Journal Article

Sulforaphane and quercetin modulate PhIP–DNA adduct formation in human HepG2 cells and hepatocytes

James R. Bacon, Gary Williamson, R. Colin Garner, Graham Lappin, Sophie Langouët and Yongping Bao

in Carcinogenesis

Volume 24, issue 12, pages 1903-1911
Published in print December 2003 | ISSN: 0143-3334
Published online December 2003 | e-ISSN: 1460-2180 | DOI:
Sulforaphane and quercetin modulate PhIP–DNA adduct formation in human HepG2 cells and hepatocytes

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The formation of DNA adducts in human HepG2 cells and human hepatocytes exposed to 14C-labelled 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) was examined using Accelerator Mass Spectrometry (AMS). PhIP generated DNA adducts in a linear dose-dependent manner between 100 pM and 20 µM. Co-treatment with the dietary isothiocyanate, sulforaphane (SFN, 1–10 µM), or the flavonoid, quercetin (5–20 µM), significantly reduced the level of PhIP–DNA adducts in a dose-dependent manner. The degree of protection was dependent on PhIP concentration, i.e. after 100 pM PhIP exposure, SFN or quercetin reduced adduct levels to below the limit of detection (0.15 amol PhIP/µg DNA) but at higher PhIP exposure (10 nM and 1 µM), the protection was 60 and 10%, respectively. The involvement of phase I, phase II and DNA repair enzymes in this protection against PhIP–DNA adduct formation was investigated using real-time RT–PCR and enzyme activity assays. In intact HepG2 cells, quercetin inhibited cytochrome P450 (CYP)1A2, the main phase I enzyme responsible for PhIP bioactivation. In contrast, SFN induced phase II detoxification enzymes, UDP-glucuronosyltransferase 1A1 and glutathione S-transferase A1 mRNA expression. SFN and quercetin showed no effect on DNA repair, neither in terms of the level of PhIP–DNA adducts, when cells were treated with phytochemicals after the carcinogen exposure, nor the regulation of mRNA expression of two DNA repair enzymes, apurinic endonuclease and DNA polymerase β. This study indicates that dietary isothiocyanates and flavonoids modulate phase I and phase II enzyme expression, hence increasing the rate of detoxification of the dietary carcinogen PhIP in human HepG2 cells but do not affect the rate of PhIP–DNA adduct repair. The formation of PhIP–DNA adducts in human hepatocytes was also dose-dependent with PhIP-concentration and the levels of protection by SFN or quercetin were up to 60% after 10 nM PhIP treatment, but showed large inter-individual variation with no observed protection in some individuals.

Keywords: AMS, Accelerator Mass Spectrometry; APE, apurinic endonuclease; GST, glutathione S-transferase; HCAs, heterocyclic amines; PhIP, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine; Pol β, DNA polymerase β; SFN, sulforaphane; UGT, UDP-glucuronosyltransferase; RT–PCR, reverse transcription polymerase chain reaction

Journal Article.  6864 words.  Illustrated.

Subjects: Clinical Cytogenetics and Molecular Genetics

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