LXR alpha transactivates mouse organic solute transporter alpha and beta via IR-1 elements shared with FXR.
OBJECTIVE
Recently identified organic solute transporter (Ost) alpha and beta are located on the basolateral membrane of enterocytes and may be responsible for the intestinal absorption of many substrates including bile acids. In the present study, the mechanism governing the transcriptional regulation of their expression was investigated.


RESULTS
To clarify the transcriptional regulation of Osts, reporter gene assays were performed using mouse Ostalpha/beta promoter-luciferase reporter constructs. Co-transfection of the constructs with farnesoid X receptor (FXR) and retinoid X receptor alpha (RXRalpha) or liver X receptor alpha (LXRalpha) and RXRalpha into Caco-2 cells induced the transcriptional activities of both Ost alpha and beta and further increases were observed following treatment with each agonist. Sequence analyses indicated the presence of IR-1 regions in Ostalpha and Ostbeta promoters, which was confirmed by the finding that the deletion of IR-1 sequences abolished the response to FXR and LXRalpha. Furthermore, mutations in IR-1 reduced the FXR- and LXRalpha-dependent transactivation of Ostalpha/beta. Together with the detection of direct binding of FXR/RXRalpha and LXRalpha/RXRalpha to the IR-1 elements, the presence of functional FXRE/LXRE was revealed in the promoter region of both Ostalpha and Ostbeta. In addition, the stimulatory effect of FXR/RXRalpha and LXRalpha/RXRalpha on Ostalpha, but not on Ostbeta, was further enhanced by HNF-4alpha.


CONCLUSIONS
It was concluded that LXRalpha/RXRalpha transcriptionally regulate mouse Ostalpha/beta via IR-1 elements shared with FXR/RXRalpha. Exposure to FXR/LXRalpha modulators may affect the disposition of Ostalpha/beta substrates.
Okuwaki
Masae
M
Department of Pharmacy, The University of Tokyo Hospital, Faculty of Medicine, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-8655, Japan.
Takada
Tappei
T
Iwayanagi
Yuki
Y
Koh
Saori
S
Kariya
Yoshiaki
Y
Fujii
Hiroshi
H
Suzuki
Hiroshi
H
D024506:5' Flanking Region / Q000235:genetics; D000595:Amino Acid Sequence; D000818:Animals; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000096:biosynthesis / Q000235:genetics; D004268:DNA-Binding Proteins / Q000235:genetics*; D024202:Electrophoretic Mobility Shift Assay; D017930:Genes, Reporter / Q000235:genetics; D005822:Genetic Vectors / Q000235:genetics; D051557:Hepatocyte Nuclear Factor 4 / Q000235:genetics; D000071518:Liver X Receptors; D008156:Luciferases / Q000235:genetics; D026901:Membrane Transport Proteins / Q000737:chemistry*; D051379:Mice; D008969:Molecular Sequence Data; D009154:Mutation / Q000502:physiology; D057093:Orphan Nuclear Receptors; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics*; D012091:Repetitive Sequences, Nucleic Acid / Q000235:genetics*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014157:Transcription Factors / Q000235:genetics*; D015533:Transcriptional Activation / Q000235:genetics*; D000099150:Receptor, Farnesoid X-Activated
D018076:DNA, Complementary; D004268:DNA-Binding Proteins; D051557:Hepatocyte Nuclear Factor 4; D000071518:Liver X Receptors; D008156:Luciferases; D026901:Membrane Transport Proteins; D057093:Orphan Nuclear Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D014157:Transcription Factors; D000099150:Receptor, Farnesoid X-Activated; C000605214:Nr1h3 protein, mouse; C476576:organic solute transporter alpha, mouse; C476578:organic solute transporter beta, mouse