A small GTP-binding protein, Rho, associates with the platelet-derived growth factor type-beta receptor upon ligand binding

A critical role for PDGFRα signaling in medial nasal process development

The primitive face is composed of neural crest cell (NCC) derived prominences. The medial nasal processes (MNP) give rise to the upper lip and vomeronasal organ, and are essential for normal craniofacial development, but the mechanism of MNP development remains largely unknown. PDGFRα signaling is known to be critical for NCC development and craniofacial morphogenesis. In this study, we show that PDGFRα is required for MNP development by maintaining the migration of progenitor neural crest cells (NCCs) and the proliferation of MNP cells. Further investigations reveal that PI3K/Akt and Rac1 signaling mediate PDGFRα function during MNP development. We thus establish PDGFRα as a novel regulator of MNP development and elucidate the roles of its downstream signaling pathways at cellular and molecular levels.

High glucose-induced upregulation of Rho/Rho-kinase via platelet-derived growth factor receptor-beta increases migration of aortic smooth muscle cells

Small GTPase Rho and Rho-kinase, the target protein of Rho, play an important role in atherosclerosis. In diabetic macroangiopathy, one of the major pathogenic changes is the migration of vascular smooth muscle cells (SMCs). Platelet-derived growth factor (PDGF) is known to stimulate the migration of SMCs. In the current study, we have investigated the involvement of the Rho/Rho-kinase pathway in the increased migration of cultured human aortic SMCs under a high glucose condition. PDGF stimulated the activation and the protein level of Rho. The protein level of PDGF receptor-beta (PDGFR-beta) was increased under the high glucose condition concomitant with the increased protein level and activation of Rho. The increased protein level and activity of Rho were suppressed by an anti-PDGF neutralizing antibody or a PDGFR-beta inhibitor, AG1433, under the high glucose condition. Furthermore, high glucose significantly increased the migration of SMCs. A specific inhibitor of Rho-kinase, Y-27632, or anti-PDGF neutralizing antibody inhibited increased migration of SMCs under the high glucose condition. The protein levels of Rho were increased in aortae of diabetic rats, which were abolished by the treatment of Imatinib, the inhibitor of PDGFR. These observations indicate that the upregulation of the PDGFR-beta / Rho / Rho-kinase pathway increases the migration of SMCs under the high glucose condition. The inhibition of Rho/Rho-kinase may be a new target for the treatment of diabetic macroangiopathy.

Different Migration of Vascular Smooth Muscle Cells from Human Coronary Artery Bypass Vessels

BACKGROUND

We examined whether vascular smooth muscle (VSMC) or endothelial cell (EC) migration from internal mammary artery (MA) differed from VSMC or EC migration from saphenous vein (SV).

METHODS AND RESULTS

Migration to PDGF-BB (1-10 ng/ml) was lower in VSMC from MA than SV; however, attachment, movement without chemokine, and chemokinesis were identical. Unlike VSMC, migration of EC was similar in response to several mediators. Expression of PDGF receptor-beta was lower in VSMC from MA than SV, while alpha-receptor expression was higher. PDGF-BB-induced RhoA activity was lower in MA than SV, while basal activity was identical. Rosuvastatin and hydroxyfasudil impaired PDGF-BB-induced migration of VSMC from MA and SV. Mevalonate and geranylgeranylpyrophosphate rescued inhibition by rosuvastatin. PDGF-BB induced less stress fiber formation in VSMC from MA than SV. A dominant negative RhoA mutant inhibited stress fiber formation to PDGF-BB, while a constitutively active mutant resulted in maximal stress fiber formation in MA and SV. Rosuvastatin and hydroxyfasudil impaired PDGF-BB-induced stress fiber formation in MA and SV.

CONCLUSIONS

VSMC migration to PDGF-BB is lower in MA than SV, which is at least in part related to lower activity of the Rho/ROCK pathway.

TGFbeta3 signaling activates transcription of the LEF1 gene to induce epithelial mesenchymal transformation during mouse palate development

Epithelial mesenchymal transformation (EMT) of the medial edge epithelial (MEE) seam creates palatal confluence. This work aims to elucidate the molecular mechanisms by which TGFβ₃ brings about palatal seam EMT. We collected mRNA for PCR analysis from individual transforming MEE cells by laser microdissection techniques and demonstrated that TGFβ₃ stimulates lymphoid-enhancing factor 1 (LEF1) mRNA synthesis in MEE cells. We show with antisense β-catenin oligonucleotides that up-regulated LEF1 is not activated by β-catenin in palate EMT. We ruled out other TGFβ₃ targets, such as RhoA and MEK1/2 pathways, and we present evidence using dominant-negative Smad4 and dominant-negative LEF1 showing that TGFβ₃ uses Smads both to up-regulate synthesis of LEF1 and to activate LEF1 transcription during induction of palatal EMT. When phospho-Smad2 and Smad4 are present in the nucleus, LEF1 is activated without β-catenin. Our paper is the first to show that the Smad2,4/LEF1 complex replaces β-catenin/LEF1 during activation of EMT in vivo by TGFβ₃.

CD38 Signaling in T Cells Is Initiated within a Subset of Membrane Rafts Containing Lck and the CD3-ζ Subunit of the T Cell Antigen Receptor

In this study we present data supporting that most CD38 is pre-assembled in a subset of Brij 98-resistant raft vesicles, which were stable at 37 degrees C, and have relatively high levels of Lck and the CD3-zeta subunit of T cell antigen receptor-CD3 complex in contrast with a Brij 98-soluble pool, where CD38 is associated with CD3-zeta, and Lck is not detected. Our data further indicate that following CD38 engagement, LAT and Lck are tyrosine phosphorylated exclusively in Brij 98-resistant rafts, and some key signaling components translocate into rafts (i.e. Sos and p85-phosphatidylinositol 3-kinase). Moreover, N-Ras results activated within rafts immediately upon CD38 ligation, whereas activated Erk was mainly found in soluble fractions with delayed kinetics respective to Ras activation. Furthermore, full phosphorylation of CD3-zeta and CD3-epsilon only occurs in rafts, whereas partial CD3-zeta tyrosine phosphorylation occurs exclusively in the soluble pool, which correlated with increased levels of c-Cbl tyrosine phosphorylation in the non-raft fractions. Taken together, these results suggest that, unlike the non-raft pool, CD38 in rafts is able to initiate and propagate several activating signaling pathways, possibly by facilitating critical associations within other raft subsets, for example, LAT rafts via its capacity to interact with Lck and CD3-zeta. Overall, these findings provide the first evidence that CD38 operates in two functionally distinct microdomains of the plasma membrane.

Contribution of Rho A and Rho kinase to platelet-derived growth factor-BB-induced proliferation of vascular smooth muscle cells

In order to identify small G protein (s) which contributes to the proliferation of vascular smooth muscle cells (VSMCs), we examined the effect of an HMG-CoA reductase inhibitor (cerivastatin), a farnesyltransferase inhibitor (FTI-277), a geranyl geranyl transferase inhibitor (GGTI-286) and a Rho kinase inhibitor (Y-27632) on the proliferation of cultured rat VSMCs stimulated with 20ng/ml platelet-derived growth factor (PDGF)-BB. Cerivastatin and GGTI-286, but not FTI-277, suppressed the PDGF-BB-induced activation of extracellular signal related kinase (ERK1/2). The inhibitory effect of cerivastatin on the PDGF-BB-induced activation of ERK1/2 was fully recovered by the addition of geranylgeranyl pyrophosphate (GGPP), but not farnesyl pyrophosphate (FPP). Cerivastatin and GGTI-286, but not FTI-277, suppressed the PDGF-BB-induced [3H] thymidine incorporation and activation of ornitine decarboxylase (ODC), both of which were fully recovered by the addition of GGPP, but not FPP. These data indicate that the PDGF-BB-induced activation of ERK1/2 and proliferation of VSMCs depend upon geranylgeranylated small G protein. Immunoblotting analysis revealed the upregulation of Rho A protein in the membrane fractions of VSMCs stimulated by PDGF-BB. Furthermore, Y-27632 suppressed the PDGF-BB-induced activation of ERK1/2 and proliferation of VSMCs. On the basis of these data, we conclude that PDGF-BB stimulates the proliferation of VSMCs via the activation of Rho A. Rho kinase plays an important role in this process as an effector of Rho A.

Formation of a beta1 integrin signaling complex in Schwann cells is independent of rho

Schwann cell adhesion to basal lamina is essential for peripheral nerve development. beta(1) integrin receptors for extracellular matrix cooperate with other receptors to transmit signals that coordinate cell cycle progression and initiation of differentiation, including myelin-specific gene expression. In Schwann cell/sensory neuron cocultures, beta(1) integrins complex with focal adhesion kinase (FAK), fyn kinase, paxillin, and schwannomin in response to basal lamina adhesion. To study the assembly of this signaling complex in Schwann cells (SCs), we induced beta(1) integrin clustering on suspended cells using an immobilized antibody and recovered a complex containing beta(1) integrin, FAK, paxillin, and schwannomin. In adherent subconfluent cells, the proteins colocalized to filopodia, ruffling membranes and focal contacts. We assessed the role of rhoGTPase in the process of integrin complex assembly by introducing C3 transferase (C3T), a rho inhibitor, into the cells. Although C3T caused dose-dependent morphological abnormalities, FAK, paxillin, and schwannomin were able to coimmunoprecipitate with beta(1) integrin. Additionally, colocalization of FAK, paxillin, and schwannomin with beta(1) integrin in filopodia and small focal contacts remained unchanged. We conclude that SCs do not require active rho to recruit signaling and structural proteins to beta(1) integrins clustered at the plasma membrane. Rho is required to establish large focal adhesions and to spread and stabilize plasma membrane extensions.

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

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

Signaling events required for transforming growth factor-beta stimulation of connective tissue growth factor expression by cultured human lung fibroblasts

It is possible that many of the fibrogenic effects of transforming growth factor-beta (TGF-beta) are mediated by connective tissue growth factor (CTGF). In the present work, we show that TGF-beta1 produces a 5- to 6-fold increase in CTGF expression by cultured human lung fibroblasts that is due mainly to increased transcription. The half-life of CTGF mRNA is 1.96 h, consistent with its role as a cytokine. In addition to requiring Smad activity, based upon the effects of specific inhibitors, the TGF-beta intracellular signaling pathway requires the activity of a phosphatidylcholine-specific phospholipase C, a protein kinase C, and one or more tyrosine kinases. It is also likely that the pathway requires a member of the Ras superfamily of small GTPases, but not trimeric G proteins. Pharmacologic inhibition of TGF-beta stimulation of CTGF expression may be an effective therapeutic approach to a variety of undesirable fibrotic reactions.

TGF-beta1 stimulation of fibronectin transcription in cultured human lung fibroblasts requires active geranylgeranyl transferase I, phosphatidylcholine-specific phospholipase C, protein kinase C-delta, and p38, but not erk1/erk2

The cytokine transforming growth factor-beta (TGF-beta) has multiple effects on a variety of cell types, modulating cell growth and differentiation as well as extracellular matrix deposition and degradation. In the present work, we demonstrate that TGF-beta1 produces a fourfold increase in transcription of the fibronectin gene in cultured human fetal lung fibroblasts with only a small increase in mRNA stability resulting in a significant increase in fibronectin mRNA steady state level. A corresponding increase in production of fibronectin protein accompanied the increase in mRNA. Through the use of specific inhibitors, we demonstrate that geranylgeranylated, but not farnesylated or acylated protein(s), protein kinase C-delta, phosphatidylcholine-specific phospholipse C, tyrosine kinase activity, and stress-activated protein kinase p38 are required for this TGF-beta1 effect. Trimeric G proteins and mitogen-activated protein kinases erk1 and erk2 do not appear to be involved. While these results emphasize the complexities involved in the control of extracellular matrix synthesis by TGF-beta, they also identify reaction sites that may be amenable to pharmacologic modulation. Such modulation could be of great advantage in the treatment of a wide variety of undesirable fibrotic reactions.

Involvement of the small GTPases XRhoA and XRnd1 in cell adhesion and head formation in early Xenopus development

The Rho family of small GTPases regulates a variety of cellular functions, including the dynamics of the actin cytoskeleton, cell adhesion, transcription, cell growth and membrane trafficking. We have isolated the first Xenopus homologs of the Rho-like GTPases RhoA and Rnd1 and examined their potential roles in early Xenopus development. We found that Xenopus Rnd1 (XRnd1) is expressed in tissues undergoing extensive morphogenetic changes, such as marginal zone cells involuting through the blastopore, somitogenic mesoderm during somite formation and neural crest cells. XRnd1 also causes a severe loss of cell adhesion in overexpression experiments. These data and the expression pattern suggest that XRnd1 regulates morphogenetic movements by modulating cell adhesion in early embryos. Xenopus RhoA (XRhoA) is a potential XRnd1 antagonist, since overexpression of XRhoA increases cell adhesion in the embryo and reverses the disruption of cell adhesion caused by XRnd1. In addition to the potential roles of XRnd1 and XRhoA in the regulation of cell adhesion, we find a role for XRhoA in axis formation. When coinjected with dominant-negative BMP receptor (tBR) in the ventral side of the embryo, XRhoA causes the formation of head structures resembling the phenotype seen after coinjection of wnt inhibitors with dominant-negative BMP receptor. Since dominant-negative XRhoA is able to reduce the formation of head structures, we propose that XRhoA activity is essential for head formation. Thus, XRhoA may have a dual role in the embryo by regulating cell adhesion properties and pattern formation.

Diverse signaling pathways activated by growth factor receptors induce broadly overlapping, rather than independent, sets of genes

We sought to explore the relationship between receptor tyrosine kinase (RTK) activated signaling pathways and the transcriptional induction of immediate early genes (IEGs). Using global expression monitoring, we identified 66 fibroblast IEGs induced by platelet-derived growth factor beta receptor (PDGFRbeta) signaling. Mutant receptors lacking binding sites for activation of the PLCgamma, PI3K, SHP2, and RasGAP pathways still retain partial ability to induce 64 of these IEGs. Removal of the Grb2-binding site further broadly reduces induction. These results suggest that the diverse pathways exert broadly overlapping effects on IEG induction. Interestingly, a mutant receptor that restores the RasGAP-binding site promotes induction of an independent group of genes, normally induced by interferons. Finally, we compare the PDGFRbeta and fibroblast growth factor receptor 1; each induces essentially identical IEGs in fibroblasts.

The IL-1 receptor and Rho directly associate to drive cell activation in inflammation

IL-1-stimulated mesenchymal cells model molecular mechanisms of inflammation. Binding of IL-1 to the type I IL-1 receptor (IL-1R) clusters a multi-subunit signaling complex at focal adhesion complexes. Since Rho family GTPases coordinately organize actin cytoskeleton and signaling to regulate cell phenotype, we hypothesized that the IL-1R signaling complex contained these G proteins. IL-1 stimulated actin stress fiber formation in serum-starved HeLa cells in a Rho-dependent manner and rapidly activated nucleotide exchange on RhoA. Glutathione S-transferase (GST) fusion proteins, containing either the full-length IL-1R cytosolic domain (GST-IL-1Rcd) or the terminal 68 amino acids of IL-1R required for IL-1-dependent signal transduction, specifically coprecipitated both RhoA and Rac-1, but not p21(ras), from Triton-soluble HeLa cell extracts. In whole cells, a small-molecular-weight G protein coimmunoprecipitated by anti-IL-1R antibody was a substrate for C3 transferase, which specifically ADP-ribosylates Rho GTPases. Constitutively activated RhoA, loaded with [gamma-32P]GTP, directly interacted with GST-IL-1Rcd in a filter-binding assay. The IL-1Rcd-RhoA interaction was functionally important, since a dominant inhibitory mutant of RhoA prevented IL-1Rcd-directed transcriptional activation of the IL-6 gene. Consistent with our previous data demonstrating that IL-1R-associated myelin basic protein (MBP) kinases are necessary for IL-1-directed gene expression, cellular incorporation of C3 transferase inhibited IL-1R-associated MBP kinase activity both in solution and in gel kinase assays. In summary, IL-1 activated RhoA, which was physically associated with IL-1Rcd and necessary for activation of cytosolic nuclear signaling pathways. These findings suggest that IL-1-stimulated, Rho-dependent cytoskeletal reorganization may cluster signaling molecules in specific architectures that are necessary for persistent cell activation in chronic inflammatory disease.

Requirement for geranylgeranyl transferase I and acyl transferase in the TGF-beta-stimulated pathway leading to elastin mRNA stabilization

The TGF-betas are multipotent in their biological activity, modulating cell growth and differentiation as well as extracellular matrix deposition and degradation. Most of these activities involve modulation of gene transcription. However, TGF-beta1 has been shown previously to substantially increase the expression of elastin by stabilization of tropoelastin mRNA through a signaling pathway which involves a phosphatidylcholine-specific phospholipase and a protein kinase C. The present results, through the use of specific inhibitors of geranylgeranyl transferase I, farnesyl transferase, and acyl transferase, demonstrate that geranylgeranylated and acylated, but not farnesyslated protein(s) is required for this TGF-beta1 effect. In addition, the general tyrosine kinase inhibitor genistein completely blocked this TGF-beta1 effect. The results suggest that the TGF-beta1 signaling pathway requires not only receptor ser/thr kinase activity, but also tyrosine kinase and small GTPase activities.

Differential effects of lovastatin on mitogen induced calcium influx in human cultured vascular smooth muscle cells

1. In this study the effect of lovastatin, an inhibitor of cholesterol and isoprenoid synthesis, on the rises in intracellular calcium concentration ([Ca2+]i) induced by platelet derived growth factor BB (PDGF-BB), angiotensin II (AII), low density lipoproteins (LDL) and foetal calf serum (FCS) was examined in human cultured vascular smooth muscle cells (VSMC) from saphenous vein. Changes in [Ca2+]i were measured in cell suspensions by the Ca2+ sensitive probe, fura 2. 2. Incubation with lovastatin for 24-26 h markedly reduced the peak rise and sustained phase of [Ca2+]i elevation in response to PDGF-BB but the responses to AII, LDL and FCS were unaffected. Further experiments showed that lovastatin pretreatment inhibited PDGF-BB induced Ca2+ influx but not intracellular Ca2+ release. This inhibition could be overcome by co-incubation with mevalonic acid. 3. Pretreatment of cells with the heterotrimeric G protein inhibitor pertussis toxin for up to 24 h completely abolished AII-induced [Ca2+]i rises but the response to PDGF-BB was unaffected. 4. The tyrosine kinase inhibitor genistein largely abolished PDGF-BB-induced [Ca2+]i elevation but had no significant effect on AII-induced responses. 5. Pre-incubation with lovastatin had no effect on the level of tyrosine phosphorylation of PDGF-beta receptors (as measured by Western blot) in response to the PDGF-BB ligand. 6. PDGF-BB elicits Ca2+ influx via a tyrosine kinase-dependent mechanism distinct from the heterotrimeric G protein coupled pathway utilized by AII. Lovastatin most likely acts by inhibition of isoprenylation (via blockade of isoprenoid synthesis) of an intermediate molecule involved in PDGF-BB-induced Ca2+ influx.

Thrombin induces apoptosis in cultured neurons and astrocytes via a pathway requiring tyrosine kinase and RhoA activities

Thrombin activity is a factor in acute CNS trauma and may contribute to such chronic neurodegenerative diseases as Alzheimer's disease. Thrombin is a multifunctional serine protease that catalyses the final steps in blood coagulation. However, increasing evidence indicates that thrombin also elicits a variety of cellular and inflammatory responses, including responses from neural cells. Most recently, high concentrations of thrombin were shown to cause cell death in both astrocyte and hippocampal neuron cultures. The purpose of this study was to determine the mechanisms underlying thrombin-induced cell death. Our data show that thrombin appears to cause apoptosis as evidenced by cleavage of DNA into oligonucleosomal-sized fragments, fragmentation of nuclei, and prevention of death by inhibition of protein synthesis. Synthetic peptides that directly activate the thrombin receptor also induced apoptosis, indicating that thrombin-induced cell death occurred via activation of the thrombin receptor. The signal transduction cascade involves tyrosine and serine/threonine kinases and an intact actin cytoskeleton. Additional study revealed the involvement of the small GTP-binding protein RhoA. Thrombin induced RhoA activity in both astrocytes and hippocampal neurons, and inhibition of RhoA activity with exoenzyme C3 attenuated cell death, indicating that thrombin activation of RhoA was necessary for thrombin-induced cell death. Tyrosine kinase inhibitors blocked thrombin induction of RhoA, indicating that tyrosine kinase activity was required upstream of RhoA. These data suggest a sequential linkage of cellular events from which we propose a model for the second messenger cascade induced by thrombin in neural cells that can lead to apoptosis.

Role of Rho family proteins in phospholipase D activation by growth factors

Treatment of fibroblasts with growth factors results in activation of phospholipase D (PLD). In order to determine the role of the Rho family of small GTPases in growth factor-mediated PLD activation, we used cells transfected with wild type and mutant Rac1. In response to epidermal growth factor (EGF), PLD activity was greatly increased in Rat1 fibroblasts expressing wild type Rac1 (wtRac1), and completely abrogated in cells expressing dominant negative N17Rac1, consistent with Rac1 mediating the action of this growth factor. In contrast, in cells treated with platelet-derived growth factor (PDGF) or phorbol ester, the wtRac1 cells showed little or no enhancement of PLD activity, and the response was not affected in the N17Rac1 cells, implying that Rac1 played a minimal role in the activation of PLD by PDGF or protein kinase C. Both growth factors produced an attenuated PLD response in cells expressing constitutively active V12Rac1, but these cells showed other changes, including altered morphology, increased basal PLD, and decreased growth factor receptor autophosphorylation. The effects of EGF and PDGF on phosphoinositide phospholipase C activity were not enhanced in cells expressing wtRac1 or inhibited in those expressing N17Rac1. In cells expressing constitutively active V12Rac1, basal phosphoinositide phospholipase C was elevated, but there were no significant effects of EGF or PDGF. We used C3 transferase of Clostridium botulinum, which ADP-ribosylates and inactivates RhoA, to investigate the involvement of RhoA in the activation of PLD by PDGF. Cells expressing wtRac1 and N17Rac1 showed a decreased PLD in response to PDGF when treated with C3 transferase, indicating a role for RhoA. In summary, these data indicate a major role for Rac1 in the activation of PLD by EGF, but not PDGF or protein kinase C.

Platelet-derived growth factor receptor tyrosine phosphorylation requires protein geranylgeranylation but not farnesylation

We have used specific inhibitors for farnesyltransferase (FTase) and geranylgeranyltransferase (GGTase) I as well as combinations of lovastatin with geranylgeraniol (GGOH) or farnesol (FOH) to investigate the role of protein prenylation in platelet-derived growth factor (PDGF)-induced PDGF receptor tyrosine phosphorylation. NIH-3T3 cells treated with the highly specific FTase inhibitor FTI-277 had no effect on PDGF receptor tyrosine phosphorylation or PDGF activation of mitogen-activated protein kinase (MAPK) at doses that completely inhibit FTase-dependent processing. In contrast, treatment of these cells with GGTase I inhibitor GGTI-298 strongly inhibited receptor tyrosine phosphorylation, and co-treatment with FTI-277 had no additional effect. Interestingly, the inhibitory effect of GGTI-298 on PDGF activation of MAPK was only partial. Furthermore, although lovastatin, which inhibits both protein geranylgeranylation and protein farnesylation, blocked PDGF receptor tyrosine phosphorylation, co-treatment with GGOH, but not FOH, reversed the lovastatin block. In addition, although lovastatin was observed to block MAPK activation by PDGF, co-treatment with GGOH, but not FOH, restored its activation. Further investigations indicated that inhibition of receptor tyrosine phosphorylation was not due to decreased expression of the receptor or to inhibition of GGTase II. Thus, these results demonstrate that PDGF receptor tyrosine phosphorylation requires protein geranylgeranylation but not protein farnesylation and that the tyrosine phosphorylation levels of the receptor are modulated by a protein that is a substrate for GGTase I.

Ca2+ channel activation by platelet-derived growth factor-induced tyrosine phosphorylation and Ras guanine triphosphate-binding proteins in rat glomerular mesangial cells

We investigated the signaling pathways mediating 1-pS Ca2+ channel activation by PDGF in cultured rat mesangial cells. In cell-attached patches, intrapipette PDGF-BB (PDGF B chain homodimer isoform) (50 ng/ml) dramatically stimulates channel activity (P < 0.003, n = 6). Tyrosine kinase inhibition (100 microM genistein or 10 microM tryphostin 9) abolished PDGF-induced channel activation (P < 0.02, n = 6). In excised patches, the effect of tyrosine kinase inhibition could be reversed by 200 microM GTPgammaS (P < 0.02, n = 4). In contrast, 200 microM GDPbetaS inhibited PDGF-induced channel activity (P < 0.04, n = 6). Pertussis toxin (250 ng/ml) had no effect on PDGF-induced channel activity (P = 0.45, n = 6). When excised patches were exposed to anti-Ras antibody (5 microg/ml), PDGF-induced channel activity was abolished (P < 0.002, n = 11). Western immunoblots revealed that PDGF-BB binding stimulates the formation of a membrane-bound complex consisting of growth factor receptor-binding protein 2, son of sevenless, and the PDGF-beta receptor. Complex formation was abolished by genistein. In mesangial cells, the intrinsic tyrosine kinase activity of the PDGF-beta receptor stimulates the formation of a membrane-bound growth factor receptor-binding protein 2/son of sevenless/PDGF-beta receptor complex and activation of the pertussis toxin-insensitive GTP-binding protein, p21-Ras, which leads to the opening of 1-pS Ca2+ channels.