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Abstract
Osteoblast‐derived IL‐6 functions in coupled bone turnover by supporting osteoclastogenesis favoring bone resorption instead of bone deposition. Gene regulation of IL‐6 is complex occurring both at transcription and post‐transcription levels. The focus of this paper is at the level of mRNA stability, which is important in IL‐6 gene regulation. Using the MC3T3‐E1 as an osteoblastic model, IL‐6 secretion was dose dependently decreased by SB203580, a p38 MAPK inhibitor. Steady state IL‐6 mRNA was decreased with SB203580 (2 µM) ca. 85% when stimulated by IL‐1β (1–5 ng/ml). These effects require de novo protein synthesis as they were inhibited by cycloheximide. p38 MAPK had minor effects on proximal IL‐6 promoter activity in reporter gene assays. A more significant effect on IL‐6 mRNA stability was observed in the presence of SB203580. Western blot analysis confirmed that SB203580 inhibited p38 MAP kinase, in response to IL‐1β in a dose dependent manner in MC3T3‐E1 cells. Stably transfected MC3T3‐E1 reporter cell lines (MC6) containing green fluorescent protein (GFP) with the 3′untranslated region of IL‐6 were constructed. Results indicated that IL‐1β, TNFα, LPS but not parathyroid hormone (PTH) could increase GFP expression of these reporter cell lines. Endogenous IL‐6 and reporter gene eGFP‐IL‐6 3′UTR mRNA was regulated by p38 in MC6 cells. In addition, transient transfection of IL‐6 3′UTR reporter cells with immediate upstream MAP kinase kinase‐3 and ‐6 increased GFP expression compared to mock transfected controls. These results indicate that p38 MAPK regulates IL‐1β‐stimulated IL‐6 at a post transcriptional mechanism and one of the primary targets of IL‐6 gene regulation is the 3′UTR of IL‐6.
