The liver X receptors (LXRs), including the 1 and 2 isoforms, are highly expressed in the liver and can be activated by natural and synthetic ligands. Previous studies on LXRs have been focused on their role in cholesterol and lipid homeostasis and inflammation. Whether or not LXRs play a role in hepatoprotection against endo- and xenobiotic toxicants has not been fully explored. Acetaminophen (APAP) or Tylenol, an over-the-counter medication, is a xenobiotic toxicant whose overdose is the leading cause of acute liver failure. Our preliminary results showed that transgenic mice expressing the activated LXR1 (VP-LXR1) in the liver had an increased expression of APAP detoxifying enzymes, including the sulfotransferase SULT2A9 and glutathione S-transferases (GSTs). Promoter analysis suggested that SULT2A9, GST M1, and MRP2 are transcriptional targets of LXR. We also showed that the VP-LXR1 transgenic mice were more resistant to APAP hepatotoxicity. The APAP-detoxifying transporter MRP4 is also likely under the positive control of LXR. We hypothesize that LXRs have a hepatoprotective role by transcriptional activation of APAP- detoxifying enzymes and/or transporters. A testable prediction is that activation of LXRs will alleviate hepatotoxicity caused by APAP. Since inflammatory responses are involved in APAP toxicity, we anticipate that the anti-inflammatory function of LXR may also contribute to the hepatoprotective role of LXRs. By using the LXR transgenic, knockout and ligand-treated wild type mice, we propose: (1) To create LXR knock-in mice that bear the expression of constitutively activated LXR1 and LXR2. (2) To determine whether the activation of LXRs alleviates APAP hepatotoxicity. (3) To determine the molecular basis by which LXRs regulate the rodent and human APAP-detoxifying glutathione S-transferases (GSTs). These studies are expected to establish a novel function for LXRs in protecting xenobiotic and endobiotic chemical insults. The APAP protective role of LXR would be opposite to the sensitizing effects that have been reported for xenobiotic receptors pregnane X receptor (PXR) and constitutive androstane receptor (CAR). It is hoped that the development of selective LXR agonists may represent a novel strategy for the prevention and treatment of APAP-induced hepatotoxicity.
The liver X receptors (LXRs) are transcriptional factors highly expressed in the liver. The goal of this study is to determine whether LXRs play a hepatoprotective role in preventing toxicity from acetaminophen (APAP), whose overdose is the most common cause of clinical drug-related liver failures. It is hoped that the development of selective LXR agonists may represent a novel strategy for the prevention and treatment of APAP-induced liver toxicity.
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