Increased mRNA expression of phospholipase D (PLD) isozymes during granulocytic differentiation of HL60 cells

The exquisite regulation of PLD2 by a wealth of interacting proteins: S6K, Grb2, Sos, WASp and Rac2 (and a surprise discovery: PLD2 is a GEF)

Phospholipase D (PLD) catalyzes the conversion of the membrane phospholipid phosphatidylcholine to choline and phosphatidic acid (PA). PLD's mission in the cell is two-fold: phospholipid turnover with maintenance of the structural integrity of cellular/intracellular membranes and cell signaling through PA and its metabolites. Precisely, through its product of the reaction, PA, PLD has been implicated in a variety of physiological cellular functions, such as intracellular protein trafficking, cytoskeletal dynamics, chemotaxis of leukocytes and cell proliferation. The catalytic (HKD) and regulatory (PH and PX) domains were studied in detail in the PLD1 isoform, but PLD2 was traditionally studied in lesser detail and much less was known about its regulation. Our laboratory has been focusing on the study of PLD2 regulation in mammalian cells. Over the past few years, we have reported, in regards to the catalytic action of PLD, that PA is a chemoattractant agent that binds to and signals inside the cell through the ribosomal S6 kinases (S6K). Regarding the regulatory domains of PLD2, we have reported the discovery of the PLD2 interaction with Grb2 via Y169 in the PX domain, and further association to Sos, which results in an increase of de novo DNA synthesis and an interaction (also with Grb2) via the adjacent residue Y179, leading to the regulation of cell ruffling, chemotaxis and phagocytosis of leukocytes. We also present the complex regulation by tyrosine phosphorylation by epidermal growth factor receptor (EGF-R), Janus Kinase 3 (JAK3) and Src and the role of phosphatases. Recently, there is evidence supporting a new level of regulation of PLD2 at the PH domain, by the discovery of CRIB domains and a Rac2-PLD2 interaction that leads to a dual (positive and negative) effect on its enzymatic activity. Lastly, we review the surprising finding of PLD2 acting as a GEF. A phospholipase such as PLD that exists already in the cell membrane that acts directly on Rac allows a quick response of the cell without intermediary signaling molecules. This provides only the latest level of PLD2 regulation in a field that promises newer and exciting advances in the next few years.

Phospholipase D2 (PLD2) shortens the time required for myeloid leukemic cell differentiation: mechanism of action

Cell differentiation is compromised in acute leukemias. We report that mammalian target of rapamycin (mTOR) and S6 kinase (S6K) are highly expressed in the undifferentiated promyelomonocytic leukemic HL-60 cell line, whereas PLD2 expression is minimal. The expression ratio of PLD2 to mTOR (or to S6K) is gradually inverted upon in vitro induction of differentiation toward the neutrophilic phenotype. We present three ways that profoundly affect the kinetics of differentiation as follows: (i) simultaneous overexpression of mTOR (or S6K), (ii) silencing of mTOR via dsRNA-mediated interference or inhibition with rapamycin, and (iii) PLD2 overexpression. The last two methods shortened the time required for differentiation. By determining how PLD2 participates in cell differentiation, we found that PLD2 interacts with and activates the oncogene Fes/Fps, a protein-tyrosine kinase known to be involved in myeloid cell development. Fes activity is elevated with PLD2 overexpression, phosphatidic acid or phosphatidylinositol bisphosphate. Co-immunoprecipitation indicates a close PLD2-Fes physical interaction that is negated by a Fes-R483K mutant that incapacitates its Src homology 2 domain. All these suggest for the first time the following mechanism: mTOR/S6K down-regulation→PLD2 overexpression→PLD2/Fes association→phosphatidic acid-led activation of Fes kinase→granulocytic differentiation. Differentiation shortening could have a clinical impact on reducing the time of return to normalcy of the white cell counts after chemotherapy in patients with acute promyelocytic leukemia.

Mechanism of increased PLD1 gene expression during early adipocyte differentiation process of mouse cell line 3T3-L1

A mouse cell line 3T3-L1 is differentiated into adipocytes when treated with an inducer cocktail (IDX) (insulin, dexametahsone, and a cAMP phosphodiesterase inhibitor of isobutyl-methylxanthine (IBMX)). Here, we report that PLD1, but not PLD2, mRNA and protein increased during the early differentiation process. Our analysis shows that IDX resulted in a sequential induction of C/EBPbeta, PLD1, and C/EBPalpha which is a key transcription factor of late adipocyte differentiation. Among the three inducers, IBMX + any other inducer induced mild adipocyte differentiation, whereas insulin + dexamethasone did not. IBMX increased PLD1 but not PLD2 mRNA. Forskolin, an adenylate cyclase activator, and dbcAMP also increased PLD1 mRNA, suggesting the cellular cAMP as the inducer of both adipocyte differentiation and PLD1 transcription. We focused on the regulatory mechanism of PLD1 transcription during this differentiation process. IDX or a combination of inducers including IBMX increased PLD1 promoter activity, which is consistent with mRNA analysis. Promoter analysis identified two adjacent C/EBP motifs located between -338 and -231 bp from the first exon as the IBMX responsive elements. Furthermore, overexpression of C/EBPbeta, but not C/EBPalpha, increased PLD1 mRNA and PLD1 5' promoter activity. EMSA and chromatin immunoprecipitation assay confirmed the direct binding of C/EBPbeta, but not C/EBPalpha, to these C/EBP motifs of PLD1 5' promoter. Our results show that PLD1 is a target gene of C/EBPbeta through the increased cellular cAMP during early adipocyte differentiation of 3T3-L1 cells.

PLD regulates myoblast differentiation through the mTOR-IGF2 pathway

A mammalian target of rapamycin (mTOR) pathway is essential for the differentiation of cultured skeletal myoblasts in response to growth factor withdrawal. Previously, phospholipase D (PLD) has been found to play a role in cell growth regulation and mitogenic activation of mTOR signaling. However, a role for PLD in the autocrine regulation of myoblast differentiation is not known. Here we show that upon induction of differentiation in mouse C2C12 satellite cells, the expression of both PLD1 and PLD2 is upregulated. C2C12 differentiation is markedly inhibited by 1-butanol, an inhibitor of the PLD-catalyzed transphosphatidylation reaction, and also by the knockdown of PLD1, but not PLD2. Further investigation has revealed that PLD1 is unlikely to regulate myogenesis through modulation of the actin cytoskeleton as previously suggested. Instead, PLD1 positively regulates mTOR signaling leading to the production of IGF2, an autocrine factor instrumental for the initiation of satellite cell differentiation. Furthermore, exogenous IGF2 fully rescues the differentiation defect resulting from PLD1 knockdown. Hence, PLD1 is critically involved in skeletal myogenesis by regulating the mTOR-IGF2 pathway.

Phospholipase D1 as a key enzyme for decidualization in human endometrial stromal cells

Using primary cell cultures of human endometrial stromal cells (ES cells), we investigated the role of phospholipase D (PLD) in 8-Br-cAMP-induced decidualization, which involves morphological and biological differentiation processes. When treated with 0.5 mM 8-Br-cAMP for 12 days, ES cells were transformed into a decidualized morphology and produced significant amounts of prolactin (PRL) and insulin-like growth factor-binding protein 1 (IGFBP1). Simultaneously, the activity and expression levels of PLD1 increased. In addition, removal of 8-Br-cAMP from decidualized ES cells restored the undifferentiated state, and this was accompanied by decreases in PLD1 promoter activity and PLD1 expression. Overexpression of dominant negative (DN)-PLD1 inhibited the morphological changes induced by 0.5 mM 8-Br-cAMP, whereas PLD1 overexpression induced morphological changes in the absence of 0.5 mM 8-Br-cAMP treatment. Moreover, knockdown of PLD1 by siRNA and blockage of PLD by treatment with 0.3% 1-butanol decreased PRL/IGFBP1 mRNA expression, whereas PLD1 overexpression increased PRL/IGFBP1 mRNA expression. Treatment of ES cells with phosphatidic acid (PA) for 3 days induced PRL mRNA expression and morphological changes, which implies that PA is an end-product of PLD activation-induced decidualization. In addition, pretreatment of ES cells with mepacrine decreased PRL/IGFBP1 expression and inhibited morphological change, whereas pretreatment with propranolol caused no changes, as compared to cAMP-treated cells, which suggests that PA induces decidualization through phospholipase A2 (PLA2G1B). Taken together, these results suggest that PLD1 regulates 8-Br-cAMP-induced decidualization through PLA2G1B, and that PLD1 upregulation is essential for the decidualization of ES cells.

1Alpha,25-dihydroxyvitamin D3-mediated stimulation of steroid sulphatase activity in myeloid leukaemic cell lines requires VDRnuc-mediated activation of the RAS/RAF/ERK-MAP kinase signalling pathway

1Alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) stimulates the activity of steroid sulphatase (STS) in myeloid cells [Hughes et al., 2001, 2005]. This was attenuated by inhibitors of phospholipase D (PLD) (n-butanol, 2,3-diphosphoglyceric acid, C(2)-ceramide) and phosphatidate phosphohydrolase (PAP) (propranolol and chlorpromazine), but was unaffected by inhibitors of phospholipase C. The 1alpha,25(OH)(2)D(3)-induced STS activity was also attenuated by inhibitors of protein kinase Calpha and protein kinase Cdelta (Go 6976, HBDDE and rottlerin), but not by an inhibitor of protein kinase Cbeta (LY379196). Additionally, 1alpha,25(OH)(2)D(3)-induced STS activity was attenuated by inhibitors of RAS (manumycin A), RAF (GW5074), MEK (PD098059 and U1026) and JNK (SP600125), but not p38 (PD169316). 1alpha,25(OH)(2)D(3) produced a rapid and long lasting stimulation of the ERK-MAP kinase signalling cascade in HL60 myeloid leukaemic cells. This 'non-genomic' effect of 1alpha,25(OH)(2)D(3) blocked by pharmacological antagonists of nuclear vitamin D receptors (VDR(nuc)) and does not appear to require hetero-dimerisation with the retinoid-X receptor (RXR). Inhibitors of the Src tyrosine kinase (PP1), RAS (manumycin A), RAS-RAF interactions (sulindac sulphide and RAS inhibitory peptide), RAF (GW5074 or chloroquine), and protein kinase Calpha (HBDDE) abrogated the 1alpha,25(OH)(2)D(3)-stimulated increase in ERK-MAP kinase activity. Taken together, these results show that 1alpha,25(OH)(2)D(3)/VDR(nuc) activation of the RAS/RAF/ERK-MAP kinase signalling pathway plays an important role in augmenting STS activity in human myeloid leukaemic cell lines.

Retinoid-mediated stimulation of steroid sulfatase activity in myeloid leukemic cell lines requires RARalpha and RXR and involves the phosphoinositide 3-kinase and ERK-MAP kinase pathways

All-trans retinoic acid and 9-cis-retinoic acid stimulate the activity of steroid sulfatase in HL60 acute myeloid leukemia cells in a concentration- and time-dependent manner. Neither of these 'natural retinoids' augmented steroid sulfatase activity in a HL60 sub-line that expresses a dominant-negative retinoic acid receptor alpha (RARalpha). Experiments with synthetic RAR and RXR agonists and antagonists suggest that RARalpha/RXR heterodimers play a role in the retinoid-stimulated increase in steroid sulfatase activity. The retinoid-driven increase in steroid sulfatase activity was attenuated by inhibition of phospholipase D (PLD), but not by inhibitors of phospholipase C. Experiments with inhibitors of protein kinase C (PKC) show that PKCalpha and PKCdelta play an important role in modulating the retinoid-stimulation of steroid sulfatase activity in HL60 cells. Furthermore, we show that pharmacological inhibition of the RAF-1 and ERK MAP kinases blocked the retinoid-stimulated increase in steroid sulfatase activity in HL60 cells and, by contrast, inhibition of the p38-MAP kinase or JNK-MAP kinase had no effect. Pharmacological inhibitors of the phosphatidylinositol 3-kinase, Akt, and PDK-1 also abrogated the retinoid-stimulated increase in steroid sulfatase activity in HL60 cells. These results show that crosstalk between the retinoid-stimulated genomic and non-genomic pathways is necessary to increase steroid sulfatase activity in HL60 cells.

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.

Phospholipase D (PLD) gene expression in human neutrophils and HL-60 differentiation

Human neutrophils exhibit a regulated phospholipase D (PLD) activity that can be measured biochemically in vitro. However, the precise expression pattern of PLD isoforms and their specific biological role(s) are not well understood. Neutrophil mRNA is intrinsically difficult to isolate as a result of the extremely high content of lytic enzymes in the cell's lysosomal granules. Reverse transcription coupled to polymerase chain reaction indicated that pure populations of human neutrophils had the CD16b(+)/CD115(-)/CD20(-)/CD3zeta(-)/interleukin-5 receptor alpha(-) phenotype. These cells expressed the following splice variants of the PLD1 isoform: PLD1a, PLD1b, PLD1a2, and PLD1b2. As for the PLD2 isoform, neutrophils expressed the PLD2a but not the PLD2b mRNA variant. The relative amount of PLD1/PLD2 transcripts exists in an approximate 4:1 ratio. The expression of PLD isoforms varies during granulocytic differentiation, as demonstrated in the promyelocytic leukemia HL-60 cell line. Further, the pattern of mRNA expression is dependent on the differentiation-inducing agent, 1.25% dimethyl sulfoxide causes a dramatic increase in PLD2a and PLD1b transcripts, and 300 nM all-trans-retinoic acid induced PLD1a expression. These results demonstrate for the first time that human neutrophils express five PLD transcripts and that the PLD genes undergo qualitative changes in transcription regulation during granulocytic differentiation.

Involvement of phospholipase D1 in melanogenesis of mouse B16 melanoma cells

In response to alpha-melanocyte-stimulating hormone (alpha-MSH) or cAMP-elevating agents (forskolin and isobutylmethylxanthine), mouse B16 melanoma cells underwent differentiation characterized by increased melanin biosynthesis. However, the mechanism(s) underlying the regulation of melanogenesis during differentiation has not yet been clearly understood. Phospholipase D (PLD) has been reported to be involved in differentiation. This enzyme cleaves phosphatidylcholine upon stimulation with stimuli to generate phosphatidic acid. In the current study, the involvement of PLD in the regulation of melanogenesis characteristic of differentiation was examined using mouse B16 melanoma cells. Treatment of B16 cells with alpha-MSH was found to cause marked decreases in the PLD1 activity concurrent with its reduced protein level. Moreover, treatment of exogenous bacterial PLD also inhibited alpha-MSH-induced melanogenesis. To further investigate the role of PLD1 in the regulation of melanogenesis, we examined the effects of overexpression of PLD1 on melanogenesis in B16 melanoma cells. The B16 cells overexpressing PLD were prepared by transfection with the vector containing the cDNA encoding PLD1. The melanin contents in PLD1-overexpressing cells (B16/PLD1) were observed to be lower compared with those in the vector control cells (B16/Vec), concomitant with the decreases in both activity and protein level of tyrosinase, a key regulatory enzyme in melanogenesis. Moreover, overexpression of PLD1 resulted in a marked inhibition of melanogenesis induced by alpha-MSH. The inhibition of melanogenesis was well correlated with the decrease in the tyrosinase activity associated with its expression. These results indicated that PLD1 negatively regulated the melanogenic signaling by modulating the expression of tyrosinase in mouse B16 melanoma cells.

Phospholipase D1 is required for efficient mating projection formation in Saccharomyces cerevisiae

Phospholipase D1 (PLD1) is an important enzyme involved in lipid signal transduction in eukaryotes. A role for PLD1 in signaling in Saccharomyces cerevisiae was examined. Pheromone response in yeast is controlled by a well-characterized protein kinase cascade. Loss of PLD1 activity was found to impair pheromone-induced changes in cellular morphology that result in formation of mating projections. The rate at which projections appeared following pheromone treatment was delayed, suggesting that PLD1 facilitates the execution of a rate-limiting step in morphogenesis. Mutants were found to be less sensitive to pheromone, again arguing that PLD1 is acting at a rate-limiting step. The fact that morphogenesis is most dramatically affected indicates that PLD1 functions primarily in the morphogenic branch of the pheromone response pathway.

Functional coupling of FcgammaRI to nicotinamide adenine dinucleotide phosphate (reduced form) oxidative burst and immune complex trafficking requires the activation of phospholipase D1

Immunoglobulin G (IgG) receptors (FcgammaRs) on myeloid cells are responsible for the internalization of immune complexes. Activation of the oxidase burst is an important component of the integrated cellular response mediated by Fc receptors. Previous work has demonstrated that, in interferon-gamma-primed U937 cells, the high-affinity receptor for IgG, FcgammaRI, is coupled to a novel intracellular signaling pathway that involves the sequential activation of phospholipase D (PLD), sphingosine kinase, and calcium transients. Here, it is shown that both known PLD isozymes, PLD1 and PLD2, were present in these cells. With the use of antisense oligonucleotides to specifically reduce the expression of either isozyme, PLD1, but not PLD2, was found to be coupled to FcgammaRI activation and be required to mediate receptor activation of sphingosine kinase and calcium transients. In addition, coupling of FcgammaRI to activation of the nicotinamide adenine dinucleotide phosphate (reduced form) (NADPH) oxidase burst was inhibited by pretreating cells with 0.3% butan-1-ol, indicating an absolute requirement for PLD. Furthermore, use of antisense oligonucleotides to reduce expression of PLD1 or PLD2 demonstrated that PLD1 is required to couple FcgammaRI to the activation of NADPH oxidase and trafficking of internalized immune complexes for degradation. These studies demonstrate the critical role of PLD1 in the intracellular signaling cascades initiated by FcgammaRI and its functional role in coordinating the response to antigen-antibody complexes.

Expression and regulation of phospholipase D during neuronal differentiation of PC12 cells

To assess a possible role for phospholipase D (PLD) in PC12 cell signal transduction and differentiation, we have investigated the expression of PLD in PC12 cells and found that the differentiation factor, nerve growth factor (NGF) increased PLD1 protein expression and phorbol 12-myristate 13 acetate (PMA)-induced PLD activity. During neuronal differentiation, this effect showed correlation to the protein expression levels of classical protein kinase C (PKC) isozymes, PKC-alpha and -beta II, but there was no significant increase in the protein level of RhoA, another regulatory factor for PLD activation. Interestingly, PLD1 was associated with PKC-alpha or beta II, and its association gradually increased as NGF-induced neuronal differentiation progressed. PKC inhibitor, Ro-31-8220, caused a significant inhibition of neurite outgrowth and PLD activity. Furthermore, PLD1 was constitutively associated with the Shc adaptor molecule, the overexpression of which is known to induce PLD activity and to induce neurite outgrowth. Taken together, the data in this study suggests that PLD1 is closely implicated in neuronal differentiation of PC12 cells.

Selective induction of phospholipase D1 in pathogen-activated human monocytes

Phospholipase D (PLD) activation is part of the complex signalling cascade induced during phagocyte activation. Two PLD isoforms have been cloned, but their role in phagocyte functions is still poorly defined. We report that resting fresh circulating human monocytes expressed PLD1. PLD1 protein expression was rapidly down-regulated during cell culture. Lipopolysaccharide and pathogen-derived agonists (Candida albicans, arabinoside-terminated lipoarabinomannan and Gram-positive bacteria, but not mannose-capped lipoarabinomannan or double-stranded RNA) strongly induced PLD1 expression at both the mRNA and protein levels. Pro-inflammatory cytokines [interleukin (IL)-1beta and tumour necrosis factor alpha] had only a weak effect, whereas immune cytokines (IL-6 and interferon gamma), anti-inflammatory cytokines (IL-13 and IL-10) and chemoattractants (fMet-Leu-Phe and macrophage chemoattractant protein 1) were inactive. None of the agonists tested induced significant changes in the basal expression of PLD2 mRNA. Consistent with PLD1 up-regulation was the observation that PLD enzymic activity was higher in monocytes treated with active-pathogen-derived agonists than in control cells, when stimulated with PMA or with chemotactic agonists (fMet-Leu-Phe and C5a). Thus PLD2 seems to be a constitutive enzyme in circulating monocytes. Conversely, PLD1 is an inducible protein, rapidly regulated during culture conditions and selectively induced during cell activation. Therefore PLD1 might have a relevant role in immune responses against pathogens and in chronic inflammation.

Changes in the expression of lipid-mediated signal-transducing enzymes in the rat liver after partial hepatectomy

Prostaglandins (PGs), metabolites of arachidonic acid, and other lipid mediators produced by phospholipases C (PLC) and D (PLD) are thought to play important roles in hepatocyte proliferation. The present study examined lipid-mediated signaling in the rat liver after partial hepatectomy (PH). Rats were killed 1-48 h after 70% PH and the remaining liver tissue was removed. The mRNA and protein levels of some signaling molecules were measured by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting, respectively. The levels of hepatocyte growth factor (HGF) mRNA showed a biphasic change, peaking 3 h and 9 h after PH. The expression of PLCdelta4 peaked at 12 h, but no significant changes in the expression of PLCbeta1 and PLCgamma1 were seen after PH. T he enzymes involved in PG production, namely, the expression of cytosolic PLA2 and cyclooxygenase 1 (COX1), remained constant after PH. However, the mRNA of COX2 increased transiently at 3 h, and Western blot analysis showed an increase in COX2 protein at 12 h. The expression of PLD1b peaked at 9 h and PLD1a at 12 h, whereas the expression of PLD2 remained consistent for 24 h. These results suggest that transcriptional controls may act for PLCdelta4, PLD1a/b, and COX2 during hepatocyte regeneration after PH.

Modifications in phospholipase D activity and isoform expression occur upon maturation and differentiation in vivo and in vitro in human myeloid cells

Activation of phospholipase D (PLD) occurs in response to various stimuli and results from the activity of two isozymes, hPLD1 and hPLD2. PLD activity appears to be involved in several myeloid cell processes during their development and activation, including proliferation of myeloblasts in the bone marrow and secretion, phagocytosis and NADPH oxidase activation, essential functions of differentiated neutrophils. The present work studies PLD characteristics, activity and both isozyme expression during maturation and differentiation of myeloid cells by using three different systems: leukemic myeloblasts at different stages of maturation, terminally differentiated neutrophils ex vivo and four human myeloid cell lines, NB4, HL-60, PLB 985 and U937, induced to differentiate with alltrans retinoic acid (ATRA), a cyclic adenosine monophosphate (cAMP) analogue or both agents together. HL-60, a bipotential cell line has also been differentiated along the granulocytic pathway with DMSO and the monocytic pathway with 1,25-dihydroxy vitamin D3. In all these systems, PLD activity increases with maturation and differentiation whatever the inducer used and the granulocytic or monocytic pathways. Increase in basal activity which reflects the expression during development of both hPLD1 and hPLD2 appears to be mainly related to the former isozyme expression. Association of PLD characteristic changes with maturation and differentiation was also confirmed using two NB4 clones resistant to these processes. Comparison between PLD characteristics in myeloblasts during maturation and differentiation ex vivo and in vitro in the different cell lines demonstrated that NB4 induced to differentiate with ATRA represents the best model for further studies on the specific roles of each PLD isoform in various functions of differentiated myeloid cells.

Localization of mRNAs for phospholipase D (PLD) type 1 and 2 in the brain of developing and mature rat

Phospholipase D (PLD) is known as one of the key enzymes in the lipid metabolism which produces several second messengers. We demonstrated the localization of the gene expression for PLD1 and PLD2 using in situ hybridization histochemistry in the brain of developing and mature rats. Whereas PLD1 mRNA expression was detected mainly in presumptive oligodendrocytes, PLD2 mRNA expression was detected mainly in presumptive astrocytes. In addition, the gene expression for PLDs were expressed in neuroepithelial cells of the ventricular/ependymal zones and the gene for PLD2 was expressed transiently in early postnatal gray matters, presumptive neurons.

Expression and regulation of phospholipase D isoforms in mammalian cell lines

Phospholipase D (PLD) is activated in mammalian cells in response to diverse stimuli that include growth factors, activators of protein kinase C, and agonists binding to G-protein-coupled receptors. Two forms of mammalian PLD, PLD1 and PLD2, have been identified. Expression of mRNA and protein for PLD1 and PLD2 was analyzed in the following cell lines: A7r5 (rat vascular smooth muscle); EL4 (mouse thymoma); HL-60 (human myeloid leukemia); Jurkat (human leukemia); PC-3 (human prostate adenocarcinoma); PC-12K (rat phaeochromocytoma); and Rat-1 HIR (rat fibroblast). All, with the exception of EL4, express agonist-activated PLD activity. PLD1 is expressed in A7r5, HL-60, PC-3, and Rat-1, while PLD2 is expressed in A7r5, Jurkat, PC12K, PC-3, and Rat-1. Neither isoform is expressed in EL4. Guanine nucleotide-independent PLD activity is present in membranes from all cells expressing PLD2. In PC12K cells, which express only PLD2, treatment with nerve growth factor causes neurite outgrowth and increases expression of PLD2 mRNA and protein within 6-12 h. A corresponding increase is observed in membrane PLD activity and in phorbol-12-myristate-13-acetate (PMA)-stimulated PLD activity in intact cells. These results show that PLD2 can be regulated both pretranslationally and posttranslationally by agonists.

Stimulation of phospholipase D activity by oxidized LDL in mouse peritoneal macrophages

Oxidation of LDL is an important factor in the development of atherosclerosis. However, the mechanisms by which oxidized LDL exerts its atherogenic actions are poorly understood. In the present work, we show that oxidized LDL stimulates phospholipase D (PLD) activity in mouse peritoneal macrophages and that this effect increases with the degree of LDL oxidation. Oxidative modification of LDL results in the production of lipid peroxides and the conversion of phosphatidylcholine to lysophosphatidylcholine. Although we found that lysophosphatidylcholine alone activates PLD, the stimulation of this enzyme activity by oxidized LDL is independent of lysophosphatidylcholine formation. Also, 7-ketocholesterol, the major oxysterol in oxidized LDL, failed to stimulate PLD activity. To determine the mechanism(s) whereby oxidized LDL activates PLD, the possible involvements of protein kinase C and tyrosine phosphorylation were investigated. Pretreatment of macrophages with the protein kinase C inhibitor Ro-32-0432 or downregulation of protein kinase C activity by prolonged incubation with 100 nmol/L 4beta-phorbol 12-myristate 13-acetate did not alter the stimulatory effect of oxidized LDL on PLD activation. However, oxidized LDL stimulated tyrosine phosphorylation of several macrophage proteins, and preincubation of the macrophages with genistein, a tyrosine kinase inhibitor, blocked the activation of PLD by oxidized LDL. In addition, pretreatment with orthovanadate, which inhibits tyrosine phosphatases, enhanced basal and oxidized LDL-stimulated PLD activity. Pretreatment of macrophages with pertussis toxin decreased the stimulatory effect of oxidized LDL, indicating that GTP-binding proteins may also be involved in the activation of PLD by oxidized LDL. We also found that the platelet-activating factor receptor antagonists WEB 2086 and L-659,989 inhibit the oxidized LDL stimulation of PLD, suggesting a role for platelet-activating factor receptor in this process. The stimulation of the PLD pathway by oxidized LDL may be of importance in atherogenesis, because PLD activation leads to generation of important second messengers such as phosphatidate, lysophosphatidate, and diacylglycerol, which are known to regulate many cellular functions.

Contribution of mitogen-activated protein kinase to stimulation of phospholipase D by the chemotactic peptide fMet-Leu-Phe in human neutrophils

Phospholipase D (PLD) plays an important role in signaling through phosphatidylcholine (PC) and in the production of superoxide (respiratory burst) by polymorphonuclear leukocytes (PMN) stimulated by the chemoattractant fMet-Leu-Phe (fMLP). However, the regulation of PLD activity by protein kinases is not fully understood. In the present study, we have used a mitogen-activated protein (MAP) kinase inhibitor (PD 98059) to investigate a possible connection between extracellular signal-regulated kinase (ERK) and PLD activity and respiratory burst. Using a range of concentrations (3-20 microM) which inhibit ERK activity, PD 98059 inhibited PLD activity induced by fMLP in cytochalasin B-primed PMN, as assessed by production-tritiated phosphatidylethanol (PEt), phosphatidic acid (PA), and hydrolysis of PC. However, the inhibition was partial (approximately 50%), while inhibition of PC hydrolysis was almost complete, suggesting a concomitant inhibition of PLA2 activity. In addition, PD 98059 reduced fMLP-induced respiratory burst by 50%, an effect which was correlated with PLD inhibition of PLD (r = 0.981, P < 0.01), and neither did PD 98059 inhibit the PLD activity and respiratory burst induced by PKC upon its direct activation by phorbol myristate acetate. These data provide the first evidence for implication of the ERK cascade in the stimulation of PLD through Gi signaling. They further indicate that PLD stimulation by fMLP receptors occurs through two pathways, dependent and independent on MAP kinase, the former pathway being linked to superoxide production.

Regulation of phospholipase D by phosphorylation-dependent mechanisms

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

Expression of phospholipase D isoforms in mammalian cells

Two mammalian isoforms of phospholipase D, PLD1 and PLD2, have recently been characterized at the molecular level. Effects of physiologic agonists on PLD activity in intact cells, as characterized in earlier studies, have generally not been attributed to specific PLD isoforms. Recent work has established that expression of PLD1 and PLD2 varies within tissues and between cell lines. A single cell type can express one, both, or neither isoform, although most cells co-express PLD1 and PLD2. Lymphocytes often lack expression of one or both isoforms of PLD. Relative levels of PLD mRNA expression vary considerably between established cell lines. Expression of transcripts for both PLD1 and PLD2 can be regulated at the transcriptional level by growth and differentiation factors in cultured cells. Thus, it is apparent that the known mammalian PLD isoforms are subject to regulation at the transcriptional level. The available data do not conclusively establish whether PLD1 and PLD2 are the only isoforms responsible for agonist-mediated PLD activation. Further studies of the regulation of expression of PLD isoforms should provide insight into the roles of PLD1 and PLD2 in physiologic responses, and may suggest whether additional forms of PLD remain to be characterized.

Down-regulation of phospholipase D during differentiation of mouse F9 teratocarcinoma cells

Phospholipase D has been recognized as playing an important role in signal transduction in many types of cells. We investigated the expression of phospholipase D during the differentiation of F9 embryonal teratocarcinoma cells. The ADP ribosylation factor-dependent phospholipase D activity, as measured by an in vitro assay, and H2O2-induced phospholipase D activity and phospholipase D protein content in whole cells were decreased during the differentiation of F9 cells induced by a combination of dibutyryl cyclic AMP and all-trans retinoic acid. In contrast, these changes were not observed when cells were induced by retinoic acid. These results suggest that down-regulation of phospholipase D protein is associated with differentiation of F9 cells to a parietal endoderm lineage.

Increased expression of phospholipase D1 mRNA during cAMP- or NGF-induced differentiation in PC12 cells

The rat pheochromocytoma PC12 cells have been employed as a useful model to study neuronal differentiation. To gain insight into the molecular events involved in extension of neurites, the differential expression of phospholipase D (PLD) isozymes were examined in cyclic AMP- or nerve growth factor (NGF)-treated PC12 cells. When the cells were incubated with dibutyryl cyclic AMP (dbcAMP), almost all the cells displayed prominent neurite outgrowth at 24 h. The neurites developed in NGF-treated cells at 24 h were shorter than those in the cells treated with dbcAMP. However, most of the NGF-treated cells extended long neurites at day 5. The rPLD1b mRNA increased within 6 h following dbcAMP treatment and maintained a high level up to 24 h. In contrast, the levels of rPLD1a and rPLD2 mRNAs were rather consistent throughout the time course examined. However, when the cells were treated with NGF, rPLD1a and rPLD1b mRNAs, but not rPLD2 mRNA, increased within 2 days and remained elevated up to 5 days. These results suggest the possible implication of PLD1 in PC12 cell differentiation.

Possible role of phospholipase D in cellular differentiation and apoptosis

Phospholipase D (PLD) is widely distributed in mammalian cells and is implicated in a variety of physiological processes that reveal it to be a member of the signal transducing phospholipases. Recently, two related PLD isozymes, PLD1 and PLD2, were cloned. The former activity is regulated in vitro by protein kinase C and small molecular weight GTP-binding proteins (Arf and Rho family). By contrast, the basal activity of the latter is high and it is unresponsive in vitro to these activators. The cellular PLD activity and mRNA levels of these PLD isozymes drastically changed during differentiation and apoptosis in several types of cells. The general trend was that the mRNA level of PLD1 increased during differentiation, as did the observed GTP gamma S-dependent PLD activity which presumably derived from PLD1-specific catalysis. In contrast, the PLD activity and mRNA level of PLD1 were down-regulated during apoptosis. In addition to these PLD isozymes, there exists another PLD isozyme which is activated by unsaturated fatty acids such as oleic acid, although its molecular nature and physiological roles are not well defined. We have observed that this type of PLD activity is drastically increased during apoptosis of Jurkat T cells, which mainly possess this kind of PLD activity. These results suggest the possibility that PLD activity is controlled at the transcriptional level in certain circumstances, and that PLD plays roles in differentiation, survival and apoptosis in mammalian cells.

Genomic analysis of murine phospholipase D1 and comparison to phospholipase D2 reveals an unusual difference in gene size

Phospholipase D (PLD) catalyzes the hydrolysis of phosphatidylcholine to generate phosphatidic acid and choline. In mammals, PLD activity is encoded by two different genes, PLD1 and PLD2. cDNAs for human, mouse and rat PLD1 and PLD2 and the mouse PLD2 genomic organization have recently been reported. In this article, we describe the genomic organization of mouse PLD1. Mouse PLD1 (mPLD1) contains 28 exons and spans approximately 147 kb. This is much larger than the closely related mouse PLD2 (mPLD2) gene (17 kb) and indicates that a rapid 20-fold extension/contraction of the set of introns for one of the genes has occurred. This change is not apparently due to PLD1-specific repetitive element mediated expansion. Similar to mPLD2, most PLD1 exons are less than 200 bp in length. Only the final exon, containing in part the 3' UTR, is significantly longer (881 bp). Comparison of the mPLD1 and mPLD2 exon-intron boundaries revealed that almost all of the sites are conserved. The mPLD1 5' UTR is interrupted by two large introns, 16 and 30 kb in length. A PLD1-specific sequence known as the 'loop' region is encoded by two non-conserved exons. The boundary between the loop-encoding exons matches precisely the site at which an alternately spliced isoform had been proposed to be initiated.

Changes in the expression of protein kinase C (PKC), phospholipases C (PLC) and D (PLD) isoforms in spleen, brain and kidney of the aged rat: RT-PCR and Western blot analysis

The age-dependent changes of expression of protein kinase C (PKC), phospholipase C (PLC) and phospholipase D (PLD) isozymes were analyzed in spleen, brain and kidney of young-adult (12-16 week-old) and aged (82-88 week-old) rats. The activities of spleen cPKC and nPKC were significantly decreased by nearly 35 and 30% in aged rats compared to those of young adults, respectively (P < 0.05). The level of PKC beta1 was significantly decreased in aged rats as assessed by RT-PCR and Western blot analyses. In aged rat brain where the activity of cPKC was significantly decreased by nearly 25% (P < 0.05), PKC alpha and beta1 isozymes were significantly down-regulated. In kidney, the level of PKC beta2 was decreased. In spleen the both mRNA and protein levels of PLC beta2 and gamma2 were significantly down-regulated in aged rat (P < 0.05). PLC beta1 was also significantly lower in aged rat brain (P < 0.05) as assessed by RT-PCR and Western blotting. Moreover, PLC beta1 was significantly down-regulated in both mRNA and protein levels in aged rat kidney (P < 0.05). In contrast, the tissues examined, the expressions of PLD isozymes (PLD1a, 1b and 2) were rather stable in the course of aging. These results indicate that mRNAs of PLD isozymes were rather stable but that particular PKC and PLC isozymes were down-regulated in different tissues during aging, suggesting age-dependent decline of specific PKC and PLC isozymes in organs which may, at least in part, be implicated in tissue dysfunction with aging.