Résumé : Molecular chaperones and co-chaperones, such as heat-shock proteins (Hsp's), play a pivotal role in the adequate folding and the stability of steroid hormone receptors. As shown by immunofluorescence staining and immunoblot analysis, the Hsp90 inhibitor radicicol induced a rapid (within hours) depletion of estrogen receptor-α (ER) in MCF-7 and IBEP-2 breast carcinoma cells. Inhibition of proteasomes (MG-132, LLnL) or of protein synthesis (cycloheximide), which both suppressed E2-induced downregulation of ER, failed to modify ER degradation caused by radicicol. On the other hand, partial antiestrogens, such as hydroxytamoxifen (a triphenylethylene) and LY 117,018 (a benzothiophene) stabilized ER, making it immune to radicicol-induced degradation. Furthermore, radicicol did not interfere with ER upregulation induced by hydroxytamoxifen. Thus, the current study points to possible variation in the mechanism/pathway of ER breakdown. Besides, the protective effect of partial antiestrogens suggests that ER stability is only compromized by Hsp90 disruption when the receptor is in its native, unliganded form. © 2004 Elsevier Ireland Ltd. All rights reserved.