Metal induction of heme oxygenase

The experimental data that have been reviewed support the following conclusions regarding metal induction of microsomal heme oxygenase activity: 1. Induction of heme oxygenase in liver, kidney, and other organs of rodents is a nonspecific, toxic response to parenteral administration of numerous metal compounds. 2. The Co2+ and Cd2+ ions are especially potent for induction of heme oxygenase in rat liver; Sn2+, Ni2+, and As3+ are especially potent for induction of the enzyme in rat kidney; Hg2+ is especially potent for induction of the enzyme in rat adrenal. 3. Rat spleen, testis, and brain are relatively refractory to metal induction of heme oxygenase activity; in testicular microsomes from Cd2+-treated rats, heme oxygenase activity is markedly inhibited. 4. Metal induction of heme oxygenase requires de novo synthesis of mRNA and protein, based on 1) experiments with metabolic inhibitors (actinomycin D, puromycin, and cycloheximide) and 2) translation assays of heme oxygenase mRNA. 5. Heme oxygenase induction by metals is generally suppressed by treatments with SH compounds (for example, cysteine and glutathione) and enhanced by agents that deplete tissue SH levels (for example, diethyl maleate), suggesting that the induction mechanism may involve binding of metal ions to SH-containing regulatory molecules. 6. Administration of DDC exerts a pronounced synergistic effect on Ni2+ induction of heme oxygenase activity in rat tissues, attributable in part to enhanced cellular uptake of nickel. 7. Induction of heme oxygenase is not sustained during repeated daily treatments of rats with NiCl2, pointing to an adaptive mechanism for tolerance to the enzyme inducer. Investigations of metal induction of heme oxygenase activity have commonly involved sc or ip administration of the test compounds to rats. The paucity of studies in other species and the dearth of experiments with oral or respiratory exposures hinder extrapolations of the animal data to human environmental or occupational exposures to metal compounds.