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BackgroundFarnesoid X receptor/retinoid X receptor-alpha (FXR/RXRα) is the master transcriptional regulator of bile salt synthesis and transport in liver and intestine. FXR is activated by bile acids, RXRα by the vitamin A–derivative 9-cis retinoic acid (9cRA). Remarkably, 9cRA inhibits binding of FXR/RXRα to its response element, an inverted repeat-1 (IR-1). Still, most FXR/RXRα target genes are maximally expressed in the presence of both ligands, including the small heterodimer partner (SHP). Here, we revisited the FXR/RXRα-mediated regulation of human SHP.MethodsA 579-bp hSHP promoter element was analyzed to locate FXR/chenodeoxycholic acid (CDCA)- and RXRα/9cRA-responsive elements. hSHP promoter constructs were analyzed in FXR/RXRα-transfected DLD-1, HEK293 and HepG2 cells exposed to CDCA, GW4064 (synthetic FXR ligand) and/or 9cRA. FXR-DNA interactions were analyzed by in vitro pull down assays.Results hSHP promoter elements lacking the previously identified IR-1 (−291/−279) largely maintained their activation by FXR/CDCA, but were unresponsive to 9cRA. FXR-mediated activation of the hSHP promoter was primarily dependent on the −122/−69 region. Pull down assays revealed a direct binding of FXR to the −122/−69 sequence, which was abrogated by site-specific mutations in a binding site for the liver receptor homolog-1 (LRH-1) at −78/−70. These mutations strongly impaired the FXR/CDCA-mediated activation, even in the context of a hSHP promoter containing the IR-1. LRH-1 did not increase FXR/RXRα-mediated activation of hSHP promoter activity.ConclusionFXR/CDCA-activated expression of SHP is primarily mediated through direct binding to an LRH-1 binding site, which is not modulated by LRH-1 and unresponsive to 9cRA. 9cRA-induced expression of SHP requires the IR-1 that overlaps with a direct repeat-2 (DR-2) and DR-4. This establishes for the first time a co-stimulatory, but independent, action of FXR and RXRα agonists.

The farnesoid X receptor/retinoid X receptor-alpha (FXR/RXR␣) complex regulates bile salt homeostasis, in part by modulating transcription of the bile salt export pump (BSEP/ ABCB11) and small heterodimer partner (SHP/NR0B2). FXR is activated by bile salts, RXR␣ by the vitamin A derivative 9-cis retinoic acid (9cRA). Cholestasis is associated with vitamin A malabsorption. Therefore, we evaluated the role of vitamin A/9cRA in the expression of human and mouse bile salt export pump (hBSEP/mBsep), small heterodimer partner (hSHP/mShp), and mouse sodium-dependent taurocholate co-transporting polypeptide (mNtcp). HBSEP and hSHP transcription were analyzed in FXR/RXR␣-transfected HepG2 cells exposed to chenodeoxycholic acid (CDCA) and/or 9cRA.