Cyclin-Dependent Kinase Activity Is Required for Progesterone Receptor Function: Novel Role for Cyclin A/Cdk2 as a Progesterone Receptor Coactivator

Abstract

Our studies examining the role of the cell cycle-regulated kinase cyclin A/Cdk2 in progesterone receptor (PR) action have demonstrated that cyclin-dependent kinase activity is required for PR function and that cyclin A/Cdk2 functions as a PR coactivator. Although Cdk2 can phosphorylate PR, elimination of these phosphorylation sites has little effect on the ability of cyclin A/Cdk2 to stimulate PR activity. PR interacts with cyclin A and recruits cyclin A/Cdk2 to progestin-responsive promoters, stimulating transcription. Inhibition of Cdk2 activity abolishes progesterone-dependent activation of PR target genes in part through inhibition of PR-dependent recruitment of steroid receptor coactivator 1 (SRC-1) and subsequent histone H4 acetylation at the target promoter. In vitro studies revealed that the interaction between SRC-1 and PR is dependent upon phosphorylation of SRC-1. This heretofore-unknown mechanism provides a potential means for integrating the regulation of PR activity with cell cycle progression. Moreover, the ability of PR to recruit cyclin A/Cdk2 to target promoters provides locally elevated levels of kinase, which can preferentially facilitate phosphorylation-dependent interactions and enzymatic activities of coactivators at the target promoter

Linkage of progestin and epidermal growth factor signaling: Phosphorylation of progesterone receptors mediates transcriptional hypersensitivity and increased ligand-independent breast cancer cell growth

“…Labriola et al (18) demonstrated a 2-4 fold activation of a PRE-reporter by heregulin in human breast cancer cells. CDK2 synergistically activated liganded PRs (17,27,56), and induced robust ligand-independent PR transcriptional activity in p27-null mouse embryonic fibroblast or breast cancer cells or during p27 knock-down of p27+ human breast cancer cells (17). We found that EGF could mimic the actions of CDK2 at PRE-driven promoters, but only when p27 levels remained low (17).…”

“…Signals from growth factors, including EGF, may enhance the activity of liganded PRs by altering PR location (19) or association with co-regulatory molecules (27). EGF induces the rapid nuclear translocation of unliganded PRs that are suspected to regulate selected promoters (15).…”

“…Other phosphorylation sites on PR-B, such as MAPK consensus site Ser345, contribute to transcriptional synergy (14). In addition, kinase inputs to PR action also target co-activator recruitment (27).…”

Integration of progesterone receptor action with rapid signaling events in breast cancer models

“…Progestins activate CDK2 [17], and PRs are predominantly phosphorylated by CDK2 at proline-directed (S/TP) sites [35,41], perhaps allowing for the coordinate regulation of PR transcriptional activity during cell cycle progression. In support of this idea, Narayanan and co-workers [43,51] found that PR activity is highest in S phase and lower in the G0/G1 phases of the cell cycle, but this activity is impaired during G2/M phases, concomitant with lowered PR phosphorylation. Overexpression of either Cyclin A or CDK2 enhanced PR transcriptional activity; while cyclin A interacts with the N-terminus of PR, CDK2 seems to alter PR function indirectly by increasing the phosphorylation and recruitment of SRC-1 to liganded PR.…”

“…While the function of PR phosphorylation is incompletely understood, it might influence aspects of transcriptional regulation, such as interaction with co-regulators, as reported for ER-α [42] and recently for PR [43]. PR phosphorylation is also involved in the regulation of ligand-dependent [39] and -independent [44,45] PR nuclear localization, receptor turnover, hormone sensitivity, and transcriptional activities [38,39,46,47].…”