Petasites japonicus bakkenolide B inhibits lipopolysaccharide-induced pro-inflammatory cytokines via AMPK/Nrf2 induction in microglia

Abnormal neuroinflammatory responses have diverse roles in neuronal death, oxidative stress and neurodegenerative diseases, such as Alzheimer's and Parkinson's disease. Microglia regulate these responses via molecular signaling cascades that involve inflammatory cytokines and complement proteins. Bakkenolide B from Petasites japonicus exhibits significant anti-inflammatory and anti-allergic bioactivities. The present study investigated the anti-neuroinflammatory effects and underlying molecular mechanisms of bakkenolide B on the lipopolysaccharide (LPS)-mediated neuroinflammatory response in microglia. The results indicated that bakkenolide B pretreatment significantly reduced microglial production of interleukin (IL)-1 beta, IL-6, IL-12, and tumor necrosis factor (TNF)-alpha. Furthermore, this effect was associated with reduced production of reactive oxygen species. The role of bakkenolide B was then evaluated in the upregulation of nuclear factor erythroid 2-related factor 2 (Nrf2)/antioxidant response element (ARE) signaling pathways. The results suggested that bakkenolide B significantly upregulated Nrf2/ARE pathway-related downstream factors, such as NADPH dehydrogenase quinone-1 (NQO-1) and heme oxygenase-1 (HO-1). Silencing of Nrf2, HO-1 and NQO-1 diminished the anti-neuroinflammatory properties of bakkenolide B. AMP-activated protein kinase (AMPK) activates the Nrf2/ARE signaling pathway, and the results of the present study demonstrated that bakkenolide B increased AMPK phosphorylation in microglia. In addition, an AMPK inhibitor abolished the bakkenolide B-induced increase in nuclear Nrf2, NQO-1 and HO-1 protein expression. Finally, an AMPK inhibitor diminished the bakkenolide B-mediated inhibition of LPS-stimulated TNF-alpha production. Taken together, the present results demonstrate that bakkenolide B may be an effective and therapeutically relevant AMPK/Nrf2 pathway activator for suppressing abnormal neuro-inflammation in neurodegenerative diseases.