Biological screen for transcriptional inducers of p21. A, HT22 cells were stably transfected with a firefly luciferase reporter driven by 2.4 kb or 60 bp of the p21 proximal promoter. The 2.4 kb reporter contains the p53 binding sites (arrows) necessary for activation by DNA damage. Both constructs are responsive to TSA, which activates the Sp1 sites (arrow heads). B, A 2,000 compound library of FDA-approved drugs and natural product small molecules were screened using the 60 bp promoter. Compounds that induced reporter activity by ≥ 2 SD (above light grey region) were further investigated. C, p21 mRNA levels from primary cortical neurons treated with neuroprotective hits (10 μM). TSA was included as a positive control.