Thyroid stimulating hormone increases angiogenic growth factor expression in rat thyrocytes

INTRODUCTION: Goitrogenesis is thought to be due to hyperstimulation of the thyroid stimulating hormone (TSH) receptor and is associated with dramatic andiogenesis within the thyroid. The angiogenic growth factors, fibroblast growth factor (FGF-2) and its receptor (FGFR-1) as well as Tie-2 (the receptor for the angiopoietins) are elevated in both the human goitre and in rat goitre. We have investigated the role of TSH in the expression of FGF-2, FGFR-1 and Tie-2 in FRTL-5 cells, which are a functional rat thyroid cell line. METHODS: Cells were cultured in medium containing 5% serum, insulin and antibodies and exposed to a variety of experimental conditions. Proteins were separated by electrophoresis and analysed by Western blotting using polyclonal antibodies to FGFR-1 (extracellular domain, Santa Cruz), Tie-2 (Santa Cruz) and FGF-2 (Sigma). RESULTS: TSH increased expression of FGF-2 reaching a maximum at 0.3 mU/ml returning to control levels at higher concentrations. A dominant fragment of its receptor (FGFR-1) at >> 57 kDa was dose-dependently increased by TSH. Tie-2 was thought to be predominantly expressed on endothelial cells but we now show that it is expressed as a full-length receptor on FRTL-5 cells with maximal expression at 0.3 mU/ml. This regulation is mediated by cAMP as forskolin and 8-Bromo-cAMP also increased Tie-2 expression. CONCLUSION: Both FGF-2 and its receptor, as well as Tie-2 are increased in rat thyroid cells by elevated TSH. These mechanisms may be central to the angiogenesis that occurs in goitrogenesis, and alteration of these growth factors may lead to new treatments for goitre.