Regulation of antioxidant enzyme expression by NGF

The rapid decreases in viability seen in H₂O₂-treated PC12 cells reflect enhanced susceptibility of neural cell types to oxidant injury. The dose-response relationship between NGF concentration and survival after H₂O₂ treatment resembles that for NGF effects on PC12 survival in serumless medium. Previously we have shown that NGF treatment enhances the activity of GSH-Px and catalase which catalyze the degradation of H₂O₂. Here in order to ascertain whether NGF stimulates transcription, affects mRNA stability, or acts post-transcriptionally, we measured catalase and GSH-Px mRNA half- lives. While both catalase and GSH-Px transcripts are stable with a relatively long half life and a gradual decay in mRNA levels, NGF had different effects on their stability. NGF had marked effects on catalase mRNA stability. The catalase gene has a 3' flanking region with T-rich clusters and CA repeats known to be susceptible to regulation by destabilization or ubiquination. NGF maintained catalase mRNA levels of actinomycin D (ACT-D) treated PC12 cells at twice that of cells exposed to ACT-D alone, delaying the rate of decay for catalase mRNA for 24 h. The NGF induction of GSH-Px and catalase mRNA was inhibited by cycloheximide (CHX) treatment with a slight decrease in their mRNA levels due to prolonged exposure to CHX. When the CHX treatment was delayed relative to the NGF treatment there was no effect on NGF effects on catalase and GSH-Px. The GSH-Px gene has conserved sequences in the open reading frame and 3' untranslated region which forms a stem-loop structure necessary for the incorporation of Se into this selenoprotein. While Se is important in stabilizing GSH-Px transcripts, it did not affect transcription rates or mRNA stability. These results are consistent with the hypothesis that NGF regulates catalase and GSH-Px expression via a primary effect on transcription factor pathways.