Oncogenic Ras decreases mitochondrial membrane potential in a p53-dependent manner. (A and B) NIH3T3 cells were infected with a control or Ha-RasV12–expressing retrovirus together with a control or p53R175H-expressing retrovirus. p53^−/− MEFs were infected with a control or Ha-RasV12–expressing retrovirus. (A) Confocal images of cells stained with JC-1. The ratio of red fluorescence (J-aggregate) to green fluorescence (monomer) is correlated with mitochondrial membrane potential. Bars, 20 µm. (B) The ratio of red fluorescence to green fluorescence at peripheral mitochondria was quantified from images in A. Data represent the mean ± SD of more than 20 cells. *, P < 0.01. (C) Ras-transformed p53^−/− MEFs were treated with CCCP (20 µM) for 4 h. After subcellular fractionation of the cytosol (Cyto) and mitochondria (Mito), the distribution of p38 MAPK was evaluated by immunoblotting. COX IV and α-tubulin were used as mitochondrial and cytosolic markers, respectively.