Reactive blue 2 inhibition of cyclic AMP-dependent differentiation of rat C6 glioma cells by purinergic receptor-independent inactivation of phosphatidylinositol 3-kinase

Revalidation of DNA Fragmentation Analyses for Human Sperm-Measurement Principles, Comparative Standards, Calibration Curve, Required Sensitivity, and Eligibility Criteria for Test Sperm

(1) Background: Double-strand breaks (DSBs) in a single nucleus are usually measured using the sperm chromatin structure assay (SCSA), sperm chromatin dispersion (SCD) test, and comet assay (CA). Mono-dimensional single-cell pulsed-field gel electrophoresis (1D-SCPFGE) and angle-modulated two- dimensional single-cell pulsed-field gel electrophoresis (2D-SCPFGE) were developed to observe DNA fragmentation in separated motile sperm. (2) Methods: Comparative standards, calibration curves, required sensitivity levels, and eligibility criteria for test sperm were set up to validate the measurement principles of these tests. (3) Results: The conventional methods overlooked the interference of nucleoproteins in their measurements. In-gel proteolysis improves the measurement accuracies of 1D- and 2D-SCPFGE. Naked DNA is suitable for comparative standards and test specimens. Moreover, several dysfunctions that might induce DNA damage are observed in the separated motile sperm. Overall, the discussion highlights the need to revisit the conventional univariable analyses based on the SCSA, SCD test, and CA. (4) Conclusions: Human infertility is a complex syndrome, and the aim of quality control in intracytoplasmic sperm injection is to identify the underlying dysfunctions remaining in the separated motile sperm that render them ineligible for injection. Multivariable analyses with special consideration to confounding factors are necessary in future cohort studies.

Cibacron blue 3G-A is a novel inhibitor of Otopetrin 1 (OTOP1), a proton channel

Otopetrin 1 (OTOP1) is a proton (H⁺) channel which detects acidic stimuli in sour taste receptor cells and plays some sort of role in the formation of otoconia in the inner ear. Although it is known that zinc ion (Zn²⁺) inhibits OTOP1, Zn²⁺ requires high concentrations (mM order) to inhibit OTOP1 sufficiently, and no other inhibitors have been found. Therefore, to identify a novel inhibitor, we screened a chemical library (LOPAC¹²⁸⁰) by whole-cell patch clamp recordings, measuring proton currents of heterologously-expressed mouse OTOP1. From the screening, we found that reactive blue 2 inhibited OTOP1 currents. Further evaluations of three analogues of reactive blue 2 revealed that cibacron blue 3G-A potently inhibited OTOP1 currents. Cibacron blue 3G-A inhibited OTOP1 currents in a concentration-dependent manner, and its 50% inhibitory concentration (IC50) and the Hill coefficient were 5.0 μM and 1.1, respectively. The inhibition of OTOP1 currents by cibacron blue 3G-A was less affected by extracellular anion compositions, membrane potentials, and low pH than the inhibition by Zn²⁺. These results suggest that the inhibition of OTOP1 by cibacron blue 3G-A is neither likely to be a pore-blocking inhibition nor a competitive inhibition. Furthermore, our findings revealed that cibacron blue 3G-A can be used as a novel inhibitor of OTOP1 especially under the conditions in which OTOP1 activity is evaluated such as low pH.

P2Y12 Purinergic Receptor and Brain Tumors: Implications on Glioma Microenvironment

Gliomas are the most common malignant brain tumors in adults, characterized by a high proliferation and invasion. The tumor microenvironment is rich in growth-promoting signals and immunomodulatory pathways, which increase the tumor's aggressiveness. In response to hypoxia and glioma therapy, the amounts of adenosine triphosphate (ATP) and adenosine diphosphate (ADP) strongly increase in the extracellular space, and the purinergic signaling is triggered by nucleotides' interaction in P2 receptors. Several cell types are present in the tumor microenvironment and can facilitate tumor growth. In fact, tumor cells can activate platelets by the ADP-P2Y₁₂ engagement, which plays an essential role in the cancer context, protecting tumors from the immune attack and providing molecules that contribute to the growth and maintenance of a rich environment to sustain the protumor cycle. Besides platelets, the P2Y₁₂ receptor is expressed by some tumors, such as renal carcinoma, colon carcinoma, and gliomas, being related to tumor progression. In this context, this review aims to depict the glioma microenvironment, focusing on the relationship between platelets and tumor malignancy.

Cross-Talk in Nucleotide Signaling in Glioma C6 Cells

The chapter is focused on the mechanism of action of metabotropic P2Y nucleotide receptors: P2Y₁, P2Y₂, P2Y₁₂, P2Y₁₄ and the ionotropic P2X₇ receptor in glioma C6 cells. P2Y₁ and P2Y₁₂ both respond to ADP, but while P2Y₁ links to PLC and elevates cytosolic Ca²⁺ concentration, P2Y₁₂ negatively couples to adenylate cyclase, maintaining cAMP at low level. In glioma C6, these two P2Y receptors modulate activities of ERK1/2 and PI3K/Akt signaling and the effects depend on physiological conditions of the cells. During prolonged serum deprivation, cell growth is arrested, the expression of the P2Y₁ receptor strongly decreases and P2Y₁₂ becomes a major player responsible for ADP-evoked signal transduction. The P2Y₁₂ receptor activates ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulates Akt activity, contributing to glioma invasiveness. In contrast, P2Y₁ has an inhibitory effect on Akt pathway signaling. Furthermore, the P2X₇ receptor, often responsible for apoptotic fate, is not involved in Ca²⁺elevation in C6 cells. The shift in nucleotide receptor expression from P2Y₁ to P2Y₁₂ during serum withdrawal, the cross talk between both receptors and the lack of P2X₇ activity shows the precise self-regulating mechanism, enhancing survival and preserving the neoplastic features of C6 cells.

P2Y1 Receptors - Properties and Functional Activities

In this chapter we try to show a comprehensive image of current knowledge of structure, activity and physiological role of the P2Y₁ purinergic receptor. The structure, distribution and changes in the expression of this receptor are summarized, as well as the mechanism of its signaling activity by the intracellular calcium mobilization. We try to show the connection between the components of its G protein activation and cellular or physiological effects, starting from changes in protein phosphorylation patterns and ending with such remote effects as receptor-mediated apoptosis. The special emphasis is put on the role of the P2Y₁ receptor in cancer cells and neuronal plasticity. We concentrate on the P2Y₁ receptor, it is though impossible to completely abstract from other aspects of nucleotide signaling and cross-talk with other nucleotide receptors is here discussed. Especially, the balance between P2Y₁ and P2Y₁₂ receptors, sharing the same ligand but signaling through different pathways, is presented.

Purinergic signalling and cancer

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

Glial cells decipher synaptic competition at the mammalian neuromuscular junction

It is now accepted that glial cells actively interact with neurons and modulate their activity in many regions of the nervous system. Importantly, modulation of synaptic activity by glial cells depends on the proper detection and decoding of synaptic activity. However, it remains unknown whether glial cells are capable of decoding synaptic activity and properties during early postdevelopmental stages, in particular when different presynaptic nerve terminals compete for the control of the same synaptic site. This may be particularly relevant because a major determinant of the outcome of synaptic competition process is the relative synaptic strength of competing terminals whereby stronger terminals are more likely to occupy postsynaptic territory and become stabilized while weaker terminals are often eliminated. Hence, because of their ability to decode synaptic activity, glial cells should be able to integrate neuronal information of competing terminals. Using simultaneous glial Ca(2+) imaging and synaptic recordings of dually innervated mouse neuromuscular junctions, we report that single glial cells decipher the strength of competing nerve terminals. Activity of single glial cells, revealed by Ca(2+) responses, reflects the synaptic strength of each competing nerve terminal and the state of synaptic competition. This deciphering is mediated by functionally segregated purinergic receptors and intrinsic properties of glial cells. Our results indicate that glial cells decode ongoing synaptic competition and, hence, are poised to influence its outcome.

Cross-talk in nucleotide signaling in glioma C6 cells

The chapter is focused on the mechanism of action of metabotropic P2Y nucleotide receptors: P2Y(1), P2Y(2), P2Y(12), P2Y(14) and the ionotropic P2X(7) receptor in glioma C6 cells. P2Y(1) and P2Y(12) both respond to ADP, but while P2Y(1) links to PLC and elevates cytosolic Ca(2+) concentration, P2Y(12) negatively couples to adenylate cyclase, maintaining cAMP at low level. In glioma C6, these two P2Y receptors modulate activities of ERK1/2 and PI3K/Akt signaling and the effects depend on physiological conditions of the cells. During prolonged serum deprivation, cell growth is arrested, the expression of the P2Y(1) receptor strongly decreases and P2Y(12) becomes a major player responsible for ADP-evoked signal transduction. The P2Y(12) receptor activates ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulates Akt activity, contributing to glioma invasiveness. In contrast, P2Y(1) has an inhibitory effect on Akt pathway signaling. Furthermore, the P2X(7) receptor, often responsible for apoptotic fate, is not involved in Ca(2+)elevation in C6 cells. The shift in nucleotide receptor expression from P2Y(1) to P2Y(12) during serum withdrawal, the cross talk between both receptors and the lack of P2X(7) activity shows the precise self-regulating mechanism, enhancing survival and preserving the neoplastic features of C6 cells.

Higenamine reduces HMGB1 during hypoxia-induced brain injury by induction of heme oxygenase-1 through PI3K/Akt/Nrf-2 signal pathways

Growing lines of evidence suggests that high mobility group box-1 (HMGB1) plays an important role for promoting inflammation and apoptosis in brain ischemia. Previously, we demonstrated that inducers of heme oxygenase-1 (HO-1) significantly reduce HMGB1 release in inflammatory conditions in vitro and in vivo. Thus, we tested our hypothesis that higenamine protects brain injury by inhibition of middle cerebral artery occlusion (MCAO)-mediated HMGB1 release in vivo, and glucose/glucose oxidase (GOX)-induced apoptosis in C6 cells in vitro due to HO-1 induction. Higenamine increased HO-1 expression in C6 cells in both hypoxia and normoxia, in which the former was much more significant than the latter. Higenamine increased Nrf-2 luciferase activity, translocated Nrf-2 to nucleus, and increased phosphorylation of Akt in C6 cells. Consistent with this, LY 294002, a PI3K inhibitor, inhibited HO-1 induction by higenamine and apoptosis induced by glucose/GOX in C6 cells was prevented by higenamine, which effect was reversed by LY 294002. Importantly, administration of higenamine (i.p) significantly reduced brain infarct size, mortality rate, MPO activity and tissue expression of HMGB1 in MCAO rats. In addition, recombinant high mobility group box 1 induced apoptosis in C6 cells by increasing ratio of Bax/bcl-2 and cleaved caspase c, which was inhibited by higenamine, and all of these effects were reversed by co-treatment with ZnPPIX. Therefore, we conclude that higenamine, at least in part, protects brain cells against hypoxic damages by up-regulation of HO-1. Thus, higenamine may be beneficial for the use of ischemic injuries such as stroke.

Cyclic AMP-dependent regulation of differentiation of rat C6 glioma cells by panaxydol

OBJECTIVES

Preliminary works have indicated that panaxydol possesses growth inhibition and induces differentiation in rat C6 glioma cells. However, the molecular mechanism underlying this differentiation remains unknown. We sought to investigate the role of cyclic adenosine monophosphate (cAMP) in cellular differentiation induced by panaxydol.

METHODS

C6 cells were treated with panaxydol and various specific inhibitors, and the inhibition of cell growth was assessed by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl tetrazolium bromide assay and homotransplantation in nude mice. Astrocytic processes were quantified under a phase-contrasted microscope. Glial fibrillary acidic protein expression and cell migration were carried out by Western blot and scratch-wound test, respectively. In addition, the intracellular cAMP concentration was measured by immunoassay.

RESULTS

Panaxydol induces the elevation of intracellular cAMP concentration in C6 cells. The effects of growth inhibition in vitro and in vivo and induction of differentiation in C6 cells by panaxydol could be inhibited by the cAMP inhibitor, Rp-adenosine 3',5'-cyclic monophosphothioate, but not by protein kinase A or protein kinase C specific inhibitors.

CONCLUSION

These results suggest that the cAMP-dependent pathway may regulate cellular proliferation, migration and differentiation in C6 glioma cells by panaxydol.

Akt-dependent heme oxygenase-1 induction by NS-398 in C6 glial cells: A potential role for CO in prevention of oxidative damage from hypoxia

We investigated whether increased heme oxygenase (HO)-1 activity by NS-398 is responsible for protection against hypoxia-induced damage in C6 cells. The expression of HO-1 was analyzed by Western blot and cell viability was analyzed by lactate dehydroxygease (LDH) activity. NS-398 increased HO-1 expression in a concentration- and time-dependent manner during both normoxia and hypoxia (95% N(2)/5% CO(2)), but the latter was much more sensitive. Because induction of HO-1 occurred due to hypoxia itself, NS-398 seemed to potentiate the expression of HO-1. The reduced cell viability due to hypoxia was significantly reversed by either NS-398 or [Ru(CO)(3)(Cl)(2)](2), a CO-donor. Zinc protophorphrin (ZnPPIX), a HO-1 inhibitor, inhibited the protective effect of NS-398 against hypoxia. Treatment with glucose oxidase (GOX, 20 mU/ml) increased ROS production and caused apoptotic death, as assayed by DCFH-DA and TUNEL, respectively. NS-398 significantly reduced GOX-induced cell death and ROS production; these effects were reversed by pre-treatment with oxyhemoglobin (HbO(2)), a CO/NO scavenger, or ZnPPIX. Finally, NS-398 increased PPAR-gamma luciferase activity in transiently PPAR-gamma transfected C6 cells, which was antagonized by ZnPPIX. NS-398 increased phosphorylation of Akt, and LY-294002, a specific PI(3) kinase inhibitor, inhibited NS-398-induced HO-1 expression. Taken together, we conclude that therapeutic use of NS-398 in the treatment of oxidative stress-oriented neuronal disorders would be beneficial through dual actions: HO-1 induction and COX-2 inhibition.

Purinergic receptor antagonism prevents cold preservation-induced cell death independent of cellular ATP levels

BACKGROUND

Purinergic (P2Y) receptors play an important role in intracellular Ca(2+) regulation in hepatocytes. Prevention of mitochondrial Ca(2+) (mCa(2+)) overload during ischemic conditions prevents cellular cell death during the early reperfusion period. P2Y antagonists are cytoprotective in other settings. We studied the effect of P2Y receptor antagonism on mitochondrial associated cell death during the period of cold storage.

METHODS

HepG2 cells were stored in UW with or without 300 muM reactive blue 2 (RB2) or 10 muM ruthenium red (RR) under either normoxic-hypothermic or hypoxic-hypothermic conditions. Cytoplasmic cytochrome c levels were studied by transfection of cytochrome c-GFP. Immunofluorescence determined the intracellular, spatio-temporal distribution of Bax, and terminal deoxynucleotidyl transferase mediated dUTP nick end labeling staining was used to evaluate cell death. Intracellular compartmental ATP levels were assayed by transfecting with luciferase vectors specific for cytoplasm (PcDNA3-luciferase-LL/V) and mitochondria (PcDNA3-COX8-luciferase).

RESULTS

Bax translocation to the mitochondria occurred immediately following cold storage and was followed by cytochrome c-GFP redistribution to the cytosol during rewarming. RB2 treatment significantly attenuated Bax translocation, cytochrome c-GFP redistribution, and cell death following both storage conditions. Both RR and RB2 provided cytoprotection despite ongoing cytoplasmic ATP consumption during cold ischemia.

CONCLUSION

These data indicate that the cytoprotective effects of mCa(2+) uptake inhibition and P2Y receptor antagonism are independent of cytoplasmic ATP levels during cold ischemia.

Expression and functional characterization of P2Y1 and P2Y12 nucleotide receptors in long-term serum-deprived glioma C6 cells

We characterized the expression and functional properties of the ADP-sensitive P2Y(1) and P2Y(12) nucleotide receptors in glioma C6 cells cultured in medium devoid of serum for up to 96 h. During this long-term serum starvation, cell morphology changed from fibroblast-like flat to round, the adhesion pattern changed, cell-cycle arrest was induced, extracellular signal-regulated kinase (ERK1/2) phosphorylation was reduced, Akt phosphorylation was enhanced, and expression of the P2Y(12) receptor relative to P2Y(1) was increased. These processes did not reflect differentiation into astrocytes or oligodendrocytes, as expression of glial fibrillary acidic protein and NG2 proteoglycan (standard markers of glial cell differentiation) was not increased during the serum deprivation. Transfer of the cells into fresh medium containing 10% fetal bovine serum reversed the changes. This demonstrates that serum starvation caused only temporary growth arrest of the glioma C6 cells, which were ready for rapid division as soon as the environment became more favorable. In cells starved for 72 and 96 h, expression of the P2Y(1) receptor was low, and the P2Y(12) receptor was the major player, responsible for ADP-evoked signal transduction. The P2Y(12) receptor activated ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulated Akt activity. These effects were reduced by AR-C69931MX, a specific antagonist of the P2Y(12) receptor. On the other hand, Akt phosphorylation increased in parallel with the low expression of the P2Y(1) receptor, indicating the inhibitory role of P2Y(1) in Akt pathway signaling. The shift in nucleotide receptor expression from P2Y(1) to P2Y(12) would appear to be a new and important self-regulating mechanism that promotes cell growth rather than differentiation and is a defense mechanism against effects of serum deprivation.

P2Y12 receptor signalling towards PKB proceeds through IGF-I receptor cross-talk and requires activation of Src, Pyk2 and Rap1

Previously it was shown that stimulation of the P2Y12 receptor activates PKB signalling in C6 glioma cells [K. Van Kolen and H. Slegers, J. Neurochem. 89, 442.]. In the present study, the mechanisms involved in this response were further elucidated. In cells transfected with the Gbetagamma-scavenger beta-ARK1/GRK2 or Rap1GAPII, stimulation with 2MeSADP failed to enhance PKB phosphorylation demonstrating that the signalling proceeds through Gbetagamma-subunits and Rap1. Moreover, Rap1-GTP pull-down assays revealed that P2Y12 receptor stimulation induced a rapid activation of Rap1. Treatment of cells with the Ca2+ chelator BAPTA-AM and inhibition of Src and PLD2 with PP2 or 1-butanol, respectively, abrogated P2Y12 receptor-mediated activation of Rap1 and PKB. In addition inhibition of PKCzeta decreased basal and 2MeSADP-stimulated phosphorylation of PKB indicating a role for this PKC isoform in PKB signalling. Although the increased PKB phosphorylation was abolished in the presence of the IGF-I receptor tyrosine kinase inhibitor AG 1024, 2MeSADP did not significantly increase receptor phosphorylation. Nevertheless, phosphorylation of a 120 kDa IGF-I receptor-associated protein was observed. The latter protein was identified by MALDI-TOF/TOF-MS as the proline-rich tyrosine kinase 2 (Pyk2) that co-operates with Src in a PLD2-dependent manner. Consistent with the signalling towards Rap1 and PKB, activation of Pyk2 was abrogated by Ca2+ chelation, inhibition of PLD2 and IGF-I receptor tyrosine kinase activity. In conclusion, the data reveal a novel type of cross-talk between P2Y12 and IGF-I receptors that proceeds through Gbetagamma-, Ca2+-and PLD2-dependent activation of the Pyk2/Src pathway resulting in GTP-loading of Rap1 required for an increased PKB phosphorylation.

Presynaptic inhibition of spontaneous acetylcholine release mediated by P2Y receptors at the mouse neuromuscular junction

At the neuromuscular junction, ATP is co-released with the neurotransmitter acetylcholine (ACh) and once in the synaptic space, it is degraded to the presynaptically active metabolite adenosine. Intracellular recordings were performed on diaphragm fibers of CF1 mice to determine the action of extracellular ATP (100 muM) and the slowly hydrolysable ATP analog 5'-adenylylimidodiphosphate lithium (betagamma-imido ATP) (30 muM) on miniature end-plate potential (MEPP) frequency. We found that application of ATP and betagamma-imido ATP decreased spontaneous secretion by 45.3% and 55.9% respectively. 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective A(1) adenosine receptor antagonist and alpha,beta-methylene ADP sodium salt (alphabeta-MeADP), which is an inhibitor of ecto-5'-nucleotidase, did not prevent the inhibitory effect of ATP, demonstrating that the nucleotide is able to modulate spontaneous ACh release through a mechanism independent of the action of adenosine. Blockade of Ca(2+) channels by both, Cd(2+) or the combined application of nitrendipine and omega-conotoxin GVIA (omega-CgTx) (L-type and N-type Ca(2+) channel antagonists, respectively) prevented the effect of betagamma-imido ATP, indicating that the nucleotide modulates Ca(2+) influx through the voltage-dependent Ca(2+) channels related to spontaneous secretion. betagamma-Imido ATP-induced modulation was antagonized by the non-specific P2 receptor antagonist suramin and the P2Y receptor antagonist 1-amino-4-[[4-[[4-chloro-6-[[3(or4)-sulfophenyl] amino]-1,3,5-triazin-2-yl]amino]-3-sulfophenyl] amino]-9,10-dihydro-9,10-dioxo-2-anthracenesulfonic acid (reactive blue-2), but not by pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) tetrasodium salt (PPADS), which has a preferential antagonist effect on P2X receptors. Pertussis toxin and N-ethylmaleimide (NEM), which are blockers of G(i/o) proteins, prevented the action of the nucleotide, suggesting that the effect is mediated by P2Y receptors coupled to G(i/o) proteins. The protein kinase C (PKC) antagonist chelerythrine and the calmodulin antagonist N-(6-aminohexil)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) occluded the effect of betagamma-imido ATP, while the protein kinase A (PKA) antagonist KT-5720 and the inhibitor of the calcium/calmodulin-dependent protein kinase II (CAMKII) KN-62 failed to do so. betagamma-Imido ATP did not affect 10, 15 and 20 mM K(+)-evoked release and application of reactive blue-2 before incubation in high K(+) induced a higher asynchronous secretion. Thus, our results show that at mammalian neuromuscular junctions, ATP induces presynaptic inhibition of spontaneous ACh release due to the modulation of Ca(2+) channels related to tonic secretion through the activation of P2Y receptors coupled to G(i/o) proteins. We also demonstrated that at increasing degrees of membrane depolarization evoked by K(+), endogenously released ATP induces presynaptic inhibition as a means of preventing excessive neurotransmitter secretion.

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

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

Atypical PKCzeta is involved in RhoA-dependent mitogenic signaling by the P2Y12 receptor in C6 cells

When nucleotide hydrolysis is prevented, agonists of the P2Y(12) receptor enhance the proliferation of C6 glioma cells by RhoA-dependent, protein kinase C (PKC)-dependent activation of the extracellular signal-regulated kinase (ERK) pathway [Claes P, Grobben B, Van Kolen K, Roymans D & Slegers H (2001) Br J Pharmacol134, 402-408; Grobben B, Claes P, Van Kolen K, Roymans D, Fransen P, Sys SU & Slegers H (2001) J Neurochem78, 1325-1338]. In this study, we show that ERK1/2 phosphorylation was not affected by transfection of the cells with the Gbetagamma-subunit-scavenging adrenergic receptor kinase peptide [betaARK1-(495-689)] or with Rap1GAPII, indicating that P2Y(12) receptor stimulation enhances ERK1/2 phosphorylation by G(i)alpha subunit-mediated signaling independently of Rap1 activation. Inhibition of the RhoA downstream effector Rho-associated coiled-coil-containing kinase (ROCK) with Y-27632 did not affect the P2Y(12) receptor-induced increase in ERK1/2 phosphorylation but abrogated the mitogenic response. Involvement of growth factor receptor transactivation in the signaling towards ERK phosphorylation could be ruled out by the lack of an effect of PP2, AG1024, AG1296 or SU1498, inhibitors of Src, insulin-like growth factor receptor, platelet-derived growth factor receptor and vascular endothelial growth factor receptor kinase activity, respectively. Experiments with bisindolylmaleimide I and IX indicated the requirement of PKC activity. Classical and novel PKC isoforms could be excluded by treatment of the cells with Gö6976 and calphostin C, whereas addition of a myristoylated PKCzeta pseudosubstrate inhibitor completely abolished P2Y(12) receptor-induced ERK1/2 activation. Moreover, coimmunoprecipitation experiments revealed PKCzeta/Raf1 and PKCzeta/ERK association, indicating the involvement of PKCzeta. From the data presented, we can conclude that the P2Y(12) receptor enhances cell proliferation by a G(i)alpha-dependent, RhoA-dependent PKCzeta/Raf1/MEK/ERK pathway that requires activation of ROCK, which is not involved in ERK1/2 signaling.

Extracellular ATP activates ERK1/ERK2 via a metabotropic P2Y1 receptor in a Ca2+ independent manner in differentiated human skeletal muscle cells

ATP is released at the neuromuscular junction to regulate development and proliferation. The sequential expression of P2X and P2Y receptors has been correlated to these effects in many species and cell lines. We have therefore investigated ATP mediated signalling in differentiated primary human skeletal muscle cells. ATP was capable to trigger Ca2+ transients in these cells via P2Y receptors which were not attributable to Ca2+ influx via P2X receptors. Instead, ATP propagated the formation of inositol phosphate (IP) with an EC50 of 21.3 microM. The Ca2+ transient provoked by ATP was abrogated roughly 75% by the phospholipase C (PLC) inhibitor, U73122. Interestingly, the ryanodine sensitive Ca2+ pool was not involved in ATP triggered Ca2+ release. On mRNA level and by a pharmacological approach we confirmed the presence of the P2Y1, P2Y2, P2Y4 and P2Y6 receptors. Substantially, ATP activated IP formation via a P2Y1 receptor. In addition, ATP elicited extracellular signal regulated kinase (ERK)1/2 phosphorylation in a time and concentration dependent manner, again mainly via P2Y1 receptors. The ATP mediated ERK1/2 phosphorylation was strictly dependent on phospholipase C and PI3 kinase activity. Importantly, ATP mediated ERK1/2 phosphorylation was Ca2+ independent. This observation was corroborated by the finding that conventional protein kinase C inhibitors did not suppress ATP triggered ERK1/2 phosphorylation. Taken together, these observations highlight the importance of ATP as a co-neurotransmitter at the neuromuscular junction via dual signalling, i.e. IP3 receptor mediated Ca2+ transients and Ca2+ insensitive phosphorylation of ERK1/2.

Estrogen prevents glutamate-induced apoptosis in C6 glioma cells by a receptor-mediated mechanism

Estrogen-mediated neuroprotection is well established; however, no single mechanism of action for this effect has yet been established. As glial cells are integral for both the intact and injured nervous system, we hypothesized that estrogen-mediated neuroprotection may partly be attributed to attenuation of glial cell apoptosis, allowing them to protect neurons following injury. To assess the protective effects of estrogen on glia, C6 rat glioma cells were treated for 24 h with 500 microM glutamate. Cell viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, and apoptosis was confirmed by cell morphology and DNA fragmentation. Pretreatment with 10 nM 17beta-estradiol (estrogen) increased cell viability and attenuated apoptosis. Treatment with the stereoisomer 17alpha-estradiol, or estrogen plus estrogen receptor antagonist ICI 182,780, was significantly less effective, indicating that cytoprotection was receptor-mediated. Estrogen treatment upregulated expression of estrogen receptor alpha. Cell impermeable bovine serum albumin-conjugated estrogen was also protective, indicating activation of estrogen receptors on the cell membrane. Intracellular free [Ca2+] was increased after glutamate treatment. This increase was attenuated in cells pretreated with estrogen. Glutamate increased the activity of pro-apoptotic proteases, such as calpain and caspase-3, and these protease activities were significantly attenuated by estrogen. The mechanism by which estrogen decreased intracellular Ca2+ was examined by assaying cell viability after using inhibitors that either blocked extracellular Ca2+ influx or prevented the release of intracellular Ca2+ stores. While several inhibitors increased cell viability in glutamate-treated cells, none were as protective as estrogen, and estrogen co-treatment significantly increased cell viability. These findings indicate that estrogen-mediated cytoprotection may be related to effects on Ca2+ entry but that these effects are not limited to any one of these Ca2+ entry points alone.

Hydrogen peroxide and epidermal growth factor activate phosphatidylinositol 3-kinase and increase sodium transport in A6 cell monolayers

Activation of phosphatidylinositol 3-kinase (PI 3-kinase) is required for insulin stimulation of sodium transport in A6 cell monolayers. In this study, we investigate whether stimulation of the PI 3-kinase by other agents also provoked an increase in sodium transport. Both epidermal growth factor (EGF) and H2O2provoked a rise in sodium transport that was inhibited by LY-294002, an inhibitor of PI 3-kinase activity. PI 3-kinase activity was estimated in extracts from A6 cell monolayers directly by performance of a PI 3-kinase assay. We also estimated the relative importance of the PI 3-kinase pathway by two different methods: 1) coprecipitation of the p85 regulatory subunit with anti-phosphotyrosine antibodies and 2) phosphorylation of PKB on both Ser 473 and Thr 308 residues observed by Western blotting. Since the mitogen-activated protein kinase (MAPK) pathway has also been implicated in the regulation of sodium transport, we also investigated whether this pathway is turned on by insulin, H2O2, or EGF. Phosphorylation of ERK1/2 was increased only transiently by insulin and H2O2but quite sustainedly by EGF. Inhibitors of this pathway (U-0126 and PD-98059) failed to affect the insulin and H2O2stimulation of sodium transport but increased substantially the stimulation induced by EGF. The latter effect was associated with an increase in PKB phosphorylation, thus suggesting that the stimulation of the MAPK pathway prevents, in part, the stimulation of the PI 3-kinase pathway in the transport of sodium stimulated by EGF.

Phosphatidylinositol 3,4,5-trisphosphate: an early mediator of insulin-stimulated sodium transport in A6 cells

Insulin stimulates sodium transport across A6 epithelial cell monolayers. Activation of phosphatidylinositol 3-kinase (PI 3-kinase) was suggested as an early step in the insulin-stimulated sodium reabsorption (Ref. 35). To establish that the stimulation of the PI 3-kinase signaling cascade is causing stimulation of apical epithelial Na channel, we added permeant forms of phosphatidylinositol (PI) phosphate (P) derivatives complexed with a histone carrier to A6 epithelium. Only PIP3and PI( 3 , 4 )P2but not PI( 4 , 5 )P2stimulated sodium transport, although each of them penetrated into A6 cell monolayers as assessed using fluorescent permeant phosphoinositides derivatives. By Western blot analysis of A6 cell extracts, the inositol 3-phosphatase PTEN and the protein kinase B PKB were both detected. To further establish that the stimulation of sodium transport induced by insulin is related to PIP3levels, we transfected A6 cells with human PTEN cDNA and observed a 30% decrease in the natriferic effect of insulin. Similarly, the increase in sodium transport observed by addition of permeant PIP3was also reduced by 30% in PTEN-overexpressing cells. PKB, a main downstream effector of PI 3-kinase, was phosphorylated at both Thr 308 and Ser 473 residues upon insulin stimulation of the A6 cell monolayer. PKB phosphorylation in response to insulin stimulation was reduced in PTEN-overexpressing cells. Permeant PIP3also increased PKB phosphorylation. Taken together, the present results establish that the d-3-phosphorylated phosphoinositides PIP3and PI( 3 , 4 )P2mediate the effect of insulin on sodium transport across A6 cell monolayers.