Mechanisms of Inhibition of IL-6-Mediated Immunoglobulin Secretion by Dexamethasone and Suramin in Human Lymphoid and Myeloma Cell Lines1

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Abstract
The cytokine IL-6 has been proposed as an autocrine growth factor in multiple myeloma, and is also required for stimulation of immunoglobulin production and secretion in normal plasma cells and myeloma cells. In this study, we show that secreted IL-6 is detectable by Western blot analysis in a panel of lymphoid and myeloma cell lines. Previous studies in our laboratory have shown that dexamethasone and suramin inhibit cell proliferation and IL-6-mediated immunoglobulin secretion in various lymphoblastoid and myeloma cell lines. In the present study, we present data to examine mechanisms by which dexamethasone and suramin inhibit IL-6-mediated immunoglobulin secretion in the lymphoid cell line SKW 6.4. Cells treated with rIL-6 or the IC10 concentration of dexamethasone respectively undergo a doubling of intracellular IgM. Moreover, rIL-6 and dexamethasone additively stimulate cells to accumulate intracellular IgM. In contrast, cells treated with the IC10 concentration of suramin undergo no significant alteration of total cellular IgM, and do not respond to IL-6 with an increase in intracellular IgM. Northern blot analysis demonstrates that cells treated with exogenous rIL-6 and/or dexamethasone respectively undergo a coordinate one to three fold increase of k and μ chain mRNA expression, while there is a 30–40% decrease of k and μ chain mRNA when cells are treated with suramin or suramin plus rIL-6. Western blot analysis shows that levels of intracellular IL-6 modestly increase when cells are treated with exogenous rIL-6, whereas treatment with dexamethasone plus rIL-6 causes a 70% decrease of immunoreactive IL-6 protein in comparison with untreated cells. An rtPCR analysis of IL-6 mRNA expression shows an abolished signal in response to dexamethasone or rIL-6 plus dexamethasone. Using a flow cytometric assay, it is demonstrated that suramin inhibits IL-6 binding to its receptor. Taken together, these results indicate that SKW 6.4 cells treated with rIL-6 and/or dexamethasone undergo increased expression of IgM mRNA leading to increased intracellular IgM levels. Treatment with suramin or suramin plus rIL-6 does not alter the IL-6 protein level or the mRNA levels for IL-6 and IL-6 receptor. Suramin treatment causes a moderate decrease in IgM mRNA, and this is associated with a decreased intracellular level of IgM in SKW 6.4 cells. Overall these findings support the concept that IL-6 is an autocrine factor for immunoglobulin production and secretion in myeloma cells. Suramin interferes with IL-6 binding to its receptor and/or decreases IL-6 receptor expression. Dexamethasone has neither of these effects on IL-6 receptor expression or IL-6 binding to its receptor, and we postulate that it acts through a block in secretion or in degradation of intracellular immunoglobulin by decreasing IL-6 mRNA expression and IL-6 protein content. These studies suggest that the combination of suramin and dexamethasone not only synergistically growth inhibit myeloma cells but also act in concert to inhibit immunoglobulin secretion and represent a therapeutic approach worthy of further investigation