Omega-3 polyunsaturated fatty acid supplementation attenuates microglial-induced inflammation by inhibiting the HMGB1/TLR4/NF-κB pathway following experimental traumatic brain injury

Background: Microglial activation and the subsequent inflammatory response in the central nervous system play important roles in secondary damage after traumatic brain injury (TBI). High-mobility group box 1 (HMGB1) protein, an important mediator in late inflammatory responses, interacts with transmembrane receptor for advanced glycation end products (RAGE) and toll-like receptors (TLRs) to activate downstream signaling pathways, such as the nuclear factor (NF)-kappa B signaling pathway, leading to a cascade amplification of inflammatory responses, which are related to neuronal damage after TBI. Omega-3 polyunsaturated fatty acid (omega-3 PUFA) is a commonly used clinical immunonutrient, which has antioxidative and anti-inflammatory effects. However, the effects of omega-3 PUFA on HMGB1 expression and HMGB1-mediated activation of the TLR4/NF-kappa B signaling pathway are not clear. Methods: The Feeney DM TBI model was adopted to induce brain injury in rats. Modified neurological severity scores, brain water content, and Nissl staining were employed to determine the neuroprotective effects of omega-3 PUFA supplementation. Assessment of microglial activation in lesioned sites and protein markers for proinflammatory, such as tumor necrosis factor (TNF)-alpha, interleukin (IL)-1 beta, IL-6, interferon (IFN)-gamma, and HMGB1 were used to evaluate neuroinflammatory responses and anti-inflammation effects of omega-3 PUFA supplementation. Immunofluorescent staining and western blot analysis were used to detect HMGB1 nuclear translocation, secretion, and HMGB1-mediated activation of the TLR4/NF-kappa B signaling pathway to evaluate the effects of omega-3 PUFA supplementation and gain further insight into the mechanisms underlying the development of the neuroinflammatory response after TBI. Results: It was found that omega-3 PUFA supplementation inhibited TBI-induced microglial activation and expression of inflammatory factors (TNF-alpha, IL-1 beta, IL-6, and IFN-gamma), reduced brain edema, decreased neuronal apoptosis, and improved neurological functions after TBI. We further demonstrated that omega-3 PUFA supplementation inhibited HMGB1 nuclear translocation and secretion and decreased expression of HMGB1 in neurons and microglia in the lesioned areas. Moreover, omega-3 PUFA supplementation inhibited microglial activation and the subsequent inflammatory response by regulating HMGB1 and the TLR4/NF-kappa B signaling pathway. Conclusions: The results of this study suggest that microglial activation and the subsequent neuroinflammatory response as well as the related HMGB1/TLR4/NF-kappa B signaling pathway play essential roles in secondary injury after TBI. Furthermore, omega-3 PUFA supplementation inhibited TBI-induced microglial activation and the subsequent inflammatory response by regulating HMGB1 nuclear translocation and secretion and also HMGB1-mediated activation of the TLR4/NF-kappa B signaling pathway, leading to neuroprotective effects.