Interindividual variation in biotransformation and cytotoxicity of bromobenzene as determined in primary hepatocyte cultures derived from monkey and human liver

1. Bromobenzene-evoked hepatotoxicity resulting from cytochrome P450-mediated epoxidation has been studied extensively in rodents in vivo and in rodent hepatocytes. In this paper we present data concerning the formation of bromphenols, glutathione (GSH) depletion and cytotoxicity observed in primate hepatocytes in primary culture after exposure to bromobenzene (BrB). 2. After pre-incubation for 2 or 24 h, hepatocytes were exposed to BrB in concentrations up to 2 mM for 4 or 24 h. 3. In both human and cynomolgus monkey hepatocytes BrB cytotoxicity and GSH depletion were found after exposure to 2 mM BrB. The degree of the observed effects was not influenced by the duration of pre-incubation and/or exposure periods. 4. Major inter-individual differences were observed, which could not be attributed to differences in cytochrome P450-mediated bioactivation rates. This suggests that the variation in individual susceptibility to BrB may be related to inter-individual differences in the activity of de-activating (metabolic) pathways. 5. The study of the background of these inter-individual differences may contribute to a more complete understanding of the factors ruling sensitivity to BrB or related chemicals.

Acid reaction products of indole-3-carbinol and their effects on cytochrome P450 and phase II enzymes in rat and monkey hepatocytes

The effects of three acid condensation products of indole-3-carbinol (I3C), i.e. 3,3'-diindolylmethane (DIM), 5,6,11,12,17,18-hexahydrocyclonona[1,2-b:4,5-b':7,8-b"]tri-indole (CTI) and 2,3-bis[3-indolylmethyl]indole (BII), on cytochrome P450 and phase II enzymes were studied in primary cultures of rat and cynomolgus monkey liver cells. In rat hepatocytes all three indole derivatives dose-relatedly induced the ethoxyresorufin O-dealkylation (EROD) activity (to 24-fold) and 7 alpha-hydroxylation of testosterone (to 4-fold), whereas all three decreased the 16 alpha- and 2 alpha-testosterone hydroxylation (DIM to 60%, CTI and BII to a mere 5% of the control cells). Treatment of monkey hepatocytes with DIM and BII enhanced the EROD activity to 6- and 9-fold, respectively. Furthermore, BII decreased the 6 beta-hydroxylation of testosterone (to 60% of the untreated cultures) in monkey cells. Phase II enzymes were also affected. In rat hepatocytes DIM, CTI and BII enhanced DT-diaphorase (DTD) (= NAD(P)H-quinone reductase) activity, and DIM and BII the glucuronidation of 1-naphthol. In monkey cells BII only enhanced DTD, and no changes were observed in the glucuronidation of 1-naphthol after treatment with either DIM or BII. The indole derivatives did not affect glutathione S-transferase activity and sulfation of 1-naphthol in either rat or monkey hepatocytes. These results identify two novel acid condensation products of I3C, CTI and BII, as potent compounds in affecting biotransformation in rat as well as in monkey hepatocytes.

Effects of cooked brussels sprouts on cytochrome P-450 profile and phase II enzymes in liver and small intestinal mucosa of the rat

Male Wistar rats were given semi-synthetic diets supplemented with 0, 2.5, 5 and 20% cooked Brussels sprouts for 2, 7, 14 or 28 days. The effects on several cytochrome P-450 enzymes and phase II enzymes (glutathione S-transferase (GST), glucuronyl transferases 1 and 2 (GT1 and GT2) and DT-diaphorase (DTD)) in the liver and small intestinal mucosa were investigated. From 2 days of exposure onwards Brussels sprouts induced P4501A2 and--to a lesser extent--P4501A1 apoprotein levels in the liver, whereas in the small intestine markedly enhanced P4502B apoprotein levels could be detected. No enhanced P4503A apoprotein levels were observed. The 5 and 20% sprouts diets increased the intestinal pentoxyresorufin depentylation (PROD, 4.5-9-fold), and the hydroxylation of testosterone at the 16 alpha- and 16 beta-site (2.6-4.2-fold) after 2 days of exposure. In addition, the 20% sprouts died also enhanced the intestinal ethoxyresorufin deethylation (EROD) activity (c. 5-fold), the hepatic EROD and PROD activities (c. 2-fold) and the formation of 6 beta-hydroxytestosterone (c. 1.6-fold); the formation of 2 alpha-hydroxytestosterone in the liver was decreased (to c. 70% of the control value). GST activity was induced both in the liver (5 and 20% diet) and intestine (20% diet only) throughout the experiment. The 20% sprouts diet enhanced the hepatic DTD and GT1 activities, whereas the GT2 activity was decreased. The induction of DTD in the small intestine after 2 days (2.5-3.2-fold with 5 and 20% sprouts diets, respectively) diminished during the experiment. These results indicate that dietary exposure to cooked Brussels sprouts for only 2 days can change the metabolic activities of several phase II enzymes and cytochrome P-450 enzymes, of which P4502B is the predominant form induced in the small intestine.

Comparison of cytochrome P450 isoenzyme profiles in rat liver and hepatocyte cultures. The effects of model inducers on apoproteins and biotransformation activities

The metabolic profile of seven subfamilies of cytochrome P450 (P450IA, IIA, IIB, IIC, IIE, IIIA, IVA) was studied in rat liver (in vivo) and in primary hepatocyte cultures (in vitro) after treatment with various inducers. The dealkylation of 7-ethoxyresorufin (EROD) and 7-pentoxyresorufin (PROD), aniline 4-hydroxylation and the regio- and stereoselective hydroxylation of testosterone were measured to characterize the isoenzyme pattern in intact hepatocytes and in liver microsomes. Occurrence of isoenzyme apoproteins was determined using Western blotting. Primary cultures of rat hepatocytes retain the capacity to respond to inducers of isoenzymes belonging to six different subfamilies (P450IA, IIA, IIB, IIC, IIIA and IVA). Treatment of cells with beta-naphthoflavone revealed a P450-activity profile similar to in vivo, namely a highly induced EROD (P450IA1), a small enhancement of testosterone 7 alpha-hydroxylation (P450IIA) and a marked reduction in 2 alpha- and 16 alpha-hydroxylation (P450IIC11). Exposure of cultured cells to phenobarbital resulted in a higher testosterone 16 beta-hydroxylation (reflecting P450IIB), though to a lesser extent than in vivo. The induction of P450IIIA due to both phenobarbital and dexamethasone, as mirrored by 6 beta- and 15 beta-hydroxylation of testosterone, was the same in cultured hepatocytes and in vivo. Treatment of cells with clofibric acid resulted in an induction profile similar to the one observed in liver microsomes from clofibrate-treated rats: the apoprotein P450IVA as well as the apoprotein P450IIB1/2 and its associated activities (PROD and testosterone 16 beta-hydroxylation) were induced. Isoniazid, a known in vivo inducer of P450IIE1 and aniline 4-hydroxylation, did not change any of the determined P450-dependent activities in vitro.

The isoenzyme pattern of cytochrome P450 in rat hepatocytes in primary culture, comparing different enzyme activities in microsomal incubations and in intact monolayers

Changes in the isoenzyme pattern of cytochrome P450 during culture were investigated in primary cultures of rat hepatocytes, measuring specific enzyme activities in microsomes prepared from cultured cells as well as in intact monolayers. Assays of 7-ethoxyresorufin O-deethylation (EROD), 7-pentoxyresorufin O-depentylation (PROD), aniline 4-hydroxylation (AH) and the specific regioselective hydroxylation of testosterone were used as representatives of the activities of seven isoenzymes of cytochrome P450. The isoenzyme profile expressed as catalytic activities was qualitatively and quantitatively similar in microsomes obtained from freshly isolated hepatocytes in comparison with microsomes obtained from whole livers of untreated rats. There was a relatively high activity in EROD, AH and the oxidation of testosterone at the 7 alpha, 2 alpha, 6 beta, 16 alpha and 17 sites (androstenedione). During culture, these microsomal enzyme activities declined at a similar rate to ca. 50% of the activities of microsomes prepared from freshly isolated hepatocytes after 24 hr and to 15% after 96 hr. The overall decline of cytochrome P450-dependent activities during culture was not accompanied with gross changes in catalytic profile. Determining the same drug-metabolizing activities directly in intact hepatocyte monolayers revealed a much higher metabolic rate for all measured P450-dependent activities. The profile of the catalytic activities was essentially the same as measured in microsomes prepared from cultured hepatocytes. The relatively low activity towards the 7 alpha site of testosterone measured in intact hepatocytes, however, remained constant during culture. Determination of enzyme activities directly in intact hepatocytes is a convenient way of studying changes in monooxygenase activities of different P450 isoenzymes in vitro.

The effect of beclobric acid and clofibric acid on peroxisomal beta-oxidation and peroxisome proliferation in primary cultures of rat, monkey and human hepatocytes

The peroxisome-proliferating effects of clofibric acid and beclobric acid were studied in primary cultures of hepatocytes derived from rat, monkey (Macaca fascicularis) and human liver. Determination of peroxisomal fatty acid beta-oxidation and morphometrical analysis of the peroxisomal compartment were performed after incubation of 1-day-old hepatocyte cultures for 3 days with either compound. In rat liver cell cultures both compounds gave a 10-fold increase in peroxisomal beta-oxidation, a 3-fold increase in the relative number of peroxisomes and a 1.5-fold increase in the mean size of peroxisomes. Beclobric acid gave its maximal effect at a concentration of 10 microM, which is at least one order of magnitude lower than the maximum-effect concentration of clofibric acid. At concentrations greater than 300 microM beclobric acid was cytotoxic. No stimulation of peroxisomal fatty acid beta-oxidation was found in either monkey or human hepatocyte cultures. Morphometrical analysis also showed no increase in the peroxisomal compartment in cultures derived from these species, as indicated by the lack of increase in both relative number and size of peroxisomes. In all three species tested beclobric acid was equally cytotoxic for hepatocytes in vitro. These results are of relevance for the interpretation of the peroxisome-proliferating effects of clofibrate and similar compounds in rats. Since peroxisome proliferation may be correlated to increased hepatic tumour incidences in the rat, the absence of peroxisome proliferation in primates suggests the absence of tumourogenic activity by hypolipidemic compounds in these species.

Effects of 1,2-dibromoethane on isolated hepatocytes: functional alterations and induction of lipid peroxidation

1. Isolated rat hepatocytes were exposed to the fumigant 1,2-dibromoethane (DBE) and cytotoxicity was evaluated by studying parameters of cellular function and lipid peroxidation. 2. DBE caused plasma membrane damage, as determined by leakage of lactate dehydrogenase, and was more severe in shaken suspensions than stationary suspensions, suggesting that cells were more fragile after DBE exposure. 3. DBE decreased hepatocyte glycogen content and stimulated albumin synthesis in hepatocyte suspensions. 4. Lipid peroxidation resulted from DBE exposure and was greater in cells isolated from phenobarbital-pretreated rats. Shaking the suspensions enhanced lipid peroxidation. Ethane production did not parallel formation of thiobarbituric acid reactants, suggesting that these parameters of lipid peroxidation reflect different mechanisms of molecular interaction of DBE.

Triorganotin-induced cytotoxicity to rat thymus, bone marrow and red blood cells as determined by several in vitro assays

To further investigate the immunotoxic effects of tri-n-propyltin chloride (TPTC), tri-n-butyltin chloride (TBTC) and triphenyltin chloride (TPhTC) several cytotoxicity tests with a series of trialkyltin chlorides and TPhTC were carried out, using isolated rat thymocytes as target cells. Thymocytes, cultured in a serum-supplemented medium, were exposed to organotin concentrations ranging from 0.01 to 10 microM for periods up to 30 h. Parameters such as cell count, trypan blue exclusion, chromium release, thymidine incorporation and cyclic AMP production were used to evaluate the cytotoxicity of these compounds. The more lipophilic compounds TPTC, TBTC, tri-n-hexyltin chloride (THTC) and TPhTC appeared most cytotoxic, reducing thymidine incorporation at concentrations as low as 0.05-1 microM. Membrane damage as determined by trypan blue exclusion and chromium release occurred at higher levels (1-10 microM). The water soluble homologue trimethyltin chloride (TMTC) was least effective in all test models. When phosphate-buffered saline supplemented with glucose was used as incubation medium, TBTC appeared more cytotoxic to thymocytes. Using this medium in 5-h incubations the cytotoxicity of TBTC to thymus, bone marrow and red blood cells was compared. Bone marrow cells were slightly less sensitive than thymocytes, while red cells were relatively resistant. In conclusion, of the triorganotin compounds especially the lipophilic homologues are cytotoxic in vitro.

Toxicity of triorganotin compounds: comparative in vivo studies with a series of trialkyltin compounds and triphenyltin chloride in male rats

In 2-week feeding studies, a series of trialkyltin chlorides and triphenyltin chloride were fed to male weanling rats at different dietary concentrations to evaluate their toxic effects, especially on the brains and the lymphoid organs, thymus and spleen. The lower trialkyltin homologs, trimethyltin chloride (TMTC) and triethyltin chloride (TETC), were neurotoxic, causing neuronal degradation and cerebral edema, respectively, at dietary concentrations of 15 ppm. The intermediate homologs, tri-n-propyltin chloride (TPTC) and tri-n-butyltin chloride (TBTC), and the aromatic compound, triphenyltin chloride (TPhTC), caused a dose-related reduction of thymus weight. At a dietary concentration of 150 ppm decreases in thymus weight to 53, 39, and 81% of controls were found following treatment with TPTC, TBTC, and TPhTC, respectively. Microscopically, thymus atrophy was associated with a lymphocyte depletion in the thymic cortex. Only 16% of the total number of nucleated thymocytes could be isolated from rats fed 150 ppm TBTC. These effects were completely reversed within 2 weeks. Slight thymus atrophy was observed after feeding a relatively high dose of 150 mg tri-n-hexyltin chloride (THTC)/kg diet, whereas tri-n-octyltin chloride (TOTC) was ineffective. A dose-related decrease in spleen weight was noticed after 2 weeks feeding of TPTC, TBTC, and TPhTC. Liver weights were increased in rats fed TBTC, THTC, and TPhTC for 2 weeks. Nevertheless, no enlarged livers and normal spleen weights were found upon feeding 100 ppm TPTC or TBTC for 4 weeks, whereas thymus weight was severely decreased. Therefore, atrophy of the thymus was considered to be the predominant effect of the intermediate trialkyltins (TPTC, TBTC). From this study it is concluded that the lower trialkyltins (TMTC, TETC) are essentially neurotoxic, the intermediate trialkytins (TPTC, TBTC) and triphenyltin are primarily immunotoxic, and the higher homologs (THTC, TOTC) are only slightly toxic or not toxic at all.

NADH-ferrihemoglobin reductase in avian erythrocytes

The assay for NADH-ferrihemoglobin reductase (NADH-FR) was optimized for avian blood samples. In this assay the pH optimum for Japanese quail red cell NADH-FR was 5.5, which was close to the enzyme's pI. Enzyme kinetic parameters were determined for quail, chicken and turkey NADH-FR. Preparation of erythrocyte ghost-cells and subsequent fractionation showed that the enzyme was present in the plasma membrane as well as in the nuclear membrane, while Triton X-100 treatment gave a release of enzyme activity from the membrane. In the cytosolar fraction of avian red cells no NADH-FR could be detected.