SIRT1 and Parkinson disease: Similarly, SIRT1 was also downregulated by oxidative stress when treated with 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and H2O2 in PD-NSCs (Fig. 2b), suggesting that downregulation of SIRT1 was a general stress response specific for PD-NSCs.