Regulation of sulfotransferase gene expression by glucocorticoid hormones and xenobiotics in primary rat hepatocyte culture

Abstract

In the rat liver, hydroxysteroid sulfotransferase-a (HST-a) and aryl sulfotransferase IV (ASTIV) represent two major rat hepatic sulfotransferases that are important to xenobiotic metabolism. Prototypic CYP1A1 and CYP2B/3A inducers regulate rat hepatic sulfotransferase gene expression although not necessarily in a coordinate direction. It has been previously reported that in vivo treatment with CYP1A1 inducer 3-methylcholanthrene (3-MC) suppresses rat hepatic HST-a mRNA expression in a dose-dependent manner. Similarly, HST-a and ASTIV mRNA levels become suppressed or induced, respectively, following in vivo treatment with phenobarbital (PB)-like CYP2B/3A inducers or prototypic CYP3A inducers such as glucocorticoid hormones. In the whole animal, sulfotransferase gene expression is modulated by members of the hypothalamic/pituitary–adrenal–gonadal hormone axis. However, studies in primary rat hepatocyte culture suggest that prototypic P450 inducers regulate HST-a and ASTIV gene expression directly at the level of the hepatocyte. Glucocorticoid-mediated sulfotransferase expression was compared with the regulation of tyrosine amino transferase (TAT), a gene that is transcriptionally regulated by ligand bound glucocorticoid receptor. It was found that lower doses of dexamethasone (DEX, 10 −7 M) produced concomitant increases in ASTIV and TAT mRNA expression, whereas HST-a mRNA expression continued to rise as the DEX dose was increased through 10 −5 M. When hepatocytes were co-incubated with DEX and antiglucocorticoid/antiprogestin RU-486, DEX-stimulated HST-a mRNA expression was not significantly inhibited by RU-486, but ASTIV and TAT mRNA expression were inhibited to a similar extent. The results suggested that ASTIV, like TAT, is likely regulated by a classical glucocorticoid receptor mediated mechanism, whereas HST-a is probably regulated by glucocorticoids via an alternative mechanism. In contrast to the positive effects of glucorticoid hormones, HST-a and ASTIV mRNA expression was negatively regulated by xenobiotics such as PB-like CYP2B/3A inducers or aryl hydrocarbon receptor ( AhR) agonist CYP1A1 inducers. Incubation of primary cultured rat hepatocytes with PB or structurally dissimilar PB-like inducers clotrimazole, diphenylhydantoin, heptachlor, γ-hexachlorocyclohexane or 2,2′,4,4′,5,5′-hexachlorobiphenyl suppressed HST-a and ASTIV mRNA levels. Also, incubation of primary cultured rat hepatocytes with CYP1A1 inducer β-naphthoflavone or with archetypic AhR agonist, 2,3,7,8-tetrachloro- p-dioxin (TCDD) markedly suppressed HST-a and ASTIV mRNA expression. These data suggest that the rat HST-a and ASTIV genes are positively regulated by glucocorticoid hormones and negatively regulated by xenobiotics as a result of molecular and cellular mechanisms that act directly on the hepatocyte.