PIK3CB/p110 beta is a selective survival factor for glioblastoma

Glioblastoma (GBM) is difficult to treat. Phosphoinositide 3-kinase (PI3K) is an attractive therapeutic target for GBM; however, targeting this pathway to effectively treat GBM is not successful because the roles of PI3K isoforms remain to be defined. The aim of this study is to determine whether PIK3CB/p110 beta, but not other PI3K isoforms, is a biomarker for GBM recurrence and important for cell survival. Gene expression and clinical relevance of PI3K genes in GBM patients were analyzed using online databases. Expression/activity of PI3K isoforms was determined using immunoblotting. PI3K genes were inhibited using short hairpin RNAs or isoform-selective inhibitors. Cell viability/growth was assessed by the MTS assay and trypan blue exclusion assay. Apoptosis was monitored using the caspase activity assay. Mouse GBM xenograft models were used to gauge drug efficacy. PIK3CB/p110 beta was the only PI3K catalytic isoform that significantly correlated with high incidence rate, risk, and poor survival of recurrent GBM. PIK3CA/p110 alpha, PIK3CB/p110 beta, and PIK3CD/p110 delta were differentially expressed in GBM cell lines and primary tumor cells derived from patient specimens, whereas PIK3CG/p110 gamma was barely detected. PIK3CB/p110 beta protein levels presented a stronger association with the activities of PI3K signaling than other PI3K isoforms. Blocking p110 beta deactivated PI3K signaling, whereas inhibition of other PI3K isoforms had no effect. Specific inhibitors of PIK3CB/p110 beta, but not other PI3K isoforms, remarkably suppressed viability and growth of GBM cells and xenograft tumors in mice, with minimal cytotoxic effects on astrocytes. PIK3CB/p110 beta is a biomarker for GBM recurrence and selectively important for GBM cell survival.