Phosphorylation and activation of p42 and p44 mitogen-activated protein kinase are required for the P2 purinoceptor stimulation of endothelial prostacyclin production

Purinergic signaling: A gatekeeper of blood-brain barrier permeation

This review outlined evidence that purinergic signaling is involved in the modulation of blood-brain barrier (BBB) permeability. The functional and structural integrity of the BBB is critical for maintaining the homeostasis of the brain microenvironment. BBB integrity is maintained primarily by endothelial cells and basement membrane but also be regulated by pericytes, neurons, astrocytes, microglia and oligodendrocytes. In this review, we summarized the purinergic receptors and nucleotidases expressed on BBB cells and focused on the regulation of BBB permeability by purinergic signaling. The permeability of BBB is regulated by a series of purinergic receptors classified as P2Y₁, P2Y₄, P2Y₁₂, P2X4, P2X7, A₁, A_2A, A_2B, and A₃, which serve as targets for endogenous ATP, ADP, or adenosine. P2Y₁ and P2Y₄ antagonists could attenuate BBB damage. In contrast, P2Y₁₂-mediated chemotaxis of microglial cell processes is necessary for rapid closure of the BBB after BBB breakdown. Antagonists of P2X4 and P2X7 inhibit the activation of these receptors, reduce the release of interleukin-1 beta (IL-1β), and promote the function of BBB closure. In addition, the CD39/CD73 nucleotidase axis participates in extracellular adenosine metabolism and promotes BBB permeability through A₁ and A_2A on BBB cells. Furthermore, A_2B and A₃ receptor agonists protect BBB integrity. Thus, the regulation of the BBB by purinergic signaling is complex and affects the opening and closing of the BBB through different pathways. Appropriate selective agonists/antagonists of purinergic receptors and corresponding enzyme inhibitors could modulate the permeability of the BBB, effectively delivering therapeutic drugs/cells to the central nervous system (CNS) or limiting the entry of inflammatory immune cells into the brain and re-establishing CNS homeostasis.

Difference in the response to angiotensin II between left and right ventricular endocardial endothelial cells

Previous studies focused on the right ventricular endocardial endothelial cells (EECRs) and showed that angiotensin II (Ang II) induced increase in cytosolic and nuclear calcium via AT₁ receptor activation. In the present study, we verified whether the response of left EECs (EECLs) to Ang II is different than that of EECRs. Our results showed that the EC₅₀ of the Ang II-induced increase of cytosolic and nuclear calcium in EECLs was 10× higher (around 2 × 10^-13 mol/L) than in EECRs (around 8 × 10^-12 mol/L). The densities of both AT₁ and AT₂ receptors were also higher in EECLs than those previously reported in EECRs. The effect of Ang II was mediated in both cell types via the activation of AT₁ receptors. Treatment with Ang II induced a significant increase of cytosolic and nuclear AT₁ receptors in EECRs, whereas the opposite was found in EECLs. In both cell types, there was a transient increase of cytosolic and nuclear AT₂ receptors following the Ang II treatment. In conclusion, our results showed that both AT₁ and AT₂ receptors densities are higher in both EECLs compared to what was reported in EECRs. The higher density of AT₁ receptors in EECLs compared to REECs may explain, in part, the higher sensitivity of EECLs to Ang II.

Circulating purine compounds, uric acid, and xanthine oxidase/dehydrogenase relationship in essential hypertension and end stage renal disease

Purine nucleotide liberation and their metabolic rate of interconversion may be important in the development of hypertension and its renal consequences. In the present study, blood triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP) breakdown pathway was evaluated in relation to uric acid concentration and xanthine dehydrogenase/xanthine oxidase (XDH/XO) in patients with essential hypertension, patients with chronic renal diseases on dialysis, and control individuals. The pattern of nucleotide catabolism was significantly shifted toward catabolic compounds, including ADP, AMP, and uric acid in patients on dialysis program. A significant fall of ATP was more expressed in a group of patients on dialysis program, compared with the control value (p<0.001), while ADP and AMP were significantly increased in both groups of patients compared with control healthy individuals (p<0.001), together with their final degradation product, uric acid (p<0.001). The index of ATP/ADP and ATP/uric acid showed gradual significant fall in both the groups, compared with the control value (p<0.001), near five times in a group on dialysis. Total XOD was up-regulated significantly in a group with essential hypertension, more than in a group on dialysis. The activity of XO, which dominantly contributes reactive oxygen species (ROS) production, significantly increased in dialysis group, more than in a group with essential hypertension. In conclusion, the examination of the role of circulating purine nucleotides and uric acid in pathogenesis of hypertension and possible development of renal disease, together with XO role in ROS production, may help in modulating their liberation and ROS production in slowing progression from hypertension to renal failure.

Activation of ERK1/2 by protein kinase C-alpha in response to hydrogen peroxide-induced cell death in human gingival fibroblasts

Hydrogen peroxide (H(2)O(2)) increases protein tyrosine phosphorylation of numerous proteins in human gingival fibroblasts (HGFs). Two main proteins, with an apparent molecular weight of 44 and 42kDa, were phosphorylated after hydrogen peroxide stimulation of the human gingival fibroblasts. Further analysis identified these two proteins as ERK1/2. Maximum phosphorylation was detected at 10min post-H(2)O(2) treatment. Pretreatment with an MEK inhibitor, PD98059, inhibited H(2)O(2)-stimulated ERK1/2 phosphorylation in a dose-dependent manner. Treatment with H(2)O(2) also induced phosphorylation of protein kinase C-alpha (PKCalpha). Staurosporine, a PKC inhibitor, blocked ERK1/2 phosphorylation induced by H(2)O(2). In addition, H(2)O(2)-induced cell death was prevented by PD98059, SB203580, and calphostin C, which are MEK, p38 and PKC inhibitors, respectively. These results suggest that H(2)O(2) leads to the phosphorylation and activation of ERK1/2 in a PKC-dependent manner. These findings demonstrate that the MAPK signaling pathway plays an active role in mediating the H(2)O(2)-induced decrease in HGF cell viability and ATP depletion.

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.

Signaling pathways involved in adenosine triphosphate-induced endothelial cell barrier enhancement

Endothelial barrier dysfunction caused by inflammatory agonists is a frequent underlying cause of vascular leak and edema. Novel strategies to preserve barrier integrity could have profound clinical impact. Adenosine triphosphate (ATP) released from endothelial cells by shear stress and injury has been shown to protect the endothelial barrier in some settings. We have demonstrated that ATP and its nonhydrolyzed analogues enhanced barrier properties of cultured endothelial cell monolayers and caused remodeling of cell-cell junctions. Increases in cytosolic Ca2+ and Erk activation caused by ATP were irrelevant to barrier enhancement. Experiments using biochemical inhibitors or siRNA indicated that G proteins (specifically Galphaq and Galphai2), protein kinase A (PKA), and the PKA substrate vasodilator-stimulated phosphoprotein were involved in ATP-induced barrier enhancement. ATP treatment decreased phosphorylation of myosin light chain and specifically activated myosin-associated phosphatase. Depletion of Galphaq with siRNA prevented ATP-induced activation of myosin phosphatase. We conclude that the mechanisms of ATP-induced barrier enhancement are independent of intracellular Ca2+, but involve activation of myosin phosphatase via a novel G-protein-coupled mechanism and PKA.

Endostatin induces acute endothelial nitric oxide and prostacyclin release

Chronic exposure to endostatin (ES) blocks endothelial cell (EC) proliferation, and migration and induces EC apoptosis thereby inhibiting angiogenesis. Nitric oxide (NO) and prostacyclin (PGI(2)), in contrast, play important roles in promoting angiogenesis. In this study, we examined the acute effects of ES on endothelial NO and PGI(2) production. Unexpectedly, a cGMP reporter cell assay showed that ES-induced acute endothelial NO release in cultured bovine aortic endothelial cells (BAECs). Enzyme immunoassay showed that ES also induced an acute increase in PGI(2) production in BAECs. These results were confirmed by ex vivo vascular ring studies that showed vascular relaxation in response to ES. Immunoblot analysis showed that ES stimulated acute phosphorylation of endothelial nitric oxide synthase (eNOS) at Ser116, Ser617, Ser635, and Ser1179, and dephosphorylation at Thr497 in BAECs, events associated with eNOS activation. Short-term exposure of EC to ES, therefore, unlike long-term exposure which is anti-angiogenic, may be pro-angiogenic.

MEK/MAPK as a signaling element in ATP control of endothelial myosin light chain

Phosphorylation of endothelial myosin light chains (MLC) is a key mechanism in control of endothelial contractile machinery. Extracellular ATP influences endothelial MLC phosphorylation by either activation of Ca(2+)-dependent MLC kinase or Ca(2+)-independent MLC phosphatase. Here, the role of the MEK/MAPK pathway in this signaling was investigated in porcine aortic endothelial cells. Phosphorylation of ERK2 and phosphorylation of MLC were analyzed in cultured aortic endothelial cells. ATP (10 microM) increased ERK2 phosphorylation from basal 17 +/- 3 to 53 +/- 4%, an effect suppressed in the presence of the MEK inhibitors PD-98059 (20 microM) or U0126 (10 microM). Phosphorylation of ERK2 was not dependent on the ATP-induced cytosolic Ca(2+) rise, because it was unaltered when this was suppressed by the Ca(2+) chelator BAPTA (10 microM) or xestospongin C (3 microM), an inhibitor of the inositol 1,4,5-trisphosphate-sensitive Ca(2+) release mechanism of the endoplasmic reticulum. Phosphorylation of ERK2 was neither induced by the adenosine analog 5'-(N-ethylcarboxamido)adenosine (1 microM) nor inhibited in the presence of the adenosine receptor antagonist 8-phenyltheophylline (10 microM). ATP increased MLC kinase activity, and this was blocked in presence of PD-98059. ATP also increased MLC phosphatase activity, which was not inhibited by PD-98059. The MEK/MAPK pathway is a Ca(2+)-independent part of ATP signaling toward MLC kinase but not of ATP signaling toward MLC phosphatase.

Regulation of epididymal principal cell functions by basal cells: role of transient receptor potential (Trp) proteins and cyclooxygenase-1 (COX-1)

The epithelia lining the epididymides of many species including the human are known to consist of several cell types. Among them, the principal cells are the most abundant and their functions most extensively studied. There are other cell types such as the narrow cells, clear cells, halo cells and basal cells which are scattered along the duct in lesser number. Although these minority cell types have not been studied to the same extent as the principal cells, it is conceivable that their presence are essential to the integrated functions of the epididymis. In the intact epididymis, basal cells can be seen adhering to the basement membrane forming close contact with the principal cells above them. Work in our laboratory has provided evidence that through local formation of prostaglandins, basal cells may regulate electrolyte and water transport by the principal cells. This regulatory process involves two proteins which are exclusively expressed by the basal cells. They are the transient receptor potential (Trp) proteins, which serve as transmembrane pathways for Ca(2+) influx, and cyclooxygenase 1 (COX-1), a key enzyme in the formation of prostaglandins. The role of the two proteins in the integrated functions of the basal cells as humoral regulators of principal cells is discussed.

Eicosanoids in cirrhosis and portal hypertension

In the last decade, the knowledge of the pathogenesis of portal hypertension and cirrhosis has increased dramatically. In portal hypertension, almost all the known vasoactive systems/substances are activated or increased and the most recent studies have stressed the importance of the endothelial factors, in particular, prostaglandins. Prostaglandins are formed following the oxygenation of arachidonic acid by the cyclooxygenase (Cox) pathway. An important consideration in portal hypertension and cirrhosis in the periphery is the altered hemodynamic profile and its contributory role in controlling endothelial release of these vasoactive substances. Prostaglandins are released from the endothelium in response to both humoral and mechanical stimuli and can profoundly affect both intrahepatic and peripheral vascular resistance. Within the liver, intrahepatic resistance is altered due to a diminution in sinusoidal responsiveness to vasodilators and an increase in prostanoid vasoconstrictor responsiveness. This review will examine the contributory role of both hormonal and/or hemodynamic force-induced changes in prostaglandin production and signaling in cirrhosis and portal hypertension and the consequence of these changes on the structural and functional response of both the vasculature and the liver.

Cyclic adenosine monophosphate-dependent vascular responses to purinergic agonists adenosine triphosphate and uridine triphosphate in the anesthetized mouse

The mechanism by which purinergic agonist adenosine triphosphate (ATP) and uridine triphosphate (UTP) decrease systemic arterial pressure in the anesthetized mouse was investigated. Intravenous injections of adenosine triphosphate (ATP) and uridine triphosphate (UTP) produced dose-dependent decreases in systemic blood pressure in the mouse. The order of potency was ATP > UTP. Vasodilator responses to ATP and UTP were altered by the cyclic adenosine monophosphate (cAMP) phosphodiesterase inhibitor rolipram. The vascular responses to ATP and UTP were not altered by a nitric oxide synthase inhibitor, a cyclooxygenase inhibitor, a cGMP phosphodiesterase inhibitor, or a particular P2 receptor antagonist. These data suggest that ATP and UTP cause a decrease in systemic arterial pressure in the mouse via a cAMP-dependent pathway via a novel P2 receptor linked to adenylate cyclase and that nitric oxide release, prostaglandin synthesis, cGMP, and P2X1, P2Y1, and P2Y4 receptors play little or no role in the vascular effects of these purinergic agonists in the mouse.

In vitro responses of equine colonic arterial and venous rings to adenosine triphosphate

OBJECTIVE

To evaluate the in vitro effects of adenosine tryphosphate (ATP) on vasomotor tone of equine colonic vasculature.

SAMPLE POPULATION

Arteries and veins from the left ventral colon of 14 mixed-breed horses euthanatized for reasons unrelated to cardiovascular or gastrointestinal tract disease.

PROCEDURES

Endothelium-intact and -denuded arterial and venous rings were precontracted with 10(-7) and 1.8 x 10(-8) M endothelin-1, respectively. In 1 trial, endothelium-intact rings were also incubated with 10(-4) M N omega-nitro-L-arginine methyl ester (L-NAME) to inhibit nitric oxide (NO) production. Adenosine triphosphate (10(-8) to 10(-3) M) was added in a noncumulative manner, and relaxation percentage versus time curves were generated. Areas under the curves (ie, percentage of relaxation time) were calculated.

RESULTS

Relaxation response of arterial and venous rings to ATP was dose-dependent. Percentage of relaxation time in response to 10(-4) and 10(-3) MATP was significantly greater, compared with that for rings not treated with ATP Removal of endothelium attenuated but did not eliminate the relaxation response. Addition of L-NAME did not attenuate the relaxation response in arteries. At higher concentrations, the vascular response to ATP was biphasic.

CONCLUSIONS AND CLINICAL RELEVANCE

ATP applied to equine colonic arterial and venous rings with and without intact endothelium induced a biphasic response characterized by transient contraction followed by slow, substantial, and sustained relaxation. This ATP-induced response is possibly mediated by a mechanism other than NO. Adenosine triphosphate may be a useful treatment to modulate colonic vasomotor tone in horses with strangulating volvulus of the ascending colon.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

Adenosine nucleotides acting at the human P2Y1 receptor stimulate mitogen-activated protein kinases and induce apoptosis

For the widely distributed P2Y receptors for nucleotides, the transductional and functional responses downstream of their coupling to G proteins are poorly characterized. Here we describe apoptotic induction and the associated differential stimulation of mitogen-activated protein (MAP) kinase family members by the human P2Y(1) receptor. The potent P2Y(1) receptor agonist, 2-methylthio-ADP (2-MeSADP), stimulated the extracellular-signal regulated kinases (ERK1/2) (EC(50) approximately 5 nm) as well as several, but not all isoforms detected, of the stress-activated protein kinase (SAPK) family. Phospho-isoforms of p38 were unaffected. The induced kinase activity was blocked by the P2Y(1) receptor-selective antagonist, adenosine-2'-phosphate-5'-phosphate, but unaffected by pertussis toxin. In addition, the endogenous ligand ADP, and significantly also 2-MeSATP, induced concentration-dependent phosphorylation changes in the same MAP kinase family members. The sustained activation of ERK1/2 was associated with Elk-1 phosphorylation that was abolished by the MEK1 inhibitor, PD 98059. However, the concomitant transient activation of the SAPKs was not sufficient to induce c-Jun or ATF-2 phosphorylation. The transient phase of the ERK activity was partially inhibited either by the phosphatidylinositol 3-kinase inhibitor, LY 294002, or the PKC inhibitor, Gö 6976. In addition, the Src inhibitor, PP1, or expression of dominant negative Ras also attenuated the transient phase of ERK phosphorylation. In contrast, inhibition of Ras or Src had no effect on the sustained ERK activity, which was critically dependent on phosphatidylinositol 3-kinase. The transient SAPK activity was suppressed by expression of a dominant negative form of MKK4. Furthermore, this kinase-deficient mutant inhibited 2-MeSADP-induced caspase-3 stimulation and the associated decrease in cell number. In conclusion, adenosine di- and triphosphate stimulation of the human P2Y(1) receptor can transiently activate the Ras-ERK cascade via the cooperative effects of phosphatidylinositol 3-kinase, Src and PKC. The sustained ERK stimulation, via a Ras-insensitive pathway, culminates in Elk-1 activation without inducing a proliferation effect. The transient SAPK activity did not evoke transcription factor phosphorylation but was required for the P2Y(1) receptor-mediated apoptotic function.

P2Y(2) nucleotide receptor signaling in human monocytic cells: activation, desensitization and coupling to mitogen-activated protein kinases

Activation of P2Y(2) receptors by extracellular nucleotides has been shown to induce phenotypic differentiation of human promonocytic U937 cells that is associated with the inflammatory response. The P2Y(2) receptor agonist, UTP, induced the phosphorylation of the MAP kinases MEK1/2 and ERK1/2 in a sequential manner, since ERK1/2 phosphorylation was abolished by the MEK1/2 inhibitor PD 098059. Other results indicated that P2Y(2) receptors can couple to MAP kinases via phosphatidylinositol 3-kinase (PI3K) and c-src. Accordingly, ERK1/2 phosphorylation induced by UTP was inhibited by the PI3K inhibitors, wortmannin and LY294002, and the c-src inhibitors, radicicol and PP2, but not by inhibitors of protein kinase C (PKC). The phosphorylation of ERK1/2 was independent of the ability of P2Y(2) receptors to increase the concentration of intracellular free calcium, since chelation of intracellular calcium by BAPTA did not diminish the phosphorylation of ERK1/2 induced by UTP. A 5-minute treatment with UTP reduced U937 cell responsiveness to a subsequent UTP challenge. UTP-induced desensitization was characterized by an increase in the EC(50) for receptor activation (from 0.44 to 9.3 microM) and a dramatic ( approximately 75%) decrease in the maximal calcium mobilization induced by a supramaximal dose of UTP. Phorbol ester treatment also caused P2Y(2) receptor desensitization (EC(50) = 12.3 microM UTP and maximal calcium mobilization reduced by approximately 33%). The protein kinase C inhibitor GF 109203X failed to significantly inhibit the UTP-induced desensitization of the P2Y(2) receptor, whereas the protein phosphatase inhibitor okadaic acid blocked receptor resensitization. Recovery of receptor activity after UTP-induced desensitization was evident in cells treated with agonist for 5 or 30 min. However, P2Y(2) receptor activity remained partially desensitized 30 min after pretreatment of cells with UTP for 1 h or longer. This sustained desensitized state correlated with a decrease in P2Y(2) receptor mRNA levels. Desensitization of ERK1/2 phosphorylation was induced by a 5-minute pretreatment with UTP, and cell responsiveness did not return even after a 30-minute incubation of cells in the absence of an agonist. Results suggest that desensitization of the P2Y(2) receptor may involve covalent modifications (i.e., receptor phosphorylation) that functionally uncouple the receptor from the calcium signaling pathway, and that transcriptional regulation may play a role in long-term desensitization. Our results indicate that calcium mobilization and ERK1/2 phosphorylation induced by P2Y(2) receptor activation are independent events in U937 monocytes.

Involvement of MAP kinases in the control of cPLA2 and arachidonic acid release in endothelial cells

Cytosolic Phospholipase A(2) (cPLA(2)) has been implicated in receptor-mediated release of arachidonic acid from membrane phospholipids, the limiting step in prostacyclin and other eicosanoid production. Its activity is controlled by Ca(++) levels and enzymatically regulated phosphorylation. The purpose of this study was to assess the importance of phosphorylation of cPLA(2) in human umbilical vein endothelial cells and to identify the kinases involved. Inhibitors were used to study the pathways leading to phosphorylation and activation of mitogen activated protein kinases (MAP-kinases) and cPLA(2), as well as release of arachidonic acid and prostacyclin production after stimulation with different agonists. We have found that agonists that release arachidonic acid, including histamine, thrombin, AlF(4)(-), and pervanadate, all activate the MAP kinases ERK, p38 and JNK and cause phosphorylation of cPLA(2). Agonist specific differences in the signal transduction pathways included variable contribution of tyrosine phosphorylation, protein kinase C and ERK activity, and different effects of pertussis toxin. Treatment with PD98059 (inhibitor of ERK-activation) or SB203580 (inhibitor of p38) caused partial decrease in arachidonic acid release and cPLA(2) activity. In contrast the nonspecific protein kinase inhibitor staurosporin completely inhibited cPLA(2) activity. We conclude that in endothelial cells arachidonic acid release is largely mediated by cPLA(2) through agonist-specific pathways. The MAP kinases ERK and p38 both have demonstrable but not major effect on agonist stimulated arachidonic acid release and the data suggest that an additional unidentified kinase also has a role.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Involvement of prostanoid release in the mediation of UTP-induced cerebrovascular contraction in the rat

The interaction between uridine-5'-triphosphate (UTP) and prostanoids was studied in isolated rat middle cerebral arteries (MCAs). The strong contractions in MCA segments induced by UTP were weakened significantly by indomethacin and more markedly by the thromboxane receptor antagonist ICI 192605. Thromboxane A(2) (TXA(2)) release by MCAs was below the detection limit of the chemiluminescence enzyme immunoassay, but increased TXA(2) formation was detected in basilar arteries in the presence of UTP. Prostacyclin (PGI(2)) formation by MCAs also increased in the presence of UTP. These results suggest that UTP stimulates the release of both TXA(2) and PGI(2) from the rat MCA but the vascular effect of TXA(2) is dominant.

Endothelial dysfunction in cirrhosis and portal hypertension

Portal hypertension (PHT) is a common clinical syndrome associated with chronic liver diseases; it is characterized by a pathological increase in portal pressure. Pharmacotherapy for PHT is aimed at reducing both intrahepatic vascular tone and elevated splanchnic blood flow. Due to the altered hemodynamic profile in PHT, dramatic changes in mechanical forces, both pressure and flow, may play a pivotal role in controlling endothelial and vascular smooth muscle cell signaling, structure, and function in cirrhotics. Nitric oxide, prostacyclin, endothelial-derived contracting factors, and endothelial-derived hyperpolarizing factor are powerful vasoactive substances released from the endothelium in response to both humoral and mechanical stimuli that can profoundly affect both the function and structure of the underlying vascular smooth muscle. This review will examine the contributory role of hormonal- and mechanical force-induced changes in endothelial function and signaling and the consequence of these changes on the structural and functional response of the underlying vascular smooth muscle. It will focus on the pivotal role of hormonal and mechanical force-induced endothelial release of vasoactive substances in dictating the reactivity of the underlying vascular smooth muscle, i.e., whether hyporeactive or hyperreactive, and will examine the extent to which these substances may exert a protective and/or detrimental influence on the structure of the underlying vascular smooth muscle in both a normal hemodynamic environment and following hemodynamic perturbations typical of PHT and cirrhosis. Finally, it will discuss the intracellular processes that regulate the release/expression of these vasoactive substances and that control the transformation of this normally protective cell to one that may promote the development of vasculopathy in PHT.

Inactivation of JNK activity by mitogen-activated protein kinase phosphatase-2 in EAhy926 endothelial cells is dependent upon agonist-specific JNK translocation to the nucleus

We have investigated the termination of agonist-stimulated mitogen-activated protein (MAP) kinase activity in EAhy926 cells by MAP kinase phosphatase-2 (MKP-2). In cells expressing either wild-type (WT) or catalytically inactive (CI)-MKP-2, there was no significant differences in TNFalpha-stimulated JNK or p38 MAP kinase activity, however hydrogen peroxide (H2O2)-stimulated JNK activity was substantially reduced in WT-MKP-2 expressing clones and enhanced in cells expressing CI-MKP-2. Consistent with these findings, we observed substantial nuclear translocation of JNK occurred in response to H2O2 but not TNFalpha. Using a phosphospecific anti-JNK antibody, we found that TNFalpha-stimulated JNK activity was associated principally with the cytosol while in response to H2O2, JNK activity was found within the nucleus. These results show that the role of MKP-2 in terminating JNK activity is determined by the translocation of JNK to the nucleus, which is under agonist-specific regulation and not a universal cellular response to stimulation.

Osteoblast response to titanium surface roughness and 1?,25-(OH)2D3 is mediated through the mitogen-activated protein kinase (MAPK) pathway

When osteoblasts are cultured on surfaces of increasing microroughness, they exhibit decreases in proliferation, increases in differentiation and local factor production, and enhanced response to 1alpha,25(OH)(2)D(3). The cells interact with surfaces through integrins, which signal by the same pathways used by 1alpha,25(OH)(2)D(3), including protein kinase C via phospholipase C and protein kinase A via phospholipase A(2). This provides opportunities for crosstalk that may contribute to the synergistic effects of surface roughness and the vitamin D metabolite. Because these pathways converge at mitogen-activated protein kinase (MAPK), we tested the hypothesis that the extracellular signal-regulated kinase (ERK1/2) subclass of MAPKs mediates the effects of surface roughness and 1alpha,25(OH)(2)D(3). MG63 osteoblast-like osteosarcoma cells were cultured on commercially pure Ti disks with various surface roughnesses: pretreatment (PT; 0.6 microm average roughness [Ra]), coarse grit-blasted and acid-etched (SLA; 4 microm RA), and titanium plasma-sprayed (TPS; 5.2-microm R(a)). At confluence, cells were treated for 24 h with control media or media containing 10(-7) M 1alpha,25(OH)(2)D(3). One-half of the cultures received 1 microm or 10 microm PD98059, a specific inhibitor of the ERK family of MAPKs. PD98059 alone did not affect proliferation, osteocalcin production, or production of transforming growth factor-beta1 or nitric oxide, regardless of the surface roughness. Alkaline phosphatase was reduced by the inhibition of the ERK family kinases on all surfaces to a comparable extent. However, when PD98059 was added to the cultures with 1alpha,25(OH)(2)D(3), the effects of the seco-steroid were blocked, including the synergistic increases seen in MG63 cells cultured on SLA or TPS. These results indicate that ERK1/2 MAPK is required for the maintenance of alkaline phosphatase at control levels and that the effects of 1alpha,25(OH)(2)D(3) are mediated by ERK1/2. However, the effects of surface roughness are not due to the ERK family of MAPKs. This suggests that alternative pathways may be used, including those mediated by other MAPK subclasses.

Advances in signalling by extracellular nucleotides. the role and transduction mechanisms of P2Y receptors

Nucleotides are ubiquitous intercellular messengers whose actions are mediated by specific receptors. Since the first clonings in 1993, it is known that nucleotide receptors belong to two families: the ionotropic P2X receptors and the metabotropic P2Y receptors. Five human P2Y receptor subtypes have been cloned so far and a sixth one must still be isolated. In this review we will show that they differ by their preference for adenine versus uracil nucleotides and triphospho versus diphospho nucleotides, as well as by their transduction mechanisms and cell expression.

The P2Y1 receptor mediates ADP-induced p38 kinase-activating factor generation in human platelets

U46619, a thromboxane A2 mimetic, but not ADP, caused activation of p38 mitogen activated protein (MAP) kinase in aspirin-treated platelets. In nonaspirinated human platelets ADP activated p38 MAP kinase in both a time-and concentration-dependent manner, suggesting that ADP-induced p38 MAP kinase activation requires generation of thromboxane A2. However, neither a thromboxane A2/prostaglandin H2 receptor antagonist SQ29548 and a thromboxane synthase inhibitor, furegrelate, either alone or together, nor indomethacin blocked ADP-induced p38 kinase activation in nonaspirinated platelets. Other cycloxygenase products, PGE2, PGD2, and PGF2alpha, failed to activate p38 kinase in aspirin-treated platelets. Hence, ADP must be generating an agonist, other than thromboxane A2, via an aspirin-sensitive pathway, which is capable of activating p38 kinase. AR-C66096, a P2TAC (platelet ADP receptor coupled to inhibition of adenylate cyclase) antagonist, did not inhibit ADP-induced p38 MAP kinase activation. The P2X receptor selective agonist, alpha, beta-methylene ATP, failed to activate p38 MAP kinase. On the other hand, the P2Y1 receptor selective antagonist, adenosine-2'-phosphate-5'-phosphate inhibited ADP-induced p38 kinase activation in a concentration-dependent manner, indicating that the P2Y1 receptor alone mediates ADP-induced generation of the p38 kinase-activating factor. These results demonstrate that ADP causes the generation of a factor in human platelets, which can activate p38 kinase, and that this response is mediated by the P2Y1 receptor. Neither the P2TAC receptor nor the P2X1 receptor has any significant role in this response.

P(2Y) purinoceptor subtypes recruit different mek activators in astrocytes

Extracellular ATP can function as a glial trophic factor as well as a neuronal transmitter. In astrocytes, mitogenic signalling by ATP is mediated by metabotropic P(2Y) receptors that are linked to the extracellular signal regulated protein kinase (Erk) cascade, but the types of P(2Y) receptors expressed in astrocytes have not been defined and it is not known whether all P(2Y) receptor subtypes are coupled to Erk by identical or distinct signalling pathways. We found that the P(2Y) receptor agonists ATP, ADP, UTP and 2-methylthioATP (2MeSATP) activated Erk and its upstream activator MAP/Erk kinase (Mek). cRaf-1, the first kinase in the Erk cascade, was activated by 2MeSATP, ADP and UTP but, surprisingly, cRaf-1 was not stimulated by ATP. Furthermore, ATP did not activate B-Raf, the major isoform of Raf in the brain, nor other Mek activators such as Mek kinase 1 (MekK1) and MekK2/3. Reverse transcriptase-polymerase chain reaction (RT - PCR) studies using primer pairs for cloned rat P(2Y) receptors revealed that rat cortical astrocytes express P(2Y(1)), a receptor subtype stimulated by ATP and ADP and their 2MeS analogues, as well as P(2Y(2)) and P(2Y(4)), subtypes in rats for which ATP and UTP are equipotent. Transcripts for P(2Y(6)), a pyrimidine-preferring receptor, were not detected. ATP did not increase cyclic AMP levels, suggesting that P(2Y(11)), an ATP-preferring receptor, is not expressed or is not linked to adenylyl cyclase in rat cortical astrocytes. These signal transduction and RT - PCR experiments reveal differences in the activation of cRaf-1 by P(2Y) receptor agonists that are inconsistent with properties of the P(2Y(1)), P(2Y(2)) and P(2Y(4)) receptors shown to be expressed in astrocytes, i.e. ATP=UTP; ATP=2MeSATP, ADP. This suggests that the properties of the native P(2Y) receptors coupled to the Erk cascade differ from the recombinant P(2Y) receptors or that astrocytes express novel purine-preferring and pyrimidine-preferring receptors coupled to the ERK cascade.

Purinoceptor-mediated calcium mobilization and cellular proliferation in cultured bovine corneal endothelial cells

In the present study, we investigated the effect of adenosine triphosphate (ATP) on cytosolic free calcium mobilization and mitogenic activity in cultured bovine corneal endothelial cells (BCEC). The [Ca2+]i was determined using a Ca2+ sensitive indicator, Fura-2/AM, and cell proliferation was evaluated by counting the cell number. ATP, its metabolites and analogs caused transient increase in [Ca2+]i in a concentration-dependent manner (10(-7) M-10(-3) M) and the potency of agonists was ordered as follows: 2-methylthio-ATP > uridine triphosphate > ATP > adenosine diphosphate. Adenosine monophosphate and adenosine did not affect [Ca2+]i. ATP (10(-4) M) also promoted the accumulation of inositol trisphosphate (IP3). The ATP-induced transient [Ca2+]i increase and IP3 accumulation were attenuated by pretreatment with a phospholipase C inhibitor, U-73122 (5 microM), for 30 min. ATP (10(-5) M) significantly enhanced the proliferation of BCEC. ATP-induced [Ca2+]i increase and cell proliferation were inhibited by a purinoceptor antagonist, suramin (10(-4) M). Thus, the present study indicates that BCEC contain P2 purinoceptors that regulate their proliferation.

Osmotic cell swelling-induced ATP release mediates the activation of extracellular signal-regulated protein kinase (Erk)-1/2 but not the activation of osmo-sensitive anion channels

Human intestine 407 cells respond to hypo-osmotic stress by the rapid release of ATP into the extracellular medium. A difference in the time course of activation as well as in the sensitivity to cytochalasin B treatment and BAPTA-AM [1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester] loading suggests that ATP leaves the cell through a pathway distinct from volume-regulated anion channels. To evaluate a putative role for nucleotides as autocrinic/paracrinic factors in osmotic signalling, the effects of extracellular ATP on the regulation of volume-sensitive anion channels as well as on the hypotonicity-induced activation of extracellular signal-regulated protein kinases (Erk-1/2) were investigated. Micromolar concentrations of ATP were unable to elicit an isotope efflux from (125)I(-)-loaded cells by itself, but strongly potentiated the hypotonicity-provoked anion efflux through a Ca(2+)-dependent mechanism. The order of potency of nucleotides (ATP = UTP = ATP[S] > ADP = AMP >> adenosine = cAMP) indicated the involvement of P2Y(2) receptors. In contrast, millimolar concentrations of ATP markedly inhibited both the osmotically induced isotope efflux and whole-cell Cl(-) currents. Inhibition of whole-cell Cl(-) currents, not only by millimolar ATP but also by the purinoceptor antagonists suramin and reactive blue, was observed most prominently at depolarizing holding potentials, suggesting a direct interaction with volume-sensitive Cl(-) channels rather than interaction with purinoceptors. Both ATP and UTP, at submicromolar levels, were found to act as potent activators of Erk-1/2 in intestine 407 cells. Addition of the ATP hydrolase apyrase to the bath greatly reduced the hypotonicity-induced Erk-1/2 activation, but did not affect the swelling-induced isotope efflux or whole-cell Cl(-) currents. Furthermore, pre-treatment with suramin or reactive blue almost completely prevented the hypo-osmotic activation of Erk-1/2. The results indicate that extracellularly released ATP functions as an autocrinic/paracrinic factor that mediates hypotonicity-induced Erk-1/2 activation but does not serve as an activator of volume-sensitive compensatory Cl(-) currents.

Regulatory effects of HDL on smooth muscle cell prostacyclin release

One mechanism by which high density lipoproteins (HDLs) exert their protective effect against coronary artery disease could be related to the induction of prostacyclin (PGI(2)) release in the vessel wall. We have recently shown that HDL increases PGI(2) production in rabbit smooth muscle cells (RSMCs) and that this increase is dependent on cyclooxygenase-2 (Cox-2). Here we analyze the mechanism by which rabbit HDL induces PGI(2) release in RSMCs. Our results show that although HDL(2) and HDL(3) share a similar capacity to induce Cox-2 protein levels, HDL(3) stimulates a higher PGI(2) release than does HDL(2), probably because of their relative arachidonate contents. Acetylsalicylic acid pretreatment (300 micromol/L, 30 minutes) significantly reduced the HDL-induced PGI(2) release, suggesting that both preexisting and induced Cox-2 activities were involved in the HDL effect. Ca(2+)-dependent cytosolic phospholipase A(2) (cPLA(2)) and Cox-1 protein levels were not altered by HDL. Dexamethasone (2 micromol/L), which also inhibited the HDL-induced PGI(2) release, reduced significantly both Cox-2 mRNA and protein levels without affecting cPLA(2) and Cox-1 protein levels. In addition, methylarachidonyl fluorophosphonate, a potent inhibitor of cPLA(2), did not produce any effect on HDL-induced PGI(2) release. In the presence of cycloheximide, Cox-2 mRNA levels were induced by HDL and inhibited by dexamethasone, suggesting that HDL and dexamethasone work in the absence of de novo protein synthesis. These results indicate an early effect of HDL on PGI(2) biosynthesis, specifically increasing Cox-2. PD98059, an inhibitor of mitogen-activated protein kinase kinase, completely inhibited HDL-induced PGI(2) release, whereas GF109203X, a protein kinase C inhibitor, had no effect. Thus, HDL induces PGI(2) synthesis by a mechanism dependent on the mitogen-activated protein kinase pathway but independent of protein kinase C.

Protein kinase C epsilon-dependent pathway of extracellular signal-regulated protein kinase activation by P2Y1 and P2Y2 purinoceptors that activate cytosolic phospholipase A2 in endothelial cells

The aim of this study was to investigate the stimulating effects on arachidonic acid release of P2Y1 and P2Y2 receptor-selective agonists, 2-methylthio-ATP (2MeSATP) and UTP, respectively, in bovine pulmonary artery endothelial cells. Exposure of cells to 2MeSATP and UTP led to the release of arachidonic acid, a response which was abolished by the removal of extracellular Ca2+ and methyl arachidonyl fluorophosphonate. Phorbol 12-myristate 13-acetate (PMA) itself not only stimulated arachidonic acid release but also played a permissive role in the response to UTP. However, PMA failed to enhance the arachidonic acid response induced by 2MeSATP, probably due to greater attenuation of the [Ca2+]i increase caused by 2MeSATP than UTP. Inhibition of protein kinase C with Ro 31-8220 (1-[3-(amidinothio) propyl-1H-indoyl-3-yl]-3-(1-methyl-1H-indoyl-3-yl)-maleimide -methane sulphate) and staurosporine, but not with Go 6976 (12-(-2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-indolo(2, 3-a)pyrrolo(3,4-c)carbazole), reduced the arachidonic acid response of 2MeSATP, UTP and PMA. PMA-induced potentiation of the UTP response reached a maximum after a 1-h preincubation, then declined and eventually lost its effect when the preincubation lasted up to 8 h. Among the protein kinase C isoforms present in endothelial cells, betaI and epsilon could be down-regulated by treatment with PMA for 4-24 h. PD 098059 (2-(2-Amino-3-methoxyphenyl)-4H-1-benzopyran-4-one) inhibited extracellular signal-regulated protein kinase activation, cytosolic phospholipase A2 phosphorylation and arachidonic acid release caused by 2MeSATP, UTP and PMA. Taken together, our results demonstrate that P2Y1 and P2Y2 purinoceptors mediate arachidonic acid release by activating cytosolic phospholipase A2 through an elevation of [Ca2+]i and protein kinase C epsilon-, extracellular signal-regulated protein kinase-dependent phosphorylation.

Signaling in human osteoblasts by extracellular nucleotides. Their weak induction of the c-fos proto-oncogene via Ca2+ mobilization is strongly potentiated by a parathyroid hormone/cAMP-dependent protein kinase pathway independently of mitogen-activated protein kinase

Extracellular nucleotides acting through specific P2 receptors activate intracellular signaling cascades. Consistent with the expression of G protein-coupled P2Y receptors in skeletal tissue, the human osteosarcoma cell line SaOS-2 and primary osteoblasts express P2Y1 and P2Y2 receptors, respectively. Their activation by nucleotide agonists (ADP and ATP for P2Y1; ATP and UTP for P2Y2) elevates [Ca2+]i and moderately induces expression of the c-fos proto-oncogene. A synergistic effect on c-fos induction is observed by combining ATP and parathyroid hormone, a key bone cell regulator. Parathyroid hormone elevates intracellular cAMP levels and correspondingly activates a stably integrated reporter gene driven by the Ca2+/cAMP-responsive element of the human c-fos promoter. Nucleotides have little effect on either cAMP levels or this reporter, instead activating luciferase controlled by the full c-fos promoter. This induction is reproduced by a stably integrated serum response element reporter independently of mitogen-activated protein kinase activation and ternary complex factor phosphorylation. This novel example of synergy between the cAMP-dependent protein kinase/CaCRE signaling module and a non-mitogen-activated protein kinase/ternary complex factor pathway that targets the serum response element shows that extracellular ATP, via P2Y receptors, can potentiate strong responses to ubiquitous growth and differentiative factors.

P2 receptor subtypes in the cardiovascular system

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

Human adenosine A1 receptor and P2Y2-purinoceptor-mediated activation of the mitogen-activated protein kinase cascade in transfected CHO cells

1. The mitogen-activated protein (MAP) kinase signalling pathway can be activated by a variety of heterotrimeric Gi/Go protein-coupled and Gq/G11 protein-coupled receptors. The aims of the current study were: (i) to investigate whether the Gi/Go protein-coupled adenosine A1 receptor activates the MAP kinase pathway in transfected Chinese hamster ovary cells (CHO-A1) and (ii) to determine whether adenosine A1 receptor activation would modulate the MAP kinase response elicited by the endogenous P2Y2 purinoceptor. 2. The selective adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) stimulated time and concentration-dependent increases in MAP kinase activity in CHO-A1 cells (EC50 7.1+/-0.4 nM). CPA-mediated increases in MAP kinase activity were blocked by PD 98059 (50 microM; 89+/-4% inhibition), an inhibitor of MAP kinase kinase 1 (MEKI) activation, and by pre-treating cells with pertussis toxin (to block Gi/Go-dependent pathways). 3. Adenosine A1 receptor-mediated activation of MAP kinase was abolished by pre-treatment with the protein tyrosine inhibitor, genistein (100 microM; 6+/-10% of control). In contrast, daidzein (100 microM), the inactive analogue of genistein had no significant effect (96+/-12 of control). MAP kinase responses to CPA (1 microM) were also sensitive to the phosphatidylinositol 3-kinase inhibitors wortmannin (100 nM; 55+/-8% inhibition) and LY 294002 (30 microM; 40+/-5% inhibition) but not to the protein kinase C (PKC) inhibitor Ro 31-8220 (10 microM). 4. Activation of the endogenous P2Y2 purinoceptor with UTP also stimulated time and concentration-dependent increases in MAP kinase activity in CHO-A1 cells (EC50=1.6+/-0.3 microM). The MAP kinase response to UTP was partially blocked by pertussis toxin (67+/-3% inhibition) and by the PKC inhibitor Ro 31-8220 (10 microm; 45+/-5% inhibition), indicating the possible involvement of both Gi/Go protein and Gq protein-dependent pathways in the overall response to UTP. 5. CPA and UTP stimulated concentration-dependent increases in the phosphorylation state of the 42 kDa and 44 kDa forms of MAP kinase as demonstrated by Western blotting. 6. Co-activation of CHO-A1 cells with CPA (10 nM) and UTP (1 microM) produced synergistic increases in MAP kinase activity which were not blocked by the PKC inhibitor Ro 31-8220 (10 microM). 7. Adenosine A1 and P2Y2 purinoceptor activation increased the expression of luciferase in CHO cells transfected with a luciferase reporter gene containing the c-fos promoter. However, co-activating these two receptors produced only additive increases in luciferase expression. 8. In conclusion, our studies have shown that the transfected adenosine A1 receptor and the endogenous P2Y2 purinoceptor couple to the MAP kinase signalling pathway in CHO-A1 cells. Furthermore, co-stimulation of the adenosine A1 receptor and the P2Y2 purinoceptor produced synergistic increases in MAP kinase activity but not c-fos mediated luciferase expression.

Tumour necrosis factor stimulates stress-activated protein kinases and the inhibition of DNA synthesis in cultures of bovine aortic endothelial cells

In this study, we examined the ability of tumour necrosis factor-alpha (TNF) to stimulate the mitogen-activated protein (MAP) kinase homologues p42/44 MAP kinase, c-Jun NH2-terminal kinase (JNK) and p38 MAP kinase and its effect upon DNA synthesis in primary cultures of bovine aortic endothelial cells (BAECs). TNF strongly stimulated p38 MAP kinase and JNK activity in both a time- and concentration-dependent manner. By contrast, TNF was a very poor activator of p42/44 MAP kinase relative to the known activator of p42/44 MAP kinase in endothelial cells, adenosine triphosphate (ATP). TNF-stimulated activation of p38 MAP kinase, and MAPKAP kinase-2, a known downstream target of p38 MAP kinase, was strongly inhibited by pre-incubation with the p38 MAP kinase inhibitor SB203580, whereas the minor activation of p42/44 MAP kinase was abolished by pre-incubation of the cell with the novel MAP kinase kinase 1 inhibitor PD098059. Addition of TNF resulted in a 50-60% decrease in DNA synthesis in BAECs. Pre-incubation with PD098059 or co-incubation with ATP failed to modify the inhibitory effect of TNF upon DNA synthesis. SB203580 reduced basal DNA synthesis by approximately 50%; however, if failed to modify the inhibition mediated by TNF. These results indicate that TNF strongly activates both p38 MAP kinase, JNK and, to a minor extent, p42/p44 MAP kinase. It is likely that only one of these kinases, JNK, plays a role in the regulation of DNA synthesis in these cells.

Evidence that P2Y4 nucleotide receptors are involved in the regulation of rat aortic smooth muscle cells by UTP and ATP

1. Previous studies have shown that ATP and UTP are able to stimulate phospholipase C (PLC) and proliferation in cultured aortic smooth muscle cells. Here we set out to characterize the receptor responsible, and investigate a possible role for p42 and p44 mitogen activated protein kinase (MAPK) in the proliferative response. 2. The phospholipase C response of spontaneously hypertensive rat (SHR) derived aortic smooth muscle cells in culture showed that the response to ATP was partial compared to the response to UTP. 3. Further studies characterized the responses of the SHR derived cells. UTP was the only full agonist with the SHR cells; UDP gave a partial response while ADP, 2-methythio-ATP and alpha,beta-methylene ATP were essentially ineffective. The response to UDP was almost lost in the presence of hexokinase, consistent with this being due to extracellular conversion to UTP. These observations are inconsistent with the response being mediated by either P2Y1 or P2Y6 receptors. 4. When increasing concentrations of ATP were present with a maximally effective concentration of UTP, the size of the response diminished, consistent with UTP and ATP acting at a single population of receptors for which ATP was a partial agonist. This is inconsistent with a response mainly at P2Y2 receptors. 5. 1321N1 cells transfected with human P2Y4 receptors gave a similar agonist response profile, with ATP being partial compared to UTP, loss of response to UDP with hexokinase treatment, and with the response to UTP diminishing in the presence of increasing concentrations of ATP. 6. Use of the reverse transcriptase-polymerase chain reaction confirmed the presence of mRNA encoding P2Y4 receptors in SHR derived vascular smooth muscle cells. Transcripts for P2Y2, P2Y4 and P2Y6 receptors, but not P2Y1 receptors, were detected. 7. Stimulation of SHR derived cells with UTP enhanced the tyrosine phosphorylation of both p42 and p44 MAPK, and the incorporation of [3H]-thymidine into DNA. Both these responses were diminished in the presence of an inhibitor of activation of MAPK. 8 These results lead to the conclusion that in SHR derived cultured aortic smooth muscle cells, PLC responses to extracellular UTP and ATP are predominantly at P2Y4 receptors, and suggest that these receptors are coupled to mitogenesis via p42/p44 MAPK.

Diverse signal transduction pathways mediated by endogenous P2 receptors in cultured rat cerebral cortical neurons

The present study was conducted to assess the intracellular signaling pathways mediated by receptors for ATP, uridine triphosphate (UTP), and 2-methylthio ATP (2-MeSATP), by monitoring patch-clamp currents and intracellular calcium mobilization in cultured rat cortical cerebral neurons. All three agonists evoked potassium currents and increased the intracellular free Ca2+ concentration ([Ca2+]i), and these effects were inhibited by the broad G-protein inhibitor guanosine-5'-O-(2-thiodiphosphate) (GDPbetaS) but not by the Gi/o-protein inhibitor pertussis toxin (PTX). UTP-evoked currents were inhibited by either the phospholipase C inhibitor neomycin or the selective protein kinase C (PKC) inhibitor GF109203X, and the rise in cytosolic Ca2+ was inhibited by either neomycin or the inositol 1,4,5-trisphosphate (IP3) receptor antagonist heparin, indicating that the UTP receptor involved phospholipase C-mediated phosphatidylinositol signaling. In contrast, 2-MeSATP-induced currents and rise in cytosolic Ca2+ were not inhibited by either neomycin, or GF109203X, or heparin. 2-MeSATP elicited single-channel currents in the cell-attached patch-clamp configuration and also in excised patches. The G-protein activator GTP gamma S induced single-channel currents in a fashion that mimicked the effect of 2-MeSATP. These data suggest that 2 MeSATP activated potassium channels by a direct action of G-protein beta gamma subunits and increased [Ca2+]i by a mechanism independent of phospholipase C stimulation and IP3 production. ATP-evoked currents were partially inhibited by either neomycin or GF109203X, although the rise in cytosolic Ca2+ was not affected by these inhibitors. ATP produced single-channel currents with two major classes of the slope conductance (86 and 95 pS) in cell-attached patches, each of which is consistent with that achieved by 2-MeSATP (85 pS) or UTP (96 pS); the currents with the lower conductance were observed in the outside-out patch-clamp configuration. These results indicate that P2 receptors for UTP and 2-MeSATP are linked to a PTX-insensitive G-protein involving different signal transduction pathways and that ATP responses are mediated by both of these P2 receptors.

Differential regulation of extracellular signal-regulated kinase and nuclear factor-kappa B signal transduction pathways by hydrogen peroxide and tumor necrosis factor

Reactive oxygen metabolites are increasingly recognized for their ability to stimulate signal transduction pathways. This is important because these oxidants are frequently generated at sites of inflammation. However, little is known about the manner in which reactive oxygen species may selectively stimulate distinct signaling pathways. We have examined this question by stimulating mesothelial cells with hydrogen peroxide (H2O2) as a model oxidant stimulus. The response to H2O2 was examined by measuring the activation of the extracellular signal-regulated kinase (ERK1/2) and the nuclear factor-kappa B (NF-kappa B) signal transduction pathways. We found that H2O2 stimulated activity of the ERK1/2 pathway in a dose- and time-dependent manner. The ability of H2O2 to activate ERK1/2 was similar to that found with tumor necrosis factor (TNF) stimulation. The oxidant effect was inhibited by various reactive oxygen scavengers. An inhibitor of mitogen-activated protein kinase/extracellular signal-regulated kinase, the upstream kinase that activates ERK1/2, inhibited the oxidant effect. The superoxide anion (O2-) also stimulated ERK1/2 activity. In contrast, H2O2 did not stimulate proteolysis of I kappa B-alpha and induced only a small degree of NF-kappa B nuclear translocation. Stimulation of the cells with O2- also induced a minimal degree of NF-kappa B activation. TNF was a potent stimulus for I kappa B-alpha proteolysis and NF-kappa B activation, demonstrating that the cells did have a functional NF-kappa B pathway. These results suggest that oxidants may selectively stimulate certain pathways, thereby preserving some specificity of the signaling process. Furthermore, different cell types and distinct signaling pathways within cells may demonstrate unique profiles in the manner in which they respond to oxidant stimulation.

The regulation of vascular function by P2 receptors: multiple sites and multiple receptors

Although the effects of nucleotides in the cardiovascular system have been known for almost 70 years, it is only in the past few years that some of the P2 receptors at which they act have been cloned and characterized. It is now clear that the control of cardiovascular function by nucleotides is complex, involving multiple receptors and multiple effects in the different cell types of importance. In this review Mike Boarder and Susanna Hourani summarize the P2 receptors that are present in endothelial cells, platelets, smooth muscle and nerves, the signalling pathways that they activate and the responses that are produced. They also discuss the important role of nucleotides in the interactions between the different cell types, and the implications of this in vascular disease.

Cl- currents activated via purinergic receptors in Xenopus follicles

Ionic currents elicited via purinergic receptors located in the membrane of Xenopus follicles were studied using electrophysiological techniques. Follicles responded to ATP-activating inward currents with a fast time course (F(in)). In Ringer solution, reversal potential (Erev) of F(in) was -22 mV, which did not change with external substitutions of Na- or K+, whereas solutions containing 50 or 5% of normal Cl- concentration shifted Erev to about +4 and +60 mV, respectively, and decreased F(in) amplitude, indicating that F(in) was carried by Cl-.F(in) had an onset delay of approximately 400 ms, measured by application of a brief jet of ATP from a micropipette positioned near the follicle (50 microns). F(in) was inhibited by 50% in follicles pretreated with pertussis toxin. This suggests a G protein-mediated receptor channel pathway. F(in) was mimicked by 2-MeSATP and UTP, the potency order (half-maximal effective concentration) was 2-MeSATP (194 nM) > UTP (454 nM) > ATP (1,086 nM). All agonists generated Cl- currents and displayed cross-inhibition on the others. F(in) activation by acetylcholine also cross-inhibited F(in)-ATP responses, suggesting that all act on a common channel-activation pathway.

Regulation of brain capillary endothelial cells by P2Y receptors coupled to Ca2+, phospholipase C and mitogen-activated protein kinase

1. The blood-brain barrier is formed by capillary endothelial cells and is regulated by cell-surface receptors, such as the G protein-coupled P2Y receptors for nucleotides. Here we investigated some of the characteristics of control of brain endothelial cells by these receptors, characterizing the phospholipase C and Ca2+ response and investigating the possible involvement of mitogen-activated protein kinases (MAPK). 2. Using an unpassaged primary culture of rat brain capillary endothelial cells we showed that ATP, UTP and 2-methylthio ATP (2MeSATP) give similar and substantial increases in cytosolic Ca2+, with a rapid rise to peak followed by a slower decline towards basal or to a sustained plateau. Removal of extracellular Ca2+ had little effect on the peak Ca2+-response, but resulted in a more rapid decline to basal. There was no response to alpha,beta-MethylATP (alpha,beta MeATP) in these unpassaged cells, but a response to this P2X agonist was seen after a single passage. 3. ATP (log EC50 -5.1+/-0.2) also caused an increase in the total [3H]-inositol (poly)phosphates ([3H]-InsPx) in the presence of lithium with a rank order of agonist potency of ATP=UTP=UDP>ADP, with 2MeSATP and alpha,beta MeATP giving no detectable response. 4. Stimulating the cells with ATP or UTP gave a rapid rise in the level of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), with a peak at 10 s followed by a decline to a sustained plateau phase. 2MeSATP gave no detectable increase in the level of Ins(1,4,5)P3. 5. None of the nucleotides tested affected basal cyclic AMP, while ATP and ATPgammaS, but not 2MeSATP, stimulated cyclic AMP levels in the presence of 5 microM forskolin. 6. Both UTP and ATP stimulated tyrosine phosphorylation of p42 and p44 mitogen-activated protein kinase (MAPK), while 2MeSATP gave a smaller increase in this index of MAPK activation. By use of a peptide kinase assay, UTP gave a substantial increase in MAPK activity with a concentration-dependency consistent with activation at P2Y2 receptors. 2MeSATP gave a much smaller response with a lower potency than UTP. 7. These results are consistent with brain endothelial regulation by P2Y2 receptors coupled to phospholipase C, Ca2+ and MAPK; and by P2Y1-like (2MeSATP-sensitive) receptors which are linked to Ca2+ mobilization by a mechanism apparently independent of agonist stimulated Ins(1,4,5)P3 levels. A further response to ATP, acting at an undefined receptor, caused an increase in cyclic AMP levels in the presence of forskolin. The differential MAPK coupling of these receptors suggests that they exert fundamentally distinct influences over brain endothelial function.

Angiotensin II responses of vascular smooth muscle cells from hypertensive rats: enhancement at the level of p42 and p44 mitogen activated protein kinase

1. Stimulation of the AT1 receptor by angiotensin II (AII) gives a larger mitogenic response in vascular smooth muscle cells from spontaneously hypertensive rats (SHR) compared to those from normotensive (WKY) controls. Here we investigated whether the p42 and p44 mitogen activated protein kinase (MAPK) pathway is differentially regulated in these cells by AT1 receptors. 2. We showed that there is a similar level of p42 and p44 MAPK immunoreactivity in the SHR and WKY derived cells. 3. However, by use of an antiserum specific for the tyrosine phosphorylated form of MAPK, and an assay with a nonapeptide MAPK substrate, we showed that AII (100 nM)-stimulated phosphorylation and activation of p42mapk and p44mapk are enhanced in the SHR derived cells. 4. This increased MAPK activity in SHR derived cells was also seen on protein kinase C activation with 100 nM phorbol myristate acetate (PMA). The size and time course of the response to PMA was the same as that to AII in each cell type. 5. The protein kinase C inhibitor Ro 31-8220 attenuated the early (2 min) phase of AII stimulation of MAPK activity and the entire stimulation caused by PMA. At longer times of AII stimulation both p42mapk and p44mapk were activated by an Ro 31-8220-insensitive mechanism. 6. Agonist or PMA stimulation of MAPK activity was inhibited by the tyrosine kinase inhibitor genistein. AII stimulated tyrosine protein phosphorylation to a greater degree in SHR than WKY cells. 7. These results show that the MAPK response of SHR derived cells is increased over that of WKY cells by mechanisms independent of the enhanced stimulation of phospholipase C; amplification at the level of sequential protein kinase C and tyrosine kinase steps leads to the enhanced responsiveness of MAPK in the SHR derived cells.