Regulation of Plasma Membrane Localization of the Na+-Taurocholate Co-Transporting Polypeptide by Glycochenodeoxycholate and Tauroursodeoxycholate
Patrick G. K. Mayer Natalia Qvartskhava Annika Sommerfeld Boris Görg Dieter Häussinger
Clinic for Gastroenterology, Hepatology, and Infectious Diseases, Heinrich-Heine-University, Düsseldorf, Germany
Key Words
TUDC • GCDC • Ntcp • Integrins • Reactive oxygen species
Abstract
Background/Aims: Hydrophobic bile salts, such as glycochenodeoxycholate (GCDC) can trigger hepatocyte apoptosis, which is prevented by tauroursodesoxycholate (TUDC), but the effects of GCDC and TUDC on sinusoidal bile salt uptake via the Na+-taurocholate transporting polypeptide (Ntcp) are unclear. Methods: The effects of GCDC and TUDC on the plasma membrane localization of Ntcp were studied in perfused rat liver by means of immunofluorescence analysis and super-resolution microscopy. The underlying signaling events were investigated by Western blotting and inhibitor studies. Results: GCDC (20 µmol/l) induced within 60 min a retrieval of Ntcp from the basolateral membrane into the cytosol, which was accompanied by an activating phosphorylation of the Src kinases Fyn and Yes. Both, Fyn activation and the GCDC-induced Ntcp retrieval from the plasma membrane were sensitive to the NADPH oxidase inhibitor apocynin, the antioxidant N-acetylcysteine and the Src family kinase inhibitors SU6656 and PP-2, whereas PP-2 did not inhibit GCDC-induced Yes activation. Internalization of Ntcp by GCDC was also prevented by the protein kinase C (PKC) inhibitor Gö6850. TUDC (20 µmol/l) reversed the GCDC-induced retrieval of Ntcp from the plasma membrane and prevented the activation of Fyn and Yes in GCDC-perfused rat livers. Reinsertion of Ntcp into the basolateral membrane in GCDC-perfused livers by TUDC was sensitive to the protein kinase A (PKA) inhibitor H89 and the integrin-inhibitory peptide GRGDSP, whereas the control peptide GRADSP was ineffective. Exposure of cultured rat hepatocytes to GCDC (50 µmol/l, 15min) increased the fluorescence intensity of the reactive oxygen fluorescent indicator DCF to about 1.6-fold of untreated controls in a TUDC (50 µmol/l)-sensitive way. GCDC caused a TUDC-sensitive canalicular dilatation without evidence for Bsep retrieval from the canalicular membrane. Conclusions: The present study suggests that GCDC triggers the retrieval of Ntcp from the basolateral membrane into the cytosol through an oxidative stress-dependent activation of Fyn. TUDC prevents the GCDC-induced Fyn activation and Ntcp retrieval through integrin-dependent activation of PKA.
Introduction
Liver cells express various bile salt transport proteins in order to maintain bile salt homeostasis. When the transcellular bile salt transport becomes impaired, bile salts accumulate in the hepatocyte which can trigger hepatocyte apoptosis and liver dysfunction [1-4]. The sodium taurocholate cotransporting polypeptide (Ntcp) is the major uptake system for conjugated bile salts from blood into liver parenchymal cells [5]. Secretion of bile salts at the canalicular membrane is mediated by the bile salt export pump (Bsep) [6] and by the multidrug resistance associated protein 2 (Mrp2) [7].
The expression of Bsep and Mrp2 in the apical membrane has been shown to be regulated by oxidative stress [8-10] and osmolarity [9, 11-16]. The ambient osmolarity can also regulate the cellular distribution of Ntcp in the hepatocyte. In this regard, hepatocyte swelling triggers Ntcp insertion into the membrane via activation of the phosphatidylinositide-3 kinase (PI3K)-signaling pathway and increases taurocholate (TC) uptake in hepatocytes [17], whereas hyperosmolarity induces the retrieval of Ntcp from the plasma membrane [15, 16]. This latter effect is mediated by the Src family kinase Fyn [15, 16], which is activated by a protein kinase C (PKC) ζ-dependent and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-driven formation of reactive oxygen species (ROS) [15, 16].
Cholestatic liver diseases lead to a retention of hydrophobic bile salts, such as glycochenodeoxycholate (GCDC) in the liver, which plays an important role in the pathogenesis of cholestatic liver injury [18-22].
Cholestatic disorders also trigger gene expression changes of bile salt transporters [23, 24]. While organic anion transporting protein 2 (OATP2) and Ntcp expression decreases with disease progression, MRP3 and P-glycoprotein become up-regulated. Expression of the basolateral Ntcp and Oatp2, as well as canalicular BSEP and MRP2 are also reduced in patients with acute inflammation-induced cholestasis. Retrieval of Bsep from the canalicular membrane in response to hyperosmotic hepatocyte shrinkage is accompanied by Ntcp retrieval, which is thought to counteract hepatocyte overload with bile salts when canalicular excretion fails [11, 15, 16].
Ursodeoxycholic acid (UDCA), which in vivo is rapidly conjugated with taurine (TUDC) has hepatoprotective and choleretic effects in liver and is widely used for the treatment of cholestatic disorders, such as primary biliary cholangitis and intrahepatic cholestasis of pregnancy [25-28]. TUDC stimulates insertion of canalicular transporters into the canalicular membrane via dual activation of the MAP kinases p38MAPK and Erk-1/-2 [29, 30] and exerts antiapoptotic activity [21, 31]. TUDC also counteracts internalization of Bsep [32] and Mrp2 [33] in taurolithocholate (TLC)-induced cholestasis. PKAα has been proposed to mediate the anti-cholestatic and antiapoptotic effects of TUDC [34]. In line with this, cyclic adenosine monophosphate (cAMP) increases Ntcp in the basolateral membrane as well as Bsep and Mrp2 in the canalicular membrane. β1-integrins were identified as receptors for TUDC [16] and β1-integrin activation not only triggers choleresis via dual MAP kinase activation, but also is antiapoptotic through formation of cAMP and activation of protein kinase A (PKA) [31]. This prevents hyperosmotic ROS formation and Fyn activation and is associated with decreased serine phosphorylation of the Ntcp [15].
The effects of the hydrophobic bile salt GCDC on the subcellular localization of Ntcp in hepatocytes are unknown and were investigated in the present study. The results show, that GCDC triggers an oxidative stress-, and Fyn-dependent internalization of Ntcp, which is counteracted by TUDC in a β1-integrin- and PKA-dependent way.
Materials and Methods
Materials
Materials used in the present study were purchased as follows: apocynin, SU6656, PP-2, H-Gly-Arg-Gly-Asp-Ser-Pro-OH (GRGDSP) and H-Gly-Arg-Ala-Asp-Ser-Pro-OH (GRADSP) were from Merck-Millipore (Darmstadt, Germany); N-acetylcystein (NAC), Dibutyryl-cAMP (DB-cAMP) sodium salt, collagenase, insulin and TUDC were from Sigma Aldrich (Munich, Germany). H89 dihydrochloride, penicillin/streptomycin and Fluoromount-G were from Tocris/Biozol (Eching, Germany). Fetal bovine serum (FBS), G418 geneticin and Dulbecco’s modified Eagle’s medium Nutrimix F12 were from Life Technologies GmbH (Darmstadt, Germany). Gö6850, complete-protease inhibitor cocktail tablets and PhosSTOP-phosphatase inhibitor cocktail tablets were from Roche Diagnostics (Mannheim, Germany) and William’s Medium E was from Biochrom (Berlin, Germany). The Ntcp antibody (K4) [5] was a generous gift from Prof. Dr. B. Stieger (Kantonsspital Zürich, Switzerland). The Bsep antibody was a generous gift from Professor Dr. P. J. Meier and Professor Dr. B. Stieger (University Hospital Zurich, Switzerland) [6]. The monoclonal antibody directed against ZO-1 was from Invitrogen (Karlsruhe, Germany). Antibodies recognizing Yes (immunoprecipitation, IP) and Fyn (IP) were from Santa Cruz Biotechnology (Heidelberg, Germany) and antibodies directed against c-Src were from Life Technologies GmbH (Darmstadt, Germany). The anti-phospho-serine antibody was from Enzo Life Sciences GmbH (Lörrach, Germany). The antibodies raised against phospho-Src family-Tyr-418 were from Cell Signaling Technology, Inc. (Danvers, USA), against Na+/K+-ATPase, Yes (Western blot, WB), Fyn (WB), phospho-cSrc, Cy3-conjugated donkey anti-rabbit IgG and FITC-conjugated donkey anti-mouse IgG from Merck-Millipore (Darmstadt, Germany). Horseradish peroxidase-conjugated anti-mouse IgG and anti-rabbit IgG were from Bio-Rad Laboratories (Munich, Germany) and Dako (Hamburg, Germany), respectively. All other chemicals were from Merck-Millipore (Darmstadt, Germany) at the highest quality available.
Liver perfusion
Livers from male Wistar rats (140-160 g, fed ad libitum with standard diet) were perfused in a non-recirculating manner with the bicarbonate-buffered Krebs-Henseleit saline plus L-lactate (2.1 mmol/l) and pyruvate (0.3 mmol/l) and equilibrated with 5% CO2 and 95% O2 at 37°C, as described previously [35]. Addition of bile salts, inhibitors and DB-cAMP to the influent perfusate was made by dissolution into the Krebs-Henseleit buffer.
Preparation and culture of primary rat hepatocytes
Hepatocytes were prepared from livers of male Wistar rats (160-180 g) by a collagenase perfusion technique in an adapted version as described previously [36]. Aliquots of rat hepatocytes were plated on collagen-coated culture plates and maintained in bicarbonate-buffered Krebs-Henseleit medium (115 mmol/l NaCl, 25 mmol/l NaHCO3, 5.9 mmol/l KCl, 1.18 mmol/l MgCl2, 1.23 mmol/l NaH2PO4, 1.2 mmol/l Na2SO4, 1.25 mmol/l CaCl2), supplemented with 6 mmol/l glucose in a humidified atmosphere of 5% CO2 and 95% air at 37°C. After 2 h, the medium was removed and the cells were washed twice. Subsequently the culture was continued for 24 h in William`s Medium E, supplemented with 2 mmol/l glutamine, 100 nmol/l insulin, 100 U/ml penicillin, 0.1 mg/ml streptomycin, 100nmol/l dexamethasone and 5% FBS. After 24 h, experimental treatments were performed using William`s Medium E that contained 2 mmol/l glutamine and 100 nmol/l dexamethasone. The viability of hepatocytes was more than 95% as assessed by trypan blue exclusion.
Immunofluorescence staining
Immunofluorescence analysis was performed as described recently [15, 16]. Tissue samples from perfused liver were cut at -20 °C in 7µm sections using a Leica Cryotom CM 3050 (Leica, Bensheim, Germany) and placed on Superfrost plus slides (Menzel, Braunschweig, Germany). Slides were air-dried (1 h) and kept at -20 °C until staining. After fixation with pure methanol (-20 °C, 2 min), sections were washed with PBS and blocked with a solution containing 5% fetal calf serum in PBS for 30min at room temperature. Then, liver sections were incubated for 2 h in a wet chamber with combinations of primary antibodies against Ntcp (1:200) and Na+/K+-ATPase (1:200) (overnight, 4°C), washed thrice, and stained with an anti-mouse-FITC and an anti-rabbit Cy3-conjugated antibody (1:500 fetal calf serum 5% in PBS, 2 h, RT, respectively). The samples were covered with Fluoromount-G and visualized by confocal laser scanning microscopy (LSM) using LSM510 META, the LSM880 or by super resolution structured illumination microscopy (SR-SIM) using the ELYRA microscope (Zeiss, Oberkochen, Germany).
Densitometric fluorescence intensity analysis
Densitometric fluorescence intensity analysis was performed as described recently [15, 16]. Cryosections of perfused rat liver for the analysis at the basolateral membrane were stained for Ntcp, Bsep and for Na+/K+-ATPase (plasma membrane marker protein) and ZO-1 which lines the border between the canalicular and the sinusoidal membrane. For negative controls, primary antibodies were omitted in each experiment. ZO-1 and Na+/K+-ATPase profiles were selected according apparent integrity and comparability. Acceptable Na+/K+-ATPase intensity profiles have a sufficiently high peak fluorescence in the central part (corresponding to the basolateral membrane) and low intracellular fluorescence. Densitometric analysis was performed as described previously [9, 13] using Image-Pro Plus (Media Cybernetics, Rockville, USA) software. Briefly, the fluorescence intensity profile was measured over a thick line at a right angle to the membrane. The length of the line was always 8 µm. The mean fluorescence intensity of each pixel over the line perpendicular to the length was calculated by Image-Pro Plus. The pixel intensity data (pixel positions with the associated pixel intensities, red and green channel) were transferred to an Excel data sheet (Microsoft Excel for Windows, Redmond, USA). Each measurement was normalized to the sum of all intensities of the respective measurement. Values are given as means ± SEM. Data from at least 10 different areas per tissue sample and from at least 3 independent liver preparations were processed. Peak fluorescence intensity values were statistically compared using one way analysis of variance (ANOVA) followed by Dunnet´s post hoc test. A p-value <0.05 was considered statistically significant.
Western blot analysis
Western blot analysis was performed as described recently [15, 16]. Liver samples were immediately lysed at 4°C by using a lysis buffer containing 20 mmol/l Tris-HCl (pH 7.4), 140 mmol/l NaCl, 10 mmol/l NaF, 10 mmol/l sodium pyrophosphate, 1% (v/v) Triton X-100, 1 mmol/l EDTA, 1 mmol/l EGTA, 1 mmol/l sodium vanadate, 20 mmol/l β-glycerophosphate, protease inhibitor and phosphatase inhibitor cocktail. The lysates were kept on ice for 10 min and then centrifuged at 8, 000 rpm for 8 min at 4°C, and aliquots of the supernatant were taken for protein determination using the Bio-Rad protein assay (Bio-Rad Laboratories, Munich, Germany). Equal amounts of protein were subjected to sodium dodecyl sulfate/ polyacrylamide gel electrophoresis, and transferred onto nitrocellulose membranes using a semidry transfer apparatus (GE Healthcare, Freiburg, Germany). Membranes were blocked for 60 min in 5% (w/v) bovine serum albumin containing 20 mmol/l Tris (pH 7.5), 150 mmol/l NaCl, and 0.1% Tween 20 (TBS-T) and exposed to primary antibodies overnight at 4°C. After washing with TBS-T and incubation at RT for 2 h with horseradish peroxidase-coupled anti-mouse or anti-rabbit IgG antibody, respectively (all diluted 1:10, 000), the immunoblots were washed extensively and bands were visualized using the ChemiDocTM Touch Imaging System from Bio-Rad (Munich, Germany). Semi-quantitative evaluation was carried out by densitometry using the Image Lab Touch Software from Bio-Rad. Protein phosphorylation is given as the ratio of detected phospho-protein/total protein.
Realtime PCR analysis
Realtime PCR analysis was performed as described recently [15, 16]. Total RNA was isolated using the RNAeasy mini kit (Qiagen, Hilden, Germany) according to the manufacturers´ protocol. RNA was quantified using NanoDrop1000 System (Thermo Scientific, Wilmington, USA) and first strand cDNA was synthesized from RNA using the RevertAid H Minus First Strand cDNA Synthesis Kit (Fermentas, St. Leon-Rot, Germany). Gene expression levels were quantified using SensiMix SYBR No-ROX Kit (Bioline, Luckenwalde, Germany) on a TOptical Real-Time qPCR thermal cycler (Biometra, Göttingen, Germany). Hypoxanthine-guanine phosphoribosyltransferase 1 (HPRT1) was used as reference gene for the normalization of the results obtained by the 2(−ΔΔCt) method. PCR-primer sequences: Ntcp 5’-TCAAGTCCAAAAGGCCACACT-3’ and 5’-AGGGAGGAGGTAGCCAGTAAG-3’, HPRT1 5’-TGCTCGAGATGTCATGAAGGA-3’ and 5’-CAGAGGGC CACAATGTGATG-3’. Statistical analysis was performed using one way analysis of variance (ANOVA).
Detection of Reactive Oxygen Species (ROS) in cultures hepatocytes
For detecting reactive oxygen species (ROS), hepatocytes were loaded with 2′, 7′-dichlorodihydrofluorescein diacetate (H2DCF-DA). Upon oxidation by ROS, the non-fluorescent H2DCF is converted into the highly fluorescent 2',7'-dichlorofluorescein (DCF). Isolated rat hepatocytes were seeded on collagen-coated 6-well culture plates and cultured for 24 h. Cells were loaded with PBS containing 5µmol/L of H2DCF-DA for 30 min at 37 °C and 5% CO2. At the end of the incubation time, excess H2DCF-DA was removed by washing the cells 3 times with culture medium (William‘s Medium E). Hepatocytes were pre-incubated with 50µmol/L tauroursodeoxycholic acid (TUDC, 30min, 5% CO2, 37°C) before cells were exposed to glycochenodeoxycholic acid (GCDC, 50µmol/l, 15min, 5% CO2, 37°C). Cells were washed 3 times with PBS and mounted on an epifluorescence microscope (Cell Observer Z1, ZEISS, Oberkochen, Germany). DCF was excited at 488nm and fluorescence emission was collected at 565nm. Measurements were performed in triplicate for each experimental condition and mean fluorescence intensities were corrected for background fluorescence. Emission intensity in GCDC-exposed hepatocytes is expressed relative to untreated controls. Statistical analysis was performed using one way analysis of variance (ANOVA).
Statistical analysis
Results from at least 3 independent experiments are expressed as mean values ± SEM. n refers to the number of independent experiments. Differences between experimental groups were analyzed by student`s t-test, or one-way analysis of variance (ANOVA) followed by Dunnett´s, Tukey´s or Bonferroni´s multiple comparison post hoc test where appropriate (GraphPad Prism; GraphPad, La Jolla, USA; Microsoft Excel for Windows). A p-value < 0.05 was considered statistically significant.
Results
GCDC triggers Ntcp retrieval from the basolateral membrane in the perfused rat liver
Subcellular Ntcp distribution in control and GCDC (20 µmol/l)-perfused livers was analyzed by confocal laser-scanning microscopy and quantified by densitometric analysis of fluorescence intensities as described in the methods section. For plasma membrane labelling, liver sections were stained with an antibody directed against the plasma membrane protein Na+/K+-ATPase. The immunofluorescence analysis shows that the fluorescence profiles of Ntcp shifted to significantly lower peak values and became more broadened (p < 0.05, Fig. 1A) in response to treatment with GCDC (20 µmol/l, 60 and 90 min), suggestive for internalization of Ntcp. In contrast, GCDC had no effect on the Na+/K+-ATPase localization (Fig. 1A). As shown in Fig. 1A by immunofluorescence microscopy, GCDC triggered the appearance of punctate Ntcp-staining, which was enriched underneath the plasma membrane, supporting the view that GCDC triggers Ntcp retrieval from the plasma membrane.
When cultured hepatocytes were exposed to GCDC, Ntcp mRNA levels remained unchanged over a 9h-period GCDC treatment (0.88 ± 0.27-fold of untreated controls, n.s., n = 4, Fig. 1B). However, after 24h Ntcp mRNA levels tended to decrease in GCDC-exposed hepatocytes but without reaching statistical significance (0.45 ± 0.07-fold, n = 4, Fig. 1B).
The subcellular distribution of Ntcp was also analysed by super-resolution structured-illumination microscopy (SR-SIM) in liver slices co-stained with Ntcp and Na+/K+-ATPase (Fig. 2). In GCDC-perfused livers (20 µmol/l, 60 or 90 min), intracellular vesicular Ntcp fluorescence intensity was markedly enhanced compared to control livers. On the other hand, Na+/K+-ATPase expression at the basolateral membrane remained largely unchanged in GCDC-perfused livers (Figs. 1, 2).
These results indicate that GCDC triggers the internalisation of Ntcp in the intact rat liver within 60 min of GCDC exposure, lasting for at least 90 min.
Role of reactive oxygen species for GCDC-induced activation of Yes and Fyn in perfused rat liver
Hyperosmolarity was shown to trigger NADPH oxidase-dependent formation of reactive oxygen species (ROS) [37] and to activate the Src family kinases Yes and Fyn in the perfused rat liver [11]. Also GCDC activates NADPH oxidase and triggers the formation of ROS in cultured hepatocytes [4]. We therefore analysed effects of GCDC on the activation of Src family kinases Yes, Fyn and c-Src in perfused rat liver.
As shown in Fig. 3, the Src family kinases Yes and Fyn, but not c-Src became activated within 30 min of GCDC perfusion. The GCDC-induced Fyn and Yes activation was strongly blunted by the NADPH oxidase inhibitor apocynin (20 µmol/l) and the antioxidant NAC (10 mmol/l). GCDC-induced Fyn activation was largely abolished in presence of the Src kinase inhibitors SU6656 (1 µmol/l) and PP-2 (250 nmol/l), whereas GCDC-induced Yes activation was sensitive to inhibition by SU6656, but not PP-2 (Fig. 3).
The data suggest that the Src family kinases Fyn and Yes but not c-Src become activated by GCDC in a NADPH oxidase and ROS-dependent way.
Role of Fyn and Yes for the GCDC-induced retrieval of Ntcp from the basolateral membrane
The plasma membrane localization of Ntcp was analysed in GCDC (20 µmol/l)-perfused livers by laser scanning microscopy and fluorescence intensity analysis in the absence or presence of inhibitors of oxidative stress (apocynin and NAC) or Src kinases (PP-2, SU6656).
Fig. 4A depicts the liver perfusion plans chosen for these experiments. As shown in Fig. 4A and B, NAC (10 mmol/l) and apocynin (20 µmol/l) completely abolished the GCDC (20 µmol/l, 60 min)-induced Ntcp retrieval from the basolateral membrane, which was otherwise observed within 60 min (see Fig. 1A). As expected, the Na+/K+-ATPase distribution remained unchanged under these conditions. Likewise, the Src kinase inhibitors SU6656 (1 µmol/l) and PP-2 (250 nmol/l) and the broad spectrum protein kinase C (PKC) inhibitor Gö6850 (10 µmol/l) prevented the GCDC-induced Ntcp retrieval from the basolateral membrane (Fig. 4B).
These findings suggest that the retrieval of Ntcp from the plasma membrane depends on activation of NADPH oxidase, ROS formation, PKC and Src kinases. Because PP-2 prevented both, GCDC-induced Ntcp retrieval and Fyn (but not Yes) activation, it is concluded that GCDC-induced Ntcp internalization is mediated by Fyn, but not by Yes. Similar observations were made regarding the hyperosmolarity-induced Ntcp retrieval [15, 16].
Tauroursodesoxycholate (TUDC) reverses GCDC-induced Ntcp internalization via activation of integrins and protein kinase A
The bile salt TUDC triggers via β1-integrin activation the insertion of intracellularly stored Bsep and Mrp2 into the canalicular membrane, thereby mediating a choleretic effect [12, 16]. We therefore investigated the effects of TUDC on GCDC-induced retrieval of Ntcp from the basolateral membrane in perfused rat liver.
As shown by immunofluorescence analysis, the GCDC (20 µmol/l)-induced retrieval of Ntcp is reversed by TUDC (20 µmol/l) within 30 min (Fig. 5A, B). This inhibitory effect of TUDC on GCDC-induced Ntcp retrieval was fully prevented by the β1-integrin-inhibitory peptide GRGDSP but not by the inactive control peptide GRADSP (both 10 µmol/l) (Fig. 5B).
As shown recently, TUDC activates protein kinase A (PKA) via β1-integrins [31]. As shown in Fig. 5B, the PKA-inhibitor H89 (2 µmol/l) largely prevented the inhibitory effect of TUDC on GCDC-induced Ntcp retrieval. Na+/K+-ATPase immunostaining remained unchanged under all conditions.
These data show that TUDC inhibits the GCDC-induced Ntcp internalization by activating integrins and PKA.
TUDC prevents GCDC-induced activation of Fyn and Yes via β1-integrins and PKA
Recent studies suggested, that TUDC counteracts the hyperosmolarity-induced retrieval of Ntcp through β1-integrin and PKA-dependent inhibition of Fyn activation [15].
As shown in Fig. 6A, the inhibitory effect of TUDC on the activation of Fyn and Yes by GCDC was fully abrogated by the inhibitory integrin antagonist GRGDSP but not by the inactive control peptide GRADSP (both 10 µmol/l). Furthermore, the inhibitory effect of TUDC on the GCDC-induced activation of Fyn and Yes was fully prevented by the PKA inhibitor H89 (2 µmol/l). The GCDC-induced activation of Fyn was also abolished in presence of dibutyryl-cAMP (50 µmol/l) (Fig. 6B). The inhibitory effect of dibutyryl-cAMP on GCDC-induced Fyn activation was fully prevented by the PKA inhibitor H89 (2 µmol/l).
The data suggest that TUDC inhibits the GCDC-induced activation of Fyn and Yes in an integrin and PKA-dependent way in the perfused rat liver.
TUDC prevents the GCDC-induced ROS formation in cultured rat hepatocytes
Effects of TUDC on GCDC-induced ROS formation were studied in in H2DCF-DA-loaded rat hepatocytes by fluorescence microscopy.
As shown in Fig. 7, exposure of hepatocytes to GCDC (50 µmol/l, 15 min) strongly increased DCF fluorescence by about 2-fold compared to untreated controls. Pretreatment of the hepatocytes with TUDC (50 µmol/l, 30 min) fully prevented the GCDC-induced DCF fluorescence increase (Fig. 7). The data suggest that TUDC blocks GCDC-induced formation of reactive oxygen species in cultured rat hepatocytes.
GCDC triggers dilatation of the canaliculi in the perfused rat liver
Subcellular distribution of Bsep was analysed in perfused livers by confocal laser-scanning microscopy (Fig. 8A) and quantified by densitometric analysis of fluorescence intensities (Fig. 8B, C). For labelling of the canalicular membrane, liver sections were stained with an antibody directed against the tight junction protein zonula occludens 1 (ZO-1). Fluorescence profiles of Bsep shifted to significantly lower peak values and became more broadened in GCDC-perfused livers (p<0.05, Fig. 8B) in response to treatment with GCDC (20 µmol/l, 90 min), which may at first glance suggest an internalization of Bsep. However, GCDC also significantly increased the distance between the ZO-1 immunoreactivities lining the canalicular membrane (Fig. 8A, B, C). However, no punctate Bsep-staining was detected in the cytosol underneath the canalicular membrane in GCDC-perfused livers in confocal microscopy (Supplementary Fig. 1). Thus, the flattening of the Bsep fluorescence profile is due to a dilatation of the canaliculus, whereas retrieval of Bsep from the canalicular membrane was not observed. Both, the GCDC-induced broadening of the Bsep profile as well as the dilatation of the canaliculus were fully reversed by TUDC (20 µmol/l) (Fig. 8A-C). These inhibitory effects of TUDC were fully prevented by the integrin inhibitory peptide GRGDSP but not by its inactive analogon GRADSP (10 µmol/l) (Fig. 8A-C).
These results indicate that GCDC triggers a dilatation of the canaliculus, which is counteracted by TUDC through activation of β1-integrins.
Discussion
Bile salt homeostasis is achieved through adaptive activity changes of bile salt transporters in the hepatocyte membrane [38]. Short-term regulation of bile salt transporters involves substrate availability [39], covalent modifications [40, 41] or rapid insertion/retrieval of transporters into/from the sinusoidal and canalicular membrane [9, 29, 42, 43], whereas long-term adaption involves transcriptional or translational changes in transporter expression [44-47]. During cholestasis bile salt transporters can undergo a hepatoprotective adaptive regulation [48, 49]. In the present study, we analysed short-term effects of GCDC on the cellular localization of the bile salt transporter Na+-taurocholate cotransporting polypeptide (Ntcp) in the perfused rat liver. The data show, that GCDC triggers the rapid retrieval of Ntcp from the basolateral membrane. This process involves PKC, NADPH oxidase-derived ROS and activation of the Src family kinase Fyn. Whereas GCDC-induced Ntcp internalization and Fyn activation were prevented by both, PP-2 and SU6656, Yes activation was only sensitive to SU6656. This suggests that Fyn, but not Yes is involved in retrieval of Ntcp from the basolateral membrane. Activated Fyn in turn may phosphorylate Ntcp in the GCDC-perfused liver and thereby trigger internalization of the transporter [15, 16].
Similar to the recently described role of ROS for activation of Fyn during hyperosmotic hepatocyte shrinkage [15, 16], also GCDC-induced Fyn activation is the consequence of NADPH oxidase-dependent ROS formation in cultured hepatocytes [4, 50]. In line with this, inhibition of NADPH oxidase by apocynin and the antioxidant N-acetylcysteine prevented Fyn activation and the GCDC-induced Ntcp internalization. Retrieval of Ntcp from the plasma membrane was also recently found in response to the hydrophobic bile salt taurochenodesoxycholate (TCDC), but the underlying mechanism may differ from the one identified for GCDC in the present study. The TCDC-induced internalization depends on activation of PKC and protein phosphatase 2A, is inhibited by PI3K and importantly ROS are not involved [51].
Interestingly, the GCDC-induced internalization of Ntcp was fully counteracted by the anti-cholestatic bile salt TUDC in a β1-integrin and PKA-dependent way. As shown in the present study, TUDC prevented the GCDC-induced ROS formation (as assessed by DCF fluorescence) and Fyn activation, as it was previously also observed for hyperosmotic ROS formation [15, 16]. This, however, is at variance to our recent finding showing that TUDC had no effect on the GCDC-induced p47phox phosphorylation [31]. The reason for this is unclear, but may reflect different approaches for assessment of oxidative stress (DCFDA fluorescence vs. p47phox phosphorylation) and apart from Nox further ROS generating compartments such as mitochondria may be picked up when measuring TUDC effects using DCFDA fluorescence. In line with this, it was shown that ursodesoxycholate can counteract the deoxycholic acid-induced ROS response by preventing mitochondrial membrane permeability transition [52]. Clearly, TUDC prevents GCDC-induced Fyn activation in a β1-integrin and PKA-dependent way, but it is unclear, whether these responses are primarily due to an antioxidative action of TUDC.
As shown recently, TUDC directly activates β1-integrins and thereby stimulates the formation of cAMP [31], whose choleretic effect is well-known. cAMP increases transcellular bile salt transport by upregulating the Ntcp in the basolateral membrane [53] and increasing the levels of Mrp2 [54] and Bsep in the canalicular membrane [55]. The choleretic effect of cAMP may be due to a PKA-mediated PI3K activation [56], which leads to the activation of Akt and PKC isoforms [56-59] and induces Ntcp targeting to the plasma membrane. Moreover, the cAMP-induced insertion of Ntcp into the plasma membrane is associated with a serine dephosphorylation of Ntcp [41, 60] and the cAMP-induced activation of protein phosphatase 2B can directly dephosphorylate Ntcp [61]. Thus, Ntcp targeting to the plasma membrane by TUDC may not only reside in a prevention of GCDC-induced Fyn phosphorylation, but also in a β1-integrin-dependent cAMP formation. In line with this, TUDC-induced reinsertion of Ntcp into the plasma membrane in GCDC perfused livers was sensitive to inhibition of β1-integrins as well as to inhibition of PKA. These data also corroborate earlier findings showing that TUDC stimulates Ntcp reinsertion into the plasma membrane under hyperosmotic conditions in a β1-integrin-dependent way [15, 16]. Interestingly, the mechanisms underlying the GCDC-induced internalization of Ntcp, as shown in the present study, strongly resembles the ones triggering Ntcp retrieval from plasma membrane in hyperosmotically-perfused livers [15, 16]. Therefore it seems reasonable to speculate that GCDC-induced hepatocyte shrinkage [4] may contribute to Ntcp internalization. As shown recently, hyperosmolarity triggers a coordinated Fyn-dependent Ntcp and Bsep retrieval from the respective membranes. Because GCDC also activates Fyn, not only Ntcp retrieval, but also Bsep retrieval under the influence of GCDC may be anticipated. However, in contrast to hyperosmolarity [11], GCDC induced not only a broadening of the Bsep fluorescence profile, but also a significant dilatation of the canaliculi, which by itself may already explain the flattening of the Bsep fluorescence profile. In line with this, no Bsep retrieval from the canalicular membrane was observed in response to GCDC. Canalicular dilatation has also been described in response to taurolithocholate [62-64], to ischemia-reperfusion injury [65], chlorpromazine and estradiol-17ß-D-glucuronide [66]. The mechanisms underlying the GCDC-induced widening of the canaliculi remain speculative and may involve oxidative stress-effects on the cytoskeleton of hepatocytes. Interestingly, GCDC-induced canalicular dilatation was prevented by TUDC and similar observations were reported in drug-induced cholestasis and after ischemia-reperfusion [65, 66].
Conclusion
Taken together, the results of the present study suggest that the cholestatic bile salt GCDC not only triggers a proapoptotic state [38], but also inhibits the uptake of bile salts by hepatocytes, which may reflect a hepatoprotective response (Supplementary Fig. 2).
Abbreviations
Bsep (bile salt export pump); cAMP (cyclic adenosine monophosphate); DB-cAMP (dibutyryl-cAMP ); H2DCFDA (dihydro-2',7'-dichlorofluorescein-diacetate ); DCF (2',7'-dichlorofluorescein); GCDC (glycochenodeoxycholate); GRGDSP (RGD motif containing hexapeptide); GRADSP (inactive control peptide of RAD motif containing hexapeptide); HPRT (hypoxanthine-guanine phosphoribosyltransferase); LSM (laser scanning microscopy); MAPK (mitogen activated protein kinase); Mrp2 (multidrug resistance associated protein 2 ); NAC (N-acetylcysteine); NADPH (reduced form of nicotinamide adenine dinucleotide phosphate); Ntcp (Na+-taurocholate cotransporting polypeptide); OATP2 (organic anion transporting polypeptide 2); PBC (primary biliary cholangitis); PI3K (phosphatidylinositide 3 kinase); PKA (protein kinase A); PKC (protein kinase C); PSC (primary sclerosing cholangitis); ROS (reactive oxygen species); SR-SIM (Super-resolution structured-illumination microscopy); TC (taurocholate); TCDC (taurochenodeoxycholate ); TLC (taurolithocholate ); TUDC (tauroursodeoxycholate); ZO-1 (zonula occludens 1).
Acknowledgements
The authors are grateful to Nicole Eichhorst, Lisa Knopp and Janina Thies for expert technical assistance. Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Projektnummer 190586431 – SFB 974 „Communication and Systems Relevance during Liver Damage and Regeneration“ (Düsseldorf). All animal experiments were approved by the responsible local authorities of the “Zentrale Einrichtung für Tierforschung und wissenschaftliche Tierschutzaufgaben” (ZETT) of the University of Düsseldorf and the “Landesamt für Natur, Umwelt und Verbraucherschutz Nordrhein-Westfalen” (LANUV, NRW).
Disclosure Statement
The authors declare that they have no conflicts of interest with the content of this article.
References
|
1
Elferink RO, Groen AK: Genetic defects in hepatobiliary transport. Biochim
Biophys Acta 2002;1586:129-145. |
|
|
|
|
|
2
Watanabe M, Houten SM, Mataki C, Christoffolete MA, Kim BW, Sato H, Messaddeq
N, Harney JW, Ezaki O, Kodama T, Schoonjans K, Bianco AC, Auwerx J: Bile
acids induce energy expenditure by promoting intracellular thyroid hormone
activation. Nature 2006;439:484-489. |
|
|
|
|
|
3
Graf D, Kohlmann C, Haselow K, Gehrmann T, Bode JG, Häussinger D: Bile acids
inhibit interleukin-6 signaling via gp130 receptor-dependent and -independent
pathways in rat liver. Hepatology 2006;44:1206-1217. |
|
|
|
|
|
4
Becker S, Reinehr R, Graf D, vom Dahl S, Häussinger D: Hydrophobic bile salts
induce hepatocyte shrinkage via NADPH oxidase activation. Cell Physiol
Biochem 2007;19:89-98. |
|
|
|
|
|
5
Stieger B, Hagenbuch B, Landmann L, Hochli M, Schroeder A, Meier PJ: In situ
localization of the hepatocytic Na+/Taurocholate cotransporting polypeptide
in rat liver. Gastroenterology 1994;107:1781-1787. |
|
|
|
|
|
6
Gerloff T, Stieger B, Hagenbuch B, Madon J, Landmann L, Roth J, Hofmann AF,
Meier PJ: The sister of P-glycoprotein represents the canalicular bile salt
export pump of mammalian liver. J Biol Chem 1998;273:10046-10050. |
|
|
|
|
|
7
Akita H, Suzuki H, Ito K, Kinoshita S, Sato N, Takikawa H, Sugiyama Y:
Characterization of bile acid transport mediated by multidrug resistance
associated protein 2 and bile salt export pump. Biochim Biophys Acta
2001;1511:7-16. |
|
|
|
|
|
8
Schmitt M, Kubitz R, Wettstein M, vom Dahl S, Häussinger D: Retrieval of the
mrp2 gene encoded conjugate export pump from the canalicular membrane
contributes to cholestasis induced by tert-butyl hydroperoxide and
chloro-dinitrobenzene. Biol Chem 2000;381:487-495. |
|
|
|
|
|
9
Schmitt M, Kubitz R, Lizun S, Wettstein M, Häussinger D: Regulation of the
dynamic localization of the rat Bsep gene-encoded bile salt export pump by
anisoosmolarity. Hepatology 2001;33:509-518. |
|
|
|
|
|
10
Perez LM, Milkiewicz P, Elias E, Coleman R, Sanchez Pozzi EJ, Roma MG:
Oxidative stress induces internalization of the bile salt export pump, Bsep,
and bile salt secretory failure in isolated rat hepatocyte couplets: a role
for protein kinase C and prevention by protein kinase A. Toxicol Sci
2006;91:150-158. |
|
|
|
|
|
11
Cantore M, Reinehr R, Sommerfeld A, Becker M, Häussinger D: The Src family
kinase Fyn mediates hyperosmolarity-induced Mrp2 and Bsep retrieval from
canalicular membrane. J Biol Chem 2011;286:45014-45029. |
|
|
|
|
|
12
Häussinger D, Kurz AK, Wettstein M, Graf D, Vom Dahl S, Schliess F:
Involvement of integrins and Src in tauroursodeoxycholate-induced and
swelling-induced choleresis. Gastroenterology 2003;124:1476-1487. |
|
|
|
|
|
13
Kubitz R, D'Urso D, Keppler D, Häussinger D: Osmodependent dynamic
localization of the multidrug resistance protein 2 in the rat hepatocyte
canalicular membrane. Gastroenterology 1997;113:1438-1442. |
|
|
|
|
|
14
Warskulat U, Kubitz R, Wettstein M, Stieger B, Meier PJ, Häussinger D:
Regulation of bile salt export pump mRNA levels by dexamethasone and
osmolarity in cultured rat hepatocytes. Biol Chem 1999;380:1273-1279. |
|
|
|
|
|
15
Sommerfeld A, Mayer PG, Cantore M, Häussinger D: Regulation of plasma
membrane localization of the Na+-taurocholate cotransporting polypeptide
(Ntcp) by hyperosmolarity and tauroursodeoxycholate. J Biol Chem
2015;290:24237-24254. |
|
|
|
|
|
16
Gohlke H, Schmitz B, Sommerfeld A, Reinehr R, Häussinger D: alpha5
beta1-integrins are sensors for tauroursodeoxycholic acid in hepatocytes.
Hepatology 2013;57:1117-1129. |
|
|
|
|
|
17
Webster CR, Blanch CJ, Phillips J, Anwer MS: Cell swelling-induced
translocation of rat liver Na(+)/taurocholate cotransport polypeptide is
mediated via the phosphoinositide 3-kinase signaling pathway. J Biol Chem
2000;275:29754-29760. |
|
|
|
|
|
18
Kaplan MM, Gershwin ME: Primary biliary cirrhosis. N Engl J Med
2005;353:1261-1273. |
|
|
|
|
|
19
Zollner G, Trauner M: Mechanisms of cholestasis. Clin Liver Dis 2008;12:1-26,
vii. |
|
|
|
|
|
20
Fickert P, Moustafa T, Trauner M: Primary sclerosing cholangitis-the
arteriosclerosis of the bile duct? Lipids Health Dis 2007;6:3. |
|
|
|
|
|
21
Reinehr R, Becker S, Wettstein M, Häussinger D: Involvement of the Src family
kinase yes in bile salt-induced apoptosis. Gastroenterology
2004;127:1540-1557. |
|
|
|
|
|
22
Reinehr R, Graf D, Häussinger D: Bile salt-induced hepatocyte apoptosis
involves epidermal growth factor receptor-dependent CD95 tyrosine
phosphorylation. Gastroenterology 2003;125:839-853. |
|
|
|
|
|
23
Zollner G, Fickert P, Zenz R, Fuchsbichler A, Stumptner C, Kenner L, Ferenci
P, Stauber RE, Krejs GJ, Denk H, Zatloukal K, Trauner M: Hepatobiliary
transporter expression in percutaneous liver biopsies of patients with
cholestatic liver diseases. Hepatology 2001;33:633-646. |
|
|
|
|
|
24
Zollner G, Fickert P, Silbert D, Fuchsbichler A, Marschall HU, Zatloukal K,
Denk H, Trauner M: Adaptive changes in hepatobiliary transporter expression
in primary biliary cirrhosis. J Hepatol 2003;38:717-727. |
|
|
|
|
|
25
Beuers U, Spengler U, Kruis W, Aydemir U, Wiebecke B, Heldwein W, Weinzierl
M, Pape GR, Sauerbruch T, Paumgartner G: Ursodeoxycholic acid for treatment
of primary sclerosing cholangitis: a placebo-controlled trial. Hepatology
1992;16:707-714. |
|
|
|
|
|
26
Leuschner U, Fischer H, Kurtz W, Guldutuna S, Hubner K, Hellstern A, Gatzen
M, Leuschner M: Ursodeoxycholic acid in primary biliary cirrhosis: results of
a controlled double-blind trial. Gastroenterology 1989;97:1268-1274. |
|
|
|
|
|
27
Poupon R: Ursodeoxycholic acid and bile-acid mimetics as therapeutic agents
for cholestatic liver diseases: an overview of their mechanisms of action.
Clin Res Hepatol Gastroenterol 2012;36:S3-12. |
|
|
|
|
|
28
Poupon RE, Balkau B, Eschwege E, Poupon R: A multicenter, controlled trial of
ursodiol for the treatment of primary biliary cirrhosis. UDCA-PBC Study
Group. N Engl J Med 1991;324:1548-1554. |
|
|
|
|
|
29
Kurz AK, Graf D, Schmitt M, Vom Dahl S, Häussinger D:
Tauroursodesoxycholate-induced choleresis involves p38(MAPK) activation and
translocation of the bile salt export pump in rats. Gastroenterology
2001;121:407-419. |
|
|
|
|
|
30
Schliess F, Kurz AK, vom Dahl S, Häussinger D: Mitogen-activated protein
kinases mediate the stimulation of bile acid secretion by
tauroursodeoxycholate in rat liver. Gastroenterology 1997;113:1306-1314. |
|
|
|
|
|
31
Sommerfeld A, Reinehr R, Häussinger D: Tauroursodeoxycholate Protects Rat
Hepatocytes from Bile Acid-Induced Apoptosis via β1-Integrin- and
Protein Kinase A-Dependent Mechanisms. Cell Physiol Biochem 2015;36:866-883. |
|
|
|
|
|
32
Dombrowski F, Stieger B, Beuers U: Tauroursodeoxycholic acid inserts the bile
salt export pump into canalicular membranes of cholestatic rat liver. Lab
Invest 2006;86:166-174. |
|
|
|
|
|
33
Beuers U, Bilzer M, Chittattu A, Kullak-Ublick GA, Keppler D, Paumgartner G,
Dombrowski F: Tauroursodeoxycholic acid inserts the apical conjugate export
pump, Mrp2, into canalicular membranes and stimulates organic anion secretion
by protein kinase C-dependent mechanisms in cholestatic rat liver. Hepatology
2001;33:1206-1216. |
|
|
|
|
|
34
Beuers U, Denk GU, Soroka CJ, Wimmer R, Rust C, Paumgartner G, Boyer JL:
Taurolithocholic acid exerts cholestatic effects via phosphatidylinositol
3-kinase-dependent mechanisms in perfused rat livers and rat hepatocyte
couplets. J Biol Chem 2003;278:17810-17818. |
|
|
|
|
|
35
Sies H: The use of perfusion of liver and other organs for the study of
microsomal electron-transport and cytochrome P-450 systems. Methods Enzymol
1978;52:48-59. |
|
|
|
|
|
36
Meijer AJ, Gimpel JA, Deleeuw GA, Tager JM, Williamson JR: Role of anion
translocation across the mitochondrial membrane in the regulation of urea
synthesis from ammonia by isolated rat hepatocytes. J Biol Chem
1975;250:7728-7738. |
|
|
|
|
|
37
Reinehr R, Becker S, Braun J, Eberle A, Grether-Beck S, Häussinger D:
Endosomal acidification and activation of NADPH oxidase isoforms are upstream
events in hyperosmolarity-induced hepatocyte apoptosis. J Biol Chem
2006;281:23150-23166. |
|
|
|
|
|
38
Hylemon PB, Zhou H, Pandak WM, Ren S, Gil G, Dent P: Bile acids as regulatory
molecules. J Lipid Res 2009;50:1509-1520. |
|
|
|
|
|
39
Häussinger D, Schmitt M, Weiergraber O, Kubitz R: Short-term regulation of
canalicular transport. Semin Liver Dis 2000;20:307-321. |
|
|
|
|
|
40
Gottesman MM, Pastan I: Biochemistry of multidrug resistance mediated by the
multidrug transporter. Annu Rev Biochem 1993;62:385-427. |
|
|
|
|
|
41
Mukhopadhyay S, Ananthanarayanan M, Stieger B, Meier PJ, Suchy FJ, Anwer MS:
Sodium taurocholate cotransporting polypeptide is a serine, threonine
phosphoprotein and is dephosphorylated by cyclic adenosine monophosphate.
Hepatology 1998;28:1629-1636. |
|
|
|
|
|
42
Dranoff JA, McClure M, Burgstahler AD, Denson LA, Crawford AR, Crawford JM,
Karpen SJ, Nathanson MH: Short-term regulation of bile acid uptake by
microfilament-dependent translocation of rat ntcp to the plasma membrane.
Hepatology 1999;30:223-229. |
|
|
|
|
|
43
Kubitz R, Warskulat U, Schmitt M, Häussinger D: Dexamethasone- and
osmolarity-dependent expression of the multidrug-resistance protein 2 in
cultured rat hepatocytes. Biochem J 1999;340:585-591. |
|
|
|
|
|
44
Dawson PA, Lan T, Rao A: Bile acid transporters. J Lipid Res
2009;50:2340-2357. |
|
|
|
|
|
45
Trauner M, Boyer JL: Bile salt transporters: molecular characterization,
function, and regulation. Physiol Rev 2003;83:633-671. |
|
|
|
|
|
46
Anwer MS, Stieger B: Sodium-dependent bile salt transporters of the SLC10A
transporter family: more than solute transporters. Pflugers Arch
2014;466:77-89. |
|
|
|
|
|
47
Claro da Silva T, Polli JE, Swaan PW: The solute carrier family 10 (SLC10):
beyond bile acid transport. Mol Aspects Med 2013;34:252-269. |
|
|
|
|
|
48
Cuperus FJ, Claudel T, Gautherot J, Halilbasic E, Trauner M: The role of
canalicular ABC transporters in cholestasis. Drug Metab Dispos
2014;42:546-560. |
|
|
|
|
|
49
Roma MG: Dynamic localization of hepatocellular transporters in health and
disease. World Journal of Gastroenterology 2008;14:6786. |
|
|
|
|
|
50
Reinehr R, Becker S, Keitel V, Eberle A, Grether-Beck S, Häussinger D: Bile
salt-induced apoptosis involves NADPH oxidase isoform activation.
Gastroenterology 2005;129:2009-2031. |
|
|
|
|
|
51
Muhlfeld S, Domanova O, Berlage T, Stross C, Helmer A, Keitel V, Häussinger
D, Kubitz R: Short-term feedback regulation of bile salt uptake by bile salts
in rodent liver. Hepatology 2012;56:2387-2397. |
|
|
|
|
|
52
Rodrigues CM, Fan G, Wong PY, Kren BT, Steer CJ: Ursodeoxycholic acid may
inhibit deoxycholic acid-induced apoptosis by modulating mitochondrial
transmembrane potential and reactive oxygen species production. Mol Med 1998;4:165-178. |
|
|
|
|
|
53
Mukhopadhayay S, Ananthanarayanan M, Stieger B, Meier PJ, Suchy FJ, Anwer MS:
cAMP increases liver Na+-taurocholate cotransport by translocating transporter
to plasma membranes. Am
J Physiol 1997;273:G842-848. |
|
|
|
|
|
54
Roelofsen H, Soroka CJ, Keppler D, Boyer JL: Cyclic AMP stimulates sorting of
the canalicular organic anion transporter (Mrp2/cMoat) to the apical domain
in hepatocyte couplets. J
Cell Sci 1998;111:1137-1145. |
|
|
|
|
|
55
Misra S, Varticovski L, Arias IM: Mechanisms by which cAMP increases bile
acid secretion in rat liver and canalicular membrane vesicles. Am J Physiol
Gastrointest Liver Physiol 2003;285:G316-324. |
|
|
|
|
|
56
Park SW, Schonhoff CM, Webster CR, Anwer MS: Protein kinase Cdelta
differentially regulates cAMP-dependent translocation of NTCP and MRP2 to the
plasma membrane. Am J Physiol Gastrointest Liver Physiol 2012;303:G657-665. |
|
|
|
|
|
57
Webster CR, Srinivasulu U, Ananthanarayanan M, Suchy FJ, Anwer MS: Protein
kinase B/Akt mediates cAMP- and cell swelling-stimulated Na+/taurocholate
cotransport and Ntcp translocation. J Biol Chem 2002;277:28578-28583. |
|
|
|
|
|
58
McConkey M, Gillin H, Webster CR, Anwer MS: Cross-talk between protein
kinases Czeta and B in cyclic AMP-mediated sodium taurocholate
co-transporting polypeptide translocation in hepatocytes. J Biol Chem
2004;279:20882-20888. |
|
|
|
|
|
59
Sarkar S, Bananis E, Nath S, Anwer MS, Wolkoff AW, Murray JW: PKCzeta is
required for microtubule-based motility of vesicles containing the ntcp
transporter. Traffic
2006;7:1078-1091. |
|
|
|
|
|
60
Anwer MS, Gillin H, Mukhopadhyay S, Balasubramaniyan N, Suchy FJ,
Ananthanarayanan M: Dephosphorylation of Ser-226 facilitates plasma membrane
retention of Ntcp. J
Biol Chem 2005;280:33687-33692. |
|
|
|
|
|
61
Webster CR, Blanch C, Anwer MS: Role of PP2B in cAMP-induced
dephosphorylation and translocation of NTCP. Am J Physiol Gastrointest Liver Physiol
2002;283:G44-50. |
|
|
|
|
|
62
Layden TJ, Schwarz, Boyer JL: Scanning electron microscopy of the rat liver.
Studies of the effect of taurolithocholate and other models of cholestasis.
Gastroenterology 1975;69:724-738. |
|
|
|
|
|
63
Miyai K, Richardson AL, Mayr W, Javitt NB: Subcellular pathology of rat liver
in cholestasis and choleresis induced by bile salts. 1. Effects of
lithocholic, 3beta-hydroxy-5-cholenoic, cholic, and dehydrocholic acids. Lab
Invest 1977;36:249-258. |
|
|
|
|
|
64
Jung W, Gebhardt R, Robenek H: Primary cultures of rat hepatocytes as a model
system of canalicular development, biliary secretion, and intrahepatic cholestasis.
II. Taurolithocholate-induced alterations of canalicular morphology and of
the distribution of filipin-cholesterol complexes. Eur J Cell Biol
1982;29:77-82. |
|
|
|
|
|
65
Baiocchi L, Tisone G, Russo MA, Longhi C, Palmieri G, Volpe A, Almerighi C,
Telesca C, Carbone M, Toti L, De Leonardis F, Angelico M: TUDCA prevents
cholestasis and canalicular damage induced by ischemia-reperfusion injury in
the rat, modulating PKCalpha-ezrin pathway. Transpl Int 2008;21:792-800. |
|
|
|
|
|
66
Abernathy CO, Zimmerman HJ, Ishak KG, Utili R, Gillespie J: Drug-induced
cholestasis in the perfused rat liver and its reversal by
tauroursodeoxycholate: an ultrastructural study. Proc Soc Exp Biol Med
1992;199:54-58. |
|