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

Phosphoinositide 3-kinase inhibition in cancer treatment

Over the past ten years, our knowledge of the integral role that the phospho-inositide 3-kinases (PI3Ks) and their 3'-phosphorylated lipid products (3'-phosphorylated phosphoinositides; 3P-PIs) play in the mediation of signal transduction, cytoskeletal rearrangements and membrane trafficking has expanded considerably. They are now known to be involved in the regulation of cell growth, differentiation, mobility, proliferation and survival and hence they have become a potential target for the control of the growth and spread of cancer cells. More recently, the correlation of the multiplicity of isomers (both catalytic and regulatory) within the different classes of the PI3Ks with their functional relevance has become possible. This, combined with our further understanding of the protein recognition patterns for their different 3P-PIs and the newly-described pathways in the control of the levels of these by dephosphorylation, has provided new aspects and areas for interference in these multiple PI3K signalling pathways. However, in the search for effective, non-toxic, drugs for use in the treatment of cancers, these individual targets for PI3K inhibition need to be further correlated with the specific in vivo effects on cell survival, invasivity and metastatic potential. Here, the range of PI3K inhibition targets are discussed in the light of recent experimental findings, with a view to the exploitation of their specificities in new approaches to effective cancer treatments based on PI3K activity inhibition.

Natural biology of polyomavirus middle T antigen

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

Developmental roles of platelet-derived growth factors

Platelet-derived growth factor (PDGF) was originally identified in platelets and in serum as a mitogen for fibroblasts, smooth muscle cells (SMC) and glia cells in culture. PDGF has since expanded to a family of dimers of at least four gene products, whose biological actions are mediated through two receptor tyrosine kinases, PDGFRs. The present review summarizes and discusses the biological functions of PDGFs and PDGFRs in developmental processes, mainly as revealed through genetic analysis in mice. Such studies have demonstrated multiple critical roles of PDGFs and PDGFRs in embryonic and postnatal development. PDGFs seem to act upon specific populations of progenitor cells that give rise to several different cell types with distinct functions in a variety of developmental processes. Analogies are seen between the cell functions and the developmental processes controlled by PDGFs. This suggests that ancestral PDGF and PDGFR expression patterns and functions may have been iterated in related sets of morphogenetic processes in the course of evolution.

Prostaglandin E2 increases growth and motility of colorectal carcinoma cells

Chronic use of nonsteroidal anti-inflammatory drugs results in a significant reduction of risk and mortality from colorectal cancer in humans. All of the mechanism(s) by which nonsteroidal anti-inflammatory drugs exert their protective effects are not completely understood, but they are known to inhibit cyclooxygenase activity. The cyclooxygenase enzymes catalyze a key reaction in the conversion of arachidonic acid to prostaglandins, such as prostaglandin E(2) (PGE(2)). Here we demonstrate that PGE(2) treatment of LS-174 human colorectal carcinoma cells leads to increased motility and changes in cell shape. The prostaglandin EP(4) receptor signaling pathway appears to play a role in transducing signals which regulate these effects. PGE(2) treatment results in an activation of phosphatidylinositol 3-kinase/protein kinase B pathway that is required for the PGE(2)-induced changes in carcinoma cell motility and colony morphology. Our results suggest that PGE(2) might enhance the invasive potential of colorectal carcinoma cells via activation of major intracellular signal transduction pathways not previously reported to be regulated by prostaglandins.

Dolichol-phosphate-mannose-3 (DPM3)/prostin-1 is a novel phospholipase C-gamma regulated gene negatively associated with prostate tumor invasion

The most ominous development in tumor progression is the transition to an invasive and metastatic phenotype. Little is known, however, about the molecular alterations that cause a tumor to become invasive. In view of this, we have used microarray expression analysis to evaluate the expression profiles of a unique panel of human DU145 prostate cancer sublines that vary in their invasive potential. The three DU145 sublines expressed epidermal growth factor (EGF) receptors that differed in their ability to activate phospholipase C-gamma (PLC gamma). Three-way analyses yielded 11 genes out of 4608 genes screened that associated directly or inversely with invasive potential. The gene whose expression correlated most strongly with lack of invasion was identified as a potential invasion suppressor and called prostin-1. Pharmacological inhibition of PLC gamma (U73122) confirmed that PLC gamma signaling suppressed prostin-1 in that U73122 treatment caused induction of prostin-1 in PLC gamma competent cells. The prostin-1 gene, conserved through phylogeny, is induced by androgen in LNCaP cells and encodes a 92 amino acid protein. The protein shares no extensive homologies with other known genes, yet was recently identified as a small stabilizer subunit of the dolichol-phosphate-mannose (DPM) synthase complex. That DPM3/prostin-1 might suppress tumor progression was supported by the finding that exogenous expression in COS cells leads to apoptosis. These findings support the use of model cell lines to identify putative tumor suppressors and promoters.

Cripto-1 enhances migration and branching morphogenesis of mouse mammary epithelial cells

Cripto-1 is an EGF-CFC protein that performs an important role during early vertebrate development and is overexpressed in several types of human cancer. In the present study mouse EpH4, NMuMG, and TAC-2 mammary epithelial cells that are negative for endogenous cripto-1 expression were transfected with the murine cripto-1 cDNA. Cripto-1-transfected cell lines exhibited functional and physiological differences from the original cell lines including enhanced anchorage-independent growth in soft agar (EpH4 cells), growth in serum-free medium, increased proliferation, and formation of branching, duct-like structures when grown in a three-dimensional collagen type I matrix. Furthermore, cripto-1-expressing cell lines showed elevated migration in vitro in Boyden chamber and wound-healing assays. These results indicate that cripto-1 can function through an autocrine pathway that enables mammary epithelial cells to undergo an epithelial to mesenchymal transition.

Tumor invasion as dysregulated cell motility

Investigations across a range of disciplines over the past decade have brought the study of cell motility and its role in invasion to an exciting threshold. The biophysical forces proximally involved in generating cell locomotion, as well as the underlying signaling and genomic regulatory processes, are gradually becoming elucidated. We now appreciate the intricacies of the many cellular and extracellular events that modulate cell migration. This has enabled the demonstration of a causal role of cell motility in tumor progression, with various points of 'dysregulation' of motility being responsible for promoting invasion. In this paper, we describe key fundamental principles governing cell motility and branch out to describe the essence of the data that describe these principles. It has become evident that many proposed models may indeed be converging into a tightly-woven tapestry of coordinated events which employ various growth factors and their receptors, adhesion receptors (integrins), downstream molecules, cytoskeletal components, and altered genomic regulation to accomplish cell motility. Tumor invasion occurs in response to dysregulation of many of these modulatory points; specific examples include increased signaling from the EGF receptor and through PLC gamma, altered localization and expression of integrins, changes in actin modifying proteins and increased transcription from specific promoter sites. This diversity of alterations all leading to tumor invasion point to the difficulty of correcting causal events leading to tumor invasion and rather suggest that the underlying common processes required for motility be targeted for therapeutic intervention.

Growth factor-dependent signaling and cell cycle progression

There are three central ideas contained within this review. Firstly, growth factor-stimulated signaling is not restricted to a 30-60 min window, but occurs at a much later time as well. Secondly, the second wave of signaling overlaps temporally with the cell cycle program and may be directly responsible for engaging it. Thirdly, the G1 to S interval appears to encompass two distinct phases of the cell cycle, during which the coordinated activation of distinct sets of signaling enzymes drives cell cycle progression. Each of these concepts is likely to initiate new investigation and hence provide additional insight into the fundamental question of how growth factors drive cell proliferation.

A differential role of extracellular signal-regulated kinase in stimulated PC12 pheochromocytoma cell movement

Rat pheochromocytoma PC12 cells have been widely used as a cell system for study of growth factor-stimulated cell functions. We report here that nerve growth factor (NGF) stimulated both chemotaxis (directional migration) and chemokinesis (random migration) of PC12 cells. Treatment with a MEK1/2-specific inhibitor (PD98059) or expression of a dominant negative variant of Ras differentially inhibited NGF-stimulated chemotaxis but not chemokinesis of PC12 cells. Priming of PC12 cells with NGF resulted in reduced extracellular signal-regulated kinase (ERK) activation and loss of chemotactic, but not chemokinetic, response. In addition, NGF stimulation of ERK is known to involve an early transient phase of activation followed by a late sustained phase of activation; in contrast, epidermal growth factor (EGF) elicits only early transient ERK activation. We observed that like NGF, EGF also stimulated both chemotaxis and chemokinesis, and treatment with PD98059 abolished the EGF-stimulated chemotaxis. Therefore, the early transient phase of ERK activation functioned in signaling chemotaxis; the late sustained phase of ERK activation did not seem to have an essential role. In addition, our results suggested that chemotactic signaling required a threshold level of ERK activation; at below threshold level of ERK activation, chemotaxis would not occur.

Phosphatidylinositol 3-kinases in tumor progression

Many cellular processes have been identified in which phosphatidylinositol 3-kinase has a key regulatory function. As an oncogene, it is also involved in the development of cancer. The transformation and progression of normal cells towards an advanced stage tumor and/or towards metastatic lesions involves a complex series of events, including genetic alterations, leading to aberrant cell cycle progression, altered adhesion and motility characteristics, inhibition of apoptosis and induction of angiogenesis. This review highlights the processes involved in the pathogenesis of cancer in which phosphatidylinositol 3-kinase is involved and provides an overview of the possible mechanisms by which the enzyme exerts its oncogenic action.

Guidance of cell migration by EGF receptor signaling during Drosophila oogenesis

Directed cell migration is important for many aspects of normal animal development, but little is known about how cell migrations are guided or the mechanisms by which guidance cues are translated into directed cell movement. Here we present evidence that signaling mediated by the epidermal growth factor receptor (EGFR) guides dorsal migration of border cells during Drosophila oogenesis. The transforming growth factor-alpha (TGF-alpha)-like ligand Gurken appears to serve as the guidance cue. To mediate this guidance function, EGFR signals via a pathway that is independent of Raf-MAP kinase and receptor-specific.

Retention of PDGFR-beta function in mice in the absence of phosphatidylinositol 3'-kinase and phospholipase Cgamma signaling pathways

Signal transduction by the platelet-derived growth-factor receptor beta (PDGFR-beta) tyrosine kinase is required for proper formation of vascular smooth muscle cells (VSMC). However, the importance of individual PDGFR-beta signal transduction pathways in vivo is not known. To investigate the role of two of the pathways believed to be critical for PDGF signal transduction, we have generated mice that bear a PDGFR-beta that can no longer activate PI3kinase or PLCgamma. Although these mutant mice have normal vasculature, we provide multiple lines of evidence in vivo and from cells derived from the mutant mice that suggest that the mutant PDGFR-beta operates at suboptimal levels. Our observations indicate that although loss of these pathways can lead to attenuated PDGF-dependent cellular function, certain PDGFR-beta-induced signal cascades are not essential for survival in mice.

Spatial sensing in fibroblasts mediated by 3' phosphoinositides

The directed movement of fibroblasts towards locally released platelet-derived growth factor (PDGF) is a critical event in wound healing. Although recent studies have implicated polarized activation of phosphoinositide (PI) 3-kinase in G protein-mediated chemotaxis, the role of 3' PI lipids in tyrosine kinase-triggered chemotaxis is not well understood. Using evanescent wave microscopy and green fluorescent protein-tagged Akt pleckstrin homology domain (GFP-AktPH) as a molecular sensor, we show that application of a shallow PDGF gradient triggers a markedly steeper gradient in 3' PI lipids in the adhesion zone of fibroblasts. Polar GFP-AktPH gradients, as well as a new type of radial gradient, were measured from front to rear and from the periphery to the center of the adhesion zone, respectively. A strong spatial correlation between polarized 3' PI production and rapid membrane spreading implicates 3' PI lipids as a direct mediator of polarized migration. Analysis of the temporal changes of 3' PI gradients in the adhesion zone revealed a fast diffusion coefficient (0.5 microm(2)/s) and short lifetime of 3' PIs of <1 min. Together, this study suggests that the tyrosine kinase-coupled directional movement of fibroblasts and their radial membrane activity are controlled by local generation and rapid degradation of 3' PI second messengers.

Differential stimulation of c-Kit mutants by membrane-bound and soluble Steel Factor correlates with leukemic potential

The authors investigated the roles of PI3-kinase and PLC-γ in stimulation by Steel Factor (SLF) through c-Kit. c-Kit mutants YF719, YF728, and a YF719/YF728 double mutant were expressed in 32D myelomonocytic cells. KitYF719 fails to recruit PI3-kinase after stimulation with SLF, whereas KitYF728 fails to stimulate PLC-γ phosphorylation or mobilize Ca++. Both single mutants responded mitogenically to soluble SLF (sSLF) in a manner indistinguishable from wild type (WT), although sSLF failed to stimulate or promote the survival of cells expressing the double mutant. In contrast, although cells expressing WT or YF719 were mitogenically stimulated by membrane-bound SLF (mSLF), stimulation of cells expressing KitYF728 was impaired. Similarly, cells expressing WT or YF719 receptors were stimulated by plate-bound anti-Kit antibodies, whereas cells expressing the YF728 receptor were not stimulated. Neomycin sulfate, a PLC antagonist, inhibited cells expressing YF719 receptors stimulated by sSLF. Neomycin also inhibited cells expressing the WT receptor that were stimulated by mSLF or immobilized anti-Kit antibodies but did not inhibit stimulation of cells expressing WT or YF719 receptors by sSLF. 32D cells expressing KitWT, KitYF719, or KitYF728 were injected into mice and the presence of cells was evaluated by colony assays 6 to 7 weeks later. Although both KitWT and KitYF719 expressing cells could be recovered from the spleen and bone marrow, recovery of KitYF728 cells from these organs was severely reduced. These results indicate that Kit tyrosine 728 is of particular importance for mitogenic stimulation by mSLF or immobilized ligand and is required for full maintenance of cells in vivo, likely through activation of PLC-γ.

Differential stimulation of c-Kit mutants by membrane-bound and soluble Steel Factor correlates with leukemic potential

The authors investigated the roles of PI3-kinase and PLC-γ in stimulation by Steel Factor (SLF) through c-Kit. c-Kit mutants YF719, YF728, and a YF719/YF728 double mutant were expressed in 32D myelomonocytic cells. KitYF719 fails to recruit PI3-kinase after stimulation with SLF, whereas KitYF728 fails to stimulate PLC-γ phosphorylation or mobilize Ca++. Both single mutants responded mitogenically to soluble SLF (sSLF) in a manner indistinguishable from wild type (WT), although sSLF failed to stimulate or promote the survival of cells expressing the double mutant. In contrast, although cells expressing WT or YF719 were mitogenically stimulated by membrane-bound SLF (mSLF), stimulation of cells expressing KitYF728 was impaired. Similarly, cells expressing WT or YF719 receptors were stimulated by plate-bound anti-Kit antibodies, whereas cells expressing the YF728 receptor were not stimulated. Neomycin sulfate, a PLC antagonist, inhibited cells expressing YF719 receptors stimulated by sSLF. Neomycin also inhibited cells expressing the WT receptor that were stimulated by mSLF or immobilized anti-Kit antibodies but did not inhibit stimulation of cells expressing WT or YF719 receptors by sSLF. 32D cells expressing KitWT, KitYF719, or KitYF728 were injected into mice and the presence of cells was evaluated by colony assays 6 to 7 weeks later. Although both KitWT and KitYF719 expressing cells could be recovered from the spleen and bone marrow, recovery of KitYF728 cells from these organs was severely reduced. These results indicate that Kit tyrosine 728 is of particular importance for mitogenic stimulation by mSLF or immobilized ligand and is required for full maintenance of cells in vivo, likely through activation of PLC-γ.

Connecting signaling and cell cycle progression in growth factor-stimulated cells

A widely used model system to investigate cell proliferation is stimulation of serum-arrested cells with growth factors. Recent data suggest that there are two waves of growth factor-dependent signaling events required for a proliferative response. One is an acute burst of signaling, which occurs immediately after growth factor stimulation and lasts for 30 - 60 min. The other occurs in a different time frame (8 - 12 h post stimulation), and involves activation of cyclin dependent kinases (Cdks). In addition to a general overview of growth factor-dependent signaling, we present our 'two wave' hypothesis for how signaling and cell cycle progression are linked.

Binding of factor VIIa to tissue factor on human fibroblasts leads to activation of phospholipase C and enhanced PDGF-BB–stimulated chemotaxis

Tissue factor (TF) is the cellular receptor for factor FVIIa (FVIIa), and the complex is the principal initiator of blood coagulation. The effects of FVIIa binding to TF on cell migration and signal transduction of human fibroblasts, which express high amounts of TF, were studied. Fibroblasts incubated with FVIIa migrated toward a concentration gradient of PDGF-BB at approximately 100 times lower concentration than do fibroblasts not ligated with FVIIa. Anti-TF antibodies inhibited the increase in chemotaxis induced by FVIIa/TF. Moreover, a pronounced suppression of chemotaxis induced by PDGF-BB was observed with active site-inhibited FVIIa (FFR-FVIIa). The possibility that hyperchemotaxis was induced by a putative generation of FXa and thrombin activity was excluded. FVIIa/TF did not induce increased levels of PDGF β-receptors on the cell surface. Thus, the hyperchemotaxis was not a result of this mechanism. FVIIa induced the production of inositol-1,4,5-trisphosphate to the same extent as PDGF-BB; the effects of FVIIa and PDGF-BB were additive. FFR-FVIIa did not induce any release of inositol-1,4,5,-trisphosphate. Thus, binding of catalytically active FVIIa to TF can, independent of coagulation, modulate cellular responses, such as chemotaxis.

Binding of factor VIIa to tissue factor on human fibroblasts leads to activation of phospholipase C and enhanced PDGF-BB–stimulated chemotaxis

Tissue factor (TF) is the cellular receptor for factor FVIIa (FVIIa), and the complex is the principal initiator of blood coagulation. The effects of FVIIa binding to TF on cell migration and signal transduction of human fibroblasts, which express high amounts of TF, were studied. Fibroblasts incubated with FVIIa migrated toward a concentration gradient of PDGF-BB at approximately 100 times lower concentration than do fibroblasts not ligated with FVIIa. Anti-TF antibodies inhibited the increase in chemotaxis induced by FVIIa/TF. Moreover, a pronounced suppression of chemotaxis induced by PDGF-BB was observed with active site-inhibited FVIIa (FFR-FVIIa). The possibility that hyperchemotaxis was induced by a putative generation of FXa and thrombin activity was excluded. FVIIa/TF did not induce increased levels of PDGF β-receptors on the cell surface. Thus, the hyperchemotaxis was not a result of this mechanism. FVIIa induced the production of inositol-1,4,5-trisphosphate to the same extent as PDGF-BB; the effects of FVIIa and PDGF-BB were additive. FFR-FVIIa did not induce any release of inositol-1,4,5,-trisphosphate. Thus, binding of catalytically active FVIIa to TF can, independent of coagulation, modulate cellular responses, such as chemotaxis.

Role of the PI3K regulatory subunit in the control of actin organization and cell migration

Cell migration represents an important cellular response that utilizes cytoskeletal reorganization as its driving force. Here, we describe a new signaling cascade linking PDGF receptor stimulation to actin rearrangements and cell migration. We demonstrate that PDGF activates Cdc42 and its downstream effector N-WASP to mediate filopodia formation, actin stress fiber disassembly, and a reduction in focal adhesion complexes. Induction of the Cdc42 pathway is independent of phosphoinositide 3-kinase (PI3K) enzymatic activity, but it is dependent on the p85alpha regulatory subunit of PI3K. Finally, data are provided showing that activation of this pathway is required for PDGF-induced cell migration on collagen. These observations show the essential role of the PI3K regulatory subunit p85alpha in controlling PDGF receptor-induced cytoskeletal changes and cell migration, illustrating a novel signaling pathway that links receptor stimulation at the cell membrane with actin dynamics.

Structure, function, and control of phosphoinositide-specific phospholipase C

Phosphoinositide-specific phospholipase C (PLC) subtypes beta, gamma, and delta comprise a related group of multidomain phosphodiesterases that cleave the polar head groups from inositol lipids. Activated by all classes of cell surface receptor, these enzymes generate the ubiquitous second messengers inositol 1,4, 5-trisphosphate and diacylglycerol. The last 5 years have seen remarkable advances in our understanding of the molecular and biological facets of PLCs. New insights into their multidomain arrangement and catalytic mechanism have been gained from crystallographic studies of PLC-delta(1), while new modes of controlling PLC activity have been uncovered in cellular studies. Most notable is the realization that PLC-beta, -gamma, and -delta isoforms act in concert, each contributing to a specific aspect of the cellular response. Clues to their true biological roles were also obtained. Long assumed to function broadly in calcium-regulated processes, genetic studies in yeast, slime molds, plants, flies, and mammals point to specific and conditional roles for each PLC isoform in cell signaling and development. In this review we consider each subtype of PLC in organisms ranging from yeast to mammals and discuss their molecular regulation and biological function.

Effects of extracellular matrix on phenotype modulation and MAPK transduction of rat aortic smooth muscle cells in vitro

The transition of arterial smooth muscle cells (SMCs) from a contractile to a synthetic phenotype may play an essential role in the formation of atherosclerotic and restenotic lesions. This process includes a prominent structural reorganization and allows cells to acquire the ability to migrate, proliferate, and secrete extracellular matrix components. According to Western blotting analysis and immunohistochemical and morphological observations, laminin not only retains SMCs in a contractile state but also possibly stimulates cells to transform a synthetic to a contractile phenotype at an early stage, mediated by P38 MAPK signal transduction. However, fibronectin promotes SMCs to transform from a contractile to a synthetic phenotype, mediated by the ERK MAPK signal pathway. The localization of smooth muscle alpha -actin, myosin heavy chain isoform SM2, and vimentin in explant-isolated rat SMCs was affected by a substrate of fibronectin and laminin and also by ERK MAP kinase inhibitor (PD098059) and P38 MAPK inhibitor (SB203580). Furthermore, vimentin may play a much more important role in differentiation than desmin in phenotype modulation in rat aortic smooth muscle cells.

Src-mediated activation of alpha-diacylglycerol kinase is required for hepatocyte growth factor-induced cell motility

Diacylglycerol kinases are involved in cell signaling, either as regulators of diacylglycerol levels or as intracellular signal-generating enzymes. However, neither their role in signal transduction nor their biochemical regulation has been elucidated. Hepatocyte growth factor (HGF), upon binding to its tyrosine kinase receptor, activates multiple signaling pathways stimulating cell motility, scattering, proliferation and branching morphogenesis. Herein we demonstrate that: (i) the enzymatic activity of alpha-diacylglycerol kinase (alphaDgk) is stimulated by HGF in epithelial, endothelial and alphaDgk-transfected COS cells; (ii) cellular expression of an alphaDgk kinase-defective mutant inhibits activation of endogenous alphaDgk acting as dominant negative; (iii) specific inhibition of alphaDgk prevents HGF-induced cell movement of endothelial cells; (iv) HGF induces the association of alphaDgk in a complex with Src, whose tyrosine kinase activity is required for alphaDgk activation by HGF; (v) Src wild type stimulates alphaDgk activity in vitro; and (vi) alphaDgk can be tyrosine phosphorylated in intact cells.

Insulin activates p38 mitogen-activated protein (MAP) kinase via a MAP kinase kinase (MKK) 3/MKK 6 pathway in vascular smooth muscle cells

BACKGROUND

Induction of stress-activated mitogen-activated protein (MAP) kinases such as p38 could be important for the development of cardiovascular diseases since p38 MAP kinase activation stimulates apoptosis, cell growth, prostanoid formation and other cellular dysfunctions when induced by oxidants, hyperosmolarity, or pro-inflammatory cytokines. On the other hand, insulin resistance is one of the most important factors promoting atherogenesis, including cardiovascular diseases, but it is not clear how these different factors transmit their signals intracellularly at the cytosolic and nuclear levels. In this study, we investigated the effect of insulin on p38 mitogen-activated protein (MAP) kinase activation in cultured rat vascular smooth muscle cells (VSMC).

MATERIALS AND METHODS

VSMC were obtained from the aortas of male Wistar rats by the media explant technique. After being stimulated by insulin with SB-203580, PD-98059, or GF109203X, the cells were solubilized and the expressions of MAP kinases, MAP kinase kinases and p70 S6 kinase were examined by immunoblot analysis. The amount of DNA synthesis was measured by [3H]thymidine incorporation.

RESULTS

Insulin activated p38 MAP kinase phosphorylation in a dose-dependent manner, and the phosphorylation was specifically inhibited by SB-203580, a p38 MAP kinase-specific inhibitor, but not by PD-98059, a specific inhibitor of upstream kinase (MEK), of extracellular signal-regulated kinase (ERK), or GF209203X, a protein kinase C-specific inhibitor. Insulin also activated MAP kinase kinase (MKK) 3/MKK 6 phosphorylation, the upstream kinase of p38 MAP kinase, but neither stress-activated protein kinase (SAPK)/ERK kinase (SEK1/MKK4) nor SAPK/c-jun NH2-terminal protein kinase. Surprisingly, phosphorylation of p70 S6 kinase and an increase of DNA synthesis by insulin were suppressed dose dependently by SB-203580.

CONCLUSION

These results have established that insulin activates the p38 MAP kinase cascade via an MKK 3/6 pathway in rat VSMC, independently of a MEK-ERK cascade, and partly regulates cell growth.

A repressor sequence in the juxtamembrane domain of Flt-1 (VEGFR-1) constitutively inhibits vascular endothelial growth factor-dependent phosphatidylinositol 3'-kinase activation and endothelial cell migration

Vascular endothelial growth factor (VEGF) has two highly homologous tyrosine kinase receptors: Flt-1 (VEGFR-1) and KDR (VEGFR-2). KDR is strongly phosphorylated on tyrosines and can transmit mitogenic and motogenic signals following VEGF binding, while Flt-1 is markedly less effective in mediating such functions. To dissect the regions that account for the differences between the two receptors, we generated a series of chimeric Flt-1-KDR molecules. We found that the juxtamembrane region of Flt-1 prevents key signaling functions. When the juxtamembrane region of Flt-1 is replaced by that of KDR, Flt-1 becomes competent to mediate endothelial cell migration and phosphatidylinositol 3'-kinase activation in response to VEGF. Further mutational analysis shows that a short divergent sequence is responsible for such repressor function. However, mutant Flt-1 receptors lacking this sequence do not transmit effective proliferative signals, suggesting that this receptor function is regulated separately. These results define a novel functional domain that serves to repress Flt-1 activity in endothelial cells.

Involvement of Ras and Ral in chemotactic migration of skeletal myoblasts

In skeletal myoblasts, Ras has been considered to be a strong inhibitor of myogenesis. Here, we demonstrate that Ras is involved also in the chemotactic response of skeletal myoblasts. Expression of a dominant-negative mutant of Ras inhibited chemotaxis of C2C12 myoblasts in response to basic fibroblast growth factor (bFGF), hepatocyte growth factor (HGF), and insulin-like growth factor 1 (IGF-1), key regulators of limb muscle development and skeletal muscle regeneration. A dominant-negative Ral also decreased chemotactic migration by these growth factors, while inhibitors for phosphatidylinositol 3-kinase and mitogen-activated protein kinase kinase (MEK) showed no effect. Activation of the Ras-Ral pathway by expression of an activated mutant of either Ras, the guanine-nucleotide dissociation stimulator for Ral, or Ral resulted in increased motility of myoblasts. The ability of Ral to stimulate motility was reduced by introduction of a mutation which prevents binding to Ral-binding protein 1 or phospholipase D. These results suggest that the Ras-Ral pathway is essential for the migration of myoblasts. Furthermore, we found that Ras and Ral are activated in C2C12 cells by bFGF, HGF and IGF-1 and that the Ral activation is regulated by the Ras- and the intracellular Ca(2+)-mediated pathways. Taken together, our data indicate that Ras and Ral regulate the chemotactic migration of skeletal muscle progenitors.

Dual functions of the heartless fibroblast growth factor receptor in development of the Drosophila embryonic mesoderm

The Drosophila embryonic mesoderm forms by invagination of the ventral-most blastoderm cells. Subsequent development of this germ layer involves the dorsolateral migration of the internalized cells and expansion by cell division, followed by the specification of particular cell fates through the coordinate actions of both intrinsic and extrinsic regulatory mechanisms. The latter include several intercellular signals that function across germ layers. These processes combine to generate a diversity of mesodermal sub-types, including the cardial and pericardial cells of the heart or dorsal vessel, a complete set of somatic muscle founders each with its unique identity, a population of cells that form the visceral musculature, and other cells that develop into hemocytes and the fat body. Here, we review recent evidence for the involvement of a fibroblast growth factor receptor (FGFR) encoded by the heartless (htl) gene in early directional migration of the Drosophila mesoderm. In addition, we provide new data that 1) demonstrate a second role for Htl in promoting the specification of the precursors to certain cardiac and somatic muscle cells in the Drosophila embryo, independent of its cell migration function, 2) suggest that Ras and at least one other signal transduction pathway act downstream of Htl, and 3) establish a functional relationship between the Ras pathway and Tinman (Tin), a homeodomain factor that is essential for specifying some of the same dorsal mesodermal cells that are dependent on Htl. Finally, parallels between requirements for FGFR signaling in Drosophila and vertebrate mesoderm development are considered.

Developmental regulation of cell migration. Insight from a genetic approach in Drosophila

Cell movements are fascinating and dramatic features of normal animal development. Moreover, failures in cell migration can lead to birth defects, and inappropriate cell migration can lead to cancer metastasis. Genetic approaches are beginning to provide some insights into the molecular basis for the developmental regulation of cell migration. This review discusses the progress that has been made in understanding the regulation of cell migration during Drosophila development, using a molecular genetic approach. In particular, these studies have implicated signaling through a receptor tyrosine kinase in the spatial control of migration. Reorganization of the cytoskeleton, under the control of the guanosine triphosphatase, Rac, is also critical for cell migration. Finally, genetic studies have demonstrated that the timing of cell migration is under transcriptional control.

Effect of platelet-activating factor receptor expression on CHO cell motility

Tumor cell migration may favor local mass expansion and metastasis dissemination. Several tumors were found to express the receptor for platelet-activating factor (PAF), a potent mediator of leukocyte chemotaxis and endothelial cell migration. However, its functional role on tumor cells is largely unexplored. In the present study, we evaluated the motogenic effect of PAF on Chinese hamster ovarian (CHO) cancer cells transfected with the human PAF-receptor cDNA (CHO PAF-R). By using time-lapse recording, we detected a rapid motogenic response to PAF stimulation on CHO PAF-R, whereas no effect was evident on vector-only transfected cells. Such an effect was observed on scattered cell motility, on cells seeded on a fibronectin- or collagen-coated surface, and on migration of confluent monolayer cells. Cell speed increased at 1 h and was maximal 6-8 h after PAF stimulation on CHO PAF-R. Concomitantly, PAF induced marked changes in cytoskeleton actin distribution with cell contraction, assembling of stress fibers, and polar foci of adhesion. In conclusion, the present study demonstrates that PAF is a potent inducer of tumor cell motility, thus suggesting a role for this mediator in tumor growth and dissemination.

Platelet-derived growth factor receptor beta regulates migration and DNA synthesis in metanephric mesenchymal cells

Platelet-derived growth factor (PDGF) B-chain and PDGF receptor beta (PDGFR beta) are essential for glomerulogenesis. Mice deficient in PDGF B-chain or PDGFR beta exhibit an abnormal glomerular phenotype characterized by total lack of mesangial cells. In this study, we localized PDGFR beta in the developing rat kidney and explored the biological effects of PDGF in metanephric mesenchymal cells in an attempt to determine the mechanism by which PDGF regulates mesangial cell development. Immunohistochemical and in situ hybridization studies of rat embryonic kidneys reveal that PDGFR beta localizes to undifferentiated metanephric mesenchyme and is later expressed in the cleft of the comma-shaped and S-shaped bodies and in more mature glomeruli in a mesangial distribution. We also isolated and characterized cells from rat metanephric mesenchyme. Metanephric mesenchymal cells express vimentin and alpha-smooth muscle actin but not cytokeratin. These cells also express functional PDGFR beta, as demonstrated by autophosphorylation of the receptor as well as activation of phosphatidylinositol 3 kinase in response to PDGF B-chain homodimer. PDGF B-chain also induces migration and proliferation of metanephric mesenchymal cells. Taken together with the fact that PDGF B-chain is expressed in the glomerular epithelium and mesangial area, as demonstrated in the human embryonic kidney, we suggest that PDGF B-chain acts in a paracrine fashion to stimulate the migration and proliferation of mesangial cell precursors from undifferentiated metanephric mesenchyme to the mesangial area. PDGF B-chain also likely stimulates proliferation of mesangial cell precursors in an autocrine fashion once these cells migrate to the glomerular tuft.

A natural ErbB4 isoform that does not activate phosphoinositide 3-kinase mediates proliferation but not survival or chemotaxis

ErbB4 is a member of the epidermal growth factor receptor (ErbB) family that mediates cellular responses activated by neuregulins (NRG) and other epidermal growth factor-like growth factors. Two naturally occurring ErbB4 isoforms, ErbB4 CYT-1 and ErbB4 CYT-2, have previously been identified. Unlike ErbB4 CYT-1, ErbB4 CYT-2 lacks a phosphoinositide 3-kinase (PI3-K)-binding site and is incapable of activating PI3-K. We have now examined the consequences of the inability of this isoform to activate PI3-K on cell proliferation, survival, and chemotaxis in response to NRG-1beta: (i) NRG-1beta stimulated proliferation of cells expressing either ErbB4 CYT-1 or ErbB4 CYT-2. Consistent with the mitogenic responsiveness, analysis of downstream signaling showed that Shc and MAPK were phosphorylated after stimulating either isoform with NRG-1beta. (ii) NRG-1beta protected cells expressing ErbB4 CYT-1 but not cells expressing ErbB4 CYT-2 from starvation-induced apoptosis as measured by effects on cell number and 4', 6-diamidino-2-phenylindole staining. Furthermore, in cells expressing ErbB4 CYT-2, Akt, a protein kinase that mediates cell survival, was not phosphorylated. (iii) NRG-1beta stimulated chemotaxis and membrane ruffling in cells expressing ErbB4 CYT-1 but not in cells expressing ErbB4 CYT-2. In summary, ErbB4 CYT-2 can mediate proliferation but not chemotaxis or survival. These results suggest a novel mechanism by which cellular responses such as chemotaxis and survival may be regulated by the expression of alternative receptor-tyrosine kinase isoforms that differ in their coupling to PI3-K signaling.

Nerve growth factor functions as a chemoattractant for mast cells through both mitogen-activated protein kinase and phosphatidylinositol 3-kinase signaling pathways

Despite being a well-characterized neurotrophic factor, nerve growth factor (NGF) influences survival, differentiation, and functions of mast cells. We investigated whether NGF was able to induce directional migration of rat peritoneal mast cells (PMCs). NGF clearly induced chemotactic movement of PMCs in a dose-dependent manner with the drastic morphological change and distribution of F-actin, which was completely blocked by pretreatment with Clostridium botulinumC2 toxin, an actin-polymerization inhibitor. Because PMCs constitutively express the NGF high-affinity receptor (TrkA) with a tyrosine kinase domain, we focused on downstream effectors in signaling cascades following the TrkA. NGF rapidly activated both mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K), and the addition of inhibitors specific for MAPK kinase and PI3K suppressed cell migration and these signals. In the coculture system with PMCs and fibroblasts, which produce biologically active NGF, directional migration of PMCs to fibroblasts was observed, and the addition of anti-NGF polyclonal antibodies significantly suppressed the migration of PMCs. These findings suggested that NGF initiated chemotactic movement of PMCs through both MAPK and PI3K signaling pathways following TrkA activation. Thus, locally produced NGF may play an important role in mast cell accumulation in allergic and nonallergic inflammatory conditions.

Activation of PLC and PI 3 kinase by PDGF receptor alpha is not sufficient for mitogenesis and migration in mesangial cells

BACKGROUND

Platelet-derived growth factor (PDGF) isoforms act through two distinct cell surface alpha and beta receptors. Glomerular mesangial cells express both receptors. PDGF BB and AB are potent mitogens for glomerular mesangial cells, and PDGF BB stimulates cell migration in a phosphatidylinositol 3 (PI 3) kinase-dependent manner. In this study, we investigated the effect of PDGF AA on cell migration, PI 3 kinase and phospholipase C (PLC) activation, and the role of these two enzymes in mediating biological responses in these cells in response to all three isoforms.

METHODS

3H-thymidine incorporation and modified Boyden chamber assay were used to determine DNA synthesis and directed migration, respectively, in response to all three PDGF isoforms. Differential activation of alpha and beta receptors was studied by immunecomplex tyrosine kinase assay of corresponding receptor immunoprecipitates. PLC gamma 1 activity was determined by measuring total inositol phosphates in response to different PDGF isoforms. PI 3 kinase activity was determined in antiphosphotyrosine or PDGF receptor immunoprecipitates.

RESULTS

Both PDGF BB and AB resulted in stimulation of DNA synthesis and directed migration of mesangial cells. AA was neither chemotactic nor mitogenic. However, all three isoforms increased tyrosine phosphorylation of a 180 kD protein in antiphosphotyrosine immunoprecipitates, suggesting activation of respective receptors. Direct immunecomplex tyrosine kinase assay of alpha and beta receptors demonstrated significant activation of both of these receptors when cells are treated with PDGF BB or AB. PDGF AA increased tyrosine kinase activity of the alpha receptor but not the beta receptor. All three isoforms significantly stimulated the production of inositol phosphates with order of potency being BB > AB > AA. PDGF AA also dose dependently stimulated PI 3 kinase activity measured in antiphosphotyrosine immunoprecipitates of treated cells. A comparison of PI 3 kinase activity in antiphosphotyrosine immunoprecipitates from mesangial cells stimulated with three different PDGF isoforms showed significant activation of this enzyme with a decreasing order of activity: BB > AB > AA.

CONCLUSION

Taken together, these data demonstrate that all three isoforms of PDGF significantly stimulate PLC gamma 1 and PI 3 kinase, two enzymes necessary for both DNA synthesis and directed migration. However, activation of alpha receptor by PDGF AA with a subsequent increase in PLC and PI 3 kinase activities is not sufficient to induce these biological responses in mesangial cells. These data indicate that the extent of activation of signal transduction pathways may be a major determinant of the biological activity of different PDGF isoforms in mesangial cells.

Phosphatidylinositide 3-kinase localizes to cytoplasmic lipid bodies in human polymorphonuclear leukocytes and other myeloid-derived cells

Phosphatidylinositide 3-kinase (PI3K) is a key enzyme implicated in intracellular signaling of diverse cellular responses including receptor-mediated responses and neutrophil activation. Several PI3K subunits have been cloned and shown to be localized to plasma membrane receptors, the cytosol, or intracellular vesicles or caveolae. We report the localization of PI3K to a distinct intracellular site, cytoplasmic lipid bodies, in leukocytes. In U937 monocyte cells, PI3K p85 regulatory and p110β catalytic subunits were localized to lipid bodies by immunocytochemistry and/or immunoblotting and enzyme assays of subcellular fractions. In RAW murine macrophages, p55, p85, and p85β PI3K subunits were present at isolated lipid bodies. PI3K p85 was also shown to colocalize and, by co-immunoprecipitation, to be physically associated with phosphorylated Lyn kinase in lipid bodies induced to form in human polymorphonuclear leukocytes. These findings, therefore, indicate a novel site for PI3K compartmentalization and suggest that PI3K-mediated signaling is active within cytoplasmic lipid bodies in leukocytes.

Synergistic role of cAMP and IP(3) in corticotropin-releasing hormone-induced cell shape changes in invertebrate immunocytes

Immunocytes from the mollusc Mytilus galloprovincialis express corticotropin-releasing hormone (CRH) receptor subtype (CRH-R1 and CRH-R2)-like mRNAs. Using computer-assisted microscopic image analysis, we have found that exogenous CRH provokes changes in the cellular shape of immunocytes, and that this response is extracellular Ca(2+)-dependent. The various inhibitors of transduction signaling pathways, i.e. suramin sodium, 2', 5'-dideoxyadenosine, neomycin sulfate, calphostin C, H-89, and wortmannin, completely or partially inhibit these changes. The present findings demonstrate that PKA, PKC, and PKB/Akt are involved in CRH-induced cell shape changes in immunocytes, and that the cellular effect of CRH needs the synergistic action of the two second messengers, cAMP and IP(3).

Platelet-derived growth factor-stimulated migration of murine fibroblasts is associated with epidermal growth factor receptor expression and tyrosine phosphorylation

Previous studies have shown that epidermal growth factor (EGF) synergizes with various extracellular matrix components in promoting the migration of B82L fibroblasts expressing wild-type EGF receptors and that functional EGF receptors are critical for the conversion of B82L fibroblasts to a migratory cell type (). In the present study, we examined the effects of platelet-derived growth factor (PDGF) on the motility of B82L fibroblasts using a microchemotaxis chamber. We found that PDGF can enhance fibronectin-induced migration of B82L fibroblasts expressing wild-type EGF receptors (B82L-clone B3). However, B82L cells that lack the EGF receptor (B82L-parental) or that express an EGF receptor that is kinase-inactive (B82L-K721M) or C-terminally truncated (B82L-c'973) exhibit little PDGF-stimulated migration. In addition, none of these three cell lines exhibit the capacity to migrate to fibronectin alone. These observations indicate that, similar to cell migration toward fibronectin, PDGF-induced cell migration of B82L fibroblasts is augmented by the expression of an intact EGF receptor kinase. The loss of PDGF-stimulated motility in B82L cells that do not express an intact EGF receptor does not appear to result from a gross dysfunction of PDGF receptors, because ligand-stimulated tyrosine phosphorylation of the PDGF-beta receptor and the activation of mitogen-activated protein kinases are readily detectable in these cells. Moreover, an interaction between EGF and PDGF receptor systems is supported by the observation that the EGF receptor exhibits an increase in phosphotyrosine content in a time-dependent fashion upon the addition of PDGF. Altogether, these studies demonstrate that the expression of EGF receptor is critical for PDGF-stimulated migration of murine B82L fibroblasts and suggest a role for the EGF receptor downstream of PDGF receptor activation in the signaling events that lead to PDGF-stimulated cell motility.

Motility and invasion are differentially modulated by Rho family GTPases

Cell migration in vivo often requires invasion through tissue matrices. Currently little is known regarding the signaling pathways that regulate cell invasion through three-dimensional matrices. The small GTPases Cdc42, Rac and Rho are key regulators of actin cytoskeletal and adhesive structures. We now show that expression of dominant negative forms of either Cdc42, Rac or Rho inhibited PDGF-BB-stimulated Rat1 fibroblast invasion into 3D collagen matrices, indicating that the activity of each of these GTPases is necessary for cell invasion. In contrast, only Rac activation was required for PDGF-BB-stimulated locomotion across a planar substrate in the Boyden chamber. Interestingly, PDGF-induced invasion was also strongly inhibited by expression of constitutively active forms of Cdc42 or Rho, and to a lesser extent by constitutively active Rac. We also show that constitutively active V12-Rac significantly stimulated basal Rat1 fibroblast invasion, independent of PI-3-kinase activity, and that this effect was suppressed by the effector mutant V12/H40-Rac. These results suggest that cellular invasion may require an optimal level of activation of Cdc42, Rho and Rac, and that migration and invasion are differentially modulated by Rho family GTPases.

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

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

Replacing two conserved tyrosines of the EphB2 receptor with glutamic acid prevents binding of SH2 domains without abrogating kinase activity and biological responses

Eph receptor tyrosine kinases play key roles in pattern formation during embryonic development, but little is known about the mechanisms by which they elicit specific biological responses in cells. Here, we investigate the role of tyrosines 605 and 611 in the juxtamembrane region of EphB2, because they are conserved Eph receptor autophosphorylation sites and demonstrated binding sites for the SH2 domains of multiple signaling proteins. Mutation of tyrosines 605 and 611 to phenylalanine impaired EphB2 kinase activity, complicating analysis of their function as SH2 domain binding sites and their contribution to EphB2-mediated signaling. In contrast, mutation to the negatively charged glutamic acid disrupted SH2 domain binding without reducing EphB2 kinase activity. By using a panel of EphB2 mutants, we found that kinase activity is required for the changes in cell-matrix and cell - cell adhesion, cytoskeletal organization, and activation of mitogen-activated protein (MAP) kinases elicited by EphB2 in transiently transfected cells. Instead, the two juxtamembrane SH2 domain binding sites were dispensable for these effects. These results suggest that phosphorylation of tyrosines 605 and 611 is critical for EphB2-mediated cellular responses because it regulates EphB2 kinase activity.

Phosphatidylinositol 3-kinase is required for insulin-like growth factor-I-induced vascular smooth muscle cell proliferation and migration

Insulin-like growth factor-I (IGF-I) plays an important role in regulating vascular smooth muscle cell (VSMC) proliferation and directed migration. The mitogenic and chemotactic actions of IGF-I are mediated through the IGF-I receptor, but how the activation of the IGF-I receptor leads to these biological responses is poorly understood. In this study, we examined the role of phosphatidylinositol 3-kinase (PI3 kinase) in mediating the mitogenic and chemotactic signals of IGF-I. IGF-I treatment resulted in a significant increase in phosphotyrosine-associated PI3 kinase activity in cultured primary VSMCs. To determine whether insulin receptor substrate (IRS)-1, -2, or both are involved in IGF-I signaling in VSMCs, cell lysates were immunoprecipitated with either an anti-IRS-1 or an anti-IRS-2 antibody, and the associated PI3 kinase activity was determined. IGF-I stimulation resulted in a significant increase in IRS-1- but not IRS-2-associated PI3 kinase activity, suggesting that IGF-I primarily utilizes IRS-1 to transmit its signal in VSMCs. The IGF-I-induced increase in IRS-I-associated PI3 kinase activity was concentration dependent. At the maximum concentration (50 ng/mL), IGF-I induced a 60-fold increase. This activation occurred within 5 minutes and was sustained at high levels for at least 6 hours. IGF-I also caused a concentration-dependent and long-lasting activation of protein kinase B (PKB/Akt). Inhibition of PI3 kinase activation by LY294002 or wortmannin abolished IGF-I-stimulated VSMC proliferation and reduced IGF-I-directed VSMC migration by approximately 60%. These results indicate that activation of PI3 kinase is required for both IGF-I-induced VSMC proliferation and migration.

Tyrosine phosphorylation of focal adhesion kinase by PDGF is dependent on ras in human hepatic stellate cells

Focal adhesion kinase (FAK) is a widely expressed nonreceptor tyrosine kinase found in focal adhesions. FAK has been indicated as a point of convergence of other signaling pathways including platelet-derived growth factor (PDGF) receptors, and recently, FAK tyrosine phosphorylation has been shown to be stimulated by PDGF. In the present study we assessed the role of Ras as a possible intermediate protein regulating PDGF-induced FAK tyrosine phosphorylation in human hepatic stellate cells (HSCs), liver-specific pericytes primarily involved in the pathogenesis of liver fibrosis. For this purpose, cells were first subjected to retroviral-mediated gene transfer with a dominant-negative mutant of Ras (N17Ras). This resulted in a marked inhibition of PDGF-induced FAK tyrosine phosphorylation together with the expected reduction of PDGF-induced extracellular signal-regulated kinase activity (ERK). Afterward, the effects of pharmacological agents potentially affecting Ras isoprenylation were evaluated. PDGF-induced FAK tyrosine phosphorylation, ERK activity and intracellular calcium increase, as well as the biological effects of this growth factor, (i.e., mitogenesis and cell migration) were effectively blocked by GGTI-298, an inhibitor of geranylgeranyltransferase I. Inhibition of Ras processing obtained with FTI-277, an inhibitor of farnesyltransferase, resulted in detectable effects only at high doses. Taken together, these results establish that Ras operates as a protein-linking PDGF-beta receptor to FAK in human HSCs, and that signaling molecules requiring geranylgeranylation may also be involved in this process.

Expression cloning of protein targets for 3-phosphorylated phosphoinositides

The phosphatidylinositol 3-kinase (PI 3'-K) family of lipid kinases play a critical role in cell proliferation, survival, vesicle trafficking, motility, cytoskeletal rearrangements, and oncogenesis. To identify downstream effectors of PI 3'-K, we developed a novel screen to isolate proteins that bind to the major products of PI 3'-K: phosphatidylinositol-3,4-bisphosphate (PtdIns-3,4-P(2)) and PtdIns-3,4,5-trisphosphate (PtdIns-3,4,5-P(3)). This screen uses synthetic biotinylated analogs of these lipids in conjunction with libraries of radiolabeled proteins that are produced by coupled in vitro transcription/translation reactions. The feasibility of the screen was initially demonstrated using avidin-coated beads prebound to biotinylated PtdIns-3,4-P(2) and PtdIns-3,4,5-P(3) to specifically isolate the pleckstrin homology domain of the serine/threonine kinase Akt. We then demonstrated the utility of this technique in isolating novel 3'-phosphorylated phosphatidylinositol (3'-PPI)-binding proteins through the preliminary screening of in vitro transcribed/translated cDNAs from a small pool expression library derived from mouse spleen. Three proteins were isolated that bound specifically to 3'PPIs. Two of these proteins have been previously characterized as PIP3BP/p42(IP4) and the PtdIns-3,4,5-P(3)-dependent serine/threonine kinase phosphoinositide-dependent kinase 1. The third protein is a novel protein that contains only a Src homology 2 domain and a pleckstrin homology domain; this protein has a higher specificity for both PtdIns-3,4,5-P(3) and PtdIns-3,4-P(2) than for PtdIns-4, 5-bisphosphate. Transcripts of this novel gene are present in every tissue analyzed but are most prominently expressed in spleen. We have renamed this new protein PHISH for 3'-phosphoinositide-interacting Src homology-containing protein. This report demonstrates the utility of this technique for isolating and characterizing 3'-PPI-binding proteins and has broad applicability for the isolation of binding domains for other lipid products.

Activation of the MAPK signal cascade by the neural cell adhesion molecule L1 requires L1 internalization

L1-mediated axon growth involves intracellular signaling, but the precise mechanisms involved are not yet clear. We report a role for the mitogen-activated protein kinase (MAPK) cascade in L1 signaling. L1 physically associates with the MAPK cascade components Raf-1, ERK2, and the previously identified p90(rsk) in brain. In vitro, ERK2 can phosphorylate L1 at Ser(1204) and Ser(1248) of the L1 cytoplasmic domain. These two serines are conserved in the L1 family of cell adhesion molecules, also being found in neurofascin and NrCAM. The ability of ERK2 to phosphorylate L1 suggests that L1 signaling could directly regulate L1 function by phosphorylation of the L1 cytoplasmic domain. In L1-expressing 3T3 cells, L1 cross-linking can activate ERK2. Remarkably, the activated ERK localizes with endocytosed vesicular L1 rather than cell surface L1, indicating that L1 internalization and signaling are coupled. Inhibition of L1 internalization with dominant-negative dynamin prevents activation of ERK. These results show that L1-generated signals activate the MAPK cascade in a manner most likely to be important in regulating L1 intracellular trafficking.

Growth factor signaling pathways in vascular development

Recent research on the formation and maintenance of the vasculature in the embryo and in the adult has provided a greater understanding of the cellular signals involved in these processes. With this understanding comes the potential means of controlling vascularization in pathological situations such as tumorigenesis and wounding. For the purpose of this review, we will discuss the key receptor tyrosine kinases involved in vascular function and the molecules which relay signals downstream of receptor activation. The receptor tyrosine kinases discussed include the vascular endothelial cell growth factor receptors, Eph receptors, Tie1, and Tie2, all of which are expressed on vascular endothelial cells. We also discuss the roles of the platelet derived growth factor receptors which are expressed on vascular smooth muscle cells. While all of these receptor tyrosine kinases activate many similar effector molecules, some of the signals initiated appear to be distinct. This may explain, at least in part, how different receptor tyrosine kinases expressed in overlapping patterns on the developing vasculature, direct unique biological functions.

Role of PI 3-kinase in angiopoietin-1-mediated migration and attachment-dependent survival of endothelial cells

Angiopoietin-1 is a unique growth factor which induces Tie2 receptor autophosphorylation and interaction with signal transduction molecules, GRB2 and p85 subunit of PI 3-kinase, but no detectable mitogenic response. Here we show that PI 3-kinase-dependent activation of Akt and attachment to extracellular matrix are required for angiopoietin-1-mediated endothelial cell survival. Apoptosis of growth factor-deprived cells grown in monolayer was decreased by angiopoietin-1 and correlated with Akt activation. In contrast, angiopoietin-1, bFGF or VEGF failed to protect cells in suspension culture. Ceramide, an intermediate of several apoptotic pathways, interferes with growth factor-mediated Akt activation. Ceramide induced endothelial cell death and abolished angiopoietin-1-mediated activation of Akt and the effect on cell survival. In addition, we found that PI 3-kinase activity is necessary for migration of endothelial cells in response to Angiopoietin-1. A transient activation of MAPK/ERKs was also detected within 10 min after stimulation with angiopoietin-1. In contrast to VEGF-mediated biological effects, inhibition of MAPK/ERKs by PD98059 in endothelial cells did not affect angiopoietin-1 mediated survival or migration. These findings indicate significant differences in intracellular signaling between VEGF and angiopoietin-1 and that PI 3-kinase lipid products are key mediators of the biological effects of angiopoietin-1.

Consequences of Stat6 deletion on Sis/PDGF- and IL-4-induced proliferation and transcriptional activation in murine fibroblasts

Aberrant communication among growth factors and cytokines that regulate tissue homeostasis often results in malignancy. Among the many cell types that participate in this process, stromal fibroblasts communicate in a paracrine and juxtracrine manner with cells of epithelial, endothelial, and hematopoietic origin. For fibroblasts, platelet-derived growth factor (PDGF) is a major proliferative and differentiation agent. Interleukin-4 (IL-4), however, possesses only modulating functions in this cell type. Here, we investigated the consequences of deleting Stat6 on PDGF and IL-4 signaling, proliferation, and transcriptional activation by establishing and characterizing early passage fibroblasts from wild-type and Stat6 null mice. Both wild-type and Stat6-/- fibroblasts showed nearly identical PDGFR and IL-4R activation, gross substrate tyrosine phosphorylation, PI 3-kinase activation, as well as Stat1, 3 and 5 DNA binding activities. Unexpectedly, IL-4's enhancement of PDGF-induced [3H]thymidine incorporation was greatly diminished in Stat6-/-, but not wild-type fibroblasts. PDGF-induced [3H]thymidine uptake was largely unaffected. Strikingly, IL-4, but not PDGF induction of the proinflammatory gene products, IL-6 and MCP-1 was markedly reduced in Stat6-/- fibroblasts. Thus, Stat6 is an important and specific mediator of IL-4-enhanced PDGF-induced proliferation as well as IL-4's transcriptional activation of IL-6 and MCP-1.

Epidermal and hepatocyte growth factors stimulate chemotaxis in an intestinal epithelial cell line

The migration of intestinal cells is important in the development and maintenance of normal epithelium, in a process that may be regulated by growth factors and cytokines. Although a number of growth factor receptors are expressed by intestinal cells, little progress has been made toward assignment of functional roles for these ligand-receptor systems. This study compares several growth factors and cytokines for their chemoattraction of the mouse small intestinal epithelial cell line. Epidermal and hepatocyte growth factors stimulated a rapid 30-fold chemotaxis of cells with delayed threefold migration toward transforming growth factor-beta1. Despite stimulating proliferation, keratinocyte, fibroblast, or insulin-like growth factors did not stimulate directed migration. Chemotaxis required tyrosine kinase and phosphatidylinositol phospholipase C activities but not protein kinase C or mitogen-activated protein kinase activity. These findings suggest that the repertoire of growth factors capable of regulating directed intestinal epithelial cell migration is limited and that a divergence exists in the signal transduction pathways for directed vs. nondirected migration.

The CXC Chemokine Stromal Cell-Derived Factor Activates a Gi-Coupled Phosphoinositide 3-Kinase in T Lymphocytes

The cellular effects of stromal cell-derived factor-1 (SDF-1) are mediated primarily by binding to the CXC chemokine receptor-4. We report in this study that SDF-1 and its peptide analogues induce a concentration- and time-dependent accumulation of phosphatidylinositol-(3,4,5)-trisphosphate (PtdIns(3,4,5)P3) in Jurkat cells. This SDF-1-stimulated generation of D-3 phosphoinositide lipids was inhibited by pretreatment of the cells with an SDF-1 peptide antagonist or an anti-CXCR4 Ab. In addition, the phosphoinositide 3 (PI 3)-kinase inhibitors wortmannin and LY294002, as well as the Gi protein inhibitor pertussis toxin, also inhibited the SDF-1-stimulated accumulation of PtdIns(3,4,5)P3. The effects of SDF-1 on D-3 phosphoinositide lipid accumulation correlated well with activation of the known PI 3-kinase effector protein kinase B, which was also inhibited by wortmannin and pertussis toxin. Concentrations of PI 3-kinase inhibitors, sufficient to inhibit PtdIns(3,4,5)P3 accumulation, also inhibited chemotaxis of Jurkat and peripheral blood-derived T lymphocytes in response to SDF-1. In contrast, SDF-1-stimulated actin polymerization was only partially inhibited by PI 3-kinase inhibitors, suggesting that while chemotaxis is fully dependent on PI 3-kinase activation, actin polymerization requires additional biochemical inputs. Finally, SDF-1-stimulated extracellular signal-related kinase (ERK)-1/2 mitogen-activated protein kinase activation was inhibited by PI 3-kinase inhibitors. In addition, the mitogen-activated protein/ERK kinase inhibitor PD098059 partially attenuated chemotaxis in response to SDF-1. Hence, it appears that ERK1/2 activation is dependent on PI 3-kinase activation, and both biochemical events are involved in the regulation of SDF-1-stimulated chemotaxis.