Here, we show that estrogen receptor ␣ (ER␣) coimmunoprecipitates with CSN5/Jab1, a subunit of the COP9 signalosome (CSN), and that overexpression of CSN5/Jab1 causes an increase in ligand-induced ER␣ degradation. Inhibition of either the kinase activity associated with the CSN complex by curcumin or of nuclear export by leptomycin B (LMB) impaired estradiol-induced ER␣ degradation by the proteasome. Degradation of ER␣ induced by the pure antagonist ICI 182,780 (ICI) was blocked by curcumin but not by LMB, indicating that in the presence of ICI, ER␣ is degraded by a nuclear fraction of the proteasome. In addition, we observed that curcumin inhibited estradiol-induced phosphorylation of ER␣. The use of three inhibitors of ER␣ degradation that target different steps of the estrogen response pathway (inhibition of the CSN-associated kinase, nuclear export, and proteasome) suggests that a phosphorylation event inhibited by curcumin is necessary for ER␣ binding to its cognate DNA target. Our results demonstrate that transcription per se is not required for ER␣ degradation and that assembly of the transcription-initiation complex is sufficient to target ER␣ for degradation by the proteasome.Estradiol (E2) regulates target cell proliferation and gene transcription through series of molecular events initiated by the hormone-dependent binding of the estrogen receptor ␣ (ER␣), a member of the nuclear receptor superfamily, to its cognate DNA target. In mammary cells, the effects of estradiol can be antagonized by compounds such as 4-hydroxy-tamoxifen (OH-Tam), a tamoxifen metabolite that is a selective estrogen receptor modulator, and ICI 182,780 (ICI), a pure antiestrogen. Both OH-Tam and ICI antagonize estrogen action primarily by competing with estradiol for receptor binding. However, OH-Tam has a partial agonist activity, depending on the tissue and response examined (14), and ER␣-OH-Tam complexes accumulate in nuclei (12, 37). In contrast, ICI compounds are totally devoid of agonist activity in the models studied to date (32), and ICI treatment provokes a rapid relocalization of ER␣-ICI complexes to a salt-resistant nuclear fraction, followed by rapid degradation by the proteasome (12, 36). A complex pathway leading to transcriptional activation of target genes involves ER␣ structural changes upon hormone binding, allowing dynamic interactions of the hormone-receptor complex with DNA and coactivators. This will allow chromatin remodeling and the subsequent recruitment of coactivators and the formation of preinitiation complexes (for a review, see reference 24). ER␣ is downregulated in the presence of E2, its cognate ligand, through the ubiquitin/proteasome (Ub/26S) pathway (1). It has been proposed that ER␣-mediated transcription and proteasome-mediated degradation are linked (17) and act to continuously turn over ER␣ on responsive promoters (20). Recently, it was proposed that the p160 coactivator steroid receptor coactivator 3 (SRC3)/AIB1 mediates agonist-induced ER␣ degradation (29).The 26S proteasome is a multipro...

In this perspective we consider new aspects of ligand-induced estrogen receptor α (ERα) degradation. What are the possible roles of CSN5/Jab1 and the CSN complex in this process? We compare hormone (estrogen) or pure antagonist (fulvestrant) induced degradation of ERα and review the effects of kinase-inhibitors and CRM1-dependent nuclear export on ERα degradation and transcription activation. A model for ERα action integrating these new actors is proposed and the relation between hormone-induced ERα degradation and transcription-activation is discussed. IntroductionOver the last few years, the involvement of the proteasome pathway in ligand-dependent nuclear receptor degradation has been established. This pathway entails first a polyubiquitination of the substrate, catalyzed by three factors (E1, E2 and E3), followed by its proteolysis by the proteasome. Recently, the role of the CSN complex that regulates the activity of a class of E3 ubiquitin ligases, the cullin RING ubiquitin ligase superfamily, was demonstrated. The cullin subunits of these ubiquitin ligases are modified by the conjugation of an ubiquitin-like protein, NEDD8. The neddylated subunit Cul1 is selectively degraded. Recent data demonstrate that CSN, via its subunit CSN5/Jab1, dennedylates cullin, thus increasing the activity of the E3 ligases [Wee et al., 2005].Several lines of evidence suggest that CSN is involved in ligand-dependant nuclear receptor degradation. First, estrogen receptor α (ERα) degradation is dependant on the neddylation pathway [Fan et al., 2003]; second, CSN5/Jab1 interacts with both the progesterone receptor and the coactivator SRC1 and is itself a coactivator of the nuclear receptors [Fan et al., 2003]; third, Jab1/CSN5 increases hormone-induced ERα degradation [Fan et al., 2003].The nature of the ligand affects ERα degradation differently: estradiol and the pure antagonist fulvestrant induce ERα degradation by the proteasome, whereas the mixed antagonist tamoxifen stabilizes Erα [Wijayaratne and McDonnell, 2001]. Thus, degradation may play an important role in ERα function and/or the action of its antagonists. ERα undergoes post translational modifications such as phosphorylation [Lannigan, 2003], acetylation [Wang et al., 2001] or sumoylation [Sentis et al., 2005]. However, the role of such modifications in targeting ERα for degradation remains unclear. A kinase activity, inhibited by curcumin, is associated to CSN. At least two curcumin-sensitive kinases, CKII and PKD, which co-purify with CSN, could contribute to this activity [Uhle et al., 2003]. CKII phosphorylates ERα on Ser167 in response to estradiol [Lannigan, 2003]. The inhibition of ERα degradation by curcumin and the co-immunoprecipitation of Jab1/CSN5 with ERα in the presence of curcumin, suggest that this kinase activity could participate in targeting ERα for degradation. In addition, the inhibition by curcumin of the interaction of ERα with its DNA target, points towards a role of this complex in transcription activation [Callige et al., 2005].The discove...