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Ferrata Storti Foundation
Haematologica 2020 Volume 105(9):2316-2326
Plasma Cell Disorders
RAL GTPases mediate multiple myeloma cell survival and are activated independently of oncogenic RAS
Marcel Seibold,1 Thorsten Stühmer,2 Nadine Kremer,2 Anja Mottok,3 Claus-Jürgen Scholz,4 Andreas Schlosser,5 Ellen Leich,6 Ulrike Holzgrabe,7 Daniela Brünnert,2 Santiago Barrio,8 K. Martin Kortüm,1
Antonio G. Solimando,1 Manik Chatterjee,2 Hermann Einsele,1
Andreas Rosenwald,6 Ralf C. Bargou2 and Torsten Steinbrunn1
1Department of Medicine II, University Hospital of Würzburg, Würzburg, Germany; 2Comprehensive Cancer Center Mainfranken, Chair of Translational Oncology, University Hospital of Würzburg, Würzburg, Germany; 3Institute of Human Genetics, University of Ulm, Ulm, Germany; 4Core Unit Systems Medicine, University of Würzburg, Würzburg, Germany; 5Rudolf Virchow Center for Experimental Biomedicine, University of Würzburg, Würzburg, Germany; 6Institute of Pathology, University of Würzburg, Würzburg, Germany; 7Institute of Pharmacy and Food Chemistry, University of Würzburg, Würzburg, Germany and 8Hematology Department, Hospital 12 de Octubre, Complutense University, Madrid, Spain
ABSTRACT
Oncogenic RAS provides crucial survival signaling for up to half of multiple myeloma (MM) cases, but has so far remained a clinically undruggable target. RAS-like protein (RAL) is a member of the RAS superfamily of small GTPases and is considered to be a potential mediator of oncogenic RAS signaling. In primary MM, we found RAL to be overex- pressed in the vast majority of samples when compared with pre-malignant monoclonal gammopathy of undetermined significance or normal plasma cells. We analyzed the functional effects of RAL abrogation in myeloma cell lines and found that RAL is a critical mediator of survival. RNAi-mediated knockdown of RAL resulted in rapid induction of tumor cell death, an effect which was independent from signaling via mitogen-activated protein kinase, but appears to be partially dependent on Akt activity. Notably, RAL activation was not correlated with the presence of activating RAS muta- tions and remained unaffected by knockdown of oncogenic RAS. Furthermore, transcriptome analysis yielded distinct RNA expression signa- tures after knockdown of either RAS or RAL. Combining RAL depletion with clinically relevant anti-myeloma agents led to enhanced rates of cell death. Our data demonstrate that RAL promotes MM cell survival inde- pendently of oncogenic RAS and, thus, this pathway represents a potential therapeutic target in its own right.
Introduction
Mutated RAS is one the most frequent oncogenic drivers in human cancers, yet it has so far confounded efforts to render it a clinically exploitable drug target.1–4 Consequently, the identification and targeting of RAS effector pathways has been pursued to establish therapeutic approaches that counter RAS-driven tumors.5–7 Multiple myeloma (MM) harbors oncogenic NRAS or KRAS mutations in up to half of the cases and we have shown that RNA-mediated knockdown of oncogenic RAS induces apoptosis in MM cell lines.8–10
In vitro, the so-called classical RAS-associated pathways which signal via RAF/MAPK and PI3K/Akt, respectively, have been studied at different levels in MM cells and have been shown to be crucial for MM cell survival.11–19 In addition, we have demonstrated that although inhibition of one or both of these pathways can strongly affect MM cell growth and survival in vitro,8,14 their constitutive activation appears not to be directly correlated with the presence of oncogenic RAS.8,11 However, early clinical trials including MM patients treated with pharmacological inhibitors of either one of these pathways have shown only limited efficacy,20–22
Correspondence:
TORSTEN STEINBRUNN
steinbrunn_t@ukw.de
Received: March 26, 2019. Accepted: October 10, 2019. Pre-published: October 10, 2019.
doi:10.3324/haematol.2019.223024 ©2020 Ferrata Storti Foundation
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