Identification of phosphatidylcholine-selective and phosphatidylinositol-selective phospholipases D in Madin-Darby canine kidney cells

ITPK1 mediates the lipid-independent synthesis of inositol phosphates controlled by metabolism

Inositol phosphates (IPs) are a class of signaling molecules regulating cell physiology. The best-characterized IP, the calcium release factor IP₃, is generated by phospholipase C hydrolysis of phosphoinositides lipids. For historical and technical reasons, IPs synthesis is believed to originate from the lipid-generated IP₃. While this is true in yeast, our work has demonstrated that other organisms use a “soluble” (nonlipid) route to synthesize IPs. This soluble pathway depends on the metabolic status of the cells, and is under the control of the kinase ITPK1, which phosphorylates inositol monophosphate likely generated from glucose. The data shed light on the evolutionary origin of IPs, signaling and tightening the link between these small molecules and basic metabolism.

Inositol phosphates (IPs) comprise a network of phosphorylated molecules that play multiple signaling roles in eukaryotes. IPs synthesis is believed to originate with IP₃ generated from PIP₂ by phospholipase C (PLC). Here, we report that in mammalian cells PLC-generated IPs are rapidly recycled to inositol, and uncover the enzymology behind an alternative “soluble” route to synthesis of IPs. Inositol tetrakisphosphate 1-kinase 1 (ITPK1)—found in Asgard archaea, social amoeba, plants, and animals—phosphorylates I(3)P₁ originating from glucose-6-phosphate, and I(1)P₁ generated from sphingolipids, to enable synthesis of IP₆. We also found using PAGE mass assay that metabolic blockage by phosphate starvation surprisingly increased IP₆ levels in a ITPK1-dependent manner, establishing a route to IP₆ controlled by cellular metabolic status, that is not detectable by traditional [³H]-inositol labeling. The presence of ITPK1 in archaeal clades thought to define eukaryogenesis indicates that IPs had functional roles before the appearance of the eukaryote.

Phospholipase D-mTOR signaling is compromised in a rat model of depression

Depression is associated with structural and neurochemical changes in limbic structures, including the hippocampus, that control emotion and mood. Structural abnormalities such as decrease in hippocampal cell proliferation, neurogenesis and hippocampal volume, and loss of neurons and glial cells have been widely reported in physical and psychosocial stress paradigms and animal model of depression, but corresponding neurochemical changes are largely unknown. Using neonatal clomipramine (CL)-treated rats as a model to elucidate the association of phospholipase D (PLD) and mammalian target of rapamycin (mTOR) signaling with depressive pathology, we found that the hippocampus of CL-treated rats showed significantly down-regulation of PLD1 expression and attenuation of PLD activity which leads to the less formation of phosphatidic acid (PA), an activator of mTOR, and free choline, a potential biomarker for depression. With lower PA levels which could affect mTOR signaling, we further observed that the phosphorylation of p70S6 kinase, one of the downstream effectors of mTOR, was also significantly decreased in the hippocampus of CL-treated rats compared to the controls. Down-regulation of PLD1 expression, PLD activity and p70S6 phosphorylation was also found in the hypothalamus and frontal cortex with CL-treated rats. Our results indicate that PLD-mTOR signaling is associated with depressive disorder.

Shear stress induces cell apoptosis via a c-Src-phospholipase D-mTOR signaling pathway in cultured podocytes

The glomerular capillary wall, composed of endothelial cells, the glomerular basement membrane and the podocytes, is continually subjected to hemodynamic force arising from tractional stress due to blood pressure and shear stress due to blood flow. Exposure of glomeruli to abnormal hemodynamic force such as hyperfiltration is associated with glomerular injury and progressive renal disease, and the conversion of mechanical stimuli to chemical signals in the regulation of the process is poorly understood in podocytes. By examining DNA fragmentation, apoptotic nuclear changes and cytochrome c release, we found that shear stress induced cell apoptosis in cultured podocytes. Meanwhile, podocytes exposed to shear stress also stimulated c-Src phosphorylation, phospholipase D (PLD) activation and mammalian target of rapamycin (mTOR) signaling. Using the antibodies against c-Src, PLD(1), and PLD(2) to perform reciprocal co-immunoprecipitations and in vitro PLD activity assay, our data indicated that c-Src interacted with and activated PLD(1) but not PLD(2). The inhibition of shear stress-induced c-Src phosphorylation by PP(2) (a specific inhibitor of c-Src kinase) resulted in reduced PLD activity. Phosphatidic acid, produced by shear stress-induced PLD activation, stimulated mTOR signaling, and caused podocyte hypertrophy and apoptosis.

Sphingosine 1-phosphate: a novel stimulator of aldosterone secretion

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid capable of regulating critical physiological and pathological functions. Here, we report for the first time that S1P stimulates aldosterone secretion in cells of the zona glomerulosa of the adrenal gland. Regulation of aldosterone secretion is important because this hormone controls electrolyte and fluid balance and is implicated in cardiovascular homeostasis. S1P-stimulated aldosterone secretion was dependent upon the protein kinase C (PKC) isoforms alpha and delta and extracellular Ca2+, and it was inhibited by pertussis toxin (PTX). S1P activated phospholipase D (PLD) through a PTX-sensitive mechanism, also involving PKC alpha and delta and extracellular Ca2+. Primary alcohols, which attenuate the formation of phosphatidic acid (the product of PLD), and cell-permeable ceramides, which inhibit PLD activity, blocked S1P-stimulated aldosterone secretion. Furthermore, propranolol, chlorpromazine, and sphingosine, which are potent inhibitors of phosphatidate phosphohydrolase (PAP) (the enzyme that produces diacylglycerol from phosphatidate), also blocked aldosterone secretion. These data suggest that the PLD/PAP pathway plays a crucial role in the regulation of aldosterone secretion by S1P and that Gi protein-coupled receptors, extracellular Ca2+, and the PKC isoforms alpha and delta are all important components in the cascade of events controlling this process.

Mechanical force-activated phospholipase D is mediated by Galpha12/13-Rho and calmodulin-dependent kinase in renal epithelial cells

The renal glomerulus, the site of plasma ultrafiltration, is exposed to mechanical force in vivo arising from capillary blood pressure and fluid flow. Studies of cultured podocytes demonstrate that they respond to stretch by altering the structure of the actin cytoskeleton, but the mechanisms by which physical force triggers this architectural change and the signaling pathways that lead to generation of second messengers are not defined. In the present study, we found that in renal epithelial cells [podocytes and Madin-Darby canine kidney (MDCK) cells], application of mechanical force to the cell surface through fibronectin-coated ferric beads and exposure of the cells to magnetic force lead to Rho translocation and actin cytoskeleton reorganization. This application of force recruited Rho and filamentous actin (F-actin) to bead loci and subsequently stimulated phospholipase D (PLD), a downstream effector of Rho. Using MDCK cells that stably express regulators of G protein-signaling (RGS) proteins [RGS4 attenuates Galpha(i) and Galpha(q), and the p115RhoGEF-RGS domain (p115-RGS) attenuates Galpha(12/13)] to define the signaling pathway, we found that mechanical force induced Galpha(12/13)-Rho activation and increased F-actin to stimulate PLD activity. The activation can be partially prevented by the C(3) exoenzyme. Pretreatment of the cells with chemical inhibitors of several kinases showed that calmodulin-dependent kinase is also involved in stretch-induced PLD activation by a separate pathway. Taken together, our data demonstrate that in cultured podocytes and MDCK cells, mechanical force leads to actin cytoskeleton reorganization and PLD activation. The signaling pathways for PLD activation involve Galpha(12/13)/Rho/F-actin and calmodulin-dependent kinase.

Angiotensin II-stimulated cortisol secretion is mediated by phospholipase D

Angiotensin II (Ang-II) regulates a variety of cellular functions including cortisol secretion. In the present report, we demonstrate that Ang-II activates phospholipase D (PLD) in zona fasciculata (ZF) cells of bovine adrenal glands, and that this effect is associated to the stimulation of cortisol secretion by this hormone. PLD activation was dependent upon extracellular Ca2+, and was blocked by inhibition of protein kinase C (PKC). Using the reverse transcription-polymerase chain reaction technique, we demonstrated that ZF cells express both PLD-1 and PLD-2 isozymes. Primary alcohols, which attenuate the formation of phosphatidate (the product of PLD), and cell-permeable ceramides, which inhibit PLD potently, blocked Ang-II-stimulated cortisol secretion. Furthermore, propranolol or chlorpromazine, which are potent inhibitors of phosphatidate phosphohydrolase (PAP) (the enzyme that produces diacylglycerol from phosphatidate), also blocked cortisol secretion. These data suggest that the PLD/PAP pathway plays an important role in the regulation of cortisol secretion by Ang-II in ZF cells.

Sphingosine-1-phosphate stimulates cortisol secretion

We show here for the first time that sphingosine-1-phosphate (Sph-1-P) stimulates cortisol secretion in zona fasciculata cells of bovine adrenal glands. This effect was dependent upon protein kinase C (PKC) and extracellular Ca2+, and was inhibited by pertussis toxin. Sph-1-P activated phospholipase D (PLD) through a pertussis toxin-sensitive mechanism, also involving extracellular Ca2+ and PKC. Primary alcohols, which attenuate formation of phosphatidic acid (the product of PLD), and cell-permeable ceramides, which inhibit PLD, blocked Sph-1-P-induced cortisol secretion. In conclusion, Sph-1-P stimulates cortisol secretion through a mechanism involving Gi/o protein-coupled receptors, extracellular Ca2+, PKC and PLD.

Parallel activation of phosphatidylinositol 4-kinase and phospholipase C by the extracellular calcium-sensing receptor

The calcium-sensing receptor (CaR) is a G protein-coupled receptor that regulates physiological processes including Ca(2+) metabolism, Na(+), Cl(-), K(+), and H(2)0 balance, and the growth of some epithelial cells through diverse signaling pathways. Although many effects of CaR are mediated by the heterotrimeric G proteins Galpha(q) and Galpha(i), not all signaling pathways regulated by CaR have been identified. We used human embryonic kidney (HEK)-293 cells that stably express human CaR to study the regulation of inositol lipid metabolism by CaR. The nonfunctional mutant CaR(R796W) was used as a negative control. We found that CaR regulates phosphatidylinositol (PI) 4-kinase, the first step in inositol lipid biosynthesis. In cells pretreated with to inhibit phospholipase C activation and to block the degradation of PI 4,5-bisphosphate to form [(3)H]inositol trisphosphate (IP(3)), CaR stimulated the accumulation of [(3)H]PI monophosphate (PIP). Additionally, wortmannin, an inhibitor of both PI 3-kinase and type III PI 4-kinase, blocked CaR-stimulated accumulation of [(3)H]PIP and inhibited [(3)H]IP(3) production. CaR-stimulated inositol lipid synthesis was attributable to PI 4-kinase and not PI 3-kinase because CaR did not activate Akt, a downstream target of PI 3-kinase. CaR associates with PI 4-kinase based on the findings that CaR and the 110-kDa PI 4-kinase beta can be co-immunoprecipitated with antibodies against either CaR or PI 4-kinase. The PI-4 kinase in co-immunoprecipitates with anti-CaR antibody was activated in Ca(2+)-stimulated HEK-293 cells, which stably express the wild type CaR. Pertussis toxin did not affect the formation of [(3)H]IP(3) or the rise in intracellular Ca(2+) (Handlogten, M. E., Huang, C. F., Shiraishi, N., Awata, H., and Miller, R. T. (2001) J. Biol. Chem. 276, 13941-13948). RGS4, an accelerator of GTPase activity of members of the Galpha(i) and Galpha(q) families, attenuated the CaR-stimulated PLC activation and IP(3) accumulation, which is mediated by Galpha(q), but did not inhibit CaR-stimulated [(3)H]PIP formation. In HEK-293 cells, which express wild type CaR, Rho was enriched in immune complexes co-immunoprecipitated with the anti-CaR antibody. C(3) toxin, an inhibitor of Rho, also inhibited the CaR-stimulated [(3)H]IP(3) production but did not lead to CaR-stimulated [(3)H]PIP formation, reflecting inhibition of PI 4-kinase. Taken together, our data demonstrate that CaR stimulates PI 4-kinase, the first step in inositol lipid biosynthesis conversion of PI to PI 4-P by Rho-dependent and Galpha(q)- and Galpha(i)-independent pathways.

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

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

Presence of a phospholipase D (PLD) distinct from PLD1 or PLD2 in human neutrophils: immunobiochemical characterization and initial purification

Utilizing the transphosphatidylation reaction catalyzed by phospholipase D (PLD) in the presence of a primary alcohol and the short-chain phospholipid PC8, we have characterized the enzyme from human neutrophils. A pH optimum of 7.8-8.0 was determined. PIP(2), EDTA/EGTA, and ATP were found to enhance basal PLD activity in vitro. Inhibitory elements were: oleate, Triton X-100, n-octyl-beta-glucopyranoside, divalent cations, GTPgammaS and H(2)O(2). The apparent K(m) for the butanol substrate was 0.1 mM and the V(max) was 6.0 nmol mg(-1) h(-1). Immunochemical analysis by anti-pan PLD antibodies revealed a neutrophil PLD of approximately 90 kDa and other bands recognized minimally by anti-PLD1 or anti-PLD2 antibodies. The 90-kDa protein is tyrosine-phosphorylated upon cell stimulation with GM-CSF and formyl-Met-Leu-Phe. Protein partial purification using column liquid chromatography was performed after cell subfractionation. Based on the enzyme's regulatory and inhibitory factors, and its molecular weight, these data indicate an enzyme isoform that might be different from the mammalian PLD1/2 forms described earlier. The present results lay the foundation for further purification of this granulocyte PLD isoform.

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

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

Phospholipase D (PLD) is present in Leishmania donovani and its activity increases in response to acute osmotic stress

We report here that the signaling molecule phospholipase D (PLD) is present in the parasitic protozoan Leishmania donovani. In vitro enzymatic activity is dependent on Ca2+ and Mg2+ ions, its basal activity is stimulated by phosphatidyl-inositol-4,5-bisphosphate (PIP2) and its pH optima are pH 8.0 and pH 6.0. PLD activity increases 3-fold about 5 min after an abrupt decrease in osmolality from 317 mOsm (isosmotic) to 155 mOsm and increases 1.5-fold in response to an abrupt increase in osmolality to 617 mOsM. Cells grown for > 24 h under the anisosmotic conditions showed only marginal changes in activity compared to the controls grown under isosmotic conditions, indicating an adaptation to long-term exposure to hypo- or hyper-osmolarity. Immunologically, two isoforms, PLD1 and PLD2, are present. An analysis of in vitro PLD activity in anti-PLD immunocomplexes revealed that either hypotonic (cell swelling) or hypertonic stress (cell shrinking) causes an increase in PLD1 activation but a reduction in PLD2 activity. The interplay between these two isoforms results in a predominance for PLD1 in the observed increase when measuring total PLD activity. Finally, the increase in enzymatic activity in acute hyposmotic shock is accompanied by tyrosyl phosphorylation of the PLD1 isoform, suggesting a role for protein tyrosine kinase in the control of PLD activity in response to osmotic stress.

Overexpression of phospholipase D1 in human breast cancer tissues

Phospholipase D (PLD) catalyzes the hydrolysis of phosphatidylcholine (PC) to produce phosphatidic acid (PA) and choline. PLD is a major enzyme implicated in important cellular processes, such as cell proliferation. We designed this study to investigate the expression of PLD in human breast carcinomas and non-malignant tissues using RT-PCR, Western blot analysis, immunohistochemistry and an Arf-dependent PLD activity assay. We examined about 550 bp of PCR product and 120 kDa of PLD protein. Our results showed that PLD protein and mRNA levels were overexpressed in 14 of 17 breast cancer tissues. We also observed increased expression by immunohistochemistry and Arf-dependent PLD activity in microsomes of human breast tumors, which correlated well with PLD expression. PLD expression was elevated in human breast tumors compared with normal breast tissues. These results implicate a possible role of PLD in human breast tumorigenesis and suggest that PLD may be useful as a marker for malignant disease in the breast.

Phospholipase D, tumor promoters, proliferation and prostaglandins

Phosphatidylcholine hydrolysis by phospholipase D is a widespread response to cellular stimulation. However, the downstream signaling events subsequent to phosphatidylcholine hydrolysis are just beginning to be determined. Initially it was proposed that diglyceride formation by phospholipase D and phosphatidate phosphohydrolase resulted in long-term stimulation of protein kinase C. However, recent studies indicate that phosphatidic acid is the relevant signaling molecule in some signaling pathways. The present review will summarize studies of phospholipase D in the response of cells to the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate, which causes cells to mimic the phenotype of oncogenic transformation. The role of phospholipase D in stimulation of Raf-1 and prostaglandin H synthase type-2 is emphasized.

Localization and possible functions of phospholipase D isozymes

The activation of PLD is believed to play an important role in the regulation of cell function and cell fate by extracellular signal molecules. Multiple PLD activities have been characterized in mammalian cells and, more recently, several PLD genes have been cloned. Current evidence indicates that diverse PLD activities are localized in most, if not all, cellular organelles, where they are likely to subserve different functions in signal transduction, membrane vesicle trafficking and cytoskeletal dynamics.

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.

Phosphatidic acid is a potent and selective inhibitor of protein phosphatase 1 and an inhibitor of ceramide-mediated responses

In the present study, we report that phosphatidic acid (PA) functions as a novel, potent, and selective inhibitor of protein phosphatase 1 (PP1). The catalytic subunit of PP1alpha was inhibited by PA dose-dependently in a noncompetitive manner with a K(i) value of 80 nM. The inhibition by PA was specific to PP1 as PA failed to inhibit protein phosphatase 2A (PP2A) or PP2B. Furthermore, PA was the most effective and potent inhibitor of PP1 compared with other phospholipids. Because we recently showed that ceramides activated PP1, we next examined the effects of PA on ceramide stimulation of PP1. PA inhibited both basal and ceramide-stimulated PP1 activities, and ceramide showed potent and stereoselective activation of PP1 in the presence of PA. Next, the effects of PA on ceramide-induced responses were examined. Molt-4 cells took up PA dose- and time-dependently such that by 1 and 3 h, uptake of PA was 0.37 and 0. 65% of total PA added, respectively. PA at 30 microM and calyculin A at 10 nM (an inhibitor of PP1 and PP2A at low concentrations), but not okadaic acid at 10 nM (a PP2A inhibitor at low concentrations) prevented poly(ADP-ribose) polymerase proteolysis induced by C(6)-ceramide. Moreover, the combination of PA with okadaic acid prevented retinoblastoma gene product dephosphorylation induced by C(6)-ceramide. These data suggest that PA functions as a specific regulator of PP1 and may reverse or counteract those effects of ceramide that are mediated by PP1, such as apoptosis and retinoblastoma gene product dephosphorylation.

Identification of multiple phosphoinositide-specific phospholipases D as new regulatory enzymes for phosphatidylinositol 3,4, 5-trisphosphate

In the course of delineating the regulatory mechanism underlying phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) metabolism, we have discovered three distinct phosphoinositide-specific phospholipase D (PI-PLD) isozymes from rat brain, tentatively designated as PI-PLDa, PI-PLDb, and PI-PLDc. These enzymes convert [3H]PI(3,4,5)P3 to generate a novel inositol phosphate, D-myo-[3H]inositol 3,4,5-trisphosphate ([3H]Ins(3,4,5)P3) and phosphatidic acid. These isozymes are predominantly associated with the cytosol, a notable difference from phosphatidylcholine PLDs. They are partially purified by a three-step procedure consisting of DEAE, heparin, and Sephacryl S-200 chromatography. PI-PLDa and PI-PLDb display a high degree of substrate specificity for PI(3,4, 5)P3, with a relative potency of PI(3,4,5)P3 >> phosphatidylinositol 3-phosphate (PI(3)P) or phosphatidylinositol 4-phosphate (PI(4)P) > phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) > phosphatidylinositol 3,4-bisphosphate (PI(3,4)P2). In contrast, PI-PLDc preferentially utilizes PI(3)P as substrate, followed by, in sequence, PI(3,4,5)P3, PI(4)P, PI(3,4)P2, and PI(4,5)P2. Both PI(3, 4)P2 and PI(4,5)P2 are poor substrates for all three isozymes, indicating that the regulatory mechanisms underlying these phosphoinositides are different from that of PI(3,4,5)P3. None of these enzymes reacts with phosphatidylcholine, phosphatidylserine, or phosphatidylethanolamine. All three PI-PLDs are Ca2+-dependent. Among them, PI-PLDb and PI-PLDc show maximum activities within a sub-microM range (0.3 and 0.9 microM Ca2+, respectively), whereas PI-PLDa exhibits an optimal [Ca2+] at 20 microM. In contrast to PC-PLD, Mg2+ has no significant effect on the enzyme activity. All three enzymes require sodium deoxycholate for optimal activities; other detergents examined including Triton X-100 and Nonidet P-40 are, however, inhibitory. In addition, PI(4,5)P2 stimulates these isozymes in a dose-dependent manner. Enhancement in the enzyme activity is noted only when the molar ratio of PI(4,5)P2 to PI(3,4, 5)P3 is between 1:1 and 2:1.

Antagonistic Roles for Phospholipase D Activities in B Cell Signaling: While the Antigen Receptors Transduce Mitogenic Signals Via a Novel Phospholipase D Activity, Phosphatidylcholine-Phospholipase D Mediates Antiproliferative Signals

Cross-linking of the Ag receptors on B cells induces DNA synthesis and proliferation. Butanol trap experiments suggest that one or more phospholipase D activities play a key role in this process. Although phosphatidylcholine-phospholipase D has been shown to play a central role in the transduction of proliferative responses for a wide variety of calcium-mobilizing receptors, we show that the Ag receptors are not coupled to this phospholipase. In addition, phosphatidylcholine-phospholipase D is not stimulated under conditions that mimic T cell-dependent B cell activation. In contrast, ATP, which inhibits surface Ig (sIg)-mediated DNA synthesis in murine B cells via P2-purinoceptors, activates phosphatidylcholine-phospholipase D. Phosphatidylcholine-phospholipase D is therefore associated with antiproliferative signal transduction in mature B cells, but it does not transduce early signals associated with sIg-mediated growth arrest or apoptosis in immature B cells. Mitogenic stimulation of sIg is, however, coupled to a novel nonphosphatidylcholine-hydrolyzing phospholipase D activity. The resultant sIg-generated phosphatidic acid, unlike the phosphatidylcholine-derived phosphatidic acid generated via the purinoceptors, is converted to diacylglycerol. These data provide the first evidence that while the novel sIg-coupled phospholipase D and resultant diacylglycerol generation may play a role in B cell survival and proliferation, phosphatidylcholine-phospholipase D may transduce, via phosphatidic acid, negative immunomodulatory signals in mature B lymphocytes.

Phospholipid signalling in the nucleus. Een DAG uit het leven van de inositide signalering in de nucleus

Diverse methodologies, ranging from activity measurements in various nuclear subfractions to electron microscopy, have been used to demonstrate and establish that many of the key lipids and enzymes responsible for the metabolism of inositol lipids are resident in nuclei. PtdIns(4)P, PtdIns(4,5)P2 and PtdOH are all present in nuclei, as well as the corresponding enzyme activities required to synthesise and metabolise these compounds. In addition other non-inositol containing phospholipids such as phosphatidylcholine constitute a significant percentage of the total nuclear phospholipid content. We feel that it is pertinent to include this lipid in our discussion as it provides an alternative source of 1, 2-diacylglycerol (DAG) in addition to the hydrolysis of PtdIns(4, 5)P2. We discuss at length data related to the sources and possible consequences of nuclear DAG production as this lipid appears to be increasingly central to a number of general physiological functions. Data relating to the existence of alternative pathways of inositol phospholipid synthesis, the role of 3-phosphorylated inositol lipids and lipid compartmentalisation and transport are reviewed. The field has also expanded to a point where we can now also begin to address what role these lipids play in cellular proliferation and differentiation and hopefully provide avenues for further research.

Stimulation of phospholipase D via alpha1-adrenergic receptors in Madin-Darby canine kidney cells is independent of PKCalpha and -epsilon activation

We have demonstrated previously that protein kinase Calpha (PKCalpha) plays a key role in regulating phospholipase D (PLD) activation by nucleotides and the phorbol ester phorbol-12-myristate-13-acetate in Madin-Darby canine kidney (MDCK-D1) cells. In the current work, we investigated PLD activation in MDCK-D1 cells triggered by the adrenergic receptor agonist epinephrine and its mechanism of activation. Epinephrine, acting through the alpha1-adrenergic receptor subtype, promoted transient translocation of PKCalpha and more prolonged translocation of PKCepsilon to the membrane fraction, indicating activation of these two isoforms. In addition, epinephrine promoted activation of PLD, as shown by a sustained accumulation of phosphatidylethanol. All of these events were blocked by pretreatment of cells with the alpha1-adrenergic antagonist prazosin. D609, an inhibitor of phosphatidylcholine hydrolysis, blocked translocation of PKCalpha and PKCepsilon but did not inhibit PLD activation. Unlike results with PMA, or with the P2 purinergic receptor agonist ATP, epinephrine-stimulated PLD activity was not inhibited in MDCK-D1 cells in which PKCalpha expression is attenuated by an antisense cDNA construct or in cells in which PKC activity was inhibited by 1 microM GF 109203X. However, PLD activation by epinephrine was abolished by concomitant incubation of cells with the calcium chelator EGTA. These data, together with previous results, are consistent with the hypothesis that in MDCK-D1 cells, epinephrine acting on alpha1-adrenergic receptors, promotes a rapid increase in cytosolic Ca2+ that promotes activation of PLD through an as-yet poorly defined mechanism. The data demonstrate that different types of G protein-linked receptors that activate PLD can mediate this activation in either a PKC activation-dependent or -independent manner within a single cell type.

Phosphatidylcholine-specific phospholipase C regulates thapsigargin-induced calcium influx in human lymphocytes

The involvement of phosphatidylcholine-specific phospholipase C (PC-PLC) and D (PC-PLD) in the regulation of the thapsigargin-induced Ca2+ increase was investigated. Pretreatment of human lymphocytes with the PC-PLC inhibitors D609 or U73122 enhanced the thapsigargin-induced Ca2+ influx. By contrast, no effect was observed in the presence of phospholipase D inhibitor butanol. Addition of exogenous PC-PLC but not PC-PLD to lymphocytes prestimulated with thapsigargin led to a decrease of intracellular Ca2+. In addition, thapsigargin was shown to release diacylglycerol (DAG) from cellular phosphatidylcholine pools. The thapsigargin-induced DAG formation was inhibited by U73122 and D609 but not by butanol. Moreover, no formation of the PC-PLD activity marker phosphatidylbutanol was detected. Thapsigargin-induced DAG formation was dependent on the Ca2+ entry, as it was abolished in the absence of extracellular Ca2+ or in the presence of Ni2+. Further investigations demonstrated that the inhibition of the cellular DAG target, protein kinase C (PKC), enhanced thapsigargin-induced Ca2+ increase, whereas direct PKC activation had an inhibitory effect. Taken together, our results reveal the involvement of PC-PLC in the regulation of the thapsigargin-induced Ca2+ increase and point to the existence of a physiologic feedback mechanism activated by Ca2+ influx and acting via consecutive activation of PC-PLC and PKC to limit the rise of intracellular Ca2+.

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

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

Activation of intestinal mitochondrial phospholipase D by polyamines and monoamines

Intestinal mitochondria have a phospholipase D (PLD) activity which was stimulated by polyamines and monoamines resulting in the formation of phosphatidic acid (PA) from endogenous phospholipids. When stimulated by polyamines, mitochondrial PLD utilized endogenous phosphatidylethanolamine (PE) as substrate whereas stimulated by monoamines, both PE and phosphatidylcholine (PC) were hydrolysed. Stimulation of PA formation by spermine was enhanced by the presence of calcium. Since polyamines are known to alter the calcium transport by mitochondria and PA is known to possess an ionophore effect, stimulation of PA formation in mitochondria by polyamines suggests that polyamine-induced alteration in calcium homeostasis might involve a PA related mechanism.

Selective stimulation of a cAMP-specific phosphodiesterase (PDE4A5) isoform by phosphatidic acid molecular species endogenously formed in rat thymocytes

We have previously reported that concanavalin A (ConA) stimulation of rat thymocytes induces an increase in the cellular phosphatidic acid mass as well as a change in its fatty acid composition. An increase in phosphodiesterase (PDE) activity, mostly due to cAMP-specific (PDE4) isoforms, has also been observed in thymocytes stimulated by ConA. Furthermore, phosphatidic acid was able to stimulate PDE4 activity in vitro. In the present study, cAMP levels have been shown to decrease upon ConA stimulation of thymocytes. Decreasing phosphatidic acid level using diacylglycerol kinase inhibitors induced a parallel decrease of the ConA-stimulated cAMP-specific PDE activity in these cells. Analyses of phosphatidic acid molecular species in cells stimulated for 5 min by ConA revealed a significant increase in 1-stearoyl-2-arachidonoyl-sn-glycerol-3-phosphate and a relative decrease in the other molecular species of phosphatidic acid, mainly species containing palmitate. On the other hand, phosphatidic acid extracted from ConA-stimulated cells activated more efficiently the recombinant PDE4A5 isoform in vitro, as compared to phosphatidic acid extracted from unstimulated cells. In addition, phosphatidic acid species containing unsaturated fatty acids were stimulatory, while those containing two saturated fatty acids had only a marginal effect on the enzyme activity. Taken together, these data suggest that the mitogenic stimulation of thymocytes is accompanied by the synthesis of peculiar phosphatidic acid molecular species able to activate a PDE4 isoform. This activation might be of physiological relevance since cAMP is a major negative effector of the mitogenic response.

Regulation of early endosome fusion by phospholipase D activity

In vitro or cell-free assays of homotypic fusion between early endosomes have provided several insights into the means by which an intracellular membrane fusion event can be regulated. In this report we show that homotypic fusion between early endosomes from baby-hamster kidney cells is partially blocked by 340 mM ethanol (2% v/v) and by 100 mM butan-1-ol, but not by the secondary alcohol butan-2-ol. We ascribe the effect of primary alcohols to their participation in a well-characterised transphosphatidylation reaction catalysed by phospholipase D activity, which results in the production of phasphatidylalcohol at the expense of phosphatidic acid. In accordance with this interpretation, we find that addition of exogenous phospholipase D results in stimulation of early endosome fusion.

Nuclear translocation of RhoA mediates the mitogen-induced activation of phospholipase D involved in nuclear envelope signal transduction

In this paper we demonstrate for the first time a mitogen-induced activation of a nuclear acting phosphatidylcholine-phospholipase D (PLD) which is mediated, at least in part, by the translocation of RhoA to the nucleus. Addition of alpha-thrombin to quiescent IIC9 cells results in an increase in PLD activity in IIC9 nuclei. This is indicated by an increase in the alpha-thrombin-induced production of nuclear phosphatidylethanol in quiescent cells incubated in the presence of ethanol as well as an increase in PLD activity in isolated nuclei. Consistent with our previous report (Wright, T. M., Willenberger, S., and Raben, D. M. (1992) Biochem. J. 285, 395-400), the presence of ethanol decreases the alpha-thrombin-induced production of phosphatidic acid without affecting the induced increase in nuclear diglyceride, indicating that the increase in nuclear PLD activity is responsible for the effect on phosphatidic acid, but not that on diglyceride. Our data further demonstrate that RhoA mediates the activation of nuclear PLD. RhoA translocates to the nucleus in response to alpha-thrombin. Additionally, PLD activity in nuclei isolated from alpha-thrombin-treated cells is reduced in a concentration-dependent fashion by incubation with RhoGDI and restored by the addition of prenylated RhoA in the presence of guanosine 5'-3-O-(thio)triphosphate. Western blot analysis indicates that this RhoGDI treatment results in the extraction of RhoA from the nuclear envelope. These data support a role for a RhoA-mediated activation of PLD in our recently described hypothesis, which proposes that a signal transduction cascade exists in the nuclear envelope and represents a novel signal transduction cascade that we have termed NEST (nuclear envelope signal transduction).

Identification of a novel Ca2+-dependent, phosphatidylethanolamine-hydrolyzing phospholipase D in yeast bearing a disruption in PLD1

We have previously reported the identification and partial characterization of a gene encoding a phospholipase D activity (PLD1) in the yeast, Saccharomyces cerevisiae. Here we report the existence of a second phospholipase D activity, designated PLD2, in yeast cells bearing disruption at the PLD1 locus. PLD2 is a Ca2+-dependent enzyme which preferentially utilizes phosphatidylethanolamine over phosphatidylcholine as a substrate. In contrast to PLD1, the activity of PLD2 is insensitive to phosphatidylinositol 4,5-bisphosphate, and the enzyme is incapable of catalyzing the transphosphatidylation reaction with short chain alcohols as acceptors. Subcellular fractionation shows that PLD2 localizes mainly to the cytosol, but could also be detected in the particulate fraction. Thus, the biochemical properties of PLD2 appear to be substantially different from those of PLD1. PLD2 activity is significantly and transiently elevated upon exit of wild type yeast cells from stationary phase, suggesting that it may play a role in the initiation of mitotic cell division in yeast. In view of the significantly different properties of PLD1 and PLD2, and because the yeast genome contains PLD1 as the sole member of the recently defined PLD gene family, it may be concluded that PLD2 is structurally unrelated to PLD1. Thus, the novel PLD2 activity described herein is likely to represent the first identified member of a new PLD gene family.

Regulation of eukaryotic phosphatidylinositol-specific phospholipase C and phospholipase D

This review focuses on two phospholipase activities involved in eukaryotic signal transduction. The action of the phosphatidylinositol-specific phospholipase C enzymes produces two well-characterized second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This discussion emphasizes recent advances in elucidation of the mechanisms of regulation and catalysis of the various isoforms of these enzymes. These are especially related to structural information now available for a phospholipase C delta isozyme. Phospholipase D hydrolyzes phospholipids to produce phosphatidic acid and the respective head group. A perspective of selected past studies is related to emerging molecular characterization of purified and cloned phospholipases D. Evidence for various stimulatory agents (two small G protein families, protein kinase C, and phosphoinositides) suggests complex regulatory mechanisms, and some studies suggest a role for this enzyme activity in intracellular membrane traffic.

Mechanisms of prostanoid synthesis in human synovial cells: cytokine-peptide synergism

Bradykinin (BK)2 and interleukin-1 (IL-1) interact synergistically to stimulate prostaglandin synthesis in human synovial fibroblast-like cells. The effect of BK is rapid and correlates with its capacity to elevate cytosolic levels of calcium ([Ca2+]i), while IL-1's effect is slow and s dependent upon de novo protein synthesis. The mechanism of this synergistic interaction was investigated. In the basal state, high levels of arachidonic acid (AA) were spontaneously released from synovial cells but near absent levels of cyclooxygenase activity prevented metabolism of AA to prostanoid. BK was a potent stimulus for elevating AA, but not prostaglandins, above basal levels. IL-1, in contrast, increased prostaglandins but not AA, above basal levels. IL-1 treatment was not associated with a loss or redistribution of AA among phospholipid classes. These results are consistent with high basal phospholipase activity in synovial cells and demonstrate the ability of BK, presumably via its ability to raise [Ca2+]i, to further elevate this activity(ies). Metabolism of AA to prostanoid is minimal in resting and BK-stimulated synovial cells, however, without the concomitant induction of cyclooxygenase activity by IL-1. These studies clarify the different, but synergistic, mechanisms of action of a peptide and cytokine in stimulating prostanoid synthesis in synovial cells. In addition, these data extend the results of previous investigations in demonstrating that basal phospholipase activity provides sufficient AA substrate for IL-1 induced prostanoid synthesis without invoking the concomitant induction of phospholipase activity by IL-1.

Role of PLA2, PLC, and PLD in bradykinin-induced release of arachidonic acid in MDCK cells

The role of cytosolic phospholipase A2 (cPLA2), phosphatidylcholine-specific phospholipase C (PC-PLC) and phospholipase D (PLD) in the bradykinin (BK)-stimulated release of arachidonic acid (AA) was examined in Madin-Darby canine kidney (MDCK) cells. Release of AA, phosphorylcholine, choline, and phosphatidic acid (PA) or the transphosphatidylation product, phosphatidylethanol, was detected after 1 min of BK stimulation. A role for PC-PLC was confirmed with D609, which reduced BK-stimulated AA by 70%. Ethanol (EtOH), which blunts PA formation, diminished BK-stimulated AA release by 50%. Together, D609 and EtOH inhibited this release almost completely. Evidence indicated that diacylglycerol and PA can enhance PLA2 activity when added to cytosol extracts. The enzyme responsible for AA release was characterized as cPLA2, since PLA2 activity assayed in cell extracts was largely inhibited by an antibody to this enzyme. The membrane fraction PLA2 activity increased significantly in BK-stimulated cells. We conclude that BK signaling in MDCK cells is mediated by the lipid products of PC-PLC and PLD, increasing cPLA2 activity, possibly by causing perturbations in the bilayer structure of its substrate, by a direct effect on the enzyme or by activation of protein kinases such as protein kinase C.

Two phosphatidylethanol classes separated by thin layer chromatography are produced by phospholipase D in rat brain hippocampal slices

Noradrenaline- and ionomycin-stimulated as well as basal phospholipase D activity from rat hippocampus produced, in the presence of ethanol, two different classes of [32P]phosphatidylethanol (designated I and II), which were separated by thin layer chromatography. Endogenous labeling experiments using 3H-fatty acids showed that two different classes of phosphatidylcholine, separated by two-dimensional TLC, one enriched with high incorporation of [3H]arachidonic acid (B) and the other with [3H]myristic acid (A), were the most likely sources for the two classes of phosphatidylethanol. Experiments where individual 32P-phospholipids extracted from [32P]Pi-labeled hippocampal slices were incubated with cabbage phospholipase D, in the presence of ethanol, showed that each class of [32P]phosphatidylcholine, i.e. A and B, produced a different band of [32P]phosphatidylethanol, with the same mobility in TLC as phosphatidylethanol II and I, respectively.

Phospholipase D: role in signal transduction and membrane traffic

The activation of phospholipase D (PLD) in response to cell stimulation by extracellular signal molecules is a widespread phenomenon. A variety of extracellular signal molecules cause a rapid and dramatic stimulation of PLD activity. G proteins and protein kinases appear to be involved in the receptor-mediated regulation of PLD. There is indirect evidence for the existence of multiple PLD subtypes, both membrane-associated and cytosolic. Recent studies indicate that PLD activities require a lipid cofactor, phosphatidylinositol 4,5-bisphosphate (PIP2). Addition of PIP2 at physiological concentrations stimulates both membrane-associated and partially purified PLD activity. Other acidic phospholipids have little or no effect. Neomycin, a high affinity ligand of PIP2, inhibits membrane PLD activity, presumably by binding to endogenous PIP2. A monoclonal antibody to phosphatidylinositol 4-kinase inhibits PIP2 synthesis in permeabilized U937 cells and blocks PLD activation by GTP gamma S and TPA. These results indicate that PIP2 synthesis is required for G protein- and protein kinase C-mediated activation of PLD in the cells. Recent evidence has implicated PLD and phosphoinositide kinases in vesicular trafficking. The main lipid mediator produced by PLD, phosphatidic acid, could regulate membrane traffic events by direct regulation of target proteins involved in vesicle targeting, docking and fusion. In addition, under certain circumstances, the formation of phosphatidic acid may lead to changes in lipid bilayer properties that would facilitate vesicle budding and fusion events in the course of intracellular membrane traffic.

The influence of acute ethanol ingestion on phospholipase D activity in rat pancreas. An in vitro and in vivo study

CONCLUSION

Since phosphatidic acid (PA), a product of phospholipase D(PLD), is known as a second messenger probably involved in cell proliferation and differentiation, our results potentially suggest a new mechanism for pancreatic tissue injury after ethanol ingestion.

BACKGROUND

The mechanisms by which ethanol causes pancreatic injury are still not clear. In vitro studies have suggested a relationship of PLD to ethanol metabolism. This study was undertaken to establish the involvement of PLD in ethanol metabolism in isolated pancreatic acini and to determine the influence of acute ethanol ingestion on PLD activity in pancreas and pancreatic growth after cerulein (Ce) infusion.

METHODS

Dispersed pancreatic acini prelabeled with 3H myristic acid were incubated with 500 pM Ce in the presence of different concentrations of ethanol; then labeled PA and phosphatidylethanol (PEt) production were measured under the same experimental conditions. For in vivo study, male rats were infused with Ce (0.25 microgram/kg/h) or saline; 1 h before infusion, animals were treated with 40% ethanol (5 g/kg p.o.) or saline, respectively. After 1, 2, and 48 h of Ce infusion, rats were killed; dispersed pancreatic acini were then prepared and PLD activity was measured. Pancreatic weight, protein, RNA, and DNA content were also established.

RESULTS

The production of PEt in vitro after Ce stimulation was significantly elevated with 1% ethanol in the medium. In the presence of different concentrations of ethanol (0.5-2%), a significant inhibition of PA accumulation in in vitro experiments was observed. The decrease of PA accumulation with ethanol was parallel to the increase of PEt production under the same experimental conditions. PLD activity was significantly elevated after 1 and 2 h of Ce infusion (116 and 105%, respectively), reaching control value after 48 h. Acute ethanol ingestion significantly diminished PLD activity after 1 and 2 h. After 48 h of Ce infusion, a significant increase in pancreatic weight, protein, RNA, and DNA content in pancreatic tissue was found. Ethanol was not able to influence pancreatic weight, proteins and RNA content. However, it had the potency to diminish DNA content after 48 h of Ce infusion.

Enzymology of mammalian phospholipases D: in vitro studies

The existence of multiple forms of phopholipase D was clearly established in a large number of biochemical studies that described and characterized the enzymological properties of the different PLD activities. This review summarizes the in vitro evidence showing differential subcellular localization and chromatographic properties of putative PLD isozymes, their phospholipid and alcohol substrate specificities, their modulation by various divalent cations, small G proteins and protein kinase c isozymes, and the role of phosphatidylinositol 4,5-bisphosphate as a cofactor of phospholipase D.

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.

Regulation of phospholipase D by protein kinase C

In nearly all mammalian cells and tissues examined, protein kinase C (PKC) has been shown to serve as a major regulator of a phosphatidylcholine-specific phospholipase D (PLD) activity. At least 12 distinct isoforms of PKC have been described so far; of these enzymes only the alpha- and beta-isoforms were found to regulate PLD activity. While the mechanism of this regulation has remained unknown, available evidence suggests that both phosphorylating and non-phosphorylating mechanisms may be involved. A phosphatidylcholine-specific PLD activity was recently purified from pig lung, but its possible regulation by PKC has not been reported yet. Several cell types and tissues appear to express additional forms of PLD which can hydrolyze either phosphatidylethanolamine or phosphatidylinositol. It has also been reported that at least one form of PLD can be activated by oncogenes, but not by PKC activators. Similar to activated PKC, some of the primary and secondary products of PLD-mediated phospholipid hydrolysis, including phosphatidic acid, 1,2-diacylglycerol, choline phosphate and ethanolamine, also exhibit mitogenic/co-mitogenic effects in cultured cells. Furthermore, both the PLD and PKC systems have been implicated in the regulation of vesicle transport and exocytosis. Recently the PLD enzyme has been cloned and the tools of molecular biology to study its biological roles will soon be available. Using specific inhibitors of growth regulating signals and vesicle transport, so far no convincing evidence has been reported to support the role of PLD in the mediation of any of the above cellular effects of activated PKC.

Biochemistry and cell biology of phospholipase D in human neutrophils

Neutrophils play a major role host defense against invading microbes. Recent studies have emphasized the importance of the phospholipase D (PLD) in the signalling cascade leading to neutrophil activation. Phospholipase D catalyzes the hydrolysis of phospholipids to generate phosphatidic acid with secondarily generation of diradylglycerol; both of these products have been implicated as second messengers. Herein, we discuss the regulation and the biochemistry of the receptor-regulated PLD in human neutrophils. In vivo and in vitro studies suggest an activation mode in which initial receptor-linked activation of phospholipase C generates diacylglycerol and inositol trisphosphate. The resulting calcium flux along with the diacylglycerol activate a conventional isoform of protein kinase C (PKC), probably PKC beta 1. This PKC, in turn phosphorylates a plasma membrane component resulting in PLD activation and a second outpouring of diradylglycerol. The small GTP-binding proteins, RhoA and ARF, also participate in this process, and synergize with a 50 kDa cytosolic regulatory factor.

Multiple forms of phospholipase D inhibitor from rat brain cytosol. Purification and characterization of heat-labile form

Rat brain cytosol contains proteins that markedly inhibit the activity of partially purified brain membrane phospholipase D (PLD) stimulated by ADP-ribosylation factor (Arf) and phosphatidylinositol 4,5-bisphosphate (PIP2). Sequential chromatography of the brain cytosol yielded four inhibitor fractions, which exhibited different kinetics to heat treatment at 70 degrees C. Purification of the most heat-labile inhibitor to homogeneity yielded two preparations, which displayed apparent molecular masses of 150 kDa and 135 kDa, respectively, on SDS-polyacrylamide gels. Tryptic digests of the 150- and 135-kDa proteins yielded similar elution profiles on a C18 reverse-phase column, suggesting that the 135-kDa form is a truncated form of the 150-kDa form. Sequences of two tryptic peptides were determined. A data base search revealed no proteins with these sequences. The purified 150-kDa inhibitor negated the PLD activity stimulated by Arf, RhoA, or Cdc42. The concentration required for half-maximal inhibition was 0.4 nM. Concentration dependence on the 150-kDa inhibitor was not affected by changes in the concentrations of Arf, PIP2, or phosphatidylcholine used in the assays, suggesting that the inhibition is not due to competition with the activators or substrate for PLD. The purified inhibitor did not affect the PIP2-hydrolyzing activity of a phospholipase C isozyme that was measured with substrate vesicles of lipid composition identical with that used for the PLD assay. Thus, the mechanism of inhibition appears to be a specific allosteric modification of PLD rather than disruption of substrate vesicles.

Electromagnetic field induced changes in lipid second messengers

Initial studies with a human hematopoietic cell line, TF-1, suggest multifarious effects of electromagnetic fields on lipid signal transduction. We have examined the effects of pulsed magnetic fields (2 T, 84 microseconds zero-to-peak haversine, 91 V/m induced electric field) on the cell cycle by flow cytometry. A 31% increase of cells in the G1 phase occurred concurrently with a 35% decrease of cells in S-phase, which suggests that doses of 30 or 40 pulses have an anti-proliferative effect. Changes in the lipid second messengers, diacylglycerol (DAG) and phosphatidic acid (PA) with stimuli of 2 T intensity were also dependent on the number of pulses. DAG production doubled with 30 pulses and tripled with 40 pulses, and PA levels were reduced to one third and one tenth of the original levels. Phospholipase D (PLD) up-regulation was assessed directly by the capacity of PLD to catalyze transphosphatidylation in the presence of alcohol. [3H]Phosphatidylethanol formed rapidly and continued to increase with concomitant decreases in [3H]PA and parallel generation of [3H]DAG. Propranolol, an inhibitor of PA phosphohydrolase, inhibited the formation of DAG in a dose-dependent manner with a marked increase in PA production. Examination of the kinetics of formation of [3H]choline and [3H]phosphocholine at different times after stimulation showed a rapid and consistent increase in [3H]choline, whereas [3H]phosphocholine increase was evident only 60 min after stimulation. Magnetic exposure also caused a shift in some molecular species patterns of DAG and PA which could be correlated with phosphatidylinositol, phosphatidylethanolamine and phosphatidylcholine molecular species decreases. Therefore, we propose that the PC-PLC pathway may be temporarily inactivated for a short period of time by exposure to pulsed stimuli, and the PC-PLD pathway is up-regulated based on: (1) cellular release of [3H]choline; (2) rapid intracellular formation of [3H]PA followed by [3H]DAG; (3)active transphosphatidylation; and (4) blockade of DAG formation by propranolol.

Raf-1 kinase possesses distinct binding domains for phosphatidylserine and phosphatidic acid. Phosphatidic acid regulates the translocation of Raf-1 in 12-O-tetradecanoylphorbol-13-acetate-stimulated Madin-Darby canine kidney cells

Previous studies demonstrated that the cysteine-rich amino-terminal domain of Raf-1 kinase interacts selectively with phosphatidylserine (Ghosh, S., Xie, W. Q., Quest, A. F. G., Mabrouk, G. M., Strum, J. C., and Bell, R. M. (1994) J. Biol. Chem. 269, 10000-10007). Further analysis showed that full-length Raf-1 bound to both phosphatidylserine and phosphatidic acid (PA). Specifically, a carboxyl-terminal domain of Raf-1 kinase (RafC; residues 295 648 of human Raf-1) interacted strongly with phosphatidic acid. The binding of RafC to PA displayed positive cooperativity with Hill numbers between 3.3 and 6.2; the apparent Kd ranged from 4.9 +/- 0.6 to 7.8 +/- 0.9 mol % PA. The interaction of RafC with PA displayed a pH dependence distinct from the interaction between the cysteine-rich domain of Raf-1 and PA. Also, the RafC-PA interaction was unaffected at high ionic strength. Of all the lipids tested, only PA and cardiolipin exhibited high affinity binding; other acidic lipids were either ineffective or weakly effective. By deletion mutagenesis, the PA binding site within RafC was narrowed down to a 35-amino acid segment between residues 389 and 423. RafC did not bind phosphatidyl alcohols; also, inhibition of PA formation in Madin-Darby canine kidney cells by treatment with 1% ethanol significantly reduced the translocation of Raf-1 from the cytosol to the membrane following stimulation with 12-O-tetradecanoylphorbol-13-acetate. These results suggest a potential role of the lipid second messenger, PA, in the regulation of translocation and subsequent activation of Raf-1 in vivo.

The antitumor phospholipid analog, hexadecylphosphocholine, activates cellular phospholipase D

Hexadecylphosphocholine (HePC), a glycerol-free phospholipid analog, belongs to a new class of drugs that demonstrate selective anticancer activity. The mechanisms underlying the anticancer activity are unclear. To investigate possible signal transduction relationships we examined the influence of HePC on cellular phospholipid metabolism. When HePC was added to cultured human breast fibroblasts (CCD-986-SK cells) that had been radiolabeled with fatty acid, phosphatidylethanol (PEt, the transphosphatidylation product of phospholipase D (PLD)) formation was stimulated as early as 5 min after addition. In cells labeled with [3H]choline, HePC treatment caused release of choline-containing metabolites to the culture medium, concurrent with PEt formation. HePC also elicited formation of diacylglycerol (DG) which, after 30 min increased 3.5-fold over control. As little is known regarding HePC and PLD, attention was directed towards studies on PC metabolism by PLD. PEt formation was shown to be optimal at 20-50 microM HePC, and structure-activity studies showed HePC to be more potent than either lyso-phosphatidylcholine or 1-hexadecyl-2-O-methyl-rac-glycero-3-phosphocholine for PLD activation. PLD activity induced by HePC was totally inhibited by cellular pretreatment with phorbol dibutyrate, and 59% diminished by pretreatment of cells with staurosporine, a protein kinase C (PKC) inhibitor. Our results demonstrate for the first time that HePC activates PLD, and suggest that PKC participates in this response. The relationship of PLD to the anticancer properties of HePC may be clinically relevant to drug actions.

Nuclear phospholipase D in Madin-Darby canine kidney cells. Guanosine 5'-O-(thiotriphosphate)-stimulated activation is mediated by RhoA and is downstream of protein kinase C

We have recently demonstrated the existence of an ATP-activated phospholipase D (PLD) in the nuclei of MDCK-D1 cells (Balboa, M. A., Balsinde, J., Dennis, E. A., and Insel, P. A. (1995) J. Biol. Chem. 270, 11738-11740). We have now found that nuclear PLD is synergistically activated by guanosine 5'-O-(thiotriphosphate) (GTP gamma S) and ATP in a time- and concentration-dependent manner, but these compounds do not alter the sensitivity of the enzyme to activation by Ca2+. The synergistic stimulation of PLD activity could be blocked by addition of the protein kinase C inhibitors chelerythrine and calphostin C. Stimulation by GTP gamma S was abolished by guanosine 5'-O-(2-thiodiphosphate). Incubation of isolated nuclei with Clostridium botulinum C3 exoenzyme inhibited the potentiating effect of GTP gamma S on ATP-dependent nuclear PLD activity. Moreover, use of the Rho GDP dissociation inhibitor to extract Rho family G proteins from cell nuclei also inhibits PLD activity. Western blot analyses of isolated nuclei revealed the presence of the small G protein RhoA, but not of RhoB or the ADP-ribosylation factor. GTP gamma S-stimulated ATP-dependent PLD activity could be reconstituted in Rho GDP dissociation inhibitor-washed nuclei by addition of recombinant prenylated RhoA, but not by addition of non-prenylated RhoA. Taken together, these results indicate that nuclear PLD activity is modulated via a RhoA-dependent activation that occurs downstream of protein kinase C. Nuclear PLD, which appears to be a previously unrecognized effector regulated by protein kinase C and G proteins, may be involved in the regulation of nuclear function or structure.