Ron receptor regulates Kupffer cell‐dependent cytokine production and hepatocyte survival following endotoxin exposure in mice

Acute liver failure (ALF) is an often fatal condition resulting in hepatocellular apoptosis and hemorrhagic necrosis. The most frequent cause of ALF in adults is due to drug toxicity, with a wide spectrum of etiologies responsible for the remaining cases (1). The cascade of events that leads to ALF is complex and not well understood. An established model for studying acute hepatocellular injury in mice is by the co-administration of the hepatocyte-specific transcriptional inhibitor galactosamine (GalN) and the bacterial endotoxin, lipopolysaccharide (LPS) (2). This model is principally a macrophage/monocyte-mediated model of shock and liver injury with secreted tumor necrosis factor alpha (TNFα) required for hepatic injury (3–4). In this model, LPS stimulates the release of TNFα, a pleiotropic cytokine that is capable of inducing proliferation or apoptosis in hepatocytes and other cell types (5), depending on the physiologic conditions, and numerous other cytokines and chemokines present in the microenvironment secreted from Kupffer cells, the resident tissue macrophage in the liver. After partial hepatectomy, TNFα is crucial for tissue regeneration, while in the setting of a toxic insult, TNFα induces cell death. The transcription factor NF-κB is reported to play an important role in determining which way the TNFα balance will tilt (6). Ron is a cell surface receptor tyrosine kinase that participates in divergent processes, including modulation of inflammatory responses (7). Ron is expressed in a variety of cells but is most abundant in epithelial cells and macrophages (8). Ron is activated by its ligand, hepatocyte growth factor-like protein (HGFL), also known as macrophage-stimulating protein (9). HGFL is an 85 kDa circulating protein produced and secreted primarily by hepatocytes (10). Activation of Ron in peritoneal macrophages has been shown to stimulate macrophage shape changes, chemotaxis, adhesion, and phagocytosis (x11). Ron has also been shown in alveolar and peritoneal macrophages to limit select cytokine responses in inflammatory cells through attenuation of NF-κB by a mechanism that has yet to be identified (12–13). Previous studies from our laboratory showed increased inflammatory responses and shortened survival times in mice with a deleted Ron tyrosine kinase domain (TK−/−) compared to wild-type control mice during the induction of bacterial peritonitis and in a lung injury model (14–15). Paradoxically, utilizing the well-characterized model of LPS/GalN induced ALF in mice, though serum levels of TNFα were elevated, livers from TK−/− mice exhibited marked hepatocyte protection compared with controls (16). To investigate the function of Ron in regulating hepatocyte survival, purified populations of Kupffer cells and hepatocytes from wild-type and TK−/− mice were isolated. Utilizing purified cells, we recapitulated ex vivo the protected hepatocyte phenotype and exaggerated cytokine production observed in the TK−/− mice in vivo. Furthermore, by using mice with targeted deletions of Ron in hepatocytes and macrophages, we were able to substantiate our findings ex vivo. In total, our data suggests that Ron loss selectively in hepatocytes provides a survival benefit during ALF despite increased cytokine production by deregulated Kupffer cell activation.