Differential stability of drug-metabolizing enzyme activities in primary rat hepatocytes, cultured in the absence or presence of dexamethasone

Sandwich-cultured hepatocytes: utility for in vitro exploration of hepatobiliary drug disposition and drug-induced hepatotoxicity

INTRODUCTION

The sandwich-cultured hepatocyte (SCH) model has become an invaluable in vitro tool for studying hepatic drug transport, metabolism, biliary excretion and toxicity. The relevant expression of many hepatocyte-specific functions together with the in vivo-like morphology favor SCHs over other preclinical models for evaluating hepatobiliary drug disposition and drug-induced hepatotoxicity.

AREAS COVERED

In this review, the authors highlight recommended procedures required for reproducibly culturing hepatocytes in sandwich configuration. It also provides an overview of the SCH model characteristics as a function of culture time. Lastly, the article presents a summary of the most prominent applications of the SCH model, including hepatic drug clearance prediction, drug-drug interaction potential and drug-induced hepatotoxicity.

EXPERT OPINION

When human (cryopreserved) hepatocytes are used to establish sandwich cultures, the model appears particularly valuable to quantitatively investigate clinically relevant mechanisms related to in vivo hepatobiliary drug disposition and hepatotoxicity. Nonetheless, the SCH model would largely benefit from better insight into the fundamental cell signaling mechanisms that are critical for long-term in vitro maintenance of the hepatocytic phenotype. Studies systematically exploring improved cell culture conditions (e.g., co-cultures or extracellular matrix modifications), as well as in vitro work identifying key transcription factors involved in hepatocyte differentiation are currently emerging.

Primary hepatocyte cultures for pharmaco-toxicological studies: at the busy crossroad of various anti-dedifferentiation strategies

Continuously increasing understanding of the molecular triggers responsible for the onset of diseases, paralleled by an equally dynamic evolution of chemical synthesis and screening methods, offers an abundance of pharmacological agents with a potential to become new successful drugs. However, before patients can benefit of newly developed pharmaceuticals, stringent safety filters need to be applied to weed out unfavourable drug candidates. Cost effectiveness and the need to identify compound liabilities, without exposing humans to unnecessary risks, has stimulated the shift of the safety studies to the earliest stages of drug discovery and development. In this regard, in vivo relevant organotypic in vitro models have high potential to revolutionize the preclinical safety testing. They can enable automation of the process, to match the requirements of high-throughput screening approaches, while satisfying ethical considerations. Cultures of primary hepatocytes became already an inherent part of the preclinical pharmaco-toxicological testing battery, yet their routine use, particularly for long-term assays, is limited by the progressive deterioration of liver-specific features. The availability of suitable hepatic and other organ-specific in vitro models is, however, of paramount importance in the light of changing European legal regulations in the field of chemical compounds of different origin, which gradually restrict the use of animal studies for safety assessment, as currently witnessed in cosmetic industry. Fortunately, research groups worldwide spare no effort to establish hepatic in vitro systems. In the present review, both classical and innovative methodologies to stabilize the in vivo-like hepatocyte phenotype in culture of primary hepatocytes are presented and discussed.

Comparison of Basal gene expression in cultured primary rat hepatocytes and freshly isolated rat hepatocytes

Cultured primary hepatocytes are one of the most suitable in vitro models for hepatic toxicological studies. Unfortunately, there is a temporal loss of liver-specific function in culture that limits their utility for some applications. Plating hepatocytes on a substratum has been shown to stabilize the differentiated phenotype for short-term culture. In order to identify the substratum that best supports in vivo basal hepatocyte gene expression profiles in vitro, the gene expression profiles of primary rat hepatocytes plated on collagen I in hepatocyte maintenance medium (HMM) or hepatocyte culture medium (HCM), or on matrigel in HMM medium for 2 h, 16 h, or 72 h were compared to the expression profiles of freshly isolated rat hepatocytes using the Atlas rat stress array. After 16 h in culture, there were differences in gene expression between cultured primary hepatocytes and freshly isolated hepatocytes, but no apparent substratum effects. At 72 h, the expression of 9 genes was altered in hepatocytes plated on either substratum compared to expression in freshly isolated hepatocytes. However, there were an additional 13 genes with increased expression in hepatocytes plated on collagen I that were expressed at low or non-detectable levels in freshly isolated hepatocytes or primary hepatocytes plated on matrigel. In summary, after 72 h, primary hepatocytes plated on matrigel had basal gene expression patterns more similar to patterns in freshly isolated hepatocytes than did hepatocytes cultured on collagen. In addition, culture on matrigel suppressed the expression of atypical genes in culture. These preliminary studies suggest that culture on matrigel may be preferable for longer-term in vitro toxicological studies.

Variability in free mycophenolic acid exposure in adult liver transplant recipients during the early posttransplantation period

Mycophenolate mofetil (MMF) pharmacokinetics variability in liver transplant recipients during the early posttransplantation period may be related to changes in mycophenolic acid (MPA) protein binding. This study aimed at characterizing the variation of free MPA exposure with respect to time since transplantation. Three groups (A, B, C) were compared. The median posttransplantation time was 12 days (A, n = 26 pharmacokinetic sessions), 36 days (B, n = 25), and 867 days (C, n = 21). The median MPA AUC(0-12) in group A (26.8 mg x h/L) was significantly lower than in groups B (45.2 mg x h/L, P = .031) and C (43.5 mg x h/L, P = .004). Free MPA AUC(0-12) was comparable whatever the time (0.41, 0.34, and 0.33 mg x h/L, respectively). MPA apparent clearance (CL/F) was significantly correlated with MPA free fraction (r = 0.60, P < .0001) and approximately 1.7-fold higher in group A compared to groups B and C (P < .05). Enhanced CL/F in relation with an increase in MPA free fraction results in a low AUC of total MPA during the first postoperative month, but on average, at the population level, the exposure to free MPA is not altered, suggesting that total MPA AUC should not be used to adapt MMF dosing during this period.

Induction of phase I and II drug metabolism in rat small intestine and colon in vitro

The aim of this study was to evaluate drug metabolism in rat small intestinal and colon precision-cut slices during 24 h of incubation and the applicability of these slices for enzyme induction studies. Various parameters were evaluated: intracellular levels of ATP (general viability marker), alkaline phosphatase activity (specific epithelial marker), villin expression (specific epithelial marker), and metabolic rates of 7-ethoxycoumarin (CYP1A), testosterone (CYP3A and CYP2B), and 7-hydroxycoumarin (glucuronide and sulfate conjugation) conversions. ATP and villin remained constant up to, respectively, 5 and 8 h in small intestine and up to 24 h in colon. The metabolic rate remained constant in small intestinal slices up to 8 h and decreased afterward to 24 to 92%, depending on the substrate studied. The inducibility of metabolism in small intestinal and colon slices was tested with several inducers at various concentrations and incubation times. The following inducers were used: 3-methylcholanthrene, beta-naphthoflavone, indirubin, and tert-butylhydroquinone (aryl hydrocarbon receptor ligands), dexamethasone (glucocorticoid receptor/pregnane X receptor ligand) and phenobarbital (constitutive androstane receptor ligand). After incubation with inducers, metabolic rates were evaluated with 7-ethoxycoumarin and testosterone (phase I) and 7-hydroxycoumarin (phase II) as substrate. All inducers elevated the metabolic rates consistent with the available published in vivo induction data. Induction of enzyme activity was already detectable after 5 h (small intestine) and after 8 h (colon) for 3-methylcholanthrene and beta-naphthoflavone and was clearly detectable for all tested inducers after 24 h (up to 20-fold compared with noninduced controls). In conclusion, small intestinal and colon precision-cut slices are useful for metabolism and enzyme induction studies.

Empirical validation of a rat in vitro organ slice model as a tool for in vivo clearance prediction

Tissue slices have been shown to be a valuable tool to predict metabolism of novel drugs. However, besides the numerous advantages of their use for this purpose, some potential drawbacks also exist, including reported poor penetration of drugs into the inner cell layers of slices and loss of metabolic capacity during prolonged incubation, leading to underprediction of metabolic clearance. In the present study, we empirically identified (and quantified) sources of underprediction using rat tissue slices of lung, intestine, kidney, and liver and found that thin liver slices (+/-100 mum) metabolized model substrates (7-hydroxycoumarin, testosterone, warfarin, 7-ethoxycoumarin, midazolam, haloperidol, and quinidine) as rapidly as isolated hepatocytes. Furthermore, it was found that organ slices remain metabolically active for sufficient periods of incubation, enabling study of the kinetics of low clearance compounds. In addition, we determined the influence of albumin on the clearance prediction of six model substrates. For three of these substrates, the intrinsic clearance in the presence of albumin was approximately 3 times higher than that obtained from incubations without albumin, but corrected for unbound fraction. This resulted in a much more accurate prediction of in vivo whole body metabolic clearance for these compounds. Collectively, these results show that drawbacks of the use of slices for clearance prediction are largely surmountable. Provided that thin liver slices and physiological albumin concentration are used, whole body metabolic clearance is predicted with acceptable (2-fold) accuracy with organ slices. These results emphasize the applicability of organ slices in this field of research.

Utility of physiologically based pharmacokinetic models to drug development and rational drug discovery candidate selection

The present paper proposes a modeling and simulation strategy for the prediction of pharmacokinetics (PK) of drug candidates by using currently available in silico and in vitro based prediction tools for absorption, distribution, metabolism and excretion (ADME). These methods can be used to estimate specific ADME parameters (such as rate and extent of absorption into portal vein, volume of distribution, metabolic clearance in the liver). They can also be part of a physiologically based pharmacokinetic (PBPK) model to simulate concentration-time profiles in tissues and plasma resulting from the overall PK after intravenous or oral administration. Since the ADME prediction tools are built only on commonly generated in silico and in vitro data, they can be applied already in early drug discovery, prior to any in vivo study. With the suggested methodology, the following advantages of the mechanistic PBPK modeling framework can now be utilized to explore potential clinical candidates already in drug discovery: (i) prediction of plasma (blood) and tissue PK of drug candidates prior to in vivo experiments, (ii) supporting a better mechanistic understanding of PK properties, as well as helping the development of more rationale PK-PD relationships from tissue kinetic data predicted, and hence facilitating a more rational decision during clinical candidate selection, and (iii) the extrapolation across species, routes of administration and dose levels.

Optimization of culture conditions for determining hepatobiliary disposition of taurocholate in sandwich-cultured rat hepatocytes

This study was undertaken to examine the influence of time and volume of collagen overlay, type of media, and media additives on taurocholate (TC) accumulation and biliary excretion in hepatocytes cultured in a collagen-sandwich configuration. Hepatocytes were isolated from male Wistar rats by in situ perfusion with collagenase, seeded onto collagen-coated 60-mm dishes, overlaid with gelled collagen, and cultured for 4 d. Experiments to examine the influence of time and volume of collagen overlay were conducted in Dulbecco's modified Eagle's medium (DMEM) + 1.0 microM dexamethasone (DEX) + 5% fetal bovine serum (FBS). Hepatocytes were overlaid at 0 h with 0.1 or 0.2 ml collagen, or at 24 h with 0.1 or 0.2 ml collagen. The influence of media type and additives was examined in hepatocytes overlaid at 0 h with 0.2 ml collagen and incubated in DMEM + 0.1 microM DEX, DMEM +/- 0.1 microM DEX + 5% FBS, Williams' medium E + 0.1 microM DEX + 1% ITS+, DMEM + 1.0 microM DEX, DMEM + 1.0 microM DEX + 5% FBS, or modified Chee's medium (MCM) + 0.1 microM DEX + 1% ITS+. [3H] TC accumulation by hepatocytes in Hank's balanced salt solution (HBSS) and Ca2+-free HBSS was measured, and the biliary-exeretion index (BEI: percentage of accumulated TC localized in the canalicular compartment) was calculated. Light microscopy and carboxydichlorofluorescein fluorescence were employed to examine the cellular and canalicular morphologies. The volume of collagen used for both the substratum and the overlay did not affect TC accumulation or biliary excretion. The BEI tended to be higher in cells overlaid at 24 h (BEI = 0.649 [0.1 ml collagen]; BEI = 0.659 [0.2 ml collagen]) compared with those overlaid at 0 h after seeding (BEI = 0.538 [0.1 ml collagen]; BEI = 0.517 [0.2 ml collagen]), although the differences were not statistically significant. Hepatocytes cultured in MCM produced consistently the lowest BEI of TC (BEI = 0.396). Differing DEX concentrations (0.1 microM versus 1.0 microM) with or without 5% FBS did not appear to have a significant effect on the BEI of TC.

Characterization of T cell responses to fragrances

Fragrances are worldwide a major cause of allergic contact dermatitis (ACD), a delayed-type hypersensitivity reaction mediated by T lymphocytes. We investigated T cell responses to fragrances using peripheral blood mononuclear cells (PBMC) and T cells from skin lesions of fragrance-allergic patients. The components of a fragrance mixture (eugenol, isoeugenol, geraniol, oak moss, alpha-amyl cinnamic aldehyde, cinnamic aldehyde, cinnamic alcohol, and hydroxycitronellal) that is commonly used in the patch test were studied in vitro in the lymphocyte transformation test (LTT). PBMC from fragrance-allergic patients (n = 32) showed significant stimulations to all eight fragrances. The calculated stimulation indices (SI) varied between 2.1 and 21.8. The influence of metabolic enzymes on T cell stimulation was studied for two fragrances. Interestingly, stimulation of eugenol and isoeugenol was increased in the presence of antigen-modified human liver microsomes (CYP450) or recombinant CYP1A1 in five of seven cases. Furthermore, we established 18 T cell clones (TCC) from a skin lesion reacting specifically to eugenol. FACS analysis revealed that the majority (n = 15, 83%) of TCC were CD3(+), CD4(+), and HLA-DR(+). Seventeen percent (n = 3) of the clones were CD8(+). TCC (n = 4) released significant amounts of IL-2 and IFN-gamma but no IL-4 and IL-5. In addition, CD4(+) TCC (n = 3) showed antigen-induced cytotoxic activities against autologous B cells. In summary, we demonstrated for the first time that fragrance-specific CD4(+) and CD8(+) T lymphocytes are present in fragrance-allergic individuals. In addition, our results suggest that CYPs can be involved in the formation of the nominative antigen.

Influence of culture system and medium enrichment on sulfotransferase and sulfatase expression in male rat hepatocyte cultures

The expression of sulfotransferase and steroid sulfatase was studied in rat liver using the most promising culture models of hepatocytes, including monolayer culture with a pyruvate (30 mM) enriched medium, co-culture with rat epithelial cells from primitive biliary origin and collagengel sandwich culture. In the latter, addition of dexamethasone (1 microM) to the medium was examined. Phenol sulfotransferase enzymes (SULT1) were studied by measuring activities towards 4-methylphenol and estradiol, hydroxysteroid sulfotransferase (SULT2A) activity was determined towards dehydroepiandrosterone (DHEA). Microsomal steroid sulfatase activity was measured towards estrone sulfate. Western blot analysis was carried out using polyclonal antibodies raised against rat phenol sulfotransferase SULT1A1 (ASTIV), estrogen sulfotransferase SULT1E1 (EST) and hydroxysteroid sulfotransferase (HST). SULT2A activity towards DHEA was maintained at a high level during the whole culture time. In the co-culture it even reached the level of freshly isolated cells. Addition of pyruvate had no positive effect on the activity measured in monolayer cultures. High SULT1A1 activity towards 4-methylphenol was found in the co-culture system. In the monolayer culture, the activity initially decreased with 35% but was then kept at a constant level, while in the sandwich culture low activities were measured. For dexamethasone, an inducing effect on the various SULT activities could not be detected. Independently of the culture model used, the SULT1E1 activity towards estradiol decreased to 20% and 5% of the initial activity after four and seven days of culture, respectively. Microsomal steroid sulfatase activity was best maintained in collagengel sandwich cultures. During the first four days in culture it retained 73% of the initial activity, afterwards it decreased to 40% of the activity found in freshly isolated hepatocytes, irrespective of the culture conditions. High expectations exist for collagengel sandwich cultures, however, in our study the results were rather disappointing. Monolayer is a suitable culture model for short-term purposes. For long-term in vitro biotransformation studies, co-culture is preferred but is rather complex.

Xenobiotic metabolism by cultured primary porcine hepatocytes

Considering the large yield of viable cells comparable to human liver, primary porcine hepatocytes offer a valuable resource for constructing a bioartificial liver device. In this study, the ability of cultured primary porcine hepatocytes to detoxify xenobiotics has been examined using various known substrates of cytochrome P450 isoenzymes and UDP-glucuronosyltransferases. Present investigation demonstrated the stability of the isoenzymes responsible for the metabolism of diazepam in native state and stabilization of other isoenzymes, as judged by ethoxycoumarin o-dealkylase (ECOD), ethoxyresorufin o-dealkylase (EROD), benzyloxyresorufin o-dealkylase (BROD), and pentoxyresorufin o-dealkylase (PROD) activities following induction in culture environment, for a period of 8 days. Resorufin O-dealkylase activities were found to be the most unstable and deteriorated within first 5 days in culture. These activities were restored following induction with 3-methylcholanthrene (3-MC) or sodium phenobarbital (PB) to 20-fold of 1 activity for EROD, and 60 and 174% of day 1 activity for PROD and BROD on day 8, respectively. Metabolism of methoxyresorufin was most strikingly increased following induction with 3-MC to approximately 60-fold of day 1 activity, on day 8. UDP-glucuronosyltransferase-dependent glucuronidation of phenol red, however, stayed intact during the course of our study without induction. Our study indicated that porcine hepatocytes in vitro maintain many important liver-specific functions including detoxification (steady state and inducibility).

Ups and downs of aging studies in vitro: the crooked path of science

Different approaches using cell culture techniques to study the biology of aging are critically described. Most of the studies concerned the relationship between cell division potential and aging. The growth potential of cells is fundamental for aging of the organism, since it relates to phenomena such as the regeneration of tissues, wound healing, the immune response, and stem cell renewal. Unfortunately many experiments were misinterpreted disregarding the physiology of the mammalian organism. The terminal postmitotic cell, on which most research has been concentrated, seems irrelevant for aging of the organism. Nevertheless, some experiments yielded important contributions to the understanding of the biology of cell division. Future research should ascertain such interesting suggestions as the terminal differentiation hypothesis of the human fibroblast life cycle. It is important to elucidate the significance of the increased number of postmitotic cells in pathological processes. A neglected area should be further explored: the relationships between structural modifications of the cell, decreased probability of activating energy barriers, and decline of the division potential.

Development and utilization of primary hepatocyte culture systems to evaluate metabolism, DNA binding, and DNA repair of xenobiotics

The use of isolated hepatocytes as an approach to evaluate hepatotoxic and hepatocarcinogenic compounds and investigate mechanisms by which chemicals induce liver lesions is well established. This review discusses techniques developed in the author's laboratory describing (1) isolation and primary culture of rodent hepatocytes detailing methods which are optimal for obtaining large numbers of viable cells, (2) DNA damage induced by physical and chemical agents in rodent hepatocytes measured as unscheduled DNA synthesis, and (3) metabolic activation of model hepatocarcinogens, their binding to DNA, and identification of individual adducts thought to be responsible for induction of DNA repair.

Area under the plasma concentration-time curve for total, but not for free, mycophenolic acid increases in the stable phase after renal transplantation: a longitudinal study in pediatric patients. German Study Group on Mycophenolate Mofetil Therapy in Pediatric Renal Transplant Recipients

Mycophenolate mofetil, an ester prodrug of the immunosuppressant mycophenolic acid (MPA), is widely used for maintenance immunosuppressive therapy in pediatric renal transplant recipients. However, little is known about the pharmacokinetics of MPA in this patient population in the stable transplant phase, and dosage guidelines are preliminary. The authors therefore compared the pharmacokinetics of MPA, free MPA, and the renal metabolite MPA glucuronide (MPAG) in the initial (sampling at 1 and 3 weeks) and stable phases (sampling at 3 and 6 months) posttransplant in 17 children (age, 12.0 +/- 0.77 years; range, 5.9 to 15.8 years), receiving the currently recommended dose of 600 mg MMF/m2 body surface area (BSA) twice a day. Plasma concentrations of MPA and MPAG were measured by reverse phase HPLC. Because MPA is extensively bound to serum albumin and only the free drug is presumed to be pharmacologically active, the authors also analyzed the MPA free fraction by HPLC after separation by ultrafiltration. The intraindividual variability of the area under the concentration-time curves (AUC0-12) of MPA throughout the 12-hour dosing interval was high in the immediate posttransplant period, but declined in the stable phase, whereas the interindividual variability remained unchanged. The median MPA-AUC0-12 values increased 2-fold from 32.4 (range, 13.9 to 57.0) mg x h/L at 3 weeks to 65.1 (range, 32.6 to 114) mg x h/L at 3 months after transplantation, whereas the median AUC0-12 values of free MPA did not significantly change over time. This discrepancy can be attributed to a 35% decline of the MPA free fraction from 1.4% in the initial phase posttransplant to 0.9% (p < 0.01) in the stable phase. In conclusion, pediatric renal transplant recipients given a fixed MMF dose exhibit a 2-fold increase of the AUC0-12 of total MPA in the stable phase posttransplant and a 35% decrease of the MPA free fraction, whereas the AUC0-12 of free MPA remains unchanged over time. Because the latter pharmacokinetic variable is theoretically best predictive of the clinical immunosuppressive efficacy of MMF, these findings may have consequences for the dosing recommendations of MMF in renal transplant recipients.

The nature of halogen substitution determines the mode of cytotoxicity of halopropanols

The cytochrome P450-dependent generation of reactive metabolites from 1,3-dichloropropanol and 1,3-dibromopropanol was assessed in a microsomal thiol depletion assay, while the toxicity of these compounds was assessed in rat hepatocyte cultures and in the 3T3 cell line. Thiol-depleting metabolites of both compounds were generated in the microsomal assay; however, only dibromopropanol extensively depleted glutathione when glutathione S-transferase was used as the enzyme source. The cytotoxicity of dichloropropanol was both cytochrome P450- and glutathione-dependent, whereas that of dibromopropanol was glutathione-dependent but largely independent of cytochrome P450. These results indicate that the mechanisms underlying the cytotoxicity of halopropanols are dependent on the nature of the halogen substitution and that microsomal and cellular assays for reactive metabolite generation may yield conflicting results.

Activity and inducibility of drug-metabolizing enzymes in immortalized hepatocyte-like cells (mhPKT) derived from a L-PK/Tag1 transgenic mouse

This report describes the establishment and characterization of the mhPKT cell line derived from the liver of a transgenic mouse harboring the simian virus (SV40) large T and small t antigens placed under the control of the 5' regulatory sequence of the rat L-type pyruvate kinase (L-PK) gene. mhPKT cells had a prolonged life span, expressed the SV40-encoded nuclear large T antigen when grown in glucose-enriched medium, and induced tumors when injected subcutaneously into athymic (nu-nu) mice. Growth on petri dishes or filters yielded multiple layers of cuboid cells, with numerous spaces between adjacent cells that were closed by junctional complexes. These bile canaliculi-like structures exhibited numerous microvilli in which villin, an actin-binding brush-border protein, colocalized with actin. These bile canaliculi-like structures appeared to be functional as they accumulated fluorescein. mhPKT cells conserved the expression of the liver-specific transcription factors HNF1, HNF3, HNF4, and DBP together with substantial levels of L-PK and albumin but not alpha-fetoprotein mRNA transcripts. mhPKT cells mainly metabolized testosterone into androstenedione and 6beta-hydroxytestosterone, as in vivo. 3-Methylcholanthrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) markedly increased ethoxyresorufin-O-deethylase activity and the related cytochrome P450 (CYP) 1A1/2 protein, whereas alpha-naphtoflavone antagonized the TCDD-elicited induction. Phenobarbital slightly increased the CYP2B-mediated activities of pentoxyresorufin-O-depentylase, 2beta- and 16beta-testosterone hydroxylase. mhPKT cells also had substantial sulfotransferase, UDP-glucuronyltransferase, and glutathione S-transferase activities. This model may serve as a tool for long-term in vitro studies of xenobiotic metabolism, potent CYP inducers, and hepatocyte damage due to drugs and other factors.

Sulfotransferase gene expression in primary cultures of rat hepatocytes

Hepatocyte cultures have been used in pharmacotoxicological studies, and sulfotransferases (ST) are important drug-metabolizing enzymes in liver. The expression of sulfotransferases in hepatocyte cultures has not been examined systematically. In the present study, the mRNA levels of different sulfotransferases in male and female rat hepatocytes were examined by northern-blot analyses. Various culture conditions such as different matrices (collagen, matrigel, collagen sandwich, or co-culture with epithelial cells), medium (Way-mouth's MB 752/1 and Modified Chee's Medium) and glucocorticoid supplementation (dexamethasone, 0.1 microM) were compared. Phenol ST (ST1A1) mRNA levels decreased to about 50% of initial mRNA levels within 10 hr of culture. At 96 hr, ST1A1 mRNA levels were approximately 20% of initial values when cultured on collagen, matrigel or co-culture. The two media did not differ in ability to maintain ST1A1 mRNA levels in the absence of dexamethasone (DEX); however, DEX addition to either medium resulted in ST1A1 mRNA levels greater than 100% of the initial mRNA levels at 96 hr, with the greatest increase observed using the matrigel substratum and Chee's medium. In the absence of DEX, the mRNA levels of N-hydroxy-2-acetylaminoflurene sulfortransferase (ST1C1), estrogen sulfotransferase (ST1E2) and hydroxysteroid sulfotransferase (ST-20/21, ST-40/41, ST-60) fell to approximately 20% of their initial levels within 24 hr, and to less than 5% at 96 hr. The loss of expression of these sulfotransferases was observed with all culture conditions. Addition of DEX to the media resulted in ST-40/41 and ST-60 mRNA expression at 20 and 35% of their initial values, respectively, in cultures maintained on matrigel and Chee's medium at 96 hr. These data suggest that sulfotransferases lose their constitutive expression in hepatocyte culture, but retain their inducibility.

Hormone- and endotoxin-modulated gene expression of a long-term organ culture system of adult rat liver

Precision-cut slices of normal adult rat liver maintained in serum-free medium remain hormone- and endotoxin-responsive for at least 48 h. They respond to glucocorticoid (dexamethasone) with the induction of the gluconeogenic enzyme tyrosine aminotransferase (TAT), as determined by enzymatic activity and by the increase in enzyme protein. Furthermore, endotoxin (LPS) induced nitric oxide synthase II (i-NOS), and this induction is repressed, similarly to the in vivo situation, by dexamethasone (DEX). All increases are inhibited by cycloheximide (CHX). The length of the period of responsiveness suggests that this organ culture system might be generally useful for studying the modulation of liver gene expression by physiological and pathological influences.

Genotoxicity of tacrine in primary hepatocytes isolated from B6C3F1 mice and aged ad libitum and calorie restricted Fischer 344 rats

Tacrine (1,2,3,4-tetrahydro-9-aminoacridine; THA), a reversible centrally acting anticholinesterase, has been shown to be potentially useful for treatment of patients with Alzheimer's disease. However, currently available forms of THA may be therapeutically limited by the fact that high doses have resulted in liver and kidney damage. To determine if THA is hepatotoxic via a genotoxic mechanism, we evaluated its ability to induce unscheduled DNA synthesis (UDS) in primary cultures of rodent hepatocytes. Positive dose-dependent increases in UDS were observed in hepatocytes derived from male B6C3F1 mice and from young, middle-aged, old, and old Aroclor-induced (ARO) male F344 rats maintained on either an ad libitum (AL) or a caloric restricted (CR) diet (60% of AL) and exposed to 0.05-1000.0 micrograms/ml of THA. Hepatocytes from old AL rats, treated with THA, exhibited significant age-related decreases in DNA repair compared to young and middle-aged AL rats. By contrast, cultures from CR rats exhibited age- and diet-related decreases in UDS from the AL and young CR animals, respectively. Moreover, ARO-induced old AL- and CR-derived hepatocytes exhibited significant increases in UDS compared to uninduced old AL and CR animals. No cytotoxicity was observed in the uninduced old AL- or any CR-derived hepatocytes. These data indicate that the aged and CR fed animal is less susceptible to the cytotoxic and genotoxic effects of THA; while the younger AL fed and enzyme induced old AL or CR fed animals were more susceptible. The data suggest that THA may be a genotoxic rodent carcinogen. At present, the relationship of these findings to the clinical use of THA are unclear and further study is required.

Expression and induction of drug-metabolizing enzymes in cultured fetal rat hepatocytes

An in vitro experimental model, fetal rat hepatocytes in culture, was metabolically characterized. Several enzymatic activities were expressed in these hepatocytes, namely, testosterone hydroxylations. Hepatocytes cultured up to 3 weeks in the presence of dexamethasone and phenobarbital still expressed some drug-metabolizing enzyme activities (e.g., ECOD). The enzymatic activities were measured both directly on monolayers during culture and on the corresponding harvested and homogenized cells. The results correlate perfectly with each other. The 'on cell' procedure allows us to repeat the assay or to measure several activities on the same cells at different time intervals. The presence of dexamethasone in the culture medium allows the expression and the induction of several cytochrome P450 isoenzymes, namely, those hydroxylating testosterone. This makes the model particularly attractive for induction experiments as well as for metabolic or toxicological studies needing longer treatments.

Effects of caloric restriction on rodent drug and carcinogen metabolizing enzymes: implications for mutagenesis and cancer

Caloric restriction in rodents results in increased longevity and a decreased rate of spontaneous and chemically induced neoplasia. The low rates of spontaneous neoplasia and other pathologies have made calorically restricted rodents attractive for use in chronic bioassays. However, caloric restriction also alters hepatic drug metabolizing enzyme (DME) expression and so may also alter the biotransformation rates of test chemicals. These alterations in DME expression may be divided into two types: (1) those that are the direct result of caloric restriction itself and are detectable from shortly after the restriction is initiated; (2) those which are the result of pathological conditions that are delayed by caloric restriction. These latter alterations do not usually become apparent until late in the life of the organism. In rats, the largest direct effect of caloric restriction on liver DMEs is an apparent de-differentiation of sex-specific enzyme expression. This includes a 40-70% decrease in cytochrome P450 2C11 (CYP2C11) expression in males and a 20-30% reduction of corticosterone sulfotransferase activity in females. Changes in DME activities that occur late in life in calorically restricted rats include a stimulation of CYP2E1-dependent 4-nitrophenol hydroxylase activity and a delay in the disappearance of male-specific enzyme activities in senescent males. It is probable that altered DME expression is associated with altered metabolic activation of chemical carcinogens. For example the relative expression of hepatic CYP2C11 in ad libitum-fed or calorically restricted rats of different ages is closely correlated with the amount of genetic damage in 2-acetylaminofluorene- or aflatoxin B1-pretreated hepatocytes isolated from rats of the same age and caloric intake. This suggests that altered hepatic drug and carcinogen metabolism in calorically restricted rats can influence the carcinogenicity of test chemicals.

Cryopreservation and long-term storage of primary rat hepatocytes: effects on substrate-specific cytochrome P450-dependent activities and unscheduled DNA synthesis

The effects of cryopreservation and long-term storage on substrate-specific cytochrome P450-dependent activities and unscheduled DNA synthesis were studied in freshly isolated and cryopreserved hepatocytes derived from adult male Fischer 344 and Sprague-Dawley rats. Primary rat hepatocytes were isolated via an in situ collagenase perfusion technique, cryopreserved at -196 degrees C, and thawed at 5 weeks and 104 and 156 weeks post-freezing. In Fischer 344 and Sprague-Dawley rats, cryopreserved hepatocytes were equivalent or similar to freshly isolated hepatocytes in substrate-specific activities for 7-ethoxyresorufin-O-deethylase and dimethylnitrosamine-N-demethylase and unscheduled DNA synthesis responses. No significant differences in activities toward 7-ethoxyresorufin-O-deethylase and dimethylnitrosamine-N-demethylase, the substrate-specific activities for cytochromes P4501A1 and P4501A2 and cytochrome P4502E1, respectively, were observed between freshly isolated and cryopreserved hepatocytes. Similar unscheduled DNA synthesis responses, a measure of DNA damage and repair, were observed after exposure to the genotoxic carcinogens 2-acetylamino-fluorene, 7,12-dimethylbenz[a]anthracene, and dimethylnitrosamine; although some decreases were also observed in Fischer 344 hepatocytes after 104 weeks and Sprague-Dawley hepatocytes after 156 weeks in the highest concentrations tested. These results suggest that cryopreserved hepatocytes, stored for extended periods of time in liquid nitrogen, are metabolically equivalent to freshly isolated hepatocytes in their ability to activate precarcinogens.

Effects of aging and caloric restriction on the genotoxicity of four carcinogens in the in vitro rat hepatocyte/DNA repair assay

The effects of aging and chronic caloric restriction (CR) on the genotoxicity of four carcinogens, representing four different classes of chemicals, in the in vitro rat hepatocyte/DNA repair assay were investigated. Hepatocyte cultures were isolated from young, middle-aged, and old male Fischer (F344) rats which were maintained on either an ad libitum (AL) or a CR diet (60% of AL). Hepatocyte cultures from old AL rats, treated with 2-acetylaminofluorene (2-AAF), aflatoxin B1 (AFB1), 7,12-dimethylbenz[a]anthracene (DMBA) and dimethylnitrosamine (DMN), exhibited age-related decreases in DNA repair as compared to young AL rats. By contrast, cultures from young CR rats exhibited significant diet-related decreases in DNA repair with 2-AAF, AFB1, DMBA and DMN, when compared to results from young AL diet-fed rats. Old CR F344 rat derived cultures exhibited no significant age-related dose-dependent decrease in the DNA repair response with any of the chemicals tested. However, in cultures from old CR rats 10.0 microM AFB1 produced an age-related decrease in DNA repair from the response observed in young CR rats. When hepatocytes were isolated from Aroclor 1254-induced rats, increases in DNA repair were observed. These data indicate an age- and diet-related decrease in DNA repair and/or DNA damage and suggest that this decrease is due to a decrease in metabolic activation of these carcinogens to genotoxic species.

Mutation of V79 cells by N-dialkylnitrosamines after activation by hamster pancreas duct cells

Pancreas duct epithelial cells (DEC), isolated from hamsters and cultured for up to 25 days, were able to metabolize N-nitrosobis(2-oxopropyl)amine (BOP) to species that were mutagenic in V79 cells. There was no decline in the nitrosamine-activating ability of DEC over the period of observation (25 d). DEC activated N-nitrosobis(2-hydroxypropyl)amine (BHP), N-nitrosodiethylamine (DEN), N-nitrosodimethylamine (DMN) and N-nitrosomethyl(2-oxopropyl)amine (MOP) and BOP in the same assay, although the mutation frequencies for BHP, DEN and DMN were barely different from that for the controls (4 +/- 1 mutants/10(6) cells). The mutation frequencies for a dose of 0.1 mM were BHP, 2 +/- 1; BOP, 113 +/- 7; DEN, 8 +/- 1; DMN, 5 +/- 2; and MOP, 18 +/- 3 (mutants/10(6) cells; means +/- SE). When hepatocytes were used the mutation frequencies were BHP, 3 +/- 1; BOP, 60 +/- 3; DEN, 8 +/- 2; DMN, 8 +/- 2; and MOP, 121 +/- 10. BOP was toxic to the DEC at doses above 0.1 mM. Experiments in which co-factors were omitted from the medium suggested that an isoform(s) of the cytochrome P-450 IIIA family was involved, directly or indirectly, in BOP activation.