Association of ligand-receptor complexes with actin filaments in human neutrophils: a possible regulatory role for a G-protein

The FPR2-specific ligand MMK-1 activates the neutrophil NADPH-oxidase, but triggers no unique pathway for opening of plasma membrane calcium channels

Human neutrophils express formyl peptide receptor 1 and 2 (FPR1 and FPR2), two highly homologous G-protein-coupled cell surface receptors important for the cellular recognition of chemotactic peptides. They share many functional as well as signal transduction features, but some fundamental differences have been described. One such difference was recently presented when the FPR2-specific ligand MMK-1 was shown to trigger a unique signal in neutrophils [S. Partida-Sanchez, P. Iribarren, M.E. Moreno-Garcia, et al., Chemotaxis and calcium responses of phagocytes to formyl peptide receptor ligands is differentially regulated by cyclic ADP ribose, J. Immunol. 172 (2004) 1896-1906]. This signal bypassed the emptying of the intracellular calcium stores, a route normally used to open the store-operated calcium channels present in the plasma membrane of neutrophils. Instead, the binding of MMK-1 to FPR2 was shown to trigger a direct opening of the plasma membrane channels. In this report, we add MMK-1 to a large number of FPR2 ligands that activate the neutrophil superoxide-generating NADPH-oxidase. In contrast to earlier findings we show that the transient rise in intracellular free calcium induced by MMK-1 involves both a release of calcium from intracellular stores and an opening of channels in the plasma membrane. The same pattern was obtained with another characterized FPR2 ligand, WKYMVM, and it is also obvious that the two formyl peptide receptor family members trigger the same type of calcium response in human neutrophils.

Developmental expression of the alpha-skeletal actin gene

Background

Actin is a cytoskeletal protein which exerts a broad range of functions in almost all eukaryotic cells. In higher vertebrates, six primary actin isoforms can be distinguished: alpha-skeletal, alpha-cardiac, alpha-smooth muscle, gamma-smooth muscle, beta-cytoplasmic and gamma-cytoplasmic isoactin. Expression of these actin isoforms during vertebrate development is highly regulated in a temporal and tissue-specific manner, but the mechanisms and the specific differences are currently not well understood. All members of the actin multigene family are highly conserved, suggesting that there is a high selective pressure on these proteins.

Results

We present here a model for the evolution of the genomic organization of alpha-skeletal actin and by molecular modeling, illustrate the structural differences of actin proteins of different phyla. We further describe and compare alpha-skeletal actin expression in two developmental stages of five vertebrate species (mouse, chicken, snake, salamander and fish). Our findings confirm that alpha-skeletal actin is expressed in skeletal muscle and in the heart of all five species. In addition, we identify many novel non-muscular expression domains including several in the central nervous system.

Conclusion

Our results show that the high sequence homology of alpha-skeletal actins is reflected by similarities of their 3 dimensional protein structures, as well as by conserved gene expression patterns during vertebrate development. Nonetheless, we find here important differences in 3D structures, in gene architectures and identify novel expression domains for this structural and functional important gene.

NHERF-1 and the cytoskeleton regulate the traffic and membrane dynamics of G protein-coupled receptors

The sodium-hydrogen exchange regulatory factor 1 (NHERF-1/EBP50) interacts with the C terminus of several G protein-coupled receptors (GPCRs). We examined the role of NHERF-1 and the cytoskeleton on the distribution, dynamics, and trafficking of the beta(2)-adrenergic receptor (beta(2)AR; a type A receptor), the parathyroid hormone receptor (PTH1R; type B), and the calcium-sensing receptor (CaSR; type C) using fluorescence recovery after photobleaching, total internal reflection fluorescence, and image correlation spectroscopy. beta(2)AR bundles were observed only in cells that expressed NHERF-1, whereas the PTH1R was localized to bundles that parallel stress fibers independently of NHERF-1. The CaSR was never observed in bundles. NHERF-1 reduced the diffusion of the beta(2)AR and the PTH1R. The addition of ligand increased the diffusion coefficient and the mobile fraction of the PTH1R. Isoproterenol decreased the immobile fraction but did not affect the diffusion coefficient of the beta(2)AR. The diffusion of the CaSR was unaffected by NHERF-1 or the addition of calcium. NHERF-1 reduced the rate of ligand-induced internalization of the PTH1R. This phenomenon was accompanied by a reduction of the rate of arrestin binding to PTH1R in ligand-exposed cells. We conclude that some GPCRs, such as the beta(2)AR, are attached to the cytoskeleton primarily via the binding of NHERF-1. Others, such as the PTH1R, bind the cytoskeleton via several interacting proteins, one of which is NHERF-1. Finally, receptors such as the CaSR do not interact with the cytoskeleton in any significant manner. These interactions, or the lack thereof, govern the dynamics and trafficking of the receptor.

Cytoskeletal remodeling in leukocyte function

PURPOSE OF REVIEW

This review focuses on recent developments in understanding the roles and regulation of the cytoskeleton in the function of leukocytes.

RECENT FINDINGS

New studies have shed light on the regulation and dynamics of actin and microtubules in leukocytes relevant both to cell motility generally and to immune function specifically. The roles of cytoskeletal dynamics in processes such as cell activation, cell migration, and phagocytosis are being elucidated. Dramatic progress has been made recently in understanding the mechanisms of leukocyte directional sensing, polarization, and chemotaxis.

SUMMARY

Leukocytes need to be activated, polarize, change shape, move, or phagocytose in response to their environment. Leukocytes accomplish these processes by remodeling their cytoskeleton, the active musculoskeletal system of the cell that is not just the ultimate effector of motile responses but is also a dynamic framework for subcellular organization and regional signaling. Active areas of research include the direct and indirect reciprocal interactions between the cytoskeleton and the membrane and among cytoskeletal elements. The pervasive and multi-layered roles played by small GTPases of the Rho family and phosphoinositides in leukocyte function are also becoming clearer.

Altered tropomyosin expression in essential hypertension

Abnormal erythrocyte sodium-lithium countertransport is common in a subgroup of patients with essential hypertension and a strong family history of hypertension and cardiovascular disease. We have previously shown that the abnormality in sodium-lithium countertransport is associated with tropomyosin, a cytoskeletal protein required to stabilize actin filament formation. Leukocyte trafficking events, which depend on cytoskeletal reorganization, are also altered in patients with essential hypertension with abnormal sodium-lithium countertransport. The aim of this study was to determine whether there is an abnormality in isoforms of tropomyosin that are common to erythrocytes and leukocytes. Analysis of reticulocyte RNA by reverse transcription (RT) and polymerase chain reaction (PCR) showed expression of TPMN and TPM5b isoforms of tropomyosin. No other isoforms were expressed. These isoforms were also detected in RNA from leukocytes. In patients with essential hypertension with abnormal erythrocyte sodium-lithium countertransport compared with normal control subjects, there was a higher TPMN/TPM5b ratio of protein in erythrocytes (median 3.8 [range 1.8 to 6.6] versus 2.9 [1.9 to 4.0], P<0.001) and of RNA in leukocytes (3.7 [1.7 to 8.2] versus 2.6 [1.2 to 4.3], P<0.01). Furthermore, the protein ratio of TPMN/TPM5b in erythrocytes showed significant correlation with the V(max)/K(m) ratio of sodium-lithium countertransport across the patient groups (r=-0.42; P<0.01). Therefore, altered tropomyosin expression may be the underlying abnormality associated with blood cell membrane changes in essential hypertension and implicates the cytoskeleton in the pathogenesis of the disease in a major subgroup of patients.

Two pathways through Cdc42 couple the N-formyl receptor to actin nucleation in permeabilized human neutrophils

We developed a permeabilization method that retains coupling between N-formyl-methionyl-leucyl-phenylalanine tripeptide (FMLP) receptor stimulation, shape changes, and barbed-end actin nucleation in human neutrophils. Using GTP analogues, phosphoinositides, a phosphoinositide-binding peptide, constitutively active or inactive Rho GTPase mutants, and activating or inhibitory peptides derived from neural Wiskott-Aldrich syndrome family proteins (N-WASP), we identified signaling pathways leading from the FMLP receptor to actin nucleation that require Cdc42, but then diverge. One branch traverses the actin nucleation pathway involving N-WASP and the Arp2/3 complex, whereas the other operates through active Rac to promote actin nucleation. Both pathways depend on phosphoinositide expression. Since maximal inhibition of the Arp2/3 pathway leaves an N17Rac inhibitable alternate pathway intact, we conclude that this alternate involves phosphoinositide-mediated uncapping of actin filament barbed ends.

Regulation of F-actin and endoplasmic reticulum organization by the trimeric G-protein Gi2 in rat hepatocytes. Implication for the activation of store-operated Ca2+ inflow

The roles of the heterotrimeric G-protein, G(i2), in regulating the actin cytoskeleton and the activation of store-operated Ca(2+) channels in rat hepatocytes were investigated. Galpha(i2) was principally associated with the plasma membrane and microsomes. Both F-actin and Galpha(i2) were detected by Western blot analysis in a purified plasma membrane preparation, the supernatant and pellet obtained by treating the plasma membrane with Triton X-100, and after depolymerization and repolymerization of F-actin in the Triton X-100-insoluble pellet. Actin in the Triton X-100-soluble supernatant co-precipitated with Galpha(i2) using either anti-Galpha(i2) or anti-actin antibodies. The principally cortical location of F-actin in hepatocytes cultured for 0.5 h changed to a pericanalicular distribution over a further 3.5 h. Some Galpha(i2) co-localized with F-actin at the plasma membrane. Pretreatment with pertussis toxin ADP-ribosylated 70-80% of Galpha(i2) in the plasma membrane and microsomes, prevented the redistribution of F-actin, caused redistribution and fragmentation of the endoplasmic reticulum, and inhibited vasopressin-stimulated Ca(2+) inflow. It is concluded that (i) a significant portion of hepatocyte Galpha(i2) associates with, and regulates the arrangement of, cortical F-actin and the endoplasmic reticulum and (ii) either or both of these regulatory roles are likely to be required for normal vasopressin activation of Ca(2+) inflow.

Disruption of beta(2)-integrin-cytoskeleton coupling abolishes the signaling capacity of these integrins on granulocytes

Integrin-dependent adhesion and dynamic modulations of the actin network are prerequisites for normal cell locomotion. To investigate whether the actin microfilamentous system does play a role in regulation of beta(2)-integrin-induced signalling, we pretreated granulocytes with staurosporine, a well-known protein kinase inhibitor that has also been shown to disrupt the cytoskeleton of intact cells. Pretreatment with staurosporine completely inhibited the beta(2)-integrin-induced Ca(2+) signal and also its ability to trigger actin polymerisation. This inhibition was not related to phosphorylation of the CD18-chain of the beta(2)-integrin, nor to inhibition of protein kinases. Instead, association of beta(2)-integrins with the cortical cytoskeleton, which was observed in untreated cells, was abolished after exposure to staurosporine, indicating that beta(2)-integrin signalling depends on integrin-cytoskeleton interaction. These results suggest not only that the actin network provides an adhesive link to the extracellular matrix and a driving force for the locomotory response, but also that it participates in regulation of beta(2)-integrin signalling during granulocyte locomotion.

Priming of phosphatidic acid production by staurosporine in f-Met-Leu-Phe-stimulated human neutrophils--correlation with respiratory burst

Staurosporine, a microbial alkaloid known as a potent though non specific PKC inhibitor, enhances the production of superoxide anion (respiratory burst) of human polymorphonuclear leukocytes (PMN) stimulated by chemoattractants such as f-Met-Leu-Phe (fMLP). To gain insights into the mechanisms of this priming, we analysed staurosporine effects on formation of second messengers issued from phospholipase D (PLD), i.e., phosphatidic acid (PA) and its dephosphorylated form, diglycerides (DG). PA and DG were measured by two methods, in mass and after the labelling of PMN with a phosphatidylcholine precursor, [3H]-1-O-alkyl-2-lyso-3-phosphatidylcholine. Treatment of labelled PMN with low concentrations of staurosporine (12.5 and 50 nM) which prime respiratory burst had no significant effect on basal amounts of tritiated PA and DG, but potentiated fMLP-mediated formation of [3H]PA and phosphatidylethanol (PEt) pointing to a priming of PLD activity. PA mass in resting PMN increased (approximately 80 +/- 7%) in the presence of high drug concentrations only (250-500 nM), with no change in basal DAG mass. Low staurosporine concentrations (6.25-25 nM) markedly potentiated PA mass formation induced by fMLP and positive correlation (R = 0.95) was found between enhanced superoxide formation and generation of PA but not DG. Furthermore, cytochalasin B, which is known to prime PA production induced by fMLP, synergised the priming of respiratory burst by staurosporine, which further suggests a functional role of PA. In contrast to staurosporine, the more selective PKC inhibitor GF109203X neither stimulated PLD nor primed fMLP-induced PLD or respiratory burst. These data indicate that priming of fMLP-mediated PMN respiratory burst by staurosporine correlates with PA formation. This priming may be linked to alteration of early signalling events upstream of PLD rather than to feedback inhibition of PKC.

The complex regulation of receptor-coupled G-proteins

Heterotrimeric G-proteins are associated with the cytoplasmic surface of the cell membrane as oligomeric structures. The oligomeric structures were deduced from a variety of studies including target (irradiation) analysis, hydrodynamic evaluation of detergent extracted material, and cross-linking of G-proteins in their membrane environment. From the functional mass determined by target analysis, it was estimated that one receptor (for glucagon) is associated with 8-10 units of Gs, the heterotrimeric G-protein that stimulates adenylyl cyclase. It is proposed that the receptor associates with each monomer of the chain via weak and strong binding forces that are dictated according to whether either GTP or GDP is bound to the alpha-subunits (weak forces) or, due to the hormone-induced release of the nucleotides during the exchange reaction, these subunits become transiently devoid of nucleotides (strong forces). The hormone-induced changes in type and degree of nucleotide binding allow for movement of the receptor along the oligomeric chain and filling of the nucleotide binding sites with the activating nucleotide, GTP. In this manner, the receptor catalytically activates Gs. It is suggested that the dynamic instability of the oligomeric chain produced by the asymmetric distribution of GTP and GDP along the chain results in release of a GTP-monomer from one end and association of a GDP-monomer at the opposite end. Adenylyl cyclase associates with the released GTP-monomer inducing a transient state of the coupled proteins. In a Mg-dependent fashion, hydrolysis of GTP occurs resulting in re-organization of the coupled proteins such that alpha and beta gamma interact with distinct domains of the cyclase molecule. The final state of the coupled process determines the degree of cyclase activity. Release of Pi from its binding site restores association of alpha and beta gamma to the GDP-bound form of the heterotrimer. The latter associates with the oligomeric structure of G-proteins to complete the cycle of events in the overall process of hormonal activation of the system.

Role of gelsolin in actin depolymerization of adherent human neutrophils

Human neutrophils generally function adherent to an extracellular matrix. We have previously reported that upon adhesion to laminin- or fibronectin-coated, but not uncoated, plastic there is a depolymerization of actin in neutrophils. This phenomenon was not affected by inhibitors of the more well-studied components of the signal transduction pathway, specifically, pertussis toxin, an inhibitor of G-proteins, H-7 or staurosporine, inhibitors of protein kinase C, or herbimycin A, an inhibitor of nonreceptor tyrosine kinase. We therefore focused our attention on actin-binding proteins and measured the changes in the partitioning of gelsolin between the Triton X-100-soluble and -insoluble cellular fractions which occur upon neutrophil adhesion by means of quantitating anti-gelsolin antibody binding to aliquots of these fractions. It was found that approximately 90% of the total cellular gelsolin was found in the Triton X-100-soluble fraction in suspended cells, but that upon adherence to either fibronectin- or laminin-coated plastic about 40% of the soluble gelsolin could be detected in the insoluble fraction. This effect was not observed in cells adherent to uncoated plastic, wherein more than 90% of the gelsolin was found in the soluble fraction. Results of immunofluorescence microscopy of these cell preparations was consistent with this data. A gelsolin translocation to the insoluble cellular actin network may account for a part of the observed actin depolymerization.

Activation of phospholipase D is an early event in integrin-mediated signalling leading to phagocytosis in human neutrophils

Integrin receptors on human neutrophils mediate adhesion and phagocytosis. These functions are linked to a signal-transduction cascade that rearranges the cytoskeleton. The intention of this study was to clarify how activation of phospholipase D (PLD) is coupled to the complement receptor three (CR3, CD18/CD11b)-mediated ingestion process. Carbobenzyloxy-leucine-tyrosine-choloromethylketone (zLYCK) inhibited PLD activation induced by complement-opsonized yeast particles (COYP) by 39%. Phagocytosis of these particles was reduced by zLYCK to the same extent. Anti-CD18-antibodies bound to protein A-positive Staphylococcus aureus bacteria induced a significant PLD activation. These particles were not ingested which implicates that CR-mediated ingestion per se is not required to induce PLD activity. Cytochalasin B-treatment, which blocks actin reorganization, partly reduced COYP-mediated PLD activity, but had no effect on activity caused by anti-CD18-coated particles. This excludes activation of PLD to be a secondary event, but rather an early signal in the phagocytic uptake prior to actin reorganization. These data suggest an important and early role for PLD in integrin-mediated phagocytosis.

Biomaterial-induced alterations of human neutrophils under fluid shear stress: scanning electron microscopical study in vitro

Morphological changes of human polymorphonuclear neutrophils (PMN) adhering to hydrophilic (glass) and hydrophobic (FEP-Teflon, polyethylene, polypropylene) surfaces were studied in a parallel-plate flow chamber at the light and scanning electron microscopical levels. The PMN were exposed to a shear stress of 0.19 Pa (1.9 dynes cm-2) or were allowed to adhere without the stress component (static control) during 30 min for all four biomaterials. Observation by light microscopy was performed in situ in the flow chamber at 1, 5, 10, 15, 20, 25 and 30 min. The total number of adherent cells as a function of time and the activation status of the population on the basis of morphological criteria were determined. On the hydrophilic material adhesion of activated PMN was significantly higher (P < 0.05) than on the more hydrophobic surfaces. This effect was most pronounced for the adhesion of neutrophils to glass and polypropylene (PP). Polyethylene (PE) showed only minor adhesion rates. Scanning electron microscopy revealed details of cell shape changes and permitted a more precise classification of populations of neutrophils based on distinctive shapes. As PMN were exposed to shear stress on glass, the majority of cells exhibited surface veils, ridges and ruffles, suggesting a high level of cell migration. In this case, on polymeric surfaces the presence of filopodial networks (FEP-Teflon) and ameoboid cell shapes (PP and PE) was noted. The results suggest that a low shear stress, as well as various chemical and physical properties of biomaterial surfaces, are together responsible for differentiation of PMN populations on solid substrata.

Direct or C5a-induced activation of heterotrimeric Gi2 proteins in human neutrophils is associated with interaction between formyl peptide receptors and the cytoskeleton

The binding of ligands to N-formyl peptide chemoattractant receptors in human neutrophils results in a rapid association of these receptors with a cytoskeletal fraction and a specific activation and release of Gi2 alpha-subunits from this fraction. In the present study we could show that pretreating neutrophils with GDPbetaS prevented the fMet-Leu-Phe-induced association of its receptor with a cytoskeletal fraction and also blocked the release of Gi2 alpha-subunits from the same cytoskeletal fraction. In contrast, direct activation of Gi2 proteins by addition of GTPgammaS or AlF4- not only caused a release of Gi2 alpha-subunits from the cytoskeleton but also an association of formyl peptide receptors with the cytoskeleton. The receptor for complement fragment 5a, which transduces its signaling through the same Gi2 protein, triggers both a release of Gi2 alpha-subunits from the cytoskeleton fraction and, of even greater interest, an association between formyl peptide receptors and the cytoskeleton. The close relationship between the activation and release of Gi2 alpha-subunits from the cytoskeleton and the association of formyl peptide receptors with the cytoskeleton might, however, not be a matter of protein-protein exchange, since the increased binding of formyl peptide receptors to the cytoskeleton occurs more rapidly than the release of Gi2 alpha-subunits from the cytoskeleton. The present findings suggest a possible mechanism for the initiation of formyl peptide receptor desensitization during neutrophil locomotion.

Leukotriene D4-induced mobilization of intracellular Ca2+ in epithelial cells is critically dependent on activation of the small GTP-binding protein Rho

We have previously shown that the leukotriene D4 (LTD4)-induced mobilization of intracellular Ca2+ in epithelial cells is mediated by a G-protein that is distinctly different from the pertussis toxin-sensitive G-protein that regulates the subsequent influx of Ca2+. In the present study, we attempted to gain further knowledge about the mechanisms involved in the LTD4-induced mobilization of intracellular Ca2+ in epithelial cells by investigating the effects of compactin, an inhibitor of the isoprenylation pathway, on this signalling event. In cells preincubated with 10 microM compactin for 48 h, the LTD4-induced mobilization of intracellular Ca2+ was reduced by 75% in comparison with control cells. This reduction was reversed by co-administration of mevalonate (1 mM). The effect of compactin occurred regardless of whether or not Ca2+ was present in the extracellular medium, suggesting that isoprenylation must occur before Ca2+ is released from intracellular stores. In accordance with this, we also found that both the LTD4-induced formation of inositol 1,4,5-trisphosphate and the LTD4-induced phosphorylation of phospholipase C gamma 1 (PLC gamma 1) on tyrosine residues were significantly reduced in compactin-pretreated cells. These results open up the possibility that the activation of PLC gamma 1 is related to a molecule that is sensitive to impaired activity of the isoprenylation pathway, such as a small monomeric G-protein. This idea was supported by the observation that Clostridium botulinum C3 exoenzyme-induced inhibition of Rho proteins abolished the LTD4-induced intracellular mobilization of Ca2+. A regulatory role of Rho proteins in the LTD4-induced activation of PLC gamma 1 is unlikely to be indirectly mediated via an effect on the cytoskeleton, since cytochalasin D had no major effect on the LTD4-induced mobilization of Ca2+. Although the mechanism of interaction remains to be elucidated, the present findings indicate an important role of an isoprenylated protein such as Rho in the LTD4-induced Ca2+ signal.

Translocation of microfilament-associated inhibitory guanine-nucleotide-binding proteins to the plasma membrane in myeloid differentiated human leukemia (HL-60) cells

The cytoskeletal localization of inhibitory guanine-nucleotide-binding (Gi) proteins and the coupling of these proteins to formyl peptide receptors were studied in myeloid differentiated human leukemia (HL-60) cells. Treatment of HL-60 cells with cytochalasin B or botulinum C2 toxin, which leads to the disruption of microfilaments, increased the binding of the stable GTP analogue guanosine 5'[gamma-thio]triphosphate (GTPS[S]) to permeabilized cells by about 30%. In contrast, the microtubule-disrupting agents colchicine and vinblastine, and cytochalasin B treatment of isolated HL-60 membranes did not affect GTP[S] binding. The stimulatory effect of cytochalasin B treatment was concentration and time dependent, with maximal increases observed at 5 micrograms/ml cytochalasin B and an incubation time of 10 min, and was counteracted by the F-actin-stabilizing toxin phalloidin. Cytochalasin B treatment increased the amount of G proteins activated by chemoattractant receptors by about 25%. Furthermore, the number of Gi-protein-coupled receptors for the chemoattractant, N-formyl-Met-Leu-Phe, was increased by about 25% upon cytochalasin B treatment. Based on these functional data, which suggest an association of G proteins with actin filaments, the Triton X-100 (1%)-insoluble cytoskeleton was analyzed for the presence of G proteins. Gia subunits were detected in the cytoskeleton preparations, both by specific antisera and by pertussis-toxin -catalyzed ADP-ribosylation. Cytochalasin B pretreatment depleted the cytoskeleton in Gialpha, with an approximately 20% concomitant increase in membrane Gialpha content. In conclusion, evidence is presented that part of the cellular Gia is localized at actin filaments in HL-60 cells. After filament disruption, these Gia subunits seem to be translocated to the plasma membrance, where they can productively interact with chemoattractant receptors.

Effect of cytochalasin B on intracellular free calcium concentration in human polymorphonuclear leukocytes after repeated stimulation with n-formyl-methionyl-leucyl-phenylalanine

Cytochalasin B can influence various functions of human polymorphonuclear leukocytes, including chemotaxis, lysosomal enzyme release, and reactive oxygen species generation. In this study we investigated the effect of cytochalasin B on the increase in intracellular free calcium concentration after three consecutive additions of 10(-7) M N-formyl-methionyl-leucyl-phenylalanine. The interval between stimulations was 5 min. Intracellular free calcium concentration was monitored using the fluorescent calcium indicator FURA-2AM. Cytochalasin B (3.3 micrograms/ml) added 60 s before the cell stimulation enhanced all three polymorphonuclear leukocyte calcium responses by increasing the N-formyl-methionyl-leucyl-phenylalanine-induced calcium influx from the extracellular space. Cytochalasin B increased the peak intracellular free calcium concentration and elevated the plateau phase level, but had no influence on its shape. In addition, pretreatment with cytochalasin B of polymorphonuclear leukocytes suspended in low calcium medium restored their capacity to respond to a third stimulation with N-formyl-methionyl-leucyl-phenylalanine. Finally, in resting cells cytochalasin B caused a moderate increase in intracellular free calcium concentration which was independent of extracellular calcium.

Chemoattractant-induced NADPH oxidase activity in human monocytes is terminated without any association of receptor-ligand complex to cytoskeleton

When the chemotactic peptide formylmethionyl-leucyl-phenylalanine binds to its cell surface receptor, a transmembrane signal is generated that activates the superoxide-producing NADPH oxidase of human phagocytes. Comparing monocytes and neutrophils with regard to the production of superoxide anion induced by the peptide, we found a similar time-course for both types of cells. In neutrophils, ligand binding induced a conversion of the receptor to a high-affinity form, a change suggested to be due to an association of the receptor-ligand complex to the Triton X-100-insoluble cytoskeleton. This event has been hypothesized to terminate the signal that activates the NADPH oxidase and thereby results in cessation of the cellular production of superoxide anion. Neutrophils preincubated with the cytoskeleton-disrupting drug cytochalasin B showed an increased and prolonged superoxide anion production after activation with the peptide, thus indicating that the cytoskeleton is involved in terminating this response. Formylmethionyl-leucyl-phenylalanine was also found to induce polymerization of actin in monocytes; however, cytochalasin B had no effect on the peptide-induced generation of superoxide anion in these cells. Furthermore, also in monocytes, ligand binding induced a conversion of the receptor to a high-affinity form; however, the receptor-ligand complex did not coisolate with the Triton X-100-insoluble cytoskeleton. These results indicate that, in monocytes, the NADPH oxidase activating pathway is terminated without any association of the receptor-ligand complex to the Triton X-100-insoluble cytoskeleton.

Moesin, ezrin, and p205 are actin-binding proteins associated with neutrophil plasma membranes

Actin-binding proteins in bovine neutrophil plasma membranes were identified using blot overlays with 125I-labeled F-actin. Along with surface-biotinylated proteins, membranes were enriched in major actin-binding polypeptides of 78, 81, and 205 kDa. Binding was specific for F-actin because G-actin did not bind. Further, unlabeled F-actin blocked the binding of 125I-labeled F-actin whereas other acidic biopolymers were relatively ineffective. Binding also was specifically inhibited by myosin subfragment 1, but not by CapZ or plasma gelsolin, suggesting that the membrane proteins, like myosin, bind along the sides of the actin filaments. The 78- and 81-kDa polypeptides were identified as moesin and ezrin, respectively, by co-migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoprecipitation with antibodies specific for moesin and ezrin. Although not present in detectable amounts in bovine neutrophils, radixin (a third and closely related member of this gene family) also bound 125I-labeled F-actin on blot overlays. Experiments with full-length and truncated bacterial fusion proteins localized the actin-binding site in moesin to the extreme carboxy terminus, a highly conserved sequence. Immunofluorescence micrographs of permeabilized cells and cell "footprints" showed moesin co-localization with actin at the cytoplasmic surface of the plasma membrane, consistent with a role as a membrane-actin-linking protein.

Neutrophil control of formylmethionyl-leucyl-phenylalanine induced mobilization of secretory vesicles and NADPH-oxidase activation: effect of an association of the ligand-receptor complex to the cytoskeleton

The stimulus formylmethionyl-leucyl-phenylalanine (FMLP) interacts with neutrophils and generates signal(s) in the cells that induces mobilization of the secretory vesicles as well as activation of the superoxide anion/hydrogen peroxide generating NADPH-oxidase. Binding, at 15 degrees C, of FMLP to its neutrophil surface receptor is followed by an association of the ligand-receptor complex to the cell cytoskeleton, and this association occurs concomitant with a desensitization of the cells with respect to activation of the NADPH-oxidase. Other stimuli can still activate the oxidase (in fact even induce a primed response), indicating that the observed phenomenon is stimulus specific and could not be accounted for by an effect on the oxidase itself, but rather that the association of the ligand-receptor complex to the cytoskeleton eliminates the capacity of the complex to generate the signal(s) that activates the NADPH-oxidase. The cytoskeleton associated ligand-receptor complex generates, however, the signal(s) responsible for mobilization of the secretory vesicles, to the plasma membrane, and this mobilization occurs without any increase in the intracellular concentration of free Ca2+.

Sequences responsible for intracellular localization of beta-actin messenger RNA also affect cell phenotype

We have characterized the structure and function of RNA sequences that direct beta-cytoplasmic actin mRNA to the cell periphery were mapped to two segments of 3'-untranslated region by expression of LacZ/beta-actin chimeric mRNAs in chicken embryo fibroblasts (CEFs). A 54-nt segment, the "RNA zipcode," and a homologous but less active 43-nt segment each localized beta-galactosidase activity to the leading lamellae. This zipcode contains the full activity, and mutations or deletions within it reduce, but do not eliminate, its activity, indicating that several motifs contribute to the activity. Two of these motifs, when multimerized, can regenerate almost full activity. These sequences are highly conserved in evolution, since the human beta-actin zipcode, positioned identically in the 3'UTR localizes equally well in chicken cells. Complementary phosphorothioate oligonucleotides against the zipcode delocalized endogenous beta-actin mRNA, whereas those complementary to the region just outside the zipcode, or sense oligonucleotides, did not. Actin mRNA or protein levels were unaffected by the antisense treatments, but a dramatic change in lamellipodia structure, and actin stress fiber organization was observed using the same antizipcode oligonucleotides which delocalized the mRNA. Hence, discrete 3'UTR sequences direct beta-actin isoform synthesis to the leading lamellae and affect cell morphology, presumably through the actin cytoskeleton.

Glycoprotein IIb-IIIa and the translocation of Rap2B to the platelet cytoskeleton

The stimulation of human platelets with physiological agonists results in the incorporation of several proteins into the cytoskeleton, fibrinogen binding, and platelet aggregation. We recently demonstrated that the Ras-related low molecular weight GTP-binding protein Rap2B associates with the cytoskeleton in activated platelets and that this interaction requires platelet aggregation. In the present study we demonstrate that agonist-induced actin polymerization is necessary for the translocation of Rap2B to the cytoskeleton, suggesting that Rap2B interacts with the newly formed actin filaments. Moreover, the association of Rap2B with Triton X-100-insoluble material from platelets was totally blocked by treatment of intact platelets with monoclonal antibodies against the fibrinogen receptor glycoprotein IIb-IIIa. Platelets from patients affected by Glanzmann thrombastenia, a genetic disorder in which platelet plasma membranes lack glycoprotein IIb-IIIa but possess normal levels of Ras-related proteins, failed to incorporate Rap2B into the cytoskeleton upon activation by thrombin. Comparative immunoblotting revealed that the translocation of Rap2B to the cytoskeleton during platelet aggregation was accompanied by the simultaneous translocation of glycoprotein IIb-IIIa. Moreover, the cytoskeleton from aggregated platelets contained Rap2B and glycoprotein IIb-IIIa in comparable amounts. These results demonstrate the association of Rap2B and glycoprotein IIb-IIIa and their translocation to the cytoskeleton in aggregated human platelets.

Neutrophil chemoattractant receptors and the membrane skeleton

Signal transduction via receptors for N-formylmethionyl peptide chemoattractants (FPR) on human neutrophils is a highly regulated process which involves participation of cytoskeletal-elements. Evidence exists suggesting that the cytoskeleton and/or the membrane skeleton controls the distribution of FPR in the plane of the plasma membrane, thus controlling the accessibility of FPR to different proteins in functionally distinct domains. In desensitized cells, FPR are restricted to domains which are depleted of G proteins but enriched in cytoskeletal proteins such as actin and fodrin. Thus, the G protein signal transduction partners of FPR become inaccessible to the agonist-occupied receptor, preventing cell activation. The mechanism of interaction of FPR with the membrane skeleton is poorly understood but evidence is accumulating that suggests a direct binding of FPR (and other receptors) to cytoskeletal proteins such as actin.

Effects of fibronectin on actin organization and respiratory burst activity in neutrophils, monocytes, and macrophages

Previous studies have shown that fibronectin (Fn) enhances phagocytosis and killing of antibody-coated bacteria by neutrophils and macrophages. In an attempt to understand the mechanism of this enhancement, we have investigated the effects of Fn on phagocytosis-related actin organization as well as respiratory burst activity in neutrophils, monocytes and culture-derived macrophages. Employing an NBD-phallacidin flow cytometric analysis of filamentous actin formation, we found that Fn promotes rapid actin polymerization within 30 seconds in neutrophils, monocytes, and macrophages, but not lymphocytes. Enhancement of actin polymerization by Fn was concentration-dependent and mediated by a pertussis toxin- but not cholera toxin-sensitive G protein. Inhibition of protein kinase C by sphingosine (20 microM), calcium influx by verapamil (0.1 mM), or intracellular calcium mobilization by 8-(N,N-diethyl-amino) octyl-3,4,5-trimethoxybenzoate HCl (TMB-8; 0.1 mM) did not block Fn-enhanced actin polymerization in phagocytes. Incubation of neutrophils and macrophages on microtiter plates precoated with Fn suppressed superoxide (O2-) production induced by IgG- and IgA- opsonized group B streptococci. In contrast, Fn significantly enhanced IgA- and IgG-mediated O2- production by freshly isolated monocytes. These data suggest that Fn enhances phagocytosis, presumably through G protein-coupled cytoskeleton reorganization and augments O2- production by circulating monocytes. In contrast, it appears to suppress O2- production by the active phagocytic cells, neutrophils and macrophages. This may result in enhanced phagocytosis and intracellular killing of microorganisms without damaging interstitial tissues.

Beta 2 integrin engagement triggers actin polymerization and phosphatidylinositol trisphosphate formation in non-adherent human neutrophils

Beta 2 integrins are involved in the adhesion of leukocytes to other cells and surfaces. Although adhesion is required for cell locomotion, little is known regarding the way beta 2 integrin-receptors affect the actin network in leukocytes. In the present study filamentous actin (F-actin) levels in non-adherent human neutrophils have been measured by phalloidin staining after antibody cross-linking of beta 2 integrins. Antibody engagement of beta 2 integrins resulted in a rapid and sustained (146 and 131% after 30 and 300 s, respectively) increase in the neutrophil F-actin content. This is in contrast to stimulation with N-formyl-l-methionyl-l-leucyl-l-phenylalanine (fMLP), which causes a prompt and pronounced but rapidly declining rise in F-actin (214 and 127% after 15 and 300 s, respectively). Priming neutrophils with 1 nM PMA, a low concentration that did not influence the F-actin content per se, increased the magnitude of the beta 2 integrin-induced response but had no effect on the kinetics (199% after 30 s and 169% after 300 s). Removal of extracellular Ca2+ only marginally affected the beta 2 integrin-induced F-actin response for cells that were pretreated with PMA whereas the response for nonprimed cells was reduced by half. This suggests that even though extracellular Ca2+ has a modulatory effect it is not an absolute requirement for beta 2 integrin-induced actin polymerization. beta 2 integrin engagement did not affect the resting cellular level of cAMP arguing against a role of cAMP in beta 2 integrin-induced actin assembly.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytoskeletal regulation of chemotactic receptors: molecular complexation of N-formyl peptide receptors with G proteins and actin

Signal transduction via receptors for N-formylmethionyl peptide chemoattractants (FPR) on human neutrophils is a highly regulated process. It involves direct interaction of receptors with heterotrimeric G-proteins and may be under the control of cytoskeletal elements. Evidence exists suggesting that the cytoskeleton and/or the membrane skeleton determines the distribution of FPR in the plane of the plasma membrane, thus controlling FPR accessibility to different proteins in functionally distinct membrane domains. In desensitized cells, FPR are restricted to domains which are depleted of G proteins but enriched in cytoskeletal proteins such as actin and fodrin. Thus, the G protein signal transduction partners of FPR become inaccessible to the agonist-occupied receptor, preventing cell activation. We are investigating the molecular basis for the interaction of FPR with the membrane skeleton, and our results suggest that FPR, and possibly other receptors, may directly bind to cytoskeletal proteins such as actin.

Identification and localization of G-proteins in the clonal adipocyte cell lines HGFu and Ob17

HGFu and Ob17 are cell lines derived from adipose tissue of lean (+/?) and ob/ob mice, respectively. Neither adenylyl cyclase activity nor G protein abundance and subcellular distribution have been assessed previously in these cells. Cyclase activity was low and resistant to catecholamine stimulation in both cell lines. However, the enzyme could be stimulated to high levels by forskolin and Mn2+. Gs alpha (largely the long isoform), Gi alpha 2, and G beta were the major G protein subunits identified. The levels of G protein mRNA expression were similar in both cell lines and, unlike actin expression, did not change as a result of differentiation. Immunoblotting and ADP-ribosylation of the G peptides corroborated these results. Assessment of the subcellular localization of the subunits by indirect epifluorescence and scanning confocal microscopy showed that each of the subunits had a characteristic subcellular pattern. Gs alpha showed vesicular cytoplasmic and nuclear staining; Gi alpha 2 colocalized with actin stress fibers and disruption of these structures altered the distribution of Gi alpha 2; beta subunits showed some colocalization with the stress fibers as well as a cytoplasmic vesicular and nuclear pattern. As a result of differentiation, there was reorganization of the actin, together with the Gi alpha 2 and beta fibrous patterns. Both cell lines showed similar modifications. The induction of differentiation in these cells is therefore not associated with changes in adenylyl cyclase activity nor of the abundance of G-protein subunits, although reorganization of some of these subunits does accompany actin reorganization.

Stimulus-induced dissociation of alpha subunits of heterotrimeric GTP-binding proteins from the cytoskeleton of human neutrophils

Previous studies on the mechanism responsible for terminating the generation of second messengers induced by chemotactic factor-receptor complexes have, on one hand, suggested a direct role of a GTP-binding protein(s) (G protein), and, on the other hand, proposed that there is a lateral segregation of the ligand-receptor complexes into G protein-depleted domains of the plasma membrane. In the present investigation, which addresses these apparently contradictory findings, we found that a substantial part of the alpha subunits of the Gn protein (Gn alpha) in unstimulated neutrophils were associated with a cytoskeletal fraction and that release of these subunits occurred upon stimulation with the chemotactic factor fMet-Leu-Phe. An identical Gn alpha release could also be induced by direct activation of G proteins with guanosine 5'-[gamma-thio]triphosphate or AIF4-. In contrast, the alpha subunits of the stimulatory G protein (Gs alpha) also found associated with the cytoskeletal fraction of unstimulated cells were not released by fMet-Leu-Phe stimulation. However, they were effectively released by direct G-protein activation with guanosine 5'-[gamma-thio]triphosphate. In addition, inhibition of the fMet-Leu-Phe-stimulated modulation of the actin network by pertussis toxin did not affect the fMet-Leu-Phe-induced release of Gn alpha from the cytoskeletal fraction. These observations indicate that fMet-Leu-Phe-induced activation of neutrophils involves a specific dissociation of Gn alpha from the cytoskeleton and that this release is not a consequence of the well-known effect of fMet-Leu-Phe on the cytoskeleton of neutrophils. The present data contribute ideas concerning the transducing properties of G proteins in cellular signaling and seem to reconcile the apparently contradictory concepts of how the cytoskeleton participates in the termination of the chemotactic-factor-induced generation of second messengers in human neutrophils.

Pertussis toxin-induced redistribution of cortical actomyosin and inhibition of phagocytosis in rat Kupffer cells

The mechanism of phagocytosis by Kupffer cells has been shown to be related to the Ca(2+)-calmodulin and the actomyosin systems. However the role of the transmembrane signal transmitter, G-protein, is still unknown. In this study, a quantitative evaluation of phagocytosis by Kupffer cells of rats in culture and the effects of pertussis toxin, a G-protein inhibitor, on the phagocytic function and morphology of Kupffer cells were investigated. Pertussis toxin inhibited phagocytosis of Kupffer cells with dysfunction of the actomyosin system. The inhibitory effects of pertussis toxin suggest that G-protein may be involved in the mechanism of transmembrane signalling in phagocytosis by Kupffer cells.

Protein kinase C activity is not involved in N-formylmethionyl-leucyl-phenylalanine-induced phospholipase D activation in human neutrophils, but is essential for concomitant NADPH oxidase activation: studies with a staurosporine analogue with improved selectivity for protein kinase C

Stimulation of human neutrophils by the receptor agonist N-formylmethionyl-leucyl-phenylalanine (fMLP) results in a respiratory burst, catalysed by an NADPH oxidase. Concomitantly, phospholipase D (PLD) is activated. To investigate the role of protein kinase C (PKC) in these neutrophil responses, we have compared the effects of staurosporine and a structural analogue of staurosporine (cgp41251), that reflects a higher selectivity towards PKC [Meyer, Regenass, Fabbro, Alteri, Rösel, Müller, Caravatti and Matter (1989) Int. J. Cancer 43, 851-856]. Both staurosporine and cgp41251 dose-dependently inhibited the production of superoxide induced by phorbol 12-myristate 13-acetate (PMA). Both compounds also caused inhibition of the fMLP-induced respiratory burst, but with a lower efficacy during the initiation phase of this response. This latter observation cannot be taken as evidence against PKC involvement in the activation of the respiratory burst, because pretreatment of neutrophils with ionomycin before PMA stimulation also results in a lower efficacy of inhibition. Activation of PLD by fMLP was enhanced in the presence of staurosporine, but not in the presence of cgp41251. Enhancement of PLD activation was also observed in the presence of H-89, an inhibitor of cyclic-AMP-dependent protein kinase (PKA). Both staurosporine and H-89 reversed the dibutyryl-cyclic-AMP-induced inhibition of PLD activation, whereas cgp41251 was without effect. These results indicate that the potentiating effect of staurosporine on PLD activation induced by fMLP does not reflect a feedback inhibition by PKC activation, but instead a feedback inhibition by PKC activation. Taken together, our results indicate that in human neutrophils: (i) PKC activity is not essential for fMLP-induced activation of PLD; (ii) PKC activity does play an essential role in the activation of the respiratory burst by fMLP, other than mediating or modulating PLD activation; (iii) there exists a negative-feedback mechanism on fMLP-induced PLD activation by concomitant activation of PKA.

N-formyl peptide receptors in human neutrophils display distinct membrane distribution and lateral mobility when labeled with agonist and antagonist

Receptors for bacterial N-formyl peptides are instrumental for neutrophil chemotactic locomotion and activation at sites of infection. As regulatory mechanisms for signal transduction, both rapid coupling of the occupied receptor to cytoskeletal components, and receptor lateral redistribution, have been suggested (Jesaitis et al., 1986, 1989). To compare the distribution and lateral diffusion of the nonactivated and activated neutrophil N-formyl-peptide receptor, before internalization, we used a new fluorescent N-formyl-peptide receptor antagonist, tertbutyloxycarbonyl-Phe(D)-Leu-Phe(D)-Leu-Phe-OH (Boc-FLFLF, 0.1-1 microM), and the fluorescent receptor agonist formyl-Nle-Leu-Phe-Nle-Tyr-Lys (fnLLFnLYK, 0.1-1 microM). Fluorescent Boc-FLFLF did not elicit an oxidative burst in the neutrophil at 37 degrees C, as assessed by chemiluminescence and reduction of p-nitroblue tetrazolium chloride, but competed efficiently both with formyl-methionyl-leucyl-phenylalanine (fMLF) and fnLLFnLYK. It was not internalized, as evidenced by confocal microscopy and acid elution of surface bound ligand. The lateral mobility characteristics of the neutrophil fMLF receptor were investigated with the technique of FRAP. The diffusion coefficient (D) was similar for antagonist- and agonist-labeled receptors (D approximately 5 x 10(-10) cm2/s), but the fraction of mobile receptors was significantly lower in agonist- compared to antagonist-labeled cells, approximately 40% in contrast to approximately 60%. This reduction in receptor mobile fraction was slightly counteracted, albeit not significantly, by dihydrocytochalasin B (dhcB, 5 microM). To block internalization of agonist-labeled receptors, receptor mobility measurements were done at 14 degrees C. At this temperature, confocal microscopy revealed clustering of receptors in response to agonist binding, compared to a more uniform receptor distribution in antagonist-labeled cells. The pattern of agonist-induced receptor clustering was less apparent after dhcB treatment. To summarize, this work shows that activated N-formyl peptide receptors aggregate and immobilize in the plane of the neutrophil plasma membrane before internalization, a process that is affected, but not significantly reversed, by cytochalasin. The results are consistent with a model where arrested receptors are associated mainly with a cytochalasin-insensitive pool of cytoskeletal elements.

Assembly dynamics of actin in adherent human neutrophils

We have extended our previous studies of adherent neutrophils and compared actin depolymerization and intracellular calcium changes induced by adherence to laminin and fibronectin. In order to accurately assess cellular actin changes, F-actin depolymerization in the cell lysates must be inhibited. We found that phalloidin or 3.7% formaldehyde treatment effectively inhibited the depolymerization of F-actin fragments following cell lysis. Formaldehyde and phalloidin treatment reduced G-actin levels 75-80% in suspended cells, 35-73% in cells adherent for 1 min, and about 50% for cells adherent for 3 min. When the actin was fixed, there were highly significant differences in G-actin levels between the suspended and adherent cells as compared with unfixed cells. Adhesion to both laminin and fibronectin initiated a rapid rise in G-actin with a corresponding decrease in F-actin. However, the changes were more pronounced in cells adherent to laminin. The peak of depolymerization occurred by 1 min and, thereafter, G-actin decreased and F-actin increased reaching a steady state at 5 min. Adhesion to both laminin- and fibronectin-coated surfaces was accompanied by an increase of [Ca2+]i with a peak at 3 min, followed by a decrease from 3-5 min and a steady state attained between 5 and 10 min. The rise of [Ca2+]i in laminin-adherent cells was about twice that in fibronectin-adherent cells at 3 min (P < 0.02). Pertussis toxin, H-7, and staurosporin treatments did not alter the dynamic changes of actin in adherent cells, suggesting that these metabolic events are transduced by a G-protein and Protein Kinase C independent mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

A GTPase controls cell-substrate adhesion in Xenopus XTC fibroblasts

Cell-substrate adhesion is crucial at various stages of development and for the maintenance of normal tissues. Little is known about the regulation of these adhesive interactions. To investigate the role of GTPases in the control of cell morphology and cell-substrate adhesion we have injected guanine nucleotide analogs into Xenopus XTC fibroblasts. Injection of GTP gamma S inhibited ruffling and increased spreading, suggesting an increase in adhesion. To further investigate this, we made use of GRGDSP, a peptide which inhibits binding of integrins to vitronectin and fibronectin. XTC fibroblasts injected with non-hydrolyzable analogs of GTP took much more time to round up than mock-injected cells in response to treatment with GRGDSP, while GDP beta S-injected cells rounded up in less time than controls. Injection with GTP gamma S did not inhibit cell rounding induced by trypsin however, showing that cell contractility is not significantly affected by the activation of GTPases. These data provide evidence for the existence of a GTPase which can control cell-substrate adhesion from the cytoplasm. Treatment of XTC fibroblasts with the phorbol ester 12-o-tetradecanoylphorbol-13-acetate reduced cell spreading and accelerated cell rounding in response to GRGDSP, which is essentially opposite to the effect exerted by non-hydrolyzable GTP analogs. These results suggest the existence of at least two distinct pathways controlling cell-substrate adhesion in XTC fibroblasts, one depending on a GTPase and another one involving protein kinase C.

Tumor cell adhesion to the extracellular matrix and signal transduction mechanisms implicated in tumor cell motility, invasion and metastasis

The relationship between tumor cell adhesion to the extracellular matrix (ECM) and invasion and metastasis formation is one of the most intensively studied topics in cancer biology within the last 10-15 years. The aberrant molecular relationships between malignant tumor cells and their surrounding ECM have been implicated at virtually every stage of the metastatic process; ranging from steps that involve the local invasion of tumor cells away from the primary tumor to those that are involved in mediating extravasation through microvessel-associated basement membranes at the site(s) of metastasis formation. The complexity of tumor metastasis has required that a reductionist approach be taken in order to identify and relate specific molecular mechanisms involved in tumor cell adhesion to various aspects of tumor metastasis. The intensive research efforts into cell adhesion and tumor cell biology have generated many significant new concepts towards our understanding of the molecular aspects of tumor cell adhesion and metastasis. Our purpose in this article is to briefly summarize the relationship of ECM-stimulated tumor cell adhesion to the processes of tumor cell motility and invasion. This is followed by a discussion of certain aspects of signal transduction pathways that may impact on cell motility, with an emphasis on the relationship between phosphatidylinositol hydrolysis and actin polymerization, as well as certain GTP-binding protein-(G-protein) mediated events that could influence cytoskeletal organization and cell motility. Our emphasis is based on increasing evidence that implicates members of the signal transduction G-proteins in the motility and invasion of many normal and transformed cells.

The epidermal growth factor receptor is associated with actin filaments

In this paper we describe our investigations on the association of receptors for the epidermal growth factor (EGF) with the cytoskeleton of A431 cells. In order to determine which filamentous system the EGF receptors are associated to, the cytoskeletal fraction to which these receptors bind was isolated. Second, the possible colocalization of EGF receptors with different cytoskeletal elements was examined in A431 cells. By selective extractions of the A431 cytoskeletons, it is shown that more than 90% of the cytoskeleton-associated EGF receptors are removed from the cytoskeletons together with the actin filamentous system. During several cycles of poly- and depolymerization of actin isolated from A431 cells, the EGF receptor precipitates together with the actin containing filaments, indicating that EGF receptors are able to bind in vitro to actin filaments. With immunofluorescence studies we show that EGF receptors especially colocalize with actin filaments. These results demonstrate that the EGF receptor is associated specifically with actin filaments in A431 cells.

Stimulus-response uncoupling in the neutrophil. Adenosine A2-receptor occupancy inhibits the sustained, but not the early, events of stimulus transduction in human neutrophils by a mechanism independent of actin-filament formation

Generation of superoxide anion (O2-) in response to occupancy of neutrophil chemoattractant receptors requires both early events ('triggering') and sustained signals ('activation'). We have previously demonstrated that occupancy of adenosine A2 receptors inhibits O2- generation by neutrophils. In parallel, adenosine-receptor occupancy promotes association of bound N-formylmethionyl-leucyl-phenylalanine (fMLP) receptors with the cytoskeleton, a process associated with termination of neutrophil activation (stimulus-response uncoupling). We undertook this study to determine whether inhibition of neutrophil function by adenosine-receptor occupancy requires intact actin filaments and to examine the effect of adenosine-receptor occupancy on the stimulated generation of intracellular signals involved in neutrophil triggering and activation. Occupancy of adenosine A2 receptors by 5'-N-ethylcarboxamidoadenosine (NECA, 1 microM) significantly increased (130 +/- 1% of control, P less than 0.001, n = 3) association of [3H]fMLP with cytoskeletal preparations. Cytochalasin B (5 micrograms/ml), an agent which disrupts actin filaments, completely blocked association of [3H]fMLP with cytoskeletal preparations, as previously reported. However, NECA markedly increased association of [3H]fMLP with the cytoskeleton even in the presence of cytochalasin B (P less than 0.0002). Moreover, NECA did not significantly affect either the early (30s) or the late (5 min) formation of actin filaments after stimulation by chemoattractant (fMLP, 0.1-100 nM). Cytochalasin B markedly inhibited actin-filament formation by stimulated neutrophils, and NECA did not reverse the effect of cytochalasin B on actin-filament formation. Adenosine-receptor occupancy did not affect the rapid peak in diacylglycerol generation (less than or equal to 15 s) from either [3H]arachidonate- or [14C]glycerol-labelled phospholipid pools. However, as would be predicted if occupancy of the adenosine receptor was a signal for early termination of cell activation, NECA (1 microM) markedly diminished the slow sustained generation of diacylglycerol. These results suggest that adenosine-A2-receptor occupancy does not affect triggering of the neutrophil, but that occupancy of adenosine receptors is an early signal for the termination of neutrophil activation, i.e. the 'premature' finish of signal transduction. Moreover, these data indicate that at least two pathways are available for increasing the association of ligated chemoattractant receptors with the cytoskeleton of neutrophils: F-actin-dependent and -independent.

Neomycin induces stimulatory and inhibitory effects on leukotriene generation, guanine triphosphatase activity, and actin polymerization within human neutrophils

The effects of neomycin on human neutrophils (PMN) were studied with respect to the generation of leukotrienes, the involvement of guanine triphosphate binding proteins (G proteins) and the polymerization of actin. Incubation of neutrophils with neomycin induced the generation of low amounts of leukotrienes. Co-incubation of neutrophils with neomycin and the direct G-protein activator sodium fluoride (NaF) resulted in an enhanced leukotriene formation at 0.5 mM neomycin and an inhibition at a concentration of 10 mM. Simultaneous incubation with neomycin and formyl-methionyl-leucyl-phenylalanine (FLMP) did not affect the FMLP-induced leukotriene formation. However, pretreatment of neutrophils with 10 mM neomycin followed by the addition of NaF or FMLP resulted in an enhanced generation of leukotrienes. Crude membrane fractions of PMN incubated with neomycin at different concentrations showed an enhanced (0.5 mM) as well as a reduced (10 mM) guanine triphosphatase activity. Furthermore, incubation of neutrophils with neomycin above a concentration of 0.5 mM led to the depolymerization of actin. The presented results show inhibitory and stimulatory effects of neomycin on various cell functions, which may reflect differences in transmembrane signalling.

A comprehensive analysis of the developmental and tissue-specific expression of the isoactin multigene family in the rat

The present study represents the first comprehensive analysis of isoactin gene expression in the developing rat. Our results clearly demonstrate that the developmental and tissue-specific expression of the actin multigene family is a highly integrated and complex process involving a variety of regulatory paradigms. The distinct temporal patterns of expression reported in this study indicate that there are three key phases in the regulation of expression of the actin multigene family during development. These include early embryonic development, late fetal development, and early postnatal development. The specific spatial patterns of expression observed in this study demonstrate that the expression of the actin multigene family is much more permissive than previously reported. This permissive expression includes a wide range of "ectopic" expression of the striated muscle isoactins as well as an extended expression of the alpha-smooth muscle isoactin. These findings expand our current understanding of the expression of the actin multigene family in development and provide a fundamental basis for future studies directed at investigating these processes.

Impaired secretion and increased insolubilization of IgE-receptor complexes in mycophenolic acid-treated (guanine nucleotide-depleted) RBL-2H3 mast cells

In RBL-2H3 rat leukemic mast cells, cross-linking anti-DNP IgE-receptor complexes with multivalent antigen (DNP-BSA) activates a signal transduction pathway leading to Ca2+ influx and secretion. Cross-linking IgE-receptor complexes also stimulates a pathway that inactivates (desensitizes) receptors; this pathway becomes important at high concentrations of cross-linking antigen. Recent evidence that antigen-induced secretion is impaired by mycophenolic acid (MPA), an inhibitor of guanine nucleotide synthesis de novo, has implicated a GTP-binding protein (G protein) in the signaling pathway. Other recent studies have indicated that the conversion of cross-linked receptors to a detergent-insoluble (cytoskeleton-associated) form at high antigen concentrations is correlated with the loss of signaling activity. Here we show that secretion elicited by an optimal concentration of antigen (0.05 micrograms/ml DNP-BSA) is only inhibited by about 25% in guanine nucleotide-depleted cells, whereas secretion elicited by 5 micrograms/ml DNP-BSA, a concentration in the range that causes the high-dose inhibition of secretion, is inhibited by more than 60%. We also show that IgE-receptor complexes are insolubilized in response to 5 but not 0.05 micrograms/ml DNP-BSA in both control and guanine nucleotide-depleted cells. Importantly, the extent of insolubilization elicited by 5 micrograms/ml DNP-BSA is increased by more than 60% in the guanine nucleotide-depleted samples. These results raise the possibility that guanine nucleotide depletion reduces the secretory response to high antigen concentrations in two ways: by inhibiting the G protein-coupled signaling pathway and by increasing the availability of receptors to the pathway leading to receptor insolubilization and inactivation.

Relationship of IgE receptor topography to secretion in RBL-2H3 mast cells

In RBL-2H3 rat leukemic mast cells, cross-linking IgE-receptor complexes with anti-IgE antibody leads to degranulation. Receptor cross-linking also stimulates the redistribution of receptors on the cell surface, a process observed here by labeling the anti-IgE with 15 nm protein A-gold particles that are visible by back-scattered electron imaging in the scanning electron microscope. We report that anti-IgE binding stimulates the redistribution of IgE-receptor complexes at 37 degrees C from a dispersed topography to distributions dominated sequentially by short chains, small clusters, and large aggregates of cross-linked receptors. Cells incubated with 1 microgram/ml anti-IgE, a concentration that stimulates maximum net secretion, redistribute receptors into chains and small clusters during a 15 min incubation period. At 3 and 10 micrograms/ml anti-IgE, net secretion is reduced and the majority of receptors redistribute rapidly into clusters and large aggregates. The addition of Fab fragments with the high anti-IgE concentrations, to reduce cross-linking, delays receptor aggregation and enhances secretion. The progression of receptors from small clusters to large aggregates is prevented in cells treated with dihydrocytochalasin B to prevent F-actin assembly. These results establish that characteristic patterns of receptor topography are correlated with receptor activity. In particular, they link the formation of large receptor aggregates to reduced signalling activity. Cytoskeleton-membrane interaction is implicated in the formation or stabilization of the large receptor clusters.

Down-regulation of receptor antigen in leukotriene B4-induced chemotactic deactivation of human polymorphonuclear leucocytes

Pretreatment of suspensions of human polymorphonuclear leucocytes (PMNL) with leukotriene B4 (LTB4) induces chemotactic deactivation, characterized by diminished expression of high-affinity LTB4 receptors and selectively decreased chemotactic responsiveness of the PMNL to LTB4. Rabbit anti-idiotypic antibodies (a-Id) to mouse monoclonal anti-LTB4, which bind to 60,000-80,000 molecular weight (MW) membrane protein of PMNL receptors for LTB4 in Western blots and block binding of [3H]LTB4 to high-affinity receptors of PMNL, detected a reduction in LTB4 receptor antigen during chemotactic deactivation. Loss of high-affinity receptors for LTB4 from the surface of PMNL deactivated by incubation with 10 nM LTB4 was significant after 1 min and after 20 min reached a mean maximum of 82% and 61%, respectively, as assessed by binding of [3H]LTB4 and a-Id. Inhibitors of PMNL proteases did not prevent the deactivation-induced decreases in surface receptors for LTB4. Disruption of deactivated PMNL and solubilization of membrane proteins failed to expose intracellular LTB4 receptors. Incubation of membranes isolated from PMNL with 100 nM LTB4 resulted in a loss of LTB4 receptors similar to that observed in intact PMNL. Changes in LTB4 receptor protein structure or membrane localization, rather than endocytosis or proteolysis, thus appear to explain the rapidly decreased expression of LTB4 receptors, which results from stimulus-specific deactivation.

Auranofin dissociates chemotactic peptide-induced generation of inositol 1,4,5-trisphosphate from the subsequent mobilization of intracellular calcium in intact human neutrophils

Auranofin, an antiarthritic gold compound, modulates a number of chemotactic factor-induced inflammatory responses in human neutrophils. In order to unravel the mechanism involved, the present study investigated the effects of auranofin on early signal transduction events in these cells. Auranofin did not affect the chemotactic peptide (fMetLeuPhe)-induced formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), neither in the presence nor in the absence of extracellular calcium ions. In contrast, there was a progressive inhibition by auranofin on the fMet-Leu-Phe-induced mobilization of intracellular calcium. This demonstrates that auranofin can dissociate the generation of Ins(1,4,5)P3 from the subsequent release of intracellular calcium, perhaps by interfering with the intracellular binding of Ins(1,4,5)P3 to its receptor. In experiments performed in electro-permeabilized cells, however, a relatively high concentration of the drug failed to abolish the specific binding of Ins(1,4,5)P3. In addition, in the same system, auranofin also failed to abolish the Ins(1,4,5)P3-induced release of Ca2+. Consequently, auranofin-mediated dissociation of fMLP-induced Ins(1,4,5)P3 formation and intracellular calcium release can not be explained merely by an antagonistic effect of auranofin on the Ins(1,4,5)P3 receptor. Instead the interaction between auranofin and the plasma membrane seems to be an initial and important part of the mechanism by which this drug interferes with the transduction signalling system.

Octyl glucoside extracts GTP-binding regulatory proteins from rat brain "synaptoneurosomes" as large, polydisperse structures devoid of beta gamma complexes and sensitive to disaggregation by guanine nucleotides

GTP-binding regulatory proteins are generally purified from cholate-extracted membranes in the form of heterotrimers (G proteins) consisting of a GTP-binding subunit (alpha protein) complexed with a tightly interacted heterodimer termed beta gamma. In this study we extracted the proteins from rat brain "synaptoneurosomes" using the neutral detergent 1-octyl beta-D-glucopyranoside (octyl glucoside). Using specific antibodies for detection by immunoblotting and sucrose gradients for analyzing hydrodynamic properties, we found that each species of alpha protein (alpha subunits of stimulatory, inhibitory, and brain GTP-binding proteins) exhibited a broad range (4 S to greater than 12 S) of polydisperse structures with peak values (5 S to 7 S) considerably greater than that of heterotrimeric G proteins. The beta subunit proteins, for example, appeared as a homogeneous peak at 4.4 S within which only a fraction of the total alpha proteins can be associated. Incubation of octyl glucose extracts at 30 degrees C rapidly sedimented the alpha proteins but not the beta proteins. Incubation at 30 degrees C with guanosine 5'[gamma-thio]triphosphate (10-100 microM) prevented rapid sedimentation. Hydrodynamic analysis revealed that all alpha proteins were converted to approximately 4 S structures by the actions of guanosine 5'-[gamma-thio]triphosphate without change in the hydrodynamic properties of the beta proteins. Extraction of the membranes with sodium cholate instead of octyl glucoside resulted in complete loss of the large, polydisperse structures of the alpha proteins; the S values were approximately 4 S, in the range for beta proteins. These findings suggest that the transducing GTP-binding proteins in synaptoneurosomes exist as polydisperse, possibly multimer, structures of various size that are stable in octyl glucoside but destroyed by cholate. The polydisperse structures are not associated with beta gamma complexes and are sensitive to the disaggregating effects of guanosine 5'-[gamma-thio]triphosphate.