Hypoxia reverses the early anti-inflammatory microglial response to the β-amyloid peptide: mitochondrial involvement and beneficial roles of melatonin and naringenin.

Background: Early Alzheimer's disease (AD) is characterized by anti-inflammatory microglial responses to the beta amyloid peptide (Aβ), which later switch to pro-inflammatory. Such transition is relevant to disease progression and can be affected by concurrent insults, such as hypoxia (HY). This study explored whether a mild hypoxic stimulus could anticipate the microglial phenotypic switch, focusing in particular on involvement of SIRT1 and mitochondrial function.

Methods: HMC3 human microglia were polarized to an anti-inflammatory phenotype by 3 h of exposure to 0.2 μM of Aβ42 to mimic early AD and transferred to a hypoxic chamber with 3% of O₂ for 1 h. Effects on microglial activation were investigated by analysis of the SIRT1-BDNF axis activation and enzymatic and ELISA assays of inflammatory markers. Mitochondrial function and morphology were analyzed by high resolution respirometry and laser scanning confocal microscopy.

Results: Hypoxia (HY) prevented the Aβ42-induced early induction of SIRT1 translocation and BDNF release and significantly increased caspase 1 and NF-kB activity. Moreover, mitochondrial oxygen flows evaluated by high resolution respirometry were significantly reduced, while mitochondrial area, perimeter and branching were increased by Aβ42 + HY, compared to Aβ alone. These changes were contrasted by both melatonin (1 μM) and naringenin (10 μM), natural substances able to induce SIRT1. However, use of the selective SIRT1 inhibitor EX-527 (5 μM) suggested only a partial involvement for SIRT1 in the observed effects, prevalent for naringenin.

Conclusions: Our results suggest that mild hypoxic insults during early asymptomatic stages of AD can pose as a risk factor for an accelerated progression of the disease and show the benefits of SIRT1 induction strategies, including use of natural substances like melatonin and naringenin.