Glioblastoma (GBM) is the most common cancer arising in the adult brain, yet despite years of intensive research, it remains a fatal disease. Major barriers to improving GBM treatment include the cancer stem cell (CSC) phenotype, a self-renewing behavior that facilitates tumor recurrence and therapy resistance, and the profoundly immunosuppressive microenvironment of GBM. Activation of receptor tyrosine kinases (RTKs) is implicated in both of these processes. A better understanding of what triggers RTK signaling, and how it leads to the CSC phenotype and immunosuppression in GBM, is essential for improving patient care. Our preliminary and published data have established the importance of Tissue Factor (TF) in GBM malignancy. TF is a protein involved in blood clotting, but it can also bind and activate protease-activated receptor 2 (PAR2), a transmembrane G protein-coupled receptor that promotes cell proliferation, migration, and angiogenesis. This TF-induced process exists to facilitate wound healing, but can be harnessed by cancers to increase their malignancy. In many cancers, including glioma, high TF expression correlates with worse patient survival, and our recently published work demonstrated that suppression of TF greatly attenuates the aggressiveness of GBM. In our preliminary and published data, we found that TF is an upstream activator of multiple classes of RTKs, including class I (EGFR), II (IGF-IR), III (PDGFR?), IV (VEGFR-2), and X (Axl). We also found that TF acts through an intracellular pathway to do so, and that TF is a major contributor to the CSC phenotype in GBM. Furthermore, we found that TF can upregulate the expression of PD-L1, which is a protein found on GBM and myeloid cells that when bound can suppress antitumor immunity by inhibiting the activity and proliferation of T cells. However, the mechanisms whereby TF exerts these effects in GBM are still unclear. Our overarching hypothesis is that TF promotes CSC subpopulations, and suppresses antitumor immunity, through PAR2- mediated activation of the intracellular domain of RTKs. In the first Specific Aim of the proposal, we will investigate the mechanisms by which TF stimulates RTK signaling by determining what constitutes the binding partner complex that activates RTKs, and by examining the detailed role of TF and PAR2 in RTK activation. The second Specific Aim will focus on the role that TF-mediated RTK activation has on the CSC phenotype in GBM. The third Specific Aim will investigate the role of TF-PAR2-RTK signaling in immunosuppression in GBM, and if therapeutics can inhibit this process and prolong survival of mice. The long-term goals of the proposed work are to improve the understanding of how TF promotes malignant behavior, elucidate a new mechanism by which RTKs are activated in cancer, and to demonstrate the therapeutic potential of blocking TF-PAR2 signaling.
Major barriers to improving treatment of glioblastoma include its cancer stem cell phenotype and profound immunosuppression, which may be caused by RTK activation. These processes may be mediated through Tissue Factor (TF) activation of protease-activated receptor 2 (PAR2) that in turn activates RTKs. Through investigation of TF-dependent mechanisms of RTK activation, novel therapeutic targets may be identified that can improve patient care.
