Cyclic AMP-dependent signal transduction leading to mitogenesis in thyroid: implication of the mammalian target of rapamycin

During the first part of this thesis work, we have collaborated to a study of the effects of the TSH/cAMP pathway on small G proteins of the Rho family and their impact on the actin cytoskeleton and thyroid cell function. This study, performed in canine thyrocytes, showed for the first time, that the TSH/cAMP/PKA pathway inactivates the three small G proteins RhoA, Rac1 and Cdc42 and the RhoA/ROCK/LIMK/cofilin pathway. Inactivation of the latter appeared both necessary and sufficient to mediate the action of TSH and PKA on the reorganization of actin microfilaments and its morphological impact. Moreover, this inactivation by PKA of Rho-mediated actin polymerization also played an important role in the cAMP-dependent expression of thyroid differentiation genes. On the other hand, a residual RhoA activity appeared to be required for mitogenesis. This dependence of DNA synthesis on RhoA activity was not mediated by ROCK-dependent events nor by the integrity of the actin cytoskeleton. Indeed, DNA synthesis induction was unexpectedly resistant to actin depolymerisation in canine thyrocytes, which explains how it could be compatible with the cAMP-dependent microfilament disruption.

This first study did not provide new insights on how the cAMP/PKA-dependent mitogenic stimulus can trigger cell cycle progression, which, in thyrocytes, depends on the phosphorylation of pRb by the cyclin D3-CDK4 complex. In the various in vitro thyroid models, the only convergent early signaling event found in response to TSH/cAMP, insulin and growth factors was the phosphorylation and activation of p70 S6K1 which largely depends on mTOR (mammalian Target Of Rapamycin). The main part of the work in this thesis has been devoted to investigation of the action of TSH/cAMP on the mTOR pathway. mTOR is a therapeutic target for a wide variety proliferative disorders and rapamycin derivates are now considered in anti-cancer treatments.

We have shown for the first time in PC Cl3 rat thyroid cells that TSH, through cAMP, activates mTORC1, leading to phosphorylation of S6K1 and 4E-BP1. mTORC1-dependent S6K1 phosphorylation in response to both insulin and cAMP required amino acids, whereas inhibition of AMPK and GSK3 enhanced insulin but not cAMP effects. Unlike insulin, TSH/cAMP did not activate PKB, nor induce TSC2 phosphorylation at Thr1462 and Tyr1571. However, like insulin, TSH/cAMP produced a stable increase in mTORC1 kinase activity associated with augmented 4E-BP1 binding to raptor. This could be caused in part by Thr246-phosphorylation of PRAS40, which was found as an in vitro substrate of PKA, but other regulatory events likely remain to be uncovered. Both in PC Cl3 cells and primary dog thyrocytes, rapamycin inhibited DNA synthesis and pRb phosphorylation induced by TSH and insulin. Rapamycin reduced cyclin D3 accumulation but not the abundance of cyclin D3-CDK4 complexes. However, rapamycin inhibited the activity of these complexes and the activating Thr172-phosphorylation of CDK4 stimulated by both TSH and insulin. We propose that mTORC1 activation by TSH, at least in part through PKA-dependent phosphorylation of PRAS40, crucially contributes to mediate cAMP-dependent mitogenesis by regulating CDK4 Thr172-phosphorylation.