Inhibition by phospholipids of the interaction between R-ras, rho, and their GTPase-activating proteins

TRPV4 mediates tumor-derived endothelial cell migration via arachidonic acid-activated actin remodeling

Changes in intracellular calcium [Ca(2+)](i) levels control critical cytosolic and nuclear events that are involved in the initiation and progression of tumor angiogenesis in endothelial cells (ECs). Therefore, the mechanism(s) involved in agonist-induced Ca(2+)(i) signaling is a potentially important molecular target for controlling angiogenesis and tumor growth. Several studies have shown that blood vessels in tumors differ from normal vessels in their morphology, blood flow and permeability. We had previously reported a key role for arachidonic acid (AA)-mediated Ca(2+) entry in the initial stages of tumor angiogenesis in vitro. In this study we assessed the mechanism involved in AA-induced EC migration. We report that TRPV4, an AA-activated channel, is differentially expressed in EC derived from human breast carcinomas (BTEC) as compared with 'normal' EC (HMVEC). BTEC display a significant increase in TRPV4 expression, which was correlated with greater Ca(2+) entry, induced by AA or 4αPDD (a selective TRPV4 agonist) in the tumor-derived ECs. Wound-healing assays revealed a key role of TRPV4 in regulating cell migration of BTEC but not HMVEC. Knockdown of TRPV4 expression completely abolished AA-induced BTEC migration, suggesting that TRPV4 mediates the pro-angiogenic effects promoted by AA. Furthermore, pre-incubation of BTEC with AA induced actin remodeling and a subsequent increase in the surface expression of TRPV4. This was consistent with the increased plasma membrane localization of TRPV4 and higher AA-stimulated Ca(2+) entry in the migrating cells. Together, the data presented herein demonstrate that: (1) TRPV4 is differentially expressed in tumor-derived versus 'normal' EC; (2) TRPV4 has a critical role in the migration of tumor-derived but not 'normal' EC migration; and (3) AA induces actin remodeling in BTEC, resulting in a corresponding increase of TRPV4 expression in the plasma membrane. We suggest that the latter is critical for migration of EC and thus in promoting angiogenesis and tumor growth.

Aiming drug discovery at lysophosphatidic acid targets

Lysophosphatidic acid (LPA, 1-radyl-2-hydroxy-sn-glycero-3-phosphate) is the prototype member of a family of lipid mediators and second messengers. LPA and its naturally occurring analogues interact with G protein-coupled receptors on the cell surface and a nuclear hormone receptor within the cell. In addition, there are several enzymes that utilize LPA as a substrate or generate it as a product and are under its regulatory control. LPA is present in biological fluids, and attempts have been made to link changes in its concentration and molecular composition to specific disease conditions. Through their many targets, members of the LPA family regulate cell survival, apoptosis, motility, shape, differentiation, gene transcription, malignant transformation and more. The present review depicts arbitrary aspects of the physiological and pathophysiological actions of LPA and attempts to link them with select targets. Many of us are now convinced that therapies targeting LPA biosynthesis and signalling are feasible for the treatment of devastating human diseases such as cancer, fibrosis and degenerative conditions. However, successful targeting of the pathways associated with this pleiotropic lipid will depend on the future development of as yet undeveloped pharmacons.

Novel role of cPLA(2)alpha in membrane and actin dynamics

Actin-directed processes such as membrane ruffling and cell migration are regulated by specific signal transduction pathways that become activated by growth factor receptors. The same signaling pathways that lead to modifications in actin dynamics also activate cPLA(2)alpha. Moreover, arachidonic acid, the product of cPLA(2)alpha activity, is involved in regulation of actin dynamics. Therefore, it was investigated whether cPLA(2)alpha plays a role in actin dynamics, more specifically during growth factor-induced membrane ruffling and cell migration. Upon stimulation of ruffling and cell migration by growth factors, endogenous cPLA(2)alpha and its active phosphorylated form were shown to relocate at protrusions of the cell membrane involved in actin and membrane dynamics. Inhibition of cPLA(2)alpha activity with specific inhibitors blocked growth factor-induced membrane and actin dynamics, suggesting an important role for cPLA(2)alpha in these processes.

Phospholipids can switch the GTPase substrate preference of a GTPase-activating protein

The major cellular inhibitors of the small GTPases of the Ras superfamily are the GTPase-activating proteins (GAPs), which stimulate the intrinsic GTP hydrolyzing activity of GTPases, thereby inactivating them. The catalytic activity of several GAPs is reportedly inhibited or stimulated by various phospholipids and fatty acids in vitro, indicating a likely physiological role for lipids in regulating small GTPases. We find that the p190 RhoGAP, a potent GAP for the Rho and Rac GTPases, is similarly sensitive to phospholipids. Interestingly, however, several of the tested phospholipids were found to effectively inhibit the RhoGAP activity of p190 but stimulate its RacGAP activity. Thus, phospholipids have the ability to "switch" the GTPase substrate preference of a GAP, thereby providing a novel regulatory mechanism for the small GTPases.

Phosphatidic acid regulates tyrosine phosphorylating activity in human neutrophils: enhancement of Fgr activity

In human neutrophils, the activation of phospholipase D and the Tyr phosphorylation of proteins are early signaling events upon cell stimulation. We found that the pretreatment of neutrophils with ethanol (0.8%) or 1-butanol (0.3%), which results in the accumulation of phosphatidylalcohol at the expense of phosphatidic acid (PA), decreased the phorbol myristate acetate-stimulated Tyr phosphorylation of endogenous proteins (42, 115 kDa). When neutrophil cytosol was incubated in the presence or absence of PA, these and other endogenous proteins became Tyr-phosphorylated in a PA-dependent manner. In contrast, phosphatidylalcohols exhibited only 25% (phosphatidylethanol) or 5% (phosphatidylbutanol) of the ability of PA to stimulate Tyr phosphorylation in the cell-free assay. Similarly, other phospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylserine, phosphatidylinositol, polyphosphoinositides, and sphingosine 1-phosphate) showed little ability to stimulate Tyr phosphorylation. These data suggest that PA can function as an intracellular regulator of Tyr phosphorylating activity. Gel filtration chromatography of leukocyte cytosol revealed a peak of PA-dependent Tyr phosphorylating activity distinct from a previously described PA-dependent phosphorylating activity (Waite, K. A., Wallin, R., Qualliotine-Mann, D., and McPhail, L. C. (1997) J. Biol. Chem. 272, 15569-15578). Among the protein Tyr kinases expressed in neutrophils, only Fgr eluted exclusively in the peak of PA-dependent Tyr phosphorylating activity. Importantly, Fgr isolated from unstimulated neutrophil lysates showed increased activity in the presence of PA but not phosphatidylbutanol. Moreover, the pretreatment of neutrophils with 1-butanol decreased Fgr activity in cells stimulated with formyl-methionyl-leucyl phenylalanine plus dihydrocytochalasin B. Together, these results suggest a new second messenger role for PA in the regulation of Tyr phosphorylation.

The cAMP-specific phosphodiesterase PDE4D3 is regulated by phosphatidic acid binding. Consequences for cAMP signaling pathway and characterization of a phosphatidic acid binding site

Hormones and growth factors induce in many cell types the production of phosphatidic acid (PA), which has been proposed to play a role as a second messenger. We have previously shown in an acellular system that PA selectively stimulates certain isoforms of type 4 cAMP-phosphodiesterases (PDE4). Here we studied the effect of endogenous PA on PDE activity of transiently transfected MA10 cells overexpressing the PA-sensitive isoform PDE4D3. Cell treatment with inhibitors of PA degradation, including propranolol, induced an accumulation of endogenous PA accompanied by a stimulation of PDE activity and a significant decrease in both cAMP levels and protein kinase A activity. Furthermore, in FRTL5 cells, which natively express PDE4D3, pretreatment with compounds inducing PA accumulation prevented both cAMP increase and cAMP-responsive element-binding protein phosphorylation triggered by thyroid-stimulating hormone. To determine the mechanism of PDE stimulation by PA, endogenous phospholipids were labeled by preincubating MA10 cells overexpressing PDE4D3 with [(32)P]orthophosphate. Immuno- precipitation experiments showed that PA was specifically bound to PDE4D3, supporting the hypothesis that PDE4D3 activation occurs through direct binding of PA to the protein. PA binding site on PDE4D3 was characterized by engineering deletions of selected regions in the N-terminal regulatory domain of the enzyme. Deletion of amino acid residues 31-59 suppressed both PA-activating effect and PA binding, suggesting that this region rich in basic and hydrophobic residues contains the PA binding site. These observations strongly suggest that endogenous PA can modulate cAMP levels in intact cells, through a direct activation of PDE4D3.

Role of actin cytoskeleton in prostaglandin-induced protection against ethanol in an intestinal epithelial cell line

Prostaglandins (PGs) protect a variety of gastrointestinal cells against injury induced by ethanol and other noxious agents. This investigation attempted to discern the mechanism of cytoprotection as it relates to the relationship between actin and PGs in IEC-6 cells (a rat intestinal epithelial cell line). IEC-6 cells were incubated in Dulbecco's modified Eagle's medium +/- 16,16-dimethyl prostaglandin E(2) (dmPG, 2.6 microM) for 15 min and subsequently incubated in medium containing 1, 2.5, 5, 7.5, and 10% ethanol (EtOH). Cells were then processed for immunocytochemistry using FITC-phalloidin in order to stain the actin cytoskeleton, and cell viability was determined by trypan blue exclusion. Quantitative Western immunoblotting of fractioned G-actin (nonpolymerized; S1) and F-actin (polymerized; S2) was also carried out. EtOH concentrations equal to and greater than 5% led to the collapse of the actin cytoskeleton as depicted by extensive disorganization and fragmentation. In addition, these same EtOH concentrations significantly decreased the S2 fraction and increased the S1 pool of actin. Preincubation with dmPG prevented collapse of the actin cytoskeleton, significantly increased the S2 polymerized fraction as determined by quantitative immunoblotting, and increased cell viability in EtOH-treated cultures. Prior incubation with cytochalasin D, an actin disruptive agent, not only reduced cell viability but also prevented the cytoprotective effects of dmPG. Phalloidin, an actin stabilizing agent, had effects similar to that of dmPG as demonstrated by stability of the actin cytoskeleton and increased cellular viability. Such findings indicate that PGs are important in the organization and stability of actin under in vitro conditions. These effects on actin may play an essential role in the mechanism of PG-induced cytoprotection.

Disruption of a gene encoding phosphatidic acid phosphatase causes abnormal phenotypes in cell growth and abnormal cytokinesis in Saccharomyces cerevisiae

Phosphatidic acid phosphatase (PAP) is an enzyme involved in lipid metabolism. Diacylglycerol (DG) and phosphatidic acid (PA) are a substrate and a product of PAP, respectively, and function as second messengers in several signal transduction pathways in animals. To investigate the function of PAP in Saccharomyces cerevisiae, we analyzed changes in cellular phenotypes of a mutant that has a disrupted PAP gene. Two putative genes for PAP (ScPAP1 and ScPAP2) are in the S. cerevisiae genome. We generated a ScPAP1-null mutant and observed its cellular phenotypic changes. The ScPAP1-null mutant cells aggregated in liquid culture, and microscopical analyses showed that these mutant cells have an abnormal cell shape and abnormal cytokinesis during cell division. The ScPAP1 is possibly involved in cell growth and cytokinesis in S. cerevisiae. Yeast phosphatidic acid phosphatase is possibly involved in cell growth and cytokinesis.

Epidermal growth factor receptor stimulation of diacylglycerol kinase

The epidermal growth factor receptor (EGFR) activates formation of the phospholipid signal messenger phosphatidic acid (PA) on ligand binding. We explored the effects of chronic EGF stimulation on cellular PA in NIH3T3 cells expressing intact EGFR a mutant EGFR (EGFRvIII). The presence of EGFRvIII increased PA levels to twice those induced by chronic EGFR activation. Fatty acid methyl ester analysis revealed a marked increase in oleic acid containing PA. No apparent increase in phospholipase D (PLD) activity was detected, and diacylglycerol (DAG) kinase assays demonstrated a marked preference for dioleoyl DAG in the presence of activated EGFR or EGFRvIII. Levels of PA which were lower than would be predicted by DAG kinase activation are explained by increased phosphatidate phosphohydrolase activity. Specific inhibitors of EGFR kinase and DAG kinase suppressed DAG kinase activation and PA production by EGFRvIII. EGFR kinase activation by chronic exposure to ligand or by deletional mutation stimulates formation of a specific form of signalling PA.

Sphingoid bases and phospholipase D activation

There is increased interest in physiological functions and mechanisms of action of sphingolipids metabolites, ceramide, sphingosine, and sphingosine-1-phosphate (SPP), members of a new class of lipid second messengers. This review summarizes current knowledge regarding the role of these sphingolipids metabolites in the actions of growth factors and focuses on the second messenger roles of sphingosine and its metabolite, SPP, in the regulation of cell growth. We also discuss possible interactions with intermediates of the well known glycerophospholipid cycle. Sphingosine and SPP generally provide positive mitogenic signals whereas ceramide has been reported to induce apoptosis and cell arrest in several mammalian cell lines. Stimulation of phospholipase D leading to an increase in phosphatidic acid, a positive regulator of cell growth, by sphingosine and SPP, and its inhibition by ceramide, might be related to their opposite effects on cell growth. This also indicates that sphingolipid turnover could regulate the diacylglycerol cycle. Cross-talk between sphingolipid turnover pathways and the diacylglycerol cycle increases complexity of signaling pathways leading to cellular proliferation and adds additional sites of regulation.

Effect of acidic phospholipids on sphingosine kinase

Sphingosine-1-phosphate (SPP) is a unique sphingolipid metabolite involved in cell growth regulation and signal transduction. SPP is formed from sphingosine in cells by the action of sphingosine kinase, an enzyme whose activity can be stimulated by growth factors. Little is known of the mechanisms by which sphingosine kinase is regulated. We found that acidic phospholipids, particularly phosphatidylserine, induced a dose-dependent increase in sphingosine kinase activity due to an increase in the apparent Vmax of the enzyme. Other acidic phospholipids, such as phosphatidylinositol, phosphatidic acid, phosphatidylinositol bisphosphate, and cardiolipin stimulated sphingosine kinase activity to a lesser extent than phosphatidylserine, whereas neutral phospholipids had no effect. Diacylglycerol, a structurally similar molecule which differs from phosphatidic acid in the absence of the phosphate group, failed to induce any changes in sphingosine kinase activity. Our results suggest that the presence of negative charges on the lipid molecules is important for the potentiation of sphingosine kinase activity, but the effect does not directly correlate with the number of negative charges. These results also support the notion that the polar group confers specificity in the stimulation of sphingosine kinase by acidic glycerophospholipids. The presence of a fatty acid chain in position 2 of the glycerol backbone was not critical since lysophosphatidylserine also stimulated sphingosine kinase, although it was somewhat less potent. Dioleoylphosphatidylserine was the most potent species, including a fourfold stimulation, whereas distearoyl phosphatidylserine was completely inactive. Thus, the degree of saturation of the fatty acid chain of the phospholipids may also play a role in the activation of sphingosine kinase.

Mechanism of inhibition by arachidonic acid of the catalytic activity of Ras GTPase-activating proteins

Ras is a guanine nucleotide-binding protein that acts as a molecular switch controlling cell growth. The Ras GTPase-activating proteins (GAPs) p120-GAP and neurofibromin are candidates as Ras effectors. The GTPase-activating activity of both proteins is inhibited by mitogenic lipids, such as arachidonic acid and phosphatidic acid, and differential inhibition of the two GAPs led to the hypothesis that both were effectors in a Ras-controlled mitogenic pathway (Bollag, G., and McCormick, F. (1991) Nature 351, 576-579). We have studied the mechanism of inhibition by arachidonic acid in three ways: first, by measurements of catalytic activity under multiple turnover conditions; second, using p-((6-phenyl)-1,3,5-hexatrienyl)benzoic acid as a fluorescent probe for ligands binding to GAPs; and third, by using a scintillation proximity assay to measure direct binding of Ras to neurofibromin. We found no significant differential inhibition between p120-GAP and neurofibromin by arachidonic acid. The inhibition by arachidonic acid included a major component that is competitive with Ras GTP. These data suggest that insomuch as the mitogenic effects of lipids are mediated via inhibition of GAPs, GAPs are not Ras effector proteins. Additionally, lipids can exert a non-competitive type effect, consistent with a protein denaturing activity, making difficult extrapolations from in vitro data to the situation within cells, and possibly explaining the variability of literature data on inhibition by lipids.

A small GTP-binding protein, Rho, associates with the platelet-derived growth factor type-beta receptor upon ligand binding

Ligand binding to the platelet-derived growth factor (PDGF) receptor initiates a complex and diverging cascade of signaling pathways. GTP-binding proteins with intrinsic GTPase activity (G-proteins) frequently link cell surface receptors to intracellular signaling pathways, but no close associations of the PDGF receptor and any small G-proteins, nor any such associations activated by ligand binding to the receptor have been previously reported. We demonstrate that a small GTP-binding protein binds specifically to the murine and human PDGF type-beta receptor. In response to PDGF-BB stimulation, there is an increase in the amount of labeled small G-protein associated with the PDGF type-beta receptor. The GTP-binding protein did not undergo ligand-induced association with a mutant receptor protein that was unable to bind ATP. Proteolytic cleavage analysis, together with two-dimensional separation techniques, identified the small G-protein specifically associating with the PDGF type-beta receptor after ligand binding as a member of the Rho family. This was confirmed by demonstration that the small G-protein coimmunoprecipitated by the anti-PDGF receptor antibody was a substrate for the ADP-ribosyltransferase C3 exoenzyme. Thus, the PDGF type-beta receptor may form a complex with one or more small G-proteins upon binding PDGF-BB, and the Rho small G-protein is likely to be an important component of the proteins making up the multimeric signaling complex of the PDGF type-beta receptor.

Rac mediates growth factor-induced arachidonic acid release

Growth factor-induced stress fiber formation involves signal transduction through Rac and Rho proteins and production of leukotrienes from arachidonic acid metabolism. In exploring the relationship between these pathways, we found that Rac is essential for EGF-induced arachidonic acid production and subsequent generation of leukotrienes and that Rac V12, a constitutively activated mutant of Rac, generates leukotrienes in a growth factor-independent manner. Leukotrienes generated by EGF or Rac V12 are necessary and sufficient for stress fiber formation. Furthermore, leukotriene-dependent stress fiber formation requires Rho proteins. We have therefore identified elements of a pathway from growth factor receptors that includes Rac, arachidonic acid production, arachidonic acid metabolism to leukotrienes, and leukotriene-dependent Rho activation. This appears to be the major pathway by which Rac influences Rho-dependent cytoskeleton rearrangements.

Role of calcium/calmodulin-dependent protein kinase II in the regulation of vascular smooth muscle cell migration

BACKGROUND

The migration of vascular smooth muscle cells (VSMCs) is a key event in the pathogenesis of many vascular diseases. We have previously shown that VSMC migration in response to platelet-derived growth factor (PDGF) is suppressed when cultured cells are growth-arrested and induced to differentiate. The present study was undertaken to elucidate the mechanism of this suppression.

METHODS AND RESULTS

While both proliferating and growth-arrested VSMCs upregulated expression of the immediate early response genes, c-fos and JE (monocyte chemoattractant protein 1), growth-arrested VSMCs exhibited much smaller changes in intracellular calcium in response to PDGF and failed to activate the calcium/calmodulin-dependent protein kinase II (CaM kinase II). Blocking calcium-calmodulin interactions (50 mumol/L W7) or the activation of CaM kinase II (10 mumol/L KN62) in proliferating cells blocked their migration by more than 90%, whereas inhibition of protein kinase C activation had no significant effect on migration. Pretreatment of growth-arrested VSMCs with the calcium ionophore ionomycin resulted in an approximately 2.5-fold activation of CaM kinase II and increased migration of growth-arrested cells to 84 +/- 6% that of proliferating cells. These effects of ionomycin were blocked by inhibitors of CaM kinase II. Constitutively activated (ie, calcium/calmodulin-independent) CaM kinase II introduced by gene transfection into growth-arrested cells significantly increased migration toward PDGF from < 20% to > 70% that of proliferating cells.

CONCLUSIONS

These results demonstrate that activation of CaM kinase II is required for VSMC migration, that its activation in response to PDGF is suppressed in growth-arrested VSMCs, and that this suppression of CaM kinase II activation is responsible, in large part, for the failure of growth-arrested VSMCs to migrate toward PDGF.

ras may mediate mammary cancer promotion by high fat

High fat consumption has been implicated as a risk factor for breast cancer. Experimental mammary carcinogenesis studies have demonstrated that the effect of high fat consumption is mainly exerted on the postinitiation stage of the disease process. We report data that have recalled in the formulation of a new hypothesis about the effect of dietary fat on mammary carcinogenesis, namely, that it promotes the development of a subpopulation of cells lacking a specific pathogenetic characteristic. In comparison with animals fed a low-fat diet, female Sprague-Dawley rats fed high-fat diets during the promotional stage developed significantly more (number and proportion) 1-methyl-1-nitrosourea-induced mammary adenocarcinomas that did not contain a codon 12 GGA-->GAA mutation in the c-Ha-ras protooncogene. The effect was independent of the types of fat fed, i.e., corn oil vs. fish oil. A model is presented to account for the preferential promotional effect of high fat consumption on 1-methyl-1-nitrosourea-initiated mammary epithelial cells. The hypothesis that the level of dietary fat consumed affects the proportion of mammary carcinomas that occur with a particular pathogenetic characteristic, in this case, the presence or absence of a Ha-ras point mutation, has important implications on the direction of future investigations concerning fat and cancer risk.

Signal transduction by Ras-like GTPases: a potential target for anticancer drugs

Members of the ras family of GTPases are involved in a wide variety of cellular processes including cell proliferation, differentiation, apoptosis, and transformation. The ras oncogene is one of the most frequently mutated genes in human cancer. In addition, other oncogene and tumor suppressor gene products are components of the signal transduction pathways in which Ras or other Ras-like GTPases play key regulatory functions. Current progress in the elucidation of these signal transduction pathways will be reviewed and the potential use of these insights for the development of novel therapeutic agents for the treatment of cancer will be discussed.

Phospholipases A2 in ras-transformed and immortalized human mammary epithelial cells

Phospholipase A2 (PLA2) activities of non-tumorigenic and tumorigenic human mammary epithelial cells, 184B5 cells (immortalized cell line from a reduction mammoplasty) and B5KTu cells (cells from a tumor induced by ras-transformed 184B5 cells), are characterized, with emphasis on lipid biomediator-related phospholipases A2. Phospholipases A2 associated with regulation of arachidonic acid metabolism include the high molecular mass cytosolic PLA2 (cPLA2) and group II PLA2. The major PLA2 activity in the mammary epithelial cells has the characteristics of cPLA2; this activity is higher in the B5KTu cells. In contrast, the 184B5 and B5KTu cells have similar levels of a Ca(2+)-independent, cytosolic PLA2 activity and low levels of a particulate fraction PLA2 activity, which does not have the properties of group II PLA2. Thus, cPLA2 activity is selectively elevated in the tumorigenic human mammary epithelial cells and this may result in increased generation of lipid biomediators such as arachidonic acid metabolites.

Membrane ruffling and signal transduction

One of the earliest structural changes observed in cells in response to many extracellular factors is membrane ruffling: the formation of motile cell surface protrusions containing a meshwork of newly polymerized actin filaments. It is becoming clear that actin reorganization is an integral part of early signal transduction pathways, and that many signalling molecules interact with the actin cytoskeleton. The small GTP-binding protein Rac is a key regulator of membrane ruffling, and proteins that can regulate Rac activity, such as Bcr, are likely to act on this signalling pathway. In addition, several previously characterized signal transducing molecules are implicated in the membrane-ruffling response, including Ras, the adaptor protein Grb2, phosphatidyl inositol 3-kinase, phospholipase A2 and phorbol ester-responsive proteins. Changes in polyphosphoinositide metabolism and intracellular Ca2+ levels may also play a role. A number of actin-binding and organizing proteins localize to membrane ruffles and are potential targets for these signal transducing molecules.

Elevated levels and mitogenic activity of lysophosphatidylinositol in k-ras-transformed epithelial cells

In cell lines stably (KiKi) or reversibly (Ts) transformed by the k-ras oncogene originated from a differentiated rat thyroid line (FRTL5 cells), k-ras-induced transformation has been associated with an increased phospholipase A2 activity. Here we provide evidence that this enzymic activity is phosphoinositide specific and leads to the formation of lysophosphatidylinositol. The levels of this lysolipid increased by 2-3-fold in ras-transformed cells (KiKi cells and Ts cells at the permissive temperature of 33 degrees C) as compared to differentiated cells (FRTL5) or to Ts cells maintained at 39 degrees C, i.e. at the temperature where ras-p21, the product of the ras oncogene, is inactive. Since another lysoderivative, lysophosphatidic acid, has been shown to be a mitogen, we have tested whether lysophosphatidylinositol could have a similar activity on thyroid cells. Lysophosphatidylinositol (10-100 microM) induced a 5-10-fold increase in [3H]thymidine incorporation in both FRTL5 and KiKi cells, whereas lysophosphatidic acid was active only in differentiated cells. Lysophosphatidylinositol (approximately 25 microM) and lysophosphatidic acid (50-100 microM) acted synergistically with insulin in increasing [3H]thymidine incorporation. Moreover, lysophosphatidylinositol at concentrations three-fold higher than those found to be mitogenic, inhibited the activity of the GTPase-activating protein. We conclude that lysophosphatidylinositol is a mitogen that might play a role in the modulation of k-ras transformed cell proliferation.

Effect of lysophosphatidic acid on motility, polarisation and metabolic burst of human neutrophils

The effect of lysophosphatidic acid (LPA) on human neutrophil activation was examined by a combination of automated tracking assays, cell shape measurements and assays of the metabolic burst by means of 7-dimethylamino-naphthalene-1,2-dicarbonic acid hydrazide (DNDH)-dependent chemiluminescence. LPA powerfully stimulated polarisation and motility. Polarisation became detectable at 2 microM LPA and virtually 100% of cells were polarised at 20 microM LPA. Cell motility increased with the degree of polarisation, and was diminished at high LPA concentration, but this decrease was reversed by albumin. LPA also inhibited the metabolic burst response to both n-formyl-methionyl-leucyl-phenylalanine (fMLP) and phorobol 12-myristate 13-acetate (PMA). Inhibition of the PMA-induced metabolic burst by LPA was not affected by pertussis toxin, showing that the effect was not mediated by the pertussis toxin-sensitive heterotrimeric G protein, and that inhibition of the PMA-stimulated metabolic burst by LPA could result from a direct action of LPA on the small cytosolic GTP-binding proteins. These results indicate that lysophosphatidic acid production by thrombin-activated platelets could play a significant role in the regulation of the inflammatory response.

A brain serine/threonine protein kinase activated by Cdc42 and Rac1

A new brain serine/threonine protein kinase may be a target for the p21ras-related proteins Cdc42 and Rac1. The kinase sequence is related to that of the yeast protein STE20, implicated in pheromone-response pathways. The kinase complexes specifically with activated (GTP-bound) p21, inhibiting p21 GTPase activity and leading to kinase autophosphorylation and activation. Autophosphorylated kinase has a decreased affinity for Cdc42/Rac, freeing the p21 for further stimulatory activities or downregulation by GTPase-activating proteins. This bimolecular interaction provides a model for studying p21 regulation of mammalian phosphorylation signalling pathways.

Epidermal growth factor-induced actin remodeling is regulated by 5-lipoxygenase and cyclooxygenase products

In a number of cell types, epidermal growth factor (EGF) evokes dramatic morphological changes, cortical actin polymerization, and stress fiber breakdown. The molecular processes by which increased EGF receptor tyrosine kinase activity results in actin reorganization and morphological changes are unresolved. Recently, we demonstrated that arachidonic acid metabolites function in EGF signal transduction. We now report that in A431 cells, HeLa cells, and rat-1 fibroblasts, the EGF-induced cortical actin polymerization is produced by lipoxygenase metabolism, whereas in these cells stress fiber breakdown is mediated by cyclooxygenase metabolites. Also, the EGF-provoked rounding up in A431 cells is dependent on arachidonic acid metabolism. We conclude that leukotrienes and prostaglandins act in concert, as second messengers, to produce morphological effects and actin reorganization, providing a novel mechanism for directing growth factor-induced cytoskeletal changes.

Mechanism of GTP hydrolysis by p21N-ras catalyzed by GAP: studies with a fluorescent GTP analogue

The mechanism of the hydrolysis of GTP by p21N-ras and its activation by the catalytic domain of p120 GTPase activating protein (GAP) have been studied using a combination of chemical and fluorescence measurements with the fluorescent GTP analogue, 2'(3')-O-(N-methylanthraniloyl)GTP (mantGTP). Since the concentration of active p21 is important in these measurements, various assays for both total protein and active p21 were investigated. All assays gave good agreement except the filter binding assay of [3H]-GDP bound to p21, which gave values of 35-40% compared to the other methods. Concentrations of p21 were thus based on the absorbance of the mant-chromophore of the p21-mant-nucleotide complexes. The rate constants of the elementary steps of the p21 intrinsic GTPase activity and the GAP activated activity were similar between GTP and mantGTP. Incubation of a stoichiometric complex of p21.mantGTP results in a biphasic decrease in fluorescence. The second phase occurs with the same rate constant as the cleavage step and is accelerated by GAP. No other steps of the mechanism are affected by GAP. Incubation of a stoichiometric complex of p21.mantGpp[NH]p also results in a biphasic decrease in fluorescence even though cleavage does not occur. This is interpreted that the cleavage step of p21.GTP is preceded by and controlled by an isomerization of the p21.GTP complex. GAP accelerates the rate constant of the second fluorescence phase occurring with p21.mantGpp[NH]p. This result shows that GAP accelerates the proposed isomerization which limits GTP cleavage rather than the cleavage step itself.

Enhancement of phospholipase A2 activation by phosphatidic acid endogenously formed through phospholipase D action in rat peritoneal mast cell

Contribution of phosphatidic acid (PA) generated by activated phospholipase (PL) D to PLA2 activation was studied in rat peritoneal mast cells. Exogenous didecanoyl PA induced arachidonate liberation in the permeabilized cells which was inhibited by p-bromophenacyl bromide. Upon exposure of the cells to ethanol in a high enough concentration to prevent PA formation, A23187-induced arachidonate liberation was suppressed by 50% and the rest was completely inhibited by p-bromophenacyl bromide. In contrast, propranolol, which enhanced PA accumulation, significantly increased the arachidonate liberation. These results suggest that A23187-induced PLA2 activation may be potentiated, at least in part, by PA generated through PLD action.

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.

Crosstalk among multiple signal-activated phospholipases

Transduction of extracellular signals across the plasma membrane often involves activation of several phospholipases that generate multiple, sometimes interconvertible, lipid-derived messengers. Coordination and integration of these signal-activated phospholipases may require crosstalk between both the messengers and target protein constituents of these pathways.

Phospholipase C-beta 1 is a GTPase-activating protein for Gq/11, its physiologic regulator

Purified M1 muscarinic cholinergic receptor and Gq/11 were coreconstituted in lipid vesicles. Addition of purified phospholipase C-beta 1 (PLC-beta 1) further stimulated the receptor-promoted steady-state GTPase activity of Gq/11 up to 20-fold. Stimulation depended upon receptor-mediated GTP-GDP exchange. Addition of PLC-beta 1 caused a rapid burst of hydrolysis of Gq/11-bound GTP that was at least 50-fold faster than in its absence. Thus, PLC-beta 1 stimulates hydrolysis of Gq/11-bound GTP and acts as a GTPase-activating protein (GAP) for its physiologic regulator, Gq/11. GTPase-stimulating activity was specific both for PLC-beta 1 and Gq/11. Such GAP activity by an effector coupled to a trimeric G protein can reconcile slow GTP hydrolysis by pure G proteins in vitro with fast physiologic deactivation of G protein-mediated signaling.

The small GTP-binding protein rho regulates the assembly of focal adhesions and actin stress fibers in response to growth factors

Actin stress fibers are one of the major cytoskeletal structures in fibroblasts and are linked to the plasma membrane at focal adhesions. rho, a ras-related GTP-binding protein, rapidly stimulated stress fiber and focal adhesion formation when microinjected into serum-starved Swiss 3T3 cells. Readdition of serum produced a similar response, detectable within 2 min. This activity was due to a lysophospholipid, most likely lysophosphatidic acid, bound to serum albumin. Other growth factors including PDGF induced actin reorganization initially to form membrane ruffles, and later, after 5 to 10 min, stress fibers. For all growth factors tested the stimulation of focal adhesion and stress fiber assembly was inhibited when endogenous rho function was blocked, whereas membrane ruffling was unaffected. These data imply that rho is essential specifically for the coordinated assembly of focal adhesions and stress fibers induced by growth factors.

Differential regulation of rasGAP and neurofibromatosis gene product activities

The ras-encoded p21ras proteins bind GTP very tightly, but catalyse hydrolysis to GDP very slowly. In humans, two genes encode proteins that stimulate this GTPase activity (GAP, or GTPase-activating proteins), one of relative molecular mass 120,000, referred to as p120-GAP, and another NF1-GAP, which is encoded by the neurofibromatosis type-1 gene. Both GAPs are widely expressed in mammalian tissues. Here we show that although they will both bind oncogenic mutants of p21ras, neither will stimulate their GTPase activity. NF1-GAP binds to the p21ras proteins up to 300 times more efficiently than p120-GAP. The two GAPs are inhibited to different extents by certain lipids: micromolar concentrations of arachidonate, phosphatidate and phosphatidylinositol-4,5-bisphosphate affect only NF1-GAP. This inhibition does not compete with p21ras, and lipid-inactivated NF1-GAP can still bind p21ras. We used the detergent dodecyl maltoside, which inhibits only NF1-GAP, to distinguish between the two activities in cell extracts and found both types present together in several mammalian cell lines. In contrast, GAP activity in extracts of Xenopus oocytes was not affected by dodecyl maltoside. By these criteria, the mammalian cells contain both GAP activities and the oocytes have only p120-like GAP activity. These results indicate that more than one GAP regulates p21ras in the same cell.

Interaction of phospholipids with proteins, peptides and amino acids. New advances 1987-1989

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins, peptides and amino acids. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. The effect of the interaction on the structure and biological activity of the interacting biomolecules is discussed.