The orphan nuclear receptors Pregnane X Receptor (PXR) and Constitutive Androstane Receptor (CAR) have been proposed to play an important role in the detoxification of xeno- and endobiotics by regulating the expression of detoxifying enzymes and transporters. We showed that the combined loss of PXR and CAR resulted in a significantly heightened sensitivity to bile acid toxicity in a sex-specific manner. The increased sensitivity in males was associated with genotype-specific suppression of bile acid transporters and loss of bile acid-mediated down regulation of small heterodimer partner, whereas the transporter suppression was modest or absent in the female DKO mice.
The liver X receptors (LXRs), including the alpha and beta isoforms were identified as sterol sensors that regulate cholesterol and lipid homeostasis and macrophage functions. We found that activation of LXRÑ in transgenic mice or with LXR ligands confers a female-specific resistance to lithocholic acid (LCA)-induced hepatotoxicity and bile duct ligation (BDL)-induced cholestasis. In contrast, LXR alpha and beta double knockout mice (LXR DKO) exhibited heightened cholestatic sensitivity. The LCA and BDL resistance in transgenic mice was associated with an increased expression of bile acid detoxifying sulfotransferase 2A (SULT2A) and selected members of the bile acid transporters.
We also showed that genetic or pharmacological activation of the orphan nuclear receptor liver X receptor (LXR) sensitized mice to cholesterol gallstone disease (CGD) induced by a high cholesterol lithogenic diet. LXR-promoted CGD was associated with increased expression of several canalicular transporters that efflux cholesterol and phospholipids, leading to higher biliary concentrations of cholesterol and phospholipids. The biliary bile salt concentration was reduced in these mice, resulting in increased cholesterol saturation index (CSI). Interestingly, the lithogenic effect of LXR was completely abolished in the low-density lipoprotein receptor (LDLR) null background or when the mice were treated with Ezetimibe, a cholesterol-lowering drug that blocks the intestinal dietary cholesterol absorption. We propose that LXRs have evolved to have dual function in maintaining cholesterol and bile acid homeostasis.
| Identifer | oai:union.ndltd.org:PITT/oai:PITTETD:etd-04022007-144214 |
| Date | 05 April 2007 |
| Creators | Uppal, Hirdesh |
| Contributors | Shivendra V. Singh, Allan Z. Zhao, Raman Venkataramanan, Wen Xie, Donald B. DeFranco |
| Publisher | University of Pittsburgh |
| Source Sets | University of Pittsburgh |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.library.pitt.edu/ETD/available/etd-04022007-144214/ |
| Rights | restricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University of Pittsburgh or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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