Role of stimulation of arachidonic acid release in the proliferative response of 3T3 mouse fibroblasts to platelet-derived growth factor

Activation of NF-kappaB is required for PDGF-B chain to transform NIH3T3 cells

Elucidating the secondary signaling molecules that are necessary for platelet-derived growth factor (PDGF) to stimulate tumor development will be crucial to the understanding and treatment of a variety of cancers. Several lines of evidence have indicated that the transcription factor NF-kappaB plays a central role in transformation induced by Ha-ras and Bcr-abl, but nothing is known concerning its role in transformation by PDGF. Here we demonstrate that transcription from a promoter containing NF-kappaB binding sequences as well as the DNA binding activity of NF-kappaB were increased in PDGF-B-chain-transformed mouse fibroblast cells. Focus formation of PDGF-B-chain-transformed mouse fibroblasts was suppressed by treatment with acetylsalicylic acid (ASA) and salicylic acid, which are known inhibitors of NF-kappaB activation, but other nonsteroidal anti-inflammatory drugs that do not have an effect on NF-kappaB activity did not affect focus formation in these cells. Furthermore, expression of a dominant negative mutant of IkappaBalpha, pMEIkappaBalpha67CJ, and a dominant negative mutant of p65, p65DeltaC, resulted in decreased focus formation and NF-kappaB activity. Therefore, the transcription factor NF-kappaB plays a vital role in PDGF-B chain transformation of mouse fibroblast cells, and the NF-kappaB activity is sensitive to treatment with ASA.

Interaction of substance P with epidermal growth factor and fibroblast growth factor in cyclooxygenase-dependent proliferation of human skin fibroblasts

Substance P (SP), fibroblast growth factor (FGF), and epidermal growth factor (EGF) are mitogens for fibroblasts. EGF acts as a progression factor, whereas FGF and SP have competence factor activity. The ability of eicosanoids to regulate proliferation of fibroblasts and the increased production of prostaglandins by fibroblasts in response to the growth factors, led us to investigate the involvement of cyclooxygenase-dependent arachidonic acid metabolites in the mitogenic response of serum-starved human skin fibroblasts to SP, FGF, and EGF. We tested the interaction of a submaximal concentration of SP(10(-9)M) with baFGF(40 micrograms/ml) and EGF(0.01 microgram/ml) both on fibroblast proliferation and release of arachidonic acid metabolites. A combination of SP and EGF synergistically stimulated fibroblast proliferation and prostaglandin E2 release, whereas addition of SP to FGF-containing cultures did not affect cell growth. Inhibition of cyclooxygenase by acetylsalicylic acid augmented the growth response of fibroblasts to all: SP, FGF, and EGF. In the presence of acetylsalicylic acid, SP combined with FGF enhanced fibroblasts proliferation, whereas a combination with EGF inhibited cellular growth with respect to growth induced by EGF alone. Thus, interactions of SP with FGF and EGF differently affected the mitogenic response depending on the formation of arachidonic acid metabolites. The findings indicate that eicosanoids may be important mediators of competence and progression factor activities that may determine the effects of substance P on fibroblast proliferation in a cytokine network.

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.

The functional interaction of EGF and PDGF with bradykinin in the proliferation of human gingival fibroblasts

Epidermal growth factor (EGF) and platelet-derived growth factor (PDGF)-BB are both involved in periodontal wound healing. Each of these growth factors exerts a positive proliferative effect on cells of the periodontium in vitro. However, in vivo the peptide bradykinin is one of a complex array of mediators present in addition to these growth factors. The purposes of this investigation were to: 1) evaluate bradykinin interactions with EGF and PDGF-BB altering cell proliferation in cultured human gingival fibroblasts (HGF), periodontal ligament cells (HPDL), and cells derived from alveolar bone (HOB); and 2) determine at the signal transduction level the mechanism of interaction between EGF and bradykinin in HGF. EGF and PDGF-BB stimulated DNA synthesis in a concentration-dependent manner, as measured by [3H] thymidine incorporation. Bradykinin alone did not alter significantly based DNA synthesis values; however, bradykinin in combination with EGF reduced DNA synthesis to nearly basal levels and bradykinin in combination with PDGF reduced the DNA synthesis over 50%. Examination of the interactions between bradykinin and EGF signal transduction pathways revealed that PGE2 release was increased in the presence of bradykinin and EGF (167 +/- 33% to 317 +/- 29%). The bradykinin-stimulated PGE2 release was completely abolished by indomethacin. Indomethacin also was found to block the bradykinin inhibition of EGF-induced DNA synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiation by cholesterol and vitamin D3 oxygenated derivatives of arachidonic acid release and prostaglandin E2 synthesis induced by the epidermal growth factor in NRK 49F cells: the role of protein kinase C

We have previously demonstrated that oxysterols and calcitriol potentiate arachidonic acid (AA) release and prostaglandin (PG) synthesis when NRK cells (fibroblastic clone 49F) are activated by foetal calf serum. As serum is essential for a full oxysterol effect, we hypothesized that these compounds could act on one or more of the events triggered by serum growth factor binding to their specific receptors and leading to PLA2 activation; we showed that the oxysterol effect on AA release is synergistic with, but not fully dependent on, protein kinase C (PKC) activity and Ca2+ ion fluxes, suggesting that oxysterols could effect early events in the cell signalling pathway. In the present paper, we investigated the effect of some oxysterols and calcitriol on epidermal growth factor (EGF)-induced AA release and PGE2 synthesis in NRK cells. The clear potentiation of EGF effect by most of the oxygenated sterols--chiefly when polyoxidized--cannot be explained by a modification of EGF high affinity binding site number which was only moderately increased after a 4 h incubation of cells with these compounds, and moreover was not related to the ability of a given oxysterol to increase PLA2 activity; whatever the compound, the dissociation constant (Kd) of either a high or low affinity binding site was unchanged (respectively, 3.5 x 10(-11) M and 4.4 x 10(-10) M). Genistein, a known inhibitor of EGF receptor tyrosine kinase, changed neither the EGF effect on AA release nor its potentiation by oxysterol, whereas it inhibited PGE2 synthesis in both situations. PKC activation by phorbol ester TPA increased the effect of EGF alone as well as the oxysterol potentiating effect, whereas PKC down-regulation strongly decreased both of these effects, showing that both are dependent on PKC activity. Nevertheless staurosporine, a PKC inhibitor, did not reproduce the effects of PKC down-regulation on EGF activation: stimulatory when AA release was induced by EGF alone, inhibitory when AA release is induced by TPA alone, this compound did not modify the oxysterol potentiating effect. In conclusion, the potentiating effect of oxysterols on AA release seems to be exerted downstream to the growth factor receptor (as demonstrated here with EGF) and probably at the PKC level, but not exclusively.

Differential effect of growth- and differentiation-inducing factors on the release of eicosanoids and phospholipids from ML-1 human myeloblastic leukemia cells

In the absence of serum, growth of ML-1 human myeloblastic leukemia cells is induced by the insulin-like growth factor-1 (IGF1) together with transferrin (Tf), whereas monocytic differentiation is initiated by the transforming growth factor-beta (TGF-beta) in combination with Tf. Initiation of growth was followed by the rapid release of arachidonic acid (AA), hydroxyeicosatetraenoic acids (HETEs) and phospholipids into the culture medium. In contrast, induction of differentiation occurred without the release of these lipids beyond the level present in control. Inhibitors of enzymes involved in the formation of AA and of HETEs, including phospholipase A2 and lipoxygenases, caused interference with growth but not with differentiation, and an inhibitor of the cyclooxygenase path affected neither growth nor differentiation. These results indicate that the initiation of ML-1 cell growth but not of cell differentiation is dependent upon the increased formation of AA and its derivatives formed primarily via the lipoxygenase path.

Stimulation of prostaglandin production by hepatocyte growth factor in human gastric carcinoma cells

Hepatocyte growth factor (HGF), a protein with pleiotropic biological activity affecting cell growth and motility, was found to markedly activate prostaglandin production in human gastric carcinoma TMK-1 cells. HPLC analysis revealed that HGF stimulated the production of prostaglandin E2 (PGE2), which is the major prostaglandin produced in these cells. HGF maximally stimulated PGE2 production at a concentration of 10 ng/ml, and it was a more potent stimulator of PGE2 production than epidermal growth factor (EGF), which is known to stimulate prostaglandin production in various cell lines. The simultaneous addition of HGF and EGF caused no further stimulation of the PGE2 production observed in HGF-treated cells. We showed also that HGF increased the arachidonate release from TMK-1 cells, which release was completely suppressed by the addition of phospholipase A2 (PLA2) inhibitors. Further studies in vitro showed that HGF enhanced cellular activities of cytosolic PLA2 and cyclooxygenase 1.5-fold each. These results indicate that HGF stimulates prostaglandin production through increases in both cytosolic PLA2 and cyclooxygenase activities.

Mechanisms of platelet-derived growth factor-induced arachidonic acid release in Swiss 3T3 fibroblasts: the role of a localized increase in free Ca2+ concentration beneath the plasma membrane and the activation of protein kinase C

Stimulation of Swiss 3T3 fibroblasts with platelet-derived growth factor (PDGF) results in a transient increase in intracellular free Ca2+ concentration ([Ca2+]i) and a phospholipase A2 (PLA2)-dependent release of arachidonic acid (AA) of 500% over control values. In the absence of extracellular Ca2+, both the PDGF-induced transient increase in [Ca2+]i and AA release were markedly reduced. Buffering the increase in [Ca2+]i with EGTA, introduced into the cells in the form of EGTA acetoxymethylester (AM), abolished the PDGF-induced transient increase in [Ca2+]i, but potentiated the AA release by at least 2-fold compared to cells without EGTA. The EGTA potentiated PDGF-induced AA release was sensitive to extracellular Ca2+ and inhibited to various degrees by both receptor-mediated as well as voltage-operated Ca2+ channel blockers, suggesting that the release of AA may be tightly coupled to the influx of Ca2+. Activation of protein kinase C (PKC) by the phorbol ester, phorbol 12-myristate 13-acetate (TPA) had little effect in promoting AA release by itself. Down-regulation of PKC in Swiss 3T3 fibroblasts by chronic stimulation with 300 nM TPA for 24 h, markedly inhibited the PDGF-stimulated AA release in both the EGTA-loaded and control cells. In conditions where PDGF-induced AA release was inhibited or potentiated, the production of inositol phosphates was unaffected. Thus, PDGF-induced PLA2 dependent AA release in Swiss 3T3 fibroblast is regulated by both PKC-dependent and -independent mechanisms, and is activated by high concentrations of free Ca2+ in the microenvironment beneath the plasma membrane during Ca2+ influx via plasma-membrane Ca2+ channels, despite buffering by EGTA of [Ca2+]i in the bulk cytoplasm of the cell.

Bradykinin inhibition of EGF- and PDGF-induced DNA synthesis in human fibroblasts

Bradykinin exhibits proliferative influences in several types of cells; however, in the present study, bradykinin did not promote DNA synthesis but actually inhibited the DNA synthesis induced by epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) in human gingival fibroblasts (HGF). This dose-dependent inhibitory effect was a specific intracellular interaction in that increasing concentrations of EGF did not counteract the inhibitory actions of bradykinin when added at 100 nM. The phosphoinositide-calcium signaling cascade is a likely point of interaction for the inhibitory influences of bradykinin; however, no interactions between bradykinin and EGF were observed with the generation of inositol phosphates or intracellular calcium fluxes. The inhibitory influences of bradykinin do not appear to be the result of a transmodulation of the EGF receptor, since EGF-mediated autophosphorylation was not negatively affected by bradykinin. Bradykinin-stimulated prostaglandin E2 (PGE2) release was potentiated by EGF, and, in the presence of indomethacin, the inhibition of the EGF-induced DNA synthesis by bradykinin was minimized. The results presented demonstrate that bradykinin can inhibit EGF- and PDGF-induced DNA synthesis and suggest that PGE2 synthesis is responsible for the observed bradykinin inhibition of EGF-induced DNA synthesis.

Membrane structure, toxins and phospholipase A2 activity

The phospholipid-hydrolyzing enzyme phospholipase A2 (PLA2) (EC 3.1.1.4) exists in several forms which can be located in the cytosol or on cellular membranes. We review briefly cellular regulatory mechanisms involving covalent modification by protein kinase C and the action of Ca2+, cytokines, G proteins and other cellular proteins. The major focus is the role of phospholipid structure on PLA2 activity, including (1) the mechanism of PLA2 action on synthetic phospholipid bilayers, (2) perturbation of synthetic and cellular membranes with lipophilic agents and membrane-interactive peptides and (3) the ability of these agents to activate endogenous PLA2 activity, with emphasis on the venom and plant toxins melittin, cardiotoxin and Pyrularia thionein.

Growth factor and oncogene signalling pathways as targets for rational anticancer drug development

There is a critical need for new targets, in addition to DNA, for anticancer drug development. A recently discovered target is the intracellular signalling pathways that mediate the actions of growth factors and oncogenes on cell proliferation. Two important pathways, the myo-inositol and protein tyrosine kinase signalling pathways are reviewed. Three classes of compounds that modulate myo-inositol signalling are discussed. These are: 1) the D-3-substituted-3-deoxy-myo-inositol analogues that act as antimetabolites of myo-inositol and show selective growth inhibition of some transformed cells; 2) the alkaloid staurosporine that acts as a potent inhibitor of protein kinase C and of platelet-derived growth factor (PDGF) receptor protein tyrosine kinase activity; 3) the ether lipid analogues that block growth factor signalling at several points by acting as inhibitors of protein kinase C, phosphoinositide specific phospholipase C and inositol(1,4,5)trisphosphate-induced Ca2+ release. It is suggested that inhibition of signalling pathways may explain the growth inhibitory effects of these compounds. Other potential signalling target sites for anticancer drug development are discussed.

Activation of signal transduction pathways protects quiescent Balb/c-3T3 fibroblasts against death due to serum deprivation

Platelet-derived growth factor (PDGF), epidermal growth factor (EGF), insulin-like growth factor-1 (IGF-1), and insulin protect density-inhibited murine Balb/c-3T3 fibroblasts against death by distinctive mechanisms. Determination of the cell survival-enhancing activity of growth factors by cell enumeration and neutral red uptake measurement gives equivalent results. PDGF displays a steep dose-response relationship in the 1-5 ng/ml range. The other factors display shallow log-linear relationships in the following ranges: EGF: 0.2-5 ng/ml; IGF-1: 2-80 ng/ml; and insulin: 57-4,500 ng/ml. Agonists that lead to the activation of protein kinase A, including forskolin, 8-bromoadenosine 3':5'-cyclic monophosphate (Br-cAMP) and N6,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate (db-cAMP), markedly increase both short-term (5-h) and long-term (20-h) survival of cells. 2-Isobutyl-1-methylxanthine (IBMX) markedly enhances short-term survival, but its effect decays with time. The protein kinase C agonist 12-O-tetradecanoyl phorbol-13-acetate (TPA) has a moderate protective effect at concentrations of 16-32 nM, and 64 nM TPA is highly effective. The synthetic diaclglycerols 1,2-dioctanoylglycerol (DiC8) and 1-oleoyl-2-acetylglycerol (OAG) and the calcium ionophore ionomycin show low activity. Supplementation of EGF with a protein kinase A or C agonist results in a varying additive increase in short-term (5-h) cell survival and supplementation of EGF + insulin or PDGF + EGF + insulin increases further the already high level of protection given by the growth factor combinations. Combining a protein kinase A and a protein kinase C agonist in the absence of growth factors gives an approximately additive increase in cell survival. Results obtained with kinase, RNA, and protein synthesis inhibitors suggest that: 1) activated protein kinase C catalyzes one or more phosphorylation events in quiescent Balb/c-3T3 cells that lead to gene expression with the protein product(s) mediating protection of quiescent cells against death, and 2) phosphorylation events catalyzed by protein kinase A largely serve to protect cells by a mechanism not requiring de novo RNA and protein biosynthesis.

Multiple signal transduction pathways lead to extracellular ATP-stimulated mitogenesis in mammalian cells: II. A pathway involving arachidonic acid release, prostaglandin synthesis, and cyclic AMP accumulation

We have previously shown that extracellular ATP acts as a mitogen via protein kinase C (PKC)-dependent and independent pathways (Wang, D., Huang, N., Gonzalez, F.A., and Heppel, L.A. Multiple signal transduction pathways lead to extracellular ATP-stimulated mitogenesis in mammalian cells. I. Involvement of protein kinase C-dependent and independent pathways in the mitogenic response of mammalian cells to extracellular ATP. J. Cell. Physiol., 1991). The present aim was to determine if metabolism of arachidonic acid, resulting in prostaglandin E2 (PGE2) synthesis and elevation of cAMP levels, plays a role in mitogenesis mediated by extracellular ATP. Addition of ATP caused a marked enhancement of cyclic AMP accumulation in 3T3, 3T6, and A431 cells. Aminophylline, an antagonist of the adenosine A2 receptor, had no effect on the accumulation of cyclic AMP elicited by ATP, while it inhibited the action of adenosine. The accumulation of cyclic AMP was concentration dependent, which corresponds to the stimulation of DNA synthesis by ATP. The maximal accumulation was achieved after 45 min, with an initial delay period of about 15 min. That the activation of arachidonic acid metabolism contributed to cyclic AMP accumulation and mitogenesis stimulated by ATP in 3T3, 3T6, and A431 cells was supported by the following observations: (a) extracellular ATP stimulated the release of [3H]arachidonic acid and PGE2 into the medium; (b) inhibition of arachidonic acid release by inhibitors of phospholipase A2 blocked PGE2 production, cyclic AMP accumulation, and DNA synthesis activated by ATP, and this inhibition could be reversed by adding exogenous arachidonic acid; (c) cyclooxygenase inhibitors, such as indomethacin and aspirin, diminished the release of PGE2 and blocked cyclic AMP accumulation as well as [3H]thymidine incorporation in response to ATP; (d) PGE2 was able to restore [3H]thymidine incorporation when added together with ATP in the presence of cyclooxygenase inhibitors; (e) pertussis toxin inhibited ATP-stimulated DNA synthesis in a time- and dose-dependent fashion as well as arachidonic acid release and PGE2 formation. Other evidence for involvement of a pertussis toxin-sensitive G protein(s) in ATP-stimulated DNA synthesis as well as in arachidonic acid release is presented. In A431 cells, the enhancement of arachidonic acid and cyclic AMP accumulation by ATP was partially blocked by PKC down-regulation, implying that the activation of PKC may represent an additional pathway in ATP-stimulated metabolism of arachidonic acid. In all of these studies, ADP and AMP-PNP, but not adenosine, were as active as ATP.(ABSTRACT TRUNCATED AT 400 WORDS)

Signal transduction by interferon-alpha through arachidonic acid metabolism

Molecular mechanisms that mediate signal transduction by growth inhibitory cytokines are poorly understood. Type I (alpha and beta) interferons (IFNs) are potent growth inhibitory cytokines whose biological activities depend on induced changes in gene expression. IFN-alpha induced the transient activation of phospholipase A2 in 3T3 fibroblasts and rapid hydrolysis of [3H]arachidonic acid (AA) from prelabeled phospholipid pools. The phospholipase inhibitor, bromophenacyl bromide (BPB), specifically blocked IFN-induced binding of nuclear factors to a conserved, IFN-regulated enhancer element, the interferon-stimulated response element (ISRE). BPB also caused a dose-dependent inhibition of IFN-alpha-induced ISRE-dependent transcription in transient transfection assays. Specific inhibition of AA oxygenation by eicosatetraynoic acid prevented IFN-alpha induction of factor binding to the ISRE. Treatment of intact cells with inhibitors of fatty acid cyclooxygenase or lipoxygenase enzymes resulted in amplification of IFN-alpha-induced ISRE binding and gene expression. Thus, IFN-alpha receptor-coupled AA hydrolysis may function in activation of latent transcription factors by IFN-alpha and provides a system for studying the role of AA metabolism in transduction of growth inhibitory signals.

Oxysterol activation of arachidonic acid release and prostaglandin E2 biosynthesis in NRK 49F cells is partially dependent on protein kinase C activity [corrected]

We previously demonstrated that the oxysterol potentiation of arachidonic acid release and prostaglandin biosynthesis induced by foetal calf serum activation of normal rat kidney (NRK) cells (fibroblastic clone 49F) was not related to a direct effect of oxysterols on cell free Ca2+ level. Since both Ca2+ variations and protein kinase C are involved in arachidonic acid release in some models, we looked for a possible modulation by protein kinase C in the oxysterol effect on arachidonic acid release. We show that when the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA), a protein kinase C activator, was added to the culture medium, the oxysterol effect on arachidonic acid release and prostaglandin synthesis clearly increased. Moreover, the effect of TPA was dose-dependent and TPA EC50 (4 x 10(-9) M) was unchanged in the presence of the oxysterol. Preincubation of cells with TPA for 24 h prevented the arachidonic acid release induced by TPA alone, whereas the oxysterol effect was decreased but not abolished. In the absence of serum, TPA and ionomycin added together induced the same noticeable (arachidonic acid) release and PGE2 synthesis as serum alone. Nevertheless, the potentiating effect of cholest-5-ene-3 beta, 25-diol was much higher when serum itself was used to activate NRK cells than it was in the present serum-mimicking experimental conditions. Thus, the presence of growth factors is probably required to obtain a full oxysterol effect. We conclude that the oxysterol effect was synergistic with, but not fully dependent on, protein kinase C and Ca2+ ion fluxes, therefore oxysterols could affect earlier events triggered by serum growth factor binding to their cell membrane receptors.

Phospholipase activation and arachidonic acid release in intestinal epithelial cells from patients with Crohn's disease

A method for studying the mobilization of free arachidonic acid (AA) in viable isolated human intestinal epithelial cells has been developed and applied to the study of patients with Crohn's disease. Cells were isolated from morphologically unaffected parts of the distal ileum and incubated with 14C-AA; most of the incorporated 14C-AA was then found in phospholipids (mainly phosphatidylcholine) and in a pool of neutral lipids (mainly triacylglycerols). Cells from patients with Crohn's disease incorporated more 14C-AA into their neutral lipids than did cells from control patients. When the labeled cells were stimulated with phospholipase C from Clostridium perfringens or with the calcium ionophore A23187, they released significant amounts of AA, mainly from phosphatidylcholine. There was no difference between cells from Crohn patients and controls in the 14C-AA amounts released, but unstimulated and phospholipase C-stimulated cells from prednisolone-treated Crohn patients released less AA than cells from control patients. The A23187-stimulated AA release was completely inhibited by the phospholipase A2 inhibitor 4-bromophenacyl bromide, whereas the phospholipase C-stimulated release was not. These findings suggest that AA release in human small-intestinal epithelial cells may be caused by calcium-mediated phospholipase A2 activation or by products of microbial phospholipase C activity and that prednisolone reduces the mobilization of free AA in intestinal epithelial cells. They also illustrate the potential use of isolated epithelial cells for revealing mechanisms underlying AA release in the intestinal mucosa in different disease states.

Mechanism of human lymphotoxin and tumor necrosis factor induced destruction of cells in vitro: phospholipase activation and deacylation of specific-membrane phospholipids

The role of phospholipase (PLase) activation and lipid metabolism in lymphotoxin (LT)- and tumor necrosis factor (TNF)-mediated destruction of murine L929 cells was examined. At the levels of LT and TNF employed, cell destruction began at 4-6 h and was 99% complete by 30 h. Cell membrane phospholipids (PL), labelled in situ at the C2 position with 14C arachidonic acid, were analyzed by two-dimensional thin-layer chromatography and quantitated over a 30 h time course after LT or TNF treatment. The ratio of radiolabel incorporation relative to the actual amount of each PL present was determined by inorganic phosphate analysis. Radiolabelled arachidonic acid, eicosanoids, and neutral lipids were released into the medium prior to the onset of cell death (4-6 h) and continued to accumulate linearly throughout the destructive reaction. There was a quantitative relationship between the appearance of radiolabelled metabolites in the media and the loss of radiolabelled cellular PL. Cellular phosphatidylethanolamine was the primary PL deacylated by PLase action, showing a 75% reduction in radiolabel. The PLase inhibitors--quinacrine, hydrocortisone, dexamethasone, and indomethacin--were potent inhibitors of LT- and TNF-mediated cell destruction, suggesting that selective deacylation of specific membrane PL by PLase activation is an important step in the events that lead to LT- and TNF-mediated cellular destruction in vitro.

Inhibitors of lipoxygenase have antiproliferative effects on P815 murine mastocytoma cells

The effect of a variety of inhibitors of prostaglandin synthase, lipoxygenase and phospholipase on the growth of P815 murine mastocytoma cells was examined. Only lipoxygenase inhibitors substantially reduced growth, presumably by inhibiting the production of arachidonic acid metabolites rather than causing arachidonate accumulation since excess arachidonic acid did not reverse growth inhibition. Evidence is presented that production of leukotrienes B4, C4, D4 or E4 was not involved. Other metabolites of arachidonic acid were not excluded. A role for lipoxygenase in growth signal transduction in these and other cells is suggested.

Phospholipase activation in the cytotoxic mechanism of thionin purified from nuts of Pyrularia pubera

Treating NIH3T3 fibroblast cells with Pyrularia pubera thionin (100 micrograms/ml) stimulated release of labelled free fatty acids from phospholipids biosynthetically labelled by incorporation of [3H]arachidonic acid. Since Pyrularia thionin exhibited no detectable phospholipase activity, it was concluded that the release response represented activation of endogenous phospholipases in the cells. The phospholipase activated by Pyrularia thionin (100 micrograms/ml) stimulated the release of 61% of the incorporated [3H]arachidonate in the presence of 1.8 mM extracellular calcium with maximum activation at 90 min following an initial lag period of about 20 min. The release response exhibited little dependence on extracellular calcium at this thionin concentration, but at concentrations 20 micrograms/ml, extracellular calcium appeared to be inhibitory to phospholipase activation. Some characteristics of the fatty acid release response are consistent with activation of a lysosomal phospholipase being part of the cellular response to Pyrularia thionin. Activation of a lysosomal enzyme can occur independently or as a result of coordinate activation of the whole lysosome, which would expose other cellular components of degradative lysosomal enzymes. Consistent with coordinate activation of lysosomal enzymes, Pyrularia thionin also stimulates the production of small, trichloroacetic acid-soluble peptides and nucleic acid fragments from biosynthetically-labelled RNA and proteins in treated cells. It is not clear from the results obtained what role, if any, activation of lysosomal enzymes plays in the overall toxic response to Pyrularia thionin in NIH3T3 cells. However, Pyrularia pubera thionin may represent a useful tool for studying the regulation of lysosomal enzymes and their roles in cells.

Possible role of prostaglandins as negative regulators in growth stimulation by tumor necrosis factor and epidermal growth factor in human fibroblasts

Recombinant tumor necrosis factor (TNF), epidermal growth factor (EGF), and transforming growth factor beta (TGF-beta) stimulated growth of confluent human diploid fibroblasts (FS-4 cells) in the presence of fetal calf serum. TGF-beta synergistically enhanced both the TNF- and EGF-stimulated cell growth, whereas synergism between the mitogenic action of EGF and that of TNF was not observed. When indomethacin or acetylsalicylic acid, an inhibitor of prostaglandin production, was added to FS-4 cells, cell growth stimulated by EGF or TNF was increased, suggesting that prostaglandins induced by these mitogens antagonize their growth stimulatory actions. In contrast, neither indomethacin nor acetylsalicylic acid had a significant effect on the TGF-beta-induced growth of FS-4 cells. Mitogenic responses of indomethacin-treated cells to EGF, TNF, and TGF-beta were similarly suppressed by the addition of exogenous prostaglandin D2 (PGD2). Other prostaglandins such as PGE2 and PGF2 produced less inhibition of the cell growth.

Heat shock stimulates the release of arachidonic acid and the synthesis of prostaglandins and leukotriene B4 in mammalian cells

Heat shock has a profound influence on the metabolism and behavior of eukaryotic cells. We have examined the effects of heat shock on the release from cells of arachidonic acid and its bioactive eicosanoid metabolites, the prostaglandins and leukotrienes. Heat shock (42-45 degrees) increased the rate of arachidonic acid release from human, rat, murine, and hamster cells. Arachidonate accumulation appeared to be due, at least partially, to stimulation of a phospholipase A2 activity by heat shock and was accompanied by the accumulation of lysophosphatidyl-inositol and lysophosphatidylcholine in membranes. Induction of arachidonate release by heat did not appear to be mediated by an increase in cell Ca++. Stimulation of arachidonate release by heat shock in hamster fibroblasts was quantitatively similar to the receptor-mediated effects of alpha thrombin and bradykinin. The effects of heat shock and alpha thrombin on arachidonate release were inhibited by glucocorticoids. Increased arachidonate release in heat-shocked cells was accompanied by the accelerated accumulation of cyclooxygenase products prostaglandin E2 and prostaglandin F2 alpha and by 5-lipoxygenase metabolite leukotriene B4. Elevated concentrations of arachidonic acid and metabolites may be involved in the cytotoxic effects of hyperthermia, in homeostatic responses to heat shock, and in vascular and inflammatory reactions to stress.

Cellular responses to Pyrularia thionin are mediated by Ca2+ influx and phospholipase A2 activation and are inhibited by thionin tyrosine iodination

Pyrularia thionin, isolated from nuts of Pyrularia pubera, is a strongly basic peptide of 47 amino acids. The amino acid sequence and configuration of its four disulfide bonds place this plant peptide, known to be hemolytic, cytotoxic, and neurotoxic, among the thionins. We report and compare several cellular responses mediated by Pyrularia thionin: hemolysis of human erythrocytes, activation of an endogenous phospholipase A2 in Swiss 3T3 cells, cytotoxicity toward HeLa and mouse B16 melanoma cells in culture, viability of rat hepatocytes and lymphocytes measured by trypan blue exclusion, and lethality in mice. Cellular responses related to ion movement include a toxin-mediated influx of Ca2+ into mouse P388 cells measured by Fura-2 fluorescence, depolarization of mouse P388 plasma membrane measured by fluorescence of bis(1,3-diethylthiobarbituric acid)trimethine oxonol (bisoxonol), and depolarization of frog (Rana pipiens) sartorius muscle determined by direct measurement of membrane potential. Graded iodination of Pyrularia thionin leads to a related loss of activity for hemolysis, phospholipase A2 activation, cytotoxicity, and lethality in mice. The mediated Ca2+ influx into and depolarization of P388 cells require Ca2+ in the external medium and are inhibited by 100 microM Ni2+. Depolarization of sartorius muscle by Pyrularia thionin also requires a functional Ca2+ channel, as shown by verapamil inhibition. This muscle depolarization also involves phospholipase A2 activation because dexamethasone and quinacrin, but not indomethacin, protect against depolarization. The IC50 values for viability of rat hepatocytes and splenic lymphocytes measured by trypan blue exclusion were 0.17 and 40 microM, respectively. The general response of cells to Pyrularia thionin involves a membrane alteration leading to depolarization and a channel-mediated influx of Ca2+. There is a related activation of phospholipase A2 that results in loss of membrane integrity, hemolysis in the case of erythrocytes, and eventually cell death. Iodination of Pyrularia thionin leads to a corresponding inhibition of all three cellular responses, which indicates an essential role for tyrosine in either maintenance of peptide structure or interaction of the peptide with cellular membranes.

Growth of epithelium from a preneoplastic mammary outgrowth in response to mammary adipose tissue

We investigated the effects of conditioned media derived from mouse mammary fat pads on the proliferation of CL-S1 cells, an epithelial cell line originally isolated from a preneoplastic mammary outgrowth line. Cell proliferation in vitro in serum-free defined medium was compared to that in this medium conditioned using intact mammary fat pad pieces or isolated fat pad adipocytes. Culture medium was conditioned by incubating the conditioning material in defined culture medium for 24 h at 37 degrees C. Conditioned medium induced CL-S1 proliferation as much as 10- to 20-fold above the minimal levels of growth in control cultures after 13 d of culture. The growth-stimulatory factor(s) had an apparent molecular weight of greater than 10 kDa. This growth-stimulatory activity was both heat and trypsin stable. Because the role of adipose tissue is to store and release lipids, we next tested whether lipids are released during medium conditioning. The lipid composition of the fat pad conditioned medium was characterized using both thin layer and gas liquid chromatography. These lipid analyses indicated that the fat pad pieces released significant amounts of fatty acids and phospholipids into the medium during the conditioning period. The free fatty acid composition included both saturated and unsaturated molecules, and about 80% of the total fatty acids consisted of palmitate, stearate, oleate, and linoleate. These same fatty acids were a structural component of the majority of phospholipid found in the medium. The addition of palmitate or stearate to defined medium had no effect or was inhibitory for CL-S1 proliferation, depending on the concentration used. Defined medium supplemented with oleate, arachidonate, or linoleate induced CL-S1 proliferation, and the inhibitory effects of palmitate and stearate were overcome by addition of oleate and linoleate. These data indicate that both unsaturated and saturated fatty acids are released from intact adipose cells of the mouse mammary fat pad and that fatty acids can influence the growth of preneoplastic mouse mammary epithelium. Thus, unsaturated fatty acids, perhaps in conjunction with other substances released simultaneously, are candidate molecules for the substances that mediate the effect of adipose tissue on growth of epithelium.

Arachidonic acid release and prostaglandin biosynthesis in synchronized rat embryo fibroblasts

Arachidonic acid (AA) release and prostaglandin (PG) biosynthesis were studied in rat embryo fibroblasts (R 129) synchronized by double thymidine-excess block. Whatever the culture medium was (medium 199 supplemented with 10% fetal calf serum (FCS) or 1% FCS plus 0.1% bovine serum albumin (BSA], AA release rapidly increased until the 4th hour of the cell cycle (S phase), remained on a plateau in G2M and G1 phases and did not increase again in the S phase of the following cell cycle. Time course and amplitude of AA release in synchronized cells did not differ from what it was observed after the simple renewal of the culture medium in asynchronous cells. So AA release seemed to be independent of the cell cycle. By contrast, PGE2 and PGF2 alpha biosynthesis clearly increased in the S phase of two consecutive cell cycles, indicating that cyclooxygenase activity and not phospholipase A2 activity vary according to the cell cycle.

cDNA-cloning, sequencing and expression in glucocorticoid-stimulated quiescent Swiss 3T3 fibroblasts of mouse lipocortin I

We isolated and sequenced mouse lipocortin I cDNA clones from a lambda gt10 cDNA library prepared from Swiss 3T3 mRNA. The homology with human lipocortin I at the amino acid level is 86%. When confluent layers of Swiss 3T3 cells were stimulated with 10% fetal calf serum, expression of lipocortin I was strongly stimulated. In parallel, DNA synthesis was induced with a peak at 24 hours after glucocorticoid treatment indicating induction of cell proliferation. In the absence of serum glucocorticoid treatment provoked neither induction of DNA synthesis nor expression of lipocortin I. We conclude that serum contains an unidentified factor, which acts synergistically with glucocorticoids on cell proliferation and lipocortin I expression.

Enhancement of human dental cyst PGI2 formation by platelet derived growth factor and its role in cyst growth and bone resorption

Various prostaglandins, particularly PGE2 and PGI2, appear to play a major role in osteolytic processes. Radiochromatographic studies have demonstrated that 6-oxo-PGF1-alpha is a major product of exogenously added arachidonic acid in human dental cysts. As platelets may also act as inflammatory cells, platelet-derived growth factor might also have a PGI2-stimulating influence in such cysts. Eleven human dental cysts were examined by a radioimmunoassay and bioassay which can show PGI2 synthesis in human dental cysts without addition of PDGF. Incremental PDGF addition caused a highly significant increase in the rate of PGI2 synthesis. PGDF thus stimulates PGI2 synthesis in chronic inflammatory processes in vitro and may thereby elicit or accelerate osteolysis.

Increased prostacyclin and thromboxane A2 biosynthesis in atherosclerosis

It has been proposed that atherosclerotic arteries produce less prostacyclin (PGI2) than nonatherosclerotic arteries do, thereby predisposing arteries to vasospasm and thrombosis in vivo. We reexamined this concept by measuring spontaneous as well as arachidonate-induced PGI2 biosynthesis in aortic segments from nonatherosclerotic and cholesterol-fed atherosclerotic New Zealand White rabbits. Thromboxane A2 (TXA2) generation was also measured. Formation of PGI2, as well as TXA2, as measured by radioimmunoassay (RIA) of their metabolites, was increased in atherosclerotic aortic segments relative to nonatherosclerotic segments (P less than or equal to 0.05) at 0, 5, 10, 15, and 30 min of incubation with arachidonate. Pretreatment of arterial segments with indomethacin inhibited PGI2 as well as TXA2 formation, whereas pretreatment with the selective TXA2 inhibitor OKY-046 inhibited only TXA2 release, thus confirming the identity of icosanoids. To confirm the RIA data, aortic segments were incubated with [14C]arachidonate prior to stimulation with unlabeled arachidonate. The uptake of arachidonate was similar, but the release of incorporated [14C]arachidonate was significantly (P less than or equal to 0.05) greater in atherosclerotic segments than in nonatherosclerotic ones. Conversions of released [14C]arachidonate to 6-keto[14C]prostaglandin F1 alpha and [14C]thromboxane B2 were similar in the two types of aortic segments. Thus, synthesis of PGI2 as well as TXA2 is increased in atherosclerosis, and this alteration in arachidonate metabolism is related to increased release of arachidonate.

Linoleic acid is a potent activator of protein kinase C type III-alpha isoform in pancreatic acinar cells; its role in amylase secretion

Linoleic acid, an unsaturated-long chain fatty acid, was found to maximally activate protein kinase C (PKC) more effectively than arachidonic or linolenic acid, while the saturated fatty acids palmitic or arachidic had no stimulatory effect. Treatment of intact pancreatic acinar cells with linoleic acid resulted in dose-dependent phosphorylation of endogenous substrate proteins for this kinase and simultaneously stimulated amylase secretion in a dose- and time-dependent fashion. During chromatographic separation of pancreas protein kinase C activity, utilizing hydroxylapatite (HTP), Type III-alpha PKC isoform was detected. These data are consistent with a role for PKC in the regulation of pancreatic exocrine secretion.

Localization of phospholipase A2 in normal and ras-transformed cells

The cellular localization of phospholipase A2 (PLA2) was examined in normal and ras-transformed rat fibroblasts using immunohistochemical techniques. Polyclonal antibodies were generated against porcine pancreatic PLA2 and were affinity purified for use in this study. The antibodies detected a 16-kD band on immunoblots of total cellular proteins from fibroblasts. In cell-free assays of phospholipase A2 activity, the purified antibodies inhibited the bulk of the enzyme activity whereas control IgG preparations had no effect. Immunofluorescence microscopy indicated that PLA2 was diffusely distributed throughout the cell. Increased concentration of PLA2 was detected under membrane ruffles in normal and ras-transformed cells. Specific immunofluorescence staining was also detected on the outer surface of the normal cells. Immunoelectron microscopy demonstrated the increased accumulation of PLA2 in membrane ruffles and also revealed the presence of the enzyme in microvilli and its association with intracellular vesicles. Ultrastructural localization of PLA2 and the ras oncogene protein, using a double immunogold labeling technique, indicated a spatial proximity between PLA2 and ras proteins in the ruffles of ras-transformed cells. The possible role of PLA2 in the structural rearrangements that underlie membrane ruffling is discussed.

Role of stress in the initial injury stages of cell killing by altered intracellular calcium

A variety of cultured cell types are effectively killed by Ca2+-carrying ionophores (A23187, ionomycin and lasalocid) in the presence of adequate extracellular concentrations of Ca2+ and Na+, although cell killing mechanisms independent of these ions also exist. Previous studies identified 2 injury stages (termed stage I and stage II injury) at which the A23187-induced killing process in 3T3 fibroblasts is interrupted in the presence of low extracellular Ca2+ concentrations and in the absence of extracellular Na+, respectively. The present studies confirm the generality of stage II injury in Ca2+-mediated cell killing, but demonstrate a requirement for concomitant stress conditions (e.g., osmotic or oxidative stress) for expression of stage I injury.

The action of erythropoietin is mediated by lipoxygenase metabolites in murine fetal liver cells

Erythroid progenitor cells synthesize 12-hydroxyeicosatetraenoic acid (12-HETE) and 15-hydroxyeicosatetraenoic acid (15-HETE) when stimulated by erythropoietin (Ep). Maximal stimulation of 12-HETE production occurred at one hour, whereas 15-HETE activity remained constant in response to Ep for 24 hours. Lipoxygenase-selective inhibitors of arachidonic acid metabolism blocked HETE production and Ep-stimulated growth and differentiation of erythroid progenitor cell-derived colonies (CFU-E). On the other hand, specific inhibitors of cyclooxygenase (aspirin and meclofenamate) did not significantly inhibit Ep-induced erythroid colony formation. It is hypothesized that the stimulation of HETE production from arachidonic acid (AA) is an essential step in the mechanism of action of Ep.

Role of platelets and thrombosis in mechanisms of acute occlusion and restenosis after angioplasty

The vascular disruption produced by angioplasty initiates platelet deposition through the processes of platelet adhesion and recruitment of circulating platelets to form an enlarging mural platelet thrombus. Thrombin produced by simultaneous activation of the coagulation cascade by subendothelial connective tissue structures enhances platelet deposition and stabilizes the forming thrombus with enmeshing fibrin. Platelet recruitment involves the expression of the glycoprotein IIb/IIIa receptor for fibrinogen and other cytoadhesive proteins including fibronectin, thrombospondin and von Willebrand factor. Platelet deposition and thrombus formation caused by angioplasty appear to be important in the development of 2 complications: acute thrombotic occlusion and restenosis. Experimental mechanical vascular injury produces a predictable, although rather variable, amount of vascular narrowing due to transient smooth muscle cell proliferative intimal lesion formation. This intimal thickening by proliferating smooth muscle cells is in part mediated by platelet mitogens, particularly platelet-derived growth factor, which are released into the damaged vessel from platelets at the time of angioplasty. Platelet-derived growth factor may also be released from other associated vascular and blood cells in response to mechanical injury, e.g., endothelium, monocyte/macrophage and smooth muscle cells themselves. The actual mitogens, and their cells of origin, that mediate restenosis after therapeutic angioplasty remain to be established. Various oral antiplatelet agents have been shown to reduce arterial thrombotic occlusion in a number of controlled clinical trials, e.g., aspirin in transient ischemic attacks and unstable angina, aspirin and dipyridamole in saphenous vein coronary artery bypass and progression of peripheral vascular disease and dipyridamole in artificial heart valves. Acute arterial thrombosis may require more potent, immediate and transient intervention, e.g., monoclonal antibody to platelet receptor expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Opposing effects of a ras oncogene on growth factor-stimulated phosphoinositide hydrolysis: desensitization to platelet-derived growth factor and enhanced sensitivity to bradykinin

Expression of a transforming Harvey or Kirsten ras gene caused opposing effects in the ability of platelet-derived growth factor (PDGF) and bradykinin to activate phospholipase C-mediated phosphoinositide hydrolysis. In [3H]inositol-labeled rat-1 fibroblasts, PDGF (5 ng/ml) resulted in a 2-fold increase in the level of [3H]inositol trisphosphate (InsP3) after 2 min and, in the presence of LiCl, a 3- to 8-fold increase in the level of [3H]inositol monophosphate (InsP1) after 30 min. However, in EJ-ras-transfected rat-1 cells, which exhibit near normal levels of PDGF receptors, PDGF resulted in little or no accumulation of either [3H]InsP3 or [3H]InsP1. Similarly, marked stimulations by PDGF were observed in NIH 3T3 cells, as well as in v-src-transformed 3T3 cells, but not in 3T3 cells transformed by Kirsten sarcoma virus or by transfection with v-Ha-ras DNA. This diminished phosphoinositide response in ras-transformed cells was associated with a markedly attenuated mitogenic response to PDGF. On the other hand, both phosphoinositide metabolism and DNA synthesis in ras-transformed fibroblasts were stimulated several-fold by serum. In NIH 3T3 cells carrying a glucocorticoid-inducible v-Ha-ras gene, a close correlation was found between the expression of p21ras and the loss of PDGF-stimulated [3H]InsP1 accumulation. In contrast to this ras-induced desensitization to PDGF, ras-transformed NIH 3T3 cells exhibited an enhanced sensitivity to bradykinin; this effect was associated with an elevated level of high-affinity [3H]bradykinin binding. We propose that a ras gene product (p21) can, directly or indirectly, influence growth factor-stimulated phosphoinositide hydrolysis, as well as DNA synthesis, via alterations in the properties of specific growth factor receptors.

Relationship of thrombin-stimulated arachidonic acid release and metabolism to mitogenesis and phosphatidylinositol synthesis

Thrombin and certain prostaglandins are both capable of stimulating the proliferation of cultured cells. Since thrombin stimulates the release and metabolism of arachidonic acid, the precursor of prostaglandins, we examined the relationship between this release and metabolism and the stimulation of cell division in cultured fibroblasts. We also examined the role of prostaglandin synthesis in thrombin-stimulated phosphatidylinositol synthesis. The data in this report demonstrate that the release and metabolism of arachidonic acid are not necessary for thrombin-stimulated cell division. The presence of a low concentration of chymotrypsin prevented thrombin-stimulated arachidonic acid release and metabolism without affecting the stimulation of cell division. Furthermore, thrombin-stimulated cell division occurred in the presence of indomethacin concentrations that prevented cyclooxygenase-mediated metabolism of arachidonic acid. The following experiments showed that thrombin-stimulated phosphatidylinositol synthesis was brought about by a cyclooxygenase-mediated metabolite(s) of arachidonic acid. Indomethacin inhibited the cyclooxygenase-mediated metabolism of arachidonic acid without affecting the thrombin-stimulated release of arachidonic acid. Indomethacin also inhibited thrombin-stimulated phosphatidylinositol synthesis. The dose dependence of this inhibition paralleled the inhibition by indomethacin of cyclooxygenase-mediated metabolism of arachidonic acid. In addition, prostaglandin F2 alpha stimulated phosphatidylinositol synthesis in the presence of indomethacin concentrations which prevented thrombin-stimulated phosphatidylinositol synthesis.

Early signals in the mitogenic response

Polypeptide growth factors, regulatory peptides, and a variety of pharmacological agents acting alone or synergistically induce mitogenesis in cultured fibroblasts. The early signals in the membrane, cytosol, and nucleus promoted by these extracellular factors, together with their mitogenic effectiveness, are integrated in a unified hypothesis for the regulation of fibroblast growth.

Induction of membrane ruffling and fluid-phase pinocytosis in quiescent fibroblasts by ras proteins

Expression of the ras oncogene is thought to be one of the contributing events in the initiation of certain types of human cancer. To determine the cellular activities that are directly triggered by ras proteins, the early consequences of microinjection of the human H-ras proteins into quiescent rat embryo fibroblasts were investigated. Within 30 minutes to 1 hour after injection, cells show a marked increase in surface ruffles and fluid-phase pinocytosis. The rapid enhancement of membrane ruffling and pinocytosis is induced by both the proto-oncogenic and the oncogenic forms of the H-ras protein. The effects produced by the oncogenic protein persist for more than 15 hours after injection, whereas the effects of the proto-oncogenic protein are short-lived, being restricted to a 3-hour interval after injection. The stimulatory effect of the ras oncogene protein on ruffling and pinocytosis is dependent on the amount of injected protein and is accompanied by an apparent stimulation of phospholipase A2 activity. These rapid changes in cell membrane activities induced by ras proteins may represent primary events in the mechanism of action of ras proteins.

Stimulation of aortic smooth muscle cell mitogenesis by serotonin

Bovine aortic smooth muscle cells in vitro responded to 1 nM to 10 microM serotonin with increased incorporation of [3H]thymidine into DNA. The mitogenic effect of serotonin was half-maximal at 80 nM and maximal above 1 microM. At a concentration of 1 microM, serotonin stimulated smooth muscle cell mitogenesis to the same extent as human platelet-derived growth factor (PDGF) at 12 ng/ml. Tryptamine was approximately 1/10th as potent as serotonin as a mitogen for smooth muscle cells. Other indoles that are structurally related to serotonin (D- and L-tryptophan, 5-hydroxy-L-tryptophan, N-acetyl-5-hydroxytryptamine, melatonin, 5-hydroxyindoleacetic acid, and 5-hydroxytryptophol) and quipazine were inactive. The stimulatory effect of serotonin on smooth muscle cell DNA synthesis required prolonged (20-24 hr) exposure to the agonist and was attenuated in the presence of serotonin D receptor antagonists. When smooth muscle cells were incubated with submaximal concentrations of serotonin and PDGF, synergistic rather than additive mitogenic responses were observed. These data indicate that serotonin has a significant mitogenic effect on smooth muscle cells in vitro, which appears to be mediated by specific plasma membrane receptors.

Early changes in inositol lipids and their metabolites induced by platelet-derived growth factor in quiescent Swiss mouse 3T3 cells

Inositol lipid turnover was studied in quiescent Swiss mouse 3T3 cells stimulated by platelet-derived growth factor (PDGF). Stimulation of the cells by PDGF for 10 min at 37 degrees C induced the following changes in lipids: in cells prelabelled with [32P]Pi, a 28% decrease in [32P]phosphatidylinositol 4,5-bisphosphate, a 41% decrease in [32P]phosphatidylinositol 4-phosphate and a 1.7-fold increase in the 32P-labelling of phosphatidic acid; in cells prelabelled with [3H8]arachidonic acid, a 17.9-fold increase in [3H]phosphatidic acid, a 20% decrease in [3H]phosphatidylinositol (PtdIns), an 8.6-fold increase in [3H]arachidonic acid released into the medium, a 57-fold increase in [3H]prostaglandin E2 in the medium, and a 5.3-fold increase in [3H]monoacylglycerol released into the medium (the last was identified as the 2-acyl derivative); in cells prelabelled with [2-3H]glycerol, a 1.7-fold increase in [3H]diacylglycerol, a 6.7-fold increase in [3H]phosphatidic acid, a 1.6-fold increase in [3H]lysophosphatidylcholine (lysoPtdCho), a 9% decrease in [3H]PtdIns, and a 1.6-fold increase in [3H]monoacylglycerol released into the medium. PDGF stimulated the formation of inositol tris-, bis- and mono-phosphates in the cells prelabelled with myo-[2-3H]inositol. These results indicate that, in Swiss 3T3 cells stimulated by PDGF, diacylglycerol produced by the hydrolysis of inositol lipids is partly degraded to 2-acylglycerol and partly converted into phosphatidic acid. The increase in lysoPtdCho indicates that a portion of arachidonic acid released from the stimulated cells is formed by the hydrolysis of PtdCho with a phospholipase A2. Different values of half-maximal doses of the partially purified PDGF used in this study were found for the various responses of quiescent Swiss 3T3 cells to PDGF. The values for half-maximal doses suggest that activation of a fraction of the cell-surface receptor for PDGF is sufficient for mitogenesis and for an increase in the cytoplasmic free Ca2+ concentration, and that the PGDF-stimulated lipid metabolism is probably proportional to the number of receptor sites activated by PDGF.

Phosphoinositides in mitogenesis: neomycin inhibits thrombin-stimulated phosphoinositide turnover and initiation of cell proliferation

Thrombin stimulates 32Pi incorporation into phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bis-phosphate (PIP2), and phosphatidylinositol (PI), and initiates DNA synthesis in hamster (NIL) fibroblasts at a half-maximal concentration of 125 ng/ml. Neomycin, which binds PIP2 and PIP, inhibits both thrombin-stimulated initiation of cell proliferation and 32P pI incorporation into at concentrations above 2 mM without affecting thrombin binding, thymidine uptake, or cellular protein synthesis. At lower concentrations, neomycin inhibits thrombin-stimulated release of inositol 1,4,5-trisphosphate (IP3), by selectively binding PIP2, but does not inhibit 32P incorporation into PI or initiation of DNA synthesis. Phosphoinositide recycling and diacylglycerol release therefore appear necessary for initiation of cell proliferation by thrombin. IP3-stimulated Ca++ mobilization may not be required for thrombin mitogenesis, however, since neomycin can block IP3 release without inhibiting initiation.