The factors that cause normally harmless episodes of inflammation to culminate in disease are largely unknown but likely include genetic and environmental influences. Exposure to environmental chemicals such as aryl hydrocarbon receptor (AhR) ligands could contribute to individual susceptibility to disease by enhancing inflammatory responses or by providing a stimulus that precipitates injury in the presence of inflammation. Indeed, in preliminary studies, mice developed liver injury when coexposed to TCDD (2,3,7,8- tetrachlorodibenzo-p-dioxin) and the inflammatory stimulus, lipopolysaccharide (LPS), each at doses that alone did not cause liver damage. The goal of the proposed research is to test the hypothesis that Ah receptor ligands enhance inflammatory responses that can lead to tissue injury. Consistent with the overall goal of the Program Project, the tissue of interest for this project is liver. This hypothesis will be tested by first evaluating the development of LPS-induced inflammation in mice exposed to AhR ligands and by delineating the dose-response relationship for AhR ligand-induced liver injury in the absence and presence of inflammation. The AhR ligands to be used are TCDD and 3,3',4,4',5-pentachlorobiphenyl (PCB 126). Mice will be treated with AhR ligands alone or in combination at doses that are not hepatotoxic. They will then be treated with a small dose of LPS that induces inflammation but alone does not cause liver injury. Markers of inflammation and liver injury will be assessed and time-courses and dose-response relationships established. Global changes in hepatic gene expression and DMA methylation in these dose-response and time-course studies will be determined by microarray analysis and will be compared to biochemical and histological changes in liver as well as to markers of inflammation. Both neutrophils and the hemostatic system, including plasminogen activator inhibitor-1 (PAI-1), are critical to liver injury in many models of chemical-inflammation interactions. To begin to understand inflammatory mediators that contribute to liver injury during AhR-inflammation interactions, the roles of neutrophils and components of the hemostatic system will be investigated in mice exposed to TCDD in the presence of inflammation. Tumor necrosis factor alpha (TNF) is a proinflammatory cytokine that is reported to mediate LPS-induced increases in expression of PAI-1 in vivo. In preliminary studies treatment of mice with either TCDD or LPS increased serum concentration of PAI-1 expression, but the combination had a synergistic effect. Accordingly, a computational description of the biochemical pathways by which AhR ligands induce changes in expression of PAI-1 in the absence and presence of inflammation will be developed. The results of these studies collectively will provide information about consequences of the interactions between inflammatory responses and AhR ligands and about molecular mechanisms involved in this interaction.
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