1,1-Dichloroethylene hepatotoxicity: effect of altered thyroid function and evidence for the subcellular site of injury

Derangement of Kupffer cell functioning and hepatotoxicity in hyperthyroid rats subjected to acute iron overload

Liver oxidative stress, Kupffer cell functioning, and cell injury were studied in control rats and in animals subjected to L-3,3',5-tri-iodothyronine (T3) and/or acute iron overload. Thyroid calorigenesis with increased rates of hepatic O2 uptake was not altered by iron treatment, whereas iron enhanced serum and liver iron levels independently of T3. Liver thiobarbituric acid reactants formation increased by 5.8-, 5.7-, or 11.0-fold by T3, iron, or their combined treatment, respectively. Iron enhanced the content of protein carbonyls independently of T3 administration, whereas glutathione levels decreased in T3- and iron-treated rats (54%) and in T3Fe-treated animals (71%). Colloidal carbon infusion into perfused livers elicited a 109% and 68% increase in O2 uptake in T3 and iron-treated rats over controls. This parameter was decreased (78%) by the joint T3Fe administration and abolished by gadolinium chloride (GdCl3) pretreatment in all experimental groups. Hyperthyroidism and iron overload did not modify the sinusoidal efflux of lactate dehydrogenase, whereas T3Fe-treated rats exhibited a 35-fold increase over control values, with a 54% reduction by GdCl3 pretreatment. Histological studies showed a slight increase in the number or size of Kupffer cells in hyperthyroid rats or in iron overloaded animals, respectively. Kupffer cell hypertrophy and hyperplasia with presence of inflammatory cells and increased hepatic myeloperoxidase activity were found in T3Fe-treated rats. It is concluded that hyperthyroidism increases the susceptibility of the liver to the toxic effects of iron, which seems to be related to the development of a severe oxidative stress status in the tissue, thus contributing to the concomitant liver injury and impairment of Kupffer cell phagocytosis and particle-induced respiratory burst activity.

Effect of hyperthyroidism on the in vitro metabolism and covalent binding of 1,1-dichloroethylene in rat liver microsomes

Hyperthyroidism potentiates the in vivo hepatotoxicity of 1,1-dicholoroethylene (DCE) in rats, with a concomitant increase in [14C]-DCE covalent binding. The enhanced injury produced in hyperthyroid livers by DCE could be due to alterations in either the bioactivation or detoxication phases of DCE metabolism. Previous in vitro studies suggested that hyperthyroidism did not potentiate DCE hepatotoxicity by increasing DCE oxidation to intermediates which were able to covalently bind. Several factors, however, that could contribute to the magnitude of DCE bioactivation or covalent binding were not examined. Our objectives were to characterize the effects of hyperthyroidism in male Sprague-Dawley rats on: (1) covalent binding of [14C]-DCE to microsomes and other subcellular fractions, (2) microsomal mixed-function oxidase (MFO) and glutathione S-transferase (GST) activities, and (3) inactivation of microsomal enzyme activities by presumptive DCE reactive intermediates. Hyperthyroid (HYPERT) and euthyroid (EUT) rats received 3 s.c. injections of thyroxine (100 micrograms/100 g) or vehicle, respectively, at 48-h intervals; microsomes and other subcellular fractions were isolated from HYPERT and EUT livers 24 h after the last injection. [14C]-DCE-derived covalent binding was consistently greater in EUT than HYPERT microsomes. The absence of NADH, and the addition of low concentrations (0.1 and 0.5 mM), but not higher concentrations (> 1 mM), of glutathione (GSH) diminished covalent binding to a greater extent in HYPERT than EUT microsomes. Covalent binding in mitochondrial, nuclear, and cytosolic fractions of EUT and HYPERT livers was equivalent. Regression analysis of covalent binding to liver cell fractions from both EUT and HYPERT rats showed a significant correlation with P-450 content. Hyperthyroidism decreased microsomal, but not mitochondrial, cytochrome P-450 content, and MFO activities for 7-ethoxycoumarin and benzphermine were similarly decreased. Hyperthyroidism also diminished microsomal GST activity, and altered GST kinetics for both GSH and 1-chloro-2,4-dinitrobenzene (CDNB). The magnitude of inactivation of MFO and GST activities in the presence of DCE (presumably by DCE reactive intermediates) was comparable between EUT and HYPERT microsomes. When covalent binding was standardized to cytochrome P-450 concentrations in microsomes and mitochondria, HYPERT fractions exhibited slightly greater covalent binding than EUT fractions, suggesting that hyperthyroidism does not reduce the capacity of P-450 hemoproteins to bioactive DCE. Thus, potentiation of DCE hepatotoxicity by hyperthyroidism may be predominantly due to diminished Phase II constituents, and major increases in reactive intermediate/conjugates that covalently bind to and impair critical cellular molecules.

Hyperthyroidism increases covalent binding and biliary excretion of 1,1-dichloroethylene in rats

Distribution, covalent binding, and biliary excretion of 1,1-dichloroethylene (DCE) were examined in euthyroid (EuT) and hyperthyroid (HyperT) rats, which are more vulnerable to DCE hepatotoxicity. Male Sprague-Dawley rats were made hyperthyroid by 3 sc injections of thyroxine at 48-h intervals prior to experiments; euthyroid controls received vehicle injections. A time course study monitored the circulation and excretion of 14C-DCE label for 24 h after administration of 14C-labeled DCE (50 mg/kg in mineral oil) in serial blood and urine samples. At 24 h, total and covalently bound 14C-label were measured in liver, kidney, and lung. Hepatotoxicity of DCE was enhanced in the HyperT rats, as evidenced by elevated serum activities of aminotransferase and histopathology, and was associated with increases in circulating metabolite, and in metabolite bound to red blood cells and liver but not to kidney or lung. Hyperthyroidism had little effect on in vitro capacity of hepatic microsomes to convert DCE to reactive intermediates as reflected by covalent binding. A biliary excretion study in pentobarbital-anesthetized rats showed a striking, but transient, increase in toxicant metabolite excretion in bile of HyperT rats during the first 2 h after toxicant administration (14C-DCE, 100 mg/kg). During the next 2 h, biliary metabolite excretion by HyperT rats decreased while there was a rise in circulating amounts of total and bound 14C-label. Thus, although hyperthyroidism had little effect on the total extent of DCE metabolized, this hormonal disturbance may have transiently enhanced metabolite formation and definitely was associated with a lesser ability to detoxify reactive DCE metabolites capable of injuring hepatic cell constituents by covalent binding reactions.

1,1-Dichloroethylene hepatotoxicity: hypothyroidism decreases metabolism and covalent binding but not injury in the rat

Our objective was to determine if the previously reported protective effect of hypothyroidism against 1,1-dichloroethylene hepatotoxicity was associated with a change in distribution and covalent binding. Sprague-Dawley male rats were made hypothyroid (HypoT) by surgical thyroidectomy 2 weeks prior to studies and compared to euthyroid (EuT) rats. Hypothyroidism decreased body weights and liver to body weight ratios while mitochondrial non-protein sulfhydryl groups and cytosolic alcohol dehydrogenase activities were increased by 50%. Rats received a single oral dose of 100 mg [14C]1,1-dichloroethylene (DCE)/kg in mineral oil and were killed at 2, 4, 12 or 24 h; controls received mineral oil only. More rapid liver injury, as measured by serum alanine aminotransferase activity and histology, was present at 2 and 4 h after DCE in HypoT than EuT rats, but a similar magnitude of injury was evident at 12 and 24 h. DCE decreased liver non-protein sulfhydryl groups to a comparable extent in HypoT and EuT rats. Cytosolic glutathione S-transferase and alcohol dehydrogenase activities were decreased only in HypoT rats after DCE. HypoT rats excreted approximately 30% less total [14C]DCE-derived label in urine and their livers, kidneys and lungs consistently contained slightly less covalently bound [14C]DCE-derived label. In contrast, between 1 and 4 h after DCE, greater amounts of acid-soluble and acid-precipitable [14C]DCE-derived label were recovered in red blood cells of HypoT rats. Our results indicate that hypothyroidism did not protect against oral DCE hepatotoxicity but was associated with a more rapid injury at early times. Concurrently, hypothyroidism was found to change the fate of [14C]DCE with higher amounts of 14C-label recovered at early times in red blood cells while less 14C-label was excreted in urine and bound to liver.

Potentiation of 1,1-dichloroethylene hepatotoxicity: comparative effects of hyperthyroidism and fasting

The responses of fed, fasted, and hyperthyroid (T4) Sprague-Dawley male rats to 50 mg 1,1-dichloroethylene (1,1-DCE)/kg were compared. Hyperthyroid rats received three sc injections of thyroxine (100 micrograms/100 g) at 48-hr intervals; all other rats were sham-injected. 1,1-DCE was given po in mineral oil 24 hr after the last T4 dose; controls received only mineral oil. Animals were killed at 2, 4, and 8 hr. Liver GSH contents were lowered about 55% by both fasting and T4 while GSH transferase activities were lowered about 20% by fasting and 35% by T4. Only T4 pretreatment lowered alcohol dehydrogenase activities. Liver injury (i.e., serum glutamate pyruvate transaminase, histology) after 1,1-DCE was minimal in fed rats, moderate in fasted rats, and intermediate in T4 rats. Fasted rats showed a more pronounced depletion of liver GSH after 1,1-DCE than T4 rats and only in fasted rats did the toxicant decrease activities of the detoxification enzymes. Hypoglycemia after 1,1-DCE occurred in fed rats, but more rapidly in T4 rats. In contrast, fasted rats unexpectedly became hyperglycemic after the toxicant. Patterns of body temperature change after the toxicant, which might be due to its metabolites, were dissimilar. Hypothermia was not observed in fed rats, was only transiently evident in T4 rats, but occurred rapidly within 1 hr in fasted rats and steadily became more severe. The dissimilar patterns of liver enzyme and body temperature and serum glucose change after the toxicant in the three groups are indicative of different pathways of injury potentiation by fasting and hyperthyroidism.

Effects of piperonyl butoxide on halothane hepatotoxicity and metabolism in the hyperthyroid rat

A series of experiments were conducted to examine the potential role of phase I metabolism in halothane-induced liver injury in the hyperthyroid rat. The metabolism of halothane was determined in both hyperthyroid (triiodothyronine, 3 mg/kg per day, for 6 days) and euthyroid rats and in animals pre-treated with the cytochrome P-450 inhibitor piperonyl butoxide (75-100 mg/kg, i.p.). It was found that the hyperthyroid state, which is associated with a substantial increase in sensitivity to the hepatotoxic effects of halothane, decreases both oxidative and reductive routes of halothane metabolism in the rat. The production of trifluoroacetic acid (TFA), an oxidative metabolite, as well as that of chlorodifluoroethylene (CDF) and chlorotrifluoroethane (CTF), 2 reductive metabolites, was significantly reduced in hyperthyroid animals. Consistent with these findings serum and urinary bromide levels resulting from the formation of TFA, CDF or CTF were significantly reduced. The only route of halothane metabolism significantly increased by the hyperthyroid condition was the defluorination of halothane. Piperonyl butoxide administration did not render euthyroid animals sensitive to the halothane-induced hepatotoxicity and had no effect on the defluorination of halothane in euthyroid animals. However, piperonyl butoxide markedly increased the hepatotoxicity of halothane in hyperthyroid rats and, except for a modest increase in debromination reactions, decreased all measured indices of halothane metabolism including the defluorination of halothane. Thus, none of the observed changes in halothane metabolism produced by triiodothyronine or piperonyl butoxide treatment could be consistently correlated to the increases in hepatotoxicity linked to these 2 treatments. Based on these studies we suggest that the halothane hepatotoxicity induced in the hyperthyroid rat results from effects produced by either the parent compound or an as yet unidentified metabolite. In addition, these studies further demonstrate that considerable mechanistic differences exist for halothane-induced hepatotoxicity when comparing euthyroid and hyperthyroid animal models.

Characterization of hyperthyroidism enhancement of halothane-induced hepatotoxicity

Administration of anesthetic doses of halothane to hyperthyroid male rats results in the development of hepatic necrosis. The severity of the hepatic lesion was dependent on the dose of triiodothyronine (T3) and the length of time it was administered. Pretreatment of rats with iodinated metabolites of thyroxin which do not induce hyperthyroidism did not result in any signs of hepatotoxicity after halothane exposure. The administration of halothane to hyperthyroid female rats or mice of either sex did not result in the development of any overt hepatotoxicity. Likewise, hyperthyroidism did not enhance the hepatotoxicity of another hepatotoxin bromobenzene. The in vitro enzymatic activities associated with cytochrome P-450-dependent metabolism and glutathione S-transferase conjugation activity were markedly altered in hyperthyroid rats. Cytochrome P-450 levels, aminopyrine N-demethylase activity, glutathione levels and glutathione S-transferase activity were all significantly lower in hyperthyroid rats. However, other enzyme activities were stimulated by T3 pretreatment; aniline hydroxylase activity was increased by 45% and cytochrome c reductase activity was increased by 54% in hyperthyroid rats. Glutathione levels were also reduced significantly in hyperthyroid male rats. Maximal changes in both the cytochrome P-450 system and in the glutathione detoxification system were required before halothane demonstrated its hepatotoxic effects. Thus, a new balance between cytochrome P-450-dependent bioactivation and glutathione conjugation of halothane may be necessary for the exaggerated hepatotoxicity of halothane seen in hyperthyroid male rats.

Halothane hepatotoxicity in hyperthyroid rats as compared to the phenobarbital-hypoxia model

Halothane hepatotoxicity was observed after exposing hyperthyroid rats to 0.625% halothane for 4 hr under hypoxic conditions (10% O2). In this model, increases in serum enzyme activities of the alanine aminotransferase (GPT) and the sorbitol dehydrogenase (SDH) were evident immediately following exposure and were six-fold higher than in the phenobarbital-hypoxic model. Plasma free-fluoride levels estimated immediately after exposure to halothane were increased twofold in halothane-exposed hyperthyroid rats under hypoxic conditions as were increased twofold in halothane-exposed hyperthyroid rats under hypoxic conditions as compared to a sixfold increase in the phenobarbital-hypoxic model. The concentration of glutathione in liver was more markedly decreased in hyperthyroid rats than in phenobarbital-induced rats. The fact that no clear-cut correlation was found between defluorination and hepatotoxicity in both models may favor the hypothesis that a non-defluorinated metabolite of halothane, e.g., 2-chloro-1,1,1-trifluoroethyl radical, is the reactive intermediate responsible for the liver lesions. On the other hand, intracellular hypoxia due to hypermetabolism during the hyperthyroid state may be the reason for the higher sensitivity of hyperthyroid rats.

Response of isolated hepatocytes to organic and inorganic cytotoxins

Thirty-four chemicals-diverse in structure, postulated mechanisms of action, and primary target organs--were tested for cytotoxic response in isolated hepatocyte suspensions from young male Sprague-Dawley rats. Hepatocytes were incubated in the presence and absence of the test chemicals in closed vessels fitted with side arms for serial sampling for up to 5 h at 37 degrees C with gentle shaking under an O2:CO2 (95:5) atmosphere. The parameters evaluated were glutamate-oxaloacetate transaminase and lactate dehydrogenase release from the cells, Trypan blue exclusion, cell count, urea synthesis capability, and steady-state ATP levels. All chemicals cytotoxic in animals following single or short-term repeated exposures caused statistically significant changes in one or more of these parameters in the 0.01-10-mM concentration range. Dimethylnitrosamine and thioacetamide were not as potent in the isolated cell system as expected from their in vivo hepatotoxicity, and the quantitative changes produced with thioacetamide in the hepatocytes were marginal, even at 10 mM. The solvents tested--ethanol, acetone, dimethyl sulfoxide (DMSO), and propylene glycol--were without effect. These results indicate that isolated hepatocyte suspensions are useful for the identification of cytotoxins in general and hepatotoxins in particular, but that their capability for yielding a quantitative index of cytotoxic potential for diverse chemical species remains to be demonstrated.