Involvement of corepressor complex subunit GPS2 in transcriptional pathways governing human bile acid biosynthesis

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Abstract
Coordinated regulation of bile acid biosynthesis, the predominant pathway for hepatic cholesterol catabolism, is mediated by few key nuclear receptors including the orphan receptors liver receptor homolog 1 (LRH-1), hepatocyte nuclear factor 4alpha (HNF4alpha), small heterodimer partner (SHP), and the bile acid receptor FXR (farnesoid X receptor). Activation of FXR initiates a feedback regulatory loop via induction of SHP, which suppresses LRH-1- and HNF4alpha-dependent expression of cholesterol 7alpha hydroxylase (CYP7A1) and sterol 12alpha hydroxylase (CYP8B1), the two major pathway enzymes. Here we dissect the transcriptional network governing bile acid biosynthesis in human liver by identifying GPS2, a stoichiometric subunit of a conserved corepressor complex, as a differential coregulator of CYP7A1 and CYP8B1 expression. Direct interactions of GPS2 with SHP, LRH-1, HNF4alpha, and FXR indicate alternative coregulator recruitment strategies to cause differential transcriptional outcomes. In addition, species-specific differences in the regulation of bile acid biosynthesis were uncovered by identifying human CYP8B1 as a direct FXR target gene, which has implications for therapeutic approaches in bile acid-related human disorders.