A glioma-derived analog to platelet-derived growth factor: demonstration of receptor competing activity and immunological crossreactivity

Involvement of platelet-derived growth factor ligands and receptors in tumorigenesis

Platelet-derived growth factor (PDGF) isoforms and their receptors have important roles during embryogenesis, particularly in the development of various mesenchymal cell types in different organs. In the adult, PDGF stimulates wound healing and regulates tissue homeostasis. However, overactivity of PDGF signalling is associated with malignancies and other diseases characterized by excessive cell proliferation, such as fibrotic conditions and atherosclerosis. In certain tumours, genetic or epigenetic alterations of the genes for PDGF ligands and receptors drive tumour cell proliferation and survival. Examples include the rare skin tumour dermatofibrosarcoma protuberance, which is driven by autocrine PDGF stimulation due to translocation of a PDGF gene, and certain gastrointestinal stromal tumours and leukaemias, which are driven by constitute activation of PDGF receptors due to point mutations and formation of fusion proteins of the receptors, respectively. Moreover, PDGF stimulates cells in tumour stroma and promotes angiogenesis as well as the development of cancer-associated fibroblasts, both of which promote tumour progression. Inhibitors of PDGF signalling may thus be of clinical usefulness in the treatment of certain tumours.

Platelet-derived growth factor in glioblastoma-driver or biomarker?

The platelet-derived growth factor (PDGF) family of mitogens exerts vital functions during embryonal development, e.g. in the central nervous system, where PDGF drives the proliferation of oligodendrocyte precursors. PDGF and PDGF receptors are co-expressed in human glioblastoma (GBM). Whether an aberrant activation of the PDGF receptor pathway is a driving force in glioma development has remained an open question. In experimental animals, overexpression of PDGF has convincingly been shown to induce tumors, both in wild-type animals (marmoset, rat, mouse) and in mice with targeted deletions of suppressor genes, e.g. Tp53 or Ink4A. Targeting the PDGF receptor in tumor-bearing mice leads to growth inhibition and reversion of the transformed phenotype. Findings of PDGF receptor amplification or mutations in human GBM are strong indicators of a causative role of the PDGF receptor pathway. However, clinical trials using PDGF receptor antagonists have been disappointing. In conclusion, a PDGF receptor profile may be a biomarker for a subgroup of GBM originating from a PDGF receptor-responsive cell. Although compelling experimental and clinical evidence supports the notion that the PDGF receptor pathway is a driver in GBM, formal proof is still missing.

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

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

PDGF and PDGF receptors in glioma

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

Glioma pathophysiology: insights emerging from proteomics

Proteomics is increasingly employed in both neurological and oncological research to provide insight into the molecular basis of disease but rarely has a coherent, novel pathophysiological insight emerged. Gliomas account for >50% of adult primary intracranial tumors, with malignant gliomas (anaplastic astrocytomas and glioblastoma multiforme) being the most common. In glioma, the application of proteomic technology has identified altered protein expression but without consistency of these alterations or their biological significance being established. A systematic review of multiple independent proteomic analyses of glioma has demonstrated alterations of 99 different proteins. Importantly 10 of the 99 proteins found differentially expressed in glioma [PHB, Hsp20, serum albumin, epidermal growth factor receptor (EGFR), EA-15, RhoGDI, APOA1, GFAP, HSP70, PDIA3] were identified in multiple publications. An assessment of protein-protein interactions between these proteins compiled using novel web-based technology, revealed a robust and cohesive network for glioblastoma. The protein network discovered (containing TP53 and RB1 at its core) compliments recent findings in genomic studies of malignant glioma. The novel perspective provided by network analysis indicates that the potential of this technology to explore crucial aspects of glioma pathophysiology can now be realized but only if the conceptual and technical limitations highlighted in this review are addressed.

RNAi screen for rapid therapeutic target identification in leukemia patients

Targeted therapy has vastly improved outcomes in certain types of cancer. Extension of this paradigm across a broad spectrum of malignancies will require an efficient method to determine the molecular vulnerabilities of cancerous cells. Improvements in sequencing technology will soon enable high-throughput sequencing of entire genomes of cancer patients; however, determining the relevance of identified sequence variants will require complementary functional analyses. Here, we report an RNAi-assisted protein target identification (RAPID) technology that individually assesses targeting of each member of the tyrosine kinase gene family. We demonstrate that RAPID screening of primary leukemia cells from 30 patients identifies targets that are critical to survival of the malignant cells from 10 of these individuals. We identify known, activating mutations in JAK2 and K-RAS, as well as patient-specific sensitivity to down-regulation of FLT1, CSF1R, PDGFR, ROR1, EPHA4/5, JAK1/3, LMTK3, LYN, FYN, PTK2B, and N-RAS. We also describe a previously undescribed, somatic, activating mutation in the thrombopoietin receptor that is sensitive to down-stream pharmacologic inhibition. Hence, the RAPID technique can quickly identify molecular vulnerabilities in malignant cells. Combination of this technique with whole-genome sequencing will represent an ideal tool for oncogenic target identification such that specific therapies can be matched with individual patients.

Structural and functional aspects of platelet-derived growth factor and its receptors

Platelet-derived growth factor (PDGF) is a dimeric molecule that exists as homodimers or heterodimers of related polypeptide chains (A and B). Two types of PDGF receptor have been identified. The PDGF alpha-receptor binds all three isoforms with high affinity whereas the beta-receptor binds only PDGF-BB with high affinity, PDGF-AB with low affinity and does not appear to bind PDGF-AA. The alpha- and beta-receptors are structurally related, each having an intracellular protein tyrosine kinase domain. Ligand-induced functional activation of the receptors appears to involve receptor dimerization. Binding of PDGF to its receptor is followed by internalization and degradation of the ligand-receptor complex. Experiments with mutant receptors have shown that ligand-induced internalization is not absolutely dependent on the kinase activity of the beta-receptor. The v-sis oncogene of simian sarcoma virus (SSV) is a retroviral version of the PDGF B chain gene and SSV-transformation is mediated by an autocrine PDGF-like growth factor. Formal evidence that the expression of the PDGF beta-receptor is sufficient to confer susceptibility to SSV-transformation has been obtained using porcine endothelial cells expressing a recombinant human beta-receptor. PDGF is a chemotactic agent for several cell types. Recent experiments have shown that the PDGF beta-receptor mediates a chemotactic response and that this effect requires an intact protein tyrosine kinase activity.

AAV Serotype 8-Mediated Gene Delivery of a Soluble VEGF Receptor to the CNS for the Treatment of Glioblastoma

The presence of the blood-brain barrier complicates drug delivery in the development of therapeutic agents for the treatment of glioblastoma multiforme (GBM). The use of local gene transfer in the brain has the potential to overcome this delivery barrier by allowing the expression of therapeutic agents directly at the tumor site. In this study, we describe the development of a recombinant adeno-associated (rAAV) serotype 8 vector that encodes an optimized soluble inhibitor, termed sVEGFR1/R2, of vascular endothelial growth factor (VEGF). VEGF is an angiogenic factor highly up-regulated in GBM tumor tissue and correlates with disease progression. In subcutaneous models of GBM, VEGF inhibition following rAAV-mediated gene transfer significantly reduces overall tumor volume and increases median survival time following a single administration of vector. Using orthotopic brain tumor models of GBM, we find that direct intracranial administration of the rAAV-sVEGFR1/R2 vector to the tumor site demonstrates anti-tumor efficacy at doses that are not efficacious following systemic delivery of the vector. We propose that rAAV-mediated gene transfer of a potent soluble VEGF inhibitor in the CNS represents an effective antiangiogenic treatment strategy for GBM.

Kinase inhibitors in cancer therapy: a look ahead

The most essential kinases involved in cell membrane receptor activation, signal transduction and cell cycle control or programmed cell death and their interconnections are reviewed. In tumours, the genes of many of those kinases are mutated or amplified or the proteins are overexpressed. The use of key kinases offers the possibility to screen in vitro for synthetic small molecule kinase inhibitors. In view of the many interconnections of cellular kinases, their role in preventing or inducing programmed cell death and the possibility that a considerable number of signal transducing proteins are still unknown, cellular test systems are recommended in which the respective key kinase or one of its main partner molecules are overexpressed.

Secretion of ribonucleases by normal and immortalized cells grown in serum-free culture conditions

The requirement of serum in cell culture is a major limitation for studies on secreted ribonucleases (RNases) because serum contains a high amount of ribonucleolytic activity. Defined culture condition is thus of interest to improve our knowledge of the RNase biology. We report here that cells from three different types and origins, Chinese hamster lung fibroblasts, bovine smooth muscle cells, and human endothelium-derived EA.hy926 cells, proliferate consistently in the presence of a basal medium supplemented with bovine serum albumin, high-density lipoproteins, basic fibroblast growth factor, insulin, and transferrin. Using a new quantitative radio-RNase inhibitor assay, two distinct ribonucleolytic assays, and a radioimmunoassay against angiogenin, it is shown that RNases became apparent in media conditioned by cell monolayers. Both the hamster lung fibroblast and the EA.hy926 cell lines secreted larger amounts of RNase inhibitor-interacting factors and RNase activity than normal smooth muscle cells. The serum-free medium represents an alternative way to grow these cells and allows investigation of biosynthesis and functions of RNases in culture. It should be useful to identify and quantitate unambiguously specific members of the RNase family secreted by normal versus tumor cells in culture.

A strategy for selective anti-cancer drug concentration increase in rat glioma tissue with Ca(2+)-channel blocker co-administration: calcium kinetics in intra-glioma arteriolar smooth muscle cells

A rat glioma model was employed to estimate the Ca2+ kinetics in the tumor arteriolar smooth muscle cells. Electron microcytochemistry revealed that the density of intracellular Ca2+ deposits in the intra-tumor arteriolar smooth muscle cells was significantly greater, with slightly higher membrane Ca(2+)-adenosine triphosphatase (ATPase) activity, compared to the contralateral cerebral arterioles. Furthermore, the administration of tyrphostin, a tyrosine kinase inhibitor, specifically increased only the intra-tumor blood flow. These findings suggest that the condition of the intra-tumor arteriole alters the susceptibility to contraction by the accelerated Ca2+ influx into the cytoplasm mediated through the tyrosine kinase pathway. After the administration of diltiazem, which also has a blocking effect on the Ca(2+)-channel mediated through this pathway, the local intra-tumor blood flow showed an increase of 39% with a marked decrease of intracellular Ca2+ concentration of the arteriolar smooth muscle cells in the tumor, while the blood flow in the basal ganglia increased by only 8%. The intra-tumor concentration of Nimustine-HCl (ACNU) with co-administration of diltiazem was significantly increased compared to that without the co-administration. Co-administration of diltiazem may be a valuable strategy in chemotherapy for glioma in affording the selective increase of intra-tumor concentration of the anti-cancer drug.

Signals controlling the expression of PDGF

PDGF is an important polypeptide growth factor that plays an essential role during early vertebrate development and is associated with tissue repair and wound healing in the adult vertebrate. Moreover, PDGF is thought to play a role in a variety of pathological phenomena, such as cancer, fibrosis and atherosclerosis. PDGF is expressed as a dimer of A and/or B chains, the precursors of which are encoded by two single copy genes. Although the PDGF genes are expressed coordinately in a number of cell types, they are independently expressed in a majority of cell types. The expression of either PDGF gene can be affected by very diverse extracellular stimuli and the type of response is dependent on the cell type that is exposed to the stimulus. Expression of the PDGF chains can be modulated at every imaginable level: by regulating accessibility of the transcription start site, by varying the transcription initiation rate, by using alternative transcription start sites, by alternative splicing, by using alternative polyadenylation signals, by varying mRNA decay rates, by regulating efficiency of translation, by protein modification, and by regulating secretion. Even upon secretion, the activity of PDGF can be modulated by non-specific or specific PDGF-binding proteins. This review provides an overview of the cell types in which the PDGF genes are expressed, of the factors that are known to affect the expression of PDGF, and of the various levels at which the expression of PDGF genes can be regulated.

Platelet-derived growth factor in human glioma

Platelet-derived growth factor (PDGF) is a 30 kDa protein consisting of disulfide-bonded dimers of A- and B-chains. PDGF receptors are of two types, alpha- and beta-receptors, which are members of the protein-tyrosine kinase family of receptors. The receptors are activated by ligand-induced dimerization, whereby the receptors become phosphorylated on tyrosine residues. These form attachment sites for signalling molecules, which inter alia activate the Ras.Raf pathway. PDGF has important functions in development and is required for a proper timing of oligodendrocyte differentiation. The v-sis oncogene of simian sarcoma virus (SSV) is a retroviral homolog of the B-chain gene, and induces transformation by an autocrine activation of PDGF receptors at the cell surface. SSV induces malignant glioma in experimental animals, suggesting a role for autocrine PDGF in glioma development. PDGF and PDGF receptors are frequently coexpressed in human glioma cell lines. Specific and nonspecific PDGF antagonists block the growth of some glioma cell lines in vitro and in vivo, suggesting that autocrine PDGF is involved in transformation and tumorigenesis. In situ studies of human gliomas show overexpression of alpha-receptors in glioma cells of high-grade tumors. In a few cases, overexpression is caused by receptor amplification. Since high-grade glioma cells also express the PDGF A-chain, an autocrine activation of the alpha-receptor may drive the proliferation of glioma cells in vivo.

Inhibition of proliferation of human cerebral meningioma cells by suramin: effects on cell growth, cell cycle phases, extracellular growth factors, and PDGF-BB autocrine growth loop

The growth of human cerebral meningiomas depends on various growth factors, including epidermal growth factor (EGF), transforming growth factor (TGF)-alpha and TGF-beta, platelet-derived growth factor (PDGF)-BB, insulin-like growth factor (IGF)-I and IGF-II, and acidic and basic fibroblast growth factors. The latter three have been shown to form autocrine loops that are thought to be a major component of uncontrolled growth in meningioma tissue. Suramin is known to prevent binding of a variety of growth factors to their receptors in mammalian tissue, thus abolishing para- and/or autocrine-mediated cell growth. The authors therefore tested the effect of suramin on the proliferation of cultured human meningioma cells. Suramin (10(-5) to 10(-4) M) significantly inhibited the growth of meningioma cells in culture. The maximum effect observed was with the higher dose (10(-4) M), which resulted in a 40% to 70% reduction in cellular proliferation. This effect was observed in all 15 tumor samples studied and was confirmed by [3H]thymidine uptake. In studies using DNA flow cytometry, suramin inhibited meningioma cell proliferation in five tumor samples by arresting cells in the S and G2/M phases of the cell cycle. Growth factor (EGF, IGF-I, and PDGF-BB)-induced cell proliferation was completely abolished in five tumor samples when 10(-4) M suramin was applied to meningioma cells. Western blot analysis of three tumor samples showed that the intracellular PDGF-BB content of meningioma cells was significantly reduced after treating the cells with 10(-4) M suramin. Binding of iodinated growth factors (that is, [125I]EGF, [125I]IGF-I, and [125I]PDGF-BB) to their receptor sites was prevented by suramin in a dose-dependent manner in 10 meningioma membrane fractions. Lowering of the intracellular PDGF content and prevention of extracellular growth factor receptor binding demonstrates that suramin disrupts autocrine loops and paracrine growth stimulation in meningioma tissue. These data provide evidence that growth of cerebral meningiomas in culture is strongly inhibited by suramin at a concentration of 10(-4) M. Suramin acts as a scavenger neutralizing exogenous growth factors; thus it can interrupt autocrine loops and paracrine stimulation of human meningioma cell growth. The evidence favors suramin as a therapeutic option for controlling meningioma proliferation in patients with inoperable and recurrent high-grade meningiomas.

Expression of platelet-derived growth factor and c-myc in atherosclerotic lesions in cholesterol-fed chickens: immunohistochemical and in situ hybridization study

Immunohistochemical examination showed no significant expression of platelet-derived growth factor-A (PDGF-A), PDGF-B, PDGF receptors, or of c-myc in the thoracic and abdominal aortas of normal roosters. In cholesterol-fed roosters, intense immunohistochemical reaction for PDGF-B, PDGF receptor, and c-myc was seen in the lipid-rich thickened intimal lesions of the thoracic and abdominal aortas while no significant immunoreaction for PDGF-A was demonstrated in the same lesions. In accordance with immunohistochemical findings, in situ hybridization demonstrated a significant level of expression of PDGF-B, PDGF-A receptor, PDGF-B receptor, and c-myc genes in proliferating intimal cells of the thoracic and abdominal aortas. These results suggest that coordinate actions of PDGF-B and c-myc play an important role in proliferation of intimal cells in the developing atherosclerotic lesions in chickens.

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.

Basic fibroblast growth factor expression is required for clonogenic growth of human glioma cells

Basic fibroblast growth factor (bFGF) is a heparin-binding protein, expressing potent mitogenic and angiogenic properties. Elevated levels of bFGF have been identified in human gliomas and glioma cell lines, suggesting that bFGF expression is involved in the aberrant growth patterns associated with these tumors. In the present study, the influence of bFGF on additional parameters of glioma cell malignancy was evaluated utilizing three distinct methods to suppress bFGF expression or activity including antisense oligonucleotide primers, a neutralizing monoclonal antibody or an inhibitor of the agonist action of bFGF: (1) The addition of 30 microM bFGF-specific antisense oligonucleotide primer to the human glioma cell line SNB-19 resulted in a 55% inhibition in colony formation in soft agar. This effect was dose-dependent and specific, as sense strand primer was ineffective in suppressing growth. In addition to exhibiting fewer colonies, antisense treatment significantly altered colony morphology. (2) SNB-19 cell growth in culture was suppressed in the presence of a neutralizing bFGF-specific monoclonal antibody. (3) Inositolhexakisphosphate, a newly identified antagonist of FGF binding and activity, suppressed SNB-19 cell growth in soft agar culture. These results demonstrate that bFGF may regulate glioma growth and progression independent of its role in tumor angiogenesis and that bFGF release or secretion may be required for these actions.

Ruffling and locomotion: role in cell resistance to growth factor-induced proliferation

It has long been known that the growth rate of cells in vitro can be retarded by providing substrates of restricted area. Such experiments were performed with adhesive islets, made by depositing metals onto agarose layers through templates of various sizes. Since normal cells are unable to adhere to agarose, they become confined to the metallic surface. Using such haptotactic islets, we have studied the role of membrane ruffling and cell locomotion in the resistance of AG1523 human fibroblasts to growth factor-induced mitogenesis. Cells plated on small substrates, i.e., 2,150 microns 2 in area, initially showed vigorous ruffling, which was suppressed by 8 h after plating but had resumed again by 12 h. In contrast, cells on larger-size islets showed a rapid decline and stabilization of ruffling activity. When the growth rate was measured for single cells cultured on haptotactic islets, it was found to increase linearly from areas of 4,280 microns 2 up to 425,000 microns 2. Since the area needed to saturate the growth response was approximately 50-fold larger than the area occupied by a single cell, the growth inhibition was attributed in part to an interference with locomotion. The implication that locomotion provided positive input into growth control mechanisms was subjected to a direct test by evaluating the effect of nine polypeptide growth factors on the motility of serum-starved cells. All except TGF-beta 1 stimulated movement. Finally, the mitogenic effect of growth factors was measured by [3H]thymidine incorporation and found to be proportional to motile activities, as quantitatively assayed. We conclude that locomotion suppression is a factor in AG1523 cell resistance to growth factor-induced mitogenesis.

A novel monoclonal antibody dependent on domain 5 of the platelet-derived growth factor beta receptor inhibits ligand binding and receptor activation

Platelet derived growth factor (PDGF) induces activation of the protein tyrosine kinase domain of the PDGF receptor, resulting in receptor dimerization and the initiation of mitogenesis in responsive cells. In order to identify domains of the receptor involved in these processes, a panel of monoclonal antibodies (MAbs) against the extracellular region of the human PDGF receptor was developed and screened to identify which of these specifically block PDGF binding. One of these, MAb 2A1E2, binds PDGF beta receptor with high affinity and blocks PDGF BB binding in a whole cell binding assay with an IC 50 of 0.1 nM. Inhibition of binding results in the inhibition of ligand-induced receptor phosphorylation, dimerization and mitogenesis in cells expressing the PDGF beta receptor. MAb 2A1E2 has been mapped to the fifth Ig domain of the PDGF beta receptor, implying that this domain is important for ligand binding, dimerization and/or activation. The potency of MAb 2A1E2 for inhibiting PDGF BB binding indicates that this antibody is ideally suited to identify and characterize PDGF BB-induced biological responses.

Growth in serum-free medium of NIH3T3 cells transformed by the EJ-H-ras oncogene: evidence for multiple autocrine growth factors

Rodent fibroblastic cells transformed by ras oncogenes can grow in serum-free (S-) medium. We have studied clonal lines of mouse NIH3T3 fibroblasts transfected with the EJ-H-ras oncogene, and observed that practically all become independent of exogenous growth and attachment factors shortly after transfection. Moreover, all the clones tested soon form anchorage-independent (AI) colonies in S- medium, and most give rise to spheroids able to grow in suspension. The cell-conditioned S- medium of the transformed (TR) cells stimulates autocrinally the AI and anchored growth of these cells, in the absence of serum, and it contains growth factors related to TGF-alpha (or EGF), PDGF and bFGF, and other uncharacterized factors. Some of these factors are not found, or are found only in very small amounts, in the S- medium of non-transformed NIH3T3 cells, which also stimulates the growth of the TR cells, in the absence of serum. In addition, the TR cells contain 4-6 times more cell-associated bFGF than the non-transformed cells and release more latent TGF-beta activatable by acid treatments. However, no active TGF-beta is secreted by either cell type. Activated TGF-beta and pure TGF-beta 1 stimulate the growth of the anchored TR and NIH3T3 cells, but inhibit the AI growth of the TR cells. Another inhibitor of this growth is also found in the concentrated medium of the NIH3T3 cells.

Astroglial growth factors in normal human brain and brain tumors: comparison with embryonic brain

Aqueous extracts of 18-day embryonic chicken brains, 15-day embryonic and adult rat brains and human brain tumors, as well as control histologically-normal adult human brain taken from around brain tumors or around arteriovenous malformations each stimulated the growth of cultured chick astrocytes. Eight mitogenic fractions were separated reproducibly by Bio-Gel P-10 molecular seive chromatography. They had apparent molecular weights (M.W.) of 24, 17, 12, 9, 5, 2.8, 1.4 and 1.2 kD. The activity of each fraction was concentration dependent. The fractions did not appear to be artifactually derived by proteolysis from a larger mitogen since (i) protease inhibitors were added at the time of homogenization to prevent degradation, (ii) protease treatment did not produce large quantities of the lower molecular weight fractions, (iii) incubation of brain extracts for up to four hours at 30 degrees C did not alter the activity of the various mitogenic fractions and (iv) addition of albumin to inhibit protease activity similarly did not change the profile of the factors. In contrast, treatment with protease reduced the activity of all the factors although those with M.W. of 5 and 1.2 kD were inactivated more slowly than the others. The various fractions were stable when rechromatographed. This suggested they were not chance aggregates derived artifactually during extraction but rather might have physiological and pathological roles. The activities of each mitogenic fraction were significantly higher in brain extracts from embryonic rats than in those from adult rats. In brain extracts of rat and chicken embryos the fractions of lower M.W. 5 kD to 1.2 kD were relatively abundent. In contrast in brain extracts from adult rats the predominant mitogenic fractions had apparent M.W. of 24, 17 and 12 kD. In histologically normal adult human brain taken from around the tumors or around arteriovenous malformations the 5 kD fraction was present in small amounts and the fractions of lower molecular weight were present in very small amounts. In human glial brain tumors there was a preponderance of the 5 kD activity and more of the 2.8 and 1.4 kD activity fractions than in histologically normal adult human brain. But there was relatively less activity in the 24 and 17 kD fractions. The growth factor profile of human meningiomas was quite different from that of histologically normal human brain or human glial brain tumors. The fraction from meningiomas that was most mitogenic for astrocytes had a molecular weight of 12 kD.(ABSTRACT TRUNCATED AT 400 WORDS)

Receptors for platelet-derived growth factor on microvascular endothelial cells

Endothelial cells are widely thought to be unresponsive to platelet-derived growth factor (PDGF) and devoid of PDGF receptors. However, in examining the growth factor responses of microvascular endothelial cells isolated from human omental adipose tissue, we detected PDGF-induced tyrosine phosphorylation of an 180-kD glycoprotein, subsequently identified as the cellular receptor for PDGF by specific immunoprecipitation. Scatchard analysis of 125I-PDGF binding to human microvascular endothelial cells revealed 30,000 PDGF receptors/cell with a kD of 0.14 nM. PDGF stimulated tyrosine phosphorylation of PDGF and other cellular proteins in a dose- and time-dependent manner, with half-maximal receptor phosphorylation occurring at 0.3 nM recombinant human PDGF-BB within 1 min of PDGF exposure. Normal cellular consequences of receptor activation were also observed, including tyrosine phosphorylation of a 42-kD protein and serine phosphorylation of ribosomal protein S6. Furthermore, PDGF was mitogenic for these cells. The finding of functional PDGF receptors on human microvascular endothelial cells suggests an important direct role for PDGF in neovascularization.

Density-dependent regulation of cell surface ?-glutamyl transpeptidase in cultured glial cells

A decline in cell surface gamma-glutamyl transpeptidase specific activity was previously observed to be concomitant with C6 glial cell proliferation. To elucidate the underlying factor(s) mediating gamma-glutamyl transpeptidase down-regulation, the effects of C6 cell density and culture conditions on cell surface transpeptidase activity levels were investigated. After 24 h of culture, the transpeptidase specific activities were inversely related to the initial plating densities. The lower-density cultures showed an induction within 24 h of plating. As the cultures proliferated, the specific transpeptidase activities declined to a common low level at post-confluency. The gamma-glutamyl transpeptidase down-regulation was unrelated to cell growth rate and was most pronounced during logarithmic proliferation. Induction and down-regulation of gamma-glutamyl transpeptidase activity at low cell densities were not a result of trypsinization. Supplementation of low-density cultures with conditioned medium, use of matrix-coated wells, or periodic replacement of growth media to prevent conditioning had minor effects on the decline of cell surface activity. Kinetic analysis showed that the Michaelis constants and the reaction mechanism were unaltered by cell density, indicating that down-regulation was not due to allosteric factors or an alteration in enzyme character. A reduction in the maximal velocity of cell surface transpeptidation at higher cell densities suggested that gamma-glutamyl transpeptidase down-regulation is related to the concentration of enzyme at the cell surface. Immunocytochemical localization of gamma-glutamyl transpeptidase demonstrated that gamma-glutamyl transpeptidase antigen levels decrease as C6 cell density increases. These results led us to propose that cell-cell contact stimulates the disappearance of gamma-glutamyl transpeptidase from the surface of cultured C6 glial cells.

Glioma-derived PDGF-related protein presents as 17 kd intracellularly and assembled form induces actin reorganization

We have electrophoretically obtained platelet-derived growth factor (PDGF)-related protein from human glioma (glioma derived PDGF-related protein: GD-PDGF) and produced rabbit antiserum against the monomer of GD-PDGF. By methods of immunoaffinity chromatography and Western blotting, we analyzed GD-PDGF in cultured human glioma cells and conditioned medium. The intracellular GD-PDGF was only detected at 17 kd molecular weight by the purified rabbit antibody. When the intracellular 17 kd monomer was purified by the IgG-coupled immunoaffinity chromatography, the eluted protein was not detected at 17 kd but at 52 kd. The 52 kd GD-PDGF was spontaneously and immediately converted to 56 kd, which was partly degraded to 32 and 35 kd within 24 hours. On the other hand, in the conditioned media of glioma cell lines GD-PDGF presents mainly as 56 kd. The assembled forms of GD-PDGF exhibited a powerful activity to induce membrane ruffle formation and reorganization of actin filaments in cultured glial cells and glioma cells. These results indicated that GD-PDGF is intracellularly stored as 17 kd monomer and exists extracellularly as assembled forms, which may act as an autocrine and paracrine effect on the surrounding cells.

Autocrine control of human meningioma proliferation: secretion of platelet-derived growth-factor-like molecules

We have used cell-culture techniques to investigate growth-factor production by human meningioma cells. Meningioma tissue was dispersed with collagenase and the cells grown to high density in tissue-culture flasks. The cultures were used to generate conditioned medium (MEN-CM), which was used to cultivate IMR32 cells (a human neuroblastoma line) and freshly dispersed primary meningioma cells. MEN-CM profoundly stimulated the in vitro growth of both IMR32 and meningioma cells. In addition, H3-thymidine uptake by cultured meningioma cells was increased in a dose-dependent manner by varying concentrations of MEN-CM. A neutralizing anti-body against platelet-derived growth factor (PDGF) completely abolished the stimulatory effects of MEN-CM, whereas an antibody against TGF-alpha was without effect. The mitogenic activity of MEN-CM, as assayed by promotion of H3-thymidine uptake by cultured meningioma cells, eluted from a Sephadex G-100 column in 3 peaks corresponding to molecular weights of greater than or equal to 150, 56 and 28 kDa. Our results show that proliferation of human meningiomas may be under autocrine control via secretion of PDGF-like molecules.

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.

Human microvascular endothelial cells express receptors for platelet-derived growth factor

Endothelial cells have been widely thought to be unresponsive to platelet-derived growth factor (PDGF, a major growth factor released from stimulated platelets at the sites of vascular insults) and devoid of PDGF receptors. Nevertheless, in examining the growth-factor responses of microvascular endothelial cells isolated from human omental adipose tissue, we were surprised to detect PDGF-induced tyrosine phosphorylation of a 180-kDa glycoprotein, subsequently identified as the cellular receptor for PDGF by specific immunoprecipitation. Scatchard analysis of 125I-labeled PDGF binding to human microvascular endothelial cells revealed 30,000 PDGF receptors per cell with a Kd of 0.14 nM. PDGF stimulated tyrosine phosphorylation of PDGF receptors and other cellular proteins in a dose- and time-dependent manner, with half-maximal receptor phosphorylation occurring at 0.3 nM recombinant human PDGF (B chain) and a less than or equal to 1-min exposure to PDGF. Normal cellular consequences of receptor activation were also observed, including tyrosine phosphorylation of a 42-kDa protein and serine phosphorylation of ribosomal protein S6. Furthermore, PDGF was mitogenic for these cells. Microvascular endothelial cells play a central role in neovascularization required for wound healing and solid tumor growth. Thus, the discovery of functional PDGF receptors on human microvascular endothelial cells suggests a direct role for PDGF in this process.

Coexpression of the platelet-derived growth factor (PDGF) B chain and the PDGF beta receptor in isolated pancreatic islet cells stimulates DNA synthesis

Suspensions rich in pancreatic beta cells were transfected by means of electroporation or by using the liposome technique with DNA constructs coding for the B chain of platelet-derived growth factor (PDGF) and the PDGF alpha and beta receptors to induce a mitotic response in this slowly replicating cell type. Transfection with the B-chain construct induced synthesis of the PDGF B-chain homodimer (PDGF-BB) as assessed by the presence of 125I-labeled PDGF-BB competing activity in the conditioned medium of the transfected islet cells. Moreover, islet cells transfected with the PDGF beta-receptor construct exhibited increased immunofluorescence staining with a PDGF beta-receptor antibody. These cells also displayed increased 125I-labeled PDGF-BB binding compared with control transfected cells. Cotransfection with the B-chain construct or the addition of 10% fetal bovine serum or purified PDGF all induced DNA synthesis in islet cells transfected with the PDGF beta-receptor construct. Islet cells transfected with the PDGF alpha-receptor construct did not respond with stimulation of [3H]thymidine incorporation to any of the PDGF isoforms (PDGF-AA, -AB, or -BB). Cotransfection of the PDGF alpha- and beta-receptor constructs resulted in a loss of the DNA synthesis response to PDGF. The beta cells exhibited elevated levels of [3H]inositol trisphosphate after transfection with the B-chain and beta-receptor constructs, indicating activation of phospholipase C. Islet cells transfected with the different receptor constructs exhibited different patterns of tyrosine phosphorylation upon ligand activation. The results demonstrate that pancreatic islet cells can be stimulated to increase DNA synthesis by transfection with the PDGF beta-receptor gene, whereas cotransfection with the alpha-receptor gene may attenuate the growth response.

Oncogenic proteins new targets for chemotherapeutic agents against cancer

Over the past 10 years, more than 40 potentially oncogenic genes, termed protooncogenes, have been identified in the human genome. Little is known of the physiological role of the proteins encoded by these genes, but they seem to be involved in the reception and transmission of hormonal and other environmental information from the cell membrane to the nucleus. These proteins may acquire transforming properties when over-expressed or if structurally altered following partial deletions or point mutations. Cytogenetic analysis shows loss of genetic material from specific chromosomal loci in many human tumors, suggesting that the absence of a functional gene at these loci may permit tumor development. The genes involved have been termed "anti-oncogenes". Understanding the control mechanisms of cell proliferation is essential in order to understand how cancer cells escape from this control. To this end, numerous oncogenes have been cloned, permitting the production of modified forms of oncogenic proteins and identification of the regions essential for their biological activity. Availability of large amounts of protein also allows the production of specific antibody which can be used to verify whether blockage of a given protein results in reversion of the transformed phenotype. If it can be shown that the expression of an oncogenic protein is essential for transformation, it should be possible to search for molecules that inhibit its action or which mimic the effects of an anti-oncogene. This type of research is already well advanced for the oncogenic ras proteins, and models have been established that permit both screening for potential inhibitors and design of specific antagonists.

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.

Growth-regulatory factors for normal, premalignant, and malignant human cells in vitro

Normal human cells, cells from nonmalignant proliferative lesions, and primary and metastatic tumor cells can be maintained in vitro and analyzed for requirements for growth in chemically defined media. The human melanocytic cell system with normal melanocytes, precursor nevus cells, and primary and metastatic melanoma cells has been extensively studied for the phenotypic properties of the cells, including their requirements for exogenous growth factors and other mitogens. In high calcium-containing W489 medium, normal melanocytes require four supplements: IGF-I (or insulin); bFGF, TPA, and alpha-MSH. Nevus cells are largely independent of bFGF. Depletion of TPA from medium is not as detrimental to nevus cells as it is to melanocytes, but the phorbol ester is still essential for maintenance of the typical nevic phenotype. Primary melanoma cells require at least one growth factor, IGF-I (or insulin), for continuous proliferation. On the other hand, metastatic cells of melanoma as well as of carcinomas of colon and rectum, bladder, ovary, and cervix are able to proliferate after a short adaptation period in medium depleted of any growth factors and other proteins. Doubling times of metastatic tumor cells in protein-free medium are only 30-60% longer than in FCS-containing medium. The growth autonomy of human tumor cells is apparently due to the endogenous production of growth factors. Likely candidates for autocrine growth stimulation of human tumor cells are TGF-alpha, TGF-beta, and PDGF. Melanoma and colorectal carcinoma cells express functional EGF/TGF-alpha receptors, and produce TGF-alpha, indicating that this growth factor is produced for autocrine stimulation. In addition to the use of anti-growth factor antibodies, other strategies for the inhibition of autocrine growth stimulation include mAbs to growth factor receptors, soluble receptors, receptor-mimicking antiidiotype antibodies, and active immunization against growth factors. Whether any of these therapeutic approaches is clinically feasible will need to be determined in extensive preclinical investigations.

C17, a retrovirally immortalized neuronal cell line, inhibits the proliferation of astrocytes and astrocytoma cells by a contact-mediated mechanism

We have investigated the ability of various cell lines to effect contact-mediated inhibition of astrocytic cells. Of the lines tested, only C17, a mouse cell line from postnatal day 0 cerebellum immortalized by infection with a retroviral construct containing the avian myc gene, and U251, a human astrocytoma line, were able to inhibit the proliferation of astrocytic cells. When co-cultured with either primary astrocytes from rat cerebellum or the U251 line, the C17 cells induced a rapid cessation of glial cell division as well as complex astrocytic process extension. The effects on glial mitosis were cell-dose-dependent, with ten C17 cells/glial cell being the optimal ratio. At this ratio [3H]thymidine incorporation into the U251 cells was reduced by greater than 80% and there was a virtual stasis in glial cell number at 48 hours. Fixed C17 cells as well as partially purified C17 membranes were also potent inhibitors of astrocytic proliferation, suggesting that the gliastatic effect of the C17 cell line is membrane associated. However, neither of these preparations induced astrocytic process formation. We also confirmed earlier reports that U251 cells inhibited their own proliferation in a density-dependent manner but at a lower efficiency than the C17 cells.

Localization of basic fibroblast growth factor, a mitogen and angiogenic factor, in human brain tumors

Fibroblast growth factor (FGF) is a potent angiogenic factor and a mitogen for a variety of mesoderm- and neuroectoderm-derived cell types (e.g., fibroblasts, endothelial cells, astrocytes, oligodendrocytes). After application of a monospecific polyclonal antiserum, we localized basic FGF on frozen sections of 73 human brain tumors using immunohistochemistry. FGF was present in a variable number of tumor cells (16/16 astrocytomas, 5/5 ependymomas, 0/3 benign and 4/7 anaplastic oligodendrogliomas, 11/12 glioblastomas, 11/11 meningiomas, 6/6 neurilemmomas, 0/3 pituitary adenomas, 2/2 choroid plexus papillomas, 0/1 neurocytoma, 2/2 benign fibrous histiocytomas, 2/5 metastatic carcinomas). FGF was detected in vascular cells of 59 tumors and in fibroblasts of connective tissue stroma from all papillomas and metastases. These results tend to indicate FGF involvement in the malignant progression of gliomas due to an autocrine or paracrine action. Histopathological aspects of malignant gliomas (e.g., pseudopalisading or pathological vessels) could be related to FGF activity.

Isolation and partial purification of growth factors with TGF-like activity from human malignant gliomas

The effect of concentrated conditioned medium from each of eight human malignant glioma cell lines on the growth of indicator cells (normal rat kidney fibroblasts (NRK), clone 14) was determined in monolayer and in soft agar assay systems. The conditioned medium from all cell lines was mitogenic in the monolayer assay, but only SF-210, U-343 MG-A, and U-251 MG produced soluble factors that caused NRK cells to grow in soft agar. The soluble growth-promoting factors from these three cell lines were acid- and heat-stable (60 degrees C for 30 minutes) but were inactivated by trypsin (100 microns/ml) and dithiothreitol (50 microM). The growth factors from SF-210 and U-343 MG-A were further purified by molecular-sieve chromatography. The partially purified growth factor from U-343 MG-A retained transforming growth factor (TGF)-like activity, had a molecular weight of 9 kD, was potentiated by TGF-beta in the soft agar assay, competed effectively with 125I-epidermal growth factor (EGF) radiolabeled for the EGF receptor on A 431 epidermoid carcinoma cells, and was completely inhibited by monoclonal antibodies to TGF-alpha. The partially purified growth factor from SF-210 had a molecular weight of 17 kD, was not inhibited by monoclonal antibodies to platelet-derived growth factor (PDGF) or TGF-alpha, and did not bind to a heparin-Sepharose column. These results imply that U-343 MG-A secretes a growth factor with TGF-alpha-like activity, and SF-210 secretes a TGF with neither TGF-alpha nor TGF-beta activity.

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

Expression of the c-sis oncogene, the gene encoding the B chain of platelet-derived growth factor (PDGF), may be related to initiation and/or progression of glial cell tumorigenesis by PDGF-mediated autocrine growth stimulation. As the mechanism for activation of expression of the c-sis gene in gliomas is not known, we searched for possible structural alterations of c-sis DNA in these tumors. Genomic Southern blots of DNA from 7 different cultured human glioblastoma cell lines and 15 different solid human brain tumors revealed no significant change in either the gross structure or the copy number of the c-sis gene in tumor cells vs. control cells. Activation of glioma c-sis gene expression is therefore not the result of a gross rearrangement or amplification of the c-sis gene. Expression of c-sis mRNA was detected in all of 12 different solid human brain tumors, 11 of which were of glial cell origin. However, in tissue adjacent to 5 different tumors, approximately the same level of c-sis mRNA was seen.

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.

Growth factors derived from a human malignant glioma cell line, U-251MG

A human malignant glioma cell line, U-251 Mg, cultured under serum free conditions, was shown to produce a growth factor for BALB/c 3T3 cells (glioma-derived growth factor-1, GDGF-1). The biological activity of GDGF-1 resided in a heat- and acid-resistant protein with a molecular weight (MW) of 25 kDa estimated by gel permeation chromatography. GDGF-1 activity was neutralized by a goat anti-human platelet derived growth factor (PDGF) antibody, indicating that the two factors were immunologically related. Furthermore, U-251 Mg cells constitutively expressed c-sis mRNA. When U-251 Mg cells were stimulated with bacterial lipopolysaccharide, 2 novel growth factors (GDGF-2 and GDGF-3) were produced in addition to the PDGF-like substance. GDGF-2 was determined to be greater than 100 kDa MW and was not neutralized by the goat anti-PDGF antiserum. The biological activity of GDGF-3 was also heat- and acid-resistant with an apparent 14 kDa MW. This factor also did not show any common antigenicity with PDGF. GDGF-2 and GDGF-3 are currently under investigation and evidence as to their natures will be published elsewhere. Our findings with this glioma cell line provide further evidence that inappropriate expression of growth factor-related genes could play important autocrine role(s) in the processes leading to malignant transformation and/or uncontrolled proliferation and may provide a paracrine stimulus for such processes as glioma neovascularization.

Expression and purification of biologically active v-sis/platelet-derived growth factor B protein by using a baculovirus vector system

Malignant transformation induced by simian sarcoma virus is mediated by its v-sis protein, the monkey homolog of the platelet-derived growth factor (PDGF) B chain. By use of an appropriately engineered baculovirus expression vector, the v-sis protein was expressed in the insect cell line Spodoptera frugiperda (Sf9) at a level 50- to 100-fold higher than that observed with overexpression in mammalian-cell transfectants. The sis protein produced by Sf9 cells underwent processing similar to that observed in mammalian cells, including efficient disulfide-linked dimer formation. Moreover, the recombinant sis protein was capable of binding PDGF receptors and inducing DNA synthesis as efficiently as PDGF-B synthesized by mammalian cells. A significant fraction of sis protein was released from Sf9 cells, which made possible a one-step immunoaffinity purification to near homogeneity with a 40% recovery of biological activity. These results demonstrate that a protein whose normal processing requires both intrachain and interchain disulfide-bridge formation can be efficiently expressed in a biologically active form in insect cells by using a baculovirus vector system.

Regulation of expression of the c-sis proto-oncogene

Regulation of expression of platelet derived growth factor polypeptide B encoded by the c-sis proto-oncogene is important in a number of physiological and pathological conditions. Sequences in the 1028 nucleotide long 5' untranslated region of the c-sis mRNA were found to inhibit protein synthesis. The inhibition is relieved by deletion of nucleotides 154-378 or 398-475. Sequences within 375 nucleotides upstream of the RNA initiation site are important for transcriptional activity. Sequences in two portions of this region, between -375 and -235 nucleotides and between -235 and -99 nucleotides relative to the RNA CAP site are important for full activity. A transcriptional enhancer activity is demonstrated by its ability to increase the activity of the human T lymphotropic virus type (HTLV) I promoter at a distance and in an orientation-independent manner. Furthermore, sequences upstream of the c-sis RNA CAP site respond to the HTLV I transactivator protein to increase RNA synthesis from either the c-sis or HTLV I promoter.

Purification and amino acid analysis of two human glioma-derived monocyte chemoattractants

Two chemoattractants for human monocytes were purified to apparent homogeneity from the culture supernatant of a glioma cell line (U-105MG) by sequential chromatography on Orange A-Sepharose, an HPLC cation exchanger, and a reverse phase HPLC column. On SDS-PAGE gels under reducing or nonreducing conditions, the molecular masses of the two peptides glioma-derived chemotactic factor 1 and 2 were 15 and 13 kD, respectively. Amino acid composition of these molecules was almost identical, and differed from other cytokines that have been reported. The NH2 terminus of each peptide was apparently blocked. When tested for chemotactic efficacy, the peptides attracted approximately 30% of the monocytes added to chemotaxis chambers, at the optimal concentration of 10(-9) M. Potency and efficacy were comparable with that of FMLP, which is often used as a reference attractant. The activity was chemotactic rather than chemokinetic. In contrast to their interaction with human monocytes, the pure peptides did not attract neutrophils. These pure tumor-derived chemoattractants can now be compared with attractants produced by normal cells and evaluated for their biological significance in human neoplastic disease.