Silencing hypoxia-inducible factor-1α inhibits cell migration and invasion under hypoxic environment in malignant gliomas

alignant gliomas are characterized by active invasiveness, necrosis, and vascular proliferation. These pathological features have been speculated to be caused by tissue hypoxia. Hypoxia-inducible factor-1 (HIF-1), which is controlled by rapid stabilization of the HIF-1α subunit, is a pivotal transcriptional factor in the cellular response to hypoxia. Although many studies have described the relationship between tumor angiogenesis and hypoxic environment, the roles of HIF-1 in cell invasion have been barely elucidated in malignant gliomas. We investigated the role of HIF-1α in the motile and invasive activities of human glioma cells under hypoxia. Four malignant glioma cell lines, U87MG, U251MG, U373MG, and LN18, were cultured under 21 and 1% oxygen concentration. Expression of HIF-1α under hypoxia was observed to be much higher than that under normoxia in all cell lines. Introducing HIF-1α-targeted small interfering RNA (HIF-1α siRNA) into the glioma cell lines resulted in downregulation of HIF-1α expression, and significantly suppressed glioma cell migration in vitro. Furthermore, invasiveness was significantly reduced in the cells transfected with HIF-1α siRNA compared with those transfected with the control siRNA. Co-culture of glioma spheroids and rat brain slices showed that HIF-1α siRNA-transfected glioma cells failed to invade the surrounding normal brain tissue in an organotypic brain slice model. These effects of HIF-1α siRNA were more conspicuous under hypoxia than under normoxia. In addition, under hypoxic conditions, the level of matrix metalloproteinase (MMP)-2 mRNA was upregulated, and that of tissue inhibitor of metalloproteinase (TIMP)-2 was downregulated in all glioma cell lines. Treatment with HIF-1α siRNA resulted in downregulation of MMP-2 mRNA and upregulation of TIMP-2 mRNA. Furthermore, the enzyme activities of MMP-2 and MMP-9, both of which were activated by hypoxia, decreased with the introduction of HIF-1α siRNA. These findings suggest that overexpression of HIF-1α induced by hypoxic stress is an essential event in the activation of glioma cell motility through alteration of invasion-related molecules. Targeting the HIF-1α molecule may be a novel therapeutic strategy for malignant gliomas.