Acetylcholine Regulation of Synoviocyte Cytokine Expression by the α7 Nicotinic Receptor

Objective. The central nervous system can regulate peripheral inflammation, but the efferent neuronal routes and the mediators remain poorly defined. One candidate is the cholinergic pathway, which releases acetylcholine (ACh). This neurotransmitter can bind to the alpha 7 cholinergic receptor (alpha 7R) expressed by nonneuronal cells and reduce inflammation. To test this possibility, we evaluated the expression of alpha 7R and its potential role as a target in rheumatoid arthritis (RA). Methods. The expression of alpha 7R in human synovium and fibroblast-like synoviocytes (FLS) was determined using immunohistochemical, Western blot, and quantitative polymerase chain reaction (PCR) analyses. The effects of ACh in vitro were determined in interieukin-1. (IL-1)-stimulated FLS using immunoassays for protein, quantitative PCR for messenger RNA (mRNA), luciferase reporter constructs for IL-6 and NF-kappa B promoter activity, and electrophoretic mobility shift assays. Expression of alpha 7R was knocked down with small interfering RNA (siRNA) or was inhibited with the selective alpha 7R antagonist methyllycaconitine (MLA). Results. Protein and mRNA for alpha 7R were demonstrated in RA and osteoarthritis synovium and cultured synoviocytes. Expression in synovium was mainly in the intimal lining. ACh significantly reduced the production of IL-6, CXCL8, CCL2, CCL3, CCL5, and granulocyte colony-stimulating factor by IL-1-stimulated FLS. This effect was blocked by the alpha 7R antagonist MLA or by using alpha 7R siRNA to knock down receptor expression. The selective alpha 7R agonist PNU-282,987 decreased the production of IL-6 by IL-1-stimulated FLS. ACh did not reduce IL-6 transcription, but it decreased IL-6 mRNA half-life and reduced IL-6 mRNA steady-state levels. Conclusion. The alpha 7 receptor is expressed in the synovium and by synoviocytes. Receptor ligation inhibits cytokine expression in FLS through a posttranscriptional mechanism. Therefore, alpha 7R is a potential therapeutic target for inflammatory diseases.