Abstract LB188: PEA-15 phosphorylation mediates pro-tumor effects by promoting β-catenin activity in TNBC

Abstract Triple negative breast cancer (TNBC) is a subtype of breast cancer that is defined by a lack of estrogen receptor, progesterone receptor, and HER2-neu gene product expression. Previously, we found that TNBC cells with phosphorylated PEA-15 adaptor protein exhibited decreased clonogenic and anchorage-independent growth in vitro, and decreased growth and lung metastasis in vivo. Further, we found lower levels of phosphorylated β-catenin at S675 in PEA-15-AA (non-phosphorylatable) mutants than in PEA-15-DD (phosphomimetic) mutants. We therefore hypothesized that phosphorylated PEA-15 enhances the aggressiveness of TNBC by regulating the phosphorylation of β-catenin at S675. We first screened a panel of TNBC cells for β-catenin levels via western blot. Next to assess the biological role of β-catenin we knocked down its expression in TNBC cells (SUM149, BT549). In the β-catenin knockouts, proliferation decreased by 20%(P<0.0001) mammosphere formation by 69% (P<0.001), and clonogenic growth by 65% (P<0.0001), when compared to the vector control. To determine if PEA-15 mediates pro-tumor effects by phosphorylating β-catenin at S675, we created a nonphosphorylatable (S675A) and a phosphomimetic (S675D) form of β-catenin. We transfected BT549 cells with S675A and observed a decrease of 26% (P=0.011) in proliferation, 65% (P<0.001) in mammospheres, and 54% (P=0.0001) in clonogenic formation when compared with WT β-catenin transfected cells. Next, we transfected SUM149 cells with S675D and found an increase of 58% (P<0.001) in proliferation, 79% (P=0.001) in mammospheres, and 23% (P=0.0303) in clonogenic formation when compared with WT β-catenin transfected cells. To test if PEA-15 phosphorylation mediates β-catenin activity, we transfected the S675A and S675D form of β-catenin into MDA-MB-468 cell lines which stably express PEA-15-AA and PEA-15-DD. When we performed a rescue experiment with PEA-15-AA cells transfected with S675D we found a 78% (P=0.0008) increase in mammospheres when compared to vector transfected cells. Conversely, when PEA-15-DD cells were transfected with S675A we found a 53% (P=0.0004) decrease in mammospheres when compared to vector transfected cells. Next, we did a multiplex staining of a tissuemicroarray (n= 102 human TNBC tumors). We observed that in patients with grade 3 tumors, the percentages of cells expressing total PEA-15(P=0.005), P-PEA-15-S116(P=0.038), and p-β-catenin (P=0.0005) were significantly higher than those in patients with grade 2 tumors. Together, these results show that PEA-15 phosphorylation mediates tumor aggressiveness in TNBC by promoting β-catenin activity. Future studies will evaluate the clinical significance of PEA-15 and β-catenin and their phosphorylated forms in TNBC patients. These results will establish β-catenin and PEA-15 phosphorylation as potential markers of, and targets against TNBC progression. Citation Format: Alex W. Tan, Mohd Mughees, Wendy A. Woodward, Debu Tripathy, Natalie Fowlkes, Chandra Bartholomeusz. PEA-15 phosphorylation mediates pro-tumor effects by promoting β-catenin activity in TNBC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB188.