Tumor Suppressors RB1 and CDKN2a Cooperatively Regulate Cell-Cycle Progression and Differentiation During Cardiomyocyte Development and Repair.

RATIONALE: Although rare cardiomyogenesis is reported in the adult mammalian heart, whether this results from differentiation or proliferation of cardiomyogenic cells remains controversial. The tumor suppressor genes RB1 (retinoblastoma) and CDKN2a (cyclin-dependent kinase inhibitor 2a) are critical cell-cycle regulators, but their roles in human cardiomyogenesis remains unclear. OBJECTIVE: We hypothesized that developmental activation of RB1 and CDKN2a cooperatively cause permanent cell-cycle withdrawal of human cardiac precursors (CPCs) driving terminal differentiation into mature cardiomyocytes, and that dual inactivation of these tumor suppressor genes promotes myocyte cell-cycle reentry. METHODS AND RESULTS: Directed differentiation of human pluripotent stem cells (hPSCs) into cardiomyocytes revealed that RB1 and CDKN2a are upregulated at the onset of cardiac precursor specification, simultaneously with GATA4 (GATA-binding protein 4) homeobox genes PBX1 (pre-B-cell leukemia transcription factor 1) and MEIS1 (myeloid ecotropic viral integration site 1 homolog), and remain so until terminal cardiomyocyte differentiation. In both GATA4+ hPSC cardiac precursors and postmitotic hPSC-cardiomyocytes, RB1 is hyperphosphorylated and inactivated. Transient, stage-specific, depletion of RB1 during hPSC differentiation enhances cardiomyogenesis at the cardiac precursors stage, but not in terminally differentiated hPSC-cardiomyocytes, by transiently upregulating GATA4 expression through a cell-cycle regulatory pathway involving CDKN2a. Importantly, cytokinesis in postmitotic hPSC-cardiomyocytes can be induced with transient, dual RB1, and CDKN2a silencing. The relevance of this pathway in vivo was suggested by findings in a porcine model of cardiac cell therapy post-MI, whereby dual RB1 and CDKN2a inactivation in adult GATA4+ cells correlates with the degree of scar size reduction and endogenous cardiomyocyte mitosis, particularly in response to combined transendocardial injection of adult human hMSCs (bone marrow-derived mesenchymal stromal cells) and cKit+ cardiac cells. CONCLUSIONS: Together these findings reveal an important and coordinated role for RB1 and CDKN2a in regulating cell-cycle progression and differentiation during human cardiomyogenesis. Moreover, transient, dual inactivation of RB1 and CDKN2a in endogenous adult GATA4+ cells and cardiomyocytes mediates, at least in part, the beneficial effects of cell-based therapy in a post-MI large mammalian model, a finding with potential clinical implications.

HBeAg-induced miR-106b promotes cell growth by targeting the retinoblastoma gene.

Chronic HBV infection is a major cause of hepatocellular carcinoma (HCC). The association between hepatitis B "e" antigen (HBeAg) and HCC is well-established by epidemiological studies. Nonetheless, the biological role of HBeAg in HCC remains enigmatic. We investigate the role of HBeAg in HBV-related HCC. Our findings suggest that HBeAg enhances cell proliferation and accelerates progression from G0/G1 phase to the S phase of the cell cycle in Huh7 cells. Examination of host gene expression and miRNA expression profiles reveals a total of 21 host genes and 12 host miRNAs that were differentially regulated in cells expressing HBeAg. Importantly, HBeAg induced the expression of miR-106b, an oncogenic miRNA. Interestingly, HBeAg-expression results in a significant reduction in the expression of retinoblastoma (Rb) gene, an experimentally validated target of miR-106b. Inhibition of miR-106b significantly increased the expression of the Rb gene, resulting in reduced cell proliferation and slowing of cell cycle progression from the G0/G1 phase to S phase. These observations suggest that the up-regulation of miR-106b by HBeAg contributes to the pathogenesis of HBV-related HCC by down-regulating the Rb gene. Our results highlight a role for HBeAg in HCC and provide a novel perspective on the molecular mechanisms underlying HBV-related HCC.

Concomitant inactivation of Rb and E2f8 in hematopoietic stem cells synergizes to induce severe anemia.

The retinoblastoma (Rb) tumor suppressor plays important roles in regulating hematopoiesis, particularly erythropoiesis. In an effort to understand whether Rb function can be mediated by E2F transcription factors in a BM-derived hematopoietic system in mice, we uncovered a functional synergy between Rb and E2F8 to promote erythropoiesis and to prevent anemia. Specifically, whereas Mx1-Cre-mediated inactivation of Rb or E2f8 in hematopoietic stem cells only led to mild erythropoietic defects, concomitant inactivation of both genes resulted in marked ineffective erythropoiesis and mild hemolysis, leading to severe anemia despite the presence of enhanced extramedullary erythropoiesis. Interestingly, although ineffective erythropoiesis was already present in the RbΔ/Δ mice and exacerbated in the RbΔ/Δ;E2f8Δ/Δ mice, hemolysis was exclusively manifested in the double-knockout mice. Using an adoptive transfer system and an erythroid-specific knockout system, we have shown that the synergy of Rb and E2f8 deficiency in triggering severe anemia is intrinsic to the erythroid lineage. Surprisingly, concomitant inactivation of Rb and E2f7, a close family member of E2f8, did not substantially worsen the erythropoietic defect resulted from Rb deficiency. The results of the present study reveal the specificity of E2F8 in mediating Rb function in erythropoiesis and suggest critical and overlapping roles of Rb and E2f8 in maintaining normal erythropoiesis and in preventing hemolysis.

The retinoblastoma gene Rb and its family member p130 suppress lung adenocarcinoma induced by oncogenic K-Ras.

Mutations of the retinoblastoma tumor suppressor gene RB are frequently observed in human cancers, but rarely in non-small cell lung carcinomas (NSCLCs). Emerging evidence also suggests that the RB-related gene p130 is inactivated in a subset of human NSCLCs. To directly test the specific tumor suppressor roles of RB and p130 in NSCLC, we crossed Rb and p130 conditional mutant mice to mice carrying a conditional oncogenic K-Ras allele. In this model, controlled oncogenic K-Ras activation leads to the development of adenocarcinoma, a major subtype of NSCLC. We found that loss of p130 accelerated the death of mice, providing direct evidence in vivo that p130 is a tumor suppressor gene, albeit a weak one in this context. Loss of Rb increased the efficiency of lung cancer initiation and resulted in the development of high-grade adenocarcinomas and rapid death. Thus, despite the low frequency of RB mutations in human NSCLCs and reports that K-Ras activation and loss of RB function are rarely found in the same human tumors, loss of Rb clearly cooperates with activation of oncogenic K-Ras in lung adenocarcinoma development in mice.

Retinoblastoma tumor suppressor gene expression determines the response to sequential flavopiridol and doxorubicin treatment in small-cell lung carcinoma.

PURPOSE: Small-cell lung cancers (SCLC) are defective in many regulatory mechanisms that control cell cycle progression, i.e., functional retinoblastoma protein (pRb). Flavopiridol inhibits proliferation and induces apoptosis in SCLC cell lines. We hypothesized that the sequence flavopiridol followed by doxorubicin would be synergistic in pRb-deficient SCLC cells. EXPERIMENTAL DESIGN: A H69 pRb-deficient SCLC cell line, H865, with functional pRb and H865 pRb small interfering RNA (siRNA) knockdown cells were used for in vitro and in vivo experiments. The in vivo efficiencies of various sequential combinations were tested using nude/nude athymic mice and human SCLC xenograft models. RESULTS: Flavopiridol then doxorubicin sequential treatment was synergistic in the pRB-negative H69 cell line. By knocking down pRb with specific siRNA, H865 clones with complete pRb knockdown became sensitive to flavopiridol and doxorubicin combinations. pRb-deficient SCLC cell lines were highly sensitive to flavopiridol-induced apoptosis. pRb-positive H865 cells arrested in G0-G1 with flavopiridol exposure, whereas doxorubicin and all flavopiridol/doxorubicin combinations caused a G2-M block. In contrast, pRb-negative SCLC cells did not arrest in G0-G1 with flavopiridol exposure. Flavopiridol treatment alone did not have an in vivo antitumor effect, but sequential flavopiridol followed by doxorubicin treatment provided tumor growth control and a survival advantage in Rb-negative xenograft models, compared with the other sequential treatments. CONCLUSIONS: Flavopiridol and doxorubicin sequential treatment induces potent in vitro and in vivo synergism in pRb-negative SCLC cells and should be clinically tested in tumors lacking functional pRB.

Molecular genetics of RB1--the retinoblastoma gene.

Thirty-seven years ago Alfred Knudson proposed his "two-hit" theory of the molecular etiology of retinoblastoma, establishing an elegant conceptual paradigm for tumorigenesis in general. A great body of work has subsequently elucidated the structure and function of the RB1 gene and the biology of its protein product, pRB. Multiple categories of mutations have been characterized and correlated with phenotypic expression, including translocations, deletions, insertions, and point mutations. The purpose of this review is to provide a concise overview of the molecular genetics and genotype-phenotype correlations in retinoblastoma.

A negative feedback signaling network underlies oncogene-induced senescence.

Oncogene-induced senescence functions to limit tumor development. However, a complete understanding of the signals that trigger this type of senescence is currently lacking. We found that mutations affecting NF1, Raf, and Ras induce a global negative feedback response that potently suppresses Ras and/or its effectors. Moreover, these signals promote senescence by inhibiting the Ras/PI3K pathway, which can impact the senescence machinery through HDM2 and FOXO. This negative feedback program is regulated in part by RasGEFs, Sprouty proteins, RasGAPs, and MKPs. Moreover, these signals function in vivo in benign human tumors. Thus, the ultimate response to the aberrant activation of the Ras pathway is a multifaceted negative feedback signaling network that terminates the oncogenic signal and participates in the senescence response.

Regulation of the G1/S phase of the cell cycle and alterations in the RB pathway in human lung cancer.

The retinoblastoma (RB)-Cyclin (CCN)D1-p16 cell cycle pathway has a crucial role in lung tumorigenesis. Impairment of the RB pathway has been shown to occur in almost all lung tumors. A deregulation at any level of this core RB pathway seems to make cells insensitive to the mitogenic signaling that is required for cell cycle progression. To date, almost all participants in this pathway have been shown to be altered to a various degree in lung tumors. Some of the alterations are mutually exclusive, including RB and p16INK4A . In small cell lung cancer, the RB tumor suppressor gene is inactivated in almost 90% of the tumors, whereas in non-small cell lung cancer, the cyclin-dependent kinase (CDK)4 inhibitor p16INK4A is inactivated in 40-60% of the tumors. Many mechanisms may be responsible for activating the RB-Cyclin D1 pathway, including activating (CDK4) and inactivating mutations (p16INK4A ), deletions (RB and p16INK4A ), amplifications (CCND1 and CDK4), silencing methylation (p16INK4A and RB), and hyper-phosphorylation (RB). As some of these alterations, such as p16INK4A methylation, can also be detected in bronchial lavage and serum, they could potentially serve as useful markers for the early detection of lung cancer. This review summarizes recent experiments describing the variable roles of key-player molecules of the RB pathway and different mechanisms by which the RB pathway can be altered in lung cancer.

The role of human papillomavirus in squamous carcinoma of the head and neck.

Human papillomavirus type-16 infection is associated with a significant portion of squamous carcinoma of the head and neck, particularly for the oropharynx and for those lacking the other risk factors of tobacco and alcohol. The link between human papillomavirus type-16 and carcinoma of the oropharynx is based on the identification of human papillomavirus type-16 in oropharyngeal tumors and the association of human papillomavirus type-16 with the risk of oropharyngeal cancer estimated in case-control epidemiologic studies. This review highlights the molecular mechanism of human papillomavirus carcinogenesis and the association of human papillomavirus type-16 as a risk factor for squamous cell carcinoma of the oropharynx as well as recent research efforts utilizing human papillomavirus as a biomarker of clinical outcomes.

A systematic search for downstream mediators of tumor suppressor function of p53 reveals a major role of BTG2 in suppression of Ras-induced transformation.

Factors that mediate p53 tumor suppressor activity remain largely unknown. In this study we describe a systematic approach to identify downstream mediators of tumor suppressor function of p53, consisting of global gene expression profiling, focused short hairpin RNA (shRNA) library creation, and functional selection of genetic elements cooperating with oncogenic Ras in cell transformation. This approach is based on our finding that repression of gene expression is a major event, occurring in response to p53 inactivation during transformation and immortalization of primary cells. Functional analysis of the subset of genes universally down-regulated in the cells that lacked functional p53 revealed BTG2 as a major downstream effector of p53-dependent proliferation arrest of mouse and human fibroblasts transduced with oncogenic Ras. shRNA-mediated knockdown of BTG2 cooperates with oncogenic Ras to transform primary mouse fibroblasts containing wild-type transcriptionally active p53. Repression of BTG2 results in up-regulation of cyclins D1 and E1 and phosphorylation of Rb and, in cooperation with other oncogenic elements, induces neoplastic transformation of primary human fibroblasts. BTG2 expression was found to be significantly reduced in a large proportion of human kidney and breast carcinomas, suggesting that BTG2 is a tumor suppressor that links p53 and Rb pathways in human tumorigenesis.

Tumour suppressor retinoblastoma protein Rb: a transcriptional regulator.

Rb was the first tumour suppressor identified through human genetic studies. The most significant achievement after almost twenty years since its cloning is the revelation that Rb possesses functions of a transcription regulator. Rb serves as a transducer between the cell cycle machinery and promoter-specific transcription factors. In this capacity, Rb is best known as a repressor of the E2F/DP family of transcription factors, which regulate expression of genes involved in cell proliferation and survival. An equally important aspect of Rb as a transcription regulator is that Rb also activates certain differentiation transcription factors to promote cellular differentiation. The molecular mechanisms behind the repressive effects of Rb on E2Fs have come to light in significant details, while those relating to Rb activation of differentiation transcription factors are much less understood. Finally, it has become clear that there are other aspects to Rb function that are not immediately related to transcription regulation.

Induction of G1 cell cycle arrest in human umbilical vein endothelial cells by flavone's inhibition of the extracellular signal regulated kinase cascade.

Dietary flavonoids have demonstrated anti-carcinogenic activity in several animal models, but their mechanisms of action have not yet been clearly established. Here, we show that flavone, a parent compound of flavonoids, inhibits the proliferation, migration, and capillary tube formation of human umbilical vein endothelial cells (HUVECs). Flow cytometric analysis showed that flavone arrests the cell cycle progression at G(1) phase in HUVECs. We observed the down-regulation of the hyperphosphorylated form of retinoblastoma gene product and cyclin-dependent kinases 2 and 4 in flavone-treated cells, but it had no affect on the expression of p53 and cyclin-dependent kinase inhibitors p21(CIP/Waf1) and p27(Kip). Flavone almost completely inhibited the activation of extracellular signal regulated kinase 1. The present results suggest that the flavone moiety of flavonoids is required for anti-proliferative activity of flavonoids and that anti-carcinogenic action of flavonoids in vivo was mediated, at least in part, by inhibiting angiogenesis.

[pRB, p53, p16INK4a, senescence and malignant transformation]. / PRB, p53, p16INK4a/ARF, sénescence cellulaire et transformation maligne.

Recent works aimed at clarifying the respective roles of p16INKa and p14ARF (both located on the same INK4a locus on chromosome 9p21 in man) in malignant transformation come to the conclusion that p16INK4a is the true tumor suppressor gene in man. In mouse, it is the p19ARF knockout that suppresses the barrier protecting cells from malignant transformation. This situation is in agreement with p19ARF- and p16-mediated senescence induced by oncogenic mutated ras (Ras*) in mouse and man respectively. Other results have shown that senescence in human diploid fibroblasts is associated with heterochromatin occurrence that maintains in repressed state E2F1-induced gens required for G1 to S phases transition. Since RB protein is responsible for this chromatin modification, cells with any impaired RB pathway cannot enter into senescence.

Discrete signaling pathways participate in RB-dependent responses to chemotherapeutic agents.

The retinoblastoma (RB) tumor suppressor has been proposed to function as a key mediator of cell cycle checkpoints induced by chemotherapeutic agents. However, these prior studies have relied on embryonic fibroblasts harboring chronic loss of RB, a condition under which compensation of RB functions is known to occur. Here we utilized primary adult fibroblasts derived from mice harboring loxP sites flanking exon 3 of the Rb gene to delineate the action of RB in the chemotherapeutic response. In this system we find that targeted disruption of Rb leads to little overt change in cell cycle distribution. However, these cells exhibited deregulation of RB/E2F target genes and became aneuploid following culture in the absence of RB. When challenged with both DNA damaging and antimetabolite chemotherapeutics, RB was required for primary adult cells to undergo DNA damage checkpoint responses and loss of RB resulted in enhanced aneuploidy following challenge. In contrast, following spontaneous immortalization and the loss of functional p53 signaling, the antimetabolite 5-fluorouracil (5-FU) failed to induce arrest despite the presence of RB. In these immortal cultures RB/E2F targets were deregulated in a complex, gene-specific manner and RB was required for the checkpoint response to camptothecin (CPT). Mechanistic analyses of the checkpoint responses in primary cells indicated that loss of RB leads to increased p53 signaling and decreased viability following both CPT and 5-FU treatment. However, the mechanism through which these agents act to facilitate cell cycle inhibition through RB were distinct. These studies underscore the critical role of RB in DNA-damage checkpoint signaling and demonstrate that RB mediates chemotherapeutic-induced cell cycle inhibition in adult fibroblasts by distinct mechanisms.

New roles for the RB tumor suppressor protein.

For a gene whose existence was first postulated in 1971, was cloned in 1986 and whose functions have been extensively characterized ever since, you might be inclined to think there was not much new to report regarding the retinoblastoma tumor suppressor gene (RB)--but you would be wrong to make such an assumption. RB is still piquing our interest with several activities defined over the past year that reveal new and exciting roles for this key tumor suppressor gene. These functions include regulation of senescence through specific gene silencing mechanisms, control of developmental processes in extra-embryonic tissues, maintaining tissue homeostasis and determining survival responses to chemotherapy.

The cell cycle and its relevance to the urologist.

PURPOSE: We surveyed fundamental concepts of the cell cycle to help the average urologist better understand the molecular basis for specific aspects of urological disease. MATERIALS AND METHODS: Important publications that have shaped our current understanding of the cell cycle were selected for review. Definitions of key terms are provided in a glossary. RESULTS: Cell proliferation, survival and programmed cell death (apoptosis) are the net result of a complex interaction of molecular signals that regulate DNA and protein synthesis. Many of the abnormal patterns of cell behavior that contribute to the pathology of malignant urological disease arise from disruptions in the molecular controls that normally regulate the cell cycle. Benign urological conditions, including cystic diseases and hypertrophy, also reflect abnormal growth that results from the disruption of cell cycle controls. CONCLUSIONS: This review is designed for the clinician and for the nonspecialist who is interested in the science of the cell cycle and its regulation as it broadly pertains to urological disease. Recent advances in the understanding of cell cycle regulation are presented with clinical correlations illustrating how these processes are involved in coordinating cell growth and cell death at the molecular level.