Effect of inhibitors of microsomal enzymes on aflatoxin B1-induced cytotoxicity and inhibition of RNA synthesis in isolated rat hepatocytes

Dietary modulation of the biotransformation and genotoxicity of aflatoxin B(1)

Diet and its various components are consistently identified as among the most important 'risk factors' for cancer worldwide, yet great uncertainty remains regarding the relative contribution of nutritive (e.g., vitamins, calories) vs. non-nutritive (e.g., phytochemicals, fiber, contaminants) factors in both cancer induction and cancer prevention. Among the most potent known human dietary carcinogens is the mycotoxin, aflatoxin B(1) (AFB). AFB and related aflatoxins are produced as secondary metabolites by the molds Aspergillus flavus and Aspergillus parasiticus that commonly infect poorly stored foods including peanuts, pistachios, corn, and rice. AFB is a potent hepatocarcinogenic agent in numerous animal species, and has been implicated in the etiology of human hepatocellular carcinoma. Recent research has shown that many diet-derived factors have great potential to influence AFB biotransformation, and some efficiently protect from AFB-induced genotoxicity. One key mode of action for reducing AFB-induced carcinogenesis in experimental animals was shown to be the induction of detoxification enzymes such as certain glutathione-S-transferases that are regulated through the Keap1-Nrf2-ARE signaling pathway. Although initial studies utilized the dithiolthione drug, oltipraz, as a prototypical inducer of antioxidant response, dietary components such as suforaphane (SFN) are also effective inducers of this pathway in rodent models. However, human GSTs in general do not appear to be extensively induced by SFN, and GSTM1 - the only human GST with measurable catalytic activity toward aflatoxin B(1)-8,9-epoxide (AFBO; the genotoxic metabolite of AFB), does not appear to be induced by SFN, at least in human hepatocytes, even though its expression in human liver cells does appear to offer considerable protection against AFB-DNA damage. Although induction of detoxification pathways has served as the primary mechanistic focus of chemoprevention studies, protective effects of chemoprotective dietary components may also arise through a decrease in the rate of activation of AFB to AFBO. Dietary consumption of apiaceous vegetables inhibits CYP1A2 activity in humans, and it has been demonstrated that some compounds in those vegetables act as potent inhibitors of human CYP1A2 and cause reduced hCYP1A2-mediated mutagenicity of AFB. Other dietary compounds of different origin (e.g., constituents of brassica vegetables and hops) have been shown to modify expression of human hepatic enzymes involved in the oxidation of AFB. SFN has been shown to protect animals from AFB-induced tumors, to reduce AFB biomarkers in humans in vivo and to reduce efficiently AFB adduct formation in human hepatocytes, although it appears that this protective effect is the result of repression of human hepatic CYP3A4 expression, rather than induction of protective GSTs, at least in human hepatocytes. If this mechanism were to occur in vivo in humans, it would raise safety concerns for the use of SFN as a chemoprotective agent as it may have important implications for drug-drug interactions in humans. A dietary chemoprevention pathway that is independent of AFB biotransformation is represented by the potential for dietary components, such as chlorophyllin, to tightly bind to and reduce the bioavailability of aflatoxins. Chlorophyllin has been shown to significantly reduce genotoxic AFB biomarkers in humans, and it therefore holds promise as a practical means of reducing the incidence of AFB-induced liver cancer. Recent reports have demonstrated that DNA repair mechanisms are inducible in mammalian systems and some diet-derived compounds elevated significantly the gene expression of enzymes potentially involved in nucleotide excision repair of AFB-DNA adducts. However, these are initial observations and more research is needed to determine if dietary modulation of DNA repair is a safe and effective approach to chemoprevention of AFB-induced liver cancer.

Interindividual differences in response to chemoprotection against aflatoxin-induced hepatocarcinogenesis: implications for human biotransformation enzyme polymorphisms

It is now evident that most, if not all, of the remarkable species differences in susceptibility to AFB hepatocarcinogenesis is due in large part, if not exclusively, to differences in biotransformation. Certainly the relative rate of oxidative formation of the proximate carcinogen, AFB-8,9-exo-epoxide, is an important determinant of species and interindividual differences in susceptibility to AFB. However, mice produce relatively large amounts of exo-AFBO, yet are highly resistant to AFB-hepatocarcinogenesis because they express a particular form of GST with remarkably high catalytic activity toward the exo-epoxide of AFB. Rats, which are highly susceptible to AFB hepatocarcinogenesis,can be made resistant through dietary induction of an orthologous form of GST that is normally expressed in only very small amounts. Based on these findings in laboratory animal models, there is great interest in identifying chemicals and/or specific dietary constituents that could offer protection against AFB-hepatocarcinogenesis to humans. Current experimental strategies have focused on the antiparasitic drug, oltipraz, which induces protection in rats and has also shown some promise in humans. The mechanism of protection in rats appears to be via induction of an alpha class GST with high catalytic activity toward AFBO (rGSTA5-5). vet human alpha class GST proteins that are constitutively expressed in the liver (hGSTA1 and hGSTA2) have little, if any activity toward AFBO. Rather, it appears that mu class GSTs may be responsible for the very low, but potentially significant, detoxification activity toward AFBO. Oltipraz and certain dietary constituents may induce mu class GSTs in human liver, and this could afford some protection against the genotoxic effects of AFBO. However, it also appears that oltipraz, and perhaps certain dietary constituents, act as competitive inhibitors of human CYP1A2. As CYP1A2 appears to mediate most of the activation of AFB to exo-AFBO in human liver at low dietary concentrations of AFB encountered in the human diet, much of the putative protective effects of oltipraz could be mediated via inhibition of CYP1A2 rather than induction of GSTs. There is now evidence that human microsomal epoxide hydrolase (mEH) could play a role in protecting human DNA from the genotoxic effects of AFB, although the importance of this detoxification pathway, relative to mu class GSTs, remains to be elucidated. Oltipraz is an effective inducer of mEH in rats (Lamb Franklin, 2000), and thus induction of this pathway in humans could also potentially contribute to the protective effects of this drug toward AFB genotoxicity. Because the dihydrodiol of AFB may contribute indirectly to the carcinogenic effects of AFB via protein adduction and subsequent hepatotoxicity, the recently characterized human aflatoxin aldehyde reductase (AFAR) may also offer some protection against AFB-induced carcinogenicity in humans. Current and future dietary and/or chemointervention strategies aimed at reducing the carcinogenic effects of AFB in humans should consider all of the possible mechanistic approaches for modifying AFB-induced genotoxicity.

Variability in aflatoxin B(1)-macromolecular binding and relationship to biotransformation enzyme expression in human prenatal and adult liver

Studies of transplacental transfer of aflatoxin B(1) (AFB(1)) suggest that the developing human fetus may be a sensitive target for AFB(1) injury. Because AFB(1) requires metabolic activation to the reactive AFB(1)-8,9-exo-epoxide (AFBO) to exert its carcinogenic effects, ontogenic and interindividual differences in AFB(1) biotransformation enzymes may underlie susceptibility to AFB(1)-induced cell injury. The present study was initiated to compare the rates of in vitro AFB(1)-DNA and AFB(1)-protein adduct formation among a panel of 10 adult and 10 second-trimester prenatal livers and to examine the relationship among AFB(1) metabolizing enzyme expression and AFB(1) binding. Mixtures of cytosolic and microsomal proteins from prenatal and adult livers catalyzed the formation of AFB(1)-DNA and AFB(1)-protein adducts at relatively similar rates, although greater individual variability in AFB(1) adduct formation was observed in adult tissues. Extensive interindividual variation among adult tissues was observed in the expression of the AFB(1) activation enzymes cytochrome P4501A2 (CYP1A2), CYP3A4/5, and lipoxygenase (LO). Prenatal CYP3A7 expression was also highly variable. LO expression was eightfold higher in prenatal liver tissues than adults, whereas the expression of the AFBO detoxification enzyme microsomal epoxide hydrolase was twofold higher in adult liver. The levels of the polymorphic glutathione S-transferase M1 (hGSTM1-1), which may potentially protect against AFBO injury, were higher in the hGSTM1-1-expressing tissues of adults in relation to prenatal livers. In general, there was not a strong relationship among AFB(1)-DNA or AFB(1)-protein adduct formation and expression levels of individual AFB(1) metabolizing enzymes. In summary, despite the presence of marked individual and ontogenic differences in the expression of AFB(1) metabolizing enzymes, human second trimester prenatal liver tissues compared to adults do not exhibit a marked sensitivity to the in vitro formation of macromolecular AFB(1) adducts.

Early influence of aflatoxin B1 on the functional state of isolated rat hepatocytes

The early effects (60 min) of aflatoxin B1 (AFB1) on membrane permeability and carbohydrate metabolism of liver cells were studied in fresh suspensions of rat hepatocytes. Evaluation by trypan blue exclusion, enzyme leakage, glycogen synthesis or degradation, and glyconeogenesis were chosen as viability tests. The results obtained showed an increase of lactate dehydrogenase (LDH), alanine aminotransferase (GPT) and aspartate aminotransferase (GOT) released into the medium and also an increase in the number of stained cells. These changes were significant at about 18 nmol/10(6) cells of AFB1, while a remarkable effect of the toxin on glyconeogenesis and glycogen synthesis or degradation was observed at 9 nmol/10(6) cells, doses commonly used for in vitro studies.

Modification of protein synthetic components by aflatoxin B1

Molecular sites of perturbation by the hepatocarcinogen aflatoxin B1 (AFB1) in the protein synthesis initiation complex were assessed using isolated hepatocytes and a cell-free activating system containing microsomes and cytoplasmic ribonucleoprotein complexes (cRPC). Ribosomal proteins showed no detectable modification by the toxin in either system. With hepatocytes, initiation factors demonstrated only slight modification by AFB1. RNAs from both hepatocytes and the cell-free system with microsomes and cRPC were modified, with poly(A)-containing RNA exhibiting at least a 5-fold higher modification than poly(A)-lacking RNA. The poly(A)-lacking RNAs were modified in the order 28S rRNA greater than 18S rRNA greater than 5-6S rRNA greater than 4S tRNA. Guanine was the target base of AFB1, but only 10% of the AFB1-GMP adducts were on guanines located in a poly(G) region. These results suggest that guanine modification in RNAs may be responsible for the observed inhibition of translational initiation by AFB1 to a greater extent than modification of either ribosomal intrinsic or associated proteins.

Hepatocellular uptake of aflatoxin B1 by non-ionic diffusion. Inhibition of bile acid transport by interference with membrane lipids

Aflatoxin B1 permeates isolated rat hepatocytes by non-ionic diffusion. Its uptake is neither saturable nor influenced by metabolic energy and not inhibited by treatment of cells with proteases. The initial rate of aflatoxin B1 uptake measured at 7 degrees C is between 40 and 50% compared to that at 37 degrees C. However, after an incubation period of 7 minutes identical equilibrium uptake is reached at both temperatures. The apparent activation energies, calculated for aflatoxin B1 uptake by Arrhenius diagrams ranged between 1.69 and 4.5 kcal/mol. A Q10 value of 1.34 was calculated for a temperature interval of 7-17 degrees C but decreased to 1.05 for the interval of 27-37 degrees C. Liposomes or lipoproteins added to the cell suspension inhibited the aflatoxin B1 uptake into hepatocytes. Liposomes mainly composed of unsaturated fatty acids bind twice as much aflatoxin B1 as those composed of saturated ones, indicating that the lipophilicity of the mycotoxin is crucial in the determination of its uptake into liver cells. At concentrations above 5 micrograms/ml, aflatoxin B1 inhibited the carrier-mediated uptake of cholic acid and of phalloidin into hepatocytes. This effect was reversible and abolished by washing the cells after preincubation with aflatoxin. In concentrations below 5 micrograms/ml the uptake of phallotoxin and cholic acid was however stimulated by 15-25%. These results indicate, that a carrier-mediated uptake into hepatocytes via the multispecific bile salt transporter is not responsible for the organoselective clearance of aflatoxins by the liver. On the other hand, the cholestatic effect of aflatoxin B1 results at least partially from the inhibition of the multispecific bile acid transport system. This inhibition may arise from affinity of aflatoxins to lipid domains of the cell membrane.

Comparative cytopathology of primary rat hepatocyte cultures exposed to aflatoxin B1, acetaminophen, and other hepatotoxins

The cytomorphological alterations in primary monolayer cultures of Fischer 344 rat hepatocytes exposed continuously to lethal concentrations of various hepatotoxins were compared. Aflatoxin B1 (AFB1) at concentrations of 1 and 10 microM killed 67 and 97% hepatocytes, respectively, at 48 hr, as determined by release of lactate dehydrogenase (LDH) into culture medium, or by trypan blue uptake. AFB1 produced numerous radial finger-like projections (blebs) at attached margins of cytoplasm in almost all hepatocytes between 6 and 24 hr. Formation of blebs was dose dependent and preceded release of LDH and trypan blue uptake. Similar marginal blebs were produced by cycloheximide, 2-acetylaminofluorene, N-hydroxy-2-acetylaminofluorene, and senecionine. By comparison, acetaminophen, at equivalently lethal concentrations (4 or 16 mM) did not cause marginal blebs, but resulted in a similar time course of LDH release and trypan blue uptake. Carbon tetrachloride and bromobenzene also failed to produce blebs at lethal concentrations. Phalloidin and cytochalasin B rapidly produced smaller spherical blebs over the entire surface of all hepatocytes, distinct from the finger-like blebs produced only at the free attached margins of cells exposed to AFB1. Blebs and lethal injury by AFB1 were reduced by phenobarbitone or 3-methylcholanthrene pretreatments and by SKF-525-A in culture. The demonstration of morphological prelethal changes in hepatocytes injured by different classes of hepatotoxins in culture provides a means of differentiating the early biochemical mechanisms by which hepatotoxins and hepatocarcinogens lethally injure hepatocytes before membrane permeability to LDH and trypan blue is detectable.

The distribution and intracellular translocation of aflatoxin B1 in isolated hepatocytes

The distribution and intracellular translocation of AFB1 in various subcellular fractions was investigated in isolated hepatocytes by pulse-chase experiments. After labeling the hepatocytes with [3H]-AFB1 (14.5 nM) for 15 min, the highest concentration of [3H]-AFB1 was found in the cytosolic fraction where 66% was bound noncovalently and 1.5% covalently. The lowest concentration of [3H]-AFB1 was found in the nuclear fraction; 36% and 4.9% were bound noncovalently and covalently respectively. When the [3H]-AFB1 loaded cells were chased with unlabeled AFB1 (1 microM), the radioactivity of [3H]-AFB1 in the cell lysate and cytosolic fraction decreased in time with an apparent rate of elimination (t1/2) of 93 min and 66 min, respectively. The levels of covalently bound AFB1 increased with time and reached a maximum at 60 min in nuclei (270%), and at 120 min in mitochondria (220%) and cytosol (430%) as compared to the zero time. Only in the microsomal fraction was there no significant increase with time in covalently bound AFB1. These results suggest that the toxin after activation by the microsomal mixed function oxidases was either detoxified or transported to other cellular organelles where covalent binding of macromolecules occurred.