Phospholipase D activation by P2Z-purinoceptor agonists in human lymphocytes is dependent on bivalent cation influx

P2 receptor subtypes in the cardiovascular system

Extracellular nucleotides have been implicated in a number of physiological functions. Nucleotides act on cell-surface receptors known as P2 receptors, of which several subtypes have been cloned. Both ATP and ADP are stored in platelets and are released upon platelet activation. Furthermore, nucleotides are also released from damaged or broken cells. Thus during vascular injury nucleotides play an important role in haemostasis through activation of platelets, modulation of vascular tone, recruitment of neutrophils and monocytes to the site of injury, and facilitation of adhesion of leucocytes to the endothelium. Nucleotides also moderate these functions by generating nitric oxide and prostaglandin I2 through activation of endothelial cells, and by activating different receptor subtypes on vascular smooth muscle cells. In the heart, P2 receptors regulate contractility through modulation of L-type Ca2+ channels, although the molecular mechanisms involved are still under investigation. Classical pharmacological studies have identified several P2 receptor subtypes in the cardiovascular system. Molecular pharmacological studies have clarified the nature of some of these receptors, but have complicated the picture with others. In platelets, the classical P2T receptor has now been resolved into three P2 receptor subtypes: the P2Y1, P2X1 and P2TAC receptors (the last of these, which is coupled to the inhibition of adenylate cyclase, is yet to be cloned). In peripheral blood leucocytes, endothelial cells, vascular smooth muscle cells and cardiomyocytes, the effects of classical P2X, P2Y and P2U receptors have been found to be mediated by more than one P2 receptor subtype. However, the exact functions of these multiple receptor subtypes remain to be understood, as P2-receptor-selective agonists and antagonists are still under development.

Activation by P2X7 agonists of two phospholipases A2 (PLA2) in ductal cells of rat submandibular gland. Coupling of the calcium-independent PLA2 with kallikrein secretion

Isolated ductal cells of rat submandibular gland phospholipid pools were labeled with [3H]arachidonic acid (AA). The tracer was incorporated preferentially to phosphatidylcholine (46% of the lipidic fraction). Extracellular ATP induced the release of [3H]AA to the extracellular medium in a time- and dose-dependent manner (EC50 = 220 microM). Among other agents tested, only 2', 3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (Bz-ATP) was able to mimic the effect of ATP (EC50 = 15 microM), without activation of phospholipase C. The purinergic antagonists oxidized ATP, suramin, and Coomassie Blue partly inhibited the response to 1 mM ATP and 100 microM Bz-ATP; the response was also blocked by the addition of Mg2+ or Ni2+. Expression of P2X7 receptor mRNA in these cells was confirmed by reverse transcription-polymerase chain reaction. In the presence of extracellular calcium, the phospholipase A2 inhibitor 2-(p-amylcinnamoyl)amino-4-chlorobenzoic acid (a nonspecific inhibitor), arachidonyl trifluoromethylketone (AACOCF3, an inhibitor of the calcium-dependent cytosolic PLA2 (cPLA2)), and bromoenol lactone (an inhibitor of the calcium-independent PLA2 (iPLA2)) inhibited the release of [3H]AA induced by ATP and Bz-ATP. In the absence of extracellular calcium, the release of [3H]AA in response to the purinergic agonists was still observed; this response was not affected by AACOCF3 and completely blocked by bromoenol lactone. ATP and Bz-ATP stimulated a calcium-independent secretion of kallikrein, which could be blocked by BEL but which was enhanced by AACOCF3. It is concluded that the P2X7 receptor in ductal cells is coupled to kallikrein secretion through a calcium-dependent cPLA2 and a calcium-independent iPLA2.

Purinergic receptor modulation of lipopolysaccharide signaling and inducible nitric-oxide synthase expression in RAW 264.7 macrophages

Previous studies have suggested that the P2Z/P2X7 purinergic receptor can participate in nucleotide-induced modulation of lipopolysaccharide (LPS) stimulated inflammatory mediator production. To test this hypothesis, we evaluated whether antagonism of the P2Z/P2X7 receptor can influence LPS signaling and expression of the inducible form of nitric-oxide synthase (iNOS) in RAW 264.7 macrophages. In the present study, we demonstrate that pretreatment of RAW 264.7 macrophages with a P2Z/P2X7 receptor antagonist, periodate oxidized adenosine 5'-triphosphate (o-ATP), substantially inhibits LPS-stimulated NO production and iNOS expression without altering cell viability. This effect on LPS-induced iNOS expression is mimicked by a pyridoxal-phosphate-based antagonist (pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid) of the P2Z/P2X7 purinergic receptor, indicating that these results are not unique to o-ATP. Additionally, o-ATP prevents cell death induced by P2Z/P2X7 receptor agonists. To ascertain how P2Z/P2X7 receptor antagonists influence LPS signaling, we evaluated the capacity of o-ATP to regulate LPS-mediated activation of the transcription factor, nuclear factor-kappaB, and the mitogen-activated protein kinases, extracellular signal-regulated kinase (ERK) 1 and ERK2. These experiments reveal that pretreatment of RAW 264.7 cells with o-ATP attenuates the LPS stimulation of a nuclear factor-kappaB-like binding activity. Moreover, the activation of ERK1 and ERK2 by LPS, but not by the phorbol ester, phorbol 12-myristate 13-acetate, is also blocked in RAW 264.7 cells by o-ATP pretreatment. In summary, these data suggest that the P2Z/P2X7 receptor modulates LPS-induced macrophage activation as assessed by iNOS expression and NO production. This report implicates the P2Z/P2X7 receptor in the control of protein kinase cascades and transcriptional processes, and these observations are likely to be important for the development of selective purinergic receptor antagonists for the treatment of septic shock.

Adenosine Triphosphate–Induced Shedding of CD23 and L-Selectin (CD62L) From Lymphocytes Is Mediated by the Same Receptor but Different Metalloproteases

CD23 is a transmembrane protein expressed on the surface of B-lymphocytes that binds IgE, CD21, CD11b, and CD11c. High concentrations of soluble CD23 and L-selectin are found in the serum of patients with B-chronic lymphocytic leukemia (B-CLL). Because extracellular adenosine triphosphate (ATP) causes shedding of L-selectin via activation of P2Z/P2X7 receptors expressed on B-CLL lymphocytes, we studied the effect of ATP on shedding of CD23. ATP-induced shedding of CD23 at an initial rate of 12% of that for L-selectin, whereas the EC50 for ATP was identical (35 μmol/L) for shedding of both molecules. Furthermore, benzoylbenzoyl ATP also produced shedding of CD23 and L-selectin with the same agonist EC50 values for both (10 μmol/L). Inactivation of the P2Z/P2X7 receptor by preincubation with oxidized ATP abolished ATP-induced shedding of both molecules. Moreover, KN-62, the most potent inhibitor for the P2Z/P2X7 receptor, inhibited ATP-induced shedding of both CD23 and L-selectin with the same IC50 (12 nmol/L). Ro 31-9790, a membrane permeant zinc chelator that inhibits the phorbol-ester-stimulated shedding of L-selectin, also inhibited shedding of CD23 from B-CLL lymphocytes. However, the IC50 for this inhibition by Ro31-9790 was different for L-selectin and CD23 (83 v 6 μmol/L, respectively). Although L-selectin was completely shed by incubation of cells with phorbol-ester, CD23 was not lost under these conditions. The data show that extracellular ATP induces shedding of L-selectin and CD23 from B-CLL lymphocytes by an action mediated by the P2Z/P2X7 receptor. However, different membrane metalloproteases seem to mediate the shedding of L-selectin and CD23.

© 1998 by The American Society of Hematology.

Adenosine Triphosphate–Induced Shedding of CD23 and L-Selectin (CD62L) From Lymphocytes Is Mediated by the Same Receptor but Different Metalloproteases

CD23 is a transmembrane protein expressed on the surface of B-lymphocytes that binds IgE, CD21, CD11b, and CD11c. High concentrations of soluble CD23 and L-selectin are found in the serum of patients with B-chronic lymphocytic leukemia (B-CLL). Because extracellular adenosine triphosphate (ATP) causes shedding of L-selectin via activation of P2Z/P2X7 receptors expressed on B-CLL lymphocytes, we studied the effect of ATP on shedding of CD23. ATP-induced shedding of CD23 at an initial rate of 12% of that for L-selectin, whereas the EC50 for ATP was identical (35 μmol/L) for shedding of both molecules. Furthermore, benzoylbenzoyl ATP also produced shedding of CD23 and L-selectin with the same agonist EC50 values for both (10 μmol/L). Inactivation of the P2Z/P2X7 receptor by preincubation with oxidized ATP abolished ATP-induced shedding of both molecules. Moreover, KN-62, the most potent inhibitor for the P2Z/P2X7 receptor, inhibited ATP-induced shedding of both CD23 and L-selectin with the same IC50 (12 nmol/L). Ro 31-9790, a membrane permeant zinc chelator that inhibits the phorbol-ester-stimulated shedding of L-selectin, also inhibited shedding of CD23 from B-CLL lymphocytes. However, the IC50 for this inhibition by Ro31-9790 was different for L-selectin and CD23 (83 v 6 μmol/L, respectively). Although L-selectin was completely shed by incubation of cells with phorbol-ester, CD23 was not lost under these conditions. The data show that extracellular ATP induces shedding of L-selectin and CD23 from B-CLL lymphocytes by an action mediated by the P2Z/P2X7 receptor. However, different membrane metalloproteases seem to mediate the shedding of L-selectin and CD23.

© 1998 by The American Society of Hematology.

Neuromedin B activates phospholipase D through both PKC-dependent and PKC-independent mechanisms

The actions of neuromedin B (NMB), a recently discovered mammalian bombesin-related peptide, are mediated by interacting with a distinct receptor; however, little is known about its cellular basis of action. Recent studies show activation of phospholipase D (PLD) is an important transduction cascade for a number of GI hormones, especially for stimulation of growth and protein sorting. The purpose of the present study was to determine whether activation of the NMB receptor causes activation of PLD and to explore whether this activation was coupled to PLC activation. Rat C6 glioblastoma cells (C6 cells), which contain a low density of native NMB receptors and BALB 3T3 cells stably transfected with rat NMB receptors, were used. NMB caused a 3-fold increase in C6 cells and an 11-fold increase in rNMB-R transfected cells in PLD activity. Increases in PLD activity were rapid and NMB was 100-fold more potent than gastrin-releasing peptide (GRP). NMB caused a half-maximal increase in [Ca2+]i at 0.2 nM, in [3H]IP and PLD at 1 nM, and half-maximal receptor occupation at 1.2 nM. TPA increased PLD dose-dependently with a half-maximal effect at 60 nM. The calcium ionophore A23187 (1 microM) alone did not increase PLD activity but potentiated the effect of TPA. The Ca2+-ATPase inhibitor, thapsigargin, did not affect NMB- or TPA-stimulated PLD activities, although it blocked completely the NMB-induced increase in [Ca2+]i. The PKC inhibitor GF109203X completely abolished TPA-induced PLD activity, however, it only inhibited NMB-induced PLD activity by 20%. The combination of thapsigargin and GF109203X had the same effect as GF109203X alone. These data indicate that NMB receptor activation is coupled to both PLC and PLD. In contrast to a number of other phospholipase C-coupled receptors, NMB receptor stimulated changes in [Ca2+]i do not contribute to PLD activation. Both PKC-dependent and PKC-independent mechanisms are involved in the NMB-stimulated PLD activation with the PKC-independent pathway predominating.

Extracellular ATP in the lymphohematopoietic system: P2Z purinoceptors off membrane permeabilization

The effects of extracellular nucleosides and nucleotides on many organs and systems have been recognized for almost 50 years. The effects of extracellular ATP (ATPo), UTPo, ADPo, and other agonists are mediated by P2 purinoceptors. One of the most dramatic effects of ATPo is the permeabilization of plasma membranes to low molecular mass solutes of up to 900 Da. This effect is evident in several cells of the lymphohematopoietic system and is supposed to be mediated by P2Z, an ATP(4-)-activated purinoceptor. Here, we review some basic information concerning P2 purinoceptors and focus our attention on P2Z-associated phenomena displayed by macrophages. Using fluorescent dye uptake, measurement of free intracellular Ca2+ concentration and electrophysiological recordings, we elucidate some of the events that follow the application of ATP to the extracellular surface of macrophages. We propose a regulatory mechanism for the P2Z-associated permeabilization pore. The presence of P2 purinoceptors in cells of the lymphohematopoietic system makes them potential candidates to mediate immunoregulatory events.

Phosphatidylcholine-specific phospholipase C regulates thapsigargin-induced calcium influx in human lymphocytes

The involvement of phosphatidylcholine-specific phospholipase C (PC-PLC) and D (PC-PLD) in the regulation of the thapsigargin-induced Ca2+ increase was investigated. Pretreatment of human lymphocytes with the PC-PLC inhibitors D609 or U73122 enhanced the thapsigargin-induced Ca2+ influx. By contrast, no effect was observed in the presence of phospholipase D inhibitor butanol. Addition of exogenous PC-PLC but not PC-PLD to lymphocytes prestimulated with thapsigargin led to a decrease of intracellular Ca2+. In addition, thapsigargin was shown to release diacylglycerol (DAG) from cellular phosphatidylcholine pools. The thapsigargin-induced DAG formation was inhibited by U73122 and D609 but not by butanol. Moreover, no formation of the PC-PLD activity marker phosphatidylbutanol was detected. Thapsigargin-induced DAG formation was dependent on the Ca2+ entry, as it was abolished in the absence of extracellular Ca2+ or in the presence of Ni2+. Further investigations demonstrated that the inhibition of the cellular DAG target, protein kinase C (PKC), enhanced thapsigargin-induced Ca2+ increase, whereas direct PKC activation had an inhibitory effect. Taken together, our results reveal the involvement of PC-PLC in the regulation of the thapsigargin-induced Ca2+ increase and point to the existence of a physiologic feedback mechanism activated by Ca2+ influx and acting via consecutive activation of PC-PLC and PKC to limit the rise of intracellular Ca2+.

ATP, a partial agonist for the P2Z receptor of human lymphocytes

1. Although extracellular adenosine 5'-triphosphate (ATP) is the natural ligand for the P2Z receptor of human lymphocytes it is less potent than 3'-O-(4-benzoylbenzoyl)-ATP (BzATP) in opening the associated ion channel, which conducts a range of permeants including Ba2+ and ethidium+. We have quantified the influx of ethidium+ into lymphocytes produced by BzATP, ATP, 2-methylthio-ATP (2MeSATP) and ATPgammaS, studied competition between ATP and BzATP and investigated the effects of KN-62, a new and potent inhibitor of the P2Z receptor. 2. BzATP and ATP stimulated ethidium+ influx with EC50 values of 15.4+/-1.4 microM (n=5) and 85.6+/-8.8 microM (n=5), respectively. The maximal response to ATP was only 69.8+/-1.9% of that for BzATP. Hill analysis gave nH of 3.17+/-0.24 (n=3) and 2.09+/-0.45 (n=4) for BzATP and ATP, suggesting greater positive cooperativity for BzATP than for ATP in opening the P2Z receptor-operated ion channel. 3. A rank order of agonist potency of BzATP>ATP=2MeSATP>ATPgammaS was observed for agonist-stimulated ethidium+ influx, while maximal influxes followed a rank order of BzATP>ATP>2MeSATP>ATPgammaS. 4. Preincubation with 30-50 microM oxidized ATP (ox-ATP), an irreversible P2Z inhibitor, reduced the maximal response but did not change the steepness of the Ba2+ influx-response curve produced by BzATP (nH 3.2 and 2.9 for 30 and 50 microM ox-ATP, respectively (n=2)). 5. ATP (300-1000 microM) added simultaneously with 30 microM BzATP (EC90) inhibited both ethidium+ and Ba2+ fluxes to a maximum of 30-40% relative to the values observed with BzATP alone. Moreover, ATP (300 microM) shifted the concentration-response curve to the right for BzATP-stimulated Ba2+ influx, confirming competition between ATP and BzATP. 6. KN-62, a new and powerful inhibitor of the lymphocyte P2Z receptor, showed less potency in antagonizing BzATP-mediated fluxes than ATP-induced fluxes when maximal concentrations of both agonists (BzATP, 50 microM; ATP, 500 microM) were used. 7. These data suggest that the natural ligand, ATP, is a partial agonist for the P2Z receptor while BzATP is a more efficacious agonist. Moreover the competitive studies show that only a single class of P2-receptor (P2Z class) is expressed on human leukaemic lymphocytes.

The Ca2+-sensing receptor (CaR) activates phospholipases C, A2, and D in bovine parathyroid and CaR-transfected, human embryonic kidney (HEK293) cells

The extracellular Ca2+ (Ca2+(o))-sensing receptor (CaR) is a G protein-coupled receptor that activates phospholipase C (PLC). In the present studies, we assessed Ca2+(o)-dependent changes in the generation of inositol phosphates (IP), free arachidonic acid (AA), and phosphatidylbutanol (PtdBtOH) by PLC, phospholipase A2 (PLA2), and phospholipase D (PLD), respectively, in bovine parathyroid cells as well as in wild-type or CaR-transfected human embryonic kidney (HEK293) cells (HEK-WT and HEK-CaR, respectively). Elevated Ca2+(o) increased the formation of IPs in parathyroid cells as well in HEK-CaR but not in HEK-WT cells. High Ca2+(o) also elicited time- and dose-dependent increases in PtdBtOH in parathyroid cells and HEK-CaR but not in HEK-WT cells. Brief treatment of parathyroid and HEK-CaR cells with an activator of protein kinase C (PKC), phorbol 12-myristate,13-acetate (PMA), stimulated PLD activity at both low and high Ca2+(o). Moreover, high Ca2+(o)-stimulated PLD activity was abolished following down-regulation of PKC by overnight phorbol myristate acetate (PMA) pretreatment, suggesting that CaR-mediated activation of PLD depends largely upon stimulation of PKC. High Ca2+(o) likewise increased the release of free AA in parathyroid and HEK-CaR but not in HEK-WT cells. Mepacrine, a general PLA2 inhibitor, and AACOCF3, an inhibitor of cytosolic PLA2, reduced AA release in parathyroid cells at high Ca2+(o), suggesting a major role for PLA2 in high Ca2+(o)-elicited AA release. Pretreatment of parathyroid cells with PMA stimulated release of AA at low and high Ca2+(o), while a PKC inhibitor, chelerythrine, reduced AA release at high Ca2+(o) to the level observed with low Ca2+(o) alone. Thus, PKC contributes importantly to the high Ca2+(o)-evoked, CaR-mediated activation of not only PLD but also PLA2. Finally, high Ca2+(o)-stimulated production of IP, PtdBtOH, and AA all decreased substantially in parathyroid cells cultured for 4 days, in which expression of the CaR decreases by 80% or more, consistent with mediation of these effects by the receptor. Thus, the CaR activates, directly or indirectly, at least three phospholipases in bovine parathyroid and CaR-transfected HEK293 cells, providing for coordinate, receptor-mediated regulation of multiple signal transduction pathways in parathyroid and presumably other CaR-expressing cells.

The isoquinoline derivative KN-62 a potent antagonist of the P2Z-receptor of human lymphocytes

1. Extracellular adenosine 5'-triphosphate (ATP) is an agonist for a P2Z receptor on human lymphocytes which mediates opening of a cation-selective ion channel, activation of phospholipase D and shedding of the adhesion molecule, L-selectin, from the cell surface. The isoquinolinesulphonamides, KN-62, (1-[N, O-bis(5-isoquinolinesulphonyl)-N-methyl-L-tyrosyl]-4-phenylpiperaz ine), a selective antagonist of Ca2+/calmodulin-dependent protein kinase II (CaMKII), and KN-04, (N-[1-[N-methyl-p-(5 isoquinoline sulphonyl)benzyl]-2-(4 phenylpiperazine)ethyl]-5-isoquinolinesulphonamide) an inactive analogue, were used to investigate the possible role of CaMKII in these diverse effects of extracellular ATP. 2. KN-62 potently antagonized ATP-stimulated Ba2+ influx into fura-2 loaded human lymphocytes with an IC50 of 12.7 +/- 1.5 nM (n = 3) and complete inhibition of the flux at a concentration of 500 nM. Similarly, KN-62 inhibited ATP-stimulated ethidium+ uptake, measured by time resolved flow cytometry, with an IC50 of 13.1 +/- 2.6 nM (n = 4) and complete inhibition of the flux at 500 nM. 3. KN-04 antagonized ATP-stimulated Ba2+ influx with an IC50 of 17.3 +/- 2.7 nM (n = 3). Similarly, KN-04 inhibited ATP-stimulated ethidium+ uptake with an IC50 of 37.2 +/- 8.9 nM (n = 4). Both fluxes were completely inhibited at 500 nM KN-04. 4. ATP-stimulated phospholipase D activity, measured in [3H]-oleic acid-labelled lymphocytes by the transphosphatidylation reaction, was antagonized by KN-62 and KN-04, with 50% inhibition at 5.9 +/- 1.2 and 9.7 +/- 2.8 nM (n = 3), respectively. Both KN-62 and KN-04 inhibited ATP-stimulated shedding of L-selectin, measured by flow cytometric analysis of cell surface L-selectin, with IC50 values of 31.5 +/- 4.5 and 78.7 +/- 10.8 nM (n = 3), respectively. Neither of the isoquinolinesulphonamides (500 nM) inhibited phorbol ester- or ionomycin-stimulated phospholipase D activity or phorbol ester-induced shedding of L-selectin. 5. The inhibitory effect of KN-62 or KN-04 on P2Z-mediated responses was slow in onset (5 min) and only partially reversed by washing the cells. 6. Both KN-62 and KN-04 (at 500 nM) had no effect on uridine 5'-triphosphate (UTP)-stimulated Ca2+ transients in fura-2 loaded human neutrophils, a response which is mediated by the P2Y2 receptor. 7. Thus, KN-62 and KN-04 are potent antagonists of the P2Z receptor and at nanomolar concentrations inhibit all known responses mediated by the P2Z receptor of human lymphocytes. In contrast, KN-62 and KN-04 had no effect on responses mediated by the P2Y2 receptor of neutrophils. Moreover, since KN-62 and KN-04 are almost equipotent, the P2Z-mediated responses do not involve CaMKII, but indicate that the isoquinolinesulphonamides are potent and direct inhibitors of the P2Z-receptor.