Effect of Vitamin E supplementation on xenobiotic-metabolizing and antioxidant enzymes in rat

Effect of Vitamin E supplementation on xenobiotic-metabolizing and antioxidant enzymes in rat. F. Vivarelli1 D. Canistro1, A. Sapone1, S. Filippi2, I.C. Antonazzo1, C. Babot Marquillas1 M. Paolini1 1 Molecular Toxicology Unit, Department of Pharmacy and Biotechnology, Alma-Mater Studiorum, University of Bologna, 40127 Bologna, Italy. 2 Interdepartmental Laboratory of Functional and Cellular Pharmacology of Reproduction, Department of NEUROFARBA and Department of Experimental, Clinical and Biomedical Sciences, University of Florence, 50139 Florence, Italy. Since its discovery and isolation Vitamin E (VE) has been considered a “safe” agent. VE has a number of biological functions being the antioxidant one, as peroxyl radical scavenger, the best known. The relationship between VE and cancer risk has been investigated in epidemiological studies and clinical trials with conflicting results. Recently, Selenium and Vitamin E Cancer Prevention Trial (SELECT) unexpectedly showed an increased risk of all primary cancers among healthy man supplemented with VE. As the literature reports that VE induce the P450 in human hepatoma cell line (HepG2), and being also known that such induction is always linked to an increased free radical and metabolite generation (cocarcinogenesis), we hypothesized that a generalized CYP upregulation migth have a role in the SELECT outcomes. Sprague Dawley male rats were intraperitoneally daily treated (for 7 or 14 consecutive days) with VE (α-tocopherol) at 100 or 200 mg/kg b.w. doses. The putative effects of VE on hepatic and renal microsomal CYP-linked monooxygenases as well as on antioxidant and post-oxidative enzymes were investigated. The almost neutral effect exerted by VE on hepatic phase I and II xenobiotic metabolizing enzymes, and the appreciable increase of catalase (up to 73% after 7 day treatment at lowest dose, p<0.01) would stick once again to VE the label of “protective agent”. On the contrary, data from kidney draw a picture that appears long far from that recorded in the liver. CYP2E1 p-nitrophenol associated mooxygenase shown a significant increase at all tested doses; in particular, at 200 mg/kg b.w., a significant (p<0.01) increase (2 and 2.6-fold, after 7 and 14 days, respectively) was seen. VE treatment significantly (p<0.01) boosted O-dealkylation of pentoxyresorufin; the phenomenon reached 2-fold induction (p<0.01) with the highest dose (both treatment periods) used. The CYP 1A1-dependent ethoxyresorufin O-deethylation was significantly (p<0.01) enanched in subcellular preparations from rats receiving 7 day regimens at the highest dose (up to 62%). A general up-regulation trend for CYP1A2-linked activity, in this instance more than doubled after 7 day treatment at 200 mg/kg b.w., was recorded. Gluthatione S-trasferase was inactivated in all cases (ranging from 19% to 28% loss, p<0.01). A phenomenon which was further exacerbated in UDP-glucuronosyl transferase (from 47% to 57% loss; p<0.01) in every experimental groups. Also the antioxidant machinery was affected, VE decreased NAD(P)H: quinone reductase in all cases (from 25% to 44% loss p<0.01). Because of the widely recognized role of reactive radical species as factors advancing cancer risk in human, the measure of free radical generation in kidney was performed by means of electron paramagnetic resonance (EPR) spectroscopy using the bis(1-hydroxy-2,2,6,6-tetra-methyl-4-piperidinyl)decandioate as radical probe. The detection of nitroxide radical yielded by the reaction of hydroxylamine with nitrogen, carbon and oxygen radicals was recorded. A notable and significant (p<0.05) radical production was found in all kidney groups as compared with relative controls (ranging from 113% to 184% increase). Taken together, these findings seem to be in accordance with the hypothesized co-carcinogenicity potential of VE. If reproduced in humans, these results may contribute to explain the harmful effects observed in the SELECT study.