p53 Deficiency in liver reduces local control of survival and proliferation, but does not affect apoptosis after DNA damage

Andrew David Hamilton Wyllie. 24 January 1944—26 May 2022

Andrew Wyllie graduated from the University of Aberdeen, becoming an academic pathologist in Aberdeen, Edinburgh and Cambridge. He was the co-discoverer of apoptotic cell death, having observed single cells dying following carcinogen exposure. Together with Alastair Currie and John Kerr, he realized the profound importance of this novel mode of cell death that showed a distinctive series of morphological changes, which he first described as a new cell death process. Wyllie and Currie introduced the term ‘apoptosis’ for this cell death process in a seminal paper in 1972. Another landmark discovery was of chromatin fragmentation in apoptosis, due to activation of an endogenous endonuclease that caused internucleosomal DNA cleavage (‘chromatin laddering’), which was the first biochemical mechanism of apoptosis described. He further characterized chromatin fragmentation in the 1980s, followed by investigations of cell surface changes to produce ‘eat-me’ signals to trigger rapid phagocytosis of the apoptotic cells and bodies, intracellular calcium ion signalling, caspase activation and other mechanisms of apoptosis. His cancer research helped identify the location of APC and generated his demonstration that apoptosis was regulated by oncogenes MYC and RAS and by tumour suppressor genes, such as TP53 . He showed how apoptosis occurred in response to DNA damage and was a key process influencing both carcinogenesis and tumour growth. Andrew made a major scientific observation that changed the understanding of how cells die in health and disease, although it took time for the scientific establishment to understand its fundamental importance. Andrew Wyllie is widely known as the ‘Father of Apoptosis’.

The oncogenic role of hepatitis B virus X gene in hepatocarcinogenesis: recent updates

Hepatocellular carcinoma (HCC) is the most prevalent form of primary liver cancers with high mortality rate. Among its various etiological factors, one of the major risk factors for HCC is a chronic infection of hepatitis B virus (HBV). HBV X protein (HBx) has been identified to play an important role in the HBV-induced HCC pathogenesis since it may interfere with several key regulators of many cellular processes. HBx localization within the cells may be beneficial to HBx multiple functions at different phases of HBV infection and associated hepatocarcinogenesis. HBx as a regulatory protein modulates cellular transcription, molecular signal transduction, cell cycle, apoptosis, autophagy, protein degradation pathways, and host genetic stability via interaction with various factors, including its association with various non-coding RNAs. A better understanding on the regulatory mechanism of HBx on various characteristics of HCC would provide an overall picture of HBV-associated HCC. This article addresses recent data on HBx role in the HBV-associated hepatocarcinogenesis.

p53-mediated redox control promotes liver regeneration and maintains liver function in response to CCl4

The p53 transcription factor coordinates wide-ranging responses to stress that contribute to its function as a tumour suppressor. The responses to p53 induction are complex and range from mediating the elimination of stressed or damaged cells to promoting survival and repair. These activities of p53 can modulate tumour development but may also play a role in pathological responses to stress such as tissue damage and repair. Using a p53 reporter mouse, we have previously detected strong induction of p53 activity in the liver of mice treated with the hepatotoxin carbon tetrachloride (CCl4). Here, we show that p53 functions to support repair and recovery from CCl4-mediated liver damage, control reactive oxygen species (ROS) and limit the development of hepatocellular carcinoma (HCC), in part through the activation of a detoxification cytochrome P450, CYP2A5 (CYP2A6 in humans). Our work demonstrates an important role for p53-mediated redox control in facilitating the hepatic regenerative response after damage and identifies CYP2A5/CYP2A6 as a mediator of this pathway with potential prognostic utility in human HCC.

Tackling hepatitis B virus-associated hepatocellular carcinoma--the future is now

Hepatocellular carcinoma (HCC) is one of the most lethal and prevalent cancers in many developing countries including India. Among the various etiological factors being implicated in the cause of HCC, the most important cause, however, is hepatitis B virus (HBV) infection. Among all HBV genes, HBx is the most critical carcinogenic component, the molecular mechanisms of which have not been completely elucidated. Despite its clinical significance, there exists a very elemental understanding of the molecular, cellular, and environmental mechanisms that drive disease pathogenesis in HCC infected with HBV. Furthermore, there are only limited therapeutic options, the clinical benefits of which are insignificant. Therefore, the quest for novel and effective therapeutic regimen against HBV-related HCC is of paramount importance. This review attempts to epitomize the current state of knowledge of this most common and dreaded liver neoplasm, highlighting the putative treatment avenues and therapeutic research strategies that need to be implemented with immediate effect for tackling HBV-related HCC that has plagued the medical and scientific fraternity for decades. Additionally, this review proposes a novel "five-point" management algorithm for HBV-related HCC apart from portraying the unmet needs, principal challenges, and scientific perspectives that are relevant to controlling this accelerating global health crisis.

Induction of p53-dependent p21 limits proliferative activity of rat hepatocytes in the presence of hepatocyte growth factor

Background

Hepatocyte growth factor (HGF), a potent mitogen for hepatocytes, enhances hepatocyte function without stimulating proliferation, depending on the physiological conditions. p53, a transcription factor, suppresses the cell proliferation by expressing p21^WAF1/CIP1 in various tissues.

Aim

To investigate the mechanism through which the hepatocytes maintain mitotically quiescent even in the presence of HGF.

Methods

We studied the relationship between p53 and p21 expression and the effect of p53-p21 axis on hepatocyte proliferation in primary cultured rat hepatocytes stimulated by HGF. Hepatic p21 levels are determined serially after partial hepatectomy or sham operation in rats.

Results

DNA synthesis was markedly increased by HGF addition in rat hepatocytes cultured at low density but not at high density. Cellular p53 levels increased in the hepatocytes cultured at both the densities. p21 levels were increased and correlated with cellular p53 levels in hepatocytes cultured at high density but not at low density. When the activity of p53 was suppressed by a chemical inhibitor for p53, cellular p21 levels were reduced, and DNA synthesis was increased. Similarly, p21 antisense oligonucleotide increased the DNA synthesis. In rats after partial hepatectomy, transient elevation of hepatic p21 levels was observed. In contrast, in sham-operated rats, hepatic p21 levels were increased on sustained time scales.

Conclusion

p53-related induction of p21 may suppress hepatocyte proliferation in the presence of HGF in the setting that mitogenic activity of HGF is not elicitable.

Splicing factor SRSF3 is crucial for hepatocyte differentiation and metabolic function

SR family RNA binding proteins regulate splicing of nascent RNAs in vitro but their physiological role in vivo is largely unexplored, as genetic deletion of many SR protein genes results in embryonic lethality. Here we show that SRSF3HKO mice carrying a hepatocyte-specific deletion of Srsf3 (homologous to human SRSF3/SRp20) have a disrupted hepatic architecture and show pre- and postnatal growth retardation. SRSF3HKO mice exhibit impaired hepatocyte maturation with alterations in glucose and lipid homeostasis characterized by reduced glycogen storage, fasting hypoglycemia, increased insulin sensitivity and reduced cholesterol synthesis. We identify various splicing alterations in the SRSF3HKO liver that explain the in vivo phenotype. In particular, loss of SRSF3 causes aberrant splicing of Hnf1α, Ern1, Hmgcs1, Dhcr7 and Scap genes, which are critical regulators of glucose and lipid metabolism. Our study provides the first evidence for a SRSF3-driven genetic programme required for morphological and functional differentiation of hepatocytes that may have relevance for human liver disease and metabolic dysregulation.

G1 cell cycle arrest signaling in hepatic injury after intraperitoneal sepsis in rats

OBJECTIVE AND DESIGN

Hepatocytes emerge from a quiescent state into a proliferative state to recover from septic injury. We hypothesize that hepatocyte cell cycle regulation after sepsis potentially contributes to the recovery of liver function.

METHODS

An animal model of sepsis was induced by cecal ligation and puncture (CLP) in rats. At serial time points after CLP, hepatocyte expression of p21, P53, cyclin D1, cyclin E, CDK2, CDK4 and PCNA was determined by immunoblot analysis, and the DNA content of isolated hepatocytes was analyzed using flow cytometry.

RESULTS

Sepsis-induced liver injury of rats was associated with G1 cell cycle arrest. Recovery of liver function was related to cell cycle progression 48 h after CLP. The upregulation of p53 and p21 correlated with G1 cell arrest 48 h after CLP. The upregulation of cyclin D1/CDK4 and cyclin E/CDK2 also correlated with the G1/S transition 48 h after CLP, resulting in PCNA expression.

CONCLUSIONS

The data suggests that G1 cell cycle arrest and p53, p21, CDKs, cyclins and PCNA expression may be involved in the injury/recovery of liver function after intraperitoneal sepsis.

Hepatitis B virus x protein in the pathogenesis of hepatitis B virus-induced hepatocellular carcinoma

Currently available evidence supports a role for the hepatitis B virus (HBV) x gene and protein in the pathogenesis of HBV-induced hepatocellular carcinoma (HCC). HBx gene is often included, and remains functionally active, in the HBV DNA that is frequently integrated into cellular DNA during hepatocellular carcinogenesis. HBx protein promotes cell cycle progression, inactivates negative growth regulators, and binds to and inhibits the expression of p53 tumour suppressor gene and other tumour suppressor genes and senescence-related factors. However, the molecular mechanisms responsible for HBx protein-induced HCC remain uncertain. Only some of the more fully documented or more recently recognised mechanisms are reviewed. During recent years evidence has accumulated that HBx protein modulates transcription of methyl transferases, causing regional hypermethylation of DNA that results in silencing of tumour suppressor genes, or global hypomethylation that results in chromosomal instability, thereby playing a role in hepatocarcinogenesis. HBx protein has both anti-apoptotic and pro-apoptotic actions, apparently contradictory effects that have yet to be explained. Particularly important among the anti-apoptotic properties is inhibition of p53. Recent experimental observations suggest that HBx protein may increase the expression of TERT and telomerase activity, prolonging the life-span of hepatocytes and contributing to malignant transformation. The protein also interferes with nucleotide excision repair through both p53-dependent and p53- independent mechanisms. Carboxy-terminal truncated HBx protein loses its inhibitory effects on cell proliferation and pro-apoptotic properties, and it may enhance the protein's ability to transform oncogenes. Dysregulation of IGF-II enhances proliferation and anti-apoptotic effects of oncogenes, resulting in uncontrolled cell growth.

Phosphorylation of Ser312 contributes to tumor suppression by p53 in vivo

The tumor suppressor p53 is a master sensor of stress, and posttranslational modifications are key in controlling its stability and transcriptional activities. p53 can be phosphorylated on at least 23 Ser/Thr residues, the majority of which are phosphorylated by stress-related kinases. An exception is Ser315 in human p53 (Ser312 in mouse), which is predominantly phosphorylated by cell cycle-related kinases. To understand the biological importance of Ser312 phosphorylation in vivo, we generated p53Ser312Ala knock-in mice. We show here that, although Ser312 is not essential for mouse life span under normal physiological conditions, Ser312Ala mutation dampens p53's activity during embryonic development. This is evident from its partial rescue of embryonic lethality caused by Mdm4 deletion. In agreement with the notion that Ser312 mutation weakens p53 function, Ser312Ala mice are also more susceptible to tumorigenesis following a sublethal ionizing radiation dose. Importantly, in the cohort studied, Ser312 mutation predisposes mice to develop thymic lymphomas and liver tumors, partly due to p53Ser312Ala's inability to fully induce a set of p53 target genes including p21 and cyclin G1. Thus, we demonstrate that phosphorylation of Ser312 is required for p53 to function fully as a tumor suppressor in vivo.

Defective DNA strand break repair causes chromosomal instability and accelerates liver carcinogenesis in mice

Chromosomal instability is a characteristic feature of hepatocellular carcinoma (HCC) but its origin and role in liver carcinogenesis are undefined. We tested whether a defect in the nonhomologous end-joining (NHEJ) DNA repair gene Ku70 was associated with chromosomal abnormalities and enhanced liver carcinogenesis. Male Ku70 NHEJ-deficient (Ku70-/-), heterozygote (Ku70 +/-), and wild-type (WT) mice were injected with diethylnitrosamine (DEN), a liver carcinogen, at age 15 days. Animals were killed at 3, 6, and 9 months for assessment of tumorigenesis and hepatocellular proliferation. For karyotype analysis, primary liver tumor cell cultures were prepared from HCCs arising in Ku70 mice of all genotypes. Compared to WT littermates, Ku70-/- mice injected with DEN displayed accelerated HCC development. Ku70-/- HCCs harbored clonal increases in numerical and structural aberrations of chromosomes 4, 5, 7, 8, 10, 14, and 19, many of which recapitulated the spectrum of equivalent chromosomal abnormalities observed in human HCC. Ku70-/- HCCs showed high proliferative activity with increased cyclin D1 and proliferating cell nuclear antigen expression, Aurora A kinase activity, enhanced ataxia telangiectasia mutated kinase and ubiquitination, and loss of p53 via proteasomal degradation, features which closely resemble those of human HCC.

CONCLUSION

These findings demonstrate that defects in the NHEJ DNA repair pathway may participate in the disruption of cell cycle checkpoints leading to chromosomal instability and accelerated development of HCC.

Microarray analysis of gene expression of mouse hepatocytes of different ploidy

Polyploidisation in hepatocytes has been associated with many physiologic and pathologic processes such as proliferation, metabolism, regeneration, aging, and cancer. We studied gene expression patterns in hepatocytes of different ploidy. Primary hepatocytes were obtained from mice of different ages: young (4-6 weeks old), adult (8-10 weeks old), and older (22-24 weeks old). Diploid (2N), tetraploid (4N), and octoploid (8N) hepatocytes were isolated for studies using a high-density mouse genome microarray. No major changes of gene expression patterns between hepatocytes of different ploidy were found. Fifty genes were identified as differentially expressed in the diploid and tetraploid populations, but the changes were less than twofold either way. Four genes (Gas2, Igfbp2, Nr1i3, and Ccne2) were differentially expressed in tetraploid and octoploid cells. This was confirmed in two age groups, "adult" and "older," but once again the factors were less than twofold and the expressions of Gas2 and Igfbp2 were more different between age groups than between ploidy classes. Our results show that polyploid hepatocytes are stable and "normal" without aberrant gene expression, unlike what is thought for cancer cells. By contrast to megakaryocytes, hepatocyte polyploidisation is not a differentiation step associated with major changes in gene expression. Our data support the hypothesis that hepatocyte polyploidisation is a protective mechanism against oxidative stress that occurs via a controlled process throughout growth and aging where binucleation is important.

Hepatocyte cell lines: their use, scope and limitations in drug metabolism studies

Gaining knowledge on the metabolism of a drug, the enzymes involved and its inhibition or induction potential is a necessary step in pharmaceutical development of new compounds. Primary human hepatocytes are considered a cellular model of reference, as they express the majority of drug-metabolising enzymes, respond to enzyme inducers and are capable of generating in vitro a metabolic profile similar to what is found in vivo. However, hepatocytes show phenotypic instability and have a restricted accessibility. Different alternatives have been explored in the past recent years to overcome the limitations of primary hepatocytes. These include immortalisation of adult or fetal human hepatic cells by means of transforming tumour virus genes, oncogenes, conditionally immortalised hepatocytes, and cell fusion. New strategies are currently being used to upregulate the expression of drug-metabolising enzymes in cell lines or to derive hepatocytes from progenitor cells. This paper reviews the features of liver-derived cell lines, their suitability for drug metabolism studies as well as the state-of-the-art of the strategies pursued in order to generate metabolically competent hepatic cell lines.

Cooperative interactions of p53 mutation, telomere dysfunction, and chronic liver damage in hepatocellular carcinoma progression

Hepatocellular carcinoma is among the most common and lethal cancers in humans. Hepatocellular carcinoma is commonly associated with physical or functional inactivation of the p53 tumor suppressor, high levels of chromosomal instability, and disease conditions causing chronic cycles of hepatocyte death and regeneration. Mounting evidence has implicated regeneration-induced telomere erosion as a potential mechanism fueling genome instability. In mouse models of hepatocellular carcinoma, telomere dysfunction has been shown to enhance initiation of hepatic neoplasias yet constrain full malignant progression of these neoplasms possibly due to activation of a p53-dependent checkpoint and/or intolerable levels of genomic instability. Here, in a hepatocellular carcinoma-prone model brought about through toxin-induced hepatocyte injury and regeneration, we sought to determine the cooperative interactions of germ line p53 mutation and telomere dysfunction [produced by telomerase reverse transcriptase (mTERT) gene knockout]. In the setting of intact telomeres, p53 mutation had no effect on hepatocarcinogenesis, whereas in the setting of telomere dysfunction, p53 mutation enabled advanced hepatocellular carcinoma disease. Notably, there was no evidence of deletion or mutation of the wild-type p53 allele in the late generation mTert(-/-)p53(+/-) mice, suggesting that reduced levels of p53 potently enable hepatocellular carcinoma progression in the setting of telomere dysfunction. Thus, this study supports a model that, in the face of chronic liver damage, attenuated p53 function and telomere-induced chromosomal instability play critical and cooperative roles in the progression of hepatocellular carcinoma.

Dual action of the inhibitors of cyclin-dependent kinases: targeting of the cell-cycle progression and activation of wild-type p53 protein

The inhibition of cyclin-dependent kinases (CDKs) represents a novel approach to the therapy of human malignancies. Already in clinical trials, recently developed CDK inhibitors very efficiently target the rapidly proliferating cancer cells and inhibit their cell-cycle progression. Interestingly, some CDK inhibitors additionally affect the stability and activity of the tumour-suppressor protein p53, thereby enhancing their antiproliferative action towards cancer cells. Considering the fact that the p53 protein is mutated or inactivated in approximately 50% of all human cancers, the efficacy of CDK inhibitor therapy could differ between cancer cells depending on their p53 status. Moreover, recent reports demonstrating that some cancer cells can proliferate despite CDK2 inhibition questioned the central role of CDK2 in the cell-cycle control and suitability of CDK2 as a therapeutic target; however, the p53 activation that is mediated by CDK inhibitors could be essential for the efficacy of CDK inhibitors in therapy of CDK2-independent cancers. Furthermore, there is also reason to believe that CDK2 inhibitors could be used for another purpose, to protect normal cells from the effects of chemotherapy.

Pifithrin-alpha induced p53 inhibition does not affect liver regeneration after partial hepatectomy in mice

BACKGROUND/AIMS

Beside its well-known function as tumour suppressor gene, p53 is supposed to positively regulate cell division and cell differentiation. Because hepatocyte proliferation has been reported to be reduced by blockade of p53 function in vitro, we examined in the present study the impact of p53 inhibition on hepatocyte proliferation in vivo.

METHODS

Mice treated with either pifithrin-alpha (PFT), a p53-inactivating agent, or the equivalent volume of vehicle, were subjected to 70% hepatectomy. In addition to assessment of liver mass restitution we examined p53 and p21 protein expression as well as PCNA expression and BrdU incorporation by using Western blot and immunohistochemical techniques. Extent of apoptosis was assessed by TUNEL assay.

RESULTS

PFT lowered nuclear but not cytoplasmic p53, and did not inhibit protein expression of regeneration-associated p21. PCNA protein expression as well as PCNA and BrdU immunohistochemistry did not differ between regenerating livers of either PFT- or vehicle-treated animals. Moreover, TUNEL analysis of regenerated liver tissue revealed comparable numbers of apoptotic cells in both groups.

CONCLUSIONS

Pharmacological inhibition of p53 did not impair liver regeneration in mice, implying that p53 is functionally redundant in that p53-independent pathways compensate for the blockade of p53 and sufficiently support the process of hepatocyte replication in liver regeneration.

The hepatitis B virus X protein sensitizes HepG2 cells to UV light-induced DNA damage

Various reports have implicated the virally encoded HBx protein as a cofactor in hepatocarcinogenesis. However, direct evidence of the role of HBx as a promoter of oncogenesis in response to an initiating factor such as DNA damage remains inadequate. Here, we report the effects of HBx in HepG2 cells exposed to UV light-induced DNA damage. HBx expression was found not to affect the morphology, viability, and cell cycle/apoptotic profiles or DNA repair machinery of untreated cells. Nonetheless, upon UV treatment, HBx protein levels increased concomitantly with p53 levels. Both HBx and p53 proteins were found to interact and colocalize primarily in the nucleus. The binding of HBx to p53 modulated (but did not inhibit) the transcriptional activation function of p53. Notably, HBx-expressing cells exhibited increased sensitivity to UV damage, resulting in greater G2/M arrest and apoptosis of these cells. Additionally, these cells displayed a reduced DNA repair capacity in response to UV damage. In conclusion, this work suggests that DNA damage may be an initiating factor in hepatocarcinogenesis and that HBx may act as the promoting factor by inhibiting DNA repair. In hepatitis B virus-infected hepatocytes, a chronic infection may present the opportunity for such a DNA-damaging event to occur, and accumulated errors caused by the inhibition of DNA repair by HBx may result in oncogenesis.

Cell and tissue responses to genotoxic stress

Cancers arise as a consequence of the accumulation of multiple genetic mutations in a susceptible cell, resulting in perturbation of regulatory networks that control proliferation, survival, and cellular function. Here, the sources of cellular stress that can cause oncogenic mutations and the responses of cells to DNA damage are reviewed. The role of different repair pathways and the potential for cell- and tissue-specific reliance on individual repair mechanisms are discussed. Evidence for cell- and tissue-specific activation of p53-mediated growth arrest and apoptosis after exposure to an individual genotoxin is assessed and some of the potential mediators of these different responses are provided. These cell- and tissue-specific responses to particular forms of DNA damage are likely to be key determinants of tissue-specific tumour susceptibility, and there is good evidence for genetic variations in these responses. The role that genotoxic agents play in altering the microenvironment to produce indirect effects on tumourigenesis through altered production of free radicals and cytokines that are characteristic of inflammatory-type processes is also evaluated. Changes to the microenvironment as direct or indirect effects of genotoxic stress can be involved in both tumour initiation and progression and may even be a prerequisite for tumourigenesis. Therefore, tumour susceptibility after endogenous or exogenous genotoxic stress represents a balance between cell-intrinsic responses of target cells and changes to the microenvironment. A fuller understanding of cell- and tissue-specific responses, alterations to the microenvironment, and genetic modifiers of these responses could lead to novel prevention and therapeutic strategies for common forms of human malignancy.

Can you be large and not obese? The distinction between body weight, body fat, and abdominal fat in occupational standards

Weight control is an important early intervention in diabetes, but the nature of the association between weight and disordered metabolism has been confused because fat mass and its distribution are only partly associated with increasing body size. Weight, fat, and regional fat placement, specifically in the abdominal site, may each have distinctly different associations with diabetes risk. Abdominal circumference may be the common marker of poor fitness habits and of increased risk for metabolic diseases such as diabetes. This is an important question for public health policy as well as for occupational standards such as those of the military, which are intended to promote fitness for military missions and include strength and aerobic capacity, as well as military appearance considerations. U.S. soldiers are heavier than ever before, reflecting both increased muscle and fat components. They also have better health care than ever before and are required to exercise regularly, and even the oldest soldiers are required to remain below body fat limits that are more stringent than the current median values of the U.S. population over age 40. The body fat standards assessed by circumference-based equations are 20-26% and 30-36%, for various age groups of men and women, respectively, and the upper limits align with threshold values of waist circumference recommended in national health goals. The basis and effects of the Army standards are presented in this paper. U.S. Army body fat standards may offer practical and reasonable health guidelines suitable for all active Americans that might help stem the increasing prevalence of obesity that is predicted to increase the prevalence of Type 2 diabetes.

Expression of Sonic hedgehog pathway genes is altered in colonic neoplasia

The Hedgehog (Hh) signalling pathway is crucial for normal development and patterning of numerous human organs including the gut. Hh proteins are also expressed during gastric gland development and gastric epithelial differentiation in adults. Recently, dysregulation of these developmentally important genes has been implicated in cancer, leading to the present study of the expression of Hh signalling proteins in colon cancer. In this study, normal colon and colonic lesions (hyperplastic polyp, adenoma, and colonic adenocarcinoma) were examined by immunohistochemistry using antibodies against Hh signalling molecules: the secreted protein Sonic hedgehog (SHH), its receptor Patched (PTCH), and the PTCH-associated transmembrane protein Smoothened (SMOH). The study shows that Hh signalling pathway members are expressed in normal colonic epithelium. SHH was expressed at the top of the crypts and in a few basally located cells, while PTCH was detected in the neuroendocrine cells and SMOH at the brush border of superficial epithelium. RT-PCR analysis of laser-microdissected crypts from normal human colon confirmed that mRNAs encoding these proteins were expressed in colonic epithelium. Expression of SHH, PTCH, and SMOH was up-regulated in hyperplastic polyps, adenomas, and adenocarcinomas of the colon, and SHH expression correlated with increased expression of the proliferation marker Ki-67 in all lesions examined. To address whether the Hh signalling pathway is functional in the gut, the effect of Shh on epithelial cells in vitro was explored by treating primary murine colonocytes with either Shh peptide or neutralizing anti-Shh antibody. The proportion of cells in the S-phase was assessed by bromodeoxyuridine (BrdU) incorporation. It was found that exogenous Shh promotes cell proliferation in colonocytes, while anti-Shh inhibits proliferation, suggesting that Shh is required during proliferation of epithelial cells in vitro. It is suggested that SHH is required during epithelial proliferation in the colon and that there is a possible role for Hh signalling in epithelial colon tumour progression in vivo.

Strong induction of p73 protein in vivo coincides with the onset of apoptosis in rat liver after treatment with the hepatocarcinogen N-nitrosomorpholine (NNM)

Treatment of rats with genotoxic hepatocarcinogens such as N-nitrosomorpholine (NNM) causes severe hepatotoxicity associated with apoptosis of hepatocytes beginning after 12 h. Previously, we reported that after a single administration of high NNM dose p53 protein level increased in liver but not in testis and that the first wave of apoptosis preceded the induction of p53 indicating that apoptosis in liver was driven by a p53-independent pathway. We now show a pronounced upregulation of p73 protein, a p53-related gene product. The increase of p73 alpha and beta occurred already 6 h after NNM administration and preceded the onset of apoptosis by 6 h. Very strong p73 signals appeared 20 and 40 h post-treatment and persisted for a few days, whereas p53 was induced only transiently at 20 and 40 h post-treatment. Immunohistochemical analysis revealed that unlike p53, p73 was detected in the nuclei of hepatocytes undergoing apoptosis. Following the upregulation of p73 levels, the products of several genes regulating DNA repair, e.g., GADD-45 and p53R2 and mediating apoptosis such as apoptosis inducing factor (AIF) were rapidly induced, whereas transient elevation of MDM-2 protein was delayed and coincided temporary with activation of p53 protein. Interestingly, NF-kappaB another transcription factor responding to cellular stress was activated at 20 h after NNM administration and reached a maximum after an additional 20 h. Our data indicate that activated p73 protein may positively affect the induction and execution of apoptosis in response to genotoxic action of NNM.

The infant mouse as a in vivo model for the detection and study of DNA damage-induced changes in the liver

The present work describes the use of the infant (4-wk-old) mouse as an animal model for the study of DNA damage-induced G(1) checkpoint response, changes in p53 protein levels, and multiple gene expression changes after DNA damage has been induced in the liver. Hepatocytes in the infant B6C3F1 mouse had a proliferation index that was 27 times greater than that of the 12-wk-old B6C3F1 mouse (57.4 vs. 2.1%, respectively). Eight hours after infant mice were exposed to the DNA damaging agents bleomycin (100 mg/kg, i.p.) or 10 Gy of whole body gamma irradiation, the G(1)/S ratio significantly increased from 21 (control) to 66 and 75, respectively, because of the induction of the G(1)/S checkpoint response. One hour after whole body irradiation of infant mice the levels of the p53 protein, phosphoserine 18-p53 and phosphoserine 23-p53 increased dramatically and tended to peak at 1 h in the liver, whereas the p21(WAF1) protein increased more slowly and tended to peak at 2 h after irradiation. The mRNA expression of the p53-response genes p21, murine double minute clone 2 (mdm2), and cyclin G was increased at 2 h after irradiation but was decreased by 8 h postirradiation, relative to the 2-h time-point. The expression of insulin-like growth factor binding protein-1 (IGFBP-1) and growth-regulated oncogene 1 (GRO1) increased at 2 and 8 h postirradiation. This work characterizes various parameters in the infant mouse, thus validating the use of this model to study in vivo DNA damage-induced cell-cycle-related changes.

Phorbol myristate induces apoptosis of taxol-resistant sarcoma cells in vitro

Taxol was found to induce polyploidization and apoptosis in cultured methylcholanthrene-induced sarcoma cells (Meth-A cells), but some of the cells in G1 phase were not affected. We refer to these cells as taxol-resistant cells. Phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) regulator, was used to test the taxol-resistant cells. Many of the taxol-resistant cells disappeared after treatment with taxol in the presence of PMA. To explore the mechanism of this effect, we employed flow cytometry to determine levels of p53, p21, bcl-2 and caspase proteins in the taxol-resistant cells, and found that the expression of the bcl-2 protein was markedly decreased and the expression of the caspase protein markedly increased after treatment with taxol in the presence of PMA. These findings suggest that PMA enhances the sensitivity of taxol-resistant cells to taxol, and taxol treatment in the presence of PMA induces the apoptosis of taxol-resistant cells by inhibiting the expression of the bcl-2 protein and increasing the expression of the caspase protein.

p73alpha is a candidate effector in the p53 independent apoptosis pathway of cisplatin damaged primary murine colonocytes

AIMS

Colonocytes were derived from wild-type (wt) and p53 deficient mice to investigate p53 dependent and independent death pathways after cisplatin treatment, and the role of p53 in growth regulation of primary, untransformed epithelial cells.

METHODS

Wt and p53 null colonocytes were exposed to cisplatin and DNA synthesis, apoptosis, and p53, p21, and p73 expression were investigated after six, 12, and 24 hours. Major p73 isoforms were identified by reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

Cisplatin treated wt cells exhibited cell cycle arrest, whereas p53 null cells continued to synthesise DNA, although both cell types died. Apoptosis was significantly higher in cisplatin treated wt and p53 null colonocytes than in controls at all timepoints, although apoptosis was lower in cisplatin treated p53 null colonocytes than in wt cells. p53 expression was upregulated in cisplatin treated wt colonocytes. p21 expression was high and remained unchanged in cisplatin treated wt cells, although it was reduced in the absence of p53. p73 was investigated because it could account for p53 independent p21 expression and p53 independent death. RT-PCR detected full length p73alpha. p73 transcript levels remained unchanged, whereas p73 protein accumulated in the nucleus of cisplatin treated cells, irrespective of genotype.

CONCLUSIONS

p53 is essential for cell cycle arrest, but not apoptosis in primary murine colonocytes. Apoptosis is reduced in cisplatin treated p53 null cells. Nuclear accumulation of endogenous p73 after cisplatin treatment suggests a proapoptotic role for p73alpha in the absence of p53 and collaboration with p53 in wt colonocytes.

Additive effect of p53, p21 and Rb deletion in triple knockout primary hepatocytes

Using Cre-Lox technology to inducibly delete Rb from wild-type, p21- and/or p53-deficient primary hepatocytes, we investigated the role of p53, p21 and pRb in the regulation of liver cell proliferation, polyploidization and death. These cellular decisions are critical to maintaining liver cell replacement in disease, and in determining the likelihood of carcinogenesis in chronic liver injury. Clearly, the present study shows a complex interplay between p53, p21 and pRb, which regulates the likelihood of hepatocytes stimulated from quiescence, to proliferate, undergo polyploidy or die. It reveals that these proteins act both in concert and independently, demonstrating that a small set of key cellular players is common to diverse cell decisions of fundamental importance to disease.

Overcoming antiapoptotic responses to promote chemosensitivity in metastatic colorectal cancer to the liver

BACKGROUND

Metastatic colon cancer is highly resistant to chemotherapy. A variety of mechanisms by which cancer cells resist chemotherapy have been described including enhanced export of drug from cancer cells and alterations in drug metabolism. In addition, the response of cancer cells to genotoxic therapies may be diminished by acquired defects in either the response mechanisms to DNA damage or cell cycle regulatory pathways. Recently, attention has focused on mechanisms that are activated by treatment exposure and subsequently promote resistance by rescuing cancer cells from apoptosis. The objective of this review is to examine the role of antiapoptotic mechanisms of chemotherapy resistance and to determine the potential utility of therapeutic strategies that target these mechanisms.

METHODS

To accomplish the objectives, a brief overview of mechanisms of chemotherapy resistance is provided. The concept of inducible chemotherapy resistance is introduced by examination of a specific antiapoptotic mechanism, mediated by the transcription factor, nuclear factor kappa B (NF-kappa B). The ability to use inhibitors of NF-kappa B to promote chemosensitivity is examined in vitro and in vivo.

RESULTS

Inhibition of chemotherapy-induced NF-kappa B activation enhances apoptosis and augments chemotherapy sensitivity.

CONCLUSIONS

NF-kappa B inhibition may overcome cancer cell defense against apoptosis. Molecular therapies that target this resistance mechanism may be useful adjuncts to conventional chemotherapy.

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.

Absence of p53 in Clara cells favours multinucleation and loss of cell cycle arrest

BACKGROUND

The p53 oncosuppressor protein is a critical mediator of the response to injury in mammalian cells and is mutationally inactivated in the majority of lung malignancies. In this analysis, the effects of p53-deficiency were investigated in short-term primary cultures of murine bronchiolar Clara cells. Clara cells, isolated from gene-targeted p53-deficient mice, were compared to cells derived from wild type littermates.

RESULTS

p53 null cultures displayed abnormal morphology; specifically, a high incidence of multinucleation, which increased with time in culture. Multinucleated cells were proficient in S phase DNA synthesis, as determined by BrdU incorporation. However, multinucleation did not reflect altered rates of S phase synthesis, which were similar between wild type and p53-/- cultures. Nucleation defects in p53-/- Clara cells associated with increased centrosome number, as determined by confocal microscopy of pericentrin-stained cultures, and may highlight a novel role of p53 in preserving genomic integrity in lung epithelial cells. Effects of p53-deficiency were also studied following exposure to DNA damage. A p53-dependent reduction in the BrdU index was observed in Clara cells following ionizing radiation. The reduction in BrdU index in wild type cells displayed serum-dependency, and occurred only in the absence of serum. Taken together, these findings demonstrate that in murine primary Clara cell culture, cell cycle arrest is a p53-mediated response to DNA damage, and that extracellular factors, such as serum, influence this response.

CONCLUSION

These findings highlight functions of wild type p53 protein in bipolar spindle formation, centrosome regulation, and growth control in bronchiolar Clara cells.

p53 May positively regulate hepatocyte proliferation in rats

p53, known as a tumor suppressor gene, is a transcription factor that regulates various cellular functions. Recently, several growth factor gene promoters, including that of transforming growth factor alpha (TGF-alpha), were shown to be direct targets of p53-mediated transcription. Hepatic p53 mRNA is up-regulated during liver regeneration in rats. The aim of this study is to examine the role of p53 in hepatocyte proliferation. p53 protein levels were examined in rat hepatocytes cultured in the medium containing hepatocyte growth factor (HGF). p53 levels began to increase after 6 hours of incubation, reached a maximum at 18 hours, and decreased thereafter. DNA synthesis increased at 12 hours and peaked at 30 hours. When hepatocytes were incubated with p53 antisense oligonucleotide in addition to HGF, increases of p53 and TGF-alpha levels were suppressed, and DNA synthesis was reduced. The increases of TGF-alpha levels and DNA synthesis were also suppressed by a chemical inhibitor of p53, pifithrin-alpha. In rats after two-thirds partial hepatectomy, hepatic p53 increased and reached maximal levels around 16 hours when hepatic HGF levels have been shown to reach a maximum followed by an increase in hepatic TGF-alpha levels or hepatocyte proliferation. In contrast, sham-operated rats showed minor elevations of hepatic p53 levels. In conclusion, p53 production is stimulated by HGF and may contribute to the proliferation of rat hepatocytes. Considering previous findings indicating the importance of endogenous TGF-alpha for the proliferation of hepatocytes stimulated by HGF, TGF-alpha might play a role in HGF-p53 mediated hepatocyte proliferation.

Establishment and characterization of immortal hepatocytes derived from various transgenic mouse lines

The potential of three genetic changes introduced into mice by the transgenic or knockout technology aimed at immortalizing hepatocytes in vitro and concomitantly preserving their differentiated hepatic functions was analyzed. Six hepatocyte lines were isolated from neonatal and adult transgenic mice expressing either IgEGF (a secretable variant of hEGF) or SV40 T antigen in the liver and from neonatal and adult p53 knockout (KO) mice and have been subcultured >150 times in serum-free, arginine-deficient medium. Only in SV40 T antigen transgenic lines profiles of mRNAs encoding serum proteins, transcription factors, and liver-specific enzymes were similar to those found in livers and primary hepatocytes. Accordingly, these cells displayed basal and inducible expression of CYP proteins as well as testosterone metabolizing activities. Thus, either knockout of the p53 gene or expression of SV40 T antigen or of IgEGF imparts immortality to hepatocytes in vitro, but only SV40 T antigen expression is compatible with the concomitant long-term preservation of differentiated liver functions.

Functional analysis of mouse hepatocytes differing in DNA content: volume, receptor expression, and effect of IFNgamma

Polyploidy and binuclearity are characteristics of the mammalian liver. Increasing polyploidisation occurs with age and after administration of various drugs and chemicals. This study was designed to examine the function of ploidy by addressing several questions: (1) Does the increase in size of polyploid hepatocytes have any physiological function by altering surface receptor expression such as intercellular adhesion molecule-1 (ICAM-1, CD54) or IFNgammaR? and (2) Do polyploid cells respond differently to inflammatory cytokines such as interferon gamma (IFNgamma)? We have developed a method to accurately measure the volume of live isolated hepatocytes using confocal microscopy and image analysis. Using flow cytometry, we have shown that the expression of ICAM-1 increases with increasing DNA content and IFNgammaR is not detectable on isolated mouse hepatocytes. Diploid (2n), tetraploid (4n) and octoploid (8n) hepatocytes were found to be equally susceptible to IFNgamma-induced apoptosis in vitro. Although the function of polyploidy remains unanswered, we have described some of the characteristics of polyploidy in isolated hepatocytes and in vitro.

Expression of presumed specific early and late factors associated with liver regeneration in different rat surgical models

Experiments performed on the portal branch ligation (PBL) model indicate that early changes observed after surgery are not related to the regenerative process because they also occur in atrophying lobes. To further confirm the lack of specificity of the early events and to exclude the influence of circulatory factors released by proliferating lobes on their occurrence, we investigated this response after sham operation (SO) and portacaval shunt (PCS), a model characterized by liver atrophy. We also attempted to determine expression of later events associated specifically with regeneration, ie, expression of p53 or c-Ha-ras, or inhibition of proliferation, ie, interleukin-1beta (IL-1beta) and transforming growth factor-beta1 (TGF-beta1) after partial (PH) and temporary partial (TPH) hepatectomy, SO and PCS. Nuclear factor-kappaB (NF-kappaB) and signal transducer and activator of transcription 3 (STAT3) DNA binding were assessed by electrophoretic mobility shift assay (EMSA), interleukin-6 (IL-6) mRNA by reverse transcription-polymerase chain reaction (RT-PCR), c-myc and c-jun mRNAs by Northern blot analysis at 0.5 and 2 hours, p53 and c-Ha-ras mRNAs by Northern blot analysis at 8 and 24 hours, and IL-1beta and TGF-beta1 by RT-PCR at 24 hours. The early response including an increase of NF-kappaB, STAT3, IL-6, and immediate-early genes expression was present after PH, PCS, and SO. In SO, slight differences were observed in comparison with PH: no NF-kappaB p65/p50 DNA binding was observed, only three of six SO rats were positive for IL-6, and immediate-early genes induction showed differences in the intensity of the response. At later times, p53 mRNA increased at 8 hours after PH and TPH, c-Ha-ras mRNA at 24 hours after PH, and IL-1beta mRNA at 24 hours after PCS. Early events are not specifically associated with the reduction of liver mass or with the regenerative process, are not predictive of future cell fate, and are most likely related to surgical stress. p53 and c-Ha-ras induction is closely associated with cell cycle progression whereas IL-1beta, but not TGF-beta1, appears to be one of the negative growth regulators that might play an important role in atrophy.

Adenovirus-mediated Cre deletion of floxed sequences in primary mouse cells is an efficient alternative for studies of gene deletion

This study evaluates the utility of Cre-expressing adenovirus for deletion of floxed genes in primary cells using primary murine hepatocytes. Adenovirus infection was very efficient, even at very low MOI (>95% infection at a MOI of 6) and did not reduce viability. High level LacZ expression was cytotoxic to hepatocytes but Cre expression had no effect on viability. Cre-mediated recombination was completed within a timespan that permits experimentation during primary culture (>95% recombination after 24 h), independently of the number of floxed alleles per cell. Recombination did not induce p53 or produce cytological nuclear abnormalities (even in polyploid cells). Contrary to expectation, deletion of DNA ligase 1 did not alter cell cycle progression, although Cre expression hastens entry to S phase from G(1), independently of the presence of floxed sequences. We conclude that adenovirus-mediated deletion of floxed alleles in primary cells is a straightforward and highly efficient tool for conducting preliminary studies of conditional gene targeting. Primary cells have advantages of differentiation, relative purity and ease of experimentation within controlled conditions, while avoiding confounding problems encountered in vivo (i.e. target cell specificity, kinetics and level of recombination, and elicitation of inflammatory and immune responses). This system could help identify important phenotypic effects and design and interpret in vivo studies.

Phenobarbitone-induced liver response in wild type and in p53 deficient mice

The tumour suppressor protein, p53, is involved in the regulation of apoptosis and growth arrest following DNA damage. Mutations of the p53 gene are found in 50-55% of all human cancers (Hollstein et al. Nucl. Acid Res. 22 (1994) 3551), including hepatocellular carcinomas. Phenobarbitone (PB) is a non-genotoxic hepatocarcinogen in rats and mice. With commercial availability of mice where one or both alleles of p53 have been removed we have examined the effect of PB in wild type C57BL/6J mice (p53 +/+), and p53 deficient mice (+/- and -/- p53) to determine whether p53 plays a role in the PB induced liver response. In each strain of mice, chronic administration caused liver enlargement, which was associated with centrilobular hepatocyte hypertrophy and a transient hyperplasia. In addition, an increase in centrilobular epidermal growth factor receptor and its ligand, transforming growth factor alpha and a decrease in mannose-6-phosphate receptor and its mitoinhibitory ligand, TGFbeta1 was also observed immunohistochemically. The similar response in all three strains indicates that p53 probably plays no role in the early PB induced liver effects of hypertrophy and changes in growth factor expression.

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.

Polyploidization: a Janus-faced mechanism

Normal human somatic cells are diploid. But sometimes certain tissues of the human body contain elevated numbers of tetraploid cells. This tetraploid cell population seems to represent the first step of an ongoing process of polyploidization. All tissues containing tetraploid cells have in common the fact that they are subjected to stress, which is caused by a variety of circumstances like inflammation, elevated metabolism, ageing, repair processes or selection pressure. Tetraploid cells are supposed to play a beneficial role in these stress situations, because they are known to be more resistant in general and because they are characterized by an elevated biosynthetic activity. In contrast to their beneficial character, they have a big potential concerning the malignant development of a tissue: they play a crucial role in early morphological stages of the pathway hyperplasia-metaplasia-dysplasia-carcinoma. This report links several intracellular mechanisms with each other, which potentially determine the real fate of tetraploid cells.

Synergistic activation of p53-dependent transcription by two cooperating damage recognition pathways

High level activation of p53-dependent transcription occurs following cellular exposure to genotoxic damaging agents such as UV-C, while ionizing radiation damage does not induce a similarly potent induction of p53-dependent gene expression. Reasoning that one of the major differences between UV-C and ionizing radiation damage is that the latter does not inhibit general transcription, we attempted to reconstitute p53-dependent gene expression in ionizing irradiated cells by co-treatment with selected transcription inhibitors that alone do not activate p53. p53-dependent transcription can be dramatically enhanced by the treatment of ionizing irradiated cells with low doses of DRB, which on its own does not induce p53 activity. The mechanism of ionizing radiation-dependent activation of p53-dependent transcription using DRB is more likely due to inhibition of gene transcription rather than prolonged DNA damage, as the non-genotoxic and general transcription inhibitor Roscovitine also synergistically activates p53 function in ionizing irradiated cells. These results identify two distinct signal transduction pathways that cooperate to fully activate p53-dependent gene expression: one responding to lesions induced by ionizing radiation and the second being a kinase pathway that regulates general RNA Polymerase II activity.

The consequence of p53 overexpression for liver tumor development and the response of transformed murine hepatocytes to genotoxic agents

To analyse the effect of p53 on liver tumor development, we generated transgenic mice overexpressing wild-type p53 in the liver and crossed them with transgenic mice in which the expression of the SV40 large T antigen (TAg) induces hepatic tumors. Remarkably, whereas preneoplastic TAg liver exhibited anisocaryosis and anisocytosis, TAg/p53 liver never presented any dysplastic cells. Moreover, whereas expression of p53 did not affect hepatic development, its constitutive expression in tumorigenic livers resulted in a significantly enhanced apoptosis once nodules had appeared. In contrast, p53 overexpression did not modify the elevated proliferation of TAg-transformed hepatocytes and had no effect on hepatocarcinoma progression. In vitro analysis of primary hepatocytes exposed to various genotoxic agents showed that p53 failed to sensitize normal or TAg-transformed hepatocytes to apoptosis, except when high doses of doxorubicin, UV-B and UV-C radiation were used. Our results confirmed that the hepatocyte cell type is very resistant to genotoxic agents and showed that constitutive expression of p53 failed to improve their responsiveness. In addition, our results showed that suppression of dysplastic cells, probably by restoring normal cytokinesis and karyokinesis, and enhancement of apoptosis by means of p53 overexpression were insufficient to counteract or delay the TAg-induced liver tumoral progression. Oncogene (2000) 19, 3498 - 3507

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.

Metallothionein and apoptosis during differentiation of myoblasts to myotubes: protection against free radical toxicity

The changes in subcellular localization of metallothionein (MT) during differentiation were studied in two muscle cell lines, L6 and H9C2, myoblasts in order to understand the nuclear presence of MT and its antiapoptotic property. In myoblasts, MT and zinc were localized mainly in the cytoplasm but were translocated into the nucleus of newly formed myotubes during early stage of differentiation, which was initiated by lowering FBS from 10% to 1%. In fully differentiated myotubes, metallothionein content was decreased with a cytoplasmic localization. These changes in subcellular localization of MT and Zn were accompanied by increased apoptosis in myotubes. The changes in the apoptosis at different stages of differentiation were measured by both DNA ladder formation and TUNEL technique. The results also show that the apoptosis may be initiated by free radical generation and may be accompanied by p53 expression. The H9C2 cells contained high levels of MT, differentiated slowly, and had low incidence of apoptotic bodies compared to L6 cell line.

Centrosome multiplication accompanies a transient clustering of polyploid cells during tissue repair

Cells from different human wounds were analyzed concerning their degree of ploidy. The experiments showed an increased tetraploidization rate in well-healing wounds especially during inflammation and proliferation. Recent data described a polyploidization in different tissues, which is accompanied and maybe caused by the multiplication of the centrosome. We show here for the first time that cells from nonmalignant tissue, namely human wound cells, are characterized by an extensive centrosome multiplication. In an effort to identify a certain mechanism, by which the centrosome may act as a modulator of the cells' ploidy, we focused our interest on p53, whose interaction with the centrosome was recently described. Applying a wound model onto p53-wildtype (wt) and p53-knockout (ko) mice, we could show that polyploidization was not reversible in p53-ko mice during wound healing. The lack of p53, the centrosome multiplication, and the polyploidization therefore may contribute to the physiological process of tissue repair in physiologically "normal" tissue.

IRF-1 is an essential mediator in IFN-gamma-induced cell cycle arrest and apoptosis of primary cultured hepatocytes

IFN-gamma induces cell cycle arrest and p53-independent apoptosis in primary cultured hepatocytes. However, it is not yet understood what molecules regulate the mechanism. We report here that interferon regulatory factor 1 (IRF-1) is an essential molecule in these phenomena. Hepatocytes from IRF-1-deficient mice were completely resistant to IFN-gamma in apoptosis indicated by three different hallmarks such as LDH release, DNA fragmentation and the activation of caspase-3 family. Caspase-1 expression was little detected in hepatocytes, and constitutive and IFN-gamma-induced mRNA expression of Fas or caspase-3 did not change in between wild type and IRF-1-deficient hepatocytes. Expression of IFN-gamma-inducible caspase, caspase-11, did not change either. Thus, it is unlikely that these molecules directly regulate the mechanisms. Interestingly, IRF-1-deficient hepatocytes were also resistant to IFN-gamma-induced cell cycle arrest despite IFN-gamma-induced cell cycle arrest and apoptosis are regulated by independent pathways. Results by Northern blot analysis showed that IFN-gamma-induced but not constitutive p53 mRNA expression was regulated by IRF-1. In fact, IFN-gamma did not induce cell cycle arrest in p53-deficient hepatocytes. Taken together, IRF-1 mediates IFN-gamma signaling into primary hepatocytes for cell cycle arrest via p53 expression and for apoptosis.

Cleavage of poly(ADP-ribose) transferase during p53-independent apoptosis in rat liver after treatment with N-nitrosomorpholine and cyproterone acetate

The aim of this work was to study the role of the tumor suppressor p53 and of poly(ADP-ribose) transferase (pADPRT) in the control of hepatocyte apoptosis in two different in vivo models, i.e., during the process of tumor initiation by the genotoxin and cytotoxin N-nitrosomorpholine (NNM) and after withdrawal of the hepatomitogen cyproterone acetate (CPA). Treatment with NNM induces apoptosis followed by necrosis and regenerative DNA synthesis. At the first wave of apoptosis 12 h after NNM application, no p53 expression could be detected by immunohistochemical analysis and immunoblotting. However, 24 h after treatment, numerous p53-positive hepatocyte nuclei were detected, whereas hepatocytes in early and later stages of apoptosis were always negative. Simultaneously with the increased p53 levels, p21 protein was induced. This was accompanied by a block in replicative DNA synthesis, as detected by proliferating-cell nuclear antigen immunostaining. Concomitantly with the increase in apoptosis, dramatic degradation of the nuclear enzyme pADPRT was observed, as evidenced by immunoblotting and activity blotting. The decrease in pADPRT enzymatic activity observed 12 h after treatment coincided with the greatest extent of pADPRT cleavage. One prominent cleavage product was 64 kDa, suggesting that granzyme B was involved in pADPRT degradation. In the second in vivo model we used, i.e., withdrawal of treatment with the hepatomitogen CPA, apoptosis of excessive hepatocytes but no necrosis occurs. Again, no induction of p53 expression could be detected in the liver even at the maximum level of apoptosis, whereas a strong correlation between induction of apoptosis and cleavage of pADPRT to a 64-kDa fragment was observed. These results from whole-animal experiments strongly suggest that the induction of apoptosis in rat liver after genotoxic and cytotoxic damage and during regression of hyperplasia is driven by a p53-independent pathway but is accompanied by cleavage of pADPRT.

Alteration of G1 cell-cycle protein expression and induction of p53 but not p21/waf1 by the DNA-modifying carcinogen 2-acetylaminofluorene in growth-stimulated hepatocytes in vitro

2-Acetylaminofluorene (AAF) is a potent tumor promoter in rat liver carcinogenesis models. In the resistant hepatocyte model, AAF is combined with a growth stimulus for efficient promotion of preneoplastic lesions. The promoting property of AAF in this model is closely associated with mito-inhibition of normal hepatocytes, an effect to which initiated cells are resistant. How AAF induces growth arrest is not known, but genotoxic as well as non-genotoxic effects have been implicated. To elucidate the mechanisms of AAF-induced mito-inhibition, we studied the expression of the tumor suppressor protein p53 and the cyclin-dependent kinase (cdk) complexes mediating G1 progression and S-phase entry. Hepatocytes were isolated from male Fisher 344 rats fed either a control diet or a diet supplemented with 0.02% AAF for 1 wk and cultured in a defined serum-free medium containing epidermal growth factor, insulin, and dexamethasone. Thymidine labeling revealed a profound inhibition of DNA synthesis in AAF-exposed cells compared with control cells. The retinoblastoma protein did not become hyperphosphorylated in AAF-exposed cells. Thus, inhibition of G1 cyclin-cdk activity was implied as a cause of growth arrest. Indeed, G1 cell-cycle arrest was accompanied by reduced induction and nuclear accumulation of the cyclin D1-cdk4 complex and inhibited nuclear translocation of cdk2. Furthermore, the growth arrest was not mediated through p21/waf1 upregulation, although nuclear levels of p53 were increased. Thus, carcinogen-induced mito-inhibition may be effected by altered levels and localization of G1 cyclin-cdk complexes, independent of the upregulation of cdk inhibitory proteins.

Life and death in the genesis of the tumour cell

Most tumours arise because of an aberrant response of cells following exposure to chemicals deliberately ingested, for example cigarette smoke, or present as an environmental pollutant, for example dietary aflatoxin. Recent evidence has highlighted the importance of tumour suppressor genes and oncogenes in determining the response of a cell to potentially mutagenic or growth disrupting events. Many toxicants in vivo can cause apoptosis in a dose dependent manner. At low dose apoptosis is engaged, but with high exposure cells may undergo necrosis as cellular metabolism is catastrophically overwhelmed preventing the ordered set of events that constitute apoptosis from occurring. Mutations in genes that control deletion of potentially damaged cells result in overriding of death signals and may result in survival of a cell that otherwise should have been deleted. This gave rise to the concept of the 'undead' cell--the aberrant cell that has escaped normal growth controls taking the first step towards cancer. However, not all cell lineages respond to injury in the same ways, and even the same gene may have quite varied effects depending on the cellular and tissue environment.

Hepatitis B x protein inhibits p53-dependent DNA repair in primary mouse hepatocytes

The mechanisms by which the hepatitis B x protein (HBx) contributes to hepatocarcinogenesis remain unclear. However, interaction with the tumor suppressor gene p53 and inhibition of p53-dependent cellular functions, including nucleotide excision repair, could be central to this process. We studied the levels of global repair (removal of cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts) and transcription-coupled repair (removal of CPDs in both strands of the dihydrofolate reductase gene) in primary wild-type and p53-null mouse hepatocytes. We show that global repair of CPDs appears to be more efficient in mouse hepatocytes than in other commonly studied rodent cells and approaches the levels of human cells and that p53 is required for global genomic DNA repair of CPDs but not for transcription-coupled repair. We then investigated the effect of HBx expression on hepatocyte nucleotide excision repair. We demonstrate that HBx expression affects DNA repair in a p53-dependent manner. Transient HBx expression reduces global DNA repair in wild-type cells to the level of p53-null hepatocytes and has no effect on the repair of a transfected damaged plasmid. Therefore, in viral hepatitis, the hepatitis B virus could inhibit the p53-dependent component of global repair leading, over time, to accumulation of genetic defects and fostering carcinogenesis.