OR22-05 Progesterone Receptor-Grb2 Interaction Is Associated With Better Outcomes In Breast Cancer

Disclosure: N. Wittayavimol: None. E. Iwabuchi: None. P. Pateetin: None. Y. Miki: None. Y. Onodera: None. H. Sasano: None. V. Boonyaratanakornkit: None. The presence of progesterone receptor (PR) is usually associated with better clinical outcomes for several endocrine-related cancers. However, the molecular mechanism of how PR mediates growth inhibition remains unclear. In addition to its role as a nuclear transcription, PR also possesses an extranuclear function, mediated mainly through PR polyproline domain (PPD) interaction with the SH3 domains of cytoplasmic signaling molecules. Our previous study suggested a possible role of PR-PPD in mediating SH3-domain interaction. We showed that PR-PPD and SH3 domain interaction inhibited EGF-mediated signaling and decreased lung cancer cell proliferation. An essential adapter protein that helps transduce growth factor signaling is Grb2 which has an SH2 domain flanked by two SH3 domains. In this study, we hypothesized that PR interferes with cytoplasmic signaling through interaction between PR-PPD and Grb2-SH3. We used several protein-protein interaction assays in vitro and in cells to demonstrate PR-Grb2 interaction. GST-pulldown analysis showed that PR-PPD interacts specifically with SH3 domains of Grb2. No interaction was detected when key prolines in the PR-PPD to alanines. Immunofluorescence staining showed colocalization of PR and Grb2 in both the nucleus and cytoplasm of BT474 breast cancer cells. Using Bimolecular Fluorescence Complementation (BiFC) analysis, we showed that PR and Grb2 interacted in breast cancer cells, specifically through the Grb2-SH3 domain. No BiFC signal was detected when Grb2-SH3 domains were deleted. Furthermore, we used Proximity Ligation Assay (PLA) analysis to examine protein-protein interaction of a distance less than 40nm in cells. We found PR-Grb2 interaction in PR-positive breast cancer tissue sections. No PLA signal was detected in PR-negative breast cancer cells. Importantly, we examined 43 PR-positive breast cancer specimens and showed that PR-Grb2 interaction was associated with low histological stage and negatively correlated with lymph node invasion and metastasis of breast cancer. Our results provide a molecular mechanism for how PR can inhibit growth factor signaling by interfering with Grb2 signaling and a basis for developing novel therapeutic strategies for future cancer treatment. Presentation: Saturday, June 17, 2023