Role of protein kinase C isoforms in bile formation and cholestasis

Protein kinase C: A potential therapeutic target for endothelial dysfunction in diabetes

Protein kinase C (PKC) is a family of serine/threonine protein kinases that play an important role in many organs and systems and whose activation contributes significantly to endothelial dysfunction in diabetes. The increase in diacylglycerol (DAG) under high glucose conditions mediates PKC activation and synthesis, which stimulates oxidative stress and inflammation, resulting in impaired endothelial cell function. This article reviews the contribution of PKC to the development of diabetes-related endothelial dysfunction and summarizes the drugs that inhibit PKC activation, with the aim of exploring therapeutic modalities that may alleviate endothelial dysfunction in diabetic patients.

Ginsenosides Restore Lipid and Redox Homeostasis in Mice with Intrahepatic Cholestasis through SIRT1/AMPK Pathways

Intrahepatic cholestasis (IC) occurs when the liver and systemic circulation accumulate bile components, which can then lead to lipid metabolism disorders and oxidative damage. Ginsenosides (GS) are pharmacologically active plant products derived from ginseng that possesses lipid-regulation and antioxidation activities. The purpose of this study was to evaluate the possible protective effects of ginsenosides (GS) on lipid homeostasis disorder and oxidative stress in mice with alpha-naphthylisothiocyanate (ANIT)-induced IC and to investigate the underlying mechanisms. A comprehensive strategy via incorporating pharmacodynamics and molecular biology technology was adopted to investigate the therapeutic mechanisms of GS in ANIT-induced mice liver injury. The effects of GS on cholestasis were studied in mice that had been exposed to ANIT-induced cholestasis. The human HepG2 cell line was then used in vitro to investigate the molecular mechanisms by which GS might improve IC. The gene silencing experiment and liver-specific sirtuin-1 (SIRT1) knockout (SIRT1^LKO) mice were used to further elucidate the mechanisms. The general physical indicators were assessed, and biological samples were collected for serum biochemical indexes, lipid metabolism, and oxidative stress-related indicators. Quantitative PCR and H&E staining were used for molecular and pathological analysis. The altered expression levels of key pathway proteins (Sirt1, p-AMPK, Nrf2) were validated by Western blotting. By modulating the AMPK protein expression, GS decreased hepatic lipogenesis, and increased fatty acid β-oxidation and lipoprotein lipolysis, thereby improving lipid homeostasis in IC mice. Furthermore, GS reduced ANIT-triggered oxidative damage by enhancing Nrf2 and its downstream target levels. Notably, the protective results of GS were eliminated by SIRT1 shRNA in vitro and SIRT1^LKO mice in vivo. GS can restore the balance of the lipid metabolism and redox in the livers of ANIT-induced IC models via the SIRT1/AMPK signaling pathway, thus exerting a protective effect against ANIT-induced cholestatic liver injury.

The role of protein kinase C in diabetic microvascular complications

Protein kinase C (PKC) is a family of serine/threonine protein kinases, the activation of which plays an important role in the development of diabetic microvascular complications. The activation of PKC under high-glucose conditions stimulates redox reactions and leads to an accumulation of redox stress. As a result, various types of cells in the microvasculature are influenced, leading to changes in blood flow, microvascular permeability, extracellular matrix accumulation, basement thickening and angiogenesis. Structural and functional disorders further exacerbate diabetic microvascular complications. Here, we review the roles of PKC in the development of diabetic microvascular complications, presenting evidence from experiments and clinical trials.

Regulation of the HBV Entry Receptor NTCP and its Potential in Hepatitis B Treatment

Hepatitis B virus (HBV) is a globally prevalent human DNA virus responsible for more than 250 million cases of chronic liver infection, a condition that can lead to liver inflammation, cirrhosis, and hepatocellular carcinoma. Sodium taurocholate co-transporting polypeptide (NTCP), a transmembrane protein highly expressed in human hepatocytes and a mediator of bile acid transport, has been identified as the receptor responsible for the cellular entry of both HBV and its satellite, hepatitis delta virus (HDV). This has led to significant advances in our understanding of the HBV life cycle, especially the early steps of infection. HepG2-NTCP cells and human NTCP-expressing transgenic mice have been employed as the primary cell culture and animal models, respectively, for the study of HBV, and represent valuable approaches for investigating its basic biology and developing treatments for infection. However, the mechanisms involved in the regulation of NTCP transcription, translation, post-translational modification, and transport are still largely elusive. Improvements in our understanding of NTCP biology would likely facilitate the design of new therapeutic drugs for the prevention of the de novo infection of naïve hepatocytes. In this review, we provide critical findings regarding NTCP biology and discuss important questions that remain unanswered.

The myristoylated alanine-rich C-kinase substrates (MARCKS): A membrane-anchored mediator of the cell function

The myristoylated alanine-rich C-kinase substrate (MARCKS) and the MARCKS-related protein (MARCKSL1) are ubiquitous, highly conserved membrane-associated proteins involved in the structural modulation of the actin cytoskeleton, chemotaxis, motility, cell adhesion, phagocytosis, and exocytosis. MARCKS includes an N-terminal myristoylated domain for membrane binding, a highly conserved MARCKS Homology 2 (MH2) domain, and an effector domain (which is the phosphorylation site). MARCKS can sequester phosphatidylinositol-4, 5-diphosphate (PIP2) at lipid rafts in the plasma membrane of quiescent cells, an action reversed by protein kinase C (PKC), ultimately modulating the immune function. Being expressed mostly in innate immune cells, MARCKS promotes the inflammation-driven migration and adhesion of cells and the secretion of cytokines such as tumor necrosis factor (TNF). From a clinical point of view, MARCKS is overexpressed in patients with schizophrenia and bipolar disorders, while the brain level of MARCKS phosphorylation is associated with Alzheimer's disease. Furthermore, MARCKS is associated with the development and progression of numerous types of cancers. Data in autoimmune diseases are limited to rheumatoid arthritis models in which a connection between MARCKS and the JAK-STAT pathway is mediated by miRNAs. We provide a comprehensive overview of the structure of MARCKS, its molecular characteristics and functions from a biological and pathogenetic standpoint, and will discuss the clinical implications of this pathway.

Cell Culture Models for the Study of Hepatitis D Virus Entry and Infection

Chronic hepatitis D is one of the most severe and aggressive forms of chronic viral hepatitis with a high risk of developing hepatocellular carcinoma (HCC). It results from the co-infection of the liver with the hepatitis B virus (HBV) and its satellite, the hepatitis D virus (HDV). Although current therapies can control HBV infection, no treatment that efficiently eliminates HDV is available and novel therapeutic strategies are needed. Although the HDV cycle is well described, the lack of simple experimental models has restricted the study of host–virus interactions, even if they represent relevant therapeutic targets. In the last few years, the discovery of the sodium taurocholate co-transporting polypeptide (NTCP) as a key cellular entry factor for HBV and HDV has allowed the development of new cell culture models susceptible to HBV and HDV infection. In this review, we summarize the main in vitro model systems used for the study of HDV entry and infection, discuss their benefits and limitations and highlight perspectives for future developments.

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.

Protective effects of n-Butanol extract and iridoid glycosides of Veronica ciliata Fisch. Against ANIT-induced cholestatic liver injury in mice

ETHNOPHARMACOLOGICAL RELEVANCE

Veronica ciliata Fisch. is a traditional medical herb that present in more than 100 types of Tibetan medicine prescriptions, most of which are used for liver disease therapy. Iridoid glycosides have been identified as the major active components of V.ciliata with a variety of biological activities.

AIMS OF THE STUDY

The aim of this study is to explore the protective effect and potential mechanism of n-Butanol extract (BE) and iridoid glycosides (IG) from V.ciliata against ɑ-naphthyl isothiocyanate (ANIT)-induced hepatotoxicity and cholestasis in mice.

MATERIALS AND METHODS

Mice were intragastrically (i.g.) given BE and IG at different dose or positive control ursodeoxycholic acid (UCDA) once a day for 14 consecutive days, and were treated with ANIT to cause liver injury on day 12th. Serum levels of hepatic injury markers and cholestasis indicators, liver index and liver histopathology were measured to evaluate the effect of BE and IG on liver injury caused by ANIT. The protein levels of tumor necrosis factor-α (TNF-α), nuclear factor kappa B(NF-κB), interleukin-6 (IL-6), Na+/taurocholate cotransporting polypeptide (NTCP), bile salt export pump (BSEP), multidrug resistance-associated protein 2 (MRP2), and the levels of oxidative stress indicators in liver tissue were investigated to reveal the underlying protective mechanisms of BE and IG against ANIT-induced hepatotoxicity and cholestasis.

RESULTS

The n-Butanol extract (BE) and iridoid glycosides (IG) isolated from V.ciliata significantly decreased serum level of cholestatic liver injury markers aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), γ-glutamyl transferase (GGT), total bile acid (TBA), total bilirubin (TBIL), and direct bilirubin (DBIL) in ANIT-treated mice. Histopathology of the liver tissue showed that pathological damages were relieved upon BE and IG treatment. Meanwhile, the results indicated BE and IG notably restored relative liver weights, inhibited oxidative stress induced by ANIT through increasing hepatic level of superoxide dismutase (SOD), reduced glutathione (GSH), catalase (CAT) and decreasing hepatic content of malondialdehyde (MDA). Western blot revealed that BE and IG inhibited the expression of pro-inflammatory factors TGF-α, IL-6 and NF-κB. Furthermore, the decreased protein expression of bile acid transporters NTCP, BSEP, MRP2 were upregulated by BE and IG in a dose-dependent manner.

CONCLUSION

The results have demonstrated that BE and IG exhibited a dose-dependently protective effect against ANIT-induced liver injury with acute intrahepatic cholestasis in mice, which might be related to the regulation of oxidative stress, inflammatory response and bile acid transport. In addition, these findings pointed out that iridoid glycosides as main active components of V.ciliata play a critical role in hepatoprotective effect of V.ciliata.

Picroside II alleviates liver injury induced by alpha-naphthylisothiocyanate through AMPK-FXR pathway

Alpha-naphthylisothiocyanate (ANIT) is a typical hepatotoxicant that causes cholestasis, which causes toxic bile acid accumulation in the liver and leads to liver injury. Picroside II (PIC), one of the dominant effective components extracted from Picrorhiza scrophulariiflora Pennell, exhibits many pharmacological effects. However, the role of AMP-activated protein kinase (AMPK)-Farnesoid X receptor (FXR) pathway in the hepatoprotective effect of PIC against ANIT-induced cholestasis remains largely unknown. This study aimed to investigate the mechanisms of PIC on ANIT-induced cholestasis in vivo and in vitro. Our results showed that PIC protected against ANIT-induced liver injury in primary mouse hepatocytes, and decreased serum biochemical markers and lessened histological injuries in mice. ANIT inhibited FXR and its target genes of bile acid synthesis enzymes sterol-12α-hydroxylase (CYP8B1), and increase bile acid uptake transporter Na + -dependent taurocholate transporter (NTCP), efflux transporter bile salt export pump (BSEP) and bile acid metabolizing enzymes UDP-glucuronosyltransferase 1a1 (UGT1A1) expressions. PIC prevented its downregulation of FXR, NTCP, BSEP and UGT1A1, and further reduced CYP8B1 by ANIT. Furthermore, ANIT activated AMPK via ERK1/2-LKB1 pathway. PIC inhibited ERK1/2, LKB1 and AMPK phosphorylation in ANIT-induced cholestasis in vivo and in vitro. AICAR, an AMPK agonist, blocked PIC-mediated changes in FXR, CYP8B1 and BSEP expression in vitro. Meanwhile, U0126, an ERK1/2 inhibitor, further repressed ERK1/2-LKB1-AMPK pathway phosphorylation. In conclusion, PIC regulated bile acid-related transporters and enzymes to protect against ANIT-induced liver injury, which related to ERK1/2-LKB1-AMPK pathway. Thus, this study extends the understanding of the anti-cholestasis effect of PIC and provides new therapeutic targets for cholestasis treatment.

Formulation of Liver-Specific PLGA-DY-635 Nanoparticles Loaded with the Protein Kinase C Inhibitor Bisindolylmaleimide I

Bisindolylmaleimide I (BIM-I) is a competitive pan protein kinase C inhibitor with anti-inflammatory and anti-metastatic properties, suggested to treat inflammatory diseases and various cancer entities. However, despite its therapeutic potential, BIM-I has two major drawbacks, i.e., it has a poor water solubility, and it binds the human ether-à-go-go-related gene (hERG) ion channels, potentially causing deadly arrhythmias. In this case, a targeted delivery of BIM-I is imperative to minimize peripheral side effects. To circumvent these drawbacks BIM-I was encapsulated into nanoparticles prepared from poly(lactic-co-glycolic acid) (PLGA) functionalized by the near-infrared dye DY-635. DY-635 served as an active targeting moiety since it selectively binds the OATP1B1 and OATP1B3 transporters that are highly expressed in liver and cancer cells. PLGA-DY-635 (BIM-I) nanoparticles were produced by nanoprecipitation and characterized using dynamic light scattering, analytical ultracentrifugation, and cryogenic transmission electron microscopy. Particle sizes were found to be in the range of 20 to 70 nm, while a difference in sizes between the drug-loaded and unloaded particles was observed by all analytical techniques. In vitro studies demonstrated that PLGA-DY-635 (BIM-I) NPs prevent the PKC activation efficiently, proving the efficacy of the inhibitor after its encapsulation, and suggesting that BIM-I is released from the PLGA-NPs. Ultimately, our results present a feasible formulation strategy that improved the cytotoxicity profile of BIM-I and showed a high cellular uptake in the liver as demonstrated in vivo by intravital microscopy investigations.

Current insights in the complexities underlying drug-induced cholestasis

Drug-induced cholestasis (DIC) poses a major challenge to the pharmaceutical industry and regulatory agencies. It causes both drug attrition and post-approval withdrawal of drugs. DIC represents itself as an impaired secretion and flow of bile, leading to the pathological hepatic and/or systemic accumulation of bile acids (BAs) and their conjugate bile salts. Due to the high number of mechanisms underlying DIC, predicting a compound's cholestatic potential during early stages of drug development remains elusive. A profound understanding of the different molecular mechanisms of DIC is, therefore, of utmost importance. Although many knowledge gaps and caveats still exist, it is generally accepted that alterations of certain hepatobiliary membrane transporters and changes in hepatocellular morphology may cause DIC. Consequently, liver models, which represent most of these mechanisms, are valuable tools to predict human DIC. Some of these models, such as membrane-based in vitro models, are exceptionally well-suited to investigate specific mechanisms (i.e. transporter inhibition) of DIC, while others, such as liver slices, encompass all relevant biological processes and, therefore, offer a better representation of the in vivo situation. In the current review, we highlight the principal molecular mechanisms associated with DIC and offer an overview and critical appraisal of the different liver models that are currently being used to predict the cholestatic potential of drugs.

Down-regulation of cell membrane localized NTCP expression in proliferating hepatocytes prevents hepatitis B virus infection

Hepatocyte proliferation could result in the loss of covalently closed circular DNA (cccDNA) and the emergence of cccDNA-cleared nascent hepatocytes, which appear refractory to hepatitis B virus (HBV) reinfection with unknown mechanism(s). Sodium taurocholate cotransporting polypeptide (NTCP) is the functional receptor for HBV entry. In this study, down-regulation of cell membrane localized NTCP expression in proliferating hepatocytes was found to prevent HBV infection in HepG2-NTCP-tet cells and in liver-humanized mice. In patients, lower NTCP protein expression was correlated well with higher levels of hepatocyte proliferation and less HBsAg expression in HBV-related focal nodular hyperplasia (FNH) tissues. Clinically, significantly lower NTCP protein expression was correlated with more active hepatocyte proliferation in CHB patients with severe active necroinflammation and better antiviral treatment outcome. Mechanistically, the activation of cell cycle regulatory genes p53, S-phase kinase-associated protein 2 (SKP2) and cyclin D1 during cell proliferation, as well as proliferative and inflammatory cytokine Interleukin-6 (IL-6) could transcriptionally down-regulate NTCP expression. From these aspects, we conclude that within the milieu of hepatocyte proliferation, down-regulation of cell membrane localized NTCP expression level renders nascent hepatocytes resistant to HBV reinfection. This may accelerate virus clearance during immune-mediated cell death and compensatory proliferation of survival hepatocytes.

Novel Mechanisms of Valproate Hepatotoxicity: Impaired Mrp2 Trafficking and Hepatocyte Depolarization

Drug-induced liver injury (DILI) remains a major challenge in drug development. Although numerous mechanisms for DILI have been identified, few studies have focused on loss of hepatocyte polarization as a DILI mechanism. The current study investigated the effects of valproate, an antiepileptic drug with DILI risk, on the cellular mechanisms responsible for loss of hepatocyte polarization. Fully polarized collagen sandwich-cultured rat hepatocytes were treated with valproate (1-20mM) for specified times (3-24hr). Hepatocyte viability was significantly decreased by 10mM and 20mM valproate. Valproate depolarized hepatocytes, even at non-cytotoxic concentrations (=5mM). Depolarization was associated with significantly decreased canalicular levels of multidrug resistance-associated protein 2 (Mrp2) resulting in reduced canalicular excretion of the Mrp2 substrate carboxydichlorofluorescein. The decreased canalicular Mrp2 was associated with intracellular accumulation of Mrp2 in Rab11-positive recycling endosomes and early endosomes. Mechanistic studies suggested that valproate inhibited canalicular trafficking of Mrp2. This effect of valproate on Mrp2 appeared to be selective in that valproate had less impact on canalicular levels of the bile salt export pump (Bsep) and no detectable effect on P-glycoprotein (P-gp) canalicular levels. Treatment with valproate for 24hr also significantly downregulated levels of tight junction-associated protein, zonula occludens 2 (ZO2), but appeared to have no effect on the levels of tight junction proteins claudin 1, claudin 2, occludin, ZO1 and ZO3. These findings reveal that two novel mechanisms may contribute to valproate hepatotoxicity: impaired canalicular trafficking of Mrp2 and disruption of ZO2-associated hepatocyte polarization.

Melatonin ameliorates ANIT‑induced cholestasis by activating Nrf2 through a PI3K/Akt‑dependent pathway in rats

Cholestasis is a devastating liver condition which is increasing in prevalence worldwide; however, its underlying pathogenic mechanisms remain to be fully elucidated. It was hypothesised that melatonin may alleviate the hepatic injury associated with cholestasis due to its established antioxidant effects. Therefore, the effect and potential anticholestatic properties of melatonin were investigated in rats with α-naphthylisothiocyanate (ANIT)-induced liver injury, a common animal model that mimics the cholestasis-associated liver injury in humans. The rats received intraperitoneal injection of ANIT with or without subsequent treatment with melatonin, and were sacrificed 24 h later. The serum biochemistry parameters of the liver were measured using conventional laboratory assays, and the liver tissue was subjected to conventional histological examination, reverse transcription-quantitative polymerase chain reaction analysis and western blotting. The levels of alanine transaminase, aspartate transaminase, total bilirubin, direct bilirubin, total bile acids, alkaline phosphatase, γ-glutamyl transferase and glutathione were restored in rats treated with melatonin. Histological examination provided further evidence supporting the protective effect of melatonin against ANIT-induced cholestasis. In addition, the mRNA and protein expression levels of glutamate cysteine ligase, phosphorylated Akt and nuclear factor-erythroid 2-related factor-2 were restored in rats treated with melatonin. These findings indicate that melatonin is a natural agent that appears to be promising for the treatment of cholestasis, and that the anticholestatic effects of melatonin involve the alleviation of oxidative stress.

PKCζ Phosphorylates SIRT6 to Mediate Fatty Acid β-Oxidation in Colon Cancer Cells

Protein kinase C (PKC) has critical roles in regulating lipid anabolism and catabolism. PKCζ, a member of atypical PKC family, has been reported to mediate glucose metabolism. However, whether and how PKCζ regulates tumor cells fatty acid β-oxidation are unknown. Here, we report that the phosphorylation of SIRT6 is significantly increased after palmitic acid (PA) treatment in colon cancer cells. PKCζ can physically interact with SIRT6 in vitro and in vivo, and this interaction enhances following PA treatment. Further experiments show that PKCζ is the phosphorylase of SIRT6 and phosphorylates SIRT6 at threonine 294 residue to promote SIRT6 enrichment on chromatin. In the functional study, we find that the expression of ACSL1, CPT1, CACT, and HADHB, the genes related to fatty acid β-oxidation, increases after PA stimulation. We further confirm that PKCζ mediates the binding of SIRT6 specifically to the promoters of fatty acid β-oxidation–related genes and elicits the expression of these genes through SIRT6 phosphorylation. Our findings demonstrate the mechanism of PKCζ as a new phosphorylase of SIRT6 on maintaining tumor fatty acid β-oxidation and define the new role of PKCζ in lipid homeostasis.

The functional role of sodium taurocholate cotransporting polypeptide NTCP in the life cycle of hepatitis B, C and D viruses

Chronic hepatitis B, C and D virus (HBV, HCV and HDV) infections are a major cause of liver disease and cancer worldwide. Despite employing distinct replication strategies, the three viruses are exclusively hepatotropic, and therefore depend on hepatocyte-specific host factors. The sodium taurocholate co-transporting polypeptide (NTCP), a transmembrane protein highly expressed in human hepatocytes that mediates the transport of bile acids, plays a key role in HBV and HDV entry into hepatocytes. Recently, NTCP has been shown to modulate HCV infection of hepatocytes by regulating innate antiviral immune responses in the liver. Here, we review the current knowledge of the functional role and the molecular and cellular biology of NTCP in the life cycle of the three major hepatotropic viruses, highlight the impact of NTCP as an antiviral target and discuss future avenues of research.

Mechanism of inhibition of taurolithocholate-induced retrieval of plasma membrane MRP2 by cyclic AMP and tauroursodeoxycholate

Taurolithocholate (TLC) produces cholestasis by inhibiting biliary solute secretion in part by retrieving MRP2 from the plasma membrane (PM). Tauroursodeoxycholate (TUDC) and cAMP reverse TLC‐induced cholestasis by inhibiting TLC‐induced retrieval of MRP2. However, cellular mechanisms for this reversal are incompletely understood. Recently, we reported that TLC decreases PM‐MRP2 by activating PKCε followed by phosphorylation of myristoylated alanine‐rich C kinase substrate (MARCKS). Thus, cAMP and TUDC may reverse TLC‐induced cholestasis by inhibiting the TLC/PKCε/MARCKS phosphorylation pathway. We tested this hypothesis by determining whether TUDC and/or cAMP inhibit TLC‐induced activation of PKCε and phosphorylation of MARCKS. Studies were conducted in HuH‐NTCP cell line and rat hepatocytes. Activation of PKCε was determined from the translocation of PKCε to PM using a biotinylation method. Phosphorylation of MARCKS was determined by immunoblotting with a phospho‐MARCKS antibody. TLC, but not cAMP and TUDC, activated PKCε and increased MARCKS phosphorylation in HuH‐NTCP as well in rat hepatocytes. Treatment with TUDC or cAMP inhibited TLC‐induced activation of PKCε and increases in MARCKS phosphorylation in both cell types. Based on these results, we conclude that the reversal of TLC‐induced cholestasis by cAMP and TUDC involves, at least in part, inhibition of TLC‐mediated activation of the PKCε/MARCKS phosphorylation pathway.

Phospholipid flippase ATP11C is endocytosed and downregulated following Ca2+-mediated protein kinase C activation

We and others showed that ATP11A and ATP11C, members of the P4-ATPase family, translocate phosphatidylserine (PS) and phosphatidylethanolamine from the exoplasmic to the cytoplasmic leaflets at the plasma membrane. PS exposure on the outer leaflet of the plasma membrane in activated platelets, erythrocytes, and apoptotic cells was proposed to require the inhibition of PS-flippases, as well as activation of scramblases. Although ATP11A and ATP11C are cleaved by caspases in apoptotic cells, it remains unclear how PS-flippase activity is regulated in non-apoptotic cells. Here we report that the PS-flippase ATP11C, but not ATP11A, is sequestered from the plasma membrane via clathrin-mediated endocytosis upon Ca²⁺-mediated PKC activation. Importantly, we show that a characteristic di-leucine motif (SVRPLL) in the C-terminal cytoplasmic region of ATP11C becomes functional upon PKC activation. Moreover endocytosis of ATP11C is induced by Ca²⁺-signaling via Gq-coupled receptors. Our data provide the first evidence for signal-dependent regulation of mammalian P4-ATPase.

ATP11C is a flippase that uses ATP hydrolysis to translocate phospholipids at the plasma membrane. Here, the authors show that the activation of Ca2+-dependent protein kinase C increases ATP11C endocytosis thus downregulating phospholipid translocation.

Acute regulation of multidrug resistance-associated protein 2 localization and activity by cAMP and estradiol-17β-D-glucuronide in rat intestine and Caco-2 cells

Multidrug resistance-associated protein 2 (MRP2) is an ATP-dependent transporter expressed at the brush border membrane of the enterocyte that confers protection against absorption of toxicants from foods or bile. Acute, short-term regulation of intestinal MRP2 activity involving changes in its apical membrane localization was poorly explored. We evaluated the effects of dibutyryl-cAMP (db-cAMP), a permeable analog of cAMP, and estradiol-17β-D-glucuronide (E₂17G), an endogenous derivative of estradiol, on MRP2 localization and activity using isolated rat intestinal sacs and Caco-2 cells, a model of human intestinal epithelium. Changes in MRP2 localization were studied by Western blotting of plasma membrane (PM) vs. intracellular membrane (IM) fractions in both experimental models, and additionally, by confocal microscopy in Caco-2 cells. After 30 min of exposure, db-cAMP-stimulated sorting of MRP2 from IM to PM both in rat jejunum and Caco-2 cells at 10 and 100 µM concentrations, respectively, with increased excretion of the model substrate 2,4-dinitrophenyl-S-glutathione. In contrast, E₂17G (400 µM) induced internalization of MRP2 together with impairment of transport activity. Confocal microscopy analysis performed in Caco-2 cells confirmed Western blot results. In the particular case of E₂17G, MRP2 exhibited an unusual pattern of staining compatible with endocytic vesiculation. Use of selective inhibitors demonstrated the participation of cAMP-dependent protein kinase and classic calcium-dependent protein kinase C in db-cAMP and E₂17G effects, respectively. We conclude that localization of MRP2 in intestine may be subjected to a dynamic equilibrium between plasma membrane and intracellular domains, thus allowing for rapid regulation of MRP2 function.

SRT1720 Alleviates ANIT-Induced Cholestasis in a Mouse Model

Intrahepatic cholestasis is a kind of clinical syndrome along with hepatotoxicity which caused by intrahepatic and systemic accumulations of bile acid. There are several crucial generating factors of the pathogenesis of cholestasis, such as inflammation, dysregulation of bile acid transporters and oxidative stress. SIRT1 is regarded as a class III histone deacetylase (HDAC). According to a set of researches, SIRT1 is one of the most important factors which can regulate the hepatic bile acid metabolism. SRT1720 is a kind of activator of SIRT1 which is 1000 times more potent than resveratrol, and this paper is aimed to study its protective influence on hepatotoxicity and cholestasis induced by alpha-naphthylisothiocyanate (ANIT) in mice. The findings revealed that SRT1720 treatment increased FXR and Nrf2 gene expressions to shield against hepatotoxicity and cholestasis induced by ANIT. The mRNA levels of hepatic bile acid transporters were also altered by SRT1720. Furthermore, SRT1720 enhanced the antioxidative system by increasing Nrf2, SOD, GCLc, GCLm, Nqo1, and HO-1 gene expressions. In conclusion, a protective influence could be provided by SRT1720 to cure ANIT-induced hepatotoxicity and cholestasis, which was partly through FXR and Nrf2 activations. These results indicated that SIRT1 could be regarded as a therapeutic target to cure the cholestasis.

Protein Kinases C-Mediated Regulations of Drug Transporter Activity, Localization and Expression

Drug transporters are now recognized as major actors in pharmacokinetics, involved notably in drug–drug interactions and drug adverse effects. Factors that govern their activity, localization and expression are therefore important to consider. In the present review, the implications of protein kinases C (PKCs) in transporter regulations are summarized and discussed. Both solute carrier (SLC) and ATP-binding cassette (ABC) drug transporters can be regulated by PKCs-related signaling pathways. PKCs thus target activity, membrane localization and/or expression level of major influx and efflux drug transporters, in various normal and pathological types of cells and tissues, often in a PKC isoform-specific manner. PKCs are notably implicated in membrane insertion of bile acid transporters in liver and, in this way, are thought to contribute to cholestatic or choleretic effects of endogenous compounds or drugs. The exact clinical relevance of PKCs-related regulation of drug transporters in terms of drug resistance, pharmacokinetics, drug–drug interactions and drug toxicity remains however to be precisely determined. This issue is likely important to consider in the context of the development of new drugs targeting PKCs-mediated signaling pathways, for treating notably cancers, diabetes or psychiatric disorders.

Dioscin Protects ANIT-Induced Intrahepatic Cholestasis Through Regulating Transporters, Apoptosis and Oxidative Stress

Intrahepatic cholestasis, a clinical syndrome, is caused by excessive accumulation of bile acids in body and liver. Proper regulation of bile acids in liver cells is critical for liver injury. We previously reported the effects of dioscin against α-naphthylisothio- cyanate (ANIT)-induced cholestasis in rats. However, the pharmacological and mechanism data are limited. In our work, the animals of rats and mice, and Sandwich-cultured hepatocytes (SCHs) were caused by ANIT, and dioscin was used for the treatment. The results showed that dioscin markedly altered relative liver weights, restored ALT, AST, ALP, TBIL, GSH, GSH-Px, MDA, SOD levels, and rehabilitated ROS level and cell apoptosis. In mechanism study, dioscin not only significantly regulated the protein levels of Ntcp, OAT1, OCT1, Bsep and Mrp2 to accelerate bile acids excretion, but also regulated the expression levels of Bak, Bcl-xl, Bcl-2, Bax, Caspase 3 and Caspase 9 in vivo and in vitro to improve apoptosis. In addition, dioscin markedly inhibited PI3K/Akt pathway and up-regulated the levels of Nrf2, GCLc, GCLm, NQO1 and HO-1 against oxidative stress (OS) caused by bile acids. These results were further validated by inhibition of PI3K and Akt using the inhibitors of wortmannin and perifosine in SCHs. Our data showed that dioscin had good action against ANIT-caused intrahepatic cholestasis through regulating transporters, apoptosis and OS. This natural product can be considered as one active compound to treat intrahepatic cholestasis in the future.

Nonalcoholic Steatohepatitis Modulates Membrane Protein Retrieval and Insertion Processes

Interindividual variability in drug response in nonalcoholic steatohepatitis (NASH) can be mediated by altered regulation of drug metabolizing enzymes and transporters. Among these is the mislocalization of multidrug resistance-associated protein (MRP2)/Mrp2 away from the canalicular membrane, which results in decreased transport of MRP2/Mrp2 substrates. The exact mechanism of this mislocalization is unknown, although increased activation of membrane retrieval processes may be one possibility. The current study measures the activation status of various mediators implicated in the active membrane retrieval or insertion of membrane proteins to identify which processes may be important in rodent methionine and choline deficient diet-induced NASH. The mediators currently known to be associated with transporter mislocalization are stimulated by oxidative stressors and choleretic stimuli, which play a role in the pathogenesis of NASH. The activation of protein kinases PKA, PKCα, PKCδ, and PKCε and substrates radixin, myristoylated alanine-rich C-kinase substrate, and Rab11 were measured by comparing the expression, phosphorylation, and membrane translocation between control and NASH. Many of the mediators exhibited altered activation in NASH rats. Consistent with membrane retrieval of Mrp2, NASH rats exhibited a decreased phosphorylation of radixin and increased membrane localization of PKCδ and PKCε, thought to be mediators of radixin dephosphorylation. Altered activation of PKCδ, PKA, and PKCα may impair the Rab11-mediated active insertion of Mrp2. Overall, these data suggest alterations in membrane retrieval and insertion processes that may contribute to altered localization of membrane proteins in NASH.

Up-regulation of BSEP and MRP2 by Calculus Bovis administration in 17α-ethynylestradiol-induced cholestasis: Involvement of PI3K/Akt signaling pathway

ETHNOPHARMACOLOGY RELEVANCE

Calculus Bovis, also known as Niuhuang, is a rare traditional Chinese medicine that has been widely used in China for 2000 years in pharmacology for sedation, anti-spasm, relieving fever, diminishing inflammation and recovering gallbladder functions.

AIM OF THE STUDY

This study aimed to investigate the choleretic potential and molecular responses in rats to Calculus Bovis (CB) administration after 17α-ethynylestradiol (EE)-induced cholestasis.

MATERIAL AND METHODS

CB (50 and 100mg/kg per day) was intragastrically (i. g.) given to experimental rats for five consecutive days in coadministration with EE (5mg/kg daily for five days, s.c.). The levels of serum biomarkers were determined biochemically. The histopathology of the liver tissue was evaluated. Expression of bile salt export pump (BSEP) and multidrug resistance-associated protein 2 (MRP2) were studied by western blot and immunohistochemical assay. The expression of Akt and phospho-Akt (pAkt) were also measured by western blot.

RESULTS

In response to EE, CB treatment significantly prevented an increase in serum levels of alanine aminotransferase (ALT), alkaline phosphatase (ALP), gamma glutamyltransferase (GGT) and total bilirubin (TBIL). CB treatment also repaired tissue lesions caused by EE. Western blots showed that EE significantly decreased the protein expression of BSEP and MRP2. EE also dramatically increased levels of pAkt and decreased levels of Akt. Compared to the EE group, CB treatment increased levels of hepatic BSEP and MRP2 while pAkt levels decreased and Akt levels increased. Immunohistochemistry also indicated that EE decreased the expression of BSEP and MRP2. LY294002 is a selective PI3K inhibitor and showed similar beneficial effects as CB. Decreased expression of BSEP and MRP2 caused by EE were also prevented by LY294002 treatment.

CONCLUSION

Calculus Bovis administration can alleviate liver injury and up-regulate the expression of BSEP and MRP2 in 17α-ethynylestradiol-induced cholestasis by a mechanism that may involve inhibiting the activated PI3K/Akt signaling pathway.

p38 MAPK α and β isoforms differentially regulate plasma membrane localization of MRP2

In hepatocytes, cAMP both activates p38 mitogen-activated protein kinase (MAPK) and increases the amount of multidrug resistance-associated protein-2 (MRP2) in the plasma membrane (PM-MRP2). Paradoxically, taurolithocholate (TLC) activates p38 MAPK but decreases PM-MRP2 in hepatocytes. These opposing effects of cAMP and TLC could be mediated via different p38 MAPK isoforms (α and β) that are activated differentially by upstream kinases (MKK3, MKK4, and MKK6). Thus we tested the hypothesis that p38α MAPK and p38β MAPK mediate increases and decreases in PM-MRP2 by cAMP and TLC, respectively. Studies were conducted in hepatocytes isolated from C57BL/6 wild-type (WT) and MKK3-knockout (MKK3(-/-)) mice and in a hepatoma cell line (HuH7) that overexpresses sodium-taurocholate cotransporting polypeptide (NTCP) (HuH-NTCP). Cyclic AMP activated MKK3, p38 MAPK, and p38α MAPK and increased PM-MRP2 in WT hepatocytes, but failed to activate p38α MAPK or increase PM-MRP2 in MKK3(-/-) hepatocytes. In contrast to cAMP, TLC activated total p38 MAPK but decreased PM-MRP2, and did not activate MKK3 or p38α MAPK in WT hepatocytes. In MKK3(-/-) hepatocytes, TLC still decreased PM-MRP2 and activated p38 MAPK, indicating that these effects are not MKK3-dependent. Additionally, TLC activated MKK6 in MKK3(-/-) hepatocytes, and small interfering RNA knockdown of p38β MAPK abrogated TLC-mediated decreases in PM-MRP2 in HuH-NTCP cells. Taken together, these results suggest that p38α MAPK facilitates plasma membrane insertion of MRP2 by cAMP, whereas p38β MAPK mediates retrieval of PM-MRP2 by TLC.

Insights into the diagnosis of hepatocellular carcinomas with hepatobiliary MRI

The incidence of hepatocellular carcinomas (HCCs) has increased worldwide in line with an improved screening by high-resolution imaging of cirrhotic livers. Besides abdominal ultrasonography and computerised tomography, magnetic resonance imaging (MRI) is an important tool to detect HCCs. With commercialisation of MR hepatobiliary contrast agents that cross membrane transporters in hepatocytes or tumour cells, MRI adds new information to detect and characterise HCCs. When tumour cells lose organic anion transporting polypeptides (OATP1B1/B3) in cell membranes facing sinusoidal blood, tumours appear hypointense (decreased contrast agent concentrations) in comparison to surrounding normal or cirrhotic liver that retains OATP1B1/B3 expression. However, expression, regulation, and prognostic significance of transporter evolution along carcinogenesis are not completely known. Moreover, understanding signal intensities in focal lesions also relies on transport functions of cellular efflux transporters. This manuscript reviews all the publications that associate liver imaging with hepatobiliary contrast agents and expression of transporters. The regulation of transporters along carcinogenesis to anticipate the prognosis of focal lesions is also included.

Canalicular membrane MRP2/ABCC2 internalization is determined by Ezrin Thr567 phosphorylation in human obstructive cholestasis

BACKGROUND & AIMS

Multidrug resistance-associated protein 2 (MRP2) excretes conjugated organic anions including bilirubin and bile acids. Malfunction of MRP2 leads to jaundice in patients. Studies in rodents indicate that Radixin plays a critical role in determining Mrp2 canalicular membrane expression. However, it is not known how human hepatic MRP2 expression is regulated in cholestasis.

METHODS

We assessed liver MRP2 expression in patients with obstructive cholestasis caused by gallstone blockage of bile ducts, and investigated the regulatory mechanism in HepG2 cells.

RESULTS

Western blot detected that liver MRP2 protein expression in obstructive cholestatic patients (n=30) was significantly reduced to 25% of the non-cholestatic controls (n=23). Immunoprecipitation identified Ezrin but not Radixin associating with MRP2 in human livers, and the increased amount of phospho-Ezrin Thr567 was positively correlated with the amount of co-precipitated MRP2 in cholestatic livers, whereas Ezrin and Radixin total protein levels were unchanged in cholestasis. Further detailed studies indicate that Ezrin Thr567 phosphorylation plays an important role in MRP2 internalization in HepG2 cells. Since increased expression of PKCα, δ and ε were detected in these cholestatic livers, we further confirmed that these PKCs stimulated Ezrin phosphorylation and reduced MRP2 membrane expression in HepG2 cells. Finally, we identified GP78 as the key ubiquitin ligase E3 involved in MRP2 proteasome degradation.

CONCLUSIONS

Activation of liver PKCs during cholestasis leads to Ezrin Thr567 phosphorylation resulting in MRP2 internalization and degradation where ubiquitin ligase E3 GP78 is involved. This process provides a mechanistic explanation for jaundice seen in patients with obstructive cholestasis.

Protein kinase C-dependent regulation of human hepatic drug transporter expression

Hepatic drug transporters are now recognized as major actors of hepatobiliary elimination of drugs. Characterization of their regulatory pathways is therefore an important issue. In this context, the present study was designed to analyze the potential regulation of human hepatic transporter expression by protein kinase C (PKC) activation. Treatment by the reference PKC activator phorbol 12-myristate 13-acetate (PMA) for 48h was shown to decrease mRNA expression of various sinusoidal transporters, including OATP1B1, OATP2B1, NTCP, OCT1 and MRP3, but to increase that of OATP1B3, whereas mRNA expression of canalicular transporters was transiently enhanced (MDR1), decreased (BSEP and MRP2) or unchanged (BCRP) in human hepatoma HepaRG cells. The profile of hepatic transporter mRNA expression changes in PMA-treated HepaRG cells was correlated to that found in PMA-exposed primary human hepatocytes and was similarly observed in response to the PKC-activating marketed drug ingenol mebutate. It was associated with concomitant repression of OATP1B1 and OATP2B1 protein expression and reduction of OATP, OCT1, NTCP and MRP2 activity. The use of chemical PKC inhibitors further suggested a contribution of novel PKCs isoforms to PMA-mediated regulations of transporter mRNA expression. PMA was finally shown to cause epithelial-mesenchymal transition (EMT) in HepaRG cells and exposure to various additional EMT inducers, i.e., hepatocyte growth factor, tumor growth factor-β1 or the HNF4α inhibitor BI6015, led to transporter expression alterations highly correlated to those triggered by PMA. Taken together, these data highlight PKC-dependent regulation of human hepatic drug transporter expression, which may be closely linked to EMT triggered by PKC activation.

Paeonia lactiflora Pall. protects against ANIT-induced cholestasis by activating Nrf2 via PI3K/Akt signaling pathway

Background

Paeonia lactiflora Pall. (PLP), a traditional Chinese herbal medicine, has been used for hepatic disease treatment over thousands of years. In our previous study, PLP was shown to demonstrate therapeutic effect on hepatitis with severe cholestasis. The aim of this study was to evaluate the antioxidative effect of PLP on alpha-naphthylisothiocyanate (ANIT)-induced cholestasis by activating NF-E2-related factor 2 (Nrf2) via phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway.

Materials and methods

Liquid chromatography-mass spectrometry (LC-MS) was performed to identify the main compounds present in PLP. The mechanism of action of PLP and its therapeutic effect on cholestasis, induced by ANIT, were further investigated. Serum indices such as total bilirubin (TBIL), direct bilirubin (DBIL), aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), γ-glutamyl transpeptidase (γ-GT), and total bile acid (TBA) were measured, and histopathology of liver was also performed to determine the efficacy of treatment with PLP. Moreover, in order to illustrate the underlying signaling pathway, liver glutathione (GSH) content and mRNA or protein levels of glutamate-cysteine ligase catalytic subunit (GCLc), glutamate-cysteine ligase modulatory subunit (GCLm), Akt, heme oxygenase-1 (HO-1), NAD(P)H/quinone oxidoreductase 1 (Nqo1), and Nrf2 were further analyzed. In addition, validation of PLP putative target network was also performed in silico.

Results

Four major compounds including paeoniflorin, albiflorin, oxypaeoniflorin, and benzoylpaeoniflorin were identified by LC-MS analysis in water extract of PLP. Moreover, PLP could remarkably downregulate serum levels of TBIL, DBIL, AST, ALT, ALP, γ-GT, and TBA, and alleviate the histological damage of liver tissue caused by ANIT. It enhanced antioxidative system by activating PI3K/Akt/Nrf2 pathway through increasing Akt, Nrf2, HO-1, Nqo1, GCLc, and GCLm expression. The putative targets network validation also confirmed the important role of PLP in activating Akt expression.

Conclusion

The potential mechanism of PLP in alleviating ANIT-induced cholestasis could to be related to the induction of GSH synthesis by activating Nrf2 through PI3K/Akt-dependent pathway. This indicates that PLP might be a potential therapeutic agent for cholestasis.

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.

Biliary complications after liver transplantation

Biliary complications (BCs) remain one of the most outstanding factors influencing long-term results after orthotopic liver transplantation. The authors carried out a systematic overview of 1720 papers since 2008, and focused on 45 relevant ones. Among 14,411 transplanted patients the incidence of BCs was 23%. Biliary leakage occurred in 8.5%, biliary stricture in 14.7%, mortality rate was 1-3%.

RISK FACTORS

preoperative sodium level; p = 0.037, model of end-stage liver disease score >25; p = 0.048, primary sclerosing cholangitis; p = 0.001, malignancy; p = 0.026, donor age >60, macrovesicular graft steatosis; p = 0.001, duct-to-duct anastomosis; p = 0.004, long anhepatic phase; p = 0.04, cold ischemic time >12 h; p = 0.043, use of T-tube; p = 0.032, insufficient flush of bile ducts; p = 0.001, acute rejection; p = 0.003, cytomegalovirus infection; p = 0.004 and hepatic artery thrombosis; p = 0.001. The management was surgical in case of biliary leakage, and interventional radiology or endoscopic retrograde cholangiopancreatography in case of biliary stricture. Mapping of miRNA profile is a new field of research. Nemes-Doros score is a useful tool in the estimation of hepatic artery thrombosis. Management of BCs requires a multidisciplinary expert team.

Protein kinase Cδ protects against bile acid apoptosis by suppressing proapoptotic JNK and BIM pathways in human and rat hepatocytes

Retained bile acids, which are capable of inducing cell death, activate protein kinase Cδ (PKC-δ) in hepatocytes. In nonhepatic cells, both pro- and antiapoptotic effects of PKC-δ are described. The aim of this study was to determine the role of PKC-δ in glycochenodeoxycholate (GCDC)-induced apoptosis in rat hepatocytes and human HUH7-Na-taurocholate-cotransporting polypeptide (Ntcp) cells. Apoptosis was monitored morphologically by Hoechst staining and biochemically by immunoblotting for caspase 3 cleavage. The role of PKC-δ was evaluated with a PKC activator (phorbol myristate acetate, PMA) and PKC inhibitors (chelerythrine, H-7, or calphostin), PKC-δ knockdown, and wild-type (WT) or constitutively active (CA) PKC-δ. PKC-δ activation was monitored by immunoblotting for PKC-δ Thr505 and Tyr311 phosphorylation or by membrane translocation. JNK and Akt phosphorylation and the amount of total bisindolylmaleimide (BIM) were determined by immunoblotting. GCDC induced the translocation of PKC-δ to the mitochondria and/or plasma membrane in rat hepatocytes and HUH7-Ntcp cells and increased PKC-δ phosphorylation on Thr505, but not on Tyr311, in HUH7-Ntcp cells. GCDC-induced apoptosis was attenuated by PMA and augmented by PKC inhibition in rat hepatocytes. In HUH-Ntcp cells, transfection with CA or WT PKC-δ attenuated GCDC-induced apoptosis, whereas knockdown of PKC-δ increased GCDC-induced apoptosis. PKC-δ silencing increased GCDC-induced JNK phosphorylation, decreased GCDC-induced Akt phosphorylation, and increased expression of BIM. GCDC translocated BIM to the mitochondria in rat hepatocytes, and knockdown of BIM in HUH7-Ntcp cells decreased GCDC-induced apoptosis. Collectively, these results suggest that PKC-δ does not mediate GCDC-induced apoptosis in hepatocytes. Instead PKC-δ activation by GCDC stimulates a cytoprotective pathway that involves JNK inhibition, Akt activation, and downregulation of BIM.

Rab11, but not Rab4, facilitates cyclic AMP- and tauroursodeoxycholate-induced MRP2 translocation to the plasma membrane

Rab proteins (Ras homologous for brain) play an important role in vesicle trafficking. Rab4 and Rab11 are involved in vesicular trafficking to the plasma membrane from early endosomes and recycling endosomes, respectively. Tauroursodeoxycholate (TUDC) and cAMP increase bile formation, in part, by increasing plasma membrane localization of multidrug resistance-associated protein 2 (MRP2). The goal of the present study was to determine the role of these Rab proteins in the trafficking of MRP2 by testing the hypothesis that Rab11 and/or Rab4 facilitate cAMP- and TUDC-induced MRP2 translocation to the plasma membrane. Studies were conducted in HuH-NTCP cells (HuH7 cells stably transfected with human NTCP), which constitutively express MRP2. HuH-NTCP cells were transfected with Rab11-WT and GDP-locked dominant inactive Rab11-GDP or with Rab4-GDP to study the role of Rab11 and Rab4. A biotinylation method and a GTP overlay assay were used to determine plasma membrane MRP2 and activation of Rab proteins (Rab11 and Rab4), respectively. Cyclic AMP and TUDC increased plasma membrane MRP2 and stimulated Rab11 activity. Plasma membrane translocation of MRP2 by cAMP and TUDC was increased and inhibited in cells transfected with Rab11-WT and Rab11-GDP, respectively. Cyclic AMP (previous study) and TUDC increased Rab4 activity. However, cAMP- and TUDC-induced increases in MRP2 were not inhibited by Rab4-GDP. Taken together, these results suggest that Rab11 is involved in cAMP- and TUDC-induced MRP2 translocation to the plasma membrane.