title: Key structural features of ligands for activation of human pregnane X receptor.
abstract: The ligand-binding domain of human pregnane X receptor (hPXR) is highly hydrophobic and flexible, allowing promiscuity in accepting structurally diverse ligands. However, little information is available regarding the critical substituents of compounds involved in the activation of hPXR. The aim of this study was to determine the structure-activity relationships for hPXR-mediated transactivation by barbiturates, hydantoins, and macrolide antibiotics. Most of the barbiturates studied (mephobarbital, pentobarbital, phenobarbital, etc.) activated hPXR. However, barbital, which has a low hydrophobic moiety at the 5-position, and primidone, which has no carbonyl moiety at the 2-position, did not activate hPXR. Therefore, a hydrophobic moiety at the 5-position and a hydrogen-bond acceptor being sufficiently separated from the phenyl-ring are responsible for activation of hPXR by barbiturates. In the case of hydantoins, only mephenytoin and ethotoin, which have an alkylchain at the R1-position, strongly activated hPXR at 300 microM. Phenytoin and 5-(4-methylphenyl)-5-phenylhydantoin, which contain a phenyl or methylphenyl group at both R2- and R3-positions, also activated hPXR, whereas 5-(4-hydroxyphenyl)-5-phenylhydantoin did not activate the receptor. These results suggest that the presence of an alkyl-chain at the R1-position and the presence of bulky and hydrophobic moieties at both R2- and R3-positions are important factors for activation of hPXR by hydantoins. In the case of macrolide antibiotics, troleandomycin, but not oleandomycin, showed significant activation of hPXR. Therefore, triacetate esterification of oleandomycin might increase the hydrophobicity and enhance the activation of hPXR. These findings suggest that hydrophobicity of the ligand and adequate distance between the hydrogen-bond acceptor and the hydrophobic group are important for hPXR activation.
authors.affiliation: Laboratory of Pharmacology and Toxicology, Graduate School of Pharmaceutical Sciences, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan. kaoruk@p.chiba-u.ac.jp
authors.forename: Kaoru
authors.initials: K
authors.lastname: Kobayashi
authors.forename: Saeko
authors.initials: S
authors.lastname: Yamagami
authors.forename: Tomoaki
authors.initials: T
authors.lastname: Higuchi
authors.forename: Masakiyo
authors.initials: M
authors.lastname: Hosokawa
authors.forename: Kan
authors.initials: K
authors.lastname: Chiba
mesh_terms: D000900:Anti-Bacterial Agents / Q000737:chemistry / Q000494:pharmacology; D001463:Barbiturates / Q000737:chemistry / Q000145:classification / Q000494:pharmacology; D045744:Cell Line, Tumor; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D006801:Humans; D006827:Hydantoins / Q000737:chemistry / Q000145:classification / Q000494:pharmacology; D008024:Ligands; D008156:Luciferases / Q000235:genetics / Q000378:metabolism; D018942:Macrolides / Q000737:chemistry / Q000145:classification / Q000494:pharmacology; D008956:Models, Chemical; D008968:Molecular Conformation; D015394:Molecular Structure; D000077297:Pregnane X Receptor; D012333:RNA, Messenger; D018160:Receptors, Cytoplasmic and Nuclear / Q000187:drug effects* / Q000378:metabolism*; D011987:Receptors, Steroid / Q000187:drug effects* / Q000378:metabolism*; D047488:Retinoid X Receptors / Q000187:drug effects / Q000235:genetics; D012293:Rifampin / Q000494:pharmacology; D013329:Structure-Activity Relationship
chemical_list: D000900:Anti-Bacterial Agents; D001463:Barbiturates; D006827:Hydantoins; D008024:Ligands; D018942:Macrolides; D000077297:Pregnane X Receptor; D012333:RNA, Messenger; D018160:Receptors, Cytoplasmic and Nuclear; D011987:Receptors, Steroid; D047488:Retinoid X Receptors; D003577:Cytochrome P-450 Enzyme System; D008156:Luciferases; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; D012293:Rifampin