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IL-6–Induced Stimulation of c-Myc Translation in Multiple Myeloma Cells Is Mediated by Myc Internal Ribosome Entry Site Function and the RNA-Binding Protein, hnRNP A1

¹ Department of Medicine, Greater Los Angeles VA Healthcare System and ² University of California at Los Angeles Medical School, Los Angeles, California

Requests for reprints: Alan Lichtenstein, W111H, Hematology, Building 500, Room 4237, VA West LA Hospital, 11301 Wilshire Boulevard, Los Angeles, CA 90073. E-mail: alan.lichtenstein{at}med.va.gov.

Key Words: multiple myeloma • interleukin-6 • c-myc • hnRNP A1 • IRES

Prior work indicates that c-myc translation is up-regulated in multiple myeloma cells. To test a role for interleukin (IL)-6 in myc translation, we studied the IL-6–responsive ANBL-6 and IL-6–autocrine U266 cell lines as well as primary patient samples. IL-6 increased c-myc translation, which was resistant to rapamycin, indicating a mechanism independent of mammalian target of rapamycin (mTOR) and cap-dependent translation. In contrast, the cytokine enhanced cap-independent translation via a stimulatory effect on the myc internal ribosome entry site (IRES). As known IRES-trans–activating factors (ITAF) were unaffected by IL-6, we used a yeast-three-hybrid screen to identify novel ITAFs and identified hnRNP A1 (A1) as a mediator of the IL-6 effect. A1 specifically interacted with the myc IRES in filter binding assays as well as EMSAs. Treatment of myeloma cells with IL-6 induced serine phosphorylation of A1 and increased its binding to the myc IRES in vivo in myeloma cells. Primary patient samples also showed binding between A1 and the IRES. RNA interference to knock down hnRNP A1 prevented an IL-6 increase in myc protein expression, myc IRES activity, and cell growth. These data point to hnRNP A1 as a critical regulator of c-myc translation and a potential therapeutic target in multiple myeloma. [Cancer Res 2008;68(24):10215–22]