Expression of the mRNA of heme-binding protein 23 is coordinated with that of heme oxygenase-1 by heme and heavy metals in primary rat hepatocytes and hepatoma cells.

A 23-kDa protein with high affinity for heme (KD = 55 nM), therefore termed heme-binding protein 23 kDa (HBP23), was purified from rat liver cytosol [Iwahara, S., et al. (1995) Biochemistry 34, 13398-13406]. Homology search of the cloned HBP23 cDNA revealed that this protein belongs to a recently recognized class of thiol peroxidases, the antioxidant peroxiredoxin family. Since HBP23 gene expression was highest in the liver, HBP23 mRNA regulation by heme and heavy metals was investigated in cultures of primary rat hepatocytes and mouse hepatoma Hepa 1-6 cells. In both cell cultures HBP23 mRNA levels were upregulated in a time- and dose-dependent manner by heme. Heme-dependent induction of HBP23 mRNA occurred coordinately with that of the heme-metabolizing enzyme heme oxygenase-1, which was recently identified as inducible by oxidative stress. Treatment of primary rat hepatocyte or hepatoma cell cultures with the heavy metals CdCl2 (10 microM) and CoCl2 (300 microM) induced in parallel HBP23 and HO-1 mRNA levels, in the case of CdCl2 to even higher levels than heme. By contrast, mRNA expression of another heme binding protein, hemopexin, was not induced in hepatocyte cell cultures by heme or heavy metals. The data suggest that the expression of HBP23 and HO-1 mRNA is regulated by (a) similar mechanism(s) in liver and that both genes could play a common physiological role as antioxidants and/or in heme metabolism.