Phosphatidic acid that accumulates in platelet-derived growth factor-stimulated Balb/c 3T3 cells is a potential mitogenic signal

Lysophosphatidic acid signalling

Lysophosphatidic acid is an intercellular phospholipid messenger that is released from platelets (and probably other cells) and evokes multiple biological responses, ranging from induction of mitogenesis to neurite retraction, by activating a specific G protein coupled receptor. Recent studies indicate that the lysophosphatidic acid receptor acts via the small GTP-binding proteins Ras and Rho to stimulate cell proliferation and to trigger actin-based cytoskeletal events, respectively.

Phosphatidylinositol 4,5-bisphosphate synthesis is required for activation of phospholipase D in U937 cells

Phospholipase D (PLD) has been implicated in signal transduction and membrane traffic. We have previously shown that phosphatidylinositol 4,5-bisphosphate (PtdIns-4,5-P2) stimulates in vitro partially purified brain membrane PLD activity, defining a novel function of PtdIns-4,5-P2 as a PLD cofactor. In the present study we extend these observations to permeabilized U937 cells. In these cells, the activation of PLD by guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) is greatly potentiated by MgATP. We have utilized this experimental system to test the hypothesis that MgATP potentiates PLD activation by G proteins because it is required for PtdIns-4,5-P2 synthesis by phosphoinositide kinases. As expected, MgATP was absolutely required for maintaining elevated phosphatidylinositol 4-phosphate (PtdIns-4-P) and PtdIns-4,5-P2 levels in the permeabilized cells. In the presence of MgATP, GTP gamma S further elevated the levels of the phosphoinositides. The importance of PtdIns-4,5-P2 for PLD activation was examined by utilizing a specific inhibitory antibody directed against phosphatidylinositol 4-kinase (PtdIns 4-kinase), the enzyme responsible for the first step in the synthesis of PtdIns-4,5-P2. Anti-PtdIns 4-kinase completely inhibited PtdIns 4-kinase activity in vitro and reduced by 75-80% PtdIns-4-P and PtdIns-4,5-P2 levels in the permeabilized cells. In parallel, the anti-PtdIns 4-kinase fully inhibited the activation of PLD by GTP gamma S and caused a 60% inhibition of PLD activation by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, indicating that elevated PtdIns-4,5-P2 levels are required for PLD activation. This conclusion is supported by the fact that neomycin, a high affinity ligand of PtdIns-4,5-P2, also blocked PLD activation. Furthermore, the activity of PLD in U937 cell lysate was stimulated by PtdIns-4,5-P2 in a dose-dependent manner. The current results indicate that PtdIns-4,5-P2 synthesis is required for PLD activation in permeabilized U937 cells and strongly support the proposed function of PtdIns-4,5-P2 as a cofactor for PLD. In addition, the results further establish PtdIns-4,5-P2 as a key component in the generation of second messengers via multiple pathways including phosphoinositide-phospholipase C, phosphoinositide 3-kinase and PLD.

Cell signalling pathway involved in PACAP-induced AR4-2J cell proliferation

The novel 38-amino acid neuropeptide PACAP (pituitary adenylate activating peptide) has recently been shown to induce the pancreatic acinar tumour AR4-2J cell growth. This growth promoting effect of PACAP was, however, independent of adenylate cyclase activation but suppressed by pertussis toxin and the somatostatin analog SMS 201-995. This study was undertaken to search for potential cell signalling pathways involved in the growth promoting effect of PACAP on AR4-2J cells. The AR4-2J cells were grown in Dulbecco's Modified Eagle's Medium containing 10% foetal calf serum. For studies on cell signalling pathways, all experiments were carried out on cells which have reached 50 to 75% confluency. At that point, they were transferred to serum free medium overnight with or without 1 microCi/ml myristic acid. The next morning, cells were harvested, washed and used for tyrosine kinase and phospholipase D (PLD) activities. For studies on growth, cells were grown for 2 days in the presence of 1 nM PACAP +/- the different inhibitors of tyrosine kinase and PLD. PACAP-38 and -27 caused a dose-dependent and parallel activation of tyrosine kinase and PLD an effect prevented by the antagonist PACAP 7-38. PACAP-38-stimulated tyrosine kinase and PLD activation are both dose-dependently inhibited by SMS 201-995. Finally, PACAP-stimulated tyrosine kinase and PLD activities are both inhibited by cell's preincubation with genistein and pertussis toxin. After 2 days, the PACAP-induced increase in AR4-2J cell growth was significantly inhibited by increasing concentrations of genistein and wortmannin, inhibitors of tyrosine kinase, PLD and phosphatidylinositol 3-kinase, respectively. PACAP can induce concomitant activation of tyrosine kinase and PLD; this finding and the observation that inhibition of these two enzymes inhibited PACAP-induced AR4-2J cell growth strongly suggests that they are intimately involved in the overall process of PACAP-induced AR4-2J cell proliferation.

Inhibitors of phospholipid intracellular signaling as antiproliferative agents

The improved understanding of oncogenesis and the involvement of oncogenes and tumor suppressor genes, has led to a rational approach of specific target-directed anti-cancer drug development. Cancer genes have been found to be important not only in the control of cell proliferation but also in the mediation of processes such as drug resistance, metastasis, neo-vascularization (angiogenesis), and apoptosis. These are all important targets in their own right and the development of drugs against specific "upstream" targets in oncogenic or growth factor signal transduction cascades it may be possible to inhibit multiple "downstream" targets. Ultimately, to test the hypothesis that signaling pathways offer good targets for anticancer drug development will take several years of careful clinical study and we cannot say at this time whether the approach will work. There are a small number of compounds in the early stages of clinical development as anticancer agents that may act by inhibiting growth factor signaling pathways. In all cases the activity of the compounds on intracellular signaling pathways was discovered after their identification as antiproliferative agents. There are also compounds in preclinical development that have been specifically developed as inhibitors of growth factor signaling, although their selectivity for tumor cells compared to normal tissue remains to be investigated fully in appropriate animal tumor models. It is possible that a single antisignaling drug by itself may not have the power to completely inhibit tumor growth and a combination of drugs may be needed. It may also take a combination of drugs to prevent the emergence of resistance. Clearly there are several challenges to developing this new class of anticancer drugs, and there will undoubtedly be others that must be faced.

Interleukin-11 induces phosphatidic acid formation and activates MAP kinase in mouse 3T3-L1 cells

Interleukin-11 (IL-11) stimulated [3H]phosphatidic acid (PA) formation in [3H]arachidonic acid (AA) prelabelled quiescent mouse 3T3-L1 cells. When IL-11 stimulated 3T3-L1 cells were incubated with NaF, a phosphatidic acid phosphohydrolase (PAP) inhibitor, increased PA formation was observed. In the presence of ethanol, phosphatidylethanol accumulated at the expense of PA. These results indicated that the formation of PA upon IL-11 stimulation was a result of phospholipase D (PLD) activation. Endogenous accumulation of PA by NaF treatment or exogenously added PA enhanced tyrosine phosphorylation of two proteins of 44 KDa (p44) and 47 KDa (p47) whereas tyrosine phosphorylation of other proteins was not affected. Among various PA species, dipalmitoyl PA was found to be most effective in enhancing tyrosine phosphorylation of these proteins. p44 and p47 cross reacted with anti-MAP kinase monoclonal antibody (MoAb) in both immunoprecipitation and western blot analysis. Lysates from IL-11-induced or PA-induced cells stimulated phosphorylation of a synthetic peptide substrate for MAP kinase, indicating the activation of MAP kinase in the induced cells. These studies suggest that one of the cellular signalling mechanisms of IL-11 in 3T3-L1 cells involves the activation of phospholipase D to produce the second messenger PA. The increased level of PA enhances tyrosine phosphorylation of p44 and p47 which belong to the members of MAP kinase family and thus transduces some of the mitogenic signals of IL-11 in this cell line.

Role of phospholipase D in laminin-induced production of gelatinase A (MMP-2) in metastatic cells

Metastatic spread depends critically upon the invasiveness of tumor cells, i.e. their ability to breach basement membranes by elaborating and secreting specific proteolytic enzymes such as gelatinase A (MMP-2). Laminin is a major constituent of the extracellular matrix that can trigger production of MMP-2 in metastatic cells, but not in non-metastatic cells. The present study was designed to examine the role of phospholipase D (PLD) and its product, phosphatidic acid, in the intracellular signal transduction mechanisms that mediate induction of MMP-2 by laminin. Here we show that stimulation of tumor cells with laminin results in a time- and dose-dependent activation of PLD. Laminin-induced production of MMP-2 is attenuated by 1-butanol, a competitive substrate of PLD that reduces PLD-catalyzed production of PA. Moreover, phosphatidic acid itself can induce production of MMP-2 in metastatic tumor cells. MMP-2 can also be induced by exposing the cells to exogenous bacterial PLD. Elevated cellular phosphatidic acid induces MMP-2 in metastatic ras-transformed 3T3 fibroblasts but, like laminin, fails to do so in normal cells. These data indicate that laminin-induced activation of PLD and consequent generation of phosphatidic acid are involved in a signal propagation pathway leading to induction of MMP-2 and enhanced invasiveness of metastatic tumor cells.

Regulation of PLC-mediated signalling in vivo by CDP-diacylglycerol synthase

CDP-diacylglycerol synthase (CDS) is an enzyme required for the regeneration of the signalling molecule phosphatidylinositol-4,5-bisphosphate (PtdlnsP2) from phosphatidic acid. A photo-receptor cell-specific isoform of CDS from Drosophila is a key regulator of phototransduction, a G-protein-coupled signalling cascade mediated by phospholipase C. cds mutants cannot sustain a light-activated current as a result of depletion of PtdlnsP2. Overexpression of CDS increases the amplitude of the light response, demonstrating that availability of PtdlnsP2 is a determinant in the gain of this pathway. cds mutants undergo light-dependent retinal degeneration which can be suppressed by a mutation in phospholipase C. Thus, enzymes involved in PtdlnsP2 metabolism regulate phosphoinositide-mediated signalling cascades in vivo.

Tamoxifen elicits rapid transmembrane lipid signal responses in human breast cancer cells

The antiestrogen tamoxifen competes with estrogen for receptor occupancy, although reports indicate that not all effects of tamoxifen are mediated via this specific interaction. In the present study we sought to determine whether tamoxifen can initiate transmembrane lipid signals. Lipid signaling is a prominent mode by which hormones, growth factors, and phorbol diesters transduce messages. Using the human mammary carcinoma cell line MDA-MB-231, phospholipid metabolism was analyzed in cells prelabeled with 3H-fatty acid. After short-term (10 min) exposure to tamoxifen (10 microM), cellular phosphatidic acid (PA) increased by approximately 50%. Dose-response kinetics for PA formation were obtained over a tamoxifen range of 2.5-20 microM. Treatment of MDA-MB-231 cells with phorbol diester (12-O-tetradecanoylphorbol-13-acetate, TPA) also elicited PA generation (60% above control). Interestingly, addition of tamoxifen, a purported protein kinase C inhibitor, to TPA-treated cells, caused further increase in PA (approximately 100% above control). PA, a second messenger lipid produced upon effector-receptor coupling, shares a prominent role in signal transduction events that govern cellular proliferation. It is therefore suggested that some actions of tamoxifen are mediated by promoting production of second messenger lipids that elicit transmembrane signal transduction cascades. This view is in line with ideas on non-estrogen receptor associated actions of tamoxifen by way of alternate binding sites.

Angiotensin II induces phosphatidic acid formation in neonatal rat cardiac fibroblasts: evaluation of the roles of phospholipases C and D

Phosphatidic acid has been proposed to contribute to the mitogenic actions of various growth factors. In 32P-labeled neonatal rat cardiac fibroblasts, 100 nM [Sar1]angiotensin II was shown to rapidly induce formation of 32P-phosphatidic acid. Levels peaked at 5 min (1.5-fold above control), but were partially sustained over 2 h. Phospholipase D contributed in part to phosphatidic acid formation, as 32P- or 3H-phosphatidylethanol was produced when cells labeled with [32P]H3PO4 or 1-O-[1,2- 3H]hexadecyl-2-lyso-sn-glycero-3-phosphocholine were stimulated in the presence of 1% ethanol. [Sar1]angiotensin II-induced phospholipase D activity was transient and mainly mediated through protein kinase C (PKC), since PKC downregulation reduced phosphatidylethanol formation by 68%. Residual activity may have been due to increased intracellular Ca2+, as ionomycin also activated phospholipase D in PKC-depleted cells. Phospholipase D did not fully account for [Sar1]angiotensin II-induced phosphatidic acid: 1) compared to PMA, a potent activator of phospholipase D, [Sar1]angiotensin II produced more phosphatidic acid relative to phosphatidylethanol, and 2) PKC downregulation did not affect [Sar1]angiotensin II-induced phosphatidic acid formation. The diacylglycerol kinase inhibitor R59949 depressed [Sar1]angiotensin II-induced phosphatidic acid formation by only 21%, indicating that activation of a phospholipase C and diacylglycerol kinase also can not account for the bulk of phosphatidic acid. Thus, additional pathways not involving phospholipases C and D, such as de novo synthesis, may contribute to [Sar1]angiotensin II-induced phosphatidic acid in these cells. Finally, as previously shown for [Sar1]angiotensin II, phosphatidic acid stimulated mitogen activated protein (MAP) kinase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Tamoxifen stimulates phospholipase D activity by an estrogen receptor-independent mechanism

The effects of tamoxifen (TAM), a widely used agent in the treatment of breast cancer, were examined on phospholipase D (PLD)-mediated phospholipid hydrolysis. In drug-sensitive MCF-7 human breast carcinoma cells TAM, similar to several well-established activators of PLD, had no effect on phospholipid hydrolysis. In an estrogen receptor-deficient multidrug-resistant subline of MCF-7 cells, TAM preferentially stimulated the hydrolysis of phosphatidylethanolamine; two-fold stimulation required 2.5 or 5 microM TAM in the absence or presence of serum, respectively. In NIH 3T3 fibroblasts significant (4- to 4.8-fold) stimulation of phosphatidylethanolamine and phosphatidylcholine hydrolysis in the presence of serum required 10 microM TAM. These data establish that TAM can stimulate PLD activity by an estrogen receptor-independent mechanism.

Effect of R59022, an inhibitor of diacylglycerol kinase, on IgE-mediated histamine release from human lung mast cells and basophils

We have examined the effect of the diacylglycerol kinase inhibitor R59022 on histamine release from human lung mast cells and basophils. At 1 microM the drug increased the IgE-dependent release of histamine from human basophils from 19 +/- 5% to 60 +/- 13% (n = 5, p < 0.01). The increase in histamine release was dose dependent with maximum enhancement between 1 and 10 microM. 1 microM R59022 also increased f-met peptide-induced histamine release from 18 +/- 4% to 55 +/- 11% (n = 5, p < 0.05). However, the drug did not significantly increase the release of histamine when the non-physiologic stimulus PMA was used to initiate release. The effect of the drug on anti-IgE-induced release was most marked at lower concentrations of anti-IgE and declined when superoptimal concentrations of anti-IgE were used. As anticipated there was a strong negative correlation (r = 0.764, p < 0.05) between anti-IgE-induced histamine release and the percentage enhancement in the presence of 10 microM R59022. In contrast, to these potent effects on the human basophil the drug failed to affect the anti-IgE-induced release of histamine from human lung mast cells. The data suggest that the R59022 increases the release of histamine induced by anti-IgE in human basophils but not in human lung mast cells. Furthermore, the ability of R59022 to potentiate basophil histamine release is restricted to receptor-mediated stimuli such as anti-IgE and does not extend to non-physiologic stimuli such as the phorbol ester PMA.

A rapid bioassay for platelet-derived growth factor beta-receptor tyrosine kinase function

We have extended a melanophore-based bioassay for G-protein coupled receptors to include the functional expression of the murine platelet-derived growth factor (PDGF) beta-receptor. The homodimeric ligand PDGF-BB induced activation of the transiently expressed receptor in melanophore cells. This led to dose dependent pigment dispersion whereas it did not induce pigment dispersion in wild type cells. The effective concentration of PDGF-BB giving half-maximal pigment dispersion (EC50) was 1nM after 30 minutes exposure. PDGF-AA had no ability to induce pigment dispersion in melanophore cells transiently expressing the beta-PDGF receptor. PDGF-BB-induced pigment dispersion could be blocked by the bis-indolylmaleimide Ro 31-8220 which is an inhibitor of protein kinase C isoenzymes. Functional expression of the PDGF beta-receptor extends the use of the pigment translocation assay to include transmembrane signaling receptor tyrosine kinases. It opens the opportunity for the discovery of potent agonists and antagonists through massive drug screening and investigations of functional ligand-receptor interactions for single transmembrane domain receptors.

Quantitation of the fatty acid composition of phosphatidic acid by capillary gas chromatography electron-capture detection with picomole sensitivity

We describe a relatively simple and sensitive method to measure femtomole amounts of phosphatidic acid in cells. Phosphatidic acid was extracted from cells in the presence of 1-heptadecanoyl-2-heptadecanoyl-sn-glycero-3-phosphate as an internal standard, purified by two-dimensional thin-layer chromatography, and hydrolyzed to its constituent free fatty acids which were then derivatized to the corresponding pentafluorobenzyl esters. Pentafluorobenzyl esters of fatty acids were analyzed by gas chromatography with electron-capture detection. Long-chain fatty acids were resolved with excellent signal-to-noise ratios. Using heptadecanoic acid as an internal standard for quantitation, as little as 1 fmol of pentafluorobenzyl ester of stearic acid was detected with a linear response up to 10 pmol. Linear detector responses were obtained for all major classes of fatty acids. For phosphatidic acid measurement, the detection limit was at least 50 fmol thus achieving a 1000-fold increase in sensitivity compared to the most sensitive of the previously described methods. An example is provided of quantitating phosphatidic acid from minute amounts of biological samples such as islets of Langerhans.

Lysophosphatidic acids induce proliferation of cultured vascular smooth muscle cells from rat aorta

Lysophosphatidic acids (LPA) with a C18 fatty acyl group accelerated thymidine incorporation into cultured rat aortic vascular smooth muscle cells and stimulated their cell division. LPA acted synergistically with epidermal growth factor and fibroblast growth factor but additively with platelet-derived growth factor. The stimulatory actions of LPA were suggested to be rather specific from the following findings: 1) their stimulation of DNA synthesis increased with an increase in their acyl moiety; 2) lysophosphatidylcholine, a neutral lysophospholipid, had no mitogenic action but was cytotoxic at high concentrations; and 3) LPA induced a rapid external Ca(2+)-independent increase in intracellular Ca2+ concentration ([Ca2+]i) in single fura 2-loaded cells that resembled the receptor-mediated increases in [Ca2+]i triggered by different agonists, whereas lysophosphatidylcholine provoked a slow sustained increase in [Ca2+]i in an external Ca(2+)-dependent manner. These results are discussed in relation to the possible pathophysiological role of LPA.

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)

Signalling pathways as targets for anticancer drug development

Intracellular signalling pathways mediating the effects of oncogenes on cell growth and transformation offer novel targets for the development of anticancer drugs. With this approach, it may be sufficient to target a component of the signalling pathway activated by the oncogene rather than the oncogene product itself. In this review, the abilities of some antiproliferative drugs to inhibit signalling targets are considered. There are some anticancer drugs already in clinical trial that may act by inhibiting signalling targets, as well as drugs in preclinical development. Some problems that may be encountered in developing this new class of anticancer drugs are discussed.

A role for phospholipase D in control of mitogenesis

How do growth factors that act on G protein-coupled cell-surface receptors communicate with the nucleus? These receptors commonly activate phospholipase C, and it has been assumed that the consequent rise in cytosolic Ca2+ concentrations and activation of protein kinase C mediates the mitogenic response. Recent evidence has demonstrated that phospholipase D (PLD) might be capable of eliciting mitogenesis. This enzyme is stimulated by a variety of growth factors, including those that act on receptors that possess intrinsic tyrosine kinase activity as well as those acting on G protein-coupled receptors. In this review, Michael Boarder considers the evidence that PLD, activated downstream of tyrosine protein kinases by both classes of cell-surface growth factor receptor, is implicated in the mitogenic response. This evidence is related to the possibility of PLD involvement in the regulation of vascular smooth muscle cell proliferation by endothelin-1 and platelet-derived growth factor.

Sphingosine-1-phosphate, a novel second messenger involved in cell growth regulation and signal transduction, affects growth and invasiveness of human breast cancer cells

This review will focus on the role of sphingosine and its phosphorylated derivative sphingosine-1-phosphate (SPP) in cell growth regulation and signal transduction. We will show that many of the effects attributed to sphingosine in quiescent Swiss 3T3 fibroblasts are mediated via its conversion to SPP. We propose that SPP has appropriate properties to function as an intracellular second messenger based on the following: it elicits diverse cellular responses; it is rapidly produced from sphingosine by a specific kinase and rapidly degraded by a specific lyase; its concentration is low in quiescent cells but increases rapidly and transiently in response to the growth factors, fetal calf serum (FCS) and platelet derived growth factor (PDGF); it releases Ca2+ from internal sources in an InsP3-independent manner; and finally, it may link sphingolipid signaling pathways to cellular ras-mediated signaling pathways by elevating phosphatidic acid levels. The effects of this novel second messenger on growth, differentiation and invasion of human breast cancer cells will be discussed.

Enhanced phospholipase D activity in vascular smooth muscle cells derived from spontaneously hypertensive rats

When cultured in the presence of fetal calf serum, aortic vascular smooth muscle cells (VSMC) derived from spontaneously hypertensive rats (SHR) grow faster than those from normotensive control Wistar-Kyoto (WKY) rats. In order to investigate the mechanism underlying this growth abnormality, we measured phospholipase D (PLD) activity in VSMC taken from both SHR and WKY rats. Upon stimulation with serum, platelet-derived growth factor (PDGF) and porbol 12-myristate 13-acetate (TPA), phosphatidylethanol (PEt) was produced in the presence of ethanol. The responses of the VSMC from SHR (SHR-cells) to all stimuli were significantly greater than those of the VSMC from WKY rats (WKY-cells), which suggests an enhanced PLD activity in the SHR-cells. Since PLD is regarded as an enzyme involved in signal transduction leading to cell proliferation, this PLD hyper-reactivity in the SHR-cells may account at least partially for the growth abnormality in the SHR-cells.

Induction of phosphatidic acid by fibroblast growth factor in cultured baby hamster kidney fibroblasts

Basic fibroblast growth factor (bFGF/FGF-2) is a strong mitogenic inducer of cultured baby hamster kidney (BHK) cells. When cultured BHK cells were stimulated with FGF-2, phosphatidic acid (PA) was induced within 2 min, peaked at 5 min and gradually decreased. Phospholipase D (PLD) was also mitogenic for cultured BHK cells and this effect was mediated via PA. The possibility that PA induction by FGF-2 is an essential signaling step for BHK cell proliferation is discussed.

Sphingosine-1-phosphate as second messenger in cell proliferation induced by PDGF and FCS mitogens

Growth signalling networks that use glycerophospholipid metabolites as second messengers have been well characterized, but less is known of the second messengers derived from sphingolipids, another major class of membrane lipids. A tantalizing link between sphingolipids and cellular proliferation has emerged from the discovery that the sphingolipid metabolites sphingosine and sphingosine-1-phosphate stimulate growth of quiescent Swiss 3T3 fibroblasts by a pathway that is independent of protein kinase C. Sphingosine-1-phosphate is rapidly produced from sphingosine and may mediate its biological effects. Furthermore, sphingosine-1-phosphate triggers the dual signal transduction pathways of calcium mobilization and activation of phospholipase D, prominent events in the control of cellular proliferation. Here we report that activation of sphingosine kinase and the formation of sphingosine-1-phosphate are important in the signal transduction pathways activated by the potent mitogens platelet-derived growth factor (PDGF) and fetal calf serum (FCS).

Sphingolipids as mediators of effects of platelet-derived growth factor in vascular smooth muscle cells

The role of sphingolipids in mediating the action of platelet-derived growth factor (PDGF) has been investigated in the vascular smooth muscle-derived A7r5 cell line. L-Cycloserine (2 mM), an inhibitor of sphingolipid synthesis, caused time-dependent inhibition of [3H]serine incorporation into [3H]sphingomyelin in A7r5 cells. PDGF-AB (10 ng/ml), PDGF-BB (10 ng/ml), or sphingosine (10 microM) independently stimulated [3H]thymidine incorporation into DNA in A7r5 cells. L-Cycloserine (2 mM) inhibited stimulation of DNA synthesis by both PDGF-AB and PDGF-BB. L-Cycloserine (2 mM, 16 h) did not affect the ability of PDGF or sphingosine to increase intracellular free calcium ([Ca2+]i) in A7r5 cells loaded with the fluorescent indicator fura 2. Measurement of adenine nucleotide levels in A7r5 cell extracts by reverse-phase high-performance liquid chromatography indicated that treatment with L-cycloserine did not adversely affect cellular metabolism. To determine directly whether PDGF activates sphingolipid metabolism, A7r5 cells were labeled with [3H]serine for 48 h and then treated with PDGF-AB (10 ng/ml) for 1 h. Sphingolipids were separated by thin-layer chromatography and quantified by liquid scintillation counting. PDGF-AB stimulated an increase in [3H]sphingosine from 25.5 +/- 3.0 to 37.5 +/- 4.1 counts.min-1 (cpm).micrograms protein-1 and a concomitant decrease in [3H]ceramide from 24.3 +/- 3.2 to 18.5 +/- 2.9 cpm/micrograms protein. These data suggest that the PDGF-stimulated increase in [Ca2+]i is not sufficient for induction of DNA synthesis and that mitogenic effects of PDGF in vascular smooth muscle cells are mediated by sphingolipid metabolism.

Signal transduction pathways of angiotensin II--induced c-fos gene expression in cardiac myocytes in vitro. Roles of phospholipid-derived second messengers

Angiotensin II (Ang II) causes a rapid induction of immediate-early genes and hypertrophy in the cardiac myocyte. However, the signaling mechanism of Ang II-induced immediate-early gene expression in cardiac myocytes has not been characterized. Therefore, we examined signal transduction of Ang II in neonatal rat cardiac myocytes, using c-fos gene expression as a model system. Transient transfection of c-fos reporter gene constructs indicated that the serum response element is not only required but also sufficient for Ang II-induced activation of the c-fos promoter. Ang II is known to cause an increase in [Ca2+]i. We found that Ang II also causes a small increase in cAMP in cardiac myocytes. However, the Ca2+/cAMP response element of the c-fos gene was not sufficient to confer Ang II responsiveness to the c-fos promoter, and inhibitors of protein kinase A had no effects on Ang II-induced c-fos expression. On the other hand, chelating intracellular Ca2+ with BAPTA-AM inhibited Ang II-induced c-fos expression in a dose-dependent manner, suggesting that Ca2+ is required for Ang II-induced signaling. Measurements of phospholipid-derived second messengers revealed that Ang II increased production of inositol trisphosphate, diacylglycerol, phosphatidic acid, and arachidonic acids, resulting in a sustained increase in protein kinase C activity. This and other evidence suggest that Ang II activates phospholipase C, phospholipase D, and possibly phospholipase A2. All of these second-messenger systems are activated through the AT1 receptor. Pharmacological inhibition of phospholipase C or downregulation of protein kinase C significantly suppressed Ang II-induced c-fos expression. In conclusion, Ang II activates multiple phospholipid-derived second-messenger systems via the AT1 receptor in cardiac myocytes. Among these second-messenger systems, phospholipase C and protein kinase C seem essential for Ang II-induced c-fos gene expression, whereas Ca2+ may play a permissive role. Finally, the "Ang II response element" of the c-fos gene maps to the protein kinase C-dependent portion of the serum response element.

Protein kinase C-dependent stimulation of phospholipase D in phospholipase C-treated fibroblasts

Treatment of [14C]choline- or [14C]ethanolamine-labeled NIH 3T3 fibroblasts with Bacillus cereus phosphatidyl-choline-specific phospholipase C (PLC) enhanced phospholipase D (PLD)-mediated hydrolysis of the respective 14C-labeled phospholipids. PLD activity was stimulated by 1.5 U/mL of PLC and by 100 nM of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) to similar extents. Treatment of [14C]palmitic acid-labeled fibroblasts with PLC in the presence of ethanol also enhanced PLD-mediated formation of phosphatidylethanol; the effects of PLC and PMA were nonadditive. PLC had no effect on PLD activity in fibroblasts in which PKC was down-regulated by prolonged (24 h) treatment with 300 nM PMA. These data indicate that treatment of fibroblasts with exogenous PLC results in PKC-dependent activation of PLD.

Endothelin-1 stimulated phospholipase D in A10 vascular smooth muscle derived cells is dependent on tyrosine kinase. Evidence for involvement in stimulation of mitogenesis

The mechanism whereby endothelin stimulates mitogenesis of vascular smooth muscle cells is not understood. Here we show that endothelin-1 stimulates phospholipase D by a protein kinase C and tyrosine kinase dependent mechanism, and present evidence that implicate the phosphatidic acid formed by phospholipase D in the mitogenic response.

Phosphatidic acid stimulates inositol 1,4,5-trisphosphate production in adult cardiac myocytes

The cellular content of phosphatidic acid can increase in response to several agonists either by phosphorylation of diacylglycerol after phospholipase C-catalyzed hydrolysis of phospholipids or directly through activation of phospholipase D. Although previous findings indicated that the generation of phosphatidic acid was exclusively a means of regulation of the cellular concentration of diacylglycerol, more recent studies have indicated that phosphatidic acid may also directly regulate several cellular functions. Accordingly, the present study was performed to assess whether phosphatidic acid could stimulate cardiac phospholipase C in intact adult rabbit ventricular myocytes. The mass of inositol 1,4,5-trisphosphate [Ins (1,4,5)P3] was determined by a specific and sensitive binding protein assay and by direct mass measurement using anion exchange chromatography for separation of selected inositol phosphates and gas chromatography and mass spectrometry for quantification of inositol monophosphate (IP1), inositol bisphosphate (IP2), inositol trisphosphate (IP3), and inositol tetrakisphosphate (IP4). Phosphatidic acid (10(-9)-10(-6) M) elicited a rapid concentration-dependent increase in Ins (1,4,5)P3 accumulation, with the peak fourfold to fivefold increase at 30 seconds of stimulation; the concentration required for 50% of maximal stimulation was 4.4 x 10(-8) M. The time course of individual inositol phosphates indicated a successive increase in the mass of IP3, IP4, IP2, and IP1 in response to stimulation with phosphatidic acid. The production of Ins (1,4,5)P3 in response to phosphatidic acid was not altered in the absence of extracellular calcium or in the presence of extracellular EGTA (10(-3) M). Thus, these findings indicate that phosphatidic acid is a potent activator of inositol phosphate production in adult ventricular myocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipase D and cell signaling

Phospholipase D, which hydrolyzes phospholipids (primarily phosphatidylcholine) to generate phosphatidic acid, has emerged as a critical component in cellular signal transduction. Research during the past year has confirmed and extended the view that phosphatidic acid and its dephosphorylated product, sn-1,2-diacylglycerol, are important intracellular second messengers and that the coupling of phospholipase D to specific receptors occurs through multiple mechanisms involving protein kinase C, protein tyrosine kinase, Ca2+ and GTP-binding proteins.