The role of the MSP/RON signaling pathway in CR3 activation and the regulation of inflammation in vitro and in vivo

We have previously identified the RON receptor tyrosine kinase as a key regulator of macrophage activation and cell-mediated immune responses. The RON receptor tyrosine kinase is expressed on a subset of tissue-resident macrophages and previous studies have shown that macrophage stimulating protein (MSP), the ligand for RON, suppresses the production of nitric oxide by primary murine peritoneal macrophages in response to IFN-gamma and LPS stimulation, through inhibition of inducible nitric oxide synthase (iNOS) expression. Mice with a targeted deletion in exon 1 of the gene encoding RON exhibit increased sensitivity to endotoxic shock, associated with elevated serum levels of NO. Therefore, we proposed a model in which the expression of RON on tissue-resident macrophages limits the ability of those cells to produce NO, thus protecting host tissues from damage during an immune response. Here, we show that Kupffer cells express RON and that LPS administration results in increased NOS activity in the livers of RON-deficient mice compared to wild-type littermate controls. In order to quantitate the effects of the absence of RON on liver damage in response to endotoxin, we injected RON-deficient mice and their littermate controls with variable doses of LPS in the presence of D-galactosamine, and measured serum levels of the liver enzymes aspartate aminotransferase (AST) and alanine aminotransferase (ALT). In these studies, RON-deficient animals exhibited elevated levels of AST and ALT in the serum in response to low doses of LPS, and the NO inhibitor, aminoguanidine, was able to ameliorate this response. Histological analysis revealed acute multifocal hepatocellular necrosis in the livers of RON-deficient animals. These data support an essential role for RON in protecting the liver from NO-mediated inflammatory damage in response to endotoxin. Additionally, in an in vivo infection model we demonstrate that, although MSP activation of RON inhibits NO production by macrophages in response to heat-killed Listeria monocytogenes, RON-deficient mice exhibit increased susceptibility to infection with Listeria. (Abstract shortened by UMI.)...