Expression of milk fat globule EGF-factor 8 (MFG-E8) mRNA and protein in the human endometrium and its regulation by prolactin

Milk fat globule epidermal growth-factor 8 (MFG-E8) has not been previously linked to endometrial physiology. We reported on MFG-E8 mRNA up-regulation in the human endometrium during the window of implantation (WOI) using microarrays. Prolactin (PRL) secreted by stromal cells has been suggested to modulate protein expression. The objective of this study was to characterize the endometrial expression of MFG-E8 and its ligand αvβ3 integrin during the menstrual cycle and its possible regulation by PRL. MFG-E8 mRNA (real-time RT-PCR) and protein expression (immunohistochemistry and immunoblotting) were analyzed in human endometrial biopsies at different times of the menstrual cycle, as well as in primary endometrial cell cultures. In primary cultures of epithelial cells, MFG-E8 intracellular protein expression was evaluated in absence or presence of PRL (0.2 and 1 μg/ml). The results show that MFG-E8 protein is almost exclusively localized to the epithelium in whole endometrial biopsies. Both MFG-E8 mRNA and protein expression increased in the luteal phase and were highest during the WOI; epithelial protein location of αvβ3 integrin also peaked on cycle Day 24. Cultured epithelial cells showed a diffuse staining of MFG-E8 over the cytoplasmic area; however, some cells presented a punctuated staining pattern. PRL treatment of epithelial cells for 72 h in vitro significantly increased MFG-E8 protein intracellular expression. This is the first report on MFG-E8 protein localization to the human endometrial epithelium and its up-regulation during the WOI. The pattern of glandular expression of its ligand αvβ3 integrin was remarkably similar. In vitro data support a modulatory role for PRL as a stromal/epithelial paracrine factor controlling MFG-E8.