| Kupffer cells play an important role in the progression of acute and chronic chemically-mediated liver injury. Upon exposure to a hepatotoxin, this cell produces and releases various biologically active mediators, such as reactive oxygen species, cytokines, and eicosanoids. These mediators, in turn, propagate various events in the liver that promote liver injury including inflammation and fibrosis. The role of Kupffer cells during liver injury has most notably been recognized during administration of hepatotoxins that produce oxidative stress-initiated membrane lipid peroxidation. Therefore, studies were conducted to determine the role of Kupffer cells in vivo and in vitro, during chemically-mediated lipid peroxidation. Initial studies evaluated the biochemical and molecular changes in Kupffer cells during acute and chronic liver injury due to administration of carbon tetrachloride (CCl4). Induction of pro-inflammatory and profibrogenic cytokine gene expression in Kupffer cells indicate CCl4 administered to rats results in activation of Kupffer cells during the early stages of liver injury. Additionally, reduction in glutathione levels and increased mRNA levels of IκBα, cyclooxygenase-2, CD14, and other metabolic enzymes correlates with increases in liver inflammation and fibrosis. Since activation of these cells occurs during liver injury involving lipid peroxidation, additional studies investigated the cellular metabolism and quantitated the metabolic products of a major aldehydic product of lipid peroxidation, 4-hydroxynonenal (4-HNE), in isolated Kupffer cells. Metabolism of 4-HNE was demonstrated to occur primarily through conjugation and oxidation by the glutathione- S-transferase (GST) and aldehyde dehydrogenase (ALDH) enzymatic systems, respectively. The metabolism of this aldehyde was incomplete and coincided with the depletion of cellular glutathione suggesting that Kupffer cells may be susceptible to increases in lipid peroxidation. Consistent with this proposal, additional investigations established that low concentrations of 4-HNE inhibited interleukin-6 (IL-6) gene expression and the associated protein production. This inhibition was demonstrated to occur at the transcriptional level as NF-κB activity was prevented by exposure to 4-HNE. Finally, 4-HNE treatment inhibited IκBα, phosphorylation correlating with increased cellular levels of IκBα. Collectively, these studies establish the role of Kupffer cells during liver injury and demonstrate that specific Kupffer bioactivities are affected by low levels of 4-HNE possibly due to their low capacity to metabolize this aldehyde. |
