Analysis of the IL-6-mediated intracellular signaling molecules in myeloma cell growth

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A cytokine, interleukin-6 (IL-6) is a growth factor for human myeloma cells. Upon IL-6 stimulation, signal transducer and activator of transcription (STAT) 3 and extracellular signal-regulated kinase (ERK) 1/2 are activated. We recently found that in myeloma cell lines the activation of both STAT3 and ERK1/2 is not sufficient for the IL-6-induced proliferation, which further requires the activation of the src family protein-tyrosine kinases (PTKs), such as Lyn, associated with the CD45 protein-tyrosine phosphatase (PTP). To explore the molecular mechanism that governs how IL-6 stimulation induces the proliferation of myeloma cells, such as downstream molecules of Lyn, we have established the myeloma cell lines, ILKM2 and ILKM8 overexpressing Lyn. We have found that both the Lyn-overexpressed ILKM2 and ILKM8 have the higher proliferative rate in response to IL-6 than that of their mock counterpart. In contrast, in the absence of IL-6 these Lyn-overexpressed cell lines showed higher susc … More eptibility to cell death. Cell surface expression of IL-6 receptor a and CD45 was not influenced by the Lyn-overexpression. We have also found that the enhanced activation of the phosphatidylinositol 3-kinase (PI3-K) and Akt in the Lyn-overexpressed cells with IL-6, whereas the activation levels of two other major IL-6-responsive molecules, STAT3 and ERK1/2 remained unchanged, indicating that these are the independent signaling pathways of the Lyn-PI3K-Akt. In the Lyn-overexpressed cells, the Lyn PTK seems to be constitutively active because the positive regulatory tyrosine residue in the catalytic domain was highly phosphorylated, while the negative regulatory tyrosine residue in the COOH-terminus was not phosphorylated, presumably due to its dephosphorylation by CD45 PTP. The PI3-K was co-precipitated with Lyn in the immunoprecipitation assay, and specific inhibition of PI3-K or Akt activities by the pharmacologic reagents suppressed the IL-6-induced proliferation of the Lyn-overexpressed cells, suggesting that the activation of the PI3K-Akt pathways associated with the activated Lyn is indeed related to their concomitant augmentation of myeloma cell growth although the downstream signaling molecules are still need to be explicated. Experiments are also underway to clarify the possible mechanisms of the accelerated cell death of the Lyn-overexpressed myeloma cells as a consequence of the IL-6 withdrawal, our work will provide an understanding of Lyn's role in the IL-6 receptor-mediated signaling. Less