Unique impact of RB loss on hepatic proliferation: tumorigenic stresses uncover distinct pathways of cell cycle control

The retinoblastoma homolog RBR1 mediates localization of the repair protein RAD51 to DNA lesions in Arabidopsis

The retinoblastoma protein (Rb), which typically functions as a transcriptional repressor of E2F‐regulated genes, represents a major control hub of the cell cycle. Here, we show that loss of the Arabidopsis Rb homolog RETINOBLASTOMA‐RELATED 1 (RBR1) leads to cell death, especially upon exposure to genotoxic drugs such as the environmental toxin aluminum. While cell death can be suppressed by reduced cell‐proliferation rates, rbr1 mutant cells exhibit elevated levels of DNA lesions, indicating a direct role of RBR1 in the DNA‐damage response (DDR). Consistent with its role as a transcriptional repressor, we find that RBR1 directly binds to and represses key DDR genes such as RADIATION SENSITIVE 51 (RAD51), leaving it unclear why rbr1 mutants are hypersensitive to DNA damage. However, we find that RBR1 is also required for RAD51 localization to DNA lesions. We further show that RBR1 is itself targeted to DNA break sites in a CDKB1 activity‐dependent manner and partially co‐localizes with RAD51 at damage sites. Taken together, these results implicate RBR1 in the assembly of DNA‐bound repair complexes, in addition to its canonical function as a transcriptional regulator.

Hepatitis C virus core protein modulates pRb2/p130 expression in human hepatocellular carcinoma cell lines through promoter methylation

Background

Hepatitis C Virus (HCV) infection is associated with chronically evolving disease and development of hepatocellular carcinoma (HCC), albeit the mechanism of HCC induction by HCV is still controversial. The nucleocapsid (core) protein of HCV has been shown to be directly implicated in cellular transformation and immortalization, enhancing the effect of oncogenes and decreasing the one of tumor suppressor genes, as RB1 and its protein product pRB. With the aim of identifying novel molecular mechanisms of hepatocyte transformation by HCV, we examined the effect of HCV core protein on the expression of the whole Retinoblastoma (RB) family of tumor and growth suppressor factors, i.e. pRb, p107 and pRb2/p130.

Methods

We used a model system consisting of the HuH-7, HCV-free, human hepatocellular carcinoma cell line and of the HuH-7-CORE cells derived from the former and constitutively expressing the HCV core protein. We determined pRb, p107 and pRb2/p130 protein and mRNA amount of the respective genes RB1, RBL1 and RBL2, RBL2 promoter activity and methylation as well as DNA methyltransferase 1 (DNMT1) and 3b (DNMT3b) expression level. The effect of pRb2/p130 over-expression on the HCV core-expressing HuH-7-CORE cells was also evaluated.

Results

We found that the HCV core protein expression down-regulated pRb2/p130 protein and mRNA levels in HuH-7-CORE cells by inducing promoter hyper-methylation with the concomitant up-regulation of DNMT1 and DNMT3b expression. When pRb2/p130 expression was artificially re-established in HuH-7-CORE cells, cell cycle analysis outlined an accumulation in the G0/G1 phase, as expected.

Conclusions

HCV core appears indeed able to significantly down-regulate the expression and the function of two out of three RB family tumor and growth suppressor factors, i.e. pRb and pRb2/p130. The functional consequences at the level of cell cycle regulation, and possibly of more complex cell homeostatic processes, may represent a plausible molecular mechanism involved in liver transformation by HCV.

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

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.

Reduced expression of the retinoblastoma protein shows that the related signaling pathway is essential for mediating the antineoplastic activity of erufosine

Erufosine is a new antineoplastic agent of the group of alkylphosphocholines, which interferes with signal transduction and induces apoptosis in various leukemic and tumor cell lines. The present study was designed to examine for the first time the mechanism of resistance to erufosine in malignant cells with permanently reduced expression of the retinoblastoma (Rb) protein. Bearing in mind the high number of malignancies with reduced level of this tumor-suppressor, this investigation was deemed important for using erufosine, alone or in combination, in patients with compromised RB1 gene expression. For this purpose, clones of the leukemic T-cell line SKW-3 were used, which had been engineered to constantly express differently low Rb levels. The alkylphosphocholine induced apoptosis, stimulated the expression of the cyclin dependent kinase inhibitor p27^Kip1 and inhibited the synthesis of cyclin D3, thereby causing a G₂ phase cell cycle arrest and death of cells with wild type Rb expression. In contrast, Rb-deficiency impeded the changes induced by eru-fosine in the expression of these proteins and abrogated the induction of G₂ arrest, which was correlated with reduced antiproliferative and anticlonogenic activities of the compound. In conclusion, analysis of our results showed for the first time that the Rb signaling pathway is essential for mediating the antineoplastic activity of erufosine and its efficacy in patients with malignant diseases may be predicted by determining the Rb status.

Suppression of hepatitis C virus replication by cyclin-dependent kinase inhibitors

Hepatitis C virus (HCV) is a causative agent of chronic hepatitis. Although the standard therapy for HCV-infected patients consists of pegylated interferon plus ribavirin, this treatment is associated with serious side effects and high costs, and fails in some patients infected with specific HCV genotypes. To address this problem, we are developing small-molecule inhibitors of cyclin-dependent kinases (CDKs) as novel anti-HCV drug candidates. Previous data showed that HCV replication is inhibited by retinoblastoma protein, which is itself inactivated by CDK-mediated phosphorylation. Here, we report that CDK inhibitors suppress HCV replication in vitro and in vivo, and that CDK4 is required for efficient HCV replication. These findings shed light on the development of novel anti-HCV drugs that target host factors.

RB tumor suppressive function in response to xenobiotic hepatocarcinogens

Diverse etiologic events are associated with the development of hepatocellular carcinoma. During hepatocarcinogenesis, genetic events likely occur that subsequently cooperate with long-term exposures to further drive the progression of hepatocellular carcinoma. In this study, the frequent loss of the retinoblastoma (RB) tumor suppressor in hepatocellular carcinoma was modeled in response to diverse hepatic stresses. Loss of RB did not significantly affect the response to a steatotic stress as driven by a methionine- and choline-deficient diet. In addition, RB status did not significantly influence the response to peroxisome proliferators that can drive hepatomegaly and tumor development in rodents. However, RB loss exhibited a highly significant effect on the response to the xenobiotic1,4-Bis-[2-(3,5-dichloropyridyloxy)] benzene. Loss of RB yielded a unique proliferative response to this agent, which was distinct from both regenerative stresses and genotoxic carcinogens. Long-term exposure to 1,4-Bis-[2-(3,5-dichloropyridyloxy)] benzene yielded profound tumor development in RB-deficient livers that was principally absent in RB-sufficient tissue. These data demonstrate the context specificity of RB and the key role RB plays in the suppression of hepatocellular carcinoma driven by xenobiotic stress.

Targeting the p53 pathway in retinoblastoma with subconjunctival Nutlin-3a

Retinoblastoma is a rare childhood cancer of the retina that begins in utero and is diagnosed in the first years of life. The goals of retinoblastoma treatment are ocular salvage, vision preservation, and reduction of short- and long-term side effects without risking mortality because of tumor dissemination. To identify better chemotherapeutic combinations for the treatment of retinoblastoma, several groups have developed genetic mouse models and orthotopic xenograft models of human retinoblastoma for preclinical testing. Previous studies have implicated the MDMX protein in the suppression of the p53 pathway in retinoblastoma and shown that the MDM2/MDMX antagonist, Nutlin-3a, can efficiently induce p53-mediated cell death in retinoblastoma cell lines. However, Nutlin-3a cannot be administered systemically to treat retinoblastoma, because it has poor penetration across the blood-ocular barrier. Therefore, we developed an ocular formulation of Nutlin-3a, Nutlin-3a(OC), and tested the pharmacokinetics and efficacy of this new formulation in genetic and human retinoblastoma orthotopic xenograft models of retinoblastoma. Here, we show that Nutlin-3a(OC) specifically and efficiently targets the p53 pathway and that the combination of Nutlin-3a(OC) with systemic topotecan is a significantly better treatment for retinoblastoma than currently used chemotherapy in human orthotopic xenografts. Our studies provide a new standardized approach to evaluate and prioritize novel agents for incorporation into future clinical trials for retinoblastoma.

Overexpression of Aurora-A in primary cells interferes with S-phase entry by diminishing Cyclin D1 dependent activities

Background

Aurora-A is a bona-fide oncogene whose expression is associated with genomic instability and malignant transformation. In several types of cancer, gene amplification and/or increased protein levels of Aurora-A are a common feature.

Results

In this report, we describe that inhibition of cell proliferation is the main effect observed after transient overexpression of Aurora-A in primary human cells. In addition to the known cell cycle block at the G2/M transition, Aurora-A overexpressing cells fail to overcome the restriction point at the G1/S transition due to diminished RB phosphorylation caused by reduced Cyclin D1 expression. Consequently, overexpression of Cyclin D1 protein is able to override the Aurora-A mediated G1 block. The Aurora-A mediated cell cycle arrest in G2 is not influenced by Cyclin D1 and as a consequence cells accumulate in G2. Upon deactivation of p53 part of the cells evade this premitotic arrest to become aneuploid.

Conclusion

Our studies describe that an increase of Aurora-A expression levels on its own has a tumor suppressing function, but in combination with the appropriate altered intracellular setting it might exert its oncogenic potential. The presented data indicate that deactivation of the tumor suppressor RB is one of the requirements for overriding a cell cycle checkpoint triggered by increased Aurora-A levels.

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

The retinoblastoma tumor suppressor (RB) is functionally inactivated at high frequency in nearly all tumor types. Herein the acute deletion of RB in the liver reveals an immediate and profound dysregulation of spatiotemporal coordination in cell-cycle phases, resulting in robust DNA damage and aneuploidy that is not found in other tissues.

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.

Cyclins, Cdks, E2f, Skp2, and more at the first international RB Tumor Suppressor Meeting

The RB1 gene was cloned because its inactivation causes the childhood ocular tumor, retinoblastoma. It is widely expressed, inactivated in most human malignancies, and present in diverse organisms from mammals to plants. Initially, retinoblastoma protein (pRB) was linked to cell cycle regulation, but it also regulates senescence, apoptosis, autophagy, differentiation, genome stability, immunity, telomere function, stem cell biology, and embryonic development. In the 23 years since the gene was cloned, a formal international symposium focused on the RB pathway has not been held. The "First International RB Tumor Suppressor Meeting" (Toronto, Canada, November 19-21, 2009) established a biennial event to bring experts in the field together to discuss how the RB family ("pocket proteins"), as well as its regulators and effectors, influence biology and human disease. We summarize major new breakthroughs and emerging trends presented at the meeting.

What's so special about RB?

RB, p107, and p130 are highly related proteins, each capable of regulating cellular proliferation. However, only RB is frequently mutated in cancer. In this issue of Cancer Cell, Chicas et al. shed new light on this conundrum, defining a "special," nonredundant role for RB in promoting cellular senescence.