Crucial Roles for Protein Kinase C Isoforms in Tumor-Specific Killing by Apoptin

Semisynthesis reveals apoptin as a tumour-selective protein prodrug that causes cytoskeletal collapse

Apoptin is a small viral protein capable of inducing cell death selectively in cancer cells. Despite its potential as an anticancer agent, relatively little is known about its mechanism of toxicity and cancer-selectivity. Previous experiments suggest that cancer-selective phosphorylation modulates apoptin toxicity, although a lack of chemical tools has hampered the dissection of underlying mechanisms. Here, we describe structure-function studies with site-specifically phosphorylated apoptin (apoptin-T108ph) in living cells which revealed that Thr108 phosphorylation is the selectivity switch for apoptin toxicity. Mechanistic investigations link T108ph to actin binding, cytoskeletal disruption and downstream inhibition of anoikis-resistance as well as cancer cell invasion. These results establish apoptin as a protein pro-drug, selectively activated in cancer cells by phosphorylation, which disrupts the cytoskeleton and promotes cell death. We anticipate that this mechanism provides a framework for the design of next generation anticancer proteins with enhanced selectivity and potency.

TLR2 dimer-specific ligands selectively activate protein kinase C isoforms in Leishmania infection

Of the thirteen Toll-like receptors (TLRs) in mice, TLR2 has a unique ability of forming heterodimers with TLR1 and TLR6. Such associations lead to selective cellular signalling and cellular responses such as cytokine expression. One of the signalling intermediates is protein kinase C (PKC); of which, eight isoforms are expressed in macrophages. Leishmania-a protozoan parasite that resides and replicates in macrophages-selectively modulates PKC-α, PKC-β, PKC-δ and PKC-ζ isoforms in macrophages. As TLR2 plays significant roles in Leishmania infection, we examined whether these PKC isoforms play selective roles in TLR2 signalling and TLR2-induced anti-leishmanial functions. We observed that the TLR2 ligands-Pam₃ CSK₄ (TLR1/2), PGN (TLR2/2) and FSL (TLR2/6)-differentially phosphorylated and translocated PKC-α, PKC-β, PKC-δ and PKC-ζ isoforms to cell membrane in uninfected and L. major-infected macrophages. The PKC isoform-specific inhibitors differentially altered IL-10 and IL-12 expression, Th1 and Th2 responses and anti-leishmanial effects in macrophages and in BALB/c mice. While PKC isoforms' inhibitors had insignificant effects on the Pam3CSK4-induced anti-leishmanial functions, PGN-induced pro-leishmanial effects were enhanced by PKC-(α + β) inhibitors, whereas PKC-(δ + ζ) inhibitors enhanced the anti-leishmanial effects of FSL. These results indicated that the ligand-induced TLR2 dimerization triggered differential dose-dependent and kinetic profiles of PKC isoform activation and that selective targeting of PKC isoforms using their respective inhibitors in combination significantly modulated TLR2-induced anti-leishmanial functions. To the best of our knowledge, this is the first demonstration of TLR2 dimer signalling through PKC isoforms and TLR2-induced PKC isoform-targeted anti-leishmanial therapy.

Apoptosis Triggered by ORF3 Proteins of the Circoviridae Family

Apoptosis, a form of the programmed cell death, is an indispensable defense mechanism regulating cellular homeostasis and is triggered by multiple stimuli. Because of the regulation of apoptosis in cellular homeostasis, viral proteins with apoptotic activity are particular foci of on antitumor therapy. One representative viral protein is the open reading frame 3 (ORF3) protein, also named as apoptin in the Circoviridae chicken anemia virus (CAV), and has the ability to induce tumor-specific apoptosis. Proteins encoded by ORF3 in other circovirus species, such as porcine circovirus (PCV) and duck circovirus (DuCV), have also been reported to induce apoptosis, with subtle differences in apoptotic activity based on cell types. This article is aimed at reviewing the latest research advancements in understanding ORF3 protein-mediated apoptosis mechanisms of Circoviridae from three perspectives: subcellular localization, interactions with host proteins, and participation in multiple apoptotic signaling pathways, providing a scientific basis for circovirus pathogenesis and a reference on its potential anticancer function.

The Role of Apoptin in Chicken Anemia Virus Replication

Apoptin is the Vp3 protein of chicken anemia virus (CAV), which infects the thymocytes and erythroblasts in young chickens, causing chicken infectious anemia and immunosuppression. Apoptin is highly studied for its ability to selectively induce apoptosis in human tumor cells and, thus, is a protein of interest in anti-tumor therapy. CAV apoptin is known to localize to different subcellular compartments in transformed and non-transformed cells, depending on the DNA damage response, and the phosphorylation of several identified threonine residues. In addition, apoptin interacts with molecular machinery such as the anaphase promoting complex/cyclosome (APC/C) to inhibit the cell cycle and induce arrest in G2/M phase. While these functions of apoptin contribute to the tumor-selective effect of the protein, they also provide an important fundamental framework to apoptin’s role in viral infection, pathogenesis, and propagation. Here, we reviewed how the regulation, localization, and functions of apoptin contribute to the viral life cycle and postulated its importance in efficient replication of CAV. A model of the molecular biology of infection is critical to informing our understanding of CAV and other related animal viruses that threaten the agricultural industry.

Metabotropic signaling cascade involved in α4β2 nicotinic acetylcholine receptor-mediated PKCβII activation

Nicotinic acetylcholine receptors (nAChRs) belong to the ionophore receptor family, which regulates plasma membrane conductance to Na⁺, K⁺, and Ca²⁺ ions. Some studies, however, have shown that nAChRs also employ second messengers for intracellular signaling. We previously showed that α4β2 nAChR mediates the translocation of protein kinase CβII (PKCβII) from the cytoplasm to the plasma membrane, which is a typical activation marker for PKCβII. In this study, we investigated the molecular mechanisms underlying PKCβII activation through α4β2 nAChR. α4β2 nAChR is the most abundant nAChR subtype and is implicated in various brain functions and diseases. Putative α4β2 nAChR signaling components were identified by knockdown or chemical inhibition of candidate proteins, and the signaling cascade was deduced by protein interactions in predicted cellular components. α4β2 nAChR-mediated PKCβII translocation was found to occur in an ionophore activity-independent manner. Nicotinic stimulation of α4β2 nAChR activated Src in a β-arrestin1 and 14-3-3η-dependent manner. Activated Src phosphorylated the tyrosine residue(s) on Syk molecules, which in turn interacted with phospholipase C γ1 to trigger the translocation of PKCβII to the cell membrane by elevating cellular diacylglycerol levels. The activated PKCβII in turn exerted a positive feedback effect on Src activation, suggesting that α4β2 nAChR signaling is amplified by a positive feedback loop. These findings provide novel information for unveiling the previously unclear metabotropic second messenger-based signal transduction pathway of nAChRs.

Insights into the mechanism of Apoptin's exquisitely selective anti-tumor action from atomic level characterization of its conformation and dynamics

Apoptin is a 121 residue protein which forms large, soluble aggregates and possesses an exceptionally selectively cytotoxic action on cancer cells. In the accompanying paper, we described the design, production and initial characterization of an Apoptin truncated variant called H₆-ApopΔProΔLeu. Whereas both the variant and wild type protein possess similar selective cytotoxicity against cancer cells following transfection, only the variant is cytotoxic when added externally. Remarkably, as observed by gel filtration chromatography and dynamic light scattering, H₆-ApopΔProΔLeu lacks the tendency of wild type Apoptin to form large aggregates, which greatly facilitated the study of its biological properties. Here, we characterize the conformation and dynamics of H₆-ApopΔProΔLeu. Using a battery of 2D, 3D and (4,2)D NMR spectra, the essentially complete ¹H, ¹³C and ¹⁵N resonance assignments of H₆-ApopΔProΔLeu were obtained. The analysis of these data shows that the variant is an intrinsically disordered protein, which lacks a preferred conformation. This conclusion is corroborated by a lack of protection against proteolytic cleavage and hydrogen/deuterium exchange. Moreover, the CD spectra are dominated by random coil contributions. Finally, ¹H-¹⁵N NOE ratios are low, which indicates flexibility on the ps-ns time scale. Interestingly, H₆-ApopΔProΔLeu's intrinsically disordered ensemble is not significantly altered by the redox state of its Cys residues or by Thr phosphorylation, which has been proposed to play a key role in Apoptin's selective cytotoxicity. These results serve to better comprehend Apoptin's remarkably selective anticancer action and provide a framework for the future design of improved Apoptin variants.

Activation of the Chicken Anemia Virus Apoptin Protein by Chk1/2 Phosphorylation Is Required for Apoptotic Activity and Efficient Viral Replication

Chicken anemia virus (CAV) is a single-stranded circular DNA virus that carries 3 genes, the most studied of which is the gene encoding VP3, also known as apoptin. This protein has been demonstrated to specifically kill transformed cells while leaving normal cells unharmed in a manner that is independent of p53 status. Although the mechanistic basis for this differential activity is unclear, it is evident that the subcellular localization of the protein is important for the difference. In normal cells, apoptin exists in filamentous networks in the cytoplasm, whereas in transformed cells, apoptin is present in the nucleus and appears as distinct foci. We have previously demonstrated that DNA damage signaling through the ataxia telangiectasia mutated (ATM) pathway induces the translocation of apoptin from the cytoplasm to the nucleus, where it induces apoptosis. We found that apoptin contains four checkpoint kinase consensus sites and that mutation of either threonine 56 or 61 to alanine restricts apoptin to the cytoplasm. Furthermore, treatment of tumor cells expressing apoptin with inhibitors of checkpoint kinase 1 (Chk1) and Chk2 causes apoptin to localize to the cytoplasm. Importantly, silencing of Chk2 rescues cancer cells from the cytotoxic effects of apoptin. Finally, treatment of virus-producing cells with Chk inhibitor protects them from virus-mediated toxicity and reduces the titer of progeny virus. Taken together, our results indicate that apoptin is a sensor of DNA damage signaling through the ATM-Chk2 pathway, which induces it to migrate to the nucleus during viral replication.

IMPORTANCE

The chicken anemia virus (CAV) protein apoptin is known to induce tumor cell-specific death when expressed. Therefore, understanding its regulation and mechanism of action could provide new insights into tumor cell biology. We have determined that checkpoint kinase 1 and 2 signaling is important for apoptin regulation and is a likely feature of both tumor cells and host cells producing virus progeny. Inhibition of checkpoint signaling prevents apoptin toxicity in tumor cells and attenuates CAV replication, suggesting it may be a future target for antiviral therapy.

Apoptin interacts with and regulates the activity of protein kinase C beta in cancer cells

Apoptin, the VP3 protein from chicken anaemia virus (CAV), induces tumour cell-specific cell death and represents a potential future anti-cancer therapeutic. In tumour but not in normal cells, Apoptin is phosphorylated and translocates to the nucleus, enabling its cytotoxic activity. Recently, the β isozyme of protein kinase C (PKCβ) was shown to phosphorylate Apoptin in multiple myeloma cell lines. However, the exact mechanism and nature of interaction between PKCβ and Apoptin remain unclear. Here we investigated the physical and functional link between PKCβ and CAV-Apoptin as well as with the recently identified Apoptin homologue derived from human Gyrovirus (HGyV). In contrast to HCT116 colorectal cancer cells the normal colon mucosa cell lines expressed low levels of PKCβI and showed reduced Apoptin activation, as evident by cytoplasmic localisation, decreased phosphorylation and lack of cytotoxic activity. Co-immunoprecipitation and proximity ligation assay studies identified binding of both CAV- and HGyV-Apoptin to PKCβI in HCT116 cells. Using Apoptin deletion constructs the N-terminal domain of Apoptin was found to be required for interacting with PKCβI. FRET-based PKC activity reporter assays by fluorescence lifetime imaging microscopy showed that expression of Apoptin in cancer cells but not in normal cells triggers a significant increase in PKC activity. Collectively, the results demonstrate a novel cancer specific interplay between Apoptin and PKCβI. Direct interaction between the two proteins leads to Apoptin-induced activation of PKC and consequently activated PKCβI mediates phosphorylation of Apoptin to promote its tumour-specific nuclear translocation and cytotoxic function.

Viral genes as oncolytic agents for cancer therapy

Many viruses have the ability to modulate the apoptosis, and to accomplish it; viruses encode proteins which specifically interact with the cellular signaling pathways. While some viruses encode proteins, which inhibit the apoptosis or death of the infected cells, there are viruses whose encoded proteins can kill the infected cells by multiple mechanisms, including apoptosis. A particular class of these viruses has specific gene(s) in their genomes which, upon ectopic expression, can kill the tumor cells selectively without affecting the normal cells. These genes and their encoded products have demonstrated great potential to be developed as novel anticancer therapeutic agents which can specifically target and kill the cancer cells leaving the normal cells unharmed. In this review, we will discuss about the viral genes having specific cancer cell killing properties, what is known about their functioning, signaling pathways and their therapeutic applications as anticancer agents.

Antitumor effects of a dual cancer-specific oncolytic adenovirus on colorectal cancer in vitro and in vivo

The efficacy and specificity of treatment are major challenges for cancer gene therapy. Oncolytic virotherapy is an attractive drug delivery platform for cancer gene therapy. In the present study, the dual-specific antitumor oncolytic adenovirus, Ad-Apoptin-hTERT-E1a, was used to infect SW1116 human colorectal carcinoma (CRC) cell lines and CT26 mouse-CRC-cell bearing BALB/c mouse models for testing antitumor effects in vitro and in vivo. The in vitro assays revealed that infection with Ad-Apoptin-hTERT-E1a induced a significant cytotoxic effect on the CRC cell line, SW1116; however, the normal human cell line, GES, was only slightly inhibited by the recombinant adenovirus. Acridine orange and ethidium bromide staining and an annexin V assay indicated that infection of SW1116 cells with Ad-Apoptin-hTERT-E1a resulted in a significant induction of apoptosis. Furthermore, western blotting and flow cytometry revealed a decrease in the mitochondrial membrane potential (MMP), the release of cytochrome c and the activation of caspase 3, 6 and 7 in Ad-Apoptin-hTERT-E1a-infected SW1116 cells. In the animal models, Ad-Apoptin-hTERT-E1a was shown to significantly inhibit tumor growth and extend the survival times of the animals. Therefore, the experimental results indicated that Ad-Apoptin-hTERT-E1a has potential for application in tumor gene therapy.

Signalling of Apoptin

The virus-derived protein Apoptin has the ability to induce p53-independent apoptosis in a variety of human cancer cells while leaving normal cells unharmed. It thus represents a potential anti-cancer therapeutic agent of the future but a proper understanding of Apoptin-induced signalling events is necessary prior to clinical application. The tumor-specific nuclear translocation and phosphorylation of Apoptin by a cellular kinase such as protein kinase C seem to be required for its function but otherwise the mode of tumor selectivity remains unknown. Apoptin has been shown to interact with several cellular proteins including Akt and the anaphase-promoting complex that regulate its activity and promote caspase-dependent apoptosis. This chapter summarizes the available data on tumor-specific pathways sensed by Apoptin and the mechanism of Apoptin-induced cell death.

pH-sensitive degradable nanoparticles for highly efficient intracellular delivery of exogenous protein

BACKGROUND

Encapsulating exogenous proteins into a nanosized particulate system for delivery into cells is a great challenge. To address this issue, we developed a novel nanoparticle delivery method that differs from the nanoparticles reported to date because its core was composed of cross-linked dextran glassy nanoparticles which had pH in endosome-responsive environment and the protein was loaded in the core of cross-linked dextran glassy nanoparticles.

METHODS

In this study, dextran in a poly(ethylene glycol) aqueous two-phase system created a different chemical environment in which proteins were encapsulated very efficiently (84.3% and 89.6% for enhanced green fluorescent protein and bovine serum albumin, respectively) by thermodynamically favored partition. The structures of the nanoparticles were confirmed by confocal laser scanning microscopy and scanning electron microscopy.

RESULTS

The nanoparticles had a normal size distribution and a mean diameter of 186 nm. MTT assays showed that the nanoparticles were nontoxic up to a concentration of 2000 μg/mL in human hepatocarcinoma cell line SMMC-7721, HeLa, and BRL-3A cells. Of note, confocal laser scanning microscopy studies showed that nanoparticles loaded with fluorescein isothiocyanate-bovine serum albumin were efficiently delivered and released proteins into the cytoplasm of HeLa cells. Flow cytometry and terminal deoxynucleotidyl transferase dUTP nick end labeling assays showed that nanoparticles with a functional protein (apoptin) efficiently induced significant tumor cell apoptosis, which was confirmed by DAPI staining.

CONCLUSION

Our findings indicate that these nanoparticles meet the high demands for delivering protein medicines and have great potential in protein therapy.

The role of CDK1 in apoptin-induced apoptosis in hepatocellular carcinoma cells

Apoptin, a small protein derived from the chicken anemia virus, specifically induces apoptosis in transformed cells or tumor cells but not in normal cells. Thus, apoptin is involved in a general, tumor-specific pathway. Apoptin-induced apoptosis presumably requires additional interaction partners that activate specific signaling pathways in cancer cells. A number of molecules interact with apoptin and play an important role in the nuclear localization of apoptin or its tumor-selective cytotoxicity. Our data indicated that apoptin selectively kills HepG2 hepatocellular carcinoma (HCC) cells but has no effect on the normal liver cell line HL-7702. Analyses of human HCC tissue samples confirmed that CDK1 (cyclin-dependent kinase 1) activity was detected in primary malignancies but not in healthy paraneoplastic tissues. shRNA knockdown of CDK1 significantly reduced the tumor-specific killing effects of apoptin, suggesting that CDK1 plays an important role in the regulation of apoptin-induced apoptosis. Furthermore, the majority of apoptin translocated to the cytoplasm from the nucleus after knockdown of CDK1. Collectively, our results revealed for the first time that apoptin interacts with CDK1 in the complex process of tumorigenesis. The link between CDK1 and apoptin may be a novel cellular signaling pathway to modulate apoptosis in cancer; therefore, apoptin may have pharmacological potential to be directly employed for cancer therapy.

Apoptin induces apoptosis by changing the equilibrium between the stability of TAp73 and ΔNp73 isoforms through ubiquitin ligase PIR2

Apoptin, a protein derived from the chicken anaemia virus, induces cell death in various cancer cells but shows little or no cytotoxicity in normal cells. The mechanism of apoptin-induced cell death is currently unknown but it appears to induce apoptosis independent of p53 status. Here we show that p73, a p53 family member, is important in apoptin-induced apoptosis. In p53 deficient and/or mutated cells, apoptin induced the expression of TAp73 leading to the induction of apoptosis. Knockdown of p73 using siRNA resulted in a significant reduction in apoptin-induced cytotoxicity. The p53 and p73 pro-apoptotic target PUMA plays an important role in apoptin-induced cell death as knockdown of PUMA significantly reduced cell sensitivity to apoptin. Importantly, apoptin expression resulted in a marked increase in TAp73 protein stability. Investigation into the mechanisms of TAp73 stability showed that apoptin induced the expression of the ring finger domain ubiquitin ligase PIR2 which is involved in the degradation of the anti-apoptotic ∆Np73 isoform. Collectively, our results suggest a novel mechanism of apoptin-induced apoptosis through increased TAp73 stability and induction of PIR2 resulting in the degradation of ∆Np73 and activation of pro-apoptotic targets such as PUMA causing cancer cell death.

Human Gyrovirus Apoptin shows a similar subcellular distribution pattern and apoptosis induction as the chicken anaemia virus derived VP3/Apoptin

The chicken anaemia virus-derived protein Apoptin/VP3 (CAV-Apoptin) has the important ability to induce tumour-selective apoptosis in a variety of human cancer cells. Recently the first human Gyrovirus (HGyV) was isolated from a human skin swab. It shows significant structural and organisational resemblance to CAV and encodes a homologue of CAV-Apoptin/VP3. Using overlapping primers we constructed a synthetic human Gyrovirus Apoptin (HGyV-Apoptin) fused to green fluorescent protein in order to compare its apoptotic function in various human cancer cell lines to CAV-Apoptin. HGyV-Apoptin displayed a similar subcellular expression pattern as observed for CAV-Apoptin, marked by translocation to the nucleus of cancer cells, although it is predominantly located in the cytosol of normal human cells. Furthermore, expression of either HGyV-Apoptin or CAV-Apoptin in several cancer cell lines triggered apoptosis at comparable levels. These findings indicate a potential anti-cancer role for HGyV-Apoptin.

Modeling of molecular interaction between apoptin, BCR-Abl and CrkL--an alternative approach to conventional rational drug design

In this study we have calculated a 3D structure of apoptin and through modeling and docking approaches, we show its interaction with Bcr-Abl oncoprotein and its downstream signaling components, following which we confirm some of the newly-found interactions by biochemical methods. Bcr-Abl oncoprotein is aberrantly expressed in chronic myelogenous leukaemia (CML). It has several distinct functional domains in addition to the Abl kinase domain. The SH3 and SH2 domains cooperatively play important roles in autoinhibiting its kinase activity. Adapter molecules such as Grb2 and CrkL interact with proline-rich region and activate multiple Bcr-Abl downstream signaling pathways that contribute to growth and survival. Therefore, the oncogenic effect of Bcr-Abl could be inhibited by the interaction of small molecules with these domains. Apoptin is a viral protein with well-documented cancer-selective cytotoxicity. Apoptin attributes such as SH2-like sequence similarity with CrkL SH2 domain, unique SH3 domain binding sequence, presence of proline-rich segments, and its nuclear affinity render the molecule capable of interaction with Bcr-Abl. Despite almost two decades of research, the mode of apoptin's action remains elusive because 3D structure of apoptin is unavailable. We performed in silico three-dimensional modeling of apoptin, molecular docking experiments between apoptin model and the known structure of Bcr-Abl, and the 3D structures of SH2 domains of CrkL and Bcr-Abl. We also biochemically validated some of the interactions that were first predicted in silico. This structure-property relationship of apoptin may help in unlocking its cancer-selective toxic properties. Moreover, such models will guide us in developing of a new class of potent apoptin-like molecules with greater selectivity and potency.

Development and application of an in vitro apoptin kinase assay

Apoptin, a protein derived from chicken anemia virus (CAV), induces apoptosis selectively in human tumor cells as compared with normal cells. This activity depends on phosphorylation and relocation of apoptin to the nucleus of cancer cells. Here, we describe an in vitro kinase assay that allows the biochemical characterization of apoptin kinase activity in tumor cells. The kinase phosphorylates apoptin in a strictly ATP-dependent fashion and in a broad salt range. The kinase activity is present constitutively in both cytoplasm and nucleus of various human tumor cells. Q-column chromatography showed that both cytoplasmic and nuclear fractions have identical fractionation characteristics, suggesting that the same kinase is present in both cellular compartments. Kinase activity derived from positive Q-column fractions bound to amylose-maltose-binding protein (MBP)-apoptin and could be eluted with ATP only in the presence of the cofactor Mg(2+). Apparently, unphosphorylated apoptin interacts with the kinase and is released only after phosphorylation has occurred, proving that our assay recognizes the genuine apoptin kinase. This is further corroborated by the finding that apoptin is phosphorylated in vitro at positions Thr108 and Thr107, in concert with earlier in vivo observations. Our assay excludes cyclin-dependent kinase 2 (CDK2) and protein kinase C beta (PKC-β), previously nominated by two separate studies as being the genuine apoptin kinase.

DNA damage response signaling triggers nuclear localization of the chicken anemia virus protein Apoptin

The chicken anemia virus (CAV) protein Apoptin is a small, 13.6-kDa protein that has the intriguing activity of inducing G(2)/M arrest and apoptosis specifically in cancer cells by a mechanism that is independent of p53. The activity of Apoptin is regulated at the level of localization. Whereas Apoptin is cytoplasmic in primary cells and does not affect cell growth, in transformed cells it localizes to the nucleus, where it induces apoptosis. The properties of cancer cells that are responsible for activating the proapoptotic activities of Apoptin remain unclear. In the current study, we show that DNA damage response (DDR) signaling is required to induce Apoptin nuclear localization in primary cells. Induction of DNA damage in combination with Apoptin expression was able to induce apoptosis in primary cells. Conversely, chemical or RNA interference (RNAi) inhibition of DDR signaling by ATM and DNA-dependent protein kinase (DNA-PK) was sufficient to cause Apoptin to localize in the cytoplasm of transformed cells. Furthermore, the nucleocytoplasmic shuttling activity of Apoptin is required for DDR-induced changes in localization. Interestingly, nuclear localization of Apoptin in primary cells was able to inhibit the formation of DNA damage foci containing 53BP1. Apoptin has been shown to bind and inhibit the anaphase-promoting complex/cyclosome (APC/C). We observe that Apoptin is able to inhibit formation of DNA damage foci by targeting the APC/C-associated factor MDC1 for degradation. We suggest that these results may point to a novel mechanism of DDR inhibition during viral infection.

Naturally occurring, tumor-specific, therapeutic proteins

The emerging approach to cancer treatment known as targeted therapies offers hope in improving the treatment of therapy-resistant cancers. Recent understanding of the molecular pathogenesis of cancer has led to the development of targeted novel drugs such as monoclonal antibodies, small molecule inhibitors, mimetics, antisense and small interference RNA-based strategies, among others. These compounds act on specific targets that are believed to contribute to the development and progression of cancers and resistance of tumors to conventional therapies. Delivered individually or combined with chemo- and/or radiotherapy, such novel drugs have produced significant responses in certain types of cancer. Among the most successful novel compounds are those which target tyrosine kinases (imatinib, trastuzumab, sinutinib, cetuximab). However, these compounds can cause severe side-effects as they inhibit pathways such as epidermal growth factor receptor (EGFR) or platelet-derived growth factor receptor, which are also important for normal functions in non-transformed cells. Recently, a number of proteins have been identified which show a remarkable tumor-specific cytotoxic activity. This toxicity is independent of tumor type or specific genetic changes such as p53, pRB or EGFR aberrations. These tumor-specific killer proteins are either derived from common human and animal viruses such as E1A, E4ORF4 and VP3 (apoptin) or of cellular origin, such as TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) and MDA-7 (melanoma differentiation associated-7). This review aims to present a current overview of a selection of these proteins with preferential toxicity among cancer cells and will provide an insight into the possible mechanism of action, tumor specificity and their potential as novel tumor-specific cancer therapeutics.

Targeting the anaphase promoting complex: common pathways for viral infection and cancer therapy

INTRODUCTION

The anaphase promoting complex/cyclosome (APC/C) is a ubiquitin ligase involved in regulation of the cell cycle through ubiquitination-dependent substrate proteolysis. Many viral proteins have been shown to interact with the APC/C, derailing cell cycle progression in order to facilitate their own replication. Induction of G(2)/M arrest by viral APC/C inhibition can lead to apoptotic cell death. Some viral proteins cause cytotoxicity specifically in tumour cells, providing evidence that targeting the APC/C could be exploited to selectively eliminate cancer cells.

AREAS COVERED

In this review, we provide a summary of studies from viral APC/C interactions over the last decade, as well as recent discoveries identifying the APC/C as a promising target in the context of cancer therapy.

EXPERT OPINION

Current therapeutic strategies inducing mitotic arrest rely on activation of the spindle assembly checkpoint (SAC) for their function. Many cancer cells have a weakened SAC and escape apoptosis through mitotic slippage. Recent evidence has demonstrated that targeting the APC/C, particularly the co-activator Cdc20, might be a better alternative. Tumour cells display greater dependency on APC/C function than normal cells and oncogenic transformation can lead to increased mitotic stress, rendering cancer cells more vulnerable to APC/C inhibition.