PTTG promotes a novel VEGF‐KDR‐ID3 autocrine mitogenic pathway in thyroid cancer

Abstract

Background. VEGF exerts its effects by binding to tyrosine kinase receptors, KDR and VEGFR1. KDR is critical for transmitting signals for proliferation of endothelial cells but is also expressed in several non‐endothelial cells suggesting existence of autocrine stimulatory pathways. Study‐design. We investigated VEGF and KDR expression in human thyroid epithelial cells in vitro and thyroid cancers samples ex vivo. Results. Expression of KDR was demonstrated using Taqman RT‐PCR and Western blotting in a human follicular thyroid cancer line (FTC133) and primary thyroid epithelial cells. Further, VEGF‐specific and dose‐dependent activation of KDR‐dependent MAPK signalling and KDR‐dependent mitogenesis was demonstrated. KDR mRNA expression was elevated in thyroid cancers (2.5‐fold, n = 38, P < 0.001), and immunohistochemistry demonstrated stronger phosphorylated‐KDR staining in thyroid cancer cells. Further, we showed that PTTG, an oncogene known to promote angiogenesis, up‐regulates KDR (2.2‐fold, P = 0.006) and VEGF (2.4‐fold, P < 0.001) expression in FTC133 thyroid cells. Next, we examined expression of ID3, known to be important in VEGF‐dependent angiogenesis, and VEGF mRNA expression in a series of thyroid cancers. We observed a strong positive correlation between expression of these genes (R2 = 0.62, P < 0.001). Stimulation of FTC133 cells with exogenous VEGF increased ID3 expression (2.1‐fold, P < 0.001), an effect abrogated by a KDR‐specific inhibitor, suggesting VEGF regulation of ID3 is KDR‐dependent. Conclusion. We suggest the presence of a VEGF‐KDR‐ID3 dependent autocrine pathway in thyroid cells. By up‐regulating both VEGF and KDR expression, we propose that PTTG may promote this autocrine proliferative pathway which may in turn be critical to thyroid cancer progression.