PDGF-BB and the metastatic brain microenvironment

PROJECT SUMMARY Recent studies by my group have highlighted the clinical relevance of platelet-derived growth factor-B (PDGFB) secreted by malignant breast epithelial cells and its receptor, platelet-derived growth factor receptor beta (PDGFRβ), expressed on cells of the mesenchymal lineage (e.g., fibroblasts, pericytes, astrocytes) in the promotion of breast cancer metastasis to the brain (BCBM). Our studies revealed: (1) PDGFB ligand promotes primary and intracranial breast tumor growth, (2) mesenchymal-specific PDGFRβ hyperactivity (Fsp1- cre;PdgfrbD849V/+) promotes BCBM in mice, (3) PDGFB ligand expression in the primary breast tumor is prognostic of brain metastases in human breast cancer patients, and (4) inhibition with a small molecule selective PDGFR inhibitor (crenolanib) reduces intracranial tumor growth in mouse models of BCBM. This published work discovered PDGFB-to-PDGFRβ signaling as a clinically relevant signaling node for predicting, and potentially treating, BCBM. Our continued investigation into how breast cancer-derived PDGFB mechanistically modulates the brain TME has further revealed previously unknown immuno-modulatory and vascular effects. The current application will directly test the overarching hypothesis that breast cancer cells expressing PDGFB completely transform the brain metastatic niche, both by creating a pro-tumorigenic immune microenvironment and by invoking vascular changes. In Aim 1, the interaction between breast cancer cells with/without PDGFB and the brain immune microenvironment will be evaluated in vitro (microglial co- cultures and conditioned media experiments) and in vivo (intracardiac injections followed by brain histopathological evaluation and high-plex spectral cytometry). This aim will also evaluate if PDGFB causes changes in the microglial phagocytic response as well as shifts in anti-tumorigenic/pro-tumorigenic myeloid cell phenotypes. These studies will be the first to directly test whether breast cancer-derived PDGFB drives immune evasion in the brain metastatic microenvironment. In Aim 2, the interaction between breast cancer cells with/without PDGFB and the brain microvasculature will be evaluated in vitro (BBB spheroids, microfluidics) and in vivo (intracardiac injections followed by brain histopathological evaluation). This aim will also evaluate a putative mechanism by which PDGFB functions in an autocrine manner to upregulate the pro- angiogenic factor Angiopoietin-1, and whether these vascular changes can be ablated with small molecule inhibitors of PDGFRβ and/or Tie2 (angiopoietin receptor). These studies will be the first to directly test the hypothesis that breast cancer-derived PDGFB alters the brain microvasculature indirectly through an autocrine PDGFB-PDGFRβ-Ang1 axis. Upon completion of the proposed study, we will have determined potentially targetable mechanism(s) by which PDGFB pre-conditions the brain microenvironment allowing for metastasis to this site.