The role and mechanism of progesterone receptor activation of extra-nuclear signaling pathways in regulating gene transcription and cell cycle progression

Human progesterone receptor (PR) contains a polyproline motif in the amino-terminal domain that interacts with the SH3 domain of Src and mediates rapid activation of c-Src and downstream MAPK (Erk-1/-2) independent of the transcriptional activity of PR. Forcedly target PR to different locations in the cell by use of mutations or tags for different cell compartments showed that progestin activation of Src/MAPK is mediated by PR outside the nucleus. No distinction could be made between the cytoplasm and cell membrane as the site of PR activation of Src. Therefore we can only conclude that this is an extra-nuclear action of PR. Interestingly, the B isoform of PR which is naturally distributed between cytoplasm and nucleus mediated progestin activation of Src/MAPK, whereas PR-A that is predominantly nuclear failed to do so indicating that the two PR isoforms have distinct abilities to mediate rapid activation of signaling pathways. Due to distinct cellular locations, progestin activation of Src/MAPK signaling can regulate selected target genes such as cyclin D1 (CCND1) that lack direct PR binding response elements (PREs). Progestin induction of CCND1 was observed in cells expressing PR-B but not PR-BΔSH3 or PR-A and induction in the presence of PR-B was dramatically reduced in the presence of inhibitors of Src or MAPK. In contrast progestin induction of Sgk (serum and glucocorticoid regulated kinase) gene, which contains a classical PRE, was observed with both PR isoforms as well as PR-BΔSH3 and was unaffected by Src and MAPK inhibitors. PR bound to enhancer region of Sgk in a progestin dependent manner as detected by chromatin co-immunoprecipitation (ChIP) whereas no PR binding to CCDN1 was observed. Consistent with CCND1 data, progestin stimulation of cell cycle progression was only observed in cells expressing PR-B but not cells expressing PR-BΔSH3 or PR-A. These results demonstrate the importance of PR activation of extra-nuclear signaling pathways in regulating selected target genes and cell cycle progression.