Regulation of vascular endothelial growth factor expression by insulin-like growth factor I in thyroid carcinomas

Abstract

Vascular endothelial growth factor (VEGF) produced by tumor cells potently stimulates endothelial cell proliferation and angiogenesis and plays a key role in the pathophysiology of several neoplasias. Hypoxia activates the VEGF promoter via response elements that bind the transcription factors hypoxia-inducible factor-1 alpha (HIF-1 alpha) and activator protein-1 (AP-1). Yet, the paracrine signaling pathways regulating VEGF production and angiogenesis in thyroid cancer have not been fully elucidated. In this study, we, therefore, investigated the regulation of VEGF production by the thyroid carcinoma cell line SW579. We found that IGF-I up-regulated VEGF mRNA expression and protein secretion. Furthermore, transfection of SW579 cells with vector expressing a constitutively active form of Akt, a major mediator of IGF-I signaling, also stimulated VEGF expression. The IGF-I-induced up-regulation of VEGF production was associated with activation of AP-1 and HIF-1 alpha and was abrogated by phosphatidylinositol 3-kinase inhibitors (wortmannin and LY294002); Jun kinase inhibitor (SP600125); HIF-1 alpha antisense oligonucleotide; or geldanamycin, an inhibitor of the heat shock protein 90 molecular chaperone, which regulates the three-dimensional conformation and function of IGF-I-receptor and Akt. These data indicate that IGF-I stimulates VEGF synthesis in thyroid carcinomas in an Akt-dependent pathway via AP-1 and HIF-1 alpha and provide the framework for clinical use of small-molecule inhibitors, including geldanamycin analogs, to abrogate proangiogenic cascades in thyroid cancer.