The regulation of phospholipase D activity and its role in sn-1,2-diradylglycerol formation in bombesin- and phorbol 12-myristate 13-acetate-stimulated Swiss 3T3 cells

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Intrathecal bombesin is sympathoexcitatory and pressor in rat

Bombesin, a 14 amino-acid peptide, is pressor when administered intravenously in rat and pressor and sympathoexcitatory when applied intracerebroventricularly. To determine the spinal effects of bombesin, the peptide was administered acutely in the intrathecal space at around thoracic spinal cord level six of urethane-anesthetized, paralyzed, and bilaterally vagotomized rats. Blood pressure, heart rate, splanchnic sympathetic nerve activity (sSNA), phrenic nerve activity, and end-tidal CO(2) were monitored to evaluate changes in the cardiorespiratory systems. Bombesin elicited a long-lasting excitation of sSNA associated with an increase in blood pressure and tachycardia. There was a mean increase in arterial blood pressure of 52 ± 5 mmHg (300 μM; P < 0.01). Heart rate and sSNA also increased by 40 ± 4 beats/min (P < 0.01) and 162 ± 33% (P < 0.01), respectively. Phrenic nerve amplitude (PNamp, 73 ± 8%, P < 0.01) and phrenic expiratory period (+0.16 ± 0.02 s, P < 0.05) increased following 300 μM bombesin. The gain of the sympathetic baroreflex increased from -2.8 ± 0.7 to -5.4 ± 0.9% (P < 0.01), whereas the sSNA range was increased by 99 ± 26% (P < 0.01). During hyperoxic hypercapnia (10% CO(2) in O(2), 90 s), bombesin potentiated the responses in heart rate (-25 ± 5 beats/min, P < 0.01) and sSNA (+136 ± 29%, P < 0.001) but reduced PNamp (from 58 ± 6 to 39 ± 7%, P < 0.05). Finally, ICI-216,140 (1 mM), an in vivo antagonist for the bombesin receptor 2, attenuated the effects of 300 μM bombesin on blood pressure (21 ± 7 mmHg, P < 0.01). We conclude that bombesin is sympathoexcitatory at thoracic spinal segments. The effect on phrenic nerve activity may the result of spinobulbar pathways and activation of local motoneuronal pools.

International Union of Pharmacology. LXVIII. Mammalian bombesin receptors: nomenclature, distribution, pharmacology, signaling, and functions in normal and disease states

The mammalian bombesin receptor family comprises three G protein-coupled heptahelical receptors: the neuromedin B (NMB) receptor (BB(1)), the gastrin-releasing peptide (GRP) receptor (BB(2)), and the orphan receptor bombesin receptor subtype 3 (BRS-3) (BB(3)). Each receptor is widely distributed, especially in the gastrointestinal (GI) tract and central nervous system (CNS), and the receptors have a large range of effects in both normal physiology and pathophysiological conditions. The mammalian bombesin peptides, GRP and NMB, demonstrate a broad spectrum of pharmacological/biological responses. GRP stimulates smooth muscle contraction and GI motility, release of numerous GI hormones/neurotransmitters, and secretion and/or hormone release from the pancreas, stomach, colon, and numerous endocrine organs and has potent effects on immune cells, potent growth effects on both normal tissues and tumors, potent CNS effects, including regulation of circadian rhythm, thermoregulation; anxiety/fear responses, food intake, and numerous CNS effects on the GI tract as well as the spinal transmission of chronic pruritus. NMB causes contraction of smooth muscle, has growth effects in various tissues, has CNS effects, including effects on feeding and thermoregulation, regulates thyroid-stimulating hormone release, stimulates various CNS neurons, has behavioral effects, and has effects on spinal sensory transmission. GRP, and to a lesser extent NMB, affects growth and/or differentiation of various human tumors, including colon, prostate, lung, and some gynecologic cancers. Knockout studies show that BB(3) has important effects in energy balance, glucose homeostasis, control of body weight, lung development and response to injury, tumor growth, and perhaps GI motility. This review summarizes advances in our understanding of the biology/pharmacology of these receptors, including their classification, structure, pharmacology, physiology, and role in pathophysiological conditions.

Sustained diacylglycerol accumulation resulting from prolonged G protein-coupled receptor agonist-induced phosphoinositide breakdown in hepatocytes

Studies in various cells have led to the idea that agonist-stimulated diacylglycerol (DAG) generation results from an early, transient phospholipase C (PLC)-catalyzed phosphoinositide breakdown, while a more sustained elevation of DAG originates from phosphatidylcholine (PC). We have examined this issue further, using cultured rat hepatocytes, and report here that various G protein-coupled receptor (GPCR) agonists, including vasopressin (VP), angiotensin II (Ang.II), prostaglandin F2alpha, and norepinephrine (NE), may give rise to a prolonged phosphoinositide hydrolysis. Preincubation of hepatocytes with 1-butanol to prevent conversion of phosphatidic acid (PA) did not affect the agonist-induced DAG accumulation, suggesting that phospholipase D-mediated breakdown of PC was not involved. In contrast, the GPCR agonists induced phosphoinositide turnover, assessed by accumulation of inositol phosphates, that was sustained for up to 18 h, even under conditions where PLC was partially desensitized. Pretreatment of hepatocytes with wortmannin, to inhibit synthesis of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PIP2), prevented agonist-induced inositol phosphate and DAG accumulation. Upon VP stimulation the level of PIP) declined, but only transiently, while increases in inositol 1,4,5-trisphosphate (InsP3) and DAG mass were sustained, suggesting that efficient resynthesis of PIP2 allowed sustained PLC activity. This was confirmed when cells were pretreated with wortmannin to prevent resynthesis of PIP2. Furthermore, metabolism of InsP3 was rapid, compared to that of DAG, with a more than 20-fold difference in half-life. Thus, rapid metabolism of InsP3 and efficient resynthesis of PIP2 may account for the larger amount of DAG generated and the more sustained time course, compared to InsP3. The results suggest that DAG accumulation that is sustained for many hours in response to VP, Ang.II, NE, and prostaglandin F2alpha in hepatocytes is mainly due to phosphoinositide breakdown.

The regulation of phospholipase D by inositol phospholipids and small GTPases

Phospholipase D1 and D2 (PLD1, PLD2) both have PX and PH domains in their N-terminal regions with these inositol lipid binding domains playing key roles in regulating PLD activity and localisation. The activity of PLD1 is also regulated by protein kinase C and members of the Rho and Arf families of GTPases. Each of these proteins binds to unique sites; however, there appears to be little in vitro discrimination between individual family members. In agonist-stimulated cells, however, there is specificity, with, for example in RBL-2H3 cells, antigen stimulating the activation of PLD1 by association with Arf6, Rac1 and protein kinase Calpha. PLD2 appears to be less directly regulated by GTPases and rather is primarily controlled through interaction with phosphatidylinositol 4-phosphate 5-kinase that generates the activating phosphatidylinositol 4,5-bisphosphate.

Mechanism of angiotensin II-induced phospholipase D activation in bovine adrenal glomerulosa cells

Based on previous data demonstrating activation of phospholipase D (PLD) in response to angiotensin II (AngII), we have hypothesized a role for PLD in mediating aldosterone secretion from bovine adrenal glomerulosa cells. In this study we demonstrate that a PLD-generated signal(s) is required for the AngII-elicited secretory response, since interfering with lipid second messenger formation using a primary alcohol inhibited AngII-induced aldosterone secretion, but not that elicited by incubation with a hydrophilic cholesterol analog, 22(R)-hydroxycholesterol, which bypasses signaling pathways. Three mechanisms for hormonal activation of PLD have been described in other systems: direct receptor coupling, activation through protein kinase C (PKC) and a combination of these two mechanisms. Our results indicate that the PKC activator, phorbol 12-myristic 13-acetate (PMA), is able to activate PLD, and that receptor engagement is apparently not necessary for PLD activation in response to this agent. Maximal doses of AngII and PMA produced no additive effect on PLD activation, suggesting that these two agents function through a common PKC pathway. This interpretation was confirmed by the ability of a PKC inhibitor, Gö 6976, to inhibit partially AngII-induced PLD activation. Finally, treatment with the calcium ionophores A23187 or ionomycin or the calcium channel agonist BAY K8644 had no effect on PLD activity. Likewise, inhibiting calcium influx with high-dose nitrendipine affected neither basal PLD activity nor that stimulated by AngII. Thus, our results suggest a role for PKC, independent of calcium influx, in mediating AngII-induced PLD activation in glomerulosa cells.

Functional coupling of rat metabotropic glutamate 1a receptors to phospholipase D in CHO cells: involvement of extracellular Ca2+, protein kinase C, tyrosine kinase and Rho-A

We report here that metabotropic glutamate 1a (mGlu1a) receptors, stably expressed in CHO cells, stimulate phospholipase D (PLD) activity. Several mGlu receptor agonists were found to exert this effect, with a rank order of potency of: L-quisqualate>L-glutamate>(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD]=(S)-3,5-dihydroxyphenylglycine [(S)-DHPG]. Both L-glutamate- and (1S,3R)-ACPD-stimulated PLD activity were attenuated by the selective mGlu receptor antagonist (S)-alpha-methyl-4-carboxyphenylglycine. mGlu1a receptor-stimulated PLD was inhibited either by the selective protein kinase C (PKC) inhibitor, GF109203X, or via PKC downregulation. MGlu1a receptor-PLD coupling required extracellular Ca2+ and was sensitive to La3+ and Zn2+, inhibitors of intracellular Ca2+ store-operated Ca2+ influx. mGlu1a receptor-PLD coupling was inhibited by the selective tyrosine kinase inhibitor, genistein. In addition, mGlu1a receptor-PLD coupling was also inhibited by cell transfection with the selective Rho (small GTP-binding protein) inhibitors: C3-exoenzyme and dominant negative mutant RhoA constructs. Brefeldin A, a selective ADP-ribosylation factor (ARF) inhibitor, and a dominant negative ARF6 mutant, failed to significantly influence mGlu1a receptor-stimulated PLD activity. We conclude that mGlu1a receptors activate PLD via a mechanism that is dependent on extracellular Ca2+, PKC, tyrosine kinase and RhoA but independent of ARF.

G protein-coupled receptor-mediated mitogen-activated protein kinase activation through cooperation of Galpha(q) and Galpha(i) signals

G protein-coupled receptors (GPCRs) have been shown to stimulate extracellular regulated kinases (ERKs) through a number of linear pathways that are initiated by G(q/11) or G(i) proteins. We studied signaling to the ERK cascade by receptors that simultaneously activate both G protein subfamilies. In HEK293T cells, bradykinin B(2) receptor (B(2)R)-induced stimulation of ERK2 and transcriptional activity of Elk1 are dependent on Galpha(q)-mediated protein kinase C (PKC) and on Galpha(i)-induced Ras activation, while they are independent of Gbetagamma subunits, phosphatidylinositol 3-kinase, and tyrosine kinases. Similar results were obtained with m(1) and m(3) muscarinic receptors in HEK293T cells and with the B(2)R in human and mouse fibroblasts, indicating a general mechanism in signaling toward the ERK cascade. Furthermore, the bradykinin-induced activation of ERK is strongly reduced in Galpha(q/11)-deficient fibroblasts. In addition, we found that constitutively active mutants of Galpha(q/11) or Galpha(i) proteins alone poorly stimulate ERK2, whereas a combination of both led to synergistic effects. We conclude that dually coupled GPCRs require a cooperation of Galpha(i)- and G(q/11)-mediated pathways for efficient stimulation of the ERK cascade. Cooperative signaling by multiple G proteins thus might represent a novel concept implicated in the regulation of cellular responses by GPCRs.

Tyrosine 766 in the fibroblast growth factor receptor-1 is required for FGF-stimulation of phospholipase C, phospholipase D, phospholipase A(2), phosphoinositide 3-kinase and cytoskeletal reorganisation in porcine aortic endothelial cells

Fibroblast growth factor-mediated signalling was studied in porcine aortic endothelial cells expressing either wild-type fibroblast growth factor receptor-1 or a mutant receptor (Y766F) unable to bind phospholipase C-(γ). Stimulation of cells expressing the wild-type receptor resulted in activation of phospholipases C, D and A(2) and increased phosphoinositide 3-kinase activity. Stimulation of the wild-type receptor also resulted in stress fibre formation and a cellular shape change. Cells expressing the Y766F mutant receptor failed to stimulate phospholipase C, D and A(2) as well as phosphoinositide 3-kinase. Furthermore, no stress fibre formation or shape change was observed. Both the wild-type and Y766F receptor mutant activated MAP kinase and elicited proliferative responses in the porcine aortic endothelial cells. Thus, fibroblast growth factor receptor-1 mediated activation of phospholipases C, D and A(2) and phosphoinositide 3-kinase was dependent on tyrosine 766. Furthermore, whilst tyrosine 766 was not required for a proliferative response, it was required for fibroblast growth factor receptor-1 mediated cytoskeletal reorganisation.

Characterization of the regulation of phospholipase D activity in the detergent-insoluble fraction of HL60 cells by protein kinase C and small G-proteins

Phospholipase D (PLD) activity has been shown to be GTP-dependent both in vivo and in vitro. One protein that confers GTP sensitivity to PLD activity in vitro is the low-molecular-mass G-protein ADP-ribosylation factor (Arf). However, members of the Rho family and protein kinase C (PKC) have also been reported to activate PLD in various cell systems. We have characterized the stimulation of PLD in HL60 cell membranes by these proteins. The results demonstrate that a considerable proportion of HL60 PLD activity is located in a detergent-insoluble fraction of the cell membrane that is unlikely to be a caveolae-like domain, but is probably cytoskeletal. This PLD activity required the presence of Arf1, a Rho-family member and PKC for efficient catalysis of the lipid substrate, suggesting that the activity represents PLD1. We show that recombinant human PLD1b is regulated in a similar manner to HL60-membrane PLD, and that PKCalpha and PKCdelta are equally effective PLD activators. Therefore maximum PLD activity requires Arf, a Rho-family member and PKC, emphasizing the high degree of regulation of this enzyme.

Diacylglycerol--when is it an intracellular messenger?

Distinct, structurally different forms of sn-1,2-diacylglycerol are found in cells, these are polyunsaturated, mono- or di-unsaturated and saturated. The pathways that generate or metabolise sn-1, 2-diacylglycerol are reviewed. The evidence that it is the polyunsaturated forms of sn-1,2-diacylglycerol, but the more saturated forms of phosphatidate which function as intracellular signals is considered.

Protein phosphatase inhibitors exert specific and nonspecific effects on calcium influx in thapsigargin-treated human neutrophils

C2-ceramide but not inhibitors of phosphatase types 1 and 2A (okadaic acid, calyculin A, tautomycin) blocked store-regulated Ca2+ entry induced in human neutrophils by thapsigargin. This contrasts with previous results showing that both types of compounds inhibit Ca2+ influx in fmet-leu-phe-treated cells. In present studies, phosphatase inhibitors increased the rate of secondary Ca2+ influx in a temperature-dependent manner. Their mechanism of action appeared to be independent of phosphatase inhibition since the inactive congeners, norokadaone and tetraacetyl okadaic acid, also potentiated Ca2+ influx at similar concentrations. When Ca2+ stores were predischarged by thapsigargin, okadaic acid but not norokadaone acted synergistically with fMLP to inhibit subsequent Ca2+ entry. Results suggest that blockade of Ca2+ influx in neutrophils is mediated by a phosphorylation reaction that is prolonged by phosphatase inhibitors. The requisite phosphorylation occurs in fMLP-activated cells but may be absent in cells incubated with thapsigargin.

Activation of phospholipase D signaling pathway by epidermal growth factor in osteoblastic cells

The receptor-mediated activation of phospholipase D (PLD) is a major signaling pathway in several cell systems. This study determined the effects of epidermal growth factor (EGF) on PLD activity in normal rat osteoblastic cells. Primary cultures were obtained from fetal rat calvaria by sequential collagenase digestion and seeded in BGJb media supplemented with 10% fetal calf serum. PLD activity was assayed by the transphosphatidylation reaction in [H3]myristic acid (5 microCi/ml)-labeled cells treated with EGF in the presence of 5% ethanol and measuring the production of phosphatidylethanol (PEtOH). Lipids were extracted and separated by thin-layer chromatography, detected by iodine staining, and the areas of interest were scraped off and transferred to vials for scintillation counting. EGF significantly increased PEtOH production in a dose-dependent manner and at short (10-60 s) and long (up to 30 minutes) incubation periods (p < 0.05). Phosphatidic acid levels were also significantly increased (p < 0.05) compared with unstimulated controls, but the levels were approximately 60% less than those of PEtOH. 4b-phorbol 12-myristate, 13-acetate (PMA) also produced a significant increase in PEtOH levels when compared with unstimulated control cultures, but when PMA was added together with EGF, the production of PEtOH was reduced about 30%. Pretreatment of cells with the protein kinase C (PKC) inhibitor H-7 caused a significant increase in PEtOH levels, compared with cells stimulated with EGF alone. Preincubation of cells with pertussis toxin produced a partial decrease in PEtOH levels. This study demonstrates that EGF activates the PLD signaling cascade in normal rat osteoblastic cells and that the pathway appears to involve, at least in part, a PKC- and Gi protein-dependent mechanism.

Phosphatidic acid effects on cytosolic calcium and proliferation in osteoblastic cells

Our previous studies show that epidermal growth factor (EGF) stimulates phospholipase D (PLD)-induced phosphatidic acid (PA) formation in rat calvarial osteoblastic cells. This study investigated the effects of PA on cytosolic calcium ([Ca2+]i) and proliferation, and the possible involvement of the PLD pathway in EGF effects on [Ca2+]i and proliferation in rat calvarial osteoblastic cells. PA markedly increased [Ca2+]i. This response was unaffected by thapsigargin, which depletes [Ca2+]i pools, blocked by verapamil, a calcium channel blocker, and enhanced by propanolol, an inhibitor of PA-phosphohydrolase. PA also reduced the EGF dependent-[Ca2+]i increase by 60%, while a PLD inhibitor blocked these effects. Furthermore, PA significantly increased cell proliferation (P < 0.05) which was inhibited by verapamil and enhanced by H-7 (PKC inhibitor). The PLD inhibitor significantly (P < 0.05) reduced the EGF-induced increase in proliferation. In summary, PA stimulates rat calvarial osteoblastic cell proliferation and mobilization of [Ca2+]i using extracellular pools, and EGF's mitogenic effect on these cells requires activation of PLD.

Distinct phospholipase C-regulated signalling pathways in Swiss 3T3 fibroblasts induce the rapid generation of the same polyunsaturated diacylglycerols

Prostaglandin F2alpha, platelet-derived growth factor (PDGF) and calcium ionophore A23187 stimulated the rapid (within 25 s) generation of polyunsaturated 1,2-diacylglycerol (DAG) species, in particular 18:0/20:3n-9, 18:0/20:4n-6 and 18:0/20:5n-3, in Swiss 3T3 fibroblasts. This was followed by a second sustained phase characterised by saturated, monounsaturated and diunsaturated DAG species derived, at least partially, from a phospholipase D/phosphatidate phosphohydrolase-linked pathway. This could be directly activated by phorbol ester. Assay of rat brain protein kinase C (PKC) in lipid vesicles showed that first phase, polyunsaturated-enriched DAG isolated from Swiss 3T3 cells was a more potent activator of kinase activity compared to that achieved with DAG from control or 5 min stimulated cells. Thus activation of distinct members of the phospholipase C family leads to the rapid and almost identical generation of polyunsaturated DAG species which are capable of preferentially activating protein kinase C (PKC).

Neuromedin B activates phospholipase D through both PKC-dependent and PKC-independent mechanisms

The actions of neuromedin B (NMB), a recently discovered mammalian bombesin-related peptide, are mediated by interacting with a distinct receptor; however, little is known about its cellular basis of action. Recent studies show activation of phospholipase D (PLD) is an important transduction cascade for a number of GI hormones, especially for stimulation of growth and protein sorting. The purpose of the present study was to determine whether activation of the NMB receptor causes activation of PLD and to explore whether this activation was coupled to PLC activation. Rat C6 glioblastoma cells (C6 cells), which contain a low density of native NMB receptors and BALB 3T3 cells stably transfected with rat NMB receptors, were used. NMB caused a 3-fold increase in C6 cells and an 11-fold increase in rNMB-R transfected cells in PLD activity. Increases in PLD activity were rapid and NMB was 100-fold more potent than gastrin-releasing peptide (GRP). NMB caused a half-maximal increase in [Ca2+]i at 0.2 nM, in [3H]IP and PLD at 1 nM, and half-maximal receptor occupation at 1.2 nM. TPA increased PLD dose-dependently with a half-maximal effect at 60 nM. The calcium ionophore A23187 (1 microM) alone did not increase PLD activity but potentiated the effect of TPA. The Ca2+-ATPase inhibitor, thapsigargin, did not affect NMB- or TPA-stimulated PLD activities, although it blocked completely the NMB-induced increase in [Ca2+]i. The PKC inhibitor GF109203X completely abolished TPA-induced PLD activity, however, it only inhibited NMB-induced PLD activity by 20%. The combination of thapsigargin and GF109203X had the same effect as GF109203X alone. These data indicate that NMB receptor activation is coupled to both PLC and PLD. In contrast to a number of other phospholipase C-coupled receptors, NMB receptor stimulated changes in [Ca2+]i do not contribute to PLD activation. Both PKC-dependent and PKC-independent mechanisms are involved in the NMB-stimulated PLD activation with the PKC-independent pathway predominating.

Diacylglycerol molecular species in plasma membrane and microsomes change transiently with endothelin-1 treatment of glioma cells

Agonist-induced intracellular signal transduction often involves activation of protein kinase C by diacylglycerol (DAG) released from membrane phospholipids by phospholipases. Using either DAG kinase or HPLC assays to quantitatively determine DAG mass, we observed a time-dependent increase in DAG accumulation upon incubation of rat C6 glioma cells with 200 nM endothelin-1 (ET-1). Total cell DAG rapidly increased by 25-35% from a basal level of 4.5 +/- 0.3 nmol/mg protein during one min of ET-1 treatment and remained constant or slightly decreased between 1 and 2 min. Thereafter, DAG increased to a maximum (1.6-fold above basal) by 5-10 min. and remained elevated to 30 min. Resolution of DAG molecular species by HPLC after incubation of cells with ET-1 revealed that accumulation of DAG species differed in total cell lysate and subcellular compartments. In plasma membrane, major DAG species increased at 1 min. followed by a decrease at 10 min. whereas in microsomes DAG species did not change at 1 min. and decreased at 10 min. Although phospholipid sources of DAG species were not identified specifically, there was preferential hydrolysis of molecular species of phospholipid for DAG production. We propose that molecular species of DAG produced at the plasma membrane may be transferred to the endoplasmic reticulum so that phospholipid resynthesis can replenish molecular species initially utilized in signal transduction.

Phorbol ester effects in atypical protein kinase C zeta overexpressing NIH3T3 cells: possible evidence for crosstalk between protein kinase C isoforms

To examine whether multiple protein kinase C (PKC) isoforms could interact or crosstalk, phorbol ester-insensitive atypical PKC (aPKC) zeta isoform was overexpressed in NIH3T3 cells, and the cells were stimulated with phorbol ester to activate diacylglycerol-dependent conventional (cPKC) and novel PKC (nPKC) isoforms. Treatment of cells with phorbol ester which activates PKCalpha, gamma, delta, and epsilon isoforms in NIH3T3 cells significantly reduced proliferation of cells. Overexpression of aPKCzeta and subsequent phorbol ester treatment abolished phorbol ester-induced reduction in cell proliferation. Overexpression of aPKCzeta also potentiated phorbol ester-induced mitogen-activated protein (MAP) kinase activation in a PKC-dependent manner. The effects of PKCzeta overexpression on proliferation and MAP kinase activation were proportional to the levels of aPKCzeta expression. Since aPKCzeta cannot be activated by phorbol ester, modulation of cell proliferation and MAP kinase activation by phorbol ester in aPKCzeta overexpressing cells might be due to the activation of cPKCs and/or nPKCs by phorbol ester. Thus, the results provide possible evidence for either direct or indirect crosstalk between PKC isoforms.

Overproduction of urokinase-type plasminogen activator is regulated by phospholipase D- and protein kinase C-dependent pathways in murine mammary adenocarcinoma cells

Urokinase-type plasminogen activator (uPA) initiates a proteolytic cascade with which invasive cells eliminate barriers to movement. The signaling pathways regulating uPA production in tumor cells remain unclear. We first studied the effects of n-butanol, a phospholipase D (PLD) and protein kinase C (PKC) inhibitor, on the production of uPA in murine mammary adenocarcinoma cells. Tumor cell monolayers treated during 24 h with 0.3% v/v n-butanol, secreted 45-50% less uPA to the culture medium than control monolayers (P < 0.001) as determined by radial caseinolysis, zymography and western blot. This inhibition occurred also with 5-h treatments and remained up to 5 h after the removal of the alcohol. Treatment with the phorbol ester PMA or with EGF, strongly increased uPA production (P < 0.001). Interestingly, a mild inhibition of uPA production was observed when PMA stimulation was assayed in cotreatments with n-butanol. In contrast EGF was unable to reverse the inhibition induced by n-butanol. H7 significantly inhibited uPA activity (P < 0.001) secreted to the culture media. Furthermore, phosphatidic acid significantly stimulated uPA production meanwhile propranolol, which blocks phosphatidic acid availability, reduced it, suggesting a main regulatory role for this intermediary metabolite. These results suggest for the first time that uPA production is regulated by PLD and PKC signal transduction pathways in murine mammary adenocarcinoma cells.

The selective protein kinase C inhibitor, Ro-31-8220, inhibits mitogen-activated protein kinase phosphatase-1 (MKP-1) expression, induces c-Jun expression, and activates Jun N-terminal kinase

The role of protein kinase C (PKC) in inflammation, mitogenesis, and differentiation has been deduced in part through the use of a variety of PKC inhibitors. Two widely used inhibitors are the structurally related compounds GF109203X and Ro-31-8220, both of which potently inhibit PKC activity and are believed to be highly selective. While using GF109203X and Ro-31-8220 to address the role of PKC in immediate early gene expression, we observed striking differential effects by each of these two compounds. Growth factors induce the expression of the immediate early gene products MAP kinase phosphatase-1 (MKP-1), c-Fos and c-Jun. Ro-31-8220 inhibits growth factor-stimulated expression of MKP-1 and c-Fos but strongly stimulated c-Jun expression, even in the absence of growth factors. GF109203X displays none of these properties. These data suggest that Ro-31-8220 may have other pharmacological actions in addition to PKC inhibition. Indeed, Ro-31-8220 strongly stimulates the stress-activated protein kinase, JNK1. Furthermore, Ro-31-8220 apparently activates JNK in a PKC-independent manner. Neither the down-regulation of PKC by phorbol esters nor the inhibition of PKC by GF109203X affected the ability of Ro-31-8220 to activate JNK1. These data suggest that, in addition to potently inhibiting PKC, Ro-31-8220 exhibits novel pharmacological properties which are independent of its ability to inhibit PKC.

Biochemistry and cell biology of phospholipase D in human neutrophils

Neutrophils play a major role host defense against invading microbes. Recent studies have emphasized the importance of the phospholipase D (PLD) in the signalling cascade leading to neutrophil activation. Phospholipase D catalyzes the hydrolysis of phospholipids to generate phosphatidic acid with secondarily generation of diradylglycerol; both of these products have been implicated as second messengers. Herein, we discuss the regulation and the biochemistry of the receptor-regulated PLD in human neutrophils. In vivo and in vitro studies suggest an activation mode in which initial receptor-linked activation of phospholipase C generates diacylglycerol and inositol trisphosphate. The resulting calcium flux along with the diacylglycerol activate a conventional isoform of protein kinase C (PKC), probably PKC beta 1. This PKC, in turn phosphorylates a plasma membrane component resulting in PLD activation and a second outpouring of diradylglycerol. The small GTP-binding proteins, RhoA and ARF, also participate in this process, and synergize with a 50 kDa cytosolic regulatory factor.

Stimulation of actin stress fibre formation mediated by activation of phospholipase D

BACKGROUND

Agonist-stimulated phospholipase D (PLD) catalyzes the hydrolysis of phosphatidylcholine, generating the putative messenger phosphatidate (PA). Proposed functions for PA, and hence for PLD, include kinase activation, the regulation of small molecular weight GTP-binding proteins, actin polymerization and secretion. It has not been possible to define a physiological function for PLD activation as it is generally stimulated together with other signalling pathways, such as those involving phospholipases A2 and C, phosphatidylinositide (PI) 3-kinase and the p21(ras)/mitogen-activated protein (MAP) kinase cascade.

RESULTS

We report that, in porcine aortic endothelial (PAE) cells, lysophosphatidic acid (LPA) stimulated PLD activity and rapidly generated PA in the absence of other phospholipase, PI 3-kinase or MAP kinase activities. PLD activation was controlled by a tyrosine kinase-regulated pathway. LPA also stimulated actin stress fibre formation, but was inhibited by butan-1-ol; the alcohol also reduced the accumulation of PA. The addition of PA to cells did not stimulate PLD activity, but did cause stress fibre formation in a manner that was insensitive to butan-1-ol. Stimulation of stress fibre formation by LPA and PA was sensitive to genistein, and was inhibited by micro-injection of the Rho-inhibiting C3 exotoxin into PAE cells.

CONCLUSIONS

This study provides the first clear demonstration of a physiological role for PLD activity. In PAE cells, the stimulation of actin stress fibre formation was a consequence of PA generation and, therefore, PLD activation. The results suggest that PA generation is upstream of Rho activation, and imply a role for PLD in the regulation of Rho-mediated pathways.

Muscarinic regulation of phospholipase D and its role in arachidonic acid release in rat submandibular acinar cells

The characteristics of muscarinic cholinergic-induced phospholipase D (PLD) activation, and the involvement of the enzyme in the release of arachidonic acid were examined in rat submandibular acinar cells. Carbachol produced a dose-related activation of PLD to around fivefold control values at 100 microM agonist concentration. This was associated with the appearance of free choline, phosphatidic acid and arachidonic acid, indicating that the PLD substrate was phosphatidylcholine. The response to carbachol was inhibited by 60% by U73122, a blocker of a phospholipase C (PLC) specific to phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], suggesting that the cleavage of phosphatidylcholine by PLD was, at least in part, secondary to agonist-coupled hydrolysis of PtdIns(4,5)P2 by PLC. Consistent with this, PLD was also activated to levels comparable to those induced by carbachol, by the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), and the Ca2+ mobilizer, thapsigargin, two agents that respectively mimic the activation of protein kinase C (PKC) by diacylglycerol and the elevation of cytosolic Ca2+ by inositol 1,4,5-triphosphate [Ins(1,4,5)P3] in the phosphoinositide effect. The cell-permeant Ca2+ chelator 1,2-bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester (BAPTA/AM) abolished the thapsigargin-induced activation of PLD and inhibited the responses of PLD to carbachol and TPA by 60%. The PKC inhibitor, Ro-31-8220, also inhibited the activation of PLD by carbacol and TPA to a level of approximately double control values, but had no effect on the thapsigargin-induced elevation of PLD. A role for both the PKC-associated and Ca(2+)-mobilizing arms of the PtdIns(4,5)P2-PLC pathway in PLD regulation is thus suggested. Pretreatment of cells with the phosphatidate phosphohydrolase blocker, propranolol, significantly enhanced the carbachol-induced elevation of phosphatidic acid, but decreased agonist-stimulated production of diacylglycerol and arachidonic acid, indicating that phosphatidlycholine was the likely source of arachidonic acid. We therefore propose that, in submandibular mucous acinar cells, muscarinic activation of the PtdIns(4,5)P2-PLC pathway regulates phosphatidylcholine-specific PLD through both the PKC- and Ca(2+)-mobilizing arms of the phosphoinositide response, and that diacylglycerol, derived from phosphatidylcholine via phosphatidic acid, is a source of free arachidonic acid.

Wortmannin and its structural analogue demethoxyviridin inhibit stimulated phospholipase A2 activity in Swiss 3T3 cells. Wortmannin is not a specific inhibitor of phosphatidylinositol 3-kinase

Wortmannin and its structural analogue demethoxyviridin (DMV) have been reported to be specific inhibitors of phosphatidylinositol 3-kinase activity. Here we report that these compounds are not as selective as assumed and demonstrate inhibition of bombesin-stimulated phospholipase A2 activity by both wortmannin and DMV with an IC50 (2 nM) which is slightly more potent than the inhibition of insulin-stimulated phosphatidylinositol 3,4,5-trisphosphate generation in these cells (approximately 10nM). While it has not been possible to fully block in vitro phospholipase A2 activity with wortmannin, inhibition cannot be a consequence of inhibition of PI 3-kinase activity since bombesin fails to generate 3-phosphorylated lipids in the intact cell. Therefore, while wortmannin is indeed a PI 3-kinase inhibitor, it is not as specific as previously reported, and experimental conclusions based solely on its use should be treated with caution.

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.

Activation of human endometrial phospholipase D by bradykinin

Bradykinin may act as a promoter of endometrial regeneration. In [3H]myristate-labelled endometrial stromal cells, bradykinin and tetradecanoylphorbol acetate (TPA) mediated activation of phospholipase D (PLD) as measured by the accumulation of [3H]phosphatidylbutanol ([3H]PtdBut). Kinetics of bradykinin-evoked PLD activation was rapid and transient, whereas the TPA response was relatively slow in onset. Bradykinin induced a dose-dependent (EC50 0.11 nM) [3H]PtdBut accumulation at concentrations at which it stimulated DNA synthesis. In [3H]inositol-labelled cells, bradykinin evoked a rapid increase in inositol phosphates which preceded the increase in [3H]PtdBut formation. Chronic pretreatment with 400 nM TPA abolished PLD activation to subsequent treatment with either TPA and bradykinin. Staurosporine, an inhibitor of protein kinase C, strongly inhibited (IC50 96 nM) TPA-induced [3H]PtdBut formation, but bradykinin-stimulated [3H]PtdBut accumulation was only partially inhibited (IC50 65 microM). The effect of bradykinin and TPA on PLD activity was synergistic, suggesting that the two agents may act via different mechanisms. These results suggest PKC-dependent and independent pathways are involved in bradykinin-induced PLD activation and that the mitogenic activity of this vasoactive peptide on endometrial stromal cells may in part be mediated via the PLD pathway. This may have significance both to implantation and endometrial cancer.

Increased receptor-mediated phospholipase D activation and Ca2+ mobilization in peritoneal polymorphonuclear leukocytes from streptozotocin-induced diabetic rats

Phospholipase D activation was investigated in peritoneal polymorphonuclear leukocytes from streptozotocin-induced diabetic rats. Stimulation of the cells with formyl-Met-Leu-Phe resulted in a time- and dose-dependent increase in phosphatidylethanol in the presence of ethanol, and this lipid formation in cells prepared from diabetic rats was enhanced as compared to that in the case of nondiabetic rats. Furthermore, the increase in the intracellular Ca2+ concentration was also enhanced in the stimulated cells from diabetic rats. Under the present conditions, N-acetyl-beta-glucosaminidase release and superoxide generation, which are known to be dependent on phospholipase D activation, were higher in the cells from diabetic rats than those in the cells from control rats. However, there was no difference in the dissociation constant and the number of binding sites for formyl-Met-Leu-Phe between the cells from diabetic and control rats. Phosphatidylethanol formation, N-acetyl-beta-glucosaminidase release and superoxide generation in response to ionomycin or 4 beta-phorbol 12-myristate 13-acetate were not enhanced in diabetic rat cells, as compared with those in control rat cells. These results suggest that receptor-mediated phospholipase D activation and Ca2+ mobilization are enhanced in diabetic rat polymorphonuclear leukocytes, which might be due to acceleration of receptor-mediated signaling.

Differential modulation of bombesin-stimulated phospholipase C beta and mitogen-activated protein kinase activity by [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P

Mitogenic stimulation of Swiss 3T3 fibroblasts with bombesin results in receptor-mediated activation of a complex array of effectors, including phospholipase C beta and mitogen-activated protein (MAP) kinase. Incubation of Swiss 3T3 fibroblasts with the 11-amino acid [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P peptide inhibited bombesin-stimulated cell proliferation and phospholipase C beta activation even at high bombesin concentrations. The peptide did not inhibit the activation of phospholipase C beta by a GTPase-deficient form of the Gq-like protein, G16, indicating that the peptide does not inhibit phospholipase C beta and is acting at a point upstream of the activated form of the G protein alpha subunit. The peptide inhibited MAP kinase activation at low bombesin concentrations, but unlike phospholipase C beta, this inhibition could be overcome with 30 nM bombesin. In control Swiss 3T3 cells, bombesin did not measurably activate Ras or Raf-1 above basal levels. Following incubation of the cells with the [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P peptide, 50 nM bombesin activated Raf-1 4-6-fold over basal levels. Platelet-derived growth factor-stimulated activities of PLC, Ras, Raf-1, and MAP kinase were unaltered after incubation of Swiss 3T3 cells with the [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P peptide, as was platelet-derived growth factor-stimulated growth of the Swiss 3T3 cells. Thus, the peptide behaves as an antagonist that differentially inhibited phospholipase C beta and MAP kinase signal transduction pathways. The growth arrest observed with the peptide indicates that the bombesin-stimulated activation of MAP kinase is not sufficient to support mitogenesis in Swiss 3T3 cells.

Evidence that the bradykinin-induced activation of phospholipase D and of the mitogen-activated protein kinase cascade involve different protein kinase C isoforms

The effect of alkylglycerol supplementation on protein kinase C (PKC)-mediated signaling events has been studied in fibroblasts from Zellweger patients (SF 3271 cells). Western blotting analysis established that Zellweger fibroblasts express PKC alpha, epsilon, and zeta. Incubation with bradykinin induced a rapid transient translocation of PKC alpha and a more sustained translocation of PKC epsilon to the particulate fraction; translocation of PKC zeta was unaffected. Bradykinin-induced translocation and activation of PKC alpha, but not translocation of PKC epsilon, was blocked in SF 3271 cells which had been incubated with 1-O-hexadecylglycerol (1-O-HDG; 20 micrograms/ml) for 24 h and then incubated in the absence of 1-O-HDG and serum for a further 24 h. Supplementation with 1-O-HDG increased the mass of ether-linked phospholipid. Bradykinin initiated a transient increase in cytosolic Ca2+ concentration in both control and 1-O-HDG supplemented cells, indicating that the initial receptor linked events were not affected by 1-O-HDG supplementation. Bradykinin also caused a rapid activation of phospholipase D (PLD), measured by phosphatidylbutanol accumulation, and mitogen-activated protein kinase (MAPK) determined by myelin basic protein phosphorylation of Mono Q fractions. Both events were blocked by preincubation of the cells with 12-O-tetradecanoylphorbol-13-acetate for 24 h to deplete PKC protein. 1-O-HDG supplementation prevented the bradykinin-induced activation of PLD, but had no effect on the stimulation of MAPK activity. These results establish that modulation of the ether lipid composition of membranes can alter PKC isozyme translocation and indicate that a PKC isozyme other than PKC alpha, most likely PKC epsilon, is involved in MAPK activation.

Heterologous desensitization of bombesin- and vasopressin-stimulated phospholipase D activity in Swiss 3T3 fibroblasts

Bombesin- and vasopressin-stimulated phospholipase D (PLD) activities are rapidly desensitized in 3T3 cells, in addition both agonists are subject to heterologous desensitization. Binding studies showed that homologous desensitization was partly a result of loss of cell surface receptors, whilst heterologous desensitization was independent of receptor changes. Pretreatment with either agonist reduced subsequent GTP gamma S-stimulated PLD activity by 50% whereas a pretreatment with GTP gamma S did not attenuate the response, suggesting that the G-protein or downstream effector systems were affected by receptor activation resulting in desensitization. The desensitization of receptor-stimulated PLD activation provides support for the phospholipase functioning in a key signalling pathway.

The roles of multiple pathways in regulating bombesin-stimulated phospholipase D activity in Swiss 3T3 fibroblasts

The regulation of bombesin-stimulated phospholipase D (PLD) activity in Swiss 3T3 fibroblasts was examined. Increasing protein-tyrosine phosphorylation by using pervanadate to inhibit tyrosine phosphatases was found to stimulate protein kinase C (PKC)-independent [3H]phosphatidylbutanol ([3H]PtdBut) accumulation within 5 min, which continued to increase up to 30 min. The stimulation of PLD activity in response to submaximal [bombesin] could be decreased by approx. 50% by the tyrosine kinase inhibitor genistein, whereas pretreatment with genistein and the PKC inhibitor Ro-31-8220 completely abolished the generation of [3H]PtdBut in response to a maximal concentration of bombesin. The addition of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) into permeabilized cells resulted in an increase in [3H]PtdBut, which was abolished by depletion of cellular ATP. The additional presence of 30 microM GTP[S] did not increase the stimulation of PLD activity by any [bombesin] tested, whereas it was synergistic with that stimulated in response to phorbol 12-myristate 13-acetate. These findings suggest that bombesin-stimulated PLD activity is indirectly regulated by G-proteins, possibly through a kinase intermediate. Furthermore, activation of protein tyrosine kinases is proposed to account for the PKC-independent arm of bombesin-stimulated PLD activity. No evidence was obtained for a form of PLD directly regulated by tyrosine phosphorylation.

Ultraviolet radiation stimulates a biphasic pattern of 1,2-diacylglycerol formation in cultured human melanocytes and keratinocytes by activation of phospholipases C and D

Ultraviolet radiation (UVR) induces melanin synthesis by human epidermal melanocytes, and phospholipid-derived 1,2-diacylglycerols (DAGs) have been implicated in mediating this response. In previous experiments, addition of the synthetic DAG 1-oleoyl-2-acetylglycerol to cultured pigment cells stimulated melanogenesis. The purpose of the present study was to analyse the effects of UVR on the endogenous generation of DAGs. It was found that in a number of cultured cell types, including human melanocytes and B16 mouse melanoma cells, but also human keratinocytes and Swiss 3T3 fibroblasts, exposure to a single dose of UVR stimulated a biphasic increase in endogenous DAG formation. An early transient rise, over seconds, was followed by a more sustained delayed rise over minutes. The early rise in DAG levels was accompanied by a transient rise in inositol trisphosphate formation, indicating activation of phosphatidylinositol-specific phospholipase C. The delayed rise was accompanied by activation of phospholipase D. This endogenous DAG formation by pigment cells is further evidence for the involvement of DAGs in UVR-induced epidermal melanin synthesis. Since DAG formation is also seen in other cells types, it is possible that DAGs may be involved in an array of UVR-induced responses.

Localisation of bradykinin-like immunoreactivity and modulation of bradykinin-evoked phospholipase D activity by 17 beta-oestradiol in human endometrium

Bradykinin may act as a promoter of endometrial regeneration. Bradykinin-like immunoreactivity was detected immunocytochemically in the glandular epithelium and stroma of human endometrium. The staining was localized around the stroma and especially in the cells undergoing mitosis. Relatively weak staining was seen in the stromal cells of secretory endometrium, which was predominantly localised around the basal vacuoles of endometrial glands. During the late secretory phase, the intensity of staining was diminished throughout the endometrium: the glandular epithelium showed weak staining and stroma appeared negative. As phosphatidate, the product of phospholipase D pathway, may mediate cell proliferation, the effect of 17 beta-oestradiol on bradykinin-evoked phospholipase D activity assayed as accumulation of [3H]phosphatidylbutanol ([3H]PtdBut) was examined in [3H]myristic acid-labelled primary cultures of human endometrial stromal cells. Bradykinin induced a rapid accumulation of [3H]PtdBut in a time-dependent manner, indicating phospholipase D activation. Pretreatment of stromal cells with 17 beta-oestradiol enhanced the bradykinin-evoked phospholipase D activity. These results suggest that bradykinin-like immunoreactivity is strongly associated with proliferative stromal cells undergoing mitosis, a process that may be mediated by phospholipase D activation as the magnitude of this enzyme's activation in vitro appears to be regulated by 17 beta-oestradiol.

The mitogenic action of recombinant basic FGF in Swiss 3T3 cells is independent of early diradylglycerol production and downregulatable protein kinase C activity

In this study we have investigated the requirement for phosphoinositide metabolism, diradylglycerol (DG) production and protein kinase C (PKC) activation in recombinant basic fibroblast growth factor (rbFGF)-mediated reinitiation of DNA synthesis in Swiss 3T3 cells. We have assessed the involvement of PKC activation in rbFGF-induced DNA synthesis by two approaches; enzymic inhibition by H7 and down-regulation by prolonged phorbol-ester treatment. In both conditions we observed that rbFGF was able to sustain a significant component of its mitogenic response, therefore denying an exclusive role for the activation of downregulatable and H7-sensitive PKC isoforms in rbFGF-induced reinitiation of DNA synthesis. Moreover, we have found no evidence for diacylglycerol accumulation in response to rbFGF by 3T3 cells. In previous studies, we observed that rbFGF caused a moderate and slow accumulation of total inositol phosphates. This effect was significant only after a 60 min incubation. It is our contention that rbFGF, in our culture system, does not exert a direct effect on phosphoinositide metabolism.

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)

ras-p21 activates phospholipase D and A2, but not phospholipase C or PKC, in Xenopus laevis oocytes

Xenopus laevis oocytes are a powerful tool for the characterization of signal transduction pathways leading to the induction of DNA synthesis. Since activation of PLA2, PLC, or PLD has been postulated as a mediator of ras function, we have used the oocyte system to study the putative functional relationship between ras-p21 and these phospholipases. A rapid generation of PA and DAG was observed after ras-p21 microinjection, suggesting the activation of both PLC and PLD enzymes. However, production of DAG was sensitive to inhibition of the PA-hydrolase by propranolol, indicating that PLD is the enzyme responsible for the generation of both PA and DAG. Microinjection of PLD or ras-p21 induced the late production of lysophosphatidylcholine on a p42MAPK-dependent manner, an indication of the activation of a PLA2. Inhibition of this enzyme by quinacrine does not inhibit PLD- or ras-induced GVBD, suggesting that PLA2 activation is not needed for ras or PLD function. Contrary to 3T3 fibroblasts, where ras-p21 is functionally dependent for its mitogenic activity on TPA- and staurosporine-sensitive PKC isoforms, in Xenopus oocytes, induction of GVBD by ras-p21 was independent of PKC, while PLC-induced GVBD was sensitive to PKC inhibition. Thus, our results demonstrate the activation of PLD and PLA2 by ras-p21 proteins, while no effect on PLC was observed.

Epidermal growth factor stimulates distinct diradylglycerol species generation in Swiss 3T3 fibroblasts: evidence for a potential phosphatidylcholine-specific phospholipase C-catalysed pathway

Stimulation of 3T3 fibroblasts with epidermal growth factor (EGF) results in an increase in 1,2-diacylglycerol (DAG) mass which is maximal at 25 s, declining at 1 min and returning to basal levels by 30 min. No changes in alkylacylglycerol or alkenylacylglycerol were detected. Three species account for most of this mass increase: 18:0/20:5,n-3, 18:0/20:4,n-6 and 18:0/20:3,n-9. These species are characteristic of the phosphoinositides; however, previous work failed to detect any EGF-stimulated rise in inositol phosphates in these cells [Cook and Wakelam (1992) Biochem. J. 285, 247-253]. This ruled out phosphoinositide hydrolysis by phospholipase C, but raised the possibility of phospholipase D/phosphatidate phosphohydrolase-catalysed hydrolysis of phosphatidylinositol. The inclusion of butanol in the incubation medium failed to block the diacylglycerol changes, indicating that the phospholipase D pathway is not involved and that DAG must be derived from another source, probably via phospholipase C-catalysed hydrolysis of a phosphatidylcholine pool that is particularly rich in these species. The tyrosine kinase inhibitor ST-271 almost abolished the elevation in 18:0/20:5,n-3, 18:0/20:4, n-6 and 18:0/20:3,n-9 at 25 s, but only reduced the rise in total DAG mass by about 50%. The protein kinase C (PKC) inhibitor Ro-31-8220 increased DAG levels at all time points but had no effect on the species profiles. This provides additional evidence for PKC-mediated regulation of cell-surface EGF receptors, since the inhibition of PKC would increase the availability and/or ligand binding affinity of receptors at the plasma membrane and hence increase and prolong the response to EGF.

Prostaglandin F2 alpha-stimulated phospholipase D activation in osteoblast-like MC3T3-E1 cells: involvement in sustained 1,2-diacylglycerol production

In [3H]myristic acid-labelled osteoblast-like MC3T3-E1 cells, prostaglandin F2 alpha (PGF2 alpha)-induced PLD activity was assessed by measuring the [3H]phosphatidylethanol (PEt) formation in the presence of ethanol. Inhibition of the increase in intracellular Ca2+ concentration ([Ca2+]i) by U73122, an inhibitor of phosphoinositide-specific phospholipase C (PI-PLC), or chelation of extracellular Ca2+ with EGTA or of intracellular Ca2+ with BAPTA, suppressed PGF2 alpha-induced phospholipase D (PLD) activation. Neither protein kinase C (PKC) inhibitors nor PKC down-regulation with phorbol 12-myristate 13-acetate affected PGF2 alpha-induced [3H]PEt formation. In permeabilized cells, guanosine 5'-[gamma-thio]triphosphate enhanced PGF2 alpha 's potency in [3H]PEt formation in the presence of Ca2+. The pretreatment of intact cells with pertussis toxin failed to inhibit PGF2 alpha-induced [3H]PEt formation. PGF2 alpha caused a biphasic production of [3H]1,2-diacylglycerol ([3H]1,2-DAG) in [3H]glycerol-labelled cells. The initial transient phase was decreased by U73122, whereas the late sustained phase was decreased by ethanol and the phosphatidic acid phosphohydrolase inhibitor, propranolol. From these results, it was suggested that PGF2 alpha-induced PLD activation was mediated by the dual control of the [Ca2+]i increase due to PI-PLC activation and activation of pertussis-toxin-insensitive G-protein, but not mediated by PKC, and also that PLD activation was involved in the late sustained 1,2-DAG generation in MC3T3-E1 cells.

Rapid desensitization and resensitization of bombesin-stimulated phospholipase D activity in Swiss 3T3 cells

The kinetics of bombesin-stimulated phospholipase D (PLD) activity were examined in Swiss 3T3 fibroblasts. The stimulated activity was found to rapidly desensitize, being completely absent after 40 s. This activity then quickly, but incompletely, resensitized, with PLD being detectable after a 4.5 min wash of the desensitized cells and 75-80% of the activity being recovered after 10 min. The desensitization was dose-dependent; however, the half-maximal stimulatory concentration of bombesin was an order of magnitude lower than that required for bombesin-stimulated second messenger generation and the KD for bombesin receptor binding. This suggested that desensitization was stimulated by a 'downstream' effect, but experiments have ruled out changes in protein kinase C activity and Ca2+ concentration. Binding experiments suggested that part of the desensitization is due to receptor internalization, and the requirement for an extracellular agonist for resensitization implies that receptor recycling plays a role. Over an extended time course, cycles of desensitization and resensitization of bombesin-stimulated PLD activity were apparent which may be relevant to mitogenic signalling. These studies add further evidence for a second messenger pathway of PLD activation, and the disparity between the kinetics of diacylglycerol generation and PLD activation supports the possibility that phosphatidic acid may have a messenger role in stimulated cells.

Colocalization of acidic and basic fibroblast growth factor (FGF) in human placenta and the cellular effects of bFGF in trophoblast cell line JEG-3

The placenta undergoes extensive angiogenesis and cellular proliferation to establish adequate blood supply to the fetus. The aim of this study was to compare and contrast the immunolocalization of acidic and basic fibroblast growth factor (FGF) in both first trimester and term placenta and gestational decidua. Human choriocarcinoma cell line JEG-3 were employed as a model of cytotrophoblast and the effect of basic FGF on cell proliferation and phospholipase C and D activation investigated. Basic FGF-immunoreactivity (IR) was detected in or around cytotrophoblast cells and in extravillous trophoblast in first trimester placenta by immunohistochemistry using primary polyclonal rabbit antibodies. Identical staining patterns were produced by acidic FGF antibodies indicating colocalization of acidic FGF and basic FGF. At term, weaker and more diffuse staining was seen in the syncytiotrophoblast surrounding the placenta villi and strong staining was present in the smooth muscle cells of mid and large size placental vessels and in some endothelial cells. Endothelial cells and extravillous trophoblast stained strongly within the decidua at first trimester, whereas the glandular epithelium was weakly stained. Basic FGF induced [3H]thymidine incorporation in JEG-3 cells in a dose dependent manner and caused an increase in inosital phosphate accumulation in cells pre-labelled with myo-[3H]inosital at similar concentrations, suggesting a role of phospholipase C in JEG-3 cell proliferation. However, basic FGF failed to stimulate phospholipase D activity in cells pre-labelled with [3H]myristic acid. The detection of acid FGF and basic FGF on both maternal and fetal side of the placenta during early pregnancy suggests a role for FGF in angiogenesis, whereas localisation of the growth factor at term, when extensive angiogenesis has diminished, would indicate that FGF may be associated with more differentiated functions of the trophoblast. The nuclear localization of basic FGF in dividing but not non-dividing placental cells together with the effect of basic FGF on JEF-3 cells, strongly supports a role for basic FGF in cytotrophoblast proliferation in vivo.

Two bombesin analogues discriminate between neuromedin B- and bombesin-induced calcium flux in a lung cancer cell line

We examined the profile of two bombesin (BN) antagonists, (CH3)2CHCO-His-Trp-Ala-Val-D-Ala-His-Leu-NHCH3] (ICI 216140) and [D-Phe6,des-Met14]BN(6-14)ethylamide (DPDM-BN EA), against neuromedin B-induced Ca2+ mobilization in the small cell lung cancer (SCLC) line NCI-H345. Neuromedin B (NMB), a BN-like peptide sharing sequence homology with ranatensin, elicited a concentration-dependent Ca2+ release (in part) from intracellular stores. Sequential addition of NMB attenuated Ca2+ mobilization. Desensitization occurred between BN and NMB; depletion of intracellular Ca2+ is a likely mechanism because thapsigargin stimulated Ca2+ release after a maximally desensitizing dose of NMB. ICI 216140 and DPDM-BN EA competitively inhibited BN-induced Ca2+ transients. In contrast, these compounds antagonized NMB-stimulated Ca2+ transients in a noncompetitive manner. The pharmacological profiles obtained support receptor heterogeneity for BN-like peptides on this SCLC line, underscoring the need for thorough examination of dose-response relationships when investigating effects of BN analogues on intact cells.

Phospholipase-induced maturation of Xenopus laevis oocytes: mitogenic activity of generated metabolites

Signal transduction induced by generation of second messengers from membrane phospholipids is considered a major regulatory mechanism in control of cell proliferation. We report here that in the Xenopus laevis oocytes model, microinjection of the three most relevant types of phospholipases acting on membrane phospholipids (A2, C, and D) are capable of inducing oocyte maturation with similar efficiencies. This effect is mediated by the generation of known second messengers such as lyso-phospholipids, arachidonic acid, diacylglycerol, and phosphatidic acid. Specific inhibitors of protein kinase C made it possible to identify alternative independent signalling pathways for induction of oocyte maturation. Our results indicate that while phospholipase C seems to be dependent on protein kinase C (PKC), phospholipase A2, and phospholipase D are completely independent of protein kinase C function. Thus, the oocyte system is a powerful tool for the analysis of the potential mitogenic activity of lipid metabolites. It is also an excellent tool for unravelling the different routes involved in the regulation of cell growth.

Distinct mechanisms of phospholipase D activation and attenuation utilized by different mitogens in NIH-3T3 fibroblasts

The activation of phospholipase D (PLD) by platelet-derived growth factor (PDGF), prostaglandin F2 alpha and 12-O-tetradecanoylphorbol 13-acetate (TPA) was studied in NIH-3T3 fibroblasts. PLD activation was determined by measuring the production of both [3H]phosphatidic acid and [3H]phosphatidylpropanol (products of the PLD-catalyzed hydrolysis and transphosphatidylation reactions, respectively), in cells that were metabolically pre-labeled with [3H]oleic acid. All mitogens caused a rapid (within 2 min) activation of PLD. Activation of PLD by prostaglandin F2 alpha and PDGF was transient and declined to near basal levels by 15 min and 55 min, respectively. In contrast, TPA-induced activation of PLD was sustained for at least 60 min of incubation. A combination of maximally effective concentrations of PDGF and TPA stimulated PLD activity in a non-additive manner, while the effect of prostaglandin F2 alpha was additional to that of either PDGF or TPA. The protein kinase inhibitor staurosporine inhibited PLD activation by PDGF or TPA with almost identical dose/response curves. In contrast, staurosporine potentiated prostaglandin-F2 alpha-induced PLD activation. The specific protein kinase C inhibitor GF109203X (a bisindolylmaleimide) inhibited PLD activation by prostaglandin F2 alpha and PDGF at concentrations higher than those required for inhibition of PLD activation induced by TPA. Depletion of cellular protein kinase C abolished PLD activation by all three mitogens without affecting in vitro activity of membrane-bound PLD. The distinct kinetics of PLD activation and its differential susceptibility to protein kinase inhibitors suggest the existence of agonist-specific activation and/or inactivation mechanisms. The results indicate also that protein kinase C participates in the mechanism of PLD activation via PDGF, while the effect of prostaglandin F2 alpha involves a pathway independent of protein kinase C.

Insulin and platelet-derived growth factor acutely stimulate glucose transport in 3T3-L1 fibroblasts independently of protein kinase C

Insulin and platelet-derived growth factor (PDGF) are mitogenic for murine 3T3-L1 fibroblasts. Both these mitogens acutely stimulate glucose transport by 2-4-fold in these cells, evident within minutes of agonist exposure. The tumour promoter and protein kinase C activator, phorbol 12-myristate 13-acetate (PMA) also stimulates glucose transport by 2-3-fold over a similar time frame, suggesting that protein kinase C may be involved in the mitogenic action of insulin and PDGF in this cell line. In an attempt to address this, we have measured intracellular sn-1,2-diacylglycerol (DAG) levels in response to insulin, PDGF and PMA. We show that PDGF and PMA induce a rapid elevation in intracellular diacylglycerol levels, but insulin was without effect. In addition, we have shown that PMA and PDGF, but not insulin, stimulate protein kinase C activity. However, depletion of protein kinase C by overnight exposure to PMA, abolished PMA-stimulated glucose transport but had no effect on insulin- and PDGF-stimulated glucose transport, suggesting that the stimulation of glucose transport by these mitogens does not involve protein kinase C. The use of the selective protein kinase C inhibitor, Roche 31-8220, which inhibited PMA-stimulated glucose transport, but was without effect on insulin- and PDGF-stimulated glucose transport further supports this conclusion. Taken together, these data argue against a role for protein kinase C in the stimulation of glucose transport in 3T3-L1 fibroblasts caused by acute exposure to insulin or PDGF.

Evidence for a nucleotide receptor on adrenal medullary endothelial cells linked to phospholipase C and phospholipase D

1. We have investigated whether the 'atypical' P2-purinoceptor previously described on adrenal microvasculature endothelial cells is a nucleotide receptor (responds to pyrimidines and purines) and is linked to phospholipase D as well as phospholipase C. 2. Cultured bovine adrenal medullary endothelial (BAME) cells responded to the pyrimidine UTP, as well as the purines. The total [3H]-inositol phosphate responses were with a rank order of UTP > ATP- = adenosine 5'-O-(3-thio-triphosphate) (ATP gamma S) >> 2MeSATP. The selective P2x agonist beta, gamma-methylene ATP was inactive. 3. Construction of dose-response curves to ATP, ATP gamma S and UTP in the presence and absence of additional agonists showed that responses to ATP gamma S and UTP were not additive, nor were those to UTP and ATP. This suggests that purines and pyrmidines acted via a common nucleotide receptor. 4. 32P-labelled BAME cells, in the presence of butanol, produced [32P]-phosphatidylbutanol (PBut) when stimulated with ATP gamma S or the protein kinase C activator, tetradecanoyl phorbol acetate (TPA). 5. Cells labelled with [3H]-palmitate and stimulated in the presence of butanol generated [3H]-PBut with the same order of agonist potencies seen for inositol phosphate responses. 6. The protein kinase C inhibitor, Ro 31-8220, abolished TPA and agonist stimulation of [3H]-PBut production. 7. These observations, and our related studies on bovine aortic endothelial cells, provide the first demonstration of a phospholipase C linked nucleotide receptor on vascular endothelial cells. It is concluded that BAME cells express a nucleotide receptor linked to phospholipase C and phospholipase D, but that activation of phospholipase D is probably down-stream of phospholipase C.

Bombesin stimulates distinct time-dependent changes in the sn-1,2-diradylglycerol molecular species profile from Swiss 3T3 fibroblasts as analysed by 3,5-dinitrobenzoyl derivatization and h.p.l.c. separation

We have developed procedures for the analysis of endogenous diradylglycerol (DRG) molecular species using derivatization with 3,5-dinitrobenzoyl chloride. The introduction of this strong chromatophore enabled us to separate less than 1 nmol of DRG into its three classes (diacylglycerol, alkylacylglycerol and alkenylacylglycerol) using a combination of h.p.l.c. and t.l.c. followed by reversed-phase h.p.l.c. to resolve these classes into their component molecular species. When applied to Swiss 3T3 mouse fibroblasts stimulated with bombesin for 25 s, 5 min or 30 min, subtle time-dependent changes in the DRG patterns were observed, with only certain polyunsaturated 1,2-diacyglycerol species [18:0/20:3(n-9), 18:0/20:4(n-6), 18:0/20:4(n-3), 18:0/20:5(n-3), 18:1(n-9)/20:3(n-9), 18:1(n-9)/20:4(n-6), 16:0/22:6(n-3), 18:0/20:3(n-6) and 16:0/20:5(n-3)] showing significant agonist-stimulated increases. The amounts of the first six species were all raised at 25 s, whereas all except the latter two were elevated at 5 min. By 30 min these last species were also increased but 18:0/20:3(n-9) had returned to basal levels. Overall DRG levels, as measured by total molecular-species peak area, remained effectively constant. No changes in the amount or species profile of 1-alkyl-2-acylglycerol were observed. Comparison of these species with the acyl-chain structure of phospholipids supports the idea that inositol lipids could be the source of DRG at early stimulation times, but phosphatidylcholine appears to be a phospholipase substrate at all times. These results indicate sequential activation of several phospholipases with different substrate specificities and/or access to different phospholipid pools. They also suggest that only polyunsaturated DRGs act as second messengers and that changes in the relative amounts of these species may trigger activation of different proteins and/or isoforms (e.g. the different isoforms of protein kinase C).

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 lithium-sensitive inositol 4,5-bisphosphate accumulation in muscarinic cholinoceptor-stimulated cerebral-cortex slices

Stimulation of [3H]inositol-prelabelled rat cerebral-cortex slices with carbachol results in the accumulation of four [3H]inositol bisphosphate isomeric species, Ins(1,3)P2, Ins(1,4)P2, Ins(3,4)P2 and Ins(4,5)P2. Although the last isomer ran as a minor peak on h.p.l.c., its accumulation was dramatically enhanced in the presence of Li+ (1 mM), such that at 30 min it represented almost 35% of the total bisphosphate fraction. The accumulation of Ins(4,5)P2 appeared to be very sensitive to Li+ (EC50 = 94 +/- 3 microM), strongly implicating a Li(+)-sensitive metabolism. Evidence for this is provided from the rapid but Li(+)-sensitive decay of Ins(4,5)P2 when muscarinic-receptor stimulation is antagonized by atropine at a time when accumulations have reached a new steady state. Manipulation of phospholipase D by activators and inhibitors of protein kinase C did not suggest a role for phospholipase D hydrolysis of PtdInsP2 in the formation of Ins(4,5)P2. Attempts to reveal Ins(4,5)P2 metabolism, or indeed its synthesis from Ins(1,4,5)P3, were not successful with broken cell preparations and strongly suggest discrete compartmentation of inositol phosphate metabolism in the intact cell.