Activation of the RON Receptor Tyrosine Kinase by Macrophage-stimulating Protein Inhibits Inducible Cyclooxygenase-2 Expression in Murine Macrophages

The RON receptor tyrosine kinase is activated by macrophage-stimulating protein, which regulates macrophage migration, phagocytosis, and nitric oxide production. We report here the inhibitory effect of RON on lipopolysaccharide (LPS)-induced cyclooxygenase (Cox)- 2 expression in mouse macrophages. In RON-expressing macrophages treated with macrophage stimulating protein, LPS-induced prostaglandin E2 (PGE2) production was significantly reduced. The inhibition was accompanied by reduction of Cox-2 protein and mRNA expression. Transcriptional studies indicated that RON activation inhibits LPS-induced luciferase activity driven by the Cox-2 gene promoter. To determine whether RON activation affects LPS-induced NF-κB pathway, which is important for Cox-2 expression. Western blot analyses were performed showing that RON activation inhibits LPS-induced IκBα degradation. The decreased IκBα degradation was due to reduced IκBα phosphorylation at Ser-32 as determined by IκBα (Ser-32) phosphor-antibody. Moreover, we found that LPS-induced IKKβ activity, an enzyme responsible for phosphorylation of IκBα, was inhibited upon RON activation. Interestingly, these inhibitory effects were not regulated by RON-mediated phosphatidylinositol-3 kinase. These results suggest that RON activation inhibits LPS-induced macrophage Cox-2 expression. The inhibitory effect is mediated by impairing LPS-activated cascade enzymes that activate NF-κB. The inhibition of Cox-2 expression might represent a novel mechanism for the inhibitory functions of RONin vivo against LPS-induced inflammation and septic shock.