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Laboratory of Histology and Experimental Cytology, Faculty of Medicine and Pharmacy, Université de Mons-Hainaut, Pentagone 1B-Avenue du Champ de Mars 6, B7000 Mons, Belgium
¹ Laboratory of Endocrinology-Bone Diseases and
² Laboratory J-C Heuson of Breast Cancer Research, Department of Internal Medicine, Institut Jules Bordet, Université Libre de Bruxelles, Rue Heger-Bordet 1, B1000 Brussels, Belgium

(Requests for offprints should be addressed to G Laurent; Email: guy.laurent{at}umh.ac.be)

(D Nonclercq and F Journé contributed equally to this study)

We used the Crm1 inhibitor leptomycin B (LMB) to examine a possible involvement of nuclear export in estrogen receptor (ER) level and function in MCF-7 breast carcinoma cells. As revealed by immunofluorescence microscopy and western blotting with anti-ER antibodies, LMB produced an accumulation of ER in cell nuclei. LMB also partly abrogated ER elimination resulting from Hsp90 disruption and 17ß-estradiol (E₂)-induced ER downregulation. By contrast, it was ineffective on ER downregulation caused by the pure anti-estrogen fulvestrant. Finally, LMB inhibited E₂-induced progesterone receptor expression and the expression of an estrogen response element-driven luciferase reporter gene in unstimulated and E₂-stimulated cells. Altogether, the data reported here suggest that: i) ER undergoes nuclear export directly or indirectly involving exportin Crm1; ii) degradation of unliganded and of agonist-bound ER probably occurs in an extranuclear compartment, while it is not the case for ER bound to a pure anti-estrogen; and iii) optimal ER-mediated gene transactivation seems to require nucleocytoplasmic shuttle of the receptor.