Characteristics of aryl hydrocarbon hydroxylase activity in rat mammary epithlial cells grown in primary culture

Abstract

Rat mammary epithelial cells grown in primary culture contain the microsomal enzyme, aryl hydrocarbon (benzo[ a]pyrene) hydroxylase (AHH), which catalyses the oxidative conversion of polycyclic aromatic hydrocarbons (PAH) to more polar derivatives. Constitutive AHH activity, measured with an established fluorometric method, was 46 pmol/mg protein/h in homogenates of rat mammary epithelial cells after 5 days in culture. The addition of dimethylbenz[ a]anthracene (DMBA), benz[ a]anthracene (BA), or 3-methylcholanthrene (3-MC) to the cell culture medium increased AHH activity 5.3-, 4.7- and 2.4-fold, respectively. Kinetic studies revealed that maximal hydroxylase induction occurred 16 h after 1 μM DMBA was added to the culture medium. The decay of the DMBA-induced hydroxylase was biphasic: one component had a t 1 2 of 15–30 min and another a t 1 2 of 4 h. Norepinephrine, 17β-estradiol and 5,6-benzoflavone also increased AHH activity in mammary epithelial cells in vitro, however, sodium phenobarbital had no effect. Fetal bovine serum (FBS), previously shown to be a potent in vitro inducer of AHH activity, had no effect on either constitutive or DMBA-induced mammary epithelial hydroxylase activities following treatment with 1% activated charcoal. Metyrapone and 7,8-benzoflavone, inhibitors of microsomal mixed function oxidase activity, reduced both constitutive and DMBA-induced AHH activities when added to homogenates of untreated and DMBA-treated mammary epithelial cells. The addition of 7,8-benzoflavone reduced both constitutive and DMBA-induced hydroxylase activities by approx. 80%, whereas metyrapone addition inhibited these activities by 20%. The study demonstrates several in vitro factors which alter AHH activity in primary cultures of rat mammary epithelial cells.