Abstract
Direct therapeutic targeting of oncogenic RAS is currently still impossible due to lack of suitable pharmacological inhibitors. Because specific blockade of druggable RAS effectors might represent an alternative treatment approach, we evaluated the role of the Raf complex for multiple myeloma (MM) pathobiology. We found frequent overexpression of the Raf isoforms (A-, B- and C-Raf) and downstream activation of MEK1,2/ERK1,2 in MM cells. Concomitant inhibition of all Raf isoforms (pan-Raf inhibition) by RNAi or pharmacological inhibitors was required to strongly induce apoptosis in human MM cell lines (HMCLs), in primary MM cells in vitro, and in a syngeneic MM mouse model in vivo. The anti-MM effect of pan-Raf inhibition did not correlate with the RAS mutation status, and functionally appeared to involve both MEK-dependent and -independent mechanisms. Furthermore, transcriptome analyses revealed that pan-Raf activity affects PI3K-dependent signalling, thus highlighting a functional link between the RAS/Raf and PI3K/mTOR/Akt pro-survival pathways. Accordingly, pharmacological inhibition of PI3K strongly enhanced the anti-MM effect of pan-Raf inhibition in MM cell lines and in primary MM cells in vitro and in vivo. Concomitant pan-Raf/PI3K inhibition was also effective in carfilzomib- and lenalidomide-resistant MM models underscoring that this attractive therapeutic anti-MM strategy is suitable for immediate clinical translation.
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Acknowledgements
This work was supported in parts by grants from the Deutsche Forschungsgemeinschaft (KFO 216), and from the Interdisciplinary Center for Clinical Research of the University of Würzburg (B-188 and Z 6).
Author contributions
EM and SB designed the research, cloned shRNA expression vectors, performed viability assays and western blot analyses and wrote the manuscript. EM and CJS performed the GEP experiments and analysed the data. SK, DB, ABr and ABe conducted the in vivo experiments. SK and HS kept the cell culture, performed viability assays and western blot analyses. AM and AR performed and analysed immunohistochemical and immunofluorescent analyses. TStü contributed to the construction of shRNA expression vectors, performed viability assays with primary MM cells and wrote the manuscript. TSte analysed the RAS mutation status of MM cells. AB facilitated in vivo experiments and analysed the data. RCB wrote and approved the final version of the manuscript. MC conceived and designed the research, supervised the project, wrote the manuscript and approved the data and final version of the manuscript.
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Müller, E., Bauer, S., Stühmer, T. et al. Pan-Raf co-operates with PI3K-dependent signalling and critically contributes to myeloma cell survival independently of mutated RAS. Leukemia 31, 922–933 (2017). https://doi.org/10.1038/leu.2016.264
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DOI: https://doi.org/10.1038/leu.2016.264