Metal ion-mediated regulation of heme oxygenase induction in cultured avian liver cells

Abstract

The induction of heme oxygenase (EC 1.14.99.3) in response to various metal treatments was investigated in monolayer cultures of chick embryo liver cells maintained in a chemically defined serum-free medium. The most potent heme oxygenase-inducing action was exhibited by CO2+, Cd2+, Sb3+, As3+, and Au1+ followed by lesser induction observed with Cu2+, Fe2+, and Fe3+. Mn2+, Ni2+, Se4+, Sn2+, and Zn2+ were without effect. In contrast to the marked inducing effect of Co2+ on heme oxygenase, Co-protoporphyrin IX decreased the enzyme activity in a dose-dependent manner. Addition of Zn2+ (20 microM) to Co2+-treated liver cell cultures revealed a striking ability of Zn2+ to block completely Co2+-induced heme oxygenase. Simultaneous addition of Mn2+ (50 microM) to Co2+-treated cells also blocked Co2+-induced heme oxygenase (approximately 50%). These findings in tissue culture confirm those made earlier in whole animals (Drummond, G. S., and Kappas, A. (1979) Proc. Natl. Acad. Sci. U. S. A. 76, 5331-5335) and indicate that these effects of Zn2+ and Mn2+ are exerted directly in liver cells. Addition of cysteine (400 microM) to the cultures also inhibited heme oxygenase induction by Co2+ substantially. Cycloheximide and actinomycin D blocked the induction of heme oxygenase, indicating that increased heme oxygenase activity by metal treatment is dependent on fresh RNA and protein synthesis. The half-life of the enzyme was calculated to be approximately 15 h after treatment with cycloheximide. These findings provide further evidence that metal ions can regulate heme oxygenase synthesis directly in isolated liver cells and that the metal-metal interactions which lead to blockade of the enzyme induction do not involve extrahepatic tissues.