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.

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.

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.

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.

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.

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.

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.

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.

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.

Oltipraz chemoprevention trial in Qidong, People's Republic of China: study design and clinical outcomes

In 1995, 234 adults from Qidong, Jiangsu Province, People's Republic of China, where hepatocellular carcinoma is the leading cause of cancer deaths and exposure to dietary aflatoxins is widespread, were enrolled and followed in a Phase II chemoprevention trial. The goals of the study were to define a dose and schedule of oltipraz for reducing levels of validated aflatoxin biomarkers and to characterize dose-limiting toxicities. Healthy eligible individuals, including those infected with hepatitis B virus, were randomized to receive either 125 mg of oltipraz daily, 500 mg of oltipraz weekly, or placebo. Blood and urine specimens were collected to monitor toxicities and evaluate biomarkers over the 8-week intervention period and subsequent 8-week follow-up period. Unique trial aspects included a synchronous follow-up schedule, daily observed administration of all medications, timely international data transference, and use of biomarkers as outcomes. One hundred thirty-two participants took their medications without interruptions, approximately 77% contributed all nine urine samples, and 78% contributed all seven blood samples. Fifty-one participants (21.8%) reported clinical adverse events. An extremity syndrome, developing soon after the start of treatment, was the only event that occurred more frequently (P = 0.002) among the active groups (18.4 and 14.1% of the daily 125 and weekly 500 mg arms, respectively) compared with placebo (2.5%). The oltipraz arms did not differ in symptom type or severity, and there were no indications of exacerbated drug intolerance among the few participants infected with hepatitis B virus. The good compliance with an intense follow-up schedule shows that chemoprevention trials with biomarker end points may be conducted in such populations.

Generation of DNA base modification following treatment of cultured murine keratinocytes with benzoyl peroxide

Benzoyl peroxide (BzPO) is a free radical generating compound that acts as a tumor promoter and progressor in mouse skin. BzPO is cleaved in the presence of copper to produce benzoyloxyl and phenyl radicals. Treatment of mutation reporter plasmids with BzPO and copper yields predominantly single-strand breaks and G-->T transversion mutations. To explore the role of base modifications in the possible mammalian mutagenicity of BzPO the formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG) within the DNA of cultured murine keratinocytes was investigated. Treatment with 10 microM BzPO produced a maximum 3-fold increase in levels of 8-OHdG versus vehicle controls within 1-2 h, with significant levels of 8-OHdG persisting 6 h after initial exposure to BzPO. Pretreatment with the copper chelator bathocuproine disulfonic acid reduced the levels of 8-OHdG generated by BzPO to near background. However, treatment with the iron chelator desferal did not. The stable metabolic product of BzPO benzoic acid was ineffective in producing 8-OHdG. Depletion of cellular glutathione with L-buthionine-(S,R)-sulfoximine increased the amount of BzPO-generated 8-OHdG, while supplementation with glutathione monoethyl ester reduced the number of 8-OHdG molecules formed. Collectively, these results suggest that BzPO at non-cytotoxic concentrations undergoes copper-dependent activation to a reactive product to generate 8-OHdG within cultured murine keratinocytes.

Intermittent dosing with oltipraz: relationship between chemoprevention of aflatoxin-induced tumorigenesis and induction of glutathione S-transferases

Oltipraz [5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione] protects against chemical carcinogenesis in several animal models and is currently under evaluation as a possible chemopreventive agent in humans. Ideally, clinical chemopreventive interventions use dosing regimens that maximize efficacy while minimizing toxicity. Toward this end, the chemopreventive efficacy achieved by administration of intermittent doses of oltipraz was evaluated in rats. F344 rats were treated with oltipraz (0.5 mmol/kg, p.o.) once weekly, twice weekly, or daily over a 5-week period. After the first week, all rats were gavaged with 20 micrograms/kg of aflatoxin B1 for 28 consecutive days. Livers were analyzed 2 months after the last aflatoxin B1 dose, and the volume of liver occupied by glutathione S-transferase (GST)-P positive foci, a presumptive marker of neoplasia, was observed to be decreased > 95%, > 97%, or > 99% in livers of rats receiving once-, twice-weekly or daily oltipraz treatments, respectively. The chemopreventive actions of oltipraz have been associated with increases in the levels of phase 2 detoxifying enzymes, such as the glutathione S-transferase isozymes. Accordingly, GST conjugation activity measured with 1-chloro-2,4-dinitrobenzene as substrate increased 1.5-, 1.8-, or 2.4-fold for the once-weekly, twice-weekly or daily treatments, respectively, throughout a 7-day period. Quantitative HPLC analyses of GST subunits 24 h after 2 or 7 daily administrations of oltipraz showed that the levels of subunits Yb1, Yp, Yc2, and Ya2 were increased with maximum elevations of 5.6-, 11.1-, 6.4-, and 10.4-fold, respectively. In comparison, levels of subunits Yb2 and Yc1 were modestly elevated 1.8- to 2.6-fold, respectively, whereas subunit Ya1 was not induced. Remarkably, the levels of subunit Yp and Ya2 remained elevated approximately 2.3-fold 7 days after a single dose of oltipraz. In contrast, the levels of subunits Yb1 and Yc2 diminished to approximate control levels within 7 days after a single dose of oltipraz. GST mRNA levels for Ya, Yb, and Yp were measured by Northern blot analysis and were found to be elevated maximally to 13.7-, 13.5-, and 3.9-fold, respectively, after two daily oltipraz doses. Interestingly, GST Ya and Yb mRNA diminished to constitutive levels after 7 daily doses of oltipraz, with no corresponding decreases in GST subunit or activity levels. The levels of GST Ya and Yb mRNA decreased to constitutive levels within 4 days after a single oltipraz administration, whereas GST Yp mRNA levels remained elevated throughout the 7-day follow-up period.(ABSTRACT TRUNCATED AT 400 WORDS)

Levels of aflatoxin-albumin biomarkers in rat plasma are modulated by both long-term and transient interventions with oltipraz

The validation process for biomarkers to be used for monitoring the efficacy of preventive interventions in humans includes assessments of whether levels of the biomarker can be modulated in experimental models. From this perspective, the influence of two intervention protocols with the chemopreventive agent oltipraz on rates of formation and disappearance of aflatoxin-albumin adducts has been evaluated in rats chronically exposed to aflatoxin B1. Male F344 rats were treated daily with 20 micrograms aflatoxin B1, p.o. for 35 days. The first strategy employed a standard, long-term intervention in which basal AIN-76A diet was supplemented with 0.05% oltipraz beginning one week before AFB1 treatment and continuing throughout the period of carcinogen exposure. In this setting, treatment with oltipraz reduced the rate of formation of aflatoxin-albumin adducts such that steady-state levels were lowered by > 50% from control values of 400 pmol aflatoxin adducts/mg albumin. The time-course for reaching respective steady-state levels was unchanged, with or without oltipraz intervention. The second intervention strategy utilized a delayed, transient protocol in which oltipraz was fed for 2 weeks beginning 1 week after AFB1 dosing began and ending 2 weeks before AFB1 dosing was completed. This second strategy, which models clinical interventions in chronically exposed individuals, produced a steady decline in aflatoxin-albumin adduct levels that approached a 50% reduction by the end of the AFB1 exposure period. Development of smooth curve functions allowed for the estimation of the ratio of effects between the non-intervention and intervention groups as well as the simultaneous 90% confidence intervals for the aflatoxin-albumin adduct levels. These analyses indicated that long-term intervention with oltipraz produced a statistically significant reduction in levels of the aflatoxin-albumin biomarker at all times throughout aflatoxin exposure. By contrast, a statistically significant decrease in biomarker levels was not seen in the delayed, transient intervention protocol until the ninth day of the intervention. However, once achieved, significant differences from the control group were maintained for the remainder of the aflatoxin exposure period. These changes in aflatoxin biomarker levels are consistent with the cancer chemopreventive outcomes of these intervention protocols in rats. Collectively, these results support the utility of measuring this biomarker as a means for assessing the efficacy or chemopreventive interventions in individuals at high risk for aflatoxin exposure and development of hepatocellular carcinoma.

Myeloperoxidase as a biomarker of skin irritation and inflammation

Topical exposure to many occupational and environmental chemicals is often accompanied by the induction of a cutaneous inflammatory response characterized by the influx of polymorphonuclear leucocytes (PMNs). In this study, the accumulation of myeloperoxidase (MPO), an enzyme found at high content in PMNs, was used as a biomarker to assess the inflammatory response to a number of well characterized skin irritants and tumour promoters. Of the chemicals examined, there was a good agreement between MPO accumulation and morphometric indicators of PMNs. Therefore, it was suggested that MPO accumulation could be an additional and useful toxicological tool for determining the dermal irritancy of chemicals.

Regulation of phase 2 enzyme induction by oltipraz and other dithiolethiones

4-Methyl-5-pyrazinyl-3H-1,2-dithiole-3-thione (oltipraz) and several other dithiolethiones protect against the acute toxicities of many xenobiotics and are effective inhibitors of experimental carcinogenesis. These protective effects are mediated, in part, through elevation of glutathione S-transferase, NAD(P)H: quinone reductase and UDP-glucuronosyltransferase activities in the liver and other target tissues. The induction of these phase 2 enzymes by oltiprax results from enhanced transcription. In the present study, the molecular mechanisms of these inductions were analyzed utilizing a construct containing a 41 bp enhancer element derived from the 5'-upstream region of the mouse liver glutathione S-transferase Ya subunit gene ligated to the 5' end of the isolated promoter region of this gene, and inserted into a plasmid containing a human growth hormone reporter gene. When this construct was transfected into murine Hepa 1c1c7 hepatoma cells, the concentrations of 25 dithiolethiones and related analogs required to double growth hormone production were determined and spanned a range nearly three orders of magnitude. Concentrations of dithiolethiones required to double the specific activity of NAD(P)H: quinone reductase were also determined in Hepa 1c1c7 cells. There was a positive correlation (r = 0.78) between the potencies of the 21 active compounds as inducers of both NAD(P)H: quinone reductase activity and growth hormone production. Moreover, no dithiolethiones were inactive in only one system. It is probable, therefore, that the induction of NAD(P)H: quinone reductase and other phase 2 enzymes by oltipraz and other dithiolethiones is mediated entirely through the 41 bp enhancer element.

Molecular dosimetry of urinary aflatoxin-N7-guanine and serum aflatoxin-albumin adducts predicts chemoprotection by 1,2-dithiole-3-thione in rats

Hepatocellular carcinoma has one of the poorest 5 year survival rates of any human cancer. Preventive measures offer the best possibility of ameliorating this disease and chemoprotective agents are being developed for this purpose. The dithiolethiones, including oltipraz and the unsubstituted molecule 1,2-dithiole-3-thione, have been shown to be potent inhibitors of aflatoxin-induced hepatic tumorigenesis in rats. However, subsequent evaluation of dithiolethiones or other chemoprotective agents in human clinical trials will require the development of intermediate, non-invasive biomarkers to evaluate the efficacy of these interventions. In this study, levels of molecular dosimetry biomarkers for determining genotoxic damage caused by aflatoxin B1 have been measured in a chronic exposure model with male F344 rats wherein half the animals were fed a diet supplemented with 0.03% 1,2-dithiole-3-thione to lower their risk for tumors and the other half were fed unsupplemented AIN-76A diet and were at high risk for tumor development. Levels of hepatic aflatoxin-DNA adducts, serum aflatoxin-albumin adducts and excreted aflatoxin-N7-guanine adducts in urine were determined following multiple administrations of 250 micrograms aflatoxin B1/kg body wt on days 0-4 and 7-11 to assess the use of the serum and urinary biomarkers as indices of chemoprotective efficacy. In the rats fed 1,2-dithiole-3-thione, the overall diminutions in the levels of hepatic DNA adducts, urinary aflatoxin-N7-guanine and serum aflatoxin-albumin adducts over the 2 week exposure period were 76, 62 and 66% respectively. This parallelism in reductions of levels of biomarkers relative to target organ DNA adduct burden suggests that these biomarkers are predictive short-term, non-invasive measures for assessing the efficacy of chemoprotective interventions in experimental studies and can be applied to human clinical trials directed at populations at high risk for aflatoxin exposure and primary hepatocellular carcinoma.

Transcriptional control of glutathione S-transferase gene expression by the chemoprotective agent 5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione (oltipraz) in rat liver

The substituted 1,2-dithiole-3-thione oltipraz [5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione] protects against the acute and chronic toxicities of many xenobiotics, including aflatoxin B1, in rodents. These protective effects are mediated, in part, through elevation of glutathione S-transferase (GST) activities. Because studies by Coles et al. [Carcinogenesis (Lond.), 6: 693-697, 1985] suggested that the detoxication of aflatoxin through conjugation with glutathione is principally catalyzed by GST homodimer YaYa, we have investigated the regulation of the gene coding for the Ya subunit in the liver of F344 rats following dietary administration of oltipraz. Overall GST activity, as measured by conjugation with 1,2-dichloro-4-nitrobenzene or 1-chloro-2,4-dinitrobenzene, as well as the levels of GST Ya protein, was elevated 1.5-fold by 24 h and maximally (2.7- to 3.5-fold) and persistently after 5 days on a purified diet supplemented with 0.075% oltipraz. Steady state mRNA levels for GST subunit Ya, as quantified by slot blot analysis using rat liver GST complementary DNA clone pGTB38, were also elevated by 24 h, with a maximal elevation of 3-fold observed at 3 days. However, mRNA levels decreased thereafter, despite continued feeding of oltipraz. Northern blot analyses demonstrated that oltipraz did not alter the size of GST mRNA. Transcriptional activity of the GST Ya gene, as determined by nuclear run-off analysis, was increased 2-fold after 24-h feeding of oltipraz, was maximally induced 2.4-fold at 3 days, and returned to near control levels at 7 days, despite sustained feeding of oltipraz. Modulation of GST activity by oltipraz was not accompanied by changes in the methylation pattern at internal sites of the GST Ya gene. These results show that the initial induction of hepatic GST activity during oltipraz exposure correlates with changes in steady state levels of GST mRNA and rates of GST gene transcription; however, the continued elevation of GST enzymatic activities and GST Ya protein levels in the face of declining GST Ya mRNA levels and transcription rates suggests that additional mechanisms may be involved in regulating GST Ya expression by oltipraz.

Role of inflammatory cells in the metabolic activation of polycyclic aromatic hydrocarbons in mouse skin

Oxidants, such as those generated by activated polymorphonuclear leukocytes (PMNs) during inflammation, have been implicated in the metabolic activation of procarcinogens to their ultimate carcinogenic form. In this study we examined the effect of inflammation on the metabolic activation of (+/-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (BP 7,8-dihydrodiol) to a covalent binding species in mouse epidermis. Interaction of BP 7,8-dihydrodiol with 12-O-tetradecanoylphorbol-13-acetate (TPA)-stimulated murine leukocytes resulted in the generation of both a chemiluminescent intermediate and one that covalently bound to the DNA of cocultured epidermal keratinocytes. Topical treatment of mouse skin with TPA led to an influx of PMNs into the skin beginning several hours after application. Myeloperoxidase activity, a marker for neutrophils, increased 15-fold in the skin by 16 hr after TPA treatment. Dual applications of TPA at both 16 hr before and concurrently with administration of [3H]BP 7,8-dihydrodiol led to a 50% enhancement of the level of carcinogen that was covalently bound to epidermal DNA. However, a single application of TPA, either 16 hr before or concurrently with BP 7,8-dihydrodiol administration, had no enhancing effect, suggesting that both initial recruitment of PMNs into the skin and subsequent stimulation of oxidant production by the PMNs were required to enhance carcinogen binding. By contrast, no enhancement of benzo[a]pyrene binding was observed by TPA treatments in vivo. However, TPA-stimulated neutrophils did not activate this procarcinogen to a chemiluminescent metabolite in vitro. These results suggest that oxidants generated by metabolically stimulated PMNs can activate penultimate polycyclic aromatic hydrocarbons, such as BP 7,8-dihydrodiol, to potentially genotoxic metabolites in vivo and further define a role for inflammation in carcinogenesis.

Mechanism of protection against aflatoxin tumorigenicity in rats fed 5-(2-pyrazinyl)-4-methyl-1,2-dithiol-3-thione (oltipraz) and related 1,2-dithiol-3-thiones and 1,2-dithiol-3-ones

1,2-Dithiol-3-thiones, reported constituents of cruciferous vegetables, are five-membered cyclic sulfur-containing compounds with antioxidant, chemotherapeutic, and chemoprotective activities. The effects of dietary administration of a substituted 1,2-dithiol-3-thione, oltipraz [5-(2-pyrazinyl)-4-methyl-1,2-dithiol-3-thione], a potent antischistosomal agent, on aflatoxin B1 (AFB1) metabolism, DNA adduct formation, and hepatic tumorigenesis were examined in male F344 rats. Rats were fed graded doses of oltipraz (0.01-0.1%) for 4 wk. During the second and third wk of oltipraz feeding rats were gavaged with 250 micrograms of AFB1/kg five times a wk. Rats were finally restored to control diet 1 wk after cessation of AFB1 dosing. At 4 months focal areas of hepatocellular alteration were identified and quantitated by staining sections of liver for gamma-glutamyl transpeptidase activity. Treatment with oltipraz at all doses reduced by greater than 90% the volume of liver occupied by gamma-glutamyl transpeptidase-positive foci. Levels of AFB1 bound to hepatic DNA were reduced between 40 and 80% in animals fed increasing doses of dietary oltipraz (0.01-0.1%) for 1 wk prior to a single exposure to AFB1. Feeding of the higher levels of oltipraz led to marked increases in the specific activity of glutathione S-transferases, presumably serving to facilitate the detoxication of the ultimate electrophilic form of AFB1, the 8,9-oxide. At low dietary concentrations of oltipraz (0.01%), the only inductive effects seen were on the activities of selected cytochrome P-450 monooxygenases. Therefore, the protection afforded by oltipraz may be due to both the enhancement of electrophile detoxication pathways as well as modified oxidative metabolism of AFB1. In in vitro metabolism studies with hepatic post-mitochondrial supernatant, low-dose oltipraz pretreatment facilitated the oxidative production of aflatoxins P1 and Q1, but not M1, from AFB1. High-dose (0.1%) oltipraz pretreatment enhanced the primary metabolism of AFB1 to aflatoxins P1, M1, and Q1 as well as the formation of chloroform-insoluble metabolites. Feeding studies with a series of 1,2-dithiol-3-thione and 1,2-dithiol-3-one derivatives of oltipraz demonstrated that the inductive activity for cytochrome P-450-dependent monooxygenases and electrophile detoxication enzymes, such as glutathione S-transferases, could be readily separated by minor modifications of the 1,2-dithiol-3-thione structure. The unsubstituted 1,2-dithiol-3-thione nucleus strongly induced electrophile detoxication enzymes, but not the monooxygenases, and was the most effective inhibitor of the binding of AFB1 to hepatic DNA in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of aflatoxin metabolism, aflatoxin-N7-guanine formation, and hepatic tumorigenesis in rats fed ethoxyquin: role of induction of glutathione S-transferases

The effects of dietary administration of ethoxyquin (EQ) on aflatoxin B1 (AFB1) metabolism, DNA adduct formation and removal, and hepatic tumorigenesis were examined in male Fischer rats. Rats were fed a semipurified diet containing 0.4% EQ for 1 wk, gavaged with 250 micrograms of AFB1 per kg 5 times a wk during the next 2 wk, and, finally, restored to the control diet 1 wk after cessation of dosing. At 4 mo, focal areas of hepatocellular alteration were identified and quantitated by staining sections of liver for gamma-glutamyl transpeptidase. Treatment with EQ reduced by greater than 95% both area and volume of liver occupied by gamma-glutamyl transpeptidase-positive foci. Utilizing the same multiple dosing protocol, patterns of covalent modifications of DNA by AFB1 were determined. EQ produced a dramatic reduction in the binding of AFB1 to hepatic DNA: 18-fold initially and 3-fold at the end of the dosing period. Although binding was detectable at 3 and 4 mo postdosing, no effect of EQ was observed, suggesting that these persistent adducts are not of primary relevance to AFB1 carcinogenesis. Analysis of nucleic acid bases by high-performance liquid chromatography revealed no qualitative differences in adduct species between treatment groups. The inhibitory effect of EQ on AFB1 binding to DNA and tumorigenesis appears related to induction of detoxication enzymes. Rats fed 0.4% EQ for 7 days showed a 5-fold increase in hepatic cytosolic glutathione S-transferase (GST)-specific activities. Multiple molecular forms of GST were induced, and concomitant elevations in messenger RNA levels coding for the synthesis of GST subunits were observed. Correspondingly, biliary elimination of AFB1-glutathione conjugate was increased 4.5-fold in animals on the EQ diet during the first 2 h following p.o. administration of 250 micrograms of AFB1 per kg. Thus, induction by EQ of enzymes important to AFB1 detoxication, such as GST, can lead to enhanced carcinogen elimination, as well as reductions of AFB1-DNA adduct formation and subsequent expression of preneoplastic lesions, and, ultimately, neoplasia.

Effects of a biomimetic superoxide dismutase on complete and multistage carcinogenesis in mouse skin

The effects of a selective detoxifier of the proximate oxygen radical, superoxide anion, on the induction of tumors in the skin of CD-1 mice by either the initiation-promotion regimen or the complete carcinogenesis process were investigated. The principle agent of interest, copper (II) (3,5-diisopropylsalicylate)2 (CuDIPS), is a low molecular weight, lipophilic copper coordination complex that catalytically disproportionates superoxide anion at a rate comparable to native Cu-Zn superoxide dismutase (SOD). The protocols used to elicit tumors were: (i) a single application of 0.2 mumol of 7,12-dimethylbenz[a]anthracene (DMBA) followed by twice-weekly applications of 12-O-tetradecanoylphorbol-13-acetate (TPA) in an initiation-promotion study, and (ii) either a single application of 3.6 mumol DMBA followed by no further treatment or weekly applications of 0.2 mumol DMBA in complete carcinogenesis protocols. Application of 2 mumol CuDIPS 15 min prior to the initiating dose of DMBA was without significant effect on tumor yield or incidence, whereas application prior to each dose of TPA substantially reduced tumor incidence and yield. This anti-promoting property of CuDIPS can be attributed to its SOD-mimetic activity in as much as the corresponding zinc coordination complex lacking in SOD activity, zinc (II) (3,5-diisopropylsalicylate)2, was non-inhibitory. Significant reductions in tumor yield were also observed when CuDIPS was applied prior to DMBA in either of the complete carcinogenesis protocols. Additionally, covalent binding of [3H]DMBA to epidermal DNA was markedly reduced by CuDIPS pre-treatment, suggesting that the anti-carcinogenic properties may reflect a perturbation in superoxide anion-dependent metabolic activation of DMBA. The induction by DMBA of ornithine decarboxylase activity, a biochemical marker of tumor promoter activity, was not affected by CuDIPS; however, induction of ornithine decarboxylase by TPA was potently blocked. Collectively, these effects of a biomimetic SOD further implicate reactive oxygen species at multiple stages in chemical carcinogenesis.

Modification of aflatoxin B1 binding to DNA in vivo in rats fed phenolic antioxidants, ethoxyquin and a dithiothione

The effects of dietary administration of 3,5-di-tert-butyl-4-hydroxytoluene (BHT), 2(3)-tert-butyl-4-hydroxyanisole (BHA), ethoxyquin (EQ) and 5-(2-pyrizinyl)-4-methyl-1,2-dithiol-3-thione (oltipraz) on aflatoxin B1 (AFB1) - DNA adduct formation in vivo in livers and kidneys of rats were investigated. Male F344 rats were treated with 1 mg/kg AFB1 by i.p. administration and nucleic acids isolated 2 h post dosing. Animals were fed a semipurified diet supplemented with either 0.5% EQ, 0.45% BHT, 0.45% BHA or 0.1% oltipraz for 2 weeks prior to AFB1 treatment. Analysis of nucleic acid bases by h.p.l.c. showed that several AFB1 metabolite-DNA adducts were formed in both tissues. The principal and related adducts of 8,9-dihydro-8-(N7-guanyl)-9-hydroxyaflatoxin B1 represented approximately 80-90% of all adducts in both tissues and in all treatment groups. However, inclusion of the antioxidants in the diet resulted in substantial reductions in overall AFB1 modified DNA levels. EQ, BHT, BHA and oltipraz reduced the covalent binding of AFB1 to liver DNA by 91, 85, 65 and 76% and to kidney DNA by 80, 35, 62 and 64%, respectively. Concordantly, the specific activities of hepatic enzymes of presumed importance to AFB1 detoxification, epoxide hydrase, and glucuronyl and glutathione transferases were significantly elevated by all antioxidants. Reduced glutathione levels were unchanged except by oltipraz, although activities of enzymes contributing to the maintenance of reduced glutathione pools, glutathione reductase and glucose-6-phosphate dehydrogenase, were elevated in most treatment groups. An excellent correlation (r = 0.95) was observed between the degree of inhibition of DNA binding by AFB1 and the induction of hepatic glutathione S-transferase activities by the four antioxidants.