Protection of chlordecone-potentiated carbon tetrachloride hepatotoxicity and lethality by partial hepatectomy

Bromotrichloromethane Hepatotoxicity. The Role of Stimulated Hepatocellular Regeneration in Recovery: Biochemical and Histopathological Studies in Control and Chlordecone Pretreated Male Rats

It has been shown that BrCCl3 is a more potent hepatotoxin than CCl4. Pretreatment with nontoxic dietary levels of chlordecone (CD) results in amplification of BrCCl3 hepatotoxicity. The objective of this research was to investigate and compare the histopathological alterations during a time course after a low dose of BrCCl3 alone and in combination with dietary CD. Male Sprague-Dawley rats were maintained on 10 ppm dietary CD or normal diet for 15 days. On day 16, they received a single ip dose (30 μ1/kg) of BrCCl3 in corn oil (CO) vehicle or corn oil alone. Blood and liver samples were collected at 0, 3, 6, 12, 24, 36, 48, 72, 96, and 120 hr for serum enzymes and histopathological examination, respectively. Serum enzymes (SDH, ALT, AST) were significantly ( p < 0.05) elevated in rats receiving the CD + BrCCl3 combination in comparison to BrCCl3 alone. For 48 hr, a continuous increase in serum enzyme activities was detected in rats treated with CD + BrCCl3 combination, but not in the rats receiving other treatments (ND + BrCCl3, ND + CO, or CD + CO). The most extensive hepatolobular necrosis was observed in rats treated with the CD + BrCCl3 combination. Thirty-six hr after the administration of BrCCl3 to rats maintained on normal diet, high mitotic activity was observed, which continued through 72 hr resulting in complete restoration of hepatolobular structure. In contrast, rats receiving the combination of CD + BrCCl3 exhibited minimal and belated hepatomitotic activity for a short period of time, resulting in progressive hepatic failure, culminating in animal death. In conclusion, hepatotoxicity of a low dose of BrCCl3 alone appeared to be overcome via stimulated hepatocellular regeneration and hepatolobular restoration. CD appears to amplify BrCCl3 hepatotoxicity via interference with this hormetic mechanism, permitting a progressive and continued hepatic injury leading to complete hepatic failure, culminating in animal death.

Polyamine Protection Against Chemically Induced Hepatotoxicity

The protective effect of putrescine (a polyamine) on chemically induced hepatotoxicity in male Sprague-Dawley rats was assessed by mortality, clinical pathological changes (specifically alanine aminotransferase and aspartate aminotransferase activities), and liver histopathological changes. A reduction in hepatotoxicant-induced mortality by 20% to 25% was observed when putrescine (100 mg/kg/day) was administered intraperitoneally for 3 days prior to hepatotoxicant administration (either carbon tetrachloride or allyl alcohol at dose levels approximating the LD50). Putrescine significantly reduced the hepatoxicant-induced increases in serum alanine aminotransferase and aspartate aminotransferase activities. Histological assessment revealed that putrescine pretreatment also reduced the severity and frequency of hepatotoxicant-induced liver necrosis. Administration of putrescine at 0.5 and 3 hours following hepatotoxicant treatment decreased both hepatoxicant-induced mortality and hepatoxicant-induced increases in serum alanine aminotransferase and aspartate aminotransferase activities, with the 0.5 hour postdose treatment being more effective than the 3 hours postdose treatment. Early intervention reduced the mortality rate in the allyl alcohol plus putrescine group by 20% and by 10% in the carbon tetrachloride as well as the carbon tetrachloride plus putrescine groups. However, the effectiveness of postdose putrescine treatment was less than when putrescine was administered prior to the hepatotoxicant.

Acetaminophen-NAPQI hepatotoxicity: a cell line model system genome-wide association study

Acetaminophen is the leading cause of acute hepatic failure in many developed nations. Acetaminophen hepatotoxicity is mediated by the reactive metabolite N-acetyl-p-benzoquinonimine (NAPQI). We performed a "discovery" genome-wide association study using a cell line-based model system to study the possible contribution of genomics to NAPQI-induced cytotoxicity. A total of 176 lymphoblastoid cell lines from healthy subjects were treated with increasing concentrations of NAPQI. Inhibiting concentration 50 values were determined and were associated with "glutathione pathway" gene single nucleotide polymorphisms (SNPs) and genome-wide basal messenger RNA expression, as well as with 1.3 million genome-wide SNPs. A group of SNPs in linkage disequilibrium on chromosome 3 was highly associated with NAPQI toxicity. The p value for rs2880961, the SNP with the lowest p value, was 1.88 × 10(-7). This group of SNPs mapped to a "gene desert," but chromatin immunoprecipitation assays demonstrated binding of several transcription factor proteins including heat shock factor 1 (HSF1) and HSF2, at or near rs2880961. These chromosome 3 SNPs were not significantly associated with variation in basal expression for any of the genome-wide genes represented on the Affymetrix U133 Plus 2.0 GeneChip. We have used a cell line-based model system to identify a SNP signal associated with NAPQI cytotoxicity. If these observations are validated in future clinical studies, this SNP signal might represent a potential biomarker for risk of acetaminophen hepatotoxicity. The mechanisms responsible for this association remain unclear.

Upregulation of calpastatin in regenerating and developing rat liver: role in resistance against hepatotoxicity

Acute liver failure induced by hepatotoxic drugs results from rapid progression of injury. Substantial research has shown that timely liver regeneration can prevent progression of injury leading to a favorable prognosis. However, the mechanism by which compensatory regeneration prevents progression of injury is not known. We have recently reported that calpain released from necrotic hepatocytes mediates progression of liver injury even after the hepatotoxic drug is cleared from the body. By examining expression of calpastatin (CAST), an endogenous inhibitor of calpain in three liver cell division models known to be resistant to hepatotoxicity, we tested the hypothesis that increased CAST in the dividing hepatocytes affords resistance against progression of injury. Liver regeneration that follows CCl(4)-induced liver injury, 70% partial hepatectomy, and postnatal liver development were used. In all three models, CAST was upregulated in the dividing/newly divided hepatocytes and declined to normal levels with the cessation of cell proliferation. To test whether CAST overexpression confers resistance against hepatotoxicity, CAST was overexpressed in the livers of normal SW mice using adenovirus before challenging them with acetaminophen (APAP) overdose. These mice exhibited markedly attenuated progression of liver injury and 57% survival. Whereas APAP-bioactivating enzymes and covalent binding of the APAP-derived reactive metabolites remained unaffected, degradation of calpain specific target substrates such as fodrin was significantly reduced in these mice. In conclusion, CAST overexpression could be used as a therapeutic strategy to prevent progression of liver injury where liver regeneration is severely hampered.

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.

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.

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.

Chlorinated methanes and liver injury: highlights of the past 50 years

The chlorinated methanes, particularly carbon tetrachloride and chloroform, are classic models of liver injury and have developed into important experimental hepatoxicants over the past 50 years. Hepatocellular steatosis and necrosis are features of the acute lesion. Mitochondria and the endoplasmic reticulum as target sites are discussed. The sympathetic nervous system, hepatic hemodynamic alterations, and role of free radicals and biotransformation are considered. With carbon tetrachloride, lipid peroxidation and covalent binding to hepatic constituents have been dominant themes over the years. Potentiation of chlorinated methane-induced liver injury by alcohols, aliphatic ketones, ketogenic compounds, and the pesticide chlordecone is discussed. A search for explanations for the potentiation phenomenon has led to the discovery of the role of tissue repair in the overall outcome of liver injury. Some final thoughts about future research are also presented.

Evaluation of sex difference in tissue repair following acute carbon tetrachloride toxicity in male and female Sprague-Dawley rats

Cellular regeneration and tissue repair greatly influence the outcome of acute carbon tetrachloride (CCl4) hepatotoxicity. This study examined the temporal kinetics of cellular regeneration and tissue repair processes in male and female Sprague Dawley (SD) rats following an acute CCl4 exposure (0.8 ml/kg, i.p.). In female rats, hepatic damage peaked at 24 h following the treatment and was approximately 2.5-fold (AST 2.7-fold, ALT 2.3 fold) greater than the damage observed in male rats. The hepatic damage in male rats appeared to peak by 3 h post-exposure and did not significantly change through the 36-h time-point. The activity of cytochrome P 4502E1 was 20% greater in male rats and did not correlate with the magnitude of hepatic damage. Morphometric analysis of cell cycle indices revealed that cellular regeneration was significantly greater in female rats as compared to male rats at 48 h and corresponded proportionally to the extent of liver damage. This study demonstrated that female SD rats respond more severely to acute CCl4 hepatotoxicity than male SD rats and the extent of tissue repair and cellular regeneration was greater in female rats. Furthermore, our results suggest that tissue repair is unlikely to result in accounting for the different responses exhibited by male and female SD rats to CCl4 hepatotoxicity.

Mechanisms of signal transduction in the stress response of hepatocytes

Adaptation of animals to stress is a unique property of life which allows the survival of the species. The stress response of hepatocytes is a very complex phenomenon, sometimes involving a cascade of events. The general stress signals are elucidated by mobilization of carbohydrate stores and akin to the insulin mediators. Oxidative signals are generated by pesticides, heavy metals, drugs, and alcohol which may or may not be under the purview of peroxisomes. Peroxisomal responses are well-defined involving specific receptors, whereas nonperoxisomal responses may be signaled by calcium, the Ah receptor, or built-in antioxidant systems. The intoxication signals are generally thought to be membrane defects induced by xenobiotics which then lead to highly nonspecific responses of hepatocytes. Detoxication signals, on the other hand, are specific responses of hepatocytes triggering de novo syntheses of detoxifier proteins or enzymes. Evidence reveals the existence of two distinct mechanisms of signal transduction in stressed hepatocytes--one involving the peroxisome and the other the plasma membrane.

A review of the role of tissue repair as an adaptive strategy: why low doses are often non-toxic and why high doses can be fatal

The role of tissue repair as an adaptive strategy by species is important to consider in both evolutionary and toxicological perspectives. This paper assesses the distinct and integrative roles of early phase regeneration (EPR) (i.e. arrested G2 hepatocytes chemically activated to proceed through mitosis) and secondary phase regeneration (SPR) (i.e. hepatocytes mobilized principally from G0/G1 to proceed through mitosis) in the repair of carbon tetrachloride (CCl4)-induced liver damage. The role of EPR as a triage system facilitating repair of minor toxic insults as well as providing an essential role in autoprotection as an initial step to augment and sustain SPR is proposed. The function of EPR is then compared with that of SPR in tissue recovery following more massive injury. The interrelationships of these two repair processes with EPR invoking and accelerated SPR following low-to-modest degrees of toxicant-induced hepatotoxicity as well as in auto- or hetero-protection supports the theory that the two responses are co-ordinated in time and functionality. The integration of these two repair processes as shown through experimental manipulation provides a new mechanistic framework to account for the previously reported profound (67-fold) potentiation of acute CCl4 hepatotoxicity by chlordecone (kepone) in adult male Sprague-Dawley rats as well as important interspecies variation in susceptibility to hepatotoxic agents in general and CCl4 in particular. On the basis of the distinct and integrative roles of EPR and SPR in liver responses to toxic injury, a generalized framework is presented that facilitates prediction of both toxic outcome, including shape of dose-response functions and interspecies variation to chemically induced liver damage.

ATSDR evaluation of health effects of chemicals. II. Mirex and chlordecone: health effects, toxicokinetics, human exposure, and environmental fate

This document provides public health officials, physicians, toxicologists, and other interested individuals and groups with an overall perspective of the toxicology of mirex and chlordecone. It contains descriptions and evaluations of toxicological studies and epidemiological investigations and provides conclusions, where possible, on the relevance of toxicity and toxicokinetic data to public health. Additional substances will be profiled in a series of manuscripts to follow.

Decrease in hepatotoxicity by lead exposure is not explained by its mitogenic response

The present research was conducted to evaluate the effect of mitogen pre-exposure on CCl4-induced hepatotoxicity. Male Wistar rats were administered a single i.p. injection of CCl4 (0.3 ml kg-1 in corn oil) 48 h following either a single dose of lead nitrate (0.33 mg kg-1) or distilled water via i.v. injection. Hepatotoxicity, as measured by serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels, was monitored 6, 24, 48, 72 and 120 h after CCl4 exposure. The lead nitrate-pretreated rats displayed markedly lower serum ALT and AST levels at 24, 48 and 72 h than rats pretreated with distilled water. However, treatment with the antimitotic agent colchicine did not alter the lead-induced protection. These findings suggest that the lead-induced protection is not associated with the major mitogenic response of lead, despite its strong temporal association. A critical review of the available toxicological data also argues against the lead protection being a function of its capacity to inhibit cytochrome P-450.

Amplified interactive toxicity of chemicals at nontoxic levels: mechanistic considerations and implications to public health

It is widely recognized that exposure to combinations or mixtures of chemicals may result in highly exaggerated toxicity even though the individual chemicals might not be toxic. Assessment of risk from exposure to combinations of chemicals requires the knowledge of the underlying mechanism(s). Dietary exposure to a nontoxic dose of chlordecone (CD; 10 ppm, 15 days) results in a 67-fold increase in lethality of an ordinarily inconsequential dose of CCl4 (100 microliters/kg, ip). Toxicity of closely related CHCl3 and BrCCl3 is also enhanced. Phenobarbital (PB, 225 ppm, 15 days) and mirex (10 ppm, 15 days) do not share the propensity of CD in this regard. Exposure to PB + CCl4 results in enhanced liver injury similar to that observed with CD, but the animals recover and survive in contrast to the greatly amplified lethality of CD + CCl4. Investigations have revealed that neither enhanced bioactivation of CCl4 nor increased lipid peroxidation offers a satisfactory explanation of these findings. Additional studies indicate that exposure to a low dose of CCl4 (100 microliters/kg, ip) results in limited injury, which is accompanied by a biphasic response of hepatocellular regeneration (6 and 36 hr) and tissue repair, which enables the animals to recover from injury. Exposure to CD + CCl4 results in suppressed tissue repair owing to an energy deficit in hepatocytes as a consequence of excessive intracellular influx of Ca2+ leading initially to a precipitous decline in glycogen and ultimately to hypoglycemia. Supplementation of cellular energy results in restoration of the tissue repair and complete recovery from the toxicity of CD + CCl4 combination. In contrast, only the early-phase hepatic tissue repair (6 hr) is affected in PB + CCl4 treatment, but this is adequately compensated for by a greater stimulation of tissue repair at 24 and 48 hr resulting in recovery from liver injury and animal survival. A wide variety of additional experimental evidence confirms the central role of stimulated tissue repair as a decisive determinant of the final outcome of liver injury inflicted by CCl4. For instance, a 35-fold greater CCl4 sensitivity of gerbils compared to rats is correlated with the very sluggish tissue repair in gerbils. These findings are consistent with a two-stage model of toxicity, where tissue injury is inflicted by the well described "mechanisms of toxicity," but the outcome of this injury is determined by whether or not sustainable tissue repair response accompanies this injury.(ABSTRACT TRUNCATED AT 400 WORDS)

Autoprotection: stimulated tissue repair permits recovery from injury

Autoprotection is a phenomenon whereby prior exposure to a small dose of a chemical results in protection against a subsequently administered lethal dose of the same compound. While CCl4 autoprotection has been studied the most, it has also been demonstrated for other chemicals. Recent studies indicate that the prevailing concept of decreased bioactivation of the normally lethal dose of CCl4 owing to decreased hepatic microsomal cytochrome P-450 content cannot be supported by direct end points of liver injury such as necrosis. These findings suggest a pivotal role for hepatocellular division and tissue healing processes stimulated by the protective dose in the mechanism of autoprotection. Augmentation of hepatocellular regeneration and tissue repair, stimulated by the protective dose, appears to permit timely recovery and restoration of hepatic structure and function. In the absence of the protective dose, hepatocellular division is substantially deficient and it occurs too late to tip the delicate balance between recovery from injury and progression of massive injury in favor of recovery. Abolition of autoprotection by colchicine antimitosis, under conditions where metabolism and disposition of CCl4 are not altered, is supportive of this concept. Selective colchicine antimitotic suppression of the early phase of hepatocellular division and tissue repair induced by a low dose of CCl4 results in progression of toxic liver injury, leading to hepatic failure and mortality. Studies have shown that pretreatment with phenobarbital results in postponed low-dose CCl4-stimulated cell division by 24 hours, which accordingly postpones the optimal autoprotection.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of phenobarbital and mirex pretreatments on CCl4 autoprotection

Male Sprague-Dawley rats maintained on either normal diet (N) or on a diet containing phenobarbital (PB; 225 ppm) or mirex (M; 10 ppm) for 15 days received either corn oil or 1 single administration of a protective dose of CCl4 (0.3 ml/kg, po) on day 16. At 24, 48, 72, 96, or 144 hr after the protective dose, a high dose of CCl4 (5 ml/kg, po) was administered to rats of all the groups, and they were observed for 14-day lethality. In a second experiment, in rats maintained on N, PB, or M diet, liver microsomal cytochromes P-450, aminopyrine demethylase, and aniline hydroxylase were measured at various time points after the administration of the protective dose of CCl4. Serum aspartate transaminase, alanine transaminase, and sorbitol dehydrogenase elevations and histopathological changes observed under a light microscope were used as toxic end points to assess hepatotoxicity. Autoprotection was 100% when the high dose was given at 24 hr after the protective dose in N rats, whereas it was only 55% in PB- or M-pretreated rats. For later time points of 48, 72, and 96 hr, autoprotection was only around 50% in N rats, whereas it was almost 100% in PB- and M-pretreated rats. When the high dose was administered at 144 hr after the protective dose, autoprotection further declined to 25% in N rats and to 75% in M-treated rats, but it remained at 100% in PB-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine triphosphate protection of chlordecone-amplified CCl4 hepatotoxicity and lethality

Dietary exposure to a nontoxic level of chlordecone (10 ppm for 15 days) followed by a single exposure to a subtoxic dose of CCl4 (100 microliters/kg, ip) is known to result in a 67-fold amplification of CCl4 toxicity. The hypothesis that the underlying mechanism is due to incapacitation of hepatocytes leading to an ablation of the early-phase hormetic response of tissue repair as a consequence of precipitous decline in hepatic glycogen and ATP, received experimental support from Mehendale in 1990. The present study was designed to investigate if direct administration of ATP to rats maintained on the chlordecone diet would result in protection from the hepatotoxic and lethal effects of the chlordecone+CCl4 combination. Male Sprague-Dawley rats (125-150 g) were maintained either on a diet containing no added contaminants (control) or on a diet containing 10 ppm chlordecone for 15 days, and were challenged with CCl4 (100 microliters/kg, ip) on day 16. Without ATP administration all rats died within 72 h, while administration of ATP (100 mg/rat, sc) to chlordecone-pretreated rats at -1, +1, 3, 5, 12, 24 and 36 h of CCl4 injection resulted in 100% survival. Injection of ATP, at -1, +1, 3 and 5 h of CCl4 administration to chlordecone pretreated rats decreased plasma enzyme elevations (alanine and aspartate aminotransferase, sorbitol dehydrogenase) as well as substantially preventing elevation of plasma bilirubin levels at 6, 12 and 24 h. Hepatic ATP levels were also elevated at 6 and 12 h, but not at 24 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of antimitotic agent colchicine on carbon tetrachloride toxicity

A single administration of a subtoxic dose of CCl4 (100 microliters/kg, i.p.) is known to induce hepatocellular regeneration and tissue repair at 6 and 48 h in rats, permitting prompt recovery from the limited liver injury associated with that dose of CCl4. Substantial evidence has accumulated to indicate that the early-phase hepatocellular regeneration and tissue repair are critical for recovery from halomethane hepatotoxicity. The objective of these studies was to test this concept in an experimental framework, wherein a selective ablation of the early-phase cell division should result in prolongation of liver injury followed by recovery. The studies were designed to evaluate the influence of the antimitotic agent colchicine (1 mg/kg, i.p. in saline) on CCl4 toxicity. Colchicine was administered 2 h prior to CCl4 or corn oil injection. Toxicological end points and markers of hepatocellular regeneration were assessed at various time points (2, 6, 12, 24, 48 and 72 h) after the injection of CCl4 to male Sprague-Dawley rats. Hepatocellular injury was assessed through elevations of serum alanine and aspartate aminotransferase and by histopathological examination of the liver. Incorporation of 3H-thymidine in hepatocellular nuclear DNA and mitotic index were used as indices of hepatocellular regeneration. Hepatocellular regeneration stimulated by CCl4 at 2-6 h was blocked by colchicine as evidenced by the decreased 3H-thymidine incorporation and mitotic index,without any significant effect on the second phase of cell division at 48 h. Ablation of this early phase of tissue repair resulted in prolongation of CCl4 hepatoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Pivotal role of hepatocellular regeneration in the ultimate hepatotoxicity of CCl4 in chlordecone-, mirex-, or phenobarbital-pretreated rats

Our earlier histomorphometric and biochemical studies suggested that the progressive phase of the interactive toxicity of chlordecone (CD) + CCl4 involves suppression of hepatocellular regeneration. The objective of the present work was to correlate hepatocellular regeneration with CCl4 (100 microliters/kg)-induced hepatotoxicity in rats maintained for 15 days on a normal (N) diet, relative to the regenerative response in rats maintained on a diet containing either 10 ppm CD, 225 ppm phenobarbital (PB), or 10 ppm mirex (M). Hepatocellular regeneration was assessed by measuring DNA and 3H-thymidine (3H-T) incorporation, followed by autoradiographic analysis of liver sections. Hepatotoxicity was assessed by measuring plasma transaminases (aspartate and alanine) followed by histopathological observations of liver sections for necrotic, swollen, and lipid-laden cells. Lethality studies were also carried out to assess the consequence of hepatotoxicity on animal survival. Dietary 10 ppm CD potentiated the hepatotoxicity of CCl4 to a greater extent than PB or M, as evidenced by elevations in plasma enzymes. Although the serum enzymes were significantly elevated in PB rats in contrast to the slight elevations in N and M rats, they returned to normal levels by 96 hr. However, serum enzyme elevations in CD rats were progressive with time until death of the animals. Actual liver injury by CCl4 was greater in PB- than in CD-pretreated rats, as evidenced by histopathological observations. A 100% mortality occurred in CD-pretreated rats at 60 hr after CCl4 administration, whereas no mortality occurred in either N-, M-, or PB-pretreated rats, indicating recovery from liver injury. Hepatocellular nuclear DNA levels were significantly decreased starting at 6 hr after CCl4 administration to CD-pretreated rats, but not in M- or PB-pretreated rats. 3H-T incorporation into nuclear DNA as well as percentage of labeled cells showed a biphasic increase in N rats: 1 at 1-2 hr, and the other at 36-48 hr after CCl4 administration. However, only 1 peak of 3H-T incorporation at 36-48 hr was observed in the CD + CCl4 combination, which was also significantly lower when compared to that observed after the M or PB + CCl4 combination treatments. These findings suggest that there is recovery in N-, PB-, or M-pretreated rats from CCl4-induced injury by virtue of the stimulated hepatocellular regeneration and tissue repair.(ABSTRACT TRUNCATED AT 400 WORDS)

Colchicine antimitosis abolishes CCl4 autoprotection

A subtoxic dose of CCl4 is known to destroy liver microsomal cytochrome P-450 and this is widely accepted as the mechanism of CCl4 autoprotection. Circumstantial evidence suggests that while cytochrome P-450 is significantly decreased, this mechanism alone cannot explain the phenomenon of autoprotection. Previous studies have established that hepatocellular regeneration is stimulated as early as 6 hr after the administration of a low dose of CCl4. If the early phase stimulation of hepatocellular regeneration by the protective dose is indeed the mechanism of autoprotection, then ablation of this early phase of tissue healing by colchicine should result in an abolishment of autoprotection. Present studies were conducted to test this conceptual premise. The protection afforded by a low dose of CCl4 (LCCl4, 100 microliter/kg, ip) on the toxic effects of a subsequently administered moderately toxic dose of CCl4 (HCCL4, 2.5 ml/kg, ip) was established in male Sprague-Dawley rats. The protective dose provided 100% protection, whereas only 62.5% survival was observed when the corn oil vehicle was administered instead of the protective dose of LCCl4. Colchicine administration (1 mg/kg, ip in saline) 2 hr prior to the injection of LCCl4 led to a complete loss of autoprotection resulting in 100% mortality in rats given the HCCl4. Earlier studies have established that colchicine selectively suppresses the early phase of hepatocellular regeneration at 6 hr without influencing the second phase at 36-48 hr. The consequence of colchicine antimitosis on the toxicological endpoints of liver injury was evaluated by serum enzyme elevations and by histopathological examination of the liver during a time course of 6, 24, 48, 72, and 96 hr after the administration of HCCl4. In the autoprotection regime, after only a transient and modest elevation of serum alanine and aspartate transaminases, complete recovery occurred by 96 hr. Hepatocellular necrosis was consistently lower compared to all other groups. Colchicine preadministration in the autoprotection regime resulted in significantly greater and progressive elevation of the serum enzymes and a correspondingly commensurate progression of hepatic lesion. Toxic effects of HCCl4 alone were more rapidly and maximally augmented by colchicine preadministration. The role of hepatocellular regeneration in autoprotection was evaluated by 3H-thymidine incorporation in hepatocellular nuclear DNA and by morphometric estimation of mitotic index. While HCCl4 alone resulted in some stimulation of 3H-thymidine incorporation and mitosis, the regenerative activity observed with prior LCCl4 administration was remarkably greater, particularly at 48 hr. Colchicine preadministration in either of these 2 protocols decisively obtunded the stimulated regenerative activities essentially abolishing the tissue healing mechanisms.(ABSTRACT TRUNCATED AT 400 WORDS)

Potentiation of BrCCl3 hepatotoxicity by chlordecone: biochemical and ultrastructural study

Previous work has established that chlordecone (CD) potentiates the hepatotoxicity of BrCCl3. This interaction occurs at nontoxic levels of CD and BrCCl3. The present research was designed to investigate the mechanism governing the pathogenesis of potentiated hepatic injury and lethality induced by a low dose of BrCCl3 after dietary pretreatment with 10 ppm of CD for 15 days. On Day 16, a single dose of BrCCl3 (30 microliters/kg) was administered ip to rats maintained either on normal diet (ND) or on a diet contaminated with 10 ppm CD. Blood and liver samples were collected at 0, 3, 6, 12, 24, 36, and 48 hr after the halomethane administration for biochemical (ATP, bilirubin, glycogen) and for ultrastructural studies. A continuous increase in serum bilirubin and decrease in hepatic ATP and glycogen were observed in CD + BrCCl3 combination, indicating progressive injury, but not in other treatment groups. In ND + BrCCl3 combination, all biochemical indices were either normal or close to normal after 36 hr, suggesting complete recovery from hepatotoxicity. The most extensive ultrastructural changes characteristic of halomethane hepatotoxicity (necrosis, ballooned cells, and dilation of rough endoplasmic reticulum) were observed after the CD + BrCCl3 combination treatment. The progressive and early depletion of hepatic ATP and glycogen, and the progressive increase in toxicity along with decreased cell division in CD + BrCCl3-treated rats, indicate the association of compromised energy status and suppression of cell division and tissue repair in CD-potentiated BrCCl3 toxicity. These findings suggest that the suppression of stimulated hepatocellular regeneration results in the loss of the essential mechanism of tissue repair leading to continuation of the toxic liver injury associated with the CD + BrCCl3 combination treatment.

Protection from CCl4 toxicity by prestimulation of hepatocellular regeneration in partially hepatectomized gerbils

The present investigation was undertaken to test our hypothesis that the slow responses of hepatocellular regeneration and tissue repair after CCl4-induced liver injury are responsible for the high sensitivity of gerbils to the hepatotoxic and lethal effects of CCl4. These studies were conducted in normal and actively regenerating livers using male gerbils 5 or 15 days after partial (2/3) hepatectomy (PH5 and PH15, respectively), or those undergoing sham operation (SH). An LD50 dose of CCl4 (80 microL/kg, i.p.) resulted in a mortality (21%) significantly (P less than 0.05) less than 50% in PH5 gerbils 48 hr after CCl4 administration, whereas the mortality observed in PH15 or SH gerbils was not significantly different from 50%. The elevations of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels were significantly (P less than 0.05) less in PH5 gerbils than in PH15 or SH groups after the administration of either the LD50 dose or a low dose (15 microL/kg) of CCl4. Histopathological and histomorphometric examinations also indicated that CCl4-induced liver injury was less severe in PH5 gerbils than in the PH15 and SH groups. The hepatic microsomal cytochrome P450 content measured before CCl4 administration in the PH5 gerbils was decreased (26%) significantly (P less than 0.05) as compared with the SH group, but was not significantly different from that of PH15 gerbils. In vivo metabolism of 14CCl4 and lipid peroxidation in liver tissue were not significantly different among the various groups. Therefore, the protection against CCl4 toxicity observed in PH5 gerbils is unlikely to be due to decreased bioactivation of CCl4 or lipid peroxidation in that group. [3H]Thymidine incorporation into hepatocellular nuclear DNA was 4- to 5-fold higher in PH5 gerbils than in the PH15 and SH groups, indicating active hepatocellular proliferation in PH5 gerbils. [3H]Thymidine incorporation was further increased significantly (P less than 0.05) 24 hr after challenge with a low dose of CCl4 in PH5 gerbils, whereas it remained low until 48 hr after the CCl4 injection in the PH15 or SH group. The protection against CCl4 toxicity afforded by partial hepatectomy was closely associated with active hepatocellular regeneration. The overall results confirm the concept that the high sensitivity of gerbils to CCl4 is due to very sluggish hepatocellular regeneration and tissue repair response to the CCl4-induced liver injury.

Hepatotoxicity and lethality of halomethanes in Mongolian gerbils pretreated with chlordecone, phenobarbital or mirex

The hepatotoxic and lethal effects of CBrCl3, CCl4 and CHCl3 were investigated in gerbils with or without prior exposure to dietary chlordecone (CD), phenobarbital (PB) and mirex (MX) at 10, 225 and 10 ppm, respectively, for 15 days. Gerbils were quite sensitive to these halomethanes (48 h LD50: 20, 80 and 400 microliters/kg, respectively). CD, known to potentiate hepatotoxic and lethal effects of halomethanes in rats, failed to potentiate the toxic effects of any of these three halomethanes in gerbils. PB and MX were also ineffective. Since stimulation of early hepatocellular regeneration has been shown to be responsible for the recovery from the toxicity of a low dose of CCl4, liver cell regeneration and tissue repair were studied in gerbils after CCl4 administration. The objectives of these studies were to investigate the possible reasons for the high sensitivity of gerbils to halomethane toxicity and to investigate the mechanism for their refractoriness to CD-potentiated halomethane toxicity. A low and a high dose of CCl4 (15 and 80 microliters/kg, i.p. respectively) were used to study the time-course of liver injury in gerbils pretreated with or without CD. The low dose of CCl4 stimulated cellular regeneration as indicated by the increase of 3H-thymidine (3H-T) incorporation in hepatic nuclear DNA. The cellular regeneration and tissue repair activities resulted in complete recovery from the limited liver injury in both CD-pretreated and control gerbils. In contrast to rats, however, the process of cell division in gerbils occurred much later, 2 days after CCl4 administration. Evidence from histomorphometric studies was consistent with serum enzyme and 3H-T incorporation data.(ABSTRACT TRUNCATED AT 250 WORDS)

Protection from chlordecone-amplified carbon tetrachloride toxicity by cyanidanol: biochemical and histological studies

Chlordecone (CD) pretreatment is well known to greatly potentiate CCl4 toxicity. Previous work has shown that suppression of hepatocellular regeneration permits an ordinarily limited liver injury to progress in an irreversible manner. Insufficient hepatocellular energy has been proposed as a mechanism for suppressed hepatocellular regeneration. Since cyanidanol reportedly increases cellular ATP, this compound was employed to test the above hypothesis. The present study was designed to investigate the sequential biochemical and histological changes over a time course of 120 hr after CCl4 administration. Male Sprague-Dawley rats (125-150 g) were maintained on 10 ppm CD diet for 15 days and were challenged with either a standard protocol dose (100 microliters/kg) or a low (50 microliters/kg, L) dose of CCl4. Cyanidanol pretreatment at 48, 24, and 2 hr before CCl4 administration to rats maintained on CD diet resulted in 100 or 70% animal survival, for CCl4 (L) or the standard dose of CCl4, respectively. Preliminary studies indicated that neither simultaneous nor subsequent administration of cyanidanol with CCl4 challenge affords such protection. Prior treatment with cyanidanol and a latency period were found necessary for protection. Without cyanidanol, CD + CCl4 combination caused 50 and 100% lethality after CCl4 (L) and the standard dose, respectively, while the same doses of CCl4 alone did not cause lethal effects. Plasma enzymes (alanine aminotransferase, aspartate aminotransferase, sorbitol dehydrogenase) in control rats showed only moderate and transient increases after CCl4 challenge. The combination of CD + standard dose of CCl4 resulted in progressive and marked elevations of all three serum enzymes at all time intervals until the death of animals. Cyanidanol pretreatment resulted in significant decline in the plasma enzyme elevations at later time points. Cyanidanol pretreatment increased hepatic ATP synthesis in control or CD rats. CCl4 administration to control rats did not alter hepatic ATP levels, while in CD-fed rats hepatic ATP levels were significantly decreased. Cyanidanol pretreatment to CD + CCl4 combination-treated rats did not significantly prevent the decline in hepatic ATP and glycogen levels. However, in the surviving rats a recovery in these parameters was observed. Light microscopic examination of livers from animals that received CCl4 alone revealed only marginal cellular injury, at early time points only. However, CCl4 challenge to rats maintained on CD resulted in progressive injury, characterized by the appearance of ballooned cells, necrotic cells, and cells with lipid droplets in the liver. Cyanidanol pretreatment to these rats caused decreased vacuolation and significantly reduced the progression of liver necrosis.(ABSTRACT TRUNCATED AT 400 WORDS)

Protection from chlordecone-amplified carbon tetrachloride toxicity by cyanidanol: regeneration studies

Previous work has shown that chlordecone (CD)-amplified CCl4 hepatotoxicity and lethality can be mitigated by pretreatment with cyanidanol. These studies also revealed that stimulated hepatocellular regeneration might play an important role in the cyanidanol protection of CD-amplified CCl4 toxicity. The present studies conducted over a time course of 0 to 120 hr after CCl4 challenge describe sequential changes in hepatic [3H]thymidine incorporation into hepatocellular nuclear DNA, polyamines and related enzymes, and histomorphometry of liver sections from variously treated rats. Male Sprague-Dawley rats (125-150 g) were maintained on a control diet or on a diet contaminated with CD (10 ppm) for 15 days and/or pretreated with cyanidanol (250 mg/kg, ip) at 48, 24, and 2 hr before a single ip injection of either a standard protocol dose (100 microliters/kg) or a low dose (50 microliters/kg, L) of CCl4 on Day 16 of the dietary protocol. Cyanidanol pretreatment significantly stimulated the hepatic [3H]thymidine incorporation into hepatocellular nuclear DNA of control rats irrespective of CD pretreatment. Similarly, polyamine metabolism was altered favorably for cell division, although mitotic index (metaphase) was not increased. Cyanidanol-stimulated [3H]thymidine incorporation was highly suppressed in rats receiving the CD + CCl4 standard dose combination treatment up to 36 hr, but after this time point a marked increase was observed. Hepatocellular regeneration, quantified histomorphometrically as volume density of cells in metaphase, was progressively increased in rats protected from CD + CCl4 interaction by cyanidanol, starting at 36 hr and lasting until 72 hr. Favorably altered polyamine metabolism was evident from the stimulated ornithine decarboxylase, as well as from the stimulated interconversion of the higher polyamines to maintain increased concentration of putrescine. Challenge by the same dose of CCl4 (100 microliters/kg) to CD-pretreated rats not protected by cyanidanol failed to cause any increase in [3H]thymidine incorporation up to 36 hr and resulted in animal death starting at 36 hr. In the surviving rats, [3H]thymidine incorporation at 48 hr was increased, but was less than 50% of the increase observed in the cyanidanol group. In these rats, attenuation in the stimulation of cell division and insufficiently increased putrescine levels were observed, which are consistent with the inadequate level of hepatocellular regeneration. With rats receiving CD + CCl4(L) combination, the [3H]thymidine incorporation at 48 hr was less than 50% of the increase of cyanidanol-protected rats. Cyanidanol pretreatment to the CD + CCl4 group of rats prevented the decrease in the hepatic DNA levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Amplification of CCl4 toxicity by chlordecone: destruction of rat hepatic microsomal cytochrome P-450 subpopulation

Previous work has established marked amplification of CCl4 hepatotoxicity by prior exposure to chlordecone (CD). Since CCl4 is toxic by virtue of its bioactivation by the hepatomicrosomal cytochrome P-450 (cyt P-450) system, which is in turn destroyed, our first interest was to determine if cyt P-450 isozymes were selectively destroyed in this interaction. CoCl2 also decreased hepatic P-450 contents, so our other interest was to observe whether CoCl2 selectively decreased or spared CCl4 metabolizing P-450 enzymes. Solubilized hepatic microsomes from variously treated rats were used. The treatment protocol was dietary CD (10 ppm, for 15 d), and CCl4 (100 microliters/kg, ip). The treatments were CD alone, CCl4 alone, CD + CCl4 and with or without CoCl2 (60 mg/kg/d, sc for 2 d) treatment on d 13 and 14 of the dietary protocol. The control group received normal diet and corn oil vehicle. The key mixed-function oxidase (MFO) parameters measured were microsomal protein, cyt P-450 content, and aminopyrine demethylase (APD). Decrease of P-450 levels ranged from 2.2-fold (CD + CCl4) to 1.3-fold (CD + CoCl2). APD activity decreased by 48 and 26.6% in CD + CCl4 and CD + CoCl2 treatments, respectively. Using an anion-exchange high-performance liquid chromatography (HPLC) column, solubilized microsomal hemoproteins were resolved into five peaks. The P-450 content associated with each peak was determined. In CD rats there was slight increase in peak heights, whereas peak heights in CCl4 and control treatments were similar. CoCl2 decreased all peaks, the decrease of peak I being maximal. In CD + CCl4 treatment, absence of peaks II and III was noted. Microsomal proteins stained for heme showed decreased staining intensity of hemo-protein bands, particularly band 4 (MW 52,000), which was absent in CD + CCl4 interaction. These findings suggest that (1) CoCl2 does not selectively decrease or spare any P-450 isozymes and (2) CD + CCl4 interaction does destroy specific P-450 isozymes.

Hepatoprotective agent (+)-cyanidanol increases the synthetic phase of hepatocellular regeneration

1. (+)-Cyanidanol (250 mg/kg) administration to male rats resulted in a concentration-dependent increase in [3H]-thymidine incorporation into hepatic nuclear DNA as well as a corresponding increase in the per cent of labelled cells. 2. The increase in [3H]-thymidine incorporation and per cent labelled cells was significant by 24 hr, maximal between 48 and 96 hr, and declined very slowly to normal by 15 days (360 hr). 3. Administration of (+)-cyanidanol resulted in an increase in heptic putrescine levels and ornithine decarboxylase activity at 6 hr but not at 24 hr. However, S-adenosylmethionine decarboxylase and spermidine acetyltransferase activities were unaltered. 4. Inspite of these favorable conditions, for cell division, mitotic index (per cent cells in metaphase) was not increased by (+)-cyanidanol. 5. These results along with previous findings indicate that (+)-cyanidanol stimulates the S-phase activity of hepatocellular regeneration, but the commitment to M-phase depends on the occurrence of liver injury.

Role of hepatocellular regeneration in CCl4 autoprotection

The destruction of liver microsomal cytochromes P450 by a previously administered low dose of CCl4 has been widely accepted as the mechanism of CCl4 autoprotection. However, circumstantial evidence suggests that this mechanism cannot completely explain the phenomenon of autoprotection. The protective effect of a low dose of CCl4 (0.3 ml/kg, po) on the lethal effect of a subsequently administered high dose (5 ml/kg, po) was established in male Sprague Dawley rats. The protective dose permitted 100% survival, whereas only 15% survival was observed without it. Hepatotoxicity, measured by serum enzyme elevations (aspartate transaminase, alanine transaminase, and sorbitol dehydrogenase) and histopathological changes 24 hr after the treatment with high dose, was similar in both the groups, even though the protective dose had significantly decreased liver microsomal cytochromes P450 (to 62% of normal) and associated enzymes, aminopyrine demethylase and aniline hydroxylase. Rats pretreated with CoCl2 to decrease hepatic microsomal cytochrome P450 to 44% of normal levels did not show a significant protection from the hepatotoxicity of high dose of CCl4. Previous studies have established that hepatocellular regeneration is stimulated within 6 hr after the administration of a low dose of CCl4. Based on this observation, a premise that autoprotection results from augmented recovery from injury rather than decreased injury appears likely. Hence, the role of hepatocellular regeneration was evaluated by following 3H-thymidine incorporation in hepatocellular nuclear DNA, labelling index by autoradiography, and by morphometric estimation of mitotic index. After administration of the protective dose of CCl4, stimulated nuclear DNA synthesis measured by 3H-thymidine incorporation into nuclear DNA was increased and this remained high even after subsequent administration of high dose of CCl4. Forty-eight hr after the administration of a lethal dose of CCl4 alone (5 ml/kg, po), labelling index was slightly increased, but mitotic index was not increased. In the surviving rats (15%), both labelling index and mitotic index were significantly elevated after an additional 24 hr. In rats receiving the protective dose, a significantly greater elevation of labelling index as well as mitotic index occurred 48 hr after the administration of the same lethal dose of CCl4. These results suggest that hepatocellular regeneration stimulated by the protective dose, as a biological response recruited to overcome the accompanying limited injury, may augment and sustain tissue repair processes to permit tissue restoration even after the massive liver injury elicited by the subsequent large dose of CC14.

Potentiation of halomethane hepatotoxicity by chlordecone: a hypothesis for the mechanism

A major toxicological issue today is the possibility of unusual toxicity due to interaction of toxic chemicals upon environmental or occupational exposures to two or more chemicals, at ordinarily harmless levels individually. While some laboratory models exist for such interactions for the simplest case of only two chemicals, progress in this area has suffered for want of a model where the two interactants are individually nontoxic. One such model is available, where prior exposure to nontoxic levels of the pesticide Kepone (chlordecone) results in a 67-fold amplication of CCl4 lethality in rats. Extensive hepatotoxicity observed in this interaction is characterized by histopathological alterations, perturbation of related biochemical parameters and is followed by complete hepatic failure. This propensity for chlordecone to potentiate hepatotoxicity of halomethanes such as CCl4, CHCl3, and BrCCl3 has been a subject of intense study to unravel the underlying mechanism. Mechanisms such as induction of microsomal cytochrome P-450 by chlordecone and greater lipid peroxidation are inadequate to explain the remarkably powerful potentiation of halomethane toxicity. Compelling experimental evidence supports the hypothesis that hepatocellular division during early time points after the administration of CCl4 is an important determinant of the progression (or repair of it) of the liver injury and consequent destruction (or restoration) of the hepatolobular architecture and function. This paper advances a hypothesis for the mechanism of hepatotoxic and lethal effect of CCl4 as being primarily related to the accelerated progression of liver injury due to suppressed hepatocellular regeneration and hepatolobular restoration. This is in contrast to the widely accepted putative mechanism, one which invokes only bioactivation followed by runaway lipid peroxidation as the events determining the course of the progressive phase of liver injury. The concept being advanced in this paper accepts bioactivation (and perhaps lipid peroxidation) as the primary initiating events of cell injury, but maintains that they are not the determinants of the progressive phase of liver injury. The biological issue of whether the cells are incapacitated from regenerating is the determinant of the progression of liver injury, and therefore, the ultimate outcome of hepatotoxicity and lethality.

Altered hepatic energy status in chlordecone (Kepone)-potentiated CCl4 hepatotoxicity

Previous studies have demonstrated that increased intracellular calcium, depletion of glycogen, and suppressed hepatocellular division resulting in progression of hepatic lesion without recovery are associated with chlordecone (CD)-potentiated CCl4 hepatotoxicity. Since these phenomena are indicative of compromised hepatic energy status, the present studies were designed to investigate this possibility. Neither hepatic ATP content nor mitochondrial Mg2(+)-ATPase was altered significantly in rats maintained on diets contaminated with either CD (10 ppm), or phenobarbital (PB; 225 ppm) alone for 15 days. Similarly, CCl4 (100 microL/kg) administration alone did not alter hepatic ATP levels or mitochondrial Mg2(+)-ATPase activity in rats maintained on a normal diet. However, CCl4 administration to CD pretreated rats resulted in significantly decreased hepatic ATP content as early as 1 hr (36%), and this decrease was irreversibly progressive with time (81% at 6 hr). Oligomycin-sensitive Mg2(+)-ATPase was decreased significantly only starting at 6 hr (21%) after CCl4 administration, indicating that depletion of ATP at early time points was most likely due to rapid utilization consequent to toxic events. CCl4 administration to mirex or PB pretreated rats resulted in a smaller decrease in ATP levels (18-24%) only at 24 hr, returning to normal levels by 36-48 hr, in accord with rapid recovery from limited liver injury. These findings indicate that CCl4 administration to CD but not to PB or mirex pretreated rats results in a severely compromised energy status of the liver. The progressive and early depletion of liver ATP and the inhibition of Mg2(+)-ATPase in CD + CCl4 treated rats indicate the association of compromised energy status with altered Ca2+ homeostasis, depletion of glycogen, and suppressed cell division in CD-potentiated CCl4 toxicity.

Bromotrichloromethane hepatotoxicity. The role of stimulated hepatocellular regeneration in recovery: biochemical and histopathological studies in control and chlordecone pretreated male rats

It has been shown that BrCCl3 is a more potent hepatotoxin than CCl4. Pretreatment with nontoxic dietary levels of chlordecone (CD) results in amplification of BrCCl3 hepatotoxicity. The objective of this research was to investigate and compare the histopathological alterations during a time course after a low dose of BrCCl3 alone and in combination with dietary CD. Male Sprague-Dawley rats were maintained on 10 ppm dietary CD or normal diet for 15 days. On day 16, they received a single ip dose (30 microliters/kg) of BrCCl3 in corn oil (CO) vehicle or corn oil alone. Blood and liver samples were collected at 0, 3, 6, 12, 24, 36, 48, 72, 96, and 120 hr for serum enzymes and histopathological examination, respectively. Serum enzymes (SDH, ALT, AST) were significantly (p less than 0.05) elevated in rats receiving the CD + BrCCl3 combination in comparison to BrCCl3 alone. For 48 hr, a continuous increase in serum enzyme activities was detected in rats treated with CD + BrCCl3 combination, but not in the rats receiving other treatments (ND + BrCCl3, ND + CO, or CD + CO). The most extensive hepatolobular necrosis was observed in rats treated with the CD + BrCCl3 combination. Thirty-six hr after the administration of BrCCl3 to rats maintained on normal diet, high mitotic activity was observed, which continued through 72 hr resulting in complete restoration of hepatolobular structure. In contrast, rats receiving the combination of CD + BrCCl3 exhibited minimal and belated hepatomitotic activity for a short period of time, resulting in progressive hepatic failure, culminating in animal death. In conclusion, hepatotoxicity of a low dose of BrCCl3 alone appeared to be overcome via stimulated hepatocellular regeneration and hepatolobular restoration. CD appears to amplify BrCCl3 hepatotoxicity via interference with this hormetic mechanism, permitting a progressive and continued hepatic injury leading to complete hepatic failure, culminating in animal death.

Perturbations in polyamines and related enzymes following chlordecone-potentiated bromotrichloromethane hepatotoxicity

The mechanism by which chlordecone (CD) amplifies the hepatotoxicity of halomethanes such as CCl4, CHCl3, and BrCCl3 has been a subject of intense study. Recent work has shown that suppression of hepatocellular regeneration leads to accelerated progression of liver injury leading to complete hepatic failure due to an unusual interaction between individually nontoxic low-dose combination of CD and CCl4. Since polyamines are involved in cell division, their levels reflect the extent to which there is suppression of hepatocellular regeneration during CD and CCl4 interaction. The present studies were designed to investigate the polyamine levels and associated enzymes in livers of rats treated with BrCCl3 alone or CD and BrCCl3 low-dose combination in order to confirm whether the sequence of events of hepatotoxicity is similar to that seen in CCl4 toxicity or that seen during CD and CCl4 interaction. The extent of liver toxicity in rats fed 10 ppm chlordecone (CD) for 15 days prior to the injection of a single low dose of BrCCl3 (15 microL/kg body weight) or after exposure to a high dose of BrCCl3 (80 microL/kg body weight) without CD pretreatment, was similar 6 and 24 hr later as assessed by plasma transaminase levels. There was also an increase in transaminase levels, in rats exposed to a single low dose of BrCCl3 alone (15 microL/kg body weight) but this increase was far below the high-dose exposure alone or the combination treatment. Hepatic levels of ornithine decarboxylase, S-adenosylmethionine decarboxylase, N1-acetylputrescine, N1-acetylspermidine, putrescine, spermidine, and spermine at the end of 24 hr increased after exposure to a low dose of BrCCl3 alone as compared to exposure to a high dose alone or the low-dose combination of CD and BrCCl3. Liver spermidine N1-acetyltransferase was elevated at 2, 6, and 24 hr after exposure to a high dose of BrCCl3 alone as compared to treatment with a low-dose combination of CD and BrCCl3 suggesting decreased synthesis of this enzyme, in spite of a greater need as seen from liver transaminase levels. In general, it was observed that there is significant elevation in some polyamines and related enzymes during toxicity of a low dose of BrCCl3 which seemed to stabilize within 24 hr. This was not observed with the other two groups of rats exposed either to BrCCl3 high dose alone or the low-dose combination of CD and BrCCl3.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of the lethal interaction of chlordecone and CCl4 at non-toxic doses

There is significant interest in the possibility of unusual toxicity due to interaction of toxic chemicals upon environmental or occupational exposures even though such exposures may involve levels ordinarily considered harmless individually. While many laboratory and experimental models exist for such interactions, progress in this area of toxicology has suffered for want of a model where the interactants are individually non-toxic. We developed such a model where prior exposure to non-toxic levels of the pesticide Kepone (chlordecone) results in a 67-fold amplification of CCl4 lethality in experimental animals. The mechanism(s) by which chlordecone amplifies the hepatotoxicity of halomethanes such as CCl4, CHCl3, and BrCCl3 has been a subject of intense study. The biological effects of this interaction include extensive hepatotoxicity characterized by histopathological alterations, hepatic dysfunction, and perturbation of related biochemical parameters. Close structural analogs of chlordecone such as mirex and photomirex do not share the propensity of chlordecone to potentiate halomethane toxicity. Mechanisms such as induction of microsomal cytochrome P-450 by chlordecone and greater lipid peroxidation are inadequate to explain the remarkably powerful potentiation of toxicity and lethality. Time-course studies in which liver tissue was examined 1-36 h after CCl4 administration were conducted. While animals receiving a normally nontoxic dose of CCl4 alone show limited hepatocellular necrosis by 6 h, proceeding to greater injury after 12 h, recovery phase ensues as revealed by greatly increased number of mitotic figures. Such regeneration and hepatic tissue repair processes are totally suppressed in animals exposed to chlordecone prior to CCl4. Thus, the arrested hepatocellular repair and renovation play a key role in the potentiation of CCl4 liver injury by chlordecone. These findings have allowed us to propose a novel hypothesis for the mechanism of chlordecone amplification of halomethane toxicity and lethality. While limited injury is initiated by the low dose of CCl4 by bioactivation followed by lipid peroxidation, this normally recoverable injury permissively progresses due to arrested hepatocellular regeneration and tissue repair processes. Recent studies designed to test this hypothesis have provided additional supporting evidence. Hepatocellular regeneration stimulated by partial hepatectomy was unaffected by 10 ppm dietary chlordecone, while these animals were protected from the hepatotoxic and lethal actions of CCl4 if administered at the time of maximal hepatocellular regeneration. The protection was abolished when CCl4 was administered upon cessation of hepatocellular regeneration.

Chlordecone (Kepone)-potentiated carbon tetrachloride hepatotoxicity in partially hepatectomized rats--a histomorphometric study

Our previous studies indicated the involvement of some unidentified mechanisms, apart from the bioactivation phenomenon, in chlordecone (CD)-potentiated CCl4 hepatotoxicity and lethality. Recent studies revealed that hepatocellular regeneration is suppressed in CD + CCl4 toxicity. The present work is a continuation of our earlier work employing a partial hepatectomy model for stimulating hepatocellular division, in normal (N) or CD-treated (10 ppm for 15 days) rats. Male Sprague-Dawley rats maintained on an appropriate dietary protocol and undergoing sham (SH) or partial hepatectomies (PH) were employed. Hepatocellular regeneration was assessed by measuring the percentage mitotic figures and by autoradiography of liver sections from rats given 3H-thymidine in vivo. Hepatotoxicity was assessed by examining liver sections for necrotic cells, swollen cells and cells having lipid droplets. CCl4 (100 microliters kg-1)-induced histopathological alterations in CD-pretreated rats were significantly decreased in rats 2 days post-PH (PH2) as compared to SH rats or rats 7 days post-PH (PH7), indicating that amplification of CCl4 toxicity is significantly reduced when there is a greater regenerative activity. The percentage of mitoses as well as the percentage of labelled cells were significantly elevated at 2-6 h after CCl4 administration in N rats but remained suppressed in CD rats. In CD-pretreated PH2 rats where the percentage of mitoses and the percentage of labelled cells were many-fold greater when compared to SH or PH7 rats, a portion of the stimulated hepatocellular division decreased significantly at 2-6 h after CCl4 administration, but remained significantly greater when compared to basal level of regeneration.(ABSTRACT TRUNCATED AT 250 WORDS)

Protective role of fructose 1,6-bisphosphate during CCl4 hepatotoxicity in rats

Rats were injected intraperitoneally with CCl4 (2.5 ml/kg body wt.) and the hepatotoxicity was compared with that of rats receiving the same dose of CCl4 and an intraperitoneal injection of fructose 1,6-bisphosphate (2 g/kg body wt.). A 50-70% decrease in plasma aspartate aminotransferase and alanine aminotransferase activities was observed in the latter treatment, indicating a protective role of the sugar bisphosphate in CCl4 hepatotoxicity. The protection was accompanied by elevated hepatic activities of ornithine decarboxylase at 2, 6 and 24 h, S-adenosylmethionine decarboxylase at 6 h, and spermidine N1-acetyltransferase at 2 h. The increase in the enzymes involved in polyamine metabolism was shown in our previous work [Rao, Young & Mehendale (1989) J. Biochem. Toxicol. 4, 55-63] to correlate with increased polyamine synthesis or interconversion, which was related to the extent of hepatocellular regeneration. The hepatic contents of fructose 1,6-bisphosphate and ATP significantly decreased after CCl4 treatment, and administration of the sugar bisphosphate increased hepatic ATP. Fructose 1,6-bisphosphate, an intermediary metabolite of the glycolytic pathway, may decrease CCl4 toxicity by increasing the ATP in the hepatocytes. The ATP generated is useful for hepatocellular regeneration and tissue repair, events which enable the liver to overcome CCl4 injury.

Role of hepatocellular regeneration in chlordecone potentiated hepatotoxicity of carbon tetrachloride

Previous histomorphometric studies led us to hypothesize that suppression of hepatocellular regeneration and the repair of the hepatolobular architecture was involved besides bioactivation phenomenon in the progressive and irreversible phase of toxicity resulting from CD + CCl4 interaction. We have recently observed significant protection from CD potentiated CCl4 toxicity in animals which are stimulated for active hepatocellular regeneration. The present work is an extension of our earlier histomorphometric investigation, taking 3H-thymidine (3H-T) incorporation as a biochemical parameter to assess hepatocellular regeneration followed by autoradiographic analysis of liver sections in normal (N) or chlordecone (CD) treated (10 ppm in diet for 15 days) male rats undergoing sham (SH) or partial hepatectomies (PH). Initial experiments established that in normal (N) rats, greatest 3H-T incorporation into hepatocellular nuclear DNA occurs at 2 days post-PH which returns to basal levels by 7 days. CD treatment alone did not change this phenomenon. 3H-T incorporation into nuclear DNA and the percentage of labelled cells as evidenced by autoradiography of liver sections were significantly elevated in N rats at 1-2 h after CCl4 (100 microliters/kg) administration and returned to basal level by 6 h. Serum enzymes (AST and ALT) in N rats undergoing SH and PH were not altered, but were significantly elevated in CD rats following CCl4 (100 microliters/kg) administration. CCl4-induced serum enzyme elevations were significantly lower in 2 days post-PH (PH2) rats when compared to SH rats or 7 days post-PH (PH7) rats maintained on CD diet, indicating that CD potentiated CCl4 hepatotoxicity is significantly reduced in livers stimulated for regenerative activity by PH.(ABSTRACT TRUNCATED AT 250 WORDS)

The time course of liver injury and [3H]thymidine incorporation in chlordecone-potentiated CHCl3 hepatotoxicity

The mechanism by which chlordecone (CD) potentiates CHCl3 hepatotoxicity and lethality remains unknown. We examined the time course of the hepatotoxicity by following serum enzymes, liver histopathology, hepatocellular regeneration, and tissue repair by morphometric analysis and [3H]thymidine (3H-T) incorporation into nuclear DNA. Male mice fed control, or CD (10 ppm), mirex (Mx. 10 ppm), or phenobarbital (PB. 225 ppm) diets for 15 days and receiving a single ip dose of 0.1 ml CHCl3/kg in corn oil vehicle were used. Liver damage was assessed by plasma alanine and aspartate transaminases and by histopathology at 4, 12, 24, 36, 48, 72, and 96 hr after CHCl3 administration. None of the dietary pretreatments caused plasma transaminase elevations, any liver necrosis, or any increase in 3H-T incorporation in nuclear DNA at any time. CHCl3 alone caused only limited hepatocellular necrosis without any increase in plasma transaminases. The same dose of CHCl3 given to CD-pretreated mice resulted in greatly increased liver injury. Plasma transaminases were elevated starting at 4 hr, reaching a maximum value at 12 hr and a decline starting at 48 hr. Centrilobular and midzonal necroses were evident at 12 hr onward. PB pretreatment caused some increase in CHCl3-induced necrosis and a moderate rise in transaminases at 24 hr, but Mx pretreatment caused neither effect. 3H-T incorporation was increased at 72 and 96 hr after CHCl3 alone. The same dose of CHCl3 caused only a modest increase in PB and Mx and a significant and maximal biphasic increase at 36 and 72 hr CD-pretreated mice. Morphometry of liver sections indicated that hepatocellular regeneration is stimulated at 72 hr after CHCl3 alone. The same dose of CHCl3 results in a greater stimulation of hepatocellular regeneration in CD-pretreated mice, and this event is pushed forward at 48 hr, continuing through 96 hr to compensate for greater hepatocellular necrosis associated with this treatment. Lesser stimulation of hepatocellular regeneration was observed in PB + CHCl3 and Mx + CHCl3 groups of mice consonance with much lesser hepatotoxicity. These results suggest that the critical decisive event in the recovery from limited hepatocellular injury is the hepatocellular regeneration and tissue repair, which appear to be stimulated in proportion to the injury.

Protection of hepatotoxic and lethal effects of CCl4 by partial hepatectomy

CCl4 is a hepatotoxic haloalkane, capable of producing hepatocellular fatty degeneration and centrilobular necrosis. Previous reports indicate induction of liver regeneration after 36-48 hr of CCl4 treatment, which is considered as a secondary effect. The present investigation was undertaken to evaluate the primary effects of CCl4 on hepatic DNA synthesis and to correlate liver regeneration with CCl4 toxicity. These studies were conducted in normal and actively regenerating livers using male Sprague-Dawley rats undergoing sham operation (SH), or partial (70%) hepatectomy (PH). Incorporation of 3H-thymidine (3H-T) in hepatocellular nuclear DNA and autoradiographic analyses of liver sections served as indices for hepatocellular regeneration. Initial experiments established that peak regeneration occurs at 2 days post-PH (PH2) and liver regeneration phases out by 7 days post-PH (PH7). SH and PH rats were challenged with a single ip dose of either corn oil vehicle or CCl4 at either 0.1 ml/kg (to represent subtoxic dose) or 2.5 ml/kg (to represent toxic dose). The low dose of CCl4 was not toxic and did not alter 3H-T incorporation and percentage labelled cells at 6 or 24 hours after administration to SH, PH2 or PH7 groups, indicating that there was no interference with PH-stimulated hepatocellular regeneration. The high dose of CCl4 was significantly hepatotoxic and lethal in SH rats, while in PH2 rats both hepatotoxic and lethal effects were significantly decreased. 3H-T incorporation as well as percentage labelled cells, highly stimulated by PH, were significantly decreased by high dose of CCl4. However, hepatocellular regeneration in PH2 rats treated with high dose of CCl4 was still significantly higher than SH or PH7 groups by virtue of the stronger stimulatory effect of PH. In PH7 rats, where hepatocellular regeneration had returned to the SH level, the hepatotoxic and lethal effects of the large dose of CCl4 were also restored. These findings show that the progressive phase of a single high dose of CCl4 injury which normally culminates in hepatotoxic and lethal effects is significantly mitigated by previously stimulated hepatocellular regeneration. High dose of CCl4 suppresses hepatocellular regeneration at early time points after administration in contrast to the smaller subtoxic dose of CCl4. By virtue of the much stronger stimulatory effect, PH results in the protection against the hepatotoxic and lethal effects of CCl4 despite the obtunding effects of the high dose on hepatocellular regeneration.

Carbon tetrachloride metabolism in partially hepatectomized and sham-operated rats pre-exposed to chlordecone (Kepone)

The potentiation of CCl4 toxicity by pre-exposure to chlordecone (CD) is well established. Chlordecone-induced metabolism of CCl4 and suppressed hepatocellular repair have been offered as possible mechanisms for this potentiation. Recent work using the partially hepatectomized (PH) rat as a model for an actively regenerating liver has provided supportive evidence for the latter hypothesis. The present study was initiated to determine if metabolism and disposition of 14CC14 is altered in the PH rat, and if this is a contributing factor to the reported protective effect afforded by the PH procedure. Male Sprague-Dawley rats (150-175 g) maintained on dietary CD (10 ppm) for 15 days were partially hepatectomized or sham-operated (SH) on day 15. Another group of CD-pretreated rats received 0.9% CoCl2 (60 mg/kg, sc, qd for 2 days) in lieu of the surgical procedure. On day 16 the rats were challenged with a single dose of CCl4 (100 microL/kg, ip) containing 20 muCi 14CCl4. A radiolabel inventory consisting of exhaled 14CCl4, 14CO2 production, total hepatic 14C, free 14CCl4 and covalently bound 14C was taken over a 6-hr time period. Lipid peroxidation and serum enzyme activities [aspartate aminotransferase (AST) and alanine aminotransferase (ALT)] were measured in indices of toxicity. Neither CD pretreatment alone nor CoCl2 treatment alone produced significant alterations in metabolism of low dose (100 microliters/kg) CCl4. No significant difference in 14CCl4 recovery or 14CO2 production was detected for PH versus SH rats. Hepatic 14CCl4-derived 14C (per gram tissue) was greater in PH rats. Values for free 14CCl4, covalently bound 14C, and lipid peroxidation were similar for SH and PH rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic polyamines and related enzymes following chlordecone-potentiated carbon tetrachloride toxicity in rats

Chlordecone potentiation of the hepatotoxic and lethal effects of CCL4 has been well established. Recent studies have shown that the suppression of hepatocellular regeneration results in an accelerated progression of liver injury leading to complete hepatic failure. Since polyamines are involved in cell division, these studies were designed to investigate the polyamine levels and associated enzymes in the livers of rats treated with a low-dose combination of CD and CCl4. For comparison, a large toxic dose of CCl4 was also employed. The extent of liver toxicity in rats fed 10 parts per million chlordecone (CD) for 15 days and subsequently injected with a single dose of CCl4 (100 microL/kg body weight) or a high dose of CCL4 alone (2.5 mL/kg body weight) was similar 6 and 24 hr later as assessed by plasma transaminase levels. There was significant elevation in liver ornithine decarboxylase, S-adenosylmethionine decarboxylase, and putrescine at 24 hr and spermidine N1-acetyltransferase, N1-acetylputrescine, putreanine, putrescine, and N1-acetylspermidine at 6 hr in rats treated with the high dose of CCl4 alone compared to the combination treatment. Spermidine levels decreased up to 6 hr and then increased up to 24 hr for both treatments. Spermine continuously decreased up to 24 hr for the CD and CCl4 low-dose combination treatment compared to rats treated with a high dose of CCl4 alone. Spermidine levels were lower than in controls and rose towards control value between 6 and 24 hr after the combination treatment and the high dose of CCl4. Results indicate that the CD and CCl4 low-dose combination treatment increased liver toxicity, resulting in compromised polyamine metabolism that is coincidental with suppressed hepatocellular regeneration, which leads to an accelerated progressive phase of liver injury and culminates in complete hepatic failure.

Protection from chlordecone (Kepone)-potentiated CCl4 hepatotoxicity in rats by fructose 1,6-diphosphate

1. The extent of liver injury assessed as elevation of plasma transaminases was decreased 40-50% by administration of fructose 1,6-diphosphate to rats receiving the highly hepatotoxic combination of chlordecone and CCl4. 2. This protection was accompanied by significantly higher sustenance of ATP levels in the liver. 3. Polyamine synthesis as well as interconversion were stimulated in favor of maintaining higher levels of polyamines. 4. These events are consistent with the concept that suppressed hepatocellular regeneration which leads to progression of otherwise limited injury observed in chlordecone potentiation of CCl4 hepatotoxicity is due to lack of cellular energy.