Rho proteins, PI 3-kinases, and monocyte/macrophage motility

Versatile Roles of R-Ras GAP in Neurite Formation of PC12 Cells and Embryonic Vascular Development

Ras GTPase-activating proteins (GAP) are negative regulators of Ras that convert active Ras-GTP to inactive Ras-GDP. R-Ras GAP is a membrane-associated molecule with stronger GAP activity for R-Ras, an activator of integrin, than H-Ras. We found that R-Ras GAP is down-regulated during neurite formation in rat pheochromocytoma PC12 cells by nerve growth factor (NGF), which is blocked by the transient expression of R-Ras gap or dominant negative R-ras cDNA. By establishing a PC12 subclone that stably expresses exogenous R-Ras GAP, it was found that NGF reduced endogenous R-Ras GAP but not exogenous R-Ras GAP, suggesting that down-regulation of R-Ras GAP occurs at the transcription level. To clarify the physiological role of R-Ras GAP, we generated mice that express mutant Ras GAP with knocked down activity. While heterozygotes are normal, homozygous mice die at E12.5-13.5 of massive subcutaneous and intraparenchymal bleeding, probably due to underdeveloped adherens junctions between capillary endothelial cells. These results show essential roles of R-Ras GAP in development and differentiation: its expression is needed for embryonic development of blood vessel barriers, whereas its down-regulation facilitates NGF-induced neurite formation of PC12 cells via maintaining activated R-Ras.

Abr and Bcr, two homologous Rac GTPase-activating proteins, control multiple cellular functions of murine macrophages

Small GTPases of the Rho family are key regulators of phagocytic leukocyte function. Abr and Bcr are homologous, multidomain proteins. Their C-terminal domain has GTPase-activating protein (GAP) activity that, in vitro, is specific for Rac and Cdc42. To address the in vivo relevance of these entire proteins, of which little is known, the current study examined the effect of the genetic ablation of Abr and Bcr in murine macrophages. The concomitant loss of Abr and Bcr induced multiple alterations of macrophage cellular behavior known to be under the control of Rac. Macrophages lacking both Abr and Bcr exhibited an atypical, elongated morphology that was reproduced by the ectopic expression of GAP domain mutant Abr and Bcr in a macrophage cell line and of constitutively active Rac in primary macrophages. A robust increase in colony-stimulating factor 1 (CSF-1)-directed motility was observed in macrophages deficient for both proteins and, in response to CSF-1 stimulation, Abr and Bcr transiently translocated to the plasma membrane. Phagocytosis of opsonized particles was also increased in macrophages lacking both proteins and correlated with sustained Rac activation. Bcr and Abr GAP mutant proteins localized around phagosomes and induced distinct phagocytic cup formation. These results identify Abr and Bcr as the only GAPs to date that specifically negatively regulate Rac function in vivo in primary macrophages.

The role of the lens actin cytoskeleton in fiber cell elongation and differentiation

The vertebrate ocular lens is a fascinating and unique transparent tissue that grows continuously throughout life. During the process of differentiation into fiber cells, lens epithelial cells undergo dramatic morphological changes, membrane remodeling, polarization, transcriptional activation and elimination of cellular organelles including nuclei, concomitant with migration towards the lens interior. Most of these events are presumed to be influenced in large part, by dynamic reorganization of the cellular actin cytoskeleton and by intercellular and cell: extracellular matrix interactions. In light of recent and unprecedented advancement in our understanding of the mechanistic bases underlying regulation of actin cytoskeletal dynamics and the role of the actin cytoskeleton in cell function, this review attempts to summarize current knowledge regarding the role of the cellular actin cytoskeleton, in lens fiber cell elongation and differentiation, and regulation of actin cytoskeletal organization in the lens.

Epac1-Rap1 signaling regulates monocyte adhesion and chemotaxis

Extravasation of leukocytes is a crucial process in the immunological defense. In response to a local concentration of chemokines, circulating leukocytes adhere to and migrate across the vascular endothelium toward the inflamed tissue. The small guanosinetriphosphatase Rap1 plays an important role in the regulation of leukocyte adhesion, polarization, and chemotaxis. We investigated the role of a guanine nucleotide exchange protein for Rap1 directly activated by cAMP (Epac1) in adhesion and chemotaxis in a promonocytic cell line and in primary monocytes. We found that Epac1 is expressed in primary leukocytes, platelets, CD34-positive hematopoietic cells, and the leukemic cell lines U937 and HL60. Epac activation with an Epac-specific cAMP analog induced Rap1 activation, beta1-integrin-dependent cell adhesion, and cell polarization. In addition, activated Epac1 enhanced chemotaxis of U937 cells and primary monocytes. Similar to activation of Epac1, stimulation of cells with serotonin to induce cAMP production resulted in Rap1 activation, increased cell adhesion and polarization, and enhanced chemotaxis. The effects of serotonin on U937 cell adhesion were dependent on cAMP production but could not be blocked by a protein kinase A inhibitor, implicating Epac in the regulation of serotonin-induced adhesion. In summary, our work reveals the existence of previously unrecognized cAMP-dependent signaling in leukocytes regulating cell adhesion and chemotaxis through the activation of Epac1.

Radiation enhances the invasive potential of primary glioblastoma cells via activation of the Rho signaling pathway

Glioblastoma multiforme (GBM) is among the most treatment-refractory of all human tumors. Radiation is effective at prolonging survival of GBM patients; however, the vast majority of GBM patients demonstrate progression at or near the site of original treatment. We have identified primary GBM cell lines that demonstrate increased invasive potential upon radiation exposure. As this represents a novel mechanism by which radiation-treated GBMs can fail therapy, we further investigated the identity of downstream signaling molecules that enhance the invasive phenotype of irradiated GBMs. Matrigel matrices were used to compare the extent of invasion of irradiated vs. non-irradiated GBM cell lines UN3 and GM2. The in vitro invasive potential of these irradiated cells were characterized in the presence of both pharmacologic and dominant negative inhibitors of extracellular matrix and cell signaling molecules including MMP, uPA, IGFR, EGFR, PI-3K, AKT, and Rho kinase. The effect of radiation on the expression of these signaling molecules was determined with Western blot assays. Ultimately, the in vitro tumor invasion results were confirmed using an in vivo 9L GBM model in rats. Using the primary GBM cell lines UN3 and GM2, we found that radiation enhances the invasive potential of these cells via activation of EGFR and IGFR1. Our findings suggest that activation of Rho signaling via PI-3K is required for radiation-induced invasion, although not required for invasion under physiologic conditions. This report clearly demonstrates that radiation-mediated invasion is fundamentally distinct from invasion under normal cellular physiology and identifies potential therapeutic targets to overcome this phenomenon.

Small Rho GTPases are important for acinus formation in a human salivary gland cell line

Rho GTPases participate in a wide variety of signal transduction pathways regulating the actin cytoskeleton, gene expression, cellular migration and proliferation. The aim of this study was to evaluate the role of Rho GTPases in signal transduction pathways during acinus formation in a human salivary gland (HSG) cell line initiated by extracellular matrix (ECM; Matrigel) alone or in combination with epidermal growth factor, basic fibroblast growth factor and lysophosphatidic acid (LPA). Immunohistochemical and Western blotting analyses showed that HSG cells contained RhoA, RhoB, Rac1 and Cdc42 proteins. All growth factors enhanced the effects of ECM on acinus formation, in a pathway dependent on PI3-kinase and Rho GTPases. The role of ROCK, a major RhoA effector, seemed limited to cortical actin polymerization. LPA stimulated cell migration and acinus formation in a PI3-kinase-independent pathway. The results suggest that Rho proteins are important for epithelial-mesenchymal interactions during salivary gland development.

Role of small GTPases in endothelial cytoskeletal dynamics and the shear stress response

Fluid shear stress caused by blood flow is a major determinant of vascular remodeling and arterial tone and can lead to development of atherosclerosis. The endothelial monolayer in vivo acts as a signal transduction interface for hemodynamic forces; these forces determine the shape, cytoskeletal organization, and function of endothelial cells, allowing the vessels to cope with physiological or pathological conditions. The Ras superfamily of GTPases have been revealed to be master regulators of many cellular activities. In particular, the GTPases RhoA, Rac1, and Cdc42 are known to regulate cell shape changes through effects on the cytoskeleton, but their ability to influence polarity, microtubule dynamics, and transcription factor activity is just as significant. Shear stress modulates the activity of small GTPases, which are critical for both cytoskeletal reorganization and changes in gene expression in response to shear stress. The goal of this article is to review what is known about Ras and more so about Rho GTPases in mechanotransduction and the responses of cells to fluid flow. Several distinct signaling pathways can be coordinately activated by flow, and small GTPases are strongly implicated in some of them; thus possible connections will be explored and a unifying hypothesis offered.

A WAVE2-Abi1 complex mediates CSF-1-induced F-actin-rich membrane protrusions and migration in macrophages

Colony-stimulating factor 1 (CSF-1) is an important physiological chemoattractant for macrophages. The mechanisms by which CSF-1 elicits the formation of filamentous actin (F-actin)-rich membrane protrusions and induces macrophage migration are not fully understood. In particular, very little is known regarding the contribution of the different members of the Wiskott-Aldrich Syndrome protein (WASP) family of actin regulators in response to CSF-1. Although a role for WASP itself in macrophage chemotaxis has been previously identified, no data was available regarding the function of WASP family verprolin-homologous (WAVE) proteins in this cell type. We found that WAVE2 was the predominant isoform to be expressed in primary macrophages and in cells derived from the murine monocyte/macrophage RAW264.7 cell line (RAW/LR5). CSF-1 treatment of macrophages resulted in WAVE2 accumulation in F-actin-rich protrusions induced by CSF-1. Inhibition of WAVE2 function by expressing a dominant-negative mutant or introducing anti-WAVE2 antibodies in RAW/LR5 cells, as well as reduction of endogenous WAVE2 expression by RNA-mediated interference (RNAi), resulted in a significant reduction of CSF-1-elicited F-actin protrusions. WAVE2 was found in a protein complex together with Abelson kinase interactor 1 (Abi1) in resting or stimulated cells. Both WAVE2 and Abi1 were recruited to and necessary for the formation of F-actin protrusions in response to CSF-1. Reducing the levels of WAVE2, directly or by targeting Abi1, resulted in an impaired cell migration to CSF-1. Altogether these data identify a WAVE2-Abi1 complex crucial for the normal actin cytoskeleton reorganization and migration of macrophages in response to CSF-1.

Lipoxin A4 Redistributes Myosin IIA and Cdc42 in Macrophages: Implications for Phagocytosis of Apoptotic Leukocytes

Lipoxins (LXs) are endogenously produced anti-inflammatory agents that modulate leukocyte trafficking and stimulate nonphlogistic macrophage phagocytosis of apoptotic neutrophils, thereby promoting the resolution of inflammation. Previous data suggest a role for altered protein phosphorylation and cytoskeletal rearrangement in LX-stimulated phagocytosis but the exact mechanisms remain unclear. In this study we examine the effects of LXA4 on the protein phosphorylation pattern of THP-1 cells differentiated into a macrophage-like phenotype. THP-1 cells stimulated with LXA4 (1 nM) exhibit dephosphorylation of a 220-kDa protein. Using mass spectrometry, this protein was identified as MYH9, a nonmuscle myosin H chain II isoform A, which is involved in cytoskeleton rearrangement. THP-1 cells treated with LXA4 adopt a polarized morphology with activated Cdc42 localized toward the leading edge and MYH9 localized at the cell posterior. Polarized distribution of Cdc42 is associated with Akt/PKB-mediated Cdc42 activation. Interestingly, the annexin-derived peptide Ac2-26, a recently described agonist for the LXA4 receptor, also stimulates macrophage phagocytosis, MYH9 dephosphorylation, and MYH9 redistribution. In addition, we demonstrate that LXA4 stimulates the phosphorylation of key polarity organization molecules: Akt, protein kinase Czeta, and glycogen synthase kinase-3beta. Inhibition of LXA4-induced Akt and protein kinase Czeta activity with specific inhibitors prevented LXA4-stimulated phagocytosis of both apoptotic polymorphonuclear neutrophils and lymphocytes, highlighting a potential use for LXA4 in the treatment of autoimmune diseases. Furthermore, phosphorylation and subsequent inactivation of glycogen synthase kinase-3beta resulted in an increase in phagocytosis similar to that of LXA4. These data highlight an integrated mechanism whereby LXA4 regulates phagocytosis through facilitative actin cytoskeleton rearrangement and cell polarization.

Differential sensitivity of P-Rex1 to isoforms of G protein betagamma dimers

P-Rex1 is a specific guanine nucleotide exchange factor (GEF) for Rac, which is present in high abundance in brain and hematopoietic cells. P-Rex1 is dually regulated by phosphatidylinositol (3,4,5)-trisphosphate and the Gbetagamma subunits of heterotrimeric G proteins. We examined which of the multiple G protein alpha and betagamma subunits activate P-Rex1-mediated Rac guanine nucleotide exchange using pure, recombinant proteins reconstituted into synthetic lipid vesicles. AlF(-)(4) activated G(s),G(i),G(q),G(12), or G(13) alpha subunits were unable to activate P-Rex1. Gbetagamma dimers containing Gbeta(1-4) complexed with gamma(2) stimulated P-Rex1 activity with EC(50) values ranging from 10 to 20 nm. Gbeta(5)gamma(2) was not able to stimulate P-Rex1 GEF activity. Dimers containing the beta(1) subunit complexed with a panel of different Ggamma subunits varied in their ability to stimulate P-Rex1. The beta(1)gamma(3), beta(1)gamma(7), beta(1)gamma(10), and beta(1)gamma(13HA) dimers all activated P-Rex1 with EC(50) values ranging from 20 to 38 nm. Dimers composed of beta(1)gamma(12) had lower EC(50) values (approximately 112 nm). The farnesylated gamma(11) subunit is highly expressed in hematopoietic cells; surprisingly, dimers containing this subunit (beta(1)gamma(11)) were also less effective at activating P-Rex1. These findings suggest that the composition of the Gbetagamma dimer released by receptor activation may differentially activate P-Rex1.

Involvement of the Rho-kinase/myosin light chain kinase pathway on human monocyte chemotaxis induced by ATL-1, an aspirin-triggered lipoxin A4 synthetic analog

Lipoxins (LX) are arachidonic acid metabolites able to induce monocyte chemotaxis in vitro and in vivo. Nonetheless, the signaling pathways mediating this process are yet unclear. In this study, we have investigated the mechanisms associated with human monocyte activation in response to 15-epi-16-(para-fluoro)-phenoxy-LXA4 (ATL-1), a stable 15-epi-LXA4 analog. Our results demonstrate that ATL-1-induced monocyte chemotaxis (10-300 nM) is inhibited by pertussis toxin, suggesting an effect via the G-protein-linked LXA4 receptor. Monocytes stimulated with the analog presented an increased ERK-2 phosphorylation, which was reduced by PD98059, a selective inhibitor of the MEK 1/2 pathway. After exposure of the cells to ATL-1, myosin L chain kinase (MLCK) phosphorylation was evident and this effect was inhibited by PD98059 or Y-27632, a specific inhibitor of Rho kinase. In addition, Y-27632 abolished ERK-2 activation, suggesting that the MAPK pathway is downstream of Rho/Rho kinase in MLCK activation induced by ATL-1. The specific MLCK inhibitor ML-7, as well as Y-27632, abrogated monocyte chemotaxis stimulated by the analog, confirming the central role of the Rho kinase/MLCK pathway on ATL-1 action. Together, these results indicate that ATL-1 acts as a potent monocyte chemoattractant via Rho kinase and MLCK. The present study clarifies some of the mechanisms involved on the activation of monocytes by LXs and opens new avenues for investigation of these checkpoint controllers of inflammation.

Prolonged lesional expression of RhoA and RhoB following spinal cord injury

Inhibition of the small GTPase ras homology protein (Rho) or its downstream target, the Rho-associated kinase (ROCK), has been shown to promote axon regeneration and to improve functional recovery following spinal cord injury (SCI) in the adult rat. Here, we have analyzed the expression of RhoA and RhoB following spinal cord injury in order to assess whether Rho is a possible target for late pharmacological intervention. In control spinal cords, RhoA(+) cells were almost absent, whereas RhoB was localized to some ependymal cells, a few microglia, and some dissociated neurons. In injured spinal cords, RhoA(+) and RhoB(+)cells accumulated at perilesional areas and in the developing necrotic core early after injury at day 1. After reaching their maximum levels (RhoA at day 3; RhoB at day 1), RhoA(+) and RhoB(+) cell numbers remained significantly elevated until day 28. In areas remote from the lesion (> or =0.75 mm), a more discrete accumulation of RhoA(+) and RhoB(+) cells was observed, primarily in areas of ongoing Wallerian degeneration. RhoA and RhoB were predominantly expressed by polymorphonuclear granulocytes, ED1(+) microglia/macrophages, oligodendrocytes, some neurons, and swollen axons/neurites. Furthermore, expression was located to lesional, reactive astrocytes and fibroblastoid cells confined to areas of scar formation. Our experiments have determined that most RhoA(+) and RhoB(+) cells (>70%) are of mononuclear origin. The persistent presence of lesional RhoA(+) and RhoB(+) axon/neurite fibers over a period of 4 weeks after injury suggests that Rho inhibition is a putative therapeutic concept also for delayed intervention after SCI.

Vav activation and function as a rac guanine nucleotide exchange factor in macrophage colony-stimulating factor-induced macrophage chemotaxis

Signal transduction mediated by phosphatidylinositol 3-kinase (PI 3-kinase) is regulated by hydrolysis of its products, a function performed by the 145-kDa SH2 domain-containing inositol phosphatase (SHIP). Here, we show that bone marrow macrophages of SHIP(-/-) animals have elevated levels of phosphatidylinositol 3,4,5-trisphosphate [PI (3,4,5)P(3)] and displayed higher and more prolonged chemotactic responses to macrophage colony-stimulating factor (M-CSF) and elevated levels of F-actin relative to wild-type macrophages. We also found that the small GTPase Rac was constitutively active and its upstream activator Vav was constitutively phosphorylated in SHIP(-/-) macrophages. Furthermore, we show that Vav in wild-type macrophages is recruited to the membrane in a PI 3-kinase-dependent manner through the Vav pleckstrin homology domain upon M-CSF stimulation. Dominant inhibitory mutants of both Rac and Vav blocked chemotaxis. We conclude that Vav acts as a PI 3-kinase-dependent activator for Rac activation in macrophages stimulated with M-CSF and that SHIP regulates macrophage M-CSF-triggered chemotaxis by hydrolysis of PI (3,4,5)P(3).

Spatial analysis of 3' phosphoinositide signaling in living fibroblasts, III: influence of cell morphology and morphological Polarity

Activation of phosphoinositide (PI) 3-kinase is a required signaling pathway in fibroblast migration directed by platelet-derived growth factor. The pattern of 3' PI lipids in the plasma membrane, integrating local PI 3-kinase activity as well as 3' PI diffusion and turnover, influences the spatiotemporal regulation of the cytoskeleton. In fibroblasts stimulated uniformly with platelet-derived growth factor, visualized using total internal reflection fluorescence microscopy, we consistently observed localized regions with significantly higher or lower 3' PI levels than adjacent regions (hot and cold spots, respectively). A typical cell contained multiple hot spots, coinciding with apparent leading edge structures, and at most one cold spot at the rear. Using a framework for finite-element modeling with actual cell contact area geometries, we find that although the 3' PI pattern is affected by irregular contact area shape, cell morphology alone cannot explain the presence of hot or cold spots. Our results and analysis instead suggest that these regions reflect different local 3' PI dynamics, specifically through a combination of mechanisms: enhanced PI 3-kinase activity, reduced 3' PI turnover, and possibly slow/constrained 3' PI diffusion. The morphological polarity of the cell may thus bias 3' PI signaling to promote persistent migration in fibroblasts.

Dynamic reorganization of the astrocyte actin cytoskeleton elicited by cAMP and PACAP: a role for phosphatidylInositol 3-kinase inhibition

Cyclic AMP (cAMP)-raising agents induce astrocytes grown in vitro to adopt a stellate morphology resembling their in vivo appearance, through the depolymerization of actomyosin stress fibres. The signalling pathways responsible for cAMP-induced astrocyte stellation have thus far remained largely elusive. We showed in this study that the neurotrophic peptide PACAP (pituitary adenylate cyclase-activating polypeptide) mimicked the effect of forskolin, a direct activator of adenylate cyclase, on the actin cytoskeleton of primary rat astrocytes. The depolymerization of stress fibres induced by PACAP or forskolin was prevented by the expression of a constitutively active mutant of RhoA, but not by a protein kinase A (PKA) blocker, indicating that cAMP-raising agents act upstream of RhoA, in a PKA-independent manner. In addition, PACAP and forskolin inhibited basal Akt phosphorylation, and basal and epidermal growth factor (EGF)-stimulated phosphatidylinositol 3-kinase (PI 3-K) activities. Incubation with a PI 3-K blocker resulted in the depolymerization of stress fibres. This effect was blocked by the expression of a constitutively active mutant of RhoA, indicating that PI 3-K inhibition acted upstream of RhoA. Together, these data demonstrate for the first time that depolymerization of stress fibres, and the resulting astrocyte stellation, induced by stimulation of cAMP production involves the inhibition of the PI 3-K-RhoA pathway.

The unique N-terminus of R-ras is required for Rac activation and precise regulation of cell migration

The Ras family GTPase, R-Ras, elicits important integrin-dependent cellular behaviors such as adhesion, spreading and migration. While oncogenic Ras GTPases and R-Ras share extensive sequence homology, R-Ras induces a distinct set of cellular behaviors. To explore the structural basis for these differences, we asked whether the unique N-terminal 26 amino acid extension of R-Ras was responsible for R-Ras-specific signaling events. Using a 32D mouse myeloid cell line, we show that full-length R-Ras activates Rac and induces Rac-dependent cell spreading. In contrast, truncated R-Ras lacking its first 26 amino acids fails to activate Rac, resulting in reduced cell spreading. Truncated R-Ras also stimulates more beta3 integrin-dependent cell migration than full-length R-Ras, suggesting that the N-terminus may negatively regulate cell movement. However, neither the subcellular localization of R-Ras nor its effects on cell adhesion are affected by the presence or absence of the N-terminus. These results indicate that the N-terminus of R-Ras positively regulates specific R-Ras functions such as Rac activation and cell spreading but negatively regulates R-Ras-mediated cell migration.

How adhesion, migration, and cytoplasmic calcium transients influence interleukin-1beta mRNA stabilization in human monocytes

We investigated the mechanisms by which primary human monocyte migration and the production of important cytokines are co-regulated. Motile monocytes underwent cyclic morphologic and adhesive changes that were associated with intracellular free calcium changes; in such cells, cytokine transcripts were unstable and translationally repressed. Agents that activate monocytes, including lipopolysacharrides (LPS), cytomegalovirus (CMV), and tumor necrosis factor (TNFalpha), have been shown to de-repress translation and these agents stabilize adhesion-induced transcripts for IL-lbeta and IL-8 and markedly diminish cell migration in the presence of autologous serum. LPS suppressed Rho A activity and either this agent or C3 transferase elevated intracellular free calcium, stabilized transcripts, and, in tandem, inhibited cell migration by preventing tail retraction, a prerequisite for cell translocation. These results, therefore, suggest that monocyte activating agents inhibit the RhoA pathway and continuously elevate intracellular calcium leading to a concomitant decrease in monocyte migration and stabilization of cytokine transcripts prior to translation.

Involvement of phosphoinositide 3-kinase gamma, Rac, and PAK signaling in chemokine-induced macrophage migration

In macrophages, chemotactic stimuli cause the activation of Rac and PAK, but little is known about the signaling pathways involved and their role in chemotactic gradient sensing. Herein, we report that in macrophages, the chemokine RANTES (regulated on activation normal T cell expressed and secreted)/CCL5 activates the small GTPase Rac and its downstream target PAK2 within seconds. This response depends on Gi activation and largely on the subsequent triggering of phosphoinositide 3-kinase gamma (PI3Kgamma) and Rac. Retroviral transduction of tagged Rac1 and -2 indicates that RANTES/CCL5-mediated activation of PI3Kgamma triggers Rac1 but not Rac2. In agreement, silencing of Rac1 by shRNA blocks PAK2 activity and inhibits RANTES/CCL5-induced macrophage polarization and directional migration. On the other hand, the tyrosine kinase receptor agonist CSF-1 activates PAK2 independently of PI3Kgamma and Rac. Our results thus demonstrate a chemokine-specific signaling pathway in which Gi and PI3Kgamma coordinate to drive Rac1 and PAK2 activation that eventually controls the chemotactic response.

The role of the guanine nucleotide exchange factor Tiam1 in cellular migration, invasion, adhesion and tumor progression

While advances in molecular genetics have provided new insights into molecular alterations that lead to the development of many tumors, including breast carcinoma, the genetic and epigenetic alterations that result in metastatic spread of the disease, from which afflicted patients ultimately succumb, are much more poorly understood. Important biologic processes in the development of metastasis include increased migration and invasion of tumor cells. While the regulation of these processes is complex, they are controlled in part by small G proteins of the Rho family, including Rho, Rac, and Cdc42, that are involved in cytoskeletal organization. These proteins, active when bound to GTP, are, in turn, regulated by guanine nucleotide exchange factors (GNEFs) and guanine nucleotide activating proteins. The GNEF Tiam1 catalyzes nucleotide exchange for Rac in vivo, and Rac, Cdc42 and Rho in vitro. Tiam1 was identified first in 1994 by in vitro selection for invasiveness in T-lymphoma cells. Accordingly, Tiam1 has been shown to increase invasion in T-lymphoma cells, as well as to increase cellular migration in fibroblasts, and to promote motility in some neuronal cells. In contrast, Tiam1 has been demonstrated to increase cellular adhesion in some epithelial cell populations. Thus, Tiam1 has multiple roles in regulating cellular functions, likely dependent on the cell type, the substratum, transformation status of the cells, and the activation state of small G proteins in a given cell. Increasing evidence has focused on Tiam1's regulation, as well as Tiam1's role in cancer progression and metastasis. Recent results from other laboratories and ours have demonstrated that increased Tiam1 expression correlates with grade of breast cancer in humans and metastatic potential of human breast carcinoma cell lines in nude mice. This review will discuss Tiam1's cellular functions and methods of regulation, and will highlight Tiam1's contribution to cancer progression and metastasis.

Lesional Expression of RhoA and RhoB following Traumatic Brain Injury in Humans

Inhibition of the small GTPase Rho or of its downstream target Rho-associated kinase (ROCK) has been shown to promote axon regeneration and to improve functional recovery following traumatic CNS lesions in the adult rat. In order to determine the expression pattern of RhoA and RhoB following human traumatic brain injury (TBI) and to assess whether Rho is a possible target for pharmacological intervention in humans, we investigated expression patterns of RhoA and RhoB in brain specimens from 25 patients who died after closed TBI in comparison to brain tissue derived from four neuropathologically unaffected control patients by immunohistochemistry. A highly significant lesional upregulation of both RhoA and RhoB was observed beginning several hours after the traumatic event and continuing for months after TBI. The cellular sources of both molecules included polymorphonuclear granulocytes, monocytes/macrophages, and reactive astrocytes. Additionally, expression of RhoA was also detected in neuronal cells in some of the cases. From our data, we conclude that inhibition of Rho is a promising mechanism for the development of new pharmacological interventions in human TBI. As the observed upregulation of RhoA and RhoB was still detectable months after TBI, we speculate that even delayed treatment with Rho inhibitors might be a therapeutic option.

Cell-cycle-dependent regulation of cell motility and determination of the role of Rac1

To study cell motility in different phases of the cell cycle, time-lapse recording by computer-assisted microscopy of unsynchronised cells from three mammalian cell lines (L929, BT4Cn, HeLa) was used for the determination of the displacements of individual cells. The displacements were used for calculation of three key parameters describing cell motility: speed, persistence time and rate of diffusion. All investigated cell lines demonstrated a lower cell displacement in the G2 phase than in the G1/S phases. This was caused by a decrease in speed and/or persistence time. The decrease in motility was accompanied by changes in morphology reflecting the larger volume of cells in G2 than in G1. Furthermore, L-cells and HeLa-cells appeared to be less adherent in the G2 phase. Transfection of L-cells with constitutively active Rac1 led to a general increase in the speed and rate of diffusion in G2 to levels comparable to those of control cells in G1. In contrast, transfection with dominant-negative Rac1 reduced cell speed and resulted in cellular displacements, which were identical in G1 and G2. These observations indicate that migration of cultured cells is regulated in a cell-cycle-dependent manner, and that an enhancement of Rac1 activity is sufficient for a delay of the reduced cell displacement otherwise seen in G2.

HCMV activates PI(3)K in monocytes and promotes monocyte motility and transendothelial migration in a PI(3)K-dependent manner

Human cytomegalovirus (HCMV) is a leading cause of morbidity and mortality in immunocompromised hosts. In immunocompetent hosts, HCMV is associated with chronic inflammatory diseases including atherosclerosis. Monocytes and macrophages are proposed to play key roles in HCMV dissemination to host tissue, and their infection provides a biological link between the lifecycle of HCMV and disease pathology. We hypothesize that viral spread occurs via a mechanism in which infected peripheral blood monocytes, which are nonpermissive for viral replication, extravasate into host tissue and subsequently differentiate into permissive macrophages. Supporting this hypothesis, we recently showed that HCMV specifically induced the differentiation of monocytes into macrophages that become permissive for viral replication. To expand our understanding of HCMV pathogenesis, we next examined monocyte activation and migration, the first events in viral pathogenesis. We show here that HCMV up-regulates phosphatidylinositol 3,4,5 triphosphate kinase [PI(3)K] activity and that this increased PI(3)K activity is essential for infected monocyte-transendothelial migration. This increase in migration occurs through the up-regulation of cell motility in a PI(3)K-dependent process. Last, we show that these activated monocytes express a number of inflammatory mediators via PI(3)K signaling. We propose that the up-regulation of monocyte migration and immune mediators by HCMV infection is required for the hematogenous dissemination of the virus and as a consequence, could promote chronic inflammatory diseases associated with HCMV infection.

The nucleotide receptor P2X7 mediates actin reorganization and membrane blebbing in RAW 264.7 macrophages via p38 MAP kinase and Rho

Extracellular nucleotides regulate macrophage function via P2X nucleotide receptors that form ligand-gated ion channels. In particular, P2X7 activation is characterized by pore formation, membrane blebbing, and cytokine release. P2X7 is also linked to mitogen-activated protein kinases (MAPK) and Rho-dependent pathways, which are known to affect cytoskeletal structure in other systems. As cytoskeletal function is critical for macrophage behavior, we have tested the importance of these pathways in actin filament reorganization during P2X7 stimulation in RAW 264.7 macrophages. We observed that the P2X7 agonists adenosine 5'-triphosphate (ATP) and 3'-O-(4-benzoylbenzoyl) ATP (BzATP) stimulated actin reorganization and concomitant membrane blebbing within 5 min. Disruption of actin filaments with cytochalasin D attenuated membrane blebbing but not P2X7-dependent pore formation or extracellular-regulated kinase (ERK)1/ERK2 and p38 activation, suggesting that these latter processes do not require intact actin filaments. However, we provide evidence that p38 MAPK and Rho activation but not ERK1/ERK2 activation is important for P2X7-mediated actin reorganization and membrane blebbing. First, activation of p38 and Rho was detected within 5 min of BzATP treatment, which is coincident with membrane blebbing. Second, the p38 inhibitors SB202190 and SB203580 reduced nucleotide-induced blebbing and actin reorganization, whereas the MAPK kinase-1/2 inhibitor U0126, which blocks ERK1/ERK2 activation, had no discernable effect. Third, the Rho-selective inhibitor C3 exoenzyme and the Rho effector kinase, Rho-associated coiled-coil kinase, inhibitor Y-27632, markedly attenuated BzATP-stimulated actin reorganization and membrane blebbing. These data support a model wherein p38- and Rho-dependent pathways are critical for P2X7-dependent actin reorganization and membrane blebbing, thereby facilitating P2X7 involvement in macrophage inflammatory responses.

Signaling through the Leukocyte Integrin LFA-1 in T Cells Induces a Transient Activation of Rac-1 That Is Regulated by Vav and PI3K/Akt-1

Integrin LFA-1 is a receptor that is able to transmit multiple intracellular signals in leukocytes. Herein we show that LFA-1 induces a potent and transient increase in the activity of the small GTPase Rac-1 in T cells. Maximal Rac-1 activity peaked 10-15 min after LFA-1 stimulation and rapidly declined to basal levels at longer times. We have identified Vav, a guanine nucleotide exchange factor for Rac-1, and PI3K/Akt, as regulators of the activation and inactivation phases of the activity of Rac-1, respectively, in the context of LFA-1 signaling based on the following experimental evidence: (i) LFA-1 induced activation of Vav and PI3K/Akt with kinetics consistent with a regulatory role for these molecules on Rac-1, (ii) overexpression of a constitutively active Vav mutant induces activation of Rac independently of LFA-1 stimulation whereas overexpression of a dominant-negative Vav mutant blocks LFA-1-mediated Rac activation, (iii) pharmacological inhibition of PI3K/Akt prevented the fall in the activity of Rac-1 after its initial activation but had no effect on Vav activity, and (iv) overexpression of a dominant-negative or a constitutively active Akt-1 induced or inhibited, respectively, Rac-1 activity. Finally, we show that T cells with a sustained Rac activity have impaired capacity to elongate onto ICAM-1. These results demonstrate that down-regulation of the activity of this GTPase is a requirement for the regulation of T cell morphology and motility and highlight the importance of temporal regulation of the signaling triggered from this integrin.

C-Peptide Colocalizes with Macrophages in Early Arteriosclerotic Lesions of Diabetic Subjects and Induces Monocyte Chemotaxis In Vitro

Objective— Increased levels of C-peptide, a cleavage product of proinsulin, circulate in patients with insulin resistance and early type 2 diabetes, a high-risk population for the development of a diffuse and extensive pattern of arteriosclerosis. This study tested the hypothesis that C-peptide might participate in atherogenesis in these patients.

Method and Results— We demonstrate significantly higher intimal C-peptide deposition in thoracic artery specimens from young diabetic subjects compared with matched nondiabetic controls as determined by immunohistochemical staining. C-peptide colocalized with monocytes/macrophages in the arterial intima of artery specimen from diabetic subjects. In vitro, C-peptide stimulated monocyte chemotaxis in a concentration-dependent manner with a maximal 2.3±0.4-fold increase at 1 nmol/L C-peptide. Pertussis toxin, wortmannin, and LY294002 inhibited C-peptide–induced monocyte chemotaxis, suggesting the involvement of pertussis toxin-sensitive G-proteins as well as a phosphoinositide 3-kinase (PI3K)-dependent mechanism. In addition, C-peptide treatment activated PI3K in human monocytes, as demonstrated by PI3K activity assays.

Conclusion— C-peptide accumulated in the vessel wall in early atherogenesis in diabetic subjects and may promote monocyte migration into developing lesions. These data support the hypothesis that C-peptide may play an active role in atherogenesis in diabetic patients and suggest a new mechanism for accelerated arterial disease in diabetes.

Cell migration: Rho GTPases lead the way

Rho GTPases control signal transduction pathways that link cell surface receptors to a variety of intracellular responses. They are best known as regulators of the actin cytoskeleton, but in addition they control cell polarity, gene expression, microtubule dynamics and vesicular trafficking. Through these diverse functions, Rho GTPases influence many aspects of cell behavior. This review will focus specifically on their role in cell migration.

Simvastatin suppresses LPS-induced Akt phosphorylation in the human monocyte cell line THP-1

BACKGROUND

Activation of the small GTPase, Rac, requires post-translational modification by isoprenylation. Statins interfere with this process by blocking the synthesis of isoprenoid intermediates. The protein kinase Akt is a multifunctional regulator of cell behavior that has been linked to Rac activation. We have shown that lipopolysaccharide (LPS) stimulation leads to Rac activation in THP-1 cells. Therefore, we hypothesized that LPS stimulation would also activate Akt, a downstream effector of Rac, and that this may be blocked by statin pretreatment.

MATERIALS AND METHODS

THP-1 cells were maintained in 1% fetal calf serum with or without 20 microM simvastatin for 24 h, followed by LPS stimulation for increasing time. Cytoskeletal changes were observed using Alexa-Phalloidin. Akt was immunoprecipitated from total cell lysate. Activated Akt was detected by immunoblotting with a phospho-Akt antibody and was quantified by image densitometry.

RESULTS

LPS stimulation of THP-1 cells results in membrane ruffling and cell polarization. Furthermore, LPS increased Akt activation in THP-1 cells when compared with the nonstimulated controls. Akt phosphorylation peaked after 15 min of LPS stimulation and was suppressed by pretreatment with simvastatin.

CONCLUSIONS

These data demonstrate that LPS stimulation leads to increased Akt phosphorylation, which can be suppressed with simvastatin pretreatment. This suggests one possible mechanism through which simvastatin could modulate LPS-induced signaling events in monocytes to improve the host response to Gram-negative infections.

The GTP/GDP cycling of rho GTPase TCL is an essential regulator of the early endocytic pathway

Rho GTPases are key regulators of actin dynamics. We report that the Rho GTPase TCL, which is closely related to Cdc42 and TC10, localizes to the plasma membrane and the early/sorting endosomes in HeLa cells, suggesting a role in the early endocytic pathway. Receptor-dependent internalization of transferrin (Tf) is unaffected by suppression of endogenous TCL by small interfering RNA treatment. However, Tf accumulates in Rab5-positive uncoated endocytic vesicles and fails to reach the early endosome antigen-1-positive early endosomal compartments and the pericentriolar recycling endosomes. Moreover, Tf release upon TCL knockdown is significantly slower. Conversely, in the presence of dominant active TCL, internalized Tf accumulates in early endosome antigen-1-positive early/sorting endosomes and not in perinuclear recycling endosomes. Tf recycles directly from the early/sorting endosomes and it is normally released by the cells. The same phenotype is generated by replacing the C terminus of dominant active Cdc42 and TC10 with that of TCL, indicating that all three proteins share downstream effector proteins. Thus, TCL is essential for clathrin-dependent endocytosed receptors to enter the early/sorting endosomes. Furthermore, the active GTPase favors direct recycling from early/sorting endosomes without accumulating in the perinuclear recycling endosomes.

Rho GTPases and phosphoinositide 3-kinase organize formation of branched dendrites

Neurons receive information from other neurons via their dendritic tree. Dendrites and their branches result from alternating outgrowth and retraction. The Rho GTPases Rac and Cdc42 (cell division cycle 42) facilitate the outgrowth of branches, whereas Rho attenuates it. The mechanism of neurite retraction is unknown. Because the adenylyl cyclase activator forskolin causes numerous branched extensions in NG108-15 cells, we have investigated the underlying mechanism in this cell line. In additional studies, we used cultured hippocampal neurons in which forskolin induces branched dendrites. In both cell types, forskolin enhanced the activity of Cdc42, but not that of Rac, although both GTPases were necessary for the formation of branched extensions. Time lapse microscopy showed that forskolin did not increase the rate of addition of new extensions or branches, but it reduced the rate of the retraction so that more branched extensions persisted. Inhibition of phosphoinositide 3-kinase activity by wortmannin or LY294002 also reduced the rate of retraction and thus facilitated dendritic arborization. Forskolin diminished the activity of phosphoinositide 3-kinases. Inhibitors of phosphoinositide 3-kinases not only reduced the retraction but also the addition of new dendrites and branches. This reduction was no longer present when Rho kinase was simultaneously inactivated, suggesting an interaction of phosphoinositide 3-kinases and Rho kinase. The present results show a central role of phosphoinositide 3-kinases in dendrite formation. In neuronal cells, increased levels of cAMP can support dendritic arborization by modulating the activity of the lipid kinase.

GC-GAP, a Rho family GTPase-activating protein that interacts with signaling adapters Gab1 and Gab2

Gab1 and Gab2 are scaffolding proteins acting downstream of cell surface receptors and interact with a variety of cytoplasmic signaling proteins such as Grb2, Shp-2, phosphatidylinositol 3-kinase, Shc, and Crk. To identify new binding partners for GAB proteins and better understand their functions, we performed a yeast two-hybrid screening with hGab2-(120-587) as bait. This work led to identification of a novel GTPase-activating protein (GAP) for Rho family GTPases. The GAP domain shows high similarity to the recently cloned CdGAP and displays activity toward RhoA, Rac1, and Cdc42 in vitro. The protein was named GC-GAP for its ability to interact with GAB proteins and its activity toward Rac and Cdc42. GC-GAP is predominantly expressed in the brain with low levels detected in other tissues. Antibodies directed against GC-GAP recognized a protein of approximately 200 kDa. Expression of GC-GAP in 293T cells led to a reduction in active Rac1 and Cdc42 levels but not RhoA. Suppression of GC-GAP expression by siRNA inhibited proliferation of C6 astroglioma cells. In addition, GC-GAP contains several classic proline-rich motifs, and it interacts with the first SH3 domain of Crk and full-length Nck in vitro. We propose that Gab1 and Gab2 in cooperation with other adapter molecules might regulate the cellular localization of GC-GAP under specific stimuli, acting to regulate precisely Rac and Cdc42 activities. Given that GC-GAP is specifically expressed in the nervous system and that it is localized to the dendritic processes of cultured neurons, GC-GAP may play a role in dendritic morphogenesis and also possibly in neural/glial cell proliferation.

VEGF‐A and α V β 3 integrin synergistically rescue angiogenesis via N‐Ras and PI3‐K signaling in human microvascular endothelial cells

We recently showed that normal fibroblasts mediate capillary-like differentiation of human microvascular endothelial cells (HMVEC) in a 3-D angiogenesis model. Here, we show that a collaborative effect of VEGF-A and alphaVbeta3 integrin is critical in fibroblast-mediated angiogenesis because enhancement of both VEGF production by fibroblasts and beta3 integrin expression in HMVEC can rescue capillary-like endothelial differentiation under reduced serum conditions. To investigate the downstream signaling mechanisms, we compared N-Ras and Rho/Rac/Cdc42, as well as phosphatidylinositol 3-kinase (PI3-K) and Akt, for their involvement in the capillary-like network formation. The dominant-negative mutant of N-Ras (N-RasN17), but not the mutants of Rho/Rac/Cdc42, suppressed network formation. Overexpression of a constitutively active form of PI3-K rescued the network formation, which was inhibited by a dominant-negative >beta3 integrin; however, an active form of Akt failed to rescue the inhibition but induced a phenotypic change in HMVEC. Moreover, PI3-K is a downstream target of N-Ras because it could be co-immunoprecipitated with N-Ras, and its active form could rescue the inhibitory effect of N-Ras N17. Thus, our data indicate the existence of N-Ras- and PI3-K-dependent but Rho/Rac/Cdc42- and Akt-independent signaling mechanisms for the synergistic effect of VEGF-A and alphaVbeta3 on fibroblast-mediated microvascular network formation.

Mevastatin suppresses lipopolysaccharide-induced Rac activation in the human monocyte cell line THP-1

BACKGROUND

Rac is a member of the Rho family of small guanosine triphosphatases that regulate the actin cytoskeleton. Activation of Rac requires lipid modification that can be blocked by statins. Lipopolysaccharide-induced rearrangements in the actin cytoskeleton lead to changes in cell adhesion and motility that play a role in the cellular response to infection. The lipid products of phosphatidylinositol-3 kinase (PI3K) modulate Rac effector pathways and have been linked to the activation of the Rac-guanosine triphosphatase. We hypothesize that lipopolysaccharide stimulation leads to Rac activation and that this may be inhibited by statin pretreatment. Furthermore, signaling downstream of Rac is linked to PI3K; therefore, we hypothesize that a signaling complex between PI3K and Rac may be involved.

METHODS

THP-1 cells were maintained in 1% fetal calf serum with or without 20 micromol/L mevastatin for 24 hours, followed by lipopolysaccharide stimulation. Active Rac was precipitated from THP-1 total cell lysate and then detected by immunoblotting. The PI3K-Rac complex was immunoprecipitated from total cell lysate, and the p85 regulatory subunit of PI3K was detected by immunoblotting.

RESULTS

Lipopolysaccharide stimulation activated Rac. Rac activation was suppressed by pretreatment with mevastatin. The p85 subunit of PI3K was associated with Rac.

CONCLUSIONS

Lipopolysaccharide stimulation leads to Rac activation in THP-1 cells, which may be suppressed with mevastatin pretreatment. There is an association between Rac and PI3K that demonstrates a role for PI3K in the activation of downstream Rac effector pathways.

The SH4-Unique-SH3-SH2 domains dictate specificity in signaling that differentiate c-Yes from c-Src

c-Src and c-Yes are highly homologous members of the Src family of non-receptor tyrosine kinases. The overall sequence similarity between c-Src and c-Yes allows them to perform many overlapping functions. However, the phenotypes of the c-src and c-yes knockout mice, and cells derived from them, are quite different, indicating functional specificity between the two proteins. Specifically, c-src-/- cells are deficient in several processes that require dynamic regulation of the actin cytoskeleton. In order to begin to understand why c-Yes is unable to compensate for c-Src signaling, we used a series of Src/Yes chimeras in which the non-catalytic functional domains of Src527F were replaced by those of c-Yes. Using chicken embryo fibroblasts as a model system, our results indicate that the c-Yes N-terminal SH4-Unique domains are sufficient to inhibit the ability of Src527F to alter cell morphology, induce actin filament rearrangements or stimulate motility or invasive potential. The data also indicate that the SH4-Unique-SH3-SH2 domains of c-Yes work cooperatively and prevent activation of signaling proteins associated with Src527F transformation, including activation of phosphatidylinositol 3-kinase, phosphorylation of c-Raf and Akt and downregulation of RhoA-GTP. These data indicate that c-Yes may not modulate signals associated with c-Src-induced changes in actin filament integrity and may explain why c-Yes fails to compensate for c-Src signaling in src-/- cells.

Androgen-stimulated DNA synthesis and cytoskeletal changes in fibroblasts by a nontranscriptional receptor action

In NIH3T3 cells, 0.001 nM of the synthetic androgen R1881 induces and stimulates association of androgen receptor (AR) with Src and phosphatidylinositol 3-kinase (Pl3-kinase), respectively, thereby triggering S-phase entry. 10 nM R1881 stimulates Rac activity and membrane ruffling in the absence of the receptor-Src-PI3-kinase complex assembly. The antiandrogen Casodex and specific inhibitors of Src and PI3-kinase prevent both hormonal effects, DNA synthesis and cytoskeletal changes. Neither low nor high R1881 concentration allows receptor nuclear translocation and receptor-dependent transcriptional activity in fibroblasts, although they harbor the classical murine AR. The very low amount of AR in NIH3T3 cells (7% of that present in LNCaP cells) activates the signaling pathways, but apparently is not sufficient to stimulate gene transcription. This view is supported by the appearance of receptor nuclear translocation as well as receptor-mediated transcriptional activity after overexpression of AR in fibroblasts. In addition, AR-negative Cos cells transiently transfected with a very low amount of hAR cDNA respond to low and high R1881 concentrations with signaling activation. Interestingly, they do not show significant transcriptional activation under the same experimental conditions. Fibroblasts are the first example of cells that respond to steroid hormones with activation of signaling pathways in the absence of endogenous receptor transcriptional activity. The data reported also show that hormone concentration can be crucial in determining the type of cell responsiveness.

Cyclin D1 governs adhesion and motility of macrophages

The cyclin D1 gene encodes the regulatory subunit of a holoenzyme that phosphorylates and inactivates the retinoblastoma protein, thereby promoting cell-cycle progression. Cyclin D1 is overexpressed in hematopoetic and epithelial malignancies correlating with poor prognosis and metastasis in several cancer types. Because tumor-associated macrophages have been shown to enhance malignant progression and metastasis, and cyclin D1-deficient mice are resistant to oncogene-induced malignancies, we investigated the function of cyclin D1-/- bone marrow-derived macrophages. Cyclin D1 deficiency increased focal complex formation at the site of substratum contact, and enhanced macrophage adhesion, yielding a flattened, circular morphology with reduced membrane ruffles. Migration in response to wounding, cytokine-mediated chemotaxis, and transendothelial cell migration of cyclin D1-/- bone marrow-derived macrophages were all substantially reduced. Thus, apart from proliferative and possible motility defects in the tumor cells themselves, the reduced motility and invasiveness of cyclin D1-/- tumor-associated macrophages may contribute to the tumor resistance of these mice.

Rho kinase blockade prevents inflammation via nuclear factor kappa B inhibition: evidence in Crohn's disease and experimental colitis

BACKGROUND & AIMS

Rho proteins are involved in the regulation of several cellular functions. Data from in vitro studies suggest that RhoA could be involved in the inflammatory response. We investigated the role of RhoA and its downstream effector Rho kinase in intestinal inflammation.

METHODS

Activation of RhoA was assessed by pull-down assays. A specific inhibitor of Rho kinase, Y-27632, was used to examine the role of Rho kinase in inflammatory response in vivo and in vitro by molecular biology and by immunological and biochemical approaches.

RESULTS

Increased activation of RhoA was found in inflamed intestinal mucosa of patients with Crohn's disease and of rats with 2,4,6-trinitrobenzene sulfonic acid-induced colitis. Oral administration of Y-27632 in rats significantly reduced the colonic inflammation. In vitro, activation of RhoA alone was sufficient to induce tumor necrosis factor production. Y-27632 inhibited production of tumor necrosis factor-alpha and interleukin-1 beta by lamina propria and peripheral blood mononuclear cells. Rho kinase inhibition prevented nuclear factor kappa B activation and I-kappa B phosphorylation and degradation. We showed that Rho kinase associates with and activates I-kappa B kinase alpha and that Y-27632 prevents I-kappa B kinase activation.

CONCLUSIONS

Our study provides the first evidence that Rho kinase activates I-kappa B kinase and, thus, nuclear factor kappa B, suggesting a key role of Rho kinase in inflammatory responses and intestinal inflammation. Specific inhibition of Rho kinase may be a promising approach for the treatment of patients with Crohn's disease.

Shear stress-induced endothelial cell polarization is mediated by Rho and Rac but not Cdc42 or PI 3-kinases

Shear stress induces endothelial polarization and migration in the direction of flow accompanied by extensive remodeling of the actin cytoskeleton. The GTPases RhoA, Rac1, and Cdc42 are known to regulate cell shape changes through effects on the cytoskeleton and cell adhesion. We show here that all three GTPases become rapidly activated by shear stress, and that each is important for different aspects of the endothelial response. RhoA was activated within 5 min after stimulation with shear stress and led to cell rounding via Rho-kinase. Subsequently, the cells respread and elongated within the direction of shear stress as RhoA activity returned to baseline and Rac1 and Cdc42 reached peak activation. Cell elongation required Rac1 and Cdc42 but not phosphatidylinositide 3-kinases. Cdc42 and PI3Ks were not required to establish shear stress-induced polarity although they contributed to optimal migration speed. Instead, Rho and Rac1 regulated directionality of cell movement. Inhibition of Rho or Rho-kinase did not affect the cell speed but significantly increased cell displacement. Our results show that endothelial cells reorient in response to shear stress by a two-step process involving Rho-induced depolarization, followed by Rho/Rac-mediated polarization and migration in the direction of flow.

Role of G protein and protein kinase signalling in influenza virus budding in MDCK cells

Recently, we have shown that influenza virus budding in MDCK cells is regulated by metabolic inhibitors of ATP and ATP analogues (Hui & Nayak, Virology 290, 329-341, 2001 ). In this report, we demonstrate that G protein signalling stimulators such as sodium fluoride, aluminium fluoride, compound 48/80 and mastoparan stimulated the budding and release of influenza virus. In contrast, G protein signalling blockers such as suramin and NF023 inhibited virus budding. Furthermore, in filter-grown lysophosphatidylcholine-permeabilized virus-infected MDCK cells, membrane-impermeable GTP analogues, such as guanosine 5'-O-(3-thiotriphosphate) or 5'-guanylylimidodiphosphate caused an increase in virus budding, which could be competitively inhibited by adding an excess of GTP. These results suggest that the G protein is involved in the regulation of influenza virus budding. We also determined the role of different protein kinases in influenza virus budding. We observed that specific inhibitors or activators of protein kinase A (H-89 and 8-bromoadenosine 3',5'-cyclic monophosphate) or of protein kinase C (bisindolylmaleimide I and Ro-32-0432) or of phosphatidylinositol 3-kinase (LY294002 and wortmannin) did not affect influenza virus budding. However, the casein kinase 2 (CK2) inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole decreased virus budding. We further observed an increase in the CK2 activity during the replication cycle of influenza virus, although Western blot analysis did not reveal any increase in the amount of CK2 protein in virus-infected cells. Also, in digitonin-permeabilized MDCK cells, the introduction of CK2 substrate peptides caused a down-regulation of virus budding. These results suggest that CK2 activity also regulates influenza virus budding.

Imaging antigen-induced PI3K activation in T cells

Activation of phosphoinositide 3-kinase (PI3K) at the immunological synapse between a T cell and an antigen-presenting cell (APC) has not been demonstrated. Using fluorescent-specific probes, we show here that the formation of an immunological synapse led to sustained production of 3'-phosphoinositides in the T cell, whereby phosphatidylinositol-3,4,5-trisphosphate (PIP3) but not phosphatidylinositol-3,4-bisphosphate was localized to the cell membrane. The accumulation of PIP3 after T cell activation preceded the increase in intracellular calcium. Neither the formation of conjugates between T cells and APCs nor signaling events such as phosphotyrosine accumulation and calcium increase changed substantially when PI3K was inhibited, and only a limited reduction in synthesis of interleukin 2 occurred. In T cell-APC conjugates, PIP3 accumulated at the T cell-APC synapse as well as in the rest of the T cell plasma membrane, which indicated unusual regulation of PI3K activity during antigen presentation.

Intestinal barrier function

Intestinal barrier function regulates transport and host defense mechanisms at the mucosal interface with the outside world. Transcellular and paracellular fluxes are tightly controlled by membrane pumps, ion channels and tight junctions, adapting permeability to physiological needs. Food and microbial antigens are under constant surveillance of the mucosal immune system. Tolerance against commensals and immunity against pathogens require intact antigen uptake, recognition, processing and response mechanisms. Disturbance at any level, but particularly bacterial translocation due to increased permeability and breakdown of oral tolerance due to compromised epithelial and T cell interaction, can result in inflammation and tissue damage. New therapeutic approaches including probiotics and peptides to restore disrupted barrier function are evolving.

Regulation of substrate adhesion dynamics during cell motility

The movement of a metazoan cell entails the regulated creation and turnover of adhesions with the surface on which it moves. Adhesion sites form as a result of signaling between the extracellular matrix on the outside and the actin cytoskeleton on the inside, and they are associated with specific assembles of actin filaments. Two broad categories of adhesion sites can be distinguished: (1) "focal complexes" associated with lamellipodia and filopodia that support protrusion and traction at the cell front; and (2) "focal adhesions" at the termini of stress fibre bundles that serve in longer term anchorage. Focal complexes are signaled via Rac1 or Cdc42 and can either turnover on a minute scale or differentiate, via intervention of the RhoA pathway, into longer-lived focal adhesions. All classes of adhesion sites depend on the stress in the actin cytoskeleton for their formation and maintenance. Different cell types use different adhesion strategies to move, in terms of the relative engagement of filopodia and lamellipodia in focal complex formation and protrusion and the extent of focal adhesion formation. These differences can be attributed to variations in the relative activities of Rho family members. However, the Rho GTPases alone are unable to signal asymmetry in the actin cytoskeleton, necessary for polarisation and movement. Polarisation requires the collaboration of the microtubule cytoskeleton. Changes in the polymerisation state of microtubules influences the activities of both Rac1 and RhoA and microtubules interact directly with adhesion foci and promote their turnover. Possible mechanisms of cross-talk between the microtubule and actin cytoskeletons in determining polarity are discussed.

Role of Akt signaling in vascular homeostasis and angiogenesis

Akt is a serine/threonine protein kinase that is activated by a number of growth factors and cytokines in a phosphatidylinositol-3 kinase-dependent manner. Although antiapoptotic activity of Akt is well known, it also regulates other aspects of cellular functions, including migration, glucose metabolism, and protein synthesis. In this review, Akt signaling in endothelial cells and its critical roles in the regulation of vascular homeostasis and angiogenesis will be discussed.

Lipoxins induce actin reorganization in monocytes and macrophages but not in neutrophils: differential involvement of rho GTPases

Lipoxins (LXs) are endogenously produced eicosanoids that inhibit neutrophil trafficking and stimulate nonphlogistic phagocytosis of apoptotic neutrophils by monocyte-derived macrophages. In this study we assessed the effect of LXs on cell ultrastructure and actin reorganization in human leukocytes and investigated the signaling events that subserve LX bioactivity in this context. LXA(4) (10(-9) mol/L), the stable synthetic analogues 15-(R/S)-methyl-LXA(4) and 16-phenoxy-LXA(4) (10(-11) mol/L), but not the LX precursor 15-(S)-HETE, induced marked changes in ultrastructure and rearrangement of actin in monocytes and macrophages. In contrast, LXA(4) did not modify actin distribution in neutrophils under basal conditions and after stimulation with leukotriene B(4). Blockade of Rho kinases by the inhibitor Y-27632 prevented LXA(4)-triggered actin reorganization in macrophages. To investigate the role of the specific small GTPases in LX-induced actin rearrangement we used THP-1 cells differentiated to a macrophage-like phenotype. THP-1 cells stimulated with LXs, but not with 15-(S)-HETE, showed an increase in membrane-associated RhoA and Rac by immunoblotting. Additionally, a twofold increase in Rho activity was seen in response to LXA(4). LX-induced actin rearrangement and RhoA activation were inhibited by the cell permeable cAMP analogue 8-Br-cAMP, whereas Rp-cAMP, an inhibitor of protein kinase A, mimicked the effect of LXA(4). These data demonstrate that LXs stimulate RhoA- and Rac-dependent cytoskeleton reorganization, contributing to the potential role of LXs in the resolution of inflammation.

Roles of PI3Ks in leukocyte chemotaxis and phagocytosis

Through selective disruption of phosphoinositide 3-kinase (PI3K) activity and the use of green fluorescent protein tagged derivatives of domains capable of specifically binding the lipid products of PI3Ks in vivo, it has been shown that this family of signalling enzymes have vital and distinct roles in chemotaxis, phagocytosis and phagosome maturation in leukocytes.