Inositol 1,4,5-trisphosphate receptor type II (InsP3R-II) is reduced in obese mice, but metabolic homeostasis is preserved in mice lacking InsP3R-II.

Inositol 1,4,5-trisphosphate receptor type II (InsP3R-II) is the most prevalent isoform of the InsP3R in hepatocytes and is concentrated under the canalicular membrane, where it plays an important role in bile secretion. We hypothesized that altered calcium (Ca(2+)) signaling may be involved in metabolic dysfunction, as InsP3R-mediated Ca(2+) signals have been implicated in the regulation of hepatic glucose homeostasis. Here, we find that InsP3R-II, but not InsP3R-I, is reduced in the livers of obese mice. In our investigation of the functional consequences of InsP3R-II deficiency, we found that organic anion secretion at the canalicular membrane and Ca(2+) signals were impaired. However, mice lacking InsP3R-II showed no deficits in energy balance, glucose production, glucose tolerance, or susceptibility to hepatic steatosis. Thus, our results suggest that reduced InsP3R-II expression is not sufficient to account for any disruptions in metabolic homeostasis that are observed in mouse models of obesity. We conclude that metabolic homeostasis is maintained independently of InsP3R-II. Loss of InsP3R-II does impair secretion of bile components; therefore, we suggest that conditions of obesity would lead to a decrease in this Ca(2+)-sensitive process.

Type 2 inositol 1,4,5-trisphosphate receptor modulates bile salt export pump activity in rat hepatocytes.

UNLABELLED: Bile salt secretion is mediated primarily by the bile salt export pump (Bsep), a transporter on the canalicular membrane of the hepatocyte. However, little is known about the short-term regulation of Bsep activity. Ca(2+) regulates targeting and insertion of transporters in many cell systems, and Ca(2+) release near the canalicular membrane is mediated by the type II inositol 1,4,5-trisphosphate receptor (InsP3R2), so we investigated the possible role of InsP3R2 in modulating Bsep activity. The kinetics of Bsep activity were monitored by following secretion of the fluorescent Bsep substrate cholylglycylamido-fluorescein (CGamF) in rat hepatocytes in collagen sandwich culture, an isolated cell system in which structural and functional polarity is preserved. CGamF secretion was nearly eliminated in cells treated with Bsep small interfering RNA (siRNA), demonstrating specificity of this substrate for Bsep. Secretion was also reduced after chelating intracellular calcium, inducing redistribution of InsP3R2 by depleting the cell membrane of cholesterol, or reducing InsP3R function by either knocking down InsP3R2 expression using siRNA or pharmacologic inhibition using xestospongin C. Confocal immunofluorescence showed that InsP3R2 and Bsep are in close proximity in the canalicular region, both in rat liver and in hepatocytes in sandwich culture. However, after knocking down InsP3R2 or inducing its dysfunction with cholesterol depletion, Bsep redistributed intracellularly. Finally, InsP3R2 was lost from the pericanalicular region in animal models of estrogen- and endotoxin-induced cholestasis. CONCLUSION: These data provide evidence that pericanalicular calcium signaling mediated by InsP3R2 plays an important role in maintaining bile salt secretion through posttranslational regulation of Bsep, and suggest that loss or redistribution of InsP3R2 may contribute to the pathophysiology of intrahepatic cholestasis.

Succinate is a paracrine signal for liver damage.

BACKGROUND/AIMS: A G-protein-coupled succinate receptor has recently been identified in several tissues, including the liver. The objectives of this work were to determine the hepatic cell types that express this receptor and to determine its physiological role. METHODS: Expression and distribution of the succinate receptor was determined by RT-PCR and confocal immunofluorescence. Biochemical assays were used to measure succinate and cAMP. Cytosolic Ca2+ was monitored in single cells by time-lapse imaging. Western blot was used to study the effect of succinate on activation of hepatic stellate cells. RESULTS: The succinate receptor was expressed in quiescent hepatic stellate cells, and expression decreased with activation. Ischemia induced release of succinate in isolated perfused livers. In contrast to what is observed in cell expression systems, succinate did not inhibit cAMP production or increase cytosolic Ca2+ in primary hepatic stellate cells. However, succinate accelerated stellate cell activation. CONCLUSIONS: Hepatic stellate cells express the succinate receptor. Succinate may behave as a paracrine signal by which ischemic hepatocytes trigger stellate cell activation.

The spatial distribution of inositol 1,4,5-trisphosphate receptor isoforms shapes Ca2+ waves.

Cytosolic Ca(2+) is a versatile second messenger that can regulate multiple cellular processes simultaneously. This is accomplished in part through Ca(2+) waves and other spatial patterns of Ca(2+) signals. To investigate the mechanism responsible for the formation of Ca(2+) waves, we examined the role of inositol 1,4,5-trisphosphate receptor (InsP3R) isoforms in Ca(2+) wave formation. Ca(2+) signals were examined in hepatocytes, which express the type I and II InsP3R in a polarized fashion, and in AR4-2J cells, a nonpolarized cell line that expresses type I and II InsP3R in a ratio similar to what is found in hepatocytes but homogeneously throughout the cell. Expression of type I or II InsP3R was selectively suppressed by isoform-specific DNA antisense in an adenoviral delivery system, which was delivered to AR4-2J cells in culture and to hepatocytes in vivo. Loss of either isoform inhibited Ca(2+) signals to a similar extent in AR4-2J cells. In contrast, loss of the basolateral type I InsP3R decreased the sensitivity of hepatocytes to vasopressin but had little effect on the initiation or spread of Ca(2+) waves across hepatocytes. Loss of the apical type II isoform caused an even greater decrease in the sensitivity of hepatocytes to vasopressin and resulted in Ca(2+) waves that were much slower and delayed in onset. These findings provide evidence that the apical concentration of type II InsP3Rs is essential for the formation of Ca(2+) waves in hepatocytes. The subcellular distribution of InsP3R isoforms may critically determine the repertoire of spatial patterns of Ca(2+) signals.

Molecular basis for calcium signaling in hepatic stellate cells.

Progressive liver fibrosis (with the resultant cirrhosis) is the primary cause of chronic liver failure. Hepatic stellate cells (HSCs) are critically important mediators of liver fibrosis. In the healthy liver, HSCs are quiescent lipid-storing cells limited to the perisinusoidal endothelium. However, in the injured liver, HSCs undergo myofibroblastic transdifferentiation (activation), which is a critical step in the development of organ fibrosis. HSCs express P2Y receptors linking extracellular ATP to inositol (1,4,5)-trisphosphate-mediated cytosolic Ca(2+) signals. Here, we report that HSCs express only the type I inositol (1,4,5)-trisphosphate receptor and that the receptor shifts into the nucleus and cell extensions upon activation. These cell extensions, furthermore, express sufficient machinery to enable local application of ATP to evoke highly localized Ca(2+) signals that induce localized contractions. These autonomous units of subcellular signaling and response reveal a new level of subcellular organization, which, in turn, establishes a novel paradigm for the local control of fibrogenesis in the liver.

Adenosine inhibits cytosolic calcium signals and chemotaxis in hepatic stellate cells.

Adenosine is produced during cellular hypoxia and apoptosis, resulting in elevated tissue levels at sites of injury. Adenosine is also known to regulate a number of cellular responses to injury, but its role in hepatic stellate cell (HSC) biology and liver fibrosis is poorly understood. We tested the effect of adenosine on the cytosolic Ca2+ concentration, chemotaxis, and upregulation of activation markers in HSCs. We showed that adenosine did not induce an increase in the cytosolic Ca2+ concentration in LX-2 cells and, in addition, inhibited increases in the cytosolic Ca2+ concentration in response to ATP and PDGF. Using a Transwell system, we showed that adenosine strongly inhibited PDGF-induced HSC chemotaxis in a dose-dependent manner. This inhibition was mediated via the A(2a) receptor, was reversible, was reproduced by forskolin, and was blocked by the adenylate cyclase inhibitor 2,5-dideoxyadenosine. Adenosine also upregulated the production of TGF-beta and collagen I mRNA. In conclusion, adenosine reversibly inhibits Ca2+ fluxes and chemotaxis of HSCs and upregulates TGF-beta and collagen I mRNA. We propose that adenosine provides 1) a "stop" signal to HSCs when they reach sites of tissue injury with high adenosine concentrations and 2) stimulates transdifferentiation of HSCs by upregulating collagen and TGF-beta production.

Prevention of liver fibrosis by the purinoceptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS).

BACKGROUND: Hepatic stellate cells (HSC) are important mediators of liver fibrosis. HSC express purinergic receptors for extracellular ATP that induce fibrogenesis. Pyridoxal-phosphate-6-azophenyl-2', 4'-disulfonate (PPADS) is a highly bioavailable purinoceptor inhibitor. We sought to determine whether PPADS could prevent experimental liver fibrosis in rats. MATERIALS AND METHODS: The effect of PPADS as an inhibitor of HSC purinoceptors was compared to the effect of suramin using confocal video microscopy. Rats were treated with CCl4, dimethylnitrosamine, or common bile duct ligation in the presence or absence of PPADS. Fibrosis in liver sections was assessed using Trichrome and Sirius red stains. In HSC isolated from experimental animals, proliferation was determined by bromodeoxyuridine uptake, apoptosis was determined using Annexin V flow cytometry, and transcription of alpha(1)-procollagen and fibronectin were determined using quantitative RT-PCR. RESULTS: Both PPADS and suramin inhibited HSC purinoceptor activation, but PPADS had a more durable effect. PPADS completely blocked the development of cirrhosis due to CCl4 or dimethylnitrosamine but not due to bile duct ligation. PPADS inhibited HSC proliferation, but had no effect on HSC apoptosis. PPADS inhibited transcription of alpha(1)-procollagen and fibronectin by HSC. CONCLUSION: Blockade of purinergic receptors is a novel approach to prevention of non- biliary liver fibrosis. The primary action of PPADS is to inhibit HSC proliferation and fibrogenesis. Future design of purinergic receptor inhibitors may be an effective pharmacologic treatment to prevent liver fibrosis.

Secretion of MCP-1/CCL2 by bile duct epithelia induces myofibroblastic transdifferentiation of portal fibroblasts.

Portal fibroblasts (PF) are fibrogenic liver cells distinct from hepatic stellate cells (HSC). Recent evidence suggests that PF may be important mediators of biliary fibrosis and cirrhosis. The cytokine monocyte chemoattractant protein-1 (MCP-1)/CCL2 is upregulated in biliary fibrosis by bile duct epithelia (BDE) and induces functional responses in HSC. Thus we hypothesized that release of MCP-1 may mediate biliary fibrosis. We report that PF express functional receptors for MCP-1 that are distinct from the receptor CCR2. MCP-1 induces proliferation, increase and redistribution of alpha-smooth muscle (alpha-SMA) expression, loss of the ectonucleotidase NTPDase2, and upregulation of alpha(1)-procollagen production in PF. BDE secretions induce alpha-SMA levels in PF, and this is inhibited by MCP-1 blocking antibody. Together, these data suggest that BDE regulate PF proliferation and myofibroblastic transdifferentiation in a paracrine fashion via release of MCP-1.

Protein 4.1N does not interact with the inositol 1,4,5-trisphosphate receptor in an epithelial cell line.

Cytosolic Ca2+ regulates a variety of cell functions, and the spatial patterns of Ca2+ signals are responsible in part for the versatility of this second messenger. The subcellular distribution of the inositol 1,4,5-trisphosphate receptor (IP3R) is thought to regulate Ca2+-signaling patterns but little is known about how the distribution of the IP3R itself is regulated. Here we examined the relationship between the IP3R and the cytoskeletal linker protein 4.1N in the polarized WIF-B cell line because protein 4.1N regulates targeting of the type I IP3R in neurons, but WIF-B cells do not express this cytoskeletal protein. WIF-B cells expressed all three isoforms of the IP3R, and each isoform was distributed throughout the cell. These cells did not express the ryanodine receptor. Photorelease of microinjected, caged IP3 induced a rapid rise in cytosolic Ca2+, but the increase began uniformly throughout the cell rather than at a specific initiation site. Expression of protein 4.1N was not associated with redistribution of the IP3R or changes in Ca2+-signaling patterns. These findings are consistent with the hypothesis that the subcellular distribution of IP3R isoforms regulates the formation of Ca2+ waves, and the finding that interactions between protein 4.1N and the IP3R vary among cell types may provide an additional, tissue-specific mechanism to shape the pattern of Ca2+ waves.

The anti-apoptotic protein Mcl-1 inhibits mitochondrial Ca2+ signals.

Apoptosis contributes to the regulation of cell growth and regeneration and to the development of neoplasia. Mcl-1 is an anti-apoptotic protein that is particularly important for the development of hematological and biliary malignancies, but the mechanism of action of Mcl-1 is unknown. A number of pro- and anti-apoptotic proteins exhibit their effects by modulating Ca2+ signals, so we examined the effects of Mcl-1 on components of the Ca2+ signaling pathway that are known to regulate apoptosis. Expression of Mcl-1 did not affect expression of the inositol 1,4,5-trisphosphate receptor or the size of endoplasmic reticulum Ca2+ stores. However, mitochondrial Ca2+ signals induced by either Ca2+ agonists or apoptotic stimuli were decreased in cells overexpressing Mcl-1 and increased in cells in which Mcl-1 expression was inhibited. These findings provide evidence that Mcl-1 directly inhibits Ca2+ signals within mitochondria, which may provide a novel mechanism to inhibit apoptosis and thereby promote neoplasia.

Portal fibroblasts regulate the proliferation of bile duct epithelia via expression of NTPDase2.

Bile duct epithelia are the target of a number of "cholangiopathies" characterized by disordered bile ductular proliferation. Although mechanisms for bile ductular proliferation are unknown, recent evidence suggests that extracellular nucleotides regulate cell proliferation via activation of P2Y receptors. Portal fibroblasts may regulate bile duct epithelial P2Y receptors via expression of the ecto-nucleotidase NTPDase2. Thus, we tested the hypothesis that portal fibroblasts regulate bile duct epithelial proliferation via expression of NTPDase2. We generated a novel co-culture model of Mz-ChA-1 human cholangiocarcinoma cells and primary portal fibroblasts. Cell proliferation was measured by bromodeoxyuridine uptake. NTPDase2 expression was assessed by immunofluorescence and quantitative real-time reverse transcription PCR. NTPDase2 expression in portal fibroblasts was blocked using short interfering RNA. NTPDase2 overexpression in portal myofibroblasts isolated from bile duct-ligated rats was achieved by cDNA transfection. Co-culture of Mz-ChA-1 cells with portal fibroblasts decreased their proliferation to 26% of control. Similar decreases in Mz-ChA-1 proliferation were induced by the soluble ecto-nucleotidase apyrase and the P2 receptor inhibitor suramin. The proliferation of Mz-ChA-1 cells returned to baseline when NTPDase2 expression in portal fibroblasts was inhibited using NTPDase2-specific short interfering RNA. Untransfected portal myofibroblasts lacking NTPDase2 had no effect on Mz-ChA-1 proliferation, yet portal myofibroblasts transfected with NTPDase2 cDNA inhibited Mz-ChA-1 proliferation. We conclude that portal fibroblasts inhibit bile ductular proliferation via expression of NTPDase2 and blockade of P2Y activation. Loss of NTPDase2 may mediate the bile ductular proliferation typical of obstructive cholestasis. This novel cross-talk signaling pathway may mediate pathologic alterations in bile ductular proliferation in other cholangiopathic conditions.

Expression of P2Y nucleotide receptors and ectonucleotidases in quiescent and activated rat hepatic stellate cells.

Extracellular nucleotides regulate a variety of cellular activities, including proliferation of fibrogenic cells outside of the liver. However, the expression of receptors for extracellular nucleotides in hepatic stellate cells (HSC) is unknown. Thus our aims were to investigate the expression of mediators of nucleotide signaling in HSC and to determine whether extracellular nucleotides regulate HSC function. Confocal video microscopy was used to observe nucleotide-induced changes in cytosolic Ca(2+) (Ca(i)(2+)) in live HSC. P2Y receptor subtype expression and ectonucleotidase expression in quiescent and activated HSC were determined using RT-PCR, Northern blot, immunoblot, and confocal immunofluorescence. Functional ectonucleotidase activity was assessed using a colorimetric method. Nucleotide-sensitive procollagen-1 mRNA expression in activated HSC was assessed using real-time RT-PCR. Extracellular ATP increased Ca(i)(2+) in HSC; this was inhibited by the P2 receptor inhibitor suramin. Quiescent HSC expressed the P2Y subtypes P2Y(2) and P2Y(4) and were activated by ATP and UTP, whereas activated HSC expressed the P2Y subtype P2Y(6) and were activated by UDP and ATP. Activated but not quiescent HSC expressed the ectonucleotidase nucleoside triphosphate diphosphohydrolase 2, extracellular UDP tripled procollagen-1 mRNA expression in activated HSC, and this was inhibited by the P2Y receptor inhibitor suramin. HSC express functional P2Y receptors and switch the expression of P2Y receptor subtypes on activation. Moreover, HSC differentially regulate nucleoside triphosphate diphosphohydrolase expression after activation. Because activation of P2Y receptors in activated HSC regulates procollagen-1 transcription, P2Y receptors may be an attractive target to prevent or treat liver fibrosis.

Ectonucleotidase NTPDase2 is selectively down-regulated in biliary cirrhosis.

BACKGROUND: Portal fibroblasts are newly identified, potentially fibrogenic liver cells that are distinct from hepatic stellate cells. The ectonucleotidase* nucleoside triphosphate diphosphohydrolase 2 (NTPDase2) is restricted to portal fibroblasts in the normal liver. However, the fate of NTPDase2 after bile duct ligation (BDL) is unknown. AIMS: The aim of this study was to assess the effect of experimental rat and disease-mediated human biliary cirrhosis on NTPDase2 expression in the liver. METHODS: Cirrhosis was induced in experimental rats via BDL and carbon tetrachloride (CCl4) administration. Archived human liver biopsy specimens from normal liver, primary biliary cirrhosis, or hepatitis C cirrhosis were examined. Changes in expression of NTPDase2 were determined using confocal immunofluorescence, immunoblot, and real-time polymerase chain reaction. RESULTS: Confocal immunofluorescence demonstrated a decrease in NTPDase2 expression after BDL. Immunoblot and real-time polymerase chain reaction demonstrated a decrease in NTPDase2 expression by portal fibroblasts after BDL. No decrease in NTPDase2 protein was noted after CCl4 administration, and NTPDase2 messenger ribonucleic acid was markedly up-regulated after CCl4 administration. Confocal immunofluorescence demonstrated a shift of NTPDase2 expression from portal areas to central areas that colocalized with alpha-smooth muscle actin after CCl4 administration. In human biopsy specimens, NTPDase2 expression was lost in cirrhosis owing to primary biliary cirrhosis, whereas NTPDase2 expression was shifted to bridging fibrous bands in cirrhosis owing to hepatitis C. CONCLUSIONS: Loss of NTPDase2 is a common pathway in both rat and human manifestations of biliary cirrhosis. Conversely, in non-biliary cirrhosis, NTPDase2 is shifted from the portal area to bridging fibrous bands. Elucidations of the mechanisms regulating NTPDase2 expression may lead to new therapeutic approaches to fibrotic liver disease.

The ecto-nucleoside triphosphate diphosphohydrolase NTPDase2/CD39L1 is expressed in a novel functional compartment within the liver.

Extracellular nucleotides regulate diverse biological functions and are important in the regulation of liver metabolism, hepatic blood flow, and bile secretion. Ecto-nucleoside triphosphate diphosphohydrolases (NTPDases) hydrolyze extracellular nucleotides and are therefore potential regulators of nucleotide-mediated signaling. To examine this, we have contrasted the structural and functional distributions of the 2 characterized membrane-bound NTPDases NTPDase1 and NTPDase2 within the rat liver. Hepatic expression of NTPDase2 was determined and contrasted to NTPDase1 using confocal immunofluorescence, immunoelectron microscopy, reverse-transcription polymerase chain reaction, Northern blot analysis, Western blot analysis, and functional assays. NTPDase2 was expressed in the periportal region surrounding intrahepatic bile ducts, whereas NTPDase1 was found in hepatic arteries, portal veins, and hepatic central veins, consistent with its known vascular distribution. Functional and molecular expression of NTPDase2 was shown in portal fibroblasts near basolateral membranes of bile duct epithelia. In conclusion, NTPDase2 is expressed in a novel cellular compartment surrounding intrahepatic bile ducts, namely portal fibroblasts. This distribution may represent a previously unrecognized mechanism for regulation of nucleotide signaling in bile ducts and other epithelia.

Isolation of primary rat liver fibroblasts.

INTRODUCTION: One of the major advances in liver research in the past decade was the ability to isolate distinct liver cell populations. Although there are established methods of isolating hepatocytes, cholangiocytes, and stellate cells, before this study no technique for liver fibroblast isolation had been devised. Consequently, we developed a technique to isolate primary rat liver fibroblasts. METHODS: Fibroblasts were isolated from a freshly perfused rat liver with a modification of the procedure for isolation of rat cholangiocytes. Cell markers were assessed with the use of confocal immunofluorescence. Cell morphology was assessed with transmission electron microscopy. Expression of procollagen-1 was assessed by reverse transcription polymerase chain reaction. RESULTS: The appearance of cells with fibroblast morphology was first noted at 48 hours, and almost all cells in culture had fibroblast morphology at 96 hours. Putative fibroblasts stained for vimentin, but not for smooth muscle actin, von Willebrand factor, or cytokeratins. Cell morphology was consistent with that of fibroblasts and showed no features of epithelial, endothelial, or smooth muscle cells. Liver fibroblasts expressed procollagen-1 mRNA. CONCLUSION: Primary isolated rat fibroblasts can be produced from a freshly perfused rat liver with a modification of standard cell culture methods. The role of fibroblasts in liver physiology can now be studied directly.