Glioblastoma (GBM), a highly aggressive brain tumor, contains a subpopulation of glioblastoma stem-like cells (GSCs) that play roles in tumor maintenance, invasion, and therapeutic resistance. GSCs are therefore a promising target for GBM treatment. Our group identified the cellular prion protein (PrP(C)) and its partner, the co-chaperone Hsp70/90 organizing protein (HOP), as potential target candidates due to their role in GBM tumorigenesis and in neural stem cell maintenance.
GSCs expressing different levels of PrP(C) were cultured as neurospheres with growth factors, and characterized with stem cells markers and adhesion molecules markers through immunofluorescence and flow cytometry. We than evaluated GSC self-renewal and proliferation by clonal density assays and BrdU incorporation, respectively, in front of recombinant HOP treatment, combined or not with a HOP peptide which mimics the PrP(C) binding site. Stable silencing of HOP was also performed in parental and/or PrP(C)-depleted cell populations, and proliferation in vitro and tumor growth in vivo were evaluated. Migration assays were performed on laminin-1 pre-coated glass.
We observed that, when GBM cells are cultured as neurospheres, they express specific stemness markers such as CD133, CD15, Oct4, and SOX2; PrP(C) is upregulated compared to monolayer culture and co-localizes with CD133. PrP(C) silencing downregulates the expression of molecules associated with cancer stem cells, upregulates markers of cell differentiation and affects GSC self-renewal, pointing to a pivotal role for PrP(C) in the maintenance of GSCs. Exogenous HOP treatment increases proliferation and self-renewal of GSCs in a PrP(C)-dependent manner while HOP knockdown disturbs the proliferation process. In vivo, PrP(C) and/or HOP knockdown potently inhibits the growth of subcutaneously implanted glioblastoma cells. In addition, disruption of the PrP(C)-HOP complex by a HOP peptide, which mimics the PrP(C) binding site, affects GSC self-renewal and proliferation indicating that the HOP-PrP(C) complex is required for GSC stemness. Furthermore, PrP(C)-depleted GSCs downregulate cell adhesion-related proteins and impair cell migration indicating a putative role for PrP(C) in the cell surface stability of cell adhesion molecules and GBM cell invasiveness, respectively.
In conclusion, our results show that the modulation of HOP-PrP(C) engagement or the decrease of PrP(C) and HOP expression may represent a potential therapeutic intervention in GBM, regulating glioblastoma stem-like cell self-renewal, proliferation, and migration.