Induction of cytochrome P4501A1 by 2,3,7,8-tetrachlorodibenzo-p-dioxin or indolo(3,2-b)carbazole is associated with oxidative DNA damage.

Abstract

Induction of cytochrome P4501A1 (CYPlA1) in the hepatoma Hepalclc7 cell line results in an elevation in the excretion rate of 8-oxoguanine (oxo8Gua), a biomarker of oxidative DNA damage and the major repair product of 8-oxo-2'-deoxyguanosine (oxo8dG) residues in DNA. Treatment of this cell line with 2,3,7,8-tetrachloro-p-dibenzodioxin (TCDD), a nonmetabolized environmental contaminant, and indolo(3,2-b)carbazole (ICZ), a metabolite of a natural pesticide found in cruciferous vegetables, is shown to both induce CYPlAl activity and elevate the excretion rate of oxo8Gua; 7,8-benzoflavone (7,8-BF or ai-naphthoflavone), an inhibitor of CYPlAl activity and an antagonist of the aryl hydrocarbon (Ah) receptor, reduced the excretion rate of oxo8Gua. The essential role of Ah-receptor, which mediates the induction of CYPlAl, is shown by the inability of TCDD to induce CYPlAl and to increase excretion of oxo8Gua in Ah receptor-defective c4 mutant cells. While there was a significant 7.0-fold increase over 2 days in the excretion rate of oxo8Gua into the growth medium of TCDD-treated Hepalclc7 cells compared to control, no significant increase was detected in the steady-state level of oxo8dG in the DNA presumably due to efficient DNA repair. Thus, the induction of CYPlAl appears to lead to a leak ofoxygen radicals and consequent oxidative DNA damage that could lead to mutation and cancer.Cytochrome P450 (CYP) is a family of enzymes involved in the oxidative metabolism of both synthetic and natural compounds (1, 2). Humans are exposed to many CYP inducers (3-6) from eating plant chemicals such as indolo(3,2-b)carbazole (ICZ), from broccoli and other cruciferous vegetables (5), and also from environmental contaminants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). It is possible that the carcinogenic effects of TCDD (7,8) and phenobarbital (9) may, in part, reside in the capacity of the induced CYP enzymes to leak oxidants and thus promote cell division (10) and oxidative DNA damage.The mechanisms of CYPlAl induction by TCDD (11-13) and its involvement in the metabolism of xenobiotics (4, 14, 15) has been investigated extensively. The CYP catalytic cycle involves binding of the substrate followed by binding and activation of molecular oxygen to a reactive intermediate, which hydroxylates the substrate. Most studies designed to investigate the toxicological consequences of CYP induction have been directed toward understanding pathways by which substrate molecules are bioactivated to reactive and genotoxic compounds. The formation of hydrogen peroxide and water as end products of CYP catalysis has been well described (16-21), but the biological consequences of this oxidant leakage from CYP in vivo has received relatively little attention. A postulated mechanism of production of activated 02 is the autoxidation of the oxycytochrome P450 complex, generating superThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance...

Accumulation of M1dG DNA adducts after chronic exposure to PCBs, but not from acute exposure to polychlorinated aromatic hydrocarbons

“…Additionally, it has been proposed that DNA repair proteins can be impaired by mutations caused by the slow accumulation of DNA adducts over a long period of time [37]. This argument is supported by the findings from previous studies that reported sustained DNA adduct (i.e., 8-OH-dG) accumulation, toxicity, and carcinogenesis [12,36,37,41].…”

“…It has been hypothesized by a number of investigators [12,37] that DNA repair in healthy control animals is sufficient to repair DNA adducts formed by an acute exposure to PHAH. Unlike acute exposures, however, long-term exposures to PHAH may change the capacity of DNA repair by modifying the expression of proteins involved in DNA repair, analogous to how PHAH changes the expression of metabolic enzymes [38][39][40].…”

“…2). This increased ROS formation may result from CYP1A1 induction by PHAH [12] or from the formation of catechol estrogens [13][14][15].…”

Effects of indole-3-carbinol on metabolic parameters and on lipogenesis and lipolysis in adipocytes 182

“…Activated P450 enzymes are involved in carcinogen activation, increasing the production of reactive oxygen species (ROS). The induction of CYP1A1 by indole-3-carbinol has been associated with an increase in oxidative DNA damage in a cell culture, which suggests that the induction can lead to a leak of oxygen radicals (Park et al, 1996). These properties of I3C are considered to contribute to the known ambiguous property (anti-and pro-oxidative) of this compound.…”

Protein function analysis: rapid, cell-based siRNA-mediated ablation of endogenous expression with simultaneous ectopic replacement

“…Long-term exposure to AHR ligands including benzo-a-pyrene (B[a]P) has been shown to lead to cell death through AHR-mediated B[a]P metabolism to reactive intermediates (Park et al 1996;Hankinson 1979). To utilize this concept in our system, MCF-7 cells were transfected with control siRNA, AHR targeted siRNA, or co-transfected with an AHR Fig.…”