Although BHB may inhibit neuro-inflammation to play beneficial roles in CNS, the protective effects of BHB against microglia activation still need clarification. In this study, BHB is revealed to inhibit microglia activation, suppress inflammatory IL-6 and TNF-α, and promote protective BDNF and TGF-β in vivo and in vitro, attenuating behavioral abnormality of mice.
Microglia activation is the major contributor to neuro-inflammation and the related neuro-disorders (Saxena et al. 2021; McFarland and Chakrabarty 2022; Shao et al. 2022). Generally, microglia can be activated by LPS, which is also used in the study to make neuro-inflammation mice model (Yang et al. 2020; Wu et al. 2022a). Behavioral abnormality, activated microglia in the hippocampus, and increased generation of inflammatory IL-6 and TNF-α; decreased generation of protective BDNF and TGF-β in the brain, were observed in LPS-treated mice. These results confirmed that LPS induced microglia activation and behavioral abnormality of mice. Considering the central role of microglia activation in neuro-disorders, a growing number of studies have conducted to search for effective therapies against microglia activation and neuro-inflammation (Gudoityte et al. 2021; Medrano-Jiménez et al. 2022; Zhang et al. 2022).
Besides energy supply, ketone body BHB plays various protective roles in CNS as a signaling molecule (Poff et al. 2019; Pérez-Liébana et al. 2020; Wang et al. 2021; Yao et al. 2021). It’s reported that BHB has the ability to inhibit the heat stress-induced neuro-inflammation (Huang et al. 2022). And BHB could also ameliorate inflammation after spinal cord injury and inhibits inflammasome activation in Alzheimer's disease (Shippy et al. 2020; Wu et al. 2020; Kong et al. 2021). In this study, BHB is verified to inhibit microglia activation in the hippocampus and attenuate the behavioral abnormality of LPS-treated mice. However, BHB exhibited little effect on the indicators of normal mice, indicating its specific protective effects against the LPS-stimulated microglia activation. Overall, no better effect of high dose BHB (3 d/kg/day) was observed than that of low dose BHB (300 mg/kg/day) in LPS-treated mice, suggesting proper dosage selection for BHB in clinical application.
A series of studies have been conducted to investigate the underlying mechanism of BHB’s beneficial roles (Wu et al. 2020; Li et al. 2021b; Wang et al. 2021). Generally, BHB acts as an inhibitor of histone deacetylase to elevate histone acetylation and promote transcription of protective genes (Li et al. 2021a; Ji et al. 2022; Wu et al. 2022b). For example, the neuro-protective BDNF has been proved to be up-regulated by BHB (Hu et al. 2020; Norgren et al. 2021). In the study, BHB was also found to promote the generation of protective BDNF and TGF-β in the brain of LPS-treated mice and the LPS-stimulated BV-2 cells.
For suppression of neuro-inflammation, BHB is reported to inhibit NLRP3 inflammasome or pro-inflammatory signaling, such as NF-κB pathways (Shippy et al. 2020; Kong et al. 2021; Huang et al. 2022). However, there exits inconsistency about the effects of BHB on inflammatory factors (Fu et al. 2015; Benito et al. 2020). In this study, BHB was found to suppress the generation of inflammatory IL-6 and TNF-α in the brain in LPS-treated mice. Besides, BHB concentration-dependently suppressed IL-6 and TNF-α generations, and reduced the ROS level with attenuation of cellular changes in LPS-stimulated BV-2 cells, with 4 mM BHB exhibiting the most dominant effects. Collaborating with other reports (Qi et al. 2021; Huang et al. 2022), the results suggested that BHB with proper physiological concentration may be sufficient and better for its protective effects. However, the intrinsic suppression mechanism of LPS-stimulated IL-6 and TNF-α by BHB still needs further clarification. Probably, BHB up-regulates some core protective controllers against microglia activation, such as Nrf2 and Sirtuins (Fernando and Wijayasinghe 2021; Saha et al. 2022).
Overall, these results demonstrated that BHB inhibited the LPS-stimulated microglia activation, suppressed inflammatory IL-6 and TNF-α, and meanwhile, promoted protective BDNF and TGF-β, antagonizing neuro-inflammation and playing protective roles. However, in view of the various cell types in the brain, the results limiting to microglia, might partially explain the protective effects of BHB in the CNS.
In conclusion, BHB exhibited protective effects against microglia activation in LPS-treated mice and BV-2 cells, antagonizing neuro-inflammation and the behavioral abnormality of mice.