G2 subpopulation in rat liver induced into mitosis by low-level exposure to carbon tetrachloride: an adaptive response

The Dose Determines the Stimulation (and Poison): Development of A Chemical Hormesis Database

A comprehensive effort was undertaken to identify articles demonstrating chemical hormesis. Nearly 4000 potentially relevant articles were retrieved from preliminary computer searches utilizing various keyword descriptors and extensive cross-referencing. A priori evaluation criteria were established including study design features (e.g., number of doses, dose range), statistical analysis, and reproducibility of results. Evidence of chemical hormesis was judged to have occurred in approximately 350 of the 4000 studies evaluated. Chemical hormesis was observed in a wide range of taxonomic groups and involved agents representing highly diverse chemical classes, many of potential environmental relevance. Numerous biologic endpoints were assessed, with growth responses the most prevalent, followed by metabolic effects, longevity, reproductive responses, and survival. Hormetic responses were generally observed to be of limited magnitude with the average low-dose maximum stimulation approximately 50% greater than controls. The hormetic dose-response range was generally limited to about one order of magnitude with the upper end of the hormetic curve approaching the estimated no-observed-effect level (NOEL) for the particular endpoint. Based on the evaluation criteria, high to moderate evidence of hormesis was observed in studies comprised of ≥ doses with <3 doses in the hormetic zone. The present analysis suggests that chem ical hormesis is a reproducible and generalizable biologic phenomenon. Over the last decade advances have been made providing mechanistic insight helpful in explaining the phenomenon of chemical hormesis in multiple biologic systems with various endpoints. The reason for the uncertainty surrounding the existence of hormesis as a “real phenomenon” is believed to be the result of its relatively infrequent observation in the literature due to experimental design considerations, especially with respect to the number of doses, range of doses, and endpoint selection.

EXTL2 controls liver regeneration and aortic calcification through xylose kinase-dependent regulation of glycosaminoglycan biosynthesis

The gene products of two members of the EXT gene family, EXT1 and EXT2, function together as a polymerase in the biosynthesis of heparan sulfate. EXTL2, one of the three EXT-like genes in the human genome that are homologous to EXT1 and EXT2, encodes an N-acetylhexosaminyltransferase. However, both the role of EXTL2 in glycosaminoglycan (GAG) biosynthesis and the biological significance of EXTL2 remain unclear. Interestingly, EXTL2 can transfer a GlcNAc residue to the tetrasaccharide linkage region when this region is phosphorylated by a xylose kinase 1 (FAM20B) and thereby terminate chain elongation. Production of GAGs was significantly higher in EXTL2-knockout mice than in wild-type mice. EXTL2-knockout mice are viable and apparently healthy during development and after birth. Therefore, EXTL2-knockout mice were analyzed following the experimental induction of two separate pathological conditions. Carbon tetrachloride (CCl4) was used to induce liver failure, and 5/6th nephrectomy in combination with a high-phosphate diet was used to induce chronic kidney disease (CKD). Under conditions of CCl4-induced liver failure, hepatocyte proliferation following CCl4 treatment was lower in EXTL2-knockout mice than in wild-type mice; consequently, liver regeneration was impaired in EXTL2-knockout mice. This reduction in hepatocyte proliferation resulted partially because EXTL2-knockout mice experienced less hepatocyte-growth-factor-mediated signaling than did wild-type mice. Under conditions of induced CKD, matrix mineralization in vascular smooth muscle cells (VSMCs) in aortic rings of EXTL2-knockout mice was enhanced relative to that in wild-type mice. Altered biosynthesis of GAGs in EXTL2-knockout mice affected bone-morphogenetic-protein signaling, and consequently enhanced the differentiation of VSMCs into osteoblasts. Taken together, these results indicated that the EXTL2-dependent mechanism that regulates GAG biosynthesis is important for the maintenance of tissue homeostasis under pathological conditions, that is, lack of EXTL2 causes GAG overproduction and structural changes of GAGs associated with pathological processes.

Roles of EXTL2, a member of the EXT family of tumour suppressors, in liver injury and regeneration processes

The gene products of two members of the EXT (exostosin) gene family, EXT1 and EXT2, function together as a polymerase in the biosynthesis of heparan sulfate. EXTL2 (EXT-like 2), one of the three EXTL genes in the human genome that are homologous to EXT1 and EXT2, encodes an N-acetylhexosaminyltransferase. We have demonstrated that EXTL2 terminates chain elongation of GAGs (glycosaminoglycans), and thereby regulates GAG biosynthesis. The abnormal GAG biosynthesis caused by loss of EXTL2 had no effect on normal development or normal adult homoeostasis. Therefore we examined the role of EXTL2 in CCl4 (carbon tetrachloride)-induced liver failure, a model of liver disease. On the fifth day after CCl4 administration, the liver/body weight ratio was significantly smaller for EXTL2-knockout mice than for wild-type mice. Consistent with this observation, hepatocyte proliferation following CCl4 treatment was lower in EXTL2-knockout mice than in wild-type mice. EXTL2-knockout mice experienced less HGF (hepatocyte growth factor)-mediated signalling than wild-type mice specifically because GAG synthesis was altered in these mutant mice. In addition, GAG synthesis in hepatic stellate cells was up-regulated during liver repair in EXTL2-knockout mice. Taken together, the results of the present study indicated that EXTL2-mediated regulation of GAG synthesis was important to the tissue regeneration processes that follow liver injury.

Tolerance of aged Fischer 344 rats against chlordecone-amplified carbon tetrachloride toxicity

We have investigated the effects of chlordecone 1(CD)+CCl4 combination in adult (3 months), middle aged (14 months), and old aged (24 months) male Fischer 344 (F344) rats. After a non-toxic dietary regimen of CD (10 ppm) or normal powdered diet for 15 days, rats received a single non-toxic dose of CCl4 (100 microl/kg, i.p., 1:4 in corn oil) or corn oil (500 microl/kg, i.p.) alone on day 16. Liver injury was assessed by plasma ALT, AST, and histopathology during a time course of 0-96 h. Liver tissue repair was measured by [3H-CH3]-thymidine (3H-T) incorporation into hepatic nuclear DNA and proliferating cell nuclear antigen (PCNA) immunohistochemistry. Hepatomicrosomal CYP2E1 protein, enzyme activity, and covalent binding of 14CCl4-derived radiolabel were measured in normal and CD fed rats. Exposure to CCl4 alone caused modest liver injury only in 14- and 24-month-old rats but neither progression of injury nor mortality. The CD+CCl4 combination led to 100% mortality in 3-month-old rats by 72 h, whereas none of the 14- and 24-month-old rats died. Both 3- and 14-month-old rats exposed to CD+Cl4 had identical liver injury up to 36 h indicating that bioactivation-mediated CCl4 injury was the same in the two age groups. Thereafter, liver injury escalated only in 3-month-old while it declined in 14-month-old rats. In 24-month-old rats initial liver injury at 6 h was similar to the 3- and 14-month-old rats and thereafter did not develop to the level of the other two age groups, recovering from injury by 96 h as in the 14-month-old rats. Neither hepatomicrosomal CYP2E1 protein nor the associated p-nitrophenol hydroxylase activity or covalent binding of 14CCl4-derived radiolabel to liver tissue differed between the age groups or diet regimens 2 h after the administration of 14CCl4. Compensatory liver tissue repair (3H-T, PCNA) was prompt and robust soon after CCl4 liver injury in the 14- and 24-month-old rats. In stark contrast, in the 3-month-old rats it failed allowing unabated progression of liver injury. These findings suggest that stimulation of early onset and robust liver tissue repair rescue the 14- and 24-month-old F344 rats from the lethal effect of the CD+CCl4 combination.

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.

Modulation of gene and protein expression by carbon tetrachloride in the rat liver

The gene and protein expression changes after exposure to a toxic compound might help elucidate its mechanism of action. In this paper we investigated the effect of carbon tetrachloride (CCl(4)) on the gene and protein expression in rat livers. Adult Wistar rats were administered CCl(4) and livers were harvested 6 or 24 h thereafter. The analysis of mitochondrial proteins on 2D gels showed the upregulation of two proteins involved in stress (catalase and uricase). Among the downregulated proteins, enzymes related to the metabolism of lipids and aminoacids were affected. Additionally, alpha-2-macroglobulin and senescence marker protein, two proteins whose decrease in expression has been connected to hepatocyte damage, were decreased. Several of the upregulated genes are involved in stress response, DNA and protein damage, and repair. Genes coding for several enzymes involved in different metabolic pathways, including some P450, were downregulated in the treated animals. In conclusion, a single dose of CCl(4) caused gene and protein expression changes that can be related to its mechanism of toxicity. Results from both technologies support previous publications and provide possible new toxicity markers. However, the correlation between gene and protein expression at a given time point is less apparent, partly as a result of different regulatory mechanisms between gene and protein expression.

Decrease of platelet-endothelial cell adhesion molecule 1-gene-expression in inflammatory cells and in endothelial cells in the rat liver following CCl(4)-administration and in vitro after treatment with TNFalpha

Platelet-endothelial cell adhesion molecule (PECAM-1), a member of the Ig superfamily is strongly expressed at endothelial cell-cell junctions, on most leukocytes and on monocytes. PECAM-1 has been implicated as a key mediator of the transendothelial migration of leukocytes and monocytes. To further define the physiological role of PECAM-1, we studied the modulation of PECAM-1-expression in a model of liver inflammation in both mononuclear cells (MCs) and sinusoidal endothelial cells (SECs). In normal rat liver sections, PECAM-1 immunohistology indicated a sinusoidal pattern similar to the ICAM-1 staining. Both, SECs, small and large MCs isolated from control rats express PECAM-1 as demonstrated by immunocytochemistry, flow cytometry, and Northern blot analysis. Immunohistochemical studies on liver sections from CCl(4)-treated animals indicated, that in the areas of necrosis 24-48 h after a single administration of the toxin, there was an accumulation of LFA-1-, ED1- (marker for rat monocytes) and ICAM-1-positive, but ED2-(marker for tissue macrophages)-negative inflammatory cells. Most of these cells were PECAM-1-negative. In situ hybridization indicated that there is no accumulation of PECAM-1 specific transcripts after CCl(4) treatment within the pericentral region. Immunocytology, flow cytometry, and Northern blot analysis of MCs and SECs isolated at different times after the administration of CCl(4) revealed a decrease of PECAM-1 gene expression in MCs and in SECs, whereas ICAM-1 expression increased. As TNFalpha has been shown to be upregulated early after CCl(4) administration, the influence of TNFalpha on PECAM-gene-expression was analyzed. TNFalpha treatment of cultured rat SECs and of small and large MCs from normal liver decreased PECAM-1 specific transcript level in parallel to the increase of ICAM-1 transcript level.

CONCLUSIONS

Early production of TNFalpha after liver injury could induce an increased ICAM-1-expression and a decreased PECAM-1 expression, which might be essential for the transmigration of inflammatory cells into the parenchyma.

Age-related changes on parameters of experimentally-induced liver injury and regeneration

Age-dependent changes related to liver injury and regeneration were studied in rats aged 2, 12, and 30 months in a time period of 96 hr following a sublethal dose of thioacetamide (6.6 mmoles/kg body wt). Serum aspartate aminotransferase activity increased earlier in young rats, but the severity of injury was higher in those aged 12 months when compared to young and to old. Microsomal hepatocyte FAD monooxygenase activity was induced earlier in 2-month-old rats following intoxication and the increase was significantly lower both in the youngest and in the oldest groups when compared to adults. As a parameter of hepatocellular postnecrotic regeneration, DNA synthesis (2C --> 4C) was evaluated. The population of hepatocytes in S phase peaked more sharply and earlier in young rat hepatocytes, and was 8 to 12 times higher than the initial in hepatocytes from 2- and 12-month-old rats, while the rise was only 3 times in the oldest group. At 96 hr of intoxication the restoration towards normal in all these parameters was complete in young, incomplete in adult, and slightly detected in the oldest. Serum proliferative activity, assayed on mouse NIH 3T3 fibroblast cultures, increased preceding the necrosis and this increase was higher in 2- and 12-month-old (171% and 224%, respectively), while in the oldest the increase was only 110%. This mitogenic activity decreased in all groups during necrosis, showing a second peak, nondetectable in rats aged 30 months, parallel to regeneration. Serum TNFalpha level was absent in untreated animals and increased markedly following intoxication, the highest values being recorded at 72 hr of intoxication in serum from rats aged 12 months (347 +/- 30 pg/ml) and the lowest at 30 months (4.1 +/- 0.3 pg/ml). The serum ability to induce nitric oxide synthase activity on peritoneal macrophages ex vivo showed significant time- and age-dependent changes in nitric oxide release: a decrease throughout necrosis and an increase during regeneration. We conclude that the main age-related changes in the sequenced process of liver injury and regeneration are the delayed response in the development of cell killing and regeneration and the decreased regenerative ability, which significantly delays the restoration of liver function.

Effect of Salmonella assay negative and positive carcinogens on intrachromosomal recombination in S-phase arrested yeast cells

A wide variety of carcinogens including Ames assay (Salmonella) positive as well as Salmonella negative carcinogens induce intrachromosomal recombination (DEL recombination) in Saccharomyces cerevisiae. We have shown previously that the Salmonella positive carcinogens, ethyl methanesulfonate (EMS), methyl methanesulfonate (MMS) and 4-Nitroquinoline-N-oxide (4-NQO, and the Salmonella negative carcinogens, safrole, benzene, thiourea, carbon tetrachloride, and urethane, induced DEL recombination in growing, in G1 and in G2 arrested yeast cells. Since we found interesting differences in response between dividing and arrested cells, we wanted to find out whether these differences were due to the difference between cell division versus cell cycle arrest or to any particular cell cycle phase. In the present paper we incubated cells in the presence of hydroxyurea (HU) for cell cycle arrest in S-phase and exposed them to the above carcinogens, and plated them onto selective medium to determine DEL and interchromosomal recombination (ICR) frequencies. It was surprising that carbon tetrachloride had no effect on DEL recombination or ICR in HU treated cells even though it induced DEL recombination in G1 and G2 arrested as well as dividing cells. Further experiments are in agreement with the interpretation that carbon tetrachloride was responsible for prematurely pushing G1 cells into S-phase. The consequence of this may be replication on a damaged template which may be responsible for the action of carbon tetrachloride. EMS, MMS, 4-NQO and urethane were more recombinagenic in HU treated cells than in previous experiments with G2 arrested cells. None of the carcinogens appeared to be activated by S9 for either DEL recombination or ICR induction. Furthermore, we only detect cytochrome P-450 in dividing but not in arrested cells, arguing that possible differences in the ability to metabolize the compounds does not explain the observed differences for DEL recombination induction in the different cell cycle phases. We discuss these data in terms of the mechanism of induced DEL recombination and the possible biological activities of these carcinogens.

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.

Necrogenic and regenerative responses of liver of newly weaned rats against a sublethal dose of thioacetamide

The hepatocellular necrogenic and regenerative responses of newly weaned rats (21 days old) to a sublethal dose of thioacetamide (6.6 mmol kg-1) were studied in comparison to adult (6-month old rats), in terms of liver injury, antioxidant defense systems and cell proliferation. Hepatocellular necrosis, detected by serum aspartate aminotransferase, was less severe in newly weaned rats than in adult animals and was parallel to previous changes in the activity of microsomal FAD monooxygenase system responsible for thioacetamide biotransformation. Liver damage in hepatocytes from newly weaned rats was also detected by the decreased levels of glutathione and protein thiol groups (47%, p < 0.001 and 52%, p < 0.001 vs. untreated, respectively) and by the enhanced malondialdehyde production (334%, p < 0.001) and glutathione S-transferase activity (384%, p < 0.001). No significant differences were detected in these values when compared to adults. Changes in cytosolic and mitochondrial superoxide dismutase and catalase activities in hepatocytes from newly weaned rats at 24 h, following thioacetamide (49%, p < 0.001; 50% and 53%, p < 0.001 vs. untreated, respectively), were less severe against those in adult hepatocytes at 48 h of intoxication, and the increases in glutathione peroxidase and glutathione reductase activities were significantly lowered: 25% (p < 0.001) and 41% (p < 0.001), respectively. Post-necrotic DNA synthesis in hepatocytes from newly weaned rats peaked at 48 h of intoxication, while in adults a more intense peak appeared at 72 h preceded by a sharp decrease in tetraploid population. These differences indicate that the lower necrogenic response against the same dose of thioacetamide in newly weaned rats may be due to the lower rate of thioacetamide biotransformation and to the earlier onset of cell division. Accordingly, the growing liver from newly weaned rats presents advantages against the necrogenic aggression of thioacetamide, first, because the diminished activity of its specific microsomal detoxification system, and second because the earlier increase in the proliferative response prevents the progression of injury permitting an earlier restoration of liver function.

Protection against carbon tetrachloride hepatotoxicity by oleanolic acid is not mediated through metallothionein

Oleanolic acid is a triterpenoid compound that has been shown to protect against liver injury produced by some hepatotoxicants. This study was designed to characterize the protective effects of oleanolic acid on carbon tetrachloride-induced hepatotoxicity, and the role of metallothionein in the protection. Oleanolic acid pretreatment (100-400 micromol/kg, s.c.) protected Sprague-Dawley rats and mice from carbon tetrachloride-induced liver injury in a dose- and time-dependent manner, as evidenced by serum alanine aminotransferase and sorbitol dehydrogenase activities, as well as by histopathology. The protection against carbon tetrachloride hepatotoxicity was not evident until animals were pretreated with oleanolic acid 12 h, and lasted for 72 h after a single injection. This suggests that the protection might be due to induction of some adaptive mechanisms. Metallothionein (MT), an acute-phase protein proposed to decrease carbon tetrachloride-induced liver injury, was dramatically induced following oleanolic acid treatment. To examine whether oleanolic acid protection is mediated through MT, MT-I and II knock-out (MT-null) mice were utilized. Oleanolic acid pretreatment increased MT levels in control mice (20-fold), but not in MT-null mice, however, it protected equally against carbon tetrachloride-induced hepatotoxicity in both control and MT-null mice. These data indicate that oleanolic acid is effective in protecting rats and mice from the hepatotoxicity produced by carbon tetrachloride, and the protection is not mediated through induction of MT.

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.

Colcemid alters S phase and other parameters in skin during chronic exposure to benzo(a)pyrene

The administration of Colcemid for collecting mitotic figures in a carcinogenesis study, using benzo(a)pyrene (BaP), diminished the experimental differences between exposed and control mice. A dose-related increase in noncollected mitotic index (n-mitotic index) was seen in keratinocytes in the dorsal epidermis of mice which received four weekly treatments of BaP at 16, 32 and 64 micrograms in 50 microliters of acetone. In contrast, the number of mitotic figures collected for 4 hr by Colcemid block (c-mitotic index) was depressed at 16 micrograms, unchanged at 32 micrograms, and elevated at 64 micrograms of BaP. Weekly treatments with 4,8 or 16 micrograms BaP for 3-8 months induced an elevation in both n-mitotic and c-mitotic indices. The differences in results produced by the two methods of determining mitotic index depended upon dose and duration of treatment with BaP. The administration of Colcemid to acetone-treated mice increased the labeling index (number of labeled cells) and reduced the rate of DNA synthesis (low grain count per keratinocyte nucleus). After chronic application of BaP, Colcemid abrogated the increase in labeling index, but produced no additional effect on the number of grains per labeled keratinocyte. The modifying effect of Colcemid was greatest when administered during the peak of the tissue response to BaP. A number of significant changes in morphology of the skin associated with chronic exposure to BaP were attenuated by the use of Colcemid.

Role of nutrition in the survival after hepatotoxic injury

Nutritional status is an important factor in determining susceptibility to toxic chemicals. While macro and micronutrients may affect many aspects of Stage I and Stage II of toxicity, in this paper, the influence of macronutrients as sources of energy required for cell division and tissue repair mechanisms on the outcome of hepatic injury is discussed. Male Sprague-Dawley rats maintained on normal rodent chow and 15% glucose (as a source of energy for the centrilobular hepatocytes) in drinking water for 7 days experienced an increased lethality from structurally and mechanistically different centrilobular hepatotoxicants (acetaminophen, thioacetamide, chloroform and carbon tetrachloride), while male Sprague-Dawley (S-D) rats fed rat chow containing palmitic acid (PA, 8% w/w, as a source of energy for the periportal hepatocytes) and L-carnitine (LC, 2 mg/ml, as a mitochondrial carrier for the supplemented fatty acids) in drinking water for 7 days were protected from a LD100 dose (600 mg/kg, i.p.) of thioacetamide (TA). Indices of cell division revealed that cell cycle progression in the liver played a very critical role in determining the final outcome of hepatotoxic injury. These results confirmed our hypothesis that cell division and tissue repair play a critical role in survival after life-threatening hepatotoxic injury. Any manipulation directed towards altering a prompt and exacting compensatory cell division and tissue repair responses after hepatotoxic injury would also alter the final outcome of the toxicity. These studies indicate that the source of cellular energy can decisively influence the compensatory response of the target tissue to alter the outcome of hepatotoxic injury. Since nutritional status is known to vary widely among human populations, these could contribute enormously to susceptibility of human populations to toxic chemicals.

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.

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

The survival of patients suffering from severe liver damage depends heavily on the ability of the remaining hepatocytes to regenerate and replace the dead or dying cells; death usually occurs when the regenerating ability of the liver is compromised owing to heavy damage to the liver. The current approach to therapy aims only to block additional liver injury from hepatotoxicants or hepatic disease. If hepatocellular regeneration and tissue repair could be stimulated after hepatic damage by a therapeutically compatible mechanism, then it might be possible to prevent death arising from serious liver injury.

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

Because of the complexity of studying the toxicological effects of mixtures of chemicals, much of the mechanistic information has become available through work with binary mixtures of toxic chemicals. Mechanisms derived from studies employing chemicals at individually nontoxic doses are more useful than the mechanisms of interactive toxicity at high doses from the perspective of environmental and public health. Several examples of chemical combinations and interactive toxicity at low doses are now available. Chlordecone-potentiated halomethane hepatotoxicity, where suppression of cell division and tissue repair response permits very high amplification of CCl4 injury culminating in animal mortality, is one such model. Phenobarbital-potentiated CCl4 injury does not lead to animal mortality in spite of much higher liver injury in comparison to the chlordecone+CCl4 model. Much higher stimulation of tissue repair allows the animals to survive despite higher liver injury. Similar interactions have been reported between alcohols and halomethane toxicants. These and other studies have revealed that infliction of toxicant-induced injury is accompanied by a parallel but opposing tissue repair stimulation response which allows the animals to overcome that injury up to a threshold dose. Beyond this threshold, tissue repair response is both diminished and delayed allowing unrestrained progression of injury. Large doses of chemicals can be predictably lethal owing to these two latter effects on tissue repair. Dose-response paradigms in which tissue repair response is measured as a parallel but opposing effect to toxic injury might be useful in more precise prediction of the ultimate outcome of toxic injury in risk assessment. Autoprotection experiments with CCl4, thioacetamide, 2-butoxyethanol and related chemicals as well as heteroprotection against acetaminophen-induced lethality with thioacetamide are examples where tissue repair stimulation has been shown to rescue the animals from massive and normally lethal liver injury. The concept of toxicodynamic interaction between inflicted injury and stimulated tissue repair offers mechanistic opportunity to fine-tune other aspects of human health risk assessment procedure. Tissue repair mechanisms may also offer a mechanistic basis to explain species and strain differences as well as to more accurately assess inter-individual differences in human sensitivity to toxic chemicals. Because tissue repair is affected by nutritional status, assessment of risk from exposure to chemicals without attention to nutritional status may be misleading. Finally, the concept of using maximum tolerated doses (MTDs) in long-term toxicity studies such as cancer bioassays may need to be re-examined. MTDs might be predictably expected to maximally stimulate cell division and it is known that increased cell division is likely to lead to increased number of errors in DNA replication thereby predisposing these animals to cancer. It is clear that detailed studies of toxicodynamic interaction between tissue injury and stimulated tissue repair are likely to yield significant dividends in fine-tuning risk assessment.

Injury and repair as opposing forces in risk assessment

Recent advances in our understanding of the toxicodynamic events that follow infliction of injury have helped us to bridge the link between the tissue injury and the final outcome of that injury. In addition to infliction of tissue injury, toxic chemicals induce a biological compensatory response of tissue repair intended to overcome tissue injury through healing. Since stimulation of tissue repair is a simultaneous response accompanying injury, measuring this response in addition to quantifying injury might be helpful in tomorrow's risk assessment. Studies with model hepatotoxicants such as thioacetamide and CCl4, where tissue repair as well as injury were measured, reveal that endogenous mechanisms that drive the tissue repair response are responsible for more than just compensation for tissue injury. Up to a threshold dose, tissue repair is stimulated in a dose-dependent manner, and above this threshold it is both delayed and diminished. During this delay, tissue injury progresses unabated leading to tissue destruction and animal death. While dose-related stimulation of tissue repair leads to recovery, delayed and diminished tissue repair seen at the high doses leads to tissue destruction and animal death. These findings impact on the currently used maximum tolerated doses (MTDs) in cancer bioassays. MTDs represent maximal stimulation of cell proliferation thereby enhancing the likelihood of errors in DNA replication. Measuring tissue repair and injury as simultaneous biological responses to toxic agents might increase the usefulness of dose-response paradigms in risk assessment.

Proliferation of hepatocytes and attenuation from carbon tetrachloride hepatotoxicity by gadolinium chloride in rats

Intravenous injection of gadolinium chloride (GdCl3) at a dose of 10 mg/kg caused an increase in proliferating cell nuclear antigen labeling index and the grade of pyronin positivity (RNA level) in rat liver. In CCl4-exposed rats, pretreatment with GdCl3 also showed a preventive effect of the liver injury both biochemically and histologically. Moreover, the proliferative action preceded the attenuative effect of the liver injury. Results suggest that GdCl3 induces hepatocyte proliferation, and this action of GdCl3 may modify the development of CCl4-induced liver injury.

Hepatocellular regeneration: key to thioacetamide autoprotection

Low doses of thioacetamide stimulate cell division and tissue repair in the liver. The objective of this study was to develop an autoprotection model for thioacetamide and investigate if a low dose of thioacetamide (50 mg/kg orally) protects against lethality of a subsequently administered lethal dose (400 mg/kg orally) of the same compound. The extent of cell division was investigated to test if autoprotection results from augmented tissue repair and recovery from injury rather than decreased injury itself. After a single administration of the protective dose of thioacetamide, hepatocellular nuclear DNA synthesis as measured by 3H-thymidine incorporation into hepatocellular nuclear DNA peaked at 36 hr indicating maximum level of S-phase stimulation. Pretreatment with the antimitotic colchicine abolished autoprotection and this was associated with a significantly decreased 3H-thymidine incorporation. Preadministration of the protective dose of thioacetamide did not result in an altered infliction of injury from the subsequently administered lethal dose. Colchicine intervention in the autoprotected group resulted in injury that followed a pattern similar to the group that received the high dose alone, ultimately resulting in animal death. These findings suggest that cell division stimulated by the protective low dose of thioacetamide is the critical mechanism in thioacetamide autoprotection.

Comparison of acute hepatocellular proliferating cell nuclear antigen labeling indices and growth fractions, p34cdc2 kinases, and serum enzymes in carbon tetrachloride-treated rats

We evaluated various biomarkers associated with cell proliferation immediately following insult with the classic hepatotoxicant carbon tetrachloride (CCl4). Rats were administered a single necrogenic dose of CCl4 and euthanized at either t = 4, 8, 12, 16, or 24 hr postdose. Parameters evaluated included the following: immunohistochemical detection of hepatocellular proliferating cell nuclear antigen labeling indices (PCNA-LIs; percentage of cells in S phase) and growth fractions (PCNA-GFs; percentage of cells in the cell cycle); PCNA and the cyclin-dependent kinase p34cdc2 (CDK) protein in S-9 fractions by Western blot and enzyme-linked immunosorbent assay (ELISA); and liver-related serum enzymes. An increase in PCNA-GF was observed at t = 4 hr, concomitant with elevations in CDK and PCNA protein (Western blot). PCNA-LIs were increased by t = 24 hr, as were CDK and PCNA by ELISA. Sorbitol dehydrogenase was the most sensitive enzyme, with increases observed at t = 4 hr. Our results indicate that PCNA-GF, CDK, and PCNA levels reflect hepatocellular regeneration as early as 4 hr following CCl4 insult. We conclude that these assays are early and sensitive indicators of acute hepatotoxicity that may be advantageous to evaluate in the early stages of exploratory studies.

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.

Cocaine-induced liver injury in mice elicits specific changes in DNA ploidy and induces programmed death of hepatocytes

Liver injury was induced by a single dose (60 mg/kg) of cocaine in male albino Swiss mice untreated or pretreated with phenobarbital (in drinking water 1 gm/L), for 5 days before cocaine administration. One parameter of liver injury, serum isocitrate dehydrogenase activity, showed sharp increases at 24 hr of cocaine treatment; we also noted decrease hepatic levels of ATP, GSH, cytochrome P-450 and NADPH/NADP+ ratio and increases in malondialdehyde concentration. Histopathological study of liver slices showed perivenous and periportal necrosis induced by cocaine in untreated mice and mice pretreated with phenobarbital, respectively. A regenerative postnecrotic response, which peaked at 48 hr, was demonstrated by the appearance of mitotic cells. Mitotic index analysis showed that proliferative cells appear to be unevenly distributed in the hepatic acinus and were mainly located in the vicinity of the damaged acinar region. Genomic DNA ploidy and the distribution of DNA in the phases of the cell cycle were studied in nuclei of isolated hepatocytes. At 12 hr of cocaine administration, both in untreated and phenobarbital-pretreated mice, the following changes were observed: a sharp decrease in tetraploid (4N) cells (40% to 17% and 25% to 6%, respectively) and octoploid (8N) cells (5% to 2% and 2% to 1%, respectively), together with the appearance of a hypodiploid population (13% and 31%, respectively). Hypodiploid population was characterized as apoptotic cells by detection of DNA fragmentation in agarose gel. These results suggest that a significant percentage of cell death induced by cocaine occurs by means of the apoptosis death program. Comparison of the initial values of DNA ploidy with those obtained at 7 days of cocaine administration showed remarkable increases in polyploid populations (4N and 8N) and a decrease in diploid cells (2N), indicating that the process of differentiation occurs when liver restores its functionality.

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)