Abstract
Glioblastoma stem cells (GSCs) are tumor-initiating cells and contribute to chemo-radiation resistance of glioblastoma multiforme (GBM) with poor prognosis. Cell-cell interactions in GBM niche regulate and maintain GSCs leading to tumor progression. Secretory molecules from different cell types in GBM niche play significant roles in intercellular communications. Therefore, we utilized phage screening technology to identify the secretory molecules regulating GSCs-microenvironment interactions and target their communications. Phage display library consisting of 109 different combinations of 7-amino acid-length peptide sequences was screened for binding to GSCs both in vitro and in vivo. In vitro screening was performed against GSCs isolated from patient GBM tumors grown in culture following negative selection on non-GSCs. In vivo screening was performed by intravenous injections of the phage library into immunocompromised mice with intracranial primary GBM xenograft. Phage peptides that preferentially bound to the GSC population were recovered. BLAST analysis of the recovered peptides identified angiotensin I converting enzyme (ACE), a key protein in renin-angiotensin system (RAS). RT-qPCR, RNA-seq, ChIP-seq, and in silico data showed that GSCs highly expressed renin and angiotensinogen, components of RAS, and differentiated cancer cells and endothelial cells expressed ACE. TCGA and histologic analysis were also consistent with these findings. High expression of these RAS components correlated with poor patient survivals. To study the effect of RAS inhibitors on GSC proliferation, GSCs were treated with captopril, ramipril, lisinopril, and telmisartan, respectively. Interestingly, RAS inhibitors stimulated cell proliferation and tumorsphere formation. In conclusion, this study demonstrates that renin-angiotensin system as a potential therapeutic target for the treatment of glioblastoma.