Sex change in cytochrome P-450 phenotype by growth hormone treatment of adult rat hepatocytes maintained in a culture system on matrigel

Extracellular matrix proteins in construction and function of in vitro blood-brain barrier models

The blood-brain barrier (BBB) is a highly impermeable barrier separating circulating blood and brain tissue. A functional BBB is critical for brain health, and BBB dysfunction has been linked to the pathophysiology of diseases such as stroke and Alzheimer’s disease. A variety of models have been developed to study the formation and maintenance of the BBB, ranging from in vivo animal models to in vitro models consisting of primary cells or cells differentiated from human pluripotent stem cells (hPSCs). These models must consider the composition and source of the cellular components of the neurovascular unit (NVU), including brain microvascular endothelial cells (BMECs), brain pericytes, astrocytes, and neurons, and how these cell types interact. In addition, the non-cellular components of the BBB microenvironment, such as the brain vascular basement membrane (BM) that is in direct contact with the NVU, also play key roles in BBB function. Here, we review how extracellular matrix (ECM) proteins in the brain vascular BM affect the BBB, with a particular focus on studies using hPSC-derived in vitro BBB models, and discuss how future studies are needed to advance our understanding of how the ECM affects BBB models to improve model performance and expand our knowledge on the formation and maintenance of the BBB.

Structural basis of human pregnane X receptor activation by the hops constituent colupulone

Hops extracts are used to alleviate menopausal symptoms and as an alternative to hormone replacement therapy, but they can produce potentially harmful drug-drug interactions. The nuclear xenobiotic receptor pregnane X receptor (PXR) is promiscuously activated by a range of structurally distinct chemicals. It has a key role in the transcriptional regulation of genes that encode xenobiotic metabolism enzymes. In this study, hops extracts are shown to induce the expression of numerous drug metabolism and excretion proteins. The beta-bitter acid colupulone is demonstrated to be a bioactive component and direct activator of human PXR. The 2.8-A resolution crystal structure of the ligand binding domain of human PXR in complex with colupulone was elucidated, and colupulone was observed to bind in a single orientation stabilized by both van der Waals and hydrogen bonding contacts. The crystal structure also indicates that related alpha- and beta-bitter acids have the capacity to serve as PXR agonists as well. Taken together, these results reveal the structural basis for drug-drug interactions mediated by colupulone and related constituents of hops extracts.

Sex and the Liver – A Journey Through Five Decades

Metabolism of steroids and drugs in rodents is sexually differentiated. The reason for this turned out to be the sexually differentiated growth hormone (GH) secretory pattern regulating the expression of a number of hepatic cytochrome P-450 genes. Although not fully resolved, it is clear that several signaling pathways and transcription factors are involved in mediating the effects of GH. It may be argued that such a well-controlled physiological system should have an important biological role and we speculate that the demands of a robust hepatic steroid metabolism during pregnancy has led to the development of this sexually differentiated hypothalamo-pituitary-liver axis.

Inducibility of male-specific isoforms of cytochrome p450 by sex-dependent growth hormone profiles in hepatocyte cultures from male but not female rats

Although in vivo expression levels of the male-specific hepatic isoforms of cytochrome P450 (P450) (CYP2C11, CYP2C13, CYP2A2, and CYP3A2) are determined by the episodic growth hormone profile secreted by male rats, these isoforms have been completely refractory to growth hormone regulation in hepatocyte culture. By using species-specific rat growth hormone, at subphysiologic in vivo concentrations administered in two daily episodic pulses, we successfully induced CYP2C11 and CYP2A2 to near normal concentrations. Whereas inductive levels of CYP2C13 were subnormal, CYP3A2 was unresponsive to all hormonal treatments, quickly declining to undetectable concentrations. In agreement with in vivo findings, we observed that induction levels of the isoforms were always greatest when the male hepatocytes were exposed to the masculine-like episodic growth hormone profile and least stimulated by the continuous feminine-like hormone profile. When administered alone, dexamethasone consistently increased isoform levels. However, when administered with growth hormone, the glucocorticoid was always antagonistic, suppressing growth hormone induction of CYP2C11, CYP2C13, and CYP2A2. Finally, the P450 isoforms were completely unresponsive to all treatments when the hepatocytes were derived from female rats, supporting earlier findings that expression levels of sexually dimorphic P450 isoforms are inherently irreversible between sexes.

Transfection of adult primary rat hepatocytes in culture

The use of adult primary hepatocytes in culture is of importance for the understanding of hepatic processes at the cellular and molecular levels, and the possibility to employ transient transfection of reporter constructs is invaluable for mechanistic studies on hepatic gene regulation. Although frequently used, there is a lack of reports addressing optimization and characterization of transfection of primary rodent hepatocytes. Here, we have shown that the efficiency of biochemical transfection reagents varies significantly and that Lipofectamine2000 was a superior transfection reagent for adult primary rat hepatocytes when using luciferase reporter vectors. The efficiency increased when the cells were allowed ample time to adapt to the in vitro milieu. Cotransfection of a second reporter gene indicated a risk for promoter competition, and we found that relating reporter activity to total cellular protein content gave consistent and reliable results. Differentiation of the cells, achieved by including biomatrix from the Engelbreth-Holm-Swarm mouse sarcoma in the culture system, was to a larger extent required for hormonal/drug responses of transfected constructs than for responses of endogenous genes and assured responses of transfected constructs. Dexamethasone (Dex) is most often included in hepatocyte culture media, but we could not demonstrate a general beneficial effect of Dex on expression of luciferease reporter contructs. Using the established protocol, we have demonstrated responses of transfected constructs to growth hormone, glucocorticoid and LXR stimuli.

Intrinsic sex differences determine expression of growth hormone-regulated female cytochrome P450s

The masculine profile of cytochrome P450s found in male liver is determined by the episodic secretion of growth hormone characteristic of males. In turn, the female pattern of P450s observed in female rat liver is regulated by the continuous secretion of growth hormone characteristic of the female. In order to determine if intrinsic and possibly permanent sex differences exist in the response of hepatic P450s to growth hormone regulation, we compared the effects of the episodic and continuous growth hormone profiles on the expression of female-dependent isoforms in cultured hepatocytes isolated from both sexes. We observed that female-specific CYP2C12 as well as female-predominant CYP2A1, 3A1, and 2C6 could be induced by growth hormone concentrations equal to as little as 6, 0.6, and 0.06% of the mean circulating hormone profile found in normal females. Irrespective of sex, all four female-dependent isoforms were far more responsive to the continuous growth hormone profile than the episodic pattern. Lastly, female-derived hepatocytes in general responded with strikingly greater induction levels of P450s than male hepatocytes exposed to the same growth hormone profiles. The present findings demonstrate intrinsic, irreversible sex differences in growth hormone-regulated female-dependent P450s.

Effects of rGH and G118RrGH on the induction of CYP2C12 and IGF-I in primary rat hepatocytes

We have investigated the induction of the CYP2C12 and IGF-I genes by rGH and a "binding site 2 mutant", G118RrGH, in primary hepatocytes derived from male and female rats. Both the basal and the induced levels of CYP2C12, but not of IGF-I, were markedly lower in male derived than in female derived hepatocytes. A lower degree of receptor occupancy appears needed to elicit the CYP2C12 than the IGF-I response in cells obtained from both gender. At 7h of GH exposure high doses of rGH caused a bell-shaped induction of IGF-1 and a plateau was reached for CYP2C12 in female derived cells. G118RrGH did not antagonize the rGH effect; in contrast G118RrGH had an agonistic effect on CYP2C12 induction in female derived cells that in dose-response experiments was bell-shaped. The difficulty in achieving bell-shaped dose-response curves with rGH, together with the lack of an antagonistic effect of G118RrGH, suggests that both binding sites of rGH have similar affinity for the rGHR. The agonistic and bell-shaped dose-related effect of G118RrGH on CYP2C12 expression indicates that G118RrGH interacts with two receptors. The results could also imply that rGH, via its site 2, can interact with other receptor molecules than rGHR.

Insulin-like growth factor-I increases astrocyte intercellular gap junctional communication and connexin43 expression in vitro

Connexin43 (cx43) forms gap junctions in astrocytes, and these gap junctions mediate intercellular communication by providing transport of low-molecular-weight metabolites and ions. We have recently shown that systemic growth hormone increases cx43 in the brain. One possibility was that local brain insulin-like growth factor-I (IGF-I) could mediate the effect by acting directly on astrocytes. In the present study, we examined the effects of direct application of recombinant human IGF-I (rhIGF-I) on astrocytes in primary culture concerning cx43 protein expression and gap junctional communication (GJC). After 24 hr of stimulation with rhIGF-I under serum-free conditions, the GJC and cx43 protein were analyzed. Administration of 30 ng/ml rhIGF-I increased the GJC and the abundance of cx43 protein. Cell proliferation of the astrocytes was not significantly increased by rhIGF-I at this concentration. However, a higher concentration of rhIGF-I (150 ng/ml) had no effect on GJC/cx43 but increased cell proliferation. Because of the important modulatory role of IGF binding proteins (IGFBPs) on IGF-I action, we analyzed IGFBPs in conditioned media. In cultures with a low abundance of IGFBPs (especially IGFBP-2), the GJC response to 30 ng/ml rhIGF-I was 81%, compared with the average of 25%. Finally, as a control, insulin was given in equimolar concentrations. However, GJC was not affected, which suggests that rhIGF-I acted via IGF-I receptors. In summary, the data show that rhIGF-I may increase GJC/cx43, whereas a higher concentration of rhIGF-I--at which stimulation of proliferation occurred--did not affect GJC/cx43. Furthermore, IGFBP-2 appeared to modulate the action of rhIGF-I on GJC in astrocytes by a paracrine mechanism.

Sexual dimorphic expression of ADH in rat liver: importance of the hypothalamic-pituitary-liver axis

Hepatic alcohol dehydrogenase (ADH) activity is higher in female than in male rats. Although sex steroids, thyroid, and growth hormone (GH) have been shown to regulate hepatic ADH, the mechanism(s) for sexual dimorphic expression is unclear. We tested the possibility that the GH secretory pattern determined differential expression of ADH. Gonadectomized and hypophysectomized male and female rats were examined. Hepatic ADH activity was 2.1-fold greater in females. Because protein and mRNA content were also 1.7- and 2.4-fold greater, results indicated that activity differences were due to pretranslational mechanisms. Estradiol increased ADH selectively in males, and testosterone selectively decreased activity and mRNA levels in females. Effect of sex steroids on ADH was lost after hypophysectomy; infusion of GH in males increased ADH to basal female levels, supporting a role of the pituitary-liver axis. However, GH and L-thyroxine (T4) replacements alone in hypophysectomized rats did not restore dimorphic differences for either ADH activity or mRNA levels. On the other hand, T4 in combination with intermittent administration of GH reduced ADH activity and mRNA to basal male values, whereas T4 plus GH infusion replicated female levels. These results indicate that the intermittent male pattern of GH secretion combined with T4 is the principal determinant of low ADH activity in male liver.

GH regulation of IGF-I and suppressor of cytokine signaling gene expression in C2C12 skeletal muscle cells

GH is required for normal postnatal growth and metabolism. GH stimulates postnatal growth through induction of IGF-I gene expression. Although the liver is the major site of GH-regulated IGF-I, recent evidence indicates that GH-regulated IGF-I expression in nonhepatic tissues is sufficient for normal postnatal growth. One potentially important nonhepatic site of GH-stimulated IGF-I expression is skeletal muscle, as injection of GH into animals leads to increased IGF-I mRNA in this tissue. Nevertheless, direct effects of GH in skeletal muscle cells in culture have not been reported. We therefore tested the C2C12 myogenic cell line for its response to GH and demonstrate that C2C12 skeletal muscle cells rapidly respond to physiological levels of GH with increased tyrosine phosphorylation of the GH receptor, Janus kinase 2, signal transducer and activator of transcription-5a and -5b, insulin receptor substrate-1, and activation of MAPKs/ERKs and protein kinase B/Akt. In these cells, GH stimulates the expression of IGF-I and two members of the suppressors of cytokine signaling family, cytokine-inducible SH2-containing protein and suppressor of cytokine signaling-2. Treatment of C2C12 myoblasts with either the MAPK kinase inhibitor PD98059 or the PI3K inhibitor wortmannin results in higher levels of GH-induced IGF-I and suppressor of cytokine signaling-2 mRNA expression, suggesting that activation of MAPK and PI3K pathways has an inhibitory role in IGF-I and suppressor of cytokine signaling-2 gene regulation. Therefore, C2C12 cells provide the first in vitro model system to study various aspects of GH action in skeletal muscle.

Analysis of cytochrome P450 and phase II conjugating enzyme expression in adult male rat hepatocytes

The induction of cytochrome P450 (CYP450) and Phase II conjugating enzymes by prototypical hepatic enzyme inducers was studied in adult male rat hepatocytes. Hepatocytes were suspended and cultured in diluted Matrigel in a basal serum-free Dulbecco's modified Eagle medium and exposed to the prototypical liver enzyme inducers, 3-methylcholanthrene, phenobarbital, hydrocortisone, and clofibrate for 48 h. Total RNA and microsomes were isolated and prepared, respectively, at 72 h. The expression of CYP1A1, CYP1A2, CYP2B1, CYP2C11, CYP2E1, CYP3A1, CYP3A2, CYP4A1, fatty acyl-CoA oxidase, uridine diphosphate-glucuronosyltransferase, glutathione-S-transferase, and sulfotransferase was determined at the mRNA level with reverse transcriptase polymerase chain reaction (RT-PCR). The expression of CYP1A1, CYP2B1, CYP2C11, CYP2E1, and CYP4A1 was also measured at the apoprotein level by Western immunoblotting. Using these culture and expression analysis techniques, we have found that the expression of these metabolic enzymes can be maintained in culture for up to 7 d at the mRNA and apoprotein levels. In addition, hepatocytes were found to respond to chemical enzyme inducers with marked increases in enzyme expression at either the mRNA or protein level and in a concentration-related fashion. Cells were responsive to enzyme induction as early as 24 h after initial plating. The results obtained from this investigation indicate that the presence of diluted Matrigel (at a concentration of 0.35 mg/ml), the use of low concentrations of insulin (1 microM), hydrocortisone (0.1 microM), and serum-free culture medium can maintain the differentiated phenotype and responsiveness of cultured hepatocytes to chemical-induced metabolic enzyme expression. Under the conditions used in this study, enzyme induction in adult male rat hepatocytes shows close agreement with enzyme induction observed in the livers of rats exposed to these or similar prototypical enzyme inducers. Rat hepatocytes cultured in the presence of diluted Matrigel coupled with enzyme mRNA expression analysis with RT-PCR are proven to be a valuable and important in vitro toxicological approach to assess the chemical-induced changes in expression of liver CYP450 and Phase II conjugating enzymes.

The effect of hormones on the expression of five isoforms of UDP-glucuronosyltransferase in primary cultures of rat hepatocytes

PURPOSE

To investigate the direct effects of sex hormones, growth hormone, thyroid hormones and dexamethasone on the regulation of UDP-glucuronosyltransferase (UGT).

METHODS

Rat hepatocytes were cultured on matrigel and treated with various hormones. Northern blot analysis was carried out using cDNA probes to family 1 and family 2 isoforms.

RESULTS

Treatment with 10(-5) M testosterone increased the mRNA levels of UGT 2B1 by 29% and UGT2B3 by 32%. Incubation of growth hormone (10 mU) with hepatocytes suppressed the expression of UGT2B1 and UGT2B3 by 17% and 38%, respectively. T3 administration resulted in a time and dose-dependent effect on the expression of UGT 1 isoforms, with increased UGT1A6 by 70%, and decreased UGT1A1 by 38% and UGT1A5 by 35%. All UGT isoforms except UGT 1A6 studied in this assay were up-regulated by dexamethasone, but to different degrees. The regulation of UGT1A1 and UGT2B1 by dexamethasone was dose and time dependent, and the induction of dexamethasone in the expression of UGT1A1 and UGT2B1 was blocked by cycloheximide but not dichloro-1-D-ribofuranosylbenzimidazole.

CONCLUSIONS

This study demonstrates that multiple hormones take part in the regulation of UGT mRNA expression in the rat and individual genes can be differentially modulated.

Bile acid synthesis in primary cultures of rat and human hepatocytes

The regulation of hepatic bile acid formation is incompletely understood. Primary cultures of mammalian hepatocytes offer an opportunity to examine putative regulatory factors in relative isolation. Using rat and human hepatocytes in primary culture, we examined bile acid composition and the expression of the rate-limiting enzyme of formation, cholesterol 7alpha-hydroxylase. Control rat hepatocytes showed a declining bile acid production over 4 days, from 156 +/- 24 ng/mL (67% cholic acid) on day 1 to 55 +/- 11 ng/mL (55% cholic acid) on day 4. In addition to cholic acid, chenodeoxycholic acid, alpha-muricholic acid, and beta-muricholic acid were formed. Treatment with triidothyronine (T3) or dexamethasone alone had no significant effect on bile acid production. A combination of T3 and dexamethasone significantly increased the total bile acid production on day 4 (224 +/- 54 ng/mL) and resulted in a marked change in composition to 23% cholic acid and 77% non-12alpha-hydroxylated bile acids. Control rat hepatocytes had a cholesterol 7alpha-hydroxylase activity of 3.3 +/- 0.6 pmol/mg protein/min after 4 days in culture. Cells treated with the combination of dexamethasone and T3 had an activity of 16.4 +/- 3.6 pmol/mg protein/min. The cholesterol 7alpha-hydroxylase messenger RNA (mRNA) levels, determined by solution hybridization after 4 days of culture, showed results similar to those for the activity data; control cells had 5.3 +/- 0.9 cpm/microg total nucleic acids (tNAs). T3 or dexamethasone-treated cells did not differ from control cells, whereas the combination of T3 and dexamethasone increased the mRNA levels to 20.6 +/- 2.8 cpm/microg tNAs. In human hepatocytes, isolated from donor liver, bile acid formation increased from 206 +/- 79 ng/mL on day 2 to 1490 +/- 594 ng/mL on day 6 and then declined slightly. Cholic acid and chenodeoxycholic acid were formed, constituting about 80% and 20%, respectively. The combined addition of T3 and dexamethasone had a tendency to decrease rather than increase bile acid formation. Also, mRNA levels of the cholesterol 7alpha-hydroxylase increased severalfold in the human hepatocytes from day 2 to day 4 and then declined. The addition of T3 or dexamethasone did not effect the mRNA levels in any consistent way. It is noteworthy that the capacity of the cultured human hepatocytes to produce bile acids was higher than that of cultured rat hepatocytes, in spite of the fact that the production of bile acids in rat liver is 3- to 5-fold higher than that in human liver in vivo. It is also evident that while hormonal factors appear to regulate bile acid synthesis in the rat, no evidence for this was found in human hepatocytes. As the composition of bile acids secreted by human hepatocytes in primary culture closely resembles that found in vivo, this represents a useful model for further studies of the synthesis and regulation of bile acids.

Effect of reduced low-density lipoprotein receptor level on HepG2 cell cholesterol metabolism

Low-density lipoproteins (LDL) are taken up by both LDL receptor (LDLr)-dependent and -independent pathways. In order to determine the importance of these pathways in the activity of the various enzymes that are important in maintaining the cellular cholesterol level in hepatic cells, we created HepG2 cells expressing lower levels of LDLr. Thus HepG2 cells were transfected with a constitutive expression vector (pRc/CMV) containing a fragment of LDLr cDNA inserted in an antisense manner. Stable transformants were obtained that showed significant reductions of 42, 72 and 85% of LDLr protein levels compared with the control, as demonstrated by immunoblotting and confirmed by the LDL binding assay. The best inactivation was achieved with the construct containing the first 0.7 kb of LDLr cDNA. Incubating the different HepG2 cell subtypes with LDL showed similar association of apolipoprotein B (apo B) or cholesteryl esters from LDL with the cells, indicating that the LDLr deficiency did not significantly affect LDL uptake by the cell. However, apoB degradation was reduced significantly by 71-82% in the most LDLr-deficient HepG2 cells. We also found that 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCoA red) activity is significantly increased by 32-35% in HepG2 cells expressing very low levels of LDLr that also demonstrate a significant decrease of 20% in acyl-CoA:cholesterol acyltransferase (ACAT) activity. However, these effects are moderate compared with those observed when cells were incubated in lipoprotein-depleted medium, where a >900% increase in HMGCoA red activity and a loss of 60% of ACAT activity was observed. Thus, in HepG2 cells, different levels of LDLr affect LDL-apoB degradation, but have very little effect on LDL association, HMGCoA red and ACAT activities, revealing that LDLr is more important in the clearance of LDL-apoB than in HepG2 cell cholesterol homoeostasis, a role that should be attributable to both LDLr-dependent and -independent pathways.

Zonation of hepatic cytochrome P-450 expression and regulation

The CYP genes encode enzymes of the cytochrome P-450 superfamily. Cytochrome P-450 (CYP) enzymes are expressed mainly in the liver and are active in mono-oxygenation and hydroxylation of various xenobiotics, including drugs and alcohols, as well as that of endogenous compounds such as steroids, bile acids, prostaglandins, leukotrienes and biogenic amines. In the liver the CYP enzymes are constitutively expressed and commonly also induced by chemicals in a characteristic zonated pattern with high expression prevailing in the downstream perivenous region. In the present review we summarize recent studies, mainly based on rat liver, on the factors regulating this position-dependent expression and induction. Pituitary-dependent signals mediated by growth hormone and thyroid hormone seem to selectively down-regulate the upstream periportal expression of certain CYP forms. It is at present unknown to what extent other hormones that also affect total hepatic CYP activities, i.e. insulin, glucagon, glucocorticoids and gonadal hormones, act zone-specifically. The expression and induction of CYP enzymes in the perivenous region probably have important toxicological implications, since many CYP-activated chemicals cause cell injury primarily in this region of the liver.

Characterization of a pretranscriptional pathway for induction by phenobarbital of cytochrome P450 3A23 in primary cultures of adult rat hepatocytes

Our laboratory has proposed that phenobarbital (PB), a typical lipophilic agent that induces some members of the supergene family of liver microsomal cytochromes P450 (e.g., CYP2B1/2 and CYP3A23), acts through a complex process inhibitable by the presence of growth hormone (GH), the absence of some components of the extracellular matrix, or a disrupted cytoskeleton. To verify that these manipulations of the culture environment block specific steps in the PB induction pathway rather than simply exerting nonspecific or toxic effects on CYP2B1/2 gene transcription, we have now examined PB induction of CYP3A23, a gene known to also be transcriptionally activated by dexamethasone (DEX) through a "nonclassical" pathway apparently involving the glucocorticoid receptor. We found that in primary cultures of adult rat hepatocytes treated with PB, induction of CYP3A23 mRNA, just as we reported for induction of CYP2B1/2 mRNA, required the use of Matrigel (a reconstituted basement membrane) and was blocked by the presence of cytoskeletal inhibitors (colchicine or cytochalasins) or of physiologic concentrations of GH in the culture medium. Moreover, PB induction of CYP3A23 and of CYP2B1/2 mRNAs was greatly diminished by inhibitors of cAMP-dependent protein kinase (PKA). In striking contrast, induction of CYP3A23 mRNA by DEX was unaffected by any of these alterations of the culture conditions that block its induction by PB. We conclude that the effects of extracellular matrix, GH, disruption of the cytoskeleton, and activation of cAMP-dependent protein kinase, pharmacologically define multiple, pretranscriptional steps in the pathway(s) for PB induction of liver cytochromes P450.

Extended primary culture of human hepatocytes in a collagen gel sandwich system

To develop a strategy for extended primary culture of human hepatocytes, we placed human hepatocytes between two layers of collagen gel, called a "collagen gel sandwich." Maintenance of hepatocellular functions in this system was compared with that of identical hepatocyte preparations cultured on dry-collagen coated dishes or cocultured with rat liver epithelial cells. Human hepatocytes in a collagen gel sandwich (five separate cultures) survived for more than 4 wk, with the longest period of culture being 78 d. They maintained polygonal morphology with bile canaliculuslike structures and high levels of albumin secretion throughout the period of culture. In contrast, hepatocytes on dry-collagen became feature-less, and albumin secretion could not be detected after 14 d of culture. This loss of albumin secretion was partially recovered by overlaying one layer of collagen gel. Ethoxyresorufin O-deethylase activity, associated with cytochrome P450 1A2, was detected basally up to 29 d in collagen gel sandwich culture. These activities were induced four- to eightfold after induction with dibenz(a,h)anthracene. Cocultures also maintained basal activity up to 29 d. However, their inducibility was lower than that of hepatocytes in collagen gel sandwich. No ethoxyresorufin O-deethylase activity was detected in hepatocytes cultured on dry-collagen at 7 d. Thus, the collagen gel sandwich system preserves differentiated morphology and functions of human hepatocytes in primary culture for a prolonged period of time. This system is a promising model for studying human hepatocellular function, including protein synthesis and drug metabolism in vitro.

Lipid-mediated transfection of normal adult human hepatocytes in primary culture

The aim of this work was to develop a procedure for the lipid-mediated transfection of DNA into normal adult human hepatocytes in culture. Cells were plated in a serum-free culture medium at various cell densities, on plastic or collagen-coated dishes, both in the absence and in the presence of epidermal growth factor (EGF). The cells were incubated for various periods of time with mixtures of DNA-lipofectin or DNA-3 beta[N-(N',N'-dimethylaminoethane)-carbamoyl] cholesterol (DC-chol) liposomes, and the efficiency of transfection was assessed by measuring the activity of reporter genes, beta-galactosidase or chloramphenicol acetyl-transferase (CAT). For comparison, similar experiments were carried out with human cell lines including HepG2, Caco-2, and WRL68. The efficiency of transfection (in percentage of cells) was not significantly different after transfection with lipofectin or DC-chol and comprised between 0.04 and 1.7% (extreme values) for different cultures. The efficiency of transfection decreased as the age or density of the culture increased and increased in cultures treated with EGF. Direct measurement of the rate of DNA synthesis suggested that the efficiency of transfection was related to the number of cells entering the S phase. Under the same conditions, the efficiency of transfection was one to two orders of magnitude greater in the three cell lines. A plasmid harboring 660 bp of the 5'-flanking region of CYP1A1 (containing two xenobiotic enhancer elements) fused upstream of the promoter of thymidine kinase and the CAT reporter gene was constructed. When this plasmid was transfected in human hepatocytes, CAT activity was induced as expected. We conclude that normal adult human hepatocytes can be transfected with exogenous DNA and that the transfected construct is regulated in the manner expected from in vivo studies.

Interaction of a novel sex-dependent, growth hormone-regulated liver nuclear factor with CYP2C12 promoter

CYP2C12 is a steroid hydroxylase cytochrome P450 whose female-specific expression in adult rat liver is transcriptionally activated by the continuous plasma growth hormone (GH) profile characteristic of adult female rats. DNase I footprinting and gel mobility shift analysis of the 5'-flank of the CYP2C12 gene were carried out to identify cis-acting elements and trans-acting factors that may contribute to the GH-regulated, sex-dependent transcription of this P450 gene. DNase I footprinting analysis revealed sex- and GH-regulated DNase I hypersensitivity sites at the boundaries of several protein binding sites detected along a 1560-nucleotide upstream segment of CYP2C12. Five distinct sites bound a novel continuous GH-regulated nuclear factor, GHNF, which is enriched in adult female and continuous GH-treated male liver nuclear extracts compared to untreated male liver nuclear extracts. Two other footprinted sites correspond to binding sites for the liver transcription factors C/EBP and albumin D element-binding protein and a third to an HNF1 binding site. A specific binding site for GHNF was also found in the 5'-proximal promoter of CYP2C11, an adult male-specific liver P450 gene, suggesting that GHNF may contribute to the down-regulation of that gene by continuous GH. GHNF was distinguished from the nuclear factors that bind to a GH response element upstream of the rat Spi 2.1 gene and is also distinct from the GH-activatable latent cytoplasmic transcription factors STAT 1, STAT 3, and STAT 5. These findings support the hypothesis that continuous GH-activated transcription of CYP2C12 in adult female rat liver (a) involves the activation of a novel GH-regulated nuclear factor which binds to multiple sites along the 5'-flank of this cytochrome P450 gene, and (b) proceeds via a signaling pathway distinct from the GH pulse-activated STAT5 pathway proposed to induce CYP2C11 and other male-expressed liver genes.

In vitro techniques for studying drug metabolism

A considerable amount of information has been obtained about human enzymes involved in drug metabolism. Cytochrome P450 is presented as a paradigm in order to illustrate the experimental techniques now available. In vitro assays can be done with tissue slices, microsomes, or even short-term cell cultures. In addition, recombinant enzymes can be produced in a variety of different vector systems. Information about catalytic selectivity regarding new drugs can be obtained, as well as important information about potential drug-drug interactions and bioavailability. Such studies play a role in the drug development process as well as metabolism and safety assessment.

Metabolism of pimobendan in long-term human hepatocyte culture: in vivo-in vitro comparison

The aim of this study was to investigate further the potential of a new hepatocyte culture based on the hypothesis that liver cells in an appropriate in vitro environment (immobilizing gel technique) maintain high metabolic activity comparable with that in vivo. Pimobendan (UD-CG 115), a pyridazinone derivative, is a cardiotonic vasodilator that increases myocardial contractility through calcium sensitization and relaxation of vascular smooth muscle, probably due to phosphodiesterase inhibition. In man, pimobendan is O-demethylated to UD-CG 212. This latter is metabolized to O- and N-glucuronides. Pimobendan itself is also glucuronidated to a N-glucuronide. Human hepatocytes immobilized in collagen gel were incubated with pimobendan to investigate their metabolic activity in the long-term and to compare the results to the data from clinical trials. 14C-labelled pimobendan was incubated at two concentrations (10 and 100 microM) at day 3, 11 and 22 of culture, and samples were analysed after 4, 24 and 48-h incubation. Metabolic patterns were evaluated by hplc with radioactivity-, diode array-, and mass spectral-detection. In vitro, pimobendan was O-demethylated and subsequently O-glucuronidated. The rate of metabolism of pimobendan could be maintained in this culture system for > 3 weeks. However, the relative amount of a putative N-glucuronide under in vitro conditions was lower than in vivo.

Characterization of the proximal promoter and two silencer elements in the CYP2C11 gene expressed in rat liver

The cytochrome P450 gene CYP2C11, expressed in the liver of male rats, is transcriptionally regulated in a dual fashion by the sexually dimorphic secretion pattern of growth hormone. To enable analysis of transcriptional regulatory DNA elements, rat genomic sequences were cloned. DNase I hypersensitivity analysis of rat liver nuclei revealed the existence of two hypersensitive sites whose presence in the vicinity of the transcription start site correlates to high transcriptional activity of the gene. Deletion mutants of the 5' flank were fused to reporter genes and transiently transfected into HepG2 cells or into primary adult rat hypatocytes. Transfection experiments in combination with DNase I footprinting analysis in vitro led to the identification of two negative regulatory regions spanning nucleotides -1,230 to -1,188 and -409 to -368 and designated (SIL1200) and (SIL400), respectively. When placed in front of the heterologous thymidine kinase promoter, SIL1200 and SIL400 reduced the activity of the chloramphenicol acetyl transferase reporter gene to 13% and 23% of the control value, respectively. No sex-dependent binding of liver nuclear extracts to the two silencers could be detected by in vitro footprinting or gel retardation assays. However, a sex-dependent footprint consistently stronger with male liver nuclear extracts than with female extracts was observed in the -320 to -294 region. A significant level of identity was found between the DNA sequence corresponding to this footprint and that of orphan steroid receptor elements as well as with that of a basal transcription element common to several CYP2C genes. However, the identity of a potential trans-acting factor binding between -320 and -294 or response of this element to growth hormone is as yet unknown. A sex- and GH secretory profile-dependent protein-DNA interaction in vitro was observed in the -107 to -95 region. In spite of the sequence similarity that exists between this region and the consensus binding site for HNF-1, this region does not bind HNF-1 alpha. This element acted as a repressor on the heterologous thymidine kinase promoter. To date, the two silencer elements and possibly also the HNF-1-like element are the only functional elements defined in the CYP2C11 gene, and it is conceivable that induction of the gene involves derepression of the silencer elements.

Expression of liver functions in immortalised rat hepatocyte cell lines

The differentiated hepatic function of two rat liver cell lines, P9 and SV40RH1, immortalised by transfection with SV40 DNA has been investigated in terms of the glutathione synthesis, and the activities of gamma-glutamyltransferase, glutathione-S-transferase and UDP-glucuronosyltransferase. SV40RH1 is a highly differentiated cell line at early passage, but the expression of some aspects of its differentiated phenotype is unstable and some functions are lost by passage 12-13. P9 is a less-well differentiated cell line, with relatively stable expression of functions between passages 4 and 13. In terms of differentiated function both cell lines represent a marked improvement over primary cultures of rat hepatocytes which de-differentiate rapidly within 24-48 h in culture. This retention of liver function in proliferating cell lines offers the opportunity to use such cells in in vitro toxicological studies.

Collagen gel immobilization: a useful cell culture technique for long-term metabolic studies on human hepatocytes

1. Primary cultures of human hepatocytes have already been employed in various applications for the study of xenobiotic metabolism. Most of these approaches were performed either on freshly isolated cells or on short-term primary cultures. Standard culture techniques do not maintain functional stability of P450 enzymes for > 1 week in vitro. 2. The aim of this study was to demonstrate the beneficial effect of an easy to apply, extracellular matrix configuration on the long-term performance of cultured human liver cells. Light microscopical examination of the cultures indicated that the cells remained viable over 1 month. As revealed by electron microscopy, hepatocytes exhibited bile canaliculi and desmosomes and were rich in mitochondria and endoplasmatic reticulum, indicating metabolic activity. 3. An early culture phase (3 days after isolation) could be described with decreasing DNA content of the cultures, peak values of alanine-amino-transferase (ALAT), and increasing albumin synthesis. After this adaptive period stable levels for DNA content and albumin synthesis were noted; ALAT returned to low values. 4. Functional activity was monitored by measurements of P450 1A1-dependent O-demethylation of p-nitroanisole to p-nitrophenol, which appeared to be constant over 3 weeks and weakly inducible by 1 mM phenobarbital. Another set-up examined conjugation of acetaminophen at subtoxic concentrations: acetaminophen was metabolized to its glucuronide and sulphate; 3-(glutathione-S-yl)-acetaminophen was not detected. Almost identical metabolism was found, comparing day 3 with 16 of culture. 5. We concluded that collagen gel immobilization not only provides mechanical support to cultured hepatocytes, but also supports long-term differentiated function of the cells for metabolic studies.

Primary cultures of rat hepatocytes in hollow fiber chambers

Hepatocyte culture may represent an alternative to the use of animals to study drug detoxification by the liver. An ideal in vitro system should closely mimic the in vivo environment by providing continuous media perfusion and oxygenation, and should facilitate sampling of cells and culture media. To meet these criteria, a hollow fiber bioreactor seeded with isolated rat hepatocytes was developed and tested by measuring the formation of three products of the oxidative metabolism of diazepam and the glucuronidation of phenolsulfonphthalein (PSP). To compare the performance of conventional monolayer culture to that of the bioreactor system, diazepam metabolism was studied for 45 days in both systems. The oxygen dependency of diazepam metabolism was evaluated by perfusing the bioreactor in an oxygen-rich atmosphere (30%). Total diazepam metabolism was twofold higher in the O2-rich perfused hollow fiber cultures than in the cultures perfused under normal conditions, reflecting an increase in temazepam and oxazepam production. Diazepam detoxification activity was significantly enhanced by oxygen (P < or = 0.001) over the life of the perfused cultures. PSP metabolism was similar in all three culture systems. By Day 10, diazepam metabolism in the oxygenated bioreactor system was 44% of the in vivo activity of rat hepatocytes. This activity dropped to 30% by Day 25 of culture. These results justify the use of perfused culture systems for in vitro detoxification studies as an alternative to animal use and emphasize the capacity of a culture device perfused under O2-enriched conditions to maintain long-term P450 activity of rat hepatocytes.

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.

Importance of cell aggregation for expression of liver functions and regeneration demonstrated with primary cultured hepatocytes

Adult rat hepatocytes aggregated to form floating multicellular spheroids when cultured in Primaria dishes, which have a positively charged surface, in serum-free Williams' medium E (WE) supplemented with insulin and epidermal growth factor (EGF). These hormones were essential for maintenance of the spheroids, whereas the size of the spheroids depended on the inoculum cell density. The spheroids retained in vivo levels of expressions of albumin and glucokinase and synthesized scarcely any DNA even in the presence of insulin and EGF. On transfer to type I collagen-coated dishes, the spheroids gradually disaggregated and the cells formed monolayers, in which the expressions of albumin and glucokinase were suppressed and DNA synthesis and hexokinase activity were increased. DNA synthesis of hepatocytes in monolayer culture was maximal 24 hr after transfer of the spheroids, approximately 80% of the hepatocyte nuclei were labelled with bromodeoxyuridine during culture for 48 hr, and the mitotic index was approximately 70% after 60 hr. These results suggest that, in spheroids, hepatocytes remained in the G0 phase, but that when they formed monolayers, they progressed to the G1 phase and proceeded through the cell cycle in the presence of insulin and EGF. This work shows that the cell cycle of hepatocytes in culture can be manipulated by providing conditions for quiescence as spheroids or growth as monolayers and that the shape of hepatocytes is important for regulating their growth and liver-specific functions.

Sulphadimidine metabolism in vitro: I. Sex differences in acetylation and hydroxylation in cultured rat hepatocytes

The hydroxylation and acetylation of 0.5 mM sulphadimidine (SDD) was studied in primary cultures of hepatocytes from male and female rats, and from castrated male and sham operated male rats. In addition, SDD metabolism was investigated in hepatocytes from castrated male rats treated with testosterone, prior to liver cell isolation. In male rat hepatocytes a significantly higher hydroxylation activity was observed than in hepatocytes from female and castrated male rats. Acetylation activity was higher in females. Testosterone induced hydroxylation but did not affect acetylation. These results correlate well with data from previous in vivo studies, showing the relevance of this in vitro model.

Dependency of the in vitro stabilization of differentiated functions in liver parenchymal cells on the type of cell line used for co-culture

The differentiation status in cultures of primary rat liver parenchymal cells was determined by measuring the activities of various xenobiotic metabolizing enzymes. Most enzyme activities dropped rather rapidly in monocultures of parenchymal cells. The protein content and the activities of cytosolic epoxide hydrolase, glutathione S-transferase, and alpha-naphthol UDP-glucuronosyl transferase were, however, well stabilized in 7-day-old co-cultures of parenchymal cells with two different lines of rat liver nonparenchymal epithelial cells (NEC1 and NEC2). Phenol sulfotransferase and microsomal epoxide hydrolase activity were reduced in this coculture system after 7 days to about 30 and 20% of the initial activity. Generally, higher enzyme activities were measured in co-cultures with one specific epithelial cell line (NEC2) as compared to those with the other line (NEC1). C3H 10T1/2 mouse embryo fibroblasts supported the parenchymal cells even better than the two epithelial lines, because the activity of microsomal epoxide hydrolase was also stabilized. Glutathione transferase activity was increased over time in this co-culture system. Our results show that the differentiation status of liver parenchymal cells was much better stabilized in co-cultures than in monocultures but that, depending on the type of cells used for co-culture, great quantitative differences existed. The entire pattern of xenobiotic metabolizing enzyme activities could not be stabilized at the kind of levels found in freshly isolated parenchymal cells.

Sulfation and glucuronidation of acetaminophen by cultured hepatocytes reproducing in vivo sex-differences in conjugation on Matrigel and type 1 collagen

The sulfate and glucuronide conjugation of acetaminophen (APAP) by hepatocytes cultured on Matrigel or type 1 collagen was compared to APAP metabolism in vivo. The metabolic fate of low (15 mg/kg), medium (125 mg/kg), and high (300 mg/kg) doses of APAP injected intraperitoneally were determined in male and female rats. Males excreted more APAP as the sulfate conjugate than females, which correlated with the twofold greater APAP sulfotransferase activity in the male vs. females (301 +/- 24 vs. 156 +/- 18 pmol.mg-1 protein.min-1). Also, as sulfate conjugation became saturated, there was a dose-related shift in APAP metabolism from sulfate to glucuronide conjugation in both sexes. After death, the livers of the same animals were perfused with collagenase and the hepatocytes cultured in modified Waymouth's medium on either Matrigel or rat-tail collagen, with various doses of APAP (0, 0.125, 0.25, 0.5, and 1.0 mM). Sex differences in APAP sulfation and glucuronidation persisted in culture for up to 4 days, with sulfation predominating in the male similar to in vivo. With increasing APAP concentration (dose), there was a saturation of sulfate conjugation and a shift to glucuronidation as observed in vivo. Sex differences in APAP sulfation and glucuronidation were no longer significant by Day 4 in culture. Sulfation, and to a lesser extent, glucuronidation, were more stable on Matrigel than collagen. We concluded that APAP metabolism of freshly isolated hepatocytes could replicate in vivo sex differences in conjugation, and that Matrigel was superior to collagen as substrate.

Differential stabilization of cytochrome P-450 isoenzymes in primary cultures of adult rat liver parenchymal cells

Cytochrome P-450 dependent hydroxylation of testosterone was measured in 7-day-old cultures of primary rat liver parenchymal cells. Determinations were carried out in monocultures of parenchymal cells and co-cultures of parenchymal cells with rat liver nonparenchymal epithelial cells, or mouse embryo fibroblasts. In the monoculture system, testosterone metabolism was drastically reduced and hardly measurable after 7 days in culture. In the co-culture systems, individual P-450 isoenzymes were stabilized on different levels. P-450s p and presumably c were well preserved, P-450 a was reduced but clearly measurable, P-450 h was totally lost whereas P-450s b and e were not measurable after 7 days (the activities of these isoenzymes however were already low in freshly isolated parenchymal cells). The results were independent of the cell line used for co-cultivation and of the method of parenchymal cell isolation, that is whether collagenase or EDTA was used as the agent for dissociating the cells from the liver. The results showed that the co-cultivation of liver parenchymal cells with other nonparenchymal cells significantly improved the differentiated status of the former. In this cell culture system however, not every parameter was equally well stabilized.

Xenobiotic induction of P-450 PB-4 (IIB1) and P-450c (IA1) and associated monooxygenase activities in primary cultures of adult rat hepatocytes

1. The long-term maintenance of metabolism of representative drugs and steroid hormone substrates by cytochromes P-450, and their inducibility, was investigated in primary cultures of adult rat hepatocytes. Collagenase-isolated cells were seeded on collagen-coated tissue culture dishes and cultured in Chee's essential media in the presence or absence of phenobarbital (PB, 0.75 mM, 96 h or continuously) and 3-methylcholanthrene (MC, 5 microM, 48 h) for up to 45 days. 2. Hepatic P-450-dependent metabolism of diazepam to its primary oxidized metabolite was inducible by PB both in vivo (monitored in isolated liver microsomes) and in cultured cells (up to 100% and 400% increases in the formation of temazepam and nordiazepam, respectively, after 25 days in culture). Hepatocyte microsomal androstenedione 16 beta-hydroxylase activity was also induced by PB treatment of the hepatocytes (350-650% increase in 20-day-old cells). 3. Western blot analysis revealed that immunoreactive P-450 form PB-4 (IIB1), which catalysed the N-demethylation of diazepam to yield nordiazepam as well as androstenedione 16 beta-hydroxylation when assayed in a purified enzyme system, was substantially elevated following PB treatment of the cultured cells. Similarly, MC induced 7-ethoxycoumarin O-deethylase activity (up to 2000% increase from 5 to 45 days) as well as immunoreactive P-450c (IA1) in the hepatocyte cultures. 4. These studies demonstrate that cytochrome P-450 activities can be maintained, and also induced, after extended periods of time in hepatocytes cultured using a simple collagen mixture as substrate and a commercially available tissue culture media. This culture system should provide an important tool for further studies of P-450-dependent xenobiotic metabolism in a well-defined, liver-derived cellular system.

Evidence from dwarf rats that growth hormone may not regulate the sexual differentiation of liver cytochrome P450 enzymes and steroid 5 alpha-reductase

Differences in the pattern of growth hormone (GH) secretion in mature rats (i.e., "continuous" secretion in females versus "pulsatile" secretion in males) are thought to be the underlying cause of sex-dependent differences in a subpopulation of liver microsomal P450 enzymes and steroid 5 alpha-reductase. A new strain of dwarf rats (NIMR/AS) has recently been shown to have low or undetectable levels of circulating GH due to a selective defect in pituitary GH synthesis. We have measured the levels and/or activity of IIA1 (P450a), IIA2 (P450m), IIC11 (P450h), IIC12 (P450i), IIIA2 (a P450p isozyme), and steroid 5 alpha-reductase in liver microsomes from male and female dwarf rats, to test the hypothesis that the expression of these sexually dimorphic enzymes is regulated by GH. In mature rats, the levels of liver microsomal IIA2, IIC11, and IIIA2 were higher in male than in female dwarf rats, whereas the levels of activity of IIA1, IIC12, and steroid 5 alpha-reductase were greater in female than in male dwarf rats. These sex differences resulted from age-related changes in either male dwarf rats (i.e., an increase in IIC11 and IIA2 and a decrease in IIA1) or female dwarf rats (i.e., an increase in IIC12 and 5 alpha-reductase and a decrease in IIIA2). The magnitudes of these sex-dependent, age-related changes were essentially indistinguishable from those observed in normal rats. These unexpected results suggest that GH is not the pituitary factor responsible for regulating the levels of sexually dimorphic, steroid-metabolizing enzymes in rat liver. Alternatively, it is possible that these enzymes are regulated by extremely low levels of GH. In either case, the current model of how steroid-metabolizing enzymes are regulated in rats must be revised to account for the normal sexual differentiation of these enzymes in dwarf rats.

Regulation of the expression of the sex-specific isoforms of cytochrome P-450 in rat liver

The hepatic metabolism of steroid hormones and of xenobiotics frequently depends on the expression of the sex-specific isoforms of cytochrome P-450 and on differences in sex hormones. Following biochemical, immunological and molecular biological investigations, it was shown that in adult rat liver there exist at least four male-specific and one female-specific isoforms of cytochrome P-450. The designation of these sex-specific genes is IIC11, IIIA2, IIC13 and IIA2 in males, and IIC12 in females. The irreversible programming of the expression of these isoforms of cytochrome P-450 in adulthood occurs during the perinatal period of life, and is named enzyme imprinting. One of the main factors that regulates the expression of the sex-specific isoforms of cytochrome P-450 is the level of androgens in the blood. Castration of adult rats decreased the level of the male isoforms of cytochrome P-450 and the activity of the monooxygenase enzyme system that remained higher than in intact females. The mechanism of enzyme imprinting can be explained as follows: neonatal androgens program the secretion of hypothalamic hormones, somatostatin and growth-hormone-releasing factor. These factors determine the type of growth hormone secretion in adult rats, and this controls the type of sex-specific isoforms of cytochrome P-450 expressed in adulthood. Metabolic regulation similar to that outlined above was shown to occur for several metabolism-dependent chemical carcinogens. Such a pathway may explain the different sensitivity displayed by male and female rats to treatment with these carcinogenic agents. One possible way of modulating the expression of some isoforms of cytochrome P-450 in adult rats is by treating neonates with specific xenobiotics that change the constitutive expression of neonatal androgens. It appears that this enzyme imprinting plays an important role in determining the individual sensitivity to the carcinogenic effects of chemicals.

A different cytochrome P450 form is induced in primary cultures of rat hepatocytes

A 49-kDa protein (P49) was discovered in the primary cultures of rat hepatocytes. P49 cross-reacted with the antibodies against purified P450IIC11 [formerly P-450(M-1)]. P49 was located in microsomes and highly induced after plating of isolated hepatocytes on collagen-coated culture dishes. To characterize P49, cDNA clones were screened from a rat liver lambda gt11 expression library. From sequence analysis of the cloned cDNAs, the amino acid sequence of P49 was deduced, and the protein was identified as a previously uncharacterized form of cytochrome P450. P49 consists of 489 amino acids and shows approximately 60% similarity with the members of class IIC subfamily of rat cytochrome P450, such as P450IIC11 and P450IIC12 [formerly P-450(F-1)]. RNA blot analysis indicates that the mRNA translating P49 was induced approximately 20- to 30-fold at 70 hr in the primary cultures compared with the liver of adult rats. Induction of P49 was not affected by density of the plated cells and the presence or absence of several hormones, serum, or antibiotics in the culture medium. On the other hand, lower induction of P49 was seen when the hepatocytes were cultured on Matrigel-coated plates. Expression of P49 mRNA was low in the liver of adult rats and was not detectable in the livers of 1- and 2-week-old male and female rats. P49 is an additional form of cytochrome P450, which is induced in the primary culture of rat hepatocytes.

Phenobarbital induction of cytochromes P-450. High-level long-term responsiveness of primary rat hepatocyte cultures to drug induction, and glucocorticoid dependence of the phenobarbital response

The induction of hepatic cytochromes P-450 by phenobarbital (PB) was studied in rat hepatocytes cultured for up to 5 weeks on Vitrogen-coated plates in serum-free modified Chee's medium then exposed to PB (0.75 mM) for an additional 4 days. Immunoblotting analysis indicated that P-450 forms PB4 (IIB1) and PB5 (IIB2) were induced dramatically (greater than 50-fold increase), up to levels nearly as high as those achieved in PB-induced rat liver in vivo. The newly synthesized cytochrome P-450 was enzymically active, as shown by the major induction of the P-450 PB4-dependent steroid 16 beta-hydroxylase and pentoxyresorufin O-dealkylase activities in the PB-induced hepatocyte microsomes (up to 90-fold increase). PB induction of these P-450s was markedly enhanced by the presence of dexamethasone (50 nM-1 microM), which alone was not an affective inducing agent, and was inhibited by greater than 90% by 10% fetal bovine serum. The PB response was also inhibited (greater than 85%) by growth hormone (250 ng/ml), indicating that this hormone probably acts directly on the hepatocyte when it antagonizes the induction of P-450 PB4 in intact rats. In untreated hepatocytes, P-450 RLM2 (IIA2), P-450 3 (IIA1) and NADPH P-450 reductase levels were substantially maintained in the cultures for 10-20 days. The latter two enzymes were also inducible by PB to an extent (3-4 fold elevation) that is comparable with that observed in the liver in vivo. Moreover, P-450c (IA1) and P-450 3 (IIA1) were highly inducible by 3-methylcholanthrene (5 microM; 48 h exposure) even after 3 weeks in culture. In contrast, the male-specific pituitary-regulated P-450 form 2c (IIC11) was rapidly lost upon culturing the hepatocytes, suggesting that supplementation of appropriate hormonal factors may be necessary for its expression. The present hepatocyte culture system exhibits a responsiveness to drug inducers that is qualitatively and quantitatively comparable with that observed in vivo, and should prove valuable for more detailed investigations of the molecular and mechanistic basis of the response to PB and its modulation by endogenous hormones.