Circannual rhythms in progesterone receptor levels and functions

Area of Science:

• Reproductive biology
• Endocrinology
• Molecular biology

Background:

• The progesterone receptor (P-R) plays a crucial role in reproductive functions.
• Nuclear binding sites (acceptors) are essential for P-R activity.
• Previous studies suggested potential variations in receptor levels and function.

Purpose of the Study:

• To investigate seasonal variations in progesterone receptor binding to nuclear acceptor sites in the chick oviduct.
• To determine if these variations affect P-R binding to pure DNA versus protein-DNA complexes.
• To explore the underlying mechanisms of observed seasonal rhythms.

Main Methods:

• Collected chick oviduct cytosol and nuclei during different seasons.
• Assessed [3H]progesterone receptor ([3H]P-R) binding to cytosol preparations, whole chromatin, purified acceptor proteins, and pure DNA in vitro.
• Analyzed nuclear binding in vivo by injecting [3H]progesterone and measuring oviduct nuclear radioactivity.
• Utilized computer analysis (least squares method) to fit binding data to cosine curves for rhythm detection.

Main Results:

• Reduced P-R levels and impaired binding capacity to nuclear acceptor sites were observed during winter/spring (January-May).
• A circannual rhythm was detected in P-R binding to acceptor protein-DNA complexes and whole chromatin, but not to pure DNA.
• In vivo nuclear binding of progesterone also exhibited a similar seasonal rhythm.
• The binding defect during winter/spring aligns with the two-subunit hypothesis of P-R, suggesting subunit absence or inactivity.

Conclusions:

• Nuclear acceptor sites for progesterone in the chick oviduct are protein-DNA complexes, not pure DNA.
• Seasonal variations significantly influence P-R binding to these nuclear sites, impacting reproductive physiology.
• The findings support a model where seasonal changes in P-R subunit availability or activity regulate its function in the chick oviduct.