Transforming growth factor beta as a potential tumor progression factor among hyperdiploid glioblastoma cultures: evidence for the role of platelet-derived growth factor

Glioblastoma multiforme: a review of therapeutic targets

Glioblastoma is the commonest primary brain tumor, as well as the deadliest. Malignant gliomas such as glioblastoma multiforme (GBM) present some of the greatest challenges in the management of cancer patients worldwide, despite notable recent achievements in oncology. Even with aggressive surgical resections using state-of-the-art preoperative and intraoperative neuroimaging, along with recent advances in radiotherapy and chemotherapy, the prognosis for GBM patients remains dismal: survival after diagnosis is about 1 year. Established prognostic factors are limited, but include age, Karnofsky performance status, mini-mental status examination score, O6-methylguanine methyltransferase promoter methylation and extent of surgery. Standard treatment includes resection of > 95% of the tumor, followed by concurrent chemotherapy and radiotherapy. Nevertheless, GBM research is being conducted worldwide at a remarkable pace, in the laboratory and at the bedside, with some of the more recent promising studies focused on identification of aberrant genetic events and signaling pathways to develop molecular-based targeted therapies, tumor stem cell identification and characterization, modulation of tumor immunological responses and understanding of the rare long-term survivors. With this universally fatal disease, any small breakthrough will have a significant impact on survival and provide hope to the thousands of patients who receive this diagnosis annually. This review describes the epidemiology, clinical presentation, pathology and tumor immunology, with a focus on understanding the molecular biology that underlies the current targeted therapeutics being tested.

Glioblastoma Multiforme Oncogenomics and Signaling Pathways

In the adult population, glioblastoma multiforme is one of the most common primary brain tumors encountered. Unfortunately, this highly malignant tumor represents over 50% of all types of primary central nervous system gliomas. The vast majority of GBMs develops quite rapidly without clinical, radiological, or morphologic evidence of a less malignant precursor lesion (primary or de novo GBMs), as compared to secondary GBMs that develop slowly by progression from diffuse low-grade astrocytomas. These GBM subtypes must be kept in mind because they may constitute distinct disease entities. Even though they look histologically quite similar, they likely involve different genetic alterations and signaling pathways. Decades of surgical therapy, radiotherapy, and chemotherapy have failed to drastically change survival. Clearly, we do not fully understand this tumor; however, the exciting genetic revolution in glioma research over the past decade is providing a promising outlook for exploring this tumor at the genetic level. Science has begun to elucidate the numerous genetic alterations and critical signaling pathways, and it has opened new exciting areas of research such as glioma stem cell biology and neoangiogenesis. This work has already begun to improve our understanding of GBM cell proliferation, migration, and invasion. Indeed, exciting novel targeted therapies are making their way to clinical trials based on this increased knowledge. This review provides the current understanding of GBM oncogenomics, signaling pathways, and glioma stem cell biology and discusses the potential new therapeutic targets on the horizon.

Role of platelet-derived growth factors in physiology and medicine

Platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) have served as prototypes for growth factor and receptor tyrosine kinase function for more than 25 years. Studies of PDGFs and PDGFRs in animal development have revealed roles for PDGFR-alpha signaling in gastrulation and in the development of the cranial and cardiac neural crest, gonads, lung, intestine, skin, CNS, and skeleton. Similarly, roles for PDGFR-beta signaling have been established in blood vessel formation and early hematopoiesis. PDGF signaling is implicated in a range of diseases. Autocrine activation of PDGF signaling pathways is involved in certain gliomas, sarcomas, and leukemias. Paracrine PDGF signaling is commonly observed in epithelial cancers, where it triggers stromal recruitment and may be involved in epithelial-mesenchymal transition, thereby affecting tumor growth, angiogenesis, invasion, and metastasis. PDGFs drive pathological mesenchymal responses in vascular disorders such as atherosclerosis, restenosis, pulmonary hypertension, and retinal diseases, as well as in fibrotic diseases, including pulmonary fibrosis, liver cirrhosis, scleroderma, glomerulosclerosis, and cardiac fibrosis. We review basic aspects of the PDGF ligands and receptors, their developmental and pathological functions, principles of their pharmacological inhibition, and results using PDGF pathway-inhibitory or stimulatory drugs in preclinical and clinical contexts.

High TGFbeta-Smad activity confers poor prognosis in glioma patients and promotes cell proliferation depending on the methylation of the PDGF-B gene

TGFbeta acts as a tumor suppressor in normal epithelial cells and early-stage tumors and becomes an oncogenic factor in advanced tumors. The molecular mechanisms involved in the malignant function of TGFbeta are not fully elucidated. We demonstrate that high TGFbeta-Smad activity is present in aggressive, highly proliferative gliomas and confers poor prognosis in patients with glioma. We discern the mechanisms and molecular determinants of the TGFbeta oncogenic response with a transcriptomic approach and by analyzing primary cultured patient-derived gliomas and human glioma biopsies. The TGFbeta-Smad pathway promotes proliferation through the induction of PDGF-B in gliomas with an unmethylated PDGF-B gene. The epigenetic regulation of the PDGF-B gene dictates whether TGFbeta acts as an oncogenic factor inducing PDGF-B and proliferation in human glioma.

TGF-β, Neuronal Stem Cells and Glioblastoma

Transforming growth factor beta (TGF-beta) signaling leads to a number of biological end points involving cell growth, differentiation, and morphogenesis. Typically, the cellular effect accompanies an induction of mesodermal cell fate and inhibition of neural cell differentiation. However, during pathological conditions, these defined effects of TGF-beta can be reversed; for example, the growth-inhibitory effect is replaced with its tumor promoting ability. A multitude of factors and cross-signaling pathways have been reported to be involved in modulating the dual effects of TGF-beta. In this review, we focus on the potential role of TGF-beta signal transduction during development of neural progenitor cells and its relation to glioblastoma development from neural stem cells.

Integration of Smad and Forkhead Pathways in the Control of Neuroepithelial and Glioblastoma Cell Proliferation

FoxO Forkhead transcription factors are shown here to act as signal transducers at the confluence of Smad, PI3K, and FoxG1 pathways. Smad proteins activated by TGF-beta form a complex with FoxO proteins to turn on the growth inhibitory gene p21Cip1. This process is negatively controlled by the PI3K pathway, a known inhibitor of FoxO localization in the nucleus, and by the telencephalic development factor FoxG1, which we show binds to FoxO-Smad complexes and blocks p21Cip1 expression. We suggest that the activity of this network confers resistance to TGF-beta-mediated cytostasis during the development of the telencephalic neuroepithelium and in glioblastoma brain tumor cells.

Molecular neuro-oncology and development of targeted therapeutic strategies for brain tumors. Part 1: Growth factor and Ras signaling pathways

Brain tumors are a diverse group of malignancies that remain refractory to conventional treatment approaches, including radiotherapy and cytotoxic chemotherapy. Molecular neuro-oncology has now begun to clarify the transformed phenotype of brain tumors and identify oncogenic pathways that may be amenable to targeted therapy. Growth factor signaling pathways are often upregulated in brain tumors and may contribute to oncogenesis through autocrine and paracrine mechanisms. Excessive growth factor receptor stimulation can also lead to overactivity of the Ras signaling pathway, which is frequently aberrant in brain tumors. Receptor tyrosine kinase inhibitors, antireceptor monoclonal antibodies and antisense oligonucleotides are targeted approaches under investigation as methods to regulate aberrant growth factor signaling pathways in brain tumors. Several receptor tyrosine kinase inhibitors, including imatinib mesylate (Gleevec), gefitinib (Iressa) and erlotinib (Tarceva), have entered clinical trials for high-grade glioma patients. Farnesyl transferase inhibitors, such as tipifarnib (Zarnestra), which impair processing of proRas and inhibit the Ras signaling pathway, have also entered clinical trials for patients with malignant gliomas. Further development of targeted therapies and evaluation of these new agents in clinical trials will be needed to improve survival and quality of life of patients with brain tumors.

Interaction of transforming growth factor-beta (TGF-beta) and epidermal growth factor (EGF) in human glioma cells

Gliomas are characterized by a deregulation of growth factor production and growth factor receptors expression, e.g. overproduction of the cytokine transforming growth factor-beta (TGF-beta) and overexpression/constitutive activation of receptors for the epidermal growth factor (EGF). Potential interactions of such growth factors and their signaling cascades could enhance the malignancy of these tumors. Therefore, we investigated the effects of TGF-beta and EGF alone and in combination on the proliferation of glioma cells cultivated from eight solid human WHO grade IV gliomas and one glioma cell line, analyzed the expression and intactness of the TGF-beta-signaling molecules Samd-4 and -2, and the phosphorylation of the EGF-signaling kinases ERK 1/2. The effects were divergent and complex: Whereas EGF mostly stimulated glioma cell proliferation, TGF-beta either enhanced, inhibited or had no significant effect on proliferation. In combination, co-stimulation and inhibition of the EGF-induced mitogenic activity could be observed. Smad-4/-2 were expressed in all glioma cells, one point mutation at base 1595 in Smad-4 did not affect its protein sequence. In part of the glioma cells, reduced phosphorylation of ERK 1/2 and expression of cyclin-dependent kinase inhibitor 1 or p21 was observed in co-stimulation experiments. These experiments show that TGF-beta can inhibit EGF-mediated effects only in some gliomas, whereas it enhances it in others. The interaction of both factors is very complex and varies between different gliomas.

Colony-stimulating factor 1 promotes progression of mammary tumors to malignancy

In human breast carcinomas, overexpression of the macrophage colony-stimulating factor (CSF-1) and its receptor (CSF-1R) correlates with poor prognosis. To establish if there is a causal relationship between CSF-1 and breast cancer progression, we crossed a transgenic mouse susceptible to mammary cancer with mice containing a recessive null mutation in the CSF-1 gene (Csf1(op)) and followed tumor progression in wild-type and null mutant mice. The absence of CSF-1 affects neither the incidence nor the growth of the primary tumors but delayed their development to invasive, metastatic carcinomas. Transgenic expression of CSF-1 in the mammary epithelium of both Csf1(op)/Csf1(op) and wild-type tumor-prone mice led to an acceleration to the late stages of carcinoma and to a significant increase in pulmonary metastasis. This was associated with an enhanced infiltration of macrophages into the primary tumor. These studies demonstrate that the growth of mammary tumors and the development to malignancy are separate processes and that CSF-1 selectively promotes the latter process. CSF-1 may promote metastatic potential by regulating the infiltration and function of tumor-associated macrophages as, at the tumor site, CSF-1R expression was restricted to macrophages. Our data suggest that agents directed at CSF-1/CSF-1R activity could have important therapeutic effects.

Growth factors in glioma angiogenesis: FGFs, PDGF, EGF, and TGFs

It has become well accepted that solid tumors must create a vascular system for nutrient delivery and waste removal in order to grow appreciably. This process, angiogenesis, is critical to the progression of gliomas, with vascular changes accompanying the advancement of these tumors. The cascade of events in this process of blood vessel formation involves a complex interplay between tumor cells, endothelial cells, and their surrounding basement membranes in which enzymatic degradation of surrounding ground substance and subsequent endothelial cell migration, proliferation, and tube formation occurs. It is likely that a host of growth factors is responsible for mediating these key events. To date, a role for Vascular Endothelial Growth Factor (VEGF) in glioma angiogenesis has been convincingly demonstrated. This review explores the contribution of other growth factors--Fibroblast Growth Factors (FGFs), Platelet-Derived Growth Factor (PDGF), Epidermal Growth Factor (EGF), and Transforming Growth Factors (TGFs)--to glioma angiogenesis. These growth factors may influence glioma angiogenesis by directly stimulating endothelial cell proliferation, by mediating the expression of key proteases on endothelial cells necessary for angiogenesis, or by regulating the expression of VEGF and of each other.

Amplification of the platelet-derived growth factor receptor-A (PDGFRA) gene occurs in oligodendrogliomas with grade IV anaplastic features

Activation of the platelet-derived growth factor (PDGF) signaling system has been implicated in the development and malignant progression of diffuse gliomas. Overexpression of PDGF system components, particularly the alpha subtype receptor (PDGFRA), is common in glial tumors, and PDGFRA gene amplification has been reported in glioblastomas. In order to address the incidence of PDGFRA gene amplification in a broad set of diffuse gliomas, we used Southern and fluorescence in situ hybridization (FISH) analyses to examine 167 astrocytic gliomas (20 grade III and 147 grade IV), 41 anaplastic oligodendrogliomas, and 29 anaplastic oligoastrocytomas. PDGFRA gene amplification was identified in 4 anaplastic oligodendrogliomas and in a single case of anaplastic oligoastrocytoma, but in none of the malignant astrocytomas. Each of the 5 tumors with PDGFRA amplification displayed features generally associated with grade IV malignancy in astrocytic tumors. Consequently, our data indicate that this gene alteration is restricted to tumors having oligodendroglial differentiation and highly anaplastic features.