The tumor suppressor p53 exerts its anti-neoplastic activity primarily through the induction of apoptosis. We found that cytosolic localization of endogenous wild-type or trans-activation-deficient p53 was necessary and sufficient for apoptosis. p53 directly activated the proapoptotic Bcl-2 protein Bax in the absence of other proteins to permeabilize mitochondria and engage the apoptotic program. p53 also released both proapoptotic multidomain proteins and BH3-only proteins [Proapoptotic Bcl-2 family proteins that share only the third Bcl-2 homology domain (BH3)] that were sequestered by Bcl-xL. The transcription-independent activation of Bax by p53 occurred with similar kinetics and concentrations to those produced by activated Bid. We propose that when p53 accumulates in the cytosol, it can function analogously to the BH3-only subset of proapoptotic Bcl-2 proteins to activate Bax and trigger apoptosis.

Using a Bax-dependent membrane-permeabilization assay, we show that peptides corresponding to the BH3 domains of Bcl-2 family "BH3-only" proteins have dual functions. Several BH3 peptides relieved the inhibition of Bax caused by the antiapoptotic Bcl-x(L) and/or Mcl-1 proteins, some displaying a specificity for either Bcl-x(L) or Mcl-1. Besides having this derepression function, the Bid and Bim peptides activated Bax directly and were the only BH3 peptides tested that could potently induce cytochrome c release from mitochondria in cultured cells. Furthermore, Bax activator molecules (cleaved Bid protein and the Bim BH3 peptide) synergistically induced cytochrome c release when introduced into cells along with derepressor BH3 peptides. These observations support a unified model of BH3 domain function, encompassing both positive and negative regulation of other Bcl-2 family members. In this model, the simple inhibition of antiapoptotic functions is insufficient to induce apoptosis unless a direct activator of Bax or Bak is present.

Summary During apoptosis, the BCL-2 protein family controls Mitochondrial Outer Membrane Permeabilization (MOMP), but the dynamics of this regulation remains controversial. We employed chimeric proteins composed of exogenous BH3 domains inserted into a tBID backbone that can activate the pro-apoptotic effectors BAX and BAK to permeabilize membranes without being universally sequestered by all anti-apoptotic BCL-2 proteins. We thus identified two “modes” whereby pro-survival BCL-2 proteins can block MOMP, by sequestering direct activator BH3-only proteins (“MODE 1”) or by binding active BAX and BAK (“MODE 2”). Notably, we found that MODE 1 sequestration is less efficient and more easily de-repressed to promote MOMP than MODE 2. Further, MODE 2 sequestration prevents mitochondrial fusion. We provide a unified model of BCL-2 family function that helps to explain otherwise paradoxical observations relating to MOMP, apoptosis, and mitochondrial dynamics.

The Trp53 tumor suppressor gene product (p53) functions in the nucleus to regulate proapoptotic genes, whereas cytoplasmic p53 directly activates proapoptotic Bcl-2 proteins to permeabilize mitochondria and initiate apoptosis. Here, we demonstrate that a tripartite nexus between Bcl-xL, cytoplasmic p53, and PUMA coordinates these distinct p53 functions. After genotoxic stress, Bcl-xL sequestered cytoplasmic p53. Nuclear p53 caused expression of PUMA, which then displaced p53 from Bcl-xL, allowing p53 to induce mitochondrial permeabilization. Mutant Bcl-xL that bound p53, but not PUMA, rendered cells resistant to p53-induced apoptosis irrespective of PUMA expression. Thus, PUMA couples the nuclear and cytoplasmic proapoptotic functions of p53.