mTOR pathway in papillary thyroid carcinoma: different contributions of mTORC1 and mTORC2 complexes for tumor behavior and SLC5A5 mRNA expression

The mTOR pathway is overactivated in a wide variety of human neoplasias, including thyroid carcinomas. The mTOR can associate with distinct proteins and form two distinct complexes: mTORC1 and mTORC2, each with different effectors and different physiological functions. pS6 is one of the effectors of mTORC1 complex, while pAKT is one of the effectors of the mTORC2 complex. A recent study of our group demonstrated that pmTOR is a marker of aggressiveness in papillary thyroid carcinomas (PTC), its expression was associated with clinicopathological features of aggressiveness, including distant metastases, resistance to ¹³¹I therapy and, consequently, worse prognosis. In the same study, we observed that the expression of pS6 was associated with clinicopathological characteristics of low aggressiveness and did not find a significant correlation between the expression of pmTOR and pS6 in the same tumors. The absence of correlation between the two proteins and their divergent behavior led us to hypothesized that, in PTCs, the activation of pmTOR could be contributing preferentially to the activation of the mTORC2 complex and, consequently, to the phosphorylation of AKT (pAKT).

In this work, we evaluated the expression of pAKT in the same series of PTCs in which we evaluated the expression of pmTOR and pS6. In addition, we performed the pharmacological inhibition of the mTORC1 complex (with RAD001) and the simultaneous inhibition of mTORC1 and C2 (Torin2) in PTC cell lines, and then evaluate the expression of the SLC5A5 gene (gene encoding the co-transporter of sodium iodine-NIS), which is believed to be the central molecule for therapeutic success with 131I.

The expression of pAKT positively correlated with the expression of pmTOR was associated with the presence of the BRAFV600E mutation, and pAKT presence in the cell nucleus was associated with the presence of distant metastases. Inhibition of the mTORC1 complex did not cause changes in SLC5A5 gene expression, but simultaneous inhibition of mTORC1 and C2 complexes caused a six-fold increase in SLC5A5 expression.

These results suggest that, in CPT, the mTORC2 complex may be preferentially activated (pAKT), and may be implicated in distance metastasis, decreased SLC5A5 gene expression, and resistance to therapy.

Autores e Afiliações:

Catarina Tavares^1,2,3, Catarina Eloy^1,2,3, Miguel Melo^1,2,4,5, Adriana Gaspar da Rocha^1,2,6, Ana Pestana^1,2,3, Rui Batista^1,2,3, Luciana Bueno Ferreira^1,2,3,7, Elisabete Rios^1,2,3,8,9, Manuel Sobrinho Simões^1,2,3,8,9 and Paula Soares^1,2,3,8

1-Instituto de Investigação e Inovação em Saúde (i3S), Universidade do Porto;

2-Institute of Molecular Pathology and Immunology of the University of Porto (IPATIMUP);

3-Medical Faculty of the University of Porto;

4-Department of Endocrinology, Diabetes and Metabolism, University and Hospital Center of Coimbra;

5-Medical Faculty, University of Coimbra;

6-Public Health Unit, ACeS Baixo Mondego;

7- Programa de Oncobiologia Celular e Molecular, Instituto Nacional de Câncer, Rio de Janeiro, Brasil;

8-Department of Pathology, Medical Faculty of the University of Porto;

9- Department of Pathology, Hospital de S.João.

Abstract:

mTOR pathway is overactivated in thyroid cancer (TC). We previously demonstrated that phospho-mTOR expression is associated with tumor aggressiveness, therapy resistance and lower mRNA expression of SLC5A5 in papillary thyroid carcinoma (PTC), while phospho-S6 (mTORC1 effector) expression was associated with less aggressive clinicopathological features. The distinct behavior of the two markers led us to hypothesize that mTOR activation may be contributing to a preferential activation of mTORC2 complex.

To approach this question, we performed immunohistochemistry for phospho-AKT Ser473 (mTORC2 effector) in a series of 182 PTCs previously characterized for phospho-mTOR and phospho-S6 expression. We evaluated the impact of each mTOR complex on SLC5A5 mRNA expression by treating cell lines with RAD001 (mTORC1 blocker) and Torin 2 (mTORC1 and mTORC2 blocker).

Phospho-AKT Ser473 expression was positively correlated with phospho-mTOR expression. Nuclear expression of phospho-AKT Ser473 was significantly associated with the presence of distant metastases. Treatment of cell lines with RAD001 did not increase SLC5A5 mRNA levels, whereas Torin2 caused a ~6fold increase of SLC5A5 mRNA expression in TPC1 cell line.

In PTC, phospho-mTOR activation may be leading to the activation of mTORC2 complex. Its downstream effector, phospho-AKT Ser473 may be implicated in distant metastization, therapy resistance and downregulation of SLC5A5 mRNA expression.

Revista: International Journal of Molecular Sciences

Link: https://www.ncbi.nlm.nih.gov/pubmed/29757257