Ellipticine and benzo(a)pyrene increase their own metabolic activation via modulation of expression and enzymatic activity of cytochromes P450 1A1 and 1A2.

Dagmar Aimová,Volker M Arlt,Marie Stiborová,Michaela Moserová,Jitka Poljaková,Věra Kotrbová,Eva Frei

doi:10.2478/v10102-010-0033-z

Abstract

Two compounds known to covalently bind to DNA after their activation with cytochromes P450 (CYPs), carcinogenic benzo(a)pyrene (BaP) and an antineoplastic agent ellipticine, were investigated for their potential to induce CYP and NADPH:CYP reductase (POR) enzymes in rodent livers, the main target organ for DNA adduct formation. Two animal models were used in the study: (i) rats as animals mimicking the fate of ellipticine in humans and (ii) mice, especially wild-type (WT) and hepatic POR null (HRN™) mouse lines. Ellipticine and BaP induce expression of CYP1A enzymes in livers of experimental models, which leads to increase in their enzymatic activity. In addition, both compounds are capable of generating DNA adducts, predominantly in livers of studied organisms. As determined by 32P postlabelling analysis, levels of ellipticine-derived DNA adducts formed in vivo in the livers of HRN™ mice were reduced (by up to 65%) relative to levels in WT mice, indicating that POR mediated CYP enzyme activity is important for the activation of ellipticine. In contrast to these results, 6.4 fold higher DNA binding of BaP was observed in the livers of HRN™ mice than in WT mice. This finding suggests a detoxication role of CYP1A in BaP metabolism in vivo. In in vitro experiments, DNA adduct formation in calf thymus DNA was up to 25 fold higher in incubations of ellipticine or BaP with microsomes from pretreated animals than with controls. This stimulation effect was attributed to induction of CYP1A1/2 enzymes, which are responsible for oxidative activation of both compounds to the metabolites generating major DNA adducts in vitro. Taken together, these results demonstrate that by inducing CYP1A1/2, ellipticine and BaP modulate their own enzymatic metabolic activation and detoxication, thereby modulating their either pharmacological (ellipticine) and/or genotoxic potential (both compounds).

Highlights

Ellipticine and benzo[a]pyrene (BaP) are compounds exhibiting significant biological activities
As determined by 32P postlabelling analysis, levels of ellipticine-derived DNA adducts formed in vivo in the livers of HRNTM mice were reduced relative to levels in WT mice, indicating that POR mediated CYP enzyme activity is important for the activation of ellipticine
The animal model found to be suitable to mimic the fate of ellipticine in humans (Stiborová et al, 2003a; 2006), and two mouse lines, namely,.(i) „Hepatic Reductase Null“ (HRNTM ) mice based on a C57BL/6 background (CXR Bioscience Ltd, Dundee, UK), in which NADPH:cytochrome P450 reductase (POR) is spe‐ cifically deleted in the liver (Porlox/lox + CreALB) (Henderson et al, 2003, 2006) and (ii) mice homozygous for loxP sites at the Por locus (Porlox/lox) as wild‐type (WT) mice, were used in this study

Summary

Introduction

Ellipticine and benzo[a]pyrene (BaP) are compounds exhibiting significant biological activities. Ellipticine is an efficient anticancer agent (for a summary, see Stiborová et al, 2006b), while BaP is a strong carcinogen (for a summary, see Arlt et al, 2008). Both two compounds were employed by us for studies concerning their phatmacologi‐ cal and toxicological effects. Ellipticine, an alkaloid isolated from Apocyanaceae plants, and its derivatives exhibits significant antitumor and anti‐HIV activities, characterized by high efficiencies against several types of cancer and rather limited toxic side effects, including complete lack of hematological toxicity. We have shown that ellipticine binds covalently to DNA in vitro and in vivo, after being enzymatically activated with cytochromes

Methods

Results

Conclusion