Reprograming Macrophages and Targeting Glioma Stem Cells in Glioblastoma

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Project Summary Glioblastoma (GBM) is the most lethal primary brain tumor with poor prognosis. GBM contains heterogeneous cancer cells including glioma stem cells (GSCs) and harbors abundant tumor-associated macrophages (TAMs). Because the majority of TAMs are tumor-promoting macrophages (pTAMs, M2) that support malignant growth and augment immune suppression, reprograming pTAMs into tumor-suppressive TAMs (sTAMs, M1) to activate macrophage phagocytosis of tumor cells represents an attractive therapeutic strategy. As pTAMs actively interact with GSCs to promote tumor growth and therapeutic resistance, redirecting pTAMs into sTAMs may synergize with targeting GSCs to suppress GBM growth. To discover small molecules that can reprogram pTAMs into sTAMs to stimulate macrophage phagocytosis of glioma cells, we designed a phagocytosis fluorescent screening assay, using GFP-labeled human iPSC-derived macrophages and tdTomato-expressing glioma cells including GSCs to identify drug candidates and the potential molecular targets. To this end, we identified several inhibitors of BACE1 (β-site amyloid precursor protein cleaving enzyme 1) as top candidates, and thus defined BACE1 as a molecular target to redirect pTAMs into sTAMs. Our studies demonstrated that BACE1 is preferentially expressed by pTAMs in human GBMs and required for maintaining pTAM polarization. Inhibiting BACE1 by its inhibitor MK-8931 potently reprogramed pTAMs into sTAMs and promoted macrophage phagocytosis of glioma cells to suppress GBM growth. Moreover, we found that low-dose radiation (IR) markedly enhanced TAM infiltration into GBM and synergized with MK-8931 treatment. As MK-8931, initially developed for Alzheimer's disease, has been shown to be safe for patients in clinical trials, MK-8931 can be potentially streamlined for the macrophage-based tumor therapy. In addition, we previously found that the non- receptor tyrosine kinase BMX maintains GSC tumorigenic potential by mediating STAT3 hyper-activation, and demonstrated that targeting BMX with ibrutinib potently suppressed GBM growth and impaired radioresistance. Because both ibrutinib and MK-8931 penetrate the blood-brain barrier (BBB) or the blood-tumor barrier (BTB) very well, repurposing ibrutinib and MK-8931 for GBM treatment should be straightforward and have promising potential. Based on these studies, we hypothesize that redirecting pTAMs into sTAMs by MK-9831 synergizes with targeting GSCs by ibrutinib to suppress malignant growth and thus improves GBM treatment. We will accomplish our objectives through the following Aims: (1) We will assess the effect of reprograming pTAMs into sTAMs on cytokine profile, GSCs, and GSC-derived pericytes in GBM; and (2) We will evaluate the therapeutic impact of reprograming pTAMs to sTAMs and targeting GSCs for GBM treatment. The outcomes from the proposed pre-clinical studies will determine whether synergistically reprograming TAMs and targeting GSCs can serve as a novel therapeutic strategy to effectively improve GBM treatment, which will inform future clinical trials.