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Submitted on December 18, 2007
Accepted on February 25, 2008
Departments of Molecular Biology, Pharmacology, and Biochemistry, and the Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, Texas 75390; and GlaxoSmithKline Research and Development, Research Triangle Park, North Carolina 27709
* To whom correspondence should be addressed. E-mail: steven.kliewer{at}utsouthwestern.edu.
Liver receptor homolog 1 (LRH-1), an orphan nuclear receptor, is highly expressed in liver and intestine, where it is implicated in the regulation of cholesterol, bile acid, and steroid hormone homeostasis. Among the proposed LRH-1 target genes in liver are those encoding cholesterol 7
-hydroxylase (CYP7A1) and sterol 12
-hydroxylase (CYP8B1), which catalyze key steps in bile acid synthesis. In vitro studies suggest that LRH-1 may be involved both in stimulating basal CYP7A1 and CYP8B1 transcription and in repressing their expression as part of the nuclear bile acid receptor (FXR)-small heterodimer partner (SHP) signaling cascade, which culminates in SHP binding to LRH-1 to repress gene transcription. However, in vivo analysis of LRH-1 actions has been hampered by the embryonic lethality of Lrh-1 knockout mice. To overcome this obstacle, mice were generated in which Lrh-1 was selectively disrupted in either hepatocytes or intestinal epithelium. LRH-1 deficiency in either tissue changed mRNA levels of genes involved in cholesterol and bile acid homeostasis. Surprisingly, LRH-1 deficiency in hepatocytes had no significant effect on basal Cyp7a1 expression or its repression by FXR. While Cyp8b1 repression by FXR was also intact in mice deficient for LRH-1 in hepatocytes, basal CYP8B1 mRNA levels were significantly decreased, and there were corresponding changes in the composition of the bile acid pool. Taken together, these data reveal a broad role for LRH-1 in regulating bile acid homeostasis but demonstrate that LRH-1 is either not involved in the feedback regulation of bile acid synthesis or is compensated for by other factors.
-hydroxylase
sterol 12
-hydroxylase
knockout mice
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