Endocrine Abstracts

The tyrosine kinase receptor, Tie-2, is expressed on endothelial cells where it has a role in angiogenesis in physiological and tumorigenic processes. We have shown that Tie-2 is also expressed on differentiated human thyroid follicular cells, and that its expression is increased in hyperplasia. This study aimed to elucidate the role of the Tie-2 axis in thyroid cells by examining regulation of Tie-2 expression in primary cultures of human thyroid follicular cells and the differentiated human follicular thyroid carcinoma cell line, FTC-133. The ligands for this receptor are the Angiopoietins (Ang-1 and -2), whose presence in both cell lines was confirmed by RT-PCR. In both cell lines, Tie-2 expression was increased by elevations in intracellular cAMP, as demonstrated by Western blotting. TSH stimulated Tie-2 expression in normal thyroid cells, but not in FTC-133 cells. This may be related to FTC-133 cells' dysfunctional TSH receptor. Ang-2 secretion in both cell lines was examined by ELISA. In contrast to Tie-2, Ang-2 expression was not regulated by cAMP. Incubation with TPA, an activator of protein kinase C, for 72 hours increased Ang-2 secretion in primary thyroid follicular cells from 199.5 to 815.0 picograms per millilitre (p<0.0001), but reduced levels of secreted Ang-2 from 223.5 to 162.0 picograms per millilitre (p<0.05) in FTC-133 cells. Forskolin, the phosphodiesterase inhibitor IBMX, and TSH did not alter secreted Ang-2 levels in either cell line. We conclude that Tie-2 and its ligands are regulated by different signalling pathways in human thyroid follicular cells. The regulation by TSH of Tie-2 in human thyroid follicular cells, but not of Ang-1 and -2, suggests that Tie-2 has an autocrine role. The regulation of Angiopoietin production by growth factors suggests that subsequent to a goitrogenic stimulus mediated by growth factors, angiogenic growth factor production will be increased, thus allowing increased tissue growth.