Tumor-specific nuclear targeting: promises for anti-cancer therapy?

Targeting Proteins in Nucleus through Dual-Regulatory Pathways Acting in Cytoplasm

Nuclear proteins have been regarded as attractive targets for exploiting therapeutic agents. However, those agents cannot efficiently pass through nuclear pores and it is also difficult to overcome the crowded nuclear environment to react with proteins. Herein, we propose a novel strategy acting in the cytoplasm to regulate nuclear proteins based on their signaling pathways, instead of directly entering into nuclei. A multifunctional complex PKK-TTP/hs carries human telomerase reverse transcriptase (hTERT) small interfering RNA (defined as hs) for gene silencing in the cytoplasm, which reduced the import of nuclear protein. At the same time, it could generate reactive oxygen species (ROS) under light irradiation, which raised the export of nuclear proteins by promoting proteins translocation. Through this dual-regulatory pathway, we successfully reduced nuclear protein (hTERT proteins) in vivo (42.3%). This work bypasses the challenge of directly entering into the nucleus and provides an effective strategy for regulating nuclear proteins.

A versatile endosome acidity-induced sheddable gene delivery system: increased tumor targeting and enhanced transfection efficiency

BACKGROUND

Polycation carriers show great foreground in the developing efficient and safe gene delivery; nevertheless, they are cytotoxic and unstable in vivo because of the excess cationic charge. PEGylation improves the biocompatibility and stability of polycation, whereas PEGylation restrains the endosomal escape to some extent.

MATERIALS AND METHODS

To address this issue and promote the transfection in vivo, a pH-sensitive conjugate folate-polyethylene glycol-carboxylated chitosan (shorten as FA-PEG-CCTS) was designed and coated on the surface of PEI/NLS/pDNA (PNDs), forming a versatile gene carrier FA-PEG-CCTS/PEI/NLS/pDNA (FPCPNDs). The novel carrier exhibited a few picturesque characteristics, including (i) neutral surface charge to restrain nonspecific interactions; (ii) folate receptors (FR)-mediated endocytosis to augment cellular uptake; (iii) dual proton sponge effect to realize endosome escape, and (iv) nuclear localization sequences (NLS) to enhance the transfection of pDNA.

RESULTS

FPCPNDs could compress and protect pDNA from degradation. FPCPNDs energetically targeted tumor cells because of their high binding affinity between FA and highly expressed FR on the tumor surface, accordingly enhancing the cellular uptake. In the acidic endosomes, FA-PEG-CCTS segment dissociated from PNDs. Then, PNDs realized endosomal escape through the proton sponge effect of PEI. Furthermore, FPCPNDs showed admirable transfection efficiency with the aid of NLS peptides. What's more, in vivo studies revealed that FPCPNDs had supreme antitumor activity among the whole preparations.

CONCLUSION

In vitro and in vivo assays thus demonstrate that FPCPNDs is a hopeful strategy for gene delivery.

Bioactive Heterometallic CuII-ZnII Complexes with Potential Biomedical Applications

A series of multinuclear heterometallic Cu-Zn complexes of molecular formula [(CuL)₂Zn(dca)₂] (1), [(CuL)₂Zn(NO₃)₂] (2), [(CuL)₂Zn₂(Cl)₄] (3), and [(CuL)₂Zn₂(NO₂)₄] (4) have been synthesized by reacting [CuL] as a "metalloligand (ML)" (where HL = N,N'-bis(5-chloro-2-hydroxybenzylidene)-2,2-dimethylpropane-1,3-diamine) and by varying the anions or coligands using the same molar ratios of the reactants. All of the four products including the ML have been characterized by infrared and UV-vis spectroscopies and elemental and single-crystal X-ray diffraction analyses. By varying the anions, different structures and topologies are obtained which we have tried to rationalize by means of thorough density functional theory calculations. All of the complexes (1-4) have now been applied for several biological investigations to verify their therapeutic worth. First, their cytotoxicity properties were assessed against HeLa human cervical carcinoma along with the determination of IC₅₀ values. The study was extended with extensive DNA and protein binding experiments followed by detailed fluorescence quenching study with suitable reagents to comprehend the mechanistic pathway. From all of these biological studies, it has been found that all of these heterometallic complexes show more than a few fold improvement of their therapeutic values as compared to the similar homometallic ones probably because of the simultaneous synergic effect of copper and zinc. Among all of the four heterometallic complexes, complex 3 exhibits highest binding constants and IC₅₀ values suggest for their better interaction toward the biological targets and hence have better clinical importance.

Apoptin Gene Delivery by the Functionalized Polyamidoamine Dendrimer Derivatives Induces Cell Death of U87-MG Glioblastoma Cells

Malignant glioma is the most common and aggressive form of primary brain tumor in adults. In this study, we describe the efficacy of nonviral gene delivery carriers, histidine- and arginine- or histidine- and lysine-grafted polyamidoamine (PAMAM) dendrimers (PAMAM-H-R and PAMAM-H-K), in delivering a therapeutic and a tumor-selective killer gene, apoptin, using human glioma cells (U87-MG) and newborn human dermal fibroblast cells. We analyzed transfection efficiency using luciferase and a plasmid DNA encoding for enhanced green fluorescent protein and assessed cell viability in both cells. The results show that transfection efficiency of PAMAM-H-R and PAMAM-H-K was greatly increased compared with that of native PAMAM. Moreover, among PAMAM derivatives, cytotoxicity of PAMAM-H-K was very low. We treated both cells with complexes of PAMAM-H-R or PAMAM-H-K and apoptin and analyzed their cellular uptake by flow cytometry and localization by confocal microscopy. Furthermore, cell cycle distribution, caspase 3 activity assay, and JC-1 analysis showed cell death induced by apoptin in U87-MG cells. The present study demonstrates that a PAMAM-H-R/apoptin complex is an effective gene carrier system in glioma cell culture.

Dipeptide-functionalized polyamidoamine dendrimer-mediated apoptin gene delivery facilitates apoptosis of human primary glioma cells

Glioblastoma multiform (GBM) is the most frequent and aggressive form of brain tumors in adults. However, the development of more efficient and safe nonviral vector gene therapy represents a promising therapeutic approach, using a tumor-specific killer gene, named apoptin. In this study, we describe the efficacy of non-viral gene delivery vectors, the amino acid-conjugated PAMAM derivatives (PAMAM-H-R and PAMAM-H-K) in delivering a therapeutic gene, displaying affinity toward human primary glioma cells (GBL-14 cells) and dermal fibroblasts. We analyzed transfection efficiency, using luciferase (Luci) and a pDNA encoding for enhanced fluorescent protein (EGFP), and cytotoxicity in both cells. The results show that transfection efficiency of PAMAM-H-R improved compared to native PAMAM dendrimer, but cytotoxicity of PAMAM-H-R and PAMAM-H-K were very low. We treated both cells with a polyplex formation of PAMAM-H-R or PAMAM-H-K/apoptin, and analyzed their cellular uptake and localization by flow cytometry and confocal microscopy. Furthermore, we analyzed the endosomal escape effect using TEM images, and found that PAMAM-H-R showed very fast escape from endosome to the cytosol. Caspase 3 activity assay, cell cycle distribution, and JC-1 analysis showed apoptosis induced by apoptin in GBL-14 cells. This indicates that PAMAM-H-R can be a potential nonviral vector gene delivery carrier for brain tumor therapy. The present study demonstrates that PAMAM-H-R/apoptin gene polyplex can be used as an effective therapeutic candidate for GBM due to its selective induction of apoptosis in primary glioma cells as a potential nonviral gene delivery carrier for brain tumor therapy.

Delivery of cancer therapeutics to extracellular and intracellular targets: Determinants, barriers, challenges and opportunities

Advances in molecular medicine have led to identification of worthy cellular and molecular targets located in extracellular and intracellular compartments. Effectiveness of cancer therapeutics is limited in part by inadequate delivery and transport in tumor interstitium. Parts I and II of this report give an overview on the kinetic processes in delivering therapeutics to their intended targets, the transport barriers in tumor microenvironment and extracellular matrix (TME/ECM), and the experimental approaches to overcome such barriers. Part III discusses new concepts and findings concerning nanoparticle-biocorona complex, including the effects of TME/ECM. Part IV outlines the challenges in animal-to-human translation of cancer nanotherapeutics. Part V provides an overview of the background, current status, and the roles of TME/ECM in immune checkpoint inhibition therapy, the newest cancer treatment modality. Part VI outlines the development and use of multiscale computational modeling to capture the unavoidable tumor heterogeneities, the multiple nonlinear kinetic processes including interstitial and transvascular transport and interactions between cancer therapeutics and TME/ECM, in order to predict the in vivo tumor spatiokinetics of a therapeutic based on experimental in vitro biointerfacial interaction data. Part VII provides perspectives on translational research using quantitative systems pharmacology approaches.

Novel delivery approaches for cancer therapeutics

Currently, a majority of cancer treatment strategies are based on the removal of tumor mass mainly by surgery. Chemical and physical treatments such as chemo- and radiotherapies have also made a major contribution in inhibiting rapid growth of malignant cells. Furthermore, these approaches are often combined to enhance therapeutic indices. It is widely known that surgery, chemo- and radiotherapy also inhibit normal cells growth. In addition, these treatment modalities are associated with severe side effects and high toxicity which in turn lead to low quality of life. This review encompasses novel strategies for more effective chemotherapeutic delivery aiming to generate better prognosis. Currently, cancer treatment is a highly dynamic field and significant advances are being made in the development of novel cancer treatment strategies. In contrast to conventional cancer therapeutics, novel approaches such as ligand or receptor based targeting, triggered release, intracellular drug targeting, gene delivery, cancer stem cell therapy, magnetic drug targeting and ultrasound-mediated drug delivery, have added new modalities for cancer treatment. These approaches have led to selective detection of malignant cells leading to their eradication with minimal side effects. Lowering multi-drug resistance and involving influx transportation in targeted drug delivery to cancer cells can also contribute significantly in the therapeutic interventions in cancer.

Stepwise-acid-active multifunctional mesoporous silica nanoparticles for tumor-specific nucleus-targeted drug delivery

In this paper, a novel stepwise-acid-active multifunctional mesoporous silica nanoparticle (MSN-(SA)TAT&(DMA)K11) was developed as a drug carrier. The MSN-(SA)TAT&(DMA)K11 is able to reverse its surface charge from negative to positive in the mildly acidic tumor extracellular environment. Then, the fast endo/lysosomal escape and subsequent nucleus targeting as well as intranuclear drug release can be realized after cellular internalization. Because of the difference in acidity between the tumor extracellular environment and that of endo/lysosomes, this multifunctional MSN-(SA)TAT&(DMA)K11 exhibits a stepwise-acid-active drug delivery with a tumor-specific nucleus-targeted property.

Regulated transport into the nucleus of herpesviridae DNA replication core proteins

The Herpesvirdae family comprises several major human pathogens belonging to three distinct subfamilies. Their double stranded DNA genome is replicated in the nuclei of infected cells by a number of host and viral products. Among the latter the viral replication complex, whose activity is strictly required for viral replication, is composed of six different polypeptides, including a two-subunit DNA polymerase holoenzyme, a trimeric primase/helicase complex and a single stranded DNA binding protein. The study of herpesviral DNA replication machinery is extremely important, both because it provides an excellent model to understand processes related to eukaryotic DNA replication and it has important implications for the development of highly needed antiviral agents. Even though all known herpesviruses utilize very similar mechanisms for amplification of their genomes, the nuclear import of the replication complex components appears to be a heterogeneous and highly regulated process to ensure the correct spatiotemporal localization of each protein. The nuclear transport process of these enzymes is controlled by three mechanisms, typifying the main processes through which protein nuclear import is generally regulated in eukaryotic cells. These include cargo post-translational modification-based recognition by the intracellular transporters, piggy-back events allowing coordinated nuclear import of multimeric holoenzymes, and chaperone-assisted nuclear import of specific subunits. In this review we summarize these mechanisms and discuss potential implications for the development of antiviral compounds aimed at inhibiting the Herpesvirus life cycle by targeting nuclear import of the Herpesvirus DNA replicating enzymes.

Molecularly modified VP3 (30-121) induces apoptosis in human bladder cancer (EJ) cells but not in normal (3T3) cells

The chicken anaemia virus protein 3 (VP3 or apoptin) induces apoptosis specifically in tumour and transformed cells but not in normal cells. This selective apoptosis is ideal for therapeutic purposes. However, VP3, a heterologous protein, is immunogenic in vivo. Such a drawback limits its clinical usage. To diminish its potential immunogenicity, we modified the sequence of VP3. The VP3 genes functional sequence starts at 33 AA (amino acids) from the N-terminal side, and the first protein structural domain consists of 30 AA. We found that the first domain of VP3 contains no functional sequences. Therefore, this domain was removed to test whether its absence affects apoptosis. Transfection of EGFP (enhance green fluorescent protein)-modified-VP3 or HA-modified-VP3 in bladder cancer cell lines (EJ) resulted in its expression, successful localization to the nucleus and efficient induction of apoptosis. Expression of EGFP-modified-VP3 or HA-modified-VP3 had no influence on mouse fibroblast cells (3T3). The modified VP3 (30-121), like the wild-type VP3, induced EJ cell apoptosis without affecting 3T3 cells. This study increases our understanding of modified VP3 (30-121) as a possible substitute for the wild-type VP3, which makes VP3 (30-121) an interesting candidate for the development of novel therapeutic strategies.

Systemic delivery of lentivirus-mediated secretable TAT-apoptin eradicates hepatocellular carcinoma xenografts in nude mice

Apoptin, a chicken anemia virus-derived protein, has been shown to induce apoptosis in various human cancer cell lines, but not in normal cells, thus making it a candidate for the development of novel therapeutic strategies. To enable the efficient transduction of tumor cells with apoptin, we have developed a novel mammalian expression system for the secretion of apoptin in vitro. We have previously shown the efficient and tumor-specific killing of cells by adding a secretory signal peptide (SP) to the N terminus of transacting activator of transcription (TAT)-apoptin (SP-TAT-apoptin). In addition, our report showed the successful secretion of high levels of TAT-apoptin/GFP into the culture medium from HUVEC cells infected by lentivirus LV-SP-TAT-apoptin/GFP. To obtain sustained apoptin-induced tumor cell death in vivo, we injected the LV-SP-TAT-apoptin viruses via the tail vein for systemic delivery of the viruses; viruses expressing LV-SP-TAT-GFP were used as a negative control. Markedly, almost all the hepatocellular carcinoma xenograft tumors disappeared following the treatment while the xenografts that received the control LV-SP-TAT-GFP viruses continued to grow. Moreover, the animal studies presented in this paper demonstrate a low toxicity of SP-TAT-apoptin in vivo, confirming and extending the results of the in vitro studies. Taken together, our data strongly suggest that systemic delivery of lentivirus-mediated secretable TAT-apoptin is feasible to eradicate liver cancer in vivo.

PP2A inactivation is a crucial step in triggering apoptin-induced tumor-selective cell killing

Apoptin (apoptosis-inducing protein) harbors tumor-selective characteristics making it a potential safe and effective anticancer agent. Apoptin becomes phosphorylated and induces apoptosis in a large panel of human tumor but not normal cells. Here, we used an in vitro oncogenic transformation assay to explore minimal cellular factors required for the activation of apoptin. Flag-apoptin was introduced into normal fibroblasts together with the transforming SV40 large T antigen (SV40 LT) and SV40 small t antigen (SV40 ST) antigens. We found that nuclear expression of SV40 ST in normal cells was sufficient to induce phosphorylation of apoptin. Mutational analysis showed that mutations disrupting the binding of ST to protein phosphatase 2A (PP2A) counteracted this effect. Knockdown of the ST-interacting PP2A-B56γ subunit in normal fibroblasts mimicked the effect of nuclear ST expression, resulting in induction of apoptin phosphorylation. The same effect was observed upon downregulation of the PP2A-B56δ subunit, which is targeted by protein kinase A (PKA). Apoptin interacts with the PKA-associating protein BCA3/AKIP1, and inhibition of PKA in tumor cells by treatment with H89 increased the phosphorylation of apoptin, whereas the PKA activator cAMP partially reduced it. We infer that inactivation of PP2A, in particular, of the B56γ and B56δ subunits is a crucial step in triggering apoptin-induced tumor-selective cell death.

Delivery of nanomedicines to extracellular and intracellular compartments of a solid tumor

Advances in molecular medicines have led to identification of promising targets on cellular and molecular levels. These targets are located in extracellular and intracellular compartments. The latter include cytosol, nucleus, mitochondrion, Golgi apparatus and endoplasmic reticulum. This report gives an overview on the barriers to delivering nanomedicines to various target sites within a solid tumor, the experimental approaches to overcome such barriers, and the potential utility of nanotechnology.

Multiple phosphorylation sites at the C-terminus regulate nuclear import of HCMV DNA polymerase processivity factor ppUL44

The processivity factor of human cytomegalovirus DNA polymerase, phosphoprotein ppUL44, is essential for viral replication. During viral infection ppUL44 is phosphorylated by the viral kinase pUL97, but neither the target residues on ppUL44 nor the effect of phosphorylation on ppUL44's activity are known. We report here that ppUL44 is phosphorylated when transiently expressed in mammalian cells and coimmunoprecipitates with cellular kinases. Of three potential phosphorylation sites (S413, S415, S418) located upstream of ppUL44's nuclear localization signal (NLS) and one (T427) within the NLS itself, protein kinase CK2 (CK2) specifically phosphorylates S413, to trigger a cascade of phosphorylation of S418 and S415 by CK1 and CK2, respectively. Negative charge at the CK2/CK1 target serine residues facilitates optimal nuclear accumulation of ppUL44, whereas negative charge on T427, a potential cyclin-dependent 1 phosphorylation site, strongly decreases nuclear accumulation. Thus, nuclear transport of ppUL44 is finely tuned during viral infection through complex phosphorylation events.

PTD4-apoptin protein therapy inhibits tumor growth in vivo

Apoptin protein harbors tumor-selective cell death activity, which makes it a potential anticancer therapy candidate. This study reports an apoptin therapy approach based on protein transduction domain 4 (PTD4)-mediated transduction of recombinant apoptin protein. In vitro, the PTD4-apoptin fusion protein is located in the nucleus and induces cell death in, e.g., human hepatocarcinoma HepG2 cells. In normal human L-02 hepatocytes, PTD4-apoptin protein retained mainly cytoplasmic and did not induce detectable levels of cell death, illustrating that the PTD4 domain does not affect apoptin's tumor-selective characteristics. In vivo, liver, cervix and gastric carcinoma xenografts treated with PTD4-apoptin protein for 6 days via the tumor epidermis exhibited a significant tumor growth inhibition because of apoptin-mediated cell death. In addition, treatment of human hepatocarcinoma xenografts during 3 weeks showed that PTD4-apoptin protein has significant anticancer activity, whereas control treatment with PTD4-enhanced green fluorescence protein or saline did not. Cell death and disruption of the tumor integrity were apparent in the PTD4-apoptin transduced xenografted tumors. As important, although PTD4-apoptin protein could be detected in the epidermal tissue covering the subcutaneous tumor tissue and in several organs, such as liver and brain, of the treated mice, no tissue disruption or signs of cell death could be detected. Our in vivo data reveal that apoptin protein delivery constitutes a novel powerful and safe anticancer therapy.

Construction and identification of the recombinant adenovirus expressing Apoptin gene of chicken anemia virus

AIM: To construct a recombinant adenovirus carrying Apoptin gene so as to provide a basis for further studying the molecular mechanism of Apoptin gene in inducing tumor cell apoptosis.

METHODS: The plasmid pVAX1-Apoptin was digested by endonuclease BamHⅠ and SpeⅠ, and the obtained Apoptin segment was inserted into vector pacAd5 CMV K-N pA to construct a shuttle plasmid pacAd5-Apoptin. After PacⅠ digestion and linearized process, the plasmid pacAd5-Apoptin and pAD (genome plasmid) were co-transfected into AAV-293 cells by liposome mediation. The DNA containing Apoptin gene of the recombinant adenovirus was identified by plaque purification, reverse transcription-polymerase chain reaction (RT-PCR) and Western blot. The titer of the obtained adenovirus was also examined.

RESULTS: The recombinant adenovirus expressed Apoptin gene and the molecular weight of the protein was about 13 kDa, which was consistent with the CVA-positive control. The protein of Apoptin could be effectively expressed in the recombinant adenovirus, and this protein had response to the CAV-positive serum. The titer of the recombinant virus was 10¹¹ PFU/L.

CONCLUSION: The adenovirus containing Apoptin gene is successfully constructed, and the virus titer is able to meet the requirements of in vitro and in vivo experiments.

Drug-resistant AML cells and primary AML specimens are killed by 111In-anti-CD33 monoclonal antibodies modified with nuclear localizing peptide sequences

Multidrug resistance (MDR) is a major challenge to the successful treatment of acute myeloid leukemia (AML). Our purpose was to determine whether (111)In-HuM195 anti-CD33 antibodies modified with peptides harboring nuclear localizing sequences (NLS) could kill drug-resistant AML cell lines and primary AML patient specimens expressing MDR transporters through the emission of Auger electrons.

METHODS

HuM195, M195, and irrelevant mouse IgG (mIgG) were conjugated to 10+/-3 NLS peptides and then labeled with (111)In by diethylenetriaminepentaacetic acid substitution to a specific activity of 1-8 MBq/microg. The binding affinity of HuM195 and M195 was determined for HL-60 and mitoxantrone-resistant HL-60-MX-1 cells. Nuclear localization of (111)In-NLS-HuM195, (111)In-NLS-M195, (111)In-HuM195, and (111)In-M195 was measured by subcellular fractionation. The antiproliferative effects of (111)In-NLS-HuM195, (111)In-NLS-M195, (111)In-HuM195, and (111)In-M195 (2.5-250 kBq/well) on HL-60 and HL-60-MX-1 were studied using the WST-1 assay. Clonogenic survival of HL-60 and HL-60-MX-1 leukemic cells and 10 primary AML specimens with MDR phenotype (assessed by flow cytometry) was determined after exposure for 3 h at 37 degrees C to 2.5-250 mBq/cell of (111)In-NLS-HuM195, (111)In-HuM195, or (111)In-NLS-mIgG. Clonogenic survival versus the amount of radioactivity incubated with the cells (mBq/cell) was plotted, and the mean lethal amount of radioactivity and the lower asymptote of the curve (plateau) were determined.

RESULTS

The (111)In-labeled anti-CD33 monoclonal antibodies HuM195 and M195 modified with NLS were efficiently routed to the nucleus of HL-60 cells and their mitoxantrone-resistant clone after CD33-mediated internalization. The following are the principal findings of our study: (111)In-NLS-HuM195 was more effective at killing HL-60 and HL-60-MX-1 cells than was (111)In-HuM195, a strong correlation between the specific activity of the (111)In-labeled radioimmunoconjugates and their cytotoxicity toward AML cells existed, and leukemic cells from patients were killed by (111)In-NLS-M195 or (111)In-M195, but the cytotoxic response among specimens was heterogeneous.

CONCLUSION

NLS conjugation enhanced the nuclear uptake and cytotoxicity of (111)In-HuM195 and (111)In-M195 toward drug-resistant AML cell lines as well as patient specimens expressing a diversity of MDR phenotypes, including Pgp-170, BCRP1, or MRP1 transporters. Targeted Auger electron radioimmunotherapy using (111)In-labeled anti-CD33 monoclonal antibodies modified with NLS may be able to overcome MDR and provide a means of treating chemotherapy-resistant myeloid leukemias in patients.

Secretory Transactivating Transcription-apoptin fusion protein induces apoptosis in hepatocellular carcinoma HepG2 cells

AIM: To determine whether SP-TAT-apoptin induces apoptosis and also maintains its tumor cell specificity.

METHODS: In this study, we designed a secretory protein by adding a secretory signal peptide (SP) to the N terminus of Transactivating Transcription (TAT)-apoptin (SP-TAT-apoptin), to test the hypothesis that it gains an additive bystander effect as an anti-cancer therapy. We used an artificial human secretory SP whose amino acid sequence and corresponding cDNA sequence were generated by the SP hidden Markov model.

RESULTS: In human liver carcinoma HepG2 cells, SP-TAT-apoptin expression showed a diffuse pattern in the early phase after transfection. After 48 h, however, it translocated into the nuclear compartment and caused massive apoptotic cell death, as determined by 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and annexin-V binding assay. SP-TAT-apoptin did not, however, cause any cell death in non-malignant human umbilical vein endothelial cells (HUVECs). Most importantly, the conditioned medium from Chinese hamster ovary (CHO) cells transfected with SP-TAT-apoptin also induced significant cell death in HepG2 cells, but not in HUVECs.

CONCLUSION: The data demonstrated that SP-TAT-apoptin induces apoptosis only in malignant cells, and its secretory property might greatly increase its potency once it is delivered in vivo for cancer therapy.

Akt is transferred to the nucleus of cells treated with apoptin, and it participates in apoptin-induced cell death

OBJECTIVES

The phosphatidylinositol 3-kinase (PI3-K)/Akt pathway is well known for the regulation of cell survival, proliferation, and some metabolic routes.

MATERIALS AND METHODS

In this study, we document a novel role for the PI3-K/Akt pathway during cell death induced by apoptin, a tumour-selective inducer of apoptosis.

RESULTS

We show for the first time that apoptin interacts with the p85 regulatory subunit, leading to constitutive activation of PI3-K. The inhibition of PI3-K activation either by chemical inhibitors or by genetic approaches severely impairs cell death induced by apoptin. Downstream of PI3-K, Akt is activated and translocated to the nucleus together with apoptin. Direct interaction between apoptin and Akt is documented. Co-expression of nuclear Akt significantly potentiates cell death induced by apoptin. Thus, apoptin-facilitated nuclear Akt, in contrast to when in its cytoplasmic pool, appears to be a positive regulator, rather than repressor of apoptosis.

CONCLUSIONS

Our observations indicate that PI3-K/Akt pathways have a dual role in both survival and cell death processes depending on the stimulus. Nuclear Akt acts as apoptosis stimulator rather than as a repressor, as it likely gains access to a new set of substrates in the nucleus. The implicated link between survival and cell death pathways during apoptosis opens new pharmacological opportunities to modulate apoptosis in cancer, for example through the manipulation of Akt's cellular localization.

Interaction with PI3-kinase contributes to the cytotoxic activity of apoptin

Apoptin, a small protein from the chicken anemia virus, has attracted attention because of its specificity in killing tumor cells. Localization of apoptin in the nucleus of tumor cells has been shown to be vital for proapoptotic activity, however, targeted expression of apoptin in the nucleus of normal cells does not harm the cells, indicating that nuclear localization of apoptin is insufficient for its cytotoxicity. Here, we demonstrate for the first time that apoptin interacts with the SH3 domain of p85, the regulatory subunit of phosphoinositide 3-kinase (PI3-K), through its proline-rich region. Apoptin derivatives devoid of this proline-rich region do not interact with p85, are unable to activate PI3-K, and show impaired apoptosis induction. Moreover, apoptin mutants containing the proline-rich domain are sufficient to elevate PI3-K activity and to induce apoptosis in cancer cells. Downregulation of p85 leads to nuclear exclusion of apoptin and impairs cell death induction, indicating that interaction with the p85 PI3-K subunit essentially contributes to the cytotoxic activity of apoptin.

Viral protein apoptin as a molecular tool and therapeutic bullet: implications for cancer control

The chicken anemia virus-derived protein apoptin induces apoptosis in human tumor cells via a p53-independent pathway, while leaving normal cells intact. Moreover, apoptin treatment in preclinical animal studies leads to reduced tumor growth or remission without a detectable effect on healthy tissues. Apoptin is activated by a still unknown tumor-specific kinase activity. The mode of action of apoptin is under intense investigation, as certain features make it a promising tool for discovering early events in tumorigenesis, identifying druggable targets for antitumor treatment and possibly serving as an antitumor therapy in itself.

Regulated nucleocytoplasmic trafficking of viral gene products: a therapeutic target?

The study of viral proteins and host cell factors that interact with them has represented an invaluable contribution to understanding of the physiology as well as associated pathology of key eukaryotic cell processes such as cell cycle regulation, signal transduction and transformation. Similarly, knowledge of nucleocytoplasmic transport is based largely on pioneering studies performed on viral proteins that enabled the first sequences responsible for the facilitated transport through the nuclear pore to be identified. The study of viral proteins has also enabled the discovery of several nucleocytoplasmic regulatory mechanisms, the best characterized being through phosphorylation. Recent delineation of the mechanisms whereby phosphorylation regulates nuclear import and export of key viral gene products encoded by important human pathogens such as human cytomegalovirus dengue virus and respiratory syncytial virus has implications for the development of antiviral therapeutics. In particular, the development of specific and effective kinase inhibitors makes the idea of blocking viral infection by inhibiting the phosphorylation-dependent regulation of viral gene product nuclear transport a real possibility. Additionally, examination of a chicken anemia virus (CAV) protein able to target selectively into the nucleus of tumor but not normal cells, as specifically regulated by phosphorylation, opens the exciting possibility of cancer cell-specific nuclear targeting. The study of nucleoplasmic transport may thus enable the development not only of new antiviral approaches, but also contribute to anti-cancer strategies.

An importin alpha/beta-recognized bipartite nuclear localization signal mediates targeting of the human herpes simplex virus type 1 DNA polymerase catalytic subunit pUL30 to the nucleus

Although the 1235 amino acids human herpes simplex virus type 1 (HSV-1) DNA polymerase catalytic subunit, pUL30, is essential for HSV-1 replication in the nucleus of host cells, little information is available regarding its nuclear import mechanism. The present study addresses this issue directly, characterizing pUL30's nuclear import pathway for the first time using quantitative confocal laser scanning microscopy (CLSM) on living cells, and fluorescent binding assays. In addition to a previously described nuclear localization signal (NLS) located within the pUL30 binding site for the polymerase accessory protein (PAP) pUL42, that appears to be dispensable for nuclear targeting, pUL30 possesses three putative basic NLSs. Intriguingly, the core of pUL30-NLS2 (residues 1114-1120) is highly homologous to that of the recently described NLS, similarly located upstream of the PAP binding site, of the human cytomegalovirus (HCMV) DNA polymerase catalytic subunit, pUL54. Here we show for the first time that pUL30-NLS2 itself is only partially functional in terms of nuclear import due to residue P1118 present in position 3 of the NLS core. Intriguingly, pUL30-NLS2 together with pUL30-NLS3 (residues 1133-1136) represents a fully functional bipartite NLS (pUL30-NLSbip), required for nuclear targeting of pUL30, and able to confer nuclear localization on heterologous proteins by conferring high-affinity interaction with the importin (IMP) alpha/beta heterodimer. Since nuclear targeting of HSV-1 proteins forming the replication fork is crucial for viral replication, the pUL30-NLSbip emerges for the first time as a viable therapeutic target.

Dynein light chain association sequences can facilitate nuclear protein import

Nuclear localization sequence (NLS)-dependent nuclear protein import is not conventionally held to require interaction with microtubules (MTs) or components of the MT motor, dynein. Here we report for the first time the role of sequences conferring association with dynein light chains (DLCs) in NLS-dependent nuclear accumulation of the rabies virus P-protein. We find that P-protein nuclear accumulation is significantly enhanced by its dynein light chain association sequence (DLC-AS), dependent on MT integrity and association with DLCs, and that P-protein-DLC complexes can associate with MT cytoskeletal structures. We also find that P-protein DLC-AS, as well as analogous sequences from other proteins, acts as an independent module that can confer enhancement of nuclear accumulation to proteins carrying the P-protein NLS, as well as several heterologous NLSs. Photobleaching experiments in live cells demonstrate that the MT-dependent enhancement of NLS-mediated nuclear accumulation by the P-protein DLC-AS involves an increased rate of nuclear import. This is the first report of DLC-AS enhancement of NLS function, identifying a novel mechanism regulating nuclear transport with relevance to viral and cellular protein biology. Importantly, this data indicates that DLC-ASs represent versatile modules to enhance nuclear delivery with potential therapeutic application.

Additive cytotoxic effect of apoptin and chemotherapeutic agents paclitaxel and etoposide on human tumour cells

Gene therapy experiments in animal models have shown that apoptin expression results in tumour regression without any significant side effects. Therefore, apoptin is regarded as a potential anticancer drug for clinical applications. In this study, we analysed whether chemotherapeutic agents combined with apoptin treatment could result in enhanced cytotoxicity in human tumour cell cultures. Combined treatment with recombinant adenovirus AdAptVP3 expressing apoptin and etoposide clearly showed an additive cytotoxic effect on human osteosarcoma U2OS cells. Paclitaxel treatment combined with apoptin expression significantly inhibited the survival of p53-positive human osteosarcoma U2OS and non-small lung carcinoma A549 cells, p53-negative human osteosarcoma Saos-2 cells and p53-mutant human prostate cancer Du145 cells, already at low doses of the chemotherapeutic agent. Our results indicate that the cytotoxicity-enhancing action by the tumour-specific apoptin in combination with chemotherapeutic agents might offer an effective and safe antitumour therapeutics.