Control of mouse hepatocyte proliferation and ploidy by p53 and p53ser246 mutation in vivo

A Boolean model explains phenotypic plasticity changes underlying hepatic cancer stem cells emergence

In several carcinomas, including hepatocellular carcinoma, it has been demonstrated that cancer stem cells (CSCs) have enhanced invasiveness and therapy resistance compared to differentiated cancer cells. Mathematical-computational tools could be valuable for integrating experimental results and understanding the phenotypic plasticity mechanisms for CSCs emergence. Based on the literature review, we constructed a Boolean model that recovers eight stable states (attractors) corresponding to the gene expression profile of hepatocytes and mesenchymal cells in senescent, quiescent, proliferative, and stem-like states. The epigenetic landscape associated with the regulatory network was analyzed. We observed that the loss of p53, p16, RB, or the constitutive activation of β-catenin and YAP1 increases the robustness of the proliferative stem-like phenotypes. Additionally, we found that p53 inactivation facilitates the transition of proliferative hepatocytes into stem-like mesenchymal phenotype. Thus, phenotypic plasticity may be altered, and stem-like phenotypes related to CSCs may be easier to attain following the mutation acquisition.

p53/MDM2 signaling pathway in aging, senescence and tumorigenesis

A wealth of evidence has emerged that there is an association between aging, senescence and tumorigenesis. Senescence, a biological process by which cells cease to divide and enter a status of permanent cell cycle arrest, contributes to aging and aging-related diseases, including cancer. Aging populations have the higher incidence of cancer due to a lifetime of exposure to cancer-causing agents, reduction of repairing DNA damage, accumulated genetic mutations, and decreased immune system efficiency. Cancer patients undergoing cytotoxic therapies, such as chemotherapy and radiotherapy, accelerate aging. There is growing evidence that p53/MDM2 (murine double minute 2) axis is critically involved in regulation of aging, senescence and oncogenesis. Therefore, in this review, we describe the functions and mechanisms of p53/MDM2-mediated senescence, aging and carcinogenesis. Moreover, we highlight the small molecular inhibitors, natural compounds and PROTACs (proteolysis targeting chimeras) that target p53/MDM2 pathway to influence aging and cancer. Modification of p53/MDM2 could be a potential strategy for treatment of aging, senescence and tumorigenesis.

Mutant p53 Gains Its Function via c-Myc Activation upon CDK4 Phosphorylation at Serine 249 and Consequent PIN1 Binding

TP53 missense mutations significantly influence the development and progression of various human cancers via their gain of new functions (GOF) through different mechanisms. Here we report a unique mechanism underlying the GOF of p53-R249S (p53-RS), a p53 mutant frequently detected in human hepatocellular carcinoma (HCC) that is highly related to hepatitis B infection and aflatoxin B1. A CDK inhibitor blocks p53-RS's nuclear translocation in HCC, whereas CDK4 interacts with p53-RS in the G1/S phase of the cells, phosphorylates it, and enhances its nuclear localization. This is coupled with binding of a peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (PIN1) to p53-RS, but not the p53 form with mutations of four serines/threonines previously shown to be crucial for PIN1 binding. As a result, p53-RS interacts with c-Myc and enhances c-Myc-dependent rDNA transcription key for ribosomal biogenesis. These results unveil a CDK4-PIN1-p53-RS-c-Myc pathway as a novel mechanism for the GOF of p53-RS in HCC.

Deletion of the MAD2L1 spindle assembly checkpoint gene is tolerated in mouse models of acute T-cell lymphoma and hepatocellular carcinoma

Chromosome instability (CIN) is deleterious to normal cells because of the burden of aneuploidy. However, most human solid tumors have an abnormal karyotype implying that gain and loss of chromosomes by cancer cells confers a selective advantage. CIN can be induced in the mouse by inactivating the spindle assembly checkpoint. This is lethal in the germline but we show here that adult T cells and hepatocytes can survive conditional inactivation of the Mad2l1 SAC gene and resulting CIN. This causes rapid onset of acute lymphoblastic leukemia (T-ALL) and progressive development of hepatocellular carcinoma (HCC), both lethal diseases. The resulting DNA copy number variation and patterns of chromosome loss and gain are tumor-type specific, suggesting differential selective pressures on the two tumor cell types.

Polyploidization of liver cells

Eukaryotic organisms usually contain a diploid complement of chromosomes. However, there are a number of exceptions. Organisms containing an increase in DNA content by whole number multiples of the entire set of chromosomes are defined as polyploid. Cells that contain more than two sets of chromosomes were first observed in plants about a century ago and it is now recognized that polyploidy cells form in many eukaryotes under a wide variety of circumstance. Although it is less common in mammals, some tissues, including the liver, show a high percentage of polyploid cells. Thus, during postnatal growth, the liver parenchyma undergoes dramatic changes characterized by gradual polyploidization during which hepatocytes of several ploidy classes emerge as a result of modified cell-division cycles. This process generates the successive appearance of tetraploid and octoploid cell classes with one or two nuclei (mononucleated or binucleated). Liver cells polyploidy is generally considered to indicate terminal differentiation and senescence and to lead both to the progressive loss of cell pluripotency and a markedly decreased replication capacity. In adults, liver polyploidization is differentially regulated upon loss of liver mass and liver damage. Interestingly, partial hepatectomy induces marked cell proliferation followed by an increase in liver ploidy. In contrast, during hepatocarcinoma (HCC), growth shifts to a nonpolyploidizing pattern and expansion of the diploid hepatocytes population is observed in neoplastic nodules. Here we review the current state of understanding about how polyploidization is regulated during normal and pathological liver growth and detail by which mechanisms hepatocytes become polyploid.

Rhythmic CLOCK-BMAL1 binding to multiple E-box motifs drives circadian Dbp transcription and chromatin transitions

Mammalian circadian rhythms are based on transcriptional and post-translational feedback loops. Essentially, the activity of the transcription factors BMAL1 (also known as MOP3) and CLOCK is rhythmically counterbalanced by Period (PER) and Cryptochrome (CRY) proteins to govern time of day-dependent gene expression. Here we show that circadian regulation of the mouse albumin D element-binding protein (Dbp) gene involves rhythmic binding of BMAL1 and CLOCK and marked daily chromatin transitions. Thus, the Dbp transcription cycle is paralleled by binding of BMAL1 and CLOCK to multiple extra- and intragenic E boxes, acetylation of Lys9 of histone H3, trimethylation of Lys4 of histone H3 and a reduction of histone density. In contrast, the antiphasic daily repression cycle is accompanied by dimethylation of Lys9 of histone H3, the binding of heterochromatin protein 1alpha and an increase in histone density. The rhythmic conversion of transcriptionally permissive chromatin to facultative heterochromatin relies on the presence of functional BMAL1-CLOCK binding sites.

Probing p53 biological functions through the use of genetically engineered mouse models

The p53 tumor suppressor gene is rendered dysfunctional in the majority of human cancers. To model the effects of p53 dysfunction in an experimentally manipulable organismal context, genetically engineered inbred mice have been the models of choice. Transgenic and knock-out technologies have been utilized to generate an array of different p53 germ line alterations. As expected, many (though not all) of the mutant p53 mouse models are susceptible to enhanced spontaneous and carcinogen-induced tumors of a variety of types. A number of different variables affect the incidence and spectrum of tumors in p53 mutant mice. These include strain background, the nature of the p53 mutation, the presence of wild-type p53 (in addition to mutant p53), exposure to physical and chemical mutagens, or introduction of other cancer-associated genes into the mutant p53 background. In addition to their role in furthering our understanding of the mechanisms of cancer initiation and progression, these models have led to unexpected insights into p53 function in embryogenesis and aging. With the development of ever more sophisticated methods for manipulating the mouse genome, new p53 models are on the horizon, which should deliver advances that will provide not only important mechanistic insights but also discoveries of great clinical relevance.

Inhibition of urethane-induced genotoxicity and cell proliferation in CYP2E1-null mice

Urethane is a multi-site animal carcinogen and was classified as "reasonably anticipated to be a human carcinogen." Urethane is a fermentation by-product and found at appreciable levels in alcoholic beverages and foods such as bread and cheese. Recent work in this laboratory demonstrated for the first time that CYP2E1 is the principal enzyme responsible for urethane metabolism. The current studies were undertaken to assess the relationships between CYP2E1-mediated metabolism and urethane-induced genotoxicity and cell proliferation as determined by induction of micronucleated erythrocytes (MN) and expression of Ki-67, respectively, using CYP2E1-null and wild-type mice. Urethane was administered at 0 (vehicle), 1, 10, or 100mg/kg/day (p.o.), 5 days/week for 6 weeks. A significant dose-dependent increase in MN was observed in wild-type mice; however, a slight increase was measured in the MN-polychromatic erythrocytes in CYP2E1-null mice treated with 100mg/kg. A significant increase in the expression of Ki-67 was detected in the livers and the lungs (terminal bronchioles, alveoli, and bronchi) of wild-type mice administered 100mg urethane/kg in comparison to controls. In contrast, CYP2E1-null mice administered this dose exhibited negligible alterations in Ki-67 expression in the livers and lungs compared to controls. Interestingly, while Ki-67 expression in the forestomach decreased in wild-type mice, it increased in CYP2E1-null mice. Subsequent comparative metabolism studies demonstrated that total urethane-derived radioactivity in the plasma, liver, and lung was significantly higher in CYP2E1-null versus wild-type mice and un-metabolized urethane constituted greater than 83% of the radioactivity in CYP2E1-null mice. Un-metabolized urethane was not detectable in the plasma, liver, and lung of wild-type mice. In conclusion, these data demonstrated that CYP2E1-mediated metabolism of urethane, presumably via epoxide formation, is necessary for the induction of genotoxicity, and cell proliferation in the liver and lung of wild-type mice.

Exogenous cadherin microdisplay can interfere with endogenous signaling and reprogram gene expression in cultured hepatocytes

We recently found that the basal micro substrate presentation of E-cadherin, a key cell-cell adhesion molecule in the liver, can modulate hepatocellular proliferative potential and differentiated function (Brieva and Moghe, in press). In the current study, we established a similar experimental model involving rat hepatocytes cultured on collagen and incorporated 5 microm polystyrene microbeads functionalized with Protein A-anchored E-cadherin/human lgG Fc chimeric fusion constructs. We investigated the cadherin governed dose-response of cell proliferative potential and quantified the underlying changes in intracellular gene signaling processes. Hepatocellular proliferative potential was found to be intensified with an increase in the microdisplay of acellular cadherins and this effect was offset by increased cell seeding density. Notably, we report that following overnight exposure to acellular cadherins, the expression of genes known to mediate the control of cell proliferation, cyclin D1 and c-myc, was upregulated, while the expression of differentiation-related genes, namely albumin and cytochrome p450 II B1, was reduced. The exposure of cell cultures to exogenous cadherins was found to markedly disrupt the localization of endogenous E-cadherin and beta-catenin to junctions at cell-cell contacts and cause a quantitative decrease in the endogenous cadherin protein levels. Based on all of our observations, we propose that the acellular presentation of E-cadherin chimeras competitively disrupts endogenous cadherin containing complexes at cell-cell junctions and increases intracellular cadherin turnover, thereby promoting beta-catenin mediated signaling, which ultimately engenders an increase in cell proliferative potential and a decrease in differentiated function.

Liver cell polyploidization: a pivotal role for binuclear hepatocytes

Polyploidy is a general physiological process indicative of terminal differentiation. During liver growth, this process generates the appearance of tetraploid (4n) and octoploid (8n) hepatocytes with one or two nuclei. The onset of polyploidy in the liver has been recognized for quite some time; however, the cellular mechanisms that govern it remain unknown. In this report, we observed the sequential appearance during liver growth of binuclear diploid (2 x 2n) and mononuclear 4n hepatocytes from a diploid hepatocyte population. To identify the cell cycle modifications involved in hepatocyte polyploidization, mitosis was then monitored in primary cultures of rat hepatocytes. Twenty percent of mononuclear 2n hepatocytes failed to undergo cytokinesis with no observable contractile movement of the ring. This process led to the formation of binuclear 2 x 2n hepatocytes. This tetraploid condition following cleavage failure did not activate the p53-dependent checkpoint in G1. In fact, binuclear hepatocytes were able to proceed through S phase, and the formation of a bipolar spindle during mitosis constituted the key step leading to the genesis of two mononuclear 4n hepatocytes. Finally, we studied the duplication and clustering of centrosomes in the binuclear hepatocyte. These cells exhibited two centrosomes in G1 that were duplicated during S phase and then clustered by pairs at opposite poles of the cell during metaphase. This event led only to mononuclear 4n progeny and maintained the tetraploidy status of hepatocytes.

E2F1 blocks and c-Myc accelerates hepatic ploidy in transgenic mouse models

Previously, we have shown that over-expression of either E2F1 or c-Myc promotes hepatocarcinogenesis and that E2F1 mice acquire HCC more rapidly than c-Myc transgenic mice. We also found that co-expression of E2F1/c-Myc further accelerates liver cancer development. Here we describe that the deregulated expression of these two transcription factors also affects hepatic ploidy during post-natal liver growth and before the onset of tumors. Oncogenic activity of E2F1 and/or c-Myc was associated with a persistent increase in hepatocyte proliferation. However, E2F1-mediated cell proliferation favored the predominance of diploid cells characteristic of pre-neoplastic type of liver growth whereas c-Myc functioned to accelerate age-related hepatocyte polyploidization. Similarly, proliferative advantage conferred by co-expression of E2F1 and c-Myc increased the frequency of diploid cells at a young age. Thus, the opposing effects of E2F1 and c-Myc on hepatocyte ploidy suggest that these two transcription factors have different mechanisms by which they control liver proliferation/maturation and ultimately, carcinogenesis.

Reduced expression of p27kip1 and increased hepatocyte proliferation in p53-deficient mice

Livers from wild-type and p53-deficient mice were analyzed for the expression of cell-cycle regulatory proteins in an attempt to determine the mechanism for the increased proliferation of liver cells in p53-deficient mice associated with enhanced susceptibility to aflatoxin-induced liver cancer. The most striking difference found was a significant reduction of the cyclin-dependent kinase inhibitor p27(kip1) in the livers of 3-mo-old p53-/- mice, whereas only small changes were found in the expression of cyclins, cyclin-dependent kinases, and the inhibitors p21(cip1) and p16(ink4a). Relative to wild-type liver, the amounts of p27(kip1) mRNA were reduced at both 1 and 3 mo, whereas the levels of p27(kip1) protein were decreased only at 3 mo. These results identify an uncharacterized link between the expression of p53 and p27(kip1) that may involve both transcriptional and post-transcriptional regulation and allow hepatocytes to continue to proliferate after 3 wk of age. We postulate that this increased proliferation leads to increased susceptibility to aflatoxin-induced hepatocarcinogenesis.

Hepatocellular carcinoma in a hepatitis B 'x' transgenic mouse model: A sequential pathological evaluation

BACKGROUND

The introduction of transgenic technology has made it possible to study the steps of carcinogenesis and directly establish the link between viral subgenomic fragments and specific types of cancer. Research directed at hepatitis B virus (HBV)-related carcinogenesis has benefited from this technology. We present a detailed pathological evaluation of the sequential steps of hepatocarcinogenesis in a hepatitis B 'x' (HBx) transgenic mouse model. In this model, the transgene incorporates the region encoding amino acids 58-154 of the HBV X protein and the murine c-myc gene. This model demonstrated changes in the liver from birth with foci of multicentric dysplasia evolving into nodules and overt hepatocellular carcinoma between 20 and 28 weeks.

METHODS AND RESULTS

The hepatocytes were mitotically active and showed increased proliferative capacity soon after birth, with exponential increase thereafter. This was accompanied by a high rate of apoptosis, which later declined as the tumors developed. Other functional and immunophenotypic characteristics included a high c-myc expression in the neoplastic lesions, no alteration in p53 expression, and no alteration in the expression of hepatic enzymes except for diffuse expression of succinic dehydrogenase.

CONCLUSION

The entire process illustrates the disturbances of cell growth and death because of the collaborative influence of HBx and c-myc genes that result in the development of hepatocellular carcinoma after a prolonged latent period.

Proliferating cell nuclear antigen assessed by a computer-assisted image analysis system in patients with chronic viral hepatitis and cirrhosis

BACKGROUND

Assessment of liver cell proliferation by immunodetection of proliferating cell nuclear antigen may predict regenerative potential and survival of liver and hepatocellular carcinoma risk in patients with chronic viral hepatitis.

AIM

To evaluate proliferating cell nuclear antigen status and its clinical significance in a large cohort of patients with chronic viral hepatitis and different degree of liver damage by a computer assisted imaging analysis system.

MATERIALS

Liver biopsies from 358 patients with chronic hepatitis (259 males, 49 years, 63% with hepatitis C infection, 27% with hepatitis B virus, 10% with multiple infections) were studied.

METHODS

Proliferating cell nuclear antigen was localised by immunoperoxidase on microwave oven pre-treated formalin-fixed, paraffin embedded sections using PC10 monoclonal antibody. Proliferating cell nuclear antigen labelling index was calculated by an automated imaging system (Immagini e Computers, Milan, Italy).

RESULTS

Mean proliferating cell nuclear antigen labelling index ranged from 0.1% for patients with minimal changes to 3.6% for those with cirrhosis and hepatocellular carcinoma. Overall, proliferating cell nuclear antigen labelling index was higher in males, in older patients, in multiple infections and in hepatitis C virus compared to hepatitis B virus related cases. By linear regression analysis, proliferating cell nuclear antigen labelling index correlated with older age, male gender; higher transaminase levels, hepatitis C virus, higher histological gradIng and staging: by multivariate analysis male gender, hepatitis C virus, higher grading and staging resulted as independent variables. Both hepatitis C virus or hepatitis B virus cirrhotics had similar liver cell proliferation rate but those with hepatitis B virus had higher prevalence of liver cell dysplasia with respect to those with hepatitis C virus.

CONCLUSIONS

Proliferating cell nuclear antigen labelling index was a reliable assay for assessing liver cell proliferation rate in patients with chronic viral hepatitis and correlated with liver disease severity

Phenobarbitone-induced ploidy changes in liver occur independently of p53

Liver polyploidisation, characterised by accumulation of tetraploid and octaploid cells, is found with increasing age and after administration of various drugs. The significance and mechanisms controlling polyploidisation are not understood but p53 is a candidate gene to be involved. We have investigated the effect of p53 on sodium phenobarbitone (PB)-induced liver proliferation and polyploidisation. Using p53 wild type (+/+), heterozygous (+/-) and homozygous (-/-) C57BL/6J mice, we measured ploidy and proliferation (BrdU incorporation) after 21 days oral administration of PB. Administration of PB caused a striking ploidy change compared with untreated controls, with an increase in 8n cells, and no difference noted comparing the p53 genotypes. BrdU positivity also increased significantly compared with controls, with the increase in BrdU positivity occurring in 8n cells. Our results confirm that PB is a hepatic mitogen that causes liver polyploidisation with a striking increase in 8n cells within the liver. p53 status does not appear to have any effect on this PB-induced ploidy change.

The nongenotoxic hepatocarcinogens diethylhexylphthalate and methylclofenapate induce DNA synthesis preferentially in octoploid rat hepatocytes

Diethylhexylphthalate (DEHP), a rodent carcinogen, and 1,4-dichlorobenzene (DCB), a noncarcinogen in rat liver, are potent hepatomitogens. We have reported previously that 7-day dosing with DEHP induced a higher bromodeoxyuridine labeling index (LI) in binuclear octoploid (2x4N) rat hepatocytes than did DCB, suggesting that induction of DNA synthesis in 2x4N hepatocytes might represent a more substantial carcinogenic risk. We compared 2 additional rodent hepatocarcinogens, methylclofenapate (MCP) and phenobarbitone, with ethylene thiourea (ETU), a noncarcinogenic hepatomitogen in rat. All 3 chemicals increased hepatic LI; the 8N population had the highest LI, but only the carcinogens increased LI in the 2x4N and 4N populations. To identify the target population for induction of DNA synthesis, we used a 1-hour pulse label at the peak of induction. The results were consistent with the 7-day data, and again the highest LI was in the 8N population. The nongenotoxic rodent carcinogens MCP and DEHP induced a significant increase in the LI in the 2x4N population, whereas ETU and DCB did not. These data support the hypothesis that increased DNA synthesis within the minority 2x4N population may be more significant for subsequent hepatocarcinogenesis.

Hepatic polyploidy and liver growth control

The onset of cellular polyploidy is recognized in all differentiated mammalian tissues. Polyploidy has been noted frequently in the normal liver, as well as in pathophysiological states of the liver. As insights into the significance of polyploidy accumulate gradually, it is becoming clear that cells belonging to high ploidy classes exhibit advancement toward terminal differentiation and cellular senescence with greater probabilities of apoptosis. Involvement of specific genetic abnormalities, such as impaired DNA repair, may lead to hepatocellular polyploidy. Working models indicate that extensive polyploidy could lead to organ failure, as well as to oncogenesis with activation of precancerous cell clones.

p53 represses CAAT enhancer-binding protein (C/EBP)-dependent transcription of the albumin gene. A molecular mechanism involved in viral liver infection with implications for hepatocarcinogenesis

p53 is a transcription factor that is activated by genotoxic stress and mediates cell cycle arrest and apoptosis. Here we demonstrate that infection of mouse liver with recombinant E1/E3-deleted adenovirus leads to p53 activation and simultaneously to the down-regulation of albumin gene expression. In vitro transcription assays indicate that transcriptional mechanisms mediated through the albumin promoter are responsible for reduced albumin mRNA levels during viral infection. Albumin expression is maintained in the liver by a combination of liver-enriched transcription factors such as CAAT enhancer-binding protein (C/EBP)alpha and C/EBPbeta. We show that p53 wild type and tumor-derived p53 mutations repress C/EBP-mediated transactivation of the albumin promoter. The binding of C/EBPalpha or -beta to its cognate sequence in the albumin promoter is not inhibited by p53 expression. Deletion analysis and domain swapping experiments show that repression of C/EBPbeta-mediated transactivation is dependent on the N-terminal domain of p53 and the transactivation domain, leucine zipper domain, and the inhibitory domain II (amino acids 163-191) of C/EBPbeta. Our results provide a molecular explanation for the p53-mediated down-regulation of liver-specific gene expression after viral infection. Additionally, as overexpression of p53 mutants is frequently found in undifferentiated hepatocellular carcinomas, the same mechanisms may contribute to the lack of liver-specific gene transcription in these tumors.

Codon 249 mutations of p53 gene in non-neoplastic liver tissues

AIM: To study the significance of p53 gene in hepatocarcino genesis through analyzing codon 249 mutations of p53 gene in non-neoplastic liver tissues.

METHODS: Codon 249 mutation was detected using single-strande d conformational polymorphism analysis and allele-specific PCR in liver tissues from 10 cases of chronic hepatitis, 5 cases of cirrhosis and 20 cases of HCCs.

RESULTS: The detection rate of codon 249 mutation in chronic hepatitis, cirrhosis and pericancerous tissues was 70% (7/10), 100% (5/5) and 70% (14/20), respectively by AS-PCR. These mutations could not be detected b y SSCP analysis. The detection rates were 65% (13/20) and 45% (9/20) in cancerous tissues by AS-PCR and SSCP analysis.

CONCLUSION: Codon 249 mutations of p53 gene were very popular in non-neoplastic liver tissues though the number of those mutant cells was only in subsection. Those mutations in cancerous tissues might take place in the stage before the formation of tumor.