Within the basolateral membrane of thyroid follicular epithelial cells, two transporter proteins are central to thyroid hormone (TH) biosynthesis and secretion. The sodium iodide symporter (NIS) delivers iodide from the bloodstream into the thyroid, and after TH biosynthesis, monocarboxylate transporter 8 (MCT8) mediates TH secretion from the thyroid gland. Pituitary tumor-transforming gene-binding factor (PBF; PTTG1IP) is a protooncogene that is up-regulated in thyroid cancer and that binds

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... and modulates its subcellular localization and function. We now show that PBF binds MCT8 in vitro, eliciting a marked shift in MCT8 subcellular localization and resulting in a significant reduction in the amount of MCT8 at the plasma membrane as determined by cell surface biotinylation assays. Colocalization and interaction between PBF and Mct8 was also observed in vivo in a mouse model of thyroid-specific PBF overexpression driven by a bovine thyroglobulin (Tg) promoter (PBF-Tg). Thyroidal Mct8 mRNA and protein expression levels were similar to wild-type mice. Critically, however, PBF-Tg mice demonstrated significantly enhanced thyroidal TH accumulation and reduced TH secretion upon TSH stimulation. Importantly, Mct8-knockout mice share this phenotype. These data show that PBF binds and alters the subcellular localization of MCT8 in vitro, with PBF overexpression leading to an accumulation of TH within the thyroid in vivo. Overall, these studies identify PBF as the first protein to interact with the critical TH transporter MCT8 and modulate its function in vivo. Furthermore, alongside NIS repression, PBF may thus represent a new regulator of TH biosynthesis and secretion. (Endocrinology 153: 3526 -3536, 2012) Endocrinology, July 2012, 153(7):3526 -3536 endo.endojournals.org 3528 Smith et al. Regulation of MCT8 by PBF Endocrinology, July 2012, 153(7):3526 -3536 FIG. 2. MCT8 and PBF localization in COS-7 cells. A, Confocal images of MCT8-HA after cotransfection with a VO control into COS-7 cells, and endogenous PBF, both detected by immunofluorescent analysis. MCT8-HA (green) was located predominantly within the plasma membrane, whereas endogenous PBF (red) was found in the nucleus and within intracellular vesicles. B, Representative fluorescent immunocytochemistry experiments examining staining of PBF (red) and MCT8-HA (green) after transient cotransfection into COS-7 cells using both epifluorescent (i) and confocal (ii) microscopy. PBF was predominantly expressed within cytoplasmic vesicles. MCT8-HA demonstrated increased staining within intracellular vesicles where it strongly colocalized with PBF, as seen in yellow in the merged images. Scale bars, 20 m. 3530 Smith et al. Regulation of MCT8 by PBF Endocrinology, July 2012, 153(7):3526 -3536 3534 Smith et al. Regulation of MCT8 by PBF Endocrinology, July 2012, 153(7):3526 -3536 Downloaded from https://academic.oup.com/endo/article-abstract/153/7/3526/2424142 by guest on 29 July 2018 http://www.endo-society.org/translational 3536 Smith et al. Regulation of MCT8 by PBF Endocrinology, July 2012, 153(7):3526 -3536