Endotoxin-Induced IL-6 Promoter Activation in Skeletal Muscle Requires an NF-κB Site.

Previous studies in monocytes and other cell types have provided evidence of a role for the NF-κB pathway in IL-6 induction. The purpose of the present study was to examine the involvement of NF-κB in the induction of the IL-6 promoter in skeletal muscle cells by endotoxin (LPS), TNFα or IL-1α. Transfection of C2C12 mouse myocytes with a luciferase reporter under the control of the IL-6 promoter indicated each immunomodulator enhanced IL-6 promoter activity. Mutation and inhibitor studies indicate this response was dependent on the IL-6 NF-κB binding site, but independent of NF-IL6, AP-1, CREB or C/EBP. Cotransfection with an expression vector which constitutively activates the RelA pathway increased IL-6 promoter activity, and activity could not be further enhanced by cytokines or LPS. However, cotransfecting various dominant negative upstream NF-κB kinase expression vectors which inhibited RelA or RelB pathways either individually or in combination had no effect on LPS-induced activation of the IL-6 promoter, but abolished induction from a NF-κB-based promoter. This lack of effect was not due to a lack of NF-κB pathway activation in C2C12 myocytes because both Western analysis and EMSA supershifting showed an LPS-induced increase in nuclear RelA and RelA phosphorylation. However, another protein was observed bound to the IL-6 NF-κB site that does not bind to a consensus NF-κB site. The present findings provide novel insights regarding inflammation-induced stimulation of IL-6 promoter activity in skeletal muscle which is an important but non-traditional component of the innate immune system.