Subnanomolar concentrations of thrombin enhance the volume-sensitive efflux of taurine from human 1321N1 astrocytoma cells

Translating the Force - mechano-sensing GPCRs

Incorporating mechanical cues into cellular responses allows us to experience our direct environment. Specialized cells can perceive and discriminate between different physical properties such as level of vibration, temperature, or pressure. Mechanical forces are abundant signals that also shape general cellular responses such as cytoskeletal rearrangement, differentiation, or migration and contribute to tissue development and function. The molecular structures that perceive and transduce mechanical forces are specialized cytoskeletal proteins, cell junction molecules, and membrane proteins such as ion channels and metabotropic receptors. G protein-coupled receptors (GPCRs) have attracted attention as metabotropic force receptors as they are among the most important drug targets. This review summarizes the function of mechano-sensitive GPCRs, specifically, the angiotensin II type 1 receptor and adrenergic, apelin, histamine, parathyroid hormone 1, and orphan receptors, focusing particularly on the advanced knowledge gained from adhesion-type GPCRs. We distinguish between shear stress and cell swelling/stretch as the two major types of mechano-activation of these receptors and contemplate the potential contribution of the force-from-lipid and force-from-tether models that have previously been suggested for ion channels.

A Novel Compound Targeting Protease Receptor 1 Activators for the Treatment of Glioblastoma

Data from human biopsies, in-vitro and in-vivo models, strongly supports the role of thrombin, and its protease-activated receptor (PAR1) in the pathology and progression of glioblastoma (GBM), a high-grade glial tumor. Activation of PAR1 by thrombin stimulates vasogenic edema, tumor adhesion and tumor growth. We here present a novel six amino acid chloromethyl-ketone compound (SIXAC) which specifically inhibits PAR1 proteolytic activation and counteracts the over-activation of PAR1 by tumor generated thrombin. SIXAC effects were demonstrated in-vitro utilizing 3 cell-lines, including the highly malignant CNS-1 cell-line which was also used as a model for GBM in-vivo. The in-vitro effects of SIXAC on proliferation rate, invasion and thrombin activity were measured by XTT, wound healing, colony formation and fluorescent assays, respectively. The effect of SIXAC on GBM in-vivo was assessed by measuring tumor and edema size as quantified by MRI imaging, by survival follow-up and brain histopathology. SIXAC was found in-vitro to inhibit thrombin-activity generated by CNS-1 cells (IC50 = 5 × 10-11M) and significantly decrease proliferation rate (p < 0.03) invasion (p = 0.02) and colony formation (p = 0.03) of these cells. In the CNS-1 GBM rat animal model SIXAC was found to reduce edema volume ratio (8.8 ± 1.9 vs. 4.9 ± 1, p < 0.04) and increase median survival (16 vs. 18.5 days, p < 0.02 by Log rank Mental-Cox test). These results strengthen the important role of thrombin/PAR1 pathway in glioblastoma progression and suggest SIXAC as a novel therapeutic tool for this fatal disease.

Cell Volume Control in Healthy Brain and Neuropathologies

Regulation of cellular volume is a critical homeostatic process that is intimately linked to ionic and osmotic balance in the brain tissue. Because the brain is encased in the rigid skull and has a very complex cellular architecture, even minute changes in the volume of extracellular and intracellular compartments have a very strong impact on tissue excitability and function. The failure of cell volume control is a major feature of several neuropathologies, such as hyponatremia, stroke, epilepsy, hyperammonemia, and others. There is strong evidence that such dysregulation, especially uncontrolled cell swelling, plays a major role in adverse pathological outcomes. To protect themselves, brain cells utilize a variety of mechanisms to maintain their optimal volume, primarily by releasing or taking in ions and small organic molecules through diverse volume-sensitive ion channels and transporters. In principle, the mechanisms of cell volume regulation are not unique to the brain and share many commonalities with other tissues. However, because ions and some organic osmolytes (e.g., major amino acid neurotransmitters) have a strong impact on neuronal excitability, cell volume regulation in the brain is a surprisingly treacherous process, which may cause more harm than good. This topical review covers the established and emerging information in this rapidly developing area of physiology.

Thrombin and protein C pathway in peripheral nerve Schwann cells

Thrombin and activated protein C (aPC) bound to the endothelial protein C receptor (EPCR) both activate protease-activated receptor 1 (PAR1) generating either harmful or protective signaling respectively. In the present study we examined the localization of PAR-1 and EPCR and thrombin activity in Schwann glial cells of normal and crushed peripheral nerve and in Schwannoma cell lines. In the sciatic crush model nerves were excised 1h, 1, 4, and 7days after the injury. Schwannoma cell lines produced high levels of prothrombin which is converted to active thrombin and expressed both EPCR and PAR-1 which co-localized. In the injured sciatic nerve thrombin levels were elevated as early as 1h after injury, reached their peak 1day after injury which was significantly higher (24.4±4.1mU/ml) compared to contralateral uninjured nerves (2.6±7mU/ml, t-test p<0.001) and declined linearly reaching baseline levels by day 7. EPCR was found to be located at the microvilli of Schwann cells at the node of Ranvier and in cytoplasm surrounding the nucleus. Four days after sciatic injury, EPCR levels increased significantly (57,785±16602AU versus 4790±1294AU in the contralateral uninjured nerves, p<0.001 by t-test) mainly distal to the site of injury, where axon degeneration is followed by proliferation of Schwann cells which are diffusely stained for EPCR. EPCR seems to be located to cytoplasmic component of Schwann cells and not to compact myelin component, and is highly increased following injury.

Thrombin Activity and Thrombin Receptor in Rat Glioblastoma Model: Possible Markers and Targets for Intervention?

High-grade gliomas constitute a group of aggressive CNS cancers that have high morbidity and mortality rates. Despite extensive research, current therapeutic approaches enable survival beyond 2 years in rare cases only. Thrombin and its main CNS target, protease-activated receptor-1, have been implicated in tumor progression and brain edema. Our aim was to study protease-activated receptor-1 (PAR-1) protein expression and thrombin-like activity levels in both in vitro and in vivo models of glioblastoma and correlate them with the volume of the surrounding edema. We measured the presence of PAR-1 protein using fluorescence immunohistochemistry and assessed thrombin activity in various glial and non-glial cell lines and in a CNS-1 glioma rat model using a thrombin-specific fluorescent assay. Thrombin activity was found to be highly elevated in various high-grade glioma cell lines as well as in non-glial malignant cell lines. In the CNS-1 glioma model, the level of PAR-1 fluorescence in the tumor was significantly elevated compared to adjacent regions of reactive gliosis or distant brain areas. The elevated level of thrombin activity observed in the high-grade glioma positively correlated with tumor-induced brain edema. In conclusion, thrombin is secreted from glioma cells and PAR-1 may be a new biological marker for high-grade gliomas.

LRRC8A protein is indispensable for swelling-activated and ATP-induced release of excitatory amino acids in rat astrocytes

In mammals, cellular swelling activates release of small organic osmolytes, including the excitatory amino acids (EAA) glutamate and aspartate, via a ubiquitously expressed volume-regulated chloride/anion channel (VRAC). Pharmacological evidence suggests that VRAC plays plural physiological and pathological roles, including excitotoxic release of glutamate in stroke. However, the molecular identity of this pathway was unknown. Two recent studies discovered that LRRC8 gene family members encode heteromeric VRAC composed of LRRC8A plus LRRC8B-E, which mediate swelling-activated Cl(-) currents and taurine release in human non-neural cells (Z. Qiu et al. Cell 157: 447, 2014; F.K. Voss et al. Science 344: 634, 2014). Here, we tested the contribution of LRRC8A to the EAA release in brain glia. We detected and quantified expression levels of LRRC8A-E in primary rat astrocytes with quantitative RT-PCR and then downregulated LRRC8A with gene-specific siRNAs. In astrocytes exposed to hypo-osmotic media, LRRC8A knockdown dramatically reduced swelling-activated release of the EAA tracer D-[(3)H]aspartate. In parallel HPLC assays, LRRC8A siRNA prevented hypo-osmotic media-induced loss of the endogenous intracellular L-glutamate and taurine. Furthermore, downregulation of LRRC8A completely ablated the ATP-stimulated release of D-[(3)H]aspartate and [(14)C]taurine from non-swollen astrocytes. Overall, these data indicate that LRRC8A is an indispensable component of a permeability pathway that mediates both swelling-activated and agonist-induced amino acid release in brain glial cells.

Receptor regulation of osmolyte homeostasis in neural cells

The capacity of cells to correct their volume in response to hyposmotic stress via the efflux of inorganic and organic osmolytes is well documented. However, the ability of cell-surface receptors, in particular G-protein-coupled receptors (GPCRs), to regulate this homeostatic mechanism has received much less attention. Mechanisms that underlie the regulation of cell volume are of particular importance to cells in the central nervous system because of the physical restrictions of the skull and the adverse impact that even small increases in cell volume can have on their function. Increases in brain volume are seen in hyponatraemia, which can arise from a variety of aetiologies and is the most frequently diagnosed electrolyte disorder in clinical practice. In this review we summarize recent evidence that the activation of GPCRs facilitates the volume-dependent efflux of osmolytes from neural cells and permits them to more efficiently respond to small, physiologically relevant, reductions in osmolarity. The characteristics of receptor-regulated osmolyte efflux, the signalling pathways involved and the physiological significance of receptor activation are discussed. In addition, we propose that GPCRs may also regulate the re-uptake of osmolytes into neural cells, but that the influx of organic and inorganic osmolytes is differentially regulated. The ability of neural cells to closely regulate osmolyte homeostasis through receptor-mediated alterations in both efflux and influx mechanisms may explain, in part at least, why the brain selectively retains its complement of inorganic osmolytes during chronic hyponatraemia, whereas its organic osmolytes are depleted.

Extracellular osmolarity modulates G protein-coupled receptor-dependent ATP release from 1321N1 astrocytoma cells

We previously reported that ATP release from 1321N1 human astrocytoma cells could be stimulated either by activation of G protein-coupled receptors (GPCR) or by hypotonic stress. Cheema et al. (Cheema TA, Ward CE, Fisher SK. J Pharmacol Exp Ther 315: 755-763, 2005) have demonstrated that thrombin activation of protease-activated receptor 1 (PAR1) in 1321N1 cells and primary astrocytes acts synergistically with hypotonic stress to gate the opening of volume-sensitive organic osmolyte and anion channels (VSOAC) and that hypertonic stress strongly inhibits PAR1 gating of VSOAC. We tested the hypothesis that a VSOAC-type permeability might comprise a GPCR-regulated pathway for ATP export by determining whether PAR1-sensitive ATP release from 1321N1 cells is similarly potentiated by hypotonicity but suppressed by hypertonic conditions. Strong hypotonic stress by itself elicited ATP release and positively modulated the response to thrombin. Thrombin-dependent ATP release was also potentiated by mild hypotonic stress that by itself did not stimulate ATP export. Notably, PAR1-sensitive ATP export was greatly inhibited in hypertonic medium. Neither the potency nor efficacy of thrombin as an activator of proximal PAR1 signaling was affected by hypotonicity or hypertonicity. 1,9-Dideoxyforskolin and carbenoxolone similarly attenuated PAR1-dependent ATP release and suppressed the PAR1-independent ATP elicited by strong hypotonic stress. Probenecid attenuated PAR1-stimulated ATP release under isotonic but not mild hypotonic conditions and had no effect on PAR1-independent release stimulated by strong hypotonicity. PAR1-dependent ATP export under all osmotic conditions required concurrent signaling by Ca(2+) mobilization and Rho-GTPase activation. In contrast, PAR1-independent ATP release triggered by strong hypotonicity required neither of these intracellular signals. Thus, we provide the new finding that GPCR-regulated ATP release from 1321N1 astrocytoma cells is remarkably sensitive to both positive and negative modulation by extracellular osmolarity. This supports a model wherein GPCR stimulation and osmotic stress converge on an ATP release pathway in astrocytes that exhibits several features of VSOAC-type channels.

Muscarinic receptor regulation of osmosensitive taurine transport in human SH-SY5Y neuroblastoma cells

The ability of G protein-coupled receptors to regulate osmosensitive uptake of the organic osmolyte, taurine, into human SH-SY5Y neuroblastoma cells has been examined. When monitored under isotonic conditions and in the presence of physiologically relevant taurine concentrations (1-100 microM), taurine influx was mediated exclusively by a Na(+)-dependent, high-affinity (K(m) = 2.5 microM) saturable transport mechanism (V(max) = 0.087 nmol/mg protein/min). Reductions in osmolarity of > 20% (attained under conditions of a constant NaCl concentration) resulted in an inhibition of taurine influx (> 30%) that could be attributed to a reduction in V(max), whereas the K(m) for uptake remained unchanged. Inclusion of the muscarinic cholinergic agonist, oxotremorine-M (Oxo-M), also resulted in an attenuation of taurine influx (EC(50) approximately 0.7 microM). Although Oxo-M-mediated inhibition of taurine uptake could be observed under isotonic conditions (approximately 25-30%), the magnitude of inhibition was significantly enhanced by hypotonicity (approximately 55-60%), a result that also reflected a reduction in the V(max), but not the K(m), for taurine transport. Oxo-M-mediated inhibition of taurine uptake was dependent upon the availability of extracellular Ca(2+) but was independent of protein kinase C activity. In addition to Oxo-M, inclusion of either thrombin or sphingosine 1-phosphate also attenuated volume-dependent taurine uptake. The ability of Oxo-M to inhibit the influx of taurine was attenuated by 4-[(2-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]butanoic acid, an inhibitor of the volume-sensitive organic osmolyte and anion channel. 4-[(2-Butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]butanoic acid also prevented receptor-mediated changes in the efflux and influx of K(+) under hypoosmotic conditions. The results suggest that muscarinic receptor activation can regulate both the volume-dependent efflux and uptake of taurine and that these events may be functionally coupled.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

Volume-dependent osmolyte efflux from neural tissues: regulation by G-protein-coupled receptors

The CNS is particularly vulnerable to reductions in plasma osmolarity, such as occur during hyponatremia, the most commonly encountered electrolyte disorder in clinical practice. In response to a lowered plasma osmolarity, neural cells initially swell but then are able to restore their original volume through the release of osmolytes, both inorganic and organic, and the exit of osmotically obligated water. Given the importance of the maintenance of cell volume within the CNS, mechanisms underlying the release of osmolytes assume major significance. In this context, we review recent evidence obtained from our laboratory and others that indicates that the activation of specific G-protein-coupled receptors can markedly enhance the volume-dependent release of osmolytes from neural cells. Of particular significance is the observation that receptor activation significantly lowers the osmotic threshold at which osmolyte release occurs, thereby facilitating the ability of the cells to respond to small, more physiologically relevant, reductions in osmolarity. The mechanisms underlying G-protein-coupled receptor-mediated osmolyte release and the possibility that this efflux can result in both physiologically beneficial and potentially harmful pathophysiological consequences are discussed.

Activation of muscarinic cholinergic receptors on human SH-SY5Y neuroblastoma cells enhances both the influx and efflux of K+ under conditions of hypo-osmolarity

The ability of receptor activation to regulate osmosensitive K+ fluxes (monitored as 86Rb+) in SH-SY5Y neuroblastoma has been examined. Incubation of SH-SY5Y cells in buffers rendered increasingly hypotonic by a reduction in NaCl concentration resulted in an enhanced basal efflux of Rb+ (threshold of release, 200 mOsM) but had no effect on Rb(+) influx. Addition of the muscarinic cholinergic agonist, oxotremorine-M (Oxo-M), potently enhanced Rb+ efflux (EC50 = 0.45 microM) and increased the threshold of release to 280 mOsM. Oxo-M elicited a similarly potent, but osmolarity-independent, enhancement of Rb+ influx (EC50 = 1.35 microM). However, when incubated under hypotonic conditions in which osmolarity was varied by the addition of sucrose to a fixed concentration of NaCl, basal- and Oxo-M-stimulated Rb+ influx and efflux were demonstrated to be dependent upon osmolarity. Basal- and Oxo-M-stimulated Rb+ influx (but not Rb+ efflux) were inhibited by inclusion of ouabain or furosemide. Both Rb+ influx and efflux were inhibited by removal of intracellular Ca2+ and inhibition of protein kinase C activity. In addition to Oxo-M, agonists acting at other cell surface receptors previously implicated in organic osmolyte release enhanced both Rb+ efflux and influx under hypotonic conditions. Oxo-M had no effect on cellular K+ concentration in SH-SY5Y cells under physiologically relevant reductions in osmolarity (0-15%) unless K+ influx was blocked. Thus, although receptor activation enhances the osmosensitive efflux of K+, it also stimulates K+ influx, and the latter permits retention of K+ by the cells.

On the role of G-protein coupled receptors in cell volume regulation

Cell volume is determined genetically for each cell lineage, but it is not a static feature of the cell. Intracellular volume is continuously challenged by metabolic reactions, uptake of nutrients, intracellular displacement of molecules and organelles and generation of ionic gradients. Moreover, recent evidence raises the intriguing possibility that changes in cell volume act as signals for basic cell functions such as proliferation, migration, secretion and apoptosis. Cells adapt to volume increase by a complex, dynamic process resulting from the concerted action of volume sensing mechanisms and intricate signaling chains, directed to initiate the multiple adaptations demanded by a change in cell volume, among others adhesion reactions, membrane and cytoskeleton remodeling, and activation of the osmolyte pathways leading to reestablish the water balance between extracellular/intracellular or intracellular/intracellular compartments. In multicellular organisms, a continuous interaction with the external milieu is fundamental for the dynamics of the cell. It is in this sense that the recent surge of interest about the influence on cell volume control by the most extended family of signaling elements, the G proteins, acquires particular importance. As here reviewed, a large variety of G-protein coupled receptors (GPCRs) are involved in this interplay with cell volume regulatory mechanisms, which amplifies and diversifies the volume-elicited signaling chains, providing a variety of routes towards the multiple effectors related to cell volume changes.

Cholesterol regulates volume-sensitive osmolyte efflux from human SH-SY5Y neuroblastoma cells following receptor activation

The ability of cholesterol to modulate receptor-mediated increases in the volume-dependent release of the organic osmolyte, taurine, has been examined. Depletion of cholesterol from SH-SY5Y neuroblastoma by preincubation of the cells with 5 mM methyl-beta-cyclodextrin (CD) for 10 min resulted in a 40 to 50% reduction in cholesterol and an enhancement of the ability of proteinase-activated receptor (PAR) 1, muscarinic cholinergic receptor (mAChR), and sphingosine 1-phosphate receptor to stimulate taurine efflux, when monitored under hypoosmotic conditions. Basal (swelling-induced) release of taurine was also enhanced by cholesterol depletion, but less markedly. Both basal- and receptor-mediated increases in taurine efflux were mediated via a volume-sensitive organic osmolyte and anion channel in control and cholesterol-depleted cells. Studies with the PAR-1 and mAChR receptor subtypes indicated that the stimulatory effect of CD pretreatment could be reversed by incubation of the cells with either CD/cholesterol or CD/5-cholesten-3alpha-ol donor complexes and that cholesterol depletion increased agonist efficacy, but not potency. The ability of cholesterol depletion to promote the PAR-1 receptor-mediated stimulation of osmolyte release was most pronounced under conditions of isotonicity or mild hypotonicity. In contrast to CD pretreatment, preincubation of the cells with cholesterol oxidase, a condition under which lipid microdomains are also disrupted, had no effect on either basal- or receptor-stimulated taurine efflux. Taken together, the results suggest that cholesterol regulates receptor-mediated osmolyte release via its effects on the biophysical properties of the plasma membrane, rather than its presence in lipid microdomains.

Thrombin increases hyposmotic taurine efflux and accelerates % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbbjxAHX % garmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy0Hgip5wz % aebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbbf9v8qqaq % Fr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qq % Q8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaeaaakeaaca % qGjbGaae4qaiaabMeadaqhaaWcbaGaae4CaiaabEhacaqGLbGaaeiB % aiaabYgaaeaacqGHsislaaaaaa!3FBE! $$ {\text{ICI}}^{ - }_{{{\text{swell}}}} $$ and RVD in 3T3 fibroblasts by a src-dependent EGFR transactivation

The present study in Swiss3T3 fibroblasts examines the effect of thrombin on hyposmolarity-induced osmolyte fluxes and RVD, and the contribution of the src/EGFR pathway. Thrombin (5 U/ml) added to a 30% hyposmotic medium markedly increased hyposmotic 3H-taurine efflux (285%), accelerated the volume-sensitive Cl- current (ICI-swell) and increased RVD rate. These effects were reduced (50-65%) by preventing the thrombin-induced intracellular Ca2+ [Ca2+]i rise with EGTA-AM, or with the phospholipase C (PLC) blocker U73122. Ca2+calmodulin (CaM) and calmodulin kinase II (CaMKII) also participate in this Ca2+-dependent pathway. Thrombin plus hyposmolarity increased src and EGFR phosphorylation, whose blockade by PP2 and AG1478, decreased by 30-50%, respectively, the thrombin effects on hyposmotic taurine efflux, ICI-swell and RVD. Ca2+- and src/EGFR-mediated pathways operate independently as shown by (1) the persistence of src and EGFR activation when [Ca2+]i rise is prevented and (2) the additive effect on taurine efflux, ICI-swell or RVD by simultaneous inhibition of the two pathways, which essentially suppressed these events. PLC-Ca2+- and src/EGFR-signaling pathways operate in the hyposmotic condition and because thrombin per se failed to increase taurine efflux and ICI-swell under isosmotic condition it seems that it is merely amplifying these previously activated mechanisms. The study shows that thrombin potentiates hyposmolarity-induced osmolyte fluxes and RVD by increasing src/EGFR-dependent signaling, in addition to the Ca2+-dependent pathway.

Thrombin potently enhances swelling-sensitive glutamate efflux from cultured astrocytes

High concentrations of thrombin (Thr) have been linked to neuronal damage in cerebral ischemia and traumatic brain injury. In the present study we found that Thr markedly enhanced swelling-activated efflux of (3)H-glutamate from cultured astrocytes exposed to hyposmotic medium. Thr (0.5-5 U/mL) elicited small (3)H-glutamate efflux under isosmotic conditions and increased the hyposmotic glutamate efflux by 5- to 10-fold, the maximum effect being observed at 15% osmolarity reduction. These Thr effects involve its protease activity and are fully mimicked by SFFLRN, the synthetic peptide activating protease-activated receptor-1. Thr potentiation of (3)H-glutamate efflux was largely dependent on a Thr-elicited increases in cytosolic Ca(2+) (Ca(2+) (i)) concentration ([Ca(2+)](i)). Preventing Ca(2+) (i) rise by treatment with EGTA-AM or with the phospholipase C blocker U73122 reduced the Thr-increased glutamate efflux by 68%. The protein kinase C blockers Go6976 or chelerythrine reduced the Thr effect by 19%-22%, while Ca/calmodulin blocker W7 caused a 63% inhibition. In addition to this Ca(2+)-sensitive pathway, Thr effect on glutamate efflux also involved activation of phosphoinositide-3 kinase (PI3K), since it was reduced by the PI3K inhibitor wortmannin (51% inhibition). Treating cells with EGTA-AM plus wortmannin essentially abolished Thr-dependent glutamate efflux. Thr-activated glutamate release was potently inhibited by the blockers of the volume-sensitive anion permeability pathway, NPPB (IC(50) 15.8 microM), DCPIB (IC(50) 4.2 microM), and tamoxifen (IC(50) 6.6 microM. These results suggest that Thr may contribute to the excitotoxic neuronal injury by elevating extracellular glutamate release from glial cells. Therefore, this work may aid in search of neuroprotective strategies for treating cerebral ischemia and brain trauma.

Receptor regulation of the volume-sensitive efflux of taurine and iodide from human SH-SY5Y neuroblastoma cells: differential requirements for Ca(2+) and protein kinase C

The basal (swelling-induced) and receptor-stimulated effluxes of (125)I(-) and taurine have been monitored to determine whether these two osmolytes are released from human SH-SY5Y cells under hypotonic conditions via common or distinct mechanisms. Under basal conditions, both (125)I(-) (used as a tracer for Cl(-)) and taurine were released from the cells in a volume-dependent manner. The addition of thrombin, mediated via the proteinase-activated receptor-1 (PAR-1) subtype, significantly enhanced the release of both (125)I(-) and taurine (3-6-fold) and also increased the threshold osmolarity for efflux of these osmolytes ("set-point") from 200 to 290 mOsM. Inclusion of a variety of broad-spectrum anion channel blockers and of 4-[(2-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]butanoic acid attenuated the release of both (125)I(-) and taurine under basal and receptor-stimulated conditions. Basal release of (125)I(-) and taurine was independent of Ca(2+) or the activity of protein kinase C (PKC). However, although PAR-1-stimulated taurine efflux was attenuated by either a depletion of intracellular Ca(2+) or inhibition of PKC by chelerythrine, the enhanced release of (125)I(-) was independent of both parameters. Stimulated efflux of (125)I(-) after activation of muscarinic cholinergic receptors was also markedly less dependent on Ca(2+) availability and PKC activity than that observed for taurine release. These results indicate that, although the osmosensitive release of these two osmolytes from SH-SY5Y cells may occur via pharmacologically similar membrane channels, the receptor-mediated release of (125)I(-) and taurine is differentially regulated by PKC activity and Ca(2+) availability.

Prostanoid receptors regulate the volume-sensitive efflux of osmolytes from murine fibroblasts via a cyclic AMP-dependent mechanism

The ability of prostanoid receptors to regulate the volume-dependent efflux of the organic osmolyte taurine from murine fibroblasts (L cells) via a cAMP-dependent mechanism has been examined. Incubation of L cells under hypoosmotic conditions resulted in a time-dependent efflux of taurine, the threshold of release occurring at 250 mOsM. Addition of prostaglandin E(1) (PGE(1)) potently (EC(50) = 2.5 nM) enhanced the volume-dependent efflux of taurine at all time points examined and increased the threshold for osmolyte release to 290 mOsM. Maximal PGE(1) stimulation (250-300% of basal) of taurine release was observed at 250 mOsM. Of the PGE analogs tested, only the EP(2)-selective agonist butaprost (9-oxo-11alpha,16S-dihydroxy-17-cyclobutyl-prost-13E-en-1-oic acid) was able to enhance taurine efflux. Inclusion of 1,9-dideoxyfoskolin, 5-nitro-2-(3-phenylpropylamino) benzoic acid, or 4-[(2-butyl-6,7-dicloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]-butanoic acid blocked the ability of PGE(1) to enhance taurine release, indicating the mediation of a volume-sensitive organic osmolyte and anion channel. The ability of PGE(1) to increase osmolyte release from L cells was mimicked by the addition of agents that inhibit cAMP breakdown, directly activate adenylyl cyclase, or are cell-permeant analogs of cAMP. Taurine release elicited by either PGE(1) or 8-(4-chlorophenylthio)-cAMP was attenuated by >70% in L cells that had been stably transfected with a mutant regulatory subunit of cAMP-dependent protein kinase (PKA). PGE(1) stimulation of taurine efflux was not attenuated by either depletion of intracellular calcium or inhibition of protein kinase C. The results indicate that activation of prostanoid receptors on murine fibroblasts enhances osmolyte release via a cAMP and PKA-dependent mechanism.

Regulation of volume-sensitive osmolyte efflux from human SH-SY5Y neuroblastoma cells following activation of lysophospholipid receptors

The ability of the lysophospholipids sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) to promote the release of the organic osmolyte taurine in response to hypoosmotic stress has been examined. Incubation of SH-SY5Y neuroblastoma cells under hypoosmotic conditions (230 mOsM) resulted in a time-dependent release of taurine that was markedly enhanced (3-7-fold) by the addition of micromolar concentrations of either S1P or LPA. At optimal concentrations, the effects of S1P and LPA on taurine efflux were additive and mediated via distinct receptors. Inclusion of 1,9-dideoxyfoskolin, 5-nitro-2-(3-phenylpropylamino benzoic acid, or 4-[(2-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]-butanoic acid blocked the ability of both lysophospholipids to enhance taurine release, indicating the mediation of a volume-sensitive organic osmolyte and anion channel. Both S1P and LPA elicited robust increases in intracellular calcium concentration that were attenuated by the removal of extracellular calcium, abolished by the depletion of intracellular calcium with thapsigargin, and were independent of phosphoinositide turnover. Taurine efflux mediated by S1P and LPA was unaffected by the removal of extracellular calcium but was attenuated by depletion of intracellular calcium (34-38%) and by inhibition of protein kinase C (PKC) with chelerythrine (38-72%). When intracellular calcium was depleted and PKC was inhibited, S1P- or LPA-stimulated taurine efflux was inhibited by 80%. Pretreatment of the cells with pertussis toxin, toxin B, or cytochalasin D had no effect on lysophospholipid-stimulated taurine efflux. The results indicate that both S1P and LPA receptors facilitate osmolyte release via a phospholipase C-independent mechanism that requires the availability of intracellular calcium and PKC activity.