Regulation of chemotaxis by the platelet-derived growth factor receptor-beta

Epidermal Growth Factor Induces Fibroblast Contractility and Motility via a Protein Kinase C δ-dependent Pathway

Myosin-based cell contractile force is considered to be a critical process in cell motility. However, for epidermal growth factor (EGF)-induced fibroblast migration, molecular links between EGF receptor (EGFR) activation and force generation have not been clarified. Herein, we demonstrate that EGF stimulation increases myosin light chain (MLC) phosphorylation, a marker for contractile force, concomitant with protein kinase C (PKC) activity in mouse fibroblasts expressing human EGFR constructs. Interestingly, PKCdelta is the most strongly phosphorylated isoform, and the preferential PKCdelta inhibitor rottlerin largely prevented EGF-induced phosphorylation of PKC substrates and MARCKS. The pathway through which EGFR activates PKCdelta is suggested by the fact that the MEK-1 inhibitor U0126 and the phosphatidylinositol 3-kinase inhibitor LY294002 had no effect on PKCdelta activation, whereas lack of PLCgamma signaling resulted in delayed PKCdelta activation. EGF-enhanced MLC phosphorylation was prevented by a specific MLC kinase inhibitor ML-7 and the PKC inhibitors chelerythrine chloride and rottlerin. Further indicating that PKCdelta is required, a dominant-negative PKCdelta construct or RNAi-mediated PKCdelta depletion also prevented MLC phosphorylation. In the absence of PLC signaling, MLC phosphorylation and cell force generation were delayed similarly to PKCdelta activation. All of the interventions that blocked PKCdelta activation or MLC phosphorylation abrogated EGF-induced cell contractile force generation and motility. Our results suggest that PKCdelta activation is responsible for a major part of EGF-induced fibroblast contractile force generation. Hence, we identify here a new pathway helping to govern cell motility, with PLC signaling playing a role in activation of PKCdelta to promote the acute phase of EGF-induced MLC activation.

Expression of a naturally occurring constitutively active variant of the epidermal growth factor receptor in mouse fibroblasts increases motility

Tumor cell motility is one of the rate-limiting steps of invasion, which defines progression toward a more malignant phenotype. Elevated expression of epidermal growth factor receptor (EGFR) in many cancers is associated with progression of superficial to invasive forms of the disease. The naturally occuring type III mutant epidermal growth factor receptor (EGFRvIII) is a tumor-specific, ligand-independent, constitutively active variant of the epidermal growth factor receptor. EGFRvIII is expressed frequently by a number of human solid tumours including those of the lung, breast, prostate, brain and ovary. Our study was designed to investigate the effect of EGFRvIII expression on cell motility and compare it to that of ligand-activated EGFR using transfected fibroblasts. We show here using time-lapse video recording that expression of EGFRvIII greatly enhances the motility of fibroblasts independently of ligand stimulation. In addition, expression of EGFRvIII caused a marked increase in the number of cellular protrusions (lamellipodia) and a reduction in the number of stress fibers and focal adhesions. The EGFR tyrosine kinase inhibitor, AG1478, and the MEK inhibitor, U0126, blocked these cellular effects of EGFRvIII. Two cell lines expressing different levels of EGFR were used for comparison. The low-expressing cell line responded to EGF treatment by increasing motility in a manner very similar to the motility induced by EGFRvIII. In contrast, the high-expressing cell line responded to EGF by detachment from the extracellular matrix and decreased motility. Cellular detachment was correlated to a high phosphorylation of PLC-gamma, whereas increased motility was correlated to a high level of ERK phosphorylation. Overall these results indicate that tumor-associated EGFR mutations might be critical for tumor cell motility, invasion and thus progression of disease.

Additive effects of PDGF receptor beta signaling pathways in vascular smooth muscle cell development

The platelet-derived growth factor β receptor (PDGFRβ) is known to activate many molecules involved in signal transduction and has been a paradigm for receptor tyrosine kinase signaling for many years. We have sought to determine the role of individual signaling components downstream of this receptor in vivo by analyzing an allelic series of tyrosine–phenylalanine mutations that prevent binding of specific signal transduction components. Here we show that the incidence of vascular smooth muscle cells/pericytes (v/p), a PDGFRβ-dependent cell type, can be correlated to the amount of receptor expressed and the number of activated signal transduction pathways. A decrease in either receptor expression levels or disruption of multiple downstream signaling pathways lead to a significant reduction in v/p. Conversely, loss of RasGAP binding leads to an increase in this same cell population, implicating a potential role for this effector in attenuating the PDGFRβ signal. The combined in vivo and biochemical data suggest that the summation of pathways associated with the PDGFRβ signal transduction determines the expansion of developing v/p cells.

Using both in vivo and biochemical approaches, the summation of pathways associated with the PDGFRβ signal transduction is shown to determine the expansion of a specific PDGFRβ-dependent cell type

Insulin-like growth factor I induces migration and invasion of human multiple myeloma cells

Multiple myeloma (MM) is an incurable form of cancer characterized by accumulation of malignant plasma cells in the bone marrow. During the course of this disease, tumor cells cross endothelial barriers and home to the bone marrow. In latter stages, myeloma cells extravasate through blood vessels and may seed a variety of organs. Insulin-like growth factor I (IGF-I) is one of several growth factors shown to promote the growth of MM cells. In the current study, we have assessed the ability of IGF-I to serve additionally as a chemotactic factor affecting the mobility and invasive properties of these cells. Results indicate that IGF-I promotes transmigration through vascular endothelial cells and bone marrow stromal cell lines. Analysis of endogenous signaling pathways revealed that protein kinase D/protein kinase Cmicro (PKD/PKCmicro) and RhoA were both activated in a phosphatidylinositol 3-kinase (PI-3K)-dependent manner. Inhibition of PI-3K, PKCs, or Rho-associated kinase by pharmacologic inhibitors abrogated migration, whereas mitogen-activated protein kinase (MAPK), Akt, and p70S6 kinase inhibitors had no effect. These results suggest that IGF-I promotes myeloma cell migration by activation of PI-3K/PKCmicro and PI-3K/RhoA pathways independent of Akt. The identification of IGF-I as both a proliferative and migratory factor provides a rational basis for the development of targeted therapeutic strategies directed at IGF-I in the treatment of MM.

Retinoic acid signaling through PI 3-kinase induces differentiation of human endometrial adenocarcinoma cells

The specific signals required for actin polymerization in response to extracellular factors remain unknown. However, in many cell types, there is a correlation between actin polymerization, activation of phosphatidylinositol 3-kinase (PI 3-kinase), and the production of the second messenger phosphatidylinositol-3,4,5-triphosphate. Increased levels of PI 3-kinase have been detected during cell growth and transformation. However, PI 3-kinase is also activated during differentiation, suggesting that PI 3-kinase and its lipid products also play a role in the regulation of cellular differentiation. The newly characterized CAC-1 cell line established from a poorly differentiated human endometrial adenocarcinoma (Exp. Mol. Pathol. 69 (2000), 175) was used as a model to investigate the role of PI 3-kinase in differentiation induction. CAC-1 cells differentiated upon treatment with pharmacological doses of retinoids (1 micro M of 13-cis or all-trans), evidenced by actin filament reorganization, and cell enlargement. PI 3-kinase staining is primarily localized to perinuclear regions in untreated cells. However, retinoic acid treatment induced PI 3-kinase to relocalize throughout the cytoplasm. Subcellular fractionation and Western blotting confirmed that PI 3-kinase decreased in the particulate fraction, concurrent with retinoid-induced differentiation. Interestingly, pretreatment with the PI 3-kinase inhibitor wortmannin (100 nM) prior to retinoic acid treatment prevented retinoic acid-induced actin reorganization and cell enlargement. To distinuish whether retinoid regulation of PI 3-kinase is mediated through traditional nuclear retinoic acid receptors, the levels of retinoic acid receptor-beta (RAR-beta) protein were evaluated. Retinoid treatment did not alter RAR-beta protein levels compared to controls. These data suggest that PI 3-kinase activity and cytoplasmic relocalization are required for retinoid-induced differentiation of poorly differentiated human endometrial adenocarcinoma cells.

Directional motility induced by epidermal growth factor requires Cdc42

Cell motility is actuated by a host of intracellular signaling cascades that result in movement of the cell in one direction, even without an external gradient. Phospholipase C-gamma (PLCgamma) has been shown to be important for growth factor-induced lamellipodial protrusion at the front of the cell while Cdc42 has been implicated in both filopodium formation at the leading edge and control of polarity of migrating cells. We asked whether these asymmetries in effector molecules may be linked. When we overexpressed either constitutively active, dominant negative, or GFP-tagged Cdc42, wild-type NR6 fibroblasts lost directionality, as expected. On epidermal growth factor (EGF) exposure these cells produced multiple, transient protrusions in every direction; these extensions failed to result in productive motility. GFP-tagged Cdc42 appeared transiently at edges of newly formed protrusions in EGF-stimulated cells while they moved haphazardly. While PLCgamma is distributed throughout the cell, the ratio of active, tyrosyl-phosphorylated PLCgamma was increased at the leading edge, where phosphatidylinositol (4,5)-bisphosphate (PIP(2)) hydrolysis is concentrated. This co-localization of activities may be due to Cdc42 directing PLCgamma to the cell front, as PLCgamma associated with Cdc42 in an EGF-dependent manner. We conclude that Cdc42 controls cell polarity, likely in part, through its binding to active PLCgamma.

Genetic evidence for convergence of c-Kit- and alpha4 integrin-mediated signals on class IA PI-3kinase and the Rac pathway in regulating integrin-directed migration in mast cells

Mast cells play a critical role in host defense against a number of pathogens. Increased mast cell infiltration has been described in allergic asthma, in rheumatoid arthritis, and during helminthes infection. Despite the importance of mast cells in allergic disease and defense against infection, little is known about the mechanisms by which mast cells migrate to various tissues under steady state conditions or during infection or inflammation. Here, we show that activation of c-Kit by its ligand, stem cell factor (SCF), cooperates with alpha4 integrin in inducing directed migration of mast cells on fibronectin. A reduction in migration and activation of a small G protein, Rac, was observed in mast cells derived from class IA phosphoinositide-3 kinase (PI-3kinase)-deficient mice in response to SCF stimulation and in mast cells expressing the dominant-negative Rac (RacN17), as well as in mast cells deficient in the hematopoietic-specific small G protein, Rac2. In addition, a PI-3kinase inhibitor inhibited alpha4- as well as SCF-induced migration in a dose-dependent fashion. In contrast, a mitogen-activated protein kinase (MAPK) inhibitor had little effect. Consistent with the pharmacologic results, abrogating the binding of the p85alpha subunit of class IA PI-3kinase to c-Kit also resulted in inhibition of SCF-induced migration on fibronectin. These genetic and biochemical data demonstrate that both c-Kit and alpha4 integrin signaling are linked to class IA PI-3kinase and Rac pathways and regulate integrin-directed (haptotactic) migration in mast cells.

A new link between the c-Abl tyrosine kinase and phosphoinositide signalling through PLC-gamma1

The c-Abl tyrosine (Tyr) kinase is activated after platelet-derived-growth factor receptor (PDGFR) stimulation in a manner that is partially dependent on Src kinase activity. However, the activity of Src kinases alone is not sufficient for activation of c-Abl by PDGFR. Here we show that functional phospholipase C-gamma1 (PLC-gamma1) is required for c-Abl activation by PDGFR. Decreasing cellular levels of phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) by PLC-gamma1-mediated hydrolysis or dephosphorylation by an inositol polyphosphate 5-phosphatase (Inp54) results in increased Abl kinase activity. c-Abl functions downstream of PLC-gamma1, as expression of kinase-inactive c-Abl blocks PLC-gamma1-induced chemotaxis towards PDGF-BB. PLC-gamma1 and c-Abl form a complex in cells that is enhanced by PDGF stimulation. After activation, c-Abl phosphorylates PLC-gamma1 and negatively modulates its function in vivo. These findings uncover a newly discovered functional interdependence between non-receptor Tyr kinase and lipid signalling pathways.

Matrix metalloproteinase-12 gene expression in human vascular smooth muscle cells

BACKGROUND

Matrix metalloproteinases (MMPs) play an important role in smooth muscle cell (SMC) migration and proliferation during vascular remodelling. To investigate the expression of MMP-12 by SMCs, we examined the protein secretion and mRNA expression of MMP-12 by cultured medial SMCs and intimal SMCs derived from human aortic atherosclerotic lesions. To further elucidate the molecular mechanism for MMP-12 expression in SMCs, we determined the sequence requirements for MMP-12 gene transcriptional activity.

RESULTS

Cultured medial SMCs and intimal SMCs showed substantial MMP-12 expression at both the protein and mRNA levels. A series of 5'-deletion and site-directed mutants of the human MMP-12 promoter demonstrated that an AP-1 site spanning -81 to -75 bp was critical for the MMP-12 promoter activity in SMCs. An electrophoretic mobility shift assay confirmed the AP-1 binding activity in SMCs and showed that the protein bound to the AP-1 site consisted predominantly of c-Jun, JunD and Fra-1. Two structurally different inhibitors of phosphatidylinositol 3-kinase, wortmannin and LY294002, inhibited MMP-12 transcriptional activity and AP-1 binding.

CONCLUSION

These results indicated the expression of MMP-12 in vascular SMCs and showed that the MMP-12 gene expression was dependent on the AP-1 binding activity. Phosphatidylinositol 3-kinase signalling may be involved in MMP-12 transcriptional activation through AP-1 binding activity.

Keratinocyte growth factor promotes cell motility during alveolar epithelial repair in vitro

Epithelia play a key role as protective barriers, and mechanisms of repair are crucial for restoring epithelial barrier integrity, especially in the lung. Cell spreading and migration are the first steps of reepithelialization. Keratinocyte growth factor (KGF) plays a key role in lung epithelial repair and protects against various injuries. We hypothesized that KGF may protect the lung not only by inducing proliferation but also by promoting epithelial repair via enhanced epithelial cell migration. In an in vitro wound-healing model, we found that KGF enhanced wound closure by 33%. KGF acted primarily by inducing lamellipodia emission (73.2 +/- 3.9% of KGF-treated cells had lamellipodia vs 61.3 +/- 3.4% of control cells) and increasing their relative surface area (59 +/- 2.7% with KGF vs 48 +/- 2.0% in controls). KGF reduced cytoskeleton stiffness as measured by magnetic twisting cytometry and increased cell motility (5.8 +/- 0.42 microm/h with KGF vs 3.7 +/- 0.41 microm/h in controls). KGF-increased cell motility was associated with increased fibronectin deposition during wound closure and with fibronectin reorganization into fibrils at the rear of the cells. Taken together, our findings strongly suggest that KGF may promote epithelial repair through several mechanisms involved in cell migration.

Differential roles of signaling pathways for proliferation and migration of rat pancreatic stellate cells

Activated pancreatic stellate cells (PSCs) have recently been implicated in the pathogenesis of pancreatic fibrosis and inflammation. Accumulation of PSCs is a fundamental feature of pancreatic fibrosis, and platelet-derived growth factor (PDGF)-BB is the most potent mitogen for PSCs. But, the molecular mechanisms responsible for PDGF's actions in PSCs are largely unknown. In hepatic stellate cells, it has been established that activation of both phosphatidylinositol (PI) 3-kinase and extracellular-signal regulated kinase (ERK) pathways is required for PDGF-BB-induced proliferation and migration. The aim of this study was to elucidate the signaling pathways mediating PDGF-BB's actions in PSCs. PSCs were isolated from rat pancreas tissue and used in their culture-activated, myofibroblast-like phenotype. Culture-activated PSCs expressed PDGF alpha- and beta-receptors. PDGF-BB induced autophosphorylation of its receptor, followed by the activation of PI 3-kinase, Akt, and ERK pathways. Activation of PI 3-kinase was not required for PDGF-BB-induced ERK activation. PDGF-BB induced approximately five-fold increase in proliferation and chemotaxis of PSCs. Inhibition of ERK pathway with PD98059 completely blocked proliferation, whereas PD98059 had a modest inhibitory effect on cell migration (approximately 50%). On the other hand, inhibition of PI 3-kinase pathway with wortmannin or LY294002 almost completely inhibited migration, but did not affect proliferation of PSCs. In conclusion, our results suggest that ERK pathway regulates proliferation and migration in response to PDGF-BB, whereas PI 3-kinase mediates cellular migration, but not proliferation of PSCs.

Development and characterisation of tetracycline-regulated phosphoinositide 3-kinase mutants: assessing the role of multiple phosphoinositide 3-kinases in chemokine signaling

A combination of pharmacological, biochemical, molecular and genetic evidence supports a key role for phosphoinositide 3-kinase (PI3K) and its associated signalling cascade in cell migration in response to members of the chemokine family. PI3Ks can be divided into three main classes on the basis of their in vitro lipid substrate specificity, structure and likely mode of regulation. The prototypical class I PI3Ks are heterodimers consisting of the class I(A) 85-kDa regulatory/adaptor subunit and a catalytic 110-kDa subunit and the class I(B) PI3K (PI3Kgamma), which is stimulated by G protein betagamma subunits. Whilst genetic evidence supports a key role for PI3Kgamma in mediating chemotactic responses, it is clear that other PI3K isoforms can be activated by chemokines and can potentially contribute to the chemotactic responses to chemokines. In order to get a more accurate picture of the precise role of individual PI3Ks in functional responses to chemokines, we report development of tetracycline-inducible dominant-negative constructs of the class I(A) and class I(B) PI3Ks and their expression in the leukemic T cell line Jurkat. SDF-1/CXCR4-mediated chemotaxis of Jurkat cells is strongly, but incompletely abrogated (e.g. approximately 60-70%) in clones expressing the dominant-negative PI3Kgamma construct. Interestingly, Jurkat cells expressing a dominant-negative mutant of class I(A) PI3K also exhibited marked abrogation of chemotactic responses to SDF-1, albeit to lesser extent (e.g. approximately 30-40% inhibition) than observed with the class I(B) mutant. These data suggests that both class I(A) and class I(B) isoforms can contribute to chemotactic responses, and both are required for optimal migratory responses to SDF-1. Furthermore, neither isoform alone is able to support optimal migration in the absence of the other. This may reflect an important interplay between the two different forms of PI3K that has yet to be fully elucidated. The use of inducible expression systems such as that described here will be an important approach in assessing the role of not only individual PI3Ks, but also their downstream effector proteins, in supporting actin polymerisation and cytoskeletal rearrangements as well as chemotaxis and adhesion molecule up-regulation.

High glucose levels inhibit focal adhesion kinase-mediated wound healing of rat peritoneal mesothelial cells

BACKGROUND

The peritoneum is progressively denuded of its mesothelial cell monolayer in patients on continuous ambulatory peritoneal dialysis (CAPD). These alterations of the mesothelium cause membrane dysfunction and progressive peritoneal fibrosis. Integrins regulate cell motility and play an important role in wound healing. We investigated the effects of high glucose on the regeneration process of the peritoneal mesothelial cell monolayer using cultured rat peritoneal mesothelial cells (RPMC).

METHODS

The effects of glucose or mannitol on the regeneration of RPMC and formation of focal adhesions were examined by in vitro wound healing assay and immunocytochemistry, respectively. Activities of focal adhesion kinase (FAK) and its downstream p130Cas were examined by Western blotting. Effects of wild-type and dominant-negative FAK on RPMC migration were examined by a transient transfection assay.

RESULTS

Cell migration over fibronectin (FN) was clearly inhibited in culture media containing high glucose (28 to 140 mmol/L). RPMC formed focal adhesions on FN in the presence of a regular glucose concentration (5.6 mmol/L); however, tyrosine phosphorylation of FAK and p130Cas and formation of focal adhesions observed by FAK and vinculin staining were substantially inhibited by high glucose. Mannitol also induced significant inhibitory effects, but these were milder than those of glucose. Transfection of dominant-negative FAK inhibited cell migration in a regular glucose concentration, whereas overexpression of wild-type FAK abrogated glucose-induced inhibition of cell migration.

CONCLUSIONS

Our results demonstrate that high glucose concentrations as well as high osmolarity inhibit FAK-mediated migration of mesothelial cells, and suggest that dialysates containing high glucose concentrations may cause peritoneal damage by inhibiting wound healing of the mesothelial cell monolayer.

Functional studies of the PI(3)-kinase signalling pathway employing synthetic and expressed siRNA

RNA interference (RNAi) is a RNA-mediated sequence-specific gene silencing mechanism. Recently, this mechanism has been used to down-regulate protein expression in mammalian cells by applying synthetic- or vector-generated small interfering RNAs (siRNAs). However, for the evaluation of this new knockdown technology, it is crucial to demonstrate biological consequences beyond protein level reduction. Here, we demonstrate that this new siRNA-based technology is suitable to analyse protein functions using the phosphatidylinositol (PI) 3-kinase signal transduction pathway as a model system. We demonstrate stable and transient siRNA-mediated knockdown of one of the PI 3-kinase catalytic subunits, p110beta, which leads to inhibition of invasive cell growth in vitro as well as in a tumour model system. Importantly, this result is consistent with loss-of-function phenotypes induced by conventional RNase H-dependent antisense molecules or treatment with the PI 3-kinase inhibitor LY294002. RNAi knockdown of the downstream kinases Akt1 and Akt2 does not reduce cell growth on extracellular matrix. Our data show that synthetic siRNAs, as well as vector-based expression of siRNAs, are a powerful new tool to interfere with signal transduction processes for the elucidation of gene function in mammalian cells.

Implications of growth factor alterations in the treatment of pancreatic cancer

Pancreatic cancer ranks fifth as a cause of cancer-related death in the world with an overall 5-year survival rate of less than 1% and a median survival of less than a year after tumour detection. Most of these patients have already metastases at the time of diagnosis. The oncologic strategies such as chemotherapy, radiotherapy, antihormonal modalities or the systemic use of specific monoclonal antibodies have not achieved a significant improvement in the survival of pancreatic cancer patients. Recent studies suggest that alterations in molecular pathways, particularly in growth factor mediated mechanisms, that regulate cell proliferation and differentiation play a pivotal role in the pathogenesis of this cancer. The molecular knowledge regarding changes in the expression of growth factors in pancreatic cancer has the potential to improve diagnostic and therapeutic treatment strategies in the near future.

Beta-adrenergic receptor activation inhibits keratinocyte migration via a cyclic adenosine monophosphate-independent mechanism

There is increasing evidence that G-protein-coupled receptors cross-talk with growth factor receptor-mediated signal transduction in a variety of cell types. We have investigated mechanisms by which the activation of beta-adrenergic receptors, classically GTP-binding proteins coupled receptors, influence the migration of cultured human keratinocytes. We found that iso-proterenol, a beta-adrenergic receptor-selective agonist, inhibited cell migration stimulated by either epidermal growth factor, or extracellular Ca2+ in a concentration-dependent manner. This was prevented by pretreatment of the cells with the beta-adrenergic receptor-selective antagonist timolol. Interestingly, isoproterenol, at a concentration of 1 nm, did not measurably increase intracellular cyclic adenosine monophosphate concentrations yet inhibited cell migration by 50%. To test further if isoproterenol's actions were mediated via activation of adenylyl cyclase, two inhibitors of its activity, 2'5'-dideoxyadenosine and SQ22536, were used. Both compounds significantly diminished iso-proterenol-induced increases in intracellular cyclic adenosine monophosphate concentrations but did not attenuate isoproterenol-induced inhibition of cell migration. Also, forskolin (1 microm) markedly increased intracellular cyclic adenosine monophosphate concentrations but did not significantly inhibit cell migration. As mitogen-activated protein kinases are known to signal growth factor-stimulated cell migration, we examined whether beta-adrenergic receptor-mediated inhibition of keratinocyte migration might occur via inactivation of mitogen-activated protein kinases. We found that isoproterenol inhibited phosphorylation of extracellular signal-regulated kinase mitogen-activated protein kinase in a concentration-dependent manner but had no effect on the phosphorylation of the stress mitogen-activated protein kinases c-jun N-terminal kinase and stress-activated protein kinase-2. Neither forskolin nor a membrane permeable cyclic adenosine monophosphate analog inhibited phosphorylation of any of these mitogen-activated protein kinases. These findings suggest that beta-adrenergic receptor-induced inhibition of keratinocyte migration is mediated through inhibition of the extracellular signal-regulated kinase mitogen-activated protein kinase signaling in a cyclic adenosine monophosphate-independent manner.

Overexpression of G protein-coupled receptor kinase-2 in smooth muscle cells reduces neointimal hyperplasia

The activation of vascular smooth muscle cells (SMCs) in neointimal hyperplasia involves signaling through receptor tyrosine kinases as well as G protein-coupled receptors. Overexpression of G protein-coupled receptor kinase-2 (GRK2) in SMCs can attenuate mitogenic signaling and proliferation in response to not only several G protein-coupled receptor agonists, but also platelet-derived growth factor (PDGF). To test whether overexpression of GRK2 could inhibit other SMC responses implicated in neointimal hyperplasia, we assessed SMC chemotaxis and mitogenic signaling evoked by PDGF and G(q)-coupled receptor agonists. To test the effects of GRK2 overexpression on neointimal hyperplasia in vivo, we employed a rabbit autologous vein graft model system. GRK2 overexpression reduced PDGF-promoted SMC chemotaxis by 85% (P<0.01), but had no effect on chemotaxis promoted by epidermal growth factor (EGF). Congruently, GRK2 overexpression reduced by approximately 50% (P<0.05) the [(3)H]thymidine incorporation induced by combinations of PDGF and Gq-coupled receptor agonists, but had no effect on that induced by PDGF plus EGF. PDGF-, but not EGF-promoted phosphoinositide 3-kinase activity in SMCs was also inhibited by GRK2 overexpression. In rabbit vein grafts, we achieved GRK2 overexpression in medial SMCs, reduced cell proliferation during the first week after graft implantation, and reduced steady state neointimal thickness by 29% (P<0.01), without affecting medial thickness or potentiating SMC apoptosis. Because of its ability to dampen chemotactic and mitogenic signaling through PDGF and Gq-coupled receptors, GRK2 overexpression in SMCs may be a useful therapeutic approach for neointimal hyperplasia.

Cysteinyl leukotrienes promote human airway smooth muscle migration

Cysteinyl leukotrienes promote airway smooth muscle (ASM) contraction and proliferation. Little is known about their role in ASM migration. We investigated this using cultured human ASMs (between the second and fifth passages) obtained from the large airways of resected nonasthmatic lung. Platelet-derived growth factor-BB (1 ng/ml) promoted significant (3.5-fold) ASM migration of myocytes across collagen-coated 8- micro m polycarbonate membranes in Transwell culture plates. Leukotriene E(4) (10(-7), 10(-8), 10(-9) M) did not demonstrate a chemotactic effect; it did promote chemokinesis. Priming by leukotriene E(4) (10(-7) M) significantly augmented the directional migratory response to platelet-derived growth factor (1.5-fold, p < 0.05). This was blocked by montelukast (10(-6) M), demonstrating the effect to be mediated by the cysteinyl leukotriene receptor. The "priming effect" was also partially attenuated by prostaglandin E(2) (10(-7) M). Whereas both the chemokinetic and the chemotactic "primed" responses were equally attenuated by a p38 mitogen-activated protein kinase inhibitor (SB203580, 25 micro M) and by a Rho-kinase inhibitor (Y27632, 10 micro M), the chemotactic response showed greater inhibition than chemokinesis by a phosphatidylinositol-3 kinase inhibitor (LY294002, 50 micro M). These experiments suggest that cysteinyl leukotrienes play an augmentary role in human ASM migration. The phosphatidylinositol-3 kinase pathway is a key signaling mechanism in the chemotactic migration of ASM cells in response to cysteinyl leukotrienes.

Inhibitory effect of a selective cyclooxygenase-2 inhibitor on liver metastasis of colon cancer

COX-2 overexpression is recognized in various cancers, but the role of COX-2 in the progression of cancer, including the liver metastasis of colon cancer, is not clearly understood. We examined the role of COX-2 in the mechanism of liver metastasis of colon cancer, using a highly metastasizable colon carcinoma cell line, LM-H3. A COX-2 inhibitor, JTE-522, inhibited cell proliferation and invasion of LM-H3 in vitro and clearly reduced the number of metastatic nodules on the surface of nude mouse livers in vivo. We also examined the effects of JTE-522 on the production of growth factors and MMPs through the use of ELISA and gelatin zymography, respectively. JTE-522 downregulated PDGF production by LM-H3 but had no influence on VEGF production. JTE-522 also inhibited MMP-2 secretion by LM-H3. JTE-522 downregulated PGE(2) production, but the associated changes in PGE(2) did not affect PDGF and VEGF production by LM-H3. We conclude that JTE-522 downregulated the cell proliferation and invasive potential of LM-H3 by reducing the production of PDGF and MMP-2 and hypothesize that these inhibitory effects on the production of PDGF and MMP-2 can lead to inhibition of liver metastasis of colon cancer. These data indicate that the COX-2 inhibitor JTE-522 has a high potential for use as a clinical agent for the treatment of liver metastasis of colon cancer.

Motility is rate-limiting for invasion of bladder carcinoma cell lines

Induced migration of tumor cells is generally considered to be one critical step in cancer progression to the invasive and metastatic stage. The implicit caveat of studies that show this is that other, unknown, signaling pathways and biophysical events are actually the operative rate-limiting steps, and not motility per se. Thus, to examine the hypothesis that motility is a single, but overall rate-limiting function required for invasion, disparate motility processes need be blocked with concordant effects on tumor invasion. Recently, we and others have described two signaling pathways that are critical to growth factor-induced motility but not mitogenesis. The key molecular switches are phospholipase C-gamma (PLCgamma) and calpain for cytoskeletal reorganization and rear detachment, respectively. We examined this hypothesis in a highly invasive tumor, bladder carcinoma. Three different human tumor cell lines, 253J-B-V, UMUC and T-24, were tested for invasiveness in vitro by transmigration of a Matrigel barrier. Inhibiting PLCgamma with the pharmacologic agent U73122 or the molecular dominant-negative PLCz construct reduced both invasiveness and motility. The same was noted when calpain was blocked using calpain inhibitor I (ALLN). These results demonstrate that one interventional target for limiting invasion is not necessarily an individual motility pathway but rather cell migration per se.

Distribution of gelsolin and phosphoinositol 4,5-bisphosphate in lamellipodia during EGF-induced motility

During induced cell motility the actin cytoskeleton at the leading edge must undergo constant reorganization. Recently, phosphoinositides have been shown to be central to cytoskeleton-membrane linkages and actin organization and turnover. Epidermal growth factor (EGF) receptor (EGFR)-mediated cell motility requires phospholipase C-gamma (PLCgamma), hydrolysis of phosphoinsotide 4,5-bisphosphate (PIP(2)) and subsequent release of gelsolin. We hypothesized this led to the mobilization of PIP(2)-binding proteins which modify the actin cytoskeleton and thus sought to determine whether the leading edge was a site of active PIP(2) hydrolysis and gelsolin redistribution to cytoskeleton. Herein, we report that during EGF-induced motility, the leading edge's submembranous region constitutes a distinct subcellular locale. The relevant phosphoinositide composition of this space was determined by probing with an antibody to PIP(2) and a green fluorescence protein (GFP)-tagged pleckstrin homology (PH) domain of PLCdelta (GFP-PH) that recognizes both PIP(2) and inositol 1,4,5-trisphosphate (IP(3)). PIP(2) was absent from leading lamellipodia despite an increase in IP(3) generation, suggesting an increase in PIP(2) hydrolysis at the leading edge. Visualized with immunofluorescence, gelsolin preferentially concentrated near the leading edge in a punctate fashion. Examining the Triton X-insoluble actin cytoskeleton fractions, we observe a PLCgamma-dependent increase of gelsolin incorporation upon EGF stimulation. At a molecular level, field emission scanning electron microscopy (FE-SEM) shows that gelsolin incorporates preferentially into the submembranous actin arcs at the leading edge of the lamellipodia. Together these data suggest a model of PIP(2) hydrolysis at the leading edge causing a localized release of PIP(2)-binding proteins-particularly gelsolin-that drives cytoskeletal rearrangement and protrusion.

EBV-expressing AGS gastric carcinoma cell sublines present increased motility and invasiveness

Tumor invasion marks a critical point in cancer progression; it is a harbinger of morbidity and mortality. Thus, the cellular events that enable the invasive phenotype are under intense investigation. Epstein-Barr virus (EBV) is associated with a number of cancers, including Burkitt lymphoma (BL) and nasopharyngeal carcinoma (NPC) and is suspected to contribute to their tumorigenesis. On average, 8% of gastric carcinomas have been shown to carry this virus. To explore whether the presence of EBV in gastric carcinoma contributes to tumor progression in this predominantly invasive carcinoma, we examined a panel of 2 in vitro EBV-infected human gastric cancer cell line sublines and their mock-infected AGS parental control line. We found EBV infection caused a marked increase in transmigration of a Matrigel barrier (415% and 303%, p < 0.05, for the 2 infected lines). This correlated with increased motility of these sublines (233% and 140%, p < 0.05). As this pattern of increased motility leading to a more pronounced enhancement of invasion has been noted in other tumor cells, we explored the roles of autocrine signaling pathways previously implicated in carcinoma motility and invasion. Inhibitors to the epidermal growth factor receptor (EGFR) (PD153035), phospholipase C (PLC) (U73122), extracellular-signal regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) (PD089035) and PI-3 kinase (Wortmannin) were not informative. These data suggest that EBV increases migration of AGS cells by a mechanism independent of these autocrine growth factor-induced pathways. Instead, we found that the EBV-infected cells presented increased focal adhesion kinase (FAK) phosphorylation. These findings suggest a role for integrin-mediated signaling in promoting EBV-associated invasiveness.

Macrophage/microglia-specific protein Iba1 enhances membrane ruffling and Rac activation via phospholipase C-gamma -dependent pathway

Iba1 is a macrophage/microglia-specific calcium-binding protein that is involved in RacGTPase-dependent membrane ruffling and phagocytosis. In this study, we introduced Iba1 into Swiss 3T3 fibroblasts and demonstrated the enhancement of platelet-derived growth factor (PDGF)-induced membrane ruffling and chemotaxis. Wortmannin treatment did not completely suppressed this enhanced membrane ruffling in Iba1-expressing cells, whereas it did in Iba1-nonexpressing cells, suggesting that the enhancement is mediated through a phosphatidylinositol 3-kinase (PI3K)-independent signaling pathway. Porcine aorta endothelial cells transfected with expression constructs of Iba1 and PDGF receptor add-back mutants were used to analyze the signaling pathway responsible for the Iba1-induced enhancement of membrane ruffling. In the absence of Iba1 expression, PDGF did not induced membrane ruffling in cells expressing the Tyr-1021 receptor mutant, which is capable of activating phospholipase C-gamma (PLC-gamma) but not PI3K. In contrast, in the presence of Iba1 expression, membrane ruffling was formed in cells expressing the Tyr-1021 mutant. In addition, Rac was shown to be activated during membrane ruffling in cells expressing Iba1 and the Tyr-1021 mutant. Furthermore, dominant negative forms of PLC-gamma completely suppressed PDGF-induced Iba1-dependent membrane ruffling and Rac activation. These results indicate the existence of a novel signaling pathway where PLC-gamma activates Rac in a manner dependent on Iba1.

Regulatory role for SRC and phosphatidylinositol 3-kinase in initiation of fibronectin matrix assembly

Fibronectin (FN) matrix assembly is a tightly regulated stepwise process that is initiated by interactions between FN and cell surface integrin receptors. These interactions activate many intracellular signaling pathways that regulate processes such as cell adhesion, migration, and survival. Here we demonstrate that cells lacking Src family kinases showed reduced ability to assemble FN fibrils as detected by immunofluorescence and by analysis of detergent extracts. The amount of FN matrix was further reduced by treatment with the phosphatidylinositol 3 (PI 3-kinase) inhibitor, wortmannin. CHOalpha5 cells, which are dependent on exogenous FN to initiate fibril formation, also showed significant reductions in matrix when treated with inhibitors of Src and PI 3-kinase. Combination of both inhibitors showed an additive inhibitory effect on assembly, which was concomitant with a loss of focal adhesion kinase phosphorylation. Decreased binding of the 70-kDa amino-terminal FN fragment at matrix assembly sites further supports a role for these kinases early during the process. We propose that these two signaling molecules, which lie downstream of integrins and focal adhesion kinase, are essential for efficient initiation of FN matrix assembly.

Red wine polyphenols inhibit vascular smooth muscle cell migration through two distinct signaling pathways

BACKGROUND

Red wine polyphenols (RWPs) have been shown to have an antiatherogenic activity mainly through antioxidative effects on LDL oxidation. Although vascular smooth muscle cell (SMC) migration is critical to atherosclerosis formation, the effect of RWPs on SMC migration has not been elucidated. In this study, we investigated whether RWPs could affect the migration of cultured SMCs stimulated by growth factors.

METHODS AND RESULTS

RWP concentration dependently inhibited platelet-derived growth factor (PDGF)-BB-induced and serum-induced SMC migration in wounding assay and Boyden chamber assay. However, these inhibitory effects of RWPs were not seen in serum-stimulated vascular endothelial cell migration in either assay. Moreover, specific inhibitors of phosphatidylinositol-3' kinase (PI3K) and p38 mitogen-activated protein kinase (p38(MAPK)), but not of extracellular signal-regulated protein kinases 1 and 2 (ERK1/2), reduced PDGF-BB-induced SMC migration. To elucidate the signaling mechanism underlying the RWP effects, we investigated the effects of RWPs on the activity of PI3K and the phosphorylation of MAPK pathways in PDGF-BB-stimulated SMCs. RWPs inhibited the PI3K activity and p38(MAPK) phosphorylation, but not ERK1/2 phosphorylation, in a concentration-dependent manner. Moreover, the phosphorylation of MKK3/6, an upstream kinase of p38(MAPK), was also inhibited by RWP treatment in a concentration-dependent manner, suggesting that the inhibitory effect of RWPs on the p38(MAPK) pathway works upstream of MKK3/6. The concentration-effect relationship of RWPs necessary for the inhibition of PI3K and p38(MAPK) pathways was similar to that of cell migration assays.

CONCLUSIONS

RWPs inhibit the SMC migration through the inhibition of 2 distinct signaling pathways and thus exert antiatherogenic actions.

Critical roles of c-Kit tyrosine residues 567 and 719 in stem cell factor-induced chemotaxis: contribution of src family kinase and PI3-kinase on calcium mobilization and cell migration

Stem cell factor (SCF) has crucial roles in proliferation, survival, and differentiation of hematopoietic stem cells and mast cells through binding to c-Kit receptor (KIT). Chemotaxis is another unique function of SCF. However, little is known about the intracellular signaling pathway of SCF/KIT-mediated cell migration. To investigate the signaling cascade, we made a series of 22 KIT mutants, in which tyrosine (Y) residue was substituted for phenylalanine (F) in the cytoplasmic domain, and introduced into BAF3 cells or 293T cells. On stimulation with SCF, BAF3 expressing KIT(WT)(WT) showed cell migration and Ca(2+) mobilization. Among 22 YF mutants, Y567F, Y569F, and Y719F showed significantly reduced cell migration and Ca(2+) mobilization compared to WT. In Y567F, Lyn activation on SCF stimulation decreased and C-terminal Src kinase (Csk) suppressed KIT-mediated Ca(2+) influx and cell migration, suggesting that Y567-mediated Src family kinase (SFK) activation leads to Ca(2+) influx and migration. Furthermore, we found that p38 mitogen-activated protein kinase (p38 MAPK) and Erk1/2 were also regulated by Y567/SFK and involved in cell migration, and that p38 MAPK induced Ca(2+) influx, thereby leading to Erk1/2 activation. In Y719F, the binding of phosphatidylinositol 3'-kinase (PI3K) to KIT was lost and KIT-mediated cell migration and Ca(2+) mobilization were suppressed by PI3K chemical inhibitors or dominant-negative PI3K, suggesting the involvement of Y719-mediated PI3K pathway in cell migration. Combination of Csk and the PI3K inhibitor synergistically reduced cell migration, suggesting the cooperation of SFK and PI3K. Taken together, these results indicate that 2 major KIT signaling pathways lead to cell migration, one is Y567-SFK-p38 MAPK-Ca(2+) influx-Erk and the other is Y719-PI3K-Ca(2+) influx.

Enhanced expression of the LDL receptor family member LR11 increases migration of smooth muscle cells in vitro

BACKGROUND

LR11, a member of the LDL receptor family, is highly expressed in vascular smooth muscle cells (SMCs) of the hyperplastic intima but not media. To further clarify the involvement of LR11 in the process of atherosclerosis, we have characterized the migration and invasion activities of LR11-overexpressing SMCs.

METHODS AND RESULTS

LR11 cDNA was transfected into the rat SMC line A7r5. Compared with mock cells (C-1), in the presence of platelet-derived growth factor-BB, the transfected cells (R-1 and R-2) showed 3.5- to 4.0-fold higher expression of LR11 protein, 1.7- to 1.8-fold increased migration, and 2.0- to 2.2-fold elevated invasion activities, respectively. The increases were essentially abolished by the addition of receptor-associated protein, anti-LR11 antibodies, or apolipoprotein E. Immunological analyses showed that urokinase-type plasminogen activator receptor (uPAR) levels were increased in LR11-overexpressing cells. Anti-urokinase-type plasminogen activator (uPA) and anti-uPAR antibodies reduced the migration and invasion activities of R-1 and R-2 cells to baseline levels. Receptor-associated protein, anti-LR11 antibodies, and apolipoprotein E decreased uPAR expression in the LR11-overexpressing cells by approximately 50%. Cellular catabolism of uPAR was significantly decreased in R-1 and R-2 cells compared with control. Cultured SMCs isolated from intima of atherosclerotic rabbit aortas showed increased expression levels of LR11 and uPAR and enhanced migration and invasion compared with SMCs from medial layers.

CONCLUSIONS

Overexpression of LR11 induces enhanced migration and invasion activities of intimal SMCs in vitro, probably through its regulation of the uPA/uPAR system.

Phosphatidylinositol 3-kinase but not tuberin is required for PDGF-induced cell migration

The loss of function of the tumor suppressor gene TSC2 and its protein product tuberin promotes the development of benign lesions by stimulating cell growth, although the role of tuberin in regulating cell migration and metastasis has not been characterized. In addition, the role of phosphatidylinositol 3-kinase (PI 3-kinase), an important signaling event regulating cell migration, in modulating tuberin-deficient cell motility remains unknown. Using a tuberin-deficient rat smooth muscle cell line, ELT3, we demonstrate that platelet-derived growth factor (PDGF) stimulates cell migration by 3.2-fold, whereas vascular endothelial growth factor (VEGF), transforming growth factor (TGF)-alpha, and basic fibroblast growth factor (bFGF) increase migration by 2.1-, 2.1-, and 2.6-fold, respectively. Basal and PDGF-induced migration in tuberin-deficient ELT3, ELT4, and ERC15 cells was not significantly different from that of tuberin-positive transformed rat kidney epithelial 2, airway smooth muscle, and pulmonary arterial vascular smooth muscle cells. Expression of tuberin in tuberin-deficient ELT3 cells also had little effect on cell migration. In parallel experiments, the role of PI 3-kinase activation in ELT3 cell migration was investigated. LY-294002, a PI 3-kinase inhibitor, decreased PDGF-induced migration in a concentration-dependent manner with an IC(50) of approximately 5 microM. LY-294002 also abrogated ELT3 cell migration stimulated by bFGF and TGF-alpha but not by VEGF and phorbol 12-myristate 13-acetate. Furthermore, transient expression of constitutively active PI 3-kinase (p110*) was sufficient to induce ELT3 cell migration. However, the migration induced by p110* was less than that induced by growth factors, suggesting other signaling pathways are also critically important in modulating growth factor-induced cell migration. These data suggest that PI 3-kinase is required for growth factor-induced cell migration and loss of tuberin appears to have little effect on cell migration.

Cyclase-associated proteins: CAPacity for linking signal transduction and actin polymerization

Many extracellular signals elicit changes in the actin cytoskeleton, which are mediated through an array of signaling proteins and pathways. One family of proteins that plays a role in regulating actin remodeling in response to cellular signals are the cyclase-associated proteins (CAPs). CAPs are highly conserved monomeric actin binding proteins present in a wide range of organisms including yeast, fly, plants, and mammals. The original CAP was isolated as a component of the Saccharomyces cerevisiae adenylyl cyclase complex that serves as an effector of Ras during nutritional signaling. CAPs are multifunctional molecules that contain domains involved in actin binding, adenylyl cyclase association in yeast, SH3 binding, and oligomerization. Genetic studies in yeast have implicated CAPs in vesicle trafficking and endocytosis. CAPs play a developmental role in multicellular organisms, and studies of Drosophila have illuminated the importance of the actin cytoskeleton during eye development and in establishing oocyte polarity. This review will highlight the critical structural and functional domains of CAPs, describe recent studies that have implied important roles for these proteins in linking cell signaling with actin polymerization, and highlight their roles in vesicle trafficking and development.

Calcium-sensing receptor-mediated activation of phospholipase C-gamma1 is downstream of phospholipase C-beta and protein kinase C in MC3T3-E1 osteoblasts

Elevated extracellular calcium (Ca(e)) stimulates both chemotaxis and mitogenesis of MC3T3-E1 osteoblasts via a calcium-sensing receptor (CasR). Ca(e)-mediated chemotaxis of these bone-forming cells is dependent on phospholipase C (PLC) and blocked by the Gi-protein inhibitor pertussis toxin. In this study, we examine the signaling mechanisms by which the CasR stimulates PLC activity in MC3T3-E1 osteoblasts. We found that elevated Ca(e) stimulated PLC-gamma1 tyrosine phosphorylation in a time-dependent and Ca(e)-concentration-dependent manner. The maximal increase in PLC-gamma1 tyrosine phosphorylation was observed 3-5 min after increasing Ca(e) by 3.2 mmol/L from 1.8 mmol/L. Elevated Ca(e) also promoted a rapid increase in both inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], a second messenger formed by PLC-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate, and cytosolic free calcium ([Ca+2]i). The kinetics of the CasR-mediated increases in Ins(1,4,5)P3 and [Ca+2]i and the sensitivity of the Ca(e)-stimulated elevation in [Ca+2]i to U73122 (a PLC inhibitor) together suggest that the osteoblast CasR is coupled via Gq to PLC-beta. U73122 blocked the Ca(e)-promoted, but not PDGF-promoted, PLC-gamma1 tyrosine phosphorylation, suggesting that the activation of PLC-beta is upstream of PLC-gamma1 activation. Inhibition of protein kinase C (PKC) disrupted Ca(e)-stimulated tyrosine phosphorylation of PLC-gamma1. In addition, exposure to pertussis toxin or exogenous activation of protein kinase A (PKA) inhibited PLC-gamma1 tyrosine phosphorylation in response to Ca(e). The results indicate that: (a) the osteoblast CasR activates PLC-gamma1 downstream of PLC-beta in a PKC-dependent manner; (b) PKA is a negative regulator of Ca(e)-promoted PLC-gamma1 phosphorylation; and (c) Gq and Gi are both involved in the CasR-mediated phosphorylation of PLC-gamma1.

Different effects of high and low shear stress on platelet-derived growth factor isoform release by endothelial cells: consequences for smooth muscle cell migration

In the present study, we analyzed the effect of conditioned media (CM) from bovine aortic endothelial cells exposed to laminar shear stress (SS) of 5 dyne/cm2 (SS5) or 15 dyne/cm2 (SS15) for 16 hours on smooth muscle cell (SMC) migration. In response to CM from bovine aortic endothelial cells exposed to SS5 (CMSS5) and SS15 (CMSS15), migration was 45 +/- 5.5 and 30 +/- 1.5 cells per field, respectively (P<0.05). Similar results were obtained with SS of 2 versus 20 dyne/cm2 and also when SS of 5 and 15 dyne/cm2 lasted 24 hours. Platelet-derived growth factor (PDGF)-AA levels in CMSS5 and CMSS15 were 9 +/- 7 and 18 +/- 5 ng/10(6) cells for 16 hours, respectively (P<0.05); PDGF-BB levels in CMSS5 and CMSS15 were 38 +/- 10 and 53 +/- 10 ng/10(6) cells for 16 hours, respectively (P<0.05). PDGF receptor alpha (PDGFRalpha) and PDGF receptor beta (PDGFRbeta) in SMCs were phosphorylated by CMSS15>CMSS5. In response to CMSS15, a neutralizing antibody against PDGF-AA enhanced SMC migration to a level comparable to that of CMSS5; in contrast, antibodies against PDGF-BB abolished SMC migration. Transfection of SMCs with a dominant-negative PDGFRalpha or PDGFRbeta increased or inhibited, respectively, SMC migration in response to CMSS15. Overexpression of wild-type PDGFRalpha inhibited SMC migration in response to CMSS5, CMSS15, or recombinant PDGF-BB (P<0.001). These results suggest that the ability of high SS to inhibit arterial wall thickening in vivo may be related to enhanced activation of PDGFRalpha in SMCs by PDGF isoforms secreted by the endothelium.

Effect of platelet-derived growth factor isoforms in rat metanephric mesenchymal cells

Platelet-derived growth factor (PDGF) B-chain or PDGF beta-receptor-deficient mice lack mesangial cells. To explore potential mechanisms for failure of PDGF A-chain to rescue mesangial cell phenotype, we investigated the biological effects and signaling pathways of PDGF AA and PDGF BB in metanephric mesenchymal (MM) cells isolated from rat kidney. PDGF AA caused modest cell migration but had no effect on DNA synthesis, unlike PDGF BB, which potently stimulated migration and DNA synthesis. PDGF AA and PDGF BB significantly increased the activities of phosphatidylinositol 3-kinase (PI 3-K) and mitogen-activated protein kinase (MAPK). PDGF BB was more potent than PDGF AA in activating PI 3-K or MAPK in these cells. Pretreatment of MM cells with the MAPK kinase (MEK) inhibitor PD-098059 abrogated PDGF BB-induced DNA synthesis, whereas the PI 3-K inhibitor wortmannin had a very modest inhibitory effect on DNA synthesis (approximately Delta20%). On the other hand, wortmannin completely blocked PDGF AA- and PDGF BB-induced migration, whereas PD-098059 had a modest inhibitory effect on cell migration. These data demonstrate that activation of MAPK is necessary for the mitogenic effect of PDGF BB, whereas PI 3-K is required for the chemotactic effect of PDGF AA and PDGF BB. Although PDGF AA stimulates PI 3-K and MAPK activity, it is not mitogenic and only modestly chemotactic. Collectively, the data may have implications related to the failure of PDGF AA to rescue mesangial cell phenotype in PDGF B-chain or PDGF-beta-receptor deficiency.

Somatostatin is a selective chemoattractant for primitive (CD34(+)) hematopoietic progenitor cells

OBJECTIVE

Somatostatin (SST) is a regulatory peptide with a wide variety of activities in different tissues. SST activates G(alpha i)-protein-coupled receptors of a family comprising five members (SSTR1-5). Despite the broad use of SST and its analogs in clinical practice, the spectrum of activities of SST is incompletely defined. Here, we examined the role of SST and its receptors in hematopoiesis.

MATERIALS AND METHODS

SSTR expression on human and mouse hematopoietic cells was analyzed by flow cytometry and reverse transcriptase polymerase chain reaction. The effects of SST on cell migration were measured in transwell assays. Using selective inhibitors, signaling mechanisms involved in SSTR2-mediated migration were studied in 32D cell transfectants expressing SSTR2.

RESULTS

Human hematopoietic cells exclusively expressed SSTR2, whereas mouse bone marrow cells expressed SSTR2 and SSTR4. SSTR levels were high on primitive (CD34(+), Lin(-)) but low or absent on more mature (CD34(-), Lin(+)) cell types. Both SST and its analog octreotide acted as chemoattractants for primitive hematopoietic cells. Despite the presence of SSTR4, bone marrow cells from SSTR2-deficient mice failed to migrate toward SST gradients, suggesting that SSTR2 and SSTR4 are functionally different in this respect. SST activated phosphatidylinositol 3-kinase and the MAP kinases Erk1/2 and p38 in 32D[SSTR2] cells. While chemical inhibitors of these kinases had some effect, SST-induced migration was most strongly affected by blocking G(alpha i) activity or by elevating intracellular cAMP levels.

CONCLUSIONS

Somatostatin acts as a selective chemoattractant for immature hematopoietic cells via activation of multiple intracellular pathways.

Chemokine signalling: pivoting around multiple phosphoinositide 3-kinases

The role of chemokines in mediating directional cell migration is well established, but more recently it has become evident that chemokines are able to couple to distinct signalling pathways that are involved in not only chemotaxis, but also cell growth and transcriptional activation. The signalling pathway controlled by the phosphoinositide 3-kinase (PI3K) family of lipid kinases has been the focus of much attention with respect to their role in chemokine-mediated functional responses. Indeed, there now exists convincing biochemical, pharmacological and genetic evidence that both CC and CXC chemokines stimulate PI3K-dependent chemotaxis of inflammatory cells such as eosinophils, macrophages, neutrophils and T lymphocytes. This review considers the role of individual PI3Ks (e.g. the p85/p110 heterodimer, PI3Kgamma and PI3KC2alpha) as well their downstream effector targets in mediating chemokine-stimulated cell migration.

Phosphatidylinositol 3-kinase and NF-kappaB regulate motility of invasive MDA-MB-231 human breast cancer cells by the secretion of urokinase-type plasminogen activator

Cell migration is a fundamental aspect of the neoplastic cell metastasis. Here, we show that phosphatidylinositol (PI) 3-kinase is constitutively active and controls cell motility of highly invasive breast cancer cells by the activation of transcription factor, NF-kappaB. The urokinase-type plasminogen activator (uPA) promoter contains an NF-kappaB binding site, and uPA expression in MDA-MB-231 cells is induced by the constitutively active NF-kappaB. Thus, motility was inhibited by overexpression of a dominant negative p85alpha regulatory subunit of PI 3-kinase (p85DN), as well as by pretreatment of cells with specific inhibitors of the p110 catalytic subunit of PI 3-kinase, wortmannin and LY294002. The involvement of gene transcription in cell motility was suggested because treatment with actinomycin D and cycloheximide, which inhibit transcription and new protein synthesis, respectively, abolished endogenous migration of MDA-MB-231 cells. Although wortmannin, Ly294002, or overexpression of p85DN did not significantly reduce DNA binding activity of NF-kappaB in nuclear extracts, wortmannin, Ly294002, and the overexpression of p85DN or IkappaBalpha inhibited constitutive activation of NF-kappaB in a reporter gene assay. Highly invasive MDA-MB-231 cells constitutively secreted uPA in amounts significantly higher than poorly invasive MCF-7 cells. Furthermore, inhibition of NF-kappaB markedly attenuated endogenous migration, and inhibition of PI 3-kinase and NF-kappaB reduced secretion of uPA. Our data suggest a link between constitutively active PI 3-kinase, NF-kappaB, and secretion of uPA, which is responsible for the migration of highly invasive breast cancer cells. Thus, constitutively active PI 3-kinase controls cell motility by the regulation of expression of uPA through the activation of NF-kappaB.

Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer

The phosphoinositide 3-kinase (PI3K) family of enzymes is recruited upon growth factor receptor activation and produces 3' phosphoinositide lipids. The lipid products of PI3K act as second messengers by binding to and activating diverse cellular target proteins. These events constitute the start of a complex signaling cascade, which ultimately results in the mediation of cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. Therefore, PI3Ks play a central role in many cellular functions. The factors that determine which cellular function is mediated are complex and may be partly attributed to the diversity that exists at each level of the PI3K signaling cascade, such as the type of stimulus, the isoform of PI3K, or the nature of the second messenger lipids. Numerous studies have helped to elucidate some of the key factors that determine cell fate in the context of PI3K signaling. For example, the past two years has seen the publication of many transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated. These models have helped to build a picture of the role of PI3K in physiology and indeed there have been a number of surprises. This review uses such models as a framework to build a profile of PI3K function within both the cell and the organism and focuses, in particular, on the role of PI3K in cell regulation, immunity, and development. The evidence for the role of deregulated PI3K signaling in diseases such as cancer and diabetes is reviewed.

An allelic series at the PDGFalphaR locus indicates unequal contributions of distinct signaling pathways during development

A central issue in signal transduction is the physiological contribution of different growth factor-initiated signaling pathways. We have generated knockin mice harboring mutations in the PDGFalpha receptor (PDGFalphaR) that selectively eliminate its capacity to activate PI3 kinase (alpha(PI3K)) or Src family kinases (alpha(Src)). The alpha(PI3K) mutation leads to neonatal lethality due to impaired signaling in many cell types, but the alpha(Src) mutation only affects oligodendrocyte development. A third knockin line containing mutations that eliminate multiple docking sites does not increase the severity of the alpha(PI3K) mutation. However, embryos with mutations in the PI3K binding sites of both PDGFRs (alpha and beta) recapitulate the PDGFalphaR null phenotype. Our results indicate that PI3K has a predominant role in PDGFalphaR signaling in vivo and that RTK-activated signaling pathways execute both specific and overlapping functions during mammalian development.

Epidermal growth factor induces acute matrix contraction and subsequent calpain-modulated relaxation

During wound healing, dermal fibroblasts switch from a migratory, repopulating phenotype to a contractile, matrix-reassembling phenotype. The mechanisms controlling this switch are unknown. A possible explanation is suggested by the finding that chemokines that appear late in wound repair prevent growth factor-induced cell-substratum de-adhesion by blocking calpain activation. In this study, we tested the specific hypothesis that fibroblast contraction of the matrix is promoted by a pro-repair growth factor, epidermal growth factor, and is modulated by calpain-mediated release of adhesions. We employed an isometric force transduction system designed to measure the contraction of a collagen matrix under tension by a population of NR6 fibroblasts transfected with the human epidermal growth factor receptor. By maintaining a fixed level of strain, we could monitor both the initial contraction and subsequent relaxation of the matrix. Epidermal growth factor stimulated a transient, dose-dependent increase in matrix contraction that peaked within 60 minutes and then decayed over the ensuing 3 to 6 hours. Calpain inhibitor I (ALLN) prevented epidermal growth factor-stimulated cell de-adhesion and resulted in a significantly slower decay of matrix contraction, with only a slight decrease of the peak magnitude of contraction. The mitogen-activated protein kinase kinase-1-selective inhibitor PD 98059 that blocks signaling through the extracellular signal-regulated kinase/mitogen-activated protein kinase pathway, required for epidermal growth factor receptor-mediated activation of calpain and de-adhesion, does not significantly affect the magnitude of matrix contraction within minutes of epidermal growth factor addition, but slows the decay similarly to calpain inhibition. Epidermal growth factor receptor signaling thus stimulates the complementary mechanisms of intracellular contractile force generation and calpain-mediated de-adhesion, which are known to coordinately facilitate cell migration. These findings suggest that calpain can act as a functional switch for transmission of intracellular contractile force to the surrounding matrix, with calpain-mediated de-adhesion reducing this transmission and corresponding matrix contraction. Countervailing processes that down-regulate calpain activation can, accordingly, direct the transition of cell function from locomotion to matrix contraction.

Evidence against high glucose as a mediator of ERK1/2 or p38 MAPK phosphorylation in rat skeletal muscle

Hyperglycemia leads to multiple changes in insulin signaling in skeletal muscle from people with type 2 diabetes. We hypothesized that mitogen-activated protein kinase (MAPK) signaling cascades may be directly activated by an acute exposure to high extracellular glucose concentrations. We determined whether an elevation in the extracellular glucose concentration would induce signal transduction in skeletal muscle via MAPK cascades. Epitrochlearis muscles were incubated in the presence of 5 or 25 mM glucose. Exposure of muscle to either hyperosmosis (600 mM mannitol) or insulin (6 nM) led to a marked increase in extracellular signal-regulated protein kinase (ERK)1/2 phosphorylation. Hyperosmosis elicited a 5.2-fold increase in p38 phosphorylation (P < 0.05), whereas insulin was without effect. ERK1/2 phosphorylation was not increased by high glucose exposure. After a 20-min exposure to 25 mM glucose, a tendency toward repressed (23%) p38 phosphorylation was observed (P = 0.06). No effect of high glucose was noted on signal transduction to signal transducer and activator of transcription 3 and Akt. In conclusion, short-term exposure of skeletal muscle to high levels of glucose does not appear to alter ERK1/2 or p38 MAPK phosphorylation.

Platelet-derived growth factor inhibits smooth muscle cell adhesion to fibronectin by ERK-dependent and ERK-independent pathways

The adhesion of cells to the extracellular matrix plays a major role in cell migration. Pretreatment with platelet-derived growth factor (PDGF) inhibited the adhesion of smooth muscle cells to fibronectin by 80%. This inhibition decreased as concentrations of fibronectin increased. In the presence of 200 microm GRGDS peptide, only 45% of PDGF-treated cells adhered to fibronectin compared with 80% of control cells. This indicates that a decrease in integrin avidity was induced by PDGF. Cell adhesion was partially restored when the activation of the extracellular signal-regulated kinase (ERK) was inhibited with PD98059. The remaining inhibition of adhesion (50%) was independent of the fibronectin concentration, suggesting that the ERK pathway is involved in the decrease in integrin avidity. This was confirmed by depleting ERK protein levels by treatment with ERK antisense oligonucleotide. The adhesion of ERK control oligonucleotide-treated cells decreased by 41% when the concentration of GRGDS peptide was increased from 50 to 200 microm but only decreased by 11% in ERK antisense oligonucleotide-treated cells. Treatment with PDGF also delayed focal complex assembly and inhibited stress fiber formation. Consistent with a delay in tyrosine phosphorylation of paxillin, PDGF treatment caused a lag in focal complex formation, although this was not associated with any change in Src family tyrosine kinase activity. Our results indicate that PDGF inhibits smooth muscle cells adhesion by two pathways. The first involves an ERK-dependent decrease in integrin avidity; the second involves the ERK-independent inhibition of focal complex assembly.

Guidance of cell migration by the Drosophila PDGF/VEGF receptor

Directed cell migrations are important for development, but the signaling pathways and mechanisms responsible for guiding cell migration in vivo are poorly understood. Migration of border cells during Drosophila oogenesis is a simple and attractive model system in which to address these questions. We demonstrate that PVR, a receptor tyrosine kinase related to mammalian PDGF and VEGF receptors, acts in border cells to guide them to the oocyte. The oocyte is the source of a ligand for PVR, PDGF/VEGF factor 1 (PVF1). Intriguingly, the guidance function of PVR is largely redundant with that of EGFR. We present evidence implicating Rac and the Rac activator Mbc/DOCK180/CED-5 as mediators of the guidance signal.

Two signals are better than one: border cell migration in Drosophila

Recent studies of border cell migration during Drosophila oogenesis demonstrate that the EGFR and PDGFR signaling pathways act in a partially redundant manner to guide this process. Evidence presented shows that PDGFR signaling directs border cell migration via Rac and the Rac activator Mbc/CED-5/Dock180.

Nck/Dock: an adapter between cell surface receptors and the actin cytoskeleton

In response to extracellular signals, cell surface receptors engage in connections with multiple intracellular signaling pathways, leading to the cellular responses such as survival, migration, proliferation and differentiation. The 'pY-->SH2/SH3-->effector' connection is a frequently used scheme by many cell surface receptors, in which SH2/SH3-containing adapters connect protein tyrosine phosphorylation to a variety of downstream effector pathways. Following the initial landmark finding that Grb2 adapter links the receptors to the Ras pathway leading to DNA synthesis, recent studies have revealed that the biological function of the SH2/SH3 adapter Nck/Dock is to link cell surface receptors to the actin cytoskeleton. For example, in the evolutionarily-conserved signaling network, GEF-Rac-Nck-Pak, Nck 'fixes up' the interaction of Pak with its upstream activator, Rac. The activated Pak then regulates the cytoskeletal dynamics. The fact that the majority of the more than 20 Nck-SH3-associated effectors are regulators of the actin cytoskeleton suggests that Nck/Dock regulates, via binding to distinct effectors, various cell type-specific motogenic responses. This review focuses on our current understanding of Nck/Dock function. Due to the number and complexity of the terminologies used in this review, a 'Glossary of Terms' is provided to help reduce confusions.

Myosin II heavy chain isoforms are phosphorylated in an EGF-dependent manner

To explore the involvement and regulation of the nonmuscle myosin II heavy chains isoforms, MHC-A and MHC-B in the chemotaxis of metastatic tumor cells,we analyzed the changes in phosphorylation and cellular localization of these isoforms upon stimulation of prostate tumor cells with epidermal growth factor(EGF). EGF stimulation of prostate tumor cells resulted in transient increases in MHC-A and MHC-B phosphorylation and subcellular localization with quite different kinetics. Furthermore, the kinetics of subcellular localization correlated with the in vivo kinetics of MHC-B phosphorylation but not of MHC-A phosphorylation, suggesting different modes of regulation for these myosin II isoforms. We further showed that protein kinase C (PKC) is involved in the EGF-dependent phosphorylation of MHC-A and MHC-B. To our knowledge, this is the first report demonstrating that MHC phosphorylation might regulate its subcellular localization and that the EGF signal is transmitted to MHC-A and MHC-B via PKC. The correlation between MHC-B phosphorylation and localization in response to EGF stimulation might suggest that MHC-B is the myosin II isoform that is involved in chemotaxis.

Phospholipase C-gamma inhibition prevents EGF protection of intestinal cytoskeleton and barrier against oxidants

Loss of intestinal barrier integrity is associated with oxidative inflammatory GI disorders including inflammatory bowel disease. Using monolayers of human intestinal epithelial (Caco-2) cells, we recently reported that epidermal growth factor (EGF) protects barrier integrity against oxidants by stabilizing the microtubule cytoskeleton, but the mechanism downstream of the EGF receptor (EGFR) is not established. We hypothesized that phospholipase C (PLC)-gamma is required. Caco-2 monolayers were exposed to oxidant (H2O2) with or without pretreatment with EGF or specific inhibitors of EGFR tyrosine kinase (AG-1478, tyrphostin 25) or of PLC (L-108, U-73122). Other Caco-2 cells were stably transfected with a dominant negative fragment for PLC-gamma (PLCz) to inhibit PLC-gamma activation. Doses of EGF that enhanced PLC activity also protected monolayers against oxidant-induced tubulin disassembly, disruption of the microtubule cytoskeleton, and barrier leakiness as assessed by radioimmunoassay, quantitative Western blots, high-resolution laser confocal microscopy, and fluorometry, respectively. Pretreatment with either type of inhibitor abolished EGF protection. Transfected cells also lost EGF protection and showed reduced PLC-gamma phosphorylation and activity. We conclude that EGF protection requires PLC-gamma signaling and that PLC-gamma may be a useful therapeutic target.

PI 3-kinases and Src kinases regulate spreading and migration of cultured VSMCs

Pulmonary artery smooth muscle cell (PASMC) adhesion, spreading, and migration depend on matrix-stimulated reorganization of focal adhesions. Platelet-derived growth factor (PDGF) activates intracellular signal transduction cascades that also regulate adhesion, spreading, and migration, but the signaling molecules involved in these events are poorly defined. We hypothesized that phosphatidylinositol (PI) 3-kinases and Src tyrosine kinases translate matrix and PDGF-initiated signals into cell motility. In experiments with cultured canine PASMCs, inhibition of PI 3-kinases with wortmannin (0.3 microM) and LY-294002 (50 microM) and of Src kinase with PP1 (30 microM) did not decrease spontaneous (nonstimulated) or PDGF-stimulated (10 ng/ml) adhesion onto collagen. PI 3-kinase and Src kinase activities, however, were necessary for cell spreading: PP1 inhibited cell spreading and Src Tyr-418 phosphorylation in a concentration-dependent manner. Inhibition of PI 3-kinase and Src partially reduced cell migration, while at 10 and 30 microM, PP1 eliminated migration, likely due to inhibition of PDGF receptors. In conclusion, both PI 3-kinases and Src tyrosine kinases are components of pathways that mediate spreading and migration of cultured PASMCs on collagen.

Integrin alpha(v)beta(3) is involved in stimulated migration of vascular adventitial fibroblasts by basic fibroblast growth factor but not platelet-derived growth factor

We examined the effects of basic fibroblast growth factor (bFGF) and platelet-derived growth factor (PDGF) on the migration of vascular adventitial fibroblasts (VAFs) isolated from rat aortic adventitiae. Both bFGF and PDGF significantly stimulated VAF migration in vitro. An antibody to rat beta(3) integrin reduced bFGF-stimulated migration in a dose dependent manner. Moreover, VAF migration was inhibited in the presence of cyclic RGD (cRGD) peptide. However, PDGF-directed migration was blocked only by equivalent cRGD peptide but not by antibody to beta(3) integrin. These data suggest that alpha(v)beta(3) integrin mediates VAF migration stimulated by bFGF and that chemoattractant directed migration may be through distinct integrins.

Effect of p27 deficiency and rapamycin on intimal hyperplasia: in vivo and in vitro studies using a p27 knockout mouse model

SUMMARY

Rapamycin, an immunosuppressant and antiproliferative agent, reduces intimal hyperplasia after arterial injury in animal models and in a preliminary study in humans. Rapamycin treatment reportedly increases expression of p27, a cyclin-dependent kinase inhibitor. This mechanism was tested using a p27-deficient (p27 -/-) murine model. Aortic smooth muscle cells from wild-type (WT) and p27 -/- mice were isolated and cultured. Cell proliferation, assessed by cell count and (3)H-thymidine incorporation, was inhibited significantly by rapamycin in WT and p27 -/- cells at concentrations of 1 ng/ml, 10 ng/ml, and 100 ng/ml (p < 0.05, versus control). The in vivo effect on intimal hyperplasia was studied in p27 -/- and WT mice after femoral artery transluminal injury. Rapamycin treatment was started 2 days before injury and maintained for 2 weeks (1 mg/kg per 48 hours, ip). No significant differences in intima-to-media ratio were found between WT (1.1 +/- 0.1) and p27 -/- mice (1.0 +/- 0.1) 4 weeks after injury. Rapamycin significantly (p < 0.05) reduced intima-to-media ratios in both WT (0.7 +/- 0.1) and p27 -/- mice (0.5 +/- 0.1), compared with untreated mice. p27 deficiency did not alter the arterial wall proliferative response to injury. The inhibitory effect of rapamycin on intimal hyperplasia occurred via a p27-independent mechanism. The in vitro data showed that this effect was mediated through decreased proliferation and enhanced apoptosis.