Abstract 1787: Selective activation of a PI3K catalytic isoform by G protein-coupled receptors in glioblastoma

Abstract Objective: To determine how PI3K catalytic isoforms become dysregulated in glioblastoma. Background: Recurrence in glioblastoma (GBM) is common and even more resistant to treatment than initial GBM. The Sheng Lab has previously identified that PIK3CB expression levels, but not other PI3K subtypes, are higher in recurrent GBM and are associated with GBM recurrence in newly diagnosed individuals. PI3KCB encodes for PI3K catalytic subunit p110β, and PI3K/AKT pathway activation is a well-known promoter of cell survival. Understanding mechanisms of PIK3CB activation in GBM may identify new therapeutic targets and improve GBM survival. Current knowledge of PI3K isoform activity in GBM is insufficient and contradictory. Given previous studies, it was hypothesized that G-Coupled Protein Receptor (GPCR) promoters preferentially activate PI3K/AKT in PIK3CBhigh GBM cells. Further, that knockdown of PIK3CB, but not other PI3K isoforms, inhibits PI3K/AKT reactivation in PIK3CBhigh cell lines. Design/Methods: U87MG cells, an established p110βhigh GBM cell line, were starved overnight in serum free media, and then treated with growth factors, promoters or inhibitors of several previously hypothesized pathways. Cell lysates were collected and PI3K/AKT pathway activity was measured with immunoblotting. The ratio of pAKT/AKT/ACTB quantified PI3K/AKT pathway activity. Results: Overnight serum starvation depleted pAKT, and incubation with 1% FBS restored pAKT signaling in a time dependent manner. Neither EGF nor FGF-2 activated PI3K/AKT signaling. Gefitinib, an EGFR inhibitor, failed to inhibit reactivation when cells were co-treated with 1% FBS. However, GPCR stimulation by Lysophosphatidic acid (LPA) increased pAKT/AKT ratios in a dose dependent manner. More significantly, Gallein, a GPCR inhibitor, suppressed PI3K/AKT signaling when cells were co-treated with 1% FBS. Selective PI3K inhibitors showed little PI3K/AKT reactivation at multiple doses. Conclusions: These results suggest that GPCR signaling is responsible for PI3K/AKT pathway activation in p110βhigh cells. Citation Format: Abigail R. Winn, Zhi Sheng. Selective activation of a PI3K catalytic isoform by G protein-coupled receptors in glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1787.