Abstract
RAS mutations occur frequently in multiple myeloma (MM), but apart from driving progression, they can also stimulate antitumor effects by activating tumor-suppressive RASSF proteins. Although this family of death effector molecules are often silenced in cancers, functional data about RASSF proteins in MM are lacking. Here, we report that RASSF4 is downregulated during MM progression and correlates with a poor prognosis. Promoter methylation analysis in human cell lines revealed an inverse correlation between RASSF4 mRNA levels and methylation status. Epigenetic modulating agents restored RASSF4 expression. Enforced expression of RASSF4 induced G2-phase cell-cycle arrest and apoptosis in human cell lines, reduced primary MM cell viability, and blocked MM growth in vivo Mechanistic investigations showed that RASSF4 linked RAS to several pro-death pathways, including those regulated by the kinases MST1, JNK, and p38. By activating MST1 and the JNK/c-Jun pathway, RASSF4 sensitized MM cells to bortezomib. Genetic or pharmacological elevation of RASSF4 levels increased the anti-MM effects of the clinical relevant MEK1/2 inhibitor trametinib. Kinome analysis revealed that this effect was mediated by concomitant activation of the JNK/c-Jun pathway along with inactivation of the MEK/ERK and PI3K/mTOR/Akt pathways. Overall, our findings establish RASSF4 as a tumor-suppressive hub in MM and provide a mechanistic rationale for combining trametinib with HDAC inhibitors or bortezomib to treat patients with tumors exhibiting low RASSF4 expression.Significance: These findings provide a mechanistic rationale for combining trametinib with HDAC inhibitors or bortezomib in patients with multiple myeloma whose tumors exhibit low RASSF4 expression. Cancer Res; 78(5); 1155-68. ©2017 AACR.
Highlights
Multiple myeloma (MM) is a plasma cell malignancy that mainly resides in the bone marrow (BM) and remains most often incurable
This analysis revealed that RASSF2 and RASSF4 expression was significantly reduced in MM cells compared to normal BMPC, whereas RASSF1, RASSF5 and RASSF6 expression was significantly upregulated in MM cells (Fig. 1A)
Methylation analysis of the RASSF4 promoter in human myeloma cell lines (HMCL) shows an inverse correlation between RASSF4 expression levels and methylation status and treatment with the histone deacetylase inhibitor (HDACi) quisinostat and/or DNA methyltransferase inhibitor (DNMTi) decitabine restores RASSF4 transcription both in HMCL and the 5T33MM model
Summary
Multiple myeloma (MM) is a plasma cell malignancy that mainly resides in the bone marrow (BM) and remains most often incurable. An important feature of MM is its genetic and subclonal heterogeneity [1]. Mutations in RAS represent the most common mutations in cancer [2]. In MM, RAS is frequently mutated (20%–45.9%) and the incidence increases during disease progression [3, 4]. In a refractory setting up to 72% of the patients harbor a mutation in NRAS, KRAS and/or BRAF [5]. The MEK/ERK and PI3K/Akt pathway are the 2 main effector pathways
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