Hepatic cell division and tissue repair: a key to survival after liver injury

Mechanistic Modelling of Drug-Induced Liver Injury: Investigating the Role of Innate Immune Responses

Drug-induced liver injury (DILI) remains an adverse event of significant concern for drug development and marketed drugs, and the field would benefit from better tools to identify liver liabilities early in development and/or to mitigate potential DILI risk in otherwise promising drugs. DILIsym software takes a quantitative systems toxicology approach to represent DILI in pre-clinical species and in humans for the mechanistic investigation of liver toxicity. In addition to multiple intrinsic mechanisms of hepatocyte toxicity (ie, oxidative stress, bile acid accumulation, mitochondrial dysfunction), DILIsym includes the interaction between hepatocytes and cells of the innate immune response in the amplification of liver injury and in liver regeneration. The representation of innate immune responses, detailed here, consolidates much of the available data on the innate immune response in DILI within a single framework and affords the opportunity to systematically investigate the contribution of the innate response to DILI.

High Glucose Concentration Alters Cell Proliferation Dynamics in Human Hepatoma Cells

The aim of this study was to develop an in vitro model to investigate the molecular mechanisms of glucose-induced inhibition of cell proliferation. HuH7 cells were grown in the presence or absence of glucose for 7 days and cell proliferation was stimulated by exposure to thioacetamide. Lactate dehydrogenase leakage and 3H-thymidine incorporation were used as indices of toxicity and DNA synthesis, respectively. Cell cycle progression and protooncogene expression was monitored by flow cytometry and slot-blot analyses. Toxicity caused by thioacetamide regressed with time in the presence of 11 mM glucose (control). However, in the presence of 28 mM glucose, sustained toxicity was evident as mirrored by lactate dehydrogenase leakage. Peak DNA synthesis noted at 48 hours in the thioacetamide-treated group (11 mM glucose) was significantly diminished in the presence of 28 mM glucose. Increased c-myc expression was observed as early as 30 minutes in the thioacetamide-treated group. When cells were exposed to 28 mM glucose, c-myc expression was delayed and diminished. Methylation profile studies revealed no appreciable changes, but c-myc was significantly amplified in the control, thioacetamide-, and in the presence of 28 mM of glucose-treated groups which correlated with mRNA changes in these groups. In the glucose-pretreated group (28 mM) significant amplification of the c-myc gene was observed at later time points but there was no change in the mRNA expression, indicating that the expression was delayed. This study shows that high glucose concentrations diminish DNA synthesis and cell cycle progression normally stimulated by thioacetamide. It is concluded that high glucose concentration causes cell cycle arrest via perturbation in protooncogene expression and hence the use of high glucose concentrations in therapy should be carefully examined in situations where postsurgical healing and healing after xenobiotic-induced injury are encountered.

Pregnane X Receptor-Humanized Mice Recapitulate Gender Differences in Ethanol Metabolism but Not Hepatotoxicity

Both human and rodent females are more susceptible to developing alcoholic liver disease following chronic ethanol (EtOH) ingestion. However, little is known about the relative effects of acute EtOH exposure on hepatotoxicity in female versus male mice. The nuclear receptor pregnane X receptor (PXR; NR1I2) is a broad-specificity sensor with species-specific responses to toxic agents. To examine the effects of the human PXR on acute EtOH toxicity, the responses of male and female PXR-humanized (hPXR) transgenic mice administered oral binge EtOH (4.5 g/kg) were analyzed. Basal differences were observed between hPXR males and females in which females expressed higher levels of two principal enzymes responsible for EtOH metabolism, alcohol dehydrogenase 1 and aldehyde dehydrogenase 2, and two key mediators of hepatocyte replication and repair, cyclin D1 and proliferating cell nuclear antigen. EtOH ingestion upregulated hepatic estrogen receptor α, cyclin D1, and CYP2E1 in both genders, but differentially altered lipid and EtOH metabolism. Consistent with higher basal levels of EtOH-metabolizing enzymes, blood EtOH was more rapidly cleared in hPXR females. These factors combined to provide greater protection against EtOH-induced liver injury in female hPXR mice, as revealed by markers for liver damage, lipid peroxidation, and endoplasmic reticulum stress. These results indicate that female hPXR mice are less susceptible to acute binge EtOH-induced hepatotoxicity than their male counterparts, due at least in part to the relative suppression of cellular stress and enhanced expression of enzymes involved in both EtOH metabolism and hepatocyte proliferation and repair in hPXR females.

Therapeutic efficacy of Wuzhi tablet (Schisandra sphenanthera Extract) on acetaminophen-induced hepatotoxicity through a mechanism distinct from N-acetylcysteine

Acetaminophen (APAP) hepatotoxicity is the most common cause of drug-induced liver injury and N-acetylcysteine (NAC) is the primary antidote of APAP poisoning. Wuzhi tablet (WZ), the active constituents well identified and quantified, is a preparation of an ethanol extract of Schisandra sphenanthera and exerts a protective effect toward APAP-induced hepatotoxicity in mice. However, the clinical use of WZ to rescue APAP-induced acute liver injury and the mechanisms involved in the therapeutic effect of WZ remain unclear. Therefore, the effect of WZ on APAP hepatotoxicity was compared with NAC in mice, and molecular pathways contributing to its therapeutic action were investigated. Administration of WZ 4 hours after APAP treatment significantly attenuated APAP hepatotoxicity and exerted much better therapeutic effect than NAC, as revealed by morphologic, histologic, and biochemical assessments. Both WZ and NAC prevented APAP-induced c-Jun N-terminal protein kinase activation and mitochondrial glutathione depletion in livers. The protein expression of nuclear factor erythroid 2-related factor 2 target genes including Gclc, Gclm, Ho-1, and Nqo1 was increased by WZ administration. Furthermore, p53 and p21 levels were upregulated upon APAP exposure, which were completely reversed by postdosing of WZ 4 hours after APAP treatment over 48 hours. In comparison with NAC, WZ significantly increased the expression of cyclin D1, cyclin D-dependent kinase 4, proliferating cell nuclear antigen, and augmenter of liver regeneration in APAP-injured livers. This study demonstrated that WZ possessed a therapeutic efficacy against APAP-induced liver injury by inhibiting oxidative stress and stimulating a regenerative response after liver injury. Thus WZ may represent a new therapy for APAP-induced acute liver injury.

HMGB1 neutralization is associated with bacterial translocation during acetaminophen hepatotoxicity

BACKGROUND

Acetaminophen (APAP) hepatotoxicity is associated with a high rate of gram-negative enteric bacterial infection; however, the underlying mechanism is still unknown. APAP overdose induces massive hepatocyte necrosis, necrotic tissue releases high mobility group B1 (HMGB1) and exogenous HMGB1 is able to induce gut bacterial translocation (BT) in normal mice; therefore, it is possible that HMGB1 mediates gut BT in APAP hepatotoxicity. This study aims to test this hypothesis by using anti-HMGB1 neutralizing antibody to treat APAP overdose for 24-48 hours.

METHODS

Male C57BL/6 mice were intraperitoneally (i.p.) injected with a single dose of APAP (350 mg/kg dissolved in 1 mL sterile saline). 2 hrs after APAP injection, the APAP challenged mice were randomized to receive treatment with either anti-HMGB1 antibody (400 μg per dose) or non-immune (sham) IgG every 24 h for a total of 2 doses.

RESULTS

24 and 48 hrs after APAP challenge, anti-HMGB1 treatment instead of sham IgG therapy significantly decreased serum HMGB1 concentrations and reduced BT by 85%; serum HMGB1 levels were positively correlated with the amount of BT; anti-HMGB1 therapy decreased hepatic BT at 48 h, which was associated with better recovered liver structure and better restored hepatic immune system that was shown by enhanced hepatic mRNA expression of TNF-α, IL-6 and extensive proliferation of inflammatory and reticuloendothelial cells; however, anti-HMGB1 treatment did not decrease gut mucosal permeability as compared to the sham IgG therapy at either 24 or 48 hrs.

CONCLUSION

HMGB1 neutralization is associated with bacterial translocation during APAP hepatotoxicity.

Role and regulation of β-catenin signaling during physiological liver growth

Wnt/β-catenin signaling plays key roles not only during development but also in adult tissue homeostasis. This is also evident in liver biology where many temporal roles of β-catenin have been identified during hepatic development, where, in hepatic progenitors or hepatoblasts, it is a key determinant of proliferation and eventually differentiation to mature hepatocytes, while also playing an important role in bile duct homeostasis. β-Catenin signaling cascade is mostly quiescent in hepatocytes in an adult liver except in the centrizonal region of a hepatic lobule. This small rim of hepatocytes around the central vein show constitutive β-catenin activation that in turn regulates expression of genes whose products play an important role in ammonia and xenobiotic metabolism. Intriguingly, β-catenin can also undergo activation in hepatocytes after acute liver loss secondary to surgical or toxicant insult. Such activation of this progrowth protein is observed as nuclear translocation of β-catenin and formation of its complex with the T-cell factor (TCF) family of transcription factors. Expression of cyclin-D1, a key inducer of transition from the G1 to S phase of cell cycle, is regulated by β-catenin-TCF complex. Thus, β-catenin activation is absolutely critical in the normal regeneration process of the liver as shown by studies in several models across various species. In the current review, the temporal role and regulation of β-catenin in liver development, metabolic zonation in a basal adult liver, and during the liver regeneration process will be discussed. In addition, the probability of therapeutically regulating β-catenin activity as a possible future treatment strategy for liver insufficiency will also be discussed.

High mobility group B1 impairs hepatocyte regeneration in acetaminophen hepatotoxicity

Background

Acetaminophen (APAP) overdose induces massive hepatocyte necrosis. Necrotic tissue releases high mobility group B1 (HMGB1), and HMGB1 contributes to liver injury. Even though blockade of HMGB1 does not protect against APAP-induced acute liver injury (ALI) at 9 h time point, the later time points are not studied and the role of HMGB1 in APAP overdose is unknown, it is possible that neutralization of HMGB1 might improve hepatocyte regeneration. This study aims to test whether blockade of HMGB1 improves hepatocyte regeneration after APAP overdose.

Methods

Male C57BL/6 mice were treated with a single dose of APAP (350 mg/kg). 2 hrs after APAP administration, the APAP challenged mice were randomized to receive treatment with either anti-HMGB1 antibody (400 μg per dose) or non-immune (sham) IgG every 24 hours for a total of 2 doses.

Results

24 hrs after APAP injection, anti-HMGB1 therapy instead of sham IgG therapy significantly improved hepatocyte regeneration microscopically; 48 hrs after APAP challenge, the sham IgG treated mice showed 14.6% hepatic necrosis; in contrast, blockade of HMGB1 significantly decreased serum transaminases (ALT and AST), markedly reduced the number of hepatic inflammatory cells infiltration and restored liver structure to nearly normal; this beneficial effect was associated with enhanced hepatic NF-κB DNA binding and increased the expression of cyclin D1, two important factors related to hepatocyte regeneration.

Conclusion

HMGB1 impairs hepatocyte regeneration after APAP overdose; Blockade of HMGB1 enhances liver recovery and may present a novel therapy to treat APAP overdose.

Ethyl pyruvate reduces liver injury at early phase but impairs regeneration at late phase in acetaminophen overdose

Introduction

Inflammation may critically affect mechanisms of liver injury in acetaminophen (APAP) hepatotoxicity. Kupffer cells (KC) play important roles in inflammation, and KC depletion confers protection at early time points after APAP treatment but can lead to more severe injury at a later time point. It is possible that some inflammatory factors might contribute to liver damage at an early injurious phase but facilitate liver regeneration at a late time point. Therefore, we tested this hypothesis by using ethyl pyruvate (EP), an anti-inflammatory agent, to treat APAP overdose for 24-48 hours.

Methods

C57BL/6 male mice were intraperitoneally injected with a single dose of APAP (350 mg/kg dissolved in 1 mL sterile saline). Following 2 hours of APAP challenge, the mice were given 0.5 mL EP (40 mg/kg) or saline treatment every 8 hours for a total of 24 or 48 hours.

Results

Twenty-four hours after APAP challenge, compared to the saline-treated group, EP treatment significantly lowered serum transaminases (ALT/AST) and reduced liver injury seen in histopathology; however, at the 48-hour time point, compared to the saline therapy, EP therapy impaired hepatocyte regeneration and increased serum AST; this late detrimental effect was associated with reduced serum TNF-α concentration and decreased expression of cell cycle protein cyclin D1, two important factors in liver regeneration.

Conclusions

Inflammation likely contributes to liver damage at an early injurious phase but improves hepatocyte regeneration at a late time point, and prolonged anti-inflammation therapy at a late phase is not beneficial.

Ringer's lactate improves liver recovery in a murine model of acetaminophen toxicity

Background

Acetaminophen (APAP) overdose induces massive hepatocyte necrosis. Liver regeneration is a vital process for survival after a toxic insult. Since hepatocytes are mostly in a quiescent state (G₀), the regeneration process requires the priming of hepatocytes by cytokines such as TNF-α and IL-6. Ringer's lactate solution (RLS) has been shown to increase serum TNF-α and IL-6 in patients and experimental animals; in addition, RLS also provides lactate, which can be used as an alternative metabolic fuel to meet the higher energy demand by liver regeneration. Therefore, we tested whether RLS therapy improves liver recovery after APAP overdose.

Methods

C57BL/6 male mice were intraperitoneally injected with a single dose of APAP (300 mg/kg dissolved in 1 mL sterile saline). Following 2 hrs of APAP challenge, the mice were given 1 mL RLS or Saline treatment every 12 hours for a total of 72 hours.

Results

72 hrs after APAP challenge, compared to saline-treated group, RLS treatment significantly lowered serum transaminases (ALT/AST) and improved liver recovery seen in histopathology. This beneficial effect was associated with increased hepatic tissue TNF-α concentration, enhanced hepatic NF-κB DNA binding and increased expression of cell cycle protein cyclin D1, three important factors in liver regeneration.

Conclusion

RLS improves liver recovery from APAP hepatotoxicity.

Stochastic drift in mitochondrial DNA point mutations: a novel perspective ex silico

The mitochondrial free radical theory of aging (mFRTA) implicates Reactive Oxygen Species (ROS)-induced mutations of mitochondrial DNA (mtDNA) as a major cause of aging. However, fifty years after its inception, several of its premises are intensely debated. Much of this uncertainty is due to the large range of values in the reported experimental data, for example on oxidative damage and mutational burden in mtDNA. This is in part due to limitations with available measurement technologies. Here we show that sample preparations in some assays necessitating high dilution of DNA (single molecule level) may introduce significant statistical variability. Adding to this complexity is the intrinsically stochastic nature of cellular processes, which manifests in cells from the same tissue harboring varying mutation load. In conjunction, these random elements make the determination of the underlying mutation dynamics extremely challenging. Our in silico stochastic study reveals the effect of coupling the experimental variability and the intrinsic stochasticity of aging process in some of the reported experimental data. We also show that the stochastic nature of a de novo point mutation generated during embryonic development is a major contributor of different mutation burdens in the individuals of mouse population. Analysis of simulation results leads to several new insights on the relevance of mutation stochasticity in the context of dividing tissues and the plausibility of ROS ”vicious cycle” hypothesis.

Aging is characterized by a systemic decline of an organism's capacity in responding to internal and external stresses, leading to increased mortality. The mitochondrial Free Radical Theory of Aging (mFRTA) attributes this decline to the accumulation of damages, in the form of mitochondrial DNA (mtDNA) mutations, caused by free radical byproducts of metabolism. However, there is still a great deal of uncertainty with this theory due to the difficulties in quantifying mtDNA mutation burden. In this modeling study, we have shown that a random drift in mtDNA point mutation during life, in combination with the experimental sampling can explain the variability seen in some of the reported experimental data. Particularly, we found that while the average mutation increases in a linear fashion, the variability in the mutation load data increases over time, and thus a low number of data replicates can often lead to a deceptive inference of the mutation burden dynamics. The model also predicted a significant contribution from the embryonic developmental phase to the accumulation of mtDNA mutation burden. Furthermore, the model revealed that the replication rate of mtDNA is a major determinant of new mutations during development and in fast-dividing tissues.

Prolonged treatment with N-acetylcystine delays liver recovery from acetaminophen hepatotoxicity

Introduction

Acetaminophen (APAP) toxicity is the most common cause of acute liver failure in the US and Europe. Massive hepatocyte necrosis is the predominant feature of APAP-induced acute liver injury (ALI). Liver regeneration is a vital process for survival after a toxic insult, it occurs at a relative late time point after the injurious phase. Currently, N-acetylcysteine (NAC), a glutathione precursor, is the antidote for acetaminophen overdose. However, NAC is effective only for patients who present within hours of an acute overdose, and is less effective for late-presenting patients. It is possible that in delayed patients, previously reduced endogenous glutathione (GSH) level has restored and prolonged treatment with NAC might be toxic and impair liver regeneration. Therefore, we hypothesize that prolonged treatment with NAC impairs liver regeneration in ALI induced by APAP.

Methods

ALI was induced in C57BL/6 male mice by a single dose of APAP (350 mg/kg) by intraperitoneal injection. After two hours of APAP challenge, the mice were given 100 mg/kg NAC dissolved in 0.6 mL saline, or saline treatment every 12 hours for a total of 72 hours.

Results

Seventy-two hours after APAP challenge, compared with saline treatment, NAC treatment significantly increased serum transaminases (alanine transaminase/aspartate aminotransferase), induced evident hepatocyte vacuolation in the periportal area and delayed liver regeneration seen in histopathology. This detrimental effect was associated with reduced hepatic nuclear factor (NF)-κB DNA binding and decreased expression of cell cycle protein cyclin D1, two important factors in liver regeneration.

Conclusions

Prolonged treatment with NAC impairs liver regeneration in ALI induced by APAP.

Undernutrition enhances alcohol-induced hepatocyte proliferation in the liver of rats fed via total enteral nutrition

To assess the relative contributions of undernutrition and ethanol (EtOH) exposure to alcohol-induced hepatotoxicity, female Sprague-Dawley rats were intragastrically infused liquid diets containing 187 or 154 kcal.kg(-3/4).day(-1) with or without 11 g.kg(-1).day(-1) EtOH. EtOH clearance was impaired in the 154 kcal.kg(-3/4).day(-1) EtOH group (P < or = 0.05). A combination of undernutrition and EtOH also increased the induction of hepatic cytochrome P-450 (CYP)2E1 and CYP4A1 mRNA, apoprotein, and activities (P < or = 0.05). This was accompanied by increased oxidative stress (P < or = 0.05). The severity of liver steatosis, macrophage infiltration, and focal necrosis was comparable in both EtOH groups. Alanine aminotransferase levels were elevated (P < or = 0.05) but did not significantly differ between the two EtOH groups. TUNEL analysis also demonstrated a comparable increase in apoptosis in the two EtOH groups (P < or = 0.05). The development of alcohol-induced liver pathology was accompanied by little change in fatty acid (FA) synthesis or degradation at 187 kcal.kg(-3/4).day(-1) but at 154 kcal.kg(-3/4).day(-1) was accompanied by decreased expression of FA synthesis genes and increased expression of peroxisome proliferator-activated receptor-alpha (PPAR-alpha)-regulated FA degradation pathways (P < or = 0.05). In addition, 154 kcal.kg(-3/4).day(-1) EtOH group livers exhibited greater hepatocyte proliferation (P < or = 0.05). We conclude that undernutrition does not exacerbate alcoholic steatohepatitis despite additional oxidative stress produced by an increased induction of CYP2E1 and CYP4A1. However, enhanced ethanol-induced cellular proliferation, perhaps as a result of enhanced PPAR-alpha signaling, may contribute to an increased risk of hepatocellular carcinoma in undernourished alcoholics.

Prior administration of a low dose of thioacetamide protects type 1 diabetic rats from subsequent administration of lethal dose of thioacetamide

Previously, we reported that an ordinarily non-lethal dose of thioacetamide (TA, 300 mg/kg) causes 90% mortality in type 1 diabetic rats due to inhibited liver tissue repair, whereas 30 mg TA/kg allows 100% survival due to stimulated although delayed tissue repair. Objective of this investigation was to test whether prior administration of a low dose of TA (30 mg/kg) would lead to sustainable stimulation of liver tissue repair in type 1 diabetic rats sufficient to protect from a subsequently administered lethal dose of TA. Therefore, in the present study, the hypothesis that preplacement of tissue repair by a low dose of TA (30 mg TA/kg, ip) can reverse the hepatotoxicant sensitivity (autoprotection) in type 1 diabetic rats was tested. Preliminary studies revealed that a single intraperitoneal (ip) administration of TA causes 90% mortality in diabetic rats with as low as 75 mg/kg. To establish an autoprotection model in diabetic condition, diabetic rats were treated with 30 mg TA/kg (priming dose). Administration of priming dose stimulated tissue repair that peaked at 72h, at which time these rats were treated with a single ip dose of 75 mg TA/kg. Our results show that tissue repair stimulated by the priming dose enabled diabetic rats to overexpress, calpastatin, endogenous inhibitor of calpain, to inhibit calpain-mediated progression of liver injury induced by the subsequent administration of lethal dose, resulting in 100% survival. Further investigation revealed that protection observed in these rats is not due to decreased bioactivation. These studies underscore the importance of stimulation of tissue repair in the final outcome of liver injury (survival/death) after hepatotoxicant challenge. Furthermore, these results also suggest that it is possible to stimulate tissue repair in diabetics to overcome the enhanced sensitivity of hepatotoxicants.

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.

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.

Role of tumor necrosis factor receptor 1 (p55) in hepatocyte proliferation during acetaminophen-induced toxicity in mice

Hepatocyte proliferation represents an important part of tissue repair. In these studies, TNF receptor 1 (TNFR1) knockout mice were used to analyze the role of TNF-alpha in hepatocyte proliferation during acetaminophen-induced hepatotoxicity. Treatment of wild-type (WT) mice with acetaminophen (300 mg/kg) resulted in centrilobular hepatic necrosis. This was associated with proliferation of hepatocytes surrounding the damaged areas, which was evident at 24 h. The cell cycle regulatory proteins, cyclin D1 and cyclin A, were also up regulated in hepatocytes. In contrast, in TNFR1-/- mice, which exhibit exaggerated acetaminophen hepatotoxicity, hepatocyte proliferation, and expression of cyclin D1 and cyclin A, as well as the cyclin dependent kinases, Cdk4 and Cdk2, were reduced. The cyclin-dependent kinase inhibitor p21 was also induced in the liver following acetaminophen administration. This was greater in TNFR1-/- mice compared to WT mice. To investigate mechanisms mediating the reduced hepatic proliferative response of TNFR1-/- mice, we analyzed phosphatidyl inositol-3-kinase (PI-3K) signaling. In both WT and TNFR1-/- mice, acetaminophen caused a rapid increase in total PI-3K within 3 h. Acetaminophen also increased phosphorylated PI-3K, but this was delayed 6-12 h in TNFR1-/- mice. Expression of Akt, a downstream target of PI-3K, was increased in both WT and TNFR1-/- mice in response to acetaminophen. However, the increase was greater in WT mice. Acetaminophen-induced expression of phosphorylated STAT3, a key regulator of cytokine-induced hepatocyte proliferation, was also delayed in TNFR1-/- mice relative to WT. These data suggest that TNF-alpha signaling through TNFR1 is important in regulating hepatocyte proliferation following acetaminophen-induced tissue injury. Delayed cytokine signaling may account for reduced hepatocyte proliferation and contribute to exaggerated acetaminophen-induced hepatotoxicity in TNFR1-/- mice.

Scavenging peroxynitrite with glutathione promotes regeneration and enhances survival during acetaminophen-induced liver injury in mice

Acetaminophen (AAP) overdose causes formation of peroxynitrite in centrilobular hepatocytes. Treatment with glutathione (GSH) after AAP accelerated recovery of mitochondrial GSH levels, which scavenged peroxynitrite and protected against liver injury at 6 h. The objective of this investigation was to evaluate whether GSH treatment has a long-term protective effect against AAP-induced injury and whether it promotes liver regeneration. AAP (300 mg/kg) induced severe centrilobular necrosis and increased plasma alanine aminotransferase (ALT) activities (24 h: 3680 +/- 320 U/liter) in fasted C3Heb/FeJ mice. Only 53% of the animals survived for 24 h. Hepatic glutathione levels were still suppressed by 62% at 24 h compared with untreated controls (19.7 +/- 2.6 micromol/g). Glutathione disulfide (GSSG) concentrations were elevated by 455% compared with controls (74 +/- 3 nmol/g liver). Treatment with GSH at 1.5 h after AAP treatment attenuated liver necrosis and plasma ALT activities by 62 to 66% at 24 h. All animals survived up to 7 days. The hepatic GSH content recovered to control values; however, the GSSG levels were still elevated at 48 h (252 +/- 26 nmol/g). Expression of proliferating cell nuclear antigen (PCNA) and cell cycle proteins cyclin D1 and p21 were not detectable in controls or after AAP alone. Treatment with GSH after AAP induced expression of cyclin D1, p21, and PCNA (12-48 h). Thus, GSH treatment after AAP provided long-term hepatoprotection and promotes progression of cell cycle activation in hepatocytes.

Stem cell factor attenuates liver damage in a murine model of acetaminophen-induced hepatic injury

Acute liver injury is a common cause of intensive care unit visits. In these studies, we used a murine model of acetaminophen poisoning to examine the role of stem cell factor (SCF) on liver damage. In the initial studies, we identified that the liver produces relatively high constitutive levels of SCF. Upon administration of acetaminophen, the levels of SCF fell dramatically, correlating to damage within the liver. When the liver was allowed to regenerate, the levels of SCF again correlated with the liver regeneration. We next treated mice with anti-SCF before sublethal doses of acetaminophen and significantly increased lethality in anti-SCF-treated animals. When exogenous SCF was given to mice, the lethality was significantly reduced compared with the control acetaminophen-treated animals and the damage within the liver tissue was attenuated. The administration of rSCF reduced the level of steady-state mRNA for cytochrome P450 cyp2E1 enzyme both in vitro and in vivo. These data suggest that SCF functions as an important factor that protects livers from acute damage.

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.

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.

Down-regulation by troglitazone of hepatic tumor necrosis factor-alpha and interleukin-6 mRNA expression in a murine model of non-insulin-dependent diabetes

Troglitazone, a novel thiazolidinedione drug used to treat non-insulin-dependent diabetes mellitus, is a selective ligand for the peroxisome proliferator-activated receptor-gamma (PPARgamma). Recent results indicate that PPARgamma activation by thiazolidinediones regulates adipose tissue- and monocyte/peritoneal macrophage-derived cytokine expression in vitro. We evaluated whether troglitazone may also negatively regulate cytokine expression in the liver, which harbors the majority of the body's resident macrophages but which only weakly expresses PPARgamma. Lean C57BL6 mice and genetically obese KKA(y) mice were chronically treated with troglitazone (100 mg/kg/day for 2 weeks). At the end of treatment, hepatic expression of tumor necrosis factor (TNF)-alpha and interleukin (IL)-6 mRNA was quantitatively determined by kinetic polymerase chain reaction both under basal conditions and after stimulation with lipopolysaccharide (LPS). Both untreated lean and obese mice exhibited low levels of baseline TNF-alpha and IL-6 mRNA expression and responded with a dramatic increase in hepatic cytokine transcripts and TNF-alpha protein expression following a challenge with LPS. Similar to the effects on white adipose tissue, troglitazone not only down-regulated the baseline levels of hepatic TNF-alpha and IL-6, but also greatly attenuated the inducing effects of LPS. The extent of this inhibitory effect of troglitazone was higher in obese KKA(y) mice than in lean mice and was also reflected by markedly down-regulated hepatic TNF-alpha protein expression. These data demonstrate that chronic administration of troglitazone is associated with a greatly attenuated responsiveness towards inducers of hepatic TNF-alpha and IL-6 production. The possible biological consequences of these effects, however, have not yet been assessed.

Inhibition of tumour necrosis factor alpha does not prevent experimental paracetamol-induced hepatic necrosis

Paracetamol-induced hepatic necrosis is the most common form of toxic liver injury experienced in clinical practice in the UK and USA. Recently, reports have described prevention of hepatic necrosis, induced by other hepato-toxins, by inhibiting tumour necrosis factor alpha (TNFalpha). The aim of the present study was to determine the role of TNFalpha in paracetamol-induced hepatic necrosis. Six-week-old CBA/J female mice were given 300 mg/kg paracetamol by intraperitoneal (IP) injection after an 8-h fast. Hepatic expression of TNFalpha was measured by enzyme-linked immunoassay (ELISA) and reverse transcriptase-polymerase chain reaction (RT-PCR). Serum TNFalpha was measured by ELISA. One hour prior to paracetamol injection, mice were also given blocking anti-TNFalpha antibodies, soluble TNFalpha receptor, interleukin 10 (IL-10), and dexamethasone. Hepatic injury was measured by serum aspartate aminotransferase and histological assessment on haematoxylin and eosin (H&E)-stained liver sections. There was a significant increase in serum TNFalpha at 6 h (control 0.002+/-0.002 ng/ml, n=7; paracetamol-treated 0.022+/-0.007 ng/ml, n=5, p<0.05), but hepatic TNFalpha expression did not change up to 24 h following paracetamol injection. Histologically severe centrilobular hepatic necrosis was noted at 3 h and progressed for 24 h after paracetamol poisoning. Death rate, serum aspartate aminotransferase, and hepatic histology were not significantly different between the groups treated with blocking anti-TNFalpha antibodies, soluble TNFalpha receptor, IL-10, and dexamethasone, compared with controls. In conclusion, there is no evidence to suggest that modulation of TNFalpha expression affects hepatic injury following experimental paracetamol poisoning; anti-TNFalpha therapies are therefore unlikely to be effective in the corresponding clinical situation.

Differential protooncogene expression in Sprague Dawley and Fischer 344 rats during 1,2-dichlorobenzene-induced hepatocellular regeneration

Significant differences in hepatotoxic injury of 1,2-dichlorobenzene (o-DCB) have been reported (Gunawardhana, L., Sipes, I.G., 1991. Dichlorobenzene hepatotoxicity strain differences and structure activity relationships. Adv. Exp. Med. Biol. 283, 731-734; Stine, E.R., Gunawardhana, L., Sipes, I.G., 1991. The acute hepatotoxicity of the isomers of dichlorobenzene in Fischer 344 and Sprague-Dawley rats: isomer specific and strain-specific differential toxicity. Toxicol. Appl. Pharmacol. 109, 472-481; Valentovic, M.A., Ball, J. G., Anestis, D., Madan E., 1993a. Acute hepatic and renal toxicity of dichlorobenzene isomers in Fischer 344 rats. J. Appl. Toxicol. 13, 1-7; Kulkarni, S.G., Duong, H., Gomila, R., Mehendale, H.M., 1996. Strain differences in tissue repair response to 1,2-dichlorobenzene. Arch. Toxicol. 70, 714-723. Kulkarni, S.G., Warbritton, A., Bucci, T., Mehendale, H.M., 1997. Antimitotic intervention with colchicine alters the outcome of o-DCB-induced hepatotoxicity in Fischer 344 rats. Toxicology. 120, 79-88). Although, hepatotoxic injury of o-DCB is greater in Fischer 344 (F344) when compared with Sprague Dawley (S-D) rats, this interstrain difference does not transcend into any difference in lethal effects of o-DCB. Interstrain difference in compensatory tissue repair has been suggested as the underlying mechanism for the lack of strain differences in lethality (Kulkarni et al., 1996; Kulkarni et al., 1997, see these refs. above). However, the mechanism(s) for this interstrain difference in tissue repair is (are) not currently understood. The objectives of the present study were (1) to investigate if the differences in compensatory tissue repair are reflected in differential protooncogene expression in S-D versus F344 rat livers and (2) to investigate if changes in protooncogene expression could explain the decrease and delay in tissue repair response beyond a threshold of 0.6 ml o-DCB/kg. Male S-D and F344 rats (8/9 weeks old) were administered either 0.6 or 1.2 ml o-DCB/kg and changes in expression of protooncogenes c-myc (immediate early) and Ha-ras (delayed early) were examined over a time course. Findings of this study indicate that the timing and extent of c-myc and Ha-ras expression varies in the two strains following administration of o-DCB. Thus, the timing and extent of compensatory liver regeneration that ensues following o-DCB administration in S-D and F344 rats is temporally concordant with the protooncogene expression in the two strains.

Effects of Corylus avellana in acetaminophen and CCl4 induced toxicosis

Our study investigated the effects of Corylus avellana extract on acetaminophen and carbon tetrachloride intoxicated liver of young rats. Hepatocytolysis was determined by measuring the level of serum transaminases (GPT and GOT), steatosis by Sudan black staining, histological structures by hamatoxylineosin staining and the activity of enzymes such as SDH, GtDH, G-6-Pase and ATPase. Comparatively, the most serious lesions appeared in CCl4 intoxication. Corylus avellana extract had some beneficial effects in CCl4 toxicosis: it reduced hepatocytolysis as well as histological lesions and returned the activity of some enzymes to normal values.

Tissue injury and repair as parallel and opposing responses to CCl4 hepatotoxicity: a novel dose-response

Recent studies indicate that the rate and extent of tissue repair, elicited as an endogenous response to toxic insult, are critical determinants in the ultimate outcome of hepatic injury. Therefore, the objective of this study was to develop a dose-response relationship for CCl4 measuring liver injury and tissue repair as two simultaneous but opposing responses. Male Sprague-Dawley rats were injected with a 40-fold dose range of CCl4 (0.1-4 ml/kg i.p.) in corn oil vehicle. Liver injury was assessed by serum enzyme elevations and histopathology, and tissue repair was measured by [3H]thymidine incorporation into hepatonuclear DNA and proliferating cell nuclear antigen immunohistochemistry over a time course of 0 to 96 h. Stimulation of cell division, evident even after a subtoxic dose of CCl4, increased in a dose-dependent manner until a threshold (2 ml/kg) was reached. Doses above this threshold yielded no further increase in tissue repair. Instead, tissue repair response was significantly delayed and diminished. Injury was markedly accelerated above the threshold indicating an unrestrained progression of injury. Although 4 ml CCl4/kg consistently caused 80% lethality by 48 h, tissue repair response in the 20% surviving rats was increased by about 5-fold, aptly demonstrating the critical role of tissue repair in overcoming injury and enabling these animals to survive. This study suggests that, in addition to the extent of tissue repair, the time of onset of tissue repair also determines the extent of hepatic injury and inter-individual differences in the magnitude of tissue repair may contribute significantly to inter-individual differences in susceptibility to toxic chemicals. Thus, while dose-related and prompt stimulation of tissue regeneration leads to recovery, delayed and attenuated repair response, occurring at higher doses, leads to progression of injury and animal mortality. Such dose-response relationships may lead to a better understanding of the underlying cellular mechanisms of injury inflicted by chemical toxicants and aid in fine-tuning risk assessment.