BIPHASIC REGULATION OF CYTOCHROME-P450 2B1 2 MESSENGER-RNA EXPRESSIONBY DEXAMETHASONE IN PRIMARY CULTURES OF ADULT-RAT HEPATOCYTES MAINTAINED ON MATRIGEL/

We have demonstrated recently that although rat hepatocytes rapidly lo se their cytochrome P450 mRNA content following their introduction int o primary culture, hepatocytes cultured on Matrigel, a reconstituted b asement membrane, subsequently spontaneously ''reexpress'' the mRNAs o f some constitutive P450 forms (Kocarek et al., Mol Pharmacol 13: 328- 334, 1993). In the present study, we used the Matrigel cell culture sy stem to examine the dose-dependent effects of dexamethasone (DE)() tre atments on the mRNAs for two of the P450 forms that are reexpressed sp ontaneously between days 3 and 5 in culture, 2B1/2 and 2C6. Treatment of cultured hepatocytes with low doses of DEX (10(-9) to 10(-8) M) tha t induced the mRNA for tyrosine aminotransferase, a model glucocortico id-inducible gene, suppressed the spontaneous appearance of 2B1/2 mRNA while having little or no effect on the level of 2C6 mRNA or on beta- actin mRNA. However, treatment of the hepatocyte cultures with high do ses of DEW (10(-6) to 10(-5) M) that induced P450 3A1 mRNA increased t he amounts of the 2B1/2 and 2C6 mRNAs (4.1- and 2.4-fold, respectively , at 10(-5) M DEX). In contrast to the suppressive effects on the spon taneous increases in 2B1/2 mRNA, low doses of DEX (10(-8) to 10(-7) M) enhanced the induction of 2B1/2 mRNA by phenobarbital (2.5-fold at 10 (-7) M DEX). Treatment of the hepatocyte cultures with triamcinolone a cetonide, another potent glucocorticoid, suppressed spontaneous 2B1/2 mRNA expression at low doses, but did not induce 2B1/2 mRNA at high do ses. Treatments with steroids of other classes, including dihydrotesto sterone, 17 alpha-ethinylestradiol, fludrocortisone or R-5020, failed to suppress 2B1/2 mRNA levels at low doses. Additionally, treatment wi th RU-486, a glucocorticoid/progestin receptor antagonist, induced 2B1 /2 mRNA at high doses (10(-6) to 10(-5) M), The suppressive effects of DEN on spontaneous 2B1/2 mRNA expression observed at low doses are co nsistent with a classical glucocorticoid-mediated mechanism, while the high-dose inductive effects of DEX appear to be exerted through a non classical mechanism, perhaps akin to that for induction of 3A1.