Ernst Petzinger

The solute carrier family SLC10: more than a family of bile acid transporters regarding function and phylogenetic relationships

The solute carrier family 10 (SLC10) comprises two sodium-dependent bile acid transporters, i.e. the Na+/taurocholate cotransporting polypeptide (NTCP; SLC10A1) and the apical sodium-dependent bile acid transporter (ASBT; SLC10A2). These carriers are essentially involved in the maintenance of the enterohepatic circulation of bile acids mediating the first step of active bile acid transport through the membrane barriers in the liver (NTCP) and intestine (ASBT). Recently, four new members of the SLC10 family were described and referred to as P3 (SLC10A3), P4 (SLC10A4), P5 (SLC10A5) and sodium-dependent organic anion transporter (SOAT; SLC10A6). Experimental data supporting carrier function of P3, P4, and P5 is currently not available. However, as demonstrated for SOAT, not all members of the SLC10 family are bile acid transporters. SOAT specifically transports steroid sulfates such as oestrone-3-sulfate and dehydroepiandrosterone sulfate in a sodium-dependent manner, and is considered to play an important role for the cellular delivery of these prohormones in testes, placenta, adrenal gland and probably other peripheral tissues. ASBT and SOAT are the most homologous members of the SLC10 family, with high sequence similarity (∼70%) and almost identical gene structures. Phylogenetic analyses of the SLC10 family revealed that ASBT and SOAT genes emerged from a common ancestor gene. Structure–activity relationships of NTCP, ASBT and SOAT are discussed at the amino acid sequence level. Based on the high structural homology between ASBT and SOAT, pharmacological inhibitors of the ASBT, which are currently being tested in clinical trials for cholesterol-lowering therapy, should be evaluated for their cross-reactivity with SOAT.

Bile acid derived HMG-CoA reductase inhibitors

The target organ for HMG-CoA reductase inhibitors to decrease cholesterol biosynthesis in hypercholesterolemic patients is the liver. Since bile acids undergo an enterohepatic circulation showing a strict organotropism for the liver and the small intestine, the structural elements of an inhibitor for HMG-CoA reductase were combined with those for specific molecular recognition of a bile acid molecule for selective uptake by hepatocytes. Either, the HMG-CoA reductase inhibitors HR 780 and mevinolin were covalently attached to 3 xi-(omega-aminoalkoxy)-7 alpha, 12 alpha-dihydroxy-5 beta-cholan-24-oic acids to obtain bile acid prodrugs, or the side chain of bile acids at C-17 was replaced by 3,5-dihydroxy-heptanoic acid--a structural element essential for inhibition of HMG-CoA reductase--to obtain hybrid bile acid: HMG-CoA reductase inhibitors. The prodrugs could, as expected, not inhibit rat liver HMG-CoA reductase to a significant extent, whereas the hybrid inhibitors showed a stereospecific inhibition of HMG-CoA reductase from rat liver microsomes with an IC50-value of 0.7 microM for the most potent compound S 2467 and 6 microM for its diastereomere S 2468. Uptake measurements with isolated rat hepatocytes and ileal brush-border membrane vesicles from rabbit small intestine revealed a specific interaction of both classes of bile acid-derived HMG-CoA reductase inhibitors with the hepatocyte and ileocyte bile acid uptake systems. Photoaffinity labeling studies using 3-azi- or 7-azi-derivatives of taurocholate with freshly isolated rat hepatocytes or rabbit ileal brush-border membrane vesicles revealed a specific interaction of bile acid derived HMG-CoA reductase inhibitors with the respective putative bile acid transporters in the liver and the ileum demonstrating the bile acid character of these derivatives, both for the prodrugs and the hybrids. Cholesterol biosynthesis in Hep G2 cells was inhibited by the bile acid prodrugs with IC50-values in the range of 68 nM to 600 nM compared to 13 nM for HR 780 and 130 nM for mevinolin. Among the hybrid inhibitors, S 2467 was the most active compound with an IC50-value of 16 microM compared to 55 microM for its diastereomere S 2468. Preliminary in vivo experiments showed an inhibition of hepatic cholesterol biosynthesis after oral dosage only with prodrugs such as S 3554, whereas the hybrid molecules were inactive after oral application.(ABSTRACT TRUNCATED AT 400 WORDS)

Cloning and Functional Characterization of Human Sodium-dependent Organic Anion Transporter (SLC10A6)

We have cloned human sodium-dependent organic anion transporter (SOAT) cDNA, which consists of 1502 bp and encodes a 377-amino acid protein. SOAT shows 42% sequence identity to the ileal apical sodium-dependent bile acid transporter ASBT and 33% sequence identity to the hepatic Na+/taurocholate-cotransporting polypeptide NTCP. Immunoprecipitation of a SOAT-FLAG-tagged protein revealed a glycosylated form at 46 kDa that decreased to 42 kDa after PNGase F treatment. SOAT exhibits a seven-transmembrane domain topology with an outside-to-inside orientation of the N-terminal and C-terminal ends. SOAT mRNA is most highly expressed in testis. Relatively high SOAT expression was also detected in placenta and pancreas. We established a stable SOAT-HEK293 cell line that showed sodium-dependent transport of dehydroepiandrosterone sulfate, estrone-3-sulfate, and pregnenolone sulfate with apparent Km values of 28.7, 12.0, and 11.3 μm, respectively. Although bile acids, such as taurocholic acid, cholic acid, and chenodeoxycholic acid, were not substrates of SOAT, the sulfoconjugated bile acid taurolithocholic acid-3-sulfate was transported by SOAT-HEK293 cells in a sodium-dependent manner and showed competitive inhibition of SOAT transport with an apparent Ki value of 0.24 μm. Several nonsteroidal organosulfates also strongly inhibited SOAT, including 1-(ω-sulfooxyethyl)pyrene, bromosulfophthalein, 2- and 4-sulfooxymethylpyrene, and α-naphthylsulfate. Among these inhibitors, 2- and 4-sulfooxymethylpyrene were competitive inhibitors of SOAT, with apparent Ki values of 4.3 and 5.5 μm, respectively, and they were also transported by SOAT-HEK293 cells.

Drug transporters in pharmacokinetics

This review deals with the drug transporters allowing drugs to enter and leave cells by carrier-mediated pathways. Emphasis is put on liver transporters but systems in gut, kidney, and blood-brain barrier are mentioned as well. Drug-drug interactions on carriers may provoke significant modification in pharmacokinetics as do carrier gene polymorphisms yielding functional carrier protein mutations. An integrated phase concept should reflect the interplay between drug metabolism and drug transport.

Inhibition of 3H-demethylphalloin uptake in isolated rat hepatocytes under various experimental conditions

Summary3H-Demethylphalloin (3H-DMP) a cyclopeptide very similar to phalloidin is taken up by isolated hepatocytes in vitro. Hepatocytes prepared from newborn animals are less sensitive to phalloidin. Their uptake of 3H-DMP is about one tenth of that of cells from adult animals. Ascites hepatoma cells, known to be insensitive to phalloidin took up negligible amounts of 3H-DMP. Cells prepared from regenerating livers took up insignificantly lower amounts of the toxin than in hepatocytes from adult animals.Treatment of hepatocytes with low concentrations of trypsin was found to switch off the phalloidin sensitivity in a reversible manner. This inhibition is due to a reduced uptake of 3H-DMP. Pretreatment of animals with CCl4, known to reduce the sensitivity to phalloidin, also decreases the uptake of 3H-DMP in isolated hepatocytes.Various agents, drugs and reagents were found to inhibit the response of isolated hepatocytes to phalloidin. All these compounds (bile acids, rifampicin, silybin, DIDS, glutardialdehyde, bromosulphophthalein, fusidic acid, antamanide, novobiocin) inhibit also the uptake of 3H-DMP in isolated hepatocytes. The results confirm our working hypothesis, presented in several previous papers, that decreased sensitivity to phalloidin is probably due to a reduced or blocked uptake of the toxin.

Driving forces in hepatocellular uptake of phalloidin and cholate

Active uptake of phalloidin and cholate in isolated rat liver cells depends upon both Na+ gradient and membrane potential. Omission of Na+ or inhibition of the (Na+ + K+)-ATPase diminished both phalloidin and cholate uptake. Dissipation of the sodium, potassium or proton gradient by monensin, nigericin, gramicidin and valinomycin blocked phalloidin uptake and also caused reduction of cholate transport. Chelation of Ca2+ and Mg2+ by EGTA or incubation of liver cells with NH4Cl neither influenced phalloidin nor cholate uptake. Hyperpolarization of liver cells by the lipophilic anions NO3- or SCN- enhanced phalloidin but reduced cholate uptake. Depolarization induced by a reversed K+ gradient reduced both kinds of transport. The results indicate that sodium ions and the membrane potential are driving forces for phalloidin and cholate uptake in hepatocytes.

Decreased sensitivity of isolated hepatocytes from baby rats, from regenerating and from poisoned livers to phalloidin

SummaryIsolated hepatocytes, prepared from 5 day old rats, from regenerating livers or from livers after poisoning with carbon tetrachloride, are less sensitive to phalloidin in vitro than hepatocytes from untreated adult controls. The time course of the reduced susceptibility to phalloidin was compared with the ability of hepatocytes to take up bile acids under various conditions. SDS-electrophoresis of cell lysates gave no evidence for decreased levels of actin in cells with reduced sensitivity to phalloidin. In contrast, there was a good relationship between the active uptake of bile acids and the sensitivity of hepatocytes to phalloidin. The decreased response of hepatocytes from baby rats, from regenerating livers or from poisoned livers to phalloidin is more probably related to differences in phalloidin uptake than to a reduced endowment with microfilamentous structures.

The Role of Bile Acids in Phalloidin Poisoning

SummaryGlycocholate and other bile acids inhibit the response of isolated hepatocytes to phalloidin in a concentration dependent manner. It is suggested that the inhibition is due to a block of phalloidin uptake. This interaction might explain the high specificity of phalloidin for liver tissue.

Breed distribution of the nt230(del4) MDR1 mutation in dogs.

A 4-bp deletion mutation associated with multiple drug sensitivity exists in the canine multidrug resistance (MDR1) gene. This mutation has been detected in more than 10 purebred dog breeds as well as in mixed breed dogs. To evaluate the breed distribution of this mutation in Germany, 7378 dogs were screened, including 6999 purebred and 379 mixed breed dogs. The study included dog breeds that show close genetic relationship or share breeding history with one of the predisposed breeds but in which the occurrence of the MDR1 mutation has not been reported. The breeds comprised Bearded Collies, Anatolian Shepherd Dog, Greyhound, Belgian Tervuren, Kelpie, Borzoi, Australian Cattle Dog and the Irish Wolfhound. The MDR1 mutation was not detected is any of these breeds, although it was found as expected in the Collie, Longhaired Whippet, Shetland Sheepdog, Miniature Australian Shepherd, Australian Shepherd, Wäller, White Swiss Shepherd, Old English Sheepdog and Border Collie with varying allelic frequencies for the mutant MDR1 allele of 59%, 45%, 30%, 24%, 22%, 17%, 14%, 4% and 1%, respectively. Allelic frequencies of 8% and 2% were determined in herding breed mixes and unclassified mixed breeds, respectively. Because of its widespread breed distribution and occurrence in many mixed breed dogs, it is difficult for veterinarians and dog owners to recognise whether MDR1-related drug sensitivity is relevant for an individual animal. This study provides a comprehensive overview of all affected dog breeds and many dog breeds that are probably unaffected on the basis of ∼15,000 worldwide MDR1 genotyping data.

Silibinin protects OTA-mediated TNF-α release from perfused rat livers and isolated rat Kupffer cells

We studied the inhibitory effect of silibinin on ochratoxin A (OTA) and LPS-mediated tumor necrosis factor alpha (TNF-alpha) release and the leakage of cytotoxic markers glutamate dehydrogenase (GLDH) and lactate dehydrogenase (LDH), from isolated blood-free perfused rat livers, and from isolated pure rat Kupffer cells. In the recirculation perfusion model at the end point 90 min, 2.5 micromol/L OTA released 2600 pg/mL TNF-alpha without effects on liver vitality. LPS at 0.1 microg/mL induced 3000 pg TNF-alpha/mL with slight leakage of GLDH and LDH. Under similar experimental conditions, the addition of silibinin 10 min prior to OTA and LPS showed dose-dependent protection against OTA or LPS-induced hepatic TNF-alpha release. High-dose of silibinin (12.5 microg/mL) also completely restored GLDH and LDH levels in the perfusate. Pretreatment of isolated Kupffer cells with 0.02, 0.1, 0.5, 2.5, and 12.5 microg silibinin/mL 30 min prior to OTA reduced OTA-induced TNF-alpha levels to 90, 70, 25, 25, and 25% at 4 h, respectively, and abrogated any TNF-alpha release at 24 h. Similarly, in the presence of silibinin LPS-induced TNF-alpha levels decreased at 4 h to 71, 57, 18, 22, and 18%, respectively. However, after 24 h of LPS exposition the protection by silibinin vanished and TNF-alpha partially recurred into the incubation medium under LPS. In summary, silibinin had hepatoprotective effects against OTA- or LPS-mediated TNF-alpha release and also reduced the cytotoxicity of both toxins. Isolated Kupffer cells were even more sensitive to the protective effect than perfused livers and responded to very low concentrations of silibinin with a strong inhibition of toxins-mediated TNF-alpha release.