Neuroprotective effect of naturally occurring RXR agonists isolated from Sophora tonkinensis Gagnep. on amyloid-β-induced cytotoxicity in PC12 cells.

Neuronal cell death induced by amyloid-β (Aβ) oligomers is implicated in neuronal degeneration and is a leading cause of Alzheimer's disease (AD). Therefore, to identify effective therapeutic agents for AD, we investigated the neuroprotective effects of two naturally occurring retinoid X receptor (RXR) agonists (SPF1 and SPF2), isolated from the root of Sophora tonkinensis Gagnep., on the Aβ25-35-induced cytotoxicity against nerve growth factor-differentiated rat pheochromocytoma (PC12) cells. Pretreatment with SPFs significantly prevented Aβ25-35-induced apoptosis in PC12 cells, similarly to the synthetic RXR agonist bexarotene. These effects were blocked by the RXR antagonist PA452. When the effects of SPFs were studied in the presence of the liver X receptor (LXR) agonist T0901317, the protective effects of SPFs were enhanced, suggesting that RXR/LXR heterodimers may play a key role in the neuroprotective effects of SPFs. SPFs and T0901317 induced ATP-binding cassette transporter 1 (ABCA1) protein expression in PC12 cells when administered alone or in combination. Intriguingly, a functional inhibitor of ABCA1 cyclosporine A negated the neuroprotective effects of SPFs or T0901317. Taken together, these results demonstrate that the RXR agonists SPF1 and SPF2 protect PC12 cells from Aβ25-35-induced neurotoxicity in an RXR-dependent manner and that their effects are markedly enhanced by the LXR agonist T0901317, in part related to ABCA1 function. These results suggest a novel approach to the treatment or prevention of AD.