Abstract 3265: PI3Kδ inhibitor, GS-1101, impacts transcription and translation in mantle cell lymphoma.
Authors: Yang Qingshan , Chen Lisa S. , Neelapu Sattva S. , Lanutti Brian J. , Gandhi Varsha 
Published: August 2014

Abstract
PI3Kα, β, γ and δ are the most ubiquitous and important PI3K family class I member isoforms that contribute to cell growth and survival processes in cancer. Among these isoforms, PI3Kα and β are broadly expressed, where PI3Kγ and δ are uniquely expressed in T- and B-cells, respectively, making them ideal candidates for targeted therapies in hematological malignancies. GS-1101, also known as CAL-101, is a potent and selective PI3Kδ inhibitor (IC50=2.5 nM) that is currently in Phase II/III clinical trials in B-cell diseases. Previous studies have elucidated disruption of survival factors and cytokine signaling mediated by PI3K/Akt pathways as major mechanism of activity in B-cell malignancies.
It has been established that oncoprotein PI3Kα isoform deregulates transcription and translation processes in solid tumors, and based on the knowledge of PI3Kδ and GS-1101, we hypothesized that 1) GS-1101 treatment will inhibit gene transcription and protein translation, 2) will impact transcription and translation regulators that are downstream of PI3K/Akt pathways and 3) will reduce short-lived mRNA and proteins such as Mcl-1, c-Myc and cyclin D1, which are important in mantle cell lymphoma (MCL), a B-cell lymphoma. In JeKo-1 and Mino MCL cell lines, PI3Kδ is the most prominent isoform. In these cell lines and in primary MCL cells, the level of apoptosis was modest (less than 15%) when cells were treated with 5 μM GS-1101 for 24 hr. Consistent with Annexin/PI data, slight levels of PARP cleavage was detected. Of the two Akt phosphorylation sites, Thr308 and Ser473, phosphorylation of Akt (Thr308) showed dose-dependent decrease in both cell lines when treated by GS-1101 for 1 hr following serum starvation; however, there was no change in Akt (Ser347) levels following 24 hr of GS-1101 treatment in full serum conditions. Global RNA synthesis was substantially reduced in MCL primary samples, reaching 80% of control at 1 μM and 50% or less at 5 μM for 24 hr treatment, while only minor changes (80-90% of control) were observed in JeKo-1 and Mino. Global translation inhibition measured by leucine incorporation was observed in JeKo-1, Mino and primary MCL cells. Phosphorylation levels of translation regulators p70S6K (Ser9) and S6 (Ser235/236) showed dose-dependent decrease when treated with GS-1101 for 24 hr in JeKo-1. Meanwhile, minor decrease of 4E-BP1 (Ser65) phosphorylation level was observed in JeKo-1 under the same conditions. In Jeko-1 cells, early response genes such as short-lived MCL1 mRNA level was reduced following 24 hr of GS-1101 treatment, however, other (PIM2, MYC, CCND1) mRNA levels decreased to a lesser extent. At the protein level, both Mcl-1 and cyclin D1 expression decreased in JeKo-1 cells. In conclusion, similar to inhibition of other PI3K isoforms, PI3Kδ inhibition impacted transcription and translation, and reduced cyclin D1 and Mcl-1 levels which are important for MCL cell survival and proliferation.
Citation Format: Qingshan Yang, Lisa S. Chen, Sattva S. Neelapu, Brian J. Lanutti, Varsha Gandhi. PI3Kδ inhibitor, GS-1101, impacts transcription and translation in mantle cell lymphoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3265. doi:10.1158/1538-7445.AM2013-3265