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ORIGINAL RESEARCH ARTICLE

Effect of Dietary Selenium on the Promotion of Hepatocarcinogenesis by 3,3', 4,4'-Tetrachlorobiphenyl and 2,2', 4,4', 5,5'-Hexachlorobiphenyl

Divinia N. Stemm^*, Job C. Tharappel, Hans-Joachim Lehmler, Cidambi Srinivasan, J. Steven Morris^||, Vickie L. Spate^||, Larry W. Robertson, Brett T. Spear^¶ and Howard P. Glauert^*,,1

^* Graduate Center for Toxicology, University of Kentucky, Lexington, Kentucky 40506; Graduate Center for Nutritional Sciences, University of Kentucky, Lexington, Kentucky 40506; Department of Occupational and Environmental Health, University of Iowa, Iowa City, Iowa 52242; Department of Statistics, University of Kentucky, Lexington, Kentucky 40506;^|| Research Reactor Center, University of Missouri, Columbia, Missouri 65201; and^¶ Department of Microbiology, Immunology, and Molecular Genetics, University of Kentucky, Lexington, Kentucky 40506

To whom requests for reprints should be addressed at ¹ Graduate Center for Nutritional Sciences, 222 Funkhouser Building, University of Kentucky, Lexington, KY 40506-0054. E-mail: hglauert{at}uky.edu

Polychlorinated biphenyls (PCBs) are persistent organic pollutants that have promoting activity in the liver. PCBs induce oxidative stress, which may influence carcinogenesis. Epidemiological studies strongly suggest an inverse relationship between dietary selenium (Se) and cancer. Despite evidence linking Se deficiency to hepatocellular carcinoma and liver necrosis, the underlying mechanisms for Se cancer protection in the liver remain to be determined. We examined the effect of dietary Se on the tumor promoting activities of two PCBs congeners, 3,3', 4,4'-tetrachlorobiphenyl (PCB-77) and 2,2', 4,4', 5,5'-hexachlorobiphenyl (PCB-153) using a 2-stage carcinogenesis model. An AIN-93 torula yeast-based purified diet containing 0.02 (deficient), 0.2 (adequate), or 2.0 mg (supplemental) selenium/kg diet was fed to Sprague-Dawley female rats starting ten days after administering a single dose of diethylnitrosamine (150 mg/kg). After being fed the selenium diets for 3 weeks, rats received four i.p. injections of either PCB-77 or PCB-153 (150 µ mol/kg) administered every 14 days. The number of placental glutathione S-transferase (PGST)-positive foci per cm³ and per liver among the PCB-77–treated rats was increased as the Se dietary level increased. Unlike PCB-77, rats receiving PCB-153 did not show the same Se dose-response effect; nevertheless, Se supplementation did not confer protection against foci development. However, the 2.0 ppm Se diet reduced the mean focal volume, indicating a possible protective effect by inhibiting progression of preneoplastic lesions into larger foci. Cell proliferation was not inhibited by Se in the liver of the PCB-treated groups. Se did not prevent the PCB-77–induced decrease of hepatic Se and associated reduction in glutathione peroxidase (GPx) activity. In contrast, thioredoxin reductase (TrxR) activity was not affected by the PCBs treatment or by Se supplementation. These findings indicate that Se does not inhibit the number of PGST-positive foci induced during promotion by PCBs, but that the size of the lesions may be inhibited. The effects of Se on altered hepatic foci do not correlate with its effects on GPx and TrxR.

Key Words: selenium • polychlorinated biphenyls (PCBs) • glutathione peroxidase • thioredoxin reductase • tumor promotion • cell proliferation • altered hepatic foci