Internalisation and recycling of seven trans-membrane domain receptors is a critical regulatory event for their signalling. The mechanism(s) by which membrane progesterone receptor-α (mPRα) number is regulated on the cell surface is unclear. In this study, we investigated the cellular distribution of mPRα and mechanisms of mPRα trafficking using a cell line derived from a primary culture of human myometrial cells (M11) as an experimental model. RT-PCR and immunofluorescent analysis demonstrated expression of mPRα in M11 cells with mPRα primarily distributed on the cell surface under basal conditions. For the first time, plasma membrane localisation of mPRα was confirmed using immuno-gold transmission electron microscopy. Stimulation of M11 cells with progesterone (P4, 100 nM) resulted in internalisation of mPRα from the plasma membrane to the cytoplasm (10 min) and subsequent partial translocation back to the cell surface (20 min). We investigated potential endocytotic pathways involved in trafficking of mPRα after its internalisation. Partial co-localisation of clathrin with mPRα was obvious after 10 min of P4 treatment. Of note, chlorpromazine (inhibitor of clathrin-mediated pathway) inhibited the endocytosis of mPRα, whereas treatment with nystatin (inhibitor of caveolae-mediated pathway) did not affect internalisation. Collectively, these data suggest that mPRα is expressed on the cell surface of M11 cells and undergoes endocytosis after P4 stimulation primarily via a clathrin-mediated pathway.

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