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

Crosstalk Between Cardiac Cells and Macrophages Postmyocardial Infarction: Insights from In Vitro Studies

Cardiovascular disease, including myocardial infarction (MI), is the leading cause of death in the western world. Following MI, a large number of cardiomyocytes are lost and inflammatory cells such as monocytes and macrophages migrate into the damaged region to remove dead cells and tissue. These inflammatory cells secrete growth factors to induce degradation of the extracellular matrix in the myocardium and recruit cardiac fibroblasts. However, the contribution of specific macrophage subsets on cardiac cell function and survival in the steady state as well as in the diseased state is not well known. There is an increasing demand for in vitro cardiac disease models to bridge the critical missing link in the existing experimental methods. In this review, studies using in vitro models to examine the interaction between macrophages and cardiac cells, including cardiomyocytes, endothelial cells, and fibroblasts, are summarized to better understand the complex inflammatory cascade post-MI. The current challenges and the future directions of in vitro cardiac models are also discussed. Detailed and more mechanistic insights into macrophages and cardiac cell interactions during the multiphase repair process could potentially revolutionize the development of treatments and diagnostic alternatives. Impact statement The inflammatory cascade postmyocardial infarction (MI) is very complex. In vitro cardiac disease model studies bridge the critical missing link in the existing experimental methods and provide insights, including multicellular interaction post-MI. Detailed and more mechanistic insights into macrophages and cardiac cell interactions during the multiphase repair process could potentially revolutionize in developing treatments and diagnostic alternatives.

Curcumin Promotes Collagen Type I, Keratinocyte Growth Factor-1, and Epidermal Growth Factor Receptor Expressions in the In Vitro Wound Healing Model of Human Gingival Fibroblasts

OBJECTIVE

 Curcumin promotes oral wound healing; however, the underlying mechanism remains unknown. We hypothesized that curcumin may regulate gene expression in human gingival fibroblasts (hGFs). This study investigated the effect of curcumin on the expression of wound healing-related genes, collagen type I (COL1), keratinocyte growth factor (KGF)-1, and epidermal growth factor receptor (EGFR), in the in vitro wound healing model of hGFs, as well as the signaling pathway involved in the regulation of these genes by curcumin.

MATERIALS AND METHODS

 The hGFs were treated with curcumin in the unwounded condition and in the in vitro wound healing model (scratch assay). Gene expression was determined by quantitative polymerase chain reaction. PD98059 was used to elucidate whether extracellular signal regulated kinase (ERK) signaling is involved in the curcumin-regulated gene expression in hGFs. Cell migration was also analyzed by the scratch assay.

STATISTICAL ANALYSIS

 Data were analyzed by independent t-test or one-way analysis of variance (ANOVA) followed by Tukey's Honestly Significant Difference ( HSD) test.

RESULTS

 In unwounded hGFs, curcumin significantly increased KGF-1 and EGFR expressions but not COL1 mRNA expression. Interestingly, curcumin significantly upregulated COL1, KGF-1, and EGFR expressions in the in vitro wound healing model. Furthermore, PD98059 significantly decreased the curcumin-induced COL1 and EGFR expressions, but did not significantly affect KGF-1 upregulation by curcumin. However, hGF migration was not affected by curcumin treatment.

CONCLUSION

 Curcumin induced KGF-1 and EGFR expressions in unwounded hGFs. In the in vitro wound healing model, curcumin upregulated COL1 and EGFR expression via the ERK pathway and increased KGF-1 expression, possibly by an ERK-independent mechanism.

Tissue-specific role of Nrf2 in the treatment of diabetic foot ulcers during hyperbaric oxygen therapy

Hyperbaric oxygen (HBO) therapy is proven to be very successful for diabetic foot ulcer (DFU) treatment due to its antimicrobial effect, increased angiogenesis and enhanced collagen synthesis. The molecular mechanism underlying HBO therapy particularly the involvement of Nrf2 in the wound healing process was investigated in the present study. In addition, we have studied the levels of angiogenic markers in ulcer tissues and their correlation with Nrf2 during HBO therapy compared with standard therapy (Non-HBO) for DFU. A total of 32 Patients were recruited and randomized to standard wound care procedure alone (n = 17) or HBO therapy in combination with standard wound care procedure (n = 15) for 20 days. Our results showed that the tissue levels of Nrf2 along with its downstream targets were significantly increased in patients who underwent HBO therapy when compared to Non-HBO therapy. Further, HBO therapy induced angiogenesis as assessed by increased levels of angiogenesis markers such as EGF, VEGF, PDGF, FGF-2 and CXCL10 in the tissue samples. The expressions of eNOS and nitrite concentrations were also significantly increased in HBO therapy when compared to Non-HBO therapy subjects. Moreover, HBO therapy sensitises the macrophages to release FGF-2 and EGF thereby promotes angiogenesis. Further, it increased the levels of neutrophil attractant CXCL-8 thereby promotes the release of chemokine CCL2, a well-known mediator of neovascularization. The Pearson correlation showed that Nrf2 has a positive correlation with EGF, VEGF and PDGF. In conclusion, the findings of the present study suggest that HBO therapy promotes wound healing by increasing oxygen supply and distribution to damaged tissues, stimulating angiogenesis, decreasing inflammation, and increasing the nitrite levels. Increased levels of Nrf2 transiently regulate the expression of angiogenic genes in wound biopsies, which may result in accelerated healing of chronic wounds.

Far-infrared radiation stimulates platelet-derived growth factor mediated skeletal muscle cell migration through extracellular matrix-integrin signaling

Despite increased evidence of bio-activity following far-infrared (FIR) radiation, susceptibility of cell signaling to FIR radiation-induced homeostasis is poorly understood. To observe the effects of FIR radiation, FIR-radiated materials-coated fabric was put on experimental rats or applied to L6 cells, and microarray analysis, quantitative real-time polymerase chain reaction, and wound healing assays were performed. Microarray analysis revealed that messenger RNA expressions of rat muscle were stimulated by FIR radiation in a dose-dependent manner in amount of 10% and 30% materials-coated. In 30% group, 1,473 differentially expressed genes were identified (fold change [FC] > 1.5), and 218 genes were significantly regulated (FC > 1.5 and p < 0.05). Microarray analysis showed that extracellular matrix (ECM)-receptor interaction, focal adhesion, and cell migration-related pathways were significantly stimulated in rat muscle. ECM and platelet-derived growth factor (PDGF)-mediated cell migration-related genes were increased. And, results showed that the relative gene expression of actin beta was increased. FIR radiation also stimulated actin subunit and actin-related genes. We observed that wound healing was certainly promoted by FIR radiation over 48 h in L6 cells. Therefore, we suggest that FIR radiation can penetrate the body and stimulate PDGF-mediated cell migration through ECM-integrin signaling in rats.

PDGFRA mRNA is overexpressed in oral cancer patients as compared to normal subjects with a significant trend of overexpression among tobacco users

BACKGROUND

Platelet-derived growth factors alpha (PDGFA) is a tyrosine kinase receptor activator which is known to be amplified in the malignancies, and their expression levels are correlated to tumor progression and reduced overall survival. The expression of PDGFRA is different among the tumors and normal tissues; furthermore, their expression level is site specific. Under a physiological condition, PDGFRA and its ligand are expressed in distinct cell populations and activated in a paracrine manner. Nevertheless, heterodimer characteristic of PDGFRA allows it to be trans-activated by non-specific ligands or via autocrine manner. The future of cancer therapy can be based on PDGFRA receptor blockade and therefore warrants further investigation to determine the differing expression of PDGFRA between controls and patients with oral squamous cell carcinoma (OSCC).

METHODS

We performed a case-control study of 111 patients with newly diagnosed tongue squamous cell carcinoma and 111 control subjects without a cancer diagnosis, matched for age and gender, to evaluate the association between PDGFRA expression levels in oral mucosa. We then performed smoking stratification in each cohort. Independent t test analysis was applied for case-control comparisons.

RESULTS

Mean value of PDGFRA mRNA level (-ΔCt) for normal cohort is -30.242, whereas mean value of PDGFRA mRNA level for patients with OSCC is -11.516. PDGFRA mRNA level (-ΔCt) was significantly higher in oral cancer cohort, P<.001. Smokers have a significantly higher PDGFRA mRNA expression in comparison with non-smokers (P=.002) among the non-cancer group. Likewise, this trend is observed in cancer cohort too, P=.044.

CONCLUSION

PDGFRA expression is significantly higher in oral cancer cohort with or without the establishment of tobacco risk factor.

Anti-staphylococcal Activity of Injectable Nano Tigecycline/Chitosan-PRP Composite Hydrogel Using Drosophila melanogaster Model for Infectious Wounds

Compared to the current treatment modalities, the use of an injectable hydrogel system, loaded with antibiotic encapsulated nanoparticles for the purpose of treating Staphylococcus aureus (S. aureus) chronic wound infections have several advantages. These include adhesiveness to infection site, reduced frequency of dressings, sustained drug release, inhibition of bacterial growth, and increased healing. In the present work tigecycline nanoparticles were loaded into chitosan-platelet-rich plasma (PRP) hydrogel. The tigecycline nanoparticles (95 ± 13 nm) were synthesized through ionic cross-linking method using chitosan, tripolyphosphate, and tigecycline and characterized by dynamic light scattering (DLS), scanning electron microscope (SEM), and Fourier transform infrared spectroscopy (FT-IR). The synthesized nanoparticles and activated PRP powder were mixed with chitosan hydrogel to form a homogeneous gel. Rheology studies have confirmed the shear thinning property, thermal stability, and injectability of the prepared gel systems. The gel system was further assessed for its drug release property and found that it was released in a sustained manner. Hemolysis and blood-clotting assays demonstrated that the gel system was neither a hemolysin nor a hamper to the clotting cascade. Cell viability results showed that these nanoparticles were cyto-compatible. The bioactivity of PRP loaded chitosan gel toward fibroblast cell line was studied using cell proliferation and migration assay. In vitro antibacterial studies revealed that the gel system inhibited bacterial growth to a great extent. The antibacterial activity was further analyzed using ex vivo porcine skin assay. In vivo anti-Staphylococcal activity of the prepared hydrogels was studied using a Drosophila melanogaster infection model. The tigecycline and tigecycline nanoparticle incorporated chitosan gel showed a significant antibacterial activity against S. aureus. Thus, the gel system is an effective medium for antibiotic delivery and can be applied on the infection sites to effectively forestall various skin infections caused by S. aureus.

Epidermal growth factor inhibition, a novel pathway to prevent chronic allograft injury

BACKGROUND

Chronic allograft injury remains a major problem in clinical kidney transplantation and different growth factors participate in its development. Epidermal growth factor (EGF) affects cell proliferation and mitogenesis through its tyrosine kinase receptor. Erlotinib is an orally administered tyrosine kinase inhibitor used in clinical oncology to inhibit EGF signaling. We investigated its effect on the development of chronic allograft injury in an experimental kidney transplantation model.

METHODS

Kidney transplantations were performed between Dark Agouti and Wistar Furth rats. Recipients were immunosuppressed either with cyclosporine A (CsA, 1.5 mg/kg/day subcutaneously) or with CsA and erlotinib (10 mg/kg/day orally). Kidney grafts were harvested after 5 and 90 days for histology and immunohistochemistry. Aorta denudation model was used for the erlotinib dose response study to define the optimal dose for the transplantation study.

RESULTS

Epidermal growth factor expression was increased in CsA-treated allografts which developed intense chronic changes on day 90. Erlotinib ameliorated neointimal formation in the dose response study. In addition, erlotinib decreased chronic rejection changes and maintained better graft function in kidney transplantation model. Late posttransplant EGF and EGF receptor levels were reduced with erlotinib.

CONCLUSION

Based on these findings, EGF mediates in part the development of chronic allograft injury. Its inhibition with erlotinib prevents chronic rejection and maintains better allograft function. Therefore, EGF blocking by erlotinib provides a novel pathway to prevent chronic allograft injury.

Honey exposure stimulates wound repair of human dermal fibroblasts

Honey is widely used for treating burns, ulcers and wounds, but the mechanisms of action are poorly known and the product is mainly used as an antimicrobial. We have examined here the wound healing properties of honey on human fibroblasts, using an in vitro scratch wound healing model. Three kinds of widely used monofloral honeys were used, viz. acacia (Robinia pseudacacia), buckwheat (Fagopyrum sp.), and manuka (Leptospermum scoparium). Data displayed an increased wound healing activity in fibroblasts, but with different efficiency and mechanisms of action among honeys. The effects of acacia and buckwheat emerged in both scratch wound and chemotaxis assays, while the effect of manuka was significant but lower. The use of inhibitors indicated on the whole an essential role of cytosolic calcium, an important role of ERK and p38, and a secondary role of PI3K. Acacia and buckwheat, but not manuka, induced significant increases in the release of interleukin-4 (IL-4), IL-6, and IL-8, indicating a correlation between interleukin upregulation and wound closure efficiency. This is consistent with our previous findings suggesting a higher ability of acacia and buckwheat to activate keratinocyte reepithelialization, with respect to manuka honey. In conclusion, our data indicate that acacia and buckwheat honeys are particularly efficient in facilitating fibroblast wound closure activities, suggesting new therapeutic possibilities for this natural product.

Multifarious functions of PDGFs and PDGFRs in tumor growth and metastasis

Platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) are frequently expressed in various tumors and their expression levels correlate with tumor growth, invasiveness, drug resistance, and poor clinical outcomes. Emerging experimental evidence demonstrates that PDGFs exhibit multiple functions in modulation of tumor growth, metastasis, and the tumor microenvironment by targeting malignant cells, vascular cells, and stromal cells. Understanding PDGF-PDGFR-mediated molecular signaling may provide new mechanistic rationales for optimizing current cancer therapies and the development of future novel therapeutic modalities.

The abluminal endothelial membrane in neurovascular remodeling in health and disease

After brain injury, blood-brain barrier (BBB) integrity can be compromised as a consequence of the breakdown of cell-cell interactions in the neurovascular unit, resulting in the loss of the characteristic endothelial luminal-to-abluminal structure. During the process of restoration of the BBB and vascularization, the endothelial cells are continuously reshaped, with both the luminal and abluminal membranes serving as sites of signaling. Here, we focus on the bidirectional signaling processes that are rapidly initiated between endothelial and perivascular cells and occur in certain brain diseases or in response to injury. The goal of these processes is (i) the reemergence of endothelial cell polarity, (ii) the remodeling of extracellular matrix interactions, (iii) the realignment of pericytes and astrocytic endfeet with endothelial cells, and (iv) the restitution of a well-organized and stable BBB. This abluminal membrane exemplifies how the brain vasculature responds to stressors and may represent promising targets for therapeutic interventions of brain diseases.

Differential effect of growth factors on invasion and proliferation of endocrine resistant breast cancer cells

We have established several breast cancer cell lines that exhibit a permanent ER-depleted phenotype, induced by shRNA transfection of MCF-7 cells, which afford a useful model for studying acquired endocrine resistance. Previously we showed that MDA-231 as well as ER-silenced cells could invade through simulated extracellular matrix components. However, the contribution of individual serum components responsible for cell invasion was not determined. In the present study, an under-agarose gel assay was used to quantitatively assess the invasive movement of two ER-silenced cell lines (pII and YS2.5) in comparison to the parental MCF-7, the ER negative MDA-231, and normal HBL100 cells, as well as a line that was ER-shRNA transfected but failed to exhibit ER down-regulation (YS1.2). We also examined the effect of the growth factors EGF, IGF-1, TGFβ, PDGFC and RANTES on pII cell invasion and proliferation. All breast cancer cell lines which had reduced ER expression exhibited a serum-dependent invasive ability related to the degree of induced ER loss. TGFβ treatment inhibited pII cell proliferation and enhanced their invasive ability but at a relatively high dose. IGF-1 and EGF enhanced pII cell proliferation, with the latter playing the major role in promoting cell invasion. PDGFC did not affect either process although it is highly expressed in pII cells. Differential effects were observed on activation of Akt and ERK1/2 suggesting their involvement as intracellular mediators of EGF induced invasion, in part through the regulation of matrix metalloproteinase activity. Targeting EGF receptor tyrosine kinase activity by erlotinib resulted in significant inhibition of both pII cell proliferation and directional invasion towards EGF suggesting that this drug has potential therapeutic usefulness for preventing spread of particularly endocrine resistant breast cancer.

Sunitinib induces PTEN expression and inhibits PDGFR signaling and migration of medulloblastoma cells

We previously showed that inhibition of the platelet-derived growth factor receptor (PDGFR) blocks the survival and migration of medulloblastoma cells. Identification of in vitro PDGFR-targeting pharmacologic agents that are suitable for preclinical testing in medulloblastoma models in vivo will be critical for efficiently translating these agents to clinical investigation in children with medulloblastoma. In this study, we investigated whether the multi-tyrosine kinase inhibitor sunitinib, effectively inhibits PDGFR signaling required for medulloblastoma cell migration. Daoy and D556 human medulloblastoma cells pre-treated for 1 h with 0.2 μM sunitinib demonstrated induction of PTEN expression and significant inhibition of PDGFR signaling activity and transactivation of EGFR, in a RAS-independent manner, in response to PDGF-BB stimulation. Sunitinib pre-treatment markedly reduced medulloblastoma cell migration in response to both PDGF-BB and 10% serum at 4 and 24 h after treatment. Pre-treatment with sunitinib for 1 h also resulted in detachment and decreased viability of D556, but not Daoy, cells and only after 48 h following treatment. However, sunitinib did not induce apoptosis in either cell line at any time point, indicating that the anti-migratory effects of sunitinib were not due to impeding cell survival. Sunitinib similarly inhibited PDGFR signaling and migration of primary murine Smo/Smo medulloblastoma cells, suggesting that the Smo/Smo mouse is an appropriate model for preclinical testing of sunitinib. These results indicate that sunitinib may be an important pharmacologic agent for the treatment of invasive medulloblastoma, particularly given evidence of its ability to cross the blood-brain barrier to target tumor cells, and thus warrants further in vivo testing for confirmation of efficacy.

Platelet lysate promotes in vitro wound scratch closure of human dermal fibroblasts: different roles of cell calcium, P38, ERK and PI3K/AKT

There is a growing interest for the clinical use of platelet derivates in wound dressing. Platelet beneficial effect is attributed to the release of growth factors and other bioactive substances, though mechanisms are mostly unknown. We studied wound-healing processes of human primary fibroblasts, by exposing cells to a platelet lysate (PL) obtained from blood samples. Crystal violet and tetrazolium salt (MTS) assays showed dose-response increase of cell proliferation and metabolism. In scratch wound and transwell assays, a dose of 20% PL induced a significant increase of wound closure rate at 6 and 24 hrs, and had a strong chemotactic effect. BAPTA-AM, SB203580 and PD98059 caused 100% inhibition of PL effects, whereas wortmannin reduced to about one third the effect of PL on wound healing and abolished the chemotactic response. Confocal imaging showed the induction by PL of serial Ca2(+) oscillations in fibroblasts. Data indicate that cell Ca2(+) plays a fundamental role in wound healing even without PL, p38 and ERK1/2 are essential for PL effects but are also activated by wounding per se, PI3K is essential for PL effects and its downstream effector Akt is activated only in the presence of PL. In conclusion, PL stimulates fibroblast wound healing through the activation of cell proliferation and motility with different patterns of involvement of different signalling pathways.

Injury-induced platelet-derived growth factor receptor-alpha expression mediated by interleukin-1beta (IL-1beta) release and cooperative transactivation by NF-kappaB and ATF-4: IL-1beta facilitates HDAC-1/2 dissociation from promoter

Platelet-derived growth factors are a family of potent mitogens and chemoattractants for fibroblasts and other cells of mesenchymal origin. Platelet-derived growth factor (PDGF) dimeric ligands (composed of A-, B-, C-, and D-chains) exert their biological activity through high affinity interactions with cell surface receptor subunits (alpha and beta). PDGF-receptor-alpha is widely implicated in the pathogenesis of hyperplastic fibrotic disease, yet the molecular mechanisms controlling its expression in response to injury are poorly understood. Here we show that PDGF-R alpha expression is induced in fibroblasts by mechanical injury and interleukin (IL)-1beta, which was abolished by neutralizing IL-1beta antibodies in the culture supernatant or inhibitors of NF-kappaB. Chromatin immunoprecipitation and electrophoretic mobility shift assays revealed the existence of a new NF-kappaB binding site at -531/-521 bp in the PDGF-R alpha promoter. We have recently shown that ATF-4 is also induced by injury (Malabanan, K. P., Kanellakis, P., Bobik, A., and Khachigian, L. M. (2008) Circ. Res. 103, 378-387), and we demonstrate here that ATF-4 binds a novel element -259/-254 and stimulates PDGF-R alpha transcription. ATF-4 and NF-kappaB interact, occupy the PDGF-R alpha promoter, and induce PDGF-R alpha transcription in a cooperative manner. IL-1beta facilitates the dissociation of histone deacetylase (HDAC)-1/2 from the PDGF-R alpha promoter, whereas the HDAC inhibitors suberoylanilide hydroxamic acid and trichostatin A potentiate IL-1beta induction of PDGF-R alpha transcription. These findings, taken together, demonstrate that injury stimulates IL-1beta secretion by fibroblasts, which activates NF-kappaB and ATF-4 and stimulates interaction with the PDGF-R alpha promoter, triggering PDGF-R alpha transcription. Physical and functional interactions between NF-kappaB and ATF-4 have not been reported in any gene. This is also the first report of HDAC regulation of PDGF-R alpha transcription.

Imatinib blocks migration and invasion of medulloblastoma cells by concurrently inhibiting activation of platelet-derived growth factor receptor and transactivation of epidermal growth factor receptor

Platelet-derived growth factor (PDGF) receptor (PDGFR) expression correlates with metastatic medulloblastoma. PDGF stimulation of medulloblastoma cells phosphorylates extracellular signal-regulated kinase (ERK) and promotes migration. We sought to determine whether blocking PDGFR activity effectively inhibits signaling required for medulloblastoma cell migration and invasion. DAOY and D556 human medulloblastoma cells were treated with imatinib mesylate (Gleevec), a PDGFR tyrosine kinase inhibitor, or transfected with small interfering RNA (siRNA) to PDGFRB to test the effects of blocking PDGFR phosphorylation and expression, respectively. PDGFR cell signaling, migration, invasion, survival, and proliferation following PDGF-BB stimulation, with and without PDGFR inhibition, were measured. PDGF-BB treatment of cells increased PDGFRB, Akt and ERK phosphorylation, and transactivated epidermal growth factor receptor (EGFR), which correlated with enhanced migration, survival, and proliferation. Imatinib (1 μmol/L) treatment of DAOY and D556 cells inhibited PDGF-BB- and serum-mediated migration and invasion at 24 and 48 h, respectively, and concomitantly inhibited PDGF-BB activation of PDGFRB, Akt, and ERK but increased PTEN expression and activity. Imatinib treatment also induced DAOY cell apoptosis at 72 h and inhibited DAOY and D556 cell proliferation at 48 h. siRNA silencing of PDGFRB similarly inhibited signaling, migration, and survival and both siRNA and imatinib treatment inhibited PDGF-BB-mediated EGFR transactivation, indicating that the effects of imatinib treatment are specific to PDGFRB target inhibition. These results indicate that PDGFRB tyrosine kinase activity is critical for migration and invasion of medulloblastoma cells possibly by transactivating EGFR; thus, imatinib may represent an important novel therapeutic agent for the treatment of medulloblastoma.

Vascular endothelial growth factor activates the Tie family of receptor tyrosine kinases

The ability of cells to respond appropriately to changes in their environment requires integration and cross-talk between relevant signalling pathways. The vascular endothelial growth factor (VEGF) and angiopoietin families of ligands are key regulators of blood vessel formation. VEGF binds to receptor tyrosine kinases of the VEGF-receptor family to activate signalling pathways leading to endothelial migration, proliferation and survival whereas the angiopoietins interact with the Tie receptor tyrosine kinases to control vessel stability, survival and maturation. Here we show that VEGF can also activate the angiopoietin receptor Tie2. Activation of human endothelial cells with VEGF caused a four-fold stimulation of tyrosine phosphorylation of Tie2. This stimulation was not due to VEGF-induction of Tie2 ligands as soluble ligand binding domain of Tie2 failed to inhibit VEGF activation of the receptor. Immunoprecipitation analysis demonstrated no physical interaction between VEGF receptors and Tie2. However Tie2 does interact with the related receptor tyrosine kinase Tie1 and this receptor was found to be essential for VEGF activation of Tie2. VEGF stimulated proteolytic cleavage of Tie1 generating a truncated Tie1 intracellular domain. Similarly, phorbol ester also both stimulated Tie1 truncation and activated Tie2 phosphorylation. Inhibition of Tie1 cleavage with the metalloprotease inhibitor TAPI-2 suppressed VEGF- and phorbol ester-induced phosphorylation of Tie2. Truncated Tie1 formed in response to VEGF was also found to be tyrosine phosphorylated and this was independent of Tie2, though Tie2 could enhance Tie1 intracellular domain phosphorylation. Together these data demonstrate that VEGF activates Tie2 via a mechanism involving proteolytic cleavage of the associated tyrosine kinase Tie1 leading to trans-phosphorylation of Tie2. This novel mechanism of receptor tyrosine kinase activation is likely to be important in integrating signalling between two of the key receptor groups regulating angiogenesis.

Constitutive EGFR signaling in oligodendrocyte progenitors leads to diffuse hyperplasia in postnatal white matter

Gliogenesis requires the careful orchestration of migration, differentiation, and proliferation of progenitors. Signaling through the epidermal growth factor receptor (EGFR) has been implicated in regulating these processes in a variety of cell types, including neural progenitors. By retroviral infection, we constitutively expressed an EGFR-GFP fusion protein in white matter glial progenitors at postnatal day 3 of the rat forebrain in vivo and analyzed the development of these cells over the subsequent 15 weeks. EGFR-GFP+ cells remained proliferative and migratory, gradually populating the brains ipsilateral and contralateral to the side of viral infection, but never differentiated into mature glia. The accumulation of these cells doubled the total cell density in white matter and led to a 10-fold increase in the abundance of glial progenitors, giving rise to a progenitor "hyperplasia." The marker profile of infected cells, NG2+, olig2+, PDGFR-alpha+, nestin+, GFAP-, and CC1-, indicated a close resemblance to oligodendrocyte progenitors. Positive immunostaining for phosphorylated EGFR colocalized with punctate accumulation of EGFR-GFP, indicating that a subset of receptors was engaged in active signaling. Furthermore, EGF was required to observe phospho-tyrosine EGFR immunostaining of glial progenitors in culture. These observations suggest that constitutive EGFR expression can inhibit glial differentiation, but requires ligand as well.

Ras activation in response to phorbol ester proceeds independently of the EGFR via an unconventional nucleotide-exchange factor system in COS-7 cells

Ras is a major mediator of PE (phorbol ester) effects in mammalian cells. Various mechanisms for PE activation of Ras have been reported [Downward, Graves, Warne, Rayter and Cantrell (1990) Nature (London) 346, 719-723; Shu, Wu, Mosteller and Broek (2002) Mol. Cell. Biol. 22, 7758-7768; Roose, Mollenauer, Gupta, Stone and Weiss (2005) Mol. Cell. Biol. 25, 4426-4441; Grosse, Roelle, Herrlich, Höhn and Gudermann (2000) J. Biol. Chem. 275, 12251-12260], including pathways that target GAPs (GTPase-activating proteins) for inactivation and those that result in activation of GEFs (guanine nucleotide-exchange factors) Sos (son of sevenless homologue) or RasGRP (RAS guanyl releasing protein). However, a biochemical link between PE and GAP inactivation is missing and GEF stimulation is hard to reconcile with the observation that dominant-negative S17N-Ras does not compromise Ras-dependent ERK (extracellular-signal-regulated kinase) activation by PE. We have addressed this controversy and carried out an in-depth biochemical study of PE-induced Ras activation in COS-7 cells. Using a cell-permeabilization approach to monitor nucleotide exchange on Ras, we demonstrate that PE-induced Ras-GTP accumulation results from GEF stimulation. Nucleotide exchange stimulation by PE is prevented by PKC (protein kinase C) inhibition but not by EGFR [EGF (epidermal growth factor) receptor] blockade, despite the fact that EGFR inhibition aborts basal and PE-induced Shc (Src homology and collagen homology) phosphorylation and Shc-Grb2 (growth-factor-receptor-bound protein 2) association. In fact, EGFR inhibition ablates basal nucleotide exchange on Ras in growth-arrested COS-7 cells. These data disclose the existence of two separate GEF systems that operate independently from each other to accomplish PE-dependent formation of Ras-GTP and to maintain resting Ras-GTP levels respectively. We document that COS-7 cells do not express RasGRP and present evidence that the PE-responsive GEF system may involve PKC-dependent phosphorylation of Sos. More fundamentally, these observations shed new light on enigmatic issues such as the inefficacy of S17N-Ras in blocking PE action or the role of the EGFR in heterologous agonist activation of the Ras/ERK pathway.

PKCdelta mediates up-regulation of NOX1, a catalytic subunit of NADPH oxidase, via transactivation of the EGF receptor: possible involvement of PKCdelta in vascular hypertrophy

NADPH oxidase is the major source of superoxide production in cardiovascular tissues. We reported previously that PG (prostaglandin) F2alpha caused hypertrophy of vascular smooth muscle cells by induction of NOX1, a catalytic subunit of NADPH oxidase. PGF2alpha-induced NOX1 expression was mediated by transactivation of the EGF (epidermal growth factor) receptor and subsequent activation of ERK (extracellular-signal-regulated kinase) 1/2, PI3K (phosphoinositide 3-kinase) and ATF-1 (activating transcription factor-1), a member of the CREB (cAMP-response-element-binding protein)/ATF family. As the receptor for PGF2alpha is known to activate PKC (protein kinase C), involvement of PKC in up-regulation of NOX1 expression was investigated in A7r5 cells. GF109203x, a non-selective inhibitor of PKC, dose-dependently suppressed the induction of NOX1 mRNA by PGF2alpha. Whereas an inhibitor of the conventional PKC, Gö 6976, and a PKCeta translocation-inhibitor peptide had no effect, an inhibitor of PKCdelta, rottlerin, significantly attenuated the PGF2alpha-induced increase in NOX1 mRNA. Gene silencing of PKCdelta by RNA interference significantly suppressed the PGF2alpha-induced increase in NOX1 mRNA, as well as phosphorylation of the EGF receptor, ERK1/2 and ATF-1. Silencing of the PKCdelta gene also attenuated the PDGF (platelet-derived growth factor)- induced increase in NOX1 mRNA and transactivation of the EGF receptor. Moreover, the augmented synthesis of the protein induced by PGF2alpha or PDGF was abolished by gene silencing of PKCdelta. These results suggest that PKCdelta-mediated transactivation of the EGF receptor is elicited not only by PGF2alpha, but also by PDGF, and that the subsequent activation of ERK1/2 and ATF-1 leads to up-regulation of NOX1 gene expression and ensuing hypertrophy in the vascular cell lineage.

Uncoupling between epidermal growth factor receptor and downstream signals defines resistance to the antiproliferative effect of Gefitinib in bladder cancer cells

Activation of the epidermal growth factor receptor (EGFR) and downstream signaling pathways, such as phosphatidylinositol-3 kinase/Akt and Ras/mitogen-activated protein kinase (MAPK), have been implicated in causing resistance to EGFR-targeted therapy in solid tumors, including the urogenital tumors. To investigate the mechanism of resistance to EGFR inhibition in bladder cancer, we compared EGFR tyrosine kinase inhibitor (Gefitinib, Iressa, ZD1839) with respect to its inhibitory effects on three kinases situated downstream of EGFR: MAPK, Akt, and glycogen synthase kinase-3beta (GSK-3beta). We found that the resistance to the antiproliferative effects of gefitinib, in vitro as well as in vivo in nude mice models, was associated with uncoupling between EGFR and MAPK inhibition, and that GSK-3beta activation and degradation of its target cyclin D1 were indicators of a high cell sensitivity to gefitinib. Further analysis of one phenotypic sensitive (253J B-V) and resistant (UM-UC13) cell lines revealed that platelet-derived growth factor receptor-beta (PDGFRbeta) activation was responsible for short circuiting the EGFR/MAPK pathway for mitogenic stimuli. However, invasion as well as actin dynamics were efficiently reduced by EGFR inhibition in UM-UC13. Chemical disruption of signaling pathways or of PDGFR kinase activity significantly reduced the inactive pool of cellular GSK-3beta in UM-UC13 cells. In conclusion, our data show that the uncoupling of EGFR with mitogenic pathways can cause resistance to EGFR inhibition in bladder cancer. Although this uncoupling may arise through different mechanisms, we suggest that the resistance of bladder cancer cells to EGFR blockade can be predicted early in the course of treatment by measuring the activation of GSK-3beta and of nuclear cyclin D1.

An MT1-MMP-PDGF receptor-beta axis regulates mural cell investment of the microvasculature

Platelet-derived growth factor (PDGF)/PDGFRbeta-dependent investment of the vascular endothelium by mural cells (i.e., pericytes and vascular smooth muscle cells; VSMCs) is critical for normal vessel wall structure and function. In the developing vasculature, mural cell recruitment is associated with the functionally undefined expression of the type I transmembrane proteinase, membrane-type 1 matrix metalloproteinase (MT1-MMP). In this paper, using VSMCs and tissues isolated from gene-targeted mice, we identify MT1-MMP as a PDGF-B-selective regulator of PDGFRbeta-dependent signal transduction and mural cell function. In VSMCs, catalytically active MT1-MMP associates with PDGFRbeta in membrane complexes that support the efficient induction of mitogenic signaling by PDGF-B in a matrix metalloproteinase inhibitor-sensitive fashion. In contrast, MT1-MMP-deficient VSMCs display PDGF-B-selective defects in chemotaxis and proliferation as well as ERK1/2 and Akt activation that can be rescued in tandem fashion following retroviral transduction with the wild-type protease. Consistent with these in vitro findings, MT1-MMP-deficient brain tissues display a marked reduction in mural cell density as well as abnormal vessel wall morphology similar to that reported in mice expressing PDGF-B or PDGFRbeta hypomorphic alleles. Together, these data identify MT1-MMP as a novel proteolytic modifier of PDGF-B/PDGFRbeta signal transduction that cooperatively regulates vessel wall architecture in vivo.

Calcitriol inhibits growth response to Platelet-Derived Growth Factor-BB in human prostate cells

Calcitriol, a hormonal form of Vitamin D, regulates growth of normal and cancer cells of various origins by modulation of peptide growth factors signaling. Platelet-Derived Growth Factor (PDGF) signaling pathway is involved in prostate cancer progression. We studied the expression of PDGF receptors in human prostate primary stromal cells and cancer epithelial cell lines and growth response to PDGF-BB isoform. We found that the expression of PDGF receptors and PDGF-BB-mediated cell growth are regulated by calcitriol in prostate cells. Quantitative RT-PCR analysis revealed a lower level of mRNA for PDGF receptors in LNCaP and PC-3 cells than in primary stromal cells. Western blotting showed a high amount of PDGFRalpha and beta proteins in primary stromal cells that could not be detected in LNCaP, which may explain the resistance of LNCaP cells to growth-promoting effect of PDGF-BB. Addition of Epidermal Growth Factor (EGF) to the culture medium induces the expression of PDGFRbeta and restores responsiveness of LNCaP to PDGF-BB to some extent. Calcitriol down-regulates PDGFRbeta expression and negatively regulates PDGF-mediated cell growth. Calcitriol does not affect PDGFRalpha and PDGF-B mRNA expression. We suggest that inhibition of PDGFRbeta expression by calcitriol might reduce responsiveness of prostate cells to mitogenic action of PDGF-BB.

The interplay between Src family kinases and receptor tyrosine kinases

Src family tyrosine kinases (SFKs) are involved in a diverse array of physiological processes, as highlighted in this review. An overview of how SFKs interact with, and participate in signaling from, receptor tyrosine kinases (RTKs) is discussed. And also, how SFKs are activated by RTKs, and how SFKs, in turn, can activate RTKs, as well as how SFKs can promote signaling from growth factor receptors in a number of ways including participation in signaling pathways required for DNA synthesis, control of receptor turnover, actin cytoskeleton rearrangements and motility, and survival are discussed.

The interface between ErbB and non-ErbB receptors in tumor invasion: clinical implications and opportunities for target discovery

The molecular switches by which malignant cancer cells evolve from a confined to an invasive state are poorly understood, but seem to involve a progressive activation of a signaling network shared by several growth factor receptors and non-receptor molecules. Abnormal expression of ErbB tyrosine kinase receptors, commonly seen in cancer, is an early event in the invasive process, which makes these receptors exciting targets for drug discovery. The past few years have been full of promise for ErbB targeting in the context of receptor overexpression, but also fraught with disappointment as clinical efficacy has often been hampered by potential problems such as the heterogeneity of receptor expression within the same tumor, and the extensive cooperative signaling among ErbB and non-ErbB receptors. Cooperative signaling is a common characteristic of invasive cancer cells, and is believed to dictate the genetic program that controls invasion switches. Molecular studies on the combinatorial signaling involved in tumor invasion are becoming a fertile area for target discovery in cancer. This review discusses how cooperative signaling between ErbB and non-ErbB receptors regulates tumor invasion and hence provides multiple opportunities for drug discovery, and how current therapies and investigational drugs could pave the way to even more potent alternative combinatorial therapeutic approaches for invasive cancers.

Caveolin-1 phosphorylation in human squamous and epidermoid carcinoma cells: dependence on ErbB1 expression and Src activation

Previous studies have shown that EGF can induce the tyrosine phosphorylation of caveolin-1 in murine fibroblasts following ErbB1 (EGF receptor) mutation or overexpression, but the cell signaling events linking EGF action with caveolin phosphorylation are not fully established. In this regard, we examined multiple human carcinoma cell lines that express various ErbB family members, including A431 epidermoid carcinoma cells and several squamous carcinoma cell lines. In all cases, EGF treatment induced the tyrosine phosphorylation of caveolin-1 in a time- and EGF dose-dependent manner, and immunoblotting analysis revealed that this phosphorylation occurred at tyrosine-14. The EGF-dependent phosphorylation of caveolin-1 was observed at low temperatures (4 degrees C) and was enhanced by caveolae-disrupting agents (cyclodextrin), suggesting that this EGF-dependent system is in a low temperature-stable arrangement that allows for their interaction under conditions where mobility in the membrane is altered. To further assess the events linking EGF action with caveolin phosphorylation, we evaluated the ligand specificity of these responses and their dependence on known effectors of EGF receptor function. We observed that EGF and HB-EGF, but not heregulin, promoted caveolin-1 phosphorylation in A431 cells, suggesting that these responses are linked to EGF receptor activation and not solely occurring via the activation of other endogenous ErbB family members. In addition, the EGF-induced phosphorylation of caveolin-1 in A431 cells was blocked by the Src kinase antagonists PP1 and PP2, but not by the MEK inhibitor PD98059, the phosphoinositide 3-kinase inhibitors LY294002 and wortmannin, or cytoskeleton-disrupting agents, such as cytochalasin D, colchicine, and nocadazole. Altogether, these data indicate that multiple human carcinoma cells exhibit an EGF receptor-dependent tyrosine phosphorylation of caveolin-1 and that this process is sensitive to Src family kinase inhibitors. These observations support a role for caveolin tyrosine phosphorylation in the profile of cellular responses by which Src potentiates cancer progression following EGF receptor overexpression.

Elastase-released epidermal growth factor recruits epidermal growth factor receptor and extracellular signal-regulated kinases to down-regulate tropoelastin mRNA in lung fibroblasts

Elastase/anti-elastase imbalance is a hallmark of emphysema, a chronic obstructive pulmonary disease associated with the rupture and inefficient repair of interstitial elastin. We report that neutrophil elastase (NE) at low physiologic concentrations, ranging from 35 nm to 1 microm, invokes transient, peaking at 15 min, activation of extracellular signal-regulated kinases 1 and 2 (ERK) in elastogenic lung fibroblasts. ERK activation is preceded by the release of soluble 25-26-kDa forms of epidermal growth factor (EGF) and transactivation of EGF receptor (EGFR) in NE-exposed cells. The stimulatory effect of NE on ERK is abrogated in the presence of anti-EGF-neutralizing antibodies, EGFR tyrosine kinase inhibitor (AG1478), and ERK kinase inhibitor (PD98059), as well as abolished in both EGFR-desensitized and endocytosis-arrested fibroblasts. Nuclear accumulation of activated ERK is associated with transient, peaking at 30 min, induction of c-Fos and sustained, observed at 24-48 h, decrease of tropoelastin mRNA levels in NE-challenged cells. Pretreatment of fibroblasts with AG1478 or PD98059 abrogates the NE-initiated tropoelastin mRNA suppression. We conclude that proteolytically released EGF signals directly via EGFR and ERK to down-regulate tropoelastin mRNA in NE-challenged lung fibroblasts.

EGFRs mediate chemotactic migration in the developing telencephalon

Epidermal growth factor receptors (EGFRs) have been implicated in the control of migration in the telencephalon, but the mechanism underlying their contribution is unclear. We show that expression of a threshold level of EGFRs confers chemotactic competence in stem cells, neurons and astrocytes in cortical explants. This level of receptor expression is normally achieved by a subpopulation of cells during mid-embryonic development. Cells that express high levels of EGFR are located in migration pathways, including the tangential pathway to the olfactory bulb via the rostral migratory stream (RMS), the lateral cortical stream (LCS) leading to ventrolateral cortex and the radial pathway from proliferative zones to cortical plate. The targets of these pathways express the ligands HB-EGF and/or TGFα. To test the idea that EGFRs mediate chemotactic migration these pathways, we increased the size of the population of cells expressing threshold levels of EGFRs in vivo by viral transduction. Our results suggest that EGFRs mediate migration radially to the cortical plate and ventrolaterally in the LCS, but not tangentially in the RMS. Within the bulb, however, EGFRs also mediate radial migration. Our findings suggest that developmental changes in EGFR expression, together with changes in ligand expression regulate the migration of specific populations of cells in the telencephalon by a chemoattractive mechanism.

Receptor heterodimerization: essential mechanism for platelet-derived growth factor-induced epidermal growth factor receptor transactivation

Previous studies showed that the epidermal growth factor receptor (EGFR) can be transactivated by platelet-derived growth factor (PDGF) stimulation and that EGFR transactivation is required for PDGF-stimulated cell migration. To investigate the mechanism for cross talk between the PDGF beta receptor (PDGFbetaR) and the EGFR, we stimulated rat aortic vascular smooth muscle cells (VSMC) with 20 ng of PDGF/ml. Transactivation of the EGFR, defined by receptor tyrosine phosphorylation, occurred with the same time course as PDGFbetaR activation. Basal formation of PDGFbetaR-EGFR heterodimers was shown by coimmunoprecipitation studies, and interestingly, disruption of this receptor heterodimer abolished EGFR transactivation. Breakdown of the heterodimer was observed when VSMC were pretreated with antioxidants or with a Src family kinase inhibitor. Disruption of heterodimers decreased ERK1 and ERK2 activation by PDGF. Although PDGF-induced PDGFbetaR activation was abolished after pretreatment with 1 microM AG1295 (a specific PDGF receptor kinase inhibitor), EGFR transactivation was still observed, indicating that PDGFbetaR kinase activity is not required. In conclusion, our data demonstrate that the PDGFbetaR and the EGFR form PDGFbetaR-EGFR heterodimers basally, and we suggest that heterodimers represent a novel signaling complex which plays an important role in PDGF signal transduction.

Platelet-derived growth factor requires epidermal growth factor receptor to activate p21-activated kinase family kinases

The platelet-derived growth factor (PDGF) receptor (PDGFR) transactivates the epidermal growth factor (EGF) receptor (ErbB1) to stimulate the cell migration of fibroblasts through an unknown mechanism (Li, J., Kim, Y. N. & Bertics, P. (2000) J. Biol. Chem. 275, 2951-2958). In this paper we provide evidence that the transactivation of the EGF receptor (EGFR) by PDGFR is essential for PDGF to activate p21-activated kinase (PAK) family kinases. Fetal calf serum (10%) transiently stimulates the PAK activity in NIH 3T3 fibroblasts. The activation of PAK was completely inhibited by either PDGFR-specific inhibitor (AG1295) or EGFR-specific inhibitor (AG1478), suggesting that serum requires either the PDGF- or EGF-dependent pathway or the combination of both to activate PAK. PDGF-induced activation of PAK is completely inhibited by either AG1295 or AG1478, indicating that PDGF requires both PDGFR and EGFR for PAK activation. In support of this notion, a mouse embryo fibroblast cell line derived from the EGFR -/- mouse (from Dr. Erwin Wagner) doesn't activate PAK in response to PDGF. Expression of human EGFR in this cell line restores the ability of the PDGF to induce PAK activation. Our results indicate that PDGF activates PAK through transactivation of ErbB1.