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Genome-wide CRISPR screens are an effective discovery tool for genes that underlie diverse cellular mechanisms that can be scored through cell fitness. Loss-of-function screens are particularly challenging compared to gain-of-function because of the limited dynamic range of decreased sgRNA sequence detection. Here we describe Guide-Only control CRISPR (GO-CRISPR), an improved loss-of-function screening workflow, and its companion software package, Toolset for the Ranked Analysis of GO-CRISPR Screens (TRACS). We demonstrate a typical GO-CRISPR workflow in a non-proliferative 3D spheroid model of dormant high grade serous ovarian cancer and demonstrate superior performance to standard screening methods. The unique integration of the pooled sgRNA library quality and guide-only controls allows TRACS to identify novel molecular pathways that were previously unidentified in tumor dormancy and undetectable to analysis packages that lack the guide only controls. Together, GO-CRISPR and TRACS can robustly improve the discovery of essential genes in challenging biological scenarios such as growth arrested cells., Competing Interests: “I have read the journal’s policy and the authors of this manuscript have the following competing interests: PP is a co-founder of Copoly.ai, a company focused on applying artificial intelligence to drive healthcare innovations. We have received no funding from commercial sources for this research from Copoly, or any other company. The findings and data from this report are freely available to all, and there is no overlap with regards to patents, products in development, or marketed products from any commercial entity. JIM and FAD have no competing interests to declare.”, (Copyright: © 2024 Perampalam et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Fibrosing interstitial lung diseases (ILDs) encompass a diverse range of scarring disorders that lead to progressive lung failure. Previous gene expression profiling studies focused on idiopathic pulmonary fibrosis (IPF) and bulk tissue samples. We employed digital spatial profiling to gain new insights into the spatial resolution of gene expression across distinct lung microenvironments (LMEs) in IPF, chronic hypersensitivity pneumonitis (CHP) and non-specific interstitial pneumonia (NSIP). We identified differentially expressed genes between LMEs within each condition, and across histologically similar regions between conditions. Uninvolved regions in IPF and CHP were distinct from normal controls, and displayed potential therapeutic targets. Hallmark LMEs of each condition retained distinct gene signatures, but these could not be reproduced in matched lung tissue samples. Based on these profiles and unsupervised clustering, we grouped previously unclassified ILD cases into NSIP or CHP. Overall, our work uniquely dissects gene expression profiles between LMEs within and across different types of fibrosing ILDs., (© 2024. The Author(s).)

Dormancy in cancer is a clinical state in which residual disease remains undetectable for a prolonged duration. At a cellular level, rare cancer cells cease proliferation and survive chemotherapy and disseminate disease. We created a suspension culture model of high-grade serous ovarian cancer (HGSOC) dormancy and devised a novel CRISPR screening approach to identify survival genes in this context. In combination with RNA-seq, we discovered the Netrin signaling pathway as critical to dormant HGSOC cell survival. We demonstrate that Netrin-1, -3, and its receptors are essential for low level ERK activation to promote survival, and that Netrin activation of ERK is unable to induce proliferation. Deletion of all UNC5 family receptors blocks Netrin signaling in HGSOC cells and compromises viability during the dormancy step of dissemination in xenograft assays. Furthermore, we demonstrate that Netrin-1 and -3 overexpression in HGSOC correlates with poor outcome. Specifically, our experiments reveal that Netrin overexpression elevates cell survival in dormant culture conditions and contributes to greater spread of disease in a xenograft model of abdominal dissemination. This study highlights Netrin signaling as a key mediator HGSOC cancer cell dormancy and metastasis., Competing Interests: PP, JM, KZ, DP, SW, YR, MH, RR, BG, RC, TS, FD No competing interests declared, PM Founder of Netris Pharma, (© 2023, Perampalam et al.)

Androgen receptor inhibitors are commonly used for prostate cancer treatment, but acquired resistance is a significant problem. Codeletion of RB and p53 is common in castration resistant prostate cancers, however they are difficult to target pharmacologically. To comprehensively identify gene loss events that contribute to enzalutamide response, we performed a genome-wide CRISPR knockout screen in LNCaP prostate cancer cells. This revealed novel genes implicated in resistance that are largely unstudied. Gene loss events that confer enzalutamide sensitivity are enriched for GSEA categories related to stem cell and epigenetic regulation. We investigated the myeloid lineage stem cell factor HOXA9 as a candidate gene whose loss promotes sensitivity to enzalutamide. Cancer genomic data reveals that HOXA9 overexpression correlates with poor prognosis and characteristics of advanced prostate cancer. In cell culture, HOXA9 depletion sensitizes cells to enzalutamide, whereas overexpression drives enzalutamide resistance. Combination of the HOXA9 inhibitor DB818 with enzalutamide demonstrates synergy. This demonstrates the utility of our CRISPR screen data in discovering new approaches for treating enzalutamide resistant prostate cancer.

Mammalian cells repress expression of repetitive genomic sequences by forming heterochromatin. However, the consequences of ectopic repeat expression remain unclear. Here we demonstrate that inhibitors of EZH2, the catalytic subunit of the Polycomb repressive complex 2 (PRC2), stimulate repeat misexpression and cell death in resting splenic B cells. B cells are uniquely sensitive to these agents because they exhibit high levels of histone H3 lysine 27 trimethylation (H3K27me3) and correspondingly low DNA methylation at repeat elements. We generated a pattern recognition receptor loss-of-function mouse model, called RIC, with mutations in Rigi (encoding for RIG-I), Ifih1 (MDA5), and Cgas. In both wildtype and RIC mutant B cells, EZH2 inhibition caused loss of H3K27me3 at repetitive elements and upregulated their expression. However, NF-κB-dependent expression of inflammatory chemokines and subsequent cell death was suppressed by the RIC mutations. We further show that inhibition of EZH2 in cancer cells requires the same pattern recognition receptors to activate an interferon response. Together, the results reveal chemokine expression induced by EZH2 inhibitors in B cells as a novel inflammatory response to genomic repeat expression. Given the overlap of genes induced by EZH2 inhibitors and Epstein-Barr virus infection, this response can be described as a form of viral mimicry., (© 2023 The Authors. Published under the terms of the CC BY NC ND 4.0 license.)

The Farnesoid X Receptor (FXR) belongs to the nuclear receptor superfamily and is an essential bile acid (BA) receptor that regulates the expression of genes involved in the metabolism of BAs. FXR protects the liver from BA overload, which is a major etiology of hepatocellular carcinoma. Herein, we investigated the changes in gene expression and chromatin accessibility in hepatocytes by performing RNA-seq in combination with the Assay for Transposase-Accessible Chromatin with high-throughput sequencing (ATAC-seq) using a novel FXR knockout mouse model ( Fxr ex5Δ : Nr1h4 ex5Δ/ex5Δ ) generated through CRISPR/Cas9. Consistent with previous Fxr knockout models, we found that Fxr ex5Δ mice develop late-onset HCC associated with increased serum and hepatic BAs. FXR deletion was associated with a dramatic loss of chromatin accessibility, primarily at promoter-associated transcription factor binding sites. Importantly, several genes involved in BA biosynthesis and circadian rhythm were downregulated following loss of FXR, also displayed reduced chromatin accessibility at their promoter regions. Altogether, these findings suggest that FXR helps to maintain a transcriptionally active state by regulating chromatin accessibility through its binding and recruitment of transcription factors and coactivators.

Clear evidence supports a causal link between Merkel cell polyomavirus (MCPyV) and the highly aggressive human skin cancer called Merkel cell carcinoma (MCC). Integration of viral DNA into the human genome facilitates continued expression of the MCPyV small tumor (ST) and large tumor (LT) antigens in virus-positive MCCs. In MCC tumors, MCPyV LT is truncated in a manner that renders the virus unable to replicate yet preserves the LXCXE motif that facilitates its binding to and inactivation of the retinoblastoma tumor suppressor protein (pRb). We previously developed a MCPyV transgenic mouse model in which MCC tumor-derived ST and truncated LT expression were targeted to the stratified epithelium of the skin, causing epithelial hyperplasia, increased proliferation, and spontaneous tumorigenesis. We sought to determine if any of these phenotypes required the association between the truncated MCPyV LT and pRb. Mice were generated in which K14-driven MCPyV ST/LT were expressed in the context of a homozygous RbΔLXCXE knock-in allele that attenuates LT-pRb interactions through LT's LXCXE motif. We found that many of the phenotypes including tumorigenesis that develop in the K14-driven MCPyV transgenic mice were dependent upon LT's LXCXE-dependent interaction with pRb. These findings highlight the importance of the MCPyV LT-pRb interaction in an in vivo model for MCPyV-induced tumorigenesis., Competing Interests: I have read the journal’s policy and the authors of this manuscript have the following competing interests: J.A.D. received research funding from Rain Therapeutics. The remaining authors have declared that no competing interests exist.

In cancer, dormancy refers to a clinical state in which microscopic residual disease becomes non-proliferative and is largely refractory to chemotherapy. Dormancy was first described in breast cancer where disease can remain undetected for decades, ultimately leading to relapse and clinical presentation of the original malignancy. A long latency period can be explained by withdrawal from cell proliferation (cellular dormancy), or a balance between proliferation and cell death that retains low levels of residual disease (tumor mass dormancy). Research into cellular dormancy has revealed features that define this state. They include arrest of cell proliferation, altered cellular metabolism, and unique cell dependencies and interactions with the microenvironment. These characteristics can be shared by dormant cells derived from disparate primary disease sites, suggesting common features exist between them.High-grade serous ovarian cancer (HGSOC) disseminates to locations throughout the abdominal cavity by means of cellular aggregates called spheroids. These growth-arrested and therapy-resistant cells are a strong contributor to disease relapse. In this review, we discuss the similarities and differences between ovarian cancer cells in spheroids and dormant properties reported for other cancer disease sites. This reveals that elements of dormancy, such as cell cycle control mechanisms and changes to metabolism, may be similar across most forms of cellular dormancy. However, HGSOC-specific aspects of spheroid biology, including the extracellular matrix organization and microenvironment, are obligatorily disease site specific. Collectively, our critical review of current literature highlights places where HGSOC cell dormancy may offer a more tractable experimental approach to understand broad principles of cellular dormancy in cancer., (© 2022. The Author(s).)

Despite extensive analysis of pRB phosphorylation in vitro, how this modification influences development and homeostasis in vivo is unclear. Here, we show that homozygous Rb ∆K4 and Rb ∆K7 knock-in mice, in which either four or all seven phosphorylation sites in the C-terminal region of pRb, respectively, have been abolished by Ser/Thr-to-Ala substitutions, undergo normal embryogenesis and early development, notwithstanding suppressed phosphorylation of additional upstream sites. Whereas Rb ∆K4 mice exhibit telomere attrition but no other abnormalities, Rb ∆K7 mice are smaller and display additional hallmarks of premature aging including infertility, kyphosis, and diabetes, indicating an accumulative effect of blocking pRb phosphorylation. Diabetes in Rb ∆K7 mice is insulin-sensitive and associated with failure of quiescent pancreatic β-cells to re-enter the cell cycle in response to mitogens, resulting in induction of DNA damage response (DDR), senescence-associated secretory phenotype (SASP), and reduced pancreatic islet mass and circulating insulin level. Pre-treatment with the epigenetic regulator vitamin C reduces DDR, increases cell cycle re-entry, improves islet morphology, and attenuates diabetes. These results have direct implications for cell cycle regulation, CDK-inhibitor therapeutics, diabetes, and longevity., (© 2022 The Authors. Published under the terms of the CC BY NC ND 4.0 license.)

DREAM (Dp, Rb-like, E2F, and MuvB) is a transcriptional repressor complex that regulates cell proliferation, and its loss causes neonatal lethality in mice. To investigate DREAM function in adult mice, we used an assembly-defective p107 protein and conditional deletion of its redundant family member p130. In the absence of DREAM assembly, mice displayed shortened survival characterized by systemic amyloidosis but no evidence of excessive cellular proliferation. Amyloid deposits were found in the heart, liver, spleen, and kidneys but not the brain or bone marrow. Using laser-capture microdissection followed by mass spectrometry, we identified apolipoproteins as the most abundant components of amyloids. Intriguingly, apoA-IV was the most detected amyloidogenic protein in amyloid deposits, suggesting apoA-IV amyloidosis (AApoAIV). AApoAIV is a recently described form, whereby WT apoA-IV has been shown to predominate in amyloid plaques. We determined by ChIP that DREAM directly regulated Apoa4 and that the histone variant H2AZ was reduced from the Apoa4 gene body in DREAM's absence, leading to overexpression. Collectively, we describe a mechanism by which epigenetic misregulation causes apolipoprotein overexpression and amyloidosis, potentially explaining the origins of nongenetic amyloid subtypes.

The retinoblastoma tumor suppressor protein (RB) plays an important role in biological processes such as cell cycle control, DNA damage repair, epigenetic regulation, and genome stability. The canonical model of RB regulation is that cyclin-CDKs phosphorylate and render RB inactive in late G1/S, promoting entry into S phase. Recently, monophosphorylated RB species were described to have distinct cell-cycle-independent functions, suggesting that a phosphorylation code dictates diversity of RB function. However, a biologically relevant, functional role of RB phosphorylation at non-CDK sites has remained elusive. Here, we investigated S838/T841 dual phosphorylation, its upstream stimulus, and downstream functional output. We found that mimicking T-cell receptor activation in Jurkat leukemia cells induced sequential activation of downstream kinases including p38 MAPK and RB S838/T841 phosphorylation. This signaling pathway disrupts RB and condensin II interaction with chromatin. Using cells expressing a WT or S838A/T841A mutant RB fragment, we present evidence that deficiency for this phosphorylation event prevents condensin II release from chromatin., Competing Interests: Conflict of interest The authors declare that they have no conflicts of interest with the contents of this article., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Background: The use of RNA-sequencing (RNA-seq) in molecular biology research and clinical settings has increased significantly over the past decade. Despite its widespread adoption, there is a lack of simple and interactive tools to analyze and explore RNA-seq data. Many established tools require programming or Unix/Bash knowledge to analyze and visualize results. This requirement presents a significant barrier for many researchers to efficiently analyze and present RNA-seq data., Results: Here we present BEAVR, a Browser-based tool for the Exploration And Visualization of RNA-seq data. BEAVR is an easy-to-use tool that facilitates interactive analysis and exploration of RNA-seq data. BEAVR is developed in R and uses DESeq2 as its engine for differential gene expression (DGE) analysis, but assumes users have no prior knowledge of R or DESeq2. BEAVR allows researchers to easily obtain a table of differentially-expressed genes with statistical testing and then visualize the results in a series of graphs, plots and heatmaps. Users are able to customize many parameters for statistical testing, dealing with variance, clustering methods and pathway analysis to generate high quality figures., Conclusion: BEAVR simplifies analysis for novice users but also streamlines the RNA-seq analysis process for experts by automating several steps. BEAVR and its documentation can be found on GitHub at https://github.com/developerpiru/BEAVR. BEAVR is available as a Docker container at https://hub.docker.com/r/pirunthan/beavr.

Interphase chromosomes are organized into topologically associated domains in order to establish and maintain integrity of transcriptional programs that remain poorly understood. Here, we show that condensin II and TFIIIC are recruited to bidirectionally transcribed promoters by a mechanism that is dependent on the retinoblastoma (RB) protein. Long-range chromosome contacts are disrupted by loss of condensin II loading, which leads to altered expression at bidirectional gene pairs. This study demonstrates that mammalian condensin II functions to organize long-range chromosome contacts and regulate transcription at specific genes. In addition, RB dependence of condensin II suggests that widespread misregulation of chromosome contacts and transcriptional alterations are a consequence of RB mutation., (Copyright © 2020 American Society for Microbiology.)

Proliferative control in cancer cells is frequently disrupted by mutations in the retinoblastoma protein (RB) pathway. Intriguingly, RB1 mutations can arise late in tumorigenesis in cancer cells whose RB pathway is already compromised by another mutation. In this study, we present evidence for increased DNA damage and instability in cancer cells with RB pathway defects when RB1 mutations are induced. We generated isogenic RB1 mutant genotypes with CRISPR/Cas9 in a number of cell lines. Cells with even one mutant copy of RB1 have increased basal levels of DNA damage and increased mitotic errors. Elevated levels of reactive oxygen species as well as impaired homologous recombination repair underlie this DNA damage. When xenografted into immunocompromised mice, RB1 mutant cells exhibit an elevated propensity to seed new tumors in recipient lungs. This study offers evidence that late-arising RB1 mutations can facilitate genome instability and cancer progression that are beyond the preexisting proliferative control deficit., (Copyright © 2019 American Society for Microbiology.)

Two recent reports describe pharmacologic approaches to specifically treat RB1 -mutant cancers. The basis of this approach is a synthetic lethal relationship between RB1 mutations and inhibition of Aurora kinases A or B. See related article by Oser et al., p. 230 . See related article by Gong et al., p. 248 ., (©2019 American Association for Cancer Research.)

RB-E2F transcriptional control plays a key role in regulating the timing of cell cycle progression from G1 to S-phase in response to growth factor stimulation. Despite this role, it is genetically dispensable for cell cycle exit in primary fibroblasts in response to growth arrest signals. Mice engineered to be defective for RB-E2F transcriptional control at cell cycle genes were also found to live a full lifespan with no susceptibility to cancer. Based on this background we sought to probe the vulnerabilities of RB-E2F transcriptional control defects found in Rb1R461E,K542E mutant mice (Rb1G) through genetic crosses with other mouse strains. We generated Rb1G/G mice in combination with Trp53 and Cdkn1a deficiencies, as well as in combination with KrasG12D. The Rb1G mutation enhanced Trp53 cancer susceptibility, but had no effect in combination with Cdkn1a deficiency or KrasG12D. Collectively, this study indicates that compromised RB-E2F transcriptional control is not uniformly cancer enabling, but rather has potent oncogenic effects when combined with specific vulnerabilities., Competing Interests: The authors have declared that no competing interests exist.

PD-L1 plays a central role in immune recognition of cancer cells. In this issue of Molecular Cell, Jin et al. (2019) report that a phosphorylated retinoblastoma protein contacts the DNA-binding domain of p65 NF-κB, thereby blocking transcription of PD-L1., (Copyright © 2018 Elsevier Inc. All rights reserved.)

The canonical model of RB-mediated tumour suppression developed over the past 30 years is based on the regulation of E2F transcription factors to restrict cell cycle progression. Several additional functions have been proposed for RB, on the basis of which a non-canonical RB pathway can be described. Mechanistically, the non-canonical RB pathway promotes histone modification and regulates chromosome structure in a manner distinct from cell cycle regulation. These functions have implications for chemotherapy response and resistance to targeted anticancer agents. This Opinion offers a framework to guide future studies of RB in basic and clinical research.

Background: Brain metastasis is becoming increasingly prevalent in breast cancer due to improved extra-cranial disease control. With emerging availability of modern image-guided radiation platforms, mouse models of brain metastases and small animal magnetic resonance imaging (MRI), we examined brain metastases' responses from radiotherapy in the pre-clinical setting. In this study, we employed half brain irradiation to reduce inter-subject variability in metastases dose-response evaluations., Methods: Half brain irradiation was performed on a micro-CT/RT system in a human breast cancer (MDA-MB-231-BR) brain metastasis mouse model. Radiation induced DNA double stranded breaks in tumors and normal mouse brain tissue were quantified using γ-H2AX immunohistochemistry at 30 min (acute) and 11 days (longitudinal) after half-brain treatment for doses of 8, 16 and 24 Gy. In addition, tumor responses were assessed volumetrically with in-vivo longitudinal MRI and histologically for tumor cell density and nuclear size., Results: In the acute setting, γ-H2AX staining in tumors saturated at higher doses while normal mouse brain tissue continued to increase linearly in the phosphorylation of H2AX. While γ-H2AX fluorescence intensities returned to the background level in the brain 11 days after treatment, the residual γ-H2AX phosphorylation in the radiated tumors remained elevated compared to un-irradiated contralateral tumors. With radiation, MRI-derived relative tumor growth was significantly reduced compared to the un-irradiated side. While there was no difference in MRI tumor volume growth between 16 and 24 Gy, there was a significant reduction in tumor cell density from histology with increasing dose. In the longitudinal study, nuclear size in the residual tumor cells increased significantly as the radiation dose was increased., Conclusions: Radiation damages to the DNAs in the normal brain parenchyma are resolved over time, but remain unrepaired in the treated tumors. Furthermore, there is a radiation dose response in nuclear size of surviving tumor cells. Increase in nuclear size together with unrepaired DNA damage indicated that the surviving tumor cells post radiation had continued to progress in the cell cycle with DNA replication, but failed cytokinesis. Half brain irradiation provides efficient evaluation of dose-response for cancer cell lines, a pre-requisite to perform experiments to understand radio-resistance in brain metastases.

The retinoblastoma protein (pRB) plays a key role in proliferative control and genome stability. For these reasons its functions are considered to be tumor suppressive. Its functional status offers critical insight into proliferative control signaling in tissues and in developing malignancies. In this chapter, we outline basic procedures to detect the retinoblastoma protein in formalin fixed, paraffin embedded tissue sections. In addition, we provide protocols to detect phosphorylation levels of pRB in tissues and offer controls to ensure fidelity of measurement. Importantly, these staining methods utilize broadly available reagents and equipment making them accessible to most biomedical research laboratories.

Organization of chromatin structure is indispensible to the maintenance of genome integrity. The retinoblastoma tumor suppressor protein (pRB) mediates both transcriptional repression and chromatin organization, but the independent contributions of these functions have been difficult to study. Here, we utilize a synthetic Rb1 mutant allele (F832A) that maintains pRB association at cell cycle gene promoters, but disrupts a cyclin-dependent kinase (CDK)-resistant interaction with E2F1 to reduce occupancy of pRB on intergenic chromatin. Reduced pRB chromatin association increases spontaneous γH2AX deposition and aneuploidy. Our data indicates that the CDK-resistant pRB-E2F1 scaffold recruits Condensin II to major satellite repeats to stabilize chromatin structure in interphase and mitosis through mechanisms that are distinct from silencing of repetitive sequence expression.

Epithelial ovarian cancer (EOC) generates multicellular aggregates called spheroids that detach from the primary tumor and disseminate through ascites. Spheroids possess a number of characteristics of tumor dormancy including withdrawal from the cell cycle and resistance to chemotherapeutics. This report systematically analyzes the effects of RNAi depletion of 21 genes that are known to contribute to negative regulation of the cell cycle in 10 ovarian cancer cell lines. Interestingly, spheroid cell viability was compromised by loss of some cyclin-dependent kinase inhibitors such as p57 Kip2 , as well as Dyrk1A, Lin52, and E2F5 in most cell lines tested. Many genes essential for EOC spheroid viability are pertinent to the mammalian DREAM repressor complex. Mechanistically, the data demonstrate that DREAM is assembled upon the induction of spheroid formation, which is dependent upon Dyrk1A. Loss of Dyrk1A results in retention of the b-Myb-MuvB complex, elevated expression of DREAM target genes, and increased DNA synthesis that is coincident with cell death. Inhibition of Dyrk1A activity using pharmacologic agents Harmine and INDY compromises viability of spheroids and blocks DREAM assembly. In addition, INDY treatment improves the response to carboplatin, suggesting this is a therapeutic target for EOC treatment. Implications: Loss of negative growth control mechanisms in cancer dormancy lead to cell death and not proliferation, suggesting they are an attractive therapeutic approach. Mol Cancer Res; 15(4); 371-81. ©2016 AACR ., (©2016 American Association for Cancer Research.)

Background: The G1-S phase transition is critical to maintaining proliferative control and preventing carcinogenesis. The retinoblastoma tumor suppressor is a key regulator of this step in the cell cycle., Results: Here we use a structure-function approach to evaluate the contributions of multiple protein interaction surfaces on pRB towards cell cycle regulation. SAOS2 cell cycle arrest assays showed that disruption of three separate binding surfaces were necessary to inhibit pRB-mediated cell cycle control. Surprisingly, mutation of some interaction surfaces had no effect on their own. Rather, they only contributed to cell cycle arrest in the absence of other pRB dependent arrest functions. Specifically, our data shows that pRB-E2F interactions are competitive with pRB-CDH1 interactions, implying that interchangeable growth arrest functions underlie pRB's ability to block proliferation. Additionally, disruption of similar cell cycle control mechanisms in genetically modified mutant mice results in ectopic DNA synthesis in the liver., Conclusions: Our work demonstrates that pRB utilizes a network of mechanisms to prevent cell cycle entry. This has important implications for the use of new CDK4/6 inhibitors that aim to activate this proliferative control network.

The mammalian G 1 -S phase transition is controlled by the opposing forces of cyclin-dependent kinases (CDK) and the retinoblastoma protein (pRB). Here, we present evidence for systems-level control of cell cycle arrest by pRB-E2F and p27-CDK regulation. By introducing a point mutant allele of pRB that is defective for E2F repression (Rb1 G ) into a p27 KIP1 null background (Cdkn1b -/- ), both E2F transcriptional repression and CDK regulation are compromised. These double-mutant Rb1 G/G ; Cdkn1b -/- mice are viable and phenocopy Rb1 +/- mice in developing pituitary adenocarcinomas, even though neither single mutant strain is cancer prone. Combined loss of pRB-E2F transcriptional regulation and p27 KIP1 leads to defective proliferative control in response to various types of DNA damage. In addition, Rb1 G/G ; Cdkn1b -/- fibroblasts immortalize faster in culture and more frequently than either single mutant genotype. Importantly, the synthetic DNA damage arrest defect caused by Rb1 G/G ; Cdkn1b -/- mutations is evident in the developing intermediate pituitary lobe where tumors ultimately arise. Our work identifies a unique relationship between pRB-E2F and p27-CDK control and offers in vivo evidence that pRB is capable of cell cycle control through E2F-independent effects., (Copyright © 2017 American Society for Microbiology.)

Repetitive genomic regions include tandem sequence repeats and interspersed repeats, such as endogenous retroviruses and LINE-1 elements. Repressive heterochromatin domains silence expression of these sequences through mechanisms that remain poorly understood. Here, we present evidence that the retinoblastoma protein (pRB) utilizes a cell-cycle-independent interaction with E2F1 to recruit enhancer of zeste homolog 2 (EZH2) to diverse repeat sequences. These include simple repeats, satellites, LINEs, and endogenous retroviruses as well as transposon fragments. We generated a mutant mouse strain carrying an F832A mutation in Rb1 that is defective for recruitment to repetitive sequences. Loss of pRB-EZH2 complexes from repeats disperses H3K27me3 from these genomic locations and permits repeat expression. Consistent with maintenance of H3K27me3 at the Hox clusters, these mice are developmentally normal. However, susceptibility to lymphoma suggests that pRB-EZH2 recruitment to repetitive elements may be cancer relevant., (Copyright © 2016 Elsevier Inc. All rights reserved.)

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., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

A fundamental need in the analysis of the cell cycle is the ability to isolate relatively homogeneous populations of cells in different phases. This is complicated by the variable proliferative properties and responses to synchronizing methods of different cancer-derived cell lines. Paradoxically, cell lines with genetic defects in cell cycle control are sometimes chosen because they are amenable to chemical synchronization. Embryonic fibroblasts from mice present the opportunity to study the effects of defined genetic modifications on a normal cell cycle. However, synchronization of these cells has often been challenging. In this chapter we outline three basic protocols for isolating mouse fibroblasts at the G1-to-S-phase transition, in S phase, and during mitosis.

The retinoblastoma tumor suppressor pathway is frequently inactivated in human cancer, enabling unrestrained proliferation. Most cancers, however, maintain expression of a wild-type (WT) retinoblastoma tumor suppressor protein (pRB). It is generally in a hyperphosphorylated state (ppRB) because of mutations in upstream regulators such as p16 and cyclin D. Hyperphosphorylated ppRB is considered inactive, although data are emerging that suggest it can retain some function. To test the clinical relevance of pRB status, we obtained archival tissue sections from 91 cases of lung adenocarcinoma resected between 2003 and 2008. All cases received platinum doublet chemotherapy, and the median survival was 5.9 years. All cases were assessed for pRB and ppRB using immunohistochemistry and quantified based on intensity of signal and proportion of positive cells. pRB expression was lost in 15% of lung adenocarcinoma cases. In tumors that did not express pRB, the survival rate was significantly improved (hazard ratio, 0.21; 95% confidence interval, 0.06-0.69; P = .01) in comparison to tumors that express pRB. pRB status was found to be an independent predictor of overall survival on multivariate analysis (hazard ratio, 0.22; 95% confidence interval, 0.07-0.73; P = .01) along with increased stage and age. pRB status did not alter baseline levels of apoptotic or proliferative markers in these tumors, but the DNA damage response protein 53BP1 was higher in cancers with high levels of pRB. In summary, loss of pRB expression is associated with improved survival in patients treated with surgical resection and chemotherapy. This may be useful in classifying patients at greatest benefit for aggressive treatment regimes., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Purpose: Small animal immobilization devices facilitate positioning of animals for reproducible imaging and accurate focal radiation therapy. In this study, the authors demonstrate the use of three-dimensional (3D) printing technology to fabricate a custom-designed mouse head restraint. The authors evaluate the accuracy of this device for the purpose of mouse brain irradiation., Methods: A mouse head holder was designed for a microCT couch using cad software and printed in an acrylic based material. Ten mice received half-brain radiation while positioned in the 3D-printed head holder. Animal placement was achieved using on-board image guidance and computerized asymmetric collimators. To evaluate the precision of beam localization for half-brain irradiation, mice were sacrificed approximately 30 min after treatment and brain sections were stained for γ-H2AX, a marker for DNA breaks. The distance and angle of the γ-H2AX radiation beam border to longitudinal fissure were measured on histological samples. Animals were monitored for any possible trauma from the device., Results: Visualization of the radiation beam on ex vivo brain sections with γ-H2AX immunohistochemical staining showed a sharp radiation field within the tissue. Measurements showed a mean irradiation targeting error of 0.14±0.09 mm (standard deviation). Rotation between the beam axis and mouse head was 1.2°±1.0° (standard deviation). The immobilization device was easily adjusted to accommodate different sizes of mice. No signs of trauma to the mice were observed from the use of tooth block and ear bars., Conclusions: The authors designed and built a novel 3D-printed mouse head holder with many desired features for accurate and reproducible radiation targeting. The 3D printing technology was found to be practical and economical for producing a small animal imaging and radiation restraint device and allows for customization for study specific needs.

Mutations in the retinoblastoma tumor suppressor gene Rb are involved in many forms of human cancer. In this study, we investigated the early consequences of inactivating Rb in the context of cellular reprogramming. We found that Rb inactivation promotes the reprogramming of differentiated cells to a pluripotent state. Unexpectedly, this effect is cell cycle independent, and instead reflects direct binding of Rb to pluripotency genes, including Sox2 and Oct4, which leads to a repressed chromatin state. More broadly, this regulation of pluripotency networks and Sox2 in particular is critical for the initiation of tumors upon loss of Rb in mice. These studies therefore identify Rb as a global transcriptional repressor of pluripotency networks, providing a molecular basis for previous reports about its involvement in cell fate pliability, and implicate misregulation of pluripotency factors such as Sox2 in tumorigenesis related to loss of Rb function., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Recent work demonstrates that retention of a single functional retinoblastoma susceptibility (RB1) allele is insufficient to maintain genome stability. Haploinsufficiency of RB1 accelerates cancer pathogenesis in concert with inactivation of tumor protein p53. Collectively, multiple lines of evidence suggest revision of the '2-hit' model to include conditional haploinsufficiency of RB1.

Unlabelled: Genome instability is a characteristic of malignant cells; however, evidence for its contribution to tumorigenesis has been enigmatic. In this study, we demonstrate that the retinoblastoma protein, E2F1, and Condensin II localize to discrete genomic locations including major satellite repeats at pericentromeres. In the absence of this complex, aberrant replication ensues followed by defective chromosome segregation in mitosis. Surprisingly, loss of even one copy of the retinoblastoma gene reduced recruitment of Condensin II to pericentromeres and caused this phenotype. Using cancer genome data and gene-targeted mice, we demonstrate that mutation of one copy of RB1 is associated with chromosome copy-number variation in cancer. Our study connects DNA replication and chromosome structure defects with aneuploidy through a dosage-sensitive complex at pericentromeric repeats., Significance: Genome instability is inherent to most cancers and is the basis for selective killing of cancer cells by genotoxic therapeutics. In this report, we demonstrate that instability can be caused by loss of a single allele of the retinoblastoma gene that prevents proper replication and condensation of pericentromeric chromosomal regions, leading to elevated levels of aneuploidy in cancer., (©2014 American Association for Cancer Research.)

Mammalian DREAM is a conserved protein complex that functions in cellular quiescence. DREAM contains an E2F, a retinoblastoma (RB)-family protein, and the MuvB core (LIN9, LIN37, LIN52, LIN54, and RBBP4). In mammals, MuvB can alternatively bind to BMYB to form a complex that promotes mitotic gene expression. Because BMYB-MuvB is essential for proliferation, loss-of-function approaches to study MuvB have generated limited insight into DREAM function. Here, we report a gene-targeted mouse model that is uniquely deficient for DREAM complex assembly. We have targeted p107 (Rbl1) to prevent MuvB binding and combined it with deficiency for p130 (Rbl2). Our data demonstrate that cells from these mice preferentially assemble BMYB-MuvB complexes and fail to repress transcription. DREAM-deficient mice show defects in endochondral bone formation and die shortly after birth. Micro-computed tomography and histology demonstrate that in the absence of DREAM, chondrocytes fail to arrest proliferation. Since DREAM requires DYRK1A (dual-specificity tyrosine phosphorylation-regulated protein kinase 1A) phosphorylation of LIN52 for assembly, we utilized an embryonic bone culture system and pharmacologic inhibition of (DYRK) kinase to demonstrate a similar defect in endochondral bone growth. This reveals that assembly of mammalian DREAM is required to induce cell cycle exit in chondrocytes., (Copyright © 2014, American Society for Microbiology. All Rights Reserved.)

The retinoblastoma protein (pRB) is best known for regulating cell proliferation through E2F transcription factors. In this report, we investigate the properties of a targeted mutation that disrupts pRB interactions with the transactivation domain of E2Fs. Mice that carry this mutation endogenously (Rb1(ΔG)) are defective for pRB-dependent repression of E2F target genes. Except for an accelerated entry into S phase in response to serum stimulation, cell cycle regulation in Rb1(ΔG/ΔG) mouse embryonic fibroblasts (MEFs) strongly resembles that of the wild type. In a serum deprivation-induced cell cycle exit, Rb1(ΔG/ΔG) MEFs display a magnitude of E2F target gene derepression similar to that of Rb1(-/-) cells, even though Rb1(ΔG/ΔG) cells exit the cell cycle normally. Interestingly, cell cycle arrest in Rb1(ΔG/ΔG) MEFs is responsive to p16 expression and gamma irradiation, indicating that alternate mechanisms can be activated in G1 to arrest proliferation. Some Rb1(ΔG/ΔG) mice die neonatally with a muscle degeneration phenotype, while the others live a normal life span with no evidence of spontaneous tumor formation. Most tissues appear histologically normal while being accompanied by derepression of pRB-regulated E2F targets. This suggests that non-E2F-, pRB-dependent pathways may have a more relevant role in proliferative control than previously identified.

The G1/S-phase restriction point is an important landmark in the mammalian cell division cycle. The key regulator of the G1/S transition is the retinoblastoma gene product (pRB). It prevents the transcription of genes required for S-phase progression by repressing E2F transcription factors. An increase in Cdk phosphorylation of pRB causes the release of E2F transcription factors and advancement into S phase. Here we describe two simple techniques used to assess pRB phosphorylation and E2F transcription during G1/S progression.

Cellular senescence is characterized by silencing of genes involved in DNA replication and cell cycle progression. Stable repression is crucial for preventing inappropriate DNA synthesis and the maintenance of a prolonged senescent state. Many of these genes are targets for E2F transcription factors. The pRB pathway plays a major role in senescence by directly repressing E2Fs and also by regulating chromatin at the promoters of E2F target genes using its LXCXE cleft-dependent interactions. In this study, we sought to investigate the mechanisms by which pRB stably silences E2F target gene transcription during cellular senescence. We report that in mouse embryonic fibroblasts, endogenous promyelocytic leukemia protein (PML) associates with E2F target genes in a pRB LXCXE-dependent manner during HrasV12-induced senescence. Furthermore, using a PML-IV-induced senescence model, we show that the pRB LXCXE binding cleft is essential for PML association with gene promoters, silencing of E2F target genes, and stable cell cycle exit. Binding assays show that pRB can interact with PML specifically during senescence, suggesting that signaling events in senescence regulate assembly of PML and pRB to establish heterochromatin and create a permanent cell cycle arrest.

Background: The Retinoblastoma protein (pRB) is a key tumor suppressor that is functionally inactivated in most cancers. pRB regulates the cell division cycle and cell cycle exit through protein-protein interactions mediated by its multiple binding interfaces. The LXCXE binding cleft region of pRB mediates interactions with cellular proteins that have chromatin regulatory functions. Chromatin regulation mediated by pRB is required for a stress responsive cell cycle arrest, including oncogene induced senescence. The in vivo role of chromatin regulation by pRB during senescence, and its relevance to cancer is not clear., Methodology/principal Findings: Using gene-targeted mice, uniquely defective for pRB mediated chromatin regulation, we investigated its role during transformation and tumor progression in response to activation of oncogenic ras. We report that the pRB(∆L) mutation confers susceptibility to escape from HrasV12 induced senescence and allows transformation in vitro, although these cells possess high levels of DNA damage. Intriguingly, LSL-Kras, Rb1 (∆L/∆L) mice show delayed lung tumor formation compared to controls. This is likely due to the increased apoptosis seen in the early hyperplastic lesions shortly following ras activation that inhibits tumor progression. Furthermore, DMBA treatment to induce sporadic ras mutations in other tissues also failed to reveal greater susceptibility to cancer in Rb1 (∆L/∆L) mice., Conclusions/significance: Our data suggests that chromatin regulation by pRB can function to limit proliferation, but its loss fails to contribute to cancer susceptibility in ras driven tumor models because of elevated levels of DNA damage and apoptosis.

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.

Inactivation of the RB protein is one of the most fundamental events in cancer. Coming to a molecular understanding of its function in normal cells and how it impedes cancer development has been challenging. Historically, the ability of RB to regulate the cell cycle placed it in a central role in proliferative control, and research focused on RB regulation of the E2F family of transcription factors. Remarkably, several recent studies have found additional tumour-suppressor functions of RB, including alternative roles in the cell cycle, maintenance of genome stability and apoptosis. These advances and new structural studies are combining to define the multifunctionality of RB.

The human papillomavirus (HPV) E7 oncoprotein binds cellular factors, preventing or retargeting their function and thereby making the infected cell conducive for viral replication. A key target of E7 is the product of the retinoblastoma susceptibility locus (pRb). This interaction results in the release of E2F transcription factors and drives the host cell into the S phase of the cell cycle. E7 binds pRb via a high-affinity binding site in conserved region 2 (CR2) and also targets a portion of cellular pRb for degradation via the proteasome. Evidence suggests that a secondary binding site exists in CR3, and that this interaction influences pRb deregulation. Additionally, evidence suggests that CR3 also participates in the degradation of pRb. We have systematically analyzed the molecular mechanisms by which CR3 contributes to deregulation of the pRb pathway by utilizing a comprehensive series of mutations in residues predicted to be exposed on the surface of HPV16 E7 CR3. Despite differences in the ability to interact with cullin 2, all CR3 mutants degrade pRb comparably to wild-type E7. We identified two specific patches of residues on the surface of CR3 that contribute to pRb binding independently of the high-affinity CR2 binding site. Mutants within CR3 that affect pRb binding are less effective than the wild-type E7 in overcoming pRb-induced cell cycle arrest. This demonstrates that the interaction between HPV16 E7 CR3 and pRb is functionally important for alteration of the cell cycle.

The retinoblastoma tumor suppressor protein (pRB) plays an integral role in G1-S checkpoint control and consequently is a frequent target for inactivation in cancer. The RB protein can function as an adaptor, nucleating components such as E2Fs and chromatin regulating enzymes into the same complex. For this reason, pRB's regulation by posttranslational modifications is thought to be critical. pRB is phosphorylated by a number of different kinases such as cyclin dependent kinases (Cdks), p38 MAP kinase, Chk1/2, Abl, and Aurora b. Although phosphorylation of pRB by Cdks has been extensively studied, activities regulated through phosphorylation by other kinases are just starting to be understood. As well as being phosphorylated, pRB is acetylated, methylated, ubiquitylated, and SUMOylated. Acetylation, methylation, and SUMOylation play roles in pRB mediated gene silencing. Ubiquitinylation of pRB promotes its degradation and may be used to regulate apoptosis. Recent proteomic data have revealed that pRB is posttranslationally modified to a much greater extent than previously thought. This new information suggests that many unknown pathways affect pRB regulation. This review focuses on posttranslational modifications of pRB and how they influence its function. The final part of the review summarizes new phosphorylation sites from accumulated proteomic data and discusses the possibilities that might arise from this data.

Commitment to divide is one of the most crucial steps in the mammalian cell division cycle. It is critical for tissue and organismal homeostasis, and consequently is highly regulated. The vast majority of cancers evade proliferative control, further emphasizing the importance of the commitment step in cell cycle regulation. The Retinoblastoma (RB) tumor suppressor pathway regulates this decision-making step. Since being the subject of Knudson's 'two hit hypothesis', there has been considerable interest in understanding pRB's role in cancer. It is best known for repressing E2F dependent transcription of cell cycle genes. However, pRB's role in controlling chromatin structure is expanding and bringing it into new regulatory paradigms. In this review we discuss pRB function through protein-protein interactions, at the level of transcriptional regulation of individual promoters and in organizing higher order chromatin domains.

The concept that aneuploidy is a characteristic of malignant cells has long been known; however, the idea that aneuploidy is an active contributor to tumorigenesis, as opposed to being an associated phenotype, is more recent in its evolution. At the same time, we are seeing the emergence of novel roles for tumor suppressor genes and oncogenes in genome stability. These include the adenomatous polyposis coli gene (APC), p53, the retinoblastoma susceptibility gene (RB1), and Ras. Originally, many of these genes were thought to be tumor suppressive or oncogenic solely because of their role in proliferative control. Because of the frequency with which they are disrupted in cancer, chromosome instability caused by their dysfunction may be more central to tumorigenesis than previously thought. Therefore, this review will highlight how the proper function of cell cycle regulatory genes contributes to the maintenance of genome stability, and how their mutation in cancer obligatorily connects proliferation and chromosome instability.

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

E2F transcription can lead to cell proliferation or apoptosis, indicating that E2Fs control opposing functions. In a similar manner, DNA double-strand breaks can signal to induce cell cycle arrest or apoptosis. Specifically, pRB is activated following DNA damage, allowing it to bind to E2Fs and block transcription at cell cycle promoters; however, E2F1 is simultaneously activated, leading to transcription at proapoptotic promoters. We examined this paradoxical control of E2F transcription by studying how E2F1's interaction with pRB is regulated following DNA damage. Our work reveals that DNA damage signals create multiple forms of E2F1 that contain mutually exclusive posttranslational modifications. Specifically, E2F1 phospho-serine 364 is found only in complex with pRB, while E2F1 phosphorylation at serine 31 and acetylation function to create a pRB-free form of E2F1. Both pRB-bound and pRB-free modifications on E2F1 are essential for the activation of TA-p73 and the maximal induction of apoptosis. Chromatin immunoprecipitation demonstrated that E2F1 phosphorylated on serine 364 is also present at proapoptotic gene promoters during the induction of apoptosis. This indicates that distinct populations of E2F1 are organized in response to DNA damage signaling. Surprisingly, these complexes act in parallel to activate transcription of proapoptotic genes. Our data suggest that DNA damage signals alter pRB and E2F1 to engage them in functions leading to apoptotic induction that are distinct from pRB-E2F regulation in cell cycle control.

The regulation of cell proliferation is central to tissue morphogenesis during the development of multicellular organisms. Furthermore, loss of control of cell proliferation underlies the pathology of diseases like cancer. As such there is great need to be able to investigate cell proliferation and quantitate the proportion of cells in each phase of the cell cycle. It is also of vital importance to indistinguishably identify cells that are replicating their DNA within a larger population. Since a cell's decision to proliferate is made in the G1 phase immediately before initiating DNA synthesis and progressing through the rest of the cell cycle, detection of DNA synthesis at this stage allows for an unambiguous determination of the status of growth regulation in cell culture experiments. DNA content in cells can be readily quantitated by flow cytometry of cells stained with propidium iodide, a fluorescent DNA intercalating dye. Similarly, active DNA synthesis can be quantitated by culturing cells in the presence of radioactive thymidine, harvesting the cells, and measuring the incorporation of radioactivity into an acid insoluble fraction. We have considerable expertise with cell cycle analysis and recommend a different approach. We Investigate cell proliferation using bromodeoxyuridine/fluorodeoxyuridine (abbreviated simply as BrdU) staining that detects the incorporation of these thymine analogs into recently synthesized DNA. Labeling and staining cells with BrdU, combined with total DNA staining by propidium iodide and analysis by flow cytometry offers the most accurate measure of cells in the various stages of the cell cycle. It is our preferred method because it combines the detection of active DNA synthesis, through antibody based staining of BrdU, with total DNA content from propidium iodide. This allows for the clear separation of cells in G1 from early S phase, or late S phase from G2/M. Furthermore, this approach can be utilized to investigate the effects of many different cell stimuli and pharmacologic agents on the regulation of progression through these different cell cycle phases. In this report we describe methods for labeling and staining cultured cells, as well as their analysis by flow cytometry. We also include experimental examples of how this method can be used to measure the effects of growth inhibiting signals from cytokines such as TGF-β1, and proliferative inhibitors such as the cyclin dependent kinase inhibitor, p27KIP1. We also include an alternate protocol that allows for the analysis of cell cycle position in a sub-population of cells within a larger culture. In this case, we demonstrate how to detect a cell cycle arrest in cells transfected with the retinoblastoma gene even when greatly outnumbered by untransfected cells in the same culture. These examples illustrate the many ways that DNA staining and flow cytometry can be utilized and adapted to investigate fundamental questions of mammalian cell cycle control.

The Hippo pathway coordinates organ size and cell proliferation. The retinoblastoma family of proteins regulates progression through the G0/G1 phase of the cell cycle. Disruption of either pathway contributes to cancer formation. Three recent studies in Genes & Development reveal how cellular proliferation is coordinated between these pathways. Here we discuss the implications of these studies and the new questions that they raise.

The pRB (retinoblastoma protein) has a central role in the control of the G(1)-S phase transition of the cell cycle that is mediated in part through the regulation of E2F transcription factors. Upon S-phase entry pRB is phosphorylated extensively, which in turn releases bound E2Fs to drive the expression of the genes required for S-phase progression. In the present study, we demonstrate that E2F1-maintains the ability to interact with ppRB (hyperphosphorylated pRB). This interaction is dependent upon the 'specific' E2F1-binding site located in the C-terminal domain of pRB. A unique region of the marked box domain of E2F1 contacts the 'specific' site to mediate the interaction with ppRB. The mechanistic basis of the interaction between E2F1 and ppRB is subtle. A single substitution between valine and proline residues in the marked box distinguishes E2F1's ability to interact with ppRB from the inability of E2F3 to bind to the 'specific' site in ppRB. The E2F1-pRB interaction at the 'specific' site also maintains the ability to regulate the transcriptional activation of E2F1 target genes. These data reveal a mechanism by which E2F1 regulation by pRB can persist, when pRB is hyperphosphorylated and presumed to be inactive., (© The Authors Journal compilation © 2011 Biochemical Society)

Background: The ability to respond to anti-growth signals is critical to maintain tissue homeostasis and loss of this negative growth control safeguard is considered a hallmark of cancer. Negative growth regulation generally occurs during the G0/G1 phase of the cell cycle, yet the redundancy and complexity among components of this regulatory network has made it difficult to discern how negative growth cues protect cells from aberrant proliferation., Methodology/principal Findings: The retinoblastoma protein (pRB) acts as the final barrier to prevent cells from entering into the cell cycle. By introducing subtle changes in the endogenous mouse Rb1 gene (Rb1(ΔL)), we have previously shown that interactions at the LXCXE binding cleft are necessary for the proper response to anti-growth signals such as DNA damage and TGF-β, with minimal effects on overall development. This disrupts the balance of pro- and anti-growth signals in mammary epithelium of Rb1(ΔL/ΔL) mice. Here we show that Rb1(ΔL/ΔL) mice are more prone to mammary tumors in the Wap-p53(R172H) transgenic background indicating that negative growth regulation is important for tumor suppression in these mice. In contrast, the same defect in anti-growth control has no impact on Neu-induced mammary tumorigenesis., Conclusions/significance: Our work demonstrates that negative growth control by pRB acts as a crucial barrier against oncogenic transformation. Strikingly, our data also reveals that this tumor suppressive effect is context-dependent.

The phosphorylation state and corresponding activity of the retinoblastoma tumor suppressor protein (Rb) are modulated by a balance of kinase and phosphatase activities. Here we characterize the association of Rb with the catalytic subunit of protein phosphatase 1 (PP1c). A crystal structure identifies an enzyme docking site in the Rb C-terminal domain that is required for efficient PP1c activity toward Rb. The phosphatase docking site overlaps with the known docking site for cyclin-dependent kinase (Cdk), and PP1 competition with Cdk-cyclins for Rb binding is sufficient to retain Rb activity and block cell-cycle advancement. These results provide the first detailed molecular insights into Rb activation and establish a novel mechanism for Rb regulation in which kinase and phosphatase compete for substrate docking.