Human progesterone receptor displays cell cycle-dependent changes in transcriptional activity

The human progesterone receptor (PR) contains multiple Ser-Pro phosphorylation sites that are potential substrates for cyclin-dependent kinases, suggesting that PR activity might be regulated during the cell cycle. Using T47D breast cancer cells stably transfected with an mouse mammary tumor virus (MMTV) chloramphenicol acetyltransferase reporter (Cat0) synchronized in different phases of the cell cycle, we found that PR function and phosphorylation is remarkably cell cycle dependent, with the highest activity in S phase. Although PR expression was reduced in the G2/M phase, the activity per molecule of receptor was markedly reduced in both G1 and G2/M phases compared to the results seen with the S phase of the cell cycle. Although PR is recruited to the MMTV promoter equivalently in the G1 and S phases, recruitment of SRC-1, SRC-3, and, consequently, CBP is reduced in G1 phase despite comparable expression levels of SRC-1 and SRC-3. In G2/M phase, site-specific phosphorylation of PR at Ser162 and at Ser294, a site previously reported to be critical for transcriptional activity and receptor turnover, was abolished. Treatment with the histone deacetylase inhibitor trichostatin A elevated G1 and G2/M activity to that of the S phase, indicating that the failure to recruit sufficient levels of active histone acetyltransferase is the primary defect in PR-mediated transactivation.