DEXAMETHASONE REGULATION OF THE RAT 3-ALPHA-HYDROXYSTEROID DIHYDRODIOL DEHYDROGENASE GENE

Rat liver 3 alpha-hydroxysteroid/dihydrodiol dehydrogenase (3 alpha-HS D/DD), a member of the aldo-keto reductase superfamily, inactivates ci rculating steroid hormones and may contribute to the carcinogenicity o f polycyclic aromatic hydrocarbons (PAHs) by oxidizing trans-dihydrodi ols to reactive o-quinones with the concomitant generation of reactive oxygen species. The 3 alpha-HSD/DD gene has been cloned, and its 5'-f lanking region contains a negative response element (NRE; -797 to -498 bp) that may repress constitutive expression by binding to Oct transc ription factors. Upstream from the NRE are three distal imperfect gluc ocorticoid response elements (GRE1, GRE2, and GRE3); in addition, a pr oximal imperfect GRE (GRE4) is adjacent to an Oct binding site in the NRE. When rat hepatocytes were cultured on Matrigel and exposed to dex amethasone (Dex), steady state levels of 3 alpha-HSD/DD mRNA were incr eased 4-fold in a dose-dependent manner, yielding an EC50 value of 10 nM, Time to maximal response was 24 hr, and the effect was blocked wit h the anti-glucocorticoid RU486. Measurement of the half-life of 3 alp ha-HSD/DD mRNA, with and without Dex treatment, indicated that the inc rease in steady state mRNA levels was not due to increased mRNA stabil ity. By contrast, nuclear run-off experiments using nuclei obtained fr om Dex-stimulated hepatocytes indicated that Dex increased transcripti on of the rat 3 alpha-HSD/DD gene. Tandem repeats of the imperfect GRE 1, GRE2, GRE3, and GRE4 were inserted into thymidine kinase-chloramphe nicol acetyl-transferase vectors and cotransfected with the human gluc ocorticoid receptor into human hepatoma cells, On treatment with Dex, maximal trans-activation of the chloramphenicol acetyl-transferase rep orter gene activity was mediated via the proximal GRE (GRE4). These da ta imply that GRE4 is a functional cis-element and that binding of the occupied glucocorticoid receptor to this element increases 3 alpha-HS D/DD gene transcription, A model is proposed for the positive and nega tive regulation of the rat 3 alpha-HSD/DD gene by the glucocorticoid r eceptor and Oct transcription factors, respectively.