Folate receptor 1 activation suppresses high glucose-induced amyloidogenesis in neurons via STAT3/Nrf2 pathway-dependent mitigation of mitochondrial oxidative stress

Diabetes is a major risk factor for diabetic encephalopathy (DE), which is closely associated with sporadic Alzheimer's disease. Folic acid (FA) receptor signaling can suppress generation of neuropathogenic amyloid-beta (Aβ) induced by high extracellular glucose, suggesting that enhanced activation of this pathway could be a therapeutic strategy against DE-associated dementia, but the precise molecular signaling mechanisms are unclear. We report that high glucose levels increased the expression of amyloid precursor protein (APP) and β-secretase (BACE1) in cultured neurons and concomitantly induced amyloidogenesis, while FA treatment suppressed high glucose-stimulated expression of APP and BACE1, Aβ release, and accumulation of mitochondrial reactive oxygen species. Expression of nuclear factor erythroid 2-related factor 2 (Nrf2) was minimal under high glucose conditions, but was significantly upregulated together with downstream antioxidant enzymes following FA co-treatment. High glucose stimulation also increased folate receptor 1 (FOLR1) mRNA expression, suggesting a compensatory protective response. While treatment with 5-methyltetrahydrofolate (5-MTHF), the activated form of folate, did not significantly alter high glucose-induced upregulation of APP and BACE1, knockdown of FOLR1 mRNA reduced high glucose-stimulated Nrf2 expression and further augmented APP and BACE1 expression under high glucose conditions. Treatment with the STAT3 inhibitor 5'15-DPP also abolished high glucose-stimulated Nrf2 expression and increased APP and BACE1 expression levels. These findings indicate that FA/FOLR1 activation suppresses high glucose-induced amyloidogenesis by mitigating mitochondrial oxidative stress via STAT3/Nrf2 pathway signaling. In conclusion, present study suggests that the FA/FOLR1/STAT3/Nrf2 pathway is an effective therapeutic target for DE.