Activation of the phospholipase C signaling pathway in nerve growth factor-treated neurons by carbon nanotubes

Low concentrations of carbon nanotubes (CNTs) promoted the number of nerve growth factor (NGF)-treated neurons with neurite outgrowth by activating extracellular signal-regulated kinase (ERK), even when MEK inhibitor was added to the neuron culture medium. We speculated that CNTs may activate ERK through the phospholipase C (PLC) signaling pathway independent of the Ras/Raf/MEK cascade involved in the ERK signaling pathway. CNTs enhanced phosphorylation of PLC-γ1 in NGF-treated neurons but failed to increase the number and length of neurites of NGF-treated neurons with neurite outgrowth when a PLC inhibitor, an inositol triphosphate receptor (IP3R) inhibitor, or an inhibitor of protein kinase C (PKC) in the PLC signaling pathway were added to the neuron culture medium. Furthermore, intracellular Ca++ levels of cells treated with CNTs + NGF were higher than those of cells treated with NGF alone. Although the combination of CNTs and NGF increased the concentration of phosphorylated ERK (p-ERK) in MEK inhibitor-treated neurons, CNTs did not induce phosphorylation of ERK in PLC inhibitor-treated neurons. These data suggest that PKC in the PLC signaling pathway may activate ERK independent of the Ras/Raf/MEK cascade. In summary, we identified a role of PLC signaling in mediating neurite outgrowth of NGF-treated neurons in the presence of CNTs.