c-myc gene amplification during hepatocarcinogenesis by a choline-devoid diet

Dietary Plant Metabolites Induced Epigenetic Modification as a Novel Strategy for the Management of Prostate Cancer

Prostate cancer is a widespread malignancy among men, with a substantial global impact on morbidity and mortality. Despite advances in conventional therapies, the need for innovative and less toxic treatments remains a priority. Emerging evidence suggests that dietary plant metabolites possess epigenetic-modifying properties, making them attractive candidates for prostate cancer treatment. The present work reviews the epigenetic effects of dietary plant metabolites in the context of prostate cancer therapy. We first outline the key epigenetic mechanisms involved in prostate cancer pathogenesis, including histone modifications, DNA methylation, and miRNA or Long Noncoding RNA (lncRNA) dysregulation. Next, we delve into the vast array of dietary plant metabolites that have demonstrated promising anti-cancer effects through epigenetic regulation. Resveratrol, minerals, isothiocyanates, curcumin, tea polyphenols, soy isoflavones and phytoestrogens, garlic compounds, anthocyanins, lycopene, and indoles are among the most extensively studied compounds. These plant-derived bioactive compounds have been shown to influence DNA methylation patterns, histone modifications, and microRNA expression, thereby altering the gene expression allied with prostate cancer progression, cell proliferation, and apoptosis. We also explore preclinical and clinical studies investigating the efficacy of dietary plant metabolites as standalone treatments or in combination with traditional treatments for people with prostate cancer. The present work highlights the potential of dietary plant metabolites as epigenetic modulators to treat prostate cancer. Continued research in this field may pave the way for personalized and precision medicine approaches, moving us closer to the goal of improved prostate cancer management.

Genetically Engineered Mouse Models for Liver Cancer

Liver cancer is the fourth leading cause of cancer-related death globally, accounting for approximately 800,000 deaths annually. Hepatocellular carcinoma (HCC) is the most common type of liver cancer, comprising approximately 80% of cases. Murine models of HCC, such as chemically-induced models, xenograft models, and genetically engineered mouse (GEM) models, are valuable tools to reproduce human HCC biopathology and biochemistry. These models can be used to identify potential biomarkers, evaluate potential novel therapeutic drugs in pre-clinical trials, and develop molecular target therapies. Considering molecular target therapies, a novel approach has been developed to create genetically engineered murine models for HCC, employing hydrodynamics-based transfection (HT). The HT method, coupled with the Sleeping Beauty transposon system or the CRISPR/Cas9 genome editing tool, has been used to rapidly and cost-effectively produce a variety of HCC models containing diverse oncogenes or inactivated tumor suppressor genes. The versatility of these models is expected to broaden our knowledge of the genetic mechanisms underlying human hepatocarcinogenesis, allowing the study of premalignant and malignant liver lesions and the evaluation of new therapeutic strategies. Here, we review recent advances in GEM models of HCC with an emphasis on new technologies.

Dietary factors and epigenetic regulation for prostate cancer prevention

The role of epigenetic alterations in various human chronic diseases has gained increasing attention and has resulted in a paradigm shift in our understanding of disease susceptibility. In the field of cancer research, e.g., genetic abnormalities/mutations historically were viewed as primary underlying causes; however, epigenetic mechanisms that alter gene expression without affecting DNA sequence are now recognized as being of equal or greater importance for oncogenesis. Methylation of DNA, modification of histones, and interfering microRNA (miRNA) collectively represent a cadre of epigenetic elements dysregulated in cancer. Targeting the epigenome with compounds that modulate DNA methylation, histone marks, and miRNA profiles represents an evolving strategy for cancer chemoprevention, and these approaches are starting to show promise in human clinical trials. Essential micronutrients such as folate, vitamin B-12, selenium, and zinc as well as the dietary phytochemicals sulforaphane, tea polyphenols, curcumin, and allyl sulfur compounds are among a growing list of agents that affect epigenetic events as novel mechanisms of chemoprevention. To illustrate these concepts, the current review highlights the interactions among nutrients, epigenetics, and prostate cancer susceptibility. In particular, we focus on epigenetic dysregulation and the impact of specific nutrients and food components on DNA methylation and histone modifications that can alter gene expression and influence prostate cancer progression.

Ginkgo biloba extract induces gene expression changes in xenobiotics metabolism and the Myc-centered network

The use of herbal dietary supplements in the United States is rapidly growing, and it is crucial that the quality and safety of these preparations be ensured. To date, it is still a challenge to determine the mechanisms of toxicity induced by mixtures containing many chemical components, such as herbal dietary supplements. We previously proposed that analyses of the gene expression profiles using microarrays in the livers of rodents treated with herbal dietary supplements is a potentially practical approach for understanding the mechanism of toxicity. In this study, we utilized microarrays to analyze gene expression changes in the livers of male B6C3F1 mice administered Ginkgo biloba leaf extract (GBE) by gavage for 2 years, and to determine pathways and mechanisms associated with GBE treatments. Analysis of 31,802 genes revealed that there were 129, 289, and 2,011 genes significantly changed in the 200, 600, and 2,000 mg/kg treatment groups, respectively, when compared with control animals. Drug metabolizing genes were significantly altered in response to GBE treatments. Pathway and network analyses were applied to investigate the gene relationships, functional clustering, and mechanisms involved in GBE exposure. These analyses indicate alteration in the expression of genes coding for drug metabolizing enzymes, the NRF2-mediated oxidative stress response pathway, and the Myc gene-centered network named "cell cycle, cellular movement, and cancer" were found. These results indicate that Ginkgo biloba-related drug metabolizing enzymes may cause herb-drug interactions and contribute to hepatotoxicity. In addition, the outcomes of pathway and network analysis may be used to elucidate the toxic mechanisms of Ginkgo biloba.

Choline and betaine intake and risk of breast cancer among post-menopausal women

Background:

Choline and betaine, similar to folate, are nutrients involved in one-carbon metabolism and hypothesised to reduce breast cancer risk. No prospective study among post-menopausal women has examined choline and betaine intakes in relation to breast cancer risk.

Methods:

We examined the intake of choline and betaine and breast cancer risk among 74 584 post-menopausal women in the Nurses’ Health Study. Nutrient intake was assessed using a validated food-frequency questionnaire six times since 1984. During 20 years of follow-up from 1984 until 2004, we documented 3990 incident cases of invasive breast cancer.

Results:

Overall, choline (mean±s.d.; 326±61 mg per day) and betaine (104±33 mg per day) intake was not associated with a reduced risk of post-menopausal breast cancer. Participants in the highest quintile of intakes had multivariate relative risks of 1.10 (95% confidence interval (95% CI): 0.99–1.22; P-value, test for trend=0.14) for choline and 0.98 (95% CI: 0.89–1.09; P-value, test for trend=0.96) for betaine, compared with those in the lowest quintiles of intakes. The results were similar in breast cancer stratified by hormone receptor (oestrogen receptor/progesterone receptor) status. The association between choline intake and breast cancer risk did not differ appreciably by alcohol intake (non-drinker, <15 or 15+ g per day) or several other breast cancer risk factors, including family history of breast cancer, history of benign breast disease, body mass index, post-menopausal hormone use, and folate intake.

Conclusion:

We found no evidence that higher intakes of choline and betaine reduce risk of breast cancer among post-menopausal women.

Hepatotoxin-induced changes in the adult murine liver promote MYC-induced tumorigenesis

Background

Overexpression of the human c-MYC (MYC) oncogene is one of the most frequently implicated events in the pathogenesis of hepatocellular carcinoma (HCC). Previously, we have shown in a conditional transgenic mouse model that MYC overexpression is restrained from inducing mitotic cellular division and tumorigenesis in the adult liver; whereas, in marked contrast, MYC induces robust proliferation associated with the very rapid onset of tumorigenesis in embryonic and neonatal mice.

Methodology/Principal Findings

Here, we show that non-genotoxic hepatotoxins induce changes in the liver cellular context associated with increased cellular proliferation and enhanced tumorigenesis. Both 5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) and carbon tetrachloride (CCl₄) cooperate with MYC to greatly accelerate the onset of liver cancer in an adult host to less than 7 days versus a mean latency of onset of over 35 weeks for MYC alone. These hepatotoxin-enhanced liver tumors grossly and histologically resemble embryonic and neonatal liver tumors. Importantly, we found that MYC overexpression is only capable of inducing expression of the mitotic Cyclin B1 in embryonic/neonatal hosts or adult hosts that were treated with either carcinogen.

Conclusion/Significance

Our results suggest a model whereby oncogenes can remain latently activated, but exposure of the adult liver to hepatotoxins that promote hepatocyte proliferation can rapidly uncover their malignant potential.

Experimental models of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a common and deadly cancer whose pathogenesis is incompletely understood. Comparative genomic studies from human HCC samples have classified HCCs into different molecular subgroups; yet, the unifying feature of this tumor is its propensity to arise upon a background of inflammation and fibrosis. This review seeks to analyze the available experimental models in HCC research and to correlate data from human populations with them in order to consolidate our efforts to date, as it is increasingly clear that different models will be required to mimic different subclasses of the neoplasm. These models will be instrumental in the evaluation of compounds targeting specific molecular pathways in future preclinical studies.

Disturbance of DNA methylation patterns in the early phase of hepatocarcinogenesis induced by a choline-deficient L-amino acid-defined diet in rats

The authors investigated the DNA methylation patterns of the E-cadherin, Connexin 26 (Cx26), Rassf1a and c-fos genes in the early phase of rat hepatocarcinogenesis induced by a choline-deficient L-amino acid-defined (CDAA) diet. Six-week-old F344 male rats were continuously fed with the CDAA diet, and three animals were then killed at each of 4 and 8 days and 3 weeks. Genomic DNA was extracted from livers for assessment of methylation status in the 5' upstream regions of E-cadherin, Cx26, Rassf1a and c-fos genes by bisulfite sequencing, compared with normal livers. The livers of rats fed the CDAA diet for 4 and 8 days and 3 weeks were methylated in E-cadherin, Cx26 and Rassf1a genes, while normal livers were all unmethylated. In contrast, normal livers were highly methylated in c-fos gene. Although the livers at 4 days were weakly methylated, those at 8 days and 3 weeks were markedly unmethylated. Methylation patterns of CpG sites in E-cadherin, Cx26 and Rassf1a were sparse and the methylation was not associated with gene repression. These results indicate that gene-specific DNA methylation patterns were found in livers of rats after short-term feeding of the CDAA diet, suggesting gene-specific hypermethylation might be involved in the early phase of rat hepatocarcinogenesis induced by the CDAA diet.

Dietary choline and betaine and the risk of distal colorectal adenoma in women

BACKGROUND

Choline and betaine are involved in methyl-group metabolism as methyl-group donors; thus, like folate, another methyl-group donor, they may be associated with a reduced risk of colorectal adenomas. No epidemiologic study has examined the association of intake of these nutrients and colorectal adenoma risk.

METHODS

We investigated the relationship between intakes of choline and betaine and risk of colorectal adenoma in US women enrolled in the Nurses' Health Study. Dietary intake was measured by food-frequency questionnaires, and individual intakes of choline and betaine were calculated by multiplying the frequency of consumption of each food item by its choline and betaine content and summing the nutrient contributions of all foods. Logistic regression models were used to calculate adjusted odds ratios (as approximations for relative risks) and 95% confidence intervals (CIs) of colorectal adenoma. All statistical tests were two-sided.

RESULTS

Among 39246 women who were initially free of cancer or polyps and who had at least one endoscopy from 1984 through 2002, 2408 adenoma cases were documented. Increasing choline intake was associated with an elevated risk of colorectal adenoma; the multivariable relative risks (95% CIs) for increasing quintiles of intake, relative to the lowest quintile, were 1.03 (0.90 to 1.18), 1.01 (0.88 to 1.16), 1.23 (1.07 to 1.41), and 1.45 (1.27 to 1.67; P(trend)<.001). Betaine intake had a nonlinear inverse association with colorectal adenoma; the multivariable relative risks (95% CIs) for increasing quintiles of intake were 0.94 (0.83 to 1.07), 0.85 (0.75 to 0.97), 0.86 (0.75 to 0.98), and 0.90 (95% CI = 0.78 to 1.04; P(trend) = .09). Among individual sources of choline, choline from phosphatidylcholine and from sphingomyelin were each positively related to adenoma risk.

CONCLUSIONS

Our findings do not support an inverse association between choline intake and risk of colorectal adenoma. The positive association between choline intake and colorectal adenoma that we observed could represent effects of other components in the foods from which choline was derived and should be investigated further.

CpG site hypermethylation of E-cadherin and Connexin26 genes in hepatocellular carcinomas induced by a choline-deficient L-Amino Acid-defined diet in rats

We investigated DNA methylation patterns of E-cadherin and Connexin26 (Cx26) genes in rat hepatocellular carcinomas (HCCs) induced by a choline-deficient L-Amino Acid-defined (CDAA) diet. Six-wks-old F344 male rats were continuously fed with a CDAA diet for 75 wks, and were then killed. A total of five HCCs were obtained, and genomic DNA was extracted from each HCC for assessment of methylation status in the 5' upstream regions of E-cadherin and Cx26 genes by bisulfite sequencing, comparing to two normal liver tissues. The five HCCs showed highly methylated E-cadherin and Cx26 genes, while these genes in two normal liver tissues were all unmethylated. For analysis of gene expression, real-time quantitative reverse transcription (RT)-polymerase chain reaction (PCR) was performed. Expressions of E-cadherin and Cx26 genes were significantly reduced in the five HCCs (P < 0.0001 and P < 0.001, respectively) compared to normal liver tissues, correlating with their methylation statuses. These results suggested that hypermethylation of E-cadherin and Cx26 genes may be involved in the development of HCCs induced by a CDAA diet in rats.

JTE-522, a cyclooxygenase-2 inhibitor, is an effective chemopreventive agent against rat experimental liver fibrosis1

BACKGROUND & AIMS

The aim of this study was to assess the effects of cyclooxygenase (COX)-2 inhibition on rat experimental liver fibrogenesis.

METHODS

We investigated the inhibitory effects of a selective COX-2 inhibitor, JTE-522, on liver fibrosis induced by a choline-deficient, l-amino acid-defined diet (CDAA). Inhibitory effect was also tested in a second model of thioacetamide (TAA)-induced liver fibrosis.

RESULTS

CDAA induced liver fibrosis and preneoplastic foci at 12 weeks and cirrhosis at 36 weeks. Hepatocellular carcinoma was noted in 13 of 15 rats (87%). JTE-522 significantly inhibited fibrosis and development of preneoplastic lesions in a dose-dependent manner and completely inhibited generation of cirrhosis and hepatocellular carcinoma at both low and high doses (10 and 30 mg/kg body wt/day, respectively). JTE-522 administrated only from 12 weeks to 36 weeks also prevented cirrhosis and formation of hepatocellular carcinoma. JTE-522 itself did not cause local or systemic gross or histopathologic changes at 36 weeks. Mechanistic studies indicated that the CDAA model displayed up-regulation of several biomarkers, including COX-2, arachidonate metabolite (prostaglandin E(2)), serum aspartate aminotransferase, and c-myc expression. The model also showed an increased proportion of activated hepatic stellate cells, proliferating cell nuclear antigen index, and CD45-positive inflammatory cells in the liver. JTE-522 effectively diminished these changes. JTE-522 exhibited similar antifibrosis effects in the TAA model.

CONCLUSIONS

Our results suggest that COX-2 is involved in CDAA- and TAA-induced liver fibrosis. Our data also indicate that JTE-522 is a potent chemopreventive agent of rat liver fibrosis with low toxicity.

Cell cycle deregulation in liver lesions of rats with and without genetic predisposition to hepatocarcinogenesis

Preneoplastic and neoplastic hepatocytes undergo c-Myc up-regulation and overgrowth in rats genetically susceptible to hepatocarcinogenesis, but not in resistant rats. Because c-Myc regulates the pRb-E2F pathway, we evaluated cell cycle gene expression in neoplastic nodules and hepatocellular carcinomas (HCCs), induced by initiation/selection (IS) protocols 40 and 70 weeks after diethylnitrosamine treatment, in susceptible Fisher 344 (F344) rats, and resistant Wistar and Brown Norway (BN) rats. No interstrain differences in gene expression occurred in normal liver. Overexpression of c-myc, Cyclins D1, E, and A, and E2F1 genes, at messenger RNA (mRNA) and protein levels, rise in Cyclin D1-CDK4, Cyclin E-CDK2, and E2F1-DP1 complexes, and pRb hyperphosphorylation occurred in nodules and HCCs of F344 rats. Expression of Cdk4, Cdk2, p16(INK4A), and p27(KIP1) did not change. In nodules and/or HCCs of Wistar and BN rats, low or no increases in c-myc, Cyclins D1, E, and A, and E2F1 expression, and Cyclin-CDKs complex formation were associated with p16(INK4A) overexpression and pRb hypophosphorylation. In conclusion, these results suggest deregulation of G1 and S phases in liver lesions of susceptible rats and block of G1-S transition in lesions of resistant strains, which explains their low progression capacity.

Amplification of c-myc gene and overexpression of c-Myc protein in breast cancer and adjacent non-neoplastic tissue

BACKGROUND

Deregulated c-Myc expression and alterations of c-myc oncogene have been reported to play an important role in breast cancer tumorigenesis. We examined the relationship between c-Myc protein level, amplification of c-myc oncogene and commonly used clinical and pathologic factors.

METHODS

The studies were conducted on 94 ductal and lobular cancers. Amplification of c-Myc was assessed by the semiquantitative multiplex PCR assay. The amount of c-Myc protein was estimated by the densitometry analysis of Western blots.

RESULTS

Amplification of c-Myc was found in 21% of examined cancers. There was no association of c-myc amplification with established risk factors. Overexpression of c-Myc protein without c-myc amplification was associated with negative status of axillary lymph node. The size of lobular carcinoma displaying overexpression of c-Myc and the normal copy number of c-myc gene was significantly smaller than the size of tumor with elevated c-Myc and amplification of c-myc gene (p < 0.01). Within tumors displaying overexpression of c-Myc protein and c-myc gene amplification the size of ductal carcinoma was smaller than the size of lobular carcinoma (p < 0.007).

CONCLUSION

Data presented in this study suggest that alterations of c-myc gene and c-Myc protein level might be related to breast cancer progression. The prognostic utility of elevated level of c-Myc protein associated with normal status of c-myc gene for patients with lobular carcinoma requires further studies.

Enhanced expression of mRNAs of antisecretory factor-1, gp96, DAD1 and CDC34 in human hepatocellular carcinomas

To identify differentially expressed genes in hepatocarcinogenesis, we performed differential display analysis using surgically resected hepatocellular carcinoma (HCC) and adjacent non-tumorous liver tissues. We identified four cDNA fragments upregulated in HCC samples, encoding antisecretory factor-1 (AF), gp96, DAD1 and CDC34. Northern blot analysis demonstrated that these mRNAs were expressed preferentially in HCCs compared with adjacent non-tumorous liver tissues or normal liver tissues from non-HCC patients. The expression of these mRNAs was increased along with the histological grading of HCC tissues. These mRNA levels were also high in three human HCC cell lines (HuH-7, HepG2 and HLF), irrespective of the growth state. We also demonstrate that sodium butyrate, an inducer of differentiation, downregulated the expression of AF and gp96 mRNAs, supporting in part our pathological observation. Immunohistochemical analysis revealed that gp96 and CDC34 proteins were preferentially accumulated in cytoplasm and nuclei of HCC cells, respectively. Overexpression of these genes could be an important manifestation of HCC phenotypes and should provide clues to understand the molecular basis of hepatocellular carcinogenesis.

Apoptosis and the liver

Regulation of the homeostatic balance between cell proliferation and programmed cell death, apoptosis, is essential for development and maintenance of multicellular organisms. Apoptosis is a genetically and evolutionarily highly conserved process. Analysis of the molecular mechanisms of apoptosis has led to a better understanding of many human diseases. Notably in cancer, but also in infectious or autoimmune disease, a deficiency in apoptosis is one of the key events in pathophysiology. On the other hand, overefficient apoptosis, as observed in fulminant liver failure, may be equally harmful for the organism indicating that a tight regulation of the apoptotic machinery is essential for survival. The execution of apoptosis may be initiated by many different signals, either from within or outside the cell involving ligand-receptor interactions, as has been shown for Fas/Fas-ligand, TNF-alpha/TNF-receptor or TGF-beta/TGF-receptor, or potentially by more unspecific signals such as ceramide or DNA damage. During the modulation phase of apoptosis many different genes such as p53, c-myc or Bcl-2/Bax have been shown to able to shift the balance either to cell survival or cell death.

Endogenous liver carcinogenesis in the rat

Carcinogenesis may be effected not only through exposure to exogenous stimuli but also by genetic and epigenetic influences derived from endogenous factors. In the latter case, the mechanisms are still largely obscure because of the limited availability of appropriate in vivo experimental models. However, continuous feeding of a diet deficient in choline and methionine is well known to cause hepatocellular carcinomas (HCC) in rats in the absence of any known exogenous carcinogens and can serve as a good research model. A semi-synthetic, choline-deficient, L-amino acid-defined (CDAA) diet, containing practically no choline and low methionine, induces HCC with a background of fatty liver and hepatocyte death, subsequent regeneration and fibrosis resulting in cirrhosis. Using the CDAA diet, we have revealed the participation of oxidative injury to DNA and other subcellular components and of alteration in intrahepatic signal transduction pathways in the mechanisms underlying this rat liver carcinogenesis model. In the present paper, the current understanding of endogenous rat liver carcinogenesis, due to dietary choline deficiency, is reviewed.

Histopathology and gene expression changes in rat liver during feeding of fumonisin B1, a carcinogenic mycotoxin produced by Fusarium moniliforme

Fumonisin B1 (FB1) is a carcinogenic mycotoxin produced by the fungus Fusarium moniliforme in corn. Feeding of FB1 to rats causes acute liver injury, chronic liver injury progressing to cirrhosis, and sometimes terminates in hepatocellular carcinoma or cholangiocarcinoma. This study describes the histolopathology and changes in gene expression in the rat liver during short-term feeding of FB1. Male Fischer rats were fed either FB1 250 mg/kg or control diet, and were killed weekly for 5 weeks. FB1 caused a predominantly zone 3 'toxic' liver injury, with hepatocyte death due to necrosis and apoptosis. Hepatocyte injury and death were mirrored by hepatic stellate cell proliferation and marked fibrosis, with progressive disturbance of architecture and formation of regenerative nodules. Despite ongoing hepatocyte mitotic activity, oval cell proliferation was noted from week 2, glutathione S-transferase pi-positive hepatic foci and nodules developed and, at later time points, oval cells were noted inside some of the 'atypical' nodules. Northern blot (mRNA) analysis of liver specimens from weeks 3 to 5 showed a progressive increase in gene expression for alpha-fetoprotein, hepatocyte growth factor, transforming growth factor alpha (TGF-alpha) and especially TGF-beta1 and c-myc. Immunostaining with LC(1-30) antibody demonstrated a progressive increase in expression of mature TGF-beta1 protein by hepatocytes over the 5 week feeding period. The overexpression of TGF-beta1 may be causally related to the prominent apoptosis and fibrosis seen with FB1-induced liver injury. Increased expression of c-myc may be involved in the cancer promoting effects of FB1.

Hepatocyte growth factor-induced expression of ornithine decarboxylase, c-met, and c-myc is differently affected by protein kinase inhibitors in human hepatoma cells HepG2

Binding of hepatocyte growth factor (HGF) to its receptor Met induces autophosphorylation and activation of the tyrosine kinase activity. In HGF-treated HepG2 cells, we studied: (i) the expression patterns of early (c-myc, c-jun, and c-fos) and delayed-early (ornithine decarboxylase and c-met) response genes and (ii) the possible involvement of protein kinase transducers in the control of the expression of c-met and of other genes eventually induced downstream. c-met and c-myc mRNAs peaked 1-2 h after HGF, while c-jun and c-fos mRNAs slightly increased at 1 h. Ornithine decarboxylase activity was induced earlier (4 h) than the mRNA (8-10 h). The transducers involved in HGF-triggered gene inductions were investigated using different protein kinase inhibitors: genistein for the receptor tyrosine kinase, herbimycin A for the nonreceptor tyrosine kinase (pp60(c-src)), wortmannin for phosphatidylinositol 3-kinase (PI3K) and H7 for protein kinase C (PKC). The similarity of responses to PKC inhibition led to suppose that c-myc and ornithine decarboxylase mRNAs were induced sequentially along the same transduction pathway triggered by HGF. Ornithine decarboxylase activity seemed to be largely regulated by phosphorylation(s). The mRNA expression of c-jun was likely to undergo a negative regulation through a mechanism involving PI3K, while that of c-met seemed to be almost independent from various protein kinases (PI3K, pp60(c-src), and PKC).

Interaction of c-myc with transforming growth factor alpha and hepatocyte growth factor in hepatocarcinogenesis

Double transgenic mice bearing fusion genes consisting of mouse albumin enhancer/promoter-mouse c-myc cDNA and mouse metallothionein 1 promoter-human TGF-alpha cDNA were generated to investigate the interaction of these genes in hepatic oncogenesis and to provide a general paradigm for characterizing the interaction of nuclear oncogenes and growth factors in tumorigenesis. Coexpression of c-myc and TGF-alpha as transgenes in the mouse liver resulted in a tremendous acceleration of neoplastic development in this organ as compared to expression of either of these transgenes alone. The two distinct cellular reactions that occurred in the liver of the double transgenic mice prior to the appearance of liver tumors were dysplastic and apoptotic changes in the existing hepatocytes followed by emergence of multiple focal lesions composed of both hyperplastic and dysplastic cell populations. These observations suggest that the interaction of c-myc and TGF-alpha, during development of hepatic neoplasia contributes to the selection and expansion of the preneoplastic cell populations which consequently increases the probability of malignant conversion. These studies have now been extended to examine the interaction of hepatocyte growth factor (HGF) with c-myc during hepatocarcinogenesis in the transgenic mouse model. While sustained overexpression of c-myc in the liver leads to cancer, coexpression of HGF and c-myc in the liver delayed the appearance of preneoplastic lesions and prevented malignant conversion. Similarly, tumor promotion by phenobarbital was completely inhibited in the c-myc/HGF double transgenic mice whereas phenobarbital was an effective tumor promoter in the c-myc single transgenic mice. The results indicate that HGF may function as a tumor suppressor during early stages of liver carcinogenesis, and suggest the possibility of therapeutic application for this cytokine. Furthermore, we show for the first time that interaction of c-myc with HGF or TGF-alpha results in profoundly different outcomes of the neoplastic process in the liver.

Overexpression of prothymosin alpha, concomitant with c-myc, during rat hepatic carcinogenesis

By using a subtraction-enhanced display technique, we identified a cDNA clone representing alpha-prothymosin, from rat hepatocellular carcinoma (HCC). alpha-Prothymosin has been reported to be involved in cell proliferation and regulated by c-myc gene in vitro. In the present study, we investigated the gene expression pattern of alpha-prothymosin and analyzed its correlation with c-myc during rat hepatic carcinogenesis and liver regeneration. Hepatic alpha-prothymosin mRNA levels, concomitant with c-myc, were increased at the early stage of hepatic carcinogenesis (6 weeks), and remained nearly 10 fold higher as the tumor progressed. In comparison, alpha-prothymosin mRNA levels were only slightly and moderately increased at early (3-6 hr) and at later stage (24-30 hr) of liver regeneration after 70% partial hepatectomy. In situ hybridization revealed that overexpressed alpha-prothymosin mRNA was restricted to the tumor nodules and to tumor cells invading blood vessels. These data provide evidence that overexpression of alpha-prothymosin, concomitant with c-myc, is related to rat hepatic carcinogenesis.

Overexpression of hepatic prothymosin alpha, a novel marker for human hepatocellular carcinoma

Identification of gene products exclusively or abundantly expressed in cancer may yield novel tumour markers. We recently isolated a number of cDNA clones, including alpha-prothymosin, from rat hepatocellular carcinoma (HCC) using a subtraction-enhanced display technique. Alpha-Prothymosin is involved in cell proliferation and is regulated by the oncogene c-myc in vitro. In the present study, we analysed alpha-prothymosin gene expression and its correlation with c-myc in patients with HCC, cirrhosis and adenoma and in normal controls. Hepatic alpha-prothymosin messenger RNA (mRNA) levels were two- to 9.2-fold higher in tumoral tissues than in adjacent non-tumoral tissues in 14 of 17 patients with HCC, regardless of coexisting cirrhosis and viral hepatitis. No marked difference in alpha-prothymosin mRNA levels was present in patients with adenoma and hepatic cirrhosis and in healthy controls. The c-myc mRNA amounts were two- to fivefold increased in 11 of 17 patients with HCC and correlated significantly with those of alpha-prothymosin (P < 0.001). In situ hybridization revealed that increased alpha-prothymosin mRNA was localized in the tumour nodules of the patients with HCC. These data suggest that overexpression of alpha-prothymosin in HCC patients, correlated with c-myc, is possibly involved in the tumorigenic process and may be a novel molecular marker for human HCC.

Hepatic tumor induction in c-myc mono-transgenic and TGF-alpha/c-myc double-transgenic mice

Double transgenic mice bearing fusion genes consisting of mouse albumin enhancer/promoter-mouse c-myc cDNA and mouse metallothionein 1 promoter-human TGF-alpha cDNA were generated to investigate the interaction of these genes in hepatic oncogenesis and to provide a general paradigm for characterizing both the interaction of nuclear oncogenes and growth factors in tumorigenesis as well as to produce an experimental model to test how environmental chemicals might interact with these genes during the neoplastic process. Coexpression of c-myc and TGF-alpha as transgenes in the mouse liver resulted in a tremendous acceleration of neoplastic development in this organ as compared to expression of either of these transgenes alone. The two distinct cellular reactions that occurred in the liver of the double transgenic mice prior to the appearance of liver tumors were dysplastic and apoptotic changes in the existing hepatocytes followed by emergence of multiple focal lesions composed of both hyperplastic and dysplastic cell populations. These observations suggest that the interaction of c-myc and TGF-alpha, during development of hepatic neoplasia contributes to the selection and expansion of the preneoplastic cell populations which consequently increases the probability of malignant conversion. Treatment of the double transgenic mice with both genotoxic agents such as diethylnitrosamine and IQ as well as the tumor promoter phenobarbital greatly accelerated the neoplastic process. These results suggest that selective transgenic mouse models may provide important tools for testing both the carcinogenic potential of environmental chemicals and the interaction/cooperation of these compounds with specific genes during the neoplastic process.

c-myc amplification in pre-malignant and malignant lesions induced in rat liver by the resistant hepatocyte model

We have investigated by restriction fragment analysis genomic abnormalities involving the c-myc gene in DNA isolated from adenomas and hepatocellular carcinomas (HCCs). Adenomas and HCCs were induced by the "resistant hepatocyte" protocol in diethylnitrosamine-initiated male F344 rats. Southern-blot analysis of EcoRI-restricted DNA from normal liver, early and late adenomas, 12 weeks (EAs) and 30 weeks (LAs) after initiation, and HCCs, showed 2 bands of 18 and 3.2 kb hybridizing with c-myc, in all tissues. c-myc amplification occurred in almost all HCCs, and in the majority of EAs and LAs. These results were confirmed by dilution analysis. c-myc amplification was also seen in adenomas and HCCs by Southern analysis with HindIII-restricted DNA, and in HCCs by differential PCR. c-myc mRNA increase occurred in all adenomas and HCCs, but it was higher in the lesions showing gene amplification. Moreover, a 13-kb DNA extraband, hybridizing with c-myc, was found in the HindIII-restricted DNA from HCCs, but not in normal liver and adenomas, and a 7.1-kb extra band was present in EcoRI-digested DNA from one LA. EcoRI-restricted DNA from some adenomas exhibited a decrease in intensity of the 18-kb fragment, and an increase in intensity of the 3.2-kb fragment. No alteration in banding pattern occurred in the beta-actin gene in adenomas. These results provide evidence of amplification and some other rearrangements involving the c-myc gene, in pre-malignant and malignant liver lesions, induced by the RH protocol, and suggest a role of c-myc rearrangement in the progression of adenomas to malignancy.

Inhibitory effects of N,N'-diphenyl-p-phenylenediamine on the early stage of the enhanced hepatocarcinogenesis caused by coadministration of ethionine and a choline-deficient L-amino acid-defined diet in rats

Effects of N,N'-diphenyl-p-phenylenediamine (DPPD), an antioxidant, on liver carcinogenesis caused by a choline-deficient L-amino acid-defined (CDAA) diet containing ethionine were studied in Fischer 344 rats. Male animals, 6 weeks old, were fed a CDAA diet, a choline-supplemented L-amino acid-defined (CSAA) diet or a CDAA diet containing 0.05% ethionine with or without 0.2% DPPD. Histological changes and lesions positive for gamma-glutamyltransferase (GGT) were analyzed 12 weeks after the beginning of the experiment. The levels of 8-hydroxyguanine (8-OHGua) in DNA and 2-thiobarbituric acid-reacting substances (TBARS) were measured as the parameters for cellular oxidative damage after 4 and 11 days of treatment. Expression of c-myc and c-Ha-ras was also investigated in relation to cell proliferation after 2, 4, 8 and 11 days. Histologically, development of diffuse fatty liver observed in rats fed a CDAA diet was inhibited, while massive oval cell proliferation and cholangiofibrosis resulted from the addition of ethionine with/without DPPD. The sizes but not numbers of GGT-positive lesions seen in the liver of rats fed a CDAA diet were increased and the levels of 8-OHGua formation and TBARS generation were also increased by the ethionine supplement. Both numbers and sizes of GGT-positive lesions were decreased and the level of TBARS, but not 8-OHGua, was decreased by adding DPPD. The increased expression of c-myc and c-Ha-ras detected in the liver of rats fed a CDAA diet was further increased by addition of ethionine and again reduced by DPPD. These results indicate that an antioxidant DPPD can inhibit the early stage of enhanced hepatocarcinogenesis caused by coadministration of ethionine and a CDAA diet, by blocking cellular oxidative damage as well as c-myc and c-Ha-ras expression.

Hepatocyte proliferation induced by a single dose of a peroxisome proliferator

In compensatory hyperplasia after partial hepatectomy or liver cell injury, hepatocyte proliferation is triggered by coordinated actions of growth factor such as hepatocyte growth factor and transforming growth factor-alpha and -beta. Initiation of hepatocyte DNA synthesis is preceded by the activation of the set of early growth response genes mediated by enhanced nuclear factor-kappa B binding to DNA. Using an experimental model to induce hepatocyte DNA synthesis in vivo by a single dose of a peroxisome proliferator, which does not induce liver cell necrosis (direct hyperplasia), we investigated whether peroxisome proliferator-induced hepatocyte proliferation involved an induction of known growth factors, an activation of early growth response genes, and nuclear factor-kappa B. A single intragastric administration of 250 mg/kg BR931 (4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio-(N-beta-hydroxyethyl) acetamide) to male wistar rats induced a wave of hepatocyte DNA synthesis starting after 12 hours and peaking at approximately 24 to 36 hours. The response was dose dependent. The treatment also induced the expression of the mRNA for the peroxisomal bifunctional enzyme, one of the peroxisome-related fatty acid beta-oxidation enzymes. Pretreatment of rats with dexamethasone (2 mg/kg) inhibited both hepatocyte DNA synthesis and the induction of the peroxisomal bifunctional enzyme gene. Northern blot analyses of liver RNA during a period preceding the onset of DNA synthesis revealed no induction of hepatocyte growth factor, transforming growth factor-alpha, or tumor necrosis factor-alpha mRNAs. No induction of early growth response genes, liver regeneration factor-1, or c-myc was detected. Furthermore, gel mobility shift assays showed no enhanced nuclear factor-kappa B binding to its DNA consensus sequence after BR931 treatment, whereas control studies demonstrated a distinct increase in binding after partial hepatectomy or lead nitrate treatment. The results suggest that peroxisome-proliferator-induced hepatocyte proliferation may be triggered by signal transduction pathways different from those after partial hepatectomy and that the binding of peroxisome proliferators to their nuclear receptors may play a role in stimulation of DNA synthesis and peroxisome proliferation.

Prevention by methionine of enhancement of hepatocarcinogenesis by coadministration of a choline-deficient L-amino acid-defined diet and ethionine in rats

The effects of methionine on hepatocarcinogenesis induced by coadministration of a choline-deficient L-amino acid-defined (CDAA) diet and ethionine were examined. F344 male rats were divided into 4 experimental groups. Groups 1 and 2 received the CDAA diet and a choline-supplemented L-amino acid-defined (CSAA)++ diet, respectively. Group 3 received the CDAA diet containing 0.05% ethionine, and group 4 the CDAA diet containing 0.05% ethionine and 0.47% methionine. Animals were killed after 12 weeks of treatment. Histologically, the CDAA diet induced intracellular fat accumulation and foci. In contrast, ethionine caused not only foci, but also hyperplastic nodules, cholangiofibrosis and the proliferation of oval cells without such fat accumulation. Methionine abolished the development of all of the liver lesions induced by coadministration of the CDAA diet and ethionine. To investigate the effects of methionine on induction of c-myc and c-Ha-ras expression, as well as generation of 8-hydroxyguanine (8-OHGua) and 2-thiobarbituric acid-reacting substances (TBARS), by coadministration of the CDAA diet and ethionine, subgroups of 3 to 5 animals were killed at 2, 4, 8, or 11 days after the beginning of the experiment. Coadministration of the CDAA diet and ethionine markedly enhanced the level of expression of c-myc and c-Ha-ras, 8-OHGua formation and TBARS generation as compared with the CDAA or CSAA diet within 11 days, and methionine blocks these actions. These results indicate that addition of methionine prevents the induction of c-myc and c-Ha-ras expression, 8-OHGua formation and TBARS generation, as well as hepatocellular lesions, by coadministration of the CDAA diet and ethionine in rats, and suggest a possible involvement of oxidative stress and gene expression in hepatocarcinogenesis by these agents.

Expression of c-myc in altered hepatic foci induced in rats by various single doses of diethylnitrosamine and promotion by 0.05% phenobarbital

Among the proto-oncogenes examined by northern blot analysis, c-myc, c-Ha-ras, c-fos, and c-raf-1 have been reported to be activated in rat liver cell carcinomas. However, there are relatively few reports on protooncogene expression in altered hepatic foci (AHF) early during hepatocarcinogenesis in the rat. In this study, diethylnitrosamine (DEN) at doses ranging from 10 to 200 mg/kg was used to initiate and phenobarbital (0.05%) to promote AHF in rats. AHF were detected by the presence of the marker enzymes glutathione s-transferase, placental form (GST-P); gamma-glutamyltranspeptidase (GGT); glucose-6-phosphatase (G6Pase); and canalicular adenosine triphosphatase (ATPase). Proto-oncogene expression in individual AHF was investigated by in situ hybridization (ISH). ISH for the mRNAs of c-Ha-ras, c-fos, and c-raf-1 revealed little or no expression in AHF. However, the levels of c-myc mRNA were increased in about 10% of the AHF initiated by the highest dose of DEN (200 mg/kg). Thus, altered expression of proto-oncogenes was not seen in AHF initiated by nonnecrogenic doses of DEN and promoted by phenobarbital. However, at the necrogenic dose of 200 mg/kg DEN, c-myc expression was found mostly in AHF in which abnormal expression of GST-P, GGT, G6Pase, and ATPase was also present, indicating that c-myc expression is correlated with phenotypically greater complexity of the AHF, a characteristic of malignant hepatic neoplasms in the rat.

New prolyl 4-hydroxylase inhibitor reduces procollagen gene expression and enzyme-altered lesions in rat liver cirrhosis

A choline deficient L-amino acid defined (CDAA) diet led to the development of liver cirrhosis in male Wistar rats after 16 weeks. A new prolyl 4-hydroxylase inhibitor, 2,4-pyridine dicarboxylic acid bis [(2-methoxyethyl amide)] (HOE 077), prevented liver fibrosis in a dose-dependent manner without a reduction in increased serum alanine aminotransferase and aspartate aminotransferase in parallel with a reduction in preneoplastic enzyme-altered lesions stained with anti-glutathione S-transferase placental form antibody. HOE 077 reduced the increase in serum procollagen III peptide (PIIIP) in a dose-dependent manner and in proportion to the reduction in mRNA expression of type III procollagen in the liver of rats fed a CDAA diet.

Expression of the liver-enriched transcription factors C/EBP alpha, C/EBP beta, HNF-1, and HNF-4 in preneoplastic nodules and hepatocellular carcinoma in rat liver

The expression patterns of the liver-enriched transcription factors CCAAT/enhancer-binding protein (C/EBP) alpha and beta and hepatocyte nuclear factor (HNF)-1 and HNF-4 were studied in liver nodules and hepatocellular carcinomas from male rats treated according to the resistant hepatocyte (RH) model. C/EBP alpha expression was lower at the transcriptional, mRNA, and protein levels in persistent nodules than in the respective surrounding livers. Expression was further decreased in the tumors. Transcriptional downregulation of C/EBP alpha gene expression was observed already in very early nodules, isolated 3 wk after partial hepatectomy in the RH model. However, no detectable changes were observed in preneoplastic nodules in the transcription or in steady-state mRNA levels of C/EBP beta, HNF-1, and HNF-4. A slight decrease in C/EBP beta protein and a more pronounced attenuation of HNF-1 and HNF-4 levels was observed in nodules, being 67%, 37%, and 46% of the levels in the corresponding surrounding livers, respectively. In conclusion, differential regulation of several transcription factors that are associated with the maintenance of the differentiated state of the hepatocytes was observed in preneoplastic and neoplastic liver lesions. This could have an impact on the regulation of a wide array of genes during liver carcinogenesis. Furthermore, the attenuation of C/EBP alpha expression, regarded as a negative growth regulator, could contribute to the proliferative advantage of nodules during liver carcinogenesis.

The molecular pathogenesis of hepatocellular carcinoma

Some of the multiple factors involved in the molecular pathogenesis of hepatocellular carcinoma have been elucidated in recent years but no clear picture of how and in what sequence these factors interact at the molecular level has emerged yet. Transformation of hepatocytes to the malignant phenotype may occur irrespective of the aetiological agent through a pathway of chronic liver injury, regeneration and cirrhosis. The activation of cellular oncogenes, the inactivation of tumour suppressor genes and overexpression of certain growth factors contribute to the development of HCC. There is increasing evidence that the hepatitis B virus may play a direct role in the molecular pathogenesis of HCC. Aflatoxins have been shown to induce specific mutations of the p53 tumour suppressor gene thus providing a clue to how an environmental factor may contribute to tumour development at the molecular level.

Modulation of expression of fos and Ha-ras oncogenes and ornithine decarboxylase activity in mammary gland and liver of young female rats by the absence of dietary lipotropes

There is evidence that diets deficient in lipotropes [methionine, choline, pteroylmonoglutamic acid (folic acid), and cyanocobalamin (vitamin B12)] induce and enhance hepatocarcinogenesis. This research examined the extent to which dietary lipotropes modify cellular oncogene expression and ornithine decarboxylase activity in mammary gland and liver of rats. Eighteen female Sprague-Dawley rats (8 wk old) were fed 3 wk on one of three diets: 1) a control synthetic diet; 2) a methyl-deficient diet lacking choline, methionine, pteroylmonoglutamic acid, and cyanocobalamin; or 3) a diet supplemented with twice the amount of each lipotrope as in the control synthetic diet. The group fed the methyl-deficient diet gained less body weight than groups fed the control or methyl-supplemented diet. The group fed the methyl-deficient diet had approximately 5- and 11-fold greater fos transcription in mammary gland and liver, respectively, than did the control group. The expression of the Ha-ras gene in mammary gland and liver of the group fed the methyl-deficient diet was increased by 4- and 6-fold compared with that of the control. Ornithine decarboxylase activity, considered to be a developmental marker, was higher in liver and mammary gland of the group fed the methyl-deficient diet than in either the group fed control synthetic diet or the group fed the methyl-supplemented diet. The methyl-deficient diet may have caused activation of the transcription factor fos and thus the activation of the transcription regulatory complex, AP-1. In turn, AP-1 may regulate genes, such as ornithine decarboxylase, which are responsible for cell proliferation and differentiation.

Different roles of 8-hydroxyguanine formation and 2-thiobarbituric acid-reacting substance generation in the early phase of liver carcinogenesis induced by a choline-deficient, L-amino acid-defined diet in rats

The present study was performed to assess the roles of hepatocellular oxidative damage to DNA and constituents other than DNA in rat liver carcinogenesis caused by a choline-deficient, L-amino acid-defined (CDAA) diet by examining the effects of the antioxidant N,N'-diphenyl-p-phenylenediamine (DPPD). The parameters used for cellular oxidative damage were the level of 8-hydroxy-guanine (8-OHGua) for DNA and that of 2-thiobarbituric acid-reacting substance (TBARS) for constituents other than DNA. A total of 40 male Fischer 344 rats, 6 weeks old, were fed the CDAA diet for 12 weeks with or without DPPD (0.05, 0.10 or 0.20%) or butylated hydroxytoluene (BHT, 0.25%). In the livers of the rats, the numbers and sizes of glutathione S-transferase (EC 2.5.1.18) placental form (GSTP)- and/or gamma-glutamyltransferase (GGT, EC 2.3.2.2)-positive lesions and levels of 8-OHGua and TBARS were determined. The GSTP-positive lesions of 0.08 mm2 or larger were all stained positively for GGT as well in cross-sectional area, whereas the smaller lesions were generally negative for GGT. DPPD and BHT reduced the size of the GSTP-positive lesions without affecting their total numbers. At the same time, they reduced TBARS generation without affecting 8-OHGua formation in DNA. The present results indicate that oxidative DNA damage (represented by 8-OHGua formation) and damage to constituents other than DNA (represented by TBARS generation) may play different roles in rat liver carcinogenesis caused by the CDAA diet; the former appears to be involved in the induction of phenotypically altered hepatocyte populations while the latter may be related to the growth of such populations.

Current pathogenetic and molecular concepts in viral liver carcinogenesis

Hepatocellular carcinoma (HCC) is one of the most frequent malignancies in humans and in most cases a consequence of chronic infection of the liver by hepatotropic viruses (Hepatitis B Virus (HBV) and possibly Hepatitis C Virus (HCV)). Formation of HCC results from a stepwise process involving different preneoplastic lesions that reflect multiple genetic events, like protooncogene activation, tumor suppressor gene inactivation, and growth factor over- or reexpression. Recent investigations have gained new insights into how these factors are activated and may interact. In addition, improved knowledge of the molecular biology of HBV has led to better understanding of its pleiotropic effects on induction and progression in hepatocarcinogenesis.

Differential activation of myc gene family members in hepatic carcinogenesis by closely related hepatitis B viruses

Woodchucks infected with woodchuck hepatitis virus (WHV) and ground squirrels infected with ground squirrel hepatitis virus (GSHV) both develop hepatocellular carcinoma (HCC), but WHV-associated tumors arise more frequently and much earlier in life. These differences are preserved when the oncogenic potentials of the two viruses are examined in the same host (woodchucks). We examined RNA and genomic DNA from tumors arising from WHV- and GSHV-infected woodchucks to determine whether these viruses use the same oncogenic pathway. N-myc RNA was not expressed in normal liver but was expressed in 10 of 13 WHV-associated HCCs examined. Southern blot analysis showed that 7 of 17 WHV-induced tumors (41%) contained rearrangements at N-myc loci due to viral genomic integration. Six of these seven inserts affected N-myc2, and most of these were at the 5' end of the gene. In contrast, only two of seven GSHV-induced woodchuck HCCs expressed N-myc RNA, and only 1 of the 16 tumors (6%) contained a rearranged N-myc allele. The GSHV-associated HCCs all contained numerous viral insertions, so the low frequency of integration into N-myc loci by GSHV was not due to a general block to integration. Four of sixteen GSHV-induced tumors harbored amplified c-myc alleles, and five of seven GSHV tumors tested contained elevated c-myc RNA levels. By contrast, enhanced c-myc RNA levels were observed in only 2 of 13 WHV-induced HCC. We conclude that N-myc overexpression is a regular feature of WHV- but not GSHV-associated hepatocarcinogenesis in a common host. In contrast, c-myc transcriptional deregulation is rarely encountered in WHV-induced HCC but is frequent in GSHV-induced HCC.

Low frequency of retinoblastoma gene alterations in rat hepatocellular carcinomas

Abnormalities of the retinoblastoma (Rb) gene have been reported in some human cancers, including hepatocellular carcinomas (HCCs). We examined by Southern blotting the status of the Rb gene in HCCs induced in rats in four experimental models. A low frequency of Rb gene alterations, detected as novel hybridizing bands unique to each tumor, was observed. Expression of the Rb protein product was examined in the HCCs and in seven established rat hepatoma cell lines studied. It appears, therefore, that alterations in the structure or expression of the Rb gene do occur but probably do not contribute in a major way to hepatocarcinogenesis in the rat.

Differential activation of myc gene family members in hepatic carcinogenesis by closely related hepatitis B viruses

Woodchucks infected with woodchuck hepatitis virus (WHV) and ground squirrels infected with ground squirrel hepatitis virus (GSHV) both develop hepatocellular carcinoma (HCC), but WHV-associated tumors arise more frequently and much earlier in life. These differences are preserved when the oncogenic potentials of the two viruses are examined in the same host (woodchucks). We examined RNA and genomic DNA from tumors arising from WHV- and GSHV-infected woodchucks to determine whether these viruses use the same oncogenic pathway. N-myc RNA was not expressed in normal liver but was expressed in 10 of 13 WHV-associated HCCs examined. Southern blot analysis showed that 7 of 17 WHV-induced tumors (41%) contained rearrangements at N-myc loci due to viral genomic integration. Six of these seven inserts affected N-myc2, and most of these were at the 5' end of the gene. In contrast, only two of seven GSHV-induced woodchuck HCCs expressed N-myc RNA, and only 1 of the 16 tumors (6%) contained a rearranged N-myc allele. The GSHV-associated HCCs all contained numerous viral insertions, so the low frequency of integration into N-myc loci by GSHV was not due to a general block to integration. Four of sixteen GSHV-induced tumors harbored amplified c-myc alleles, and five of seven GSHV tumors tested contained elevated c-myc RNA levels. By contrast, enhanced c-myc RNA levels were observed in only 2 of 13 WHV-induced HCC. We conclude that N-myc overexpression is a regular feature of WHV- but not GSHV-associated hepatocarcinogenesis in a common host. In contrast, c-myc transcriptional deregulation is rarely encountered in WHV-induced HCC but is frequent in GSHV-induced HCC.

Distribution of glucose-6-phosphatase activity in normal, hyperplastic, and preneoplastic rat liver

The significance of glucose-6-phosphatase (G6P) expression by bile duct-like cells proliferating during hepatocarcinogenesis in the histogenesis of hepatocellular carcinoma is not clear. To this end, we measured the histochemical and biochemical activity of G6P in normal rat liver, and in rat livers in which bile duct-like proliferation was induced by either hyperplastic (bile duct ligation for 14 days or feeding alpha-naphthylisothiocyanate for 28 days) or neoplastic (feeding a choline-devoid diet containing 0.1% ethionine for 60 days) regimens. In normal, hyperplastic, and preneoplastic livers, G6P histochemical activity was confined to the hepatocytes; proliferated bile duct-like cells, like normal bile ducts, did not display visible G6P staining. When the enzyme activity was determined biochemically, however, hydrolysis of glucose-6-phosphate was observed in both parenchymal and nonparenchymal liver cells isolated from all experimental animals. In elutriated nonparenchymal fractions, G6P activity was directly proportional to the number of cells positive for gamma-glutamyl transpeptidase and cytokeratin no. 19 (markers of bile duct cells) and inversely proportional to the number of cells positive for vimentin (marker of mesenchymal cells). These results indicate that, while by light microscopy hepatic G6P histochemical activity is detectable only in the hepatocytes, the biochemical activity is also expressed in proliferating bile duct-like cells. However, the nonparenchymal activity is observed during both neoplastic and hyperplastic liver growth, thus indicating that the presence of this enzyme in bile duct-like cells proliferating during hepatocarcinogenesis should not necessarily be construed as supporting their stem cell nature nor their neoplastic commitment.

Choline: an important nutrient in brain development, liver function and carcinogenesis

Choline is required to make certain phospholipids which are essential components of all membranes. It is a precursor for biosynthesis of the neurotransmitter acetylcholine and also is an important source of labile methyl groups. Much attention has been given to the effect of supplemental choline upon brain function, i.e., enhancement of acetylcholine synthesis and release. In addition, choline supplements administered to rats in utero or shortly after birth permanently after brain function. The mechanisms for this effect is unknown and under investigation at this time. Healthy humans fed diets deficient in choline, and humans fed parenterally have decreased plasma choline concentrations and develop liver dysfunction that is similar to that seen in choline-deficient animals. In experimental animals, fatty liver occurs in choline deficiency because phosphatidylcholine synthesis is required for very low-density lipoprotein secretion. This accumulation of lipids in liver may explain why choline-deficient rats spontaneously develop hepatocarcinoma. We found that choline deficiency was associated with the accumulation of 1,2-diacylglycerol, an activator of protein kinase C. Several lines of evidence indicate that cancers might develop secondary to abnormalities in protein kinase C-mediated signal transduction.

Distribution of albumin and alpha-fetoprotein mRNAs in normal, hyperplastic, and preneoplastic rat liver

The nature of bile duct-like (oval) cells proliferating during chemical hepatocarcinogenesis has been controversial. To investigate this issue further, the authors compared the hepatic distribution of albumin (ALB) and alpha-fetoprotein (AFP) mRNAs in rats in which oval cell proliferation was induced by feeding a choline-devoid diet containing 0.1% ethionine (CDE, a hepatocarcinogenic diet) with that in normal rats and in rats in which biliary epithelial cell hyperplasia was induced by either bile duct ligation or feeding alpha-naphthylisothiocyanate (ANIT). Northern blot analysis in parenchymal and nonparenchymal liver cells isolated from these animals demonstrated that ALB mRNA was present in the hepatocytes of both control and experimental animals, whereas this transcript was detected in nonparenchymal epithelial cells only in CDE-fed rats. Alpha-fetoprotein mRNA was not seen in either parenchymal or nonparenchymal cells isolated from normal or hyperplastic livers induced by bile duct ligation or ANIT feeding. In CDE-fed rats, however, both parenchymal and nonparenchymal cell populations displayed AFP message. In situ hybridization directly demonstrated nonparenchymal cell expression of both ALB and AFP transcripts in CDE-fed rats. Most surprisingly, ALB and AFP mRNAs were also detected by in situ hybridization in occasional nonparenchymal cells located in portal tracts near the limiting plate in normal liver, as well as under conditions associated with bile duct hyperplasia. Immunohistochemical studies of intermediate filament proteins, cytokeratin 19 (a marker of glandular epithelia), vimentin (a marker of mesenchymal lineage), and desmin (a marker of muscle cell differentiation) demonstrated that oval cells, as well as normal and hyperplastic bile duct cells, were positive for cytokeratin 19 and negative for both vimentin and desmin. Cytokeratin-positive oval cells formed duct profiles and were connected to preexisting ductules and ducts. These results are construed to suggest that oval cells proliferating during CDE hepatocarcinogenesis are derived from epithelial cells within the biliary tree. The presence of cells with similar morphologic appearance, periportal location, and AFP and ALB expression in normal liver suggests that these cells may be the progenitors of oval cells induced by some carcinogenic regimens.

Frequent amplification of c-myc in ground squirrel liver tumors associated with past or ongoing infection with a hepadnavirus

Persistent infection with hepatitis B virus (HBV) is a major cause of hepatocellular carcinoma (HCC) in humans. HCC has also been observed in animals chronically infected with two other hepadnaviruses: ground squirrel hepatitis virus (GSHV) and woodchuck hepatitis virus (WHV). A distinctive feature of WHV is the early onset of woodchuck tumors, which may be correlated with a direct role of the virus as an insertional mutagen of myc genes: c-myc, N-myc, and predominantly the woodchuck N-myc2 retroposon. In the present study, we searched for integrated GSHV DNA and genetic alterations of myc genes in ground squirrel HCCs. Viral integration into host DNA was detected in only 3/14 squirrel tumors and did not result in insertional activation of myc genes, despite the presence of a squirrel locus homologous to the woodchuck N-myc2 gene. This suggests that GSHV may differ from WHV in its reduced ability to induce mutagenic integration events. However, the high frequency of c-myc amplification (6/14) observed in ground squirrel HCCs indicates that myc genes might be preferential effectors in the tumorigenic processes associated with rodent hepadnaviruses, a feature not reported so far in HBV-induced carcinogenesis. Together with previous observations, our results suggest that hepadnaviruses, despite close genetic and biological properties, may use different pathways in the genesis of liver cancer.