Nitric oxide mediates the lipopolysaccharide dependent upregulation of the heme oxygenase-1 gene expression in cultured rat Kupffer cells

Background/Aims: Heme oxygenase catalyzes the rate-limiting enzymatic step of heme degradation. The inducible isoform of heme oxygenase, heme oxygenase-1, is expressed at a low level in most tissues and is upregulated by its substrate heme and various stress stimuli. Kupffer cells which represent the largest population of the body's tissue macrophages serve physiological functions in the defense against various pathogens such as lipopolysaccharide. The goal of the present study was to investigate the heme oxygenase-1 gene expression in Kupffer cells of rat liver and in isolated Kupffer cell cultures during treatment with lipopolysaccharide. Methods: Cryostat sections of normal rat liver were investigated by immunofluorescence double-staining using specific antibodies for rat heme oxygenase-1 and ED2. Isolation and cell culture of Kupffer cells and primary hepatocytes from rat liver, as well as Northern and Western blot analysis, were performed with standard protocols. Results: Heme oxygenase-1 protein was highly expressed in large sinusoidal cells of normal rat liver, which were identified as Kupffer cells by staining with the macrophage surface marker ED2. By contrast, no expression of heme oxygenase-1 was detected in liver parenchymal cells. High expression of heme oxygenase-1 was also found in isolated Kupffer cells in culture by immunocytochemical staining as well as by Western and Northern blot analysis. After treatment of Kupffer cells cultures with lipopolysaccharide, heme oxygenase-1 was upregulated on the protein and mRNA level in a time- and dose-dependent manner, This increase in heme oxygenase-1 expression by lipopolysaccharide was prevented by the nitric oxide inhibitor N-G-monomethyl-L-arginine which was reversed by an excess of L-arginine. Various nitric oxide donors up-regulated heme oxygenase-1 mRNA expression in Kupffer cells. Conclusions: The lipopolysaccharide-dependent upregulation of the heme oxygenase-1 gene which is highly expressed in Kupffer cells is mediated by a nitric oxide-dependent mechanism.