The simultaneous production of phosphatidic acid and diacylglycerol is essential for the translocation of protein kinase Cepsilon to the plasma membrane in RBL-2H3 cells

Saturated fatty acid- and/or monounsaturated fatty acid-containing phosphatidic acids selectively interact with heat shock protein 27

Diacylglycerol kinase (DGK) α, which is a key enzyme in the progression of cancer and, in contrast, in T-cell activity attenuation, preferentially produces saturated fatty acid (SFA)- and/or monounsaturated fatty acid (MUFA)-containing phosphatidic acids (PAs), such as 16:0/16:0-, 16:0/18:0-, and 16:1/16:1-PA, in melanoma cells. In the present study, we searched for the target proteins of 16:0/16:0-PA in melanoma cells and identified heat shock protein (HSP) 27, which acts as a molecular chaperone and contributes to cancer progression. HSP27 more strongly interacted with PA than other phospholipids, including phosphatidylcholine, phosphatidylserine, phosphatidylglycerol, cardiolipin, phosphatidylinositol, phosphatidylinositol 4-monophosphate, and phosphatidylinositol 4,5-bisphosphate. Moreover, HSP27 is more preferentially bound to SFA- and/or MUFA-containing PAs, including 16:0/16:0- and 16:0/18:1-PAs, than PUFA-containing PAs, including 18:0/20:4- and 18:0/22:6-PA. Furthermore, HSP27 and constitutively active DGKα expressed in COS-7 cells colocalized in a DGK activity-dependent manner. Notably, 16:0/16:0-PA, but not phosphatidylcholine or 16:0/16:0-phosphatidylserine, induced oligomer dissociation of HSP27, which enhances its chaperone activity. Intriguingly, HSP27 protein was barely detectable in Jurkat T cells, while the protein band was intensely detected in AKI melanoma cells. Taken together, these results strongly suggest that SFA- and/or MUFA-containing PAs produced by DGKα selectively target HSP27 and regulate its cancer-progressive function in melanoma cells but not in T cells.

PKCε controls the fusion of secretory vesicles in mast cells in a phosphatidic acid-dependent mode

PKCε is highly expressed in mast cells and plays a fundamental role in the antigen-triggered activation of the allergic reaction. Although its regulation by diacylglycerols has been described, its regulation by acidic phospholipids and how this regulation leads to the control of downstream vesicle secretion is barely known. Here, we used structural and evolutionary studies to find the molecular mechanism that explains the selectivity of the C1B domain of PKCε by Phosphatidic Acid (PA). This resided in a collection of Arg residues that form a specific rim on the outer surface of the C1B domain, around the diacylglycerol binding cleft. In RBL-2H3 cells, this basic rim allowed the kinase to respond specifically to phosphatidic acid signals that induced its translocation to the plasma membrane and subsequent activation. Further experiments in cells that overexpress PKCε and a mutant of the PA binding site, showed that PA-dependent PKCε activation increased vesicle degranulation in RBL-2H3 cells, and this correlated with increased SNAP23 phosphorylation. Over-expression of PKCε in these cells also induced an increase in the number of docked vesicles containing SNAP23, when stimulated with PA. This accumulation could be attributed to the stabilizing effect of phosphorylation on the formation of the SNARE complex, which ultimately led to increased release of content in the presence of Ca²⁺ during the fusion process. Therefore, these findings reinforce the importance of PA signaling in the activation of PKCε, which could be an important target to inhibit the exacerbated responses of these cells in the allergic reaction.

The binding of different model membranes with PKCε C2 domain is not dependent on membrane curvature but affects the sequence of events during unfolding

The C2 domain of novel protein kinases C (nPKC) binds to membranes in a Ca²⁺-independent way contributing to the activation of these enzymes. We have studied the C2 domain of one of these nPKCs, namely PKCε, and confirmed that it establishes a strong interaction with POPA, which is clearly visible through changes in chemical shifts detected through ³¹P-MAS-NMR and the protection that it exerts on the domain against thermal denaturation seen through DSC and FT-IR. In this study, using two-dimensional correlation analysis (2D-COS) applied to infrared spectra, we determined the sequence of events that occur during the thermal unfolding of the domain and highlighted some differences when phosphatidic acid or cardiolipin are present. Finally, by means of FRET and DLS experiments, we wanted to determine the effect of membrane curvature on the domain/membrane interaction by using lysophosphatidylcholine to introduce positive curvature as a control and we observed that the effect of these phospholipids on the protein binding is not exerted through the change of membrane curvature.

New Era of Diacylglycerol Kinase, Phosphatidic Acid and Phosphatidic Acid-Binding Protein

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol (DG) to generate phosphatidic acid (PA). Mammalian DGK consists of ten isozymes (α–κ) and governs a wide range of physiological and pathological events, including immune responses, neuronal networking, bipolar disorder, obsessive-compulsive disorder, fragile X syndrome, cancer, and type 2 diabetes. DG and PA comprise diverse molecular species that have different acyl chains at the sn-1 and sn-2 positions. Because the DGK activity is essential for phosphatidylinositol turnover, which exclusively produces 1-stearoyl-2-arachidonoyl-DG, it has been generally thought that all DGK isozymes utilize the DG species derived from the turnover. However, it was recently revealed that DGK isozymes, except for DGKε, phosphorylate diverse DG species, which are not derived from phosphatidylinositol turnover. In addition, various PA-binding proteins (PABPs), which have different selectivities for PA species, were recently found. These results suggest that DGK–PA–PABP axes can potentially construct a large and complex signaling network and play physiologically and pathologically important roles in addition to DGK-dependent attenuation of DG–DG-binding protein axes. For example, 1-stearoyl-2-docosahexaenoyl-PA produced by DGKδ interacts with and activates Praja-1, the E3 ubiquitin ligase acting on the serotonin transporter, which is a target of drugs for obsessive-compulsive and major depressive disorders, in the brain. This article reviews recent research progress on PA species produced by DGK isozymes, the selective binding of PABPs to PA species and a phosphatidylinositol turnover-independent DG supply pathway.

Substrate channeling in the glycerol-3-phosphate pathway regulates the synthesis, storage and secretion of glycerolipids

The successive acylation of glycerol-3-phosphate (G3P) by glycerol-3-phosphate acyltransferases and acylglycerol-3-phosphate acyltransferases produces phosphatidic acid (PA), a precursor for CDP-diacylglycerol-dependent phospholipid synthesis. PA is further dephosphorylated by LIPINs to produce diacylglycerol (DG), a substrate for the synthesis of triglyceride (TG) by DG acyltransferases and a precursor for phospholipid synthesis via the CDP-choline and CDP-ethanolamine (Kennedy) pathways. The channeling of fatty acids into TG for storage in lipid droplets and secretion in lipoproteins or phospholipids for membrane biogenesis is dependent on isoform expression, activity and localization of G3P pathway enzymes, as well as dietary and hormonal and tissue-specific factors. Here, we review the mechanisms that control partitioning of substrates into lipid products of the G3P pathway.

Phosphatidic Acid: From Pleiotropic Functions to Neuronal Pathology

Among the cellular lipids, phosphatidic acid (PA) is a peculiar one as it is at the same time a key building block of phospholipid synthesis and a major lipid second messenger conveying signaling information. The latter is thought to largely occur through the ability of PA to recruit and/or activate specific proteins in restricted compartments and within those only at defined submembrane areas. Furthermore, with its cone-shaped geometry PA locally changes membrane topology and may thus be a key player in membrane trafficking events, especially in membrane fusion and fission steps, where lipid remodeling is believed to be crucial. These pleiotropic cellular functions of PA, including phospholipid synthesis and homeostasis together with important signaling activity, imply that perturbations of PA metabolism could lead to serious pathological conditions. In this mini-review article, after outlining the main cellular functions of PA, we highlight the different neurological diseases that could, at least in part, be attributed to an alteration in PA synthesis and/or catabolism.

Ethanol-Induced Changes in PKCε: From Cell to Behavior

The long-term binge intake of ethanol causes neuroadaptive changes that lead to drinkers requiring higher amounts of ethanol to experience its effects. This neuroadaptation can be partly attributed to the modulation of numerous neurotransmitter receptors by the various protein kinases C (PKCs). PKCs are enzymes that control cellular activities by regulating other proteins via phosphorylation. Among the various isoforms of PKC, PKCε is the most implicated in ethanol-induced biochemical and behavioral changes. Ethanol exposure causes changes to PKCε expression and localization in various brain regions that mediate addiction-favoring plasticity. Ethanol works in conjunction with numerous upstream kinases and second messenger activators to affect cellular PKCε expression. Chauffeur proteins, such as receptors for activated C kinase (RACKs), cause the translocation of PKCε to aberrant sites and mediate ethanol-induced changes. In this article, we aim to review the following: the general structure and function of PKCε, ethanol-induced changes in PKCε expression, the regulation of ethanol-induced PKCε activities in DAG-dependent and DAG-independent environments, the mechanisms underlying PKCε-RACKε translocation in the presence of ethanol, and the existing literature on the role of PKCε in ethanol-induced neurobehavioral changes, with the goal of creating a working model upon which further research can build.

The Multirole of Liposomes in Therapy and Prevention of Infectious Diseases

Liposomes are closed bilayer structures spontaneously formed by hydrated phospholipids that are widely used as efficient delivery systems for drugs or antigens, due to their capability to encapsulate bioactive hydrophilic, amphipathic, and lipophilic molecules into inner water phase or within lipid leaflets. The efficacy of liposomes as drug or antigen carriers has been improved in the last years to ameliorate pharmacokinetics and capacity to release their cargo in selected target organs or cells. Moreover, different formulations and variations in liposome composition have been often proposed to include immunostimulatory molecules, ligands for specific receptors, or stimuli responsive compounds. Intriguingly, independent research has unveiled the capacity of several phospholipids to play critical roles as intracellular messengers in modulating both innate and adaptive immune responses through various mechanisms, including (i) activation of different antimicrobial enzymatic pathways, (ii) driving the fusion–fission events between endosomes with direct consequences to phagosome maturation and/or to antigen presentation pathway, and (iii) modulation of the inflammatory response. These features can be exploited by including selected bioactive phospholipids in the bilayer scaffold of liposomes. This would represent an important step forward since drug or antigen carrying liposomes could be engineered to simultaneously activate different signal transduction pathways and target specific cells or tissues to induce antigen-specific T and/or B cell response. This lipid-based host-directed strategy can provide a focused antimicrobial innate and adaptive immune response against specific pathogens and offer a novel prophylactic or therapeutic option against chronic, recurrent, or drug-resistant infections.

Identification and characterization of the phosphatidic acid-binding A. thaliana phosphoprotein PLDrp1 that is regulated by PLDα1 in a stress-dependent manner

Phospholipase D (PLD) and its cleavage product phosphatidic acid (PA) are crucial in plant stress-signalling. Although some targets of PLD and PA have been identified, the signalling pathway is still enigmatic. This study demonstrates that the phosphoprotein At5g39570, now called PLD-regulated protein1 (PLDrp1), from Arabidopsis thaliana is directly regulated by PLDα1. The protein PLDrp1 can be divided into two regions with distinct properties. The conserved N-terminal region specifically binds PA, while the repeat-rich C-terminal domain suggests interactions with RNAs. The expression of PLDrp1 depends on PLDα1 and the plant water status. Water stress triggers a pldα1-like phenotype in PLDrp1 mutants and induces the expression of PLDrp1 in pldα1 mutants. The regulation of PLDrp1 by PLDα1 and environmental stressors contributes to the understanding of the complex PLD regulatory network and presents a new member of the PA-signalling chain in plants.

The tumor promoter-activated protein kinase Cs are a system for regulating filopodia

Different protein kinase C (PKC) isoforms have distinct roles in regulating cell functions. The conventional (α, β, γ) and novel (δ, ɛ, η, θ) classes are targets of phorbol ester tumor promoters, which are surrogates of endogenous second messenger, diacylglycerol. The promoter-stimulated disappearance of filopodia was investigated by use of blocking peptides (BPs) that inhibit PKC maturation and/or docking. Filopodia were partially rescued by a peptide representing PKC ɛ hydrophobic sequence, but also by a myristoylated PKC α/β pseudosubstrate sequence, and an inhibitor of T-cell protein tyrosine phosphatase (TC-PTP). The ability to turn over filopodia was widely distributed among PKC isoforms. PKC α and η hydrophobic sequences enhanced filopodia in cells in the absence of tumor promoter treatment. With transcriptional knockdown of PKC α, the content of PKC ɛ predominated over other isoforms. PKC ɛ could decrease filopodia significantly in promoter-treated cells, and this was attributed to ruffling. The presence of PKC α counteracted the PKC ɛ-mediated enhancement of ruffling. The results showed that there were two mechanisms of filopodia downregulation. One operated in the steady-state and relied on PKC α and η. The other was stimulated by tumor promoters and relied on PKC ɛ. Cycles of protrusion and retraction are characteristic of filopodia and are essential for the cell to orient itself during chemotaxis and haptotaxis. By suppressing filopodia, PKC ɛ can create a long-term "memory" of an environmental signal that may act in nature as a mnemonic device to mark the direction of a repulsive signal.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

Phospholipase D2 is involved in the formation of Golgi tubules and ArfGAP1 recruitment

Lipids and lipid-modifying enzymes play a key role in the biogenesis, maintenance and fission of transport carriers in the secretory and endocytic pathways. In the present study we demonstrate that phosphatidic acid generated by phospholipase D2 (PLD2) is involved in the formation of Golgi tubules. The main evidence to support this is: 1) inhibitors of phosphatidic acid formation and PLD2 depletion inhibit the formation of tubules containing resident enzymes and regulators of intra-Golgi transport in a low temperature (15°C) model of Golgi tubulation but do not affect brefeldin A-induced tubules, 2) inhibition of PLD2 enzymatic activity and PLD2 depletion in cells cultured under physiological conditions (37°C) induce the formation of tubules specifically containing Golgi matrix proteins, and, 3) over-expression of PLD2 induces the formation of a tubular network. In addition, it was found that the generation of this lipid by the isoenzyme is necessary for ArfGAP1 recruitment to Golgi membranes. These results suggest that both proteins are involved in the molecular mechanisms which drive the formation of different types of Golgi tubules.

Signaling through C2 domains: more than one lipid target

C2 domains are membrane-binding modules that share a common overall fold: a single compact Greek-key motif organized as an eight-stranded anti-parallel β-sandwich consisting of a pair of four-stranded β-sheets. A myriad of studies have demonstrated that in spite of sharing the common structural β-sandwich core, slight variations in the residues located in the interconnecting loops confer C2 domains with functional abilities to respond to different Ca(2+) concentrations and lipids, and to signal through protein-protein interactions as well. This review summarizes the main structural and functional findings on Ca(2+) and lipid interactions by C2 domains, including the discovery of the phosphoinositide-binding site located in the β3-β4 strands. The wide variety of functions, together with the different Ca(2+) and lipid affinities of these domains, converts this superfamily into a crucial player in many functions in the cell and more to be discovered. This Article is Part of a Special Issue Entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Phosphatidic acid-mediated signaling

Phosphatidic acid (PA) is recognized as an important class of lipid messengers. The cellular PA levels are dynamic; PA is produced and metabolized by several enzymatic reactions, including different phospholipases, lipid kinases, and phosphatases. PA interacts with various proteins and the interactions may modulate enzyme catalytic activities and/or tether proteins to membranes. The PA-protein interactions are impacted by changes in cellular pH and other effectors, such as cations. PA is involved in a wide range of cellular processes, including vesicular trafficking, cytoskeletal organization, secretion, cell proliferation, and survival. Manipulations of different PA production reactions alter cellular and organismal response to a wide range of abiotic and biotic stresses. Further investigations of PA's function and mechanisms of action will advance not only the understanding of cell signaling networks but also may lead to biotechnological and pharmacological applications.

Diacylglycerol kinase θ couples farnesoid X receptor-dependent bile acid signalling to Akt activation and glucose homoeostasis in hepatocytes

DGKs (diacylglycerol kinases) catalyse the conversion of diacylglycerol into PA (phosphatidic acid), a positive modulator of mTOR (mammalian target of rapamycin). We have found that chenodeoxycholic acid and the synthetic FXR (farnesoid X receptor) ligand GW4064 induce the mRNA and protein expression of DGKθ in the HepG2 cell line and in primary human hepatocytes. Reporter gene studies using 1.5 kB of the DGKθ promoter fused to the luciferase gene revealed that bile acids increase DGKθ transcriptional activity. Mutation of putative FXR-binding sites attenuated the ability of GW4046 to increase DGKθ luciferase activity. Consistent with this finding, ChIP (chromatin immunoprecipitation) assays demonstrated that bile acid signalling increased the recruitment of FXR to the DGKθ promoter. Furthermore, GW4064 evoked a time-dependent increase in the cellular concentration of PA. We also found that GW4064 and PA promote the phosphorylation of mTOR, Akt and FoxO1 (forkhead box O1), and that silencing DGKθ expression significantly abrogated the ability of GW4046 to promote the phosphorylation of these PA-regulated targets. DGKθ was also required for bile-acid-dependent decreased glucose production. Taken together, our results establish DGKθ as a key mediator of bile-acid-stimulated modulation of mTORC2 (mTOR complex 2), the Akt pathway and glucose homoeostasis.

PKC-ε pseudosubstrate and catalytic activity are necessary for membrane delivery during IgG-mediated phagocytosis

In RAW 264.7 cells, PKC-ε regulates FcγR-mediated phagocytosis. BMDM behave similarly; PKC-ε concentrates at phagosomes and internalization are reduced in PKC-ε⁻/⁻ cells. Two questions were asked: what is the role of PKC-ε? and what domains are necessary for PKC-ε concentration? Function was studied using BMDM and frustrated phagocytosis. On IgG surfaces, PKC-ε⁻/⁻ macrophages spread less than WT. Patch-clamping revealed that the spreading defect is a result of the failure of PKC-ε⁻/⁻ macrophages to add membrane. The defect is specific for FcγR ligation and can be reversed by expression of full-length (but not the isolated RD) PKC-ε in PKC-ε⁻/⁻ BMDM. Thus, PKC-ε function in phagocytosis requires translocation to phagosomes and the catalytic domain. The expression of chimeric PKC molecules in RAW cells identified the εPS as necessary for PKC-ε targeting. When placed into (nonlocalizing) PKC-δ, εPS was sufficient for concentration, albeit to a lesser degree than intact PKC-ε. In contrast, translocation of δ(εPSC1B) resembled that of WT PKC-ε. Thus, εPS and εC1B cooperate for optimal phagosome targeting. Finally, cells expressing εK437W were significantly less phagocytic than their PKC-ε-expressing counterparts, blocked at the pseudopod-extension phase. In summary, we have shown that εPS and εC1B are necessary and sufficient for targeting PKC-ε to phagosomes, where its catalytic activity is required for membrane delivery and pseudopod extension.

A GEF-to-phospholipase molecular switch caused by phosphatidic acid, Rac and JAK tyrosine kinase that explains leukocyte cell migration

Phospholipase D2 (PLD2) is a cell-signaling molecule that bears two activities: a guanine-nucleotide exchange factor (GEF) and a lipase that reside in the PX/PH domains and in two HKD domains, respectively. Upon cell stimulation, the GEF activity yields Rac2-GTP and the lipase activity yields phosphatidic acid (PA). In the present study, we show for the first time that these activities regulate one another. Upon cell stimulation, both GEF and lipase activities are quickly (within ∼3 min) elevated. As soon as it is produced, PA positively feeds back on the GEF and further activates it. Rac2-GTP, on the other hand, is inhibitory to the lipase activity. PLD2 would remain downregulated if it were not for the contribution of the tyrosine kinase Janus kinase 3 (JAK3), which restores lipase action (by phosphorylation at Y415). Conversely, the GEF is inhibited upon phosphorylation by JAK3 and is effectively terminated by this action and by the increasing accumulation of PA at >15 min of cell stimulation. This PA interferes with the ability of the GEF to bind to its substrate (Rac2-GTP). Thus, both temporal inter-regulation and phosphorylation-dependent mechanisms are involved in determining a GEF-lipase switch within the same molecule. Human neutrophils stimulated by interleukin-8 follow a biphasic pattern of GEF and lipase activation that can be explained by such an intramolecular switch. This is the first report of a temporal inter-regulation of two enzymatic activities that reside in the same molecule with profound biological consequences in leukocyte cell migration.

Autophosphorylation of the C2 domain inhibits translocation of the novel protein kinase C (nPKC) Apl II

Protein kinase Cs (PKCs) are critical signaling molecules controlled by complex regulatory pathways. Herein, we describe an important regulatory role for C2 domain phosphorylation. Novel PKCs (nPKCs) contain an N-terminal C2 domain that cannot bind to calcium. Previously, we described an autophosphorylation site in the Aplysia novel PKC Apl II that increased the binding of the C2 domain to lipids. In this study, we show that the function of this phosphorylation is to inhibit PKC translocation. Indeed, a phosphomimetic serine-glutamic acid mutation reduced translocation of PKC Apl II while blocking phosphorylation with a serine-alanine mutation enhanced translocation and led to the persistence of the kinase at the membrane longer after the end of the stimulation. Consistent with a role for autophosphorylation in regulating kinase translocation, inhibiting PKC activity using bisindolymaleimide 1 increased physiological translocation of PKC Apl II, whereas inhibiting phosphatase activity using calyculin A inhibited physiological translocation of PKC Apl II in neurons. Our results suggest a major role for autophosphorylation-dependent regulation of translocation.

The role of C2 domains in PKC signaling

More than two decades ago, the discovery of the first C2 domain in conventional Protein Kinase Cs (cPKCs) and of its role as a calcium-binding motif began to shed light on the activation mechanism of this family of Serine/Threonine kinases which are involved in several critical signal transduction pathways. In this chapter, we review the current knowledge of the structure and the function of the different C2 domains in PKCs. The C2 domain of cPKCs is a calcium sensor and its calcium-dependent binding to phospholipids is crucial for kinase activation. While the functional role of the cPKC C2 domain is better understood, phylogenetic analysis revealed that the novel C2 domain is more ancient and related to the C2 domain in the fungal PKC family, while the cPKC C2 domain is first associated with PKC in metazoans. The C2 domain of novel PKCs (nPKCs) does not contain a calcium-binding motif but still plays a critical role in nPKCs activation by regulating C1-C2 domain interactions and consequently C2 domain-mediated inhibition in both the nPKCs of the epsilon family and the nPKCs of the delta family. Moreover, the C2 domain of the nPKCs of the delta family was shown to recognize phosphotyrosines in a novel mode different from the ones observed for the Src Homology 2 (SH2) and the phosphotyrosine binding domains (PTB). By binding to phosphotyrosines, the C2 domain regulates the activation of this subclass of PKCs. The C2 domain was also shown to be involved in protein-protein interactions and binding to the receptor for activated C-kinase (RACKs) thus contributing to the subcellular localization of PKCs. In summary, the C2 domain is a critical player that can sense the activated signaling pathway in response to external stimuli to specifically regulate the different conventional and novel PKC isoforms.

Protein kinase Cθ C2 domain is a phosphotyrosine binding module that plays a key role in its activation

Protein kinase Cθ (PKCθ) is a novel PKC that plays a key role in T lymphocyte activation. To understand how PKCθ is regulated in T cells, we investigated the properties of its N-terminal C2 domain that functions as an autoinhibitory domain. Our measurements show that a Tyr(P)-containing peptide derived from CDCP1 binds the C2 domain of PKCθ with high affinity and activates the enzyme activity of the intact protein. The Tyr(P) peptide also binds the C2 domain of PKCδ tightly, but no enzyme activation was observed with PKCδ. Mutations of PKCθ-C2 residues involved in Tyr(P) binding abrogated the enzyme activation and association of PKCθ with Tyr-phosphorylated full-length CDCP1 and severely inhibited the T cell receptor/CD28-mediated activation of a PKCθ-dependent reporter gene in T cells. Collectively, these studies establish the C2 domain of PKCθ as a Tyr(P)-binding domain and suggest that the domain may play a major role in PKCθ activation via its Tyr(P) binding.

A novel class of PTEN protein in Arabidopsis displays unusual phosphoinositide phosphatase activity and efficiently binds phosphatidic acid

PTEN (phosphatase and tensin homologue deleted on chromosome ten) proteins are dual phosphatases with both protein and phosphoinositide phosphatase activity. They modulate signalling pathways controlling growth, metabolism and apoptosis in animals and are implied in several human diseases. In the present paper we describe a novel class of PTEN pro-teins in plants, termed PTEN2, which comprises the AtPTEN (Arabidopsis PTEN) 2a and AtPTEN2b proteins in Arabidopsis. Both display low in vitro tyrosine phosphatase activity. In addition, AtPTEN2a actively dephosphorylates in vitro the 3' phosphate group of PI3P (phosphatidylinositol 3-phosphate), PI(3,4)P2 (phosphatidylinositol 3,4-bisphosphate) and PI(3,5)P2 (phosphatidylinositol 3,5-bisphosphate). In contrast with animal PTENs, PI(3,4,5)P3 (phosphatidylinositol 3,4,5-trisphosphate) is a poor substrate. Site-directed mutagenesis of AtPTEN2a and molecular modelling of protein-phosphoinositide interactions indicated that substitutions at the PTEN2 core catalytic site of the Lys267 and Gly268 residues found in animals, which are critical for animal PTEN activity, by Met267 and Ala268 found in the eudicot PTEN2 are responsible for changes in substrate specificity. Remarkably, the AtPTEN2a protein also displays strong binding activity for PA (phosphatidic acid), a major lipid second messenger in plants. Promoter::GUS (β-glucuronidase) fusion, transcript and protein analyses further showed the transcriptional regulation of the ubiquitously expressed AtPTEN2a and AtPTEN2b by salt and osmotic stress. The results of the present study suggest a function for this novel class of plant PTEN proteins as an effector of lipid signalling in plants.

Genetically encoded probes for phosphatidic acid

In addition to forming bilayers to separate cellular compartments, lipids participate in vesicular trafficking and signal transduction. Among others, phosphatidic acid (PA) is emerging as an important signaling molecule. The spatiotemporal distribution of cellular PA appears to be tightly regulated by localized synthesis and a rapid metabolism. Although PA has been long proposed as a pleiotropic bioactive lipid, when and where PA is produced in the living cells have only recently been explored using biosensors that specifically bind to PA. The probes that we have generated are composed of the PA-binding domains of either Spo20p or Raf1 directly fused to GFP. In this chapter, we will describe the expression and purification of GST-fusion proteins of these probes, and the use of phospholipid strips to validate the specificity of their interaction with PA. We will then illustrate the use of GFP-tagged probes to visualize the synthesis of PA in the neurosecretory PC12 cells and RAW 267.4 macrophages. Interestingly, the two probes show a differential distribution in these cell types, indicating that they may have different affinities for PA or recognize different pools of PA. In conclusion, the development of a broader choice of probes may be required to adequately follow the complex dynamics of PA in different cell types, in order to determine the cellular distribution of PA and its role in various cellular processes.

Understanding of the roles of phospholipase D and phosphatidic acid through their binding partners

Phospholipase D (PLD) is a phosphatidyl choline (PC)-hydrolyzing enzyme that generates phosphatidic acid (PA), a lipid second messenger that modulates diverse intracellular signaling. Through interactions with signaling molecules, both PLD and PA can mediate a variety of cellular functions, such as, growth/proliferation, vesicle trafficking, cytoskeleton modulation, development, and morphogenesis. Therefore, systemic approaches for investigating PLD networks including interrelationship between PLD and PA and theirs binding partners, such as proteins and lipids, can enhance fundamental knowledge of roles of PLD and PA in diverse biological processes. In this review, we summarize previously reported protein-protein and protein-lipid interactions of PLD and PA and their binding partners. In addition, we describe the functional roles played by PLD and PA in these interactions, and provide PLD network that summarizes these interactions. The PLD network suggests that PLD and PA could act as a decision maker and/or as a coordinator of signal dynamics. This viewpoint provides a turning point for understanding the roles of PLD-PA as a dynamic signaling hub.

A new mechanism of action of a C2 domain-derived novel PKC inhibitor peptide

Novel protein kinase Cs (nPKCs) contain an N-terminal C2 domain that cannot bind to calcium. We have previously shown that the Aplysia novel PKC Apl II's C2 domain inhibits binding of diacylglycerol (DAG) to the C1 domain and that this inhibition is removed by phosphatidic acid (PA) binding to the C1b domain. Another model for C2 domain regulation of nPKCs suggests that the C2 domain binds to receptors for activated C kinase (RACKs) to assist in kinase translocation and activation. In the present study, we examined how a pharmacological peptide derived from RACK-binding site in the vertebrate novel PKCɛ regulates translocation of PKC Apl II from the cytosol to the plasma membrane. We found that a C2 domain-derived inhibitor peptide inhibited PKC Apl II translocation. This inhibition was removed by R273H mutation in the C1b domain and by phosphatidic acid, which can both remove C2-domain mediated inhibition suggesting that the peptide can regulate C1-C2 domain interactions.

Molecular, cellular, and physiological responses to phosphatidic acid formation in plants

Phosphatidic acid (PA) is an essential phospholipid involved in membrane biosynthesis and signal transduction in all eukaryotes. This review focuses on its role as lipid second messenger during plant stress, metabolism, and development. The contribution of different individual isoforms of enzymes that generate and break down PA will be discussed and the downstream responses highlighted, with particular focus on proteins that bind PA. Through characterization of several of these PA targets, a molecular and genetic basis for PA's role in plant stress and development is emerging.

Membrane-surface anchoring of charged diacylglycerol-lactones correlates with biological activities

Synthetic diacylglycerol-lactones (DAG-lactones) are effective modulators of critical cellular signaling pathways, downstream of the lipophilic second messenger diacylglycerol, that activate a host of protein kinase C (PKC) isozymes and other nonkinase proteins that share similar C1 membrane-targeting domains with PKC. A fundamental determinant of the biological activity of these amphiphilic molecules is the nature of their interactions with cellular membranes. This study examines the biological properties of charged DAG-lactones exhibiting different alkyl groups attached to the heterocyclic nitrogen of an α-pyridylalkylidene chain, and particularly the relationship between membrane interactions of the substituted DAG-lactones and their respective biological activities. Our results suggest that bilayer interface localization of the N-alkyl chain in the R(2) position of the DAG-lactones inhibits translocation of PKC isoenzymes onto the cellular membrane. However, the orientation of a branched alkyl chain at the bilayer surface facilitates PKC binding and translocation. This investigation emphasizes that bilayer localization of the aromatic side residues of positively charged DAG-lactone derivatives play a central role in determining biological activity, and that this factor contributes to the diversity of biological actions of these synthetic biomimetic ligands.

The C2 domains of classical and novel PKCs as versatile decoders of membrane signals

The C2 domains of classical and novel protein kinases C play a very important role in decoding signals, which trigger the translocation of these enzymes to the plasma membrane and/or other membrane subcellular compartments. The C2 domain of classical PKCs has a long reputation as a paradigm of protein responding to intracytosolic Ca2+ elevations through a calcium-binding region, where this cation acts as a bridge with the phosphatidylserine located in the inner leaflet of the plasma membrane. However, more recently, it has been discovered that a second site on the C2 domain interacts specifically with the phosphoinositide, PtdIns(4,5)P(2). Furthermore, several in vivo studies have shown that both calcium and PtdIns(4,5)P(2)-interacting regions are essential for the translocation of classical PKCs to the membrane. Other molecules like arachidonic and retinoic acid have also been observed to bind to these domains, modulating the activity of classical PKCs. The C2 domains of novel PKCs, on the other hand, were supposed to play only a secondary role with respect to the C1 domain in the activation process of these enzymes. New insights reveal that these C2 domains may also receive regulatory inputs and play an important role in the localization and activation of these enzymes. In this way, the C2 domain of PKCepsilon has been observed to respond to phosphatidic acid and to act together with the C1 domain in the membrane anchorage and activation of the protein. These domains are also regulated by lipid-independent events like protein-protein interactions and phosphorylation. In this review we will focus in describing the recent findings on structural and functional properties of the C2 domains of PKCs, mainly as lipid-interacting modules able to integrate a wide variety of signals in the cell.

A comparison of the membrane binding properties of C1B domains of PKCgamma, PKCdelta, and PKCepsilon

The C1 domains of classical and novel PKCs mediate their diacylglycerol-dependent translocation. Using fluorescence resonance energy transfer, we studied the contribution of different negatively charged phospholipids and diacylglycerols to membrane binding. Three different C1B domains of PKCs were studied (the classical gamma, and the novel delta and epsilon), together with different lipid mixtures containing three types of acidic phospholipids and three types of activating diacylglycerols. The results show that C1Bgamma and C1Bepsilon exhibit a higher affinity to bind to vesicles containing 1-palmitoyl-2-oleoyl-sn-phosphatidic acid, 1-palmitoyl-2-oleoyl-sn-phoshatidylserine, or 1-palmitoyl-2-oleoyl-sn-phosphatidylglycerol, with C1Bepsilon being the most relevant case because its affinity for POPA-containing vesicles increased by almost two orders of magnitude. When the effect of the diacylglycerol fatty acid composition on membrane binding was studied, the C1Bepsilon domain showed the highest binding affinity to membranes containing 1-stearoyl-oleoyl-sn-glycerol or 1,2-sn-dioleoylglycerol with POPA as the acidic phospholipid. Of the three diacylglycerols used in this study, 1,2-sn-dioleoylglycerol and 1-stearoyl-oleoyl-sn-glycerol showed the highest affinities for each isoenzyme, whereas 1,2-sn-dipalmitoylglycerol; showed the lowest affinity. DSC experiments showed this to be a consequence of the nonfluid conditions of 1,2-sn-dipalmitoylglycerol;-containing systems.

A 25-amino acid sequence of the Arabidopsis TGD2 protein is sufficient for specific binding of phosphatidic acid

Genetic analysis suggests that the TGD2 protein of Arabidopsis is required for the biosynthesis of endoplasmic reticulum derived thylakoid lipids. TGD2 is proposed to be the substrate-binding protein of a presumed lipid transporter consisting of the TGD1 (permease) and TGD3 (ATPase) proteins. The TGD1, -2, and -3 proteins are localized in the inner chloroplast envelope membrane. TGD2 appears to be anchored with an N-terminal membrane-spanning domain into the inner envelope membrane, whereas the C-terminal domain faces the intermembrane space. It was previously shown that the C-terminal domain of TGD2 binds phosphatidic acid (PtdOH). To investigate the PtdOH binding site of TGD2 in detail, the C-terminal domain of the TGD2 sequence lacking the transit peptide and transmembrane sequences was fused to the C terminus of the Discosoma sp. red fluorescent protein (DR). This greatly improved the solubility of the resulting DR-TGD2C fusion protein following production in Escherichia coli. The DR-TGD2C protein bound PtdOH with high specificity, as demonstrated by membrane lipid-protein overlay and liposome association assays. Internal deletion and truncation mutagenesis identified a previously undescribed minimal 25-amino acid fragment in the C-terminal domain of TGD2 that is sufficient for PtdOH binding. Binding characteristics of this 25-mer were distinctly different from those of TGD2C, suggesting that additional sequences of TGD2 providing the proper context for this 25-mer are needed for wild type-like PtdOH binding.

Phosphatidic acid signaling regulation of Ras superfamily of small guanosine triphosphatases

Phosphatidic acid (PA) has been increasingly recognized as an important signaling lipid regulating cell growth and proliferation, membrane trafficking, and cytoskeletal reorganization. Recent studies indicate that the signaling PA generated from phospholipase D (PLD) and diacylglycerol kinase (DGK) plays critical roles in regulating the activity of some members of Ras superfamily of small guanosine triphosphatases (GTPases), such as Ras, Rac and Arf. Change of PA levels regulates the activity of small GTPases by modulating membrane localization and activity of small GTPase regulatory proteins, guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs). In addition, PA also targets some small GTPases to membranes by direct binding. This review summarizes the roles of PLD and DGK in regulating the activity of several Ras superfamily members and cellular processes they control. Some future directions and the implication of PA regulation of Ras small GTPases in pathology are also discussed.

Enzymatic measurement of phosphatidic acid in cultured cells

In this work, we developed a novel enzymatic method for measuring phosphatidic acid (PA) in cultured cells. The enzymatic reaction sequence of the method involves hydrolysis of PA to produce glycerol-3-phosphate (G3P), which is then oxidized by G3P oxidase to generate hydrogen peroxide. In the presence of peroxidase, hydrogen peroxide reacted with Amplex Red to produce highly fluorescent resorufin. We found that lipase from Pseudomonas sp. can completely hydrolyze PA to G3P and FAs. The calibration curve for PA measurement was linear between 20 and 250 microM, and the detection limit was 5 microM (50 pmol in the reaction mixture). We also modified the method for the enzymatic measurement of lysophosphatidic acid. By this new method, we determined the PA content in the lipid extract from HEK293 cells. The cellular content of PA was decreased with increasing cell density but not correlated with the proliferation rate. The diacylglycerol kinase inhibitor R59949 markedly reduced the cellular PA content, suggesting the diacylglycerol kinase activity was involved in a large part of the PA production in HEK293 cells. This novel method for PA quantification is simple, rapid, specific, sensitive, and high-throughput and will help to study the biological functions of PA and its related enzymes.

Biophysics and function of phosphatidic acid: a molecular perspective

Phosphatidic acid is the simplest (diacyl)glycerophospholipid present in cells and is now a well established second messenger with direct biological functions. It is specifically recognized by diverse proteins and plays an important role in cellular signaling and membrane dynamics in all eukaryotes. An important determinant of the biological functions of phosphatidic acid is its anionic headgroup. In this review we will focus on the peculiar ionization properties of phosphatidic acid and their crucial role in lipid-protein interactions. We will take a molecular approach focusing entirely on the physical chemistry of the lipid and develop a model explaining the ionization properties of phosphatidic acid, termed the electrostatic-hydrogen bond switch model. Diverse examples from recent literature in support of this model will be presented and the broader implications of our findings will be discussed.

Spatiotemporal dynamics of lipid signaling: protein kinase C as a paradigm

The lipid second messenger diacylglycerol (DAG) controls the rate, amplitude, duration, and location of protein kinase C (PKC) activity in the cell. There are three classes of PKC isozymes and, of these, the conventional and novel isozymes are acutely controlled by DAG. The kinetics of DAG production at various intracellular membranes, the intrinsic affinity of specific isoforms for DAG-containing membranes, the coordinated use of additional membrane-binding modules, the intramolecular regulation of DAG sensitivity, and the competition from other DAG-responsive proteins together result in a unique, context-dependent activation signature for each isoform. This review focuses on the spatiotemporal dynamics of PKC activation and how it is controlled by lipid second messengers.

Protein kinase C as an effector of lipid-derived second messengers

Members of the protein kinase C family are major effectors of lipid second messengers. We describe three protocols to assess protein kinase C activity in polymorphonuclear leukocytes (neutrophils). These methods are useful to study the activation and function of protein kinase C in these immune cells. Since neutrophils provide a ready source of human primary tissue, these methods are also useful for pharmacological studies on the protein kinase C system and for evaluation of protein kinase C activators and inhibitors in the context of human primary cells. Furthermore, since protein kinase C activity is determined by a number of lipid-generating signaling systems, the methods described here can also be employed to study the pharmacology of these "upstream" signaling systems.

Structural basis of protein kinase C isoform function

Protein kinase C (PKC) isoforms comprise a family of lipid-activated enzymes that have been implicated in a wide range of cellular functions. PKCs are modular enzymes comprised of a regulatory domain (that contains the membrane-targeting motifs that respond to lipid cofactors, and in the case of some PKCs calcium) and a relatively conserved catalytic domain that binds ATP and substrates. These enzymes are coexpressed and respond to similar stimulatory agonists in many cell types. However, there is growing evidence that individual PKC isoforms subserve unique (and in some cases opposing) functions in cells, at least in part as a result of isoform-specific subcellular compartmentalization patterns, protein-protein interactions, and posttranslational modifications that influence catalytic function. This review focuses on the structural basis for differences in lipid cofactor responsiveness for individual PKC isoforms, the regulatory phosphorylations that control the normal maturation, activation, signaling function, and downregulation of these enzymes, and the intra-/intermolecular interactions that control PKC isoform activation and subcellular targeting in cells. A detailed understanding of the unique molecular features that underlie isoform-specific posttranslational modification patterns, protein-protein interactions, and subcellular targeting (i.e., that impart functional specificity) should provide the basis for the design of novel PKC isoform-specific activator or inhibitor compounds that can achieve therapeutically useful changes in PKC signaling in cells.

Physiological role for phosphatidic acid in the translocation of the novel protein kinase C Apl II in Aplysia neurons

In Aplysia californica, the serotonin-mediated translocation of protein kinase C (PKC) Apl II to neuronal membranes is important for synaptic plasticity. The orthologue of PKC Apl II, PKCepsilon, has been reported to require phosphatidic acid (PA) in conjunction with diacylglycerol (DAG) for translocation. We find that PKC Apl II can be synergistically translocated to membranes by the combination of DAG and PA. We identify a mutation in the C1b domain (arginine 273 to histidine; PKC Apl II-R273H) that removes the effects of exogenous PA. In Aplysia neurons, the inhibition of endogenous PA production by 1-butanol inhibited the physiological translocation of PKC Apl II by serotonin in the cell body and at the synapse but not the translocation of PKC Apl II-R273H. The translocation of PKC Apl II-R273H in the absence of PA was explained by two additional effects of this mutation: (i) the mutation removed C2 domain-mediated inhibition, and (ii) the mutation decreased the concentration of DAG required for PKC Apl II translocation. We present a model in which, under physiological conditions, PA is important to activate the novel PKC Apl II both by synergizing with DAG and removing C2 domain-mediated inhibition.

GPR30 estrogen receptor agonists induce mechanical hyperalgesia in the rat

We evaluated the signalling pathway by which estrogen acts in peripheral tissue to produce protein kinase Cepsilon (PKCepsilon)-dependent mechanical hyperalgesia. Specific agonists for the classical estrogen receptors (ER), ERalpha and ERbeta, did not result in activation of PKCepsilon in neurons of dissociated rat dorsal root ganglia. In contrast, G-1, a specific agonist of the recently identified G-protein-coupled estrogen receptor, GPR30, induced PKCepsilon translocation. Involvement of GPR30 and independence of ERalpha and ERbeta was confirmed using the GPR30 agonist and simultaneous ERalpha and ERbeta antagonist ICI 182,780 (fulvestrant). The GPR30 transcript could be amplified from dorsal root ganglia tissue. We found estrogen-induced as well as GPR30-agonist-induced PKCepsilon translocation to be restricted to the subgroup of nociceptive neurons positive for isolectin IB4 from Bandeiraea simplicifolia. Corroborating the cellular results, both GPR30 agonists, G-1 as well as ICI 182,780, resulted in the onset of PKCepsilon-dependent mechanical hyperalgesia if injected into paws of adult rats. We therefore suggest that estrogen acts acutely at GPR30 in nociceptors to produce mechanical hyperalgesia.

Protein kinase Cepsilon mediates polymeric fibronectin assembly on the surface of blood-borne rat breast cancer cells to promote pulmonary metastasis

Malignant breast cancer cells that have entered the blood circulation from primary mammary fat pad tumors or are grown in end-over-end suspension culture assemble a characteristic, multi-globular polymeric fibronectin (polyFn) coat on their surfaces. Surface polyFn is critical for pulmonary metastasis, presumably by facilitating lung vascular arrest via endothelial dipeptidylpeptidase IV (CD26). Here, we show that cell-surface polyFn assembly is initiated by the state of suspension, is dependent upon the synthesis and secretion of cellular Fn, and is augmented in a dose- and time-dependent manner by plasma Fn. PolyFn assembly is regulated by protein kinase Cepsilon (PKCepsilon), which translocates rapidly and in increasing amounts from the cytosol to the plasma membrane and is phosphorylated. PolyFn assembly is impeded by select inhibitors of this kinase, i.e. bisindolylmaleimide I, Ro-32-0432, Gö6983, and Rottlerin, by the phorbol 12-myristate 13-acetate-mediated and time-dependent loss of PKCepsilon protein and decreased plasma membrane translocation, and more specifically, by stable transfection of lung-metastatic MTF7L breast cancer cells with small interfering RNA-PKCepsilon and dominant-negative PKCepsilon constructs (e.g. RD-PKCepsilon). The inability to assemble a cell surface-associated polyFn coat by knockdown of endogenous Fn or PKCepsilon impedes cancer cells from metastasis to the lungs. The present studies identify a novel regulatory mechanism for polyFn assembly on blood-borne breast cancer cells and depict its effect on pulmonary metastasis.

Lipin1 regulation by estrogen in uterus and liver: implications for diabetes and fertility

Estrogens are essential for fertility and also have important effects on regulation of adiposity and the euglycemic state. We report here that lipin1, a candidate gene for lipodystrophy and obesity that is a phosphatidic acid phosphatase critical in regulation of cellular levels of diacylglycerol and triacylglycerol and a key regulator of lipid utilization, is rapidly and robustly down-regulated in the uterus by estradiol via the estrogen receptor. Lipin1 is expressed predominantly in the uterine luminal and glandular epithelium, and during the estrous cycle, lipin1 is lowest when blood levels of estrogen are highest. Lipin1 is expressed throughout all cells in the liver of ovariectomized female mice, and a sustained down-regulation is observed at the mRNA, protein and immunohistochemical levels after estrogen administration. Because the coupling of proper energy use and availability is central to reproduction, we also investigated expression of lipin1 in the uterus and liver of several mouse models of diabetes. Nonobese diabetic (NOD) mice, which have high blood levels of estrogen and impaired fertility, were severely deficient in lipin1 in the uterus and liver, which, interestingly, could be restored by insulin treatment. By contrast, nonobese diabetic/severe combined immunodeficient (NOD-SCID) mice, which do not develop diabetes, showed normal levels of lipin1. Our findings of lipin1 regulation by estrogen in two key target organs suggest a new role for this lipid-regulating phosphatase not only in central metabolic regulation but also in uterine function and reproductive biology. Estrogen regulation of lipin1 may provide a mechanistic link between estrogens, lipid metabolism, and lipid signaling.

Diacylglycerol kinases: Why so many of them?

Diacylglycerol (DAG) kinase (DGK) modulates the balance between the two signaling lipids, DAG and phosphatidic acid (PA), by phosphorylating DAG to yield PA. To date, ten mammalian DGK isozymes have been identified. In addition to the C1 domains (protein kinase C-like zinc finger structures) conserved commonly in all DGKs, these isoforms possess a variety of regulatory domains of known and/or predicted functions, such as a pair of EF-hand motifs, a pleckstrin homology domain, a sterile alpha motif domain and ankyrin repeats. Beyond our expectations, recent studies have revealed that DGK isozymes play pivotal roles in a wide variety of signal transduction pathways conducting development, neural and immune responses, cytoskeleton reorganization and carcinogenesis. Moreover, there has been rapidly growing evidence indicating that individual DGK isoforms exert their specific roles through interactions with unique partner proteins such as protein kinase Cs, Ras guanyl nucleotide-releasing protein, chimaerins and phosphatidylinositol-4-phosphate 5-kinase. Therefore, an emerging paradigm for DGK is that the individual DGK isoforms assembled in their own signaling complexes should carry out spatio-temporally segregated tasks for a wide range of biological processes via regulating local, but not global, concentrations of DAG and/or PA.

alpha(1)-Adrenergic receptor stimulation of cell motility requires phospholipase D-mediated extracellular signal-regulated kinase activation

Phospholipase D is suspected to play a role in tumorigenesis, and the inhibition of phospholipase D has been associated with changes in several cellular events including invasion and migration. We report here that the specific alpha(1)-adrenergic receptor agonist, phenylepherine, signals to a growth factor pathway in a manner that requires phospholipase D activity in CCL39 fibroblasts. Phenylepherine increased extracellular signal-regulated kinase phosphorylation eightfold and promoted stress fiber formation threefold. Stress fiber formation was blocked when extracellular signal-regulated kinase activation was inhibited. Stimulation of CCL39 fibroblasts by phenylepherine increased the rate of wound healing fourfold in a wounding assay, while treatment with the MEK inhibitor, PD98059 reduced the closure of phenylepherine-induced wound healing to control levels. Addition of 1-butanol but not 2-butanol inhibited extracellular signal-regulated kinase activation by phenylepherine, presumably by blocking the formation of phosphatidic acid. Exogenously added cell permeable phosphatidic acid increased extracellular signal-regulated kinase activation in a time- and dose-dependent manner as well as stimulated the formation of stress fibers. 1-butanol also significantly inhibited the ability of phenylepherine to stimulate stress fiber formation and wound healing. Taken together, these results indicate a novel role for phospholipase D in the activation of the extracellular signal-regulated kinase growth factor pathway to stimulate early cellular events induced by phenylepherine.

The C2 domains of classical PKCs are specific PtdIns(4,5)P2-sensing domains with different affinities for membrane binding

C2 domains are conserved protein modules in many eukaryotic signaling proteins, including the protein kinase (PKCs). The C2 domains of classical PKCs bind to membranes in a Ca(2+)-dependent manner and thereby act as cellular Ca(2+) effectors. Recent findings suggest that the C2 domain of PKCalpha interacts specifically with phosphatidylinositols 4,5-bisphosphate (PtdIns(4,5)P(2)) through its lysine rich cluster, for which it shows higher affinity than for POPS. In this work, we compared the three C2 domains of classical PKCs. Isothermal titration calorimetry revealed that the C2 domains of PKCalpha and beta display a greater capacity to bind to PtdIns(4,5)P(2)-containing vesicles than the C2 domain of PKCgamma. Comparative studies using lipid vesicles containing both POPS and PtdIns(4,5)P(2) as ligands revealed that the domains behave as PtdIns(4,5)P(2)-binding modules rather than as POPS-binding modules, suggesting that the presence of the phosphoinositide in membranes increases the affinity of each domain. When the magnitude of PtdIns(4,5)P(2) binding was compared with that of other polyphosphate phosphatidylinositols, it was seen to be greater in both PKCbeta- and PKCgamma-C2 domains. The concentration of Ca(2+) required to bind to membranes was seen to be lower in the presence of PtdIns(4,5)P(2) for all C2 domains, especially PKCalpha. In vivo experiments using differentiated PC12 cells transfected with each C2 domain fused to ECFP and stimulated with ATP demonstrated that, at limiting intracellular concentration of Ca(2+), the three C2 domains translocate to the plasma membrane at very similar rates. However, the plasma membrane dissociation event differed in each case, PKCalpha persisting for the longest time in the plasma membrane, followed by PKCgamma and, finally, PKCbeta, which probably reflects the different levels of Ca(2+) needed by each domain and their different affinities for PtdIns(4,5)P(2).

Phosphorylation at Ser729 specifies a Golgi localisation for protein kinase C epsilon (PKCepsilon) in 3T3 fibroblasts

We demonstrate that GFP-PKCepsilon concentrates at a perinuclear site in living fibroblasts and that cell passage induces rapid translocation of PKCepsilon to the periphery where it appears to colocalise with F-actin. When newly passaged cells have adhered and are proliferating again, GFP-PKCepsilon returns to its perinuclear site. GFP-PKCepsilon co-localises with wheat germ agglutinin suggesting that it is associated with the Golgi at the perinuclear site. In support, PKCepsilon is detected in a Golgi-enriched fraction in pre-passage cells but is lost from the fraction after passage. PKCepsilon at the perinuclear Golgi site is phosphorylated at Ser729 but cell passage induces the loss of the phosphate at this site as reported previously [England et al. (2001) J. Biol. Chem. 276, 10437-10442]. PKCepsilon S729A, S729E and S729T mutants, which are not recognised by a specific antiphosphoPKCepsilon (Ser729) antibody, do not concentrate at a perinuclear/Golgi site in proliferating fibroblasts. This suggests that both phosphorylation and serine rather than threonine are needed at position 729 to locate PKCepsilon at its perinuclear/Golgi site. Phorbol ester induced translocation of PKCepsilon to the nucleus also requires dephosphorylation at Ser729; after translocation nuclear PKCepsilon lacks a phosphate at Ser729. Sulphation and secretion of glycosaminoglycan (GAG) chains from fibroblasts increases on passage and returns to basal as cells proliferate showing that cell passage influences secretory events at the Golgi. The results indicate that Ser729 phosphorylation plays a role in determining PKCepsilon localisation in fibroblasts.