Lipopolysaccharide (LPS or endotoxin) from gram-negative bacteria produces a multitude of adverse biological effects in humans and experimental animals. Exposure to LPS occurs normally in humans because of leakage of bacterial products from the gastrointestinal tract into the portal circulation; however, under some conditions, this translocation increases, subjecting the liver to exposure to greater concentrations of LPS. This exposure to LPS can influence toxic responses to xenobiotic agents. For example, the investigator's recent studies demonstrate that small doses of LPS markedly enhance the hepatotoxicity of allyl alcohol (AA) and of monocrotaline (MCT), and similar potentiation of toxicity by LPS has been documented for other hepatotoxicants. Thus, the amount of LPS to which the liver is exposed may be a determinant of the magnitude of response to chemical insult. The overall goal of this application is to explore mechanisms by which LPS influences chemical-induced hepatotoxicity, and initial studies will focus on AA as a model periportal hepatotoxicant. Some studies will also be performed with MCT, a centrilobular hepatotoxicant. Recent results indicate that Kupffer cells (KCs) play a critical role in enhancement of AA-induced injury by LPS. LPS activates KCs to release a number of soluble mediators, including prostaglandin D2 (PGD2), and preliminary results indicate that PGD2 increases the cytotoxicity of AA toward hepatic parenchymal cells (HCs) in vitro. The hypothesis to be tested is that PGD2, released by KCs stimulated with LPS, increases the susceptibility of HCs to subsequent insult by hepatotoxicants such as AA and MCT. A combination of in vivo and in vitro approaches will be used in these studies. Studies in Aim 1 will test whether exposure of liver to PGD2 increases the hepatotoxicity response to AA or MCT by addressing the following hypotheses: a) exposure of HCs to PGD2 increases AA and MCT cytotoxicity, b) LPS-activated KCs increases cytotoxicity in HCs by a mechanism dependent on PGD2, c) inhibition of PGD2 synthesis in vivo attenuates LPS-induced enhancement of hepatotoxicity, and d) administration of PGD2 into the hepatic circulation increases hepatotoxicity. Studies in Aim 2 will reveal mechanisms by which PGD2 increases sensitivity of HCs to toxicity of AA or MCT by testing whether any of the following contribute to LPS enhancement of hepatotoxicity: a) PGD2-induced alterations in energy production, b) PGD2-induced inhibition of protein synthesis, and/or c) a PGD2 induced increase in intracellular calcium concentration. Results of these experiments will increase the understanding of how KC-derived inflammatory mediators affect the liver during exposure to hepatotoxicants. Furthermore, they will begin to reveal the mechanisms by which small doses of LPS determine sensitivity to chemical insult.
