The cooked food derived carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b] pyridine is a potent oestrogen: a mechanistic basis for its tissue-specific carcinogenicity

Abstract

The cooked meat carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) induces tumours of the breast, colon and prostate in rats. Here we show that in addition to its well-established genotoxicity, which can be detected at concentrations >10(-6) M, PhIP is also oestrogenic. In COS-1 cells transiently transfected with an oestrogen-responsive reporter gene, PhIP (10(-10)-10(-6) M) mediated transcription through oestrogen receptor (ER) alpha, but not ER-beta, and inhibition by the pure ER antagonist ICI 182 780 demonstrated a requirement for a functional ER. In contrast, the structurally related food-derived carcinogen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) failed to induce reporter gene transcription. Additionally, we show that in a hormonally responsive breast cancer cell line (MCF-7 cells), PhIP induced transcriptional activation using endogenously expressed ER. Examination of the genotoxic potential of PhIP using a model mammalian cell mutation assay (hprt(-) locus) demonstrated that the genetic toxicology of PhIP was readily detectable, but separate, in terms of effective concentration, from its oestrogenic activity. To determine whether the oestrogenicity of PhIP could mediate oestrogen-dependent responses such as cell growth, we examined the growth of hormonally responsive cells (MCF-7 cells). We show that PhIP can stimulate cell proliferation and, again, this was dependent upon a functional ER. Using ligand blotting, we further show that PhIP can stimulate the expression of progesterone receptor (PR-A and PR-B) and c-MYC and activate the MAPK signal transduction pathway. These responses were similar to that produced by oestradiol, in terms of temporal aspects, potency and a requirement for a functional ER. Each of these dose-dependent mitogenic responses occurred at concentrations of PhIP ( approximately 10(-9)-10(-11)M) that are likely to be equivalent to systemic human exposure via consumption of cooked meat. Thus PhIP can induce cellular responses that encompass altered gene expression and mitogenesis. We suggest that the combination of genetic toxicology and oestrogen-like promotion of genomic and cellular events provide a mechanism for the tissue-specific tumorigenicity of this compound.

Bcrp1;Mdr1a/b;Mrp2 Combination Knockout Mice: Altered Disposition of the Dietary Carcinogen PhIP (2-Amino-1-Methyl-6-Phenylimidazo[4,5-b]Pyridine) and Its Genotoxic Metabolites

“…As PhIP is heavily metabolized in the body (Supplemental Fig. 1), and, as only some of the known PhIP metabolites are potentially carcinogenic (Schut and Snyderwine, 1999;Frandsen and Alexander, 2000;Gooderham et al, 2001Gooderham et al, , 2002Lauber et al, 2004;Nakagama et al, 2005;Chen et al, 2007;Gooderham, 2007, 2011), we developed an LC-MS/MS assay for the quantification of PhIP and N2-OH-PhIP in mouse matrices (Teunissen et al, 2010) and expanded this assay for detection of various phase 1 and phase 2 metabolites of PhIP (Teunissen et al, 2011) We used this method to determine the plasma and tissue concentrations of PhIP and its carcinogenic metabolites 30 minutes after oral or intravenous administration of PhIP (1 mg/kg) to wild-type, Bcrp1;Mdr1a/b;Mrp2 2/2 and Bcrp1;Mrp2;Mrp3 2/2 mice. Overviews of all metabolites detected in the tissues of the three strains are presented in Supplemental Table 2 (intravenous administration) and Supplemental Table 3 (oral administration).…”

“…1) is considered a detoxification pathway. On the other hand, besides PhIP itself, N2-OH-PhIP and the short-lived species PhIP-N-sulfate and N-acetyl-PhIP are (pre)carcinogenic (Schut and Snyderwine, 1999;Frandsen and Alexander, 2000;Gooderham et al, 2001Gooderham et al, , 2002Lauber et al, 2004;Nakagama et al, 2005;Chen et al, 2007;Gooderham, 2007, 2011). 5-OH-PhIP and PhIP-5-sulfate are surrogate markers for the levels of the ultimate genotoxic nitrenium radical cation that forms DNA adducts (Supplemental Fig.…”

“…PhIP is rapidly taken up after ingestion and heavily metabolized, leading to detoxification products but also to activated, potentially carcinogenic metabolites (Supplemental Fig. 1; Gooderham et al, 2001Gooderham et al, , 2002Lauber et al, 2004;Nakagama et al, 2005). Here, we focused on the precarcinogen N2-OH-PhIP (2-hydroxyamino-1-methyl-6-phenylimidazo [4,5-b]pyridine), and the derivatives of 5-OH-PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]-5-hydroxypyridine) and PhIP-5-sulfate, which represent breakdown products of the ultimate genotoxic nitrenium radical cation that also forms mutagenic PhIP-DNA adducts.…”

“…PhIP-N-sulfate and N-acetyl-PhIP, the primary precursors of the genotoxic nitrenium radical, are too shortlived to be detectable in pharmacokinetic studies. Furthermore, part of the carcinogenic potential of PhIP may also be explained by the fact that PhIP possesses estrogenic activity and could therefore stimulate cell proliferation (Schut and Snyderwine, 1999;Frandsen and Alexander, 2000;Gooderham et al, 2002;Lauber et al, 2004;Chen et al, 2007;Gooderham, 2007, 2011).…”

The mutagenic effects of 2-amino-1-methyl-6-phenylimidazo[4,5-b] pyridine in Muta™Mouse colon is attenuated by resveratrol

“…Moreover, PhIP like resveratrol has been shown to act as a potent oestrogen 31 and as such might be expected to affect the genders differently.…”