Physical interaction of the retinoblastoma protein with human D cyclins

CCND2 Modified mRNA Activates Cell Cycle of Cardiomyocytes in Hearts With Myocardial Infarction in Mice and Pigs

BACKGROUND

Experiments in mammalian models of cardiac injury suggest that the cardiomyocyte-specific overexpression of CCND2 (cyclin D2, in humans) improves recovery from myocardial infarction (MI). The primary objective of this investigation was to demonstrate that our specific modified mRNA translation system (SMRTs) can induce CCND2 expression in cardiomyocytes and replicate the benefits observed in other studies of cardiomyocyte-specific CCND2 overexpression for myocardial repair.

METHODS

The CCND2-cardiomyocyte-specific modified mRNA translation system (cardiomyocyte SMRTs) consists of 2 modRNA constructs: one codes for CCND2 and contains a binding site for L7Ae, and the other codes for L7Ae and contains recognition elements for the cardiomyocyte-specific microRNAs miR-1 and miR-208. Thus, L7Ae suppresses CCND2 translation in noncardiomyocytes but is itself suppressed by endogenous miR-1 and -208 in cardiomyocytes, thereby facilitating cardiomyocyte-specific CCND2 expression. Experiments were conducted in both mouse and pig models of MI, and control assessments were performed in animals treated with an SMRTs coding for the cardiomyocyte-specific expression of luciferase or green fluorescent protein (GFP), in animals treated with L7Ae modRNA alone or with the delivery vehicle, and in Sham-operated animals.

RESULTS

CCND2 was abundantly expressed in cultured, postmitotic cardiomyocytes 2 days after transfection with the CCND2-cardiomyocyte SMRTs, and the increase was accompanied by the upregulation of markers for cell-cycle activation and proliferation (eg, Ki67 and Aurora B kinase). When the GFP-cardiomyocyte SMRTs were intramyocardially injected into infarcted mouse hearts, the GFP signal was observed in cardiomyocytes but no other cell type. In both MI models, cardiomyocyte proliferation (on day 7 and day 3 after treatment administration in mice and pigs, respectively) was significantly greater, left-ventricular ejection fractions (days 7 and 28 in mice, days 10 and 28 in pigs) were significantly higher, and infarcts (day 28 in both species) were significantly smaller in animals treated with the CCND2-cardiomyocyte SMRTs than in any other group that underwent MI induction.

CONCLUSIONS

Intramyocardial injections of the CCND2-cardiomyocyte SMRTs promoted cardiomyocyte proliferation, reduced infarct size, and improved cardiac performance in small and large mammalian hearts with MI.

Crystal structure of active CDK4-cyclin D and mechanistic basis for abemaciclib efficacy

Despite the biological and therapeutic relevance of CDK4/6 for the treatment of HR+, HER2- advanced breast cancer, the detailed mode of action of CDK4/6 inhibitors is not completely understood. Of particular interest, phosphorylation of CDK4 at T172 (pT172) is critical for generating the active conformation, yet no such crystal structure has been reported to date. We describe here the x-ray structure of active CDK4-cyclin D3 bound to the CDK4/6 inhibitor abemaciclib and discuss the key aspects of the catalytically-competent complex. Furthermore, the effect of CDK4/6 inhibitors on CDK4 T172 phosphorylation has not been explored, despite its role as a potential biomarker of CDK4/6 inhibitor response. We show mechanistically that CDK4/6i stabilize primed (pT172) CDK4-cyclin D complex and selectively displace p21 in responsive tumor cells. Stabilization of active CDK4-cyclin D1 complex can lead to pathway reactivation following alternate dosing regimen. Consequently, sustained binding of abemaciclib to CDK4 leads to potent cell cycle inhibition in breast cancer cell lines and prevents rebound activation of downstream signaling. Overall, our study provides key insights demonstrating that prolonged treatment with CDK4/6 inhibitors and composition of the CDK4/6-cyclin D complex are both critical determinants of abemaciclib efficacy, with implications for this class of anticancer therapy.

Targeting Wnt/β-catenin signaling pathway in triple-negative breast cancer by benzylic organotrisulfides: Contribution of the released hydrogen sulfide towards potent anti-cancer activity

The potent anti-cancer activity of naturally occurring organopolysulfides has attracted wide research attention over the last two decades. Sustained donation of hydrogen sulfide (H₂S) from organopolysulfides is found to be beneficial for the treatment of several organ-specific cancers. In the present study, for the first time, the mechanism of action for the potent anti-cancer activity of bis(3,5-dimethoxybenzyl) trisulfide 4 against highly aggressive triple-negative breast cancer cells (MDA-MB-231) is described. Preliminary in vitro studies revealed potent anti-proliferative activity of the trisulfide 4 against triple-negative breast cancer cells with an IC₅₀ value of 1.0 μM. Mechanistic studies reveal that the compound exhibited anti-cancer activity, primarily by targeting and suppressing the Wnt/β-catenin signaling pathway. The inactivation of the β-catenin level was associated with the cell cycle arrest in the G2/M phase and the significant down-regulation of downstream signaling genes such as Cyclin D1 and c-Myc expression. Several control experiments with analogous organosulfur compounds and the key enzyme inhibitors reveal that the presence of a trisulfide unit in the compound is crucial for the desired inactivation of β-catenin expression, which is promoted by GSK-3β-induced phosphorylation of β-catenin and its proteasomal degradation. Moreover, the trisulfide unit or the released H₂S induced down-regulation of the p53 expression with the possible S-sulfhydration process led to p53-independent up-regulation of p21 expression. Therefore, the key results of this study highlighting the potency of synthetic benzylic organotrisulfide and the released H₂S towards the growth inhibition of triple-negative breast cancer via Wnt/β-catenin signaling pathway would certainly be helpful for further studies and developing small-molecule anti-cancer therapeutics in future.

Unraveling the links between neurodegeneration and Epstein-Barr virus-mediated cell cycle dysregulation

The Epstein-Barr virus is a well-known cell cycle modulator. To establish successful infection in the host, EBV alters the cell cycle at multiple steps via antigens such as EBNAs, LMPs, and certain other EBV-encoded transcripts. Interestingly, several recent studies have indicated the possibility of EBV's neurotrophic potential. However, the effects and outcomes of EBV infection in the CNS are under-explored. Additionally, more and more epidemiological evidence implicates the cell-cycle dysregulation in neurodegeneration. Numerous hypotheses which describe the triggers that force post-mitotic neurons to re-enter the cell cycle are prevalent. Apart from the known genetic and epigenetic factors responsible, several reports have shown the association of microbial infections with neurodegenerative pathology. Although, studies implicating the herpesvirus family members in neurodegeneration exist, the involvement of Epstein-Barr virus (EBV), in particular, is under-evaluated. Interestingly, a few clinical studies have reported patients of AD or PD to be seropositive for EBV. Based on the findings mentioned above, in this review, we propose that EBV infection in neurons could drive it towards neurodegeneration through dysregulation of cell-cycle events and induction of apoptosis.

Proteomic profiling reveals CDK6 upregulation as a targetable resistance mechanism for lenalidomide in multiple myeloma

The immunomodulatory drugs (IMiDs) lenalidomide and pomalidomide are highly effective treatments for multiple myeloma. However, virtually all patients eventually relapse due to acquired drug resistance with resistance-causing genetic alterations being found only in a small subset of cases. To identify non-genetic mechanisms of drug resistance, we here perform integrated global quantitative tandem mass tag (TMT)-based proteomic and phosphoproteomic analyses and RNA sequencing in five paired pre-treatment and relapse samples from multiple myeloma patients. These analyses reveal a CDK6-governed protein resistance signature that includes myeloma high-risk factors such as TRIP13 and RRM1. Overexpression of CDK6 in multiple myeloma cell lines reduces sensitivity to IMiDs while CDK6 inhibition by palbociclib or CDK6 degradation by proteolysis targeting chimeras (PROTACs) is highly synergistic with IMiDs in vitro and in vivo. This work identifies CDK6 upregulation as a druggable target in IMiD-resistant multiple myeloma and highlights the use of proteomic studies to uncover non-genetic resistance mechanisms in cancer.

Acquired resistance to immunomodulatory drugs is common in multiple myeloma patients, but rarely attributed to genetic alterations. Here, proteomic, phosphoproteomic and RNA sequencing analysis in five paired pre-treatment and relapse samples reveals a CDK6-regulated protein resistance signature.

Cyclin E/CDK2: DNA Replication, Replication Stress and Genomic Instability

DNA replication must be precisely controlled in order to maintain genome stability. Transition through cell cycle phases is regulated by a family of Cyclin-Dependent Kinases (CDKs) in association with respective cyclin regulatory subunits. In normal cell cycles, E-type cyclins (Cyclin E1 and Cyclin E2, CCNE1 and CCNE2 genes) associate with CDK2 to promote G1/S transition. Cyclin E/CDK2 complex mostly controls cell cycle progression and DNA replication through phosphorylation of specific substrates. Oncogenic activation of Cyclin E/CDK2 complex impairs normal DNA replication, causing replication stress and DNA damage. As a consequence, Cyclin E/CDK2-induced replication stress leads to genomic instability and contributes to human carcinogenesis. In this review, we focus on the main functions of Cyclin E/CDK2 complex in normal DNA replication and the molecular mechanisms by which oncogenic activation of Cyclin E/CDK2 causes replication stress and genomic instability in human cancer.

Computational and structural based approach to identify malignant nonsynonymous single nucleotide polymorphisms associated with CDK4 gene

Cycline-dependent kinase 4 (CDK4), an enzyme of the cycline dependent or Ser/Thr protein kinase family, plays a role in cell cycle progression (G1 phase) by phosphorylating a tumor suppressor protein called pRB. Alteration of this enzyme due to missense mutation/ nonsynonymous single nucleotide polymorphisms (nsSNPs) are responsible for various types of cancer progression, e.g. melanoma, lung cancer, and breast cancer. Hence, this study is designed to identify the malignant missense mutation of CDK4 from the single nucleotide polymorphism database (dbSNP) by incorporating computational algorithms. Out of 239 nsSNPs; G15S, D140Y and D140H were predicted to be highly malignant variants which may have a devastating impact on protein structure or function. We also found defective binding motif of these three mutants with the CDK4 inhibitor ribociclib and ATP. However, by incorporating molecular dynamic simulation, our study concludes that the superiority of G15S than the other two mutants (D140Y and D140H) in destabilizing proteins nature.

Cell Cycle Regulatory CCND1 G870A Gene Polymorphism and Periodontitis-Induced Oral Cancer: A Risk Analysis

Strong association has been recently observed between periodontitis/gingivitis and Oral squamous cell carcinoma (OSCC). A high incidence of oral cancer has been reported in the case of chronic periodontitis. Recently Cell cycle regulatory /Senescence genes have been associated with Gingivitis/ Periodontitis susceptibility. Cyclin D1 is one such cell cycle regulatory gene. Several findings have reported that Cyclin D1 (CCND1) G870A Single nucleotide polymorphism is associated with oral cancer (OC) risk, but yielded inconsistent data across different studies. This meta-analysis explores the precise relationship between CCND1 G870A polymorphism and OC risk. PubMed (Medline), EMBASE, & Google Scholar databases were searched for eligible studies and pooled odds ratios (ORs) and 95% confidence intervals (CI) were calculated. Newcastle-Ottawa analysis was done for selected articles quality assessment, bias in publication (if any) was estimated through Funnel plots and Egger’s test. Pooled analysis from eleven eligible studies suggests that CCND1 G870A polymorphism is not significantly associated with OC risk. Sub-group analysis by ethnicity failed to show any association. Sequential single study omission was performed to determine the credibility and resilience of the inferences drawn.

Feedback, Crosstalk and Competition: Ingredients for Emergent Non-Linear Behaviour in the PI3K/mTOR Signalling Network

The PI3K/mTOR signalling pathway plays a central role in the governing of cell growth, survival and metabolism. As such, it must integrate and decode information from both external and internal sources to guide efficient decision-making by the cell. To facilitate this, the pathway has evolved an intricate web of complex regulatory mechanisms and elaborate crosstalk with neighbouring signalling pathways, making it a highly non-linear system. Here, we describe the mechanistic biological details that underpin these regulatory mechanisms, covering a multitude of negative and positive feedback loops, feed-forward loops, competing protein interactions, and crosstalk with major signalling pathways. Further, we highlight the non-linear and dynamic network behaviours that arise from these regulations, uncovered through computational and experimental studies. Given the pivotal role of the PI3K/mTOR network in cellular homeostasis and its frequent dysregulation in pathologies including cancer and diabetes, a coherent and systems-level understanding of the complex regulation and consequential dynamic signalling behaviours within this network is imperative for advancing biology and development of new therapeutic approaches.

Proline-Rich Motifs Control G2-CDK Target Phosphorylation and Priming an Anchoring Protein for Polo Kinase Localization

The hydrophobic patch (hp), a docking pocket on cyclins of CDKs (cyclin-dependent kinases), has been thought to accommodate a single short linear motif (SLiM), the "RxL or Cy" docking motif. Here we show that hp can bind different motifs with high specificity. We identify a PxxPxF motif that is necessary for G2-cyclin Clb3 function in S. cerevisiae, and that mediates Clb3-Cdk1 phosphorylation of Ypr174c (proposed name: Cdc5 SPB anchor-Csa1) to regulate the localization of Polo kinase Cdc5. Similar motifs exist in other Clb3-Cdk1 targets. Our work completes the set of docking specificities for the four major cyclins: LP, RxL, PxxPxF, and LxF motifs for G1-, S-, G2-, and M-phase cyclins, respectively. Further, we show that variations in motifs can change their specificity for human cyclins. This diversity could provide complexity for the encoding of CDK thresholds to achieve ordered cell-cycle phosphorylation.

LxCxD motif of the APC/C coactivator subunit FZR1 is critical for interaction with the retinoblastoma protein

Retinoblastoma protein (pRB) regulates cell cycle by utilizing different regions of its pocket domain for interacting with E2F family of transcription factors and with cellular and viral proteins containing an LxCxE motif. An LxCxE-like motif, LxCxD, is present in FZR1, an adaptor protein of the multi-subunit E3 ligase complex anaphase-promoting complex/cyclosome (APC/C). The APC/C^FZR1 complex regulates the timely degradation of multiple cell cycle proteins for mitotic exit and maintains G1 state. We report that FZR1 interacts with pRB via its LxCxD motif. By using point mutations, we found that the cysteine residue in the FZR1 LxCxD motif is critical for direct interaction with pRb. The direct binding of the LxCxD motif of FZR1 to the pRB LxCxE binding pocket is confirmed by using human papillomavirus protein E7 as a competitor, both in vitro and in vivo. While mutation of the cysteine residue significantly disrupts FZR1 interaction with pRB, this motif does not affect FZR1 and core APC/C association. Expression of the FZR1 point mutant results in accumulation of S-phase kinase-associated protein 2 (SKP2) and Polo-like kinase 1 (PLK1), while p27^Kip1 and p21^Cip1 proteins are downregulated, indicating a G1 cell cycle defect. Consistently, cells containing point mutant FZR1 enter the S phase prematurely. Together our results suggest that the LxCxD motif of FZR1 is a critical determinant for the interaction between FZR1 and pRB and is important for G1 restriction.

Dissection of Anti-tumor Activity of Histone Deacetylase Inhibitor SAHA in Nasopharyngeal Carcinoma Cells via Quantitative Phosphoproteomics

Suberoylanilide hydroxamic acid (SAHA), a pan HDAC inhibitor, has been approved by the Food and Drug Administration (FDA) to treat cutaneous T cell lymphoma (CTCL). Nevertheless, the mechanisms underlying the therapeutic effects of SAHA on tumors are yet not fully understood. Protein phosphorylation is one of the most important means to regulate key biological processes (BPs), such as cell division, growth, migration, differentiation, and intercellular communication. Thus, investigation on the impacts of SAHA treatment on global cellular phosphorylation covering major signaling pathways deepens our understanding on its anti-tumor mechanisms. Here we comprehensively identified and quantified protein phosphorylation for the first time in nasopharyngeal carcinoma (NPC) cells upon SAHA treatment by combining tandem mass tags (TMTs)-based quantitative proteomics and titanium dioxide (TiO₂)-based phosphopeptide enrichment. In total, 7,430 phosphorylation sites on 2,456 phosphoproteins were identified in the NPC cell line 5-8F, of which 1,176 phosphorylation sites on 528 phosphoproteins were significantly elevated upon SAHA treatment. Gene ontology (GO) analysis showed that SAHA influenced several BPs, including mRNA/DNA processing and cell cycle. Furthermore, signaling pathway analysis and immunoblotting demonstrated that SAHA activated tumor suppressors like p53 and Rb1 via phosphorylation and promoted cell apoptosis in NPC cells but inactivated energetic pathways such as AMPK signaling. Overall, our study indicated that SAHA exerted anti-tumor roles in NPC cells, which may serve as novel therapeutic for NPC patients.

Beyond What Your Retina Can See: Similarities of Retinoblastoma Function between Plants and Animals, from Developmental Processes to Epigenetic Regulation

The Retinoblastoma protein (pRb) is a key cell cycle regulator conserved in a wide variety of organisms. Experimental analysis of pRb's functions in animals and plants has revealed that this protein participates in cell proliferation and differentiation processes. In addition, pRb in animals and its orthologs in plants (RBR), are part of highly conserved protein complexes which suggest the possibility that analogies exist not only between functions carried out by pRb orthologs themselves, but also in the structure and roles of the protein networks where these proteins are involved. Here, we present examples of pRb/RBR participation in cell cycle control, cell differentiation, and in the regulation of epigenetic changes and chromatin remodeling machinery, highlighting the similarities that exist between the composition of such networks in plants and animals.

Cyclin B1-Cdk1 facilitates MAD1 release from the nuclear pore to ensure a robust spindle checkpoint

How the cell rapidly and completely reorganizes its architecture when it divides is a problem that has fascinated researchers for almost 150 yr. We now know that the core regulatory machinery is highly conserved in eukaryotes, but how these multiple protein kinases, protein phosphatases, and ubiquitin ligases are coordinated in space and time to remodel the cell in a matter of minutes remains a major question. Cyclin B1-Cdk is the primary kinase that drives mitotic remodeling; here we show that it is targeted to the nuclear pore complex (NPC) by binding an acidic face of the kinetochore checkpoint protein, MAD1, where it coordinates NPC disassembly with kinetochore assembly. Localized cyclin B1-Cdk1 is needed for the proper release of MAD1 from the embrace of TPR at the nuclear pore so that it can be recruited to kinetochores before nuclear envelope breakdown to maintain genomic stability.

Exploring the Role of Novel Medical Therapies for Aggressive Pituitary Tumors: A Review of the Literature-"Are We There Yet?"

Aggressive pituitary tumors account for up to 10% of pituitary tumors and are characterized by resistance to medical treatment and multiple recurrences despite standard therapies, including surgery, radiotherapy, and chemotherapy. They are associated with increased morbidity and mortality, particularly pituitary carcinomas, which have mortality rates of up to 66% at 1 year after diagnosis. Novel targeted therapies under investigation include mammalian target of rapamycin (mTOR), tyrosine kinase, and vascular endothelial growth factor (VEGF) inhibitors. More recently, immune checkpoint inhibitors have been proposed as a potential treatment option for pituitary tumors. An increased understanding of the molecular pathogenesis of aggressive pituitary tumors is required to identify potential biomarkers and therapeutic targets. This review discusses novel approaches to the management of aggressive pituitary tumors and the role of molecular profiling.

Regression of a nonfunctioning pituitary macroadenoma on the CDK4/6 inhibitor palbociclib: case report

A 71-year-old female patient was referred in 2013 for evaluation of an asymptomatic nonsecreting pituitary adenoma. The adenoma, measuring 13 mm in height by 10 mm in width, was discovered incidentally on imaging in 2012. Biochemical testing demonstrated a nonfunctioning adenoma. Given the relatively small lesion size and the lack of symptoms, observation was preferred over surgical intervention. The patient was monitored with routine MRI, which until 2016 demonstrated minimal growth. In early 2016, the patient developed recurrence of metastatic breast cancer and was treated with palbociclib, a cyclin-dependent kinase (CDK) 4/6 inhibitor. This inhibitor acts on a pathway believed to be involved in pituitary adenoma tumorigenesis. One year after starting palbociclib, routine imaging demonstrated significant regression of her pituitary adenoma. The authors hypothesize that inhibition of the CDK4/6 pathway by palbociclib contributed to adenoma regression in this patient, and that palbociclib may represent a possible adjuvant therapy for the treatment of nonfunctioning pituitary adenomas.

Transmembrane Channel-Like 5 (TMC5) promotes prostate cancer cell proliferation through cell cycle regulation

BACKGROUND

In this study, we aimed to investigate the biological functions of Transmembrane Channel-Like 5 (TMC5) by bioinformatics and molecular biology methods in prostate cancer (PCa).

METHODS

We assessed the mRNA expression level of TMC5 in PCa with public database the Cancer Genome Atlas (TCGA) and Oncomine. The biological functions were demonstrated by bioinformatics methods and siRNA mediated knockdown experiments. Reverse transcription polymerase chain reaction (RT-PCR), immunohistochemical (IHC) experiments and microarray analysis were performed to confirm the results.

RESULTS

TMC5 expression level was significantly up-regulated in 4 independent PCa cohorts compared to normal group. Moreover, TMC5 has higher diagnostic efficiency than PSA-KLK3 (AUC (Area Under Curve) = 0.772, P < 0.001). The high expression of TMC5 was associated with clinical Gleason score, prostate-specific antigen (PSA) level, androgen receptor (AR) activity score and the genes which were known frequently mutated in PCa progression (P < 0.05). Functionally, Gene Otology (GO) analysis suggested that TMC5 was related to cell development; TMC5 knockdown significantly inhibited PCa cells proliferation by arresting cell cycle at G1 phase. Drug sensitivity experiments showed TMC5 knockdown significantly enhanced cells sensitivity to 5-Fluorouracil. Microarray analysis showed TMC5 knockdown significantly inhibited cell cycle and tumor progression.

CONCLUSION

Our findings revealed that TMC5 promoted PCa cell proliferation through cell cycle regulation and could be a powerful and hopeful target for PCa treatment.

Cyclin D-Cdk4,6 Drives Cell-Cycle Progression via the Retinoblastoma Protein's C-Terminal Helix

The cyclin-dependent kinases Cdk4 and Cdk6 form complexes with D-type cyclins to drive cell proliferation. A well-known target of cyclin D-Cdk4,6 is the retinoblastoma protein Rb, which inhibits cell-cycle progression until its inactivation by phosphorylation. However, the role of Rb phosphorylation by cyclin D-Cdk4,6 in cell-cycle progression is unclear because Rb can be phosphorylated by other cyclin-Cdks, and cyclin D-Cdk4,6 has other targets involved in cell division. Here, we show that cyclin D-Cdk4,6 docks one side of an alpha-helix in the Rb C terminus, which is not recognized by cyclins E, A, and B. This helix-based docking mechanism is shared by the p107 and p130 Rb-family members across metazoans. Mutation of the Rb C-terminal helix prevents its phosphorylation, promotes G1 arrest, and enhances Rb's tumor suppressive function. Our work conclusively demonstrates that the cyclin D-Rb interaction drives cell division and expands the diversity of known cyclin-based protein docking mechanisms.

Essential role of O-GlcNAcylation in stabilization of oncogenic factors

A reversible post-translational protein modification which involves addition of N-acetylglucosamine (GlcNAc) onto hydroxyl groups of serine and/or threonine residues which is known as O-GlcNAcylation, has emerged as a potent competitor of phosphorylation. This glycosyltransfer reaction is catalyzed by the enzyme O-linked β-N-acetylglucosamine transferase (OGT). This enzyme uses uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), the end product of hexosamine biosynthetic pathway, to modify numerous nuclear and cytosolic proteins. O-GlcNAcylation influences cancer cell metabolism in such a way that hyper-O-GlcNAcylation is considered as a prominent trait of many cancers, and is proposed as a major factor enabling cancer cell proliferation and progression. Growing evidence supports a connection between O-GlcNAcylation and major oncogenic factors, including for example, c-MYC, HIF-1α, and NF-κB. A comprehensive study of the roles of O-GlcNAc modification of oncogenic factors is warranted as a thorough understanding may help drive advances in cancer diagnosis and therapy. The focus of this article is to highlight the interplay between oncogenic factors and O-GlcNAcylation along with OGT in cancer cell proliferation and survival. The prospects for OGT inhibitors will also be discussed.

Structural insights into the functional diversity of the CDK-cyclin family

Since their characterization as conserved modules that regulate progression through the eukaryotic cell cycle, cyclin-dependent protein kinases (CDKs) in higher eukaryotic cells are now also emerging as significant regulators of transcription, metabolism and cell differentiation. The cyclins, though originally characterized as CDK partners, also have CDK-independent roles that include the regulation of DNA damage repair and transcriptional programmes that direct cell differentiation, apoptosis and metabolic flux. This review compares the structures of the members of the CDK and cyclin families determined by X-ray crystallography, and considers what mechanistic insights they provide to guide functional studies and distinguish CDK- and cyclin-specific activities. Aberrant CDK activity is a hallmark of a number of diseases, and structural studies can provide important insights to identify novel routes to therapy.

The retinoblastoma 1 gene (RB1) modulates the proliferation of chicken preadipocytes

1. The objective of this study was to reveal the role of chicken RB1 (Gallus gallus RB1, gRB1) in the proliferation of preadipocytes. 2. To measure gene expression of gRB1 in the proliferation of chicken preadipocyte, quantitative real-time PCR was used. The expression levels of gRB1 transiently increased during this process. 3. To detect the effect of gRB1 on the proliferation of chicken preadipocyte, MTT assay and cell-cycle analysis were performed. MTT assay showed that overexpression of gRB1 significantly suppressed (P < 0.05) the proliferation of chicken preadipocytes, and knockdown of gRB1 promoted the proliferation of chicken preadipocytes. Cell-cycle analysis showed that the proportion of preadipocytes in the G1 and G2 phases significantly increased (P < 0.05), and the proportion of preadipocytes in the S phase significantly decreased (P < .05) after up-regulation of the expression of gRB1. The proportion of preadipocytes in the S phase significantly increased (P < 0.05) after down-regulation of gRB1. 4. Quantitative real-time PCR was used to detect the effect of gRB1 on the expression of genes related to proliferation of chicken preadipocytes. Gene expression analysis showed that gRB1 knockdown promoted markers indicating proliferation of Ki-67 (MKi67) expression at 96 h (P < 0.05), and overexpression of gRB1 reduced MKi67 expression at 72 h (P < 0.05). 5. This study demonstrated that gRB1 inhibited preadipocyte proliferation at least in part by inhibiting the G1 to S phase transition.

A Code of Mono-phosphorylation Modulates the Function of RB

Hyper-phosphorylation of RB controls its interaction with E2F and inhibits its tumor suppressor properties. However, during G1 active RB can be mono-phosphorylated on any one of 14 CDK phosphorylation sites. Here, we used quantitative proteomics to profile protein complexes formed by each mono-phosphorylated RB isoform (mP-RB) and identified the associated transcriptional outputs. The results show that the 14 sites of mono-phosphorylation co-ordinate RB's interactions and confer functional specificity. All 14 mP-RBs interact with E2F/DP proteins, but they provide different shades of E2F regulation. RB mono-phosphorylation at S811, for example, alters RB transcriptional activity by promoting its association with NuRD complexes. The greatest functional differences between mP-RBs are evident beyond the cell cycle machinery. RB mono-phosphorylation at S811 or T826 stimulates the expression of oxidative phosphorylation genes, increasing cellular oxygen consumption. These results indicate that RB activation signals are integrated in a phosphorylation code that determines the diversity of RB activity.

Hyper-phosphorylation of Rb S249 together with CDK5R2/p39 overexpression are associated with impaired cell adhesion and epithelial-to-mesenchymal transition: Implications as a potential lung cancer grading and staging biomarker

Prediction of lung cancer metastasis relies on post-resection assessment of tumor histology, which is a severe limitation since only a minority of lung cancer patients are diagnosed with resectable disease. Therefore, characterization of metastasis-predicting biomarkers in pre-resection small biopsy specimens is urgently needed. Here we report a biomarker consisting of the phosphorylation of the retinoblastoma protein (Rb) on serine 249 combined with elevated p39 expression. This biomarker correlates with epithelial-to-mesenchymal transition traits in non-small cell lung carcinoma (NSCLC) cells. Immunohistochemistry staining of NSCLC tumor microarrays showed that strong phospho-Rb S249 staining positively correlated with tumor grade specifically in the squamous cell carcinoma (SCC) subtype. Strong immunoreactivity for p39 positively correlated with tumor stage, lymph node invasion, and distant metastases, also in SCC. Linear regression analyses showed that the combined scoring for phospho-Rb S249, p39 and E-cadherin in SCC is even more accurate at predicting tumor staging, relative to each score individually. We propose that combined immunohistochemistry staining of NSCLC samples for Rb phosphorylation on S249, p39, and E-cadherin protein expression could aid in the assessment of tumor staging and metastatic potential when tested in small primary tumor biopsies. The intense staining for phospho-Rb S249 that we observed in high grade SCC could also aid in the precise sub-classification of poorly differentiated SCCs.

DNA replication stress and its impact on chromosome segregation and tumorigenesis

Genome instability and cell cycle dysregulation are commonly associated with cancer. DNA replication stress driven by oncogene activation during tumorigenesis is now well established as a source of genome instability. Replication stress generates DNA damage not only during S phase, but also in the subsequent mitosis, where it impacts adversely on chromosome segregation. Some regions of the genome seem particularly sensitive to replication stress-induced instability; most notably, chromosome fragile sites. In this article, we review some of the important issues that have emerged in recent years concerning DNA replication stress and fragile site expression, as well as how chromosome instability is minimized by a family of ring-shaped protein complexes known as SMC proteins. Understanding how replication stress impacts on S phase and mitosis in cancer should provide opportunities for the development of novel and tumour-specific treatments.

Acetylation and deacetylation of Cdc25A constitutes a novel mechanism for modulating Cdc25A functions with implications for cancer

The dual specificity phosphatase Cdc25A is a key regulator of the cell cycle that promotes cell cycle progression by dephosphorylating and activating cyclin-dependent kinases. In response to genotoxicants, Cdc25A undergoes posttranslational modifications which contribute to its proteasome-mediated degradation and consequent cell cycle checkpoint arrest. The most thoroughly studied Cdc25A modification is phosphorylation. We now provide the first evidence that Cdc25A can be acetylated and that it directly interacts with the ARD1 acetyltransferase which acetylates Cdc25A both biochemically and in cultured cells. When acetylated, Cdc25A has an extended half-life. We have also identified the class IV histone deacetylase, HDAC11, as a Cdc25A deacetylase. We further show that DNA damage, such as exposure to methyl methanesulfonate (MMS), etoposide or arsenic, increases Cdc25A acetylation. Importantly, this acetylation modulates Cdc25A phosphatase activity and its function as a cell cycle regulator, and may reflect a cellular response to DNA damage. Since Cdc25A, ARD1, and HDAC11 are frequently dysregulated in multiple types of cancer, our findings may provide insight into a novel mechanism in carcinogenesis.

A parapoxviral virion protein targets the retinoblastoma protein to inhibit NF-κB signaling

Poxviruses have evolved multiple strategies to subvert signaling by Nuclear Factor κB (NF-κB), a crucial regulator of host innate immune responses. Here, we describe an orf virus (ORFV) virion-associated protein, ORFV119, which inhibits NF-κB signaling very early in infection (≤ 30 min post infection). ORFV119 NF-κB inhibitory activity was found unimpaired upon translation inhibition, suggesting that virion ORFV119 alone is responsible for early interference in signaling. A C-terminal LxCxE motif in ORFV119 enabled the protein to interact with the retinoblastoma protein (pRb) a multifunctional protein best known for its tumor suppressor activity. Notably, experiments using a recombinant virus containing an ORFV119 mutation which abrogates its interaction with pRb together with experiments performed in cells lacking or with reduced pRb levels indicate that ORFV119 mediated inhibition of NF-κB signaling is largely pRb dependent. ORFV119 was shown to inhibit IKK complex activation early in infection. Consistent with IKK inhibition, ORFV119 also interacted with TNF receptor associated factor 2 (TRAF2), an adaptor protein recruited to signaling complexes upstream of IKK in infected cells. ORFV119-TRAF2 interaction was enhanced in the presence of pRb, suggesting that ORFV119-pRb complex is required for efficient interaction with TRAF2. Additionally, transient expression of ORFV119 in uninfected cells was sufficient to inhibit TNFα-induced IKK activation and NF-κB signaling, indicating that no other viral proteins are required for the effect. Infection of sheep with ORFV lacking the ORFV119 gene led to attenuated disease phenotype, indicating that ORFV119 contributes to virulence in the natural host. ORFV119 represents the first poxviral protein to interfere with NF-κB signaling through interaction with pRb.

pRb and CyclinD1 Complement p16 as Immunohistochemical Surrogate Markers of HPV Infection in Head and Neck Cancer

Identification of human papillomavirus (HPV) association in head and neck squamous cell carcinoma (HNSCC) is important to identify patients with favorable disease course. However, molecular HPV detection is not universally available. p16 has been proposed as a surrogate marker for HPV infection in HNSCC but, use on its own may result in wrong assignment of some cases to the group of HPV-associated tumors. We have therefore studied 424 HNSCC cases with known p16 and HPV DNA polymerase chain reaction (PCR) status for expression of retinoblastoma protein (pRb) and CyclinD1 by immunohistochemistry using 6-tiered scales (0 to 5) and a combined score (0 to 10). Sixty-one of 424 cases showed overexpression of p16. Of these, 52 cases were HPV DNA-PCR-positive. HPV association strongly correlated with low expression scores for pRb and CyclinD1 individually (scores ≤2) or combined (score sum ≤4), whereas HPV-negative carcinomas showed widely distributed expression scores. High expression scores for pRb or for pRb/CyclinD1 were observed exclusively in HPV DNA-PCR-negative cases. Three of 9 p16-positive/HPV DNA-PCR-negative cases showed high expression of pRb and displayed a high combined pRb/CyclinD1 score. We conclude that HPV-positive HNSCC are characterized by p16 overexpression and low scores for pRb, CyclinD1, and a low combined pRb/CyclinD1 score. High pRb or combined pRb/CyclinD1 scores are strong indicators for HPV-negativity and may justify excluding these cases from further molecular HPV testing. Furthermore p16-positive/HPV DNA-PCR-negative cases show heterogeneous expression of pRb and CyclinD1, including high pRb or high combined pRb/CyclinD1 scores suggesting that at least some of these cases are truly HPV negative.

High glucose concentration induces endothelial cell proliferation by regulating cyclin-D2-related miR-98

Cyclin D2 is involved in the pathology of vascular complications of type 2 diabetes mellitus (T2DM). This study investigated the role of cyclin‐D2‐regulated miRNAs in endothelial cell proliferation of T2DM. Results showed that higher glucose concentration (4.5 g/l) significantly promoted the proliferation of rat aortic endothelial cells (RAOECs), and significantly increased the expression of cyclin D2 and phosphorylation of retinoblastoma 1 (p‐RB1) in RAOECs compared with those under low glucose concentration. The cyclin D2‐3′ untranslated region is targeted by miR‐98, as demonstrated by miRNA analysis software. Western blot also confirmed that cyclin D2 and p‐RB1 expression was regulated by miR‐98. The results indicated that miR‐98 treatment can induce RAOEC apoptosis. The suppression of RAOEC growth by miR‐98 might be related to regulation of Bcl‐2, Bax and Caspase 9 expression. Furthermore, the expression levels of miR‐98 decreased in 4.5 g/l glucose‐treated cells compared with those treated by low glucose concentration. Similarly, the expression of miR‐98 significantly decreased in aortas of established streptozotocin (STZ)‐induced diabetic rat model compared with that in control rats; but cyclin D2 and p‐RB1 levels remarkably increased in aortas of STZ‐induced diabetic rats compared with those in healthy control rats. In conclusion, this study demonstrated that high glucose concentration induces cyclin D2 up‐regulation and miR‐98 down‐regulation in the RAOECs. By regulating cyclin D2, miR‐98 can inhibit human endothelial cell growth, thereby providing novel therapeutic targets for vascular complication of T2DM.

CDK6-a review of the past and a glimpse into the future: from cell-cycle control to transcriptional regulation

The G1 cell-cycle kinase CDK6 has long been thought of as a redundant homolog of CDK4. Although the two kinases have very similar roles in cell-cycle progression, it has recently become apparent that they differ in tissue-specific functions and contribute differently to tumor development. CDK6 is directly involved in transcription in tumor cells and in hematopoietic stem cells. These functions point to a role of CDK6 in tissue homeostasis and differentiation that is partially independent of CDK6's kinase activity and is not shared with CDK4. We review the literature on the contribution of CDK6 to transcription in an attempt to link the new findings on CDK6's transcriptional activity to cell-cycle progression. Finally, we note that anticancer therapies based on the inhibition of CDK6 kinase activity fail to take into account its kinase-independent role in tumor development.

Androgen Receptor-Mediated Growth Suppression of HPr-1AR and PC3-Lenti-AR Prostate Epithelial Cells

The androgen receptor (AR) mediates the developmental, physiologic, and pathologic effects of androgens including 5α-dihydrotestosterone (DHT). However, the mechanisms whereby AR regulates growth suppression and differentiation of luminal epithelial cells in the prostate gland and proliferation of malignant versions of these cells are not well understood, though they are central to prostate development, homeostasis, and neoplasia. Here, we identify androgen-responsive genes that restrain cell cycle progression and proliferation of human prostate epithelial cell lines (HPr-1AR and PC3-Lenti-AR), and we investigate the mechanisms through which AR regulates their expression. DHT inhibited proliferation of HPr-1AR and PC3-Lenti-AR, and cell cycle analysis revealed a prolonged G₁ interval. In the cell cycle, the G₁/S-phase transition is initiated by the activity of cyclin D and cyclin-dependent kinase (CDK) complexes, which relieve growth suppression. In HPr-1AR, cyclin D1/2 and CDK4/6 mRNAs were androgen-repressed, whereas CDK inhibitor, CDKN1A, mRNA was androgen-induced. The regulation of these transcripts was AR-dependent, and involved multiple mechanisms. Similar AR-mediated down-regulation of CDK4/6 mRNAs and up-regulation of CDKN1A mRNA occurred in PC3-Lenti-AR. Further, CDK4/6 overexpression suppressed DHT-inhibited cell cycle progression and proliferation of HPr-1AR and PC3-Lenti-AR, whereas CDKN1A overexpression induced cell cycle arrest. We therefore propose that AR-mediated growth suppression of HPr-1AR involves cyclin D1 mRNA decay, transcriptional repression of cyclin D2 and CDK4/6, and transcriptional activation of CDKN1A, which serve to decrease CDK4/6 activity. AR-mediated inhibition of PC3-Lenti-AR proliferation occurs through a similar mechanism, albeit without down-regulation of cyclin D. Our findings provide insight into AR-mediated regulation of prostate epithelial cell proliferation.

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.

Cyclin D activates the Rb tumor suppressor by mono-phosphorylation

The widely accepted model of G₁ cell cycle progression proposes that cyclin D:Cdk4/6 inactivates the Rb tumor suppressor during early G₁ phase by progressive multi-phosphorylation, termed hypo-phosphorylation, to release E2F transcription factors. However, this model remains unproven biochemically and the biologically active form(s) of Rb remains unknown. In this study, we find that Rb is exclusively mono-phosphorylated in early G₁ phase by cyclin D:Cdk4/6. Mono-phosphorylated Rb is composed of 14 independent isoforms that are all targeted by the E1a oncoprotein, but show preferential E2F binding patterns. At the late G₁ Restriction Point, cyclin E:Cdk2 inactivates Rb by quantum hyper-phosphorylation. Cells undergoing a DNA damage response activate cyclin D:Cdk4/6 to generate mono-phosphorylated Rb that regulates global transcription, whereas cells undergoing differentiation utilize un-phosphorylated Rb. These observations fundamentally change our understanding of G₁ cell cycle progression and show that mono-phosphorylated Rb, generated by cyclin D:Cdk4/6, is the only Rb isoform in early G₁ phase.

DOI:http://dx.doi.org/10.7554/eLife.02872.001

Cells go through a tightly controlled, multi-step procedure before they divide. This cell division program—the cell cycle—is necessary for preventing unrestrained cellular growth, which may lead to cancer. Proteins called cyclins control the progression through each of the phases of the cell cycle, with different cyclins working during different phases.

During the G1 phase of the cell cycle, cells grow in size and produce the proteins that are required to copy DNA. Once a cell passes a checkpoint called the 'restriction point' at the end of the G1 phase, it is committed to dividing. It is therefore particularly important to keep events during G1 phase in check.

The Retinoblastoma tumor suppresor protein (Rb) is a key player in regulating the G1 phase. Rb sequesters transcription factors that are essential for the cell cycle to progress. Previously, it was thought that a complex called cyclin D added more and more phosphates to the Rb protein during the G1 phase. This process predicted a slow release of transcription factors, which attach to DNA and start the process of DNA replication. While many studies have presented data that is consistent with this model, direct biochemical evidence of these events is lacking.

Narasimha, Kaulich, Shapiro et al. now present biochemical analyses of Rb proteins that show—completely unexpectedly—that the cyclin D complex adds just one phosphate group to Rb during the G1 phase, although this group can be added to one of fourteen different sites. The resulting 'mono-phosphorylated' Rb varieties can each sequester different transcription factors and stop them working.

At the restriction point, many more phosphate groups are then rapidly added, and the Rb protein is inactivated by a different cyclin. This cyclin—called Cyclin E—then drives cells into the next phase of the cell cycle. Establishing how cyclin E is activated is a priority for future research.

DOI:http://dx.doi.org/10.7554/eLife.02872.002

Arabidopsis RETINOBLASTOMA-RELATED and Polycomb group proteins: cooperation during plant cell differentiation and development

RETINOBLASTOMA (RB) is a tumour suppressor gene originally discovered in patients that develop eye tumours. The pRb protein is now well established as a key cell-cycle regulator which suppresses G1-S transition via interaction with E2F-DP complexes. pRb function is also required for a wide range of biological processes, including the regulation of stem-cell maintenance, cell differentiation, permanent cell-cycle exit, DNA repair, and genome stability. Such multifunctionality of pRb is thought to be facilitated through interactions with various binding partners in a context-dependent manner. Although the molecular network in which RB controls various biological processes is not fully understood, it has been found that pRb interacts with transcription factors and chromatin modifiers to either suppress or promote the expression of key genes during the switch from cell proliferation to differentiation. RETINOBLASTOMA-RELATED (RBR) is the plant orthologue of RB and is also known to negatively control the G1-S transition. Similar to its animal counterpart, plant RBR has various roles throughout plant development; however, much of its molecular functions outside of the G1-S transition are still unknown. One of the better-characterized molecular mechanisms is the cooperation of RBR with the Polycomb repressive complex 2 (PRC2) during plant-specific developmental events. This review summarizes the current understanding of this cooperation and focuses on the processes in Arabidopsis in which the RBR-PRC2 cooperation facilitates cell differentiation and developmental transitions.

FRK suppresses the proliferation of human glioma cells by inhibiting cyclin D1 nuclear accumulation

The Fyn related kinase (FRK) is a noteworthy member of the Src non-receptor tyrosine kinase family for its distinctive tumor suppressive function. Recently, we have shown that FRK plays a protective role against the progression of glioma by suppressing cell migration and invasion. However, it is unclear whether the cell growth of glioma is also regulated by FRK and by which mechanism FRK alters its specific biological functions. In the current study, we found that FRK over-expression significantly suppressed the proliferation of glioma cells. In contrast, FRK knockdown by siRNA promoted glioma cell growth. In addition, FRK over-expression caused G1 phase arrest as well as apoptosis of glioma cells. Further investigation disclosed that FRK-induced G1 arrest was accompanied by down-regulation of hyperphosphorylated retinoblastoma protein (pRb), which led to the consequent suppression of E2F1. More importantly, we found that over-expression of FRK inhibited proper cyclin D1 accumulation in the nucleus of proliferating cells. Taken together, our results demonstrate a combined mechanism for the anti-proliferative effects of FRK by inhibiting cyclin D1 nucleus accumulation and pRb phosphorylation in glioma cells.

Sj7170, a unique dual-function peptide with a specific α-chymotrypsin inhibitory activity and a potent tumor-activating effect from scorpion venom

A new peptide precursor, termed Sj7170, was characterized from the venomous gland cDNA library of the scorpion Scorpiops jendeki. Sj7170 was deduced to be a 62-amino acid peptide cross-linked by five disulfide bridges. The recombinant Sj7170 peptide (rSj7170) with chromatographic purity was produced by a prokaryotic expression system. Enzyme inhibition assay in vitro and in vivo showed that rSj7170 specifically inhibited the activity of α-chymotrypsin at micromole concentrations. In addition, Sj7170 not only promoted cell proliferation and colony formation by up-regulating the expression of cyclin D1 in vitro but also enhanced tumor growth in nude mice. Finally, Sj7170 accelerated cellular migration and invasion by increasing the expression of the transcription factor Snail and then inducing the epithelial-mesenchymal transition. Moreover, Sj7170 changed cell morphology and cytoskeleton of U87 cells by the GTPase pathway. Taken together, Sj7170 is a unique dual-function peptide, i.e. a specific α-chymotrypsin inhibitor and a potent tumorigenesis/metastasis activator. Our work not only opens an avenue of developing new modulators of tumorigenesis/metastasis from serine protease inhibitors but also strengthens the functional link between protease inhibitors and tumor activators.

Overview of cyclins D1 function in cancer and the CDK inhibitor landscape: past and present

INTRODUCTION

Intensive efforts, over the last decade, have been made to inhibit the kinase activity of cyclins that act as mediators during cell-cycle progression. Activation of the cyclin D1 oncogene, often by amplification or rearrangement, is a major driver of multiple types of human tumors including breast and squamous cell cancers, B-cell lymphoma, myeloma and parathyroid adenoma.

AREAS COVERED

In this review, the authors summarize the activity of cyclins and cyclin-dependent kinases in cell-cycle progression and transcription. They focus on cyclin D1/CDK4/CDK6, a central mediator in the transition from G1 to S phase. Furthermore, the authors discuss the first generation of pan-cyclin-dependent kinase inhibitors that failed to meet expectation and discuss, in detail, the second generation of highly specific cyclin D1/CDK4/CDK6 inhibitors that are proving to be more efficacious.

EXPERT OPINION

The mechanism by which cyclin D1 drives tumorigenesis may be dependent on kinase and kinase-independent functions. Further evidence is necessary to delineate the roles of cyclin D1 in early pre-neoplastic lesions where its overexpression may promote genomic instability in a kinase-independent manner.

Mantle cell lymphoma in cyclin D1 transgenic mice with Bim-deficient B cells

Mantle cell lymphoma (MCL) is a highly aggressive B-cell lymphoma resistant to conventional chemotherapy. Although defined by the characteristic t(11;14) translocation, MCL has not been recapitulated in transgenic mouse models of cyclin D1 overexpression alone. Indeed, several genetic aberrations have been identified in MCL that may contribute to its pathogenesis and chemoresistance. Of particular interest is the frequent biallelic deletion of the proapoptotic BCL-2 family protein BIM. BIM exerts its pro-death function via its α-helical BH3 death domain that has the dual capacity to inhibit antiapoptotic proteins such as BCL-2 and MCL-1 and directly trigger proapoptotic proteins such as the mitochondrial executioner protein BAX. To evaluate a functional role for Bim deletion in the pathogenesis of MCL, we generated cyclin D1-transgenic mice harboring Bim-deficient B cells. In response to immunization, Eμ(CycD1)CD19(CRE)Bim(fl/fl) mice manifested selective expansion of their splenic mantle zone compartment. Three distinct immune stimulation regimens induced lymphomas with histopathologic and molecular features of human MCL in a subset of mice. Thus, deletion of Bim in B cells, in the context of cyclin D1 overexpression, disrupts a critical control point in lymphoid maturation and predisposes to the development of MCL. This genetic proof of concept for MCL pathogenesis suggests an opportunity to reactivate the death pathway by pharmacologic mimicry of proapoptotic BIM.

RB1 in cancer: different mechanisms of RB1 inactivation and alterations of pRb pathway in tumorigenesis

Loss of RB1 gene is considered either a causal or an accelerating event in retinoblastoma. A variety of mechanisms inactivates RB1 gene, including intragenic mutations, loss of expression by methylation and chromosomal deletions, with effects which are species-and cell type-specific. RB1 deletion can even lead to aneuploidy thus greatly increasing cancer risk. The RB1gene is part of a larger gene family that includes RBL1 and RBL2, each of the three encoding structurally related proteins indicated as pRb, p107, and p130, respectively. The great interest in these genes and proteins springs from their ability to slow down neoplastic growth. pRb can associate with various proteins by which it can regulate a great number of cellular activities. In particular, its association with the E2F transcription factor family allows the control of the main pRb functions, while the loss of these interactions greatly enhances cancer development. As RB1 gene, also pRb can be functionally inactivated through disparate mechanisms which are often tissue specific and dependent on the scenario of the involved tumor suppressors and oncogenes. The critical role of the context is complicated by the different functions played by the RB proteins and the E2F family members. In this review, we want to emphasize the importance of the mechanisms of RB1/pRb inactivation in inducing cancer cell development. The review is divided in three chapters describing in succession the mechanisms of RB1 inactivation in cancer cells, the alterations of pRb pathway in tumorigenesis and the RB protein and E2F family in cancer.

LXCXE-independent chromatin remodeling by Rb/E2f mediates neuronal quiescence

Neuronal survival is dependent upon the retinoblastoma family members, Rb1 (Rb) and Rb2 (p130). Rb is thought to regulate gene repression, in part, through direct recruitment of chromatin modifying enzymes to its conserved LXCXE binding domain. We sought to examine the mechanisms that Rb employs to mediate cell cycle gene repression in terminally differentiated cortical neurons. Here, we report that Rb loss converts chromatin at the promoters of E2f-target genes to an activated state. We established a mouse model system in which Rb-LXCXE interactions could be induciblely disabled. Surprisingly, this had no effect on survival or gene silencing in neuronal quiescence. Absence of the Rb LXCXE-binding domain in neurons is compatible with gene repression and long-term survival, unlike Rb deficiency. Finally, we are able to show that chromatin activation following Rb deletion occurs at the level of E2fs. Blocking E2f-mediated transcription downstream of Rb loss is sufficient to maintain chromatin in an inactive state. Taken together our results suggest a model whereby Rb-E2f interactions are sufficient to maintain gene repression irrespective of LXCXE-dependent chromatin remodeling.

Animal models of hyperfunctioning parathyroid diseases for drug development

BACKGROUND

Disorders of mineral and bone metabolism have been implicated as a risk factor in the high mortality in patients with chronic kidney disease (CKD). Hyperphosphatemia, disorders of vitamin D metabolism and secondary hyperparathyroidism of uremia (SHPT) are therapeutic targets in these patients to improve the mortality. Animal models for CKD are indispensable and uremic rats produced by 5/6-nephrectomies are one of the most useful animal models for the development of new therapeutic agents. As there are limitations of uremic rats such as short lifespan and less severity of secondary hyperparathyroidism distinct from CKD patients on maintenance hemodialysis, the development of new model animals is expected.

OBJECTIVE

This review discusses the molecular pathogenesis of hyperfunctioning parathyroid diseases and the applications of animal models exhibiting hyperparathyroidisms in the aspect of the development of new therapeutics.

CONCLUSION

PTH-cyclin D1 transgenic mice, with parathyroid-targeted overexpression of cyclin D1 oncogene, not only developed abnormal parathyroid cell proliferation but, notably, also developed biochemical hyperparathyroidism with characteristic abnormalities in bone. The mice exhibit age-dependent development of biochemical hyperparathyroidism, which enables testing of the drug precisely. In addition, the mice develop parathyroid cell hyperplasia, followed by monoclonal expansion, which is observed in refractory SHPT patients.

AUREOCHROME1a-mediated induction of the diatom-specific cyclin dsCYC2 controls the onset of cell division in diatoms (Phaeodactylum tricornutum)

Cell division in photosynthetic organisms is tightly regulated by light. Although the light dependency of the onset of the cell cycle has been well characterized in various phototrophs, little is known about the cellular signaling cascades connecting light perception to cell cycle activation and progression. Here, we demonstrate that diatom-specific cyclin 2 (dsCYC2) in Phaeodactylum tricornutum displays a transcriptional peak within 15 min after light exposure, long before the onset of cell division. The product of dsCYC2 binds to the cyclin-dependent kinase CDKA1 and can complement G1 cyclin-deficient yeast. Consistent with the role of dsCYC2 in controlling a G1-to-S light-dependent cell cycle checkpoint, dsCYC2 silencing decreases the rate of cell division in diatoms exposed to light-dark cycles but not to constant light. Transcriptional induction of dsCYC2 is triggered by blue light in a fluence rate-dependent manner. Consistent with this, dsCYC2 is a transcriptional target of the blue light sensor AUREOCHROME1a, which functions synergistically with the basic leucine zipper (bZIP) transcription factor bZIP10 to induce dsCYC2 transcription. The functional characterization of a cyclin whose transcription is controlled by light and whose activity connects light signaling to cell cycle progression contributes significantly to our understanding of the molecular mechanisms underlying light-dependent cell cycle onset in diatoms.

A role for p21-activated kinase 7 in the development of gastric cancer

p21-activated kinase (PAK)7 (also known as PAK5) is a member of the group B PAK family of serine/threonine protein kinases, which are effectors of the small GTPases Rac and CDC42. PAK7 can promote neurite outgrowth, induce microtubule stabilization, and activate cell survival signaling pathways. However, the role of PAK7 in cancer is still poorly understood. Here, we showed that PAK7 expression was upregulated in different gastric cancer cell lines and gastric cancer tissues, as compared with human embryonic kidney 293 cells and adjacent normal tissues, respectively. The results suggested that PAK7 expression was related to gastric cancer progression. Thus, we employed lentivirus-mediated small interfering RNA to inhibit PAK7 expression, to investigate the role of PAK7 in human gastric carcinogenesis. RNA interference efficiently downregulated expression of PAK7 in SGC-7901 and MGC-803 cells at both mRNA and protein levels. Knockdown of PAK7 inhibited human gastric cancer cell proliferation by inducing cell cycle arrest in G(0)/G(1) phase, in concordance with the downregulation of CDK2, CDC25A, and cyclin D1. Our data suggest that PAK7 is a new hallmark of gastric cancer, in which PAK7 might contribute to gain of tumor growth potential, acting by affecting the expression of cell cycle regulators. Therefore, PAK7 may be an attractive candidate as a therapeutic target in gastric cancer.

Toward system-level understanding of baculovirus-host cell interactions: from molecular fundamental studies to large-scale proteomics approaches

Baculoviruses are insect viruses extensively exploited as eukaryotic protein expression vectors. Molecular biology studies have provided exciting discoveries on virus-host interactions, but the application of omic high-throughput techniques on the baculovirus-insect cell system has been hampered by the lack of host genome sequencing. While a broader, systems-level analysis of biological responses to infection is urgently needed, recent advances on proteomic studies have yielded new insights on the impact of infection on the host cell. These works are reviewed and critically assessed in the light of current biological knowledge of the molecular biology of baculoviruses and insect cells.