The first one coincides with hepatocyte priming (i.e. G0-G1 transition), while the second one takes place at the G1-S interface. Electrophoretic mobility shift assays showed that a putative E2F sequence present in MAT2A promoter binds this factor and ChIP assays confirmed that E2F1, E2F3 and E2F4, as well as the pocket protein p130, are bound to the promoter in quiescent liver. MAT2A activation is accompanied by changes in the binding of histone-modifying enzymes to the promoter. Interestingly, p130 is not displaced from MAT2A promoter during hepatocyte priming, but it is in the late expression of the gene at the G1-S transition. Finally, the transcription factor Sp1 seems to play a decisive role in MAT2A induction, as it binds the promoter when the gene is being actively transcribed. In summary, the present work shows that the molecular mechanism of MAT2A expression is different during G0-G1 or G1-S transition and this may be related to the distinct requirements of S-adenosylmethionine during liver regeneration.

Cyclin D1/cdk4 can interact with E2F4/DP1 and disrupts its DNA-binding capacity

The E2F family of transcription factors regulate the expression of many growth-related genes in a cell cycle-dependent manner. These transcription factors can activate or, in conjunction with an Rb-related protein, repress transcription. E2F transcriptional activity is regulated at several different levels that are each linked to cell cycle progression. In many cell types, E2F4 and E2F5 are the predominant E2F species during G(0) and early G(1) and function primarily as repressors of E2F-regulated genes. In this study, co-immunoprecipitation techniques were used to demonstrate that cyclins D1, D2, and D3 are capable of interacting with E2F4, E2F5, and DP1. Overexpression of cyclin D1/cdk4 reduced E2F4-mediated transcription in a simple reporter gene assay and electrophoretic mobility shift analyses using nuclear extracts from transfected cells indicated that cyclin D1/cdk4 disrupts the DNA-binding ability of E2F4. Cell cycle analysis following stimulation of serum-starved 3T3 cells indicated that E2F4 undergoes changes in its phosphorylation pattern coincident with the synthesis of cyclin D1. Examination of a series of E2F4 deletion mutants indicated that a cyclin D1-binding site located close to the carboxyl terminus of E2F4 was critical for the disruption of DNA binding by cyclin D1/cdk4. These data support a model in which E2F4 DNA binding is abolished during mid-G(1) at the same time when E2F interactions with pRb-related proteins are disrupted by cyclin D1/cdk4.

Regulation of CDK4

Cyclin-dependent kinase (CDK)4 is a master integrator that couples mitogenic and antimitogenic extracellular signals with the cell cycle. It is also crucial for many oncogenic transformation processes. In this overview, we address various molecular features of CDK4 activation that are critical but remain poorly known or debated, including the regulation of its association with D-type cyclins, its subcellular location, its activating Thr172-phosphorylation and the roles of Cip/Kip CDK "inhibitors" in these processes. We have recently identified the T-loop phosphorylation of CDK4, but not of CDK6, as a determining target for cell cycle control by extracellular factors, indicating that CDK4-activating kinase(s) might have to be reconsidered.

E2F4 regulates a stable G2 arrest response to genotoxic stress in prostate carcinoma

The retinoblastoma (pRB) family proteins regulate the E2F transcription factors; their complexes regulate critical transitions through the cell cycle. The function of these pRB family/E2F complexes, which includes p130/E2F4, in response to genotoxic agents, is not well understood. We investigated the role of E2F4 in the genotoxic stress response. Following radiation treatment, E2F4 colocalized with p130 in the nucleus during a radiation-induced stable G(2)-phase arrest. Arrested cells had significantly decreased expression of Cyclins A2 and B1 and decreased phosphorylation of mitotic protein monoclonal-2 (MPM-2) mitotic proteins. Small interference RNA (siRNA)-mediated knockdown of E2F4 sensitized cells to subsequent irradiation, resulting in enhanced cellular DNA damage and cell death, as determined by caspase activation and decreased clonogenic cell survival. Downstream E2F4 targets potentially involved in the progression from G(2) into M phase were identified by oligonucleotide microarray expression profiling. Chromatin immunoprecipitation localized E2F4 at promoter regions of the Bub3 and Pttg1 mitotic genes following irradiation, which were among the downregulated genes identified by the microarray. These data suggest that in response to radiation, E2F4 becomes active in the nucleus, enforces a stable G(2) arrest by target gene repression, and thus provides increased cell survival ability by minimizing propagation of cells that have irreparable DNA damage.

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.

Cyclin D2 and cyclin D3 play opposite roles in mouse skin carcinogenesis

D-type cyclins are components of the cell-cycle engine that link cell signaling pathways and passage throughout G1 phase. We previously described the effects of overexpression cyclin D1, D2 or D3 in mouse epidermis and tumor development. We now asked whether cyclin D2 and/or cyclin D3 play a relevant role in ras-dependent tumorigenesis. Here, we described the effect of cyclin D3 and cyclin D2 overexpression in mouse skin tumor development. Notably, overexpression of cyclin D3 results in reduced tumor development and malignant progression to squamous cell carcinomas (SCC). Biochemical analysis of keratinocytes shows that overexpression of cyclin D3 results in strong reduction of cyclin D2 and its associated kinase activity. Furthermore, we found that reinstatement of cyclin D2 level in the cyclin D3/cyclin D2 bigenic mice results in a complete reversion of the inhibitory action of cyclin D3. Supporting these results, ablation of cyclin D2 results in reduced tumorigenesis and malignant progression. On the other hand, overexpression of cyclin D2 results in an increased number of papillomas and malignant progression. We conclude that cyclin D3 and cyclin D2 play opposite roles in mouse skin tumor development and that the suppressive activity of cyclin D3 is associated with cyclin D2 downregulation.

Regulated activating Thr172 phosphorylation of cyclin-dependent kinase 4(CDK4): its relationship with cyclins and CDK "inhibitors"

Cyclin-dependent kinase 4 (CDK4) is a master integrator of mitogenic and antimitogenic extracellular signals. It is also crucial for many oncogenic transformation processes. Various molecular features of CDK4 activation remain poorly known or debated, including the regulation of its association with D-type cyclins, its activating Thr172 phosphorylation, and the roles of Cip/Kip CDK "inhibitors" in these processes. Thr172 phosphorylation of CDK4 was reinvestigated using two-dimensional gel electrophoresis in various experimental systems, including human fibroblasts, canine thyroid epithelial cells stimulated by thyrotropin, and transfected mammalian and insect cells. Thr172 phosphorylation of CDK4 depended on prior D-type cyclin binding, but Thr172 phosphorylation was also found in p16-bound CDK4. Opposite effects of p27 on cyclin D3-CDK4 activity observed in different systems depended on its stoichiometry in this complex. Thr172-phosphorylated CDK4 was enriched in complexes containing p21 or p27, even at inhibitory levels of p27 that precluded CDK4 activity. Deletion of the p27 nuclear localization signal sequence relocalized cyclin D3-CDK4 in the cytoplasm but did not affect CDK4 phosphorylation. Within cyclin D3 complexes, T-loop phosphorylation of CDK4, but not of CDK6, was directly regulated, identifying it as a determining target for cell cycle control by extracellular factors. Collectively, these unexpected observations indicate that CDK4-activating kinase(s) should be reconsidered.

Identification of mutations that disrupt phosphorylation-dependent nuclear export of cyclin D1

Although cyclin D1 is overexpressed in a significant number of human cancers, overexpression alone is insufficient to promote tumorigenesis. In vitro studies have revealed that inhibition of cyclin D1 nuclear export unmasks its neoplastic potential. Cyclin D1 nuclear export depends upon phosphorylation of a C-terminal residue, threonine 286, (Thr-286) which in turn promotes association with the nuclear exportin, CRM1. Mutation of Thr-286 to a non-phosphorylatable residue results in a constitutively nuclear cyclin D1 protein with significantly increased oncogenic potential. To determine whether cyclin D1 is subject to mutations that inhibit its nuclear export in human cancer, we have sequenced exon 5 of cyclin D1 in primary esophageal carcinoma samples and in cell lines derived from esophageal cancer. Our work reveals that cyclin D1 is subject to mutations in primary human cancer. The mutations identified specifically disrupt phosphorylation of cyclin D1 at Thr-286, thereby enforcing nuclear accumulation of cyclin D1. Through characterization of these mutants, we also define an acidic residue within the C-terminus of cyclin D1 that is necessary for recognition and phosphorylation of cyclin D1 by glycogen synthase kinase-3 beta. Finally, through construction of compound mutants, we demonstrate that cell transformation by the cancer-derived cyclin D1 alleles correlates with their ability to associate with and activate CDK4. Our data reveal that cyclin D1 is subject to mutations in primary human cancer that specifically disrupt phosphorylation-dependent nuclear export of cyclin D1 and suggest that such mutations contribute to the genesis and progression of neoplastic growth.

The retinoblastoma gene family in cell cycle regulation and suppression of tumorigenesis

Since its discovery in 1986, as the first tumor suppressor gene, the retinoblastoma gene (Rb) has been extensively studied. Numerous biochemical and genetic studies have elucidated in great detail the function of the Rb gene and placed it at the heart of the molecular machinery controlling the cell cycle. As more insight was gained into the genetic events required for oncogenic transformation, it became clear that the retinoblastoma gene is connected to biochemical pathways that are dysfunctional in virtually all tumor types. Besides regulating the E2F transcription factors, pRb is involved in numerous biological processes such as apoptosis, DNA repair, chromatin modification, and differentiation. Further complexity was added to the system with the discovery of p107 and p130, two close homologs of Rb. Although the three family members share similar functions, it is becoming clear that these proteins also have unique functions in differentiation and regulation of transcription. In contrast to Rb, p107 and p130 are rarely found inactivated in human tumors. Yet, evidence is accumulating that these proteins are part of a "tumor-surveillance" mechanism and can suppress tumorigenesis. Here we provide an overview of the knowledge obtained from studies involving the retinoblastoma gene family with particular focus on its role in suppressing tumorigenesis.

Involvement of E2F transcription factor family in cancer

The E2F family of transcription factors is a central modulator of important cellular events, including cell cycle progression, apoptosis and DNA damage response. The role of E2F family members in various human malignancies is yet unclear and may provide vital clues to the diagnosis, prognosis and therapy of cancer patients. In this review we provide a brief but concise overview of E2F function and its putative role in the most common human tumour types.

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Are RB proteins a potential substrate of Pin1 in the regulation of the cell cycle?

RB family members are post-transductionally regulated proteins and phosphorylation at Ser/Thr residues leads to their gradual inactivation. Cyclin/cdk complexes are mainly responsible for the regulation of these pocket proteins, which is crucial for release of E2F factor. Despite the fact that E2F release is a phosphorylation-dependent process, it is still not evident how phosphorylation physically determines the shift from the active to the inactive feature of RB molecules. We would like to put forward the hypothesis that Pin1 is involved in RB proteins phosphorylation and E2F release, suggesting an additional post-translational level of control on this family of molecules.