Profiling the expression and function of the truncated oestrogen receptor isoform ER46 in human endometrium

Introduction: Oestrogen receptors (ER) are essential for reproductive function and fertility. ER46 is a 46 kDa truncated isoform of full length ERα (ER66) that binds oestradiol (E2) and can signal via nuclear or membrane-initiated signalling pathways. ER46 has been detected in breast cancer cell lines but expression in endometrial tissues has not been documented. The aims of this study were a) to determine whether endometrial cells express ER46, and b) to investigate a potential functional role for ER46 in regulating oestrogen responses in the endometrium.

Methods: Primary human endometrial (n=20) and first trimester decidual tissue biopsies (n=18) were collected using methods approved by the local institutional ethics committee (LREC/05/51104/12 and LREC/10/51402/59). Uterine Natural Killer (uNK) cells were freshly isolated from decidua (n=8) by magnetic bead sorting. The expression of oestrogen receptors (ER66, ER46 and ERβ) was assessed by qPCR, western blot and immunohistochemistry. Cell motility was measured in uNK cells by live cell imaging; cells were treated with E2-BSA (10 nM equivalent), the ERβ-selective agonist DPN (10 nM) or vehicle control (DMSO).

Results: ER46 was detected by qPCR and western blot in endometrial tissues and was the predominant ERα isoform in first trimester decidua (P<0.01). Immunohistochemistry identified putative ER46 immunolocalised to the nuclei, cytoplasm and cell membrane of endometrial cells. Analysis of decidual tissues demonstrated that uNK cells, a specialised leukocyte population which is abundant in early pregnancy, were uniquely ER66neg/ER46pos with ER46 localised to the cell membrane. ER46 expression was confirmed in isolated uNK cells where selective activation of ER46 with E2-BSA significantly increased cell velocity (P<0.001) and distance (P<0.001) compared to DPN or vehicle control.

Conclusions: These novel findings identify a role for ER46 in regulating oestrogen responsiveness of the endometrium and provide unique insight into the regulation of uNK cell activity during the establishment of pregnancy in women.