DESCRIPTION (provided by applicant): Triple-negative breast cancer (TNBC) lacks the expression of estrogen receptor (ER), progesterone receptor (PR) and Her2. TNBC can be classified into at least six subtypes including mesenchymal-like and basal-like with the majority of TNBC categorized as mesenchymal-like. TNBC makes up approximately 15-20% of all breast cancer cases and most commonly affects younger women and African-American women. Patients with TNBC generally have a poor prognosis and short-term survival. This aggressive disease does not respond to widely used targeted therapies such as trastuzumab or endocrine therapies, such as tamoxifen or aromatase inhibitors. Although women with metastatic TNBC can initially respond to conventional chemotherapeutic agents, relapse is inevitable, and oncologists have little to offer other than additional non-cross reactive systemic chemotherapy regimens. Therefore, the challenge is to develop a more effective treatment regimen for women with metastatic TNBC. We find that serine-threonine phosphatase PP2A is a rate-limiting factor (bottleneck) that regulates drug-resistance in TNBC cells. We show that PP2A inhibitors can sensitize both sensitive and resistant mesenchymal-like TNBC cells to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. TRAIL-based drugs are currently being tested for the treatment of several cancer types in clinical trials. Therefore, these considerations suggest that mesenchymal-like TNBC may be likely to benefit from TRAIL therapy or in combination with PP2A inhibitor therapy. Although mesenchymal-like TNBC cells are highly sensitive to TRAIL-induced apoptosis, basal-like TNBC cells are resistant to TRAIL. We find that the PP2A inhibitor LB100, a small molecule that has recently been approved to enter phase I clinical trials for the treatment of several cancers, increases conventional chemotherapeutic agent-induced anticancer activity in basal-like TNBC cells in vitro and effectively inhibits tumor growth of basal-like TNBC in vivo. This consideration suggests that LB100 alone or in combination with clinically used chemotherapeutic agents could be an efficacious therapy for basal-like TNBC. In addition, our preliminary data show that TRAIL treatment causes the degradation of PP2A/C, a catalytic subunit of PP2A, in TNBC cells. Mechanistically, TRAIL treatment leads to the recruitment of PP2A/C to the death-inducing signaling complex (DISC) where a Cullin-3-based E3 ligase targets PP2A/C for ubiquitination and subsequent degradation. Importantly, in TNBC cells that have developed acquired TRAIL resistance, PP2A/C protein is resistant to TRAIL-induced degradation. In this proposal, we will test the hypotheses that TRAIL alone or in combination with inhibition of PP2A-mediated survival signals or clinically used chemotherapeutic agents sufficiently causes apoptosis in mesenchymal-like TNBC cells and that inhibition of PP2A survival signals alone or in co...