The novel putative bile acid transporter SLC10A5 is highly expressed in liver and kidney

Integrated profiling identifies DXS253E as a potential prognostic marker in colorectal cancer

BACKGROUND

Increasing evidence suggests that DXS253E is critical for cancer development and progression, but the function and potential mechanism of DXS253E in colorectal cancer (CRC) remain largely unknown. In this study, we evaluated the clinical significance and explored the underlying mechanism of DXS253E in CRC.

METHODS

DXS253E expression in cancer tissues was investigated using the Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) databases. The Kaplan-Meier plot was used to assess the prognosis of DXS253E. The cBioPortal, MethSurv, and Tumor Immune Estimation Resource (TIMER) databases were employed to analyze the mutation profile, methylation, and immune infiltration associated with DXS253E. The biological functions of DXS253E in CRC cells were determined by CCK-8 assay, plate cloning assay, Transwell assay, flow cytometry, lactate assay, western blot, and qRT-PCR.

RESULTS

DXS253E was upregulated in CRC tissues and high DXS253E expression levels were correlated with poor survival in CRC patients. Our bioinformatics analyses showed that high DXS253E gene methylation levels were associated with the favorable prognosis of CRC patients. Furthermore, DXS253E levels were linked to the expression levels of several immunomodulatory genes and an abundance of immune cells. Mechanistically, the overexpression of DXS253E enhanced proliferation, migration, invasion, and the aerobic glycolysis of CRC cells through the AKT/mTOR pathway.

CONCLUSIONS

We demonstrated that DXS253E functions as a potential role in CRC progression and may serve as an indicator of outcomes and a therapeutic target for regulating the AKT/mTOR pathway in CRC.

SLC10A3 Is a Prognostic Biomarker and Involved in Immune Infiltration and Programmed Cell Death in Lower Grade Glioma

BACKGROUND

The association between SLC10A3 (solute carrier family 10 member 3) and lower grade glioma (LGG) remains unclear.

METHODS

We used public databases and bioinformatics analysis to analyze SLC10A3. These included The Cancer Genome Atlas, Genotype-Tissue Expansion, Chinese Glioma Genome Atlas, Human Protein Atlas, GeneCards, cBioPortal, Search Tool for the Retrieval of Interacting Genes/Proteins, Gene Expression Profiling Interactive Analysis, Tumor Immune Estimation Resource, Tumor-Immune System Interaction Database, receiver operating characteristic curve analysis, Kaplan-Meier analysis, Cox analysis, nomograms, calibration plots, gene ontology/Kyoto Encyclopedia of Genes and Genomes enrichment analysis, gene set enrichment analysis, single-sample gene set enrichment analysis, and Spearman's correlation analysis.

RESULTS

SLC10A3 was upregulated in adrenocortical carcinoma, glioblastoma, and LGG and was associated with good overall survival (OS) in adrenocortical carcinoma and poor OS in LGG and glioblastoma. SLC10A3 was increased with increased World Health Organization grade, upregulated in isocitrate dehydrogenase-wild type, 1p/19q (chromosome arms 1p and 19q) non-co-deleted, and higher in astrocytoma. Patients with LGG were grouped by the occurrence of the clinical outcome endpoints (i.e., OS, disease-specific survival [DSS], and progression-free interval events). Genetic alterations in SLC10A3 were associated with poor progression-free survival in LGG. Most of clinical characteristics were associated with the SLC10A3 expression level. SLC10A3 with diagnostic and prognostic value (OS, DSS, and progression-free interval) was an independent prognostic factor in LGG. Moreover, Nomograms (WHO grade, 1p/19q codeletion, age and SLC10A3) had moderately accurate predictive for OS and DSS. Functional analysis showed that SLC10A3 might participate in the transport of multiple substances, neurogenic signaling, immune response, and programmed cell death in LGG. SLC10A3 correlated with immune infiltration in LGG and moderately correlated with the gene signature of pyroptosis, lysosome-dependent cell death, necroptosis, apoptosis, ferroptosis, alkaliptosis, and autophagy-dependent cell death.

CONCLUSIONS

SLC10A3 is a potential diagnostic and prognostic biomarker for LGG and might be associated with substance transport, neurogenic signaling, immune infiltration, and programmed cell death in LGG.

Role of the Sodium-Dependent Organic Anion Transporter (SOAT/SLC10A6) in Physiology and Pathophysiology

The sodium-dependent organic anion transporter (SOAT, gene symbol SLC10A6) specifically transports 3'- and 17'-monosulfated steroid hormones, such as estrone sulfate and dehydroepiandrosterone sulfate, into specific target cells. These biologically inactive sulfo-conjugated steroids occur in high concentrations in the blood circulation and serve as precursors for the intracrine formation of active estrogens and androgens that contribute to the overall regulation of steroids in many peripheral tissues. Although SOAT expression has been detected in several hormone-responsive peripheral tissues, its quantitative contribution to steroid sulfate uptake in different organs is still not completely clear. Given this fact, the present review provides a comprehensive overview of the current knowledge about the SOAT by summarizing all experimental findings obtained since its first cloning in 2004 and by processing SOAT/SLC10A6-related data from genome-wide protein and mRNA expression databases. In conclusion, despite a significantly increased understanding of the function and physiological significance of the SOAT over the past 20 years, further studies are needed to finally establish it as a potential drug target for endocrine-based therapy of steroid-responsive diseases such as hormone-dependent breast cancer.

SLC10A7, an orphan member of the SLC10 family involved in congenital disorders of glycosylation

SLC10A7, encoded by the so-called SLC10A7 gene, is the seventh member of a human sodium/bile acid cotransporter family, known as the SLC10 family. Despite similarities with the other members of the SLC10 family, SLC10A7 does not exhibit any transport activity for the typical SLC10 substrates and is then considered yet as an orphan carrier. Recently, SLC10A7 mutations have been identified as responsible for a new Congenital Disorder of Glycosylation (CDG). CDG are a family of rare and inherited metabolic disorders, where glycosylation abnormalities lead to multisystemic defects. SLC10A7-CDG patients presented skeletal dysplasia with multiple large joint dislocations, short stature and amelogenesis imperfecta likely mediated by glycosaminoglycan (GAG) defects. Although it has been demonstrated that the transporter and substrate specificities of SLC10A7, if any, differ from those of the main members of the protein family, SLC10A7 seems to play a role in Ca²⁺ regulation and is involved in proper glycosaminoglycan biosynthesis, especially heparan-sulfate, and N-glycosylation. This paper will review our current knowledge on the known and predicted structural and functional properties of this fascinating protein, and its link with the glycosylation process.

Substrate Specificities and Inhibition Pattern of the Solute Carrier Family 10 Members NTCP, ASBT and SOAT

Three carriers of the solute carrier family SLC10 have been functionally characterized so far. Na⁺/taurocholate cotransporting polypeptide NTCP is a hepatic bile acid transporter and the cellular entry receptor for the hepatitis B and D viruses. Its intestinal counterpart, apical sodium-dependent bile acid transporter ASBT, is responsible for the reabsorption of bile acids from the intestinal lumen. In addition, sodium-dependent organic anion transporter SOAT specifically transports sulfated steroid hormones, but not bile acids. All three carriers show high sequence homology, but significant differences in substrate recognition that makes a systematic structure-activity comparison attractive in order to define the protein domains involved in substrate binding and transport. By using stably transfected NTCP-, ASBT-, and SOAT-HEK293 cells, systematic comparative transport and inhibition experiments were performed with more than 20 bile acid and steroid substrates as well as different inhibitors. Taurolithocholic acid (TLC) was identified as the first common substrate of NTCP, ASBT and SOAT with K _m values of 18.4, 5.9, and 19.3 µM, respectively. In contrast, lithocholic acid was the only bile acid that was not transported by any of these carriers. Troglitazone, BSP and erythrosine B were identified as pan-SLC10 inhibitors, whereas cyclosporine A, irbesartan, ginkgolic acid 17:1, and betulinic acid only inhibited NTCP and SOAT, but not ASBT. The HBV/HDV-derived myr-preS1 peptide showed equipotent inhibition of the NTCP-mediated substrate transport of taurocholic acid (TC), dehydroepiandrosterone sulfate (DHEAS), and TLC with IC₅₀ values of 182 nM, 167 nM, and 316 nM, respectively. In contrast, TLC was more potent to inhibit myr-preS1 peptide binding to NTCP with IC₅₀ of 4.3 µM compared to TC (IC₅₀ = 70.4 µM) and DHEAS (IC₅₀ = 52.0 µM). Based on the data of the present study, we propose several overlapping, but differently active binding sites for substrates and inhibitors in the carriers NTCP, ASBT, SOAT.

Deorphaning a solute carrier 22 family member, SLC22A15, through functional genomic studies

The human solute carrier 22A (SLC22A) family consists of 23 members, representing one of the largest families in the human SLC superfamily. Despite their pharmacological and physiological importance in the absorption and disposition of a range of solutes, eight SLC22A family members remain classified as orphans. In this study, we used a multifaceted approach to identify ligands of orphan SLC22A15. Ligands of SLC22A15 were proposed based on phylogenetic analysis and comparative modeling. The putative ligands were then confirmed by metabolomic screening and uptake assays in SLC22A15 transfected HEK293 cells. Metabolomic studies and transporter assays revealed that SLC22A15 prefers zwitterionic compounds over cations and anions. We identified eight zwitterions, including ergothioneine, carnitine, carnosine, gabapentin, as well as four cations, including MPP⁺ , thiamine, and cimetidine, as substrates of SLC22A15. Carnosine was a specific substrate of SLC22A15 among the transporters in the SLC22A family. SLC22A15 transport of several substrates was sodium-dependent and exhibited a higher Km for ergothioneine, carnitine, and carnosine compared to previously identified transporters for these ligands. This is the first study to characterize the function of SLC22A15. Our studies demonstrate that SLC22A15 may play an important role in determining the systemic and tissue levels of ergothioneine, carnosine, and other zwitterions.

Homo- and heterodimerization is a common feature of the solute carrier family SLC10 members

The solute carrier family SLC10 consists of seven members, including the bile acid transporters Na+/taurocholate co-transporting polypeptide (NTCP) and apical sodium-dependent bile acid transporter (ASBT), the steroid sulfate transporter SOAT as well as four orphan carriers (SLC10A3, SLC10A4, SLC10A5 and SLC10A7). Previously, homodimerization of NTCP, ASBT and SOAT was described and there is increasing evidence that carrier oligomerization is an important regulatory factor for protein sorting and transport function. In the present study, homo- and heterodimerization were systematically analyzed among all SLC10 carriers (except for SLC10A3) using the yeast-two-hybrid membrane protein system. Strong homodimerization occurred for NTCP/NTCP, ASBT/ASBT and SLC10A7/SLC10A7. Heterodimerization was observed for most of the SLC10 carrier combinations. Heterodimerization of NTCP was additionally investigated by co-localization of NTCP-GFP and NTCP-mScarlet with respective SLC10 carrier constructs. NTCP co-localized with SLC10A4, SLC10A5, SOAT and SLC10A7. This co-localization was most pronounced for SLC10A4 and was additionally confirmed by co-immunoprecipitation. Interestingly, SLC10 carrier co-expression decreased the taurocholate transport function of NTCP for most of the analyzed constructs, indicating that SLC10 carrier heterodimerization is of functional relevance. In conclusion, homo- and heterodimerization is a common feature of the SLC10 carriers. The relevance of this finding for regulation and transport function of the SLC10 carriers in vivo needs further investigation.

Protein-protein interactions of drug uptake transporters that are important for liver and kidney

Drug uptake transporters are membrane proteins responsible for the trans-membrane transport of endo- and xenobiotics, including numerous drugs. They are important for the uptake of drugs into target tissues or into organs for metabolism and excretion. Many drug uptake transporters have a broad spectrum of structural-independent substrates, which make them vulnerable to drug-drug interactions. Recent studies have shown more and more complex pharmacokinetics involving transporters, and regulatory agencies now require studies to be performed to measure the involvement of transporters in drug development. A better understanding of the factors affecting the expression of transporters is needed. Despite many efforts devoted to the functional characterization of different drug uptake transporters, transporter in vitro to in vivo extrapolations are far from predicting the behavior under physiological conditions. There is an increasing number of uptake transporters demonstrated to form protein-protein interactions or to oligomerize. This raises the possibility that these interactions between or among transporters could help explaining the gap between in vitro and in vivo measurement of drug transporters. In this review, we summarized protein-protein interactions of drug uptake transporters that are important for pharmacokinetics, especially those in the liver and the kidneys.

Transporter for sulfated steroid hormones in the testis - expression pattern, biological significance and implications for fertility in men and rodents

In various tissues, steroid hormones may be sulfated, glucuronidated or otherwise modified. For a long time, these hydrophilic molecules have been considered to be merely inactive metabolites for excretion via bile or urine. Nevertheless, different organs such as the placenta and breast tissue produce large amounts of sulfated steroids. After the discovery of the enzyme steroid sulfatase, which is able to re-activate sulfated steroids, these precursor molecules entered the focus of interest again as a local supply for steroid hormone synthesis with a prolonged half-life compared to their unconjugated counterparts. The first descriptions of this so-called sulfatase pathway in the placenta and breast tissue (with special regards to hormone-dependent breast cancer) were quickly followed by studies of steroid sulfate production and function in the testis. These hydrophilic molecules may not permeate the cell membrane by diffusion in the way that unbound steroids can, but need to be transported through the plasma membrane by transport systems. In the testis, a functional sulfatase pathway requires the expression of specific uptake carrier and efflux transporters in testicular cells, i.e. Sertoli, Leydig and germ cells. Main focus has to be placed on Sertoli cells, as these cells build up the blood-testis barrier. In this review, an overview of carrier expression pattern in the human as well as rodent testis is provided with special interest towards implications on fertility.

Human bile acid transporter ASBT (SLC10A2) forms functional non-covalent homodimers and higher order oligomers

The human apical sodium-dependent bile acid transporter, hASBT/SLC10A2, plays a central role in cholesterol homeostasis via the efficient reabsorption of bile acids from the distal ileum. hASBT has been shown to self-associate in higher order complexes, but while the functional role of endogenous cysteines has been reported, their implication in the oligomerization of hASBT remains unresolved. Here, we determined the self-association architecture of hASBT by site-directed mutagenesis combined with biochemical, immunological and functional approaches. We generated a cysteine-less form of hASBT by creating point mutations at all 13 endogenous cysteines in a stepwise manner. Although Cysless hASBT had significantly reduced function correlated with lowered surface expression, it featured an extra glycosylation site that facilitated its differentiation from wt-hASBT on immunoblots. Decreased protein expression was associated with instability and subsequent proteasome-dependent degradation of Cysless hASBT protein. Chemical cross-linking of wild-type and Cysless species revealed that hASBT exists as an active dimer and/or higher order oligomer with apparently no requirement for endogenous cysteine residues. This was further corroborated by co-immunoprecipitation of differentially tagged (HA-, Flag-) wild-type and Cysless hASBT. Finally, Cysless hASBT exhibited a dominant-negative effect when co-expressed with wild-type hASBT which validated heterodimerization/oligomerization at the functional level. Combined, our data conclusively demonstrate the functional existence of hASBT dimers and higher order oligomers irrespective of cysteine-mediated covalent bonds, thereby providing greater understanding of its topological assembly at the membrane surface.

Organic anion transporting polypeptide 1B3 can form homo- and hetero-oligomers

OATP1B3 is a 12 transmembrane domain protein expressed at the basolateral membrane of human hepatocytes where it mediates the uptake of numerous drugs and endogenous compounds. Previous western blot results suggest the formation of OATP1B3 multimers. In order to better understand the function of OATP1B3 under normal physiological conditions, we investigated its oligomerization status. We transiently transfected OATP1B3 with a C-terminal His-, FLAG- or HA-tag in HEK293 cells and used co-immunoprecipitation and a Proximity Ligation Assay to detect interactions between the different constructs. All three constructs retained similar transport rates as wild-type OATP1B3. Immunofluorescence experiments indicated that in contrast to wild-type, His- and FLAG-tagged OATP1B3, where the C-terminal end is on the cytoplasmic side of the membrane, the C-terminal end of HA-tagged OATP1B3 is extracellular. After cross-linking, anti-FLAG antibodies were able to pull down FLAG-tagged OATP1B3 (positive control) and co-transfected His- or HA-tagged OATP1B3, demonstrating the formation of homo-oligomers and suggesting that the C-terminal part is not involved in oligomer formation. We confirmed co-localization of His- and FLAG-tagged OATP1B3 in transfected HEK293 cells with the Proximity Ligation Assay. Transport studies with a non-functional OATP1B3 mutant suggest that the individual subunits and not the whole oligomer are the functional units in the homo-oligomers. In addition, we also detected OATP1B3-FLAG co-localization with OATP1B1-His or NTCP-His, suggesting that OATP1B3 also hetero-oligomerizes with other transport proteins. Using the Proximity Ligation Assay with transporter specific antibodies, we demonstrate close association of OATP1B3 with NTCP in frozen human liver tissue. These findings demonstrate that OATP1B3 can form homo- and hetero-oligomers and suggest a potential co-regulation of the involved transporters.

Expression, sorting and transport studies for the orphan carrier SLC10A4 in neuronal and non-neuronal cell lines and in Xenopus laevis oocytes

Background

SLC10A4 belongs to the solute carrier family SLC10 whose founding members are the Na⁺/taurocholate co-transporting polypeptide (NTCP, SLC10A1) and the apical sodium-dependent bile acid transporter (ASBT, SLC10A2). These carriers maintain the enterohepatic circulation of bile acids between the liver and the gut. SLC10A4 was identified as a novel member of the SLC10 carrier family with the highest phylogenetic relationship to NTCP. The SLC10A4 protein was detected in synaptic vesicles of cholinergic and monoaminergic neurons of the peripheral and central nervous system, suggesting a transport function for any kind of neurotransmitter. Therefore, in the present study, we performed systematic transport screenings for SLC10A4 and also aimed to identify the vesicular sorting domain of the SLC10A4 protein.

Results

We detected a vesicle-like expression pattern of the SLC10A4 protein in the neuronal cell lines SH-SY5Y and CAD. Differentiation of these cells to the neuronal phenotype altered neither SLC10A4 gene expression nor its vesicular expression pattern. Functional transport studies with different neurotransmitters, bile acids and steroid sulfates were performed in SLC10A4-transfected HEK293 cells, SLC10A4-transfected CAD cells and in Xenopus laevis oocytes. For these studies, transport by the dopamine transporter DAT, the serotonin transporter SERT, the choline transporter CHT1, the vesicular monoamine transporter VMAT2, the organic cation transporter Oct1, and NTCP were used as positive control. SLC10A4 failed to show transport activity for dopamine, serotonin, norepinephrine, histamine, acetylcholine, choline, acetate, aspartate, glutamate, gamma-aminobutyric acid, pregnenolone sulfate, dehydroepiandrosterone sulfate, estrone-3-sulfate, and adenosine triphosphate, at least in the transport assays used. When the C-terminus of SLC10A4 was replaced by the homologous sequence of NTCP, the SLC10A4-NTCP chimeric protein revealed clear plasma membrane expression in CAD and HEK293 cells. But this chimera also did not show any transport activity, even when the N-terminal domain of SLC10A4 was deleted by mutagenesis.

Conclusions

Although different kinds of assays were used to screen for transport function, SLC10A4 failed to show transport activity for a series of neurotransmitters and neuromodulators, indicating that SLC10A4 does not seem to represent a typical neurotransmitter transporter such as DAT, SERT, CHT1 or VMAT2.

The Concise Guide to PHARMACOLOGY 2013/14: transporters

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Transporters are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

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

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.

The SLC10 carrier family: transport functions and molecular structure

The SLC10 family represents seven genes containing 1-12 exons that encode proteins in humans with sequence lengths of 348-477 amino acids. Although termed solute carriers (SLCs), only three out of seven (i.e. SLC10A1, SLC10A2, and SLC10A6) show sodium-dependent uptake of organic substrates across the cell membrane. These include the uptake of bile salts, sulfated steroids, sulfated thyroidal hormones, and certain statin drugs by SLC10A1 (Na(+)-taurocholate cotransporting polypeptide (NTCP)), the uptake of bile salts by SLC10A2 (apical sodium-dependent bile acid transporter (ASBT)), and uptake of sulfated steroids and sulfated taurolithocholate by SLC10A6 (sodium-dependent organic anion transporter (SOAT)). The other members of the family are orphan carriers not all localized in the cell membrane. The name "bile acid transporter family" arose because the first two SLC10 members (NTCP and ASBT) are carriers for bile salts that establish their enterohepatic circulation. In recent years, information has been obtained on their 2D and 3D membrane topology, structure-transport relationships, and on the ligand and sodium-binding sites. For SLC10A2, the putative 3D morphology was deduced from the crystal structure of a bacterial SLC10A2 analog, ASBT(NM). This information was used in this chapter to calculate the putative 3D structure of NTCP. This review provides first an introduction to recent knowledge about bile acid synthesis and newly found bile acid hormonal functions, and then describes step-by-step each individual member of the family in terms of expression, localization, substrate pattern, as well as protein topology with emphasis on the three functional SLC10 carrier members.

The Candida albicans plasma membrane protein Rch1p, a member of the vertebrate SLC10 carrier family, is a novel regulator of cytosolic Ca2+ homoeostasis

Candida albicans RCH1 (regulator of Ca2+ homoeostasis 1) encodes a protein of ten TM (transmembrane) domains, homologous with human SLC10A7 (solute carrier family 10 member 7), and Rch1p localizes in the plasma membrane. Deletion of RCH1 confers hypersensitivity to high concentrations of extracellular Ca2+ and tolerance to azoles and Li+, which phenocopies the deletion of CaPMC1 (C. albicans PMC1) encoding the vacuolar Ca2+ pump. Additive to CaPMC1 mutation, lack of RCH1 alone shows an increase in Ca2+ sensitivity, Ca2+ uptake and cytosolic Ca2+ level. The Ca2+ hypersensitivity is abolished by cyclosporin A and magnesium. In addition, deletion of RCH1 elevates the expression of CaUTR2 (C. albicans UTR2), a downstream target of the Ca2+/calcineurin signalling. Mutational and functional analysis indicates that the Rch1p TM8 domain, but not the TM9 and TM10 domains, are required for its protein stability, cellular functions and subcellular localization. Therefore Rch1p is a novel regulator of cytosolic Ca2+ homoeostasis, which expands the functional spectrum of the vertebrate SLC10 family.

Homo- and hetero-dimeric architecture of the human liver Na+-dependent taurocholate co-transporting protein

The NTCP (Na+–taurocholate co-transporting protein)/SLC10A [solute carrier family 10 (Na+/bile acid co-transporter family)] 1 is tightly controlled to ensure hepatic bile salt uptake while preventing toxic bile salt accumulation. Many transport proteins require oligomerization for their activity and regulation. This is not yet established for bile salt transporters. The present study was conducted to elucidate the oligomeric state of NTCP. Chemical cross-linking revealed the presence of NTCP dimers in rat liver membranes and U2OS cells stably expressing NTCP. Co-immunoprecipitation of tagged NTCP proteins revealed a physical interaction between subunits. The C-terminus of NTCP was not required for subunit interaction, but was essential for exit from the ER (endoplasmic reticulum). NTCP without its C-terminus (NTCP Y307X) retained full-length wtNTCP (wild-type NTCP) in the ER in a dominant fashion, suggesting that dimerization occurs early in the secretory pathway. FRET (fluorescence resonance energy transfer) using fluorescently labelled subunits further demonstrated that dimerization persists at the plasma membrane. NTCP belongs to the SLC10A protein family which consists of seven members. NTCP co-localized in U2OS cells with SLC10A4 and SLC10A6, but not with SLC10A3, SLC10A5 or SLC10A7. SLC10A4 and SLC10A6 co-immunoprecipitated with NTCP, demonstrating that heteromeric complexes can be formed between SLC10A family members in vitro. Expression of SLC10A4 and NTCP Y307X resulted in a reduction of NTCP abundance at the plasma membrane and NTCP-mediated taurocholate uptake, whereas expression of SLC10A6 or NTCP E257N, an inactive mutant, did not affect NTCP function. In conclusion, NTCP adopts a dimeric structure in which individual subunits are functional. Bile salt uptake is influenced by heterodimerization when this impairs NTCP plasma membrane trafficking.

Study of the transport of thyroid hormone by transporters of the SLC10 family

Transport of (sulfated) iodothyronines across the plasma membrane is required for their intracellular metabolism. Rat Na(+)/taurocholate cotransporting polypeptide (Ntcp; Slc10a1) has been identified as an important transporter protein. We demonstrate that among the 7 members of the solute carrier family SLC10, only human SLC10A1 mediates sodium-dependent transport of the iodothyronine T4 and iodothyronine sulfates T3S and T4S. In contrast to SLC10A2-7, cells co-expressing SLC10A1 and the deiodinase D1 demonstrate a dramatic increase in T3S and T4S metabolism. The SLC10A1 substrates taurocholate, DHEAS and E3S inhibit T3S and T4S transport. Furthermore, co-transfection of SLC10A1 with CRYM, a well-known intracellular iodothyronine-binding protein, results in an enhanced intracellular accumulation of T3S and T4S, indicating that CRYM binds iodothyronine sulfates. The present findings indicate that the liver-specific transporter SLC10A1 transports (sulfated) iodothyronines, thereby increasing their intracellular availability. Therefore, SLC10A1 may fulfill a critical step in providing liver D1 with iodothyronine sulfates for rapid degradation.