Abstract
We reported previously that microglia decreased the growth of human brain tumor-initiating cells (BTICs). Through microarray analyses of BTICs exposed in vitro to microglia, we found the induction of several genes ascribed to have roles in cell cycle arrest, reduced cell proliferation and differentiation. Herein, we tested the hypothesis that one of these genes, growth arrest specific 1 (Gas1), is a novel growth reduction factor that is induced in BTICs by microglia. We found that microglia increased the expression of Gas1 transcript and protein in glioblastoma patient-derived BTIC lines. Using neurosphere assay we show that RNAi-induced reduction of Gas1 expression in BTICs blunted the microglia-mediated BTIC growth reduction. The role of Gas1 in mediating BTIC growth arrest was further validated using orthotopic brain xenografts in mice. When microglia-induced Gas1-expressing BTIC cells (mGas1-BTICs) were implanted intra-cranially in mice, tumor growth was markedly decreased; this was mirrored in the remarkable increase in survival of mGas1-BT025 and mGas1-BT048 implanted mice, compared to mice implanted with non-microglia-exposed BTIC cells. In conclusion, this study has identified Gas1 as a novel factor and mechanism through which microglia arrest the growth of BTICs for anti-tumor property.
Highlights
Gliomas are the most common primary brain tumors in adults
We have previously shown that stimulated microglia-conditioned medium (MCM) reduces the formation of brain tumor-initiating cells (BTICs) spheres; in correspondence, genes with ascribed roles in cell cycle arrest, proliferation, and differentiation in BTICs such as THBS1, GADD45B, IHNBA, BCL6, SOD223 are induced by MCM
We focused on GAS1 and validated its up-regulation in patient-derived BTIC cells (BT025 and BT048 lines) using Quantitative real-time PCR (qPCR) after exposure to MCM for 6 h (Fig. 1A)
Summary
Gliomas are the most common primary brain tumors in adults. The most malignant form, glioblastoma, is refractory to current therapies and is associated with a median survival of only 14.6 months despite surgery and chemoradiotherapy[1]. We have previously shown that stimulated microglia-conditioned medium (MCM) reduces the formation of BTIC spheres; in correspondence, genes with ascribed roles in cell cycle arrest, proliferation, and differentiation in BTICs such as THBS1, GADD45B, IHNBA, BCL6, SOD223 are induced by MCM. In these published microarrays[23], the expression of growth arrest specific 1 (Gas1), a putative tumor suppressor gene associated with blockade of the G0-to-S phase transition[30], was undetectable across all specimens. As Gas[1] affects growth suppression in several cancers[31,32,33], and since it has been shown that Gas[1] induces cell cycle arrest and apoptosis in conventional glioma cell lines in vitro and in vivo[34,35,36], we tested the novel hypothesis that microglia elevate Gas[1] expression in BTICs to reduce their growth
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