IGFBP3 inhibits angiogenesis through intracellular regulation of THBS1 expression

Insulin-like growth factor binding protein-3 (IGFBP3) has been postulated to be a mediator of growth suppression signaling. It was shown to function as a suppressor of invasion in epithelial ovarian cancer (EOC). In this study, we identified an angiogenesis inhibitor, thrombospondin-1 (THBS1), which correlated with IGFBP3 expression in EOC cells. After restoring IGFBP3 expression in an EOC cell line using an inducible plasmid, the transfectants showed an increase in IGFBP3 associated with a parallel increase in THBS1. IGFBP3 decreased cell capillary tube formation in HUVECs, which was reversed after anti-THBS1 treatment. IGFBP3 also decreased blood vessel development in chick embryo chorioallantoic membrane (CAM) assay, which was reversed after THBS1 silencing using THBS1 siRNA. Heterotransplantation of IGFBP3 transfectants significantly decreased tumor growth and vascular formation. Luciferase promoter assay illustrated that THBS1 promoter was activated in the presence of both intracellular and extracellular IGFBP3. The signal was stronger in intracellular IGFBP3 expression than that in extracellular IGFBP3 neutralization. In conclusion, we have identified a novel association between IGFBP3 expression and THBS1 elevation, which consequently results in a decrease in angiogenesis. IGFBP3 could activate THBS1 through promoter regulation mainly via an intracellular signaling pathway. Such angiogenesis-regulating ability could be associated with tumor progression and may represent a major function of IGFBP3 as an onco-suppressor in the pathogenesis of ovarian cancer.