Structure of antiviral drug bulevirtide bound to hepatitis B and D virus receptor protein NTCP

Cellular entry of the hepatitis B and D viruses (HBV/HDV) requires binding of the viral surface polypeptide preS1 to the hepatobiliary transporter Na+-taurocholate co-transporting polypeptide (NTCP). This interaction can be blocked by bulevirtide (BLV, formerly Myrcludex B), a preS1 derivative and approved drug for treating HDV infection. Here, to elucidate the basis of this inhibitory function, we determined a cryo-EM structure of BLV-bound human NTCP. BLV forms two domains, a plug lodged in the bile salt transport tunnel of NTCP and a string that covers the receptor's extracellular surface. The N-terminally attached myristoyl group of BLV interacts with the lipid-exposed surface of NTCP. Our structure reveals how BLV inhibits bile salt transport, rationalizes NTCP mutations that decrease the risk of HBV/HDV infection, and provides a basis for understanding the host specificity of HBV/HDV. Our results provide opportunities for structure-guided development of inhibitors that target HBV/HDV docking to NTCP.

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.

Interactions of Na+/taurocholate cotransporting polypeptide with host cellular proteins upon hepatitis B and D virus infection: novel potential targets for antiviral therapy

Na⁺/taurocholate cotransporting polypeptide (NTCP) is a member of the solute carrier (SLC) family 10 transporters (gene symbol SLC10A1) and is responsible for the sodium-dependent uptake of bile salts across the basolateral membrane of hepatocytes. In addition to its primary transporter function, NTCP is the high-affinity hepatic receptor for hepatitis B (HBV) and hepatitis D (HDV) viruses and, therefore, is a prerequisite for HBV/HDV virus entry into hepatocytes. The inhibition of HBV/HDV binding to NTCP and internalization of the virus/NTCP receptor complex has become a major concept in the development of new antiviral drugs called HBV/HDV entry inhibitors. Hence, NTCP has emerged as a promising target for therapeutic interventions against HBV/HDV infections in the last decade. In this review, recent findings on protein-protein interactions (PPIs) between NTCP and cofactors relevant for entry of the virus/NTCP receptor complex are summarized. In addition, strategies aiming to block PPIs with NTCP to dampen virus tropism and HBV/HDV infection rates are discussed. Finally, this article suggests novel directions for future investigations evaluating the functional contribution of NTCP-mediated PPIs in the development and progression of HBV/HDV infection and subsequent chronic liver disorders.

Tyrosine 146 of the Human Na+/Taurocholate Cotransporting Polypeptide (NTCP) Is Essential for Its Hepatitis B Virus (HBV) Receptor Function and HBV Entry into Hepatocytes

Na⁺/taurocholate cotransporting polypeptide (NTCP, gene symbol SLC10A1) is a hepatic bile acid uptake carrier participating in the enterohepatic circulation of bile acids. Apart from its transporter function, NTCP acts as the high-affinity liver-specific receptor for the hepatitis B virus (HBV), which attaches via its preS1-peptide domain of the large surface protein to NTCP, subsequently leading to endocytosis of the virus/NTCP-receptor complex. Although the process of NTCP-dependent HBV infection of hepatocytes has received much attention over the last decade, the precise molecular sites of the virus/NTCP interaction have not been fully identified. Inspection of the primary protein sequence of human NTCP revealed 139YIYSRGIY146 as a highly conserved tyrosine-rich motif. To study the role of Y139, Y141 and Y146 amino acids in NTCP biology, the aforementioned residues were substituted with alanine, phenylalanine or glutamate (mimicking phosphorylation) using site-directed mutagenesis. Similar to wt NTCP, the Y139A, Y141A, Y146A, Y141F, Y146F, and Y146E mutants were expressed at the plasma membrane of HEK293 cells and exhibited intact bile acid transport function. Y146A, Y146E, and Y146F demonstrated transport kinetics comparable to wild-type NTCP with K_m values of 57.3–112.4 µM and V_max values of 6683–7579 pmol/mg protein/min. Only Y141E was transport deficient, most likely due to an intracellular accumulation of the mutant protein. Most importantly, Y146A and Y146E mutation completely abrogated binding of the viral preS1-peptide to NTCP, while the Y146F mutant of NTCP showed some residual binding competence for preS1. Consequently, the NTCP mutants Y146A and Y146E, when expressed in HepG2 hepatoma cells, showed complete loss of susceptibility for in vitro HBV infection. In conclusion, tyrosine 146, and to some extent tyrosine 141, both belonging to the tyrosine-rich motif 139YIYSRGIY146 of human NTCP, are newly identified amino acid residues that play an essential role in the interaction of HBV with its receptor NTCP and, thus, in the process of virus entry into hepatocytes.

Role of the Steroid Sulfate Uptake Transporter Soat (Slc10a6) in Adipose Tissue and 3T3-L1 Adipocytes

In addition to the endocrine and paracrine systems, peripheral tissues such as gonads, skin, and adipose tissue are involved in the intracrine mechanisms responsible for the formation of sex steroids via the transformation of dehydroepiandrosterone and dehydroepiandrosterone sulfate (DHEA/DHEAS) into potent androgenic and estrogenic hormones. Numerous studies have examined the relationship between overweight, central obesity, and plasma levels of DHEA and DHEAS. The sodium-dependent organic anion transporter Soat (Slc10a6) is a plasma membrane uptake transporter for sulfated steroids. Significantly increased expression of Slc10a6 mRNA has been previously described in organs and tissues of lipopolysaccharide (LPS)-treated mice, including white adipose tissue. These findings suggest that Soat plays a role in the supply of steroids in peripheral target tissues. The present study aimed to investigate the expression of Soat in adipocytes and its role in adipogenesis. Soat expression was analyzed in mouse white intra-abdominal (WAT), subcutaneous (SAT), and brown (BAT) adipose tissue samples and in murine 3T3-L1 adipocytes. In addition, adipose tissue mass and size of the adipocytes were analyzed in wild-type and Slc10a6^−/− knockout mice. Soat expression was detected in mouse WAT, SAT, and BAT using immunofluorescence. The expression of Slc10a6 mRNA was significantly higher in 3T3-L1 adipocytes than that of preadipocytes and was significantly upregulated by exposure to lipopolysaccharide (LPS). Slc10a6 mRNA levels were also upregulated in the adipose tissue of LPS-treated mice. In Slc10a6^−/− knockout mice, adipocytes increased in size in the WAT and SAT of female mice and in the BAT of male mice, suggesting adipocyte hypertrophy. The serum levels of adiponectin, resistin, and leptin were comparable in wild-type and Slc10a6^−/− knockout mice. The treatment of 3T3-L1 adipocytes with DHEA significantly reduced lipid accumulation, while DHEAS did not have a significant effect. However, following LPS-induced Soat upregulation, DHEAS also significantly inhibited lipid accumulation in adipocytes. In conclusion, Soat-mediated import of DHEAS and other sulfated steroids could contribute to the complex pathways of sex steroid intracrinology in adipose tissues. Although in cell cultures the Soat-mediated uptake of DHEAS appears to reduce lipid accumulation, in Slc10a6^−/− knockout mice, the Soat deletion induced adipocyte hyperplasia through hitherto unknown mechanisms.

Detection of the ABCB11930_1931del TC Mutation in Two Suspected Ivermectin-Sensitive Cats and Their Relatives by a Novel TaqMan Allelic Discrimination Assay

The multidrug resistance gene MDR1 (syn. ABCB1) encodes for the multidrug efflux transporter P-glycoprotein (P-gp), which is highly expressed at the blood-brain barrier and protects the brain from potentially neurotoxic compounds, such as ivermectin. MDR1 mutation in dogs is known to be linked to dramatically increased brain accumulation of ivermectin and life-threatening neurological toxicity. The present report describes two suspected ivermectin-sensitive Maine Coon cats, which exhibited neurological toxicity following subcutaneous application of therapeutic doses of ivermectin. Both cats showed a homozygous 2-bp deletion in the MDR1/ABCB1 coding sequence (ABCB11930_1931del TC, syn. MDR1 nt1930(del2)) that had previously been associated with a drug-sensitive phenotype in cats. For cat MDR1 genotyping, a novel TaqMan allelic discrimination assay was established and validated. This assay was used for ABCB11930_1931del TC genotyping of the drug-sensitive cats as well as of more than 50 relatives. About half of them had the heterozygous MDR1(+/-) genotype, while none of these related cats with former ivermectin treatment had a history of drug-sensitivity. In conclusion: The present study supports previous findings on drug-sensitivity in cats with homozygous ABCB11930_1931del TC mutation. The newly established TaqMan allelic discrimination assay provides a useful and reliable method for routine MDR1 genotyping in cats in order to identify drug-sensitive cats prior to treatment with established P-gp substrates such as ivermectin and other macrocyclic lactones and thus to improve therapeutic safety.

Organic Cation Transporter I and Na+ /taurocholate Co-Transporting Polypeptide are Involved in Retrorsine- and Senecionine-Induced Hepatotoxicity in HepaRG cells

SCOPE

1,2-unsaturated pyrrolizidine alkaloids (PAs) are secondary plant metabolites that are found in many plant species throughout the world. They are of concern for risk assessment as consumption of contaminated foodstuff can cause severe liver damage. Of late, transporter-mediated uptake and transport has advanced as a vital determinant of PA toxicity. In this study, the authors investigate a transporter-mediated uptake of PAs and its implications in PA toxicity.

METHODS AND RESULTS

We show that transporter expression levels are significantly affected by treatment with the PAs senecionine (Sc) and retrorsine (Re) in the human hepatoma cell line HepaRG. Furthermore, the specific contribution to PA uptake of the two transporters Na⁺ /taurocholate co-transporting polypeptide (SLC10A1) and organic cation transporter I (SLC22A1), both belonging to the heterogeneous solute carrier super family, is investigated by means of a siRNA-mediated knockdown approach. Knockdown of both uptake transporters result in reduced uptake of Re and Sc in a time-dependent manner and attenuated PA-mediated cytotoxic effects in HepaRG cells.

CONCLUSION

Our results confirm previous findings of active transport mechanisms of PAs into hepatocytes and highlight the importance of toxicokinetic studies for the risk assessment of PAs.

Functional Analysis of Rare Genetic Variants in the Negative Regulator of Intracellular Calcium Signaling RCAS/SLC10A7

The solute carrier family 10 member SLC10A7 is a negative regulator of intracellular calcium signaling (RCAS). In cell culture, SLC10A7 expression is negatively correlated with store-operated calcium entry (SOCE) via the plasma membrane. SLC10A7-deficient cells have significantly increased calcium influx after treatment with thapsigargin for depletion of ER calcium stores, whereas SLC10A7/RCAS overexpression limits calcium influx. Genetic variants in the human SLC10A7 gene are associated with skeletal dysplasia and amelogenesis imperfecta and reveal loss of function on cellular calcium influx. More recently, an additional disease-related genetic variant (P303L) as well as some novel genetic variants (V235F, T221M, I136M, L210F, P285L, and G146S) have been identified. In the present study, these variants were expressed in HEK293 cells to study their subcellular localization and their effect on cellular calcium influx. All variants were properly sorted to the ER compartment and closely co-localized with the STIM protein, a functional component of SOCE. The variants P303L and L210F showed significantly reduced effects on cellular calcium influx compared to the wild type but still maintained some degree of residual activity. This might explain the milder phenotype of patients bearing the P303L variant and might indicate disease potential for the newly identified L210F variant. In contrast, all other variants behaved like the wild type. In conclusion, the occurrence of variants in the SLC10A7 gene should be considered in patients with skeletal dysplasia and amelogenesis imperfecta. In addition to the already established variants, the present study identifies another potential disease-related SLC10A7/RCAS variant, namely, L210F, which seems to be most frequent in South Asian populations.

Identification of Novel HBV/HDV Entry Inhibitors by Pharmacophore- and QSAR-Guided Virtual Screening

The hepatic bile acid transporter Na⁺/taurocholate co-transporting polypeptide (NTCP) was identified in 2012 as the high-affinity hepatic receptor for the hepatitis B and D viruses (HBV/HDV). Since then, this carrier has emerged as promising drug target for HBV/HDV virus entry inhibitors, but the synthetic peptide Hepcludex^® of high molecular weight is the only approved HDV entry inhibitor so far. The present study aimed to identify small molecules as novel NTCP inhibitors with anti-viral activity. A ligand-based bioinformatic approach was used to generate and validate appropriate pharmacophore and QSAR (quantitative structure–activity relationship) models. Half-maximal inhibitory concentrations (IC₅₀) for binding inhibition of the HBV/HDV-derived preS1 peptide (as surrogate parameter for virus binding to NTCP) were determined in NTCP-expressing HEK293 cells for 150 compounds of different chemical classes. IC₅₀ values ranged from 2 µM up to >1000 µM. The generated pharmacophore and QSAR models were used for virtual screening of drug-like chemicals from the ZINC¹⁵ database (~11 million compounds). The 20 best-performing compounds were then experimentally tested for preS1-peptide binding inhibition in NTCP-HEK293 cells. Among them, four compounds were active and revealed experimental IC₅₀ values for preS1-peptide binding inhibition of 9, 19, 20, and 35 µM, which were comparable to the QSAR-based predictions. All these compounds also significantly inhibited in vitro HDV infection of NTCP-HepG2 cells, without showing any cytotoxicity. The best-performing compound in all assays was ZINC000253533654. In conclusion, the present study demonstrates that virtual compound screening based on NTCP-specific pharmacophore and QSAR models can predict novel active hit compounds for the development of HBV/HDV entry inhibitors.

Mutational Analysis of the GXXXG/A Motifs in the Human Na+/Taurocholate Co-Transporting Polypeptide NTCP on Its Bile Acid Transport Function and Hepatitis B/D Virus Receptor Function

Homodimerization is essential for plasma membrane sorting of the liver bile acid transporter NTCP and its function as Hepatitis B/D Virus (HBV/HDV) receptor. However, the protein domains involved in NTCP dimerization are unknown. NTCP bears two potential GXXXG/A dimerization motifs in its transmembrane domains (TMDs) 2 and 7. The present study aimed to analyze the role of these GXXXG/A motifs for the sorting, function, and dimerization of NTCP. The NTCP mutants G₆₀LXXXA₆₄L (TMD2), G₂₃₃LXXXG₂₃₇L (TMD7) and a double mutant were generated and analyzed for their interaction with wild-type NTCP using a membrane-based yeast-two hybrid system (MYTH) and co-immunoprecipitation (co-IP). In the MYTH system, the TMD2 and TMD7 mutants showed significantly lower interaction with the wild-type NTCP. In transfected HEK293 cells, membrane expression and bile acid transport activity were slightly reduced for the TMD2 mutant but were completely abolished for the TMD7 and the TMD2/7 mutants, while co-IP experiments still showed intact protein-protein interactions. Susceptibility for in vitro HBV infection in transfected HepG2 cells was reduced to 50% for the TMD2 mutant, while the TMD7 mutant was not susceptible for HBV infection at all. We conclude that the GXXXG/A motifs in TMD2 and even more pronounced in TMD7 are important for proper folding and sorting of NTCP, and so indirectly affect glycosylation, homodimerization, and bile acid transport of NTCP, as well as its HBV/HDV receptor function.

First Sequencing of Caprine Mdr1 (Abcb1) mRNA Due to Suspected Neurological Adverse Drug Reaction in a Thuringian Goat Following Extra-Label Use of Doramectin

The multidrug resistance gene MDR1 encodes for an efflux transporter called P-glycoprotein (P-gp). In the canine Mdr1 gene, a nonsense mutation was identified in certain dog breeds causing increased drug sensitivity to various P-gp substrates such as antiparasitic macrocyclic lactones. Symptoms of neurologic toxicity include ataxia, depression, salivation, tremor, apparent blindness, and mydriasis. In the current report, a Thuringian goat developed similar neurological signs after treatment with doramectin, a compound from the macrocyclic lactone class. Therefore, Mdr1 might be defective in this individual goat. For diagnostic purposes, sequencing of the complete mRNA transcript coding for caprine Mdr1 was performed to investigate a potential missense mutation. The Mdr1 transcripts of two related goats without drug sensitivity were also sequenced to allow differential diagnosis and were compared to the suspected drug-sensitive goat. The only position where the Mdr1 sequence from the suspected drug-sensitive goat differed was in the 3′-untranslated region, being a heterozygous single nucleotide polymorphism c.3875C>A. It can be suspected that this variant affects the expression level, stability, or translation efficiency of the Mdr1 mRNA transcript and therefore might be associated with the suspected drug sensitivity. To clarify this, further studies are needed, particularly investigating the Mdr1 mRNA and protein expression levels from brain material of affected goats. In conclusion, Mdr1 variants may exist not only in dogs, but also in individual goats. The current report provides the first Mdr1 mRNA transcript sequence of a goat and therefore represents the basis for more detailed Mdr1 sequence and expression analyses.

Functional and Pharmacological Comparison of Human and Mouse Na+/Taurocholate Cotransporting Polypeptide (NTCP)

The Na⁺/taurocholate cotransporting polypeptide (NTCP) is located in the basolateral membrane of hepatocytes, where it transports bile acids from the portal blood back into hepatocytes. Furthermore, NTCP has a role for the hepatic transport of some drugs. Extrapolation of drug transport data from rodents to humans is not always possible, because species differences in the expression level, localization, affinity, and substrate selectivity of relevant transport proteins must be considered. In the present study, a functional comparison of human NTCP (hNTCP) and mouse Ntcp (mNtcp) showed similar K_m values of 67 ± 10 µM and 104 ± 9 µM for the probe substrate estrone-3-sulfate as well as of 258 ± 42 µM and 199 ± 13 µM for the drug rosuvastatin, respectively. IC₅₀ values for the probe inhibitor cyclosporine A were 3.1 ± 0.3 µM for hNTCP and 1.6 ± 0.4 µM for mNtcp. In a drug and pesticide inhibitory screening on both transporters, 4 of the 15 tested drugs (cyclosporine A, benzbromarone, MK571, and fluvastatin) showed high inhibitory potency, but only slight inhibition was observed for the 13 tested pesticides. Among these compounds, only four drugs and three pesticides showed significant differences in their inhibition pattern on hNTCP and mNtcp. Most pronounced was the difference for benzbromarone with a fivefold higher IC₅₀ for mNtcp (27 ± 10 µM) than for hNTCP (5.5 ± 0.6 µM).In conclusion, we found a strong correlation between the transport kinetics and inhibition pattern among hNTCP and mNtcp. However, specific compounds, such as benzbromarone, showed clear species differences. Such species differences have to be considered when pharmacokinetic data are transferred from rodent to humans.

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.

Sequencing of the Canine Cytochrome P450 CYP2C41 Gene and Genotyping of Its Polymorphic Occurrence in 36 Dog Breeds

Cytochrome P450 (CYP) drug metabolizing enzymes play an important role in efficient drug metabolism and elimination. Many CYPs are polymorphic and, thereby, drug metabolism can vary between individuals. In the case of canine CYP2C41, gene polymorphism was identified. However, as the first available canine genome sequences all were CYP2C41 negative, this polymorphism could not be clarified at the genomic level. The present study provides an exact characterization of the CYP2C41 gene deletion polymorphism at the genomic level and presents a PCR-based genotyping method that was used for CYP2C41 genotyping of 1,089 individual subjects from 36 different dog breeds. None of the Bearded Collie, Bernese Mountain, Boxer, Briard, French Bulldog or Irish Wolfhound subjects had the CYP2C41 gene in their genomes. In contrast, in the Chinese Char-Pei, Siberian Husky, Schapendoes and Kangal breeds, the CYP2C41 allele frequency was very high, with values of 67, 57, 43, and 34%, respectively. Interestingly, the site of gene deletion was identical for all CYP2C41 negative dogs, and all CYP2C41 positive dogs showed highly homologous sequence domains upstream and downstream from the CYP2C41 gene. CYP2C41 genotyping can now be routinely used in future pharmacokinetic studies in canines, in order to identify genetically-based poor or extensive drug metabolizers. This, together with more extensive in vitro drug screening for CYP2C41 substrates will help to determine the clinical relevance of CYP2C41, and to optimize drug treatment. Although the relative abundance of the CYP2C41 protein in the canine liver seems to not be very high, this CYP could substantially contribute to hepatic drug metabolism in dogs expressing CYP2C41 from both alleles and, when CYP2C41 shows higher catalytic activity to a given drug than other hepatic metabolic enzymes.

Hepatitis D Virus Entry Inhibitors Based on Repurposing Intestinal Bile Acid Reabsorption Inhibitors

Identification of Na⁺/taurocholate co-transporting polypeptide (NTCP) as high-affinity hepatic entry receptor for the Hepatitis B and D viruses (HBV/HDV) opened the field for target-based development of cell-entry inhibitors. However, most of the HBV/HDV entry inhibitors identified so far also interfere with the physiological bile acid transporter function of NTCP. The present study aimed to identify more virus-selective inhibitors of NTCP by screening of 87 propanolamine derivatives from the former development of intestinal bile acid reabsorption inhibitors (BARIs), which interact with the NTCP-homologous intestinal apical sodium-dependent bile acid transporter (ASBT). In NTCP-HEK293 cells, the ability of these compounds to block the HBV/HDV-derived preS1-peptide binding to NTCP (virus receptor function) as well as the taurocholic acid transport via NTCP (bile acid transporter function) were analyzed in parallel. Hits were subsequently validated by performing in vitro HDV infection experiments in NTCP-HepG2 cells. The most potent compounds S985852, A000295231, and S973509 showed in vitro anti-HDV activities with IC₅₀ values of 15, 40, and 70 µM, respectively, while the taurocholic acid uptake inhibition occurred at much higher IC₅₀ values of 24, 780, and 490 µM, respectively. In conclusion, repurposing of compounds from the BARI class as novel HBV/HDV entry inhibitors seems possible and even enables certain virus selectivity based on structure-activity relationships.

Long-term trans-inhibition of the hepatitis B and D virus receptor NTCP by taurolithocholic acid

Human hepatic bile acid transporter Na⁺/taurocholate cotransporting polypeptide (NTCP) represents the liver-specific entry receptor for the hepatitis B and D viruses (HBV/HDV). Chronic hepatitis B and D affect several million people worldwide, but treatment options are limited. Recently, HBV/HDV entry inhibitors targeting NTCP have emerged as promising novel drug candidates. Nevertheless, the exact molecular mechanism that NTCP uses to mediate virus binding and entry into hepatocytes is still not completely understood. It is already known that human NTCP mRNA expression is downregulated under cholestasis. Furthermore, incubation of rat hepatocytes with the secondary bile acid taurolithocholic acid (TLC) triggers internalization of the rat Ntcp protein from the plasma membrane. In the present study, the long-term inhibitory effect of TLC on transport function, HBV/HDV receptor function, and membrane expression of human NTCP were analyzed in HepG2 and human embryonic kidney (HEK293) cells stably overexpressing NTCP. Even after short-pulse preincubation, TLC had a significant long-lasting inhibitory effect on the transport function of NTCP, but the NTCP protein was still present at the plasma membrane. Furthermore, binding of the HBV/HDV myr-preS1 peptide and susceptibility for in vitro HDV infection were significantly reduced by TLC preincubation. We hypothesize that TLC rapidly accumulates in hepatocytes and mediates long-lasting trans-inhibition of the transport and receptor function of NTCP via a particular TLC-binding site at an intracellularly accessible domain of NTCP. Physiologically, this trans-inhibition might protect hepatocytes from toxic overload of bile acids. Pharmacologically, it provides an interesting novel NTCP target site for potential long-acting HBV/HDV entry inhibitors.NEW & NOTEWORTHY The hepatic bile acid transporter NTCP is a high-affinity receptor for hepatitis B and D viruses. This study shows that TLC rapidly accumulates in NTCP-expressing hepatoma cells and mediates long-lasting trans-inhibition of NTCP's transporter and receptor function via an intracellularly accessible domain, without substantially affecting its membrane expression. This domain is a promising novel NTCP target site for pharmacological long-acting HBV/HDV entry inhibitors.

Trospium Chloride Transport by Mouse Drug Carriers of the Slc22 and Slc47 Families

Background: The muscarinic receptor antagonist trospium chloride (TCl) is used for pharmacotherapy of the overactive bladder syndrome. TCl is a hydrophilic positively charged drug. Therefore, it has low permeability through biomembranes and requires drug transporters for distribution and excretion. In humans, the organic cation transporters OCT1 and OCT2 and the multidrug and toxin extrusion MATE1 and MATE2-K carriers showed TCl transport. However, their individual role for distribution and excretion of TCl is unclear. Knockout mouse models lacking mOct1/mOct2 or mMate1 might help to clarify their role for the overall pharmacokinetics of TCl. Method: In preparation of such experiments, TCl transport was analyzed in HEK293 cells stably transfected with the mouse carriers mOct1, mOct2, mMate1, and mMate2, respectively. Results: Mouse mOct1, mOct2, and mMate1 showed significant TCl transport with Km values of 58.7, 78.5, and 29.3 µM, respectively. In contrast, mMate2 did not transport TCl but showed MPP⁺ transport with Km of 60.0 µM that was inhibited by the drugs topotecan, acyclovir, and levofloxacin. Conclusion: TCl transport behavior as well as expression pattern were quite similar for the mouse carriers mOct1, mOct2, and mMate1 compared to their human counterparts.

Selective hepatitis B and D virus entry inhibitors from the group of pentacyclic lupane-type betulin-derived triterpenoids

Current treatment options against hepatitis B and D virus (HBV/HDV) infections have only limited curative effects. Identification of Na⁺/taurocholate co-transporting polypeptide (NTCP) as the high-affinity hepatic receptor for both viruses in 2012 enables target-based development of HBV/HDV cell-entry inhibitors. Many studies already identified appropriate NTCP inhibitors. However, most of them interfere with NTCP’s physiological function as a hepatic bile acid transporter. To overcome this drawback, the present study aimed to find compounds that specifically block HBV/HDV binding to NTCP without affecting its transporter function. A novel assay was conceptualized to screen for both in parallel; virus binding to NTCP (measured via binding of a preS1-derived peptide of the large HBV/HDV envelope protein) and bile acid transport via NTCP. Hits were subsequently validated by in vitro HDV infection studies using NTCP-HepG2 cells. Derivatives of the birch-derived pentacyclic lupane-type triterpenoid betulin revealed clear NTCP inhibitory potency and selectivity for the virus receptor function of NTCP. Best performing compounds in both aspects were 2, 6, 19, and 25. In conclusion, betulin derivatives show clear structure–activity relationships for potent and selective inhibition of the HBV/HDV virus receptor function of NTCP without tackling its physiological bile acid transport function and therefore are promising drug candidates.

A double-Flp-in method for stable overexpression of two genes

Overexpression of single genes in mammalian cells is widely used to investigate protein function in basic and applied biosciences and in drug research. A better understanding of interactions of two proteins is an important next step in the advancement of our understanding of complex biological systems. However, simultaneous and robust overexpression of two or more genes is challenging. The Flp-In system integrates a vector into cell lines at a specific genomic locus, but has not been used for integration of more than one gene. Here we present a modification of the Flp-In system that enables the simultaneous targeted integration of two genes. We describe the modification and generation of the vectors required and give the complete protocol for transfection and validation of correct genomic integration and expression. We also provide results on the stability and reproducibility, and we functionally validated this approach with a pharmacologically relevant combination of a membrane transporter facilitating drug uptake and an enzyme mediating drug metabolism.

Very High Dehydroepiandrosterone Sulfate (DHEAS) in Serum of an Overweight Female Adolescent Without a Tumor

Introduction: An increase of serum dehydroepiandrosterone (DHEA) sulfate (DHEAS) is observed in premature adrenarche and congenital adrenal hyperplasia. Very high DHEAS levels are typical for adrenal tumors. Approximately 74% of DHEAS is hydrolyzed to DHEA by the steroid sulfatase (STS). The reverse reaction is DHEA sulfation. Besides these two enzyme reactions, the DHEAS transported through the cell membrane is important for its distribution and excretion. Case Presentation: We present a female adolescent with overweight and a very high DHEAS. The presence of a DHEAS-producing tumor was rejected using ultrasonography, Magnetic Resonance Tomography (MRT), and dexamethasone suppression. STS deficiency was suspected. Sequence analysis revealed a heterozygous nonsense mutation which predicts a truncation of the carboxyl region of the STS that is implicated in substrate binding. No partial gene deletion outside exon 5 was detected by multiplex ligation-dependent probe amplification. The bioassay revealed normal enzyme activity in the patient's leukocytes. A defect of transporter proteins was suggested. Both efflux [multidrug-resistance protein (MRP)2 and breast cancer-resistance protein (BCRP)] and uptake [organic anion-transporting polypeptide (OATP) and organic anion transporter (OAT) carriers] transporters were studied. Sequence analysis of exons revealed a heterozygous Q141K variant for BCRP. Conclusions: A novel heterozygous nonsense mutation in the STS gene and a known heterozygous missense variant in the BCRP gene were found. The heterozygous nonsense mutation in the STS gene is not supposed to be responsible for STS deficiency. The BCRP variant is associated with reduced efflux transport activity only in its homozygous state. The combination of the two heterozygous mutations could possibly explain the observed high levels of DHEAS and other sulfated steroids.

In-vitro safety and off-target profile of the anti-parasitic arylmethylaminosteroid 1o

Parasite-mediated diseases like malaria and schistosomiasis are growing health problems worldwide and novel drug candidates are urgently needed. In this study, the in-vitro safety profile of steroid compound 1o (sc1o), effective against the parasites Plasmodium falciparum and Schistosoma mansoni with an IC₅₀ value of 5 nM, was characterized. We assessed viability/proliferation, apoptosis and cell cycle tests to determine the cytotoxic profile of sc1o in cancer cells. The mutagenic potential was determined with the AMES test. To identify off-target effects we investigated whether sc1o interacts with safety-relevant molecules such as cytochrome P450 (CYP) enzymes, phosphodiesterases (PDE), histone deacteylases (HDAC) and human ether-a-go-go related gene (hERG). Furthermore, to predict the potential bioavailability of sc1o, its effect on Caco-2 cell barrier integrity, by measurement of the transepithelial electrical resistance (TEER), was determined. Sc1o at 25 µM reduced cell viability, probably through cell-cycle arrest, but did not induce apoptosis in cancer cells. No adverse off-target effects nor mutagenic potential of sc1o were observed. Furthermore, sc1o did not disturb the integrity of the cell barrier, but exhibited low membrane permeability, apparently due to cell adherence. In conclusion, sc1o up to 10 µM showed a good in-vitro safety profile.

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.

Adverse Drug Reactions After Administration of Emodepside/Praziquantel (Profender®) in an MDR1-Mutant Australian Shepherd Dog: Case Report

A 3-year-old male Australian Shepherd was presented with signs of neurological toxicity following the administration of Profender® at the recommended dosage. Unfortunately, the owner had received the product from a veterinarian without any further instructions on fasting as recommended by the manufacturer, so the dog was fed prior to Profender® administration. Neurological toxicity included generalized tremor, agitation and panting, and required hospitalization of the dog. All neurological signs resolved after symptomatic treatment within 24 h and the dog was discharged without the need for further medication. MDR1 genotyping revealed a homozygous mutation of the MDR1 gene, which is normally important to prevent brain penetration of emodepside by an efflux-based transport mechanism at the blood brain barrier. This case indicates that Profender® can lead to serious, but transient neurological toxicity in dogs with homozygous MDR1 mutation even at therapeutic dosage, in particular when fasting recommendations are disregarded. Therefore, the case report highlights both the importance of MDR1 genotyping in predisposed dog breeds as well as strict compliance with fasting recommendations around the time of Profender® administration.

Suspected neurological toxicity after oral application of fluralaner (Bravecto®) in a Kooikerhondje dog

Background

Although the new isoxazoline drug fluralaner (Bravecto®) is generally well tolerated in dogs, adverse drug reactions involving neurological dysfunction occurred in individual dogs. However, most of these cases are documented inadequately and none of them is reported and discussed in the literature. As isoxazoline drugs target neuronal chloride channels with a clear preference for invertebrates, they are considered to have a good safety profile. However, pharmacodynamic effects in the nervous system of vertebrates cannot be ruled out completely.

Case presentation

A seven-month-old female Kooikerhondje dog was treated with Bravecto® at the recommended dose. About 24 h after administration, the dog exhibited signs of neurological toxicity, including generalized ataxia, myoclonic jerks, tremor of head and body, muscle twitching and oral dysphagia. All symptoms were transient and the dog fully recovered without any treatment after 10 h.

Conclusion

This case report describes transient occurrence of neurological dysfunction after administration of Bravecto®. It may help to better classify adverse drug reactions after application of isoxazoline drugs and documents a good prognosis even after occurrence of severe neurological dysfunction in the present case.

Electronic supplementary material

The online version of this article (10.1186/s12917-019-2016-4) contains supplementary material, which is available to authorized users.

Expression of components of the urothelial cholinergic system in bladder and cultivated primary urothelial cells of the pig

BACKGROUND

Porcine urinary bladders are widely used for uro-pharmacological examinations due to their resemblance to the human organ. However, characterisations of the porcine urothelium at the molecular level are scarce up to now. As it has become clear over the last years that this tissue plays an important role in the signaling-pathways of the bladder, we examined whether the transporter and receptor pattern (with focus on the transmitter acetylcholine) is comparable to the human urothelium. With regard to in vitro studies, we also investigated if there is a difference between the native tissue and cultivated primary urothelial cells in culture.

METHODS

Urothelium from German Landrace and Göttingen Minipig bladders was collected. One part of the German Landrace tissue was used for cultivation, and different passages of the urothelial cells were collected. The actual mRNA expression of different transporters and receptors was examined via quantitative real-time PCR. These included the vesicular acetylcholine transporter (VAChT), the choline acetyl transferase (ChAT), organic cation transporters 1-3 (OCT1-3), organic anion transporting polypeptide 1A2 (OATP1A2), P-glycoprotein (ABCB1), the carnitine acetyl-transferase (CarAT), as well as the muscarinic receptors 1-5 (M1-5).

RESULTS

There is a strong qualitative resemblance between the human and the porcine urothelium with regard to the investigated cholinergic receptors, enzymes and transporters. CarAT, OCT1-3, OATP1A2 and ABCB1 could be detected in the urothelium of both pig races. Moreover, all 5 M-receptors were prominent with an emphasis on M2 and M3. VAChT and ChAT could not be detected at all. Cultures of the derived urothelial cells showed decreased expression of all targets apart from ABCB1 and CarAT.

CONCLUSIONS

Based on the expression pattern of receptors, transporters and enzymes of the cholinergic system, the porcine urinary bladder can be regarded as a good model for pharmacological studies. However, cultivation of primary urothelial cells resulted in a significant drop in mRNA expression of the targets. Therefore, it can be concluded that the intact porcine urothelium, or the whole pig bladder, may be appropriate models for studies with anticholinergic drugs, whereas cultivated urothelial cells have some limitation due to significant changes in the expression levels of relevant targets.

Estrone-3-Sulfate Stimulates the Proliferation of T47D Breast Cancer Cells Stably Transfected With the Sodium-Dependent Organic Anion Transporter SOAT (SLC10A6)

Estrogens play a pivotal role in the development and proliferation of hormone-dependent breast cancer. Apart from free estrogens, which can directly activate the estrogen receptor (ER) of tumor cells, sulfo-conjugated steroids, which maintain high plasma concentrations even after menopause, first have to be imported into tumor cells by carrier-mediated uptake and then can be cleaved by the steroid sulfatase to finally activate ERs and cell proliferation. In the present study, expression of the sodium-dependent organic anion transporter SOAT was analyzed in breast cancer and its role for hormone-dependent proliferation of T47D breast cancer cells was elucidated. The SOAT protein was localized to the ductal epithelium of the mammary gland by immunohistochemistry. SOAT showed high expression in different pathologies of the breast with a clear ductal localization, including ductal hyperplasia, intraductal papilloma, and intraductal carcinoma. In a larger breast cancer cDNA array, SOAT mRNA expression was high in almost all adenocarcinoma specimen, but expression did not correlate with either the ER, progesterone receptor, or human epidermal growth factor receptor 2 status. Furthermore, SOAT expression did not correlate with tumor stage or grade, indicating widespread SOAT expression in breast cancer. To analyze the role of SOAT for breast cancer cell proliferation, T47D cells were stably transfected with SOAT and incubated under increasing concentrations of estrone-3-sulfate (E₁S) and estradiol at physiologically relevant concentrations. Cell proliferation was significantly increased by 10^-9 M estradiol as well as by E₁S with EC₅₀ of 2.2 nM. In contrast, T47D control cells showed 10-fold lower sensitivity to E₁S stimulation with EC₅₀ of 21.7 nM. The E₁S-stimulated proliferation of SOAT-T47D cells was blocked by the SOAT inhibitor 4-sulfooxymethylpyrene. In conclusion: The present study clearly demonstrates expression of SOAT in breast cancer tissue with ductal localization. SOAT inhibition can block the E₁S-stimulated proliferation of T47D breast cancer cells, demonstrating that SOAT is an interesting novel drug target from the group of E₁S uptake carriers for anti-proliferative breast cancer therapy.

Effects of supplementing rumen-protected niacin on fiber composition and metabolism of skeletal muscle in dairy cows during early lactation

Nicotinic acid (NA) has been shown to induce muscle fiber switching toward oxidative type I fibers and a muscle metabolic phenotype that favors fatty acid (FA) utilization in growing rats, pigs, and lambs. The hypothesis of the present study was that supplementation of NA in cows during the periparturient phase also induces muscle fiber switching from type II to type I fibers in skeletal muscle and increases the capacity of the muscle to use free FA, which may help to reduce nonesterified fatty acid (NEFA) flow to the liver, liver triglyceride (TG) accumulation, and ketogenesis. Thirty multiparous Holstein dairy cows were allocated to 2 groups and fed a total mixed ration without (control group) or with ∼55 g of rumen-protected NA per cow per day (NA group) from 21 d before expected calving until 3 wk postpartum (p.p.). Blood samples were collected on d -21, -14, -7, 7, 14, 21, 35, and 63 relative to parturition for analysis of TG, NEFA, and β-hydroxybutyrate. Muscle and liver biopsies were collected on d 7 and 21 for gene expression analysis and to determine muscle fiber composition in the musculus semitendinosus, semimembranosus, and longissimus lumborum by immunohistochemistry, and liver TG concentrations. Supplementation of NA did not affect the proportions of type I (oxidative) or the type II:type I ratio in the 3 muscles considered. A slight shift from glycolytic IIx fibers toward oxidative-glycolytic fast-twitch IIa fibers was found in the semitendinosus, and a tendency in the longissimus lumborum, but not in the semimembranosus. The transcript levels of the genes encoding the muscle fiber type isoforms and involved in FA uptake and oxidation, carnitine transport, tricarboxylic acid cycle, oxidative phosphorylation, and glucose utilization were largely unaffected by NA supplementation in all 3 muscles. Supplementation of NA had no effect on plasma TG and NEFA concentrations, liver TG concentrations, and hepatic expression of genes involved in hepatic FA utilization and lipogenesis. However, it reduced plasma β-hydroxybutyrate concentrations in wk 2 and 3 p.p. by 18 and 26% and reduced hepatic gene expression of fibroblast growth factor 21, a stress hormone involved in the regulation of ketogenesis, by 74 and 56%. In conclusion, a high dosage of rumen-protected NA reduced plasma β-hydroxybutyrate concentrations in cows during early lactation, but failed to cause an alteration in muscle fiber composition and muscle metabolic phenotype.

Characterisation of the hepatitis B virus cross-species transmission pattern via Na+/taurocholate co-transporting polypeptides from 11 New World and Old World primate species

The hepatic Na+/taurocholate co-transporting polypeptide (NTCP in man, Ntcp in animals) is the high-affinity receptor for the hepatitis B (HBV) and hepatitis D (HDV) viruses. Species barriers for human HBV/HDV within the order Primates were previously attributed to Ntcp sequence variations that disable virus-receptor interaction. However, only a limited number of primate Ntcps have been analysed so far. In the present study, a total of 11 Ntcps from apes, Old and New World monkeys were cloned and expressed in vitro to characterise their interaction with HBV and HDV. All Ntcps showed intact bile salt transport. Human NTCP as well as the Ntcps from the great apes chimpanzee and orangutan showed transport-competing binding of HBV derived myr-preS1-peptides. In contrast, all six Ntcps from the group of Old World monkeys were insensitive to HBV myr-preS1-peptide binding and HBV/HDV infection. This is basically predetermined by the amino acid arginine at position 158 of all studied Old World monkey Ntcps. An exchange from arginine to glycine (as present in humans and great apes) at this position (R158G) alone was sufficient to achieve full transport-competing HBV myr-preS1-peptide binding and susceptibility for HBV/HDV infection. New World monkey Ntcps showed higher sequence heterogeneity, but in two cases with 158G showed transport-competing HBV myr-preS1-peptide binding, and in one case (Saimiri sciureus) even susceptibility for HBV/HDV infection. In conclusion, amino acid position 158 of NTCP/Ntcp is sufficient to discriminate between the HBV/HDV susceptible group of humans and great apes (158G) and the non-susceptible group of Old World monkeys (158R). In the case of the phylogenetically more distant New World monkey Ntcps amino acid 158 plays a significant, but not exclusive role.

Sodium-dependent organic anion transporter (Slc10a6-/-) knockout mice show normal spermatogenesis and reproduction, but elevated serum levels for cholesterol sulfate

The sodium-dependent organic anion transporter SOAT (gene name SLC10A6 in man and Slc10a6 in mice) is a plasma membrane transporter for sulfated steroids, which is highly expressed in germ cells of the testis. SOAT can transport biologically inactive sulfated steroids into specific target cells, where they can be reactivated by the steroid sulfatase (STS) to biologically active, unconjugated steroids known to regulate spermatogenesis. Significantly reduced SOAT mRNA expression was previously found in different forms of impaired spermatogenesis in man. It was supposed that SOAT plays a role for the local supply of steroids in the testis and consequently for spermatogenesis and fertility. Thus, an Slc10a6^-/- Soat knockout mouse model was established by recombination-based target deletion of the Slc10a6 gene to elucidate the role of Soat in reproduction. However, the Slc10a6^-/- knockout mice were fertile, produced normal litter sizes, and had normal spermatogenesis and sperm vitality. This phenotype suggests that the loss of Soat can be compensated in the knockout mice or that Soat function is not essential for reproduction. In addition to reproductive phenotyping, a comprehensive targeted steroid analysis including a set of 9 un-conjugated and 12 sulfo-conjugated steroids was performed in serum of Slc10a6^-/- knockout and Slc10a6^+/+ wildtype mice. Only cholesterol sulfate, corticosterone, and testosterone (only in the males) could be detected in considerable amounts. Interestingly, male Slc10a6^-/- knockout mice showed significantly higher serum levels for cholesterol sulfate compared to their wildtype controls. As cholesterol sulfate has a broader impact apart from the testis, further analysis of this phenotype will include other organs such as skin and lung, which also show high Soat expression in the mouse.

Transport of steroid 3-sulfates and steroid 17-sulfates by the sodium-dependent organic anion transporter SOAT (SLC10A6)

The sodium-dependent organic anion transporter SOAT/Soat shows highly specific transport activity for sulfated steroids. SOAT substrates identified so far include dehydroepiandrosterone sulfate, 16α-hydroxydehydroepiandrosterone sulfate, estrone-3-sulfate, pregnenolone sulfate, 17β-estradiol-3-sulfate, and androstenediol sulfate. Apart from these compounds, many other sulfated steroids occur in mammals. Therefore, we aimed to expand the substrate spectrum of SOAT and analyzed the SOAT-mediated transport of eight different sulfated steroids by combining in vitro transport experiments in SOAT-transfected HEK293 cells with LC-MS/MS analytics of cell lysates. In addition, we aimed to better understand the structural requirements for SOAT substrates and so selected structural pairs varying only at specific positions: 3α/3β-sulfate, 17α/17β-sulfate, mono-sulfate/di-sulfate, and 17α-hydroxylation. We found significant and sodium-dependent SOAT-mediated transport of 17α-hydroxypregnenolone sulfate, 17β-estradiol-17-sulfate, androsterone sulfate, epiandrosterone sulfate, testosterone sulfate, epitestosterone sulfate, and 5α-dihydrotestosterone sulfate. However, 17β-estradiol-3,17-disulfate was not transported by SOAT.

IN CONCLUSION

SOAT substrates from the group of sulfated steroids are characterized by a planar and lipophilic steroid backbone in trans-trans-trans conformation of the rings and a negatively charged mono-sulfate group at positions 3' or 17' with flexibility for α- or β- orientation. Furthermore, 5α-reduction, 16α-hydroxylation, and 17α-hydroxylation are acceptable for SOAT substrate recognition, whereas addition of a second negatively charged sulfate group seems to abolish substrate binding to SOAT, and so 17β-estradiol-3,17-disulfate is not transported by SOAT.

The polymorphism L204F affects transport and membrane expression of the sodium-dependent organic anion transporter SOAT (SLC10A6)

Sodium-dependent organic anion transporter (SOAT) represents a membrane transporter specific for sulfated steroid hormones, which are supposed to participate in the regulation of reproductive processes. In man, SOAT shows predominant mRNA expression in the testis and here was localized to primary spermatocytes. SOAT mRNA expression is significantly downregulated in different disorders of spermatogenesis, including hypospermatogenesis. The resulting decline of SOAT-mediated transport of sulfated steroids may participate in the impairment of functional spermatogenesis. Apart from downregulation of SOAT mRNA expression, genetic polymorphisms affecting the transport function of SOAT may have the same negative effect on spermatogenesis. Therefore, in the present study we searched for functionally relevant SOAT polymorphisms, aiming to comparatively analyze their occurrence in patients with impaired spermatogenesis vs. patients with intact spermatogenesis. We found that the SOAT polymorphism L204F showed a significantly reduced transport function for DHEAS when expressed in HEK293 cells. Although the K_m value was identical with that of the SOAT wildtype, the V_max value dramatically declined for the SOAT-L204F variant (942.5 vs. 313.6pmol×mg protein^-1×min^-1). Although the same amount of total SOAT-L204F protein was detected in transfected HEK293 cells compared to the SOAT wildtype, plasma membrane expression was significantly reduced, which points to a plasma membrane sorting defect of the SOAT-L204F variant. Groups of 20 subjects with normal spermatogenesis and 26 subjects with hypospermatogenesis were genotyped for this polymorphism. Both groups showed nearly identical distributions of the SOAT-L204F polymorphism (∼10% heterozygous and ∼5% homozygous), indicating that this polymorphism seems not be causative for hypospermatogenesis.

Rare genetic variants in the sodium-dependent organic anion transporter SOAT (SLC10A6): Effects on transport function and membrane expression

Sulfo-conjugated steroid hormones, such as dehydroepiandrosterone sulfate (DHEAS), pregnenolone sulfate or estrone-3-sulfate are abundant in the body, but are biologically inactive at classical androgen and estrogen steroid receptors. However, after carrier-mediated import and de-conjugation by the steroid sulfatase, these compounds participate in the overall steroid regulation of reproductive organs. The sodium-dependent organic anion transporter SOAT, coded by the SLC10A6 gene, is specific for the transport of steroid sulfates and is highly expressed in testicular germ cells, including pachytene spermatocytes, secondary spermatocytes, and round spermatids. Therefore, SOAT is supposed to be involved in the regulation of spermatogenesis and male fertility. In the present study, the SLC10A6 gene was analyzed for rare genetic variants, which might affect transport function or membrane expression of SOAT. Among the 31 SOAT variants analyzed, L44P, Q75R, P107L, G109S, S112F, N113K, S133F, G241D, G263E, G294R, and Y308N showed no transport activity for DHEAS at all. In the case of P107L, G241D, G263E, and Y308N, this was most likely due to significantly reduced expression in the plasma membrane. Other variants are located directly at (Q75R, S112F, N113K) or close to (G109S, S133F, and G263E) the supposed SOAT Na⁺ binding sites and thus could disable the sodium-coupled transport cycle. If these loss-of-function SOAT variants are more frequent in men with impaired spermatogenesis or infertility needs further investigation.

Real-time transmission of 16 Tb/s over 1020km using 200Gb/s CFP2-DCO

We demonstrate real-time transmission of 16 Tb/s (80x200Gb/s) over 1020km TeraWave ULL fiber with 170km span length using the world's first 200Gb/s CFP2-DCO module with a record low power consumption less than 0.1W/Gbps.

The carnitine status does not affect the contractile and metabolic phenotype of skeletal muscle in pigs

Background

Recently, supplementation of L-carnitine to obese rats was found to improve the carnitine status and to counteract an obesity-induced muscle fiber transition from type I to type II. However, it has not been resolved if the change of muscle fiber distribution induced in obese rats and the restoration of the "normal" muscle fiber distribution, which is found in lean rats, in obese rats by supplemental L-carnitine is causally linked with the carnitine status. In the present study we hypothesized that fiber type distribution in skeletal muscle is dependent on carnitine status.

Methods

To test this, an experiment with 48 piglets which were randomly allocated to four groups (n = 12) was performed. All piglets were given orally either 60 mg sodium bicarbonate/kg body weight (group CON), 20 mg L-carnitine and 60 mg sodium bicarbonate/kg body weight (group CARN), 30 mg pivalate (dissolved in sodium bicarbonate)/kg body weight (group PIV) or 20 mg L-carnitine and 30 mg pivalate/kg body weight (group CARN + PIV) each day for a period of 4 weeks.

Results

Concentrations of total carnitine in plasma, liver and longissimus dorsi and biceps femoris muscles were 2.0-2.7 fold higher in group CARN than in group CON, whereas these concentrations were 1.9-2.5-fold lower in group PIV than in group CON. The concentrations of total carnitine in these tissues did not statistically differ between group CARN + PIV and group CON. Fiber type distribution of longissimus dorsi and biceps femoris muscles, mRNA and protein levels of molecular regulators of fiber distribution in longissimus dorsi and biceps femoris muscles and mRNA levels of genes reflecting the metabolic phenotype of longissimus dorsi and biceps femoris muscles did not differ between groups.

Conclusion

Changes in the systemic carnitine status and the muscle carnitine concentration induced by either supplementing L-carnitine or administering pivalate have no impact on the contractile and metabolic phenotype of skeletal muscles in pigs.

The role of sulfated steroid hormones in reproductive processes

Sulfated steroid hormones, such as dehydroepiandrosterone sulfate or estrone-3-sulfate, have long been regarded as inactive metabolites as they cannot activate classical steroid receptors. Some of them are present in the blood circulation at quite high concentrations, but generally sulfated steroids exhibit low membrane permeation due to their hydrophilic properties. However, sulfated steroid hormones can actively be imported into specific target cells via uptake carriers, such as the sodium-dependent organic anion transporter SOAT, and, after hydrolysis by the steroid sulfatase (so-called sulfatase pathway), contribute to the overall regulation of steroid responsive organs. To investigate the biological significance of sulfated steroid hormones for reproductive processes in humans and animals, the research group "Sulfated Steroids in Reproduction" was established by the German Research Foundation DFG (FOR1369). Projects of this group deal with transport of sulfated steroids, sulfation of free steroids, desulfation by the steroid sulfatase, effects of sulfated steroids on steroid biosynthesis and membrane receptors as well as MS-based profiling of sulfated steroids in biological samples. This review and concept paper presents key findings from all these projects and provides a broad overview over the current research on sulfated steroid hormones in the field of reproduction.

Identification of novel inhibitors of the steroid sulfate carrier 'sodium-dependent organic anion transporter' SOAT (SLC10A6) by pharmacophore modelling

The sodium-dependent organic anion transporter SOAT specifically transports sulfated steroid hormones and is supposed to play a role in testicular steroid regulation and male fertility. The present study aimed to identify novel specific SOAT inhibitors for further in vitro and in vivo studies on SOAT function. More than 100 compounds of different molecular structures were screened for inhibition of the SOAT-mediated transport of dehydroepiandrosterone sulfate in stably transfected SOAT-HEK293 cells. Twenty-five of these with IC50 values covering four orders of magnitude were selected as training set for 3D pharmacophore modelling. The SOAT pharmacophore features were calculated by CATALYST and consist of three hydrophobic sites and two hydrogen bond acceptors. By substrate database screening, compound T 0511-1698 was predicted as a novel SOAT inhibitor with an IC50 of 15 μM. This value was confirmed by cell-based transport assays. Therefore, the developed SOAT pharmacophore model demonstrated its suitability in predicting novel SOAT inhibitors.

Transfection of Sertoli cells with androgen receptor alters gene expression without androgen stimulation

Background

Androgens play an important role for the development of male fertility and gained interest as growth and survival factors for certain types of cancer. Androgens act via the androgen receptor (AR/Ar), which is involved in various cell biological processes such as sex differentiation. To study the functional mechanisms of androgen action, cell culture systems and AR-transfected cell lines are needed. Transfection of AR into cell lines and subsequent gene expression analysis after androgen treatment is well established to investigate the molecular biology of target cells. However, it remains unclear how the transfection with AR itself can modulate the gene expression even without androgen stimulation. Therefore, we transfected Ar-deficient rat Sertoli cells 93RS2 by electroporation using a full length human AR.

Results

Transfection success was confirmed by Western Blotting, immunofluorescence and RT-PCR. AR transfection-related gene expression alterations were detected with microarray-based genome-wide expression profiling of transfected and non-transfected 93RS2 cells without androgen stimulation. Microarray analysis revealed 672 differentially regulated genes with 200 up- and 472 down-regulated genes. These genes could be assigned to four major biological categories (development, hormone response, immune response and metabolism). Microarray results were confirmed by quantitative RT-PCR analysis for 22 candidate genes.

Conclusion

We conclude from our data, that the transfection of Ar-deficient Sertoli cells with AR has a measurable effect on gene expression even without androgen stimulation and cause Sertoli cell damage. Studies using AR-transfected cells, subsequently stimulated, should consider alterations in AR-dependent gene expression as off-target effects of the AR transfection itself.

Electronic supplementary material

The online version of this article (doi:10.1186/s12867-015-0051-7) contains supplementary material, which is available to authorized users.

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.

A steroidogenic pathway for sulfonated steroids: the metabolism of pregnenolone sulfate

In many tissues sulfonated steroids exceed the concentration of free steroids and recently they were also shown to fulfill important physiological functions. While it was previously demonstrated that cholesterol sulfate (CS) is converted by CYP11A1 to pregnenolone sulfate (PregS), further conversion of PregS has not been studied in detail. To investigate whether a steroidogenic pathway for sulfonated steroids exists similar to the one described for free steroids, we examined the interaction of PregS with CYP17A1 in a reconstituted in-vitro system. Difference spectroscopy revealed a Kd-value of 74.8±4.2μM for the CYP17A1-PregS complex, which is 2.5-fold higher compared to the CYP17A1-pregnenolone (Preg) complex. Mass spectrometry experiments proved for the first time that PregS is hydroxylated by CYP17A1 at position C17, identically to pregnenolone. A higher Km- and a lower kcat-value for CYP17A1 using PregS compared with Preg were observed, indicating a 40% reduced catalytic efficiency when using the sulfonated steroid. Furthermore, we analyzed whether the presence of cytochrome b5 (b5) has an influence on the CYP17A1 dependent conversion of PregS, as was demonstrated for Preg. Interestingly, with 17OH-PregS no scission of the 17,20-carbon-carbon bond occurs, when b5 is added to the reconstituted in-vitro system, while b5 promotes the formation of DHEA from 17OH-Preg. When using human SOAT-HEK293 cells expressing CYP17A1 and CPR, we could confirm that PregS is metabolized to 17OH-PregS, strengthening the potential physiological meaning of a pathway for sulfonated steroids.

Kinetics of the bile acid transporter and hepatitis B virus receptor Na+/taurocholate cotransporting polypeptide (NTCP) in hepatocytes

BACKGROUND & AIMS

The human liver bile acid transporter Na(+)/taurocholate cotransporting polypeptide (NTCP) has recently been identified as liver-specific receptor for infection of hepatitis B virus (HBV), which attaches via the myristoylated preS1 (myr-preS1) peptide domain of its large surface protein to NTCP. Since binding of the myr-preS1 peptide to NTCP is an initiating step of HBV infection, we investigated if this process interferes with the physiological bile acid transport function of NTCP.

METHODS

HBV infection, myr-preS1 peptide binding, and bile acid transport assays were performed with primary Tupaia belangeri (PTH) and human (PHH) hepatocytes as well as NTCP-transfected human hepatoma HepG2 cells allowing regulated NTCP expression, in the presence of various bile acids, ezetimibe, and myr-preS1 peptides.

RESULTS

The myr-preS1 peptide of HBV inhibited bile acid transport in PTH and PHH as well as in NTCP-expressing HEK293 and HepG2 cells. Inversely, HBV infection of PTH, PHH, and NTCP-transfected HepG2 cells was inhibited in a concentration-dependent manner by taurine and glycine conjugates of cholic acid and ursodeoxycholic acid as well as by ezetimibe. In NTCP-HepG2 cells and PTH, NTCP expression, NTCP transport function, myr-preS1 peptide binding, and HBV infection followed comparable kinetics.

CONCLUSIONS

Myr-preS1 virus binding to NTCP, necessary for productive HBV infection, interferes with the physiological bile acid transport function of NTCP. Therefore, HBV infection via NTCP may be lockable by NTCP substrates and NTCP-inhibiting drugs. This opens a completely new way for an efficient management of HBV infection by the use of NTCP-directed drugs.

Transport of the placental estriol precursor 16α-hydroxy-dehydroepiandrosterone sulfate (16α-OH-DHEAS) by stably transfected OAT4-, SOAT-, and NTCP-HEK293 cells

16α-Hydroxy-dehydroepiandrosterone sulfate (16α-OH-DHEAS) mainly originates from the fetus and serves as precursor for placental estriol biosynthesis. For conversion of 16α-OH-DHEAS to estriol several intracellular enzymes are required. However, prior to enzymatic conversion, 16α-OH-DHEAS must enter the cells by carrier mediated transport. To identify these carriers, uptake of 16α-OH-DHEAS by the candidate carriers organic anion transporter OAT4, sodium-dependent organic anion transporter SOAT, Na(+)-taurocholate cotransporting polypeptide NTCP, and organic anion transporting polypeptide OATP2B1 was measured in stably transfected HEK293 cells by LC-MS-MS. Furthermore, the study aimed to localize SOAT in the human placenta. Stably transfected OAT4-HEK293 cells revealed a partly sodium-dependent transport for 16α-OH-DHEAS with an apparent Km of 23.1 ± 5.1 μM and Vmax of 485.0 ± 39.1 pmol/mg protein/min, while stably transfected SOAT- and NTCP-HEK293 cells showed uptake only under sodium conditions with Km of 319.0 ± 59.5 μM and Vmax of 1465.8 ± 118.8 pmol/mg protein/min for SOAT and Km of 51.4 ± 9.9 μM and Vmax of 1423.3 ± 109.6 pmol/mg protein/min for NTCP. In contrast, stably transfected OATP2B1-HEK293 cells did not transport 16α-OH-DHEAS at all. Immunohistochemical studies and in situ hybridization of formalin fixed and paraffin embedded sections of human late term placenta showed expression of SOAT in syncytiotrophoblasts, predominantly at the apical membrane as well as in the vessel endothelium. In conclusion, OAT4, SOAT, and NTCP were identified as carriers for the estriol precursor 16α-OH-DHEAS. At least SOAT and OAT4 seem to play a functional role for the placental estriol synthesis as both are expressed in the syncytiotrophoblast of human placenta.