Phospholipase A2 and phospholipase C are activated by distinct GTP-binding proteins in response to alpha 1-adrenergic stimulation in FRTL5 thyroid cells

ATP-release pannexin channels are gated by lysophospholipids

Adenosine triphosphate (ATP) serves as an extracellular messenger that mediates diverse cell-to-cell communication. Compelling evidence supports that ATP is released from cells through pannexins, a family of heptameric large pore-forming channels. However, the activation mechanisms that trigger ATP release by pannexins remain poorly understood. Here, we discover lysophospholipids as endogenous pannexin activators, using activity-guided fractionation of mouse tissue extracts combined with untargeted metabolomics and electrophysiology. We show that lysophospholipids directly and reversibly activate pannexins in the absence of other proteins. Molecular docking, mutagenesis, and single-particle cryo-EM reconstructions suggest that lysophospholipids open pannexin channels by altering the conformation of the N-terminal domain. Our results provide a connection between lipid metabolism and ATP signaling, both of which play major roles in inflammation and neurotransmission.

One-Sentence Summary

Untargeted metabolomics discovers a class of messenger lipids as endogenous activators of membrane channels important for inflammation and neurotransmission.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Hepatic stellate cell senescence in liver fibrosis: Characteristics, mechanisms and perspectives

Myofibroblasts play an important role in fibrogenesis. Hepatic stellate cells are the main precursors of myofibroblasts. Cellular senescence is the terminal cell fate in which proliferating cells undergo irreversible cell cycle arrest. Senescent hepatic stellate cells were identified in liver fibrosis. Senescent hepatic stellate cells display decreased collagen production and proliferation. Therefore, induction of senescence could be a protective mechanism against progression of liver fibrosis and the concept of therapy-induced senescence has been proposed to treat liver fibrosis. In this review, characteristics of senescent hepatic stellate cells and the essential signaling pathways involved in senescence are reviewed. Furthermore, the potential impact of senescent hepatic stellate cells on other liver cell types are discussed. Senescent cells are cleared by the immune system. The persistence of senescent cells can remodel the microenvironment and interact with inflammatory cells to induce aging-related dysfunction. Therefore, senolytics, a class of compounds that selectively induce death of senescent cells, were introduced as treatment to remove senescent cells and consequently decrease the disadvantageous effects of persisting senescent cells. The effects of senescent hepatic stellate cells in liver fibrosis need further investigation.

Receptor- and cellular compartment-specific activation of the cAMP/PKA pathway by α1-adrenergic and ETA endothelin receptors

The signalling functions of many G protein-coupled receptors (GPCRs) expressed in the myocardium are incompletely understood. Among these are the endothelin receptor (ETR) family and α₁-adrenergic receptor (α₁-AR), which are thought to couple to the G protein Gαq. In this study, we used transcriptome analysis to compare the signalling networks downstream of these receptors in primary neonatal rat cardiomyocytes. This analysis indicated increased expression of target genes of cAMP responsive element modulator (CREM) after 24 h treatment with the α₁-AR agonist phenylephrine, but not the ETR agonist endothelin-1, suggesting a specific role for the α₁-AR in promoting cAMP production in cardiomyocytes. To validate the difference observed between these two GPCRs, we used heterologous expression of the receptors and genetically encoded biosensors in HEK 293 cell lines. We validated that both α_1A- and α_1B-AR subtypes were able to lead to the accumulation of cAMP in response to phenylephrine in both the nucleus and cytoplasm in a Gαs-dependent manner. However, the ETR subtype ETA did not affect cAMP levels in either compartment. All three receptors were coupled to Gαq signalling as expected. Further, we showed that activation of PKA in different compartments was α₁-AR subtype specific, with α_1B-AR able to activate PKA in the cytoplasm and nucleus and α_1A-AR only able to in the nucleus. We provide evidence for a pathway downstream of the α₁-AR, and show that distinct pools of a receptor lead to differential activation of downstream effector proteins dependent on their cellular compartment.

A feedback mechanism between PLD and deadenylase PARN for the shortening of eukaryotic poly(A) mRNA tails that is deregulated in cancer cells

The removal of mRNA transcript poly(A) tails by 3′→5′ exonucleases is the rate-limiting step in mRNA decay in eukaryotes. Known cellular deadenylases are the CCR4-NOT and PAN complexes, and poly(A)-specific ribonuclease (PARN). The physiological roles and regulation for PARN is beginning to be elucidated. Since phospholipase D (PLD2 isoform) gene expression is upregulated in breast cancer cells and PARN is downregulated, we examined whether a signaling connection existed between these two enzymes. Silencing PARN with siRNA led to an increase in PLD2 protein, whereas overexpression of PARN had the opposite effect. Overexpression of PLD2, however, led to an increase in PARN expression. Thus, PARN downregulates PLD2 whereas PLD2 upregulates PARN. Co-expression of both PARN and PLD2 mimicked this pattern in non-cancerous cells (COS-7 fibroblasts) but, surprisingly, not in breast cancer MCF-7 cells, where PARN switches from inhibition to activation of PLD2 gene and protein expression. Between 30 and 300 nM phosphatidic acid (PA), the product of PLD enzymatic reaction, added exogenously to culture cells had a stabilizing role of both PARN and PLD2 mRNA decay. Lastly, by immunofluorescence microscopy, we observed an intracellular co-localization of PA-loaded vesicles (0.1-1 nm) and PARN. In summary, we report for the first time the involvement of a phospholipase (PLD2) and PA in mediating PARN-induced eukaryotic mRNA decay and the crosstalk between the two enzymes that is deregulated in breast cancer cells.

Summary: Cell signaling enzyme phospholipase D2 (PLD2) and its reaction product, phospholipid phosphatidic acid (PA), are involved in mediating PARN-induced eukaryotic mRNA decay.

A Pluridimensional View of Biased Agonism

When studying G protein-coupled receptor (GPCR) signaling and ligand-biased agonism, at least three dimensional spaces must be considered, as follows: 1) the distinct conformations that can be stabilized by different ligands promoting the engagement of different signaling effectors and accessory regulators; 2) the distinct subcellular trafficking that can be conferred by different ligands, which results in spatially distinct signals; and 3) the differential binding kinetics that maintain the receptor in specific conformation and/or subcellular localization for different periods of time, allowing for the engagement of distinct signaling effector subsets. These three pluridimensional aspects of signaling contribute to different faces of functional selectivity and provide a complex, interconnected way to define the signaling profile of each individual ligand acting at GPCRs. In this review, we discuss how each of these aspects may contribute to the diversity of signaling, but also how they shed light on the complexity of data analyses and interpretation. The impact of phenotype variability as a source of signaling diversity, and the influence of novel and more sensitive assays in the detection and analysis of signaling pluridimensionality, is also discussed. Finally, we discuss perspectives for the use of the concept of pluridimensional signaling in drug discovery, in which we highlight future predictive tools that may facilitate the identification of compounds with optimal therapeutic and safety properties based on the signaling signatures of drug candidates.

Protein expression, biochemical pharmacology of signal transduction, and relation to intraocular pressure modulation by bradykinin B₂ receptors in ciliary muscle

PURPOSE

To examine the bradykinin (BK) B₂-receptor system in human and monkey ciliary muscle (CM) using immunohistochemical techniques, and to pharmacologically characterize the associated biochemical signal transduction systems in human CM (h-CM) cells. BK-induced modulation of intraocular pressure (IOP) in pigmented Dutch-Belt rabbits and cynomolgus monkeys was also studied.

METHODS

Previously published procedures were used throughout these studies.

RESULTS

The human and monkey ciliary bodies expressed high levels of B₂-receptor protein immunoreactivity. Various kinins differentially stimulated [Ca²⁺](i) mobilization in primary h-CM cells (BK EC₅₀=2.4±0.2 nM > Hyp³,β-(2-thienyl)-Ala⁵,Tyr(Me)⁸-(®)-Arg⁹-BK (RMP-7) > Des-Arg⁹-BK EC₅₀=4.2 µM [n=3-6]), and this was blocked by B₂-selective antagonists, HOE-140 (IC₅₀=1.4±0.1 nM) and WIN-63448 (IC₅₀=174 nM). A phospholipase C inhibitor (U73122; 10-30 µM) and ethylene glycol tetraacetic acid (1-2 mM) abolished the BK-induced [Ca²⁺](i) mobilization. Total prostaglandin (primarily PGE₂) secretion stimulated by BK and other kinins in h-CM cells was attenuated by the cyclooxygenase inhibitors bromfenac and flurbiprofen, and by the B₂-antagonists. BK and RMP-7 (100 nM) induced a twofold increase in extracellular signal-regulated kinase-1/2 phosphorylation, and BK (0.1-1 µM; at 24 h) caused a 1.4-3.1-fold increase in promatrix metalloproteinases-1-3 release. Topical ocular BK (100 µg) failed to alter IOP in cynomolgus monkeys. However, intravitreal injection of 50 µg of BK, but not Des-Arg⁹-BK, lowered IOP in rabbit eyes (22.9±7.3% and 37.0±5.6% at 5 h and 8 h post-injection; n=7-10).

CONCLUSIONS

These studies have provided evidence of a functional endogenously expressed B₂-receptor system in the CM that appears to be involved in modulating IOP.

Mechanisms in bradykinin stimulated arachidonate release and synthesis of prostaglandin and platelet activating factor

Regulatory mechanisms in bradykinin (BK) activated release of arachidonate (ARA) and synthesis of prostaglandin (PG) and platelet activating factor (PAF) were studied in bovine pulmonary artery endothelial cells (BPAEC). A role for GTP binding protein (G-protein) in the binding of BK to the cells was determined. Guanosine 5-O- (thiotriphosphate), (GTPtauS), lowered the binding affinity for BK and increased the Kd for the binding from 0.45 to 1.99 nM. The Bmax remained unaltered at 2.25 x 10(-11) mole. Exposure of the cells to aluminium fluoride also reduced the affinity for BK. Bradykinin-induced release of ARA proved pertussis toxin (PTX) sensitive, with a maximum sensitivity at 10 ug/ml PTX. GTPtauS at 100 muM increased the release of arachidonate. The effect of GTPtauS and BK was additive at suboptimal doses of BK up to 0.5 nM but never exceeded the levels of maximal BK stimulation at 50 nM. PTX also inhibited the release of ARA induced by the calcium ionophore, A23187. Phorbol 12-myristate 13-acetate or more commonly known as tetradecanoyl phorbol acetate (TPA) itself had little effect on release by the intact cells. However, at 100 nM it augmented the BK activated release. This was downregulated by overnight exposure to TPA and correlated with down-regulation of protein kinase C (PKC) activity. The down-regulation only affected the augmentation of ARA release by TPA but not the original BK activated release. TPA displayed a similar, but more potent amplification of PAF synthesis in response to both BK or the calcium ionophore A23187. These results taken together point to the participation of G-protein in the binding of BK to BPAEC and its activation of ARA release. Possibly two types of G-protein are involved, one associated with the receptor, the other activated by Ca(2+) and perhaps associated with phospholipase A(2) (PLA(2)). Our results further suggest that a separate route of activation, probably also PLA(2) related, takes place through a PKC catalysed phosphorylation.

Allosteric modulation of the M₃ muscarinic receptor by amiodarone and N-ethylamiodarone: application of the four-ligand allosteric two-state model

We have reported previously that amiodarone interacts with muscarinic receptors via a novel allosteric site. This study presents mechanistic details on the nature of that interaction. Amiodarone enhanced the maximal level of agonist-stimulated release of arachidonic acid (AA) from Chinese hamster ovary cells that expressed M₃ muscarinic receptors; this enhancement was observed for acetylcholine and for the partial agonist pilocarpine. A similar effect of amiodarone was observed when pilocarpine was used to stimulate inositol phosphate (IP) metabolism, but not when acetylcholine was used. Subsequent studies showed that the IP response exhibited a much larger receptor reserve than the AA response, and reduction of that reserve by receptor alkylation unmasked amiodarone's enhancement of the maximal IP response to acetylcholine. Modulating the receptor reserve also revealed acetylcholine's greater affinity (K(A)) for the conformation of the receptor that mediates the AA response. The amiodarone analog N-ethylamiodarone (NEA) did not alter maximal agonist response but merely reduced agonist potency (that is, it appeared to be an antagonist). However, the action of NEA could be clearly distinguished from the action of the orthosteric antagonist NMS. Demonstration of this point was facilitated by an elaboration of Hall's allosteric two-state model; this new model represents a system composed of two ligands that compete with each other at the orthosteric site and two ligands that compete with each other at the allosteric site. In conclusion, amiodarone competes with NEA at a novel, extracellular, allosteric site to enhance the maximal stimulation evoked by acetylcholine and pilocarpine in two different responses.

Effects of prostaglandin E2 and progesterone on rat brain synaptosomal plasma membranes

The lipid fluidity of rat brain synaptosomal plasma membranes (SPM) labelled with 1,6-diphenyl-1,3,5-hexatriene (DPH) was increased by prostaglandin E2 (PGE2) and decreased by progesterone, as indicated by steady-state fluorescence anisotropy [(ro/r)-1]-1. Arrhenius-type plots of [(ro/r)-1]-1 indicated a lipid phase separation of SPM at approximately 23.5 degrees C which was reduced to approximately 18.1 degrees C by PGE2 and increased to approximately 34.6 degrees C by progesterone. Treatment of SPM by PGE2 and progesterone caused an increase of the lipid phase separation to approximately 32.4 degrees C. Arrhenius plots of Na+/K(+)-ATPase activity in control SPM exhibited a break point at approximately 23.1 degrees C which was reduced to approximately 17.8 degrees C by PGE2 and increased to approximately 32.6 degrees C by progesterone. SPM treated with PGE2 plus progesterone showed an increased break point at approximately 29.3 degrees C. Na+/K(+)-ATPase activity was increased at a PGE2 concentration range between 0.1 and 3 microM; higher concentrations (up to 10 microM) led to a gradual inhibition of enzyme activity. Progesterone (0.1-10 microM) and PGE2 plus progesterone both produced a gradual decrease in enzyme activity. The allosteric inhibition of Na+/K(+)-ATPase by fluoride (F-) (as reflected by changes in the Hill coefficient) was modulated by PGE2 and progesterone. The perturbations of membrane lipid structure and changes in membrane fluidity provide a basis for suggesting an independent non-genomic mechanism for the progesterone-induced alterations in the effects of PGE2 on brain function.

Calcitonin gene-related peptide activates different signaling pathways in mesenteric lymphatics of guinea pigs

The effects of calcitonin gene-related peptide (CGRP) on constriction frequency, smooth muscle membrane potential (V(m)), and endothelial V(m) of guinea pig mesenteric lymphatics were examined in vitro. CGRP (1-100 nM) caused an endothelium-dependent decrease in the constriction frequency of perfused lymphatic vessels. The endothelium-dependent CGRP response was abolished by the CGRP-1 receptor antagonist CGRP-(8-37) (1 microM) and pertussis toxin (100 ng/ml). This action of CGRP was also blocked by the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine (L-NNA; 10 microM), an action that was reversed by the addition of L-arginine (100 microM). cGMP, adenylate cyclase, cAMP-dependent protein kinase (PKA), and ATP-sensitive K+ (K+(ATP)) channels were all implicated in the endothelium-dependent CGRP response because it was abolished by methylene blue (20 microM), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 microM), dideoxyadenosine (10 microM), N-[2-(p-bromociannamylamino)-ethyl]-5-isoquinolinesulfonamide-dichloride (H89; 1 microM) and glibenclamide (10 microM). CGRP (100 nM), unlike acetylcholine, did not alter endothelial intracellular Ca2+ concentration or V(m). CGRP (100 nM) hyperpolarized the smooth muscle V(m), an effect inhibited by L-NNA, H89, or glibenclamide. CGRP (500 nM) also caused a decrease in constriction frequency. However, this was no longer blocked by CGRP-(8-37). CGRP (500 nM) also caused smooth muscle hyperpolarization, an action that was now not blocked by L-NNA (100 microM). It was most likely mediated by the activation of the cAMP/PKA pathway and the opening of K+(ATP) channels because it was abolished by H89 or glibenclamide. We conclude that CGRP, at low to moderate concentrations (i.e., 1-100 nM), decreases lymphatic constriction frequency primarily by the stimulation of CGRP-1 receptors coupled to pertussis toxin-sensitive G proteins and the release of NO from the endothelium or enhancement of the actions of endogenous NO. At high concentrations (i.e., 500 nM), CGRP also directly activates the smooth muscle independent of NO. Both mechanisms of activation ultimately cause the PKA-mediated opening of K+(ATP) channels and resultant hyperpolarization.

PLA2 and secondary metabolites of arachidonic acid control filopodial behavior in neuronal growth cones

The neuronal growth cone provides the sensory and motor structure that guides neuronal processes to their target. The ability of a growth cone to navigate correctly depends on its filopodia, which sample the environment by continually extending and retracting as the growth cone advances. Several second messengers systems that are activated upon contact with extracellular cues have been reported to affect growth cone morphology by changing the length and number of filopodia. Because recent studies have suggested that guidance cues can signal via G-protein coupled receptors to regulate phospholipases, we here investigated whether phospholipase A2 (PLA2) may control filopodial dynamics and could thereby affect neuronal pathfinding. Employing identified Helisoma neurons in vitro, we demonstrate that inhibition of PLA2 with 2 microM BPB caused a 40.3% increase in average filopodial length, as well as a 37.3% reduction in the number of filopodia on a growth cone. The effect of PLA2 inhibition on filopodial length was mimicked by the inhibition of G-proteins with 500 ng/ml pertussis toxin and was partially blocked by the simultaneous activation of PLA2 with 50 nM melittin. We provide evidence that PLA2 acts via production of arachidonic acid (AA), because (1) the effect of inhibition of PLA2 could be counteracted by supplying AA exogenously, and (2) the inhibition of cyclooxygenase, which metabolizes AA into prostaglandins, also increased filopodial length. We conclude that filopodial contact with extracellular signals that alter the activity of PLA2 can control growth cone morphology and may affect neuronal pathfinding by regulating the sensory radius of navigating growth cones.

A complex signaling cascade links the serotonin2A receptor to phospholipase A2 activation: the involvement of MAP kinases

Previous studies in our laboratory have shown that in NIH3T3-5HT2A cells, 5-HT-induced AA release is PLA2-coupled and independent of 5-HT2A receptor-mediated PLC activation. Although 5-HT2A receptor-mediated PLC activation is known to be Galphaq-coupled, much less is understood about 5-HT2A receptor-mediated PLA2 activation. Therefore, the studies presented here were aimed at elucidating the signal transduction pathway linking stimulation of the 5-HT2A receptor to PLA2 activation. By employing various selective inhibitors, toxins, and antagonistic peptide constructs, we propose that the 5-HT2A receptor can couple to PLA2 activation through two parallel signaling cascades. Initial experiments were designed to examine the role of pertussis toxin-sensitive G proteins, namely Galphai/o, as well as pertussis toxin-insensitive G proteins, namely Galpha12/13, in 5-HT-induced AA release. Furthermore, inactivation of both Gbetagamma heterodimers and Rho proteins resulted in decreased agonist-induced AA release, without having any effect on PLC-IP accumulation. We also demonstrated 5-HT2A receptor-mediated phosphorylation of ERK1,2 and p38. Moreover, pretreatment with selective ERK1,2 and p38 inhibitors resulted in decreased 5-HT-induced AA release. Taken together, these results suggest that the 5-HT2A receptor expressed in NIH3T3 cells can couple to PLA2 activation though a complex signaling mechanism involving both Galphai/o-associated Gbetagamma-mediated ERK1,2 activation and Galpha12/13-coupled, Rho-mediated p38 activation.

Regulated interactions of the alpha 2A adrenergic receptor with spinophilin, 14-3-3zeta, and arrestin 3

The present studies demonstrate that no single stretch of sequence in the third intracellular (3i) loop of the alpha(2A) adrenergic receptor (alpha(2A)-AR) can fully account for its previously described interactions with spinophilin (Richman, J. G., Brady, A. E., Wang, Q., Hensel, J. L., Colbran, R. J., and Limbird, L. E. (2001) J. Biol. Chem. 276, 15003-15008), 14-3-3zeta (Prezeau, L., Richman, J. G., Edwards, S. W., and Limbird, L. E. (1999) J. Biol. Chem. 274, 13462-13469), and arrestin 3 (Wu, G., Krupnick, J. G., Benovic, J. L., and Lanier, S. M. (1997) J. Biol. Chem. 272, 17836-17842), suggesting that a three-dimensional surface, rather than a linear sequence, provides the basis for these interactions as proposed for 3i loop tethering of the alpha(2A)-AR to the basolateral surface of Madin-Darby canine kidney cells (Edwards, S. W., and Limbird, L. E. (1999) J. Biol. Chem. 274, 16331-16336). Sequences at the extreme N-terminal and C-terminal ends of the 3i loop are critical for interaction with spinophilin but not for interaction with 14-3-3zeta or arrestin 3, for which the C-terminal half of the loop is more important. Competition binding for (35)S-labeled alpha(2A)-AR 3i loop binding to glutathione S-transferase (GST)-spinophilin amino acids 151-444 revealed a relative affinity of spinophilin congruent with arrestin > 14-3-3zeta for the unphosphorylated alpha(2A)-AR 3i loop. Agonist occupancy of the alpha(2A)-AR increases receptor association with spinophilin, and arrestin 3 appears to compete for this enrichment. However, when the G protein-coupled receptor kinase 2 substrate sequence was deleted from the 3i loop, arrestin 3 could not compete for the agonist-enriched binding of spinophilin to the mutant alpha(2A)-AR. These data are consistent with a model where sequential or competitive interactions among spinophilin, arrestin, and/or 14-3-3zeta play a role in alpha(2A)-AR functions.

ATP-induced endothelium-independent enhancement of lymphatic vasomotion in guinea-pig mesentery involves P2X and P2Y receptors

1. The present study has investigated mechanisms underlying ATP-induced endothelium-independent enhancement of vasomotion in guinea-pig mesenteric lymphatic vessels. 2. Lymphatic vasomotion, vessel tone and smooth muscle [Ca(2+)](i) showed similar ATP concentration-response curves. 3. ATP, at 0.1 mM, caused a biphasic increase in tonic [Ca(2+)](i) and superimposed vasomotion-associated Ca(2+) transients. All ATP-induced [Ca(2+)](i) changes were abolished by incubating the smooth muscle with suramin (0.1 mM). 4. alpha,beta-MeATP (0.1 mM) and UTP (0.1 mM) caused similar changes in [Ca(2+)](i) but the responses to these agonists were smaller than to ATP. 5. The actions of alpha,beta-MeATP (0.1 mM) were inhibited by suramin (0.1 mM) and PPADS (30 micro M) but not by reactive blue 2 (30 micro M). 6. In the presence of alpha,beta-MeATP (0.1 mM), the increases in tonic [Ca(2+)](i) and vasomotion-associated Ca(2+) transients induced by ATP (0.1 mM) were inhibited by U73122 (5 micro M), CPA (20 micro M) and heparin, whereas U73343 (5 micro M) and pre-treatment with PTx (100 ng ml(-1)) had no significant effects. 7. Depletion of the intracellular stores with CPA (20 micro M) caused an increase in [Ca(2+)](i), which was not blocked by desensitization of P(2X) receptors with alpha,beta-MeATP. 8. The data indicate that ATP, at relatively high concentrations increases lymphatic smooth muscle [Ca(2+)](i) and vasomotion through activation of P(2X1) and P(2Y2) purinoceptors present on lymphatic smooth muscle. The increase in [Ca(2+)](i) is likely to result from Ca(2+) release from inositol-1,4,5-trisphosphate-sensitive stores as well as Ca(2+) influx through store-operated channels and P(2X)-gated channels.

Effect of Glutamate and Ionomycin on the Release of Arachidonic Acid, Prostaglandins and HETEs from Cultured Neurons and Astrocytes

The release of arachidonic acid (ArA) metabolites from mouse neurons and astrocytes in primary culture has been studied in response to ionomycin or glutamate stimulation. Cells were preincubated with [3H]ArA for 24 h and the radioactivity released was examined by HPLC. In striatal, cortical and hippocampal neurons, glutamate and ionomycin strongly stimulated the release of ArA, but neither prostaglandins (PGs) nor hydroxyeicosatetraenoic acids (HETEs) could be detected. If they were released, these latter compounds represented < 0.02% of the amount of ArA. In contrast, in astrocyte cultures, ionomycin (but not glutamate) strongly stimulated the release of PGs and HETEs as well as ArA. Reversed- and straight-phase HPLC analysis revealed the presence of PGD2, PGE2, PGF2alpha, 12-hydroxyheptadeca-5,8,10-trienoic acid (HHT) and HETEs (15-HETE, 11-HETE and 5-HETE). Indomethacin inhibited the release of PGs and HHT, but also that of 11- and 15-HETE, indicating that these two HETEs may be produced through the cyclooxygenase pathway. Metabolism of [3H]ArA was also examined in cellular homogenates. Although > 50% of the [3H]ArA was metabolized to PGF2alpha, PGE2, PGD2, HHT, 15- and 11-HETE in cultured astrocyte homogenates, no [3H]ArA metabolism could be detected in cultured striatal neuron homogenates. Moreover, neuronal homogenates did not inhibit the metabolism of [3H]ArA observed in either astrocyte or platelet homogenates. These results indicate that central neurons in primary culture possess very low lipoxygenase and cyclooxygenase activities. They emphasize the need to identify the cellular source of ArA metabolites in the brain, particularly when considering the multiple new messenger roles proposed for these molecules, such as that of retrograde messengers involved in synaptic plasticity phenomena.

Endothelin-evoked Release of Arachidonic Acid from Mouse Astrocytes in Primary Culture

In striatal astrocytes, receptors for the vasoactive peptide endothelin (ET) are associated with several intracellular signalling pathways: ET-1 increases the breakdown of phosphoinositides, induces a sustained influx of Ca2+ and inhibits the isoproterenol-induced formation of cAMP (Marin et al., J. Neurochem., 56, 1270 - 1275, 1991). In the present study, it will be shown that ET-1 and ET-3 markedly stimulate the release of arachidonic acid (AA) from cultured astrocytes from the mouse striatum (EC50=3 and 7 nM for ET-1 and ET-3, respectively), mesencephalon and cerebral cortex. The ET-1-evoked release of AA probably resulted from the activation of a phospholipase A2, since it required extracellular Ca2+ and was prevented by mepacrine but not by RHC 80267, an inhibitor of diacylglycerol lipase. The ET-1-induced release of AA was shown to be partially mediated by a guanine nucleotide-binding protein sensitive to pertussis toxin but not to cholera toxin. A cAMP-dependent process is not involved since the ET-1-evoked release of AA was not affected when cells were incubated with either isoproterenol or 8-bromo-cAMP. The ET-1-evoked release of AA could be mimicked by the co-application of a calcium ionophore and a protein kinase C activator. However, staurosporine, a potent inhibitor of protein kinase C, which blocked the release of AA induced by the combined application of ionomycin and phorbol 12-myristate 12-acetate (PMA), was without effect on the ET-1-evoked response, indicating that protein kinase C is not directly involved in the ET-1-induced release of AA. Furthermore, the responses induced by ET-1 and by PMA were found to be additive. These results suggest that (1) ET-1 receptors are coupled to the release of AA by a mechanism independent of both protein kinase C activation and the adenylate cyclase pathway, possibly via the activation of phospholipase A2, (2) different mechanisms (or different phospholipase A2 subtypes) are involved in the control of AA release in astrocytes.

Modulation of intracellular Ca(2+) via alpha(1B)-adrenoreceptor signaling molecules, G alpha(h) (transglutaminase II) and phospholipase C-delta 1

We characterized the alpha(1B)-adrenoreceptor (alpha(1B)-AR)-mediated intracellular Ca(2+) signaling involving G alpha(h) (transglutaminase II, TGII) and phospholipase C (PLC)-delta 1 using DDT1-MF2 cell. Expression of wild-type TGII and a TGII mutant lacking transglutaminase activity resulted in significant increases in a rapid peak and a sustained level of intracellular Ca(2+) concentration ([Ca(2+)](i)) in response to activation of the alpha(1B)-AR. Expression of a TGII mutant lacking the interaction with the receptor or PLC-delta 1 substantially reduced both the peak and sustained levels of [Ca(2+)](i). Expression of TGII mutants lacking the interaction with PLC-delta 1 resulted in a reduced capacitative Ca(2+) entry. Reduced expression of PLC-delta 1 displayed a transient elevation of [Ca(2+)](i) and a reduction in capacitative Ca(2+) entry. Expression of the C2-domain of PLC-delta 1, which contains the TGII interaction site, resulted in reduction of the alpha(1B)-AR-evoked peak increase in [Ca(2+)](i), while the sustained elevation in [Ca(2+)](i) and capacitative Ca(2+) entry remained unchanged. These findings demonstrate that stimulation of PLC-delta 1 via coupling of the alpha(1B)-AR with TGII evokes both Ca(2+) release and capacitative Ca(2+) entry and that capacitative Ca(2+) entry is mediated by the interaction of TGII with PLC-delta 1.

Adenosine A2Aand A2Breceptor activation of erythropoietin production

First published July 12, 2001; 10.1152/ajprenal.0083.2001.—We have examined the effects of adenosine receptors and protein kinases A and C in the regulation of erythropoietin (Epo) production using hepatocellular carcinoma (Hep3B) cells in culture and in vivo in normal mice under normoxic and hypoxic conditions. CGS-21680, a selective adenosine A2Aagonist, significantly increased levels of Epo in normoxic Hep3B cell cultures and in serum of normal mice under both normoxic and hypoxic conditions. CGS-21680 also produced a significant increase in Epo mRNA levels in Hep3B cell cultures. SCH-58261, a selective adenosine A2Areceptor antagonist, significantly inhibited the increase in medium levels of Epo in Hep3B cell cultures exposed to hypoxia (1% O2). Enprofylline, a selective adenosine A2Breceptor antagonist, significantly inhibited the increase in plasma levels of Epo in normal mice exposed to hypoxia. Chelerythrine chloride, an antagonist of protein kinase C activation, significantly inhibited hypoxia-induced increases in serum levels of Epo in normal mice. A model is presented for adenosine in hypoxic regulation of Epo production that involves kinases A and C and phospholipase A2pathways.

Adenosine A(2A) and A(2B) receptor activation of erythropoietin production

We have examined the effects of adenosine receptors and protein kinases A and C in the regulation of erythropoietin (Epo) production using hepatocellular carcinoma (Hep3B) cells in culture and in vivo in normal mice under normoxic and hypoxic conditions. CGS-21680, a selective adenosine A(2A) agonist, significantly increased levels of Epo in normoxic Hep3B cell cultures and in serum of normal mice under both normoxic and hypoxic conditions. CGS-21680 also produced a significant increase in Epo mRNA levels in Hep3B cell cultures. SCH-58261, a selective adenosine A(2A) receptor antagonist, significantly inhibited the increase in medium levels of Epo in Hep3B cell cultures exposed to hypoxia (1% O(2)). Enprofylline, a selective adenosine A(2B) receptor antagonist, significantly inhibited the increase in plasma levels of Epo in normal mice exposed to hypoxia. Chelerythrine chloride, an antagonist of protein kinase C activation, significantly inhibited hypoxia-induced increases in serum levels of Epo in normal mice. A model is presented for adenosine in hypoxic regulation of Epo production that involves kinases A and C and phospholipase A(2) pathways.

Bradykinin and electrical stimulation increase prostaglandin production in the rat vas deferens

The epididymal portion of the rat vas deferens produced prostaglandins (PG) E(2), F(2alpha)and 6-keto F(1alpha). Electrical stimulation (ES, 0.1 Hz, 1 ms) increased such production by 100%, and similar results were obtained in the presence of 1.0 microM bradykinin (Bk). When both stimuli were applied simultaneously, the increases in PG production were 1100% for PGE(2), 800% for PGF(2alpha)and 400% for PG6-keto F(1alpha). Prazosin abolished the effect of ES on PG production. A selective Bk B(2)-receptor antagonist abolished the increase in PG production induced by Bk, both in non-stimulated and in ES tissues. Bk (1.0 microM) elicited contractile responses in non-stimulated as well as in ES tissues, responses that were not modified in the presence of 10 microM indomethacin. In conclusion, the effects of Bk on prostaglandin production appears to depend on the activation of B(2) receptors, while the increase in prostaglandin release induced by ES, and the effects observed with both stimuli simultaneously, should be mediated by the release of noradrenaline and the subsequent activation of alpha(1) adrenoceptors.

Receptor-stimulated phospholipase A(2) liberates arachidonic acid and regulates neuronal excitability through protein kinase C

Type B photoreceptors in Hermissenda exhibit increased excitability (e.g., elevated membrane resistance and lowered spike thresholds) consequent to the temporal coincidence of a light-induced intracellular Ca(2+) increase and the release of GABA from presynaptic vestibular hair cells. Convergence of these pre- and postsynaptically stimulated biochemical cascades culminates in the activation of protein kinase C (PKC). Paradoxically, exposure of the B cell to light alone generates an inositol triphosphate-regulated rise in diacylglycerol and intracellular Ca(2+), co-factors sufficient to stimulate conventional PKC isoforms, raising questions as to the unique role of synaptic stimulation in the activation of PKC. GABA receptors on the B cell are coupled to G proteins that stimulate phospholipase A(2) (PLA(2)), which is thought to regulate the liberation of arachidonic acid (AA), an "atypical" activator of PKC. Here, we directly assess whether GABA binding or PLA(2) stimulation liberates AA in these cells and whether free AA potentiates the stimulation of PKC. Free fatty-acid was estimated in isolated photoreceptors with the fluorescent indicator acrylodan-derivatized intestinal fatty acid-binding protein (ADIFAB). In response to 5 microM GABA, a fast and persistent increase in ADIFAB emission was observed, and this increase was blocked by the PLA(2) inhibitor arachidonyltrifluoromethyl ketone (50 microM). Furthermore, direct stimulation of PLA(2) by melittin (10 microM) increased ADIFAB emission in a manner that was kinetically analogous to GABA. In response to simultaneous exposure to the stable AA analogue oleic acid (OA, 20 microM) and light (to elevate intracellular Ca(2+)), B photoreceptors exhibited a sustained (>45 min) increase in excitability (membrane resistance and evoked spike rate). The excitability increase was blocked by the PKC inhibitor chelerythrine (20 microM) and was not induced by exposure of the cells to light alone. The increase in excitability in the B cell that followed exposure to light and OA persisted for > or =90 min when the pairing was conducted in the presence of the protein synthesis inhibitor anisomycin (1 microm), suggesting that the synergistic influence of these signaling agents on neuronal excitability did not require new protein synthesis. These results indicate that GABA binding to G-protein-coupled receptors on Hermissenda B cells stimulates a PLA(2) signaling cascade that liberates AA, and that this free AA interacts with postsynaptic Ca(2+) to synergistically stimulate PKC and enhance neuronal excitability. In this manner, the interaction of postsynaptic metabotropic receptors and intracellular Ca(2+) may serve as the catalyst for some forms of associative neuronal/synaptic plasticity.

Cyclooxygenases: structural, cellular, and molecular biology

The prostaglandin endoperoxide H synthases-1 and 2 (PGHS-1 and PGHS-2; also cyclooxygenases-1 and 2, COX-1 and COX-2) catalyze the committed step in prostaglandin synthesis. PGHS-1 and 2 are of particular interest because they are the major targets of nonsteroidal anti-inflammatory drugs (NSAIDs) including aspirin, ibuprofen, and the new COX-2 inhibitors. Inhibition of the PGHSs with NSAIDs acutely reduces inflammation, pain, and fever, and long-term use of these drugs reduces fatal thrombotic events, as well as the development of colon cancer and Alzheimer's disease. In this review, we examine how the structures of these enzymes relate mechanistically to cyclooxygenase and peroxidase catalysis, and how differences in the structure of PGHS-2 confer on this isozyme differential sensitivity to COX-2 inhibitors. We further examine the evidence for independent signaling by PGHS-1 and PGHS-2, and the complex mechanisms for regulation of PGHS-2 gene expression.

Alpha 1-adrenoceptors: subtypes, signaling, and roles in health and disease

Alpha 1-adrenoceptors mediate some of the main actions of the natural catecholamines, adrenaline, and noradrenaline. They participate in many essential physiological processes, such as sympathetic neurotransmission, modulation of hepatic metabolism, control of vascular tone, cardiac contraction, and the regulation of smooth muscle activity in the genitourinary system. It is now clear that alpha 1-adrenoceptors mediate, in addition to immediate effects, longer term actions of catecholamines such as cell growth and proliferation. In fact, adrenoceptor genes can be considered as protooncogenes. Over the past years, considerable progress has been achieved in the molecular characterization of different alpha 1-adrenoceptor subtypes. Three main subtypes have been characterized pharmacologically and in molecular terms. Splice variants, truncated isoforms, and polymorphisms have also been detected. Similarly, it is now clear that these receptors are coupled to several classes of G proteins that, therefore, are capable of modulating different signaling pathways. In the present article, some of these aspects are reviewed, together with the distribution of the subtypes in different tissues and some of the known roles of these receptors in health and disease.

Thrombin releases calcium from internal stores of ultraviolet C-treated V79 fibroblasts independent of phosphatidylinositol bisphosphate hydrolysis: role of oxidative stress

V79 fibroblasts were treated with ultraviolet (UV) C radiation alone as well as in conjunction with chronic oxidative stress. The effects of these treatments on calcium signaling were observed at 30 min post-irradiation. In the absence of extracellular calcium, thrombin released calcium from internal stores of UVC-irradiated V79 fibroblasts even after exposure to neomycin. In neomycin-treated control and chronic oxidative stress cells, no calcium release by thrombin was observed after chelation of external calcium. Calcium release by thrombin from internal stores of UV-irradiated and neomycin-treated cells was completely abolished by pretreatment with N-acetyl cysteine and dexamethasone. Cellular total soluble thiol content which is a good indicator of cellular reduced glutathione (GSH) level was significantly elevated 30 min after ultraviolet radiation, indicating an adaptive response after oxidative stress. Chronic oxidative stress alone resulted in a much smaller increase in GSH but chronic oxidative stress in conjunction with UVC produced a very prominent elevation in GSH levels. Our data suggest that thrombin can cause calcium release from internal stores of ultraviolet-irradiated fibroblasts which is independent of phosphatidylinositol bisphosphate hydrolysis and is directly related to the level of oxidative stress. Involvement of phospholipase A2 and a role for its products as possible mediators of calcium release from intracellular stores, is strongly indicated.

ADP-induced platelet aggregation and inhibition of adenylyl cyclase activity stimulated by prostaglandins: signal transduction mechanisms

ADP is the oldest and one of the most important agonists of platelet activation. ADP induces platelet shape change, exposure of fibrinogen binding sites, aggregation, and influx and intracellular mobilization of Ca2+. ADP-induced platelet aggregation is important for maintaining normal hemostasis, but aberrant platelet aggregation manifests itself pathophysiologically in myocardial ischemia, stroke, and atherosclerosis. Another important aspect of ADP-induced platelet activation is the ability of ADP to antagonize adenylyl cyclase activated by prostaglandins. ADP-induced inhibition of the stimulated adenylyl cyclase activity does not appear to play a role in ADP-induced platelet aggregation in vitro or in vivo. It is believed that a single ADP receptor mediates the above two ADP-induced platelet responses in platelets. The ADP receptor mediating ADP-induced platelet aggregation and inhibition of the stimulated adenylyl cyclase activity has not been purified. Therefore, the nature of molecular mechanisms underlying the two seemingly unrelated ADP-induced platelet responses remains either unclear or less well understood. The purpose of this commentary is to examine and make suggestions concerning the role of phospholipases and G-proteins in the molecular mechanisms of signal transduction underlying the two ADP-induced platelet responses. It is hoped that such discussion would stimulate thinking and invite future debates on this subject, and energize investigators in their efforts to advance our knowledge of the details of the molecular mechanisms of ADP-induced platelet activation.

GTP-binding protein mediated phospholipase A2 activation in rat liver during the progression of sepsis

Effects of GTP-binding proteins on the activation of secretory phospholipaseA2 (sPLA2) and cytosolic phospholipaseA2 (cPLA2) in rat liver during two different phases of sepsis were studied. Sepsis was induced by cecal ligation and puncture (CLP). Experiments were divided into three groups: control, early sepsis, and late sepsis. Early and late sepsis refers to those animals sacrificed at 9 and 18 h, respectively, after CLP. The results show that in the absence of G-protein modulator, hepatic sPLA2 and cPLA2 activities were activated by 40.8-46 and 91.6-105.8%, respectively, during early and late phases of sepsis. GTPgammaS and fluoroaluminate (AlF4-) stimulated sPLA2 and cPLA2 activities within each experimental group, i.e., control, early sepsis, and late sepsis. The GTPgammaS and AlF4(-)-stimulated sPLA2 and cPLA2 activities remained significantly elevated during early phase (22.3-65.6% increase) and late phase (32.5-109.1% increase) of sepsis. Further analyses demonstrate that cholera toxin significantly stimulated sPLA2 and cPLA2 activities within each experimental group, and that the cholera toxin stimulated sPLA2 and cPLA2 activities remained significantly higher during early phase (23.5-37% increase) and late phase (56.7-70% increase) of sepsis. In contrast, pertussis toxin significantly inhibited sPLA2 and cPLA2 activities within each experimental group, and that the pertussis toxin-inhibited sPLA2 and cPLA2 activities remained significantly higher in early septic (57-68.5% increase) and late septic (34.6-45.5% increase) experiments. These data demonstrate that cholera toxin-sensitive G alpha s and pertussis toxin-sensitive G alpha i were both involved in the activation of sPLA2 and cPLA2 activities in rat liver during the progression of sepsis.

Phospholipase A2: its role in ADP- and thrombin-induced platelet activation mechanisms

ADP and thrombin are two of the most important agonists of platelet aggregation--a cellular response that is critical for maintaining normal hemostasis. However, aberrant platelet aggregation induced by these agonists plays a central role in the pathogenesis of cardiovascular and cerebrovascular diseases. Agonist-induced primary or secondary activation of phospholipases leads to generation of the second messengers that participate in biochemical reactions essential to a number of platelet responses elicited by ADP and thrombin. Phospholipase A2 (PLA2) has been linked to cardiovascular diseases. However, the mechanism(s) of activation of PLA2 in platelets stimulated by ADP and thrombin has remained less well defined and much less appreciated. The purpose of this review is to examine and compare the molecular mechanisms of activation of PLA2 in platelets stimulated by ADP and thrombin.

Enhanced in vivo alpha1- and alpha2-adrenoceptor-mediated venoconstriction with indomethacin in humans

Vasodilator prostaglandins are released in vitro from endothelium during adrenergic stimulation. We hypothesized that indomethacin would block this production in vivo and increase venoconstriction to alpha1- and alpha2-stimulation but not to the nonadrenergic agonist PGF2alpha. Hand vein distension was measured in 24 normal subjects (23.0 +/- 0.5 yr) during local infusions of phenylephrine (8-12,000 ng/min), clonidine (3-7,000 ng/min), or PGF2alpha (1-2,048 ng/min) plus indomethacin (3 microg/min) versus saline on two separate days. Dose-dependent venoconstriction to phenylephrine occurred in all subjects, with a parallel shift to the left with indomethacin (P = 0. 003) and a decrease in the phenylephrine 50% effective dose (1,009 vs. 241 ng/min, geometric means, P = 0.012). Venoconstriction to clonidine was more variable, with most subjects eliciting a biphasic response (initial venoconstriction followed by attenuation). With indomethacin, the dose-response curve was displaced up and to the left (P = 0.005), and peak venoconstriction was increased (51.1 +/- 6.8 vs. 27.2 +/- 5.3% of control, P = 0.018) without a biphasic response. In all subjects, PGF2alpha elicited dose-dependent venoconstriction that was not altered by indomethacin. Thus venous alpha1- and alpha2-stimulation results in release of vasodilator prostaglandins that antagonize the venoconstrictor response. This modulates the sympathetic response of venous smooth muscle and may be important in diseases with endothelial dysfunction.

Ubiquitous molecular substrates for associative learning and activity-dependent neuronal facilitation

Recent evidence suggests that many of the molecular cascades and substrates that contribute to learning-related forms of neuronal plasticity may be conserved across ostensibly disparate model systems. Notably, the facilitation of neuronal excitability and synaptic transmission that contribute to associative learning in Aplysia and Hermissenda, as well as associative LTP in hippocampal CA1 cells, all require (or are enhanced by) the convergence of a transient elevation in intracellular Ca2+ with transmitter binding to metabotropic cell-surface receptors. This temporal convergence of Ca2+ and G-protein-stimulated second-messenger cascades synergistically stimulates several classes of serine/threonine protein kinases, which in turn modulate receptor function or cell excitability through the phosphorylation of ion channels. We present a summary of the biophysical and molecular constituents of neuronal and synaptic facilitation in each of these three model systems. Although specific components of the underlying molecular cascades differ across these three systems, fundamental aspects of these cascades are widely conserved, leading to the conclusion that the conceptual semblance of these superficially disparate systems is far greater than is generally acknowledged. We suggest that the elucidation of mechanistic similarities between different systems will ultimately fulfill the goal of the model systems approach, that is, the description of critical and ubiquitous features of neuronal and synaptic events that contribute to memory induction.

Protein tyrosine phosphorylation and calcium signaling in thyroid FRTL-5 cells

We examined the importance of tyrosine kinase(s) on the ATP-evoked Ca2+ entry and DNA synthesis of thyroid FRTL-5 cells. ATP rapidly and transiently tyrosine phosphorylated a 72-kDa protein(s). This phosphorylation was abolished by pertussis toxin and by the tyrosine kinase inhibitor genistein, and was dependent on Ca2+ entry. Pretreatment of the cells with genistein did not affect the release of sequestered Ca2+, but the capacitative Ca2+ or Ba2+ entry evoked by ATP or thapsigargin was attenuated. Pretreatment of the cells with orthovanadate enhanced the increase in intracellular free Ca2+ ([Ca2+]i), whereas the Ba2+ entry was not increased. Phorbol 12-myristate 13-acetate (PMA) phosphorylated the same protein(s) as did ATP. Genistein inhibited the ATP-evoked phosphorylation of MAP kinase and attenuated both the ATP- and the PMA-evoked DNA synthesis. However, genistein did not inhibit the ATP-evoked expression of c-fos. Furthermore, genistein enhanced the ATP-evoked release of arachidonic acid. Thus, ATP activates a tyrosine kinase via a Ca2+-dependent mechanism. A genistein-sensitive mechanism participates, in part, in the ATP-evoked activation of DNA synthesis. Genistein inhibits only modestly capacitative Ca2+ entry in FRTL-5 cells.

Leukotriene B4 Activates the NADPH Oxidase in Eosinophils by a Pertussis Toxin-Sensitive Mechanism That Is Largely Independent of Arachidonic Acid Mobilization

Experiments were designed to investigate whether leukotriene (LTB4) receptors can couple directly to phospholipase A2 (PLA2) in guinea pig eosinophils and the role of endogenous arachidonic acid (AA) in LTB4-induced activation of the NADPH oxidase. LTB4 (EC50 ∼ 16 nM) and AA (EC50 ∼ 6 μM) generated hydrogen peroxide (H2O2) in a concentration-dependent manner and at an equivalent maximum rate (5–6 nmol/min/106 cells). LTB4 stimulated PLA2 over a similar concentration range that activated the NADPH oxidase, although kinetic studies revealed that the release of [3H]AA (t1/2 ∼ 2 s) preceded H2O2 generation (t1/2 &gt; 30 s). Pretreatment of eosinophils with pertussis toxin abolished the increase in inositol(1,4,5)trisphosphate mass, [Ca2+]c, [3H]AA release, and H2O2 generation evoked by LTB4. Qualitatively identical results were obtained in eosinophils in which phospholipase C (PLC) was desensitized by 4β-phorbol 12,13-dibutyrate with the exception that [3H]AA release was largely unaffected. Additional studies performed with the protein kinase C inhibitor, Ro 31-8220, and under conditions in which Ca2+ mobilization was abolished, provided further evidence that LTB4 released [3H]AA independently of signal molecules derived from the hydrolysis of phosphatidylinositol(4,5)bisphosphate by PLC. Pretreatment of eosinophils with the PLA2 inhibitor, mepacrine, abolished LTB4-induced [3H]AA release at a concentration that inhibited H2O2 by only 36%. Collectively, the results of this study indicate that agonism of LTB4 receptors on guinea pig eosinophils mobilizes AA by a mechanism that does not involve the activation of PLC. In addition, although LTB4 effectively stimulated PLA2, a central role for AA in the activation of the NADPH oxidase was excluded.

Mapping of neural and signal transduction pathways for lordosis in the search for estrogen actions on the central nervous system

Estrogen can act on the brain to regulate various biological functions and behavior. In attempts to elucidate the estrogen action, the rodent female reproductive behavior, lordosis, was used as a model. Lordosis is an estrogen-dependent reflexive behavior and, hence, is mediated by discrete neural pathways that are modulated by estrogen. Therefore, a strategy of mapping the pathways, both neural and biochemical, and examining them for estrogen effect was used to localize and subsequently analyze the central action of estrogen. Using various experimental approaches, an 'inverted Y-shaped' neural pathway both sufficient and essential for mediating lordosis was defined. The top portion is a descending pathway conveying the permissive estrogen influence which originated from hypothalamic ventromedial nucleus relayed via midbrain periaqueductal grey down to medullary reticular formation, the top of the spino-bulbo-spinal reflex arc at the bottom. This estrogen influence alters the input-output relationship, shifting the output toward more excitation. With this shift in output, estrogen can enable the otherwise ineffective lordosis-triggering sensory stimuli to elicit lordosis. In the ventromedial nucleus, the origin of the estrogen influence, a multidisciplinary approach was used to map intracellular signaling pathways. A phosphoinositide pathway involving a specific G protein and the activation of protein kinase C was found to be involved in the mediation of lordosis as well as a probable target of the permissive estrogen action. The action of estrogen on this signal transduction pathway, a potentiation, is consistent with and, hence, may be an underlying mechanism for the estrogen influenced shift toward excitation. Thus, further investigation on this specific signal transduction pathway should be helpful in elucidating the action of estrogen on the brain.

Long-term effect of norepinephrine on iodide uptake in FRTL-5 cells

The sympathetic nervous system plays a role in the regulation of thyroid function. In FRTL-5 rat thyroid cells, norepinephrine (NE) acutely depresses intracellular I- by increasing I- efflux. The present study was undertaken to determine the effect of NE on iodide transport after a longer time period. NE inhibited the ability of thyrotropin (TSH) to induce iodide uptake by FRTL-5 cells after 48 or 72 hours, but not after 24 hours. The effect of NE was more evident with increasing concentrations of TSH. NE did not modify the rate of I- efflux. Inhibition was associated with a decrease in the Vmax and no change in the Km for iodide influx. To determine if this was a generalized effect of NE on thyroid cell membrane, the uptake of alpha-aminoisobutyric acid (a nonmetabolizable aminoacid) and of 2-deoxyglucose was measured. NE did not inhibit TSH stimulation of the uptake of the two compounds. NE inhibited the action of dibutyryl cAMP (dbcAMP) on iodide uptake in a similar manner to TSH, but did not alter the cyclic adenosine monophosphate (cAMP) levels increased by TSH. The effects of different adrenoreceptor agonists and antagonists demonstrated that norepinephrine acts through an alpha1-adrenergic receptor.

Cystic fibrosis gene mutation (deltaF508) is associated with an intrinsic abnormality in Ca2+-induced arachidonic acid release by epithelial cells

The mechanism(s) of chronic airway inflammation in cystic fibrosis (CF) remains poorly understood. We studied Ca2+-induced release of arachidonic acid (AA), a precursor of proinflammatory lipid mediators, in epithelial cell lines with the deltaF508 mutation in CF transmembrane conductance regulator (CFTR) gene and in those lacking this mutation or cells in which this mutation was corrected by a functional CFTR gene transfer. We found that: (i) the mutant cells manifested an abnormally high Ca2+-induced AA release as compared to controls, (ii) AA release appeared to be catalyzed by a phospholipase A2 (PLA2) but not by phospholipase C followed by diacylglycerol lipase, and (iii) either correction of the CFTR-mutation or inhibition of PLA2 activity rectified this AA release abnormality. Taken together, our results suggest that CFTR mutation is associated with an intrinsic abnormality in AA release by epithelial cells carrying the deltaF508 mutation and suggest that the mechanism of chronic airway inflammation in CF, at least in part, involves this abnormality. These results also partly explain the effectiveness of high-dose ibuprofen therapy in arresting the progression of destructive lung disease in CF. Furthermore, they raise the possibility that correction of abnormal AA release by inhibiting PLA2 activity may improve the therapeutic benefits of ibuprofen.

Endothelin-1-evoked arachidonic acid release: a Ca(2+)-dependent pathway

Endothelins (ET) are potent vasoconstricting peptides with 21 amino acid residues. Endothelin-1 (ET-1) stimulates arachidonic acid (AA) release in human pericardial smooth muscle cells (HPSMC), which is primarily mediated through the ETA receptor. Manoalide, an inhibitor for phospholipase A2, inhibited the ET-1-evoked response by 50% at 1 microM. RHC-80267, an inhibitor for diacylglycerol lipase, did not have a significant effect. The Ca2+ ionophore A-23187 at 2 microM greatly stimulated AA release in the presence of extracellular Ca2+. ET-1 (10 nM) evoked a robust Ca2+ response. The intracellular Ca2+ concentration reached a peak after 10 s and gradually decreased to a new plateau level in the presence of extracellular Ca2+. ET-1-evoked AA release closely followed the change in the intracellular Ca2+ concentration. Removal of extracellular Ca2+ or treating cells with 250 microM bis(2-amino-5-methylphenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester (MAPTAM; an intracellular Ca2+ chelator) greatly reduced ET-1-stimulated AA release. The protein kinase C (PKC) inhibitors, staurosporine (1 microM) and chelerythrine chloride (2.5 microM), inhibited ET-1-evoked AA release by 70%. Phorbol 12-myristate 13-acetate, a PKC activator, potentiated the effect of ET on AA release. The data suggest that the effect of ET on AA release in HPSMC is via phospholipase A2, which is modulated by Ca2+ and PKC.

Acrosome reaction stimulated by the GTP non-hydrolizable analogue GTP gamma S is blocked by phospholipase A2 inhibitors in human spermatozoa

Phospholipase A2 (PLA2, EC 3.1.1.4) is involved in the cascade of signalling events leading to the acrosome reaction in human spermatozoa. In order to study the role of PLA2 in the acrosome reaction triggered by GTP gamma S, a non-hydrolizable analogue of GTP, two well-known PLA2 inhibitory reagents were used: dexamethasone (1 mM, a synthetic glucocorticoid), and 2-(p-amylcinnamoyl)amino-4-chlorobenzoic acid (ONO-RS-082, 320 micrograms/ml). Normal human spermatozoa were incubated for 3 h under capacitating conditions and treated with several reagents [GTP gamma S, dexamethasone, ONO-RS-082, arachidonic acid (AA) and lysophosphatidylcholine (LPC)], alone or in different combinations. In confirmation of earlier reports, GTP gamma S induced the acrosome reaction. On the other hand, dexamethasone and ONO-RS-082 were both able to inhibit the acrosome reaction induced by GTP gamma S. However, when AA or LPC was added after dexamethasone or ONO-RS-082, the acrosome reaction reached values close to those obtained using GTP gamma S alone. It is concluded that PLA2 probably plays an active role in the acrosome reaction triggered by GTP-binding proteins.

Biosynthesis of the endogenous cyclic adenosine monophosphate (AMP) antagonist, prostaglandylinositol cyclic phosphate (cyclic PIP), from prostaglandin E and activated inositol polyphosphate in rat liver plasma membranes

The endogenous cyclic adenosine monophosphate (AMP) antagonist, cyclic PIP, has been identified as a prostaglandylinositol cyclic phosphate. It inhibits protein kinase A 100% and activates protein serine phosphatase about sevenfold. It is biosynthesized by an enzyme of the plasma membrane when the assay mixture contains adenosine triphosphate (ATP), Mg2+, prostaglandin E and a novel inositol polyphosphate, which cannot be substituted by commercially available inositol phosphates. This novel inositol polyphosphate is a very labile compound. On anion exchange chromatography it elutes in the range of ATP, which may indicate the presence of three phosphate groups. It adsorbs on charcoal, which suggests the presence of a hydrophobic component, possibly a guanosine. Pyrophosphates obtained from inositol 1,4- and inositol 2,4-bisphosphate are accepted by cyclic PIP synthetase for the synthesis of cyclic PIP. The biosynthesis is characterized by enzyme kinetic parameters like dependence on time, enzyme and substrate concentration. The pH optimum of the enzyme is in the range 7.5-8. The enzyme functions optimally with prostaglandin E and poorly with prostaglandin A as the substrate. The presence of fluoride in the assay causes a three- to fourfold increase in cyclic PIP synthesis, which may be correlated with activation via G proteins. These data support previous reports on the chemical structure and action of cyclic PIP. With respect to the possible isomers of cyclic PIP, these indicate that it is most likely the C4-hydroxyl group of the inositol which binds the C15-hydroxyl group of prostaglandin E. A model of hormone-stimulated synthesis of cyclic PIP is proposed: phospholipase A2 and phospholipase C, activated by G proteins upon alpha-adrenergic stimulation, liberate either unsaturated fatty acids or inositol phosphates, which are transformed to prostaglandins and to novel inositol polyphosphate with an energy-rich bond. The cyclic PIP synthetase combines these two substrates to cyclic PIP.

Studies on fenestral contraction in rat liver endothelial cells in culture

Liver endothelial cells possess fenestrae, which are pores supported by a cytoskeleton ring composed of actin and myosin. Fenestrae are dynamic structures that can contract or dilate, although the mechanism for this phenomenon remains to be elucidated. Staining of actin and/or of myosin permitted measurement of fenestral diameter and area in cultured rat liver endothelial cells using digitized video-intensified fluorescence microscopy with image analysis. Within 1 minute of incubation with 0.1 micromol/L serotonin, fenestral diameter and area decreased by 24 +/- 5% and 56 +/- 7%, respectively. Contraction of fenestrae by serotonin was inhibited by chelation of extracellular Ca2+ with EGTA and by addition of Ca2+ channel blockers, such as dilthiazem and verapamil. The response of fenestrae to serotonin was mimicked by addition of a Ca2+ ionophore, A23187. Serotonin inhibited cAMP production, had no effect on inositol phosphate production, and activated phospholipase A2, causing release of arachidonic acid. These results suggest that contraction of fenestrae is associated with Ca2+ influx. In response to 0.1 micromol/serotonin, intracellular Ca2+ levels increased within 3 to 5 seconds from 150 nmol/L to >400 nmol/l followed by rapid phosphorylation of the 20-kd subunit of myosin light chain; both events dependent on extracellular Ca2+.

Angiotensin II-elicited signal transduction via AT1 receptors in endothelial cells

1. Angiotensin II (AII) actions are mediated by two distinct types of receptors: AT1, which includes two subtypes, AT1A and AT1B, and AT2. AII produces vasoconstriction on the vascular wall acting directly on smooth muscle cells via AT1 receptors. AII receptors have recently been demonstrated on endothelial cells. But the pharmacological characteristics of these receptors and the intracellular signal pathways coupled to them remain unclear. 2. The aim of this work was to characterize the AII receptor subtypes in rat aortic endothelial cells (RAEC) in primary culture and to evaluate the signal pathways coupled to these receptors by measuring the activation of phospholipase C (PLC) and phospholipase A2 (PLA2). 3. Labelled AII bound to RAEC in a specific, saturable manner. Scatchard analysis showed a Kd of 1.87 +/- 0.49 nM and a Bmax of 50.2 +/- 10.9 x 10(3) sites per cell. AII was displaced by the AT1-specific antagonist, DuP753 with a Ki of 17.37 +/- 1.49 nM, but not by the AT2 receptor analogues CGP42771B or PD123177. These data were confirmed by the finding of AT1 mRNA in endothelial cells. Analysis of RNA expression by RT-PCR showed the presence of both subtypes, AT1A and AT1B in endothelial cells, whereas smooth muscle cells express only AT1A. 4. The activation of PLC and PLA2 in response to AII was evaluated by measuring inositol phosphate production and arachidonic acid release, respectively. Both were enhanced by AII in a dose-dependent manner, and inhibited by DuP753, but not by PD123177. 5. We conclude that AT1 receptors are expressed by endothelial cells in primary culture and that phospholipase C and phospholipase A2 activated via this receptor.

Increase by lysophosphatidylcholines of smooth muscle Ca2+ sensitivity in alpha-toxin-permeabilized small mesenteric artery from the rat

1. Pharmacological characterization of different lysophosphatidylcholines was performed based on their effect on the Ca2+ sensitivity of contraction in alpha-toxin-permeabilized rat mesenteric arteries. Furthermore, the effect of noradrenaline on [3H]-myristate-labelled lysophosphatidylcholine levels was assessed, to investigate whether lysophosphatidylcholines could be second messengers. 2. Palmitoyl or myristoyl L-alpha-lysophosphatidylcholine increased the sensitivity to Ca2+, whereas lysophosphatidylcholines containing other fatty acids had less or no effect. 3. L-alpha-phosphatidylcholine, L-alpha-glycerophosphorylcholine, palmitic acid, myristic acid and choline, potential metabolites of lysophosphatidylcholines, did not affect contractions. 4. Noradrenaline (GTP was required) and GTP gamma S increased the sensitivity to Ca2+, and GDP-beta-S inhibited the effect of noradrenaline. Lysophosphatidylcholines, however, had no requirement for GTP and caused sensitization in the presence of GDP-beta-S. 5. Calphostin C, a relatively specific protein kinase C inhibitor, did not affect contraction induced by Ca2+, but abolished the sensitizing effect of lysophosphatidylcholine. 6. Noradrenaline caused no measurable changes in the levels of [3H]-myristate-labelled phosphatidylcholine and lysophosphatidylcholine at 30 s and 5 min stimulation. 7. These results suggest that lysophosphatidylcholines can increase Ca2+ sensitivity through a G-protein-independent, but a protein kinase C-dependent mechanism. However, the role for lysophosphatidylcholines as messengers causing Ca2+ sensitization during stimulation with noradrenaline remains uncertain because no increase in [3H]-myristate labelled lysophosphatidylcholine could be measured during noradrenaline stimulation.

Interaction of the GTP-binding protein Gi2 with a protein kinase A-like kinase in mouse fibroblasts

We have previously shown that the GTP-binding protein, Gi2 of mouse Balb/c3T3 cells is linked to a serine kinase which phosphorylates the alpha-subunit of Gi itself. In this report we show that Gi is coupled to a second protein kinase. This kinase does not phosphorylate G but phosphorylates another protein bound non-covalently to G. Phosphorylation of the Gi-linked protein induces its release from Gi. Kinase activity is slightly enhanced by GTPyS, suggesting that this kinase may be physiologically regulated by Gi. In an attempt to identify the kinase we have examined the effect of peptide substrates and inhibitors on kinase activity. We found that the protein kinase A inhibitory peptide, PK1 5-24, inhibited the kinase activity, but at concentrations above those usually required to block protein kinase A. The protein kinase A substrate peptide, kemptide, acted as a substrate of the kinase, and was an inhibitor of the phosphorylation of the Gi-linked protein. However, a protein kinase A, catalytic subunit antibody failed to react with any proteins linked to Gi., A protein kinase C inhibitory peptide had no effect on phosphorylation of the Gi-linked protein. Thus, the identity of this kinase has not been resolved, but it may form part of the signalling system of activated Gi in fibroblasts.