Receptor synergy of interleukin-6 (IL-6) and insulin-like growth factor-I in myeloma cells that highly express IL-6 receptor α

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Abstract
Interleukin-6 (IL-6) is a growth and antiapoptotic factor for human myeloma cells. The autocrine loop and increased expression of the growth factor receptors have been postulated as the mechanisms of tumorigenesis. Here we show that IL-6 stimulation induced the phosphorylation of insulin-like growth factor-I (IGF-I) receptors in a human myeloma cell line, NOP2, highly expressing IL-6 receptor α (IL-6Rα) and in the IL-6Rα–transfected U266 cell line. IL-6–dependent complex formation of IL-6Rα with IGF-I receptor β was found in NOP2 where IL-6Rα colocalized with IGF-I receptors at lipid rafts. Moreover, the IL-6–induced phosphorylation of IGF-I receptor β was not blocked by a Janus kinase 2 (Jak2) inhibitor. In addition to the activation of the signal transducer and activator of transcription 3 and extracellular signal-regulated kinase 1/2, IL-6 stimulation led to the activation of Akt, presumably following the phosphorylation of IGF-I receptors. Thus, our results suggest that in NOP2, IL-6Rα and IGF-I receptors exist on the plasma membrane in close proximity, facilitating the efficient assembly of 2 receptors in response to IL-6. The synergistic effects of highly expressed IL-6Rα on IGF-I receptor–mediated signals provide a novel insight into the Jak-independent IL-6 signaling mechanism of receptor cross-talk in human myeloma cells.