Modulation of benzo[a]pyrene-DNA adduct formation by CYP1 inducer and inhibitor

Abstract

Benzo[a]pyrene (BaP) is a well-studied pro-carcinogen that is metabolically activated by cytochrome P450 enzymes. Cytochrome P4501A1 (CYP1A1) has been considered to play a central role in the activation step, which is essential for the formation of DNA adducts. This enzyme is strongly induced by many different chemical agents, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which binds to the aryl hydrocarbon receptor (AhR). Therefore, AhR activators are suspected to have the potential to aggravate the toxicity of BaP through the induction of CYP1A1. Besides, CYP1A1 inhibitors, including its substrates, are estimated to have preventive effects against BaP toxicity. However, strangely, increased hepatic BaP-DNA adduct levels have been reported in Cyp1a1 knockout mice. Moreover, numerous reports describe that concomitant treatment of AhR activators reduced BaP-DNA adduct formation. In an experiment using several human cell lines, TCDD had diverse modulatory effects on BaP-DNA adducts, both enhancing and inhibiting their formation. In this review, we focus on the factors that could influence the BaP-DNA adduct formation. To interpret these complicated outcomes, we propose a hypothesis that CYP1A1 is a key enzyme for both generation and reduction of (±)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE), the major carcinogenic intermediate of BaP. Conversely, CYP1B1 is thought to contribute only to the metabolic activation of BaP related to carcinogenesis.

Keywords: Aryl hydrocarbon receptor; Benzo[a]pyrene; Cytochrome P450; DNA adduct.

Fig. 1

Alteration of BaP–DNA adduct formation by CYP subtypes and AhR activators. Effect of AhR activator on BaP–DNA adduct formation in HepG2 cells. HepG2 cells were concomitantly treated with BaP (5 or 10 μM), indirubin (IND, 100 nM), quinizarin (QNZ, 10 μM), omeprazole (OME, 100 μM), or TCDD (10 nM) for 16 h. After incubation, DNA was purified and BaP–DNA adducts were analyzed. This figure is adapted from data reported previously [24]

Fig. 2

TCDD-mediated modulation of BaP adducts in various human cell lines. Hepatoma cell line HepG2 (a, b), breast carcinoma cell line MCF7 (c, d), and lung carcinoma cell line A549 (e, f) were co-treated with BaP (0.5–10 μM; a, c, e) or BPDE (0.1–5 μM; b, d, f) with or without TCDD. Closed columns represent adducts by 10 nM TCDD treatment and open columns represent adducts by solvent control (DMSO). After 16 h of incubation, genomic DNA was isolated and DNA adducts were detected by ³²P-postlabeling and PAGE. G: CYP1A1 and 1B1 mRNA expression in the three cell lines. Each cell line was exposed to 10 nM TCDD or DMSO for 24 h and total RNA was extracted. For the quantitation of specific transcripts, real-time PCR was performed. These figures are adapted from data reported previously [24]

Fig. 3

Scheme of the relationship between AhR modulators and BaP–DNA adduct formation. Broken arrows indicate the pathway of BaP metabolism. Solid arrows indicate the activation or inductive commitment, whereas the T-shaped lines indicate inhibition. The “protective pathway” and “aggravating pathway” are indicated in blue and red, respectively. AhR modulators influence BaP adducts through CYP1A1/1B1 gene induction (a), reduction (b), and/or inhibition of enzyme activity (c). Note that only CYP1A1 has a BPDE metabolic detoxification indicated by a blue arrow