IL-6 plays an important role in various inflammatory ocular diseases, including diabetic retinopathy. Muller cells are the major source of inflammatory mediators, including IL-6, in the retina. However, the mechanism of regulating IL-6 production in these cells remains unclear. Examination of signaling pathways in human retinal Muller cells (MID-MI cell line) cultured with IL-10, TNF-a, 1-6, IL-8, VEGF, IFN-y, glucose or mannitol showed that IL-1f3 was the most potent stimulator of IL-6 production. In addition, IL-1 p also increased NF-x13 p50 protein level and phosphorylation of p38 MAPK, ERK1/2 and c-Jun. Induction of IL-6 production by IL-10 was significantly reduced by addition of p38 MAPK (SB203580), MEK1/2 (U0126) or NF-0 (BAY117082) inhibitors, with the highest effect being observed with SB203580. To explore the specific elements in IL-6 promoter responsible for IL-l3-induction of IL-6 expression, a series of plasmids bearing various IL-6 promoter mutations were transiently expressed in MID-MI cells cultured in the presence or absence of IL-lp (10 ng/ml) and/or SB203580 (1011M). Results showed that IL-6 promoter activity of the parent pIL-6-Luc651 was significantly enhanced by IL-l13, but the level was significantly attenuated by SB203580. Furthermore, the IL-6 promoter activity was also reduced upon deletion of NF-1(13, AP-1 or C/EBP binding sites, with NF-KB deletion being the greatest. These results are the first demonstration that IL-l1 induces IL-6 production in Muller cells by activation of IL-6 promoter activity predominantly through the p38 MAPK/NF-KB pathway. (C) 2014 Elsevier Inc. All rights reserved.