Abstract PR02: RSK1 targeting impedes oncogenic driver and inflammatory cytokine signaling to attenuate myeloid neoplasms
Authors: Kong Tim , Laranjeira Angelo , Oh Stephen 
Published: May 2023

Abstract
Myeloproliferative neoplasms (MPNs), chronic myelomonocytic leukemia (CMML), and acute myeloid leukemia (AML) exist among a spectrum of myeloid malignancies, governed by critical genetic dependencies, and propagated by a ripe pro-inflammatory niche. We previously identified elevated RPS6KA1 (RSK1) expression in myeloid malignancies associated with poor overall survival. Here, we demonstrate that RSK1 plays a fundamental role in mediating inflammation and serves as a central signaling node driving leukemogenesis. We further evaluate a novel therapeutic regimen utilizing a first-in-class RSK1 inhibitor, PMD-026, currently evaluated in Phase 1/1b clinical trials for breast cancer, for use in myeloid malignancies. Plasma analysis of 144 MPN patients and RNA-seq of CD14+ monocytes from 54 MPN patients revealed enrichment of inflammatory cytokines and hyperactive NFκB signaling. RSK1 inhibition by shRNA or PMD-026 in THP-1 monocytic AML cells potently suppressed this NFκB node, decreased TNF and NFKB1 mRNA, and reduced luminescence in cells expressing a NFκB-luciferase reporter. Immunoblotting and cytokine mass cytometry revealed suppression of RelA/p65 phosphorylation with reduction of TNF, IL-6, IL-8, CCL3, and CCL4 in MPN and CMML CD14+ monocytes. We also observed that RSK1 inhibition attenuated NFκB activation by TNF, LPS, and Pam3CSK4, while also suppressing monocyte differentiation to M1 macrophages. In the MPL W515L myelofibrosis (MF) model, mice treated with PMD-026 had significant reduction in TNF, IL-6, IL-1b, CCL3, and CCL5 by multiplex cytokine profiling and diminished disease burden (reduction of leukocytosis, splenomegaly, and bone marrow fibrosis). Finally, amelioration of disease was observed across MF and sAML patient-derived xenograft (PDX) models with PMD-026 treatment. We extended our findings to FLT3-ITD de novo AML models, in which RSK1 inhibition led to lethality in FLT3-ITD cells and PMD-026 potently suppressed leukemic engraftment in a MV4-11 mouse xenograft. Both genetic and pharmacological inhibition of RSK1 reduced FLT3 phosphorylation and revealed a novel, bi-directional regulatory network. Moreover, we found that RSK1 perturbation also attenuated FLT3 expression and altered protein stability, which was not observed with FLT3 inhibitor quizartinib. Mechanistically, RSK1 modulated FLT3 dynamics through deubiquitinase USP1, which was downregulated upon RSK1 perturbation. USP1 perturbation led to cell death and phenocopied FLT3 suppression observed with RSK1 inhibition. We further identified elevated USP1 expression associated with worse survival outcomes across the TCGA LAML, BeatAML, and Leucegene cohorts, thus indicating the potential of USP1 as a novel biomarker. We uncover a role for targeting RSK1 as a two-pronged approach in reducing disease burden across myeloid malignancies via: 1) suppressing driver signaling, and 2) dampening the pro-inflammatory milieu. These findings highlight a critical dependency along the RSK1-USP1-FLT3 axis that is potentially exploitable with RSK1 inhibitors such as PMD-026.
Citation Format: Tim Kong, Angelo Laranjeira, Stephen Oh. RSK1 targeting impedes oncogenic driver and inflammatory cytokine signaling to attenuate myeloid neoplasms [abstract]. In: Proceedings of the AACR Special Conference: Acute Myeloid Leukemia and Myelodysplastic Syndrome; 2023 Jan 23-25; Austin, TX. Philadelphia (PA): AACR; Blood Cancer Discov 2023;4(3_Suppl):Abstract nr PR02.