Background: Mammalian target of rapamycin (mTOR) is a downstream effector of the PI3K/AKT pathway. It can be activated by diverse stimuli, such as growth factors, nutrients, energy, stress signals and signaling pathways such as PI3K, MAPK and AMPK, in order to control cell growth, proliferation and survival. The active form of mTOR (pmTOR) may form two distinct complexes: mTORC1 that activates S6K1 and 4EBP1 which are involved in mRNA translation; and mTORC2 that activates PKC-α and AKT, regulating actin cytoskeleton organization, cell migration and survival1. mTOR deregulation is observed in multiple cancers2. In papillary thyroid carcinoma (PTC), mTOR pathway was reported to be overactivated in PTC3, particularly in those with BRAF mutation compared to BRAF wt4. Sodium iodide symporter (NIS) is a transmembrane glycoprotein that transports iodine from bloodstream to the interior of follicular cells to participate in the normal process of thyroid hormone synthesis5. Thyroid cancer therapy is based on surgery followed by 131I to treat possible tumor remnants and metastases. NIS is a vital protein in this process because it transports the radioactive iodine to the interior of cancer cells, resulting in their death. Approximately 20% of well differentiated thyroid tumors lose NIS expression becoming refractory to therapy6. The mechanisms of loss of NIS expression remain poorly understood, but it has been described that mTOR pathway activation decreases iodine uptake by thyrocytes5 and that inhibition of the pathway in thyroid tumor cell lines increases differentiation resulting in higher iodine uptake6.
Aim and methods: In order to study mTOR pathway status and possible associations with clinicopathological (age, gender, tumor capsule, tumor capsule invasion, extrathyroidal invasion, multifocality, lymphocytic infiltration, lymph node metastases, vascular invasion, status of tumor margins, staging, distant metastases, persistence of disease) and molecular features (BRAF, NRAS, TERT promoter mutations, RET/PTC rearrangements) as well as with NIS expression, we studied by immunohistochemistry pmTOR and pS6 in a series of 192 PTCs with detailed clinicopathological informations, addressing NIS expression in 44 of them by real time PCR.
Results: pmTOR expression was significantly higher in PTCs presenting absence of capsule, distant metastases and persistence of disease. Furthermore, pmTOR expression was significantly correlated with higher number of 131I therapies, higher cumulative dosis and with lower NIS expression. Additionally, pmTOR revealed to be an independent risk factor for distant metastases. The expression of pS6 was significantly associated with presence of capsule and absence of extrathyroid extension and of lymphocytic infiltration, and with BRAF wt status. There was no correlation between pmTOR and pS6 expression.
Conclusions: Despite pS6 being a downstream effector of pMTOR in the mTOR pathway, we observed, in this work, a different behavior of both markers. pmTOR expression is associated with aggressiveness and worse prognosis, while pS6 associates with less aggressive clinicopathological features. The discordant results obtained with both markers and the lack of correlation of their expression, lead us to hypothesize that mTOR activation preferentially induces the formation of mTORC2 complex, activating other downstream effectors than pS6.
Further studies are needed to confirm this hypothesis, namely regarding the dissection of which mTOR complex (1 or 2) is being preferentially activated in PTCs, so that we can use the most appropriate pharmacological strategy to block the pathway. Such blockage may be very important to overcome tumor progression and refractoriness to radioactive iodine therapy.
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