Receptor-mediated activation of phospholipase A2 via GTP-binding proteins: arachidonic acid and its metabolites as second messengers

G protein regulation of phospholipase A2

Many neurotransmitters and hormones activate receptors that are known to be coupled to their effectors by GTP-binding regulatory proteins, G proteins. Activation of many of these same receptors elicits arachidonate release and metabolism. During the past few years, novel experimental techniques have revealed that in many cells arachidonate release is independent of generation of other second messengers, including inositol phosphates, diacylglycerols, and elevation in free intracellular calcium. Much evidence has accumulated to implicate phospholipase A2 as the enzyme catalyzing arachidonate release, and suggesting that this effector enzyme, too, is activated by G proteins. In neural tissues as well as epithelium, endothelium, contractile and connective tissues, and blood cells, G proteins coupled to receptors for a variety of peptide and nonpeptide neurotransmitters and hormones have been shown to directly activate phospholipase A2. In retinal rod outer segments, transducin is the coupling G protein, but the G proteins coupling receptor activation to phospholipase A2 in other cell types is less clear. Some are pertussis toxin-sensitive, whereas others are not, and evidence exists that the ras gene product G protein may also be coupled to and regulate phospholipase A2.

Protein kinase C activation alters the sensitivity of agonist-stimulated endothelial-cell prostacyclin production to intracellular Ca2+

Agonist-stimulated release of prostacyclin (PGI2) from endothelial cells requires elevation of the concentration of intracellular ionized calcium ([Ca2+]i) above a threshold value, and raised [Ca2+]i provides a sufficient transduction signal to account for the extent of PGI2 production. However, chronic activation of protein kinase C has been reported separately to potentiate PGI2 release, but to depress agonist-induced elevations of [Ca2+]i. We show here that pretreatment with phorbol 12-myristate 13-acetate (PMA) dose-dependently induces PGI2 release over many minutes after a significant lag period without any change in [Ca2+]i. In addition, PMA potentiates the transient release of PGI2 in response to agonists in a complex manner depending on the time of pre-incubation and the concentrations of both PMA and agonist. Concomitant measurement of [Ca2+]i and PGI2 release demonstrates that PMA pretreatment dose-dependently inhibits both the peak [Ca2+]i transient and the subsequent steady-state elevation of [Ca2+]i in response to agonists. Determination of the quantitative [Ca2+]i/PGI2 dose/response relationship, when PGI2 release is driven purely by elevating [Ca2+]i with ionomycin, demonstrates that PMA also enhances the Ca2+-sensitivity of PGI2 release. The observed effects of PMA on PGI2 release can be explained quantitatively by its abilities to lower the threshold [Ca2+]i required for PGI2 synthesis and to depress the peak [Ca2+]i evoked by agonist. We propose that these effects are due respectively to actions of PMA on phospholipase A2 and on a G-protein (Gp) that couples activated receptors to phospholipase C.

G protein-dependent activation of a phosphoinositide-specific phospholipase C in UMR-106 osteosarcoma cell membranes

Recent evidence suggests that guanyl nucleotide binding (G) proteins are involved in receptor-mediated bone resorption and in osteoblastic function, but the nature of the G protein coupled to effectors that are involved in these skeletal effects is unknown. The purposes of this study were to determine (1) whether a G protein mediates activation of phosphoinositide-specific phospholipase C in UMR-106 rat osteosarcoma cells, and (2) whether parathyroid hormone (PTH) and a PTH-like protein (PLP) associated with humoral hypercalcemia of malignancy promote GTP-dependent PIP2 hydrolysis. Addition of GTP (10(-4) M) or guanosine 5'-0-(3-thiotriphosphate, GTP gamma S, 10(-5) M) to membranes prepared from UMR-106 cells labeled with [3H]myo-inositol increased both [3H]inositol trisphosphate (IP3) and [3H]inositol bisphosphate (IP2) formation. The increases in [3H]IP2 and [3H]IP3 produced by GTP were 8.6- and 4.3-fold, respectively. GTP gamma S produced a 17.6- and 11.9-fold increase in [3H]IP2 and [3H]IP3, respectively. The stimulatory effects of GTP and GTP gamma S were dose dependent (GTP ED50 = 3.9 x 10(-6) M; GTP gamma S ED50 = 2.5 x 10(-7) M) and progressive over 10 minutes and required the presence of Mg2+.GTP (10(-4) M) and GTP gamma S (10(-5) M) decreased membrane [3H]phosphoinositides concomitantly with increased [3H]IP2 and [3H]IP3. The GDP analog guanosine 5'-O-(2-thiodiphosphate, GDP beta S) alone did not alter [3H]IP2 or [3H]IP3 production but at 10(-4) M blocks the stimulatory effects of GTP and GTP gamma S. NaF (3 x 10(-2)M) produced a 2.8- and 2.0-fold stimulation of [3H]IP2 and [3H]IP3, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Arachidonic acid affects membrane ionic conductances of GH3 pituitary cells

Arachidonic acid (AA) stimulates prolactin release from pituitary cells, by mechanisms not yet understood. In this work, we analyzed the effects of AA on membrane ionic conductances in a clonal line of anterior pituitary cells (GH3/B6), finding time- and dose-dependent effects of AA on their membrane ionic conductances. The predominant response at concentrations between 100 nM and 10 microM was a prolongation of the action potential (AP) and an increase in the transient after-hyperpolarization potential. Voltage clamp studies showed that this was associated with a decrease in a voltage-dependent potassium current and an increase in a voltage-dependent calcium current. In some cells (30%) the effect of AP duration was less important, but spike firing was enhanced. For the highest concentrations used (1 and 10 microM) the effects described above were preceded by hyperpolarization of the cell membrane; in voltage clamp it was shown that this hyperpolarization resulted from the activation of a calcium-dependent potassium conductance suspected to be due to the release of intracellular calcium. The calcium store affected by AA was, at least in part, insensitive to vanadate and heparin. These data suggest that AA may enhance intracellular calcium concentration by increasing calcium entry during each voltage-dependent calcium AP, by increasing the spike frequency, or by releasing calcium from an intracellular compartment. The resulting rise in cytosolic free calcium concentration may be a key link in the process by which AA stimulates prolactin release in GH3/B6 pituitary cells.

Pertussis toxin inhibits 1-methyladenine-induced maturation in starfish oocytes

Starfish oocytes injected with pertussis toxin (3-6 micrograms/ml) or its catalytically active A-subunit (1 microgram/ml) did not undergo germinal vesicle breakdown in response to 1-methyladenine (1-10 microM). The pertussis block could be bypassed by transfer of cytoplasm that contained maturation-promoting factor (MPF). After insemination, pertussis-blocked, MPF-rescued oocytes underwent cortical vesicle exocytosis and cleavage. These results suggest the involvement of a pertussis sensitive G-protein in the pathway coupling 1-methyladenine action at the cell surface to the reinitiation of meiosis.

Alpha-adrenergic activation of the muscarinic K+ channel is mediated by arachidonic acid metabolites

Phenylephrine (10 microM), added in the bathing solution, stimulated the cardiac muscarinic K+ channel (IK.ACh) in the cell-attached patch. The pipette solution contained 10 microM atropine and 100 microM theophylline to block the muscarinic acetylcholine and adenosine receptors, respectively. The channel activation induced by phenylephrine was blocked by prazosin, an alpha 1-antagonist, indicating that alpha 1-adrenergic receptor mediates the response. Phenylephrine-induced activation was prevented by nordihydroguaiaretic acid, a lipoxygenase inhibitor, and AA-861, a 5-lipoxygenase inhibitor, but was not affected by indomethacin, a cyclooxygenase inhibitor. These observations suggest that 5-lipoxygenase metabolites of arachidonic acid may be involved in the alpha-adrenergic activation of IK.ACh.

An investigation into the mechanisms of inhibition of calcium channel currents in cultured sensory neurones of the rat by guanine nucleotide analogues and (-)-baclofen

1. The mechanism of inhibition of calcium channel currents by the guanine nucleotide analogue guanosine 5'-O-3 thiotriphosphate (GTP-gamma-S) and by the GABAB agonist (-)-baclofen has been studied in cultured dorsal root ganglion neurones of the rat. The inhibition by GTP-gamma-S is particularly characterized by an abolition of the transient component of calcium channel currents carried either by Ba2+ (IBa) or by Ca2+ (ICa). 2. The effect of agents increasing intracellular cyclic AMP levels has been examined. Neither internal cyclic AMP nor forskolin prevented the inhibition of IBa by baclofen. Neither forskolin nor pretreatment of cells with cholera toxin prevented the inhibition of the transient component of IBa by GTP-gamma-S. However, both these treatments increased the amplitude of the sustained IBa in the presence of GTP-gamma-S. The ATP analogue adenosine imido-diphosphate which inhibits many ATP requiring enzymes did not prevent the effect of GTP-gamma-S although it reduced the amplitude of IBa. 3. Baclofen (100 microM) produced a 22 +/- 2% increase in inositol phosphate production in 30 s, whereas the increase produced by bradykinin (1 microM) was 70 +/- 14%. However, unlike baclofen, bradykinin did not inhibit IBa or ICa in these cells. 4. The effect of protein kinase C inhibitors was examined. Polymixin B (20 microM in patch pipette) had no effect on the inhibition of IBa by baclofen or GTP-gamma-S. A higher concentration (100 microM) alone inhibited IBa and no further inhibition by baclofen was observed. Neither H7 (50 microM) nor staurosporine (100 nM), applied extracellularly, prevented the response to GTP-gamma-S. 5. The protein kinase C activator di-octanoyl glycerol (20 microM) did not inhibit IBa. Arachidonic acid (100 microM) also produced no inhibition of IBa. 6. In conclusion we have obtained no evidence that a second messenger system mediates the inhibition of calcium channel currents by GTP-gamma-S or baclofen in dorsal root ganglion neurones. These results support the hypothesis that GABAB receptors are directly coupled to calcium channels by G proteins.

Influence of PLA2-PG system on purine release and cAMP content in dissociated primary glial cultures from rat striatum

Purine release and prostaglandin (PG) outflow were simultaneously evaluated from untreated glial primary cultures of rat striatum, at rest and under field electrical stimulation. Purine release was also assayed from sister cultured cells in which a suitable pharmacological treatment with 1 x 10(-6) M dexamethasone or 1 x 10(-4) M indomethacin had produced a complete inhibition of the phospholipase A2-prostaglandin (PLA2-PG) system. Purine release from untreated cells seems to be regulated by specific receptor sites for released adenosine (Ado); A1 receptors exert an inhibitory control on purine release while A2 receptors facilitate it. PG release appears to be related to A1-mediated Ado activity, since culture treatment with 1 x 10(-10) M 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) or 1 x 10(-4) M N-ethylmaleimide (NEM), A1 receptor inhibitory agents able to increase purine release, induced a significant reduction of the evoked PG outflow. Purine amount, released from glial cells with inhibited PLA2-PG system, was remarkably greater than that one assayed from control cultured cells. In so treated cultures, no additive effect, NEM-induced, was detected, while the addition of a mixture of PGs partially reduced the increased purine outflow. An electrically evoked cAMP accumulation, significantly greater than that found in controls, was even detected in cultured cells with inhibited PLA2-PG system. Since 10 micrograms/ml adenosine deaminase (ADA) reduced while DPCPX enhanced the evoked cAMP accumulation, it seems partially due to released Ado and accounts for a prevalent A2-stimulating rather than an A1-inhibitory control on adenylate cyclase activity. Thus, in cultured glial cells, the PLA2-PG system, likely linked to A1 receptor sites, concurs to control purine release and seems to affect less directly cAMP accumulation.

Arachidonic acid metabolites as intracellular modulators of the G protein-gated cardiac K+ channel

Arachidonic acid is released from cell membranes in response to receptor-dependent as well as receptor-independent stimulation in various cells, including cardiac myocytes. Arachidonic acid is converted to prostaglandins by cyclooxygenase and to leukotrienes by 5-lipoxygenase, metabolites which are very biologically active and modulate cellular functions such as platelet aggregation, smooth muscle contraction and neural excitation. The molecular mechanisms underlying their modulations are, however, still badly understood. Here, we report that the 5-lipoxygenase metabolites of arachidonic acid activate the pertussis toxin-sensitive G protein-gated muscarinic K+ channel (IK.ACh): arachidonic acid activation of IK.ACh was prevented by the lipoxygenase inhibitors, nordihydroguaiaretic acid and AA-861; leukotriene A4 and C4 activated IK.ACh. The activation occurred in pertussis toxin-treated atrial cells and ceased when inside-out patches were formed but the patches were still susceptible to stimulation by GTP and to inhibition by GDP-beta-S. These results indicate that arachidonic acid metabolites may stimulate the G-protein in a receptor-independent way.

Arachidonic acid release in rabbit neutrophils

[3H]Arachidonic acid is released after stimulation of rabbit neutrophils with fMet-Leu-Phe or platelet-activating factor (PAF). The release is rapid and dose-dependent, and is inhibited in phorbol 12-myristate 13-acetate (PMA)-treated rabbit neutrophils. The protein kinase C (PKC) inhibitor 1-(5-isoquinoline-sulphonyl)-2-methylpiperazine (H-7) prevents this inhibition. In addition, PMA increases arachidonic acid release in H-7-treated cells stimulated with fMet-Leu-Phe. [3H]Arachidonic acid release, but not the rise in the concentration of intracellular Ca2+, is inhibited in pertussis-toxin-treated neutrophils stimulated with PAF. The diacylglycerol kinase inhibitor R59022 increases the concentration of diacylglycerol and potentiates [3H]arachidonic acid release in neutrophils stimulated with fMet-Leu-Phe. This potentiation is not inhibited by H-7. These results suggest several points. (1) A rise in the intracellular concentration of free Ca2+ is not sufficient for arachidonic acid release in rabbit neutrophils stimulated by physiological stimuli. (2) A functional pertussis-toxin-sensitive guanine nucleotide regulatory protein and/or one or more of the changes produced by phospholipase C activation are necessary for arachidonic acid release produced by physiological stimuli. (3) Agents that stimulate PKC potentiate arachidonic acid release, and this potentiation is not inhibited by H-7. These agents produce their actions in part by direct membrane perturbation.

Phospholipase A2--regulation and inhibition

Phospholipase A2 (PLA2) has been implicated in the pathogenesis of different diseases. Thus, the pharmacological intervention of PLA2 activity by specific inhibitors is of great therapeutical value in ameliorating pathological conditions. Despite a great number of published data regarding PLA2 inhibitors none has reached clinical application. Since enzyme activity can be greatly influenced by the experimental conditions of the test system used, a potent in vitro enzyme inhibitor does not indicate therapeutic effectiveness per se. In order to enhance the predictable value of an in vitro screening system for PLA2 inhibitors, a battery of test systems each measuring certain parameters should be applied. Considering the complex mechanism(s) of PLA2 it is extremely important to elucidate the exact inhibition mechanism of those compounds, which have passed these first filters. True inhibitors of PLA2 should then be evaluated in suitable ex vivo, in vivo models.

Multiple signaling pathways control stimulus-secretion coupling in rat peritoneal mast cells

Fura-2 and membrane capacitance measurements were performed to investigate intracellular Ca2+ concentration [( Ca2+]i) and secretory responses of rat peritoneal mast cells following secretagogue stimulation. Compound 48/80 and internally applied guanosine 5'-[gamma-thio]triphosphate (GTP[gamma-S]) induced transient rises in [Ca2+]i and caused membrane capacitance increases as secretion occurred. The 48/80-induced Ca2+ transients and secretory responses were blocked by guanosine 5'-[beta-thio]diphosphate and neomycin, indicating that inositolphospholipid breakdown mediated by guanine nucleotide-binding regulatory protein (G protein) plays an important role in stimulus-secretion coupling. However, pertussis toxin did not block Ca2+ transients induced by 48/80 or GTP[gamma-S], whereas secretory responses were either abolished (48/80) or developed only after a considerable delay (GTP[gamma-S]). Similar effects were obtained by perfusing cells with cAMP: (i) Ca2+ transients following stimulation with 48/80 remained unaffected by cAMP, but secretory responses were abolished; (ii) GTP[gamma-S] induced normal Ca2+ transients and degranulation in the presence of cAMP. Pretreatment of mast cells with phorbol 12-myristate 13-acetate (PMA) abolished 48/80- and GTP[gamma-S]-induced Ca2+ transients (but not inositol trisphosphate-induced Ca2+ transients), whereas secretion still occurred. At the same time, the Ca2+ requirement for secretion was reduced by PMA. These results indicate that secretion in mast cells is under control of an as yet unidentified signaling pathway that involves a G protein. This pathway is distinct from inositolphospholipid turnover and may provide the triggering mechanism for secretion, whereas the inositolphospholipid pathway serves to increase [Ca2+]i and renders the secretory process more sensitive to [Ca2+]i by activating protein kinase C. Persistent activation of protein kinase C through phorbol ester imposes negative feedback control on the inositolphospholipid pathway, whereas cAMP may inhibit the unidentified signaling pathway.

Stimulation of arachidonic acid release and inhibition of mitogenesis by cloned genes for muscarinic receptor subtypes stably expressed in A9 L cells

A family of genes encoding four distinct muscarinic receptors (designated m1-m4) has been cloned and stably expressed in A9 L cells. When the m1 and m3 receptors were stimulated with carbachol, there was a rapid rise of liberated arachidonic acid, inositol phosphates, and cAMP, while m2 and m4 receptor stimulation had no detectable stimulation of these second messengers. Pretreatment with phorbol 12-myristate 13-acetate (PMA) caused a marked acceleration and amplification of m1 and m3 receptor-mediated arachidonic acid release. In contrast, m1- and m3-mediated inositol phosphate formation was inhibited by the same PMA pretreatment. Arachidonic acid release was unaffected by manipulations of cAMP levels. Arachidonic acid production was inhibited by calcium-free medium and 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8; an inhibitor of cytosolic calcium mobilization) yet was unaffected by verapamil, a calcium-channel blocker. These experiments show that arachidonic acid release induced by the m1 and m3 receptors is regulated independently of phospholipase C and cAMP accumulation. Carbachol stimulation of the m1 and m3 cAMP accumulation. Carbachol stimulation of the m1 and m3 receptors also markedly decreased mitogenesis as measured by thymidine incorporation. The m1 receptor-mediated inhibition of mitogenesis could be partially blocked by indomethacin, a cyclooxygenase inhibitor. The inhibition of mitogenesis could be mimicked by cAMP elevation.

Role of two different guanine nucleotide-binding proteins in the antagonistic modulation of the S-type K+ channel by cAMP and arachidonic acid metabolites in Aplysia sensory neurons

The role of guanine nucleotide-binding proteins (G proteins) in the cAMP-dependent action of serotonin (5-HT) and the antagonistic action of the neuropeptide Phe-Met-Arg-Phe-NH2 (FMRF-amide), mediated by the lipoxygenase metabolites of arachidonic acid, was investigated in Aplysia sensory neurons. Intracellular injection of guanosine 5'-[gamma-thio]triphosphate (GTP[gamma-S]) mimics the hyperpolarizing action of FMRF-amide due to activation of the S K+ current and alters the transient response to FMRF-amide into an irreversible (or only partially reversible) response. At higher concentrations, GTP[gamma-S] occludes the response to FMRF-amide. Injection of activated pertussis toxin inhibits the response to FMRF-amide but not to 5-HT. Injection of guanosine 5'-[beta-thio]diphosphate inhibits the response to FMRF-amide by approximately equal to 50% and completely blocks the response to 5-HT. Three lines of evidence suggest that the FMRF-amide-activated G protein is involved at an early stage of the arachidonic acid cascade, prior to the release of arachidonate. (i) Pertussis toxin injection blocks the hyperpolarizing response to FMRF-amide but not to exogenously applied arachidonic acid. (ii) Two blockers of the arachidonic acid cascade inhibit the hyperpolarizing responses to both FMRF-amide and GTP[gamma-S] (and unmask a 5-HT-like depolarizing response to the nucleotide). (iii) Concentrations of GTP[gamma-S] that alter the kinetics of the FMRF-amide response have no effect on the hyperpolarizing response to arachidonic acid. We conclude that a pertussis toxin-sensitive G protein most likely acts to couple the FMRF-amide receptor to phospholipase activation and arachidonic acid release, whereas a pertussis toxin-insensitive G protein couples the 5-HT receptor to adenylate cyclase.