G-protein coupled receptor kinase (GRK)-5 regulates proliferation of glioblastoma-derived stem cells

Glioblastoma multiforme (GBM) is a grade IV malignant brain tumor with high mortality and has been well known to involve many molecular pathways, including G-protein coupled receptor (GPCR)-mediated signaling (such as epithelial growth factor receptor [EGFR] and platelet derived growth factor receptor [PDGFR]). G protein-coupled receptor kinases (GRK) directly regulate GPCR activity by phosphorylating activated agonist-bound receptors to desensitize signaling and internalize receptors through beta-arrestins. Recent studies in various cancers, including prostate and breast cancer, have highlighted the role of change in GRK expression to oncogenesis and tumor proliferation. In this study, we evaluated the expression of GRK5 in grade II to grade IV glioma specimens using immunohistochemistry and found that GRK5 expression levels are highly correlated with aggressiveness of glioma. We used culture conditions to selectively promote the growth of either glioblastoma cells with stem cell markers (GSC) or differentiated glioblastoma cells (DGC) from fresh GBM specimens. GSC are known to be highly invasive and mobile, and have the capacity to self-renew and are more resistant to chemotherapy and radiation compared to differentiated populations of GBM. We examined the expression of GRK5 in these two sets of culturing conditions for GBM cells and found that GRK5 expression is upregulated in GSC compared to differentiated GBM cells. To better understand the role of GRK5 in GBM-derived stem cells, we created stable GRK5 knockdown and evaluated the proliferation rate. Using an ATP chemiluminescence assay, we show, for the first time, that knocking down the expression of GRK5 decreased the proliferation rate of GSC in contrast to control.