IL-6-induced Enhancement of c-Myc Translation in Multiple Myeloma Cells

Abstract

Prior work indicates that IL-6 can stimulate c-Myc expression in multiple myeloma (MM) cells, which is independent of effects on transcription and due to enhanced translation mediated by an internal ribosome entry site in the 5-UTR of the c-Myc RNA. The RNA-binding protein hnRNP A1 (A1) was also critical to IL-6-stimulated translation. Because A1 shuttles between nucleus and cytoplasm, we investigated whether the ability of IL-6 to enhance Myc translation was mediated by stimulation of A1 shuttling. In MM cell lines and primary specimens, IL-6 increased A1 cytoplasmic localization. In contrast, there was no effect on the total cellular levels of A1. Use of a dominant negative A1 construct, which prevents endogenous A1 from nucleus-to-cytoplasm transit, prevented the ability of IL-6 to enhance Myc internal ribosome entry site activity, Myc protein expression, and MM cell growth. IL-6-stimulated cytoplasmic localization was mediated by alterations in the C-terminal M9 peptide of A1, and this correlated with the ability of IL-6 to induce serine phosphorylation of this domain. A p38 kinase inhibitor prevented IL-6-induced A1 phosphorylation. Thus, IL-6 activates c-Myc translation in MM cells by inducing A1 phosphorylation and cytoplasmic localization in a p38-dependent fashion. These data suggest A1 as a potential therapeutic target in MM.Our previous work (1) demonstrated that IL-6 could enhance c-Myc protein expression in multiple myeloma (MM) 2 cells independent of any effect on Myc transcription. This was not due to altered c-Myc protein stability or the mammalian target of rapamycin-dependent stimulation of cap-dependent translation. In contrast, it resulted from an up-regulation of Myc IRES activity and cap-independent translation. IRESes are structures in the 5Ј-UTR of transcripts that can promote translation and are especially important when RNA leaders are highly structured or when cap-dependent translation is inhibited (2, 3). The three-dimensional IRES structure recruits the 40 S ribosomal subunit to the mRNA for translation initiation. The Myc IRES is well characterized (4, 5). In addition, there are trans-acting proteins that facilitate Myc IRES function via binding to the RNA at sites in the 5Ј-UTR and inducing conformational changes that facilitate recruitment of the IRES to the ribosome (4). Several publications (6, 7) document a myeloma-specific up-regulation of Myc IRES function.The ability of IL-6 to stimulate Myc IRES function in MM cells, up-regulate rapamycin-resistant Myc translation and expression, and stimulate MM cell proliferation were all prevented by knockdown of hnRNP A1, an RNA-binding protein (1). Because other studies (8 -10) demonstrate that hnRNP A1 (A1) can function as an IRES-trans-activating factor (ITAF), regulating Myc IRES activity, these results collectively indicate that IL-6-up-regulated c-Myc translation in MM cells was dependent on enhanced A1 ITAF function.hnRNP A1 is a shuttling protein with movement from nucleus to cytoplasm (11), and a recent study (12) demonstrated ...

Selective targeting of IRF4 by synthetic microRNA-125b-5p mimics induces anti-multiple myeloma activity in vitro and in vivo

“… 3 In this regard, we found that exogenous interleukin-6 (IL-6) or insulin-like growth-factor-1 (IGF-1) or hepatocyte growth-factor (HGF) significantly reduced miR-125b-5p expression in all MM cell lines except U266 cells ( Figure 6a ), which express L-Myc instead of c-Myc. Since c-Myc suppresses miR-125b transcription 39 , 40 and is upregulated by IL-6, IGF-1 and HGF, 40 , 41 , 42 we investigated whether these factors downregulate miR-125b in MM cells through c-Myc induction. Specifically, we treated c-Myc-expressing SK-MM-1 and c-Myc-defective U266 cells with the 10058-F4 small molecule inhibitor of Myc–Max heterodimerization 38 and with the JQ1 BET-bromodomain inhibitor, which is reported to inhibit c-Myc transcription.…”

MMSET stimulates myeloma cell growth through microRNA-mediated modulation of c-MYC

“…25 Furthermore, interleukin-6, a critical cytokine for myeloma growth, also stimulates c-MYC translation. 26 In this work, we describe a novel mechanism to control the expression of c-MYC: the regulation of its translation by the HMT MMSET.…”

Interactions between Myc and Mediators of Inflammation in Chronic Liver Diseases

“…IL-6 can enhance the translation of c-Myc in multiple myeloma cells [ 42 ]. Moreover, IL-6 can promote c-Myc expression and cultured vascular smooth muscle cell proliferation [ 43 ].…”

“…STAT3 activation transduces IL-6 signaling, which induces the production of acute phase proteins such as fibrinogen and haptoglobin. IL-6 could enhance c-Myc protein expression in multiple myeloma cells independent of any effect on Myc transcription [ 42 ]. Also, IL-6 can reverse CD33 expression by upregulating Myc and subsequently downregulating CCAAT/enhancer binding protein (CEBPA) expression in myeloma cells [ 43 ].…”