Death Signal-induced Localization of p53 Protein to Mitochondria

TLDR

The mechanisms by which p53 triggers apoptosis after cellular stress are poorly understood, with evidence that p53 can induce cell death through multiple pathways, including transcription‑independent signaling, and that mitochondria play a central role in this process. The study proposes a model in which p53 directly signals at mitochondria to amplify its transcription‑dependent apoptotic activity. The proposed mechanism is that p53 directly signals at mitochondria to amplify transcription‑dependent apoptosis. The study found that p53 rapidly accumulates in mitochondria at the onset of p53‑dependent apoptosis—within one hour—preceding mitochondrial membrane potential loss, cytochrome c release, and caspase‑3 activation, with most mitochondrial p53 residing in the membrane and some associated with mtHsp70; this accumulation is blocked by Bcl‑2/Bcl‑xL, and forcing p53 to mitochondria alone can trigger apoptosis in p53‑deficient cells.

Abstract

The mechanism of p53-mediated apoptosis after cellular stress remains poorly understood. Evidence suggests that p53 induces cell death by a multitude of molecular pathways involving activation of target genes and transcriptionally independent direct signaling. Mitochondria play a key role in apoptosis. We show here that a fraction of p53 protein localizes to mitochondria at the onset of p53-dependent apoptosis but not during p53-independent apoptosis or p53-mediated cell cycle arrest. The accumulation of p53 to mitochondria is rapid (within 1 h after p53 activation) and precedes changes in mitochondrial membrane potential, cytochrome c release, and procaspase-3 activation. Immunoelectron microscopy and immuno-fluorescence-activated cell sorter analysis of isolated mitochondria show that the majority of mitochondrial 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 again partially localizes to mitochondria, preceding the onset of apoptosis. Overexpression of anti-apoptotic Bcl-2 or Bcl-xL abrogates stress signal-mediated mitochondrial p53 accumulation and apoptosis but not cell cycle arrest, suggesting a feedback signaling loop between p53 and mitochondrial apoptotic regulators. Importantly, bypassing the nucleus by targeting p53 to mitochondria using import leader fusions is sufficient to induce apoptosis in p53-deficient cells. We propose a model where p53 can contribute to apoptosis by direct signaling at the mitochondria, thereby amplifying the transcription-dependent apoptosis of p53.