Abstract 4604: PTP4A3 is oncogenic and modulates triple negative breast cancer growth

Triple negative breast cancers (TNBCs) are among the most aggressive and heterogenous breast cancers characterized by a high propensity to invade, metastasize, and relapse; the exact mechanisms by which TNBCs mediate progression is unclear and thus, new insights into the biology of TNBC are desperately needed. We have developed a novel anti-cancer compound (AMPI-109) that is selectively efficacious against TNBC cell-lines. We conducted a genome-wide functional genomic shRNA screen to identify modifiers of AMPI-109 sensitivity and identified PTP4A3 (P3), a protein tyrosine phosphatase implicated in invasive, node positive TNBC tumors and associates with poor survival outcome, as a mediator of AMPI-109 action. In silico modeling predicted that AMPI-109 binds the catalytic site of P3 with favorable energetics. To investigate whether AMPI-109 could impact the activity of P3, we performed an in vitro phosphatase assay and demonstrated that AMPI-109 impaired the catalytic activity of P3. To investigate an oncogenic role of P3 in TNBC, we conducted stable lentiviral knockdown of P3 (ShP3) in BT-20 TNBC cells and demonstrated that loss of P3 leads to substantial growth inhibition (48%, p = 0.0055), similar in magnitude to AMPI-109 at 100 nM dose (66%, p = 0.00005). AMPI-109 also decreased P3 mRNA and protein levels. Accordingly, we blocked de novo protein synthesis and observed enhanced P3 protein degradation in the presences of AMPI-109. This degradation was reversible in the presence of the proteasome inhibitor MG-132, suggesting that AMPI-109 may induce P3 degradation through a proteasome-dependent mechanism. Functionally, high expression of P3 has been implicated in driving metastatic phenotypes in colorectal cancer but its role in TNBC cell migration has not been investigated. Wound-healing assays demonstrated that both AMPI-109 and ShP3 significantly impeded the migratory capacity of BT-20 cells (AMPI-109: 85% reduction, shP3: 80% reduction). AMPI-109 and ShP3 also conferred a higher rate of apoptosis induction as measured by activation of pro-apoptotic caspases-3 and 7 suggesting that the growth inhibitory phenotypes observed could potentially be attributed to increases in apoptotic cell death. To determine how P3 mechanistically exerts its oncogenic effect in the context of modulating cell signaling pathways, we carried out co-expression analysis of mRNAs that tracked with P3 expression across a suite of TNBC cell lines. We identified TRAPPC9, an activator