Effect of perilla flavonoid apigenin on tryptophan-NAD metabolic key enzymes expression in LPS-stimulated microglial cells and its mechanism

Background and objectives: Flavonoid apigenin (AP) is abundantly present in various vegetables such as perilla. AP has anti-oxidant and anti-inflammatory effects. 2-Amino-3-carboxymuconate-6-semialdehyde decarboxylase (ACMSD) and Indole amine 2,3 dioxygenase (IDO) are metabolic key enzymes that play pivotal role in tryptophan degradation via kynurenine pathway. Tryptophan is converted to various metabolites such as NAD and quinolinate (QUIN). Excess production of QUIN leads to pathogenesis of neurodegenerative diseases. ACMSD and IDO affect QUIN production. Therefore, regulation of ACMSD and IDO may be effective to suppress QUIN production and the progression of neurodegeneration. In this study, we examined the effect of AP on ACMSD and IDO expression, inflammatory mediators and related inflammatory signal pathways including NF-κB pathways in lipopolysaccharide (LPS)-stimulated microglial cells. Methods: The MG6 cell line, derived from mouse microglial cells was used. Cells were cultured in dishes in Dulbecco's modified Eagle's medium with fetal bovine serum. MG6 cells were treated with LPS and/or AP for the detection of ACMSD expression, nitric oxide (NO) secretion and inflammatory cytokine production. NO concentration in the medium was measured by the Griess reaction method. Subsequently, ACMSD and IDO mRNA expression levels were measured by quantitative polymerase chain reaction. Related inflammatory mediators were also measured by enzyme-linked immunosorbent assay (ELISA). Protein expression levels were detected by Western Blot analysis. Results: LPS suppressed ACMSD and enhance IDO expression, which may lead to increase QUIN concentration. However, AP recovered LPS-stimulated ACMSD and IDO expression. Furthermore, it also inhibited interleukin-6 (IL-6) and NO production. AP also inhibited degradation of IκBα. Conclusions: AP recovered LPS-stimulated ACMSD and IDO mRNA expression and suppressed some inflammatory mediators, which may be mediated by inhibition of the NF-κB pathway in microglial cells. These results suggest that AP may be helpful for avoiding central nerve system inflammation.