Cardamonin from Alpinia rafflesiana Wall. ex Baker as inhibitor of inflammatory responses in IFN-Gamma/LPS-Stimulated microglial cell line (BV2) via NF-κB signalling pathway

The prevalence of neurodegenerative diseases affecting the worldwide population is showing an elevating trend over the last two decades. Chronic neuroinflammation has been proven to contribute to the pathogenesis of these diseases and over-activated microglial cells as well as the inflammatory mediators produced play a crucial role in sustaining the progression of neuroinflammation. In the anti-inflammatory approach, non-steroidal anti-inflammatory drugs (NSAIDs) are among the therapeutic choices and are anticipated to be useful drugs in treating neuroinflammation. Unfortunately, NSAIDs-associated adverse effects had hampered the long term usage of these drugs and thus prompted the investigations for discovering innovative therapeutics for neurodegenerative diseases. Previously, cardamonin isolated from Alpinia rafflesiana was shown to possess promising anti-inflammatory property in macrophages and hence potential to be used as a therapeutic agent for various inflammatory diseases. However, the mechanism of action and molecular basis of cardamonin on microglial cells, which are the central nervous system (CNS)-specific macrophages, still remains unknown. Hence, the main objectives of this study were to investigate the effects of cardamonin on microglial inflammatory responses as well as its mechanism of action in cellular model of neuroinflammation. In this study, effects of cardamonin on nitric oxide (NO) production in recombinant mouse interferon- (IFN-) and Escherichia coli lipopolysaccharide (LPS)-stimulated microglial cell line (BV2) were evaluated using Griess assay while immunoassays were used for determination of prostaglandin E2 (PGE2), tumour necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-6 (IL-6) secretion. To decipher the mechanism in which cardamonin possibly regulates microglia, effects of cardamonin on inducible nitric oxide synthase (iNOS) and cyclooxygenases (COX) protein expression as well as the gene expression of TNF-α, IL-1β and IL-6 were determined. In order to understand the effects of cardamonin on cell signal transduction, NF-κB DNA binding activity as well as cell surface expression of cluster of differentiation 14 (CD14) and toll-like receptor 4 (TLR4) of cardamonin-treated BV2 cells were investigated. Results obtained demonstrated that cardamonin inhibited NO and PGE2 production in IFN-γ/LPS-stimulated BV2 cells dose-dependently, with IC50 values of 27.45 ± 0.46 µM and 2.52 ± 0.12 µM, respectively. This inhibitory effect was contributed by the suppression of cardamonin on the protein expression of iNOS and COX-2, respectively, without affecting COX-1 protein expression. Besides, cardamonin also reduced the production of TNF-α, IL-1β and IL-6 with the IC50 values of 40.02 ± 4.01 M, 5.25 ± 0.85 M and 13.36 ± 1.52 M, respectively as well as inhibited the gene expression of these three cytokines. The results further verified the interruption of cardamonin on NF-κB signalling pathway by attenuation of NF-κB DNA binding activity. Interestingly, cardamonin was found to demonstrate suppressive effect on the cell surface expression of CD14 in IFN-γ/LPS-stimulated BV2 cells as well. In conclusion, these experimental data have provided mechanistic insights for the anti-inflammatory actions of cardamonin on BV2 cells via suppression of CD14 signalling followed by interruption of NF-κB activation. With these results, it could be projected that by limiting the inflammatory responses of microglia, the tissue injury and neuron death associated with neuroinflammatory diseases may be lessened by cardamonin. Thus, cardamonin could be a potential lead compound for developing anti-inflammatory drugs for neuroinflammatory diseases.

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