The expression of pro-prion, a transmembrane isoform of the prion protein, leads to the constitutive activation of the canonical Wnt/β-catenin pathway to sustain the stem-like phenotype of human glioblastoma cells.

BACKGROUND

Cellular prion protein (PrP) is a widely expressed membrane-anchored glycoprotein, which has been associated with the development and progression of several types of human malignancies, controlling cancer stem cell activity. However, the different molecular mechanisms regulated by PrP in normal and tumor cells have not been characterized yet.

METHODS

To assess the role of PrP in patient-derived glioblastoma stem cell (GSC)-enriched cultures, we generated cell lines in which PrP was either overexpressed or down-regulated and investigated, in 2D and 3D cultures, its role in cell proliferation, migration, and invasion. We evaluated the role of PrP in supporting GSC stemness and the intracellular signaling involved using qRT-PCR, immunocytofluorescence, and Western blot.

RESULTS

Stable PrP down-regulation leads to a significant reduction of GSC proliferation, migration, and invasiveness. These effects were associated with the inhibition of the expression of stemness genes and overexpression of differentiation markers. At molecular level PrP down-regulation caused a significant inhibition of Wnt/β-catenin pathway, through a reduced expression of Wnt and Frizzled ligand/receptor subtypes, resulting in the inhibition of β-catenin transcriptional activity,  as demonstrated by the reduced expression of its target genes. The specificity of PrP in these effects was demonstrated by rescuing the phenotype and the biological activity of PrP down-regulated GSCs by re-expressing the protein. To get insights into the distinct mechanisms by which PrP regulates proliferation in GSCs, but not in normal astrocytes, we analyzed structural features of PrP in glioma stem cells and astrocytes using Western blot and immunofluorescence techniques. Using Pi-PLC, an enzyme that cleaves GPI anchors, we show that, in GSCs, PrP is retained within the plasma membrane in an immature Pro-PrP isoform whereas in astrocytes, it is expressed in its mature PrP form, anchored on the extracellular face of the plasma membrane.

CONCLUSIONS

The persistence of Pro-PrP in GSCs is an altered cellular mechanism responsible of the aberrant, constitutive activation of Wnt/β-catenin pathway, which contributes to glioblastoma malignant features. Thus, the activity of Pro-PrP may represent a targetable vulnerability in glioblastoma cells, offering a novel approach for differentiating and eradicating glioblastoma stem cells.