Nobiletin Reduces LPS-Induced Neuroinflammation through TLR4/MyD88/NF-κB and Oxidative Stress via Nrf2/HO-1 Signaling in Human Microglial HMC3 Cells.

Neuroinflammation and oxidative stress (OS) are the major contributors to the onset and progression of neurodegenerative diseases (NDs), where microglial activation and dysregulated inflammatory signaling exacerbate neuronal injury. Nobiletin (NOB), a polymethoxylated flavonoid abundant in citrus fruits, has been reported to possess excellent bioactivities; however, its effects in combating inflammation and OS in human microglial cells (HMC3) have not been comprehensively examined. In this study, we investigated the effects of NOB on lipopolysaccharide (LPS)-induced inflammatory and oxidative responses in HMC3 cells. The HMC3 cells exposed to LPS (1 µg/mL) in the presence/absence of NOB (5, 10, 20, and 40 µM) for 24 h showed that NOB could attenuate LPS-induced cytotoxicity. NOB treatment attenuated LPS-induced upregulation of pro-inflammatory cytokines including interleukin (IL)-1β, and IL-6, and suppressed activation of the toll-like receptor 4/myeloid differentiation primary response 88/nuclear factor kappa-light-chain-enhancer of activated B (TLR4/MyD88/NF-κB) pathway. NOB enhanced the protein expression levels of TLR10, a negative regulator of TLR4-mediated inflammatory signaling. In addition, NOB increased the nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1) expression, along with other antioxidants including catalase (CAT), glutathione peroxidase (GPx), and superoxide dismutase (SOD), leading to reduced intracellular reactive oxygen species (ROS). These findings suggest that NOB has promising anti-inflammatory and antioxidant effects in an in vitro model of LPS-induced neuroinflammation, potentially through modulation of TLR4/MyD88/NF-κB and Nrf2/HO-1 signaling pathways. However, further in vivo studies are needed to validate these effects and explore NOB's potential as a candidate for therapeutic development in NDs.