Expression and transport properties of the human ileal and renal sodium-dependent bile acid transporter

Abstract

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.

The solute carrier family SLC10: more than a family of bile acid transporters regarding function and phylogenetic relationships

“…Further differences between the substrate specificities of NTCP and ASBT exist for sulfated bile acids, which are structurally very similar to the sulfated sex steroids. Chenodeoxycholate-3-sulfate is not transported by ASBT, but is a substrate of NTCP (Craddock et al 1998). Similarly, taurolithocholate-3-sulfate had a much higher inhibitory potency to rabbit Ntcp (IC 50 =0.8 μM) than to rabbit Asbt (IC 50 =9.1 μM) .…”

“…taurochenodeoxycholate, taurodeoxycholate) were normally higher than for the trihydroxy bile acids (i.e. cholate, taurocholate and glycocholate) (Craddock et al 1998;Schroeder et al 1998;Kramer et al 1999;KullakUblick et al 2000a;Hata et al 2003). Exceptionally, ASBT has a lower affinity for the dihydroxy bile acid tauroursodeoxycholate than for taurocholate (Craddock et al 1998).…”

“…However, sequence identity to the NTCP is relatively low, at 35% (see Table 1). Similar to NTCP/Ntcp, ASBT/Asbt transports conjugated bile acids with high affinity in a sodium-dependent manner (Wong et al 1994Craddock et al 1998). This transport is electrogenic and shows a 2:1 Na + /bile acid coupling stoichiometry (Weinman et al 1998).…”

“…cholate, taurocholate and glycocholate) (Craddock et al 1998;Schroeder et al 1998;Kramer et al 1999;KullakUblick et al 2000a;Hata et al 2003). Exceptionally, ASBT has a lower affinity for the dihydroxy bile acid tauroursodeoxycholate than for taurocholate (Craddock et al 1998). Due to their transport characteristics and expression pattern, NTCP and ASBT are essentially involved in the maintenance of the enterohepatic circulation of bile acids mediating the first step in the cellular uptake of bile acids through the membrane barriers in the liver (NTCP) and intestine (ASBT).…”

“…NTCP/Ntcp also transports steroid sulfates such as oestrone-3-sulfate and DHEAS, bromosulfophthalein and the drug-conjugate chlorambuciltaurocholate (Kullak-Ublick et al 1997;Craddock et al 1998;Schroeder et al 1998;Kullak-Ublick et al 2000b;Hata et al 2003) (see Table 2). However, in comparison with the transport of bile acids, Ntcp-mediated uptake of steroid sulfates is relatively low Schroeder et al 1998;Kullak-Ublick et al 2000b).…”