P2u purinoceptor modulation of intracellular Ca2+ in a human lung adenocarcinoma cell line: down-regulation of Ca2+ influx by protein kinase C

Autocrine and paracrine purinergic signaling in the most lethal types of cancer

Cancer comprises a collection of diseases that occur in almost any tissue and it is characterized by an abnormal and uncontrolled cell growth that results in tumor formation and propagation to other tissues, causing tissue and organ malfunction and death. Despite the undeniable improvement in cancer diagnostics and therapy, there is an urgent need for new therapeutic and preventive strategies with improved efficacy and fewer side effects. In this context, purinergic signaling emerges as an interesting candidate as a cancer biomarker or therapeutic target. There is abundant evidence that tumor cells have significant changes in the expression of purinergic receptors, which comprise the G-protein coupled P2Y and AdoR families of receptors and the ligand-gated ion channel P2X receptors. Tumor cells also exhibit changes in the expression of nucleotidases and other enzymes involved in nucleotide metabolism, and the concentrations of extracellular nucleotides are significantly higher than those observed in normal cells. In this review, we will focus on the potential role of purinergic signaling in the ten most lethal cancers (lung, breast, colorectal, liver, stomach, prostate, cervical, esophagus, pancreas, and ovary), which together are responsible for more than 5 million annual deaths.

Dexamethasone reduces airway epithelial Cl- secretion by rapid non-genomic inhibition of KCNQ1, KCNN4 and KATP K+ channels

Basolateral membrane K⁺ channels play a key role in basal and agonist stimulated Cl^- transport across airway epithelial cells by generating a favourable electrical driving force for Cl^- efflux. The K⁺ channel sub-types and molecular mechanisms of regulation by hormones and secretagoues are still poorly understood. Here we have identified the type of K⁺ channels involved in cAMP and Ca²⁺ stimulated Cl^- secretion and uncovered a novel anti-secretory effect of dexamethasone mediated by inhibition of basolateral membrane K⁺ channels in a human airway cell model of 16HBE14o^- cells commonly used for ion transport studies. Dexamethasone produced a rapid inhibition of transepithelial chloride ion secretion under steady state conditions and after stimulation with cAMP agonist (forskolin) or a Ca²⁺ mobilizing agonist (ATP). Our results show three different types of K⁺ channels are targeted by dexamethasone to reduce airway secretion, namely Ca²⁺-activated secretion via KCNN4 (KCa3.1) channels and cAMP-activated secretion via KCNQ1 (Kv7.1) and KATP (Kir6.1,6.2) channels. The down-regulation of KCNN4 and KCNQ1 channel activities by dexamethasone involves rapid non-genomic activation of PKCα and PKA signalling pathways, respectively. Dexamethasone signal transduction for PKC and PKA activation was demonstrated to occur through a rapid non-genomic pathway that did not implicate the classical nuclear receptors for glucocorticoids or mineralocorticoids but occurred via a novel signalling cascade involving sequentially a G_i-protein coupled receptor, PKC, adenylyl cyclase Type IV, cAMP, PKA and ERK1/2 activation. The rapid, non-genomic, effects of dexamethasone on airway epithelial ion transport and cell signalling introduces a new paradigm for glucocorticoid actions in lung epithelia which may serve to augment the anti-inflammatory activity of the steroid and enhance its therapeutic potential in treating airway hypersecretion in asthma and COPD.

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.

Purinergic signaling in the airways

Evidence for a significant role and impact of purinergic signaling in normal and diseased airways is now beyond dispute. The present review intends to provide the current state of knowledge of the involvement of purinergic pathways in the upper and lower airways and lungs, thereby differentiating the involvement of different tissues, such as the epithelial lining, immune cells, airway smooth muscle, vasculature, peripheral and central innervation, and neuroendocrine system. In addition to the vast number of well illustrated functions for purinergic signaling in the healthy respiratory tract, increasing data pointing to enhanced levels of ATP and/or adenosine in airway secretions of patients with airway damage and respiratory diseases corroborates the emerging view that purines act as clinically important mediators resulting in either proinflammatory or protective responses. Purinergic signaling has been implicated in lung injury and in the pathogenesis of a wide range of respiratory disorders and diseases, including asthma, chronic obstructive pulmonary disease, inflammation, cystic fibrosis, lung cancer, and pulmonary hypertension. These ostensibly enigmatic actions are based on widely different mechanisms, which are influenced by the cellular microenvironment, but especially the subtypes of purine receptors involved and the activity of distinct members of the ectonucleotidase family, the latter being potential protein targets for therapeutic implementation.

Ca2+-dependent ATP release from A549 cells involves synergistic autocrine stimulation by coreleased uridine nucleotides

Extracellular ATP is a potent surfactant secretagogue but its origin in the alveolus, its mechanism(s) of release and its regulatory pathways remain unknown. Previously, we showed that hypotonic swelling of alveolar A549 cells induces Ca(2+)-dependent secretion of several adenosine and uridine nucleotides, implicating regulated exocytosis. In this study, we examined sources of Ca(2+) for the elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) evoked by acute 50% hypotonic stress and the role of autocrine purinergic signalling in Ca(2+)-dependent ATP release. We found that ATP release does not directly involve Ca(2+) influx from extracellular spaces, but depends entirely on Ca(2+) mobilization from intracellular stores. The [Ca(2+)](i) response consisted of slowly rising elevation, representing mobilization from thapsigargin (TG)-insensitive stores and a superimposed rapid spike due to Ca(2+) release from TG-sensitive endoplasmic reticulum (ER) Ca(2+) stores. The latter could be abolished by hydrolysis of extracellular triphospho- and diphosphonucleotides with apyrase; blocking P2Y(2)/P2Y(6) receptors of A549 cells with suramin; blocking UDP receptors (P2Y(6)) with pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid (PPADS); emptying TG-sensitive stores downstream with TG or caffeine in Ca(2+)-free extracellular solution; or blocking the Ca(2+)-release inositol 1,4,5-triphosphate receptor channel of the ER with 2-aminoethyldiphenylborinate. These data demonstrate that the rapid [Ca(2+)](i) spike results from the autocrine stimulation of IP(3)/Ca(2+)-coupled P2Y, predominantly P2Y(6), receptors, accounting for approximately 70% of total Ca(2+)-dependent ATP release evoked by hypotonic shock. Our study reveals a novel paradigm in which stress-induced ATP release from alveolar cells is amplified by the synergistic autocrine/paracrine action of coreleased uridine and adenosine nucleotides. We suggest that a similar mechanism of purinergic signal propagation operates in other cell types.

Tyrosine kinase modulation of protein kinase C activity regulates G protein-linked Ca2+ signaling in leukemic hematopoietic cells

We have used a recombinant mouse pre-B cell line (TonB210.1, expressing Bcr/Abl under the control of an inducible promoter) and several human leukemia cell lines to study the effect of high tyrosine kinase activity on G protein-coupled receptor (GPCR) agonist-stimulated cellular Ca(2+) release and store-operated Ca(2+) entry (SOCE). After induction of Bcr/Abl expression, GPCR-linked SOCE increased. The effect was reverted in the presence of the specific Abl inhibitor imatinib (1microM) and the Src inhibitor PP2 (10microM). In leukemic cell lines constitutively expressing high tyrosine kinase activity, Ca(2+) transients were reduced by imatinib and/or PP2. Ca(2+) transients were enhanced by specific inhibitors of PKC subtypes and this effect was amplified by tyrosine kinase inhibition in Bcr/Abl expressing TonB210.1 and K562 cells. Under all conditions Ca(2+) transients were essentially blocked by the PKC activator PMA. In Bcr/Abl expressing (but not in native) TonB210.1 cells, tyrosine kinase inhibitors enhanced PKCalpha catalytic activity and PKCalpha co-immunoprecipitated with Bcr/Abl. Unlike native TonB210.1 cells, Bcr/Abl expressing cells showed a high rate of cell death if Ca(2+) influx was reduced by complexing extracellular Ca(2+) with BAPTA. Our data suggest that tonic inhibition of PKC represents a mechanism by which high tyrosine kinase activity can enhance cellular Ca(2+) transients and thus exert profound effects on the proliferation, apoptosis and chemotaxis of leukemic cells.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Rapid non-genomic inhibition of ATP-induced Cl- secretion by dexamethasone in human bronchial epithelium

A non-genomic antisecretory role for dexamethasone at low concentrations (0.1 nM to1 microM) is described in monolayers of human bronchial epithelial cells in primary culture and in a continuous cell line (16HBE14o- cells). Dexamethasone produced a rapid decrease of [Ca(2+)](i) (measured with fura-2 spectrofluorescence) to a new steady-state concentration. After 15 min exposure to dexamethasone (1 nM), [Ca(2+)](i) was reduced by 32 +/- 11 nM (n = 7, P < 0.0001) from a basal value of 213 +/- 36 nM (n = 7). We have shown previously that aldosterone (1 nM) also produces a rapid fall in [Ca(2+)](i); however, after the decrease in [Ca(2+)](i) induced by dexamethasone, subsequent addition of aldosterone did not produced any further lowering of [Ca(2+)](i). The rapid response to dexamethasone was insensitive to pretreatment with cycloheximide and unaffected by the glucocorticoid type II and mineralocorticoid receptor antagonists RU486 and spironolactone, respectively. The rapid [Ca(2+)](i) decrease induced by dexamethasone was inhibited by the Ca(2+)-ATPase pump inhibitor thapsigargin (1 microM), the adenylate cyclase inhibitor MDL hydrochloride (500 microM) and the protein kinase A inhibitor Rp-adenosine 3',5'-cyclic monophosphorothioate (200 microM), but was not affected by the protein kinase C inhibitor, chelerythrine chloride (0.1 microM). Treatment of 16HBE14o- cell monolayers with dexamethasone (1 nM) inhibited the large and transient [Ca(2+)](i) increase induced by apical exposure to ATP (10(-4) M). Dexamethasone (1 nM) also reduced by 30 % the Ca(2+)-dependant Cl(-) secretion induced by apical exposure to ATP (measured as the Cl(-)-sensitive short-circuit current across monolayers mounted in Ussing chambers). Our results demonstrate, for the first time, that dexamethasone at low concentrations inhibits Cl(-) secretion in human bronchial epithelial cells. The rapid inhibition of Cl(-) secretion induced by the synthetic glucocorticoid is associated with a rapid decrease in [Ca(2+)](i) via a non-genomic mechanism that does not involve the classical glucocorticoid or mineralocorticoid receptor. Rather, it is a result of rapid non-genomic stimulation of thapsigargin-sensitive Ca(2+)-ATPase, via adenylate cyclase and protein kinase A signalling.

Uncoupling of ATP-induced inositol phosphate formation and Ca(2+) mobilization by phorbol ester in canine cultured tracheal epithelial cells

The regulation of the increase in inositol phosphates (IPs) production and intracellular Ca(2+) concentration ([Ca(2+)](i)) by protein kinase C (PKC) was investigated in canine cultured tracheal epithelial cells (TECs). Pretreatment of TECs with phorbol 12-myristate 13-acetate (PMA, 1 microM) for 30 min attenuated the ATP- and UTP-induced IPs formation and Ca(2+) mobilization. The concentrations of PMA that gave half-maximal (EC(50)) inhibition of ATP- and UTP-induced IPs accumulation and an increase in [Ca(2+)](i) were 5-10 and 4-12 nM, respectively. Prior treatment of TECs with staurosporine (1 microM), a PKC inhibitor, partially inhibited the ability of PMA to attenuate ATP- and UTP-induced responses, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. Furthermore, analysis of cell extracts by Western blotting with antibodies against different PKC isozymes revealed that TECs expressed PKC-alpha, -betaI, -betaII, -gamma, -delta, -epsilon, -theta, and -zeta. With PMA treatment of the cells for various times, translocation of PKC-alpha, -betaI, -betaII, -gamma, -delta, -epsilon, and -theta from the cytosol to the membrane was seen after 5- and 30-min and 2- and 4-h treatment. However, 6-h treatment caused a partial down-regulation of these PKC isozymes. PKC-zeta was not significantly translocated and down-regulated at any of the times tested. In conclusion, these results suggest that activation of PKC may inhibit the phosphoinositide (PI) hydrolysis and consequently attenuate the [Ca(2+)](i) increase or inhibit independently both responses to ATP and UTP. The translocation of PKC-alpha, -betaI, -betaII, -delta, -epsilon, -gamma, and -theta induced by PMA caused an attenuation of ATP- and UTP-induced IPs accumulation and Ca(2+) mobilization in TECs.

Adenosine triphosphate-evoked cytosolic calcium oscillations in human granulosa-luteal cells: role of protein kinase C

ATP has been shown to modulate progesterone production in human granulosa-luteal cells (hGLCs) in vitro. After binding to a G protein-coupled P2 purinergic receptor, ATP stimulates phospholipase C. The resultant production of diacylglycerol and inositol triphosphate activates protein kinase C (PKC) and intracellular calcium [Ca(2+)](i) mobilization, respectively. In the present study, we examined the potential cross-talk between the PKC and Ca(2+) pathway in ATP signal transduction. Specifically, the effect of PKC on regulating ATP-evoked [Ca(2+)](i) oscillations were examined in hGLCs. Using microspectrofluorimetry, [Ca(2+)](i) oscillations were detected in Fura-2 loaded hGLCs in primary culture. The amplitudes of the ATP-triggered [Ca(2+)](i) oscillations were reduced in a dose-dependent manner by pretreating the cells with various concentrations (1 nM to 10 microM) of the PKC activator, phorbol-12-myristate-13-acetate (PMA). A 10 microM concentration of PMA completely suppressed 10 microM ATP-induced oscillations. The inhibitory effect occurred even when PMA was given during the plateau phase of ATP evoked [Ca(2+)](i) oscillations, suggesting that extracellular calcium influx was inhibited. The role of PKC was further substantiated by the observation that, in the presence of a PKC inhibitor, bisindolylmaleimide I, ATP-induced [Ca(2+)](i) oscillations were not completely suppressed by PMA. Furthermore, homologous desensitization of ATP-induced calcium oscillations was partially reversed by bisindolylmaleimide I, suggesting that activated PKC may be involved in the mechanism of desensitization. These results demonstrate that PKC negatively regulates the ATP-evoked [Ca(2+)](i) mobilization from both intracellular stores and extracellular influx in hGLCs and further support a modulatory role of ATP and P2 purinoceptor in ovarian steroidogenesis.

Effect of erythromycin on ATP-induced intracellular calcium response in A549 cells

ATP induced a biphasic increase in the intracellular Ca(2+)concentration ([Ca(2+)](i)), an initial spike, and a subsequent plateau in A549 cells. Erythromycin (EM) suppressed the ATP-induced [Ca(2+)](i) spike but only in the presence of extracellular calcium (Ca(2+)(o)). It was ineffective against ATP- and UTP-induced inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] formation and UTP-induced [Ca(2+)](i) spike, implying that EM perturbs Ca(2+) influx from the extracellular space rather than Ca(2+)release from intracellular Ca(2+) stores via the G protein-phospholipase C-Ins(1,4,5)P(3) pathway. A verapamil-sensitive, KCl-induced increase in [Ca(2+)](i) and the Ca(2+) influx activated by Ca(2+) store depletion were insensitive to EM. 3'-O-(4-benzoylbenzoyl)-ATP evoked an Ca(2+)(o)-dependent [Ca(2+)](i) response even in the presence of verapamil or the absence of extracellular Na(+), and this response was almost completely abolished by EM pretreatment. RT-PCR analyses revealed that P2X(4) as well as P2Y(2), P2Y(4), and P2Y(6) are coexpressed in this cell line. These results suggest that in A549 cells 1) the coexpressed P2X(4) and P2Y(2)/P2Y(4) subtypes contribute to the ATP-induced [Ca(2+)](i) spike and 2) EM selectively inhibits Ca(2+) influx through the P2X channel. This action of EM may underlie its clinical efficacy in the treatment of airway inflammation.

Evidence that Ca2+-waves in Xenopus melanotropes depend on calcium-induced calcium release: a fluorescence correlation microscopy and linescanning study

The neuroendocrine melanotrope cell displays Ca2+ oscillations that are build up by several discrete Ca2+ rises ('steps'). Each step is linked to Ca2+-entry across the plasma membrane via voltage-operated calcium channels and associated with a fast Ca2+-wave travelling from the plasma membrane to the central parts of the cell. Previously, linescanning with confocal laser scanning microscopy (CLSM) supported that these waves have high speeds (between 30 and 80 microm/s), which is considered indicative of the involvement of a calcium-induced calcium release (CICR) mechanism in fast-wave propagation. However, to firmly establish the presence of a CICR mechanism one must rule out the possibility that the Ca2+ signal is artifactually accelerated by the presence of a highly mobile Ca2+ probe and also eliminate imaging artifacts inherent to single wavelength imaging. In the present study both problems are addressed. Mobility and intracellular distribution of a generally used Ca2+ probe, Oregon-green 488 BAPTA-1 (O-green-1), were established using fluorescence correlation microscopy. We then used the ratio signal of co-loaded O-green-1 and Fura-Red to quantify the relative [Ca2+]i during linescanning. It was found that O-green-1 displays different diffusion times when regions near the plasma membrane and in the center of the cell are compared. However, the calculated diffusion constant of the probe was too low to account for the observed high speed of the Ca2+ wave. In conclusion, we established the authenticity of the high speed of Ca2+-waves in Xenopus melanotropes, providing evidence for the involvement of a CICR mechanism in wave propagation.

Culture substrate-specific expression of P2Y2 receptors in distal lung epithelial cells isolated from foetal rats

ATP and UTP did not evoke [Ca2+]i signals in rat foetal lung epithelial cells grown on glass but elicited clear responses in cells grown into functionally polarised epithelia on permeable supports. Moreover, P2Y2 receptor mRNA could not be detected in cells on glass by the polymerase chain reaction but this mRNA species was clearly expressed by polarised cells. P2Y2 receptor expression thus appears to be a feature of the polarised phenotype.

Protein kinase C mediates P2U purinergic receptor inhibition of K+ channel in apical membrane of strial marginal cells

Strial marginal cells (SMC) electrogenically secrete K+ via slowly activating K+ (I[sK]) channels, consisting of I(sK) regulatory and KvLQT1 channel subunits, and the associated short circuit current (I[sc]) is inhibited by agonists of the apical P2U receptor [Liu et al., Audit. Neurosci. 2 (1995) 331-340]. Measurements of relative K+ flux (JK) with a self-referencing K+-selective probe demonstrated a decrease in JK after apical perfusion of 100 microM ATP. On-cell macro patch recordings from the apical membrane of gerbil SMC showed a decrease of the I(sK) channel current (I[IsK]) by 88 +/- 8% during pipette perfusion of 100 microM ATP. The magnitude of the decrease of L(sc) by ATP was diminished in the presence of inhibitors of phospholipase C (PLC) and protein kinase C (PKC), U-73122 and GF109203X. Activation of PKC by phorbol 12-myristate 13-acetate (20 nM) decreased I(IsK) (gerbil: by 62 +/- 10%; rat: by 72 +/- 6%) in perforated-patch whole-cell recordings while the inactive analog, 4alphaPMA, had no effect. By contrast, elevation of cytosolic [Ca2+] by A23187 increased the whole-cell I(IsK). The expression of the isk gene transcript was confirmed and the serine responsible for the species-specific response to PKC was found to be present in the gerbil I(sK) sequence. These data provide evidence consistent with a direct effect of the PKC branch of the PLC pathway on the I(sK) channel of SMC in response to activation of the apical P2U receptor and predict that the secretion of endolymph in the human cochlea may be controlled by PKC in the same way as in our animal model.

P2U purinergic receptor inhibits apical IsK/KvLQT1 channel via protein kinase C in vestibular dark cells

Vestibular dark cells (VDC) are known to electrogenically secrete K+ via slowly activating K+ (IsK) channels, consisting of IsK regulatory and KvLQT1 channel subunits, and the associated short-circuit current (Isc) is inhibited by agonists of the apical P2U (P2Y2) receptor (J. Liu, K. Kozakura, and D. C. Marcus. Audit. Neurosci. 2: 331-340, 1995). Measurements of relative K+ flux (JK) with a self-referencing K(+)-selective probe demonstrated a decrease in JK after apical perfusion of 100 microM ATP. On-cell macropatch recordings from gerbil VDC showed a decrease of the IsK channel current (IIsK) by 83 +/- 7% during pipette perfusion of 10 microM ATP. The magnitude of the decrease of Isc by ATP was diminished in the presence of inhibitors of phospholipase C (PLC) and protein kinase C (PKC), U-73122 and GF109203X. Activation of PKC by phorbol 12-myristate 13-acetate (PMA, 20 nM) decreased IIsK by 79 +/- 3% in perforated-patch whole cell recordings, whereas the inactive analog, 4 alpha-PMA, had no effect. In contrast, elevation of cytosolic Ca2+ concentration by A-23187 increased the whole cell IIsK. The expression of the isk gene transcript was confirmed, and the serine responsible for the species-specific response to PKC was found to be present in the gerbil IsK sequence. These data provide evidence consistent with a direct effect of the PKC branch of the PLC pathway on the IsK channel of VDC in response to activation of the apical P2U receptor and predict that the secretion of endolymph in the human vestibular system may be controlled by PKC in the same way as in our animal model.