Prostaglandin F2 alpha-stimulated phospholipase D activation in osteoblast-like MC3T3-E1 cells: involvement in sustained 1,2-diacylglycerol production

Phospholipase D1 activity is crucial for cytosolic phospholipase A2 -dependent prostaglandin E2 formation in murine osteoblastic MC3T3-E1 cells

In bone, prostaglandin E2 (PGE2) is highly osteogenic and formed by osteoblasts, a key modulatory event in the regulation of bone cell activity. MC3T3-E1 cells are widely used as an in vitro model of osteoblast function. It is still not clear which pathways contribute to the release of AA in these cells. In this study we have focussed on the contribution of phospholipase D (PLD) enzymes to osteoblastic PGE2 formation after stimulation with endothelin-1 (ET-1). Using specific inhibitors of PLD1 and PLD2 we could show that PGE2 formation was strictly dependent on PLD1 but not PLD2 activity and cytosolic phospholipase A2 (cPLA2) was activated by triggering through PLD1. We have identified diacyl glycerol (DAG) as a possible effector molecule which may serve as a triggering signal for PKC activation and subsequent cPLA2 phosphorylation.

Functional Role of Phospholipase D in Bone Metabolism

Phospholipase D (PLD) proteins are major enzymes that regulate various cellular functions, such as cell growth, cell migration, membrane trafficking, and cytoskeletal dynamics. As they are responsible for such important biological functions, PLD proteins have been considered promising therapeutic targets for various diseases, including cancer and vascular and neurological diseases. Intriguingly, emerging evidence indicates that PLD1 and PLD2, 2 major mammalian PLD isoenzymes, are the key regulators of bone remodeling; this suggests that these isozymes could be used as potential therapeutic targets for bone diseases, such as osteoporosis and rheumatoid arthritis. PLD1 or PLD2 deficiency in mice can lead to decreased bone mass and dysregulated bone homeostasis. Although both mutant mice exhibit similar skeletal phenotypes, PLD1 and PLD2 play distinct and nonredundant roles in bone cell function. This review summarizes the physiological roles of PLD1 and PLD2 in bone metabolism, focusing on recent findings of the biological functions and action mechanisms of PLD1 and PLD2 in bone cells.

Molecular characterisation of group IVA (cytosolic) phospholipase A2 in murine osteoblastic MC3T3-E1 cells

Formation of the powerful osteogenic prostaglandin E2 by osteoblasts, a key modulatory event in the paracrine and autocrine regulation of bone cell activity, is preceded by release of the precursor arachidonic acid from phospholipid stores. The main routes of arachidonate liberation may involve phospholipase enzymes such as group IVA phospholipase A2 which is believed to be the main effector in many cell system due to its preference for arachidonate-containing lipids. MC3T3-E1 cells are non-transformed osteoblasts and are widely used as an in vitro model of osteoblast function. In these cells there is still no clarity about the main release pathway of arachidonic acid. Besides cytosolic phospholipase A2, phospholipase C and D pathways may play a key role in arachidonate release. Despite the crucial role of osteoblastic prostgalandin synthesis information on the occurrence of involved enzymes at the molecular level is scarse in MC3T3-E1 cells. We have characterised group IVA phospholipase A2 at the mRNA in these cells as a constitutively expressed enzyme which is cytosolic and translocates to the membrane upon endothelin-1 stimulation. Using immunopurification combined with Western blotting and high-resolution mass spectrometry, the enzyme was also identified at the protein level. Using specific gene silencing we were able to show that osteoblastic cytosolic phospholipase A2 is crucially involved in ET-1-induced prostaglandin formation.

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

Effects of prostaglandin F2α on small intestinal interstitial cells of Cajal

AIM: To explore the role of prostaglandin F_2α (PGF_2α)) on pacemaker activity in interstitial cells of Cajal (ICC) from mouse small intestine.

METHODS: In this study, effects of PGF_2α in the cultured ICC cells were investigated with patch clamp technology combined with Ca²⁺ image analysis.

RESULTS: Externally applied PGF_2α (10 μmol/L) produced membrane depolarization in current-clamp mode and increased tonic inward pacemaker currents in voltage-clamp mode. The application of flufenamic acid (a non-selective cation channel inhibitor) or niflumic acid (a Cl^- channel inhibitor) abolished the generation of pacemaker currents but only flufenamic acid inhibited the PGF_2α-induced tonic inward currents. In addition, the tonic inward currents induced by PGF_2α were not inhibited by intracellular application of 5’-[-thio]diphosphate trilithium salt. Pretreatment with Ca²⁺ free solution, U-73122, an active phospholipase C inhibitor, and thapsigargin, a Ca²⁺-ATPase inhibitor in endoplasmic reticulum, abolished the generation of pacemaker currents and suppressed the PGF_2α-induced tonic inward currents. However, chelerythrine or calphostin C, protein kinase C inhibitors, did not block the PGF_2α-induced effects on pacemaker currents. When recording intracellular Ca²⁺ ([Ca²⁺]_i) concentration using fluo-3/AM, PGF_2α broadly increased the spontaneous [Ca²⁺]_i oscillations.

CONCLUSION: These results suggest that PGF_2α can modulate pacemaker activity of ICC by acting non-selective action channels through phospholipase C-dependent pathway via [Ca²⁺]i regulation

Redox-active antioxidant modulation of lipid signaling in vascular endothelial cells: vitamin C induces activation of phospholipase D through phospholipase A2, lipoxygenase, and cyclooxygenase

We have earlier reported that the redox-active antioxidant, vitamin C (ascorbic acid), activates the lipid signaling enzyme, phospholipase D (PLD), at pharmacological doses (mM) in the bovine lung microvascular endothelial cells (BLMVECs). However, the activation of phospholipase A(2) (PLA(2)), another signaling phospholipase, and the modulation of PLD activation by PLA(2) in the ECs treated with vitamin C at pharmacological doses have not been reported to date. Therefore, this study aimed at the regulation of PLD activation by PLA(2) in the cultured BLMVECs exposed to vitamin C at pharmacological concentrations. The results revealed that vitamin C (3-10 mM) significantly activated PLA(2) starting at 30 min; however, the activation of PLD resulted only at 120 min of treatment of cells under identical conditions. Further studies were conducted utilizing specific pharmacological agents to understand the mechanism(s) of activation of PLA(2) and PLD in BLMVECs treated with vitamin C (5 mM) for 120 min. Antioxidants, calcium chelators, iron chelators, and PLA(2) inhibitors offered attenuation of the vitamin C-induced activation of both PLA(2) and PLD in the cells. Vitamin C was also observed to significantly induce the formation and release of the cyclooxygenase (COX)- and lipoxygenase (LOX)-catalyzed arachidonic acid (AA) metabolites and to activate the AA LOX in BLMVECs. The inhibitors of PLA(2), COX, and LOX were observed to effectively and significantly attenuate the vitamin C-induced PLD activation in BLMVECs. For the first time, the results of the present study revealed that the vitamin C-induced activation of PLD in vascular ECs was regulated by the upstream activation of PLA(2), COX, and LOX through the formation of AA metabolites involving oxidative stress, calcium, and iron.

Activation of phospholipase D1 by surface roughness of titanium in MG63 osteoblast-like cell

Although it is recognized that the surface roughness of titanium (Ti) promotes the osteogenic differentiation, the related mechanisms and factors remain elusive. The purpose of this study was to explore the potential correlation among phospholipase D (PLD) activity, Ti surface roughness and subsequent osteoblast differentiation. The machined Ti disks were sandblasted with aluminum oxide particles to produce surfaces of varying roughness (n = 160). Normal or transfected MG63cells with PLD genes were cultured on roughened Ti specimens and assayed for PLD, alkaline phosphatase (ALP) and osteocalcin. The statistical significance was evaluated by analysis of variance. The activity, mRNA and protein levels of PLD significantly increased in MG63 cells with a roughness-dependent pattern (P < 0.05). The ALP activity and osteocalcin production, promoted by Ti surface roughness, were enhanced by the PLD activator and inhibited by the PLD blocker. It was also found that the PLD1 isoform responds to Ti surface roughness and regulates selectively the ALP activity. These observations strongly suggest that PLD1 mediates the cellular signaling of and modulates osteoblast differentiation induced by Ti surface roughness in MG63 osteoblast-like cell.

Role of protein kinase A, phospholipase C and phospholipase D in parathyroid hormone receptor regulation of protein kinase Cα and interleukin-6 in UMR-106 osteoblastic cells

Parathyroid hormone (PTH) stimulates both bone formation and resorption by activating diverse osteoblast signalling pathways. Upstream signalling for PTH stimulation of protein kinase C-alpha (PKCalpha) membrane translocation and subsequent expression of the pro-resorptive cytokine interleukin-6 (IL-6) was investigated in UMR-106 osteoblastic cells. PTH 1-34, PTH 3-34, PTHrP and PTH 1-31 stimulated PKCalpha translocation and IL-6 promoter activity. Pharmacologic intervention at the adenylyl cyclase (AC) pathway (forskolin, IBMX, PKI) failed to alter PTH 1-34- or PTH 3-34-stimulated PKCalpha translocation. The phosphoinositol-phospholipase C (PI-PLC) antagonist U73122 slightly decreased PTH 1-34-stimulated PKCalpha translocation; however, the control analogue U73343 acted similarly. Propranolol, an inhibitor of phosphatidic acid (PA) phosphohydrolase, decreased diacylglycerol (DAG) formation and attenuated PTH 1-34- and PTH 3-34-stimulated PKCalpha translocation and IL-6 promoter activity, suggesting a phospholipase D (PLD)-dependent mechanism. This is the first demonstration that PLD-mediated signalling leads to both PKC-alpha translocation and IL-6 promoter activation in osteoblastic cells.

PGE(2) is essential for gap junction-mediated intercellular communication between osteocyte-like MLO-Y4 cells in response to mechanical strain

We have observed, in our previous studies, that fluid flow increases gap junction-mediated intercellular coupling and the expression of a gap junction protein, connexin 43, in osteocyte-like MLO-Y4 cells. Interestingly, this stimulation is further enhanced during the poststress period, indicating that a released factor(s) is likely to be involved. Here, we report that the conditioned medium obtained from the fluid flow-treated MLO-Y4 cells increased the number of functional gap junctions and connexin 43 protein. These changes are similar to those observed in MLO-Y4 cells directly exposed to fluid flow. Fluid flow was found to induce PGE(2) release and increase cyclooxygenase 2 expression. Treatment of the cells with PGE(2) had the same effect as fluid flow, suggesting that PGE(2) could be responsible for these autocrine effects. When PGE(2) was depleted from the fluid flow-conditioned medium, the stimulatory effect on gap junctions was partially, but significantly, decreased. Addition of the cyclooxygenase inhibitor, indomethacin, partially blocked the stimulatory effects of mechanical strain on gap junctions. Taken together, these studies suggest that the stimulatory effect of fluid flow on gap junctions is mediated, in part, by the release of PGE(2). Hence, PGE(2) is an essential mediator between mechanical strain and gap junctions in osteocyte-like cells.

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.

U73122 and U73343 inhibit receptor-mediated phospholipase D activation downstream of phospholipase C in CHO cells

The aminosteroid 1-(6-¿[17beta-3-methoxyestra- 1,3,5(10)-trien- 17-yl]-amino¿hexyl)- 1H-pyrrole-2,5-dione (U73122) and its inactive analogue 1-(6-¿[17beta-3-methoxyestra-1,3,5(10)-trien- 17-yl]-amino¿hexyl-2,5-pyrrolidine-dione (U73343) are widely used to study the involvement of G protein-coupled 1-phosphatidylinositol-phosphodiesterase, or phospholipase C, in receptor-mediated cell activation. The present work shows that both aminosteroids inhibit cholecystokinin-(26-33)-peptide amide (CCK-8)-induced phospholipase D activation equipotently in Chinese hamster ovary cells expressing the cholecystokinin-A receptor (CHO-CCK(A) cells). In addition, the two aminosteroids virtually completely inhibited thapsigargin- and 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced phospholipase D activation. Since the latter two drugs mimic inositol 1,4,5-trisphosphate-mediated Ca2+ mobilisation and 1,2-diacylglycerol-mediated protein kinase C activation. respectively, this suggests that both U73122 and U73343 act downstream of phospholipase C to inhibit receptor-mediated phospholipase D activation. U73122, but not U73343. effectively inhibited both TPA/Ca2+-stimulated phospholipase D activation and TPA/phosphatidylserine-stimulated protein kinase C activation in a homogenate of CHO-CCK(A) cells. The data presented suggest that U73122 may act at the level of protein kinase C to inhibit activation of phospholipase D. The exact site of action of U73343 is presently unknown.

Thrombin regulates interleukin-6 synthesis through phosphatidylcholine hydrolysis by phospholipase D in osteoblasts

We previously reported that thrombin stimulates Ca2+ influx and activates phosphatidylcholine-hydrolyzing phospholipase D in osteoblast-like MC3T3-E1 cells. In this study, we investigated the effect of thrombin on interleukin-6 (IL-6) synthesis in these cells. Thrombin stimulated IL-6 synthesis dose-dependently in the range between 0.01 and 1 U/ml. The depletion of extracellular Ca2+ by EGTA suppressed the thrombin-induced IL-6 synthesis. TMB-8, an inhibitor of intracellular Ca2+ mobilization, also inhibited the IL-6 synthesis by thrombin. Propranolol, a phosphatidic acid phosphohydrolase inhibitor, enhanced the IL-6 synthesis by thrombin. Calphostin C, a highly potent and specific inhibitor for protein kinase C, significantly amplified the IL-6 synthesis by thrombin. The thrombin-induced IL-6 synthesis was enhanced in PKC down-regulated MC3T3-E1 cells. These results strongly suggest that thrombin stimulates IL-6 synthesis, which depends on intracellular Ca2+ mobilization mainly from extracellular space in osteoblasts, and that the IL-6 synthesis by thrombin is regulated due to thrombin-activated protein kinase C through phosphatidylcholine-hydrolyzing phospholipase D.

Involvement of Rho family proteins in prostaglandin F2 alpha-induced phospholipase D activation in the osteoblast-like cell line MC3T3-E1

To examine the role of Rho family proteins in prostaglandin F2 alpha (PGF2 alpha)-mediated phospholipase D (PLD) activation of osteoblast-like cell line MC3T3-E1 cells, we used Toxin-B from Clostridium difficile, which inhibits Rho family proteins by monoglucosylation. Pretreatment of [3H]myristic acid-labeled MC3T3-E1 cells with Toxin B induced rounding-up of the cells and inhibited the PGF2 alpha-induced PLD activation by 60%, but not the phospholipase C (PLC) activation. Cytochalasin D also induced rounding the cells, but showed a small inhibition in the PLD activation. Brefeldin A (BFA) had marginal inhibitory effect on the PGF2 alpha-induced PLD activation. In digitonin-permeabilized MC3T3-E1 cells, [3H]P But formation was stimulated by guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) or 4 beta-phorbol 12-myristate 13-acetate (PMA) in the presence of Ca2+ (1 microM) and ATP (1 mM), and phosphatidylinositol 4,5-bisphosphate (PIP2) was also required for its full PLD activation. Pretreatment of the digitonin-permeabilized MC3T3-E1 cells with Toxin B reduced the GTP gamma S- and PMA-stimulated PLD activities by 80% and 60%, respectively. On the other hand, C3 toxin which inhibits Rho by ADP-ribosylation, exerted a partial inhibitory effect on the GTP gamma S-stimulated PLD activity. These results suggest that Cdc42 as well as RhoA appear to be involved in the PLD activation mediated by PGF2 alpha and also that the PLD activation may be independent of actin cytoskeleton in MC3T3-E1 cells.

Characterization of phospholipase D activation by muscarinic receptors in human neuroblastoma SH-SY5Y cells

The cholinergic regulation of phospholipase D activity was studied in SH-SY5Y human neuroblastoma cells with phosphatidylethanol formation as a specific marker for the enzyme activity. The muscarinic antagonists, hexahydrosiladifenidol and pirenzepine, inhibited carbachol-induced phosphatidylethanol formation in a concentration-dependent manner and the inhibitory constants indicated that muscarinic M1 receptors are responsible for the major part of the phospholipase D activation. The mechanism of receptor-mediated phospholipase D activation varies between different cell types and receptors. In SH-SY5Y cells, the carbachol-induced phospholipase D activity was inhibited by protein kinase C inhibitors. Since both phospholipases D and C are activated by muscarinic stimulation in SH-SY5Y cells, most of the phospholipase D activation is probably secondary to the protein kinase C activation that follows phospholipase C-mediated increase in diacylglycerols. Other kinases may be involved in the regulation since also a tyrosine kinase inhibitor decreased the phosphatidylethanol formation. Stimulation of G-protein(s) and increase in the intracellular Ca2+ concentration activated phospholipase D and may be additional mechanisms for the muscarinic regulation of phospholipase D in SH-SY5Y cells. Propranolol, an inhibitor of phosphatidic acid phosphohydrolase, increased the carbachol-induced formation of phosphatidic acid at the expense of 1,2-diacylglycerol. This indicates that phospholipase D contributes to the formation of 1,2-diacylglycerol after carbachol stimulation in SH-SY5Y cells.

Arf and RhoA regulate both the cytosolic and the membrane-bound phospholipase D from human placenta

In this paper we demonstrate for the first time that human placenta contains a cytosolic phospholipase D (PLD) activity. This activity had a pH optimum of 7.0 and was stimulated by PIP2 and inhibited by oleate. Furthermore, cytosolic PLD was stimulated by 30 microM GTP gamma S (6-14-fold) and by the small G proteins 1 microM mArf3 (2-fold) and 0.37 nM RhoA (2-fold). This is the first report to show RhoA activation of a cytosolic PLD. The activation by mArf3 was maintained after partial purification on DEAE Sepharose of the enzyme. We have previously reported the existence of a membrane-bound PLD from human placenta, which is stimulated by PIP2, but not by oleate (Vinggaard, A. M. & Hansen, H. S. (1995) Biochim. Biophys. Acta 1258, 169-176). Here we show that oleic acid and alpha-linolenic acid both dose-dependently inhibited solubilized membrane PLD (65% inhibition at 4 mM), whereas stearic acid (4 mM) had no effect. Thus, the presence of double bonds in the fatty acid is important for the inhibitory effect. Furthermore, placental membrane PLD was activated by 30 microM GTP gamma S (4-fold) and by mArf3 (1 microM) and RhoA (0.37 nM) by a factor of 3 and 2, respectively. The solubilized membrane phospholipase D was partially purified to a basal specific activity of 25-37 nmol/min/mg. This preparation was devoid of endogenous RhoA and Arf and could not be stimulated by GTP gamma S. However, mArf3 (1 microM) still activated this partially purified membrane PLD, whereas RhoA (0.37 nM) was not able to activate this PLD fraction. In conclusion, our results suggest that the human placenta contains a PLD that is located both in the cytosol and the membranes, and that is activated by PIP2, mArf3 and RhoA but inhibited by oleate.

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.

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.

PLD activation in Chinese hamster ovary (CHO) cells transfected with PGF2 alpha receptor cDNA

Pertussis toxin-insensitive GTP-binding protein was observed to be involved in prostaglandin F2 alpha (PGF2 alpha)-induced phosphoinositide metabolism in Chinese hamster ovary (CHO) cells transfected with PGF2 alpha receptor cDNA (CHO-PGF2 alpha R cells) (Ito, S. et al. Biochem. Biophys. Res. Commun. 200: 756,1994). In the present study, we investigated PGF2 alpha-induced PLD activation in CHO-PGF2 alpha R cells. PLD activation was examined by measuring the production of [3H]phosphatidylbutanol ([3H]PBut), a specific product of the PLD-catalyzed transphosphatidylation reaction. PGF2 alpha-induced [3H]PBut formation was concentration-dependent with the maximal level obtained at 1 microM PGF2 alpha. The maximal [3H]PBut formation was observed at 2 min after addition of PGF2 alpha. Depletion of extracellular Ca2+ with EGTA suppressed PGF2 alpha-induced PLD activation by 50%. PKC inhibitors Ro31-8425 and calphostin C inhibited PGF2 alpha-induced [3H]PBut formation by 50%. PTK inhibitors genistein and herbimycin A failed to inhibit PGF2 alpha-induced PLD activation. A combination of maximal effective concentrations of PGF2 alpha (1 microM) and PMA (100 nM) enhanced PLD activation in an additive manner. Pretreatment of the cells with PMA for 2 h down-regulated PKC alpha and decreased PGF2 alpha-induced PLD activation. These results suggest that PLD activation by PGF2 alpha is mediated by both PKC-dependent and -independent pathways and that PKC alpha is involved in the former pathway.

Potentiated bradykinin-induced increase of 1,2-diacylglycerol generation and phospholipase D activity in human senescent fibroblasts

1. The comparative study of the effect of bradykinin (BK) in young and old IMR-90 human fibroblasts shows that old cells are characterized by a reduced increase in 1,2-diacylglycerol (1,2-DAG) generation upon stimulation after short-term treatment and a significant higher increase after long-term agonist treatment. BK-induced activation of phospholipase D (PLD), the major enzyme involved in sustained 1,2-DAG generation, was 2.5-fold higher in old cells, strongly suggesting that it is involved in the potentiated increase of 1,2-DAG formation. The increased activation of PLD by BK in old cells was specific, since in parallel experiments the effect of thrombin was not significantly different in young and old cells. PLD activity in old cells was only reduced by down-regulation of protein kinase C (PKC) activity, in contrast to what was observed in young cells where it was completely abolished. This indicates that the enzyme activity in old cells was partially PKC-independent. BK was also able to increase the release of [14C]ethanolamine, a water-soluble product of hydrolysis of phosphatidylethanolamine (PtdEtn), through PLD activation in young and old cells. The BK effect was significantly higher in old cells and, very likely, PKC-independent, since phorbol 12-myristate 13-acetate failed to induce PtdEtn hydrolysis. 2. The present results indicate that the PLD/1,2-DAG pathway is specifically potentiated by BK in old fibroblasts, demonstrating that the formation of positive effectors of PKC activation is not necessarily decreased in cellular senescence. It remains to be established whether the increased generation of DAG upon BK stimulation plays any role in the altered PKC signalling pathway which characterizes old fibroblasts.

Activation of phospholipase D by prostaglandin F2 alpha in rat luteal cells and effects of inhibitors of arachidonic acid metabolism

In rat luteal cells labeled with [3H]oleic acid, PGF2 alpha-stimulated phospholipase D (PLD) activation was investigated. The PLD activity was detected by measuring the accumulation of [3H]phosphatidylethanol (PtdEt) in the presence of ethanol. PGF2 alpha stimulated PtdEt accumulation at concentrations of more than 100 nM in the presence of ethanol. However, PtdEt accumulation did not change in the absence of ethanol. PGF2 alpha (1 microM) increased PtdEt accumulation after 1 min, and the accumulation reached a plateau by 2-3 min. These results indicate that PGF2 alpha activates PLD in rat luteal cells. U-73122, a phospholipase C (PLC) inhibitor, and staurosporine, a protein kinase C (PKC) inhibitor, did not inhibit PGF2 alpha-stimulated [3H]PtdEt accumulation. These results suggest that PGF2 alpha-induced PLD activation is different from PLC-PKC systems. We reported previously that PGF2 alpha stimulated the release of arachidonic acid. The effects of indomethacin, nordihydroguaiaretic acid (NDGA), and 5,8,11,14-eicosatetraynoic acid (ETYA), inhibitors of arachidonic acid metabolism, on PGF2 alpha-stimulated PtdEt accumulation were examined. Pretreatment with indomethacin enhanced PGF2 alpha-induced PtdEt accumulation. In contrast, pretreatment with NDGA and ETYA inhibited PGF2 alpha-induced PtdEt accumulation. It is suggested that PGF2 alpha-stimulated PLD activation is mediated via lipoxygenase products.

Purinergic agonist and G protein stimulation of phospholipase D in rat liver plasma membranes. Independence from phospholipase C activation

Hormonal regulation of phospholipase D (PLD) was studied in isolated rat liver plasma membranes. Purinergic agents and a submaximal concentration of guanosine 5'-0-(3-thiotriphosphate) (GTP gamma S), a non-hydrolyzable analog of GTP, synergistically stimulate phosphatidylethanol formation, a measure of PLD activity. The rank order of efficacy for stimulation of PLD activity in the presence of 0.2 microM GTP gamma S was beta, gamma-methylene-ATP > adenosine 5'-0-(3-thiotriphosphate) = ATP = ADP = 2-methylthio-ATP > alpha, beta-methylene-ATP = UTP. This pattern of activation does not conform to the series at known P2 receptors. GTP gamma S stimulated PLD activity in a dose-dependent manner, and the GTP gamma S dose-response curve for phosphatidylethanol formation was shifted to the left by an analog of ATP. Activation of PLD by purinergic agents in the presence of GTP gamma S supports the involvement of a purinergic receptor of the P2 class and a GTP-binding protein. Purinergic agents competitively inhibited [35S]adenosine 5'-0-(3-thiotriphosphate) binding to plasma membranes in the rank order adenosine 5'-0'(3-thiotriphosphate) > ATP > alpha,beta-methylene-ATP = UTP >> beta, gamma-methylene-ATP = ADP. Stimulation of phosphoinositide phospholipase C (PI-PLC) by purinergic agents, as measured by release of radioactivity from endogenously myo[3H]inositol-labeled plasma membranes, occurred in the order alpha, beta-methylene-ATP >> 2-methylthio-ATP. Beta, gamma-methylene-ATP had little effect on PI-PLC activity. Different dose-response relationships for agonist-stimulation of PI-PLC and PLD indicate that activation of PI-PLC is not involved in stimulation of PLD in rat liver plasma membranes, and suggest that purinergic activation of PLD occurs via a pathway involving a G protein and a heretofore uncharacterized P2 receptor.