Role of osteopontin in hepatic neutrophil infiltration during alcoholic steatohepatitis.
Alcoholic liver disease (ALD) is a major complication of heavy alcohol (EtOH) drinking and is characterized by three progressive stages of pathology: steatosis, steatohepatitis, and fibrosis/cirrhosis. Alcoholic steatosis (AS) is the initial stage of ALD and consists of fat accumulation in the liver accompanied by minimal liver injury. AS is known to render the hepatocytes increasingly sensitive to toxicants such as bacterial endotoxin (LPS). Alcoholic steatohepatitis (ASH), the second and rate-limiting step in the progression of ALD, is characterized by hepatic fat accumulation, neutrophil infiltration, and neutrophil-mediated parenchymal injury. However, the pathogenesis of ASH is poorly defined. It has been theorized that the pathogenesis of ASH involves interaction of increased circulating levels of LPS with hepatocytes being rendered highly sensitive to LPS due to heavy EtOH consumption. We hypothesize that osteopontin (OPN), a matricellular protein (MCP), plays an important role in the hepatic neutrophil recruitment due to its enhanced expression during the early phase of ALD (AS and ASH). To study the role of OPN in the pathogenesis of ASH, we induced AS in male Sprague-Dawley rats by feeding EtOH-containing Lieber-DeCarli liquid diet for 6 weeks. AS rats experienced extensive fat accumulation and minimal liver injury. Moderate induction in OPN was observed in AS group. ASH was induced by feeding male Sprague-Dawley rats EtOH-containing Lieber-DeCarli liquid diet for 6 weeks followed by LPS injection. The ASH rats had substantial neutrophil infiltration, coagulative oncotic necrosis, and developed higher liver injury. Significant increases in the hepatic and circulating levels of OPN was observed in the ASH rats. Higher levels of the active, thrombin-cleaved form of OPN in the liver in ASH group correlated remarkably with hepatic neutrophil infiltration. Finally, correlative studies between OPN and hepatic neutrophil infiltration was corroborated in a simple rat peritoneal model where enhanced peritoneal fluid neutrophil infiltration was noted in rats injected OPN intraperitoneally. Taken together these data indicate that OPN expression induced during ASH may play a significant role in the pathogenesis of ASH by stimulating neutrophil transmigration.