The solute carrier family 10 (SLC10): beyond bile acid transport

Abstract

The solute carrier (SLC) family 10 (SLC10) comprises influx transporters of bile acids, steroidal hormones, various drugs, and several other substrates. Because the seminal transporters of this family, namely, sodium/taurocholate cotransporting polypeptide (NTCP; SLC10A1) and the apical sodium-dependent bile acid transporter (ASBT; SLC10A2), were primarily bile acid transporters, the term "sodium bile salt cotransporting family" was used for the SLC10 family. However, this notion became obsolete with the finding of other SLC10 members that do not transport bile acids. For example, the sodium-dependent organic anion transporter (SOAT; SLC10A6) transports primarily sulfated steroids. Moreover, NTCP was shown to also transport steroids and xenobiotics, including HMG-CoA inhibitors (statins). The SLC10 family contains four additional members, namely, P3 (SLC10A3; SLC10A3), P4 (SLC10A4; SLC10A4), P5 (SLC10A5; SLC10A5) and SLC10A7 (SLC10A7), several of which were unknown or considered hypothetical until approximately a decade ago. While their substrate specificity remains undetermined, great progress has been made towards their characterization in recent years. Explicitly, SLC10A4 may participate in vesicular storage or exocytosis of neurotransmitters or mastocyte mediators, whereas SLC10A5 and SLC10A7 may be involved in solute transport and SLC10A3 may have a role as a housekeeping protein. Finally, the newly found role of bile acids in glucose and energy homeostasis, via the TGR5 receptor, sheds new light on the clinical relevance of ASBT and NTCP. The present mini-review provides a brief summary of recent progress on members of the SLC10 family.

Figure 1. Schematic tissue distribution and activity of established members SLC10 family

Tissues involved in bile acid (BA) or sulfated (S⁻) substrates are highlighted. Top right square: Sodium-dependent transport of sulfated substrates (S⁻) as well as tissues with high SOAT expression in humans. Efflux transporters of SOAT substrates are not fully elucidated but may include the breast cancer resistant protein (BCRP, ABCG2) in the placenta, for efflux of sulfated estrogens from the fetus. Top left square: Bile acids are synthesized and conjugated to taurine or glycine in the liver and are actively effluxed from the liver into bile by the canalicular ATP-binding cassete (ABC) transporters BSEP, MRP2 and possibly, MDR1; they travel down the biliary tract (cholangiocytes) and are stored in the gallbladder. In response to meal ingestion, BA are secreted into the duodenum and travel down the intestine, where they are absorbed passively until they reach the distal ileum and are actively reclaimed by ASBT in the brush border membrane of ileal enterocytes (ileocytes). BA that escape ASBT absorption reach the colon, where they are modified by enterobacteria and are absorbed passively into the portal circulation (not shown). In the ileocytes (bottom left square), BA are shuttled to the basolateral membrane by the cytosolic ILBP, and are effluxed into the portal blood by OST/and MRP3. Because of the high electrochemical gradient generated by BA influx, here the facilitative transporter OST/functions as a BA efflux transporter, although it can function bi-directionally. Next, BA reach the liver through the portal circulation and are taken up, both by sodium-dependent NTCP, as well as via sodium-independent OATP1B1 and OATP1B3, in the sinusoidal membrane of hepatocytes. A fraction of BA that escape canalicular efflux spill over into the systemic circulation assisted by the sinusoidal efflux transporters MRP3, MRP4 and OST/. From the systemic circulation, BA reach tissues where they act as signaling molecules, e.g., by binding and activating specific receptors such as nuclear receptors and the membrane-bound TGR5 receptor. Systemically circulating BA also reach the kidney (bottom right square), undergo glomerular filtration, are reclaimed by ASBT in the apical membrane of proximal tubule cells, and are effluxed back into the systemic circulation by the basolaterally-expressed MRP3 and OST/. They return to the liver where, again, canalicular BA efflux in the hepatocytes direct BA to travel down the bile ducts and reach the gallbladder. In the bile ducts, a small fraction of BA undergo cholehepatic shunt, i.e., are absorbed actively - putatively through ASBT - or passively into cholangiocytes (center left square) and return to the liver through the periductular capillary plexus. Efflux from cholangiocytes is mediated by MRP3, MRP4 and OST/. Additional transporters, e.g., MRP2 in hepatocytes and MRP2/3 in enterocytes may possibly be involved in efflux of modified BA, such as sulfates and glucuronides. NTCP expression in the luminal membrane of rat pancreatic acinar cells (center right square) suggests another possible mechanism of BA salvage, whereby BA that reach the lumen through OATP1 uptake and MRP3 and MDR1 efflux, or by spillage from the bile ducts into the terminal acini, are reclaimed by NTCP.

Figure 2. Secondary structure of hASBT and 3ZUY (ASBT(NM)), highlighting primary stucture similarities and differences between hASBT and mammalian ASBT species, as well as between proteins reported by Hu and colleagues (Hu et al., 2011)

Transmembrane (TM) domains, extracellular (EL), intracellular (IL), N- and C-terminal loops are indicated. Amino acids putatively relevant for transport are indicated by arrows, with full arrows pointing directly to the amino acid, whereas dotted arrows point to regions in the proximity to the amino acid. Amino acid sequences employed to build secondary structure representations were retrieved from public databases, namely, from the National Center for Biotechnology Information (NCBI): human ASBT (RefSeq ID: NP_000443.1); from the Universal Protein Resource Knowledgebase: Neisseria meningitidis serogroup B strain MC58 (UniProtKB/Swiss-Prot ID: Q9K0A9); from the Worldwide Protein Data Bank: 3ZUY (PDB ID: 3ZUY_A) and 3ZUX (PDB ID: 3ZUX_A). Primary sequence alignments were performed in ClustalW NPS@: Network Protein Sequence Analysis (Combet et al., 2000). A, hASBT topology highlighting its 7 TMs, N_exo/C_cyt orientation, glycosylation site (Y) and amino acids reputedly relevant for transport of sodium (N27, L38, M46, D122, F287, F278 and E261) and bile acid (N2, D124, P142, E282, L283, P234, G237, G241, and the TM7 hydrophilic cleft formed by F287 – Q297). Amino acids for which topology was confirmed by N-glycosylation (★) and dual label epitope insertion mutagenesis ( ) are highlighted. Alignment of hASBT with other mammalian species, namely, orangutan, dog, wild boar, mouse, rat, hamster and rabbit were compared and are represented here as one letter amino acid codes, enclosed in black circles for identical, gray circles for highly similar and white circles for nonconserved residues. These sequences were also retrieved from NCBI, under the RefSeq ID’s: orangutan (Pongo abelii; NP_001125080.1), dog (Canis lupus familiaris; NP_001002968.1), wild boar (Sus scrofa; NP_001231392.1), mouse (Mus musculus; NP_035518.1), rat (Rattus norvegicus; NP_058918.1), hamster (Cricetulus griseus; NP_001233749.1) and rabbit (Oryctolagus cuniculus; NP_001076233.1). B, Secondary structure of 3ZUY (derived from (Hu et al., 2011) supplementary Fig. 5), highlighting its 10 TMs, N_cyt/C_cyt orientation and amino acids reputedly relevant for transport of sodium (Q77, E260 and Q264) and bile acid (N295). Amino acids that differ between 3ZUY, Q9K0A9 (i.e., native ASBT in MC58 Neisseria meningitis), and the mutant employed to generate the crystal structure, 3ZUX, based on ClustalW alignment, are also indicated. Amino acids in 3ZUY were aligned with hASBT, and are indicated by black circles for identical, gray circles for highly similar and white circles for non-conserved residues.

Figure 3. Bile acid activation of the TGR5 receptor, possible effects on SLC10 proteins and potential clinical implications

Bile acids are natural ligands of the membrane-bound receptor TGR5, which is a GPCR expressed in various tissues and whose activation elevates cAMP levels. In brown adipose and muscle tissue, this increase in cAMP was shown to activate the thyroid hormone (T3) and increase energy expenditure, with potential implications for obesity (Pols et al., 2011). In cholangiocytes, it is possible that cAMP-induced ASBT translocation to the cell surface and increased BA uptake will reduce biliary BA concentrations during cholestasis. In mouse liver and ileum, TGR5 activation reduced NTCP, elevated ASBT levels and increased the risk of cholelithiasis. In the colon and ileum, cAMP stimulates GLP-1 secretion with downstream improved glycemia (Pols et al., 2011). Arrows denote activation of downstream pathways, while flat-end arrows indicate inhibition. Gray clouds indicate pathophysiological conditions that may be positively affected by TGR5 activation, while gray hexagons denote potential negative outcomes of TGR5 activation. Dashed arrows indicate the possible mechanism involved in ASBT inhibition by 264W94, which elevates ileal and colonic BA levels resulting in TGR5 activation and GLP-1 secretion (Chen et al.). Interestingly, insulin secretion consequent to GLP-1 activation was shown to reduce ASBT expression (Annaba et al.), and may potentially function as a positive feedback mechanism of ASBT inhibition with 264W94 treatment.