Abstract 316: Effect of schweinfurthins on malignant plasma cells

Abstract Multiple myeloma (MM), a bone marrow neoplasm is currently incurable and effective alternative therapies are needed. Schweinfurthin-3114 (S-3114) purified from a plant extract, inhibits growth of established MM cell lines. The mechanism(s) for this effect are not fully characterized. To advance understanding, MM cell lines RPMI-8226 and MM.1s were incubated with increasing concentrations of S-3114 for up to 48 hours. S-3114 decreased cell viability (trypan blue assay) and mitochondrial activity (MTT assay) in a concentration and time-dependent manner (p≤0.05). RPMI-8226 cells were more sensitive (EC50 2.36 nM) than MM.1s (EC50 11.1 nM) at 48 hours. Flow cytometry analyses revealed increased apoptosis in a concentration and time-dependent manner in both cell lines (64.0% in RPMI-8226 with 20 nM S-3114; 86.3% in MM.1s with 270 nM S-3114, both at 48 hours). S-3114 also induced cell cycle arrest at G2/M. In RPMI-8226, the percent of cells arrested in G2/M increased from 45.9 to 66.1 with 3.5 nM S-3114 at 24 and 48 hours, respectively. In RPMI-8226, 20 nM S-3114 for 24 and 48 hours caused a 100% G2/M arrest. In both cell lines, S-3114 reduced protein kinase B (Akt) activation (Western blot) in a concentration and time-dependent manner. In MM.1s the apoptotic extrinsic pathway (caspase 8), and not the intrinsic pathway (caspase 9), is highly involved. In fact, S-3114 (20 nM for 48 hours, and 270 nM for 24 and 48 hours) caused a major cleavage for both 43 and 18 KDa caspase 8 fragments. Also cleaved Poly ADP-Ribose Polymerase 1 (PARP-1) increased at S-3114 concentrations of 20 and 270 nM at 24 and 48 hours. S-3114 influence on Golgi-mediated secretion was evaluated by measuring the soluble chemokine Macrophage Inflammatory Protein-1 alpha (MIP-1α), which is secreted by the Golgi and it is involved in the progression of MM disease. S-3114 treatment for 48 hours stimulated the MIP-1α negative cell line (RPMI-8226) to significantly increase MIP-1α RNA levels and to secrete MIP-1α at low concentrations. On the contrary, despite that prolonged S-3114 treatment significantly increased MIP-1α RNA levels, its secretion is decreased in the MIP-1α positive cell line (MM.1s). These in vitro studies demonstrate that S-3114 impairs the Phosphoinositide 3-Kinases (PI3K)/Akt signaling pathway resulting in cell death and/or cell cycle arrest. Data also shows an increased gene expression of the pro-survival MIP-1α by S-3114. Our ongoing studies are focused on schweinfurthin effects on MIP-1 α and Golgi secretion. We postulate that the cytotoxic impact of S-3114 requires disruption of MIP-1α signaling to regulate cell cycle, survival, and impairment of Golgi architecture and secretion in malignant plasma cells. In aggregate, these results reveal mechanism(s) for S-3114 effects and support further work advancing S-3114 as a novel therapy for MM. Citation Format: Barbara Manfredi, Raymond J. Hohl. Effect of schweinfurthins on malignant plasma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 316.