Endothelial cell hyperplasia in human glioblastoma: coexpression of mRNA for platelet-derived growth factor (PDGF) B chain and PDGF receptor suggests autocrine growth stimulation

Understanding Neovascularization in Glioblastoma: Insights from the Current Literature

Glioblastomas (GBMs), among the most aggressive and resilient brain tumors, characteristically exhibit high angiogenic potential, leading to the formation of a dense yet aberrant vasculature, both morphologically and functionally. With these premises, numerous expectations were initially placed on anti-angiogenic therapies, soon dashed by their limited efficacy in concretely improving patient outcomes. Neovascularization in GBM soon emerged as a complex, dynamic, and heterogeneous process, hard to manage with the classical standard of care. Growing evidence has revealed the existence of numerous non-canonical strategies of angiogenesis, variously exploited by GBM to meet its ever-increasing metabolic demand and differently involved in tumor progression, recurrence, and escape from treatments. In this review, we provide an accurate description of each neovascularization mode encountered in GBM tumors to date, highlighting the molecular players and signaling cascades primarily involved. We also detail the key architectural and functional aspects characteristic of the GBM vascular compartment because of an intricate crosstalk between the different angiogenic networks. Additionally, we explore the repertoire of emerging therapies against GBM that are currently under study, concluding with a question: faced with such a challenging scenario, could combined therapies, tailored to the patient's genetic signatures, represent an effective game changer?

The relationships of platelet-derived growth factor, microvascular proliferation, and tumor cell proliferation in canine high-grade oligodendrogliomas: Immunohistochemistry of 45 tumors and an AFOB-01 xenograft mouse model

High-grade oligodendroglioma (HGOG) is the most common type of glioma in dogs and expresses platelet-derived growth factor receptor-α (PDGFR-α). Microvascular proliferation is often observed in HGOG. Therefore, the present study investigated the functional relationships between PDGFR-α, microvascular proliferation, and tumor cell proliferation in canine HGOG. The expression of PDGFR-α and PDGF-subunit A (PDGF-A) in tumor cells, as well as endothelial cells and pericytes of tumor-associated microvascular proliferations, in 45 canine HGOGs were examined immunohistochemically. Microvascular proliferation was observed in 24/45 cases (53%). PDGFR-α expression in tumor cells and microvascular proliferations was observed in 45/45 (100%) and 2/24 cases (8%), respectively. Furthermore, PDGF-A expression in tumor cells and microvascular proliferations was detected in 13/45 (29%) and 24/24 cases (100%), respectively. In vitro, stimulation of the canine HGOG cell line AOFB-01 with PDGF-A showed that the doubling time of AOFB-01 cells was significantly shorter with PDGF-A than without PDGF-A. Crenolanib (a PDGFR inhibitor) inhibited AOFB-01 cell proliferation. In vivo, the AOFB-01 xenograft mouse model was treated with crenolanib. Tumor xenografts were smaller in crenolanib-treated mice than in untreated control mice. PDGFR-α expression in tumor cells and PDGF-A expression in microvascular proliferations and tumor cells suggest autocrine and paracrine effects of PDGF-A in canine HGOG. The results of in vitro assays indicate that canine HGOG expresses functional PDGFR-α, which responds to PDGF-A. Therefore, PDGF-A produced by microvascular proliferations and tumor cells may promote the proliferation of PDGFR-α-expressing tumor cells in canine HGOG. PDGFR-α signaling has potential as a therapeutic target.

Tumor Niches: Perspectives for Targeted Therapies in Glioblastoma

Significance: Glioblastoma (GBM), the most common and lethal primary brain tumor with a median survival rate of only 15 months and a 5-year survival rate of only 6.8%, remains largely incurable despite the intensive multimodal treatment of surgical resection and radiochemotherapy. Developing effective new therapies is an unmet need for patients with GBM. Recent Advances: Targeted therapies, such as antiangiogenesis therapy and immunotherapy, show great promise in treating GBM based upon increasing knowledge about brain tumor biology. Single-cell transcriptomics reveals the plasticity, heterogeneity, and dynamics of tumor cells during GBM development and progression. Critical Issues: While antiangiogenesis therapy and immunotherapy have been highly effective in some types of cancer, the disappointing results from clinical trials represent continued challenges in applying these treatments to GBM. Molecular and cellular heterogeneity of GBM is developed temporally and spatially, which profoundly contributes to therapeutic resistance and tumor recurrence. Future Directions: Deciphering mechanisms of tumor heterogeneity and mapping tumor niche trajectories and functions will provide a foundation for the development of more effective therapies for GBM patients. In this review, we discuss five different tumor niches and the intercellular and intracellular communications among these niches, including the perivascular, hypoxic, invasive, immunosuppressive, and glioma-stem cell niches. We also highlight the cellular and molecular biology of these niches and discuss potential strategies to target these tumor niches for GBM therapy. Antioxid. Redox Signal. 39, 904-922.

Multiparametric MRI radiomics for the differentiation of brain glial cell hyperplasia from low-grade glioma

BACKGROUND

Differentiating between low-grade glioma and brain glial cell hyperplasia is crucial for the customized clinical treatment of patients.

OBJECTIVE

Based on multiparametric MRI imaging and clinical risk factors, a radiomics-clinical model and nomogram were constructed for the distinction of brain glial cell hyperplasia from low-grade glioma.

METHODS

Patients with brain glial cell hyperplasia and low-grade glioma who underwent surgery at the First Affiliated Hospital of Soochow University from March 2016 to March 2022 were retrospectively included. In this study, A total of 41 patients of brain glial cell hyperplasia and 87 patients of low-grade glioma were divided into training group and validation group randomly at a ratio of 7:3. Radiomics features were extracted from T1-weighted imaging (T1WI), T2-weighted imaging (T2WI), diffusion-weighted imaging (DWI), contrast-enhanced T1-weighted imaging (T1-enhanced). Then, LASSO, SVM, and RF models were created in order to choose a model with a greater level of efficiency for calculating each patient's Rad-score (radiomics score). The independent risk factors were identified via univariate and multivariate logistic regression analysis to filter the Rad-score and clinical risk variables in turn. A radiomics-clinical model was next built of which effectiveness was assessed.

RESULTS

Brain glial cell hyperplasia and low-grade gliomas from the 128 cases were randomly divided into 10 groups, of which 7 served as training group and 3 as validation group. The mass effect and Rad-score were two independent risk variables used in the construction of the radiomics-clinical model, and their respective AUCs for the training group and validation group were 0.847 and 0.858. The diagnostic accuracy, sensitivity, and specificity of the validation group were 0.821, 0.750, and 0.852 respectively.

CONCLUSION

Combining with radiomics constructed by multiparametric MRI images and clinical features, the radiomics-clinical model and nomogram that were developed to distinguish between brain glial cell hyperplasia and low-grade glioma had a good performance.

PDGF signaling inhibits mitophagy in glioblastoma stem cells through N6-methyladenosine

Dysregulated growth factor receptor pathways, RNA modifications, and metabolism each promote tumor heterogeneity. Here, we demonstrate that platelet-derived growth factor (PDGF) signaling induces N6-methyladenosine (m6A) accumulation in glioblastoma (GBM) stem cells (GSCs) to regulate mitophagy. PDGF ligands stimulate early growth response 1 (EGR1) transcription to induce methyltransferase-like 3 (METTL3) to promote GSC proliferation and self-renewal. Targeting the PDGF-METTL3 axis inhibits mitophagy by regulating m6A modification of optineurin (OPTN). Forced OPTN expression phenocopies PDGF inhibition, and OPTN levels portend longer survival of GBM patients; these results suggest a tumor-suppressive role for OPTN. Pharmacologic targeting of METTL3 augments anti-tumor efficacy of PDGF receptor (PDGFR) and mitophagy inhibitors in vitro and in vivo. Collectively, we define PDGF signaling as an upstream regulator of oncogenic m6A regulation, driving tumor metabolism to promote cancer stem cell maintenance, highlighting PDGF-METTL3-OPTN signaling as a GBM therapeutic target.

A review of gliomas-related proteins. Characteristics of potential biomarkers

Brain tumors are one of the most commonly diagnosed cancers of the central nervous system. Of all diagnosed malignant tumors, 80% are gliomas. An unequivocal diagnosis of gliomas is not always simple, and there is a great need for research to find new treatment options and diagnostic approaches. This paper is focused on the glioma-related protein profiles as compared to healthy brain tissue, which is reflected in multiple correlations between biological aspects that influence proliferation, apoptosis evasion and the invasiveness of neoplastic cells. The work presents the possibilities of facilitating clinical practice with proteomic biomarkers, which offer a wider diagnostic spectrum and reduce the margin of mistake in histopathological or imaging diagnostic methods. In fact, many changes in the body's homeostasis can be overlooked due to the lack of symptoms or their non-specificity. Nevertheless, a single marker has limited reliability in distinguishing a particular tumor subtype, since the increased or decreased level of the protein of interest may differ between the stages or locations of the tumor. Moreover, the correlations between proposed proteins - presented in this paper - may help clinicians to choose the most optimal therapy, and estimate its effectiveness, or indicate new therapeutic targets affecting disrupted biochemical pathways.

Tumor Development and Angiogenesis in Adult Brain Tumor: Glioblastoma

Angiogenesis is the growth of new capillaries from the preexisting blood vessels. Glioblastoma (GBM) tumors are highly vascularized tumors, and glioma growth depends on the formation of new blood vessels. Angiogenesis is a complex process involving proliferation, migration, and differentiation of vascular endothelial cells (ECs) under the stimulation of specific signals. It is controlled by the balance between its promoting and inhibiting factors. Various angiogenic factors and genes have been identified that stimulate glioma angiogenesis. Therefore, attention has been directed to anti-angiogenesis therapy in which glioma proliferation is inhibited by inhibiting the formation of new tumor vessels using angiogenesis inhibitory factors and drugs. Here, in this review, we highlight and summarize the various molecular mediators that regulate GBM angiogenesis with focus on recent clinical research on the potential of exploiting angiogenic pathways as a strategy in the treatment of GBM patients.

VEGF overexpression is associated with optic nerve involvement and differentiation of retinoblastoma: A PRISMA-compliant meta-analysis

BACKGROUND

Vascular endothelial growth factor (VEGF) plays an important role in the pathogenesis of cancer. Although numerous studies have investigated the association between VEGF expression and pathogenesis of retinoblastoma, the results remained inconsistent. To illuminate the association, we performed a meta-analysis study.

METHODS

According to the PRISMA guideline, eligible studies were searched in the Medicine, Embase, Web of Science, Chinese National Knowledge Infrastructure, and Wanfang databases. Stata 14.0 software was used to calculate the relevant statistical parameters.

RESULTS

Seventeen studies with 296 controls and 470 patients with retinoblastoma were included from 17 eligible literatures. Overall, significant association between VEGF overexpression and susceptibility of retinoblastoma was observed in Chinese population (odds ratio [OR] = 21.67, 95% confidence interval [CI] = 13.96-33.62). Subgroup analysis based on control sample type showed that VEGF overexpression was significantly associated with the risk of retinoblastoma (Normal retina tissue, OR = 23.97, 95% CI = 9.67-59.42; retinoblastoma adjacent tissue, OR = 20.85, 95% CI = 12.64-34.37). Significant associations of VEGF overexpression with optic nerve involvement and differentiation of retinoblastoma were found (Optic nerve involvement, OR = 6.90, 95% CI = 4.01-11.88; Differentiation, OR = 0.18, 95% CI = 0.12-0.28). In addition, only 1 study was included to analyze the role of VEGF protein expression in the prognosis of retinoblastoma, and the result showed that VEGF expression was significantly associated with the prognosis of retinoblastoma, which should be verified in the future studies.

CONCLUSIONS

Our findings demonstrated that VEGF overexpression was significantly associated with the risk of retinoblastoma. Besides, the results suggested that VEGF overexpression might have a crucial effect on the optic nerve involvement and differentiation of retinoblastoma.

The role of inflammation in subventricular zone cancer

The adult subventricular zone (SVZ) stem cell niche has proven vital for discovering neurodevelopmental mechanisms and holds great potential in medicine for neurodegenerative diseases. Yet the SVZ holds a dark side - it can become tumorigenic. Glioblastomas can arise from the SVZ via cancer stem cells (CSCs). Glioblastoma and other brain cancers often have dismal prognoses since they are resistant to treatment. In this review we argue that the SVZ is susceptible to cancer because it contains stem cells, migratory progenitors and unusual inflammation. Theoretically, SVZ stem cells can convert to CSCs more readily than can postmitotic neural cells. Additionally, the robust long-distance migration of SVZ progenitors can be subverted upon tumorigenesis to an infiltrative phenotype. There is evidence that the SVZ, even in health, exhibits chronic low-grade cellular and molecular inflammation. Its inflammatory response to brain injuries and disease differs from that of other brain regions. We hypothesize that the SVZ inflammatory environment can predispose cells to novel mutations and exacerbate cancer phenotypes. This can be studied in animal models in which human mutations related to cancer are knocked into the SVZ to induce tumorigenesis and the CSC immune interactions that precede full-blown cancer. Importantly inflammation can be pharmacologically modulated providing an avenue to brain cancer management and treatment. The SVZ is accessible by virtue of its location surrounding the lateral ventricles and CSCs in the SVZ can be targeted with a variety of pharmacotherapies. Thus, the SVZ can yield aggressive tumors but can be targeted via several strategies.

Temporal and spatial specificity of PDGF alpha receptor promoter in transgenic mice

Aberrant expression of the platelet-derived growth factor alpha receptor (PDGF alpha R) has been linked to developmental abnormalities in vertebrate models, and has been implicated in multiple disease states in humans. To identify cis-acting regulatory elements that dictate expression of this receptor, we generated transgenic mice bearing the reporter gene beta-galactosidase (lacZ) under the control of a 6-kb promoter sequence. Expression of lacZ was monitored throughout embryonic development, with special focus on nervous tissue, skeleton, and several organ systems wherein PDGF alpha R expression is thought to play a pivotal role. In several independent transgenic mouse strains, lacZ expression recapitulated predominant features of PDGF alpha R gene expression during mouse development. These results demonstrate that critical tissue-specific regulatory elements for PDGF alpha R expression are located within a 6-kb upstream region of the PDGF alpha R gene.

Characterizing Glioblastoma Heterogeneity via Single-Cell Receptor Quantification

Dysregulation of tyrosine kinase receptor (RTK) signaling pathways play important roles in glioblastoma (GBM). However, therapies targeting these signaling pathways have not been successful, partially because of drug resistance. Increasing evidence suggests that tumor heterogeneity, more specifically, GBM-associated stem and endothelial cell heterogeneity, may contribute to drug resistance. In this perspective article, we introduce a high-throughput, quantitative approach to profile plasma membrane RTKs on single cells. First, we review the roles of RTKs in cancer. Then, we discuss the sources of cell heterogeneity in GBM, providing context to the key cells directing resistance to drugs. Finally, we present our provisionally patented qFlow cytometry approach, and report results of a "proof of concept" patient-derived xenograft GBM study.

Chronic platelet-derived growth factor receptor signaling exerts control over initiation of protein translation in glioma

Using phospho-proteomics in a new model of malignant glioma, we reveal that clinically relevant, chronic PDGFRα signaling differs considerably from acute receptor stimulation and unveils previously unrecognized control over key elements of the translation initiation machinery.

Activation of the platelet-derived growth factor receptors (PDGFRs) gives rise to some of the most important signaling pathways that regulate mammalian cellular growth, survival, proliferation, and differentiation and their misregulation is common in a variety of diseases. Herein, we present a comprehensive and detailed map of PDGFR signaling pathways assembled from literature and integrate this map in a bioinformatics protocol designed to extract meaningful information from large-scale quantitative proteomics mass spectrometry data. We demonstrate the usefulness of this approach using a new genetically engineered mouse model of PDGFRα-driven glioma. We discovered that acute PDGFRα stimulation differs considerably from chronic receptor activation in the regulation of protein translation initiation. Transient stimulation activates several key components of the translation initiation machinery, whereas the clinically relevant chronic activity of PDGFRα is associated with a significant shutdown of translational members. Our work defines a step-by-step approach to extract biologically relevant insights from global unbiased phospho-protein datasets to uncover targets for therapeutic assessment.

PDGF Family Expression in Glioblastoma Multiforme: Data Compilation from Ivy Glioblastoma Atlas Project Database

Glioblastoma Multiforme (GBM) is the most frequent and lethal primary brain cancer. Due to its therapeutic resistance and aggressiveness, its clinical management is challenging. Platelet-derived Growth Factor (PDGF) genes have been enrolled as drivers of this tumour progression as well as potential therapeutic targets. As detailed understanding of the expression pattern of PDGF system in the context of GBM intra- and intertumoral heterogeneity is lacking in the literature, this study aims at characterising PDGF expression in different histologically-defined GBM regions as well as investigating correlation of these genes expression with parameters related to poor prognosis. Z-score normalised expression values of PDGF subunits from multiple slices of 36 GBMs, alongside with clinical and genomic data on those GBMs patients, were compiled from Ivy Glioblastoma Atlas Project – Allen Institute for Brain Science data sets. PDGF subunits show differential expression over distinct regions of GBM and PDGF family is heterogeneously expressed among different brain lobes affected by GBM. Further, PDGF family expression correlates with bad prognosis factors: age at GBM diagnosis, Phosphatase and Tensin Homolog deletion and Isocitrate Dehydrogenase 1 mutation. These findings may aid on clinical management of GBM and development of targeted curative therapies against this devastating tumour.

Platelet-derived growth factor-C and -D in the cardiovascular system and diseases

The cardiovascular system is among the first organs formed during development and is pivotal for the formation and function of the rest of the organs and tissues. Therefore, the function and homeostasis of the cardiovascular system are finely regulated by many important molecules. Extensive studies have shown that platelet-derived growth factors (PDGFs) and their receptors are critical regulators of the cardiovascular system. Even though PDGF-C and PDGF-D are relatively new members of the PDGF family, their critical roles in the cardiovascular system as angiogenic and survival factors have been amply demonstrated. Understanding the functions of PDGF-C and PDGF-D and the signaling pathways involved may provide novel insights into both basic biomedical research and new therapeutic possibilities for the treatment of cardiovascular diseases.

Platelet-Rich Plasma as an Autologous and Proangiogenic Cell Delivery System

Angiogenesis is a key factor in early stages of wound healing and is crucial for the repair of vascularized tissues such as the bone. However, supporting timely revascularization of the defect site still presents a clinical challenge. Tissue engineering approaches delivering endothelial cells or prevascularized constructs may overcome this problem. In the current study, we investigated platelet-rich plasma (PRP) gels as autologous, injectable cell delivery systems for prevascularized constructs. PRP was produced from human thrombocyte concentrates. GFP-expressing human umbilical vein endothelial cells (HUVECs) and human bone marrow-derived mesenchymal stem cells (MSCs) were encapsulated in PRP gels in different proportions. The formation of cellular networks was assessed over 14 days by time-lapse microscopy, gene expression analysis, and immunohistology. PRP gels presented a favorable environment for the formation of a three-dimensional (3D) cellular network. The formation of these networks was apparent as early as 3 days after seeding. Networks increased in complexity and branching over time but were only stable in HUVEC-MSC cocultures. The high cell viability together with the 3D capillary-like networks observed at early time points suggests that PRP can be used as an autologous and proangiogenic cell delivery system for the repair of vascularized tissues such as the bone.

Silencing platelet-derived growth factor receptor-β enhances the radiosensitivity of C6 glioma cells in vitro and in vivo

Platelet-derived growth factor receptor (PDGFR)-β is an important tyrosine kinase and its downregulation has been reported to alter the radiosensitivity of glioma cells, although the underlying mechanism is unclear. In order to investigate the effect of PDGFR-β on the radiosensitivity of glioblastoma, the present study transfected C6 glioma cells with a PDGFR-β-specific small interfering (si)RNA expression plasmid, and downregulation of the expression of PDGFR-β in C6 glioma cells was confirmed by western blotting and immunohistochemical analysis. Clone formation assays and xenograft growth curves demonstrated that PDGFR-β-siRNA enhanced the radiosensitivity of C6 glioma cells in vitro and in vivo. Furthermore, MTT and xenograft growth curves demonstrated that PDGFR-β-siRNA inhibited the proliferation of C6 glioma cells in vitro and in vivo, and terminal deoxynucleotidyl transferase dUTP nick end-labeling and immunohistochemical analyses demonstrated that PDGFR-β-siRNA induced apoptosis and inhibited the expression of Ki-67, cyclin B1 and vascular endothelial growth factor in C6 glioma cell xenografts. Taken together, these results suggested that PDGFR-β may be used as a target for the radiosensitization of glioblastoma.

Fit for the Eye: Aptamers in Ocular Disorders

For any new class of therapeutics, there are certain types of indications that represent a natural fit. For nucleic acid ligands in general, and aptamers in particular, the eye has historically been an attractive site for therapeutic intervention. In this review, we recount the discovery and early development of three aptamers designated for use in ophthalmology, one approved (Macugen), and two in late-stage development (Fovista and Zimura). Every one of these molecules was originally intended for other indications. Key improvements in technology, specifically with regard to libraries used for in vitro selection and subsequent chemical optimization of aptamers, have played an important role in allowing the identification of development candidates with suitable properties. The lessons learned from the selection of these molecules are valuable for informing us about the many remaining opportunities for aptamer-based therapeutics in ophthalmology as well as for identifying additional indications for which aptamers as a class of therapeutics have distinct advantages.

TRF2 acts as a transcriptional regulator in tumor angiogenesis

We recently showed that telomeric repeat-binding factor 2 (TRF2) regulates gene expression to promote angiogenesis. We found that TRF2 is highly expressed in tumor vessels and transcriptionally activates platelet-derived growth factor receptor β to promote endothelial cell angiogenic properties independently of its function in telomere protection. This work identifies TRF2 as a promising dual target for cancer therapy.

Most human non-GCIMP glioblastoma subtypes evolve from a common proneural-like precursor glioma

To understand the relationships between the non-GCIMP glioblastoma (GBM) subgroups, we performed mathematical modeling to predict the temporal sequence of driver events during tumorigenesis. The most common order of evolutionary events is 1) chromosome (chr) 7 gain and chr10 loss, followed by 2) CDKN2A loss and/or TP53 mutation, and 3) alterations canonical for specific subtypes. We then developed a computational methodology to identify drivers of broad copy number changes, identifying PDGFA (chr7) and PTEN (chr10) as driving initial nondisjunction events. These predictions were validated using mouse modeling, showing that PDGFA is sufficient to induce proneural-like gliomas and that additional NF1 loss converts proneural to the mesenchymal subtype. Our findings suggest that most non-GCIMP mesenchymal GBMs arise as, and evolve from, a proneural-like precursor.

Angiogenesis and expression of PDGF-C, VEGF, CD105 and HIF-1α in human glioblastoma

Glioblastoma (GBM), the most frequent and aggressive brain tumor, is characterized by marked angiogenesis directly related to invasiveness and poor prognosis. Hypoxia is considered to be an important stimulus for angiogenesis by inducing hypoxia-inducible factor 1-alpha (HIF-1α) overexpression that activates platelet-derived growth factor (PDGF) and VEGF. The aim of this study is to analyze the expression of PDGF-C, VEGF in endothelial and tumor cells of GBM and their relation to HIF-1α expression. Two hundred and eight GBM cases were studied by tissue microarray immunohistochemical preparation. Expression of HIF-1α, VEGF and PDGF-C was observed in 184 (88.5%), 131 (63%) and 160 (76.9%) tumor cases, respectively. The numbers of vessels were quantified by CD34, PDGF-C, VEGF and CD105 staining, and were in median 20, 16, 5 and 6, respectively. The GBMs that showed positive or negative expression for HIF-1α showed a median vascular density of 30 and 14, respectively, for CD34 (P < 0.015). Positive expression for HIF-1α was correlated with VEGF and PDGF-C expression in tumors (P < 0.001). There was a significant correlation between VEGF and PDGF-C expression in the cytoplasm of GBM tumor cells (P < 0.0001). We showed that VEGF expression in tumor cells was correlated with its expression in blood vessels (P < 0.0001). Endothelial cells with PDGF-C and VEGF positive expression were also positive for CD105 and their nuclei for Ki-67, confirming the neoangiogenic and proliferative influence of VEGF and PDGF-C. VEGF nuclear staining in tumor cells (P = 0.002) as well as nuclear staining for HIF-1α and VEGF (P = 0.005) correlated with survival. In summary, our present findings of the concomitant upregulation of PDGF-C with VEGF in GBM tumor cells and vessels further reinforce the benefit of using combined anti-angiogenic approaches to potentially improve the therapeutic response for GBM.

Platelet-derived growth factor-C (PDGF-C) induces anti-apoptotic effects on macrophages through Akt and Bad phosphorylation

PDGF-C, which is abundant in the malignant breast tumor microenvironment, plays an important role in cell growth and survival. Because tumor-associated macrophages (TAMs) contribute to cancer malignancy, macrophage survival mechanisms are an attractive area of research into controlling tumor progression. In this study, we investigated PDGF-C-mediated signaling pathways involved in anti-apoptotic effects in macrophages. We found that the human malignant breast cancer cell line MDA-MB-231 produced high quantities of PDGF-C, whereas benign MCF-7 cells did not. Recombinant PDGF-C induced PDGF receptor α chain phosphorylation, followed by Akt and Bad phosphorylation in THP-1-derived macrophages. MDA-MB-231 culture supernatants also activated macrophage PDGF-Rα. PDGF-C prevented staurosporine-induced macrophage apoptosis by inhibiting the activation of caspase-3, -7, -8, and -9 and cleavage of poly(ADP-ribose) polymerase. Finally, TAMs isolated from the PDGF-C knockdown murine breast cancer cell line 4T1 and PDGF-C knockdown MDA-MB-231-derived tumor mass showed higher rates of apoptosis than the respective WT controls. Collectively, our results suggest that tumor cell-derived PDGF-C enhances TAM survival, promoting tumor malignancy.

A multi-parametric imaging investigation of the response of C6 glioma xenografts to MLN0518 (tandutinib) treatment

Angiogenesis, the development of new blood vessels, is essential for tumour growth; this process is stimulated by the secretion of numerous growth factors including platelet derived growth factor (PDGF). PDGF signalling, through its receptor platelet derived growth factor receptor (PDGFR), is involved in vessel maturation, stimulation of angiogenesis and upregulation of other angiogenic factors, including vascular endothelial growth factor (VEGF). PDGFR is a promising target for anti-cancer therapy because it is expressed on both tumour cells and stromal cells associated with the vasculature. MLN0518 (tandutinib) is a potent inhibitor of type III receptor tyrosine kinases that demonstrates activity against PDGFRα/β, FLT3 and c-KIT. In this study a multi-parametric MRI and histopathological approach was used to interrogate changes in vascular haemodynamics, structural response and hypoxia in C6 glioma xenografts in response to treatment with MLN0518. The doubling time of tumours in mice treated with MLN0518 was significantly longer than tumours in vehicle treated mice. The perfused vessel area, number of alpha smooth muscle actin positive vessels and hypoxic area in MLN0518 treated tumours were also significantly lower after 10 days treatment. These changes were not accompanied by alterations in vessel calibre or fractional blood volume as assessed using susceptibility contrast MRI. Histological assessment of vessel size and total perfused area did not demonstrate any change with treatment. Intrinsic susceptibility MRI did not reveal any difference in baseline R2* or carbogen-induced change in R2*. Dynamic contrast-enhanced MRI revealed anti-vascular effects of MLN0518 following 3 days treatment. Hypoxia confers chemo- and radio-resistance, and alongside PDGF, is implicated in evasive resistance to agents targeted against VEGF signalling. PDGFR antagonists may improve potency and efficacy of other therapeutics in combination. This study highlights the challenges of identifying appropriate quantitative imaging response biomarkers in heterogeneous models, particularly considering the multifaceted roles of angiogenic growth factors.

Platelet-derived growth factor signaling in human malignancies

Platelet-derived growth factors (PDGFs) and their receptors were identified and purified decades ago. PDGFs are important during normal development and in human cancers. In particular, autocrine PDGF signaling has been implicated in various types of malignancies such as gliomas and leukemia. In contrast, paracrine signaling was found in cancers that originate from epithelial cells, where it may be involved in stromal cell recruitment, metastasis, and epithelial-mesenchymal transition. This editorial briefly discusses autocrine and paracrine PDGF signaling and their roles in human cancers, and introduces a series of review articles in this issue that address the possible roles of PDGFs in various processes involved in different types of cancers.

Platelet-derived growth factor receptor alpha in glioma: a bad seed

Recent collaborative, large-scale genomic profiling of the most common and aggressive brain tumor glioblastoma multiforme (GBM) has significantly advanced our understanding of this disease. The gene encoding platelet-derived growth factor receptor alpha (PDGFRα) was identified as the third of the top 11 amplified genes in clinical GBM specimens. The important roles of PDGFRα signaling during normal brain development also implicate the possible pathologic consequences of PDGFRα over-activation in glioma. Although the initial clinical trials using PDGFR kinase inhibitors have been predominantly disappointing, diagnostic and treatment modalities involving genomic profiling and personalized medicine are expected to improve the therapy targeting PDGFRα signaling. In this review, we discuss the roles of PDGFRα signaling during development of the normal central nervous system (CNS) and in pathologic conditions such as malignant glioma. We further compare various animal models of PDGF-induced gliomagenesis and their potential as a novel platform of pre-clinical drug testing. We then summarize our recent publication and how these findings will likely impact treatments for gliomas driven by PDGFRα overexpression. A better understanding of PDGFRα signaling in glioma and their microenvironment, through the use of human or mouse models, is necessary to design a more effective therapeutic strategy against gliomas harboring the aberrant PDGFRα signaling.

Autocrine platelet-derived growth factor-vascular endothelial growth factor receptor-related (Pvr) pathway activity controls intestinal stem cell proliferation in the adult Drosophila midgut

A dynamic pool of undifferentiated somatic stem cells proliferate and differentiate to replace dead or dying mature cell types and maintain the integrity and function of adult tissues. Intestinal stem cells (ISCs) in the Drosophila posterior midgut are a well established model to study the complex genetic circuitry that governs stem cell homeostasis. Exposure of the intestinal epithelium to environmental toxins results in the expression of cytokines and growth factors that drive the rapid proliferation and differentiation of ISCs. In the absence of stress signals, ISC homeostasis is maintained through intrinsic pathways. In this study, we uncovered the PDGF- and VEGF-receptor related (Pvr) pathway as an essential regulator of ISC homeostasis under unstressed conditions in the posterior midgut. We found that Pvr is coexpressed with its ligand Pvf2 in ISCs and that hyperactivation of the Pvr pathway distorts the ISC developmental program and drives intestinal dysplasia. In contrast, we show that mutant ISCs in the Pvf/Pvr pathway are defective in homeostatic proliferation and differentiation, resulting in a failure to generate mature cell types. Additionally, we determined that extrinsic stress signals generated by enteropathogenic infection are epistatic to the hypoplasia generated in Pvf/Pvr mutants, making the Pvr pathway unique among all previously studied intrinsic pathways. Our findings illuminate an evolutionarily conserved signal transduction pathway with essential roles in metazoan embryonic development and direct involvement in numerous disease states.

Autocrine PDGF stimulation in malignancies

Platelet-derived growth factor (PDGF) isoforms are important mitogens for different types of mesenchymal cells, which have important functions during the embryonal development and in the adult during wound healing and tissue homeostasis. In tumors, PDGF isoforms are often over-expressed and contribute to the growth of both normal and malignant cells. This review focuses on tumors expressing PDGF isoforms together with their tyrosine kinase receptors, thus resulting in autocrine stimulation of growth and survival. Patients with such tumors could benefit from treatment with inhibitors of either PDGF or PDGF receptors.

PDGF and PDGF receptors in glioma

The family of platelet-derived growth factors (PDGFs) plays a number of critical roles in normal embryonic development, cellular differentiation, and response to tissue damage. Not surprisingly, as it is a multi-faceted regulatory system, numerous pathological conditions are associated with aberrant activity of the PDGFs and their receptors. As we and others have shown, human gliomas, especially glioblastoma, express all PDGF ligands and both the two cell surface receptors, PDGFR-α and -β. The cellular distribution of these proteins in tumors indicates that glial tumor cells are stimulated via PDGF/PDGFR-α autocrine and paracrine loops, while tumor vessels are stimulated via the PDGFR-β. Here we summarize the initial discoveries on the role of PDGF and PDGF receptors in gliomas and provide a brief overview of what is known in this field.

Concomitant intraventricular colloid cyst and low-grade astrocytoma of the brainstem in a 16-year-old boy

Multiple primary brain tumors are commonly observed in patients with a history of brain radiation therapy or neurofibromatosis. The concomitant presence of 2 different types of brain tumors in a single location or chamber is a very rare clinical presentation in the absence of such a predisposing factor. The authors report on the case of a 16-year-old boy presenting with different types of brain tumors in 2 ventricular chambers concomitantly. This boy had a medium-sized colloid cyst of the third ventricle and a large fibrillary astrocytoma fungating from the brainstem into the floor of the fourth ventricle. The lesions were successfully excised in 2 separate surgeries. Radiotherapy was used as the adjuvant mode of therapy. There has been no sign of tumor recurrence after 16 months of follow-up. Clinical awareness and recognition of such a combination of tumors is important because they will dictate special treatment strategies depending on the individual biological aggressiveness of each tumor.

Weak association of the platelet-derived growth factor beta (PDGFB) and PDGF receptor beta (PDGFRB) genes with schizophrenia and schizoaffective disorder

Schizophrenia is a severe neuropsychiatric disorder with diverse characterization of symptoms. Extensive research has been performed to elucidate the etiology of schizophrenia. One of the most convincing hypotheses comes from the dopaminergic system although none of the core genes has been consistently positive in association studies.

OBJECTIVE

In this investigation, we explored the possibility that the genes for platelet-derived growth factor beta (PDGFB) and its receptor (PDGFRB) might play an important role in the development of schizophrenia based on previous reports pointing to their ability to interact with the dopamine D(2)/D(4) and NMDA receptors as well as their role in neurite outgrowth.

METHODS

We investigated the association of variants around these genes with schizophrenia and schizoaffective disorder in 104 small nuclear families using the Sib-Transmission Disequilibrium Test (TDT-STDT). Furthermore, quantitative trait analysis using family-based association test was applied to determine possible association of age at onset (AAO).

RESULTS

Allele G in PDGFRB(rs758588) was associated with AAO (P=0.019). An over-transmission of allele T in PDGFB(rs130650) polymorphism (P=0.043) and an over-transmission of allele A in PDGFRB(rs6865659) polymorphism (P=0.046) were observed. Furthermore, the combined TDT-STDT yielded consistent results.

CONCLUSION

Overall, PDGFB and PDGFRB genes might play a role in the etiology of schizophrenia.

Endothelial progenitor cells induce a phenotype shift in differentiated endothelial cells towards PDGF/PDGFRβ axis-mediated angiogenesis

Background

Endothelial Progenitor Cells (EPC) support neovascularization and regeneration of injured endothelium both by providing a proliferative cell pool capable of differentiation into mature vascular endothelial cells and by secretion of angiogenic growth factors.

Objective

The aim of this study was to investigate the role of PDGF-BB and PDGFRβ in EPC-mediated angiogenesis of differentiated endothelial cells.

Methods and Results

Conditioned medium from human EPC (EPC-CM) cultured in hypoxic conditions contained substantially higher levels of PDGF-BB as compared to normoxic conditions (P<0.01). EPC-CM increased proliferation (1.39-fold; P<0.001) and migration (2.13-fold; P<0.001) of isolated human umbilical vein endothelial cells (HUVEC), as well as sprouting of vascular structures from ex vivo cultured aortic rings (2.78-fold increase; P = 0.01). The capacity of EPC-CM to modulate the PDGFRβ expression in HUVEC was assessed by western blot and RT-PCR. All the pro-angiogenic effects of EPC-CM on HUVEC could be partially inhibited by inactivation of PDGFRβ (P<0.01). EPC-CM triggered a distinct up-regulation of PDGFRβ (2.5±0.5; P<0.05) and its phosphorylation (3.6±0.6; P<0.05) in HUVEC. This was not observed after exposure of HUVEC to recombinant human PDGF-BB alone.

Conclusion

These data indicate that EPC-CM sensitize endothelial cells and induce a pro-angiogenic phenotype including the up-regulation of PDGFRβ, thereby turning the PDGF/PDGFRβ signaling-axis into a critical element of EPC-induced endothelial angiogenesis. This finding may be utilized to enhance EPC-based therapy of ischemic tissue in future.

Phase II trial of pazopanib (GW786034), an oral multi-targeted angiogenesis inhibitor, for adults with recurrent glioblastoma (North American Brain Tumor Consortium Study 06-02)

The objective of this phase II single-arm study was to evaluate the efficacy and safety of pazopanib, a multi-targeted tyrosine kinase inhibitor, against vascular endothelial growth factor receptor (VEGFR)-1, -2, and -3, platelet-derived growth factor receptor-alpha and -beta, and c-Kit, in recurrent glioblastoma. Patients with < or =2 relapses and no prior anti-VEGF/VEGFR therapy were treated with pazopanib 800 mg daily on 4-week cycles without planned interruptions. Brain magnetic resonance imaging and clinical reassessment were made every 8 weeks. The primary endpoint was efficacy as measured by 6-month progression-free survival (PFS6). Thirty-five GBM patients with a median age of 53 years and median Karnofsky performance scale of 90 were accrued. Grade 3/4 toxicities included leukopenia (n = 1), lymphopenia (n = 2), thrombocytopenia (n = 1), ALT elevation (n = 3), AST elevation (n = 1), CNS hemorrhage (n = 1), fatigue (n = 1), and thrombotic/embolic events (n = 3); 8 patients required dose reduction. Two patients had a partial radiographic response by standard bidimensional measurements, whereas 9 patients (6 at the 8-week point and 3 only within the first month of treatment) had decreased contrast enhancement, vasogenic edema, and mass effect but <50% reduction in tumor. The median PFS was 12 weeks (95% confidence interval [CI]: 8-14 weeks) and only 1 patient had a PFS time > or =6 months (PFS6 = 3%). Thirty patients (86%) had died and median survival was 35 weeks (95% CI: 24-47 weeks). Pazopanib was reasonably well tolerated with a spectrum of toxicities similar to other anti-VEGF/VEGFR agents. Single-agent pazopanib did not prolong PFS in this patient population but showed in situ biological activity as demonstrated by radiographic responses. ClinicalTrials.gov identifier: NCT00459381.

Enlarged lateral ventricles and aberrant behavior in mice overexpressing PDGF-B in embryonic neural stem cells

Platelet-derived growth factor (PDGF) is important in central nervous system (CNS) development, and aberrant expression of PDGF and its receptors has been linked to developmental defects and brain tumorigenesis. We previously found that neural stem and progenitor cells in culture produce PDGF and respond to it by autocrine and/or paracrine signaling. We therefore aimed to examine CNS development after PDGF overexpression in neural stem cells in vivo. Transgenic mice were generated with PDGF-B under control of a minimal nestin enhancer element, which is specific for embryonic expression and will not drive adult expression in mice. The resulting mouse showed increased apoptosis in the developing striatum, which suggests a disturbed regulation of progenitor cells. Later in neurodevelopment, in early postnatal life, mice displayed enlarged lateral ventricles. This enlargement remained into adulthood and it was more pronounced in male mice than in transgenic female mice. Nevertheless, there was an overall normal composition of cell types and numbers in the brain and the transgenic mice were viable and fertile. Adult transgenic males, however, showed behavioral aberrations and locomotor dysfunction. Thus, a tightly regulated expression of PDGF during embryogenesis is required for normal brain development and function in mice.

Collision tumor of meningioma and malignant astrocytoma

The authors report a 12-year-old boy who was presented with headache, nausea, vomiting and seizure. His magnetic resonance imaging showed 2 adjacent tumors in the region of the left ventricular trigone. The pathology of tumors reported collision tumors composed of meningioma and malignant astrocytoma.

Angiogenesis inhibition in cancer therapy: platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) and their receptors: biological functions and role in malignancy

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen in vitro and an angiogenic inducer in a variety of in vivo models. VEGF gene transcription is induced in particular in hypoxic cells. In developmental angiogenesis, the role of VEGF is demonstrated by the finding that the loss of a single VEGF allele results in defective vascularization and early embryonic lethality. Substantial evidence also implicates VEGF as a mediator of pathological angiogenesis. In situ hybridization studies demonstrate expression of VEGF mRNA in the majority of human tumors. Platelet-derived growth factor (PDGF) is mainly believed to be an important mitogen for connective tissue, and also has important roles during embryonal development. Its overexpression has been linked to different types of malignancies. Thus, it is important to understand the physiology of VEGF and PDGF and their receptors as well as their roles in malignancies in order to develop antiangiogenic strategies for the treatment of malignant disease.

Recent insights into PDGF-induced gliomagenesis

Gliomas are aggressive and almost incurable glial brain tumors which frequently display abnormal platelet-derived growth factor (PDGF) signaling. Evidence gained from studies on several in vivo animal models has firmly established a causal connection between aberrant PDGF signaling and the formation of some gliomas. However, only recently has significant knowledge been gained regarding crucial issues such as the glioma cell of origin and the relationship between the transforming stimulus and the cellular characteristics of the resulting tumor. Based on recent evidence, we propose that PDGF can bias cell-fate decisions, driving the acquisition of cell type-specific features by the progeny of multipotent neural progenitors, thus determining the shape and direction of the transformation path. Furthermore, recent data about the cellular mechanisms of PDGF-driven glioma progression and maintenance indicate that PDGF may be required, unexpectedly, to override cell contact inhibition and promote glioma cell infiltration rather than to stimulate cell proliferation.

Expression, mutation and copy number analysis of platelet-derived growth factor receptor A (PDGFRA) and its ligand PDGFA in gliomas

BACKGROUND

Malignant gliomas are the most prevalent type of primary brain tumours but the therapeutic armamentarium for these tumours is limited. Platelet-derived growth factor (PDGF) signalling has been shown to be a key regulator of glioma development. Clinical trials evaluating the efficacy of anti-PDGFRA therapies on gliomas are ongoing. In this study, we intended to analyse the expression of PDGFA and its receptor PDGFRA, as well as the underlying genetic (mutations and amplification) mechanisms driving their expression in a large series of human gliomas.

METHODS

PDGFA and PDGFRA expression was evaluated by immunohistochemistry in a series of 160 gliomas of distinct World Health Organization (WHO) malignancy grade. PDGFRA-activating gene mutations (exons 12, 18 and 23) were assessed in a subset of 86 cases by PCR-single-strand conformational polymorphism (PCR-SSCP), followed by direct sequencing. PDGFRA gene amplification analysis was performed in 57 cases by quantitative real-time PCR (QPCR) and further validated in a subset of cases by chromogenic in situ hybridisation (CISH) and microarray-based comparative genomic hybridisation (aCGH).

RESULTS

PDGFA and PDGFRA expression was found in 81.2% (130 out of 160) and 29.6% (48 out of 160) of gliomas, respectively. Its expression was significantly correlated with histological type of the tumours; however, no significant association between the expression of the ligand and its receptor was observed. The absence of PDGFA expression was significantly associated with the age of patients and with poor prognosis. Although PDGFRA gene-activating mutations were not found, PDGFRA gene amplification was observed in 21.1% (12 out of 57) of gliomas. No association was found between the presence of PDGFRA gene amplification and expression, excepting for grade II diffuse astrocytomas.

CONCLUSION

The concurrent expression of PDGFA and PDGFRA in different subtypes of gliomas, reinforce the recognised significance of this signalling pathway in gliomas. PDGFRA gene amplification rather than gene mutation may be the underlying genetic mechanism driving PDGFRA overexpression in a portion of gliomas. Taken together, our results could provide in the future a molecular basis for PDGFRA-targeted therapies in gliomas.

PDGF-C induces maturation of blood vessels in a model of glioblastoma and attenuates the response to anti-VEGF treatment

Background

Recent clinical trials of VEGF inhibitors have shown promise in the treatment of recurrent glioblastomas (GBM). However, the survival benefit is usually short-lived as tumors escape anti-VEGF therapies. Here we tested the hypothesis that Platelet Derived Growth Factor-C (PDGF-C), an isoform of the PDGF family, affects GBM progression independent of VEGF pathway and hinders anti-VEGF therapy.

Principal Findings

We first showed that PDGF-C is present in human GBMs. Then, we overexpressed or downregulated PDGF-C in a human GBM cell line, U87MG, and grew them in cranial windows in nude mice to assess vessel structure and function using intravital microscopy. PDGF-C overexpressing tumors had smaller vessel diameters and lower vascular permeability compared to the parental or siRNA-transfected tumors. Furthermore, vessels in PDGF-C overexpressing tumors had more extensive coverage with NG2 positive perivascular cells and a thicker collagen IV basement membrane than the controls. Treatment with DC101, an anti-VEGFR-2 antibody, induced decreases in vessel density in the parental tumors, but had no effect on the PDGF-C overexpressing tumors.

Conclusion

These results suggest that PDGF-C plays an important role in glioma vessel maturation and stabilization, and that it can attenuate the response to anti-VEGF therapy, potentially contributing to escape from vascular normalization.

Antiangiogenic therapy in brain tumors

Angiogenesis, the recruitment of new blood vessels, is an essential component of tumor progression. Malignant brain tumors are highly vascularized and their growth is angiogenesis-dependent. As such, inhibition of the sprouting of new capillaries from pre-existing blood vessels is one of the most promising antiglioma therapeutic approaches. Numerous classes of molecules have been implicated in regulating angiogenesis and, thus, novel agents that target and counteract angiogenesis are now being developed. The therapeutic trials of a number of angiogenesis inhibitors as antiglioma drugs are currently under intense investigation. Preliminary studies of angiogenic blockade in glioblastoma have been promising and several clinical trials are now underway to develop optimum treatment strategies for antiangiogenic agents. This review will cover state-of-the-art antiangiogenic targets for brain tumor treatment and discuss future challenges. An increased understanding of the angiogenic process, the diversity of its inducers and mediators, appropriate drug schedules and the use of these agents with other modalities may lead to radically new treatment regimens to achieve maximal efficacy.

Vascular changes in merkel cell carcinoma based on a histopathological study of 92 cases

Although prominent vascular proliferation is a known feature of various neuroendocrine tumors, it has not been systematically studied in Merkel cell carcinoma (MCC) of the skin. The purpose of this study was to fully characterize the light microscopic, immunohistochemical, and ultrastructural features of vascular changes associated with MCC and to determine their frequency and differential diagnostic implications. Additionally, the presence of human herpesvirus 8 DNA in the lesional tissue was investigated. Of 92 studied cases of MCC, 18 cases (20%) were found to exhibit foci of prominent vascular changes which were classified into the following 6 patterns: pericyte hyperplasia, pyogenic granuloma-like, hemangioendothelioma-like, epithelioid hemangioma-like, peliosis-like, and follicular dendritic cell tumor-like pattern. In addition, Azzopardi phenomenon was observed. These changes occurred singly or in combination. Human herpesvirus 8 DNA was identified by polymerase chain reaction in none of the 18 cases. It is concluded that prominent vascular proliferations may be seen in 20% of MCC, and thereby, MCC resembles neuroendocrine tumors in other organs. When unduly prominent and encountered in a limited biopsy specimen, vascular alterations may represent a potential diagnostic pitfall, but, on the other hand, they themselves may serve as a clue to the correct diagnosis. Human herpesvirus 8 does not play a role in angiogenesis in MCC.

Angiogenesis in brain tumours

Despite aggressive surgery, radiotherapy and chemotherapy, malignant gliomas remain uniformly fatal. To progress, these tumours stimulate the formation of new blood vessels through processes driven primarily by vascular endothelial growth factor (VEGF). However, the resulting vessels are structurally and functionally abnormal, and contribute to a hostile microenvironment (low oxygen tension and high interstitial fluid pressure) that selects for a more malignant phenotype with increased morbidity and mortality. Emerging preclinical and clinical data indicate that anti-VEGF therapies are potentially effective in glioblastoma--the most frequent primary brain tumour--and can transiently normalize tumour vessels. This creates a window of opportunity for optimally combining chemotherapeutics and radiation.

Planning for intracavitary anti-EGFR radionuclide therapy of gliomas. Literature review and data on EGFR expression

Targeting with radionuclide labelled substances that bind specifically to the epidermal growth factor receptor, EGFR, is considered for intracavitary therapy of EGFR-positive glioblastoma multiforme, GBM. Relevant literature is reviewed and examples of EGFR expression in GBM are given. The therapeutical efforts made so far using intracavitary anti-tenascin radionuclide therapy of GBM have given limited effects, probably due to low radiation doses to the migrating glioma cells in the brain. Low radiation doses might be due to limited penetration of the targeting agents or heterogeneity in the expression of the target structure. In this article we focus on the possibilities to target EGFR on the tumour cells instead of an extracellular matrix component. There seems to be a lack of knowledge on the degree of intratumoral variation of EGFR expression in GBM, although the expression seemed rather homogeneous over large areas in most of the examples (n=16) presented from our laboratory. The observed homogeneity was surprising considering the genomic instability and heterogeneity that generally characterises highly malignant tumours. However, overexpression of EGFR is, at least in primary GBMs, one of the steps in the development of malignancy, and tumour cells that lose or downregulate EGFR will probably be outgrown in an expanding tumour cell population. Thus, loss of EGFR expression might not be the critical factor for successful intracavitary radionuclide therapy. Instead, it is likely that the penetration properties of the targeting agents are critical, and detailed studies on this are urgent.

Angiogenesis in malignant glioma--a target for antitumor therapy?

The prognosis of malignant gliomas is still dismal despite aggressive treatment attempts. Thus, alternative therapy strategies are needed. Malignant gliomas are upon the best vascularized tumors in humans and their proliferation is hallmarked by a distinct proliferative vascular component. Hence it seems to be a logical consequence to apply anti-angiogenic treatment strategies to malignant gliomas. These treatment strategies have shown promising effects in animal models and some experimental clinical studies. This review gives a short introduction into the molecules involved in angiogenesis of malignant gliomas, it provides an overview of the latest experimental developments of glioma angiogenesis inhibition and discusses the results of clinical anti-angiogenic trials in patients with high grade glioma. Additionally the problem of monitoring the treatment success of an anti-angiogenic therapy is addressed.

Platelet-derived growth factor receptor-beta promotes early endothelial cell differentiation

Platelet-derived growth factor BB (PDGF-BB) has been assigned a critical role in vascular stability by promoting the recruitment of PDGF receptor-beta-expressing perivascular cells. Here we present data indicating that early hematopoietic/endothelial (hemangio) precursors express PDGFR-beta based on coexpression with CD31, vascular endothelial growth factor receptor-2, and CD41 in 2 models: mouse yolk sac (embryonic day 8 [E8]) and differentiating mouse embryonic stem cells (embryoid bodies). Expression of PDGFR-beta on hemangioprecursor cells in the embryoid bodies gradually disappeared, and, at E14, expression appeared on perivascular cells. Activation of the PDGFR-beta on the hemangioprecursors accelerated the differentiation of endothelial cells, whereas differentiation of the hematopoietic lineage was suppressed. In E9.5 yolk sacs derived from recombinant mice expressing kinase-active PDGFR-beta with an aspartic acid to asparagine (D894N) replacement in the kinase activating loop and from mice with ubiquitous expression of PDGF-BB driven by the Rosa26 locus, the number of CD41-expressing early hematopoietic cells decreased by 36% and 34%, respectively, compared with staged wild-type littermates. Moreover, enhanced vascular remodeling was evident in the Rosa26-PDGF-BB yolk sacs. We conclude that PDGFR-beta is expressed on early hemangioprecursor cells, regulating vascular/hematopoietic development.

Mechanisms of angiogenesis in gliomas

Gliomas are the most frequent primary tumors of the central nervous system in adults. Glioblastoma multiforme, the most aggressive form of astrocytic tumors, displays a rapid progression that is accompanied by particular poor prognosis of patients. Intense angiogenesis is a distinguishing pathologic characteristic of these tumors and in fact, glioblastomas are of the most highly vascularized malignant tumors. For this reason, research and therapy strategies have focused on understanding the mechanisms leading to the origin of tumor angiogenic blood vessels in order to develop new approaches that effectively block angiogenesis and cause tumor regression. We discuss here some important features of glioma angiogenesis and we present molecules and factors and their possible functions and interactions that play a role in neovascularization. In spite of the great progress that molecular biology has achieved on investigating tumor angiogenesis, many aspects remain obscure and the complexity of the angiogenic process stands for an obstacle in identifying the exact and complete molecular pathways orchestrating new blood vessels formation, which are necessary for the survival and expansion of these tumors.

Characterization of an imatinib-sensitive subset of high-grade human glioma cultures

High-grade gliomas, including glioblastomas, are malignant brain tumors for which improved treatment is urgently needed. Genetic studies have demonstrated the existence of biologically distinct subsets. Preliminary studies have indicated that platelet-derived growth factor (PDGF) receptor signaling contributes to the growth of some of these tumors. In this study, human high-grade glioma primary cultures were analysed for sensitivity to treatment with the PDGF receptor inhibitor imatinib/Glivec/Gleevec/STI571. Six out of 15 cultures displayed more than 40% growth inhibition after imatinib treatment, whereas seven cultures showed less than 20% growth inhibition. In the sensitive cultures, apoptosis contributed to growth inhibition. Platelet-derived growth factor receptor status correlated with imatinib sensitivity. Supervised analyses of gene expression profiles and real-time PCR analyses identified expression of the chemokine CXCL12/SDF-1 (stromal cell-derived factor 1) as a predictor of imatinib sensitivity. Exogenous addition of CXCL12 to imatinib-insensitive cultures conferred some imatinib sensitivity. Finally, coregulation of CXCL12 and PDGF alpha-receptor was observed in glioblastoma biopsies. We have thus defined the characteristics of a novel imatinib-sensitive subset of glioma cultures, and provided evidence for a functional relationship between imatinib sensitivity and chemokine signaling. These findings will assist in the design and evaluation of clinical trials exploring therapeutic effects of imatinib on malignant brain tumors.

Selective kinase inhibition with daily imatinib intensifies toxicity of chemotherapy in patients with solid tumours

The aim of this study was to determine the safety and maximum-tolerated doses of imatinib combined with cytotoxic chemotherapy (either gemcitabine or doxorubicin). Patients with advanced solid tumours were enrolled separately in two different combinations of imatinib with chemotherapy (imatinib + gemcitabine or imatinib + doxorubicin). A standard modified Fibonacci inter-cohort dose escalation was planned for each combination. Sixteen patients were accrued. Seven patients received gemcitabine and imatinib. A separate cohort of nine patients received imatinib and doxorubicin. In both groups, dose-limiting toxicity (DLT) was observed at the initial dose level requiring dose reductions for subsequent cohorts. Further DLTs were observed necessitating closure of the protocol. Daily dosing of imatinib with concurrent administration of cytotoxic chemotherapy (either gemcitabine or doxorubicin) at standard doses was associated with toxicity that was clinically unacceptable. It remains unclear whether addition of growth factors might improve tolerability for imatininb in combination with cytotoxic chemotherapy.