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Tesis doctoral.-- Universidad Autónoma de Madrid. Departamento de Bioquímica., TAZ is a mediator of the Hippo signaling pathway, which has emerged recently as an important regulator of tissue homeostasis. As a cofactor, TAZ does not directly bind DNA but needs to partner with other transcription factors to modulate its transcriptional output. TAZ has been described to interact with PAX8, a key transcription factor governing thyroid follicular differentiation and polarization. In this work, we demonstrated that TAZ functions as a co-repressor of PAX8 in the modulation of the sodium iodide symporter (NIS) gene by decreasing PAX8 nuclear localization and its binding to the NIS upstream enhancer. NIS is a fundamental protein for thyroid hormone synthesis and radioactive iodine therapy of thyroid carcinomas and their metastases. Our experimental analysis indicated that TAZ is a potent repressor of NIS expression and function. TAZ silencing can restore NIS downregulation by TGFβ, another well-known inhibitor of NIS. TGFβ induces elevated levels of the cofactor in the nucleus, thus reinforcing its impairment of NIS expression, although TAZ mediates independent repressor effects too. We also identified that TSH/cAMP, the main stimulator of thyroid follicular cells differentiation, triggers Hippo kinases activation, stimulating TAZ nuclear exit and its degradation in the cytoplasm. Hence, decreasing TAZ activity as a transcriptional co-factor is another mechanism by which TSH upregulates NIS expression. Moreover, this association between elevated serum TSH and decreased TAZ nuclear levels was confirmed in hypothyroidism mice. The Hippo pathway is frequently dysregulated in human cancer. We confirmed higher TAZ mRNA and protein levels in thyroid carcinoma-derived cell lines. Furthermore, we observed that TAZ localizes to the primary cilia. Mutations in the MAPK pathway constitute the principal trigger of thyroid carcinoma, and we characterized this axis to be responsible for TAZ upregulation. However, we did not find any association between increased TAZ expression and altered cell migration nor proliferation in thyroid carcinoma-derived cell lines. Papillary thyroid carcinoma patients from The Cancer Genome Atlas show low TAZ levels, but high expression of the cofactor is associated with poor survival. Furthermore, our preliminary results suggest that YAP/TAZ confer resistance to MAPK inhibition in thyroid carcinoma-derived cell lines. A combinatorial treatment impairing YAP/TAZ transcriptional activity could be an appealing therapeutic strategy to overcome the emergence of resistance to small kinase inhibitors, a major concern for the treatment of advanced thyroid carcinomas. Altogether our data illuminates a novel role for TAZ in thyroid differentiation, working as a integrator of the Hippo, TSH, and TGFβ signaling pathways. Considering that more aggressive thyroid carcinomas involve a dedifferentiation state, it would be interesting to approach TAZ silencing as a double strategy to reinduce NIS expression and avoid the emergence of resistance mechanisms to kinase inhibitors

© 2021 López-Márquez, Carrasco-López, Fernández-Méndez and Santisteban. Thyroid differentiation of progenitor cells occurs during embryonic development and in the adult thyroid gland, and the molecular bases of these complex and finely regulated processes are becoming ever more clear. In this Review, we describe the most recent advances in the study of transcription factors, signaling molecules and regulatory pathways controlling thyroid differentiation and development in the mammalian embryo. We also discuss the maintenance of the adult differentiated phenotype to ensure the biosynthesis of thyroid hormones. We will focus on endoderm-derived thyroid epithelial cells, which are responsible for the formation of the thyroid follicle, the functional unit of the thyroid gland. The use of animal models and pluripotent stem cells has greatly aided in providing clues to the complicated puzzle of thyroid development and function in adults. The so-called thyroid transcription factors – Nkx2-1, Foxe1, Pax8 and Hhex – were the first pieces of the puzzle identified in mice. Other transcription factors, either acting upstream of or directly with the thyroid transcription factors, were subsequently identified to, almost, complete the puzzle. Among them, the transcription factors Glis3, Sox9 and the cofactor of the Hippo pathway Taz, have emerged as important players in thyroid differentiation and development. The involvement of signaling molecules increases the complexity of the puzzle. In this context, the importance of Bmps, Fgfs and Shh signaling at the onset of development, and of TSH, IGF1 and TGFβ both at the end of terminal differentiation in embryos and in the adult thyroid, are well recognized. All of these aspects are covered herein. Thus, readers will be able to visualize the puzzle of thyroid differentiation with most – if not all – of the pieces in place. This work was supported by grants PID2019-105303RB-I00 from Ministerio de Ciencia e Innovación (MICIN), Spain, S2017/BMD-3724 from Comunidad de Madrid, and GCB14142311CRES from Fundación Española Contra el Cáncer (AECC).

Resumen del póster presentado al 6th Symposium on Biomedical Research: Advances and Perspectives in Molecular Endocrinology "In Homage to Gabriella Morreale", celebrado en el Instituto de Investigaciones Biomédicas Alberto Sols (IIBM-CSIC) el 31 de mayo de 2019., TAZ is a well-known coactivator downstream of the Hippo signaling pathway. Together with its paralogous YAP and the transcription factors of the TEAD family, TAZ plays a key role in the regulation of differentiation, among other cellular processes. Little is known about the involvement of this signaling pathway in the function of the thyroid gland, although TAZ has been described as a coactivator of Pax8 on the thyroglobulin promoter. Pax8 has a predominant role in thyroid differentiation by regulating the transcription of many crucial genes, such as sodium-iodide symporter (NIS). The aim of this work was to study the role of the Hippo pathway, and particularly its mediator TAZ, in NIS expression, and hence in thyroid differentiation. By western blot and immunofluorescence, we show TAZ nuclear expression is promoted by TGFβ signalling through its downstream mediator p38. We demonstrated that nuclear TAZ exerts a marked role as NIS repressor, decreasing its expression levels, affecting its presence in the celular membrane and thus impairing iodide uptake. By chromatin immunoprecipitation (ChIP) and luciferase reporter assays, we demonstrated that TAZ strongly suppresses Pax8 activation of the NIS promoter by decreasing its binding to the NIS Upstream Enhancer (NUE). In fact, the absence of TAZ directly increases Pax8 nuclear levels. RNAi assays of other Hippo effectors were performed in order to check their participation in this process. Although YAP and TEAD1 could have additive effects, TAZ seems to be the main requirement for NIS downregulation. TGFβ role in thyroid differentiation is well establish, however here we describe a novel crosstalk with the Hippo Pathway and its mediator TAZ, remarkably involved in the control of the sodium iodide symporter expression., SAF2016-75531-R

Resumen del trabajo presentado al 1st PhD Research Symposium in Health Sciences and Biomedicine, celebrado en la Universidad Autónoma de Madrid el 18 de mayo de 2018., SAF2016-75531-R