Abstract 329: Selinexor or KPT-8602 mediated XPO1 inhibition synergizes with dexamethasone to repress convergent pathways in the mTORC1 signaling network and drive cell death in multiple myeloma (2017)

Abstract Background: Dexamethasone (DEX), a synthetic glucocorticoid (GC), is administered to nearly all multiple myeloma (MM) patients as a single agent and in combination with other chemotherapies or targeted agents. DEX and other GCs bind to glucocorticoid receptors (GR) in the cytoplasm, induce nuclear translocation and regulate GR-dependent gene expression networks in the nucleus. Selective Inhibitor of Nuclear Export (SINE) compounds (selinexor and KPT-8602) exhibit potent anti-tumor activity in MM especially when combined with DEX. SINE compounds enhance nuclear localization of tumor suppressor proteins (TSPs) through inhibition of the nuclear export protein, XPO1. We discovered that inhibition of the mechanistic Target of Rapamycin Complex 1 (mTORC1) pathway is a primary driver of the combination effect. Here we further elucidate the molecular mechanism of action of the SINE-DEX synergy in MM. Methods: GR+ MM.1S and GRnull MM.1R MM cell lines were treated with SINE compounds and/or DEX for 24 hours. Whole cell lysates were subjected to SDS-PAGE and western blot analysis. Gene expression and GR transcriptional activity was analyzed using qPCR and ELISA, respectively. Cell viability was examined using the Celltiter-Fluor assay. Results: We found that MM cell lines treated with SINE compounds (selinexor or KPT-8602) have increased basal GR protein levels. Consistent with these results, the SINE-DEX combination shows enhanced GR transcriptional activity. Several GR-DEX target genes are known to inhibit the GTPase, Ras homolog enriched in brain (RHEB), which is required for mTORC1 activation. We discovered that the SINE-DEX combination not only reduces RHEB protein but also induces the RHEB inhibitory pathways containing REDD1 and the KLF15-BCAT2 axis. Although SINE compound-mediated inhibition of mTORC1 (i.e. reduced phosphorylation of S6K1 and 4E-BP1) is GR independent, SINE-DEX inhibition is more robust in GR+ MM.1S cell line when compared to the GRnull MM.1R cells. The combination resulted in the selinexor IC50 in MM.1S cells shifting from 40 nM to 11 nM in the presence of low dose DEX. As expected, DEX did not modulate the SINE compounds IC50s in MM.1R cells. Conclusion: We show that SINE compound inhibition of MM cell viability is enhanced with DEX. Our results indicate that this combinatorial effect is due to convergent suppression of mTORC1 signaling by GR targets. The findings provide mechanism of action data around the SINE-DEX combination in MM with suggestive biomarkers (REDD1, KLF15, BCAT2 and GR) that may predict best response to the combination. Therefore, these data may translate directly to the current clinical development of SINE compounds. Citation Format: Christian Argueta, Trinayan Kashyap, Boris Klebanov, Hua Chang, Sharon Friedlander, Erkan Baloglu, Yosef Landesman, Margaret Lee, Humphrey Gardner, Sharon Shacham, William Senapedis. Selinexor or KPT-8602 mediated XPO1 inhibition synergizes with dexamethasone to repress convergent pathways in the mTORC1 signaling network and drive cell death in multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 329. doi:10.1158/1538-7445.AM2017-329