CHARACTERIZATION OF CYP1B1 AND CYP1A1 EXPRESSION IN HUMAN MAMMARY EPITHELIAL-CELLS - ROLE OF THE ARYL-HYDROCARBON RECEPTOR IN POLYCYCLIC AROMATIC HYDROCARBON METABOLISM

CYP1B1 and CPP1A1 expression and metabolism of 9,12-dimethyl-benz(a)an thracene (DMBA) have been characterized in early-passage human mammary epithelial cells (HMECs) isolated from reduction mammoplasty tissue o f seven individual donors. The level of constitutive microsomal CYP1B1 protein expression mas donor dependent (<0.01-1.4 pmol/mg microsomal protein), CYP1B1 expression was substantially induced by exposure of t he cells to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to levels rangi ng from 2.3 to 16.6 pmol/mg among the seven donors. Extremely low, rep roducible levels of constitutive CYP1A1 expression were detectable in three donors (0.03-0.16 pmol/mg microsomal protein). TCDD inductions w ere larger for CYP1A1, as compared to CYP1B1, demonstrating substantia l variability in the induced levels among the donors (0.8-16.5 pmol/mg ). Northern and reverse transcriptase PCR analyses corroborate the don or-dependent differences in protein expression, whereby CYP1B1 mRNA (5 .2 kb) was constitutively expressed and was highly induced by TCDD (33 -fold), The contributions of CYP1B1 and CYP1A1 to the metabolism of DM BA were analyzed using recombinant human CYP1B1 and CPP1A1, as referen ces, in conjunction with antibody-specific inhibition analyses (anti-C YP1B1 and anti-CYP1A1), Constitutive microsomal activity exhibited a p rofile of regioselective DMBA metabolism that was characteristic of hu man CYP1B1 (increased proportions of 5,6- and 10,11-DMBA-dihydrodiols) , which was inhibited by anti-CYP1B1 (84%) but not by anti-CYP1A1. TCD D-induced HMEC microsomal DMBA metabolism generated the 8,9-dihydrodio l of DMBA as the predominant metabolite, with a regioselectivity simil ar to that of recombinant human CYP1A1, which was subsequently inhibit ed by anti-CYP1A1 (79%), A CYP1B1 contribution was indicated by the re gioselectivity of residual metabolism and by anti-CYP1B1 inhibition (2 5%). DMBA metabolism analyses of one of three donors ex-pressing measu rable basal expression of CYP1A1 confirmed DMBA. metabolism levels equ ivalent to that from CYP1B1. The HMECs of all donors expressed similar , very high levels of the aryl hydrocarbon receptor and the aryl hydro carbon nuclear translocator protein, suggesting that aryl hydrocarbon receptor and aryl hydrocarbon nuclear translocator protein expression are not responsible for differences in cytochrome P450 expression. Thi s study indicates that CYP1B1 is an important activator of polycyclic aromatic hydrocarbons in the mammary gland when environmental chemical exposures minimally induce CYP1A1, additionally. certain individuals express low levels of basal CYP1A1 in HMECs, representing a potential risk factor of mammary carcinogenesis through enhanced polycyclic arom atic hydrocarbon bioactivation.