Phosphoinositide 3-kinase lipid products regulate ATP-dependent transport by sister of P-glycoprotein and multidrug resistance associated protein 2 in bile canalicular membrane vesicles

Intrahepatic Cholestasis Is a Clinically Significant Feature Associated with Natural History of X-Linked Myotubular Myopathy (XLMTM): A Case Series and Biopsy Report

X-linked myotubular myopathy (XLMTM) is a rare, life-threatening congenital myopathy characterized by profound skeletal muscle weakness, respiratory distress, and motor dysfunction. However, pathology is not limited to muscle and can be associated with life-threatening hepatic peliosis. Hepatobiliary disease has been reported in up to 17% of XLMTM patients but has not been extensively characterized. We report on five XLMTM patients who experienced intrahepatic cholestasis in their disease natural history, illustrating the need to further investigate these manifestations. These patients shared presentations that included pruritus, hypertransaminemia, and hyperbilirubinemia with normal gamma-glutamyl transferase, following infection or vaccination. Three patients who had genetic testing showed no evidence of genetic mutations associated with familial cholestasis. In one patient, progression to cirrhotic, decompensated liver disease occurred. Further investigations into the molecular pathomechanism underpinning these clinical observations in XLMTM patients will be important for informing patient care.

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.

P-Glycoprotein: One Mechanism, Many Tasks and the Consequences for Pharmacotherapy of Cancers

P-glycoprotein or multidrug resistance protein (MDR1) is an adenosine triphosphate (ATP) binding cassette transporter (ABCB1) intensely investigated because it is an obstacle to successful pharmacotherapy of cancers. P-glycoprotein prevents cellular uptake of a large number of structurally and functionally diverse compounds, including most cancer therapeutics and in this way causes multidrug resistance (MDR). To overcome MDR, and thus improve cancer treatment, an understanding of P-glycoprotein inhibition at the molecular level is required. With this goal in mind, we propose rules that predict whether a compound is a modulator, substrate, inhibitor, or inducer of P-glycoprotein. This new set of rules is derived from a quantitative analysis of the drug binding and transport properties of P-glycoprotein. We further discuss the role of P-glycoprotein in immune surveillance and cell metabolism. Finally, the predictive power of the proposed rules is demonstrated with a set of FDA approved drugs which have been repurposed for cancer therapy.

Transforming growth factor β1 (TGFβ1) regulates CD44V6 expression and activity through extracellular signal-regulated kinase (ERK)-induced EGR1 in pulmonary fibrogenic fibroblasts

The appearance of myofibroblasts is generally thought to be the underlying cause of the fibrotic changes that underlie idiopathic pulmonary fibrosis. However, the cellular/molecular mechanisms that account for the fibroblast-myofibroblast differentiation/activation in idiopathic pulmonary fibrosis remain poorly understood. We investigated the functional role of hyaluronan receptor CD44V6 (CD44 containing variable exon 6 (v6)) for differentiation of lung fibroblast to myofibroblast phenotype. Increased hyaluronan synthesis and CD44 expression have been detected in numerous fibrotic organs. Previously, we found that the TGFβ1/CD44V6 pathway is important in lung myofibroblast collagen-1 and α-smooth-muscle actin synthesis. Because increased EGR1 (early growth response-1) expression has been shown to appear very early and nearly coincident with the expression of CD44V6 found after TGFβ1 treatment, we investigated the mechanism(s) of regulation of CD44V6 expression in lung fibroblasts by TGFβ1. TGFβ1-mediated CD44V6 up-regulation was initiated through EGR1 via ERK-regulated transcriptional activation. We showed that TGFβ1-induced CD44V6 expression is through EGR1-mediated AP-1 (activator protein-1) activity and that the EGR1- and AP-1-binding sites in the CD44v6 promoter account for its responsiveness to TGFβ1 in lung fibroblasts. We also identified a positive-feedback loop in which ERK/EGR1 signaling promotes CD44V6 splicing and found that CD44V6 then sustains ERK signaling, which is important for AP-1 activity in lung fibroblasts. Furthermore, we identified that HAS2-produced hyaluronan is required for CD44V6 and TGFβRI co-localization and subsequent CD44V6/ERK1/EGR1 signaling. These results demonstrate a novel positive-feedback loop that links the myofibroblast phenotype to TGFβ1-stimulated CD44V6/ERK/EGR1 signaling.

The inhibition of Akt-Pdpk1 interaction efficiently suppresses the growth of murine primary liver tumor cells

The lack of primary liver tumor cells has hampered testing of potential chemotherapeutic agents in vitro. To overcome this issue we developed a primary mouse liver tumor cell line K07074. The K07074 cells were immortal, exhibited a biliary phenotype, formed colonies in soft agar and displayed an increase in Hedgehog, Notch and Akt signaling. To study the effect of single and combined inhibition of the liver tumor-related pathways on the growth of K07074 cells we treated these with small-molecule antitumor agents. While the inhibition of Akt and Notch pathways strongly inhibited the growth of K07074 cells the inhibition of Wnt and Hedgehog pathways was less efficient in cell growth suppression. Interestingly, the inhibition of Akt pathway at the level of Akt-Pdpk1 interaction was sufficient to suppress the growth of tumor cells and no significant additive effect could be detected when co-treated with the inhibitors of Wnt, Hedgehog or Notch pathways. Only when suboptimal doses of Akt-Pdpk1 interaction inhibitor NSC156529 were used an additive effect with Notch inhibition was seen. We conclude that the Akt pathway inhibitor NSC156529 is potentially useful as single treatment for liver tumors with hyperactivated Akt signaling.

Structural and functional hepatocyte polarity and liver disease

Hepatocytes form a crucially important cell layer that separates sinusoidal blood from the canalicular bile. They have a uniquely organized polarity with a basal membrane facing liver sinusoidal endothelial cells, while one or more apical poles can contribute to several bile canaliculi jointly with the directly opposing hepatocytes. Establishment and maintenance of hepatocyte polarity is essential for many functions of hepatocytes and requires carefully orchestrated cooperation between cell adhesion molecules, cell junctions, cytoskeleton, extracellular matrix and intracellular trafficking machinery. The process of hepatocyte polarization requires energy and, if abnormal, may result in severe liver disease. A number of inherited disorders affecting tight junction and intracellular trafficking proteins have been described and demonstrate clinical and pathophysiological features overlapping those of the genetic cholestatic liver diseases caused by defects in canalicular ABC transporters. Thus both structural and functional components contribute to the final hepatocyte polarity phenotype. Many acquired liver diseases target factors that determine hepatocyte polarity, such as junctional proteins. Hepatocyte depolarization frequently occurs but is rarely recognized because hematoxylin-eosin staining does not identify the bile canaliculus. However, the molecular mechanisms underlying these defects are not well understood. Here we aim to provide an update on the key factors determining hepatocyte polarity and how it is affected in inherited and acquired diseases.

Involvement of a cyclic adenosine monophosphate-dependent signal in the diet-induced canalicular trafficking of adenosine triphosphate-binding cassette transporter g5/g8

The adenosine triphosphate-binding cassette (ABC) half-transporters Abcg5 and Abcg8 promote the secretion of neutral sterol into bile. Studies have demonstrated the diet-induced gene expression of these transporters, but the regulation of their trafficking when the nutritional status changes in the liver remains to be elucidated. Here, we generated a novel in vivo kinetic analysis that can monitor the intracellular trafficking of Abcg5/Abcg8 in living mouse liver by in vivo transfection of the genes of fluorescent protein-tagged transporters and investigated how hypernutrition affects the canalicular trafficking of these transporters. The kinetic analysis showed that lithogenic diet consumption accelerated the translocation of newly synthesized fluorescent-tagged transporters to intracellular pools in an endosomal compartment and enhanced the recruitment of these pooled gene products into the bile canalicular membrane in mouse liver. Because some ABC transporters are reported to be recruited from intracellular pools to the bile canaliculi by cyclic adenosine monophosphate (cAMP) signaling, we next evaluated the involvement of this machinery in a diet-induced event. Administration of a protein kinase A inhibitor, N-(2-{[3-(4-bromophenyl)-2-propenyl]amino}ethyl)-5-isoquinolinesulfonamide, decreased the canalicular expression of native Abcg5/Abcg8 in lithogenic diet-fed mice, and injection of a cAMP analog, dibutyryl cAMP, transiently increased their levels in standard diet-fed mice, indicating the involvement of cAMP signaling. Indeed, canalicular trafficking of the fluorescent-tagged Abcg5/Abcg8 was enhanced by dibutyryl cAMP administration.

CONCLUSION

These observations suggest that diet-induced lipid loading into liver accelerates the trafficking of Abcg5/Abcg8 to the bile canalicular membrane through cAMP signaling machinery.

Evaluation of 11C-acetate and 18F-FDG PET/CT in mouse multidrug resistance gene-2 deficient mouse model of hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) remains a global health problem with unique diagnostic and therapeutic challenges, including difficulties in identifying the highest risk patients. Previous work from our lab has established the murine multidrug resistance-2 mouse (MDR2) model of HCC as a reasonable preclinical model that parallels the changes seen in human inflammatory associated HCC. The purpose of this study is to evaluate modalities of PET/CT in MDR2(-/-) mice in order to facilitate therapeutic translational studies from bench to bedside.

METHODS

18F-FDG and 11C-acetate PET/CT was performed on 12 m MDR2(-/-) mice (n = 3/tracer) with HCC and 12 m MDR2(-/+) control mice (n = 3/tracer) without HCC. To compare PET/CT to biological markers of HCC and cellular function, serum alpha-fetoprotein (AFP), lysophosphatidic acid (LPA), cAMP and hepatic tumor necrosis factor α (TNFα) were quantified in 3-12 m MDR2(-/-) (n = 10) mice using commercially available ELISA analysis. To translate results in mice to patients 11C-acetate PET/CT was also performed in 8 patents suspected of HCC recurrence following treatment and currently on the liver transplant wait list.

RESULTS

Hepatic18F-FDG metabolism was not significantly increased in MDR2(-/-) mice. In contrast, hepatic 11C-acetate metabolism was significantly elevated in MDR2(-/-) mice when compared to MDR2(-/+) controls. Serum AFP and LPA levels increased in MDR2(-/-) mice contemporaneous with the emergence of HCC. This was accompanied by a significant decrease in serum cAMP levels and an increase in hepatic TNFα. In patients suspected of HCC recurrence there were 5 true positives, 2 true negatives and 1 suspected false 11C-acetate negative.

CONCLUSIONS

Hepatic 11C-acetate PET/CT tracks well with HCC in MDR2(-/-) mice and patients with underlying liver disease. Consequently 11C-acetate PET/CT is well suited to study (1) HCC emergence/progression in patients and (2) reduce animal numbers required to study new chemotherapeutics in murine models of HCC.

Unique functional and structural properties of the LRRK2 protein ATP-binding pocket

Pathogenic mutations in the LRRK2 gene can cause late-onset Parkinson disease. The most common mutation, G2019S, resides in the kinase domain and enhances activity. LRRK2 possesses the unique property of cis-autophosphorylation of its own GTPase domain. Because high-resolution structures of the human LRRK2 kinase domain are not available, we used novel high-throughput assays that measured both cis-autophosphorylation and trans-peptide phosphorylation to probe the ATP-binding pocket. We disclose hundreds of commercially available activity-selective LRRK2 kinase inhibitors. Some compounds inhibit cis-autophosphorylation more strongly than trans-peptide phosphorylation, and other compounds inhibit G2019S-LRRK2 more strongly than WT-LRRK2. Through exploitation of structure-activity relationships revealed through high-throughput analyses, we identified a useful probe inhibitor, SRI-29132 (11). SRI-29132 is exquisitely selective for LRRK2 kinase activity and is effective in attenuating proinflammatory responses in macrophages and rescuing neurite retraction phenotypes in neurons. Furthermore, the compound demonstrates excellent potency, is highly blood-brain barrier-permeant, but suffers from rapid first-pass metabolism. Despite the observed selectivity of SRI-29132, docking models highlighted critical interactions with residues conserved in many protein kinases, implying a unique structural configuration for the LRRK2 ATP-binding pocket. Although the human LRRK2 kinase domain is unstable and insoluble, we demonstrate that the LRRK2 homolog from ameba can be mutated to approximate some aspects of the human LRRK2 ATP-binding pocket. Our results provide a rich resource for LRRK2 small molecule inhibitor development. More broadly, our results provide a precedent for the functional interrogation of ATP-binding pockets when traditional approaches to ascertain structure prove difficult.

Isothiocyanate analogs targeting CD44 receptor as an effective strategy against colon cancer

Inflammatory pathway plays an important role in tumor cell progression of colorectal cancers. Although colon cancer is considered as one of the leading causes of death worldwide, very few drugs are available for its effective treatment. Many studies have examined the effects of specific COX-2 and 5-LOX inhibitors on human colorectal cancer, but the role of isothiocyanates (ITSCs) as COX-LOX dual inhibitors engaged in hyaluronan-CD44 interaction has not been studied. In the present work, we report series of ITSC analogs incorporating bioisosteric thiosemicarbazone moiety. These inhibitors are effective against panel of human colon cancer cell lines including COX-2 positive HCA-7, HT-29 cells lines, and hyaluronan synthase-2 (Has2) enzyme over-expressing transformed intestinal epithelial Apc10.1Has2 cells. Specifically, our findings indicate that HA-CD44v6-mediated COX-2/5-LOX signaling mediate survivin production, which in turn, supports anti-apoptosis and chemo-resistance leading to colon cancer cell survival. The over-expression of CD44v6shRNA as well as ITSC treatment significantly decreases the survival of colon cancer cells. The present results thus offer an opportunity to evolve potent inhibitors of HA synthesis and CD44v6 pathway and thus underscoring the importance of the ITSC analogs as chemopreventive agents for targeting HA/CD44v6 pathway.

Inherited disorders of bilirubin transport and conjugation: new insights into molecular mechanisms and consequences

Inherited disorders of bilirubin metabolism might reduce bilirubin uptake by hepatocytes, bilirubin conjugation, or secretion of bilirubin into bile. Reductions in uptake could increase levels of unconjugated or conjugated bilirubin (Rotor syndrome). Defects in bilirubin conjugation could increase levels of unconjugated bilirubin; the effects can be benign and frequent (Gilbert syndrome) or rare but severe, increasing the risk of bilirubin encephalopathy (Crigler-Najjar syndrome). Impairment of bilirubin secretion leads to accumulation of conjugated bilirubin (Dubin-Johnson syndrome). We review the genetic causes and pathophysiology of disorders of bilirubin transport and conjugation as well as clinical and therapeutic aspects. We also discuss the possible mechanisms by which hyperbilirubinemia protects against cardiovascular disease and the metabolic syndrome and the effects of specific genetic variants on drug metabolism and cancer development.

Ursodeoxycholic acid stabilizes the bile salt export pump in the apical membrane in MDCK II cells

BACKGROUND

Ursodeoxycholic acid (UDCA) partly exerts choleretic effects by modifying the function of the bile salt export pump (Bsep, ABCB11). UDCA induces insertion of Bsep into the canalicular membrane of hepatocytes; however, underlying mechanisms remain unknown. We aimed to elucidate molecular mechanisms behind UDCA-induced Bsep activation.

METHODS

We established MDCK II cells stably expressing both Bsep and Na(+)-taurocholate cotransporting polypeptide, and investigated the effect of UDCA on activity and protein expression of Bsep using these cells. We performed inhibitor study to know the molecules involved in UDCA-induced Bsep activation, and also tested the influence of UDCA on Bsep having a disease-associated mutation.

RESULTS

UDCA activated Bsep in a dose-dependent manner. UDCA did not affect Bsep protein expression in whole cell lysates but increased its apical surface expression by extending the half-life from 2.4 to 5.0 h. This effect was specific to Bsep because UDCA did not affect other apical and basolateral proteins, and was independent of protein kinase A, adenylate cyclase, p38(MAPK), phosphatidylinositide 3-kinase, Ca(2+), and microtubules. NorUDCA activated Bsep similar to UDCA; however, cholic acid, taurocholic acid, and tauroUDCA had no effect. UDCA significantly increased the activity of Bsep with a benign recurrent intrahepatic cholestasis 2 mutation (A570T) but did not affect Bsep with a progressive familial intrahepatic cholestasis 2 mutation (G982R or D482G).

CONCLUSIONS

We demonstrated that UDCA stabilizes Bsep protein in the apical membrane and increases its activity in MDCK II cells, presumably by retarding the endocytotic process.

Overexpression of c-Met and CD44v6 receptors contributes to autocrine TGF-β1 signaling in interstitial lung disease

The hepatocyte growth factor (HGF) and the HGF receptor Met pathway are important in the pathogenesis of interstitial lung disease (ILD). Alternatively spliced isoforms of CD44 containing variable exon 6 (CD44v6) and its ligand hyaluronan (HA) alter cellular function in response to interaction between CD44v6 and HGF. TGF-β1 is the crucial cytokine that induces fibrotic action in ILD fibroblasts (ILDFbs). We have identified an autocrine TGF-β1 signaling that up-regulates both Met and CD44v6 mRNA and protein expression. Western blot analysis, flow cytometry, and immunostaining revealed that CD44v6 and Met colocalize in fibroblasts and in tissue sections from ILD patients and in lungs of bleomycin-treated mice. Interestingly, cell proliferation induced by TGF-β1 is mediated through Met and CD44v6. Further, cell proliferation mediated by TGF-β1/CD44v6 is ERK-dependent. In contrast, action of Met on ILDFb proliferation does not require ERK but does require p38(MAPK). ILDFbs were sorted into CD44v6(+)/Met(+) and CD44v6(-)/Met(+) subpopulations. HGF inhibited TGF-β1-stimulated collagen-1 and α-smooth muscle cell actin expression in both of these subpopulations by interfering with TGF-β1 signaling. HGF alone markedly stimulated CD44v6 expression, which in turn regulated collagen-1 synthesis. Our data with primary lung fibroblast cultures with respect to collagen-1, CD44v6, and Met expressions were supported by immunostaining of lung sections from bleomycin-treated mice and from ILD patients. These results define the relationships between CD44v6, Met, and autocrine TGF-β1 signaling and the potential modulating influence of HGF on TGF-β1-induced CD44v6-dependent fibroblast function in ILD fibrosis.

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

Conjugated bile acids activate the sphingosine-1-phosphate receptor 2 in primary rodent hepatocytes

Bile acids have been shown to be important regulatory molecules for cells in the liver and gastrointestinal tract. They can activate various cell signaling pathways including extracellular regulated kinase (ERK)1/2 and protein kinase B (AKT) as well as the G-protein-coupled receptor (GPCR) membrane-type bile acid receptor (TGR5/M-BAR). Activation of the ERK1/2 and AKT signaling pathways by conjugated bile acids has been reported to be sensitive to pertussis toxin (PTX) and dominant-negative Gα(i) in primary rodent hepatocytes. However, the GPCRs responsible for activation of these pathways have not been identified. Screening GPCRs in the lipid-activated phylogenetic family (expressed in HEK293 cells) identified sphingosine-1-phosphate receptor 2 (S1P(2) ) as being activated by taurocholate (TCA). TCA, taurodeoxycholic acid (TDCA), tauroursodeoxycholic acid (TUDCA), glycocholic acid (GCA), glycodeoxycholic acid (GDCA), and S1P-induced activation of ERK1/2 and AKT were significantly inhibited by JTE-013, a S1P(2) antagonist, in primary rat hepatocytes. JTE-013 significantly inhibited hepatic ERK1/2 and AKT activation as well as short heterodimeric partner (SHP) mRNA induction by TCA in the chronic bile fistula rat. Knockdown of the expression of S1P(2) by a recombinant lentivirus encoding S1P(2) shRNA markedly inhibited the activation of ERK1/2 and AKT by TCA and S1P in rat primary hepatocytes. Primary hepatocytes prepared from S1P(2) knock out (S1P(2) (-/-) ) mice were significantly blunted in the activation of the ERK1/2 and AKT pathways by TCA. Structural modeling of the S1P receptors indicated that only S1P(2) can accommodate TCA binding. In summary, all these data support the hypothesis that conjugated bile acids activate the ERK1/2 and AKT signaling pathways primarily through S1P(2) in primary rodent hepatocytes.

Liver dysfunction and phosphatidylinositol-3-kinase signalling in early sepsis: experimental studies in rodent models of peritonitis

BACKGROUND

Hepatic dysfunction and jaundice are traditionally viewed as late features of sepsis and portend poor outcomes. We hypothesized that changes in liver function occur early in the onset of sepsis, yet pass undetected by standard laboratory tests.

METHODS AND FINDINGS

In a long-term rat model of faecal peritonitis, biotransformation and hepatobiliary transport were impaired, depending on subsequent disease severity, as early as 6 h after peritoneal contamination. Phosphatidylinositol-3-kinase (PI3K) signalling was simultaneously induced at this time point. At 15 h there was hepatocellular accumulation of bilirubin, bile acids, and xenobiotics, with disturbed bile acid conjugation and drug metabolism. Cholestasis was preceded by disruption of the bile acid and organic anion transport machinery at the canalicular pole. Inhibitors of PI3K partially prevented cytokine-induced loss of villi in cultured HepG2 cells. Notably, mice lacking the PI3Kγ gene were protected against cholestasis and impaired bile acid conjugation. This was partially confirmed by an increase in plasma bile acids (e.g., chenodeoxycholic acid [CDCA] and taurodeoxycholic acid [TDCA]) observed in 48 patients on the day severe sepsis was diagnosed; unlike bilirubin (area under the receiver-operating curve: 0.59), these bile acids predicted 28-d mortality with high sensitivity and specificity (area under the receiver-operating curve: CDCA: 0.77; TDCA: 0.72; CDCA+TDCA: 0.87).

CONCLUSIONS

Liver dysfunction is an early and commonplace event in the rat model of sepsis studied here; PI3K signalling seems to play a crucial role. All aspects of hepatic biotransformation are affected, with severity relating to subsequent prognosis. Detected changes significantly precede conventional markers and are reflected by early alterations in plasma bile acids. These observations carry important implications for the diagnosis of liver dysfunction and pharmacotherapy in the critically ill. Further clinical work is necessary to extend these concepts into clinical practice. Please see later in the article for the Editors' Summary.

The role of the sodium-taurocholate cotransporting polypeptide (NTCP) and of the bile salt export pump (BSEP) in physiology and pathophysiology of bile formation

Bile formation is an important function of the liver. Bile salts are a major constituent of bile and are secreted by hepatocytes into bile and delivered into the small intestine, where they assist in fat digestion. In the small intestine, bile salts are almost quantitatively reclaimed and transported back via the portal circulation to the liver. In the liver, hepatocytes take up bile salts and secrete them again into bile for ongoing enterohepatic circulation. Uptake of bile salts into hepatocytes occurs largely in a sodium-dependent manner by the sodium taurocholate cotransporting polypeptide NTCP. The transport properties of NTCP have been extensively characterized. It is an electrogenic member of the solute carrier family of transporters (SLC10A1) and transports predominantly bile salts and sulfated compounds, but is also able to mediate transport of additional substrates, such as thyroid hormones, drugs and toxins. It is highly regulated under physiologic and pathophysiologic conditions. Regulation of NTCP copes with changes of bile salt load to hepatocytes and prevents entry of cytotoxic bile salts during liver disease. Canalicular export of bile salts is mediated by the ATP-binding cassette transporter bile salt export pump BSEP (ABCB11). BSEP constitutes the rate limiting step of hepatocellular bile salt transport and drives enterohepatic circulation of bile salts. It is extensively regulated to keep intracellular bile salt levels low under normal and pathophysiologic situations. Mutations in the BSEP gene lead to severe progressive familial intrahepatic cholestasis. The substrates of BSEP are practically restricted to bile salts and their metabolites. It is, however, subject to inhibition by endogenous metabolites or by drugs. A sustained inhibition will lead to acquired cholestasis, which can end in liver injury.

Bile salts and cholestasis

Bile salts have a crucial role in hepatobiliary and intestinal homeostasis and digestion. Primary bile salts are synthesized by the liver from cholesterol, and may be modified by the intestinal flora to form secondary and tertiary bile salts. Bile salts are efficiently reabsorbed from the intestinal lumen to undergo enterohepatic circulation. In addition to their function as a surfactant involved in the absorption of dietary lipids and fat-soluble vitamins bile salts are potent signaling molecules in both the liver and intestine. Under physiological conditions the bile salt pool is tightly regulated, but the adaptive capacity may fall short under cholestatic conditions. Elevated serum and tissue levels of potentially toxic hydrophobic bile salts during cholestasis may cause mitochondrial damage, apoptosis or necrosis in susceptible cell types. Therapeutic nontoxic bile salts may restore impaired hepatobiliary secretion in cholestatic disorders. The hydrophilic bile salt ursodeoxycholate is today regarded as the effective standard treatment of primary biliary cirrhosis and intrahepatic cholestasis of pregnancy, and is implicated for use in various other cholestatic conditions. Novel therapeutic bile salts that are currently under evaluation may also prove valuable in the treatment of these diseases.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Sex-dependent disposition of acetaminophen sulfate and glucuronide in the in situ perfused mouse liver

Breast cancer resistance protein (BCRP, ABCG2) is expressed in the hepatic canalicular membrane and mediates biliary excretion of xenobiotics including sulfate and glucuronide metabolites of some compounds. Hepatic Bcrp expression is sex-dependent, with higher expression in male mice. The hypothesis that sex-dependent Bcrp expression influences the hepatobiliary disposition of phase II metabolites was tested in the present study using acetaminophen (APAP) and the generated APAP glucuronide (AG) and sulfate (AS) metabolites in single-pass in situ perfused livers from male and female wild-type and Abcg(-/-) (Bcrp-deficient) mice. Pharmacokinetic modeling was used to estimate parameters governing the hepatobiliary disposition of APAP, AG, and AS. In wild-type mice, the biliary excretion rate constant was 2.5- and 7-fold higher in males than in females for AS and AG, respectively, reflecting male-predominant Bcrp expression. Sex-dependent differences in AG biliary excretion were not observed in Bcrp-deficient mice, and AS biliary excretion was negligible. Interestingly, sex-dependent basolateral excretion of AG (higher in males) and AS (higher in females) was noted in wild-type mice with a similar trend in Bcrp-deficient mouse livers, reflecting an increased rate constant for AG formation in male and AS formation in female mouse livers. In addition, the rate constant for AS basolateral excretion was increased significantly in female mouse livers compared with that in male mouse livers. It is interesting to note that multidrug resistance-associated protein 4 was higher in female than in male mouse livers. In conclusion, sex-dependent differences in conjugation and transporter expression result in profound differences in the hepatobiliary disposition of AG and AS in male and female mouse livers.

Glucagon induces the gene expression of aquaporin-8 but not that of aquaporin-9 water channels in the rat hepatocyte

Glucagon stimulates the vesicle trafficking of aquaporin-8 (AQP8) water channels to the rat hepatocyte canalicular membranes, a process thought to be relevant to glucagon-induced bile secretion. In this study, we investigated whether glucagon is able to modulate the gene expression of hepatocyte AQP8. Glucagon was administered to rats at 0.2 mg/100 g body wt ip in 2, 3, or 6 equally spaced doses for 8, 16, and 36 h, respectively. Immunoblotting analysis showed that hepatic 34-kDa AQP8 was significantly increased by 79 and 107% at 16 and 36 h, respectively. Hepatic AQP9 protein expression remained unaltered. AQP8 mRNA expression, assessed by real-time PCR, was not modified over time, suggesting a posttranscriptional mechanism of AQP8 protein increase. Glucagon effects on AQP8 were directly studied in primary cultured rat hepatocytes. Immunoblotting and confocal immunofluorescence microscopy confirmed the specific glucagon-induced AQP8 upregulation. The RNA polymerase II inhibitor actinomycin D was unable to prevent glucagon effect, providing additional support to the nontranscriptional upregulation of AQP8. Cycloheximide also showed no effect, suggesting that glucagon-induced AQP8 expression does not depend on protein synthesis but rather on protein degradation. Inhibitory experiments suggest that a reduced calpain-mediated AQP8 proteolysis could be involved. The action of glucagon on hepatocyte AQP8 was mimicked by dibutyryl cAMP and suppressed by PKA or phosphatidylinositol-3-kinase (PI3K) inhibitors. In conclusion, our data suggest that glucagon induces the gene expression of rat hepatocyte AQP8 by reducing its degradation, a process that involves cAMP-PKA and PI3K signal pathways.

Aquaporins: Their role in cholestatic liver disease

This review focuses on current knowledge on hepatocyte aquaporins (AQPs) and their significance in bile formation and cholestasis. Canalicular bile secretion results from a combined interaction of several solute transporters and AQP water channels that facilitate water flow in response to the osmotic gradients created. During choleresis, hepatocytes rapidly increase their canalicular membrane water permeability by modulating the abundance of AQP8. The question was raised as to whether the opposite process, i.e. a decreased canalicular AQP8 expression would contribute to the development of cholestasis. Studies in several experimental models of cholestasis, such as extrahepatic obstructive cholestasis, estrogen-induced cholestasis, and sepsis-induced cholestasis demonstrated that the protein expression of hepatocyte AQP8 was impaired. In addition, biophysical studies in canalicular plasma membranes revealed decreased water permeability associated with AQP8 protein downregulation. The combined alteration in hepatocyte solute transporters and AQP8 would hamper the efficient coupling of osmotic gradients and canalicular water flow. Thus cholestasis may result from a mutual occurrence of impaired solute transport and decreased water permeability.

Bile acid transporters in health and disease

In recent years the discovery of a number of major transporter proteins expressed in the liver and intestine specifically involved in bile acid transport has led to improved understanding of bile acid homeostasis and the enterohepatic circulation. Sodium (Na(+))-dependent bile acid uptake from portal blood into the liver is mediated primarily by the Na(+) taurocholate co-transporting polypeptide (NTCP), while secretion across the canalicular membrane into the bile is carried out by the bile salt export pump (BSEP). In the ileum, absorption of bile acids from the lumen into epithelial cells is mediated by the apical Na(+) bile salt transporter (ASBT), whereas exit into portal blood across the basolateral membrane is mediated by the organic solute transporter alpha/beta (OSTalpha/beta) heterodimer. Regulation of transporter gene expression and function occurs at several different levels: in the nucleus, members of the nuclear receptor superfamily, regulated by bile acids and other ligands are primarily involved in controlling gene expression, while cell signalling events directly affect transporter function, and subcellular localization. Polymorphisms, dysfunction, and impaired adaptive responses of several of the bile acid transporters, e.g. BSEP and ASBT, results in liver and intestinal disease. Bile acid transporters are now understood to play central roles in driving bile flow, as well as adaptation to various pathological conditions, with complex regulation of activity and function in the nucleus, cytoplasm, and membrane.

High glucose induced translocation of Aquaporin8 to chicken hepatocyte plasma membrane: involvement of cAMP, PI3K/Akt, PKC, MAPKs, and microtubule

Aquaporin8 (AQP8) is a transmembrane water channel that is found mainly in hepatocytes. The direct involvement of AQP8 in high glucose condition has not been established. Therefore, this study examined the effects of high glucose on AQP8 and its related signal pathways in primary cultured chicken hepatocytes. High glucose increased the movement of AQP8 from the intracellular membrane to plasma membrane in a 30 mM glucose concentration and in a time- (> or =10 min) dependent manner. On the other hand, 30 mM mannitol did not affect the translocation of AQP8, which suggested the absence of osmotic effect. Thirty millimolar glucose increased intracellular cyclic adenosine 3, 5-monophosphate (cAMP) level. Moreover, high glucose level induced Akt phosphorylation, protein kinase C (PKC) activation, p44/42 mitogen-activated protein kinases (MAPKs), p38 MAPK, and c-jun NH2-terminal kinase (JNK) phosphorylation. On the other hand, inhibition of each pathway by SQ 22536 (adenylate cyclase inhibitor), LY 294002 (PI3-K phosphatidylinositol 3-kinase inhibitor), Akt inhibitor, staurosporine (PKC inhibitor), PD 98059 (MEK inhibitor), SB 203580 (p38 MAPK inhibitor), or SP 600125 (JNK inhibitor) blocked 30 mM glucose-induced AQP8 translocation, respectively. In addition, inhibition of microtubule movement with nocodazole blocked high glucose-induced AQP8 translocation. High glucose level also increased the level of kinesin light chain and dynein protein expression. In conclusion, high glucose level stimulates AQP8 via cAMP, PI3-K/Akt, PKC, and MAPKs pathways in primary cultured chicken hepatocytes.

Phenotypic differences in PFIC2 and BRIC2 correlate with protein stability of mutant Bsep and impaired taurocholate secretion in MDCK II cells

Progressive familial cholestasis (PFIC) 2 and benign recurrent intrahepatic cholestasis (BRIC) 2 are caused by mutations in the bile salt export pump (BSEP, ABCB11) gene; however, their prognosis differs. PFIC2 progresses to cirrhosis and requires liver transplantation, whereas BRIC2 is clinically benign. To identify the molecular mechanism(s) responsible for the phenotypic differences, eight PFIC2 and two BRIC2 mutations were introduced in rat Bsep, which was transfected in MDCK II cells. Taurocholate transport activity, protein expression, and subcellular distribution of these mutant proteins were studied in a polarized MDCK II monolayer. The taurocholate transport activity was approximately half of the wild-type (WT) in BRIC2 mutants (A570T and R1050C), was substantially less in two PFIC2 mutants (D482G and E297G), and was almost abolished in six other PFIC2 mutants (K461E, G982R, R1153C, R1268Q, 3767-3768insC, and R1057X). Bsep protein expression levels correlated closely with transport activity, except for R1057X. The half-life of the D482G mutant was shorter than that of the WT (1.35 h vs. 3.49 h in the mature form). BRIC2 mutants and three PFIC mutants (D482G, E297G, and R1057X) were predominantly distributed in the apical membrane. The other PFIC2 mutants remained intracellular. The R1057X mutant protein was stably expressed and trafficked to the apical membrane, suggesting that the COOH-terminal tail is required for transport activity but not for correct targeting. In conclusion, taurocholate transport function was impaired in proportion to rapid degradation of Bsep protein in the mutants, which were aligned in the following order: A570T and R1050C > D482G > E297G > K461E, G982R, R1153C, R1268Q, 3767-3768insC, and R1057X. These results may explain the phenotypic difference between BRIC2 and PFIC2.

Role of hepatic opioid receptors in the regulation of bile excretion

Experiments on isolated rat liver perfused with Ringer-Krebs bicarbonate buffer showed that stimulation of delta-opiate receptors in this organ increases bile flow rate and taurocholate secretion. Stimulation of micro-opiate receptors decelerated bile production and inhibited taurocholate secretion. Acceleration of bile production and stimulation of taurocholate secretion under the influence of dalargin is probably related to its interaction with delta-opiate receptors in the liver.

Two N-linked glycans are required to maintain the transport activity of the bile salt export pump (ABCB11) in MDCK II cells

The aim of this study was to determine the role of N-linked glycosylation in protein stability, intracellular trafficking, and bile acid transport activity of the bile salt export pump [Bsep (ATP-binding cassette B11)]. Rat Bsep was fused with yellow fluorescent protein, and the following mutants, in which Asn residues of putative glycosylation sites (Asn(109), Asn(116), Asn(122), and Asn(125)) were sequentially replaced with Gln, were constructed by site-directed mutagenesis: single N109Q, double N109Q + N116Q, triple N109Q + N116Q + N122Q, and quadruple N109Q + N116Q + N122Q + N125Q. Immunoblot and glycosidase cleavage analysis demonstrated that each site was glycosylated. Removal of glycans decreased taurocholate transport activity as determined in polarized MDCK II cells. This decrease resulted from rapid decay of the mutant Bsep protein; biochemical half-lives were 3.76, 3.65, 3.24, 1.35, and 0.52 h in wild-type, single-mutant, double-mutant, triple-mutant, and quadruple-mutant cells, respectively. Wild-type and single- and double-mutant proteins were distributed exclusively along the apical membranes, whereas triple- and quadruple-mutant proteins remained intracellular. MG-132 but not bafilomycin A(1) extended the half-life, suggesting a role for the proteasome in Bsep degradation. To determine whether a specific glycosylation site or the number of glycans was critical for protein stability, we studied the protein expression of combinations of N-glycan-deficient mutants and observed that Bsep with one glycan was considerably unstable compared with Bsep harboring two or more glycans. In conclusion, at least two N-linked glycans are required for Bsep protein stability, intracellular trafficking, and function in the apical membrane.

The bile salt export pump

Canalicular secretion of bile salts mediated by the bile salt export pump Bsep constitutes the major driving force for the generation of bile flow. Bsep is a member of the B-family of the super family of ATP-binding cassette transporters and is classified as ABCB11. Bsep has a narrow substrate specificity, which is largely restricted to bile salts. Bsep is extensively regulated at the transcriptional and posttranscriptional level, which directly modulates canalicular bile formation. Pathophysiological alterations of Bsep by either inherited mutations or acquired processes such as inhibition by drugs or disease-related down regulation may lead to a wide spectrum of mild to severe forms of liver disease. Furthermore, many genetic variants of Bsep are known, some of which potentially render individuals susceptible to acquired forms of liver disease.

Hepatobiliary transporters and drug-induced cholestasis

Drug-induced liver injury is an important clinical problem with significant morbidity and mortality. Whereas for most hepatocellular forms of drug-induced hepatic injury the underlying pathophysiological mechanism is poorly understood, there is increasing evidence that cholestatic forms of drug-induced liver damage result from a drug- or metabolite-mediated inhibition of hepatobiliary transporter systems. In addition to their key role in determining hepatic drug exposure and clearance, the coordinated action of these transport systems is essential for bile formation and the biliary secretion of cholephilic compounds and xenobiotics. Any drug-mediated functional disturbance of these processes can lead to an intracellular accumulation of potentially harmful bile constituents and result in the development of cholestatic liver cell damage. In addition to direct drug-mediated inhibition of hepatocellular transport, function of these transporters can be altered by pre-existing hepatic disease and genetic factors, which contribute to the development of drug-induced cholestasis in susceptible individuals. This review summarizes current knowledge about the function of hepatobiliary uptake and efflux systems and discusses factors that might predispose to drug-induced cholestasis.

Role of ABCG2/BCRP in biology and medicine

The protein variously named ABCG2/BCRP/MXR/ABCP is a recently described ATP-binding cassette (ABC) transporter originally identified by its ability to confer drug resistance that is independent of Mrp1 (multidrug-resistance protein 1) and Pgp (P-glycoprotein). Unlike Mrp1 and Pgp, ABCG2 is a half-transporter that must homodimerize to acquire transport activity. ABCG2 is found in a variety of stem cells and may protect them from exogenous and endogenous toxins. ABCG2 expression is upregulated under low-oxygen conditions, consistent with its high expression in tissues exposed to low-oxygen environments. ABCG2 interacts with heme and other porphyrins and protects cells and/or tissues from protoporphyrin accumulation under hypoxic conditions. Individuals who carry ABCG2 alleles that have impaired function may be more susceptible to porphyrin-induced toxicity. Abcg2 knock-out models have allowed in vivo studies of Abcg2 function in host and cellular defense. In combination with immunohistochemical analyses, these studies have revealed how ABCG2 influences the absorption, distribution, and excretion of drugs and cytotoxins.

Tauroursodeoxycholic acid inserts the bile salt export pump into canalicular membranes of cholestatic rat liver

Ursodeoxycholic acid exerts anticholestatic effects in chronic cholestatic liver disease in humans as well as in experimental animal models of cholestasis. Its taurine conjugate, TUDCA, was recently shown to stimulate insertion of the apical conjugate export pump, Mrp2 (ABCC2), into canalicular membranes of rat hepatocytes made cholestatic by exposure to taurolithocholic acid (TLCA). The aim of this immunoelectronmicroscopic study was to test whether TLCA and TUDCA modulate the canalicular density of the other key apical transporter, the bile salt export pump, Bsep (ABCB11), in a similar way. Immunoelectronmicroscopic analysis of Bsep density on canalicular membranes, microvilli, and pericanalicular area of hepatocytes was performed in rat liver tissue prepared after liver perfusion with bile acids or carrier medium only. TLCA (10 micromol/l for 50 min) decreased Bsep density in canalicular membranes to 31% of controls (P<0.05) when bile flow was reduced to 35% of controls (P<0.05). Concomitantly, Bsep density in a 1 microm pericanalicular zone increased to 202% (P<0.05) indicating effective retrieval of Bsep from the canalicular membrane induced by TLCA. Coadministration of TUDCA (25 micromol/l) led to a 3.2-fold increase of Bsep density in canalicular membranes equal to control liver (P<0.05 vs TLCA) in association with a 3.8-fold increase of bile flow (P<0.05 vs TLCA). Stimulation of apical membrane insertion of key transporters like the bile salt export pump, Bsep, and-as previously shown-the conjugate export pump, Mrp2, may contribute to the anticholestatic action of UDCA amides in cholestatic conditions.

Phosphoinositide 3-kinase is involved in the glucagon-induced translocation of aquaporin-8 to hepatocyte plasma membrane

BACKGROUND INFORMATION

PI3K (phosphoinositide 3-kinase) mediates several signal transduction pathways in hepatocytes, including some involved in the regulation of vesicle trafficking. Hepatocytes express the water channel AQP8 (aquaporin-8) predominantly in an intracellular location, and it redistributes to the canalicular membrane, upon stimulation with the hormone glucagon, by a cAMP/protein kinase A-dependent mechanism. Since glucagon is capable of stimulating PI3K activity in hepatocytes and a cross talk between cAMP and PI3K has been suggested, in the present study, we examine whether PI3K activation is involved in the glucagon-induced translocation of AQP8.

RESULTS

By quantitative immunoblotting of purified hepatocyte plasma membranes, we found that the preincubation of cells with two structurally different PI3K inhibitors, wortmannin or LY294002, prevented the glucagon-induced translocation of AQP8 to hepatocyte plasma membrane. Confocal immunofluorescence microscopy in cultured hepatocytes confirmed the dependence of the hormone-induced redistribution of AQP8 on PI3K activity. Functional studies showed that the PI3K inhibitors were also capable of preventing the glucagon-induced increase in hepatocyte osmotic membrane water permeability.

CONCLUSIONS

Our results suggest that PI3K activation is involved in the glucagon-dependent signal transduction pathways leading to hepatocyte AQP8 translocation.

Characterization of organic anion transporter regulation, glutathione metabolism and bile formation in the obese Zucker rat

BACKGROUND/AIMS

Alterations in hepatobiliary transporters may render fatty livers more vulnerable against various toxic insults.

METHODS

We therefore studied expression and function of key organic anion transporters and their transactivators in 8-week-old obese Zucker rats, an established model for non-alcoholic fatty liver disease.

RESULTS

Compared to their heterozygous littermates, obese animals showed a significant reduction in canalicular bile salt secretion, which was paralleled by significantly diminished Oatp2 mRNA and protein levels together with reduced nuclear HNF3beta, while expression of bile salt export pump, organic anion transporter (Oatp) 1 and multidrug resistance-associated protein (Mrp) 4 were unchanged. Impaired bile salt-independent bile flow in obese rats was associated with a 50% reduction of biliary secretion of the Mrp 2 model-substrates glutathione disulfide and S-(2,4-dinitrophenyl)glutathione. In line Mrp2 protein expression was reduced by 50% in obese rats.

CONCLUSIONS

Oatp2 and Mrp2 expression is decreased in fatty liver and may impair metabolism and biliary secretion of numerous xenobiotics. Reduction of bile salt secretion and absence of biliary GSH excretion may contribute to impaired bile flow and posthepatic disorders associated with biliary GSH depletion.

Hepatic pharmacokinetics of taurocholate in the normal and cholestatic rat liver

The disposition kinetics of [3H]taurocholate ([3H]TC) in perfused normal and cholestatic rat livers were studied using the multiple indicator dilution technique and several physiologically based pharmacokinetic models. The serum biochemistry levels, the outflow profiles and biliary recovery of [3H]TC were measured in three experimental groups: (i) control; (ii) 17 alpha-ethynylestradiol (EE)-treated (low dose); and (iii) EE-treated (high dose) rats. EE treatment caused cholestasis in a dose-dependent manner. A hepatobiliary TC transport model, which recognizes capillary mixing, active cellular uptake, and active efflux into bile and plasma described the disposition of [3H]TC in the normal and cholestatic livers better than the other pharmacokinetic models. An estimated five- and 18-fold decrease in biliary elimination rate constant, 1.7- and 2.7-fold increase in hepatocyte to plasma efflux rate constant, and 1.8- and 2.8-fold decrease in [3H]TC biliary recovery ratio was found in moderate and severe cholestasis, respectively, relative to normal. There were good correlations between the predicted and observed pharmacokinetic parameters of [3H]TC based on liver pathophysiology (e.g. serum bilirubin level and biliary excretion of [3H]TC). In conclusion, these results show that altered hepatic TC pharmacokinetics in cholestatic rat livers can be correlated with the relevant changes in liver pathophysiology in cholestasis.

Short-term regulation of multidrug resistance-associated protein 3 in rat and human hepatocytes

The short-term regulation of multidrug resistance-associated protein 3 (Mrp3/MRP3) by cAMP and PKC was investigated in sandwich-cultured rat and human hepatocytes and isolated perfused rat livers. The modulator glucagon (500 nM) and the phorbol ester PMA (0.1 muM) were utilized to increase intracellular cAMP and PKC levels, respectively. In glucagon-treated rat hepatocytes, efflux of the Mrp3 substrate 5-(6)-carboxy-2',7'-dichlorofluorescein (CDF) increased approximately 1.5-fold, even in hepatocytes treated with the organic anion transporter (Oatp) inhibitor sulfobromophthalein (BSP). Confocal microscopy revealed more concentrated Mrp3 fluorescence in the basolateral membrane (less diffuse staining pattern) with glucagon treatment. PMA had no effect on Mrp3 activity or localization in sandwich-cultured rat hepatocytes. Glucagon and PMA treatment in isolated perfused rat livers resulted in a threefold increase (14 +/- 4.6 mul.min(-1).g liver(-1)) and a fourfold decrease (1.3 +/- 0.3 mul.min(-1).g liver(-1)) in CDF basolateral clearance compared with control livers (4.7 +/- 2.3 mul.min(-1).g liver(-1)), whereas CDF biliary clearance was not statistically different. In sandwich-cultured human hepatocytes, glucagon treatment resulted in a 1.3-fold increase in CDF efflux and a concomitant increase in MRP3 fluorescence in the basolateral membrane. In summary, cAMP and PKC appear to be involved in the short-term regulation of Mrp3/MRP3, as demonstrated by alterations in activity and localization in rat and human hepatocytes.

Mrp2/Abcc2 transport activity is stimulated by protein kinase Calpha in a baculo virus co-expression system

Cholestatic and choleretic effect are well known for protein kinase C activator and inhibitor, respectively. However, post-translational regulation, especially the effect of phosphorylation status of the biliary transporters on their intrinsic transport activity has not been fully understood. In this study, effect of phosphorylation on the transport activity of Mrp2, a biliary organic anion transporter, was examined in membrane vesicles isolated from Sf9 cells co-expressing excess amount of protein kinase Calpha (PKCalpha). Mrp2-mediated transport activity was enhanced to three-fold by co-expressing PKCalpha. At the same time, phosphorylation of Mrp2 was also detected. The Km and Vmax values for the transport of [3H]estradiol-17beta-D-glucuronide exhibited a 1.5-fold decrease and a 1.9-fold increase, respectively. Probenecid (100 microM) and benzylpenicillin (1 mM), both are activator of Mrp2, did not stimulated the transport activity of phosphorylated Mrp2. On the other hand, transport activity was further stimulated by Estron-3-sulfate and taurocholic acid. Similar mechanism that occurred in the presence of probenecid and benzylpenicillin, but different from that occurred in the presence of Estron-3-sulfate and taurocholic acid seems to be involved in the stimulation. Considering the discrepancy between the previous in vivo inhibitory effect of PKC activators and our in vitro stimulatory effect of PKCalpha on Mrp2 transport activity, direct modulation of Mrp2-transport activity may be minor if any under in vivo condition.

Intracellular Localization and Trafficking of Fluorescein-Labeled Cisplatin in Human Ovarian Carcinoma Cells

Purpose: We sought to identify the subcellular compartments in which cisplatin [cis-diamminedichloroplatinum (DDP)] accumulates in human ovarian carcinoma cells and define its export pathways.

Experimental Design: Deconvoluting digital microscopy was used to identify the subcellular location of fluorescein-labeled DDP (F-DDP) in 2008 ovarian carcinoma cells stained with organelle-specific markers. Drugs that block vesicle movement were used to map the traffic pattern.

Results: F-DDP accumulated in vesicles and were not detectable in the cytoplasm. F-DDP accumulated in the Golgi, in vesicles belonging to the secretory export pathway, and in lysosomes but not in early endosomes. F-DDP extensively colocalized with vesicles expressing the copper efflux protein, ATP7A, whose expression modulates the cellular pharmacology of DDP. Inhibition of vesicle trafficking with brefeldin A, wortmannin, or H89 increased the F-DDP content of vesicles associated with the pre-Golgi compartments and blocked the loading of F-DDP into vesicles of the secretory pathway. The importance of the secretory pathway was confirmed by showing that wortmannin and H89 increased whole cell accumulation of native DDP.

Conclusions: F-DDP is extensively sequestered into vesicular structures of the lysosomal, Golgi, and secretory compartments. Much of the distribution to other compartments occurs via vesicle trafficking. F-DDP detection in the vesicles of the secretory pathway is consistent with a major role for this pathway in the efflux of F-DDP and DDP from the cell.

Insulin-like growth factor-i regulation of hepatic scavenger receptor class BI

High-density lipoprotein mediates a normal physiological process called reverse cholesterol transport. This process enables the transfer of cholesterol from peripheral tissues to the liver for further metabolism and eventual secretion in the form of bile. The scavenger receptor of the B class (SR-BI), human homolog of SR-BI, and CD36 and LIMPII analogous-1 (CLA-1) are different names for the same receptor that facilitates hepatocellular uptake of cholesterol from high-density lipoprotein. The pivotal role of this receptor in enterohepatic circulation of cholesterol and bile salts underlies our interest to study the regulation of hepatic SR-BI gene in response to the actions of IGF-I. The results of our studies showed that endogenous expression of SR-BI/CLA-1 was suppressed by exposure to GH or IGF-I in cultured HepG2 cells. This observation extended to a whole animal model of rats continuously infused with IGF-I. IGF-I decreased transcriptional activity of the SR-BI promoter. However, the inhibitory effect of IGF-I on SR-BI/CLA-1 promoter activity was abrogated by wortmannin, a specific inhibitor of phosphoinositide 3-kinase (PI3-K). Exposure of HepG2 cells to IGF-I elicited a rapid phosphorylation of Akt. We also demonstrated that the constitutively active form of both p110, a subunit of PI3-K, and Akt inhibited activity of the human SR-BI/CLA-1 promoter. Furthermore, the dominant-negative mutant of Akt abolished the ability of IGF-I to suppress activity of the SR-BI/CLA-1 promoter. In conclusion, PI3-K/Akt pathways participate in IGF-I-suppression of SR-BI/CLA-1 expression, which suggests that the activation of Akt plays an important role in cholesterol metabolism in liver.

Separation of insulin signaling into distinct GLUT4 translocation and activation steps

GLUT4 (glucose transporter 4) plays a pivotal role in insulin-induced glucose uptake to maintain normal blood glucose levels. Here, we report that a cell-permeable phosphoinositide-binding peptide induced GLUT4 translocation to the plasma membrane without inhibiting IRAP (insulin-responsive aminopeptidase) endocytosis. However, unlike insulin treatment, the peptide treatment did not increase glucose uptake in 3T3-L1 adipocytes, indicating that GLUT4 translocation and activation are separate events. GLUT4 activation can occur at the plasma membrane, since insulin was able to increase glucose uptake with a shorter time lag when inactive GLUT4 was first translocated to the plasma membrane by pretreating the cells with this peptide. Inhibition of phosphatidylinositol (PI) 3-kinase activity failed to inhibit GLUT4 translocation by the peptide but did inhibit glucose uptake when insulin was added following peptide treatment. Insulin, but not the peptide, stimulated GLUT1 translocation. Surprisingly, the peptide pretreatment inhibited insulin-induced GLUT1 translocation, suggesting that the peptide treatment has both a stimulatory effect on GLUT4 translocation and an inhibitory effect on insulin-induced GLUT1 translocation. These results suggest that GLUT4 requires translocation to the plasma membrane, as well as activation at the plasma membrane, to initiate glucose uptake, and both of these steps normally require PI 3-kinase activation.

Subcellular localization of the ABCG2 transporter in normal and malignant human gallbladder epithelium

Epithelium of the gallbladder and biliary tract is exposed to high concentrations of potentially harmful exogenous and endogenous compounds excreted into primary bile. As the ATP-dependent efflux pump ABCG2 can prevent cellular accumulation of anticancer drugs, estrogen sulfate, xenobiotics, porphyrins, and sterols, its expression in the biliary tract might mediate protection by hindering their penetration. We therefore investigated the expression and subcellular distribution of ABCG2 in normal and malignant human gallbladder. After demonstrating ABCG2 expression in gallbladder epithelium by RT-PCR and Western blotting, we analyzed the subcellular localization of ABCG2 by indirect immunofluorescence in gallbladder adenocarcinoma specimens, and compared it to that in cholelithiasis, and normal gallbladder samples (n = 54). In control, cholelithiasis, and well-differentiated tumor samples (grade 1, T1-3), ABCG2 is present at the luminal membrane of epithelial cells, which was proven by colocalization of apical-bound TRITC-labeled lectin (wheat germ agglutinin). In poorly differentiated gallbladder adenocarcinomas, intracellular ABCG2, in addition to luminal ABCG2 immunoreactivity, was found in 13/21 carcinoma samples (grade 2 and 3, T2-4, P < 0.01). In 3/11 of grade 3 tumors, ABCG2 was present in the cytoplasmatic compartment only (P < 0.01). In proliferating bile ducts of cholangiocarcinomas, ABCG2 showed an analogous staining pattern with presence in cytosolic compartments. However, the apical marker enzyme neutral endopeptidase remained on the membrane in all samples. To study whether phosphatidylinositol 3-kinase (PI3K) signaling might be necessary for ABCG2 membrane insertion, we treated freshly isolated human gallbladder epithelial cells with the PI3K inhibitor wortmannin. As assessed by indirect immunofluorescence, this maneuver redistributes ABCG2 to intracellular compartments. In conclusion, our data suggest a protective role for ABCG2 in well-differentiated gallbladder epithelial cells. Cytoplasmatic accumulation of ABCG2 in poorly differentiated carcinomas might coincide with malfunctioning of PI3K-signaling pathways during tumor progression.

Hepatic canalicular membrane transport of bile salt in C57L/J and AKR/J mice: implications for cholesterol gallstone formation

C57L/J (gallstone-susceptible) and AKR/J (gallstone-resistant) mice have been utilized for quantitative trait loci (QTL) analysis to identify the Lith 1 locus for cholelithiasis. Abcb11 encodes for the liver canalicular membrane bile salt export pump (BSEP), which maps to this QTL and is a candidate gene for Lith 1. We investigated the transmembrane transport of taurocholate in canalicular liver membrane vesicles isolated from these murine strains. Canalicular liver plasma membranes (cLPM) and RNA were isolated from C57L/J and AKR/J mice livers, and were utilized for Northern and Western blot analysis and functional (3)H-taurocholate uptake studies. ATP-dependent (3)H-taurocholate uptake was significantly higher in AKR/J, compared to C57L/J mice. V(max) was 127 vs. 42 pmol TC/mg/s in the murine strains, respectively, while K(m) was unchanged. In contrast, gene and protein expression of hepatic Abcb11 was increased three-fold in C57L/J, compared to AKR/J mice. Thus, Abcb11 bile salt transport activity per unit protein was reduced nine-fold in the C57L/J, compared to AKR/J mice. In contrast, canalicular membrane cholesterol:phospholipid content was also significantly higher in the C57L/J mice. We conclude that gallstone-susceptible C57L/J mice demonstrate increased gene and canalicular membrane expression of Abcb11, however, taurocholate transport is functionally diminished. The latter may be due to the increased cholesterol membrane content of the cLPM in C57L/J mice. These findings may be important for the pathogenesis of gallstone formation.

Cross-talk between protein kinases Czeta and B in cyclic AMP-mediated sodium taurocholate co-transporting polypeptide translocation in hepatocytes

Cyclic AMP stimulates taurocholate (TC) uptake and sodium taurocholate co-transporting polypeptide (Ntcp) translocation in hepatocytes via the phosphoinositide-3 kinase (PI3K) signaling pathway. The aim of the present study was to determine whether protein kinase (PK) Czeta, one of the downstream mediators of the PI3K signaling pathway, is involved in cAMP-mediated stimulation of TC uptake. Studies were conducted in isolated rat hepatocytes and in HuH-7 cells stably transfected with rat liver Ntcp (HuH-Ntcp cells). Studies in hepatocytes showed that cAMP activates PKCzeta in a PI3K-dependent manner without inducing translocation of PKCzeta to the plasma membrane. Inhibition of cAMP-induced PKCzeta activity by myristoylated PKC (zeta/lambda) pseudosubstrate, a specific inhibitor of PKCzeta, and Gö 6850, a PKC inhibitor, resulted in inhibition of cAMP-induced increases in TC uptake and Ntcp translocation. Studies in HuH-Ntcp cells showed that inhibition of cAMP-induced PKCzeta activation by dominant-negative (DN) PKCzeta resulted in inhibition of cAMP-induced increases in TC uptake and Ntcp translocation. DN PKCzeta also inhibited wild-type PKCzeta-induced increases in PKCzeta activity, TC uptake, and Ntcp translocation. Myristoylated PKC (zeta/lambda) pseudosubstrate and DN PKCzeta also inhibited cAMP-induced activation of PKB in hepatocytes and HuH-Ntcp cells, respectively. Neither DN PKB nor constitutively active PKB affected cAMP-induced activation of PKCzeta, and wild-type PKCzeta did not activate PKB. Taken together, these results suggest that cAMP-induced activation of PKB is dependent on cAMP-induced stimulation of PKCzeta. It is proposed that cAMP-induced Ntcp translocation involves the activation of the PI3K/PKCzeta signaling pathway followed by the activation of the PI3K/PKB signaling pathway.

Enterohepatic bile salt transporters in normal physiology and liver disease

The vectorial transport of bile salts from blood into bile is essential for the generation of bile flow, solubilization of cholesterol in bile, and emulsification of lipids in the intestine. Major transport proteins involved in the enterohepatic circulation of bile salts include the hepatocellular bile salt export pump (BSEP, ABCB11), the apical sodium-dependent bile salt transporter (ASBT, SLC10A2) in cholangiocytes and enterocytes, the sodium-dependent hepatocyte bile salt uptake system NTCP (SLC10A1), the organic anion transporting polypeptides OATP-C (SLC21A6), OATP8 (SLC21A8) and OATP-A (SLC21A3), and the multidrug resistance protein MRP3 (ABCC3). Synthesis and transport of bile salts are intricately linked processes that undergo extensive feedback and feed-forward regulation by transcriptional and posttranscriptional mechanisms. A key regulator of hepatocellular bile salt homeostasis is the bile acid receptor/farnesoid X receptor FXR, which activates transcription of the BSEP and OATP8 genes and of the small heterodimer partner 1 (SHP). SHP is a transcriptional repressor that mediates bile acid-induced repression of the bile salt uptake systems rat Ntcp and human OATP-C. A nuclear receptor that activates rodent Oatp2 (Slc21a5) and human MRP2 (ABCC2) is the pregnane X receptor/steroid X receptor PXR/SXR. Intracellular trafficking and membrane insertion of bile salt transporters is regulated by lipid, protein, and extracellular signal-related kinases in response to physiologic stimuli such as cyclic adenosine monophosphate or taurocholate. Finally, dysfunction of individual bile salt transporters such as BSEP, on account of genetic mutations, steric inhibition, suppression of gene expression, or disturbed signaling, is an important cause of cholestatic liver disease.

Akt signaling regulates side population cell phenotype via Bcrp1 translocation

Akt is an important regulator of cell survival, growth, and glucose metabolism in many cell types, but the role of this signaling molecule in hematopoietic stem cells is poorly defined. Side population (SP) cells are enriched for hematopoietic stem cell activity and are defined by their ability to efficiently efflux Hoechst 33342. Bone marrow from Akt1-null mice exhibited a reduced SP fraction. However, bone marrow cellularity, growth factor-responsive progenitor cultures, and engraftable stem cells were normal in these mice. Treatment of bone marrow with LY294002, an inhibitor of the Akt effector protein phosphatidylinositol 3-kinase, led to a reversible loss of the SP fraction. Bcrp1, which encodes the Hoechst dye transporter, was translocated from the membrane to the intracellular compartment under conditions that promote the SP-depleted state. Lentivirus-mediated overexpression of Akt1 in bone marrow markedly increased the SP fraction, whereas there was no effect on bone marrow from Bcrp(-/-) mice. These data suggest that Akt signaling modulates the SP cell phenotype by regulating the expression of Bcrp1.