Iodinated TG in Thyroid Follicular Lumen Regulates TTF-1 and PAX8 Expression via TSH/TSHR Signaling Pathway.

Our previous study showed that highly iodinated thyroglobulin (TG) inhibited thyroid transcription factor-1 (TTF-1) and paired box gene 8 (PAX8) expression, but the potential mechanism remains unclear. In this study, we constructed a thyroid follicle model in vitro to mimic its natural physiological structure and explored how iodinated TG in the follicular lumen tuned TTF-1 and PAX8 expression. Our data showed that lowly iodinated TG enhanced PKA activity while upregulation of both TTF-1 and PAX8 expression; and that highly iodinated TG triggered PKC activity while suppression of TTF-1 and PAX8 expression. Further, PKA agonist alone could increase TTF-1 and PAX8 expression while PKC agonist decreased TTF-1 and PAX8 level. If blocking PLC-PKC pathway using PKC-specific inhibitor, highly iodinated TG significantly promoted the expressions of TTF-1 and PAX8, and similarly PKA-specific blocker moderately inhibited TTF-1 and PAX8 expression. And opposite tendencies of TTF1 and PAX8 aberrant expression were observed in the condition of low iodinated TG when blocking PLC-PKC and cAMP-PKA signaling pathways. Our results indicated that iodinated TG manipulated TTF-1 and PAX8 expression through PLC-PKC and cAMP-PKA pathways, and highly iodinated TG played inhibitory role via PLC-PKC pathway from the TTF1 and PAX8 perspective while low level of iodinated TG was an activator through cAMP-PKA pathway. Our findings proved that iodinated TG in thyroid follicular lumen regulated TTF-1 and PAX8 expression through thyroid stimulating hormone/thyroid stimulating hormone receptor (TSH/TSHR) mediated cAMP-PKA and PLC-PKC signaling pathways. J. Cell. Biochem. 118: 3444-3451, 2017.