Stimulation by phospholipids of a protein-tyrosine-phosphatase containing two src homology 2 domains

SHP-1 tyrosine phosphatase binding to c-Src kinase phosphor-dependent conformations: A comparative structural framework

SHP-1 is a cytosolic tyrosine phosphatase that is primarily expressed in hematopoietic cells. It acts as a negative regulator of numerous signaling pathways and controls multiple cellular functions involved in cancer pathogenesis. This study describes the binding preferences of SHP-1 (pY536) to c-Srcopen (pY416) and c-Srcclose (pY527) through in silico approaches. Molecular dynamics simulation analysis revealed more conformational changes in c-Srcclose upon binding to SHP-1, as compared to its active/open conformation that is stabilized by the cooperative binding of the C-SH2 domain and C-terminal tail of SHP-1 to c-Src SH2 and KD. In contrast, c-Srcclose and SHP-1 interaction is mediated by PTP domain-specific WPD-loop (WPDXGXP) and Q-loop (QTXXQYXF) binding to c-Srcclose C-terminal tail residues. The dynamic correlation analysis demonstrated a positive correlation for SHP-1 PTP with KD, SH3, and the C-terminal tail of c-Srcclose. In the case of the c-Srcopen-SHP-1 complex, SH3 and SH2 domains of c-Srcopen were correlated to C-SH2 and the C-terminal tail of SHP-1. Our findings reveal that SHP1-dependent c-Src activation through dephosphorylation relies on the conformational shift in the inhibitory C-terminal tail that may ease the recruitment of the N-SH2 domain to phosphotyrosine residue, resulting in the relieving of the PTP domain. Collectively, this study delineates the intermolecular interaction paradigm and underlying conformational readjustments in SHP-1 due to binding with the c-Src active and inactive state. This study will largely help in devising novel therapeutic strategies for targeting cancer development.

Alterations in cellular and organellar phospholipid compositions of HepG2 cells during cell growth

The human hepatoblastoma cell line, HepG2, has been used for investigating a wide variety of physiological and pathophysiological processes. However, less information is available about the phospholipid metabolism in HepG2 cells. In the present report, to clarify the relationship between cell growth and phospholipid metabolism in HepG2 cells, we examined the phospholipid class compositions of the cells and their intracellular organelles by using enzymatic fluorometric methods. In HepG2 cells, the ratios of all phospholipid classes, but not the ratio of cholesterol, markedly changed with cell growth. Of note, depending on cell growth, the phosphatidic acid (PA) ratio increased and phosphatidylcholine (PC) ratio decreased in the nuclear membranes, the sphingomyelin (SM) ratio increased in the microsomal membranes, and the phosphatidylethanolamine (PE) ratio increased and the phosphatidylserine (PS) ratio decreased in the mitochondrial membranes. Moreover, the mRNA expression levels of enzymes related to PC, PE, PS, PA, SM and cardiolipin syntheses changed during cell growth. We suggest that the phospholipid class compositions of organellar membranes are tightly regulated by cell growth. These findings provide a basis for future investigations of cancer cell growth and lipid metabolism.

Dysregulated Lipid Metabolism and Its Role in α-Synucleinopathy in Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disease, the main pathological hallmark of which is the accumulation of α-synuclein (α-syn) and the formation of filamentous aggregates called Lewy bodies in the brainstem, limbic system, and cortical areas. Lipidomics is a newly emerging field which can provide fresh insights and new answers that will enhance our capacity for early diagnosis, tracking disease progression, predicting critical endpoints, and identifying risk in pre-symptomatic persons. In recent years, lipids have been implicated in many aspects of PD pathology. Biophysical and lipidomic studies have demonstrated that α-syn binds preferentially not only to specific lipid families but also to specific molecular species and that these lipid-protein complexes enhance its interaction with synaptic membranes, influence its oligomerization and aggregation, and interfere with the catalytic activity of cytoplasmic lipid enzymes and lysosomal lipases, thereby affecting lipid metabolism. The genetic link between aberrant lipid metabolism and PD is even more direct, with mutations in GBA and SMPD1 enhancing PD risk in humans and loss of GALC function increasing α-syn aggregation and accumulation in experimental murine models. Moreover, a number of lipidomic studies have reported PD-specific lipid alterations in both patient brains and plasma, including alterations in the lipid composition of lipid rafts in the frontal cortex. A further aspect of lipid dysregulation promoting PD pathogenesis is oxidative stress and inflammation, with proinflammatory lipid mediators such as platelet activating factors (PAFs) playing key roles in arbitrating the progressive neurodegeneration seen in PD linked to α-syn intracellular trafficking. Lastly, there are a number of genetic risk factors of PD which are involved in normal lipid metabolism and function. Genes such as PLA2G6 and SCARB2, which are involved in glycerophospholipid and sphingolipid metabolism either directly or indirectly are associated with risk of PD. This review seeks to describe these facets of metabolic lipid dysregulation as they relate to PD pathology and potential pathomechanisms involved in disease progression, while highlighting incongruous findings and gaps in knowledge that necessitate further research.

CD22: A Regulator of Innate and Adaptive B Cell Responses and Autoimmunity

CD22 (Siglec 2) is a receptor predominantly restricted to B cells. It was initially characterized over 30 years ago and named “CD22” in 1984 at the 2nd International workshop in Boston (1). Several excellent reviews have detailed CD22 functions, CD22-regulated signaling pathways and B cell subsets regulated by CD22 or Siglec G (2–4). This review is an attempt to highlight recent and possibly forgotten findings. We also describe the role of CD22 in autoimmunity and the great potential for CD22-based immunotherapeutics for the treatment of autoimmune diseases such as systemic lupus erythematosus (SLE).

Crystal structure of human protein tyrosine phosphatase SHP-1 in the open conformation

SHP-1 belongs to the family of non-receptor protein tyrosine phosphatases (PTPs) and generally acts as a negative regulator in a variety of cellular signaling pathways. Previously, the crystal structures of the tail-truncated SHP-1 and SHP-2 revealed an autoinhibitory conformation. To understand the regulatory mechanism of SHP-1, we have determined the crystal structure of the full-length SHP-1 at 3.1 Å. Although the tail was disordered in current structure, the huge conformational rearrangement of the N-SH2 domain and the incorporation of sulfate ions into the ligand-binding site of each domain indicate that the SHP-1 is in the open conformation. The N-SH2 domain in current structure is shifted away from the active site of the PTP domain to the other side of the C-SH2 domain, resulting in exposure of the active site. Meanwhile, the C-SH2 domain is twisted anticlockwise by about 110°. In addition, a set of new interactions between two SH2 domains and between the N-SH2 and the catalytic domains is identified, which could be responsible for the stabilization of SHP-1 in the open conformation. Based on the structural comparison, a model for the activation of SHP-1 is proposed.

Compartmentalized CDK2 is connected with SHP-1 and β-catenin and regulates insulin internalization

The cyclin-dependant kinase Cdk2 is compartmentalized in endosomes but its role is poorly understood. Here we show that Cdk2 present in hepatic endosome fractions is strictly located in a Triton X-100-resistant environment. The endosomal Cdk2 was found to be associated with the protein tyrosine phosphatase SHP-1, a regulator of insulin clearance, and the actin anchor β-catenin, a known substrate for both Cdk2 and SHP-1. In the plasma membranes and endosome fractions, β-catenin is associated with CEACAM1, also known as regulator of insulin clearance. We show that β-catenin, not CEACAM1, is a substrate for Cdk2. Partial down-modulation of Cdk2 in HEK293 cells increased the rate of insulin internalization. These findings reveal that Cdk2 functions, at least in part, via a Cdk2/SHP-1/β-catenin/CEACAM1 axis, and show for the first time that Cdk2 has the capacity to regulate insulin internalization.

Insulin modifies aging-related inhibition of 1-stearoyl, 2-arachidonoylglycerol phosphorylation in rat synaptic terminals

The purpose of the present study was to analyze diacylglycerol kinase (DAGK) activity in synaptic terminals from cerebral cortex (CC) and hippocampus (Hp) from adult (3-4 month-old) and aged (26-28 month-old) rats. The effect of insulin through DAGK activity on synaptosomes from adult and aged rats was also analyzed under conditions favoring saturated or unsaturated phosphatidic acid (PA) formation, using exogenous di-palmitoil glycerol (DPG) or 1-stearoyl-2-arachidonoylglycerol (SAG) as substrates. Results showed that the enzymatic activity preferentially uses SAG as substrate, thus indicating the presence of ɛ-type DAGK. A significant decrease in DAGK activity transforming SAG into PA was also observed in both tissues from aged rats. Western blot detection of DAGKɛ showed that enzyme content undergoes no changes with aging. [3H] inositol incorporation into phosphoinosites was also analyzed to evaluate the role of DAGKɛ in their synthesis. Data obtained from 3H-inositol incorporation into phosphoinositides revealed that in synaptosomes from aged rats phosphatidylinositol (PI) synthesis is lower than in adult animals. Interestingly, in the presence of SAG, PI synthesis was restored to adult values. DAGK activity over SAG was more highly stimulated by insulin in CC and Hp synaptosomes of aged rats with respect to adult rats. On the other hand, insulin exerted a stimulatory effect on PI and phosphatidylinositol 4 phosphate (PI(4)P) synthesis in synaptosomal CC from aged rats. Taken together, our findings indicate that in aged rats insulin triggers a stimulatory mechanism that reverts the diminished synaptosomal ability to synthesize arachidonoyl phosphatidic acid (20:4 PA). The recovery of this PA species indicates that insulin positively regulates phosphoinositide synthesis.

Characterization of a highly effective protein substrate for analysis of JAK2(V617F) Activity

OBJECTIVE

Identification of JAK2V617F in myeloproliferative disorders makes JAK2 an important marker for disease diagnosis and a highly attractive target for therapeutic drug development. This study is intended to identify a sensitive and specific substrate for assays of the JAK2 enzymatic activity.

METHODS

We expressed a glutathione S-transferase (GST) fusion protein designated GST-JAKS, which carries a peptide sequence derived from the autophosphorylation sites of human JAK2. The protein was purified from Escherichia coli cells and was used to analyze to tyrosine kinase activities of purified enzymes and crude cell extracts from cells, including mononuclear cells of JAK2V617F -positive polycythemia vera blood. It was also used to perform JAK2 kinase assays to screen inhibitors of JAK2.

RESULTS

GST-JAKS is strongly phosphorylated by activated forms of JAK2 including JAK2V617F and recombinant protein containing its catalytic domain alone. It showed minimal responses to wild-type JAK2 and was not phosphorylated by the epidermal growth receptor and the insulin receptor tyrosine kinases. Kinase assays with GST-JAKS provide a sharp contrast between wild-type and mutant JAK2,V617F and are sensitive enough to detect minute amounts of JAK2V617F found in crude cell extracts. Assays can be scaled up to screen for inhibitors of JAK2 in a dot blot format.

CONCLUSION

GST-JAKS is sensitive and specific protein substrate for JAK2 assays. It may have clinical applications in diagnosis of diseases related to abnormal JAK2 activity. It is also an excellent substrate for development of large scale assays to screen JAK2 inhibitors.

Identification of a novel lipid raft-targeting motif in Src homology 2-containing phosphatase 1

The tyrosine phosphatase Src homology 2-containing phosphatase 1 (SHP-1) is a key negative regulator of TCR-mediated signaling. Previous studies have shown that in T cells a fraction of SHP-1 constitutively localizes to membrane microdomains, commonly referred to as lipid rafts. Although this localization of SHP-1 is required for its functional regulation of T cell activation events, how SHP-1 is targeted to the lipid rafts was unclear. In this study, we identify a novel, six-amino acid, lipid raft-targeting motif within the C terminus of SHP-1 based on several biochemical and functional observations. First, mutations of this motif in the context of full-length SHP-1 result in the loss of lipid raft localization of SHP-1. Second, this motif alone restores raft localization when fused to a mutant of SHP-1 (SHP-1 DeltaC) that fails to localize to rafts. Third, a peptide encompassing the 6-mer motif directly binds to phospholipids whereas a mutation of this motif abolishes lipid binding. Fourth, whereas full-length SHP-1 potently inhibits TCR-induced tyrosine phosphorylation of specific proteins, expression of a SHP-1-carrying mutation within the 6-mer motif does not. Additionally, although SHP-1 DeltaC was functionally inactive, the addition of the 6-mer motif restored its functionality in inhibiting TCR-induced tyrosine phosphorylation. Finally, this 6-mer mediated targeting of SHP-1 lipid rafts was essential for the function of this phosphatase in regulating IL-2 production downstream of TCR. Taken together, these data define a novel 6-mer motif within SHP-1 that is necessary and sufficient for lipid raft localization and for the function of SHP-1 as a negative regulator of TCR signaling.

Insulin promotes diacylglycerol kinase activation by different mechanisms in rat cerebral cortex synaptosomes

The mechanism by which insulin increases diacylglycerol kinase (DAGK) activity has been studied in cerebral cortex (CC) synaptosomes from adult (3-4 months of age) rats. The purpose of this study was to identify the role of phospholipases C and D (PLC and PLD) in DAGK activation by insulin. Neomycin, an inhibitor of PLC phosphatidylinositol-bisphosphate (PIP2) specific; ethanol, an inhibitor of phosphatidic acid (PA) formation by the promotion of a transphosphatidyl reaction of phosphatidylcholine phospholipase D (PC-PLD); and DL propranolol, an inhibitor of phosphatidate phosphohydrolase (PAP), were used in this study. Insulin (0.1 microM) shielded an increase in PA synthesis by [32P] incorporation using [gamma-32P]ATP as substrate and endogenous diacylglycerol (DAG) as co-substrate. This activated synthesis was strongly inhibited either by ethanol or DL propranolol. Pulse chase experiments also showed a PIP2-PLC activation within 1 min exposure to insulin. When exogenous unsaturated 18:0-20:4 DAG was present, insulin increased PA synthesis significantly. However, this stimulatory effect was not observed in the presence of exogenous saturated (di-16:0). In the presence of R59022, a selective DAGK inhibitor, insulin exerted no stimulatory effect on [32P]PA formation, suggesting a strong relationship between increased PA formation by insulin and DAGK activity. These data indicate that the increased synthesis of PA by insulin could be mediated by the activation of both a PC-PLD pathway to provide DAG and a direct DAGK activation that is associated to the use of 18:0-20:4 DAG species. PIP2-PLC activation may contribute at least partly to the insulin effect on DAGK activity.

A SHPing tale: perspectives on the regulation of SHP-1 and SHP-2 tyrosine phosphatases by the C-terminal tail

Protein tyrosine phosphorylation is a ubiquitous signalling mechanism and is regulated by a balance between the action of kinases and phosphatases. The SH2 domain-containing phosphatases SHP-1 and SHP-2 are the best studied of the classical non-receptor tyrosine phosphatases, but it is intriguing that despite their close sequence and structural homology these two phosphatases play quite different cellular roles. In particular, whereas SHP-1 plays a largely negative signalling role suppressing cellular activation, SHP-2 plays a largely positive signalling role. Major sequence differences between the two molecules are apparent in the approximately 100 amino acid residues at the extreme C-terminus of the proteins, beyond the phosphatase catalytic domain. Here we review how the differences in the tails of these proteins may regulate their activities and explain some of their functional differences.

Phosphatidic acid and diacylglycerol generation is regulated by insulin in cerebral cortex synaptosomes from adult and aged rats

Insulin receptor associated with the cerebral cortex (CC) has been shown to be involved in brain cognitive functions. Furthermore, deterioration of insulin signaling has been associated with age-related brain degeneration. We have reported previously that aging stimulates phospholipase D/phosphatidate phosphohydrolase 2 (PLD/PAP2) pathway in CC synaptosomes from aged rats, generating a differential availability of their reaction products: diacylglycerol (DAG) and phosphatidic acid (PA). The aim of this work was to determine the effect of aging on DAG kinase (DAGK), as an alternative pathway for PA generation, and to evaluate the effect of insulin on PLD/PAP2 pathway and DAGK. PLD, PAP2, and DAGK activities were measured using specific radiolabeled substrates in CC synaptosomes from adult (4 months old) and aged rats (28 months old). In adult animals, in the presence of the tyrosine phosphatase inhibitor (sodium o-vanadate), insulin stimulated PLD activity at 5 min incubation. DAGK activity was also increased at the same time of incubation and PAP2 was inhibited. In aged animals, PLD activity was not modified by the presence of insulin plus vanadate, PAP2 was inhibited, and DAGK was stimulated by the hormone. Insulin, vanadate, and the combination of both induced protein tyrosine phosphorylation in adult CC synaptosomes. Aged rats showed a lower level of protein phosphorylation with respect to adult rats. Our results show that insulin modulates PA and DAG availability through the regulation of PLD/PAP2 and DAGK pathways in adult rat CC synaptosomes. Additionally, we demonstrated that PA and DAG generation is regulated differentially by insulin during aging.

Phosphatidylinositol phosphate kinase type Igamma directly associates with and regulates Shp-1 tyrosine phosphatase

Tyrosine phosphorylation plays a critical role in many regulatory aspects of cellular signaling, and dephosphorylation of phosphotyrosine residues is crucial for termination of signals initiated by tyrosine kinases. Previous work has shown that the tyrosine kinase Src phosphorylates Tyr644 on phosphatidylinositol phosphate kinase type I (PIPKI) gamma661 in a focal adhesion kinase-dependent manner. Phosphorylation of this residue is essential for high affinity binding of PIPKI gamma661 to the focal adhesion protein talin and for targeting of PIPKI gamma661 to focal adhesions. A yeast two-hybrid screen performed with the C-terminal 178-amino acid tail of PIPKI gamma661 identified an interaction with the phosphatase domain of the tyrosine phosphatase Shp-1. The interaction between PIPKI gamma661 and Shp-1 was confirmed via co-immunoprecipitation from HEK293 cell lysates. In addition, Src-phosphorylated PIPKI gamma661 is a substrate for Shp-1, and Shp-1 modulates both the association between PIPKI gamma661 and talin and the targeting of PIPKI gamma661 to focal adhesions in mammalian cells. Finally, we showed that Shp-1 phosphatase activity is inhibited by the product of PIPKI gamma661, phosphatidylinositol 4,5-bisphosphate, in vitro. These combined results suggest a model in which the reciprocal actions of Src tyrosine kinase and Shp-1 tyrosine phosphatase dynamically regulate the association between PIPKI gamma661 and talin.

Localization of Src homology 2 domain-containing phosphatase 1 (SHP-1) to lipid rafts in T lymphocytes: functional implications and a role for the SHP-1 carboxyl terminus

The protein tyrosine phosphatase Src homology 2 domain-containing phosphatase 1 (SHP-1) has previously been shown to be a negative regulator of signaling mediated via the TCR. A growing body of evidence indicates that the regulated localization of proteins within certain membrane subdomains, referred to as lipid rafts, is important for the successful transduction of signaling events downstream of the TCR. However, considerably less is known about the localization of negative regulators during these lipid raft-dependent signaling events. In this study we have investigated the subcellular localization of SHP-1 and its role in regulation of TCR-mediated signaling. Our studies demonstrate that in a murine T cell hybridoma as well as in primary murine thymocytes, a fraction of SHP-1 localizes to the lipid rafts, both basally and after TCR stimulation. Interestingly, although SHP-1 localized in the nonraft fractions is tyrosine phosphorylated, the SHP-1 isolated from the lipid rafts lacks the TCR-induced tyrosine phosphorylation, suggesting physical and/or functional differences between these two subpopulations. We identify a requirement for the C-terminal residues of SHP-1 in optimal localization to the lipid rafts. Although expression of SHP-1 that localizes to lipid rafts potently inhibits TCR-mediated early signaling events and IL-2 production, the expression of lipid raft-excluded SHP-1 mutants fails to elicit any of the inhibitory effects. Taken together these studies reveal a key role for lipid raft localization of SHP-1 in mediating the inhibitory effects on T cell signaling events.

Leishmania donovani engages in regulatory interference by targeting macrophage protein tyrosine phosphatase SHP-1

Protozoan parasites of the genus leishmania are obligate intracellular parasites of monocytes and macrophages. These pathogens have evolved to invade the mammalian immune system and typically survive for long periods of time. Leishmania have developed a variety of remarkable strategies to prevent their elimination by both innate and acquired immune effector mechanisms. One particular strategy of interest involves manipulation of host cell regulatory pathways so as to prevent macrophage activation required for efficient microbicidal activity. These interference mechanisms are the main focus of this review. Several lines of evidence have been developed to show that the Src homology-2 domain containing tyrosine phosphatase-1 (SHP-1) becomes activated in leishmania-infected cells and that this contributes to disease pathogenesis. Recent studies aimed at understanding the mechanism responsible for the change in activation state of SHP-1 led to the identification of leishmania EF-1alpha as an SHP-1 binding protein and SHP-1 activator. This was a surprising finding given that this ubiquitous and highly conserved protein plays an essential role in protein translation in both prokaryotic and eukaryotic cells. The role of leishmania EF-1alpha as an SHP-1 activator and its contribution to pathogenesis are reviewed with particular attention to the properties that distinguish it from host EF-1alpha.

AtDGK2, a novel diacylglycerol kinase from Arabidopsis thaliana, phosphorylates 1-stearoyl-2-arachidonoyl-sn-glycerol and 1,2-dioleoyl-sn-glycerol and exhibits cold-inducible gene expression

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol (DAG) to generate phosphatidic acid (PA). Both DAG and PA are implicated in signal transduction pathways. DGKs have been widely studied in animals, but their analysis in plants is fragmentary. Here, we report the cloning and biochemical characterization of AtDGK2, encoding DGK from Arabidopsis thaliana. AtDGK2 has a predicted molecular mass of 79.4 kDa and, like AtDGK1 previously reported, harbors two copies of a phorbol ester/DAG-binding domain in its N-terminal region. AtDGK2 belongs to a family of seven DGK genes in A. thaliana. AtDGK3 to AtDGK7 encode approximately 55-kDa DGKs that lack a typical phorbol ester/DAG-binding domain. Phylogenetically, plant DGKs fall into three clusters. Members of all three clusters are widely expressed in vascular plants. Recombinant AtDGK2 was expressed in Escherichia coli and biochemically characterized. The enzyme phosphorylated 1,2-dioleoyl-sn-glycerol to yield PA, exhibiting Michaelis-Menten type kinetics. Estimated K(m) and V(max) values were 125 microm for DAG and 0.25 pmol of PA min(-1) microg(-1), respectively. The enzyme was maximally active at pH 7.2. Its activity was Mg(2+)-dependent and affected by the presence of detergents, salts, and the DGK inhibitor R59022, but not by Ca(2+). AtDGK2 exhibited substrate preference for unsaturated DAG analogues (i.e. 1-stearoyl-2-arachidonoyl-sn-glycerol and 1,2-dioleoyl-sn-glycerol). The AtDGK2 gene is expressed in various tissues of the Arabidopsis plant, including leaves, roots, and flowers, as shown by Northern blot analysis and promoter-reporter gene fusions. We found that AtDGK2 is induced by exposure to low temperature (4 degrees C), pointing to a role in cold signal transduction.

Blocking the function of tyrosine phosphatase SHP-2 by targeting its Src homology 2 domains

SHP-2 is an Src homology 2 (SH2) domain-containing tyrosine phosphatase with crucial functions in cell signaling and major pathological implications. It stays inactive in the cytosol and is activated by binding through its SH2 domains to tyrosine-phosphorylated receptors on the cell surface. One such cell surface protein is PZR, which contains two tyrosine-based inhibition motifs responsible for binding of SHP-2. We have generated a glutathione S-transferase fusion protein carrying the tandem tyrosine-based inhibition motifs of PZR, and the protein was tyrosine-phosphorylated by co-expressing c-Src in Escherichia coli cells. The purified phosphoprotein displays a strong binding to SHP-2 and causes its activation in vitro. However, when introduced into NIH 3T3 cells by using a protein delivery reagent, it effectively inhibited the activation of ERK1/2 induced by growth factors and serum but not by phorbol ester, in reminiscence of the effects caused by expression of dominant negative SHP-2 mutants and deletion of functional SHP-2. The data suggest that the exogenously introduced PZR protein specifically binds SHP-2, blocks its translocation, and renders it functionally incompetent. This is further supported by the fact that the phosphorylated PZR protein had no inhibitory effects on fibroblasts derived from mice expressing only a mutant SHP-2 protein lacking most of the N-terminal SH2 domain. This study thus provides a novel and highly specific method to interrupt the function of SHP-2 in cells.

Specific interaction of protein tyrosine phosphatase-MEG2 with phosphatidylserine

Protein tyrosine phosphatase (PTP)-MEG2 is an intracellular tyrosine phosphatase that contains a Sec14 homology domain. We have purified the full-length and truncated forms of the enzyme from recombinant adenovirus-infected human 293 cells. By using lipid-membrane overlay and liposome binding assays, we demonstrated that PTP-MEG2 specifically binds phosphatidylserine among over 20 lipid compounds tested. The binding is mediated by its N-terminal Sec14 domain. In intact cells, the Sec14 domain is responsible for localization of PTP-MEG2 to the perinuclear region, and uploading of PS into the cell membrane causes translocation of PTP-MEG2 to the plasma membrane. Phosphatidylserine is a relatively abundant cell membrane phospholipid non-symmetrically distributed in the outer layer and inner layer of cell membranes. It has recently been defined as an important ligand for clearance of apoptotic cells. By specifically binding phosphatidylserine, PTP-MEG2 may play an important role in regulating signaling processes associated with phagocytosis of apoptotic cells.

Pulmonary phosphatidic acid phosphatase and lipid phosphate phosphohydrolase

The lung contains two distinct forms of phosphatidic acid phosphatase (PAP). PAP1 is a cytosolic enzyme that is activated through fatty acid-induced translocation to the endoplasmic reticulum, where it converts phosphatidic acid (PA) to diacylglycerol (DAG) for the biosynthesis of phospholipids and neutral lipids. PAP1 is Mg(2+) dependent and sulfhydryl reagent sensitive. PAP2 is a six-transmembrane-domain integral protein localized to the plasma membrane. Because PAP2 degrades sphingosine-1-phosphate (S1P) and ceramide-1-phosphate in addition to PA and lyso-PA, it has been renamed lipid phosphate phosphohydrolase (LPP). LPP is Mg(2+) independent and sulfhydryl reagent insensitive. This review describes LPP isoforms found in the lung and their location in signaling platforms (rafts/caveolae). Pulmonary LPPs likely function in the phospholipase D pathway, thereby controlling surfactant secretion. Through lowering the levels of lyso-PA and S1P, which serve as agonists for endothelial differentiation gene receptors, LPPs regulate cell division, differentiation, apoptosis, and mobility. LPP activity could also influence transdifferentiation of alveolar type II to type I cells. It is considered likely that these lipid phosphohydrolases have critical roles in lung morphogenesis and in acute lung injury and repair.

Leishmania EF-1alpha activates the Src homology 2 domain containing tyrosine phosphatase SHP-1 leading to macrophage deactivation

The human leishmaniasis are persistent infections of macrophages caused by protozoa of the genus Leishmania. The chronic nature of these infections is in part related to induction of macrophage deactivation, linked to activation of the Src homology 2 domain containing tyrosine phosphatase-1 (SHP-1) in infected cells. To investigate the mechanism of SHP-1 activation, lysates of Leishmania donovani promastigotes were subjected to SHP-1 affinity chromatography and proteins bound to the matrix were sequenced by mass spectrometry. This resulted in the identification of Leishmania elongation factor-1alpha (EF-1alpha) as a SHP-1-binding protein. Purified Leishmania EF-1alpha, but not host cell EF-1alpha, bound directly to SHP-1 in vitro leading to its activation. Three independent lines of evidence indicated that Leishmania EF-1alpha may be exported from the phagosome thereby enabling targeting of host SHP-1. First, cytosolic fractions prepared from macrophages infected with [(35)S]methionine-labeled organisms contained Leishmania EF-1alpha. Second, confocal, fluorescence microscopy using Leishmania-specific antisera detected Leishmania EF-1alpha in the cytosol of infected cells. Third, co-immunoprecipitation showed that Leishmania EF-1alpha was associated with SHP-1 in vivo in infected cells. Finally, introduction of purified Leishmania EF-1alpha, but not the corresponding host protein into macrophages activated SHP-1 and blocked the induction of inducible nitric-oxide synthase expression in response to interferon-gamma. Thus, Leishmania EF-1alpha is identified as a novel SHP-1-binding and activating protein that recapitulates the deactivated phenotype of infected macrophages.

Mechanisms regulating the expression, self-maintenance, and signaling-function of the bradykinin B2 and B1 receptors

Bradykinin (BK) is a potent short-lived effector belonging to a class of peptides known as kinins. It participates in inflammatory and vascular regulation and processes including angioedema, tissue permeability, vascular dilation, and smooth muscle contraction. BK exerts its biological effects through the activation of the bradykinin B2 receptor (BKB2R) which is G-protein-coupled and is generally constitutively expressed. Upon binding, the receptor is activated and transduces signal cascades which have become paradigms for the actions of the Galphai and Galphaq G-protein subunits. Following activation the receptor is then desensitized, endocytosed, and resensitized. The bradykinin B1 (BKB1R) is a closely related receptor. It is activated by desArg(10)-kallidin or desArg(9)-BK, metabolites of kallidin and BK, respectively. This receptor is induced following tissue injury or after treatment with bacterial endotoxins such as lipopolysacharide or cytokines such as interleukin-1 or tumor necrosis factor-alpha. In this review we will summarize the BKB2R and BKB1R mediated signal transduction pathways. We will then emphasize the relevance of key residues and domains of the intracellular regions of the BKB2R as they relate to modulating its function (signal transduction) and self-maintenance (desensitization, endocytosis, and resensitization). We will examine the features of the BKB1R gene promoter and its mRNA as these operate in the expression and self-maintenance of this inducible receptor. This communication will not cover areas discussed in earlier reviews pertaining to the actions of peptide analogs. For these we refer you to earlier reviews (Regoli and Barabé, 1980, Pharmacol Rev 32:1-46; Regoli et al., 1990, J Cardiovasc Pharmacol 15(Suppl 6):S30-S38; Regoli et al., 1993, Can J Physiol Pharmacol 71:556-557; Marceau, 1995, Immunopharmacology 30:1-26; Regoli et al., 1998, Eur J Pharmacol 348:1-10).

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.

Phospholipase D and immune receptor signalling

Immune receptors are coupled to the activation of phosphatidylcholine phospholipase D (PC-PLD) that hydrolyses phosphatidylcholine to generate phosphatidic acid and choline. As these receptors are also coupled to other signalling cascades, it has been difficult to define the precise cell activation events resulting from PLD activation in the absence of specific inhibitors. There is increasing evidence that phosphatidic acid acts as an intracellular signalling molecule regulating release of calcium from intracellular stores, sphingosine kinase and protein kinase C activation and membrane budding. Phosphatidic acid can also be rapidly converted into lysophosphatidic acid, diacylglycerol and arachidonates.

Purification and characterization of protein tyrosine phosphatase PTP-MEG2

PTP-MEG2 is an intracellular protein tyrosine phosphatase with a putative lipid-binding domain at the N-terminus. The present study reports expression, purification, and characterization of the full-length form of the enzyme plus a truncated form containing the catalytic domain alone. Full-length PTP-MEG2 was expressed with an adenovirus system and purified from cytosolic extracts of human 293 cells infected with the recombinant adenovirus. The purification scheme included chromatographic separation of cytosolic extracts on fast flow Q-Sepharose, heparin-agarose, l-histidyldiazobenzylphosphonic acid agarose, and hydroxylapatite. The enrichment of PTP-MEG2 from the cytosol was about 120-fold. The truncated form of PTP-MEG2 was expressed in E. coli cells as a non-fusion protein and purified by using a chromatographic procedure similar to that used for the full-length enzyme. The purified full-length and truncated enzymes showed single polypeptide bands on SDS-polyacrylamide gel electrophoresis under reducing conditions and behaved as monomers on gel exclusion chromatography. With para-nitrophenylphosphate and phosphotyrosine as substrates, both forms of the enzyme exhibited classical Michaelis-Menten kinetics. Their responses to pH, ionic strength, metal ions, and protein phosphatase inhibitors are similar to those observed with other characterized tyrosine phosphatases. Compared with full-length PTP-MEG2, the truncated DeltaPTP-MEG2 displayed significantly higher V(max) and lower K(m) values, suggesting that the N-terminal putative lipid-binding domain may have an inhibitory role. The full-length and truncated forms of PTP-MEG2 were also expressed as GST fusion proteins in E. coli cells and purified to near homogeneity through affinity columns. However, the specific phosphatase activities of the GST fusion proteins were 10-25-fold below those obtained with the correspondent non-fusion proteins.

Tyrosine phosphatases SHP-1 and SHP-2 are associated with distinct tyrosine-phosphorylated proteins

SHP-1 and SHP-2 are two SH2 domain-containing tyrosine phosphatases. They share significant overall sequence identity but their functions are often opposite. The mechanism underlying this is not well understood. In this study, we have investigated the association of SHP-1 and SHP-2 with tyrosine-phosphorylated proteins in mouse tissues and in cultured cells treated with a potent tyrosine phosphatase inhibitor, pervanadate. Pervanadate was introduced into mice by intravenous injection. It induced robust tyrosine phosphorylation of cellular proteins in a variety of tissues. Both SHP-1 and SHP-2 were phosphorylated on tyrosyl residues upon pervanadate treatment, and they became associated with distinct tyrosine-phosphorylated proteins in different tissues and cells. Among these proteins, PZR and PECAM were identified as major SHP-2-binding proteins while LAIR-1 was shown to be a major SHP-1-binding protein. A number of other proteins are to be identified. We believe that the different binding proteins may determine the distinct physiological functions of SHP-1 and SHP-2. The present study also provides a general method to induce tyrosine phosphorylation of cellular proteins and to study protein-protein interactions involving tyrosine phosphorylation in vivo and in vitro.

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

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

Differential activation of phospholipases by mitogenic EGF and neurogenic PDGF in immortalized hippocampal stem cell lines

In several neuronal systems, nerve growth factor (NGF) and platelet-derived growth factor (PDGF) act as neurogenic agents, whereas epidermal growth factor (EGF) acts as a mitogenic agent. Hippocampal stem cell lines (HiB5) immortalized by the expression of a temperature-sensitive SV40 large T antigen also respond differentially to EGF and PDGF. While EGF treatment at the permissive temperature induces proliferation, the addition of PDGF induces differentiation at the non-permissive temperature. However, the mechanism responsible for these different cellular fates has not been clearly elucidated. In order to clarify possible critical signaling events leading to these distinct cellular outcomes, we examined whether either EGF or PDGF differentially induces the activation of phospholipases, such as phospholipase A(2) (PLA(2)), C (PLC), or D (PLD). Although EGF stimulation did not induce phospholipases, PDGF caused a rapid and transient activation of PLC and PLD, but not PLA(2). When the activation of PLC or PLD was blocked, the neurite outgrowth induced by PDGF was significantly inhibited. Although the activation of PLD occurred faster than PLC, blocking of PLD activity by transient expression of lipase-inactive mutants did not inhibit the induction of PLC activity by PDGF. These results suggest that the differential activation of phospholipases may play an important role in signal transduction by mitogenic EGF and neurotrophic PDGF in HiB5 neuronal hippocampal stem cells. In particular, the activation of phospholipase C and D may contribute to neuronal differentiation by neurogenic PDGF in the HiB5 cells.

Phosphatidic acid regulates tyrosine phosphorylating activity in human neutrophils: enhancement of Fgr activity

In human neutrophils, the activation of phospholipase D and the Tyr phosphorylation of proteins are early signaling events upon cell stimulation. We found that the pretreatment of neutrophils with ethanol (0.8%) or 1-butanol (0.3%), which results in the accumulation of phosphatidylalcohol at the expense of phosphatidic acid (PA), decreased the phorbol myristate acetate-stimulated Tyr phosphorylation of endogenous proteins (42, 115 kDa). When neutrophil cytosol was incubated in the presence or absence of PA, these and other endogenous proteins became Tyr-phosphorylated in a PA-dependent manner. In contrast, phosphatidylalcohols exhibited only 25% (phosphatidylethanol) or 5% (phosphatidylbutanol) of the ability of PA to stimulate Tyr phosphorylation in the cell-free assay. Similarly, other phospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylserine, phosphatidylinositol, polyphosphoinositides, and sphingosine 1-phosphate) showed little ability to stimulate Tyr phosphorylation. These data suggest that PA can function as an intracellular regulator of Tyr phosphorylating activity. Gel filtration chromatography of leukocyte cytosol revealed a peak of PA-dependent Tyr phosphorylating activity distinct from a previously described PA-dependent phosphorylating activity (Waite, K. A., Wallin, R., Qualliotine-Mann, D., and McPhail, L. C. (1997) J. Biol. Chem. 272, 15569-15578). Among the protein Tyr kinases expressed in neutrophils, only Fgr eluted exclusively in the peak of PA-dependent Tyr phosphorylating activity. Importantly, Fgr isolated from unstimulated neutrophil lysates showed increased activity in the presence of PA but not phosphatidylbutanol. Moreover, the pretreatment of neutrophils with 1-butanol decreased Fgr activity in cells stimulated with formyl-methionyl-leucyl phenylalanine plus dihydrocytochalasin B. Together, these results suggest a new second messenger role for PA in the regulation of Tyr phosphorylation.

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.

Properties and functions of diacylglycerol kinases

Diacylglycerol kinases (DGKs) phosphorylate the second-messenger diacylglycerol (DAG) to phosphatidic acid (PA). The family of DGKs is well conserved among most species. Nine mammalian isotypes have been identified, and are classified into five subgroups based on their primary structure. DGKs contain a conserved catalytic domain and an array of other conserved motifs that are likely to play a role in lipid-protein and protein-protein interactions in various signalling pathways dependent on DAG and/or PA production. DGK is therefore believed to be activated at the (plasma) membrane where DAG is generated. Some isotypes are found associated with and/or regulated by small GTPases of the Rho family, presumably acting in cytoskeletal rearrangements. Others are (also) found in the nucleus, in association with other regulatory enzymes of the phosphoinositide cycle, and have an effect on cell cycle progression. Most DGK isotypes show high expression in the brain, often in distinct brain regions, suggesting that each individual isotype has a unique function.

Src-mediated activation of alpha-diacylglycerol kinase is required for hepatocyte growth factor-induced cell motility

Diacylglycerol kinases are involved in cell signaling, either as regulators of diacylglycerol levels or as intracellular signal-generating enzymes. However, neither their role in signal transduction nor their biochemical regulation has been elucidated. Hepatocyte growth factor (HGF), upon binding to its tyrosine kinase receptor, activates multiple signaling pathways stimulating cell motility, scattering, proliferation and branching morphogenesis. Herein we demonstrate that: (i) the enzymatic activity of alpha-diacylglycerol kinase (alphaDgk) is stimulated by HGF in epithelial, endothelial and alphaDgk-transfected COS cells; (ii) cellular expression of an alphaDgk kinase-defective mutant inhibits activation of endogenous alphaDgk acting as dominant negative; (iii) specific inhibition of alphaDgk prevents HGF-induced cell movement of endothelial cells; (iv) HGF induces the association of alphaDgk in a complex with Src, whose tyrosine kinase activity is required for alphaDgk activation by HGF; (v) Src wild type stimulates alphaDgk activity in vitro; and (vi) alphaDgk can be tyrosine phosphorylated in intact cells.

Phospholipase D2 activity suppresses hydrogen peroxide-induced apoptosis in PC12 cells

Phospholipase D (PLD) plays an important role as an effector in the membrane lipid-mediated signal transduction. However, the precise physiological functions of PLD are not yet well understood. In this study, we examined the role of PLD activity in hydrogen peroxide (H(2)O(2))-induced apoptosis in rat pheochromocytoma (PC12) cells. Treatment of PC12 cells with H(2)O(2) resulted in induction of apoptosis in these cells, which is accompanied by the activation of PLD. This H(2)O(2)-induced apoptosis was enhanced remarkably when phosphatidic acid production by PLD was selectively inhibited by pretreating the PC12 cells with 1-butanol. Expression of PLD2, but not of PLD1, correlated with increased H(2)O(2)-induced PLD activity in a concentration- and time-dependent manner. Concomitant with PLD activation, the PLD2 activity suppressed H(2)O(2)-induced apoptosis in PC12 cells. Expression of PLD2 lipase-inactive mutant (K758R) had no effect on either PLD activity or apoptosis. PLD2 activity also suppressed H(2)O(2)-induced cleavage and activation of caspase-3. Taken together, the results suggest that PLD2 activity is specifically up-regulated by H(2)O(2) in PC12 cells and that it plays a suppressive role in H(2)O(2)-induced apoptosis.

Identification and characterization of leukocyte-associated Ig-like receptor-1 as a major anchor protein of tyrosine phosphatase SHP-1 in hematopoietic cells

SHP-1, an SH2 domain-containing tyrosine phosphatase, has a crucial role in hematopoiesis. Here we report that SHP-1 is associated with two major tyrosine-phosphorylated proteins in hematopoietic cells treated with the tyrosine phosphatase inhibitor, pervanadate. One of the proteins corresponds to leukocyte-associated Ig-like receptor-1 (LAIR-1), a recently cloned transmembrane protein. Molecular cloning revealed four isoforms of the protein. LAIR-1 is hyper-phosphorylated on tyrosyl residues in cells overexpressing a catalytically inactive mutant form of SHP-1 as well as in pervanadate-treated cells. An antibody against the extracellular domain of the protein also induced its tyrosine phosphorylation. Tyrosine-phosphorylated LAIR-1 specifically interacts with SHP-1 but not with SHP-2, a structurally related tyrosine phosphatase. Using site-specific mutagenesis, we demonstrated that Tyr(233) and Tyr(263), each embedded in an immunoreceptor tyrosine-based inhibitory motif, are responsible for tyrosine phosphorylation of LAIR-1 and recruitment of SHP-1. Both tyrosyl residues are required for SHP-1 binding. Protein kinases responsible for tyrosine phosphorylation of LAIR-1 may belong to the Src family since PP1, a Src family kinase inhibitor, significantly inhibited its phosphorylation. As a major binding protein of SHP-1 on the plasma membrane, LAIR-1 may play an important role in hematopoietic cell signaling.

Endothelin-1 effect on tyrosine phosphorylation and on tyrosine phosphatase (PTP-1C) translocation in rabbit platelets

This study examined the temporal relationships of endothelin-1-stimulated rabbit platelets tyrosine phosphorylated proteins. The effect of endothelin-1 on tyrosine phosphorylation was dose- and time-dependent and caused a rapid tyrosine phosphorylation of three groups of proteins in the molecular mass range 70-100 kDa, 100-150 kDa and 150-200 kDa. Significant protein tyrosine phosphatase activity and amount were found to be associated with the cytoskeleton of endothelin-1-stimulated rabbit platelets. Under our experimental conditions, translocation from the cytosolic fraction to the cytoskeleton reached its highest levels within 10-20 sec of endothelin-1 stimulation. Endothelin-1-induced translocation of protein tyrosine phosphatase, associated with the increase in its activity was demonstrated by immunoblotting and immunoelectron microscopy.

Regulation of acidification and apoptosis by SHP-1 and Bcl-2

Recruitment of the SH2 domain containing cytoplasmic protein-tyrosine phosphatase SHP-1 to the membrane by somatostatin (SST) is an early event in its antiproliferative signaling that induces intracellular acidification-dependent apoptosis in breast cancer cells. Fas ligation also induces acidification-dependent apoptosis in a manner requiring the presence of SHP-1 at the membrane. Moreover, we have recently reported that SHP-1 is required not only for acidification, but also for apoptotic events that follow acidification (Thangaraju, M., Sharma, K., Liu, D., Shen, S. H., and Srikant, C. B. (1999) Cancer Res. 59, 1649-1654). Here we show that ectopically expressed SHP-1 was predominantly membrane-associated and amplified the cytotoxic signaling initiated upon SST receptor activation and Fas ligation. The catalytically inactive mutant of SHP-1 (SHP-1C455S) abolished the ability of the SST agonists to signal apoptosis by preventing the recruitment of wild type SHP-1 to the membrane. Overexpression of the anti-apoptotic protein Bcl-2 in MCF-7 cells inhibited SST-induced apoptosis upstream of acidification by inhibiting p53-dependent induction of Bax as well as by raising the resting pH(i) and attenuating SST-induced decrease in pH(i). By contrast, Bcl-2 failed to prevent apoptosis triggered by direct acidification. These data demonstrate that (i) membrane-associated SHP-1 is required for receptor-mediated cytotoxic signaling that causes intracellular acidification and apoptosis, and (ii) Bcl-2 acts distal to SHP-1 and p53 to prevent SST-induced acidification but cannot inhibit the apoptotic events that ensue intracellular acidification.

Differential phospholipase D activation by bradykinin and sphingosine 1-phosphate in NIH 3T3 fibroblasts overexpressing gelsolin

Gelsolin, an actin-binding protein, shows a strong ability to bind to phosphatidylinositol 4,5-bisphosphate (PIP(2)). Here we showed in in vitro experiments that gelsolin inhibited recombinant phospholipase D1 (PLD1) and PLD2 activities but not the oleate-dependent PLD and that this inhibition was not reversed by increasing PIP(2) concentration. To investigate the role of gelsolin in agonist-mediated PLD activation, we used NIH 3T3 fibroblasts stably transfected with the cDNA for human cytosolic gelsolin. Gelsolin overexpression suppressed bradykinin-induced activation of phospholipase C (PLC) and PLD. On the other hand, sphingosine 1-phosphate (S1P)-induced PLD activation could not be modified by gelsolin overexpression, whereas PLC activation was suppressed. PLD activation by phorbol myristate acetate or Ca(2+) ionophore A23187 was not affected by gelsolin overexpression. Stimulation of control cells with either bradykinin or S1P caused translocation of protein kinase C (PKC) to the membranes. Translocation of PKC-alpha and PKC-beta1 but not PKC-epsilon was reduced in gelsolin-overexpressed cells, whereas phosphorylation of mitogen-activated protein kinase was not changed. S1P-induced PLC activation and mitogen-activated protein kinase phosphorylation were sensitive to pertussis toxin, but PLD response was insensitive to such treatment, suggesting that S1P induced PLD activation via certain G protein distinct from G(i) for PLC and mitogen-activated protein kinase pathway. Our results suggest that gelsolin modulates bradykinin-mediated PLD activation via suppression of PLC and PKC activities but did not affect S1P-mediated PLD activation.

Activation of phosphotyrosine phosphatase activity attenuates mitogen-activated protein kinase signaling and inhibits c-FOS and nitric oxide synthase expression in macrophages infected with Leishmania donovani

Intracellular protozoan parasites of the genus Leishmania antagonize host defense mechanisms by interfering with cell signaling in macrophages. In this report, the impact of Leishmania donovani on mitogen-activated protein (MAP) kinases and nitric oxide synthase (NOS) expression in the macrophage cell line RAW 264 was investigated. Overnight infection of cells with leishmania led to a significant decrease in phorbol-12-myristate-13-acetate (PMA)-stimulated MAP kinase activity and inhibited PMA-induced phosphorylation of the MAP kinase substrate and transcription factor Elk-1. Simultaneously, leishmania infection markedly attenuated the induction of c-FOS and inducible nitric oxide synthase (iNOS) expression in response to PMA and gamma interferon (IFN-gamma), respectively. These effects correlated with decreased phosphorylation of p44 and p42 MAP kinases on tyrosine residues. Consistent with the latter finding, lysates prepared from leishmania-infected cells contained an activity that dephosphorylated MAP kinase in vitro, suggesting the possibility of a phosphatase acting in vivo. Attenuation of both MAP kinase activity and c-FOS and iNOS expression was reversed by treatment of macrophages with sodium orthovanadate prior to infection. It was also found that the specific activity of the Src homology 2 domain containing tyrosine phosphatase (SHP-1) toward MAP kinase was markedly increased in leishmania-infected cells. These findings indicate that infection with L. donovani attenuates MAP kinase signaling and c-FOS and iNOS expression in macrophages by activating cellular phosphotyrosine phosphatases. This may represent a novel mechanism of macrophage deactivation during intracellular infection.

Regulation of phospholipase D by phosphorylation-dependent mechanisms

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

Regulation and function of PLDs in yeast

While yeast contain multiple phospholipase D activities, only one, encoded by SPO14, appears to be a member of the phosphatidylcholine-specific phospholipase D gene family. Genetic analyses have revealed a role for this enzyme in regulated membrane trafficking events.

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.

Amphiphilic and hydrophilic nature of sheep and human platelet phosphotyrosine phosphatase forms

To date, although at least 75 different PTPases (protein-tyrosine-phosphate-phosphohydrolase, EC 3.1.3.48) have been identified, those detected in platelets are rather scarce. Based on previous results from our laboratory, we investigated the existence of new PTPases in platelets. Triton X-114 phase partitioning of Triton X-100-solubilized human and sheep platelet membranes allowed PTPase to be recovered in the detergent-rich (40-35%, respectively) and -poor phases (60-65%, respectively). Sedimentation analyses of both phases from the sheep species revealed hydrophilic 6S and 3.7S, and amphiphilic 7.5S and 10.3S PTPase forms. Sedimentation analyses of human platelet membrane-associated or cytosolic PTPase revealed hydrophilic 6.7S and 4.3S, and amphiphilic 5.5S and 10.8S forms, or hydrophilic 4S, 5.9S and 6.9S forms, respectively. Western blot analysis using monoclonal antibodies (MoAb) against human PTP1B, PTP1C, PTP1D and RPTPalpha (mouse anti-human PTPase MoAbs) showed that RPTPalpha was not present in platelets and that the PTP1C type and PTP1D type (but probably not the PTP1B type) were expressed in sheep species. Immunoblots also revealed that all PTPases detected were mainly membrane-associated, with similar percentages of cellular distribution in both species. All PTPases were mainly recovered in the detergent-poor phases from the Triton X-114 phase partitioning, although PTP1D from human species was also significantly present (30%) in the detergent-rich phase. Additionally, all PTPases sedimented within the same PTPase peak in sucrose gradients (sedimentation coefficients around 4S). These findings indicate that amphiphilic and hydrophilic PTPases different from PTP1B, PTP1C, PTP1D or RPTPalpha, with higher sedimentation coefficients and with higher activity when O-phosphotyrosine or a synthetic peptide phosphorylated on tyrosine were used as substrates, are present in platelets.

Tyrosine phosphatase SHP-2 dephosphorylates the platelet-derived growth factor receptor but enhances its downstream signalling

SHP-2 is a widely distributed Src homology 2 (SH2) domain-containing tyrosine phosphatase that is recruited to growth factor receptors on stimulation. We have transiently co-expressed several catalytically active and inactive forms of the enzyme with the platelet-derived growth factor (PDGF) receptor in human embryonic kidney 293 cells. The catalytically active forms of SHP-2 decreased the tyrosine phosphorylation of the receptor, whereas the catalytically inactive forms increased the phosphorylation. However, PDGF-induced activation of the mitogen-activated protein (MAP) kinase pathway was enhanced by the active forms of SHP-2 but decreased by the inactive forms. The results suggest that the PDGF receptor is a physiological substrate of SHP-2 and that SHP-2 has a positive role in the PDGF-stimulated activation of MAP kinase. The dissociation of the receptor phosphorylation from the activation of MAP kinase suggests that signalling through growth factor receptors does not depend merely on their tyrosine phosphorylation.

Phospholipase D and its product, phosphatidic acid, mediate agonist-dependent raf-1 translocation to the plasma membrane and the activation of the mitogen-activated protein kinase pathway

The primary known function of phospholipase D (PLD) is to generate phosphatidic acid (PA) via the hydrolysis of phosphatidylcholine. However, the functional role of PA is not well understood. We report here evidence that links the activation of PLD by insulin and the subsequent generation of PA to the activation of the Raf-1-mitogen-activated protein kinase (MAPK) cascade. Brefeldin A (BFA), an inhibitor of the activation of ADP-ribosylation factor proteins, inhibited insulin-dependent production of PA and MAPK phosphorylation. The addition of PA reversed the inhibition of MAPK activation by BFA. Overexpression of a catalytically inactive variant of PLD2, but not PLD1, blocked insulin-dependent activation of PLD and phosphorylation of MAPK. Real time imaging analysis showed that insulin induced Raf-1 translocation to cell membranes by a process that was inhibited by BFA. PA addition reversed the effects of BFA on Raf-1 translocation. However, PA did not activate Raf-1 in vitro or in vivo, suggesting that the primary function of PA is to enhance the recruitment of Raf-1 to the plasma membrane where other factors may activate it. Finally, we found that the recruitment of Raf-1 to the plasma membrane was transient, but Raf-1 remained bound to endocytic vesicles.

Cell signaling by protein tyrosine phosphorylation

The above data, and others not described herein, indicate the following: First, that phosphatases are not scavenger enzymes, simply there to remove the phosphate groups introduced by the kinases. They cannot be viewed simply as providing an 'off' switch in an 'on/off' kinase/phosphatase system. Kinases and phosphatases do not carry out one-way and opposing reactions. The same enzyme, depending on where it localizes within the cell, or the molecule with which it might interact, can serve either as a positive or negative determinant in defining cell behavior. In many instances, it can act synergistically with the kinases to enhance the phosphorylation reaction. Second, the factors that determine whether a phosphatase would enhance or oppose a kinase reaction would seem to depend less on its state of activity than on its subcellular localization. This would suggest that if one wanted to call upon it to control transformation, one should try to tamper with its localization segments or whatever binding proteins it might be attached to--rather than with its catalytic domains. Displacement of these enzymes from where they are meant to bind would seem a more promising approach than trying to modulate their catalytic activity. Finally, their architectural features are so basically different from those of the kinases, with receptor tyrosine phosphatases displaying all the structural characteristics of cell adhesion molecules, that they must also have a mission of their own in cell development, survival and death, quite apart from that of the kinases.