Death signal-induced localization of p53 protein to mitochondria - A potential role in apoptotic signaling

The mechanism of p53-mediated apoptosis after cellular stress remains poorl y understood. Evidence suggests that p53 induces cell death by a multitude of molecular pathways involving activation of target genes and transcriptio nally independent direct signaling. Mitochondria play a key role in apoptos is, We show here that a fraction of p53 protein localizes to mitochondria a t the onset of p53-dependent apoptosis but not during p53-independent apopt osis or p53-mediated cell cycle arrest. The accumulation of p53 to mitochon dria is rapid (within 1 h after p53 activation) and precedes changes in mit ochondrial membrane potential, cytochrome c release, and procaspase-3 activ ation. Immunoelectron microscopy and immuno-fluorescence-activated cell sor ter analysis of isolated mitochondria show that the majority of mitochondri al p53 localizes to the membranous compartment, whereas a fraction is found in a complex with the mitochondrial import motor mt hsp70, After induction of ectopic p53 without additional DNA damage in p53-deficient cells, p53 a gain partially localizes to mitochondria, preceding the onset of apoptosis, Overexpression of anti-apoptotic Bcl-2 or Bcl-xL abrogates stress signal-m ediated mitochondrial p53 accumulation and apoptosis but not cell cycle arr est, suggesting a feedback signaling loop between p53 and mitochondrial apo ptotic regulators. Importantly, bypassing the nucleus by targeting p53 to m itochondria using import leader fusions is sufficient to induce apoptosis i n p53-deficient cells. We propose a model where p53 can contribute to apopt osis by direct signaling at the mitochondria, thereby amplifying the transc ription-dependent apoptosis of p53.