Parkin functions as a repressor of p53 transcription and protein levels . A-B, p53 promoter activity measured by luciferase assay in HEK-293 and SH-SY5Y cells transiently transfected with parkin or the control vector pcDNA. C, p53 promoter activity in SH-SY5Y cell line stably expressing parkin (P-SY5Y). D-E, Western blot analyses of parkin, p53 and actin in SH-SY5Y cells transiently ( D ) or stably ( E ) expressing parkin. Quantification of p53 signal (right panels) was made relative to actin and expressed in arbitrary units (a.u.). All values are mean + SEM, n = 6 ( A ), 12 ( B ), 18 (C), 3 (D) and 9 (E); * p < 0.01.