Bile acids are essential for the solubilization and transport of dietary lipids and are the major products of cholesterol catabolism. Results presented here show that bile acids are physiological ligands for the farnesoid X receptor (FXR), an orphan nuclear receptor. When bound to bile acids, FXR repressed transcription of the gene encoding cholesterol 7alpha-hydroxylase, which is the rate-limiting enzyme in bile acid synthesis, and activated the gene encoding intestinal bile acid-binding protein, which is a candidate bile acid transporter. These results demonstrate a mechanism by which bile acids transcriptionally regulate their biosynthesis and enterohepatic transport.

Summary The emergence of the novel SARS coronavirus 2 (SARS-CoV-2) in 2019 has triggered an ongoing global pandemic of severe pneumonia-like disease designated as coronavirus disease 2019 (COVID-19) 1 . The development of a vaccine is likely to require at least 12-18 months, and the typical timeline for approval of a novel antiviral therapeutic can exceed 10 years. Thus, repurposing of known drugs could significantly accelerate the deployment of novel therapies for COVID-19. Towards this end, we profiled a library of known drugs encompassing approximately 12,000 clinical-stage or FDA-approved small molecules. We report the identification of 100 molecules that inhibit viral replication, including 21 known drugs that exhibit dose response relationships. Of these, thirteen were found to harbor effective concentrations likely commensurate with achievable therapeutic doses in patients, including the PIKfyve kinase inhibitor apilimod 2 – 4 , and the cysteine protease inhibitors MDL-28170, Z LVG CHN2, VBY-825, and ONO 5334. Notably, MDL-28170, ONO 5334, and apilimod were found to antagonize viral replication in human iPSC-derived pneumocyte-like cells, and the PIKfyve inhibitor also demonstrated antiviral efficacy in a primary human lung explant model. Since most of the molecules identified in this study have already advanced into the clinic, the known pharmacological and human safety profiles of these compounds will enable accelerated preclinical and clinical evaluation of these drugs for the treatment of COVID-19.

The estrogen-related receptors (ERR␣, ERR␤, and ERR␥) form a family of orphan nuclear receptors that share significant amino acid identity with the estrogen receptors, but for which physiologic roles remain largely unknown. By using a peptide sensor assay, we have identified the stilbenes diethylstilbestrol (DES), tamoxifen (TAM), and 4-hydroxytamoxifen (4-OHT) as high-affinity ligands for ERR␥. In direct binding assays, 4-OHT had a Kd value of 35 nM, and both DES and TAM displaced radiolabeled 4-OHT with Ki values of 870 nM. In cell-based assays, 4-OHT binding caused a dissociation of the complex between ERR␥ and the steroid receptor coactivator-1, and led to an inhibition of the constitutive transcriptional activity of ERR␥. ERR␣ did not bind 4-OHT, but replacing a single amino acid predicted to be in the ERR␣ ligand-binding pocket with the corresponding ERR␥ residue allowed high-affinity 4-OHT binding. These results demonstrate the existence of highaffinity ligands for the ERR family of orphan receptors, and identify 4-OHT as a molecule that can regulate the transcriptional activity of ERR␥. N uclear receptors are ligand-activated transcription factors that play critical roles in many aspects of development and adult physiology. Common structural features of nuclear receptors include a central, highly conserved DNA-binding domain (DBD) and a carboxyl-terminal ligand-binding domain (LBD) that contains both a hydrophobic ligand-binding pocket and a transcriptional activation function known as AF-2. Ligand binding induces a conformational change in the LBD, which, in turn, allows the binding of coactivator proteins (1). The binding of coactivators depends on the presence of a short leucine-rich domain with the consensus sequence motif LXXLL (2). Indeed, peptides that contain the LXXLL motif and are as short as eight amino acids can bind to nuclear receptors in a ligand-dependent fashion (3). Many issues concerning the formation and function of nuclear receptor-coactivator complexes remain unresolved, but one emerging theme is that coactivator recruitment brings histone acetyltransferase activity to the transcription complex (4). This activity presumably alters chromatin structure and allows for efficient expression of target genes.The same general concepts of ligand activation and coactivator interaction apply not only to the classical nuclear receptors but also to a growing list of orphan receptors for which natural or synthetic ligands have recently been identified. This list includes the peroxisome proliferator-activated receptors, the liver X receptors, and the farnesoid X receptor, which activate transcription in response to binding fatty acids (5), oxysterols (6, 7), and bile acids (3, 8), respectively. In addition, two other orphan nuclear receptors, the constitutive androstane receptor and the pregnane X receptor, have been shown to regulate the expression of cytochrome-P450 genes in response to a variety of ligands, including the planar hydrocarbon 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), t...