The chemistry and biology of aflatoxin B(1): from mutational spectrometry to carcinogenesis

Binding of aflatoxins to the 20S proteasome: effects on enzyme functionality and implications for oxidative stress and apoptosis

Aflatoxins (AF) are contaminants of improperly stored foods; they are potent genotoxic and carcinogenic compounds, exerting their effects through damage to DNA. They can also induce mutations that increase oxidative damage. The goal of this study was to evaluate the possibility that a third mechanism could be involved in the carcinogenic action of aflatoxins, namely, direct binding to key enzymes involved in the regulatory pathways of the cell cycle, thereby modulating enzyme functionality. The 20S constitutive and immunoproteasome peptidase and proteolytic activities were assayed in the presence of aflatoxins B1, G1 and M1. All three toxins activated multiple peptidase activities of the proteasome. Aflatoxin (AF) M1 was the most potent activator of proteasome activity, while the constitutive 20S proteasome was specifically stimulated by AFG1. Furthermore, the effects of AFB1 on cultured hepatoma cells were investigated and the various proteasomal activities determined with cell lysates were differently affected. Taking into account the key role of the proteasome in cellular defense against oxidative stress, the carbonyl group content and the activities of antioxidant enzymes in cell lysates were analyzed. The proapoptotic effect of AFB1 was also investigated by measuring caspase-3 activity and cellular levels of p27 and IkappaBalpha.

Depurinating acylfulvene-DNA adducts: characterizing cellular chemical reactions of a selective antitumor agent

Acylfulvenes (AFs) are a class of semisynthetic agents with high toxicity toward certain tumor cells, and for one analogue, hydroxymethylacylfulvene (HMAF), clinical trials are in progress. DNA alkylation by AFs, mediated by bioreductive activation, is believed to contribute to cytotoxicity, but the structures and chemical properties of corresponding DNA adducts are unknown. This study provides the first structural characterization of AF-specific DNA adducts. In the presence of a reductive enzyme, alkenal/one oxidoreductase (AOR), AF selectively alkylates dAdo and dGuo in reactions with a monomeric nucleoside, as well as in reactions with naked or cellular DNA, with 3-alkyl-dAdo as the apparently most abundant AF-DNA adduct. Characterization of this adduct was facilitated by independent chemical synthesis of the corresponding 3-alkyl-Ade adduct. In addition, in naked or cellular DNA, evidence was obtained for the formation of an additional type of adduct resulting from direct conjugate addition of Ade to AF followed by hydrolytic cyclopropane ring-opening, indicating the potential for a competing reaction pathway involving direct DNA alkylation. The major AF-dAdo and AF-dGuo adducts are unstable under physiologically relevant conditions and depurinate to release an alkylated nucleobase in a process that has a half-life of 8.5 h for 3-alkyladenine and less than approximately 2 h for dGuo adducts. DNA alkylation further leads to single-stranded DNA cleavage, occurring exclusively at dGuo and dAdo sites, in a nonsequence-specific manner. In AF-treated cells that were transfected with either AOR or control vectors, the DNA adducts identified match those from in vitro studies. Moreover, a positive correlation was observed between DNA adduct levels and cell sensitivity to AF. The potential contributing roles of AOR-mediated bioactivation and adduct stability to the cytotoxicity of AF are discussed.

Molecular pathogenesis of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most common life-threatening malignancies in the world. This cancer generally arises within the boundaries of well-defined causal factors, of which viral hepatitis infection, aflatoxin exposure, chronic alcohol abuse, and nonalcoholic steatohepatitis are the major risk factors. Despite the identification of these etiological agents, hepatocarcinogenesis remains poorly understood. The molecular mechanisms leading to the development of HCC appear extremely complex and only recently have begun to be elucidated. Currently, surgical resection or liver transplantation offer the best chance of cure for the patient with HCC; however, these therapies are hindered by inability of many of these patients to undergo liver resection, by tumor recurrence and by donor shortages. A lack of suitable therapeutic strategies has led to a greater focus on prevention of HCC using antiviral agents and vaccination. Overall, the current outlook for patients with HCC is bleak; however, a better understanding of the molecular and genetic basis of this cancer should lead to the development of more efficacious therapies.

Aflatoxin B1 and polycyclic aromatic hydrocarbon adducts, p53 mutations and p16 methylation in liver tissue and plasma of hepatocellular carcinoma patients

Elevated aflatoxin B(1)-albumin adducts (AFB(1)-Alb) have been associated with an increased risk for HCC development. However, there are no studies in humans, correlating albumin adducts in blood with liver DNA adducts. Forty frozen tumor tissues and 39 paired plasma samples from HCC patients were collected in Taiwan, to determine the relationship between albumin adducts in blood and DNA adducts in liver tissue as well as mutations in p53 and methylation of p16. AFB(1)- and polycyclic aromatic hydrocarbon (PAH)-DNA adducts in tissue and albumin adducts in plasma were determined by immunohistochemistry and competitive ELISA, respectively. Plasma AFB(1)-Alb adducts in subjects with low, medium and high levels of AFB(1)-DNA adducts in tumor tissues were 51.0 +/- 36.5, 70.5 +/- 48.1 and 84.9 +/- 48.2 fmol/mg, respectively (p(trend) = 0.05). No significant correlation was found for PAH. Fourteen of 40 (36%) tissues were positive for mutant p53 protein by immunohistochemistry; 11 of 40 tissue DNA samples (28%) were positive for p53 mutations, but not their corresponding plasma DNAs. p16 was methylated in 24 of 40 (62%) tissues and 12 of 39 (32%) plasma DNAs. Significant correlations were observed between AFB(1)-Alb adducts and p53 mutations and p16 methylation. These data suggest that genetic, epigenetic and environmental exposure biomarkers in plasma may help in estimating the risk for the development of HCC.

Chronic inflammation and oxidative stress in the genesis and perpetuation of cancer: role of lipid peroxidation, DNA damage, and repair

BACKGROUND AND AIMS

Chronic inflammation, induced by biological, chemical, and physical factors, was associated with increased risk of human cancer at various sites. Chronic inflammatory processes induce oxidative/nitrosative stress and lipid peroxidation (LPO), thereby generating excess reactive oxygen species (ROS), reactive nitrogen species (RNS), and DNA-reactive aldehydes. Miscoding etheno- and propano-modified DNA bases are generated inter alia by reaction of DNA with these major LPO products. Steady-state levels of LPO-derived (etheno-) DNA adducts in organs affected by persistent inflammatory processes were investigated as potential lead markers for assessing progression of inflammatory cancer-prone diseases.

RESULTS

Using ultrasensitive and specific detection methods for the analysis of human tissues, cells, and urine, etheno-DNA adduct levels were found to be significantly elevated in the affected organs of subjects with chronic pancreatitis, ulcerative colitis, and Crohn's disease. Patients with alcohol-related liver diseases showed excess hepatic DNA damage progressively increasing from hepatitis, fatty liver, to liver cirrhosis. Ethenodeoxyadenosine excreted after DNA repair in urine of hepatitis B virus-related chronic hepatitis and liver cirrhosis patients was increased up to 90-fold. Putative mechanisms that may control DNA damage in inflamed tissues including impaired or imbalanced DNA repair pathways are reviewed.

CONCLUSION

Persistent oxidative/nitrosative stress and excess LPO are induced by inflammatory processes in a self-perpetuating process and cause progressive accumulation of DNA damage in target organs. Together with deregulation of cell homeostasis, the resulting genetic changes act as driving force in chronic inflammation-associated human disease pathogenesis. Thus steady-state levels of DNA damage caused by ROS, RNS, and LPO end products provide promising molecular signatures for risk prediction and potential targets and biomarkers for preventive measures.

Dichotomies in cancer research: some suggestions for a new synthesis

Continuing high cancer incidence and mortality raise concern about the prevailing overall approach to the control of this disease. The purpose of this article is to elaborate on fundamental dichotomies between traditional and revisionist viewpoints and then to attempt a synthesis of these contrasting perspectives. Topics considered include the importance of controlling carcinogenesis in its earliest stages; consideration of epigenetic, as well as genetic, factors in cancer; development of appropriate genetic animal models of carcinogenesis; the need for multifunctional agents to prevent and treat cancer; and the limits of reductionism. The need for development of new preventive and therapeutic measures that will maintain quality of life, not merely extend life, is stressed. Finally, the importance of context in cancer biology is emphasized, as epitomized in Walt Whitman's famous quotation that "Nothing out of its place is good and nothing in its place is bad."

Genetics of hepatocellular tumors

Numerous genetic alterations are accumulated during the process of hepatocarcinogenesis. These genetic alterations can be divided into two groups. The first set of genetic alterations is specific of hepatocellular tumor risk factors. It includes integration of hepatitis B virus (HBV) DNA, R249S TP53 (tumor protein p53) mutation in aflatoxin B1-exposed patients, KRAS mutations related to vinyl chloride exposure, hepatocyte nuclear factor 1alpha (HNF1alpha) mutations associated to hepatocellular adenomas and adenomatosis polyposis coli (APC) germline mutations predisposing to hepatoblastomas. The second set of genetic alterations are etiological nonspecific, it includes recurrent gains and losses of chromosomes, alteration of TP53 gene, activation of WNT/beta-catenin pathway through CTNNB1/beta-catenin and AXIN (axis inhibition protein) mutations, inactivation of retinoblastoma and IGF2R (insulin-like growth factor 2 receptor) pathways through inactivation of RB1 (retinoblastoma 1), P16 and IGF2R. Comprehensive analyses of these genetic alterations have defined two pathways of hepatocarcinogenesis according to the presence or the absence of chromosomal instability. Hepatitis B virus and poorly differentiated tumors are related to chromosome instable tumors associated with frequent TP53 mutations, whereas non-HBV and well-differentiated tumors are related to chromosomal stable samples that are frequently beta-catenin activated. These classifications have clinical relevance as genetic alterations may also be related to prognosis.

Probiotic supplementation reduces a biomarker for increased risk of liver cancer in young men from Southern China

BACKGROUND

In vitro and in vivo studies suggest that selected strains of probiotic bacteria can form tight complexes with aflatoxin B(1) and other carcinogens.

OBJECTIVE

The aim of the present study was to determine whether administration of probiotic bacteria could block the intestinal absorption of aflatoxin B(1) and thereby lead to reduced urinary excretion of aflatoxin B(1)-N(7)-guanine (AFB-N(7)-guanine), a marker for a biologically effective dose of aflatoxin exposure. Elevated urinary excretion of this aflatoxin-DNA adduct is associated with an increased risk of liver cancer.

DESIGN

Ninety healthy young men from Guangzhou, China, were randomly assigned to 2 groups; one group received a mixture of Lactobacillus rhamnosus LC705 and Propionibacterium freudenreichii subsp. shermanii strains 2 times/d for 5 wk, and the other group received a placebo preparation. The subjects provided 4 urine samples: at baseline, at 3 and 5 wk after starting the supplementation, and at the end of the 5-wk postintervention period.

RESULTS

The percentage of samples with negative AFB-N(7)-guanine values tended to be higher in the probiotic group than in the placebo group during the 5-wk intervention period (odds ratio: 2.63, P = 0.052), and a statistically significant decrease in the concentration of urinary AFB-N(7)-guanine was observed in the probiotic group. The reduction was 36% at week 3 and 55% at week 5. The geometric means for the probiotic and placebo groups were 0.24 and 0.49 ng AFB-N(7)-guanine/mL, respectively, during the intervention period (P = 0.005).

CONCLUSION

A probiotic supplement reduces the biologically effective dose of aflatoxin exposure and may thereby offer an effective dietary approach to decrease the risk of liver cancer.

Proteome analysis of hepatocellular carcinoma by laser capture microdissection

Hepatocellular carcinoma (HCC) is one of the most frequent visceral neoplasia worldwide and is a multifactorial and multistage pathogenesis that finally leads to the deregulation of cell homeostasis. Laser capture microdissection (LCM) may allow a more ready identification of differences in protein expression in selected cell types or areas of tissue, and microscopic regions as small as 3-5 microm in diameter can be sampled. Here we applied the LCM to the proteomic study of hepatitis B-related HCC and surrounding non-tumor tissues. Proteome alterations were observed using 2-DE and ESI-MS/MS, and alterations in the proteome were examined. Twenty protein spots were selected, of which 11 proteins were significantly altered in the HCC compared with the surrounding non-tumor tissues. Of the proteins that were selected, peroxiredoxin 2, apolipoprotein A-I precursor, 3-hydroxyacyl-CoA dehydrogenase type II, and 14.5-kDa translational inhibitor protein appear to be novel candidates as useful hepatitis B-related HCC markers. This study indicates that LCM is a useful technological method in the proteomic study of cancer tissue. The proteins revealed in this experiment can be used in the future for studies pertaining to hepatocarcinogenesis, or as diagnostic markers and therapeutic targets for HCC associated with hepatitis B virus infection.

Molecular pathways in hepatocellular carcinoma

Research over the past decade has unraveled important molecular pathways involved in hepatocellular carcinoma (HCC), and several chromosomal and genetic aberrations have been identified to be responsible for initiation of the carcinogenic process. HBx protein and HCV core protein appear to play a pivotal role in hepatocarcinogenesis related to hepatitis B virus and hepatitis C virus, respectively. These viral oncoproteins allow cells to bypass some of the multi-steps in hepatocarcinogenesis, accounting for the etiological role of the two viruses in HCC. Understanding of the molecular pathways of HCC facilitates the development of novel molecular strategies for chemoprevention and therapy of HCC.

249(ser) TP53 mutation in plasma DNA, hepatitis B viral infection, and risk of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) from regions with high dietary exposure to aflatoxins and endemic for hepatitis B virus (HBV) often contain a specific mutation at codon 249 in TP53 (249(ser); AGG to AGT, Arg to Ser). This mutation is also detectable in circulating cell-free DNA from the plasma of HCC patients and healthy subjects in these regions. We have examined the joint effect of plasma 249(ser) and HBV infection in a case-control study design involving 348 control, 98 cirrhotic, and 186 HCC participants from The Gambia, West Africa, an area of high HCC incidence. The 249(ser) mutation was detected in 3.5% of controls, 15.3% of cirrhotics, and 39.8% of HCC cases (adjusted odds ratios (OR): 4.83, (95% confidence interval (CI): 1.71-13.7) for cirrhosis and 20.3 (8.19-50.0) for HCC). HBsAg positivity along with plasma 249(ser) was observed in 45/183 (24.6%) HCC cases compared to only one (0.3%) control. Risk for HCC was associated with markers of HBV alone (OR: 10.0, 95% CI: 5.16-19.6), 249(ser) alone (OR: 13.2, 95% CI: 4.99-35.0), and both markers present (OR: 399, 95% CI: 48.6-3270). These results suggest a multiplicative effect on HCC risk resulting from the mutational effect of aflatoxin on TP53, as monitored by detection of plasma 249(ser), with concomitant chronic infection with HBV.

Alleviation of aflatoxin B1-induced oxidative stress in HepG2 cells by volatile extract from Allii Fistulosi Bulbus

The volatile extract from Allii Fistulosi Bulbus (VEAF) was isolated by steam distillation under reduced pressure, followed by continuous liquid-liquid extraction, and its effects on aflatoxin B1 (AFB1)-induced oxidative stress were investigated in human hepatoma cells (HepG2). The main constituents of the VEAF, identified by gas chromatography/mass spectrometry, were 2-octyl-5-methyl-3(2H)-furanone, 2-hexyl-5-methyl-3(2H)-furanone, 2,5-dimethylthiophene, 3,5-diethyl-1,2,4-trithiolane and 3,4-dimethyl-2,5-dihydro-thiophene-2-one. VEAF significantly inhibited the formation of intracellular reactive oxygen species caused by AFB1 in a dose-dependent manner, concomitant with a significant decrease in the AFB1-induced cytotoxicity. VEAF pretreatment significantly reduced the levels of thiobarbituric acid reactive substances, an indicator of lipid peroxidation, whereas increased the level of reduced glutathione. The level of 8-hydroxy-2'-deoxyguanosine, a DNA oxidative stress marker, was also decreased by 49-59% with pretreatment of VEAF. With respect to the activity of AFB1 metabolizing enzymes, VEAF significantly increased the activity of glutathione S-transferase, and significantly decreased the cytochrome (CYP) P450 3A4 activity, but had a little effect on the CYP1As. These results suggest that VEAF may be selectively effective in alleviating the AFB1-induced oxidative stress, and lead to cytoprotection against AFB1 exposure.

Chronic toxicological evaluation of dietary NovaSil clay in Sprague-Dawley rats

NovaSil (NS) clay, a common anti-caking agent in animal feeds, has been shown to sorb aflatoxins in the GI tract and diminish their bioavailability and adverse effects in short-term animal studies. Based on this evidence, it is hypothesized that clay-based enterosorption of aflatoxins may be a useful strategy for the prevention of aflatoxicosis in human populations. However, the potential toxicity of long-term dietary exposure to NS has not been determined. In this research, 5-6-week-old male and female Sprague-Dawley rats were fed rations containing 0, 0.25, 0.5, 1.0, or 2.0% (w/w) levels of NS for 28 weeks. Analysis of the NS showed negligible levels of dioxin and furan contaminants. Total feed consumption, cumulative feed consumption, body weight, total body weight gain, feed conversion efficiency, cumulative feed conversion efficiency, and relative organ weights were unaffected in either sex at the doses tested. No NS-dependent differences in relative organ weights or gross or histopathological changes were observed. Analysis of hematological parameters, clinical chemistry, and selected vitamin and mineral levels revealed isolated significant differences between some treatments and control groups (mean corpuscular hemoglobin, serum Ca, serum vitamin A, and serum Fe). However, the differences observed in each case were not dose-dependent. These results suggest that dietary inclusion of NS at levels as high as 2.0% (w/w) does not result in overt toxicity. These findings (as well as others) support the use of NS clay for dietary intervention studies in human populations at high risk for aflatoxicosis.

A calcium-based theory of carcinogenesis

Most human cancers are initiated by chronic injuries that repeatedly kill cells and must, therefore, repeatedly raise cell calcium within nearby survivors. They may also raise calcium in distant cells via calcium waves. Here it is argued that these calcium increases initiate oncogenesis by breaking gap junctions and thus disorganizing tissues and by activating proto-oncogenes. It is also argued that these calcium increases become self-perpetuating in part through the development of an ability of cells to divide in reduced extracellular calcium, i.e., habituation to reduced extracellular calcium. I propose to test these calcium-based theories by using aequorinated mice.

p53 Mutations in human cholangiocarcinoma: a review

The reported mortality from intrahepatic bile duct tumours is increasing markedly in industrialised countries, for reasons that remain unknown. Inactivation of the tumour suppressor gene p53, is the commonest genetic abnormality in human cancer and has been implicated in the genesis of cholangiocarcinoma in various immunohistochemical and molecular epidemiological investigations, including gene sequencing studies. The structure and function of p53 and its role in linking cancer to specific carcinogens by way of mutational signatures is reviewed. The findings of previous p53 studies and their relevance in human cholangiocarcinoma are summarised.

Somatic mutations in human cancer: applications in molecular epidemiology

The tumour suppressor protein p53 mediates cell-cycle arrest, DNA repair and apoptosis after activation by multiple forms of cellular stresses. When activated, this "master protein" modulates its response depending on the type and intensity of the stress. The TP53 gene with its nearly 20,000 described mutations is the most mutated gene in cancer. Most mutations are missense and occur at over 200 codons within the central portion of the gene. In several cancers, the distribution of mutation types and sites follow a specific pattern reflecting the effects of environmental mutagens. An example for such a "mutagen fingerprint" is TP53 mutation at codon 249 in hepatocellular carcinoma in regions of the world characterised by high levels of the mutagen aflatoxin B1 and endemic HBV infection. Recently, TP53 mutations have been detected in surrogate sources of genetic material such as free circulating DNA isolated from plasma. Plasma TP53 mutations can be detected in the blood of pre-cancer and cancer patients, with potential application for early cancer detection. Thus, TP53 mutations have multiple applications as markers of mutagenic exposures, or as intermediate end-points in assessment of cancer occurrence and progression.

Pathogenesis of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most common malignant tumours worldwide. The major aetiologies and risk factors for the development of HCC are well defined and some of the multiple steps involved in hepatocarcinogenesis have been elucidated in recent years. However, no clear picture of how and in what sequence these factors interact at the molecular level has emerged yet. Malignant transformation of hepatocytes may occur as a consequence of various aetiologies, such as chronic viral hepatitis, alcohol, and metabolic disorders, in the context of increased cellular turnover induced by chronic liver injury, regeneration and cirrhosis. Activation of cellular oncogenes, inactivation of tumour suppressor genes, genomic instability, including DNA mismatch repair defects and impaired chromosomal segregation, overexpression of growth and angiogenic factors, and telomerase activation may contribute to the development of HCC. Overall, HCCs are genetically very heterogeneous tumours. New technologies, including gene expression profiling and proteomic analyses, should allow us to further elucidate the molecular events underlying HCC development and identify novel diagnostic markers as well as therapeutic targets.

A fluorimetric assay for the spontaneous release of an N7-alkylguanine residue from duplex DNA

A fluorimetric assay for monitoring depurination of the N7-alkylguanine adduct derived from the anticancer natural product leinamycin is described. This general approach could potentially provide the foundation for a high throughput assay that detects DNA-alkylating agents or a convenient continuous fluorimetric assay for base excision repair enzymes.

CYP2K6 from zebrafish (Danio rerio): cloning, mapping, developmental/tissue expression, and aflatoxin B1 activation by baculovirus expressed enzyme

A full-length zebrafish (Danio rerio) cytochrome P450 (CYP) 2K6 cDNA, was obtained (GenBank accession No. AF283813) through polymerase chain reaction cloning using degenerated primers based on a consensus CYP2 sequence and the heme-binding domain. This first CYP2K family member cloned from zebrafish had 1861 bp which contained 27 bp of 5'-untranslated region (5'-UTR), an open reading frame (ORF) of 1518 bp, and a 300 bp 3'-UTR with a poly A tail. The deduced 506 amino acid sequence of CYP2K6 had 63%, 62% and 59% identity with rainbow trout CYP2K1, CYP2K4 and CYP2K3, respectively; and 45%, 42%, and 42% identity with rabbit CYP2C1, human CYP2C19 and mouse CYP2C39, respectively. CYP2K6 mapped to 107.49cR on LG3 using the LN54 radiation hybrid panel. Its mRNA was detected at 5 days post-fertilization and in the adult liver and ovary among nine tissues examined. The ORF, including the 27 bp of the 5'-UTR, was cloned into pFastBac donor vector and then transferred into the baculovirus genome (bacmid DNA) in DH10Bac competent cells. The recombinant bacmid DNA was used to infect Spodoptera frugiperda insect cells to express the CYP2K6 protein (Bv-2K6). As its ortholog, rainbow trout Bv-2K1 [Yang, Y.H., Miranda, C.L., Henderson, M.C., Wang-Buhler, J.-L., Buhler, D.R., 2000. Heterologous expression of CYP2K1 and identification of the expressed protein (Bv-2K1) as lauric acid (omega-1)-hydroxylase and aflatoxin B1 exo-epoxidase. Drug Metab. Disp. 28,1279-83.], Bv-2K6 also catalyzed the conversion of aflatoxin B1 (AFB1) to its exo-8,9-epoxide as assessed by the trapping of a glutathione (GSH) adduct in the presence of a specific mouse alpha class glutathione S-transferase. The identity of the AFB1-GSH adduct was verified by liquid chromatography-mass spectrometry (LC-MS) and mass spectrometry-mass spectrometry (MS-MS) analysis. Although rainbow trout Bv-2K1 was capable of oxidizing lauric acid, zebrafish Bv-2K6 protein showed no activity against this substrate.

Health Effects of Mycotoxins: A Toxicological Overview

Diseases caused by fungi are spread by direct implantation or inhalation of spores. Fungi can cause adverse human health effects to many organ systems. In addition to infection and allergy, fungi can produce mycotoxins and organic chemicals that are responsible for various toxicologic effects. We reviewed the published literature on important mycotoxins and systemic effects of mycotoxins. Scientific literature revealed a linkage between ingesting mycotoxin contaminated food and illness, especially hepatic, gastrointestinal, and carcinogenic diseases. Issues related to mycotoxin exposure, specific diseases, and management are discussed. Although there is agreement that diet is the main source of mycotoxin exposure, specific health effects and risk assessment from indoor nonagricultural exposure are limited by the paucity of scientific evidence currently available. Further research on the health effects of inhaling mycotoxins in indoor settings is needed.

Trans-4-hydroxy-2-nonenal inhibits nucleotide excision repair in human cells: a possible mechanism for lipid peroxidation-induced carcinogenesis

Lipid peroxidation (LPO) is a cellular process that commonly takes place under normal physiological conditions. Under excessive oxidative stress, the level of LPO becomes very significant, and a growing body of evidence has shown that excessive LPO may be involved in carcinogenesis. Trans-4-hydroxy-2-nonenal (4-HNE) is a major product of LPO, and its level becomes relatively high in cells under oxidative stress. 4-HNE is able to react readily with various cellular components, including DNA and proteins. We previously found that the 4-HNE-DNA adduct is a potent mutagen in human cells and is preferentially formed at codon 249 of the p53 gene, a mutational hotspot in human cancers. To further understand the role of 4-HNE in carcinogenesis, we addressed the question of whether 4-HNE affects DNA repair in human cells. We found that the repair capacity for benzo[a]pyrene diol epoxide and UV light-induced DNA damage was greatly compromised in human cells or human cell extracts treated with 4-HNE, which is mainly through interaction of 4-HNE with cellular repair proteins. We also found that 4-HNE greatly sensitizes cells to benzo[a]pyrene diol epoxide- and UV-induced killing. Together these results strongly suggest that this LPO metabolite damages not only DNA but also DNA repair mechanisms in human cells. We propose that these two detrimental effects of LPO may contribute synergistically to human carcinogenesis.

Discriminating different classes of toxicants by transcript profiling

Male rats were treated with various model compounds or the appropriate vehicle controls. Most substances were either well-known hepatotoxicants or showed hepatotoxicity during preclinical testing. The aim of the present study was to determine if biological samples from rats treated with various compounds can be classified based on gene expression profiles. In addition to gene expression analysis using microarrays, a complete serum chemistry profile and liver and kidney histopathology were performed. We analyzed hepatic gene expression profiles using a supervised learning method (support vector machines; SVMs) to generate classification rules and combined this with recursive feature elimination to improve classification performance and to identify a compact subset of probe sets with potential use as biomarkers. Two different SVM algorithms were tested, and the models obtained were validated with a compound-based external cross-validation approach. Our predictive models were able to discriminate between hepatotoxic and nonhepatotoxic compounds. Furthermore, they predicted the correct class of hepatotoxicant in most cases. We provide an example showing that a predictive model built on transcript profiles from one rat strain can successfully classify profiles from another rat strain. In addition, we demonstrate that the predictive models identify nonresponders and are able to discriminate between gene changes related to pharmacology and toxicity. This work confirms the hypothesis that compound classification based on gene expression data is feasible.

Transcriptional response of yeast to aflatoxin B1: recombinational repair involving RAD51 and RAD1

The potent carcinogen aflatoxin B(1) is a weak mutagen but a strong recombinagen in Saccharomyces cerevisiae. Aflatoxin B(1) exposure greatly increases frequencies of both heteroallelic recombination and chromosomal translocations. We analyzed the gene expression pattern of diploid cells exposed to aflatoxin B(1) using high-density oligonucleotide arrays comprising specific probes for all 6218 open reading frames. Among 183 responsive genes, 46 are involved in either DNA repair or in control of cell growth and division. Inducible growth control genes include those in the TOR signaling pathway and SPO12, whereas PKC1 is downregulated. Eleven of the 15 inducible DNA repair genes, including RAD51, participate in recombination. Survival and translocation frequencies are reduced in the rad51 diploid after aflatoxin B(1) exposure. In mec1 checkpoint mutants, aflatoxin B(1) exposure does not induce RAD51 expression or increase translocation frequencies; however, when RAD51 is constitutively overexpressed in the mec1 mutant, aflatoxin B(1) exposure increased translocation frequencies. Thus the transcriptional profile after aflatoxin B(1) exposure may elucidate the genotoxic properties of aflatoxin B(1).

Ser-249TP53 mutation in tumour and plasma DNA of hepatocellular carcinoma patients from a high incidence area in the Gambia, West Africa

Hepatocellular carcinoma (HCC) is frequent in areas of high exposure to aflatoxin and high prevalence of HBV infection, such as western Africa and south-east China. A selective mutation in TP53 (AGG-->AGT at codon 249, Arg-->Ser) has been identified as a hotspot in HCCs from such areas, reflecting DNA damage caused by aflatoxin metabolites. Recent studies have shown that circulating free DNA can be retrieved from human plasma, and it is hypothesised that plasma DNA may serve as a source for biomarkers of tumorigenic processes. In our study, we have determined the prevalence of Ser-249 mutation, using a PCR-restriction digestion method, with selective use of short oligonucleotide mass spectrometry analysis (SOMA), in a series of 29 biopsy specimens of HCC from The Gambia in West Africa. Overall, we identified the Ser-249 mutation in 35% (10/29) of the tumours. In parallel, we tested 17 plasma samples from HCC patients with matching tumour tissue. The 249 status concordance between tumour tissues and matched plasma was 88.5%. These results indicate that the Ser-249 mutation is common in HCC in The Gambia (35%), although a higher prevalence has been reported in other regions with high population exposure to aflatoxin (e.g., eastern China: >50%). Moreover, our studies indicate that plasma is a convenient source of liver tumour-derived DNA, thus holding promise for earlier detection and diagnosis of cancer.

Specific mutations of hepatitis B virus in plasma predict liver cancer development

A major risk factor for hepatocellular carcinoma (HCC) is hepatitis B virus (HBV), whose pathogenesis is exacerbated by the acquisition of mutations that accelerate carcinogenesis. We examined, with mass spectrometry, the temporality of an HBV 1762(T)/1764(A) double mutation in plasma and tumors. Initial studies found that 52 of 70 (74.3%) tumors from patients residing in Qidong, People's Republic of China, contained this HBV mutation. Paired plasma samples were available for six of the tumor specimens; four tumors had the HBV 1762(T)/1764(A) mutation, whereas three of the paired plasma samples were also positive. The potential predictive value of this biomarker was explored by using stored plasma samples from a study of 120 residents of Qidong who had been monitored for aflatoxin exposure and HBV infection. After 10 years of passive follow-up, there were six cases of major liver disease including HCC (four cases), hepatitis (one case), and cirrhosis (one case). All six cases had detectable levels of the HBV 1762(T)/1764(A) mutation up to 8 years before diagnosis. Finally, 15 liver cancers were selected from a prospective cohort of 1,638 high-risk individuals in Qidong on the basis of available plasma samples spanning the years before and after diagnosis. The HBV 1762(T)/1764(A) mutation was detected in 8 of the 15 cases (53.3%) before cancer. The persistence of detection of this mutation was statistically significant (P = 0.022, two-tailed). We therefore found that a prediagnosis biomarker of specific HBV mutations can be measured in plasma and suggest this marker for use as an intermediate endpoint in prevention and intervention trials.

In vivo identification of aflatoxin-induced free radicals in rat bile

Aflatoxin B1 (AFB1) is a potent hepatocarcinogen. We have recently detected [via electron spin resonance (ESR) spectroscopy] free radicals in vivo in rat bile following AFB1 metabolism using the spin trapping [alpha-(4-pyridyl-1-oxide)-N-tert-butyl nitrone (4-POBN)] technique. The aim of the present study was to identify the trapped free radical intermediates from the in vivo hepatic metabolism of AFB1. Rats were treated simultaneously with AFB1 (3 mg/kg i.p.) and the spin trapping agent 4-POBN (1 g/kg i.p.), and bile was collected over a period of 1 h at 20 min intervals. On-line high performance liquid chromatography (HPLC) coupled to ESR was used to identify an arachidonic acid-derived radical adduct of 4-POBN in rat bile, and a methyl adduct of 4-POBN from the reaction of hydroxyl radicals with carbon-13-labeled dimethyl sulfoxide ((13)C-DMSO). The effect of metabolic inhibitors, such as desferoxamine mesylate (DFO), an iron chelator, 2-dimethylaminoethyl-2,2-diphenylvalerate hydrochloride (SKF) 525A, a cytochrome P-450 inhibitor, and gadolinium chloride (GdCl(3)), a Kupffer cell inactivator, on in vivo aflatoxin-induced free radical formation were also studied. It was found that there was a significant decrease in radical formation as a result of DFO, SKF525A and GdCl(3) inhibition. Trapped 4-POBN radical adducts were also detected in rat bile following the in vivo metabolism of aflatoxin-M1, one of the hydroxylated metabolites of AFB1.

Oxidative DNA damage in peripheral leukocytes and its association with expression and polymorphisms of hOGG1: A study of adolescents in a high risk region for hepatocellular carcinoma in China

AIM: To study the oxidative DNA damage to adolescents of hepatocellular carcinoma (HCC) families in Guangxi Zhuang Autonomous Region, China.

METHODS: Peripheral leukocytes’ DNA 7,8-dihydro-8-oxoguanine (8-oxoG) and repair enzyme hOGG₁ were quantified by flow-cytometry. hOGG₁-Cys326Ser single nucleotide polymorphism (SNP) was distinguished by polymerase chain reaction-single strand conformational polymorphism (PCR-SSCP) assay.

RESULTS: There was a positive correlation between 8-oxoG and repair enzyme hOGG₁ expression (P < 0.001). HCC children (n = 21) in Fusui county had a higher level of hOGG₁ (P < 0.01) and a lower level of 8-oxoG (P < 0.05) than the controls (n = 63) in Nanning city. Children in Nanning exposed to passive-smoking had a higher hOGG₁ expression (P < 0.05) than the non-exposers. 8-oxoG and hOGG₁ were negatively correlated with body mass index, while hOGG₁ was positively correlated with age. There was a peak of 8-oxoG level nearby the 12 year point. Individuals with the hOGG₁ 326Ser allele had a significantly marginal higher concentration of leukocyte 8-oxoG level than hOGG₁ 326Cys allele.

CONCLUSION: This is the first report using flow-cytometry to simultaneously quantify both the DNA oxidative damage and its repairing enzyme hOGG1. The results provide new insights towards a better understanding of the mechanisms of oxidative stress in a population highly susceptible to hepatocarcinogenesis.

Indoor mold, toxigenic fungi, and Stachybotrys chartarum: infectious disease perspective

Damp buildings often have a moldy smell or obvious mold growth; some molds are human pathogens. This has caused concern regarding health effects of moldy indoor environments and has resulted in many studies of moisture- and mold-damaged buildings. Recently, there have been reports of severe illness as a result of indoor mold exposure, particularly due to Stachybotrys chartarum. While many authors describe a direct relationship between fungal contamination and illness, close examination of the literature reveals a confusing picture. Here, we review the evidence regarding indoor mold exposure and mycotoxicosis, with an emphasis on S. chartarum. We also examine possible end-organ effects, including pulmonary, immunologic, neurologic, and oncologic disorders. We discuss the Cleveland infant idiopathic pulmonary hemorrhage reports in detail, since they provided important impetus for concerns about Stachybotrys. Some valid concerns exist regarding the relationship between indoor mold exposure and human disease. Review of the literature reveals certain fungus-disease associations in humans, including ergotism (Claviceps species), alimentary toxic aleukia (Fusarium), and liver disease (Aspergillys). While many papers suggest a similar relationship between Stachybotrys and human disease, the studies nearly uniformly suffer from significant methodological flaws, making their findings inconclusive. As a result, we have not found well-substantiated supportive evidence of serious illness due to Stachybotrys exposure in the contemporary environment. To address issues of indoor mold-related illness, there is an urgent need for studies using objective markers of illness, relevant animal models, proper epidemiologic techniques, and examination of confounding factors.

Hepatocellular carcinoma: the high-risk patient

Hepatocellular carcinoma (HCC) is the most common hepatic malignancy worldwide. The primary risk factor for the development of HCC is cirrhosis. Even patients without cirrhosis who develop HCC are typically found to have some underlying hepatic abnormality, such as steatohepatitis or chronic viral hepatitis. Although cirrhosis of any cause increases the risk of developing HCC, cirrhosis associated with chronic hepatitis B or C virus infection or hemochromatosis carries the greatest risk. Additional factors such as patient age and sex, duration and severity of liver disease, concurrent alcohol or aflatoxin exposure, liver histology, and alpha-fetoprotein levels also contribute to the relative risk of developing HCC. Vaccination programs aimed at preventing hepatitis B virus infection have been very successful in lowering the incidence of HCC in some areas of the world. Interferon-based therapy, which may control the inflammatory activity in chronic hepatitis C, also holds promise in preventing HCC. Other novel chemopreventative agents, such as glycyrrhizin and polyprenoic acid, may also have a role in preventing HCC, but they require further study before they can be recommended for widespread use.

Molecular pathogenesis of human hepatocellular carcinoma

Hepatocarcinogenesis is a slow process during which genomic changes progressively alter the hepatocellular phenotype to produce cellular intermediates that evolve into hepatocellular carcinoma. During the long preneoplastic stage, in which the liver is often the site of chronic hepatitis, cirrhosis, or both, hepatocyte cycling is accelerated by upregulation of mitogenic pathways, in part through epigenetic mechanisms. This leads to the production of monoclonal populations of aberrant and dysplastic hepatocytes that have telomere erosion and telomerase re-expression, sometimes microsatellite instability, and occasionally structural aberrations in genes and chromosomes. Development of dysplastic hepatocytes in foci and nodules and emergence of hepatocellular carcinoma are associated with the accumulation of irreversible structural alterations in genes and chromosomes, but the genomic basis of the malignant phenotype is heterogeneous. The malignant hepatocyte phenotype may be produced by the disruption of a number of genes that function in different regulatory pathways, producing several molecular variants of hepatocellular carcinoma. New strategies should enable these variants to be characterized.

The aflatoxin B(1) formamidopyrimidine adduct plays a major role in causing the types of mutations observed in human hepatocellular carcinoma

A G to T mutation has been observed at the third position of codon 249 of the p53 tumor-suppressor gene in over 50% of the hepatocellular carcinoma cases associated with high exposure to aflatoxin B(1) (AFB(1)). Hypotheses have been put forth that AFB(1), in concert with hepatitis B virus (HBV), may play a role in the formation of, and/or the selection for, this mutation. The primary DNA adduct of AFB(1) is 8,9-dihydro-8-(N(7)-guanyl)-9-hydroxyaflatoxin B(1) (AFB(1)-N7-Gua), which is converted naturally to two secondary lesions, an apurinic site and an AFB(1)-formamidopyrimidine (AFB(1)-FAPY) adduct. AFB(1)-FAPY is detected at near maximal levels in rat DNA days to weeks after AFB(1) exposure, underscoring its high persistence in vivo. The present study reveals two striking properties of this DNA adduct: (i) AFB(1)-FAPY was found to cause a G to T mutation frequency in Escherichia coli approximately 6 times higher than that of AFB(1)-N7-Gua, and (ii) one proposed rotamer of AFB(1)-FAPY is a block to replication, even when the efficient bypass polymerase MucAB is used by the cell. Taken together, these characteristics make the FAPY adduct the prime candidate for both the genotoxicity of aflatoxin, because mammalian cells also have similar bypass mechanisms for combating DNA damage, and the mutagenicity that ultimately may lead to liver cancer.