The Src family kinase Fyn mediates hyperosmolarity-induced Mrp2 and Bsep retrieval from canalicular membrane

NADPH oxidase-generated reactive oxygen species are involved in estradiol 17ß-d-glucuronide-induced cholestasis

The endogenous metabolite of estradiol, estradiol 17β-D-glucuronide (E17G), is considered the main responsible of the intrahepatic cholestasis of pregnancy. E17G alters the activity of canalicular transporters through a signaling pathway-dependent cellular internalization, phenomenon that was attributed to oxidative stress in different cholestatic conditions. However, there are no reports involving oxidative stress in E17G-induced cholestasis, representing this the aim of our work. Using polarized hepatocyte cultures, we showed that antioxidant compounds prevented E17G-induced Mrp2 activity alteration, being this alteration equally prevented by the NADPH oxidase (NOX) inhibitor apocynin. The model antioxidant N-acetyl-cysteine prevented, in isolated and perfused rat livers, E17G-induced impairment of bile flow and Mrp2 activity, thus confirming the participation of reactive oxygen species (ROS) in this cholestasis. In primary cultured hepatocytes, pretreatment with specific inhibitors of ERK1/2 and p38MAPK impeded E17G-induced ROS production; contrarily, NOX inhibition did not affect ERK1/2 and p38MAPK phosphorylation. Both, knockdown of p47phox by siRNA and preincubation with apocynin in sandwich-cultured rat hepatocytes significantly prevented E17G-induced internalization of Mrp2, suggesting a crucial role for NOX in this phenomenon. Concluding, E17G-induced cholestasis is partially mediated by NOX-generated ROS through internalization of canalicular transporters like Mrp2, being ERK1/2 and p38MAPK necessary for NOX activation.

Role of Bile Acids and Nuclear Receptors in Acupuncture in Improving Crohn's Disease

Nuclear receptors (NRs) are ligand-dependent transcription factors that regulate the transcription of target genes. Bile acids (BAs) can be used as effector molecules to regulate physiological processes in the gut, and NRs are important receptors for bile acid signaling. Relevant studies have shown that NRs are closely related to the occurrence of Crohn's disease (CD). Although the mechanism of NRs in CD has not been clarified completely, growing evidence shows that NRs play an important role in regulating intestinal immunity, mucosal barrier, and intestinal flora. NRs can participate in the progress of CD by mediating inflammation, immunity, and autophagy. As the important parts of traditional Chinese medicine (TCM) therapy, acupuncture and moxibustion in the treatment of CD curative mechanism can get a lot of research support. At the same time, acupuncture and moxibustion can regulate the changes of related NRs. Therefore, to explore whether acupuncture can regulate BA circulation and NRs expression and then participate in the disease progression of CD, a new theoretical basis for acupuncture treatment of CD is provided.

MRCK-Alpha and Its Effector Myosin II Regulatory Light Chain Bind ABCB4 and Regulate Its Membrane Expression

ABCB4, is an adenosine triphosphate-binding cassette (ABC) transporter localized at the canalicular membrane of hepatocytes, where it mediates phosphatidylcholine secretion into bile. Gene variations of ABCB4 cause different types of liver diseases, including progressive familial intrahepatic cholestasis type 3 (PFIC3). The molecular mechanisms underlying the trafficking of ABCB4 to and from the canalicular membrane are still unknown. We identified the serine/threonine kinase Myotonic dystrophy kinase-related Cdc42-binding kinase isoform α (MRCKα) as a novel partner of ABCB4. The role of MRCKα was explored, either by expression of dominant negative mutant or by gene silencing using the specific RNAi and CRISPR-cas9 strategy in cell models. The expression of a dominant-negative mutant of MRCKα and MRCKα inhibition by chelerythrine both caused a significant increase in ABCB4 steady-state expression in primary human hepatocytes and HEK-293 cells. RNA interference and CRISPR-Cas9 knockout of MRCKα also caused a significant increase in the amount of ABCB4 protein expression. We demonstrated that the effect of MRCKα was mediated by its downstream effector, the myosin II regulatory light chain (MRLC), which was shown to also bind ABCB4. Our findings provide evidence that MRCKα and MRLC bind to ABCB4 and regulate its cell surface expression.

Oxidative Stress and Localization Status of Hepatocellular Transporters: Impact on Bile Secretion and Role of Signaling Pathways

Significance: Most hepatopathies are primarily or secondarily cholestatic in nature. Oxidative stress (OS) is a frequent trait among them, and impairs the machinery to generate bile by triggering endocytic internalization of hepatocellular transporters, thus causing cholestasis. This is critical, since it leads to accelerated transporter degradation, which could explain the common post-transcriptional downregulation of transporter expression in human cholestatic diseases. Recent Advances: The mechanisms involved in OS-induced hepatocellular transporter internalization are being revealed. Filamentous actin (F-actin) cytoskeleton disorganization and/or detachment of crosslinking actin proteins that afford transporter stability have been characterized as causal factors. Activation of redox-sensitive signaling pathways leading to changes in phosphorylation status of these structures is involved, including Ca²⁺-mediated activation of "classical" and "novel" protein kinase C (PKC) isoforms or redox-signaling cascades downstream of NADPH oxidase. Critical Issues: Despite the well-known occurrence of hepatocellular transporter internalization in human hepatopathies, the cholestatic implications of this phenomenon have been overlooked. Accordingly, no specific treatment has been established in the clinical practice for its prevention/reversion. Future Directions: We need to improve our knowledge on the pro-oxidant triggering factors and the multiple signaling pathways that mediate this oxidative injury in each cholestatic hepatopathy, so as to envisage tailor-made therapeutic strategies for each case. Meanwhile, administration of antioxidants or heme oxygenase-1 induction to elevate the hepatocellular levels of the endogenous scavenger bilirubin are promising alternatives that need to be re-evaluated and implemented. They may complement current treatments in cholestasis aimed to enhance transcriptional carrier expression, by providing membrane stability to the newly synthesized carriers. Antioxid. Redox Signal. 35, 808-831.

The many facets of bile acids in the physiology and pathophysiology of the human liver

Bile acids perform vital functions in the human liver and are the essential component of bile. It is therefore not surprising that the biology of bile acids is extremely complex, regulated on different levels, and involves soluble and membrane receptors as well as transporters. Hereditary disorders of these proteins manifest in different pathophysiological processes that result in liver diseases of varying severity. In this review, we summarize our current knowledge of the physiology and pathophysiology of bile acids with an emphasis on recently established analytical approaches as well as the molecular mechanisms that underlie signaling and transport of bile acids. In this review, we will focus on ABC transporters of the canalicular membrane and their associated diseases. As the G protein-coupled receptor, TGR5, receives increasing attention, we have included aspects of this receptor and its interaction with bile acids.

Liver cell hydration and integrin signaling

Liver cell hydration (cell volume) is dynamic and can change within minutes under the influence of hormones, nutrients, and oxidative stress. Such volume changes were identified as a novel and important modulator of cell function. It provides an early example for the interaction between a physical parameter (cell volume) on the one hand and metabolism, transport, and gene expression on the other. Such events involve mechanotransduction (osmosensing) which triggers signaling cascades towards liver function (osmosignaling). This article reviews our own work on this topic with emphasis on the role of β₁ integrins as (osmo-)mechanosensors in the liver, but also on their role in bile acid signaling.

Molecular Regulation of Canalicular ABC Transporters

The ATP-binding cassette (ABC) transporters expressed at the canalicular membrane of hepatocytes mediate the secretion of several compounds into the bile canaliculi and therefore play a key role in bile secretion. Among these transporters, ABCB11 secretes bile acids, ABCB4 translocates phosphatidylcholine and ABCG5/G8 is responsible for cholesterol secretion, while ABCB1 and ABCC2 transport a variety of drugs and other compounds. The dysfunction of these transporters leads to severe, rare, evolutionary biliary diseases. The development of new therapies for patients with these diseases requires a deep understanding of the biology of these transporters. In this review, we report the current knowledge regarding the regulation of canalicular ABC transporters' folding, trafficking, membrane stability and function, and we highlight the role of molecular partners in these regulating mechanisms.

Evidence for functional selectivity in TUDC- and norUDCA-induced signal transduction via α5β1 integrin towards choleresis

Functional selectivity is the ligand-specific activation of certain signal transduction pathways at a receptor and has been described for G protein-coupled receptors. However, it has not yet been described for ligands interacting with integrins without αI domain. Here, we show by molecular dynamics simulations that four side chain-modified derivatives of tauroursodeoxycholic acid (TUDC), an agonist of α₅β₁ integrin, differentially shift the conformational equilibrium of α₅β₁ integrin towards the active state, in line with the extent of β₁ integrin activation from immunostaining. Unlike TUDC, 24-nor-ursodeoxycholic acid (norUDCA)-induced β₁ integrin activation triggered only transient activation of extracellular signal-regulated kinases and p38 mitogen-activated protein kinase and, consequently, only transient insertion of the bile acid transporter Bsep into the canalicular membrane, and did not involve activation of epidermal growth factor receptor. These results provide evidence that TUDC and norUDCA exert a functional selectivity at α₅β₁ integrin and may provide a rationale for differential therapeutic use of UDCA and norUDCA.

A Critical Evaluation of Liver Pathology in Humans with Danon Disease and Experimental Correlates in a Rat Model of LAMP-2 Deficiency

Danon disease is a genetic deficiency in lysosome-associated membrane protein 2 (LAMP-2), a highly glycosylated constituent of the lysosomal membrane and characterized by a cardiomyopathy, skeletal muscle myopathy, and cognitive impairment. Patients, however, often manifest hepatic abnormalities, but liver function has not been well evaluated and the syndrome is relatively uncommon. Hence, we have taken advantage of a rat that has been deleted of LAMP-2 to study the relative role of LAMP-2 on liver function. Interestingly, rats deficient in LAMP-2 develop a striking increase in serum alkaline phosphatase (ALP) and a decrease in bile flow compared with wild-type littermates. Importantly and by ultrastructural analysis, deficient rats manifest dilated canaliculi that lack microvilli with evidence of bile-containing bodies. Moreover, following bile duct ligation, LAMP-2-deficient rats develop rapid and severe evidence of advanced cholestasis, with an increase in serum bilirubin, as early as 6 h later. In wild-type control rats, multidrug resistance-associated protein 2 (Mrp2) normally concentrates at the bile canalicular membranes to secrete conjugated bilirubin into bile. However, in LAMP-2^y/- rats, Mrp2 was detected in hepatocytes compared with other canalicular proteins including P-glycoproteins, dipeptidyl peptidase IV (CD26), and aminopeptidase (CD13). Our data further suggest that LAMP-2 interacts with the membrane cytoskeletal proteins radixin and F-actin in determining the localization of integral membrane proteins.

Fyn deficiency attenuates renal fibrosis by inhibition of phospho-STAT3

The hallmark of renal tubulointerstitial fibrosis is the accumulation of myofibroblasts and extracellular matrix proteins. Fyn, a member of the Src family of kinases, has diverse biological functions including regulation of mitogenic signaling and proliferation and integrin-mediated interaction. Src family proteins promote pulmonary fibrosis by augmenting transforming growth factor-β signaling, but their role in renal fibrosis is less understood. We observed upregulation of Fyn in a renal fibrosis model induced by unilateral ureteral obstruction. Upon ureteral obstruction, Fyn-deficient mice exhibited attenuated renal fibrosis relative to wild-type mice. Furthermore, obstruction-induced renal expression of type I collagen, fibronectin, α-smooth muscle actin, and plasminogen activator inhibitor-1 was suppressed. Pharmacologic inhibition of Fyn blocked induction of extracellular matrix proteins in kidney cell lines. Importantly, the attenuation of renal fibrosis by Fyn deficiency was not accompanied by changes in the Smad pathway. Rather, the antifibrotic effect of Fyn deficiency was associated with downregulation of signal transducer and activator of transcription 3 (STAT3). Small, interfering RNA targeting STAT3 in Fyn-deficient cells further suppressed α-smooth muscle actin expression, whereas a STAT3 activator partially restored plasminogen activator inhibitor-1 expression, indicating that STAT3 signaling is critically involved in this process. Thus, Fyn plays an important role in renal fibrosis. Hence, Fyn kinase inhibitors may be therapeutically useful against renal fibrosis.

Autoimmune BSEP disease: disease recurrence after liver transplantation for progressive familial intrahepatic cholestasis

Severe cholestasis may result in end-stage liver disease with the need of liver transplantation (LTX). In children, about 10 % of LTX are necessary because of cholestatic liver diseases. Apart from bile duct atresia, three types of progressive familial intrahepatic cholestasis (PFIC) are common causes of severe cholestasis in children. The three subtypes of PFIC are defined by the involved genes: PFIC-1, PFIC-2, and PFIC-3 are due to mutations of P-type ATPase ATP8B1 (familial intrahepatic cholestasis 1, FIC1), the ATP binding cassette transporter ABCB11 (bile salt export pump, BSEP), or ABCB4 (multidrug resistance protein 3, MDR3), respectively. All transporters are localized in the canalicular membrane of hepatocytes and together mediate bile salt and phospholipid transport. In some patients with PFIC-2 disease, recurrence has been observed after LTX, which mimics a PFIC phenotype. It could be shown by several groups that inhibitory anti-BSEP antibodies emerge, which most likely cause disease recurrence. The prevalence of severe BSEP mutations (e.g., splice site and premature stop codon mutations) is very high in this group of patients. These mutations often result in the complete absence of BSEP, which likely accounts for an insufficient auto-tolerance against BSEP. Although many aspects of this "new" disease are not fully elucidated, the possibility of anti-BSEP antibody formation has implications for the pre- and posttransplant management of PFIC-2 patients. This review will summarize the current knowledge including diagnosis, pathomechanisms, and management of "autoimmune BSEP disease."

Regulation of plasma membrane localization of the Na+-taurocholate cotransporting polypeptide (Ntcp) by hyperosmolarity and tauroursodeoxycholate

In perfused rat liver, hepatocyte shrinkage induces a Fyn-dependent retrieval of the bile salt export pump (Bsep) and multidrug resistance-associated protein 2 (Mrp2) from the canalicular membrane (Cantore, M., Reinehr, R., Sommerfeld, A., Becker, M., and Häussinger, D. (2011) J. Biol. Chem. 286, 45014-45029) leading to cholestasis. However little is known about the effects of hyperosmolarity on short term regulation of the Na(+)-taurocholate cotransporting polypeptide (Ntcp), the major bile salt uptake system at the sinusoidal membrane of hepatocytes. The aim of this study was to analyze hyperosmotic Ntcp regulation and the underlying signaling events. Hyperosmolarity induced a significant retrieval of Ntcp from the basolateral membrane, which was accompanied by an activating phosphorylation of the Src kinases Fyn and Yes but not of c-Src. Hyperosmotic internalization of Ntcp was sensitive to SU6656 and PP-2, suggesting that Fyn mediates Ntcp retrieval from the basolateral membrane. Hyperosmotic internalization of Ntcp was also found in livers from wild-type mice but not in p47(phox) knock-out mice. Tauroursodeoxycholate (TUDC) and cAMP reversed hyperosmolarity-induced Fyn activation and triggered re-insertion of the hyperosmotically retrieved Ntcp into the membrane. This was associated with dephosphorylation of the Ntcp on serine residues. Insertion of Ntcp by TUDC was sensitive to the integrin inhibitory hexapeptide GRGDSP and inhibition of protein kinase A. TUDC also reversed the hyperosmolarity-induced retrieval of bile salt export pump from the canalicular membrane. These findings suggest a coordinated and oxidative stress- and Fyn-dependent retrieval of sinusoidal and canalicular bile salt transport systems from the corresponding membranes. Ntcp insertion was also identified as a novel target of β1-integrin-dependent TUDC action, which is frequently used in the treatment of cholestatic liver disease.

Genetic variations of bile salt transporters

Bile salt transporters directly or indirectly influence biological processes through physicochemical or signalling properties of bile salts. The coordinated action of uptake and efflux transporters in polarized epithelial cells of the liver, biliary tree, small intestine and kidney determine bile salt concentrations in different compartments of the body. Genetic variations of bile salt transporters lead to clinical relevant phenotypes of varying severity ranging from a predisposition for drug-induced liver injury to rapidly progressing end-stage liver disease. This review focuses on the impact of genetic variations of bile salt transporters including BSEP, NTCP, ASBT and OSTα/β and discusses approaches for transporter analysis.

EGFR participates downstream of ERα in estradiol-17β-D-glucuronide-induced impairment of Abcc2 function in isolated rat hepatocyte couplets

Estradiol-17β-D-glucuronide (E17G) induces acute endocytic internalization of canalicular transporters, including multidrug resistance-associated protein 2 (Abcc2) in rat, generating cholestasis. Several proteins organized in at least two different signaling pathways are involved in E17G cholestasis: one pathway involves estrogen receptor alpha (ERα), Ca(2+)-dependent protein kinase C and p38-mitogen activated protein kinase, and the other pathway involves GPR30, PKA, phosphoinositide 3-kinase/AKT and extracellular signal-related kinase 1/2. EGF receptor (EGFR) can potentially participate in both pathways since it interacts with GPR30 and ERα. Hence, the aim of this study was to analyze the potential role of this receptor and its downstream effectors, members of the Src family kinases in E17G-induced cholestasis. In vitro, EGFR inhibition by Tyrphostin (Tyr), Cl-387785 or its knockdown with siRNA strongly prevented E17G-induced impairment of Abcc2 function and localization. Activation of EGFR was necessary but not sufficient to impair the canalicular transporter function, whereas the simultaneous activation of EGFR and GPR30 could impair Abcc2 transport. The protection of Tyr was not additive to that produced by the ERα inhibitor ICI neither with that produced by Src kinase inhibitors, suggesting that EGFR shared the signaling pathway of ERα and Src. Further analysis of ERα, EGFR and Src activations induced by E17G, demonstrated that ERα activation precedes that of EGFR and EGFR activation precedes that of Src. In conclusion, activation of EGFR is a key factor in the alteration of canalicular transporter function and localization induced by E17G and it occurs before that of Src and after that of ERα.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Short-term feedback regulation of bile salt uptake by bile salts in rodent liver

The sodium taurocholate cotransporting polypeptide (Ntcp) is the major bile salt uptake transporter at the sinusoidal membrane of hepatocytes. Short-term feedback regulation of Ntcp by primary bile salts has not yet been investigated in vivo. Subcellular localization of Ntcp was analyzed in Ntcp-transfected HepG2-cells by flow cytometry and in immunofluorescence images from tissue sections by a new automated image analysis method. Net bile salt uptake was investigated in perfused rat liver by a pulse chase technique. In Flag-Ntcp-EGFP (enhanced green fluorescent protein) expressing HepG2-cells, taurochenodeoxycholate (TCDC), but not taurocholate (TC), induced endocytosis of Ntcp. TCDC, but not TC, caused significant internalization of Ntcp in perfused rat livers, as shown by an increase in intracellular Ntcp immunoreactivity, whereas Bsep distribution remained unchanged. These results correlate with functional studies. Rat livers were continuously perfused with 100 μmol/L of TC. 25 μmol/L of TCDC, taurodeoxycholate (TDC), tauroursodeoxycholate (TUDC), or TC were added for 30 minutes, washed out, followed by a pulse of (3) [H]-TC. TCDC, but not TDC, TUDC, or TC significantly increased the amount of (3) [H]-TC in the effluent, indicating a reduced sinusoidal net TC uptake. This effect was sensitive to chelerythrine (protein kinase C inhibitor) and cypermethrin (protein phosphatase 2B inhibitor). Phosphoinositide 3-kinase (PI3K) inhibitors had an additive effect, whereas Erk1/2 (extracellular signal activated kinase 1/2), p38MAPK, protein phosphatase 1/2A (PP1/2A), and reactive oxygen species (ROS) were not involved.

CONCLUSION

TCDC regulates bile salt transport at the sinusoidal membrane by protein kinase C- and protein phosphatase 2B-mediated retrieval of Ntcp from the plasma membrane. During increased portal bile salt load this mechanism may adjust bile salt uptake along the acinus and protect periportal hepatocytes from harmful bile salt concentrations.

The bile salt export pump (BSEP) in health and disease

The bile salt export pump (BSEP) is the major transporter for the secretion of bile acids from hepatocytes into bile in humans. Mutations of BSEP are associated with cholestatic liver diseases of varying severity including progressive familial intrahepatic cholestasis type 2 (PFIC-2), benign recurrent intrahepatic cholestasis type 2 (BRIC-2) and genetic polymorphisms are linked to intrahepatic cholestasis of pregnancy (ICP) and drug-induced liver injury (DILI). Detailed analysis of these diseases has considerably increased our knowledge about physiology and pathophysiology of bile secretion in humans. This review focuses on expression, localization, and function, short- and long-term regulation of BSEP as well as diseases association and treatment options for BSEP-associated diseases.

Hyperosmotic stress regulates the distribution and stability of myocardin-related transcription factor, a key modulator of the cytoskeleton

Hyperosmotic stress initiates several adaptive responses, including the remodeling of the cytoskeleton. Besides maintaining structural integrity, the cytoskeleton has emerged as an important regulator of gene transcription. Myocardin-related transcription factor (MRTF), an actin-regulated coactivator of serum response factor, is a major link between the actin skeleton and transcriptional control. We therefore investigated whether MRTF is regulated by hyperosmotic stress. Here we show that hypertonicity induces robust, rapid, and transient translocation of MRTF from the cytosol to the nucleus in kidney tubular cells. We found that the hyperosmolarity-triggered MRTF translocation is mediated by the RhoA/Rho kinase (ROK) pathway. Moreover, the Rho guanine nucleotide exchange factor GEF-H1 is activated by hyperosmotic stress, and it is a key contributor to the ensuing RhoA activation and MRTF translocation, since siRNA-mediated GEF-H1 downregulation suppresses these responses. While the osmotically induced RhoA activation promotes nuclear MRTF accumulation, the concomitant activation of p38 MAP kinase mitigates this effect. Moderate hyperosmotic stress (600 mosM) drives MRTF-dependent transcription through the cis-element CArG box. Silencing or pharmacological inhibition of MRTF prevents the osmotic stimulation of CArG-dependent transcription and renders the cells susceptible to osmotic shock-induced structural damage. Interestingly, strong hyperosmolarity promotes proteasomal degradation of MRTF, concomitant with apoptosis. Thus, MRTF is an osmosensitive and osmoprotective transcription factor, whose intracellular distribution is regulated by the GEF-H1/RhoA/ROK and p38 pathways. However, strong osmotic stress destabilizes MRTF, concomitant with apoptosis, implying that hyperosmotically induced cell death takes precedence over epithelial-myofibroblast transition, a potential consequence of MRTF-mediated phenotypic reprogramming.