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

Immunopharmacologic agents in the amelioration of hepatic injuries

A number of immunomodulating agents of different origin have been shown to reduce liver injury of various etiologies. Immunostimulants like levamisole, BCG, a protein polysaccharide from myceria Coriolus vesicolor PS-K, a streptoccocal preparation OK-432 and immunomodulators like N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) and its analogs. Selective T-cell suppressors like the polypeptide cyclosporine A (CsA) and the macrolide FK 506 (tacrolimus) have also been claimed to possess hepatoprotrophic or hepatoprotective properties at low doses. The aim of this review article is to highlight the interplay between the administration of immunomodulating agents and the amelioration of hepatic injuries. Hepatic effects of exogenous immunomodulators are discussed with special focus on the most widely used immunosuppressive agents, CsA and tacrolimus. An important question exists as to whether these potential hepatoprotective effects are related mechanistically to the immune system or are working at different levels. Due to the differences in effects and modes of actions of various immunoactive substances presented herein, a common mechanism for their cytoprotective effects cannot be formulated at this stage. Levamisole and cyanidanol may protect cells against necrosis by acting as free radical scavengers. MDP and its analogs reduce carbon tetrachloride-elevated (CCl4) lipid peroxides and their protective effects are primarily on hepatic cytoplasmic membranes where lipid peroxidation and calcium homeostasis interact. MDP reduced CCl4-elevated calcium in both intact hepatocytes and in the post microsomal supernatant suggest that the influx of extracellular calcium across plasma membrane is affected. Elevations of intracellular calcium above a threshold are involved in: the stimulation of Ca2+-sensitive enzymes such as phospholipase A2, endonucleases and proteases, the conversion of xanthine dehydrogenase to xanthine oxidase and the formation of free radicals, all of which disturb biomembranes. MDP and its analogs, in a specific dose range, may act to maintain intracellular calcium within physiological ranges. Highly complex cellular signalling systems, including calcium, are involved in the explanation of the mechanism of the immunosuppressive effect of CsA and tacrolimus. The hepatoprotective effects of these selective immunosuppressive agents, however, are independent of the inhibition of T-cell activation. The cyclophilin and tacrolimus binding proteins of the mitochondria are the receptors for these compounds and play a key role in the regulation of mitochondrial permeability transition pores. CsA or tacrolimus inhibition of mitochondrial permeability transition pores does not require interaction with calcineurin, indicating a dissociation between immunosuppression and mitochondrial protection. The involvement of intracellular or intramitochondrial proteins in the modulation of mitochondrial permeability transition pores with the creation of a partially impermeable state for Ca2+ movement in drug-treated mitochondria and the dissociation of this effect from immunomodulatory actions potentially offers new and promising approaches for the development of new pharmacologicals targeted at therapeutic intervention. Clinical trials of these drugs as hepatoprotective agents are limited. Use of CsA in patients with primary biliary cirrhosis and autoimmune chronic hepatitis and in cirrhotic animal models produced by chronic administration of CCl4 have yielded encouraging results. It seems that this class of compounds may be of substantial benefit in liver protection against many pathological conditions where disturbance in mitochondrial function and in Ca2+ homeostasis appear to be prerequisites for cell 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.

DNA synthesis in cultured neonatal rat islets--a comparison of two methods

To assess the effect of glucose, foetal calf serum (FCS) and iron-saturated transferrin (Fe-TRF) on DNA synthesis of fibroblast-free neonatal rat islets cultured in Ham's F-12 medium, we measured [3H]thymidine uptake in cultures and used immunohistochemistry to quantitate nuclear 5-bromo-2'-deoxyuridine staining. Addition of glucose to a concentration of 26.1 mmol/l resulted in a significant increase from baseline in DNA synthesis in islet cells as measured by both methods (22,591 counts/min per microgram DNA +/- 3628 S.E.M., vs. 9631 counts/min per microgram DNA +/- 1912 S.E.M., P < 0.002 and 19.19% positive beta-cells +/- 2.72 S.E.M., vs. 11.98 positive beta-cells +/- 0.26 S.E.M., P = 0.05). At a glucose concentration of 16.1 mmol/l we could demonstrate an increase compared with baseline only in [3H]thymidine uptake (15,700 counts/min per microgram DNA +/- 3323 S.E.M., P < 0.05). Supplementation with 10% or 15% FCS also increased [3H]thymidine uptake compared with baseline (to 15,809 counts/min per micrograms DNA +/- 136 +/- S.E.M., P < 0.05 and 23,746 counts/min per microgram DNA +/- 3114 S.E.M., P < 0.01, respectively) but did not significantly effect the BrDU labelling index. Addition of Fe-TRF to islet cultures significantly increased [3H]thymidine uptake at a concentration of 45 micrograms/ml compared with baseline (23,149 counts/min per microgram DNA +/- 6387 S.E.M., P < 0.01).(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)