Interleukin‐6 induces transcriptional activation of vascular endothelial growth factor (VEGF) in astrocytes in vivo and regulates VEGF promoter activity in glioblastoma cells via direct interaction between STAT3 and Sp1

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Abstract
Interleukin‐6 (IL‐6) expression is strongly correlated with the degree of human glioma malignancy and necessary for tumor formation in a mouse model of spontaneous astrocytomas. Yet, exactly how IL‐6 contributes to malignant progression of these brain tumors is still unclear. We have scrutinized the mechanism of transcriptional activation of vascular endothelial growth factor (VEGF) expression by IL‐6 in the mouse brain and in glioblastoma cells. We demonstrate here that IL‐6 drives transcriptional upregulation of VEGF in astrocytes in vivo using glial fibrillary acidic protein (GFAP)‐IL‐6/VEGF‐green fluorescent protein (GFP) double transgenic mice. We further show that IL‐6‐induced VEGF transcription and VEGF secretion by human glioblastoma cells is dependent on signal transducer and activator of transcription 3 (STAT3). By progressive 5′‐deletion analysis we defined the minimal VEGF promoter region for IL‐6‐responsiveness to nucleotides −88/−50. Surprisingly, this promoter region is rich in GC‐boxes and does not contain STAT3 binding elements. Electrophoretic mobility shift and supershift assays revealed binding of Sp1 and Sp3 to the −88/−50 element upon IL‐6 stimulation. Interestingly, preincubation with STAT3 antibody prevented the binding of Sp1 and Sp3 to the −88/−50 element, indicating that STAT3 is involved in IL‐6‐driven Sp1/Sp3 protein‐DNA complex formation. Physical interaction of STAT3 and Sp1 was demonstrated by coimmunoprecipitation. The functional relevance of the STAT3/Sp1 association was corroborated by transient transfection experiments, which showed that overexpression of constitutively active STAT3 increased the minimal VEGF promoter activity. Taken together, our study suggests that IL‐6 promotes tumor angiogenesis in gliomas and describes a novel transcriptional activation mechanism for STAT3 in the context of a STAT3 binding element (SBE)‐free promoter.