Dose-dependent toxicity of FTY720, a sphingosine-1-phosphate receptor agonist, on double hit lymphoma cells via PP2A-mediated dephosphorylation of c-myc and bcl-2

High grade B cell lymphoma with translocations involving MYC and rearrangements involving BCL2 and/or BCL6 define an aggressive subtype of diffuse large B cell lymphoma (DLBCL) with poor prognosis and high rates of relapse. The standard treatment for DLBCL is not considered effective and targeted molecular therapy has yet to be identified for this disease. The mechanisms that make this type of lymphoma particularly aggressive and refractory to treatment include cell growth signals from the c-myc transcriptional regulator working together with the anti-apoptotic activity of the bcl-2 protein. Protein Phosphatase 2A (PP2A) is a ubiquitous cellular enzyme that acts as a tumor suppressor through dephosphorylation of specific cellular phosphoproteins. In fact, PP2A has been shown to dephosphorylate bcl-2 at ser70 leading to inhibition of its function. Further, PP2A-mediated dephosphorylation of c-myc at ser62 promotes its degradation. In many cancers, PP2A activity is inhibited due to overexpression of binding proteins such as SET and CIP2A. Small molecules can act as PP2A activators by releasing PP2A from these inhibitory binding proteins. In the present study we investigated the efficacy of FTY720, a high-affinity agonist of the sphingosine 1-phosphate receptor-1 and PP2A activator, on the double hit lymphoma derived cell line, WSU DLCL2. WSU DLCL2 cells were treated with various concentrations of FTY720, a derivative of myriocin, a fungal metabolite of the Chinese herb Iscaria sinclarii known to promote PP2A activation. Cell viability and PP2A activity were measured 24 hours after FTY720 treatment. In addition, cell lysates were assayed by Western blot using antibodies to detect PP2A subunits, as well as total and phosphorylated c-myc and bcl-2 proteins. A dose-dependent decrease in cell viability was observed over the range of FTY720 concentrations from 1 to 10 micromolar with concomitant increase in in vitro PP2A activity. Western blots showed decreases in c-myc protein levels with FTY720 treatment as well as decreases in ser70 phosphorylation levels of bcl-2 and ser62 phosphorylation levels of c-myc. The results indicate that PP2A activation via FTY720, a small molecule activator of PP2A, leads to a dose-dependent induction of cell death in double hit lymphoma. The data are consistent with an underlying cell death mechanism that involves the targeted dephosphorylation of c-myc and bcl2. Taken together, these data suggest that FTY720 may act as a targeted treatment with therapeutic potential as a single agent or as a combination with other therapies in the treatment of this subtype of lymphoma. There is also the potential of a therapeutic role in other lymphoma subtypes with poor prognosis including transformed follicular lymphoma and relapsed burkitt9s lymphoma that rely individually on bcl-2 and c-myc, respectively, for their growth. Citation Format: Geoffrey P. Shouse, Rosalia de Necochea-Campion, Saied Mirshahidi, Chien-Shing Chen, Kimberly J. Payne. Dose-dependent toxicity of FTY720, a sphingosine-1-phosphate receptor agonist, on double hit lymphoma cells via PP2A-mediated dephosphorylation of c-myc and bcl-2 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5442.