Alpha 1-adrenergic receptor-mediated activation of phospholipase D in rat cerebral cortex

Changes in phospholipid composition of synaptic membranes in frontal lobes of cerebral hemispheres in cats at various stages of hemorrhagic shock

Phospholipid composition of synaptic membranes in the frontal lobes of cerebral hemispheres was studied in cats with hemorrhagic shock. The compensatory and adaptive mechanisms of regulation of neurotransmission in this region of the brain at the initial stage of hemorrhagic shock are associated with increased degradation of phosphatidylinositol. Accumulation of this phospholipid in synaptic membranes during severe hemorrhagic shock reflects instability of the key neuroregulatory pathway, which is mediated by phosphatidylinositol metabolites. Dysregulation of transmembrane signaling in hemorrhagic shock is related to depletion of phosphatidylcholine and phosphatidylserine in synaptic membranes and accumulation of phosphatidylethanolamine.

Effects of norepinephrine and cardiotrophin-1 on phospholipase D activity and incorporation of myristic acid into phosphatidylcholine in rat heart

The present study is part of a project on phospholipase D (PLD) in cardiac hypertrophy and analyzed effects on PLD activity of two growth stimuli, norepinephrine (NE) and cardiotrophin-1 (CT-1), in incubated rat heart. Phosphatidylcholine (PC) was labeled by (3)H-myristic acid. PLD produced (3)H-phosphatidylethanol ((3)H-PEth) from (3)H-PC in the presence of ethanol and maintained a basal formation of (3)H-PEth. Short-term and long-term exposure to NE for 2 or 13 h, respectively, enhanced the formation of (3)H-PEth, which was blocked by prazosin. Long-term pretreatment with NE or CT-1 increased the incorporation of (3)H-myristic acid into PC, which was blocked by atenolol. When the (3)H-PEth formation was expressed as a fraction of (3)H-PC, PLD activity seemingly was unchanged (NE) or markedly reduced (CT-1); the true effects, namely, stimulation by NE and nonresponsiveness towards CT-1, were unraveled by atenolol (NE) or when PLD activity was expressed as (3)H-PEth per ng protein. In conclusion, alpha-adrenoceptor activation increased PLD activity. Long-term treatment with NE (via beta-receptors) or CT-1 enhanced the (3)H-myristic acid incorporation into a PC compartment, that was not available for the alpha-receptor-mediated PLD activation. These results were discussed in regard to cellular mechanisms of cardiac hypertrophy and to the transphosphatidylation assay of PLD.

Characterization of G proteins involved in activation of nonselective cation channels and arachidonic acid release by norepinephrine/α1A-adrenergic receptors

We demonstrated recently that norepinephrine activates Ca2+-permeable nonselective cation channels (NSCCs) in Chinese hamster ovary cells stably expressing α1A-adrenergic receptors (CHO-α1A). Moreover, extracellular Ca2+through NSCCs plays essential roles in norepinephrine-induced arachidonic acid release. The purpose of the present study was to identify the G proteins involved in the activation of NSCCs and arachidonic acid release by norepinephrine. For these purposes, we used U73122, an inhibitor of phospholipase C (PLC), and dominant negative mutants of G12and G13(G12G228A and G13G225A, respectively). U73122 failed to inhibit NSCCs activation by norepinephrine. The magnitudes of norepinephrine-induced extracellular Ca2+influx in CHO-α1Amicroinjected with G13G225A were smaller than those in CHO-α1A. In contrast, the magnitudes of norepinephrine-induced extracellular Ca2+influx in CHO-α1Amicroinjected with G12G228A were similar to those in CHO-α1A. In addition, neither a Rho-associated kinase (ROCK) inhibitor nor a phosphoinositide 3-kinase inhibitor affected norepinephrine-induced extracellular Ca2+influx. G13G225A, but not G12G228A, also inhibited arachidonic acid release partially. These results demonstrate that 1) the Gq/PLC-pathway is not involved in NSCCs activation by norepinephrine, 2) G13couples with CHO-α1Aand plays important roles for norepinephrine-induced NSCCs activation, 3) neither ROCK- nor PI3K-dependent cascade is involved in NSCCs activation, and 4) G13is involved in norepinephrine-induced arachidonic acid release in CHO-α1A.

Psychoses and creativity: is the missing link a biological mechanism related to phospholipids turnover?

Recent evidence suggests that genetic and biochemical factors associated with psychoses may also provide an increased propensity to think creatively. The evolutionary theories linking brain growth and diet to the appearance of creative endeavors have been made recently, but they lack a direct link to research on the biological correlates of divergent and creative thought. Expanding upon Horrobin's theory that changes in brain size and in neural microconnectivity came about as a result of changes in dietary fat and phospholipid incorporation of highly unsaturated fatty acids, we propose a theory relating phospholipase A2 (PLA2) activity to the neuromodulatory effects of the noradrenergic system. This theory offers probable links between attention, divergent thinking, and arousal through a mechanism that emphasizes optimal individual functioning of the PLA2 and NE systems as they interact with structural and biochemical states of the brain. We hope that this theory will stimulate new research in the neural basis of creativity and its connection to psychoses.

N-terminal truncation of human alpha1D-adrenoceptors increases expression of binding sites but not protein

The role of the N-terminus of human alpha(1D)-adrenoceptors was examined by deleting the first 79 amino acids (Delta(1-79)) and epitope-tagging to facilitate immunoprecipitation and detection. Following transfection into HEK293 cells, 6- to 13-fold increases in the density of specific [125I]BE 2254 binding sites were observed for both tagged and untagged Delta(1-79)alpha(1D)- compared to full-length alpha(1D)-adrenoceptors, while agonist and antagonist affinities remained unchanged. In contrast, immunoprecipitation of tagged receptors showed that full-length alpha(1D)-adrenoceptor protein was at least twice as abundant as Delta(1-79)alpha(1D)-adrenoceptor protein. Photoaffinity labelling with [125I]arylazidoprazosin showed much more intense labelling of tagged Delta(1-79)alpha(1D)- than of full-length alpha(1D)-adrenoceptors. Substantial N-linked glycosylation of tagged Delta(1-79)alpha(1D)-adrenoceptors was observed, although full-length alpha(1D)-adrenoceptors contain two consensus glycosylation sites but are not glycosylated. These results suggest that N-terminal truncation of alpha(1D)-adrenoceptors enhances processing of a binding competent form in HEK293 cells; and show a clear dissociation between abundance of receptor protein and density of receptor binding sites.

alpha 1-Adrenergic receptor subtypes differentially control the cell cycle of transfected CHO cells through a cAMP-dependent mechanism involving p27Kip1

Three distinct subtypes of alpha(1)-adrenergic receptors (alpha(1)A-, alpha(1)B-, and alpha(1)D-AR) play a prominent role in cell growth. However, little is known about subtype-specific effects on cell proliferation. The activation of alpha(1)A- or alpha(1)B-AR inhibits serum-promoted cell proliferation, whereas alpha(1)D-AR activation does not show such an inhibitory effect. Notably, cell-cycle progression was blocked at G(1)/S transition after activation of alpha(1)A/alpha(1)B-AR but not of alpha(1)D-AR. In agreement with the differential cell proliferation effect, cAMP production was increased after activation of alpha(1)A/alpha(1)B-AR but not alpha(1)D-AR, whereas all alpha(1)-AR subtypes are associated with inositol 1,4,5-trisphosphate production and mitogen-activated protein kinase activation in a similar fashion. Furthermore, the serum-induced reduction in the levels of the cyclin-dependent kinase inhibitor, p27(Kip1), was blocked after activation of alpha(1)A/alpha(1)B-AR but not alpha(1)D-AR. These results show that alpha(1)-AR subtypes differentially activate the cAMP/p27(Kip1) pathway and thereby have differential inhibitory effects on cell proliferation. Subtype-dependent effects should be taken into consideration when assessing the physiological response of native cells where alpha(1)-AR subtypes are generally co-expressed.

Calcium and protein kinase C (PKC)-related kinase mediate alpha 1A-adrenergic receptor-stimulated activation of phospholipase D in rat-1 cells, independent of PKC

A previous study conducted in rat-1 cells expressing alpha(1A)-adrenergic receptors showed that phenylephrine (PHE) stimulates phospholipase D (PLD) activity. This study was conducted to determine the contribution of protein kinase C (PKC) to PHE-induced PLD activation in these cells. PKC inhibitors bisindolylmaleimide (BIM) I and Ro 31-8220, but not Gö 6976 or a pseudosubstrate peptide inhibitor of PKCalpha, decreased PLD activity and arachidonic acid release elicited by PHE. However, antisense oligonucleotides directed against PKC alpha, delta, epsilon, and eta reduced PKC isoform levels by about 80% but failed to alter PHE-induced PLD activation, indicating that these PKC isoforms are not involved in PLD activation elicited by alpha1A-adrenergic receptor stimulation. Ectopic expression of a kinase-deficient mutant of the PKC-related kinase PKN significantly attenuated PHE-induced PLD activation. On the other hand, BIM I and Ro 31-8220 blocked PHE-mediated increase in intracellular Ca2+ but Gö 6976 and the peptide inhibitor did not. In the absence of extracellular Ca2+, PHE failed to increase PLD activity. These results indicate that alpha1A-adrenergic receptor-stimulated PLD activation is mediated by a mechanism independent of PKCalpha, delta, epsilon, and eta, but dependent on a PKC-related kinase, PKN. Moreover, PKC inhibitors BIM I and Ro 31-8220 block PHE-induced PLD activity by inhibiting calcium signal. Caution should be used in interpreting the data obtained with PKC inhibitors in vivo.

Key amino acids for differential coupling of alpha1-adrenergic receptor subtypes to Gs

We have established that differing effects of alpha1-adrenergic receptor (AR) subtypes on cell proliferation are due to differential coupling to the Gs/cAMP pathway; thus, both alpha1A- and alpha1B-ARs couple to Gs, while alpha1D-AR does not. To identify the region responsible for this difference in subtype-specific Gs coupling, we constructed a series of chimeric and a set of point-mutated human alpha1A- and alpha1D-ARs, and examined their signaling ability. Here, we show that the amino acid residues Thr 136 and Val138 in the intracellular loop II of the human alpha1A-AR are intimately involved with Gs coupling.

Phospholipase D activation by norepinephrine is mediated by 12(s)-, 15(s)-, and 20-hydroxyeicosatetraenoic acids generated by stimulation of cytosolic phospholipase a2. tyrosine phosphorylation of phospholipase d2 in response to norepinephrine

Norepinephrine (NE) stimulates phospholipase D (PLD) through a Ras/MAPK pathway in rabbit vascular smooth muscle cells (VSMC). NE also activates calcium influx and calmodulin (CaM)-dependent protein kinase II-dependent cytosolic phospholipase A(2) (cPLA(2)). Arachidonic acid (AA) released by cPLA(2)-catalyzed phospholipid hydrolysis is then metabolized into hydroxyeicosatetraenoic acids (HETEs) through lipoxygenase and cytochrome P450 4A (CYP4A) pathways. HETEs, in turn, have been shown to stimulate Ras translocation and to increase MAPK activity in VSMC. This study was conducted to determine the contribution of cPLA(2)-derived AA and its metabolites (HETEs) to the activation of PLD. NE-induced PLD activation was reduced by two structurally distinct CaM antagonists, W-7 and calmidazolium, and by CaM-dependent protein kinase II inhibition. Blockade of cPLA(2) activity or protein depletion with selective cPLA(2) antisense oligonucleotides abolished NE-induced PLD activation. The increase in PLD activity elicited by NE was also blocked by inhibitors of lipoxygenases (baicalein) and CYP4A (17-octadecynoic acid), but not of cyclooxygenase (indomethacin). AA and its metabolites (12(S)-, 15(S)-, and 20-HETEs) increased PLD activity. PLD activation by AA and HETEs was reduced by inhibitors of Ras farnesyltransferase (farnesyl protein transferase III and BMS-191563) and MEK (U0126 and PD98059). These data suggest that HETEs are the mediators of cPLA(2)-dependent PLD activation by NE in VSMC. In addition to cPLA(2), PLD was also found to contribute to AA release for prostacyclin production via the phosphatidate phosphohydrolase/diacylglycerol lipase pathway. Finally, a catalytically inactive PLD(2) (but not PLD(1)) mutant inhibited NE-induced PLD activity, and PLD(2) was tyrosine-phosphorylated in response to NE by a MAPK-dependent pathway. We conclude that NE stimulates cPLA(2)-dependent PLD(2) through lipoxygenase- and CYP4A-derived HETEs via the Ras/ERK pathway by a mechanism involving tyrosine phosphorylation of PLD(2) in rabbit VSMC.

In vivo changes in free choline level induced by autonomic agonists in mouse organs, including three major salivary glands

Whether free choline levels are changeable in vivo in response to different types of autonomic agonists was examined in several mouse organs. Upon one subcutaneous injection of isoproterenol, phenylephrine and pilocarpine, choline levels in whole organ decreased, increased and decreased, respectively, in various organs within 30 min and returned to initial levels in a day. In the three major salivary glands, a delayed choline elevation also appeared on day 2 after one isoproterenol injection and subsided by day 6. Only in the three salivary glands more choline was accumulated after 10 once-a-day injections of isoproterenol than after one isoproterenol injection. Neither phenylephrine nor pilocarpine induced comparable choline accumulation in any organs examined. Isoproterenol injection repeated at a 2-day interval augmented the subsequent, delayed choline elevation. Examination with dobutamine and the adenylyl cyclase activator 6-(3-dimethylaminopropionyl)forskolin suggested that isoproterenol-induced immediate choline lowering was down-stream of cAMP synthesis and linked to cAMP more tightly than the choline accumulation, though both choline changes occurred via beta1-adrenergic receptors. Choline levels in the salivary glands also changed depending on the form of diet given and particularly in the parotid gland in parallel with gland weights. These results provide the first evidence for the autonomic control of intracellular choline levels; intracellular choline levels might be an integral part of the autonomic signalling pathway.

Choline release and inhibition of phosphatidylcholine synthesis precede excitotoxic neuronal death but not neurotoxicity induced by serum deprivation

N-Methyl-d-aspartate (NMDA) receptor overactivation has been proposed to induce excitotoxic neuronal death by enhancing membrane phospholipid degradation. In previous studies, we have shown that NMDA releases choline and reduces membrane phosphatidylcholine in vivo. We now observed that glutamate and NMDA induce choline release in primary neuronal cortical cell cultures. This effect is Ca(2+)-dependent and is blocked by MK-801 ((+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate). In cortical neurons, the NMDA receptor-mediated choline release precedes excitotoxic cell death but not neuronal death induced by either osmotic lysis or serum deprivation. Glutamate, at concentrations that release arachidonic acid, does not release choline in cerebellar granule cells, unless these cells are rendered susceptible to excitotoxic death by energy deprivation. The NMDA-evoked release of choline is not mediated by phospholipases A(2) or C. Moreover, NMDA does not activate phospholipase D in cortical cells. However, NMDA inhibits incorporation of [methyl-(3)H]choline into both membrane phosphatidylcholine and sphingomyelin. These results show that the increase in extracellular choline induced by NMDA receptor activation is directly related with excitotoxic cell death and indicate that choline release is an early event of the excitotoxic process produced by inhibition of phosphatidylcholine synthesis and not by activation of membrane phospholipid degradation.

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.

Expression of phospholipase D isoforms in mammalian cells

Two mammalian isoforms of phospholipase D, PLD1 and PLD2, have recently been characterized at the molecular level. Effects of physiologic agonists on PLD activity in intact cells, as characterized in earlier studies, have generally not been attributed to specific PLD isoforms. Recent work has established that expression of PLD1 and PLD2 varies within tissues and between cell lines. A single cell type can express one, both, or neither isoform, although most cells co-express PLD1 and PLD2. Lymphocytes often lack expression of one or both isoforms of PLD. Relative levels of PLD mRNA expression vary considerably between established cell lines. Expression of transcripts for both PLD1 and PLD2 can be regulated at the transcriptional level by growth and differentiation factors in cultured cells. Thus, it is apparent that the known mammalian PLD isoforms are subject to regulation at the transcriptional level. The available data do not conclusively establish whether PLD1 and PLD2 are the only isoforms responsible for agonist-mediated PLD activation. Further studies of the regulation of expression of PLD isoforms should provide insight into the roles of PLD1 and PLD2 in physiologic responses, and may suggest whether additional forms of PLD remain to be characterized.

(2R,1'S,2'R,3'S)-2-(2'-Carboxy-3'-phenylcyclopropyl)glycine (PCCG-13), the first potent and selective competitive antagonist of phospholipase D-coupled metabotropic glutamate receptors: asymmetric synthesis and preliminary biological properties

The asymmetric synthesis of (2R,1'S,2'R, 3'S)-2-(2'-carboxy-3'-phenylcyclopropyl)glycine (PCCG-13), a trisubstituted carboxycyclopropylglycine endowed with unusual stereochemical features, is described. Preliminary biological evaluation demonstrates PCCG-13 as a very potent and selective competitive antagonist for the novel class of metabotropic glutamate (mGlu) receptors coupled to the activity of phospholipase D (PLD). PCCG-13 is therefore a useful tool for the exploration of the physiopathological role of this novel class of receptors.

Alpha1-adrenoceptor subtypes

Alpha1-adrenoceptors are one of three subfamilies of receptors (alpha1, alpha2, beta) mediating responses to adrenaline and noradrenaline. Three alpha1-adrenoceptor subtypes are known (alpha1A, alpha1B, alpha1D) which are all members of the G protein coupled receptor family, and splice variants have been reported in the C-terminus of the alpha1A. They are expressed in many tissues, particularly smooth muscle where they mediate contraction. Certain subtype-selective agonists and antagonists are now available, and alpha1A-adrenoceptor selective antagonists are used to treat benign prostatic hypertrophy. All subtypes activate phospholipase C through the G(q/11) family of G proteins, release stored Ca2+, and activate protein kinase C, although with significant differences in coupling efficiency (alpha1A > alpha1B > alpha1D). Other second messenger pathways are also activated by these receptors, including Ca2+ influx, arachidonic acid release, and phospholipase D. Alpha1-adrenoceptors also activate mitogen-activated protein kinase pathways in many cells, and some of these responses are independent of Ca2+ and protein kinase C but involve small G proteins and tyrosine kinases. Direct interactions of alpha1-adrenoceptors with proteins other than G proteins have not yet been reported, however there is a consensus binding motif for the immediate early gene Homer in the C-terminal tail of the alpha1D subtype. Current research is focused on discovering new subtype-selective drugs, identifying non-traditional signaling pathways activated by these receptors, clarifying how multiple signals are integrated, and identifying proteins interacting directly with the receptors to influence their functions.

Adrenergic modulation of astroglial phospholipase D activity and cell proliferation

As phospholipase D (PLD) activation has been associated with mitogenic signalling in several cell types, we tested an association between adrenergic activation of PLD and cellular proliferation in primary cultures of rat cortical astrocytes. In 2-week old cultures, PLD activation by noradrenaline (EC50: 0.49 microM) was inhibited by prazosin, a specific antagonist at alpha1-adrenergic receptors (IC50: 0.23 microM). Adrenergic PLD activation was not affected by genistein, an inhibitor of tyrosine kinases, or by Ro 31-8220, an inhibitor of protein kinase C (PKC), but was dose-dependently depressed in the presence of brefeldin A (1-100 microg/ml), an inhibitor of ARF activation. In experiments measuring cell proliferation, noradrenaline potently (EC50: 20 nM) reduced [3H]thymidine incorporation to 20-30% of basal values. This action was mimicked by the beta-specific agonist isoprenaline and was inhibited by the beta-antagonist propranolol in a concentration-dependent manner. The alpha1-adrenergic agonists, phenylephrine and methoxamine, also reduced DNA synthesis. The adrenergic inhibition of astroglial DNA synthesis was not reduced, but further potentiated in the presence of brefeldin A, ethanol, and 1- and 2-butanol; 1-butanol, a substrate of PLD, was equally effective as 2-butanol, a non-substrate. We conclude that adrenergic PLD activation in astrocytes is not involved in mitogenic signalling. The involvement of ARF in the activation of PLD via alpha1-adrenoceptors indicates a role in protein trafficking.

Phosphorylation of the cAMP response element-binding protein and activation of transcription by alpha1 adrenergic receptors

Activation of alpha1 adrenergic receptors not only stimulates smooth muscle contraction but also modifies gene expression. We wondered if alpha1 adrenergic receptors could activate transcription of genes regulated by the cAMP response element-binding protein (CREB). Using Rat1 cells stably transfected with each of the three cloned human alpha1 adrenergic receptor subtypes, norepinephrine strongly stimulated CREB phosphorylation in alpha1A and alpha1B but more weakly in alpha1D-transfected cells. Norepinephrine increased the activity of a somatostatin cAMP-regulated enhancer-chloramphenicol acetyltransferase reporter in these cells. alpha1 adrenergic receptors are known to activate protein kinase C (PKC) and increase [Ca2+ ]i. Nonetheless, neither GF109203X, a PKC inhibitor, nor BAPTA-AM, a calcium chelator, blocked phosphorylation of CREB induced by norepinephrine. In addition, alpha1 adrenergic receptor-induced CREB phosphorylation was not mediated via the mitogen-activated protein kinase pathway because norepinephrine did not stimulate mitogen-activated protein kinase activity in these cells. Activation of alpha1 adrenergic receptors increased cAMP accumulation in these cells. Norepinephrine-induced cAMP-regulated enhancer-chloramphenicol acetyltransferase activity was inhibited either by expression of the PKA inhibitory peptide or a dominant negative PKA regulatory subunit mutant. These results demonstrate that alpha1 adrenergic receptors activate the transcription factor CREB by a PKA-dependent pathway.

Stimulation of phospholipase D via alpha1-adrenergic receptors in Madin-Darby canine kidney cells is independent of PKCalpha and -epsilon activation

We have demonstrated previously that protein kinase Calpha (PKCalpha) plays a key role in regulating phospholipase D (PLD) activation by nucleotides and the phorbol ester phorbol-12-myristate-13-acetate in Madin-Darby canine kidney (MDCK-D1) cells. In the current work, we investigated PLD activation in MDCK-D1 cells triggered by the adrenergic receptor agonist epinephrine and its mechanism of activation. Epinephrine, acting through the alpha1-adrenergic receptor subtype, promoted transient translocation of PKCalpha and more prolonged translocation of PKCepsilon to the membrane fraction, indicating activation of these two isoforms. In addition, epinephrine promoted activation of PLD, as shown by a sustained accumulation of phosphatidylethanol. All of these events were blocked by pretreatment of cells with the alpha1-adrenergic antagonist prazosin. D609, an inhibitor of phosphatidylcholine hydrolysis, blocked translocation of PKCalpha and PKCepsilon but did not inhibit PLD activation. Unlike results with PMA, or with the P2 purinergic receptor agonist ATP, epinephrine-stimulated PLD activity was not inhibited in MDCK-D1 cells in which PKCalpha expression is attenuated by an antisense cDNA construct or in cells in which PKC activity was inhibited by 1 microM GF 109203X. However, PLD activation by epinephrine was abolished by concomitant incubation of cells with the calcium chelator EGTA. These data, together with previous results, are consistent with the hypothesis that in MDCK-D1 cells, epinephrine acting on alpha1-adrenergic receptors, promotes a rapid increase in cytosolic Ca2+ that promotes activation of PLD through an as-yet poorly defined mechanism. The data demonstrate that different types of G protein-linked receptors that activate PLD can mediate this activation in either a PKC activation-dependent or -independent manner within a single cell type.

Adrenergic activation of phospholipase D in primary rat astrocytes

Phospholipase D (PLD) activity was investigated in astrocytes prepared from newborn rat cerebral cortex using the transphosphatidylation assay. Basal PLD activity was measurable and was found to be enhanced by ATP, carbachol and noradrenaline. The activation by noradrenaline (EC50, 0.68 microM) was mimicked by methoxamine (EC50, 65 microM), an alpha 1-specific adrenergic agonist, and was inhibited by prazosine, an alpha 1-specific adrenergic antagonist. Clonidin, an alpha 2-adrenergic agonist, slightly lowered PLD activity whereas beta-adrenergic drugs were without effect. Experiments with mitogens indicate that PLD activation in astrocytes may be involved in the control of astrocytic cell proliferation.

Differential involvement of calcium channels and protein kinase-C activity in GnRH-induced phospholipase-C, -A2 and -D activation in a gonadotrope cell line (alpha T3-1)

The mode of action of GnRH on pituitary gonadotropes involves metabolism of phospholipids, protein kinase-C (PKC) and voltage sensitive Ca2+ channels (VSCC) activation. We have studied the differential role of PKC and VSCC on the coupling of the GnRH receptor with phospholipases-C (PLC), -A2 (PLA2) and -D (PLD) activities in a gonadotrope cell line (alpha T3-1), by measuring the production of inositol phosphates (IPs), arachidonic acid (AA) and phosphatidylethanol (PEt) respectively. We demonstrated that in these cells GnRH stimulated through a specific receptor, IPs formation, a rapid and sustained diacylglycerol generation, consequently AA release and a delayed PEt production in a dose-dependent manner. In contrast to GnRH-induced PLC activity, the PLA2 and PLD stimulation by the neuropeptide involved Ca2+ mobilization via VSCC activation. BAY-K8644 a VSCC agonist significantly potentiated, while the VSCC antagonist nitrendipine markedly inhibited GnRH-induced AA release and PEt production. TPA, a phorbol ester which induced a rapid and important redistribution of PKC, although unable to elicit PLC or PLA2 stimulation, specifically provoked PLD activation in a PKC-dependent but Ca(2+)-independent manner. The PKC stimulation by TPA significantly inhibited the GnRH-stimulated IPs and AA formation, while it potentiated the GnRH-evoked PEt production. This negative feed-back of PKC on GnRH-Induced PLC and PLA2 activities was reversed when PKC was either down regulated after long TPA treatments or inhibited by the PKC inhibitors, staurosporine or GF109203X. The GnRH-induced PEt formation was markedly diminished in PKC depleted cells or after PKC inhibition. Under such conditions, both agonist and antagonist of VSCC became less effective in modulating the remaining GnRH-evoked PEt formation. These results suggest that PKC, in coordination with Ca2+, plays a key role in regulating the cross-talk between the multiple phospholipases implicated in the GnRH signal transduction.

Prostacyclin formation elicited by endothelin-1 in rat aorta is mediated via phospholipase D activation and not phospholipase C or A2

Endothelin-1 (ET-1) is a potent vasoconstrictor peptide that also stimulates production of prostacyclin (PGI2) from arachidonic acid. The purpose of this study was to determine the contribution of phospholipases (PLs) A2, C, and/or D in ET-1-induced PGI2 formation in the rat aorta, measured as immunoreactive 6-ketoprostaglandin (PG) F1 alpha. ET-1 increased 6-keto-PGF1 alpha formation, which was not affected by a PLA2 inhibitor, 7,7-dimethyl eicosadienoic acid (DEDA). Furthermore, ET-1 failed to stimulate PLA2 activity measured in the cytosol (cPLA2), using phosphatidylcholine, L-a-1-palmitoyl-2-arachidonyl[14C] as a substrate. However, the adrenergic agonist norepinephrine increased 6-keto-PGF1 alpha formation, which was attenuated by DEDA, and enhanced PLA2 activity. ET-1 enhanced PLC activity, as indicated by increased inositol phosphate production, which was prevented by a PLC inhibitor, U-73122. However, ET-1-induced 6-keto-PGF1 alpha production was not altered by U-73122. An inhibitor of PLD activation, C2-ceramide, attenuated ET-1-induced PLD activity, as indicated by the production of phosphatidylethanol. Furthermore, ET-1-induced 6-keto-PGF1 alpha formation was inhibited by C2-ceramide as well as by ethanol treatment. Moreover, inhibitors of phosphatidate phosphohydrolase (propranolol) and diacylglycerol lipase (RHC-80267), attenuated ET-1-induced 6-keto-PGF1 alpha formation. Finally, ET-1-induced activation of PLD was not attenuated by a selective PKC inhibitor, bisindolylmaleimide I. These data suggest a novel pathway for ET-1-induced PGI2 formation in the rat aorta involving activation of PLD but not cPLA2 and independent of PLC or PKC activation.

Pharmacological characterization of metabotropic glutamate receptors coupled to phospholipase D in the rat hippocampus

1. Phospholipase D (PLD) is the key enzyme in a signal transduction pathway leading to the formation of the second messengers phosphatidic acid and diacylglycerol. In order to define the pharmacological profile of PLD-coupled metabotropic glutamate receptors (mGluRs), PLD activity was measured in slices of adult rat brain in the presence of mGluR agonists or antagonists. Activation of the phospholipase C (PLC) pathway by the same agents was also examined. 2. The mGluR-selective agonist (1S,3R)-l-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD] induced a concentration-dependent (10-300 microM) activation of PLD in the hippocampus, neocortex, and striatum, but not in the cerebellum. The effect was particularly evident in hippocampal slices, which were thus used for all subsequent experiments. 3. The rank order of potencies for agonists stimulating the PLD response was: quisqualate > ibotenate > (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine > (1S,3R)-ACPD > L-cysteine sulphinic acid > L-aspartate > L-glutamate. L-(+)-2-Amino-4-phosphonobutyric acid and the ionotropic glutamate receptor agonists N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and kainate failed to activate PLD. (RS)-3,5-dihydroxyphenylglycine (100300 microM), an agonist of mGluRs of the first group, stimulated PLC but inhibited the PLD response elicited by 100 microM (1S,3R)-ACPD. 4. (+)-alpha-Methyl-4-carboxyphenylglycine (0.1-1 mM), a competitive antagonist of mGluRs of the first and second group, elicited a significant PLD response. L-(+)-2-Amino-3-phosphonopropionic acid (1 mM), an antagonist of mGluRs of the first group, inhibited the 100 microM (1S,3R)-ACPD-induced PLC response but produced a robust stimulation of PLD. 5. 12-O-Tetradecanoylphorbol 13-acetic acid and phorbol 12,13-dibutyrate (PDBu), activators of protein kinase C, at 1 microM had a stimulatory effect on mGluRs linked to PLD but depressed (1S,3R)-ACPD-induced phosphoinositide hydrolysis. The protein kinase C inhibitor, staurosporine (1 and 10 microM) reduced PLD activation induced by 1 microM PDBu but not by 100 microM (1S,3R)-ACPD. 6. Our results suggest that PLD-linked mGluRs in rat hippocampus may be distinct from any known mGluR subtype coupled to PLC or adenylyl cyclase. Moreover, they indicate that independent mGluRs coupled to the PLC and PLD pathways exist and that mGluR agonists can stimulate PLD through a PKC-independent mechanism.

ARF-regulated phospholipase D: a potential role in membrane traffic

Phospholipase D activity is stimulated rapidly upon occupation of cell-surface receptors. One of the intracellular regulators of phospholipase D activity has been identified as ADP ribosylation factor (ARF). ARF is a small GTP binding protein whose function has been elucidated in vesicular traffic. This review puts into context the connection between the two fields of signal transduction and vesicular transport.

Bradykinin stimulates phospholipase D in PC12 cells by a mechanism which is independent of increases in intracellular Ca2+

These experiments were designed to learn the role of bradykinin induced changes in intracellular Ca2+ in the activation of phospholipase D activity in PC12 cells. Ionomycin at a concentration of 0.1 microM caused an increase in intracellular Ca2+ comparable to bradykinin, but had no effect on phospholipase D activity. Carbachol, ATP, and thapsigargin also increased intracellular Ca2+ but had no effect on phospholipase D activity. Increases in intracellular Ca2+ may be a necessary but not a sufficient factor in the activation of phospholipase D. To investigate this issue, the bradykinin induced increase in intracellular Ca2+ was blocked by preincubating the cells in Ca(2+)-free media plus EGTA or in media containing the intracellular Ca2+ chelator BAPTA/AM. These preincubations completely blocked the bradykinin induced increase in intracellular Ca2+ but only attenuated the bradykinin mediated activation of phospholipase D. Physiological increases in intracellular Ca2+ apparently do not mediate the effect of bradykinin on phospholipase D.

Inducible expression of alpha 1B-adrenoceptors in DDT1 MF-2 cells: comparison of receptor density and response

Hamster DDT1 MF-2 smooth muscle cells natively express alpha 1B-adrenoceptors which are linked to phosphatidylinositol hydrolysis. We studied the relationship between alpha 1B-adrenoceptor density and response in this cell line by stable transfection with an isopropyl-beta-D-thiogalactoside (IPTG)-inducible vector (pOP alpha 1B) containing the hamster alpha 1B-adrenoceptor cDNA. Transfected cells showed a 2-fold increase in receptor density compared to untransfected cells due to constitutive activity of the uninduced vector. Induction of vector expression caused a time-dependent increase in receptor density, reaching a maximum 8-fold increase after 48 h. Exposure to different concentrations of inducing agent for 16 h caused a graded increase in both receptor density and norepinephrine-stimulated [3H]inositol phosphate (InsP) formation. A linear correlation between receptor density and maximum InsP response was observed. Induction of receptor expression did not alter the potency of norepinephrine in stimulating [3H]InsP formation, suggesting that there was no receptor reserve, even at very high expression levels. This inducible expression system should be useful for relating receptor density and responsiveness, and comparing the coupling efficiency of closely related subtypes in activating different signal transduction mechanisms.

Cholinergic activation of phospholipase D in lacrimal gland acini is independent of protein kinase C and calcium

To determine if rat lacrimal gland acini contain phospholipase D (PLD) activity, we took advantage of PLD's unique ability, in the presence of ethanol, to catalyze a transphosphatidylation reaction to produce phosphatidylethanol (PEth). Lacrimal gland acini were labeled for 3 h with [14C]stearic acid, preincubated for 20 min in the presence of 2% ethanol, and incubated for 20 min with or without agonists. Total cellular lipids were then extracted and analyzed by thin-layer chromatography, and the radioactivity was determined by liquid scintillation counting. Carbachol (1 mM), a cholinergic agonist, stimulated the production of both [14C]PEth and [14C]phosphatidic acid ([14C]PA) twofold. This effect was completely blocked by the muscarinic antagonist atropine (10 microM). [14C]PEth accumulation was also stimulated twofold by the active phorbol esters, 4 beta-phorbol 12-myristate 13-acetate and 4 beta-phorbol 12,13-dibutyrate at 1 microM. Ionomycin (1 microM), a Ca2+ ionophore, also stimulated the production of [14C]PEth twofold. In contrast to carbachol, neither phorbol esters nor ionomycin stimulated [14C]PA production. Neither [14C]PEth nor [14C]PA production was altered by epinephrine (1 mM), a nonselective adrenergic agonist, or phenylephrine (0.1 and 1 mM), a specific alpha 1-adrenergic agonist. We concluded that PLD activity, modulated by muscarinic receptors, protein kinase C, and Ca2+, but not by adrenergic receptors, is present in rat lacrimal gland acini. We also concluded that cholinergic activation of PLD appears to be independent of PKC and Ca2+.

Phosphatidylcholine breakdown and signal transduction

PC hydrolysis by PLA2, PLC or PLD is a widespread response elicited by most growth factors, cytokines, neurotransmitters, hormones and other extracellular signals. The mechanisms can involve G-proteins, PKC, Ca2+ and tyrosine kinase activities. Although an agonist-responsive cytosolic PLA2 has been purified, cloned and sequenced, the agonist-responsive form(s) of PC-PLC has not been identified and no form of PC-PLD has been purified or cloned. Regulation of PLA2 by Ca2+ and MAPK is well established and involves membrane translocation and phosphorylation, respectively. PKC regulation of the enzyme in intact cells is probably mediated by MAPK. The question of G-protein control of PLA2 remains controversial since the nature of the G-protein is unknown and it is not established that its interaction with the enzyme is direct or not. Growth factor regulation of PLA2 involves tyrosine kinase activity, but not necessarily PKC. It may be mediated by MAPK. The physiological significance of PLA2 activation is undoubtedly related to the release of AA for eicosanoid production, but the LPC formed may have actions also. There is much evidence that PKC regulates PC-PLC and PC-PLD and this is probably a major mechanism by which agonists that promote PI hydrolysis secondarily activate PC hydrolysis. Since no agonist-responsive forms of either phospholipase have been isolated, it is not clear that PKC exerts its effects directly on the enzymes. Although it is assumed that a phosphorylation mechanism is involved, this may not be the case, and regulation may be by protein-protein interactions. G-protein control of PC-PLD is well-established, although, again, it has not been demonstrated that this is direct, and the nature of the G-protein(s) involved is unknown. In some cell types, there is evidence of the participation of a soluble protein, which may be a low Mr GTP-binding protein. What role this plays in the activation of PC-PLD is obscure. Agonist activation of PC hydrolysis in cells is usually Ca(2+)-dependent, but the step at which Ca2+ is involved is unclear, since PC-PLD and PC-PLC per se are not influenced by physiological concentrations of the ion. Most growth factors promote PC hydrolysis and this is mainly due to activation of PKC as a result of PI breakdown. However, in some cases, PC breakdown occurs in the absence of PI hydrolysis, implying another mechanism that does not involve PI-derived DAG.(ABSTRACT TRUNCATED AT 400 WORDS)

Receptor- and phorbol ester-mediated phospholipase D activation in rat parotid involves two different pathways

We have investigated phospholipase D (PLD) activation in rat parotid acini prelabeled with [14C]stearic acid. In the presence of 2% ethanol, muscarinic and alpha-adrenergic agonists stimulated the formation of [14C]phosphatidylethanol as a result of a PLD activity. The calcium ionophore, ionomycin, and the phorbol esters, 4 beta-phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate (PDBu), also stimulated phosphatidylethanol accumulation, but 1-oleyl-2-acetyl-sn-glycerol (OAG), a permeant analogue of diacylglycerol did not. Chelerythrine and staurosporine, two inhibitors of protein kinase C, failed to affect any response. These results suggest that protein kinase C was not involved in the regulation of PLD activity. A difference between PLD regulation by PMA and receptor-mediated agonists was observed with regard to the extracellular calcium requirement. Our results strongly suggest that PLD activation in parotid acini involved different pathways: a calcium-dependent pathway activated by receptor-mediated agonists and a calcium-independent pathway activated by phorbol esters. Moreover, we observed that PLD activation did not result in any change in phosphatidic acid level. We propose that the phosphatidyl transferase activity of PLD reflected a metabolic pathway which may allow a base-exchange reaction in parotid gland.

Phospholipase C and phospholipase D are independently activated in rat hippocampal slices

In order to investigate a possible G-protein-mediated activation of phospholipase D (PLD) and its relationship to the activation of phosphoinositide-specific phospholipase C (PI-PLC), we measured the effects of aluminium fluoride and carbachol on choline release, the PLD-specific transphosphatidylation reaction (generation of phosphatidylpropanol) and the formation of inositol phosphates in rat hippocampal slices. Aluminium fluoride markedly enhanced the formation of choline and phosphatidylpropanol but failed to increase the formation of inositol phosphates. In contrast, the muscarinic agonist carbachol strongly stimulated PI-PLC but failed to activate PLD. We conclude that PLD in hippocampal slices is activated by a G-protein independently of phosphoinositide hydrolysis.

Noradrenaline stimulation unbalances the phosphoinositide cycle in rat cerebral cortical slices

Muscarinic cholinergic and alpha 1-adrenoceptor-mediated stimulation of phosphoinositide hydrolysis in rat cerebral cortex were compared by measuring carbachol- and noradrenaline-induced accumulation of various intermediates of the phosphoinositide cycle. Unlike carbachol, noradrenaline in the presence of guanosine 5'-O-(3-thiotriphosphate) did not stimulate phospholipase C activity in brain cortical membranes. In cortical slices, the efficacy of noradrenaline to stimulate accumulation of 3H-inositol phosphates and [32P]phosphatidic acid was 2.5 to threefold that of carbachol. However, noradrenaline was less effective than carbachol in stimulating accumulation of [3H]CDP-diacylglycerol and resynthesis of phosphatidylinositol. This was not due to calcium inhibition of CTP:phosphatidate cytidyltransferase or to different lithium requirements for carbachol- and noradrenaline-stimulated accumulation of [3H]CDP-diacylglycerol. The noradrenaline-induced unbalance of the phosphoinositide cycle, which was most apparent at relatively high concentrations of calcium (2.5 mM) in the incubation buffer, was qualitatively reproduced with ionomycin. The use of the alpha 1a-subtype-selective adrenoceptor antagonists WB4101 and 5-methylurapidil revealed a single alpha 1a-like component mediating the effects of noradrenaline. Our results suggest that the primary mechanism for phospholipase C activation by brain alpha 1 adrenoceptors involves an increase in intracellular calcium concentration.

Endothelin receptors stimulate both phospholipase C and phospholipase D activities in different cell lines

Endothelin (ET) receptor-binding assays using [125I]ET-1 in C6-glioma cells and in Rat-1 and Swiss 3T3 fibroblasts indicated the presence of two binding sites, one of which binds agonists at the pM range and the other at the nM range. All three cell lines exhibited the same pharmacological profile for agonist binding (ET-1 congruent to sarafotoxin-b > ET-3), which suggests that the receptor is of the ETA type. Binding of ET-1 to the receptor resulted in activation of two phospholipases, phospholipase C (PLC) and phospholipase D (PLD). The activation of PLC or PLD by endothelin in the three cell lines was mediated by the high affinity binding site (nM range) and was not significantly affected by either extracellular or intracellular Ca2+. Measurement of PLD activation by ET-1 and/or phorbol 12-myristate 13-acetate (PMA), in the presence and absence of two potent inhibitors of protein kinase C (PKC), strongly suggests that activation of PLD by ET receptor in C6 glioma cells as well as in Rat-1 and Swiss 3T3 fibroblasts involves both PKC-dependent and PKC-independent mechanisms.

Phospholipase D: regulation and functional significance

PLD is a major route for hydrolysis of PC in most tissues, consistent with it playing an important role in signal transduction. The enzyme appears to be activated by a variety of different mechanisms in different tissues, suggesting there might be several different isoforms. Little, however, is known at present about its enzymology and molecular biology. There is little direct evidence to indicate the functional significance of PLD activation but an accumulation of indirect evidence links PLD with prolonged changes in cell function. In particular, two areas where there is strong evidence for a role for PLD are mitogenesis and leukocyte hyperresponsiveness. An important area for future work will be the investigation of how products from the PLD pathway exert these effects. Current evidence suggests an important role for Ca(2+)-independent PKC isoforms and probably also for novel cellular targets for the putative second messenger PA.

Evidence for receptor-linked activation of phospholipase D in rat parotid glands. Stimulation by carbamylcholine, PMA and calcium

In order to test if phospholipase D (PLD) activity exists in the rat parotid gland, we took advantage of the fact that, in the presence of ethanol, PLD generates phosphatidylethanol (PEth) via a transphosphatidylation reaction. Lipid extracts of parotid acini prelabelled with [3H]myristic acid were analyzed by thin layer chromatography to determine [3H]phosphatidylethanol ([3H]PEth) formation. Carbamylcholine (1 mM) stimulated [3H]PEth formation in the presence of 2% ethanol, this effect was completely inhibited by atropine (10 microM). PMA (0.1-1 microM) and ionomycine (10 microM) also caused [3H]PEth generation. We conclude that a phospholipase D activity is present in the rat parotid gland and is regulated by muscarinic cholinergic receptors. Protein kinase C and calcium could also modulate this activity. This report provides the first evidence for the existence and receptor-linked regulation of phospholipase D in an exocrine gland, the rat parotid gland.

Metabotropic excitatory amino acid receptor activation stimulates phospholipase D in hippocampal slices

Metabotropic excitatory amino acid (EAA) receptors are coupled to effector systems through G proteins. Because various G protein-coupled receptors stimulate the hydrolysis of phosphatidylcholine by phospholipase D (PLD), we examined the possibility that metabotropic EAA receptors exist that are coupled to the activation of PLD. We found that the selective metabotropic glutamate receptor (mGluR) agonists 1S,3R-amino-1,3-cyclopentanedicarboxylic acid (ACPD) and 1S,3S-ACPD, but not the inactive isomer, 1R,3S-ACPD, induce a concentration-dependent increase in PLD activity in hippocampal slices. Selective ionotropic glutamate receptor (iGluR) antagonists did not block 1S,3R-ACPD-induced PLD stimulation. Furthermore, although selective iGluR agonists did not activate this response, the nonselective mGluR-iGluR agonists, ibotenate and quisqualate, caused significant increases in PLD activity (all in the presence of iGluR antagonists). L-2-Amino-3-phosphonopropionic acid, which blocks the mGluR that is coupled to phosphoinositide hydrolysis in various brain regions, activates PLD to the same extent as the active isomers of ACPD. These data suggest that metabotropic EAA receptors exist in hippocampus that are coupled to PLD activation and are pharmacologically distinct from phosphoinositide hydrolysis-coupled mGluRs.

Species heterogeneity of hepatic alpha 1-adrenoceptors: alpha 1A-, alpha 1B- and alpha 1C-subtypes

alpha 1-Adrenergic activation stimulated phosphorylase and phosphoinositide turnover in hepatocytes from guinea pigs, rats and rabbits. Chlorethylclonidine inhibited these effects in rat and rabbit cells but not in guinea pig hepatocytes; low concentrations of 5-methyl urapidil blocked the alpha 1 actions in guinea pig and rabbit liver cells, but not in rat hepatocytes. Binding competition experiments also showed high affinity for 5-methyl urapidil in liver membranes from guinea pigs and rabbits and low affinity in those from rats. The data indicated that guinea pig hepatocytes express alpha 1A-, rat hepatocytes alpha 1B- and rabbit hepatocytes alpha 1C- adrenoceptors. This was confirmed by Northern analysis using receptor subtype-selective probes.