PROJECT SUMMARY: Triple-negative breast cancer (TNBC) represents ~15% of invasive breast cancer cases, with disproportionately high prevalence in African-American and Hispanic women, and is defined by the lack of expression of estrogen receptor, progesterone receptor, and HER2 (ER-/PR-/HER2-). Due to the absence of currently targetable molecular drivers, treatment for TNBC still relies on non-specific toxic chemotherapeutics and outcomes remain poor. As a result, there is a pressing need to develop molecularly-informed, targeted therapies to treat TNBC. In this proposal, we investigate MALT1 protease as a potential new therapeutic target in a subset of TNBC. MALT1 is the enzymatic effector protein of a signaling complex composed of proteins CARMA3, BCL10 and MALT1 (CBM signalosome) that functions downstream of specific G protein-coupled receptors (GPCRs) to drive oncogenic reprogramming in a subset of carcinomas including TNBC. GPCRs known to activate MALT1 in breast cancer include PAR1, AGTR1, and the LPARs (LPAR1-3) and overexpression of these GPCRs is associated with aggressive breast cancer behavior in experimental models and worse clinical outcomes for human patients. We recently found that the GPCR/MALT1 signaling axis drives a program of epithelial-to-mesenchymal transition (EMT) in TNBC. Since tumor cells undergoing EMT are known to promote a permissive, immune- suppressed microenvironment, we propose to investigate the role of the GPCR/MALT1 signaling axis in the regulation of the tumor immune microenvironment. Our preliminary data indicate that MALT1 is a key mediator of immune suppression induced by GPCR+ TNBC, suggesting that inhibiting MALT1 protease activity in TNBC cells may have a beneficial effect. Additionally, inhibition of MALT1 protease activity in immune cells has recently been found to preferentially impair Treg function, tipping the balance between regulatory and effector T cells to promote heightened immunoreactivity. Together, these observations in TNBC cells and immune cells lead us to hypothesize that pharmaceutic MALT1 inhibition may have dual therapeutic benefit in GPCR+ TNBC by (1) preventing MALT1-mediated tumor immune suppression from within cancer cells and (2) altering the composition of tumor infiltrating immune cells in favor of anti-tumor immunity. This proposal will evaluate this hypothesis via a combination of elegant in vitro and in vivo model systems. The overarching goal of our proposal is to rigorously evaluate the role of MALT1 as a pathogenic driver of TNBC and to test pharmaceutic MALT1 inhibition as a novel therapeutic strategy in this difficult-to-treat disease.

PROJECT NARRATIVE: Triple Negative Breast Cancer (TNBC) is an aggressive cancer subtype that represents ~15-20% of all breast cancers, with disproportionately high prevalence in African-American and Hispanic women who also experience worse outcomes compared to Caucasian women. This project will advance our knowledge of basic molecular mechanisms that underlie certain TNBC cases. The proposed studies will provide a framework for the rational design of pharmacologic agents to combat specific forms of this disease by enhancing anti-tumor immune response.