Metabolic Activation of the Cooked Meat Carcinogen 2-Amino-1-Methyl-6-Phenylimidazo[4,5-b]Pyridine in Human Prostate

Abstract

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), an heterocyclic aromatic amine (HAA) formed in cooked meat, is a rodent and possible human prostate carcinogen. Recently, we identified DNA adducts of PhIP in the genome of prostate cancer patients, but adducts of 2-amino-3, 8-dimethylmidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-9 H-pyrido[2,3-b]indole (AαC), other prominent HAAs formed in cooked meats, were not detected. We have investigated the bioactivation of HAAs by Phase I and II enzymes in the human prostate (LNCaP) cell line using cytotoxicity and DNA adducts as endpoints. PhIP, MeIQx, and 2-amino-3-methylimidazo[4,5-f]quinoline, another HAA found in cooked meats, were poorly bioactivated and not toxic. The synthetic genotoxic N-hydroxylated-HAAs were also assayed in LNCaP cells with Phase II enzyme inhibitors. Notably, 2-hydroxy-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (HONH-PhIP), but not other HONH-HAAs, induced cytotoxicity. Moreover, PhIP-DNA adduct formation was 20-fold greater than adducts formed with other HONH-HAAs. Pretreatment of LNCaP cells with mefenamic acid, a specific inhibitor of sulfotransferase (SULT1A1), decreased PhIP-DNA adducts by 25%, whereas (Z)-5-(2'-hydroxybenzylidene)-2-thioxothiazolidin-4-one and pentachlorophenol, inhibitors of SULTs and N-acetyltransferases (NATs), decreased the PhIP-DNA adduct levels by 75%. NATs in cytosolic fractions of LNCaP cells and human prostate catalyzed DNA binding of HONH-PhIP by up to 100-fold greater levels than for SULT and kinase activities. Recombinant NAT2 is catalytically superior to recombinant NAT1 in the bioactivation of HONH-PhIP; however, the extremely low levels of NAT2 activity in prostate suggest that NAT1 may be the major isoform involved in PhIP-DNA damage. Thus, the high susceptibility of LNCaP cells recapitulates the DNA-damaging effect of HONH-PhIP in rodent and human prostate.

Dietary Carcinogens and DNA Adducts in Prostate Cancer

“…However, HONH-PhIP, the genotoxic metabolite of PhIP, induced a dose-dependent increase in cytotoxicity, whereas HONH-MeIQx, HONH-IQ, and HONH-AαC were not toxic [285,286]. Moreover, HONH-PhIP forms DNA adducts at levels that are 20-fold higher than other HONH-HAAs in LNCaP cells [285]. These data suggest that the initial bioactivation step of PhIP to form HONH-PhIP occurs in the liver through CYP 1A2-catalyzed N-oxidation, followed by systemic circulation to reach the prostate, where bioactivation is mediated by Phase II enzymes (Figure 6) [285].…”

“…Moreover, HONH-PhIP forms DNA adducts at levels that are 20-fold higher than other HONH-HAAs in LNCaP cells [285]. These data suggest that the initial bioactivation step of PhIP to form HONH-PhIP occurs in the liver through CYP 1A2-catalyzed N-oxidation, followed by systemic circulation to reach the prostate, where bioactivation is mediated by Phase II enzymes (Figure 6) [285]. Similar data were reported in primary human prostate epithelial cells, where HONH-PhIP formed 50-to 100-fold higher levels of DNA adducts than IQ, MeIQx, and HONH-MeIQx [207,287].…”

“…The parent HAAs, PhIP, MeIQx, IQ and AαC were not toxic at doses up to 10 μM and formed low levels of DNA adducts in LNCaP cells [285]. However, HONH-PhIP, the genotoxic metabolite of PhIP, induced a dose-dependent increase in cytotoxicity, whereas HONH-MeIQx, HONH-IQ, and HONH-AαC were not toxic [285,286]. Moreover, HONH-PhIP forms DNA adducts at levels that are 20-fold higher than other HONH-HAAs in LNCaP cells [285].…”

“…Cytotoxicity and DNA adduct formation induced by PhIP and other HAAs has been studied in primary and PC cell lines. The parent HAAs, PhIP, MeIQx, IQ and AαC were not toxic at doses up to 10 μM and formed low levels of DNA adducts in LNCaP cells [285]. However, HONH-PhIP, the genotoxic metabolite of PhIP, induced a dose-dependent increase in cytotoxicity, whereas HONH-MeIQx, HONH-IQ, and HONH-AαC were not toxic [285,286].…”

Detection of a novel, primate-specific ‘kill switch’ tumor suppression mechanism that may fundamentally control cancer risk in humans: an unexpected twist in the basic biology of TP53

“…Chimpanzees consume only a fraction of the meat that humans do, and neither did they harness fire, as humans did. Their species-specific trajectory through spacetime thus did not involve exposure to the heterocyclic amines, N-nitrosamines and polycyclic aromatic hydrocarbons that are formed in the processing of meat by heat ( Gu et al 2011 , Bellamri et al 2018 ). Consumption of such heat-processed meat is well documented to be carcinogenic ( Milton 1999 , IARC 2015 , Chiang & Quek 2017 ).…”