High expression of the estrogen receptor-related receptor (ERR)-␣ in human tumors is correlated to a poor prognosis, suggesting an involvement of the receptor in cell proliferation. In this study, we show that a synthetic compound (XCT790) that modulates the activity of ERR␣ reduces the proliferation of various cell lines and blocks the G 1 /S transition of the cell cycle in an ERR␣-dependent manner. XCT790 induces, in a p53-independent manner, the expression of the cell cycle inhibitor p21waf/cip1 at the protein, mRNA, and promoter level, leading to an accumulation of hypophosphorylated Rb. Finally, XCT790 reduces cell tumorigenicity in Nude mice.Breast cancer is the most common malignancy in women (1). In the initial phases, these tumors often depend on estrogens for their growth. At the molecular level, 17␤-estradiol (E2, 3 the main estrogen in the adult) acts by up-regulating the expression of cyclin D1, which regulates the activity of cyclin-dependent kinases (cdks) 4 and 6 (2, 3). These cdks facilitate the transition from the G 1 to the S phase of the cell cycle by hyperphosphorylating Rb (4). Adjuvant therapies aimed at counteracting the effects of estrogens are widely used (5). Anti-estrogens such as tamoxifen down-regulate the expression of cyclin D1 (3), whereas ICI182,780 (fulvestrant and faslodex) stimulates the expression of p21 waf/cip1 (hereafter referred to as p21) (6). This protein blocks the activity of the cyclin D-cdk complexes leading to hypophosphorylation of Rb and arrest in the G 1 phase of the cell cycle (4). However, a large proportion of breast cancers evolve so as to become resistant to anti-hormonal treatments through mechanisms that are not clearly characterized (1,5). This illustrates the need of new targets against which to develop innovative anti-cancer therapies.The effects of estrogens are mediated by two specific nuclear receptors (ER␣ and ER␤), which act as ligand-dependent transcription factors (7). Estrogen receptor-related receptors (ERR␣, -␤, and -␥) are other members of the nuclear receptor superfamily that share a high level of sequence identity with the ERs (8). However, no natural ligand has been identified for the ERRs, which are therefore considered as orphan receptors. Crystallographic studies have shown that ERR␣ and -␥ spontaneously adopt active conformations (9, 10). Although ERR␣ regulates transcription in a constitutive manner, some of its activities (DNA binding and contact with coactivators) can be regulated by phosphorylation (11). Furthermore, compounds such as the phytoestrogen genistein can inhibit the transcriptional activities of ERR␣ (12). In addition, based on its capacity to disrupt the interactions between ERR␣ and the PGC-1␣ coactivator, the synthetic molecule XCT790 has been identified as an ERR␣-specific inverse agonist (13). This compound down-regulates the expression of ERR␣ target genes probably by inducing an actively repressing conformation (14). At the same time, XCT790 promotes the proteasomedependent degradation of its receptor (15). ERR...

ICI182,780 (Fulvestrant) is a pure anti-estrogen used in adjuvant therapies of breast cancer. This compound not only inhibits the transcriptional activities of the estrogen receptor-alpha (ER alpha) but also induces its proteasome-dependent degradation. The latter activity is believed to be required for the antiproliferative effects of ICI182,780. Estrogen receptor-related receptor-alpha (ERR alpha) is an orphan member of the nuclear receptor superfamily that is expressed in a wide range of tissues including breast tumors, in which its high expression correlates with poor prognosis. Although not regulated by any natural ligand, ERR alpha can be deactivated by the synthetic molecule XCT790. Here we demonstrate that this compound also induces a proteasome degradation of ERR alpha. We also show that although it does not act directly on the steady-state level of ER alpha, XCT790 potentiates the ICI182,780-induced ER alpha degradation. We suggest that treatment with XCT790 could thus enhance the efficacy of ICI182,780 in ER alpha-dependent pathologies such as breast cancer.