Smooth endoplasmic reticulum proliferation and increased cell multiplication in cultured hepatocytes of the newborn rat in the presence of phenobarbital

Dose- and time-dependent effects of phenobarbital on gene expression profiling in human hepatoma HepaRG cells

Phenobarbital (PB) induces or represses a wide spectrum of genes in rodent liver. Much less is known about its effects in human liver. We used pangenomic cDNA microarrays to analyze concentration- and time-dependent gene expression profile changes induced by PB in the well-differentiated human HepaRG cell line. Changes in gene expression profiles clustered at specific concentration ranges and treatment times. The number of correctly annotated genes significantly modulated by at least three different PB concentration ranges (spanning 0.5 to 3.2 mM) at 20 h exposure amounted to 77 and 128 genes (p< or =0.01) at 2- and 1.8-fold filter changes, respectively. At low concentrations (0.5 and 1 mM), PB-responsive genes included the well-recognized CAR- and PXR-dependent responsive cytochromes P450 (CYP2B6, CYP3A4), sulfotransferase 2A1 and plasma transporters (ABCB1, ABCC2), as well as a number of genes critically involved in various metabolic pathways, including lipid (CYP4A11, CYP4F3), vitamin D (CYP24A1) and bile (CYP7A1 and CYP8B1) metabolism. At concentrations of 3.2 mM or higher after 20 h, and especially 48 h, increased cytotoxic effects were associated with disregulation of numerous genes related to oxidative stress, DNA repair and apoptosis. Primary human hepatocyte cultures were also exposed to 1 and 3.2 mM PB for 20 h and the changes were comparable to those found in HepaRG cells treated under the same conditions. Taken altogether, our data provide further evidence that HepaRG cells closely resemble primary human hepatocytes and provide new information on the effects of PB in human liver. These data also emphasize the importance of investigating dose- and time-dependent effects of chemicals when using toxicogenomic approaches.

Induction of drug-metabolizing enzymes by phenobarbitone: structural and biochemical aspects

Two aspects of the induction of microsomal monooxygenases by phenobarbitone have been investigated. First, structural associations between mitochondria and single cisternae of rough endoplasmic reticulum (mitochondria--RER complexes) may operate as functional units in the biosynthesis of cytochrome P-450. This was deduced from (i) studies on the subcellular distribution of the phenobarbitone-induced incorporation of leucine into microsomal proteins including apocytochromes P-450 and (ii) the incorporation of labelled delta-aminolaevulinic acid into the haem prosthetic group of cytochrome P-450. Secondly, in hepatocytes from chick embryo in primary monolayer culture, induction of cytochrome P-450-haemoproteins was markedly influenced by changes in the proliferative activity of hepatocytes. Inducibility of cytochrome P-450 by phenobarbitone and by beta-naphthoflavone was decreased in cultures with 'spontaneous' or experimentally increased proliferative activity of hepatocytes. Treatment with inhibitors of DNA synthesis increased the induction response.

Role for mitogen-activated protein kinases in phenobarbital-induced expression of cytochrome P450 2B in primary cultures of rat hepatocytes

Phenobarbital (PB) alters expression of numerous hepatic genes, including genes of cytochrome P450 2B1 and 2B2 (CYP2B). However, the intracellular mechanisms remain to be fully elucidated. The present study investigated the involvement of mitogen-activated protein kinases (MAPKs) in rat hepatocytes in primary culture. We showed that PB induced an early, dose-dependent activation of ERK (extracellular signal-regulated kinase), JNK (c-Jun N-terminal kinase) and p38 MAPKs. Regarding the PB (1mM) induction of CYP2B mRNA expression, while chemically inhibiting JNK had no effect, specific inhibitors of the ERK (U0-126) and p38 (SB-203580) pathways up- and down-regulated this expression, respectively. However, although such a regulation was confirmed when testing the effect of a dominant negative mutant of the ERK pathway on the CYP2B2 enhancer-promoter activity, no such transcriptional role was found with the p38 pathway. Moreover, upon arrest of transcription, the stability of CYP2B mRNA remained unaffected by SB-203580. In conclusion, we show that the ERK pathway negatively regulates CYP2B2 enhancer-promoter activity and that, despite p38 activation upon PB exposure, the sensitivity of CYP2B mRNA expression to SB-203580 appears to be unrelated to this kinase.

Influence of liver tumor promoters and structurally related chemicals on survival of normal adult rat hepatocytes in primary culture

The influence of four liver tumor promoters and two structurally related chemicals (non-promoters) on survival of normal adult rat hepatocytes was examined in primary culture. Of the four promoters, butylated hydroxytoluene (BHT), dichlorodiphenyltrichloroethane (DDT), barbital sodium and phenobarbital sodium, only phenobarbital efficiently prolonged the hepatocyte survival and maintained the morphological features of the cells. Both BHT and DDT were toxic to hepatocytes within the dose ranges tested. Barbital was also ineffective for maintenance of primary cultured hepatocytes but not toxic to the cells. Of the two non-promoters, barbituric acid and amobarbital, barbituric acid also showed no maintenance effect on hepatocytes. However, amobarbital resembled phenobarbital in its effect on the maintenance of hepatocytes in primary culture. DNA synthesis of primary cultured hepatocytes was severely suppressed by phenobarbital in a dose-dependent manner. The results clearly show that the ability to support long-term survival of primary cultured hepatocytes is a common property of some barbiturates but not of liver tumor promoters, and that the maintenance of hepatocytes by phenobarbital is not due to a counterbalance of stimulated proliferation and death of the cells.

Long-term survival of functional hepatocytes from adult rat in the presence of phenobarbital in primary culture

In the presence of phenobarbital (PB) at 3 mM, hepatocytes isolated from adult rats by a collagenase-perfusion technique survived well on plastic dishes for at least 49 days after initiation of primary culture. PB at concentrations less than 3 mM was ineffective for the maintenance of hepatocytes, and the maintenance of them was attained only in the continuous presence of 3 mM PB. The hepatocytes surviving in the presence of 3 mM PB were morphologically indistinguishable from the hepatocytes after 1-day attachment period, except for the presence of prominent nucleoli in the former. Although both the albumin secretion and tyrosine aminotransferase (TAT) activities of the cells decreased gradually up to day 7 with time in culture, both were thereafter maintained at relatively high levels at least up to day 35 of primary culture. The addition of 10 microM dexamethasone caused a 3-5-fold induction in TAT activity, and the cells were capable of responding to the hormone in this manner at least up to day 28 of primary culture. Furthermore, the cells also had glucose-6-phosphatase activity, even though the level of this enzyme activity was relatively low as compared with that of TAT activity. Survival of hepatocytes in the presence of 3 mM PB was further enhanced by simultaneous addition of dexamethasone (10 microM) and insulin (10 micrograms/ml). The sensitivity of hepatocytes to 3'-methyl-4-dimethylaminoazobenzene (0.24 mM) was remarkably reduced by treatment with PB at 3 mM. PB treatment decreased efficiently the falling rate of total cytochrome P-450 content, but did not induce P-450PB, which is the specific form of cytochrome P-450 induced by PB, in primary cultured hepatocytes. On the other hand, 3-methylcholanthrene (MC, 10 microM) caused an increase of both contents of total cytochrome P-450 and P-450MC, which is the specific form of cytochrome P-450 induced by MC, in primary cultured hepatocytes. However, MC was ineffective for the maintenance of hepatocytes in primary culture. The possible biological actions of PB on primary cultured hepatocytes are discussed on the basis of the experimental data obtained.

Prolonged maintenance of active cytochrome P-450 in adult rat hepatocytes co-cultured with another liver cell type

The cytochrome P-450 content promptly fell in adult rat hepatocytes cultured in conventional conditions whereas no obvious change was detected over a 10-day period when these cells were co-cultured with another rat liver epithelial cell type. A concomitant maintenance of the amino-pyrine N-demethylase activity and a high aflatoxin B1-induced cytotoxicity was observed, and addition of phenobarbital to the culture medium produced about a 2-fold increase in cytochrome P-450 level. These results indicate that when placed in co-culture, adult hepatocytes remain differentiated for several days and suggest that such a co-culture system is an appropriate model for pharmacotoxicological studies.

Toxicology investigations with cell culture systems

This review concerns some of the cell culture systems that are most frequently used in toxicology investigations. In particular, it sets out to evaluate the effectiveness of these cell culture systems in assessing the toxic potential of chemicals. Metabolic studies and general and specific toxicology investigations are highlighted. Specific toxicology investigations relate to the effects of the tests substances on the highly specialized functions typical of the cell systems chosen. The general toxicology investigations include most of the other studies where differentiated or undifferentiated cells have been used to evaluate the effects of the tested substances on common basic biochemical processes essential for life. Lastly, we have attempted to focus attention on the most promising applications of cell cultures in toxicology studies for the near future and to identify those areas where further research is needed. Because of the several excellent reviews that already exist, we have decided not to consider cell cultures utilized in screening potential mutagens and carcinogens. We have also excluded investigations of drug therapeutic effects and action mechanisms of drugs.