Receptor specific for certain nucleotides stimulates inositol phosphate metabolism and Ca2+ fluxes in A431 cells

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

Mechanisms for inhibition of P2 receptors signaling in neural cells

Trophic factors are required to ensure neuronal viability and regeneration after neural injury. Although abundant information is available on the factors that cause the activation of astrocytes, little is known about the molecular mechanisms underlying the regulation of this process. Nucleotides released into the extracellular space from injured or dying neural cells can activate astrocytes via P2 nucleotide receptors. After a brief historical review and update of novel P2 receptor antagonists, this article focuses on recent advancements toward understanding molecular mechanisms that regulate G protein-coupled P2Y receptor signaling. Among P2Y receptor subtypes, the heptahelical P2Y2 nucleotide receptor interacts with vitronectin receptors via an RGD sequence in the first extracellular loop, and this interaction is required for effective signal transduction to activate mitogen-activated protein kinases ERK1/2, to mobilize intracellular calcium stores via activation of phospholipase C, protein kinase C isoforms, and to activate focal adhesion kinase and other signaling events. Ligation of vitronectin receptors with specific antibodies caused an inhibition of P2Y2 receptor-induced ERK1/2 and p38 phosphorylation and P2Y2 receptor-induced cytoskeleton rearrangement and DNA synthesis. Structure-function studies have identified agonist-induced phosphorylation of the C-terminus of the P2Y2 receptor, an important mechanism for receptor desensitization. Understanding selective mechanisms for regulating P2Y2 receptor signaling could provide novel targets for therapeutic strategies in the management of brain injury, synaptogenesis, and neurological disorders.

Antitumor effects of amlodipine, a Ca 2+ channel blocker, on human epidermoid carcinoma A431 cells in vitro and in vivo

Amlodipine, a dihydropyridine Ca(2+) channel blocker, is reported to inhibit proliferation of human epidermoid carcinoma A431 cells, and specifically attenuates Ca(2+) responses evoked by thapsigargin, an inhibitor of endoplasmic reticulum Ca(2+)-ATPases. In this study, we further examined the possible mechanism of the antiproliferative action of amlodipine and its antitumor effect on A431 xenografts in nude mice. Amlodipine reduced BrdU incorporation into nucleic acids in serum-starved A431 cells, and the reduction was diminished by uridine 5'-triphosphate (UTP), a phospholipase C (PLC)-linked agonist. Fluorometric measurement of intracellular free Ca(2+) concentration revealed that amlodipine blunted the UTP-induced Ca(2+) release from the internal Ca(2+) stores and consequently Ca(2+) influx through Ca(2+)-permeable channels on the plasma membrane. Although amlodipine alone caused Ca(2+) release from thapsigargin-sensitive Ca(2+) stores, such an effect was not reproduced by other dihydropyridine Ca(2+) channel blockers, including nicardipine and nimodipine, despite their antiproliferative effects in the cells. Daily intraperitoneal administration of amlodipine (10 mg/kg) for 20 days into mice bearing A431 xenografts retarded tumor growth and prolonged the survival of mice. Our results suggest a potential antitumor action for amlodipine in vitro and in vivo, which may be in part mediated by inhibiting Ca(2+) influx evoked by the passive depletion of internal Ca(2+) stores and by PLC-linked agonist stimulation.

Differential agonist-induced desensitization of P2Y2 nucleotide receptors by ATP and UTP

The equal potency and efficacy of the agonists, ATP and UTP, pharmacologically distinguish the P2Y2 receptor from other nucleotide receptors. Investigation of the desensitization of the P2Y2 receptors is complicated by the simultaneous expression of different P2 nucleotide receptor subtypes. The co-expression of multiple P2 receptor subtypes in mammalian cells may have led to contradictory reports on the efficacy of the natural agonists of the P2Y2 receptor to induce desensitization. We decided to investigate the desensitization of human and murine isoforms of the P2Y2 receptor, and to rigorously examine their signaling and desensitization properties. For these purposes, we used 1321N1 astrocytoma cells stably transfected with the human or murine P2Y2 receptor cDNA, as well as human A431 cells that endogenously express the receptor. The mobilization of intracellular calcium by extracellular nucleotides was used as a functional assay for the P2Y2 receptors. While ATP and UTP activated the murine and human P2Y2 receptors with similar potencies (EC50 values were 1.5-5.8 microM), ATP was approximately 10-fold less potent (IC50 = 9.1-21.2 microM) than UTP (IC50 = 0.7-2.9 microM) inducing homologous receptor desensitization in the cell systems examined. Individual cell analyses of the rate and dose dependency of agonist-induced desensitization demonstrated that the murine receptor was slightly more resistant to desensitization than its human counterpart. To our knowledge, this is the first individual cell study that has compared the cellular heterogeneity of the desensitized states of recombinant and endogenously expressed receptors. This comparison demonstrated that the recombinant system conserved the cellular regulatory elements needed to attenuate receptor signaling by desensitization.

Mechanisms of agonist-dependent and -independent desensitization of a recombinant P2Y2 nucleotide receptor

UTP activates P2Y, receptors in both 1321N1 cell transfectants expressing the P2Y2 receptor and human HT-29 epithelial cells expressing endogenous P2Y, receptors with an EC50 of 0.2-1.0 microM. Pretreatment of these cells with UTP diminished the effectiveness of a second dose of UTP (the IC50 for UTP-induced receptor desensitization was 0.3-1.0 microM for both systems). Desensitization and down-regulation of the P2Y2 nucleotide receptor may limit the effectiveness of UTP as a therapeutic agent. The present studies investigated the phenomenon of P2Y2 receptor desensitization in human 1321N1 astrocytoma cells expressing recombinant wild type and C-terminal truncation mutants of the P2Y2 receptor. In these cells, potent P2Y2 receptor desensitization was observed after a 5 min exposure to UTP. Full receptor responsiveness returned 5-10 min after removal of UTP. Thapsigargin, an inhibitor of Ca2+-ATPase in the endoplasmic reticulum, induced an increase in the intracellular free calcium concentration, [Ca2+]i, after addition of desensitizing concentrations of UTP, indicating that P2Y2 receptor desensitization is not due to depletion of calcium from intracellular stores. Single cell measurements of increases in [Ca2+]i induced by UTP in 1321N1 cell transfectants expressing the P2Y2 receptor indicate that time- and UTP concentration-dependent desensitization occurred uniformly across a cell population. Other results suggest that P2Y2 receptor phosphorylation/dephosphorylation regulate receptor desensitization/resensitization. A 5 min preincubation of 1321N1 cell transfectants with the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA), reduced the subsequent response to UTP by about 50%, whereas co-incubation of PMA with UTP caused a greater inhibition in the response. The protein phosphatases-1 and -2A inhibitor, okadaic acid, partially blocked resensitization of the receptor. Furthermore, C-terminal truncation mutants of the P2Y2 receptor that eliminated several potential phosphorylation sites including two for PKC were resistant to UTP-, but not phorbol ester-induced desensitization. Down regulation of protein kinase C isoforms prevented phorbol ester-induced desensitization but had no effect on agonist-induced desensitization of wild type or truncation mutant receptors. These results suggest that phosphorylation of the C-terminus of the P2Y2 receptor by protein kinases other than protein kinase C mediates agonist-induced receptor desensitization. A better understanding of the molecular mechanisms of P2Y2 nucleotide receptor desensitization may help optimize a promising cystic fibrosis pharmacotherapy based on the activation of anion secretion in airway epithelial cells by P2Y, receptor agonists.

Lysophosphatidic acid inhibits epidermal-growth-factor-induced Stat1 signaling in human epidermoid carcinoma A431 cells

Lysophosphatidic acid (LPA) is a lipid mediator which acts on its putative G protein-coupled receptor (GPCR). Recently, activation of signal transducers and activators of transcription (STATs) mediated by GPCR has been reported. In this study, we examined the effect of LPA on STAT activation using the electrophoretic mobility shift assays and the heterologous promoter analysis in human epidermoid carcinoma A431 cells. We found that LPA inhibited epidermal growth factor (EGF)-induced Stat1 activation in a concentration-dependent manner. Other phospholipase C (PLC)-coupled GPCR agonists, bradykinin and ATP, also inhibited Stat1 activation. This inhibitory effect of LPA was completely mimicked by an activator of protein kinase C (PKC), a PLC-downstream effector. These findings suggest that the inhibitory effect on EGF-induced Stat1 activation may be a general characteristic of PLC-coupled GPCRs and PKC pathway may be mainly associated with this inhibitory effect. This is the first evidence showing that GPCR agonists inhibit the Janus kinase-independent Stat1 activation induced by receptor tyrosine kinase.

Extracellular nucleotides stimulate proliferation in MCF-7 breast cancer cells via P2-purinoceptors

Nucleotides such as ATP can act as extracellular effector molecules by interaction with specific cellular receptors known as P2-purinoceptors. Recently, we cloned the human P2U purinoceptor from osteoclastoma and demonstrated its expression in skeletal tissues. In the current study we have investigated the expression of P2U purinoceptors in human breast tumour cell lines and examined functional effects of extracellular nucleotides on these cells. By reverse transcription-linked polymerase chain reaction (RT-PCR) the expression of mRNA for P2U purinoceptors was demonstrated in four human breast cancer cell lines, Hs578T, MCF-7, SK-Br3 and T47-D. In MCF-7 cells, extracellular ATP (1-100 microM) elevated intracellular free calcium concentration [Ca2+]i, indicating that these cells express functional P2-purinoceptors. UTP elevated [Ca2+]i in an identical manner to ATP, whereas 2-methylthioATP was completely ineffective, and ADP only partially effective. This pharmacological profile suggests that the P2U subtype may be the only P2-purinoceptor expressed by these cells. The functional significance of P2U purinoceptor expression by MCF-7 cells was investigated by analysing the effects of extracellular ATP on cell proliferation. The slowly hydrolysed analogue of ATP, ATPgammaS (which was also shown to elevate [Ca2+]i), induced proliferation of MCF-7 cells when added daily to serum-free cultures over a period of 3 days. ATPgammaS-induced proliferation was demonstrated by three separate methods, detection by scintillation counting of [3H]thymidine incorporation, immunocytochemical detection of 5-bromo-2-deoxyuridine incorporation and direct counting of cell numbers. These data suggest that ATP, possibly released at sites of tissue injury or inflammation, may be capable of growth factor action in promotion of tumour proliferation or progression.

ATP- and EGF-stimulated phosphatidulinositol synthesis by two different pathways, phospholipase D and diacylglycerol kinase, in A-431 epidermoid carcinoma cells

The [(3)H]inositol incorporation into the membrane fraction of A-431 human epidermoid carcinoma cells was markedly increased by stimulation of the cells with either epidermal growth factor (EGF), ATP, bradykinin, or a calcium ionophore A23187 in the presence of 1 mM extracellular calcium ions; most incorporated [(3)H]inositol was found to have accumulated as phosphatidylinositol (PI). The EGF- and ATP-stimulated PI synthesis was inhibited by two protein kinase C inhibitors, staurosporine and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7), and an intracellular calcium chelator, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA/AM), but not by the calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7). Pretreatment of cells with pertussis toxin (IAP, islet-activating protein) inhibited the PI synthesis, [Ca(2+)]i elevation, and inositol trisphosphate (IP(3)) production by ATP, suggesting that the phospholipase C(PLC) system coupled with IAP-sensitive G protein is involved in the ATP-stimulated PI synthesis. On the other hand, the ATP stimulation increased the release of [(3)H]choline and [(32)P)phosphatidic acid (PA) from radiolabeled cells, and such release was not inhibited by IAP. In the presence of n-butyl alcohol, which prevents the production of PA by generation of phosphatidylbutanol, the ATP-stimulated PI synthesis was reduced. Because n-butyl alcohol did not inhibit IP(3) production and [Ca(2+)]i elevation, this fact suggests that the lAP-insensitive PLD system is involved in the ATP-stimulated PI synthesis. In A-431 cells, the stimulation of P(2)-purinergic receptors appears to activate the IAP-sensitive PLC system and IAP-insensitive PLD system, both of which are essential for the stimulation of PI synthesis. The present results imply the general prospect that ligand stimulation, which mobilizes second messengers and consumes their precursors, simultaneously provokes the pathway to synthesize and salvage the second messenger precursors as well.

Mystixin-7 and mystixin-11 increase cytosolic free Ca2+ and inositol trisphosphates in human A-431 cells

Mystixin-7 and mystixin-11, small peptides structurally related to corticotropin-releasing factor (CRF), have been shown to attenuate vascular leakage in injured skin. The goal of this study was to characterize changes in cytosolic Ca2+ concentration ([Ca2+]i) in human epidermoid A-431 cells treated with these two peptides and to investigate the mechanisms by which these changes occurred. The resting [Ca2+]i in A-431 cells at 37 degrees C was 76 +/- 2 nM (n = 373). When cells were treated with either peptide, [Ca2+]i increased immediately. The increase depended on the peptide concentration, with a median effective concentration of 299 +/- 9 pM for mystixin-7 and 2.23 +/- 0.04 pM for mystixin-11. The increases also depended on extracellular Ca2+ and were blocked by Cd2+, Co2+, verapamil, and nifedipine. alpha-Helical CRF-(9-41), a synthetic CRF receptor antagonist, and pertussis toxin also blocked the increase in [Ca2+]i induced by the two peptides. Taken together, these results suggest that mystixin-7 and mystixin-11 interact with CRF receptors to activate pertussis-sensitive G proteins coupled to L-type Ca2+ channels that allow an uptake of extracellular Ca2+. Because U-73122, an inhibitor of 1,4,5-inositol trisphosphate production, partially inhibited the increase in [Ca2+]i, we measured inositol trisphosphates in cells stimulated by the two peptides. Both increased inositol trisphosphate levels within 1 min. The increase was inhibited by the removal of extracellular Ca2+ or treatment with U-73122. The results suggest that the Ca2+ influx stimulated by mystixin-7 and mystixin-11 induces an increase in inositol trisphosphates, resulting in a mobilization of Ca2+ from 1,4,5-inositol trisphosphate-sensitive Ca2+ pools.

Oscillations in cytosolic free Ca2+ induced by ADP and ATP in single rat hepatocytes display differential sensitivity to application of phorbol ester

We have previously described differences in the oscillatory responses of cytosolic free Ca2+ concentration ([Ca2+]i) in hepatocytes to ADP and ATP, which we have interpreted as evidence that these two nucleotides are acting at distinct receptors. We show here that ADP- and ATP-induced oscillations are differentially sensitive to application of the phorbol ester 4 beta-phorbol 12,13-dibutyrate (PDB). ADP-induced [Ca2+]i oscillations are abolished by low concentrations of PDB (5-10 nM), whereas ATP-induced oscillations of long duration are refractory to PDB, even at greatly elevated concentrations (100 nM). The data illustrate a further difference in the actions of ADP and ATP, strengthening the argument that these agonists are not acting at the same receptor on rat hepatocytes.

Effect of extracellular ATP on breast tumor cell growth, implication of intracellular calcium

We studied the effects of purine nucleotides and particularly adenosine triphosphate (ATP) in two (one hormonosensitive, MCF7 and one hormonoinsensitive, MDA-MB 231) human breast tumor cell lines. As described in other cells, we observed that purine nucleotides produced transient elevations in intracellular calcium ions, [Ca2+]i, in both types of cells as determined from Indo-1 fluorescence of loaded cells. In the absence of external calcium the [Ca2+]i transients consisted of single narrow peaks while an extension of peak duration along with a biphasic appearance were observed in the presence of external calcium. The potency of different purine nucleotides in elevating [Ca2+]i was ATP > ADP >> AMP > adenosine (which was inefficient) proving the presence of P2 purinergic receptor subtypes. Suramin, a compound known to compete with ATP for its binding sites, nearly abolished the effect of ATP on [Ca2+]i increase. while verapamil, a calcium channel blocker, was unable to abolish such an an ATP-induced [Ca2+]i increase. The concentrations of ATP required to increase [Ca2%bdi ranged from 10(-7) M to 10(-3) M, the maximal effect being obtained with 10(-4) M ATP. At this latter concentration, ATP induced cell growth inhibition which was dose-independent as triggered only when maximal elevation of [Ca2+]i was attained. This ATP concentration also induced maximal apoptotic features in both types of cells. Together, our results highlighted an 'all or none' effect of ATP on breast tumor cell growth mediated by its effect on [Ca2+]i liberation from intracellular stores, the first rise of [Ca2+]i being further amplified by an influx of calcium from extracellular space. The attainment of sufficient [Ca2+]i level then triggers cellular events.

Elevated intracellular cyclic AMP exerts different modulatory effects on cytosolic free Ca2+ oscillations induced by ADP and ATP in single rat hepatocytes

Single aequorin-injected hepatocytes respond to agonists acting via the phosphoinositide signalling pathway by the generation of oscillations in cytosolic free Ca2+ concentration ([Ca2+]free). The duration of [Ca2+]free transients is characteristic of the stimulating agonist. We have previously reported that ADP and ATP, which are believed to act through a single P(2y)-purinoceptor species, induce very different oscillatory [Ca2+]free responses in the majority of hepatocytes. We have interpreted these data as evidence for two separate Ca(2+)-mobilizing purinoceptors for these nucleotides. We show here that the elevation of intracellular cyclic AMP concentration, by the co-application of either dibutyryl cyclic AMP or 7 beta-desacetyl-7 beta-[gamma-(N-methylpiperazino)butyryl]- forskolin (L858051), exerts different modulatory effects on [Ca2+]free oscillations induced by ADP and ATP in single rat hepatocytes. Elevated intracellular cyclic AMP levels enhance the frequency and peak [Ca2+]free of transients induced by ADP. In contrast, the elevation of intracellular cyclic AMP levels in hepatocytes producing [Ca2+]free oscillations in response to ATP stimulates either an increase in the duration of transients or a sustained rise in [Ca2+]free. The data illustrate a further difference between the oscillatory [Ca2+]free responses of hepatocytes to ADP and ATP, thus further arguing against ADP and ATP acting via a single purinoceptor species.

Corticotropin-releasing factor increases [Ca2+]i via receptor-mediated Ca2+ channels in human epidermoid A-431 cells

Corticotropin-releasing factor (CRF) has been shown to attenuate vascular leakage in injured skin, mucous membrane, muscle, and brain. Calcium is thought to play an important role in many of the physiological responses to CRF, but there has been little characterization of how calcium is involved in process by which CRF protects damaged tissues. The goal of this study was to characterize changes in cytosolic free calcium concentrations ([Ca2+]i) in human epidermoid A-431 cells exposed to human/rat-CRF and to investigate the mechanisms by which these changes occur. The resting [Ca2+]i in normal cells at 37 degrees C was 66 +/- 4 nM (n = 32). When cells were treated with CRF, [Ca2+]i increased immediately. The increase depended on CRF concentration, with a median effective concentration of 11 pM. This increase in [Ca2+]i depended on external Ca2+ but not Na+, Mg2+, or K+. La3+ (10 microM) and Co2+ (10 microM) inhibited the CRF-induced [Ca2+]i increase, whereas verapamil and nifedipine tested at concentrations up to 1 mM did not. alpha-Helical CRF-(9-41), a synthetic CRF receptor antagonist, and pertussis toxin blocked the increase in [Ca2+]i induced by CRF, which suggests that the entry of extracellular Ca2+ is mediated by receptor-operated Ca2+ channels coupled with pertussis toxin-sensitive G proteins. Although 420 pM CRF stimulated an immediate increase in [Ca2+]i, inositol trisphosphate and cellular cAMP levels did not change within 1 min either in the presence or absence of external Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth inhibition of breast cancer cells induced by exogenous ATP

Addition of ATP (> 0.1 mM) to cultures of human breast cancer T47D cells resulted in an inhibition of cell proliferation. The inhibition was found to be specific for ATP, and dependent on its concentration. Growth inhibition continued for at least three days, although ATP and its hydrolysis products were metabolized within one day. Conditioned medium from ATP-treated cultures (CM+) was found to inhibit the growth of cells that were not exposed to ATP. This is an indication that extracellular factors, besides ATP, are involved in the inhibition process. The inhibition was maintained after dialysis of the CM+, using an 8 kDa cut-off membrane. Conditioned medium from untreated cultures (CM-), however, only slightly affected cell growth. The data suggest that the CM(+)-induced cell growth inhibition is mediated by an ATP-activated growth inhibiting factor. Flow microfluorometry and thymidine incorporation experiments have shown that the growth arrest is mainly due to the elongation of the S-phase of the cell cycle.

Bradykinin-stimulated transient modulation of epidermal growth factor receptors in A-431 human epidermoid carcinoma cells

Of nine biological factors (ATP, bradykinin, vasopressin, substance P, angiotensin II, norepinephrine, epinephrine, 12-tetradecanoylphorbol 13-acetate (TPA), and A23187 calcium ionophore) examined, bradykinin, as well as ATP, TPA, and A23187, significantly increased the phosphorylation of epidermal growth factor (EGF) receptors and reduced the binding of EGF to their high-affinity site. The reduction in EGF binding by bradykinin, ATP, and TPA was similarly reversed by concomitant incubation with staurosporine, a protein kinase C inhibitor, implying that the phosphorylation of EGF receptors was catalyzed probably by a protein kinase C of the same or similar type in each case. This possibility was confirmed by the fact that the major phosphorylation site of EGF receptors by the stimulation with either bradykinin, ATP, or TPA was the same (Thr-654). Different from the stimulations with ATP and TPA, the effect of bradykinin of decreasing the high-affinity EGF binding was transient (a minimum binding at 2.5 min); the reduced EGF binding was, however, sustained for up to 30 min in the presence of calyculin A, a phosphoprotein phosphatase inhibitor. Moreover, the homogenate prepared from bradykinin-stimulated A-431 cells had stronger dephosphorylation activity for phosphorylated EGF receptors than that from control cells. These results suggest that bradykinin stimulates both the protein kinase C system and a phosphoprotein phosphatase(s) activity in A-431 cells. Such biphasic effects of bradykinin to phosphorylate and dephosphorylate EGF receptors via protein kinase C and a phosphoprotein phosphatase, respectively, imply a homeostatic control of receptor function in regulating phosphorylation level by the same bioactive factor.

Adenosine 5'-[alpha beta-methylene]triphosphate potentiates the oscillatory cytosolic Ca2+ responses of hepatocytes to ATP, but not to ADP

Single rat hepatocytes microinjected with aequorin generate oscillations in cytosolic free Ca2+ concentration ([Ca2+]i) when stimulated with agonists acting through the phosphoinositide signalling pathway. The duration of these transients has been shown to be characteristic of the stimulating agonist, so that transients of very different duration can be induced in the same individual hepatocyte by different agonists. In a previous study we have shown that ADP and ATP, which are believed to act through a single P2y-purinoceptor species, elicit very different [Ca2+]i responses in most of the hepatocytes. We have interpreted this as evidence for two Ca(2+)-mobilizing purinoceptors. The methylated derivative of ATP, adenosine 5'-[alpha beta-methylene]-triphosphate (pp[CH2]pA), is only a weak P2y-purinoceptor agonist. When 100 microM pp[CH2]pA was supplied to aequorin-injected hepatocytes, there was no effect on [Ca2+]i. However, 25 microM pp[CH2]pA co-supplied with ATP causes a potentiation of the [Ca2+]i response in most of the hepatocytes. The effect was specific for ATP-induced transients; [Ca2+]i transients induced by other agonists, and importantly by ADP, were not affected by addition of pp[CH2]pA. This further illustrates differences in the actions of ADP and ATP, strengthening the argument for separate receptors for these nucleotides.

Extracellular ATP and ADP stimulate proliferation of porcine aortic smooth muscle cells

The mitogenic effect of extracellular ATP on porcine aortic smooth muscle cells (SMC) was examined. Stimulation of [3H]thymidine incorporation by ATP was dose-dependent; the maximal effect was obtained at 100 microM. ATP acted synergistically with insulin, IGF-1, EGF, PDGF, and various other mitogens. Incorporation of [3H]thymidine was correlated with the fraction of [3H]thymidine-labeled nuclei and changes in cell counts. The stimulation of proliferation was also determined by measurement of cellular DNA using bisbenzamide and by following the increase of mitochondrial dehydrogenase protein. The effect of ATP was not due to hydrolysis to adenosine, which shows synergism with ATP. ATP acted as a competence factor. The mitogenic effect of ATP, but not adenosine, was further increased by lysophosphatidate, phosphatidic acid, or norepinephrine. The inhibitor of adenosine deaminase, EHNA, stimulated the effect of adenosine but not ATP. The adenosine receptor antagonist theophylline depressed adenosine-induced mitogenesis. ADP and the non-hydrolyzable analogue adenosine 5'-[beta, gamma-imido]triphosphate (AMP-PNP) were equally mitogenic. Thus extracellular ATP stimulated mitogenesis of SMC via P2Y purinoceptors. The mechanism of ATP acting as a mitogen in SMC was further explored. Extracellular ATP stimulated the release of [3H]arachidonic acid (AA) and prostaglandin E2 (PGE2) into the medium, and enhanced cAMP accumulation in a dose-dependent fashion similar to ATP-induced [3H]thymidine incorporation. Inhibitors of the arachidonic acid metabolism pathway, quinacrine and indomethacin, partially inhibited the mitogenic effect of ATP but not of adenosine. Pertussis toxin inhibited ATP-stimulated DNA synthesis, AA release, PGE2 formation, and cAMP accumulation. Down-regulation of protein kinase C (PKC) by long-term exposure to phorbol dibutyrate (PDBu) partially prevented stimulation of DNA synthesis and activation of the AA pathway by ATP. The PKC inhibitor, staurosporine, antagonized mitogenesis stimulated by ATP. No synergistic effect was found when PDBu and ATP were added together. Therefore, a dual mechanism, including both arachidonic acid metabolism and PKC, is involved in ATP-mediated mitogenesis in SMC. In addition, ATP acted synergistically with angiotensin II, phospholipase C, serotonin, or carbachol to stimulate DNA synthesis. Finally, the possible physiological significance of ATP as a mitogen in SMC was further studied. The effect of endothelin and heparin, which are released from endothelial cells, on ATP-dependent mitogenesis was investigated. Extracellular ATP acted synergistically with endothelin to stimulate a greater extent of [3H]thymidine incorporation than was seen with PDGF plus endothelin. Heparin, believed to have a regulatory role, partially inhibited the stimulation of DNA synthesis caused both by ATP and PDGF.(ABSTRACT TRUNCATED AT 400 WORDS)

Independent pathways regulate the cytosolic [Ca2+] initial transient and subsequent oscillations in individual cultured arterial smooth muscle cells responding to extracellular ATP

Stimulation with extracellular ATP causes a rapid initial transient rise followed by asynchronous periodic oscillations in cytosolic calcium ion activity ([Ca2+]i) in individual aortic smooth muscle cells in either HEPES-buffered or HCO3(-)-buffered saline. The dose at which one-half of the cells display an initial rise in cytosolic calcium is 0.11 microM ATP in the presence of external Ca2+ and 0.88 microM ATP in the absence of external Ca2+; the corresponding value for oscillations in the presence of external Ca2+ is 2.6 microM ATP. While the initial transient displays rapid desensitization, the oscillations persist for greater than 30 min in the continuous presence of ATP. The presence of the agonist ATP is also absolutely required for the maintenance of the oscillations, presumably to provide continuous activation of P2 purinoceptors. The average frequency of oscillation is approximately 0.9 min-1. The frequency depends only slightly on the concentration of ATP, and oscillations do not collapse into a prolonged elevated [Ca2+]i at high concentrations of ATP. Both Ca2+ influx and release from internal stores participate in the initial transient. Oscillations are not produced in the absence of external Ca2+ but are initiated upon the addition of external Ca2+ in the continued presence of ATP. Oscillations in progress are abolished by the removal of extracellular Ca2+ with one additional peak occurring after the Ca2+ removal. These data suggest that extracellular Ca2+ influx is required for the maintenance of the posttransient oscillations, presumably to provide the Ca2+ necessary for refilling intracellular Ca2+ pools that are the source of the oscillating [Ca2+]i. The Ca2+ influx is not regulated by voltage-gated Ca2+ channels. The data in this report are consistent with the view that the initial transient has contributions from two receptor-mediated pathways, and the oscillations are controlled either by a mechanism separate from the ones that control the initial transient or by steps whose control diverges before the point of desensitization.

Regulation of Cl- channels in normal and cystic fibrosis airway epithelial cells by extracellular ATP

The rate of Cl- secretion by human airway epithelium is determined, in part, by apical cell membrane Cl- conductance. In cystic fibrosis airway epithelia, defective regulation of Cl- conductance decreases the capability to secrete Cl-. Here we report that extracytosolic ATP in the luminal bath of cultured human airway epithelia increased transepithelial Cl- secretion and apical membrane Cl- permeability. Single-channel studies in excised membrane patches revealed that ATP increased the open probability of outward rectifying Cl- channels. The latter effect occurs through a receptor mechanism that requires no identified soluble second messengers and is insensitive to probes of G protein function. These results demonstrate a mode of regulation of anion channels by binding ATP at the extracellular surface. Regulation of Cl- conductance by external ATP is preserved in cystic fibrosis airway epithelia.

P2 purinergic receptors and cellular calcium metabolism in A 431 human epidermoid carcinoma cells

Stimulation of P2 purinergic receptors on A 431 human epidermoid cells with ATP rapidly mobilized intracellular calcium and increased cytosolic free Ca2+ ([Ca2+]i). Incorporation of 45Ca2+ was also stimulated by ATP at a rate less than that of [Ca2+]i elevation. Among a number of nucleosides, nucleotides, and their analogues examined, ATP, GTP, UTP, ADP, UDP, adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S), and 5'-adenylylimidodiphosphate (AMP-PNP) increased both [Ca2+]i and 45Ca2+ influx, whereas others did not; these latter two analogues (ATP gamma S and AMP-PNP) blocked the ATP-stimulated 45Ca2+ influx only very slightly, suggesting that they are not prominent antagonists but rather agonists. A high correlation between [Ca2+]i increase and 45Ca2+ influx, in terms of nucleotide specificity, suggests the involvement of [Ca2+]i in influx of 45Ca2+. It appeared that [Ca2+]i elevated by several nucleotides or nucleotide analogues opened a calcium gate, thus allowing the influx of 45Ca2+. P2 purinergic receptors on these cells had such a characteristic that they were rapidly desensitized. These nucleotides or analogues also affected epidermal growth factor (EGF) receptors by inhibiting the EGF binding. The differences of ligand or substrate specificities between P2 purinergic receptors and ecto-nucleotidases indicates that the two components are different molecules involved in different systems.

Calcium-permeable channels in rat hepatoma cells are activated by extracellular nucleotides

Extracellular ATP is known to cause uptake of Ca2+ by rat liver cells. The specific pathway permitting influx of Ca2+ has not yet been identified. In the present investigations, we studied the properties of ATP-evoked 45Ca2+ uptake in rat hepatoma cell monolayers and then used patch-clamp electrophysiology to identify the channel that may account for this uptake. The results suggest that ATP-stimulated 45Ca2+ uptake occurs as a result of P2-purinergic receptor interaction because uptake was inhibited by Reactive Blue (100 microM), a blocker of this type of receptor. Furthermore, the ability of other adenine nucleotides to stimulate 45Ca2+ uptake was related to the selectivity sequence for binding to the P2-purinergic receptor. ATP-stimulated 45Ca2+ uptake occurs primarily through a conductance pore since it was inhibited by 70% upon dissipation of the membrane potential using the K+ ionophore valinomycin. The calcium channel blockers nifedipine and verapamil failed to inhibit 45Ca2+ uptake, but gadolinium (GdCl3) was an effective blocker. In cell-attached patch-clamp experiments, a single type of channel was activated with ATP (100 microM) addition to the bath in 18 of 32 trials. The current-voltage relationship of the ATP-activated channel is identical to that of the stretch-activated channel previously characterized in this laboratory as a calcium-permeable cation-nonselective channel [Am. J. Physiol. 258 (Cell Physiol. 27): C421-C428, 1990]. There are several lines of evidence which suggest that this cation-nonselective channel may account for ATP-stimulated 45Ca2+ uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Extracellular ATP and some of its analogs induce transient rises in cytosolic free calcium in individual canine keratinocytes

Changes in intracellular free calcium ([Ca++]i) play an important role in a variety of biochemical reactions that lead to cellular responses such as proliferation and differentiation. The response of [Ca++]i to extracellular nucleotides (ATP, UTP, ITP, and AMP-PNP) was determined in individual canine keratinocytes using the fluorescent probe fura-2 and digital video fluorescence imaging microscopy. In the presence of 1.8 mM extracellular Ca++, 100 and 500 microM ATP caused a rapid (less than 9 sec) three- to twelvefold rise in [Ca++]i above resting levels of 50-150 nM followed by occasional fluctuations. Small responses were elicited with doses as low as 0.1 microM ATP. The response of cells stimulated with 500 microM ATP in Ca(++)-free medium was characterized by 1.5 to 3 times rapid initial peak followed by a decrease of [Ca++]i below resting levels. Loss of response occurred in the majority of keratinocytes preincubated for 30 min in Ca(++)-free medium. UTP was as effective as ATP in stimulating rises in [Ca++]i in keratinocytes. Smaller elevations in [Ca++]i up to four- to fivefold resting levels were noted with 100 microM AMP-PNP or 500 microM ITP. Desensitization of cells was demonstrated when a second stimulation followed the primary ATP or UTP treatment. These results are suggestive of the presence of purinergic receptors in the cytoplasmic membrane of canine keratinocytes. Experiments using the calcium channel blocker lanthanum suggest that ATP-induced initial rises and sustained levels of [Ca++]i are dependent on the release of Ca++ from intracellular stores. These intracellular Ca++ stores appear to be rapidly depleted after removal of extracellular calcium ([Ca++]e), thereby abolishing ATP-induced [Ca++]i increases.

Multiple signal transduction pathways lead to extracellular ATP-stimulated mitogenesis in mammalian cells: II. A pathway involving arachidonic acid release, prostaglandin synthesis, and cyclic AMP accumulation

We have previously shown that extracellular ATP acts as a mitogen via protein kinase C (PKC)-dependent and independent pathways (Wang, D., Huang, N., Gonzalez, F.A., and Heppel, L.A. Multiple signal transduction pathways lead to extracellular ATP-stimulated mitogenesis in mammalian cells. I. Involvement of protein kinase C-dependent and independent pathways in the mitogenic response of mammalian cells to extracellular ATP. J. Cell. Physiol., 1991). The present aim was to determine if metabolism of arachidonic acid, resulting in prostaglandin E2 (PGE2) synthesis and elevation of cAMP levels, plays a role in mitogenesis mediated by extracellular ATP. Addition of ATP caused a marked enhancement of cyclic AMP accumulation in 3T3, 3T6, and A431 cells. Aminophylline, an antagonist of the adenosine A2 receptor, had no effect on the accumulation of cyclic AMP elicited by ATP, while it inhibited the action of adenosine. The accumulation of cyclic AMP was concentration dependent, which corresponds to the stimulation of DNA synthesis by ATP. The maximal accumulation was achieved after 45 min, with an initial delay period of about 15 min. That the activation of arachidonic acid metabolism contributed to cyclic AMP accumulation and mitogenesis stimulated by ATP in 3T3, 3T6, and A431 cells was supported by the following observations: (a) extracellular ATP stimulated the release of [3H]arachidonic acid and PGE2 into the medium; (b) inhibition of arachidonic acid release by inhibitors of phospholipase A2 blocked PGE2 production, cyclic AMP accumulation, and DNA synthesis activated by ATP, and this inhibition could be reversed by adding exogenous arachidonic acid; (c) cyclooxygenase inhibitors, such as indomethacin and aspirin, diminished the release of PGE2 and blocked cyclic AMP accumulation as well as [3H]thymidine incorporation in response to ATP; (d) PGE2 was able to restore [3H]thymidine incorporation when added together with ATP in the presence of cyclooxygenase inhibitors; (e) pertussis toxin inhibited ATP-stimulated DNA synthesis in a time- and dose-dependent fashion as well as arachidonic acid release and PGE2 formation. Other evidence for involvement of a pertussis toxin-sensitive G protein(s) in ATP-stimulated DNA synthesis as well as in arachidonic acid release is presented. In A431 cells, the enhancement of arachidonic acid and cyclic AMP accumulation by ATP was partially blocked by PKC down-regulation, implying that the activation of PKC may represent an additional pathway in ATP-stimulated metabolism of arachidonic acid. In all of these studies, ADP and AMP-PNP, but not adenosine, were as active as ATP.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple signal transduction pathways lead to extracellular ATP-stimulated mitogenesis in mammalian cells: I. Involvement of protein kinase C-dependent and -independent pathways

We recently reported that extracellular ATP was mitogenic for Swiss 3T3, 3T6, and A431 cells (Huang et al.: Proc. Natl. Acad. Sci. USA, 86:7904-7908, 1989). Here we examined the possible involvement of activation of the protein kinase C (PKC) signal transduction pathway in the mechanism of action of extracellular ATP. A potent synergistic stimulation of DNA synthesis in quiescent cultures of 3T3 and 3T6 cells was observed when ATP was presented in combination with growth factors that activate PKC, such as bombesin, vasopressin, or tumor-promoting phorbol esters. This finding suggests that ATP and these mitogens do not act through a common mechanism. In contrast, ATP was unable to show synergism with phorbol esters in A431 cells. We discovered striking differences when we examined the kinetics of formation of diacylglycerol (DAG) stimulated by ATP among these cell lines. Thus, ATP stimulated a sustained biphasic increase of DAG in A431 cells, but only a rapid transient increase of DAG formation was observed in 3T3 and 3T6 cells. The breakdown of phosphatidylcholine was stimulated by ATP in A431 cells; however, a significantly reduced effect was displayed in 3T6 cells. Furthermore, we found that the diacylglycerol-kinase inhibitor, 1-monooleoylglycerol, greatly potentiated ATP-stimulated DNA synthesis in A431 cells. Finally, down-regulation of PKC by long-term exposure to phorbol dibutyrate (PDBu) prevented stimulation of DNA synthesis induced by bombesin, vasopressin, or phorbol esters in 3T3 or 3T6 cells, while it had no such effect on ATP-stimulated mitogenesis in the presence of insulin or epidermal growth factor. On the other hand, PDBu-mediated down-regulation of PKC partially inhibited [3H [thymidine incorporation stimulated by ATP in A431 cells. Taken together, we conclude that a protein kinase C-dependent pathway is partially involved in ATP-stimulated DNA synthesis in A431 cells, but a protein kinase C-independent pathway exists in 3T3 and 3T6 cells. Pertussis toxin (PTX) inhibited the sustained phase of DAG formation and the breakdown of phosphatidylcholine stimulated by ATP in A431 cells. This suggests involvement of a PTX-sensitive G protein.

Comparison of extracellular ATP and UTP signalling in rat renal mesangial cells. No indications for the involvement of separate purino- and pyrimidino-ceptors

Extracellular ATP and UTP caused a rapid formation of InsP3, with similar kinetics and dose-dependences. ITP also displayed strong agonistic properties in terms of InsP3 production, whereas CTP was almost inactive. Pretreatment of the cells with pertussis toxin attenuated ATP- and UTP-stimulated InsP3 generation to a comparable extent, indicating that both nucleotides couple to phospholipase C by a pertussis-toxin-sensitive G-protein. Short-term (15 min) treatment of the cells with phorbol 12-myristate 13-acetate (PMA) produced a dose-dependent inhibition of ATP- and UTP-induced InsP3 formation. Furthermore, down-regulation of protein kinase C by long-term (24 h) exposure of the cells to PMA resulted in a comparable potentiation of phosphoinositide hydrolysis by both nucleotides. Preincubation of mesangial cells with ATP or UTP caused a pronounced cross-desensitization of subsequent nucleotide-stimulated InsP3 production. ATP and UTP displayed no additivity in terms of InsP3 formation, when used at maximally effective concentrations. In contrast, the peptide hormone angiotensin II interacted in an additive manner with either nucleotide in stimulating phosphoinositide hydrolysis. Reactive Blue 2, a putative P2y-purinoceptor antagonist, caused a rightward shift of both the ATP and UTP dose-response curves. However, since 2-methylthio-ATP was only a partial agonist in stimulating InsP3 formation, the mesangial-cell ATP receptor appears to be different from a classic P2y-receptor. In summary, these results provide no evidence for separate purino- and pyrimidino-ceptors on mesangial cells. In contrast, ATP and UTP may use a common nucleotide receptor for transducing their signals in mesangial cells.

Activation of early events of the mitogenic response by a P2Y purinoceptor with covalently bound 3'-O-(4-benzoyl)-benzoyladenosine 5'-triphosphate

3'-O-(4-Benzoyl)benzoyl-ATP (BzATP), a photoaffinity analog of ATP, was used as a ligand for a P2Y purinoceptor (adenine nucleotide receptor) present in intact Swiss 3T3 and 3T6 cells and A-431 epidermoid carcinoma cells. Photolysis of serum-starved cells in the presence of 10-50 microM BzATP, followed by extensive washing to remove unincorporated BzATP, induced the release of arachidonic acid. A trace (less than 0.01%) of photoincorporated BzATP was as effective as when 50 microM BzATP or ATP was contained in the incubation medium during the assay. Photoincorporated BzATP also stimulated the production of prostaglandin E2 and the accumulation of cyclic AMP. In previous studies, we demonstrated that these three events are obligatory early steps in a pathway leading to DNA synthesis in the above cell lines. The evidence indicated that the purinoceptor activated by extracellular ATP or BzATP was linked to a pertussis toxin-sensitive GTP-binding protein. Consistent with these observations, we now find that pertussis toxin inhibits the effect of photoincorporated BzATP on arachidonic acid release. These results indicate that BzATP is an effective agonist for the P2Y purinoceptor concerned with stimulation of DNA synthesis in 3T3, 3T6, and A-431 cells. Furthermore, after photolysis it becomes irreversibly associated with intact cells and promotes the activation of early events required for DNA synthesis.