Extracellular ATP and UTP activate the protein kinase B/Akt cascade via the P2Y(2) purinoceptor in renal mesangial cells

P2X7R/AKT/mTOR signaling mediates high glucose-induced decrease in podocyte autophagy

Diabetic nephropathy is one of the leading causes of end-stage renal disease worldwide. In our study we found that Adenosine triphosphate (ATP) content was significantly increased in the urine of diabetic mice. We examined the expression of all purinergic receptors in the renal cortex and found that only purinergic P2X7 receptor (P2X7R) expression was significantly increased in the renal cortex of wild-type diabetic mice and that the P2X7R protein partially co-localized with podocytes. Compared with P2X7R(-/-) non-diabetic mice, P2X7R(-/-) diabetic mice showed stable expression of the podocyte marker protein podocin in the renal cortex. The renal expression of microtubule associated protein 1A/1B light chain 3 (LC-3II) in wild-type diabetic mice was significantly lower than in wild-type controls, whereas the expression of LC-3II in the kidneys of P2X7R(-/-) diabetic mice was not significantly different from that of P2X7R(-/-) non-diabetic mice. In vitro, high glucose induced an increase in p-Akt/Akt, p-mTOR/mTOR and p62 protein expression along with a decrease in LC-3II levels in podocytes, whereas after transfection with P2X7R siRNA, Phosphorylated serine/threonine kinase (p-Akt)/Akt, Phosphorylated mechanistic target of rapamycin (p-mTOR)/mTOR, and p62 expression were restored and LC-3II expression was increased. In addition, LC-3II expression was also restored after inhibition of Akt and mTOR signaling with MK2206 and rapamycin, respectively. Our results suggest that P2X7R expression is increased in podocytes in diabetes, and that P2X7R is involved in the inhibition of podocyte autophagy by high glucose, at least in part through the Akt-mTOR pathway, thereby exacerbating podocyte damage and promoting the onset of diabetic nephropathy. Targeting P2X7R may be a potential treatment for diabetic nephropathy.

Novel compounds with dual S1P receptor agonist and histamine H3 receptor antagonist activities act protective in a mouse model of multiple sclerosis

The sphingosine 1-phosphate (S1P) receptor 1 (S1P₁) has emerged as a therapeutic target for the treatment of multiple sclerosis (MS). Fingolimod (FTY720) is the first functional antagonist of S1P₁ that has been approved for oral treatment of MS. Previously, we have developed novel butterfly derivatives of FTY720 that acted similar to FTY720 in reducing disease symptoms in a mouse model of experimental autoimmune encephalomyelitis (EAE). In this study, we have synthesized a piperidine derivative of the oxazolo-oxazole compounds, denoted ST-1505, and its ring-opened analogue ST-1478, and characterised their in-vitro and in-vivo functions. Notably, the 3-piperidinopropyloxy moiety resembles a structural motif of pitolisant, a drug with histamine H₃R antagonistic/inverse agonist activity approved for the treatment of narcolepsy. Both novel compounds exerted H₃R affinities, and in addition, ST-1505 was characterised as a dual S1P₁₊₃ agonist, whereas ST-1478 was a dual S1P₁₊₅ agonist. Both multitargeting compounds were also active in mice and reduced the lymphocyte numbers as well as diminished disease symptoms in the mouse model of MS. The effect of ST-1478 was dependent on SK-2 activity suggesting that it is a prodrug like FTY720, but with a more selective S1P receptor activation profile, whereas ST-1505 is a fully active drug even in the absence of SK-2. In summary, these data suggest that the well soluble piperidine derivatives ST-1505 and ST-1478 hold promise as novel drugs for the treatment of MS and other autoimmune or inflammatory diseases, and by their H₃R antagonist potency, they might additionally improve cognitive impairment during disease.

P2Y2 purinergic receptor modulates virus yield, calcium homeostasis, and cell motility in human cytomegalovirus-infected cells

Human cytomegalovirus (HCMV) manipulates many aspects of host cell biology to create an intracellular milieu optimally supportive of its replication and spread. Our study reveals that levels of several components of the purinergic signaling system, including the P2Y2 and P2X5 receptors, are elevated in HCMV-infected fibroblasts. Knockdown and drug treatment experiments demonstrated that P2Y2 enhances the yield of virus, whereas P2X5 reduces HCMV production. The HCMV IE1 protein induces P2Y2 expression; and P2Y2-mediated signaling is important for efficient HCMV gene expression, DNA synthesis, and the production of infectious HCMV progeny. P2Y2 cooperates with the viral UL37x1 protein to regulate cystolic Ca2+ levels. P2Y2 also regulates PI3K/Akt signaling and infected cell motility. Thus, P2Y2 functions at multiple points within the viral replication cycle to support the efficient production of HCMV progeny, and it may facilitate in vivo viral spread through its role in cell migration.

Targeting Purinergic Receptor P2Y2 Prevents the Growth of Pancreatic Ductal Adenocarcinoma by Inhibiting Cancer Cell Glycolysis

PURPOSE

Extensive research has reported that the tumor microenvironment components play crucial roles in tumor progression. Thus, blocking the supports of tumor microenvironment is a promising approach to prevent cancer progression. We aimed to determine whether blocking extracellular ATP-P2RY2 axis could be a potential therapeutic approach for PDAC treatment.

EXPERIMENTAL DESIGN

Expression of P2RY2 was determined in 264 human PDAC samples and correlated to patient survival. P2RY2 was inhibited in human PDAC cell lines by antagonist and shRNA, respectively, and cell viability, clonogenicity, and glycolysis were determined. RNA sequencing of PDAC cell line was applied to reveal underlying molecular mechanisms. Multiple PDAC mouse models were used to assess the effects of the P2RY2 inhibition on PDAC progression.

RESULTS

P2RY2 was upregulated and associated with poor prognosis in PDAC. Activated P2RY2 by increased extracellular ATP in tumor microenvironment promoted PDAC growth and glycolysis. Further studies showed that the agonist-activated P2RY2 triggered PI3K/AKT-mTOR signaling by crosstalk with PDGFR mediated by Yes1, resulting in elevated expression of c-Myc and HIF1α, which subsequently enhanced cancer cell glycolysis. Genetic and pharmacologic inhibition of P2RY2 impaired tumor cell growth in subcutaneous and orthotopic xenograft model, as well as delayed tumor progression in inflammation-driven PDAC model. In addition, synergy was observed when AR-C118925XX, the selective antagonist of P2RY2 receptor, and gemcitabine were combined, resulting in prolonged survival of xenografted PDAC mice.

CONCLUSIONS

These findings reveal the roles of the P2RY2 in PDAC metabolic reprogramming, suggesting that P2RY2 might be a potential metabolic therapeutic target for PDAC.

Anti-proliferative Effects of Nucleotides on Gastric Cancer via a Novel P2Y6/SOCE/Ca2+/β-catenin Pathway

Although purinegic signaling is important in regulating gastric physiological functions, it is currently unknown for its role in gastric cancer (GC). We demonstrate for the first time that the expression of P2Y6 receptors was markedly down-regulated in human GC cells and primary GC tissues compared to normal tissues, while the expression of P2Y2 and P2Y4 receptors was up-regulated in GC cells. Moreover, the expression levels of P2Y6 receptors in GC tissues were correlated to tumor size, differentiation, metastasis to lymph nodes, and the survival rate of the patients with GC. Ncleotides activated P2Y6 receptors to raise cytosolic Ca²⁺ concentrations in GC cells through store-operated calcium entry (SOCE), and then mediated Ca²⁺-dependent inhibition of β-catenin and proliferation, eventually leading to GC suppression. Furthermore, UTP particularly blocked the G1/S transition of GC cells but did not induce apoptosis. Collectively, we conclude that nucleotides activate P2Y6 receptors to suppress GC growth through a novel SOCE/Ca²⁺/β-catenin-mediated anti-proliferation of GC cells, which is different from the canonical SOCE/Ca²⁺-induced apoptosis in other tumors.

Purinergic receptors in tubulointerstitial inflammatory cells: a pathophysiological mechanism of salt-sensitive hypertension

Recent studies have suggested that both the tubulointerstitial inflammatory cells and the activation of purinergic receptors integrate common mechanisms that result in salt-sensitive hypertension. The basis of this hypothesis is that renal endothelial cells release ATP in response to shear stress in the setting of hypertension. It has been demonstrated that the over-expression and activation of the P2X7, P2Y12 and P2X1 receptors favour the elevation of blood pressure induced by high-salt intake. In addition, the release of interleukins and inflammatory mediators in the tubulointerstitial area appears to be related to the activation of these receptors. Renal vasoconstriction and tubulointerstitial injury develop as a result, which increase sodium reabsorption by epithelial cells. Consistent with these effects, the reduction of tubulointerstitial inflammation caused by immunosuppressants, such as mycophenolate mofetil, prevents the development of salt-sensitive hypertension. Also, P2X7-receptor knockout mice develop minor renal injury when hypertension is induced via the administration of deoxycorticosterone acetate and a high-salt diet. In the setting of angiotensin II-induced hypertension, which is an early stage in the development of salt-sensitive hypertension, an acute blockade with the specific, non-selective P2 antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid prevented the renal vasoconstriction induced by angiotensin II. In addition, it normalized glomerular haemodynamics and restored sodium excretion to control values. These findings suggest that chronic administration of P2 purinergic antagonists may prevent the deleterious effects of purinergic receptors during the development of salt-sensitive hypertension.

Purinergic signalling in the kidney in health and disease

The involvement of purinergic signalling in kidney physiology and pathophysiology is rapidly gaining recognition and this is a comprehensive review of early and recent publications in the field. Purinergic signalling involvement is described in several important intrarenal regulatory mechanisms, including tuboglomerular feedback, the autoregulatory response of the glomerular and extraglomerular microcirculation and the control of renin release. Furthermore, purinergic signalling influences water and electrolyte transport in all segments of the renal tubule. Reports about purine- and pyrimidine-mediated actions in diseases of the kidney, including polycystic kidney disease, nephritis, diabetes, hypertension and nephrotoxicant injury are covered and possible purinergic therapeutic strategies discussed.

The P2Y2/Src/p38/COX-2 pathway is involved in the resistance to ursolic acid-induced apoptosis in colorectal and prostate cancer cells

One of the hallmarks of cancer is resistance to apoptosis. Elucidating the mechanisms of how cancer cells evade or delay apoptosis should lead to novel therapeutic strategies. Previously, we showed that HT-29 colorectal cancer cells undergoing apoptosis overexpressed cyclooxygenase-2 (COX-2), in a p38 dependent pathway, to delay ursolic acid-induced apoptosis. Here, we focused on elucidating the upstream signaling pathways regulating this resistance mechanism. The role of ATP as an extracellular signaling molecule took a long time to be accepted. In recent years, ATP and its analogs, via the activation of specific purinergic receptors, have been implicated in many biological processes including cell proliferation, differentiation and apoptosis. In the present report, we have demonstrated a novel role involving purinergic receptors and particularly the P2Y(2) receptor in resistance to ursolic acid-induced apoptosis in both colorectal HT-29 and prostate DU145 cancer cells. We found that ursolic acid induced an increase in intracellular ATP and P2Y(2) transcript levels. Upon activation, P2Y(2) activated Src which in turn phosphorylated p38 leading to COX-2 overexpression which induced resistance to apoptosis in both HT-29 and DU145 cells. Furthermore, Ca(2+)-independent PLA(2) (iPLA(2)) and Ca(2+)-dependent secretory PLA(2) (sPLA(2)) were responsible for arachidonic acid release, the substrate of COX-2. Our findings document that apoptosis triggering was dependent on protein kinase C (PKC) activation in both cell lines after ursolic acid treatment.

Extracellular ATP attenuates ischemia-induced caspase-3 cleavage in human endothelial cells

BACKGROUND

Apoptotic death of endothelial cells (EC) plays a crucial role for the development of ischemic injury. In the present study we investigated the impact of extracellular Adenosine-5'-triphosphate (ATP), either released from cells or exogenously added, on ischemia-induced apoptosis of human EC.

METHODS AND RESULTS

To simulate ischemic conditions, cultured human umbilical vein endothelial cells (HUVEC) were exposed to 2 h of hypoxia (Po(2)<4mm Hg) in serum-free medium. Ischemia led to a 1.7-fold (+/-0.4; P<0.05) increase in EC apoptosis compared to normoxic controls as assessed by immunoblotting and immunocytochemistry of cleaved caspase-3. Ischemia-induced apoptosis was accompanied by a 2.3-fold (+/-0.5; P<0.05) increase of extracellular ATP detected by using a luciferin/luciferase assay. Addition of the soluble ecto-ATPase apyrase, enhancing ATP degradation, increased ischemia-induced caspase-3 cleavage. Correspondingly, inhibition of ATP breakdown by addition of the selective ecto-ATPase inhibitor ARL67156 significantly reduced ischemia-induced apoptosis. Extracellular ATP acts on membrane-bound P2Y- and P2X-receptors to induce intracellular signaling. Both, ATP and the P2Y-receptor agonist UTP significantly reduced ischemia-induced apoptosis in an equipotent manner, whereas the P2X-receptor agonist αβ-me-ATP did not alter caspase-3 cleavage. The anti-apoptotic effects of ARL67156 and UTP were abrogated when P2-receptors were blocked by Suramin or PPADS. Furthermore, extracellular ATP led to an activation of MEK/ERK- and PI3K/Akt-signaling pathways. Accordingly, inhibition of MEK/ERK-signaling by UO126 or inhibition of PI3K/Akt-signaling by LY294002 abolished the anti-apoptotic effects of ATP.

CONCLUSION

The data of the present study indicate that extracellular ATP counteracts ischemia-induced apoptosis of human EC by activating a P2Y-receptor-mediated signaling reducing caspase-3 cleavage.

Aspects of astrocyte energy metabolism, amino acid neurotransmitter homoeostasis and metabolic compartmentation

Astrocytes are key players in brain function; they are intimately involved in neuronal signalling processes and their metabolism is tightly coupled to that of neurons. In the present review, we will be concerned with a discussion of aspects of astrocyte metabolism, including energy-generating pathways and amino acid homoeostasis. A discussion of the impact that uptake of neurotransmitter glutamate may have on these pathways is included along with a section on metabolic compartmentation.

Insulin and AMPK regulate FA translocase/CD36 plasma membrane recruitment in cardiomyocytes via Rab GAP AS160 and Rab8a Rab GTPase

The FA translocase cluster of differentiation 36 (CD36) facilitates FA uptake by the myocardium, and its surface recruitment in cardiomyocytes is induced by insulin, AMP-dependent protein kinase (AMPK), or contraction. Dysfunction of CD36 trafficking contributes to disordered cardiac FA utilization and promotes progression to disease. The Akt substrate 160 (AS160) Rab GTPase-activating protein (GAP) is a key regulator of vesicular trafficking, and its activity is modulated via phosphorylation. Our study documents that AS160 mediates insulin or AMPK-stimulated surface translocation of CD36 in cardiomyocytes. Knock-down of AS160 redistributes CD36 to the surface and abrogates its translocation by insulin or the AMPK agonist 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR). Conversely, overexpression of a phosphorylation-deficient AS160 mutant (AS160 4P) suppresses the stimulated membrane recruitment of CD36. The AS160 substrate Rab8a GTPase is shown via overexpression and knock-down studies to be specifically involved in insulin/AICAR-induced CD36 membrane recruitment. Our findings directly demonstrate AS160 regulation of CD36 trafficking. In myocytes, the AS160 pathway also mediates the effect of insulin, AMPK, or contraction on surface recruitment of the glucose transporter GLUT4. Thus, AS160 constitutes a point of convergence for coordinating physiological regulation of CD36 and GLUT4 membrane recruitment.

Interaction of purinergic receptors with GPCRs, ion channels, tyrosine kinase and steroid hormone receptors orchestrates cell function

Extracellular purines and pyrimidines have emerged as key regulators of a wide range of physiological and pathophysiological cellular processes acting through P1 and P2 cell surface receptors. Increasing evidence suggests that purinergic receptors can interact with and/or modulate the activity of other classes of receptors and ion channels. This review will focus on the interactions of purinergic receptors with other GPCRs, ion channels, receptor tyrosine kinases, and steroid hormone receptors. Also, the signal transduction pathways regulated by these complexes and their new functional properties are discussed.

The therapeutic potential of adenosine triphosphate as an immune modulator in the treatment of HIV/AIDS: a combination approach with HAART

Extracellular adenosine triphosphate (eATP) is a potent molecule that has the capacity to modulate various aspects of cell functions including gene expression. This element of modulation is essential to the role of ATP as a therapeutic agent. The hypothesis presented is that ATP can have an important impact on the treatment of HIV infection. This is supported in part by published research, although a much greater role for ATP is suggested than prior authors ever thought possible. ATP has the ability to enhance the immune system and could thus improve the host's own defense mechanisms to eradicate the virus-infected cells and restore normal immune function. This could provide effective therapy when used in conjunction with highly active antiretroviral therapies (HAART) to eliminate the latently infected cells. The key lies in applying ATP through the methodology described. This article presents a strategy for using ATP therapeutically along with background evidence to substantiate the importance of using ATP in the treatment of HIV infection.

Extracellular nucleotide derivatives protect cardiomyocytes against hypoxic stress

RATIONALE

Extracellular nucleotides have widespread effects and various cell responses. Whereas the effect of a purine nucleotide (ATP) and a pyrimidine nucleotide (UTP) on myocardial infarction has been examined, the role of different purine and pyrimidine nucleotides and nucleosides in cardioprotection against hypoxic stress has not been reported.

OBJECTIVE

To investigate the role of purine and pyrimidine nucleotides and nucleosides in protective effects in cardiomyocytes subjected to hypoxia.

METHODS AND RESULTS

Rat cultured cardiomyocytes were treated with various extracellular nucleotides and nucleosides, before or during hypoxic stress. The results revealed that GTP or CTP exhibit cardioprotective ability, as revealed by lactate dehydrogenase (LDH) release, by propidium iodide (PI) staining, by cell morphology, and by preserved mitochondrial activity. Pretreatment with various P2 antagonists (suramin, RB-2, or PPADS) did not abolish the cardioprotective effect of the nucleotides. Moreover, P2Y₂ -/- , P2Y₄ -/-, and P2Y₂ -/-/P2Y₄ -/- receptor knockouts mouse cardiomyocytes were significantly protected against hypoxic stress when treated with UTP. These results indicate that the protective effect is not mediated via those receptors. We found that a wide variety of triphosphate and diphosphate nucleotides (TTP, ITP, deoxyGTP, and GDP), provided significant cardioprotective effect. GMP, guanosine, and ribose phosphate provided no cardioprotective effect. Moreover, we observed that tri/di-phosphate alone assures cardioprotection. Treatment with extracellular nucleotides, or with tri/di-phosphate, administered under normoxic conditions or during hypoxic conditions, led to a decrease in reactive oxygen species production.

CONCLUSIONS

Extracellular tri/di-phosphates are apparently the molecule responsible for cardioprotection against hypoxic damage, probably by preventing free radicals formation.

Changes in cytosolic glucose level in ATP stimulated live astrocytes

Astrocytes which lie between brain capillaries and neuronal terminals are the primary site of glucose uptake and have a key role in coupling synaptic activity to glucose utilization in the central nervous system (CNS). We used a fluorescence resonance energy transfer (FRET) based approach to monitor cytosolic glucose in astrocytes. We determined the effect of increasing extracellular glucose concentrations on FRET ratio as a measure of increased cytosolic glucose in astrocytes. By briefly raising extracellular glucose concentration, astrocytes responded promptly by increased cytosolic glucose levels, which was manifested by decreased time-dependent FRET ratio. The FRET ratio fall-time recorded at low extracellular D-glucose concentration change (from 0 to 0.5 mM) was 53 s, whereas 17 s was recorded by raising extracellular concentration of D-glucose from 0 to 10 mM, which is likely due to facilitated d-glucose entry along the increased D-glucose gradient across the plasmalemma. The relationship between the extracellular glucose concentration and the FRET ratio change is limited to the maximal ratio change, where the D-glucose plasma membrane permeability is balanced by the cytosolic utilization. We measured the effect of extracellular ATP, an important extracellular messenger for astrocyte-to-astrocyte communication, on intracellular glucose concentration. The results show that stimulation of astrocytes with ATP (1 mM) decreases cytosolic glucose concentration with a time constant of ∼145 s. The mechanism of this change is discussed.

ATP stimulates the proliferation of MCF-7 cells through the PI3K/Akt signaling pathway

We studied the modulation of the PI3K/Akt signaling pathway by ATP in MCF-7 cells. Western blot analysis showed that ATP stimulated the phosphorylation of Akt in a dose- and time-dependent manner. Akt phosphorylation in response to nucleotides followed the potency order ATP=UTP=ATPgammaS>>ADP=UDP>ADPbetaS=adenosine, suggesting participation of P2Y(2/4) receptors. Inhibitors of PI3K, PLC, PKC and Src or Src antisense oligonucleotides prevented ATP-induced phosphorylation of Akt. Incubation of cells with 2-APB or in a nominally Ca(2+)-free medium plus EGTA showed that Akt phosphorylation by ATP depends on intracellular calcium release but is independent of calcium influx. The PI3K inhibitor was not effective in reducing MAPKs phosphorylation by ATP. ATP and UTP stimulated MCF-7 cell proliferation, effect that was inhibited by PI3K, PLC, PKC, Src and MAPKs inhibitors. These findings suggest that ATP modulation of P2Y(2/4) receptors increases MCF-7 cell proliferation by activation of the PI3K/Akt signaling pathway through PLC/IP(3)/Ca(2+), PKC and Src.

Glucocorticoids protect renal mesangial cells from apoptosis by increasing cellular sphingosine-1-phosphate

Neutral ceramidase (NCDase) and sphingosine kinases (SphKs) are key enzymes regulating cellular sphingosine-1-phosphate (S1P) levels. In this study we found that stress factor-induced apoptosis of rat renal mesangial cells was significantly reduced by dexamethasone treatment. Concomitantly, dexamethasone increased cellular S1P levels, suggesting an activation of sphingolipid-metabolizing enzymes. The cell-protective effect of glucocorticoids was reversed by a SphK inhibitor, was completely absent in SphK1-deficient cells, and was associated with upregulated mRNA and protein expression of NCDase and SphK1. Additionally, in vivo experiments in mice showed that dexamethasone also upregulated SphK1 mRNA and activity, and NCDase protein expression in the kidney. Fragments (2285, 1724, and 1126 bp) of the rat NCDase promoter linked to a luciferase reporter were transfected into rat kidney fibroblasts and mesangial cells. There was enhanced NCDase promoter activity upon glucocorticoids treatment that was abolished by the glucocorticoid receptor antagonist RU-486. Single and double mutations of the two putative glucocorticoid response element sites within the promoter reduced the dexamethasone effect, suggesting that both glucocorticoid response elements are functionally active and required for induction. Our study shows that glucocorticoids exert a protective effect on stress-induced mesangial cell apoptosis in vitro and in vivo by upregulating NCDase and SphK1 expression and activity, resulting in enhanced levels of the protective lipid second messenger S1P.

P2 receptor-mediated stimulation of the PI3-K/Akt-pathway in vivo

ATP acts as a growth factor as well as a toxic agent by stimulating P2 receptors. The P2 receptor-activated signaling cascades mediating cellular growth and cell survival after injury are only incompletely understood. Therefore, the aim of the present study was to identify the role of the phosphoinositide 3 kinase (PI3-K/Akt) and the mitogen-activated protein kinase/extracellular signal regulated protein kinase (MAPK/ERK) pathways in P2Y receptor-mediated astrogliosis after traumatic injury and after microinfusion of ADP beta S (P2Y(1,12,13) receptor agonist) into the rat nucleus accumbens (NAc). Mechanical damage and even more the concomitant treatment with ADP beta S, enhanced P2Y(1) receptor-expression in the NAc, which could be reduced by pretreatment with the P2X/Y receptor antagonist PPADS. Quantitative Western blot analysis indicated a significant increase in phosphorylated (p)Akt and pERK1/2 2 h after ADP beta S-microinjection. Pretreatment with PPADS or wortmannin abolished the up-regulation of pAkt by injury alone or ADP beta S-treatment. The ADP beta S-enhanced expression of the early apoptosis marker active caspase 3 was reduced by PPADS and PD98059, but not by wortmannin. Multiple immunofluorescence labeling indicated a time-dependent expression of pAkt and pMAPK on astrocytes and neurons and additionally the colocalization of pAkt, pMAPK, and active caspase 3 with the P2Y(1) receptor especially at astrocytes. In conclusion, the data show for the first time the involvement of PI3-K/Akt-pathway in processes of injury-induced astroglial proliferation and anti-apoptosis via activation of P2Y(1) receptors in vivo, suggesting specific roles of P2 receptors in glial cell pathophysiology in neurodegenerative diseases.

Involvement of UTP in protection of cardiomyocytes from hypoxic stress

Massive amounts of nucleotides are released during ischemia in the cardiovascular system. Although the effect of the purine nucleotide ATP has been intensively studied in myocardial infarction, the cardioprotective role of the pyrimidine nucleotide UTP is still unclear, especially in the cardiovascular system. The purpose of our study was to elucidate the protective effects of UTP receptor activation and describe the downstream cascade for the cardioprotective effect. Cultured cardiomyocytes and left anterior descending (LAD)-ligated rat hearts were pretreated with UTP and exposed to hypoxia-ischemia. In vitro experiments revealed that UTP reduced cardiomyocyte death induced by hypoxia, an effect that was diminished by suramin. UTP caused several effects that could trigger a cardioprotective response: a transient increase of [Ca2+]i, an effect that was abolished by PPADS or RB2; phosphorylation of the kinases ERK and Akt, which was abolished by U0126 and LY294002, respectively; and reduced mitochondrial calcium elevation after hypoxia. In vivo experiments revealed that UTP maintained ATP levels, improved mitochondrial activity, and reduced infarct size. In conclusion, UTP administrated before ischemia reduced infarct size and improved myocardial function. Reduction of mitochondrial calcium overload can partially explain the protective effect of UTP after hypoxic-ischemic injury.

Increased Uridine Adenosine Tetraphosphate Concentrations in Plasma of Juvenile Hypertensives

BACKGROUND

Uridine adenosine tetraphosphate (Up4A) was been recently characterized as a potent vasoconstrictor. Up4A occurs in plasma from healthy subjects at concentrations sufficient to cause strong vasoconstrictive effects. In this study, Up4A concentrations in plasma from juvenile hypertensives and normotensives were determined.

METHODS AND RESULTS

Up4A was purified to homogeneity by preparative reverse phase high performance liquid-chromatography (HPLC), affinity chromatography HPLC, and analytic reverse phase HPLC from deproteinized plasma of juvenile hypertensives and normotensives. Mean total plasma Up4A concentration was significantly increased in juvenile hypertensives compared with juvenile normotensives (33.0+/-25.4 versus 3.7+/-0.9 nmol/L; mean+/-SEM, n=40 and 38, respectively; P<0.005). Accordingly, Up4A showed a significant association with juvenile hypertension (OR for ln(Up4A): 1.82; 95% CI 1.12, 2.95). Plasma Up4A concentrations correlated with left ventricular mass (Kendall-tau correlation coefficient 0.220, n=40; P<0.05) and intima media wall thickness (Kendall-tau correlation coefficient 0.296, n=40; P<0.05) in the hypertensives. Because the increased intima media thickness may be related to proliferative effects of Up4A, we studied the effects of Up4A on human vascular smooth muscle cell proliferation. The maximum proliferative effect of Up4A was 80.0+/-24.0% % above control (P<0.01). The proliferative effect of Up4A on smooth muscle cells is cell cycle-dependent, involving stimulation of S phase entry.

CONCLUSION

Circulating levels of Up4A are strongly associated with juvenile hypertension. The endothelium-derived vasoconstrictor Up4A may contribute to the early development of primary hypertension and is moreover an important risk factor of juvenile hypertension.

Protein kinase C alpha-dependent phosphorylation of the mRNA-stabilizing factor HuR: implications for posttranscriptional regulation of cyclooxygenase-2

In this study, we investigated the molecular mechanisms underlying the ATP analogue adenosine-5'-O-(3-thio)triphosphate-induced nucleocytoplasmic shuttling of the mRNA stabilizing factor HuR in human (h) mesangial cells (MC). Using synthetic protein kinase C (PKC) inhibitors and small interfering RNA approaches, we demonstrated that knockdown of PKC alpha efficiently blocked the ATP-dependent nuclear HuR export to the cytoplasm. The functional importance of PKC alpha in HuR shuttling is highlighted by the high cytosolic HuR content detected in hMC stably overexpressing PKC alpha compared with mock-transfected cells. The ATP-induced recruitment of HuR to the cytoplasm is preceded by a direct interaction of PKC alpha with nuclear HuR and accompanied by increased Ser phosphorylation as demonstrated by coimmunoprecipitation experiments. Mapping of putative PKC target sites identified serines 158 and 221 as being indispensable for HuR phosphorylation by PKC alpha. RNA pull-down assay and RNA electrophoretic mobility shift assay demonstrated that the HuR shuttling by ATP is accompanied by an increased HuR binding to cyclooxygenase (COX)-2 mRNA. Physiologically, the ATP-dependent increase in RNA binding is linked with an augmentation in COX-2 mRNA stability and subsequent increase in prostaglandin E(2) synthesis. Regulation of HuR via PKC alpha-dependent phosphorylation emphasizes the importance of posttranslational modification for stimulus-dependent HuR shuttling.

Extracellular adenosine triphosphate protects oxidative stress-induced increase of p21(WAF1/Cip1) and p27(Kip1) expression in primary cultured renal proximal tubule cells: role of PI3K and Akt signaling

Oxidative stress, the result of cellular production of reactive oxygen species (ROS), has been implicated in causing many renal diseases. Adenosine triphosphate (ATP) is an important extracellular signal in the regulation of many intracellular processes in normal tubular cells as well as in the pathogenesis of cell injury. This study investigated the effect of ATP on H(2)O(2)-induced increase of cyclin kinase inhibitors (CKI) expression and its related signal molecules in primary cultured renal proximal tubule cells (PTCs). H(2)O(2) inhibited DNA synthesis in a concentration- (>50 microM) and time-dependent manner (>2 h), as determined by thymidine and BrdU incorporation, and by increase in the p21(WAF/Cip1) and p27(Kip1) expression levels. In contrast, ATP increased the level of thymidine, BrdU incorporation (>10(-5) M), and decreased the p21(WAF/Cip1) and p27(Kip1) expression levels, suggesting that ATP has a protective effect against H(2)O(2)-induced oxidative damage. Suramin, reactive blue 2 (RB-2), MRS 2159, and MRS 2179 did block the reversing effect of ATP. In addition, AMP-CPP or 2-methylthio-ATP blocked H(2)O(2)-induced inhibition of DNA synthesis, suggesting all these P2 purinoceptors may be potentially involved. ATP-induced stimulation of DNA synthesis was blocked by phosphatidylinositol 3-kinase (PI3K) and Akt inhibitors. These results suggest the involvement of P2 purinoceptors-mediated PI3K/Akt signal pathway in the protective effect of ATP against H(2)O(2)-induced oxidative damage. Indeed, pre-treatment with PI3K or Akt inhibitors did not protect H(2)O(2)-induced lipid peroxide (LPO) production and inhibition of thymidine incorporation. In conclusion, ATP, in part, blocked H(2)O(2)-induced increase of p21(WAF1/Cip1) and p27(Kip1) expression through PI3K and Akt signal pathway in renal PTCs.

Multiple P2X receptors are involved in the modulation of apoptosis in human mesangial cells: evidence for a role of P2X4

Apoptosis, a normal event in renal tissue homeostasis, has been considered as a major mechanism for either resolution of glomerular hypercellularity in glomerulonephritis or loss of cellularity and progression to glomerulosclerosis in chronic renal disease. This study was aimed at investigating the role of extracellular ATP (eATP) in mediating apoptosis in human mesangial cells (HMC) and identifying the subtype(s) of purinergic receptors involved. eATP, but not uridin-5'-triphosphate (UTP), caused dose-dependent modifications of cellular morphology, as assessed by contrast-phase microscopy, and late apoptosis, as measured by Annexin V/propidium iodide-based flow cytometry and caspase-3 activation. Both phenomena were prevented by the P2X antagonist oxidized-ATP. 2', 3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP) was less effective than ATP, whereas 1[N,O-bis (5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl] -4-phenylpiperazine (KN62), a selective inhibitor of human P2X(7), prevented morphological changes but potentiated apoptosis induced by BzATP. P2X(7) was barely expressed in HMC and showed a relatively scarce functional activity, as assessed by monitoring nucleotide-induced intracellular calcium surge and plasma membrane depolarization by Fura-2/AM and bis[1,3-diethylthiobarbiturate]trimethineoxonal uptake, respectively. These data indicated a negligible role of P2X(7) in eATP-mediated apoptosis and pointed to the involvement of other P2X receptor(s). Molecular and inhibitor studies suggested a main role for P2X(4) receptor in nucleotide-induced apoptosis in HMC, indicating a relevant role for purinergic signaling in regulating death rate in these cells.

Renal mesangial cells: moving on sphingosine kinase-1

Sphingosine kinase-1 (SphK1) catalyses the phosphorylation of sphingosine to sphingosine-1-phosphate (S1P), which acts on at least five specific G-protein-coupled receptors and also intracellularly. SphK1 has been implicated in cell proliferation, cancer growth, chemoresistance, immune cell functions and cell migration. In this issue of the British Journal of Pharmacology, Klawitter et al. demonstrate that extracellular nucleotides stimulate the migration of renal mesangial cells. The nucleotides furthermore upregulated SphK1 expression and activity, and this enzyme was required for nucleotide-induced migration. Together with previous findings, these data raise exciting questions: by which mechanism does SphK1 regulate migration in mesangial cells, how is the interplay of purinoceptors and S1P receptors organized in these cells, and how would SphK1-deficient mice respond to kidney damage?

Extracellular nucleotides induce migration of renal mesangial cells by upregulating sphingosine kinase-1 expression and activity

BACKGROUND AND PURPOSE

Extracellular nucleotides act as potent mitogens for renal mesangial cells (MC). In this study we determined whether extracellular nucleotides trigger additional responses in MCs and the mechanisms involved.

EXPERIMENTAL APPROACH

MC migration was measured after nucleotide stimulation in an adapted Boyden-chamber. Sphingosine kinase-1 (SK-1) protein expression was detected by Western blot analysis and mRNA expression quantified by real-time PCR. SK activity was measured by an in vitro kinase assay using sphingosine as substrate.

KEY RESULTS

Nucleotide stimulation caused biphasic activation of SK-1, but not SK-2. The first peak occurred after minutes of stimulation and was followed by a second delayed peak after 4-24 h of stimulation. The delayed activation of SK-1 is due to increased SK-1 mRNA steady-state levels and de novo synthesis of SK-1 protein, and depends on PKC and the classical MAPK cascade. To see whether nucleotide-stimulated cell responses require SK-1, we selectively depleted SK-1 from cells by using small-interference RNA (siRNA). MC migration is highly stimulated by ATP and UTP; this is mimicked by exogenously added S1P. Depletion of SK-1 by siRNA drastically reduced the effect of ATP and UTP on cell migration but not on cell proliferation. Furthermore, MCs isolated from SK-1-deficient mice were completely devoid of nucleotide-induced migration.

CONCLUSIONS AND IMPLICATIONS

These data show that extracellular nucleotides besides being mitogenic also trigger MC migration and this cell response critically requires SK-1 activity. Thus, pharmacological intervention of SK-1 may have impacts on situations where MC migration is important such as during inflammatory kidney diseases.

Phospholipase D and extracellular signal-regulated kinase in hepatic stellate cells: effects of platelet-derived growth factor and extracellular nucleotides

We have previously provided evidence suggesting that phosphatidic acid, possibly derived from the hydrolysis of phosphatidylcholine by phospholipase D (PLD), is involved in platelet-derived growth factor (PDGF)-mediated increases in extracellular signal-regulated kinase (ERK) activity and DNA synthesis in rat hepatic stellate cells (HSC), the primary fibrogenic cells of the liver. A recent study has shown the presence of P2Y nucleotide receptors on HSC that are coupled to contraction and synthesis of the matrix component, alpha1-procollagen, leading to the suggestion that they may represent a new therapeutic target in the treatment of liver fibrosis. However, although extracellular nucleotides have been shown to stimulate both PLD and ERK, and to elicit proliferation of fibrogenic cells outside the liver, their effect on these parameters in HSC have not yet been investigated. PLD activity was determined by [3H]choline release and [3H]phosphatidylbutanol production, ERK activity by Western blotting, and DNA synthesis by [3H]thymidine incorporation. We report here, for the first time in HSC, that extracellular nucleotides stimulate PLD activity and a sustained activation of ERK. However, in contrast to PDGF, nucleotides had negligible effects on DNA synthesis. Moreover, the effects of PDGF and nucleotides on PLD and ERK were not additive, suggesting activation of the same PLD isoform and pool of ERK. The data demonstrate that nucleotide-stimulated PLD and ERK activities are not coupled to DNA synthesis in HSC. Instead, these responses may be linked to other phenotypic changes associated with activated HSC such as increases in contraction, motility, or extracellular matrix deposition.

P2Y receptors activate MAPK/ERK through a pathway involving PI3K/PDK1/PKC-zeta in human vein endothelial cells

AIMS

In this study we investigated the effects of P2 receptors in the regulation of mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) in human umbilical vein endothelial cells (HUVEC).

METHODS

Cytosolic Ca(2+) concentration ([Ca(2+)](i)) was measured using fura-2/AM, and MAPK/ ERK phosphorylation using Western blot analysis.

RESULTS

ATP, 2-meSATP, UTP and UDP cause a rapid and transitory increase in the phosphorylation of MAPK/ERK. In contrast, negligible response was seen for a,Beta-meATP, a general P2X receptors agonist. ATP-dependent activation of MAPK/ERK was prevented by pretreatment of HUVEC with pertussis toxin or MEK inhibitor PD98059. In addition, activation of the MAPK/ ERK cascade by ATP was blocked in cells pretreated with wortmannin and LY294002, but not by U73122, BAPTA or a Ca(2+)-free medium. Furthermore, an inhibition of ATP-dependent MAPK/ERK phosphorylation was observed in HUVEC pretreated with high doses of GF109203X or myristoylated PKC- zeta pseudosubstrate. Similar results were observed when cells were pretreated with the Src tyrosine kinase inhibitor PP2. However, ATP-stimulated MAPK/ERK activation was unaffected in cells pretreated with AG1478 or perillic acid. We also found that ATP stimulates both the phosphorylation of 3- phosphoinositide-dependent protein kinase-1 (PDK1) and its translocation to plasma membrane in a time-dependent manner.

CONCLUSION

These observations suggest that the effects mediated by ATP in HUVEC occur via PTX-sensitive G-protein-coupled P2Y receptors through PI3K-dependent mechanisms, in which PDK1 and PKC-zeta are two key molecules within signal cascade leading to MAPK/ERK activation.

Distinct mono- and dinucleotide-specific P2Y receptors in A549 lung epithelial cells: different control of arachidonic acid release and nitric oxide synthase expression

P2Y nucleotide receptors activated by mono- and dinucleotides have already been found in lung tissue. Here, we compare effects of dinucleotides and mononucleotides on arachidonic acid release, intracellular calcium mobilization, and inducible nitric oxide synthase (iNOS) expression in the alveolar lung cell line A549. Both types of nucleotides were effective. Diadenosine polyphosphates (Ap(n)A, n=2 to 5) increased arachidonic acid release and raised intracellular calcium concentration ([Ca(2+)](i)), albeit with lower potency than mononucleotides (ATP, UTP, UDP). Among the dinucleotides only diadenosine tetraphosphate (Ap(4)A) was a potent agonist. Arachidonic acid release induced by Ap(4)A was almost completely abolished in the presence of the P2 receptor antagonists suramin and Reactive blue 2, whereas arachidonic acid release evoked by ATP, UTP or UDP was hardly reduced by these antagonists. Both, the mononucleotides ATP and UDP and the dinucleotide Ap(4)A induced the expression of iNOS in the cytoplasm around the nucleus, similar to the expression of iNOS evoked by lipopolysaccharide. iNOS is barely detectable in unstimulated cells. Suramin selectively blocked the capacity of Ap(4)A to induce iNOS, but not that of ATP or UDP. Thus, we find the same pharmacology for nucleotide-induced arachidonic acid release and iNOS expression. Therefore, we suggest that a distinct P2Y receptor subtype specifically activated by Ap(4)A exists in A549 cells, which is sensitive to the antagonist suramin, in contrast to other P2Y receptor subtypes activated by mononucleotides which are suramin-insensitive. Distinct P2Y receptors activated by mononucleotides or by Ap(4)A could play a role in inflammatory conditions by affecting the release of arachidonic acid and the expression of iNOS. Therefore, these receptors present a promising target in inflammatory diseases.

Connective tissue growth factor/CCN2 stimulates actin disassembly through Akt/protein kinase B-mediated phosphorylation and cytoplasmic translocation of p27(Kip-1)

Connective tissue growth factor (CTGF/CCN2) is a 38-kDa secreted protein, a prototypic member of the CCN family, which is up-regulated in many diseases, including atherosclerosis, pulmonary fibrosis, and diabetic nephropathy. We previously showed that CTGF can cause actin disassembly with concurrent down-regulation of the small GTPase Rho A and proposed an integrated signaling network connecting focal adhesion dissolution and actin disassembly with cell polarization and migration. Here, we further delineate the role of CTGF in cell migration and actin disassembly in human mesangial cells, a primary target in the development of renal glomerulosclerosis. The functional response of mesangial cells to treatment with CTGF was associated with the phosphorylation of Akt/protein kinase B (PKB) and resultant phosphorylation of a number of Akt/PKB substrates. Two of these substrates were identified as FKHR and p27(Kip-1). CTGF stimulated the phosphorylation and cytoplasmic translocation of p27(Kip-1) on serine 10. Addition of the PI-3 kinase inhibitor LY294002 abrogated this response; moreover, addition of the Akt/PKB inhibitor interleukin (IL)-6-hydroxymethyl-chiro-inositol-2(R)-2-methyl-3-O-octadecylcarbonate prevented p27(Kip-1) phosphorylation in response to CTGF. Immunocytochemistry revealed that serine 10 phosphorylated p27(Kip-1) colocalized with the ends of actin filaments in cells treated with CTGF. Further investigation of other Akt/PKB sites on p27(Kip-1), revealed that phosphorylation on threonine 157 was necessary for CTGF mediated p27(Kip-1) cytoplasmic localization; mutation of the threonine 157 site prevented cytoplasmic localization, protected against actin disassembly and inhibited cell migration. CTGF also stimulated an increased association between Rho A and p27(Kip-1). Interestingly, this resulted in an increase in phosphorylation of LIM kinase and subsequent phosphorylation of cofilin, suggesting that CTGF mediated p27(Kip-1) activation results in uncoupling of the Rho A/LIM kinase/cofilin pathway. Confirming the central role of Akt/PKB, CTGF-stimulated actin depolymerization only in wild-type mouse embryonic fibroblasts (MEFs) compared to Akt-1/3 (PKB alpha/gamma) knockout MEFs. These data reveal important mechanistic insights into how CTGF may contribute to mesangial cell dysfunction in the diabetic milieu and sheds new light on the proposed role of p27(Kip-1) as a mediator of actin rearrangement.

Integration of P2Y receptor-activated signal transduction pathways in G protein-dependent signalling networks

The role of nucleotides in intracellular energy provision and nucleic acid synthesis has been known for a long time. In the past decade, evidence has been presented that, in addition to these functions, nucleotides are also autocrine and paracrine messenger molecules that initiate and regulate a large number of biological processes. The actions of extracellular nucleotides are mediated by ionotropic P2X and metabotropic P2Y receptors, while hydrolysis by ecto-enzymes modulates the initial signal. An increasing number of studies have been performed to obtain information on the signal transduction pathways activated by nucleotide receptors. The development of specific and stable purinergic receptor agonists and antagonists with therapeutical potential largely contributed to the identification of receptors responsible for nucleotide-activated pathways. This article reviews the signal transduction pathways activated by P2Y receptors, the involved second messenger systems, GTPases and protein kinases, as well as recent findings concerning P2Y receptor signalling in C6 glioma cells. Besides vertical signal transduction, lateral cross-talks with pathways activated by other G protein-coupled receptors and growth factor receptors are discussed.

Inhibition of apoptosis by P2Y2 receptor activation: novel pathways for neuronal survival

Cell survival is an essential function in the development and maintenance of the nervous system. We demonstrate here a previously unappreciated role for extracellular nucleotide signaling through the P2Y2 receptor in the survival of neurons: PC12 (pheochromocytoma 12) cells and dorsal root ganglion neurons are protected from serum starvation-induced apoptosis by ATP, UTP, and ATPgammaS, an effect mediated via P2Y2 receptors, as demonstrated by small interfering RNA and genetic knock-out models. This protection occurs independently of neurophin signaling but requires Src activation of ERK (extracellular signal-regulated kinase) and Akt. Moreover, ATPgammaS and NGF act synergistically to enhance neuronal survival through enhanced TrkA signaling. The results, which define a novel mechanism for inhibition of apoptosis, implicate parallel, interacting systems--extracellular nucleotides/P2Y2 receptors and neurotrophin/TrkA--to sustain neuronal survival.

Lung epithelial cells release ATP during ozone exposure: signaling for cell survival

The common air pollutant ozone causes acute toxicity to human airways. In primary and transformed epithelial cells from all levels of human or rat airways, ozone levels relevant to air pollution (50-200 ppb) increased extracellular [ATP] within 7-30 min. A human bronchial epithelial cell line (16HBE14o(-)) that forms electrically resistant polarized monolayers had up to 10-fold greater apical than basolateral surface extracellular [ATP] within 7 min of ozone exposure. Increased extracellular [ATP] appeared due to ATP secretion or release because (1) inhibition of ectonucleotidase (cell surface enzyme(s) which degrade ATP) by ozone did not occur until >120 min of ozone exposure and (2) brefeldin A, a secretory inhibitor, eliminated elevation of extracellular [ATP] without affecting intracellular ATP. Extracellular ATP protected against ozone toxicity in a P2Y receptor-dependent manner as (1) removal of ATP and adenosine by apyrase and adenosine deaminase, respectively, potentiated ozone toxicity, (2) extracellular supplementation with ATP, a poorly hydrolyzable ATP analog ATPgammaS, or UTP inhibited apoptotic and necrotic ozone-mediated cell death, and (3) ATP-mediated protection was eliminated by P2 and P2Y receptor inhibitors suramin and Cibacron blue (reactive blue 2), respectively. The decline in glucose uptake caused by prolonged ozone exposure was prevented by supplemental extracellular ATP, an effect blocked by suramin. Further, Akt and ERK phosphorylation resulted from exposure to supplemental extracellular ATP. Thus, extracellularly released ATP signals to prevent ozone-induced death and supplementation with ATP or its analogs can augment protection, at least in part via Akt and /or ERK signaling pathways and their metabolic effects.

ATP stimulates Na+-glucose cotransporter activity via cAMP and p38 MAPK in renal proximal tubule cells

Extracellular ATP plays an important role in the regulation of renal function. However, the effect of ATP on the Na(+)-glucose cotransporters (SGLTs) has not been elucidated in proximal tubule cells (PTCs). Therefore, this study was performed to examine the action of ATP on SGLTs and their related signal pathways in primary cultured rabbit renal PTCs. ATP increased [(14)C]-alpha-methyl-d-glucopyranoside (alpha-MG) uptake in a time-dependent (>1 h) and dose-dependent (>10(-6) M) manner. ATP stimulated alpha-MG uptake by increasing in V(max) without affecting K(m). ATP-induced increase of alpha-MG uptake was correlated with the increase in both SGLT1 and SGLT2 protein expression levels. ATP-induced stimulation of alpha-MG uptake was blocked by suramin (nonspecific P2 receptor antagonist), RB-2 (P2Y receptor antagonist), and MRS-2179 (P2Y(1) receptor antagonist), suggesting a role for the P2Y receptor. ATP-induced stimulation of alpha-MG uptake was blocked by pertussis toxin (PTX, a G(i) protein inhibitor), SQ-22536 (an adenylate cyclase inhibitor), and PKA inhibitor amide 14-22 (PKI). ATP also increased cAMP formation, which was blocked by PTX and RB-2. However, pretreatment of adenosine deaminase did not block ATP-induced cAMP formation. In addition, ATP-induced stimulation of alpha-MG uptake was blocked by SB-203580 (p38 MAPK inhibitor), but not by PD-98059 (p44/42 MAPK inhibitor) or SP-600125 (JNK inhibitor). Indeed, ATP induced phosphorylation of p38 MAPK. In conclusion, ATP increases alpha-MG uptake via cAMP and p38 MAPK in renal PTCs.

Activity-dependent phosphorylation of Akt/PKB in adult DRG neurons

The serine/threonine kinase Akt/PKB has been implicated in cell survival signalling in many cell types, including the dorsal root ganglion (DRG). However, little is known about its role in physiological and pathophysiological conditions in the adult sensory and nociceptive system. In this study, we show that in naive animals almost all cells express Akt but only a subset of small-diameter neurons expresses a high level of phospho-Akt (p-Akt Ser 473). Activation of peripheral nociceptors in vivo using intraplantar injections of capsaicin in anaesthetized rats induced a rapid onset and time-dependent increase in p-Akt Ser 473 in small- and medium-sized DRG, predominantly TRPV1-positive neurons. In addition, electrical stimulation of 'A and C' fibres in the sciatic nerve induced an increase in the cytoplasmic staining of p-Akt Ser 473 in small- and medium-size DRG neurons. Blocking neuronal activity in the sciatic nerve using tetrodotoxin reduced the basal level of p-Akt Ser 473. Cultured DRG neurons confirmed that phosphorylation of Akt in different cellular compartments is triggered by depolarization or receptor activation, and suggested that this effect is mediated in part by phosphatidylinositol 3-kinase. Our results show that p-Akt Ser 473 is a marker of nociceptor activation and suggest a novel role for Akt in the transduction of intracellular signals in adult DRG neurons.

Purinergic modulation of mesangial extracellular matrix production: role in diabetic and other glomerular diseases

BACKGROUND

Extracellular adenosine triphosphate (ATP) (eATP) mediates several biologic activities via purinergic P2 receptors (P2Rs). This study aimed at (1) evaluating the role of the purinergic system in modulating mesangial extracellular matrix (ECM) and transforming growth factor-beta (TGF-beta) production and (2) its contribution to diabetes-induced mesangial ECM accumulation.

METHODS

Rat mesangial cells were grown in normal glucose (5.5 mmol/L) or high glucose (30 mmol/L) containing media and probed with purinergic agonists and antagonists for the assessment of the expression pattern and function of P2Rs; release of ATP and activity of ectoATPases; and changes in ECM and TGF-beta expression.

RESULTS

Cells cultured in normal glucose and high glucose expressed similar amounts of functional P2Rs of the P2X(2), P2X(3), P2X(4), P2X(5), P2X(7), P2Y(1), P2Y(2), P2Y(4), and P2Y(6) subtypes. Levels of eATP were higher in high glucose vs. normal glucose, with unchanged ectoATPase activity. The ATP-hydrolyzing enzymes hexokinase or apyrase reduced ECM and TGF-beta production from cells grown in high glucose, but not normal glucose. Under both normal glucose and high glucose conditions, ATP and the P2X(7) agonist benzoylbenzoylATP increased dose-dependently ECM and TGF-beta production, whereas the P2Y agonist uridine triphosphate (UTP) produced the opposite effect. The P2X(7) inhibitor oxidized ATP attenuated the ECM and TGF-beta up-regulation induced by ATP and, to a lesser extent, that caused by high glucose. A TGF-beta neutralizing antibody also prevented ATP-induced ECM up-regulation.

CONCLUSION

These data indicate a role for eATP in regulating ECM production via TGF-beta and suggest that P2XRs and P2YRs differentially modulate this process. An increased ATP release induced by hyperglycemia might contribute to mesangial matrix expansion occurring in diabetes.

Cellular injury induces activation of MAPK via P2Y receptors

Wound healing is a complex process that involves cell communication, migration, proliferation, and changes in gene expression. One of the first events after injury is the rapid release of Ca(2+) that propagates as a wave to neighboring cells (Klepeis et al. [2001]: J. Cell. Sci. 114:4185-4195). Our goal was to examine the signaling events induced by cellular injury and identify extracellular molecules that induce the activation of extracellular signal responsive kinase (ERK) (p42/44). In this study we demonstrated that injury induced ERK1/2 activation occurred within 2 min and was negligible by 15 min. Treatment of unwounded cells with wound media caused activation of ERK that could be inhibited by apyrase III. Stimulation with epidermal growth factor (EGF) did not mimic the injury response and it was not detected in the wound media. To identify the active component, size fractionation was performed and factor(s) less than 3 kDa that induced the release of Ca(2+) and activation of ERK1/2 were identified. Activity was not altered by heat denaturation, incubation with proteinase K but it was lost by treatment with apyrase. Adenosine triphosphate (ATP), uridine triphosphate (UTP), adenosine diphosphate (ADP), and uridine diphosphate (UDP) promoted activation by 2 min with similar profiles as that generated by injury. Preincubation with phospholipase C inhibitor, U73122, inhibited activation that was induced by injury and/or nucleotides. Lack of activation by alpha-beta-methylATP (alpha, beta-MeATP) and beta-gamma-methylATP (beta, gamma-MeATP) to purinergic (P)2X receptors further indicated that activation occurs via P2Y and not P2X purinergic receptors. These results indicate that injury-induced activation of ERK1/2 is mediated by a P2Y signaling pathway.

ERK, PKC and PI3K/Akt pathways mediate extracellular ATP and adenosine-induced proliferation of U138-MG human glioma cell line

OBJECTIVE

Extracellular nucleotides and nucleosides induce proliferation in a set of human glioma cell lines. In this study we investigate the signal transduction pathways involved in ATP and adenosine-mediated proliferation in U138-MG human glioma cells.

METHODS

Cell proliferation was accessed through [(3)H]thymidine incorporation, cell counting and flow cytometry. Protein phosphorylation was detected through Western blotting.

RESULTS

ATP or adenosine (100 microM) induced extracellular signal-regulated protein kinase (ERK), Akt and GSK3beta phosphorylation. The increase in [(3)H]thymidine incorporation induced by ATP or adenosine was decreased when cells were incubated with LY 294002 (by +/-90%), GF 109203X (by +/-76%) or PD 098059 (by +/-63%). The increase in cell numbers with ATP or adenosine was less after a 48-hour treatment of cells with ATP or adenosine plus GF 109203X (by +/-66%) or LY 294002 (by +/-83%). Percentage of cells in S phase was decreased in cells treated with LY 294002 plus ATP when compared to ATP- treated cells.

CONCLUSION

Stimulation of purinergic receptors in U138-MG cells leads to cell proliferation mediated by PI3K/Akt, ERK and PKC signaling. It may be clinically important for pharmacological intervention in gliomas to associate purinergic receptor antagonists and signal transduction pathways blockers.

Extracellular ATP-induced proliferation of adventitial fibroblasts requires phosphoinositide 3-kinase, Akt, mammalian target of rapamycin, and p70 S6 kinase signaling pathways

Extracellular nucleotides are increasingly recognized as important regulators of growth in a variety of cell types. Recent studies have demonstrated that extracellular ATP is a potent inducer of fibroblast growth acting, at least in part, through an ERK1/2-dependent signaling pathway. However, the contributions of additional signaling pathways to extracellular ATP-mediated cell proliferation have not been defined. By using both pharmacologic and genetic approaches, we found that in addition to ERK1/2, phosphatidylinositol 3-kinase (PI3K), Akt, mammalian target of rapamycin (mTOR), and p70 S6K-dependent signaling pathways are required for ATP-induced proliferation of adventitial fibroblasts. We found that extracellular ATP acting in part through G(i) proteins increased PI3K activity in a time-dependent manner and transient phosphorylation of Akt. This PI3K pathway is not involved in ATP-induced activation of ERK1/2, implying activation of independent parallel signaling pathways by ATP. Extracellular ATP induced dramatic increases in mTOR and p70 S6K phosphorylation. This activation of the mTOR/p70 S6 kinase (p70 S6K) pathway in response to ATP is because of independent contributions of PI3K/Akt and ERK1/2 pathways, which converge on the level of p70 S6K. ATP-dependent activation of mTOR and p70 S6K also requires additional signaling inputs perhaps from pathways operating through Galpha or Gbetagamma subunits. Collectively, our data demonstrate that ATP-induced adventitial fibroblast proliferation requires activation and interaction of multiple signaling pathways such as PI3K, Akt, mTOR, p70 S6K, and ERK1/2 and provide evidence for purinergic regulation of the protein translational pathways related to cell proliferation.

Activation of phosphatidylinositol 3-kinase, Akt, and mammalian target of rapamycin is necessary for hypoxia-induced pulmonary artery adventitial fibroblast proliferation

In contrast to cell types in which exposure to hypoxia causes a general reduction of metabolic activity, a remarkable feature of pulmonary artery adventitial fibroblasts is their ability to proliferate in response to hypoxia. Previous studies have suggested that ERK1/2, phosphatidylinositol 3-kinase (PI3K), Akt, and mammalian target of rapamycin (mTOR) are activated by hypoxia and play a role in a variety of cell responses. However, the pathways involved in mediating hypoxia-induced proliferation are largely unknown. Using pharmacological inhibitors, we established that PI3K-Akt, mTOR-p70 ribosomal protein S6 kinase (p70S6K), and EKR1/2 signaling pathways play a critical role in hypoxia-induced adventitial fibroblast proliferation. We found that exposure of serum-starved fibroblasts to 3% O2 resulted in a time-dependent activation of PI3K and transient phosphorylation of Akt. However, activation of PI3K was not required for activation of ERK1/2, implying a parallel involvement of these pathways in the proliferative response of fibroblasts to hypoxia. We found that hypoxia induced significant increases in mTOR, p70S6K, 4E-BP1, and S6 ribosomal protein phosphorylation, as well as dramatic increases in p70S6K activity. The activation of p70S6K/S6 pathway was sensitive to inhibition by rapamycin and LY294002, indicating that mTOR and PI3K/Akt are upstream signaling regulators. However, the magnitude of hypoxia-induced p70S6K activity and phosphorylation suggests involvement of additional signaling pathways. Thus our data demonstrate that hypoxia-induced adventitial fibroblast proliferation requires activation and interaction of PI3K, Akt, mTOR, p70S6K, and ERK1/2 and provide evidence for hypoxic regulation of protein translational pathways in cells exhibiting the capability to proliferate under hypoxic conditions.

P2Y2 receptors activate neuroprotective mechanisms in astrocytic cells

Mechanical or ischemic trauma to the CNS causes the release of nucleotides and other neurotransmitters into the extracellular space. Nucleotides can activate nucleotide receptors that modulate the expression of genes implicated in cellular adaptive responses. In this investigation, we used human 1321N1 astrocytoma cells expressing a recombinant P2Y2 receptor to assess the role of this receptor in the regulation of anti-apoptotic (bcl-2 and bcl-xl) and pro-apoptotic (bax) gene expression. Acute treatment with the P2Y2 receptor agonist UTP up-regulated bcl-2 and bcl-xl, and down-regulated bax, gene expression. Activation of P2Y2 receptors was also coupled to the phosphorylation of cyclic AMP responsive element binding protein that positively regulates bcl-2 and bcl-xl gene expression. Cyclic AMP responsive element decoy oligonucleotides markedly attenuated the UTP-induced increase in bcl-2 and bcl-xl mRNA levels. Activation of P2Y2 receptors induced the phosphorylation of the pro-apoptotic factor Bad and caused a reduction in bax/bcl-2 mRNA expression ratio. All these signaling pathways are known to be involved in cell survival mechanisms. Using cDNA microarray analysis and RT-PCR, P2Y2 receptors were found to up-regulate the expression of genes for neurotrophins, neuropeptides and growth factors including nerve growth factor 2; neurotrophin 3; glia-derived neurite-promoting factor, as well as extracellular matrix proteins CD44 and fibronectin precursor--genes known to regulate neuroprotection. Consistent with this observation, conditioned media from UTP-treated 1321N1 cells expressing P2Y2 receptors stimulated the outgrowth of neurites in PC-12 cells. Taken together, our results suggest an important novel role for the P2Y2 receptor in survival and neuroprotective mechanisms under pathological conditions.

Mechanisms of the ATP potentiation of hyposmotic taurine release in Swiss 3T3 fibroblasts

Reducing osmolarity by 35% increased (3)H-taurine efflux from Swiss 3T3 fibroblasts from 0.5% to a peak of 5.7%. The presence of ATP (10-100 microM; EC(50) 1.5 microM) increased taurine efflux up to 10%, and decreased the set point for hyposmotically stimulated taurine release (HTR). ATP potentiation was mimicked by UTP, reduced by addition of suramin and pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and unaffected by ADP, beta,gamma-methylene-ATP (beta,gamma-ATP) or 2-methylthio-ATP (Me-ATP), suggesting its mediation by purinergic P2Y(2) and P2Y(4) metabotropic receptors. Under isosmotic conditions ATP increased the cytosolic [Ca(2+)] ([Ca(2+)](i)) markedly, but did not increase taurine release. HTR was independent of external Ca(2+) but was reduced (by 56-59%) by BAPTA-AM, thapsigargin-induced depletion of intracellular Ca(2+) stores, or phospholipase C (PLC) inhibition. Blockade of calmodulin (CaM) or calmodulin kinase II (CaMKII) reduced HTR by 54% and 76%, respectively. The ATP-mediated potentiation was prevented fully by all these treatments. HTR was reduced by 30-50% by blockers of protein tyrosine kinases (AG18), phosphoinositide 3-kinase (PI3K) (wortmannin), p21rho (toxin B), p21rho-kinase (Y27632) and the stress-activated kinase p38 (PD169316). ATP-mediated potentiation was reduced similarly by these blockers. Simultaneous inhibition of PI3K and CaMKII abolished HTR. Altogether, these results suggest a modulatory effect of ATP, probably exerted by a potentiation of the Ca(2+)-dependent fraction of HTR. This fraction has as signalling elements a PLC-dependent [Ca(2+)](i) increase, resulting from Ca(2+) released from thapsigargin-sensitive internal stores, followed by activation of CaM/CaMKII reactions. The Ca(2+)/ATP effect operates only when the Ca(2+)-independent, tyrosine kinase-mediated pathway is already activated. Suggested elements of cross-talk between the two pathways are PLC, PI3K and CaMKII.

Subtype specific internalization of P2Y1 and P2Y2 receptors induced by novel adenosine 5'-O-(1-boranotriphosphate) derivatives

P2Y-nucleotide receptors represent important targets for drug development. The lack of stable and receptor specific agonists, however, has prevented successful therapeutic applications. A novel series of P-boronated ATP derivatives (ATP-alpha-B) were synthesized by substitution of a nonbridging O at P(alpha) with a BH(3) group. This introduces a chiral center, thus resulting in diastereoisomers. In addition, at C2 of the adenine ring a further substitution was made (Cl- or methylthio-). The pairs of diastereoisomers were denoted here as A and B isomers. Here, we tested the receptor subtype specificity of these analogs on HEK 293 cells stably expressing rat P2Y(1) and rat P2Y(2) receptors, respectively, both attached to the fluorescent marker protein GFP (rP2Y(1)-GFP, rP2Y(2)-GFP). We investigated agonist-induced receptor endocytosis, [Ca(2+)](i) rise and arachidonic acid (AA) release. Agonist-induced endocytosis of rP2Y(1)-GFP was more pronounced for the A isomers than the corresponding B counterparts for all ATP-alpha-B analogs. Both 2-MeS-substituted diastereoisomers induced a greater degree of agonist-induced receptor endocytosis as compared to the 2-Cl-substituted derivatives. Endocytosis results are in accordance with the potency to induce Ca(2+) release by these compounds in HEK 293 cells stably transfected with rP2Y(1). In case of rP2Y(2)-GFP, the borano-nucleotides were very weak agonists in comparison to UTP and ATP in terms of Ca(2+) release, AA release and in inducing receptor endocytosis. The different ATP-alpha-B derivatives and also the diastereoisomers were equally ineffective. Thus, the new agonists may be considered as potent and highly specific agonist drug candidates for P2Y(1) receptors. The difference in activity of the ATP analogs at P2Y receptors could be used as a tool to investigate structural differences between P2Y receptor subtypes.

Suramin inhibits death receptor–induced apoptosis in vitro and fulminant apoptotic liver damage in mice

Suramin is a polysulfonated derivative of urea and has been widely used both to treat infections and as a chemotherapeutic drug. Suramin has been shown to inhibit growth factor signaling pathways; however, its effect on apoptosis is unknown. Here we show that suramin inhibits apoptosis induced through death receptors in hepatoma and lymphoma cells. It also inhibits the proapoptotic effect of chemotherapeutic drugs. The antiapoptotic mechanism is specific to cell type and is caused by reduced activation, but not altered composition, of the death-inducing signaling complex (DISC), and by inhibition of the initiator caspases 8, 9 and 10. Suramin also shows similar effects in in vivo models: apoptotic liver damage induced by CD95 stimulation and endotoxic shock mediated by tumor-necrosis factor (TNF) are inhibited in mice, but necrotic liver damage is not inhibited in a rat model of liver transplantation. Thus, the antiapoptotic property of suramin in the liver may be therapeutically exploited.

P2Y12 receptor stimulation inhibits beta-adrenergic receptor-induced differentiation by reversing the cyclic AMP-dependent inhibition of protein kinase B

Cyclic AMP-dependent induction of differentiation by activation of the beta-adrenergic receptor is correlated with inhibition of protein kinase B activity concomitant with growth arrest and increase in glial fibrillary acidic protein (GFAP) synthesis in rat C6 glioma cells. Costimulation of the beta-adrenergic receptor with purinergic receptors activated by 2-methylthio-adenosine-5'-diphosphate (2MeSADP) increased protein kinase B (PKB) phosphorylation above the level measured in non-stimulated cells and abolished cAMP-dependent differentiation. Transfection of cells with constitutively active PKB confirmed that reactivation of PKB is involved in the 2MeSADP-dependent inhibition of GFAP synthesis. The P2Y(12) and P2Y(13) receptor antagonist AR-C69931MX [N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloro-methylene ATP] decreased PKB phosphorylation to the level in non-stimulated cells, whereas the P2Y(13) antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) and P(1),P(3)-di(adenosine-5') tetraphosphate (Ap(4)A) did not alter the 2MeSADP-induced phosphorylation of PKB, showing that enhanced PKB activity and subsequent phosphorylation of glycogen synthase kinase-3 is due to stimulation of the P2Y(12) receptor. In addition, experiments in the presence of pertussis toxin and phosphatidylinositol 3-kinase (PI 3-K) activity assays demonstrated that the P2Y(12) receptor-mediated increase in PKB phosphorylation is G(i) protein- and PI 3-K-dependent. The presented data demonstrated that a cAMP-dependent inhibition of PKB induces differentiation of C6 glioma cells and that inhibition of adenylate cyclase and reactivation of the PI 3-K/PKB pathway by the P2Y(12) receptor reverses differentiation into enhanced proliferation.

P2X7 receptors stimulate AKT phosphorylation in astrocytes

1. Emerging evidence indicates that nucleotide receptors are widely expressed in the nervous system. Here, we present evidence that P2Y and P2X receptors, particularly the P2X(7) subtype, are coupled to the phosphoinositide 3-kinase (PI3K)/Akt pathway in astrocytes. 2. P2Y and P2X receptor agonists ATP, uridine 5'-triphosphate (UTP) and 2',3'-O-(4-benzoyl)-benzoyl ATP (BzATP) stimulated Akt phosphorylation in primary cultures of rat cortical astrocytes. BzATP induced Akt phosphorylation in a concentration- and time-dependent manner, similar to the effect of BzATP on Akt phosphorylation in 1321N1 astrocytoma cells stably transfected with the rat P2X(7) receptor. Activation was maximal at 5 - 10 min and was sustained for 60 min; the EC(50) for BzATP was approximately 50 microM. In rat cortical astrocytes, the positive effect of BzATP on Akt phosphorylation was independent of glutamate release. 3. The effect of BzATP on Akt phosphorylation in rat cortical astrocytes was significantly reduced by the P2X(7) receptor antagonist Brilliant Blue G and the P2X receptor antagonist iso-pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulfonic acid, but was unaffected by trinitrophenyl-ATP, oxidized ATP, suramin and reactive blue 2. 4. Results with specific inhibitors of signal transduction pathways suggest that extracellular and intracellular calcium, PI3K and a Src family kinase are involved in the BzATP-induced Akt phosphorylation pathway. 5. In conclusion, our data indicate that stimulation of astrocytic P2X(7) receptors, as well as other P2 receptors, leads to Akt activation. Thus, signaling by nucleotide receptors in astrocytes may be important in several cellular downstream effects related to the Akt pathway, such as cell cycle and apoptosis regulation, protein synthesis, differentiation and glucose metabolism.

Differential effects of P2Y1 and P2Y12 nucleotide receptors on ERK1/ERK2 and phosphatidylinositol 3-kinase signalling and cell proliferation in serum-deprived and nonstarved glioma C6 cells

We have previously shown that, in glioma C6 cells, two nucleotide ADP-sensitive receptors coexist: P2Y1, coupled to PLC and responsible for Ca2+ release, and P2Y12, negatively coupled to adenylate cyclase. In the present study, we examined the effects of the stimulation of these two receptors on ERK1/2 and PI3-K activation, and cell proliferation in either serum-deprived or nonstarved C6 cells. In response to ADP and its analogues, in serum-starved cells, both p44 ERK1 and p42 ERK2 were activated in a time-dependent manner, as monitored by Western blot analysis using an antiphospho-p42/p44 MAPK antibody. The phosphorylation was reduced both by removal of the extracellular Ca2+ and partially or almost completely by MRS2179 or AR-C69931MX, specific antagonists of the P2Y1 and P2Y12 receptors, respectively. The inhibitory effect of antagonists was additive. These data indicate the involvement of both receptors, P2Y1 and P2Y12, in the ERK1/2 activation, but the P2Y12 receptor contribution predominates. ERK1/2 activity was positively correlated with cell proliferation of cultured glioma C6 cells. In nonstarved cells, ADP markedly decreased the PI3-K activity. In contrast, in serum-starved cells, ADP evoked an increase in the PI3-K activity. Blocking of the P2Y1 receptor by MRS2179 additionally increased this ADP response. These results suggest that the P2Y1 receptor has an inhibitory and the P2Y12 receptor a stimulatory effect on PI3-K signalling pathway. RT-PCR analysis revealed different mRNA expression of both receptors in starved and nonstarved cells. In nonstarved cells, the P2Y1 receptor mRNA predominates, whereas in serum-deprived cells the expression of P2Y12 mRNA becomes more pronounced. British Journal of Pharmacology (2004) 141, 497-507. doi:10.1038/sj.bjp.0705639

Extracellular ATP facilitates flow-induced vasodilatation in rat small mesenteric arteries

ATP can be released from endothelial cells, and this release is increased by intraluminal flow in blood vessels. In the present study, the effect of extracellular ATP (1 microM) on flow-induced vasodilatation was investigated in isolated and pressurized rat small mesenteric arteries. In the absence of extracellular ATP, only 46% of arteries developed dilatation in response to flow, and this response was both transient and unstable. In marked contrast, with ATP present, all vessels developed a prolonged and stable dilatation in response to flow. Even in the vessels that failed to respond to flow in the absence of ATP, dilatation could be stimulated once ATP was present. The ability of ATP to facilitate flow-induced vasodilatation was mimicked by UTP (1 microM), a P2Y agonist, or 3'-O-(4-benzoyl)benzoyl ATP (BzATP; 10 microM), an agonist for P2X1, P2X7, and P2Y11 purinoceptors. The involvement of P2X7 purinoceptors was further supported by the inhibitory effect of KN-62 (1 microM), a P2X7 antagonist, on the action of BzATP. P2X1 and P2X3 purinoceptors were not involved because their receptor agonist alpha,beta-methylene ATP had no effect. The facilitating effect of ATP on flow dilatation was also attenuated by the combined application of reactive blue 2 (100 microM), a P2Y antagonist, and suramin (100 microM), a nonselective P2X and P2Y antagonist. Furthermore, flow-induced dilatation obtained in the presence of ATP was reproducible. In contrast, in the additional presence of the ectonucleotidase inhibitor ARL-67156 (10 microM), although the first dilatation was normal, the responses to the second and later exposures to flow were greatly attenuated. The nonhydrolyzable ATP analogs adenosine-5'-(3-thiotriphosphate)trilithium salt (1 microM) and adenosine 5'-(beta,gamma-imido) triphosphate tetralithium salt hydrate (10 microM) had similar effects to those of ARL-67156. These data suggest that ATP acts through both P2X and P2Y purinoceptors to facilitate flow-induced vasodilatation and that ectonucleotidases prevent this effect by degrading ATP on the endothelial cell surface.

ATP potentiates interleukin-1 beta-induced MMP-9 expression in mesangial cells via recruitment of the ELAV protein HuR

Renal mesangial cells express high levels of matrix metalloproteinase 9 (MMP-9) in response to inflammatory cytokines such as interleukin (IL)-1 beta. We demonstrate here that the stable ATP analog adenosine 5'-O-(thiotriphosphate) (ATP gamma S) potently amplifies the cytokine-induced gelatinolytic content of mesangial cells mainly by an increase in the MMP-9 steady-state mRNA level. A Luciferase reporter gene containing 1.3 kb of the MMP-9 5'-promoter region showed weak responses to ATP gamma S but conferred a strong ATP-dependent increase in Luciferase activity when under the additional control of the 3'-untranslated region of MMP-9. By in vitro degradation assay and actinomycin D experiments we found that ATP gamma S potently delayed the decay of MMP-9 mRNA. Gel-shift and supershift assays demonstrated that three AU-rich elements (AREs) present in the 3'-untranslated region of MMP-9 are constitutively bound by complexes containing the mRNA stabilizing factor HuR. The RNA binding of these complexes was markedly increased by ATP gamma S. Mutation of each ARE element strongly impaired the RNA binding of the HuR containing complexes. Reporter gene assays revealed that mutation of one ARE did not affect the stimulatory effects by ATP gamma S, but mutation of all three ARE motifs caused a loss of ATP-dependent increase in luciferase activity without affecting IL-1 beta-inducibility. By confocal microscopy we demonstrate that ATP gamma S increased the nucleo cytoplasmic shuttling of HuR and caused an increase in the cytosolic HuR level as shown by cell fractionation experiments. Together, our results indicate that the amplification of MMP-9 expression by extracellular ATP is triggered through mechanisms that likely involve a HuR-dependent rise in MMP-9 mRNA stability.

ATP- and UTP-activated P2Y receptors differently regulate proliferation of human lung epithelial tumor cells

The involvement of P2Y receptors, which are activated by extracellular nucleotides, in proliferative regulation of human lung epithelial cells is unclear. Here we show that extracellular ATP and UTP stimulate bromodeoxyuridine (BrdU) incorporation into epithelial cell lines. The nucleotide efficacy profile [ATP = ADP > UDP >or= UTP > adenosine >or= 2-methylthioadenosine-5'-diphosphate, with alpha,beta-methylene adenosine 5'-triphosphate, 2',3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate, AMP, UMP, and ATPalphaS inactive] and PCR analysis indicate involvement of P2Y2 and P2Y6 receptors. The signal transduction pathway, which, via the P2Y2 receptor, transmits the proliferative activity of ATP or UTP in A549 cells downstream of phospholipase C, depends on Ca2+/calmodulin-dependent protein kinase II and nuclear factor-kappaB, but not on protein kinase C. Signaling does not involve the mitogen-activated protein kinases extracellular signal-regulated kinases-1 and -2, the phosphatidylinositol 3-kinase pathway, or Src kinases. Thus nucleotides regulate proliferation of human lung epithelial cells by a novel pathway. The stimulatory effect of UTP, but not ATP, in A549 cells is attenuated by preincubation with interleukin-1beta and interleukin-6, but not tumor necrosis factor-alpha. This indicates an important role for the pyrimidine-activated P2Y receptor in the inflammatory response of lung epithelia. ATP antagonizes the antiproliferative effect of the anticancer drugs paclitaxel and etoposide, whereas it enhances the activity of cisplatin about fourfold. Thus pathways activated by extracellular nucleotides differentially control proliferation of lung epithelial tumor cells.