Neuroprotective effect of Citrus unshiu immature peel and nobiletin inhibiting hydrogen peroxide‑induced oxidative stress in HT22 murine hippocampal neuronal cells

Background: Oxidative stress‑induced cell damage is common in the etiology of several neurobiological disorders, including Alzheimer’s disease and Parkinson’s disease. In a case study, nobiletin‑rich Citrus reticulata peels could prevent the progression of cognitive impairment in donepezil‑preadministered Alzheimer’s disease patients. Objective: In this study, we investigated the effects and underlying mechanism of nobiletin and Citrus unshiu immature peel (CUIP) water extract, which contains nobiletin as a major compound, on hydrogen peroxide‑induced oxidative stress in HT22 cells, a murine hippocampal neuronal model. Materials and Methods: HT22 cells were treated with hydrogen peroxide in the presence or absence of various concentrations of CUIP and nobiletin. Cytotoxicity and apoptotic protein levels were measured by 3‑(4,5‑dimethythiazol‑2‑yl)‑2,5‑diphenyl tetrazolium bromide assay and Western blotting. Results: Pretreatment with CUIP and nobiletin inhibited cell death due to hydrogen peroxide. Hydrogen peroxide‑induced the expression of phospho‑Jun N‑terminal kinases (p‑JNK) and p‑p38 proteins in HT22 cells; however CUIP and nobiletin suppressed p‑JNK and p‑p38 without changing JNK or p38. Regarding apoptosis, caspase 3, B‑cell lymphoma 2 (Bcl‑2), and Bax protein expression was determined. CUIP and nobiletin suppressed caspase 3 and Bax expression, but they induced Bcl‑2 expression in HT22 cells. Conclusion: These results show that CUIP and nobiletin can protect against hydrogen peroxide‑induced cell death in HT22 neurons via mitogen‑activated protein kinases and apoptotic pathways.