The ERβ5 splice variant increases oestrogen responsiveness of ERαpos Ishikawa cells.

Endometrial cancer is a common gynaeological malignancy: life time exposure to oestrogen is a key risk factor. Oestrogen action is mediated by receptors encoded by ESR1 (ER alpha) and ESR2 (ER beta): ER alpha plays a key role in regulating endometrial cell proliferation. A truncated splice variant isoform (ER beta 5) encoded by ESR2 is highly expressed in cancers. This study explored whether ER beta 5 alters oestrogen responsiveness of endometrial epithelial cells. Immunhistochemistry profiling of human endometrial cancer tissue biopsies identified epithelial cells co-expressing ER beta 5 and ER alpha in stage I endometrial adenocarcinomas and post menopausal endometrium. Induced co-expression of ER beta 5 in ER alpha(Pos) endometrial cancer cells (Ishikawa) significantly increased ligand-dependent activation of an ERE-luciferase reporter stimulated by either E2 or the ER alpha-selective agonist 1,3,5-(4-hydroxyphenyI)-4-propyl-1H-pyrazole (PPT) compared to untransfected cells. Fluorescence recovery after photobleaching (FRAP) analysis of tagged yellow fluorescent protein (YFP)-ER beta 5 transfected into Ishikawa cells revealed that incubation with E2 induced a transient reduction in intra-nuclear mobility characterised by punctate protein redistribution which phenocopied the behaviour of ER alpha following ligand activation with E2. In ER alpha(neg) MDA-MD-231 breast cancer cells, there was no E2-dependent change in mobility of YFP-ER beta 5 and no activation of the ERE reporter in cells expressing ER beta 5. In conclusion, we demonstrate that ER beta 5 can act as heterodimeric partner to ERa in Ishikawa cells and increases their sensitivity to E2. We speculate that expression of ER beta 5 in endometrial epithelial cells may increase the risk of malignant transformation and suggest that immunostaining for ER beta 5 should be included in diagnostic assessment of women with early grade cancers.