Justicidin A Inhibits the Transport of Tumor Necrosis Factor-α to Cell Surface in Lipopolysaccharide-Stimulated RAW 264.7 Macrophages

Abstract

Exposure of macrophages to lipopolysaccharide (LPS) induces release of tumor necrosis factor-alpha (TNF-alpha), which is initially synthesized as a 26-kDa pro-TNF-alpha followed by proteolytic processing to a 17-kDa secreted form. In this study, justicidin A, an arylnaphthalide lignan isolated from Justicia procumbens, was found to inhibit LPS-stimulated TNF-alpha release from RAW 264.7 macrophages in a concentration- and time-dependent manner, and the underlying mechanism was investigated. In the presence of justicidin A, challenge with LPS increased the steady-state level of the 26-kDa membrane-bound form of TNF-alpha protein, whereas justicidin A had little effect on the expression of TNF-alpha mRNA and on the synthesis of pro-TNF-alpha protein. Results of the pulse-chase experiment, revealed that the conversion of pro-TNF-alpha to mature TNF-alpha was inhibited by justicidin A. Moreover, justicidin A suppressed the transport of TNF-alpha to cell surface as analyzed by flow cytometry. The immunofluorescence analysis demonstrated that large amounts of LPS-induced TNF-alpha accumulated primarily within Golgi complex. These results indicate that justicidin A inhibits TNF-alpha release at the step of transport of pro-TNF-alpha to cell surface, and this leads to the accumulation of TNF-alpha in Golgi complex in RAW 264.7 macrophages.