Effects of a hormone-supplemented medium on cytochrome P-450 content and mono-oxygenase activities of rat hepatocytes in primary culture

Measurement of Xenobiotic Metabolising Enzyme Activities in Primary Monolayer Cultures of Immature Rainbow Trout Hepatocytes at Two Acclimatisation Temperatures

Rainbow trout hepatocytes with a high viability were isolated by two-step collagenase perfusion through the portal vein. The yield was 1-2.5 x 106cells/g body weight. Culture conditions were defined for providing enhanced attachment and long-term cell survival at 7 ± 0.5°C and 15 ± 0.5°C, respectively. The hepatocytes, attached to Primaria™ plastic and cultured in Leibowitz L-15 medium with 9% fetal calf serum, were maintained as monolayers for 6–7 days. The activities in hepatocytes from immature trout of the biotransformation enzymes ethoxyresorufin-O-deethylase (EROD), aldrine epoxidase (AEPOX), NADPH-cytochrome c reductase (NCR), glutathione-S-transferase (GST) and UDP-glucuronosyltransferase (UDPGT), were all stable during the culture period. Differences in enzyme stability and activity (particularly the activity of EROD) between hepatocytes from different fish were observed at both temperatures. The temperature did not influence the activities of EROD or NCR, whereas AEPOX showed metabolic compensation. Both GST and UDPGT exhibited inverse temperature compensation. Hepatocyte monolayers, cultured from immature trout, may provide a useful system in pharmacological and toxicological research for investigating drug metabolism.

Insulin-mediated modulation of cytochrome P450 gene induction profiles in primary rat hepatocyte cultures

In this investigation, we examined the effects of insulin on gene induction responsiveness in primary rat hepatocytes. Cells were cultured for 72 hours either in the absence or presence of 1 microM insulin and then exposed to increasing concentrations of phenobarbital (PB; 0.01-3.5 mM). Culturing in the absence of insulin produced 1.5-2-fold increases in the induction magnitude of CYP2B1 and CYP2B2 mRNA expression resulting from PB exposures, without altering the bell-shaped dose-response curve characteristic of this agent. However, for the CYP3A1 gene, insulin removal led to a pronounced shift in both the PB-induction magnitude and dose-response relationships of the induction response, with higher levels of CYP3A1 expression resulting from exposures to lower concentrations of inducer. Insulin removal also reduced the time required to attain maximal induction of CYP2B1/2 and CYP3A1 gene expression. The insulin effects were not specific for PB induction, as insulin deprivation similarly enhanced both dexamethasone- and beta-naphthoflavone-inducible CYP3A1 and CYP1A1 expression profiles, respectively. In contrast, the level of albumin mRNA expression was reduced considerably in cells deprived of insulin. We conclude that insulin is an important regulator of inducible and liver-specific gene expression in primary rat hepatocytes.

Effects of prostaglandins on ethanol damage in primary cultured rat hepatocytes

OBJECTIVES

Several reports demonstrated that ethanol administration impairs the DNA synthesis in rat hepatocytes. Also, it has been demonstrated that prostaglandin (PG) helps prevent membrane damage by hepatotoxic chemicals. In this study, the authors examined PG's effects on the toxicity of ethanol in the primary culture of rat regenerations.

METHODS

We examined two kinds of parameters, i.e., DNA synthesis and lipid peroxidation in the primary culture of rat hepatocytes. Hepatocytes were isolated by the collagenase perfusion method. The rate of DNA synthesis was determined by pulse-labelling cultured cells with [3H]-thymidine. Incorporation of (3H)-thymidine was determined by liquid scintillation spectrophotometer. DNA content was measured by the fluorescence spectrophotometer. The lipid peroxidation was assayed with spectrophotometer.

RESULTS

The results were as follows: 1) PG family (PGA1, PGD2, PGE1, PGE2, PGG2a, PGI2 & Thromboxane B2) stimulated the DNA synthesis of hepatocytes (especially PGD2 and PGE1), 2) ethanol decreased DNA synthesis by clear dose-dependent manner, 3) the combined treatment of PGD2 or PGE1, prevents the decreasing of DNA synthesis, which was induced by ethanol, 4) in ethanol treatment, lipid peroxidation was decreased significantly, but PGD2, PGE1 and PGA1 were not affected, and 5) PGD2, PGE1 and PGA1 decreased lipid peroxidation with ethanol, significantly.

CONCLUSIONS

From these results, we concluded that PG could be useful for the treatment of degenerative liver disease and alcohol-induced liver disease in the assumption that further studies on the action mechanisms of PG will continue.

The role of metabolism in 2-methoxyethanol-induced suppression of in vitro polyclonal antibody responses by rat and mouse lymphocytes

Previous studies from this laboratory have shown that the glycol ether 2-methoxyethanol (ME) produces immunosuppression in the rat but not in the mouse. To investigate possible mechanisms for this species difference in ME-induced immunotoxicity, the effects of ME and its metabolites, 2-methoxyacetic acid (MAA) and 2-methoxyacetaldehyde (MAAD), on in vitro polyclonal antibody responses by Fisher 344 rat and B6C3F1 mouse lymphocytes, were studied. MAAD and MAA suppressed IgM and IgG production by both mouse and rat lymphocytes at non-cytotoxic doses. However, ME had no effect on antibody production by either mouse or rat lymphocytes. Lower concentrations of MAA suppressed rat lymphocyte IgM and IgG production (at 0.5 and 1.0 mM MAA, respectively) compared with mouse lymphocytes (2.0 mM MAA). IgM and IgG production by both rat and mouse lymphocytes was suppressed at comparable concentrations of MAAD (0.3 mM MAAD). The role that metabolism of ME to its immunosuppressive forms plays in this in vitro suppression was demonstrated using hepatocyte-lymphocyte co-cultures. IgM production by both mouse and rat lymphocytes was suppressed at a lower concentration of ME following co-culture with mouse (12.5 mM ME) versus rat (25 and 50 mM ME) hepatocytes. These in vitro results indicate that rat lymphocytes are more sensitive to MAA than are mouse lymphocytes and that mouse hepatocytes have a greater capacity to metabolize ME to its immunosuppressive metabolites than do rat hepatocytes. In addition, MAAD is more immunotoxic than MAA, suggesting that this metabolite may be the proximate immunotoxicant. These observation may partially explain the species differences in ME-induced immunosuppression in vivo.

Effect of pyridine on the expression of cytochrome P450 isozymes in primary rat hepatocyte culture

In vivo administration of pyridine has been shown to increase the activity and content of several forms of cytochrome P450 by transcriptional and posttranscriptional mechanisms. The effect of pyridine on CYP1A and CYP2E1 isozymes was studied in a rat hepatocyte culture model. Hepatocytes were isolated from non-induced rats and seeded onto matrigel-coated dishes and incubated in William's medium E containing 10% fetal calf serum, hormones, and essential metals. Cultures were treated with 0, 10 or 25 mM pyridine for 1-3 days and microsomes were isolated to determine catalytic activity and for immunoblot analysis, and total RNA was isolated for mRNA determinations. CYP2E1 content, CYP2E 1 mRNA, and CYP2E1 catalyzed oxidation of p-nitrophenol declined during culture to values of 3, 30 and 19% that of initial, non-cultured controls by day 3 of culture. Pyridine prevented this decline of CYP2E1 protein and activity such that 60-80% original activity remained after 3 days of culture in the presence of 25 mM pyridine. However, pyridine did not prevent the fall in CYP2E1 mRNA levels, nor did pyridine increase the content or activity of CYP2E1 above initial values of microsomes from freshly isolated hepatocytes. Pyridine increased the content of CYP1A2 and the oxidation of ethoxyresorufin 2-4 fold compared to cultures incubated without pyridine over the 3 day culture period. CYP1A1 levels, which rapidly declined, were induced and maintained in the presence of pyridine. Pyridine increased CYP1A content and activity 2-3 fold over initial values of freshly isolated hepatocytes. These increases were associated with corresponding increases in CYP1A mRNA levels. CYP1A2, but not CYP1A1, mRNA levels increased in the cultures incubated in the absence of pyridine. These results indicate that pyridine has different effects on CYP1A and CYP2E1 in this hepatocyte culture model. Pyridine appears to modulate CYP2E1 levels by posttranscriptional mechanisms as CYP2E1 activity and content were maintained in the presence of pyridine under conditions in which CYP2E1 mRNA levels declined. These mechanisms may involve increased translational efficiency of existing CYP2E1 mRNA or stabilization of CYP2E1 protein against degradation. Pyridine increased CYP1A1 and CYP1A2 content, activity and mRNA levels, either inducing CYP1A transcription or stabilizing CYP1A mRNA. Hepatocyte cultures may be a useful model to study the interaction of pyridine with P450 isozymes and their associated drug-mediated toxicity.

Rat hepatocyte cultures and co-cultures in biotransformation studies of xenobiotics

Long-term cultures of hepatocytes could represent a suitable in vitro model for biotransformation studies of xenobiotics. At present however, no ideal culture system can be proposed since, in all existing models, phenotypic changes occur, affecting selectively some components of phase I and/or phase II xenobiotic metabolism. From the authors' own results and recent studies of several other investigators, carried out on rat hepatocytes, it becomes clear that four groups of factors may affect biotransformation capacity: soluble medium factors, extracellular matrix components, cell-cell interactions and factors affecting replication. For the maintenance of liver-specific functions, it seems of utmost importance that the tridimensional shape of the hepatocytes is kept. Usually, phase II enzymatic activity is better kept than that of phase I. The cytochrome P450 dependent monoxygenases, in particular, are easily lost. Interesting is the observation that co-cultures of rat hepatocytes with rat liver epithelial cells exhibit higher and much better preserved phase I and phase II biotransformation than monocultures. Clearly, further research is needed to improve this promising in vitro model.

In vitro methodologies for enhanced toxicity testing

This report will give a general overview of some of the in vitro methodologies used in toxicity testing. The use of computer-based structure-activity relationships and cell culture testing systems can provide valuable toxicological data for hazard and risk assessments. In vitro systems allow for a more rapid identification of toxic compounds and can be utilized to study mechanisms of toxicity at the cellular and subcellular level. The data derived from these types of studies can be used to improve the predictability of animal models for chemical or drug toxicity. This report focused on primary hepatocytes as an in vitro model for cytotoxicity and metabolic studies.

Transcriptional- and post-transcriptional-dependent regulation of glutathione S-transferase expression in rat hepatocytes as a function of culture conditions

Transcriptional activity of the glutathione S-transferase (GST) alpha (subunits 1 and 2), mu (subunits 3 and 4) and pi (subunit 7) gene families has been analyzed using the nuclear 'run-on' technique on adult rat hepatocytes maintained for 4 days in conventional culture and for 4 and 12 days in co-culture with rat liver epithelial cells. Several medium conditions are included in this study, namely with or without fetal calf serum and with nicotinamide or dimethylsulphoxide. Hepatocytes co-cultured for 4 days maintain approximately 30-70% of the alpha gene family transcriptional activity, whatever the medium conditions, when compared to freshly isolated hepatocytes. A marked decrease is observed after 12 days of co-culture or when hepatocytes are maintained in conventional culture. The transcriptional activity of the mu gene family is maintained at 40-160% when hepatocytes are cultured with or without fetal calf serum, and is inducible by nicotinamide (approximately 4-fold) and dimethylsulphoxide (approximately 2-fold) in conventional culture and/or in co-culture. In contrast to freshly isolated hepatocytes, GST pi gene transcriptional activity is observed in conventional and co-cultured hepatocytes, irrespective of the medium conditions. Dimethylsulphoxide treatment however, represses the expression of GST 7 in vitro. These results demonstrate that the expression of GST alpha, mu and pi genes in conventional and co-cultured rat hepatocytes is controlled primarily at the level of transcription. It cannot be excluded, however, that dimethylsulphoxide stabilizes the GST mRNA levels in vitro.

Importance of hepatocyte culture conditions in dimethylnitrosamine-induced suppression of antibody response in the mixed cultures of murine hepatocytes and splenocytes

The role of the hepatocyte culture media in dimethylnitrosamine (DMN)-induced suppression of antibody responses by splenocytes against sheep erythrocytes (SRBCs) was investigated using the mixed culture system of murine hepatocytes and murine splenocytes. It was observed that hormone-supplemented complete media was required for hepatocyte cultures to optimally activate DMN to its immunosuppressive form(s). In the absence of the hormone supplement, the concentration of DMN required to produce a 50% suppression (IC50) was increased by over 10-fold (i.e., compare the IC50 in complete media of less than 0.5 mM to the IC50 in basal media of almost 10.0 mM). In contrast, the activation of cyclophosphamide (cytoxan, CTX), which was used in these studies as a comparative control, was not affected by the absence of the hormone supplement. These results indicate that the observed effect on the activation of DMN was not due to a generalized loss of metabolic capability of hepatocytes cultured without hormones. To examine the role of drug metabolizing capabilities of hepatocytes in the differential activation of DMN, we compared phase I and phase II enzyme activities of hepatocytes cultured for 24 h in either basal media or hormone-supplemented complete media. Our results indicated that there was a significant decrease of phase I monooxygenase activities of cultured hepatocytes when compared to freshly isolated hepatocytes. However, our results failed to show any difference in the activities of hepatocytes cultured in the two media. Most notably, there was no difference in the activity of either high- or low-affinity DMN demethylase, as measured by the generation of formaldehyde. We observed a similar profile with phase II conjugative capabilities, specifically sulfotransferase and glucuronyltransferase. These results indicate that the activation of DMN to its immunosuppressive form(s) can be modulated in the co-culture system by culturing hepatocytes under different conditions. Because we failed to show any differences in the metabolic capabilities of hepatocytes cultured under the two media conditions, the results suggest that the modulation of immunosuppressive activity may not be related to a change in the generation of the immunosuppressive metabolite(s).

Maintenance of mixed-function oxidase and conjugation enzyme activities in hepatocyte cultures prepared from normal and diseased human liver

Isolated hepatocytes were prepared from normal and diseased human livers and maintained in primary monolayer culture for up to 96 h. The viability and yields of cell preparations obtained from diseased livers did not differ significantly from those obtained from normal livers. During the culture period a significant increase in cell protein/DNA ratio was observed in both normal and diseased hepatocytes. The maintenance of a number of drug metabolising enzyme activities was determined in these hepatocytes during 96 h of culture. In normal hepatocytes the maintenance pattern of mixed-function oxidase activities (ethoxycoumarin-O-deethylase and ethoxyresorufin-O-deethylase) was clearly different from that of the conjugating enzymes (sulfotransferase and glutathione transferase). Whereas ethoxycoumarin-O-deethylase and ethoxyresorufin-O-deethylase activities declined sharply over the first 24 h in culture and then either totally or partially recovered, sulfotransferase and glutathione transferase activities were found to be relatively more stable initially but thereafter decline progressively. In diseased hepatocytes mixed-function oxidase activities were maintained less well than the corresponding activities in normal hepatocytes whereas conjugation enzyme activities were maintained to a similar extent.

Characterization and induction of xenobiotic metabolizing enzyme activities in a primary culture of rainbow trout hepatocytes

1. A primary cell culture from rainbow trout (Oncorhynchus mykiss) liver was prepared and evaluated for biotransformation of xenobiotics. 2. The hepatocytes maintained cytochrome P-450 content, as well as their cytochrome P-450-dependent activities, stable for 5-6 days in serum-free medium. Protein and glutathione levels, as well as other enzyme activities important for biotransformation, were close to their fresh cell levels throughout the culture period. 3. The cells were also responsive to cytochrome P-450 inducers. Both beta-naphthoflavone (BNF) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) caused an increase in ethoxyresorufin O-deethylase (EROD) activity, which was dose-dependent over the concentration ranges of 3.6-360 nM and 2.5-100 pM, respectively. The induced activities in BNF exposed cells returned to basal levels within 48 h after replacing the medium with a BNF-free medium. Exposure of cells to TCDD (100 pM) for 48 h induced EROD activity which, in contrast to response of BNF-exposed cells, continued to increase after the medium had been replaced with TCDD-free medium. 4. The results show that trout hepatocytes in primary culture afford a reliable in vitro method for studying the regulation and functions of xenobiotic biotransformation enzymes, and for defining toxic effects of aquatic pollutants in cells.

Co-cultures of hepatocytes with epithelial-like cell lines: expression of drug-biotransformation activities by hepatocytes

To improve long-term expression of drug biotransformation activities in hepatocytes, we have examined the suitability of several epithelial-like cell lines (MDCK, MS and L-132) for supporting functional co-cultures with rat hepatocytes. Cells were selected on the basis of their compatibility with hepatocytes, formation of stable monolayers in the absence of serum and lack of drug biotransformation activities. The expression of individual elements of the biotransformation system was evaluated in these co-cultures. Co-cultured hepatocytes remained viable and showed a characteristic polygonal shape for more than a week. Depending on the cell line used, levels of aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase activities of co-cultured hepatocytes oscillated between 24-47% of their initial value after 4 days in culture. The highest levels of monooxygenase activity were found in hepatocytes co-cultured with MS cells (41-47%). In contrast, these activities decreased to 6% when hepatocytes were maintained in pure culture for the same period. The activities of the conjugating enzymes UDP-glucuronyltransferase and glutathione S-transferase were maintained at nearly the initial levels during the complete period of study, both in pure and mixed-cultures, regardless of the cell line used. MS cells adapted themselves much better to serum-free culture conditions, and the co-culture with rat hepatocyte was technically easier. After one week, total cytochrome P450 and reduced glutathione in rat hepatocytes/MS co-cultures were 31% and 127% respectively of the day O values, whereas they were undetectable in pure culture. A clear induction of monooxygenase activities by methylcholanthrene, phenobarbital and ethanol could be observed by the 5th day in MS cells/hepatocyte co-cultures. The fact that the results of our work show the suitability of MS cells, an epithelial-derived cell line, for improving the expression of biotransformation enzymes of cultured hepatocytes opens new possibilities of simplifying co-cultures for their use in drug-metabolism studies.

Drug metabolizing enzymes in rat hepatocytes co-cultured with cell lines

We have developed new co-cultures of continuous cell lines 3T3 (clone A31) and C3H/10T1/2 (clone 8) with hepatocytes as an alternative to co-cultures with noncontinuous epithelial cells. In this biological system we studied in detail the expression of the hepatic biotransformation system. After 7 d in culture, total cytochrome P-450 content and the monooxygenase activities aryl hydrocarbon hydroxylase and 7-ethoxycoumarin o-deethylase still maintained about 30% of their initial value, whereas in pure cultured hepatocytes these activities were undetectable. A significant response to induction by methylcholanthrene and phenobarbital of monooxygenase activities was observed in co-cultures for 7 d. NADPH-cytochrome c reductase activity remained unchanged for at least 7 d in co-cultured hepatocytes, whereas in pure cultures this activity was reduced to about 75% of the initial value after only 24 h. Finally, the activity of the conjugating enzymes UDP-Gt and GSH-t was maintained at nearly the initial levels during the complete period of study. The easy handling of continuous cell lines and the maintenance of the biotransformation system of hepatocytes in co-culture make this approach simpler and easier to standardize.

Phase I and phase II xenobiotic biotransformation in cultures and co-cultures of adult rat hepatocytes

The aim of this study was to measure the activity of phase I and II key enzymes in the biotransformation of xenobiotics and their inducibility by phenobarbital (2 mM) in two currently used in vitro models, namely adult rat hepatocytes, conventionally cultured or co-cultured with rat epithelial cells derived from primitive biliary duct cells. For phase I, the cytochrome P450 content and the enzymic activities of 7-ethoxycoumarin O-deethylase and aldrin epoxidase have been determined, for phase II glutathione S-transferase activity was measured. In conventional cultures, all phase I parameters investigated declined continuously as a function of culture time. Two mM phenobarbital had inducing effects on 7-ethoxycoumarin O-deethylase and glutathione S-transferases but not on aldrin epoxidase. In co-cultures, after an initial decrease, a steady state situation developed for all the parameters measured, lasting for at least 10 days. The cytochrome P450 content, the 7-ethoxycoumarin O-deethylase, aldrin epoxidase and glutathione S-transferase activities were maintained from 3 to 4 days on at 25, 100, 15 and 50%, respectively, of their corresponding value obtained for freshly isolated hepatocytes. After phenobarbital treatment, the parameters mentioned were significantly increased with the exception of the aldrin epoxidase activity of which the inducibility was nearly completely lost.

Effect of epinephrine on glycogen phosphorylase-alpha in various preparations of rat liver

1. Glycogen phosphorylase-alpha, a commonly used index of cytosolic free calcium, was compared in various preparations of rat liver in the absence and presence of 0.1 microM epinephrine. 2. Total phosphorylase in isolated perfused livers and freshly-isolated hepatocytes were the same as that observed in liver in situ; however, phosphorylase-alpha was 50% higher in perfused liver and 80% higher in hepatocytes than activities measured in situ. Total phosphorylase was reduced approximately 50% in hepatocytes maintained in primary culture for 24 hr. 3. Epinephrine increased phosphorylase-alpha approximately 2-fold in livers perfused for 30 min but only about 20% in hepatocytes incubated for 30 min. After 90 min of perfusion or incubation, epinephrine increased phosphorylase-alpha nearly 4-fold in perfused livers and only 30% in isolated hepatocytes. The results suggest that amounts of free calcium and calcium-dependent coupling of adrenergic receptors to phosphorylase-alpha differ markedly between the intact liver and isolated hepatocytes.

The maintenance of cytochrome P-450 in rat hepatocyte culture: some applications of liver cell cultures to the study of drug metabolism, toxicity and the induction of the P-450 system

Treatments affecting the loss of cytochrome P-450 in rat hepatocyte culture are reviewed and the way in which these have produced an understanding of the mechanisms involved are discussed extensively. A simple way to prevent the loss of P-450 in hepatocytes is to culture them with 0.5 mM metyrapone which appears to restore the cytochromes' synthesis and degradation to steady state values. Knowledge of this mechanism has led to the formulation of special culture medium and the application of both culture systems to the study of drug metabolism and toxicity are described. Finally the effect of these culture systems on the expression of the multiple forms of cytochrome P-450 are presented to illustrate the potential of cultured hepatocytes in induction studies.

Dependence of the induction of cytochrome P-450 by phenobarbital in primary cultures of adult rat hepatocytes on the composition of the culture medium

A chemically defined medium developed for the maintenance of differentiated adult rat hepatocytes (T1) was compared with two commercially available media (Waymouth 752/1 and Leibovitz L-15) for maintenance of cytochrome P-450 metabolic activity in cultured hepatocytes. Specific metabolic activities of initially isolated cells and 72-hr control and phenobarbital-treated cultures were determined with 7-ethoxycoumarin, 7-ethoxyresorufin, and 7-pentoxyresorufin as substrates. Control and phenobarbital-treated cultures in T1 medium had a higher metabolic activity towards each of the three substrates than comparable cultures in the other media. These studies indicated that the metabolic activity and the response to phenobarbital of the major isozyme of the phenobarbital-inducible family of cytochrome P-450 were maintained in hepatocytes in T1 medium. However, there was anomalous expression and induction by phenobarbital of the major 3-methylcholanthrene-inducible isozyme, cytochrome P-450c, in cultured hepatocytes in each of the three media tested, but this response was more pronounced in T1 medium. In conclusion, the regulation of cytochrome P-450 metabolic activity in cultured hepatocytes was shown to be dependent on the composition of the culture medium.

Changes in the quantity and activity of cytochrome P-450 isozymes in primary cultured rat hepatocytes

Hepatocytes from male Sprague-Dawley rats pretreated with a cytochrome P-450 inducer, 3-methoxy-4-aminoazobenzene (3-MeO-AAB), 3-methylcholanthrene (MC) or phenobarbital (PB), were cultured in vitro, and changes in the quantity and activity of microsomal cytochrome P-450 isozymes in the cells were determined by means of immunochemical methods and a bacterial mutation test, respectively. The results of enzyme-linked immunosorbent assay using monoclonal antibodies against rat P-450 isozymes revealed that the amount of cytochrome P-450d induced by 3-MeO-AAB or MC declined rapidly during culture and fell to 10 to 15% of the initial value after 24 h. A similar tendency was observed with PB-induced cytochrome P-450b/e. By contrast, cytochrome P-450c in MC-induced hepatocytes declined more slowly than cytochrome P-450d and remained at 45 to 60% of the initial value after 24 h. Similar quantitative changes of the individual cytochrome P-450 isozymes in culture were also observed by immunoblotting using the anti-cytochrome P-450 monoclonal antibodies. Changes in the activities of individual cytochrome P-450 isozymes in hepatocytes by culture were in accordance with the quantitative changes of the cytochromes, as determined by a mutation test using Salmonella typhimurium TA 98 and carcinogenic aromatic amines. These results indicate that microsomal cytochrome P-450c in primary cultured rat hepatocytes is more stable in culture, in terms of both quantity and activity, than cytochrome P-450d and P-450b/e.

The influence of culture medium composition on drug metabolising enzyme activities of the human liver derived Hep G2 cell line

When grown in the standard Dulbecco's medium the human liver derived Hep G2 hepatoma cell line shows only 10-20% of the cytochrome P-450-dependent mixed function oxidase (MFO) activity of freshly isolated human adult hepatocytes. However, the MFO activities and, to a lesser extent, the activities of UDP-glucuronyltransferase and glutathione-S-transferase can be increased by altering the composition of the growth medium. Modified Earle's medium was more effective in this respect than Williams' E medium and increased the O-dealkylations of ethoxyresorufin, benzyloxyresorufin and pentoxyresorufin 50-, 30- and 10-fold, respectively.

The expression and metabolic activity of cytochrome P-450 isozymes in control and phenobarbital-induced primary cultures of rat hepatocytes

The expression and activity of the phenobarbital (PB)-inducible P-450 isozymes, P-450b and P-450e, and the major 3-methylcholanthrene (MC)-inducible form, P-450c, were studied in primary cultures of adult rat hepatocytes in T1, Leibovitz L-15 (L-15), and a modification of Waymouth 752/1 (Way) media. P-450 isozymes in initially isolated hepatocytes and control and PB-treated cultures were quantitated by Western blot analysis, and activity was determined with 7,12-dimethylbenz[a]anthracene (DMBA) as substrate. Data from the Western blot analysis correlated well with the metabolic activity toward DMBA. P-450b was consistently induced by PB in hepatocytes in T1 and to a lesser extent in Way. P-450e protein was constitutive in initially isolated cells, expressed in control cultures at a reduced level, and increased or maintained by PB in all three media. DMBA metabolite formation associated with P-450b and P-450e activity was induced by PB in hepatocytes in T1 and Way and was inhibited by antibodies to P-450b. P-450c was only infrequently expressed in freshly prepared hepatocytes, but was detected in all control and PB-treated cultures although at a much higher level in T1. Thus, the amounts of P-450 isozymes, their inducibility by PB, and their activity toward DMBA were found to be dependent on the medium. We have demonstrated enzyme induction and increased activity of the major PB-inducible isozymes in hepatocytes in T1; these are also associated with a change in the control of P-450c expression leading to enhanced constitutive expression and inducibility by phenobarbital.

Expression of five forms of microsomal cytochrome P-450 in primary cultures of rabbit hepatocytes treated with various classes of inducers

In order to investigate the expression of five different forms of microsomal cytochrome P-450 including P-450 2 (P450IIBI), 3b (P450IIC3), 3c (P450IIIA4), 4 and 6 (P450IA2 and A1), hepatocytes were isolated from untreated rabbit and maintained in primary monolayer cultures in serum free modified Waymouth medium in the absence and in the presence of various classes of inducers including phenobarbital (PB), rifampicin (RIF), dexamethasone (DEX) and B-naphthoflavone (BNF). In untreated cultures the level of the various forms of P-450, determined by immunoblot with the use of specific antibodies, generally declined with time but at markedly different rates. In cultures treated with the inducers decline of the various forms was either unaffected, reduced, or even reversed, so that 96 hr after plating some of these forms appeared to be induced several-fold with respect to the untreated cultures. The forms 2 and 3c were co-induced by PB, RIF or DEX; as in vivo, BNF induced forms 4 and 6. Induction of forms 2, 3c, 4 and 6 was accompanied by stimulation of related monooxygenase activities, benzphetamine demethylase, progesterone 6B hydroxylase and benzpyrene hydroxylase and ethoxyresorufin deethylase, respectively. In all cases, induction was accompanied by an increased rate of de novo synthesis of the protein, determined by radio-immunoprecipitation assay with the use of specific antibodies on [3H]-Leu labeled cell lysate. Both induction and increased de novo synthesis were time- and inducer concentration-dependent. In cultures treated with RIF or BNF de novo synthesis of P-450 3c or of P-450 4 and 6 was correlated with the level of their specific mRNA quantitated from northern blots probed with either pLM3c-4.1 or pLM6.1, two plasmids containing inserted cDNA coding for P-450 3c or P-450 6, respectively. We conclude from these experiments that rabbit hepatocytes in primary monolayer cultures represent suitable models for studying regulation induction and pharmacological implications of the microsomal cytochromes P-450.

Effect of dexamethasone on cytochrome P-450 mediated metabolism of 2-acetylaminofluorene in cultured rat hepatocytes

The metabolism of 2-acetylaminofluorene (AAF) to its six oxidative metabolites has been used to investigate the effect of dexamethasone on cytochrome P-450 activity in cultured rat hepatocytes. In control hepatocytes the metabolism of AAF to its 1-, 5-, 7-, 9- and N-hydroxylated metabolites rapidly declined in culture over the first 24 hr while 3-hydroxylation remained relatively constant. These activities either remained unchanged or increased slightly during the next 48 hr in culture. The addition of dexamethasone (100 nM) to the culture medium had little effect in arresting the initial decline but by 72 hr the 7-, 5- and 3-hydroxylations increased to values 2.5, 16 and 21 times the respective 24-hr values. The inductive effect of dexamethasone on the 3- and 5-hydroxylations of AAF was maximal at 100 nM whereas the 7-hydroxylation increased linearly as a function of the dexamethasone concentration up to 1 microM. Cortisol and corticosterone and the non-glucocorticoids fluoxymesterone and methyltestosterone induced a pattern of AAF metabolism resembling that in dexamethasone-treated cultures, suggesting that a range of steroids not restricted to glucocorticoids may induce multiple cytochrome P-450 isozymes via related mechanisms. Pregnenolone 16 alpha-carbonitrile induced only the 7-hydroxylation of AAF probably reflecting induction of cytochrome P-450p. While dexamethasone was a strong inducer of the 3- and 5-hydroxylations of AAF in hepatocyte culture, assay of these activities in freshly isolated cells after in vivo treatment with dexamethasone showed a strong induction of 7-hydroxylation but only small effects on 3- and 5-hydroxylations. Indeed the profile of AAF metabolism induced in culture by dexamethasone resembles more closely the profile induced by 3-methylcholanthrene in vivo. These data suggest that factors yet to be identified strongly influence the steroid-induced pattern of cytochrome P-450 gene expression.

Strain differences in the maintenance of cytochrome P-450 and mixed-function-oxidase activities in cultured rat hepatocytes. Effect of prostaglandins

The mixed-function-oxidase (MFO) activities, ethoxyresorufin and pentoxyphenoxazone O-dealkylase, of cultured Hooded-Lister(HL)-rat hepatocytes declined rapidly during 72 h of culture, whereas in Sprague-Dawley(SD)-rat hepatocytes the MFO activities increased between 24 and 72 h in culture. Cytochrome P-450 content declined at the same rate in both HL- and SD-rat hepatocyte cultures. NADPH:cytochrome c reductase and NADH:cytochrome b5 reductase were more stable in SD- than in HL-rat hepatocyte cultures. 16,16-Dimethylprostaglandins E2 and F2 alpha improved the maintenance of cytochrome P-450 content, MFO activity and NADPH:cytochrome c reductase in the HL-rat hepatocyte cultures. In SD-rat hepatocytes, the prostaglandins had no effect on cytochrome P-450 content or NADPH:cytochrome c reductase activity, whereas they prevented the increase observed in MFO activities between 24 and 72 h after culture.

Studies on the maintenance of cytochromes P-450 and b5, monooxygenases and cytochrome reductases in primary cultures of rat hepatocytes

The cytochrome P-450 content of rat hepatocytes declined rapidly over 72 h in culture, due primarily to denaturation to cytochrome P-420. Six different media were investigated for their ability to conserve cytochrome P-450 during culture, and the most successful was a modified Earle's medium. After 72 h culture in this medium, cytochromes P-450 and b5, NADH-cytochrome b5- and NADPH-cytochrome c-reductases were maintained at 40, 100, 35 and 52% of fresh cell values, respectively. Cytochrome P-450 showed differential functional stability during culture with ethoxyresorufin O-deethylation being more stable than either pentoxyphenoxazone O-depentylation or biphenyl 4-hydroxylation. Monooxygenase than did cytochrome P-450 content. This discrepancy was not explained by loss of flavin nucleotides, FMN or FAD.

Activities of several phase I and phase II xenobiotic biotransformation enzymes in cultured hepatocytes from male and female rats

Hepatocytes were isolated from adult male and female rats and maintained in monolayer culture for up to 24 hr. The degree of preservation of representative phase I and phase II xenobiotic biotransformation enzymes was studied in these cells immediately after isolation, after attachment in culture, and after 24 hr in culture. Regarding phase I pathways, hepatocytes during 24 hr lost 50% of cytochrome P-450, but maintained high mixed function oxidase activities; 75% of aryl hydrocarbon hydroxylase and 65% of benzphetamine demethylase activities were preserved in hepatocytes from males, whereas in hepatocytes from females 70 and 50% of these activities, respectively, were maintained. Of phase II pathways, glutathione transferase activity after 24 hr, tested toward 1,2-dichloro-4-nitrobenzene as substrate, was diminished in male hepatocytes to 20% of the initial liver activity and in female cells, to 35%, whereas the activity tested toward 1-chloro-2,4-dinitrobenzene as substrate was stable. UDP-glucuronosyltransferase activities, tested toward p-nitrophenol and phenolphthalein as substrates, were slightly increased during 24 hr of culture of hepatocytes to levels higher than in liver before perfusion. The level of UDP-glucuronic acid, the endogenous substrate for the enzyme, was reduced after isolation to only 6% of the initial liver value, and then increased during culture to a level approximately 60% of normal. Thus, the changes in xenobiotic biotransformation enzymes and associated constituents in cultured hepatocytes were not uniform, although biotransformation capability remained reasonably intact.

Toxicity and mutagenicity of 6 anti-cancer drugs in Chinese hamster V79 cells co-cultured with rat hepatocytes

Toxicity and induction of 6-thioguanine-resistant mutants in Chinese hamster V79 cells, co-cultured with or without isolated rate hepatocytes, by 6 anti-cancer drugs (cyclophosphamide, adriamycin, methotrexate, cytosine arabinoside, 6-mercaptopurine and vincristine) were studied. The effect of hepatocyte density on the cloning efficiency and recovery of mutants was found using dimethylnitrosamine as a positive control. In the absence of hepatocytes, this compound was neither toxic nor mutagenic to V79 cells, but in their presence it was highly mutagenic and extremely toxic. The cloning efficiency and mutation frequency of control (untreated) cells was unaffected by hepatocyte density. All the drugs were toxic to V79 cells, although different responses were found for certain of them depending upon whether hepatocytes were present or not. Cyclophosphamide and adriamycin were clearly mutagenic, and 6-mercaptopurine only weakly so. A slight mutagenic effect was seen for cytosine arabinoside, but both methotrexate and vincristine were negative. Here also, the presence or absence of hepatocytes was important.

Maintenance and induction of cytochrome P-450 in cultured rat hepatocytes

Maintenance of microsomal cytochrome P-450 content by cultured rat hepatocytes has proven an elusive goal. It is reported here that exogenous heme maintains cytochrome P-450 content of cultured rat hepatocytes at high levels during the first 72 h of incubation. The maintenance studies have been expanded to demonstrate the in vitro induction of cytochrome P-450 by phenobarbital treatment. The induction of P-450 in vitro by phenobarbital required the trace element, selenium, in the presence of exogenous heme. The present findings suggest that selenium, and other trace elements, may have an essential role in the formation of holocytochrome P-450 in vitro.

Glutathione status in primary cultures of rat hepatocytes and its role in cell attachment to collagen

In primary cultures of rat hepatocytes the intracellular level of reduced glutathione (GSH) declines to approx. 50% of that observed in freshly isolated cells within 1 h of culture. Pretreatment of freshly isolated hepatocytes with diethylmaleate (DEM) to deplete GSH and inhibition of glutathione synthesis by buthionine sulfoximine (BSO) markedly decrease the proportion of cells attaching to the collagen coated culture dishes. A positive correlation between the intracellular content of GSH and the ability of hepatocytes to attach to collagen is observed. Presence of dithiothreitol (DTT) in the culture medium efficiently prevents hepatocyte attachment. A net increase in hepatocyte disulfides is also observed after the first hours of culture. The formation of disulfides seems to be essential for the attachment of hepatocytes to collagen. The depletion of GSH in the early period of culture is probably due to its regulatory function of thiol/disulfide groups in proteins and/or its involvement in the synthesis of essential cytoskeletal proteins.

Effects of butylated hydroxytoluene pretreatment on the metabolism and genotoxicity of aflatoxin B1 in primary cultures of adult rat hepatocytes: selective reduction of nucleic acid binding

To elucidate biochemical mechanisms underlying the anticarcinogenic activity of butylated hydroxytoluene (BHT), studies were undertaken to characterize the influence of BHT pretreatment on the metabolism and genotoxicity of aflatoxin B1 (AFB1) in primary cultures of rat hepatocytes. During a 10-day pretreatment period, adult male rats were fed either a control diet or a diet supplemented with 0.5% BHT. Hepatocytes were subsequently isolated from each animal and cultured in chemically defined medium. Cultures prepared from rats which had been fed BHT metabolized AFB1 more rapidly than did controls. BHT pretreatment also enhanced oxidation of AFB1 to aflatoxin M1 (AFM1), and accelerated the rate of AFM1 conjugation. Covalent binding to DNA and RNA in BHT-pretreated cultures was reduced by 91 and 82%, respectively, while protein binding decreased by only 29%. AFB1 did not stimulate detectable DNA repair synthesis in BHT-pretreated cultures, although stimulation of DNA repair was clearly evident in control cultures. In a separate experiment, consistently higher baseline concentrations of reduced glutathione were observed in BHT-pretreated cells, indicating that BHT pretreatment may enhance formation of detoxified glutathione conjugates of AFB1. These findings suggest that the anticarcinogenic activity of BHT is due in part to preferential enhancement of hepatic detoxification mechanisms, with the result that intracellular concentrations of reactive metabolites are reduced and fewer covalently bound adducts are formed.

Mutagenic activation of 3-amino-1,4-dimethyl-5H-pyrido(4,3-b)indole(Trp-P-1) and 3-amino-1-methyl-5H-pyrido(4,3-b)indole (Trp-P-2) by primary cultures of adult rat hepatocytes: effect of Aroclor induction in vitro

The mutagenic activation of tryptophan pyrolysis products, Trp-P-1 and Trp-P-2, was studied in a Salmonella TA98/hepatocyte mutagenesis assay. Adult rat hepatocytes in primary culture were either untreated or induced by the addition of Aroclor 1254 (2 micrograms/ml) 18-20 h before the mutagenesis test which was performed at day 1 and at day 2 after the isolation of hepatocytes. The mutagenic activation of Trp-P-1 and Trp-P-2 was studied as a function of the time of incubation and of the concentration of chemical. Trp-P-1 and Trp-P-2 incubated for 20 min in the presence of untreated hepatocytes and bacteria gave rise to a weak number of revertants which doubled the level of spontaneous mutants. Aroclor-induced hepatocytes became highly competent in mutagenic activation of tryptophan pyrolysis products and the induction ratio reached 4.9 and 7.1 for Trp-P-1 and Trp-P-2, respectively, after 60 min of incubation, on day 2 of the experiment. It should be noted that the induction ratio was higher on day 2 than on day 1. When conditions were standardized, i.e. Aroclor-induced hepatocytes on day 2, final concentration of cellular protein about 1 mg/ml, 20 min of incubation, the Salmonella/hepatocyte assay produced a linear concentration-dependent mutagenic response for Trp-P-1 and Trp-P-2. By comparing the results obtained with Aroclor-induced hepatocytes and Aroclor-induced liver S9 fraction in the Salmonella test, it could be estimated that hepatocytes were 3 times less active than the S9 fraction with regard to mutagenic activation of both Trp-P-1 and Trp-P-2.

7-Ethoxycoumarin deethylase activity as a convenient measure of liver drug metabolizing enzymes: regulation in cultured rat hepatocytes

Assays of 7-ethoxycoumarin O-deethylase (ECD) activity in intact cells were used as a sensitive and convenient measure of the drug-metabolizing activity of rat hepatocytes maintained for up to 4 days in primary culture. A combination of nicotinamide or other pyridines with dexamethasone was shown to maintain ECD at or above the activity of untreated livers in vivo and to potentiate induction by xenobiotics. Inductions in vivo and in culture were quantitatively similar but differed qualitatively as judged by the proportion of ECD activity inhibitable by metyrapone. A survey of possible endogenous regulators of liver monooxygenases established that: dexamethasone and other glucocorticoids induced ECD and potentiated induction by xenobiotics, particularly phenobarbitone; other steroids including testosterone, 17 beta-estradiol and pregnenolone 16 alpha-carbonitrile caused small inductions; insulin lowered both ECD activity and the proportion of activity inhibitable by metyrapone; dibutyryl cyclic AMP or glucagon lowered ECD; and high concentrations of aminolevulinate partly repressed induction by xenobiotics. Based on these findings, hepatocyte culture conditions which maintain ECD activity and inducibility at or above in vivo levels are defined.

Regulation of heme metabolism and cytochrome P-450 levels in primary culture of rat hepatocytes in a defined medium

Liver cells were prepared from adult Sprague-Dawley rats and used for the determination of delta-aminolevulinic acid synthetase (ALAS) activity and cytochrome P-450 concentrations at different time intervals in tissue culture in a serum-free synthetic medium. During the first 24 hr in culture, the level of cytochrome P-450 decreased to 30-40% of the level in isolated liver cells from untreated animals. The disappearance of cytochrome P-450 was especially fast in hepatocytes obtained from female phenobarbital-treated rats where only 40% of the original cytochrome P-450 was present after 2 hr in culture and 80% had disappeared in 2 days. The activity of ALAS increased 3- to 4-fold when measured 2 hr after plating, and it reached the maximum level in 19-24 hr when its activity was about eight times the original activity. In 2-4 days in culture, the activity of ALAS was four to five times above the original level. When the amount of delta-aminolevulinic acid (ALA) in the medium was increased from 1 to 100 microM, a decrease in ALAS was obtained, but no significant increase in cytochrome P-450 level was observed. Addition of heme to the medium gave a dose-dependent decrease in the activity of ALAS. Our data indicate that during the first 24 hr in culture the increase of ALAS activity was prevented by exogenous heme. This effect may be due to inhibition of the catalytic activity, suppression of the synthesis of the enzyme, or accelerated breakdown of the enzyme by heme.

Aroclor 1254 pretreatment enhances the DNA repair response to amino acid pyrolysate mutagens in primary cultures of rat hepatocytes

The stimulation of unscheduled DNA synthesis (UDS) by 3-amino-1,4-dimethyl -5H-pyrido[4,3-b]indole (Trp-P-1), 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2), 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1) and 2-amino-dipyrido[1,2-a:3',2'-d]imidazole (Glu-P-2) was measured in primary cultures of rat hepatocytes. Hepatocellular DNA was recovered as a cetyltrimethylammonium salt from nuclei digested with protease. The incorporation of [3H]thymidine was measured by liquid scintillation counting techniques. Although Trp-P-1 was the most potent of the pyrolysis products tested, it was considerably less active than 2-aminofluorene (2-AF) or aflatoxin B1 (AFB1). A greater induction of UDS by Trp-P-1 was observed in cultures from female rats than in cultures from male rats. Aroclor 1254 pretreatment resulted in a substantially greater UDS response to all 4 pyrolysis products.

Modulation of functional activities in cultured rat hepatocytes

Rat hepatocytes isolated by enzymatic dissociation of the liver must attach in order to survive for more than a few hours. In conventional culture conditions, they rapidly lose their highly differentiated functions, e.g. adult isozymic forms, enzyme response to specific hormones and cytochrome P-450-dependent monooxygenase activities. Incompletely differentiated cells such as perinatal and regenerating hepatocytes, can transiently exhibit a more differentiated state. Therefore, regulation of hepatic functions, particularly enzyme activities cannot be studied for more than a few days. Hepatocyte survival rate and maintenance of specific functions are dependent on nutrient composition of the medium as well as the substrate. Complex matrices, particularly that derived from the connective liver biomatrix, appear to have an important favorable effect. However, regardless of culture conditions specific functions cannot be quantitatively maintained for more than several days. Recent observations strongly suggest that such a problem may be overcome by mimicking in vivo specific cell-cell interactions. Thus when co-cultured with a liver epithelial cell line, probably derived from biliary ductular cells, adult hepatocytes remain able to synthesize high levels of albumin and to conjugate drugs. In these conditions, the cells secrete an abundant heterogeneous extracellular material. The co-cultures can be maintained in a serum-free medium and specific liver functions can be altered experimentally. Such a model could be appropriate for studying long-term induction and modulation of liver enzyme activities under defined experimental conditions.

Primary cultures and the levels of cytochrome P450 in hepatocytes from mouse, rat, hamster, and rabbit liver

Hepatocyte primary cultures (HPC) derived from rat, mouse, hamster, and rabbit liver were characterized for a variety of parameters. The conditions that maximized recovery, attachment, and survival varied between species. Hepatocytes from all four species were capable of attaching in serum-free Williams' medium E (WME), but optimal attachment as monolayer cultures was achieved for mouse and hamster HPC in medium receiving 1% calf serum supplementation. Hamster hepatocytes required additional cations, whereas rabbit and rat hepatocytes displayed maximal attachment in medium supplemented with 10% calf serum. Survival of mouse and rabbit hepatocytes after 24 h in serum supplemented media was in the order of 90%. Rat and hamster hepatocyte 24 h survival was approximately 70 and 60%, respectively, and was not significantly affected by serum supplementation. Hepatocytes from each species varied in their content of cytochrome P450 at the time of isolation and in the rate of reduction during culture. Mouse and rat hepatocytes demonstrated the most rapid decline in content during the initial 24 h in culture, whereas concentrations in rabbit hepatocytes were virtually unchanged. The rate of decline in P450 concentrations in hamster hepatocytes was intermediate between those displayed by rat and rabbit hepatocytes. These studies have delineated conditions useful for the culture of hepatocytes from four species and have documented the status of an important parameter of their functional capability.

The comparative metabolism and toxic potency of aflatoxin B1 and aflatoxin M1 in primary cultures of adult-rat hepatocytes

Both aflatoxin B1 (AFB1) and a hydroxylated metabolite, aflatoxin M1 (AFM1), were potent cytotoxins and genotoxins to primary cultures of rat hepatocytes. However, AFB1 stimulated the release of lactate dehydrogenase into the culture medium and the loss of viable cells from the monolayer at lower doses than did AFM1. The lowest toxic doses of AFB1 and AFM1 were 0.05-01 and 0.6 microgram/culture, respectively. Genotoxicity, determined by an assay for stimulation of DNA repair, was apparent at lower doses than was cytotoxicity. AFB1 was again more potent than AFM1, stimulating DNA repair at 0.025 microgram/culture, compared to the lowest genotoxic dose of AFM1 of 0.05 microgram/culture. At higher doses (1.2-2.4 microgram/culture) the responses due to both aflatoxins in the cytotoxicity and DNA-repair assays were approximately equal. The metabolism of a low dose (c. 0.17 microgram/culture) of [14C]AFB1 and [3H]AFM1 by cultured hepatocytes differed significantly. After 1 hr, 50% of the [14C]AFB1 remained unchanged in the culture medium, whereas about 18 hr were required for the same amount of [3H]AFM1 metabolism to occur [14C]AFB1 was metabolized to AFM1, to polar metabolites recovered in the aqueous phase after chloroform extraction, and to metabolites covalently bound to hepatocyte macromolecules. [3H]AFM1 was also metabolized to polar metabolites and to forms bound to macromolecules. The degree of covalent binding of the aflatoxins correlated with their cytotoxicity and genotoxicity at lower doses. After a 24-hr incubation, 12.5% of the dose of [14C]AFB1 was covalently bound to macromolecules compared to 1.5% of [3H]AFM1. Although AFM1 was less potent than AFB1 in cytotoxicity, DNA-repair and covalent-binding assays using primary cultures of hepatocytes, AFM1 was still active at relatively low doses and therefore is probably a potent hepatotoxin in vivo.

Activation of aflatoxin B1 by primary cultures of adult rat hepatocytes: effects of hepatocyte density

Primary monolayer cultures of adult rat hepatocytes readily activate aflatoxin B1 as determined by bacterial mutagenesis (Ames test) and the extent of apparent covalent binding of aflatoxin B1 residues to hepatocyte macromolecules. For intact cultures inoculated with 3 X 10(5)-3 X 10(6) cells/dish, the efficiency of activation decreases with increasing cell density whereas permeabilized hepatocytes prepared from similarly-handled monolayer cultures show with increasing protein proportional increases in the capacity to activate aflatoxin B1. The density effect observed with intact cultured hepatocytes appears not to be due to substrate (aflatoxin B1) or oxygen depletion. These findings have apparent relevance to studies of carcinogen metabolism and in the design of carcinogen/mutagen testing protocols which utilize cultured hepatocytes.