The transcription factor EGR-1 is a potential regulator of over 30 genes and plays a role in growth, development, and differentiation and, in addition, has significant transformation suppression activity. The regulatory properties are reviewed and a hypothesis for the transformation suppression activity is proposed. EGR-1 contains three "zinc-finger" motifs in the C-terminal portion of the molecule that constitute the DNA-binding domain and interact with the promoters by virtue of two classes of GC-rich elements: single GC-elements (GCEs) with the consensus 5'-T-G-C-G-T/g-G/A-G-G-C/a/t-G-G/T-3' and overlapping sites consisting of an Sp-1 binding site and the GCE consensus or close homolog of these sequences. The Wilm's tumor suppressor gene product WT1 interacts with the same GCE and. owing in part to four alternate splice products, interacts with a broader range of GC-rich elements with the consensus 5'-GNGNGGGNG-3' and 5'-TCCTCCTCCTCCTC-3'. WTI commonly but not invariably acts as repressor of transcription, whereas EGR-1, in the absence of overlapping Sp-1 binding sequences, is often an activator. The well-known rapid response of the EGR-1 gene following mitogenic stimulation together with the occurrence of GCEs in the promoters of many growth factors and protooncogenes suggests a role of EGR-1 in growth. Moreover, EGR-1 is constitutively expressed in several viral-transformed systems. On the other hand, studies of model and human tumor lines reveal that EGR-1 has significant growth and transformation suppression roles. Recent studies show that this effect can be accounted for by the ability of EGR-1 to induce the expression and secretion of TGF-β1, a potent growth suppressor of many cell types, by binding to a single GCE of the TGF-β1 promoter. Although the effects of EGR-1 at overlapping Spl/EGR-1 DNA binding sites are not predictable, known cases fall into two loose groups. Sp1 is usually activating and increasing concentrations of EGR-1 lead to displacement that results in either inhibition of transactivation or EGR-1 -dependent transactivation. Moreover, recent studies suggest that displaced Spl binds to and activates the endogenous Egr-1 gene, thereby leading to "facilitated inhibition" of Spl function by the resulting increased EGR-1. This effect may augment the growth suppressive function of EGR-1 based on induction of TGF-βl.

In the present study we evaluated the expression of the intermediate conductance calcium-activated potassium (KCa3.1) channel in human glioblastoma stem-like cells (CSCs) and investigated its role in cell motility. While the KCa3.1 channel is not expressed in neuronal- and glial-derived tissues of healthy individuals, both the KCa3.1 mRNA and protein are present in the glioblastoma tumor population, and are significantly enhanced in CSCs derived from both established cell line U87MG and a primary cell line, FCN9. Consistent with these data, voltage-independent and TRAM-34 sensitive potassium currents imputable to the KCa3.1 channel were recorded in the murine GL261 cell line and several primary human glioblastoma cells lines. Moreover, a significantly higher KCa3.1 current was recorded in U87MG-CD133 positive cells as compared to the U87MG-CD133 negative subpopulation. Further, we found that the tumor cell motility is strongly associated with KCa3.1 channel expression. Blockade of the KCa3.1 channel with the specific inhibitor TRAM-34 has in fact a greater impact on the motility of CSCs (reduction of 75%), which express a high level of KCa3.1 channel, than on the FCN9 parental population (reduction of 32%), where the KCa3.1 channel is expressed at lower level. Similar results were also observed with the CSCs derived from U87MG. Because invasion of surrounding tissues is one of the main causes of treatment failure in glioblastoma, these findings can be relevant for future development of novel cancer therapeutic drugs.

Glial cell proliferation in culture is under the control of metabotropic glutamate (mGlu) receptors. We have examined whether this control extends to human glioma cells. Primary cultures were prepared from surgically removed human glioblastomas. RT‐PCR combined with western blot analysis showed that most of the cultures (eight out of 11) expressed group‐II mGlu receptors. In two selected cultures (MZC‐12 and FCN‐9), the mGlu2/3 receptor antagonist, LY341495, slowed cell proliferation when applied to the growth medium from the second day after plating. This effect was reversible because linear cell growth was restored after washing out the drug. LY341495 reduced glioma cell proliferation at concentrations lower than 100 nm, which are considered as selective for mGlu2/3 receptors. In addition, its action was mimicked by the putative mGlu2/3 receptor antagonist (2S)‐α‐ethylglutamate. The anti‐proliferative effect of LY341495 was confirmed by measuring [methyl‐³H]‐thymidine incorporation in cultures arrested in G₀ phase of the cell cycle and then stimulated to proliferate by the addition of 10% fetal calf serum or 100 ng/mL of epidermal growth factor (EGF). In cultures treated with EGF, LY341495 was also able to reduce the stimulation of the mitogen‐activated protein kinase (MAPK) pathway, as well as the induction of cyclin D1. Both effects, as well as decreased [methyl‐³H]‐thymidine incorporation, were partially reduced by co‐addition of the potent mGlu2/3 receptor agonist, LY379268. We conclude that activation of group‐II mGlu receptors supports the growth of human glioma cells in culture and that antagonists of these receptors should be tested for their ability to reduce tumour growth in vivo.