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

The future of therapy for glioblastoma

Effective therapy for malignant gliomas has centered on traditional approaches such as surgery and radiation therapy. Over the past two decades, more innovative approaches involving the use of chemotherapy and immunotherapy have been developed. Although these techniques have improved the quality of survival for many patients, the median survival following diagnosis and adjuvant treatment still remains only about a year. Recently, genetically engineered viruses for gene transduction and targeted cell killing have been used successfully in the experimental treatment of glioblastoma multiforme. We provide a review of the current and possible future therapies for malignant glioma with the belief that molecular biologic and genetic techniques offer the greatest hope of significantly altering the course of disease.

Insulin-like growth factor (IGF)-I, -II and IGF-binding proteins in the cyst fluid of a patient with astrocytoma

Insulin-like growth factor (IGF)-I, -II and IGF-binding proteins (IGFBPs) were demonstrated in the cyst fluid of a patient with a hypothalamic astrocytoma. The astrocytoma cyst fluid was subjected to gel chromatography at low pH and the IGF-I and IGF-II levels were measured by specific radioimmunoassays. Immunoreactive IGF-I and IGF-II levels were 19 ng/ml and 78 ng/ml respectively. Several-fold higher IGF-II values were obtained when cyst fluid was not extracted or was extracted with acid ethanol before radioimmunoassay analysis. The immunoreactive IGFBP-1 concentration was 26 ng/ml. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and subsequent Western ligand blotting with [125I]IGF-II revealed bands at approximately 200, 34.5, 29.5, 24 and 21 kD as visualized by autoradiography. Binding studies demonstrated that these binding proteins bind specifically [125I]IGF-I and [125I]IGF-II. These observations suggest that IGFs as well as IGF-binding proteins are produced by astrocytoma cells and may act in a paracrine or autocrine fashion capable of modulating the growth of astrocytoma tumours.

Inhibitory effect of trapidil on human meningioma cell proliferation via interruption of autocrine growth stimulation

In a previous study, the authors demonstrated that meningioma cells secrete platelet-derived growth factor (PDGF)-like molecules that stimulate their own growth in an autocrine manner. Based on that finding, a study was undertaken to examine the effect of trapidil, a drug known to have an antagonistic action against PDGF, on cell proliferation of human meningiomas in culture. Trapidil showed a dose-dependent inhibition of meningioma cell proliferation in the absence of any exogenous mitogenic stimulation. The maximum effect was observed at a concentration of 100 micrograms/ml, with the decrease in cell growth ranging from 16% to 54% compared to control samples. Trapidil similarly inhibited the basal deoxyribonucleic acid (DNA) synthesis assessed by [3H]-thymidine incorporation in three of seven meningiomas. While the conditioned medium generated from meningioma cells remarkably stimulated the proliferation of meningioma cells (166% to 277% of control), this effect was strikingly inhibited by the addition of trapidil. Trapidil also inhibited conditioned medium-stimulated DNA synthesis, even when there was no effect on basal DNA synthesis. Furthermore, trapidil significantly inhibited the epidermal growth factor (EGF)-stimulated proliferation of meningioma cells. This inhibitory effect on EGF-stimulated cell proliferation was also observed in nontumorous fibroblasts, demonstrating that trapidil is not an antagonist specific to PDGF. The addition of trapidil (30 micrograms/ml) in combination with bromocriptine (1 microM) showed an additive inhibitory effect on the meningioma cell growth compared to trapidil or bromocriptine alone. The overall results suggest that trapidil exhibits an inhibitory effect on meningioma cell proliferation through blocking the mitogenic stimulation induced by autocrine or exogenous growth factors, and may be considered as a possible new approach to the medical treatment of meningiomas.

Effects of EGF, bFGF, NGF and PDGF(bb) on cell proliferative, migratory and invasive capacities of human brain-tumour biopsies in vitro

Spheroids initiated directly from human primary gliomas were used to investigate the effects of EGF, bFGF, NGF and PDGF(bb) on cell proliferation, migration and invasion into foetal rat brain tissue. EGF increased tumour spheroid volume in 10 of 13 glioblastomas studied, whereas 5 of 11 tumours responded to bFGF. NGF increased the spheroid volume in 2 of 5 tumours. In 8 tumours, PDGF(bb) had no effect on tumour spheroid volume. An increase in BUdR-labelling indices confirmed that cell proliferation was responsible for the volume increase observed in stimulated spheroids. EGF stimulated cell migration in 5 and bFGF in 3 of 8 tumours studied. NGF stimulated cell migration in 1 of 5 glioblastomas, whereas 1 of 3 glioblastomas responded to PDGF(bb). The effects of growth factors on the invasion of spheroids prepared from the glioblastoma biopsy specimens were also studied in vitro using foetal rat brain aggregates as target tissue. EGF stimulated invasion in 7 of 8 glioblastomas studied, whereas bFGF stimulated invasion in 2 of these tumours. NGF or PDGF(bb) did not increase the invasiveness of the glioblastoma tissue. Our results represent the net effect of the growth factors on a complex tumour-cell population. We conclude that exogenously administered growth factors, EGF in particular, increase the cell proliferation as well as migratory and invasive capacities of cultured primary brain tumour biopsies in vitro.

Growth factor modulation of surface ganglioside expression in cloned neoplastic glia

Cloned neoplastic astrocytes from a human glioma-derived cell line (IPSB-18) were grown in fetal calf serum (FCS)-supplemented culture medium in the presence of three growth factors. Basic fibroblast growth factor (bFGF), epidermal growth factor (EGF) but not platelet-derived growth factor (PDGF) induced an increase in the number of cells positive for the ganglioside-recognizing monoclonal antibody, A2B5. No such growth factor-mediated induction could be detected in cells maintained in plasma-derived serum (PDS)-supplemented medium. Small molecules, removed from PDS during dialysis, may, therefore, act synergistically with growth factors in the control of ganglioside synthesis.

Therapeutic potential of protein kinase C inhibitors

The serine/threonine protein kinase, protein kinase C (PKC) is a family of closely related isoforms which are physiologically activated by diacylglycerol generated by the binding of a variety of agonists to their cellular receptors. Free fatty acids may also play a role in activating PKC. The enzyme apparently mediates a wide range of signal transduction processes in cells and, therefore, inhibitors directed selectively against PKC may have wide-ranging therapeutic potential. This review highlights the evidence that inappropriate activation of PKC occurs in a number of disease states. Such evidence, however, is often seriously flawed because it relies on the use of phorbol esters, which are potent and direct PKC activators but may not mimic the physiological triggering of the enzyme in cells, or on the use of non-selective protein kinase inhibitors such as H7 and staurosporine. A new generation of bis-indolylmaleimides, derived from the lead provided by staurosporine, shows a high degree of selectivity for PKC over closely related protein kinases and such agents may provide more appropriate tools to investigate the role of PKC in cellular processes.

Growth retardation in glioma cells cocultured with cells overexpressing a gap junction protein

To examine the role of gap-junctional intercellular communication in controlling cell proliferation, we have transfected C6 glioma cells with connexin 43 cDNA. The growth of transfected clones was dramatically reduced compared with nontransfected glioma cells. To further characterize the role of gap junctions in controlling proliferation, we have examined the growth of C6 cells cocultured with transfected cells overexpressing connexin 43. Although C6 cells grew at their normal rate when cocultured with nontransfected C6 cells, when cocultured with connexin 43-overexpressing cells they displayed a dramatic reduction in growth rate. Furthermore, a significant, dose-dependent reduction in cell proliferation was noted when C6 cells were cultured in medium conditioned by transfected cells. This effect correlated with the level of connexin 43 expression. These results suggest that the decreased cell proliferation rate of transfected cells and C6 cells cultured with them is due to the secretion of a growth inhibitory factor(s) and that the secretion of this factor may be linked to the level of gap junctional intercellular communication.

Glioma heterogeneity in vitro: the significance of growth factors and gangliosides

Despite renewed attempts by the WHO at updating the system of classification for brain tumours, most of the dynamic biological processes which underlie both the morphological appearances which form the basis for such systems and the malignant potential of gliomas remain an enigma to the neuropathologist. One feature recognized in human gliomas is their phenotypic and genotypic heterogeneity. Such cellular heterogeneity seen in the histological section is retained in vitro, at least during early passage. It is proposed that this heterogeneity is important in the growth and maintenance of the tumour and may be related to the activity of growth factors and gangliosides. Such molecules may not only influence the histoarchitecture of glial neoplasms but may also determine malignant progression and invasive potential. Moreover, there may be an intimate relationship between growth factors and gangliosides constituting an intricate feedback mechanism upon which the biological progression of gliomas depends.

Protein kinase C activity correlates with the growth rate of malignant gliomas: Part II. Effects of glioma mitogens and modulators of protein kinase C

The proliferation rates of gliomas may be modulated by the protein kinase C (PKC) signal transduction system. The present study was undertaken to further examine the role of PKC system in growth regulation of gliomas in vitro by measurement of PKC activity over various phases of tumor growth and by assessing its potential role as a signal transduction system induced by serum mitogens and the known glioma mitogens epidermal growth factor and fibroblast growth factor. All human glioma lines examined, and the rat glioma C6, displayed high PKC activity relative to nonmalignant glial cells, which correlated with their proliferation rates over their respective growth phase. Frozen surgical human malignant glioma specimens also displayed high PKC activity. The relatively selective PKC inhibitor staurosporine (SP) reduced PKC activity and corresponding growth rates in a dose-related manner. Stimulation of PKC with phorbol esters under different concentrations of serum in the growth medium indicated that the high PKC activity, which correlated with their rapid growth rates, is highly susceptible to down-regulation by these agents. Epidermal growth factor and fibroblast growth factor increased both PKC activity and the growth rate of glioma line A172; addition of SP reduced the growth rate to levels observed in SP-treated control tumors, indicating that PKC may be a common signal transduction system induced by these mitogens. These results implicate PKC as an important signal transduction system regulating glioma growth, and offers a potential target for tumor inhibition.

IGF-II mRNA expression in LI human glioblastoma cell line parallels cell growth

A human glioblastoma cell line was found to express in vitro mRNA transcripts specific for insulin-like growth factor-II (IGF-II) and growth-hormone releasing-hormone (GHRH). In the absence of gross morphological changes, retinoic acid reduced the growth rate without major change of IGF-II mRNA expression, while alpha-difluoromethylornithine produced a complete growth arrest and a sharp decrease of IGF-II mRNA expression. Both reagents increased the expression of GHRH mRNA. Also in this glioblastoma cell line, like other neuroectodermal tumours, IGF-II mRNA is expressed independently from GHRH and seems to be parallel to growth rate.

Biology and treatment of gliomas

This review discusses some of the recent advances in glioma research and treatment. Our understanding of the characteristics of these tumors has been strengthened by the application of molecular biologic and genetic techniques to pathologic grading and therapy outcome. Newer attempts to correlate imaging modalities to pathologic grading are also discussed. It is anticipated that these developments will strengthen our ability to design improved treatment strategies, an essential goal inasmuch as current treatment schemes have limited benefit. More work needs to be done to understand the biology of these tumors especially the complex interactions of their cytokine expression, multiplicity of genetic abnormalities, and their local environment. Only then will be able to develop improved therapeutic interventions.