The effect of platelet-derived growth factor on morphology and motility of human glial cells

Aberrant expression of GTPase-activating protein ARAP1 triggers circular dorsal ruffles associated with malignancy in hepatocellular carcinoma Hep3B cells

BACKGROUND

Circular dorsal ruffles (CDRs) are large and rounded membrane ruffles that function as precursors of macropinocytosis. We recently reported that CDRs form in Hep3B hepatocellular carcinoma (HCC) cells, but not in Huh7 and HepG2 HCC cells or LO2 cells, suggesting that an unknown molecular mechanism implicates CDRs in Hep3B malignancy through macropinocytosis uptake of excessive extracellular nutrients. In this study, we investigated the cellular role and the mechanism of CDRs in Hep3B cells by focusing on the GTPase-activating protein ARAP1.

METHODS

ARAP1 knock-out (KO) cells were generated. Confocal microscopy and high-resolution scanning electron microscopy (SEM) were used for identification of the target proteins and structure analysis, respectively. Proteasome inhibitor MG132, mitochondrial function inhibitor CCCP, ARF1 inhibitor Golgicide A, and macropinocytosis inhibitor EIPA were used to investigate the molecular mechanism. Cell proliferation and Transwell migration/invasion assays were used to investigate the role of ARAP1 in cellular malignancy.

RESULTS

ARAP1 was localized to CDRs, which had reduced size following ARAP1 KO. CDRs comprised small vertical lamellipodia, the expression pattern of which was disrupted in ARAP1 KO cells. Extracellular solute uptake, rate of cell growth, and malignant potential were attenuated in KO cells. ARAP1 was also localized to mitochondria in Hep3B cells but not in the control cell lines. Mitochondrial fission protein was increased in KO cells. CCCP treatment blocked CDRs in Hep3B cells but not in controls. Surprisingly, ARAP1 expression level in Hep3B cells was lower than in Huh7, HepG2, and LO2 cells. MG132 treatment increased the ARAP1 levels in Hep3B cells, but not in Huh7 cells, revealing that ARAP1 is actively degraded in Hep3B cells.

CONCLUSIONS

These results strongly suggest that the aberrant expression of ARAP1 in Hep3B cells modulates CDRs via mitochondrial function, thereby resulting in excess uptake of nutrients as an initial event in cancer development. Based on these findings, we propose that the molecular mechanisms underlying the formation of CDRs, focusing on ARAP1, may serve as an effective therapeutic target in some types of HCC and cancers.

Mechanical stretch regulates macropinocytosis in Hydra vulgaris

Cells rely on a diverse array of engulfment processes to sense, exploit, and adapt to their environments. Among these, macropinocytosis enables indiscriminate and rapid uptake of large volumes of fluid and membrane, rendering it a highly versatile engulfment strategy. Much of the molecular machinery required for macropinocytosis has been well established, yet how this process is regulated in the context of organs and organisms remains poorly understood. Here, we report the discovery of extensive macropinocytosis in the outer epithelium of the cnidarian Hydra vulgaris. Exploiting Hydra's relatively simple body plan, we developed approaches to visualize macropinocytosis over extended periods of time, revealing constitutive engulfment across the entire body axis. We show that the direct application of planar stretch leads to calcium influx and the inhibition of macropinocytosis. Finally, we establish a role for stretch-activated channels in inhibiting this process. Together, our approaches provide a platform for the mechanistic dissection of constitutive macropinocytosis in physiological contexts and highlight a potential role for macropinocytosis in responding to cell surface tension.

GTPase-activating protein ARAP1 regulates circular dorsal ruffles as a nutrient uptake mechanism in the Hep3B hepatocellular carcinoma cell line

Circular dorsal ruffles (CDRs), large-scale rounded membrane ruffles, function as precursors of macropinocytosis. We recently reported that CDRs are exposed in the Hep3B hepatocellular carcinoma cell line, while not in other hepatocellular carcinoma cell lines, indicating that the CDRs in Hep3B are associated with malignant potential. In this study, we investigated the cellular function of CDRs in Hep3B cells by focusing on the molecular mechanisms of the GTPase-activating protein ARAP1. ARAP1 was localized to the CDRs, the sizes of which were reduced by deletion of this protein. High-resolution scanning electron micrographs revealed that CDRs comprise small vertical lamellipodia, the expression pattern of which was disrupted in ARAP1 KO cells. Extracellular solute uptake, rate of cell growth, and malignant potential were attenuated in the KO cells. ARAP1 is also localized in Hep3B cell mitochondria, although not in those of the Huh7 hepatocellular carcinoma cell line. On the basis of these findings, we propose that the aberrant expression of ARAP1 in Hep3B cells modulates CDRs, thereby resulting in an excess uptake of nutrients as an initial event in cancer development.

SUMMARY STATEMENT

ARAP1 regulates circular dorsal ruffles (CDRs) in the Hep3B HCC cell line and deletion of this protein attenuates malignant potential, thereby indicating the involvement of CDRs in cancer development.

G-Protein Gα13 Functions with Abl Kinase to Regulate Actin Cytoskeletal Reorganization

Heterotrimeric G-proteins are essential cellular signal transducers. One of the G-proteins, Gα₁₃, is critical for actin cytoskeletal reorganization, cell migration, cell proliferation, and apoptosis. Previously, we have shown that Gα₁₃ is essential for both G-protein-coupled receptor and receptor tyrosine kinase-induced actin cytoskeletal reorganization such as dynamic dorsal ruffle turnover and cell migration. However, the mechanism by which Gα₁₃ signals to actin cytoskeletal reorganization is not completely understood. Here we show that Gα₁₃ directly interacts with Abl tyrosine kinase, which is a critical regulator of actin cytoskeleton. This interaction is critical for Gα₁₃-induced dorsal ruffle turnover, endothelial cell remodeling, and cell migration. Our data uncover a new molecular signaling pathway by which Gα₁₃ controls actin cytoskeletal reorganization.

Fronts and waves of actin polymerization in a bistability-based mechanism of circular dorsal ruffles

During macropinocytosis, cells remodel their morphologies for the uptake of extracellular matter. This endocytotic mechanism relies on the collapse and closure of precursory structures, which are propagating actin-based, ring-shaped vertical undulations at the dorsal (top) cell membrane, a.k.a. circular dorsal ruffles (CDRs). As such, CDRs are essential to a range of vital and pathogenic processes alike. Here we show, based on both experimental data and theoretical analysis, that CDRs are propagating fronts of actin polymerization in a bistable system. The theory relies on a novel mass-conserving reaction–diffusion model, which associates the expansion and contraction of waves to distinct counter-propagating front solutions. Moreover, the model predicts that under a change in parameters (for example, biochemical conditions) CDRs may be pinned and fluctuate near the cell boundary or exhibit complex spiral wave dynamics due to a wave instability. We observe both phenomena also in our experiments indicating the conditions for which macropinocytosis is suppressed.

Circular dorsal ruffles (CDRs) are important for the vesicular uptake of extracellular matter, but the basis of their wave dynamics is not understood. Here, the authors propose and experimentally test a bistable reaction-diffusion system, which they show accounts for the typical CDR expansion and shrinkage and for aberrant formation of pinned waves and spirals.

Regulation, Signaling, and Physiological Functions of G-Proteins

Heterotrimeric guanine-nucleotide-binding regulatory proteins (G-proteins) mainly relay the information from G-protein-coupled receptors (GPCRs) on the plasma membrane to the inside of cells to regulate various biochemical functions. Depending on the targeted cell types, tissues, and organs, these signals modulate diverse physiological functions. The basic schemes of heterotrimeric G-proteins have been outlined. In this review, we briefly summarize what is known about the regulation, signaling, and physiological functions of G-proteins. We then focus on a few less explored areas such as the regulation of G-proteins by non-GPCRs and the physiological functions of G-proteins that cannot be easily explained by the known G-protein signaling pathways. There are new signaling pathways and physiological functions for G-proteins to be discovered and further interrogated. With the advancements in structural and computational biological techniques, we are closer to having a better understanding of how G-proteins are regulated and of the specificity of G-protein interactions with their regulators.

Neurofibromin controls macropinocytosis and phagocytosis in Dictyostelium

Cells use phagocytosis and macropinocytosis to internalise bulk material, which in phagotrophic organisms supplies the nutrients necessary for growth. Wildtype Dictyostelium amoebae feed on bacteria, but for decades laboratory work has relied on axenic mutants that can also grow on liquid media. We used forward genetics to identify the causative gene underlying this phenotype. This gene encodes the RasGAP Neurofibromin (NF1). Loss of NF1 enables axenic growth by increasing fluid uptake. Mutants form outsized macropinosomes which are promoted by greater Ras and PI3K activity at sites of endocytosis. Relatedly, NF1 mutants can ingest larger-than-normal particles using phagocytosis. An NF1 reporter is recruited to nascent macropinosomes, suggesting that NF1 limits their size by locally inhibiting Ras signalling. Our results link NF1 with macropinocytosis and phagocytosis for the first time, and we propose that NF1 evolved in early phagotrophs to spatially modulate Ras activity, thereby constraining and shaping their feeding structures.

DOI:http://dx.doi.org/10.7554/eLife.04940.001

Dictyostelium amoebae are microbes that feed on bacteria living in the soil. They are unusual in that the amoebae can survive and grow in a single-celled form, but when food is scarce, many individual cells can gather together to form a simple multicellular organism.

To feed on bacteria, the amoebae use a process called phagocytosis, which starts with the membrane that surrounds the cell growing outwards to completely surround the bacteria. This leads to the bacteria entering the amoeba within a membrane compartment called a vesicle, where they are broken down into small molecules by enzymes. The cells can also take up fluids and dissolved molecules using a similar process called macropinocytosis.

With its short and relatively simple lifestyle, Dictyostelium is often used in research to study phagocytosis, cell movement and other processes that are also found in larger organisms. For example, some immune cells in animals use phagocytosis to capture and destroy invading microbes. Most studies using Dictyostelium as a model have used amoebae with genetic mutations that allow them to be grown in liquid cultures in the laboratory without needing to feed on bacteria. The mutations allow the ‘mutant’ amoebae to take up more liquid and dissolved nutrients by macropinocytosis, but it is not known where in the genome these mutations are.

Here, Bloomfield et al. used genome sequencing to reveal that these mutations alter a gene that encodes a protein called Neurofibromin. The experiments show that the loss of Neurofibromin increases the amount of fluid taken up by the amoebae through macropinocytosis, and also enables the amoebae to take up larger-than-normal particles during phagocytosis.

The experiments suggest that Neurofibromin controls both phagocytosis and macropinocytosis by inhibiting the activity of another protein called Ras. Neurofibromin is found in animals and many other organisms so Bloomfield et al. propose that it is an ancient protein that evolved in early single-celled organisms to control the size and shape of their feeding structures.

In humans, mutations in the gene that encodes the Neurofibromin protein can lead to the development of a severe disorder—called Neurofibromatosis type 1—in which tumours form in the nervous system. Given that tumour cells can use phagocytosis and macropinocytosis to gain nutrients as they grow, understanding how this protein works in the Dictyostelium amoebae may help to inform future efforts to develop treatments for this human disease.

DOI:http://dx.doi.org/10.7554/eLife.04940.002

Dynamics of actin waves on patterned substrates: a quantitative analysis of circular dorsal ruffles

Circular Dorsal Ruffles (CDRs) have been known for decades, but the mechanism that organizes these actin waves remains unclear. In this article we systematically analyze the dynamics of CDRs on fibroblasts with respect to characteristics of current models of actin waves. We studied CDRs on heterogeneously shaped cells and on cells that we forced into disk-like morphology. We show that CDRs exhibit phenomena such as periodic cycles of formation, spiral patterns, and mutual wave annihilations that are in accord with an active medium description of CDRs. On cells of controlled morphologies, CDRs exhibit extremely regular patterns of repeated wave formation and propagation, whereas on random-shaped cells the dynamics seem to be dominated by the limited availability of a reactive species. We show that theoretical models of reaction-diffusion type incorporating conserved species capture partially the behavior we observe in our data.

Atypical protein kinase Cλ is critical for growth factor receptor-induced dorsal ruffle turnover and cell migration

Gα13, a member of the heterotrimeric G proteins, is critical for actin cytoskeletal reorganization and cell migration. Previously we have shown that Gα13 is essential for both G protein-coupled receptor and receptor tyrosine kinase-induced actin cytoskeletal reorganization such as dynamic dorsal ruffle turnover and cell migration. Ric-8A, a non-receptor guanine nucleotide exchange factor for some heterotrimeric G proteins, is critical for coupling receptor tyrosine kinases to Gα13. Here, we show that PDGF can induce phosphorylation of Ric-8A. Atypical protein kinase Cλ (aPKCλ) is required for Ric-8A phosphorylation. Furthermore, aPKCλ is required for PDGF-induced dorsal ruffle turnover and cell migration as demonstrated by both down-regulation of aPKCλ protein levels in cells by RNA interference and by studies in aPKCλ knock-out cells. Moreover, phosphorylation of Ric-8A modulates its subcellular localization. Hence, aPKCλ is critical for PDGF-induced actin cytoskeletal reorganization and cell migration.

Mechanistic insights into the regulation of circular dorsal ruffle formation

Growth factor stimulations induce dynamic changes in the cytoskeleton beneath the plasma membrane. Among them is the formation of membrane ruffles organized in a circular array, called 'circular dorsal ruffles' (CDRs). Physiological functions of CDRs include downregulation of cell growth by desensitizing the signalling from growth factor receptors as well as rearrangement of adhesion sites at the onset of cell migration. For the formation of CDRs, not only the activators of actin polymerization, such as N-WASP and the Arp2/3-complex, but also membrane deforming proteins with BAR/F-BAR domains are necessary. Small GTPases are also involved in the formation of CDRs by controlling intracellular trafficking through endosomes. Moreover, recent analyses of another circular cytoskeletal structure, podosome rosettes, have revealed common molecular features shared with CDRs. Among them, the roles of PI3-kinase and phosphoinositide 5-phosphatase may hold the key to the induction of these circular structures.

The role of endocytosis in activating and regulating signal transduction

Endocytosis is increasingly understood to play crucial roles in most signaling pathways, from determining which signaling components are activated, to how the signal is subsequently transduced and/or terminated. Whether a receptor-ligand complex is internalized via a clathrin-dependent or clathrin-independent endocytic route, and the complexes' subsequent trafficking through specific endocytic compartments, to then be recycled or degraded, has profound effects on signaling output. This review discusses the roles of endocytosis in three markedly different signaling pathways: the Wnt, Notch, and Eph/Ephrin pathways. These offer fundamentally different signaling systems: (1) diffusible ligands inducing signaling in one cell, (2) membrane-tethered ligands inducing signaling in a contacting receptor cell, and (3) bi-directional receptor-ligand signaling in two contacting cells. In each of these systems, endocytosis controls signaling in fascinating ways, and comparison of their similarities and dissimilarities will help to expand our understanding of endocytic control of signal transduction across multiple signaling pathways.

Co-localization of the PDGF β-Receptor and Actin during PDGF Stimulation in Mouse Fibroblasts

The subcellular localization of the PDGF β-receptor was investigated in relation with PDGF-induced actin and membrane dynamics in mouse C3H10T1/2 fibroblasts. Serum-starved cells exhibit a nonhomogenous distribution of PDGF β-receptors. However, the observed pattern does not resemble the localization of PDGF-induced actin structures. Interestingly, the PDGF β-receptor showed a changed subcellular distribution in relation to the formation of PDGF-BB-induced actin structures. Upon PDGF exposure, PDGF β-receptors were found to accumulate in dorsal circular ruffles. The presence of both macropinosomes and clathrin in the induced circular ruffles suggests that the accumulation of PDGF β-receptors in circular ruffles results in the efficient internalization of PDGF β-receptors.

Investigating circular dorsal ruffles through varying substrate stiffness and mathematical modeling

Circular dorsal ruffles (CDRs) are transient actin-rich ringlike structures that form on the dorsal surface of growth-factor stimulated cells. However, the dynamics and mechanism of formation of CDRs are still unknown. It has been observed that CDR formation leads to stress fibers disappearing near the CDRs. Because stress fiber formation can be modified by substrate stiffness, we examined the effect of substrate stiffness on CDR formation by seeding NIH 3T3 fibroblasts on glass and polydimethylsiloxane substrates of varying stiffnesses from 20 kPa to 1800 kPa. We found that increasing substrate stiffness increased the lifetime of the CDRs. We developed a mathematical model of the signaling pathways involved in CDR formation to provide insight into this lifetime and size dependence that is linked to substrate stiffness via Rac-Rho antagonism. From the model, increasing stiffness raised mDia1-nucleated stress fiber formation due to Rho activation. The increased stress fibers present increased replenishment of the G-actin pool, therefore prolonging Arp2/3-nucleated CDR formation due to Rac activation. Negative feedback by WAVE-related RacGAP on Rac explained how CDR actin propagates as an excitable wave, much like wave propagation in other excitable medium, e.g., nerve signal transmission.

Resistance to inhibitors of cholinesterase-8A (Ric-8A) is critical for growth factor receptor-induced actin cytoskeletal reorganization

Heterotrimeric G proteins are critical transducers of cellular signaling. In addition to their classic roles in relaying signals from G protein-coupled receptors (GPCRs), heterotrimeric G proteins also mediate physiological functions from non-GPCRs. Previously, we have shown that Gα(13), a member of the heterotrimeric G proteins, is essential for growth factor receptor-induced actin cytoskeletal reorganization such as dynamic dorsal ruffle turnover and cell migration. These Gα(13)-mediated dorsal ruffle turnover and cell migration by growth factors acting on their receptor tyrosine kinases (RTKs) are independent of GPCRs. However, the mechanism by which RTKs signal to Gα(13) is not known. Here, we show that cholinesterase-8A (Ric-8A), a nonreceptor guanine nucleotide exchange factor for some heterotrimeric G proteins, is critical for coupling RTKs to Gα(13). Down-regulation of Ric-8A protein levels in cells by RNA interference slowed down platelet-derived growth factor (PDGF)-induced dorsal ruffle turnover and inhibited PDGF-initiated cell migration. PDGF was able to increase the activity of Ric-8A in cells. Furthermore, purified Ric-8A proteins interact directly with purified Gα(13) protein in a nucleotide-dependent manner. Deficiency of Ric-8A prevented the translocation of Gα(13) to the cell cortex. Hence, Ric-8A is critical for growth factor receptor-induced actin cytoskeletal reorganization.

Tropomyosin is a tetramer under physiological salt conditions

Tropomyosin (TM) is a coiled-coil dimer of alpha-helical peptides, which self associates in a head- to-tail fashion along actin polymers, conferring stability to the microfilaments and serving a regulatory function in acto-myosin driven force generation. While the major amount of TM is associated with filaments also in non-muscle cells, it was recently reported that there are isoform-specific pools of TM multimers (not associated with F-actin), which appear to be utilized during actin polymerization and reformed during depolymerization. To determine the size of these multimers, skeletal muscle TM was studied under different salt conditions using gel-filtration and sucrose gradient sedimentation, and compared with purified non-muscle TM 1 and 5, as well as with TM present in non-muscle cell extracts and skeletal muscle TM added to such extracts. Under physiological salt conditions TM appears as a single homogenous peak with the Stokes radius 8.2 nm and the molecular weight (mw) 130,000. The corresponding values for TM 5 are 7.7 nm and 104,000, respectively. This equals four peptides, implying that native TM is a tetramer in physiological salt. It is therefore concluded that the TM multimers are tetramers.

12/15-lipoxygenase-derived lipid peroxides control receptor tyrosine kinase signaling through oxidation of protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) are regulated through reversible oxidation of the active-site cysteine. Previous studies have implied soluble reactive oxygen species (ROS), like H(2)O(2), as the mediators of PTP oxidation. The potential role(s) of peroxidized lipids in PTP oxidation have not been described. This study demonstrates that increases in cellular lipid peroxides, induced by disruption of glutathione peroxidase 4, induce cellular PTP oxidation and reduce the activity of PDGF receptor targeting PTPs. These effects were accompanied by site-selective increased PDGF beta-receptor phosphorylation, sensitive to 12/15-lipoxygenase (12/15-LOX) inhibitors, and increased PDGF-induced cytoskeletal rearrangements. Importantly, the 12/15-LOX-derived 15-OOH-eicosatetraenoic acid lipid peroxide was much more effective than H(2)O(2) in induction of in vitro PTP oxidation. Our study thus establishes that lipid peroxides are previously unrecognized inducers of oxidation of PTPs. This identifies a pathway for control of receptor tyrosine kinase signaling, which might also be involved in the etiology of diseases associated with increased lipid peroxidation.

Microtubule-dependent localization of profilin I mRNA to actin polymerization sites in serum-stimulated cells

Specific localization of messenger RNA (mRNA) appears to be a general mechanism to accumulate certain proteins to subcellular compartments for participation in local processes, thereby maintaining cell polarity under strict spatiotemporal control. Transportation of mRNA with associated protein components (RNP granules) by the actin microfilament or the microtubule systems is one important mechanism to achieve this locally distributed protein production. Here we provide evidence for a microtubule-dependent localization of mRNA encoding the actin regulatory protein profilin to sites in mouse embryonic fibroblasts, which express enhanced actin polymerization.

Cortactin branches out: roles in regulating protrusive actin dynamics

Since its discovery in the early 1990's, cortactin has emerged as a key signaling protein in many cellular processes, including cell adhesion, migration, endocytosis, and tumor invasion. While the list of cellular functions influenced by cortactin grows, the ability of cortactin to interact with and alter the cortical actin network is central to its role in regulating these processes. Recently, several advances have been made in our understanding of the interaction between actin and cortactin, providing insight into how these two proteins work together to provide a framework for normal and altered cellular function. This review examines how regulation of cortactin through post-translational modifications and interactions with multiple binding partners elicits changes in cortical actin cytoskeletal organization, impacting the regulation and formation of actin-rich motility structures.

On the Rho'd: the regulation of membrane protrusions by Rho-GTPases

Cell migration entails the formation of cellular protrusions such as lamellipodia or filopodia, the growth of which is powered by the polymerisation of actin filaments abutting the plasma membrane. Specific Rho-GTPase subfamilies are able to drive different types of protrusions. However, significant crosstalk between Rho-family members and the interplay of distinct Rho-effectors regulating or modulating actin reorganization in protrusions complicate the picture of how precisely they are initiated and maintained. Here, we briefly sketch our current knowledge on structure and dynamics of different protrusions as well as their regulation by Rho-GTPases. We also comment on topical, unresolved controversies in the field, with special emphasis on the interrelation of different protrusion types, and on the composition of the nanomachineries driving them.

The microfilament system and malignancy

Increased motile activity, increased rate of cell proliferation and removal of growth inhibiting cell-cell contacts are hallmarks of tumorigenesis. Activation of cell motility and migration is caused by activation of receptors, turning on the growth cycle. Increased expression of metalloproteinases, breaking cell:cell contacts and organ confines, allows the spread of malignant cancer cells to other sites in the organism. It has become increasingly clear that most transmembrane proteins (growth factor receptors, adhesion proteins and ion channels) are either permanently or transiently associated with the sub-membraneous system of actin microfilaments (MF), whose force generating capacity they control. Although there has been great progress in our understanding of the physiological importance of the MF-system, as will be exemplified in this issue of SCB, many aspects of actin microfilament formation and its regulation are still unclear. Redox control of the actin (MF)-system in cell motility and migration and its perturbations in pathophysiology, including cancer, is an emerging field of research.

Molecular and structural basis for redox regulation of beta-actin

An essential consequence of growth factor-mediated signal transduction is the generation of intracellular H(2)O(2). It operates as a second messenger in the control of actin microfilament dynamics, causing rapid and dramatic changes in the morphology and motile activity of stimulated cells. Little is understood about the molecular mechanisms causing these changes in the actin system. Here, it is shown that H(2)O(2) acts directly upon several levels of this system, and some of the mechanistic effects are detailed. We describe the impact of oxidation on the polymerizability of non-muscle beta/gamma-actin and compare with that of muscle alpha-actin. Oxidation of beta/gamma-actin can cause a complete loss of polymerizability, crucially, reversible by the thioredoxin system. Further, oxidation of the actin impedes its interaction with profilin and causes depolymerization of filamentous actin. The effects of oxidation are critically dependent on the nucleotide state and the concentration of Ca(2+). We have determined the crystal structure of oxidized beta-actin to a resolution of 2.6 A. The arrangement in the crystal implies an antiparallel homodimer connected by an intermolecular disulfide bond involving cysteine 374. Our data indicate that this dimer forms under non-polymerizing and oxidizing conditions. We identify oxidation of cysteine 272 in the crystallized actin dimer, likely to a cysteine sulfinic acid. In beta/gamma-actin, this is the cysteine residue most reactive towards H(2)O(2) in solution, and we suggest plausible structural determinants for its reactivity. No other oxidative modification was obvious in the structure, highlighting the specificity of the oxidation by H(2)O(2). Possible consequences of the observed effects in a cellular context and their potential relevance are discussed.

G Proteins G12 and G13 Control the Dynamic Turnover of Growth Factor-induced Dorsal Ruffles

Growth factors induce massive actin cytoskeletal remodeling in cells. These reorganization events underlie various cellular responses such as cell migration and morphological changes. One major form of actin reorganization is the formation and disassembly of dorsal ruffles (also named waves, dorsal rings, or circular ruffles). Dorsal ruffles are involved in physiological functions including cell migration, invasion, macropinocytosis, plasma membrane recycling, and others. Growth factors initiate rapid formation (within 5 min) of circular membrane ruffles, and these ruffles move along the dorsal side of the cells, constrict, close, and eventually disassemble ( approximately 20 min). Considerable attention has been devoted to the mechanism by which growth factors induce the formation of dorsal ruffles. However, little is known of the mechanism by which these ruffles are disassembled. Here we have shown that G proteins G(12) and G(13) control the rate of disassembly of dorsal ruffles. In Galpha(12)(-/-)Galpha(13)(-/-) fibroblast cells, dorsal ruffles induced by growth factor treatment remain visible substantially longer ( approximately 60 min) than in wild-type cells, whereas the rate of formation of these ruffles was the same with or without Galpha(12) and Galpha(13). Thus, Galpha(12)/Galpha(13) critically regulate dorsal ruffle turnover.

Modulation of α7-integrin-mediated adhesion and expression by platelet-derived growth factor in vascular smooth muscle cells

We showed previously that the expression of α7-integrin in aortic vascular smooth muscle cells (VSMC) is enhanced in a rat model of atherosclerosis. In the present study, we investigated the effects of platelet-derived growth factor (PDGF) on α7-integrin expression and VSMC adhesion and migration. Expression of the α7-integrin gene was determined by real-time RT-PCR, whereas protein levels were determined by fluorescence-activated cell sorting analysis. PDGF increased α7cell surface protein expression (12 and 24 h: 3.3 ± 0.8- and 3.6 ± 0.4-fold, P < 0.05 vs. control) and mRNA levels (24 h: 3.1-fold, P < 0.05 vs. control) in a time-dependent manner. Actinomycin D and cycloheximide attenuated PDGF-induced increases in α7-integrin, indicating the involvement of de novo mRNA and protein synthesis. Treatment with the MAPK inhibitors PD-98059, SP-600125, and SB-203580 attenuated PDGF-induced increases in mRNA. In contrast, PD-98059 and SP-600125, but not SB-203580, attenuated PDGF-induced increases in cell surface protein levels. PDGF-treated VSMC adhered to laminin more efficiently (42 ± 6% increase, P < 0.01), and this increase was partially inhibited by anti-α7-integrin function-blocking antibody. However, PDGF did not alter migration on laminin, and there was no effect of the anti-α7-integrin function-blocking antibody on basal or PDGF-stimulated migration. Immunofluorescence imaging revealed an increase in α7-integrin distribution along the stress fibers. Together, these observations indicate that PDGF enhances α7-integrin expression in VSMC and promotes α7-integrin-mediated adhesion to laminin.

Expression of tyrosine kinase receptors in malignant midgut carcinoid tumors

BACKGROUND

The expression of certain tyrosine kinase receptors (TKR) has been shown to have a prognostic value in many tumor entities. In recent years, inhibitors and monoclonal antibodies directed towards these receptors have been developed. Several have shown antitumoral effects and have been tested in clinical trials. We wanted to investigate whether midgut carcinoid tumors express TKR and therefore would be suitable for clinical trials with TKR inhibitors (TKRI) or monoclonal antibodies.

MATERIAL AND METHODS

Tumor tissue from 36 patients (24 women and 12 men) with a malignant midgut carcinoid tumor was obtained. The tissues were examined with immunohistochemistry, using polyclonal antibodies against platelet-derived growth factor receptor-alpha (PDGFRalpha), platelet-derived growth factor receptor-beta (PDGFRbeta), epidermal growth factor receptor (EGFR) and c-kit. Human BON1 cells were cultivated and stimulated with PDGF-BB. We also present a case report of a patient with a malignant midgut carcinoid tumor who had stabilization of tumor growth during treatment with imatinib.

RESULTS

Immunohistochemical staining for PDGFRalpha in tumor cells showed immunoreaction for the receptor in 13/34 (38%) for PDGFRbeta in 29/33 (88%), and 24/33 (73%) were immunoreactive for EGFR. No tumor tissue showed immunoreaction for c-kit. In tumor stroma PDGFRalpha was expressed in 35%, PDGFRbeta in 94% and EGFR in 9%. We show that human neuroendocrine tumor cells respond to PDGF, indicating that these tumors express functional PDGF receptors.

CONCLUSION

Malignant midgut carcinoid tumors may express three of the four TKR tested in this investigation. Therefore, these tumors might be susceptible for treatment with TKRI or monoclonal antibodies and this should be further explored in clinical trials.

Foot and mouth: podosomes, invadopodia and circular dorsal ruffles

The plasma membrane of many motile cells undergoes highly regulated protrusions and invaginations that support the formation of podosomes, invadopodia and circular dorsal ruffles. Although they are similar in appearance and in their formation--which is mediated by a highly conserved actin-membrane apparatus--these transient surface membrane distortions are distinct. Their function is to help the cell as it migrates, attaches and invades.

Platelet-derived growth factor-BB modulates membrane mobility of beta1 integrins

Platelet-derived growth factor (PDGF)-BB elicits a migratory response including reorganization of the actin cytoskeleton in different cell types. Here we have investigated the effects of PDGF-BB stimulation on beta(1) integrin containing focal adhesions in human diploid fibroblasts adhered to collagen type I. Stimulation with PDGF-BB dissociated focal adhesions and relocated beta(1) integrins from focal adhesions to the periphery of the cells. These changes were rapid and transient in character. Relocation of beta(1) integrins was prevented by inhibitors of phosphoinositide-3-kinase and protein kinase C. PDGF-BB stimulated fibroblasts exhibited an increased diffusion coefficient of cell surface beta(1) integrins as determined by fluorescence recovery of photobleaching. The cell surface expression of beta(1) integrins was not changed after stimulation with PDGF-BB. Our data suggest that PDGF-BB increases the dynamic properties of cell-surface beta(1) integrins, which most likely are important for the migratory response elicited by PDGF-BB.

Redox-dependent downregulation of Rho by Rac

Rac and Rho GTPases function as critical regulators of actin cytoskeleton remodelling during cell spreading and migration. Here we demonstrate that Rac-mediated reactive oxygen species (ROS) production results in the downregulation of Rho activity. The redox-dependent decrease in Rho activity is required for Rac-induced formation of membrane ruffles and integrin-mediated cell spreading. The pathway linking generation of ROS to downregulation of Rho involves inhibition of the low-molecular-weight protein tyrosine phosphatase (LMW-PTP) and then an increase in the tyrosine phosphorylation and activation of its target, p190Rho-GAP. Our findings define a novel mechanism for the coupling of changes in cellular redox state to the control of actin cytoskeleton rearrangements by Rho GTPases.

A dynamin-cortactin-Arp2/3 complex mediates actin reorganization in growth factor-stimulated cells

The mechanisms by which mammalian cells remodel the actin cytoskeleton in response to motogenic stimuli are complex and a topic of intense study. Dynamin 2 (Dyn2) is a large GTPase that interacts directly with several actin binding proteins, including cortactin. In this study, we demonstrate that Dyn2 and cortactin function to mediate dynamic remodeling of the actin cytoskeleton in response to stimulation with the motogenic growth factor platelet-derived growth factor. On stimulation, Dyn2 and cortactin coassemble into large, circular structures on the dorsal cell surface. These "waves" promote an active reorganization of actin filaments in the anterior cytoplasm and function to disassemble actin stress fibers. Importantly, inhibition of Dyn2 and cortactin function potently blocked the formation of waves and subsequent actin reorganization. These findings demonstrate that cortactin and Dyn2 function together in a supramolecular complex that assembles in response to growth factor stimulation and mediates the remodeling of actin to facilitate lamellipodial protrusion at the leading edge of migrating cells.

Desensitization of the PDGFbeta receptor by modulation of the cytoskeleton: the role of p21(Ras) and Rho family GTPases

Ligand-induced PDGF-type beta receptor (PDGFbeta-R) autophosphorylation is profoundly suppressed in cells transformed by activated p21(Ras). We report here that the integrity of the actin cytoskeleton is a critical regulator of PDGFbeta-R function in the presence of p21(Ras). Morphological reversion of Balb cells expressing a constitutively activated p21(Ras), with re-formation of actin stress fibers and cytoskeletal architecture, rendering them phenotypically similar to untransformed fibroblasts, allowed recovery of ligand-dependent PDGFbeta-R autophosphorylation. Conversely, disruption of the actin cytoskeleton in Balb/c-3T3 cells obliterated the normal ligand-induced phosphorylation of the PDGFbeta-R. The Rho family GTPases Rac and Rho are activated by p21(Ras) and are critical mediators of cell motility and morphology via their influence on the actin cytoskeleton. Transient expression of wild-type or constitutively active mutant forms of RhoA suppressed ligand-dependent PDGFbeta-R autophosphorylation and downstream signal transduction. These studies demonstrate the necessary role of Rho in the inhibition of PDGFbeta-R autophosphorylation in cells containing activated p21(Ras) and also demonstrate the importance of cell context and the integrity of the actin cytoskeleton in the regulation of PDGFbeta-R ligand-induced autophosphorylation.

Transforming growth factor-beta-induced mobilization of actin cytoskeleton requires signaling by small GTPases Cdc42 and RhoA

Transforming growth factor-beta (TGF-beta) is a potent regulator of cell growth and differentiation in many cell types. The Smad signaling pathway constitutes a main signal transduction route downstream of TGF-beta receptors. We studied TGF-beta-induced rearrangements of the actin filament system and found that TGF-beta 1 treatment of PC-3U human prostate carcinoma cells resulted in a rapid formation of lamellipodia. Interestingly, this response was shown to be independent of the Smad signaling pathway; instead, it required the activity of the Rho GTPases Cdc42 and RhoA, because ectopic expression of dominant negative mutant Cdc42 and RhoA abrogated the response. Long-term stimulation with TGF-beta 1 resulted in an assembly of stress fibers; this response required both signaling via Cdc42 and RhoA, and Smad proteins. A known downstream effector of Cdc42 is p38(MAPK); treatment of the cells with the p38(MAPK) inhibitor 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(pyridyl)1H-imidazole (SB203580), as well as ectopic expression of a kinase-inactive p38(MAPK), abrogated the TGF-beta-induced actin reorganization. Moreover, treatment of cells with the inhibitors of the RhoA target-protein Rho-associated coiled-coil kinase (+)-R-trans-4-(aminoethyl)-N-(4-pyridyl) cyclohexanecarboxamide (Y-27632) and 1-5(-isoquinolinesulfonyl)homopiperazine (HA-1077), as well as ectopic expression of kinase-inactive Rho coiled-coil kinase-1, abrogated the TGF-beta 1-induced formation of stress fibers. Collectively, these data indicate that TGF-beta-induced membrane ruffles occur via Rho GTPase-dependent pathways, whereas long-term effects require cooperation between Smad and Rho GTPase signaling pathways.

A comparative structural analysis of the ADF/cofilin family

Actin-depolymerizing factor (ADF) and cofilin define a family of actin-binding proteins essential for the rapid turnover of filamentous actin in vivo. Here we present the 2.0 A crystal structure of Arabidopsis thaliana ADF1 (AtADF1), the first plant crystal structure from the ADF/cofilin (AC) family. Superposition of the four AC isoform structures permits an accurate sequence alignment that differs from previously reported data for the location of vertebrate-specific inserts and reveals a contiguous, vertebrate-specific surface opposite the putative actin-binding surface. Extending the structure-based sequence alignment to include 30 additional isoforms indicates three major groups: vertebrates, plants, and "other eukaryotes." Within these groups, several structurally conserved residues that are not conserved throughout the entire AC family have been identified. Residues that are highly conserved among all isoforms tend to cluster around the tryptophan at position 90 and a structurally conserved kink in alpha-helix 3. Analysis of surface character shows the presence of a hydrophobic patch and a highly conserved acidic cluster, both of which include several residues previously implicated in actin binding.

Effects of cross-linked profilin:beta/gamma-actin on the dynamics of the microfilament system in cultured cells

There is evidence that the profilin:actin complex is the immediate precursor in the formation of actin filaments in cells. This paper describes the cell morphology and microfilament distribution after microinjection of covalently cross-linked profilin:beta/gamma-actin (PxA) in two different cell lines. Injected cells were either kept unstimulated or stimulated with platelet-derived growth factor (PDGF) before fixation and visualization of filamentous actin. After injection of low doses of PxA, the cells displayed an actin organization characterized by a clearance of diffuse fluorescence from a region immediately interior of ruffling edges and the appearance of small dots of fluorescence in the same region. At higher concentrations, PxA effectively inhibited outgrowth of lamellae and microspikes, and there was a drastic reduction of actin staining in the zone behind the advancing edge. This effect is reminiscent of the effect of cytochalasin B on fibroblasts and the growth cone of neuronal cells. As in these cases, there remained a rim of actin-dependent fluorescence on the very edge of the membrane lamella, particularly in the PxA-treated fibroblasts. The interference of PxA with the formation of surface structures was pronounced after PDGF stimulation. Here, PxA effectively eliminated the enhancement of the ruffling activity in the cell edges and on the dorsal surface of the cells. In contrast to PxA, injection of non-cross-linked profilin:beta/gamma-actin had no apparent effect on cell morphology and microfilament distribution except for an increased concentration of filamentous actin in one of the cell lines.

The PI 3-kinase isoforms p110(alpha) and p110(beta) have differential roles in PDGF- and insulin-mediated signaling

Phosphoinositide 3′-kinases constitute a family of lipid kinases implicated in signal transduction through tyrosine kinase receptors and heterotrimeric G protein-linked receptors. Phosphoinositide 3′-kinases that bind to the platelet-derived growth factor receptor are composed of two subunits: the p85 subunit acts as an adapter and couples the catalytic p110 subunit to the activated receptor. There are different isoforms of p85 as well as of p110, the individual roles of which have been elusive. Using microinjection of inhibitory antibodies specific for either p110(alpha) or p110(beta) we have investigated the involvement of the two p110 isoforms in platelet-derived growth factor- and insulin-induced actin reorganization in porcine aortic endothelial cells. We have found that antibodies against p110(alpha), but not antibodies against p110(beta), inhibit platelet-derived growth factor-stimulated actin reorganization, whereas the reverse is true for inhibition of insulin-induced actin reorganization. These data indicate that the two phosphoinositide 3′-kinase isoforms have distinct roles in signal transduction pathways induced by platelet-derived growth factor and insulin.

Stable transfectants of smooth muscle cell line lacking the expression of myosin light chain kinase and their characterization with respect to the actomyosin system

We constructed a plasmid vector having a 1.4-kilobase pair insert of myosin light chain kinase (MLCK) cDNA in an antisense direction to express antisense mRNA. The construct was then transfected to SM3, a cell line from vascular smooth muscle cells, producing a few stable transfectants. The down-regulation of MLCK expression in the transfectants was confirmed by both Northern and Western blots. The control SM3 showed chemotaxic motility to platelet-derived growth factor-BB, which was supported by lamellipodia. However, the transfectants showed neither chemotaxic motility nor developed lamellipodia, indicating the essential role of MLCK in the motility. The specificity for the targeting was assessed by a few tests including the rescue experiment. Despite this importance of MLCK, platelet-derived growth factor-BB failed to induce MLC20 phosphorylation in not only the transfectants but also in SM3. The mode in which MLCK was involved in the development of membrane ruffling is discussed with special reference to the novel property of MLCK that stimulates the ATPase activity of smooth muscle myosin without phosphorylating its light chain (Ye, L.-H., Kishi, H., Nakamura, A., Okagaki, T., Tanaka, T., Oiwa, K., and Kohama, K. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 6666-6671).

Mitogens as motogens

Numerous in vivo methodologies have documented the invasive behavior of glioma cells through normal brain parenchyma. Glioma cell locomotion has also been assessed with a number of in vitro assays including the Boyden chamber and other chemotaxis assays, colloidal gold cell tracking, analysis of migration of cells tumor cells from spheroids, confrontation cultures of glioma cells with aggregates of non-neoplastic tissue, time-lapse video microscopy, electron microscopic examination of the cytomorphologic correlates of cell motility, the radial dish assay, and quantitative enzyme immunoassay of proteins associated with invasion (e.g. laminin). Several of these techniques have been specifically modified to assess the effects of cytokines on glioma cell motility in vitro. Cytokines studied utilizing these methods include: epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), the bb dimer of platelet-derived growth factor (PDGFbb), nerve growth factor (NGF), interleukin 2 (IL-2), transforming growth factors alpha and beta 1 (TGF alpha and TGFstraat1), and tumor necrosis factor alpha (TNF alpha). This review summarizes the investigational methods used to evaluate random and directional glioma cell motility and invasion in vivo and in vitro. The roles of specific mitogens as motogens, as evaluated with these methods are then presented.

A novel type of regulatory element is required for promoter-specific activity of the PDGF-B intronic enhancer region

We have previously described a non-classical, promoter-specific enhancer for the human Platelet-Derived Growth Factor B (PDGF-B) gene. In JEG-3 choriocarcinoma cells the activity of the enhancer depends upon co-operation with a sequence (the Enhancer-Dependent cis Co-activator "EDC" element) within the promoter. The PDGF-B enhancer fails to activate heterologous promoters, indicating that promoter-specificity depends on an element within the enhancer that can recognise a target sequence within the promoter. Here we identify a sequence within the enhancer of the PDGF-B gene which directs activation of the PDGF-B promoter by distal cis-acting elements. This specifies the wild-type PDGF-B promoter as the target for the enhancer and has been designated the EDC specificity element (EDCse). The cell-type specific nature of this interaction is extended by the observation that the EDCse is also dispensable for enhancer activity in breast-cancer cells (ZR-75). Concomitant to this observation, JEG-3 and ZR-75 cells differ in the binding of nuclear factors to the EDCse. We discuss the relevance of the EDC/EDCse system in regulation of gene expression.

Mitogens as motogens

Numerous in vivo methodologies have documented the invasive behavior of glioma cells through normal brain parenchyma. Glioma cell locomotion has also been assessed with a number of in vitro assays including the Boyden chamber and other chemotaxis assays, colloidal gold cell tracking, analysis of migration of cells tumor cells from spheroids, confrontation cultures of glioma cells with aggregates of non-neoplastic tissue, time-lapse video microscopy, electron microscopic examination of the cytomorphologic correlates of cell motility, the radial dish assay, and quantitative enzyme immunoassay of proteins associated with invasion (e.g. laminin). Several of these techniques have been specifically modified to assess the effects of cytokines on glioma cell motility in vitro. Cytokines studied utilizing these methods include: epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), the bb dimer of platelet-derived growth factor (PDGFbb), nerve growth factor (NGF), interleukin 2 (IL-2), transforming growth factors alpha and beta 1 (TGF alpha and TGFstraat1), and tumor necrosis factor alpha (TNF alpha). This review summarizes the investigational methods used to evaluate random and directional glioma cell motility and invasion in vivo and in vitro. The roles of specific mitogens as motogens, as evaluated with these methods are then presented.

Involvement of phosphatidylinositide 3'-kinase and Rac in platelet-derived growth factor-induced actin reorganization and chemotaxis

Previous work has suggested a role for phosphatidylinositide 3'-kinase (PI3-kinase) in platelet-derived growth factor (PDGF)-induced actin reorganization and chemotaxis. In support of this notion, we show in this report that the PI3-kinase inhibitor wortmannin inhibits chemotaxis of PDGF beta-receptor expressing porcine aortic endothelial (PAE/PDGFR-beta) cells. Treatment with wortmannin resulted in a dose-dependent decrease in chemotaxis with an IC50 value of about 15-20 nM. Higher concentrations of wortmannin also reduced basal random migration of transfected cells in the absence of PDGF. We also investigated the role of Rac in PDGF-induced actin reorganization and cell motility. Overexpression of wt Rac in PAE/PDGFR-beta cells led to an increased cell motility and edge ruffling in response to PDGF-BB, compared to control cells. In PAE/PDGFR-beta cells transfected with inducible V12Rac (a constitutively active Rac mutant), membrane ruffling occurred in the absence of PDGF stimulation and was independent of PI3-kinase activity. On the other hand, PAE/PDGFR-beta cells transfected with inducible N17Rac (a dominant negative Rac mutant) failed to show membrane ruffling in response to PDGF stimulation. Together with previous observations, these data indicate that activation of PI3-kinase is crucial for initiation of PDGF-induced cell motility responses and that Rac has a major role downstream of PI3-kinase, in this pathway.

Characterization of a mutant profilin with reduced actin-binding capacity: effects in vitro and in vivo

We are investigating structure-function relationships in profilin and actin by site-specific mutagenesis using a yeast, Saccharomyces cerevisiae, expression system to produce wild-type and mutant proteins. This paper shows that deleting proline 96 and threonine 97, which are located close to the major actin binding site on profilin, did not significantly alter the interaction between profilin and phosphatidylinositol 4,5-bisphosphate, nor did it affect the profilin:poly(L-proline) interaction. The mutant protein, however, had a lower capacity to bind to actin in vitro than wild-type profilin, though it showed a slightly increased profilin-enhanced nucleotide exchange on the actin. When microinjected into Swiss 3T3 mouse fibroblasts or porcine aortic endothelial cells, the mutant profilin did not change the organization of the microfilament system like the wild-type profilin did. This provides further evidence that profilin controls microfilament organization in the cell by interacting directly with actin.

A Cdc42 target protein with homology to the non-kinase domain of FER has a potential role in regulating the actin cytoskeleton

BACKGROUND

Members of the Rho family of small GTPases have been shown to have a diverse role in cell signalling events. They were originally identified as proteins that, by regulating the assembly of the actin cytoskeleton, are important determinants of cell morphology, and have recently been shown to be involved in transcriptional activation by the JNK/SAPK signalling pathway. In order to understand the mechanisms underlying the effects of Rho GTPases on these processes, the yeast two-hybrid system has been used to identify proteins that bind to an activated mutant of Cdc42, a Rho-family member.

RESULTS

A cDNA encoding a previously unidentified Cdc42 target protein, CIP4, which is 545 amino-acids long and contains an SH3 domain at its carboxyl terminus, was cloned from a human B-cell library. The amino terminus of CIP4 bears resemblance to the non-kinase domain of the FER and Fes/Fps family of tyrosine kinases. In addition, similarities to a number of proteins with roles in regulating the actin cytoskeleton were noticed. CIP4 binds to activated Cdc42 in vitro and in vivo and overexpression of CIP4 in Swiss 3T3 fibroblasts reduces the amount of stress fibres in these cells. Moreover, coexpression of activated Cdc42 and CIP4 leads to clustering of CIP4 to a large number of foci at the dorsal side of the cells.

CONCLUSIONS

CIP4 is a downstream target of activated GTP-bound Cdc42, and is similar in sequence to proteins involved in signalling and cytoskeletal control. Together, these findings suggest that CIP4 may act as a link between Cdc42 signalling and regulation of the actin cytoskeleton.

The crystal structure of a major allergen from plants

BACKGROUND

Profilins are small eukaryotic proteins involved in modulating the assembly of actin microfilaments in the cytoplasm. They are able to bind both phosphatidylinositol-4,5-bisphosphate and poly-L-proline (PLP) and thus play a critical role in signaling pathways. Plant profilins are of interest because immunological cross-reactivity between pollen and human profilin may be the cause of hay fever and broad allergies to pollens.

RESULTS

The determination of the Arabidopsis thaliana profilin isoform I structure, using multiwavelength anomalous diffraction (MAD) to obtain structure-factor phases, is reported here. The structure of Arabidopsis profilin is similar to that of previously determined profilin structures. Conserved amino acid residues in profilins from plants, mammals, and lower eukaryotes are critically important in dictating the geometry of the PLP-binding site and the overall polypeptide fold. The main feature distinguishing plant profilins from other profilins is a solvent-filled pocket located in the most variable region of the fold.

CONCLUSIONS

Comparison of the structures of SH3 domains with those of profilins from three distinct sources suggests that the mode of PLP binding may be similar. A comparison of three profilin structures from different families reveals only partial conservation of the actin-binding surface. The proximity of the semi-conserved actin-binding site and the binding pocket characteristic of plant profilins suggests that epitopes encompassing both features are responsible for the cross-reactivity of antibodies between human and plant profilins thought to be responsible for type I allergies.

Immunocytochemical evidence of PDGF- and TGF-beta-like molecules in invertebrate and vertebrate immunocytes: an evolutionary approach

Immunoreactive platelet-derived growth factor-AB and transforming growth factor-beta 1 were demonstrated in invertebrate and vertebrate immunocytes by an immunocytochemical procedure. These factors are only present in phagocytic cells among invertebrate immunocytes, whereas in vertebrate immunocytes they are found in monocytes, granulocytes, lymphocytes, thrombocytes and platelets. These results, in agreement with previous reports, represent further evidence in favour of the hypothesis that Nature has followed a conservative strategy in using a common pool of signal molecules that have been highly conserved throughout evolution.

Src, ras, and rac mediate the migratory response elicited by NGF and PMA in PC12 cells

The combination of nerve growth factor (NGF) and phorbol 12-myristate 13-acetate (PMA) rapidly induced the locomotion of PC12 cells by sequentially stimulating lamellar spreading, ruffling with pinocytosis, and polarization by retraction from the substratum. During migration, cells acquired long processes as a result of several undisrupted cell-substratum attachment points. The effect of NGF on PC12 migration was blocked by K-252a, a selective inhibitor of the trk family of receptor tyrosine kinases. When PMA was added to cells expressing pp60v-src, the cells displayed the same morphological behavior as they did with NGF and PMA addition. Activated ras only partially substituted for the effects of NGF; but, when ras was inhibited, the number of migrating cells decreased significantly due to a defect in spreading and retraction. Expression of an activated form of rac stimulated spontaneous growth of lamellipodia and enhanced cell migration in response to PMA. Expression of a dominant negative form of rac inhibited cell spreading and motility. Also, as a later effect, rac-inhibited cells extended much shorter neurites than wild type cells in response to NGF alone. These results indicate that the cytoarchitectural changes induced by NGF and PMA in PC12 cells are mediated by src, ras, and rac. Whereas ras and rac activation affect lamellipodia extension and retraction but not pinocytotic ruffling, src activation is involved in all three events.

The association of focal adhesion kinase with a 200-kDa protein that is tyrosine phosphorylated in response to platelet-derived growth factor

Focal adhesion kinase (FAK) is a cytoplasmic tyrosine kinase implicated in the signal transduction pathways initiated by integrins. However, we have previously found that platelet-derived growth factor (PDGF) could stimulate the association of FAK with phosphatidylinositol 3-kinase in NIH 3T3 cells [Chen, H.-C. & Guan, J.-L. (1994) J. Biol. Chem 269, 31229-31223], suggesting that FAK might participate in some of the cellular effects of the growth factors in modulating cell morphology and migration. In this report, we describe the association of FAK with a 200-kDa protein (pp200) that is tyrosine phosphorylated in response to PDGF stimulation in NIH 3T3 cells. Although the identity of pp200 is unknown at present, we have excluded the possibilities that it is the PDGF receptor beta, tension, talin, myosin or the guanosine-triophosphatase-activating-protein-associated p190 protein. Furthermore, we found that the tyrosine phosphorylation of FAK-associated pp200 upon PDGF stimulation is largely independent of cell adhesion or the integrity of the cytoskeleton. Therefore, pp200 and its interaction with FAK may also be involved in growth-factor-induced cellular effects such as the modulation of cell adhesion or cell migration via cytoskeletal reorganization or disruption of focal adhesions.

Two GTPases, Cdc42 and Rac, bind directly to a protein implicated in the immunodeficiency disorder Wiskott-Aldrich syndrome

BACKGROUND

Members of the Rho family of small GTPases play an essential role in controlling the motile behaviour of animal cells. Specifically, Cdc42 and Rac have been shown to induce the formation of filopodia and lamellipodia, respectively, at the cell periphery of Swiss 3T3 fibroblasts. In addition, both GTPases are required for progression through G1 phase of the cell cycle, possibly by regulating the activity of the Jun N-terminal kinase (JNK) signalling pathway. In order to examine more closely the mechanisms underlying the diverse functions of Rho GTPases in mammalian cells, we searched for downstream targets of these proteins.

RESULTS

A yeast two-hybrid screen for proteins interacting with the human Cdc42 GTPase identified WASP, a protein implicated in the immunodeficiency disorder Wiskott-Aldrich syndrome (WAS). Recombinant WASP, expressed in Escherichia coli, also bound to Cdc42 and weakly to Rac, but not at all to Rho. The Cdc42/Rac-binding domain was identified in a region between amino acids 201-321 of WASP, and binding was dependent on Cdc42 and Rac being in the GTP-bound conformation. Furthermore, WASP did not catalyze GTPase activation or nucleotide exchange activity on Cdc42.

CONCLUSIONS

Positional cloning has implicated WASP in causing WAS, and the protein is defective in patients suffering from the disease. WASP is expressed exclusively in cells of hematopoietic lineage, and lymphocytes from WAS patients have a distorted cell-surface and exhibit reduced proliferative capacity. WASP has recently been found to bind to the Src-homology 3 (SH3) domain of the adapter protein Nck. This observation, and the results presented here, suggest that WAS is the result of defects in signal transduction pathways regulated by Cdc42/Rac and Nck.

Phosphorylation of the growth arrest-specific protein Gas2 is coupled to actin rearrangements during Go-->G1 transition in NIH 3T3 cells

Growth arrest-specific (Gas2) protein has been shown to be a component of the microfilament system, that is highly expressed in growth arrested mouse and human fibroblasts and is hyperphosphorylated upon serum stimulation of quiescent cells. (Brancolini, C., S. Bottega, and C. Schneider. 1992. J. Cell Biol. 117:1251-1261). In this study we demonstrate that the kinetics of Gas2 phosphorylation, during Go-->G1 transition, as induced by addition of 20% FCS to serum starved NIH 3T3 cells, is temporally coupled to the reorganization of actin cytoskeleton. To better dissect the relationship between Gas2 phosphorylation and the modification of the microfilament architecture we used specific stimuli for both membrane ruffling (PDGF and PMA) and stress fiber formation (L-alpha-lysophosphatidic acid LPA) (Ridley, A. J., and A. Hall. 1992. Cell. 70:389-399). All of them, similarly to 20% FCS, are able to downregulate Gas2 biosynthesis. PDGF and PMA induce Gas2 hyperphosphorylation that is temporally coupled with the appearance of membrane ruffling where Gas2 localizes. On the other hand LPA, a specific stimulus for stress fiber formation, fails to induce a detectable Gas2 hyperphosphorylation. Thus, Gas2 hyperphosphorylation is specifically correlated with the formation of membrane ruffling possibly implying a role of Gas2 in this process.