Luteolin Mitigates Dopaminergic Neuron Degeneration and Restrains Microglial M1 Polarization by Inhibiting Toll Like Receptor 4

Background: Luteolin is a natural flavonoid and its neuroprotective and anti-inflammatory effects have been confirmed to mitigate neurodegeneration. Despite these findings, the underlying mechanisms responsible for these effects remain unclear. Toll-like receptor 4 (TLR4) is widely distributed in microglia and plays a pivotal role in neuroinflammation and neurodegeneration. Here studies are outlined that aimed at determining the mechanisms responsible for the anti-inflammatory and neuroprotective actions of luteolin using a rodent model of Parkinson's disease (PD) and specifically focusing on the role of TLR4 in this process. Methods: The mouse model of PD used in this experiment was established through a single injection of lipopolysaccharide (LPS). Mice were then subsequently randomly allocated to either the luteolin or vehicle-treated group, then motor performance and dopaminergic neuronal injury were evaluated. BV2 microglial cells were treated with luteolin or vehicle saline prior to LPS challenge. MRNA expression of microglial specific marker ionized calcium-binding adapter molecule 1 (IBA-1) and M1/M2 polarization markers, as well as the abundance of indicated pro-inflammatory cytokines in the mesencephalic tissue and BV2 were quantified by real time-polymerase chain reaction (RT-PCR) and Enzyme-linked Immunosorbent Assay (ELISA), respectively. Cell viability and apoptosis of neuron-like PC12 cell line co-cultured with BV2 were detected. TLR4 RNA transcript and protein abundance in mesencephalic tissue and BV2 cells were detected. Nuclear factor kappa-gene binding (NF-κB) p65 subunit phosphorylation both in vitro and in vivo was evaluated by immunoblotting. Results: Luteolin treatment induced functional improvements and alleviated dopaminergic neuronal loss in the PD model. Luteolin inhibited apoptosis and promoted cell survival in PC12 cells. Luteolin treatment shifted microglial M1/M2 polarization towards an anti-inflammatory M2 phenotype both in vitro and in vivo. Finally, it was found that luteolin treatment significantly downregulated both TLR4 mRNA and protein expression as well as restraining NF-κB p65 subunit phosphorylation. Conclusions: Luteolin restrained dopaminergic degeneration in vitro and in vivo by blocking TLR4-mediated neuroinflammation.

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