Potentiation of nerve growth factor-action by picrosides I and II, natural iridoids, in PC12D cells

Natural iridoid, picroside I (beta-D-glucopyranoside, 1a,1b,2,5a,6, 6a-hexahydro-6-hydroxy-1a-(hydroxymethyl)oxireno4,5]cyclopenta1, 2-c]pyran-2-yl, 6-(3-phenyl-2-propenoate)) or II (beta-D-glucopyranoside, 1a,1b,2,5a,6, 6a-hexahydro-6-(4-hydroxy-3-methoxybenzoyl)oxy]-1a-(hydroxymethyl )ox ireno4,5]cyclopenta1,2-c]pyran-2-yl) alone did not exhibit neuritogenic activity, but caused a concentration-dependent (>0.1 microM) enhancement of nerve growth factor (NGF, 2 ng/ml)-induced neurite outgrowth from PC12D cells. The picroside-induced enhancing action of NGF was abolished by GF109203X (2-1-(3-dimethylaminopropyl)-indol-3-yl]-3-(indol-3-yl)maleimide) (0.1 microM), a protein kinase C inhibitor. Furthermore, PD98059 (2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one) (20 microM), a potent mitogen-activated protein (MAP) kinase kinase inhibitor, completely blocked the picroside-induced enhancement of neurite outgrowth in the presence of NGF (2 ng/ml), suggesting that picrosides activate the MAP kinase-dependent signaling pathway. Interestingly, no increase in the expression of phosphorylated MAP kinase was observed in picroside-treated (60 microM) PC12D cells in the presence of NGF (2 ng/ml). These results suggest that picroside I or II enhances NGF-induced neurite outgrowth from PC12D cells, probably by amplifying a down-stream step of MAP kinase in the NGF receptor-mediated intracellular MAP kinase-dependent signaling pathway.