Major structural alterations of the c-sis gene are not observed in a series of tumors of the human central nervous system

PDGF in gliomas: more than just a growth factor?

Platelet-derived growth factor B (PDGF-B) is a growth factor promoting and regulating cell migration, proliferation, and differentiation, involved in both developmental processes and in maintaining tissue homeostasis under strict regulation. What are the implications of prolonged or uncontrolled growth factor signaling in vivo, and when does a growth factor such as PDGF-B become an oncogene? Under experimental conditions, PDGF-B induces proliferation and causes tumor induction. It is not known whether these tumors are strictly a PDGF-B-driven proliferation of cells or associated with secondary genetic events such as acquired mutations or methylation-mediated gene silencing promoting neoplasia. If PDGF-B-driven tumorigenesis was only cellular proliferation, associated changes in gene expression would thus be correlated with proliferation and not associated with secondary events involved in tumorigenesis and neoplastic transformation such as cycle delay, DNA damage response, and cell death. Changes in gene expression might be expected to be reversible, as is PDGF-B-driven proliferation under normal circumstances. Since PDGF signaling is involved in oligodendrocyte progenitor cell differentiation and maintenance, it is likely that PDGF-B stimulates proliferation of a pool of cells with that phenotype, and inhibition of PDGF-B signaling would result in reduced expression of oligodendrocyte-associated genes. More importantly, inhibition of PDGF signaling would be expected to result in reversion of genes induced by PDGF-B accompanied by a decrease in proliferation. However, if PDGF-B-driven tumorigenesis is more than simply a proliferation of cells, inhibition of PDGF signaling may not reverse gene expression or halt proliferation. These fundamental questions concerning PDGF-B as a potential oncogene have not been resolved.

Autocrine growth stimulation of human meningioma cells by platelet-derived growth factor

The authors have previously shown that meningioma-derived conditioned medium profoundly stimulates the in vitro proliferation of meningioma cells. In this paper, self mitogenic agents found in the conditioned medium-autocrine growth-stimulatory factors actually secreted by human meningioma cells-are characterized as proteins related to the B chain of platelet-derived growth factor (PDGF) and possibly to the A chain of PDGF as well. The addition to conditioned medium of a neutralizing antibody against PDGF-BB caused a significant inhibition of the conditioned medium-stimulated DNA synthesis in all three meningioma cultures studied. A similar neutralizing effect was observed with an anti-PDGF-AA antibody in one meningioma culture studied. Gel filtration chromatography of concentrated conditioned medium from two different meningiomas using a Sephadex G-100 column revealed similar profiles from both conditioned media with a major peak of mitogenic activity against meningioma cells at a molecular weight (M(r)) of approximately 32 to 36 kD, accompanied by a minor peak at approximately 22 kD. The major peak mitogenic activity was significantly reduced by addition of an anti-PDGF-BB antibody. Western blot analysis of protein extracts from five meningioma specimens was performed using a monoclonal antibody against the B chain of PGDF, and a major band of PDGF-B immunoreactivity was detected at an M(r) of approximately 19 kD in all five meningiomas under both reducing and nonreducing conditions. Exogenous human and porcine PDGFs both exhibited a significant dose-dependent stimulation of DNA synthesis in two of three and three of five meningioma cultures examined, respectively. Although not all meningiomas investigated proved to share the biological activity associated with PDGF and these results may be preliminary, it seems that the autocrine growth-stimulatory loop established by PDGF-B-related molecules plays an important functional role in meningioma cell proliferation.

Genetic aberrations in human brain tumors

Over the last decade, much has been learned about the genetic changes that occur in human neoplasia and how they contribute to the neoplastic state. Oncogenes and tumor suppressor genes have been identified, and many powerful molecular genetic techniques have emerged. Brain tumors have been intensively studied as part of this process. Specific and recurring genetic alterations have been identified and are associated with specific tumor types. In astrocytomas, for example, losses of genetic material on chromosomes 10 and 17 and amplification of the epidermal growth factor receptor gene seem important in pathogenesis, with the loss of chromosome 10 and the amplification of epidermal growth factor receptor being strongly associated with glioblastoma multiforme. Meningiomas, on the other hand, have usually lost part or all of chromosome 22. Brain tumors also express growth factors and growth factor receptors that may be important in promoting tumor growth and angiogenesis. These include epidermal growth factor, transforming growth factor-alpha, platelet-derived growth factor, the fibroblast growth factors, and vascular endothelial growth factor. In this article, we review the genetic aberrations that occur in the major types of brain tumors, including glial tumors, meningiomas, acoustic neuromas, medulloblastomas, primitive neuroectodermal tumors, and pituitary tumors. Wherever possible, clinical correlations have been made concerning the prognostic and therapeutic implications of specific aberrations. We also provide some background about the cytogenetic and molecular genetic techniques that have contributed to the description and understanding of these alterations and speculate as to some clinical and basic science issues that might be explored in the future.

Expression of c-sis and c-fos genes in human meningiomas and neurinomas

The HindIII c-sis RFLP was analyzed in constitutional and tumor DNAs of 19 meningioma patients. Loss of a c-sis allele (allele I in all cases) was observed in 6 out of 10 tumors from heterozygous patients. No alteration in the structure or dosage of the c-sis gene was detected in any tumor sample. Northern analysis of c-sis evidenced sis transcripts in 7 out of 8 meningiomas which were amenable to analysis. All tumors expressing c-sis also expressed c-fos, suggesting that the autocrine loop elicited by sis activation may have c-fos as its nuclear target. Co-expression of c-sis and c-fos was also observed in 5 neuromas out of 7 analyzed. Very high levels of fos-mRNA were observed in 2 neuromas, but were not found to be caused by apparent alteration or amplification of the c-fos gene.

Amplified cellular oncogenes in neoplasms of the human central nervous system

Significant advances have recently been made in a number of areas concerning central nervous system (CNS) neoplasia. Particularly salient are the following: (1) gene amplification is related to increasing grade of human glioma malignancy and occurs in approximately 40% of the most common and most malignant variety of glioma, glioblastoma multiforme (GBM), (2) by far the most commonly amplified gene in glioblastomas is the epidermal growth factor receptor (EGFR) gene, which is amplified in about one third of GBMs, (3) a small percentage of GBMs amplify N-myc or the novel sequence gli, (4) the EGFR gene is rearranged in at least half of gliomas in which it is amplified, and (5) EGFR gene rearrangement results in external domain deletions that yield truncated EGF receptors. Antibodies specific for the mutant EGF receptor fusion junction have been successfully produced and provide stimulating new potential avenues for tumor imaging and therapy. For pediatric CNS neoplasms, only medulloblastoma has been investigated in adequate numbers; a small percentage exhibit amplification of either the N-myc or c-myc genes.

Coexpression of platelet-derived growth factor (PDGF) B chain and PDGF B-type receptor in human gliomas

In this study we analyzed the expression of mRNA for PDGF-B and PDGF B-type receptor (PDGFrecB) in 42 biopsy specimens from human astrocytic gliomas and other brain tumors. All gliomas expressed PDGF-B mRNA at higher levels than found in peritumoral and normal nervous tissues. Levels of PDGF-B transcripts correlated strongly with those of mRNA for glial fibrillary acidic protein. Thus, the production of PDGF-B mRNA may be attributed mainly to tumoral glial cells. PDGFrecB transcripts were found in 24 out of 29 gliomas, in agreement with the hypothesis of an autocrine growth stimulation in these tumors. Moreover, mean levels of PDGFrecB expression were higher in glioblastomas than in astrocytomas and over 30-fold higher in meningiomas than in gliomas. This suggests that in gliomas PDGFrecB can be expressed also by vascular elements, in agreement with the hypothesized existence of a paracrine mechanism that may be responsible for the endothelial hyperplasias.