Tie-2 Is Expressed on Thyroid Follicular Cells, Is Increased in Goiter, and Is Regulated by Thyrotropin through Cyclic Adenosine 3*,5*-Monophosphate

Angiogenesis is coordinated with follicular cell growth in goitrogenesis. The angiopoietins, Ang-1 and Ang-2, are angiogenic growth factors acting through Tie-2, a tyrosine kinase receptor. We have examined the expression and regulation of the angiopoietins and Tie-2 in human and rat thyroids. In human goiters there was increased Tie-2 immunostaining, compared with that in normal thyroids, on both follicular and endothelial cells. In an induced goiter in rats, in situ hybridization showed increased expression of messenger ribonucleic acids (mRNAs) for Tie-2 and Ang-1 in follicular cells. As Tie-2 has previously been believed to be restricted to cells of endothelial lineage in adults, we examined its expression further in isolated follicular cells. Tie-2 and Ang-1 mRNA expression in human thyrocytes was confirmed by ribonuclease protection assay. Ang-2 mRNA was not detected in human cultures or rat thyroids. In both human follicular cell cultures and FRTL-5 cells, immunoblotting showed that Tie-2 expression was increased by TSH and agents that increased intracellular cAMP. In conclusion, we have demonstrated the expression of Tie-2 and Ang-1 in thyroid epithelial and endothelial cells, and have shown the regulation of Tie-2 by TSH and cAMP in follicular cells. Tie-2 expression is increased in goiter in both humans and rats, consistent with a role in goitrogenesis.