Angiogenesis stimulated by PDGF-CC, a novel member in the PDGF family, involves activation of PDGFR-alphaalpha and -alphabeta receptors

PDGF-C mediates the angiogenic and tumorigenic properties of fibroblasts associated with tumors refractory to anti-VEGF treatment

Tumor- or cancer-associated fibroblasts (TAFs or CAFs) from different tumors exhibit distinct angiogenic and tumorigenic properties. Unlike normal skin fibroblasts or TAFs from TIB6 tumors that are sensitive to anti-VEGF treatment (TAF-TIB6), TAFs from resistant EL4 tumors (TAF-EL4) can stimulate TIB6 tumor growth even when VEGF is inhibited. We show that platelet-derived growth factor C (PDGF-C) is upregulated in TAFs from resistant tumors. PDGF-C-neutralizing antibodies blocked the angiogenesis induced by such TAFs in vivo, slowed the growth of EL4 and admixture (TAF-EL4 + TIB6) tumors, and exhibited additive effects with anti-VEGF-A antibodies. Hence, our data reveal an additional mechanism for TAF-mediated tumorigenesis and suggest that some tumors may overcome inhibition of VEGF-mediated angiogenesis through upregulation of PDGF-C.

Imatinib mesylate in combination with docetaxel for the treatment of patients with advanced, platinum-resistant ovarian cancer and primary peritoneal carcinomatosis : a Hoosier Oncology Group trial

BACKGROUND

Ovarian tumors frequently express c-Kit and/or platelet-derived growth factor receptors (PDGFRs). Imatinib mesylate blocks the growth of ovarian cancer cells in vitro and may enhance the activity of chemotherapy. This study was conducted to determine the activity of imatinib in combination with docetaxel in patients with recurrent, platinum-resistant epithelial ovarian cancer (EOC).

METHODS

Eligible patients had recurrent, platinum-resistant, or refractory EOC that expressed PDGFRalpha or c-kit, as determined by immunohistochemistry. Imatinib mesylate at a dose of 600 mg orally once daily was administered continuously with docetaxel at a dose of 30 mg/m(2) given intravenously once weekly in Weeks 1 through 4 of every 6-week cycle. The primary endpoint was objective response rate (ORR) as assessed by the Response Evaluation Criteria in Solid Tumors (RECIST).

RESULTS

Thirty-four patients were screened for PDGFRalpha and c-kit expression to enroll 23 patients between December 2003 and October 2005. Four patients had c-kit-positive/PDGFR-negative tumors, 11 patients had PDGFR-positive/c-kit-negative tumors, and 8 patients had c-kit-positive/PDGFR-positive tumors. The median patient age was 56 years (range, 33-76 years). Patients had received a median of 3 prior treatments. The ORR was 21.7% and included 1 complete and 4 partial responses. An additional 3 patients had stable disease for more than 4 months. Expression of PDGFR, c-kit, phosphatase and tensin homolog (PTEN), and phosphorylated protein kinase B (Akt) did not predict response to therapy. The most common adverse events encountered were fatigue (83%), nausea (74%), diarrhea (61%), anorexia (52%), and edema (65%), and the majority of those events were graded as grade 1 or 2.

CONCLUSIONS

The combination imatinib and docetaxel was tolerated in patients with heavily pretreated EOC that expressed c-kit or PDGFRalpha. Few patients had sustained responses or stable disease.

miR-126 regulates angiogenic signaling and vascular integrity

Precise regulation of the formation, maintenance, and remodeling of the vasculature is required for normal development, tissue response to injury, and tumor progression. How specific microRNAs intersect with and modulate angiogenic signaling cascades is unknown. Here, we identified microRNAs that were enriched in endothelial cells derived from mouse embryonic stem (ES) cells and in developing mouse embryos. We found that miR-126 regulated the response of endothelial cells to VEGF. Additionally, knockdown of miR-126 in zebrafish resulted in loss of vascular integrity and hemorrhage during embryonic development. miR-126 functioned in part by directly repressing negative regulators of the VEGF pathway, including the Sprouty-related protein SPRED1 and phosphoinositol-3 kinase regulatory subunit 2 (PIK3R2/p85-beta). Increased expression of Spred1 or inhibition of VEGF signaling in zebrafish resulted in defects similar to miR-126 knockdown. These findings illustrate that a single miRNA can regulate vascular integrity and angiogenesis, providing a new target for modulating vascular formation and function.

FOXC2 controls Ang-2 expression and modulates angiogenesis, vascular patterning, remodeling, and functions in adipose tissue

Adipogenesis is spatiotemporally coupled to angiogenesis throughout adult life, and the interplay between these two processes is communicated by multiple factors. Here we show that in a transgenic mouse model, increased expression of forkhead box C2 (FOXC2) in the adipose tissue affects angiogenesis, vascular patterning, and functions. White and brown adipose tissues contain a considerably high density of microvessels appearing as vascular plexuses, which show redistribution of vascular smooth muscle cells and pericytes. Dysfunction of these primitive vessels is reflected by impairment of skin wound healing. We further provide a mechanistic insight of the vascular phenotype by showing that FOXC2 controls Ang-2 expression by direct activation of its promoter in adipocytes. Remarkably, an Ang-2-specific antagonist almost completely reverses this vascular phenotype. Thus, the FOXC2-Ang-2 signaling system is crucial for controlling adipose vascular function, which is part of an adaptation to increased adipose tissue metabolism.

Fibroblast growth factor regulation of neovascularization

PURPOSE OF REVIEW

Fibroblast growth factors are potent angiogenic inducers; however, their precise roles in angiogenesis have not been well understood. In this review, we will focus on specific roles played by fibroblast growth factors in neovascularization.

RECENT FINDINGS

Although fibroblast growth factors promote a strong angiogenic response, it has been suggested that FGF-induced angiogenesis requires activation of the vascular endothelial growth factor system. Recent findings have endorsed the view of indirect contribution of fibroblast growth factor signaling to vascular development. A study using embryoid bodies demonstrated a nonimmediate role played by fibrobalst growth factor receptor 1 in vasculogenesis as vascular endothelial growth factor supplementation was sufficient to promote vascular development in Fgfr1-/- embryoid bodies. Moreover, another line of evidence indicated that myocardial fibroblast growth factor signaling is essential for mouse coronary development. The key role of fibroblast growth factor signaling in this process is Hedgehog activation, which induces vascular endothelial growth factor expression and formation of the coronary vasculature. In addition to vascular endothelial growth factor interaction, fibroblast growth factors can control neovascularization by influencing other growth factors and chemokines such as platelet-derived growth factor, hepatocyte growth factor and monocyte chemoattractant protein-1, contributing to development of mature vessels and collateral arteries.

SUMMARY

Although fibroblast growth factors are potent angiogenic factors, they may indirectly control neovascularization in concert with other growth factors. Thus, the unique role played by fibroblast growth factors might be organization of various angiogenic pathways and coordination of cell-cell interactions in this process.

Drug insight: antiangiogenic therapies for gastrointestinal cancers--focus on monoclonal antibodies

Tumor angiogenesis is strongly induced by vascular endothelial growth factor (VEGF), which is overexpressed in most human gastrointestinal cancers. VEGF overexpression is known to be associated with poor prognosis and survival in patients with various solid tumors. The humanized monoclonal anti-VEGF antibody bevacizumab (Avastin, Genentech Inc., South San Francisco, CA) is a prototypic antiangiogenic compound, and has proven therapeutic benefit combined with conventional chemotherapy-namely, significantly improved progression-free survival in patients with metastatic colorectal cancer. Bevacizumab is the only anti-VEGF antibody that has been approved by the FDA and the European Medicines Agency for the treatment of metastatic colorectal cancer. Several ongoing clinical studies are evaluating the potential of bevacizumab therapy for other gastrointestinal cancers, in combination with chemotherapy, other targeted therapies and/or radiation. Soluble chimeric receptors, tyrosine kinase inhibitors, and monoclonal antibodies against VEGF and molecular targets in the integrin and Delta-like protein 4-Notch pathways are being developed. As tumors acquire resistance to anti-VEGF therapy, further development of antiangiogenic and vascular targets and therapy is warranted.

Angiotensin II induction of PDGF-C expression is mediated by AT1 receptor-dependent Egr-1 transactivation

Platelet-derived growth factors are a family of mitogens and chemoattractants comprising of four ligand genes (A-, B-, C-, D-chains) implicated in many physiologic and pathophysiologic processes, including atherosclerosis, fibrosis and tumorigenesis. Our understanding of the molecular mechanisms, which regulate PDGF-C transcription remains incomplete. Transient transfection analysis, conventional and quantitative real-time PCR revealed the induction of PDGF-C transcription and mRNA expression in smooth muscle cells (SMCs) exposed to the peptide hormone angiotensin (ATII), which induces Egr-1. Occupancy of a G + C-rich element in the proximal region of the PDGF-C promoter was unaffected by ATII. Instead we discovered, using both nuclear extracts and recombinant proteins with EMSA and ChIP analyses, the existence of a second Egr-1-binding element located 500 bp upstream. ATII induction of PDGF-C transcription is mediated by the angiotensin type 1 receptor (AT1R) and Egr-1 activation through this upstream element. DNAzyme ED5 targeting Egr-1 blocked ATII-inducible PDGF-C expression. Moreover, increased PDGF-C expression after exposure to ATII depends upon the differentiation state of the SMCs. This study demonstrates the existence of this novel ATII-AT1R-Egr-1-PDGF-C axis in SMCs of neonatal origin, but not in adult SMCs, where ATII induces Egr-1 but not PDGF-C.

New approaches in angiogenic targeting for colorectal cancer

Colorectal carcinoma (CRC) is one of the leading causes of cancer death worldwide. In the last decade, the addition of irinotecan and oxaliplatin to standard fluorouracil-based chemotherapy regimens have set the new benchmark of survival for patients with metastatic CRC at approximately 20 mo. Despite these advances in the management of CRC, there is a strong medical need for more effective and well-tolerated therapies. The dependence of tumor growth and metastasis on blood vessels makes angiogenesis a rational target for therapy. One of the major pathways involved in this process is the vascular endothelial growth factor (VEGF) and its receptors (VEGFR). In 2004, the first agent targeting angiogenesis, bevacizumab (BV), was approved as an adjunct to first-line cytotoxic treatment of metastatic CRC. The role of BV as part of adjuvant treatment and in combination with other targeted therapies is the subject of ongoing trials. However, BV is associated with an increase in the risk of arterial thromboembolic events, hypertension and gastrointestinal perforations and its use must be cautious. Novel VEGFR TK inhibitors with different ranges of nanomolar potencies, selectivities, and pharmacokinetic properties are entering phase III trials for the treatment of cancer. Conversely, one of these novel agents, vatalanib, has been shown not to confer survival benefit in first and second-line treatment of advanced CRC. The basis of these findings is being extensively evaluated. Ongoing and new well-designed trials will define the optimal clinical application of the actual antiangiogenic agents, and, on the other hand, intensive efforts in basic research will identify new agents with different antiangiogenic approaches for the treatment of CRC. In this review we discuss and highlight current and future approaches in angiogenic targeting for CRC.

Angiogenic factors FGF2 and PDGF-BB synergistically promote murine tumor neovascularization and metastasis

Tumors produce multiple growth factors, but little is known about the interplay between various angiogenic factors in promoting tumor angiogenesis, growth, and metastasis. Here we show that 2 angiogenic factors frequently upregulated in tumors, PDGF-BB and FGF2, synergistically promote tumor angiogenesis and pulmonary metastasis. Simultaneous overexpression of PDGF-BB and FGF2 in murine fibrosarcomas led to the formation of high-density primitive vascular plexuses, which were poorly coated with pericytes and VSMCs. Surprisingly, overexpression of PDGF-BB alone in tumor cells resulted in dissociation of VSMCs from tumor vessels and decreased recruitment of pericytes. In the absence of FGF2, capillary ECs lacked response to PDGF-BB. However, FGF2 triggers PDGFR-alpha and -beta expression at the transcriptional level in ECs, which acquire hyperresponsiveness to PDGF-BB. Similarly, PDGF-BB-treated VSMCs become responsive to FGF2 stimulation via upregulation of FGF receptor 1 (FGFR1) promoter activity. These findings demonstrate that PDGF-BB and FGF2 reciprocally increase their EC and mural cell responses, leading to disorganized neovascularization and metastasis. Our data suggest that intervention of this non-VEGF reciprocal interaction loop for the tumor vasculature could be an important therapeutic target for the treatment of cancer and metastasis.

Phase II trial of sunitinib in patients with metastatic colorectal cancer after failure of standard therapy

PURPOSE

Sunitinib is an oral, multitargeted receptor tyrosine kinase inhibitor of the vascular endothelial growth factor receptor and multiple other growth factor receptors. We assessed the safety and efficacy of sunitinib in patients with metastatic colorectal cancer after failure of standard therapy.

PATIENTS AND METHODS

Eighty-four patients were enrolled onto this two-stage phase II trial and were stratified by whether they had received prior bevacizumab (n = 43) or not (n = 41). Treatment comprised sunitinib 50 mg orally daily for 4 weeks, followed by 2 weeks off treatment, in repeated 6-week cycles.

RESULTS

By Response Evaluation Criteria in Solid Tumors criteria, one patient, who was in the prior bevacizumab cohort, achieved a partial response, and 13 patients (two in the prior bevacizumab cohort and 11 in the no prior bevacizumab cohort) achieved stable disease lasting > or = 22 weeks. Median time to progression in the prior bevacizumab and bevacizumab-naïve cohorts was 2.2 months (95% CI, 1.9 to 2.3 months) and 2.5 months (95% CI, 2.3 to 3.1 months), respectively, whereas median overall survival time was 7.1 months (95% CI, 4.9 to 10.6 months) and 10.2 months (95% CI, 8.2 to 15.3 months), respectively. The most common adverse events were fatigue, diarrhea, nausea, vomiting, and anorexia. Twenty-six patients (32%) required dose reduction to 37.5 mg/d, and one patient required dose reduction to 25 mg/d.

CONCLUSION

Sunitinib did not demonstrate a meaningful single-agent objective response rate in colorectal cancer refractory to standard chemotherapy. However, the mechanisms of action and acceptable safety profile of sunitinib warrant further study in combination with standard regimens for metastatic colorectal cancer.

Mechanisms of action of TGF-beta in cancer: evidence for Smad3 as a repressor of the hTERT gene

Transforming growth factor-beta (TGF-beta) induces cell differentiation and suppresses cell proliferation, but the mechanisms underlying the actions of TGF-beta remain to be fully elucidated. Recent studies suggest that TGF-beta suppresses neoplastic cell development by employing Smad3 protein to repress the gene of human telomerase reverse transcriptase (hTERT). In human breast cancer cells, TGF-beta induces rapid phosphorylation and subsequent entry of Smad3 into the nucleus. In the nucleus, Smad3 binds to the hTERT gene promoter directly and inhibits hTERT gene transcription activity. By interacting with c-myc, Smad3 also represses the c-myc gene. Thus, TGF-beta prevents continuous cell proliferation by switching off telomerase activity through Smad3 repression of the hTERT gene and the action of c-myc. Modulating the interface between Smad3 and the hTERT gene, and the potential feedback loop from telomeres to Smad3 via Smurf2, may represent a novel approach to regulate cell lifespan of proliferation.

Mesenchymal stem cells and neovascularization: role of platelet-derived growth factor receptors

There is now accumulating evidence that bone marrow-derived mesenchymal stem cells (MSCs) make an important contribution to postnatal vasculogenesis, especially during tissue ischaemia and tumour vascularization. Identifying mechanisms which regulate the role of MSCs in vasculogenesis is a key therapeutic objective, since while increased neovascularization can be advantageous during tissue ischaemia, it is deleterious during tumourigenesis. The potent angiogenic stimulant vascular endothelial growth factor (VEGF) is known to regulate MSC mobilization and recruitment to sites of neovascularization, as well as directing the differentiation of MSCs to a vascular cell fate. Despite the fact that MSCs did not express VEGF receptors, we have recently identified that VEGF-A can stimulate platelet-derived growth factor (PDGF) receptors, which regulates MSC migration and proliferation. This review focuses on the role of PDGF receptors in regulating the vascular cell fate of MSCs, with emphasis on the function of the novel VEGF-A/PDGF receptor signalling mechanism.

Combinatorial protein therapy of angiogenic and arteriogenic factors remarkably improves collaterogenesis and cardiac function in pigs

Establishment of functional and stable collaterals in the ischemic myocardium is crucial to restoring cardiac function after myocardial infarction. Here, we show that only dual delivery of a combination of angiogenic and arteriogenic factors to the ischemic myocardium could significantly reestablish stable collateral networks and improve myocardial perfusion and function. A combination of FGF-2 with PDGF-BB, two factors primarily targeting endothelial cells and vascular smooth muscle cells, remarkably promotes myocardial collateral growth and stabilizes the newly formed collateral networks, which significantly restore myocardial perfusion and function. Using various members of the PDGF family together with FGF-2 in an angiogenesis assay, we demonstrate that PDGFR-alpha is mainly involved in angiogenic synergism, whereas PDGFR-beta mediates vessel stability signals. Our findings provide conceptual guidelines for the clinical development of proangiogenic/arteriogenic factors for the treatment of ischemic heart disease.

The role of integrin alpha5beta1 in the regulation of corneal neovascularization

Integrins are transmembrane receptor proteins critical for growth and stabilization of vessels, but the mechanisms by which integrin activities are involved in neoangiogenesis of the eye remain unclear. Specific inhibitors to fibronectin receptor integrin alpha(5)beta(1) impeded pathological neovascularization in vivo. Our objective was to determine whether alpha(5)beta(1) plays a role in ocular angiogenesis, and whether a novel alpha(5)beta(1)-inhibiting small molecule is able to reduce angiogenesis in a model of inflammatory corneal neovascularization. Corneal neovascularization was induced in C57Bl/6 mice by NaOH-application and debridement of the limbal epithelium. Mice were randomized into six groups receiving either no treatment, or intraperitoneal osmotic pumps delivering three different doses of integrin antagonist or control substance on day 10 after scraping. In order to quantify the neovascular response, flatmounts were stained with FITC-CD31. Integrin alpha(5) expression was determined by immunohistochemistry and quantified by semiquantitative western blot analysis. Influence of integrin antagonist treatment on the mRNA expression of VEGF, bFGF and integrin alpha(5) was quantified by real-time RT-PCR. Vascularized corneas demonstrated a strong up-regulation of integrin alpha(5) within affected areas. Animals treated systemically with alpha(5)beta(1)-inhibiting small molecule showed a significant inhibition and regression of corneal neovascularization. PCR analysis evinced a significant up-regulation of VEGF and integrin alpha(5) mRNA levels in injured animals compared to controls, and a significant reduction of integrin alpha(5) mRNA in substance-treated animals compared to control substance, but no significant differences of bFGF levels in all groups. Western blot analysis of integrin alpha(5)beta(1) protein expression showed a trend towards up-regulation in injured animals, both control substance-treated and those treated with the alpha(5)beta(1)-inhibiting small molecule. Systemic delivery of an alpha(5)beta(1)-inhibiting small molecule inhibits and regresses corneal neovascularization induced by mechanical-alkali burn corneal injury. These results suggest an essential role for the integrin alpha(5)beta(1) in pathological neovascular processes of the cornea. Integrin alpha(5)beta(1) inhibitors could become a new approach for treatment of neovascularization in the eye.

Cell-Type Localization of Platelet-Derived Growth Factors and Receptors in the Postnatal Rat Ovary and Follicle1

Intraovarian growth factors play a significant role in the regulation of follicular selection and growth. In this study, the presence and localization of all members of the family of platelet-derived growth factors (PDGF) and receptors (PDGFR) were identified and characterized in the rat ovary. In addition, a role was identified for members of this family in contributing towards growth of preantral follicles. Real-time PCR revealed the presence of mRNA for all platelet-derived growth factors (Pdgfa, Pdgfb, Pdgfc and Pdgfd) and receptors (Pdgfra and Pdgfrb) in the rat ovary from birth until 4 wk. In situ hybridization and immunohistochemistry were utilized to identify cell-type expression of PDGFs and PDGFRs in rat ovaries from birth until 4 wk. Shortly after birth, expression of PDGFRA and PDGFC was observed in and around oocyte clusters, and PDGFRB in stromal cells surrounding oocyte clusters. All members were identified in oocytes of primordial and primary follicles, and in cells of the theca layer of primordial to antral follicles. PDGFRA and PDGFA were also localized to some granulosa cells of secondary and antral follicles in ovaries from rats at Days 20 and 24. Thus, localization data suggest both theca-theca and theca-granulosa cell interactions of PDGFs and receptors. Preantral follicles cultured in vitro over 5 days in serum-free medium plus recombinant PDGFAA, PDGFAB, or PDGFBB increased in follicle diameter by 18.32%+/-2.18%, 17.72%+/-2.3%, and 17.6%+/-1.81%, respectively, representing significantly greater increases than for follicles incubated in serum-free medium alone (11%+/-1.57%), and suggesting a role for these growth factors in positively influencing early follicle growth.

Platelet-Derived Growth Factors and Receptors in the Rat Corpus Luteum: Localization and Identification of an Effect on Luteogenesis1

Platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) play a vital role in regulating cell growth and angiogenesis. In this study, the expression of the family of PDGFs and PDGFRs in the ovarian corpus luteum were identified and characterized, and an effect of their activity on development of the corpus luteum revealed. Gonadotropin-stimulated immature rats were utilized as a model of induced ovulation, luteogenesis, and pseudopregnancy. Levels of ovarian mRNA for Pdgfb and Pdgfd, and their receptor, Pdgfrb, increased significantly as early as 4 h after human chorionic gonadotropin (hCG) injection in immature rats primed with equine chorionic gonadotropin (eCG). Gonadotropin regulation of Pdgfb expression was confirmed by in vitro promoter-reporter assays, which showed a 2- to 3-fold increase in Pdgfb promoter activity in response to luteinizing hormone (LH). Inhibition studies implicated protein kinase A, phosphatidylinositol 3-kinase and mitogen activated protein kinase signaling pathways in the LH-induced upregulation. In the corpus luteum, PDGFA, PDGFB, PDGFC, and PDGFRA were localized to a population of luteal parenchymal/steroidogenic cells. PDGFRB was expressed primarily in what appeared to be cells of the luteal microvasculature. Intraovarian injection of an inhibitor of PDGF receptor activity, the tyrphostin AG1295, prior to injection of hCG in eCG-primed immature rats resulted in a significant 21.86%+/-11.15% decrease in corpora lutea per treated ovary in comparison to the contralateral vehicle-injected control ovary. In addition, the treated ovary of 3 of 16 rats showed widespread hemorrhage throughout the entire ovary, indicating a possible role for PDGF receptor activity in maintenance of the ovarian vasculature.

PDGF Receptors as Targets in Tumor Treatment

Signaling through platelet-derived growth factor (PDGF) receptors contributes to multiple tumor-associated processes. The recent introduction of clinically useful PDGF inhibitors have the last years validated PDGF receptors in malignant and stromal cells as relevant cancer drug targets. Mutational activation of PDGF receptor signaling in malignant cells has been described in some rare tumor types such as dermatofibrosarcoma protuberans, a subset of GISTs, and some hematologic malignancies. Furthermore, expression of PDGF receptors on pericytes is a common characteristic of solid tumors. The clinical efficacy of novel multikinase inhibitors, such as sunitinib and sorafenib, most likely involves targeting of PDGF receptor-dependent pericytes. Preclinical studies suggest that targeting of stromal PDGF receptors might also constitute a novel strategy to enhance tumor drug uptake. Finally, recent studies have implied both pro- and antimetastatic effects of PDGF receptors on malignant and stromal cells. The studies on the roles of PDGF receptors in cancer signaling are thus presently in a dynamic phase where collaborations between oncologists, pathologists, and tumor biologists are predicted to be highly productive.

Biology of platelet-derived growth factor and its involvement in disease

Platelet-derived growth factor (PDGF) is mainly believed to be an important mitogen for connective tissue, especially for fibroblasts that serve in wound healing. However, PDGF also has important roles during embryonal development, and its overexpression has been linked to different types of fibrotic disorders and malignancies. Platelet-derived growth factor is synthesized by many different cell types, and its expression is broad. Its synthesis is in response to external stimuli, such as exposure to low oxygen tension, thrombin, or stimulation by other cytokines and growth factors. In addition, PDGF may function in autocrine stimulation of tumor cells, regulation of interstitial fluid pressure, and angiogenesis. Recently, several drugs were developed that are potent inhibitors of the tyrosine kinase activity of PDGF receptors. Thus, it is important to understand the physiology of PDGF and its receptors and the role of PDGF in different diseases. This review summarizes the physiologic activity of PDGF, the expression of PDGF during embryonal development, and the roles of PDGF expression in nonmalignant disease and in different tumors.

Over-expression of PDGF-C using a lung specific promoter results in abnormal lung development

PDGF isoforms are a family of polypeptides that bind to cell surface receptors and induce fibroblast proliferation and chemotaxis. PDGF-A and -B chain isoforms have previously been shown to be involved in murine lung development. A new PDGF polypeptide, PDGF-C, was recently recognized and differs from the PDGF-A and -B isoforms in that it requires proteolytic cleavage before it can bind and activate the PDGF alpha receptor. In these studies PDGF-C was over-expressed during embryogenesis using the lung specific surfactant protein C promoter. PDGF-C transgenic pups died from respiratory insufficiency within minutes following birth. At E18.5, nontransgenic lungs exhibited lung morphology consistent with the saccular stage of lung development. In contrast, E18.5 transgenic lungs retained many features of the canalicular stage of lung development and had abundant numbers of large poorly differentiated mesenchymal cells. These results suggest that PDGF-C is activated during lung development and is a potent growth factor for mesenchymal cells in vivo.

Tumor-driven paracrine platelet-derived growth factor receptor alpha signaling is a key determinant of stromal cell recruitment in a model of human lung carcinoma

Activated fibroblasts are thought to play important roles in the progression of many solid tumors, but little is known about the mechanisms responsible for the recruitment of fibroblasts in tumors. Using several methods, we identified platelet-derived growth factor A (PDGFA) as the major fibroblast chemoattractant and mitogen from conditioned medium generated by the Calu-6 lung carcinoma cell line. In addition, we showed that Calu-6 tumors express significant levels of PDGFC, and that the levels of expression of these two PDGFRalpha ligands correlate strongly with the degree of stromal fibroblast infiltration into the tumor mass. The most intense expression of PDGFRalpha was observed in fibroblasts in the tumor outer rim. We subsequently showed that disrupting PDGFRalpha-mediated signaling results in significant inhibition of tumor growth in vivo. Furthermore, analysis of a compendium of microarray data revealed significant expression of PDGFA, PDGFC, and PDGFRalpha in human lung tumors. We propose that therapies targeting this stromal cell type may be effective in treating certain types of solid tumors.

Expression of platelet-derived growth factors C and D in the synovial membrane of patients with rheumatoid arthritis and osteoarthritis

OBJECTIVE

To investigate the messenger RNA (mRNA) and protein expression of the recently discovered platelet-derived growth factor C (PDGF-C) and PDGF-D in the synovial membrane (SM) of patients with rheumatoid arthritis (RA) and osteoarthritis (OA) and to assess the localization and cellular source of these proteins in the SM and their functional influence on synovial fibroblasts.

METHODS

Expression of mRNA for PDGFs A, B, C, and D as well as for PDGF receptor (PDGFR) alpha and beta chains in RA and OA SM samples was assessed by real-time reverse transcription-polymerase chain reaction. Protein levels of PDGF-C and PDGF-D were quantified by immunoblotting. Regional and cellular localization of PDGF-C and PDGF-D in the SM was investigated by double-staining immunohistochemistry. In addition, the influence of PDGF-D on the proliferation of synovial fibroblasts and their matrix metalloproteinase (MMP-1) mRNA expression were determined.

RESULTS

The expression of mRNA for PDGFs A, B, and C and for PDGFR alpha and beta chains was comparable in RA and OA SM samples; in contrast, the expression of mRNA for PDGF-D was significantly higher in OA SM. PDGF-C protein was not differentially expressed in OA and RA. The expression of PDGF-D protein was significantly higher in RA SM (full-length and activated form). PDGF-C and PDGF-D were expressed throughout the SM (lining layer, diffuse infiltrates, and stroma) by both synovial fibroblasts and macrophages. In addition, PDGF-D increased the proliferation of synovial fibroblasts and the expression of mRNA for MMP-1.

CONCLUSION

PDGF-C and PDGF-D are expressed by synovial fibroblasts and macrophages in RA and OA SMs. The levels of PDGF-D protein were significantly higher in RA SM. In addition, PDGF-D stimulated synovial fibroblast proliferation and expression of MMP-1. These findings may have pathogenetic implications for cellular transformation and matrix remodeling in the RA SM.

Involvement of cytoskelatal proteins and growth factor receptors during development of the human eye

The spatial and temporal distribution of nestin, cytokeratins (CKs), vimentin, glial fibrillary acidic protein (GFAP), neurofilaments (NFs), beta-tubulin as well as fibroblast growth factor receptors (FGFRs) and platelet-derived growth factor receptor beta (PDGF-Rbeta) were investigated in the developing human eye in eight conceptuses of 5-9 postovulatory weeks using immunostaining. Nestin was found in the neuroglial precursors and the radial glial fibres of the optic nerve. In the pigmented retina, nestin was present only in the 5th week, while at later stages (6-9th week), co-expression of CKs and vimentin was seen. Nestin, CKs, vimentin, and GFAP were observed in the precursors to various cell types in the neural retina. Additionally, their expression was also apparent in the lens epithelium, showing its gradual fading following the lens fibre elongation. They appeared in the mesenchymal cells of the cornea, the choroid, the sclera, and the corpus vitreum, too. In the corneal epithelium, co-expression of nestin and CKs was detected. NFs and beta-tubulin were confined to the differentiating retinal neuroblasts. Growth factor receptors were seen in the retina, the lens epithelium while less intensely in the lens fibres, the corneal epithelium, and the mesenchymal cells. During the early eye development (5-9th week), IFs expressing normal pattern of distribution as well as acting in concert might contribute to the normal developmental processes occurring in certain parts of the human eye. Additionally, NFs and beta-tubulin seem to have an important role in the retinal ganglion cell differentiation, while FGFRs and PDGF-Rbeta may regulate the cell proliferation, differentiation, and survival in various parts of the developing eye.

Vesicular Trafficking of Tyrosine Kinase Receptors and Associated Proteins in the Regulation of Signaling and Vascular Function

Receptor tyrosine kinases (RTKs) play a pivotal role in the development and function of the cardiovascular system. Ligand-activated RTKs promote numerous downstream signal transduction pathways that lead to vascular permeability, as well as proliferation, migration, and differentiation of vascular endothelia and smooth muscle cells. Ligand binding also promotes internalization of the activated receptors either to downregulate the signaling via degradation of the ligand/receptor complex or to signal from endosomes. However, the outcomes of receptor internalization via clathrin-dependent or caveolar pathways and trafficking mechanisms are incompletely clarified in vascular systems. Activity modulation through endocytosis and vesicular trafficking significantly impacts downstream targets of RTKs such as endothelial nitric oxide synthase (eNOS) and VE-cadherin. RTKs and their associated targets are also transported to the nucleus, where they may directly impact nuclear signaling. Although the nuclear transport pathways are just beginning to be unraveled, it appears that endocytosis and vesicular trafficking are involved. In this review, we discuss the mechanisms by which activated RTKs and the downstream targets eNOS and VE-cadherin may be internalized and transported to various intracellular compartments. How localization and interacting proteins impact protein function and influence signaling is an important theme, as is the potential for modulating signaling through therapeutic targeting of activated receptors and components of the endocytic machinery.

Nuclear localisation of endogenous SUMO-1-modified PDGF-C in human thyroid tissue and cell lines

We investigated post-translational modification and subcellular localisation of endogenous platelet-derived growth factor-C (PDGF-C) in human thyroid papillary carcinomas (PTC), non-neoplastic thyroid tissues, and a selection of cultured cell lines. PDGF-C expressed nuclear localisation in 95% of all tested cell types in culture and in 10% of the thyrocytes from both PTC and non-neoplastic tissue. The cell lines expressed two forms of full-length PDGF-C, approximately 39 and approximately 55 kDa, in cell membrane and cytosol, while the approximately 55 kDa form dominated in the nucleus where it was partly chromatin-associated. The approximately 55 kDa form was post-translationally modified by SUMO-1. The putative PDGF-C SUMOylation site is the surface exposed (314)lysine part of a positively charged loop ((312)RPKTGVRGLHK(322)) with characteristics of a nuclear localisation signal. The tissue thyrocytes expressed a non-SUMOylated approximately 43 kDa and the 55 kDa PDGF-C. The SUMO-1 modified approximately 55 kDa PDGF-C expression was low in PTC where the approximately 43 kDa PDGF-C dominated. This is in contrast to non-neoplastic tissue and cultured cells where the SUMOylated approximately 55 kDa PDGF-C was strongly expressed. Our data provide novel evidence for nuclear localisation of PDGF-C, post-translational modification by SUMOylation and the expression of a novel form of PDGF-C in human papillary thyroid carcinomas.

PDGF-C Controls proliferation and is down-regulated by retinoic acid in mouse embryonic palatal mesenchymal cells

BACKGROUND

Platelet-derived growth factor C (PDGF-C) was recently identified as a member of the PDGF ligand family. Some observation suggests that PDGF-C could play an important role in palatogenesis highlighted by the Pdgfc(-/-) mouse with cleft palate, which led us to examine the mechanism of PDGF-C signaling in palatogenesis. It is well known that retinoic acid (RA) is a teratogen that can effectively induce cleft palate in the mouse. Due to the critical roles of PDGF-C and RA in cleft palate, the link between cleft palate induced by RA and loss of PDGF-C was investigated.

METHODS

Retarded mesenchymal proliferation is an important cause for cleft palate. To clarify the mechanism of PDGF-C in palatogenesis, we evaluated the effects of PDGF-C and anti-PDGF-C neutralizing antibody on proliferation activity in mouse embryonic palatal mesenchymal (MEPM) cells.

RESULTS

Briefly, our results show PDGF-C promotes proliferation, anti-PDGF-C antibody inhibits it in MEPM cells, and RA downregulates the PDGF-C expression both at the mRNA and protein levels.

CONCLUSIONS

These demonstrate that PDGF-C is a potent mitogen for MEPM cells, implying that inactivated PDGF-C by gene-targeting or reduced PDGF-C by RA may both cause inhibition of proliferation in palatal shelves, which might account for the pathogenesis of cleft palate in Pdgfc(-/-) mouse or RA-treated mouse. In conclusion, our results suggest that PDGF-C signaling is a new mechanism of cleft palate induced by RA.

Telomerase down-regulation does not mediate PC12 pheochromocytoma cell differentiation induced by NGF, but requires MAP kinase signalling

Telomerase is a ribonucleoprotein complex that maintains chromosomal telomere homeostasis and underlies continuous renewal of stem cells and immortalization of neoplastic cells. Telomerase is down-regulated during cell differentiation, but the mechanisms of down-regulation are largely unknown. Here, we examined roles of mitogen-activated protein (MAP) kinase and phosphatidylinositol-3 (PI3) kinase signalling pathways in telomerase down-regulation triggered by nerve growth factor (NGF), and the role of telomerase down-regulation in NGF-induced neural differentiation in PC12 cells. We report that NGF-induced telomerase down-regulation requires MAP kinase signalling. While mutations of all putative Akt phosphorylation sites in telomerase reverse transcriptase (TERT) has no effect on telomerase activity, inhibition of MAP kinase signalling by PD98059 or U0126 abolishes NGF-induced telomerase down-regulation in a concentration-dependent manner. Reversal of NGF-induced telomerase down-regulation by TERT overexpression does not prevent NGF-induced neural differentiation. Down-regulation of telomerase by silencing TERT gene expression does not trigger cell differentiation in the absence of NGF, nor enhances NGF-induced differentiation. Thus, telomerase, withdraws by a mechanism at TERT gene transcription level involving MAP kinase signalling while cells cease proliferation and undergo differentiation. The withdrawal of telomerase is not required to mediate NGF-induced PC12 cell differentiation and re-establishment of telomerase activity at significant levels does not inhibit differentiation.

Structural and functional specificities of PDGF-C and PDGF-D, the novel members of the platelet-derived growth factors family

The platelet-derived growth factor (PDGF) family was for more than 25 years assumed to consist of only PDGF-A and -B. The discovery of the novel family members PDGF-C and PDGF-D triggered a search for novel activities and complementary fine tuning between the members of this family of growth factors. Since the expansion of the PDGF family, more than 60 publications on the novel PDGF-C and PDGF-D have been presented, highlighting similarities and differences to the classical PDGFs. In this paper we review the published data on the PDGF family covering structural (gene and protein) similarities and differences among all four family members, with special focus on PDGF-C and PDGF-D expression and functions. Little information on the protein structures of PDGF-C and -D is currently available, but the PDGF-C protein may be structurally more similar to VEGF-A than to PDGF-B. PDGF-C contributes to normal development of the heart, ear, central nervous system (CNS), and kidney, while PDGF-D is active in the development of the kidney, eye and brain. In adults, PDGF-C is active in the kidney and the central nervous system. PDGF-D also plays a role in the lung and in periodontal mineralization. PDGF-C is expressed in Ewing family sarcoma and PDGF-D is linked to lung, prostate and ovarian cancers. Both PDGF-C and -D play a role in progressive renal disease, glioblastoma/medulloblastoma and fibrosis in several organs.

Platelet-derived growth factor-AA is an essential and autocrine regulator of vascular endothelial growth factor expression in non-small cell lung carcinomas

It is widely accepted that angiogenesis is required for tumor progression. Vascular endothelial growth factor (VEGF) is a key molecule for tumor angiogenesis; however, its expressional regulation is not well understood during all stages of tumorigenesis. Using cell lines and surgical specimens of human non-small cell lung cancers (NSCLCs), we here show that platelet-derived growth factor-AA (PDGF-AA) is an essential autocrine regulator for VEGF expression. To directly assess the expression of PDGF-AA-dependent VEGF and its roles in tumorigenesis, we stably transfected established cell lines with their antisense genes. In addition, the levels of PDGF-AA and VEGF expression in surgical sections were measured and compared with clinicopathologic findings such as tumor size and patient prognosis. PDGF-AA tightly regulated VEGF expression and had a greater effect on tumor size and patient prognosis than did VEGF in both cell lines and surgical sections. PDGF-AA expression was not seen in the atypical adenomatous hyperplasia at all, whereas VEGF was occasionally seen. Furthermore, the frequency of VEGF expression was higher in advanced NSCLCs than in precancerous lesions, which was tightly correspondent to the results for PDGF-AA. These results indicate that PDGF-AA is an important regulator of the frequency and level of VEGF expression during the transition from a precancerous lesion to advanced cancer. The PDGF-AA/VEGF axis, therefore, may be a ubiquitous autocrine system for enhancing angiogenic signals, and PDGF-AA, and its related pathways could be a more efficient target of antiangiogenic therapy for cancers than VEGF and its pathways.

[Vascularization of the head and neck during development]

One of the earliest priorities of the embryonic vascular system is to ensure the metabolic needs of the head. This review covers some of the principles that govern the cellular assembly and localization of blood vessels in the head. In order to understand the development and organization of the cephalic vascular tree, one needs to recall the morphogenetic movements underlying vertebrate head formation and giving rise to the constituent cells of the vascular system. Some of the major signaling molecules involved in vascular development are discussed, including the angiopoietins, the endothelins, the FGFs, the Notch receptors, the PDGFs, Sonic hedgehog, the TGF family and the VEGFs, in order to underline similarities between embryonic and postnatal vascular development, even in the context of increasingly divergent form.

Structural Requirements for Activation of Latent Platelet-derived Growth Factor CC by Tissue Plasminogen Activator

Platelet-derived growth factor C (PDGF-C) is one of four members in the PDGF family of growth factors, which are known mitogens and survival factors for cells of mesenchymal origin. PDGF-C has a unique two-domain structure consisting of an N-terminal CUB and a conserved C-terminal growth factor domain that are separated by a hinge region. PDGF-C is secreted as a latent dimeric factor (PDGF-CC), which undergoes extracellular removal of the CUB domains to become a PDGF receptor alpha agonist. Recently, the multidomain serine protease tissue plasminogen activator (tPA), a thrombolytic agent used for treatment of acute ischemic stroke, was shown to cleave and activate PDGF-CC. In this study we determine the molecular mechanism of tPA-mediated activation of PDGF-CC. Using various PDGF-CC and tPA mutants, we were able to demonstrate that both the CUB and the growth factor domains of PDGF-C, as well as the kringle-2 domain of tPA, are required for the interaction and cleavage to occur. We also show that Arg231 in PDGF-C is essential for tPA-mediated proteolysis and that the released "free" CUB domain of PDGF-C can act as a competitive inhibitor of the cleavage reaction. Furthermore, we studied how the PDGF-C/tPA axis is regulated in primary fibroblasts and found that PDGF-C expression is down-regulated by hypoxia but induced by transforming growth factor (TGF)-beta1 treatment. Elucidating the regulation and the mechanism of tPA-mediated activation of PDGF-CC will advance our knowledge of the physiological function of PDGF-CC and tPA and may provide new therapeutic opportunities for thrombolytic and cardiovascular therapies.

Angiogenesis in benign and malignant thyroid disease

CONTEXT

Angiogenesis has been recognized as an important process contributing to the pathophysiology of many benign and malignant diseases. It is not surprising, therefore, that this complex process is proving to be an important regulator of both benign and malignant disease processes in the thyroid gland. This paper will review the general principles of angiogenesis and lymphangiogenesis, as well as the importance of the balance between angiogenic stimulators and inhibitors in the normal thyroid gland. We will also review how this balance is disturbed in benign and malignant thyroid conditions. Finally, we will address the role manipulation of this process may play in the development of novel treatment strategies for diseases of the thyroid.

OBJECTIVE

To review the literature concerning the role of angiogenesis in the thyroid gland.

CONCLUSIONS

Angiogenesis is an important process which has been shown to be involved in the pathophysiology of benign and malignant diseases of the thyroid gland. Manipulation of this process holds great promise for the development of novel treatments for these disorders. As the mechanisms regulating angiogenesis in the thyroid become increasingly clear, researchers will come ever closer to turning this promise into clinical reality.

Revascularization of ischemic tissues by PDGF-CC via effects on endothelial cells and their progenitors

The angiogenic mechanism and therapeutic potential of PDGF-CC, a recently discovered member of the VEGF/PDGF superfamily, remain incompletely characterized. Here we report that PDGF-CC mobilized endothelial progenitor cells in ischemic conditions; induced differentiation of bone marrow cells into ECs; and stimulated migration of ECs. Furthermore, PDGF-CC induced the differentiation of bone marrow cells into smooth muscle cells and stimulated their growth during vessel sprouting. Moreover, delivery of PDGF-CC enhanced postischemic revascularization of the heart and limb. Modulating the activity of PDGF-CC may provide novel opportunities for treating ischemic diseases.

PDGF receptors-mediators of autocrine tumor growth and regulators of tumor vasculature and stroma

PDGFs and their cognate tyrosine kinase alpha- and beta-receptors are involved in multiple tumor-associated processes including autocrine growth stimulation of tumor cells, stimulation of tumor angiogenesis and recruitment and regulation of tumor fibroblasts. The recent development of clinically useful PDGF antagonists, like STI571/Glivec, has increased the interest in PDGF receptors as cancer drug targets. Autocrine PDGF receptor signaling occurs in certain malignancies characterized by mutational activation of PDGF or PDGF receptors, for instance, dermatofibrosaracoma protuberans, gastrointestinal stromal tumors, and hypereosinophilic syndrome. The roles of PDGF in regulation of tumor angiogenesis and tumor fibroblasts are more general, and probably occur in most common solid tumors. Concerning tumor angiogenesis recent studies have predominantly focused on the importance of PDGF receptor signaling for tumor pericyte recruitment. PDGF receptors in the tumor stroma have also attracted attention as interesting drug targets because of their function as regulators of tumor interstitial fluid pressure, tumor transvascular transport and tumor drug uptake. In summary, the improved understanding of the role of PDGF signaling in tumor biology, and the introduction of PDGF antagonists, has set the stage for a continued development of PDGF antagonists as novel cancer drugs.

PDGF and cardiovascular disease

Platelet-derived growth factor (PDGF) was identified in a search for serum factors that stimulate smooth muscle cell (SMC) proliferation. During the development of lesions of atherosclerosis that can ultimately lead to vessel occlusion, SMC are stimulated by inflammatory factors to migrate from their normal location in the media. They accumulate within the forming lesion where they contribute to lesion expansion by proliferation and deposition of extracellular matrix. Different genetic manipulations in vascular cells combined with various inhibitory strategies have provided strong evidence for PDGF playing a prominent role in the migration of SMC into the neointima following acute injury and in atherosclerosis. Other activities of PDGF identified in vivo suggest additional functions for PDGF in the pathogenesis of cardiovascular disease.

The PDGF family: four gene products form five dimeric isoforms

Platelet-derived growth factors (PDGFs) were discovered more than two decades ago. Today the PDGF family of growth factors consists of five different disulphide-linked dimers built up of four different polypeptide chains encoded by four different genes. These isoforms, PDGF-AA, PDGF-AB, PDGF-BB, PDGF-CC and PDGF-DD, act via two receptor tyrosine kinases, PDGF receptors alpha and beta. The classic PDGFs, PDGF-A and PDGF-B, undergo intracellular activation during transport in the exocytic pathway for subsequent secretion, while the novel PDGFs, PDGF-C and PDGF-D, are secreted as latent factors that require activation by extracellular proteases. The classical PDGF polypeptide chains, PDGF-A and PDGF-B, are well studied and they regulate several physiological and pathophysiological processes, mainly using cells of mesenchymal or neuroectodermal origin as their targets. The discovery of two additional ligands for the two PDGF receptors suggests that PDGF-mediated cellular signaling is more complex than previously thought.

Role of the vascular endothelial growth factor pathway in tumor growth and angiogenesis

New blood vessel formation (angiogenesis) is a fundamental event in the process of tumor growth and metastatic dissemination. Hence, the molecular basis of tumor angiogenesis has been of keen interest in the field of cancer research. The vascular endothelial growth factor (VEGF) pathway is well established as one of the key regulators of this process. The VEGF/VEGF-receptor axis is composed of multiple ligands and receptors with overlapping and distinct ligand-receptor binding specificities, cell-type expression, and function. Activation of the VEGF-receptor pathway triggers a network of signaling processes that promote endothelial cell growth, migration, and survival from pre-existing vasculature. In addition, VEGF mediates vessel permeability, and has been associated with malignant effusions. More recently, an important role for VEGF has emerged in mobilization of endothelial progenitor cells from the bone marrow to distant sites of neovascularization. The well-established role of VEGF in promoting tumor angiogenesis and the pathogenesis of human cancers has led to the rational design and development of agents that selectively target this pathway. Studies with various anti-VEGF/VEGF-receptor therapies have shown that these agents can potently inhibit angiogenesis and tumor growth in preclinical models. Recently, an anti-VEGF antibody (bevacizumab), when used in combination with chemotherapy, was shown to significantly improve survival and response rates in patients with metastatic colorectal cancer and thus, validate VEGF pathway inhibitors as an important new treatment modality in cancer therapy.

The vascular endothelial growth factor family and its receptors

This article focuses on describing the biology of vascular endothelial growth factor (VEGF) and its receptors as well as the regulation of their expression. A thorough understanding of the VEGF system is paramount in optimizing antiangiogenic therapies as a component of antineoplastic regimens.

PDGF-BB induces intratumoral lymphangiogenesis and promotes lymphatic metastasis

Cancer metastases are commonly found in the lymphatic system. Like tumor blood angiogenesis, stimulation of tumor lymphangiogenesis may require the interplay of several tumor-derived growth factors. Here we report that members of the PDGF family act as lymphangiogenic factors. In vitro, PDGF-BB stimulated MAP kinase activity and cell motility of isolated lymphatic endothelial cells. In vivo, PDGF-BB potently induced growth of lymphatic vessels. Expression of PDGF-BB in murine fibrosarcoma cells induced tumor lymphangiogenesis, leading to enhanced metastasis in lymph nodes. These data demonstrate that PDGF-BB is an important growth factor contributing to lymphatic metastasis. Thus, blockage of PDGF-induced lymphangiogenesis may provide a novel approach for prevention and treatment of lymphatic metastasis.

Tissue plasminogen activator is a potent activator of PDGF-CC

Tissue plasminogen activator (tPA) is a serine protease involved in the degradation of blood clots through the activation of plasminogen to plasmin. Here we report on the identification of tPA as a specific protease able to activate platelet-derived growth factor C (PDGF-C). The newly identified PDGF-C is secreted as a latent dimeric factor (PDGF-CC) that upon proteolytic removal of the N-terminal CUB domains becomes a PDGF receptor alpha agonist. The CUB domains in PDGF-CC directly interact with tPA, and fibroblasts from tPA-deficient mice fail to activate latent PDGF-CC. We further demonstrate that growth of primary fibroblasts in culture is dependent on a tPA-mediated cleavage of latent PDGF-CC, generating a growth stimulatory loop. Immunohistochemical analysis showed similar expression patterns of PDGF-C and tPA in developing mouse embryos and in tumors, indicating both autocrine and paracrine modes of activation of PDGF receptor-mediated signaling pathways. The identification of tPA as an activator of PDGF signaling establishes a novel role for the protease in normal and pathological tissue growth and maintenance, distinct from its well-known role in plasminogen activation and fibrinolysis.

PDGF C is a selective alpha platelet-derived growth factor receptor agonist that is highly expressed in platelet alpha granules and vascular smooth muscle

OBJECTIVE

The platelet-derived growth factor (PDGF) family consists of four members, PDGF A, PDGF B, and 2 new members, PDGF C and PDGF D, which signal through the alpha and beta PDGF receptor (PDGFR) tyrosine kinases. This study was performed to determine the receptor specificity and cellular expression profile of PDGF C.

METHODS AND RESULTS

PDGF C growth factor domain (GFD) was shown to preferentially bind and activate alpha PDGFR and activate beta PDGFR when it is co-expressed with alpha PDGFR through heterodimer formation. An investigation of PDGF C mRNA and protein expression revealed that during mouse fetal development, PDGF C was expressed in the mesonephric mesenchyme, prefusion skeletal muscle, cardiac myoblasts, and in visceral and vascular smooth muscle, whereas in adult human tissues expression was largely restricted to smooth muscle. Microarray analysis of various cell types showed PDGF C expression in vascular smooth muscle cells, renal mesangial cells, and platelets. PDGF C mRNA expression in platelets was confirmed by real-time polymerase chain reaction, and PDGF C protein was localized in alpha granules by immuno-gold electron microscopy. Western blot analysis of platelets identified 55-kDa and 80-kDa PDGF C isoforms that were secreted on platelet activation.

CONCLUSIONS

Taken together, our results demonstrated for the first time to our knowledge that like PDGF A and B, PDGF C is likely to play a role in platelet biology.

Fibroblast growth factor-2 induction of platelet-derived growth factor-C chain transcription in vascular smooth muscle cells is ERK-dependent but not JNK-dependent and mediated by Egr-1

Platelet-derived growth factors (PDGFs) play an integral role in normal tissue growth and maintenance as well as many human pathological states including atherosclerosis, fibrosis, and tumorigenesis. The PDGF family of ligands is comprised of A, B, C, and D chains. Here, we provide the first functional characterization of the PDGF-C promoter. We examined 797 bp of the human PDGF-C promoter and identified several putative recognition elements for Sp1, Ets Egr-1, and Smad. The proximal region of the PDGF-C promoter bears a remarkable resemblance to a comparable region of the PDGF-A promoter (1). Binding and transient transfection analysis in primary vascular smooth muscle cells revealed that PDGF-C, like PDGF-A, is under the transcriptional control of the zinc finger nuclear protein Egr-1 (early growth response-1). Electrophoretic mobility shift analysis using both smooth muscle cell nuclear extracts and recombinant protein revealed that Egr-1 and Sp1 bind this region of the PDGF-C promoter (Oligo C, -35 to -1). Egr-1 competes with Sp1 for overlapping binding sites even when the former is at a stoichiometric disadvantage. Reverse transcriptase PCR and supershift analysis demonstrate that fibroblast growth factor-2 (FGF-2) stimulates both Egr-1 and PDGF-C mRNA expression in a time-dependent and transient manner and that FGF-2-inducible Egr-1 binds the proximal PDGF-C promoter. FGF-2-inducible PDGF-C expression was completely abrogated using catalytic DNA (DNAzymes) targeting Egr-1 but not by its scrambled counterpart. Moreover, using pharmacological inhibitors we demonstrate the critical role of ERK but not JNK in FGF-2-inducible PDGF-C expression. These findings thus demonstrate that PDGF-C transcription, activated by FGF-2, is mediated by Egr-1 and its upstream kinase ERK.

Haemangiopericytoma of the thyroid gland in combination with Hashimoto?s disease

We present a hitherto unique case of haemangiopericytoma (HP) of the thyroid gland in a 15-year-old female patient suffering from Hashimoto's disease for several months. Since angiogenesis has been discussed to play a major role in both diseases, we examined the expression of vascular endothelial growth factor (VEGF), VEGF receptors (VEGFRs) and platelet-derived growth factor receptors (PDGFRs). Most interestingly, strong expression of PDGFR alpha and beta was found in spindle-shaped tumour cells and tumour vessels in HP, while VEGF and VEGFR type I and -II were negative in these regions. In contrast, VEGF was expressed in the lymphoid infiltrate of Hashimoto's disease. Since PDGFR-beta is commonly expressed in pericytes, we suggest that the strong expression discovered in this study further supports the view that HP is derived from pericytes. The combination of HP and Hashimoto's disease is most probably a coincidental event. However, this case confirms previous reports demonstrating that in patients with Hashimoto's disease different neoplasias can occur.

Simultaneous blockade of platelet-derived growth factor-receptor and epidermal growth factor-receptor signaling and systemic administration of paclitaxel as therapy for human prostate cancer metastasis in bone of nude mice

Once prostate cancer metastasizes to bone, conventional chemotherapy is largely ineffective. We hypothesized that inhibition of phosphorylation of the epidermal growth factor receptor (EGF-R) and platelet-derived growth factor receptor (PDGF-R) expressed on tumor cells and tumor-associated endothelial cells, which is associated with tumor progression, in combination with paclitaxel would inhibit experimental prostate cancer bone metastasis and preserve bone structure. We tested this hypothesis in nude mice, using human PC-3MM2 prostate cancer cells. PC-3MM2 cells growing adjacent to bone tissue and endothelial cells within these lesions expressed phosphorylated EGF-R and PDGF-R alpha and -beta on their surfaces. The percentage of positive endothelial cells and the intensity of receptor expression directly correlated with proximity to bone tissue. Oral administration of PKI166 inhibited the phosphorylation of EGF-R but not PDGF-R, whereas oral administration of STI571 inhibited the phosphorylation of PDGF-R but not EGF-R. Combination therapy using oral PKI166 and STI571 with i.p. injections of paclitaxel induced a high level of apoptosis in tumor vascular endothelial cells and tumor cells in parallel with inhibition of tumor growth in the bone, preservation of bone structure, and reduction of lymph node metastasis. Collectively, these data demonstrate that blockade of phosphorylation of EGF-R and PDGF-R coupled with administration of paclitaxel significantly suppresses experimental human prostate cancer bone metastasis.

Platelet-derived growth factor production by B16 melanoma cells leads to increased pericyte abundance in tumors and an associated increase in tumor growth rate

Platelet-derived growth factor (PDGF) receptor signaling participates in different processes in solid tumors, including autocrine stimulation of tumor cell growth, recruitment of tumor stroma fibroblasts, and stimulation of tumor angiogenesis. In the present study, the B16 mouse melanoma tumor model was used to investigate the functional consequences of paracrine PDGF stimulation of host-derived cells. Production of PDGF-BB or PDGF-DD by tumor cells was associated with an increased tumor growth rate. Characterization of tumors revealed an increase in pericyte abundance in tumors derived from B16 cells producing PDGF-BB or PDGF-DD. The increased tumor growth rate associated with PDGF-DD production was not seen in mice expressing an attenuated PDGF beta-receptor and was thus dependent on host PDGF beta-receptor signaling. The increased pericyte abundance was not associated with an increased tumor vessel density. However, tumor cell apoptosis, but not proliferation, was reduced in tumors displaying PDGF-induced increased pericyte coverage. Our findings thus demonstrate that paracrine PDGF production stimulates pericyte recruitment to tumor vessels and suggest that pericyte abundance influences tumor cell apoptosis and tumor growth.