Luteolin blocks dopaminergic neuron degeneration and shifts microglial
M1/M2 polarization by inhibiting Toll like receptor 4
Abstract
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 expressed in microglia and plays a pivotal role in
neuroinflammation and neurodegeneration. Here we outline studies aimed
at determining the mechanisms responsible for the neuroprotective and
anti-inflammatory effects of luteolin using a mouse 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 study was
established through a single injection of LPS. Mice were subsequently
randomly assigned 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 marker IBA-1 and M1/M2
polarization markers, as well as the levels of pro-inflammatory
cytokines in the mesencephalic tissue and BV2 were quantified by RT-PCR
and ELISA, respectively. Apoptosis and cell viability of SH-SY5Y cells
co-cultured with BV2 were examined. TLR4 RNA transcript and protein
abundance in mesencephalic tissue and BV2 cells were detected. NFkB p65
subunit phosphorylation both in vivo and in vitro was evaluated by
immunoblotting. Results: Luteolin treatment induced functional
improvements and alleviated dopaminergic neuronal loss in our PD model.
Luteolin inhibited apoptosis and promoted cell survival in SH-SY5Y
cells. Luteolin treatment shifted microglial M1/M2 polarization towards
the anti-inflammatory M2 phenotype both in vivo and in vitro. Finally,
we found that luteolin treatment significantly downregulated both TLR4
mRNA and protein expression as well as restraining NFkB p65 subunit
phosphorylation. Conclusions: Luteolin promoted dopaminergic neuronal
survival in vivo and in vitro by blocking TLR4-mediated
neuroinflammation.