Selenium Regulation of Glutathione Peroxidase in Human Hepatoma Cell Line Hep3B

Abstract

Glutathione peroxidase is an important enzyme in cellular antioxidant defense systems, detoxifying peroxides and hydroperoxides. As a component of the glutathione cycle, it protects the liver from reactive oxygen metabolites. Selenocysteine is present at the catalytic site of glutathione peroxidase, and selenium availability regulates glutathione peroxidase enzyme activity. Hep3B cells, a well-differentiated human hepatoma-derived cell line, exhibited a time-dependent decrease in glutathione peroxidase activity (nmol NADPH oxidized/min/mg protein, mean ± SE) when incubated in selenium-free medium for 10 days (Day 0, 21.8 ± 7.3; Day 2, 10.9 ± 1.2; Day 4, 7.9 ± 0.8; Day 6, 4.0 ± 0.7; Day 8, 4.5 ± 0.6; Day 10, 1.6 ± 0.4). With the reintroduction of selenium, glutathione peroxidase activity returned. A second human hepatoma cell line, HepG2, demonstrated a similar pattern when depleted of and then repleted with selenium. To assess protein synthesis, glutathione peroxidase activity was measured in deficient and replete Hep3B cells incubated with and without selenium and with and without cycloheximide. Deficient cells (mean ± SE) (4.9 ± 0.2) showed an increase in glutathione peroxidase activity after 24 h in selenium-containing medium (11.6 ± 0.2), but not when cycloheximide was included in the medium (6.9 ± 0.5) or when cycloheximide and no selenium was included (5.3 ± 0.8). Replete Hep3B cells (40.1 ± 1.1) demonstrated decreased glutathione peroxidase after 24 h in medium without selenium (34.0 ± 1.4), medium with both cycloheximide and selenium (34.0 ± 2.6), and medium without selenium and containing cycloheximide (37.6 ± 1.3). These data suggest that protein synthesis is needed for selenium repletion to exert control on glutathione peroxidase activity. Using a cDNA for human glutathione peroxidase (GPx1), selenium-deficient and replete Hep3B cell RNA was analyzed by Northern blot. mRNA for GPx was quantified by densitometry. The steady-state mRNA level for glutathione peroxidase in deficient cells was 40% of that in replete cells. Nuclear run-on studies to determine the rate of GPx-specific mRNA synthesis showed no difference between nuclei from selenium-replete and selenium-deficient cells. This finding eliminated the possibility of differential transcription rates as an explanation for the observed reduction in mRNA brought about by selenium deficiency and suggested instead a stabilization of mRNA in selenium-replete cells. While selenium deficiency decreased mRNA levels by 60%, glutathione peroxidase enzyme activity decreased by 93%, suggesting a co- and/or post-translational control mechanism in addition to the effect on mRNA stability.