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[Cancer Biology & Therapy 7:7, 1-2; July 2008]; ©2008 Landes Bioscience
Commentary
The art of killing
Double stroke with apoptin and survivin as a novel approach in cancer therapy
1Department of Physiology; University of Manitoba; Winnipeg, Canada; 2Manitoba Institute of Cell Biology; Cancer Care Manitoba; Winnipeg, Canada; 3Department of Human
Anatomy and Cell Science; University Manitoba; Winnipeg, Canada; 4BioApplications Enterprises; Winnipeg, Manitoba Canada
Key words: apoptin, brevinin-2R, cancer therapy, hamlet, IAPs, R4orf4, survivin
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to caspase-3 activation and apoptotic cell death. Moreover, it also
causes defects in cell division that manifest as aneuploidy, multinucleation, and supernumerary centrosomes.8 Previous experiments
that involved targeting survivin’s expression with specific ribozymes
or with antisense nucleotides induced apoptosis in various cell lines
or broke resistance to conventional anticancer drugs.6 The dual mode
of survivin’s action has attracted attention of the pharmaceutical
industry. For example, Isis Pharmaceuticals and Abbot Laboratories
are testing antisense-based approaches that interfere with the expression of survivin.9,10
Apoptin gained significant attention as a selective killer of cancer
cells, and thus it may not only serve as a lead for the development
of novel drugs, but also as a tool to delineate the critical molecular
differences between the normal cellular state and neoplasia. Apoptin
is a small (121 amino acids long, 14 kDa) basic protein encoded
by the third open reading frame (VP3) of chicken anemia virus.3
Its selective anticancer properties were discovered by the group of
Noteborn, in 90s.11 Its cancer-selective mode of action, although still
not fully understood, appears to involve multiple mechanisms.
Apoptin selectively kills both p53-positive and negative cancer
cells.3,12 The mitochondrial/apoptosome-dependent death pathway
mediates apoptin induced cell death.13-15 Apoptin induced cell death
requires PI3-K activation, nuclear transfer of Akt, and cytoplasmic
transfer of phosphorylated Nur77.15-17 Apoptin interacts with
both p85PI3-K and Akt via its proline-rich sequence.16,18 It appears
that apoptin is able to “hijack” these cell survival pathways and
redirect them to fuel apoptotic cell death. In normal cells, apoptin
localizes to the cytoplasm, whereas in cancer cells it is predominantly found in the nucleus. The preferred nuclear localization of
apoptin in malignant cells is governed by the combined actions
of an activated nuclear localization signal and the suppression of
a nuclear export signal.19 Phosphorylation of Thr-108 at apoptin’s
C-terminus appears to play an important role in the above events.
Apoptin interacts with the components of anaphase-promoting
complex and interferes with cell division.20 Exhibiting basic properties (pK ~10.6), apoptin will interact directly with heterochromatin
and with DNA ends, thus, it may interfere with gene transcription,
DNA synthesis and DNA repair.21
The choice of combining apoptin expression with mi-RNA-based
surviving inhibition for cancer therapy is brilliant as both molecules
have diverse, multiple, yet partly overlapping targets in the cell. It is
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Despite the introduction of first cancer chemotherapies in the
early 20s of the last century cancer still remains the second most
common cause of death in developed countries. Several anticancer
drugs have been introduced to the clinic, and at last count, a few
dozen more are at various stages of development. With the accumulation of knowledge about the biology of cancer in recent years,
more researchers now aim at targeting signaling pathways that are
frequently upregulated in certain types of cancer. These so called
targeted therapies, although not always effective as a single-agent
treatment, have generally low to negligible side effects.1,2 In addition
to therapeutics with known molecular targets, experimental drugs
have emerged which are more or less selectively toxic against cancer
cells but spare normal, healthy cells. However, their exact molecular
mechanisms of action are mostly unclear. This group includes viral
proteins R4orf4, apoptin, human cytokine TRIAL, and the human
lipoprotein complex, HAMLET.3,4
In the current issue of Cancer Biololgy & Therapy, Liu and
coauthors aim to develop a new anticancer therapy by combined
delivery of apoptin and micro-RNA-based inhibition of survivin.5
Both therapeutic modalities are incorporated into a single delivery
vector. This approach assures temporary- and spatially-synchronised
expression and action of both therapeutic agents.
Survivin is an atypical member of the ‘inhibitor of apoptosis
protein’ (IAP) family of proteins that inhibits both the activation and
activity of already mobilized caspases, a family of proteases involved
primarily in the propagation and execution of apoptotic cell death.
Unlike other IAP-family members, survivin’s action is more complex.
Survivin expression is upregulated in human cancers. It is associated
with chemo- and radiotherapy resistance, and linked to poor prognosis. Survivin’s expression is highest in the late G2 and the M-phase
of the cell cycle and it appears to function both as apoptosis inhibitor
and cell cycle regulator.6,7 With the onset of mitosis, survivin binds
to microtubules of the mitotic spindle apparatus and both the cyclindependent-kinase inhibitor p21Cip1/Waf1 and caspase-3 colocalize
with survivin at centrosomes. Inhibition of survivin function leads
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This manuscript has been published online, prior to printing.Once the issue is complete and page numbers have been assigned, the citation will change accordingly.
Soumya Panigrahi,1,2,* Thomas Klonisch3 and Marek Los4
*Correspondence to: Soumya Panigrahi; AUTHOR: please complete mailing address;
Email: [email protected]
Submitted: 05/19/08; Accepted: 06/23/08
Previously published online as a Cancer Biology & Therapy E-publication:
http://www.landesbioscience.com/journals/cbt/article/6492
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Cancer Biology & Therapy
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Figure 1. AUTHOR: please provide figure legend.
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15. Maddika S, Booy EP, Johar D, Gibson SB, Ghavami S, Los M. Cancer-specific toxicity of
apoptin is independent of death receptors but involves the loss of mitochondrial membrane
potential and the release of mitochondrial cell-death mediators by a Nur77-dependent
pathway. J Cell Sci 2005; 118:4485-93.
16. Maddika S, Bay GH, Kroczak TJ, Ande SR, Maddika S, Wiechec E, Gibson SB, Los M.
Akt is transferred to the nucleus of cells treated with apoptin, and it participates in apoptininduced cell death. Cell Prolif 2007; 40:835-48.
17. Maddika S, Wiechec E, Ande SR, Poon IK, Fischer U, Wesselborg S, Jans DA, SchulzeOsthoff K, Los M. Interaction with PI3-kinase contributes to the cytotoxic activity of
Apoptin. Oncogene 2007; PMID:18059340.
18. Maddika S, Wiechec E, Ande SR, Poon IK, Fischer U, Wesselborg S, Jans DA, SchulzeOsthoff K, Los M. Interaction with PI3-kinase contributes to the cytotoxic activity of
Apoptin. Oncogene 2008; 27:3060-5.
19. Poon IK, Oro C, Dias MM, Zhang J, Jans DA. Apoptin nuclear accumulation is modulated
by a CRM1-recognized nuclear export signal that is active in normal but not in tumor cells.
Cancer Res 2005; 65:7059-64.
20. Teodoro JG, Heilman DW, Parker AE, Green MR. The viral protein Apoptin associates
with the anaphase-promoting complex to induce G2/M arrest and apoptosis in the absence
of p53. Genes Dev 2004; 18:1952-7.
21. Leliveld SR, Dame RT, Mommaas MA, Koerten HK, Wyman C, Danen-van Oorschot AA,
Rohn JL, Noteborn MH, Abrahams JP. Apoptin protein multimers form distinct higherorder nucleoprotein complexes with DNA. Nucleic Acids Res 2003; 31:4805-13.
22. Asanuma H, Torigoe T, Kamiguchi K, Hirohashi Y, Ohmura T, Hirata K, Sato M, Sato N.
Survivin expression is regulated by coexpression of human epidermal growth factor receptor
2 and epidermal growth factor receptor via phosphatidylinositol 3-kinase/AKT signaling
pathway in breast cancer cells. Cancer Res 2005; 65:11018-25.
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safe to assume that the interference with survivin expression will
facilitate apoptin’s action in many ways. (i) Apoptin-induced cell
death involves caspases, thus, the absence of survivin will facilitate
their active state and ensure completion of the apoptotic process.
(ii) Among other factors, survivin expression is regulated by the
PI3-K/Akt pathway.22 The interference of apoptin with these pathways may affect survivin expression and amplify apoptin’s action.
(iii) Apoptin interacts with the anaphase-promoting complex. The
lack of survivin’s action as a guardian of mitotic spindle formation
will facilitate apoptin-triggered cell death. These are major effects
that likely occur as a result of survivin knock-down in the presence of apoptin when applied simultaneously in cancer therapy.
The molecular mechanisms of action of both survivin and apoptin
are currently the subject of intense investigation and additional
molecular events triggered by this combined therapeutic strategy
are likely to be discovered.
Proper targeting is a major issue when new gene therapies are
designed. Although its suitability in animal models remains to be
tested, the combined and simultaneous therapeutic system of miRNA-based specific knockdown of survivin, and overexpression of
apoptin described by Liu and colleagues has built-in selective advantages: (a) survivin is predominantly (if not exclusively) expressed in
rapidly dividing cells, and (b) apoptin selectively kills cancer cells.
The therapy however may carry potential complications that need
to be carefully experimentally addressed before this vector approach
is to be moved into clinical trials. For example, the inhibition of
survivin expression may sensitize also normal cells to apoptin, and
apoptin’s interference with the cell cycle may cause survivin to induce
aneuploidy and tumorigenic dedifferentiation of normal cells.
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References
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