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

An overview of mouse models of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) has become a severe burden on global health due to its high morbidity and mortality rates. However, effective treatments for HCC are limited. The lack of suitable preclinical models may contribute to a major failure of drug development for HCC. Here, we overview several well-established mouse models of HCC, including genetically engineered mice, chemically-induced models, implantation models, and humanized mice. Immunotherapy studies of HCC have been a hot topic. Therefore, we will introduce the application of mouse models of HCC in immunotherapy. This is followed by a discussion of some other models of HCC-related liver diseases, including non-alcoholic fatty liver disease (NAFLD), hepatitis B and C virus infection, and liver fibrosis and cirrhosis. Together these provide researchers with a current overview of the mouse models of HCC and assist in the application of appropriate models for their research.

Cooperation between liver-specific mutations of pten and tp53 genetically induces hepatocarcinogenesis in zebrafish

BACKGROUND

Liver cancer, mainly hepatocellular carcinoma, is one of the deadliest cancers worldwide and has a poor prognosis due to insufficient understanding of hepatocarcinogenesis. Previous studies have revealed that the mutations in PTEN and TP53 are the two most common genetic events in hepatocarcinogenesis. Here, we illustrated the crosstalk between aberrant Pten and Tp53 pathways during hepatocarcinogenesis in zebrafish.

METHODS

We used the CRISPR/Cas9 system to establish several transgenic zebrafish lines with single or double tissue-specific mutations of pten and tp53 to genetically induce liver tumorigenesis. Next, the morphological and histological determination were performed to investigate the roles of Pten and Tp53 signalling pathways in hepatocarcinogenesis in zebrafish.

RESULTS

We demonstrated that Pten loss alone induces hepatocarcinogenesis with only low efficiency, whereas single mutation of tp53 failed to induce tumour formation in liver tissue in zebrafish. Moreover, zebrafish with double mutations of pten and tp53 exhibits a much higher tumour incidence, higher-grade histology, and a shorter survival time than single-mutant zebrafish, indicating that these two signalling pathways play important roles in dynamic biological events critical for the initiation and progression of hepatocarcinogenesis in zebrafish. Further histological and pathological analyses showed significant similarity between the tumours generated from liver tissues of zebrafish and humans. Furthermore, the treatment with MK-2206, a specific Akt inhibitor, effectively suppressed hepatocarcinogenesis in zebrafish.

CONCLUSION

Our findings will offer a preclinical animal model for genetically investigating hepatocarcinogenesis and provide a useful platform for high-throughput anticancer drug screening.

p53-Independent Induction of p21 Fails to Control Regeneration and Hepatocarcinogenesis in a Murine Liver Injury Model

BACKGROUND & AIMS

A coordinated stress and regenerative response is important after hepatocyte damage. Here, we investigate the phenotypes that result from genetic abrogation of individual components of the checkpoint kinase 2/transformation-related protein 53 (p53)/cyclin-dependent kinase inhibitor 1A (p21) pathway in a murine model of metabolic liver injury.

METHODS

Nitisinone was reduced or withdrawn in Fah^-/- mice lacking Chk2, p53, or p21, and survival, tumor development, liver injury, and regeneration were analyzed. Partial hepatectomies were performed and mice were challenged with the Fas antibody Jo2.

RESULTS

In a model of metabolic liver injury, loss of p53, but not Chk2, impairs the oxidative stress response and aggravates liver damage, indicative of a direct p53-dependent protective effect on hepatocytes. Cell-cycle control during chronic liver injury critically depends on the presence of both p53 and its downstream effector p21. In p53-deficient hepatocytes, unchecked proliferation occurs despite a strong induction of p21, showing a complex interdependency between p21 and p53. The increased regenerative potential in the absence of p53 cannot fully compensate the surplus injury and is not sufficient to promote survival. Despite the distinct phenotypes associated with the loss of individual components of the DNA damage response, gene expression patterns are dominated by the severity of liver injury, but reflect distinct effects of p53 on proliferation and the anti-oxidative stress response.

CONCLUSIONS

Characteristic phenotypes result from the genetic abrogation of individual components of the DNA damage-response cascade in a liver injury model. The extent to which loss of gene function can be compensated, or affects injury and proliferation, is related to the level at which the cascade is interrupted. Accession numbers of repository for expression microarray data: GSE156983, GSE156263, GSE156852, and GSE156252.

Ccne1 Overexpression Causes Chromosome Instability in Liver Cells and Liver Tumor Development in Mice

BACKGROUND & AIMS

The CCNE1 locus, which encodes cyclin E1, is amplified in many types of cancer cells and is activated in hepatocellular carcinomas (HCCs) from patients infected with hepatitis B virus or adeno-associated virus type 2, due to integration of the virus nearby. We investigated cell-cycle and oncogenic effects of cyclin E1 overexpression in tissues of mice.

METHODS

We generated mice with doxycycline-inducible expression of Ccne1 (Ccne1^T mice) and activated overexpression of cyclin E1 from age 3 weeks onward. At 14 months of age, livers were collected from mice that overexpress cyclin E1 and nontransgenic mice (controls) and analyzed for tumor burden and by histology. Mouse embryonic fibroblasts (MEFs) and hepatocytes from Ccne1^T and control mice were analyzed to determine the extent to which cyclin E1 overexpression perturbs S-phase entry, DNA replication, and numbers and structures of chromosomes. Tissues from 4-month-old Ccne1^T and control mice (at that age were free of tumors) were analyzed for chromosome alterations, to investigate the mechanisms by which cyclin E1 predisposes hepatocytes to transformation.

RESULTS

Ccne1^T mice developed more hepatocellular adenomas and HCCs than control mice. Tumors developed only in livers of Ccne1^T mice, despite high levels of cyclin E1 in other tissues. Ccne1^T MEFs had defects that promoted chromosome missegregation and aneuploidy, including incomplete replication of DNA, centrosome amplification, and formation of nonperpendicular mitotic spindles. Whereas Ccne1^T mice accumulated near-diploid aneuploid cells in multiple tissues and organs, polyploidization was observed only in hepatocytes, with losses and gains of whole chromosomes, DNA damage, and oxidative stress.

CONCLUSIONS

Livers, but not other tissues of mice with inducible overexpression of cyclin E1, develop tumors. More hepatocytes from the cyclin E1-overexpressing mice were polyploid than from control mice, and had losses or gains of whole chromosomes, DNA damage, and oxidative stress; all of these have been observed in human HCC cells. The increased risk of HCC in patients with hepatitis B virus or adeno-associated virus type 2 infection might involve activation of cyclin E1 and its effects on chromosomes and genomes of liver cells.

Surgical resection and radiofrequency ablation initiate cancer in cytokeratin-19+- liver cells deficient for p53 and Rb

The long term prognosis of liver cancer patients remains unsatisfactory because of cancer recurrence after surgical interventions, particularly in patients with viral infections. Since hepatitis B and C viral proteins lead to inactivation of the tumor suppressors p53 and Retinoblastoma (Rb), we hypothesize that surgery in the context of p53/Rb inactivation initiate de novo tumorigenesis.

We, therefore, generated transgenic mice with hepatocyte and cholangiocyte/liver progenitor cell (LPC)-specific deletion of p53 and Rb, by interbreeding conditional p53/Rb knockout mice with either Albumin-cre or Cytokeratin-19-cre transgenic mice.

We show that liver cancer develops at the necrotic injury site after surgical resection or radiofrequency ablation in p53/Rb deficient livers. Cancer initiation occurs as a result of specific migration, expansion and transformation of cytokeratin-19⁺-liver (CK-19⁺) cells. At the injury site migrating CK-19⁺ cells formed small bile ducts and adjacent cells strongly expressed the transforming growth factor β (TGFβ). Isolated cytokeratin-19⁺ cells deficient for p53/Rb were resistant against hypoxia and TGFβ-mediated growth inhibition. CK-19⁺ specific deletion of p53/Rb verified that carcinomas at the injury site originates from cholangiocytes or liver progenitor cells.

These findings suggest that human liver patients with hepatitis B and C viral infection or with mutations for p53 and Rb are at high risk to develop tumors at the surgical intervention site.

The Dark Side of Cell Fusion

Cell fusion is a physiological cellular process essential for fertilization, viral entry, muscle differentiation and placental development, among others. In this review, we will highlight the different cancer cell-cell fusions and the advantages obtained by these fusions. We will specially focus on the acquisition of metastatic features by cancer cells after fusion with bone marrow-derived cells. The mechanism by which cancer cells fuse with other cells has been poorly studied thus far, but the presence in several cancer cells of syncytin, a trophoblastic fusogen, leads us to a cancer cell fusion mechanism similar to the one used by the trophoblasts. The mechanism by which cancer cells perform the cell fusion could be an interesting target for cancer therapy.

Rb and p53 Liver Functions Are Essential for Xenobiotic Metabolism and Tumor Suppression

The tumor suppressors Retinoblastoma (Rb) and p53 are frequently inactivated in liver diseases, such as hepatocellular carcinomas (HCC) or infections with Hepatitis B or C viruses. Here, we discovered a novel role for Rb and p53 in xenobiotic metabolism, which represent a key function of the liver for metabolizing therapeutic drugs or toxins. We demonstrate that Rb and p53 cooperate to metabolize the xenobiotic 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC). DDC is metabolized mainly by cytochrome P450 (Cyp)3a enzymes resulting in inhibition of heme synthesis and accumulation of protoporphyrin, an intermediate of heme pathway. Protoporphyrin accumulation causes bile injury and ductular reaction. We show that loss of Rb and p53 resulted in reduced Cyp3a expression decreased accumulation of protoporphyrin and consequently less ductular reaction in livers of mice fed with DDC for 3 weeks. These findings provide strong evidence that synergistic functions of Rb and p53 are essential for metabolism of DDC. Because Rb and p53 functions are frequently disabled in liver diseases, our results suggest that liver patients might have altered ability to remove toxins or properly metabolize therapeutic drugs. Strikingly the reduced biliary injury towards the oxidative stress inducer DCC was accompanied by enhanced hepatocellular injury and formation of HCCs in Rb and p53 deficient livers. The increase in hepatocellular injury might be related to reduce protoporphyrin accumulation, because protoporphrin is well known for its anti-oxidative activity. Furthermore our results indicate that Rb and p53 not only function as tumor suppressors in response to carcinogenic injury, but also in response to non-carcinogenic injury such as DDC.

RB loss contributes to aggressive tumor phenotypes in MYC-driven triple negative breast cancer

Triple negative breast cancer (TNBC) is characterized by multiple genetic events occurring in concert to drive pathogenic features of the disease. Here we interrogated the coordinate impact of p53, RB, and MYC in a genetic model of TNBC, in parallel with the analysis of clinical specimens. Primary mouse mammary epithelial cells (mMEC) with defined genetic features were used to delineate the combined action of RB and/or p53 in the genesis of TNBC. In this context, the deletion of either RB or p53 alone and in combination increased the proliferation of mMEC; however, the cells did not have the capacity to invade in matrigel. Gene expression profiling revealed that loss of each tumor suppressor has effects related to proliferation, but RB loss in particular leads to alterations in gene expression associated with the epithelial-to-mesenchymal transition. The overexpression of MYC in combination with p53 loss or combined RB/p53 loss drove rapid cell growth. While the effects of MYC overexpression had a dominant impact on gene expression, loss of RB further enhanced the deregulation of a gene expression signature associated with invasion. Specific RB loss lead to enhanced invasion in boyden chambers assays and gave rise to tumors with minimal epithelial characteristics relative to RB-proficient models. Therapeutic screening revealed that RB-deficient cells were particularly resistant to agents targeting PI3K and MEK pathway. Consistent with the aggressive behavior of the preclinical models of MYC overexpression and RB loss, human TNBC tumors that express high levels of MYC and are devoid of RB have a particularly poor outcome. Together these results underscore the potency of tumor suppressor pathways in specifying the biology of breast cancer. Further, they demonstrate that MYC overexpression in concert with RB can promote a particularly aggressive form of TNBC.

Inhibition of wild-type p53-induced phosphatase 1 promotes liver regeneration in mice by direct activation of mammalian target of rapamycin

The liver possesses extraordinary regenerative capacity in response to injury. However, liver regeneration (LR) is often impaired in disease conditions. Wild-type p53-induced phosphatase 1 (Wip1) is known as a tumor promoter and enhances cell proliferation, mainly by deactivating antioncogenes. However, in this work, we identified an unexpected role of Wip1 in LR. In contrast to its known role in promoting cell proliferation in extrahepatic tissue, we found that Wip1 suppressed hepatocyte proliferation after partial hepatectomy (PHx). Deletion of Wip1 increased the rate of LR after PHx. Enhanced LR in Wip1-deficient mice was a result of the activation of the mammalian target of rapamycin (mTOR) complex 1 (mTORC1) pathway. Furthermore, we showed that Wip1 physically interacted with and dephosphorylated mTOR. Interestingly, inhibition of Wip1 also activated the p53 pathway during LR. Disruption of the p53 pathway further enhanced LR in Wip1-deficient mice. Therefore, inhibition of Wip1 has a dual role in LR, i.e., promoting hepatocyte proliferation through activation of the mTORC1 pathway, meanwhile suppressing LR through activation of the p53 pathway. However, the proregenerative role of mTORC1 overwhelms the antiproliferative role of p53. Furthermore, CCT007093, a Wip1 inhibitor, enhanced LR and increased the survival rate of mice after major hepatectomy.

CONCLUSION

mTOR is a new direct target of Wip1. Wip1 inhibition can activate the mTORC1 pathway and enhance hepatocyte proliferation after hepatectomy. These findings have clinical applications in cases where LR is critical, including acute liver failure, cirrhosis, or small-for-size liver transplantations.

Mechanisms of hepatocarcinogenesis in chronic hepatitis C

Infection with hepatitis C virus (HCV) is a major risk factor for hepatocellular carcinoma. The genetic changes that drive cancer development are heterogeneous and how chronic hepatitis C promotes the initiation of hepatocellular carcinoma is incompletely understood. Cancer typically arises in the setting of advanced fibrosis and/or cirrhosis where chronic immune-mediated inflammation over decades promotes hepatocyte turnover providing selective pressure that favors the malignant phenotype. As well as contributions of unresolved inflammation to carcinogenesis, evidence from transgenic mice with liver-specific expression of viral sequences suggests that some HCV-encoded proteins may directly promote cancer. Numerous in vitro studies suggest roles for HCV proteins in subversion of cellular pathways that normally act to suppress tumorigenesis. Here, we review the mechanisms by which persistent HCV infection might promote cancer in addition to the procarcinogenic effects of inflammatory liver disease.

Organ size control is dominant over Rb family inactivation to restrict proliferation in vivo

In mammals, a cell's decision to divide is thought to be under the control of the Rb/E2F pathway. We previously found that inactivation of the Rb family of cell cycle inhibitors (Rb, p107, and p130) in quiescent liver progenitors leads to uncontrolled division and cancer initiation. Here, we show that, in contrast, deletion of the entire Rb gene family in mature hepatocytes is not sufficient for their long-term proliferation. The cell cycle block in Rb family mutant hepatocytes is independent of the Arf/p53/p21 checkpoint but can be abrogated upon decreasing liver size. At the molecular level, we identify YAP, a transcriptional regulator involved in organ size control, as a factor required for the sustained expression of cell cycle genes in hepatocytes. These experiments identify a higher level of regulation of the cell cycle in vivo in which signals regulating organ size are dominant regulators of the core cell cycle machinery.

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.

The changing landscape of hepatocellular carcinoma: etiology, genetics, and therapy

Hepatocellular carcinoma (HCC) represents one of the leading causes of cancer death and has proved to be highly refractory to treatment. Extensive analysis of the disease has demonstrated that it arises predominantly in response to high-risk etiological challenges, most notably hepatitis virus. However, with evolving vaccination and the obesity epidemic, progressively more cases are associated with underlying metabolic dysfunction. Pathologically diverse forms of HCC are observed, and recent sequencing analysis has defined common events that target well-known cancer pathways including β-catenin/Axin, TP53, and RB/CDKN2A, as well as frequent aberrations in chromatin remodeling factors. However, there are a myriad of low frequency genetic events that make each HCC case unique. Gene expression profiling approaches have successfully been deployed for prognostic assessment of hepatocellular carcinoma and to detect the earliest stages of disease. Despite more extensive research, systemic treatment for HCC is exceedingly limited, with only a handful of drugs providing benefit. Ongoing clinical trials are attempting to exploit specific biological dependencies of HCC to improve the dismal prognosis. Overall, the future of HCC treatment will rely on an understanding of the interplay between etiological factors, molecular features of disease, and rational therapeutic intervention.

The Complex Relationship between Liver Cancer and the Cell Cycle: A Story of Multiple Regulations

The liver acts as a hub for metabolic reactions to keep a homeostatic balance during development and growth. The process of liver cancer development, although poorly understood, is related to different etiologic factors like toxins, alcohol, or viral infection. At the molecular level, liver cancer is characterized by a disruption of cell cycle regulation through many molecular mechanisms. In this review, we focus on the mechanisms underlying the lack of regulation of the cell cycle during liver cancer, focusing mainly on hepatocellular carcinoma (HCC). We also provide a brief summary of novel therapies connected to cell cycle regulation.

Multiple interactive factors in hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is the fifth most prevalent cancer and the third most frequent cause of cancer mortality globally. Each year there are approximately 630,000 new cases of HCC in the world and more than half of the new cases occur in China. Major risk factors of HCC include HBV or HCV infection, alcoholic liver disease, and nonalcoholic fatty liver disease. Most of these risk factors lead to chronic hepatitis and cirrhosis, which is present in 80-90% of HCC patients. Hepatocarcinogenesis has been regarded as a multi-stage process involving multiple genetic or environmental factors. Interaction and cross-regulation of distinct factors synergistically contributes to HCC occurrence. A comprehensive knowledge on the multiple factors and their interaction in hepatocarcinogenesis is necessary to improve the effectiveness of HCC intervention. In this review, we will focus on the recent progress made in understanding the mechanisms of hepatocarcinogenesis and discuss some potential issues or challenges in this area.

APE1/Ref-1 enhances DNA binding activity of mutant p53 in a redox-dependent manner

Apurinic/apyrimidinic endonuclease 1/redox factor-1 (APE1/Ref-1) is a dual function protein; in addition to its DNA repair activity, it can stimulate DNA binding activity of numerous transcription factors as a reduction-oxidation (redox) factor. APE1/Ref-1 has been found to be a potent activator of wild-type p53 (wtp53) DNA binding in vitro and in vivo. Although p53 is mutated in most types of human cancer including hepatocellular carcinoma (HCC), little is known about whether APE1/Ref-1 can regulate mutant p53 (mutp53). Herein, we reported the increased APE1/Ref-1 protein and accumulation of mutp53 in HCC by immunohistochemistry. Of note, it was observed that APE1/Ref-1 high-expression and mutp53 expression were associated with carcinogenesis and progression of HCC. To determine whether APE1/Ref-1 regulates DNA binding of mutp53, we performed electromobility shift assays (EMSAs) and quantitative chromatin immunoprecipitation (ChIP) assays in HCC cell lines. In contrast to sequence-specific and DNA structure-dependent binding of wtp53, reduced mutp53 efficiently bound to nonlinear DNA, but not to linear DNA. Notably, overexpression of APE1/Ref-1 resulted in increased DNA binding activity of mutp53, while downregulation of APE1/Ref-1 caused a marked decrease of mutp53 DNA binding. In addition, APE1/Ref-1 could not potentiate the accumulation of p21 mRNA and protein in mutp53 cells. These data indicate that APE1/Ref-1 can stimulate mutp53 DNA binding in a redox-dependent manner.

Skp2 deletion unmasks a p27 safeguard that blocks tumorigenesis in the absence of pRb and p53 tumor suppressors

pRb and p53 are two major tumor suppressors. Here, we found that p53 activates expression of Pirh2 and KPC1, two of the three ubiquitin ligases for p27. Loss of p53 in the absence of Skp2, the third ubiquitin ligase for p27, shrinks the cellular pool of p27 ubiquitin ligases to accumulate p27 protein. In the absence of pRb and p53, p27 was unable to inhibit DNA synthesis in spite of its abundance, but could inhibit division of cells that maintain DNA replication with rereplication. This mechanism blocked pRb/p53 doubly deficient pituitary and prostate tumorigenesis lastingly coexistent with bromodeoxyuridine-labeling neoplastic lesions, revealing an unconventional cancer cell vulnerability when pRb and p53 are inactivated.

Polyploidization in liver tissue

Polyploidy (alias whole genome amplification) refers to organisms containing more than two basic sets of chromosomes. Polyploidy was first observed in plants more than a century ago, and it is known that such processes occur in many eukaryotes under a variety of circumstances. In mammals, the development of polyploid cells can contribute to tissue differentiation and, therefore, possibly a gain of function; alternately, it can be associated with development of disease, such as cancer. Polyploidy can occur because of cell fusion or abnormal cell division (endoreplication, mitotic slippage, or cytokinesis failure). Polyploidy is a common characteristic of the mammalian liver. Polyploidization occurs mainly during liver development, but also in adults with increasing age or because of cellular stress (eg, surgical resection, toxic exposure, or viral infections). This review will explore the mechanisms that lead to the development of polyploid cells, our current state of understanding of how polyploidization is regulated during liver growth, and its consequence on liver function.

The retinoblastoma tumor suppressor pathway modulates the invasiveness of ErbB2-positive breast cancer

The processes that control the progression of ductal carcinoma in situ (DCIS) to invasive breast cancer remain poorly understood. Epidermal growth factor receptor 2 (ErbB2) over expression is common in DCIS, as is disruption of the retinoblastoma tumor suppressor (RB) pathway. Here we examined the cooperative impact of ErbB2 and RB deregulation on facets of disease progression. Our studies demonstrate that RB deficiency altered the expression of key molecules needed for proper cellular organization and epithelial cell-cell adhesion as part of a program related to the epithelial to mesenchymal transition (EMT). An increase in the invasive potential of ErbB2 over expressing cells was observed upon RB depletion. Furthermore, stable knockdown of RB resulted in invasive lesions in orthotopic xenograft assays, compared to DCIS-like lesions developing from RB-proficient cells. Conversely, the invasive phenotype observed in ErbB2-positive cancer models was inhibited through CDK4/6 inhibition in an RB-dependent manner. Lastly, in a cohort of DCIS cases, we show that while elevated levels of ErbB2 are associated with increased risk of a subsequent DCIS recurrence, it is not associated with progression to invasive disease. In contrast, RB loss in ErbB2 positive DCIS cases was associated with increased risk for invasive breast cancer. Taken together, these data demonstrate a key role for the RB-pathway in invasion associated with breast tumor progression, and shed light on the key molecular events that promote the progression of DCIS to invasive disease.

Molecular mechanisms underlying the development of hepatocellular carcinoma and molecular targeted therapy

Hepatocellular carcinoma is one of the most common malignant tumors worldwide and remains one of leading causes of death from cancer in China. Hepatocarcinogenesis is a complex process associated with many environmental risk factors, including cellular and molecular signaling pathways, oncogenes and tumor suppressor genes, and the differentiation of cancer stem cells. Molecular targeted therapy is a new approach to the treatment of liver cancer. The main mechanism of therapy is a type of medication that blocks the growth of cancer cells by interfering with specific targeted molecules needed for carcinogenesis and tumor growth, which can enhance the specificity and selectivity of the treatment. In this review, we discuss recent advances in the understanding of molecular mechanisms underlying the development of HCC and in the development of novel cancer therapeutics.

Endoreplication and polyploidy: insights into development and disease

Polyploid cells have genomes that contain multiples of the typical diploid chromosome number and are found in many different organisms. Studies in a variety of animal and plant developmental systems have revealed evolutionarily conserved mechanisms that control the generation of polyploidy and have recently begun to provide clues to its physiological function. These studies demonstrate that cellular polyploidy plays important roles during normal development and also contributes to human disease, particularly cancer.

Fluoro-Sorafenib (Regorafenib) effects on hepatoma cells: growth inhibition, quiescence, and recovery

To evaluate the growth-inhibitory properties of the potent multi-kinase antagonist Regorafenib (Fluoro-Sorafenib), which was synthesized as a more potent Sorafenib, a Raf inhibitor and to determine whether similar mechanisms were involved, human hepatoma cell lines were grown in the presence or absence of Regorafanib and examined for growth inhibition. Western blots were performed for Raf targets, apoptosis, and autophagy. Regorafenib inhibited growth of human Hep3B, PLC/PRF/5, and HepG2 cells in a concentration- and time-dependent manner. Multiple signaling pathways were altered, including MAP kinases phospho-ERK and phospho-JNK and its target phospho-c-Jun. There was evidence for apoptosis by FACS, cleavage of caspases and increased Bax levels; as well as induction of autophagy, as judged by increased Beclin-1 and LC3 (II) levels. Prolonged drug exposure resulted in cell quiescence. Full growth recovery occurred after drug removal, unlike with doxorubicin chemotherapy. Regorafenib is a potent inhibitor of cell growth. Cells surviving Regorafenib treatment remain viable, but quiescent and capable of regrowth following drug removal. The reversibility of tumor cell growth suppression after drug removal may have clinical implications.

Comparative analysis of SV40 17kT and LT function in vivo demonstrates that LT's C-terminus re-programs hepatic gene expression and is necessary for tumorigenesis in the liver

Transformation by Simian Virus 40 (SV40) large T antigen (LT) is mediated in large part by its interaction with a variety of cellular proteins at distinct binding domains within LT. While the interaction of LT's N-terminus with the tumor suppressor Rb is absolutely required for LT-dependent transformation, the requirement for the interaction of LT's C-terminus with p53 is less clear and cell- and context-dependent. Here, we report a line of transgenic mice expressing a doxycycline-inducible liver-specific viral transcript that produces abundant 17kT, a naturally occurring SV40 early product that is co-linear with LT for the first 131 amino acids and that binds to Rb, but not p53. Comparative analysis of livers of transgenic mice expressing either 17kT or full length LT demonstrates that 17kT stimulates cell proliferation and induces hepatic hyperplasia but is incapable of inducing hepatic dysplasia or promoting hepatocarcinogenesis. Gene expression profiling demonstrates that 17kT and LT invoke a set of shared molecular signatures consistent with the action of LT's N-terminus on Rb-E2F-mediated control of hepatocyte transcription. However, 17kT also induces a unique set of genes, many of which are known transcriptional targets of p53, while LT actively suppresses them. LT also uniquely deregulates the expression of a subset of genes within the imprinted network and rapidly re-programs hepatocyte gene expression to a more fetal-like state. Finally, we provide evidence that the LT/p53 complex provides a gain-of-function for LT-dependent transformation in the liver, and confirm the absolute requirement for LT's C-terminus for liver tumor development by demonstrating that phosphatase and tensin homolog (PTEN)-deficiency readily cooperates with LT, but not 17kT, for tumorigenesis. These results confirm independent and inter-dependent functions for LT's N- and C-terminus and emphasize differences in the requirements for LT's C-terminus in cell-type dependent transformation.

Modification of the DNA damage response by therapeutic CDK4/6 inhibition

The RB/E2F axis represents a critical node of cell signaling that integrates a diverse array of signaling pathways. Recent evidence has suggested a role for E2F-mediated gene transcription in DNA damage response and repair, as well as apoptosis signaling. Herein, we investigated how repression of E2F activity via CDK4/6 inhibition and RB activation impacts the response of triple negative breast cancer (TNBC) to frequently used therapeutic agents. In combination with taxanes and anthracyclines CDK4/6 inhibition and consequent cell cycle arrest prevented the induction of DNA damage and associated cell death in an RB-dependent manner; thereby demonstrating antagonism between the cytostatic influence of the CDK-inhibitor and cytotoxic agents. As many of these effects were secondary to cell cycle arrest, γ-irradiation (IR) was utilized to examine effects of CDK4/6 inhibition on direct DNA damage. Although E2F controls a number of genes involved in DNA repair (e.g. Rad51), CDK4/6 inhibition did not alter the overall rate of DNA repair, rather it significantly shifted the burden of this repair from homologous recombination (HR) to non-homologous end joining (NHEJ). Together, these data indicate that CDK4/6 inhibition can antagonize cytotoxic therapeutic strategies and increases utilization of error-prone DNA repair mechanisms that could contribute to disease progression.

Disruption of Trp53 in livers of mice induces formation of carcinomas with bilineal differentiation

BACKGROUND & AIMS

p53 limits the self-renewal of stem cells from various tissues. Loss of p53, in combination with other oncogenic events, results in aberrant self-renewal and transformation of progenitor cells. It is not known whether loss of p53 is sufficient to induce tumor formation in liver.

METHODS

We used AlfpCre mice to create mice with liver-specific disruption of Trp53 (AlfpCre(+)Trp53(Δ2-10/Δ2-10) mice). We analyzed colony formation and genomic features and gene expression patterns in liver cells during hepatocarcinogenesis in mice with homozygous, heterozygous, and no disruption of Trp53.

RESULTS

Liver-specific disruption of Trp53 consistently induced formation of liver carcinomas that had bilineal differentiation. In nontransformed liver cells and cultured primary liver cells, loss of p53 (but not p21) resulted in chromosomal imbalances and increased clonogenic capacity of liver progenitor cells (LPCs) and hepatocytes. Primary cultures of hepatocytes and LPCs from AlfpCre(+)Trp53(Δ2-10/Δ2-10) mice, but not Cdkn1a(-/-) mice, formed tumors with bilineal differentiation when transplanted into immunocompromised mice. Spontaneous liver tumors that developed in AlfpCre(+)Trp53(Δ2-10/Δ2-10) mice had significant but complex alterations in expression of Rb checkpoint genes compared with chemically induced liver tumors that developed mice with wild-type Trp53.

CONCLUSIONS

Deletion of p53 from livers of mice is sufficient to induce tumor formation. The tumors have bilineal differentiation and dysregulation of Rb checkpoint genes.