Regulation of Agonist--and Antagonist--Mediated Activation of Human Progesterone Receptors by Phosphorylation.

The human progesterone receptor hPR in breast cancer cells T47D is phosphorylated on multiple serine residues. I have previously reported the identification of eight phosphorylation sites. Here I show the identification of a new site, Ser2O. This site is hPR-B specific and contains a Ser-Pro consensus sequence. The role of phosphorylation in hPR and RU 486 antagonistagonist switch has also been investigated. Using a yeast system, the effect of B-specific phosphorylation on AF3 transactivation has been studied. Mutation of Ser102 to Ala nearly depleted the activity of AF3, suggesting that phosphorylation is a specific and an important regulatory step for hPR activity. I have also compared the activity of the mutant Ala400 with the wild type hPR. Surprisingly, the mutants activity is significantly higher than that of the wild type, implying that regulation of hPR by phosphorylation is complex. Recently, several co-regulators of steroid receptors have been cloned and characterized. I have just begun to study their roles in the RU 486 antagonistagonist switch. Initial results show that the PKA may potentiate the hPR activity through SRC-1. Whether SRC-1 can mediate the switch is under investigation.