Aryl hydrocarbon (benzo(a)pyrene) hydroxylase induction in cells in culture

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

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

Methylcholanthrene: a possible pseudosubstrate for adrenocortical 17 alpha-hydroxylase and aryl hydrocarbon hydroxylase

In cultured bovine adrenocortical cells, loss of 17 alpha-hydroxylase activity was observed after incubation with 3-methylcholanthrene (3-MC). The suppression of 17 alpha-hydroxylase by 3-MC was rapid (50% loss of activity in 10 hr at 1 microM 3-MC), did not exhibit a lag period, and was not affected by cycloheximide. Direct effects of 3-MC on 17 alpha-hydroxylase were observed only at high concentrations, but the concentration for 50% loss of activity was 0.3 microM when 3-MC was added for 24 hr prior to assay of 17 alpha-hydroxylase. High concentrations (to 40 microM) of substrate (progesterone), did not affect the loss of activity due to 3-MC. Loss of 17 alpha-hydroxylase activity was specific; 11 beta-hydroxylase was unaffected and cell growth was unaltered. However, 22-amino-23,24-bisnorchol-5-en-3 beta-ol, an inhibitor of 17 alpha-hydroxylase, partially prevented the loss of 17 alpha-hydroxylase at 1-30 nM. 3-MC is thought to induce cytochrome P-450s via a receptor with high affinity for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). TCDD was without effect on 17 alpha-hydroxylase over the range of 10 nM to 10 microM. Benz[a]anthracene, 7,12-dimethylbenz[a]anthracene, benzo[a]pyrene, chrysene, and methylphenanthrenes suppressed 17 alpha-hydroxylase at high concentrations (10-50 microM for 50% loss of activity). Some steroids that lack a substituent at position 17 also caused loss of 17 alpha-hydroxylase. Like 17 alpha-hydroxylase, bovine adreno-cortical cell AHH was found to be suppressed by exposure to 3-MC. Compounds that caused loss of 17 alpha-hydroxylase caused loss of AHH, with a similar order of potency and at similar concentrations. Suppression of AHH by 3-MC did not require protein synthesis and was prevented by an inhibitor of enzymatic activity, alpha-naphthoflavone. This implies a degree of similarity of the cytochrome P-450s for 17 alpha-hydroxylase and adrenal AHH, but the activities were shown to be likely due to different enzymes. The suppression of 17 alpha-hydroxylase and AHH by 3-MC appears not to occur by a receptor-mediated mechanism but to be similar to the suppression of 11 beta-hydroxylase and 21-hydroxylase by steroid pseudosubstrates previously observed.

Hormonal regulation of benzo[a]pyrene metabolism in human adrenocortical cell cultures

In cultured fetal human adrenocortical cells, metabolism of the carcinogen benzo[a]pyrene was found to be unresponsive to the xenobiotic inducers 3-methylcholanthrene, benz[a]anthracene and 2,3,7,8-tetrachlorodibenzo-p-dioxin. However, exposure of cultures to the hormone adrenocorticotropin (ACTH) for 48 hours stimulated benzo[a]pyrene metabolism 3-fold. The major metabolite was the 7,8-diol. Other compounds which stimulate the production of adrenocortical cell cyclic AMP (forskolin and cholera toxin) as well as monobutyryl cyclic AMP also increased benzo[a]pyrene metabolism. Human adrenocortical cells thus provide an unusual example of hormonal regulation of the metabolism of a carcinogen.

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

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

Polyamines as modulators of drug oxidation reactions catalyzed by cytochrome P-450 from liver microsomes

The effect of polyamines on the activity of the mixed-function oxidase (MFO) system from human, rat and rabbit liver microsomes was investigated in detail. It was shown that polyamine (spermine) stimulates NADPH-dependent activity of the MFO system several-fold whatever the substrate (foreign drug or natural), not only with microsomes but also with the reconstituted system consisting of highly purified cytochrome P-450 (LM2 isozyme), cytochrome P-450 NADPH reductase and dilauroylphosphorylcholine. Stimulation (extent and concentration dependence) appeared to be dependent on a number of parameters such as ionic strength, pH, animal species and treatment, nature of the substrate, and was stereospecific (different effect on 6 beta-and 16 alpha-testosterone hydroxylation). Further, the spermine effect was evaluated on some elementary steps of the cytochrome P-450 reaction cycle, like substrate binding, P-450 reduction and second electron transfer. Finally, it was shown that the organic peroxide dependent activity was not stimulated by spermine with microsomes nor with the purified P-450 LM2 isozyme. On the basis of this study, it was concluded that the locus of polyamine action is cytochrome P-450 and that stimulation could result either from increased stability of the oxyferrous intermediate of P-450 or from an increased rate of second electron transfer from reductase to P-450.

Dissociation between ornithine decarboxylase activity and aryl hydrocarbon hydroxylase induction in cell cultures treated with benz[a]anthracene and inhibitors of ornithine decarboxylase

The induction of aryl hydrocarbon and ornithine decarboxylase by benz[a]-anthracene in the presence or absence of ornithine decarboxylase inhibitors was studied in three different cell culture systems. An almost complete abolishment of ornithine decarboxylase activity by 1,3-diamino-2-propanol or alpha-difluoremethyl ornithine before the addition of the inducer did not affect appreciably the induction of aryl hydrocarbon hydroxylase by benz[a]anthracene in human embryo, HeLa and Rueber H-II-4-E cells in culture. These results suggest that the induction of aryl hydrocarbon hydroxylase does not require ornithine decarboxylase activity per se and can be expressed in the absence of continuous polyamine synthesis.

Activity and kinetic properties of basal aryl hydrocarbon hydroxylase during proliferation in the transformable C3H 10T1/2 CL8 cell line

Basal aryl hydrocarbon hydroxylase (AHH) activity and its kinetic properties were studied as a function of proliferation in C3H mouse embryo 10T1/2 CL8 cells. Activity was low in freshly plated cells, increased during exponential growth, peaked at confluency, and then declined. The apparent Km-values for benzo[a]pyrene (BP) and NADPH were less in proliferating (approx. 0.37 microM BP, 3.3 nM NADPH) than in confluent cells (0.74-1.39 microM BP, 33.4-53.4 nM NADPH). Cells at different growth states responded differently to benz[a]anthracene (BA) and aminophylline, an inhibitor of cyclic nucleotide phosphodiesterases. When cells were harvested at the mid log phase of growth, 12 h of exposure to aminophylline caused maximum induction, while 24 h of BA treatment were required. In contrast, at early confluence, 12 h of BA treatment gave the greatest levels of activity, while exposure to aminophylline did not induce AHH. In fact, decreases in activity were observed. These differences are indicative of different regulatory mechanisms for BA and aminophylline induction. They also suggest the regulation of basal AHH by cyclic nucleotides changes during growth. The exposure times giving maximum activity were used to determine the kinetic properties of BA-induced activity. As with basal AHH, the Km-value for BP was less in log phase (0.2-0.4 microM BP) than in confluent cells (0.64-1.05 microM BP). Moreover, the Km-values for BP and NADPH in control cultures at confluency (0.10-0.14 microM BP, 15.4-23.2 nM NADPH) were less than those for BA-treated cells (0.64 microM BP, 37.9-54.8 nM NADPH) under the same nutritional conditions. The finding that the Km-values for BP is lower in rapidly dividing cells than in confluent cells may help to explain why proliferating cells are more susceptible to transforming agents.