P2Y2 receptor-Gq/11 signaling at lipid rafts is required for UTP-induced cell migration in NG 108-15 cells.

Lipid rafts, formed by sphingolipids and cholesterol within the membrane bilayer, are believed to have a critical role in signal transduction. P2Y(2) receptors are known to couple with G(q) family G proteins, causing the activation of phospholipase C (PLC) and an increase in intracellular Ca(2+) ([Ca(2+)](i)) levels. In the present study, we investigated the involvement of lipid rafts in P2Y(2) receptor-mediated signaling and cell migration in NG 108-15 cells. When NG 108-15 cell lysates were fractionated by sucrose density gradient centrifugation, Galpha(q/11) and a part of P2Y(2) receptors were distributed in a fraction where the lipid raft markers, cholesterol, flotillin-1, and ganglioside GM1 were abundant. Methyl-beta-cyclodextrin (CD) disrupted not only lipid raft markers but also Galpha(q/11) and P2Y(2) receptors in this fraction. In the presence of CD, P2Y(2) receptor-mediated phosphoinositide hydrolysis and [Ca(2+)](i) elevation were inhibited. It is noteworthy that UTP-induced cell migration was inhibited by CD or the G(q/11)-selective inhibitor YM254890 [(1R)-1-{(3S,6S,9S,12S,18R,21S,22R)-21-acetamido-18-benzyl-3-[(1R)-1-methoxyethyl]-4,9,10,12,16, 22-hexamethyl-15-methylene-2,5,8,11,14,17,-20-heptaoxo-1,19-dioxa-4,7,10,13,16-pentaazacyclodocosan-6-yl}-2-methylpropyl rel-(2S,3R)-2-acetamido-3-hydroxy-4-methylpentanoate]. Moreover CD and YM254890 completely inhibited Rho-A activation. Downstream of Rho-A signaling, stress fiber formation and phosphorylation of cofilin were also inhibited by CD or YM254890. However, UTP-induced phosphorylation of cofilin was not affected by the expression of p115-regulator of G protein signaling, which inhibits the G(12/13) signaling pathway. This implies that UTP-induced Rho-A activation was relatively regulated by the G(q/11) signaling pathway. These results suggest that lipid rafts are critical for P2Y(2) receptor-mediated G(q/11)-PLC-Ca(2+) signaling and this cascade is important for cell migration in NG 108-15 cells.

Adenosine uptake-dependent C6 cell growth inhibition.

In C6 glioma cells, adenine nucleotides, especially AMP, and adenosine inhibited cell proliferation in time- and concentration-dependent manners. alpha,beta-methylene-ADP, an ecto-5'-nucleotidase inhibitor, suppressed the hydrolysis of AMP and reversed the inhibition of cell growth induced by AMP but not by adenosine. Adenosine deaminase eliminated both AMP- and adenosine-mediated growth inhibitions. 5'-N-ethylcarboxamidoadenosine, an adenosine receptor agonist, had little effect on the cell growth. Equilibrative nucleoside transporters, ENT-1 and ENT-2, were expressed in C6 cells by determining their mRNAs. ENT inhibitors, nitrobenzylthioinosine and dipyridamole, suppressed the uptake of [(3)H]adenosine into C6 cells, and attenuated AMP- or adenosine-mediated growth inhibition. Furthermore, an adenosine kinase inhibitor 5-iodotubercidin reversed the growth inhibition induced by AMP and adenosine. When uridine was added in the extracellular space, AMP- or adenosine-induced cell growth inhibition was completely reversed, suggesting that intracellular pyrimidine starvation would be involved in their cytostatic effects. These results indicate that extracellular adenine nucleotides inhibit C6 cell growth via adenosine, which is produced by ecto-nucleotidases including CD73 at the extracellular space and then incorporated into cells by ENT2. Intracellular AMP accumulation by adenosine kinase after adenosine uptake would induce C6 cell growth inhibition through pyrimidine starvation.

Cooperation of calcineurin and ERK for UTP-induced IL-6 production in HaCaT keratinocytes.

UTP causes IL-6 production in HaCaT keratinocytes, which is partially inhibited by PD98059, a mitogen-activated protein kinase kinase (MEK) inhibitor, suggesting that a pathway other than the extracellular signal-regulated kinase (ERK) pathway is involved in the production. In the present study, we examined the involvement of calcineurin in the UTP-induced interleukin (IL)-6 production in HaCaT keratinocytes. FK506 and cyclosporine A, calcineurin inhibitors, partially inhibited UTP-induced IL-6 mRNA expression and protein production. In addition, combined application of FK506 and PD98059 synergistically inhibited the UTP-induced IL-6 production. These results suggest that ERK and calcineurin are cooperatively involved in UTP-induced IL-6 production.

Piperlongumine, a constituent of Piper longum L., inhibits rabbit platelet aggregation as a thromboxane A(2) receptor antagonist.

Piper longum L. has been used as a crude drug for the treatment of the disorder of peripherally poor blood circulation in Asia. In the present study, we examined the effect of piperlongumine, a constituent of P. longum L., on rabbit platelet aggregation. Piperlongumine concentration-dependently inhibited platelet aggregation induced by thromboxane A(2) receptor agonist U46619, but it only slightly inhibited thrombin-induced one. Piperlongumine also inhibited U46619-induced phosphatidylinositol hydrolysis and the binding of [(3)H]SQ29548 to thromboxane A(2) receptor with a similar concentration-dependency to the aggregation. It is assumed that piperlongumine inhibits platelet aggregation as a thromboxane A(2) receptor antagonist.

Low expression of cell-surface thromboxane A2 receptor beta-isoform through the negative regulation of its membrane traffic by proteasomes.

Human thromboxane A(2) receptor (TP) consists of two alternatively spliced isoforms, TP alpha and TP beta, which differ in their cytoplasmic tails. To examine the functional difference between TP alpha and TP beta, we searched proteins bound to C termini of TP isoforms by a yeast two-hybrid system, and found that proteasome subunit alpha 7 and proteasome activator PA28 gamma interacted potently with the C terminus of TP beta. The binding of TP beta with alpha 7 and PA28 gamma was confirmed by co-immunoprecipitation and pull-down assays. MG-132 and lactacystin, proteasome inhibitors, increased cell-surface expression of TP beta, but not TP alpha. Scatchard analysis of [(3)H]SQ29548 binding revealed that the B(max) was higher in transiently TP alpha-expressing cells than TP alpha-expressing cells. In addition, TP-mediated phosphoinositide hydrolysis was clearly observed in TP alpha-, but not TP beta-expressing cells. These results suggest that TP beta binds to alpha 7 and PA28 gamma, and the cell-surface expression of TP beta is lower than that of TP alpha through the negative regulation of its membrane traffic by proteasomes.

[Role of lipid rafts in trimeric G protein-mediated signal transduction].

Lipid rafts and caveolae are microdomains in the cell membranes, which contain cholesterol, glycolipids, and sphingomyelin. While caveolae are relatively stable because caveolin, an integral protein, supports the structure, lipid rafts are considered to be unstable, being dynamically produced and degraded. Recent studies have reported that lipid rafts contain many signaling molecules, such as glycosylphosphatidylinositol-anchored proteins, acylated proteins, G-protein-coupled receptors (GPCRs), trimeric and small G-proteins and their effectors, suggesting that the lipid rafts have an important role in receptor-mediated signal transduction. Therefore drugs that modify the composition of lipid rafts might influence the efficacy of cellular signal transduction. In this review, we demonstrate the role of lipid rafts in GPCR-G-protein signaling and also present our recent results showing that the wasp toxin mastoparan modifies G(q/11)-mediated phospholipase C activation through the interaction with gangliosides in lipid rafts.

Distinct effects of z-335, a new thromboxane A2 receptor antagonist, on rabbit platelets and aortic smooth muscle.

The effect of a novel thromboxane A2 receptor (TP) antagonist, (+/-)-sodium[2-(4-chlorophenylsulfonylaminomethyl)- indan-5-yl]acetate monohydrate (Z-335), on the U46619-induced responses was compared between rabbit platelets and aorta. Z-335 inhibited platelet shape change induced by U46619 with higher efficacy than SQ29548, a common TP antagonist. The U46619-induced platelet aggregation was inhibited by Z-335 in a noncompetitive manner, while it was competitively inhibited by SQ29548. Z-335 inhibited U46619-induced vasoconstriction of rabbit aorta with higher efficacy than SQ29548. The pA2 value of Z-335 in aortic vasoconstriction was significantly higher than in platelet shape change. The competitive binding study showed the higher pKi value of Z-335 against [3H]-SQ29548 binding in rabbit aortic smooth muscle cells than in platelets. These data suggest that Z-335 has useful characteristics of TP antagonism.

Contribution of extracellular signal-regulated kinase to UTP-induced interleukin-6 biosynthesis in HaCaT keratinocytes.

UTP causes interleukin (IL)-6 production via mRNA expression through P2Y(2)/P2Y(4) receptors in human HaCaT keratinocytes. In the present study, we analyzed the mechanism of UTP-induced IL-6 production in these cells. UTP, an agonist of P2Y(2)/P2Y(4) receptors, induced phosphorylation of extracellular signal-regulated kinase (ERK) in a concentration- and time-dependent manner. PD98059, a MEK (mitogen-activated protein kinase kinase) inhibitor, and BAPTA-AM [O,O'-bis(2-aminophenyl)ethyleneglycol-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester], an intracellular Ca(2+) chelator, reduced UTP-induced ERK phosphorylation and IL-6 mRNA expression. 2-APB [(2-aminoethoxy)diphenylborane], an inositol 1,4,5-trisphosphate (IP(3))-receptor antagonist, inhibited UTP-induced IL-6 mRNA expression; and the action of A23187, a Ca(2+) ionophore, resembled the action of UTP. In contrast, protein kinase C (PKC) downregulation and pertussis toxin did not affect UTP-induced IL-6 mRNA expression, suggesting that PKC and G(i) are not involved in the UTP-induced IL-6 production. However, AG1478, an epidermal growth factor (EGF)-receptor inhibitor, partially decreased UTP-induced ERK phosphorylation and IL-6 expression. These results suggest that UTP-induced IL-6 production is in part mediated via phosphorylation of ERK through G(q/11)/IP(3)/[Ca(2+)](i) and transactivation of the EGF receptor.

Thromboxane A2 receptor-mediated G12/13-dependent glial morphological change.

Glial cells express thromboxane A(2) receptor, but its physiological role remains unknown. The present study was performed to examine thromboxane A(2) receptor-mediated morphological change in 1321N1 human astrocytoma cells. Thromboxane A(2) receptor agonists U46619 and STA(2) caused a rapid morphological change to spindle shape from stellate form of the cells pretreated with dibutyryl cyclic AMP, but neither carbachol nor histamine caused the change, suggesting that G(q) pathway may not mainly contribute to the change. Rho kinase inhibitor Y-27632 inhibited U46619-induced morphological change, and U46619 increased the GTP-bound form of RhoA accompanied with actin stress fiber formation. These responses were reduced by expression of p115-RGS that inhibits G(12)/(13) signaling pathway. U46619 also caused the phosphorylation of extracellular signal-regulated kinase (ERK) and [(3)H]thymidine incorporation mainly through G(12)/(13)-Rho pathway. These results suggest that stimulation of thromboxane A(2) receptor causes the morphological change with proliferation mainly through G(12)/(13) activation in glial cells.

ATP stimulates interleukin-6 production via P2Y receptors in human HaCaT keratinocytes.

We evaluated the role of ATP in functions of human HaCaT keratinocytes. ATP was released from HaCaT cells by changing the culture medium. Reverse transcription-polymerase chain reaction analysis revealed that HaCaT cells expressed multiple P2 purinergic receptor mRNAs. UTP was the most potent agonist to increase the intracellular Ca2+ concentration ([Ca2+]i). UTP and ATP caused the accumulation of [3H]inositol phosphates, suggesting that UTP binds to the Gq/11-coupled P2Y receptor. UTP increased IL-6 mRNA and protein levels, and the increases were inhibited by a P2 purinergic receptor antagonist (suramin, 300 microM). While a protein kinase C inhibitor (GF109203X, 10 microM) was without effect, an intracellular free Ca2+ chelator (BAPTA-AM, 50 microM) suppressed UTP-mediated IL-6 induction. These results suggest that 1) ATP is released from HaCaT cells upon physical stimulation and may act as an autocrine molecule, and 2) the stimulation of P2Y receptors causes IL-6 production via mRNA expression through [Ca2+]i elevation.

Different pathways for activation of extracellular signal-regulated kinase through thromboxane A2 receptor isoforms.

Thromboxane A2 receptor (TP) consists of two alternatively spliced isoforms, TPalpha and TPbeta, which differ in their cytoplasmic tails. In the present study, we examined the difference in signal transduction of TPalpha and TPbeta, using stably expressing cells of TPalpha and TPbeta. The cells expressing TPalpha (TPalpha-SC2) and TPbeta (TPbeta-SC15) were selected based on the similar binding sites of [3H]-SQ29548, a TP antagonist. U46619, a TP agonist, elicited phosphoinositide hydrolysis in TPalpha-SC2 and TPbeta-SC15 cells with a similar concentration-dependency. U46619 also caused the phosphorylation of extracellular signal-regulated kinase (ERK1/2) in both TPalpha-SC2 and TPbeta-SC15 cells. While the peak of the phosphorylation of ERK1/2 was observed 5 min after addition of U46619 in TPalpha-SC2 cells, the long lasting phosphorylation up to 60 min was in TPbeta-SC15 cells. U46619-induced phosphorylation of ERK1/2 at 5 min was inhibited by pertussis toxin in both cells, suggesting that G(i) is involved in the phosphorylation mediated via both TP isoforms. Interfering G(12/13) activity by overexpression of p115-RGS reduced U46619-induced ERK1/2 phosphorylation in TPbeta-SC15 cells, but not in TPalpha-SC2 cells. H89, an inhibitor of protein kinase A (PKA), reduced U46619-induced ERK1/2 phosphorylation in TPalpha-SC2 cells, but not in TPbeta-SC15 cells. These results indicate that G(i) may be involved in TP-mediated ERK1/2 phosphorylation in both isoforms. In addition, H89-sensitive kinase and G(12/13) may be involved in TP-mediated ERK1/2 phosphorylation in TPalpha and TPbeta, respectively.

Physiological significance of thromboxane A(2) receptor dimerization.

The thromboxane A(2) receptor (TP), one of the G protein-coupled receptors (GPCRs), consists of two splicing variants, TPalpha and TPbeta, which differ in their C-terminal regions. In the present study, we investigated whether TPalpha and TPbeta formed homo- or hetero-dimers and whether the dimerization changed the function of TP. The immunofluorescent analysis using human embryonic kidney (HEK) 293 cells expressing either FLAG-tagged TPalpha or TPbeta showed that TPalpha is mainly distributed on plasma membranes and TPbeta existed on plasma membranes and within the cells. Co-immunoprecipitation analysis using HEK293 cells expressing both TPalpha and TPbeta showed that TPalpha and TPbeta formed homo- and hetero-dimers. U46619, a TP agonist, caused phosphoinositide hydrolysis and elevation of [Ca(2+)](i) in a concentration-dependent manner in Chinese hamster ovary (CHO) cells expressing TPalpha or TPbeta. The responses were observed to a greater extent in the cells expressing TPalpha than TPbeta. In the cells expressing both TPalpha and TPbeta, U46619-induced responses were observed to a lesser extent than in the cells expressing TPalpha alone. Furthermore, [(3)H]SQ29548 binding showed that the level of the cell surface expression of TP was the following order: the cells expressing TPalpha > TPalpha and TPbeta > TPbeta. These results indicate that TPalpha and TPbeta formed homo- and hetero-dimers, and TP-mediated signaling may be regulated by the hetero-dimer.

Neurotrophic factor production in human astrocytoma cells by 2,5,6-tribromogramine via activation of epsilon isoform of protein kinase C.

It is known that astrocytes secrete several neurotrophic factors to promote the survival of neurons. For the treatment of neuronal disorders, low molecular weight compounds inducing neurotrophic factor synthesis are useful, because neurotrophic factors are polypeptides which cannot cross the blood brain barrier. When rat pheochromocytoma (PC-12) cells were cultivated in the medium of human astrocytoma cells (1321N1) treated with 2,5,6-tribromogramine, they differentiated to neuron-like cells possessing neurites, indicating that 2,5,6-tribromogramine released neurotrophic factors from 1321N1 cells. In fact, 2,5,6-tribromogramine increased nerve growth factor (NGF) protein synthesis and secretion through mRNA expression. 2,5,6-Tribromogramine inhibited carbachol-induced phosphoinositide hydrolysis as well as phorbol 12,13-myristate acetate did. The inhibition was recovered by bisindolylmaleimide I (GF109203X), a specific protein kinase C (PKC) inhibitor, indicating that 2,5,6-tribromogramine may activate PKC. The morphological differentiation of PC-12 cells by the medium treated with 2,5,6-tribromogramine was also reduced by GF109203X. 2,5,6-Tribromogramine translocated PKC-epsilon but not PKC-alpha or PKC-zeta, to membrane fraction from cytosol fraction. These results indicate that 2,5,6-tribromogramine promotes the synthesis and secretion of neurotrophic factors including NGF in 1321N1 cells via an activation of PKC-epsilon.

Mastoparan changes the cellular localization of Galphaq/11 and Gbeta through its binding to ganglioside in lipid rafts.

Although it is known that mastoparan, a wasp venom toxin, directly activates Gi/o, mastoparan-induced biological responses are not always explained by this mechanism. For instance, we have demonstrated previously that mastoparan suppressed phosphoinositide hydrolysis induced by carbachol in human astrocytoma cells (FEBS Lett 206:91-94, 1990). In the present study, we examined whether mastoparan affected phosphoinositide hydrolysis by interacting with lipid rafts in PC-12 cells. Mastoparan inhibited UTP-induced increase in [Ca2+]i and phosphoinositide hydrolysis in a concentration-dependent manner. UTP-induced phosphoinositide hydrolysis occurred in lipid rafts, because methyl-beta-cyclodextrin, a disrupting regent of lipid rafts, inhibited the hydrolysis. Mastoparan changed the localization of Galphaq/11 and Gbeta together with cholesterol from lipid rafts to nonraft fractions or cytosol. These changes were inhibited by ganglioside mixtures, suggesting that mastoparan interacts with gangliosides in lipid rafts. In fact, ganglioside mixtures and neuraminidase, but not sialic acid, attenuated the inhibitory effect of mastoparan on phosphoinositide hydrolysis. Furthermore, fluorescence intensity of tyrosine residue of [Tyr3]mastoparan was potentiated by ganglioside mixtures, suggesting the direct binding of mastoparan to gangliosides. Mastoparan caused cytotoxicity of PC-12 cells in a concentration-dependent manner, determined by LDH release. The mastoparan-induced cytotoxicity was significantly inhibited by neuraminidase or gangliosides. The order of inhibitory potency of gangliosides was GT1b approximately GD1b > GD1a > GM1 >> GQ1b, but asialo-GM1 and sialic acid were inactive. These results suggest that mastoparan initially binds to gangliosides in lipid rafts and then it inhibits phosphoinositide hydrolysis by changing the localization of Galphaq/11 and Gbeta in lipid rafts.

ATP- and adenosine-mediated signaling in the central nervous system: adenosine receptor activation by ATP through rapid and localized generation of adenosine by ecto-nucleotidases.

Extracellular ATP is now recognized as a neurotransmitter or neuromodilator in the nervous system, producing diverse physiological effects by activating multiple P2 receptors. Although P2-receptor signaling is terminated by hydrolysis of ATP by the ecto-nucleotidase cascade, such a metabolic step leads to adenosine generation, thereby initiating adenosine (P1)-receptor activation. Because most cells and tissues co-express P1 and P2 receptors, ecto-nucleotidase on target tissues, especially enzymes catalyzing adenosine formation, are determinants of the cellular response to ATP. Ecto-5'-nucleotidase (E-5'-NT) has been considered to play a principal role in conversion of AMP to adenosine. In addition to E-5'-NT, we have recently demonstrated that ecto-alkaline phosphatase is also involved in ATP-induced P1-receptor activation through a rapid and localized adenosine production on the membrane surface. In this minireview, we describe the pharmacological profile of ecto-nucleotidase-dependent P1-receptor activation by ATP and molecular bases of preferential delivery of metabolically generated adenosine to P1 receptors. Several lines of evidence suggest that the close association between ecto-nucleotidases and P1 receptors may constitute a functional receptor for extracellular ATP, and some physiological responses to ATP would occur through this mechanism.

A comparative study on contractile responses of rabbit and guinea pig vasa deferentia to electrical field stimulation.

Contractile responses of rabbit and guinea pig vasa deferentia to electrical field stimulation (EFS) are compared. A muscarinic receptor blocking agent, 1 microM atropine markedly reduced phasic and tonic contraction induced by EFS (20 Hz, 0.5 msec, 30 V, for 30 sec) in rabbit vas deferens, while it only slightly depressed those in guinea pig vas deferens. Further addition of an adrenergic alpha1 receptor blocking agent, 1 microM prazosin markedly depressed the second tonic contraction in both rabbit and guinea pig vasa deferentia. In the presence of atropine and prazosin, further addition of a P2X purinoceptor desensitizing agent, 10 microM alpha,beta-methylene ATP (alpha, beta-MeATP) abolished the residual phasic contractile response in guinea pig vas deferens, while it partially depressed that in rabbit vas deferens. The administration of 10 microM alpha,beta-MeATP in the absence of atropine and prazosin markedly potentiated the phasic contractile response of rabbit vas deferens to EFS, while it depressed that of guinea pig vas deferens. Contractile response of rabbit vas deferens to alpha,beta-MeATP was more potent than those of ATP and 2-methyl-thioATP (2-Me-thioATP), while these nucleotides had almost same potency in guinea pig vas deferens. These findings may indicate that contribution of cholinergic, adrenergic and purinergic neurotransmission to the contractile response of rabbit vas deferens to EFS is different from that of guinea pig vas deferens.

[Lipid rafts and their analytical methods].

Microdomains in cell membranes consist of caveolae and lipid rafts, in which cholesterol, glycolipids, and sphingomyelin are concentrated. While caveolae are relatively stable because caveolin, an integral protein, supports the structure, lipid rafts are unstable, being dynamically produced and degraded. In lipid rafts, flotillin is assumed to be one of the specifically located proteins. Since microdomains contain several signaling molecules, such as transmembrane receptors, they have an important role in receptor-medicated signal transduction. Caveolae or lipid rafts are known to be resistant to non-ionic detergents, such as Triton X-100. Because of this property, they are separated as the detergent-resistant membranes when the Triton X-100-treated cell lysate is subjected to sucrose gradient centrifugation. On the other hand, cholesterol is an essential molecule to maintain microdomain structure. When the cells are treated with cholesterol removing agents, such as methyl-beta-cyclodextrin and filipin, the microdomain in cell membranes is disrupted. Thus, the cholesterol removing agents are utilized to determine whether the microdomain is involved in certain cellular/physiological responses. Recently, green fluorescent protein-tagged protein is used to analyze the localization of the protein in lipid rafts in intact cells. Research on lipid rafts will be helpful for understanding the detailed mechanism of signal transduction and to clarify the molecular basis of several diseases.

Baicalein inhibits Raf-1-mediated phosphorylation of MEK-1 in C6 rat glioma cells.

Baicalein is a flavonoid derived from the Scutellaria root. In investigations of the inhibitors of prostaglandin synthesis in C6 rat glioma cells, we found that baicalein had a potent inhibitory activity on prostaglandin synthesis induced by either histamine or A23187, a Ca(2+) ionophore. Baicalein inhibited histamine- or A23187-induced phosphorylation of p42/p44 extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK), which causes the phosphorylation of cytosolic phospholipase A(2) (PLA(2)). Baicalein also inhibited the phosphorylation of MAPK kinase-1 (MEK-1) induced by histamine or A23187 in the cells. To examine the site of action of baicalein, MEK-1 and Raf-1 were prepared by immunoprecipitation with anti-MEK-1 and anti-Raf-1 antibodies, respectively. Baicalein inhibited the phosphorylation of exogenous MEK-1 by Raf-1 under cell-free conditions, while it did not change the phosphorylation of exogenous p42 MAPK by MEK-1. These results imply that baicalein inhibits the ERK/MAPK cascade, acting on the phosphorylation of MEK-1 by Raf-1.

Adenine nucleotide-induced activation of adenosine A(2B) receptors expressed in Xenopus laevis oocytes: involvement of a rapid and localized adenosine formation by ectonucleotidases.

We recently demonstrated that extracellular ATP effectively activates adenosine (Ade) A(2B) receptors indirectly through a localized rapid conversion to Ade by ectonucleotidases on the membrane surface of C6Bu-1 rat glioma cells. These responses were observed even in the presence of adenosine deaminase (ADA). Here, we demonstrate that such responses indeed occur in A(2B) receptor-expressing Xenopus laevis oocytes, which possess endogenous ectonucleotidase activity. In oocytes coexpressing the A(2B) receptor and cystic fibrosis transmembrane conductance regulator (CFTR), Ade induced a concentration-dependent increase in a cyclic AMP-activated CFTR current, a response that was inhibited by the P1 antagonist xanthine-amine congener (XAC). A brief application of ATP and beta,gamma-methylene ATP (beta,gamma-MeATP) also induced the CFTR current in a manner similar to that seen with Ade. Among several nucleotide agonists, ADP, AMP, and adenosine-5'-O-(3-thio)triphosphate induced the CFTR current. Although adenine nucleotide-induced CFTR currents were inhibited by XAC, they were highly resistant to ADA treatment; 5 U/ml ADA was required for inhibition of adenine nucleotide-induced CFTR current, whereas 1 U/ml ADA was sufficient to abolish the Ade-induced response. In addition, the ecto-5'-nucleotidase inhibitor alpha,beta-methylene ADP markedly inhibited the beta,gamma-MeATP-induced response but not the Ade-induced one. These results support our hypothesis that adenine nucleotides are rapidly and locally converted into Ade on the membrane surface, resulting in the activation of A(2B) receptors.

Theonezolide A, a novel marine macrolide, induces drastic shape change in rabbit platelets by reorganization of microtubules.

Theonezolide A, a marine macrolide, and thrombin caused a shape change followed by an aggregation in the rabbit platelets. Theonezolide A-induced platelet shape change, estimated by a decrease in light transmission, appeared to a greater extent than thrombin-induced one. Morphological studies using an electron microscope showed that theonezolide A changed platelet shape with various numbers of long pseudopods, loosing their discoid shape. Theonezolide A-induced shape change was inhibited by a microtubule-stabilizing agent, taxol, but not by an actin-depolymerizing agent, cytochalasin B. In contrast, thrombin-induced shape change was inhibited by cytochalasin B but not by taxol. Confocal fluorescence microscopy showed that circumferential microtubule bundle disappeared in the platelets treated with theonezolide A. Theonezolide A had no direct effect on polymerization of microtubules isolated from bovine brain, indicating that it indirectly causes microtubule reorganization. These results suggest that theonezolide A induces drastic shape change through reorganization of microtubules in rabbit platelets. Thus, theonezolide A is a useful drug to examine microtubule reorganization in the cells.