Long-term phorbol ester treatment dissociates phospholipase D activation from phosphoinositide hydrolysis and prostacyclin synthesis in endothelial cells stimulated with bradykinin

Phospholipase D2 regulates endothelial permeability through cytoskeleton reorganization and occludin downregulation

Endothelial permeability is controlled by adhesive strengths which connect cells to each other through interendothelial junctions and by contractile forces associated with cytoskeleton reorganization. Phospholipase D (PLD) activation resulting in the generation of phosphatidic acid (PA) is increasingly recognized as a key event in the initiation of various cell responses. In human umbilical vein endothelial cells (HUV-EC), enhancement of intracellular PA by a variety of approaches increased the permeability of endothelial cell monolayers and induced stress fibre formation. Using adenovirus-mediated overexpression and siRNA silencing, we showed that PLD2 but not PLD1 was involved in the enhancement of basal permeability through cytoskeleton reorganization. Furthermore, PLD2 overexpression induced ERK1/2 activation and downregulated the expression of occludin, a major component of tight junctions. A substantial part of PLD2 protein was associated with the low-density caveolin-rich fractions isolated on sucrose gradients. The Raf-1 specific inhibitor GW-5074 drastically reduced hyperpermeability induced by PLD2 overexpression, and inhibited PA-mediated increase of endothelial permeability and ERK1/2 activation. On the whole, the present results demonstrate the selective role of PLD2 isoform in the control of endothelial permeability through a mechanism involving both stress fibre formation and contraction, and occludin downregulation, possibly resulting from PA-mediated activation of Raf-1.

Wounding sheets of epithelial cells activates the epidermal growth factor receptor through distinct short- and long-range mechanisms

Wounding epithelia induces activation of the epidermal growth factor receptor (EGFR), which is absolutely required for induction of motility. ATP is released from cells after wounding; it binds to purinergic receptors on the cell surface, and the EGFR is subsequently activated. Exogenous ATP activates phospholipase D, and we show here that ATP activates the EGFR through the phospholipase D2 isoform. The EGFR is activated in cells far (>0.3 cm) from wounds, which is mediated by diffusion of extracellular ATP because activation at a distance from wounds is abrogated by eliminating ATP in the medium with apyrase. In sharp contrast, activation of the EGFR near wounds is not sensitive to apyrase. Time-lapse microscopy revealed that cells exhibit increased motilities near edges of wounds; this increase in motility is not sensitive to apyrase, and apyrase does not detectably inhibit healing of wounds in epithelial sheets. This novel ATP/PLD2-independent pathway activates the EGFR by a transactivation process through ligand release, and it involves signaling by a member of the Src family of kinases. We conclude that wounding activates two distinct signaling pathways that induce EGFR activation and promote healing of wounds in epithelial cells. One pathway signals at a distance from wounds through release of ATP, and another pathway acts locally and is independent on ATP signaling.

Activation of muscarinic receptors in porcine airway smooth muscle elicits a transient increase in phospholipase D activity

Phospholipase D (PLD) is a phosphodiesterase that catalyses hydrolysis of phosphatidylcholine to produce phosphatidic acid and choline. In the presence of ethanol, PLD also catalyses the formation of phosphatidylethanol, which is a unique characteristic of this enzyme. Muscarinic receptor-induced changes in the activity of PLD were investigated in porcine tracheal smooth muscle by measuring the formation of [3H]phosphatidic acid ([3H]PA) and [3H]phosphatidylethanol ([3H]PEth) after labeling the muscle strips with [3H]palmitic acid. The cholinergic receptor agonist acetylcholine (Ach) significantly but transiently increased formation of both [3H]PA and [3H]PEth in a concentration-dependent manner (>105-400% vs. controls in the presence of 10(-6) to 10(-4) M Ach) when pretreated with 100 mM ethanol. The Ach receptor-mediated increase in PLD activity was inhibited by atropine (10(-6) M), indicating that activation of PLD occurred via muscarinic receptors. Activation of protein kinase C (PKC) by phorbol-12-myristate-13-acetate (PMA) increased PLD activity that was effectively blocked by the PKC inhibitors calphostin C (10(-8) to 10(-6) M) and GFX (10(-8) to 10(-6) M). Ach-induced increases in PLD activity were also significantly, but incompletely, inhibited by both GFX and calphostin C. From the present data, we conclude that in tracheal smooth muscle, muscarinic acetylcholine receptor-induced PLD activation is transient in nature and coupled to these receptors via PKC. However, PKC activation is not solely responsible for Ach-induced activation of PLD in porcine tracheal smooth muscle.

Inhibition of GTP gamma S-dependent phospholipase D and Rho membrane association by calphostin is independent of protein kinase C catalytic activity

We studied the relationships between the activation of phospholipase D (PLD) by guanine nucleotides and phorbol esters in permeabilized U937 promonocytes and in solubilized extracts prepared from U937 cell membranes. Treatment of permeabilized cells with phorbol myristate acetate (PMA) strongly potentiated GTP gamma S-dependent PLD activity at free Ca2+ < 100 nM. In the absence of GTP gamma S, PMA stimulated only minor PLD activity. This suggested synergistic interaction between regulatory G-proteins and a protein kinase C (PKC) family kinase. The potential role of PKC was evaluated by testing two mechanistically distinct PKC inhibitors, bisindolylmaleimide (BIM) and calphostin. BIM inhibits PKC enzymes via competition with ATP for binding to the catalytic domain, while calphostin competes with PMA or diglyceride for binding to the regulatory domain. The ability of PMA to potentiate the GTP gamma S-dependent PLD was not inhibited by BIM. In contrast, calphostin strongly inhibited the GTP gamma S-dependent PLD activity, both in the presence and absence of PMA as a potentiating agent. Calphostin also produced complete inhibition of a GTP gamma S-dependent PLD activity, present in solubilized membrane extracts, which was assayed using phospholipid vesicles of defined composition. Treatment of reconstituted membrane/cytosol mixtures with calphostin also produced complete inhibition of the GTP gamma S-induced translocation of Rho A from cytosol to membrane. In contrast to its effects on the U937 cell PLD, calphostin did not inhibit the activity of purified PLD from cabbage. These results suggest that the assembly of active RhoA/PLD signaling complexes on membranes involves a phorbol ester/calphostin-binding protein, but is not dependent on PKC-type catalytic activity.

Cell signalling through guanine-nucleotide-binding regulatory proteins (G proteins) and phospholipases

Phospholipases are important enzymes in cell signal transduction since they hydrolyze membrane phospholipids to generate signalling molecules. Heterotrimeric guanine-nucleotide-binding regulatory proteins (G proteins) play a major role in their regulation by a variety of agonists that activate receptors with seven membrane-spanning domains. Phospholipases of the C type, which hydrolyze inositol phospholipids to yield inositol trisphosphate and diacylglycerol, are regulated by the alpha and betagamma subunits of certain heterotrimeric G proteins as well as by receptor-associated and non-receptor-associated tyrosine kinases. Phospholipases of the D type, which hydrolyze phosphatidylcholine to phosphatidic acid, are regulated by members of the ADP-ribosylation factor and Rho subfamilies of small G proteins, and by protein kinase C and other factors. This review presents recent information concerning the molecular details of G protein regulation of these phospholipases.

Regulation of bradykinin-induced Ins(1,4,5)P3 formation by protein kinase C in human fibroblasts

To better understand the molecular mechanisms that underlie the exaggerated bradykinin (BK)-stimulated release of Ins(1,4,5)P3 in fibroblasts from Alzheimer patients, the role of G-proteins, protein kinase C (PKC) and cyclic AMP in BK-induced Ins(1,4,5)P3 formation was determined. A role for G-proteins in the coupling of the BK receptor to intracellular signals was indicated by guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) enhanced BK-stimulated Ins(1,4,5)P3 release. The coupling of G-proteins to Ins(1,4,5)P3 formation was sensitive to cholera toxin (CTX), but not pertussis toxin (PTX), and was not altered by PKC activation. The inhibition by CTX appeared to be secondary to its ability to increase cyclic AMP, because forskolin also inhibited the BK-mediated Ins (1,4,5)P3 release. Activation of PKC with TPA diminished the number of BK receptors by 33% and proportionally decreased BK-mediated Ins(1,4,5)P3 formation by 28%. The latter response was abolished by PKC inhibitors. Depletion of PKC by prolonged TPA treatment did not further alter the number of BK receptors but further decreased the Ins(1,4,5)P3 response by 65%. Thus, changes in PKC probably do not underlie the enhanced BK-induced Ins(1,4,5)P3 formation in AD fibroblasts, because both activation and depletion of the PKC diminished the Ins(1,4,5)P3 response.

Mechanisms of phospholipase D stimulation by m3 muscarinic acetylcholine receptors. Evidence for involvement of tyrosine phosphorylation

In human embryonic kidney cells stably expressing the human m3 muscarinic acetylcholine receptor (mAChR) subtype, agonist (carbachol) activation stimulated phospholipase C, increased cytoplasmic calcium concentration, induced tyrosine phosphorylation of various cellular proteins and activated phospholipase D. Bypassing membrane receptors, phospholipase D was activated in these cells by direct activation of protein kinase C by phorbol esters, by direct activation of GTP-binding proteins by A1F4- and a stable GTP analogue (in permeabilized cells), by increasing cytoplasmic calcium concentration with the calcium ionophore A23187 and also apparently by tyrosine phosphorylation. In order to identify possible mechanisms by which the m3 mAChR couples to phospholipase D, various inhibitors of protein kinase C, tyrosine kinases and calcium-dependent events were studied. Prevention of an agonist-induced increase in cytoplasmic calcium concentration did not alter the mAChR-induced phospholipase D stimulation. The protein kinase C inhibitors, calphostin C and staurosporine, efficiently prevented phospholipase D activation by phorbol 12-myristate 13-acetate but only partially inhibited the activation induced by the mAChR agonist. Additionally, down-regulation of protein kinase C by prolonged exposure to phorbol 12-myristate 13-acetate abrogated phospholipase D activation by this effector but had only minor or no effects on the response to the mAChR agonist and direct activators of GTP-binding proteins. In contrast, the tyrosine kinase inhibitor genistein abolished the carbachol-induced and A1F4(-)-induced phospholipase D activation but had no effect on enzyme activation by phorbol 12-myristate 13-acetate. The data indicate that phospholipase D in m3 mAChR-expressing human embryonic kidney cells can be activated by various different mechanisms, i.e. receptor agonists, GTP-binding proteins, protein kinase C-dependent and calcium-dependent events and tyrosine phosphorylation. The coupling of m3 mAChR to phospholipase D appears to be largely independent of concomitant phospholipase C activation with subsequent increase in cytoplasmic calcium concentration and protein kinase C activity. The data instead suggest the involvement of an essential protein tyrosine phosphorylation mechanism in phopsholipase D activation by the m3 mAChR and heterotrimeric GTP-binding proteins.

Insulin-stimulated hydrolysis of phosphatidylcholine by phospholipase C and phospholipase D in cultured rat hepatocytes

We have investigated the mechanism of action by which insulin increases phosphatidate (PA) and diacylglycerol (DAG) levels in cultured rat hepatocytes. Insulin initially stimulated phosphatidylcholine-dependent phospholipase D (PC-PLD) with a significant increase in both PA and intracellular as well as extracellular choline. The involvement of phospholipase D was confirmed by the formation of PC-derived phosphatidylethanol in the presence of ethanol. The DAG increase appeared to be biphasic. Only the early phase of DAG production was inhibited by propranolol, an inhibitor of the phosphatidate phosphatase (PAP) responsible for the conversion of PA into DAG, suggesting that initially the DAG increase is due to the PLD-PAP pathway. The delayed DAG increase was in parallel with increased intracellular and extracellular phosphocholine and probably derived directly from PC-PLC activity. Experiments performed in the presence of 1 microM phorbol 12-myristate 13-acetate (PMA) indicated that protein kinase C (PKC) mediated the insulin effect on PC-PLC, but not on PC-PLD. These findings were confirmed using the PKC inhibitors calphostin, H7 and staurosporine. The dual activation of these phospholipases with a biphasic elevation of DAG levels and activation of specific PKC isoenzymes could be necessary to elicit both early and delayed effects of insulin.

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)

Protein kinase C modulates receptor-independent activation of endothelial nitric oxide synthase

The intracellular regulation of nitric oxide synthase has been the focus of intense investigation. Bioassay studies using vascular rings have suggested that protein kinase C inhibits endothelium-dependent vascular relaxation. However, information regarding the effects of protein kinase C on the synthesis of nitric oxide in endothelial cells is not available. Therefore, we investigated the effects of protein kinase C to regulate receptor-independent activation of nitric oxide synthase activity in cultured bovine pulmonary artery endothelial cells. Activation of protein kinase C by phorbol 12-myristate 13-acetate or 1,2-dioctanoyl-sn-glycerol inhibited receptor-dependent and receptor-independent nitric oxide synthase activity. The inhibition of nitric oxide synthase by protein kinase C was concentration dependent and markedly blunted by staurosporine. The inhibition of protein kinase C by staurosporine alone enhanced basal nitric oxide synthase activity. Furthermore, depletion of protein kinase C enhanced both basal and agonist-stimulated nitric oxide synthase activity. These studies indicate that protein kinase C modulates the activity of the constitutive Ca2+/calmodulin-dependent endothelial nitric oxide synthase in the basal state and following agonist stimulation through direct inhibition of the enzyme as well as receptor desensitization. These direct regulatory effects of protein kinase C on endothelial nitric oxide synthase activity may have important implications in the physiologic regulation of vascular tone.

Platelet-activating factor stimulates phosphatidic acid formation in cultured rat mesangial cells: roles of phospholipase D, diglyceride kinase, and de novo phospholipid synthesis

Platelet-activating factor (PAF) stimulates phospholipase C (PLC)-induced hydrolysis of phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P2). Yet, PAF-stimulated diglycerides (DG) are still elevated at time points where inositol polyphosphates have returned to basal levels. Thus, other signal transduction pathways that hydrolyze phosphatidylcholine (PtdCho) or phosphatidylethanolamine (PtdEth) and form DG and phosphatidic acids (PA) through either PLC or phospholipase D (PLD) may also mediate PAF-stimulated cellular responses. Initially the effects of PAF upon 32P-PA generation in mesangial cells (MC) were assessed. PA formation may be indicative of several metabolic pathways including PLD and DG kinase activities as well as de novo phospholipid synthesis. PAF (10(-7) M) increased 32P-PA formation as early as 5 seconds and this elevation persisted up to 15 minutes. When MC were pretreated with the DG kinase inhibitor-R59022, PAF-induced 32P-PA formation was diminished at early but not late time points, demonstrating that the initial component of PA formation may be due, in part, to PLC activation and subsequent phosphorylation of DG. The reciprocal reaction, PA phosphohydrolase, which dephosphorylates PA to from DG was not stimulated by PAF, suggesting that the sustained elevation of DG induced by PAF is primarily a reflection of PLC. 3H-glycerol pulse-labeling experiments suggest that PAF also stimulates de novo phospholipid synthesis which also contributes to PA formation. Conclusive proof for PLD in the generation of PA was obtained by assessing the formation of 3H-phosphatidyl-ethanol (PEt) from 3H-alkyl-lyso-glycero phosphocholine (GPC) and exogenous ethanol. PAF stimulated alkyl-PEt generation in the presence but not the absence of 0.5% ethanol. Also, PAF induced a concomitant elevation of alkyl-PA at 15 minutes and this elevation of alkyl-PA was reduced when the cells were exposed to exogenous ethanol, reflecting the formation of PEt. Corroborating evidence suggests that PAF stimulates 3H-choline and 3H-ethanolamine release, suggesting that PtdCho and PtdEth are substrates for PLD. Thus, these data demonstrate that MC respond to PAF with elevated PLD and DG kinase activities as well as with an increased rate of de novo lipid synthesis which increases PA, a potential intracellular signal.

Phospholipase D activity in phagocytic leucocytes is synergistically regulated by G-protein- and tyrosine kinase-based mechanisms

The regulation of phospholipase D (PLD)-type effector enzymes by G-proteins and protein kinases/phosphatases was characterized in the U937 human promonocytic leucocyte line. PLD activity was assayed by measuring (in the presence of 1% ethanol) the accumulation of phosphatidylethanol in cells permeabilized with beta-escin, a saponin-like detergent. Basal PLD activity was very low when cells were permeabilized and incubated in cytosol-like medium containing micromolar [Ca2+]. When this medium was supplemented with exogenous MgATP or guanosine 5'-[gamma-thio]triphosphate (GTP[S]), PLD activity increased by 9- and 14-fold respectively. Cells permeabilized in the absence of exogenously added MgATP, but in the presence of 1 microM vanadate/100 microM H2O2, also exhibited a modest 12-fold increase in PLD activity. However, the simultaneous presence of either GTP[S] plus exogenous MgATP or GTP[S] plus vanadate/H2O2 (and endogenous MgATP) induced similar 60-75-fold increases in the rate and extent of phosphatidylethanol accumulation. These latter effects of vanadate/H2O2 were strongly correlated with the very rapid accumulation of multiple tyrosine-phosphorylated proteins. Other studies utilized cells which were permeabilized in the presence of GTP[S] and then washed before assay of PLD. These cells retained approximately 60% of the MgATP-regulatable PLD activity (EC50 approximately = to 100 microM MgATP) observed in freshly permeabilized non-washed cells. In the absence of GTP[S] pre-treatment, washed cells retained minimal PLD activity. Genistein, a tyrosine kinase inhibitor, significantly attenuated the ability of MgATP to stimulate PLD activity and accumulation of tyrosine-phosphorylated proteins in the washed GTP[S]-treated cells. These data suggest that PLD activity in myeloid leucocytes involves co-ordinate regulation by both G-protein(s) and tyrosine phosphorylation.

Bradykinin stimulates phospholipase D in primary cultures of guinea-pig tracheal smooth muscle

Conditions were established for the primary culture of guinea-pig tracheal smooth muscle cells, the identity of which was confirmed by the presence of smooth muscle alpha-actin by western blotting. Cells were preincubated with [3H]palmitate which was incorporated, almost exclusively, into phosphatidylcholine. When these cells were stimulated by either bradykinin or phorbol 12-myristate 13-acetate (PMA), in the presence of butan-1-ol, the non-metabolizable product [3H]phosphatidylbutanol ([3H]PtdBut) accumulated by virtue of the phosphatidyltransferase activity of phospholipase D. The activation of phospholipase D by bradykinin was inhibited by 86 +/- 11% (N = 3 experiments) in the presence of the protein kinase C inhibitor, staurosporine (1 microM) and by 88 +/- 11% (N = 3 experiments) in cells that had been chronically treated with PMA to down-regulate their protein kinase C. PMA-stimulated phospholipase D was similarly affected (92 +/- 2% inhibited by staurosporine, 87 +/- 6% inhibited by protein kinase C down-regulation). Removal of extracellular Ca2+ markedly reduced the bradykinin-stimulated phospholipase D response (by 73 +/- 10%, N = 3 experiments) but had only a limited effect upon PMA-stimulated phospholipase D activity (by 23 +/- 6%, N = 3 experiments). [AIF4](-)-stimulation of the cells also resulted in the activation of phospholipase D, indicating the involvement of a G-protein. However, this was not Gi since pertussis-toxin pretreatment of the cells failed to abolish either bradykinin-stimulated inositol (1,4,5)trisphosphate formation or [3H]PtdBut accumulation. Western blotting revealed the presence of Gq/G11 which couples to the inositol lipid-directed phospholipase C. Indomethacin (10 microM) was without effect upon bradykinin-stimulated phospholipase D activity, suggesting that the bradykinin effects were not mediated indirectly by cyclooxygenase products. The role of phospholipase D activation in tracheal smooth muscle may be to, indirectly, produce diacylglycerol for the activation of protein kinase C which has been implicated in sustained contraction. However, the immediate product of phospholipase D, phosphatidate, has been proposed to have a number of second messenger roles and may itself, by an undefined mechanism, be involved in the sustained contraction of airway smooth muscle.

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.

Bradykinin and phorbol dibutyrate activate phospholipase D in PC12 cells by different mechanisms

Bradykinin is known to activate phospholipase D in PC12 cells. Because bradykinin may also activate protein kinase C in these cells, the possible role of this kinase in mediating the action of bradykinin was investigated. Phospholipase D activity in PC12 cells was assayed by measuring the formation of [3H]phosphatidylethanol in cells prelabeled with [3H]palmitic acid and incubated in the presence of ethanol. The phorbol ester phorbol dibutyrate mimicked the effect of bradykinin on [3H]phosphatidylethanol formation. The protein kinase C inhibitor staurosporine (1 microM) significantly attenuated the effect of phorbol dibutyrate (35-70%) but did not block bradykinin-stimulated [3H]phosphatidylethanol formation. In addition, the effect of phorbol dibutyrate was additive with that of bradykinin. Prolonged treatment of PC12 cells with phorbol dibutyrate (24 h), which depletes cells of protein kinase C, greatly attenuated bradykinin-stimulated [3H]phosphatidylethanol accumulation in intact cells. This treatment caused a 55% decrease in both fluoride-stimulated [3H]phosphatidylethanol production in the intact cell and phospholipase D activity as assessed by an in vitro assay using an exogenous substrate. Therefore, the effect of prolonged phorbol dibutyrate pretreatment on bradykinin-stimulated [3H]phosphatidylethanol production could not be attributed exclusively to the depletion of protein kinase C. Thus, although the data with phorbol ester suggest that activation of protein kinase C leads to an increase in phospholipase D activity, this kinase probably does not play a role in mediating the effect of bradykinin. Finally, although pretreatment with phorbol dibutyrate completely blocked bradykinin-stimulated [3H]phosphatidylethanol production in the intact cell, it only partially (approximately 50%) inhibited bradykinin-stimulated [3H]diacylglycerol formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term activation of protein kinase C by nicotine in bovine adrenal chromaffin cells

Previous results from our laboratory suggest that long-term treatment of primary cultured bovine adrenal medullary (BAM) chromaffin cells with nicotine or phorbol 12-myristate 13-acetate, either of which directly activates protein kinase C (PKC), increases the mRNA levels encoding catecholamine-synthesizing enzymes and proenkephalin. In the present study, we have examined the effects of nicotine on BAM cell PKC activity with special emphasis on long-term effects. Nicotine increased particulate PKC activity in a concentration-dependent manner when measured using in vitro enzyme assay with histone as the substrate. This effect is mediated through nicotinic cholinergic receptors, because 1,1-dimethylphenylpiperazinium, a nicotinic agonist, had a similar effect. In addition, chlorisondamine, a specific nicotine-receptor blocking drug, antagonized the effect of nicotine. Nicotine also increased specific [3H]phorbol 12,13-dibutyrate ([3H]PdBu) binding within 1 min, the effect of which was maximal between 3 and 12 min. This effect was reversed by chlorisondamine similarly after 12 min and after 18 h of nicotine treatment, indicating that continual nicotinic-receptor occupancy is required for persistent PKC activation. Compared to PKC activation, the onset of nicotine-stimulated diacylglycerol production was slow, and it was observed after 12 min of incubation with nicotine. The diacylglycerol levels, specific [3H]PdBu binding, and PKC activity remained significantly elevated for at least 18 h with continuous nicotine incubation. Furthermore, nicotine increased the PKC immunoreactivity of a particulate protein with a molecular mass of 82 kDa in the western blot. These results suggest that nicotinic-receptor activation increases PKC activity and immunoreactivity in BAM cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin stimulates phosphatidic acid formation in cultured rat mesangial cells: role of a protein kinase C-regulated phospholipase D

We have previously reported that endothelin-1 stimulates phospholipase C-induced hydrolysis of phosphatidylinositol-4,5-bisphosphate. Other signal transduction pathways that hydrolyze alternative phospholipids through phospholipase D may also mediate endothelin-stimulated cellular responses. We initially evaluated endothelin-dependent generation of 32P-phosphatidic acid as an indirect indication of phospholipase D activity in rat mesangial cells. Endothelin (10(-7) M) induced an elevation of phosphatidic acid that was maximal at 15 min and persisted upward of 60 min. Pretreatment with the diacylglycerol-kinase inhibitor, R59022, did not reduce formation of endothelin-stimulated 32P-phosphatidic acid, demonstrating that the sequential actions of phospholipase C/diacylglycerol kinase do not contribute to endothelin-stimulated phosphatidic acid formation. We next conclusively identified a role for phospholipase D in the generation of phosphatidic acid by assessing the formation of 3H-phosphatidylethanol from 3H-alkyl lyso glycerophosphocholine and exogenous ethanol. Endothelin stimulated 3H-alkyl phosphatidylethanol formation in the presence but not the absence of 0.5% ethanol. Also, endothelin induced a concomitant elevation of 3H-alkyl-phosphatidic acid that was significantly reduced when the cells were exposed to exogenous ethanol, reflecting the formation of phosphatidylethanol. In addition, endothelin stimulated the release of 3H-choline and 3H-ethanolamine, demonstrating that additional phospholipids may serve as substrates for phospholipase D. Phorbol esters and synthetic diglycerides mimicked the effects of endothelin to stimulate phospholipase D and inhibitors of protein kinase C significantly reduced endothelin-stimulated phospholipase D. In addition, endothelin did not stimulate phosphatidylethanol formation in protein kinase C down-regulated cells. The calcium ionophore, ionomycin, did not stimulate phospholipase D and mesangial cells pretreated with BAPTA to chelate cytosolic calcium did not show a diminished endothelin-stimulated phospholipase D. Thus these data demonstrate that mesangial cells possess a protein kinase C-regulated phospholipase D activity that can be stimulated with endothelin.

Bacterial lipopolysaccharide stimulates phospholipid synthesis and phosphatidylcholine breakdown in cultured human leukemia monocytic THP-1 cells

1. De novo synthesis of phospholipid and its catabolism in human leukemia monocytic THP-1 cells were investigated. 2. Radiolabelled precursors: [methyl-3H]chloride, [1,2-14C]ethanolamine and myo-[2-3H]inositol were readily incorporated into CHCl3-MEOH extractable lipid fraction as a function of time. 3. The radiolabels derived from choline, ethanolamine and inositol were preferentially incorporated into PC, PE and PI fraction, respectively. The data indicate that de novo PL synthesis takes place, and the CDP-choline pathway is operative as a major pathway for PC synthesized in THP-1 cells. 4. Bacterial endotoxin dose-dependently stimulated the incorporation of radiolabelled precursors. Approximately 50% stimulation in PC and PE synthesis was obtained in 20 hr, while the incorporation of [3H]inositol was rapidly stimulated by 170% within 4 hr, and the stimulation declined drastically thereafter. 5. LPS did not alter the radiolabel distribution into PL in any of the three cases. 6. In pulse-chase studies, the cells prelabelled with radioactive PL were exposed to LPS (1 micrograms/ml). The breakdown of PC was enhanced about 30% within the first 2 hr followed by a stimulated PC synthesis observed in the next 4 hr. In contrast, LPS did not induce the hydrolysis of PE and PI. 7. The data indicate that LPS produces a broad spectrum of stimulatory effects on PL synthesis and selectively stimulates the hydrolysis of PC via phospholipase C/D reaction in THP-1 cells.

Regulation of phospholipid hydrolysis and second messenger formation by protein kinase C

The binding of a variety of agonists to their receptors leads to the breakdown of membrane phospholipids and the formation of intracellular second messengers. Hydrolysis of inositol phospholipids by phospholipase C results in the formation of two second messengers, inositol-1,4,5-trisphosphate which mobilizes intracellular calcium and the neutral lipid diacylglycerol (DAG) which binds to and activates protein kinase C (PKC). PKC is actually a family of homologous serine/threonine protein kinases which play a central role in regulation of growth, differentiation and secretion reactions in a variety of cell types. In addition to these feedforward roles of PKC, it is thought to play an important feedback role, regulating early events in signal transduction. To explore these feedback functions we have examined the effect of PKC inhibitors on second messenger formation in thrombin-stimulated human platelets (a rapidly responding system) and the effect of PKC overexpression on second messenger formation and mitogenesis in rat fibroblasts (a system where sustained signaling occurs). Treatment of platelets with inhibitors of PKC potentiates DAG mass formation in response to thrombin while prior activation of PKC with phorbol esters blocks DAG mass formation, consistent with PKC playing a negative feedback role, inhibiting inositol phospholipid breakdown. DAG can also be formed by the sequential hydrolysis of phosphatidylcholine by phospholipase D and phosphatidic acid phosphohydrolase. This is a minor reaction in the rapidly responding platelet system, but may play a role in sustained signaling events. We have found that fibroblasts which overexpress the beta 1 isozyme of PKC display greatly enhanced DAG formation and phospholipase D activation in response to phorbol ester treatment. Upon stimulation of fibroblasts with thrombin, phospholipase D activation is also enhanced by PKC overexpression while formation of inositol phosphates is suppressed. These data suggest that PKC may act as a switch, terminating inositol phospholipid hydrolysis and activating the hydrolysis of phosphatidylcholine. Furthermore, we have observed a strong correlation between activation of phospholipase D and mitogenesis, suggesting an important role for this enzyme in long-term cellular responses to activation.

Influence of bradykinin on diacylglycerol and phosphatidic acid accumulation in cultured bovine adrenal chromaffin cells

Earlier studies have shown that bradykinin stimulated release of catecholamines from chromaffin cells by an influx of calcium through dihydropyridine-insensitive channels, and also that bradykinin stimulated (poly)phosphoinositide hydrolysis. To investigate membrane-bound second messengers in chromaffin cells, and to elucidate any role these may play in stimulus-secretion coupling, we have studied the influence of bradykinin on diacylglycerol and phosphatidic acid (PA). Using equilibrium labelling of primary cultures of chromaffin cells with [3H]arachidonic acid or [3H]glycerol, we found no influence of bradykinin (10 nM) on labelled diacylglycerol formation, either in the presence or absence of inhibitors of diacylglycerol lipase or kinase. However, when we used cells prelabelled with 32Pi for 2.5 h, we found that bradykinin produced a substantial stimulation of label found in PA, with an EC50 value of about 1 nM. This bradykinin stimulation of [32P]PA formation was only partially dependent on extracellular calcium, in contrast to the smaller response to nicotine, which was completely dependent on extracellular calcium. Short (10 min) pretreatment with tetradecanoylphorbol acetate (TPA) almost completely eliminated the bradykinin-stimulated formation of inositol phosphates, but failed to affect bradykinin stimulation of label in PA, suggesting that PA production in response to bradykinin is not downstream of phospholipase C activation. TPA alone failed to stimulate [32P]PA substantially, whereas long-term (24 or 48 h) treatment with TPA failed to attenuate the response to bradykinin. Diacylglycerol kinase inhibitors were also without effect on the bradykinin stimulation of [32P]PA. These results suggest that bradykinin stimulates PA production by a mechanism independent of the activation of protein kinase C.(ABSTRACT TRUNCATED AT 250 WORDS)

Guanine-nucleotide- and adenine-nucleotide-dependent regulation of phospholipase D in electropermeabilized HL-60 granulocytes

We have characterized the regulation of phospholipase D (PLD) in electropermeabilized HL-60 granulocytes in which endogenous phospholipids were pre-labelled with [3H]oleic acid. Treatment of these permeabilized cells with the non-hydrolysable GTP analogues guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and guanosine 5'-[beta gamma-imido]triphosphate induced a sustained (near-linear for up to 60 min) accumulation of phosphatidic acid (PA). In the presence of ethanol a sustained production of phosphatidylethanol (PEt) was also observed. With increasing concentrations of ethanol, PEt formation increased, whereas PA formation declined; this indicated involvement of a PLD-type effector enzyme. The ability of GTP[S] to stimulate this PLD activity was Mg(2+)-dependent and was inhibited by GDP and its non-hydrolysable beta-thio analogue. Ca2+, at concentrations less than or equal to nM, had no effect on the GTP[S]-dependent PLD activity. However, higher concentrations of Ca2+ produced a significant potentiation of this activity. Inclusion of MgATP (greater than or equal to 0.1 mM), but not other nucleoside triphosphates, also induced a large potentiation of GTP[S]-dependent PLD activation. In the absence of guanine nucleotides, MgATP elicited no significant activation of PLD. Significantly, this effect of ATP was not mimicked by adenosine 5'-[beta gamma-methylene]triphosphate, a non-hydrolysable ATP analogue. Rather, this analogue inhibited both basal and ATP-potentiated GTP[S]-dependent PLD activity. This suggests that the ability of ATP to potentiate GTP[S]-dependent PLD activity involves phosphotransferase action rather than simple allosteric effects induced by adenine nucleotide binding. The absolute magnitude of the GTP[S]-dependent PLD activity which could be potentiated by MgATP was decreased by 90% when the permeabilized cells were preincubated for various times before addition of these stimulatory agents. This time-dependent loss of MgATP-induced potentiation was prevented when the permeabilized cells were preincubated in the presence of GTP[S]. These results demonstrate that electropermeabilized HL-60 granulocytes can be used to discriminate synergistic roles for a GTP-binding protein(s) and an ATP-dependent process (kinase?) in the regulation of phospholipase D activity.

Bradykinin stimulates production of inositol (1,4,5) trisphosphate in cultured mesangial cells of the rat via a BK2-kinin receptor

1. Using [125I-Tyr0]-BK, as radiolabelled ligand, and various agonists and antagonists of bradykinin (BK) we identified a single class of specific BK2-binding sites in mesangial cell membranes (Bmax = 73 fmol mg-1 protein and Kd = 3.7 nM). 2. Following the addition of 0.1 microM BK, inositol (1,4,5) trisphosphate (IP3) formation increased within 20 s from a basal level of 64 to a maximal value of 175 pmol mg-1 protein. 3. Incubation in a Ca(2+)-free medium did not change IP3 production but a 5 min preincubation with 1 mM EGTA completely prevented the BK-induced IP3 formation, suggesting that IP3 formation is partly dependent on extracellular calcium. 4. The BK2 antagonist D-Arg-Hyp3-D-Phe7-BK (10 microM) but not the BK1 antagonist (des-Arg9-Leu8-BK) abolished IP3 production in response to 0.1 microM BK. Pretreatment of mesangial cells with pertussis toxin was without effect on BK-induced IP3 formation, whereas phorbol 12-myristate 13-acetate significantly enhanced (by 25%) BK-induced IP3 formation. 5. The present data demonstrate that inositol phosphate breakdown in rat mesangial cells can be mediated via activation of a BK2-kinin receptor and is under negative control of protein-kinase C.

Overexpression of protein kinase C beta 1 enhances phospholipase D activity and diacylglycerol formation in phorbol ester-stimulated rat fibroblasts

We are using a Rat-6 fibroblast cell line that stably overexpresses the beta 1 isozyme of protein kinase C (PKC) to study regulation of phospholipid hydrolysis by PKC. Stimulation of control (R6-C1) or overexpressing (R6-PKC3) cells with phorbol ester results in an increase in diacylglycerol (DAG) mass with no increase in inositol phosphates, indicating that DAG is not formed by inositol phospholipid breakdown. A more dramatic DAG increase occurs in R6-PKC3 cells (4.0-fold over basal) compared to R6-C1 cells (1.5-fold over basal). To further define the source of DAG, phosphatidylcholine (PC) pools were labeled with [3H]myristic acid or with [3H]- or [32P]alkyllyso-PC and formation of labeled phosphatidylethanol, an unambiguous marker of phospholipase D activation, was monitored. Phorbol ester-stimulated phosphatidylethanol formation is 5-fold greater in the R6-PKC3 cell line. Formation of radiolabeled phosphatidic acid (PA) is also enhanced by PKC overexpression. In cells double-labeled with [3H]- and [32P]-alkyl-lysoPC, the 3H/32P ratio of PA and PC are identical 15 min after stimulation, suggesting that a phospholipase D mechanism predominates. In support of this, the PA phosphohydrolase inhibitor propranolol decreased phorbol 12-myristate 13-acetate-stimulated DAG formation by 72%. Increases in DAG and phosphatidylethanol were inhibited by the PKC inhibitors K252a and staurosporine. These results indicate that phospholipase D is regulated by the action of PKC. Enhanced phospholipase D activity may contribute to the growth abnormalities seen in PKC-overexpressing cells.

Phospholipase D in cell signalling and its relationship to phospholipase C

Phospholipases C and D are phosphodiesterases which act on phospholipid head groups. Although the presence of these enzymes in living organisms has long been known, it is only recently that their role in cell signal transduction has been appreciated. The new developments on phospholipases D (PLD) are especially noteworthy, since these enzymes catalyze a novel pathway for second messenger generation. In a variety of mammalian cell systems, several biological or chemical agents have recently been shown to stimulate PLD activity. Depending on the system, activation of PLD has been suggested to be either dependent on, or independent of, Ca2+ and protein kinase C. PLD primarily hydrolyses phosphatidylcholine (PC) but phosphatidylinositol and phosphatidylethanolamine have also been reported as substrates. Different forms of endogenous PLD may also exist in cells. Exogenous addition of PLD causes alterations in cellular functions. In many instances, Ca2+ mobilizing agonists may stimulate both PLC and PLD pathways. Interestingly, several metabolites of these two enzymes are second messengers and are common to both pathways (e.g. phosphatidic acid, diglyceride). This has raised the issue of the interrelationship between these pathways. The regulation of either PLC or PLD by cellular components, e.g. guanine nucleotide binding proteins or protein kinases, is under intense investigation. These recent advances are providing novel information on the significance of phospholipase C and D mediated phospholipid turnover in cellular signalling. This review highlights some of these new discoveries and emerging issues, as well as challenges for future research on phospholipases.

Altered phospholipase activities related to alpha 1-adrenergic receptor supersensitivity of aortas from aldosterone-salt hypertensive rats

Many of the concepts presented in this paper are summarized in Fig. 7. Some aspects are well supported while others are speculative. The operation of PLC in VSM is well established, and in some hypertensive models (AHR, SHRSP) PLC assays exhibited altered activation. Currently this pathway leading to the production of IP3 and DAG is considered to be the major regulator of Ca release from sarcoplasmic reticulum (SR) and Ca entry by channels (CaC). Regulation of PKC by [Ca]i and DAG is thought to play a major role in controlling Ca entry. PKC has also been proposed to regulate PLA2 as well as PLD in conjunction with elevated [Ca]i. An important issue to be resolved is whether receptor regulation of other lipases occurs independently of the PLC-[Ca]i-PKC axis. Currently information supporting receptor regulation is lacking for VSM, but few studies have been conducted. Our observation that NE stimulation of PLD activity occurs in VSM indicates that the control of VSM by biochemical messengers is much more complicated than previously proposed. This seemingly redundant pathway may allow VSM to use alternate substrates for producing PA and DAG than are readily available to PLC. It also allows PA to be produced directly without phosphorylation of DAG. Although the role of PA in the regulation of Ca entry was proposed earlier, definitive studies establishing this linkage are still required. Any PLD activity on PIP2 would produce biochemical messengers (PA, DAG) which could stimulate Ca entry without producing the messenger, IP3, associated with Ca release (inactive IP2 would be produced). If PLC and PLD were independently regulated by receptor-guanine nucleotide-regulatory protein (G-protein) complexes, this would offer the potential for some agonists to excite VSM by Ca release and Ca entry mechanisms while others may excite by Ca entry alone. This system would also circumvent the problem of limited substrate for cellular regulation of [Ca]i if PIP2 were the primary substrate. This limitation does not exist with other phospholipids such as phosphatidylcholine which is a preferred substrate for PLD. The presence of multiple phospholipases under separate receptor regulation allows for a wider range of tissue responses to various agonists, than a system which is linked only through the PLC-[Ca]i-PKC axis. The presence of a PLD pathway also reopens the interpretation of previous studies which demonstrated a resetting between receptor occupancy and production of second messengers by PLC.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphatidylcholine hydrolysis stimulated by phorbol myristate acetate is mediated principally by phospholipase D in endothelial cells

The mechanism of phosphatidylcholine (PC) degradation stimulated by phorbol myristate acetate (PMA) was investigated in bovine pulmonary artery endothelial cells prelabeled with [methyl-3H]choline ([3H]choline) or [9,10-3H]myristic acid ([3H]myristic acid). Both labels were selectively incorporated into PC, and addition of PMA stimulated comparable losses of 3H from PC in cells prelabeled with [3H]choline or [3H]myristate. In cells prelabeled with [3H]choline, the loss of 3H from PC correlated with a rapid increase in intracellular free [3H]choline. The increase in intracellular [3H]choline stimulated by PMA was not preceded by an increase in any other 3H-labeled PC degradation product. PMA did not stimulate the formation of PC deacylation products in cells prelabeled with [3H]choline. In permeabilized cells prelabeled with [3H]choline, PMA stimulated the formation of [3H]choline but not [3H]phosphocholine. In intact cells prelabeled with [3H]myristate, the loss of 3H from PC induced by PMA correlated with the formation of [3H]phosphatidic acid ([3H]PA) and [3H]diacylglycerol. In the presence of ethanol, PMA stimulated the formation of [3H]phosphatidylethanol ([3H]PEt) at the expense of [3H]PA. The time-course of [3H]PEt formation was similar to the time-course of intracellular [3H]choline formation in cells stimulated with PMA. These data taken together support the notion that PC degradation in endothelial cells stimulated with PMA is mediated principally by phospholipase D. PC breakdown via phospholipase D was not observed in cells treated with phorbol esters incapable of interacting with protein kinase C. Activation of phospholipase D by phorbol esters was inhibited by long-term pretreatment of cells with PMA to down-regulate protein kinase C and by pretreatment of the cells with staurosporine. These data support the notion that activation of phospholipase D by phorbol esters is dependent upon protein kinase C.