Remote tissue injury primes hepatocytes for nitric oxide synthesis

Following trauma and tissue injury, patients frequently suffer infections and septic complications. Tissue injury is associated with the induction of the hepatic acute-phase response, but how this phenotypic expression by hepatocytes influences their subsequent response to endotoxin (lipopolysaccharide, LPS) or inflammatory cytokines is unknown. We have shown that both rat and human hepatocytes maximally express the enzyme-inducible nitric oxide synthase (iNOS) in response to a combination of LPS and the cytokines tumor necrosis factor (TNF), interferon-γ (IFN-γ), and interleukin-1. Furthermore, we have shown that the in vivo induction of the acute-phase response following tissue injury (hind limb turpentine injection) is not associated with hepatocyte iNOS expression. In this study, we show that the phenotypic change associated with the acute-phase response following tissue injury primes the hepatocyte to subsequently express iNOS in vitro in response to LPS alone as well as TNF and IFN-γ. This expression of iNOS can be seen as early as 3 hr following the initial injury and lasts up to 24 hr. Early postinjury changes result in maximal expression following stimulation with TNF or IFN-γ. Later (24 hr postinjury) changes reveal LPS to be the most potent inducer with as little as 0.01 μg/ml LPS being required for iNOS mRNA expression. The in vivo correlate of tissue injury (turpentine injection) followed by sepsis (intraperitoneal LPS injection) resulted in a three- to fourfold rise in plasma levels of the stable endproducts of nitric oxide production, nitrite, and nitrate (NO^-₂ + NO^-₃), over levels seen in cases of sepsis alone. These data show that hepatocytes can be primed by remote tissue injury to respond directly to LPS as well as TNF and IFN-γ. Such enhanced responses could have important influences on hepatocellular function following trauma and sepsis.