Glutathione conjugation of microsome-mediated and synthetic aflatoxin B1-8,9-oxide by purified glutathione S-transferases from rats

Chitosan Nanoparticles Alleviated the Adverse Effects of Sildenafil on the Oxidative Stress Markers and Antioxidant Enzyme Activities in Rats

Sildenafil (SF) is widely used for erectile dysfunction and other conditions, though with limitations regarding oral absorption and adverse effects. Despite nanotechnological improvements, the effect of nanocarriers on SF hepatotoxicity has not been documented to date. This study aimed at assessing the impact of chitosan nanoparticles either uncoated (CS NPs) or Tween 80-coated (T-CS NPs) on the effects of SF on oxidative stress markers and antioxidant enzyme activities in rats. Test SF-CS NPs prepared by ionic gelation were uniform positively charged nanospheres (diameter 178-215 nm). SF was administered intraperitoneally to male rats (1.5 mg/kg body weight) in free or nanoencapsulated forms as SF-CS NPs and T-SF-CS NPs for 3 weeks. Free SF significantly suppressed the activity of the antioxidant enzymes glutathione S-transferase (GST), glutathione peroxidase (GPx), glutathione reductase (GR), catalase (CAT), and superoxide dismutase (SOD), as well as the levels of glutathione (GSH) and thiobarbituric acid reactive substances (TBARS) as in an indirect measure of free radicals. Interestingly, SF-CS NPs and T-SF-CS-NPs treatments significantly attenuated the inhibitory effects of SF on the activity of these enzymes whereas, GST activity was inhibited. Moreover, the protein expression of GST was downregulated upon treatment of rats with free SF, SF-CS-NPs, and T-SF CS-NPs. In contrast, the activity and protein expression of GPx was induced by SF-CS NPs and T-SF-CS-NPs treatments. The histopathological study showed that SF induced multiple adverse effects on the rat liver architecture which were markedly suppressed particularly by T-SF-CS NPs. In conclusion, chitosan nanoencapsulation of SF counteracted the adverse effects of SF on the activity of antioxidant enzymes and liver architecture. Findings might have significant implications in improving the safety and efficacy of SF treatment of the widely expanding disease conditions.

Zinc Oxide Nanoparticles Ameliorate Dimethylnitrosamine-Induced Renal Toxicity in Rat

Dimethylnitrosamine (DMN) is an established carcinogen. It is toxic to several organs, viz., the liver, kidney, and lungs, and immune system. Several drugs have been used in the past to modulate its toxicity using experimental animal models. The present study was designed to investigate the effect of zinc oxide nanoparticles (ZnONPs) on renal toxicity caused by DMN in laboratory rat. Since oxidative mechanisms are mainly involved in its toxicity, the proposed study focuses on the amelioration of oxidative stress response by ZnONPs, if any. The present results show that administration of ZnONPs (50 mg/kg body weight/rat) to DMN (2 μl/100 g body weight/rat)-treated rats diminuted the concentration of malonaldehyde, H₂O₂, and NO in the kidney. However, reduced glutathione (GSH) concentration increased after ZnONP treatment. Results on glutathione S-transferase and glutathione peroxidase favored its antioxidative effects. These results are supported by the recovery of oxidative DNA damage and less pronounced histopathological changes in the kidney. It is hypothesized that ZnONPs might be toxic to renal tissue; however, its strong therapeutic/antioxidative potential helps in ameliorating DMN-induced renal toxicity in rat.

Molecular Docking and Dynamics Simulation Revealed Ivermectin as Potential Drug against Schistosoma-Associated Bladder Cancer Targeting Protein Signaling: Computational Drug Repositioning Approach

Urogenital schistosomiasis is caused by Schistosoma haematobium (S. haematobium) infection, which has been linked to the development of bladder cancer. In this study, three repurposing drugs, ivermectin, arteether and praziquantel, were screened to find the potent drug-repurposing candidate against the Schistosoma-associated bladder cancer (SABC) in humans by using computational methods. The biology of most glutathione S-transferases (GSTs) proteins and vascular endothelial growth factor (VEGF) is complex and multifaceted, according to recent evidence, and these proteins actively participate in many tumorigenic processes such as cell proliferation, cell survival and drug resistance. The VEGF and GSTs are now widely acknowledged as an important target for antitumor therapy. Thus, in this present study, ivermectin displayed promising inhibition of bladder cancer cells via targeting VEGF and GSTs signaling. Moreover, molecular docking and molecular dynamics (MD) simulation analysis revealed that ivermectin efficiently targeted the binding pockets of VEGF receptor proteins and possessed stable dynamics behavior at binding sites. Therefore, we proposed here that these compounds must be tested experimentally against VEGF and GST signaling in order to control SABC. Our study lies within the idea of discovering repurposing drugs as inhibitors against the different types of human cancers by targeting essential pathways in order to accelerate the drug development cycle.

Effect of eriodictyol on preneoplastic lesions, oxidative stress and bacterial enzymes in 1,2-dimethyl hydrazine-induced colon carcinogenesis

Eriodictyol, one of the strong flavonoids extracted from Eriodictyon californicum, is known for its antioxidant and anticarcinogenic properties. We estimated the chemopreventive effect of eriodictyol on 1,2 dimethylhydrazine (DMH)-induced experimental colon carcinogenesis in male albino Wistar rats. The rats were randomized into six groups. Our results evaluated the effect of eriodictyol supplementation (200 μg per kg b.w.) on DMH (20 mg per kg b.w)-induced rats (Groups 4-6). The incidence of polyps, aberrant crypt foci (ACF) and the lipid peroxidation levels were significantly decreased as compared to those in the DMH-alone treated rats (Group 2). In eriodictyol-supplemented DMH-treated rats, we observed increased activity of enzymatic and non-enzymatic antioxidants in the circulatory system, liver, and colon. The bacterial enzymes activities of mucosa and faecal were significantly decreased in the group with treatment of eriodictyol on DMH-induced rats. Moreover, in the eriodictyol-supplemented DMH-exposed rats, we observed reduced malignant glands of a histopathological appearance in both liver and colon tissue. Furthermore, we also observed reduced AgNORs counts of eriodictyol supplemented to the DMH-exposed rats. Therefore, we can conclude that eriodictyol can be used as an effective chemopreventive agent against DMH-induced colon carcinogenesis in experimental animal models.

Heavy Metal-Induced Changes in the Glutathione Levels and Glutathione Reductase/Glutathione S-Transferase Activities in the Liver of Male Mice

Glutathione S-transferases catalyze the metabolism of reactive substances of exogenous or endogenous origin and are involved in inactivation processes of xenobiotics and their metabolites. The present study aims at investigating the influence of heavy metals on the hepatic level of reduced glutathione (GSH), glutathione S-transferase, and glutathione reductase activities in the liver of male mice after single-dose (1 and 24 h) and repeated-dose treatments (three consecutive days). The hepatic level of GSH was depleted after single-dose treatments with cadmium chloride, mercuric chloride, cobalt chloride, cesium chloride, lead acetate, and silver nitrate, and percentage of GSH depletion was greater still after the repeated-dose treatments. Nickel chloride, on the other hand, did not cause any change in the level of GS H after any period of treatment. Glutathione reductase activity was increased 24 hours after treatment with cadmium chloride, mercuric chloride, lead acetate, and silver nitrate, whereas cobalt chloride decreased such activity after repeated doses. With the exception of cadmium chloride, glutathione S-transferase activity was significantly decreased 24 hours after a single-dose treatment with all of the tested heavy metals. Such alterations in the activities of phase II drug-metabolizing enzymes as a result of heavy metal treatment may change the hepatic capacity for the detoxification of many toxic compounds from endogenous or exogenous sources.

Oral supplementation with troxerutin (trihydroxyethylrutin), modulates lipid peroxidation and antioxidant status in 1,2-dimethylhydrazine-induced rat colon carcinogenesis

The present study was aimed to investigate the chemopreventive potential of troxerutin on 1,2-dimethylhydrazine (DMH) induced rat colon carcinogenesis by evaluating the antioxidant and lipid peroxidation (LPO) status. Rats were randomly divided into six groups. Group I rats served as control. Group II rats received troxerutin (50 mg/kgb.w., p.o.) for 16 weeks. Groups III-VI rats received subcutaneous injections of DMH (20 mg/kgb.w., s.c.) once a week, for the first 4 weeks. In addition to DMH, groups IV-VI rats received troxerutin at the doses of 12.5, 25 and 50 mg/kgb.w., respectively. In DMH treated rats, our results showed decreased activities of antioxidants and increased levels of LPO in the liver. Moreover, LPO and antioxidants in the colon were found to be significantly diminished in DMH the treated rats. Furthermore, enhanced activity of colonic vitamin C and vitamin E levels were observed in DMH alone treated rats (group III), which was significantly reversed on troxerutin supplementation. Troxerutin at the dose of 25 mg/kgb.w. had shown profound beneficial effects by exhibiting near normal biochemical profile and well-preserved colon histology as compared to the other two tested doses (12.5 and 50 mg/kgb.w.). These findings suggest that troxerutin could serve as a novel agent for colon cancer chemoprevention.

Effect of kaempferol on lipid peroxidation and antioxidant status in 1,2-dimethyl hydrazine induced colorectal carcinoma in rats

Colorectal cancer, a common cause of cancer related deaths in both sexes in western population is often due to persistent oxidative stress leading to DNA damage. Antioxidants scavenge free radicals and inhibit neoplastic process. Kaempferol, a flavonol widely distributed in tea, broccoli, grape fruit, brussels sprouts and apple, is claimed to have chemopreventive action in colon cancer. The aim of our study was to evaluate the effect of kaempferol on tissue lipid peroxidation and antioxidant status in 1,2-dimethyl hydrazine induced colorectal cancer in male Wistar rats and to compare its efficacy with irinotecan. Experimental colon cancer induced by 1,2-dimethyl hydrazine in rats mimic human colon cancer and therefore is an ideal model for chemoprevention studies. The rats were divided into six groups. Group 1 served as control. Group 2 received 1,2-dimethyl hydrazine (20 mg/kg body weight) subcutaneously once a week for four weeks. Group 3 received irinotecan (100 mg/kg body weight) intravenously once a week for four weeks with 1,2-dimethyl hydrazine. Groups 4 to 6 were given a daily oral dose of 50, 100, 200 mg/kg body weight of kaempferol with 1,2-dimethyl hydrazine. The total study period was 16 weeks. Kaempferol supplementation lowered 1,2-dimethyl hydrazine induced erythrocyte lysate and liver thiobarbituric acid reactive substances level and rejuvenated anti oxidant enzymes catalase, super oxide dismutase and glutathione peroxidase. The recovery of enzyme status was maximum at the dose of 200 mg/kg body weight and was comparable to irinotecan. Our study reveals that kaempferol could be safely used as a chemopreventive agent in colorectal cancer.

Ameliorating Effect of Phytoestrogens on CCl4-Induced Oxidative Stress in the Livers of Male Wistar Rats

Glutathione-S-transferases and glutathione play a key role in the detoxification of most toxic agents. In the present study, the protective effects, if any, of isoflavone phytoestrogens--genistein and daidzein on the carbon tetrachloride (CCl4) induced changes in the activity of alanine aminotransferase (ALT), aspartate aminotransferase (AST), glutathione S transferase (GSH) and levels of glutathione (GSH) and thiobarbituric acid reactive substances (TBARS)-were studied. The activities of ALT and AST were assayed in the serum, whereas the activity of GST and levels of GSH and TBARS were determined in the livers of rats. The current study involved the division of animals into two main groups: (i) rats pretreated with genistein and daidzein for three days; and (ii) non-pretreated rats. In the pretreated group, rats received oral doses of genistein (7.9 micromol/kg body weight) and daidzein (7.9 micromol/kg body weight) for three consecutive days (once daily) followed by oral dose of CCl4 on the 4th and the 5th day concurrently with the phytoestrogens-genistein or daidzein. In the non-pretreated group animals received oral dose of CCl4 (1 ml/kg body weight) for two consecutive days along with the phytoestrogens-genistein or daidzein. Treatment of male rats with CCl4 significantly elevated the activity of ALT and AST in serum and levels of TBARS in the liver. On the other hand, CCl4 resulted in decreased activity of GST and lowered the GSH levels. Coadministration of genistein and daidzein with CCl4 could not restore the alterations in the activity of ALT and AST caused by CCl4 to normal control levels. However, repeated dose treatments with genistein and daidzein for three days prior to the administration of CCl4 restored such alterations to normal levels. Our results indicate that genistein is more effective than daidzein in counteracting the inhibition of GST activity caused by CCl4 and restoring it to normal levels. Genistein was also more effective than daidzein restoring the induced TBARS levels caused by CCl4 to normal control levels when rats were pretreated with the isoflavone orally for three days. It has been observed that the tested isoflavonoids were able to antagonize the toxic effects of CCl4. Such counteracting effects were more pronounced for genistein and when the phytoestrogens were administered as repeated doses prior CCl4 administration.

N-Nitrosodimethylamine changes the expression of glutathione S-transferase in the liver of male mice: The role of antioxidants

The present study investigated the protective effect of gossypol, selenium, zinc, or glutathione (GSH) against dimethylnitrosamine (DMN)-induced hepatotoxicity in the livers of male mice. The expression and the activity of glutathione S-transferase (GST), levels of GSH, and free radicals (malondialdehyde (MDA)), as well as the activity of glutathione reductase were determined after the treatment of mice for seven consecutive days with low or high doses of gossypol, selenium, zinc, or GSH. In experimental groups, DMN was administered as a single dose for 2 h after the repeated dose treatments of mice for seven consecutive days with each antioxidant. DMN reduced the expression and inhibited the activity of GST. However, repeated treatments of mice with low-dose gossypol or high dose of either selenium or GSH followed by a single dose of DMN induced the expression and the activity of GST. In contrast, low-dose treatments of mice with zinc, selenium, or GSH followed by a single dose of DMN reduced the expression and the activity of GST compared to either control or DMN-treated groups. In addition, high-dose treatment with either gossypol or selenium markedly induced the levels of GSH compared to either control or DMN-treated groups. Interestingly, pretreatment of mice with high dose of either gossypol or selenium for seven consecutive days followed by a single dose of DMN decreased the levels of MDA, whereas DMN induced such levels. It is concluded that high dose of either gossypol or selenium is a stronger protector than zinc and GSH in ameliorating the toxic effects of DMN.

Hesperetin exerts dose dependent chemopreventive effect against 1,2-dimethyl hydrazine induced rat colon carcinogenesis

Colon cancer is still one of the leading causes of death in USA and is increasing at an alarming rate in Asia. It is one of the major causes of death in industrialized countries, and its etiology is known to be a combination of hereditary, environmental, dietary factors and lack of physical activity. Chemoprevention plays a potential role in colorectal cancer. The present study was performed to evaluate the efficacy of hesperetin supplementation on colonic aberrant crypt foci, lipid peroxidation and antioxidant defense system in 1,2-dimethylhydrazine (DMH) induced colon carcinogenesis in male Wistar rats. The rats were segregated into six groups viz., group 1, control rats received modified pellet diet; group 2 rats received modified pellet diet along with hesperetin (30 mg/kg body weight/day); groups 3-6 administrated DMH (20 mg/kg body weight) subcutaneous injection once a week for the first 4 weeks; in addition groups 4-6 received hesperetin at three different doses of 10, 20 and 30 mg/kg body weight/day for 16 weeks. All the rats were sacrificed at the end of the experimental period of 16 weeks. Increased tumor incidence and increased number aberrant crypt foci (ACF) accompanied by a decrease in the tissue lipid peroxidation, glutathione S-transferase (GST), glutathione peroxidase (GPx), superoxide dismutase (SOD), and catalase (CAT) activities were observed in DMH-treated rats. Administration of hesperetin to DMH treated rats significantly decreased the tumor incidence, the number of aberrant crypt foci with simultaneous enhancement of tissue lipid peroxidation, GST, GPx, SOD, and CAT activities. The results of this study suggest that hesperetin at a dose of 20 mg/kg body weight showed a significant beneficial effect against chemically induced colonic carcinogenesis in rats as compared to the other two doses.

Effects of Schistosoma haematobium infection on drug-metabolizing enzymes in human bladder cancer tissues

The mixed function oxidase system includes the phase I drug oxidation proteins e.g. aryl hydrocarbon hydroxylase (AHH), N-nitrosodimethylamine-N-demethylase I (NDMA-dI) and cytochrome b5 which metabolize most carcinogens and xenobiotics into less and/or more active intermediates. These were determined in human bladder tissues diagnosed as bladder cancer only (10 samples) and bladder cancer associated with Schistosoma haematobium (12 samples) and normal bladder tissues (12 samples). In addition to the above enzymes, agents involved in Phase II drug metabolism e.g. glutathione and glutathione S-transferase as well as free radicals (detected as thiobarbituric acid-reactive substances, TBARS) were also determined in these tissues samples. AAH and NDMA-dI, cytochrome b5, and glutathione S-transferase activity decreased by 42, 28, 47 and 32%, respectively, in human bladder cancer tissues. In bladder cancer tissues associated with S. haematobium infection NDMA-dI and GST activity decreased further by 65 and 56%, respectively, whereas AHH activity increased by 50% and levels of reduced glutathione also increased by 43% in cancer tissue and by 29% in schistocome infected bladder cancer tissue. The level of free radicals also increased significantly (by 57%) in infected bladder cancer tissue but not at all in non-infected cancer tissue. Alterations in the activity of phase I and II of drug-metabolizing enzymes in human bladder tissues as a result of S. haematobium infection may therefore change the bladder's capacity to detoxify many endogenous compounds and may also potentiate the deleterious effects of bladder carcinogens, (e.g. N-nitrosamines) which are known to be present in relatively large quantities in the bladder of patients with schistosomiasis. The present study thus provides new insights into mechanisms for the genesis of bladder cancer initiated in association with schistosomiasis.

Carbon tetrachloride-induced changes in the activity of phase II drug-metabolizing enzyme in the liver of male rats: role of antioxidants

Glutathione S-transferases and glutathione play an important role in the detoxification of most toxic agents. In the present study, the protective effects of some antioxidants (L-ascorbic acid (AA), vitamin E (VE) or garlic) on carbon tetrachloride-induced changes in the activity of alanine amino transferase (ALT), aspartate amino transferase (AST), glutathione S-transferase (GST), and the level of glutathione (GSH) and thiobarbituric acid reactive substances (TBARS) were studied. The activities of ALT, and AST were assayed in plasma, whereas the activity of GST and the levels of GSH and TBARS were determined in the livers of rats. The current study included two experiments. In the first experiment, animals received single oral dose of CCl4 (400 mg/kg body weight) after administration of AA (100 mg/kg b.w.), VE (100 mg/kg b.w.) or garlic (800 mg/kg b.w.) as single oral doses. In the second experiment, rats received repeated oral doses of antioxidants for 12 consecutive days followed by a single oral dose of CCl4 on the 13th day and killed after that by 24 h. Treatment of male rats with CCl4 significantly increased the activity of ALT and AST in plasma, and the levels of both GSH and TBARS in the liver. On the other hand, CCl4 inhibited the activity of GST after single dose treatment. Single-dose treatments of rats with AA, VE or garlic prior to the administration of CCl4 could not restore the alterations in the activity of ALT and AST caused by CCl4 to the normal control level. However, repeated dose treatments with these agents restored such alterations to the normal level. We observed that VE is more effective than AA and garlic in restoring the inhibition of GST activity caused by CCl4 to the normal level after single dose treatments. On the other hand, AA and VE are more effective than garlic in restoring the induced TBARS level caused by CCl4 to the normal control level after repeated dose treatments. It has been observed that the tested antioxidants were able to antagonize the toxic effects of CCl4, and such counteracting effects were more pronounced when they were administered as repeated doses prior to administration of CCl4.

Involvement of mu class glutathione S-transferase subunit M2 (rGST M2) levels in the initiation and promotion of hepatocellular carcinogenesis in old rats

Age-associated differences in the response of the initiation and promotion of hepatocellular carcinogenesis in the rat were analyzed. Male Wistar rats 5 and 18 months-old were used throughout. They underwent an experimental design of multistage model of hepatocarcinogenesis: hepatic cells were initiated with the complete carcinogen Aflatoxin B1 (0.5mg/Kg b.w.) and the promotion was performed through a combined treatment of proliferation (partial hepatectomy, 65%) and administration of the tumorigenic promoter phenobarbital (0.1% in drinking water for 21 days). After the treatment, rats were sacrificed and the following parameters were determined: activity and subunit composition of the glutathione S-transferase enzyme system, the number of liver preneoplastic foci and the proliferation cell index. The combined treatment (initiation + promotion) lowered the expression of the mu class GST (rGST M1, rGST M2). The inhibition in rGST M2 in old animals (which in basal conditions had already been lower) was significant. On the other hand, the treatment increased the alpha class GST (rGST A, rGST A3). The number of preneoplastic foci was higher in old rats (number of foci/cm(2): 6.9+/-0.3 vs 3.9+/-0.3 in young rats, p< 0.05). The proliferation cell index did not show age-related differences. Because rGST M2 deficiency coexisted with induced expression of alpha class, the livers would be resistant to some toxic insults, being selectively sensitive to potentially genotoxic substances for which M2 is an essential detoxification pathway. The transition to a rGST M2-deficient phenotype during aging could induce higher responsiveness to genotoxic effects, and might favor the likelihood of further progression, indicating a higher susceptibility of aged animals to the development of carcinogenesis.

Different levels of Schistosoma mansoni infection induce changes in drug-metabolizing enzymes

Most carcinogens and xenobiotics are metabolized primarily by the mixed function oxidase system which includes cytochrome P450, cytochrome b5, NADPH-cytochrome c reductase and aryl hydrocarbon [benzo(a)pyrene] hydroxylase. The present study investigates the influence of infection with different levels of Schistosoma mansoni cercariae on the hepatic levels of reduced glutathione, glutathione S-transferase and glutathione reductase in addition to the enzymes of mixed function oxidase. Cercariae infection levels of 60, 120, 180, 300 and 600 per mouse increased: (i) the hepatic content of cytochrome P450 by 27%, 38%, 72%, 57%, 48% respectively; (ii) the aryl hydrocarbon hydroxylase activity by 44%, 64%, 76%, 90%, 51% respectively; and (iii) the hepatic level of reduced glutathione by 67%, 83%, 103%, 60%, 38% respectively. The cytochrome b5 content did not change at the lowest level of infection but increased at the other four levels by 45%, 76%, 49% and 38% respectively. The activity of glutathione S-transferase increased at the first three levels by 42%, 40%, 27% respectively and decreased at the last two levels by 28% and 52% respectively. On the other hand, the activity of glutathione reductase did not change at any level, whereas, NADPH-cytochrome c reductase activity decreased at the last two levels by 44% and 54%. The alterations in the activities of phase I & II of drug-metabolizing enzymes as a result of infection with different levels of S. mansoni may thus change the liver's capacity to detoxify many endogenous compounds and may also potentiate the deleterious effects of aromatic hydrocarbons, e.g. benzo(a)pyrene, upon the liver and probably other organs. Such alterations may also change the therapeutic actions of drugs that are primarily metabolized by the P450 system, when administered to patients with schistosomiasis.

N-nitrosamines and their effects on the level of glutathione, glutathione reductase and glutathione S-transferase activities in the liver of male mice

The present study investigates the influence of different chemical structure of N-nitroso compounds on the hepatic level of reduced glutathione (GSH), glutathione reductase (GSH-R) and glutathione S-transferase (GST) activities in the liver of male Balb/C mice after treatment with 20 mg/kg body weight of each compound for 1 h as a single dose. The level of reduced glutathione decreased significantly between 37 and 70% after the treatment of male mice with ethylbutylnitrosamine (-37%), diphenylnitrosamine (-50%), propylbutylnitrosamine (-52%), diethylnitrosamine (-54%), ethylmethylnitrosamine (-55%), and dibutylnitrosamine (-70%), whereas, methylpropylnitrosamine increased the level of GSH by 71%. All the N-nitrosamine compounds tested increased the activity of glutathione reductase except ethylmethylnitrosamine had no effect. The activity of glutathione S-transferase activity was inhibited after treatment of the male mice with diphenylnitrosamine (-60%), dibutylnitrosamine (-60%), and methylpropylnitrosamine (-81%), while, ethylmethylnitrosamine and ethylbutylnitrosamine had no effect on such activity. On the other hand, diethylnitrosamine increased the activity of glutathione S-transferase by 50%. It can be postulated from this study that the chemical structure of N-nitrosamines plays a significant role in the alteration of GSH level and GSH metabolizing enzymes, since the substitution of different groups on the nitroso group was found to be capable of causing an alteration in such activities.

Cellular interactions and metabolism of aflatoxin: an update

The aflatoxins are a group of closely related mycotoxins that are widely distributed in nature. The most important of the group is aflatoxin B1 (AFB1), which has a range of biological activities, including acute toxicity, teratogenicity, mutagenicity and carcinogenicity. In order for AFB1 to exert its effects, it must be converted to its reactive epoxide by the action of the mixed function mono-oxygenase enzyme systems (cytochrome P450-dependent) in the tissues (in particular, the liver) of the affected animal. This epoxide is highly reactive and can form derivatives with several cellular macromolecules, including DNA, RNA and protein. Cytochrome P450 enzymes may additionally catalyse the hydroxylation (to AFQ1 and AFM1) and demethylation (to AFP1) of the parent AFB1 molecule, resulting in products less toxic than AFB1. Conjugation of AFB1 to glutathione (mediated by glutathione S-transferase) and its subsequent excretion is regarded as an important detoxification pathway in animals. Resistance to AFB1 toxicity has been interpreted in terms of levels and activities of these detoxifying pathways. This article reviews the multiple reactions and effects attributed to aflatoxin, with particular reference to the interaction of aflatoxin with nucleic acids and proteins, and the contribution this mycotoxin has in disease development and in the promotion of hepatocellular carcinoma (HCC). The anti-mutagenic properties of several dietary factors are also considered in this article. Undoubtedly, the most important aspect of aflatoxin action is its putative role in the development of human cancer, in particular, HCC. Recently, there has been a renewed interest in this aspect and experimental evidence is rapidly accumulating at the molecular level, indicating aflatoxin as an important consideration in the aetiology of human HCC.

Influences of dietary deoxycholic acid on progression of hepatocellular neoplasms and expression of glutathione S-transferases in rats

Serial magnetic resonance imaging (MRI) was used to evaluate the influences of dietary deoxycholic acid (DCA) on the rate of progression of chemically induced hepatocellular neoplasms in rats. Male Fischer-344 rats with established persistent hepatocellular nodules generated by the Solt-Farber protocol were exposed to dietary DCA (0.3%) between 6 and 12 mo of age. Growth of nodules and carcinomas in vivo was measured by morphometric quantification of tumor images obtained every 6 wk. The final stages of neoplastic progression were determined by terminal histopathological examination and by expression and functional evaluation of glutathione S-transferase (GST) isoenzyme phenotypes. Dietary DCA increased the number of hepatocellular neoplasms per rat, accelerated the rate of growth of persistent nodules, and increased the histological progression of liver tumors. Expression of immunoreactive GST subunits Yf, Ya, and Yb1 was induced in early persistent nodules, a pattern that was maintained throughout the study in both basal diet and DCA-fed groups. However, 5% of early nodules and about 75% of advanced neoplasms were partially or completely deficient in GST Yb2 expression in both groups. DCA did not alter the cytosolic activity for the GST substrates 1-chloro-2,4-dinitrobenzene (CDNB) or trans-4-phenyl-3-buten-2-one (tPBO) in tumors or surrounding liver. However, in both groups, CDNB activity was increased in the tumors relative to the surrounding nonneoplastic tissue, whereas activity for tPBO, a substrate more specific for the Yb2 subunit, was reduced in the tumors. All advanced neoplasms were similarly more resistant than surrounding liver to DNA-binding metabolites of aflatoxin B1 or benzo[a]pyrene. These data demonstrate that DCA can increase the progression of established hepatocellular nodules to larger, more advanced neoplasms but does not preferentially select for a specific GST phenotype. Preferential loss of constitutively expressed GST Yb2 in both basal diet and DCA-fed groups may be an important aspect of progression from resistant nodules to advanced cancers in this model. These studies also demonstrate that serial MRI is a useful tool for measuring the rates of enlargement and patterns of growth in established hepatocellular neoplasms.

Conjugation of microsome generated and synthetic aflatoxin B1-8,9 epoxide and styrene oxide to glutathione by purified glutathione S-transferases from hamster and mouse livers

Glutathione (GSH) conjugation of microsome-mediated and synthetic aflatoxin B1 (AFB1)-epoxide and styrene oxide has been studied with purified glutathione transferases (GSTs) from mouse and hamster liver cytosols. In hamster, with microsomally activated epoxide, the alpha group of GSTs show about 10-fold more activity than the mu group. With the synthetic AFB1 epoxide, the mu enzymes designated H3B and C show considerable activity although less than alpha, whereas H3A and D demonstrate similar ranges of activity as the alpha group. The pi class of GST could not be assayed due to its absence in the hamster liver. The mouse liver cytosols show 3.6-fold greater activity than hamster cytosol in microsome mediated assay system. The mouse alpha and mu enzymes have similar levels of activity in the microsome mediated system; this activity could not be determined with the pi GST due to shortage of this enzyme. The alpha group has 2- and 5-fold higher activity than mu and pi group of GSTs, respectively, with the synthetic epoxide of AFB1. With styrene oxide, the purified GSTs from hamster liver show total loss of activity whereas in the mouse alpha, mu and pi classes of GSTs have similar range of activity as the cytosol. The role of alpha and mu isozymes of GST in rendering these animals resistant to hepatocarcinogenecity is suggested.

Effect of glutathione levels on aflatoxin B1-DNA binding in livers and kidneys of male rats and hamsters pretreated with buthionine sulfoximine and dimethylmaleate

The effects of pretreatment of buthionine sulfoximine (BSO) alone or in combination with diethylmaleate (DEM) on glutathione (GSH) levels and aflatoxin B1 (AFB1)-DNA binding have been examined in livers and kidneys of young male Fischer rats and Syrian golden hamsters 2 h after an intraperitoneal injection of [3H]AFB1 (0.4 mg/kg body wt.). Animals were treated with BSO (4 mmol/kg body wt.) alone at 4 h and 2 h or with DEM (3 mmol/kg body wt.) at 4 h and BSO at 2 h before AFB1 injection. Hepatic AFB1-DNA binding was about 29.0 and 6.0 pmol/mg DNA in rats and hamsters, respectively. In rats, BSO increased AFB1-DNA binding by about 40% with a drop in GSH by 70%. Treatment with DEM-BSO increased AFB1-DNA binding by about 80% with a concomitant drop in GSH in both species. In hamsters, BSO increased AFB1-DNA binding by only 10% with a 50% drop in GSH. The kidneys of both species have lower GSH levels and AFB1-DNA binding than their respective liver tissues. The effect of BSO alone or in combination with DEM on both GSH levels and AFB1-DNA binding are comparable even though BSO alone is less effective in both species. The role of modulation of GSH levels on AFB1-DNA binding and hence biological effects of AFB1 in these two species is discussed.

Effects of nivalenol on hepatic drug-metabolizing activity in rats

Effects of nivalenol (NIV), a trichothecene mycotoxin, on hepatic drug-metabolizing activity and aflatoxin B1 (AFB1) metabolism were investigated in male rats. In rats fed the diets containing 6-12 ppm NIV for 2 or 4 wk, decreases in initial feed uptake, terminal weight gain and organ weights were evident. An increase in cytochrome P-450 activity was observed in the hepatic microsomes, and Western blot analysis revealed a transient increase in P4502B1/2, together with a slight induction of P4501A2. The activity of cytosolic glutathione S-transferase (GST) enzymes was also enhanced in the rats, and Western blot analysis demonstrated an elevation of GST 1-2. The formation of aflatoxin B1-DNA adducts (AFB1-DNA) was increased in experiments using hepatic microsomal preparations from rats fed the NIV diet, whereas supplementation with cytosol prepared from NIV-treated rats reduced the microsomal potential for adduct formation. In in vivo experiments, the AFB1-DNA concentration in NIV-treated rats was lower than that in the controls. These results suggest that activities of cytochrome P-450 and GST enzymes were increased in rats fed NIV for several weeks. Alteration of these phase I and phase 2 enzyme levels resulted in the modulation of AFB1 adduction to DNA in vitro and in vivo.