Regulation of Ca(2+) signaling in rat bile duct epithelia by inositol 1,4,5-trisphosphate receptor isoforms

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 type III inositol 1,4,5-trisphosphate receptor preferentially transmits apoptotic Ca2+ signals into mitochondria

There are three isoforms of the inositol 1,4,5- trisphosphate receptor (InsP(3)R), each of which has a distinct effect on Ca(2+) signaling. However, it is not known whether each isoform similarly plays a distinct role in the activation of Ca(2+)-mediated events. To investigate this question, we examined the effects of each InsP(3)R isoform on transmission of Ca(2+) signals to mitochondria and induction of apoptosis. Each isoform was selectively silenced using isoform-specific small interfering RNA in Chinese hamster ovary cells, which express all three InsP(3)R isoforms. ATP-induced cytosolic Ca(2+) signaling patterns were altered, regardless of which isoform was silenced, but in a different fashion depending on the isoform. ATP also induced Ca(2+) signals in mitochondria, which were inhibited more effectively by silencing the type III InsP(3)R than by silencing either the type I or type II isoform. The type III isoform also co-localized most strongly with mitochondria. When apoptosis was induced by activation of either the extrinsic or intrinsic apoptotic pathway, induction was reduced most effectively by silencing the type III InsP(3)R. These findings provide evidence that the type III isoform of the InsP(3)R plays a special role in induction of apoptosis by preferentially transmitting Ca(2+) signals into mitochondria.

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.

Activation of CREB by tauroursodeoxycholic acid protects cholangiocytes from apoptosis induced by mTOR inhibition

Tauroursodeoxycholic acid (TUDCA) is a cytoprotective bile acid frequently prescribed to patients with cholestatic diseases. Several mechanisms of action have been investigated, but the possibility that cyclic adenosine monophosphate responsive element binding protein (CREB), a transcription factor promoting cell survival, mediates TUDCA's protective effects has not been considered. We examined whether TUDCA activates CREB and whether this activation can protect biliary epithelial cells. Cholangiocytes were stressed by exposure to CCI-779, which inhibits signaling though the kinase mTOR (mammalian target of rapamycin), resulting in cell cycle arrest and apoptosis. Incubation of normal rat cholangiocytes (NRC) cells, with TUDCA resulted in phosphorylation of CREB (Western blotting analysis) and activation of CREB transcription activity (luciferase reporter assay). Inhibition of calcium signals and inhibition of protein kinase C prevented the TUDCA-induced activation of CREB. CCI-779 decreased the viability of rat cholangiocytes in a dose-dependent manner (MTT [3-(4,5-dimethythiazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay). TUDCA protected against CCI-779 cytotoxicity. A dominant negative form of CREB was stably transduced in NRC cells (NRC-M1). TUDCA protection was decreased in NRC-M1. While CCI-779 induced apoptosis in NRC cells as determined by caspase 3 activity, TUDCA attenuated CCI-779-induced apoptosis, an effect absent in NRC-M1. Finally, CCI-779 blocked proliferation of both NRC and NRC-M1 (thymidine incorporation) and this was unaffected by TUDCA. In conclusion, TUDCA activates CREB in cholangiocytes, reducing the apoptotic effect of CCI-779. These findings suggest a novel cytoprotective mechanism for this bile acid.

Pathophysiology of cholangiopathies

The diseases of the intrahepatic biliary tree are a large group of potentially evolutive congenital and acquired liver disorders affecting both the adult and pediatric populations. They represent a relevant cause of liver-related morbidity and mortality and an important indication for liver transplantation, particularly in children. While the practical approach to patients affected by biliary tree diseases has not significantly changed yet, the conceptual approach to the pathophysiology of cholangiopathies has witnessed important advances that will be discussed. The primary cell target of the pathogenetic sequence of these disorders is the biliary epithelium. Cholangiocytes have multifaceted functions, not limited to bile production. Their capability to secrete a range of different pro-inflammatory mediators, cytokines, and chemokines indicates a major role of cholangiocytes in the inflammatory reaction. Furthermore, paracrine secretion of growth factors and peptides mediates an extensive cross-talk with other liver cell types, including hepatocytes, stellate, and endothelial and inflammatory cells. Cholangiopathies share a number of pathogenetic mechanisms, including inflammation, cholestasis, fibrosis, apoptosis, altered development, and neoplastic transformation. These basic disease mechanisms will be discussed in detail, along with the distinct features of a number of cholangiopathies. Furthermore, an increase in the biliary cell compartment is a common response to many forms of liver injury, from cholangiopathies to viral and fulminant hepatitis. Elucidation of these pathophysiologic mechanisms will likely provide clues for future therapeutic strategies. Furthermore, understanding the role of cholangiocytes in liver regeneration/repair and the mechanisms of cholangiocyte activation and their relationship with liver progenitor cell will be of further interest.

Autocrine/paracrine regulation of the growth of the biliary tree by the neuroendocrine hormone serotonin

BACKGROUND & AIMS

The biliary tree is the target of cholangiopathies that are chronic cholestatic liver diseases characterized by loss of proliferative response and enhanced apoptosis of cholangiocytes, the epithelial cells lining the biliary tree. The endogenous factors that regulate cholangiocyte proliferation are poorly understood. Therefore, we studied the role of the neuroendocrine hormone serotonin as a modulator of cholangiocyte proliferation.

METHODS

The presence of the serotonin 1A and 1B receptors on cholangiocytes was evaluated. We then tested whether the activation of such receptors by the administration of the selective agonists modifies cholangiocyte proliferation and functional activity both in vivo and in vitro. In addition, the intracellular signal mediating the serotonin receptor action in cholangiocytes was characterized. We studied the expression and secretion of serotonin by cholangiocytes and the effects of the neutralization of the secreted hormone on the growth of the biliary tree.

RESULTS

Cholangiocytes express the serotonin 1A and 1B receptors. Their activation markedly inhibits the growth and choleretic activity of the biliary tree in the bile duct-ligated rat, a model of chronic cholestasis. Such changes are mediated by enhanced d -myo-inositol 1,4,5-triphosphate/Ca 2+ /protein kinase C signaling and the consequent inhibition of the adenosine 3',5'-cyclic monophosphate/protein kinase A/Src/extracellular signal-regulated kinase 1/2 cascade. Cholangiocytes secrete serotonin, the blockage of which enhances cholangiocyte proliferation in the course of cholestasis.

CONCLUSIONS

We observed the existence of an autocrine loop based on serotonin that limits the growth of the biliary tree in the course of chronic cholestasis. Our novel findings might open new approaches for the management of cholangiopathies.

The role of inositol 1,4,5-trisphosphate receptors in the regulation of bile secretion in health and disease

Ca2+ signaling via the inositol 1,4,5-trisphosphate receptor (InsP3R) is a ubiquitous mechanism for regulation of cell function, yet very little is known about the role of the InsP3R in specific disease states. Converging lines of evidence suggest that the liver may provide a model for the role of the InsP3R in health and disease. Ca2+ signaling is mediated entirely by the InsP3R in hepatocytes and cholangiocytes, the two types of epithelia in the liver. Here we review the role of specific InsP3R isoforms and the physiological effects of InsP3R-mediated Ca2+ signals in both of these types of epithelia. In addition, we review evidence that the InsP3R is lost from cholangiocytes in cholestatic forms of liver disease, and discuss this as a possible final common pathway for cholestasis.

Loss of inositol 1,4,5-trisphosphate receptors from bile duct epithelia is a common event in cholestasis

BACKGROUND AND AIMS

Cholestasis is one of the principal manifestations of liver disease and often results from disorders involving bile duct epithelia rather than hepatocytes. A range of disorders affects biliary epithelia, and no unifying pathophysiologic event in these cells has been identified as the cause of cholestasis. Here we examined the role of the inositol 1,4,5-trisphosphate receptor (InsP3R)/Ca(2+) release channel in Ca(2+) signaling and ductular secretion in animal models of cholestasis and in patients with cholestatic disorders.

METHODS

The expression and distribution of the InsP3R and related proteins were examined in rat cholangiocytes before and after bile duct ligation or treatment with endotoxin. Ca(2+) signaling was examined in isolated bile ducts from these animals, whereas ductular bicarbonate secretion was examined in isolated perfused livers. Confocal immunofluorescence was used to examine cholangiocyte InsP3R expression in human liver biopsy specimens.

RESULTS

Expression of the InsP3R was selectively lost from biliary epithelia after bile duct ligation or endotoxin treatment. As a result, Ca(2+) signaling and Ca(2+)-mediated bicarbonate secretion were lost as well, although other components of the Ca(2+) signaling pathway and adenosine 3',5'-cyclic monophosphate (cAMP)-mediated bicarbonate secretion both were preserved. Examination of human liver biopsy specimens showed that InsP3Rs also were lost from bile duct epithelia in a range of human cholestatic disorders, although InsP3R expression was intact in noncholestatic liver disease.

CONCLUSIONS

InsP3-mediated Ca(2+) signaling in bile duct epithelia appears to be important for normal bile secretion in the liver, and loss of InsP3Rs may be a final common pathway for cholestasis.

The type 3 inositol 1,4,5-trisphosphate receptor is concentrated at the tight junction level in polarized MDCK cells

The subcellular localization of inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ signals is important for the activation of many physiological functions. In epithelial cells the spatial distribution of InsP3 receptor is restricted to specific areas, but little is known about the relationship between the receptor's distribution and cell polarity. To investigate this relationship, the best known polarized cell model, MDCK, was examined. This cell line is characterized by a strong expression of the type 3 InsP3 receptor and the subcellular localization of this receptor was followed during cell polarization using immunofluorescence and confocal analysis. In non-polarized cells, including ras transformed f3 MDCK cells, the type 3 InsP3 receptor was found to co-localize with markers of the endoplasmic reticulum in the cytoplasm. In contrast, in polarized cells, this receptor was mostly distributed at the apex of the lateral plasma membrane with the markers of tight junctions, ZO-1 and occludin. The localization of the type 3 InsP3 receptor in the vicinity of tight junctions was confirmed by immunogold electron microscopy. The culture of MDCK cells in calcium-deprived medium, led to disruption of cell polarity and receptor redistribution in the cytoplasm. Addition of calcium to these deprived cells induced the restoration of polarity and the relocalization of the receptor to the plasma membrane. MDCK cells were stably transfected with a plasmid coding the full-length mouse type 1 InsP3 receptor tagged with EGFP at the C-terminus. The EGFP-tagged type 1 receptor and the endogenous type 3 co-localized in the cytoplasm of non-polarized cells and at the tight junction level of polarized cells. Thus, the localization of InsP3 receptor in MDCK depends on polarity.

Cholangiocyte biology

Cholangiocytes are of considerable intrinsic biologic interest, particularly with regard to their roles in the transport of water, ions, and solutes, and to their heterogeneity and proliferative capacity. Cholangiocytes represent an important target of study in the cholangiopathies, a group of genetic developmental and acquired diseases of the liver. New biologic concepts continue to evolve through the use of experimental models (eg, knockout mice and selective gene silencing) and enhanced approaches to three-dimensional modeling and microscopy. The role of the cholangiocyte cytoskeleton in transport and intracellular trafficking has been recently recognized. These paradigms provide a framework for further understanding the mechanisms modulating normal cholangiocyte growth, transport, and signaling, and the abnormalities that result in disease.

Nuclear and cytosolic calcium are regulated independently

Nuclear calcium (Ca(2+)) regulates a number of important cellular processes, including gene transcription, growth, and apoptosis. However, it is unclear whether Ca(2+) signaling is regulated differently in the nucleus and cytosol. To investigate this possibility, we examined subcellular mechanisms of Ca(2+) release in the HepG2 liver cell line. The type II isoform of the inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) was expressed to a similar extent in the endoplasmic reticulum and nucleus, whereas the type III InsP(3)R was concentrated in the endoplasmic reticulum, and the type I isoform was not expressed. Ca(2+) signals induced by low InsP(3) concentrations started earlier or were larger in the nucleus than in the cytosol, indicating higher sensitivity of nuclear Ca(2+) stores for InsP(3). Nuclear InsP(3)R channels were active at lower InsP(3) concentrations than InsP(3)R from cytosol. Enriched expression of type II InsP(3)R in the nucleus results in greater sensitivity of the nucleus to InsP(3), thus providing a mechanism for independent regulation of Ca(2+)-dependent processes in this cellular compartment.

Expression and regulation of gap junctions in rat cholangiocytes

Hepatocytes and other digestive epithelia exchange second messengers and coordinate their functions by communicating through gap junctions. However, little is known about intercellular communication in cholangiocytes. The aim of this study was to examine expression and regulation of gap junctions in cholangiocytes. Connexin expression was determined by confocal immunofluorescence in rat bile ducts and in normal rat cholangiocyte (NRC) cells, a polarized cholangiocyte cell line. Intercellular Ca(2+) signaling was monitored by fluorescent microscopy. Microinjection studies assessed regulation of gap junction permeability in NRC cells and in SKHep1 cells, a liver-derived cell line engineered to express connexin 43. Immunochemistry showed that cholangiocytes from normal rat liver as well as the NRC cells express connexin 43. Localization of apical, basolateral, and tight junction proteins confirmed that NRC cells are well polarized. Apical exposure to ATP induced Ca(2+) oscillations that were coordinated among neighboring NRC cells, and inhibition of gap junction conductance desynchronized the Ca(2+) oscillations. NRC cells transfected with a connexin 43 antisense were significantly less coupled. Transcellular dye spreading was inhibited by activation of protein kinase A or protein kinase C. The same was observed in transfected SKHep1 cells, which expressed only connexin 43. Rat cholangiocytes and NRC cells express connexin 43, which permits synchronization of Ca(2+) signals among cells. Permeability of connexin 43-gap junctions is negatively regulated by protein kinases A and C. In conclusion, cholangiocytes have the capacity for intercellular communication of second messenger signals via gap junctions in a fashion that is under hormonal control.