Regulation of phospholipase D by protein kinase C

ICAM-1-suPAR-CD11b Axis Is a Novel Therapeutic Target for Metastatic Triple-Negative Breast Cancer

Accumulating evidence demonstrates that circulating tumor cell (CTC) clusters have higher metastatic ability than single CTCs and negatively correlate with cancer patient outcomes. Along with homotypic CTC clusters, heterotypic CTC clusters (such as neutrophil-CTC clusters), which have been identified in both cancer mouse models and cancer patients, lead to more efficient metastasis formation and worse patient outcomes. However, the mechanism by which neutrophils bind to CTCs remains elusive. In this study, we found that intercellular adhesion molecule-1 (ICAM-1) on triple-negative breast cancer (TNBC) cells and CD11b on neutrophils mediate tumor cell-neutrophil binding. Consequently, CD11b deficiency inhibited tumor cell-neutrophil binding and TNBC metastasis. Furthermore, CD11b mediated hydrogen peroxide (H2O2) production from neutrophils. Moreover, we found that ICAM-1 in TNBC cells promotes tumor cells to secrete suPAR, which functions as a chemoattractant for neutrophils. Knockdown of uPAR in ICAM-1+ TNBC cells reduced lung-infiltrating neutrophils and lung metastasis. Bioinformatics analysis confirmed that uPAR is highly expressed in TNBCs, which positively correlates with higher neutrophil infiltration and negatively correlates with breast cancer patient survival. Collectively, our findings provide new insight into how neutrophils bind to CTC to facilitate metastasis and discover a novel potential therapeutic strategy by blocking the ICAM-1-suPAR-CD11b axis to inhibit TNBC metastasis.

Effects of flutamide on [methyl-(3)h]-choline uptake in human prostate cancer-3 cells: a pilot study

BACKGROUND

Positron emission tomography using [methyl-(11)C]-choline is effective in imaging many types of cancer, especially prostate cancer (PC). The antiandrogen flutamide is often used as part of the initial treatment of PC. Data on the effect of flutamide on and methylcholine incorporation into PC-3 cells are lacking in the experimental and literature work.

OBJECTIVES

The aims of this study were to assess whether human PC-3 cells are susceptible to flutamide and whether the drug modulates the uptake of [methyl-(3)H]-choline into these cells.

METHODS

PC-3 cells were treated for 3 days with flutamide (≤100 nmol/L), inhibiting growth by 20% to 70% with control cells included. Two viability tests (cytotoxic analyses), the thiazole blue assay and the trypan blue exclusion method, were used to determine the median inhibitory concentration for flutamide (10 nmol/L). Control and flutamide-treated cells were incubated with [methyl-(3)H]-choline for 10 minutes and then in nonradioactive medium for 10 minutes to simulate the rapid blood clearance of [methyl-(11)C]-choline tracer that occurs within 5 to 20 minutes, and then extracted using organic and aqueous solvents to determine the intracellular distribution of the tracer. Protein assay and flow-cytometry analysis were used to determine protein content and DNA synthesis in both control and treated cells. The uptake of [methyl-(3)H]-choline was normalized to protein content and expressed as mean (SD) dpm/1Jg protein (n = 6).

RESULTS

PC-3 cell proliferation was inhibited with flutamide treatment. After treatment of PC-3 cells with flutamide 10 nmol/L for 3 days, cells accumulated DNA during the S phase. Mean (SD) [methyl-(3)H]-choline uptake was found to be significantly lower with flutamide 10-nmol/L-treated cells compared with control cells (65.95 [0.72] vs 114.21 [0.57] dpm/1Jg protein; P < 0.001); the difference between the 5-nmol/L-treated cells and controls was nonsignificant.

CONCLUSIONS

In this pilot study, flutamide inhibited tumor cell growth and proliferation and decreased (modulated) the uptake of [methyl-(3)H]-choline into androgen receptor-negative PC-3 cells. These results suggest that flutamide might inhibit proliferation by an androgen-independent mechanism.

Protease-activated receptors differentially regulate human platelet activation through a phosphatidic acid-dependent pathway

Pathological conditions such as coronary artery disease are clinically controlled via therapeutic regulation of platelet activity. Thrombin, through protease-activated receptor (PAR) 1 and PAR4, plays a central role in regulation of human platelet function in that it is known to be the most potent activator of human platelets. Currently, direct thrombin inhibitors used to block platelet activation result in unwanted side effects of excessive bleeding. An alternative therapeutic strategy would be to inhibit PAR-mediated intracellular platelet signaling pathways. To elucidate the best target, we are studying differences between the two platelet thrombin receptors, PAR1 and PAR4, in mediating thrombin's action. In this study, we show that platelet activation by PAR1-activating peptide (PAR1-AP) requires a phospholipase D (PLD)-mediated phosphatidic acid (PA) signaling pathway. We show that this PAR1-specific PA-mediated effect is not regulated through differential granule secretion after PAR-induced platelet activation. Perturbation of this signaling pathway via inhibition of lipid phosphate phosphatase-1 (LPP-1) by propranolol or inhibition of the phosphatidylcholine-derived phosphatidic acid (PA) formation by PLD with a primary alcohol significantly attenuated platelet activation by PAR1-AP. Platelet activation by thrombin or PAR4-AP was insensitive to these inhibitors. Furthermore, these inhibitors significantly attenuated activation of Rap1 after stimulation by PAR1-AP but not thrombin or PAR4-AP. Because PA metabolites such as diacylglycerol play an important role in intracellular signaling, identifying crucial differences in PA regulation of PAR-induced platelet activation may lead to a greater understanding of the role of PAR1 versus PAR4 in progression of thrombosis.

A role of p44/42 mitogen‐activated protein kinases in formylpeptide receptor‐mediated phospholipase D activity and oxidant production

Phosphatidylcholine-specific phospholipase D (PLD) is a major cellular source of phosphatidic acid and choline, which regulate various physiopathological processes. PLD activation mediated by chemoattractants involves protein phosphorylation. This study provides pharmacological and biochemical evidence of a major role of p44/42 MAP kinases (ERK1/2) in PLD activation induced by the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP). ERK1/2 inhibition by the MEK1/2 antagonist U0126 in neutrophilic HL-60 cells or HEK 293T cells stably expressing fMLP receptors abolished fMLP-mediated PLD activity. Conversely, a constitutively activated MEK1 mutant expressed in HEK 293T cells potentiated fMLP-induced PLD activity. Expression of inactive PLD mutants showed that PLD2, but not PLD1, contributed to fMLP-mediated PLD activity. PLD2 co-immunoprecipitated with ERK1/2 and became phosphorylated on MAP kinase consensus sites in fMLP-stimulated cells. In cell-free systems, ERK2 gave rise to strong ATP-dependent PLD activity and directly phosphorylated PLD2 that generated two phosphopeptides only after tryptic digestion. Finally, pharmacological inhibition of ERK activation and the inhibition of PLD expression by antisense oligonucleotides in HL-60 cells suggest that the ERK/PLD2 pathway contributes to fMLP-mediated oxidant production. In conclusion, the fMLP-mediated PLD activity is regulated by ERK1/2, involving a predominant contribution of PLD2. The ERK/PLD2 coupling may provide potential pharmacological targets to control PLD-associated cellular dysfunctions.

The MARCKS family of phospholipid binding proteins: regulation of phospholipase D and other cellular components

Myristoylated alanine-rich C kinase substrate (MARCKS) and MARCKS-related protein (MRP) are essential proteins that are implicated in coordination of membrane-cytoskeletal signalling events, such as cell adhesion, migration, secretion, and phagocytosis in a variety of cell types. The most prominent structural feature of MARCKS and MRP is a central basic effector domain (ED) that binds F-actin, Ca2+-calmodulin, and acidic phospholipids; phosphorylation of key serine residues within the ED by protein kinase C (PKC) prevents the above interactions. While the precise roles of MARCKS and MRP have not been established, recent attention has focussed on the high affinity of the MARCKS ED for phosphatidylinositol 4,5-bisphosphate (PIP2), and a model has emerged in which calmodulin- or PKC-mediated regulation of these proteins at specific membrane sites could in turn control spatial availability of PIP2. The present review summarizes recent progress in this area and discusses how the above model might explain a role for MARCKS and MRP in activation of phospholipase D and other PIP2-dependent cellular processes.

Thyrotropin-releasing hormone increases phospholipase D activity through stimulation of protein kinase C in GH3 cells

Activation of phospholipase D was investigated after treatment of GH3 cells with thyrotropin-releasing hormone. Thyrotropin-releasing hormone treatment resulted in both time- and dose-dependent increases of phospholipase D activity, translocation of protein kinase C-alpha and -beta I isozymes from cytosol to membrane within 30 min, and approx 43-fold increase of phosphatidylinositol-specific phospholipase C activity. Intracellular calcium concentration was rapidly increased and diacyglycerol level remained high up to 3 h after the treatment. Pretreatment of the cells with U73122, a potent inhibitor of phosphatidylinositol-specific phospholipase C, inhibited thyrotropin-releasing hormone-induced phospholipase D activation. Protein kinase C activity was down-regulated by pretreatment of the GH3 cells with either protein kinase C inhibitors (RO320432, GF109203X) or preincubation of the cells with phorbol myristrate acetate (500 nM) for 24 h. This treatment largely abolished the thyrotropin-releasing hormone-induced activation of phospholipase D, thus further confirming the involvement of protein kinase C in the activation. These results suggest that thyrotropin-releasing hormone-induced phospholipase D activation may be due to phosphatidylinositol-specific phospholipase C, and activation of protein kinase C isozymes is responsible for this stimulation.

PKCδ inhibits PKCα-mediated activation of phospholipase D1 in a manner independent of its protein kinase activity

The regulation of phospholipase D1 (PLD1) by protein kinase C (PKC) isoforms was analyzed in human melanoma cell lines. 12-O-Tetradecanoylphorbol-13-acetate (TPA)-induced PLD1 activation was suppressed by the introduction of PKCdelta as well as its kinase-negative mutant in MeWo cells, which contain PKCalpha but lack PKCbeta. PLD activity was not affected by PKCdelta in G361 cells, which have PKCbeta but are deficient in PKCalpha. In MeWo cells introduced by PKCalpha and PLD1, the association of these proteins was observed, which was enhanced by the TPA treatment. In cells overexpressing PKCdelta in addition to PKCalpha and PLD1, TPA treatment increased the association of PKCdelta and PLD1, while it attenuated the association of PKCalpha and PLD1. These results indicate that PKCdelta inhibits TPA-induced PLD1 activation mediated by PKCalpha through the association with PLD1.

Protein kinase C alpha associates with phospholipase D1 and enhances basal phospholipase D activity in a protein phosphorylation-independent manner in human melanoma cells

It is well known that phospholipase D plays a crucial part in the signal transduction of many types of cells, and is activated by protein kinase C alpha when cells are stimulated. To elucidate the role of phospholipase D in melanoma, the expression of phospholipase D1 and protein kinase C alpha in primary and metastatic lesions of acral lentiginous melanoma and superficial spreading melanoma was investigated using immunohistologic techniques. In addition, the mechanism of regulation of phospholipase D1 by protein kinase C alpha was examined in a human melanoma cell line HM3KO using an adenovirus-mediated gene transfer technique. Both phospholipase D1 and protein kinase C alpha were strongly expressed in primary and metastatic lesions of superficial spreading melanoma. Conversely, in acral lentiginous melanoma lesions, the expression of these two proteins increased dramatically with tumor progression; the expression of both phospholipase D1 and protein kinase C alpha was almost negative in the radial growth phase of primary acral lentiginous melanoma lesions, and increased synchronously in a progression-related manner in advanced acral lentiginous melanoma lesions, including vertical growth phase and metastatic lesions. Immunoprecipitation study showed that phospholipase D1 and protein kinase C alpha are associated physiologically in resting melanoma cells. Further immunoprecipitation study using HM3KO cells after adenovirus-mediated simultaneous overexpression of phospholipase D1 and protein kinase C alpha, or phospholipase D1 and the kinase-negative mutant of protein kinase C alpha revealed that both protein kinase C alpha and the kinase-negative mutant of protein kinase C alpha are associated with phospholipase D1 in melanoma cells in the absence of an external signal. Overexpression of protein kinase C alpha or the kinase-negative mutant of protein kinase C alpha in melanoma cells by the adenovirus vectors resulted in the enhancement of basal phospholipase D activity in a viral concentration-dependent manner. Furthermore, enhanced basal phospholipase D activity increased the in vitro invasive potential of HM3KO cells. These results suggest that upregulation of phospholipase D1 and protein kinase C alpha plays a part in the progression of acral lentiginous melanoma from the radial growth phase to the vertical growth phase. The present results also suggest that protein kinase C alpha associates with phospholipase D1 and enhances basal phospholipase D activity in a protein phosphorylation-independent manner in melanoma cells, which contributes to the cell's high invasive potential.

Phospholipase C-protein kinase C mediated phospholipase D activation pathway is involved in tamoxifen induced apoptosis

Tamoxifen (TAM) is the endocrine therapeutic agent the most widely used in the treatment of breast cancer, and it operates primarily through the induction of apoptosis. In this study, we attempted to elucidate the non-ER mediated mechanism behind TAM treatment, involving the phospholipase C-protein kinase C (PLC-PKC) mediated phospholipase D (PLD) activation pathway, using multimodality methods. In TAM treated MCF7 cells, the PLC and PLD protein and mRNA levels increased. Phosphatidylethanol (PEt) and diacylglycerol (DAG) generation also increased, showing increased activity of PLD and PLCgamma1. Translocation of PKCalpha, from cytosol to membrane, was observed in TAM treated cells. By showing that both PKC and PLC inhibitors could reduce the effects of TAM-induced PLD activation, we confirmed the role of PKC and PLC as upstream regulators of PLD. Finally, we demonstrated that TAM treatment reduced the viability of MCF7 cells and brought about rapid cell death. From these results, we confirmed the hypothesis that TAM induces apoptosis in breast cancer cells, and that the signal transduction pathway, involving PLD, PLC, and PKC, constitutes one of the possible mechanisms underlying the non-ER mediated effects associated with TAM.

ADP-ribosylation factor-dependent phospholipase D2 activation is required for agonist-induced mu-opioid receptor endocytosis

Agonist exposure of many G protein-coupled receptors induces a rapid receptor phosphorylation and uncoupling from G proteins. Resensitization of these desensitized receptors requires endocytosis and subsequent dephosphorylation. Using a yeast two-hybrid screen, the rat mu-opioid receptor (MOR1, also termed MOP) was found to be associated with phospholipase D2 (PLD2), a phospholipid-specific phosphodiesterase located in the plasma membrane, which has been implicated in the formation of endocytotic vesicles. Coimmunoprecipitation experiments in HEK293 cells coexpressing MOR1 and PLD2 confirmed that MOR1 constitutively interacts with PLD2. Treatment with the mu receptor agonist DAMGO ([d-Ala(2), Me Phe(4), Glyol(5)]enkephalin) led to an increase in PLD2 activity, whereas morphine, which does not induce MOR1 receptor internalization, failed to induce PLD2 activation. The DAMGO-mediated PLD2 activation was inhibited by brefeldin A, an inhibitor of ADP-ribosylation factor (ARF) but not by the protein kinase C (PKC) inhibitor calphostin C indicating that opioid receptor-mediated activation of PLD2 is ARF- but not PKC-dependent. Furthermore, heterologous stimulation of PLD2 by phorbol ester led to an accelerated internalization of the mu-opioid receptor after both DAMGO and morphine exposure. Conversely the inhibition of PLD2-mediated phosphatidic acid formation by 1-butanol or overexpression of a negative mutant of PLD2 prevented agonist-mediated endocytosis of MOR1. Together, these data suggest that PLD2 play a key role in the regulation of agonist-induced endocytosis of the mu-opioid receptor.

Calcium and protein kinase C (PKC)-related kinase mediate alpha 1A-adrenergic receptor-stimulated activation of phospholipase D in rat-1 cells, independent of PKC

A previous study conducted in rat-1 cells expressing alpha(1A)-adrenergic receptors showed that phenylephrine (PHE) stimulates phospholipase D (PLD) activity. This study was conducted to determine the contribution of protein kinase C (PKC) to PHE-induced PLD activation in these cells. PKC inhibitors bisindolylmaleimide (BIM) I and Ro 31-8220, but not Gö 6976 or a pseudosubstrate peptide inhibitor of PKCalpha, decreased PLD activity and arachidonic acid release elicited by PHE. However, antisense oligonucleotides directed against PKC alpha, delta, epsilon, and eta reduced PKC isoform levels by about 80% but failed to alter PHE-induced PLD activation, indicating that these PKC isoforms are not involved in PLD activation elicited by alpha1A-adrenergic receptor stimulation. Ectopic expression of a kinase-deficient mutant of the PKC-related kinase PKN significantly attenuated PHE-induced PLD activation. On the other hand, BIM I and Ro 31-8220 blocked PHE-mediated increase in intracellular Ca2+ but Gö 6976 and the peptide inhibitor did not. In the absence of extracellular Ca2+, PHE failed to increase PLD activity. These results indicate that alpha1A-adrenergic receptor-stimulated PLD activation is mediated by a mechanism independent of PKCalpha, delta, epsilon, and eta, but dependent on a PKC-related kinase, PKN. Moreover, PKC inhibitors BIM I and Ro 31-8220 block PHE-induced PLD activity by inhibiting calcium signal. Caution should be used in interpreting the data obtained with PKC inhibitors in vivo.

Dual regulation of phospholipase D1 by protein kinase C alpha in vivo

The regulation of phospholipase D1 (PLD1), which has been shown to be activated by protein kinase C (PKC) alpha, was investigated in the human melanoma cell lines. In G361 cell line, which lacks PKCalpha, 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced PLD activation was potentiated by introducing PKCalpha by the adenovirus vector. The kinase-negative PKCalpha elevated TPA-induced PLD activity less significantly than the wild type. A PKC specific inhibitor GF109203X lowered PLD activation in the cells expressing PKCalpha, but did not prevent PLD potentiation induced by the kinase-negative PKCalpha. Expression of PKCbetaII and the kinase-negative PKCbetaII enhanced TPA-stimulated PLD activity moderately in MeWo cell line, in which PKCbetaII is absent. Furthermore, the TPA treatment increased the association of PKCalpha, PKCbetaII, and their kinase-negative mutants with PLD1 in melanoma cells. These results indicate that PLD1 is dually regulated through phosphorylation as well as through the protein-protein interaction by PKCalpha, and probably by PKCbetaII, in vivo.

Activation of phospholipase D in porcine tracheal smooth muscle: role of phosphatidylinositol 3-kinase and RhoA activation

Muscarinic receptor agonists transiently activate phospholipase D in tracheal smooth muscle. Muscarinic activation of phospholipase D in this tissue is dependent on activation of protein kinase C and an unidentified pathway that is not protein kinase C dependent. Cholinergic agents have also been shown to activate phospholipase D by pathways linked to the small G protein, RhoA. This study explores the relationship between muscarinic activation of phophatidylinositol 3-kinase and activation of RhoA, and examines whether phospholipase D activation is dependent on either pathway in tracheal smooth muscle. Wortmannin or 2-(4-morphonyl)-8-phenyl-4H-1-benzopyran-4-one (LY-294002), putative specific inhibitors of phophatidylinositol 3-kinase, significantly inhibit acetylcholine-induced formation of phosphatidylethanol and also block acetylcholine-induced translocation of RhoA to the membrane. In previous experiments calphostin C, a protein kinase C inhibitor, partially inhibited both acetylcholine-induced and phorbol-12-myristate-13-acetate (PMA)-induced phosphatidylethanol formation. In the present study calphostin C did not block acetylcholine-induced RhoA translocation to the membrane. However, the Rho kinase inhibitor, N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide (Y-27632), significantly inhibited acetylcholine-induced phosphatidylethanol formation, but had no effect on activation of phospholipase D by PMA. Acetylcholine treatment also stimulated the phosphorylation of the 110-kDa subunit of phosphatidylinositol 3-kinase. Phosphorylation of phosphatidylinositol 3-kinase 110-kDa subunit could be blocked by wortmannin in a concentration-dependent manner, and acetylcholine-induced phosphatidylinositol 3-kinase activity was significantly inhibited by wortmannin. LY-294002 also inhibited acetylcholine-induced phosphorylation of 110-kDa subunit and activation of phosphatidylinositol 3-kinase. These results suggest that acetylcholine stimulation translocates RhoA to the membrane by a phosphatidylinositol 3-kinase-dependent mechanism and acetylcholine-induced phospholipase D stimulation is at least partly mediated via phosphatidylinositol 3-kinase, however, protein kinase C appears to activate phospholipase D independent of phosphatidylinositol 3-kinase or RhoA activation in porcine tracheal smooth muscle.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Protein kinase C activation by 12-0-tetradecanoylphorbol 13-acetate in CG-4 line oligodendrocytes stimulates turnover of choline and ethanolamine phospholipids by phospholipase D and induces rapid process contraction

Treatment of [3H]-choline- or [14C]-ethanolamine-labelled undifferentiated bipolar and differentiated multipolar CG-4 line oligodendrocytes with 12-0-tetradecanoylphorbol 13-acetate (TPA) to activate protein kinase C stimulated the release of choline or ethanolamine metabolites to the medium over controls. Ro31-8220, a PKC inhibitor, reduced TPA-stimulated release of choline- and ethanolamine-metabolites to basal levels. TPA treatment of both bipolar and multipolar cells caused rapid contraction of processes leaving rounded up cells: this effect was blocked by Ro31-8220. After 12-15 h exposure to TPA, bipolar undifferentiated CG-4 line cells extended short processes again and the cells became multipolar. Nocodozole, an agent which disrupts microtubules and caused CG-4 line cells to round up, caused increased choline or ethanolamine-metabolite release to the medium over basal levels suggesting that some release during TPA-treatment might occur due to process fragmentation. However, the transphosphatidylation reaction confirmed that phospholipase D was active in these cells. Exposure of bipolar undifferentiated CG-4 line cells to TPA resulted in down-regulatation of PKC-alpha and PKC-beta which could not be detected by Western blotting after a few hours; PKC-epsilon was down-regulated much more slowly but PKCs delta, zeta and iota were not influenced by 48 h exposure of cells to TPA. Formation of phosphatidylethanol in the transphosphatidylation reaction was markedly reduced in TPA down-regulated cells indicating a role for PKCs alpha and beta in phospholipase D activation in CG-4 line oligodendrocytes.

Interaction of the type Ialpha PIPkinase with phospholipase D: a role for the local generation of phosphatidylinositol 4, 5-bisphosphate in the regulation of PLD2 activity

Phosphoinositides are localized in various intracellular compartments and can regulate a number of intracellular functions, such as cytoskeletal dynamics and membrane trafficking. Phospholipase Ds (PLDs) are regulated enzymes that hydrolyse phosphatidylcholine (PtdCho) to generate the putative second messenger phosphatidic acid (PtdOH). In vitro, PLDs have an absolute requirement for higher phosphorylated inositides, such as phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)]. Whether this lipid is able to regulate the activity of PLD in vivo is contentious. To examine this hypothesis we studied the relationship between PLD and an enzyme critical for the intracellular synthesis of PtdIns(4,5)P(2): phosphatidylinositol 4-phosphate 5-kinase alpha (Type Ialpha PIPkinase). We find that both PLD1 and PLD2 interact with the Type Ialpha PIPkinase and that PLD2 activity in vivo can be regulated solely by the expression of this lipid kinase. Moreover, PLD2 is able to recruit the Type Ialpha PIPkinase to its intracellular location. We show that the physiological requirement of PLD enzymes for PtdIns(4,5)P(2) is critical and that PLD2 activity can be regulated solely by the levels of this key intracellular lipid.

Expression of a peptide binding to receptor for activated C-kinase (RACK1) inhibits phorbol myristoyl acetate-stimulated phospholipase D activity in C3H/10T1/2 cells: dissociation of phospholipase D-mediated phosphatidylcholine breakdown from its synthesis

The C3H/10T1/2 Cl8 HAbetaC2-1 cells used in this study express a peptide with a sequence shown to bind receptor for activated C-kinase (RACK1) and inhibit cPKC-mediated cell functions. Phorbol myristoyl acetate (PMA) strongly stimulated phosphatidylcholine (PtdCho)-specific phospholipase D (PLD) activity in the C3H/10T1/2 Cl8 parental cell line, but not in Cl8 HAbetaC2-1 cells, indicating that full PLD activity in PMA-treated Cl8 cells is dependent on a functional interaction of alpha/betaPKC with RACK1. In contrast, the PMA-stimulated uptake of choline and its subsequent incorporation into PtdCho, were not inhibited in Cl8 HAbetaC2-1 cells as compared to Cl8 cells, indicating a RACK1-independent but PKC-mediated process. Increased incorporation of labelled choline into PtdCho upon PMA treatment was not associated with changes of either CDP-choline: 1,2-diacylglycerol cholinephosphotransferase activity or the CTP:phosphocholine cytidylyltransferase distribution between cytosol and membrane fractions in Cl8 and Cl8 HAbetaC2-1 cells. The major effect of PMA on the PtdCho synthesis in C3H/10T1/2 fibroblasts was to increase the cellular uptake of choline. As a supporting experiment, we inhibited PMA-stimulated PtdH formation by PLD, and also putatively PtdH-derived DAG, in Cl8 cells with 1-butanol. Butanol did not influence the incorporation of [(14)C]choline into PtdCho. The present study shows: (1) PMA-stimulated PLD activity is dependent on a functional interaction between alpha/betaPKC and RACK1 in C3H/10T1/2 Cl8 fibroblasts; and (2) inhibition of PLD activity and PtdH formation did not reduce the cellular uptake and incorporation of labelled choline into PtdCho, indicating that these processes are not directly regulated by PtdCho-PLD activity in PMA-treated C3H/10T1/2 Cl8 fibroblasts.

Phosphatidylcholine metabolism in nuclei of phorbol ester-activated LA-N-1 neuroblastoma cells

The agonist stimulation of a variety of cells results in the induction of specific lipid metabolism in nuclear membranes, supporting the hypothesis of an important role of the lipids in nuclear signal transduction. While the existence of a phosphatidylinositol cycle has been reported in cellular nuclei, little attention has been given to the metabolism of phosphatidylcholine in nuclear signaling. In the present study the metabolism of phosphatidylcholine in the nuclei of neuroblastoma cells LA-N-1 was investigated. The incubation of LA-N-1 nuclei with radioactive choline, phosphocholine or CDP-choline led to the production of labelled phosphatidylcholine. The incorporation of choline and phosphocholine but not CDP-choline was enhanced in nuclei of TPA treated cells. Moreover the presence of choline kinase, phosphocholine cytidylyltransferase and phosphocholine transferase activities were detected in the nuclei and the TPA treatment of the cells stimulated the activity of the phosphocholine cytidylyltransferase. When cells prelabelled with [3H]palmitic acid were stimulated with TPA in the presence of ethanol, an increase of labelled diacylglycerol and phosphatidylethanol in the nuclei was observed. Similarly, an increase of labelled diacylglycerol and phosphatidic acid but not of phosphatidylethanol occurred in [3H]palmitic acid prelabelled nuclei stimulated with TPA in the presence of ethanol. However the production of phosphatidylethanol was observed when the nuclei were treated with TPA in the presence of ATP and GTPgammaS. The stimulation of [3H]choline prelabelled nuclei with TPA also generated the release of free choline and phosphocholine. The results indicate the presence of PLD and probably PLC activities in LA-N-1 nuclei and the involvement of phosphatidylcholine in the production of nuclear lipid second messengers upon TPA stimulation of LA-N-1 cells. The correlation of the disappearance of phosphatidylcholine, the production of diacylglycerol and phosphatidic acid with the stimulation of phosphatidylcholine synthesis in nuclei of TPA treated LA-N-1 suggests the existence of a phosphatidylcholine cycle in these nuclei.

Regulation of phospholipase D from human hepatocarcinoma cell line by purine nucleotides and protein kinase A

The regulation of phosphatidylcholine-specific phospholipase D by purine nucleotides and protein kinase A were studied in vitro using an enzyme preparation partially purified from the membranous fraction of 7721 hepatocarcinoma cells. It was found that the enzyme activity was elevated by low concentrations of some purine nucleotides, but the activating effects were decreased when the concentrations of the nucleotides were higher. The optimal concentrations of GTP, GTPgamma[S], GDP and ATP for maximal activation were 0.1 mM, 5 microM, 1 mM and 1 mM respectively. The activation caused by 1 mM ADP was lower. The enzyme was not activated by 1 mM AMP, but significant activation was observed by the addition of 1 mM cAMP. The latter was mediated by protein kinase A, as a specific inhibitor of protein kinase A abolished the activation. There were synergic effects between ATP and GTP, ATP and PIP2, but not between ATP and GTPgamma[S], or PIP2 and GTPgamma[S]. The activating effects of GTP and ATP were abolished by neomycin, a PIP2 scavenger. These results suggest that phospholipase D is regulated by GTP-binding protein and the presence of PIP2 is required for the activation induced by GTP. Protein kinase A may be another protein kinase in addition to protein kinase C and protein tyrosine kinase which regulate the activity of phospholipase D, when the intracellular concentration of cAMP is increased.

Ethanol inhibits astroglial cell proliferation by disruption of phospholipase D-mediated signaling

The activation of phospholipase D (PLD) is a common response to mitogenic stimuli in various cell types. As PLD-mediated signaling is known to be disrupted in the presence of ethanol, we tested whether PLD is involved in the ethanol-induced inhibition of cell proliferation in rat cortical primary astrocytes. Readdition of fetal calf serum (FCS) to serum-deprived astroglial cultures caused a rapid, threefold increase of PLD activity and a strong mitogenic response; both effects were dependent on tyrosine kinases but not on protein kinase C. Ethanol (0.1-2%) suppressed the FCS-induced, PLD-mediated formation of phosphatidic acid (PA) as well as astroglial cell proliferation in a concentration-dependent manner. Moreover, exogenous bacterial PLD increased astroglial proliferation in an ethanol-sensitive manner, whereas exogenous PA or lysophosphatidic acid was less effective. Formation of PA and astroglial proliferation were strongly inhibited by 1-butanol (0.1-1%), a substrate of PLD, but were unaffected by t-butanol, a non-substrate; 2-butanol had intermediate effects. Platelet-derived growth factor and endothelin-1 mimicked the mitogenic effect of FCS; their effects were also inhibited by the butanols in the potency order 1-butanol > 2-butanol > tert-butanol. Our results, in particular, the differential effects of 1-, 2-, and tert-butanol with respect to PA formation and astroglial proliferation, strongly suggest that the antiproliferative effects of ethanol in glial cells are due to the disruption of the PLD signaling pathway. This mechanism may also contribute to the inhibition of astroglial growth and brain development observed in alcoholic embryopathy.

Cytosolic Phospholipase A2-Mediated Regulation of Phospholipase D2 in Leukocyte Cell Lines

Phospholipase D (PLD) has been implicated in a variety of cellular processes, including inflammation, secretion, and respiratory burst. Two distinct PLD isoforms, designated PLD1 and PLD2, have been cloned; however, the regulatory mechanism for each PLD isoform is not clear. In our present study we investigated how PLD2 activity is regulated in mouse lymphocytic leukemia L1210 cells, which mainly contain PLD2 , and in PLD2 -transfected COS-7 cells. Intriguingly, A23187, a calcium ionophore that induces calcium influx, potently stimulates PLD activity in these two cell lines, suggesting that Ca2+ might be implicated in the regulation of the PLD2 activity. In addition to the A23187-induced PLD2 activation, A23187 also increases PLA2-mediated arachidonic acid release, and the A23187-stimulated PLD2 and PLA2 activities could be blocked by pretreatment of the cells with cytosolic calcium-dependent PLA2 (cPLA2) inhibitors, such as arachidonyl trifluoromethyl ketone and methyl arachidonyl fluorophosphonate in these two cell lines. Moreover, the A23187-induced PLD2 and PLA2 activities could be inhibited by cotransfection with antisense cPLA2 oligonucleotide. These results suggest a role for cPLA2 in the regulation of PLD2 activity in vivo. The inhibitory effect of arachidonyl trifluoromethyl ketone on the A23187-induced PLD2 activity could be recovered by addition of exogenous lysophosphatidylcholine. This study is the first to demonstrate that PLD2 activity is up-regulated by Ca2+ influx and that cPLA2 may play a key role in the Ca2+-dependent regulation of PLD2 through generation of lysophosphatidylcholine.

Tumour phospholipid metabolism

Following the impetus of early clinical and experimental investigations, in vivo and in vitro MRS studies of tumours pointed in the eighties to the possible significance of signals arising from phospholipid (PL) precursors and catabolites as novel biochemical indicators of in vivo tumour progression and response to therapy. In the present decade, MRS analyses of individual components contributing to the 31P PME (phosphomonoester) and PDE (phosphodiester) resonances, as well as to the 1H 'choline peak', have reinforced some of these expectations. Moreover, the absolute quantification of these signals provided the basis for addressing more specific (although still open) questions on the biochemical mechanisms responsible for the formation of intracellular pools of PL derivatives in tumours, under different conditions of cell proliferative status and/or malignancy level. This article is aimed at providing an overview on: (a) quantitative MRS measurements on the contents of phosphocholine (PCho), phosphoethanolamine (PEtn) and their glycerol derivatives ģlycerol 3-phosphocholine (GPC) and glycerol 3-phosphoethanolamine (GPE)[ in human tumours and cells (with particular attention to breast and brain cancer and lymphomas), as well as in normal mammalian tissues (including developing organs and rapidly proliferating tissues); (b) possible correlations of MRS parameters like PEtn/PCho and PCho/GPC ratios with in vitro cell growth status and/or cell tumorigenicity; and (c) current and new hypotheses on the role and interplay of biosynthetic and catabolic pathways of the choline and ethanolamine cycles in modulating the intracellular sizes of PCho and PEtn pools, either in response to mitogenic stimuli or in relation to malignant transformation.

Differential phospholipase D activation by bradykinin and sphingosine 1-phosphate in NIH 3T3 fibroblasts overexpressing gelsolin

Gelsolin, an actin-binding protein, shows a strong ability to bind to phosphatidylinositol 4,5-bisphosphate (PIP(2)). Here we showed in in vitro experiments that gelsolin inhibited recombinant phospholipase D1 (PLD1) and PLD2 activities but not the oleate-dependent PLD and that this inhibition was not reversed by increasing PIP(2) concentration. To investigate the role of gelsolin in agonist-mediated PLD activation, we used NIH 3T3 fibroblasts stably transfected with the cDNA for human cytosolic gelsolin. Gelsolin overexpression suppressed bradykinin-induced activation of phospholipase C (PLC) and PLD. On the other hand, sphingosine 1-phosphate (S1P)-induced PLD activation could not be modified by gelsolin overexpression, whereas PLC activation was suppressed. PLD activation by phorbol myristate acetate or Ca(2+) ionophore A23187 was not affected by gelsolin overexpression. Stimulation of control cells with either bradykinin or S1P caused translocation of protein kinase C (PKC) to the membranes. Translocation of PKC-alpha and PKC-beta1 but not PKC-epsilon was reduced in gelsolin-overexpressed cells, whereas phosphorylation of mitogen-activated protein kinase was not changed. S1P-induced PLC activation and mitogen-activated protein kinase phosphorylation were sensitive to pertussis toxin, but PLD response was insensitive to such treatment, suggesting that S1P induced PLD activation via certain G protein distinct from G(i) for PLC and mitogen-activated protein kinase pathway. Our results suggest that gelsolin modulates bradykinin-mediated PLD activation via suppression of PLC and PKC activities but did not affect S1P-mediated PLD activation.

Effect of dietary lipids on phospholipase D activity in rat brain

Phospholipase D (PLD) is emerging as a major player in many novel signaling pathways. Based on recent studies correlating membrane composition with enzyme function, we speculated that feeding of dietary lipids to the newborns has a major impact on brain PLD activity. To test this hypothesis, the rat dams were fed fat-free powder containing either safflower oil or fish oil, and a control powdered chow. The pups were weaned onto the diet and sacrificed at 30 days of age. PLD activity was measured by transphosphatidylation assays using rat brain membranes. This study shows that microsome GTPgammaS-dependent PLD activity in rats fed safflower oil or fish oil was significantly reduced by 38% and 30% respectively compared to controls. Oleate-dependent PLD activity in the safflower oil group, however, was significantly increased by 38%. In contrast, synaptosome membrane (P2) GTPgammaS-dependent PLD activity in rats consuming safflower oil was significantly increased by 29%, but there was no difference in oleate-dependent PLD activity. Likewise, no difference was observed in microsome oleate-dependent PLD and P2 GTPgammaS-dependent PLD activity between the fish oil and the control groups. These results indicate that dietary lipid intake appears to modulate phospholipid metabolism and differential expression of PLD isozymes in the brain.

Effects of phosphatidylinositol 4,5-bisphosphate and neomycin on phospholipase D: kinetic studies

The kinetics of phosphatidylcholine-specific phospholipase D activated by phosphatidylinositol 4,5-bisphosphate (PIP2) and inhibition by neomycin were studied in an enzyme preparation partially purified from human hepatocarcinoma cell line. It was found that phospholipase D was marginally activated by phosphatidyl-4-phosphate (PIP) and phosphatidylethanolamine (PE). In contrast, it was considerably activated by PIP2 in different concentration of phosphatidylcholine (PC). Sphingomyelin (SM), lysophosphatidylcholine (LPC) and phosphatidylserine (PS) were neither substrates nor inhibitors of the phospholipase D. PIP, induced an allosteric effect on phospholipase D and a negative cooperative effect with respect to phosphatidylcholine as indicated in the Lineweaver-Burk plot. In the absence of PIP2, a straight line was obtained, whereas a downward concave curve was observed in the presence of 25 microM of PIP2. The Hill coefficient and the apparent K(m) of phosphatidylcholine in the presence of 25 microM PIP, were calculated to be 0.631 and 10.79 mM, respectively. PIP2 also increased the maximal velocity (Vmax) of the phospholipase D reaction, suggesting that the affinity of substrate to enzyme was decreased, and the turnover number of the enzyme (kcat) was increased by PIP2. The activation of phospholipase D by PIP2 was dose dependent up to 50 microM of PIP2. The Ka of PIP2 was 15.8 mM. Neomycin, a polycationic glycoside, was shown to be an uncompetitive inhibitor of phospholipase D, and revealed the formation of a neomycin-PIP2 complex. The Ki of neomycin was estimated to be 8.7 mM.

1,25-dihydroxyvitamin D3 but not TPA activates PLD in Caco-2 cells via pp60(c-src) and RhoA

In the accompanying paper [Khare et al., Am. J. Physiol. 276 (Gastrointest. Liver Physiol. 39): G993-G1004, 1999], activation of protein kinase C-alpha (PKC-alpha) was shown to be involved in the stimulation of phospholipase D (PLD) by 1,25-dihydroxyvitamin D3 [1, 25(OH)2D3] and 12-O-tetradecanoylphorbol 13-acetate (TPA) in Caco-2 cells. Monomeric or heterotrimeric G proteins, as well as pp60(c-src) have been implicated in PLD activation. We therefore determined whether these signal transduction elements were involved in PLD stimulation by 1,25(OH)2D3 or TPA. Treatment with C3 transferase, which inhibits members of the Rho family of monomeric G proteins, markedly diminished the ability of 1,25(OH)2D3, but not TPA, to stimulate PLD. Brefeldin A, an inhibitor of ADP-ribosylation factor proteins, did not, however, significantly reduce the stimulation of PLD by either of these agents. Moreover, 1,25(OH)2D3, but not TPA, activated pp60(c-src) and treatment with PP1, a specific inhibitor of the pp60(c-src) family, blocked the ability of 1,25(OH)2D3 to activate PLD. Pretreatment of cells with pertussis toxin (PTx) markedly reduced the stimulation of PLD by either agonist. PTx, moreover, inhibited the stimulation of pp60(c-src) and PKC-alpha by 1,25(OH)2D3. PTx did not, however, block the membrane translocation of RhoA induced by 1,25(OH)2D3 or inhibit the stimulation of PKC-alpha by TPA. These findings, taken together with those of the accompanying paper, indicate that although 1,25(OH)2D3 and TPA each activate PLD in Caco-2 cells in part via PKC-alpha, their stimulation of PLD differs in a number of important aspects, including the requirement for pp60(c-src) and RhoA in the activation of PLD by 1,25(OH)2D3, but not TPA. Moreover, the requirement for different signal transduction elements by 1,25(OH)2D3 and TPA to induce the stimulation of PLD may potentially underlie differences in the physiological effects of these agents in Caco-2 cells.

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.

Anandamide stimulates phospholipase D activity in PC12 cells but not in NIH 3T3 fibroblasts

The endogenous cannabinoid arachidonoylethanolamide was previously reported to have no effects on the phospholipase D activity in Chinese hamster ovary cells expressing the human brain-specific cannabinoid receptor, while in mouse peritoneal cells, delta9-tetrahydrocannabinol stimulated this enzyme. In this work, arachidonoylethanolamide (0.1-1 microM) was found to stimulate the phospholipase D-mediated phospholipid hydrolysis in rat adrenal pheochromocytoma PC12 cells, but not in mouse NIH 3T3 fibroblasts. The phospholipase D-activating effects of arachidonoylethanolamide were comparable to those elicited by phorbol ester and nerve growth factor, while arachidonic acid (1 microM) had no effects. The results show that, depending on the cell type, arachidonoylethanolamide can be an activator of the phospholipase D system.

Phospholipase D activation in endothelial cells is redox sensitive

Reactive oxygen species (ROS) are implicated in the pathophysiology of a number of vascular disorders, including atherosclerosis. Recent studies indicate that ROS modulate signal transduction in mammalian cells. Previously, we have shown that ROS (hydrogen peroxide, fatty acid hydroperoxide, diperoxovanadate, and 4-hydroxynonenal) enhance protein tyrosine phosphorylation and activate phospholipase D (PLD) in bovine pulmonary artery endothelial cells (BPAECs). In the present study, our aim was to investigate the role of exogenous thiol agents on ROS-induced PLD activation in conjunction with the role of cellular thiols--glutathione (GSH) and protein thiols--on PLD activation and protein tyrosine phosphorylation. Pretreatment of BPAECs with N-acetyl-L-cysteine (NAC) or 2-mercaptopropionylglycine (MPG) blocked ROS-induced changes in intracellular GSH and PLD activation. Also, pretreatment with NAC attenuated diperoxovanadate-induced protein tyrosine phosphorylation. Pretreatment of BPAECs with diamide or L-buthionine-(S,R)-sulfoximine (BSO), agents that lower intracellular GSH and thiols, enhanced PLD activity. Furthermore, NAC blocked diamide- or BSO-mediated changes in GSH levels, PLD activity, and protein tyrosine phosphorylation. NAC also attenuated diamide-induced tyrosine phosphorylation of proteins between 69 and 118 KDa. These results support the hypothesis that modulation of thiol-redox status (cellular nonprotein and protein thiols) may contribute to the regulation of ROS-induced protein tyrosine phosphorylation and PLD activation in vascular endothelium.

Activation of phospholipase D signaling pathway by epidermal growth factor in osteoblastic cells

The receptor-mediated activation of phospholipase D (PLD) is a major signaling pathway in several cell systems. This study determined the effects of epidermal growth factor (EGF) on PLD activity in normal rat osteoblastic cells. Primary cultures were obtained from fetal rat calvaria by sequential collagenase digestion and seeded in BGJb media supplemented with 10% fetal calf serum. PLD activity was assayed by the transphosphatidylation reaction in [H3]myristic acid (5 microCi/ml)-labeled cells treated with EGF in the presence of 5% ethanol and measuring the production of phosphatidylethanol (PEtOH). Lipids were extracted and separated by thin-layer chromatography, detected by iodine staining, and the areas of interest were scraped off and transferred to vials for scintillation counting. EGF significantly increased PEtOH production in a dose-dependent manner and at short (10-60 s) and long (up to 30 minutes) incubation periods (p < 0.05). Phosphatidic acid levels were also significantly increased (p < 0.05) compared with unstimulated controls, but the levels were approximately 60% less than those of PEtOH. 4b-phorbol 12-myristate, 13-acetate (PMA) also produced a significant increase in PEtOH levels when compared with unstimulated control cultures, but when PMA was added together with EGF, the production of PEtOH was reduced about 30%. Pretreatment of cells with the protein kinase C (PKC) inhibitor H-7 caused a significant increase in PEtOH levels, compared with cells stimulated with EGF alone. Preincubation of cells with pertussis toxin produced a partial decrease in PEtOH levels. This study demonstrates that EGF activates the PLD signaling cascade in normal rat osteoblastic cells and that the pathway appears to involve, at least in part, a PKC- and Gi protein-dependent mechanism.

Protein kinases and multidrug resistance

The role of protein kinases in the multidrug resistance phenotype of cancer cell lines is discussed with an emphasis on protein kinase C and protein kinase A. Evidence that P-glycoprotein is phosphorylated by these kinases is summarised and the relationship between P-glycoprotein phosphorylation and the multidrug-resistant phenotype discussed. Results showing that protein kinase C, particularly the alpha subspecies, is overexpressed in many MDR cell lines are described: this common but by no means universal finding seems to be drug- and cell line-dependent and in only in a few cases is there a direct correlation between protein kinase C activity and multidrug resistance. From co-immunoprecipitation results it is suggested that P-glycoprotein is a specific protein kinase C receptor, as well as being a substrate. Revertant experiments provide conflicting results as to a direct relationship between expression of P-glycoprotein and protein kinase C. Evidence that protein kinase A influences P-glycoprotein expression at the gene level is well documented and the mechanisms by which this occurs are becoming clarified. Results on the relationship between protein kinase C and multidrug resistance using many inhibitors and phorbol esters are difficult to interpret because such compounds bind to P-glycoprotein. In spite of huge effort, a direct involvement of protein kinase C in regulating multidrug resistance has not yet been firmly established. However, evidence that PKC regulates a Pgp-independent mechanism of drug resistance is accumulating.

Role of PKC and tyrosine kinase in ethanol-mediated inhibition of LPS-inducible nitric oxide synthase

Ethanol increases human and animal susceptibility to opportunistic lung infections in part by suppression of endotoxin (LPS) and bacteria-mediated upregulation of inducible nitric oxide synthase (iNOS) in alveolar macrophages (AM). LPS and cytokine-induced NOS mRNA are dependent on NF-kappaB/Rel (NFkappaB) and Activator Protein-1 (AP-1), which are regulated in turn by protein kinase C and tyrosine kinase-dependent phosphorylation. ETOH does not directly inhibit NFkappaB or AP-1, in vivo, but rather inhibits LPS-induced activation of the MEKK/MAP kinase system and inhibition of inhibitory protein IkappaBalpha required for formation of AP-1 and NFkappaB, respectively. in AM. Both transcription factors are involved iNOS mRNA transcription. LPS-induced upregulation of MEKK/MAP tyrosine kinase upregulates NADPH oxidase activity and oxygen free radical formation required for activation of NFkappaB and AP-1 and phosphorylation of IkappaBalpha. LPS downregulates endogenous calcium-sensitive PKC isozymes (PKCdelta), which repress iNOS mRNA expression. ETOH inhibits LPS-induced upregulation of iNOS mRNA by preventing its ability to decrease PKCdelta and upregulate tyrosine kinase-mediated phosphorylation. This effect of ETOH is prevented by inhibitors of PKC and tyrosine kinase. The data support the hypothesis that ETOH inhibits LPS-induced upregulation of iNOS mRNA by interfering with the phosphorylation processes involved in activation of the nuclear transcription factors NFkappaB and AP-1.

Phosphatidic acid effects on cytosolic calcium and proliferation in osteoblastic cells

Our previous studies show that epidermal growth factor (EGF) stimulates phospholipase D (PLD)-induced phosphatidic acid (PA) formation in rat calvarial osteoblastic cells. This study investigated the effects of PA on cytosolic calcium ([Ca2+]i) and proliferation, and the possible involvement of the PLD pathway in EGF effects on [Ca2+]i and proliferation in rat calvarial osteoblastic cells. PA markedly increased [Ca2+]i. This response was unaffected by thapsigargin, which depletes [Ca2+]i pools, blocked by verapamil, a calcium channel blocker, and enhanced by propanolol, an inhibitor of PA-phosphohydrolase. PA also reduced the EGF dependent-[Ca2+]i increase by 60%, while a PLD inhibitor blocked these effects. Furthermore, PA significantly increased cell proliferation (P < 0.05) which was inhibited by verapamil and enhanced by H-7 (PKC inhibitor). The PLD inhibitor significantly (P < 0.05) reduced the EGF-induced increase in proliferation. In summary, PA stimulates rat calvarial osteoblastic cell proliferation and mobilization of [Ca2+]i using extracellular pools, and EGF's mitogenic effect on these cells requires activation of PLD.

Fodrin inhibits phospholipases A2, C, and D by decreasing polyphosphoinositide cell content

Brain fodrin inhibited in a dose dependent manner the GTPgammaS-stimulated cytosolic PLA2 (cPLA2), PLC, and PLD activities in differentiated HL-60 cells permeabilized with streptolysin O. cPLA2 and PLD were inhibited by the same concentrations of fodrin (IC50=1.5-2 nM) but PLC was inhibited by lower concentrations (IC50=0.3 nM). Moreover, the rates of inhibition were different between the phospholipases. Spectrin, which shares 50% homology with fodrin, had similar effects on the three phospholipases. However, using cytosol-depleted cells or recombinant PLD1, we showed that fodrin was not a direct inhibitor. Studying the potential mechanisms of these inhibitions, we demonstrated that a major decrease in membrane phosphatidylinositol 4-monophosphate (PtdIns(4)P) and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) amounts was induced by fodrin. Exogenous PtdIns(4,5)P2 partly reversed fodrin inhibition of GTPgammaS-stimulated phospholipase C activity. Hence, inhibition of PLC, cPLA2, and PLD activities observed with fodrin could be related to the decrease of PtdIns(4,5)P2, substrate of PLC, a cofactor of PLD and an enhancer of cPLA2 activity.

Phospholipase D activity is altered in X-linked adrenoleukodystrophy heterozygous carriers, but not in hemizygous patients

Abnormalities in levels of choline and its metabolites have been reported in the lesions of brains of X-linked adrenoleukodystrophy (X-ALD) patients. We have examined the turnover of the major choline-containing phospholipid, phosphatidylcholine (PtdCho), in fibroblasts from hemizygous X-ALD, heterozygous X-ALD, Zellweger syndrome (ZW), and male and female control individuals to assess possible alterations in PtdCho metabolism mediated by activation of protein kinase C (PKC). Hydrolysis of PtdCho by phospholipase D (PLD) and resynthesis of PtdCho from labeled choline were stimulated 2- to 4-fold by PKC activation with the phorbol ester, 4beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA), in all cells except those from heterozygous X-ALD individuals. No differences in quantity or intracellular distribution of PKC activity, PKC isoforms by Western blot analysis, or of the PKC substrate, myristoylated alanine-rich C kinase substrate (MARCKS), were apparent in any of the cells. Thus, altered PtdCho metabolism was not directly linked to either of these inherited defects that result in abnormal peroxisomal functions. Further, altered responsiveness of PLD in X-ALD heterozygotes was independent of changes in PKC and MARCKS.

Overexpression of myristoylated alanine-rich C-kinase substrate enhances activation of phospholipase D by protein kinase C in SK-N-MC human neuroblastoma cells

Signal transduction can involve the activation of protein kinase C (PKC) and the subsequent phosphorylation of protein substrates, including myristoylated alanine-rich C kinase substrate (MARCKS). Previously we showed that stimulation of phosphatidylcholine (PtdCho) synthesis by PMA in SK-N-MC human neuroblastoma cells required overexpression of MARCKS, whereas PKCalpha alone was insufficient. We have now investigated the role of MARCKS in PMA-stimulated PtdCho hydrolysis by phospholipase D (PLD). Overexpression of MARCKS enhanced PLD activity 1.3-2.5-fold compared with vector controls in unstimulated cells, and 3-4-fold in cells stimulated with 100 nM PMA. PMA-stimulated PLD activity was blocked by the PKC inhibitor bisindolylmaleimide. Activation of PLD by PMA was linear with time to 60 min, whereas stimulation of PtdCho synthesis by PMA in clones overexpressing MARCKS was observed after a 15 min time lag, suggesting that the hydrolysis of PtdCho by PLD preceded synthesis. The formation of phosphatidylbutanol by PLD was greatest when PtdCho was the predominantly labelled phospholipid, indicating that PtdCho was the preferred, but not the only, phospholipid substrate for PLD. Cells overexpressing MARCKS had 2-fold higher levels of PKCalpha than in vector control cells analysed by Western blot analysis; levels of PKCbeta and PLD were similar in all clones. The loss of both MARCKS and PKCalpha expression at higher subcultures of the clones was paralleled by the loss of stimulation of PLD activity and PtdCho synthesis by PMA. Our results show that MARCKS is an essential link in the PKC-mediated activation of PtdCho-specific PLD in these cells and that the stimulation of PtdCho synthesis by PMA is a secondary response.

Phorbol ester stimulation of phosphatidylcholine synthesis requires expression of both protein kinase C-alpha and phospholipase D

The protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) stimulates both the synthesis and phospholipase D (PLD)-mediated hydrolysis of phosphatidylcholine (PtdCho). Here, attached and suspended NIH 3T3 fibroblasts as well as variants of the MCF-7 human breast carcinoma cell line expressing PKC-alpha and a PtdCho-specific PLD activity at widely different levels were used to determine the possible role of PKC-alpha, PtdCho hydrolysis, and choline uptake in the mediation of PMA effect on PtdCho synthesis. In wild-type MCF-7 cells, which express both PKC-alpha and PLD activities at very low levels, PMA had little effects on the uptake or incorporation [14C]choline into PtdCho. In multidrug resistant MCF-7/MDR1 cells, which highly express PKC-alpha but lack the PtdCho-specific PLD activity, 100-nM PMA had relatively small stimulatory effects on the uptake of [14C]choline (approximately 1.5-fold) and [14C]PtdCho synthesis (1.5- to 2-fold). In NIH 3T3 fibroblasts and MCF-7/PKC-alpha cells, both expressing PKC-alpha and PLD activities at high levels, 10-100-nM PMA enhanced [14C]choline uptake only slightly (1.7- to 2.2-fold), while it had much greater (approximately 4-9-fold) stimulatory effects on PtdCho synthesis. PMA significantly enhanced the formation of phosphatidic acid (PtdOH) in MCF-7/PKC-alpha cells (2.8-fold increase), but not in MCF-7/MDR1 cells (1.4-fold increase), while in both cell lines it had only small (1.3-1.5-fold) stimulatory effects on 1,2-diacylglycerol (1, 2-DAG) formation. In suspended NIH 3T3 cells, 200-300-mM ethanol blocked the stimulatory effect of PMA on PtdOH formation without affecting PtdCho synthesis indicating that neither PtdOH nor 1,2-DAG derived from it is a mediator of PMA effect on PtdCho synthesis. In attached NIH 3T3 cells, dimethylbenz[a]anthracene enhanced phosphocholine formation and, thus, choline uptake without increasing PtdCho synthesis or modifying the effect of PMA. While the results indicate that the stimulatory effect of PMA on PtdCho synthesis requires the expression of both PKC-alpha and a PtdCho-specific PLD, they do not support a role for 1,2-DAG, PtdOH or choline in the mediation of PMA effect.

Kinetic analysis in mixed micelles of partially purified rat brain phospholipase D activity and its activation by phosphatidylinositol 4,5-bisphosphate

A partially purified rat brain membrane phospholipase D (PLD) activity was characterized in a mixed micellar system consisting of 1-palmitoyl-2-[6-N-(7-nitrobenzo-2-oxa-1,3-diazol-4-yl)-amino]capr oyl-phosphatidylcholine (NBD-PC) and Triton X-100, under conditions where Triton X-100 has a surface dilution effect on PLD activity and the catalytic rate is dependent on the surface concentration (expressed in terms of molar ratio) of NBD-PC. PLD activity was specifically activated by phosphatidylinositol 4,5-bisphosphate (PIP2), and the curve of activation versus PIP2 molar ratio fitted a Michaelis-Menten equation with a K(act) value between molar ratios of 0.001-0.002. Maximal activation was observed at a PIP2 molar ratio of 0.01. Similar values were obtained when activities of partially purified PLD as well as membrane-bound PLD were determined towards pure NBD-PC micelles. In the mixed micellar system PIP2 was shown to elevate by 6-22 fold the specificity constant of PLD towards NBD-PC (K(A), which is proportional to Vmax/Km). Kinetic analysis of PLD trans-phosphatidylation activity towards ethanol, 1-propanol and 1-butanol revealed a Michaelis-Menten type dependence on alcohol concentration up to 1000, 200 and 80 mM, respectively. While Vmax values were similar towards all three alcohols, enzyme affinity increased as the alcohol was longer, and Km values for ethanol, 1-propanol and 1-butanol were 291, 75 and 16 mM (respectively). PLD specificity constants (K(A)) towards ethanol, 1-propanol and 1-butanol were shown to be respectively 260, 940 and 5,920 times higher than to water, the competing substrate. 1-Propanol and 1-butanol inhibited PLD activity above 400 and 100 mM, respectively. The present results indicate that partially purified PLD obeys surface dilution kinetics with regard to its phospholipid substrate PC and its cofactor PIP2, and that in the presence of alcohols, its transphosphatidylation activity may be analyzed as a competitive reaction to the hydrolysis reaction.

Synergistic effects of diacylglycerols and fatty acids on membrane structure and protein kinase C activity

The synergistic effects of diacylglycerol (DAG) and fatty acid (FA) in activating protein kinase C have been investigated by correlating their individual and combined effects on enzymatic activity and on membrane structure in phosphatidylcholine/phosphatidylserine (4:1) lipid mixtures using a combination of specific enzymatic assays and 31P and 2H NMR. Addition of DAGs and unsaturated FAs to the bilayers synergistically increased the tendency of the lipids to form nonbilayer phases with a concomitant increase in PKC activity until a maximum was achieved. Further increases in the DAG/FA concentration led to the formation of the nonbilayer lipid phases under the conditions of the PKC activity assays and correlated with decreased activity. The nonbilayer lipid phases still supported PKC activity, although with less than 50% efficiency as compared with the bilayer lipids. Long-chain saturated FA increased DAG-induced PKC activity by causing a lateral phase separation of gel (Lbeta) and liquid-crystalline (Lalpha) domains. Due to the preferential partitioning of DAGs into liquid-crystalline domains, the local DAG concentration increased in these domains, leading to an increase in PKC activity. Because a wide range of lipophilic compounds is capable of altering curvature stress, and therefore the tendency for nonbilayer phase formation in cellular membranes, these compounds would be expected to modulate PKC activity and the activities of a number of other membrane-associated enzymes that are sensitive to biophysical properties of lipid membranes.

Diacylglycerol molecular species in plasma membrane and microsomes change transiently with endothelin-1 treatment of glioma cells

Agonist-induced intracellular signal transduction often involves activation of protein kinase C by diacylglycerol (DAG) released from membrane phospholipids by phospholipases. Using either DAG kinase or HPLC assays to quantitatively determine DAG mass, we observed a time-dependent increase in DAG accumulation upon incubation of rat C6 glioma cells with 200 nM endothelin-1 (ET-1). Total cell DAG rapidly increased by 25-35% from a basal level of 4.5 +/- 0.3 nmol/mg protein during one min of ET-1 treatment and remained constant or slightly decreased between 1 and 2 min. Thereafter, DAG increased to a maximum (1.6-fold above basal) by 5-10 min. and remained elevated to 30 min. Resolution of DAG molecular species by HPLC after incubation of cells with ET-1 revealed that accumulation of DAG species differed in total cell lysate and subcellular compartments. In plasma membrane, major DAG species increased at 1 min. followed by a decrease at 10 min. whereas in microsomes DAG species did not change at 1 min. and decreased at 10 min. Although phospholipid sources of DAG species were not identified specifically, there was preferential hydrolysis of molecular species of phospholipid for DAG production. We propose that molecular species of DAG produced at the plasma membrane may be transferred to the endoplasmic reticulum so that phospholipid resynthesis can replenish molecular species initially utilized in signal transduction.

Involvement of phospholipase D and protein kinase C in phorbol ester and fatty acid stimulated turnover of phosphatidylcholine and phosphatidylethanolamine in neural cells

Hydrolysis of phosphatidylcholine (PtdCho) can provide lipid second messengers involved in sustained signal transduction. Four neural-derived cell lines (C6 rat glioma; N1E-115 mouse and SK-N-MC and SK-N-SH human neuroblastoma) express different protein kinase C (PKC) isoforms and differentially respond to 4beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA)-stimulation of PtdCho synthesis. We examined involvement of PLD and PKC in the hydrolysis and resynthesis of PtdCho and phosphatidylethanolamine stimulated by beta-TPA, bryostatin (a non-phorbol PKC activator) and oleic acid (18:1n-9) in the four cell lines. beta-TPA or bryostatin produced similar enhancement of [3H]Cho incorporation, loss of stimulated synthesis after down regulation of PKC, and activation of PLD. In C6 cells, staurosporine (STS) and bis-indolylmaleimide (BIM) only partially inhibited basal and beta-TPA-stimulated PLD activity measured as choline or ethanolamine release; phosphatidylbutanol formation after prelabeling with [9,10-3H]18:1n-9, [9,10-3H]myristic acid (14:0), [1-14C]eicosapentaenoic acid (20:5n-3) or 1-O-[alkyl-1', 2-3H]-sn-glyceryl-3-phosphorylcholine gave similar results. STS at >200 nM activated PLD in the presence or absence of beta-TPA. In SK-N-SH cells where PtdCho synthesis was stimulated by beta-TPA or bryostatin, no effect of these agents on PLD was observed. 18:1n-9 stimulated PtdCho synthesis and, to a lesser extent, hydrolysis by PLD both with and without beta-TPA present. Fatty acids had no effect on PKC activities and down regulation of PKC with beta-TPA enhanced fatty acid stimulation of PtdCho synthesis. Thus, activation of PLD hydrolysis preceding resynthesis is involved in the stimulatory effects of beta-TPA on PtdCho synthesis in some but not all of these neural derived cells. Further, PLD hydrolysis of PtdCho and PtdEtn appear to have differing aspects of regulation. Fatty acid regulation of PtdCho synthesis occurs independent of PKC activation. Accordingly, regulation of membrane phospholipid degradation and resynthesis in association with lipid second messenger generation can involve a complex interplay of PLD, PKC, and fatty acids. (c) 1998 Elsevier Science B.V.

The C2 domain of the Ca(2+)-independent protein kinase C Apl II inhibits phorbol ester binding to the C1 domain in a phosphatidic acid-sensitive manner

There are two protein kinase Cs (PKCs) in the Aplysia nervous system, PKC Apl I, which is homologous to the Ca(2+)-activated PKC family, and PKC Apl II, which is homologous to the Ca(2+)-independent PKCs epsilon and eta. Purified PKC Apl I requires much less phosphatidylserine for activation than does purified PKC Apl II, and this may explain why the neurotransmitter serotonin activates PKC Apl I but not PKC Apl II in the intact nervous system [Sossin, W. S., Fan, X., and Baseri, F. (1996) J. Neurosci. 16, 10-18]. PKC Apl II's requirement for high levels of phosphatidylserine may be mediated by its C2 domain, since removal of this domain allows PKC Apl II to be activated at lower concentrations of phosphatidylserine. To begin to understand how this inhibition is mediated, we generated fusion proteins containing the C1 and C2 domains from PKC Apl II and determined their lipid dependence for phorbol ester binding. Our results indicate that the presence of the C2 domain lowers the affinity of protein kinase C activators for the C1 domains and this inhibition can be removed by phosphatidylserine. Phosphatidic acid, however, is much more potent than phosphatidylserine in reducing C2 domain-mediated inhibition, suggesting that phosphatidic acid may be a required cofactor for the activation of PKC Apl II.

Receptor-mediated endocytosis in kidney proximal tubules: recent advances and hypothesis

Preparation of kidney proximal tubules in suspension allows the study of receptor-mediated endocytosis, protein reabsorption, and traffic of endosomal vesicles. The study of tubular protein transport in vitro coupled with that of the function of endosomal preparation offers a unique opportunity to investigate a receptor-mediated endocytosis pathway under physiological and pathological conditions. We assume that receptor-mediated endocytosis of albumin in kidney proximal tubules in situ and in vitro can be regulated, on the one hand, by the components of the acidification machinery (V-type H+-ATPase, Cl(-)-channel and Na+/H+-exchanger), giving rise to formation and dissipation of a proton gradient in endosomal vesicles, and, on the other hand, by small GTPases of the ADP-ribosylation factor (Arf)-family. In this paper we thus analyze the recent advances of the studies of cellular and molecular mechanisms underlying the identification, localization, and function of the acidification machinery (V-type H+-ATPase, Cl(-)-channel) as well as Arf-family small GTPases and phospholipase D in the endocytotic pathway of kidney proximal tubules. Also, we explore the possible functional interaction between the acidification machinery and Arf-family small GTPases. Finally, we propose the hypothesis of the regulation of translocation of Arf-family small GTPases by an endosomal acidification process and its role during receptor-mediated endocytosis in kidney proximal tubules. The results of this study will not only enhance our understanding of the receptor-mediated endocytosis pathway in kidney proximal tubules under physiological conditions but will also have important implications with respect to the functional consequences under some pathological circumstances. Furthermore, it may suggest novel targets and approaches in the prevention and treatment of various diseases (cystic fibrosis, Dent's disease, diabetes and autosomal dominant polycystic kidney disease).

Extracellular sphingosine 1-phosphate stimulates formation of ethanolamine from phosphatidylethanolamine: modulation of sphingosine 1-phosphate-induced mitogenesis by ethanolamine

In this work, we determined the effects of sphingosine 1-phosphate (S1P) on phospholipase D (PLD)-mediated hydrolysis of phosphatidylethanolamine (PtdEtn), and evaluated the effects of the water-soluble product ethanolamine on S1P-induced DNA synthesis in NIH 3T3 cells. In [14C]ethanolamine-labelled cells, S1P (0.5-5 microM) stimulated PLD-mediated hydrolysis of PtdEtn 1.5-2.1-fold. Down-regulation of protein kinase C by chronic (24 h) treatment of cells with 300 nM PMA, or pretreatments (10 min) with the cell-permeant calcium chelator 1,2-bis-(O-aminophenoxy)-ethane-N,N, N',N'-tetra-acetic acid tetra-acetoxymethyl ester led to the inhibition of S1P-induced PtdEtn hydrolysis. S1P alone was a weak inducer of DNA synthesis, but its effects were enhanced by phosphocholine (PCho), insulin, ATP or PMA. Ethanolamine (5-100 microM) did not modify the mitogenic effect of S1P alone, whereas at 50-100 microM concentrations it actually enhanced the mitogenic effect of PCho via a mitogen-activated protein (MAP) kinase-independent mechanism. In contrast, 5-20 microM concentrations of ethanolamine, which correspond to normal blood ethanolamine levels in humans, strongly inhibited DNA synthesis induced by S1P plus PCho via a MAP kinase-dependent mechanism; importantly, less or no inhibition was observed with 50-100 microM concentrations of ethanolamine. At 5-50 microM concentrations, ethanolamine also inhibited the synergistic mitogenic effects of both S1P plus insulin (22-27% inhibition) and PCho plus ATP (45-73% inhibition) but not those of S1P plus PMA or S1P plus ATP. The results indicate that S1P stimulates PLD-mediated hydrolysis of PtdEtn by a mechanism that may involve a regulatory protein kinase C isoform. Increased formation of ethanolamine by PLD-mediated PtdEtn hydrolysis or by other means may be required for maximal stimulation of DNA synthesis by S1P in the presence of insulin, and particularly PCho.

Ethanolamine, but not phosphoethanolamine, potentiates the effects of insulin, phosphocholine, and ATP on DNA synthesis in NIH 3T3 cells--role of mitogen-activated protein-kinase-dependent and protein-kinase-independent mechanisms

NIH 3T3 fibroblasts express a phospholipase D activity hydrolyzing phosphatidylethanolamine (PtdEtn) which produces ethanolamine (Etn) in response to a variety of growth regulating agents. The main objective of this work was to evaluate the effects of Etn on mitogenesis and to determine whether these effects require its metabolism to phosphoethanolamine (PEtn) or PtdEtn. To increase conversion of Etn to PEtn, an Etn-specific kinase derived from Drosophila was highly expressed in NIH 3T3 cells. Overexpression of this Etn kinase resulted in large (10-12.5-fold) increases in PEtn formation, but only in modest (1.2-1.7-fold) increases in PtdEtn synthesis. In both vector control and Etn kinase overexpressor cells, Etn had biphasic effects on insulin-induced DNA synthesis with maximal (approximately 2-fold) potentiating effects being observed at 0.5-1 mM concentrations, followed by an inhibitory phase at higher Etn concentrations. In the Etn kinase overexpressor lines, the inhibitory phase was elicited by lower Etn concentrations and it was partially blocked by 5 mM choline due to decreased formation of PEtn. In both vector control and Etn kinase overexpressor cells, phosphocholine (PCho) and insulin synergistically stimulated DNA synthesis; their effects were further enhanced by physiologically relevant (5-60 microM) concentrations of Etn by a mechanism independent of mitogen-activated protein (MAP) kinase. Concentrations of Etn >50 microM also enhanced the effects of both PCho and the synergistic effects of PCho plus ATP; however, in the latter case 20 microM Etn was inhibitory. The magnitude of both the potentiating and inhibitory effects of Etn on PCho-induced as well as PCho + ATP-induced DNA synthesis were similar in the vector control and Etn kinase overexpressor cells; they were associated with stimulation and inhibition, respectively, of p42 MAP kinase activity. The results indicate that in NIH 3T3 cells Etn exerts significant effects on DNA synthesis which, except inhibition of insulin-induced DNA synthesis by higher concentrations of Etn, do not correlate with the metabolism of Etn to PEtn or PtdEtn.

The coatomer protein beta'-COP, a selective binding protein (RACK) for protein kinase Cepsilon

Distinct subcellular localization of activated protein kinase C (PKC) isozymes is mediated by their binding to isozyme-specific RACKs (receptors for activated C-kinase). Our laboratory has previously isolated one such protein, RACK1, and demonstrated that this protein displays specificity for PKCbeta. We have recently shown that at least part of the PKCepsilon RACK-binding site on PKCepsilon lies within the unique V1 region of this isozyme (Johnson, J. A., Gray, M. O., Chen, C.-H., and Mochly-Rosen, D. (1996) J. Biol. Chem. 271, 24962-24966). Here, we have used the PKCepsilon V1 region to clone a PKCepsilon-selective RACK, which was identified as the COPI coatomer protein, beta'-COP. Similar to RACK1, beta'-COP contains seven repeats of the WD40 motif and fulfills the criteria previously established for RACKs. Activated PKCepsilon colocalizes with beta'-COP in cardiac myocytes and binds to Golgi membranes in a beta'-COP-dependent manner. A role for PKC in control of secretion has been previously suggested, but this is the first report of direct protein/protein interaction of PKCepsilon with a protein involved in vesicular trafficking.

Inhibition of phospholipase D by lysophosphatidylethanolamine, a lipid-derived senescence retardant

Phospholipid signaling mediated by lipid-derived second messengers or biologically active lipids is still new and is not well established in plants. We recently have found that lysophosphatidylethanolamine (LPE), a naturally occurring lipid, retards senescence of leaves, flowers, and postharvest fruits. Phospholipase D (PLD) has been suggested as a key enzyme in mediating the degradation of membrane phospholipids during the early stages of plant senescence. Here we report that LPE inhibited the activity of partially purified cabbage PLD in a cell-free system in a highly specific manner. Inhibition of PLD by LPE was dose-dependent and increased with the length and unsaturation of the LPE acyl chain whereas individual molecular components of LPE such as ethanolamine and free fatty acid had no effect on PLD activity. Enzyme-kinetic analysis suggested noncompetitive inhibition of PLD by LPE. In comparison, the related lysophospholipids such as lysophosphatidylcholine, lysophosphatidylglycerol, and lysophosphotidylserine had no significant effect on PLD activity whereas PLD was stimulated by lysophosphatidic acid and inhibited by lysophosphatidylinositol. Membrane-associated and soluble PLD, extracted from cabbage and castor bean leaf tissues, also was inhibited by LPE. Consistent with acyl-specific inhibition of PLD by LPE, senescence of cranberry fruits as measured by ethylene production was more effectively inhibited according to the increasing acyl chain length and unsaturation of LPE. There are no known specific inhibitors of PLD in plants and animals. We demonstrate specific inhibitory regulation of PLD by a lysophospholipid.

Ro31-8220 inhibits protein kinase C to block the phorbol ester-stimulated release of choline- and ethanolamine-metabolites from C6 glioma cells: p70 S6 kinase and MAPKAP kinase-1beta do not function downstream of PKC in activating PLD

The use of bisindolylmaleimide derivatives of staurosporine as selective inhibitors of protein kinase C (PKC) is in doubt following the report by Alessi [FEBS Lett. 402 (1997) 121-123] that Ro31-8220 and GF109203X are potent in vitro inhibitors of p70 S6 kinase and mitogen-activated protein kinase-activated protein kinase-1beta, as well as of PKC. Here we show that the phorbol ester-stimulated release of choline- and ethanolamine-metabolites from C6 glioma cells due to phospholipid hydrolysis by phospholipase D (PLD) is not inhibited by rapamycin or PD98059, specific inhibitors respectively of p70 S6 kinase and MAPKK (MEK) and thus of MAPKAP kinase-1beta but is still completely blocked by Ro31-8220. We conclude therefore that p70S6k and MAPKAP kinase-1beta as well as MAPK are not involved in signalling pathways downstream of PKC that regulate phorbol ester-stimulated phospholipid turnover and that the inhibitory action of Ro31-8220 occurs by blocking PKC which regulates at least one pathway to PLD activation. The PI-3 kinase inhibitor, wortmannin, inhibits the phorbol ester-stimulated release of ethanolamine- but not choline-metabolites from C6 cells suggesting that different PLD isoforms regulate the turnover of PtdEth and PtdCho in C6 cells. Both PLD isoforms are activated via PKC but the PtdEth-PLD is also regulated via a wortmannin-sensitive pathway.

Effects of dipalmitoylglycerol and fatty acids on membrane structure and protein kinase C activity

The individual and combined effects of the saturated diacylglycerol (DAG) dipalmitin (DP) and saturated or polyunsaturated unesterified fatty acids (PUFAs) on both the structure of phosphatidylcholine/phosphatidylserine (PC/PS; 4:1 mol/mol) bilayers and on protein kinase C (PKC) activity were studied using 2H nuclear magnetic resonance (NMR) and enzyme activity assays. In the absence of DP, PUFAs only slightly activated PKC whereas palmitic acid had no effect. In the absence of fatty acids, DP induced lateral phase separation of the bilayer into liquid-crystalline and gel phases. Under these conditions virtually all DP was sequestered into the gel phase and no activation of PKC was observed. The addition of polyunsaturated arachidonic or docosahexaenoic acids to the DP-containing bilayers significantly increased the relative amounts of DP and other lipid components in the liquid-crystalline phase, correlating with a dramatic increase in PKC activity. Furthermore, the effect was greater with PS, resulting in an enrichment of PS in the liquid-crystalline domains. In the presence of DP, palmitic acid did not decrease the amount of gel phase lipid and had no effect on PKC activity. The results explain the observed lack of PKC-activating capacity of long-chain saturated DAGs as due to the sequestration of DAG into gel domains wherein it is complexed with phospholipids and thus not available for the required interaction with the enzyme.