Phosphatidylinositol 3-kinase-independent signal transduction pathway for platelet-derived growth factor-induced chemotaxis

Microfluidic devices fitted with "flowver" paper pumps generate steady, tunable gradients for extended observation of chemotactic cell migration

Microfluidics approaches have gained popularity in the field of directed cell migration, enabling control of the extracellular environment and integration with live-cell microscopy; however, technical hurdles remain. Among the challenges are the stability and predictability of the environment, which are especially critical for the observation of fibroblasts and other slow-moving cells. Such experiments require several hours and are typically plagued by the introduction of bubbles and other disturbances that naturally arise in standard microfluidics protocols. Here, we report on the development of a passive pumping strategy, driven by the high capillary pressure and evaporative capacity of paper, and its application to study fibroblast chemotaxis. The paper pumps-flowvers (flow + clover)-are inexpensive, compact, and scalable, and they allow nearly bubble-free operation, with a predictable volumetric flow rate on the order of μl/min, for several hours. To demonstrate the utility of this approach, we combined the flowver pumping strategy with a Y-junction microfluidic device to generate a chemoattractant gradient landscape that is both stable (6+ h) and predictable (by finite-element modeling calculations). Integrated with fluorescence microscopy, we were able to recapitulate previous, live-cell imaging studies of fibroblast chemotaxis to platelet derived growth factor (PDGF), with an order-of-magnitude gain in throughput. The increased throughput of single-cell analysis allowed us to more precisely define PDGF gradient conditions conducive for chemotaxis; we were also able to interpret how the orientation of signaling through the phosphoinositide 3-kinase pathway affects the cells' sensing of and response to conducive gradients.

Activation of phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signaling and the consequent induction of transformation by overexpressed 14-3-3γ protein require specific amino acids within 14-3-3γ N-terminal variable region II

Members of the 14-3-3 superfamily regulate numerous cellular functions by binding phosphoproteins. The seven human isoforms (and the myriad of other eukaryotic 14-3-3 proteins) are highly conserved in amino acid sequence and secondary structure, yet there is abundant evidence that the various isoforms manifest disparate as well as common functions. Several of the human 14-3-3 isoforms are dysregulated in certain cancers and thus have been implicated in oncogenesis; experimentally, 14-3-3γ behaves as an oncogene, whereas 14-3-3σ acts as a tumor suppressor. In this study, we sought to localize these opposing phenotypes to specific regions of the two isoforms and then to individual amino acids therein. Using a bioinformatics approach, six variable regions (VRI-VRVI) were identified. Using this information, two sets of constructs were created in which N-terminal portions (including either VRI-IV or only VRI and VRII) of 14-3-3γ and 14-3-3σ were swapped; NIH3T3 cells overexpressing the four chimeric proteins were tested for transformation activity (focus formation, growth in soft agar) and activation of PI3K and MAPK signaling. We found that the specific phenotypes of 14-3-3γ are associated with the N-terminal 40 amino acids (VRI and VRII); in like fashion, VRI and VRII of 14-3-3σ dictated its tumor suppressor function. Using individual amino acid substitutions within the 14-3-3γ VRII, we identified two residues required for and two contributing to the γ-specific phenotypes. Our observations suggest that isoform-specific phenotypes are dictated by a relatively few amino acids within variable regions.

Phosphoinositides in chemotaxis

Phosphatidylinositol lipids generated through the action of phosphinositide 3-kinase (PI3K) are key mediators of a wide array of biological responses. In particular, their role in the regulation of cell migration has been extensively studied and extends to amoeboid as well as mesenchymal migration. Through the emergence of fluorescent probes that target PI3K products as well as the use of specific inhibitors and knockout technologies, the spatio-temporal distribution of PI3K products in chemotaxing cells has been shown to represent a key anterior polarity signal that targets downstream effectors to actin polymerization. In addition, through intricate cross-talk networks PI3K products have been shown to regulate signals that control posterior effectors. Yet, in more complex environments or in conditions where chemoattractant gradients are steep, a variety of cell types can still chemotax in the absence of PI3K signals. Indeed, parallel signal transduction pathways have been shown to coordinately regulate cell polarity and directed movement. In this chapter, we will review the current role PI3K products play in the regulation of directed cell migration in various cell types, highlight the importance of mathematical modeling in the study of chemotaxis, and end with a brief overview of other signaling cascades known to also regulate chemotaxis.

PTEN inhibitors cause a negative inotropic and chronotropic effect in mice

Inactivation of phosphatase and tensin homologue deleted on chromosome ten (PTEN) decreases cardiac contractility under basal conditions and induces cardioprotection against ischemia-reperfusion injury. However, the pharmacological effect of PTEN inhibitors on cardiac contractility has not been studied before. In the present study, we investigated the hypothesis that PTEN inhibition decreases cardiac contractility in mice. We first exposed isolated mouse hearts to the PTEN inhibitor bpV(phen) (40μM), the phosphoinositide-3 kinase inhibitor wortmannin (1μM), and the PTEN-resistant PIP3 analog 3-phosphorothioate-PtdIns(3,4,5)P3 (3-PT-PTP, 0.5μM) for 10min. Left ventricular pressure was measured by a Mikro-tip pressure catheter. We then inhibited PTEN in mice by intra-peritoneal injection of VO-OHpic (10μg/kg) 30min before ischemia and then exposed them to 30min of ischemia and 120min of reperfusion. At the end of the experiments, hearts were isolated for measurement of myocardial infarct size by 1.5% triphenyltetrazolium chloride. Left ventricular systolic pressure and heart rate were significantly decreased by bpV(phen). Consistent with the result, the maximal rate of left ventricular pressure increase or decrease was significantly decreased by bpV(phen). 3-PT-PIP3 mimicked the effect of bpV(phen), and the opposite effect on cardiac contractility was seen with wortmannin. Moreover, inhibition of PTEN in vivo by VO-OHpic decreased left ventricular systolic pressure and heart rate before ischemia, but resulted in an increase in cardiac functional recovery and a decrease in myocardial infarct size after ischemia-reperfusion. In conclusion, PTEN inhibition causes a negative inotropic and chronotropic effect while inducing cardioprotection against ischemia-reperfusion injury.

14-3-3gamma induces oncogenic transformation by stimulating MAP kinase and PI3K signaling

The 14-3-3 proteins are a set of highly conserved scaffolding proteins that have been implicated in the regulation of a variety of important cellular processes such as the cell cycle, apoptosis and mitogenic signaling. Recent evidence indicates that the expression of some of the family members is elevated in human cancers suggesting that they may play a role in tumorigenesis. In the present study, the oncogenic potential of 14-3-3γ was shown by focus formation and tumor formation in SCID mice using 14-3-3γ transfected NIH3T3 mouse fibroblast cells. In contrast, 14-3-3σ, a putative tumor suppressor, inhibited NIH3T3 transformation by H-ras and c-myc. We also report that activation of both MAP kinase and PI3K signaling pathways are essential for transformation by 14-3-3γ. In addition, we found that 14-3-3γ interacts with phosphatidylinositol 3-kinase (PI3K) and TSC2 proteins indicating that it could stimulate PI3K signaling by acting at two points in the signaling pathway. Overall, our studies establish 14-3-3γ as an oncogene and implicate MAPK and PI3K signaling as important for 14-3-3γ induced transformation.

Nck adapters are involved in the formation of dorsal ruffles, cell migration, and Rho signaling downstream of the platelet-derived growth factor beta receptor

The SH3 and SH2 domain-containing adapter proteins Nck1 and Nck2 are known to function downstream of activated tyrosine kinase receptors, such as the platelet-derived growth factor (PDGF) receptors. The SH2 domain of Nck1 binds to phosphorylated tyrosine residue 751 in PDGFbeta receptor and has been suggested to have a role in the PDGF-induced mobilization of the actin filament system. Because Tyr-751 is a site for additional receptor interactors, it has been difficult to discriminate the signaling from Nck from signaling via other molecules. For this reason we have used mouse embryonic fibroblasts derived from mice in which the genes for Nck1 and Nck2 have been inactivated by gene targeting (knock-out (KO) cells). The mutant cells had a reduced ability to form edge ruffles in response to PDGF, and the presence of Nck was obligatory for the formation of dorsal ruffles. In addition, the KO cells had a reduced chemotactic and migratory potential. Importantly, KO cells had reduced cell attachment properties and a reduced ability to form focal adhesions in response to serum stimulation. Moreover, signaling involving the Rho GTPases was defective in KO cells. In summary, our observations suggest that the Nck adapters are needed for signaling to Rho GTPases and actin dynamics downstream of the PDGFbeta receptor.

Phosphatidylinositol 3- and 4-phosphate modulate actin filament reorganization in guard cells of day flower

Phosphatidylinositol 3-kinases (PtdIns 3-kinases) that produce phosphatidylinositol (3,4,5) triphosphate (PtdIns(3,4,5)P(3)) are considered to be important regulators of actin dynamics in animal cells. In plants, neither PtdIns(3,4,5)P(3) nor the enzyme that produces this lipid has been reported. However, a PtdIns 3-kinase that produces phosphatidylinositol 3-phosphate (PtdIns3P) has been identified, suggesting that PtdIns3P, instead of PtdIns(3,4,5)P(3), regulates actin dynamics in plant cells. Phosphatidylinositol 4-kinase (PtdIns 4-kinase) is closely associated with the actin cytoskeleton in plant cells, suggesting a role for this lipid kinase and its product phosphatidylinositol 4-phosphate (PtdIns4P) in actin-related processes. Here, we investigated whether or not PtdIns3P or PtdIns4P plays a role in actin reorganization induced by a plant hormone abscisic acid (ABA) in guard cells of day flower (Commelina communis). ABA-induced changes in actin filaments were inhibited by LY294002 (LY) and wortmannin (WM), inhibitors of PtdIns3P and PtdIns4P synthesis. Expression of PtdIns3P- and PtdIns4P-binding domains also inhibited ABA-induced actin reorganization in a manner similar to LY and WM. These results suggest that PtdIns3P and PtdIns4P regulate actin dynamics in guard cells. Furthermore, we demonstrate that PtdIns3P exerts its effect on actin dynamics, at least in part, via generation of reactive oxygen species (ROS) in response to ABA.

Therapeutic effect of magnesium lithospermate B on neointimal formation after balloon-induced vascular injury

Vascular smooth muscle cell (VSMC) proliferation and migration in response to platelet-derived growth factor (PDGF) play an important role in the development of atherosclerosis and restenosis. Recent evidence indicates that PDGF increases intracellular levels of reactive oxygen species in VSMCs and that both PDGF-induced VSMC proliferation and migration are reactive oxygen species-dependent. Danshen is a representative oriental medicine used for the treatment of vascular disease. Previously, we reported that magnesium lithospermate B, an active component of Danshen, is a potent antioxidant. Thus we investigated the therapeutic potential of magnesium lithospermate B in neointimal formation after carotid artery injury in rats along with its effects on the PDGF signaling pathway for stimulating VSMC proliferation and migration in vitro. PDGF is dimeric glycoprotein composed of two A or two B chains. In this study, we used PDGF-BB, which is one of the isoforms of PDGF (i.e., PDGF-AA, PDGF-BB, and PDGF-AB). Our results demonstrated that magnesium lithospermate B directly scavenged reactive oxygen species in a xanthine/xanthine oxidase system and reduced PDGF-BB-induced intracellular reactive oxygen species generation in VSMCs. In a rat carotid artery balloon injury model, magnesium lithospermate B treatment (10 mg/kg/day, i.p) showed a significant effect on the prevention of neointimal formation compared with vehicle treatment. In cultured VSMCs, magnesium lithospermate B significantly attenuated PDGF-BB-induced cell proliferation and migration as measured by 3-[4,5-dimethyl-2-thiazolyl]-2,5-diphenyl-2-tetrazolium bromide (MTT) assay and transwell migration assays, respectively. Further, magnesium lithospermate B inhibited PDGF-BB-induced phosphorylation of phospatidylinositol 3-kinase (PI3K)/Akt and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathways by scavenging reactive oxygen species. Together, these data indicated that magnesium lithospermate B, a potent reactive oxygen species scavenger, prevented both injury-induced neointimal formation in vivo and PDGF-BB-induced VSMC proliferation and migration in vitro, suggesting that magnesium lithospermate B may be a promising agent to prevent atherosclerosis and restenosis following angioplasty.

Stromal cell-derived factor 1-alpha (SDF)-induced human T cell chemotaxis becomes phosphoinositide 3-kinase (PI3K)-independent: role of PKC-theta

Stromal cell-derived factor 1alpha (SDF-1alpha) is the exclusive ligand for the chemokine receptor CXCR4. This receptor plays a pivotal role in immune responses, the pathogenesis of infection such as HIV, and cellular trafficking. However, the signaling mechanisms regulating SDF-driven T cell migration are not well defined. In this study, we determined the role of PI3K and protein kinase C- theta (PKC-theta) in SDF-induced human T cell migration in fresh versus cultured T cells. Purified human T cells (fresh vs. 48 h in media, unstimulated or activated by anti-CD3+anti-CD28) were used. Western blots showed that SDF induced phospho-(p)-Akt [threonine (Thr)308 and serine 473], a proxy for PI3K activity, in fresh cells and p-PKC-theta in 48 h unstimulated cells. LY294002 (PI3K inhibitor) reduced SDF-induced chemotaxis in fresh cells by 51%, whereas it minimally affected chemotaxis in 48 h unstimulated or activated cells. However, a specific PKC-theta inhibitor, pseudosubstrate for PKC-theta, reduced chemotaxis in 48 h unstimulated and stimulated T cells by 72% and 87%, respectively. Thus, chemotaxis becomes independent of PI3K signaling in human T cells cultured for 48 h. Under these conditions, PKC-theta is phosphorylated (Thr538) by SDF, and chemotaxis becomes largely PKC-theta-dependent.

Mechanisms of vascular smooth muscle cell migration

Smooth muscle cell migration occurs during vascular development, in response to vascular injury, and during atherogenesis. Many proximal signals and signal transduction pathways activated during migration have been identified, as well as components of the cellular machinery that affect cell movement. In this review, a summary of promigratory and antimigratory molecules belonging to diverse chemical and functional families is presented, along with a summary of key signaling events mediating migration. Extracellular molecules that modulate migration include small biogenic amines, peptide growth factors, cytokines, extracellular matrix components, and drugs used in cardiovascular medicine. Promigratory stimuli activate signal transduction cascades that trigger remodeling of the cytoskeleton, change the adhesiveness of the cell to the matrix, and activate motor proteins. This review focuses on the signaling pathways and effector proteins regulated by promigratory and antimigratory molecules. Prominent pathways include phosphatidylinositol 3-kinases, calcium-dependent protein kinases, Rho-activated protein kinase, p21-activated protein kinases, LIM kinase, and mitogen-activated protein kinases. Important downstream targets include myosin II motors, actin capping and severing proteins, formins, profilin, cofilin, and the actin-related protein-2/3 complex. Actin filament remodeling, focal contact remodeling, and molecular motors are coordinated to cause cells to migrate along gradients of chemical cues, matrix adhesiveness, or matrix stiffness. The result is recruitment of cells to areas where the vessel wall is being remodeled. Vessel wall remodeling can be antagonized by common cardiovascular drugs that act in part by inhibiting vascular smooth muscle cell migration. Several therapeutically important drugs act by inhibiting cell cycle progression, which may reduce the population of migrating cells.

Platelet derived growth factor induced tenascin-C transcription is phosphoinositide 3-kinase/Akt-dependent and mediated by Ets family transcription factors

Previous studies have identified several cytokines as inducers of tenascin-C (TN-C) expression in various tissue culture systems. However, the signaling pathways of the regulation of TN-C expression are almost unknown. In this study, we clarified the molecular mechanism(s) underlying the regulation of the TN-C gene by platelet derived growth factor (PDGF) in cultured human dermal fibroblasts. PDGF induced the expression of TN-C protein as well as mRNA in a dose-dependent manner. Actinomycin D, an RNA synthesis inhibitor, significantly blocked the PDGF-mediated upregulation of TN-C mRNA expression, whereas cycloheximide, a protein synthesis inhibitor, did not. The PDGF-mediated induction of TN-C expression was inhibited by the treatment of fibroblasts with a selective phosphoinositide 3-kinase (PI3K) inhibitor, wortmannin, or LY294002. These results suggest that PDGF induced the expression of TN-C at a transcriptional level via phosphoinositide3-kinase/Akt signaling pathways. We performed serial 5' deletions and a transient transfection analysis to define the region in the TN-C promoter mediating the responsiveness to PDGF. Overexpression of Sp1, Ets1, or Ets2 activated the TN-C promoter and superinduced TN-C promoter activity stimulated by PDGF, whereas overexpression of Fli1 inhibited the effects of PDGF on TN-C expression. Mutation of the Sp1/3 binding sites or Ets binding sites in the TN-C promoter region responsible to PDGF abrogated the PDGF-inducible promoter activity. Immunoprecipitation analysis revealed that Sp1, Ets1, and Ets2 form a transcriptionally active complex. On the other hand, the interaction of Fli1 with Sp1 decreased after PDGF treatment. These results suggest that the upregulation of TN-C expression by PDGF involves Ets family transcription factors, co-operating with Sp1.

Shear stress-stimulated endothelial cells induce smooth muscle cell chemotaxis via platelet-derived growth factor-BB and interleukin-1alpha

OBJECTIVE

Vascular smooth muscle cell (SMC) migration is critical to the development of atherosclerosis and neointimal hyperplasia. Hemodynamic forces such as shear stress and cyclic strain stimulate endothelial cell signal-transduction pathways, resulting in the secretion of several factors, including SMC chemoattractants such as platelet-derived growth factor (PDGF). We hypothesized that mechanical forces stimulate endothelial cells to secrete SMC chemoattractants to induce migration via the mitogen-activated protein kinase (MAPK) pathway.

METHODS

Bovine aortic endothelial cells were exposed to shear stress, cyclic strain, or static conditions for 16 hours. The resulting conditioned medium was used as a SMC chemoattractant in a Boyden chamber. Activation of SMC extracellular signal-regulated protein kinase 1/2 (ERK1/2) was assessed by Western blot analysis. Pathways were inhibited with anti-PDGF-BB or anti-interleukin-1alpha (IL-1alpha) antibodies, or the ERK1/2 upstream pathway inhibitor PD98059.

RESULTS

Conditioned medium from endothelial cells exposed to shear stress corresponding to arterial levels of shear stress stimulated SMC migration but lower levels of shear stress or cyclic strain did not. Both PDGF-BB and IL-1alpha were secreted into the conditioned medium by endothelial cells stimulated with shear stress. Both PDGF-BB and IL-1alpha stimulated SMC chemotaxis but were not synergistic, and both stimulated SMC ERK1/2 phosphorylation. Inhibition of PDGF-BB or IL-1alpha inhibited SMC chemotaxis and ERK1/2 phosphorylation.

CONCLUSION

Shear stress stimulates endothelial cells to secrete several SMC chemoattractants, including PDGF-BB and IL-1alpha; both PDGF-BB and IL-1alpha stimulate SMC chemotaxis via the ERK1/2 signal-transduction pathway. These results suggest that the response to vascular injury may have a common pathway amenable to pharmacologic manipulation.

CLINICAL RELEVANCE

One difficulty in the pharmacologic treatment of atherosclerosis or neointimal hyperplasia leading to restenosis is the multiplicity of activated pathways and thus potential treatment targets. This study demonstrates that shear stress, a hemodynamic force that may be a biologically relevant stimulus to induce vascular pathology, stimulates endothelial cells to secrete PDGF-BB and IL-1alpha. Both of these mediators stimulate the SMC ERK1/2 pathway to induce migration, a critical event in the pathogenesis of atherosclerosis and neointimal hyperplasia. Therefore, this study suggests a relevant common target pathway in SMC that is amenable to manipulation for clinical treatment.

Insulin Signaling in Arteries Prevents Smooth Muscle Apoptosis

OBJECTIVE

Insulin is an antiapoptotic factor of cultured vascular cells, but it is not clear whether it also exerts antiapoptotic effects on vascular cells in vivo. We studied insulin receptor signaling in the arteries of normal and diabetic rats to establish whether insulin exhibits antiapoptotic activity toward vascular smooth muscle cells in vivo as well as in vitro.

METHODS AND RESULTS

Western blot analysis and real-time polymerase chain reaction revealed alpha- and beta-subunits of the insulin receptor in association with insulin receptor substrate-1 and phosphatidylinositol 3-kinase in the media of the aorta and carotid artery. The insulin receptor signaling pathway was partially activated under physiological conditions, further activated by intravenous insulin injection, and was attenuated in streptozotocin-induced diabetic rats. Lipopolysaccharide injection induced more apoptosis of vascular smooth muscle cells in diabetic rats than in control rats, whereas insulin prevented apoptosis in the aortic wall. An in vitro study suggested that the antiapoptotic effect of insulin was mediated by phosphatidylinositol 3-kinase.

CONCLUSIONS

Insulin is an antiapoptotic factor of vascular smooth muscle cells in vitro and in vivo. Decreased insulin activity on the artery may increase smooth muscle cell death and cause unstable plaque formation associated with diabetes.

The role of phospholipase C and phosphatidylinositol 3-kinase in vascular smooth muscle cell migration and proliferation

BACKGROUND

Vascular smooth muscle cell (SMC) proliferation and migration both contribute to the formation of intimal hyperplasia. Phospholipase C (PLC) and phosphatidylinositol 3-kinase (PI3-K) are ubiquitous signaling proteins that mediate multiple cellular events. In this study, we investigate the role of PLC and PI3-K in platelet-derived growth factor (PDGF) and extracellular matrix protein (ECM) induced SMC proliferation and migration.

MATERIAL AND METHODS

Proliferation of human saphenous vein SMC was assessed by (3)H-thymidine incorporation. SMC migration was evaluated using a microchemotaxis chamber. U-73122 was used as a general inhibitor for PLC, and D609 and ET-18-OCH3, respectively, were used to block the isotypes of PLC, phosphatidylcholine- (PC-), and phosphatidylinositol- (PI-) specific PLC. PI3-K activity was inhibited using two selective inhibitors, LY-294002 and wortmannin.

RESULTS

PDGF and Type 1 collagen (CN-I) stimulated SMC proliferation, whereas PDGF and four distinct extracellular matrix proteins CN-I, Type 4 collagen (CN-IV), fibronectin (FN), and laminin (LN) stimulated SMC migration. Both isotypes of PLC as well as PI3-K were necessary for PDGF- and CN-I-induced proliferation. Signaling for migration, however, was more specific. Of the various signaling proteins studied, only PI-PLC was necessary for PDGF-induced SMC migration. Conversely, PI3-K was the only signaling protein necessary for SMC migration in response to ECM proteins.

CONCLUSION

The signaling pathways necessary for PDGF- and CN-I-induced SMC proliferation involve both isotypes of PLC as well as PI3-K. The signaling pathways used by growth factors and ECM to stimulate SMC migration are more selective. Understanding the intracellular signaling pathways required for SMC proliferation and migration may allow the development of tools to selectively block intimal hyperplasia.

Fibroblast invasive migration into fibronectin/fibrin gels requires a previously uncharacterized dermatan sulfate-CD44 proteoglycan

After tissue injury, fibroblast migration from the peri-wound collagenous stroma into the fibrin-laden wound is critical for granulation tissue formation and subsequent healing. Recently we found that fibroblast transmigration from a collagen matrix into a fibrin matrix required the presence of fibronectin. Several integrins-alpha 4 beta 1, alpha 5 beta 1, and alpha v beta 3-with known fibronectin binding affinity were necessary for this invasive migration. Here we examined another family of cell surface receptors: the proteoglycans. We found that dermatan sulfate was required for fibroblast migration into a fibronectin/fibrin gel. This conclusion was based on beta-xyloside inhibition of glycanation and specific glycosaminoglycan degradation. CD44, a cell surface receptor known to bind hyaluronan, not infrequently exists as a proteoglycan, decorated with various glycosaminoglycan chains including heparan sulfate and chondroitin sulfate, and as such can bind fibronectin. We found that CD44H, the non-spliced isoform of CD44, was necessary for fibroblast invasion into fibronectin/fibrin gels. Resting fibroblasts expressed mostly nonglycanated CD44H core protein, which became glycanated with chondroitin sulfate and dermatan sulfate, but not heparan sulfate, after a 24 h incubation with platelet-derived growth factor, the stimulus used in the migration assay. These results demonstrate that dermatan sulfate-CD44H proteoglycan is essential for fibroblast migration into fibrin clots and that platelet-derived growth factor, the stimulus for migration, induces the production of chondroitin-sulfate- and dermatan-sulfate-glycanated CD44H.

Phosphoinositide-dependent kinase 1 and p21-activated protein kinase mediate reactive oxygen species-dependent regulation of platelet-derived growth factor-induced smooth muscle cell migration

Smooth muscle cell migration in response to platelet-derived growth factor (PDGF) is a key event in several vascular pathologies, including atherosclerosis and restenosis. PDGF increases intracellular levels of reactive oxygen species (ROS) in vascular smooth muscle cells (VSMCs), but the ROS sensitivity of migration and of the signaling pathways leading to migration are largely unknown. In VSMCs, PDGF dose-dependently increased migration compared with nonstimulated cells, with a maximum increase at 10 ng/mL. Pretreatment with the antioxidant N-acetyl-cysteine, the flavin-containing enzyme inhibitor diphenylene iodonium, or the glutathione peroxidase mimetic ebselen significantly attenuated migration (PDGF alone, 5.0+/-1.1-fold; NAC, 1.8+/-0.2-fold; diphenylene iodonium, 1.4+/-0.3-fold migration; and ebselen, 2.0+/-0.5-fold migration), as did overexpression of catalase. Pretreatment of VSMCs with the Src inhibitor PP1 or dominant-negative Rac adenovirus significantly inhibited migration, but only Src activation was attenuated by ROS inhibitors. Phosphorylation of the Src- and Rac-effector p21-activated protein kinase (PAK) 1 on Thr423 (the phosphoinositide-dependent kinase-1 [PDK1] site) was attenuated by ROS inhibition, and infection of VSMCs with dominant-negative PAK1 adenovirus attenuated migration. Moreover, kinase-inactive K111N-PDK1 inhibited PAK1 phosphorylation on Thr423, and both K111N-PDK1 and Y9F-PDK1 significantly inhibited VSMC migration. PDK1 tyrosine phosphorylation was also ROS dependent. These data indicate that PDGF-induced VSMC migration is ROS dependent and identify the Src/PDK1/PAK1 signaling pathway as an important ROS-sensitive mediator of migration. Such information is critical to understanding the role of ROS in vascular diseases in which migration of VSMCs is an important component.

Nitric Oxide Induces Hypoxia-inducible Factor 1 Activation That Is Dependent on MAPK and Phosphatidylinositol 3-Kinase Signaling

Hypoxia-inducible factor-1 (HIF-1) is a master regulator of cellular adaptive responses to hypoxia. Levels of the HIF-1alpha subunit increase under hypoxic conditions. Exposure of cells to certain nitric oxide (NO) donors also induces HIF-1alpha expression under nonhypoxic conditions. We demonstrate that exposure of cells to the NO donor NOC18 or S-nitrosoglutathione induces HIF-1alpha expression and transcriptional activity. In contrast to hypoxia, NOC18 did not inhibit HIF-1alpha hydroxylation, ubiquitination, and degradation, indicating an effect on HIF-1alpha protein synthesis that was confirmed by pulse labeling studies. NOC18 stimulation of HIF-1alpha protein and HIF-1-dependent gene expression was blocked by treating cells with an inhibitor of the phosphatidylinositol 3-kinase or MAPK-signaling pathway. These inhibitors also blocked NOC18-induced phosphorylation of the translational regulatory proteins 4E-BP1, p70 S6 kinase, and eIF-4E, thus providing a mechanism for the modulation of HIF-1alpha protein synthesis. In addition, expression of a dominant-negative form of Ras significantly suppressed HIF-1 activation by NOC18. We conclude that the NO donor NOC18 induces HIF-1alpha synthesis under conditions of NO formation during normoxia and that hydroxylation of HIF-1alpha is not regulated by NOC18.

Hepatocyte growth factor triggers signaling cascades mediating vascular smooth muscle cell migration

A key event in neointima formation and atherogenesis is the migration of vascular smooth muscle cells (VSMCs) into the intima. This is controlled by cytokines and extracellular matix (ECM) components within the microenvironment of the diseased vessel wall. At present, these signals have only been partially identified. In this study, we demonstrate that Met, the receptor tyrosine kinase for hepatocyte growth factor (HGF), is expressed on VSMCs isolated from the intima of atherosclerotic plaques of carotid arteries. Stimulation with HGF led to activation of Met as well as to activation of PI3-K, PKB/Akt, MEK, and the MAP kinases Erk1 and -2. Moreover, HGF induced lamellipodia formation, a characteristic feature of motile cells, and promoted VSMC migration across fibronectin-coated filters. The HGF-induced cell migration was mediated by beta1 integrins and required PI3-K activation. Our results suggest a role for the HGF-Met signaling pathway in the pathogenesis of atherosclerosis and restenosis.

PI3K is required for proliferation and migration of human pulmonary vascular smooth muscle cells

Human vascular smooth muscle cell proliferation and migration contribute to vascular remodeling in pulmonary hypertension and atherosclerosis. The precise mechanisms that regulate structural remodeling of the vessel wall remain unknown. This study tests the hypothesis that phosphatidylinositol 3-kinase (PI3K) activation is both necessary and sufficient to mediate human pulmonary vascular smooth muscle (PVSM) cell proliferation and migration. Microinjection of human PVSM cells with a dominant-negative class IA PI3K inhibited platelet-derived growth factor (PDGF)-induced DNA synthesis by 65% (P < 0.001; chi(2) analysis) compared with cells microinjected with control plasmid, whereas microinjection of cells with a constitutively active class IA PI3K (p110*-CA) was sufficient to induce DNA synthesis (mitotic index of p110*-CA-microinjected cells was 15% vs. 3% in control cells; P < 0.01). Transfection of PVSM cells with p110*-CA was also sufficient to promote human PVSM cell migration. In parallel experiments, stimulation of human PVSM cells with PDGF induced PI3K-dependent activation of Akt, p70 S6 kinase, and ribosomal protein S6 but not mitogen-activated protein kinase. PDGF-induced proliferation and migration was inhibited by LY-294002. These results demonstrate that PI3K signaling is both necessary and sufficient to mediate human PVSM cell proliferation and migration and suggest that the activation of PI3K may play an important role in vascular remodeling.

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.

Phosphatidylinositol 3-kinase is required for the expression but not for the induction or the maintenance of long-term potentiation in the hippocampal CA1 region

Several signal transduction pathways have been implicated in the induction of long-term potentiation (LTP), yet the signal transduction mechanisms behind the maintenance-expression phase of LTP are still poorly understood. We investigated the role of phosphatidylinositol 3-kinase (PI3-kinase) in LTP at Schaffer collateral/commissural fiber-CA1 synapses in rat hippocampal slices using biochemical approaches and extracellular electrophysiological recordings. We observed that PI3-kinase activity was induced in the CA1 region during LTP of field EPSPs (fEPSPs) and that two structurally unrelated PI3-kinase inhibitors, LY294002 and wortmannin, abated established LTP, suggesting that PI3-kinase is involved in the maintenance-expression phase of LTP. However, LTP recovered after washout of the reversible PI3-kinase inhibitor LY294002, confirming that LTP maintenance and expression are distinct events and indicating that PI3-kinase activity is required for LTP expression rather than for its maintenance. Interestingly, preincubation with LY294002 did not prevent LTP induction. In fact, if LY294002 was withdrawn 5 min after high-frequency stimulation, an LTP of fEPSP was seen. Last, a voltage-dependent calcium channel-dependent form of LTP in the CA1 could also be reversibly abated by LY294002, raising the possibility that PI3-kinase could be required for the expression of multiple forms of synaptic potentiation.

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.

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.

Coupling of the insulin-like growth factor-I receptor tyrosine kinase to Gi2 in human intestinal smooth muscle: Gbetagamma -dependent mitogen-activated protein kinase activation and growth

Endogenous insulin-like growth factor-1 (IGF-I) stimulates growth of cultured human intestinal smooth muscle by activating distinct mitogen-activated protein (MAP) kinase-dependent and phosphatidylinositol 3-kinase-dependent signaling pathways. In Rat1 and Balb/c3T3 fibroblasts and in neurons the IGF-I receptor is coupled to an inhibitory G protein, G(i), which mediates G(beta)gamma-dependent MAP kinase activation. The present study determined whether in normal human intestinal smooth muscle cells the IGF-I receptor activates a heterotrimeric G protein and the role of G protein activation in mediating IGF-I-induced growth. IGF-I elicited IGF-I receptor tyrosine phosphorylation, resulting in the specific activation of G(i2). G(beta)gamma subunits selectively mediated IGF-I-dependent MAP kinase activation; G(alpha)i2 subunits selectively mediated IGF-I-dependent inhibition of adenylyl cyclase activity. IGF-I-stimulated MAP kinase activation and growth were inhibited by pertussis toxin, an inhibitor of G(i)/G(o) activation. Cyclic AMP inhibits growth of human intestinal muscle cells. IGF-I inhibited both basal and forskolin-stimulated cAMP levels. This inhibition was attenuated in the presence of pertussis toxin. IGF-I stimulated phosphatidylinositol 3-kinase activation, in contrast to MAP kinase activation, occurred independently of G(i2) activation. These data suggest that IGF-I specifically activates G(i2), resulting in concurrent G(beta)gamma-dependent stimulation of MAP kinase activity and growth, and G(alpha)i2-dependent inhibition of cAMP levels resulting in disinhibition of cAMP-mediated growth suppression.

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.

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.

CNTFR alpha alone or in combination with CNTF promotes macrophage chemotaxis in vitro

Microchemotaxis chambers were used to investigate whether one aspect of ciliary neurotrophic factor CNTF's role as a lesion factor might be to promote the initial early recruitment of macrophages, which express the signal transducing receptor components, gp130 and LIFRbeta. CNTFRalpha alone, or in combination with CNTF, elicited concentration-dependent macrophage chemotaxis that was inhibited by a neutralizing gp 130 antibody. IL-6, but not LIF, similarly promoted gp 130-dependent macrophage chemotaxis. Stimulation of macrophages with either CNTFRalpha in combination with CNTF or IL-6 alone resulted in tyrosine phosphorylation of an approximately 130 kD protein, presumed to be gp130. Macrophage chemotaxis induced by the combination of CNTFRalpha and CNTF was inhibited in a dose-dependent fashion by wortmannin, LY294002 or PD98059, suggesting the involvement of the phosphoinositide-3 kinase and mitogen-activated protein kinase signaling proteins. As CNTFRalpha and CNTF are present, or have immediate access to nerves after injury, these data point to the possibility that this soluble receptor alone or in combination with its ligand may promote the initial early recruitment of macrophages in vivo.

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.

Cross-talk between receptors with intrinsic tyrosine kinase activity and alpha1b-adrenoceptors

The effect of epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) on the phosphorylation and function of alpha(1b)-adrenoceptors transfected into Rat-1 fibroblasts was studied. EGF and PDGF increased the phosphorylation of these adrenoceptors. The effect of EGF was blocked by tyrphostin AG1478 and that of PDGF was blocked by tyrphostin AG1296, inhibitors of the intrinsic tyrosine kinase activities of the receptors for these growth factors. Wortmannin, an inhibitor of phosphoinositide 3-kinase, blocked the alpha(1b)-adrenoceptor phosphorylation induced by EGF but not that induced by PDGF. Inhibition of protein kinase C blocked the adrenoceptor phosphorylation induced by EGF and PDGF. The ability of noradrenaline to increase [(35)S]guanosine 5'-[gamma-thio]triphosphate ([(35)S]GTP[S]) binding in membrane preparations was used as an index of the functional coupling of the alpha(1b)-adrenoceptors and G-proteins. Noradrenaline-stimulated [(35)S]GTP[S] binding was markedly decreased in membranes from cells pretreated with EGF or PDGF. Our data indicate that: (i) activation of EGF and PDGF receptors induces phosphorylation of alpha(1b)-adrenoceptors, (ii) phosphatidylinositol 3-kinase is involved in the EGF response, but does not seem to play a major role in the action of PDGF, (iii) protein kinase C mediates this action of both growth factors and (iv) the phosphorylation of alpha(1b)-adrenoceptors induced by EGF and PDGF is associated with adrenoceptor desensitization.

Phosphatidylinositol 3-kinases regulate ERK and p38 MAP kinases in canine colonic smooth muscle

In canine colon, M2/M3 muscarinic receptors are coupled to extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein (MAP) kinases. We tested the hypothesis that this coupling is mediated by enzymes of the phosphatidylinositol (PI) 3-kinase family. RT-PCR and Western blotting demonstrated expression of two isoforms, PI 3-kinase-alpha and PI 3-kinase-gamma. Muscarinic stimulation of intact muscle strips (10 microM ACh) activated PI 3-kinase-gamma, ERK and p38 MAP kinases, and MAP kinase-activated protein kinase-2, whereas PI 3-kinase-alpha activation was not detected. Wortmannin (25 microM) abolished the activation of PI 3-kinase-gamma, ERK, and p38 MAP kinases. MAP kinase inhibition was a PI 3-kinase-gamma-specific effect, since wortmannin did not inhibit recombinant activated murine ERK2 MAP kinase, protein kinase C, Raf-1, or MAP kinase kinase. In cultured muscle cells, newborn calf serum (3%) activated PI 3-kinase-alpha and PI 3-kinase-gamma isoforms, ERK and p38 MAP kinases, and stimulated chemotactic cell migration. Using wortmannin and LY-294002 to inhibit PI 3-kinase activity and PD-098059 and SB-203580 to inhibit ERK and p38 MAP kinases, we established that these enzymes are functionally important for regulation of chemotactic migration of colonic myocytes.

Platelet-derived growth factor-mediated signaling through the Shb adaptor protein: effects on cytoskeletal organization

The Src homology (SH) 2 domain adaptor protein Shb has previously been shown to interact with the platelet-derived growth factor (PDGF)-beta receptor. In this study we show an association between Shb and the PDGF-alpha receptor which is mediated by the SH2 domain of Shb and involves tyrosine residue 720 in the kinase insert domain of the receptor. To assess the role of Shb in PDGF-mediated signaling, we have overexpressed wild-type Shb or Shb carrying a mutation (R522K) which renders the SH2 domain inactive, in Patch mouse (PhB) fibroblasts expressing both PDGF receptors (PhB/Ralpha). Overexpression of wild-type Shb, but not the R522K Shb mutant, affected PDGF-mediated reorganization of the cytoskeleton by decreasing membrane ruffle formation and stimulating the generation of filopodia relative the parental control cells. In addition, the PDGF-induced receptor-associated phosphatidylinositol 3'-kinase activity and phosphorylation of Akt was similar in both PhB/Ralpha/Shb and PhB/Ralpha/ShbR522K cells compared with the parental control, whereas the activation of Rac in response to PDGF-BB was diminished only in the PhB/Ralpha/Shb cells. We conclude that Shb plays a role in PDGF-dependent regulation of certain cytoskeletal changes by modulating the ability of PDGF to activate Rac.

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.

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.

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.

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.

PSM, a mediator of PDGF-BB-, IGF-I-, and insulin-stimulated mitogenesis

PSM/SH2-B has been described as a cellular partner of the FcepsilonRI receptor, insulin receptor (IR), insulin-like growth factor-I (IGF-I) receptor (IGF-IR), and nerve growth factor receptor (TrkA). A function has been proposed in neuronal differentiation and development but its role in other signaling pathways is still unclear. To further elucidate the physiologic role of PSM we have identified additional mitogenic receptor tyrosine kinases as putative PSM partners including platelet-derived growth factor (PDGF) receptor (PDGFR) beta, hepatocyte growth factor receptor (Met), and fibroblast growth factor receptor. We have mapped Y740 as a site of PDGFR beta that is involved in the association with PSM. We have further investigated the putative role of PSM in mitogenesis with three independent experimental strategies and found that all consistently suggested a role as a positive, stimulatory signaling adapter in normal NIH3T3 and baby hamster kidney fibroblasts. (1) PSM expression from cDNA using an ecdysone-regulated transient expression system stimulated PDGF-BB-, IGF-I-, and insulin- but not EGF-induced DNA synthesis in an ecdysone dose-responsive fashion; (2) Microinjection of the (dominant negative) PSM SH2 domain interfered with PDGF-BB- and insulin-induced DNA synthesis; and (3) A peptide mimetic of the PSM Pro-rich putative SH3 domain-binding region interfered with PDGF-BB-, IGF-I-, and insulin- but not with EGF-induced DNA synthesis in NIH3T3 fibroblasts. This experiment was based on cell-permeable fusion peptides with the Drosophila antennapedia homeodomain which effectively traverse the plasma membrane of cultured cells. These experimental strategies independently suggest that PSM functions as a positive, stimulatory, mitogenic signaling mediator in PDGF-BB, IGF-I, and insulin but not in EGF action. This function appears to involve the PSM SH2 domain as well as the Pro-rich putative SH3 domain binding region. Our findings support the model that PSM participates as an adapter in various mitogenic signaling mechanisms by linking an activated (receptor) phospho-tyrosine to the SH3 domain of an unknown cellular partner.

Phosphatidylinositol 3'-kinase-mediated calcium mobilization regulates chemotaxis in phosphatidic acid-stimulated human neutrophils

Phosphatidylinositol 3'-kinase (PI 3'-kinase) plays an important role in the migration of hepatocytes, endothelial cells and neoplastic cells to agonists which activate cellular tyrosine kinases. We examined the PI 3'-kinase-dependent chemotactic responses of neutrophilic leukocytes induced by phosphatidic acid (PA) in order to clarify mechanisms by which the enzyme potentially influences cellular migration. Western analysis of immunoprecipitates indicated that PA induced the tyrosine phosphorylation of three distinct proteins involved in functional activation which co-immunoprecipitated in PA-stimulated cells. These proteins were identified as lyn, syk and the 85 kDa regulatory subunit of PI 3'-kinase. Chemotactic responses to PA but not to several other neutrophil agonists were inhibited by the PI 3'-kinase inhibitors wortmannin and LY294002. Chemotactic inhibition resulted from upstream inhibition of calcium mobilization. Chelation of extracellular calcium by ethylene glycol-bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) did not affect the PA-induced chemotaxis, whereas chelation of intracellular calcium by 1, 2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) attenuated this response. Thus, changes in intracellular Ca(2+) levels that can be effected by Ca(2+) mobilized from intracellular stores in the absence of Ca(2+) influx regulate PA-induced chemotaxis. Furthermore, PI 3'-kinase inhibition blunted the agonist-dependent generation of inositol 1,4,5-trisphosphate (IP(3)), suggesting that PI 3'-kinase exerted its effects on calcium mobilization from intracellular sources by mediating activation of phospholipase C (PLC) in PA-stimulated cells. Moreover, the PI 3'-kinase inhibitor LY294002 also inhibited phosphorylation of syk in PA-stimulated cells. We, therefore, propose that products of PI 3'-kinase confined to the inner leaflet of the plasma membrane play a role in activation of syk, calcium mobilization and induction of chemotactic migration.

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.

Autocrine FGF-2 is responsible for the cell density-dependent susceptibility to apoptosis of HUVEC : A role of a calpain inhibitor-sensitive mechanism

To elucidate the factors affecting endothelial susceptibility to apoptosis, we studied the effects of cell density on endothelial cell apoptosis induced by deprivation of serum and fibroblast growth factor-2 (FGF-2/basic FGF). On deprivation, more cells became apoptotic in a dense culture (5 x 10(2) cells/mm(2)) than in a sparse culture (1 x 10(2) cells/mm(2)) of human umbilical vein endothelial cells. FGF-2, hepatocyte growth factor, and vascular endothelial cell growth factor, but not insulin-like growth factor-I, decreased apoptosis in the dense culture to a level similar in the sparse culture. An anti-FGF-2 antibody significantly increased the apoptosis in the sparse culture, suggesting that the sparse culture was resistant to apoptosis because of the greater autocrine production of FGF-2. Western blot analysis and metabolic labeling revealed that the sparse culture has, in fact, more FGF-2 than the dense culture. The steady state level of mRNA for FGF-2 was not significantly different between the dense and sparse cultures. Among a panel of inhibitors for 2 major cytoplasmic proteolytic enzymes, calpain inhibitors increased FGF-2 in the dense culture, but proteasome inhibitors did not. Our findings demonstrate that cell density affects endothelial survival by regulating autocrine FGF-2 production through a calpain inhibitor-sensitive mechanism.

Phosphatidylinositol 3-kinase is required for growth factor-induced amino acid uptake by vascular smooth muscle cells

Although accumulating evidence suggests that phosphatidylinositol 3-kinase (PI3K) is a common signaling molecule for growth factor-induced amino acid uptake by the cell, the role of PI3K in the uptake of different amino acids was not tested under the same conditions. In this study, we asked whether PI3K mediates platelet-derived growth factor (PDGF) -stimulated uptake of different amino acids that are taken up through 3 major amino acid transporters expressed in rat vascular smooth muscle cells and other cell types and whether PI3K mediates amino acid uptake stimulated with different growth factors and vasoactive substances. PDGF increased the uptake of [(3)H]leucine, [(3)H]proline, and [(3)H]arginine in a dose- and time-dependent fashion. Two different PI3K inhibitors, wortmannin (100 nmol/L) and LY294002 (10 micromol/L), completely inhibited the amino acid uptake stimulated by PDGF. Chinese hamster ovary cells expressing both PDGF receptor-beta and a dominant-negative PI3K did not increase their leucine uptake when stimulated with PDGF, whereas the same cells expressing only PDGF receptor-beta did. Transforming growth factor-beta, as well as insulin-like growth factor-I and angiotensin II, increased leucine uptake by vascular smooth muscle cells. Wortmannin and LY294002 inhibited this increase. We also found that transforming growth factor-beta stimulated PI3K activity and the phosphorylation of Akt, a downstream signaling molecule of PI3K. A similar effect of PI3K inhibitors on amino acid uptake was observed in Swiss 3T3 cells. We conclude that PI3K mediates the uptake of different amino acids by vascular smooth muscle cells and other cell types stimulated with a variety of growth factors, including transforming growth factor-beta. Our findings suggest that PI3K may play an important role in vascular pathophysiology by regulating amino acid uptake.

Overactivation of phospholipase C-gamma1 renders platelet-derived growth factor beta-receptor-expressing cells independent of the phosphatidylinositol 3-kinase pathway for chemotaxis

We have previously shown that porcine aortic endothelial cells expressing the Y934F platelet-derived growth factor (PDGF) beta-receptor mutant respond to PDGF-BB in a chemotaxis assay at about 100-fold lower concentration than do wild-type PDGF beta-receptor-expressing cells (Hansen, K., Johnell, M., Siegbahn, A. , Rorsman, C., Engström, U., Wernstedt, C., Heldin, C.-H., and Rönnstrand, L. (1996) EMBO J. 15, 5299-5313). Here we show that the increased chemotaxis correlates with increased activation of phospholipase C-gamma1 (PLC-gamma1), measured as inositol-1,4, 5-trisphosphate release. By two-dimensional phosphopeptide mapping, the increase in phosphorylation of PLC-gamma1 was shown not to be selective for any site, rather a general increase in phosphorylation of PLC-gamma1 was seen. Specific inhibitors of protein kinase C, bisindolylmaleimide (GF109203X), and phosphatidylinositol 3-kinase (PI3-kinase), LY294002, did not affect the activation of PLC-gamma1. To assess whether increased activation of PLC-gamma1 is the cause of the hyperchemotactic behavior of the Y934F mutant cell line, we constructed cell lines expressing either wild-type or a catalytically compromised version of PLC-gamma1 under a tetracycline-inducible promoter. Overexpression and concomitant increased activation of wild-type PLC-gamma1 in response to PDGF-BB led to a hyperchemotactic behavior of the cells, while the catalytically compromised PLC-gamma1 mutant had no effect on PDGF-BB-induced chemotaxis. Furthermore, in cells expressing normal levels of PLC-gamma1, chemotaxis was inhibited by LY294002. In contrast, the increase in chemotactic response seen upon overexpression of PLC-gamma1 was not inhibited by the PI3-kinase inhibitor LY294002. These observations suggest the existence of two different pathways which mediate PDGF-induced chemotaxis; depending on the cellular context, the PI3-kinase pathway or the PLC-gamma1 pathway may dominate.

Crosstalk between protein kinase A and growth factor receptor signaling pathways in arterial smooth muscle

Crosstalk between the cyclic AMP-dependent protein kinase (PKA) and growth factor receptor signaling is one of many emerging concepts of crosstalk in signal transduction. Understanding of PKA crosstalk may have important implications for studies of crosstalk between other, less well known, signaling pathways. This review focuses on PKA crosstalk in arterial smooth muscle. Proliferation and migration of arterial smooth muscle cells (SMCs) contribute to the thickening of the blood vessel wall that occurs in many types of cardiovascular disease. PKA potently inhibits SMC proliferation by antagonizing the major mitogenic signaling pathways induced by growth factors in SMCs. PKA also inhibits growth factor-induced SMC migration. An intricate crosstalk between PKA and the mitogen-activated protein kinase (MAPK/ERK) pathway, the p70 S6 kinase pathway and cyclin-dependent kinases has been described. Further, PKA regulates expression of growth regulatory molecules. The result of PKA activation in SMCs is the potent inhibition of cell cycle traverse and SMC migration. In this review, we discuss recent advances in our understanding of the crosstalk between PKA and signaling pathways induced by growth factor receptors in SMCs, and where relevant, in other cell types in which interesting examples of PKA crosstalk have been described.

Platelet-derived growth factor activates p38 mitogen-activated protein kinase through a Ras-dependent pathway that is important for actin reorganization and cell migration

Members of the mitogen activated protein (MAP) kinase family, extracellular signal-regulated kinase, stress-activated protein kinase-1/c-Jun NH2-terminal kinase, and p38, are central elements that transduce the signal generated by growth factors, cytokines, and stressing agents. It is well known that the platelet-derived growth factor (PDGF) activates extracellular signal-regulated kinase, which leads to cellular mitogenic response. On the other hand, the role of the other MAP kinases in mediating the cellular function of PDGF remains unclear. In the present study, we have investigated the functional role of the other MAP kinases in PDGF-mediated cellular responses. We show that ligand stimulation of PDGF receptors leads to the activation of p38 but not stress-activated protein kinase-1/c-Jun NH2-terminal kinase. Experiments using a specific inhibitor of p38, SB203580, show that the activation of p38 is required for PDGF-induced cell motility responses such as cell migration and actin reorganization but not required for PDGF-stimulated DNA synthesis. Analyses of tyrosine residue-mutated PDGF receptors show that Src homology 2 domain-containing proteins including Src family kinases, phosphatidylinositol 3-kinase, the GTPase-activating protein of Ras, the Src homology 2 domain-containing phosphatase SHP-2, phospholipase C-gamma, and Crk do not play a major role in mediating the PDGF-induced activation of p38. Finally, the expression of dominant-negative Ras but not dominant-negative Rac inhibited p38 activation by PDGF, suggesting that Ras is a potent mediator in the p38 activation pathway downstream of PDGF receptors. Taken together, our present study proposes the existence of a Ras-dependent pathway for the activation of p38, which is important for cell motility responses elicited by PDGF stimulation.

A p85 subunit-independent p110alpha PI 3-kinase colocalizes with p70 S6 kinase on actin stress fibers and regulates thrombin-stimulated stress fiber formation in swiss 3T3 cells

The signaling pathways linking receptor activation to actin stress fiber rearrangements during growth factor-induced cell shape change are still to be determined. Recently our laboratory demonstrated the involvement of p70 S6 kinase (p70(s6k)) activation in thrombin-induced stress fiber formation in Swiss 3T3 cells. The present work shows that thrombin-induced p70(s6k) activation is inhibited by the PI 3-kinase inhibitors wortmannin and LY-294002. These inhibitors also significantly reduced thrombin-induced stress fiber formation, demonstrating a role for PI 3-kinase activity in this process, most likely upstream of p70(s6k). Furthermore, the p110alpha form of PI 3-kinase was localized to actin stress fibers, as was previously shown for p70(s6k), as well as to a golgi-like distribution. In contrast, PI 3-kinase p110gamma colocalized with microtubules. The PI 3-kinase p85 subunit, known to be capable of association with p110alpha, was present in a predominantly golgi-like distribution with no presence on actin filaments, suggesting the existence of distinctly localized PI 3-kinase pools. Immunodepletion of p85 from cell lysates resulted in only partial depletion of p110alpha and p110alpha-associated PI 3-kinase activity, confirming the presence of a p85-free p110alpha pool located on the actin stress fibers. Our data, therefore, point to the importance of subcellular localization of PI 3-kinase in signal transduction and to a novel action of p85 subunit-independent PI 3-kinase p110alpha in the stimulation by thrombin of p70(s6k) activation and actin stress fiber formation.

NGF activates similar intracellular signaling pathways in vascular smooth muscle cells as PDGF-BB but elicits different biological responses

The signaling pathways that regulate smooth muscle cell migration and proliferation are incompletely understood. Smooth muscle cells express at least 3 families of receptor tyrosine kinases that mediate cell migration: platelet-derived growth factor (PDGF) receptors, the trk family of neurotrophin receptors, and insulin-like growth factor 1 receptor. The neurotrophin, nerve growth factor (NGF), and insulin-like growth factor 1 induce the migration but not the proliferation of smooth muscle cells, whereas PDGF-BB stimulates both responses. To determine whether distinct signaling pathways downstream of receptor tyrosine kinases specifically mediate smooth muscle cell migration or proliferation, the ligand-induced activation of different signaling pathways in smooth muscle cells was examined. NGF induces prolonged activation of the Shc/MAP kinase pathway and phospholipase Cgamma compared with PDGF-BB. The activation of phosphatidylinositol-3 kinase, however, was 10-fold greater in response to PDGF-BB compared with NGF. Insulin-like growth factor 1 activates only phosphatidylinositol-3 kinase. Pharmacological inhibitors of phosphatidylinositol-3 kinase, Wortmannin and LY294002, inhibit PDGF-BB and NGF-induced migration, whereas an inhibitor of MAP kinase kinase, PD98059, has no effect. Our results suggest that (1) different receptor tyrosine kinases use similar patterns of activation of signaling pathways to mediate distinct biological outcomes of cell migration and proliferation, (2) NGF activates signaling proteins in smooth muscle cells similar to those activated during NGF-induced neuronal differentiation, and (3) the combinatorial effects of different signaling pathways are important for the regulation of smooth muscle cell migration and proliferation. Further studies using mutant trk receptors will help to define the signal transduction pathways mediating NGF-induced smooth muscle cell migration.

Central role of the MAPK pathway in ang II-mediated DNA synthesis and migration in rat vascular smooth muscle cells

Angiotensin II (Ang II) promotes vascular smooth muscle cell (VSMC) growth and migration, but the signaling pathways mediating these VSMC behaviors critical to restenosis and atherosclerosis are not completely known. The purpose of the present investigation was to define the role of mitogen-activated protein kinase (MAPK) in Ang II-induced DNA synthesis, migration, and c-fos induction in VSMCs. PD 98059, a synthetic inhibitor of MAPK kinase, or antisense oligodeoxynucleotides (ODNs) to deplete extracellular signal-regulated kinase (ERK)1 and ERK2 MAPKs, were used to inhibit MAPK signaling. PD 98059 at 30 micromol/L reduced Ang II-induced MAPK activity by 69% (P<0.01). Under these conditions, Ang II-induced DNA synthesis was completely inhibited (P<0.01), and Ang II-directed migration was attenuated by 76% (P<0.05). In contrast, induction of c-fos by Ang II was only partially suppressed (58% inhibition, P<0.01). Antisense ODNs against the initiation site of rat ERK1 and ERK2 MAPK mRNAs reduced corresponding protein levels by 63% (P<0.01) and completely inhibited MAPK activation by either Ang II (1 micromol/L) or 10% serum. Antisense ODNs (0.4 micromol/L) completely inhibited Ang II-induced DNA synthesis (P<0.01), decreased migration by 47% (P<0.01), and reduced c-fos induction by 40% (P<0.01 versus control ODN-transfected VSMCs). The Ang II type 1 (AT1)-receptor blocker irbesartan completely blocked DNA synthesis, migration, MAPK activation, and c-fos induction by Ang II in VSMCs. These results demonstrate that activation of MAPK plays a crucial role in Ang II-directed migration and DNA synthesis through the AT1 receptor. In contrast, Ang II-mediated c-fos induction and migration were only partially inhibited by either antisense ODNs or PD 98059, suggesting that other pathways in addition to the MAPK pathway may be involved in these actions of Ang II. We conclude that MAPK is a critical regulatory factor for Ang II-mediated migration and growth in VSMCs. Ang II-induced DNA synthesis showed a stronger MAPK dependence than did Ang II-directed migration or c-fos induction.

Signal transduction via platelet-derived growth factor receptors

Platelet-derived growth factor (PDGF) exerts its stimulatory effects on cell growth and motility by binding to two related protein tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation, allowing binding and activation of cytoplasmic SH2-domain containing signal transduction molecules. Thereby, a number of different signaling pathways are initiated leading to cell growth, actin reorganization migration and differentiation. Recent observations suggest that extensive cross-talk occurs between different signaling pathways, and that stimulatory signals are modulated by inhibitory signals arising in parallel.