Neuroprotective Effects of Triterpenoids from Camellia japonica against Amyloid β-Induced Neuronal Damage

Alzheimer's disease (AD), a neurocognitive impairment affecting human mental capacity, is related to the accumulation of amyloid-beta peptide (A beta) and the hyperphosphorylation of tau protein. In addition to modern therapies approved for AD treatment, natural products with antioxidant and anti-inflammatory properties have been studied for their potential to prevent AD pathogenesis. Six new noroleanane triterpenoids from the fruit peels of Camellia japonica were isolated, and their structures were determined by diverse spectroscopic methods. The APP neuroprotective effects of the six new compounds were tested against A beta-induced neurotoxicity and neuroinflammation in mouse hippocampal and microglial cells. In the model of HT22-transfected cells, compounds 1-4 showed strongly neuroprotective effects via antioxidant response element gene activation and decreased the level of glutamate uptake. Compounds 1-4 also appeared to have strong inhibitory effects on NO production in A beta(1-42)-transfected BV2 microglial cells. A docking simulation study was used to explain the inhibitory effects of compounds 1-4 on beta-secretase 1 (BACE1). Noroleanane triterpenoids 1 - 4 had potential neuroprotective and anti-inflammatory effects against A beta-induced neuronal damage. The structure-activity relationships of the 30 oleanane triterpenoids from C. japonica were assessed in a model of A beta(1-42)-transfected HT22 cells.