Mitochondrially targeted wild-type p53 induces apoptosis in a solid human tumor xenograft model

Elucidating the chain of command: our current understanding of critical target genes for p53-mediated tumor suppression

TP53 encodes a transcription factor that is centrally-involved in several pathways, including the control of metabolism, the stress response, DNA repair, cell cycle arrest, senescence, programmed cell death, and others. Since the discovery of TP53 as the most frequently-mutated tumor suppressor gene in cancer over four decades ago, the field has focused on uncovering target genes of this transcription factor that are essential for tumor suppression. This search has been fraught with red herrings, however. Dozens of p53 target genes were discovered that had logical roles in tumor suppression, but subsequent data showed that most were not tumor suppressive, and were dispensable for p53-mediated tumor suppression. In this review, we focus on p53 transcriptional targets in two categories: (1) canonical targets like CDKN1A (p21) and BBC3 (PUMA), which clearly play critical roles in p53-mediated cell cycle arrest/senescence and cell death, but which are not mutated in cancer, and for which knockout mice fail to develop spontaneous tumors; and (2) a smaller category of recently-described p53 target genes that are mutated in human cancer, and which appear to be critical for tumor suppression by p53. Interestingly, many of these genes encode proteins that control broad cellular pathways, like splicing and protein degradation, and several of them encode proteins that feed back to regulate p53. These include ZMAT3, GLS2, PADI4, ZBXW7, RFX7, and BTG2. The findings from these studies provide a more complex, but exciting, potential framework for understanding the role of p53 in tumor suppression.

p53 regulates mitochondrial dynamics by inhibiting Drp1 translocation into mitochondria during cellular senescence

Cellular senescence acts as an important barrier to tumorigenesis by eliminating precancerous cells. Previous studies have shown an essential role of the tumor suppressor p53 in cellular senescence, but how p53 induces cellular senescence is not fully understood. We found that p53 promoted the formation of highly interconnected and elongated mitochondria prior to the onset of cellular senescence. The inhibition of mitochondrial elongation upon p53 expression suppressed cellular senescence, suggesting that mitochondrial elongation is required for the induction of p53-dependent senescence. p53-induced mitochondrial elongation resulted in mitochondrial dysfunction and subsequent increases in intracellular reactive oxygen species (ROS) levels, an important mediator of cellular senescence. Mechanistically, the inhibitory phosphorylation of Drp1 Ser637 increased upon p53 expression, suppressing the translocation of Drp1 into mitochondria. The transcriptional function of p53 was crucial for controlling the inhibitory phosphorylation of Drp1, whereas p21 was nonessential. Protein kinase A (PKA) activity was responsible for p53-mediated Drp1 Ser637 phosphorylation and mitochondrial dysfunction. Taken together, these results suggest that p53 regulates mitochondrial dynamics through the PKA-Drp1 pathway to induce cellular senescence.

SIRT3 deregulation is linked to mitochondrial dysfunction in Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of dementia in the elderly. Despite decades of study, effective treatments for AD are lacking. Mitochondrial dysfunction has been closely linked to the pathogenesis of AD, but the relationship between mitochondrial pathology and neuronal damage is poorly understood. Sirtuins (SIRT, silent mating type information regulation 2 homolog in yeast) are NAD-dependent histone deacetylases involved in aging and longevity. The objective of this study was to investigate the relationship between SIRT3 and mitochondrial function and neuronal activity in AD. SIRT3 mRNA and protein levels were significantly decreased in AD cerebral cortex, and Ac-p53 K320 was significantly increased in AD mitochondria. SIRT3 prevented p53-induced mitochondrial dysfunction and neuronal damage in a deacetylase activity-dependent manner. Notably, mitochondrially targeted p53 (mito-p53) directly reduced mitochondria DNA-encoded ND2 and ND4 gene expression resulting in increased reactive oxygen species (ROS) and reduced mitochondrial oxygen consumption. ND2 and ND4 gene expressions were significantly decreased in patients with AD. p53-ChIP analysis verified the presence of p53-binding elements in the human mitochondrial genome and increased p53 occupancy of mitochondrial DNA in AD. SIRT3 overexpression restored the expression of ND2 and ND4 and improved mitochondrial oxygen consumption by repressing mito-p53 activity. Our results indicate that SIRT3 dysfunction leads to p53-mediated mitochondrial and neuronal damage in AD. Therapeutic modulation of SIRT3 activity may ameliorate mitochondrial pathology and neurodegeneration in AD.

Delivery of drugs and macromolecules to the mitochondria for cancer therapy

Mitochondria are organelles that have pivotal functions in producing the energy necessary for life and executing the cell death pathway. Targeting drugs and macromolecules to the mitochondria may provide an effective means of inducing cell death for cancer therapy, and has been actively pursued in the last decade. This review will provide a brief overview of mitochondrial structure and function, how it relates to cancer, and importantly, will discuss different strategies of mitochondrial delivery including delivery using small molecules, peptides, genes encoding proteins and MTSs, and targeting polymers/nanoparticles with payloads to the mitochondria. The advantages and disadvantages for each strategy will be discussed. Specific examples using the latest strategies for mitochondrial targeting will be evaluated, as well as potential opportunities for specific mitochondrial compartment localization, which may lead to improvements in mitochondrial therapeutics. Future perspectives in mitochondrial targeting of drugs and macromolecules will be discussed. Currently this is an under-explored area that is prime for new discoveries in cancer therapeutics.

Interaction of p53 with prolyl isomerases: Healthy and unhealthy relationships

BACKGROUND

The p53 protein family, comprising p53, p63 and p73, is primarily involved in preserving genome integrity and preventing tumor onset, and also affects a range of physiological processes. Signal-dependent modifications of its members and of other pathway components provide cells with a sophisticated code to transduce a variety of stress signaling into appropriate responses. TP53 mutations are highly frequent in cancer and lead to the expression of mutant p53 proteins that are endowed with oncogenic activities and sensitive to stress signaling.

SCOPE OF REVIEW

p53 family proteins have unique structural and functional plasticity, and here we discuss the relevance of prolyl-isomerization to actively shape these features.

MAJOR CONCLUSIONS

The anti-proliferative functions of the p53 family are carefully activated upon severe stress and this involves the interaction with prolyl-isomerases. In particular, stress-induced stabilization of p53, activation of its transcriptional control over arrest- and cell death-related target genes and of its mitochondrial apoptotic function, as well as certain p63 and p73 functions, all require phosphorylation of specific S/T-P motifs and their subsequent isomerization by the prolyl-isomerase Pin1. While these functions of p53 counteract tumorigenesis, under some circumstances their activation by prolyl-isomerases may have negative repercussions (e.g. tissue damage induced by anticancer therapies and ischemia-reperfusion, neurodegeneration). Moreover, elevated Pin1 levels in tumor cells may transduce deregulated phosphorylation signaling into activation of mutant p53 oncogenic functions.

GENERAL SIGNIFICANCE

The complex repertoire of biological outcomes induced by p53 finds mechanistic explanations, at least in part, in the association between prolyl-isomerases and the p53 pathway. This article is part of a Special Issue entitled Proline-directed foldases: Cell signaling catalysts and drug targets.

Mitochondrial translocation of p53 modulates neuronal fate by preventing differentiation-induced mitochondrial stress

AIMS

Apoptosis regulatory proteins, such as p53, play a pivotal role in neural differentiation, through mechanisms independent of cell death. In addition, p53 has been identified as an important regulator of mitochondrial survival response, maintaining mitochondrial DNA (mtDNA) integrity and oxidative protection. The aim of this study was to determine the role of mitochondrial p53 in organelle damage and neural differentiation.

RESULTS

Our results show that mitochondrial apoptotic events such as reactive oxygen species production, mitochondrial membrane permeabilization, and cytochrome c release are typical of early-stage mouse neural stem cell differentiation, which occurs 3-18 h after induction of differentiation, with no evidence of cell death. In addition, decreased mtDNA content, lipidated LC3 (LC3-II), colocalization of mitochondria and LC3-II puncta, and mitochondria-associated Parkin are consistent with activation of mitophagy. Importantly, at early stages of neural differentiation, p53 was actively translocated to mitochondria and attenuated mitochondrial oxidative stress, cytochrome c release, and mitophagy. Forced mitochondrial translocation of p53 increased neurogenic potential and neurite outgrowth.

INNOVATION AND CONCLUSION

In conclusion, our results reveal a novel role for mitochondrial p53, which modulates mitochondrial damage and apoptosis-related events in the context of neural differentiation, thus enhancing neuronal fate.

The cytoplasmic side of p53's oncosuppressive activities

The tumor suppressor p53 is a transcription factor that in response to a plethora of stress stimuli activates a complex and context-dependent cellular response ultimately protecting genome integrity. In the last two decades, the discovery of cytoplasmic p53 localization has driven an intense research on its extra-nuclear functions. The ability to induce apoptosis acting directly at mitochondria and the related mechanisms of p53 localization and translocation in the cytoplasm and mitochondria have been dissected. However, recent works indicate the involvement of cytoplasmic p53 also in biological processes such as autophagy, metabolism, oxidative stress and drug response. This review will focus on the mechanisms of cytoplasmic p53 activation and the pathophysiological role of p53's transcription-independent functions, highlighting possible therapeutic implications.

Nutlin-3 induces HO-1 expression by activating JNK in a transcription-independent manner of p53

A recent study reported that p53 can induce HO-1 by directly binding to the putative p53 responsive element in the HO-1 promoter. In this study, we report that nutlin-3, a small molecule antagonist of HDM2, induces the transcription of HO-1 in a transcription-independent manner of p53. Nutlin-3 induced HO-1 expression at the level of transcription in human cancer cells such as U2OS and RKO cells. This induction of HO-1 did not occur in SAOS cells in which p53 was mutated and was prevented by knocking down the p53 protein using p53 siRNA transfection, but not by PFT-α, an inhibitor of the transcriptional activity of p53. Accompanying HO-1 expression, nutlin-3 stimulated the accumulation of ROS and the phosphorylation of MAPKs such as JNK, p38 MAPK and ERK1/2. Nutlin-3-induced HO-1 expression was suppressed by TEMPO, a ROS scavenger, and chemical inhibitors of JNK and p38 MAPK but not ERK1/2. In addition, nutlin‑3-induced phosphorylation of JNK but not p38 MAPK was inhibited by TEMPO. Notably, the levels of nutlin-3-induced ROS were correlated with the mitochondrial translocation of p53 and this induction was prevented by PFT-μ, an inhibitor of the mitochondrial translocation of p53. Consistent with the effect of the ROS scavenger and MAPK inhibitors, PFT-μ reduced HO-1 expression and the phosphorylation of JNK induced by nutlin-3. In the experiments of analyzing cell death, the knockdown of HO-1 augmented nutlin-3-induced apoptosis. Collectively, these results suggest that nutlin-3 induces HO-1 expression via the activation of both JNK which is dependent on ROS generated by p53 translocated to the mitochondria and p38 MAPK which appears to be stimulated by a ROS-independent mechanism, and this HO-1 induction may inhibit nutlin-3-induced apoptosis, constituting a negative feedback loop of p53-induced apoptosis.

Identification of p53 in mitochondria

p53 is a master regulator of cell death pathways and has transcription-dependent and transcription-independent modes of action. Mitochondria are major signal transducers in apoptosis and are critical for p53-dependent cell death. Our lab and others have discovered that a fraction of stress-induced wild-type p53 protein rapidly translocates to mitochondria upon various stress stimuli and exerts p53-dependent apoptosis. Suborganellar localization by various methods shows that p53 localizes to the surface of mitochondria. Direct targeting of p53 to mitochondria is sufficient to induce apoptosis in p53-null cells, without requiring further DNA damage. Recently, p53 has been also shown to localize to other mitochondrial compartments such as the mitochondrial matrix where it plays a role in maintaining mitochondrial genome integrity. Here, we describe subcellular fractionation as a classic technique for detecting mitochondrial p53 in cell extracts. It consists of cell homogenization by hypo-osmotic swelling, removal of nuclear components by low-speed centrifugation, and mitochondrial isolation by a discontinuous sucrose density gradient. Additionally, we describe a method for submitochondrial fractionation, performed by phosphate buffer mediated swelling/shrinking. p53 and other mitochondrial proteins can then be detected by standard immunoblotting procedures. The quality of mitochondrial isolates/subfractions can be verified for purity and intactness.

The prolyl-isomerase Pin1 activates the mitochondrial death program of p53

In response to intense stress, the tumor protein p53 (p53) tumor suppressor rapidly mounts a direct mitochondrial death program that precedes transcription-mediated apoptosis. By eliminating severely damaged cells, this pathway contributes to tumor suppression as well as to cancer cell killing induced by both genotoxic drugs and non-genotoxic p53-reactivating molecules. Here we have explored the role had in this pathway by the prolyl-isomerase Pin1 (peptidylprolyl cis/trans isomerase, NIMA-interacting 1), a crucial transducer of p53's phosphorylation into conformational changes unleashing its pro-apoptotic activity. We show that Pin1 promotes stress-induced localization of p53 to mitochondria both in vitro and in vivo. In particular, we demonstrate that upon stress-induced phosphorylation of p53 on Ser46 by homeodomain interacting protein kinase 2, Pin1 stimulates its mitochondrial trafficking signal, that is, monoubiquitination. This pathway is induced also by the p53-activating molecule RITA, and we demonstrate the strong requirement of Pin1 for the induction of mitochondrial apoptosis by this compound. These findings have significant implications for treatment of p53-expressing tumors and for prospective use of p53-activating compounds in clinics.

p53 opens the mitochondrial permeability transition pore to trigger necrosis

Ischemia-associated oxidative damage leading to necrosis is a major cause of catastrophic tissue loss, and elucidating its signaling mechanism is therefore of paramount importance. p53 is a central stress sensor responding to multiple insults, including oxidative stress to orchestrate apoptotic and autophagic cell death. Whether p53 can also activate oxidative stress-induced necrosis is, however, unknown. Here, we uncover a role for p53 in activating necrosis. In response to oxidative stress, p53 accumulates in the mitochondrial matrix and triggers mitochondrial permeability transition pore (PTP) opening and necrosis by physical interaction with the PTP regulator cyclophilin D (CypD). Intriguingly, a robust p53-CypD complex forms during brain ischemia/reperfusion injury. In contrast, reduction of p53 levels or cyclosporine A pretreatment of mice prevents this complex and is associated with effective stroke protection. Our study identifies the mitochondrial p53-CypD axis as an important contributor to oxidative stress-induced necrosis and implicates this axis in stroke pathology.

Functional study of dextran-graft-poly((2-dimethyl amino)ethyl methacrylate) gene delivery vector for tumor therapy

The obstacle of gene therapy is the shortage of efficient delivery system. The development of the gene delivery system with high transfection efficiency and low toxicity appears to be crucial. Recently, we reported that the dextran-graft-poly((2-dimethyl amino)ethyl methacrylate) (DPD) can be potentially used as efficient gene vector. Herein, DPD was systematically studied for its potential in tumor gene therapy. DPD was synthesized and characterized by agarose gel electrophoresis, particle size and zeta potential. The particle size and zeta potential of the DPD/enhanced green fluorescent protein (pEGFP-C1) plasmid complexes at various N/P ratios were 130-150 nm and about 40 mV, respectively. The results showed that DPD exhibit a higher transfection effect compared with Lipofectamine 2K (Lipo 2K), a commercialized vector. The possibility of DPD in gene therapy was evaluated using p53, a gene that has been wildly applied in the research of cancer gene therapy. DPD/pEGFP-C1-p53 complex was found to be able to inhibit tumor cell proliferation through cell cycle arrest and apoptosis. Moreover, the tumor growth was found to be restrained when DPD/pEGFP-C1-p53 complex was used in a xenograft MCF7 tumor model in vivo. These observations indicated that DPD/pEGFP-C1-p53 complex may be considered to be an efficient delivery system for tumor gene therapy.

Selective targeting of c-Abl via a cryptic mitochondrial targeting signal activated by cellular redox status in leukemic and breast cancer cells

PURPOSE

The tyrosine kinase c-Abl localizes to the mitochondria under cell stress conditions and promotes apoptosis. However, c-Abl has not been directly targeted to the mitochondria. Fusing c-Abl to a mitochondrial translocation signal (MTS) that is activated by reactive oxygen species (ROS) will selectively target the mitochondria of cancer cells exhibiting an elevated ROS phenotype. Mitochondrially targeted c-Abl will thereby induce malignant cell death.

METHODS

Confocal microscopy was used to determine mitochondrial colocalization of ectopically expressed c-Abl-EGFP/cMTS fusion across three cell lines (K562, Cos-7, and 1471.1) with varying levels of basal (and pharmacologically modulated) ROS. ROS were quantified by indicator dye assay. The functional consequences of mitochondrial c-Abl were assessed by DNA accessibility to 7-AAD using flow cytometry.

RESULTS

The cMTS and cMTS/c-Abl fusions colocalized to the mitochondria in leukemic (K562) and breast (1471.1) cancer phenotypes (but not Cos-7 fibroblasts) in a ROS and PKC dependent manner.

CONCLUSIONS

We confirm and extend oxidative stress activated translocation of the cMTS by demonstrating that the cMTS and Abl/cMTS fusion selectively target the mitochondria of K562 leukemia and mammary adenocarcinoma 1471.1 cells. c-Abl induced K562 leukemia cell death when targeted to the matrix but not the outer membrane of the mitochondria.

The cancer-associated K351N mutation affects the ubiquitination and the translocation to mitochondria of p53 protein

Stress-induced monoubiquitination of p53 is a crucial event for the nuclear-cytoplasm-mitochondria trafficking and transcription-independent pro-apoptotic functions of p53. Although an intact ubiquitination pathway and a functional nuclear export sequence are required for p53 nuclear export, the role of specific residues within this region in regulating both processes remains largely unknown. Here we characterize the mechanisms accounting for the nuclear accumulation of a new point mutation (Lys-351 to Asn) in the nuclear export sequence of p53 identified in a cisplatin-resistant ovarian carcinoma cell line (A2780 CIS). We found that K351N substitution abrogates the monoubiquitination of p53 induced by both Mdm2 and MSL2 E3-ligases. As a consequence, cells expressing p53 K351N mutant showed defects in cisplatin-induced translocation of p53 to mitochondria, Bax oligomerization, and mitochondrial membrane depolarization. These data identify K351N as a critical mutation of p53 that contributes to the development and maintenance of resistance to cisplatin.

Silencing of RhoA and RhoC expression by RNA interference suppresses human colorectal carcinoma growth in vivo

Background

RhoA and RhoC have been proved to be over-expressed in many solid cancers, including colorectal cancer. The reduction of RhoA and RhoC expression by RNA interference (RNAi) resulted growth inhibition of cancer cells. The present study was to evaluate the effect of silencing of RhoA and RhoC expression by RNAi on growth of human colorectal carcinoma (CRC) in tumor-bearing nude mice in vivo.

Methods

To establish HCT116 cell transplantable model, the nude mice were subcutaneously inoculated with 1.0 × 10⁷ HCT116 cells and kept growing till the tumor xenografts reached 5-7 mm in diameter. Then the mice were randomly assigned to three groups(seven mice in each group): (1) normal saline(NS) group, (2)replication-defective recombinant adenovirus carrying the negative control shRNA (Ad-HK) group and (3)replication-defective recombinant adenovirus carrying the 4-tandem linked RhoA and RhoC shRNAs (Ad-RhoA-RhoC) group. Ad-HK (4 × 10⁸ pfu, 30 ul/mouse), Ad-RhoA-RhoC (4 × 10⁸ pfu, 30 ul/mouse) or PBS (30 ul/mouse) was injected intratumorally four times once every other day. The weight and volumes of tumor xenografts were recorded. The levels of RhoA and RhoC mRNA transcripts and proteins in tumor xenografts were detected by reverse quantitative transcription polymerase chain reaction (QRT-PCR) and immunohistochemical staining respectively. The terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay was used to detect the death of cells.

Results

The xenografts in mice could be seen at 5th day from the implantation of HCT116 cells and all had reached 5-7 mm in size at 9th day. After injection intratumorally, the growth speed of tumor xenografts in Ad-RhoA-RhoC group was significantly delayed compared with those in NS and Ad-HK group(P < 0.05). The results of QRT-PCR showed that mRNA levels of RhoA and RhoC reduced more in Ad-RhoA-RhoC group than those in NS and Ad-HK group. The relative RhoA and RhoC mRNA transcripts were decreased to 48% and 43% respectively (P < 0.05). Immunohistochemical analyses of tumor xenograft sections also revealed the decreased RhoA and RhoC expression in Ad-RhoA-RhoC group. TUNEL assay also showed higher death of tumor xenograft tissue cells in Ad-RhoA-RhoC group.

Conclusion

Recombinant adenovirus mediated RhoA and RhoC shRNA in tandem linked expression may inhibit the growth of human colorectal tumor xenografts in vivo. These results indicate that RhoA and RhoC might be potential targets for gene therapy in colorectal cancer.

Transcription-independent p53 apoptosis: an alternative route to death

Apoptosis induced by p53 is firmly established as a central mechanism of tumour suppression. In addition to its complex functions as a nuclear transcription factor, p53 can act in the cytosol and mitochondria to promote apoptosis through transcription-independent mechanisms. Recent studies have shown that physical and functional interactions of p53 with various members of the Bcl-2 family provide the basis for this alternative route of p53-mediated cell death. However, different models of how these interactions promote apoptosis have been proposed. This review focuses on the mechanisms, regulation and physiological roles of transcription-independent p53 activities and highlights recent findings suggesting that the utilisation of these activities provides a promising alternative strategy for p53-based cancer therapy.

The mitochondrial p53 pathway

p53 is one of the most mutated tumor suppressors in human cancers and as such has been intensively studied for a long time. p53 is a major orchestrator of the cellular response to a broad array of stress types by regulating apoptosis, cell cycle arrest, senescence, DNA repair and genetic stability. For a long time it was thought that these functions of p53 solely rely on its function as a transcription factor, and numerous p53 target genes have been identified [1]. In the last 8 years however, a novel transcription-independent proapoptotic function mediated by the cytoplasmic pool of p53 has been revealed. p53 participates directly in the intrinsic apoptosis pathway by interacting with the multidomain members of the Bcl-2 family to induce mitochondrial outer membrane permeabilization. Our review will discuss these studies, focusing on recent advances in the field.