Platelet-derived growth factor stimulates a biphasic mobilization of arachidonic acid in Swiss 3T3 cells. The role of phospholipase A2

Identification of c-Src tyrosine kinase substrates in platelet-derived growth factor receptor signaling

c-Src non-receptor tyrosine kinase is an important component of the platelet-derived growth factor (PDGF) receptor signaling pathway. c-Src has been shown to mediate the mitogenic response to PDGF in fibroblasts. However, the exact components of PDGF receptor signaling pathway mediated by c-Src remain unclear. Here, we used stable isotope labeling with amino acids in cell culture (SILAC) coupled with mass spectrometry to identify Src-family kinase substrates involved in PDGF signaling. Using SILAC, we were able to detect changes in tyrosine phosphorylation patterns of 43 potential c-Src kinase substrates in PDGF receptor signaling. This included 23 known c-Src kinase substrates, of which 16 proteins have known roles in PDGF signaling while the remaining 7 proteins have not previously been implicated in PDGF receptor signaling. Importantly, our analysis also led to identification of 20 novel Src-family kinase substrates, of which 5 proteins were previously reported as PDGF receptor signaling pathway intermediates while the remaining 15 proteins represent novel signaling intermediates in PDGF receptor signaling. In validation experiments, we demonstrated that PDGF indeed induced the phosphorylation of a subset of candidate Src-family kinase substrates - Calpain 2, Eps15 and Trim28 - in a c-Src-dependent fashion.

Platelet-derived growth factor reorganizes the actin cytoskeleton through 3-phosphoinositide-dependent and 3-phosphoinositide-independent mechanisms in human mesangial cells

BACKGROUND

Platelet-derived growth factor (PDGF) is a potent activator of mesangial cell proliferation and migration. Although phosphoinositide 3-kinase (PI3K) enzymes are important downstream targets of the PDGF receptor, the contribution made by their 3-phosphoinositide products in the reorganization of actin cytoskeleton and focal adhesions has been questioned.

METHODS AND RESULTS

Pharmacological inhibition of the PI3K activity blocks PDGF-induced migration of human primary mesangial cells using an in vitro scrape wound healing assay. Acute (<10 min) inhibition of the PI3K activity did not alter the effect of PDGF on either stress fibre dissolution or reorganization of focal adhesions. However, at later times (>30 min), PDGF-stimulated responses were inhibited. In contrast, PDGF-stimulated membrane ruffling remained insensitive to PI3K inhibitors throughout. Inhibition of protein kinase C and Erk also attenuated PDGF-stimulated mesangial cell migration; however, neither signaling pathway was responsible for the initial effects on filamentous actin and focal adhesions.

CONCLUSIONS

We propose that following PDGF stimulation of mesangial cells, reorganization of the actin cytoskeleton occurs in a biphasic manner. The mechanism responsible for mesangial cell migration that occurs immediately following PDGF stimulation may serve to 'prime' for the subsequent 3-phosphoinositide-, protein-kinase-C-, and Erk-dependent migration.

Histamine activates a background, arachidonic acid-sensitive K channel in embryonic chick dorsal root ganglion neurons

Histamine has been proposed to be an important modulator of developing neurons, but its mechanism of action remains unclear. In embryonic chick dorsal root ganglion neurons we found that histamine activates, through the pyrilamine-sensitive H1 receptor, a K-selective, background channel. The K channel activated by histamine was also activated by arachidonic acid in a dose-dependent way, with a KD of 4 microM and a slope of 2.5, had a unitary conductance of about 150 pS (symmetrical 140 KCl) and a moderate voltage dependence. The channel was insensitive to the classical K channel blockers tetraethylammonium, charybdotoxin, 4-aminopyridine, but inhibited by millimolar Ba2+. Channel activity could also be increased by lowering the intracellular pH from 7.2 to 5.5, or by applying negative pressure pulses through the patch pipette. Experiments aimed at delineating the metabotropic pathway leading to K channel activation by histamine indicated the involvement of a pertussis toxin-insensitive G protein, and a quinacrine-sensitive cytosolic phospholipase A2. The histamine-induced K channel activation was observed only with elevated internal Ca2+ (achieved using 0.5 microM ionomycin or elevated external KCl). An increase in the histamine-induced phosphoinositide hydrolysis was also observed upon internal Ca2+ elevation, showing the presence of a Ca2+ dependent step upstream to inositol 1,4,5-triphosphate production. In view of the functional importance of K conductances during cell differentiation, we propose that histamine activation of this K channel may have a significant role during normal development of embryonic chick neurons.

Cyclooxygenase and cytochrome P-450 pathways induced by fetal calf serum regulate wound closure in 3T6 fibroblast cultures through the effect of prostaglandin E2 and 12 and 20 hydroxyeicosatetraenoic acids

Wound-induced injury of 3T6 fibroblast cultures initiated a repair process stimulated by fetal calf serum (FCS) that restored the integrity of cell cultures. In these experimental conditions, FCS induced arachidonic acid (AA) release and eicosanoid production. Our results show that the inhibition of the cyclooxygenase (COX) and/or cytochrome P-450 pathways significantly decreases the wound closure, whereas that of the lipoxygenase pathway does not modify the wound repair process. Both EP(1) and EP(4) receptors of prostaglandin E(2) (PGE(2)) mediate PGE(2) stimulated 3T6 fibroblast wound closure. Our data suggest that calcium and cAMP are involved in the signaling event induced by PGE(2) during the 3T6 fibroblast wound repair process. On the other hand, we show that ketoconazole, a cytochrome P-450 inhibitor, hinders the wound closure induced by FCS in wounded 3T6 fibroblast cultures. 12 and 20 Hydroxyeicosatetraenoic acids (HETEs), which are key AA metabolites synthesized by cytochrome P-450, partially revert the effects of ketoconazole on the wound repair process. Thus, the COX and cytochrome P-450 pathways of the arachidonate cascade are involved in 3T6 fibroblast wound closure.

The beta1-integrin cytosolic domain optimizes phospholipase A2-mediated arachidonic acid release required for NIH-3T3 cell spreading

Inhibition of PLA2 activity and rescue by addition of exogenous AA was used to demonstrate that AA production is essential for integrin-mediated NIH-3T3 murine cell spreading. Both AA release and cell spreading after attachment to a FN substrate were inhibited by the PLA2 inhibitor mepacrine. AA release was essential for signaling spreading since the inhibition of spreading induced by mepacrine was overcome by exogenous AA. Cells ectopically expressing full-length chicken beta1-integrins both released AA and spread fully on a substrate of anti-chicken beta1-integrin monoclonal antibody, and inhibition of PLA2 by mepacrine suppressed both spreading and AA release. Exogenous AA also reversed this mepacrine-induced inhibition of spreading. The role of the beta1-integrin cytosolic domain in AA release was examined by comparing responses of cells expressing full-length chicken beta1-integrins versus cells expressing a deletion mutant chicken beta1-integrin with a truncated cytosolic domain. Cells expressing a truncated chicken beta1-integrin released significantly less AA and failed to spread on the anti-chicken beta1-integrin antibody substrate. Furthermore, clustering full-length receptors with soluble antibody stimulated greater AA release than clustering of receptors having truncated cytosolic domains. These data suggest the beta1-integrin cytosolic domain is required for optimal PLA2 activation to produce AA necessary for cell spreading.

Ketoprofen S(+) enantiomer inhibits prostaglandin production and cell growth in 3T6 fibroblast cultures

The ketoprofen S(+) enantiomer inhibits with great stereoselectivity both prostaglandin H synthase isoenzymes. Thus, the biological effects of ketoprofen on inflammation are due almost entirely to the S(+) isomer. Here, we report that the S(+) enantiomer, at doses that inhibit prostaglandin synthesis, is effective in reducing DNA synthesis and 3T6 fibroblast growth. Our data suggest that prostaglandins are involved in the control of 3T6 fibroblast growth and that the effect of the ketoprofen S(+) enantiomer on 3T6 proliferation is correlated with its effects on prostaglandin H synthase and prostaglandin production.

Regulation of the cellular expression of secretory and cytosolic phospholipases A2, and cyclooxygenase-2 by peptide growth factors

Secretory group II (sPLA2) and cytosolic (cPLA2) phospholipases A2 and cyclooxygenase-2 (Cox-2) play a pivotal role in release of proinflammatory eicosanoids. Excessive activity of sPLA2 per se can also propagate inflammation. Endogenous control of the above enzymes has not been completely elucidated. We investigated the combined impact of promoting cytokines and inhibitory peptide growth factors on the expression of mRNA of the above enzymes, on protein content and extracellular release of sPLA2 and on PGE2 production in osteoblasts (FRCO). The synthesis and release of sPLA2 were enhanced by about 20-fold by 0.5 ng/ml IL-1beta or by 50 ng/ml of TNFalpha. Coaddition of both cytokines resulted in synergistic 150-fold increase in the release of sPLA2 implying the existence of two paths of induction. IL-1beta and TNFalpha markedly enhanced the transcription of sPLA2 mRNA. Kinetic study showed that IL-1/TNF initiated sPLA2 release after 12 h, reaching maximum at 48 h. IL-1alpha was a weak stimulator of sPLA2 release, whereas IL-6, IL-8, IGF, IFN-gamma, growth hormone, insulin and GM-CSF were not stimulatory. Peptide growth hormones TGFbeta, PDGF-BB, EGF and bFGF markedly inhibited the extracellular release of sPLA2. TGFbeta and PDGF-BB significantly reduced the level of sPLA2 mRNA, thus acting upon transcription whereas EGF and bFGF were not inhibitory, acting rather upon the translational or posttranslational steps. IL-1/TNF and growth factors had no significant effect on cPLA2 mRNA expression. Cox-2 mRNA expression was markedly enhanced by IL-1/TNF and suppressed by all growth factors tested. Cytokines enhanced the extracellular release of PGE2 and further enhancement was induced by growth factors with the exception of TGFbeta. Cycloheximide abolished completely the release of sPLA2 and markedly reduced the release of PGE2 from cytokine-stimulated FRCO, regardless of whether growth factors were present or not. NS-398, a specific inhibitor of Cox-2 abolished almost completely the release of PGE2 from cytokine-stimulated cells, regardless of the presence of growth factors. Thus, different signalling mechanisms are involved in the impact of growth factors on mRNA expression of sPLA2, cPLA2 and Cox-2. The differences between the impact on FRCO sPLA2 and that reported in other cells, imply that endogenous control of arachidonic acid cascade is cell-specific.

Ets-1 is an early response gene activated by ET-1 and PDGF-BB in vascular smooth muscle cells

Ets-1 is a transcription factor that activates expression of matrix-degrading proteinases such as collagenase and stromelysin. To study the control of ets-1 gene expression in rat vascular smooth muscle cells (VSMC), cells were exposed to factors known to regulate VSMC migration and proliferation. Platelet-derived growth factor-BB (PDGF-BB), endothelin-1 (ET-1), and phorbol 12-myristate 13-acetate (PMA) induced a dose-dependent expression of ets-1 mRNA. These effects were abrogated by inhibition of protein kinase C (PKC) by H-7 or chronic PMA treatment. Ets-1 mRNA was superinduced by PDGF-BB and ET-1 in the presence of cycloheximide. The chelation of intracellular Ca2+ by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester and the depletion of endoplasmic reticulum intracellular Ca2+ concentration ([Ca2+]i) by thapsigargin inhibited PDGF-BB- and ET-1-induced ets-1 mRNA, whereas ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid had no effect. However, [Ca2+]i release alone was not sufficient to increase ets-1 mRNA. Forskolin blocked ET-1-, PDGF-BB-, and PMA-induced ets-1 mRNA, as well as inositol phosphate formation, consistent with an effect through impairment of PKC activation. Inhibitors of ets-1 gene expression, such as H-7 and herbimycin A, inhibited the ET-1 induction of collagenase I mRNA. We propose that ets-1 may be an important element in the orchestration of matrix proteinase expression and of vascular remodeling after arterial injury.

Vascular endothelial growth factor stimulates prostacyclin production and activation of cytosolic phospholipase A2 in endothelial cells via p42/p44 mitogen-activated protein kinase

Vascular endothelial growth factor (VEGF) stimulated a time- and concentration-dependent increase in PGI2 synthesis in human umbilical vein endothelial cells with a mean maximum increase of 2-fold above basal levels at 25 ng/ml after 60 min. VEGF also rapidly stimulated the release of arachidonic acid and phosphorylation and activation of cytosolic phospholipase A2 (cPLA2). The VEGF-related factor, placenta growth factor (PIGF), had little effect on PGI2 synthesis, arachidonic acid release or cPLA2 activation. PD98059, a selective inhibitor of MAP kinase kinase, caused complete inhibition of VEGF-stimulated MAP kinase activity, PGI2 synthesis and cPLA2 gel retardation, but had no effect on VEGF-induced vWF secretion. These findings provide the first evidence that VEGF can stimulate PGI2 synthesis via cPLA2-mediated arachidonic acid release and indicate that VEGF stimulation of this biosynthetic pathway may occur, at least in part, via activation of p42/p44 MAP kinases.

Aspirin enhances platelet-derived growth factor-induced vascular smooth muscle cell proliferation

PURPOSE

Aspirin is frequently used after vascular reconstruction to pharmacologically prevent graft occlusion and to suppress the development of myointimal hyperplasia in vascular surgery, but its efficacy is controversial. The purpose of this study was to examine the direct effects of aspirin on platelet-derived growth factor (PDGF)-induced vascular smooth muscle cell (SMC) proliferation.

METHODS

Human aortic SMCs were grown to confluence in 96 well plates. 3 x 10(-5) mol/L aspirin was added 24 hours previously and PDGF 10 ng/ml at the beginning of each experiment. Cell proliferation at 48 hours was determined using tritiated thymidine uptake. Supernatant 12-L-hydroxy 5,8,10,14-eicosatetraenoic acid (12-HETE) and prostaglandin E2 (PGE2) were measured by competitive enzyme immunoassay.

RESULTS

Aspirin did not change vascular SMC proliferation rates relative to controls (4665 +/- 181 counts per minute [CPM] vs 4749 +/- 155 CPM). However, aspirin pretreatment of PDGF-stimulated vascular SMCs increased proliferation (9408 +/- 237 CPM vs 7283 +/- 283 CPM; p < 0.001). 5,8,10,14-eicosatriynoic acid, a 12-lipoxygenase inhibitor, decreased basal (2037 +/- 181 CPM vs 2306 +/- 158 CPM; p < 0.05) and PDGF-stimulated vascular SMC proliferation (4909 +/- 1089 CPM vs 4310 +/- 1022 CPM; p < 0.001). Aspirin increased supernatant 12-HETE levels and decreased PGE2 levels in both basal and PDGF-stimulated cell cultures.

CONCLUSIONS

Aspirin enhances PDGF-stimulated vascular SMC proliferation. The effects of aspirin on vascular SMC proliferation may be mediated by changes in vascular SMC arachidonic acid metabolism.

Functional dissociation between glucocorticoid-induced decrease in arachidonic acid release and inhibition of adrenocorticotropic hormone secretion in AtT-20 corticotrophs

The biochemical basis of the short-term inhibitory effects of glucocorticoids on corticotropin release from pituitary corticotrophs is still obscure. A well-characterized effect of glucocorticoids in several cell types is the inhibition of arachidonic acid (AA) generation by phospholipase A2 (PLA2). Arachidonic acid and its metabolites have been implicated in the secretory process from a number of pituitary cells, such as the corticotrophs. We have thus examined the role of AA in the anti-secretagogue effects of glucocorticoids in a corticotropin-secreting clonal corticotroph line (AtT-20 D16/16). Glucocorticoids decreased AA release induced by melittin, a bee venom protein related to extracellular PLA2. When a possible role of AA in corticotropin release was studied, the following results were obtained: (a) all corticotropin secretagogues tested, including corticotropin-releasing factor (CRF), did not alter AA generation; (b) calcium and guanine nucleotides, which stimulate corticotropin release in permeabilized cells, inhibited the release of AA under the same conditions; (c) administration of melittin or of exogenous AA had no effect on basal and CRF-stimulated corticotropin release; (d) administration of large amounts of exogenous AA was unable to restore the ability to secrete corticotropin under suppression by glucocorticoids. Altogether, the data suggest that whereas glucocorticoids can inhibit both AA generation and corticotropin release, these two effects appear to be causally unrelated.

Platelet-derived growth factor BB mediated regulation of 12-lipoxygenase in porcine aortic smooth muscle cells

Platelet-derived growth factor BB (PDGF) is a potent mitogen and chemoattractant for vascular smooth muscle cells (VSMC). In the present study, we have examined the effect of PDGF on the 12-lipoxygenase (12-LO) pathway of arachidonate metabolism in porcine aortic VSMC (PVSMC). The rationale for this is previous studies showing that LO products have growth and chemotactic effects in VSMC and that another VSMC growth factor, angiotensin II, is a potent positive regulator of 12-LO activity and expression. We observed that PDGF causes a significant increase in the formation of the 12-LO product, 12-hydroxyeicosatetraenoic acid (12-HETE) in PVSMC. In addition, PDGF also markedly increased leukocyte-type 12-LO messenger RNA and protein expression. PDGF-induced PVSMC migration was inhibited significantly by two LO blockers but not by a cyclooxygenase blocker. Furthermore, although the proliferative effects of PDGF on PVSMC were not altered by cell culture under hyperglycemic conditions (25 mM glucose, HG), the chemotactic effects of PDGF as well as those of 10% fetal calf serum were significantly greater in cells cultured in HG as compared to normal glucose conditions (5.5 mM), thus indicating a potential new mechanism for the accelerated cardiovascular disease usually observed in diabetes. These results indicate a novel mechanism for the biological effects of PDGF in leading to cardiovascular disease.

Distinct roles in signal transduction for each of the phospholipase A2 enzymes present in P388D1 macrophages

Receptor-stimulated arachidonic acid (AA) mobilization in P388D1 macrophages consists of a transient phase in which AA accumulates in the cell and a sustained phase in which AA accumulates in the incubation medium. We have shown previously that a secretory group II phospholipase A2 (sPLA2) is the enzyme responsible for most of the AA released to the incubation medium. By using selective inhibitors for each of the PLA2s present in P388D1 macrophages, we demonstrate herein that the cytosolic group IV PLA2 (cPLA2) mediates accumulation of cell-associated AA during the early steps of P388D1 cell activation. The contribution of both cPLA2 and sPLA2 to AA release can be distinguished on the basis of the different spatial and temporal characteristics of activation and substrate preferences of the two phospholipase A2s (PLA2s). Furthermore, the results suggest the possibility that a functionally active cPLA2 may be necessary for sPLA2 to act. cPLA2 action precedes that of sPLA2, and overcoming cPLA2 inhibition by artificially increasing intracellular free AA levels restores extracellular AA release. Although this suggests cross-talk between cPLA2 and sPLA2, selective inhibition of one other PLA2 present in these cells, namely the Ca2+-independent PLA2, does not block, but instead enhances receptor-coupled AA release. These data indicate that Ca2+-independent PLA2 does not mediate AA mobilization in P388D1 macrophages. Collectively, the results of this work suggest that each of the PLA2s present in P388D1 macrophages serves a distinct role in cell activation and signal transduction.

Involvement of an arachidonic-acid-dependent pathway in the interferon-beta-mediated expression of C202 gene in Ehrlich-ascites-tumor cells

We have investigated the signal transduction mechanism of the expression of the C202 gene mediated by interferon beta (IFN-beta) in the murine Ehrlich's ascites tumor cell line. We have shown that treatment of cells with IFN-beta transiently enhances within minutes the release of free arachidonic acid through membrane phospholipase activity. Furthermore, prior treatment with either p-bromophenacyl bromide, an antagonist of both cytosolic and secretory phospholipase A2, or neomycin, which blocks phospholipase C activity, significantly decreased the activation of the murine IFN-beta-inducible gene, C202. Moreover, an increase of the expression of the C202 gene was observed after blocking of both the cyclooxygenase and lipoxygenase pathways. This suggests that further metabolism of arachidonic acid to epoxides via epoxygenase-catalysed pathways may be a mechanism by which second messengers for IFN-beta-mediated effects on C202 gene expression are generated. Taken together, these results indicate that lipids as second messengers may be important mediators in the IFN-beta-based activation of C202 gene expression.

Thioesterase and protein deacylase activities of porcine pancreatic phospholipase A2

The thioesterase activity of porcine pancreatic phospholipase A2 has been investigated with non-phospholipid substrates. The acyl-CoA hydrolase activity towards acyl-CoA derivatives is specific for long chain fatty acids (14 C, 16 C) but is unable to hydrolyze short chain acyl-CoA compounds (below 8 C). The same enzyme also shows protein deacylase activity liberating [3H]palmitic acid from [3H]palmitoyl-acyl carrier protein.

Platelet-derived growth factor stimulates protein kinase A through a mitogen-activated protein kinase-dependent pathway in human arterial smooth muscle cells

The abilities of platelet-derived growth factor (PDGF) and insulin-like growth factor (IGF-I) to regulate cAMP metabolism and mitogen-activated protein kinase (MAP kinase) activity were compared in human arterial smooth muscle cells (hSMC). PDGF-BB stimulated cAMP accumulation up to 150-fold in a concentration-dependent manner (EC50 approximately 0.7 nM). The peak of cAMP formation and cAMP-dependent protein kinase (PKA) activity occurred approximately 5 min after the addition of PDGF and rapidly declined thereafter. Incubating cells with PDGF and 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor) enhanced the accumulation of cAMP and PKA activity by an additional 2.5-3-fold, whereas IBMX alone was essentially without effect. The PDGF-stimulated increase in cAMP was prevented by addition of the cyclooxygenase inhibitor indomethacin, consistent with release of prostaglandins stimulating cAMP. PDGF, but not IGF-I, stimulated MAPK activity, cytosolic phospholipase A2 (cPLA2) phosphorylation, and cAMP synthesis which indicated a key role for MAP kinase in the activation of cPLA2. Further, PDGF stimulated the rapid release of arachidonic acid and synthesis of prostaglandin E2 (PGE2) which could be inhibited by a cPLA2 inhibitor (AACOCF3). Calcium mobilization was required for PDGF-induced arachidonic acid release and PGE2 synthesis but not for MAPK activation, whereas PKC was required for PGE2-mediated activation of PKA. In summary, these results demonstrated that PDGF increases cAMP formation and PKA activity through a MAP kinase-mediated activation of cPLA2, arachidonic acid release, and PGE2 synthesis in human arterial smooth muscle cells.

Calcium-mediated translocation of cytosolic phospholipase A2 to the nuclear envelope and endoplasmic reticulum

Cytosolic phospholipase A2 (cPLA2) is activated by a wide variety of stimuli to release arachidonic acid, the precursor of the potent inflammatory mediators prostaglandin and leukotriene. Specifically, cPLA2 releases arachidonic acid in response to agents that increase intracellular Ca2+. In vitro data have suggested that these agents induce a translocation of cPLA2 from the cytosol to the cell membrane, where its substrate is localized. Here, we use immunofluorescence to visualize the translocation of cPLA2 to distinct cellular membranes. In Chinese hamster ovary cells that stably overexpress cPLA2, this enzyme translocates to the nuclear envelope upon stimulation with the calcium ionophore A23187. The pattern of staining observed in the cytoplasm suggests that cPLA2 also translocates to the endoplasmic reticulum. We find no evidence for cPLA2 localization to the plasma membrane. Translocation of cPLA2 is dependent on the calcium-dependent phospholipid binding domain, as a calcium-dependent phospholipid binding deletion mutant of cPLA2 (delta CII) fails to translocate in response to Ca2+. In contrast, cPLA2 mutated at Ser-505, the site of mitogen-activated protein kinase phosphorylation, translocates normally. This observation, combined with the observed phosphorylation of delta CII, establishes that translocation and phosphorylation function independently to regulate cPLA2. The effect of these mutations on cPLA2 translocation was confirmed by subcellular fractionation. Each of these mutations abolished the ability of cPLA2 to release arachidonic acid, establishing that cPLA2-mediated arachidonic acid release is strongly dependent on both phosphorylation and translocation. These data help to clarify the mechanisms by which cPLA2 is regulated in intact cells and establish the nuclear envelope and endoplasmic reticulum as primary sites for the liberation of arachidonic acid in the cell.

Arachidonic acid release from NIH 3T3 cells by group-I phospholipase A2: involvement of a receptor-mediated mechanism

Group I pancreatic phospholipase A2 (PLA2 I) is primarily a digestive enzyme. Recently, however, in addition to its catalytic activity a receptor-mediated function has been described for this enzyme. PLA2 I binding to its receptor induces cellular chemokinesis, proliferation, and smooth muscle contraction. This enzyme also induces the production of prostaglandin E2 in certain cells and may have a proinflammatory role. However, despite its ability to hydrolyze phospholipids in in vitro assays, PLA2-I does not efficiently catalyze release of AA from intact cells. Here, we demonstrate that while short-term exposure of NIH 3T3 cells to PLA2-I is ineffective, exposure of 6 h or longer significantly increases the basal release of AA. Dose-response curve of PLA2-I-induced AA release was saturable with an EC50 of 14.01 +/- 1.36 nM (n = 3). [3H]-AA was preferentially released over [3H]-oleic acid by PLA2-I. PLA2-I, inactivated with 4-bromophenacyl bromide, was fully capable of mediating AA release. These data suggest that a non-catalytic, receptor-mediated mechanism is involved in PLA2-I-induced AA release in NIH-3T3 cells. This release of AA is not dependent on protein kinase C or Ca2+ concentration. Comparison of the effect of PLA2-I with those of ATP and platelet-derived growth factor indicates that each of these agonists regulates AA release via independent pathways. Neither the basal enzymatic activity of the 85-kDa cytosolic PLA2 nor the protein level of this enzyme was affected by treatment of cells with PLA2-I. However, the increase in basal enzymatic activity of 85 kDa PLA2 due to protein kinase C activation was further enhanced by pretreatment of cells with PLA2-I. We conclude that: (1) short-term exposure of cells to PLA2 I does not cause measurable AA release; (2) release of AA from intact cells by this enzyme requires long-term exposure; (3) AA release is not mediated by a direct catalytic effect of PLA2 I; and (4) AA release by PLA2 I is accomplished via a receptor-mediated process. Taken together, these results raise the possibility that PLA2 I, in addition to its digestive function, may also contribute to aggravate preexisting inflammatory processes and/or to initiate new ones when chronic exposure of cells to this enzyme occurs.

Cytosolic phospholipase A2

To summarize the regulation of cPLA2, we have proposed a model for the activation of cPLA2 based both on our previous studies (Clark et al., 1991; Lin et al., 1993) and the work of many others (Fig. 5). In this model, cPLA2 is tightly regulated by multiple pathways, including those that control Ca2+ concentration, phosphorylation states and cPLA2 protein levels, to exert both rapid and prolonged effects on cellular processes, such as inflammation. cPLA2 is rapidly activated by increased intracellular Ca2+ concentration and phosphorylation by MAP kinase. When cells are stimulated with a ligand for a receptor, such as ATP or PDGF, PLC is activated via either a G protein-dependent or -independent process, leading to the production of diacylglycerol (DAG) and inositol triphosphate (IP3). The rise in these intracellular messengers cause the activation of PKC and mobilization of intracellular Ca2+. Alternatively, the increase in intracellular Ca2+ can result from a Ca2+ influx. Increased Ca2+ acts through the CaLB domain to cause translocation of cPLA2 from the cytosol to the membrane where its substrate, phospholipid, is localized. This step is essential for the activation of cPLA2 and may account for the partial activation of cPLA2 in the absence of phosphorylation. MAP kinase activation can occur through both PKC-dependent and -independent mechanisms (Cobb et al., 1991; Posada and Cooper, 1992; Qiu and Leslie, 1994). In many cases, this pathway is also G protein-dependent. Activated MAP kinase phosphorylates cPLA2 at Ser-505, causing increased enzymatic activity of cPLA2, which is realized only upon translocation of cPLA2 to the membrane. Therefore, full activation of cPLA2 requires both increased cytosolic Ca2+ and cPLA2 phosphorylation at Ser-505. In a more delayed response, cPLA2 activity in the cells can be controlled by changes in its expression levels, such as in response to inflammatory cytokines and certain growth factors. Thus the expression level of cPLA2 is regulated by both transcriptional and post-transcriptional mechanisms.

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.

Arachidonic acid and free fatty acids as second messengers and the role of protein kinase C

In addition to serving as the precursor to a plethora of eicosanoids and other bioactive molecules, arachidonate may function as a bona fide second messenger. A number of studies have documented the ability of arachidonate to regulate the function of multiple targets in vitro systems. This has been particularly well established and studied with the activation of protein kinase C by arachidonate in a mechanism distinct from activation by diacylglycerol. In cells, arachidonate induces a number of activities, many of which may be independent of further metabolism to eicosanoids; suggesting possible direct action of arachidonate. This review summarizes the current state of knowledge on the possible second messenger function of arachidonate with specific emphasis on the regulation of protein kinase C.

Activation of cytosolic phospholipase A2 by basic fibroblast growth factor via a p42 mitogen-activated protein kinase-dependent phosphorylation pathway in endothelial cells

Basic fibroblast growth factor (FGF) stimulates the proliferation, differentiation, and motility of multiple cell types. Signal transduction by FGF is mediated by high affinity FGF receptors that have autophosphorylating tyrosine kinase activity and also elicit the release of low molecular weight signaling molecules, including inositol 1,4,5-trisphosphate, diacylglycerol, and arachidonate. We have shown previously that basic FGF-stimulated, phospholipase A2 (PLA2)-mediated arachidonate release regulates endothelial cell (EC) motility (Sa, G., and Fox, P.L. (1994) J. Biol. Chem. 269, 3219-3225). Here we identify the phospholipase responsible for basic FGF-mediated arachidonate release as cytosolic PLA2 (cPLA2) by demonstrating in EC lysates a requirement for micromolar Ca2+, dithiothreitol insensitivity, and inactivation by anti-cPLA2 antiserum. The role of cPLA2 is also indicated by the observed mechanisms of activation which show a requirement for p42 mitogen-activated protein kinase activity, cPLA2 phosphorylation, and cPLA2 translocation from cytosol to membranes. Phosphorylation of cPLA2, arachidonate release from prelabeled intact cells, and cell motility all have similar concentration dependencies on basic FGF. Since arachidonate release is required for basic FGF-stimulated motility of EC, our results show that p42 mitogen-activated protein kinase activation of cPLA2 may be a regulatory event in stimulation of cellular release of this important eicosanoid precursor during cellular responses to basic FGF.

Synergistic stimulation of DNA synthesis by bradykinin and vasopressin in Swiss 3T3 cells

Vasopressin and bradykinin bind to receptors coupled to GTP-binding proteins and rapidly induce polyphosphoinositide breakdown leading to Ca2+ mobilization and activation of protein kinase C. Both peptides are known to induce mitogenesis in the presence of growth factors that act through receptors with intrinsic tyrosine kinase activity. Surprisingly, addition of a combination of vasopressin and bradykinin to Swiss 3T3 cells synergistically stimulates DNA synthesis in the absence of any other growth factors. This effect is induced at nanomolar concentrations of the peptides and could be inhibited by addition of specific receptor antagonists or broad spectrum neuropeptide antagonists. Bradykinin, which stimulates transient activation of protein kinase C, induces DNA synthesis in synergy with substances that cause long-term activation of protein kinase C, like vasopressin or phorbol 12,13-dibutyrate. Down-regulation of protein kinase C inhibited the induction of mitogenesis by the combination of vasopressin and bradykinin, thus demonstrating the importance of long-term activation of this enzyme for DNA synthesis. Analysis of tyrosine phosphorylated proteins of M(r) = 110,000-130,000 and M(r) = 70,000-80,000 revealed a biphasic response after stimulation with bradykinin, whereas the response induced by vasopressin declined after the initial maximum. The combination of bradykinin with vasopressin caused an enhanced and prolonged increase in tyrosine phosphorylation of these proteins as compared with the individual peptides. Inhibition of tyrosine phosphorylation by tyrphostin was paralleled by inhibition of DNA synthesis. Together, these results demonstrate synergistic stimulation of DNA synthesis by bradykinin and vasopressin via prolonged stimulation of multiple signaling pathways and imply that the interactive effects of Ca(2+)-mobilizing peptides on mitogenesis may be more general than previously thought.