Suppression of hydroxysteroid sulfotransferase-a gene expression by 3-methylcholanthrene.

The hydroxysteroid sulfotransferases (HSTs) are phase II drug metabolizing enzymes which play a role in xenobiotic detoxication, carcinogen activation, and intratissue hormone modulation. Rat liver contains three principal HST enzymes. Of these, HST-a is most abundant in female rat liver. The phase I drug metabolizing enzyme cytochrome P450 1A1 (CYP1A1) provides a major pathway for polycyclic aromatic hydrocarbon (PAH) carcinogen activation, whereas HST-a, through enzymatic sulfation, offers an alternative pathway for PAH carcinogen activation. Transcription of the rat hepatic CYP1A1 gene is induced in response to treatment with PAH carcinogen 3-methylcholanthrene (3-MC). In view of the potential importance of enzymatic sulfation to the complex process of PAH carcinogenesis, the effect of 3-MC on HST-a gene expression was investigated. Groups of prepubescent (aged approximately 22-30 days) and young adult female (aged approximately 69-84 days) Sprague-Dawley rats were injected ip with corn oil vehicle or with 3-MC (80 mg/kg). After fasting for 24 hr, rats were terminated and hepatic HST-a gene expression was analyzed using slot blot, Northern blot, and Western blot analyses. Initial Northern blot and slot blot analyses demonstrated a substantial suppression of rat hepatic HST-a mRNA in both prepubescent and young adult female rats. Subsequent Northern blot analyses on liver tissue isolated from female rats aged approximately 55 days confirmed CYP1A1 mRNA induction in 3-MC-treated animals and demonstrated a dose-dependent suppression of HST-a mRNA expression of approximately 25, 50, and 75% in rats treated with 10, 25, and 80 mg/kg 3-MC ip, respectively. In contrast, HST-a protein levels remained unaltered 24 hr after 3-MC treatment. A time-course study revealed that hepatic HST-a mRNA levels returned to control levels by 36 hr following 3-MC treatment. These results suggest that xenobiotics such as 3-MC modulate HST-a mRNA expression and that HST-a mRNA suppression after 3-MC treatment may occur at the level of transcription.