Cytotoxicity by Lead-induced nNOS Phosphorylation in a Dopaminergic CATH.a Cells: Roles of Protein Kinase A

Neuronal cell toxicity induced by decreased nitric oxide (NO) production may be caused by modulation of constitutive neuronal NO synthase (nNOS). We used lead acetate ($Pb^{2+}$) to modulate physiological NO release and the related pathways of protein kinases like PKC, CaM-KII, and PKA in CATH.a cells, a dopaminergic cell line that has constitutive nNOS activity. In the cells treated with $Pb^{2+}$, cell viability and modulation (phosphorylation) levels of nNOS were determined by MTT assay and Western blot analysis, respectively. nNOS reductase activity (cytochrome c) was also assessed to compare the phosphorylation site-specific nNOS activity. nNOS activity was also determined by NADPH consumption rates. $Pb^{2+}$ treatment alone increased the phosphorylation of nNOS with decreased reductase activity. The phosphorylation levels increased markedly with decreased nNOS reductase activity, when $Pb^{2+}$ was combined with inhibitors for two (PKC and CaM-KII) or three (PKA, PKC and CaM-KII) protein kinases. Interestingly, when the cells were exposed to $Pb^{2+}$ plus PKC or CaM-KII inhibitor, the nNOS was phosphorylated strongly with the lowest activity. However, the levels of phosphorylated nNOS following $Pb^{2+}$ treatment decreased significantly after combined treatment with the PKA inhibitor, and $Pb^{2+}$-induced suppression of reductase activity did not occur. These results demonstrate that physiological NO release in the neuronal cells exposed to $Pb^{2+}$ can be decreased by PKA-mediated nNOS phosphorylation that may be caused by interactions with PKC and/or CaM-KII.