Oestrogen receptor (ER)-α and ER-β isoforms in normal endometrial and endometriosis-derived stromal cells

Several investigators have noted that hormone-dependent development of endometriosis implants lags behind that of simultaneously analysed eutopic endometrium. With the recent discovery of the oestrogen receptor-β (ER-β) isoform, the aim of this study was to investigate whether differences in the expression of ER-α and ER-β might explain this observation. mRNA transcripts from endometrial stromal cells isolated from normal endometrium (NE) and from endometriomas (EI) were analysed using a semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) technique. RT-PCR and Southern blot analyses of the two ER isoforms indicated that NE and EI stromal cells predominantly express ER-α mRNA, however the relative concentrations of ER isoform mRNA transcripts differed between the two cell types. Steady-state ER-α:ER-β mRNA ratios were 15.5 ± 2.8 and 5.2 ± 0.9 respectively for NE and EI cells (P = 0.02). NE and EI stromal cells expressed ER proteins with similar K(d) (~0.9 nM) and densities (~24 500 binding sites/cell) respectively. Functional ER expression was indicated by an increase in progesterone receptor concentrations of ~60% (P = 0.03) after incubation with 10 nM oestradiol. We postulate that differential transcript processing, ligand specificity and biological actions of the ER-α and -β isoforms may influence differential growth responses in normal and ectopic endometrium.