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

Role and therapeutic potential of gelsolin in atherosclerosis

Atherosclerosis is the major pathophysiological basis of a variety of cardiovascular diseases and has been recognized as a lipid-driven chronic inflammatory disease. Gelsolin (GSN) is a member of the GSN family. The main function of GSN is to cut and seal actin filaments to regulate the cytoskeleton and participate in a variety of biological functions, such as cell movement, morphological changes, metabolism, apoptosis and phagocytosis. Recently, more and more evidences have demonstrated that GSN is Closely related to atherosclerosis, involving lipid metabolism, inflammation, cell proliferation, migration and thrombosis. This article reviews the role of GSN in atherosclerosis from inflammation, apoptosis, angiogenesis and thrombosis.

Structure and regulation of human phospholipase D

Mammalian phospholipase D (PLD) generates phosphatidic acid, a dynamic lipid secondary messenger involved with a broad spectrum of cellular functions including but not limited to metabolism, migration, and exocytosis. As a promising pharmaceutical target, the biochemical properties of PLD have been well characterized. This has led to the recent crystal structures of human PLD1 and PLD2, the development of PLD specific pharmacological inhibitors, and the identification of cellular regulators of PLD. In this review, we discuss the PLD1 and PLD2 structures, PLD inhibition by small molecules, and the regulation of PLD activity by effector proteins and lipids.

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Expression of endogenous phospholipase D1, localized in mouse submandibular gland, is greater in females and is suppressed by testosterone

To clarify the signal transduction mechanism in the differentiation and secretion of salivary glandular cells, the present study was attempted to examine in the submandibular gland (SMG) of mice, the expression and localization of phospholipase D1 (PLD1), one of the important effector molecules working in response to the activation of intramembranous receptors by first messengers. In immunoblotting analysis, the expression of PLD1 was high at postnatal 4 weeks (P4W) and decreased at P8W, and it was at negligible levels at newborn stage (P0W) and postnatal 2 weeks (P2W). The expression of PLD1 was greater in females, and it was suppressed by administration of testosterone to female mice. In immuno-light microscopy, immunoreactivity for PLD1 at P4W was moderate to intense, in the forms of dots and globules mainly in the apical domains of immature granular convoluted tubule (GCT)-cells localized largely in the proximal portion of the female GCT. By P8W, it decreased in intensity and remained weak to moderate along the apical plasmalemma of cells throughout the course of the female GCT, whereas it was faint throughout the GCT of the male SMG at P4W and negligible at P8W. In immuno-electron microscopy, immature GCT-cells characterized by electron-lucent granules were immunoreactive and the immunoreactive materials were deposited close to, but not within, those granules. Typical GCT cells, characterized by electron-dense granules, were immunonegative. No significant immunoreaction for PLD1 was seen in acini of SMGs of either sex at any time point examined. It is suggested that PLD1 is involved in the signaling for secretion of immature GCT cells and influences differentiation of these cells, probably through their own secretory substances.

Targeting sphingosine-1-phosphate signaling in lung diseases

Sphingosine-1-phosphate (S1P), a simple, bioactive sphingolipid metabolite, plays a key role, both intracellularly and extracellularly, in various cellular processes such as proliferation, survival, migration, inflammation, angiogenesis, and endothelial barrier integrity. The cellular S1P level is low and is tightly regulated by its synthesis and degradation. Sphingosine Kinases (SphKs) 1 and 2, catalyze the ATP-dependent phosphorylation of sphingosine to S1P, while the degradation is mediated by the reversible dephosphorylation catalyzed by the S1P phosphatases and lipid phosphate phosphatases and the irreversible degradation to hexadecenal and ethanolamine phosphate by sphingosine-1-phosphate lyase (S1PL). As a ligand for specific G-protein-coupled receptors, S1P1-5, which are differentially expressed in different cell types, S1P generates downstream signals that play crucial role in developmental and disease related pathologies. In addition to acting extracellularly on receptors located on the plasma membrane, S1P can also act intracellularly, independently of S1P1-5, affecting calcium homeostasis and cell proliferation. The SphKs /S1P /S1PL metabolic pathway is implicated in numerous human pathologies including respiratory disorders, thereby raising the possibility that manipulating intracellular S1P levels could offer therapeutic potential in ameliorating lung diseases. This review focuses on the prospects of targeting S1P signaling and S1P metabolizing enzymes using small molecule inhibitors, receptor agonists, and antagonists in the treatment of lung diseases.

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.

Gelsolin induces promonocytic leukemia differentiation accompanied by upregulation of p21CIP1

Tumor suppressor genes have received much attention for their roles in the development of human malignancies. Gelsolin has been found to be down-regulated in several types of human cancers, including leukemias. It is, however, expressed in macrophages, which are the final differentiation derivatives for the monocytic myeloid lineage, implicating this protein in the differentiation process of such cells. In order to investigate the role of gelsolin in leukaemic cell differentiation, stable clones over-expressing ectopic gelsolin, and a control clone were established from U937 leukaemia cells. Unlike the control cells, both gelsolin-overexpressing clones displayed retarded growth, improved monocytic morphology, increased NADPH and NSE activities, and enhanced surface expression of the β-integrin receptor, CD11b, when compared with the parental U937 cells. Interestingly, RT- PCR and western blot analysis also revealed that gelsolin enhanced p21CIP1 mRNA and protein expression in the overexpressing clones. Moreover, transient transfection with siRNA silencing P21CIP1, but not the control siRNA, resulted in a reduction in monocytic differentiation, accompanied by an increase in proliferation. In conclusion, our work demonstrates that gelsolin, by itself, is capable of inducing monocytic differentiation in U937 leukaemia cells, most probably through p21CIP1 activation.

Phospholipase D family member 4, a transmembrane glycoprotein with no phospholipase D activity, expression in spleen and early postnatal microglia

BACKGROUND

Phospholipase D (PLD) catalyzes conversion of phosphatidylcholine into choline and phosphatidic acid, leading to a variety of intracellular signal transduction events. Two classical PLDs, PLD1 and PLD2, contain phosphatidylinositide-binding PX and PH domains and two conserved His-x-Lys-(x)(4)-Asp (HKD) motifs, which are critical for PLD activity. PLD4 officially belongs to the PLD family, because it possesses two HKD motifs. However, it lacks PX and PH domains and has a putative transmembrane domain instead. Nevertheless, little is known regarding expression, structure, and function of PLD4.

METHODOLOGY/PRINCIPAL FINDINGS

PLD4 was analyzed in terms of expression, structure, and function. Expression was analyzed in developing mouse brains and non-neuronal tissues using microarray, in situ hybridization, immunohistochemistry, and immunocytochemistry. Structure was evaluated using bioinformatics analysis of protein domains, biochemical analyses of transmembrane property, and enzymatic deglycosylation. PLD activity was examined by choline release and transphosphatidylation assays. Results demonstrated low to modest, but characteristic, PLD4 mRNA expression in a subset of cells preferentially localized around white matter regions, including the corpus callosum and cerebellar white matter, during the first postnatal week. These PLD4 mRNA-expressing cells were identified as Iba1-positive microglia. In non-neuronal tissues, PLD4 mRNA expression was widespread, but predominantly distributed in the spleen. Intense PLD4 expression was detected around the marginal zone of the splenic red pulp, and splenic PLD4 protein recovered from subcellular membrane fractions was highly N-glycosylated. PLD4 was heterologously expressed in cell lines and localized in the endoplasmic reticulum and Golgi apparatus. Moreover, heterologously expressed PLD4 proteins did not exhibit PLD enzymatic activity.

CONCLUSIONS/SIGNIFICANCE

Results showed that PLD4 is a non-PLD, HKD motif-carrying, transmembrane glycoprotein localized in the endoplasmic reticulum and Golgi apparatus. The spatiotemporally restricted expression patterns suggested that PLD4 might play a role in common function(s) among microglia during early postnatal brain development and splenic marginal zone cells.

Sphingosine kinase as an oncogene: autocrine sphingosine 1-phosphate modulates ML-1 thyroid carcinoma cell migration by a mechanism dependent on protein kinase C-alpha and ERK1/2

Sphingosine 1-phosphate (S1P) induces migration of the human thyroid follicular carcinoma cell line ML-1 by activation of S1P(1) and S1P(3) receptors, G(i) proteins, and the phosphatidylinositol 3-kinase-Akt pathway. Because sphingosine kinase isoform 1 (SK) recently has been implicated as an oncogene in various cancer cell systems, we investigated the functions of SK in the migration, proliferation and adhesion of the ML-1 cell line. SK overexpressing ML-1 cells show an enhanced secretion of S1P, which can be attenuated, by inhibiting SK activity and a multidrug-resistant transport protein (ATP-binding cassette transporter). Furthermore, overexpression of SK enhances serum-induced migration of ML-1 cells, which can be attenuated by blocking ATP-binding cassette transporter and SK, suggesting that the migration is mediated by autocrine signaling through secretion of S1P. Inhibition of protein kinase C alpha, with both small interfering RNA (siRNA) and small molecular inhibitors attenuates migration in SK overexpressing cells. In addition, SK-overexpressing cells show an impaired adhesion, slower cell growth, and an up-regulation of ERK1/2 phosphorylation, as compared with cells expressing a dominant-negative SK. Taken together, we present evidence suggesting that SK enhances migration of ML-1 cells by an autocrine mechanism and that the S1P-evoked migration is dependent on protein kinase C alpha, ERK1/2, and SK.

Phospholipase D2 regulates endothelial permeability through cytoskeleton reorganization and occludin downregulation

Endothelial permeability is controlled by adhesive strengths which connect cells to each other through interendothelial junctions and by contractile forces associated with cytoskeleton reorganization. Phospholipase D (PLD) activation resulting in the generation of phosphatidic acid (PA) is increasingly recognized as a key event in the initiation of various cell responses. In human umbilical vein endothelial cells (HUV-EC), enhancement of intracellular PA by a variety of approaches increased the permeability of endothelial cell monolayers and induced stress fibre formation. Using adenovirus-mediated overexpression and siRNA silencing, we showed that PLD2 but not PLD1 was involved in the enhancement of basal permeability through cytoskeleton reorganization. Furthermore, PLD2 overexpression induced ERK1/2 activation and downregulated the expression of occludin, a major component of tight junctions. A substantial part of PLD2 protein was associated with the low-density caveolin-rich fractions isolated on sucrose gradients. The Raf-1 specific inhibitor GW-5074 drastically reduced hyperpermeability induced by PLD2 overexpression, and inhibited PA-mediated increase of endothelial permeability and ERK1/2 activation. On the whole, the present results demonstrate the selective role of PLD2 isoform in the control of endothelial permeability through a mechanism involving both stress fibre formation and contraction, and occludin downregulation, possibly resulting from PA-mediated activation of Raf-1.

Sphingosine-1-phosphate receptors: biology and therapeutic potential in kidney disease

The major sphingolipid metabolite, sphingosine-1-phosphate (S1P), has important biological functions. S1P is the ligand for a family of five G-protein-coupled receptors with distinct signaling pathways that regulate angiogenesis, vascular maturation, immunity, chemotaxis, and other important biological pathways. Recently, clinical trials have targeted S1P receptors (S1PRs) for autoimmune diseases and transplantation and have generated considerable interest in developing additional, more selective compounds. This review summarizes current knowledge on the biology of S1P and S1PRs that forms the basis for future drug development and the treatment of kidney disease.

Catalytic inactivation of human phospholipase D2 by a naturally occurring Gly901Asp mutation

BACKGROUND

We previously showed that the 1814C-->T (Thr577Ile) polymorphism of the human phospholipase D2 (PLD2) gene is associated with the prevalence of colorectal cancer, with the T allele representing a risk factor for this condition. However, we failed to detect a difference in PLD activity of cell lysates or membrane fractions between cells transfected with cDNAs encoding the Thr577 or Ile577 variants of PLD2. In the present study, we have examined the possible functional relevance of other naturally occurring polymorphisms (or mutations) of the human PLD2 gene that result in amino acid substitutions.

METHODS

Human embryonic kidney cells were transfected with expression vectors for each PLD2 variant and assayed for enzyme activity in vitro and in vivo.

RESULTS AND CONCLUSIONS

The G-->A (Gly901Asp) mutation of the human PLD2 gene was found to result in catalytic inactivation of the encoded protein.

Ewing's sarcoma fusion protein, EWS/Fli-1 and Fli-1 protein induce PLD2 but not PLD1 gene expression by binding to an ETS domain of 5' promoter

It was reported that short interfering RNA (siRNA) of EWS/Fli-1 downregulated phospholipase D (PLD)2 in Ewing's sarcoma (EWS) cell line, suggesting that PLD2 is the target of aberrant transcription factor, EWS/Fli-1. Here, we further investigated the regulation of PLD2 gene expression by EWS/Fli-1 and Fli-1 in another EWS cell line, and also in EWS/Fli-1- or Fli-1-transfected cell line. EWS/Fli-1- or Fli-1-overexpressed cells showed higher PLD2 but not PLD1 protein expression and enhanced cell proliferation as compared to mock transfectant. The treatment of these cells with 1-butanol or siRNA of PLD2 inhibited cell growth, suggesting the pivotal role of PLD in cell growth promotion. PLD2 but not PLD1 mRNA level was also increased in EWS/Fli-1 or Fli-1-transfectants. After determining the transcription initiation points, we cloned the 5' promoter of both PLD1 and PLD2 and analysed promoter activities. Results showed that EWS/Fli-1 and Fli-1 increase PLD2 gene expression by binding to an erythroblast transformation-specific domain (-126 to -120 bp from the transcription initiation site) of PLD2 promoter, which is the minimal and most powerful region. Electrophoresis mobility shift assay using truncated proteins showed that both DNA-binding domain and trans-activating domain were necessary for the enhanced gene expression of PLD2.

Involvement of phospholipase D1 in collagen type I production of human dermal fibroblasts

In the current study, the involvement of phospholipase D (PLD) in the regulation of collagen type I production was examined using human dermal fibroblasts. Procollagen I production in the cells overexpressing PLD1, but not PLD2, was found to be increased compared with those in the vector control cells. To investigate the role of PLD1, we examined the effect of knockdown of endogenous PLD1 by small interference RNA (siRNA) on collagen production. The reduction of expression levels of PLD1 by siRNA transfection was accompanied by diminution of procollagen biosynthesis and also ribosomal S6 kinase 1 (S6K1) phosphorylation. The activity of mammalian target of rapamycin (mTOR) is essential for phosphorylation of S6K1 and the treatment of dermal fibroblasts with rapamycin, a potent inhibitor of mTOR abolished procollagen I production. These results suggest that PLD1 plays a crucial role in collagen type I production through mTOR signaling in human dermal fibroblast.

Involvement of calpain in melanogenesis of mouse B16 melanoma cells

In the current study, the involvement of calpain, a cysteine proteinase in the regulation of melanogenesis was examined using mouse B16 melanoma cells. In response to alpha-melanocyte-stimulating hormone (a-MSH), B16 melanoma cells underwent differentiation characterized by increased melanin biosynthesis. The total calapain activity was decreased within 2 h following alpha-MSH-treatment, and restored to the initial level in 6-12 h. To further investigate the involvement of calpain in the regulation of melanogenesis, the effect of calpain inhibitors on alpha-MSH-induced melanogenesis was examined. Inhibition of calpain by either N-acetyl-Leu-Leu-norleucinal (ALLN) or calpastatin (CS) peptide blocked alpha-MSH-induced melanogenesis. The magnitude of inhibition of melanin biosynthesis was well correlated with a decrease in the activity of tyrosinase, a key regulatory enzyme in melanogenesis. Treatment of B16 cells with ALLN caused marked decrease in both tyrosinase protein and mRNA levels. These results indicate that calpain would be involved in the melanogenic signaling by modulating the expression of tyrosinase in mouse B16 melanoma cells.

Negative regulation of melanogenesis by phospholipase D1 through mTOR/p70 S6 kinase 1 signaling in mouse B16 melanoma cells

Melanogenesis is a principal parameter of differentiation in melanocytes and melanoma cells. Our recent study has demonstrated that phospholipase D1 (PLD1) regulates the melanogenic signaling through modulating the expression of tyrosinase, the rate-limiting step enzyme in the melanin biosynthesis. The current study was designed to gain more insight into the involvement of PLD1 in the regulation of melanogenesis. To investigate the role of PLD1, we examined the effect of knockdown of endogenous PLD1 by small interference RNA (siRNA) on melanogenesis in B16 melanoma cells. It was shown that the melanin synthesis was induced in PLD1-knockdowned cells, and also that the level of melanin synthesis was well correlated with increases in expression level of tyrosinase and its related proteins (Tyrp1 and Dct). Furthermore, the reduction of expression levels of PLD1 by siRNA transfection was accompanied by diminution of ribosomal S6 kinase 1 (S6K1) phosphorylation. The activity of mammalian target of rapamycin (mTOR) is essential for phosphorylation of S6K1 and the treatment malanoma cells with rapamycin, a potent inhibitor of mTOR effectively induced melanogenesis. The results obtained here provide possible evidence that PLD1 exerts a negative regulatory role in the melanogenic process through mTOR/S6K1 signaling.

Mechanism of action of sphingolipids and their metabolites in the toxicity of fumonisin B1

Fumonisins are a group of mycotoxins produced primarily by Fusarium moniliforme. Several fumonisins have been isolated through out the years but only fumonisin B1, B2 and B3 are the ones present in naturally contaminated foods, with B1 being the most toxic between them. The structural similarity between sphinganine and fumonisin B1 suggests that the mechanism of action of this mycotoxin is mainly via disruption of sphingolipid metabolism, this is an important step in the cascade of events leading to altered cell growth, differentiation and cell injury. Sphingolipids are a second type of lipid found in cell membranes, particularly nerve cells and brain tissues. Toxicity of fumonisin B1 is given via inhibition of ceramide synthase that catalyzes the formation of dihydroceramide from sphingosine. This mechanism of action may explain the wide variety of health effects observed when this mycotoxin is ingested like high rate of human oesophageal cancer and promotion of primary liver cancer.

Arf1-dependent PLD1 is localized to oleic acid-induced lipid droplets in NIH3T3 cells

Phospholipase D (PLD) is known to play a role in vesicle transport through the hydrolysis of phosphatidylcholine (PC) to produce the bioactive lipid, phosphatidic acid. Lipid droplets (LDs) are surrounded by a monolayer of phospholipids, including PC and its lyso derivative, and exhibit a number of signaling proteins. Our recent report suggests that the association of adipose differentiation-related protein (ADRP) to LDs is regulated by an ADP-ribosylation factor 1 (Arf1)-dependent mechanism. In the present study, we found an increase in PLD activity accompanied with LD formation in oleic acid-treated NIH3T3 cells. Brefeldin A, an inhibitor of ARF-GEFs, suppressed both PLD activation and LD formation in oleic acid-treated cells. PLD1, but not PLD2, was found to exist in LDs by immunocytochemical analysis. Furthermore, co-existence of PLD1, Arf1, and ADRP was observed in the LD-enriched subcellular fractions obtained from oleic acid-treated NIH3T3 cells by Western blot analysis. PLD1 activity in the LD-enriched fractions was stimulated by exogenously added Arf1. Although LDs were induced in either PLD1- or PLD2-overexpressing CHO cells by oleic acid treatment, the stimulation of PLD activity was observed only in PLD1-CHO cells. Taken together, the data suggest that the activation of Arf1-dependent PLD1 occurs in LDs and may be involved in their physiological function.

Inhibition of platelet-derived growth factor-induced cell growth signaling by a short interfering RNA for EWS-Fli1 via down-regulation of phospholipase D2 in Ewing sarcoma cells

EWS-Fli1, a fusion gene resulting from a chromosomal translocation t(11;22, q24;q12) and found in Ewing sarcoma and primitive neuroectodermal tumors, encodes a transcriptional activator and promotes cellular transformation. However, the precise biological functions of its products remain unknown. To investigate the role of EWS-Fli1 in cell growth signaling, we transfected Ewing sarcoma TC-135 cells with short interfering RNAs for EWS-Fli1. EWS-Fli1 knockdown reduced cell growth and platelet-derived growth factor (PDGF)-BB-induced activation of the growth signaling enzymes. Interestingly, phospholipase D2 (but not the PDGF-BB receptor) showed marked down-regulation in the EWS-Fli1-knocked down TC-135 cells compared with the control cells. In Ewing sarcoma TC-135 cells, the PDGF-BB-induced phosphorylation of growth signaling involving extracellular signal-regulated kinase, Akt, p70S6K, and the expression of cyclin D3 were markedly inhibited by transfection with short interfering RNA phospholipase (PL)-D2. The PDGF-BB-induced activation of growth signaling was also suppressed by 1-butanol, which prevents the production of phosphatidic acid by phospholipase D (but not by t-butyl alcohol), thereby implicating PLD2 in PDGF-BB-mediated signaling in TC-135 cells. These results suggest that EWS-Fli1 may play a role in the regulation of tumor proliferation-signaling enzymes via PLD2 expression in Ewing sarcoma cells.

Acidic extracellular pH induces matrix metalloproteinase-9 expression in mouse metastatic melanoma cells through the phospholipase D-mitogen-activated protein kinase signaling

The extracellular pH (pHe) of tumor tissues is often acidic, which can induce the expression of several proteins. We previously showed that production of matrix metalloproteinase-9 (MMP-9) was induced by culturing cells at acidic pHe (5.4-6.5). Here we have investigated the signal transduction pathway by which acidic pHe induces MMP-9 expression. We found that acidic pHe (5.9) activated phospholipase D (PLD), and inhibition of PLD activity by 1-butanol and Myr-ARF6 suppressed the acidic pHe-induced MMP-9 expression. Exogenous PLD, but not phosphatidylinositol-specific PLC or PLA2, mimicked MMP-9 induction by acidic pHe. Western blot analysis revealed that acidic pHe increased the steady-state levels of phosphorylated extracellular signal-regulated kinases 1/2 and p38 and that the PLD inhibitors suppressed these increases. Using 5'-deletion mutant constructs of the MMP-9 promoter, we found that the acidic pHe-responsive region was located at nucleotide -670 to -531, a region containing the NF kappa B binding site. A mutation into the NF kappa B binding site reduced, but not completely, the acidic pHe-induced MMP-9 promoter activity, and NF kappa B activity was induced by acidic pHe. Pharmacological inhibitors specific for mitogen-activated protein kinase kinase 1/2 (PD098059) and p38 (SB203580) attenuated the acidic pHe-induced NF kappa B activity and MMP-9 expression. These data suggest that PLD, mitogen-activated protein kinases (extracellular signal-regulated kinases 1/2 and p38), and NF kappa B mediate the acidic pHe signaling to induce MMP-9 expression. A transcription factor(s) other than NF kappa B may also be involved in the MMP-9 expression.

Sphingosine kinase 1 is involved in dibutyryl cyclic AMP-induced granulocytic differentiation through the upregulation of extracellular signal-regulated kinase, but not p38 MAP kinase, in HL60 cells

The role of sphingosine kinase (SPHK) in the dibutyryl cyclic AMP (dbcAMP)-induced granulocytic differentiation of HL60 cells was investigated. During differentiation, SPHK activity was increased, as were mRNA and protein levels of SPHK1, but not of SPHK2. Pretreatment of HL60 cells with N,N-dimethylsphingosine (DMS), a potent SPHK inhibitor, completely blocked dbcAMP-induced differentiation. The phosphorylation of mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase 1/2 (ERK1/2), and p38 MAPK was also increased during dbcAMP-induced differentiation. Pretreatment of HL60 cells with the MEK inhibitor, U0126, but not the p38 MAPK inhibitor, SB203580, completely suppressed dbcAMP-induced ERK1/2 activation and granulocytic differentiation, but did not affect the increase in SPHK activity. DMS inhibited dbcAMP-induced ERK1/2 activation, but had little effect on p38 MAPK activation. DMS had no effect on the dbcAMP-induced membrane translocation of protein kinase C (PKC) isozymes, and PKC inhibitors had no significant effect on ERK activation. The overexpression of wild-type SPHK1, but not dominant negative SPHK1, resulted in high basal levels of ERK1/2 phosphorylation and stimulated granulocytic differentiation in HL60 cells. These data show that SPHK1 participates in the dbcAMP-induced differentiation of HL60 cells by activating the MEK/ERK pathway.

Phospholipase D

Phospholipase D catalyses the hydrolysis of the phosphodiester bond of glycerophospholipids to generate phosphatidic acid and a free headgroup. Phospholipase D activities have been detected in simple to complex organisms from viruses and bacteria to yeast, plants, and mammals. Although enzymes with broader selectivity are found in some of the lower organisms, the plant, yeast, and mammalian enzymes are selective for phosphatidylcholine. The two mammalian phospholipase D isoforms are regulated by protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families. Mammalian and yeast phospholipases D are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. This review discusses the identification, characterization, structure, and regulation of phospholipase D. Genetic and pharmacological approaches implicate phospholipase D in a diverse range of cellular processes that include receptor signaling, control of intracellular membrane transport, and reorganization of the actin cytoskeleton. Most ideas about phospholipase D function consider that the phosphatidic acid product is an intracellular lipid messenger. Candidate targets for phospholipase-D-generated phosphatidic acid include phosphatidylinositol 4-phosphate 5-kinases and the raf protein kinase. Phosphatidic acid can also be converted to two other lipid mediators, diacylglycerol and lyso phosphatidic acid. Coordinated activation of these phospholipase-D-dependent pathways likely accounts for the pleitropic roles for these enzymes in many aspects of cell regulation.

High dose of glucose promotes chondrogenesis via PKC? and MAPK signaling pathways in chick mesenchymal cells

We investigated the molecular mechanism of the glucose effect on the regulation of chondrogenesis. Exposure of chick wing bud mesenchymal cells to high concentrations of glucose stimulated chondrogenesis 2-fold to 2.5-fold without affecting cell proliferation. Glucose increased protein levels and the membrane translocation of protein kinase C alpha (PKCalpha), leading to a reduction of extracellular signal-regulated kinase (ERK) phosphorylation. Phosphorylation of p38 was also increased in a PKC-independent manner by glucose treatment. Glucose also increased cell adhesion molecules such as fibronectin, integrin beta1, and N-cadherin at early stages and then decreased these adhesion molecules at later stages of chondrogenesis. These alterations in protein level of adhesion molecules and in the phosphorylation of mitogen-activated protein kinases by glucose were blocked by inhibition of PKC or p38 but were synergistically increased by the inhibition of ERK. Therefore, high doses of glucose induce the down-regulation of ERK activity via PKCalpha and the up-regulation of p38 and result in the stimulation of chondrogenesis of chick mesenchymal cells through modulating the expression of adhesion molecules.

Overexpression of phospholipase D prevents actinomycin D-induced apoptosis through potentiation of phosphoinositide 3-kinase signalling pathways in Chinese-hamster ovary cells

To examine the roles of PLD (phospholipase D) in the regulation of the apoptotic process, PLD1 and PLD2 were stably overexpressed in S1P3-CHO cells [CHO (Chinese-hamster ovary) cells expressing the S1P (sphingosine 1-phosphate) receptor S1P3]. Treatment of S1P3-CHO cells with ActD (actinomycin D) induced apoptosis, as shown by the occurrence of nuclear fragmentation and the caspase-dependent proteolytic cleavage of PARP [poly(ADP-ribose) polymerase] and protein kinase Cd. Overexpression of either PLD1 or PLD2 protected S1P3-CHO cells from ActD-induced apoptosis, as demonstrated by an increased number of viable cells and inhibition of PARP and protein kinase Cd cleavage. However, in the early phase of apoptosis, ActD induced an increase in PLD activity and activation of key factors in the cell-survival signalling pathways, such as PI3K (phosphoinositide 3-kinase), Akt, p70S6K (p70 S6 kinase) and ERK (extracellular-signal-regulated kinase). Furthermore, the ActD-induced activation of these survival signalling enzymes was potentiated by overexpression of either PLD1 or PLD2. The PI3K inhibitor LY294002 inhibited the ActD-induced activation of Akt and p70S6K, and completely abolished the effects of PLD1 or PLD2, whereas inhibition of ERK activity by the MEK inhibitor U0126 had a milder effect. The ActD-induced activation of p70S6K and ERKs was blocked by 1-butanol, but not by t-butanol; similar to S1P, exogenous PLD suppressed the ActD-induced events in the apoptosis signalling pathways. These results show that, in S1P3-CHO cells, increased expression of PLDs prevents ActD-induced apoptosis by enhanced activation of the PI3K signalling pathways.

Selective and transient activation of protein kinase C alpha by fumonisin B1, a ceramide synthase inhibitor mycotoxin, in cultured porcine renal cells

Fumonisin B(1) (FB(1)), a potent and naturally occurring mycotoxin produced by the fungus Fusarium verticillioides, has been implicated in fatal and debilitating diseases in animals and humans. FB(1) affects a variety of cell signaling proteins including protein kinase C (PKC); a serine/threonine kinase, involved in a number of signal transduction pathways that include cytokine induction, carcinogenesis and apoptosis. The aim of this study was to investigate the short-term temporal and concentration-dependent effects of FB(1) on PKC isoforms present in LLC-PK(1) cells in relation to the FB(1)-induced accumulation of sphinganine and sphingosine utilizing various inhibitors and activators. Our studies demonstrated that FB(1) (0.1-1 microM) selectively and transiently activated PKCalpha at 5 min, without affecting PKC-delta, -epsilon and -zeta isoforms. At higher FB(1) concentrations and later time points (15-120 min), PKCalpha membrane concentrations declined to untreated levels. The observed increase in cytosol PKCalpha protein expression at 15 min was not associated with an increase in its activity or protein biosynthesis. Calphostin C, a PKC inhibitor, abrogated the FB(1)-induced translocation of PKCalpha. Pre-incubation with the PKC activator, phorbol 12-myristate 13-acetate, resulted in an additive effect on membrane translocation of PKCalpha. Intracellular sphinganine and sphingosine concentrations were unaltered at the time points tested. Myriocin, a specific inhibitor of serine palmitoyltransferase, the first enzyme in de novo sphingolipid biosynthesis, did not prevent the FB(1)-induced PKCalpha cytosol to membrane redistribution. Altering PKCalpha and its signal transduction pathways may be of importance in the ability of FB(1) to exert its toxicity via apoptosis and/or carcinogenesis.

S1P 3 ‐mediated Akt activation and crosstalk with platelet‐derived growth factor receptor (PDGFR)

Akt plays a pivotal role in cell survival and tumorigenesis. We investigated the potential interaction between sphingosine-1-phosphate (S1P) and platelet-derived growth factor (PDGF) in the Akt signaling pathway. Using mouse embryonic fibroblasts (MEFs) from S1P receptor knockout mice, we show here that S1P3 was required for S473 phosphorylation of Akt by S1P. In addition, S1P-stimulated activation of Akt, but not ERK, was blocked by a PDGF receptor (PDGFR)-specific inhibitor, AG1296, suggesting a S1P3-mediated specific crosstalk between the Akt signaling pathways of S1P and PDGFR in MEFs. We investigated this crosstalk under different conditions and found that both Akt and ERK activation induced by S1P, but not lysophosphatidic acid (LPA), in HEY ovarian cancer cells required PDGFR but not epidermal growth factor receptor (EGFR) or insulin-like growth factor-I receptor (IGFR). Importantly, S1P induced a Gi-dependent tyrosine phosphorylation of PDGFR in HEY cells. This dependence on PDGFR in S1P-induced Akt activation was also observed in A2780, T47D, and HMEC-1 cells (which express S1P3), but not in PC-3 or GI-101A cells (which do not express S1P3), further supporting that S1P3 mediates the crosstalk between S1P and PDGFR. This is the first report demonstrating a unique interaction between S1P3 and PDGFR, in addition to demonstrating a specific role for S1P3 in S1P-induced Akt activation.

Involvement of phospholipase D1 in melanogenesis of mouse B16 melanoma cells

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

Sphingoid bases and their phosphates: transient activation and delayed repression of protein kinase C isoforms and their possible involvement in fumonisin B1 cytotoxicity

Fumonisin B(1), a potent inhibitor of ceramide synthase, leads to accumulation of sphinganine, and later sphingosine, in vivo and in vitro. Fumonisin B(1) modulates the activity of protein kinase C (PKC), however, which metabolite of disrupted sphingolipid metabolism is involved, has not been ascertained. In the present study, we evaluated the modulation of PKC by sphingolipid bases and their metabolites using exogenous sphingolipid analogues in porcine renal epithelial (LLC-PK(1)) cells. In preliminary studies we found that fumonisin B(1) (1 microM) selectively and transiently activated PKCalpha, whereas fumonisin B(1) concentrations of 1-50 microM at 48 h repressed PKC-alpha, -delta, - epsilon and -zeta isoforms in a concentration-dependent manner. Addition of exogenous sphinganine-1-phosphate (1 microM for 5 min) alone stimulated cytosolic to membrane translocation of PKCalpha. Co-exposure of fumonisin B(1) with N,N-dimethylsphingosine, an inhibitor of sphingosine/sphinganine kinase, prevented the effects of fumonisin B(1) on PKCalpha. Sphinganine, sphingosine, sphingosine-1-phosphate and ceramide (all at 1 microM) added exogenously, did not alter PKCalpha cytosolic to membrane translocation at 5 min. Fumonisin B(1) (10 microM), sphinganine, sphingosine and ceramide (1 microM each) significantly repressed PKC-alpha and -delta isoforms at 48 h, whereas all the exogenously added sphingolipids significantly repressed PKC- epsilon and zeta similar to fumonisin B(1). Co-exposure of myriocin with fumonisin B(1) prevented the delayed inhibitory effects of fumonisin B(1) on PKC isoforms in LLC-PK(1) cells. This study demonstrated that selective and transient activation of PKCalpha may be due to the fumonisin B(1)-induced accumulation of the bioactive sphinganine-1-phosphate, whereas the long-term repression of PKC isoforms may be predominantly due to the accumulation of sphinganine or its metabolite, and to a lesser extent sphingosine or its metabolite in LLC-PK(1) cells. These findings suggest that the direct or indirect modulation of PKC by these sphingolipids is involved at least in part in the action of fumonisin B(1).

Gelsolin suppresses tumorigenicity through inhibiting PKC activation in a human lung cancer cell line, PC10

Gelsolin expression is frequently downregulated in lung cancer and several types of different human cancers. To examine the effects of gelsolin restoration on tumorigenicity, we here stably expressed various levels of gelsolin via gene transfer in lung cancer cells (squamous cell carcinoma line, PC10). We observed the alterations in tumorigenicity in vivo when implanted in nude mice, and the changes in growth properties in vitro. As compared to parental cells and control clones, gelsolin transfectants highly reduced tumorigenicity and repressed cell proliferation. Moreover, we investigated bradykinin-induced responses in gelsolin-overexpressing clones, because agonist-stimulated activation of the phospholipases C (PLC)/protein kinase C (PKC) signal transduction pathway is critical for cell growth and tumorigenicity. Bradykinin promotes phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis by PLC and translocation of various PKC isoforms from the cytosolic fraction to the particulate fraction. Bradykinin treatment did not increase inositoltriphosphate (IP3) production and induce the membrane fractions of PKC alpha and PKC gamma in gelsolin tranfectants, while it induced PIP2 hydrolysis and increased the fractions in parental and control clones. These results suggest that gelsolin suppressed the activation of PKCs involved in phospholipid signalling pathways, inhibiting cell proliferation and tumorigenicity.

Association of a polymorphism of the phospholipase D2 gene with the prevalence of colorectal cancer

Phospholipase D plays an important role in transmembrane signaling in a variety of cell types and its activity is increased in certain cancers, suggesting that it also contributes to tumorigenesis. A C-->T transition at nucleotide 1814 of the human phospholipase D(2) gene, which results in a Thr-->Ile substitution at amino acid 577, was noted in the GenBank database. The relationship of this polymorphism to the prevalence of cancer of the esophagus, stomach, colon-rectum, lung, and breast in Japanese was investigated in a case-control study. The genotype of the phospholipase D(2) gene was determined by the polymerase chain reaction with confronting two-pair primers. Multivariate logistic regression analysis with adjustment for age, gender, and smoking status revealed that the frequency of the T allele of the 1814C-->T polymorphism was significantly higher in individuals with colorectal cancer than in controls. A significant association of the polymorphism with the prevalence of colorectal cancer was found in analyses assuming either dominant (TT+CT vs. CC) or additive (CT vs. CC) effects of the T allele, but the T allele was not associated with the prevalence of esophageal, gastric, lung, or breast cancer. The activities of phospholipase D in cell lysates or membrane fractions did not differ between cells transfected with cDNAs encoding the Thr-577 or Ile-577 variants of phospholipase D(2). These results suggest that the phospholipase D(2) gene is a susceptibility locus for colorectal cancer in Japanese individuals, although a functional effect of the 1814C-->T (Thr577Ile) polymorphism was not detected.

Activation of phospholipase D by bradykinin and sphingosine 1-phosphate in A549 human lung adenocarcinoma cells via different GTP-binding proteins and protein kinase C delta signaling pathways

Phospholipase D (PLD) is involved in the signaling by many extracellular ligands, and its regulation appears to be quite complex. We investigated the signaling pathways initiated by bradykinin (BK) or sphingosine 1-phosphate (S1P) in A549 cells to define molecular mechanisms responsible for their additive effects on PLD activity. BK and S1P each elicited a sustained increase in phosphatidic acid content through a rapid and transient activation of PLD. The two pathways demonstrated rapid homologous downregulation, but heterologous desensitization was not observed. Action of both agonists required protein kinase C (PKC) activation and Ca(2+) influx but was mediated by different heterotrimeric G proteins. In membranes, inhibition of PKCdelta by rottlerin enhanced BK activation of PLD but inhibited that by S1P. Rottlerin inhibited activation of PLD in nuclei by both BK and S1P. By in situ immunofluorescence or cell fractionation followed by immunoblotting, PLD1 was concentrated primarily in nuclei, whereas the membrane fraction contained PLD2 and PLD1. Moreover, PKCdelta specifically phosphorylated recombinant PLD2, but not PLD1. BK and S1P similarly enhanced RhoA translocation to nuclei, whereas BK was less efficacious than S1P on RhoA relocalization to membranes. Effects of both agonists on the nuclear fraction, which contains only PLD1, are compatible with a RhoA- and PKCdelta-dependent process. In membranes, which contain both PLD1 and PLD2, the stimulatory effect of S1P on PLD activity can best be explained by RhoA- and PKCdelta-dependent activation of PLD1; in contrast, the effects of BK on RhoA translocation and enhancement of BK-stimulated PLD activity by PKC inhibition are both consistent with PLD2 serving as its primary target.

Involvement of phospholipase D in insulin-like growth factor-I-induced activation of extracellular signal-regulated kinase, but not phosphoinositide 3-kinase or Akt, in Chinese hamster ovary cells

Available evidence suggests the involvement of phospholipase D (PLD) in cell proliferation and survival. Phosphoinositide 3-kinase (PI 3-kinase)/Akt and extracellular signal-regulated kinases (ERKs) are signalling molecules that have essential roles in cell proliferation and survival. We previously demonstrated that sphingosine 1-phosphate (S1P)-induced PLD activation via the G-protein-coupled receptor endothelial differentiation gene (EDG) 3/S1P(3) was involved in S1P-induced stimulation of PI 3-kinase and Akt. In the present study, we examined the involvement of two PLD isozymes, PLD1 and PLD2, in insulin-like growth factor (IGF)-I receptor tyrosine kinase-mediated stimulation of PI 3-kinase/Akt and ERKs. IGF-I and to a lesser degree S1P stimulated PI 3-kinase activity in Chinese hamster ovary cells overexpressing EDG3/S1P(3). IGF-I-induced ERK phosphorylation was suppressed by butan-1-ol, but not butan-2-ol, whereas no effect of butanol was observed in IGF-I-induced Akt activation in S1P(3)-overexpressing Chinese hamster ovary cells. Overexpression of wild-type PLD1 and PLD2 substantially potentiated S1P-, but not IGF-I-, induced activation of PI 3-kinase and Akt, whereas overexpression of the catalytically inactive mutant of PLD1 or PLD2 did not affect the responses to either agonist. On the other hand, overexpression of wild-type PLD1 and PLD2 potentiated IGF-I- and, to much smaller extents, S1P-induced ERK stimulation. ERK activation by IGF-I as well as S1P was dependent on Ras, but Akt activation by IGF-I was not dependent on Ras. These results suggest that PLDs are involved in growth factor regulation of at least two signalling pathways, PI 3-kinase/Akt and ERKs, depending on the class of cell-surface receptors.

Regulation of phospholipase D activity by actin. Actin exerts bidirectional modulation of Mammalian phospholipase D activity in a polymerization-dependent, isoform-specific manner

Many critical cellular processes, including proliferation, vesicle trafficking, and secretion, are regulated by both phospholipase D (PLD) and the actin microfilament system. Stimulation of human PLD1 results in its association with the detergent-insoluble actin cytoskeleton, but the molecular mechanisms and functional consequences of PLD-actin interactions remain incompletely defined. Biochemical and pharmacologic modulation of actin polymerization resulted in complex bidirectional effects on PLD activity, both in vitro and in vivo. Highly purified G-actin inhibited basal and stimulated PLD activity, whereas F-actin produced the opposite effects. Actin-induced modulation of PLD activity was independent of the activating stimulus. The efficacy and potency of the effects of actin were isoform-specific but broadly conserved among actin family members. Human betagamma-actin was only 45% as potent and 40% as efficacious as rabbit skeletal muscle alpha-actin, whereas its inhibitory profile was similar to the single actin species from the yeast, Saccharomyces cerevisiae. Use of actin polymerization-specific reagents indicated that PLD1 binds both monomeric G-actin, as well as actin filaments. These data are consistent with a model in which the physical state of the actin cytoskeleton is a critical determinant of its regulation of PLD activity.

Gelsolin-induced epithelial cell invasion is dependent on Ras-Rac signaling

Gelsolin is a widely distributed actin binding protein involved in controlling cell morphology, motility, signaling and apoptosis. The role of gelsolin in tumor progression, however, remains poorly understood. Here we show that expression of green fluorescent protein (GFP)-tagged gelsolin in MDCK-AZ, MDCKtsSrc or HEK293T cells promotes invasion into collagen type I. In organ culture assays, MDCK cells expressing gelsolin-GFP invaded pre-cultured chick heart fragments. Gelsolin expression inhibited E-cadherin-mediated cell aggregation but did not disrupt the E-cadherin-catenin complex. Co-expression of dominant-negative Rac1N17, but not RhoAN19 or Cdc42N17, counteracted gelsolin-induced invasion, suggesting a requirement for Rac1 activity. Increased ARF6, PLD or PIP5K 1alpha activity canceled out gelsolin-induced invasion. Furthermore, we found that invasion induced by gelsolin is dependent on Ras activity, acting through the PI3K-Rac pathway via the Ras guanine nucleotide exchange factor Sos-1. These findings establish a connection between gelsolin and the Ras oncogenic signaling pathway.

Role of protein kinase C alpha in primary human osteoblast proliferation

Protein kinase C (PKC) isoforms have been shown to have specific expression profiles and individual isoforms are believed to play distinct roles in the cells in which they are found. The goal here was to determine which specific isoform(s) is involved in proliferation of primary human osteoblasts. In primary human osteoblasts, 10 microM of acute sphingosine-1-phosphate (S1P) treatment induced an increase in proliferation that correlated with an increase in PKCalpha and PKCiota expression. To further delineate which isoforms are involved in osteoblastic cell proliferation, the effect of low versus high serum culture conditions on PKC isoform expression was determined. Likewise, the effect of antisense oligodeoxynucleotides (ODNs) to specific PKC isoforms on proliferation and MAPK activation was studied. The effect of S1P on intracellular translocation of activated PKC isoforms was also evaluated. The results indicated that in primary human osteoblasts, PKCalpha was not expressed under conditions of low proliferative rate while PKCdelta and PKCiota expression was not affected. The specific inhibition of PKCalpha by antisense ODNs resulted in inhibition of MAPK activity leading to a significant decrease in proliferation. S1P up-regulated antisense ODN inhibited PKCalpha expression and MAPK activity and led to an increase in proliferation. Subsequent experiments using platelet-derived growth factor (PDGF) as an additional mitogen generated similar data. PDGF stimulation resulted in a significant increase in proliferation that correlated with an up-regulation of inhibited PKCalpha expression in antisense ODN-treated cells. Immunofluorescence methods showed that mitogenic stimulation of PKCa resulted in nuclear translocation. Our findings present original data that PKCalpha is the isoform specifically involved in the proliferation of primary human osteoblasts.

Involvement of phospholipases D1 and D2 in sphingosine 1-phosphate-induced ERK (extracellular-signal-regulated kinase) activation and interleukin-8 secretion in human bronchial epithelial cells

Sphingosine 1-phosphate (S1P), a metabolite of sphingomyelin degradation, stimulates interleukin-8 (IL-8) secretion in human bronchial epithelial (Beas-2B) cells. The molecular mechanisms regulating S1P-mediated IL-8 secretion are yet to be completely defined. Here we provide evidence that activation of phospholipases D1 and D2 (PLD1 and PLD2) by S1P regulates the phosphorylation of extracellular-signal-regulated kinase (ERK) and IL-8 secretion in Beas-2B cells. S1P, in a time- and dose-dependent manner, enhanced the threonine/tyrosine phosphorylation of ERK. The inhibition of S1P-induced ERK phosphorylation by pertussis toxin and PD 98059 indicated coupling of S1P receptors to G(i) and the ERK signalling cascade respectively. Treatment of Beas-2B cells with butan-1-ol, but not butan-3-ol, abrogated the S1P-induced phosphorylation of Raf-1 and ERK, suggesting that PLD is involved in this activation. The roles of PLD1 and PLD2 in ERK activation and IL-8 secretion activated by S1P were investigated by infecting cells with adenoviral constructs of wild-type and catalytically inactive mutants of PLD1 and PLD2. Infection of Beas-2B cells with the wild-type constructs resulted in the activation of PLD1 and PLD2 by S1P and PMA. Also, the enhanced production of [(32)P]phosphatidic acid and [(32)P]phosphatidylbutanol in the presence of butan-1-ol and the increased phosphorylation of ERK by S1P were blocked by the catalytically inactive mutants hPLD1-K898R and mPLD2-K758R. Transient transfection of Beas-2B cells with human PLD1 and mouse PLD2 cDNAs potentiated S1P-mediated IL-8 secretion compared with vector controls. In addition, PD 98059 attenuated IL-8 secretion induced by S1P in a dose-dependent fashion. These results demonstrate that both PLD1 and PLD2 participate in S1P stimulation of ERK phosphorylation and IL-8 secretion in bronchial epithelial cells.

Adenovirus mediated gelsolin gene therapy for orthotopic human bladder cancer in nude mice

PURPOSE

Gelsolin is an actin regulatory protein that is undetectable or reduced in human bladder tumors compared with normal epithelial cells. Whether the over expression of gelsolin could inhibit tumor growth was investigated in an orthotopic bladder cancer nude mouse model using recombinant adenovirus encoding wild-type gelsolin (Ad-GSN).

MATERIALS AND METHODS

The 2 human bladder cancer cell lines KU-7 and UMUC-2 were transduced with Ad-GSN in vitro. Flow cytometric analysis was done to examine the cell cycle after transducing the adenovirus. Cell growth was compared with control groups of these cells transduced with adenovirus containing the Escherichia coli beta-galactosidase gene Ad-betagal. In vivo KU-7 cells were introduced into the bladder of nude mice (day 0), followed by 3 injections into the urethra (days 2 to 4) with Ad-GSN or Ad-betagal (1 x 10 pfu). At 8 days after initial adenovirus exposure (day 10) each bladder was sectioned and stained, and the mass of the tumor was digitally determined.

RESULTS

Bladder cancer cell growth (KU-7 and UMUC-2) was inhibited after these cells were transduced with Ad-GSN in vitro. Based on flow cytometric analysis over expression of gelsolin may cause these cells to arrest or delay at the G2/M phase of the cell cycle. In the orthotopic bladder cancer model the mass of the tumor was approximately 90% less in Ad-GSN treated animals than in controls.

CONCLUSIONS

Ad-GSN provides a significant tumor suppressive effect on human bladder cancer cells in this orthotopic nude mouse model. Adenovirus mediated over expression of gelsolin may be useful therapy for human bladder cancer.

Phospholipase D stimulation is required for sphingosine-1-phosphate activation of actin stress fibre assembly in human airway epithelial cells

In human airway epithelial cells, sphingosine-1-phosphate (SPP) and lysophosphatidic acid (LPA) stimulated the production of phosphatidic acid (PA), which was inhibited by the primary alcohol butan-1-ol, but not by the inactive butan-2-ol, clearly indicating phospholipase D (PLD) involvement. Both SPP and LPA stimulated actin stress fibre formation, which was also butan-2-ol-insensitive and inhibited by butan-1-ol. SPP-induced PLD activation and cytoskeletal remodelling were insensitive to brefeldin A and toxin B from Clostridium difficile, which conversely blocked the effect of LPA, suggesting that the monomeric GTPases ADP ribosylation factor (ARF) and Rho are involved in LPA, but not in SPP responses. Pertussis toxin inhibited SPP- but not LPA-induced effects. PLD activation and stress fibre formation by both lysolipids were abolished by the tyrosine kinase inhibitor genistein. Addition of PA to cells caused a massive stress fibre assembly. In conclusion, PLD is one of the signalling components linking SPP-receptor activation to assembly of actin stress fibres.

A novel phospholipase D of Arabidopsis that is activated by oleic acid and associated with the plasma membrane

Oleate-dependent phospholipase D (PLD; EC 3.1.4.4) has been reported in animal systems, but its molecular nature is unkown. Multiple PLDs have been characterized in plants, but none of the previously cloned PLDs exhibits the oleate-activated activity. Here, we describe the biochemical and molecular identification and characterization of an oleate-activated PLD in Arabidopsis. This PLD, designated PLDdelta, was associated tightly with the plasma membrane, and its level of expression was higher in old leaves, stems, flowers, and roots than in young leaves and siliques. A cDNA encoding the oleate-activated PLD was identified, and catalytically active PLDdelta was expressed from its cDNA in Escherichia coli. PLDdelta was activated by free oleic acid in a dose-dependent manner, with the optimal concentration being 0.5 mM. Other unsaturated fatty acids, linoleic and linolenic acids, were less effective than oleic acid, whereas the saturated fatty acids, stearic and palmitic acids, were totally ineffective. Phosphatidylinositol 4,5-bisphosphate stimulated PLDdelta to a lesser extent than oleate. Mutation at arginine (Arg)-611 led to a differential loss of the phosphatidylinositol 4,5-bisphosphate-stimulated activity of PLDdelta, indicating that separate sites mediate the oleate regulation of PLDdelta. Oleate stimulated PLDdelta's binding to phosphatidylcholine. Mutation at Arg-399 resulted in a decrease in oleate binding by PLDdelta and a loss of PLDdelta activity. However, this mutation bound similar levels of phosphatidylcholine as wild type, suggesting that Arg-399 is not required for PC binding. These results provide the molecular information on oleate-activated PLD and also suggest a mechanism for the oleate stimulation of this enzyme.

Involvement of phospholipase D in sphingosine 1-phosphate-induced activation of phosphatidylinositol 3-kinase and Akt in Chinese hamster ovary cells overexpressing EDG3

Phospholipase D (PLD), phosphatidylinositol 3-kinase (PI3K), and Akt are known to be involved in cellular signaling related to proliferation and cell survival. In this report, we provide evidence that PLD links sphingosine 1-phosphate (S1P)-induced activation of the G protein-coupled EDG3 receptor to stimulation of PI3K and its downstream effector Akt in Chinese hamster ovary (CHO) cells. S1P stimulation of EDG3-overexpressing CHO cells but not vector-transfected cells induced activation of PLD, PI3K, and Akt in a time- and dose-dependent manner. Akt phosphorylation was prevented by the PI3K inhibitors wortmannin and LY294002 (2-(4-monrpholinyl)-8-phenyl-4H-1-benzopyran-4-one), indicating that Akt activation was dependent on PI3K. S1P-induced activation of PI3K and Akt was abrogated by 1-butanol, which inhibited S1P-induced accumulation of phosphatidic acid by serving as a phosphatidyl group acceptor in the transphosphatidylation reaction catalyzed by PLD, whereas both PI3K and Akt activation were not inhibited by 2-butanol without such reaction. Co-expression of wild-type PLD2 with myc-Akt resulted in increased Akt activation in response to S1P. In contrast, co-expression of a catalytically inactive mutant of PLD2 eliminated the S1P-induced Akt activation. The treatment of EDG3-expressing CHO cells with exogenous Streptomyces chromofuscus PLD, which caused an accumulation of phosphatidic acid, resulted in increases in PI3K activity and the phosphorylation of Akt, the latter of which was completely abolished by LY294002. Furthermore, S1P-induced membrane ruffling, which was dependent on PI3K and Rac, was inhibited by 1-butanol, but not by 2-butanol. These results demonstrate that PLD participates in the activation of PI3K and Akt stimulation of EDG3 receptor.

Regulation of phospholipase C-gamma(1) by the actin-regulatory protein villin

The actin-regulatory protein villin is tyrosine phosphorylated and associates with phospholipase C-gamma(1) (PLC-gamma(1)) in the brush border of intestinal epithelial cells. To study the mechanism of villin-associated PLC-gamma(1) activation, we reconstituted in vitro the tyrosine phosphorylation of villin and its association with PLC-gamma(1). Recombinant villin was phosphorylated in vitro by the nonreceptor tyrosine kinase c-src or by expression in the TKX1 competent cells that carry an inducible tyrosine kinase gene. Using in vitro binding assays, we demonstrated that tyrosine-phosphorylated villin associates with the COOH-terminal Src homology 2 (SH2) domain of PLC-gamma(1). The catalytic activity of PLC-gamma(1) was inhibited by villin in a dose-dependent manner with half-maximal inhibition at a concentration of 12.4 microM. Villin inhibited PLC-gamma(1) activity by sequestering the substrate phosphatidylinositol 4,5-bisphosphate (PIP(2)), since increasing concentrations of PIP(2) reversed the inhibitory effects of villin on PLC activity. The inhibition of PLC-gamma(1) activity by villin was reversed by the tyrosine phosphorylation of villin. Further, we demonstrated that tyrosine phosphorylation of villin abolished villin's ability to associate with PIP(2). In conclusion, tyrosine-phosphorylated villin associates with the COOH-terminal SH2 domain of PLC-gamma(1) and activates PLC-gamma(1) catalytic activity. Villin regulates PLC-gamma(1) activity by modifying its own ability to bind PIP(2). This study provides biochemical proof of the functional relevance of tyrosine phosphorylation of villin and identifies the molecular mechanisms involved in the activation of PLC-gamma(1) by villin.

Sphingosine-1-phosphate effects on PKC isoform expression in human osteoblastic cells

Sphingosine-1-phosphate (S1P) has been shown to participate in the proliferative process in human osteoblasts.(1) The mitogenic effect of S1P has been postulated to involve two signaling pathways, the Gi linked protein receptor pathway and the PKC pathway. To define the possible role of PKC isoforms in osteoblastic cell proliferation, the effects of S1P on PKC isoform expression was determined. While PKC lambda was minimally detected, the isoforms alpha, delta and iota were all found to be highly expressed by the human osteoblast. In human osteoblastic cells, S1P induced a 25% increase in the expression of PKC alpha and approximately a 30% increase in the expression of PKC iota. S1P did not have an effect on PKC delta expression. Pretreatment with pertussis toxin (PT) led to an inhibition of the observed S1P effects on the expression of the alpha and iota isoforms.

Dual regulation of sphingosine 1-phosphate-induced phospholipase D activity through RhoA and protein kinase C-alpha in C2C12 myoblasts

Previous studies showed that in C2C12 cells, phospholipase D (PLD) and its known regulators, RhoA and protein kinase Calpha (PKCalpha), were downstream effectors in sphingosine 1-phosphate (SPP) signalling. Moreover, the role of PKC for SPP-mediated PLD activation and the requirement of PKCalpha for RhoA translocation were reported. The present results demonstrated that inactivation of RhoA, by overexpression of RhoGDP dissociation inhibitor (RhoGDI) as well as treatment with C3 exotoxin, attenuated SPP-stimulated PLD activity, supporting the involvement of RhoA in the stimulation of PLD activity by the bioactive lipid in C2C12 myoblasts. In addition, the effect of PKCalpha inhibitor Gö6976 on the SPP-induced PLD activation in myoblasts, where RhoA function was inactivated, was consistent with a dual regulation of the enzyme through RhoA and PKCalpha. Interestingly, the subcellular distribution of PLD isoforms, RhoA and PKCalpha, in SPP-stimulated cells supported the view that the functional relationship between the two PLD regulators, demonstrated to occur in SPP signalling, represents a novel mechanism of regulation of specifically localized PLD.

Involvement of ERK and p38 MAP kinase in oxidative stress-induced phospholipase D activation in PC12 cells

Exposure to hydrogen peroxide induced considerable activation of phospholipase D (PLD) in rat pheochromocytoma PC12 cells. This PLD activation was potentiated by orthovanadate and okadaic acid, suggesting that tyrosine kinase and serine/threonine kinase are involved. Furthermore, H2O2-induced PLD activation was partially inhibited by either MEK1 inhibitor (PD98059) or p38 MAP kinase inhibitor (SB203580), but a combination of both inhibitors resulted in nearly 80% suppression. The major isozyme was found to be PLD2 in PC12 cells by Western blotting analysis. When the PLD2-transfected COS-7 cells were exposed to H2O2, the PLD activation was markedly inhibited by the combined pretreatment with PD98059 and SB203580. To our knowledge, this study is the first demonstration that both ERK1/2 and p38 MAP kinase are involved in the PLD2 activation in PC12 cells exposed to H2O2.