Diminishment of α-MSH anti-inflammatory activity in MC1r siRNA-transfected RAW264.7 macrophages

摘要

The neuropeptide α-melanocyte-stimulating hormone (α-MSH) is a powerful suppressor of inflammation mediated by macrophages, which express at least two receptors, melanocortin 1 and 3 receptors (MC1r and MC3r) that bind α-MSH. Albeit, the anti-inflammatory activity of α-MSH has been well documented in macrophages, the mechanisms of α-MSH activity in macrophages are not clearly understood. This study is to investigate which of the MCr expressed on macrophages is associated with the immunosuppressive activities of α-MSH on LPS-stimulated macrophages. To address this question, we transfected RAW264.7 macrophage cells with MC1r small interfering (si)RNA, which specifically targets mouse MC1r mRNA. The diminution of MC1r mRNA expression was 82% at 24 h and 67% at 48 h after transfection. There was a significant loss in α-MSH suppression of NO generation and TNF-α production by MC1r siRNA-transfected macrophages stimulated with LPS. There was an equally diminished α-MSH suppression of LPS-stimulated intracellular activation of NF-κB and p38 phosphorylation. In addition, the diminishment of MC1r expression by siRNA transfection had no influence on MC3r expression and function in the macrophages. These findings demonstrate that α-MSH suppression of LPS-induced inflammatory activity in macrophages requires expression of MC1r. The results imply that although all of the MCr are G-coupled proteins, they may not necessarily function through the same intracellular pathways in macrophages.