Asiaticoside but not its aglycone exhibits neuritogenicity through TrkA receptor signaling: a bridge between ERK1/2-CREB and Akt-GSK3β/RhoA

The neuritogenicity and the neuroregenerative potential of asiaticoside (AS) and its aglycone, asiatic acid (AA), has been generally reported. We recently identified the participation of extracellular signal-regulated protein kinases 1/2 (ERK1/2) and protein kinase B (Akt) in the neuritogenic mechanism of AS and AA. In this study, we further investigated the possible upstream target molecule and the associated downstream signaling of both triterpenoids in mouse neuroblastoma Neuro-2a cells. Our immunoblotting and immunofluorescence assays revealed that either AS or AA exerted neurite extension activity through inhibitory effect on glycogen synthase kinase 3beta (GSK3beta) and Ras homolog gene family member A (RhoA). AS appeared significantly more potent in promoting neurite elongation than AA, and concurrently expressed a higher degree of inhibition on GSK3beta and RhoA activations. The mediation of GSK3beta and RhoA activities in AS-treated cells involved Akt signaling. Moreover, when using GW441756, a specific tropomyosin receptor kinase A (TrkA) receptor signaling inhibitor, the ERK1/2 and Akt phosphorylation, the inhibitory effects on GSK3beta and RhoA and the neurite outgrowth induced by AS, but not AA, were totally suppressed. In conclusion, our findings supported the different upstream regulators of AS and AA in promoting neuritogenicity in Neuro-2a cells. Although both AS and AA could enhance neurite elongation through the suppression of GSK3beta and RhoA activities, only AS could modulate the effect through TrkA receptor signaling.