Activation of the mitogen-activated protein kinase cascade through nitric oxide synthesis as a mechanism of neuritogenic effect of genipin in PC12h cells

Prominent neurite outgrowth induced by genipin, a plant-derived iridoid, wa s substantially inhibited by addition of NG nitro-L-arginine methyl ester ( L-NAME), a nitric oxide (NO) synthase (NOS) inhibitor, and carboxy-PTIO, an NO scavenger, in PC12h cells. Increases of the NADPH-diaphorase activity a nd neuronal and inducible NOS proteins in cells preceded the neurite outgro wth after addition of genipin to medium. NO donors could induce the neurite outgrowth close-dependently in the cells. On the other hand, an inhibitor of soluble guanylate cyclase (SGC), which is known to be a stimulatory targ et of NO, abolished greatly the genipin-induced neurite outgrowth. Addition of extracellular signal-regulated kinase (ERK) kinase inhibitors could alm ost completely abolish the neurite induction. L-NAME remarkably depressed g enipin-stimulated phosphorylation of ERK-1 and -2. A neuritogenic effect of nerve growth factor (NGF) in PC12h cells was also remarkably inhibited by the NOS inhibitor, NO scavenger and SGC inhibitor. These findings suggest t hat induced NO production followed by cyclic GMP-mediated stimulation of th e mitogen-activated protein kinase (MAPK) cascade is implicated in the neur itogenesis by genipin and NGF in PC12h cells.