Transcriptional Activation of the Human Heme Oxygenase-1 Gene during Acute-Phase Reaction

Abstract

The effects of human interleukin-6 (hIL-6), the major acute-phase inducer, on the level of the transcript of microsomal heme oxygenase (HO-1) were examined in human hepatoma cell lines, Hep3B, HepG2 and HepG2f. While Hep3B and HepG2 cells express well-differentiated phenotype of hepatocytes, HepG2f cells do not. mRNAs encoding HO-1 and haptoglobin (Hpt) increased following hIL-6 treatment in a time- and dose-dependent manner, in Hep3B and HepG2, but not in HepG2f cells. The hIL-6-mediated induction of HO-1 mRNA in Hep3B cells was completely abrogated by simultaneous treatment of cells with actinomycin D, but not with cycloheximide, suggesting that the induction occurs at the level of transcription.

To study the molecular mechanism of activation of the HO-1 gene by hIL-6, a chimeric plasmid was constructed by fusing a 5’-flanking region of the HO-1 gene to the firefly luciferase gene, and introduced into human HepG2 hepatoma cells. The luciferase activity in HepG2 cells transfected with the chimeric plasmid was significantly increased by treatment of cells with hIL-6. Deletion analyses revealed that the region between nucleotides -120 and -80 contains a regulatory element involved in the hIL-6-mediated induction of the gene in HepG2 cells. In this region, there is a sequence homologous to the type A IL-6 responsive element, which has been shown to be involved in the activation of certain acute-phase protein genes. Gel mobility shift assays demonstrated the existence of a nuclear factor which specifically binds to this region. These findings suggest that HO-1 is a positive acute-phase reactant in human liver-derived cell lines such as Hep3B and HepG2, and that the nuclear factor specific to the IL6-RE may be involved in the activation of the HO-1 gene after hIL-6 treatment.

This work was supported in part by grants from U.S.P.H.S. DK-39264 and the Yamanouchi Molecular Medicine Fund.