Hormone-dependent processing of the avian progesterone receptor

Area of Science:

• Molecular Endocrinology
• Steroid Hormone Receptors
• Avian Physiology

Background:

• The avian progesterone receptor (PR) exists as two distinct molecular forms, A and B.
• The functional significance and interconversion of these PR forms are not fully understood.
• Hormone binding to receptors is known to be labile at elevated temperatures.

Purpose of the Study:

• To investigate the stability of avian progesterone receptor forms in cytosol.
• To examine changes in receptor forms induced by progesterone binding in vitro and in vivo.
• To determine the role of receptor phosphorylation as an early event in progesterone action.

Main Methods:

• Western blotting using monoclonal antibodies against avian PR forms A and B.
• In vivo studies involving progesterone injection into diethylstilbestrol-stimulated chicks.
• Analysis of receptor phosphorylation using [32P]orthophosphate and immune isolation.

Main Results:

• Avian PR is stable in freshly prepared oviduct cytosol for 2 hours at 37°C, but hormone binding is lost within 30 minutes.
• Progesterone treatment in vivo induces a time- and dose-dependent conversion of PR form A to a slower migrating form (A').
• Cytosolic PR is equally distributed between forms A and B, while nuclear PR is predominantly A'; receptor phosphorylation increases rapidly after progesterone treatment.

Conclusions:

• Avian progesterone receptor phosphorylation is an early event in progesterone signaling.
• Progesterone induces a conversion of PR form A to A' in vivo.
• Receptor stability and form changes are critical aspects of avian progesterone receptor function.