Rat oesophageal cytochrome P450 (CYP) monooxygenase system: comparison to the liver and relevance in N-nitrosodiethylamine carcinogenesis

Abstract

N-nitrosodiethylamine (NDEA) is able to induce tumours in the rat oesophagus. It has been suggested that this could be due to tissue specific expression of NDEA activating cytochrome P450 enzymes. We investigated this by characterizing the oesophageal monooxygenase complex of male Wistar rats and comparing it with that of the liver. Total amount of cytochrome P450, NADPH P450 reductase, cytochrome b5 and cytochrome b5 reductase of the oesophageal mucosa was approximately 7% of what was found in the liver. In addition, major differences were found in the cytochrome P450 isoenzyme composition between these organs: CYP 2B1/2B2 and CYP3A were found only in the liver, whereas CYP1A1 was constitutively expressed only in the oesophagus. Of the two well-known nitrosamine metabolizing enzymes, CYP2A3 was found only in the oesophagus whereas CYP2E1 was exclusively expressed in the liver. Catalytic studies, western blotting and RT-PCR analyses confirmed the expression of CYP2A3 in the oesophagus. CYP2A enzymes are known to be good catalysts of NDEA metabolism. Oesophageal microsomes had a K(m) for NDEA metabolism, which was about one-third of that of hepatic microsomes, but they showed similar activities when compared per nmol of total P450. NDEA activity in the oesophagus was significantly increased by coumarin (CO), which also induced oesophageal CYP2A3. Immunoinhibition of the microsomal NDEA activity showed that up to 70% of this reaction is catalysed by CYP2A3 in the oesophagus, whereas no inhibition of the hepatic NDEA activity could be achieved by the anti-CYP2A5 antibody. NDEA, but not N-nitrosodimethylamine (NDMA) inhibited the oesophageal metabolism of CO. The results of the present investigation show major differences in the enzyme composition of the oesophageal and hepatic monooxygenase complexes, and are in accordance with the hypothesis that the NDEA organotropism could, to a large extent, be due to the tissue specific expression of the activating enzymes.

Electric Pulse Stimulation of Myotubes as an In Vitro Exercise Model: Cell-Mediated and Non-Cell-Mediated Effects

“…Only the CM of contracting C2C12 myotubes was able to reduce the expression of Cyp2a3 in FAO hepatocytes. It should be noted however that this effect was not observed in primary hepatocytes and that the Cyp2a3 gene is usually not very highly expressed in rat liver 39 . The biological relevance of the down-regulation of Cyp2a3 in FAO hepatocytes by EPS-CM therefore remains to be established.…”

Garlic Oil Attenuated Nitrosodiethylamine-Induced Hepatocarcinogenesis by Modulating the Metabolic Activation and Detoxification Enzymes

“…These compounds are considered to be critical environmental factors influencing cancer risk in humans, as they widely exist in the environment, in certain occupational settings, in food stuffs such as meat products and milk, as well as an endogenous formation in the human body from dietary components 3. A variety of studies have demonstrated that N-nitroso compounds could cause a wide range of tumors in many animal species 4-6. Nitrosodiethylamine (NDEA) is one of the most potent chemical hepatocarcinogens of this class, which can induce a variety of liver lesions in rodents.…”

Risk assessment of N‐nitrosamines in food

“…The expression of NDEA metabolising CYPs has been reported as a major determinant of the development of tumours in certain tissues. For example, Ribeiro Pinto et al (2001) found that CYP2E1 was exclusively expressed in liver, while CYP2A3 was only found in the oesophagus; the Km for NDEA N-deethylase in oesophageal microsomes was about one-third of that calculated in liver microsomes (Ribeiro Pinto et al, 2001) and similar results were obtained using tissue slices (Ribeiro Pinto, 2000). Results from these studies suggest that in rat oesophagus, CYP2A3 is mainly responsible for NDEA bioactivation and may induce tumour formation even at low doses.…”