PXR (NR1I2): splice variants in human tissues, including brain, and identification of neurosteroids and nicotine as PXR activators.
To gain insight on the expression of pregnane X receptor (PXR), we analyzed PXR.1 and PXR alternatively spliced transcripts in a panel of 36 human tissues. PXR.1 was expressed in many more tissues than previously determined, including human bone marrow and select regions of the human brain. In each of these tissues, we observed alternative splicing of various exons of PXR that generated multiple distinct PXR isoforms. The most abundant PXR alternative mRNA transcripts lacked 111 nucleotides, deleting 37 amino acids from the PXR LBD (PXR.2), or lacked 123 nt, deleting 41 amino acids from the PXR LBD (PXR.3). CYP3A4, a gene transcriptionally regulated by PXR, showed incomplete overlap with PXR in its tissue distribution. Quantitation of PXR mRNAs in human liver demonstrated that PXR.2 and PXR.3 represented 6.7% and 0.32% of total PXR mRNA transcripts. Brain expression of PXR prompted analysis of whether some brain acting chemicals were PXR ligands. The neurosteroids allopregnanolone and pregnanolone activated PXR and induced transcription of a CYP3A4-luciferase reporter. Nicotine, the psychoactive and addictive chemical in cigarettes, and a known inducer of brain CYP2B6, was an efficacious activator of PXR and inducer of CYP3A4 transcription. Because nicotine activation of PXR will enhance metabolism of nicotine to the non-psychoactive cotinine, these results provide one molecular mechanism for the development of tolerance to nicotine. Moreover, the identification of PXR in many human tissues, such as brain, and activation by tissue specific ligands (such as neurosteroids) suggests additional biological roles for this receptor in these tissues.
Lamba
Vishal
V
Department of Pharmaceutical Sciences, St. Jude Children's Research Hospital, Memphis, TN 38105, USA.
Yasuda
Kazuto
K
Lamba
Jatinder K
JK
Assem
Mahfoud
M
Davila
Julio
J
Strom
Stephen
S
Schuetz
Erin G
EG
D000328:Adult; D000368:Aged; D017398:Alternative Splicing / Q000235:genetics*; D000595:Amino Acid Sequence; D000818:Animals; D005075:Biological Evolution; D001923:Brain Chemistry / Q000235:genetics*; D017124:Conserved Sequence; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D017931:DNA Primers; D004285:Dogs; D005260:Female; D005399:Fishes; D006801:Humans; D007536:Isomerism; D008099:Liver / Q000378:metabolism; D008253:Macaca mulatta; D008297:Male; D051379:Mice; D008875:Middle Aged; D008969:Molecular Sequence Data; D018377:Neurotransmitter Agents / Q000494:pharmacology*; D009538:Nicotine / Q000494:pharmacology*; D018722:Nicotinic Agonists / Q000494:pharmacology*; D010957:Plasmids / Q000235:genetics; D000077297:Pregnane X Receptor; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D011817:Rabbits; D051381:Rats; D018160:Receptors, Cytoplasmic and Nuclear / Q000819:agonists* / Q000235:genetics*; D011987:Receptors, Steroid / Q000819:agonists* / Q000235:genetics*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012907:Smoking / Q000378:metabolism; D013256:Steroids / Q000494:pharmacology*; D013552:Swine; D014018:Tissue Distribution; D014162:Transfection
D017931:DNA Primers; C000625852:NR1I2 protein, human; D018377:Neurotransmitter Agents; D018722:Nicotinic Agonists; C000625853:Nr1i2 protein, mouse; C000625854:Nr1i2 protein, rat; D000077297:Pregnane X Receptor; D012333:RNA, Messenger; D018160:Receptors, Cytoplasmic and Nuclear; D011987:Receptors, Steroid; D013256:Steroids; D009538:Nicotine; D003577:Cytochrome P-450 Enzyme System; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human