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.

Retinoblastoma makes its mark on imprinting in plants

Imprinting in flowering plants and mammals causes monoallelic expression of parental alleles, but how is this achieved? New evidence in plants implicates the retinoblastoma pathway in establishing imprints during gametogenesis.

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.

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.

[pRB, p53, p16INK4a, senescence and malignant transformation]

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.

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.

The persistence of senescence

Senescence is a potent anticancer mechanism, representing a barrier that most, if not all, would-be tumor cells must traverse on their path to malignant transformation. In this Perspective, I discuss two recent publications (1, 2) that deal with the durability of senescence. These findings are of interest not only to those who study aging, but to those who study cancer as well.

Genetic alterations of p16INK4a and p14ARF genes in human bladder cancer

PURPOSE

The growth suppressive genes p16INK4a and p14ARF located on the 9p21 gene cluster have active roles in the Rb and p53 growth control pathways, respectively. p16INK4a is a cyclin dependent kinase inhibitor functioning upstream of Rb. p14ARF restrains cell growth by abrogating Mdm2 inhibition of p53 activity, thereby, facilitating p53 mediated cell cycle arrest and apoptosis. To elucidate specific targets and aberrations affecting the 9p21 chromosomal region in bladder cancer tumorigenesis alterations in the p16INK4a and p14ARF genes were analyzed.

MATERIALS AND METHODS

A total of 53 transitional cell carcinomas from 44 patients with bladder cancer were collected. Genetic alterations of p16INK4a and p14ARF genes were analyzed by Southern hybridization, polymerase chain reaction (PCR)-single strand conformational polymorphism analysis and methylation specific PCR. In addition, mRNA expression status was detected by reverse transcriptase-PCR.

RESULTS

Homozygous deletion of p16INK4a and p14ARF genes was observed in 23% (12 of 53 samples) and 43% (23 of 53), respectively. Most deletions occurred exclusively on the E1 beta-p14ARF region. Concomitant deletion of p16INK4a and p14ARF genes was found in only 2 samples. One mutation was detected in exon 2 of p14ARF plus p16INK4a genes. Aberrant methylation of p16INK4a gene was found in 60% (24 of 40 tumors). However, no p14ARF gene methylation was detected in any case. The result of comparative reverse transcriptase-PCR showed that suppressed mRNA expression correlated with genetic alterations of p14ARF and p16INK4a genes in most tumor samples examined.

CONCLUSIONS

Results indicate that p14ARF is a primary target of homozygous deletion, whereas p16INK4a is the hot spot of hypermethylation on the 9p21 region in bladder cancer. The frequent inactivation of the p14ARF and p16INK4a genes may be an important mechanism for the dysfunction of p53 and Rb growth regulatory pathways during bladder cancer development.

Role of the RB tumor suppressor in cancer

Apart from their coordinated inactivation by DNA tumor viral oncoproteins, the pRB and p53 tumor suppressor pathways were not known to be connected ten years ago. Within the last decade, our appreciation of how these pathways are interconnected has grown substantially. The checks and balances that exist between pRB and p53 involve the regulation of the G1/S transition and its checkpoints, and much of this is under the control of the E2F transcription factor family. Following DNA damage, the p53-dependent induction of p21CIP1 regulates cyclin E/Cdk2 and cyclin A/Cdk2 complexes both of which phosphorylate pRB, leading to E2F-mediated activation. Similarly, E2F1-dependent induction of p19ARF antagonizes the ability of mdm2 to degrade p53, leading to p53 stabilization and potentially p53-mediated apoptosis or cell cycle arrest. From the existing mouse models discussed above, we also know that proliferation, cell death and differentiation of distinct tissues are also intimately linked through entrance and exit from the cell cycle, and thus through pRB and p53 pathways. Virtually all human tumors deregulate either the pRB or p53 pathway, and often times both pathways simultaneously, which is critical for crippling cellular defense against neoplasia. The next decade of cancer research will likely see these two tumor suppressor pathways only merge even more.

ARF mutation accelerates pituitary tumor development in Rb+/- mice

Mice heterozygous for the retinoblastoma (Rb) tumor suppressor gene develop pituitary and thyroid tumors with high penetrance. We demonstrate here that loss of the ARF tumor suppressor strongly accelerates intermediate lobe pituitary tumorigenesis in Rb heterozygous mice. These effects in the pituitary are greater than those conferred by p53 loss in that Rb+-;ARF-- mice display significantly more early atypical lesions than Rb+-; p53-- mice. Also, Rb+-;ARF-- compound mutants do not develop many of the novel tumors or precancerous lesions seen in Rb+-;p53-- compound mutants. Although complete loss of ARF expression is not obligatory for pituitary tumorigenesis in Rb+- mice, alterations of the ARF locus are observed in tumors from Rb+-;ARF+- mice, consistent with a selective advantage of ARF inactivation in this context. We conclude that inactivation of ARF acts more broadly than that of p53 in connecting abrogation of the Rb pathway to tumorigenesis.

Functional inactivation of pRB results in aneuploid mammalian cells after release from a mitotic block

The widespread chromosome instability observed in tumors and in early stage carcinomas suggests that aneuploidy could be a prerequisite for cellular transformation and tumor initiation. Defects in tumor suppressors and genes that are part of mitotic checkpoints are likely candidates for the aneuploid phenotype. By using flow cytometric, cytogenetic, and immunocytochemistry techniques we investigated whether pRB deficiency could drive perpetual aneuploidy in normal human and mouse fibroblasts after mitotic checkpoint challenge by microtubule-destabilizing drugs. Both mouse and human pRB-deficient primary fibroblasts resulted, upon release from a mitotic block, in proliferating aneuploid cells possessing supernumerary centrosomes. Aneuploid pRB-deficient cells show an elevated variation in chromosome numbers among cells of the same clone. In addition, these cells acquired the capability to grow in an anchorage-independent way at the same extent as tumor cells did suggesting aneuploidy as an initial mutational step in cell transformation. Normal Mouse Embryonic Fibroblasts (MEFs) harboring LoxP sites flanking exon 19 of the Rb gene arrested in G2/M with duplicated centrosomes after colcemid treatment. However, these cells escaped the arrest and became aneuploid upon pRB ablation by CRE recombinase, suggesting pRB as a major component of a checkpoint that controls cellular ploidy.

Retinoblastoma protein partners

Studies of the retinoblastoma gene (Rb) have shown that its protein product (pRb) acts to restrict cell proliferation, inhibit apoptosis, and promote cell differentiation. The frequent mutation of the Rb gene, and the functional inactivation of pRb in tumor cells, have spurred interest in the mechanism of pRb action. Recently, much attention has focused on pRb's role in the regulation of the E2F transcription factor. However, biochemical studies have suggested that E2F is only one of many pRb-targets and, to date, at least 110 cellular proteins have been reported to associate with pRb. The plethora of pRb-binding proteins raises several important questions. How many functions does pRb possess, which of these functions are important for development, and which contribute to tumor suppression? The goal of this review is to summarize the current literature of pRb-associated proteins.

Inactivation of the retinoblastoma tumor suppressor induces apoptosis protease-activating factor-1 dependent and independent apoptotic pathways during embryogenesis

Inactivation of the retinoblastoma (Rb) tumor suppressor in the mouse induces mid-gestational death accompanied by massive apoptosis in certain tissues. Herein, we analyzed the role of the apoptosis protease-activating factor Apaf-1, an essential component of the apoptosome, in mediating apoptosis in Rb-deficient mice. Analysis of compound mutant embryos lacking Rb and Apaf-1 revealed that Apaf-1 was absolutely required for apoptosis in the central nervous system and lens. In contrast, apoptosis in the peripheral nervous system and skeletal muscles only partly depended on Apaf-1 function. The dependency on Apaf-1 coincided with the requirement documented previously for E2F1 and p53 in the respective tissues. Loss of Apaf-1 specifically suppressed apoptosis but not the proliferation and differentiation defects in Rb-mutant embryos. We also show that the Apaf1+ but not the Rb+ allele is retained in pituitary tumors arising in Rb+/-:Apaf1+/- double heterozygous mice. Our results indicate that Apaf-1 plays a critical role in apoptosis in a subset of tissues and that both E2F1:p53:Apaf-1-dependent and -independent apoptotic pathways operate downstream of Rb.

Interactions of gemcitabine, carboplatin and paclitaxel in molecularly defined non-small-cell lung cancer cell lines

PURPOSE

To evaluate in vitro interactions of carboplatin, gemcitabine and paclitaxel in molecularly defined non-small-cell lung cancer lines.

MATERIALS AND METHODS

Three NSCLC lines, A549 (p16-,p53 wt, Rb wt), Calu-1 (p16-, p53-, Rb+) and H596 (p16 wt, p53 mut, Rb-) were utilized. Cells were exposed to carboplatin, gemcitabine and paclitaxel as individual drugs and in two- and three-drug combinations with various sequences of administration. Cytotoxicity was assessed with the MTT assay. Interactions between the drugs (additive, synergistic and antagonistic) were evaluated by median effect analysis.

RESULTS

Gemcitabine and carboplatin were synergistic in all three cell lines. In the A549 line, this synergy was most pronounced when gemcitabine preceded carboplatin. For three-drug combinations, paclitaxel was synergistic with gemcitabine and carboplatin regardless of sequence of administration.

CONCLUSIONS

In vitro modeling of gemcitabine and carboplatin as well as gemcitabine/carboplatin and paclitaxel demonstrates synergistic interaction regardless of p16, p53, or Rb status.

RB activation and repression of C-MYC transcription precede apoptosis of human prostate epithelial cells

OBJECTIVES

The retinoblastoma gene (Rb) encodes a transcriptional regulatory protein that functions in the regulation of cellular growth, differentiation, and survival. Many of the target genes of Rb with respect to growth regulation and differentiation have been identified. However, the identities of the Rb target genes involved in the regulation of cell survival and cell death (apoptosis) are unknown. We sought to determine whether the c-myc oncogene, a known target of Rb activity in cell cycle control, is also recruited in an apoptotic pathway uniquely regulated by Rb in prostate epithelial cells.

METHODS

We previously described a cell culture model to study the apoptosis of prostate cancer cells in which the human prostate cancer line, LNCaP, will undergo apoptosis after inducible expression and activation of the alpha isozyme of protein kinase C (PKC) or after exposure to low concentrations of the PKC activator TPA. Rb protein and c-myc mRNA and protein were evaluated in the Rb+/+ LNCaP and in the Rb-/- DU145 prostate cancer cells.

RESULTS

TPA-induced apoptosis in LNCaP cells was preceded by the rapid depletion of c-myc mRNA. The DU145 cultures were resistant to TPA-induced apoptosis and the c-myc mRNA levels remained elevated. The examination of Rb protein in the LNCaP cells revealed rapid dephosphorylation preceding both c-myc protein depletion and apoptosis. Additionally, pretreatment of LNCaP cells with staurosporine, a potent inhibitor of several isozymes of the PKC family, inhibited apoptosis in these cells and completely blocked activation of Rb and repression of c-myc.

CONCLUSIONS

On the basis of these studies, we suggest that induction of PKC-mediated apoptosis of Rb+/+ prostate cancer cells occurs by means of an intracellular pathway that involves the activation of Rb and repression of c-myc transcription.

The retinoblastoma gene regulates somatic growth during mouse development

Overexpression of the retinoblastoma gene (Rb) in mice leads to the dwarf phenotype. To explore the potential mechanism of Rb effects on the somatic growth, bitransgenic mice with tetracycline-regulated Rb expression were generated, and their phenotypes were compared with those of previously established Rb mouse models. By gestational day 12.5, embryos lacking Rb and those expressing twice the regular amount of Rb are 15% larger and 10-30% smaller, respectively, compared with their wild-type littermates. The dwarf phenotype is associated with increased plasma levels of insulin-like growth factor-I (IGF-I) but not with growth hormone and glucose concentrations. Down-regulation of the Rb transgene expression results in a reduction of the IGF-I plasma concentrations to normalcy and an increase of somatic growth prenatally and postnatally. Consistent with the in vivo results, cells overexpressing Rb require higher thresholds of IGF-I to stimulate proliferation. Thus, Rb plays an integral role for mouse somatic growth and maintenance during ontogenesis, and IGF-I pathway is likely to be a target for such regulation.

Sex hormone-induced carcinogenesis in Rb-deficient prostate tissue

The retinoblastoma (Rb) gene product is a prototypic tumor suppressor. Mice lacking the Rb gene are not viable and die in utero at approximately 13 days of gestation. In this study, we have rescued Rb-/- prostates by grafting pelvic organ rudiments from Rb-/- mouse embryos under the renal capsule of adult male nude mouse hosts. Grafts of embryonic pelvic organs developed into functional prostatic tissue. Some of the prostatic tissue generated was further used to construct chimeric prostatic tissue recombinants by combining wild-type rat urogenital mesenchyme (rUGM) with Rb-/- and Rb+/+ prostatic epithelium (PRE). The tissue recombinants were grown as subcapsular renal grafts and treated from the time of grafting with Silastic capsules containing 25 mg of testosterone plus 2.5 mg of estradiol. During 5-8 weeks of hormone treatment, rUGM+Rb+/+PRE tissue recombinants developed prostatic hyperplasia, whereas PRE in rUGM+Rb-/-PRE tissue recombinants developed hyperplasia, atypical hyperplasia, and carcinoma. During carcinogenesis in rUGM+Rb-/-PRE tissue recombinants, prostatic epithelial cells of the basal lineage disappeared, whereas the luminal cells underwent carcinogenesis. Epithelial E-cadherin almost totally disappeared. In all cases, epithelial PCNA labeling was elevated in tissue recombinants containing Rb-/- versus Rb+/+ epithelium. These epithelial changes were associated with almost total loss of smooth muscle cells in the stroma. In contrast, in untreated hosts rUGM+Rb+/+PRE tissue recombinants developed normally, and rUGM+Rb-/-PRE tissue recombinants developed mild epithelial hyperplasia. The results of this study demonstrate that Rb-/- prostatic tissue can be rescued from embryonic lethal mice and used to test its susceptibility to hormonal carcinogenesis. Deletion of the Rb gene predisposes prostatic epithelium to hyperplasia and increases proliferative activity Susceptibility to hormonal carcinogenesis in response to exogenous testosterone + estradiol is manifested in the progression from atypica hyperplasia to carcinoma. Thus, these findings demonstrate that the absence of the Rb tumor suppressor gene may predispose prostatic epithelial cells to carcinogenesis. Rescue of organs from Rb-/- embryos not only provides an opportunity to analyze the Rb gene pathway in the development and progression of prostate cancer but also provides an opportunity for specifically evaluating the role of the Rb pathway in development and carcinogenesis in other organs, such as the mammary gland and colon. Because rUGM greatly stimulates prostatic epithelial proliferation, the tissue recombinant model is a particularly useful tool for assessing the functional role of other genes in prostatic carcinogenesis through use of the appropriate transgenic or gene knockout mice.

Expression of genes involved with cell cycle control, cell growth and chromatin modification are altered in hepatoblastomas

Hepatoblastoma is a rare pediatric liver tumor. While much progress has been made in the treatment of the disease, very little is known about the moleculer events underlying the pathogenesis of this disease. We sought to investigate a series of hepatoblastomas for alterations in gene expression patterns with emphasis on important cell regulatory genes, including chromatin modifying enzymes, cyclin dependent kinase inhibitors, growth factors, oncogenes and cell cycle regulators. Total RNA was extracted from a series of sporadic hepatoblastomas with matched normal liver, some unmatched tumors and fetal livers, and gene expression was measured for various genes using RNase Protection Analysis (RPA). The results of this analysis show that the expression of many important regulatory genes are distinctly altered in these tumors, and a subset of tumors can be distinguished on the basis of these gene expression differences and histopathological features. Because the molecular events underlying the pathogenesis of this rare tumor are so poorly understood, this study represents a first step in determining some of the possible mechanisms involved which may provide future avenues of research.

Sodium butyrate modulates p53 and Bcl-2 expression in human retinoblastoma cell lines

Sodium butyrate (SB) is a potent biological modifier that can induce diverse effects including growth inhibition, differentiation, or apoptosis of many cell types including retinoblastoma (Rb), and modulation of genes such as c-fos and p53. In this study we assessed the effects of SB on cell growth and expression of p53, critical for cell cycle control, and Bcl-2, an inhibitor of apoptosis, in two human Rb cell lines (Y79 and WERI-Rb1). Attachment cultures were treated with 1 mM SB for up to 5 days and immunocytochemistry was used to examine for the expression of neural cell adhesion molecule (NCAM), p53, and Bcl-2. Suspension cultures of both cell lines were also treated with 1 and 4 mM SB, and at selected times cell extracts were prepared and the expression of p53 and Bcl-2 proteins determined by Western blot analysis. Treatment with 1 mM SB of both cell lines for 5 days inhibited growth and induced morphological changes including extension of neurite-like processes. Up to 12 h after 1 mM SB treatment, p53 and Bcl-2 expressions were similar to control levels, then gradually decreased to very low levels at 5 days. SB (4 mM) also inhibited growth associated with cell death, which was apparent at 24 h posttreatment. Expressions of p53 and Bcl-2 were decreased below control levels at 4 h, and by 24 h only very low levels of protein were detected. SB-induced modulation of p53 and Bcl-2 expression may have implications for controlling Rb growth, particularly in combination with chemotherapy drugs, which are increasingly used in the treatment of Rb.

[Mechanism of tumorigenesis caused by tumor suppressor gene]

Hereditary cancer syndrome is mainly caused by tumor suppressor genes, such as p53 gene in Li-Fraumeni syndrome and p16 gene in familial melanoma. There are 2 signal pathways of regulation of cell cycle, RB pathway and p53 pathway. Cyclin dependent kinase(CDK) was regulated by CDK inhibitor(CDKI) (p16, p15), and CDK-CDKI complex control RB by phosphorylation. p53 regulate RB pathway through p21. p53 is involved in apoptosis through bcl 2 or Bax 3. p19 coded by exon 1 beta and exons 2 and 3 of p16 gene, binds to MDM2 and regulate p53 pathway. Chromosome 9p21 locus, which p16, p15 and p19 genes are assigned, is important to regulate both RB and p53 pathways.

[Molecular pathogenesis of lung cancer]

Lung cancer is the largest cancer killer of men and women in the world. In addition to the progress made from antismoking primary prevention measures, new tools to help treat patients with lung cancer are emerging from the rapid advances in knowledge of the molecular pathogenesis of lung cancer. These tools include molecular and cellular biology and are starting to provide an insight into how the tumor cells, by altering oncogenes and tumor suppressor genes, achieves growth advantage, uncontrolled proliferation and metastatic behavior via disruption of key cell-cycle regulators and signal transduction cascades. These tools are being translated into clinical strategies to complement surgery, radiotherapy, and chemotherapy and also to assist in primary and secondary prevention efforts. From the current knowledge of the molecular pathogenesis of lung cancer we know that respiratory epithelial cells require many genetic alterations to become invasive and metastatic cancer.

Inhibition of the p53 tumor suppressor gene results in growth of human aortic vascular smooth muscle cells. Potential role of p53 in regulation of vascular smooth muscle cell growth

Loss of activity of the p53 tumor suppressor gene product has been postulated in the pathogenesis of human restenosis. Although the antioncogenes p53 and retinoblastoma (Rb) susceptibility gene have been reported to play a pivotal role in cell cycle progression in various cells, the role of p53 and Rb in the growth of human vascular smooth muscle cells (VSMC) has not yet been clarified. We used antisense strategy against p53 and Rb genes by the viral envelope-liposomal method. Transfection of antisense p53 oligodeoxynucleotides (ODN) alone resulted in an increase in DNA synthesis compared with control (P<0.01). Similarly, transfection of antisense Rb ODN alone resulted in a higher DNA synthesis rate than control (P<0.01). Moreover, increase in VSMC number was only induced by transfection of antisense p53 ODN alone or cotransfection of p53/Rb ODN (P<0.01), whereas a single transfection of antisense Rb ODN had little effect on cell number. Therefore, we hypothesized that this discrepancy is due to the induction of apoptosis mediated by p53. Interestingly, apoptotic cells were markedly increased in VSMC transfected with antisense Rb ODN alone, accompanied by the induction of p53 protein. The number of apoptotic cells was attenuated by cotransfection of antisense p53 ODN (P<0.01). We finally examined the molecular mechanisms of apoptosis induced by the absence of Rb. In VSMC transfected with antisense Rb ODN, bax, a promoter of apoptosis, was significantly increased in VSMC transfected with antisense Rb ODN (P<0.01), whereas bcl-2 and Fas did not play a pivotal role in the induction of apoptosis. Overall, these data first demonstrated that the antioncogenes p53 and Rb negatively regulated the cell cycle in VSMC, suggesting that the modulation of their activity may mediate VSMC growth such as that in restenosis and atherosclerosis. The presence of p53 plays a pivotal role in the regulation of apoptosis in human VSMC growth, probably through the bax pathway. These results provide evidence that p53 is a functional link between cell growth and apoptosis in VSMC.

Adenoviral-mediated gene transfer of a constitutively active form of the retinoblastoma gene product attenuates neointimal thickening in experimental vein grafts

PURPOSE

Inappropriate or excessive vascular smooth muscle cell proliferation leads to the development of occlusive lesions in up to 50% of vein grafts. The purpose of this study was to test the hypothesis that induced overexpression of a cytostatic nonphosphorylatable form of the retinoblastoma protein (DeltaRb) would attenuate neointimal thickening in experimental vein grafts.

METHODS

A replication-deficient adenovirus vector that encoded a nonphosphorylatable, constitutively active form of DeltaRb was constructed (AdDeltaRb) and contained an NH2-terminal epitope tag from the influenza hemagglutinin molecule (HA). Forty-eight male New Zealand white rabbits underwent surgical exposure of the external jugular vein for transfection with either 3 x 10(10) plaque-forming units/mL AdDeltaRb (n = 16), 3 x 10(10) plaque-forming units/mL control adenovirus (AdBglII, n = 15), or vehicle (n = 17) for 10 minutes at 120 mm Hg. After vector exposure, the vein was excised and interposed end-to-end into the carotid circulation. After 5 days, 12 grafts (four from each group) were excised and assayed for genomic DeltaRb DNA with the polymerase chain reaction or for hemagglutinin molecule expression and localization with immunohistochemistry. The remainder of the grafts (n = 36) were perfusion-fixed after 4 weeks, and 5 microm sections prepared for digital planimetric analysis.

RESULTS

Polymerase chain reaction results identified the DeltaRb gene only in the grafts that were transfected with AdDeltaRb. Immunohistochemical analysis results revealed transgene expression in most of the endothelial cells and in many of the smooth muscle cells. After 4 weeks, the grafts that were exposed to AdDeltaRb exhibited a 22% reduction in neointimal thickness (vehicle, 77 +/- 7 microm; AdBglII, 75 +/- 5 microm; AdDeltaRb, 60 +/- 5 microm; P =.05), and medial thickness, luminal diameter, and other parameters were unchanged (medial thickness: vehicle, 72 +/- 10 microm; AdBglII, 85 +/- 7 microm; AdDeltaRb, 69 +/- 9 microm; P = NS; luminal diameter: vehicle, 4.5 +/- 0.2 mm; AdBglII, 4.4 +/- 0.2 mm; AdDeltaRb, 4.7 +/- 0.1 mm; P = NS).

CONCLUSION

With this delivery system, adenoviral-mediated gene transfer is highly efficient and induced overexpression of DeltaRb leads to a reduction in vein graft neointimal thickening.

Loss of functional pRB is not a ubiquitous feature of B-cell malignancies

Human cancers frequently sustain genetic mutations that alter the function of their G1 cell cycle control check point. These include changes to the retinoblastoma gene and to the genes that regulate its phosphorylation, such as the cyclin-dependent kinase inhibitor p16INK4a. Altered expression of retinoblastoma protein (pRb) is associated with non-Hodgkin's lymphoma, particularly centroblastic and Burkitt's lymphomas. pRb is expressed in normal B-cells and its regulatory phosphorylation pathway is activated in response to a variety of stimuli. Since human B-lymphoma-derived cell lines are often used as in vitro model systems to analyse the downstream effects of signal transduction, we examined the functional status of pRb in a panel of human B-cell lines. We identified eleven cell lines which express the hyperphosphorylated forms of pRb. Furthermore, we suggest that the pRb protein appears to be functional in these cell lines.

pRb and p16 protein alterations in human oral tumorigenesis

Cyclin dependent kinase inhibitor 2/multiple tumour suppressor gene 1 (CDKN2/MTS1) and retinoblastoma (Rb) tumour suppressor genes play important roles in the regulation of the cell cycle. The protein products of these genes p16INK4 (p16) and pRb, respectively, like p53 protein inhibit progression from G1 to S phase. p16 exerts its function through inhibition of CDK4-mediated phosphorylation of pRb. The pRb/p16 pathway is a critical target for molecular aberration at the G1-S checkpoint in a wide range of primary human tumours. The expression of p16 and pRb proteins was analyzed by immunohistochemistry in 35 cases of oral squamous cell carcinomas (SCCs), 22 cases of premalignant oral lesions and 30 normal oral tissues. Lack of pRb expression was observed in 23/35 (66%) oral SCCs and 14/22 (64%) premalignant lesions. Lack of p16 expression was observed in 22/35 (63%) oral SCCs and 13/22 (59%) premalignant lesions. Weak p16 and pRb immunoreactivities were observed in normal oral mucosal epithelium. The status of p16 and pRb was correlated with clinicopathological characteristics of the patients. Alteration in p16 expression showed significant correlation with tumour staging and progression (P = 0.024). Alteration in pRb/p16 expression correlated with heavy consumption of betel and tobacco. Our results suggest that alterations in the p16/pRb pathway are early events in oral tumorigenesis and may be involved in the development of betel- and tobacco-related oral malignancies.

Interaction of retinoblastoma protein and D cyclins during cell-growth inhibition by hexamethylenebisacetamide in TM2H mouse epithelial cells

To explore the regulation and function of D-type cyclins in breast cancer cells, the mouse mammary hyperplastic epithelial cell line TM2H was treated with 5 mM hexamethylenebisacetamide (HMBA), a polar differentiation factor. The resulting growth-inhibitory effect of HMBA was completely reversible and was analyzed in terms of percent cells in G1; association of D-type cyclins with cyclin-dependent kinase (cdk) 4 and cdk6; G1 kinase activity; association of retinoblastoma protein (pRb) and phosphorylated pRb with D-type cyclins; and association of p16INK4a, p15INK4b, and p27Kip1 with cdk4 and cdk6. Synchronized TM2H cells were examined at 0, 3, 5, 9, 12, and 24 h after exposure to 5 mM HMBA. Inhibition of DNA synthesis, as measured by thymidine uptake, was first observed at 5 h (40%) and peaked at 24 h (80%). Flow cytometry at 9 h showed treated cells to be in G1 arrest. Western blot analysis showed weakly detectable cyclin D1 but readily detectable cyclin D2 and D3 proteins at 0 h; thereafter, cyclin D2 and D3 protein levels remained higher while cyclin D1 levels declined significantly in treated versus untreated cells. By 5 h (early G1), HMBA had markedly inhibited cdk4 and cdk6 kinase activity (67% and 75%, respectively) in treated versus untreated cells. By 9 and 12 h, pRb levels had increased 3.4-fold in treated versus untreated cells. At 5 h, cyclin D-associated pRb was totally hypophosphorylated in treated cells and hyperphosphorylated in untreated cells. The levels of pRb associated with cyclin D2 and D3 increased 2.89-fold and 4.6-fold, respectively, in treated versus untreated cells. At 5 h, treated cells showed a fivefold increase in cdk4-associated p27Kip1 and, at 9 h, a fourfold increase in cdk6-associated p27Kip1 over control levels. In confirmation of these data, HMBA was found to inhibit the growth of Rb-positive Du/145Rb cells but not their Rb-negative parental Du/145 cells. The data suggest that HMBA-induced growth inhibition is due to multifactorial mechanisms involving decreases in total cyclin D1 and inhibition of cdk4 and cdk6 kinase activities through elevation of levels of cdk4- and cdk6-associated p27Kip1 and concomitant increases in hypophosphorylated pRb and stable cyclin D2/pRb and cyclin D3/pRb complexes that help maintain pRb in a functional state.

Growth arrest associated with 12-o-tetradecanoylphorbol-13-acetate-induced hematopoietic differentiation with a defective retinoblastoma tumor suppressor-mediated pathway

The retinoblastoma tumor suppressor (Rb) gene product plays an essential role in cell-cycle regulation. However, its role in terminal differentiation of hematopoietic cells is speculative. Here we show a model of 12-o-tetradecanoylphorbol-13-acetate (TPA)-induced hematopoietic differentiation and growth arrest with a defective Rb-mediated pathway. TPA treatment arrested the cell cycle of a human hematopoietic cell line, MEG-01s, at the G1-S boundary and induced expression of p21/SDI1/WAF1/CIP1 and p27/KIP1. Both of these proteins were present in cyclin E-associated complexes, the histone H1 and Rb kinase activities of which were then inactivated. However, MEG-01s cells lacked the intact Rb protein and the Rb-mediated pathway was defective. This model raises a question about the role for Rb in terminal differentiation of hematopoietic cells.

Cyclin D1, p16, and retinoblastoma gene regulate mitogenic signaling of endothelin in rat mesangial cells

To elucidate the mechanisms by which endothelin (ET)-1 induces proliferation of mesangial cells, we investigated the involvement of the first gap phase of the cell cycle (G1) cyclin, cyclin-dependent kinase 4 (CDK4) activity, and the retinoblastoma gene product (pRb) in ET-1-stimulated cell cycle progression. In the present study, ET-1 stimulated CDK4 activity and cell cycle progression via ET A-type receptors and induced cyclin D1 mRNA and protein expression in rat mesangial cells. We also found that ET-1 stimulation of mesangial cell proliferation was inhibited by antisense oligonucleotides directed against cyclin D1 and by overexpression of a nonphosphorylatable form of pRb. To investigate the functional roles of p16INK4 and p21cip1 in ET-1-stimulated mesangial cell proliferation, we used adenovirus-mediated gene transfer. Endothelin-1-stimulated [3H]-thymidine incorporation, CDK4 kinase activity, and the percent of cells in S phase were found to be significantly inhibited by overexpression of p16INK4 and slightly inhibited by overexpression of p21cip1. Thus, ET-1 induced cyclin D1 expression and stimulated CDK4 activity and cell cycle progression via the A-type receptor in rat mesangial cells. These effects were regulated by the expression of cyclin D1, p16INK4, p21cip1, and phosphorylatable form of pRb. The results of the present study provide the basis for further investigation of basic and therapeutic approaches towards mesangial proliferative diseases.

The Knights of the Round Table hypothesis of tumour suppressor gene function--noble sacrifice or sexual dalliance: genes, including p53, BRCA1/2 and RB have evolved by horizontal and vertical transmission of mating factor genes and are involved in gametogenesis, implantation, development and tumourigenesis

The genes involved in negative cell cycle regulation and familial tumour susceptibility including APC, BRCA, p53, RB, WT1 are unique and have no homologies with other genes. Our hypothesis suggests they originated from mating factor genes, which halted cell division in response to stress to generate genetic diversity by sexual mechanisms. Some have evolved principally by vertical transmission (mismatch repair), others by horizontal transmission via mobile elements, predominantly in oocytes. We demonstrate amplification in human extra-embryonic tissues in fetus and mother in implantation; in the developing fetus, differing tissue-specific patterns are seen, especially between testis and ovary. We suggest that the fetus is susceptible to maternal transmission of infections including CMV, malaria, trypanosomes, whose sequences occur within these genes. In head and neck cancers, we demonstrate specific patterns of loss or instability involving up to seven different TSG. We suggest mechanisms of tumourigenesis involve transposable elements and episome formation, leading to loss of negative cell cycle regulation and exit from G0.

Role of the retinoblastoma protein in the pathogenesis of human cancer

The retinoblastoma gene (RB-1) was originally identified as the gene involved in hereditary retinoblastoma. However, RB-1 mutations are found in a number of common mesenchymal and epithelial malignancies. The retinoblastoma protein (pRB) acts as a transcriptional regulator of genes involved in DNA synthesis and cell-cycle control. In this regard, the functional interaction between pRB and the E2F transcription factor family appears to be critical. The pRB-E2F interaction is, in turn, regulated by a pathway that includes cyclin D1, cdk4, and p16. Mutations that affect this pathway have been documented in nearly every type of adult cancer. Thus, perturbation of pRB function may be required for the development of cancer. Insights into the biochemical functions of pRB, and its upstream regulators, may form the basis for the development of novel antineoplastic agents.

[Regulation of the cell cycle and the development of cancer: therapeutic prospects]

Several genetic alterations occur during the transformation process from normal to tumor cells, that involve the loss of fidelity of processes as replication, reparation, and segregation of the genomic material. Although normal cells have defense mechanisms against cancer progression, in tumor cells different escape pathways are activated leading to tumor progression. Recent advances have permitted cancer research to focus on the identification of some of its etiological factors. The knowledge of cell cycle reveals a precise mechanism achieved by the coordinated interactions and functions of cyclin-dependent kinases, control checkpoint, and repair pathways. Furthermore, it has been demonstrated that this coordinated function can be abrogated by specific genetic changes. These findings suggest that the molecular mechanisms responsible for cellular transformation may help to identify potential targets to improve cancer therapies.

[The inhibition effect of foreign retinoblastoma gene mediated by recombinant adenovirus vector on the growth of smooth muscle cells]

OBJECTIVE

To study the effect of foreign retinoblastoma (Rb) gene mediated by recombinant adenovirus vector on vascular smooth muscle cell (SMC) and the possibility by using Rb gene for gene therapy of atherosclerosis and artery restenosis.

METHODS

Rb gene recombinant adenovirus vector was constructed to transfect into rabbits' aortic SMC in vitro. The efficiency of transfection and expression was detected by biochemical staining, immunocytochemical analysis and polymerase chain reaction technique. The role of Rb gene for SMC proliferation, DNA synthesis and cell cycle were observed by cell counting. 3H-incorporation and cytometer respectively.

RESULTS

Adenovirus vector transfered the Rb gene into SMC effectively. The expression of Rb gene restricted the proliferation of SMC, decreased the DNA synthesis and plaaied a role in the cell cycle.

CONCLUSION

Rb gene mediated by adenovirus can be used to treat atherosclerosis.

Apoptosis and carcinogenesis

Many tumours are characterised by increased levels of apoptosis. This observation establishes significance for this process in tumour development, but it does little to elucidate the nature of this role, nor does it yield information relevant to the early stages of carcinogenesis. To gain a better understanding of the importance of apoptosis, it has been necessary to create a number of transgenic model systems wherein the apoptotic response has been modified. Using this strategy, a number of genetic lesions have been identified which affect both the apoptotic pathway and predisposition to malignancy. These lesions can operate either directly, by blocking the induction of apoptosis; or indirectly, by increasing the selective pressure for further genetic change. The consequent deregulation of growth control and increase in mutation burden represent two key steps in carcinogenesis, underlining the pivotal role played in tumour suppression by the normal induction of apoptosis.