Upregulation of Tyrosine Hydroxylase and Downregulation of Choline Acetyltransferase in Lead-Exposed PC12 Cells: The Role of PKC Activation

Abstract

The effects of lead (Pb) on the expression of tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT) were compared in relation to Pb-activation of cPKC in the PC12 cells. Exposure to 0.53 μM Pb (0.1094 ppm) increased TH but reduced ChAT activity and mRNA levels. The increase of TH activity was detectable as early as 0.5 h of exposure, reached a maximum 150% of control after 2 h, and then diminished to a steady state 135% of control between 12 and 48 h of exposure. The decrease of ChAT activity was first detectable after 2 h of Pb exposure, reached a 45% reduction after 6 h, and remained stable thereafter through 48 h of exposure. PKC activity increased 200% after 2 h and then reverted to control levels by 48 h of Pb exposure. The increase of TH activity after 2 h but not 48 h of exposure exceeded that of its mRNA. PKC inhibitor Rö32-0342 suppressed TH activity increase after 2 h of Pb exposure by 80% without affecting TH mRNA. The decrease of ChAT activity correlated with the reductions in steady-state ChAT mRNA levels at 2 and 48 h of Pb exposure and Rö32-0342 had no effect on the Pb-induced decrease of either ChAT activity or mRNA. These results demonstrate that Pb alters TH and ChAT expression in PC12 cells in a reciprocal manner, i.e., upregulates the former but downregulates the latter. PKC is not involved in Pb-induced downregulation of ChAT but does mediate the early phase of Pb-induced augmentation of TH activity, presumably through postranslational modification (phosphorylation) of the enzyme. However, this effect is short-lived due to downregulation of PKC in the course of prolonged (48 h) Pb exposure. It is concluded that, in the course of prolonged exposure, both upregulation of TH and downregulation of ChAT reflect primarily the effects of Pb at the level of gene expression through mechanisms that are not related to Pb activation of PKC.