Dexamethasone plus retinoids decrease IL‐6/IL‐6 receptor and induce apoptosis in myeloma cells

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Abstract
Interleukin 6 (IL-6) is the most important known growth factor for multiple myeloma, and IL-6 signalling pathways are potential targets for therapy. We hypothesized that interfering with the IL-6 signalling pathway at more than one level would be more effective than a single block in inhibiting proliferation of myeloma cells. Accumulating data support the concept that glucocorticoids down-regulate IL-6, whereas retinoic acid derivatives (RA) down-regulate IL-6R in myeloma. We found that all-trans RA (ATRA), 13-cis-RA and 9-cis-RA each similarly inhibited growth of RPMI 8226 myeloma cells and that addition of dexamethasone (DEX) added to RA growth inhibition. The major effects of retinoids were to reduce the proliferative fraction and induce apoptosis whereas DEX increased the apoptotic fraction. When combined, apoptosis was enhanced. Effects of RA + DEX were also least able to be overcome by exogenous IL-6. RA decreased IL-6R levels and addition of DEX to RA delayed recovery of IL-6R levels compared with RA alone. Since RPMI 8226 cells have undetectable IL-6, we investigated U266B1 cells and found that RA and DEX decreased both IL-6 secretion and IL-6 RNA levels. Mechanistically, IL-6R down-regulation by RA was enhanced by DEX, whereas IL-6 protein and RNA levels were reduced by DEX and by RA. In summary, combinations of RA + DEX were not only more effective in inhibiting myeloma cells growth by the dual mechanisms of decreasing proliferative fraction and increasing apoptotic fraction, but were also less able to be overcome by IL-6.