Uptake of the colon carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine by different segments of the rat gastrointestinal tract: Its implication in colorectal carcinogenesis

Synthesis and in vitro characterization of the genotoxic, mutagenic and cell-transforming potential of nitrosylated heme

Data from epidemiological studies suggest that consumption of red and processed meat is a factor contributing to colorectal carcinogenesis. Red meat contains high amounts of heme, which in turn can be converted to its nitrosylated form, NO-heme, when adding nitrite-containing curing salt to meat. NO-heme might contribute to colorectal cancer formation by causing gene mutations and could thereby be responsible for the association of (processed) red meat consumption with intestinal cancer. Up to now, neither in vitro nor in vivo studies characterizing the mutagenic and cell transforming potential of NO-heme have been published due to the fact that the pure compound is not readily available. Therefore, in the present study, an already existing synthesis protocol was modified to yield, for the first time, purified NO-heme. Thereafter, newly synthesized NO-heme was chemically characterized and used in various in vitro approaches at dietary concentrations to determine whether it can lead to DNA damage and malignant cell transformation. While NO-heme led to a significant dose-dependent increase in the number of DNA strand breaks in the comet assay and was mutagenic in the HPRT assay, this compound tested negative in the Ames test and failed to induce malignant cell transformation in the BALB/c 3T3 cell transformation assay. Interestingly, the non-nitrosylated heme control showed similar effects, but was additionally able to induce malignant transformation in BALB/c 3T3 murine fibroblasts. Taken together, these results suggest that it is the heme molecule rather than the NO moiety which is involved in driving red meat-associated carcinogenesis.

Gut microbial transformation of the dietary mutagen MeIQx may reduce exposure levels without altering intestinal transport

The mutagen and probable human carcinogen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) is metabolized in the colon to 9-hydroxyl-2,7-dimethyl-7,9,10,11-tetrahydropyrimido[2',1':2,3]imidazo[4,5-f]quinoxaline (MeIQx-M1) by conjugation with microbially generated acrolein. However, whether this microbiota-controlled process alters systemic exposure and hepatotoxicity of MeIQx remains unclear. The physiological relevance of this microbial transformation on the systemic exposure of MeIQx was investigated using an in vitro-in vivo extrapolation approach. To address whether microbial transformation influences intestinal transport of MeIQx, the intestinal uptake of MeIQx and its metabolite MeIQx-M1 was quantified using Ussing chambers mounted with different intestinal segments from male Fischer 344 rats. Up to 0.4% of both MeIQx and MeIQx-M1 were transported from the mucosal side to the serosal side of intestinal tissue within 90 min, suggesting that the intestinal uptake of both compounds is similar. With the uptake rates of both compounds, physiologically based pharmacokinetic (PBPK) modeling of the fate of MeIQx in the human body including microbial transformation of MeIQx was performed. Results indicate for the first time that high levels of microbe-derived acrolein would be required to significantly reduce systemic exposure of MeIQx in humans. Finally, neither MeIQx nor MeIQx-M1 were cytotoxic towards human liver HepaRG cells at dietary or higher concentrations of MeIQx. In summary, these findings suggest that gut microbial transformation of heterocyclic amines has the potential to influence systemic human exposure to some extent, but may require significant gut microbial production of acrolein and that further investigations are needed to understand physiological levels of acrolein and competing biotransformation pathways.

Gut Microbial Transformation of the Dietary Imidazoquinoxaline Mutagen MelQx Reduces Its Cytotoxic and Mutagenic Potency

The diverse community of microbes present in the human gut has emerged as an important factor for cancer risk, potentially by altering exposure to chemical carcinogens. In the present study, human gut bacteria were tested for their capacity to transform the carcinogenic heterocyclic amine 2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MelQx). Eubacterium hallii, Lactobacillus reuteri, and Lactobacillus rossiae were able to convert MelQx to a new microbial metabolite characterized on the basis of high-resolution mass spectrometry and NMR as 9-hydroxyl-2,7-dimethyl-7,9,10,11-tetrahydropyrimido[2′,1′:2,3]imidazo[4,5-f]quinoxaline (MelQx-M1), resulting from conjugation with activated glycerol. Acrolein derived from the decomposition of 3-hydroxypropionaldehyde, which is the product of bacterial glycerol/diol dehydratase activity, was identified as the active compound responsible for the formation of MelQx-M1. A complex human gut microbial community obtained from invitro continuous intestinal fermentation was found to also transform MelQx to MelQx-M1. MelQx-M1 had slightly reduced cytotoxic potency toward human colon epithelial cells invitro, and diminished mutagenic potential toward bacteria after metabolic activation. As bacterially derived acrolein also transformed 2 other HCAs, namely 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 2-amino-3-methylimidazo[4,5-f]quinoline, these results generalize the capacity of gut microbiota to detoxify HCAs in the gut, potentially modulating cancer risk.

The strict anaerobic gut microbe Eubacterium hallii transforms the carcinogenic dietary heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)

2-Amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhIP) is the most abundant food-derived heterocyclic aromatic amine in well-cooked meats and may contribute to the recognized carcinogenicity of processed meats. In this study, a panel of human gut microbes was tested for their ability to convert PhIP to a conjugate PhIP-M1. Eubacterium hallii was newly identified to catalyse the conversion of PhIP to PhIP-M1 with high efficiency. The reaction was shown to involve the metabolism of glycerol to 3-hydroxypropionaldehyde as a key pathway. The proficiency of E. hallii in transforming PhIP in the presence of a complex intestinal microbiota was confirmed using batch fermentations inoculated with effluents from a continuous intestinal fermentation model mimicking human proximal and distal colon microbiota. In batch fermentations inoculated with proximal colon microbiota, PhIP-M1 transformation corresponded to an up to 300-fold increase of E. hallii. In contrast, PhIP transformation of distal colon microbiota was low but increased by 120-fold after supplementation with E. hallii. These findings indicate for the first time the relevance of the abundant commensal strict anaerobe E. hallii in the transformation of a dietary carcinogen that could contribute to its detoxification in the human colon.

Oral treatment of rodents with soluble epoxide hydrolase inhibitor 1-(1-propanoylpiperidin-4-yl)-3-[4-(trifluoromethoxy)phenyl]urea (TPPU): Resulting drug levels and modulation of oxylipin pattern

Epoxides from polyunsaturated fatty acids (PUFAs) are potent lipid mediators. In vivo stabilization of these epoxides by blockade of the soluble epoxide hydrolase (sEH) leads to anti-inflammatory, analgesic and normotensive effects. Therefore, sEH inhibitors (sEHi) are a promising new class of drugs. Herein, we characterized pharmacokinetic (PK) and pharmacodynamic properties of a commercially available potent sEHi 1-(1-propanoylpiperidin-4-yl)-3-[4-(trifluoromethoxy)phenyl]urea (TPPU). Cell culture studies suggest its high absorption and metabolic stability. Following administration in drinking water to rats (0.2, 1, and 5mg TPPU/L with 0.2% PEG400), TPPU's blood concentration increased dose dependently within the treatment period to reach an almost steady state after 8 days. TPPU was found in all the tissues tested. The linoleic epoxide/diol ratios in most tissues were dose dependently increased, indicating significant sEH inhibition. Overall, administration of TPPU with the drinking water led to systemic distribution as well as high drug levels and thus makes chronic sEH inhibition studies possible.

Intestinal absorption and cell transforming potential of PhIP-M1, a bacterial metabolite of the heterocyclic aromatic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)

Previous studies have shown that in the rat, the colon carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is only absorbed to a limited extent in the small intestines and that a major fraction of unmetabolised PhIP reaches the colon. Moreover, PhIP is extensively metabolised when incubated with human stool samples to a major derivative, 7-hydroxy-5-methyl-3-phenyl-6,7,8,9-tetrahydropyrido [3',2':4,5]imidazo[1,2-a]pyrimidin-5-ium chloride (PhIP-M1). In the present study, the uptake and transport of PhIP-M1 in Ussing chamber experiments, its cytotoxicity in the different segments of the Fischer 344 rat gut and its transforming potential in the BALB/c 3T3 cell transformation assay were analysed. At the most, 10-20% of the PhIP-M1 amount added to the mucosal compartment of the Ussing chambers per segment were absorbed within 90min. Therefore, the amount of PhIP-M1 detected in the tissues as well as in the serosal compartment of the Ussing chambers was extremely low. Moreover, human-relevant concentrations of PhIP-M1 were not cytotoxic and did not induce the malignant transformation of BALB/c 3T3 cells. In conclusion, even if one would assume that 100% of the daily amount of PhIP ingested by a human being is converted into PhIP-M1 in the colon, this concentration most probably would not lead to cytotoxicity and/or carcinogenicity in the colorectal mucosa.