Initiation of purinergic signaling by exocytosis of ATP-containing vesicles in liver epithelium

Evidence for sustained ATP release from liver cells that is not mediated by vesicular exocytosis

Extracellular ATP regulates many important cellular functions in the liver by stimulating purinergic receptors. Recent studies have shown that rapid exocytosis of ATP-enriched vesicles contributes to ATP release from liver cells. However, this rapid ATP release is transient, and ceases in ~30 s after the exposure to hypotonic solution. The purpose of these studies was to assess the role of vesicular exocytosis in sustained ATP release. An exposure to hypotonic solution evoked sustained ATP release that persisted for more than 15 min after the exposure. Using FM1-43 (N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide) fluorescence to measure exocytosis, we found that hypotonic solution stimulated a transient increase in FM1-43 fluorescence that lasted ~2 min. Notably, the rate of FM1-43 fluorescence and the magnitude of ATP release were not correlated, indicating that vesicular exocytosis may not mediate sustained ATP release from liver cells. Interestingly, mefloquine potently inhibited sustained ATP release, but did not inhibit an increase in FM1-43 fluorescence evoked by hypotonic solution. Consistent with these findings, when exocytosis of ATP-enriched vesicles was specifically stimulated by 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), mefloquine failed to inhibit ATP release evoked by NPPB. Thus, mefloquine can pharmacologically dissociate sustained ATP release and vesicular exocytosis. These results suggest that a distinct mefloquine-sensitive membrane ATP transport may contribute to sustained ATP release from liver cells. This novel mechanism of membrane ATP transport may play an important role in the regulation of purinergic signaling in liver cells.

Regulation of purinergic signaling in biliary epithelial cells by exocytosis of SLC17A9-dependent ATP-enriched vesicles

ATP in bile is a potent secretogogue, stimulating biliary epithelial cell (BEC) secretion through binding apical purinergic receptors. In response to mechanosensitive stimuli, BECs release ATP into bile, although the cellular basis of ATP release is unknown. The aims of this study in human and mouse BECs were to determine whether ATP release occurs via exocytosis of ATP-enriched vesicles and to elucidate the potential role of the vesicular nucleotide transporter SLC17A9 in purinergic signaling. Dynamic, multiscale, live cell imaging (confocal and total internal reflection fluorescence microscopy and a luminescence detection system with a high sensitivity charge-coupled device camera) was utilized to detect vesicular ATP release from cell populations, single cells, and the submembrane space of a single cell. In response to increases in cell volume, BECs release ATP, which was dependent on intact microtubules and vesicular trafficking pathways. ATP release occurred as stochastic point source bursts of luminescence consistent with exocytic events. Parallel studies identified ATP-enriched vesicles ranging in size from 0.4 to 1 μm that underwent fusion and release in response to increases in cell volume in a protein kinase C-dependent manner. Present in all models, SLC17A9 contributed to ATP vesicle formation and regulated ATP release. The findings are consistent with the existence of an SLC17A9-dependent ATP-enriched vesicular pool in biliary epithelium that undergoes regulated exocytosis to initiate purinergic signaling.

Rho signaling regulates pannexin 1-mediated ATP release from airway epithelia

ATP released from airway epithelial cells promotes purinergic receptor-regulated mucociliary clearance activities necessary for innate lung defense. Cell swelling-induced membrane stretch/strain is a common stimulus that promotes airway epithelial ATP release, but the mechanisms transducing cell swelling into ATP release are incompletely understood. Using knockdown and knockout approaches, we tested the hypothesis that pannexin 1 mediates ATP release from hypotonically swollen airway epithelia and investigated mechanisms regulating this activity. Well differentiated primary cultures of human bronchial epithelial cells subjected to hypotonic challenge exhibited enhanced ATP release, which was paralleled by the uptake of the pannexin probe propidium iodide. Both responses were reduced by pannexin 1 inhibitors and by knocking down pannexin 1. Importantly, hypotonicity-evoked ATP release from freshly excised tracheas and dye uptake in primary tracheal epithelial cells were impaired in pannexin 1 knockout mice. Hypotonicity-promoted ATP release and dye uptake in primary well differentiated human bronchial epithelial cells was accompanied by RhoA activation and myosin light chain phosphorylation and was reduced by the RhoA dominant negative mutant RhoA(T19N) and Rho and myosin light chain kinase inhibitors. ATP release and Rho activation were reduced by highly selective inhibitors of transient receptor potential vanilloid 4 (TRPV4). Lastly, knocking down TRPV4 impaired hypotonicity-evoked airway epithelial ATP release. Our data suggest that TRPV4 and Rho transduce cell membrane stretch/strain into pannexin 1-mediated ATP release in airway epithelia.

HB-EGF synthesis and release induced by cholesterol depletion of human epidermal keratinocytes is controlled by extracellular ATP and involves both p38 and ERK1/2 signaling pathways

The heparin-binding EGF-like growth factor (HB-EGF) is an autocrine/paracrine keratinocyte growth factor, which binds to the epidermal growth factor (EGF) receptor family and plays a critical role during the re-epithelialization of cutaneous wound by stimulating the keratinocytes proliferation and migration. In this study, cellular stressing condition in autocrine cultures of human keratinocytes was induced by cholesterol depletion using methyl-beta-cyclodextrin (MβCD). MβCD treatment induces the expression and the release of HB-EGF. By analysis of the culture media, large amounts of cellular ATP were measured particularly after 1 h of MβCD treatment. To investigate whether ATP contributes to the expression of HB-EGF, the nonhydrolyzable ATP analogue, ATP-γ-S, was used to mimic the extracellular ATP released. We report that keratinocytes stimulated with ATP-γ-S induce HB-EGF expression and activate EGFR and ERK1/2. Using an antagonist of P2 purinergic receptors, we demonstrate that HB-EGF synthesis induced by lipid rafts disruption is dependent on ATP interaction with P2 purinergic receptors. Moreover, our data suggest that both MAPKs p38 and ERK1/2 are involved together or independently in the regulation of HB-EGF gene expression. These findings provide new insight into the signaling pathway by which HB-EGF is expressed after lipid rafts disruption. In summary, after lipid raft disruption, keratinocytes release large amount of extracellular ATP. ATP induces HB-EGF synthesis and release by interacting with the P2 purinergic receptor and through p38 and ERK1/2 signaling in response to a challenging environment. A release of ATP acts as an early stress response in keratinocytes.

Altered alkaline phosphatase activity in obese Zucker rats liver respect to lean Zucker and Wistar rats discussed in terms of all putative roles ascribed to the enzyme

Biliary complications often lead to acute and chronic liver injury after orthotopic liver transplantation (OLT). Bile composition and secretion depend on the integrated action of all the components of the biliary tree, starting from hepatocytes. Fatty livers are often discarded as grafts for OLT, since they are extremely vulnerable to conventional cold storage (CS). However, the insufficiency of donors has stimulated research to improve the usage of such marginal organs as well as grafts. Our group has recently developed a machine perfusion system at subnormothermic temperature (20°C; MP20) that allows a marked improvement in preservation of fatty and even of normal rat livers as compared with CS. We sought to evaluate the response of the biliary tree of fatty liver to MP20, and a suitable marker was essential to this purpose. Alkaline phosphatase (AlkP, EC 3.1.3.1), frequently used as marker of membrane transport in hepatocytes and bile ducts, was our first choice. Since no histochemical data were available on AlkP distribution and activity in fatty liver, we have first settled to investigate AlkP activity in the steatotic liver of fatty Zucker rats (fa/fa), using as controls lean Zucker (fa/+) and normal Wistar rats. The AlkP reaction in Wistar rats was in accordance with the existing data and, in particular, was present in bile canaliculi of hepatocytes in the periportal region and midzone, in the canals of Hering and in small bile ducts but not in large bile ducts. In lean ZR liver the AlkP reaction in Hering canals and small bile ducts was similar to Wistar rat liver but hepatocytes had lower canalicular activity and besides presented moderate basolateral reaction. The difference between lean Zucker and Wistar rats, both phenotypically normal animals, could be related to the fact that lean Zucker rats are genotypically heterozygous for a recessive mutated allele. In fatty liver, the activity in ductules and small bile ducts was unchanged, but most hepatocytes were devoid of AlkP activity with the exception of clusters of macrosteatotic hepatocytes in the mid-zone, where the reaction was intense in basolateral domains and in distorted canaliculi, a typical pattern of cholestasis. The interpretation of these data was hindered by the fact that the physiological role of AlkP is still under debate. In the present study, the various functions proposed for the role of the enzyme in bile canaliculi and in cholangiocytes are reviewed. Independently of the AlkP role, our data suggest that AlkP does not seem to be a reliable marker to study the initial step of bile production during OLT of fatty livers, but may still be used to investigate the behaviour of bile ductules and small bile ducts.

Regulatory volume decrease and P receptor signaling in fish cells: mechanisms, physiology, and modeling approaches

For animal cell plasma membranes, the permeability of water is much higher than that of ions and other solutes, and exposure to hyposmotic conditions almost invariably causes rapid water influx and cell swelling. In this situation, cells deploy regulatory mechanisms to preserve membrane integrity and avoid lysis. The phenomenon of regulatory volume decrease, the partial or full restoration of cell volume following cell swelling, is well-studied in mammals, with uncountable investigations yielding details on the signaling network and the effector mechanisms involved in the process. In comparison, cells from other vertebrates and from invertebrates received little attention, despite of the fact that e.g. fish cells could present rewarding model systems given the diversity in ecology and lifestyle of this animal group that may be reflected by an equal diversity of physiological adaptive mechanisms, including those related to cell volume regulation. In this review, we therefore present an overview on the most relevant aspects known on hypotonic volume regulation presently known in fish, summarizing transporters and signaling pathways described so far, and then focus on an aspect we have particularly studied over the past years using fish cell models, i.e. the role of extracellular nucleotides in mediating cell volume recovery of swollen cells. We, furthermore, present diverse modeling approaches developed on the basis of data derived from studies with fish and other models and discuss their potential use for gaining insight into the theoretical framework of volume regulation.

Myosin light chain kinase and Src control membrane dynamics in volume recovery from cell swelling

MLCK resolves membrane blebbing induced by osmotic cell swelling. Cell swelling also stimulates the formation of a Src–MLCK complex, which with cortactin and dynamin forms actin-based structures at the base of the cell to facilitate membrane retrieval for volume recovery.

 The expansion of the plasma membrane, which occurs during osmotic swelling of epithelia, must be retrieved for volume recovery, but the mechanisms are unknown. Here we have identified myosin light chain kinase (MLCK) as a regulator of membrane internalization in response to osmotic swelling in a model liver cell line. On hypotonic exposure, we found that there was time-dependent phosphorylation of the MLCK substrate myosin II regulatory light chain. At the sides of the cell, MLCK and myosin II localized to swelling-induced membrane blebs with actin just before retraction, and MLCK inhibition led to persistent blebbing and attenuated cell volume recovery. At the base of the cell, MLCK also localized to dynamic actin-coated rings and patches upon swelling, which were associated with uptake of the membrane marker FM4-64X, consistent with sites of membrane internalization. Hypotonic exposure evoked increased biochemical association of the cell volume regulator Src with MLCK and with the endocytosis regulators cortactin and dynamin, which colocalized within these structures. Inhibition of either Src or MLCK led to altered patch and ring lifetimes, consistent with the concept that Src and MLCK form a swelling-induced protein complex that regulates volume recovery through membrane turnover and compensatory endocytosis under osmotic stress.

Adenosine triphosphate release and purinergic (P2) receptor-mediated secretion in small and large mouse cholangiocytes

Adenosine triphosphate (ATP) is released from cholangiocytes into bile and is a potent secretogogue by increasing intracellular Ca²(+) and stimulating fluid and electrolyte secretion via binding purinergic (P2) receptors on the apical membrane. Although morphological differences exist between small and large cholangiocytes (lining small and large bile ducts, respectively), the role of P2 signaling has not been previously evaluated along the intrahepatic biliary epithelium. The aim of these studies therefore was to characterize ATP release and P2-signaling pathways in small (MSC) and large (MLC) mouse cholangiocytes. The findings reveal that both MSCs and MLCs express P2 receptors, including P2X4 and P2Y2. Exposure to extracellular nucleotides (ATP, uridine triphosphate, or 2',3'-O-[4-benzoyl-benzoyl]-ATP) caused a rapid increase in intracellular Ca²(+) concentration and in transepithelial secretion (I(sc)) in both cell types, which was inhibited by the Cl(-) channel blockers 5-nitro-2-(-3-phenylpropylamino)-benzoic acid (NPPB) or niflumic acid. In response to mechanical stimulation (flow/shear or cell swelling secondary to hypotonic exposure), both MSCs and MLCs exhibited a significant increase in the rate of exocytosis, which was paralleled by an increase in ATP release. Mechanosensitive ATP release was two-fold greater in MSCs compared to MLCs. ATP release was significantly inhibited by disruption of vesicular trafficking by monensin in both cell types.

CONCLUSION

These findings suggest the existence of a P2 signaling axis along intrahepatic biliary ducts with the "upstream" MSCs releasing ATP, which can serve as a paracrine signaling molecule to "downstream" MLCs stimulating Ca²(+)-dependent secretion. Additionally, in MSCs, which do not express the cystic fibrosis transmembrane conductance regulator, Ca²(+)-activated Cl(-) efflux in response to extracellular nucleotides represents the first secretory pathway clearly identified in these cholangiocytes derived from the small intrahepatic ducts.

Pathways for ATP release by bovine ciliary epithelial cells, the initial step in purinergic regulation of aqueous humor inflow

ATP release by nonpigmented (NPE) and pigmented (PE) ciliary epithelial cells is the enabling step in purinergic regulation of aqueous humor formation, but the release pathways are unknown. We measured ATP release from primary cultures of bovine mixed NPE and PE (bCE) cells and transformed bovine NPE and PE cells, using the luciferin-luciferase reaction. Hypotonicity-triggered bCE ATP release was inhibited by the relatively selective blocker of pannexin-1 (PX1) hemichannels (probenecid, 1 mM, 47 ± 2%), by a connexin inhibitor (heptanol, 1 mM, 49 ± 4%), and by an inhibitor of vesicular release (bafilomycin A1, 25 ± 2%), but not by the P2X(7) receptor (P2RX(7)) antagonist KN-62. Bafilomycin A1 acts by reducing the driving force for uptake of ATP from the cytosol into vesicles. The reducing agent dithiothreitol reduced probenecid-blockable ATP release. Similar results were obtained with NPE and PE cell lines. Pannexins PX1-3, connexins Cx43 and Cx40, and P2RX(7) were identified in native cells and cell lines by RT-PCR. PX1 mRNA expression was confirmed by Northern blots; its quantitative expression was comparable to that of Cx43 by real-time PCR. Heterologous expression of bovine PX1 in HEK293T cells enhanced swelling-activated ATP release, inhibitable by probenecid. We conclude that P2RX(7)-independent PX1 hemichannels, Cx hemichannels, and vesicular release contribute comparably to swelling-triggered ATP release. The relatively large response to dithiothreitol raises the possibility that the oxidation-reduction state is a substantial regulator of PX1-mediated ATP release from bovine ciliary epithelial cells.

The biliary HCO(3)(-) umbrella: a unifying hypothesis on pathogenetic and therapeutic aspects of fibrosing cholangiopathies

This review focuses on the hypothesis that biliary HCO(3)(-) secretion in humans serves to maintain an alkaline pH near the apical surface of hepatocytes and cholangiocytes to prevent the uncontrolled membrane permeation of protonated glycine-conjugated bile acids. Functional impairment of this biliary HCO(3)(-) umbrella or its regulation may lead to enhanced vulnerability of cholangiocytes and periportal hepatocytes toward the attack of apolar hydrophobic bile acids. An intact interplay of hepatocellular and cholangiocellular adenosine triphosphate (ATP) secretion, ATP/P2Y- and bile salt/TGR5-mediated Cl(-)/ HCO(3)(-) exchange and HCO(3)(-) secretion, and alkaline phosphatase-mediated ATP breakdown may guarantee a stable biliary HCO(3)(-) umbrella under physiological conditions. Genetic and acquired functional defects leading to destabilization of the biliary HCO(3)(-) umbrella may contribute to development and progression of various forms of fibrosing/sclerosing cholangitis.

Involvement of SLC17A9-dependent vesicular exocytosis in the mechanism of ATP release during T cell activation

Recent reports have shown that T cell receptor (TCR)-dependent ATP release from T cells is involved in production of interleukin-2 (IL-2) through activation of P2 receptors. Stimulation of TCR induces ATP release from T cells through gap junction hemichannels and maxianion channels, at least in part. However, the mechanisms of ATP release from activated T cells are not fully understood. Here, we studied the mechanisms of ATP release during TCR-dependent T cell activation by investigating the effects of various inhibitors on TCR-dependent ATP release from murine T cells. We found that not only anion channel and gap junction hemichannel inhibitors, but also exocytosis inhibitors suppressed the ATP release. These results suggest that ATP release from murine T cells is regulated by various mechanisms, including exocytosis. An inhibitor of exocytosis, bafilomycin A, significantly blocked TCR signaling, such as Ca(2+) elevation and IL-2 production. Furthermore, bafilomycin A, ectonucleotidase, and P2Y(6) receptor antagonist significantly inhibited production of pro-inflammatory cytokines from external antigen-restimulated splenocytes, indicating that vesicular exocytosis-mediated purinergic signaling has a significant role in TCR-dependent cytokine production. We also detected vesicular ATP in murine T cells and human T lymphoma Jurkat cells, both of which also expressed mRNA of SLC17A9, a vesicular nucleotide transporter. Knockdown of SLC17A9 in Jurkat cells markedly reduced ATP release and cytosolic Ca(2+) elevation after TCR stimulation, suggesting involvement of SLC17A9-dependent vesicular exocytosis in ATP release and T cell activation. In conclusion, vesicular exocytosis of ATP appears to play a role in T cell activation and immune responses.