Toxicodynamics of low level toxicant interactions of biological significance: inhibition of tissue repair

Pharmacological stimulation of Wnt/beta-catenin signaling pathway attenuates the course of thioacetamide-induced acute liver failure

Acute liver failure (ALF) is known for extremely high mortality rate, the result of widespread damage of hepatocytes. Orthotopic liver transplantation is the only effective therapy but its application is limited by the scarcity of donor organs. Given the importance in the liver biology of Wnt/beta-catenin signaling pathway, we hypothesized that its stimulation could enhance hepatocyte regeneration and attenuate the course of thioacetamide (TAA)-induced ALF in Lewis rats. Chronic treatment with Wnt agonist was started either immediately after hepatotoxic insult ("early treatment") or when signs of ALF had developed ("late treatment"). Only 23 % of untreated Lewis rats survived till the end of experiment. They showed marked increases in plasma alanine aminotransferase (ALT) activity and bilirubin and ammonia (NH3) levels; plasma albumin decreased significantly. "Early" and "late" Wnt agonist treatment raised the final survival rate to 69 % and 63 %, respectively, and normalized ALT, NH3, bilirubin and albumin levels. In conclusion, the results show that treatment with Wnt agonist attenuates the course of TAA-induced ALF in Lewis rats, both with treatment initiated immediately after hepatotoxic insult and in the phase when ALF has already developed. Thus, the pharmacological stimulation of Wnt/beta-catenin signaling pathway can present a new approach to ALF treatment.

Acute Toxic Injuries of Rat's Visceral Tissues Induced by Different Oximes

Certain AChE reactivators, asoxime, obidoxime, K027, K048, and K075, when taken in overdoses and sometimes even when introduced within therapeutic ranges, may injure the different organs. As a continuation of previously published data, in this study, Wistar rats have sacrificed 24 hrs and 7 days after single im application of 0.1LD₅₀, 0.5LD₅₀ and 1.0LD₅₀ of each reactivator, and examinated tissue samples were obtained for pathohistological and semiquantitative analysis. A severity of tissue alteration, expressed as different tissue damage scores were evaluated. Morphological structure of examinated tissues treated with of 0.1LD₅₀ of all reactivators was comparable with the control group of rats. Moderate injuries were seen in visceral tissues treated with 0.5LD₅₀ of asoxime, obidoxime and K027. Acute damages were enlarged after treatment with 0.5LD₅₀ and 1.0LD₅₀ of all reactivators during the next 7 days. The most prominent changes were seen in rats treated with 1.0LD₅₀ of K048 and K075 (P < 0.001 vs. control and asoxime-treated group). All reactivators given by a single, high, unitary dose regimen, have an adverse effect not only on the main visceral tissue, but on the whole rat as well, but the exact mechanism of cellular injury remains to be confirmed in further investigation.

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.

Hepato-protective effect of resveratrol against acetaminophen-induced liver injury is associated with inhibition of CYP-mediated bioactivation and regulation of SIRT1-p53 signaling pathways

Resveratrol (RES) has been shown to possess many pharmacological activities including protective effect against liver damage induced by hepatotoxins. In the present study, the hepato-protective effect of RES against acetaminophen (APAP)-induced liver injury in mice and the involved mechanisms was investigated. This study clearly demonstrated that administration of RES three days before APAP treatment significantly alleviated APAP-induced hepatotoxicity, as evidenced by morphological, histopathological, and biochemical assessments such as GSH content and serum ALT/AST activity. Treatment with RES resulted in significant inhibition of CYP2E1, CYP3A11, and CYP1A2 activities, and then caused significant inhibition of the bioactivation of APAP into toxic metabolite NAPQI. Pretreatment with RES significantly reduced APAP-induced JNK activation to protect against mitochondrial injury. Additionally, RES treatment significantly induced SIRT1 and then negatively regulated p53 signaling to induce cell proliferation-associated proteins including cyclin D1, CDK4, and PCNA to promote hepatocyte proliferation. This study demonstrated that RES prevents APAP-induced hepatotoxicity by inhibition of CYP-mediated APAP bioactivation and regulation of SIRT1, p53, cyclin D1 and PCNA to facilitate liver regeneration following APAP-induced liver injury.

The transcription factor FOXM1 (Forkhead box M1): proliferation-specific expression, transcription factor function, target genes, mouse models, and normal biological roles

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor, which stimulates cell proliferation and exhibits a proliferation-specific expression pattern. Accordingly, both the expression and the transcriptional activity of FOXM1 are increased by proliferation signals, but decreased by antiproliferation signals, including the positive and negative regulation by protooncoproteins or tumor suppressors, respectively. FOXM1 stimulates cell cycle progression by promoting the entry into S-phase and M-phase. Moreover, FOXM1 is required for proper execution of mitosis. Accordingly, FOXM1 regulates the expression of genes, whose products control G1/S-transition, S-phase progression, G2/M-transition, and M-phase progression. Additionally, FOXM1 target genes encode proteins with functions in the execution of DNA replication and mitosis. FOXM1 is a transcriptional activator with a forkhead domain as DNA binding domain and with a very strong acidic transactivation domain. However, wild-type FOXM1 is (almost) inactive because the transactivation domain is repressed by three inhibitory domains. Inactive FOXM1 can be converted into a very potent transactivator by activating signals, which release the transactivation domain from its inhibition by the inhibitory domains. FOXM1 is essential for embryonic development and the foxm1 knockout is embryonically lethal. In adults, FOXM1 is important for tissue repair after injury. FOXM1 prevents premature senescence and interferes with contact inhibition. FOXM1 plays a role for maintenance of stem cell pluripotency and for self-renewal capacity of stem cells. The functions of FOXM1 in prevention of polyploidy and aneuploidy and in homologous recombination repair of DNA-double-strand breaks suggest an importance of FOXM1 for the maintenance of genomic stability and chromosomal integrity.

Characterization of the human kinetic adjustment factor for the health risk assessment of environmental contaminants

A default uncertainty factor of 3.16 (√10) is applied to account for interindividual variability in toxicokinetics when performing non-cancer risk assessments. Using relevant human data for specific chemicals, as WHO/IPCS suggests, it is possible to evaluate, and replace when appropriate, this default factor by quantifying chemical-specific adjustment factors for interindividual variability in toxicokinetics (also referred to as the human kinetic adjustment factor, HKAF). The HKAF has been determined based on the distributions of pharmacokinetic parameters (e.g., half-life, area under the curve, maximum blood concentration) in relevant populations. This article focuses on the current state of knowledge of the use of physiologically based algorithms and models in characterizing the HKAF for environmental contaminants. The recent modeling efforts on the computation of HKAF as a function of the characteristics of the population, chemical and its mode of action (dose metrics), as well as exposure scenario of relevance to the assessment are reviewed here. The results of these studies, taken together, suggest the HKAF varies as a function of the sensitive subpopulation and dose metrics of interest, exposure conditions considered (route, duration, and intensity), metabolic pathways involved and theoretical model underlying its computation. The HKAF seldom exceeded the default value of 3.16, except in very young children (i.e., <≈ 3 months) and when the parent compound is the toxic moiety. Overall, from a public health perspective, the current state of knowledge generally suggest that the default uncertainty factor is sufficient to account for human variability in non-cancer risk assessments of environmental contaminants.

Chronic High Doses of Thioacetamide Followed by Vitamin A Modify Dolichol, Dolichol Isoprenoids, and Retinol Content in Rat Liver Cells

Our line of researches follows the hypothesis that dolichol and retinol metabolism might be interrelated and involved in liver fibrosis. To this end, in this study rats were subjected to chronic treatment with thioacetamide (TAA) (300 mg/L liquid diet) for 1 and 2 months and, after liver damage had occurred, supplemented with vitamin A before sacrifice. Dolichol, dolichol isoprene units, and retinol content were determined in isolated parenchymal and sinusoidal liver cells (hepatic stellate cells; Kupffer cells; sinusoidal endothelial cells). Dolichol increased in hepatocytes after TAA treatment, with or without vitamin A. Dolichol decreased in the other cells. Retinol in general decreased. In hepatocytes, retinol decreased only on normal nutrition, while the vitamin A load was taken up normally. The percentages of dolichol isoprene units (Dol-16 to Dol-20, in rats) confirm that Dol-18, which was not modified in percentage by TAA on normal nutrition, did not increase after vitamin A, as it did in control cells (7-12%). The behavior of Dol-18 was similar in all the cells studied. Vitamin A might reveal a latent damage produced by TAA on dolichol homologues. These data support previous hypotheses that the action of TAA depends on the administration modality, the dosage, and the diet, and that Dol-18 might have different functions and compartmentalization in the cells. Furthermore, the results support the hypothesis that dolichol chain length might be interrelated with retinol metabolism, perhaps through their metabolites.

Review of the toxicity of chemical mixtures: Theory, policy, and regulatory practice

An analysis of current mixture theory, policy, and practice was conducted by examining standard reference texts, regulatory guidance documents, and journal articles. Although this literature contains useful theoretical concepts, clear definitions of most terminology, and well developed protocols for study design and statistical analysis, no general theoretical basis for the mechanisms and interactions of mixture toxicity could be discerned. There is also a poor understanding of the relationship between exposure-based and internal received dose metrics. This confounds data interpretation and limits reliable determinations of the nature and extent of additivity. The absence of any generally accepted classification scheme for either modes/mechanisms of toxic action or of mechanisms of toxicity interactions is problematic as it produces a cycle in which research and policy are interdependent and mutually limiting. Current regulatory guidance depends heavily on determination of toxicological similarity concluded from the presence of a few prominent constituents, assumed from a common toxicological effect, or presumed from an alleged similar toxic mode/mechanism. Additivity, or the lack of it, is largely based on extrapolation of existing knowledge for single chemicals in this context. Thus, regulatory risk assessment protocols lack authoritative theoretical underpinnings, creating substantial uncertainty. Development of comprehensive classification schemes for modes/mechanisms of toxic action and mechanisms of interaction is needed to ensure a sound theoretical foundation for mixture-related regulatory activity and provide a firm basis for iterative hypothesis development and experimental testing.

Metallothionein and liver cell regeneration

Hepatocytes in adults are in a nonproliferative state but they have high capacity to regenerate within few hours after an injury. After partial hepatectomy or chemical injury, hepatocytes undergo a synchronized multistep process consisting of priming/initiation, proliferation, and termination. These distinct steps are essential for restoring the structure and functions of liver. The mechanisms involved in each of these steps of regeneration are well documented from various laboratories and are described in several reviews. We briefly describe these steps and the involvement of various cytokines and growth factors for cell regeneration in this short review. Liver cell regeneration may also involve stem cell proliferation. The regenerating cells require large amounts of zinc within a short time, and this requirement is met by induction of a zinc and copper binding protein, metallothionein (MT), during the priming step, soon after an injury. There are several reports on the transfer of zinc from MT to various metalloenzymes and transcription factors. Genetically modified mouse models have been used to study the involvement of interleukin (IL)-6 and tumor necrosis factor (TNF)-alpha in cell regeneration. The use of an MT-knockout mouse has enabled us to investigate the specific role of MT in liver regeneration after partial hepatectomy, chemical injury, and fibrosis. Several studies have suggested a defective liver regeneration after an injury in MT-knockout mice. There is cumulative evidence that indicates an essential role for MT in liver cell regeneration.

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.

Hepatocyte proliferation is the possible mechanism for the transient decrease in liver injury during steatosis stage of alcoholic liver disease

Steatosis is a frequent pathologic stage in alcoholic liver disease (ALD). Although the mechanisms for increased susceptibility of steatotic liver to injury have been postulated, the ability of these hepatocytes to proliferate and withstand injury is unknown. There are conflicting reports on the status of hepatocyte regeneration following chronic alcohol ingestion. Hence, the objective of this study was to investigate the temporal dynamics between the pattern of liver injury and hepatocyte proliferation during the steatosis stage of ALD. Alcoholic steatosis was induced in male Sprague-Dawley rats by feeding an ethanol (EtOH)-containing Lieber-DeCarli liquid diet for a period of 5 weeks. Microvesicular steatosis was evident in H&E sections by three weeks in the EtOH-treated rats, which further developed into panlobular macrovesicular steatosis by 5 weeks. Plasma transaminase activities indicated progressive increase in liver injury peaking at 3 weeks with significant but mild decrease at 4 and 5 weeks. CYP2E1 protein and activity was significantly increased in EtOH-fed rats as measured by Western blot and pNP hydroxylation assay. PCNA analysis of liver sections indicated that EtOH-treated rats had a significantly higher number of cells in S phase of cell division at weeks 1 (3.20 +/- 0.19), 2 (7.03 +/- 0.92), and 3 (4.23 +/- 1.41) when compared to controls (1.5 +/- 0.22). NF-kappaB DNA binding and Cyclin D1 proteins increased significantly in the EtOH-treated rats corresponding with enhanced hepatic proliferation. These data suggest the transient decline in liver injury during alcoholic steatosis is due to enhanced NF-kappaB-dependent hepatocyte proliferation.

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.

Calpain released from dying hepatocytes mediates progression of acute liver injury induced by model hepatotoxicants

Liver injury is known to progress even after the hepatotoxicant is long gone and the mechanisms of progressive injury are not understood. We tested the hypothesis that hydrolytic enzymes such as calpain, released from dying hepatocytes, destroy the surrounding cells causing progression of injury. Calpain inhibitor, N-CBZ-VAL-PHE-methyl ester (CBZ), administered 1 h after a toxic but nonlethal dose of CCl(4) (2 ml/kg, ip) to male Sprague Dawley rats substantially mitigated the progression of liver injury (6 to 48 h) and also led to 75% protection against CCl(4)-induced lethality following a lethal dose (LD75) of CCl(4) (3 ml/kg). Calpain leakage in plasma and in the perinecrotic areas increased until 48 h and decreased from 72 h onward paralleling progression and regression of liver injury, respectively, after CCl(4) treatment. Mitigation of progressive injury was accompanied by substantially low calpain in perinecrotic areas and in plasma after CBZ treatment. Normal hepatocytes incubated with the plasma collected from CCl(4)-treated rats (collected at 12 h when most of the CCl(4) is eliminated) resulted in extensive cell death prevented by CBZ. Cell-impermeable calpain inhibitor E64 also protected against progression of CCl(4)-induced liver injury, thereby confirming the role of released calpain in progression of liver injury. Following CCl(4) treatment, calpain-specific breakdown of alpha-fodrin increased, while it was negligible in rats receiving CBZ after CCl(4). Hepatocyte cell death in incubations containing calpain was completely prevented by CBZ. Eighty percent of Swiss Webster mice receiving a lethal dose (LD80) of acetaminophen (600 mg/kg, ip) survived if CBZ was administered 1 h after acetaminophen, suggesting that calpain-mediated progression of liver injury is neither species nor chemical specific. These findings suggest the role of calpain in progression of liver injury.

Effect of Sapthaparna (Alstonia scholaris Linn) in modulating the benzo(a)pyrene-induced forestomach carcinogenesis in mice

The chemopreventive effect of various doses of hydroalcoholic extract of Alstonia scholaris (ASE) was studied on the benzo(a)pyrene (BaP) induced forestomach carcinoma in female mice. The treatment of mice with different doses, i.e. 1, 2 and 4 mg/ml ASE in drinking water before, during and after the treatment with carcinogen, exhibited chemopreventive activity. The highest activity was observed for 4 mg/ml ASE, where the tumor incidence (93.33%) was reduced by 6.67%. Similarly, the tumor multiplicity reduced (61.29%) significantly (P<0.02) at 4 mg/ml in the pre-post-ASE treated group. However, the pre or post-treatment of mice with 4 mg/ml ASE did not show chemopreventive activity. These findings are corroborated by micronucleus assay, where treatment of mice with ASE before, during and after carcinogen treatment reduced the frequency of micronuclei (MN) in the splenocytes in a dose dependent manner. The MN frequency reached a nadir at 4 mg/ml ASE, the highest drug dose which showed maximum chemopreventive action. The ASE treatment not only reduced the frequency of splenocytes bearing one MN but also cells bearing multiple MN indicating the efficacy of ASE in inhibiting mutagenic changes induced by BaP. The pre or post-treatment of mice with 4 mg/ml ASE also significantly reduced the frequency of BaP-induced MN in the splenocytes of treated animals.

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.

Thioacetamide impairs retinol storage and dolichol content in rat liver cells in vivo

The aim of this paper was to ascertain whether chronic pretreatment with thioacetamide (TAA) might alter the uptake of a load of retinol and dolichol distribution in hepatocytes (HC), hepatic stellate cells (HSC) (Ito-1 and Ito-2 subfractions), Kupffer (KC) and sinusoidal endothelial cells (SEC). The reason why retinol and dolichol content was studied is that their metabolism and transport might be interrelated and that the two isoprenoids might exert different functions in the cells of the hepatic sinusoid. Rats were treated for 2 and 4 months with TAA, a known fibrogenic hepatotoxin, at a low dosage, to produce an early stage of damage. Three days before sacrifice, the rats were given a load of vitamin A, and cells were isolated to investigate its uptake. In HC, the load of retinol was taken up and accumulated, while a decrease in dolichol preceded retinol increase. In HSC, much less of the retinol load was stored than in controls, and dolichol content also decreased. Various minor modifications were seen in KC and SEC.Collectively, the results show that the distribution of these two isoprenoids, which play important roles in cellular differentiation and proliferation, is differently altered in the multiple cell types that line the hepatic sinusoid, and that both isoprenoids seem to participate in the first steps of liver damage.

Dolichol content in isolated sinusoidal liver cells after in vivo chronic treatment with thioacetamide

The content of dolichol, an isoprenoid present in all biological membranes, was determined in isolated sinusoidal liver cells after treatment of rats for 2 and 4 months with a low dosage of the hepatotoxin thioacetamide. The significant decrease in dolichol observed in hepatocytes after 2 months might be explained by peroxidation of the isoprenoid. At the same time point, retinol was retained, and decreased only after 4 months of treatment. After 4 months of treatment therefore both lipids decreased. In a subfraction of hepatic stellate cells, Ito-1 cells, the main storage site of vitamin A, dolichol decreased significantly only after 4 months. A remarkable difference from hepatocytes is that in Ito-1 cells retinol content significantly decreased after 2 months of treatment. In another subfraction, Ito-2 cells, the content of the two isoprenoids decreased in parallel. This heterogeneous subfraction might represent those transitional hepatic stellate cells that, while losing retinol, are in the process of differentiating into myofibroblasts secreting extracellular matrix components. In Kupffer cells and sinusoidal endothelial cells, impairment of dolichol might be observed later, only after 4 months of treatment, while retinol decreases uniformly over time. Starting after two months of treatment, the decrease of dolichol and the increase of retinol in hepatocytes, at the same time as retinol decreases in hepatic stellate cells, might be taken as an early index of incipient liver injury due to thioacetamide. This hypothesis is discussed with regard to a role of dolichol in the modulation of membrane fluidity for intracellular and intercellular retinol transport.

Paracetamol (acetaminophen)-induced toxicity: molecular and biochemical mechanisms, analogues and protective approaches

An overview is presented on the molecular aspects of toxicity due to paracetamol (acetaminophen) and structural analogues. The emphasis is on four main topics, that is, bioactivation, detoxication, chemoprevention, and chemoprotection. In addition, some pharmacological and clinical aspects are discussed briefly. A general introduction is presented on the biokinetics, biotransformation, and structural modification of paracetamol. Phase II biotransformation in relation to marked species differences and interorgan transport of metabolites are described in detail, as are bioactivation by cytochrome P450 and peroxidases, two important phase I enzyme families. Hepatotoxicity is described in depth, as it is the most frequent clinical observation after paracetamol-intoxication. In this context, covalent protein binding and oxidative stress are two important initial (Stage I) events highlighted. In addition, the more recently reported nuclear effects are discussed as well as secondary events (Stage II) that spread over the whole liver and may be relevant targets for clinical treatment. The second most frequent clinical observation, renal toxicity, is described with respect to the involvement of prostaglandin synthase, N-deacetylase, cytochrome P450 and glutathione S-transferase. Lastly, mechanism-based developments of chemoprotective agents and progress in the development of structural analogues with an improved therapeutic index are outlined.

Efficient tissue repair underlies the resiliency of postnatally developing rats to chlordecone + CCl4 hepatotoxicity

It is often assumed that at a younger age populations are at higher risk of toxic effects from exposure to toxic chemicals. Recent studies have demonstrated that neonate and postnatally developing rats are resilient to a wide variety of structurally and mechanistically dissimilar hepatotoxicants such as galactosamine, acetaminophen, allyl alcohol, and CCl4. Most interestingly, young rats survive exposure to the lethal combination of chlordecone (CD) + CCl4 known to cause 100% mortality in adult male and female rats. In a study where postnatally developing (20- and 45-day), and adult (60-day) male Sprague Dawley rats were used, administration of CCl4 (100 microliters/kg, i.p.) alone resulted in transient liver injury regardless of age as indicated by plasma alanine transaminase (ALT), sorbitol dehydrogenase (SDH) levels and histopathological lesions. In CD-pretreated rats, CCl4-induced toxicity progressed with time culminating in 25 and 100% mortality by 72 h after CCl4 in 45- and 60-day rats, respectively, in contrast to regression of CCl4-induced toxicity and 0% mortality in 20-day rats. [3H]Thymidine (3H-T) incorporation and proliferating cell nuclear antigen (PCNA) studies revealed an association between delayed and diminished DNA synthesis, unrestrained progression of liver injury, and animal death. Time-course studies revealed that the loss of resiliency in the two higher age groups might be due to inability to repair the injured liver rather than due to infliction of higher injury. Intervention of cell division in 45-day CD rats by colchicine (CLC, 1 mg/kg, i.p.) 30 h after CCl4 challenge increased mortality from 25 to 85%, confirming the importance of stimulated tissue repair in animal survival. In contrast, efficient and substantial DNA synthesis observed in 20-day rats allows them to limit further progression of liver injury, thereby leading to full recovery of this age group with 0% mortality. Examination of growth factors and proto-oncogene expression revealed a 3- and 3.5-fold increase in transforming growth factor-alpha (TGF-alpha) and H-ras mRNA expressions, respectively, coinciding with maximal hepatocyte DNA synthesis in 20-day normal diet (ND) rats, as opposed to only 2- and 2.5-fold increases observed in 60-day ND rats, respectively. Increased expression of c-fos (10-fold) in 20-day rats occurred 1 h after CCl4 compared to less than a 2-fold increase in 60-day rats. These findings suggest that prompt stimulation of tissue repair permits efficient recovery from injury during early postnatal development of rats.