Hypoxia and hypoxia-inducible factor-1 (HIF-1) play a critical role in glioblastoma multiforme (GBMs). CXCR4 is involved in angiogenesis and is upregulated by HIF-1alpha. CXCR4 is a chemokine receptor for stromal cell-derived factor-1 (SDF-1)alpha, also known as CXCL12. We hypothesized that CXCR4 would be upregulated by hypoxia in GBMs. First, we investigated the expression of HIF-1alpha and CXCR4 in GBMs. CXCR4 was consistently found colocalized with HIF-1alpha expression in pseudopalisading glioma cells around areas of necrosis. In addition, angiogenic tumor vessels were strongly positive for CXCR4. Next, we tested the in vitro effect of hypoxia and vascular endothelial growth factor (VEGF) on the expression of CXCR4 in glioma cell lines and in human brain microvascular endothelial cells (HBMECs). Exposure to hypoxia induced significant expression of CXCR4 and HIF-1alpha in glioma cells, whereas treatment with exogenous VEGF increased CXCR4 expression in HBMECs. We also transfected U87MG glioma cells with an HIF-1alpha construct and observed that CXCR4 was upregulated in these cells even in normoxic conditions. We then used a lentivirus-mediated shRNA expression vector directed against HIF-1alpha. When exposed to hypoxia, infected cells failed to show HIF-1alpha and CXCR4 upregulation. We performed migration assays under normoxic and hypoxic conditions in the presence or absence of AMD3100, a CXCR4 inhibitor. There was a significant increase in the migration of U87MG and LN308 glioma cells in hypoxic conditions, which was inhibited in the presence of AMD3100. These studies demonstrate the critical role played by hypoxia and CXCR4 in glioma cell migration. Based on these studies, we suggest that hypoxia regulates CXCR4 in GBMs at two levels. First, through HIF-1alpha in the pseudopalisading tumor cells themselves and, secondly, by the VEGF-stimulated angiogenic response in HBMECs. We believe this knowledge may lead to a potentially important two-pronged therapy against GBM progression using chemotherapy targeting CXCR4.
The genetic hallmark of hemangioblastomas and clear cellrenal cell carcinomas (CC-RCCs) is loss-of-function of the von Hippel-Lindau (VHL) tumor suppressor protein. VHL is required for oxygen-dependent degradation of hypoxiainducible factor-1A
Invasion into surrounding brain tissue is a fundamental feature of gliomas and the major reason for treatment failure. The process of brain invasion in gliomas is not well understood. Differences in gene expression and/or gene products between invading and noninvading glioma cells may identify potential targets for new therapies. To look for genes associated with glioma invasion, we first employed Affymetrix microarray Genechip s technology to identify genes differentially expressed in migrating glioma cells in vitro and in invading glioma cells in vivo using laser capture microdissection. We observed upregulation of a variety of genes, previously reported to be linked to glioma cell migration and invasion. Remarkably, major histocompatiblity complex (MHC) class I and II genes were significantly downregulated in migrating cells in vitro and in invading cells in vivo. Decreased MHC expression was confirmed in migrating glioma cells in vitro using RT-PCR and in invading glioma cells in vivo by immunohistochemical staining of human and murine glioblastomas for b2 microglobulin, a marker of MHC class I protein expression. To the best of our knowledge, this report is the first to describe the downregulation of MHC class I and II antigens in migrating and invading glioma cells, in vitro and in vivo, respectively. These results suggest that the very process of tumor invasion is associated with decreased expression of MHC antigens allowing glioma cells to invade the surrounding brain in a 'stealth'-like manner.
Angiogenesis is a critical step required for sustained tumor growth and tumor progression. The stimulation of endothelial cells by cytokines secreted by tumor cells such as vascular endothelial growth factor (VEGF) induces their proliferation and migration. This is a prominent feature of high-grade gliomas. The secretion of VEGF is greatly upregulated under conditions of hypoxia because of the transcription factor hypoxiainducible factor (HIF)-1alpha, which controls the expression of many genes, allowing rapid adaptation of cells to their hypoxic microenvironment. Flavopiridol, a novel cyclin-dependent kinase inhibitor, has been attributed with antiangiogenic properties in some cancer cell lines by its ability to inhibit VEGF production. Here, we show that flavopiridol treatment of human U87MG and T98G glioma cell lines decreases hypoxia-mediated HIF-1alpha expression, VEGF secretion, and tumor cell migration. These in vitro results correlate with reduced vascularity of intracranial syngeneic GL261 gliomas from animals treated with flavopiridol. In addition, we show that flavopiridol downregulates HIF-1alpha expression in the presence of a proteasome inhibitor, an agent that normally results in the accumulation and overexpression of HIF-1alpha. The potential to downregulate HIF-1alpha expression with flavopiridol treatment in combination with a proteasome inhibitor makes this an extremely attractive anticancer treatment strategy for tumors with high angiogenic activity, such as gliomas.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.