Interleukin-6 (IL-6) Enhances C-MYC Translation IN MULTIPLE MYELOMA (MM) CELLS: ROLE of IL-6-INDUCED EFFECTS On the C-MYC RNA-Binding PROTEIN, HNRNPA1.

Abstract 3839

Poster Board III-775

Our previous work (Cancer Research 68:10215, 2008) demonstrated that IL-6 enhanced c-myc protein expression in MM cells and function of the RNA-binding protein, hnRNPA1 (A1), was required. This occurred by way of enhanced cap-independent translation mediated via the internal ribosome entry site (IRES) in the 5'UTR of the myc RNA. IRES-dependent translation is the fail safe mechanism for protein expression when cap-dependent translation is suppressed by mTOR inhibition (with curtailed RNA cap-ribosome binding) and is especially important when an mRNA leader is relatively long and highly structured, such that scanning ribosomes are unlikely to efficiently initiate translation. The IRES directly recruits the ribosome to within close proximity to the start codon, bypassing the need for cap binding and ribosome scanning. HNRNPA1 (A1) is a documented IRES trans-acting factor (ITAF) for the myc IRES, facilitating translation but how its effects are enhanced by IL-6 is unknown. HNRNPA1 is a shuttling protein which binds the myc RNA in the nucleus and transports it to the cytoplasm. Initial experiments demonstrated that IL-6 stimulation of ANBL-6 and OCI-My5 MM cell lines, as well as several primary MM specimens, significantly increased the cytoplasmic localization of A1. To test if this enhanced cytoplasmic localization was critical, we stably expressed a dominant negative (DN) A1 gene that is incapable of nuclear-to-cytoplasmic shuttling and which also prevents endogenous hnRNPA1 shuttling. The DN also prevented A1-mediated transport of the shuttling protein, FUS. Expression of the DN in ANBL-6 cells prevented IL-6-induced effects on myc expression and on ANBL-6 cell growth. We also tested if IL-6 treatment affected A1 binding to the myc RNA by immunoprecipitating A1 and performing real time PCR on the immunoprecipitate for myc RNA levels. A significant increase in A1-myc RNA binding was confirmed. Mass spectroscopy demonstrated that IL-6 induced phosphorylation of A1 in its RNA-binding domain, which possibly mediated the enhanced binding. These results demonstrate that, by enhanced binding of the myc ITAF, hnRNPA1, to the myc IRES, and by enhanced transport of the complex to the translationally active cytoplasmic subcellular site, IL-6 may stimulate c-myc translation and subsequent MM cell growth.