Peroxynitrite nitration of Tyr 56 in Hsp90 induces PC12 cell death through P2X7R-dependent PTEN activation

The diffusion-limited reaction of nitric oxide (NO) and superoxide (O₂^-) produces peroxynitrite (ONOO^-), a biological oxidant that has been implicated in a number of pathological conditions, including neurodegenerative disorders. We previously reported that incubation of PC12 cells with peroxynitrite triggers apoptosis by simultaneously inhibiting the PI3K/Akt survival pathway, and activating the p38 and JNK MAP kinase pathways. We also reported that peroxynitrite-treated Heat Shock Protein 90 (Hsp90) stimulates PC12 cell death. Here, we show that nitrated Hsp90 mediates peroxynitrite-induced apoptosis by regulating specific signaling pathways triggered by activation of the purine receptor P2X7 (P2X7R) and downstream activation of PTEN. Intracellular delivery of peroxynitrite-treated Hsp90 was sufficient to stimulate PC12 cell death. In contrast, intracellular delivery of peroxynitrite-treated Hsp90 in which the five tyrosine (Tyr) residues susceptible to nitration were replaced by nitration-resistant phenylalanine had no effect on PC12 cell survival. Further, only nitration of Hsp90 at Tyr 56 was necessary and sufficient to stimulate PC12 cell apoptosis, and incubation of PC12 cells with peroxynitrite resulted in Hsp90 nitration at Tyr 56. Inhibition of P2X7R or downstream inhibition of PTEN prevented PC12 cell death stimulated by both incubation with peroxynitrite and nitrated Hsp90 (Hsp90^NY). Peroxynitrite, Hsp90^NY, and P2X7R activation all increased p38 and JNK MAP kinases activity, while inhibiting the Akt survival pathway. These results suggest that, in undifferentiated PC12 cells, peroxynitrite triggers apoptosis via nitration of Hsp90 at Tyr 56, which in turn activates P2X7R and PTEN. These results contrast with observations in motor neurons where the nitration of either Tyr 33 or Tyr 56 in Hsp90 stimulates apoptosis, suggesting that the targets of peroxynitrite may be different in different cell types. However, uncovering the pathways through which peroxynitrite triggers cell death in neurodegenerative conditions will provide new potential targets for therapeutic treatment.