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Induction and repression of the major phenobarbital-induced cytochrome P-450 measured by radioimmunoassay.

ABSTRACT: Two independent radioimmunoassay techniques for the major phenobarbital-inducible cytochrome P-450 (PB P-450) of rat liver microsomal membranes are described. The first technique employs as the source of radiolabelled antigen the products of translation in vitro labelled with [35S]methionine. The second technique employs purified antigen labelled with 125I and is quicker, less expensive and more precise. Both assays are highly specific for PB P-450 and can detect quantities of this variant as small as 1 ng. This is several orders of magnitude more sensitive than any method described previously for the quantification of cytochromes P-450, and consequently the technique is particularly well suited for the quantification of so-called constitutive cytochrome P-450 variants that are present in very low amounts. The results of the radioimmunoassays demonstrate that the apparent 2.6-fold induction of total cytochromes P-450 after phenobarbital treatment is due to a 43-fold increase in Pb P-450. Although beta-naphthoflavone increases the total content of cytochrome P-450 of microsomal membranes 1.4-fold, it actually causes a 55% decrease in the amount of PB P-450. Thus different xenobiotics can have differential effects on the expression of the genes for specific cytochrome P-450 variants.

SUBMITTER: Phillips IR

PROVIDER: S-EPMC1152009 | biostudies-other | 1983 Apr

REPOSITORIES: biostudies-other

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Project description: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.

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Induction and repression of the major phenobarbital-induced cytochrome P-450 measured by radioimmunoassay.

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