Direct induction of apoptosis using an optimal mitochondrially targeted p53

Elephant TP53-RETROGENE 9 induces transcription-independent apoptosis at the mitochondria

Approximately 20 TP53 retrogenes exist in the African and Asian elephant genomes (Loxodonta Africana, Elephas Maximus) in addition to a conserved TP53 gene that encodes a full-length protein. Elephant TP53-RETROGENE 9 (TP53-R9) encodes a p53 protein (p53-R9) that is truncated in the middle of the canonical DNA binding domain. This C-terminally truncated p53 retrogene protein lacks the nuclear localization signals and oligomerization domain of its full-length counterpart. When expressed in human osteosarcoma cells (U2OS), p53-R9 binds to Tid1, the chaperone protein responsible for mitochondrial translocation of human p53 in response to cellular stress. Tid1 expression is required for p53-R9-induced apoptosis. At the mitochondria, p53-R9 binds to the pro-apoptotic BCL-2 family member Bax, which leads to caspase activation, cytochrome c release, and cell death. Our data show, for the first time, that expression of this truncated elephant p53 retrogene protein induces apoptosis in human cancer cells. Understanding the molecular mechanism by which the additional elephant TP53 retrogenes function may provide evolutionary insight that can be utilized for the development of therapeutics to treat human cancers.

The Challenges and Prospects of p53-Based Therapies in Ovarian Cancer

It has been well established that mutations in the tumor suppressor gene, p53, occur readily in a vast majority of cancer tumors, including ovarian cancer. Typically diagnosed in stages three or four, ovarian cancer is the fifth leading cause of death in women, despite accounting for only 2.5% of all female malignancies. The overall 5-year survival rate for ovarian cancer is around 47%; however, this drops to an abysmal 29% for the most common type of ovarian cancer, high-grade serous ovarian carcinoma (HGSOC). HGSOC has upwards of 96% of cases expressing mutations in p53. Therefore, wild-type (WT) p53 and p53-based therapies have been explored as treatment options via a plethora of drug delivery vehicles including nanoparticles, viruses, polymers, and liposomes. However, previous p53 therapeutics have faced many challenges, which have resulted in their limited translational success to date. This review highlights a selection of these historical p53-targeted therapeutics for ovarian cancer, why they failed, and what the future could hold for a new generation of this class of therapies.

Recent Advances in Chemical Biology of Mitochondria Targeting

Mitochondria are vital subcellular organelles that generate most cellular chemical energy, regulate cell metabolism and maintain cell function. Mitochondrial dysfunction is directly linked to numerous diseases including neurodegenerative disorders, diabetes, thyroid squamous disease, cancer and septicemia. Thus, the design of specific mitochondria-targeting molecules and the realization of real-time acquisition of mitochondrial activity are powerful tools in the study and treatment of mitochondria dysfunction in related diseases. Recent advances in mitochondria-targeting agents have led to several important mitochondria chemical probes that offer the opportunity for selective targeting molecules, novel biological applications and therapeutic strategies. This review details the structural and physiological functional characteristics of mitochondria, and comprehensively summarizes and classifies mitochondria-targeting agents. In addition, their pros and cons and their related chemical biological applications are discussed. Finally, the potential biomedical applications of these agents are briefly prospected.

The p53 inactivators pifithrin-μ and pifithrin-α mitigate TBI-induced neuronal damage through regulation of oxidative stress, neuroinflammation, autophagy and mitophagy

Traumatic brain injury (TBI) is one of the most common causes of death and disability worldwide. We investigated whether inhibition of p53 using pifithrin (PFT)-α or PFT-μ provides neuroprotective effects via p53 transcriptional dependent or -independent mechanisms, respectively. Sprague Dawley rats were subjected to controlled cortical impact TBI followed by the administration of PFTα or PFT-μ (2 mg/kg, i.v.) at 5 h after TBI. Brain contusion volume, as well as sensory and motor functions were evaluated at 24 h after TBI. TBI-induced impairments were mitigated by both PFT-α and PFT-μ. Fluoro-Jade C staining was used to label degenerating neurons within the TBI-induced cortical contusion region that, together with Annexin V positive neurons, were reduced by PFT-μ. Double immunofluorescence staining similarly demonstrated that PFT-μ significantly increased HO-1 positive neurons and mRNA expression in the cortical contusion region as well as decreased numbers of 4-hydroxynonenal (4HNE)-positive cells. Levels of mRNA encoding for p53, autophagy, mitophagy, anti-oxidant, anti-inflammatory related genes and proteins were measured by RT-qPCR and immunohistochemical staining, respectively. PFT-α, but not PFT-μ, significantly lowered p53 mRNA expression. Both PFT-α and PFT-μ lowered TBI-induced pro-inflammatory cytokines (IL-1β and IL-6) mRNA levels as well as TBI-induced autophagic marker localization (LC3 and p62). Finally, treatment with PFT-μ mitigated TBI-induced declines in mRNA levels of PINK-1 and SOD2. Our data suggest that both PFT-μ and PFT-α provide neuroprotective actions through regulation of oxidative stress, neuroinflammation, autophagy, and mitophagy mechanisms, and that PFT-μ, in particular, holds promise as a TBI treatment strategy.

p53-Bad: A Novel Tumor Suppressor/Proapoptotic Factor Hybrid Directed to the Mitochondria for Ovarian Cancer Gene Therapy

Clinical trials involving p53 gene therapy for ovarian cancer failed due to the dominant negative inhibition of wild-type p53 and multiple genetic aberrations in ovarian cancer. To overcome this problem, we have designed a more potent chimeric gene fusion, called p53-Bad, that combines p53 with the mitochondrial pro-apoptotic factor Bad. Unlike wild-type p53, which acts as a nuclear transcription factor, this novel p53-Bad construct has multiple unique mechanisms of action including a direct and rapid apoptotic effect at the mitochondria. The mitochondrial localization, transcription activity, and apoptotic activity of the constructs were tested. The results suggest that p53 can be effectively targeted to the mitochondria by controlling the phosphorylation of pro-apoptotic Bad, which can only localize to the mitochondria when Ser-112 and Ser-136 of Bad are unphosphorylated. By introducing S112A and S136A mutations, p53-Bad fusion cannot be phosphorylated at these two sites and always localizes to the mitochondria. p53-Bad constructs also have superior activity over p53 and Bad alone. The apoptotic activity is consistent in many ovarian cancer cell lines regardless of the endogenous p53 status. Both p53 and the BH3 domain of Bad contribute to the superior activity of p53-Bad. Our data suggests that p53-Bad fusions are capable of inducing apoptosis and should be further pursued for gene therapy for ovarian cancer.

Mitochondrially targeted p53 or DBD subdomain is superior to wild type p53 in ovarian cancer cells even with strong dominant negative mutant p53

Background

While tumor suppressor p53 functions primarily as a transcription factor in the nucleus, cellular stress can cause p53 to translocate to the mitochondria and directly trigger a rapid apoptotic response. We have previously shown that fusing p53 (or its DNA binding domain, DBD, alone) to the mitochondrial targeting signal (MTS) from Bak or Bax can target p53 to the mitochondria and induce apoptosis in gynecological cancer cell lines including cervical cancer cells (HeLa; wt p53), ovarian cancer cells (SKOV-3; p53 267del non-expressing), and breast cancer cells (T47D; L194F p53 mutation). However, p53 with Bak or Bax MTSs have not been previously tested in cancers with strong dominant negative (DN) mutant p53 which are capable of inactivating wt p53 by homo-oligomerization. Since p53-Bak or Bax MTS constructs act as monomers, they are not subject to DN inhibition. For this study, the utility of p53-Bak or p53-Bax MTS constructs was tested for ovarian cancers which are known to have varying p53 statuses, including a strong DN contact mutant p53 (Ovcar-3 cells), a p53 DN structural mutant (Kuramochi cells), and a p53 wild type, low expressing cells (ID8).

Results

Our mitochondrial p53 constructs were tested for their ability to localize to the mitochondria in both mutant non-expressing p53 (Skov-3) and p53 structural mutant (Kuramochi) cell lines using fluorescence microscopy and a nuclear transcriptional activity assay. The apoptotic activity of these mitochondrial constructs was determined using a mitochondrial outer membrane depolarization assay (TMRE), caspase assay, and a late stage cell death assay (7-AAD). We also tested the possibility of using our constructs with paclitaxel, the current standard of care in ovarian cancer treatment. Our data indicates that our mitochondrial p53 constructs are able to effectively localize to the mitochondria in cancer cells with structural mutant p53 and induce apoptosis in many ovarian cancer cell lines with different p53 statuses. These constructs can also be used in combination with paclitaxel for an increased apoptotic effect.

Conclusions

The results suggest that targeting p53 to mitochondria can be a new strategy for ovarian cancer treatment.

Electronic supplementary material

The online version of this article (10.1186/s13048-019-0516-2) contains supplementary material, which is available to authorized users.

JTC-801 exerts anti-proliferative effects in human osteosarcoma cells by inducing apoptosis

BACKGROUND

The research of G protein-coupled receptors (GPCRs) is a promising strategy for drug discovery. In cancer therapy, there is a need to discover novel agents that can inhibit proliferation and induce apoptosis in cancer cells. JTC-801 is a novel GPCR antagonist with the function of reversing pain and anxiety symptoms. This study aims to investigate the antitumor effects of JTC-801 on human osteosarcoma cells (U2OS) and elucidate the underlying mechanism.

MATERIALS AND METHODS

The Cell Counting Kit-8 assay was used to detect the viability of U2OS cells treated with JTC-801 in vitro. The cell apoptosis was evaluated using a flow cytometry assay with Annexin V-FITC/PI double staining. The inhibitory effect of JTC-801 on invasion and migration of U2OS cells were determined by the Transwell assays. Western blot assay was performed to measure the levels of proteins related to cell apoptosis and its mechanism.

RESULTS

The JTC-801 significantly decreased the viability of U2OS cells (p < .05) as a result of its anti-proliferative effect through induction of apoptosis associated with activation of BAX, Caspase-3 and down-regulating BCL-2 expression. The invasive and migratory cells were obviously reduced after JTC-801 treatment (p < .05). Further, the phosphorylated AKT, mTOR and active p70 S6 protein kinase in the PI3K/AKT signaling pathway were obviously lessened in the JTC-801 treated U2OS group (p < .05).

CONCLUSIONS

JTC-801 may exert osteosarcoma cell growth inhibition by promoting cell apoptosis, through PI3K/AKT signaling pathway participation.

Inhibition of Mitochondrial p53 Accumulation by PFT-μ Prevents Cisplatin-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN), a debilitating major side effect of cancer treatment, is characterized by pain and sensory loss in hand and feet. Platinum-based chemotherapeutics like cisplatin frequently induce CIPN. The molecular mechanism underlying these neurotoxic symptoms is incompletely understood and there are no preventive or curative interventions. We hypothesized that cisplatin acts as a cellular stressor that triggers p53 accumulation at mitochondria, leading to mitochondrial dysfunction and CIPN. To test this hypothesis, we examined the effect of the small molecule pifithrin-μ (PFT-μ), an inhibitor of p53 mitochondrial association on CIPN and the associated mitochondrial dysfunction. We show here for the first time that in vivo cisplatin rapidly increases mitochondrial accumulation of p53 in dorsal root ganglia (DRG), spinal cord, and peripheral nerve without evidence for apoptosis. Cisplatin-treatment also reduced mitochondrial membrane potential and lead to abnormal mitochondrial morphology and impaired mitochondrial function in DRG neurons. Pre-treatment with PFT-μ prevented the early cisplatin-induced increase in mitochondrial p53 and the reduction in mitochondrial membrane potential. Inhibition of the early mitochondrial p53 accumulation by PFT-μ also prevented the abnormalities in mitochondrial morphology and mitochondrial bioenergetics (reduced oxygen consumption rate, maximum respiratory capacity, and adenosine triphosphate synthesis) that develop in DRG and peripheral nerve after cisplatin-treatment. Functionally, inhibition of mitochondrial p53 accumulation prevented the hallmarks of CIPN including mechanical allodynia, peripheral sensory loss (numbness) as quantified by an adhesive-removal task, and loss of intra-epidermal nerve fibers. In conclusion, PFT-μ is a potential neuroprotective agent that prevents cisplatin-induced mitochondrial dysfunction in DRG and peripheral nerves thereby protecting against CIPN through blockade of the early cisplatin-induced increase in mitochondrial p53. Notably, there is accumulating evidence that PFT-μ has anti-tumor activities and could therefore be an attractive candidate to prevent CIPN while promoting tumor cell death.

GSIV serine/threonine kinase can induce apoptotic cell death via p53 and pro-apoptotic gene Bax upregulation in fish cells

Previous studies have shown that GSIV induces apoptotic cell death through upregulation of the pro-apoptotic genes Bax and Bak in Grouper fin cells (GF-1 cells). However, the role of viral genome-encoded protein(s) in this death process remains unknown. In this study, we demonstrated that the Giant seaperch iridovirus (GSIV) genome encoded a serine/threonine kinase (ST kinase) protein, and induced apoptotic cell death via a p53-mediated Bax upregulation approach and a downregulation of Bcl-2 in fish cells. The ST kinase expression profile was identified through Western blot analyses, which indicated that expression started at day 1 h post-infection (PI), increased up to day 3, and then decreased by day 5 PI. This profile indicated the role of ST kinase expression during the early and middle phases of viral replication. We then cloned the ST kinase gene and tested its function in fish cells. The ST kinase was transiently expressed and used to investigate possible novel protein functions. The transient expression of ST kinase in GF-1 cells resulted in apoptotic cell features, as revealed with Terminal deoxynucleotidyl transferase biotin-dUTP nick-end labeling (TUNEL) assays and Hoechst 33258 staining at 24 h (37 %) and 48 h post-transfection (PT) (49 %). Then, through studies on the mechanism of cell death, we found that ST kinase overexpression could upregulate the anti-stress gene p53 and the pro-apoptotic gene Bax at 48 h PT. Interestingly, this upregulation of p53 and Bax also correlated to alterations in the mitochondria function that induced loss of mitochondrial membrane potential (MMP) and activated the initiator caspase-9 and the effector caspase-3 in the downstream. Moreover, when the p53-dependent transcriptional downstream gene was blocked by a specific transcriptional inhibitor, it was found that pifithrin-α not only reduced Bax expression, but also averted cell death in GF-1 cells during the ST kinase overexpression. Taken altogether, these results suggested that aquatic GSIV ST kinase could induce apoptosis via upregulation of p53 and Bax expression, resulting in mitochondrial disruption, which activated a downstream caspases-mediated cell death pathway.

Imiquimod Induces Apoptosis in Human Endometrial Cancer Cells In vitro and Prevents Tumor Progression In vivo

PURPOSE

The increasing incidence of endometrial cancer (EC), in younger age at diagnosis, calls for new tissue-sparing treatment options. This work aims to evaluate the potential of imiquimod (IQ) in the treatment of low-grade EC.

METHODS

Effects of IQ on the viabilities of Ishikawa and HEC-1A cells were evaluated using MTT assay. The ability of IQ to induce apoptosis was evaluated by testing changes in caspase 3/7 levels and expression of cleaved caspase-3, using luminescence assay and western blot. Apoptosis was confirmed by flow cytometry and the expression of cleaved PARP. Western blot was used to evaluate the effect of IQ on expression levels of Bcl-2, Bcl-xL, and BAX. Finally, the in vivo efficacy of IQ was tested in an EC mouse model.

RESULTS

There was a decrease in EC cell viability following IQ treatment as well as increased caspase 3/7 activities, cleaved caspase-3 expression, and Annexin-V/ 7AAD positive cell population. Western blot results showed the ability of IQ in cleaving PARP, decreasing Bcl-2 and Bcl-xL expressions, but not affecting BAX expression. In vivo study demonstrated IQ's ability to inhibit EC tumor growth and progression without significant toxicity.

CONCLUSIONS

IQ induces apoptosis in low-grade EC cells in vitro, probably through its direct effect on Bcl-2 family protein expression. In, vivo, IQ attenuates EC tumor growth and progression, without an obvious toxicity. Our study provides the first building block for the potential role of IQ in the non-surgical management of low-grades EC and encouraging further investigations.

p53 overexpression increases chemosensitivity in multidrug-resistant osteosarcoma cell lines

PURPOSE

Multidrug resistance (MDR) is a major obstacle to the successful treatment of osteosarcoma with chemotherapy. Effectiveness of cancer therapy correlates with the ability to induce a p53-dependent apoptotic response. p53 is a tumor suppressor gene that is mutated in 22 % of osteosarcomas. While impaired p53 has been implicated in the oncogenesis of osteosarcoma, it is unclear whether overexpression of wild-type p53 can increase chemosensitivity in MDR osteosarcoma cells.

METHODS

We transfected a plasmid encoding the wild-type p53 gene to MDR osteosarcoma cell lines, which have different p53 statuses, U-2OSR2 with wild-type p53 (Wt-p53) and KHOSR2 with mutant p53 (Mt-p53), and determined the effect of p53 overexpression on chemosensitivities.

RESULTS

Both of the U-2OSR2 and KHOSR2 cell lines displayed similar trends in p53-induced drug sensitivities. However, it seems that the impact of p53 overexpression is different based on the differential intrinsic p53 status in these cell lines. In the KHOSR2 cell line (Mt-p53), overexpression of p53 up-regulates the expression of pro-apoptotic protein p21 and Bax, while in the U-2OSR2 cell line (Wt-p53), overexpression of p53 down-regulates IGF-1r expression significantly.

CONCLUSIONS

These results demonstrated that transfection of wild-type p53 increases chemosensitivity either through inhibiting IGF-1r or through increasing the expression of pro-apoptotic proteins p21 and Bax in human MDR osteosarcoma cell lines.

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.

Bifunctional fused polypeptide inhibits the growth and metastasis of breast cancer

Breast cancer is the most common cancer and the leading cause of cancer-related death among women worldwide, with urgent need to develop new therapeutics. Targeted therapy is a promising strategy for breast cancer therapy. Stromal-derived factor-1/CXC chemokine receptor 4 (CXCR4) has been implicated in the metastasis of breast cancer, which renders it to be therapeutic target. This study aimed to evaluate the anticancer effect of fused TAT- DV1-BH3 polypeptide, an antagonist of CXCR4, and investigate the underlying mechanism for the cancer cell-killing effect in the treatment of breast cancer in vitro and in vivo. This results in a potent inhibitory effect of fused TAT-DV1-BH3 polypeptide on tumor growth and metastasis in nude mice bearing established MDA-MB-231 tumors. Fused TAT-DV1-BH3 polypeptide inhibited the proliferation of MDA-MB-231 and MCF-7 cells but did not affect that of HEK-293 cells. The fused TAT-DV1-BH3 polypeptide colocalized with mitochondria and exhibited a proapoptotic effect through the regulation of caspase-9 and -3. Furthermore, the fused TAT-DV1-BH3 polypeptide suppressed the migration and invasion of the highly metastatic breast cancer cell line MDA-MB-231 in a concentration-dependent manner. Notably, the DV1-mediated inhibition of the stromal-derived factor-1/CXCR4 pathway contributed to the antimetastasis effect, evident from the reduction in the level of phosphoinositide 3 kinase and matrix metalloproteinase 9 in MDA-MB-231 cells. Collectively, these results indicate that the apoptosis-inducing effect and migration- and invasion-suppressing effect explain the tumor regression and metastasis inhibition in vivo, with the involvement of caspase- and CXCR4-mediated signaling pathway. The data suggest that the fused TAT-DV1-BH3 polypeptide is a promising agent for the treatment of breast cancer, and more studies are warranted to fully elucidate the therapeutic targets and molecular mechanism.

Zinc deficiency induces apoptosis via mitochondrial p53- and caspase-dependent pathways in human neuronal precursor cells

Previous studies have shown that zinc deficiency leads to apoptosis of neuronal precursor cells in vivo and in vitro. In addition to the role of p53 as a nuclear transcription factor in zinc deficient cultured human neuronal precursors (NT-2), we have now identified the translocation of phosphorylated p53 to the mitochondria and p53-dependent increases in the pro-apoptotic mitochondrial protein BAX leading to a loss of mitochondrial membrane potential as demonstrated by a 25% decrease in JC-1 red:green fluorescence ratio. Disruption of mitochondrial membrane integrity was accompanied by efflux of the apoptosis inducing factor (AIF) from the mitochondria and translocation to the nucleus with a significant increase in reactive oxygen species (ROS) after 24h of zinc deficiency. Measurement of caspase cleavage, mRNA, and treatment with caspase inhibitors revealed the involvement of caspases 2, 3, 6, and 7 in zinc deficiency-mediated apoptosis. Down-stream targets of caspase activation, including the nuclear structure protein lamin and polyADP ribose polymerase (PARP), which participates in DNA repair, were also cleaved. Transfection with a dominant-negative p53 construct and use of the p53 inhibitor, pifithrin-μ, established that these alterations were largely dependent on p53. Together these data identify a cascade of events involving mitochondrial p53 as well as p53-dependent caspase-mediated mechanisms leading to apoptosis during zinc deficiency.

Inhibitory effects of long noncoding RNA MEG3 on hepatic stellate cells activation and liver fibrogenesis

Long noncoding RNAs (lncRNAs) are being increasingly recognized as major players in governing fundamental biological processes through diverse mechanisms. Maternally expressed gene 3 (MEG3) is an imprinted gene located at 14q32 that encodes a lncRNA correlated with several human cancers. Recently, the methylation-dependent downregulation of MEG3 has been described in liver cancers. However, its biological functional role in liver fibrosis remains unknown. In our study, MEG3 levels were remarkably decreased in CCl4-induced mouse liver fibrosis models and human fibrotic livers as demonstrated by real-time quantitative PCR. Moreover, the expression of MEG3 was downregulated in human hepatic stellate cell lines LX-2 cells in response to transforming growth factor-β1 (TGF-β1) stimulation in dose and time-dependent manner. Enforced expression of MEG3 in LX-2 cells inhibited TGF-β1-induced cell proliferation, while promoting cell apoptosis. In addition, hypermethylation of MEG3 promoter was identified by methylation-specific PCR and MEG3 expression was robustly increased by the inhibition of methylation with either 5-aza-2-deoxycytidine (5-azadC), or siRNA to DNA methyltransferase 1 (DNMT1) in TGF-β1-induced LX-2 cells. More importantly, overexpression of MEG3 could activate p53 and mediate cytochrome c release, subsequently leading to caspase-3-dependent apoptosis in TGF-β1-treated LX-2 cells. These findings suggested that MEG3 may play an important role in stellate cell activation and liver fibrosis progression and act as a novel potential therapeutic target for liver fibrosis.

Re-engineered p53 activates apoptosis in vivo and causes primary tumor regression in a dominant negative breast cancer xenograft model

Inactivation of p53 pathway is reported in more than half of all human tumors and can be correlated to malignant development. Missense mutation in the DNA binding region (DBD) of p53 is the most common mechanism of p53 inactivation in cancer cells. The resulting tumor-derived p53 variants, similar to wild-type (wt) p53, retain their ability to oligomerize via the tetramerization domain (TD). Upon hetero-oligomerization, mutant p53 enforces a dominant negative effect over active wt-p53 in cancer cells. To overcome this barrier, we have previously designed a chimeric superactive p53 (p53-CC) with an alternative oligomerization domain capable of escaping transdominant inhibition by mutant p53 in vitro. In this report, we demonstrate the superior tumor suppressor activity of p53-CC and its ability to cause tumor regression of the MDA-MB-468 aggressive p53-dominant negative breast cancer tumor model in vivo. In addition, we illustrate the profound effects of the dominant negative effect of endogenous mutant p53 over wt-p53 in cancer cells. Finally, we investigate the underlying differential mechanisms of activity for p53-CC and wt-p53 delivered using viral-mediated gene therapy approach in the MDA-MB-468 tumor model.

Re-engineered p53 chimera with enhanced homo-oligomerization that maintains tumor suppressor activity

The use of the tumor suppressor p53 for gene therapy of cancer is limited by the dominant negative inactivating effect of mutant endogenous p53 in cancer cells. We have shown previously that swapping the tetramerization domain (TD) of p53 with the coiled-coil (CC) from Bcr allows for our chimeric p53 (p53-CC) to evade hetero-oligomerization with endogenous mutant p53. This enhances the utility of this construct, p53-CC, for cancer gene therapy. Because domain swapping to create p53-CC could result in p53-CC interacting with endogenous Bcr, which is ubiquitous in cells, modifications on the CC domain are necessary to minimize potential interactions with Bcr. Hence, we investigated the possible design of mutations that will improve homodimerization of CC mutants and disfavor hetero-oligomerization with wild-type CC (CCwt), with the goal of minimizing potential interactions with endogenous Bcr in cells. This involved integrated computational and experimental approaches to rationally design an enhanced version of our chimeric p53-CC tumor suppressor. Indeed, the resulting lead candidate p53-CCmutE34K-R55E avoids binding to endogenous Bcr and retains p53 tumor suppressor activity. Specifically, p53-CCmutE34K-R55E exhibits potent apoptotic activity in a variety of cancer cell lines, regardless of p53 status (in cells with mutant p53, wild-type p53, or p53-null cells). This construct overcomes the dominant negative effect limitation of wt p53 and has high significance for future gene therapy for treatment of cancers characterized by p53 dysfunction, which represent over half of all human cancers.

Heterologous Src homology 4 domains support membrane anchoring and biological activity of HIV-1 Nef

The HIV-1 pathogenicity factor Nef enhances viral replication by modulation of multiple host cell transport and signaling pathways. Nef associates with membranes via an N-terminal Src homology 4 (SH4) domain, and membrane association is believed to be essential for its biological functions. At which subcellular site(s) Nef exerts its different functions and how kinetics of membrane interactions contribute to its biological activity are unknown. To address how specific characteristics of Nef membrane association affect its biological properties, the SH4 domain of Nef was replaced by heterologous membrane targeting domains. The use of a panel of heterologous SH4 domains resulted in chimeric Nef proteins with distinct steady state subcellular localization, membrane association efficiency, and anterograde transport routes. Irrespective of these modifications, cardinal Nef functions affecting host cell vesicular transport and actin dynamics were fully preserved. In contrast, stable targeting of Nef to the surface of mitochondria, peroxisomes, or the Golgi apparatus, and thus prevention of plasma membrane delivery, caused potent and broad loss of Nef activity. These results support the concept that Nef adopts its active conformation in the membrane-associated state but exclude that membrane-associated Nef simply acts by recruiting soluble factors independently of its local microenvironment. Rather than its steady state subcellular localization or membrane affinity, the ability to undergo dynamic anterograde and internalization cycles appear to determine Nef function. These results reveal that functional membrane interactions of Nef underlie critical spatiotemporal regulation and suggest that delivery to distinct subcellular sites via such transport cycles provides the basis for the multifunctionality of Nef.

Delivery of a monomeric p53 subdomain with mitochondrial targeting signals from pro-apoptotic Bak or Bax

PURPOSE

p53 targeted to the mitochondria is the fastest and most direct pathway for executing p53 death signaling. The purpose of this work was to determine if mitochondrial targeting signals (MTSs) from pro-apoptotic Bak and Bax are capable of targeting p53 to the mitochondria and inducing rapid apoptosis.

METHODS

p53 and its DNA-binding domain (DBD) were fused to MTSs from Bak (p53-BakMTS, DBD-BakMTS) or Bax (p53-BaxMTS, DBD-BaxMTS). Mitochondrial localization was tested via fluorescence microscopy in 1471.1 cells, and apoptosis was detected via 7-AAD in breast (T47D), non-small cell lung (H1373), ovarian (SKOV-3) and cervical (HeLa) cancer cells. To determine that apoptosis is via the intrinsic apoptotic pathway, TMRE and caspase-9 assays were conducted. Finally, the involvement of p53/Bak specific pathway was tested.

RESULTS

MTSs from Bak and Bax are capable of targeting p53 to the mitochondria, and p53-BakMTS and p53-BaxMTS cause apoptosis through the intrinsic apoptotic pathway. Additionally, p53-BakMTS, DBD-BakMTS, p53-BaxMTS and DBD-BaxMTS caused apoptosis in T47D, H1373, SKOV-3 and HeLa cells. The apoptotic mechanism of p53-BakMTS and DBD-BakMTS was Bak dependent.

CONCLUSION

Our data demonstrates that p53-BakMTS (or BaxMTS) and DBD-BakMTS (or BaxMTS) cause apoptosis at the mitochondria and can be used as a potential gene therapeutic in cancer.

The DNA binding domain of p53 is sufficient to trigger a potent apoptotic response at the mitochondria

The tumor suppressor p53 is one of the most studied proteins in human cancer.1-3 While nuclear p53 has been utilized for cancer gene therapy, mitochondrial targeting of p53 has not been fully exploited to date.4,5 In response to cellular stress, p53 translocates to the mitochondria and directly interacts with Bcl-2 family proteins including antiapoptotic Bcl-XL and Bcl-2 and proapoptotic Bak and Bax.6 Antiapoptotic Bcl-XL forms inhibitory complexes with proapoptotic Bak and Bax preventing their homo-oligomerization.7 Upon translocation to the mitochondria, p53 binds to Bcl-XL, releases Bak and Bax from the inhibitory complex and enhances their homo-oligomerization.8 Bak and Bax homotetramer formation disrupts the mitochondrial outer membrane, releases antiapoptotic factors such as cytochrome c and triggers a rapid apoptotic response mediated by caspase induction.9 It is still unclear if the MDM2 binding domain (MBD), the proline-rich domain (PRD) and/or DNA binding domain (DBD) of p53 are the domains responsible for interaction with Bcl-XL.10-17 The purpose of this work is to determine if a smaller functional domain of p53 is capable of inducing apoptosis similarly to full length p53. To explore this question, different domains of p53 (MBD, PRD, DBD) were fused to the mitochondrial targeting signal (MTS) from Bcl-XL to ensure Bcl-XL specific targeting.18 The designed constructs were tested for apoptotic activity (TUNEL, Annexin-V, and 7-AAD) in 3 different breast cancer cell lines (T47D, MCF-7, MDA-MB-231), in a cervical cancer cell line (HeLa) and in non-small cell lung adenocarcinoma cells H1373. Our results indicate that DBD-XL (p53 DBD fused to the Bcl-XL MTS) reproduces (in T47D cells) or demonstrates increased apoptotic activity (in MCF-7, MDA-MB-231, and HeLa cells) compared to p53-XL (full length p53 fused to Bcl-XL MTS). Additionally, mitochondrial dependent apoptosis assays (TMRE, caspase-9), co-IP and overexpression of Bcl-XL in T47D cells suggest that DBD fused to XL MTS may bind to and inhibit Bcl-XL. Taken together, our data demonstrates for the first time that the DBD of p53 may be the minimally necessary domain for achieving apoptosis at the mitochondria in multiple cell lines. This work highlights the role of small functional domains of p53 as a novel cancer biologic therapy.

A chimeric p53 evades mutant p53 transdominant inhibition in cancer cells

Because of the dominant negative effect of mutant p53, there has been limited success with wild-type (wt) p53 cancer gene therapy. Therefore, an alternative oligomerization domain for p53 was investigated to enhance the utility of p53 for gene therapy. The tetramerization domain of p53 was substituted with the coiled-coil (CC) domain from Bcr (breakpoint cluster region). Our p53 variant (p53-CC) maintains proper nuclear localization in breast cancer cells detected via fluorescence microscopy and shows a similar expression profile of p53 target genes as wt-p53. Additionally, similar tumor suppressor activities of p53-CC and wt-p53 were detected by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), annexin-V, 7-aminoactinomycin D (7-AAD), and colony-forming assays. Furthermore, p53-CC was found to cause apoptosis in four different cancer cell lines, regardless of endogenous p53 status. Interestingly, the transcriptional activity of p53-CC was higher than wt-p53 in 3 different reporter gene assays. We hypothesized that the higher transcriptional activity of p53-CC over wt-p53 was due to the sequestration of wt-p53 by endogenous mutant p53 found in cancer cells. Co-immunoprecipitation revealed that wt-p53 does indeed interact with endogenous mutant p53 via its tetramerization domain, while p53-CC escapes this interaction. Therefore, we investigated the impact of the presence of a transdominant mutant p53 on tumor suppressor activities of wt-p53 and p53-CC. Overexpression of a potent mutant p53 along with wt-p53 or p53-CC revealed that, unlike wt-p53, p53-CC retains the same level of tumor suppressor activity. Finally, viral transduction of wt-p53 and p53-CC into a breast cancer cell line that harbors a tumor derived transdominant mutant p53 validated that p53-CC indeed evades sequestration and consequent transdominant inhibition by endogenous mutant p53.

Controlled access of p53 to the nucleus regulates its proteasomal degradation by MDM2

The tumor suppressor p53 can be sent to the proteasome for degradation by placing its nucleo-cytoplasmic shuttling under ligand control. Endogenous p53 is ubiquitinated by MDM2 in the nucleus, and controlling the access of p53 to the nuclear compartment regulates its ubiquitination and proteasomal degradation. This was accomplished by the use of a protein switch that places nuclear translocation under the control of externally applied dexamethasone. Fluorescence microscopy revealed that sending protein switch p53 (PS-p53) to the nucleus produces a distinct punctate distribution in both the cytoplasm and nucleus. The nuclear role in accessing the proteasome was investigated by inhibiting classical nuclear export with leptomycin B. Trapping PS-p53 in the nucleus only allows this punctate staining in that compartment, suggesting that PS-p53 must translocate first to the nuclear compartment for cytoplasmic punctate staining to occur. The role of MDM2 binding was explored by inhibiting MDM2/p53 binding with nutlin-3. Inhibition of this interaction blocked both nuclear export and cytoplasmic and nuclear punctate staining, providing evidence that any change in localization after nuclear translocation is due to MDM2 binding. Further, blocking the proteolytic activity of the proteasome maintained the nuclear localization of the construct. Truncations of p53 were made to determine smaller constructs still capable of interacting with MDM2, and their subcellular localization and degradation potential was observed. PS-p53 and a smaller construct containing the two MDM2 binding regions of p53 (Box I + V) were indeed degraded by the proteasome as measured by loss of enhanced green fluorescent protein that was also fused to the construct. The influence of these constructs on p53 gene transactivation function was assessed and revealed that PS-p53 decreased gene transactivation, while PS-p53(Box I + V) did not significantly change baseline gene transactivation.

A single mutant, A276S of p53, turns the switch to apoptosis

The tumor suppressor protein p53 induces apoptosis, cell cycle arrest, and DNA repair along with other functions in a transcription-dependent manner [Vousden, K. H. Cell 2000, 103(5), 691-694]. The selection of these functions depends on sequence-specific recognition of p53 to a target decameric sequence of gene promoters [Kitayner, M.; et al. Mol. Cell 2006, 22(6), 741-753]. Amino acid residues in p53 that directly bind to DNA were analyzed, and the replacement of A276 in p53 with selected amino acids elucidated its importance in promoter transcription. For most apoptotic and cell cycle gene promoters, position 9 of the target decameric sequence is a cytosine, while for DNA repair gene promoters, thymine is found instead. Therefore, selective binding to the cytosine at the ninth position may transcribe apoptotic gene promoters and thus can induce apoptosis and cell cycle arrest. Molecular modeling with PyMOL indicated that substitution of a hydrophilic residue, A276S, would prefer binding to cytosine at the ninth position of the target decameric sequence, whereas substitution of a hydrophobic residue (A276F) would fail to do so. Correspondingly, A276S demonstrated higher transcription of PUMA, PERP, and p21(WAF1/CIP1)gene promoters containing a cytosine at the ninth position and lower transcription of GADD45 gene promoter containing a thymine at the ninth position compared to wild-type p53. Cell cycle analysis showed that A276S maintained similar G1/G0 phase arrest as wild-type p53. Additionally, A276S induced higher apoptosis than wild-type p53 as measured by DNA segmentation and 7-AAD assay. Since the status of endogenous p53 can influence the activity of the exogenous p53, we examined the activity of A276S in HeLa cells (wild-type endogenous p53) in addition to T47D cells (mutated and mislocalized endogenous p53). The same apoptotic trend in both cell lines suggested A276S can induce cell death regardless of endogenous p53 status. Cell proliferation assay depicted that A276S efficiently reduced the viability of T47D cells more than wild-type p53 over time. We conclude that the predicted preferred binding of A276S to cytosine at the ninth position better transactivates a number of apoptotic gene promoters. Higher induction apoptosis than wild-type p53 makes A276S an attractive candidate for therapy to eradicate cancer.

p53 and Ceramide as Collaborators in the Stress Response

The sphingolipid ceramide mediates various cellular processes in response to several extracellular stimuli. Some genotoxic stresses are able to induce p53-dependent ceramide accumulation leading to cell death. However, in other cases, in the absence of the tumor suppressor protein p53, apoptosis proceeds partly due to the activity of this "tumor suppressor lipid", ceramide. In the current review, we describe ceramide and its roles in signaling pathways such as cell cycle arrest, hypoxia, hyperoxia, cell death, and cancer. In a specific manner, we are elaborating on the role of ceramide in mitochondrial apoptotic cell death signaling. Furthermore, after highlighting the role and mechanism of action of p53 in apoptosis, we review the association of ceramide and p53 with respect to apoptosis. Strikingly, the hypothesis for a direct interaction between ceramide and p53 is less favored. Recent data suggest that ceramide can act either upstream or downstream of p53 protein through posttranscriptional regulation or through many potential mediators, respectively.

Solid phase synthesis of mitochondrial triphenylphosphonium-vitamin E metabolite using a lysine linker for reversal of oxidative stress

Mitochondrial targeting of antioxidants has been an area of interest due to the mitochondria's role in producing and metabolizing reactive oxygen species. Antioxidants, especially vitamin E (α-tocopherol), have been conjugated to lipophilic cations to increase their mitochondrial targeting. Synthetic vitamin E analogues have also been produced as an alternative to α-tocopherol. In this paper, we investigated the mitochondrial targeting of a vitamin E metabolite, 2,5,7,8-tetramethyl-2-(2′-carboxyethyl)-6-hydroxychroman (α-CEHC), which is similar in structure to vitamin E analogues. We report a fast and efficient method to conjugate the water-soluble metabolite, α-CEHC, to triphenylphosphonium cation via a lysine linker using solid phase synthesis. The efficacy of the final product (MitoCEHC) to lower oxidative stress was tested in bovine aortic endothelial cells. In addition the ability of MitoCEHC to target the mitochondria was examined in type 2 diabetes db/db mice. The results showed mitochondrial accumulation in vivo and oxidative stress decrease in vitro.

Enhanced and selective killing of chronic myelogenous leukemia cells with an engineered BCR-ABL binding protein and imatinib

The oncoprotein Bcr-Abl stimulates prosurvival pathways and suppresses apoptosis from its exclusively cytoplasmic locale, but when targeted to the mitochondrial compartment of leukemia cells, Bcr-Abl was potently cytotoxic. Therefore, we designed a protein construct to act as a mitochondrial chaperone to move Bcr-Abl to the mitochondria. The chaperone (i.e., the 43.6 kDa intracellular cryptic escort (iCE)) contains an EGFP tag and two previously characterized motifs: (1) an optimized Bcr-Abl binding motif that interacts with the coiled-coil domain of Bcr (ccmut3; 72 residues), and (2) a cryptic mitochondrial targeting signal (cMTS; 51 residues) that selectively targets the mitochondria in oxidatively stressed cells (i.e., Bcr-Abl positive leukemic cells) via phosphorylation at a key residue (T193) by protein kinase C. While the iCE colocalized with Bcr-Abl, it did not relocalize to the mitochondria. However, the iCE was selectively toxic to Bcr-Abl positive K562 cells as compared to Bcr-Abl negative Cos-7 fibroblasts and 1471.1 murine breast cancer cells. The toxicity of the iCE to leukemic cells was equivalent to 10 μM imatinib at 48 h and the iCE combined with imatinib potentiated cell death beyond imatinib or the iCE alone. Substitution of either the ccmut3 or the cMTS with another Bcr-Abl binding domain (derived from Ras/Rab interaction protein 1 (RIN1; 295 residues)) or MTS (i.e., the canonical IMS derived from Smac/Diablo; 49 residues) did not match the cytotoxicity of the iCE. Additionally, a phosphorylation null mutant of the iCE also abolished the killing effect. The mitochondrial toxicity of Bcr-Abl and the iCE in Bcr-Abl positive K562 leukemia cells was confirmed by flow cytometric analysis of 7-AAD, TUNEL, and annexin-V staining. DNA segmentation and cell viability were assessed by microscopy. Subcellular localization of constructs was determined using confocal microscopy (including statistical colocalization analysis). Overall, the iCE was highly active against K562 leukemia cells and the killing effect was dependent upon both the ccmut3 and functional cMTS domains.

Utilizing the estrogen receptor ligand-binding domain for controlled protein translocation to the insoluble fraction

PURPOSE

The estrogen receptor forms insoluble aggregates in the insoluble cytoskeletal subcellular fraction when bound to the antagonist fulvestrant. The ligand-binding domain was isolated and fused to signal sequences to target subcellular compartments. Sequestering a pro-apoptotic peptide tested the utility of a protein targeted to the insoluble fraction.

METHODS

The ligand-binding domain of the estrogen receptor was isolated and fused with signal sequences, either a nuclear localization signal or nuclear export signal. The subcellular localization when bound to drug fulvestrant was examined, specifically its interaction with cytokeratins 8 and 18. The ability to target a therapeutic peptide to the insoluble fraction was tested by fusing a therapeutic coiled-coil from Bcr-Abl in K562 cells.

RESULTS

The estrogen receptor ligand-binding domain responds to fulvestrant by translocating to the insoluble fraction. Adding a signal sequence significantly limited the translocation to either the nucleus or cytoplasm. The cytokeratin 8/18 status of the cell did not alter this response. The therapeutic coiled-coil fused to ERLBD was inactivated upon ligand induction.

CONCLUSIONS

Isolating the ligand-binding domain of the estrogen receptor creates a ligand-controllable protein capable of translocation to the insoluble fraction. This can be used to sequester an active peptide to alter its function.

Targeting malignant mitochondria with therapeutic peptides

The current status of peptides that target the mitochondria in the context of cancer is the focus of this review. Chemotherapy and radiotherapy used to kill tumor cells are principally mediated by the process of apoptosis that is governed by the mitochondria. The failure of anticancer therapy often resides at the level of the mitochondria. Therefore, the mitochondrion is a key pharmacological target in cancer due to many of the differences that arise between malignant and healthy cells at the level of this ubiquitous organelle. Additionally, targeting the characteristics of malignant mitochondira often rely on disruption of protein--protein interactions that are not generally amenable to small molecules. We discuss anticancer peptides that intersect with pathological changes in the mitochondrion.

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.