In vitro assay of cytotoxicity with cultured liver: accomplishments and possibilities

Comments on the Use of Diverse Cell Systems in Toxicity Testing

To initiate a discussion of the relative merits of various cell systems in acute toxicity testing we have considered the suitability of seven systems in testing three types of toxicity i.e. systemic toxicity in general, systemic toxicity to specific target organs, and local toxicity Available data indicate that systemic toxicity can be predicted by simple systems with non-differentiated cell lines, as well as by primary cultures of hepatocytes. The combined use of both cell systems is probably the method of choice. Specific, cellular, target organ toxicity is often tested in primary cultures of various specific cells. Such use of these cells seems to be irrational, and ought to be replaced by a parallel use of specific and non-specific cell cultures. An analysis of the differential cytotoxicity to both cell types would then point out specific target organ toxicity. It is uncertain whether non-specific cell lines or primary cultures of target-specific cells should be used in tests of local toxicity. Future parallel use of both cell types to produce data on differential cytotoxicity would settle the question.

Mutagenic properties of linuron and chlorbromuron evaluated by means of cytogenetic biomarkers in mammalian cell lines

Agricultural practices are usually supported by several chemical substances, such as herbicides. Linuron and chlorbromuron are phenylurea herbicides largely used to protect crops from weeds, blocking photosynthesis by inhibition of the photosystem II complex. The former, also commercially known as lorox or afalon, is selectively used to protect bean and French bean plants, fennels, and celeriacs; the second, commercially known as maloran, is selectively used for carrots, peas, potatoes, soy sprouts, and sunflowers. Considering the widespread use of herbicides and, more generally, pesticides, it is important to clarify their involvement on human health, one of them concerning the possible direct or indirect effect on the genome of exposed populations. Here, we show that these herbicides are endowed by mutagenic properties, as demonstrated by an increased number of chromosomal aberrations (CAs) in two exposed Chinese hamster cell lines derived from ovary and epithelial liver, respectively. This was also confirmed by sister chromatid exchange (SCE) and micronucleus (MN) assays. Our present and previously obtained data clearly indicate that phenylurea herbicides must be used with great caution, especially for agricultural workers who use large amounts of herbicides during their work, and particular attention should be given to residues of these herbicides and their involvement in environmental pollution.

Cytogenetic evaluation of extractable agents from airborne particulate matter generated in the city of Catania (Italy)

In order to document cytogenetic damage associated with air pollution and, possibly, with health effects in the city of Catania, Sicily (Italy), we analyzed the induction of chromosomal aberrations by extractable agents from airborne particulate matter in a Chinese hamster epithelial liver (CHEL) cells. These cells retain their metabolic competence to activate different classes of promutagens/procarcinogens into biologically active metabolites. Airborne particulate matter was obtained from two stationary samplers (stations I and II) in two areas endowed by an elevated car transit in the centre of Catania. The results obtained clearly indicated that airborne particulate matter from both stations I and II proved to be clastogens in CHEL cells but not in Chinese hamster ovary (CHO) cells without metabolic activation, indicating that airborne particulate mixtures need to be metabolically converted before exerting their genotoxic potential. On the basis of these results we can assert that the test system employed to identify the cytogenetic potential of airborne particulate matter is useful and profitable for environmental control, and helpful to plan specific actions aimed at reducing the hazards derived from exposure to polluted air.

Induction of sister-chromatid exchanges by procarcinogens in metabolically competent Chinese hamster epithelial liver cells

An epithelial cell strain has been established from the livers of male Chinese hamsters (CHEL cells). These cells, which proliferate in culture and retain their metabolic enzymatic activities during several subcultures, were used in a sister-chromatid exchange assay to evaluate the effectiveness of polycyclic aromatic hydrocarbons (PAHs), aflatoxin B1 (AFB1) and cyclophosphamide (CP). The results obtained demonstrate that CHEL cells are metabolically competent to activate different classes of procarcinogens into biologically active metabolites. Moreover, they showed a selective capacity to discriminate chemical carcinogens from noncarcinogens. Thus, the CHEL cell system appears to be a promising alternative to the short-term tests that include cell-free rodent liver homogenate to evaluate new promutagens and/or procarcinogens.

In vitro cytotoxicity of allyl alcohol and bromobenzene in a novel organ culture system

Two well-known hepatotoxicants, allyl alcohol (AA) and bromobenzene (BB), were studied using an in vitro system of cultured liver slices from control and phenobarbital-treated rats, respectively. Dose- and time-dependent increases in media lactate dehydrogenase (LDH), and decreases in slice K+ content and in protein synthesis were observed in rat liver slices incubated with either compound at concentrations between 0.1 and 1 mM over a period of 6 hr. The histopathological changes which occurred in the intoxicated slices appeared to parallel these biochemical changes. Additionally, the toxicity of either BB or AA, evaluated at 4 hr, was inhibited when slices were preincubated for 30 min with beta-ethyl-2,2-diphenylvalerate hydrochloride (SKF 525-A) (0.1 mM) or pyrazole (1.0 mM), respectively. In this in vitro incubation system the cytotoxicity of xenobiotics can be studied under conditions where the multicellular hepatic lobular architecture is partially maintained, and alterations in biochemical and functional processes may be correlated to pathological changes.

Maintenance of adult rat liver slices in dynamic organ culture

Adult rat liver slices were maintained for 20 h in a novel organ culture system with minimal loss of viability and function. Potassium and adenosine triphosphate levels were maintained at in vivo levels, following an initial recovery period (2 to 4 h), for up to 20 h. Protein synthesis and secretion were linear for 20 and 16 h, respectively. In addition, the liver slices synthesized glycogen between 4 and 12 h in culture. Finally, the liver slices were hormonally responsive during the 20 h culture period as exemplified by glucagon-stimulated glucose production. This system provides a simple and effective method for the culture and biochemical maintenance of adult rat liver for 20 h with minimal loss of biochemical function.

Identification of genotoxic and epigenetic carcinogens in liver culture systems

Liver culture systems are available for identifying the DNA reactivity of carcinogens and a cell membrane effect, which appears to be associated with neoplasm-promoting ability. Using these and other approaches, carcinogens can be categorized as genotoxic or epigenetic. This distinction has implications for risk assessment.

Effect of different collagenases on the isolation of viable hepatocytes from rat liver

The isolation of viable hepatocytes from rat liver was found to depend on the source of collagenase (EC 3.4.24.3) more than any other single factor examined. Collagenase purified from Achromobacter iophagus/Bacillus polymyxa (collagenase/dispase) gave reproducibly high viability without the use of complex perfusion protocols.

Cytotoxic effects of N-acetyl-p-benzoquinone imine, a common arylating intermediate of paracetamol and N-hydroxyparacetamol

The cytotoxic effects of N-acetyl-p-benzoquinone imine (NAPQI), a postulated ultimate reactive metabolite of paracetamol (pHAA), was studied in suspensions of isolated rat hepatocytes. Incubation of cells for 10-300 min with 0.1-0.5 mM NAPQI led to concentration dependent cell damage, as determined by increased trypan blue exclusion, lactate dehydrogenase release and glutathione (GSH) depletion. NAPQI and N-hydroxyparacetamol (N-OH-pHAA), a postulated proximate metabolite of pHAA, caused cytotoxic effects in the same concentration range. In contrast, no toxic effects of pHAA (less than or equal to 20 mM) could be demonstrated. With the short half-life of NAPQI, less than 0.5% of the NAPQI added is expected to be left in the incubation medium after a 2 min incubated period. Nevertheless, 10-120 min (depending on the concentration of NAPQI) elapsed before the cells responded with increased membrane permeability. Clearly, the initial damage caused by NAPQI must be followed by subsequent cellular steps before toxicity becomes apparent. The addition of N-acetylcysteine, GSH or ascorbate during the NAPQI exposure period fully protected the hepatocytes from NAPQI damage. Lesser effects were demonstrated when these agents were added after the 5 min NAPQI exposure period. The results presented in this study further support the hypothesis that NAPQI is the ultimate reactive formed from pHAA.

Screening of toxic compounds in mammalian cell cultures

The cytotoxic concentrations of about 100 randomly selected drugs and chemicals, tested on HeLa cells in the MIT-24 system and/or in primary cultures of fetal chicken cells, were compared with the lethal doses and/or concentrations of the agents in the mouse and in man. Most agents (80%) had a similar toxicity in vitro and in vivo, suggesting a lethal interference in man with basal functions common to all specialized human tissues as well as cultured cells, i.e., basal cytotoxicity. This high frequency of basally cytotoxic agents opens possibilities for screening chemicals for toxicity and for studying cytotoxic mechanisms with a standard battery of a few appropriate cell tests. This battery may be used in three ways: (1) to study cytotoxic mechanisms of all chemicals, and apply the resulting knowledge to understanding toxicity in man of basally cytotoxic agents; (2) to supplement conventional animal tests in acute toxicity test programs; (3) to screen chemicals and extracts for their potential basal cytotoxicity. To validate these ideas and to select suitable tests for the battery, results from cytotoxicity tests on a wide variety of chemicals in several in vitro systems must be compared with one another and with the toxicity of the agents in animals and man.

Drug metabolism activities of isolated rat hepatocytes in monolayer culture

The levels of cytochrome P-450 in hepatocytes cultured as monolayers for 22 hrs in Dulbecco's modified Eagle medium supplemented with serum and insulin was reduced to approximately 40% of initial values of freshly isolated hepatocytes. In correspondence with this the activities of the cytochrome P-450 monooxygenases aryl hydrocarbon (benzo(a)pyrene) hydroxylase (AHH) and ethylmorphine (EM) N-demethylase were reduced to 40 and 22% of their initial activities, respectively. Modifying the culture medium through omission of cysteine and cystine, and adding dexamethazone and delta-amino levulinic acid, increased the content of cytochrome P-450 to 59% and EM N-demethylase to 46% of initial values, but was without effect on AHH activity. However, further modifications by adding high concentrations of asparagine and leucine increased AHH activity to 62% of initial values, but did not further enhance the total content of cytochrome P-450 or the EM N-demethylase activity. The activities of cytochrome P-450 reductase, flavin containing monooxygenase, epoxide hydrolase and glutathione S-transferase decreased less (to about 70-80% of initial values) than cytochrome P-450 associated monooxygenase activities, whereas UDP-glucuronyl transferase decreased to about 50% of initial values. In contrast to what was observed regarding cytochrome P-450 and associated monooxygenase activities, modification of the incubation conditions did not affect the non-cytochrome P-450 enzymatic activities.

Mouse liver cell culture. II. Primary culture

Mouse hepatocytes in primary culture were characterized. Hepatocytes were isolated by the two-step hepatic portal vein perfusion method described previously. An optimal cell attachment of 43% was noted after 2 h incubation in 10% fetal bovine serum. Minimal attachment (less than 7%) occurred in serumless medium. Serum concentrations above 10% and attachment durations greater that 2 h resulted in no increased attachment of viable cells. Nonviable cells, however, progressively attached when both of these parameters were increased. Survival data of the cells in culture resembled those reported for rat hepatocytes in primary culture. A progressive decrease in survival was noted following initial attachment until only approximately 15% of initially plated cells remained viable and attached after 8 d culture. The decrease in survival was accompanied by morphologic changes including flattening and elongation of the cells, some multinucleation, and disruption of monolayer groups.

In vitro comparative toxicity of selected drugs and chemicals in HeLa cells, Chang liver cells, and rat hepatocytes

The cytotoxicity of 16 agents was compared in four different in vitro cellular systems: HeLa cells, Chang liver cells, freshly isolated rat hepatocytes in suspension, and primary monolayer cultures of postnatal rat hepatocytes. The compared cytotoxicity values had been obtained by different techniques: inhibition of spreading of HeLa cells by 50% after 24 hr incubation and leakage of lactate dehydrogenase (LDH) by 50% in Chang liver cells, isolated rat hepatocytes in suspension, and primary cultures of rat hepatocytes after 4, 1, and 3 or 24 hr incubation with the chemicals. Carbon tetrachloride, halothane, acetaminophen, and tetracycline were more toxic to the hepatocytes compared with the two cell lines. Alcohol, sodium salicylate, caffeine, papaverine, nitrofurantoin, and five tricyclic antidepressants showed similar toxicity in all compared cellular systems. Thus, the agents with a known or suspected metabolism-mediated liver toxicity indeed had a high differential hepatotoxicity in vitro.

Mouse liver cell culture. II. Primary culture

Mouse hepatocytes in primary culture were characterized. Hepatocytes were isolated by the two-step hepatic portal vein perfusion method described previously. An optimal cell attachment of 43% was noted after 2 h incubation in 10% fetal bovine serum. Minimal attachment (less than 7%) occurred in serumless medium. Serum concentrations above 10% and attachment durations greater that 2 h resulted in no increased attachment of viable cells. Nonviable cells, however, progressively attached when both of these parameters were increased. Survival data of the cells in culture resembled those reported for rat hepatocytes in primary culture. A progressive decrease in survival was noted following initial attachment until only approximately 15% of initially plated cells remained viable and attached after 8 d culture. The decrease in survival was accompanied by morphologic changes including flattening and elongation of the cells, some multinucleation, and disruption of monolayer groups.

Mouse liver cell culture. I. Hepatocyte isolation

A method for isolation of mouse liver cells by a two-step perfusion with calcium and magnesium-free Hanks' salt solution followed by a medium containing collagenase is described. Several variations of the commonly used procedure for rat liver cell isolation were quantitatively compared with respect to cell yield and viability. The optimal isolation technique involved perfusion through the hepatic portal vein and routinely produced an average of 2.3 x 10(6) viable liver cells/g body weight. Optimal perfusate collagenase concentration was found to be 100 U of enzyme activity per milliliter of perfusate. Light and electron microscopic evaluation of liver morphology after several steps of the isolation showed distinct morphologic changes in hepatocytes and other liver cells during perfusion. After perfusion with Hanks' calcium- and magnesium-free solution, many hepatocytes exhibited early reversible cell injury. These changes included vesiculation and slight swelling of the endoplasmic reticulum as well as mitochondrial matrix condensation. Subsequent to perfusion with collagenase, the majority of hepatocytes appeared connected to one another only by tight junctional complexes at the bile canaliculi. Multiple evaginations were seen on the outer membrane resembling microville and probably represented the remains of cell-to-cell interdigitations between hepatocytes and sinusoidal lining cells from the space of Disse. The cytoplasmic injury seen after Hanks' perfusion was reversed after collagenase perfusion. After mechanical dispersion, isolated mouse hepatocytes were spherical in shape and existed as individual cells; many (80 to 85%) were binucleated under hase contrast light microscopy. By electron microscopy, cells appeared morphologically similar in cytoplasmic constitution to that seen in intact nonaltered liver cells.

Morphological and biochemical criteria for evaluating cytotoxic effects of antiviral substances

To evaluate the cytotoxicity of antiviral substances in a primary screening programme both biological (viability and alterations of cell morphology) and biochemical methods. (51Cr release) are recommended. Because of their different sensitivities the examination of primary as well as permanent cell lines is necessary.

Screening of toxic compounds in tissue culture

To screen toxicity of chemicals most often easily manageable cultures of less differentiated cells have been used. This work includes 3 fields: (i) Screening of chemicals and fermentation broths for their cytoinhibitory effect, to predict antineoplastic activity. A related practical approach is to achieve optimal antitumour drug therapy by testing drugs on cultures of tumour cells from the patient. (ii) Screening of metal and plastic materials used in medicine, surgery and dentistry for their cytoinhibitory effect to predict local irritation. (iii) Screening of the mutagenicity or transformation capacity of chemicals in tissue culture, to predict their carcinogenicity. In addition, organ-specific cultures of most specialized cells (hepatocytes, ova, nerve cells, heart cells, skin cells, respiratory mucosa, and macrophages) have also been used to predict drug action on corresponding targets in the body. The author's group has focused on 2 new uses of standard cells for screening chemical toxicity: (i) Comparisons of in vitro cytotoxicity with in vivo toxicity of 85 randomly selected drugs indicated that for most drugs a systemic lethal action was brought about by cytotoxicity. A screening model is advocated by which results of cytotoxicity tests are compared with systemic toxicity in vivo to evaluate the systemic cytotoxicity of chemicals. (ii) Combinations of compounds with a cytotoxic lethal action in man indicated by the previous method have been screened in vitro for their combined systemic toxicity. By systematic comparison of results from standardized in vitro tests with in vivo toxicity, steps have been taken to resolve the question of the relevance of screening in tissue culture and to contribute to the development of an emerging subdiscipline to toxicology -- in vitro cytotoxicology.