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* Laboratory of Metabolism, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892
University of Texas Southwestern Medical Center, Dallas, TX 75390
Published, JLR Papers in Press, August 24, 2007.
1 Present address of G. L. Guo: University of Kansas Medical Center, Kansas City, KS 66160.
2 To whom correspondence should be addressed. e-mail: fjgonz{at}helix.nih.gov
Bile acid concentrations are controlled by a feedback regulatory pathway whereby activation of the farnesoid X receptor (FXR) represses transcription of both the CYP7A1 gene, encoding the rate-limiting enzyme in the classic bile acid synthesis pathway, and the CYP8B1 gene, required for synthesis of cholic acid. The tissue-specific roles of FXR were examined using liver- and intestine-specific FXR-null models. FXR deficiency in either liver (Fxr
L) or intestine (Fxr
IE) increased bile acid pool size. Treatment with the FXR-selective agonist GW4064 significantly repressed CYP7A1 in Fxr
L mice but not Fxr
IE mice, demonstrating that activation of FXR in intestine but not liver is required for short-term repression of CYP7A1 in liver. This intestinal-specific effect of FXR is likely mediated through induction of the hormone FGF15, which suppresses CYP7A1. In comparison to CYP7A1, FXR-mediated repression of CYP8B1 was more dependent on the presence of FXR in liver and less dependent on its presence in intestine. Consistent with these findings, recombinant FGF15 repressed CYP7A1 mRNA levels without affecting CYP8B1 expression. These data provide evidence that FXR-mediated repression of bile acid synthesis requires the complementary actions of FXR in both liver and intestine and reveal mechanistic differences in feedback repression of CYP7A1 and CYP8B1.
Supplementary key words FXR FGF15 CYP7A1 CYP8B1 liver-specific FXR null mice intestine-specific FXR null mice
Abbreviations: BW, body weight; CA, cholic acid; DCA, deoxycholic acid; FXR, farnesoid X receptor; LC-MS/MS, liquid chromatography tandem mass spectrometry; MCA, murocholic acid; RT-qPCR, real-time quantitative PCR; TCA, taurocholic acid; TDCA, taurodeoxycholic acid
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