Abstract:

Nerve growth factor (NGF) promotes axonal growth in PC12 cells primarily by regulating the RTK-RAS-MEK-ERK pathway. Panaxydol, a polyacetylene isolated from Panax notoginseng, can mimic the effects of NGF. Panaxydol promotes neurite outgrowth in PC12 cells, but its molecular mechanism remains unclear. Indeed, although alkynol compounds such as panaxydol can increase intracellular cyclic adenosine 3′,5′-monophosphate (cAMP) levels and the ERK inhibitor U0126 inhibits alkynol-induced axonal growth, how pathways downstream of cAMP activate ERK have not been investigated. This study observed the molecular mechanism of panaxydol-, NGF- and forskolin-induced PC12 cell axon growth using specific signaling pathway inhibitors. The results demonstrated that although the RTK inhibitor SU5416 obviously inhibited the growth-promoting effect of NGF, it could not inhibit the promoting effect of panaxydol on axonal growth of PC12 cells. The adenylate cyclase inhibitor SQ22536 and cAMP-dependent protein kinase inhibitor RpcAMPS could suppress the promoting effect of forskolin and panaxydol on axonal growth. The ERK inhibitor U0126 inhibited axonal growth induced by all three factors. However, the PKA inhibitor H89 inhibited the promoting effect of forskolin on axonal growth but could not suppress the promoting effect of panaxydol. A western blot assay was used to determine the effects of stimulating factors and inhibitors on ERK phosphorylation levels. The results revealed that NGF activates the ERK pathway through tyrosine receptors to induce axonal growth of PC12 cells. In contrast, panaxydol and forskolin increased cellular cAMP levels and were inhibited by adenylyl cyclase inhibitors. The protein kinase A inhibitor H89 completely inhibited forskolin-induced axonal outgrowth and ERK phosphorylation, but could not inhibit panaxydol-induced axonal growth and ERK phosphorylation. These results indicated that panaxydol promoted axonal growth of PC12 cells through different pathways downstream of cAMP. Considering that exchange protein directly activated by cAMP 1 (Epac1) plays an important role in mediating cAMP signaling pathways, RNA interference experiments targeting the Epac1 gene were employed. The results verified that Epac1 could mediate the axonal growth signaling pathway induced by panaxydol. These findings suggest that compared with NGF and forskolin, panaxydol elicits axonal growth through the cAMP-Epac1-Rap1-MEK-ERK-CREB pathway, which is independent of PKA.

Key words: nerve regeneration, panax notoginseng, polyacetylene, panaxydol, nerve growth factor, PC12 cells, neurite outgrowth, cAMP, PKA, Epac1, ERK, CREB, neural regeneration