Differential regulation of phosphoinositide and phosphatidylcholine hydrolysis by protein kinase C-beta 1 overexpression. Effects on stimulation by alpha-thrombin, guanosine 5'-O-(thiotriphosphate), and calcium

Protein kinase Cdelta-mediated phosphorylation of phospholipase D controls integrin-mediated cell spreading

Integrin signaling plays critical roles in cell adhesion, spreading, and migration, and it is generally accepted that to regulate these integrin functions accurately, localized actin remodeling is required. However, the molecular mechanisms that control the targeting of actin regulation molecules to the proper sites are unknown. We previously demonstrated that integrin-mediated cell spreading and migration on fibronectin are dependent on the localized activation of phospholipase D (PLD). However, the mechanism underlying PLD activation by integrin is largely unknown. Here we demonstrate that protein kinase Cδ (PKCδ) is required for integrin-mediated PLD signaling. After integrin stimulation, PKCδ is activated and translocated to the edges of lamellipodia, where it colocalizes with PLD2. The abrogation of PKCδ activity inhibited integrin-induced PLD activation and cell spreading. Finally, we show that Thr566 of PLD2 is directly phosphorylated by PKCδ and that PLD2 mutation in this region prevents PLD2 activation, PLD2 translocation to the edge of lamellipodia, Rac translocation, and cell spreading after integrin activation. Together, these results suggest that PKCδ is a primary regulator of integrin-mediated PLD activation via the direct phosphorylation of PLD, which is essential for directing integrin-induced cell spreading.

Phospholipase D

Phospholipase D catalyses the hydrolysis of the phosphodiester bond of glycerophospholipids to generate phosphatidic acid and a free headgroup. Phospholipase D activities have been detected in simple to complex organisms from viruses and bacteria to yeast, plants, and mammals. Although enzymes with broader selectivity are found in some of the lower organisms, the plant, yeast, and mammalian enzymes are selective for phosphatidylcholine. The two mammalian phospholipase D isoforms are regulated by protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families. Mammalian and yeast phospholipases D are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. This review discusses the identification, characterization, structure, and regulation of phospholipase D. Genetic and pharmacological approaches implicate phospholipase D in a diverse range of cellular processes that include receptor signaling, control of intracellular membrane transport, and reorganization of the actin cytoskeleton. Most ideas about phospholipase D function consider that the phosphatidic acid product is an intracellular lipid messenger. Candidate targets for phospholipase-D-generated phosphatidic acid include phosphatidylinositol 4-phosphate 5-kinases and the raf protein kinase. Phosphatidic acid can also be converted to two other lipid mediators, diacylglycerol and lyso phosphatidic acid. Coordinated activation of these phospholipase-D-dependent pathways likely accounts for the pleitropic roles for these enzymes in many aspects of cell regulation.

Isoenzyme-specific Translocation of Protein Kinase C (PKC)βII and not PKCβI to a Juxtanuclear Subset of Recycling Endosomes

Elucidation of isoenzyme-specific functions of individual protein kinase C (PKC) isoenzymes has emerged as an important goal in the study of this family of kinases, but this task has been complicated by modest substrate specificity and high homology among the individual members of each PKC subfamily. The classical PKCbetaI and PKCbetaII isoenzymes provide a unique opportunity because they are the alternatively spliced products of the beta gene and are 100% identical except for the last 50 of 52 amino acids. In this study, it is shown that green fluorescent protein-tagged PKCbetaII and not PKCbetaI translocates to a recently described juxtanuclear site of localization for PKCalpha and PKCbetaII isoenzymes that arises with sustained stimulation of PKC. Mechanistically, translocation of PKCbetaII to the juxtanuclear region required kinase activity. PKCbetaII, but not PKCbetaI, was found to activate phospholipase D within this time frame. Inhibitors of phospholipase D (1-butanol and a dominant negative construct) prevented the translocation of PKCbetaII to the juxtanuclear region but not to the plasma membrane, thus demonstrating a role for phospholipase D in the juxtanuclear translocation of PKCbetaII. Taken together, these results define specific biochemical and cellular actions of PKCbetaII when compared with PKCbetaI.

Participation of phospholipase D and alpha/beta-protein kinase C in growth factor-induced signalling in C3H10T1/2 fibroblasts

We have studied phospholipase D (PLD) activation in relation to protein kinase C (PKC) and the involvement of PLD in extracellularly regulated kinase 1 (MAPK) (ERK1) activation and c-fos mRNA expression in C3H/10T1/2 (Cl8) fibroblasts. In these cells, the PLD activity was significantly increased by porcine platelet-derived growth factor (PDGF-BB), phorbol 12-myristate 13-acetate (PMA), and epidermal growth factor (EGF). PLD activation by PDGF-BB and PMA, but not EGF, was inhibited in Cl8 cells expressing the HAbetaC2-1 peptide (Cl8 HAbetaC2-1 cells), with a sequence (betaC2-1) shown to bind receptor for activated C kinase 1 (RACK1) and inhibit c-PKC-mediated cell functions [Science 268 (1995) 247]. A role of alpha-PKC in PLD activation is further underscored by co-immunoprecipitation of alpha-PKC with PLD1 and PLD2 in non-stimulated as well as PMA- and PDGF-BB-stimulated Cl8 cells. However, only PKC in PLD1 precipitates was activated by these agonists, while the PKC in the PLD2 precipitates was constitutively activated. The c-fos mRNA levels in Cl8 cells increased more than 30-fold in response to either PDGF-BB, EGF, or PMA. Approximately 60% inhibition of this increase in c-fos mRNA levels was observed in Cl8 HAbetaC2-1 cells. Formation of phosphatidylbutanol (PtdBut) at the expense of phosphatidic acid (PtdH) in the presence of n-butanol inhibited ERK1 activation and c-fos mRNA expression in PDGF-BB-treated Cl8 cells. ERK activation by PMA was unaffected by n-butanol in Cl8 cells but almost abolished by n-butanol in Cl8 HAbetaC2-1 cells, showing that ERK activation by PMA is heavily dependent on PKC and PLD1. In contrast, ERK activation by EGF in both cell types was not sensitive to n-butanol. These results indicate (1) a role of a functional interaction between the RACK1 scaffolding protein and a alphaPKC-PLD complex for achieving full PLD activity in PDGF-BB- and PMA-stimulated Cl8 cells; (2) PLD-mediated PtdH formation is needed for optimal ERK1 activation by PDGF-BB and maximal increase in c-fos mRNA expression. These findings place PLD as an important component in PDGF-BB- and PMA-stimulated intracellular signalling leading to gene activation in Cl8 cells, while EGF does not require PLD.

Role of specific protein kinase C isozymes in mediating epidermal growth factor, thyrotropin-releasing hormone, and phorbol ester regulation of the rat prolactin promoter in GH4/GH4C1 pituitary cells

Epidermal growth factor (EGF) and TRH both produce enhanced prolactin (PRL) gene transcription and PRL secretion in GH4 rat pituitary tumor cell lines. These agents also activate protein kinase C (PKC) in these cells. Previous studies have implicated the PKCepsilon isozyme in mediating TRH-induced PRL secretion. However, indirect studies using phorbol ester down-regulation to investigate the role of PKC in EGF- and TRH-induced PRL gene transcription have been inconclusive. In the present study, we examined the role of multiple PKC isozymes on EGF- and TRH-induced activation of the PRL promoter by utilizing general and selective PKC inhibitors and by expression of genes for wild-type and kinase-negative forms of the PKC isozymes. Multiple nonselective PKC inhibitors, including staurosporine, bisindolylmaleimide I, and Calphostin C, inhibited both EGF and TRH induced rat PRL promoter activity. TRH effects were more sensitive to Calphostin C, a competitive inhibitor of diacylglycerol, whereas Go 6976, a selective inhibitor of Ca(2+)-dependent PKCs, produced a modest inhibition of EGF but no inhibition of TRH effects. Rottlerin, a specific inhibitor of the novel nPKCdelta isozyme, significantly blocked both EGF and TRH effects. Overexpression of genes encoding PKCs alpha, betaI, betaII, delta, gamma, and lambda failed to enhance either EGF or TRH responses, whereas overexpression of nPKCeta enhanced the EGF response. Neither stable nor transient overexpression of nPKCepsilon produced enhancement of EGF- or TRH-induced PRL promoter activity, suggesting that different processes regulate PRL transcription and hormone secretion. Expression of a kinase inactive nPKCdelta construct produced modest inhibition of EGF-mediated rPRL promoter activity. Taken together, these data provide evidence for a role of multiple PKC isozymes in mediating both EGF and TRH stimulated PRL gene transcription. Both EGF and TRH responses appear to require the novel isozyme, nPKCdelta, whereas nPKCeta may also be able to transmit the EGF response. Inhibitor data suggest that the EGF response may also involve Ca(2+)-dependent isozymes, whereas the TRH response appears to be more dependent on diacylglycerol.

Pharmacological importance of phospholipase D and phosphatidic acid in the regulation of the mitogen-activated protein kinase cascade

The stimulation of cells with many extracellular agonists leads to the activation of phospholipase (PL)D. PLD metabolizes phosphatidylcholine to generate phosphatidic acid (PA). Neither the mechanism through which cell surface receptors regulate PLD activation nor the functional consequences of PLD activity in mitogenic signaling are completely understood. PLD is activated by protein kinase C, phospholipids, and small GTPases of the ADP-ribosylation factor and Rho families, but the mechanisms linking cell surface receptors to the activation of PLD still require detailed analysis. Furthermore, the latest data on the functional consequences of the generation of cellular PA suggest an important role for this lipid in the regulation of membrane traffic and on the activation of the mitogen-activated protein kinase cascade. This review addresses these issues, examining some novel models for the physiological role of PLD and PA and discussing their potential usefulness as specific targets for the development of new therapies.

The D2s dopamine receptor stimulates phospholipase D activity: a novel signaling pathway for dopamine

The D2 dopamine receptor isoforms signal to a variety of cellular effectors in both the central nervous system and periphery. Two alternative splice forms of the D2 dopamine receptor exist, the D2s (short) and D2l (long), which has an insertion of 29 amino acids in the third intracellular loop (). In cells of the anterior lobe of the pituitary, D2 dopamine receptors (both forms) are present on lactotroph cells coupled to the inhibition of adenylyl cyclase, activation of voltage-gated calcium channels, and inhibition of potassium channels. We describe here a novel signaling pathway for the D2s, which is the activation of phospholipase D (PLD). GH4C1 cells, a clonal line derived from a rat pituitary tumor, were stably transfected with the gene encoding the D2s, generating GH4-121 cells. Treatment of GH4-121 cells with a dopaminergic agonist resulted in activation of PLD in both a dose-dependent and time-dependent manner. This signaling pathway was not inhibited by prior treatment of cells with pertussis toxin at concentrations that ablate other D2s receptor signaling in this cell line. The stimulation of PLD activity by D2s appeared to correlate with the presence of a specific protein kinase C isoform, PKCepsilon. The D2s stimulation of PLD activity was blocked by preincubation of cells with C3 exoenzyme, indicating that the stimulation of PLD may involve Rho family members. The stimulation of PLD by dopaminergic agonists took place in the absence of any detectable stimulation of phosphoinositide metabolism.

(+)-MCPG induces PKCepsilon translocation in cortical synaptosomes through a PLD-coupled mGluR

We have tested whether different agonists of metabotropic glutamate receptors could induce translocation of selective protein kinase C isozymes in nerve terminals. In rat cortical synaptosomes 1S, 3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 100 microM) induced an increase in translocation to 124.6 +/- 5.7% of basal unstimulated conditions of the Ca++-independent protein kinase Cepsilon, but not of the Ca++-dependent isozyme beta. This effect was counteracted by 1-aminoindan-1,5-dicarboxylic acid (100 microM), an antagonist of metabotropic glutamate receptor 1. On the other hand, (+)-alpha-methyl-4-carboxyphenylglycine [(+)-MCPG], an antagonist of metabotropic glutamate receptors group I and II, did not antagonize the effect of 1S,3R-ACPD, and per se induced a translocation of protein kinase Cepsilon of 164 +/- 17.7% of basal unstimulated conditions. Because the (+)-MCPG induction of protein kinase Cepsilon translocation was not antagonized by 1-aminoindan-1, 5-dicarboxylic acid, it is suggested that 1S,3R-ACPD and (+)-MCPG activate this signal transduction pathway through distinct membrane receptors. Indeed (2-[2"-carboxy-3'-phenylcyclopropyl]glycine)-13 (300 nM), a new compound known to antagonize metabotropic glutamate receptors coupled to phospholipase D, was able to antagonize protein kinase Cepsilon translocation induced by (+)-MCPG. Moreover (+)-MCPG directly induced phospholipase D activity, measured as [3H]phosphoethanol production in cortical synaptosomes. These data suggest that in cortical nerve terminals (i) distinct metabotropic glutamate receptors, coupled to different signal transduction pathways, are present, (ii) (+)-MCPG is able to induce protein kinase Cepsilon translocation, and that (iii) a metabotropic glutamate receptor associated to phospholipase D might influence translocation of protein kinase C in a calcium-independent manner.

Altered activation of phospholipase D by lysophosphatidic acid and endothelin-1 in mouse embryo fibroblasts lacking phospholipase C-gamma1

Lysophosphatidic acid (LPA) and endothelin-1 (ET-1) activate phospholipase D (PLD) in many cell types. To see if phospholipase C-gamma1 plays a role, we used embryonic fibroblasts from mice in which the PLCgamma1 gene was disrupted. Surprisingly, the effect of LPA on inositol phosphate accumulation was increased in these PLCgamma1-/- cells, whereas that of ET-1 was completely abrogated. When PLD activity was measured, the response to LPA was also enhanced and the response to ET-1 lost in the PLCgamma1-/- cells. Treatment of these cells with ionomycin and oleoyl acetyl glycerol to mimic PLC stimulation restored PLD activity. Treatment of either PLCgamma1+/+ and PLCgamma1-/- cells with tyrosine kinase inhibitors did not inhibit LPA- or ET-1-induced PLD activity. Moreover, LPA and ET-1 treatment of PLCgamma1+/+ and PLCgamma1-/- cells did not cause tyrosine phosphorylation of PLC-gamma1 or PLC-gamma2. In summary, these results show that the altered PLD responses to LPA and ET-1 in PLCgamma1-/- are due to changes in PLC activity and do not involve tyrosine kinase activity.

Regulation of phospholipase D by phosphorylation-dependent mechanisms

The rapid production of phosphatidic acid following receptor stimulation has been demonstrated in a wide range of mammalian cells. Virtually every cell uses phosphatidylcholine as substrate to produce phosphatidic acid in a controlled reaction catalyzed by specific PLD isoforms. Considerable effort has been directed at studying the regulation of PLD activities and subsequent work has characterized a family of proteins including PLD1 and PLD2. Whereas both PLD enzymes are dependent on phosphatidylinositol 4, 5-bisphosphate for activity only the PLD1 isoform was strongly stimulated by the small GTPases ARF and RhoA and by protein kinase Calpha as well. A role for tyrosine kinase activities in the membrane recruitment of small GTPases, in the synthesis of phosphatidylinositol 4,5-bisphosphate and tyrosine phosphorylation of PLD1 and PLD2 has been uncovered. However, it still not clear exactly how tyrosine phosphorylation of proteins contributes to PLD activation in cells. Here we review the data linking tyrosine phosphorylation of proteins to the activation of PLD and describe recent finding on the sites and possible mechanisms of action of tyrosine kinases in receptor-mediated PLD activation. Finally, a model illustrating the potential complex interplay linking these signaling events with the activation of PLD is presented.

Regulation of phospholipase D

Phospholipase D (PLD) is a widely distributed enzyme that is under elaborate control by hormones, neurotransmitters, growth factors and cytokines in mammalian cells. Protein kinase C (PKC) plays a major role in the regulation of the PLD1 isozyme through interaction with its N-terminus. PKC activates this isozyme by a non-phosphorylation mechanism in vitro, but phosphorylation plays a role in the action of PKC on the enzyme in vivo. Although PLD1 can be phosphorylated by PKC in vitro, it is unclear that this occurs in vivo. Small GTPases of the ADP-ribosylation factor (ARF) and Rho families directly activate PLD1 in vitro and there is evidence that Rho proteins are involved in agonist regulation of PLD1 in vivo. ARF proteins stimulate PLD activity in the Golgi apparatus, but the role of these proteins in agonist regulation of the enzyme is less clear. PLD1 undergoes tyrosine phosphorylation in response to H(2)O(2) treatment of cells. The functional consequence of this phosphorylation and soluble tyrosine kinase(s) involved are presently unknown.

1,25-dihydroxyvitamin D3 and TPA activate phospholipase D in Caco-2 cells: role of PKC-alpha

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] and 12-O-tetradecanoylphorbol 13-acetate (TPA) both activated phospholipase D (PLD) in Caco-2 cells. GF-109203x, an inhibitor of protein kinase C (PKC) isoforms, inhibited this activation by both of these agonists. 1,25(OH)2D3 activated PKC-alpha, but not PKC-beta1, -betaII, -delta, or -zeta, whereas TPA activated PKC-alpha, -beta1, and -delta. Chronic treatment with TPA (1 microM, 24 h) significantly reduced the expression of PKC-alpha, -betaI, and -delta and markedly reduced the ability of 1,25(OH)2D3 or TPA to acutely stimulate PLD. Removal of Ca2+ from the medium, as well as preincubation of cells with Gö-6976, an inhibitor of Ca2+-dependent PKC isoforms, significantly reduced the stimulation of PLD by 1,25(OH)2D3 or TPA. Treatment with 12-deoxyphorbol-13-phenylacetate-20-acetate, which specifically activates PKC-betaI and -betaII, however, failed to stimulate PLD. In addition, the activation of PLD by 1,25(OH)2D3 or TPA was markedly reduced or accentuated in stably transfected cells with inhibited or amplified PKC-alpha expression, respectively. Taken together, these observations indicate that PKC-alpha is intimately involved in the stimulation of PLD in Caco-2 cells by 1,25(OH)2D3 or TPA.

GnRH signalling pathways and GnRH-induced homologous desensitization in a gonadotrope cell line (alphaT3-1)

Exposure of the gonadotrope cells to gonadotropin-releasing hormone (GnRH) reduces their responsiveness to a new GnRH stimulation (homologous desensitization). The time frame as well as the mechanisms underlying this phenomenon are yet unclear. We studied in a gonadotrope cell line (alphaT3-1) the effects of short as well as long term GnRH pretreatments on the GnRH-induced phospholipases-C (PLC), -A2 (PLA2) and -D (PLD) activities, by measuring the production of IP3, total inositol phosphates (IPs), arachidonic acid (AA) and phosphatidylethanol (PEt) respectively. We demonstrated that although rapid desensitization of GnRH-induced IP3 formation did not occur in these cells, persistent stimulation of cells with GnRH or its analogue resulted in a time-dependent attenuation of GnRH-elicited IPs formation. GnRH-induced IPs desensitization was potentiated after direct activation of PKC by the phorbol ester TPA, suggesting the involvement of distinct mechanisms in the uncoupling exerted by either GnRH or TPA on GnRH-stimulated PI hydrolysis. The levels of individual phosphoinositides remained unchanged under any desensitization condition applied. Interestingly, while the GnRH-induced PLA2 activity was rapidly desensitized (2.5 min) after GnRH pretreatments, the neuropeptide-evoked PLD activation was affected at later times, indicating an important time-dependent contribution of these enzymatic activities in the sequential events underlying the GnRH-induced homologous desensitization processes in the gonadotropes. Under GnRH desensitization conditions, TPA was still able to induce PLD activation and to further potentiate the GnRH-evoked PLD activity. AlphaT3-1 cells possess several PKC isoforms which, except PKCzeta, were differentially down-regulated by TPA (PKCalpha, betaII, delta, epsilon, eta) or GnRH (PKCbetaII, delta, epsilon, eta). In spite of the presence of PKC inhibitors or down-regulation of PKC isoforms by TPA, the desensitizing effect of the neuropeptide on GnRH-induced IPs, AA and PEt formation remained unchanged. In conclusion, in alphaT3-1 cells the GnRH-induced homologous desensitization affects the GnRH coupling with PLC, PLA2 and PLD by mechanism(s) which do not implicate TPA-sensitive PKC isoforms, but likely reflect time-dependent modification(s) on the activation processes of the enzymes.

Protein kinase C beta modulates thrombin-induced Ca2+ signaling and endothelial permeability increase

We investigated the function of the Ca2+-dependent protein kinase C (PKC) beta1 in the regulation of endothelial barrier property. Human dermal microvascular endothelial cells (HMEC-1) were transduced with full-length PKCbeta1 antisense (AS) cDNA or control pLNCX vector to generate stable cell lines (HMEC-AS and HMEC-pLNCX, respectively). Analyses indicated that HMEC-AS expressed the antisense PKCbeta1 transcript with decreased PKCbeta protein level (without a change in PKCalpha or PKCepsilon). The baseline transendothelial 125I-albumin clearance rates of HMEC-1, HMEC-pLNCX, and HMEC-AS were 5.0+/-0.5 x 10(-2), 6.8+/-0.4 x 10(-2), and 6.9+/-0.6 x 10(-2) microl/min, respectively. Activation of HMEC-1 and HMEC-pLNCX with phorbol 12-myristate 13-acetate (PMA) increased the rates to the respective 14.5+/-1.7 x 10(-2) microl/min and 16.9+/-2.8 x 10(-2) microl/min (corresponding to 191% and 149% increases over baseline). However, in HMEC-AS, PMA increased the rate to 9.8+/-1.0 x 10(-2) microl/min (42%). When HMEC-1 and HMEC-pLNCX were activated with thrombin, the rates increased to 10.8+/-1.4 x 10(-2) and 14.0+/-1.9 x 10(-2) microl/min, respectively (116% and 106%). In contrast, thrombin stimulation of HMEC-AS more than doubled the increase to 27.2+/-3.5 x 10(-2) microl/min (294%). Furthermore, the thrombin-induced peak increase in the [Ca2+]i in HMEC-AS was greater than in control cells. Fluorescence-activated cell sorter analysis of thrombin receptor expression indicated that the augmented thrombin-induced responses were not attributable to altered receptor density in HMEC-AS. These results indicate that PKCbeta functions in a negative feedback manner to inactivate thrombin-generated signals and thereby modulates the endothelial permeability increase. Because decreased PKCbeta expression significantly reduced the PMA-induced permeability increase, PKCbeta may downregulate thrombin receptor function upstream of PKC activation (i.e., Ca2+).

Inhibition of GTP gamma S-dependent phospholipase D and Rho membrane association by calphostin is independent of protein kinase C catalytic activity

We studied the relationships between the activation of phospholipase D (PLD) by guanine nucleotides and phorbol esters in permeabilized U937 promonocytes and in solubilized extracts prepared from U937 cell membranes. Treatment of permeabilized cells with phorbol myristate acetate (PMA) strongly potentiated GTP gamma S-dependent PLD activity at free Ca2+ < 100 nM. In the absence of GTP gamma S, PMA stimulated only minor PLD activity. This suggested synergistic interaction between regulatory G-proteins and a protein kinase C (PKC) family kinase. The potential role of PKC was evaluated by testing two mechanistically distinct PKC inhibitors, bisindolylmaleimide (BIM) and calphostin. BIM inhibits PKC enzymes via competition with ATP for binding to the catalytic domain, while calphostin competes with PMA or diglyceride for binding to the regulatory domain. The ability of PMA to potentiate the GTP gamma S-dependent PLD was not inhibited by BIM. In contrast, calphostin strongly inhibited the GTP gamma S-dependent PLD activity, both in the presence and absence of PMA as a potentiating agent. Calphostin also produced complete inhibition of a GTP gamma S-dependent PLD activity, present in solubilized membrane extracts, which was assayed using phospholipid vesicles of defined composition. Treatment of reconstituted membrane/cytosol mixtures with calphostin also produced complete inhibition of the GTP gamma S-induced translocation of Rho A from cytosol to membrane. In contrast to its effects on the U937 cell PLD, calphostin did not inhibit the activity of purified PLD from cabbage. These results suggest that the assembly of active RhoA/PLD signaling complexes on membranes involves a phorbol ester/calphostin-binding protein, but is not dependent on PKC-type catalytic activity.

Antisense oligonucleotides targeting human protein kinase C-alpha inhibit phorbol ester-induced reduction of bradykinin-evoked calcium mobilization in A549 cells

Regulation of the bradykinin-evoked increase in intracellular Ca2+ concentration by protein kinase C (PKC)-alpha was investigated in A549 human lung carcinoma cells. Bradykinin, a potent and selective kinin B2 receptor agonist, induces calcium mobilization in a concentration-dependent fashion in this cell line. 12-O-Tetradecanoylphorbol-13-acetate (TPA), a potent activator of PKC, is known to reduce the amplitude of agonist-induced calcium mobilization in various cell lines. Because PKC-alpha is a major PKC isozyme in A549 cells, we investigated whether this isozyme plays a role in this process. A 20-mer phosphorothioate oligonucleotide targeting the 3'-untranslated region of the human PKC-alpha mRNA, which contains 2'-methoxyethyl modifications incorporated into the 5' and 3' segments of the oligonucleotide, was used to assess the putative role of PKC-alpha in the receptor regulation. ISIS 9606 reduced PKC-alpha mRNA for > or = 72 hr after the initial treatment and the reduction was concentration dependent, whereas the mismatch control, ISIS 13009, had no effect. Concentrations of ISIS 9606 of 150 nM specifically reduced the level of immunoreactive PKC-alpha protein by 66.3 +/- 2.5% at 72 hr after treatment, without an effect on immunoreactive PKC-delta protein. This reduction in PKC-alpha was sufficient to inhibit the reduction of bradykinin-induced calcium mobilization by TPA. This finding is corroborated by the use of staurosporine, a nonselective PKC inhibitor, that prevented the effect of TPA. These results suggest that PKC-alpha is involved in kinin B2 receptor regulation by phorbol esters in A549 cells.

Cell signalling through guanine-nucleotide-binding regulatory proteins (G proteins) and phospholipases

Phospholipases are important enzymes in cell signal transduction since they hydrolyze membrane phospholipids to generate signalling molecules. Heterotrimeric guanine-nucleotide-binding regulatory proteins (G proteins) play a major role in their regulation by a variety of agonists that activate receptors with seven membrane-spanning domains. Phospholipases of the C type, which hydrolyze inositol phospholipids to yield inositol trisphosphate and diacylglycerol, are regulated by the alpha and betagamma subunits of certain heterotrimeric G proteins as well as by receptor-associated and non-receptor-associated tyrosine kinases. Phospholipases of the D type, which hydrolyze phosphatidylcholine to phosphatidic acid, are regulated by members of the ADP-ribosylation factor and Rho subfamilies of small G proteins, and by protein kinase C and other factors. This review presents recent information concerning the molecular details of G protein regulation of these phospholipases.

Role of protein kinase C alpha, Arf, and cytoplasmic calcium transients in phospholipase D activation by sodium fluoride in osteoblast-like cells

The effect of fluoride on phospholipase D (PLD) activation was studied using in vitro culture of Saos-2, MG-63 osteosarcoma cells, and normal osteoblast-like cells derived from human bone explants. Millimolar concentrations of NaF induced a significant accumulation of phosphatidylethanol (PEt) in Saos-2 cells but not in MG-63 and normal osteoblast-like cells. PLD activation was evident at 15 mM and concentration-dependent up to 50 mM. This stimulation was inhibited by deferoxamine, a chelator of Al3+, suggesting that PLD activation involves fluoride-sensitive G proteins. A good correlation was found between the levels of intracellular free Ca2+ and the activation of PLD. The time courses of the two responses were nearly identical. The ability of NaF to induce both responses was largely dependent on the presence of extracellular calcium. The calcium ionophore A23187 reproduced the effect of NaF, and this effect was antagonized by EGTA, suggesting that PLD activation was, at least in part, a calcium-regulated event. Phorbol 12-myristate 13-acetate (PMA) also stimulated PLD activity in human bone cells. Protein kinase C alpha (PKC alpha) and epsilon were expressed in Saos-2 cells. Acute pretreatment of cells with PMA reduced concomitantly the amounts of PKC alpha, but not of PKC epsilon, and the subsequent activation of PLD elicited by PKC activators. The PLD response to NaF was not attenuated but rather enhanced by down-regulation of PKC alpha. Therefore, PKC-alpha-induced PLD activation is unlikely to mediate the effect of NaF. Moreover, PMA and NaF showed a supraadditive effect on PLD activation in Saos-2 cells. This stimulation, in contrast to NaF alone, was not reduced by EGTA. Hence, mobilization of calcium by NaF cannot account for the enhanced PLD activation in response to PMA stimulation. Membrane Arf and RhoA contents were assessed by Western immunoblot analyses. Membranes derived from NaF-stimulated Saos-2 cells contained more Arf and RhoA when compared with membranes derived from control or PMA-stimulated cells. Translocation of the small GTPases was calcium-independent. We conclude that PLD activation by NaF in Saos-2 cells includes a fluoride-sensitive G protein, increases in the levels of intracellular calcium, and Arf/RhoA redistribution to membranes. The results also indicate that the NaF-induced Arf/RhoA translocation exerts in concert with PMA-activated PKC alpha a synergistic effect on the activation of PLD in Saos-2 cells.

Thyroid hormone represses protein kinase C isoform expression and activity in rat cardiac myocytes

We have previously demonstrated that at least four isoforms of protein kinase C (PKC; alpha, delta, epsilon, and zeta) are expressed in neonatal rat ventricular myocytes and that development is associated with a decline in their expression. The mechanism(s) regulating PKC isoform expression in ventricular myocytes is completely unknown. The developmental decline in PKC expression occurs, in large part, during the first 2 weeks of postnatal life, while thyroid hormone levels are known to be progressively increasing. Accordingly, this study examined the influence of thyroid hormone on PKC isoform expression to determine whether thyroid hormone can be implicated as a potential physiological regulator of PKC gene expression during normal cardiac development. Hypothyroidism was induced in adult rats by surgical thyroidectomy; thyroid status was manipulated in cultured neonatal ventricular myocytes by growth in serum-free medium with varying triiodothyronine (T3) levels. In each case, hypothyroidism was verified by a 10- to 50-fold increase in steady state mRNA for beta-myosin heavy chain. In hypothyroid adult ventricular myocardium, there was a selective 60% increase in the expression of PKC epsilon protein that corresponded to an increase in maximally stimulated PKC enzyme activity with PKC epsilon substrate peptide (epsilon pep) but not with histone as substrate. Northern blot analysis revealed a 70% increase in PKC epsilon mRNA, indicating that the regulatory effects of thyroid hormone are mediated, at least in part, at the message level. In neonatal ventricular myocytes, there was a T3-dependent reduction in immunoreactivity for both PKC alpha and PKC epsilon that was associated with significant reductions in both histone- and epsilon pep-kinase activities. The concentration of T3 that half-maximally repressed PKC alpha and PKC epsilon expression was approximately 0.5 nmol/L. Thyroid hormone had no effect on PKC delta and PKC zeta expression in neonatal or adult ventricular myocytes. PKC isoform expression in cardiac fibroblasts was not influenced by variations in the thyroid hormone concentration during culture. These results provide evidence that thyroid hormone specifically represses PKC alpha and PKC epsilon in the neonatal heart and PKC epsilon in the adult heart. Thyroid hormone-induced changes in PKC may play an important permissive role in the modulation of autonomic responsiveness in ventricular cardiomyocytes.

Phenylarsine oxide (PAO)-mediated activation of phospholipase D in rat basophilic leukemia (RBL-2H3) cells: possible involvement of calcium and protein kinase C

Addition of phenylarsine oxide (PAO) to [3H]oleic acid-labeled rat basophilic leukemia (RBL-2H3) cells gave rise to the remarkable formation of [3H]phosphatidylbutanol (PBut), a specific product of phospholipase D (PLD) activation. Preincubation of cells with 2,3-dimercaptopropanol (DMP) or dithiothreitol (DTT), compounds containing sulfhydryls, prevented PAO-stimulated [3H]PBut formation, indicating that PAO-stimulated PLD through interacting with vicinal thiol groups. Treatment of cells with PAO resulted in increase in intracellular Ca2+ concentration without significant production of inositol phosphates. Removal of extracellular free Ca2+ by chelating with EGTA was found to inhibit [3H]PBut formation by PAO. Incubation of cells with 20 nM phorbol 12-myristate 13-acetate (PMA) for 6 h caused down-regulation of protein kinase C (PKC) alpha and beta isozymes, whereas it had no effect on PKC delta, epsilon and zeta isozymes. Under this condition, decrease in PAO-stimulated [3H]PBut formation was observed to occur with a concomitant decrease in the level of PKC alpha and beta isozymes. These results suggest that a covalent bridge between vicinal thiol groups of cell surface proteins induced by PAO potentiates PLD activation and that PAO-induced PLD activation is regulated by Ca2+ and PKC alpha and/or beta isozymes.

Small GTPase-regulated phospholipase D in granulocytes

This review examines the functional role of phospholipase D in the neutrophil. Phospholipase D is emerging as an important component in the signal transduction pathways leading to granulocyte activation. Through the second messenger it produces, phosphatidic acid, phospholipase D plays an active role in the regulation of granulocyte NADPH oxidase activation and granular secretion. Many factors from both the cytosol and the membrane are necessary for maximal phospholipase D activation. This paper will focus on the regulation of phospholipase D by low molecular weight GTP-binding proteins, tyrosine kinases, and protein kinase C.

Mechanisms of increased endothelial permeability

The increase in endothelial permeability in response to inflammatory mediators such as thrombin and histamine is accompanied by reversible cell rounding and interendothelial gap formation, suggesting that the predominant transport pathway is a diffusive one (i.e., via cellular junctions (paracellular transport)). However, vesicle-mediated transport (i.e., via albumin-binding protein gp60) may also contribute significantly to the overall increase in permeability. Regulation of paracellular transport in endothelial cells is associated with modulation of actin-based systems, which anchor the cell to its neighbor or extracellular matrix, thus maintaining endothelial integrity. At the cell-cell junctions, actin is linked indirectly to the plasma membrane by linking proteins (e.g., vinculin, catenins, alpha-actinin) to cadherins, which function in homophilic intercellular adhesion. At endothelial focal contacts, the transmembrane receptors (integrins) for matrix proteins are linked to actin via linking proteins (i.e., vinculin, talin, alpha-actinin). In response to inflammatory mediators, second messengers signal two regulatory pathways, which modulate the actin-based systems, and can thus lead to impairment of the endothelial barrier integrity. One critical signal may be based on protein kinase C isoenzyme specific phosphorylation of linking proteins at the cell-cell and cell-matrix junctions. The increased phosphorylation is associated with actin reorganization, cell rounding, and increased paracellular transport. Another important event is the activation of myosin light chain kinase (MLCK), which causes an actin-myosin-based contraction that may lead to centripetal retraction of endothelial cells. Current research is being conducted at identification of protein substrates of protein kinase C isoenzymes, the specific role of their phosphorylation in barrier function, and determination of the precise role of MLCK in modulation of endothelial barrier function. Since mechanisms by which the increased permeability is returned to normal may be regulated at multiple levels (e.g., receptor desensitization, protein kinase C mediated negative feedback pathways, activation of protein phosphatases), it is also important to determine these cellular "off-switch" mechanisms.

ARF-regulated phospholipase D: a potential role in membrane traffic

Phospholipase D activity is stimulated rapidly upon occupation of cell-surface receptors. One of the intracellular regulators of phospholipase D activity has been identified as ADP ribosylation factor (ARF). ARF is a small GTP binding protein whose function has been elucidated in vesicular traffic. This review puts into context the connection between the two fields of signal transduction and vesicular transport.

Phorbol ester stimulation of phosphatidylcholine synthesis in four cultured neural cell lines: correlations with expression of protein kinase C isoforms

Phosphatidylcholine (PtdCho) can provide lipid second messengers involved in signal transduction pathways. As a measure of phospholipid turnover in response to extracellular stimulation, we investigated differential enhancement of [3H]choline incorporation into PtdCho by phorbol esters. In C6 rat glioma and SK-N-SH human neuroblastoma cells, [3H]PtdCho synthesis was 2-4 fold stimulated by beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA) when [3H]choline was incubated simultaneously with, or 15 min prior to, beta-TPA treatment. By contrast, in N1E-115 mouse and SK-N-MC human neuroblastoma cells, phorbol esters had no appreciable effect on [3H]choline incorporation; however, in all cells, 200 microM oleic acid enhanced PtdCho synthesis, indicating a stimulable process. Alterations by thymeleatoxin (TMT), an activator of conventional PKC isoforms (alpha, beta and gamma), were similar to beta-TPA. We investigated whether expression of specific PKC isoforms might correlate with these effects of phorbol esters on PtdCho synthesis. All cell lines bound phorbol esters, had PKC activity that was translocated by phorbol esters and differentially expressed isoforms of PKC. Northern and western blot analyses, using specific cDNA and antibodies for PKC-alpha, -beta, -gamma, -delta, -epsilon, and -zeta, revealed that expression of alpha-isoform predominated in C6 and SK-N-SH cells. In contrast, TPA-responsive beta-isoform predominated in SK-N-MC cells. gamma-PKC was not detected in any cells and only in C6 cells was PKC-delta present and translocated by beta-TPA treatment. PKC-epsilon was not detected in SK-N-MC cell lines but translocated with TPA treatment in the other three cell lines. PKC-zeta was present in all cells but was unaltered by TPA treatment. Accordingly, stimulation of PtdCho turnover by phorbol esters correlated only with expression of PKC-alpha; presence of PKC-beta alone was insufficient for a TPA response.

Antigen-mediated phospholipase D activation in rat basophilic leukemia (RBL-2H3) cells: possible involvement of calcium/calmodulin

The differential implication of protein kinase C (PKC) isozymes in antigen- or PMA-induced phospholipase D (PLD) activation was investigated in rat basophilic leukemia (RBL-2H3) cells. In [3H]oleic acid-labeled cells, both antigen (100 ng/ml) and phorbol 12-myristate 13-acetate (PMA) (100 nM) produced a specific product of PLD activation, [3H]phosphatidylbutanol (PBut) in the presence of butanol. Pretreatment of cells with a selective PKC inhibitor, Ro31-8425 (1-5 microM) inhibited PMA-stimulated PLD activity by 85%. In contrast, the antigen-stimulated PLD activity was much less sensitive to the inhibitor. RBL-2H3 cells express PKC alpha, beta, delta, epsilon and zeta isozymes and down-regulation of PKC by exposure to PMA (20 nM) for 1-2 h caused rapid decrease in PKC alpha and beta isozymes, leaving PKC delta, epsilon and zeta isozymes intact. Apparent decreases in the levels of PKC alpha and beta to about 50% were observed after adding 20 nM PMA for 1 h, when PMA-stimulated PLD activity was inhibited by up to 70%. Decrease in antigen-stimulated PLD activity was evident after 2 h PMA-treatment, when PKC alpha and beta decreased by nearly 70%. These results suggest that in the antigen-mediated PLD pathway PKC may be implicated but not play such a great role as PMA-stimulated pathway which is mediated through PKC alpha or beta. Then, we have examined the involvement of calcium/calmodulin (CaM) in PLD activation by antigen, since the antigen-stimulated PLD activation showed the absolute requirement for extracellular calcium. Preincubation of RBL-2H3 cells with a CaM antagonist W-7 (20 microM) inhibited the antigen-stimulated PLD activity by 90%, but W-5, a chlorine-deficient analogue of W-7 that only weakly interact with CaM, caused little inhibitory effect. Another non-specific CaM antagonist, trifluoperazine (TFP) also inhibited PLD activation. These results suggest that calcium/CaM may be involved in the antigen-stimulated PLD activation.

Bradykinin stimulates phospholipase D in PC12 cells by a mechanism which is independent of increases in intracellular Ca2+

These experiments were designed to learn the role of bradykinin induced changes in intracellular Ca2+ in the activation of phospholipase D activity in PC12 cells. Ionomycin at a concentration of 0.1 microM caused an increase in intracellular Ca2+ comparable to bradykinin, but had no effect on phospholipase D activity. Carbachol, ATP, and thapsigargin also increased intracellular Ca2+ but had no effect on phospholipase D activity. Increases in intracellular Ca2+ may be a necessary but not a sufficient factor in the activation of phospholipase D. To investigate this issue, the bradykinin induced increase in intracellular Ca2+ was blocked by preincubating the cells in Ca(2+)-free media plus EGTA or in media containing the intracellular Ca2+ chelator BAPTA/AM. These preincubations completely blocked the bradykinin induced increase in intracellular Ca2+ but only attenuated the bradykinin mediated activation of phospholipase D. Physiological increases in intracellular Ca2+ apparently do not mediate the effect of bradykinin on phospholipase D.

Differential regulation of phospholipase D and phospholipase C by protein kinase C-beta and -delta in liver macrophages

We have studied activation of phospholipase (PL) C and PLD in liver macrophages labelled with [3H]arachidonic acid. Zymosan, phorbol 12-myristate 13-acetate (PMA), A23187 and fluoride but not arachidonic acid or lipopolysaccharide (LPS) induce an activation of PLD ([3H]phosphatidylethanol (PEt) accumulation). An activation of PLC ([3H]diacylglycerol (DAG) accumulation) is measured with zymosan, PMA and fluoride but not with A23187, LPS or arachidonic acid whereas inositol phosphates are formed with zymosan, only. Removal of extracellular calcium reduces the formation of [3H]PEt and [3H]DAG while pretreatment of the cells with dexamethasone reduces [3H]PEt formation, only. PMA- and zymosan-induced activation of PLD and PMA-induced activation of PLC both seem to be mediated by protein kinase (PK) C-beta whereas zymosan-induced activation of PLC is negatively controlled by PKC-delta. We could furthermore present evidence that the release of [3H]arachidonic acid in these cells occurs independent of an activation of PLD.

Regulation of phospholipase D by protein kinase C in human neutrophils. Conventional isoforms of protein kinase C phosphorylate a phospholipase D-related component in the plasma membrane

In a variety of intact cells, phorbol esters are known to activate phospholipase D. In a cell-free system consisting of plasma membrane and cytosol from human neutrophils, phorbol esters activated phospholipase D in an adenosine nucleotide triphosphate-dependent manner. ATP gamma S (adenosine 5'-O-(thiotriphosphate)) was 2-3-fold more effective than ATP, while ADP and AppNHp (adenyl-5'-yl imidodiphosphate) were ineffective, and activation was blocked by the kinase inhibitor staurosporine. In cytosol deplete of protein kinase C by chromatography on threnoine-Sepharose, phorbol ester-dependent activation was lost, but was restored upon addition of purified rat brain protein kinase C. The target for phosphorylation was shown to be the plasma membrane plasma membrane was phosphorylated using ATP gamma S/phorbol 12,13-dibutyrate and protein kinase C and was reisolated to remove activators. Upon adding nucleotide-depleted cytosol, activator-independent phospholipase D activity was seen. Using this prephosphorylation protocol, PKC-dependent activation of plasma membranes was found to require micromolar calcium, implicating a conventional protein kinase C. Using recombinant isoforms of protein kinase C, only the conventional isoforms showed significant activation, with the following rank order of potency: beta 1 > alpha > gamma; the beta 2, delta, epsilon, eta, and sigma isoforms showed little or no activity. Thus, conventional isoform(s) of protein kinase C activate neutrophil phospholipase D by phosphorylating a target protein located in the plasma membrane.

Stimulation of phospholipase D by epidermal growth factor requires protein kinase C activation in Swiss 3T3 cells

The proposal that epidermal growth factor (EGF) activates phospholipase D (PLD) by a mechanism(s) not involving phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) hydrolysis was examined in Swiss 3T3 fibroblasts. EGF, basic fibroblast growth factor (bFGF), bombesin, and platelet-derived growth factor (PDGF) activated PLD as measured by transphosphatidylation of butanol to phosphatidylbutanol. The increase in inositol phosphates induced by bFGF, EGF, or bombesin was significantly enhanced by Ro-31-8220, an inhibitor of protein kinase C (PKC), suggesting that PtdIns(4,5)P2-hydrolyzing phospholipase is coupled to the receptors for these agonists but that the response is down-regulated by PKC. Activation of PLD by EGF was inhibited dose dependently by the PKC inhibitors bis-indolylmaleimide and Ro-31-8220, which also inhibited the effects of bFGF, bombesin, and PDGF. Down-regulation of PKC by prolonged treatment with 4 beta-phorbol 12-myristate 13-acetate also abolished EGF- and PDGF-stimulated phosphatidylbutanol formation. EGF and bombesin induced biphasic translocations of PKC delta and epsilon to the membrane that were detectable at 15 s. In the presence of Ro-31-8220, translocation of PKC alpha became evident, and membrane association of the delta- and epsilon-isozymes was enhanced and/or sustained in response to the two agonists. The inhibitor also enhanced EGF-stimulated [3H]diacylglycerol formation in cells preincubated with [3H]arachidonic acid, which labeled predominantly phosphatidylinositol, but inhibited [3H]diacylglycerol production in cells preincubated with [3H]myristic acid, which labeled mainly phosphatidylcholine. These data support the conclusion that EGF can stimulate diacylglycerol formation from PtdIns(4,5)P2 and that PKC performs the dual role of down-regulating this response as well as mediating phosphatidylcholine hydrolysis. In summary, all of the results of the study indicate that PLD activation by EGF is downstream of PtdIns(4,5)P2-hydrolyzing phospholipase and is dependent upon subsequent PKC activation.

Homologous and heterologous regulation of receptor-stimulated phosphoinositide hydrolysis

Signal transduction at a diverse range of pharmacologically distinct receptors is effected by the enhanced turnover of inositol phospholipids, with the attendant formation of inositol 1,4,5-trisphosphate and diacylglycerol. Although considerable progress has been made in recent years towards the identification and characterization of the individual components of this pathway, much less is known of mechanisms that may underlie its regulation. In this review, evidence is presented for the potential regulation of inositol lipid turnover at the level of receptor, phosphoinositide-specific phospholipase C and substrate availability in response to either homologous or heterologous stimuli. Available data indicate that the extent of receptor-stimulated inositol lipid hydrolysis is regulated by multiple mechanisms that operate at different levels of the signal transduction pathway.

Differential effects of polychlorinated biphenyl congeners on phosphoinositide hydrolysis and protein kinase C translocation in rat cerebellar granule cells

Previous reports from our laboratory have suggested that the neuroactivity of some polychlorinated biphenyl (PCB) congeners is associated with perturbations in cellular Ca(2+)-homeostasis. We have characterized further the neurochemical effects of PCBs on signal transduction in primary cultures of cerebellar granule cells. The present experiments found that neither 2,2'-dichlorobiphenyl (DCBP), an ortho-substituted congener, nor 3,3',4,4',5-pentachlorobiphenyl (PCBP), a non-ortho-substituted congener, affected basal phosphoinositide (PI) hydrolysis in cerebellar granule cells. However, at concentrations up to 50 microM, DCBP potentiated carbachol-stimulated PI hydrolysis, while decreasing it at 100 microM. PCBP, on the other hand, had no effect on carbachol-stimulated PI hydrolysis in concentrations up to 100 microM. [3H]Phorbol ester ([3H]PDBu) binding was used to determine protein kinase C (PKC) translocation. DCBP increased [3H]PDBu binding in a concentration-dependent manner and a twofold increase was observed at 100 microM in cerebellar granule cells. PCBP had no effect on [3H]PDBu binding at concentrations up to 100 microM. The effect of DCBP on [3H]PDBu binding was time-dependent and was also dependent on the presence of external Ca2+ in the medium. To test the hypothesis that DCBP increases [3H]PDBu binding by acting on receptor-activated calcium channels, the effects of DCBP were compared to those of L-glutamate. The effects of DCBP (50 microM) and glutamate (20 microM) were additive.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in the regulation of phosphatidylinositol-specific phospholipase C in normal and neoplastic keratinocytes

The induction of epidermal differentiation by Ca2+ in vitro is associated with enhanced activity of phosphatidylinositol-specific phospholipase C (PLC). Neoplastic keratinocyte cell lines expressing a mutant c-Ha-ras gene and normal keratinocytes transformed to the neoplastic phenotype by transduction with the v-Ha-ras gene (v-Ha-ras keratinocytes) have elevated constitutive activity of PLC that increases further in response to Ca2+, but the cells do not differentiate normally. PLC-gamma 1 (145 kDa) is the major isoform detected by immunoblotting of extracts from control, v-Ha-ras, and neoplastic keratinocyte cell lines cultured in 0.05 mM Ca2+ medium. The amount of PLC-gamma 1 protein was higher in neoplastic cell lines than in normal and v-Ha-ras keratinocytes that had similar PLC-gamma 1 protein levels. Thus, higher PLC-gamma 1 protein levels cannot account for the elevated constitutive activity PLC in v-Ha-ras keratinocytes. After induction of differentiation by Ca2+, the amount of PLC-gamma 1 protein increased in all cell types, and PLC-delta 1 (85 kDa), barely detectable in 0.05 mM Ca2+, increased. PLC-beta 1 was not detected at any Ca2+ concentration. PLC-gamma 1 and PLC-delta 1 mRNA did not increase after elevation of extracellular Ca2+, suggesting that posttranscriptional mechanisms can regulate PLC-gamma 1 and PLC-delta 1 protein levels in normal and neoplastic keratinocytes. Activation of protein kinase C by treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA) inhibited the stimulation of inositol phosphate (InsP) formation by Ca2+ but did not alter basal InsP levels in normal keratinocytes. In contrast, TPA treatment reduced both Ca(2+)-stimulated and basal InsP formation in neoplastic cells lines and v-Ha-ras keratinocytes. In both normal and v-Ha-ras keratinocytes labeled with [32P]orthophosphate, antibodies against PLC-gamma 1 immunoprecipitated a complex of 32P-labeled proteins. The relative labeling of the PLC-gamma 1 band was greater in normal than in v-Ha-ras keratinocytes. Furthermore, treatment with TPA specifically increased the relative phosphorylation of PLC-gamma 1 in v-Ha-ras keratinocytes but not in normal keratinocytes. These results suggest that the negative regulation of constitutive activity of PLC by protein kinase C differs in normal and neoplastic keratinocytes and that this could be the mechanism of increased PLC activity produced by an oncogenic ras gene in keratinocytes.

Phosphatidylcholine breakdown and signal transduction

PC hydrolysis by PLA2, PLC or PLD is a widespread response elicited by most growth factors, cytokines, neurotransmitters, hormones and other extracellular signals. The mechanisms can involve G-proteins, PKC, Ca2+ and tyrosine kinase activities. Although an agonist-responsive cytosolic PLA2 has been purified, cloned and sequenced, the agonist-responsive form(s) of PC-PLC has not been identified and no form of PC-PLD has been purified or cloned. Regulation of PLA2 by Ca2+ and MAPK is well established and involves membrane translocation and phosphorylation, respectively. PKC regulation of the enzyme in intact cells is probably mediated by MAPK. The question of G-protein control of PLA2 remains controversial since the nature of the G-protein is unknown and it is not established that its interaction with the enzyme is direct or not. Growth factor regulation of PLA2 involves tyrosine kinase activity, but not necessarily PKC. It may be mediated by MAPK. The physiological significance of PLA2 activation is undoubtedly related to the release of AA for eicosanoid production, but the LPC formed may have actions also. There is much evidence that PKC regulates PC-PLC and PC-PLD and this is probably a major mechanism by which agonists that promote PI hydrolysis secondarily activate PC hydrolysis. Since no agonist-responsive forms of either phospholipase have been isolated, it is not clear that PKC exerts its effects directly on the enzymes. Although it is assumed that a phosphorylation mechanism is involved, this may not be the case, and regulation may be by protein-protein interactions. G-protein control of PC-PLD is well-established, although, again, it has not been demonstrated that this is direct, and the nature of the G-protein(s) involved is unknown. In some cell types, there is evidence of the participation of a soluble protein, which may be a low Mr GTP-binding protein. What role this plays in the activation of PC-PLD is obscure. Agonist activation of PC hydrolysis in cells is usually Ca(2+)-dependent, but the step at which Ca2+ is involved is unclear, since PC-PLD and PC-PLC per se are not influenced by physiological concentrations of the ion. Most growth factors promote PC hydrolysis and this is mainly due to activation of PKC as a result of PI breakdown. However, in some cases, PC breakdown occurs in the absence of PI hydrolysis, implying another mechanism that does not involve PI-derived DAG.(ABSTRACT TRUNCATED AT 400 WORDS)

Formation of diacylglycerol, inositol phosphates, arachidonic acid and its metabolites in macrophages

Treatment of macrophages with zymosan, 4 beta-phorbol 12-myristate 13-acetate (PMA) and fluoride but not with A 23187 or arachidonic acid (delta Ach) leads to a generation of diacylglycerol (acyl2Gro). Formation of inositol phosphates is achieved with zymosan, only. An elevation of intracellular calcium is obtained with zymosan and A 23187 but not with PMA, fluoride or delta Ach. Prior treatment of the cells with phorbol ester for 3 h which has been shown recently to result in a down-regulation of protein kinase (PK) C-beta but not PKC-delta [Duyster, J., Schwende, H., Fitzke, E., Hidaka H. & Dieter P. (1993) Biochem. J. 292, 203-207] has no effect on the zymosan-induced formation of acyl2Gro or inositol phosphates but inhibits the PMA-induced generation of acyl2Gro. Down-regulation of PKC-delta by prior phorbol ester treatment for 24 h augments the zymosan-induced generation of acyl2Gro and inositol phosphates. The acyl2Gro lipase inhibitor RG 80267 inhibits the PMA-induced and fluoride-induced generation of prostaglandin (PG) E2, reduces the zymosan-induced release of PGE2 by 50% but has no effect on PGE2 formation of unstimulated, A 23187-treated or delta Ach-treated cells. Furthermore, RG 80267 enhances accumulation of delta Ach-labeled acyl2Gro in response to zymosan, PMA and fluoride. These data indicate that zymosan activates a phosphatidylinositol 4,5-bisphosphate-specific phospholipase (PL) C, that generation of acyl2Gro by PMA and fluoride occurs via hydrolysis of other phospholipids, that PKC-beta is involved in the PMA-induced generation of acyl2Gro and PKC-delta negatively modulates the zymosan-induced activation of PLC and PMA and fluoride induce a liberation of delta Ach from acyl2Gro, A 23187 activates the PLA2 pathway and zymosan stimulates both, the acyl2Gro- and PLA2-pathway.

Keratinocyte differentiation is associated with changes in the expression and regulation of phospholipase C isoenzymes

In murine keratinocytes, Ca(++)-induced terminal differentiation is accompanied by a rapid and sustained increase of inositol phosphates and diacylglycerol. Based on Western blotting analysis, basal keratinocytes cultured in 0.05 mM Ca++ medium express phospholipase C (PLC)-gamma 1 predominantly and no detectable PLC-beta 1. Differentiating keratinocytes cultured in 1.4 mM Ca++ express two- to threefold more PLC-gamma 1 protein and PLC-delta 1, but no detectable PLC-beta 1. Although the amount of PLC-gamma 1 and -delta 1 protein increased, PLC-gamma 1 and -delta 1 mRNA decreased in differentiating cells. Thus the sustained rise of PLC activity induced by Ca++ in differentiating keratinocytes may be associated with higher amounts of both PLC-gamma 1 and -delta 1 in maturing cells, determined by a posttranscriptional mechanism. Tyrosine phosphate content in PLC-gamma 1 was low in basal cells and did not change in cells exposed to 1.4 mM Ca++. However, genistein inhibited the increase in PLC activity induced by 1.4 mM Ca++. In contrast, transforming growth factor (TGF)alpha, which stimulates both PLC activity and growth in basal keratinocytes, increased tyrosine phosphorylation of PLC-gamma 1. These results suggest that tyrosine phosphorylation of PLC-gamma 1 by the epidermal growth factor (EGF) receptor is linked to stimulated proliferation, whereas stimulation of PLC activity by Ca++ is linked to keratinocyte differentiation and involves the action of a tyrosine kinase but not tyrosine phosphorylation of PLC-gamma 1. Based on studies using the intracellular free Ca++ chelator BAPTA, a rise in intracellular free Ca++ was not required for stimulation of PLC activity by raising extracellular Ca++. Phorbol esters inhibited PLC stimulation by 1.4 mM Ca++ medium and increased serine phosphorylation of PLC-gamma 1. Exogenous phosphatidylinositol-specific and phosphatidylcholine-specific bacterial PLC also inhibited endogenous inositol phosphate formation and increased endogenous diacylglycerol (DAG). Thus, direct serine phosphorylation of PLC-gamma 1 by protein kinase C is associated with the inhibition of Ca(++)-mediated PLC stimulation. These results show that keratinocytes have multiple mechanisms to regulate PLC activity in response to a specific signal.

Phospholipase D activation regulates endothelin-1 stimulation of phosphoinositide-specific phospholipase C in SK-N-MC cells

Endothelin-1 (ET-1) is known to stimulate phospholipase C (PLC) activity in SK-N-MC human neuroblastoma/epithelioma cells: here we show that phospholipase D (PLD) is also stimulated. The generation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) by ET-1-stimulated PLC was attenuated by protein kinase C (PKC) activation and enhanced by PKC inhibition. An enhancement of ET-1-stimulated Ins(1,4,5)P3 accumulation was also seen when the product of PLD activity was either diverted into phosphatidyl butanol in the presence of butanol, or phosphatidate phosphohydrolase (PPH) activity was inhibited by DL-propranolol. We conclude that there is an inhibitory, PKC-mediated, feedback loop in these cells which is dependent, in part, on the activation of PKC by product(s) of the PLD/PPH pathway. This provides a novel role for agonist-stimulated PLD activation.

Distinct mechanisms of phospholipase D activation and attenuation utilized by different mitogens in NIH-3T3 fibroblasts

The activation of phospholipase D (PLD) by platelet-derived growth factor (PDGF), prostaglandin F2 alpha and 12-O-tetradecanoylphorbol 13-acetate (TPA) was studied in NIH-3T3 fibroblasts. PLD activation was determined by measuring the production of both [3H]phosphatidic acid and [3H]phosphatidylpropanol (products of the PLD-catalyzed hydrolysis and transphosphatidylation reactions, respectively), in cells that were metabolically pre-labeled with [3H]oleic acid. All mitogens caused a rapid (within 2 min) activation of PLD. Activation of PLD by prostaglandin F2 alpha and PDGF was transient and declined to near basal levels by 15 min and 55 min, respectively. In contrast, TPA-induced activation of PLD was sustained for at least 60 min of incubation. A combination of maximally effective concentrations of PDGF and TPA stimulated PLD activity in a non-additive manner, while the effect of prostaglandin F2 alpha was additional to that of either PDGF or TPA. The protein kinase inhibitor staurosporine inhibited PLD activation by PDGF or TPA with almost identical dose/response curves. In contrast, staurosporine potentiated prostaglandin-F2 alpha-induced PLD activation. The specific protein kinase C inhibitor GF109203X (a bisindolylmaleimide) inhibited PLD activation by prostaglandin F2 alpha and PDGF at concentrations higher than those required for inhibition of PLD activation induced by TPA. Depletion of cellular protein kinase C abolished PLD activation by all three mitogens without affecting in vitro activity of membrane-bound PLD. The distinct kinetics of PLD activation and its differential susceptibility to protein kinase inhibitors suggest the existence of agonist-specific activation and/or inactivation mechanisms. The results indicate also that protein kinase C participates in the mechanism of PLD activation via PDGF, while the effect of prostaglandin F2 alpha involves a pathway independent of protein kinase C.

Association between leukotriene B4-induced phospholipase D activation and degranulation of human neutrophils

We have explored the role of phospholipase D (PLD) activation in leukotriene B4 (LTB4)-induced Ca2+ mobilization and degranulation of human neutrophils. Stimulation of [3H]alkyl-acyl-phosphatidylcholine-labeled neutrophils with LTB4 resulted in a rapid accumulation of [3H]alkyl-phosphatidic acid (PA) as well as a somewhat slower accumulation of [3H]alkyl-diglyceride (DG). In the presence of ethanol, PLD catalyzed a transphosphatidylation reaction in which LTB4 increased [3H]alkyl-phosphatidylethanol formation and simultaneously decreased LTB4-induced PA and DG accumulation. This pattern of lipid metabolism is consistent with the conclusion that LTB4 stimulates PLD activity in human neutrophils. Additional studies in which the extracellular and intracellular concentrations of Ca2+ were varied indicated that maximal LTB4-induced PLD activation was dependent upon Ca2+ and potentiated by inhibitors of protein kinase C. The time-course and concentration-response curves for LTB4-induced PLD activation were different from those for LTB4-induced Ca2+ mobilization, as measured by fura-2 fluorescence. On the other hand, the concentration-response curve for LTB4-induced PLD activation was similar to that for LTB4-induced degranulation. Preincubation of the cells with ethanol inhibited LTB4-induced PA and DG accumulation, as well as degranulation, suggesting that one or both of these metabolites were important for this response. In contrast, ethanol had no effect on LTB4-induced Ca2+ mobilization. Propranolol, an inhibitor of phosphatidate phosphohydrolase, abolished DG accumulation in response to LTB4 but had no effect on degranulation, suggesting that PA is more important than DG as a mediator of degranulation. Taken collectively, these data indicate that LTB4-induced activation of PLD in human neutrophils is mediated by a Ca(2+)-dependent mechanism, but not by protein kinase C. In addition, PLD activation in these cells may induce degranulation, but not Ca2+ mobilization.