Small GTPase-regulated phospholipase D in granulocytes

Regulators and Effectors of Arf GTPases in Neutrophils

Polymorphonuclear neutrophils (PMNs) are key innate immune cells that represent the first line of defence against infection. They are the first leukocytes to migrate from the blood to injured or infected sites. This process involves molecular mechanisms that coordinate cell polarization, delivery of receptors, and activation of integrins at the leading edge of migrating PMNs. These phagocytes actively engulf microorganisms or form neutrophil extracellular traps (NETs) to trap and kill pathogens with bactericidal compounds. Association of the NADPH oxidase complex at the phagosomal membrane for production of reactive oxygen species (ROS) and delivery of proteolytic enzymes into the phagosome initiate pathogen killing and removal. G protein-dependent signalling pathways tightly control PMN functions. In this review, we will focus on the small monomeric GTPases of the Arf family and their guanine exchange factors (GEFs) and GTPase activating proteins (GAPs) as components of signalling cascades regulating PMN responses. GEFs and GAPs are multidomain proteins that control cellular events in time and space through interaction with other proteins and lipids inside the cells. The number of Arf GAPs identified in PMNs is expanding, and dissecting their functions will provide important insights into the role of these proteins in PMN physiology.

Differential expression and targeting of endogenous Arf1 and Arf6 small GTPases in kidney epithelial cells in situ

ADP-ribosylation factors (Arfs) are small GTPases that regulate vesicular trafficking in exo- and endocytotic pathways. As a first step in understanding the role of Arfs in renal physiology, immunocytochemistry and Western blotting were performed to characterize the expression and targeting of Arf1 and Arf6 in epithelial cells in situ. Arf1 and Arf6 were associated with apical membranes and subapical vesicles in proximal tubules, where they colocalized with megalin. Arf1 was also apically expressed in the distal tubule, connecting segment, and collecting duct (CD). Arf1 was abundant in intercalated cells (IC) and colocalized with V-ATPase in A-IC (apical) and B-IC (apical and/or basolateral). In contrast, Arf6 was associated exclusively with basolateral membranes and vesicles in the CD. In the medulla, basolateral Arf6 was detectable mainly in A-IC. Expression in principal cells became weaker throughout the outer medulla, and Arf6 was not detectable in principal cells in the inner medulla. In some kidney epithelial cells Arf1 but not Arf6 was also targeted to a perinuclear patch, where it colocalized with TGN38, a marker of the trans-Golgi network. Quantitative Western blotting showed that expression of endogenous Arf1 was 26–180 times higher than Arf6. These data indicate that Arf GTPases are expressed and targeted in a cell- and membrane-specific pattern in kidney epithelial cells in situ. The results provide a framework on which to base and interpret future studies on the role of Arf GTPases in the multitude of cellular trafficking events that occur in renal tubular epithelial cells.

Chemotactic factor-induced recruitment and activation of Tec family kinases in human neutrophils. II. Effects of LFM-A13, a specific Btk inhibitor

Tyrosine phosphorylation events play major roles in the initiation and regulation of several functional responses of human neutrophils stimulated by chemotactic factors such as the bacterially derived tripeptide formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe). However, the links between the G protein-coupled receptors, the activation of the tyrosine kinases, and the initiation of neutrophil functional responses remain unclear. In the present study we assessed the effects of a Btk inhibitor, leflunomide metabolite analog (LFM-A13), on neutrophils. LFM-A13 decreased the tyrosine phosphorylation induced by fMet-Leu-Phe and inhibited the production of superoxide anions and the stimulation of adhesion, chemotaxis, and phospholipase D activity. We observed a decreased accumulation of phosphatidylinositol-3,4,5-trisphosphate in response to fMet-Leu-Phe in LFM-A13-pretreated cells even though the inhibitor had no direct effect on the lipid kinase activity of the p110 gamma or p85/p110 phosphatidylinositol 3-kinases or on the activation of p110 gamma by fMet-Leu-Phe. The phosphorylation of Akt and of extracellular signal-regulated kinases 1/2 and p38 were similarly inhibited by LFM-A13. LFM-A13 also negatively affected the translocation of Rac-2, RhoA, ADP ribosylation factor-1, Tec, Bmx, and Btk induced by fMet-Leu-Phe. The results of this study provide evidence for an involvement of Btk and possibly other Tec kinase family members in the regulation of the functional responsiveness of human neutrophils and link these events, in part at least, to the modulation of levels of phosphatidylinositol-3,4,5-trisphosphate.

Phospholipase D activation by endogenous 5-hydroxytryptamine 2C receptors is mediated by Galpha13 and pertussis toxin-insensitive Gbetagamma subunits

Phospholipase D activation was measured in primary cultures of rat choroid plexus epithelial cells, which endogenously express the 5-hydroxytryptamine (5-HT) 2C receptor, as well as a heterologous cell line expressing the cloned receptor. In both systems, serotonin stimulation of the 5-HT(2C) receptor activates phospholipase D in addition to phospholipase C, the traditional effector. Specific inhibitors and membrane permeable blocking peptides were used to determine which heterotrimeric G-proteins were involved. Results suggest that both alpha and free betagamma subunits from G(13) heterotrimers are responsible for phospholipase D activation.

Na(+)-K(+)-ATPase gene expression in the avian eggshell gland: distinct regulation in different cell types

The avian eggshell gland (ESG) is a tissue specialized in transporting the Ca(2+) required for eggshell formation and represents a unique biological system in which the calcification process takes place in a circadian fashion. With the use of RNA fingerprinting, a set of genes differentially induced at the time of calcification was detected, one of which was identified as the alpha(1)-subunit of Na(+)-K(+)-ATPase. The gene was expressed in a circadian manner in both cell types populating the ESG, but in different temporal patterns, suggesting distinct mechanisms of regulation. Ca(2+) flux and mechanical strain were found to regulate gene expression in the inner glandular epithelium and the pseudostratified epithelium facing the lumen, respectively. Mechanical strain also affected gene expression in cell layers facing the lumen in other parts of the oviduct. Only the alpha(1)-isoform, not the alpha(2)- or alpha(3)-isoform, of Na(+)-K(+)-ATPase was expressed in the ESG. In summary, we demonstrate that the alpha(1)-subunit Na(+)-K(+)-ATPase gene is expressed in different epithelial cell types in the ESG and is regulated by various mechanisms, which may reflect the disparity in the physiological roles of the cells in the process of eggshell formation.

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

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

Characterization of rac and cdc42 activation in chemoattractant-stimulated human neutrophils using a novel assay for active GTPases

A major function of Rac2 in neutrophils is the regulation of oxidant production important in bacterial killing. Rac and the related GTPase Cdc42 also regulate the dynamics of the actin cytoskeleton, necessary for leukocyte chemotaxis and phagocytosis of microorganisms. Although these GTPases appear to be critical downstream components of chemoattractant receptor signaling in human neutrophils, the pathways involved in direct control of Rac/Cdc42 activation remain to be determined. We describe an assay that measures the formation of Rac-GTP and Cdc42-GTP based on their specific binding to the p21-binding domain of p21-activated kinase 1. A p21-binding domain glutathione S-transferase fusion protein specifically binds Rac and Cdc42 in their GTP-bound forms both in vitro and in cell samples. Binding is selective for Rac and Cdc42 versus RhoA. Using this assay, we investigated Rac and Cdc42 activation in neutrophils and differentiated HL-60 cells. The chemoattractant fMet-Leu-Phe and the phorbol ester phorbol myristate acetate stimulate formation of Rac-GTP and Cdc42-GTP with distinct time courses that parallel cell activation. We also show that the signaling pathways leading to Rac and Cdc42 activation in HL-60 cells involve G proteins sensitive to pertussis toxin, as well as tyrosine kinase and phosphatidylinositol 3-kinase activities.

Phospholipase D: a novel major player in signal transduction

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

Ro31-8220 inhibits protein kinase C to block the phorbol ester-stimulated release of choline- and ethanolamine-metabolites from C6 glioma cells: p70 S6 kinase and MAPKAP kinase-1beta do not function downstream of PKC in activating PLD

The use of bisindolylmaleimide derivatives of staurosporine as selective inhibitors of protein kinase C (PKC) is in doubt following the report by Alessi [FEBS Lett. 402 (1997) 121-123] that Ro31-8220 and GF109203X are potent in vitro inhibitors of p70 S6 kinase and mitogen-activated protein kinase-activated protein kinase-1beta, as well as of PKC. Here we show that the phorbol ester-stimulated release of choline- and ethanolamine-metabolites from C6 glioma cells due to phospholipid hydrolysis by phospholipase D (PLD) is not inhibited by rapamycin or PD98059, specific inhibitors respectively of p70 S6 kinase and MAPKK (MEK) and thus of MAPKAP kinase-1beta but is still completely blocked by Ro31-8220. We conclude therefore that p70S6k and MAPKAP kinase-1beta as well as MAPK are not involved in signalling pathways downstream of PKC that regulate phorbol ester-stimulated phospholipid turnover and that the inhibitory action of Ro31-8220 occurs by blocking PKC which regulates at least one pathway to PLD activation. The PI-3 kinase inhibitor, wortmannin, inhibits the phorbol ester-stimulated release of ethanolamine- but not choline-metabolites from C6 cells suggesting that different PLD isoforms regulate the turnover of PtdEth and PtdCho in C6 cells. Both PLD isoforms are activated via PKC but the PtdEth-PLD is also regulated via a wortmannin-sensitive pathway.