Mahadevan Brinda, Marston Charis P, Luch Andreas, Dashwood Wan-Mohaiza, Brooks Eric, Pereira Cliff, Doehmer Johannes, Baird William M
Department of Environmental and Molecular Toxicology, Oregon State University, Corvallis, OR, USA.
Int J Cancer. 2007 Mar 15;120(6):1161-8. doi: 10.1002/ijc.22466.
A complex mixture of polycyclic aromatic hydrocarbons (PAH) extracted from coal tar, the Standard Reference Material (SRM) 1597, was recently shown to decrease the levels of DNA binding of the 2 strong carcinogens benzo[a]pyrene (BP) and dibenzo[a,l]pyrene (DBP) in the human mammary carcinoma-derived cell line MCF-7 (Mahadevan et al., Chem Res Toxicol 2005;18:224-231). The present study was designed to further elucidate the biochemical mechanisms involved in this inhibition process. We examined the effects of SRM 1597 on the metabolic activation of BP and DBP toward DNA-binding derivatives in Chinese hamster cells expressing either human cytochrome P450 (CYP) 1A1 or CYP1B1. SRM 1597 inhibited BP-DNA adduct formation through the entire exposure time in cells expressing human CYP1A1, while it significantly inhibited adduct formation only up to 48 hr when co-treated with DBP. Conversely, human CYP1B1-expressing cells were unable to catalyze PAH-DNA adduct formation on treatment with SRM 1597 alone, and on co-treatment with BP or DBP. The data obtained from biochemical experiments revealed that SRM 1597 competitively inhibited the activity of both human enzymes as analyzed by 7-ethoxyresorufin O-deethylation assays. While the Michaelis-Menten constant (K(M)) was <0.4 microM in the absence of SRM 1597, this value increased up to 1.12 (CYP1A1) or 4.45 microM (CYP1B1) in the presence of 0.1 microg/ml SRM 1597. Hence the inhibitory effects of the complex mixture on human CYP1B1 were much stronger when compared to human CYP1A1. Taken together, the decreases in PAH-DNA adduct formation on co-treatment with SRM 1597 revealed inhibitory effects on the CYP enzymes that convert carcinogenic PAH into DNA-binding metabolites. The implications for the tumorigenicity of complex environmental PAH mixtures are discussed.
从煤焦油中提取的多环芳烃（PAH）复杂混合物标准参考物质（SRM）1597，最近被证明可降低人乳腺癌衍生细胞系MCF-7中两种强致癌物苯并[a]芘（BP）和二苯并[a,l]芘（DBP）的DNA结合水平（Mahadevan等人，《化学研究毒理学》2005年；18：224 - 231）。本研究旨在进一步阐明这一抑制过程中涉及的生化机制。我们研究了SRM 1597对表达人细胞色素P450（CYP）1A1或CYP1B1的中国仓鼠细胞中BP和DBP向DNA结合衍生物的代谢活化作用。SRM 1597在表达人CYP1A1的细胞整个暴露时间内抑制BP-DNA加合物形成，而与DBP共同处理时，仅在48小时内显著抑制加合物形成。相反，单独用SRM 1597处理以及与BP或DBP共同处理时，表达人CYP1B1的细胞均无法催化PAH-DNA加合物形成。生化实验获得的数据表明，通过7-乙氧基试卤灵O-脱乙基化分析，SRM 1597竞争性抑制了这两种人源酶的活性。在不存在SRM 1597时，米氏常数（K(M)）<0.4微摩尔，在存在0.1微克/毫升SRM 1597时，该值增加至1.12（CYP1A1）或4.45微摩尔（CYP1B1）。因此，与CYP1A1相比，该复杂混合物对人CYP1B1的抑制作用要强得多。综上所述，与SRM 1597共同处理时PAH-DNA加合物形成的减少揭示了对将致癌PAH转化为DNA结合代谢物的CYP酶的抑制作用。文中讨论了复杂环境PAH混合物对肿瘤发生的影响。
