Regulation of heme oxygenase gene expression by cobalt in rat liver and kidney

The effect of heavy metals such as cobalt chloride and the corresponding metalloporphyrin on the transcription of the heme oxygenase gene in tissues was examined using cDNA for rat heme oxygenase as the probe. An increase in heme oxygenase mRNA level was observed in response to cobalt chloride and cobalt protoporphyrin treatment in both liver and kidney. Quantitative evaluation of the heme oxygenase transcript was obtained by determining the intensity of mRNA bands by scanning densitometry, and indicated that cobalt chloride increased heme oxygenase mRNA by 40-60-fold after 2 h of metal exposure. Accumulation of heme oxygenase mRNA after cobalt chloride administration was prevented by co-administration of actinomycin D or cycloheximide. These results indicate that the increased expression of heme oxygenase by cobalt chloride required de novo protein synthesis and was regulated at the transcriptional level. The time course of heme oxygenase transcript accumulation following administration of cobalt protoporphyrin was different from that of cobalt chloride. There was a sharp increase in heme oxygenase mRNA after cobalt chloride administration at 2 h and cobalt protoporphyrin at 10 h. Heme, to which cobalt protoporphyrin is structurally analogous, acted as a potent inducer of heme oxygenase transcripts in both liver and kidney. Variation in heme oxygenase mRNA levels resulting from enhanced transcription of the heme oxygenase gene was evaluated by nuclear runoff assay using isolated rat liver nuclei after cobalt chloride administration. Quantification of specific nuclear RNAs labeled during the in vitro transcription revealed active heme oxygenase gene transcription in liver nuclei from cobalt-chloride-treated rats. Transcription of heme oxygenase is greatly increased within 1 h of administration of cobalt chloride in rat liver, as evidenced by the level of [alpha-32P]UTP incorporation into nuclear RNA. The transcription was increased by 40-fold after 3 h of cobalt chloride administration. The activation of the heme oxygenase gene by metal ions is the most rapid transcriptional response to heavy metals yet described and highlights the regulatory role of heme oxygenase in heme degradation during deviating environmental conditions. On the other hand, cobalt protoporphyrin and heme arginate increase transcription of the heme oxygenase gene in a similar pattern but at a slower rate than that of the heavy metal, suggesting that the heme oxygenase promotor region may contain additional elements conferring the inducing effect of these two agents.