SUN-LB129 Thyroglobulin Stimulates the Secretion of Thyroxine From Thyroid Epithelial Cells in Suspension Culture via Nuclear Factor-κB Signaling Pathway

The nuclear factor (NF)-κB signaling pathway controls a variety of biological functions such as cell growth and differentiation as well as immune and inflammatory responses. Two distinct pathways of NF-κB activation are known. NF-κB signaling via non-canonical pathway plays an important role in maintaining normal thyroid function, namely, thyroid cells survival and expression of thyroid-specific proteins, such as NIS, TPO and thyroglobulin (Tg). The primary function of thyroid cells is the production of thyroid hormones, which are cleaved out by various processing enzymes from the modified tyrosine residues of the Tg molecule. Although Tg is currently recognized as no more than a scaffold protein for thyroid hormone synthesis, our group has shown that Tg stimulates the proliferation of rat thyroid follicular FRTL-5 cells via PI3K pathway. The signaling pathways underlying various functions of thyroid cells have been extensively investigated but some discrepancies still exist, depending on the experimental model systems and the types of cells used. Recently, our group discovered that Tg may directly regulate the synthesis of thyroid hormones in thyroid cells. In this study, we performed suspension culture of porcine thyroid cells and aimed to elucidate the mechanism of the effect of Tg on thyroid cells. Porcine thyroid epithelial cells were prepared from thyroid gland by enzymatic digestion and cultured in monolayer or in suspension culture. Various concentrations of Tg were added to the medium and cultured for several days. Then, the conditioned medium was collected and the amount of thyroid hormone was measured by reversed phase chromatography; the cells were harvested and mRNA levels of hormone-processing enzymes, cathepsin B, K, L and dipeptidyl peptidase I, II (DPP I, II), were measured by quantitative PCR. The amount and activation of cell signaling molecules were also evaluated by western blotting. Tg stimulated the secretion of thyroid hormone, thyroxine, from porcine thyroid cells in suspension culture, however, no stimulation by Tg was observed in monolayer culture. The expression of mRNAs of cathepsin B, K and L were reduced and that of DPP II was increased by the treatment of Tg in suspension culture. Tg increased the amount of NF-κB p52 and Rel B dose dependently. On the other hand, TSH had little effects on the amount of these proteins. Tg and TSH both activated PDK1 and MAPK p44/42 but not Akt. These results suggest that Tg regulates thyroid hormone synthesis in totally different way from TSH by altering the expression levels of hormone-processing enzymes in thyroid cells. This stimulatory effect might be mediated by NF-κB signaling pathway, whereas the proliferative effect of Tg under these conditions might be exerted via MAPK signaling pathway. The detailed mechanisms of these effects of Tg on thyroid cells are under investigation.