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.

No abstract

No abstract

The cooked meat-derived heterocyclic amine 2-amino-3-methylimidazo [4,5-b]pyridine (PhIP) is activated by CYP1A2 to the N-hydroxy metabolite, then esterified by acetyl transferase and sulfur transferase into unstable DNA-reactive products that can lead to mutation. The genotoxicity of PhIP has been implicated in its carcinogenicity. Yet, CYP1A2-null mice are still prone to PhIP-mediated cancer, inferring that alternative mechanisms must be operative in tumor induction. PhIP induces tumors of the breast, prostate, and colon in rats and lymphoma in mice. This profile of carcinogenicity is indicative of hormonal involvement. We recently reported that PhIP has potent estrogenic activity inducing transcription of estrogen (E 2 )-regulated genes, proliferation of E 2 -dependent cells, up-regulation of progesterone receptor, and stimulation of mitogen-activated protein kinase signaling. In this report, we show for the first time that PhIP at doses as low as of 10 À11 mol/L has direct effects on a rat pituitary lactotroph model (GH3 cells) and is able to induce cell proliferation and the synthesis and secretion of prolactin. This PhIP-induced pituitary cell proliferation and synthesis and secretion of prolactin can be attenuated by an estrogen receptor (ER) inhibitor, implying that PhIP effects on lactotroph responses are ERA mediated. In view of the strong association between estrogen, progesterone, prolactin, and breast cancer, the PhIP repertoire of hormone-like activities provides further mechanistic support for the tissue-specific carcinogenicity of the chemical. Furthermore, the recent epidemiology studies that report an association between consumption of cooked red meat and premenopausal and postmenopausal human breast cancer are consonant with these observations. [Cancer Res 2007;67(19):9597-602]

No abstract