RB restricts DNA damage-initiated tumorigenesis through an LXCXE-dependent mechanism of transcriptional control.

The LXCXE peptide motif facilitates interaction between the RB tumor suppressor and a large number of cellular proteins that are expected to impinge on diverse biological processes. In vitro and in vivo analyses demonstrated that LXCXE binding function is dispensable for RB promoter association and control of basal gene expression. Dependence on this function of RB is unmasked after DNA damage, wherein LXCXE binding is essential for exerting control over E2F3 and suppressing cell-cycle progression in the presence of genotoxic stress. Gene expression profiling revealed that the transcriptional program coordinated by this specific aspect of RB is associated with progression of human hepatocellular carcinoma and poor disease outcome. Consistent with these findings, biological challenge revealed a requirement for LXCXE binding in suppression of genotoxin-initiated hepatocellular carcinoma in vivo. Together, these studies establish an essential role of the LXCXE binding motif for RB-mediated transcriptional control, response to genotoxic insult, and tumor suppression.

Retinoblastoma protein potentiates the innate immune response in hepatocytes: significance for hepatocellular carcinoma.

UNLABELLED: Cancers mediated by viral etiology must exhibit deregulated cellular proliferation and evade immune recognition. The role of the retinoblastoma tumor suppressor (RB) pathway, which is lost at relatively high frequency in hepatocellular carcinoma (HCC), has recently been expanded to include the regulation of innate immune responsiveness. In this study we investigated the coordinate impact of RB-loss on cell cycle control and immune function in the liver. We found that RB depletion in hepatoma cells resulted in a compromised immunological response to multiple stimuli and reduced the potential of these cells to recruit myeloid cells. Viral-mediated liver-specific RB deletion in vivo led to the induction of genes associated with proliferation and cell cycle entry as well as the significant attenuation of genes associated with immune function, as evidenced by decreases in cytokine and chemokine expression, leukocyte recruitment, and hepatic inflammation. To determine if these changes in gene expression were instructive in human disease, we compared our liver-specific RB-loss gene signature to existing profiles of HCC and found that this signature was associated with disease progression and confers a worse prognosis. CONCLUSION: Our data confirm that RB participates in the regulation of innate immunity in liver parenchymal cells both in vitro and in vivo and to our knowledge describes the first gene signature associated with HCC that includes both immunoregulatory and proliferative genes and that can also be attributed to the alteration of a single gene in vitro.

RB and p53 cooperate to prevent liver tumorigenesis in response to tissue damage.

BACKGROUND & AIMS: The tumor suppressors retinoblastoma (RB) and p53 are important regulators of the cell cycle. Although human cancer cells inactivate RB and p53 by many mechanisms, the cooperative roles of these proteins in tumorigenesis are complex and tissue specific. We analyzed the cooperation of RB and p53 in liver development and pathogenesis of hepatocellular carcinoma. METHODS: Spontaneous and carcinogen-induced (diethylnitrosamine) tumorigenesis were studied in mice with liver-specific deletions of Rb and/or p53 (Rbf/f;albcre+, p53f/f;albcre+ and Rbf/f; p53f/f;albcre+ mice). Genotype, histologic, immunohistochemical, microarray, quantitative polymerase chain reaction, immunoblot, and comparative genomic hybridization analyses were performed using normal and tumor samples. Comparative microarray analyses were performed against publicly available human microarray data sets. RESULTS: Deletion of RB and p53 from livers of mice deregulated the transcriptional programs associated with human disease. These changes were not sufficient for spontaneous tumorigenesis; potent quiescence mechanisms compensated for loss of these tumor suppressors. In response to hepatocarcinogen-induced damage, distinct and cooperative roles of RB and p53 were revealed; their loss affected cell cycle control, checkpoint response, and genome stability. In damaged tissue, combined loss of RB and p53 resulted in early lesion formation, aggressive tumor progression, and gene expression signatures and histologic characteristics of advanced human hepatocellular carcinoma. CONCLUSIONS: The effects RB and p53 loss are determined by the tissue environment; cell stresses that promote aggressive disease reveal the functions of these tumor suppressors.

Activation of the retinoblastoma tumor suppressor mediates cell cycle inhibition and cell death in specific cervical cancer cell lines.

High-risk human papilloma virus (HPV) encodes two oncoproteins, E6 and E7, which are vital to viral replication and contribute to the development of cervical cancer. HPV16 E7 can target over 20 cellular proteins, but is best known for inactivating the retinoblastoma (RB) tumor suppressor. RB functions by restraining cells from entering S-phase of the cell cycle, thus preventing aberrant proliferation. While it is well established that HPV16 E7 facilitates the degradation of the RB protein, the ability of the RB pathway to overcome E7 action is less well understood. In this study the RB-pathway was activated via the overexpression of the p16ink4a tumor suppressor or ectopic expression of an active allele of RB (PSM-RB). While p16ink4a had no influence on cell cycle progression, PSM-RB expression was sufficient to induce a cell cycle arrest in both SiHa and HeLa cells, HPV positive cervical cancer cell lines. Strikingly, this arrest led to the downregulation of E2F target gene expression, which was antagonized via enhanced HPV-E7 expression. Since downmodulation of E7 function is associated with chronic growth arrest and senescence, the effect of PSM-RB on proliferation and survival was evaluated. Surprisingly, sustained PSM-RB expression impeded the proliferation of SiHa cells, resulting in both cell cycle inhibition and cell death. From these studies we conclude that active RB expression can sensitize specific cervical cancer cells to cell cycle inhibition and cell death. Thus, targeted therapies involving activation of RB function may be effective in inducing cell death in cervical cancer.

Capture of associated targets on chromatin links long-distance chromatin looping to transcriptional coordination.

Here we describe a sensitive and novel method of identifying endogenous DNA-DNA interactions. Capture of Associated Targets on CHromatin (CATCH) uses efficient capture and enrichment of specific genomic loci of interest through hybridization and subsequent purification via complementary biotinylated oligonucleotide. The CATCH assay requires no enzymatic digestion or ligation, requires little starting material, provides high-quality data, has excellent reproducibility and is completed in less than 24 h. Efficacy is demonstrated through capture of three disparate loci, which demonstrate unique subsets of long-distance chromatin interactions enriched for both enhancer marks and oestrogen receptor-binding sites. In each experiment, CATCH-seq peaks representing long-distance chromatin interactions were centred near the TSS of genes, and, critically, the genes identified as physically interacting are shown to be transcriptionally coexpressed. These interactions could potentially create transcriptional hubs for the regulation of gene expression programmes.

Genomic agonism and phenotypic antagonism between estrogen and progesterone receptors in breast cancer.

The functional role of progesterone receptor (PR) and its impact on estrogen signaling in breast cancer remain controversial. In primary ER(+) (estrogen receptor-positive)/PR(+) human tumors, we report that PR reprograms estrogen signaling as a genomic agonist and a phenotypic antagonist. In isolation, estrogen and progestin act as genomic agonists by regulating the expression of common target genes in similar directions, but at different levels. Similarly, in isolation, progestin is also a weak phenotypic agonist of estrogen action. However, in the presence of both hormones, progestin behaves as a phenotypic estrogen antagonist. PR remodels nucleosomes to noncompetitively redirect ER genomic binding to distal enhancers enriched for BRCA1 binding motifs and sites that link PR and ER/PR complexes. When both hormones are present, progestin modulates estrogen action, such that responsive transcriptomes, cellular processes, and ER/PR recruitment to genomic sites correlate with those observed with PR alone, but not ER alone. Despite this overall correlation, the transcriptome patterns modulated by dual treatment are sufficiently different from individual treatments, such that antagonism of oncogenic processes is both predicted and observed. Combination therapies using the selective PR modulator/antagonist (SPRM) CDB4124 in combination with tamoxifen elicited 70% cytotoxic tumor regression of T47D tumor xenografts, whereas individual therapies inhibited tumor growth without net regression. Our findings demonstrate that PR redirects ER chromatin binding to antagonize estrogen signaling and that SPRMs can potentiate responses to antiestrogens, suggesting that cotargeting of ER and PR in ER(+)/PR(+) breast cancers should be explored.

Examining the pathogenesis of breast cancer using a novel agent-based model of mammary ductal epithelium dynamics.

The study of the pathogenesis of breast cancer is challenged by the long time-course of the disease process and the multi-factorial nature of generating oncogenic insults. The characterization of the longitudinal pathogenesis of malignant transformation from baseline normal breast duct epithelial dynamics may provide vital insight into the cascading systems failure that leads to breast cancer. To this end, extensive information on the baseline behavior of normal mammary epithelium and breast cancer oncogenesis was integrated into a computational model termed the Ductal Epithelium Agent-Based Model (DEABM). The DEABM is composed of computational agents that behave according to rules established from published cellular and molecular mechanisms concerning breast duct epithelial dynamics and oncogenesis. The DEABM implements DNA damage and repair, cell division, genetic inheritance and simulates the local tissue environment with hormone excretion and receptor signaling. Unrepaired DNA damage impacts the integrity of the genome within individual cells, including a set of eight representative oncogenes and tumor suppressors previously implicated in breast cancer, with subsequent consequences on successive generations of cells. The DEABM reproduced cellular population dynamics seen during the menstrual cycle and pregnancy, and demonstrated the oncogenic effect of known genetic factors associated with breast cancer, namely TP53 and Myc, in simulations spanning ∼40 years of simulated time. Simulations comparing normal to BRCA1-mutant breast tissue demonstrated rates of invasive cancer development similar to published epidemiologic data with respect to both cumulative incidence over time and estrogen-receptor status. Investigation of the modeling of ERα-positive (ER+) tumorigenesis led to a novel hypothesis implicating the transcription factor and tumor suppressor RUNX3. These data suggest that the DEABM can serve as a potentially valuable framework to augment the traditional investigatory workflow for future hypothesis generation and testing of the mechanisms of breast cancer oncogenesis.

RB deletion disrupts coordination between DNA replication licensing and mitotic entry in vivo.

The integrity of the retinoblastoma tumor suppressor (RB) pathway is critical for restraining inappropriate proliferation and suppressing tumor development in a plethora of tissues. Here adenovirus-mediated RB deletion in the liver of adult mice led to DNA replication in the absence of productive mitotic condensation. The replication induced by RB loss was E2F-mediated and associated with the induction of DNA damage and a nontranscriptional G2/M checkpoint that targeted the accumulation of Cyclin B1. In the context of RB deletion or E2F activation, there was an increase in hepatocyte ploidy that was accompanied by hyperphysiological assembly of prereplication complexes. In keeping with this dysregulation, initiation of DNA replication was readily observed in hepatocytes that were phenotypically in G2/M. Under such conditions, uncoupling of replication initiation from mitotic progression led to altered genome ploidy in the liver. Interestingly, these findings in hepatocytes were not recapitulated in the basally proliferative tissues of the gastrointestinal tract, where RB deletion, while increasing DNA replication, did not lead to a profound uncoupling from mitosis. Combined, these findings demonstrate the critical role of RB in controlling cell-cycle transitions and underscore the importance of intrinsic tissue environments in resultant phenotypes.

SWI/SNF deficiency results in aberrant chromatin organization, mitotic failure, and diminished proliferative capacity.

Switch (SWI)/sucrose nonfermentable (SNF) is an evolutionarily conserved complex with ATPase function, capable of regulating nucleosome position to alter transcriptional programs within the cell. It is known that the SWI/SNF complex is responsible for regulation of many genes involved in cell cycle control and proliferation, and it has recently been implicated in cancer development. The ATPase action of SWI/SNF is conferred through either the brahma-related gene 1 (Brg1) or brahma (Brm) subunit of the complex, and it is of central importance to the modification of nucleosome position. In this study, the role of the Brg1 and Brm subunits were examined as they relate to chromatin structure and organization. Deletion of the Brg1 ATPase results in dissolution of pericentromeric heterochromatin domains and a redistribution of histone modifications associated with these structures. This effect was highly specific to Brg1 and is not reproduced by the loss of Brm or SNF5/BAF47/INI1. Brg1 deficiency is associated with the appearance of micronuclei and aberrant mitoses that are a by-product of dissociated chromatin structure. Thus, Brg1 plays a critical role in maintaining chromatin structural integrity.