Regulation of the SUMO pathway sensitizes differentiating human endometrial stromal cells to progesterone.

cAMP is required for differentiation of human endometrial stromal cells (HESCs) into decidual cells in response to progesterone, although the underlying mechanism is not well understood. We now demonstrate that cAMP signaling attenuates ligand-dependent sumoylation of the progesterone receptor (PR) in HESCs. In fact, decidualization is associated with global hyposumoylation and redistribution of small ubiquitin-like modifier (SUMO)-1 conjugates into distinct nuclear foci. This altered pattern of global sumoylation was not attributable to impaired maturation of SUMO-1 precursor or altered expression of E1 (SAE1/SEA2) or E2 (Ubc9) enzymes but coincided with profound changes in the expression of E3 ligases and SUMO-specific proteases. Down-regulation of several members of the protein inhibitors of activated STAT (PIAS) family upon decidualization pointed toward a role of these E3 ligases in PR sumoylation. We demonstrate that PIAS1 interacts with the PR and serves as its E3 SUMO ligase upon activation of the receptor. Furthermore, we show that silencing of PIAS1 not only enhances PR-dependent transcription but also induces expression of prolactin, a decidual marker gene, in progestin-treated HESCs without the need of simultaneous activation of the cAMP pathway. Our findings demonstrate how dynamic changes in the SUMO pathway mediated by cAMP signaling determine the endometrial response to progesterone.