ABSTRACT: BACKGROUND:Alterations in estrogen and progesterone signaling, via their respective receptors, estrogen receptor alpha (ER?) and progesterone receptor (PR), respectively, are largely involved in the development of breast cancer (BC). The recent identification of ER?-36, a splice variant of ER?, has uncovered a new facet of this pathology. Although ER?-36 expression is associated with poor prognosis, metastasis development, and resistance to treatment, its predictive value has so far not been associated with a BC subtype and its mechanisms of action remain understudied. METHODS:To study ER?-36 expression in BC specimens, we performed immunochemical experiments. Next, the role of ER?-36 in progesterone signaling was investigated by generating KO clones using the CRISPR/CAS9 technology. PR signaling was also assessed by proximity ligation assay, Western blotting, RT-QPCR, and ChIP experiments. Finally, proliferation assays were performed with the IncuCyte technology and migration experiments using scratch assays. RESULTS:Here, we demonstrate that ER?-36 expression at the plasma membrane is correlated with a reduced disease-free survival in a cohort of 160 BC patients, particularly in PR-positive tumors, suggesting a crosstalk between ER?-36 and PR. Indeed, we show that ER?-36 interacts constitutively with PR in the nucleus of tumor cells. Moreover, it regulates PR expression and phosphorylation on key residues, impacting the biological effects of progesterone. CONCLUSIONS:ER?-36 is thus a regulator of PR signaling, interfering with its transcriptional activity and progesterone-induced anti-proliferative effects as well as migratory capacity. Hence, ER?-36 may constitute a new prognostic marker as well as a potential target in PR-positive BC.
