Coexpression of platelet-derived growth factor (PDGF) and PDGF-receptor genes by primary human astrocytomas may contribute to their development and maintenance

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.

Collision Occurrence of Meningioma and Astrocytoma: A Case Report and Literature Review

A case of collision tumors occurring between two distinct primary brain tumors is reported. A 61-year-old female without history of radiotherapy or phakomatosis presented with progressive ly increasing headache and left hemiparesis. Investigation revealed a meningioma and a Grade II astrocytoma in the right frontal lobe. Simultaneous development of a meningioma and a low-grade glioma at adjacent sites is extremely rare. This is the third case reported in the literature. Some hypotheses are proposed to explain this phenomenon but most likely represent a coincidental event.

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.

A PDGFRα-driven mouse model of glioblastoma reveals a stathmin1-mediated mechanism of sensitivity to vinblastine

Glioblastoma multiforme (GBM) is an aggressive primary brain cancer that includes focal amplification of PDGFRα and for which there are no effective therapies. Herein, we report the development of a genetically engineered mouse model of GBM based on autocrine, chronic stimulation of overexpressed PDGFRα, and the analysis of GBM signaling pathways using proteomics. We discover the tubulin-binding protein Stathmin1 (STMN1) as a PDGFRα phospho-regulated target, and that this mis-regulation confers sensitivity to vinblastine (VB) cytotoxicity. Treatment of PDGFRα-positive mouse and a patient-derived xenograft (PDX) GBMs with VB in mice prolongs survival and is dependent on STMN1. Our work reveals a previously unconsidered link between PDGFRα activity and STMN1, and highlight an STMN1-dependent cytotoxic effect of VB in GBM.

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.

The ubiquitin-proteasome pathway in adult and pediatric brain tumors: biological insights and therapeutic opportunities

Nearly 20 years ago, the concept of targeting the proteasome for cancer therapy began gaining momentum. This concept was driven by increased understanding of the biology/structure and function of the 26S proteasome, insight into the role of the proteasome in transformed cells, and the synthesis of pharmacological inhibitors with clinically favorable features. Subsequent in vitro, in vivo, and clinical testing culminated in the FDA approval of three proteasome inhibitors-bortezomib, carfilzomib, and ixazomib -for specific hematological malignancies. However, despite in vitro and in vivo studies pointing towards efficacy in solid tumors, clinical responses broadly have been evasive. For brain tumors, a malignancy in dire need of new approaches both in adult and pediatric patients, this has also been the case. Elucidation of proteasome-dependent processes in specific types of brain tumors, the evolution of newer proteasome targeting strategies, and the use of proteasome inhibitors in combination strategies will clarify how these agents can be leveraged more effectively to treat central nervous system malignancies. Since brain tumors represent a heterogeneous subset of solid tumors, and in particular, pediatric brain tumors possess distinct biology from adult brain tumors, tailoring of proteasome inhibitor-based strategies to specific subtypes of these tumors will be critical for advancing care for affected patients, and will be discussed in this review.

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.

Molecular Biology in Pediatric High-Grade Glioma: Impact on Prognosis and Treatment

High-grade gliomas are the main cause of death in children with brain tumours. Despite recent advances in cancer therapy, their prognosis remains poor and the treatment is still challenging. To date, surgery followed by radiotherapy and temozolomide is the standard therapy. However, increasing knowledge of glioma biology is starting to impact drug development towards targeted therapies. The identification of agents directed against molecular targets aims at going beyond the traditional therapeutic approach in order to develop a personalized therapy and improve the outcome of pediatric high-grade gliomas. In this paper, we critically review the literature regarding the genetic abnormalities implicated in the pathogenesis of pediatric malignant gliomas and the current development of molecularly targeted therapies. In particular, we analyse the impact of molecular biology on the prognosis and treatment of pediatric high-grade glioma, comparing it to that of adult gliomas.

PDGF receptor alpha inhibition induces apoptosis in glioblastoma cancer stem cells refractory to anti-Notch and anti-EGFR treatment

Background

Cancer stem cells (CSC) represent a rare fraction of cancer cells characterized by resistance to chemotherapy and radiation, therefore nowadays there is great need to develop new targeted therapies for brain tumors and our study aim to target pivotal transmembrane receptors such as Notch, EGFR and PDGFR, which are already under investigation in clinical trials setting for the treatment of Glioblastoma Multiforme (GBM).

Methods

MTS assay was performed to evaluate cells response to pharmacological treatments. Quantitative RT-PCR and Western blots were performed to state the expression of Notch1, EGFR and PDGFRα/β and the biological effects exerted by either single or combined targeted therapy in GBM CSC. GBM CSC invasive ability was tested in vitro in absence or presence of Notch and/or EGFR signaling inhibitors.

Results

In this study, we investigated gene expression and function of Notch1, EGFR and PDGFR to determine their role among GBM tumor core- (c-CSC) vs. peritumor tissue-derived cancer stem cells (p-CSC) of six cases of GBM. Notch inhibition significantly impaired cell growth of c-CSC compared to p-CSC pools, with no effects observed in cell cycle distribution, apoptosis and cell invasion assays. Instead, anti-EGFR therapy induced cell cycle arrest, sometimes associated with apoptosis and reduction of cell invasiveness in GBM CSC. In two cases, c-CSC pools were more sensitive to simultaneous anti-Notch and anti-EGFR treatment than either therapy alone compared to p-CSC, which were mostly resistant to treatment. We reported the overexpression of PDGFRα and its up-regulation following anti-EGFR therapy in GBM p-CSC compared to c-CSC. RNA interference of PDGFRα significantly reduced cell proliferation rate of p-CSC, while its pharmacological inhibition with Crenolanib impaired survival of both CSC pools, whose effects in combination with EGFR inhibition were maximized.

Conclusions

We have used different drugs combination to identify the more effective therapeutic targets for GBM CSC, particularly against GBM peritumor tissue-derived CSC, which are mostly resistant to treatments. Overall, our results provide the rationale for simultaneous targeting of EGFR and PDGFR, which would be beneficial in the treatment of GBM.

Electronic supplementary material

The online version of this article (doi:10.1186/1476-4598-13-247) contains supplementary material, which is available to authorized users.

Current Understanding on EGFR and Wnt/β-Catenin Signaling in Glioma and Their Possible Crosstalk

Glioblastoma multiformes (GBMs) are extensively heterogeneous at both cellular and molecular levels. Current therapeutic strategies include targeting of key signaling molecules using pharmacological inhibitors in combination with genotoxic agents such as temozolomide. In spite of all efforts, the prognosis of glioma patients remains dismal. Therefore, a proper understanding of individual molecular pathways responsible for the progression of GBM is necessary. The epidermal growth factor receptor (EGFR) pathway is probably the most significant signaling pathway clinically implicated in glioma. Not surprisingly, anti-EGFR therapies mostly prevail for therapeutic purposes. The Wnt/β-catenin pathway is well implicated in multiple tumors; however, its role in glioma has only recently started to emerge. We give a concise account of the current understanding of the role of both these pathways in glioma. Last, taking evidences from a limited literature, we outline a number of points where these pathways intersect each other and put forward the possibility of combinatorially targeting them for treatment of glioma.

Targeting the PDGF signaling pathway in tumor treatment

Platelet-derived growth factor (PDGF) isoforms and PDGF receptors have important functions in the regulation of growth and survival of certain cell types during embryonal development and e.g. tissue repair in the adult. Overactivity of PDGF receptor signaling, by overexpression or mutational events, may drive tumor cell growth. In addition, pericytes of the vasculature and fibroblasts and myofibroblasts of the stroma of solid tumors express PDGF receptors, and PDGF stimulation of such cells promotes tumorigenesis. Inhibition of PDGF receptor signaling has proven to useful for the treatment of patients with certain rare tumors. Whether treatment with PDGF/PDGF receptor antagonists will be beneficial for more common malignancies is the subject for ongoing studies.

Multifarious functions of PDGFs and PDGFRs in tumor growth and metastasis

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

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.

Platelet-derived growth factor receptors differentially inform intertumoral and intratumoral heterogeneity

Growth factor-mediated proliferation and self-renewal maintain tissue-specific stem cells and are frequently dysregulated in cancers. Platelet-derived growth factor (PDGF) ligands and receptors (PDGFRs) are commonly overexpressed in gliomas and initiate tumors, as proven in genetically engineered models. While PDGFRα alterations inform intertumoral heterogeneity toward a proneural glioblastoma (GBM) subtype, we interrogated the role of PDGFRs in intratumoral GBM heterogeneity. We found that PDGFRα is expressed only in a subset of GBMs, while PDGFRβ is more commonly expressed in tumors but is preferentially expressed by self-renewing tumorigenic GBM stem cells (GSCs). Genetic or pharmacological targeting of PDGFRβ (but not PDGFRα) attenuated GSC self-renewal, survival, tumor growth, and invasion. PDGFRβ inhibition decreased activation of the cancer stem cell signaling node STAT3, while constitutively active STAT3 rescued the loss of GSC self-renewal caused by PDGFRβ targeting. In silico survival analysis demonstrated that PDGFRB informed poor prognosis, while PDGFRA was a positive prognostic factor. Our results may explain mixed clinical responses of anti-PDGFR-based approaches and suggest the need for integration of models of cancer as an organ system into development of cancer therapies.

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.

Phase II study of Gleevec plus hydroxyurea in adults with progressive or recurrent low-grade glioma

BACKGROUND

We evaluated the efficacy of imatinib plus hydroxyurea in patients with progressive/recurrent low-grade glioma.

METHODS

A total of 64 patients with recurrent/progressive low-grade glioma were enrolled in this single-center study that stratified patients into astrocytoma and oligodendroglioma cohorts. All patients received 500 mg of hydroxyurea twice a day. Imatinib was administered at 400 mg per day for patients not on enzyme-inducing antiepileptic drugs (EIAEDs) and at 500 mg twice a day if on EIAEDs. The primary endpoint was progression-free survival at 12 months (PFS-12) and secondary endpoints were safety, median progression-free survival, and radiographic response rate.

RESULTS

Thirty-two patients were enrolled into each cohort. Eleven patients (17%) had before radiotherapy and 24 (38%) had received before chemotherapy. The median PFS and PFS-12 were 11 months and 39%, respectively. Outcome did not differ between the histologic cohorts. No patient achieved a radiographic response. The most common grade 3 or greater adverse events were neutropenia (11%), thrombocytopenia (3%), and diarrhea (3%).

CONCLUSIONS

Imatinib plus hydroxyurea was well tolerated among recurrent/progressive LGG patients but this regimen demonstrated negligible antitumor activity.

Phase II study of imatinib mesylate for recurrent meningiomas (North American Brain Tumor Consortium study 01-08)

Platelet-derived growth factor (PDGF) and its receptors (PDGFR) are frequently coexpressed in meningiomas, potentially contributing to their pathogenesis. The North American Brain Tumor Consortium conducted a phase II study to evaluate the therapeutic potential of imatinib mesylate (Gleevec), a PDGFR inhibitor, in patients with recurrent meningiomas. Patients were stratified into benign (WHO grade I) meningiomas or atypical (WHO grade II) and malignant (WHO grade III) meningiomas. The primary end point was 6-month progression-free survival (6M-PFS). Patients requiring enzyme-inducing antiepileptic drugs were ineligible. Patients received imatinib at a dose of 600 mg/day for the first 4-week cycle and then gradually increased to 800 mg/day for subsequent cycles, if there were no unacceptable toxicities. Plasma concentrations of imatinib and its active metabolite, CGP74588, were assessed. Twenty-three heavily pretreated patients were enrolled into the study (13 benign, 5 atypical, and 5 malignant meningiomas), of whom 22 were eligible. The study was closed prematurely due to slow accrual. Tissue was available only from a minority of patients, but in these specimens there was uniform distribution of PDGFR, the drug target. Imatinib was generally well tolerated. Of 19 patients evaluable for response, 10 progressed at the first scan, and 9 were stable. There were no complete or partial responses. Overall median PFS was 2 months (range, 0.7-34 months); 6M-PFS was 29.4%. For benign meningiomas, median PFS was 3 months (range, 1.1-34 months); 6M-PFS was 45%. For atypical and malignant meningiomas, median PFS was 2 months (range, 0.7-3.7 months); 6M-PFS was 0%. Cycle 1 trough concentrations of imatinib and CGP74588 were 2,129 +/- 1,600 ng/ml and 517 +/- 326 ng/ml, respectively. Single-agent imatinib was well tolerated but had no significant activity in recurrent meningiomas. Trough plasma concentrations of imatinib exceeded those associated with imatinib activity in chronic myelogenous leukemia.

Design, synthesis, and X-ray crystal structures of 2,4-diaminofuro[2,3-d]pyrimidines as multireceptor tyrosine kinase and dihydrofolate reductase inhibitors

To optimize dual receptor tyrosine kinase (RTK) and dihydrofolate reductase (DHFR) inhibition, the E- and Z-isomers of 5-[2-(2-methoxyphenyl)prop-1-en-1-yl]furo[2,3-d]pyrimidine-2,4-diamines (1a and 1b) were separated by HPLC and the X-ray crystal structures (2.0 and 1.4A, respectively) with mouse DHFR and NADPH as well as 1b with human DHFR (1.5A) were determined. The E- and Z-isomers adopt different binding modes when bound to mouse DHFR. A series of 2,4-diaminofuro[2,3-d]pyrimidines 2-13 were designed and synthesized using the X-ray crystal structures of 1a and 1b with DHFR to increase their DHFR inhibitory activity. Wittig reactions of appropriate 2-methoxyphenyl ketones with 2,4-diamino-6-chloromethyl furo[2,3-d]pyrimidine afforded the C8-C9 unsaturated compounds 2-7 and catalytic reduction gave the saturated 8-13. Homologation of the C9-methyl analog maintains DHFR inhibitory activity. In addition, inhibition of EGFR and PDGFR-beta were discovered for saturated C9-homologated analogs 9 and 10 that were absent in the saturated C9-methyl analogs.

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.

Interactions between PTEN and receptor tyrosine kinase pathways and their implications for glioma therapy

Gliomas are the most common and deadly form of malignant primary brain tumors. Loss of the tumor-suppressor PTEN and activation of the receptor tyrosine kinases (RTKs) EGF receptor, c-Met, PDGF receptor and VEGF receptor are among the most common molecular dysfunctions associated with glioma malignancy. PTEN interacts with RTK-dependent signaling at multiple levels. These include the ability of PTEN to counteract PI3K activation by RTKs, as well as possible effects of PTEN on RTK activation of the MAPK pathway and RTK-dependent gene-expression regulation. Consequently, PTEN expression affects RTK-induced malignancy. Importantly, the PTEN status was recently found to be critical for the outcome of RTK-targeted clinical therapies that have been developed recently. Combining RTK-targeted therapies with therapies aimed at counteracting the effects of PTEN loss, such as mTOR inhibition, might also have therapeutic advantage. This article reviews the known molecular and functional interactions between PTEN and RTK pathways and their implications for glioma therapy.

Differential effect of imatinib and synergism of combination treatment with chemotherapeutic agents in malignant glioma cells

Imatinib mesylate (STI571, Gleevec) is a signal transduction inhibitor and novel anti-cancer agent. It selectively inhibits aberrantly activated tyrosine kinases in malignant cells, for example, bcr-abl in leukaemia, platelet-derived growth factor receptor and stem cell factor receptor (c-Kit) in solid cancers including malignant glioma. However, recently published clinical studies with imatinib monotherapy in patients with malignant glioma demonstrated only very modest anti-tumour activity. The aim of this study was to investigate the biological activity of imatinib, its cellular mechanisms of action and its synergism with other chemotherapeutic agents in human malignant glioma cells in culture. Expression of PDGF/R and c-Kit was analyzed by RT-PCR. Proliferation was measured by MTT assays and drug synergy was assessed by the Chou-Talalay method. Cell cycle and apoptosis were analyzed by flow cytometry and migration by monolayer migration assays. Multi-immunoblot was performed on imatinib-treated and control malignant glioma cells. Results indicate that imatinib is more effective in inhibiting cell colony formation and migration rather than proliferation. Imatinib treatment caused cell cycle arrest of glioma cells in G0-G1 or G2/M, with significant elevation of a few cyclin-dependent kinases. Furthermore, imatinib acted synergistically with chemotherapy agents, such as the DNA alkylating agent, temozolomide, and riboneucleotide reductase inhibitors, for example, hydroxyurea at varied effective dose levels. In conclusion, imatinib exerts varied biological effects on malignant glioma cells in culture. Synergistic interaction of imatinib with chemotherapy agents may be related to cell cycle control mechanisms and could be potentially important in a clinical setting.

Therapeutic application of noncytotoxic molecular targeted therapy in gliomas: growth factor receptors and angiogenesis inhibitors

Growth factor receptors and angiogenesis play major roles in the oncogenesis of gliomas. Over the last several years, several noncytotoxic molecular targeted therapies have been developed against growth factor receptors and tumor angiogenesis. In gliomas, two main anti-growth factor receptor strategies have been evaluated in phase I/II clinical trials: (a) small molecule tyrosine kinase inhibitors (TKIs) and (b) monoclonal antibodies that target growth factors or growth factor receptors other than vascular endothelial growth factor (VEGF). Up to now, few glioma patients have responded to small TKIs (0%-14%) or monoclonal antibodies (three case reports) delivered as a single agent. Greater doses, combined therapies, as well as the identification of molecular biomarkers predictive of response and resistance are important in order to optimize drug delivery and improve efficacy. Antiangiogenic therapies are promising for the treatment of gliomas. Thalidomide and metronomic chemotherapy were the first antiangiogenic strategies evaluated, but they have shown only modest activity. Recent studies of bevacizumab, an anti-VEGF antibody, and irinotecan, a topoisomerase I inhibitor, have demonstrated a high response rate, suggesting that targeted antiangiogenic therapies may play a significant role in the management of high-grade gliomas in the future. However, the toxicity profiles of these agents are not fully defined and the radiological evaluation of possible tumor response is challenging. Clinical evaluation of several VEGF receptor TKIs is currently ongoing; one of these inhibitors, cediranib, has already demonstrated interesting activity as a single agent. The integrin inhibitor cilengitide represents another promising strategy.

Efficacy of the HSP90 inhibitor 17-AAG in human glioma cell lines and tumorigenic glioma stem cells

Glioblastoma multiforme (GBM) arises from genetic and signaling abnormalities in components of signal transduction pathways involved in proliferation, survival, and the cell cycle axis. Studies to date with single-agent targeted molecular therapy have revealed only modest effects in attenuating the growth of these tumors, suggesting that targeting multiple aberrant pathways may be more beneficial. Heat-shock protein 90 (HSP90) is a molecular chaperone that is involved in the conformational maturation of a defined group of client proteins, many of which are deregulated in GBM. 17-allylamino-17-demethoxygeldanamycin (17-AAG) is a well-characterized HSP90 inhibitor that should be able to target many of the aberrant signal transduction pathways in GBM. We assessed the ability of 17-AAG to inhibit the growth of glioma cell lines and glioma stem cells both in vitro and in vivo and assessed its ability to synergize with radiation and/or temozolomide, the standard therapies for GBM. Our results reveal that 17-AAG is able to inhibit the growth of both human glioma cell lines and glioma stem cells in vitro and is able to target the appropriate proteins within these cells. In addition, 17-AAG can inhibit the growth of intracranial tumors and can synergize with radiation both in tissue culture and in intracranial tumors. This compound was not found to synergize with temozolomide in any of our models of gliomas. Our results suggest that HSP90 inhibitors like 17-AAG may have therapeutic potential in GBM, either as a single agent or in combination with radiation.

Molecular pathogenesis of adult brain tumors and the role of stem cells

Primary brain tumors consist of neoplasms with varied molecular defects, morphologic phenotypes, and clinical outcomes. The genetic and signaling abnormalities involved in tumor initiation and progression of the most prevalent adult primary brain tumors, including gliomas, meningiomas, and medulloblastomas, are described in this article. The current understanding of the cell-of-origin of these neoplasms is reviewed, which suggests that the malignant phenotype is propelled by cells with stem-like qualities. A comprehensive understanding of the molecular basis of transformation and the cell-of-origin of these neoplasms will enable the formulation of more targeted treatment alternatives that could improve survival and quality of life.

Molecular analysis of pediatric brain tumors

The identification of molecular genetic abnormalities in specific types of pediatric brain tumors is beginning to play a role in the stratification of patients into treatment groups. The finding of an INI1 alteration in an atypical teratoid/rhabdoid tumor or malignant neoplasm with overlapping histologic features will be required for entry onto disease-specific protocols within the Children's Oncology Group. Refinement in the classification of medulloblastoma and malignant glioma patients will likely depend on the genetic and signaling pathways that characterize these tumors. Advances in this area will depend on the ability to identify new disease genes, validate prognostic markers, and develop biologically based therapeutic strategies to tailor treatment.

Platelet-derived growth factor-mediated gliomagenesis and brain tumor recruitment

Platelet-derived growth factor (PDGF) is a growth factor family of ligands and receptors known to activate phosphatidylinositol 3-kinase, mitogen-activated protein kinase, Jak family kinase, Src family kinase, and phospholipase Cgamma signal transduction pathways, some of which have been causally linked to glioma formation. Extensive involvement of PDGF in development and its implication in a variety of pathologic conditions, including gliomagenesis, are mediated not only by autocrine effects but by paracrine effects. Many researchers view brain tumors as clonal entities derived from the cancer stem cell; however, recent documentation of the importance of the tumor microenvironment for glioma initiation and progression as well as the ability of neural stem or progenitor cells to migrate toward the sites of injury or tumor formation reveals additional complexities in brain tumorigenesis. Paracrine effects of PDGF in animal models of gliomagenesis, continued adult neurogenesis capable of increasing in response to brain injury, and the growth factor-rich environment of brain tumors suggest that recruitment may play a role in gliomagenesis. In this view, glioma formation involves recruitment of cells from the adjacent brain and possibly other sites.

Phase I trial of imatinib in children with newly diagnosed brainstem and recurrent malignant gliomas: a Pediatric Brain Tumor Consortium report

This study estimated the maximum tolerated dose (MTD) of imatinib with irradiation in children with newly diagnosed brainstem gliomas, and those with recurrent malignant intracranial gliomas, stratified according to use of enzyme-inducing anticonvulsant drugs (EIACDs). In the brainstem glioma stratum, imatinib was initially administered twice daily during irradiation, but because of possible association with intratumoral hemorrhage (ITH) was subsequently started two weeks after irradiation. The protocol was also amended to exclude children with prior hemorrhage. Twenty-four evaluable patients received therapy before the amendment, and three of six with a brainstem tumor experienced dose-limiting toxicity (DLT): one had asymptomatic ITH, one had grade 4 neutropenia and, one had renal insufficiency. None of 18 patients with recurrent glioma experienced DLT. After protocol amendment, 3 of 16 patients with brainstem glioma and 2 of 11 patients with recurrent glioma who were not receiving EIACDs experienced ITH DLTs, with three patients being symptomatic. In addition to the six patients with hemorrhages during the DLT monitoring period, 10 experienced ITH (eight patients were symptomatic) thereafter. The recommended phase II dose for brainstem gliomas was 265 mg/m(2). Three of 27 patients with brainstem gliomas with imaging before and after irradiation, prior to receiving imatinib, had new hemorrhage, excluding their receiving imatinib. The MTD for recurrent high-grade gliomas without EIACDs was 465 mg/m(2), but the MTD was not established with EIACDs, with no DLTs at 800 mg/m(2). In summary, recommended phase II imatinib doses were determined for children with newly diagnosed brainstem glioma and recurrent high-grade glioma who were not receiving EIACDs. Imatinib may increase the risk of ITH, although the incidence of spontaneous hemorrhages in brainstem glioma is sufficiently high that this should be considered in studies of agents in which hemorrhage is a concern.

PDGFRs are critical for PI3K/Akt activation and negatively regulated by mTOR

The receptor tyrosine kinase/PI3K/Akt/mammalian target of rapamycin (RTK/PI3K/Akt/mTOR) pathway is frequently altered in tumors. Inactivating mutations of either the TSC1 or the TSC2 tumor-suppressor genes cause tuberous sclerosis complex (TSC), a benign tumor syndrome in which there is both hyperactivation of mTOR and inhibition of RTK/PI3K/Akt signaling, partially due to reduced PDGFR expression. We report here that activation of PI3K or Akt, or deletion of phosphatase and tensin homolog (PTEN) in mouse embryonic fibroblasts (MEFs) also suppresses PDGFR expression. This was a direct effect of mTOR activation, since rapamycin restored PDGFR expression and PDGF-sensitive Akt activation in Tsc1-/- and Tsc2-/- cells. Akt activation in response to EGF in Tsc2-/- cells was also reduced. Furthermore, Akt activation in response to each of EGF, IGF, and PMA was reduced in cells lacking both PDGFRalpha and PDGFRbeta, implying a role for PDGFR in transmission of growth signals downstream of these stimuli. Consistent with the reduction in PI3K/Akt signaling, in a nude mouse model both Tsc1-/- and Tsc2-/- cells had reduced tumorigenic potential in comparison to control cells, which was enhanced by expression of either active Akt or PDGFRbeta. In conclusion, PDGFR is a major target of negative feedback regulation in cells with activated mTOR, which limits the growth potential of TSC tumors.

Epidermal Growth Factor Receptor–Mediated Signal Transduction in the Development and Therapy of Gliomas

The epidermal growth factor receptor (EGFR) and its ligands figure prominently in the biology of gliomas, the most common tumors of the central nervous system (CNS). Although their histologic classification seems to be straightforward, these tumors constitute a heterogeneous class of related neoplasms. They are associated with a variety of molecular abnormalities affecting signal transduction, transcription factors, apoptosis, angiogensesis, and the extracellular matrix. Under normal conditions, these same interacting factors drive CNS growth and development. We are now recognizing the diverse molecular genetic heterogeneity that underlies tumors classified histologically into three distinct grades. This recognition is leading to new therapeutic strategies targeted directly at specific molecular subtypes. In this article, we will review the role of EGFR and related molecular pathways in the genesis of the normal CNS and their relationship to glial tumorigenesis. We will discuss barriers to effective treatment as they relate to anatomic specialization of the CNS. We will also consider the ways in which specific EGFR alterations common to glioma reflect outcomes following treatment with targeted therapies, all with an eye towards applying this understanding to improved patient outcomes.

Expression of endothelin 1 and its angiogenic role in meningiomas

Meningiomas are one of the most frequent central nervous system tumours. Although slow-growing at times, they continue to be a cause of morbidity and mortality. The endothelin (ET) family consists of three isoforms: ET-1 is the most abundant one. ET-1 may be involved in meningioma tumourigenesis in concert with other growth factors, in particular with angiogenic agents. We analysed ET-1 expression by immunohistochemistry and its activating system by reverse-transcription-polymerase chain reaction in 56 cases of meningioma. We found an association between high-grade meningiomas and high ET-1 expression levels (p=0.002). Moreover, we evaluated the potential angiogenic role of ET-1, finding an elevated microvessel count in tumours with high ET expression levels (p=0.004). ET-1 may contribute to meningioma growth by inducing formation of new blood vessels. The finding that ET-1 expression positively correlates with vascular endothelial growth factor (VEGF) expression in meningiomas (p=0.03) also supports the hypothesized modulating effect of ET-1 on angiogenesis. Thus, the influence of the ET system on the progression of meningiomas may occur through stimulation of VEGF. The association of ET-1 and meningioma represents a potential area for therapeutic intervention with selective ET inhibitors. Additional clinical studies will be needed before inhibitors can be incorporated in clinical practice.

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.

Phase I/II Study of Imatinib Mesylate for Recurrent Malignant Gliomas: North American Brain Tumor Consortium Study 99-08

PURPOSE

Phase I: To determine the maximum tolerated doses, toxicities, and pharmacokinetics of imatinib mesylate (Gleevec) in patients with malignant gliomas taking enzyme-inducing antiepileptic drugs (EIAED) or not taking EIAED. Phase II: To determine the therapeutic efficacy of imatinib.

EXPERIMENTAL DESIGN

Phase I component used an interpatient dose escalation scheme. End points of the phase II component were 6-month progression-free survival and response.

RESULTS

Fifty patients enrolled in the phase I component (27 EIAED and 23 non-EIAED). The maximum tolerated dose for non-EIAED patients was 800 mg/d. Dose-limiting toxicities were neutropenia, rash, and elevated alanine aminotransferase. EIAED patients received up to 1,200 mg/d imatinib without developing dose-limiting toxicity. Plasma exposure of imatinib was reduced by approximately 68% in EIAED patients compared with non-EIAED patients. Fifty-five non-EIAED patients (34 glioblastoma multiforme and 21 anaplastic glioma) enrolled in the phase II component. Patients initially received 800 mg/d imatinib; 15 anaplastic glioma patients received 600 mg/d after hemorrhages were observed. There were 2 partial response and 6 stable disease among glioblastoma multiforme patients and 0 partial response and 5 stable disease among anaplastic glioma patients. Six-month progression-free survival was 3% for glioblastoma multiforme and 10% for anaplastic glioma patients. Five phase II patients developed intratumoral hemorrhages.

CONCLUSIONS

Single-agent imatinib has minimal activity in malignant gliomas. CYP3A4 inducers, such as EIAEDs, substantially decreased plasma exposure of imatinib and should be avoided in patients receiving imatinib for chronic myelogenous leukemia and gastrointestinal stromal tumors. The evaluation of the activity of combination regimens incorporating imatinib is under way in phase II trials.

Widespread CXCR4 activation in astrocytomas revealed by phospho-CXCR4-specific antibodies

The chemokine receptor CXCR4 is expressed in many cancers where it may regulate tumor cell growth and migration. The role of CXCR4 in cancer will depend on it being in an activated, signaling state. To better define the significance of CXCR4 expression in cancer, we developed an antibody that can distinguish CXCR4 phosphorylated on serine 339, a residue previously identified as a site for ligand-induced phosphorylation. With this antibody, we investigated the mechanisms of CXCR4 phosphorylation and evaluated the phosphorylation status of CXCR4 in human astrocytomas. In vitro, phosphorylation of serine 339 occurred in response to CXCL12 or epidermal growth factor (EGF) treatment and was increased by protein kinase C activation. In all grades of astrocytomas, CXCR4 was expressed in tumor cells and some endothelial cells, whereas CXCL12 was present in endothelial cells and infiltrating microglia. We found that CXCR4 phosphorylated on serine 339 was present in tumor cells and vascular endothelial cells in all grades of astrocytoma. These data indicate that CXCR4 is expressed and activated in astrocytomas and that phosphorylation of CXCR4 can occur through ligand activation or transactivation via the EGF receptor. These studies extend the potential roles of CXCR4 in cancer to include functions associated with benign (grade 1) tumors. The ability to distinguish phosphorylated CXCR4 will be invaluable for the continued analysis of the role of CXCR4 in cancer and the development of CXCR4 antagonist therapy for patients suffering with primary tumors of the brain and other sites.

STI571 (Gleevec) improves tumor growth delay and survival in irradiated mouse models of glioblastoma

PURPOSE

Glioblastoma multiforme (GBM) is a devastating brain neoplasm that is essentially incurable. Although radiation therapy prolongs survival, GBMs progress within areas of irradiation. Recent studies in invertebrates have shown that STI571 (Gleevec; Novartis, East Hanover, NJ) enhances the cytotoxicity of ionizing radiation. In the present study, the effectiveness of STI571 in combination with radiation was studied in mouse models of GBM.

METHODS AND MATERIALS

Murine GL261 and human D54 GBM cell lines formed tumors in brains and hind limbs of C57BL6 and nude mice, respectively. GL261 and D54 cells were treated with 5 micromol/L of STI571 for 1 h and/or irradiated with 3 Gy. Protein was analyzed by Western immunoblots probed with antibodies to caspase 3, cleaved caspase 3, phospho-Akt, Akt, and platelet-derived growth factor receptor (PDGFR) alpha and beta. Tumor volumes were assessed in mice bearing GL261 or D54 tumors treated with 21 Gy administered in seven fractionated doses. Histologic sections from STI571-treated mice were stained with phospho-Akt and phospho-PDGFR beta antibodies. Kaplan-Meier survival curves were used to study the response of mice bearing intracranial implants of GL261.

RESULTS

STI571 penetrated the blood-brain barrier, which resulted in a reduction in phospho-PDGFR in GBM. STI571-induced apoptosis in GBM was significantly enhanced by irradiation. STI571 combined with irradiation induced caspase 3 cleavage in GBM cells. Glioblastoma multiforme response to therapy correlated with an increase in tumor growth delay and survival when STI571 was administered in conjunction with daily irradiation.

CONCLUSION

These findings suggest that STI571 has the potential to augment radiotherapy and thereby improve median survival.

Upregulation of ASCL1 and inhibition of Notch signaling pathway characterize progressive astrocytoma

Astrocytoma is the most common type of brain cancer constituting more than half of all brain tumors. With an aim to identify markers describing astrocytoma progression, we have carried out microarray analysis of astrocytoma samples of different grades using cDNA microarray containing 1152 cancer-specific genes. Data analysis identified several differentially regulated genes between normal brain tissue and astrocytoma as well as between grades II/III astrocytoma and glioblastoma multiforme (GBM; grade IV). We found several genes known to be involved in malignancy including Achaete-scute complex-like 1 (Drosophila) (ASCL1; Hash 1). As ASCL has been implicated in neuroendocrine, medullary thyroid and small-cell lung cancers, we chose to examine the role of ASCL1 in the astrocytoma development. Our data revealed that ASCL1 is overexpressed in progressive astrocytoma as evidenced by increased levels of ASCL1 transcripts in 85.71% (6/7) of grade II diffuse astrocytoma (DA), 90% (9/10) of grade III anaplastic astrocytoma (AA) and 87.5% (7/8) of secondary GBMs, while the majority of primary de novo GBMs expressed similar to or less than normal brain levels (66.67%; 8/12). ASCL1 upregulation in progressive astrocytoma is accompanied by inhibition of Notch signaling as seen by uninduced levels of HES1, a transcriptional target of Notch1, increased levels of HES6, a dominant-negative inhibitor of HES1-mediated repression of ASCL1, and increased levels of Notch ligand Delta1, which is capable of inhibiting Notch signaling by forming intracellular Notch ligand autonomous complexes. Our results imply that inhibition of Notch signaling may be an important early event in the development of grade II DA and subsequent progression to grade III AA and secondary GBM. Furthermore, ASCL1 appears to be a putative marker to distinguish primary GBM from secondary GBM.

Brain as a paradigm of organ growth: Hedgehog-Gli signaling in neural stem cells and brain tumors

The Hedgehog-Gli (Hh-Gli) signaling pathway is essential for numerous events during the development of many animal cell types and organs. In particular, it controls neural cell precursor proliferation in dorsal brain structures and regulates the number of neural stem cells in distinct embryonic, perinatal, and adult niches, such as the developing neocortex, the subventricular zone of the lateral ventricle of the forebrain, and the hippocampus. We have proposed that Hh-Gli signaling regulates dorsal brain growth during ontogeny and that its differential regulation underlays evolutionary change in the morphology (size and shape) of dorsal brain structures. It is also critically involved in sporadic brain tumorigenesis--as well as several other human cancer--suggesting that tumors derive from stem cells or progenitors maintaining an inappropriate active Hh-Gli pathway. Importantly, we and others have demonstrated that human sporadic tumors from the brain and other organs require sustained HH-GLI signaling for sustained growth and survival. Modulating HH-GLI signaling thus represents a novel rational avenue to treat, on one hand, brain degeneration and injury by inducing controlled HH-GLI-mediated regeneration and growth, and on the other hand, to combat cancer by blocking its abnormal activity in tumor cells.

Autocrine platelet-derived growth factor-dependent gene expression in glioblastoma cells is mediated largely by activation of the transcription factor sterol regulatory element binding protein and is associated with altered genotype and patient survival in human brain tumors

A complex profile of gene expression elicited by autocrine platelet-derived growth factor (PDGF) signaling was identified in U87 MG glioblastoma cells by microarray analysis. The most striking pattern observed was a PDGF-dependent activation of at least 25 genes involved with biosynthesis and/or uptake of cholesterol and isoprenoids, including mevalonate pyrophosphate decarboxylase, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase, HMG-CoA reductase, and low-density lipoprotein receptor. Activity of the HMG-CoA synthase promoter was induced by autocrine PDGF activity as indicated by significant reductions following forced expression of dominant-negative PDGF-A (88%) or treatment with the PDGF receptor antagonist CT52923 (50%). Induction of the HMG-CoA synthase promoter required a binding site for sterol regulatory element binding proteins (SRE-BP), consistent with a key role for these transcription factors in the induction of this gene network. Neither proteolytic activation nor nuclear localization of SRE-BP was affected by disruption of the PDGF autocrine loop, indicating that PDGF signaling is required for other signaling events involved in activation of SRE-BP target genes. Analysis of an expression databank derived from human glial tumors (n = 77) identified a subgroup exhibiting a profile consistent with PDGF dependence, including increased expression of SRE-BP target genes. This subgroup displayed an absence of epidermal growth factor receptor gene amplification, decreased incidence of allelic loss of 10q, increased frequency of TP53 mutations and allelic losses of 1p and 19q, and longer patient survival. This study identifies genes associated with oncogenic activity of PDGF and provides important insights into biomarkers and therapeutic targets in malignant gliomas.

Novel 5-substituted, 2,4-diaminofuro[2,3-d]pyrimidines as multireceptor tyrosine kinase and dihydrofolate reductase inhibitors with antiangiogenic and antitumor activity

Recent evidence suggests that combination therapy of cancer with receptor tyrosine kinase (RTK) inhibitors, which are usually cytostatic, with conventional chemotherapeutic agents, which are usually cytotoxic, provide an improved treatment option. We have designed, synthesized, and evaluated a series of novel 2,4-diamino-5-substituted furo[2,3-d]pyrimidines with RTK and dihydrofolate reductase (DHFR) inhibitory activity in single molecules, as potential cytostatic and cytotoxic agents with antitumor activity. These compounds were synthesized from 2,4-diamino-5-chloromethyl furo[2,3-d]pyrimidine and aryl methyl ketones using the Wittig reaction to afford the C-8-C-9 unsaturated analogs followed by catalytic reduction to the corresponding saturated compounds. The saturated and unsaturated C-8-C-9 bridged compounds were evaluated as inhibitors of vascular endothelial growth factor receptor (VEGFR-2, Flk, KDR), epidermal growth factor receptor, and platelet-derived growth factor receptor-beta (PDGFR-beta). Selected analogs were also evaluated as antiangiogenic agents in the chicken embryo chorioallantoic membrane (CAM) assay. The compounds were also evaluated as inhibitors of human (h) DHFR and Toxoplasma gondii (tg) DHFR. In each evaluation, a known standard compound was used as a comparison. Of the compounds evaluated, compound 32 was as potent as the standard compounds against VEGFR-2 and PDGFR-beta, showing dual inhibitory activity against RTK. This analog was also highly effective in the CAM assay. A second analog 18 also demonstrated dual VEGFR-2 and PDGFR-beta inhibitory activity as well as potent antiangiogenic activity in the CAM assay. Four additional analogs were also effective against PDGFR-beta and in the CAM assay. An unsaturated C-8-C-9 moiety was necessary for RTK inhibitory activity. Compound 32 also showed inhibitory activity against hDHFR and tgDHFR, illustrating the multitarget inhibitory potential of these analogs. The biological activity of these analogs also suggests the necessity of an unsaturated C-8-C-9 bridge for dual RTK and DHFR inhibitory activity. Compounds 18 and 32 were also evaluated in a B16 melanoma mouse model and were found to be more active as antitumor agents than methotrexate. In addition, both 18 and 32 were also active in decreasing lung metastases in a mouse model of B16 melanomas.

Platelet-derived growth factor (PDGF) and glial tumorigenesis

Platelet-derived growth factor (PDGF) has long been implicated in cancer and is known to be involved in many biological processes. In Central Nervous System (CNS) neoplasms, particularly gliomas, PDGF is often over-expressed. However, what role PDGF plays in tumor progression remains to be fully described. A wide range of work from in vitro studies to mouse models have implicated the PDGF pathway in various processes including proliferation, cellular migration, development, and angiogenesis. Being a secreted factor, PDGF not only has autocrine effects on producing cells but also has potential for paracrine effects on other tumor cells and the tumor microenvironment. The development of small molecules that inhibit the PDGF receptor and various subsequent signaling components promises to introduce new approaches to the treatment of gliomas.

Galanin and galanin receptors in human cancers

The increasing interest in peptides and peptide receptors in cancer is based on the possibility of receptor targeting, because peptide receptors are often expressed in different human tumors. The neuropeptide galanin has also been suggested to be involved in the development of neuroendocrine tumors based on the development of estrogen-induced tumors in estrogen-sensitive rats. This study summarizes our current knowledge on the expression of galanin peptide and galanin receptors in different human neuroendocrine tumors. The expression of both, peptide and corresponding receptor, seems to be a common feature of human gliomas, pheochromocytomas, pituitary and neuroblastic tumors. The co-expression of galanin and its receptors supports a role for galanin in tumor cell pathology via autocrine/paracrine mechanisms.

Pathology and molecular genetics of oligodendroglial tumors

Oligodendroglial gliomas are second only to astrocytic gliomas in frequency. The lack of stringent diagnostic criteria cause high interobserver variation in regard to classification and grading of these tumors. Previous studies have described oligodendrogliomas with features that overlap with those of neurocytic tumors, thus further complicating diagnostic decisions. The increasing need for standardized diagnostic criteria in this subset of gliomas is emphasized by the benefit of adjuvant therapies in patients with anaplastic oligodendrogliomas. Characteristic chromosomal aberrations have been successfully determined for oligodendroglial tumors in recent years. In contrast to astrocytomas, however, no genes in the affected regions have been clearly linked to their pathogenesis. However, the molecular findings promise to be helpful for diagnostic and therapeutic decisions. This review compiles clinical, pathological, and molecular genetic findings on WHO grades II and III oligodendrogliomas and oligoastrocytomas.

Antiangiogenic and antitumor agents. Design, synthesis, and evaluation of novel 2-amino-4-(3-bromoanilino)-6-benzylsubstituted pyrrolo[2,3-d]pyrimidines as inhibitors of receptor tyrosine kinases

Several different classes of growth factor receptors containing tyrosine kinases (RTK) are directly or indirectly involved in angiogenesis. Inhibition of these RTKs has provided a new paradigm in the treatment of tumors by restricting their growth and metastasis. We have designed, synthesized and evaluated eleven novel 2-amino-4-(3-bromoanilino)-6-substituted benzyl pyrrolo[2,3-d]pyrimidines as the first in a series of RTK inhibitors. These analogues were synthesized from appropriate alpha-bromomethylbenzyl ketones by cyclocondensation with 2,6-diamino-4-pyrimidone to afford the 2-amino-4-oxo-6-substituted benzyl pyrrolo[2,3-d]pyrimidines. Chlorination of the 4-position followed by displacement with 3-bromoaniline afforded the target compounds. In some instances, the 2-amino moiety of the pyrrolo[2,3-d]pyrimidines was protected prior to the chlorination and displacement followed by deprotection. The compounds were evaluated as inhibitors of vascular endothelial growth factor receptors VEGFR-2 (Flk-1, KDR) and VEGFR-1 (Flt-1); epidermal growth factor receptor (EGFR); and platelet-derived growth factor receptor-beta (PDGFR-beta). Selected compounds were also evaluated against the growth of A431 cells (which overexpress EGFR) in culture and as inhibitors of angiogenesis in the chicken embryo chorioallantonic membrane (CAM) assay. In each evaluation, a known standard compound was used as a comparison. Of the 11 analogues, five were more potent or equipotent as compared to standard compounds against the growth factor receptors. Two analogues showed superior inhibition of A431 cells in culture compared to the standard compounds. Three analogues were equipotent with the standard compound in the CAM assay and four of the analogues were dual inhibitors of RTKs. The structure-activity relationship for inhibition of different RTKs was quite distinct and different, and for VEGFR-2 and EGFR diametrically opposite. The inhibitory data against the RTKs in this study demonstrates that variation of the substituent(s) in the benzyl ring of these 2-amino-4-anilino 6-benzyl pyrrolo[2,3-d]pyrimidines does indeed control both the potency and specificity of inhibitory activity against RTKs.

Inhibition of platelet-derived growth factor signalling induces autophagy in malignant glioma cells

Malignant gliomas highly coexpress platelet-derived growth factor (PDGF) and its receptor, suggesting the presence of an autocrine loop. Therefore, disruption of PDGF ligand/receptor complex represents a promising strategy for the treatment of malignant gliomas. However, the mechanisms of the antitumour effect exerted by the inhibition of PDGF-mediated cell growth remain unclear. In the present study, using anti-PDGF neutralising antibody, we investigated the effect of the inhibition of PDGF signalling on malignant glioma U87-MG, D54, and T98G cells with high levels of PDGF-A and -B. As a control, normal fibroblast MRC5 cells expressing low levels of PDGF-A and -B were used. Treatment with anti-PDGF neutralising antibody did not affect the expressions of PDGF-A, PDGF-B, and Akt, but suppressed the level of phosphorylated Akt in tumour cells, indicating the inhibition of PDGF signalling. The cell viability of all malignant glioma cells tested in this study was significantly inhibited in a time-dependent manner following the treatment compared to that of fibroblast cells (P<0.02 to <0.05). The antitumour effect of anti-PDGF antibody was suppressed by the activation of Akt and enhanced by the downregulation of Akt. Interestingly, the inhibition of PDGF signalling induced the development of acidic vesicular organelles and the autophagosome membrane association of the microtubule-associated protein light chain 3, which are characteristic of autophagy, in malignant glioma cells, while apoptotic cell death was not observed. Together these findings imply a new concept of autophagy for PDGF autocrine inhibition in malignant gliomas.

Can mouse models for brain tumors inform treatment in pediatric patients?

Brain tumors represent the most common solid tumor of childhood. Although the histology of many pediatric brain tumors is similar to that of their adult counterparts, significant differences exist with regard to tumor location and response to therapy. The biological and genetic basis for this difference is poorly understood, as tumor tissue is generally unavailable for such studies. While targeted therapies directed against specific molecules active in cancer represents a new arsenal of agents for treating these tumors, such agents are generally not being developed for pediatric cancer in particular. Therefore, new agents for treatment of pediatric glioma must be obtained from compounds being tested against tumors of comparable histology in adult patients. Compounding this problem, although brain tumors are among the most lethal tumors of childhood, their absolute number is relatively small. As a consequence, trials with new agents must be prioritized based on the likelihood that a particular agent or combination of agents will have efficacy in pediatric cancer. Mouse models for brain tumors may help to identify targeted agents, and combinations of agents, effective against these tumors. Such data can be used to prioritize therapies for clinical trials in children with these tumors.

Angiogenesis-related growth factors in brain tumors

Numerous growth factors have been implicated in glioma angiogenesis. This chapter focuses on the role of scatter factor/hepatocyte growth factor, fibroblast growth factor, platelet-derived growth factor and transforming growth factor beta. We review the expression pattern of these factors in gliomas, their functional contribution to tumor angiogenesis - also in relation to vascular endothelial growth factor, and the effects resulting from their inhibition or overexpression in gliomas in vivo.

Thalidomide: a new anticancer drug?

Experimental studies have demonstrated that thalidomide (Thal), a drug developed as a sedative, has antitumoural properties. The possible antitumour mechanisms of action involve: inhibition of angiogenesis, cytokine-mediated pathways, modulation of adhesion molecules, inhibition of cyclooxygenase-2 and stimulation of immuno response. Therefore, Thal is under clinical evaluation in oncology. This paper provides an overview of the data currently available in literature regarding, in terms of activity and toxicity, the use of Thal in cancer patients. Multiple myeloma is so far the most responsive malignancy. A moderate activity has been documented in certain solid tumours: glioblastoma multiforme, renal cell carcinoma and malignant melanoma. Tolerability is generally satisfactory with peripheral neuropathy being the most relevant dose-dependent toxicity. The more frequent, but moderate side effects are: somnolence, constipation, dizziness and fatigue. More studies are needed to properly evaluate the anticancer activity of Thal alone or in combination with other anticancer treatments. Preliminary studies suggest promising results of Thal in combinations with corticosteroids and cytotoxic drugs as front-line therapy of multiple myeloma. Regarding therapy of solid tumours in the adult, combination with chemotherapy, radiation therapy and molecular-targeting compounds are under investigation.

Galanin and galanin receptors in human gliomas

Galanin-like immunoreactivity (GAL-LI) and specific GAL binding sites have been shown to be widely distributed in the central nervous system (CNS) and in CNS tumors. GAL and its receptors have also been shown to be present in glial cells, but to date it is still unknown whether human gliomas produce GAL and express GAL receptors. In this study 20 brain tumors consisting of 15 glioblastomas, 4 meningiomas and 1 gliosarcoma were investigated for the presence of GAL-LI and GAL receptors. Immunofluorescence analysis revealed a dense network of GAL-LI positive cellular processes and cell bodies in 18 of the 20 tumors. In contrast, in vitro (125)I-labeled GAL receptor autoradiography showed substantial GAL binding in only 6 glioblastoma tissues. Reverse transcription-PCR analysis detected mRNA of all three known galanin receptors in the tumor tissues, with most tumors expressing multiple receptor subtypes. Pharmacological analysis of tumor membrane homogenates with GAL and the specific GAL receptor GalR2 agonist, AR-M1896, revealed that the GAL receptor GalR1 is most likely the receptor responsible for the observed GAL binding in the glioblastomas. No correlation could be found between GAL-LI, the level of GAL binding and proliferative activity as determined by immunostaining with the cell proliferation marker Ki-67.

Molecular genetic changes in a series of neuroepithelial tumors of childhood

Amplification of the EGFR, mdm2, CDK4 and PDGFR A genes has been widely demonstrated in adult malignant gliomas, almost exclusively glioblastomas. To determine the role of these mutational events in pediatric astrocytic gliomas we investigated the presence of EGFR, mdm2, CDK4 and PDGFR A gene amplification in 38 childhood brain tumor biopsies, including 24 low-grade astrocytomas and 14 malignant tumors. We used differential PCR assay on DNA extracted either from paraffin embedded or frozen tissues. EGFR gene amplification was detected in 4 out of 14 malignant tumors; no low-grade astrocytoma showed amplification. Tumors with EGFR gene amplification were negative for the presence of p53 mutations, as observed in a previous study. One glioblastoma showed PDGFR A amplification, while no amplifications were observed for mdm2 and CDK4 genes. These data are in line with those obtained from studies on gliomas of adults and suggest the existence of two different subsets of malignant gliomas also in pediatric brain tumors: one carrying EGFR gene amplification, the other showing p53 mutations.

In vitro and in vivo growth inhibition of human malignant astrocytoma cells by the farnesyltransferase inhibitor B1620

p21-Ras, the protein product of the proto-oncogene Ras is overactivated in malignant astrocytomas despite the absence of mutation. It is known that p21-Ras participates in signaling events from membrane tyrosine kinase receptors and a variety of intracellular biochemical pathways to downstream targets. Signal transduction inhibition by targeting against Ras is now thought to be a promising therapeutic strategy for malignant astrocytomas. This study demonstrates that Ras pathway inactivation by a farnesyltransferase inhibitor, B1620, effectively inhibits in vitro and in vivo growth of human astrocytoma cells, although normal human astrocytes (NHA) derived from fetal brain are resistant to B1620. Anti-proliferative effect of B1620 on in vitro growth of astrocytoma cells was examined by MTT assays and soft agar colony formation assay. B1620 inhibited anchorage-dependent growth of six astrocytoma cell lines with a median effective dose (IC50) ranging from 2.0 to 20.7 microM. However, growth of NHA was not significantly affected by B1620 even at the concentration of 100 microM. All astrocytoma cells showed apoptotic figures after Hoechst 33258 staining, when treated for 5 days at each IC50 concentration against B1620. Anchorage-independent growth of these astrocytoma cell lines was inhibited at a much lower concentration than that of anchorage-dependent growth. Daily treatment of U87 xenograft-bearing athymic mice with B1620 at 100 or 50 mg kg(-1) resulted in significant inhibition of tumor growth. A histological study of the B1620-treated tumor tissue showed decreased vascularity with numerous TUNEL-positive apoptotic cells. These results suggest that the mechanism of the growth-inhibitory effect of B1620 is anti-angiogenesis, apoptosis induction and reversion of the transformed phenotype. The potential clinical use of B1620 could be expanded to malignant astrocytomas.