This study investigated the mechanism of baicalin on lipopolysaccharide(LPS)/interferon γ(IFN-γ)-induced inflammatory microglia based on the triggering receptor expressed on myeloid cells 2(TREM2)/Toll-like receptor 4(TLR4)/nuclear factor kappaB(NF-κB) pathway. Specifically, LPS and IFN-γ were used to induce inflammation in mouse microglia BV2 cells. Then the normal group, model group, low-dose(5 μmol·L~(-1)) baicalin group, medium-dose(10 μmol·L~(-1)) baicalin group, high-dose(20 μmol·L~(-1)) baicalin group, and minocycline(10 μmol·L~(-1)) group were designed. Cell viability was detected by CCK-8 assay and cell morphology was observed under bright field. The expression of interleukin-1β(IL-1β), interleukin-4(IL-4), inducible nitric oxide synthase(iNOS), interleukin-6(IL-6), interleukin-10(IL-10), and arginase-1(Arg-1) mRNA was detected by real-time quantitative PCR, the protein expression of tumor necrosis factor-α(TNF-α), IL-1β, TREM2, TLR4, inhibitor kappaB-alpha(IκBα), p-IκBα, NF-κB p65 and p-NF-κB p65 by Western blot, and transfer of NF-κB p65 from cytoplasm to nucleus by cellular immunofluorescence. Compared with the normal group, most of the BV2 cells in the model group tended to demonstrate the pro-inflammatory M1 amoeba morphology, and the model group showed significant increase in the mRNA levels of IL-1β, IL-6, and iNOS, decrease in the mRNA levels of IL-4, IL-10, and Arg-1(P<0.01), rise of the protein expression of TNF-α, IL-1β, TLR4, p-IκBα, and p-NF-κB p65(P<0.01), reduction in TREM2 protein expression, and increase in the expression of NF-κB p65 in nucleus. Compared with the model group, baicalin groups and minocycline group showed the recovery of BV2 cell morphology, significant decrease in the mRNA levels of IL-1β, IL-6 and iNOS, increase in the mRNA levels of IL-4, IL-10, and Arg-1(P<0.01), reduction in the protein expression of TNF-α, IL-1β, TLR4, p-IκBα, and p-NF-κB p65(P<0.05), rise of TREM2 protein expression, and decrease in the expression of NF-κB p65 in nucleus. In summary, these results suggest that baicalin can regulate the imbalance between TREM2 and TLR4 of microglia and inhibit the activation of downstream NF-κB, thus promoting the polarization of microglia from pro-inflammatory phenotype to anti-inflammatory phenotype.
Keywords: NF-κB; TLR4; TREM2; baicalin; inflammatory; microglia.