Expression of receptors for VEGFs on normal human thyroid follicular cells and their role in follicle formation.

Human thyroid follicular cells in culture expressed the mRNAs for the receptors for vascular endothelial growth factors (VEGFRs). The relative expression was neuropilin1 = neuropilin2 = VEGFR2 > VEGFR1 > VEGFR3. Western blotting for VEGFR2 showed labeling of proteins ~200-230 kDa. Clonal follicular thyroid cell lines (FRTL5 and FTC133) also expressed mRNAs for the VEGFR1 and 2 obviating concerns of endothelial cell contamination. In the primary cultures, TSH, which is essential for expression of differentiated function, reduced VEGFR2 mRNA levels by 60%. Immunostaining for VEGFRs and neuropilin2 (NRP2), showed expression on the plasma membrane but with the exception of neuropilin1 (NRP1), all VEGFRs were also found in the cytoplasm and nucleus. Antibody specific for phosphotyrosine 1214 in VEGFR2 showed that the receptor was phosphorylated in the primary cultures and the cell lines. When VEGFR signaling was blocked with a specific inhibitor, follicle formation in the primary cultures was enhanced suggesting that VEGFR activation was detrimental to follicle formation. Immunostaining of sections of normal thyroids and various pathologies showed staining for VEGFR2 and pVEGFR2. We conclude that normal thyroid follicular cell express VEGFRs. For VEGFR2 its subcellular localization suggests functions additional to that of a cell surface receptor and a role in follicular integrity.