inner mitochondrial membrane mitochondrial outer membrane permeabilization mitochondrial permeability transition permeability transition pore reactive oxygen species the notion that p53 may induce transcription - independent apoptosis in response to genotoxic stress was first suggested almost 20 y ago . in experiments with p53-null tumor cells that stably express a temperature - sensitive mutant p53 protein , caelles et  al . showed that at permissive temperatures the induction of apoptosis following dna damage was independent of new rna or protein synthesis . these findings were subsequently confirmed in experiments of p53-null cells reconstituted with transcriptionally inactive truncation mutants of p53 . the mechanistic explanation for the transcription - independent apoptotic p53 activity came from pioneering studies that linked direct p53 protein action to the intrinsic mitochondrial apoptotic pathway . we and others showed that , in response to stress , a fraction of induced p53 protein localizes to mitochondria during p-53 mediated apoptosis , where it behaves like a super bh3-only protein to induce mitochondrial outer membrane permeabilization ( momp ) , physically interacting with and inhibiting antiapoptotic members ( bcl2 , bclxl , mcl1 ) as well as activating proapoptotic members ( bak , bax ) of the bcl2 family of momp regulators and thereby triggering p53-dependent apoptosis ( see below ) . moreover , overexpression of antiapoptotic bcl2 or bclxl abrogates stress signal - mediated mitochondrial p53 accumulation and subsequent apoptosis but not cell cycle arrest , suggesting a currently unexplored feedback signaling loop between p53 and apoptotic bcl regulators . importantly , bypassing the nucleus by targeting p53 to mitochondria via fusions with an import leader peptide or the bcl2-derived transmembrane domain is sufficient to induce apoptosis in p53-deficient cells . this initial observation of stress - induced mitochondrial translocation of p53 was confirmed to occur in response to a multitude of apoptotic stimuli including genotoxic stress , deregulated oncogenes , hypoxia , transcriptional blockade , mdm2 inhibition ( fig . 1b ) , and heat stress in cultured human and mouse normal , immortal , and cancer cells , and in response to gamma irradiation in vivo in radiosensitive tissues such as thymus , spleen , and testis ( fig . figure 1.(a ) detection of stress - induced mitochondrial p53 accumulation in vivo by confocal immunofluorescence . after 2  h , tissue smears were prepared from thymi and double - stained with monoclonal anti - p53 antibody pab 246 followed by fitc - goat anti - mouse antibody and a rabbit anti - grp75 antibody followed by tritc - donkey anti - rabbit antibody . in contrast , all irradiated thymocytes exhibited prominent , finely punctated perinuclear ring staining , which exceeded the concomitant faint nuclear staining at this time point . ( b ) inhibition of mitochondrial p53 translocation by pft inhibits mdm2 inhibitor nutlin - induced apoptosis . pre - treatment of ml-1 cells with pft , a specific inhibitor of p53 's association with mitochondria ( see text ) , results in markedly reduced p53 translocation and reduced cytochrome c release in response to nutlin treatment . ml-1 cells were pre - treated with 25  m pft for 2  h before adding 10  m nutlin for 12  h , followed by mitochondrial fractionation and immunoblotting as indicated . ( a ) detection of stress - induced mitochondrial p53 accumulation in vivo by confocal immunofluorescence . after 2  h , tissue smears were prepared from thymi and double - stained with monoclonal anti - p53 antibody pab 246 followed by fitc - goat anti - mouse antibody and a rabbit anti - grp75 antibody followed by tritc - donkey anti - rabbit antibody . in contrast , all irradiated thymocytes exhibited prominent , finely punctated perinuclear ring staining , which exceeded the concomitant faint nuclear staining at this time point . ( b ) inhibition of mitochondrial p53 translocation by pft inhibits mdm2 inhibitor nutlin - induced apoptosis . pre - treatment of ml-1 cells with pft , a specific inhibitor of p53 's association with mitochondria ( see text ) , results in markedly reduced p53 translocation and reduced cytochrome c release in response to nutlin treatment . ml-1 cells were pre - treated with 25  m pft for 2  h before adding 10  m nutlin for 12  h , followed by mitochondrial fractionation and immunoblotting as indicated . pathways governing the translocation of p53 to mitochondria were found to be regulated by post - translational modifications . the first mechanistic insight came from work that demonstrated differential apoptotic potential of 2 common polymorphic variants of human p53 ( arg / pro at codon 72 ) , which perfectly correlated with their differential abilities to localize to mitochondria ( with arg72 being the more potent form for both killing and translocation ) . the authors suggested that greater interaction of the p53 arg72 variant with mdm2 might lead to enhanced ubiquitination of p53 , with enhanced nuclear export and mitochondrial localization . subsequently , we mechanistically expanded on these observations and showed that the cytoplasm contains a separate distinct p53 pool , which is the major source for p53 translocation to mitochondria upon stress - induced stabilization . we found that the cytoplasmic p53 pool is subject to a binary switch from a fate of inactivation via polyubiquitination and degradation in unstressed cells , to stress - induced mdm2-mediated monoubiquitination that masks the p53 nuclear localization sites , thereby locking it in the cytoplasm . multi - monoubiquitination of p53 on c - terminal lysine residues greatly promotes its translocation to mitochondria , thereby shifting the cell fate to apoptosis . on the other hand upon arrival at the mitochondrion , p53 undergoes rapid deubiquitination by resident mitochondrial hausp via a stress - induced mitochondrial p53-hausp complex to generate the apoptotically active non - ubiquitinated p53 . upstream , dna damage - activated homeodomain interacting protein kinase 2 ( hipk2 ) induces phosphorylation of p53 at ser46 , which recruits pin1 ( a phospho - specific peptidylprolyl cis / trans isomerase that translates phosphorylation modifications into conformation changes of its target substrates ) and triggers a phospho - dependent cis / trans isomerization of p53 . this conformational change of cytoplasmic p53 reduces its affinity for mdm2 , resulting in an increase of monoubiquitinated p53 that undergoes mitochondrial translocation . sorrentino et  al . showed that pin1 promotes stress - induced localization of p53 to mitochondria in vitro and in vivo by stimulating p53 monoubiquitination . of note , pin1 also plays an important activating role in stress kinase - mediated transcriptional activation of nuclear p53 . at the mitochondria , ser46 phosphorylation of p53 mediates its binding to resident mdm4 on the outer membrane , which in turn facilitates the interaction between p53 and bcl2 to induce momp and mitochondrial apoptosis ( fig . interestingly , the mitochondrial ser46p53-mdm4 axis might play a role in mediating a better prognosis in glioblastoma . in unstressed cells the cytoplasmic pool of p53 is inactivated via mdm2-mediated polyubiquitination and degradation . dna damage induces hipk2-mediated p53 phosphorylation at ser46 , which recruits pin1 and triggers a phospho - dependent cis / trans isomerization of p53 . this conformational change of cytoplasmic p53 reduces its affinity for mdm2 , resulting in a switch from polyubiquitination to monoubiquitination that masks the p53 nuclear localization sites and promotes mitochondrial translocation of p53 . upon arrival at the mitochondria , p53 undergoes rapid deubiquitination by mitochondrial hausp via a stress - induced mitochondrial p53-hausp complex . ser46-p53 phosphorylation at the mitochondrion also mediates its binding to resident mdm4 on the outer membrane , which in turn facilitates interaction between p53 and bcl2/bclxl to induce mitochondrial outer membrane permeabilization and mitochondrial apoptosis . dna damage induces hipk2-mediated p53 phosphorylation at ser46 , which recruits pin1 and triggers a phospho - dependent cis / trans isomerization of p53 . this conformational change of cytoplasmic p53 reduces its affinity for mdm2 , resulting in a switch from polyubiquitination to monoubiquitination that masks the p53 nuclear localization sites and promotes mitochondrial translocation of p53 . upon arrival at the mitochondria , p53 undergoes rapid deubiquitination by mitochondrial hausp via a stress - induced mitochondrial p53-hausp complex . ser46-p53 phosphorylation at the mitochondrion also mediates its binding to resident mdm4 on the outer membrane , which in turn facilitates interaction between p53 and bcl2/bclxl to induce mitochondrial outer membrane permeabilization and mitochondrial apoptosis . . showed that a lys120-acetylated form of p53 is enriched at the outer mitochondrial membrane after dna damage . lys120 acetylation is not required for p53 mitochondrial translocation , but rather enables p53 to displace the antiapoptotic mcl1 from the proapoptotic effector bak , allowing bak oligomerization and pore formation . consistent with these data , p53 acetylation at lys320/lys373/lys382 is required for its transcription - independent functions in bax activation and apoptosis in response to histone deacetylase inhibitors ( hdaci ) . as briefly alluded to above , a major outcome of stress - induced mitochondrial translocation of p53 is the induction of transcription - independent apoptosis via the intrinsic mitochondrial pathway . this pathway depends on mitochondrial outer membrane permeabilization ( momp ) , which is regulated by bcl2 family members . upon arrival at mitochondria the bcl2 family of proteins is subdivided into 3 groups according to function as follows : ( i ) antiapoptotic bclxl , bcl2 , bclw , and mcl-1 , which stabilize the outer membrane , ( ii ) the ultimate proapoptosis effectors bax and bak , which are able to form pores and/or associate with pore - forming proteins in the outer membrane ; ( iii ) small bh3 domain - only proteins , which interact with both anti- and proapoptotic bcl2 members to transmit the death signal and induce apoptosis ( puma , noxa , bad as enablers on one hand ; tbid and bim as  activators on the other hand ) . in response to stress , the cytoplasmic pool of p53 rapidly translocates to the surface of the outer mitochondrial membrane , where it behaves like a dual bh3-only protein , engaging in complexes with antiapoptotic ( bclxl , bcl2 ) and/or proapoptotic ( bak ) members of the bcl2 family . these interactions facilitate bax and bak homo - oligomerization , causing pore formation and permeabilization of the outer membrane and release of cytochrome c , smac / diablo , htra2 , and aif , which activate the caspase cascade and ultimately induce apoptosis . importantly , recombinant p53 induces momp and release of cytochrome c from isolated healthy mitochondria within minutes , confirming the direct effect of p53 on momp ( fig . computational and mutational structure / function studies revealed that the dna - binding interface of p53 is in direct contact with the bh4 domain and adjacent loops connecting  helices of bclxl ( opposite the canonical bh123 binding pocket ) to form the p53-bclxl complex . additionally , through direct binding to bak , p53 is able to disrupt the complex between bak and its antiapoptotic partner mcl1 at the outer membrane , leading to bak oligomerization and momp ( fig . 3 ) . notably , tumor - derived p53 missense mutants , although constitutively localized at the outer membrane as a result of their intrinsic cellular stabilization , are unable to bind to bcl2 and trigger momp , indicating that missense mutants are double hit loss - of - function mutants for apoptosis and suggesting that the transcription - independent direct mitochondrial apoptosis program might participate in tumor suppression . figure 3.p53 mediates 2 programmed cell death pathways , apoptosis and necrosis , directly at the mitochondria . top , apoptosis : upon arrival at the mitochondria , p53 physically interacts with antiapoptotic bcl2 family members and proapoptotic bak , facilitating bak oligomerization . this in turn causes pore formation and permeabilization of the outer membrane and release of cytochrome c and other apoptogenic factors . the inhibitory interaction of p53 with bclxl / bcl2 also liberates proapoptotic tbid from its inhibitory complex with bclxl / bcl2 . p53 interaction with bak induces a conformational change in bak , promoting bak oligomerization and pore formation . bottom , necrosis : in response to oxidative stress , p53 accumulates in the mitochondrial matrix and triggers opening of the mitochondrial permeability transition pore ( mptp ) by physical interaction with its critical regulator cyclophilin d ( cypd ) , which leads to necrosis . p53 mediates 2 programmed cell death pathways , apoptosis and necrosis , directly at the mitochondria . top , apoptosis : upon arrival at the mitochondria , p53 physically interacts with antiapoptotic bcl2 family members and proapoptotic bak , facilitating bak oligomerization . this in turn causes pore formation and permeabilization of the outer membrane and release of cytochrome c and other apoptogenic factors . the inhibitory interaction of p53 with bclxl / bcl2 also liberates proapoptotic tbid from its inhibitory complex with bclxl / bcl2 . p53 interaction with bak induces a conformational change in bak , promoting bak oligomerization and pore formation . bottom , necrosis : in response to oxidative stress , p53 accumulates in the mitochondrial matrix and triggers opening of the mitochondrial permeability transition pore ( mptp ) by physical interaction with its critical regulator cyclophilin d ( cypd ) , which leads to necrosis . an extension of this mitochondrial outer membrane - based p53 program that integrates concomitant transcription by nuclear p53 was proposed by the green laboratory . in this model , p53 in unstressed cells is sequestered in the cytoplasm by soluble bclxl . upon genotoxic stress , nuclear p53 transcriptionally induces puma , which binds to cytosolic bclxl , thereby liberating p53 . p53 can then bind and activate monomeric bax in the cytoplasm via transient formation of a  hit - and - run complex , causing bax homo - oligomerization and translocation to the outer membrane . this mechanism appears to be cell - context dependent because the p53 transcription - independent direct mitochondrial apoptotic program also occurs in puma cells . the major physiological outcome of p53-mediated momp is cytochrome c release , which leads to activation of the executioner caspases 3 , 6 , and 7 via the apoptosome . caspase-3 , the key executioner caspase , is localized both in the cytosol and at the inner and outer mitochondrial membrane . interestingly , tumor - derived mutant p53 proteins also bind procaspase-3 but interfere with its ability to be proteolytically activated by caspase-9 . these data suggest that p53 mutants may be selected for in tumor cells because of their ability to bind and inhibit activation of caspase-3 and therefore block apoptosis . the role of p53 at mitochondria in determining cell fate is not restricted to activation of apoptosis . we recently identified a transcription - independent direct mitochondrial matrix - based program of p53 that induces necrosis , a major form of cell death that is distinct from apoptosis in microscopic structure and biochemical and molecular features . necrosis is irreversible tissue destruction as a result of bioenergetic failure , and is central to the ischemia / reperfusion injury and oxidative damage that occurs in stroke and organ infarctions . the fundamental difference between necrosis and apoptosis is the rapid loss of cellular membrane potential as a result of energy depletion and ion pump / channel failure , leading to hydropic swelling , organellar rupture , and cytolysis with extracellular spilling of cellular content . the major mediators of necrosis are excess levels of cytosolic ca and reactive oxygen species ( ros ) . rather than being a passive event , in contrast to apoptosis , necrosis induces an inflammatory response to stimulate tissue repair by selectively releasing immunostimulatory factors such as hmgb1 and hdgf from dying cells . during ischemic injury , ros and calcium levels this increase triggers programmed necrosis through sudden opening of the mitochondrial permeability transition pore ( mptp ) , a normally impermeable non - selective water , ion , and small solute channel in the inner mitochondrial membrane ( imm ) that is uniquely controlled by cyclophilin d ( cypd ) . cypd , which is located in the matrix , is the essential regulator of ptp opening and the only genetically proven indispensable ptp component . cypd is the sole mitochondrial member of the cyclophilin family of peptidyl prolyl cis - trans isomerases , which have a crucial role in protein folding and adaptive conformational changes . recent studies with 4 independent knockout mouse strains definitively identified cypd as the key regulator of the mptp pore at the inner membrane , opening of which leads to necrotic cell death . cypd null mice are remarkably protected from ischemia / reperfusion - induced myocardial infacrction and stroke in vivo . conversely , cyclophilin d - overexpressing mice show mitochondrial swelling and spontaneous cell death . in agreement with these findings , mitochondria isolated from liver , heart , and brain of cypd null mice are resistant to calcium spike - induced mitochondrial swelling and permeability transition in vitro . moreover , primary hepatocytes and fibroblasts isolated from cypd null mice are largely protected from necrotic death induced by ca overload and oxidative stress . treatment with cyclosporine a , a highly specific cypd catalytic site inhibitor , reduces ischemia / reperfusion injury in the brain and heart . importantly , cypd - deficient cells die normally by apoptosis , emphasizing the existence of 2 functionally distinct mitochondrial death pathways . in healthy cells , pumping h out of the matrix creates the proton gradient m across the imm that is necessary to maintain mitochondrial respiration and atp production . in unstressed cells the ptp is closed and the imm is impermeable to ions . upon oxidative stress , sudden ptp opening causes massive ion influx that dissipates m and shuts down oxidative phosphorylation and atp production . concomitant water influx causes matrix swelling , rupture of the rigid outer mitochondrial membrane , and release of all sequestered cell death factors . mpt is triggered by mitochondrial matrix sequestration of the high levels of cytosolic ca and ros generated during oxidative damage . clinically , cypd - triggered ptp opening and mpt are the driving pathophysiological forces behind cerebral stroke , myocardial infarction , and other vascular catastrophes that are leading causes of death . the mechanism by which cypd becomes activated to induce mpt remains unclear but is known to depend on its prolyl isomerase activity . cyclosporine a ( csa ) potently and specifically prevents mpt by binding to cypd , thus inhibiting its isomerase activity and displacing it from the ptp . the first report that linked p53 with programmed necrosis showed that upon dna damage bax / bak double knockout ( dko ) mouse embryonic fibroblasts ( mefs ) undergo necrosis , which is largely controlled by p53-mediated transcription of cathepsin q in cooperation with dna damage - induced ros . interestingly , this transcriptional p53-cathepsin q axis only plays a minor role in the corresponding necrosis in wild - type cells . we subsequently found that , in contrast to the prototypical momp - inducer tbid , purified p53 protein has efficacy beyond activating the bax / bak lipid pore at the outer membrane . recombinant p53 is able to rapidly release inner membrane - tethered apoptosis inducing factor ( aif ) from healthy isolated bax / bak dko mitochondria , whereas tbid fails to do so . following up on this observation , we identified a novel transcription - independent function of p53 in the activation of necrotic cell death . in response to oxidative stress and ischemia , p53 protein accumulates in the mitochondrial matrix and triggers ptp pore opening , mpt , and necrosis by physical interaction with the critical ptp regulator cypd ( fig . 3 ) . this p53 action is transcription - independent ( refractory to pifithrin  and -amanitin ) and is inhibited by csa or by genetic cypd deletion or knockdown . similarly , p53 causes structural alterations of the ptp pore , but tbid does not . knockout and knockdown experiments of each partner in mouse and human cells demonstrated that oxidative stress induces ptp opening , mpt , collapse of m , and necrosis that all concomitantly depend on the presence of p53 and cypd , and thus are interdependent . most intriguingly , in an ischemia / reperfusion injury stroke model in mice , a robust p53-cypd complex is formed during necrosis in wt brain tissue . conversely , genetic reduction of p53 levels ( p53/ mice ) or csa - pretreatment of p53 mice is associated with effective stroke protection and a concomitant lack of pathologic p53-cypd complex formation . overall , these data imply a pathophysiologic role of the mitochondrial p53-cypd axis in ischemic stroke . prior to our mechanistic study , an intriguing study using rat neonatal brain ischemia to model perinatal hypoxia / asphyxia - induced brain damage had shown that blocking p53 translocation to mitochondria with pifithrin- ( pft- ) suppressed oxidative stress and strongly reduced tissue necrosis and apoptosis . thus pft- although not exclusively restricted to inhibiting mitochondrial p53 action , was remarkably neuroprotective , with improved functional outcome and greatly decreased lesion size . of note , the neuroprotective effect of inhibiting only p53 transcriptional activity by pft- was significantly smaller and did not involve reduced oxidative stress . together , these studies imply that the direct mitochondrial p53-based necrotic and apoptotic machineries are major pathophysiologic effectors of ischemic brain damage , and that their interception might have promising therapeutic potential . aside from p53 , very few extramitochondrial proteins are known to bind cypd and trigger pathologic ptp opening . a compelling analogy to p53 , albeit acting chronically , is the highly toxic soluble intracellular form of  amyloid peptide ( a ) , a key protein in alzheimer disease ( ad ) , that progressively accumulates in brain mitochondria of patients with ad and in ad transgenic mice . in ad , a forms a critical complex with cypd that promotes ptp opening , thereby boosting ca - induced mitochondrial damage and neuronal injury . inhibiting ptp or ablating cypd in ad mice renders their cortical brain mitochondria resistant to ptp opening and improves their cognitive functions . the mitochondrial p53-cypd - ptp signaling pathway of necrosis has since been independently confirmed and expanded to cover additional necrotic stimuli . as a model of oxidative stress - induced necrotic cell death of neurons , sh - sy5y neuroblastoma cells were subjected to ischemia / reperfusion conditions via oxygen glucose deprivation / re - oxygenation . the ensuing necrosis was associated with mitochondrial p53 translocation and cypd complex formation , whereas disruption of the complex ( by csa , cypd knockdown , or p53 deletion / knockdown ) inhibited the mptp - mediated neuronal necrosis . in addition , prolonged glucocorticoid exposure - mediated osteoporosis / osteonecrosis was modeled in cultured murine osteoblasts . dexamethasone ( dex ) induced necrotic death via mptp opening , as indicated by its suppression by csa , sanglifehrin a ( another specific cypd inhibitor ) , or cypd knockdown . dex - induced events were associated with phosphorylation of p53 and its translocation to mitochondria , where it formed a complex with cypd . moreover , glucocorticoid receptor knockdown or p53 inhibition ( by shrna silencing ) suppressed dex - induced p53-cypd complexation and necrosis . furthermore , chemotherapy - induced necrosis in p53 cancer cells also involves the p53-cypd - mptp axis . loss - of - function analysis showed that cisplatin - induced necrosis of pancreatic cancer lines aspc-1 and capan-2 and doxorubicin - induced necrosis of lung cancer cells a459 require the mitochondrial p53-cypd signaling axis . in summary , in addition to the well - characterized mitochondrial p53 program of apoptosis , a spectrum of stress stimuli including hypoxia , oxidative stress , dna damage , and excess glucocorticoid induce a distinct mitochondrial p53-cypd program of necrosis . the decision of whether to engage one and/or the other program appears to depend on cell type and the nature or magnitude of the stress . in addition , a recent study reported another type of p53-dependent , bax / bak - independent necrotic death in mefs and human tumor cells . although the precise mechanism remains to be elucidated , this process involves dna damage induced by very high doses of h2o2 and is mediated by parp-1 , suggesting that in this scenario p53 can modulate the enzymatic activity of parp-1 . interestingly , this form of cell death is not inhibited by csa and minimally affected by cypd knockout , and is thus probably ptp - independent . furthermore , necrotic signaling also uses p53-independent pathways depending on the insult ; for example , dna alkylating agents induce necrosis through parp and the tnfr / rip1 pathways . in addition to the cns ischemia studies discussed above , there is growing evidence supporting a critical role of mitochondrial p53 in death signaling upon ischemic cerebral injury . using microdissected hippocampal ca1 tissue instead of whole brain in a transient global cerebral ischemia model in rats , endo et  al . showed that mitochondrial p53 accumulation in hypoxia - sensitive hca1 neurons strongly correlates with ( i ) formation of a p53-bclxl complex in isolated ca1 mitochondria , ( ii ) cytochrome c release , and ( iii ) neuronal death within 24  h. the pan - p53 small molecule inhibitor pft suppressed p53 transcription and p53 mitochondrial translocation and blocked neuronal cell death by 70% in vivo . proper mitochondrial length is critical for optimal mitochondrial function , and its dysregulation plays a causal role in neurologic and other pathologies . p53 directly influences the regulation of mitochondrial dynamics by modulating the expression and activity of fission and fusion proteins , dynamin - related gtpases that include drp1 ( the main fission protein ) in a transcription - dependent and -independent manner . p53 transcriptionally induces mitofusin-2 ( a fusion protein ) and drp1 under apoptotic conditions in hepg2 cells and cardiomyocytes , respectively . moreover , huntington 's disease ( hd ) , which is caused by mutated huntingtin protein ( mthtt ) , exhibits several cellular dysfunctions including excessive mitochondrial fission caused by hyperactivation of drp1 at mitochondria . accumulation of p53 on mitochondria is observed in animal and cell culture models of hd , and is associated with mitochondrial depolarization and mitochondrial complex iv inactivity . an elegant study using mthtt knock - in mouse neurons and human hd - ips cell - derived striatal neurons implicates direct mitochondrial p53-drp1 complex formation as a causal event for excessive mitochondrial fragmentation and necrosis in hd neurons . cytoplasmic p53 binds to drp1 and induces co - accumulation of drp1 and p53 at mitochondria , leading to drp1 hyperactivation and subsequent excessive mitochondrial fission and dysfunction ( ros production , loss of m ) and neuronal necrosis . suppression of p53 ( by sirna ) , but not additional simultaneous drp1 inhibition , effectively blocks all of these events . in addition to huntington 's disease , a link to p53-drp1 also exists in oxidative stress - induced necrosis . using cell models , guo et  al . recently showed that under conditions of oxidative stress drp1 stabilizes p53 and is required for p53 mitochondrial translocation through the induction of p53 monoubiquitination . most notably , inhibition of drp1 hyperactivation by a peptide inhibitor ( p110 ) blocks p53 mitochondrial accumulation and strongly reduces brain infarction in a rat ischemia reperfusion stroke model . another important translational aspect of mitochondrial p53 function comes from pioneering work of the gudkov laboratory . p53-mediated apoptosis in normal rapidly dividing radiosensitive and chemosensitive organs ( e.g. , the gastrointestinal lining and bone marrow ) is a major cause of severe side effects during cancer treatment by radiation or chemotherapy , and often limits their efficacy . studies to determine the effect of the apoptotic mitochondrial p53 program on radiosensitivity of normal tissue identified the small molecule pifithrin  ( pft ) , which inhibits p53 accumulation at mitochondria by reducing its affinity to antiapoptotic bclxl and bcl2 without affecting p53-dependent transactivation . remarkably , a single intraperitoneal injection of pft 30 minutes prior to treatment with 8 - 9 gy of gamma radiation rescued otherwise lethally irradiated mice from bone marrow failure and strongly reduced radiation - induced thymocyte death in vitro . thus , the direct mitochondrial p53 program might be at least partly responsible for the adverse side effects that occur in sensitive normal tissues during cancer treatment and in acute radiation accidents . in vivo radioprotection through inhibition of the mitochondrial p53 program with pft is an encouraging possible approach to suppressing the toxic side effects of chemo / radiotherapy and increasing the efficacy of anticancer treatments . an interesting mouse model demonstrating the pathophysiological role of mitochondrial p53 function in liver was described by the george group . the liver is generally refractory to p53-mediated apoptosis after genotoxic stress , in part due to transcriptional induction of the prosurvival factor igfbp1 in the liver by stress - activated p53 . igfbp1 translocates to mitochondria , where it binds to proapoptotic bak and prevents bak activation , thus inhibiting apoptosis induction . in contrast , the liver of igfbp1 mice exhibits spontaneous apoptosis associated with mitochondrial p53 accumulation and bak oligomerization . on the other hand , blockade of p53 mitochondrial translocation by pft in wt mice significantly reduces cisplatin - induced liver cell apoptosis . along the same line , fatal liver necrosis that occurs after -amanitin poisoning from the highly toxic mushroom amanitis phalloides is caused by the direct mitochondrial p53 death program . -amanitin is an extremely potent transcriptional inhibitor ( thus excluding any participation of the nuclear transcriptional function of p53 ) that was shown to induce p53-dependent apoptosis ( and probably also necrosis , although this was not investigated ) via mitochondrial p53 translocation and p53-dependent momp in liver and other cell types . accordingly , p53 and bak mice are largely protected from -amanitin - induced liver damage , whereas wild - type mice undergo organ destruction . in summary , the pathophysiological significance of proapoptotic and pronecrotic functions of mitochondrial p53 has been demonstrated in a number of compelling in vivo studies . such studies have stimulated exploration into the translational potential of targeted interception of these pathways .
the p53 tumor suppressor network plays a fundamental surveillance role in both homeostatic and adaptive cell biology . 
 p53 is one of the most important barriers against malignant derailment of normal cells , orchestrating growth arrest , senescence , or cell death by linking many different pathways in response to genotoxic and non - genotoxic insults . 
 p53 is the key broadband sensor for numerous cellular stresses such as dna damage , hypoxia , oxidative stress , oncogenic signaling , and nucleolar stress . the crucial tumor suppressive and tissue homeostasis activity of p53 is its ability to activate cell death via multiple different pathways . 
 a well - characterized biochemical function of p53 in the regulation of apoptosis is its role as a potent transcriptional regulator . 
 p53 activates a panel of proapoptotic genes from the mitochondrial apoptotic and death receptor programs while repressing antiapoptotic bcl2 family genes . 
 in addition , over the last 10 y a growing body of evidence has also defined direct extranuclear non - transcriptional p53 activities within mitochondria - mediated cell death pathways that are based on p53 protein accumulation in cytosolic and mitochondrial compartments and protein - protein interactions . to date 
 , transcription - independent p53-mediated cell death regulation has been described for apoptosis , necrosis , and autophagy . 
 because mitochondrial dysregulation is central to the development of a number of pathologic processes such as cancer and neurodegenerative and age - related diseases , understanding the direct roles of p53 protein in mitochondria has high translational impact and could facilitate the development of novel drug targets to combat these diseases . in this review 
 we will mainly focus on mechanisms of p53-mediated transcription - independent cell death pathways at mitochondria .