Thyroid hormone receptor β1 domains responsible for the antagonism with the ras oncogene: role of corepressors

Abstract

The thyroid hormone receptor (TR) is a suppressor of rasmediated responses. To characterize the receptor domains involved in this function, we analyzed a panel of TRb1 mutants for their ability to interfere with ras-driven cyclin D1 activation, formation of transformation foci and tumor growth in nude mice. Our results show that the domains and mechanisms responsible for the anti-transforming and anti-tumorigenic actions of the receptor are divergent from those operating in classical T3-dependent transcriptional activation. TRb1 mutants that do not bind coactivators and do not transactivate retained the capacity of suppressing cellular transformation and tumor growth, whereas selective mutations in the hinge region affecting corepressors recruitment abolished these actions, while preserving ligand-dependent transcription. There was a strict parallelism between anti-transforming activity of the various mutants and their ability to antagonize cyclin D1 stimulation by ras, indicating that transrepression mechanisms may have an important function in suppression of the transforming effects of the oncogene by TRb1. The inhibitory action of T3 on transformation was further enhanced after over-expression of corepressors, while corepressor depletion by means of small-interference RNA reversed significantly hormonal action. This shows an important functional role of endogenous corepressors in suppression of ras-mediated transformation and tumorigenesis by TRb1

Autoregulatory loop of nuclear corepressor 1 expression controls invasion, tumor growth, and metastasis

“…NCoR induction appears to play a crucial role in the antitumorigenic and antimetastatic actions of the receptor (24), because these actions were reversed almost completely upon NCoR silencing. We also have shown that, in contrast with the native receptor, TRβ mutants unable to bind the corepressor cannot antagonize Ras-mediated transformation and tumorigenesis (26), reinforcing the idea that NCoR is an essential mediator of the tumor-suppressive actions of TRβ. TRβ-mediated up-regulation of NCoR could extend the scope of genes that are influenced by the receptor, because the corepressor has been shown to repress transcription through other nuclear receptors and numerous transcription factors with a role in cancer progression (1,12).…”

“…We have shown that this receptor retards tumor growth and suppresses invasion, extravasation, and metastasis formation in nude mice (23)(24)(25)(26). These tumor-suppressing effects are associated with a decreased expression of prometastatic genes (23).…”

Thyroid hormones inhibit TGF-β signaling and attenuate fibrotic responses

“…Mechanistically, we found that mutant receptors that are unable to recruit classical coactivators and to mediate T3-dependent transcriptional activation in transfection studies (35) still showed significant repression of TGF-β/SMAD transcriptional activity, although with less potency than the wild-type receptors. In addition, analysis of DBD mutants revealed that residues in this domain are required for repression of TGF-β-dependent transcription.…”

“…2 C and F). The mutations K288I and E457Q in ΤRβ and the equivalent mutation E401Q in TRα are located in the ligand-binding domain (LBD) and abolish the recruitment of coactivators and T3-dependent transcription, although they still bind T3 (35). However, these mutants maintain the ligand-independent activity and mediate significant T3-dependent repression of TGF-β-dependent transactivation, although with less potency than the WT receptors.…”

“…To explore the mechanism underlying T3-dependent inhibition of SMAD signaling, we analyzed the possibility that TRs could interact with SMADs. GST pull-down assays showed that SMAD3 and SMAD4 interacted similarly with 35 S-labeled TRα and ΤRβ, and that this interaction was reduced by T3 (Fig. 3A).…”