Toxicant-inflicted injury and stimulated tissue repair are opposing toxicodynamic forces in predictive toxicology

Comparison of liver regeneration after partial hepatectomy and acetaminophen-induced acute liver failure: A global picture based on transcriptome analysis

Liver regenerates following surgical removal and after drug-induced liver injury (DILI). However, most of the mechanisms of liver regeneration were identified using partial hepatectomy (PHX) model rather than using DILI models. We compared mechanisms of liver regeneration following PHX and after acetaminophen (APAP) overdose, a DILI model, using transcriptomic approach. Kinetics of hepatocyte proliferation and global gene expression profiles were studied in male C57BL/6J mice either subjected to PHX or following APAP overdose. Liver regeneration was much more synchronized after PHX as compared to APAP overdose. Transcriptomics analysis revealed activation of common upstream regulators in both models including growth factors HGF, EGF and VEGF; and cytokines IL6 and TNFα. However, magnitude of activation and temporality was significantly differed between the two models. HGF and VEGF showed similar activation between PHX and APAP but activation of EGF was significantly stronger in the APAP model. Activation of IL6 and TNFα transcriptional programs was delayed but remarkably higher in APAP. These dissimilarities could be attributed to inherent differences in the two models including significant injury and inflammation exclusively in the APAP model. This study highlights need to study mechanisms of liver regeneration after DILI separately from the mechanisms of regeneration PHX.

Wnt/β-Catenin Signaling Drives Thioacetamide-Mediated Heteroprotection Against Acetaminophen-Induced Lethal Liver Injury

Preplacement of compensatory tissue repair (CTR) by exposure to a nonlethal dose of a toxicant protects animals against a lethal dose of another toxicant. Although CTR is known to heteroprotect, the underlying molecular mechanisms are not completely known. Here, we investigated the mechanisms of heteroprotection using thioacetamide (TA): acetaminophen (APAP) heteroprotection model. Male Swiss Webster mice received a low dose of TA or distilled water (DW) vehicle 24 hours prior to a lethal dose of APAP. Liver injury, tissue repair, and promitogenic signaling were studied over a time course of 24 hours after APAP overdose to the TA- and DW-primed mice (TA + APAP and DW + APAP, respectively). Thioacetamide pretreatment afforded 100% protection against APAP overdose compared to 100% lethality in the DW + APAP-treated mice. Although hepatic Cyp2e1 was similar at the time of APAP administration, immediate activation of hepatic c-Jun N-terminal kinases (JNK) was observed in the TA + APAP-treated mice compared to its delayed activation in the DW + APAP group. In contrast to the DW + APAP group, the TA + APAP-treated mice exhibited extensive CTR, which was secondary to the timely activation of Wnt/β-catenin pathway. Our data indicate that rapid activation and appropriate termination of Wnt/β-catenin signaling and modulation of JNK activity underlie TA + APAP heteroprotection.

Diet Restriction as a Protective Mechanism in Noncancer Toxicity Outcomes: A Review

It is well documented that diet restriction (DR) increases life expectancy, slows aging, and decreases the incidence of a variety of age-associated diseases including cancer and chemical-induced carcinogenesis. With regard to chemical toxicity, very few studies have attempted to investigate the effects of DR on noncancer toxicity outcomes. This review summarizes the findings of how DR influences acute toxicity outcomes and mechanisms. DR-induced protection in ozone lung inflammation, acute toxicity of isoproterenol, ganciclovir-, aspirin-and acidified ethanol-induced gastric injury are discussed. Because similar physiologic mechanisms exist in humans, DR, if practiced as a fife-style option, may improve quality of life in addition to accruing savings in burgeoning health care costs. Finally, these studies may be of value in boosting research in the area of pharmacology and therapeutics in developing potential therapeutic and safety assessment tools in human and veterinary medicine.

Assessing human variability in kinetics for exposures to multiple environmental chemicals: a physiologically based pharmacokinetic modeling case study with dichloromethane, benzene, toluene, ethylbenzene, and m-xylene

The objective of this study was to compare the magnitude of interindividual variability in internal dose for inhalation exposure to single versus multiple chemicals. Physiologically based pharmacokinetic models for adults (AD), neonates (NEO), toddlers (TODD), and pregnant women (PW) were used to simulate inhalation exposure to "low" (RfC-like) or "high" (AEGL-like) air concentrations of benzene (Bz) or dichloromethane (DCM), along with various levels of toluene alone or toluene with ethylbenzene and xylene. Monte Carlo simulations were performed and distributions of relevant internal dose metrics of either Bz or DCM were computed. Area under the blood concentration of parent compound versus time curve (AUC)-based variability in AD, TODD, and PW rose for Bz when concomitant "low" exposure to mixtures of increasing complexities occurred (coefficient of variation (CV) = 16-24%, vs. 12-15% for Bz alone), but remained unchanged considering DCM. Conversely, AUC-based CV in NEO fell (15 to 5% for Bz; 12 to 6% for DCM). Comparable trends were observed considering production of metabolites (AMET), except for NEO's CYP2E1-mediated metabolites of Bz, where an increased CV was observed (20 to 71%). For "high" exposure scenarios, Cmax-based variability of Bz and DCM remained unchanged in AD and PW, but decreased in NEO (CV= 11-16% to 2-6%) and TODD (CV= 12-13% to 7-9%). Conversely, AMET-based variability for both substrates rose in every subpopulation. This study analyzed for the first time the impact of multiple exposures on interindividual variability in toxicokinetics. Evidence indicates that this impact depends upon chemical concentrations and biochemical properties, as well as the subpopulation and internal dose metrics considered.

Dietary restriction (DR) and its advantages

Dietary restriction (DR) also called dietary control or calorie restriction is reported to have many advantages with regard to human health. It leads to suppression of obesity, mitigates free radicals and increases available antioxidants which are accounted for extending the life span of individuals. DR is also reported to induce synthesis of heat shock proteins in animals as a control mechanism against stress. Further, it is known to play a significant role in decreasing toxicity and lethality due to a variety of toxic chemicals and drugs by stimulating tissue repair damaged by the toxicants leading to restoration of intact organ and its functions. Moreover, extensive work done on animals indicate DR has an important role in suppressing certain types of cancer. In this review an effort is made to highlight the various advantages of DR from the point of human health perspective.

Acute exposure to ozone exacerbates acetaminophen-induced liver injury in mice

Ozone (O(3)), an oxidant air pollutant in photochemical smog, principally targets epithelial cells lining the respiratory tract. However, changes in gene expression have also been reported in livers of O(3)-exposed mice. The principal aim of the present study was to determine if acute exposure to environmentally relevant concentrations of O(3) could cause exacerbation of drug-induced liver injury in mice. Overdose with acetaminophen (APAP) is the most common cause of drug-induced liver injury in developed countries. In the present study, we examined the hepatic effects of acute O(3) exposure in mice pretreated with a hepatotoxic dose of APAP. C57BL/6 male mice were fasted overnight and then given APAP (300 mg/kg ip) or saline vehicle (0 mg/kg APAP). Two hours later, mice were exposed to 0, 0.25, or 0.5 ppm O(3) for 6 h and then sacrificed 9 or 32 h after APAP administration (1 or 24 h after O(3) exposure, respectively). Animals euthanized at 32 h were given 5-bromo-2-deoxyuridine 2 h before sacrifice to identify hepatocytes undergoing reparative DNA synthesis. Saline-treated mice exposed to either air or O(3) had no liver injury. All APAP-treated mice developed marked centrilobular hepatocellular necrosis that increased in severity with time after APAP exposure. O(3) exposure increased the severity of APAP-induced liver injury as indicated by an increase in necrotic hepatic tissue and plasma alanine aminotransferase activity. O(3) also caused an increase in neutrophil accumulation in livers of APAP-treated animals. APAP induced a 10-fold increase in the number of bromodeoxyuridine-labeled hepatocytes that was markedly attenuated by O(3) exposure. Gene expression analysis 9 h after APAP revealed differential expression of genes involved in inflammation, oxidative stress, and cellular regeneration in mice treated with APAP and O(3) compared to APAP or O(3) alone, providing some indications of the mechanisms behind the APAP and O(3) potentiation. These results suggest that acute exposure to near ambient concentrations of this oxidant air pollutant may exacerbate drug-induced liver injury by delaying hepatic repair.

Toxicity, biomarkers, genotoxicity, and carcinogenicity of trichloroethylene and its metabolites: a review

Trichloroethylene (TCE) is a prevalent occupational and environmental contaminant that has been reported to cause a variety of toxic effects. This article reviews toxicity, mutagenicity, and carcinogenicity caused by the exposure of TCE and its metabolites in the living system as well as on their (TCE and its metabolites) toxicity biomarkers.

Mechanisms and outcomes of drug- and toxicant-induced liver toxicity in diabetes

Increase dincidences of hepatotoxicity have been observed in diabetic patients receiving drug therapies. Neither the mechanisms nor the predisposing factors underlying hepatotoxicity in diabetics are clearly understood. Animal studies designed to examine the mechanisms of diabetes-modulated hepatotoxicity have traditionally focused only on bioactivation/detoxification of drugs and toxicants. It is becoming clear that once injury is initiated, additional events determine the final outcome of liver injury. Foremost among them are two leading mechanisms: first, biochemical mechanisms that lead to progression or regression of injury; and second, whether or not timely and adequate liver tissue repair occurs to mitigate injury and restore liver function. The liver has a remarkable ability to repair and restore its structure and function after physical or chemical-induced damage. The dynamic interaction between biotransformation-based liver injury and compensatory tissue repair plays a pivotal role in determining the ultimate outcome of hepatotoxicity initiated by drugs or toxicants. In this review, mechanisms underlying altered hepatotoxicity in diabetes with emphasis on both altered bioactivation and liver tissue repair are discussed. Animal models of both marked sensitivity (diabetic rats) and equally marked protection (diabetic mice) from drug-induced hepatotoxicity are described. These examples represent a remarkable species difference. Availability of the rodent diabetic models offers a unique opportunity to uncover mechanisms of clinical interest in averting human diabetic sensitivity to drug-induced hepatotoxicities. While the rat diabetic models appear to be suitable, the diabetic mouse models might not be suitable in preclinical testing for potential hepatotoxic effects of drugs or toxicants, because regardless of type 1 or type2 diabetes, mice are resistant to acute drug-or toxicant-induced toxicities.

Characterization of a lipopolysaccharide mediated neutrophilic hepatitis model in Sprague Dawley rats

Several studies have investigated the role of neutrophils during endotoxin-mediated liver injury, yet the precise mechanism for endotoxin-mediated hepatic neutrophil transmigration is unknown. The primary objective of this study was to establish a reliable lipopolysaccharide (LPS)-mediated necro-hepatitis model to investigate the mechanisms of hepatic neutrophil infiltration following LPS administration. Male Sprague Dawley rats were administered a single (5 or 10 mg kg(-1), i.v.) or repeated injection of LPS (10 mg kg(-1), i.v., 24 h apart) with appropriate controls (i.v. saline) and were killed at various time points following LPS injection. Significant hematologic changes included neutrophilia, elevation of the neutrophil to lymphocyte ratio and toxic changes in neutrophils. Biochemical changes were observed in several liver (aspartate aminotransferase AST, gamma glutamyl transferase GGT) and kidney (blood urea nitrogen BUN) associated parameters generally at the earliest time points. Histopathology revealed a time-dependent neutrophil and mononuclear infiltration around the periportal areas in the single dose study and multifocal midzonal coagulative necrosis in the repeated dose study. The neutrophil adhesion molecule, CD 11b was up-regulated in single and repeat dose studies. Based on these studies, a reliable LPS-mediated hepatitis model with necrosis was developed by intravenous administration of LPS in a repeat dose fashion. Midzonal hepatic necrosis, peripheral neutrophilia, hepatic neutrophil infiltration and up-regulation of CD11b were the most significant and consistent markers of LPS mediated effects in this model.

Adaptive tolerance in mice upon subchronic exposure to chloroform: Increased exhalation and target tissue regeneration

The aims of the present study were to characterize the subchronic toxicity of chloroform by measuring tissue injury, repair, and distribution of chloroform and to assess the reasons for the development of tolerance to subchronic chloroform toxicity. Male Swiss Webster (SW) mice were given three dose levels of chloroform (150, 225, and 300 mg/kg/day) by gavage in aqueous vehicle for 30 days. Liver and kidney injury were measured by plasma ALT and BUN, respectively, and by histopathology. Tissue regeneration was assessed by (3)H-thymidine incorporation into hepato- and nephro-nuclear DNA and by proliferating cell nuclear antigen staining. In addition, GSH and CYP2E1 in liver and kidney were assessed at selected time points. The levels of chloroform were measured in blood, liver, and kidney during the dosing regimen (1, 7, 14, and 30 days). Kidney injury was evident after 1 day with all three doses and sustained until 7 days followed by complete recovery. Mild to moderate liver injury was observed from 1 to 14 days with all three dose levels followed by gradual decrease. Significantly higher regenerative response was evident in liver and kidney at 7 days, but the response was robust in kidney, preventing progression of injury beyond first week of exposure. While the kidney regeneration reached basal levels by 21 days, moderate liver regeneration with two higher doses sustained through the end of the dosing regimen and 3 days after that. Following repeated exposure for 7, 14, and 30 days, the blood and tissue levels of chloroform were substantially lower with all three dose levels compared to the levels observed with single exposure. Increased exhalation of (14)C-chloroform after repeated exposures explains the decreased chloroform levels in circulation and tissues. These results suggest that toxicokinetics and toxicodynamics (tissue regeneration) contribute to the tolerance observed in SW mice to subchronic chloroform toxicity. Neither bioactivation nor detoxification appears to play a decisive role in the development of this tolerance.

Stable isotopes, mass spectrometry, and molecular fluxes: Applications to toxicology

In order to meet the increasing demands for safe and affordable drugs, improvements in the efficiency and accuracy of every step in drug development are required. Accordingly, new approaches for assessing drug toxicity that are faster and more precise are in demand. Numerous approaches using -omics and systems biology are being developed to meet this demand and, while promising, they have not yet provided the improvements in toxicology promised. Other innovative methodologies for predicting and assessing toxicities should therefore be explored. Here we present a novel approach for directly measuring the in vivo response of specific metabolic pathways to toxic agents. Using stable isotopes and ultra sensitive mass spectrometry, the effect of an agent on myelin synthesis, protein synthesis, or cell proliferation can be directly measured. Examples are presented where this approach is used to detect toxicity in the liver, brain, peripheral neurons, breast, and skin. Collagen synthesis, microglia proliferation, myelin synthesis, tubulin synthesis, hepatic cell proliferation, epidermal cell proliferation and mammary epithelial cell proliferation are quantitatively determined in vivo, in a high throughput manner. This approach avoids the computationally complex approach of systems biology and allows the user to observe the emergent properties of the system directly and quantitatively.

Protective effect of type 2 diabetes on acetaminophen-induced hepatotoxicity in male Swiss-Webster mice

Type 2 diabetic (DB) mice exposed to CCl(4) (LD(50) = 1.25 ml/kg), acetaminophen (LD(80) = 600 mg/kg; APAP), and bromobenzene (LD(80) = 0.5 ml/kg) i.p. yielded 30, 20, and 20% mortality, respectively, indicating hepatotoxic resistance. Male Swiss-Webster mice were made diabetic by feeding high fat and administrating streptozotocin (120 mg/kg i.p.) on day 60. On day 71, time-course studies after APAP (600 mg/kg) treatment revealed identical initial liver injury in non-DB and DB mice, which progressed only in non-DB mice, resulting in 80% mortality. The hypothesis that decreased APAP bioactivation, altered toxicokinetics, and/or increased tissue repair are the underlying mechanisms was investigated. High-performance liquid chromatography analysis revealed no difference in plasma and urinary APAP or detoxification of APAP via glucuronidation between DB and non-DB mice. Hepatic CYP2E1 protein and activity, glutathione, and [(14)C]APAP covalent binding did not differ between DB and non-DB mice, suggesting that lower bioactivation-based injury is not the mechanism of decreased hepatotoxicity in DB mice. Diabetes increased cells in S phase by 8-fold in normally quiescent liver of these mice. Immunohistochemistry revealed overexpression of calpastatin in the newly dividing/divided cells, explaining inhibition of hydrolytic enzyme calpain in perinecrotic areas and lower progression of APAP-initiated injury in the DB mice. Antimitotic intervention of diabetes-associated cell division with colchicine before APAP administration resulted in 70% mortality in APAP-treated colchicine-intervened DB mice. These studies suggest that advancement of cells in the cell division cycle and higher tissue repair protect DB mice by preventing progression of APAP-initiated liver injury that normally leads to mortality.

Dose-dependent liver regeneration in chloroform, trichloroethylene and allyl alcohol ternary mixture hepatotoxicity in rats

The present study was designed to examine the hypothesis that liver tissue repair induced after exposure to chloroform (CF) + trichloroethylene (TCE) + allyl alcohol (AA) ternary mixture (TM) is dose-dependent similar to that elicited by exposure to these compounds individually. Male Sprague Dawley (S-D) rats (250-300 g) were administered with fivefold dose range of CF (74-370 mg/kg, ip), and TCE (250-1250 mg/kg, ip) in corn oil and sevenfold dose range of AA (5-35 mg/kg, ip) in distilled water. Liver injury was assessed by plasma alanine amino transferase (ALT) activity and liver tissue repair was measured by (3) H-thymidine incorporation into hepatonuclear DNA. Blood and liver levels of parent compounds and two major metabolites of TCE [trichloroacetic acid (TCA) and trichloroethanol (TCOH)] were quantified by gas chromatography. Blood and liver CF and AA levels after TM were similar to CF alone or AA alone, respectively. However, the TCE levels in blood and liver were substantially decreased after TM in a dose-dependent fashion compared to TCE alone. Decreased plasma and liver TCE levels were consistent with decreased production of metabolites and elevated urinary excretion of TCE. The antagonistic interaction resulted in lower liver injury than the summation of injury caused by the individual components at all three-dose levels. On the other hand, tissue repair showed a dose-response leading to regression of injury. Although the liver injury was lower and progression was contained by timely tissue repair, 50% mortality occurred only with the high dose combination, which is several fold higher than environmental levels. The mortality could be due to the central nervous system toxicity. These findings suggest that exposure to TM results in lower initial liver injury owing to higher elimination of TCE, and the compensatory liver tissue repair stimulated in a dose-dependent manner mitigates progression of injury after exposure to TM.

Tissue repair: an important determinant of final outcome of toxicant-induced injury

Tissue repair is a dynamic compensatory cell proliferation and tissue regeneration response stimulated in order to overcome acute toxicity and recover organ/tissue structure and function. Extensive evidence in rodent models using structurally and mechanistically diverse hepatotoxicants such as acetaminophen (APAP), carbon tetrachloride (CCl4), chloroform (CHCl3), thioacetamide (TA), trichloroethylene (TCE), and allyl alcohol (AA) have demonstrated that tissue repair plays a critical role in determining the final outcome of toxicity, i.e., recovery from injury and survival or progression of injury leading to liver failure and death. Tissue repair is a complex process governed by intricate cellular signaling involving a number of chemokines, cytokines, growth factors, and nuclear receptors leading to promitogenic gene expression and cell division. Tissue repair also encompasses regeneration of hepatic extracellular matrix and angiogenesis, the processes necessary to completely restore the structure and function of the liver tissue lost to toxicant-induced initiation followed by progression of injury. New insights have emerged over the last quarter century indicating that tissue repair follows a dose response. Tissue repair increases with dose until a threshold dose, beyond which it is delayed and impaired due to inhibition of cellular signaling resulting in runaway secondary events causing tissue destruction, organ failure, and death. Prompt and adequately stimulated tissue repair response to toxic injury is critical for recovery from toxic injury. Tissue repair is modulated by a variety of factors including species, strain, age, nutrition, and disease condition causing marked changes in susceptibility and toxic outcome. This review focuses on the properties of tissue repair, different factors affecting tissue repair, and the mechanisms that govern tissue repair and progression of injury. It also highlights the significance of tissue repair as a target for drug development strategies and an important consideration in the assessment of risk from exposure to toxicants.

Dose-dependent liver tissue repair in chloroform plus thioacetamide acute hepatotoxicity

The objective of this study was to test whether a binary mixture (BM) of chloroform (CHCl(3)) and thioacetamide (TA) causes a dose-dependent liver injury and an opposing tissue repair. Liver injury was assessed by plasma alanine aminotransferase (ALT) and histopathology. Tissue repair was measured by [(3)H-CH(3)]-thymidine ((3)H-T) incorporation into hepatonuclear DNA and PCNA over a time course of 0-72h. Male Sprague-Dawley (S-D) rats received six- and five-fold dose ranges of TA and CHCl(3), respectively. ALT levels and (3)H-T incorporation were in complete agreement with corresponding microscopic observations, and only ALT elevation and (3)H-T incorporation data are presented here. Liver injury observed after exposure to BM was no different than addition of injuries caused by individual compounds. Tissue repair was prompt and adequate, leading to recovery from injury and animal survival. Tissue repair is dose-dependent and plays central role in the hepatotoxic outcome.

Liver regeneration: a critical toxicodynamic response in predictive toxicology

The objective of the present review is to discuss the importance tissue repair in the mixture risk assessment. Studies have revealed the existence of two stages of toxicity: an inflictive stage (stage I) and progressive or regressive stage (stage II). While much is known about mechanisms by which injury is inflicted (stage I), very little is known about the mechanisms that lead to progression or regression of injury. A wide variety of additional experimental evidence suggests that tissue repair impacts decisively on the final toxic outcome and any modulation in this response has profound impact in the final outcome of toxicity. We designed the present research to investigate the importance of tissue repair in the final acute hepatotoxic outcome upon exposures to mixture of toxicants comprising thioacetamide (TA), allyl alcohol (AA), chloroform (CHCl(3)) and trichloroethylene (TCE). Dose response studies with individual compounds, binary mixtures (BM), ternary (TM) and quaternary mixtures (QM) have been conducted. Results of CHCl(3) + AA BM [Anand, S.S., Murthy, S.N., Vishal, V.S., Mumtaz, M.M., Mehendale, H.M., 2003. Tissue repair plays pivotal role in final outcome of supra-additive liver injury after chloroform and allyl alcohol binary mixture. Food Chem. Toxicol. 41, 1123] and CHCl(3) + AA + TA +TCE QM [Soni, M.G., Ramaiah, S.K., Mumtaz, M.M., Clewell, H., Mehendale, H.M., 1999. Toxicant-inflicted injury and stimulated tissue repair are opposing toxicodynamic forces in predictive toxicology. Regul. Phramcol. Toxicol. 19, 165], and two representative individual compounds (TA and AA) [Mangipudy, R.S., Chanda, S., Mehendale, H.M., 1995a. Tissue repair response as a function of dose in thioacetamide hepatotoxicity. Environ. Health Perspect. 103, 260; Soni, M.G., Ramaiah, S.K., Mumtaz, M.M., Clewell, H., Mehendale, H.M., 1999. Toxicant-inflicted injury and stimulated tissue repair are opposing toxicodynamic forces in predictive toxicology. Regul. Phramcol. Toxicol. 19, 165] are described in this review. In addition, modulation of tissue repair in the outcome of hepatotoxicity and its implications in the risk assessment have been discussed. Male Sprague-Dawley (S-D) rats (250-300g) received a single i.p. injection of individual toxicants as well as mixtures. Liver injury was assessed by plasma alanine amino transferase (ALT) and histopathology. Tissue regeneration response was measured by [(3)H]-thymidine ((3)H-T) incorporation into hepatocellular nuclear DNA and PCNA. Only ALT and (3)H-T data have been presented in this review for the sake of simplicity. Studies with individual hepatotoxicants showed a dose-related increase in injury as well as tissue repair up to a threshold dose. Beyond this threshold, tissue repair was inhibited, and liver injury progressed leading to mortality. Since the highest dose of individual compounds resulted in mortality, this dose was not employed for mixture studies. While CHCl(3) + AA BM caused supra-additive liver injury, QM caused additive liver injury. Due to the prompt and robust compensatory tissue repair, all the rats exposed to BM survived. With QM, the rats receiving the highest dose combination experienced some mortality consequent to the progression of liver injury attendant to suppressed tissue repair. These findings suggest that liver tissue repair, the opposing biological response that restores tissue lost to injury, may play a critical and determining role in the outcome of liver injury regardless of the number of toxicants in the mixture or the mechanism of initiation of injury. These data suggest that inclusion of this response in risk assessment might help in fine-tuning the prediction of toxic outcomes.

Potentiation of Carbon Tetrachloride Hepatotoxicity and Lethality in Type 2 Diabetic Rats

There is a need for well characterized and economical type 2 diabetic model that mimics the human disease. We have developed a type 2 diabetes rat model that closely resembles the diabetic patients and takes only 24 days to develop robust diabetes. Nonlethal doses of allyl alcohol (35 mg/kg i.p.), CCl(4) (2 ml/kg i.p.), or thioacetamide (300 mg/kg i.p.) yielded 80 to 100% mortality in diabetic rats. The objective of the present study was to investigate two hypotheses: higher CCl(4) bioactivation and/or inhibited compensatory tissue repair were the underlying mechanisms for increased CCl(4) hepatotoxicity in diabetic rats. Diabetes was induced by feeding high fat diet followed by a single dose of streptozotocin on day 14 (45 mg/kg i.p.) and was confirmed on day 24 by hyperglycemia, normoinsulinemia, and oral glucose intolerance. Time course studies (0-96 h) of CCl(4) (2 ml/kg i.p.) indicated that although initial liver injury was the same in nondiabetic and diabetic rats, it progressed only in the latter, culminating in hepatic failure, and death. Hepatomicrosomal CYP2E1 protein and activity, lipid peroxidation, glutathione, and (14)CCl(4) covalent binding to liver tissue were the same in both groups, suggesting that higher bioactivation-based injury is not the mechanism. Inhibited tissue repair resulted in progression of injury and death in diabetic rats, whereas in the nondiabetic rats robust tissue repair resulted in regression of injury and survival after CCl(4) administration. These studies show high sensitivity of type 2 diabetes to model hepatotoxicants and suggest that CCl(4) hepatotoxicity is potentiated due to inhibited tissue repair.

Tissue repair plays pivotal role in final outcome of liver injury following chloroform and allyl alcohol binary mixture

The objective of this study was to evaluate the interaction profile of chloroform (CHCl(3))+allyl alcohol (AA) binary mixture (BM)-induced acute hepatotoxic response. Plasma alanine aminotransferase (ALT) was measured to assess liver injury, and 3H-thymidine (3H-T) incorporation into hepatonuclear DNA was measured as an index of liver regeneration over a time course of 0-72 h. Male Sprague-Dawley (S-D) rats received single ip injection of 5-fold dose range of CHCl(3) (74, 185 and 370 mg/kg) in corn oil (maximum 0.5 ml/kg) and 7-fold dose range of AA (5, 20 and 35 mg/kg) in distilled water simultaneously. The doses for BM were selected from individual toxicity studies of CHCl(3) alone [Int. J. Toxicol. 22 (2003) 25], and AA alone [Reg. Pharmacol. Toxicol. 19 (1999) 165]. Since the highest dose of each treatment (CHCl(3)- 740 and AA- 50 mg/kg) yielded mortality due to the suppressed tissue repair followed by liver failure, this dose was omitted for BM. The levels of CHCl(3) (30-360 min) and AA (5-60 min) were quantified in blood and liver by gas chromatography (GC). The liver injury was more than additive after BM compared to CHCl(3) alone or AA alone at highest dose combination (370+35 mg/kg), which peaked at 24 h. The augmented liver injury observed with BM was consistent with the quantitation data. Though the liver injury was higher, the greater stimulation of tissue repair kept injury from progressing, and rescued the rats from hepatic failure and death. At lower dose combinations, the liver injury was no more than additive. Results of the present study suggest that liver tissue repair, in which liver tissue lost to injury is promptly replaced, plays a pivotal role in the final outcome of liver injury after exposure to BM of CHCl(3) and AA.

Renal injury and repair following S-1, 2 dichlorovinyl-L-cysteine administration to mice

S-(1,2-dichlorovinyl)-L-cysteine (DCVC), a metabolite of a common environmental contaminant, trichloroethylene, is a selective proximal tubular nephrotoxicant. The objective of our study was to examine the dose-response relationship of renal injury and repair following DCVC administration. Male Swiss-Webster mice were injected with DCVC [15, 30, or 75 mg/kg ip in distilled water (10 ml/kg)] and the extent of nephrotoxicity and tissue repair was assessed over a 14-day period. The renal injury due to the low and medium doses of DCVC peaked at 36 and 72 h after dosing, respectively, and then regressed over time due to a timely and adequate tissue repair response. At the highest dose tissue repair was inhibited, thereby causing progression of renal injury, which led to acute renal failure and death of the mice. The possibility that compromised tissue repair was a result of the extensive nephrotoxic injury attendant to the high dose of DCVC was investigated via an equinephrotoxicity study in which separate groups of mice received 40 (LD40) and 75 (LD90) mg DCVC/kg, respectively. Bioactivation-based renal proximal tubular injury measured in these two groups over a time course was identical but there was a marked difference in mortality due to an early and robust tissue repair in the first group relative to the second group. These results support the concept that quantitative evaluation of renal tissue repair in parallel with injury is useful in the assessment of the likely toxic outcome associated with exposure to nephrotoxic drugs and toxicants.

A single administration of adenoviral-mediated HGF cDNA permits survival of mice from acute hepatic failure

Heptatocyte growth factor (HGF) having a variety of biological activity was suggested as a protective agent against acute toxic hepatic injury or a potentially therapeutic agent. For the efficient in vivo application of this factor, we employed adenoviral-mediated HGF gene delivery system. In this study, we constructed E1-deleted recombinant adenovirus carrying cDNA of human HGF (Ad.hHGF) and elucidated that HGF was efficiently expressed in the liver of C57/BL mice. A mouse model of acute hepatic failure was induced by high dose (1000mg/kg) of thioacetamide (TA) administration. Mice infected with Ad.hHGF showed a dramatic resistance to TA-induced acute hepatic injury. Serum ALT was increased transiently and then the level was normalized in Ad.hHGF-infected mice with TA administration. Furthermore, the survival rate was remarkably enhanced in the mice infected with Ad.hHGF. In the histological examination, massive hepatic necrosis induced by TA was almost completely protected by HGF produced by Ad.hHGF. Our results indicate that a single dose of HGF-encoding adenoviral vector maintained liver function and prevented the progression of liver necrosis in a mouse model of acute hepatic failure.

Extent and timeliness of tissue repair determines the dose-related hepatotoxicity of chloroform

As a part of mixture toxicity studies, the objective of the present investigation was to validate the hypothesis that the rate and extent of liver tissue repair response to a given dose determines the end result of toxicity (death or recovery), regardless of the mechanisms by which injury is inflicted, using a well-known environmental pollutant, chloroform (CHCl(3)). In future, the data will be used to compare with the results of mixtures containing CHCl(3) to aid in characterizing the safety of chemical mixtures and to construct a physiologically based pharmacokinetic (PBPK) model for dose, route, and species extrapolation. Hepatotoxicity and tissue repair were measured in male Sprague-Dawley rats (S-D) receiving a 10-fold dose range of CHCl(3) (74, 185, 370, and 740 mg/kg, IP) during a time course of 0 to 96 hours. Liver injury, as assessed by plasma alanine aminotransferase (ALT) and sorbitol dehydrogenase (SDH) elevation, increased with dose over the 10-fold dose range. Because CHCl(3) is also known to cause kidney damage, blood urea nitrogen (BUN) and creatinine were measured to evaluate the kidney injury. With doses up to 370 mg/kg, liver injury increased in a dose-related fashion, which peaked at 24 hours and returned to normal after 48 hours, whereas at highest dose (740 mg/kg), the injury was progressive resulting in 90% mortality. Blood and liver CHCl(3) levels were quantified using gas chromatography (GC) over a time course of 30 to 360 minutes. The dose-related increase in the blood and liver CHCl(3) levels were consistent with dose-dependent liver injury. Tissue regeneration response, as measured by [(3)H]-thymidine incorporation into hepatocellular nuclear DNA peaked at 36 hours in rats treated with the lower two doses of CHCl(3) (74 and 185 mg/kg). Further increase in CHCl(3) dose to 370 mg/kg resulted in an earlier increase in [(3)H]-thymidine incorporation at 24 hours, which peaked at 36 hours. However, at the highest dose of CHCl(3) (740 mg/kg), tissue repair was delayed and attenuated, allowing for unrestrained progression of liver injury. The kidney injury markers after CHCl(3) administration were not different from controls. These results support the concept that in addition to the magnitude of tissue repair response, the time at which this response occurs is critical in restraining the progression of injury. Measuring tissue repair and injury as simultaneous biological responses to toxic agents might increase the usefulness of dose-response paradigms in predictive toxicology and risk assessment. Although the dosimetry of the present study was well beyond the environmental exposure levels of CHCl(3), a PBPK model will be developed in future based upon these data to evaluate the effects at environmental levels.

Diallyl sulfide inhibition of CYP2E1 does not rescue diabetic rats from thioacetamide-induced mortality

Previously we have shown that hepatotoxicity of thioacetamide (TA) was increased in streptozotocin (STZ)-induced diabetic (DB) rats due to combined effects of enhanced bioactivation-based liver injury of TA and compromised liver tissue repair response. We have also shown that TA is primarily bioactivated by hepatic CYP2E1. The present study was done to further investigate the importance of liver tissue repair in determining the final outcome of hepatotoxicity. STZ-induced DB rats were pretreated with a CYP2E1 inhibitor, diallyl sulfide (DAS), to decrease the bioactivation-based liver injury of TA. The treatments were as follows: DB/DAS/TA, DB/corn oil/TA, and DB/DAS/saline. Nondiabetic (non-DB) rats received the same treatments as controls. A dose of TA (300 mg/kg ip), which was nonlethal in non-DB rats, caused 92 and 90% mortality in DB/DAS/TA and DB/corn oil/TA groups, respectively. At various times (0--60 h) after treatment, liver injury was assessed by plasma alanine aminotransferase and histopathology. Cell proliferation was evaluated by [(3)H]thymidine incorporation and immunohistochemical staining of proliferating cell nuclear antigen (PCNA). In the DB/DAS/TA rats, DAS pretreatment markedly reduced the CYP2E1-dependent liver injury of TA compared to that in DB/corn oil/TA rats. However, subsequent hepatic DNA synthesis in both DB groups was inhibited approximately 50%. PCNA analysis showed a corresponding decrease in cell-cycle progression. This compromised tissue repair response in DB rats was insufficient to compensate for cell loss, resulting in progression of liver injury and culminating in high mortality in both DB groups. Furthermore, non-DB rats were pretreated with a CYP2E1 inducer, isoniazid, to increase the bioactivation-based TA liver injury equal to the liver injury observed in DB/DAS/TA rats. Despite equal injury up to 36 h following TA treatment, the tissue repair response in the non-DB rats was highly stimulated to compensate for liver injury and led to 70% survival in this group. These studies underscore the importance of adequate and timely tissue repair in compensating for liver injury and protecting from lethality.

Enhanced hepatotoxicity and toxic outcome of thioacetamide in streptozotocin-induced diabetic rats

Diabetes is known to potentiate thioacetamide (TA)-induced liver injury via enhanced bioactivation. Little attention has been given to the role of compensatory tissue repair on ultimate outcome of hepatic injury in diabetes. The objective of this study was to investigate the effect of diabetes on TA-induced liver injury and lethality and to investigate the underlying mechanisms. We hypothesized that hepatotoxicity of TA in diabetic rats would increase due to enhanced bioactivation-mediated liver injury and also due to compromised compensatory tissue repair, consequently making a nonlethal dose of TA lethal. On day 0, male Sprague-Dawley rats (250-300 g) were injected with streptozotocin (STZ, 60 mg/kg ip) to induce diabetes. On day 10 the STZ-induced diabetic rats and the nondiabetic rats received a single dose of TA (300 mg/kg ip). This normally nonlethal dose of TA caused 90% mortality in the STZ-induced diabetic rats. At various times (0-60 h) after TA administration, liver injury was assessed by plasma alanine aminotransferase (ALT), sorbitol dehydrogenase (SDH), and liver histopathology. Liver function was evaluated by plasma bilirubin. Cell proliferation and tissue repair were evaluated by [(3)H]thymidine ((3)H-T) incorporation and proliferating cell nuclear antigen (PCNA) assays. In the nondiabetic rat, liver necrosis peaked at 24 h and declined thereafter toward normal by 60 h. In the STZ-induced diabetic rat, however, liver necrosis was significantly increased from 12 h onward and progressed, culminating in liver failure and death. Liver tissue repair studies showed that, in the liver of nondiabetic rats, S-phase DNA synthesis was increased at 36 h and peaked at 48 h following TA administration. However, DNA synthesis was approximately 50% inhibited in the liver of diabetic rats. PCNA study showed a corresponding decrease of cell-cycle progression, indicating that the compensatory tissue repair was sluggish in the diabetic rats. Further investigation of tissue repair by employing equitoxic doses (300 mg TA/kg in the non-diabetic rats; 30 mg TA/kg in the diabetic rats) revealed that, despite equal injury up to 24 h following injection, the tissue repair response in the diabetic rats was much delayed. The compromised tissue repair prolonged liver injury in the diabetic rats. These studies suggest that the increased TA hepatotoxicity in the diabetic rat is due to combined effects of increased bioactivation-mediated liver injury of TA and compromised compensatory tissue repair.