Role of an mSin3A-Swi/Snf Chromatin Remodeling Complex in the Feedback Repression of Bile Acid Biosynthesis by SHP

Abstract

The orphan receptor SHP interacts with many nuclear receptors and inhibits their transcriptional activities. SHP is central to feedback repression of cholesterol 7α hydroxylase gene (CYP7A1) expression by bile acids, which is critical for maintaining cholesterol homeostasis. Using CYP7A1 as a model system, we studied the molecular mechanisms of SHP repression at the level of native chromatin. Chromatin immunoprecipitation studies showed that mSin3A and a Swi/Snf complex containing Brm as a central ATPase were recruited to the promoter. This recruitment was associated with chromatin remodeling after bile acid treatment that was blunted by inhibition of the endogenous Swi/Snf function by dominant-negative ATPase mutants. Biochemical studies indicated that SHP was associated with the mSin3A-Swi/Snf complex by direct interaction with Brm and mSin3A through its repression domain. Expression of Brm, but not an ATPase mutant, inhibited CYP7A1 promoter activity and further enhanced SHP-mediated repression. Bile acid-induced recruitment of mSin3A/Brm, chromatin remodeling, and concomitant repression of endogenous CYP7A1 expression were impaired when SHP expression was inhibited by SHP small interfering RNA. Our results suggest that SHP mediates recruitment of mSin3A-Swi/Snf to the CYP7A1 promoter, resulting in chromatin remodeling and gene repression, which may also be a mechanism for the repression by SHP of genes activated by many nuclear receptors. Our study establishes the first link between a Swi/Snf complex and regulation of cholesterol metabolism.

We thank W. Wang, R. Kingston, A. Imbalzano, S. Sif, J. Chiang, J. Auwerx, and D. Moore for providing valuable materials, including antibodies, a stable cell line, and plasmids for the study. We are grateful to S. Wu and C. Chiang from Case Western Reserve University for their advice on protein purification. We thank S. Okino and D. Burakov for technical advice on the LMPCR and ChIP assays, respectively. We also thank A. Nardulli and B. Kemper for helpful comments on the manuscript.

This study was supported by AHA grant 0130538Z and NIH DK062777 to J.K.K.