Fibroblast growth factor 8 induced downregulation of thrombospondin 1 is mediated by the MEK/ERK and PI3K pathways in breast cancer cells.

Expression of fibroblast growth factor 8 (FGF-8) is increased in several forms of hormonal cancer. It was previously shown to regulate expression of thrombospondin 1 (TSP-1), an inhibitor of angiogenesis, in S115 breast cancer cells. Here, we studied the FGF-8-activated signalling pathways mediating TSP-1 repression in S115 cells and in non-tumorigenic MCF10A cells. Inhibition of FGF receptors or of MEK1/2 and PI3K with specific inhibitors (PD173074, U0126 or LY294002, respectively) restored TSP-1 mRNA expression in the presence of FGF-8 in S115 cells. Furthermore, U0126 and LY294002 increased TSP-1 mRNA expression in S115 cells over-expressing FGF-8. In MCF10A cells, FGF-8 treatment also decreased TSP-1 expression and the effect was dependent on active MEK1/2. In conclusion, FGF-8 suppresses TSP-1 expression through two independent pathways, MEK1/2 and PI3K. Repression of TSP-1 may be an important mechanism involved in induction of an angiogenic phenotype and growth of FGF-8-expressing breast cancer.