Alien inhibits E2F1 gene expression and cell proliferation

Epigenetic involvement of Alien/ESET complex in thyroid hormone-mediated repression of E2F1 gene expression and cell proliferation

The ligand-bound thyroid hormone receptor (TR) is known to repress via a negative TRE (nTRE) the expression of E2F1, a key transcription factor that controls the G1/S phase transition. Alien has been identified as a novel interacting factor of E2F1 and acts as a corepressor of E2F1. The detailed molecular mechanism by which Alien inhibits E2F1 gene expression remains unclear. Here, we report that the histone H3 lysine 9 (H3K9) methyltransferase (HMT) ESET is an integral component of the corepressor Alien complex and the Alien/ESET complex is recruited to both sites, the E2F1 and the nTRE site of the E2F1 gene while the recruitment to the negative thyroid hormone response element (nTRE) is induced by the ligand-bound TRβ1 within the E2F1 gene promoter. We show that, overexpression of ESET promotes, whereas knockdown of ESET releases, the inhibition of TRβ1-regulated gene transcription upon T3 stimulation; and H3K9 methylation is required for TRβ1-repressed transcription. Furthermore, depletion of ESET impairs thyroid hormone-repressed proliferation as well as the G1/S transition of the cell cycle. Taken together, our data indicate that ESET is involved in TRβ1-mediated transcription repression and provide a molecular basis of thyroid hormone-induced repression of proliferation.

The corepressor Alien as a novel tumor suppressor?

Alien has been characterized as a corepressor for nuclear hormone receptors that harbor a silencing domain such as the thyroid hormone receptor (TR), the vitamin D3 receptor (VDR) and DAX-1. In addition, the androgen receptor (AR), a steroid hormone receptor, interacts with Alien. Alien enhances gene silencing mediated by TR, VDR and DAX-1, whereas Alien inhibits AR-mediated transactivation. The inhibition of AR by Alien seems to be restricted to cases where AR is bound to AR antagonists. In line with this, Alien inhibits AR target gene expression and human prostate cancer cell proliferation in an antagonist-specific manner indicating that Alien has an inhibitory role for cell cycle progression. Alien mediates gene silencing by recruitment of histone deacetylase activity and interestingly through nucleo-some assembly activity. Hereby, Alien enhances nucleosome positioning mediated by nucleosome assembly protein 1, which suggests a novel molecular mechanism of corepressor function. Using a proteomic approach to identify Alien interacting partners, we detected the cell cycle factor E2F1 to bind to Alien in vivo. The E2F1-mediated transactivation and E2F target gene expression is inhibited by Alien, and in line with this Alien is observed to repress cell cycle progression.

The coregulator Alien

Alien has characteristics of a corepressor for selected members of the nuclear hormone receptor (NHR) superfamily and also for transcription factors involved in cell cycle regulation and DNA repair. Alien mediates gene silencing and represses the transactivation of specific NHRs and other transcription factors to modulate hormone response and cell proliferation. Alien is a highly conserved protein and is expressed in a wide variety of tissues. Knockout of the gene encoding Alien in mice is embryonic lethal at a very early stage, indicating an important evolutionary role in multicellular organisms. From a mechanistic perspective, the corepressor function of Alien is in part mediated by histone deacetylase (HDAC) activity. In addition, Alien seems to modulate nucleosome assembly activity. This suggests that Alien is acting on chromatin not only through recruitment of histone-modifying activities, but also through enhancing nucleosome assembly.