American Journal of Physiology-Gastrointestinal and Liver Physiology · 1998 · 309 citations · 21 references
Life SciencesBiochemistryMolecular BiologyGene ExpressionIntracellular TransportNucleic Acid BiochemistryBiliary ExcretionEnterohepatic CirculationRenal TransportersTransport In CellsApical Sodium-dependent Bile Acid TransporterBile Acid Transport MechanismsHepatic Bile Acid TransportersIleal Bile Acid UptakeInhibition ConstantsIntestinal Bile Acid TransportersTissue-specific ExpressionTranscriptional Start Sites
The enterohepatic circulation of bile acids is maintained by Na⁺‑dependent transport mechanisms. To better understand these processes, the authors sought to identify the full‑length human ileal Na⁺‑bile acid cotransporter cDNA. They cloned the cDNA using rapid amplification of cDNA ends and genomic cloning techniques. Northern blotting revealed ileal Na⁺‑bile acid cotransporter mRNA in terminal ileum and kidney, and functional assays in transfected cells showed Na⁺‑dependent, chloride‑independent taurocholate uptake with a narrow substrate specificity limited to bile acids, unlike the broader liver transporter.
The enterohepatic circulation of bile acids is maintained by Na + -dependent transport mechanisms. To better understand these processes, a full-length human ileal Na + -bile acid cotransporter cDNA was identified using rapid amplification of cDNA ends and genomic cloning techniques. Using Northern blot analysis to determine its tissue expression, we readily detected the ileal Na + -bile acid cotransporter mRNA in terminal ileum and kidney. Direct cloning and mapping of the transcriptional start sites confirmed that the kidney cDNA was identical to the ileal Na + -bile acid cotransporter. In transiently transfected COS cells, ileal Na + -bile acid cotransporter-mediated taurocholate uptake was strictly Na + dependent and chloride independent. Analysis of the substrate specificity in transfected COS or CHO cells showed that both conjugated and unconjugated bile acids are efficiently transported. When the inhibition constants for other potential substrates such as estrone-3-sulfate were determined, the ileal Na + -bile acid cotransporter exhibited a narrower substrate specificity than the related liver Na + -bile acid cotransporter. Whereas the multispecific liver Na + -bile acid cotransporter may participate in hepatic clearance of organic anion metabolites and xenobiotics, the ileal and renal Na + -bile acid cotransporter retains a narrow specificity for reclamation of bile acids.
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