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

Oligomerization of drug transporters: Forms, functions, and mechanisms

Drug transporters are essential players in the transmembrane transport of a wide variety of clinical drugs. The broad substrate spectra and versatile distribution pattern of these membrane proteins infer their pharmacological and clinical significance. With our accumulating knowledge on the three-dimensional structure of drug transporters, their oligomerization status has become a topic of intense study due to the possible functional roles carried out by such kind of post-translational modification (PTM). In-depth studies of oligomeric complexes formed among drug transporters as well as their interactions with other regulatory proteins can help us better understand the regulatory mechanisms of these membrane proteins, provide clues for the development of novel drugs, and improve the therapeutic efficacy. In this review, we describe different oligomerization forms as well as their structural basis of major drug transporters in the ATP-binding cassette and solute carrier superfamilies, summarize our current knowledge on the influence of oligomerization for protein expression level and transport function of these membrane proteins, and discuss the regulatory mechanisms of oligomerization. Finally, we highlight the challenges associated with the current oligomerization studies and propose some thoughts on the pharmaceutical application of this important drug transporter PTM.

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.

A guide to plasma membrane solute carrier proteins

This review aims to serve as an introduction to the solute carrier proteins (SLC) superfamily of transporter proteins and their roles in human cells. The SLC superfamily currently includes 458 transport proteins in 65 families that carry a wide variety of substances across cellular membranes. While members of this superfamily are found throughout cellular organelles, this review focuses on transporters expressed at the plasma membrane. At the cell surface, SLC proteins may be viewed as gatekeepers of the cellular milieu, dynamically responding to different metabolic states. With altered metabolism being one of the hallmarks of cancer, we also briefly review the roles that surface SLC proteins play in the development and progression of cancer through their influence on regulating metabolism and environmental conditions.

Dissecting the Conformational Dynamics of the Bile Acid Transporter Homologue ASBTNM

Apical sodium-dependent bile acid transporter (ASBT) catalyses uphill transport of bile acids using the electrochemical gradient of Na⁺ as the driving force. The crystal structures of two bacterial homologues ASBT_NM and ASBT_Yf have previously been determined, with the former showing an inward-facing conformation, and the latter adopting an outward-facing conformation accomplished by the substitution of the critical Na⁺-binding residue glutamate-254 with an alanine residue. While the two crystal structures suggested an elevator-like movement to afford alternating access to the substrate binding site, the mechanistic role of Na⁺ and substrate in the conformational isomerization remains unclear. In this study, we utilized site-directed alkylation monitored by in-gel fluorescence (SDAF) to probe the solvent accessibility of the residues lining the substrate permeation pathway of ASBT_NM under different Na⁺ and substrate conditions, and interpreted the conformational states inferred from the crystal structures. Unexpectedly, the crosslinking experiments demonstrated that ASBT_NM is a monomer protein, unlike the other elevator-type transporters, usually forming a homodimer or a homotrimer. The conformational dynamics observed by the biochemical experiments were further validated using DEER measuring the distance between the spin-labelled pairs. Our results revealed that Na⁺ ions shift the conformational equilibrium of ASBT_NM toward the inward-facing state thereby facilitating cytoplasmic uptake of substrate. The current findings provide a novel perspective on the conformational equilibrium of secondary active transporters.

S-acylation status of bile acid transporter hASBT regulates its function, metabolic stability, membrane expression, and phosphorylation state

The human apical sodium-dependent bile acid transporter (hASBT, SLC10A2) is the rate-limiting step of intestinal bile acid absorption in the enterohepatic circulation system of bile acids. Therefore, the regulation and stability of hASBT is vital in maintaining bile acid and cholesterol homeostasis and may serve as a potential target for cholesterol-related disorders. We hypothesized that post-translational mechanisms that govern hASBT function and regulation will provide novel insight on intestinal bile acid transport and homeostasis. In this study, we confirm the S-acylation status of hASBT via acyl biotin exchange in COS-1 cells and its impact on hASBT expression, function, kinetics, and protein stability. Using the acylation inhibitor, 2-bromopalmitate, we show that S-acylation is an important modification which modulates the function, surface expression, and maximal transporter flux (J_max) of hASBT. By means of proteasome inhibitors, S-acylated hASBT was found to be cleared via the proteasome whereas a reduction in the palmitoylation status of hASBT resulted in rapid proteolytic degradation compared to the unmodified transporter. Screening of cysteine mutants in and or near transmembrane domains, some of which are exposed to the cytosol, confirmed Cys314 to be the predominate S-acylated residue. Lastly, we show that S-acylation was reduced in a mutant form of hASBT devoid of cytosolic facing tyrosine residues, suggestive of crosstalk between acylation and phosphorylation post-translational modification mechanisms.

Functional (un)cooperativity in elevator transport proteins

The activity of enzymes is subject to regulation at multiple levels. Cooperativity, the interconnected behavior of active sites within a protein complex, directly affects protein activity. Cooperativity is a mode of regulation that requires neither extrinsic factors nor protein modifications. Instead, it allows enzymes themselves to modulate reaction rates. Cooperativity is an important regulatory mechanism in soluble proteins, but also examples of cooperative membrane proteins have been described. In this review, we summarize the current knowledge on interprotomer cooperativity in elevator-type proteins, a class of membrane transporters characterized by large rigid-body movements perpendicular to the membrane, and highlight well-studied examples and experimental approaches.

S-acylation modulates the function of the apical sodium-dependent bile acid transporter in human cells

The ileal apical sodium-dependent bile acid transporter (ASBT) is crucial for the enterohepatic circulation of bile acids. ASBT function is rapidly regulated by several posttranslational modifications. One reversible posttranslational modification is S-acylation, involving the covalent attachment of fatty acids to cysteine residues in proteins. However, whether S-acylation affects ASBT function and membrane expression has not been determined. Using the acyl resin-assisted capture method, we found that the majority of ASBT (∼80%) was S-acylated in ileal brush border membrane vesicles from human organ donors, as well as in HEK293 cells stably transfected with ASBT (2BT cells). Metabolic labeling with alkyne-palmitic acid (100 μm for 15 h) also showed that ASBT is S-acylated in 2BT cells. Incubation with the acyltransferase inhibitor 2-bromopalmitate (25 μm for 15 h) significantly reduced ASBT S-acylation, function, and levels on the plasma membrane. Treatment of 2BT cells with saturated palmitic acid (100 μm for 15 h) increased ASBT function, whereas treatment with unsaturated oleic acid significantly reduced ASBT function. Metabolic labeling with alkyne-oleic acid (100 μm for 15 h) revealed that oleic acid attaches to ASBT, suggesting that unsaturated fatty acids may decrease ASBT's function via a direct covalent interaction with ASBT. We also identified Cys-314 as a potential S-acylation site. In conclusion, these results provide evidence that S-acylation is involved in the modulation of ASBT function. These findings underscore the potential for unsaturated fatty acids to reduce ASBT function, which may be useful in disorders in which bile acid toxicity is implicated.

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.

Intestinal Absorption of Bile Acids in Health and Disease

The intestinal reclamation of bile acids is crucial for the maintenance of their enterohepatic circulation. The majority of bile acids are actively absorbed via specific transport proteins that are highly expressed in the distal ileum. The uptake of bile acids by intestinal epithelial cells modulates the activation of cytosolic and membrane receptors such as the farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (GPBAR1), which has a profound effect on hepatic synthesis of bile acids as well as glucose and lipid metabolism. Extensive research has focused on delineating the processes of bile acid absorption and determining the contribution of dysregulated ileal signaling in the development of intestinal and hepatic disorders. For example, a decrease in the levels of the bile acid-induced ileal hormone FGF15/19 is implicated in bile acid-induced diarrhea (BAD). Conversely, the increase in bile acid absorption with subsequent overload of bile acids could be involved in the pathophysiology of liver and metabolic disorders such as fatty liver diseases and type 2 diabetes mellitus. This review article will attempt to provide a comprehensive overview of the mechanisms involved in the intestinal handling of bile acids, the pathological implications of disrupted intestinal bile acid homeostasis, and the potential therapeutic targets for the treatment of bile acid-related disorders. Published 2020. Compr Physiol 10:21-56, 2020.

Multiscale Simulations of Biological Membranes: The Challenge To Understand Biological Phenomena in a Living Substance

Biological membranes are tricky to investigate. They are complex in terms of molecular composition and structure, functional over a wide range of time scales, and characterized by nonequilibrium conditions. Because of all of these features, simulations are a great technique to study biomembrane behavior. A significant part of the functional processes in biological membranes takes place at the molecular level; thus computer simulations are the method of choice to explore how their properties emerge from specific molecular features and how the interplay among the numerous molecules gives rise to function over spatial and time scales larger than the molecular ones. In this review, we focus on this broad theme. We discuss the current state-of-the-art of biomembrane simulations that, until now, have largely focused on a rather narrow picture of the complexity of the membranes. Given this, we also discuss the challenges that we should unravel in the foreseeable future. Numerous features such as the actin-cytoskeleton network, the glycocalyx network, and nonequilibrium transport under ATP-driven conditions have so far received very little attention; however, the potential of simulations to solve them would be exceptionally high. A major milestone for this research would be that one day we could say that computer simulations genuinely research biological membranes, not just lipid bilayers.

Troglitazone Impedes the Oligomerization of Sodium Taurocholate Cotransporting Polypeptide and Entry of Hepatitis B Virus Into Hepatocytes

Current anti-hepatitis B virus (HBV) agents, which include nucleos(t)ide analogs and interferons, can significantly suppress HBV infection. However, there are limitations in the therapeutic efficacy of these agents, indicating the need to develop anti-HBV agents with different modes of action. In this study, through a functional cell-based chemical screening, we found that a thiazolidinedione, troglitazone, inhibits HBV infection independently of the compound's ligand activity for peroxisome proliferator-activated receptor γ (PPARγ). Analog analysis suggested chemical moiety required for the anti-HBV activity and identified ciglitazone as an analog having higher anti-HBV potency. Whereas, most of the reported HBV entry inhibitors target viral attachment to the cell surface, troglitazone blocked a process subsequent to viral attachment, i.e., internalization of HBV preS1 and its receptor, sodium taurocholate cotransporting polypeptide (NTCP). We also found that NTCP was markedly oligomerized in the presence of HBV preS1, but such NTCP oligomerization was abrogated by treatment with troglitazone, but not with pioglitazone, correlating with inhibition activity to viral internalization. Also, competitive peptides that blocked NTCP oligomerization impeded viral internalization and infection. This work represents the first report identifying small molecules and peptides that specifically inhibit the internalization of HBV. This study is also significant in proposing a possible role for NTCP oligomerization in viral entry, which will shed a light on a new aspect of the cellular mechanisms regulating HBV infection.

Transporter oligomerisation: roles in structure and function

Oligomerisation is a key feature of integral membrane transporters with roles in structure, function and stability. In this review, we cover some very recent advances in our understanding of how oligomerisation affects these key transporter features, with emphasis on a few groups of transporters, including the nucleobase ascorbate transporters, neurotransmitter sodium symporters and major facilitator superfamily members.

A novel bioluminescence-based method to investigate uptake of bile acids in living cells

Bile acid transporters, including the ileal apical sodium-dependent bile acid transporter (ASBT) and the hepatic sodium-taurocholate cotransporting polypeptide (NTCP), are crucial for the enterohepatic circulation of bile acids. Our objective was to develop a method for measuring bile acid transporter activity in real time to precisely evaluate rapid changes in their function. We designed a reporter system relying on a novel probe: cholic acid attached to luciferin via a disulfide-containing, self-immolating linker (CA-SS-Luc). Incubation of human embryonic kidney-293 cells coexpressing luciferase and ASBT with different concentrations of CA-SS-Luc (0.01-1 μM) resulted in bioluminescence with an intensity that was concentration- and time-dependent. The bioluminescence measured during incubation with 1 μM CA-SS-Luc was dependent on the levels of ASBT or NTCP expressed in the cells. Coincubation of CA-SS-Luc with natural bile acids enhanced the bioluminescence in a concentration-dependent manner with kinetic parameters for ASBT similar to those previously reported using conventional methods. These findings suggest that this method faithfully assesses ASBT function. Further, incubation with tyrosine phosphatase inhibitor III (PTPIII) led to significantly increased bioluminescence in cells expressing ASBT, consistent with previous studies showing an increase in ASBT function by PTPIII. We then investigated CA-SS-Luc in isolated mouse intestinal epithelial cells. Ileal enterocytes displayed significantly higher luminescence compared with jejunal enterocytes, indicating a transport process mediated by ileal ASBT. In conclusion, we have developed a novel method to monitor the activity of bile acid transporters in real time that has potential applications both for in vitro and in vivo studies. NEW & NOTEWORTHY This article reports the development of a real-time method for measuring the uptake of bile acids using a bioluminescent bile acid-based probe. This method has been validated for measuring uptake via the apical sodium-dependent bile acid transporter and the sodium-taurocholate cotransporting polypeptide in cell culture and ex vivo intestinal models.