Transmembrane domain II of the human bile acid transporter SLC10A2 coordinates sodium translocation

Structure-based virtual screening and molecular dynamics of potential inhibitors targeting sodium-bile acid co-transporter of carcinogenic liver fluke Clonorchis sinensis

Background

Clonorchis sinensis requires bile acid transporters as this fluke inhabits bile juice-filled biliary ducts, which provide an extreme environment. Clonorchis sinensis sodium-bile acid co-transporter (CsSBAT) is indispensable for the fluke’s survival in the final host, as it circulates taurocholate and prevents bile toxicity in the fluke; hence, it is recognized as a useful drug target.

Methodology and principal findings

In the present study, using structure-based virtual screening approach, we presented inhibitor candidates targeting a bile acid-binding pocket of CsSBAT. CsSBAT models were built using tertiary structure modeling based on a bile acid transporter template (PDB ID: 3zuy and 4n7x) and were applied into AutoDock Vina for competitive docking simulation. First, potential compounds were identified from PubChem (holding more than 100,000 compounds) by applying three criteria: i) interacting more favorably with CsSBAT than with a human homolog, ii) intimate interaction to the inward- and outward-facing conformational states, iii) binding with CsSBAT preferably to natural bile acids. Second, two compounds were identified following the Lipinski’s rule of five. Third, other two compounds of molecular weight higher than 500 Da (Mr > 500 Da) were presumed to efficiently block the transporter via a feasible rational screening strategy. Of these candidates, compound 9806452 exhibited the least hepatotoxicity that may enhance drug-likeness properties.

Conclusions

It is proposed that compound 9806452 act as a potential inhibitor toward CsSBAT and further studies are warranted for drug development process against clonorchiasis.

Sodium-bile acid co-transporter is crucial for survival of a carcinogenic liver fluke Clonorchis sinensis in the bile

The liver fluke Clonorchis sinensis inhabits the bile ducts, where bile concentration disparities across the fluke cell membrane can cause bile intoxication. Sodium-bile acid co-transporter (SBAT) plays a crucial role in bile acid recycling. The process by which SBAT imports bile acids is electrically coupled to sodium ion co-transportation. Here, we report that the SBAT of C. sinensis (CsSBAT) is involved in bile acid transportation. CsSBAT cDNA encoded a putative polypeptide of 546 amino acid residues. Furthermore, CsSBAT consisted of ten putative transmembrane domains, and its 3D structure was predicted to form panel and core domains. The CsSBAT had one bile acid- and three Na⁺-binding sites, enabling coordination of a symport process. CsSBAT was mainly localized in the mesenchymal tissue throughout the fluke body and sparsely localized in the basement of the tegument, intestinal epithelium, and excretory bladder wall. Bile acid permeated into the adult flukes in a short time and remained at a low concentration level. Bile acid accumulated inside the mesenchymal tissue when CsSBAT was inhibited using polyacrylic acid–tetradeoxycholic acid conjugate. The accumulated bile acid deteriorated the C. sinensis adults leading to death. CsSBAT silencing shortened the lifespan of the fluke when it was placed into bile. Taken together, we propose that CsSBAT transports bile acids in the mesenchymal tissue and coordinate with outward transporters to maintain bile acid homeostasis of C. sinensis adults, contributing to C. sinensis survival in the bile environment.

Clonorchiasis is a neglected tropical disease caused by infection with the liver fluke Clonorchis sinensis. C. sinensis is a biological carcinogen causing cholangiocarcinoma in humans. Juvenile worms inhabit and grow to adults in the bile ducts. Bile acids in the bile are double-edged molecules; they promote metabolism, but differences in their concentration across the cell membrane could lead to bile intoxication. The sodium-bile acid co-transporter of C. sinensis (CsSBAT) is indispensable for maintaining its normal physiology and bile detoxification in the bile duct. However, information related to the molecular and biological characteristics of the SBAT of liver flukes is not available. Here, we cloned CsSBAT for the first time in trematodes and characterized its tertiary structure and physiological functions. The sequential and structural properties of CsSBAT were similar to the apical sodium-bile acid co-transporter found in mammalian intestines. CsSBAT shared a mesenchymal tissue distribution with Na⁺-taurocholate co-transporting polypeptide in the hepatocytes adjacent to the bile ducts. Bile acids accumulated in C. sinensis adults when CsSBAT was inhibited, causing their death. This information might promote further studies on the physiological functions of SBAT and other trematode bile transporters and open new avenues toward developing novel anthelminthic drugs.

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.

[The inhibitors of the apical sodium-dependent bile acid transporter (ASBT) as promising drugs]

Inhibition of the apical sodium-dependent bile acid transporter (ASBT, also known as IBAT - ileal bile acid transporter, SLC10A2) leads to disruption of the enterohepatic circulation of bile acids and their excretion with fecal masses. This is accompanied by cholesterol utilization for synthesis of new bile acids. ASBT inhibitors are promising drugs for the treatment of such diseases as non-alcoholic fatty liver disease, non-alcoholic steatohepatitis, type 2 diabetes mellitus, necrotic enterocolitis, chronic constipation, atherosclerosis. To date the most known chemically synthesized inhibitors are: A3309, SHP626, A4250, 264W94, GSK2330672, SC-435. All of them are at different stages of clinical trials, which confirm the high efficacy and good tolerance of these inhibitors. Current trends in this field also include directed chemical synthesis of ASBT inhibitors, as well as their search among substances of plant origin.

Bile acid transporter-mediated oral drug delivery

Bile acids are synthesized in the liver, stored in the gallbladder, and secreted into the duodenum at meals. Apical sodium-dependent bile acid transporter (ASBT), an ileal Na⁺-dependent transporter, plays the leading role of bile acid absorption into enterocytes, where bile acids are delivered to basolateral side by ileal bile acid binding protein (IBABP) and then released by organic solute transporter OSTα/β. The absorbed bile acids are delivered to the liver via portal vein. In this process called "enterohepatic recycling", only 5% of the bile acid pool (~3 g in human) is excreted in feces, indicating the large recycling capacity and high transport efficacy of ASBT-mediated absorption. Therefore, bile acid transporter-mediated oral drug delivery has been regarded as a feasible and potential strategy to improve the oral bioavailability. This review introduces the key factors in enterohepatic recycling, especially the mechanism of bile acid uptake by ASBT, and the development of bile acid-based oral drug delivery for ASBT-targeting, including bile acid-based prodrugs, bile acid/drug electrostatic complexation and bile acid-containing nanocarriers. Furthermore, the specific transport pathways of bile acid in enterocytes are described and the recent finding of lymphatic delivery of bile acid-containing nanocarriers is discussed.

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.

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.