Correspondence of results from hepatocyte studies with in vivo response

An In Vitro Cytotoxicity Assay Using Rat Hepatocytes and MTT and Coomassie Blue Dye as Indicators

Isolated hepatocytes from male Sprague-Dawley rats suspended in culture medium supplemented with either 0.2 or 2% bovine serum albumin (BSA) were allowed to attach to collagen coated 96-well dishes. Ten test chemicals from the MEIC list and salicylic acid were added individually to the dishes, and at the end of 24 and 48 hours, cytotoxicity was determined by measuring MTT (tetrazolium salt) reduction (mitochondrial integrity) and total cellular protein using Coomassie blue dye (reflecting cell number). Total cellular lactate dehydrogenase activity was also determined in some experiments, as an indicator of plasma membrane integrity. The relative toxicities of the test chemicals were quantified by the estimation of EC10, EC20 and EC50 values for each parameter. Except for one chemical, digoxin, in the MTT assay, cytotoxic potency increased with incubation time. The hepatocytes tended to be more sensitive to the chemicals in medium containing 0.2% BSA than in medium containing 2% BSA. Simple linear regression analyses of the log transformed data from the MTT assay versus log oral LD50 in rats for the test chemicals gave the best results using EC10 at 24 hours (r2 = 0.86). With protein as the cytotoxic indicator, the best results were obtained with EC values in the medium containing 2% BSA, again at 24 hours (r2 = 0.83). These results suggest that the MTT and Coomassie blue dye assays could be useful indicators for testing the cytotoxic potential of chemicals in rat hepatocyte cultures.

Antibacterial, antifungal and cytotoxic activities of extracts from fish epidermis and epidermal mucus

Annual discards from the world fisheries are estimated to be approximately 20 million metric tonnes (25%) per year. The main objective of this work is to increase the utilisation of by-products (notably skin) from fish species in order to isolate new biologically active compounds. This study presents the results of a screening program for antifungal, antibacterial and cytotoxic activities in epidermal mucus and epidermis extracts of thirteen fish species that are commonly caught in North Atlantic waters and generate an important amount of fish waste. Antimicrobial assays used five Gram-positive bacteria five Gram-negative bacteria and five fungi. Of the 78 extracts tested, 15 showed antibacterial and/or antifungal activities. None of the aqueous fractions were active. One third of the active extracts were ethanolic fractions and three fourth of extracts were dichloromethane fractions. One third of the active fractions were of epidermal origin and the remaining came from mucus fractions. The high levels of inhibitory activity and no apparent toxicity against mouse fibroblasts of extracts of Pollachius virens (CH(3)CH(2)OH/epidermis), Labrus bergylta (CH(2)Cl(2)/mucus), Platichthys flesus (CH(3)CH(2)OH/mucus), Solea solea (CH(2)Cl(2)/mucus) and Scophtalamus rhombus (CH(2)Cl(2)/mucus) suggest they may have potential as novel active therapeutic agents.

Inhibition of marine bacteria by extracts of macroalgae: potential use for environmentally friendly antifouling paints

Although a total ban on the use of TBT coatings is not expected in the short term, there is a growing need for environmentally safe antifouling systems. A search for new non-toxic antifoulants has been carried out among marine macroalgae. Antifouling activity of aqueous, ethanolic and dichloromethane extracts from 30 marine algae from Brittany coast (France) was examined in vitro against 35 isolates of marine bacteria. About 20% of the extracts were found to be active. The high levels of inhibitory activities against bacteria recorded in some extracts and the absence of toxicity on the development of oyster and sea urchin larvae and to mouse fibroblast growth suggests a potential for novel active ingredients in antifouling preparations.

The application of in vitro models of drug metabolism and toxicity in drug discovery and drug development

In vitro models are being used increasingly during all phases of the drug development process in concert with the more traditional in vivo toxicological and pharmacokinetic evaluations. These in vitro models may be classified empirically as either validated in vitro screens, value-added screens or 'ad-hoc' mechanistic screens. The application of these screens is discussed with respect to their level of validation, standardization, uses of human tissue, level of iteration with in vivo studies, regulatory position and utility in the drug discovery and development process. The predictability and reproducibility of these screens is discussed, as well as future trends in regard to emerging technology and its application.

Hepatic peroxisome proliferation in rodents and its significance for humans

Peroxisomes are subcellular organelles found in all eukaryotic cells. In the liver they are usually round and measure about 0.5-1.0 microns; in rodents they contain a prominent crystalloid core, but this may be absent in newly formed rodent peroxisomes as well as in human peroxisomes. A major role of the peroxisomes is the breakdown of long-chain fatty acids, thereby complementing mitochondrial fatty-acid metabolism. Many chemicals are known to increase the number of peroxisomes in rat and mouse hepatocytes. This peroxisome proliferation is accompanied by replicative DNA synthesis and liver growth. No clear structure-activity relationships are apparent. Many of these peroxisome proliferators contain acid functions that can modulate fatty acid metabolism. Two mechanisms have been proposed for the induction of peroxisome proliferation. One is based on the existence of one or several specific cytosolic receptors that bind the peroxisome proliferator, facilitating its translocation to the cell nucleus and the activation of the expression of specific genes. The second, perhaps more general, hypothesis involves chemically mediated perturbation of lipid metabolism. These two hypotheses are not mutually exclusive. Many peroxisome proliferators have been shown to induce hepatocellular tumours, despite being uniformly non-genotoxic, when administered at high dose levels to rats and mice for long periods. Three mechanisms have been proposed to explain the induction of tumours. One is based on increased production of active oxygen species due to imbalanced production of peroxisomal enzymes; it has been proposed that these reactive oxygen species cause indirect DNA damage with subsequent tumour formation. In rodents, an alternative mechanism is the promotion of endogenous lesions by sustained DNA synthesis and hyperplasia. Thirdly, it is conceivable that sustained growth stimulation may be sufficient for tumour formation. Marked species differences are apparent in response to peroxisome proliferations. Rats and mice are extremely sensitive, and hamsters show an intermediate response while guinea pigs, monkeys and humans appear to be relatively insensitive or non-responsive at dose levels that produce a marked response in rodents. These species differences may be reproduced in vitro using primary culture hepatocytes isolated from a variety of species including humans. The available experimental evidence suggests a strong association and a probable casual link between peroxisome-proliferator-elicited liver growth and the subsequent development of liver tumours in rats and mice. Since humans are insensitive or unresponsive, at therapeutic dose levels, to peroxisome-proliferator-induced hepatic effects, it is reasonable to conclude that the encountered levels of exposure to these non-genotoxic agents do not present a hepatocarcinogenic hazard to humans.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of cytochrome P-450-dependent enzyme activities in cultured rat liver slices

Precision-cut liver slices were prepared from male Sprague-Dawley rats with a Krumdieck tissue slicer and cultured in RPMI 1640 medium for up to 72 hr. After 48 hr, cytochrome P-450 content in the slices declined to 36% of levels present in freshly cut rat liver slices. The addition of either beta-naphthoflavone (BNF) or Aroclor 1254 (ARO) partially prevented the loss of cytochrome P-450. Culture of liver slices with phenobarbitone (PB), BNF and ARO resulted in the induction of 7-ethoxycoumarin O-deethylase, 7-benzoxyresorufin O-debenzylase and 7-ethoxyresorufin O-deethylase activities. Generally, the induction of mixed-function oxidase enzymes was greater in 72- than in 48-hr cultured slices, and at the concentrations examined ARO produced a greater stimulation of enzyme activities than did either PB or BNF. These results demonstrate that rat liver slices may be maintained in culture for up to 72 hr, and that they respond in a similar manner to rat primary hepatocyte cultures to some inducers of xenobiotic metabolism. Precision-cut liver slices may therefore be a useful alternative in vitro system to hepatocyte cultures for screening compounds for effects on mixed-function oxidases and for assessing species differences in response.

Culture of precision-cut liver slices: effect of some peroxisome proliferators

Precision-cut rat liver slices were prepared with a Krumdieck tissue slicer and cultured in three standard hepatocyte culture media. Rat liver slices cultured in either RPMI 1640 medium or Williams Medium E could be maintained in culture for up to 72 hr. In contrast, Leibovitz's L-15 medium was unsatisfactory in that slice viability, assessed either by morphological examination or by measurement of enzyme activities, could not be maintained for periods greater than 24 hr. As a measure of functional viability liver slices were cultured with some known rodent peroxisome proliferators, namely clofibric acid, nafenopin, ciprofibrate and Wy-14,643. The peroxisome proliferators induced both palmitoyl CoA oxidation and carnitine acetyltransferase activities in 48- and 72-hr slice cultures. Ultrastructural examination of liver slices cultured with either ciprofibrate or Wy-14,643 for 72 hr revealed an increase in the number of peroxisomes. These results demonstrate that rat liver slices may be maintained in culture for up to 72 hr, and that they respond in a similar manner to rat primary hepatocyte cultures to some peroxisome proliferators. Precision-cut liver slices may therefore be a useful alternative in vitro system to hepatocyte cultures for screening compounds for effects on enzyme activities and for assessing species differences in response.

A CCl4/CHCl3 interaction study in isolated hepatocytes: non-induced and phenobarbital-pretreated cells

The purpose of this study was to evaluate an isolated hepatocyte model for predicting the in vivo hepatotoxicity of carbon tetrachloride (CCl4) and chloroform (CHCl3), alone and in combination. Response surface methodology (RSM) was used to analyze and describe the data. The interaction was evaluated for % initial K+ (cell injury) and % LDH leakage (cell death) in non-induced (untreated) and phenobarbital-pretreated suspended hepatocytes. CCl4 and CHCl3 were delivered alone and in combination in dimethyl sulfoxide (DMSO) to suspended hepatocytes. The maximum observed no-effect level (MONEL) for CCl4 in non-induced cells was 1.0 mM (LDH and K+). In induced cells, the MONEL was 0.25 mM (K+) and 0.5 mM (LDH). The MONEL for CHCl3 in non-induced cells was 5.0 mM (LDH and K+) and in induced cells was 0.5 mM (K+) and 1.0 mM (LDH). Phenobarbital pretreatment enhanced the toxicity of both CCl4 and CHCl3, alone and in combination. RSM analysis of the % initial K+ and % LDH for CCl4 and CHCl3 in combination in noninduced and induced cells showed a greater than additive interaction. The isolated hepatocyte model appears to be a promising system for evaluating the toxicity of chemical mixtures and predicting their in vivo effects.

Interference with hepatocellular substrate uptake by 1,1,1-trichloroethane and tetrachloroethylene

The effects of chlorinated aliphatic hydrocarbon solvent exposure on hepatocellular transport of some model substrates have been investigated. Exposure of isolated hepatocytes to 1,1,1-trichloroethane or tetrachloroethylene resulted in suppression of uptake of taurocholate, ouabain, and 2-aminoisobutyric acid but not CdCl2 or 3-O-methyl-D-glucose. The effect was clearly evident at noncytotoxic concentrations, as indicated by the lack of intracellular enzyme leakage and unaltered intracellular K+ ion concentration. Moreover, the ultrastructure of solvent-exposed hepatocytes was similar to that of control cells, except for a reduction in membrane microvilli. The suppression of uptake was reversible provided that sufficient time was allowed for the cells to recover. The mechanism of this inhibition may be associated with energy-linked processes, as uptake of taurocholate, ouabain, and 2-aminoisobutyric acid is energy requiring while uptake of CdCl2 and 3-O-methyl-D-glucose is not. Cellular ATP was reduced in a dose-dependent manner, but a marked depletion occurred only at cytotoxic concentrations. Na(+)-K(+)- and Mg2(+)-ATPase activities in hepatocyte plasma membrane preparations were also inhibited by solvent exposure. The data suggest that 1,1,1-trichloroethane and tetrachloroethylene interfere specifically with energy-dependent hepatic transport functions and that a decrease in ATP levels and/or inhibition of cell membrane ATPases may be the mechanism.

Potentiation of carbon tetrachloride-induced lipid peroxidation by trichloroethylene in isolated rat hepatocytes: no role in enhanced toxicity

Hepatocytes isolated from Sprague-Dawley rats were exposed to carbon tetrachloride together with various concentrations of trichloroethylene over a 40-fold range. A potentiation of carbon tetrachloride-induced lipid peroxidation by trichloroethylene and an enhanced toxicity on combined exposure were clearly demonstrated. Additionally, rats were treated 2.5 hr before isolation of hepatocytes, which were then exposed to carbon tetrachloride. Lipid peroxidation and potassium ion leakage were increased in these cells. Some incubations included the antioxidant N,N'-diphenyl-p-phenylenediamine (DPPD) while others contained dithiothreitol (DTT), a thiol reducing compound. DPPD inhibited lipid peroxidation while DTT did not. Neither, however, was able to inhibit the toxicity. Assays to estimate total and nonprotein bound sulfhydryl groups were also performed. There was no indication of a causative role for cellular sulfhydryl groups in the enhanced toxicity. Therefore, our data show that lipid peroxidation is not responsible for the trichloroethylene-induced enhancement of toxicity in hepatocytes due to carbon tetrachloride. Furthermore, there is no evidence to indicate a role for sulfhydryl groups in this response.

Subclinical response to cadmium in liver cells

Effects of cadmium (Cd) in vivo and in vitro were studied in the absence of enhanced metallothionein (MT) production and overt Cd toxicity. Such a condition was established by extended oral exposure of male rats to 0.2 mumols Cd/kg and by incubation of isolated hepatocytes with up to 25 microM for 30 min. Subsequently, mitochondrial and extramitochondrial responses to Cd were recorded. Cadmium diminished the activity of cytochrome c oxidase (CYT C OX) by 50% in vivo and by 35% in vitro. In hepatocytes, this was accompanied by increased Cd and decreased protoheme (PrH) in mitochondria. Extramitochondrial PrH and cytochrome P 450 were not significantly altered. In hepatocytes from phenobarbitone pretreated rats, 25 microM Cd decreased CYT C OX but not mitochondrial PrH. Moreover, stimultaneous incubation of hepatocytes with 25 microM Cd and either 2.5 mM dithiothreitol or 5 mM reduced glutathione diminished cellular and mitochondrial Cd and prevented the decrease in CYT C OX but not that in PrH. In contrast, coincubation with either 250 microM L-buthionine-sulfoximine or diethylmaleate, which did not alter Cd uptake, prevented the decrease in PrH but not that in CYT C OX owing to Cd. These results show that Cd exerts mitochondrial alterations in vivo and in vitro in the absence of enhanced MT production. Moreover, Cd effects on CYT C OX and PrH do not seem to be firmly linked.

Toxicity of mixtures of trichloroethylene, tetrachloroethylene and 1,1,1-trichloroethane: similarity of in vitro to in vivo responses

The toxicities of various combinations of trichloroethylene (TRI), tetrachloroethylene (TET) and 1,1,1-trichloroethane (TCE) were examined in suspensions of rat hepatocytes and in vivo. For each pair and for the three solvents together, an interactive toxicity was demonstrated in vitro, as determined by release of potassium ion and cytoplasmic enzymes. A similar pattern of response was found after administration to the intact rat for increases in plasma alanine aminotransferase and sorbitol dehydrogenase, both indices of hepatotoxicity. Plasma urea levels were significantly elevated on exposure to the three chemicals together. Thus a remarkably similar pattern of toxicity was found in vitro and in vivo, which supports the possible use of hepatocyte suspensions as a screening procedure for toxicity of mixtures.

In vitro screens from CNS, liver and kidney for systemic toxicity

The development and evaluation of in vitro systems from target organs for preliminary assessments of the potential for systemic toxic effects has been receiving increased attention. This review presents a synopsis of progress made in developing toxicity screens for three common target organs and identifies further work needed for more complete validation.