Abstract P6-12-01: Withdrawn

This abstract was withdrawn by the authors.

Citation Format: Palliyaguru DL, Chartoumpekis DV, Skoko JJ, Wendell SG, Woodcock SR, Wakabayashi N, Yagishita Y, Oesterreich S, Michalopoulos GK, Kensler TW. Withdrawn [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P6-12-01.

Activation of the astrocytic Nrf2/ARE system ameliorates the formation of demyelinating lesions in a multiple sclerosis animal model

Oxidative stress critically contributes to the pathogenesis of a variety of neurodegenerative diseases such as multiple sclerosis. Astrocytes are the main regulators of oxidative homeostasis in the brain and dysregulation of these cells likely contributes to the accumulation of oxidative damage. The nuclear factor erythroid 2-related factor 2 (Nrf2) is the main transcriptional regulator of the anti-oxidant stress defense. In this study, we elucidate the effects of astrocytic Nrf2-activation on brain-intrinsic inflammation and lesion development. Cells deficient for the Nrf2 repressor kelch-like ECH-associated protein 1 (Keap1) are characterized by hyperactivation of Nrf2-signaling. Therefore, wild type mice and mice with a GFAP-specific Keap1-deletion were fed with 0.25% cuprizone for 1 or 3 weeks. Cuprizone intoxication induced pronounced oligodendrocyte loss, demyelination and reactive gliosis in wild type animals. In contrast, astrocyte-specific Nrf2-activation was sufficient to prevent oligodendrocyte loss and demyelination, to ameliorate brain intrinsic inflammation and to counteract axonal damage. Our results highlight the potential of the Nrf2/ARE system for the treatment of neuroinflammation in general and of multiple sclerosis in particular. © GLIA 2016;64:2219-2230.

Keap1 eye on the target: chemoprevention of liver cancer

Hepatocellular carcinoma (HCC) is one of the most common cancers worldwide, causing nearly 600,000 deaths each year. Increased risk of HCC due to chronic infection with hepatitis B virus (HBV) and exposure to dietary aflatoxins is responsible for many of these deaths. Prevention strategies targeting HBV infection and aflatoxin exposure could dramatically impact the rates of HCC. Universal HBV vaccination programs have begun in some high-risk areas. Strategies to reduce aflatoxin contamination in food stores have also been implemented. However, complete elimination of aflatoxin contamination might not be possible. For this reason, chemoprevention strategies which alter aflatoxin disposition are a practical strategy to reduce the incidence of HCC in populations with high dietary aflatoxin exposure. The mechanisms of aflatoxin-induced hepatocarcinogenesis are well known. This knowledge provides the basis for evaluation of both exposures to aflatoxin, as well as modulation of aflatoxin disposition by chemopreventive agents. Products of aflatoxin DNA damage and toxicity as well as other metabolites can be used as biomarkers to evaluate modulation of aflatoxin disposition. Modulation of aflatoxin disposition can be achieved through induction of conjugating and cytoprotective enzymes. Many of these enzymes are regulated through Kelch ECH-associating protein 1 (Keap1)-NF-E2-related factor 2(Nrf2)-antioxidant response element (ARE) signaling, making this pathway an important molecular target for chemoprevention. Rodent studies have identified several classes of chemopreventive agents which induce cytoprotective genes. These inducers include phenolic antioxidants, dithiolethiones, isothiocyanates, and triterpenoids. Furthermore, clinical interventions have shown that inducers of Keap1-Nrf2- ARE signaling increase cytoprotective enzyme expression, resulting in modulation of aflatoxin disposition. Much work remains to be done in order to take promising chemopreventive agents from preclinical evaluation to application in at-risk populations. However, appropriately designed clinical trials will aid in this process, which can have profound impact on the incidence of HCC.

Chlorophyllin intervention reduces aflatoxin-DNA adducts in individuals at high risk for liver cancer

Residents of Qidong, People's Republic of China, are at high risk for development of hepatocellular carcinoma, in part from consumption of foods contaminated with aflatoxins. Chlorophyllin, a mixture of semisynthetic, water-soluble derivatives of chlorophyll that is used as a food colorant and over-the-counter medicine, has been shown to be an effective inhibitor of aflatoxin hepatocarcinogenesis in animal models by blocking carcinogen bioavailability. In a randomized, double-blind, placebo-controlled chemoprevention trial, we tested whether chlorophyllin could alter the disposition of aflatoxin. One hundred and eighty healthy adults from Qidong were randomly assigned to ingest 100 mg of chlorophyllin or a placebo three times a day for 4 months. The primary endpoint was modulation of levels of aflatoxin-N(7)-guanine adducts in urine samples collected 3 months into the intervention measured by using sequential immunoaffinity chromatography and liquid chromatography-electrospray mass spectrometry. This aflatoxin-DNA adduct excretion product serves as a biomarker of the biologically effective dose of aflatoxin, and elevated levels are associated with increased risk of liver cancer. Adherence to the study protocol was outstanding, and no adverse events were reported. Aflatoxin-N(7)-guanine could be detected in 105 of 169 available samples. Chlorophyllin consumption at each meal led to an overall 55% reduction (P = 0.036) in median urinary levels of this aflatoxin biomarker compared with those taking placebo. Thus, prophylactic interventions with chlorophyllin or supplementation of diets with foods rich in chlorophylls may represent practical means to prevent the development of hepatocellular carcinoma or other environmentally induced cancers.

Preneoplastic prostate lesions: an opportunity for prostate cancer prevention

Environmental factors, especially the diet, play a prominent role in the epidemic of prostate cancer (PCA), in the United States. Many candidate dietary components have been proposed to influence human prostatic carcinogenesis, including fat, calories, fruits and vegetables, anti-oxidants, and various micronutrients, but the specific roles dietary agents play in promoting or preventing PCA remain controversial. We have collected evidence to suggest that GSTP1, the gene encoding the pi-class glutathione S-transferase (GST), may serve a "caretaker" function for prostatic cells. Although GSTP1 can be detected in normal prostatic epithelium, in almost all PCA cases, PCA cells fail to express GSTP1 polypeptides, and lack of GSTP1 expression most often appears to be the result of somatic "CpG island" DNA methylation changes. Loss of GSTP1 function also appears to be characteristic of prostatic epithelial neoplasia (PIN) lesions, thought to represent PCA precursors. We have recently learned that a new candidate early PCA precursor lesion, proliferative inflammatory atrophy (PIA), characterized by proliferating prostatic cells juxtaposed to inflammatory cells, contains epithelial cells that express high levels of GSTP1. These findings have formed the basis for a new model of prostatic carcinogenesis, in which prostatic cells in PIA lesions, subjected to a barrage of inflammatory oxidants, induce GSTP1 expression as a defense against oxidative genome damage. When cells with defective GSTP1 genes appear amongst the PIA cells, such cells become vulnerable to oxidants and electrophiles that inflict genome damage that tends to promote neoplastic transformation to PIN and PCA cells. Subsequently, PIN and PCA cells with defective GSTPI genes remain vulnerable to similar stresses tending to promote malignant progression. This new model for prostatic carcinogenesis has implications for the design of new prostate cancer prevention strategies. Rational prevention approaches might include: (i) restoration of GSTPI expression via treatment with inhibitors of CpG methylation, (ii) compensation for inadequate GSTPI activity via treatment with inducers of general GST activity, and (iii) abrogation of genome-damaging stresses via avoidance of exogenous carcinogens and/or reduction of endogenous carcinogenic (particularly oxidant) stresses.

Antioxidative function and substrate specificity of NAD(P)H-dependent alkenal/one oxidoreductase. A new role for leukotriene B4 12-hydroxydehydrogenase/15-oxoprostaglandin 13-reductase

There are several known routes for the metabolic detoxication of alpha,beta-unsaturated aldehydes and ketones, including conjugation to glutathione and reduction and oxidation of the aldehyde to an alcohol and a carboxylic acid, respectively. In this study, we describe a fourth class of detoxication that involves the reduction of the alpha,beta-carbon=carbon double bond to a single bond. This reaction is catalyzed by NAD(P)H-dependent alkenal/one oxidoreductase (AO), an enzyme heretofore known as leukotriene B4 12-hydroxydehydrogenase, 15-oxoprostaglandin 13-reductase, and dithiolethione-inducible gene-1. AO is shown to effectively reduce cytotoxic lipid peroxidation products such as 4-hydroxy-2-nonenal (HNE) (k(cat) = 4.0 x 10(3) min(-1); k(cat)/K(m) = 3.3 x 10(7) min(-1) M(-1)) and acrolein (k(cat) = 2.2 x 10(2) min(-1); k(cat)/K(m) = 1.5 x 10(6) min(-1) M(-1)) and common industrial compounds such as ethyl vinyl ketone (k(cat) = 9.6 x 10(3) min(-1); k(cat)/K(m) = 8.8 x 10(7) min(-1) M(-1)) and 15-oxoprostaglandin E1 (k(cat) = 2.4 x 10(3) min(-1); k(cat)/K(m) = 2.4 x 10(9) min(-1) M(-1)). Furthermore, transfection of human embryonic kidney cells with a rat liver AO expression vector protected these cells from challenge with HNE. The concentration of HNE at which 50% of the cells were killed after 24 h increased from approximately 15 microM in control cells to approximately 70 microM in AO-transfected cells. Overexpression of AO also completely abolished protein alkylation by HNE at all concentrations tested (up to 30 microM). Thus, we describe a novel antioxidative activity of a previously characterized bioactive lipid-metabolizing enzyme that could prove to be therapeutically or prophylactically useful due to its high catalytic rate and inducibility.

GSTA1 expression in normal, preneoplastic, and neoplastic human prostate tissue

BACKGROUND

Glutathione S-transferases (GSTs), inducible enzymes that catalyze the detoxification of reactive electrophiles and oxidants, protect against neoplastic transformation. Prostatic adenocarcinoma and high-grade prostatic intraepithelial neoplasia (HGPIN) fail to express GSTP1, a major class of GST. This failure of expression is associated with methlyation of the GSTP1 promoter, a somatic alteration proposed to be a critical step in prostatic carcinogenesis. However, simple atrophy and post-atrophic hyperplasia-proliferative lesions associated with chronic inflammation, which we have termed "proliferative inflammatory atrophy" (PIA)-express elevated levels of GSTP1. We postulated that this increase in GSTP1 expression in PIA occurs in response to increased oxidative stress. We examined the expression of another major class of GST, GSTA1, in the human prostate.

METHODS

We performed immunohistochemistry against GSTA1 on formalin-fixed radical prostatectomies (n = 45). A stereological grid point counting method was used to estimate the percent of cells staining positive for GSTA1 in normal prostate, PIA, HGPIN, and adenocarcinoma.

RESULTS

In contrast to GSTP1, normal peripheral zone epithelium was virtually devoid of GSTA1. Strikingly, though, epithelial cells in PIA demonstrated strong staining for GSTA1 (median of percent of cells staining positive = 44) as compared to those in normal peripheral zone (median = 3.0, P <.00001), HGPIN (median = 2.9, P <.00001), and adenocarcinoma (median = 3.8, P <.00001). Variations in GSTA1 were also detected between normal anatomic zones: the central zone showed an increase in the percentage of cells staining positive (median = 20.9) as compared to the transition (median = 0.47, P <.0002) and the peripheral (P <.0001) zones.

CONCLUSIONS

Expression of GSTA1 is increased in PIA, supporting the concept that cells within these lesions are subject to localized increases in oxidative stress. Low levels of GSTA1 and GSTP1 in HGPIN and adenocarcinoma suggest a broad lack of detoxification activity in these cells, which may be associated with carcinogenesis in the prostate.

Role of phase 2 enzyme induction in chemoprotection by dithiolethiones

One of the major mechanisms of protection against carcinogenesis, mutagenesis, and other forms of toxicity mediated by carcinogens is the induction of enzymes involved in their metabolism, particularly phase 2 enzymes such as glutathione S-transferases (GSTs), UDP-glucuronosyl transferases, and quinone reductases. Animal studies indicate that induction of phase 2 enzymes is a sufficient condition for obtaining chemoprevention and can be achieved by administering any of a diverse array of naturally-occurring and synthetic chemopreventive agents. Indeed, monitoring of enzyme induction has led to the recognition or isolation of novel, potent chemopreventive agents such as 1,2-dithiole-3-thiones, terpenoids and the isothiocyanate sulforaphane. For example, oltipraz, a substituted 1,2-dithiole-3-thione originally developed as an antischistosomal agent, possesses chemopreventive activity against different classes of carcinogens targeting multiple organs. Mechanistic studies in rodent models for chemoprevention of aflatoxin B(1) (AFB(1))-induced hepatocarcinogenesis by oltipraz indicates that increased expression of phase 2 genes is of central importance, although inhibition of phase 1 activation of AFB(1) can also contribute to protection. Exposure of rodents to 1,2-dithiole-3-thiones triggers nuclear accumulation of the transcription factor Nrf2 and its enhanced binding to the "antioxidant response element" (ARE), leading to transcriptional activation of a score of genes involved in carcinogen detoxication and attenuation of oxidative stress. Nrf2-deficient mice fail to induce many of these genes in response to dithiolethiones; moreover, basal expression of these genes is typically repressed. To test the hypothesis that enzyme induction is a useful strategy for chemoprevention in humans, three key elements are necessary: a candidate agent, an at-risk population and modulatable intermediate endpoints. Towards this end, a placebo-controlled, double blind clinical trial of oltipraz was conducted in residents of Qidong, PR China who are exposed to dietary aflatoxins and who are at high risk for the development of liver cancer. Oltipraz significantly enhanced excretion of a phase 2 product, aflatoxin-mercapturic acid, a derivative of the aflatoxin-glutathione conjugate, in the urine of study participants administered 125 mg oltipraz by mouth daily. Administration of 500 mg oltipraz once a week led to a significant reduction in the excretion of the primary oxidative metabolite of AFB(1), AFM(1), when measured shortly after drug administration. While this study highlighted the general feasibility of inducing phase 2 enzymes in humans, a longer term intervention is addressing whether protective alterations in aflatoxin metabolism can be sustained for extended periods of time in this high-risk population.

A non-calcemic sulfone version of the vitamin D(3) analogue seocalcitol (EB 1089): chemical synthesis, biological evaluation and potency enhancement of the anticancer drug adriamycin

Novel side-chain diene sulfones 5, analogues of the natural hormone 1alpha,25-dihydroxyvitamin D(3) (calcitriol, 1), were designed to incorporate some of the therapeutically most favorable structural features of the Leo Pharmaceutical Company's drug candidate diene EB 1089 (seocalcitol, 4) and of the Hopkins' non-calcemic side-chain sulfone analogues 2 and 3. Synthesis of diene sulfones 5 features selective Swern oxidation of a primary silyl ether in the presence of a secondary silyl ether (9-->10) and Horner-Wadsworth-Emmons aldehyde addition by a 1-phosphonyl-3-sulfonyl stabilized carbanion regiospecifically at the 1-position to form E,E-diene sulfone 11. Sulfone diene analogue 5a with natural 1alpha,3beta-diol functionality, but not its diastereomer 5b with unnatural A-ring stereochemistry, is antiproliferative in vitro toward murine keratinocytes and malignant melanoma cells, as well as toward MCF-7 human breast cancer cells. Combining diene sulfone 5a with the currently used anticancer drug adriamycin (ADR) caused a noteworthy 3-fold enhancement of ADR antiproliferative potency in MCF-7 cells. Sulfone diene analogue 5a is weakly active transcriptionally in MCF-7 and ROS 17/2.8 cells, binds poorly but measurably to the vitamin D receptor (VDR), and desirably is non-calcemic in vivo at a daily dose (7 days) of 10 microg/kg of rat body weight.

Kinetic constraints for the thiolysis of 4-methyl-5-(pyrazin-2-yl)-1,2-dithiole-3-thione (oltipraz) and related dithiole-3-thiones in aqueous solution

This report summarizes an investigation of the reactions of biological and other thiols with the cancer chemopreventive oltipraz and other dithiolethiones. Analysis of the kinetics of reaction of 4-methyl-5-(pyrazin-2-yl)-1,2-dithiole-3-thione (oltipraz) 1 with monothiols and dithiols in the range of 0.75-20 mM in aqueous 15% ethanol, at pH 7.5 (0.1 M Tris buffer) and at 37 degrees C has been undertaken. A plot of k(obsd) against [thiol] shows that reactions of mono- and dithiols are first order in thiol concentration. The dependence on pH of these reactions shows that the active species is the thiolate anion. Specific second-order rate constants, k(2) (M(-1) s(-1)) for reaction of the thiolate anions with oltipraz have been determined to be cysteine, 0.040 +/- 0.001; 2-mercaptoethanol, 2.0 +/- 0.02; glutathione, 0.099 +/- 0.001; mercaptoacetic acid anion, 4.0 +/- 0.01; dithiothreitol, 1.33 +/- 0.02; 1,3-propanedithiol, 10 +/- 0.5; 1-mercaptopropane-3-ol, 6.5 +/- 0.1; 1-mercaptopropane-2,3-diol, 1.26 +/- 0.05. A plot of pK(a) against log k(2) for monothiols shows a linear dependence of k(2) on pK(a), beta(nuc) 1.1 +/- 0.07, which accounts for most of the reportedly enhanced reactivity of dithiols over monothiols. The pseudo-first-order rate constant for the solvolysis of oltipraz has been measured as 2.2 (+/-0.2) x 10(-8) s(-1). The kinetics of reaction of three other dithiole-3-thiones with glutathione has also been studied for comparison with oltipraz. The specific second-order rate constants, k(2) (M(-1) s(-1)) are 5-phenyl-1,2-dithiole-3-thione, 4.7 x 10(-)(4); 5-(4-methoxyphenyl)-1,2-dithiole-3-thione, 4.1 x 10(-4); and 1,2-dithiole-3-thione 0.08. Important implications for the mode of biological action of these compounds and the nature of the putative biological targets of the compounds are discussed.

Liquid chromatography electrospray-mass spectrometry of urinary aflatoxin biomarkers: characterization and application to dosimetry and chemoprevention in rats

A liquid chromatography electrospray tandem mass spectrometry (LC-ESI-MS/MS) method for the measurement of aflatoxin biomarkers in urine has been developed and validated. The two major aflatoxin-DNA adducts formed in rat tissues, aflatoxin N(7)-guanine and its imidazole ring opened derivative, 8,9-dihydro-8-(2,6-diamino-4-oxo-3,4-dihydropyrimid-5-yl formamido)-9-hydroxy-aflatoxin B(1), were detected and quantified in urine by the LC-ESI-MS/MS technique. Other metabolites derived from the conjugation and/or oxidation of aflatoxin B(1) measured in the urine of dosed rats included aflatoxin P(1), aflatoxin P(1)-glucuronide, aflatoxin Q(1), aflatoxin M(1), 8,9-dihydro-8,9-dihydroxy aflatoxin B(1), aflatoxin B(1)-mercapturic acid, the aflatoxin-cysteine glycine adduct derived from the aflatoxin-glutathione conjugate, aflatoxin M(1)P(1) and the aflatoxin B(1)-dialcohol. For in vivo studies to determine the dosimetry of certain aflatoxin metabolites, aflatoxin B(2) was used as an internal standard for recovery since this compound is not naturally produced in rats. In the final method using the internal standard, the coefficient of variation of six replicate analyses of in vivo rat urine samples for aflatoxin N(7)-guanine, aflatoxin B(1)-mercapturic acid, and aflatoxin M(1) was 12.5, 12.8, and 5.8%, respectively. Further, the LC-ESI-MS/MS method to detect aflatoxin N(7)-guanine in in vivo rat urine samples was at least 20-fold more sensitive than prior techniques. Using the LC-ESI-MS/MS technique, the dosimetry, on a weekly basis, of major urinary aflatoxin metabolites was assessed in animals chronically dosed over a 5-week period. Of particular importance was the application of this method to determine the modulation of levels of urinary aflatoxin metabolites by treatment with oltipraz, a chemopreventive agent that can completely ablate aflatoxin hepatocarcinogenesis in the rat. After 1 week, oltipraz administration diminished urinary aflatoxin N(7)-guanine, aflatoxin B(1)-mercapturic acid and aflatoxin M(1) levels by 83, 92, and 82%, respectively. The magnitude of this reduction was persistent at the day 14, 21, 28, and 35-day time points with the average decrease of aflatoxin N(7)-guanine, aflatoxin B(1)-mercapturic acid and aflatoxin M(1) being 73, 92, and 90%, respectively. Importantly, even under circumstances where the oltipraz intervention was most efficient in reducing aflatoxin metabolite levels, the LC-ESI-MS/MS method was still sensitive enough to detect the reduced biomarker content. This outcome has important translational implications for the application and analysis of the efficacy of primary and secondary prevention interventions in human populations where ambient exposure levels are low, but the toxicologic hazards of these exposures remain high.

Oltipraz chemoprevention trial in Qidong, People's Republic of China: results of urine genotoxicity assays as related to smoking habits

A Phase II chemoprevention trial was carried out in Qidong, Jiangsu Province, People's Republic of China. The recruited subjects, all of whom were positive for serum aflatoxin-albumin adducts, were divided into three treatment arms: placebo; oltipraz ([5-(2-pyrazinyl)-4-methyl-1,2-dithiol-3-thione]) given daily at 125 mg p.o.; and oltipraz given once per week at 500 mg p.o. Besides biomarkers related to aflatoxin B(1) exposure, the genotoxicity of blind-coded urine XAD-2 concentrates was evaluated in 201 subjects on the fifth and seventh week of intervention. Genotoxicity was assessed both in the Ames reversion test in strain YG1024 of Salmonella typhimurium, in the presence of an exogenous metabolic system (S9 mix), with or without beta-glucuronidase, and in a DNA repair test in Escherichia coli. Heating of concentrated urine samples or of cigarette smoke condensates was discovered to result in a significant enhancement of their mutagenicity. It was also found that the mutagenicity of condensates from the most extensively used brands of cigarettes in Qidong was much lower than that of Western cigarette brands. Urine mutagenicity was unrelated to treatment with oltipraz, intervention time, gender, and supplement of S9 mix with beta-glucuronidase. Mutagenicity was significantly but variably higher in cigarette smokers than in nonsmokers, which suggests that the urinary excretion of mutagens in the examined population was not exclusively attributable to smoking. Nevertheless, within smokers (28% of the recruited subjects; 67% of all males), the mutagenic potency was significantly correlated with the self-reported number of cigarettes smoked per day and, even more sharply, with the cotinine concentrations in urines. In conclusion, this study demonstrated the validity of urine mutagenicity assays as a biomarker of tobacco smoke exposure that can be investigated on a relatively large scale in chemoprevention trials and provided evidence that oltipraz treatment had no influence on this parameter in the examined population.

Cancer chemoprevention using natural vitamin D and synthetic analogs

Substantial epidemiologic data support a role for vitamin D in cancer prevention. However, dose-limiting hypercalcemic effects have proved a major obstacle to the development of natural vitamin D as a cancer chemopreventive. Structure-activity studies have sought to disassociate the toxicities and chemopreventive activities of vitamin D, and a number of synthetic deltanoids (vitamin D analogs) have shown considerable promise in this regard. Several such compounds have chemopreventive efficacy in preclinical studies, as does natural vitamin D. Data supporting further development of agents of this class include in vitro and in vivo evidence of antiproliferative, proapoptotic, prodifferentiating and antiangiogenic activities. Ongoing studies are aimed at further defining the molecular mechanisms through which vitamin D and synthetic deltanoids affect gene expression and cellular fate. Additional efforts are focused on establishing the chemopreventive index (efficacy vs toxicity) of each synthetic deltanoid.

Biomarkers and surrogacy: relevance to chemoprevention

Clinical cancer prevention trials that use disease as the end-point are of necessity large, lengthy and costly. While such trials will always remain the 'gold standard' for establishing efficacy, they are unwieldy and inefficient for the rapid translation of our accelerating understanding of the molecular basis of cancer into preventive strategies. The inclusion of biomarkers in the process of chemopreventive agent development is crucial for the advancement of the field. This overview highlights the types of approach that are being used in the development and application of biomarkers in chemoprevention studies. Biomarkers, which measure exposure, susceptibility or risk factors, can be used in selecting study cohorts, assessing participant compliance and/or determining agent efficacy. Key features of biomarkers include reliability, precision, accuracy and validity. Not all biomarkers are suitable for all purposes and are likely to be imperfect in any single setting. Judicious selection and matching of biomarkers with agents and study cohorts is required for their effective utilization. A critical but non-dichotomous element of risk biomarkers is their degree of surrogacy. A classification scheme is provided that relates the degree of surrogacy of risk biomarkers to their utility in preventive interventions.

Sensitivity to carcinogenesis is increased and chemoprotective efficacy of enzyme inducers is lost in nrf2 transcription factor-deficient mice

Induction of phase 2 enzymes, which neutralize reactive electrophiles and act as indirect antioxidants, appears to be an effective means for achieving protection against a variety of carcinogens in animals and humans. Transcriptional control of the expression of these enzymes is mediated, at least in part, through the antioxidant response element (ARE) found in the regulatory regions of their genes. The transcription factor Nrf2, which binds to the ARE, appears to be essential for the induction of prototypical phase 2 enzymes such as glutathione S-transferases (GSTs) and NAD(P)H:quinone oxidoreductase (NQO1). Constitutive hepatic and gastric activities of GST and NQO1 were reduced by 50-80% in nrf2-deficient mice compared with wild-type mice. Moreover, the 2- to 5-fold induction of these enzymes in wild-type mice by the chemoprotective agent oltipraz, which is currently in clinical trials, was almost completely abrogated in the nrf2-deficient mice. In parallel with the enzymatic changes, nrf2-deficient mice had a significantly higher burden of gastric neoplasia after treatment with benzo[a]pyrene than did wild-type mice. Oltipraz significantly reduced multiplicity of gastric neoplasia in wild-type mice by 55%, but had no effect on tumor burden in nrf2-deficient mice. Thus, Nrf2 plays a central role in the regulation of constitutive and inducible expression of phase 2 enzymes in vivo and dramatically influences susceptibility to carcinogenesis. Moreover, the total loss of anticarcinogenic efficacy of oltipraz in the nrf2-disrupted mice highlights the prime importance of elevated phase 2 gene expression in chemoprotection by this and similar enzyme inducers.

Chemoprotection by organosulfur inducers of phase 2 enzymes: dithiolethiones and dithiins

One of the major mechanisms of protection against carcinogenesis, mutagenesis, and other forms of toxicity mediated by carcinogens is the induction of enzymes involved in their metabolism, particularly phase 2 enzymes such as glutathione S-transferases, UDP-glucuronosyl transferases, and quinone reductases. Animal studies indicate that induction of phase 2 enzymes is a sufficient condition for obtaining chemoprevention and can be achieved by administering any of a diverse array of naturally-occurring and synthetic chemopreventive agents. Alliaceous and cruciferous plants are rich in organosulfur compounds with inducer activity. Indeed, monitoring of enzyme induction has led to the recognition or isolation of novel, potent chemopreventive agents such as 1,2-dithiole-3-thiones, dithiins and the isothiocyanate sulforaphane. For example, oltipraz, a substituted 1,2-dithiole-3-thione originally developed as an antischistosomal agent, possesses chemopreventive activity against different classes of carcinogens targeting multiple organs. Mechanistic studies in rodent models for chemoprevention of aflatoxin B1 (AFB1)-induced hepatocarcinogenesis by oltipraz indicates that increased expression of phase 2 genes is of central importance, although inhibition of phase 1 activation of aflatoxin B1 can also contribute to protection. Exposure of rodents to 1,2-dithiole-3-thiones triggers nuclear accumulation of the transcription factor Nrf2 and its enhanced binding to the Antioxidant Response Element, leading to transcriptional activation of a score of genes involved in carcinogen detoxification and attenuation of oxidative stress. Nrf2-deficient mice fail to induce many of these genes in response to oltipraz and the impact of this genotype on the chemopreventive efficacy of dithiolethiones is currently under investigation. To test the hypothesis that enzyme induction is a useful strategy for chemoprevention in humans, three key elements are necessary: a candidate agent, an at-risk population and modulatable intermediate endpoints. Towards this end, a placebo-controlled, double blind clinical trial of oltipraz was conducted in residents of Qidong, P.R. China who are exposed to dietary aflatoxins and who are at high risk for the development of liver cancer. Oltipraz significantly enhanced excretion of a phase 2 product, aflatoxin-mercapturic acid, a derivative of the aflatoxin-glutathione conjugate, in the urine of study participants administered 125 mg oltipraz by mouth daily. Administration of 500 mg oltipraz once a week led to a significant reduction in the excretion of the primary oxidative metabolite of AFB1, aflatoxin M1, when measured shortly after drug administration. While this study highlighted the general feasibility of inducing phase 2 enzymes in humans, a longer term intervention is addressing whether protective alterations in aflatoxin metabolism can be sustained for extended periods of time in this high-risk population. Food-based approaches to chemoprotection, targeted both to the general population and high-risk individuals, offer many practical advantages compared to the use of pharmaceutical agents. Thus, identification and utilization of naturally-occurring organosulfur chemoprotectors including dithiins should be a high priority.

Hepatocellular carcinoma and aflatoxin exposure in Zhuqing Village, Fusui County, People's Republic of China

Hepatocellular carcinoma (HCC) is a common cause of cancer morbidity and mortality in Asia and Africa. Epidemiological studies have found that dietary exposure to aflatoxin B1 (AFB1) and chronic infection with hepatitis B virus are two major risk factors for HCC. We have collated the incidence and mortality data of malignant tumors from 1973 to 1999 in Zhuqing Village, Fusui County, an area with very high HCC rates, and found that this cancer accounted for 64% of the total cancer incidence. Dietary intake of AFB1 was monitored for 1 week in a study group consisting of 15 males and 14 females from different households in this village. Four of 29 participants (13.8%) and 3 of 15 (20%) male participants were hepatitis B virus surface antigen positive. AFB1 was detectable in 76.7% (23 of 30) of ground corn samples (range, 0.4-128.1 ppb), 66.7% (20 of 30) of cooking peanut oil samples (range, 0.1-52.5 ppb), and 23.3% (7 of 30) of rice samples (range, 0.3-2.0 ppb) collected from each household. Mean levels of serum AFB1-albumin adducts in this group were 1.24 +/- 0.31 pmol/mg of albumin at the beginning of the study and 1.21 +/- 0.19 pmol/mg of albumin at the end of the period. Urinary AFB1 metabolites were detectable in 88.9% (24 of 27) samples (range, 0.9-3569.7 ng/24-h urine). These data provide the exposure and disease risk information for establishing intervention studies to diminish the impact of aflatoxin exposure in this high-risk population.

Role of transcription factor Nrf2 in the induction of hepatic phase 2 and antioxidative enzymes in vivo by the cancer chemoprotective agent, 3H-1, 2-dimethiole-3-thione

BACKGROUND

The induction of phase 2 enzymes by dithiolethiones such as oltipraz is an effective means for achieving protection against environmental carcinogens in animals and humans. Transcriptional control of the expression of at least some of these protective enzymes is mediated through the antioxidant response element (ARE) found in the upstream regulatory region of many phase 2 genes. The transcription factor Nrf2, which binds to the ARE, appears to be essential for the induction of proto-typical phase 2 enzymes such as glutathione S-transferase (GST) Ya, Yp, and NAD(P)H: quinone reductase (NQO1) in vivo.

MATERIALS AND METHODS

In the present study, 3H-1,2-dithiole-3-thione (D3T) was used as a potent model inducer whose effects on gene expression and chemopreventive efficacy have been extensively characterized in the rat. Over a dozen putative D3T-inducible genes were examined in wild-type and nrf2-disrupted mice by Northern blot hybridization and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis to elucidate whether loss of Nrf2 function also affects the induction of a broader representation of phase 2 and antioxidative enzymes. The effects of D3T on hepatic Nrf2 expression and localization were also examined in vivo by Northern blot hybridization, electromobility shift assay, and Western blot analysis.

RESULTS

Specific activities of hepatic GST and NQO1 were increased by D3T in wild-type mice and were largely blunted in the nrf2-deficient mice. However, changes in levels of RNA transcripts following D3T treatment of nrf2-disrupted mice were multidirectional, dependent upon the particular gene examined. Although elevation of mRNAs for GST Ya, NQO1, microsomal epoxide hydrolase and gamma-glutamylcysteine synthetase regulatory chain were blocked in the mutant mice, elevation of GST Yp mRNA was largely unimpeded. Increases in levels of mRNA for the heavy and light chains of ferritin were only seen in the nrf2-disrupted mice. Transcript levels of UDP-glucuronyl-transferase 1A6, heme oxygenase-1, maganese superoxide dismutase, which were inducible in the wild-type mice, actually decreased in the mutant mice, whereas levels of mRNA for GST Yc, aflatoxin B1 aldehyde reductase and catalase decreased following D3T treatment in the mutant mice in the absence of any inductive effect by D3T in the wild-type mice. In wild-type mice, treatment with D3T lead to 3-fold increases in hepatic Nrf2 mRNA levels within several hours following dosing as assessed by Northern blot and RT-PCR analyses. Gel shift analyses with oligonucleotide probes for human NQO1 ARE, murine GST Ya ARE, and erythroid transcription factor (NF-E2) binding site showed increased intensity of binding with nuclear extracts prepared from livers of D3T-treated mice compared to vehicle-treated controls. Antibody to Nrf2 supershifted the DNA binding bands of these nuclear extracts. Moreover, immunoblot analysis indicated accumulation of Nrf2 in extracts prepared from hepatic nuclei of D3T-treated mice at the same time points.

CONCLUSIONS

Nrf2 plays a central role in the regulation of constitutive and inducible expression of multiple phase 2 and antioxidative enzymes by chemoprotective dithiolethiones in vivo, although patterns of response vary among different genes. Knowledge of the factors controlling the specificity of actions of enzyme inducers will be exceedingly helpful in the design and isolation of more efficient and selective chemoprotective agents.

Specific p53 mutations detected in plasma and tumors of hepatocellular carcinoma patients by electrospray ionization mass spectrometry

Hepatocellular carcinoma (HCC), a common cause of cancer deaths worldwide, has several major etiological risk factors, including infection with the hepatitis viruses and exposure to aflatoxin B1. A specific missense mutation resulting from a guanine to thymine transversion at the third position of codon 249 in the p53 tumor suppressor gene has been reported in 10-70% of HCCs from areas of high dietary exposure to aflatoxin B1. Short oligonucleotide mass analysis was compared with DNA sequencing in 25 HCC samples for specific p53 mutations. Mutations were detected in 10 samples by short oligonucleotide mass analysis in agreement with DNA sequencing. Analysis of another 20 plasma and tumor pairs showed 11 tumors containing the specific mutation, and this change was detected in six of the paired plasma samples. Four of the plasma samples had detectable levels of the mutation; however, the tumors were negative, suggesting possible multiple independent HCCs. Ten plasma samples from healthy individuals were all negative. This molecular diagnostic technique has implications for prevention trials and for the early diagnosis of HCC.

Protection against 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine cytotoxicity and DNA adduct formation in human prostate by glutathione S-transferase P1

The prostate has been identified as a target for 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced carcinogenesis. Humans are exposed to PhIP through ingestion of well-done cooked meats, and there is evidence from epidemiological studies that implicates red meat consumption in prostate carcinogenesis. The alpha and pi class isoforms of glutathione S-transferases (GSTs) have been shown to inhibit adduction of activated PhIP metabolites to DNA in cell-free systems. In humans, silencing of GST pi(GSTP1) through CpG island hypermethylation is found in nearly all prostate carcinomas and is believed to be an early event in prostate carcinogenesis. We hypothesized that suppressed GSTP1 expression in prostate cells would increase their vulnerability to cytotoxicity and DNA adduct formation mediated by activated PhIP metabolites. To test this hypothesis, the human prostate adenocarcinoma cell line, LNCaP, which contains a silenced GSTP1 gene, was genetically modified to constitutively express high levels of GSTP1. Both LNCaP and LNCaP-GSTP1 cells exposed to N-OH-PhIP, but not parent PhIP, for 24 h showed a dose-dependent decrease in cell viability. GSTP1-overexpressing cells had LC50s 30-40% higher than cells transfected with the vector alone. PhIP-DNA adducts isolated from LNCaP-derived cells and primary human prostate tissue cultures exposed to N-OH-PhIP were analyzed by liquid chromatography/electrospray ionization mass spectrometry. Primary cultures of human prostate tissue and LNCaP-GSTP1 cells had approximately 50% lower adduct levels than parental LNCaP and vector control cells. Bioactivation assays using LNCaP cytosols showed that enzymatic activation of N-OH-PhIP to a DNA binding species was dependent on ATP and could be inhibited by recombinant human GSTP1 in the presence of glutathione. This evidence confirms that N-OH-PhIP can be bioactivated to a DNA binding species in human prostate and human prostate-derived cells. These observations provide the basis for using LNCaP and LNCaP-GSTP1 cells as a model system for studying the role of this enzyme in protection against N-OH-PhIP induced DNA damage in prostate carcinogenesis. Loss of GSTP1 expression in human prostate may, therefore, enhance its susceptibility to carcinogenic insult by compounds such as N-OH-PhIP. Conversely, induction of GSTs in early-stage prostate carcinogenesis may be a useful protective strategy.

Conceptually new 20-epi-22-oxa sulfone analogues of the hormone 1alpha,25-dihydroxyvitamin D(3): synthesis and biological evaluation

New C,D-ring side-chain-modified sulfone 4a, with natural 1alpha, 3beta-hydroxyl groups but lacking the 25-hydroxyl group characteristic of the natural hormone 1alpha,25-dihydroxyvitamin D(3) (1), has been prepared and characterized. Novel synthetic features include: (1) chemoselective oxidation of only a primary silyl ether in a primary-secondary bis-silyl ether intermediate and (2) smooth reductive etherification without interference by a neighboring sulfonyl group. Sulfone 4a, but not its 1beta, 3alpha-diastereomer 4b, is powerfully antiproliferative and transcriptionally active in vitro but desirably noncalcemic in vivo. Although sulfone 4a, designed to resemble Leo Pharmaceutical Co.'s KH-1060 (3), is recognized by catabolic enzymes, the selective biological profile of sulfone 4a is likely not due to its metabolites that are formed in only minor amounts.

Identification and characterization of chlorin e(4) ethyl ester in sera of individuals participating in the chlorophyllin chemoprevention trial

Chlorophyllin (CHL), a mixture of water soluble derivatives of chlorophyll, has been shown to be an effective inhibitor of aflatoxin B(1) (AFB(1)) carcinogenesis and AFB(1)-DNA adduct formation in rainbow trout and rats [Breinholt, V., Hendricks, J., Pereira, C., Arbogast, D., and Bailey, G. (1995) Cancer Res. 55, 57-62; Kensler, T. W., Groopman, J. D., and Roebuck, B. D. (1998) Mutat. Res. 402, 165-172]. The chemopreventive action of CHL has been previously attributed to molecular complexing. In 1997, a randomized, double-blind clinical trial of CHL was conducted in Qidong, Jiangsu Province, People's Republic of China. At the completion of the study, when serum samples were regrouped by subject identification number, it was noted that many of the participant samples were green in color. Using HPLC, ESI/MS, and MS/MS techniques, serum samples from individuals receiving CHL were found to contain previously unreported copper chlorin e(4) ethyl ester (CuCle(4) ethyl ester) as well as copper chlorin e(4) (CuCle(4)). Both chlorins originated in the study tablet, were absorbed into the bloodstream, and conferred a green color to the sera. This initial finding of in vivo absorption and bioavailability of two chlorin derivatives suggests that the mechanism of CHL chemoprevention may lie in the actions of these two components in vivo in addition to preventing carcinogen absorption from the gut.

A role for protein kinase C-delta in the regulation of ornithine decarboxylase expression by oxidative stress

The expression of genes that regulate cell growth, such as ornithine decarboxylase (ODC), can be modulated by oxidant tumor promoters. Treatment of murine papilloma PE cells with H2O2 led to a transient induction of ODC enzyme activity, which could be blocked by calphostin, a nonspecific inhibitor of protein kinase C (PKC). Peak activity (11-fold) occurred 5-6 h after treatment, followed by a rapid decline. The increase in ODC activity was associated with an elevation of both ODC mRNA (3-fold) and protein (7-fold). Direct involvement of PKC in the regulation of ODC by oxidants was determined by stable transfection of PE cells with a dominant-negative PKC-delta mutant. PKC-delta activity was completely inhibited in response to H2O2 in cells overexpressing mutant PKC-delta compared with cells transfected with a blank plasmid. Induction of ODC mRNA, protein, and activity was also completely inhibited in cells expressing the PKC-delta mutant after H2O2 treatment. Activation of an ODC promoter-luciferase reporter construct by H2O2 was attenuated in mutant cells compared with control cells, further confirming that ODC is regulated transcriptionally by PKC-delta. However, fold-increases in ODC mRNA and protein were much less than the increase in activity, suggesting that ODC may also undergo posttranscriptional regulation in the presence of oxidants. Taken together, these studies provide new insight into the regulation of ODC by oxidants and suggest that PKC-delta may play a critical role in this regulation.

Conceptually new deltanoids (vitamin D analogs) inhibit multistage skin tumorigenesis

Development of vitamin D analogs (deltanoids) as chemopreventive agents requires separation of desirable antiproliferative and pro-differentiating activities from the undesirable calcemic activity also found in the hormone calcitriol (1 alpha, 25-dihydroxyvitamin D(3)). Therefore, several conceptually new deltanoids were synthesized with modifications to the 1alpha- and/or 25-hydroxyl groups, positions traditionally considered essential for stimulating biological responses. In this study, 1 beta-hydroxymethyl-3-epi-25-hydroxyvitamin D(3), a non-calcemic CH(2) homolog of the natural hormone with antiproliferative activity in vitro, was ineffective as an inhibitor of 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced induction of ornithine decarboxylase activity in mouse epidermis. However, a hybrid analog incorporating not only the calcemia-ablating 1 beta-hydroxymethyl alteration, but potentiating C,D ring 16-unsaturation and side chain 24,24-fluorination and 26, 27-homologation was found to be as effective as calcitriol. Several non-calcemic 24- or 25-t-butyl sulfones, some containing side chain fluorination but all lacking the 25-hydroxyl group, were also shown to be active in this assay. Three sulfones and the 1 beta-hydroxymethyl hybrid were evaluated as inhibitors of multistage carcinogenesis in mouse skin. Female CD-1 mice were initiated with a single dose of 7,12-dimethylbenz[a]anthracene and then promoted twice weekly for 20 weeks with TPA. Deltanoids were applied topically 30 min before TPA. Unlike calcitriol, none of the atypical deltanoids affected body weight gain in these animals. Minimal effects on urinary calcium excretion were observed following chronic treatment with these analogs. All deltanoids inhibited the incidence and multiplicity of papilloma formation, with the hybrid analog showing the greatest efficacy. With this deltanoid, tumor incidence was significantly reduced by 28% and tumor multiplicity by 63%. These results, coupled with the rich chemical diversity available in side chain sulfur-containing deltanoids, particularly when combined with A ring modifications such as 1 beta-hydroxylalkyl groups, provide important new advances in the fundamental understanding of chemical structure-biological activity relationships as well as more potent and safe vitamin D analogs for cancer chemoprevention and other medicinal uses.

Reduction of aflatoxin B(1) adduct biomarkers by oltipraz in the tree shrew (Tupaia belangeri chinensis)

The risk of liver cancer is greatest in people both infected with hepatitis B virus (HBV) and highly exposed to aflatoxin B(1) (AFB(1)). The tree shrew (Tupaia belangeri chinensis) is a unique species that can be infected with human HBV, is susceptible to AFB(1)-induced liver cancer, and shows a synergistic interaction between HBV and AFB(1) for liver cancer. In this regard, the tree shrew may be useful for evaluating experimental chemoprevention strategies relevant to high-risk human populations as it mirrors the human epidemiology of liver cancer. To begin developing the model for chemoprevention study, two groups of tree shrews were fed 400 microg AFB(1)/kg b.wt. in milk daily for 4 weeks. One week prior to AFB(1) administration, one group also received oltipraz (0.5 mmol/kg, p.o.) daily for 5 weeks. At weekly intervals, 1 ml of blood and a 24-h urine sample were obtained from each animal. Aflatoxin-albumin adducts in serum were determined by a radioimmunological assay and aflatoxin-N(7)-guanine adducts in urine were measured by HPLC. Aflatoxin-albumin adducts increased rapidly in 2 weeks to plateau at 20 pmol/mg protein, and they diminished after cessation of AFB(1) exposure. Oltipraz significantly attenuated the overall burden of aflatoxin-albumin adducts throughout the exposure period with a median reduction of 80%. In a single cross-sectional analysis at the end of AFB(1) dosing, oltipraz treatment decreased urinary aflatoxin-N(7)-guanine by 93%. Collectively, these results indicate that oltipraz reduces AFB(1) risk biomarkers in the tree shrew in a manner similar to that observed in rodents and humans, and establishes a rationale to evaluate cancer chemoprevention by oltipraz in human HBV-infected, AFB(1) exposed tree shrews.

Chemoprotective 3H-1,2-dithiole-3-thione induces antioxidant genes in vivo

Several 1,2-dithiole-3-thiones are potent inhibitors of chemical-induced tumors in multiple tissues. Chemoprotection by 1, 2-dithiole-3-thiones has been associated with induction of detoxication enzymes, although several studies suggest that additional mechanisms may be involved. In this study, we examined the induction of hepatic antioxidant genes in rats treated with 3H-1, 2-dithiole-3-thione (D3T). After a 24 h D3T treatment, a 2.4-fold increase in catalase mRNA was observed, which was accompanied by a 1. 5-fold increase in catalase protein expression and a 2.3-fold increase in catalase activity. D3T also mediated 2.9-, 5.9-, and 3. 7-fold increases in the 1.0, 3.0, and 4.0 kb mRNA species of manganese superoxide dismutase (MnSOD), respectively. The induction of MnSOD mRNA by D3T was coincident with 1.7-fold and 4.6-fold increases in MnSOD protein and enzyme activity, respectively. Induction of gamma-glutamylcysteine synthetase mRNA by D3T was accompanied by an increase in glutathione levels. Nuclear run-on assays provided evidence that D3T enhances the transcription rate from MnSOD, catalase, and gamma-glutamylcysteine synthetase. In support of this view, D3T also activated an MnSOD promoter-reporter construct in transiently transfected HepG2 cells. In light of observations that antioxidant enzyme regulation may be altered during carcinogenesis, induction of these genes could provide a potentially important mechanism of action of chemoprotective 1, 2-dithiole-3-thiones.

United States-Japan workshop on New Rodent Models for the Analysis and Prevention of Carcinogenesis

In assessing the present status of rodent models for the analysis and prevention of carcinogenesis, the discussion emphasized that models exist for very few of the major cancers occurring in the United States and Japan. Almost without exception, those that do exist were invented for etiological and mechanistic studies of chemical and physical carcinogenesis. Applicability to cancer chemoprevention was not a primary objective in their development. However, they have been adapted and used for identifying and characterizing chemopreventive agents out of necessity. It is therefore not surprising that they generally fail to fulfill most or all of the requisites of "ideal" models enumerated above, and consequently, the validity of data produced through their use has been questioned. Evolution of future mechanistic models may similarly lead to their eventual adaptation to chemoprevention, but advances will be fragmentary, and the rate of progress seems likely to be slow. Animal models discussed at this workshop were designed specifically for this purpose and promise to expedite the accumulation of valuable new information. Nonetheless, recurrent discussion identified the urgent need for institution of a major, dedicated research initiative with the expressed objective of developing rodent models for organ-specific chemoprevention based on current understanding of underlying genetic and cellular processes. No concerted programs with this objective presently exist either in the United States or Japan. In addition to research specifically designed to meet this need, efforts should be made to involve investigators developing mechanistic animal models in the validation of their models through modulating cancer development by chemopreventive agents. Support for such research initiatives will be essential to continued progress toward the overall objective of identifying safe and effective cancer chemopreventive agents.

Elevated HPRT mutation frequencies in aflatoxin-exposed residents of daxin, Qidong county, People's Republic of China

Molecular biomarkers are becoming increasingly important tools to identify people who are at highest risk of developing cancer. For many years we have been studying residents of Qidong County, People's Republic of China, to examine the combined impact of aflatoxin exposure with other risk factors as contributors to the high liver cancer incidence rates in this region. This study was conducted to determine the effects of aflatoxin exposure, as measured by serum aflatoxin-albumin adduct levels, on somatic mutation frequency in the human hypoxanthine guanine phosphoribosyl transferase gene (HPRT). Subjects were assigned as low or high according to a dichotomization around the population mean of aflatoxin-albumin adducts. HPRT mutant frequency was determined in individuals by a T cell clonal assay and the samples were categorized as low or high according to mean values. Separate analyses were also conducted for the small set of hepatitis B virus surface antigen (HBsAg)-positive and the larger set of HBsAg-negative individuals, known risk factors for liver cancer. An odds ratio of 19.3 (95% confidence interval 2.0, 183) was demonstrated for a high HPRT mutation frequency in individuals with high aflatoxin exposure compared with those with low aflatoxin exposure. This association indicates that aflatoxin-induced DNA damage in T lymphocytes, assessed using the validated surrogate albumin adduct markers, leads to increased mutations reflected as elevated HPRT gene mutations. This cross-sectional study suggests the potential use of mutation frequency of the HPRT gene as a long-term biomarker of aflatoxin exposure in high risk populations.

Antimalarial, antiproliferative, and antitumor activities of artemisinin-derived, chemically robust, trioxane dimers

Nine C-10 non-acetal derivatives of the natural trioxane artemisinin (1) were prepared as dimers using some novel chemistry. As designed, each dimer was stable chemically. C-10 Olefinic dimers 7 and C-10 saturated dimers 8-13 all showed good to excellent antimalarial and antiproliferative activities in vitro. Dimers 8, 10, and 12 were especially potent and selective at inhibiting growth of some human cancer cell lines in the NCI in vitro 60-cell line assay.

Conceptually new sulfone analogues of the hormone 1alpha, 25-dihydroxyvitamin D(3): synthesis and preliminary biological evaluation

A conceptually new series of vitamin D(3)-like nonfluorinated and fluorinated 16-ene side chain tert-butyl sulfones 3-7 has been synthesized. Even though these novel C,D-ring side chain analogues of the hormone 1alpha,25-dihydroxyvitamin D(3) (1,1,25D(3)) lack a terminal OH group, thought previously to be essential for high biological activity, they are highly antiproliferative and, in several cases, transcriptionally active in vitro but desirably noncalcemic in vivo. The side chain sulfone group may be binding to the nVDR as a hydrogen-bond acceptor, in contrast to the hydrogen-bond donor function of the 25-OH group of natural 1,25D(3).

cDNA cloning, expression and activity of a second human aflatoxin B1-metabolizing member of the aldo-keto reductase superfamily, AKR7A3

The aflatoxin B1 (AFB1) aldehyde metabolite of AFB1 may contribute to the cytotoxicity of this hepatocarcinogen via protein adduction. Aflatoxin B1 aldehyde reductases, specifically the NADPH-dependent aldo-keto reductases of rat (AKR7A1) and human (AKR7A2), are known to metabolize the AFB1 dihydrodiol by forming AFB1 dialcohol. Using a rat AKR7A1 cDNA, we isolated and characterized a distinct aldo-keto reductase (AKR7A3) from an adult human liver cDNA library. The deduced amino acid sequence of AKR7A3 shares 80 and 88% identity with rat AKR7A1 and human AKR7A2, respectively. Recombinant rat AKR7A1 and human AKR7A3 were expressed and purified from Escherichia coli as hexa-histidine tagged fusion proteins. These proteins catalyzed the reduction of several model carbonyl-containing substrates. The NADPH-dependent formation of AFB1 dialcohol by recombinant human AKR7A3 was confirmed by liquid chromatography coupled to electrospray ionization mass spectrometry. Rabbit polyclonal antibodies produced using recombinant rat AKR7A1 protein were shown to detect nanogram amounts of rat and human AKR7A protein. The amount of AKR7A-related protein in hepatic cytosols of 1, 2-dithiole-3-thione-treated rats was 18-fold greater than in cytosols from untreated animals. These antibodies detected AKR7A-related protein in normal human liver samples ranging from 0.3 to 0.8 microg/mg cytosolic protein. Northern blot analysis showed varying levels of expression of AKR7A RNA in human liver and in several extrahepatic tissues, with relatively high levels in the stomach, pancreas, kidney and liver. Based on the kinetic parameters determined using recombinant human AKR7A3 and AFB1 dihydrodiol at pH 7.4, the catalytic efficiency of this reaction (k2/K, per M/s) equals or exceeds those reported for other enzymes, for example cytochrome P450s and glutathione S-transferases, known to metabolize AFB1 in vivo. These findings indicate that, depending on the extent of AFB1 dihydrodiol formation, AKR7A may contribute to the protection against AFB1-induced hepatotoxicity.

Protective alterations in phase 1 and 2 metabolism of aflatoxin B1 by oltipraz in residents of Qidong, People's Republic of China

BACKGROUND

Residents of Qidong, People's Republic of China, are at high risk for development of hepatocellular carcinoma, in part due to consumption of foods contaminated with aflatoxins, which require metabolic activation to become carcinogenic. In a randomized, placebo-controlled, double-blind phase IIa chemoprevention trial, we tested oltipraz, an antischistosomal drug that has been shown to be a potent and effective inhibitor of aflatoxin-induced hepatocarcinogenesis in animal models.

METHODS

In 1995, 234 adults from Qidong were enrolled. Healthy eligible individuals were randomly assigned to receive by mouth 125 mg oltipraz daily, 500 mg oltipraz weekly, or a placebo. Sequential immunoaffinity chromatography and liquid chromatography coupled to mass spectrometry or to fluorescence detection were used to identify and quantify phase 1 and phase 2 metabolites of aflatoxin B1 in the urine of study participants. Reported P values are two-sided.

RESULTS

One month of weekly administration of 500 mg oltipraz led to a 51% decrease in median levels of the phase 1 metabolite aflatoxin M1 excreted in urine compared with administration of a placebo (P = .030), but it had no effect on levels of a phase 2 metabolite, aflatoxin-mercapturic acid (P = .871). By contrast, daily intervention with 125 mg oltipraz led to a 2.6-fold increase in median aflatoxin-mercapturic acid excretion (P = .017) but had no effect on excreted aflatoxin M1 levels (P = .682).

CONCLUSIONS

Intermittent, high-dose oltipraz inhibited phase 1 activation of aflatoxins, and sustained low-dose oltipraz increased phase 2 conjugation of aflatoxin, yielding higher levels of aflatoxin-mercapturic acid. While both mechanisms can contribute to protection, this study highlights the feasibility of inducing phase 2 enzymes as a chemopreventive strategy in humans.

Identification of dithiolethiones with better chemopreventive properties than oltipraz

Oltipraz and related dithiolethiones are an important class of chemopreventive agents. Studies were undertaken to identify cancer chemopreventive dithiolethiones more active than oltipraz. Largely based upon enzyme induction activities in vitro, 17 dithiolethiones, including oltipraz, were analyzed for their ability to induce hepatic phase II enzyme activities in vivo. Of these compounds, 15 produced greater induction of NAD(P)H:quinone reductase and 11 yielded greater induction of glutathione S-transferase than oltipraz. All 17 dithiolethiones were then tested for their ability to inhibit acute hepatotoxicity by aflatoxin B1 (AFB1), which previously has been shown to be an intermediate predictor of chemopreventive activity. Rats were pretreated with dithiolethiones (0.3 mmol/kg body wt, three times a week per os) and challenged with two acutely toxic doses of AFB1 (0.5 mg/kg body wt, once daily for two successive days per os). Inhibition of hepatotoxicity was measured by changes in body weight gain during AFB1 challenge, reduction in levels of hepatic enzymes in serum and diminution of bile duct cell proliferation. Nine dithiolethiones spanning a range of responses in this toxicity screen were further tested for their ability to prevent AFB1-induced tumorigenicity, as assessed by a reduction in hepatic burden of putative preneoplastic foci. Six dithiolethiones were found to be considerably more effective than oltipraz in preventing AFB1-induced tumorigenesis. In general, dithiolethiones that were very effective in inhibition of acute hepatotoxicity were also found to be effective in prevention of hepatic tumorigenesis.

Noncalcemic, antiproliferative, transcriptionally active, 24-fluorinated hybrid analogues of the hormone 1alpha, 25-dihydroxyvitamin D3. Synthesis and preliminary biological evaluation

Four new hybrid analogues of 1alpha,25-dihydroxyvitamin D3 (1) have been synthesized in a convergent manner by joining A-ring and C, D-ring fragments. Each hybrid analogue, having a noncalcemic 1-hydroxymethyl group and a potentiating 16-ene 24,24-difluorinated C,D-ring side chain, was designed to be lipophilic and inert toward 24-hydroxylase enzyme catabolism. Each hybrid analogue with 1beta, 3alpha-substituent stereochemistry (i.e., analogues 3b and 4b) showed a pharmacologically desirable combination of in vitro high antiproliferative activity in two different cell lines and high transcriptional activity with also low calcemic activity in vivo.

Use of aflatoxin adducts as intermediate endpoints to assess the efficacy of chemopreventive interventions in animals and man

Clinical cancer prevention studies that use disease as an endpoint are of necessity, large, lengthy, and extremely costly. Development of the field of cancer chemoprevention is being accelerated by the application of intermediate markers to preclinical and clinical studies. Sensitive and specific analytic methods have been developed for detecting and quantifying levels of covalent adducts of aflatoxins with cellular DNA and blood proteins at ambient levels of exposure. Such biomarkers can be applied to the preselection of exposed individuals for study cohorts, thereby reducing study size requirements. Levels of these aflatoxin-DNA and albumin adducts can be modulated by chemopreventive agents such as oltipraz and chlorophyllin in experimental models. Overall, a good concordance is seen between diminution of biomarkers and reductions in tumor incidence and/or multiplicity in these settings. Thus, these markers can also be used to rapidly assess the efficacy of preventive interventions. However, the successful application of these biomarkers to clinical prevention trials will be dependent upon prior determination of the associative or causal role of the marker to the carcinogenic process, establishment of the relationship between dose and response, and appreciation of the kinetics of adduct formation and removal. The general approach that has been utilized for the development, validation and application of aflatoxin-DNA and protein adduct biomarkers to cancer chemoprevention trials is summarized.

Identification of dithiolethione-inducible gene-1 as a leukotriene B4 12-hydroxydehydrogenase: implications for chemoprevention

Cancer chemoprevention is inhibition of neoplastic disease by naturally occurring or synthetic chemical agents. Dithiolethiones inhibit production of experimentally produced tumors by elevating the expression of several genes that encode for known cytoprotective enzymes. In an effort to discover additional molecular mechanisms mediating chemoprevention, cDNA clones representing a gene that is transcriptionally activated by dithiolethiones, hence named dithiolethione-inducible gene-1 (DIG-1), were isolated from rat liver via differential hybridization screening. The deduced amino acid sequence of DIG-1 was found to have 80% identity with the human liver enzyme leukotriene B4 (LTB4) 12-hydroxydehydrogenase. DIG-1, purified >400-fold from the liver of rats dosed with 1,2-dithiole-3-dithiolethione, possessed an NADP+-dependent activity to convert LTB4 to 12-oxo-LTB4. Kinetic analysis of DIG-1 revealed apparent Km and Vmax values of 28 mM and 8.1 nmol 12-oxo-LTB4 formed/min/mg purified protein respectively. Since LTB4 is a potent chemotactic factor and stimulator of production of reactive oxygen species from neutrophils, the effects of DIG-1 on these LTB4-mediated processes were examined. Pre-incubation of LTB4 with purified rat hepatic DIG-1 greatly diminished LTB4-stimulated migration of neutrophils. In addition, pre-incubation of LTB4 with purified rat hepatic DIG-1 reduced LTB4-stimulated production of superoxide anions in neutrophils, as evidenced by decreased lucigenin-derived chemiluminescence. These results suggest that DIG-1-catalyzed dehydrogenation of LTB4 to 12-oxo-LTB4 inhibits the pro-inflammatory actions of LTB4. Consequently, elevation of LTB4 catabolism via enhanced DIG-1 activity may suppress inflammatory processes implicated in tumorigenesis.

Oltipraz chemoprevention trial in Qidong, Jiangsu Province, People's Republic of China

Oltipraz has been used clinically in many regions of the world as an antischistosomal agent and is an effective inhibitor of aflatoxin hepatocarcinogenesis in rats. This chemopreventive action of oltipraz results primarily from an altered balance in aflatoxin metabolic activation and detoxication. In 1995, a randomized, placebo-controlled, double-blind intervention was conducted in residents of Qidong, People's Republic of China, who are at high risk for exposure to aflatoxin and development of hepatocellular carcinoma. The major study objectives were to define a dose and schedule for oltipraz that would reduce levels of aflatoxin biomarkers in biofluids of the participants, and to further characterize dose-limiting side effects. Two hundred thirty-four healthy eligible individuals, including those infected with HBV, were randomized to receive either 125 mg oltipraz daily, 500 mg oltipraz weekly, or placebo. Blood and urine specimens were collected to monitor potential toxicities and evaluate biomarkers over the 8-week intervention and subsequent 8-week follow-up periods. Overall, compliance in the intervention was excellent; approximately 85% of the participants completed the study. Objective evaluation of adverse events was greatly facilitated by inclusion of a placebo arm in the study design. A syndrome involving numbness, tingling, and pain in the fingertips was the only event that occurred more frequently among the active groups (18 and 14% of the daily 125 mg and weekly 500 mg arms, respectively) compared to placebo (3%). These symptoms were reversible and could be relieved with non-steroidal antiinflammatory agents. A more complete understanding of the chemopreventive utility of oltipraz awaits completion of an assessment of the efficacy of oltipraz in modulating levels of aflatoxin biomarkers.

Inhibition of cell growth in CaCO2 cells by the polyamine analogue N1,N12-bis(ethyl)spermine is preceded by a reduction in MYC oncoprotein levels

The polyamine analogue N1,N12bis(ethyl)spermine (BESpm) is a potent inhibitor of cell proliferation and is representative of a class of agents currently in clinical trials. Previous studies have demonstrated that BESpm treatment can produce a decrease in the mRNA levels of the protooncogene c-myc resulting from decreased transcription. Investigation into the mechanism of the antiproliferative effect of BESpm in the colon cancer cell line CaCO2 indicated that significant reduction in MYC protein, but not c-myc mRNA levels, preceded cytostasis. Specificity of the downregulation of MYC expression by BESpm treatment was demonstrated by comparison to effects on the polyamine catabolic enzyme spermidine/spermine N1-acetyltransferase (SSAT) and the polyamine biosynthetic enzyme ornithine decarboxylase (ODC). SSAT activity rapidly increased while levels of ODC activity decreased after BESpm treatment. Measurement of intracellular polyamines demonstrated significant uptake of the analogue after 24 hours, which was concurrent with a reduction of spermine and spermidine levels. Thus, cellular uptake of BESpm mediated a reduction of polyamine levels that was associated with a decrease of MYC protein at the post-transcriptional level.

Oltipraz chemoprevention trial in Qidong, People's Republic of China: modulation of serum aflatoxin albumin adduct biomarkers

In 1995, 234 adults from Qidong, People's Republic of China, were enrolled and followed in a Phase IIa 4-methyl-5-(N-2-pyrazinyl)-1,2-dithiole-3-thione (oltipraz) chemoprevention trial. Residents of this area are at high risk for development of hepatocellular carcinoma, in part due to consumption of aflatoxin-contaminated foods. The intervention was a randomized, placebo-controlled, double-blind study. Elements of the study design and clinical outcomes have been recently published (Jacobson et al, Cancer Epidemiol. Biomark. Prev., 6: 257-265, 1997). The primary objective was to conduct a preliminary assessment of the ability of oltipraz to modulate levels of a validated biomarker of aflatoxin exposure and of the risk of hepatocellular carcinoma by determining levels of aflatoxin-albumin adducts in sera. Healthy eligible individuals were randomized into three arms to receive p.o. 125 mg of oltipraz daily, 500 mg of oltipraz weekly, or placebo for 8 weeks. There were no consistent changes in biomarker levels in the placebo arm over the 16-week observation period, nor was any apparent effect observed in the arm receiving 125 mg of oltipraz each day. However, individuals receiving 500 mg of oltipraz once a week for 8 weeks showed a triphasic response to oltipraz. No effect was observed during the 1st month of the intervention, whereas a significant (P = 0.001) diminution in adduct levels was observed during the 2nd month of active intervention and during the lst month of follow-up. A partial rebound in adduct levels toward baseline values was observed during the 2nd month postintervention. Linear regression models up to week 13 confirmed a significant (P = 0.008) weekly decline of biomarker levels in the group receiving 500 mg of oltipraz once a week. However, despite these effects relative to baseline values within the 500-mg weekly arm, there were no statistically significant differences in biomarker trajectories between treatment arms. The genotype for glutathione S-transferase M1, an oltipraz-inducible isoform involved in the detoxification of aflatoxin B1, did not appear to affect either baseline levels or rates of decline in the biomarker. A follow-up Phase IIb trial with a longer intervention period will be necessary to determine the full extent to which aflatoxin biomarker burden can be reduced and whether diminution of biomarkers can be sustained over the long term.

Redox regulation of genes that protect against carcinogens

Most carcinogens require activation to electrophilic metabolites or species that generate reactive oxygen in order to initiate the tumorigenic process. These reactive intermediates can, in turn, be detoxified by endogenous enzyme systems that and in the protection of cells from either toxic or mutagenic product formation. The levels of many of these enzymes are elevated by numerous compounds found in the diet, or by antioxidants. Recent evidence describes the mechanism for this induction of carcinogen detoxication enzymes to be regulated at the transcriptional level. Nuclear transcription factors bound to sites common among these carcinogen detoxication genes are activated by as yet unknown signal transduction pathways. The activity of these nuclear transcription factors are modulated by pro- and antioxidant reagents, suggesting that a redox-sensitive component governs the induction of enzymes involved in carcinogen metabolism. In this review, evidence for the redox regulation of the genes encoding carcinogen detoxication enzymes is presented. Evidence is also presented suggesting the participation of nuclear factor kappa B (NF-kappa B), mitogen-activated protein (MAP) kinase, and basic leucine zipper (bZIP) proteins and their activation pathways in this induction.

Prevention of liver cancer

Hepatocellular carcinoma (HCC) is among the most prevalent and deadly cancers worldwide. Elevated incidence of HCC is strongly associated with hepatitis B virus infection and dietary exposure to hepatotoxic contaminants. Vaccination programs against viral hepatitis hold hope for the eventual reduction of HCC incidence, and screening of cirrhotic patients may provide one mechanism for improving prognosis of HCC patients. Additional efforts have focused on preventing or retarding liver cancer development in populations at risk for HCC mortality. This review highlights recent clinical trials aimed at exploring the efficacy and practicality of chemoprevention efforts in these cohorts. Preclinical findings that provide insight into mechanisms of hepatocarcinogenesis and point to potential strategies for intervention also are discussed. Finally, novel molecular mechanisms contributing to the known ability of chemopreventive agents to inhibit liver cancer in experimental models are explored.

Predictive value of molecular dosimetry: individual versus group effects of oltipraz on aflatoxin-albumin adducts and risk of liver cancer

Studies in animals and humans have established serum aflatoxin-albumin adducts as biomarkers of exposure to aflatoxin B1 (AFB1), a food-borne hepatocarcinogen. To assess the utility of measurements of aflatoxin-albumin adducts to predict risk of hepatocellular carcinoma (HCC), 123 male F344 rats were dosed with 20 microg of AFB1 daily for 5 weeks after randomization into three groups: no intervention; delayed-transient (500 ppm of oltipraz, weeks 2 and 3 relative to AFB1); or persistent (500 ppm oltipraz, weeks -1 to 5). Serial blood samples were collected from each animal at weekly intervals throughout aflatoxin B1 exposure and assayed for levels of aflatoxin-albumin by radioimmune assay. Area under the curve (AUC) values for aflatoxin-albumin adducts decreased 20 and 39% in the delayed-transient and persistent oltipraz intervention groups, respectively, as compared to no intervention. Similarly, the total incidence of HCC dropped from 83 to 60% (P = 0.03) and 48% (P < 0.01) in these groups. Tumor multiplicity was also reduced in the two oltipraz intervention groups, whereas time to HCC was increased. Mononuclear cell leukemia, a common neoplasm in F344 rats, was seen in 39% of the control animals, whereas the two oltipraz interventions reduced incidence to 18% (P = 0.05) and 13% (P = 0.01), respectively. Overall, a significant association was seen between biomarker AUC and risk of HCC (P = 0.01). However, when the predictive value of aflatoxin-albumin adducts was assessed within treatment groups, there was no association between AUC and risk of HCC (P = 0.56). Thus, aflatoxin-albumin adducts can be useful for monitoring population-based changes induced by interventions, such as in chemoprevention trials, but have limited utility in identifying individuals destined to develop HCC. As a consequence, the use of this biomarker in quantitative risk assessment should be pursued cautiously.