Amplified, overexpressed and rearranged epidermal growth factor receptor gene in a human astrocytoma cell line

The transcriptome of the newt Cynops orientalis provides new insights into evolution and function of sexual gene networks in sarcopterygians

Amphibians evolved in the Devonian period about 400 Mya and represent a transition step in tetrapod evolution. Among amphibians, high-throughput sequencing data are very limited for Caudata, due to their largest genome sizes among terrestrial vertebrates. In this paper we present the transcriptome from the fire bellied newt Cynops orientalis. Data here presented display a high level of completeness, comparable to the fully sequenced genomes available from other amphibians. Moreover, this work focused on genes involved in gametogenesis and sexual development. Surprisingly, the gsdf gene was identified for the first time in a tetrapod species, so far known only from bony fish and basal sarcopterygians. Our analysis failed to isolate fgf24 and foxl3, supporting the possible loss of both genes in the common ancestor of Rhipidistians. In Cynops, the expression analysis of genes described to be sex-related in vertebrates singled out an expected functional role for some genes, while others displayed an unforeseen behavior, confirming the high variability of the sex-related pathway in vertebrates.

EGFR Amplification and Glioblastoma Stem-Like Cells

Glioblastoma (GBM), the most common malignant brain tumor in adults, contains a subpopulation of cells with a stem-like phenotype (GS-cells). GS-cells can be maintained in vitro using serum-free medium supplemented with epidermal growth factor, basic fibroblast growth factor-2, and heparin. However, this method does not conserve amplification of the Epidermal Growth Factor Receptor (EGFR) gene, which is present in over 50% of all newly diagnosed GBM cases. GS-cells with retained EGFR amplification could overcome the limitations of current in vitro model systems and contribute significantly to preclinical research on EGFR-targeted therapy. This review recapitulates recent methodological approaches to expand stem-like cells from GBM with different EGFR status in order to maintain EGFR-dependent intratumoral heterogeneity in vitro. Further, it will summarize the current knowledge about the impact of EGFR amplification and overexpression on the stem-like phenotype of GBM-derived GS-cells and different approaches to target the EGFR-dependent GS-cell compartment of GBM.

Erlotinib resistance in EGFR-amplified glioblastoma cells is associated with upregulation of EGFRvIII and PI3Kp110δ

BACKGROUND

The treatment efficacy of epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors like erlotinib has not met expectations for glioblastoma therapy, even for EGFR-overexpressing tumors. We determined possible mechanisms of therapy resistance using the unique BS153 glioblastoma cell line, which has retained amplification of the egfr gene and expression of EGFR variant (v)III.

METHODS

Functional effects of erlotinib, gefitinib, and cetuximab on BS153 proliferation, migration, and EGFR-dependent signal transduction were systematically compared in vitro. The tumor-initiating capacity of parental and treatment-resistant BS153 was studied in Naval Medical Research Institute/Foxn1(nu) mice. Potential mediators of resistance were knocked down using small interfering (si)RNA.

RESULTS

Erlotinib and gefitinib inhibited proliferation and migration of BS153 in a dose-dependent manner, whereas cetuximab had no effect. BS153 developed resistance to erlotinib (BS153(resE)) but not to gefitinib. Resistance was associated with strong upregulation of EGFRvIII and subsequent activation of the phosphatidylinositol-3-OH kinase (PI3K) pathway in BS153(resE) and an increased expression of the regulatory 110-kDa delta subunit of PI3K (p110δ). Knockdown of EGFRvIII in BS153(resE) largely restored sensitivity to erlotinib. Targeting PI3K pharmacologically caused a significant decrease in cell viability, and specifically targeting p110δ by siRNA partially restored erlotinib sensitivity in BS153(resE). In vivo, BS153 formed highly invasive tumors with an unusual growth pattern, displaying numerous satellites distant from the initial injection site. Erlotinib resistance led to delayed onset of tumor growth as well as prolonged overall survival of mice without changing tumor morphology.

CONCLUSIONS

EGFRvIII can mediate resistance to erlotinib in EGFR-amplified glioblastoma via an increase in PI3Kp110δ. Interfering with PI3Kp110δ can restore sensitivity toward the tyrosine kinase inhibitor.

Glioblastoma stem-like cell lines with either maintenance or loss of high-level EGFR amplification, generated via modulation of ligand concentration

PURPOSE

Despite the high incidence of epidermal growth factor receptor (EGFR) gene amplification and rearrangement in glioblastomas, no suitable cell line exists that preserves these alterations in vitro and is tumorigenic in immunocompromised mice. On the basis of previous observations that glioblastoma cells cultured with serum lose the EGFR amplification rapidly and that EGF can inhibit the growth of EGFR-amplified tumor cells, we hypothesized that serum-free and EGF-free culture conditions could promote maintenance of the EGFR amplification.

EXPERIMENTAL DESIGN

Cells from EGFR-amplified glioblastomas were taken into culture using neural stem cell conditions with modifications, including varying oxygen concentrations and omission of routine EGF supplementation.

RESULTS

High-level EGFR amplification was rapidly lost in 5 glioblastoma cultures supplemented with EGF, whereas it was preserved in cultures from the same tumors established without EGF. Cultures from 2 glioblastomas developed into pairs of cell lines, with either stable maintenance or irreversible loss of high-level EGFR amplification in the majority of cells. One EGFR-amplified cell line preserved expression of the receptor variant EGFRvIII. Cell lines with high-level EGFR amplification/EGFRvIII expression formed highly aggressive tumors in nude mice, whereas nonamplified cell lines were either nontumorigenic or grew significantly more slowly. In contrast, nonamplified cell lines proliferated faster in vitro. All cell lines responded to erlotinib, with inhibition of receptor activation and proliferation but partly different effects on downstream signaling and migration.

CONCLUSIONS

Isogenic glioblastoma cell lines maintaining stable differences in EGFR/EGFRvIII status can be derived by varying exposure to EGF ligand and reflect the intratumoral genetic heterogeneity.

Maintenance of EGFR and EGFRvIII expressions in an in vivo and in vitro model of human glioblastoma multiforme

Glioblastoma multiforme (GBM) is the most common, and most aggressive primary brain tumor among adults. A vast majority of the tumors express high levels of the epidermal growth factor receptor (EGFR) as a consequence of gene amplification. Furthermore, gene amplification is often associated with mutation of EGFR, and the constitutive activated deletion variant EGFRvIII is the most common EGFR mutation found in GBM. Activated EGFR signaling, through overexpression and/or mutation, is involved in increased tumorigenic potential. As such, EGFR is an attractive target for GBM therapy. However, clinical studies with EGFR inhibitors have shown inconsistent results, and as such, further knowledge regarding the role of EGFR and EGFRvIII in GBM is needed. For this, an appropriate in vivo/in vitro tumor model is required. Here, we report the establishment of an experimental GBM model in which the expressions of EGFR and EGFRvIII are maintained both in xenograft tumors growing subcutaneously on mice and in cell cultures established in stem cell conditions. With this model it will be possible to further study the role of EGFR and EGFRvIII, and response to targeted therapy, in GBM.

Cetuximab insufficiently inhibits glioma cell growth due to persistent EGFR downstream signaling

Overexpression and/or amplification of the epidermal growth factor receptor (EGFR) is present in 35-45% of primary glioblastoma multiforme tumors and has been correlated with a poor prognosis. In this study, we investigated the effect of cetuximab and intracellular signaling pathways downstream of EGFR, important for cell survival and proliferation. We show insufficient EGFR downregulation and competition with endogenous EGFR ligands upon cetuximab treatment. Dose-response experiments showed inhibition of EGFR phosphorylation without affecting two of the prominent downstream signaling pathways. Our results indicate that amplification and/or overexpression of EGFR is an unsatisfactory predictor for response to cetuximab.

The EGFRvIII variant in glioblastoma multiforme

Glioblastoma multiforme (GBM) is the most common brain tumour and has the worst prognosis. Epidermal growth factor receptor (EGFR) gene amplification, mutation and re-arrangement (all of which enhance tumour growth, survival, progression and resistance to therapy) are frequently observed in primary GBM. The most common EGFR variant in GBM, the EGFRvIII, is characterised by a deletion of 267 amino acids in the extracellular domain, leading to a receptor which is unable to bind ligand yet is constitutively active. Together with its impaired internalisation and degradation, the EGFRvIII enhances the tumourigenic potential of GBM by activating and sustaining mitogenic, anti-apoptotic and pro-invasive signalling pathways. This EGFRvIII-mediated enhanced tumourigenicity combined with the lack of EGFRvIII expression in normal tissue makes it an ideal candidate for targeted therapy. This review summarizes the current knowledge about the role of EGFRvIII in GBM and discusses therapeutic agents targeting EGFRvIII that are being evaluated as treatments for GBM.

Patient tumor EGFR and PDGFRA gene amplifications retained in an invasive intracranial xenograft model of glioblastoma multiforme

We have previously described a panel of serially transplantable glioblastoma multiforme xenograft lines established by direct subcutaneous injection of patient tumor tissue in the flanks of nude mice. Here we report the characterization of four of these lines with respect to their histopathologic, genetic, and growth properties following heterotopic-to-orthotopic (flank-to-intracranial) transfer. Cells from short-term cultures, established from excised flank xenografts, were harvested and injected into the brains of nude mice (10(6) cells per injection). The intracranial tumors generated from these injections were all highly mitotic as well as highly invasive, but they lacked necrotic features in most instances and failed to show endothelial cell proliferation in all instances. For mice receiving injections from a common explant culture, tumor intracranial growth rate was consistent, as indicated by relatively narrow ranges in survival time. In contrast to the loss of epidermal growth factor receptor gene (EGFR) amplification in cell culture, high-level amplification and overexpression of EGFR were retained in intracranial tumors established from two EGFR-amplified flank tumors. A third intracranial tumor retained patient tumor amplification and high-level expression of platelet-derived growth factor receptor alpha gene. Because the heterotopic-to-orthotopic transfer and propagation of glioblastoma multiforme preserves the receptor tyrosine kinase (RTK) gene amplification of patient tumors, this approach should facilitate investigations for determining the extent to which RTK amplification status influences tumor response to RTK-directed therapies. The fact that such studies were carried out by using an invasive tumor model in an anatomically appropriate context should ensure a rigorous preclinical assessment of agent efficacy.

Receptor binding specificities of Herstatin and its intron 8-encoded domain

Retention of intron 8 in alternative HER-2 mRNA generates an inhibitory secreted ligand, Herstatin, with a novel receptor-binding domain (RBD) encoded by the intron. This study examines binding interactions with several receptors and investigates sequence variations in intron 8. The RBD, expressed as a peptide, binds at nM concentrations to HER-2, the EGFR, DeltaEGFR, HER-4 and to the IGF-1 receptor, but not to HER-3 nor to the FGF-3 receptor, whereas a rare mutation in the RBD (Arg to Ile) eliminates receptor binding. The full-length Herstatin binds with 3-4-fold higher affinity than its RBD, but with approximately 10-fold lower affinity to the IGF-IR. Sequence conservation in rhesus monkey but not in rat suggests that intron 8 recently diverged as a receptor-binding module critical for the function of Herstatin.

Inverse correlation of epidermal growth factor receptor messenger RNA induction and suppression of anchorage-independent growth by OSI-774, an epidermal growth factor receptor tyrosine kinase inhibitor, in glioblastoma multiforme cell lines

OBJECT

Quantitative and qualitative alterations in the epidermal growth factor receptor (EGFR) commonly occur in many cancers in humans, including malignant gliomas. The aim of the current study was to evaluate molecular and cellular effects of OSI-774, a novel EGFR tyrosine kinase inhibitor, on nine glioblastoma multiforme (GBM) cell lines.

METHODS

The effects of OSI-774 on expression of EGFR messenger (m)RNA and protein, proliferation, anchorage-independent growth, and apoptosis were examined using semiquantitative reverse transcription-polymerase chain reaction, immunocytochemical analysis, Coulter counting, soft agar cloning, and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling/fluorescence-activated cell sorting, respectively. All p53 genes were completely and bidirectionally sequenced. Suppression of anchorage-independent growth by OSI-774 was inversely correlated to the induction of EGFR mRNA during relative serum starvation (r = -0.74) and was unrelated to p53 status. Overall, suppression of anchorage-independent growth was a considerably stronger effect of OSI-774 than inhibition of proliferation. The extent of OSI-774-induced apoptosis positively correlated with both proliferation and anchorage-independent growth of GBM cell lines (r = 0.75 and 0.79, respectively). In a single cell line derived from a secondary GBM, exposure to concentrations of greater than or equal to 1 micromol/L resulted in a substantial net cell loss during proliferation studies.

CONCLUSIONS

The induction of EGFR mRNA may constitute a cellular mechanism to counteract the inhibitory effect of OSI-774 on the anchorage-independent growth of GBM cells. In contrast, no considerable correlation could be established between baseline expression levels of EGFR (both mRNA and protein) in GBM cell lines and their biological response to OSI-774. The OSI-774 induced greater (p53-independent) apoptosis in more malignant GBM phenotypes and may be a promising therapeutic agent against secondary GBM.

Inverse correlation of epidermal growth factor receptor messenger RNA induction and suppression of anchorage-independent growth by OSI-774, an epidermal growth factor receptor tyrosine kinase inhibitor, in glioblastoma multiforme cell lines

Object

Quantitative and qualitative alterations in the epidermal growth factor receptor (EGFR) commonly occur in many cancers in humans, including malignant gliomas. The aim of the current study was to evaluate molecular and cellular effects of OSI-774, a novel EGFR tyrosine kinase inhibitor, on nine glioblastoma multiforme (GBM) cell lines.

Methods

The effects of OSI-774 on expression of EGFR messenger (m)RNA and protein, proliferation, anchorage-independent growth, and apoptosis were examined using semiquantitative reverse transcription–polymerase chain reaction, immunocytochemical analysis, Coulter counting, soft agar cloning, and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling/fluorescence-activated cell sorting, respectively. All p53 genes were completely and bidirectionally sequenced.

Suppression of anchorage-independent growth by OSI-774 was inversely correlated to the induction of EGFR mRNA during relative serum starvation (r = −0.74) and was unrelated to p53 status. Overall, suppression of anchorage-independent growth was a considerably stronger effect of OSI-774 than inhibition of proliferation. The extent of OSI-774–induced apoptosis positively correlated with both proliferation and anchorage-independent growth of GBM cell lines (r = 0.75 and 0.79, respectively). In a single cell line derived from a secondary GBM, exposure to concentrations of greater than or equal to 1 μmol/L resulted in a substantial net cell loss during proliferation studies.

Conclusions

The induction of EGFR mRNA may constitute a cellular mechanism to counteract the inhibitory effect of OSI-774 on the anchorage-independent growth of GBM cells. In contrast, no considerable correlation could be established between baseline expression levels of EGFR (both mRNA and protein) in GBM cell lines and their biological response to OSI-774. The OSI-774 induced greater (p53-independent) apoptosis in more malignant GBM phenotypes and may be a promising therapeutic agent against secondary GBM.

Contrasting in vivo and in vitro fates of glioblastoma cell subpopulations with amplified EGFR

Despite the high incidence of EGFR amplification in patient glioblastoma multiforme (GBM) tissues, only a single GBM cell line, of the many described in the literature, is known to contain and maintain amplified EGFR. Because EGFR mutations in GBM manifest primarily, if not exclusively, in amplified form, it follows that the availability of cell lines with mutation of endogenous EGFR would also be in short supply. In fact, there are no GBM cell lines harboring the common EGFR mutants described in patient GBMs. These observations suggest that in vivo environments select for EGFR amplification, whereas in vitro environments, specifically cell cultures, select against this gene alteration. To contrast directly the fates of EGFR amplification in vivo and in vitro, as well as to examine potential relationships between EGFR amplification and mutation, we have established and maintained GBM explants as xenografts by serial passaging in nude mice. Analysis of EGFR copy number and EGFR mutation status in 11 patient tumors and their corresponding xenografts, as well as the monitoring of EGFR copy number during the establishment of a GBM cell line from a xenograft with amplified EGFR, indicated that selection for EGFR amplification is an in vivo phenomenon. Furthermore, our data indicated that EGFR mutation occurs only in tumors with EGFR amplification and showed that the selection of amplified mutant EGFR over amplified wild-type EGFR as a xenograft occurred rapidly and completely during tumor propagation.

Suppression of EGFRvIII-mediated proliferation and tumorigenesis of breast cancer cells by ribozyme

EGFRvIII is a tumor specific, ligand-independent, constitutively active variant of the epidermal growth factor receptor. Its expression has been detected in many human malignancies including breast cancer. No detectable level of EGFRvIII has, however, been observed in adult tissues, including normal breast tissues. These unique features of the EGFRvIII make it an excellent target for biologically based therapies. We have designed and generated a tumor specific ribozyme targeted to EGFRvIII. This specific EGFRvIII ribozyme is able to effectively cleave EGFRvIII mRNA under physiological conditions in a cell-free system, but does not cleave wild-type EGFR and other EGF-family receptors. While expressing this EGFRvIII-ribozyme in breast cancer cells, EGFRvIII-ribozyme is capable of downregulating endogenous EGFRvIII expression at the mRNA and protein levels. Inhibition of proliferation was observed in EGFRvIII-ribozyme transfectants. In addition, downregulation of EGFRvIII in breast cancer cells significantly inhibited tumor growth in athymic nude mice. Furthermore, this ribozyme has no effect on EGF-family receptor expression or the proliferation of breast cancer cells, which do not express EGFRvIII but express wild-type EGFR and other EGF-family receptors. These results suggest that we have generated a tumor-specific, biologically functional ribozyme and further demonstrate that EGFRvIII plays a significant role in breast cancer cell proliferation. The ultimate goal of this approach is to provide a potential treatment for breast cancer by specifically targeting this receptor.

Spontaneous activation and signaling by overexpressed epidermal growth factor receptors in glioblastoma cells

Overexpressed epidermal growth receptor factor receptors (EGFRs) are thought to contribute to the malignant phenotype of human glioblastomas (GBMs), but the mechanism is not well understood. We found that SKMG-3 cells, a rare GBM cell line that maintains EGFR gene amplification in vitro, produced high levels of EGFR protein. The cells also expressed the related receptors HER2/neu and HER4, but not HER3. Immunoblots and tryptic phosphopeptide maps showed that the SKMG-3 EGFRs were intact and functional and that a subset of these receptors were spontaneously autophosphorylated. EGF treatment stimulated phosphorylation of the EGFRs as well as the downstream effectors Erk, AKT1, stat3 and c-Cbl. Under minimal growth conditions, the unstimulated SKMG-3 cells contained constitutively phosphorylated Erk and AKTI but no detectable stat3 DNA-binding complexes. The EGFR kinase inhibitor PD158780 reduced the constitutive phosphorylation of the receptor and Erk but not that of AKT1. In contrast, inhibition of phosphatidylinositol-3-kinase (PI3K) blocked the constitutive phosphorylation of Erk and AKT-1 but not the EGFR. We conclude that the SKMG-3 cells represent the subset of GBMs with amplified EGFR genes that overexpress intact receptors. The results also suggest that in some GBMs, signals from overexpressed EGFRs contribute to the constitutive phosphorylation of Erk, but these signals may not required for the constitutive activation of PI3K or AKT1.

Constitutive activation of Stat3alpha in brain tumors: localization to tumor endothelial cells and activation by the endothelial tyrosine kinase receptor (VEGFR-2)

Members of the normally latent family of transcription factors signal/inducers and activators of transcription (Stat) are activated in a number of human tumors and tumor-derived cell lines. In the case of Stat3, it is believed that this activation leads to the induction of survival signals as well as increased proliferation. In this study, we demonstrate that Stat3 is constitutively activated in glioma and medulloblastoma tumors and that the activated protein localizes predominantly to the tumor endothelial cells in the highly vascularized glioma tumors. Our efforts to elucidate potential mechanism(s) for this activated protein have shown that coexpression of Stat3alpha and the vascular endothelial growth factor receptor-2 (VEGFR-2) result in ligand-independent activation of Stat3alpha tyrosine phosphorylation and subsequent transcriptional activation in non-endothelial cells. We also show that activated Stat3alpha can increase transcription from the vascular endothelial growth factor (VEGF) gene. Taken together, these results suggest that the activated Stat3alpha found in brain tumors may be due to the endothelial tyrosine kinase VEGFR-2 and that Stat3alpha may play a central role in autocrine VEGF activation.

Evidence of high incidence of EGFRvIII expression and coexpression with EGFR in human invasive breast cancer by laser capture microdissection and immunohistochemical analysis

EGFRvIII was first reported in human glioblastomas. Subsequent reports indicated EGFRvIII protein to be frequently detected in several other human cancers, but not in normal tissues. Our previous studies suggested that EGFRvIII could induce a transformation from ligand-dependent non-tumorigenic cell line to ligand-independent malignant phenotype cells in vitro and in vivo. Transfection of EGFRvIII in MCF-7 cell line resulted in a 3-fold increase in colony formation and significantly enhanced tumorigenicity in nude mice (p < 0.001). EGFRvIII could also induce ErbB-2 phosphorylation. The existence and significance of EGFRvIII transcript in human breast cancer, however, was not reported. In our study, we detected the presence of EGFRvIII mRNA and revealed a high incidence (67.8%) of EGFRvIII transcript in human primary invasive breast cancer by utilizing laser capture microdissection (LCM)/RT-PCR to capture pure breast cancer cells. In addition, 57.1% of the infiltrating breast carcinomas expressed both EGFRwt and EGFRvIII mRNA in the same tumor. There is no detectable EGFRvIII mRNA in normal breast tissue. Evaluation of the EGFRwt and EGFRvIII protein levels in the same sample sets by immunohistochemical analysis further confirmed the LCM/RT-PCR finding. Our study provides first direct evidence of high incidence of coexpression of EGFRvIII and EGFRwt in human invasive breast cancer tissue. The unique characteristics and high prevalence of EGFRvIII in invasive human breast cancer as well as negative expression in normal breast may suggest its important role in breast carcinogenesis and make it an ideally potential target for treatment of breast cancer without interrupting normal EGFR signaling.

GBAS, a novel gene encoding a protein with tyrosine phosphorylation sites and a transmembrane domain, is co-amplified with EGFR

Amplification of the epidermal growth factor receptor gene (EGFR) has been noted in several forms of human cancer. Although various aspects of this gene alteration have been extensively studied, little is known about the genetic content of the chromosome 7p12 region that can be co-amplified with this cellular oncogene. Here we report the identification of and coding sequence for a novel gene, GBAS, that is localized to this region and was found to be amplified in 2 of 12 tumors as well as in 2 of 3 cell lines with EGFR amplification. The GBAS transcript is 2.2 kb in length and contains an 858-bp open reading frame. The associated amino acid sequence has identifiable signal peptide and transmembrane motifs, as well as two tyrosine phosphorylation sites that suggest that the encoded protein may be a substrate for tyrosine kinases.

The effect of calphostin C, a potent photodependent protein kinase C inhibitor, on the proliferation of glioma cells in vitro

Recent studies have suggested that the proliferation of malignant gliomas may result from activation of protein kinase C (PKC)-mediated pathways; conversely, inhibition of PKC may provide a strategy for blocking tumor growth. In the current studies, we examined the effect of a novel PKC inhibitor, calphostin C, which is a selective, highly potent, photo-activatable inhibitor of the PKC regulatory domain, on the proliferation and viability of three established and three low-passage malignant glioma cell lines, four low-passage low-grade glioma cell lines, and in adult human and neonatal rat non-neoplastic astrocyte cell lines in vitro. Under light-treated conditions, calphostin C consistently inhibited cell proliferation in each of the tumor cell lines and in the neonatal rat astrocyte cell line with a 50% effective concentration of 30 to 50 ng/ml (40 to 60 nm), which was comparable to the previously reported median inhibitory concentration (IC50) for PKC inhibition by calphostin C. Complete elimination of proliferation was achieved at concentrations of 50 to 100 ng/ml (60 to 125 nM). Cell viability decreased sharply with calphostin C concentrations of 100 to 300 ng/ml (125 to 380 nM). In contrast, under light-shielded conditions, calphostin C had a comparatively modest effect on cell proliferation and viability, with a median effective concentration of approximately 300 ng/ml. No significant inhibition of proliferation was noted in the non-neoplastic adult astrocyte cell line under either light-treated or light-shielded conditions. These findings provide further evidence that PKC may play an essential role in mediating the proliferation of both benign and malignant glioma cells in vitro and may also contribute to the proliferation of non-neoplastic immature astrocytes. Light-sensitive inhibition of proliferation and viability by agents such as calphostin C may provide a novel strategy for applying photodynamic therapy to the treatment of neoplastic glial cells.

Differences between phosphotyrosine accumulation and Neu/ErbB-2 receptor expression in astrocytic proliferative processes. Implications for glial oncogenesis

BACKGROUND

Previous work has shown that enhanced growth potential of malignant astrocytomas correlates with increased expression of growth factor receptor tyrosine kinases. The functional implications of increased receptor expression were addressed by analyzing possible accumulation of phosphotyrosyl proteins in neoplastic and nonneoplastic astrocytic proliferative processes. The results were compared with the expression of Neu receptor protein (also called ErbB-2 or HER-2).

METHODS

Western immunoblots and immunocytochemistry were utilized to evaluate glioma and carcinoma cell lines, neonatal astrocytic cultures, and human brain biopsies of graded gliosis and astrocytomas. The effects of three tyrosine kinase inhibitors on 3H-thymidine uptake and cell proliferation and viability were examined in cultured glioma cells.

RESULTS

Phosphotyrosine was conspicuously elevated in all three grades of astrocytoma, but remained at low levels in nonneoplastic astrocytic proliferations. Dose-dependent decreases in DNA synthesis and proliferation of cultured glioma cells occurred after inhibition of tyrosine kinase. Neu receptor protein showed increased expression in malignant astrocytomas (including glioblastomas) and severe reactive gliosis.

CONCLUSIONS

Upregulation of tyrosyl protein phosphorylation enables differentiation of neoplastic from nonneoplastic astrocytic proliferative states. Inhibition of this phosphorylation impairs growth of cells. Increased Neu receptor protein expression can distinguish malignant from low grade astrocytomas. We speculate that genetic events leading to stably increased phosphotyrosine may be critical for neoplastic transformation of astrocytes, whereas increased receptor tyrosine kinase expression could be a factor in the aggressive growth associated with malignancy.

Striatal TGF-alpha: postnatal developmental expression and evidence for a role in the proliferation of subependymal cells

Transforming growth factor alpha (TGF-alpha) is expressed in the brain and affects cells by binding to the epidermal growth factor receptor (EGF-R). Using a ribonuclease protection assay, we found that TGF-alpha steady state mRNA levels in the mouse striatum peak during the first week of postnatal life. Temporally this peak correlates with the height of gliogenesis in the subependymal layer (SEL), which lies along the striatal border of the lateral ventricle. In vitro studies demonstrate that TGF-alpha can stimulate the proliferation of astrocytes, so glial fibrillary acidic protein (GFAP) mRNA levels were measured as well and it was observed that the peak of GFAP expression followed that of TGF-alpha by 1 week. Furthermore, in a TGF-alpha deficient mouse, waved-1 (wa-1), a significant reduction of GFAP mRNA levels and immunostaining for GFAP was found in the striatum. Bromodeoxyuridine labeling combined with immunohistochemistry of normal postnatal day 6 brain showed that the proliferating cells in the SEL are EGF-R immunoreactive. In the waved-1 SEL, there were fewer BrdU positive cells and there was a reduced level of [3H]thymidine incorporation. EGF-R immunoreactive cells were found in the SEL of the adult mouse brain. Taken together, our data suggest that the TGF-alpha/EGF-R signaling pathway is involved in postnatal mitogenic events in the brain.

Receptor blockade with monoclonal antibodies as anti-cancer therapy

Human tumors express high levels of growth factors and their receptors, and many types of malignant cells appear to exhibit autocrine- or paracrine-stimulated growth. Therefore, antireceptor directed therapies have the potential of being useful anti-cancer agents. A series of murine monoclonal antibodies (MAbs) directed against human growth factor receptors and their corresponding growth factors have been produced. MAbs against the receptors for epidermal growth factor, Her2/Neu, transferrin, insulin-like growth factor, interleukin, (IL)-2 and IL-1 are currently being evaluated. MAbs directed against epidermal growth factor, transforming growth factor-alpha, bombesin, IL-2, and IL-6 also are under study. These MAbs have shown promising preclinical activity, and some of them are being tested in clinical trials. So far, anti-tumor responses have been observed with anti-IL-2 receptor, anti-bombesin and anti-IL-6 MAbs. Further research is focusing in the production of "chimeric" and "humanized" MAbs, in order to obviate the problem of host immune reactions.

A mutant epidermal growth factor receptor common in human glioma confers enhanced tumorigenicity

The development and neoplastic progression of human astrocytic tumors appears to result through an accumulation of genetic alterations occurring in a relatively defined order. One such alteration is amplification of the epidermal growth factor receptor (EGFR) gene. This episomal amplification occurs in 40-50% of glioblastomas, which also normally express endogenous receptors. Moreover, a significant fraction of amplified genes are rearranged to specifically eliminate a DNA fragment containing exons 2-7 of the gene, resulting in an in-frame deletion of 801 bp of the coding sequence of the extracellular domain. Here we used retroviral transfer of such a mutant receptor (de 2-7 EGFR) into glioblastoma cells expressing normal endogenous receptors to test whether the mutant receptor was able to augment their growth and malignancy. Western blotting analysis showed that these cells expressed endogenous EGFR of 170 kDa as well as the exogenous de 2-7 EGFR of 140-155 kDa. Although holo-EGFRs were phosphorylated on tyrosine residues only after exposure of the cells to ligand, de 2-7 EGFRs were constitutively phosphorylated. In tissue culture neither addition of EGF nor expression of the mutant EGFR affected the rate of cell growth. However, when cells expressing mutant EGFR were implanted into nude mice subcutaneously or intracerebrally, tumorigenic capacity was greatly enhanced. These results suggest that a tumor-specific alteration of the EGFR plays a significant role in tumor progression perhaps by influencing interactions of tumor cells with their microenvironment in ways not easily assayed in vitro.

TM-1 cells from an established human malignant glioma cell line produce PDGF, TGF-alpha, and TGF-beta which cooperatively play a stimulatory role for an autocrine growth promotion

We have previously established a human malignant glioma cell line, TM-1. TM-1 cells could proliferate in the serum-free medium. In the present study, immunochemical analysis demonstrated that platelet-derived growth factor (PDGF), transforming growth factor (TGF)-alpha, and TGF-beta are present in the serum-free medium conditioned by growing TM-1 cells. While the cells appeared to possess a single type of binding sites for epidermal growth factor (EGF) with properties comparable to those determined for other tumor cells, the conditioned medium did not contain EGF.PDGF, TGF-alpha, and EGF added exogenously to serum-free media stimulated thymidine incorporation into DNA of TM-1 cells. In addition, antibodies specific for PDGF and TGF-alpha suppressed this activity. These results indicate autocrine and stimulatory roles of PDGF and TGF-alpha for the proliferation of TM-1 cells. As observed for other tumor cells, TGF-beta by itself weakly suppressed thymidine incorporation by TM-1 cells. However, TGF-beta employed in combination with TGF-alpha or EGF appeared to stimulate thymidine incorporation, suggesting that a cooperative action of TGF-beta with different growth factors may be involved in the stimulatory growth regulation at least for TM-1 cells.

Urinary epidermal growth factor in patients with gliomas: significance of the factor as a glial tumor marker

Epidermal growth factor (EGF) content in urine from patients with glial tumors was examined by radioimmunoassay techniques with labeled human EGF and its rabbit EGF polyclonal antibody. There was no cross-reaction with transforming growth factor-alpha, which has a common receptor with EGF. Forty glial tumors were divided into three groups according to the clinical stage: Samples from Group A patients were obtained before therapy and/or after biopsy; in these patients a large volume of tumor was apparent on computerized tomography (CT). Group B samples were obtained after gross total removal of the tumor and/or chemo- and radiation therapy; these patients showed a small volume of residual tumor on CT. Samples from Group C patients were obtained after gross tumor total removal and/or chemo- and radiation therapy; no tumor was detected on CT scans in these patients. Urinary EGF levels in Group A samples were statistically significantly higher than in samples from healthy individuals (p < 0.001), Group B patients (p < 0.10), and Group C patients (p < 0.02). In addition, high-grade glial tumors in Group A cases showed a significantly higher level of urinary EGF than low-grade tumors in Group A patients (p < 0.05), or patients with meningioma (p < 0.02), metastatic brain tumor (p < 0.05), and cerebral infarction (p < 0.001). Longitudinal changes of urinary EGF levels in glioma patients mostly synchronized with the clinical course and therapeutic interventions. Therefore, urinary EGF, as a glial tumor marker, may be of practical value for diagnosing a malignant glioma and evaluating for the efficacy of chemo- and radiation therapy.

Receptor tyrosine kinase expression in astrocytic lesions: similar features in gliosis and glioma

The purpose of this study was to determine if the expression of receptor tyrosine kinases would distinguish astrocytosis from astrocytoma. Because the expression of this family of receptor proteins is greater in higher-grade tumors, a graded series of both reactive and neoplastic astrocytic lesions in humans was evaluated. The reactive processes included both mild gliosis and severe (intense) gliosis. The two immunocytochemically detected membrane receptor proteins, p145 and p185, are those encoded by the kit and neu genes, respectively. Semi-quantitative assessments were made independently for the frequency and intensity of astrocytic immunostaining together with corollary immunocytochemical staining to detect glial fibrillary acidic protein. It was found that both mild gliosis and low-grade astrocytomas only minimally express these receptors. In contrast, severe gliosis and high-grade neoplasms consistently express these receptors at much higher levels. In both astrocytosis and astrocytomas, a cell with abundant perikaryal cytoplasm is most likely to be immunoreactive, often with dense reaction product concentrated at the periphery of the somal cytoplasm. The extent and pattern of immunoreactivity in high-grade astrocytomas preclude the interpretation that all immunoreactive cells were merely reactive astrocytes. We conclude that the expression of these receptors per se does not differentiate astrocytic neoplasia from reactive astrocytosis. Second, we conclude that immunocytochemically detectable levels of neu or kit receptor expression is not constitutive in normal astrocytes and in some stages of astrocytic neoplasia. On the basis of these findings, we speculate that some neoplastic astrocytes maintain and manifest the capacity to respond to transitory exogenous stimuli, as do reactive astrocytes. This reaction could include the elaboration of receptor tyrosine kinases. Alternatively, even if the function of these receptors in gliosis differs from that in gliomas, the common expression could still reflect an "active" state shared by astrocytes in these two processes.

Epidermal growth factor receptor and lipid membrane components in human lung cancers

The binding of 125I-epidermal growth factor (EGF) to the plasma membranes of 54 samples of human lung tumors was determined. These included 34 squamous cell carcinomas and 20 adenocarcinomas. Twenty samples of histologically normal lung excised surgically along with the tumors were used as controls. Most of the plasma membranes showed an EGF receptor level higher than that of normal tissue. A moderate increase in the amount of 125I-EGF bound (2-5 fold) was observed in the majority of the tumors. Only a few cases (5-10% of the total) showed a large increase (> than 10 fold). The binding of 125I-EGF was compared with clinical stages and grades of differentiation. No correlation between the stage of the tumor and 125I-EGF binding was observed. However, the highest levels of EGF receptor (EGF-R) were found in poorly differentiated squamous cell carcinomas. The total amount and the distribution pattern of gangliosides and phospholipids were analyzed in individual tumors. A decrease in GD1b, GD1a and sphingomyelin and an increase in GM1 and GM3 was observed. No correlation was detected when tumors with the highest or lowest levels of gangliosides or phospholipids were compared with tumors exhibiting the highest binding of 125I-EGF.

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.

Epidermal growth factor receptor in human brain tumors

The expression of epidermal growth factor receptor (EGF-R) was examined in 27 primary human brain tumors (7 glioblastomas, 10 astrocytomas, 5 oligodendrogliomas, 1 schwannoma, 1 ganglioneuroma, 1 medulloblastoma, 1 ependymoma, 1 histiocytic lymphoma), in 6 brain metastases from lung carcinomas and in 20 meningiomas. Peritumoral tissues histologically normal excised surgically along with a large tumor were used as control. All plasma membranes from brain tissues tested showed specific EGF binding. The EGF receptor is expressed at low levels in the control human brain and at very high levels in 60% of the total intracranial tumors studied. When the various histological types of tumors were analyzed, the higher percentage of positive tumors was found with the meningiomas (85%) and the glioblastomas (71%), while the lower percentage of positivity was found with the oligodendrogliomas (40%) and the astrocytomas (30%). A good correlation between binding and total amount of EGF-R protein detected by Western Blot was also observed.

Genetic alterations in glioma and medulloblastoma

Multiple genetic changes take place during tumor development and progression. These genetic changes result in inactivation of tumor suppressor genes and activation of proto-oncogenes. Frequent genetic changes observed in gliomas are losses of chromosomal regions on 9p, 10q, 13q, 17p and on 22. Loss of 10q is seen in more than 80% of the glioblastoma multiforme (GBM) tumors suggesting the presence of a gene critical for GBM formation on this chromosome. Amplification of epidermal growth factor receptor gene and expression of platelet derived growth factor and fibroblast growth factor genes are also common among gliomas. The most common genetic abnormality found in medulloblastomas is loss of 17p. The C-myc gene is amplified in a few primary tumors, but the incidence of amplification is higher in medulloblastoma derived cell lines. These findings suggest that the same two genetic processes, gene amplification and regional chromosomal loss, which characterize other primitive childhood neuroectodermal tumors such as retinoblastoma and neuroblastoma are also important in medulloblastomas.

Response of low-passage human malignant gliomas in vitro to stimulation and selective inhibition of growth factor-mediated pathways

The proliferation of many nonglial tumors in vitro depends on the presence of nanomolar concentrations of one or more growth factors. To define the growth factor requirements of malignant glial tumors, the authors examined the response properties of four low-passage human malignant glioma lines to the following mitogens: epidermal growth factor (EGF), acidic and basic fibroblast growth factors (FGF's), insulin-like growth factor I (IGF-I), nerve growth factor (NGF), platelet-derived growth factor (PDGF), 12-O-tetradecanoyl-13-phorbol acetate (TPA), and serum. Each of the tumors showed increased deoxyribonucleic acid (DNA) synthesis (assessed by acid-precipitable [3H]-thymidine incorporation) in response to PDGF with a maximum effect at 50 ng/ml. Three tumors responded to EGF, three to IGF-I, two to acidic FGF, two to basic FGF, and two to TPA with maximum effects at 10, 50, 1, 1, and 10 ng/ml, respectively. None of the tumors responded to NGF. In the responsive tumors, optimum concentrations of EGF, IGF, TPA, acidic FGF, and basic FGF induced, at most, a two- to fourfold increase in [3H]-thymidine incorporation, which was only 30% to 50% of the response seen in 10% serum. In contrast, PDGF increased DNA synthesis eight- to 10-fold, equaling the effect of 10% serum. Measurements of cell proliferation also demonstrated a significant response to PDGF in each of the tumors. Appropriate concentrations of an anti-PDGF neutralizing antibody inhibited baseline DNA synthesis and proliferation in the absence of added growth factors, suggesting the possible role of PDGF in autocrine stimulation of these cells. However, this antibody produced only slight inhibition of serum-induced mitogenesis. Trapidil, an agent reported to inhibit the effects of PDGF, and polymyxin B, an inhibitor of protein kinase C, strongly inhibited baseline as well as PDGF- and serum-induced mitogenesis. It is concluded that, in the malignant gliomas studied, PDGF may be acting as a dominant mitogen to enhance DNA synthesis, and may function in autocrine stimulation. However, other factors contained in serum can also contribute to cell division.

A cell line of human malignant astrocytoma producing autocrine growth factor

A cell line was established from an anaplastic astrocytoma from a 69-yr-old female. The cells have been serially subcultured over 300 times for 6 yr without showing any sign of cell senescence. Their doubling time is about 36 h. The cells are fusiform and often hexagonal in sparse culture, but become spindle-shaped and formed mosaic structure in confluent culture. Under electron microscopy, intermediate filaments were randomly distributed in the cytoplasma, especially in the perinuclear space. The chromosome number was near tetraploid and varied from 86 to 94 chromosomes with a modal number of 91. The alpha and beta subunits of S-100 protein, vimentin, and glial fibrillary acidic protein (GFAP), which are reliable markers of astrocytic cells, were demonstrated in a large number of cells by immunoperoxidase staining. The results of immunoblotting showed that the expression of vimentin was much higher than that of GFAP. The tumorigenicity of the cells was revealed by xenografting into nude mice, which were X-irradiated before inoculation. Culture medium conditioned by the cells promoted growth of these cells in serum-free conditions and of normal rat glial cells in serum-depleted culture. The growth-promoting effect of conditioned medium was lost by trypsinization and reduced by boiling. These findings suggest that these cells are derived from neoplastic astrocytic cells and secrete a self-acting polypeptide growth-promoting factor into the culture medium.

Morphological, immunocytochemical and growth characteristics of three human glioblastomas established in vitro

The human glioblastoma-derived cell lines 86HG-39, 87HG-28 and 87HG-31, used for the production of monoclonal antibodies (mAbs) against glioma-associated antigens (GAA), were characterized in terms of morphology, growth behaviour, chromosomes and antigen expression. In the primary tumours, differential expression of glial fibrillary acidic protein, S100 protein, Leu-7 and GAA as defined by mAbs MUC 2-39, MUC 2-63 and MUC 8-22 was demonstrated. Receptors for epidermal growth factor (EGFr) and nerve growth factor (NGFr) were found in many cells in short-term cultures, but the transferrin receptor (Tr) was found in only a few cells of 87HG-28. In permanent cell lines, differentiation antigens and EGFr decreased and Tr increased markedly. NGFr and GAA remained stable. Transplantation tumours of 86HG-39 were partly positive for Tr and GAA. Chromosomal analysis revealed that the 86HG-39 and 87HG-28 cell lines had a hypodiploid or diploid stem line with lines in the hypotetraploid to tetraploid region for 50 in vitro passages. The 87HG-31 cell line had chromosomal patterns in the hypotriploid to triploid region. A gain of chromosomes was seen in the groups C7, C8, C10, D14, F19, F20, G21, G22. The variability of antigens in these tumours and especially during long-term cultivation probably reveals an ability to influence the growth of malignant glioma cells via the respective effector molecules.

A mathematical model describing consequences of abnormally high levels of epidermal growth factor receptor on the proliferation of neoplastic cells

Recent laboratory and clinical data suggest that some human neoplasms exhibit unusually high levels of cell-surface receptors for epidermal growth factor, and that this abnormality is associated with rapid cellular proliferation and poor prognosis. We propose that the existence of an abnormally high number of mitogen receptors is not merely correlated with rapid proliferation but is pathophysiologically responsible for such behavior. Cells with high levels of mitogen receptors may be rendered 'hypersensitive' to mitogenic stimuli, and hence may be stimulated to divide even when ambient mitogen concentrations are at a low 'background' level, insufficient to prompt the division of cells with a normal number of receptors. To investigate this hypothesis further, we have developed a mathematical model that describes proliferative behavior of cells as a function of mitogen concentration and receptor number. The model enables us to simulate the proliferative behavior of cells with various receptor levels at various mitogen concentrations and predicts a growth advantage associated with excess mitogen receptors. Computer simulations based on the model are consistent with previously published experimental data. This work provides support for the view that overexpression of genes encoding normal growth factor receptors can contribute to the inappropriate proliferation of neoplastic cells.

Epidermal growth factor receptor expression in human gliomas

The expression of epidermal growth factor receptor (EGFR) was determined in cryosections of 42 human gliomas using biotinylated epidermal growth factor (B-EGF) and two monoclonal antibodies (mAb) against EGFR. All gliomas were found to express EGFR when examined with B-EGF, whereas 33 expressed EGFR when examined with the two mAbs. The highly malignant gliomas (glioblastomas and anaplastic astrocytomas) had a more heterogeneous staining strongly with B-EGF than did the low-grade gliomas (astrocytomas, oligodendrogliomas, mixed gliomas, and ependymomas). This indicates that high-grade gliomas contain more tumour cells rich in EGFR than do the low-grade gliomas. Reactive astrocytes, ependymal cells, and many types of nerve cells (cerebral cortical pyramidal cells, pyramidal and granular hippocampal cells, Purkinje cells, cerebellar granular cells and neurons in the molecular layer of the cerebellum) expressed EGFR, whereas small neurons and normal glial cells were not found to express EGFR.

Polysomy of chromosome 7 is associated with amplification and overexpression of the EGF-receptor gene in a human carcinoma cell line derived from a brain metastasis

Overexpression of the EGF-receptor gene is associated with the malignant nature of some tumors. We have recently reported the establishment of a human carcinoma cell line (T-CAR1), derived from a brain metastasis, that had 7 million EGF receptors per cell and was growth inhibited by EGF. The present study was carried out in order to further characterize the EGF-receptor protein in T-CAR1 cells, and to see if the overexpression of the EGF-receptor gene in these cells was associated with abnormalities at the genomic level. We have compared the T-CAR1 cells with the human glioblastoma cell line T-MG1, which has 135,000 EGF-receptors and is growth stimulated by EGF. The MW of the EGF receptors in T-CAR1 cells and T-MG1 cells was estimated to be 170 kDa, equal to the normal EGF-receptor. However, in T-CAR1 cells an additional protein reacted with the monoclonal antibody directed against the internal domain of the EGF receptor. The levels of EGF receptor-related RNAs in T-CAR1 cells and T-MG1 cells reflected the number of EGF receptors in these cell lines. The EGF-receptor gene was amplified ten-fold in T-CAR1 cells, while it was not amplified in T-MG1 cells. No restriction fragment length polymorphism of DNA digested with various restriction enzymes was seen in either of the cell lines. Chromosomal analysis of T-CAR1 cells showed polysomy of chromosome 7 and marker chromosomes derived partly from chromosome 7. Thus, in the T-CAR1 cell line it was an association between polysomy of chromosome 7 and EGF-receptor gene amplification.

Cytogenetics and molecular genetics of malignant gliomas and medulloblastoma

Malignant gliomas and medulloblastomas which are the most common primary malignant brain tumours of adult and children, respectively, resemble other neurogenic tumours as they frequently contain gene amplification and show non-random loss of specific chromosomal regions. In gliomas the gene which is most often amplified, is the epidermal growth factor receptor gene. In many cases the gene is rearranged as well, producing abnormally small epidermal growth factor receptor proteins. More than 80% of tumours have lost chromosome 10 and losses of 9p13, 17p and 22 occur in subgroups of cases. 17p loss is associated with point mutations of the p53 gene, but the relevant genes in the other chromosomal regions remain to be identified. For medulloblastoma the most frequent chromosomal abnormality is i(17q). Whether or not p53 gene mutations are the targets of 17p losses in these tumours remains to be determined. Approximately 5% of medulloblastoma biopsies contain gene amplification, although the incidence in medulloblastoma cell lines is more than 80%. c-myc is the gene which is most frequently amplified in this tumour type. The relationship of these various molecular genetic abnormalities to the biology of the tumours and the course of the patients remains largely unexplored.

Cytogenetics of human brain tumors

The most frequent cytogenetic alterations in primary brain tumors are losses of chromosomes or chromosomal regions and the presence of double minute chromosomes (dmins). The regions which are lost and the genes which are amplified are distinctive for individual tumor types. Most malignant gliomas contain gains of chromosome 7 and losses of chromosome 10; losses of chromosome 22, 9p, and the sex chromosomes occur in subgroups of cases. The gene most frequently amplified in tumors with dmins is the epidermal growth factor receptor gene. Medulloblastomas have losses of 17p and most cases with dmins have c-myc gene amplification. Meningiomas have losses or deletions of chromosome 22. Identification of these specific cytogenetic abnormalities in human brain tumors has provided the framework for identifying genes which are amplified in them and has identified chromosomal regions likely to contain tumor suppressor genes, the loss or inactivation of which is important in the development of these tumors.

Response of malignant glioma cell lines to epidermal growth factor and platelet-derived growth factor in a serum-free medium

The use of a serum-free culture system for assessing the growth factor responsiveness of malignant glial cells is described. The mitogenic properties of epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) were examined in three human malignant glioma cell lines (T98G, U87, and U138). Each of the three had high-affinity EGF receptors and all responded in a dose-dependent fashion to physiological concentrations of EGF. These cell lines also showed a pronounced mitogenic response to PDGF which equaled or exceeded that achieved with EGF. Simultaneous stimulation with both factors produced an additive response, which approximated that obtained in medium supplemented with 10% fetal calf serum. The authors conclude that functional EGF and PDGF receptors were present in the human malignant glial tumors studied. The response of the human glioma lines to these growth factors in many respects parallels the response seen in fetal astrocytes tested under similar conditions. In contrast, the behavior of two chemically induced rat gliomas (9L and C6) differed significantly from that seen in the human lines, suggesting that the rat lines may not be entirely acceptable as models for studying the growth characteristics of human malignant glial tumors.

A truncated, secreted form of the epidermal growth factor receptor is encoded by an alternatively spliced transcript in normal rat tissue

Two independent cDNA clones corresponding to a 2.7-kilobase (kb) epidermal growth factor receptor (EGF-R) mRNA were isolated from a rat liver cDNA library. Sequence analysis revealed 100% homology in the external domain when compared with the full-length rat EGF-R nucleotide sequence and 80 to 90% similarity relative to the human EGF-R. However, the 3'-terminal sequence of these clones did not match EGF-R or any other known sequence(s) and was distinct from the 3' end of the 2.8-kb mRNA, which encodes a truncated EGF-R in A431 cells. The deduced amino acid sequence revealed an open reading frame which is homologous to the external domain of the EGF-R but which terminates prior to the transmembrane region. Southern blot analysis of rat genomic DNA indicated that the 3'-terminal sequence of this transcript is derived from the EGF-R gene. Analysis of a genomic clone containing the 3' end of the 2.7-kb transcript revealed that this sequence is present as a discrete exon in the mid-region of the receptor gene in proximity to the exon encoding the transmembrane domain. Introduction of an expression vector containing the truncated EGF-R cDNA into Chinese hamster ovary (CHO) cells led to the expression of a 95-kilodalton protein which was detected in conditioned media, by using antisera directed against the EGF-R. A similarly sized protein was also detected in the media of WB cells, a continuous, nontransformed line of rat hepatic epithelial cells. Northern (RNA blot) analysis established that the truncated receptor is encoded by a 2.7-kb transcript found in normal rat liver. Furthermore, Northern analysis of rat poly(A)+ RNA showed that the 2.7-kb EGF-R transcript is expressed at differing levels in various fetal and adult tissues. These data indicate that alternative splicing of the EGF-R primary transcript yields a 2.7-kb mRNA which codes for a truncated form of the receptor. This receptor is secreted by rat hepatic epithelial cells in culture, which suggests that it may be secreted by normal rat cells or tissues and perhaps serve an as yet unknown physiological function.

A truncated, secreted form of the epidermal growth factor receptor is encoded by an alternatively spliced transcript in normal rat tissue

Two independent cDNA clones corresponding to a 2.7-kilobase (kb) epidermal growth factor receptor (EGF-R) mRNA were isolated from a rat liver cDNA library. Sequence analysis revealed 100% homology in the external domain when compared with the full-length rat EGF-R nucleotide sequence and 80 to 90% similarity relative to the human EGF-R. However, the 3'-terminal sequence of these clones did not match EGF-R or any other known sequence(s) and was distinct from the 3' end of the 2.8-kb mRNA, which encodes a truncated EGF-R in A431 cells. The deduced amino acid sequence revealed an open reading frame which is homologous to the external domain of the EGF-R but which terminates prior to the transmembrane region. Southern blot analysis of rat genomic DNA indicated that the 3'-terminal sequence of this transcript is derived from the EGF-R gene. Analysis of a genomic clone containing the 3' end of the 2.7-kb transcript revealed that this sequence is present as a discrete exon in the mid-region of the receptor gene in proximity to the exon encoding the transmembrane domain. Introduction of an expression vector containing the truncated EGF-R cDNA into Chinese hamster ovary (CHO) cells led to the expression of a 95-kilodalton protein which was detected in conditioned media, by using antisera directed against the EGF-R. A similarly sized protein was also detected in the media of WB cells, a continuous, nontransformed line of rat hepatic epithelial cells. Northern (RNA blot) analysis established that the truncated receptor is encoded by a 2.7-kb transcript found in normal rat liver. Furthermore, Northern analysis of rat poly(A)+ RNA showed that the 2.7-kb EGF-R transcript is expressed at differing levels in various fetal and adult tissues. These data indicate that alternative splicing of the EGF-R primary transcript yields a 2.7-kb mRNA which codes for a truncated form of the receptor. This receptor is secreted by rat hepatic epithelial cells in culture, which suggests that it may be secreted by normal rat cells or tissues and perhaps serve an as yet unknown physiological function.

Epidermal growth factor receptor and thyrotropin response in human thyroid tissues

The content of epidermal-growth-factor receptor (EGFr) and its relation to TSH-response were examined in 27 malignant thyroid tumors (5 follicular, 6 papillary, 5 medullary, 11 anaplastic carcinomas) and in 30 tumor-like lesions (21 hyperplastic goiters and 9 toxic adenomatous goiters). Normal 12 thyroid tissues adjacent to benign tumors with no evidence of macroscopic or microscopic abnormalities were used as control. All thyroid plasma membranes tested showed specific EGF binding. In membranes from toxic adenomas (2.18 +/- 0.73 fmoles/mg protein) and papillary carcinomas (2.80 +/- 0.80) the EGF binding was similar to that of normal thyroid membranes (2.32 +/- 0.73). Hyperplastic goiters showed an EGF binding (4.4 +/- 0.82) slightly higher than normal tissue. The highest and the lowest EGF binding values were found in anaplastic (11.8 +/- 2.78) and medullary (0.50 +/- 0.39) carcinomas, respectively. An inverse correlation between EGFr content and TSH-response was found when anaplastic thyroid tumors were compared to tumor-like lesions. However no correlation was observed in medullary carcinomas which also failed to respond to TSH and in papillary carcinomas which partially respond to thyrotropin.

Patterns of proto-oncogene expression in human glioma cell lines

Previously reported studies have suggested that variations in the pattern of proto-oncogene expression within a specific tumor type may denote an underlying difference in the biology and clinical behavior of those tumors. To more sensitively characterize malignant tumors of the central nervous system, we have used Northern blot hybridization analysis to determine the patterns of expression of seven proto-oncogenes in 20 cell lines established from biopsy specimens of patients with malignant glioma. The following proto-oncogenes are expressed at detectable levels in 30 micrograms of total RNA from most glioma cell lines examined: c-myc (20/20), c-mil/raf-1 (18/18), neu (19/20), c-erbB (19/20), and c-myb (17/20). In contrast, only half of the cell lines expressed detectable c-sis (10/20). In none of the cell lines tested was N-myc (0/20) mRNA detected. Morphologic analysis of these 20 cell lines revealed three different growth patterns: bipolar, epithelial, and pleomorphic-glial. Detectable levels of c-sis mRNA typically occurred with either an epithelial or pleomorphic-glial morphology. The pleomorphic-glial subgroup was also characterized by the expression of glial fibrillary acidic protein.

Proto-oncogene abnormalities and their relationship to tumorigenicity in some human glioblastomas

Human glioblastomas are highly malignant intracranial tumors, some of which demonstrate amplification of the epidermal growth factor-receptor (EGF-R) gene. Overexpression of this gene is seen in the majority of primary tumors; however, the role of the EGF-R gene in glial tumorigenesis is unknown. The authors explored the relationship between EGF-R gene expression and glioblastoma cell growth in vitro and in vivo and found that this level of EGF-R gene expression did not correlate with tumor cell growth either in soft agar or in the nude mouse. This suggests that the EGF-R gene is not involved in effecting direct growth stimulation in glial oncogenesis. Tumorigenesis involves differentiation arrest; therefore, the expression of several proto-oncogenes in neuroectodermal tumors was investigated to evaluate the potential involvement of the EGF-R gene in glial differentiation. A nonoverlapping expression of the N-myc and EGF-R genes was found in neuronal-derived and glial-derived tumors, respectively. This suggests that the EGF-R gene may be involved in differentiation or its arrest in glia.

Chromosomal composition of malignant human gliomas through serial subcutaneous transplantation in athymic mice

The karyotypes of seven human glioblastomas were followed through serial subcutaneous passage in athymic mice. One tumor maintained the same hypodiploid stemline as seen in the original biopsy. Three tumors that originally had near-diploid stemlines showed an increase in stemline number to pseudodiploid or hyperdiploid due to gains of whole chromosomes. One tumor showed an entirely different karyotypic profile in the xenograft than had been demonstrated originally. Two tumors showed a shift from near-diploid stemlines to near-tetraploid and near-pentaploid stemlines. Structural abnormalities, including double minutes, that were present originally were maintained in the xenografts, and acquisition of new marker types was rare. Five of the seven lines showed an increased growth rate with serial passage as measured by a shortening of tumor doubling time and decreased time to 1000-mg size. There was, however, no obvious relationship between this change in growth rate and the karyotypic or histologic pattern. These studies demonstrate that most subcutaneous xenografts derived from malignant human gliomas retain karyotypes similar to those seen originally, making these systems useful models for studying the biologic significance of the chromosomal abnormalities of these tumors.

Oncogene-related growth factors and growth factor receptors in human malignant glioma-derived cell lines

Oncogenes induce malignant transformation of cells. Two oncogenes are closely related to genes coding for a mitogenic growth factor (v-sis to the PDGF gene) and a receptor for a mitogenic growth factor (v-erb B to the EGF receptor gene). We studied the possibility that cells derived from malignant gliomas produce mitogenic factors that bind to cell surface receptors, the activation of which could lead to excessive stimulation of cell proliferation. All six cell lines tested secrete into their medium factors that stimulate DNA synthesis. The factor secreted by one cell line was characterized and found to resemble PDGF. Six of 11 cell lines had receptors for PDGF demonstrable by binding and receptor autophosphorylation assays. Six of six cell lines tested had EGF receptors demonstrable by binding and receptor autophosphorylation experiments. The extremely high levels of EGF receptor in one cell line may reflect excessive expression of the erb B oncogene associated with abnormalities of chromosome 7 that occur in this cell line.

Oncogenes and neuro-oncology

The application of molecular biological techniques to the study of lympho-erythroid neoplasms, colo-rectal carcinoma and neuroblastoma has led to fundamental insights into the nature of cellular proliferation, transformation and immortalisation as well as providing prognostic information about the biological behaviour of certain tumours. The study of the molecular genetics of central nervous system tumours with particular reference to oncogenes is however in its infancy. Most of the current literature concerns studies of small numbers of glial tumours or of glial tumour cell lines. In this review the results of these studies are analysed and compared with relevant oncogene findings in experimental cerebral neoplasia, extracranial tumours and postulated mechanisms of oncogene activation. The role of proto-oncogenes in the development of the brain, and the clinical relevance of advances in molecular biology to central nervous system neoplasia are discussed.