Phospholipase D activity in PC12 cells. Effects of overexpression of alpha2A-adrenergic receptors

Characterization of Lysophospholipase D Activity in Mammalian Cell Membranes

Lysophosphatidic acid (LPA) is a lipid mediator that binds to G-protein-coupled receptors, eliciting a wide variety of responses in mammalian cells. Lyso-phospholipids generated via phospholipase A2 (PLA2) can be converted to LPA by a lysophospholipase D (lyso-PLD). Secreted lyso-PLDs have been studied in more detail than membrane-localized lyso-PLDs. This study utilized in vitro enzyme assays with fluorescent substrates to examine LPA generation in membranes from multiple mammalian cell lines (PC12, rat pheochromocytoma; A7r5, rat vascular smooth muscle; Rat-1, rat fibroblast; PC-3, human prostate carcinoma; and SKOV-3 and OVCAR-3, human ovarian carcinoma). The results show that membranes contain a lyso-PLD activity that generates LPA from a fluorescent alkyl-lyso-phosphatidylcholine, as well as from naturally occurring acyl-linked lysophospholipids. Membrane lyso-PLD and PLD activities were distinguished by multiple criteria, including lack of effect of PLD2 over-expression on lyso-PLD activity and differential sensitivities to vanadate (PLD inhibitor) and iodate (lyso-PLD inhibitor). Based on several lines of evidence, including siRNA knockdown, membrane lyso-PLD is distinct from autotaxin, a secreted lyso-PLD. PC-3 cells express GDE4 and GDE7, recently described lyso-PLDs that localize to membranes. These findings demonstrate that membrane-associated lyso-D activity, expressed by multiple mammalian cell lines, can contribute to LPA production.

Effects of active and inactive phospholipase D2 on signal transduction, adhesion, migration, invasion, and metastasis in EL4 lymphoma cells

The phosphatidylcholine-using phospholipase D (PLD) isoform PLD2 is widely expressed in mammalian cells and is activated in response to a variety of promitogenic agonists. In this study, active and inactive hemagglutinin-tagged human PLD2 (HA-PLD2) constructs were stably expressed in an EL4 cell line lacking detectable endogenous PLD1 or PLD2. The overall goal of the study was to examine the roles of PLD2 in cellular signal transduction and cell phenotype. HA-PLD2 confers PLD activity that is activated by phorbol ester, ionomycin, and okadaic acid. Proliferation and Erk activation are unchanged in cells transfected with active PLD2; proliferation rate is decreased in cells expressing inactive PLD2. Basal tyrosine phosphorylation of focal adhesion kinase (FAK) is increased in cells expressing active PLD2, as is phosphorylation of Akt; inactive PLD2 has no effect. Expression of active PLD2 is associated with increased spreading and elongation of cells on tissue culture plastic, whereas inactive PLD2 inhibits cell spreading. Inactive PLD2 also inhibits cell adhesion, migration, and serum-induced invasion. Cells expressing active PLD2 form metastases in syngeneic mice, as do the parental cells; cells expressing inactive PLD2 form fewer metastases than parental cells. In summary, active PLD2 enhances FAK phosphorylation, Akt activation, and cell invasion in EL4 lymphoma cells, whereas inactive PLD2 exerts inhibitory effects on adhesion, migration, invasion, and tumor formation. Overall, expression of active PLD2 enhances processes favorable to lymphoma cell metastasis, whereas expression of inactive PLD2 inhibits metastasis.

Direct Modulation of Phospholipase D Activity by Gβγ

Phospholipase D-mediated hydrolysis of phosphatidylcholine is stimulated by protein kinase C and the monomeric G proteins Arf, RhoA, Cdc42, and Rac1, resulting in complex regulation of this enzyme. Using purified proteins, we have identified a novel inhibitor of phospholipase D activity, Gbetagamma subunits of heterotrimeric G proteins. G protein-coupled receptor activation alters affinity between Galpha and Gbetagamma subunits, allowing subsequent interaction with distinct effectors. Gbeta1gamma1 inhibited phospholipase D1 and phospholipase D2 activity, and both Gbeta1gamma1 and Gbeta1gamma2 inhibited stimulated phospholipase D1 activity in a dosedependent manner in reconstitution assays. Reconstitution assays suggest this interaction occurs through the amino terminus of phospholipase D, because Gbeta1gamma1 is unable to inhibit an amino-terminally truncated phospholipase D construct, PLD1.d311, which like full-length phospholipase D isoforms, requires phosphatidylinositol-4,5-bisphosphate for activity. Furthermore, a truncated protein consisting of the amino-terminal region of phospholipase D containing the phox/pleckstrin homology domains was found to interact with Gbeta1gamma1, unlike the PLD1.d311 recombinant protein, which lacks this domain. In vivo, expressed recombinant Gbeta1gamma2 was also found to inhibit phospholipase D activity under basal and stimulated conditions in MDA-MB-231 cells, which natively express both phospholipase D1 and phospholipase D2. These data demonstrate that Gbetagamma directly regulates phospholipase D activity in vitro and suggest a novel mechanism to negatively regulate phospholipase D signaling in vivo.

Activation of phospholipase D is not mediated by direct phosphorylation on tyrosine residues

The activation of phospholipase D (PLD) in PC12/PC2 pheochromocytoma cells involves a tyrosine kinase. However, it is not clear whether this is due to direct phosphorylation of the enzyme or some other intermediary protein. In this manuscript, we examined this issue by two methods: (1) immunoprecipitation of phosphotyrosine containing proteins and assay of phospholipase D; (2) overexpression of HA-phospholipase D2 and susbsequent immunoprecipitation. The only agent that caused phosphorylation of phospholipase D on tyrosine residues was the phosphatase inhibitor, peroxyvanadate. Other agents that activate phospholipase D, including bradykinin, ionomycin, and phorbol dibutyrate did not cause phosphorylation of the enzyme. In addition, there was a lack of correlation between the peroxyvanadate-mediated phosphorylation and activation of phospholipase D, both in terms of time course and concentration dependence. These data demonstrate that phospholipase D is directly phosphorylated on tyrosine residues. However, phosphorylation of tyrosine residues does not correlate with activation of the enzyme.

Distinct signaling pathways mediate cardiomyocyte phospholipase D stimulation by endothelin-1 and thrombin

Several G protein-coupled receptors which stimulate phospholipase C (PLC) also activate phospholipase D (PLD) in cardiomyocytes. Here, we characterized PLD activation in neonatal rat cardiomyocytes by the PLC-stimulatory thrombin receptor PAR1, in comparison to the endothelin-1 receptor ET(A)R, which induces PLD stimulation by activation of protein kinase C (PKC) delta and epsilon. Similar to ET(A)R, activation of PAR1 induced PLD stimulation, which, however, was insensitive to PKC inhibition. Furthermore, in contrast to ET(A)R, PLD stimulation by PAR1 was suppressed by overexpression of regulators of G protein signaling specific for G(12)-type G proteins and treatment with brefeldin A, an inhibitor of guanine nucleotide exchange factors for ADP-ribosylation factor (ARF) GTPases. On the other hand, inactivation of Rho GTPases by Clostridium difficile toxin B and treatment with general tyrosine kinase inhibitors suppressed PAR1- and ET(A)R- as well as phorbol ester-induced PLD stimulation and was associated with a fall in the cellular level of phosphatidylinositol 4,5-bisphosphate (PIP(2)). We conclude that, in contrast to ET(A)R-PLD coupling, PAR1-induced cardiomyocyte PLD stimulation is PKC-independent and mediated by G(12)-type G proteins and ARF GTPases, while Rho and tyrosine kinases regulate PLD stimulation by either receptor, apparently by controlling the cellular level of PIP(2), a common regulator of PLD activity.

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.

Protein kinase C-dependent coupling of alpha(2A/D)-adrenergic receptors to phospholipase D

To clarify the role of protein kinase C (PKC) in regulating the coupling pathway of alpha(2)-adrenergic receptors, we examined receptor activation of phospholipase D (PLD) in PC12 cells overexpressing alpha(2A/D) receptors, using [(3)H]phosphatidylbutanol formation as an index of PLD activity. In intact PC12/alpha(2A/D) cells, the ability of either epinephrine or the alpha(2)-receptor-selective agonist UK14304 to stimulate PLD was completely dependent on concomitant PKC activation. Pretreatment with the PKC activator phorbol dibutyrate revealed an agonist-stimulated PLD activity which was blocked by the alpha(2)-receptor-selective antagonist rauwolscine and by pertussis toxin treatment. Removal of extracellular calcium or tyrosine kinase inhibition by genistein pretreatment also eliminated the ability of epinephrine to stimulate PLD. These results indicate that alpha(2A/D)-adrenergic receptors couple via pertussis toxin-sensitive G proteins to PLD in a PKC-requiring and tyrosine kinase regulated manner.

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.

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.

Phospholipase D: a novel major player in signal transduction

The role of the mammalian phospholipase D (PLD) in the control of key cellular responses has been recognised for a long time, but only recently have there been the reagents to properly study this very important enzyme in the signalling pathways, linking cell agonists with intracellular targets. With the recent cloning of PLD isoenzymes, their association with low-molecular-weight G proteins, protein kinase C and tyrosine kinases, the availability of antibodies and an understanding of the role of PLD product, phosphatidic acid (PA), in cell physiology, the field is gaining momentum. In this review, we will explore the molecular properties of mammalian PLD and its gene(s), the complexity of this enzyme regulation and the myriad physiological roles for PLD and PA and related metabolic products, with particular emphasis on a role in the activation of NADPH oxidase, or respiratory burst, leading to the generation of oxygen radicals.