Phosphatidic acid modulates DNA synthesis, phospholipase C, and platelet-derived growth factor mRNAs in cultured mesangial cells. Role of protein kinase C

Design and Development of Novel Urea, Sulfonyltriurea, and Sulfonamide Derivatives as Potential Inhibitors of phingosine Kinase 1

Sphingosine kinase 1 (SphK1) is one of the well-studied drug targets for cancer and inflammatory diseases. Recently discovered small-molecule inhibitors of SphK1 have been recommended in cancer therapeutics; however, selectivity and potency of first-generation inhibitors are great challenge. In search of effective SphK1 inhibitors, a set of small molecules have been designed and synthesized bearing urea, sulfonylurea, sulfonamide, and sulfonyltriurea groups. The binding affinity of these inhibitors was measured by fluorescence-binding assay and isothermal titration calorimetry. Compounds 1, 5, 6, and 7 showed an admirable binding affinity to the SphK1 in the sub-micromolar range and significantly inhibited SphK1 activity with admirable IC50 values. Molecular docking studies revealed that these compounds fit well into the sphingosine binding pocket of SphK1 and formed significant number of hydrogen bonds and van der Waals interactions. These molecules may be exploited as potent and selective inhibitors of SphK1 that could be implicated in cancer therapeutics after the required in vivo validation.

Phospholipase C and D regulation of Src, calcium release and membrane fusion during Xenopus laevis development

This review emphasizes how lipids regulate membrane fusion and the proteins involved in three developmental stages: oocyte maturation to the fertilizable egg, fertilization and during first cleavage. Decades of work show that phosphatidic acid (PA) releases intracellular calcium, and recent work shows that the lipid can activate Src tyrosine kinase or phospholipase C during Xenopus fertilization. Numerous reports are summarized to show three levels of increase in lipid second messengers inositol 1,4,5-trisphosphate and sn 1,2-diacylglycerol (DAG) during the three different developmental stages. In addition, possible roles for PA, ceramide, lysophosphatidylcholine, plasmalogens, phosphatidylinositol 4-phosphate, phosphatidylinositol 5-phosphate, phosphatidylinositol 4,5-bisphosphate, membrane microdomains (rafts) and phosphatidylinositol 3,4,5-trisphosphate in regulation of membrane fusion (acrosome reaction, sperm-egg fusion, cortical granule exocytosis), inositol 1,4,5-trisphosphate receptors, and calcium release are discussed. The role of six lipases involved in generating putative lipid second messengers during fertilization is also discussed: phospholipase D, autotaxin, lipin1, sphingomyelinase, phospholipase C, and phospholipase A2. More specifically, proteins involved in developmental events and their regulation through lipid binding to SH3, SH4, PH, PX, or C2 protein domains is emphasized. New models are presented for PA activation of Src (through SH3, SH4 and a unique domain), that this may be why the SH2 domain of PLCγ is not required for Xenopus fertilization, PA activation of phospholipase C, a role for PA during the calcium wave after fertilization, and that calcium/calmodulin may be responsible for the loss of Src from rafts after fertilization. Also discussed is that the large DAG increase during fertilization derives from phospholipase D production of PA and lipin dephosphorylation to DAG.

Lipid metabolism, apoptosis and cancer therapy

Lipid metabolism is regulated by multiple signaling pathways, and generates a variety of bioactive lipid molecules. These bioactive lipid molecules known as signaling molecules, such as fatty acid, eicosanoids, diacylglycerol, phosphatidic acid, lysophophatidic acid, ceramide, sphingosine, sphingosine-1-phosphate, phosphatidylinositol-3 phosphate, and cholesterol, are involved in the activation or regulation of different signaling pathways. Lipid metabolism participates in the regulation of many cellular processes such as cell growth, proliferation, differentiation, survival, apoptosis, inflammation, motility, membrane homeostasis, chemotherapy response, and drug resistance. Bioactive lipid molecules promote apoptosis via the intrinsic pathway by modulating mitochondrial membrane permeability and activating different enzymes including caspases. In this review, we discuss recent data in the fields of lipid metabolism, lipid-mediated apoptosis, and cancer therapy. In conclusion, understanding the underlying molecular mechanism of lipid metabolism and the function of different lipid molecules could provide the basis for cancer cell death rationale, discover novel and potential targets, and develop new anticancer drugs for cancer therapy.

STAT3 is involved in phosphatidic acid-induced Bcl-2 expression in HeLa cells

Phosphatidic acid (PA), the product of a PLD-mediated reaction, is a lipid second messenger that participates in various intracellular signaling events and is known to regulate a growing list of signaling proteins. We found that Bcl-2 was upregulated by PA treatment in HeLa cells. However, how PA upregulates Bcl-2 expression has not yet been studied. In this study, we tried to discover the mechanisms of Bcl-2 up-regulation by PA treatment in HeLa cells. Treatment with PA resulted in significantly increased expression of Bcl-2 in HeLa cells. Moreover, PA-induced Bcl-2 expression was blocked by mepacrine, an inhibitor of PLA2, but not by propranolol, an inhibitor of PA phospholyhydrolase (PAP). Treatment of 1,2-dipalmitoryl-sn-glycero-3- phosphate (DPPA) also increased Bcl-2 expression. These results indicate that Bcl-2 expression is mediated by lysophosphatidic acid (LPA), not by arachidonic acid (AA). Thereafter, we used MEK1/2 inhibitor, PD98059 to investigate the relationship between ERK1/2 MAPK and PA-induced Bcl-2 expression. PA-induced Bcl-2 expression was decreased when ERK1/2 was inhibited by PD98059. The transcription factor such as STAT3 which is controlled by ERK1/2 MAPK was increased along with Bcl-2 expression when the cells were treated with PA. Furthermore, STAT3 siRNA treatments inhibited PA-induced Bcl-2 expression, suggesting that STAT3 (Ser727) is involved in PA-induced Bcl-2 expression. Taken together, these findings indicate that PA acts as an important mediator for increasing Bcl-2 expression through STAT3 (Ser727) activation via the ERK1/2 MAPK pathway.

Dioleoyl phosphatidic acid induces morphological changes through an endogenous LPA receptor in C6 glioma cells

Previously, we suggested that dioleoyl phosphatidic acid (PA) and lysophosphatidic acid (LPA) increased [Ca(2+)](i) through endogenous LPA receptors coupled to pertussis toxin-sensitive G proteins in rat C6 glioma cells. In the present report, we investigated morphological changes and cytotoxicity induced by PA and LPA in C6 glioma cells. Isoproterenol treatment led to changes in the cell morphology of rat C6 glioma cells, which were reverted by the addition of PA and LPA. PA-and LPA-induced morphological reversions were inhibited by treatment with Ki16425, an LPA(1)/LPA(3) receptor antagonist. VPC32183, another LPA(1)/LPA(3) receptor antagonist with a different structure, only inhibited PA-induced morphological reversion but not LPA-induced reversion. However, the reversions were not inhibited by treatment with pertussis toxin, a specific inhibitor of G(i/o) proteins. In addition, cytotoxicity was only induced by LPA but not by PA in C6 glioma cells. Our results suggest that PA may act as a partial agonist at endogenous LPA receptors, which are sensitive to Ki16425 and coupled to PTX-insensitive G proteins, to evoke morphological changes in C6 glioma cells.

Dioleoyl phosphatidic acid increases intracellular Ca2+ through endogenous LPA receptors in C6 glioma and L2071 fibroblasts

Phosphatidic acid (PA) increased intracellular Ca(2+) concentration ([Ca(2+)](i)) in C6 rat glioma and L2071 mouse fibroblast cells. Dioleoyl PA (PA, 18:1) was the most efficacious, followed by dipalmitoyl PA (16:0 PA) and dimyristoyl PA (14:0 PA). Lysophosphatidic acid (LPA) also increased the [Ca(2+)](i) in the both cells. PA desensitized LPA-induced Ca(2+) response completely in C6 cells, but partly in L2071 cells. Treatment of pertussis toxin (PTX), a specific inhibitor of G(i/o)-type G proteins, completely ameliorated LPA- and PA-induced Ca(2+) response in C6 cells. However, in L2071 cells, PTX inhibited PA-induced Ca(2+) increase by 80% and LPA-induced one by 20%. Ki16425, a specific inhibitor of LPA(1)/LPA(3) receptors, completely inhibited both LPA- and PA-induced Ca(2+) responses in C6 cells. On the other hand, in L2071 cells, Ki16425 completely inhibited PA-induced Ca(2+) response, but partly LPA-induced one. VPC32183, another specific inhibitor of LPA(1)/LPA(3) receptors, completely inhibited LPA- and PA-induced Ca(2+) responses in both C6 and L2071 cells. Therefore, PA and LPA appear to increase [Ca(2+)](i) through Ki16425/VPC32183-sensitive LPA receptor coupled to PTX-sensitive G proteins in C6 cells. In L2071 cells, however, LPA increases [Ca(2+)](i) through Ki16425-insensitive LPA receptor coupled to PTX-insensitive G proteins and Ki16425-sensitive LPA receptor coupled to PTX-sensitive G protein, whereas PA utilized only the latter pathway. Our results suggest that PA acts as a partial agonist on endogenous LPA receptors, which are sensitive to Ki16425 and coupled to PTX-sensitive G protein, but not on LPA receptors, which are not sensitive to Ki16425 and coupled to PTX-insensitive G protein.

Gonadotropin-releasing hormone-induced activation of diacylglycerol kinase-zeta and its association with active c-src

Gonadotropin-releasing hormone (GnRH)-induced receptor activation has been demonstrated to entrain a wide variety of signaling modalities. Most signaling pathways are concerned with the control of serine, threonine, or tyrosine-protein kinases, however, in the current article we demonstrate that in both a model cell line and in gonadotropes, GnRH additionally mediates the activation of lipid-directed kinases. We have shown that there is a functional connection between protein-tyrosine kinase modulation and lipid kinase activation. In HEK293 cells stably expressing the Type I mammalian GnRH receptor, we employed a proteomic approach to identify novel protein binding partners for GnRH-activated c-Src. Using matrix-assisted laser desorption ionization time-of-flight mass spectrometry we identified a GnRH-induced association between c-Src and the lipid kinase, diacylglycerol kinase-zeta (DGK-zeta). Using reciprocal co-immunoprecipitation we show that there is a significant elevation of the association between catalytically active c-Src with DGK-zeta in both HEK293 cells and murine gonadotrope LbetaT2 cells. Employing lipid kinase assays we have shown that the catalytic activity of DGK-zeta is significantly heightened in both HEK293 and LbetaT2 cells by GnRH. In addition, we demonstrate that the activation of DGK-zeta exerts a functional role in the murine gonadotrope LbetaT2 cell line. Elevated expression of DGK-zeta resulted in a shortening of the time scale of ERK activation in these cells suggesting a potential role of endogenous DGK-zeta in controlling the induction of LHbeta transcription by ERK1/2.

Atrial natriuretic factor inhibits mitogen-induced growth in aortic smooth muscle cells

Atrial natriuretic factor (ANF) is a polypeptide able to affect cardiovascular homeostasis exhibiting diuretic, natriuretic, and vasorelaxant activities. ANF shows antimitogenic effects in different cell types acting through R(2) receptor. Excessive proliferation of smooth muscle cells is a common phenomenon in diseases such as atherosclerosis, but the role of growth factors in the mechanism which modulate this process has yet to be clarified. The potential antimitogenic role of ANF on the cell growth induced by growth factors appears very intriguing. Aim of the present study was to investigate the possible involvement of ANF on rat aortic smooth muscle (RASM) cells proliferation induced by known mitogens and the mechanism involved. Our data show that ANF, at physiological concentration range, inhibits RASM cell proliferation induced by known mitogens such as PDGF and insulin, and the effect seems to be elicited through the modulation of phosphatidic acid (PA) production and MAP kinases involvement.

Angiotensin II as a cardiovascular risk factor

A renin-angiotensin level that is inappropriately high for the systemic blood pressure and the state of sodium balance is now recognized to be one of the modifiable cardiovascular risk factors. Angiotensin acts both as a circulating hormone and as a locally acting paracrine/autocrine/intracrine factor. The adverse effects of angiotensin on the heart include the mechanical results of elevated resistance to the pumping function of the myocardium, as well as the effects of neurohumoral abnormalities on various cardiac structures. In addition, cardiac damage follows acute ischaemic injury or chronic energy starvation due to coronary artery disease, attributable to either mechanical obstruction (atherosclerotic and/or thrombotic) or functional stenosis (vasospasm). Activation of the renin-angiotensin system has several haemodynamic and humoral consequences, all of which may damage the myocardium. These include acute myocardial ischaemia, left-ventricular hypertrophy, arrhythmias, alterations in the coagulation-fibrinolysis equilibrium, increased oxidative stress, and pro-inflammatory activity. A brief review of some of the mechanisms by which activation of the renin-angiotensin system can inflict damage on the heart is presented.

Diacylglycerol kinase alpha activity promotes survival of CD4+ 8+ double positive cells during thymocyte development

The diacylglycerol kinases (DGK) form a family of isoenzymes that catalyse the conversion of diacylglycerol (DAG) to phosphatidic acid (PA), both powerful second messengers in the cell. DGKalpha is expressed in brain, peripheral T cells and thymocytes and has been shown to translocate to the nuclear matrix upon T-cell receptor (TCR) engagement. Here, we show that high level expression of DGKalpha is induced following a signal transmitted through the pre-TCR and the protein tyrosine kinase, lck. Activity of DGKalpha contributes to survival in CD4+ 8+ (DP) thymocytes as pharmacological inhibition of DGK activity results in death of this cell population both in cell suspension and thymic explants. DGKalpha promotes survival in these thymocytes through a Bcl-regulated pathway. A consequence of inhibition of DGKalpha is the specific down-regulation of Bcl-xl, whereas in transgenic mice that over-express Bcl-2, death induced by the inhibitor is partially blocked. Thus we report a novel activity of DGKalpha in survival of thymocytes immediately after entry into the DP stage in development.

Phospholipase D contributes to transmural pressure control of prorenin processing in juxtaglomerular cell

This study was designed to delineate the involvement of phospholipase C (PLC) and phospholipase D (PLD) in transmural pressure control of renin synthesis and secretion. Primary cultures of rat juxtaglomerular (JG) cells were applied to a transmural pressure-loading apparatus for 12 hours, and the renin secretion rate (RSR), active renin content (ARC), and total (active + inactive) renin content (TRC) were determined. Under control conditions (n=5), transmural pressure decreased RSR (78.1 +/- 3.0 and 64.6 +/- 4.4% for 0 or 40 mm Hg, respectively; P<0.05) and ARC (42.8 +/- 3.3 and 26.0 +/- 3.9 ng of angiotensin I per hour per million cells for 0 or 40 mm Hg, respectively; P<0.05) but did not have a significant effect on TRC (99.5 +/- 6.7 and 89.2 +/- 4.6 ng of angiotensin I per hour per million cells for 0 or 40 mm Hg, respectively). Treatment with PLC inhibitors, 2-nitro-4-carboxyphenyl-N,N-diphenyl-carbamate (200 micromol/L) and U73122 (10 micromol/L) did not alter RSR but prevented the RSR decrease with transmural pressure, whereas neither 2-nitro-4-carboxyphenyl-N,N-diphenyl-carbamate nor U73122 altered ARC, TRC, or the decrease in ARC with transmural pressure. Experiments were also performed using JG cells (n=5) treated with a PLD inhibitor, 4-(2-aminoethyl)-benzensulfonyl fluoride (AEBSF, 100 micromol/L). Treatment with AEBSF did not influence basal levels of RSR, ARC, and TRC or the RSR decrease with transmural pressure. However, AEBSF did inhibit the decrease in ARC with transmural pressure. These results indicate that transmural pressure inhibits renin secretion via PLC-dependent pathways and prevents conversion of inactive renin to active renin via PLD-dependent mechanisms in JG cells.

Purification of diacylglycerol kinase from Microsporum gypseum and its phosphorylation by the catalytic subunit of protein kinase A

Diacylglycerol (DG) kinase (EC 2.7.1.107) was purified to homogeneity from the soluble extract of Microsporum gypseum, a dermatophyte. Purified enzyme showed a final specific activity of 2172 pmol/min/mg protein and its apparent molecular weight on SDS-PAGE was found to be 93 kDa. The activity of purified enzyme was inhibited in a dose-dependent manner in the presence of DG-kinase inhibitor (D5919, Sigma). DG-kinase activity was found to be stimulated in the presence of phosphatidylcholine, phosphatidylethanolamine, and cardiolipin while the activity was alleviated in the presence of phosphatidic acid and arachidonic acid. Kinase activity was partially inhibited when assayed after prior treatment with alkaline phosphatase. Treatment of DG-kinase with the catalytic subunit of protein kinase A (PKA)-stimulated DG-kinase activity in a dose-dependent manner. Incubation of DG-kinase with the catalytic subunit of PKA led to the phosphorylation of DG-kinase as revealed by autoradiography. The phosphorylated band disappeared completely in the presence of specific PKA inhibitor. Increased activity of DG-kinase on incubation with the catalytic subunit of PKA was possibly due to the phosphorylation of the former by the latter. Whether this in vitro phosphorylation and activation of DG-kinase occurs under physiological conditions remains to be elucidated.

Modulation of DNA synthesis by muscarinic cholinergic receptors

Acetylcholine muscarinic receptors are a family of five G-protein-coupled receptors widely distributed in the central nervous system and in peripheral organs. Activation of certain subtypes of muscarinic receptors (M1, M3, M5) has been found to modulate DNA synthesis in a number of cell types. In several cell types acetylcholine, by activating endogenous or transfected muscarinic receptors, can indeed elicit cell proliferation. In other cell types, however, or under different experimental conditions, activation of muscarinic receptors has no effect, or inhibits DNA synthesis. A large number of intracellular pathways are being investigated to define the mechanisms involved in these effects of muscarinic receptors; these include among others, phospholipase D, protein kinases C and mitogen-activated-protein kinases. The ability of acetylcholine to modulate DNA synthesis through muscarinic receptors may be relevant in the context of brain development and neoplastic growth.

Phosphatidylcholine-specific phospholipase C-mediated induction of phospholipase D activity in Fas-expressing murine cells

We have previously reported that Fas cross-linking resulted in the activation of phosphatidylcholine-specific phospholipase C (PC-PLC) and the subsequent activation of protein kinase C (PKC) and phospholipase D (PLD) in A20 cells. In an attempt to correlate the existence of PC-PLC activity and activation of PLD by Fas activation among various Fas-expressing murine cell lines, we have investigated the effect of anti-Fas monoclonal antibody on PC-PLC and PLD activities in A20, P388D1 and YAC-1 cell lines. Upon treatment of anti-Fas monoclonal antibody to these three cell lines, the activation of PLD was only observed in A20 cells. When the effect of anti-Fas monoclonal antibody on PKC and PC-PLC activities in Fas-expressing clones were investigated, the activation of PKC and PC-PLC was detected only in A20 clones. Results presented here also show that exogenous addition of Bacillus cereus PC-PLC activates PC hydrolysis, PKC and PLD in all three murine cell lines. These findings suggest that the activation of PC-PLC is a necessary requirement for the activation of PLD by Fas cross-linking and cell lines devoid of functional PC-PLC activity could exhibit enhanced PLD activity by exogenous addition of PC-PLC.

The activation of phospholipase D by endothelin-1, angiotensin II, and platelet-derived growth factor in vascular smooth muscle A10 cells is mediated by small G proteins of the ADP-ribosylation factor family

We show here that A10 cells express the phospholipase D (PLD) isoforms PLD1b and PLD2. The activation of PLD in these cells by angiotensin II (AngII), endothelin-1 (ET-1), and platelet-derived growth factor (PDGF) was found to be sensitive to inhibitors of the activation of ADP-ribosylation factor (ARF) but not to blockers of Rho protein function. PDGF, AngII, and ET-1 induced the binding of ARF proteins to cell membranes in a permeabilized cell assay. Cells permeabilized and depleted of ARF were no longer sensitive to stimulation with AngII, ET-1, or PDGF, but the addition of recombinant myristoylated human ARF1 restored agonist-dependent PLD activity. Expression of dominant negative ARF mutants blocked receptor-dependent activation of PLD. PLD activity was also potently stimulated by treatment with phorbol esters, but this activity was only partially inhibited by brefeldin A or by the overexpression of ARF dominant negative mutants. Transient expression of catalytically inactive mutants of PLD2, but not PLD1, inhibited significantly PDGF- and AngII-dependent PLD activity. We conclude: 1) the activation of PLD by cell surface receptors occurs primarily by an ARF-dependent mechanism in A10 cells, whereas the activation of PLD by protein kinase C-dependent pathways is only partially dependent on the regulation of ARF proteins; and 2) cell surface receptors, such as AngII and PDGF, signal primarily via PLD2 in A10 cells.

The role of phosphatidic acid in platelet-derived growth factor-induced proliferation of rat hepatic stellate cells

Platelet-derived growth factor (PDGF) is the most potent mitogen for hepatic stellate cells (HSCs) in vitro. The aim of this study was to investigate the role of the lipid-derived second messenger phosphatidic acid (PA) in mediating this effect and, in particular, to determine its interaction with the extracellular signal-regulated kinase (ERK) cascade. HSCs were isolated from rat livers. PA production was determined by lipid extraction and thin-layer chromatography (TLC) after prelabeling cells with [(3)H]myristate. ERK activity was measured by an in vitro kinase assay after immunoprecipitation. Mitogenic concentrations of PDGF, but not those of the relatively less potent mitogen, transforming growth factor alpha (TGF-alpha), stimulated the sustained production of PA from HSCs. Exogenous PA stimulated HSC proliferation and a sustained increase in ERK activity, and proliferation was completely blocked by the inhibition of ERK activation with PD98059. The stimulation of ERK by PDGF was of a similar magnitude but more sustained than that caused by TGF-alpha. These results suggest that the potent mitogenic effect of PDGF in HSCs may be caused, in part, by the generation of PA and subsequently by a more sustained activation of ERK than occurs with less potent mitogens that do not induce the production of this lipid second messenger.

Ang II-stimulated superoxide production is mediated via phospholipase D in human vascular smooth muscle cells

Intracellular signaling events that mediate the long-term effects of Ang II in vascular smooth muscle cells are unclear, but oxidative stress may play an important role. This study examined the ability of Ang II to generate reactive oxygen species and investigated the putative role of phospholipase D (PLD)-dependent signaling pathways for its production in human vascular smooth muscle cells. In addition, we assessed whether redox-sensitive pathways influence Ang II-stimulated cell growth. Primary and low-passage cells (passages 1 to 4) derived from resistance arteries of subcutaneous gluteal biopsies from healthy subjects were studied. Oxidative stress was measured with the fluorescent probe 5-(and 6)-chloromethyl-2', 7'-dichlorodihydrofluorescein diacetate (CM-H(2)DCFDA) (8 micromol/L), and the role of PLD was assessed with the PLD inhibitor D-erythro-sphingosine, dihydro (sphinganine) (10 micromol/L). To determine whether NADH/NADPH oxidase contributes to production of reactive oxygen species, Ang II-stimulated cells were pretreated with the specific flavoprotein inhibitor diphenylene iodinium (DPI) (10 micromol/L). DNA and protein synthesis were determined by [(3)H]thymidine and [(3)H]leucine incorporation, respectively. Ang II increased CM-H(2)DCFDA fluorescence, and this was inhibited by catalase (350 U/mL), indicating that the fluorescence signal was derived predominantly from H(2)O(2). Ang II dose-dependently increased H(2)O(2) production (E(max)=57.6+/-1.7 nmol/L, pD(2)=7.7+/-0.06) and PLD activation (E(max)=207+/-3.3% of control, pD(2)=7.7+/-0.5). H(2)O(2) effects were evident within 1 hour, and maximal PLD activation occurred within 40 minutes after stimulation. DPI inhibited (P<0.01) Ang II-stimulated responses. PLD inhibition significantly attenuated (P<0.05) Ang II-elicited H(2)O(2) production (E(max)=29+/-5 nmol/L). DPI and sphinganine inhibited Ang II-induced DNA and protein synthesis. These data indicate that in vascular smooth muscle cells from human peripheral resistance arteries, Ang II increases H(2)O(2) generation via PLD-dependent, NADH/NADPH oxidase-sensitive pathways. These cascades may function as second messengers in long-term Ang II-mediated growth-signaling events.

Bone morphogenetic protein 2 inhibits platelet-derived growth factor-induced c-fos gene transcription and DNA synthesis in mesangial cells. Involvement of mitogen-activated protein kinase

Bone morphogenetic proteins (BMPs) play an important role in nephrogenesis. The biologic effect and mechanism of action of these proteins in the adult kidney has not yet been studied. We investigated the effect of BMP2, a member of these growth and differentiation factors, on mitogenic signal transduction pathways induced by platelet-derived growth factor (PDGF) in glomerular mesangial cells. PDGF is a growth and survival factor for these cells in vitro and in vivo. Incubation of mesangial cells with increasing concentrations of BMP2 inhibited PDGF-induced DNA synthesis in a dose-dependent manner with maximum inhibition at 250 ng/ml. Immune complex tyrosine kinase assay of PDGF receptor beta immunoprecipitates from lysates of mesangial cells treated with PDGF showed no inhibitory effect of BMP2 on PDGF receptor tyrosine phosphorylation. This indicates that the inhibition of DNA synthesis is likely due to postreceptor events. However, BMP2 significantly inhibited PDGF-stimulated mitogen-activated protein kinase (MAPK) activity that phosphorylates the Elk-1 transcription factor, a component of the ternary complex factor. Using a fusion protein-based reporter assay, we also show that BMP2 blocks PDGF-induced Elk-1-mediated transcription. Furthermore, we demonstrate that BMP2 inhibits PDGF-induced transcription of c-fos gene, a natural target of Elk-1 that normally forms a ternary complex that activates the serum response element of the c-fos gene. These data provide the first evidence that in mesangial cells, BMP2 signaling cross-talks with MAPK-based transcriptional events to inhibit PDGF-induced DNA synthesis. One target for this inhibition is the early response gene c-fos.

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.

Phospholipase D is associated in a phorbol ester-dependent manner with protein kinase C-alpha and with a 220-kDa protein which is phosphorylated on serine and threonine

Many studies have shown that phospholipase D (PLD) is activated by protein kinase C (PKC) in vivo and in vitro. In this study, a PLD isoform (rPLD1) was shown to bind to PKC-alpha in Rat1 fibroblasts treated with phorbol ester. The PKC-alpha binding domain of rPLD1 was localized to its N-terminus. The phospholipase was shown to become associated also with a 220-kDa protein (p220) in the fibroblasts and in Sf9 cells infected with recombinant baculovirus coding rPLD1. This interaction was increased by phorbol myristate acetate (PMA) treatment. p220 was phosphorylated on serine/threonine in PMA-stimulated Rat1 cells, and rPLD1 expressed in Sf9 cells was also serine/threonine phosphorylated in response to PMA treatment. These data suggest the PMA induces the formation of a RPLD1/PKC alpha/P220 complex in cells, some components of which undergo serine/threonine phosphorylation.

Phosphatidic acid-induced elevation of intracellular Ca2+ is mediated by RhoA and H2O2 in Rat-2 fibroblasts

We have investigated possible roles of RhoA and H2O2 in the elevation of intracellular Ca2+ ([Ca2+]i) by phosphatidic acid (PA) in Rat-2 fibroblasts. PA induced a transient elevation of [Ca2+]i in the presence or absence of EGTA. Lysophosphatidic acid (LPA) also increased [Ca2+]i, but the sustained Ca2+ response was inhibited by EGTA. LPA stimulated the production of inositol phosphates, but PA did not. In the presence of EGTA, preincubation with LPA completely blocked the subsequent elevation of [Ca2+]i by PA, but not vice versa. PA stimulated the translocation of RhoA to the particulate fraction as did LPA. Scrape loading of C3 transferase inhibited the transient Ca2+ response to PA, but not to LPA, suggesting an essential role of RhoA in the elevation of [Ca2+]i by PA. H2O2 also induced a transient increase of [Ca2+]i as did PA. H2O2 scavengers, catalase and N-acetyl-L-cysteine, completely blocked the rise of [Ca2+]i stimulated by PA, but not by LPA. Furthermore, preincubation with PA blocked the subsequent Ca2+ response to H2O2, and the incubation with H2O2 also blocked the PA-induced rise of [Ca2+]i. Thus, it was suggested that PA stimulated Ca2+ release from PA-sensitive, but not inositol 1,4,5-trisphosphate-sensitive, Ca2+ stores by the activation of RhoA and intracellular H2O2.

Characterization of a rat brain phospholipase D isozyme

We have recently cloned a cDNA encoding a phospholipase D (PLD) from rat brain and named it rPLD1. It shows 90% amino acid identity with the human PLD isoform hPLD1b. We have expressed rPLD1 as a histidine-tagged fusion protein in insect (Sf9) cells using the expression vector pBlueBacHis and purified the recombinant protein to homogeneity by Ni2+-agarose affinity chromatography. Phosphatidylinositol 4,5-P2 and phosphatidylinositol 3,4,5-P3 activated the PLD equipotently, but other acidic phospholipids were ineffective. The activity of rPLD1 was dependent on both Mg2+ and Ca2+. It was specific for phosphatidylcholine and showed a broad dependence on pH with optimum activity at pH 6.5-7.5. The enzyme was inhibited by oleate and activated by the small G proteins ARF3 and RhoA in the presence of guanosine 5'-3-O-(thio)triphosphate. Protein kinase C (PKC)-alpha and -betaII, but not PKC-gamma, -delta, -epsilon, or -zeta, activated rPLD1 in a manner that was stimulated by phorbol ester but did not require ATP. Neither synergistic interactions between ARF3 and RhoA nor between these G proteins and PKC-alpha or -betaII were observed. Recombinant PKC-alpha and -betaII phosphorylated purified rPLD1 to high stoichiometry in vitro, and the phosphorylated PLD exhibited a mobility shift upon electrophoresis. Phosphorylation of the PLD by PKC was correlated with inhibition of its catalytic activity. rPLD1 bound to concanavalin A-Sepharose beads, and its electrophoretic mobility was altered by treatment with endoglycosidase F. The amount of PLD bound to the beads was decreased in a concentration-dependent manner when tunicamycin was added to the Sf9 expression system. Tunicamycin also decreased membrane localization of rPLD1. These results suggest that rPLD1 is a glycosylated protein and that it is negatively regulated by phosphorylation by PKC in vitro.

Phosphatidic acid elicits calcium mobilization and actin polymerization through a tyrosine kinase-dependent process in human neutrophils: a mechanism for induction of chemotaxis

Phospholipids mediate important effects as extracellular messengers in diverse biological systems. We investigated the effects of phosphatidic acid, a biologically active phospholipid potentially involved in the inflammatory process, on calcium mobilization and actin polymerization in human neutrophils and correlated these effects with induction of chemotactic migration. Intermediate-chain length phosphatidic acid (DiC10-PA) induced a biphasic increase in intracellular Ca2+ characterized by a rapid rise commencing immediately upon addition of stimulus followed by a secondary increase which, unlike the initial response, was eliminated by chelation of extracellular Ca2+. Neither of these responses were induced by C10-lysophosphatidic acid or diacylglycerol. The tyrosine kinase inhibitor herbimycin-A (5-10 microg/ml) completely blunted the initial but not the delayed response effected by DiC10-PA. Long-chain phosphatidic acid (DiC18:1) induced only an initial rapid increase in intracellular Ca2+ and this response was similarly markedly attenuated by herbimycin-A. Among several physiologically relevant phospholipids, only phosphatidic acid was able to induce Ca2+ mobilization; phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol -- used individually or in mixed liposomes -- were without effect. Phosphatidic acid conferred calcium-mobilizing activity upon inactive liposome preparations and phosphatidic acid-enriched cellular plasma membranes possessed similar calcium-mobilizing activity. Both DiC10-PA and DiC18:1-PA induced actin polymerization in neutrophils at rates which mirrored the influence of each agent on Ca2+ mobilization. Herbimycin-A blunted the initial increase in actin polymerization effected by phosphatidic acid but had no effect on the delayed, EGTA-sensitive phase. DiC10-PA and DiC18:1-PA also induced neutrophil migration along a concentration gradient. Phospholipids that failed to induce a calcium transient, including phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol, likewise failed to induce either actin polymerization or chemotactic migration. Unlike chemotaxis induced by zymosan-activated human serum, phosphatidate-induced chemotaxis was strongly inhibited by pretreatment of cells with herbimycin-A. Consistent with these observations, phosphatidic acid induced the tyrosine phosphorylation of several proteins as early as 10 s after stimulation. Phosphorylation of two distinct proteins with approximate molecular sizes of 72 and 82 kDa was inhibited by levels of herbimycin A used to effectively inhibit calcium mobilization, actin polymerization and chemotaxis. Thus, in neutrophilic leukocytes, extracellular phosphatidic acid induces a unique tyrosine kinase-based signalling pathway that results in calcium mobilization and actin polymerization. These processes may promote directed cellular migration as a consequence of the interaction of phosphatidic acid with neutrophil plasma membranes.

Alternative splicing of the human diacylglycerol kinase zeta gene in muscle

Diacylglycerol can function as a second messenger, and one mechanism for the attenuation of this signal is its conversion to phosphatidic acid, which is catalyzed by diacylglycerol kinase (DGK). We screened a cDNA library from human skeletal muscle and isolated two DGKzeta cDNAs that differed from the 3.5-kb clone originally identified in endothelial cells. One transcript, which was 3.4 kb long, was shown to be nonfunctional; it had a 77-bp deletion that included the translation initiation site. The other was 4.1 kb long with a unique 5' sequence of 853 bp. We also isolated a genomic clone of DGKzeta and determined its organization and location; it contains 32 exons, spans approximately 50 kb of genomic sequence, and maps to chromosome 11p11.2. The protein encoded by the 4.1-kb transcript contains two cysteine-rich regions, a catalytic domain, and ankyrin repeats like the endothelial form of DGKzeta, as well as a unique N-terminal domain. The coding sequence was shown to be derived from alternative splicing of the DGKzeta gene. In cells transfected with the 4.1-kb clone, we detected a 130-kDa protein with an antibody to DGKzeta and demonstrated that it was localized predominantly in the nucleus. We conclude that alternative splicing generates tissue-specific variants of DGKzeta that share some properties but may have unique ones as well.

Dual effect of lysophosphatidic acid on proliferation of glomerular mesangial cells

Among the variety of factors able to contribute to mesangial hypertrophy by altering mesangial cell growth, lysophosphatidic acid (LPA) is the focus of increasing attention. It is produced in plasma following platelet activation, as well as by mesangial cells stimulated by secretory phospholipase A2. As mitogenic/antimitogenic properties of LPA are already described in a variety of cells, knowledge of its specific actions on mesangial cells is of potential interest regarding the pathophysiology of glomerulus damage in situ. We tested the effect of LPA on cultured rat mesangial cell growth. At 10 to 20 microM, LPA stimulated thymidine incorporation as well as phosphorylation of mitogen-activated protein kinases (MAP-kinases) p42-p44 in dose- and time-dependent manner, which demonstrated a positive effect on cell proliferation. However, higher concentrations of LPA (100 microM) were unable to stimulate thymidine incorporation and partly inhibited the proliferative effect as well as p42-p44 phosphorylation evoked by serum. Finally, whereas lysophosphatidylcholine (10 to 20 microM) was lytic for mesangial cells, no cell lysis could be detected at the highest concentrations of LPA. Taken together, these results suggest that LPA exerts a dual effect on mesangial cell proliferation, which could be due to activation of distinct specific signaling pathways, in dose-dependent fashion. Specific actions of LPA able to modify mesangial cell proliferation in a positive or negative manner are also likely to influence the pathophysiological processes involved in the progression of glomerulosclerosis in the kidney.

Stimulation of protein synthesis by phosphatidic acid in rat cardiomyocytes

Phosphatidic acid (PA) was observed to stimulate protein synthesis in adult cardiomyocytes in a time- and concentration-dependent manner. The maximal stimulation in protein synthesis (142 +/- 12% vs 100% as the control) was achieved at 10 microM PA within 60 min and was inhibited by actinomycin D (107 +/- 4% of the control) or cycloheximide (105 +/- 6% of the control). The increase in protein synthesis due to PA was attenuated or abolished by preincubation of cardiomyocytes with a tyrosine kinase inhibitor, genistein (94 +/- 9% of the control), phospholipase C inhibitors 2-nitro-4-carboxyphenyl N,N-diphenyl carbamate or carbon-odithioic acid O-(octahydro-4,7-methanol-1H-inden-5-yl (101 +/- 6 and 95 +/- 5% of the control, respectively), protein kinase C inhibitors staurosporine or polymyxin B (109 +/- 3 and 93 +/- 3% of the control), and chelators of extracellular and intracellular free Ca2+ EGTA or BAPTA/AM (103 +/- 6 and 95 +/- 6% of the control, respectively). PA at different concentrations (0.1 to 100 microM) also caused phosphorylation of a cell surface protein of approximately 24 kDa. In addition, mitogen-activated protein kinase was stimulated by PA in a concentration-dependent manner; maximal stimulation (217 +/- 6% of the control) was seen at 10 microM PA. These data suggest that PA increases protein synthesis in adult rat cardiomyocytes and thus may play an important role in the development of cardiac hypertrophy.

Phosphoinositide-derived diacylglycerol conversion to phosphatidic acid is a receptor-dependent and compartmentalized phenomenon in human neuroblastoma

We report that upon muscarinic stimulation of SK-N-BE(2) human neuroblastoma cells, the extent of phosphoinositide-derived diacylglycerol (DG) conversion to phosphatidic acid (PA), operated by a DG kinase, is dependent on the potency of receptor stimulation and correlates with the reduction of phosphatidylinositol 4,5-bisphosphate mass. Evidence is provided that agonist-evoked Ca2+ mobilisation or protein kinase activation are not key events in triggering receptor-generated DG conversion to PA; furthermore, the phenomenon is compartmentalized, namely it occurs within a topologically restricted area that is poorly accessible to DG artificially generated by cell treatment with bacterial phosphatidylinositol-specific phospholipase C. Possible mechanisms driving regulation of the DG kinase operating in the transduction system investigated are discussed.

Inhibition of the insulin receptor tyrosine kinase by phosphatidic acid

The lipid second messenger, phosphatidic acid, inhibits the intrinsic tyrosine kinase activity of the insulin receptor in detergent-lipid mixed micelles or in reconstituted membranes. Enzymatic studies revealed that this lipid second messenger inhibits the catalytic activity of partially purified insulin receptor without affecting the affinity of the receptor for insulin. Selectivity in the protein-lipid interaction is suggested by the inability of several other acidic lipids to affect the kinase activity of the receptor and by the relative insensitivity of the inhibition to increasing ionic strength and, in some cases, micelle surface charge. Lysophosphatidic acid and phosphatidic acids with short acyl chains do not affect significantly the receptor's kinase activity, suggesting that hydrophobic interactions are involved in the inhibition. Thus, both a high affinity interaction of the insulin receptor with the phosphate headgroup and a stabilizing hydrophobic interaction with the acyl chains contribute to the inhibitory protein-lipid interaction. The selective sensitivity of the insulin receptor to phosphatidic acid suggests that the receptor-mediated generation of this lipid in the plasma membrane could negatively modulate insulin receptor function.

Cloning and characterization of a glucocorticoid-induced diacylglycerol kinase

Diacylglycerol kinase (DGK) plays a key role in cellular processes by regulating the intracellular concentration of the second messenger diacylglycerol. We screened a hamster DDT1 smooth muscle cell library and isolated a unique, glucocorticoid-inducible cDNA with substantial homology to known DGKs. DGK activity was increased in lysates of insect cells infected with recombinant baculovirus containing this cDNA. Antibodies raised against expressed sequences recognized a glucocorticoid-inducible 130-140-kDa protein on immunoblots of DDT1 cell lysates. Thus, this sequence appears to be a new member of the DGK family that we refer to as DGKeta. Homology to other DGKs was apparent in domains that are thought to be important for DGK function including the cysteine-rich motifs and potential catalytic domains. DGKeta shares substantial homology with DGKdelta including the N-terminal pleckstrin homology domain. The tissue distribution of DGKeta message (determined by ribonuclease protection assays) and protein (determined by immunoblots) was broader than reported for other DGKs, indicating that DGKeta may play a more general role in regulating cellular DG levels than other DGKs. Heterogeneity among DGK family members indicates that individual DGKs may have unique functions.

Phosphatidic acid: a lipid messenger involved in intracellular and extracellular signalling

Generated during the initial phases of cell signalling, phosphatidic acid has been implicated as a messenger involved in the activation of cellular kinases and phospholipases as well as certain proto-oncogene products and low-molecular-weight G-proteins. Although many of the reported effects of phosphatidic acid can be attributed to metabolites generated by cellular hydrolases, the parent compound clearly possesses important biological activities. However, instead of acting as a ubiquitous second messenger mediating signalling events shared by a wide variety of cells, in many systems the phospholipid seems to function specifically, regulating unique functions confined to specialized groupings of cells. One such function is neutrophil superoxide generation, which is induced when phosphatidic acid, generated by activated phospholipase D (PLD), facilitates the interaction of a cytoplasmic low-molecular-weight G-protein with dormant, membrane-bound reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Positioned on the outer surface of the plasma membrane of triggering cells, phosphatidic acid potentially mediates the "juxtacrine" stimulation of cells in direct contact. This review critically evaluates the known biological effects of phosphatidic acid as opposed to functions induced by its metabolites and addresses the mechanisms by which these effects are specifically induced by this phospholipid.

Phosphatidic acid binding to human neutrophils: effects on tyrosine kinase-regulated intracellular Ca2+ mobilization

Neutrophils provide an attractive model with which to characterize cellular effects of phosphatidic acid (PA) independently of effects triggered by lysophosphatidic acid (LPA), since these cells lack LPA receptors. We developed a novel method to quantitate binding of PA to neutrophils and neutrophil plasma membranes. Intact cells or subcellular fractions were immobilized on nitrocellulose membranes and incubated in a bath containing [32P]PA under various conditions, followed by rapid rinsing with a mild detergent (0.05% Tween 20) to minimize non-specific binding. With this method, dioctanoyl PA specifically ligated plasma-membrane binding sitesin a time- and temperature-dependent manner. Specific binding of (DiC8-PA was markedly potentiated by pre-treatment of cells or membranes with ecto-phosphatidic acid phosphohydrolase (PAPase) inhibitor dimethylsphingosine (DMS). Optimum binding of DiC8-PA to PAPase-inhibited cells occurred within 10 min at room temperature, increased linearly with the cell concentration used, and was not significantly affected by alteration of pH over the range of 5.5-8.5. Of several phosphatidic acid species examined, optimal specific binding to immobilized neutrophils was observed with DiC8-PA and dicapryl (DiC10) PA; dicaproyl (DiC6) PA bound weakly, whereas dimyristoyl (DiC14) PA and dipalmitoyl (DiC16) PA did not bind. Dioleoyl (DiC18:1) PA bound to immobilized cells, but this binding was essentially non-specific, in that it was not reduced by excess non-radioactive ligand. Various LPA preparations, including [32P] lyso-octanoyl (C8) PA and [32P] lyso-oleoyl (C18:1) PA, showed very low specific binding to neutrophils in this system. Specific binding of DiC8-PA and DiC10-PA preparations correlated well with the ability of each to effect the mobilization of intracellular Ca2+ in neutrophils. Ca2+ mobilization was characterized by two distinct phases; a rapid rise that was inhibited in the presence of the tyrosine kinase inhibitor herbimycin-A, followed by a sustained increase that was eliminated in the presence of EGTA. The results are consistent with the hypothesis that neutrophils have specific binding sites for phosphatidic acid, the occupation of which leads to rapid mobilization of intracellular free Ca2+ via activation of tyrosine kinases. The methods described in this report may facilitate the identification and characterization of functional phosphatidic acid receptors on neutrophil plasma membranes.

Regulation of phospholipase D by tyrosine kinases

Activation of phospholipase D (PLD) represents part of an important signalling pathway in mammalian cells. Phospholipase D catalyzed hydrolysis of phospholipids generates phosphatidic acid (PA) which is subsequently metabolized to lyso-PA (LPA) or diacylglycerol (DAG). While DAG is an endogenous activator of protein kinase C (PKC), PA and LPA have been recognized as second messengers as well. Activation of PLD in response to an external stimulus may involve PKC, Ca2+, G-proteins and/or tyrosine kinases. In this review, we will address the role of protein tyrosine phosphorylation in growth factor-, agonist- and oxidant-mediated activation of PLD. Furthermore, a possible link between PKC, Ca2+, G-proteins and tyrosine kinases is discussed to indicate the complexity involved in the regulation of PLD in mammalian cells.

Messenger functions of phosphatidic acid

Under physiological conditions, phosphatidic acid (PA) is an anionic phospholipid with moderate biological reactivity. Some of its biological effects can be attributed to lyso-PA and diacylglycerol generated by the action of cellular hydrolases. However, it is clear that the parent compound exhibits biological activities of its own. Early studies implicated PA in the transport of Ca++ across plasma membranes as well as in the mobilization of intracellular stored calcium. Both responses may be induced as a consequence of other cellular processes activated by PA, as opposed to being directly mediated by the lipid. PA may be involved in the activation of certain functions confined to specialized groupings of cells, such as the neutrophil superoxide-generating enzyme or actin polymerization. Recent studies implicate PA as an activator of intracellular protein kinases, and a PA-dependent superfamily of kinases involved in cellular signalling has been hypothesized. Deployed on the outer surface of the plasma membrane, PA potentially provides a method of communication between cells in direct contact. This review will explore the known functions of PA as an intracellular mediator and extracellular messenger of biological activities and address ways in which these functions are potentially regulated by cellular enzymes which hydrolyse the phospholipid.

Regulation of phospholipase D by protein kinase C

In nearly all mammalian cells and tissues examined, protein kinase C (PKC) has been shown to serve as a major regulator of a phosphatidylcholine-specific phospholipase D (PLD) activity. At least 12 distinct isoforms of PKC have been described so far; of these enzymes only the alpha- and beta-isoforms were found to regulate PLD activity. While the mechanism of this regulation has remained unknown, available evidence suggests that both phosphorylating and non-phosphorylating mechanisms may be involved. A phosphatidylcholine-specific PLD activity was recently purified from pig lung, but its possible regulation by PKC has not been reported yet. Several cell types and tissues appear to express additional forms of PLD which can hydrolyze either phosphatidylethanolamine or phosphatidylinositol. It has also been reported that at least one form of PLD can be activated by oncogenes, but not by PKC activators. Similar to activated PKC, some of the primary and secondary products of PLD-mediated phospholipid hydrolysis, including phosphatidic acid, 1,2-diacylglycerol, choline phosphate and ethanolamine, also exhibit mitogenic/co-mitogenic effects in cultured cells. Furthermore, both the PLD and PKC systems have been implicated in the regulation of vesicle transport and exocytosis. Recently the PLD enzyme has been cloned and the tools of molecular biology to study its biological roles will soon be available. Using specific inhibitors of growth regulating signals and vesicle transport, so far no convincing evidence has been reported to support the role of PLD in the mediation of any of the above cellular effects of activated 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.

Phosphatidic acid generation through interleukin 2 (IL-2)-induced alpha-diacylglycerol kinase activation is an essential step in IL-2-mediated lymphocyte proliferation

Proliferation of T lymphocytes is triggered by the interaction of interleukin 2 (IL-2) with its high affinity specific receptor that is expressed on the cell surface following T lymphocyte activation. Significant advances have recently been made in identifying the multiple signals that follow IL-2 receptor occupancy, although the exact mechanism responsible for IL-2-induced proliferation remains an enigma. It has been shown previously that unique species of phosphatidic acid are rapidly produced in vivo following IL-2 binding. It was then suggested that, in contrast to other eukaryotic growth factor systems, phosphatidic acid was at least in part generated through IL-2-induced diacylglycerol (DG) kinase activation. In the present study we demonstrate IL-2-dependent activation of the alpha isoform of DG kinase. Confocal microscopy studies reveal that the enzyme is located in the cytosol and nuclei of resting T cells. Interleukin 2 stimulation induces translocation of the enzyme to the perinuclear region. Furthermore, our results indicate that inhibition of the alpha isoform of DG kinase has a profound effect on IL-2-induced T cell growth. Studies on cell cycle distribution demonstrate that the inhibition of IL-2-induced phosphatidic acid production induces arrest in late G1 phase of IL-2 dependent cells. Altogether, these results link previous observations of interleukin 2 and phosphatidic acid production to activation of an specific isoform of DG kinase and suggest that activation of this enzyme is part of a novel signaling cascade that utilizes phosphatidic acid as an effector molecule.

Signals controlling the expression of PDGF

PDGF is an important polypeptide growth factor that plays an essential role during early vertebrate development and is associated with tissue repair and wound healing in the adult vertebrate. Moreover, PDGF is thought to play a role in a variety of pathological phenomena, such as cancer, fibrosis and atherosclerosis. PDGF is expressed as a dimer of A and/or B chains, the precursors of which are encoded by two single copy genes. Although the PDGF genes are expressed coordinately in a number of cell types, they are independently expressed in a majority of cell types. The expression of either PDGF gene can be affected by very diverse extracellular stimuli and the type of response is dependent on the cell type that is exposed to the stimulus. Expression of the PDGF chains can be modulated at every imaginable level: by regulating accessibility of the transcription start site, by varying the transcription initiation rate, by using alternative transcription start sites, by alternative splicing, by using alternative polyadenylation signals, by varying mRNA decay rates, by regulating efficiency of translation, by protein modification, and by regulating secretion. Even upon secretion, the activity of PDGF can be modulated by non-specific or specific PDGF-binding proteins. This review provides an overview of the cell types in which the PDGF genes are expressed, of the factors that are known to affect the expression of PDGF, and of the various levels at which the expression of PDGF genes can be regulated.

Phosphatidic acid activation of protein kinase C in LA-N-1 neuroblastoma cells

Phosphatidic acid (PA), a hydrolytic product of phospholipase D activity, stimulated cytosolic protein kinase C (PKC) activity when LA-N-1 neuroblastoma cells in culture were treated with PA, without translocating the enzyme to the membrane. Treatment of cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) translocated and activated PKC in a dogmatic manner. Partially purified PKC activity derived from LA-N-1 neuroblastoma cells was stimulated by PA alone or in the presence of phosphatidylserine or TPA, without affecting [3H]phorbol dibutyrate binding, indicating that the site of action of PA was different from the phorbol ester or diacylglycerol binding site. These results suggest an unorthodox pattern of PKC stimulation mediated by PA which appears to be yet another mode of PA signal transduction.

Stimulation of protein and DNA synthesis in mouse C2C12 satellite cells: evidence for phospholipase D-dependent and -independent pathways

In C2C12 myoblasts, 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulated a phospholipase D (PLD) to degrade phosphatidylcholine (PC) as measured by the release of choline and an increase in the formation of phosphatidic acid (PA) (or phosphatidylbutanol [PtdBuOH] in the presence of 0.5% butanol). Exogenous PLD also stimulated choline release, PA and PtdBuOH formation. The protein kinase C (PKC) inhibitor, Ro-31-8220, and PKC downregulation significantly inhibited the effects of TPA but Ro-31-8220 had no effect on PLD action. Neither basic Fibroblast Growth Factor (bFGF) or Epidermal Growth Factor (EGF) increased PLD activity. All agonists stimulated protein synthesis during both a 90 min and a 6 hr incubation and increased RNA accretion after 6 hr. The response at 90 min was not inhibited by the transcription inhibitor, actinomycin D. Ro-31-8220 and PKC downregulation significantly inhibited all the effects of TPA. In contrast, Ro-31-8220 significantly inhibited the increase in RNA accretion elicited by PLD but had no effect on the ability of agonists other than TPA to enhance protein synthesis. All agonists also stimulated thymidine incorporation into DNA. The effects of EGF, bFGF, and PLD were rapid and transient whereas that of TPA was delayed and sustained. Ro-31-8220 and PKC downregulation significantly inhibited the response due to TPA. Furthermore, Ro-31-8220 also significantly inhibited the effects elicited by EGF and PLD but not that induced by bFGF. In differentiated myotubes, TPA and PLD, but not bFGF or EGF, again stimulated choline release and PtdBuOH formation. However, all agents failed to stimulate protein synthesis and RNA accretion. The data demonstrate the presence in C2C12 myoblasts, but not differentiated myotubes, of both a PLD-dependent and PLD-independent pathway(s) leading to the stimulation of protein synthesis, RNA accretion, and DNA synthesis.

Phosphatidic acid and lysophosphatidic acid induce haptotactic migration of human monocytes

The present study was aimed at defining the chemotactic activity of phosphatidic acid, which is rapidly produced by phagocytes in response to chemotactic agonists. Exogenously added phosphatidic acid induced human monocyte directional migration across polycarbonate filters with an efficacy (number of cell migrated) comparable to that of "classical" chemotactic factors. In lipid specificity studies, activity of phosphatidic acid decreased with increasing acyl chain length but was restored by introducing unsaturation in the acyl chain with the most active form being the natural occurring 18:0,20:4-phosphatidic acid. Lysophosphatidic acid was also active in inducing monocyte migration. No other phospholipid and lysophospholipid tested was effective in this response. Monocyte migration was regulated by a gradient of phosphatidic acid and lysophosphatidic acid bound to the polycarbonate filter, in the absence of detectable soluble chemoattractant. Migration was also observed if phospholipids were bound to fibronectin-coated polycarbonate filters. Thus, phosphatidic acid and lysophosphatidic acid, similarly to other physiological chemoattractants (e.g. C5a and interleukin-8), induce cell migration by an haptotactic mechanism. Phosphatidic acid caused a rapid increase of filamentous actin and, at higher concentrations, induced a rise of intracellular calcium concentration. Monocyte migration to phosphatidic acid and lysophosphatidic acid, but not to diacylglycerol, was inhibited in a concentration-dependent manner by Bordetella pertussis toxin, while cholera toxin was ineffective. In the chemotactic assay, phosphatidic acid and lysophosphatidic acid induced a complete homologous desensitization and only partially cross-desensitized one with each other, or with diacyl-glycerol and monocyte chemotactic protein-1. Suramine inhibited monocyte chemotaxis with a different efficiency phosphatidic acid > lysophosphatidic acid" diacyl-glycerol On the contrary, monocyte chemotactic protein-1-induced chemotaxis was not affected by the drug. Collectively, these data show that phosphatidic acid induces haptotactic migration of monocytes that is at least in part receptor-mediated. These results support a role for phosphatidic acid and lysophosphatidic acid in the regulation of leukocyte accumulation into tissues.

Activation of phospholipase D by prostaglandin F2 alpha in rat luteal cells and effects of inhibitors of arachidonic acid metabolism

In rat luteal cells labeled with [3H]oleic acid, PGF2 alpha-stimulated phospholipase D (PLD) activation was investigated. The PLD activity was detected by measuring the accumulation of [3H]phosphatidylethanol (PtdEt) in the presence of ethanol. PGF2 alpha stimulated PtdEt accumulation at concentrations of more than 100 nM in the presence of ethanol. However, PtdEt accumulation did not change in the absence of ethanol. PGF2 alpha (1 microM) increased PtdEt accumulation after 1 min, and the accumulation reached a plateau by 2-3 min. These results indicate that PGF2 alpha activates PLD in rat luteal cells. U-73122, a phospholipase C (PLC) inhibitor, and staurosporine, a protein kinase C (PKC) inhibitor, did not inhibit PGF2 alpha-stimulated [3H]PtdEt accumulation. These results suggest that PGF2 alpha-induced PLD activation is different from PLC-PKC systems. We reported previously that PGF2 alpha stimulated the release of arachidonic acid. The effects of indomethacin, nordihydroguaiaretic acid (NDGA), and 5,8,11,14-eicosatetraynoic acid (ETYA), inhibitors of arachidonic acid metabolism, on PGF2 alpha-stimulated PtdEt accumulation were examined. Pretreatment with indomethacin enhanced PGF2 alpha-induced PtdEt accumulation. In contrast, pretreatment with NDGA and ETYA inhibited PGF2 alpha-induced PtdEt accumulation. It is suggested that PGF2 alpha-stimulated PLD activation is mediated via lipoxygenase products.

Wortmannin has opposite effects on phorbol ester-induced DNA synthesis and phosphatidylcholine hydrolysis

The tumor promoter phorbol 12-myristate 13-acetate (PMA) and hormonal activators of protein kinase C (PKC) commonly stimulate phospholipase D (PLD)-mediated formation of phosphatidic acid from phosphatidylcholine (PtdCho) in fibroblasts and other cell types. On the basis that phosphatidic acid is a mitogen, PLD is often considered to have a major role in the regulation of cell growth by PKC activators. However, we found that in NIH 3T3 fibroblasts wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3K), strongly inhibited DNA synthesis induced by 100 nM PMA, while it actually enhanced PMA-stimulated PtdCho hydrolysis. These results indicate that stimulation of PLD activity is either not required or not sufficient for the mitogenic action of PMA.

Multifunctional phosphatidic acid signaling in mammary epithelial cells: stimulation of phosphoinositide hydrolysis and conversion to diglyceride

We have shown previously that phosphatidic acid esterified to polyunsaturated fatty acids is mitogenic for primary cultures of mouse mammary epithelial cells embedded within collagen gels. We hypothesized that this mitogenic competence resulted from the ability of this phospholipid to activate multiple signal transduction pathways in mammary epithelium. A closer examination of this hypothesis was undertaken by examining the effect of exogenous phosphatidic acid on phosphoinositide (PI) hydrolysis and its intracellular metabolism to diglyceride, an activator of protein kinase C. For assays of phosphoinositide-specific phospholipase C activation, mammary epithelial cells from virgin Balb/c mice were isolated by collagenase dissociation of mammary glands and cultured on the surface of Type I collagen-coated culture dishes. Phosphatidic acid (PA) stimulated a sustained increase in inositol phosphates and caused inositol phospholipid depletion when added to cells in which inositol phospholipids were prelabeled with 3H-myoinositol. This effect was specific for PA among phospholipids tested. Neither lineoleic acid, that can be released from PA, nor prostaglandin E2 affected PI hydrolysis. When mammary epithelial cells were cultured inside collagen gels in the presence of exogenous PA or phosphatidylcholine (PC) radiolabeled with 3H-glycerol, PA was found to persist intracellularly and be dephosphorylated to diglyceride (an activator of protein kinase C) to a greater extent than PC, a nonmitogenic phospholipid. In contrast to PA, epidermal growth factor (EGF) only slightly stimulated PI hydrolysis, showing that these two different growth-promoting factors do not actively couple to the same signal transduction pathways in mammary epithelial cells. These results show that PA may activate multiple pathways in mammary epithelial cells either directly or via its metabolism to diglyceride.