Evidence for the involvement of organelles in the mechanism of ketone-potentiated chloroform-induced hepatotoxicity

Ketone potentiation of haloalkane-induced hepatotoxicity: CCl4 and ketone treatment on hepatic membrane integrity

Previous results in male Sprague-Dawley rats indicate that acetone (A), methyl ethyl ketone (MEK), and methyl isobutyl ketone (MiBK) pretreatments (3 d, p.o.) at a dosage of 6.8 mmol/kg potentiate CCl4 hepatotoxicity. The potentiation potency profile observed was MiBK > A > MEK. In the present study, male Sprague-Dawley rats were treated for 3 d with 6.8 mmol/kg (p.o.) of A, MEK, or MiBK using Emulphor as vehicle (10 ml/kg). Rats were either killed 18 h after the last pretreatment or treated with CCl4 (prepared in corn oil) and then killed 48 h later. Livers were perfused; purified plasma membrane (PM), sinusoidal (SM) and basal canalicular membrane (BCM) fractions were prepared. Membrane fluidity was monitored by fluorescence polarization using 1,6-diphenyl-1,3,5-hexatriene (DPH) or 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). The following membrane enzymes were measured to monitor membrane purity and treatment effects: 5'-nucleotidase (5N), leucine aminopeptidase (LAP), and alkaline phosphatase (AP). Our results suggest that CCl4 modifies membrane integrity as indicated by a decrease in liver membrane 5N, LAP, and AP activity. CCl4 also increased the fluidity of the lipid and protein portions of the liver membranes as measured by the DPH and TMA-DPH fluorescence probes, respectively. Of the three ketones, only A altered CCl4 effects on plasma membrane enzymes and decreased BCM fluidity. The data only partially support increased susceptibility of liver membranes by ketone pretreatment as a factor implicated in the mechanism of potentiation of CCl4-induced hepatotoxicity.

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.

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.

Ketone potentiation of haloalkane-induced hepato- and nephrotoxicity. I. Dose-response relationships

Carbon tetrachloride (CCl4) induced hepatotoxicity and chloroform (CHCl3) induced nephrotoxicity were evaluated in male Sprague-Dawley rats pretreated with acetone (A), methyl ethyl ketone (MEK), and methyl isobutyl ketone (MiBK). Dose-response relationships for A, MEK, and MiBK potentiation of CCl4-induced hepatotoxicity and CHCl3-induced nephrotoxicity were compared. A, MEK, and MiBK pretreatment at a dosage of 6.8 mmol/kg, given daily for 3 d, markedly potentiated CCl4-induced liver toxicity as indicated by a decrease in the CCl4 ED50 to 3.4, 4.6, and 1.8 mmol/kg, respectively, compared to vehicle-pretreated rats (17.1 mmol/kg). Similarly, pretreatment with these ketones (13.6 mmol/kg) potentiated CHCl3 kidney toxicity but to a lesser degree; CHCl3 ED50 values for vehicle-, A-, MEK-, and MiBK-pretreated rats were 3.4, 1.6, 2.1, and 2.2 mmol/kg, respectively. Our results indicate a potency ranking profile for the potentiation of CCl4 hepatotoxicity of MiBK > A > MEK and of A > MEK > or = MiBK for CHCl3 nephrotoxicity. These dissimilar ranking profiles could be due to differences in mechanisms of action for the two target sites.

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)

Influence of agents affecting monooxygenase activity on taurolithocholic acid-induced cholestasis

In rats, pretreatment with certain ketones results in enhanced taurolithocholic acid (TLCA)-induced reduction in bile flow, whereas pretreatment with inhibitors of protein synthesis diminishes the effect on bile flow of cholestatic regimens. In the present study, the possible role of cytochrome P-450 in the ketone potentiation phenomenon was investigated. Male rats were pretreated with inducers or inhibitors of hepatic cytochrome P-450 and the impact of these pretreatments on TLCA-induced cholestasis assessed. Phenobarbital, 3-methylcholanthrene, chlordecone or mirex were used as inducers, and SKF 525-A, piperonyl butoxide, or cobaltous chloride as inhibitors of monooxygenase activity. Phenobarbital and 3-methylcholanthrene pretreatment enhanced TLCA-induced reduction of bile flow, while mirex and chlordecone were without effect. The three inhibitors of monooxygenase activity did not diminish TLCA-induced cholestasis. Instead, piperonyl butoxide and cobaltous chloride appeared to enhance the action of TLCA. Consequently, an increase in cytochrome P-450 (or specific isozymes) as a common denominator in the potentiation phenomenon appears unlikely. While hepatic proteins may play an important role in the potentiation of TLCA-induced cholestasis following pretreatment with ketones, the pattern of potentiation after pretreatment of rats with different inducers or inhibitors of cytochrome P-450 does not appear to implicate this family of proteins.

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.