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Apoptotic Signaling Pathways in Lung Cancer

      Tumor cells may respond to chemo- or radiotherapy by activation of several cellular signaling cascades that influence cell survival and cell death, including activation of cell cycle arrest, senescence or triggering of several cell death types (i.e., mitotic catastrophe, necrosis, or apoptosis).
      • Johnstone RW
      • Ruefli AA
      • Lowe SW
      Apoptosis: a link between cancer genetics and chemotherapy.
      • Brown JM
      • Attardi LD
      The role of apoptosis in cancer development and treatment response.
      However, tumor cells derived from solid tumors are often refractory to therapy or develop resistance during the treatment course. This is illustrated by non-small cell lung cancer (NSCLC), which shows a high degree of intrinsic resistance, and by small cell lung cancer (SCLC), which often develops resistance to treatment during the course of disease.
      • Joseph B
      • Lewensohn R
      • Zhivotovsky B
      Role of apoptosis in the response of lung carcinomas to anti-cancer treatment.
      In part, defective apoptotic signaling may contribute to chemo-resistance and radiotherapy resistance.
      • Johnstone RW
      • Ruefli AA
      • Lowe SW
      Apoptosis: a link between cancer genetics and chemotherapy.
      However, it is also likely that other cell death modes (e.g., necrosis, autophagy, and mitotic catastrophe) and premature senescence are of equal importance for efficient tumor cell death in response to chemo- and radiotherapy.
      • Brown JM
      • Attardi LD
      The role of apoptosis in cancer development and treatment response.
      In this article, we give a brief overview of the main apoptotic signaling pathways and their deregulation in lung cancer (LC), and we provide some examples of apoptosis-based therapies.
      Apoptosis is distinguished by some morphological characteristics (i.e., plasma membrane blebbing, cell shrinkage, condensation/fragmentation of the chromatin, and disintegration of the cell into apoptotic bodies). All these characteristics are effects of selective proteolysis of proteins involved in cell signaling, DNA repair, or structural maintenance of DNA integrity, carried out by caspases, a group of cystein-aspartate enzymes.
      • Nicholson DW
      Caspase structure, proteolytic substrates, and function during apoptotic cell death.
      Caspases are classified as initiator caspases (caspase-2, -8, -9, and -10 within human cells), which, upon activation, cleave and activate the second group, the effector caspases (mainly caspase-3, -6, and -7 within human cells), then perform selective proteolysis.
      • Nicholson DW
      Caspase structure, proteolytic substrates, and function during apoptotic cell death.
      Caspases are activated either by death receptor (DR) activation (extrinsic) or via mitochondrial release of apoptogenic proteins (e.g., cytochrome c, smac/DIABLO, and HtrA2/Omi) (intrinsic) (Figure 1). The signals propagated by the intrinsic pathway may also be generated in cell nuclei or lysosomes or within the endoplasmatic reticulum (Fig. 1).
      • Kroemer G
      • Jaattela M
      Lysosomes and autophagy in cell death control.
      • Norbury CJ
      • Zhivotovsky B
      DNA damage-induced apoptosis.
      • Orrenius S
      • Zhivotovsky B
      • Nicotera P
      Regulation of cell death: the calcium-apoptosis link.
      Figure thumbnail gr1
      FIGURE 1Apoptotic signaling pathways. Caspases are activated by extrinsic (death receptor-mediated) or intrinsic pathway (mitochondria-mediated) signaling. Death ligands bind to their receptors (Fas-L and Fas-R) and via death domain (DD) adaptor proteins (FADD) are bound. Via a death effector domain (DED), the adaptor protein recruits pro-caspase-8, which gets activated. Caspase-8 thereafter directly activates pro-caspase-3, which cleaves structural proteins and inhibitor of caspase-activated DNase (ICAD), resulting in free caspase-activated DNase (CAD), which causes fragmentation of nuclear DNA. To amplify the signal, caspase-8 may also cleave Bid into t-Bid, which can initiate mitochondria-mediated signaling. The mitochondria-mediated pathway results in increased mitochondrial outer membrane permeability (MOMP) and release of apoptogenic proteins to cytosol (cytochrome c and Smac/DIABLO). The Bcl-2 family proteins (Bcl-2, Bcl-XL, Bak, Bax, Bad, and Bid) in part control MOMP. Within cytosol, cytochrome c forms a complex together with Apaf-1 the apoptosome, in which pro-caspase-9 is activated. Activated caspase-9 then triggers pro-caspase-3 activation. At several levels, caspase processing and/or activity can be inhibited by inhibitor of apoptosis proteins (IAPs) or by heat shock proteins (HSPs). IAPs are antagonized by Smac/DIABLO. Apoptotic signaling is also influenced by growth factor receptor signaling (exemplified by the Akt-pathway), which blocks Bad function by inducing binding to 14-3-3 proteins. Apoptotic signals can also be initiated at other places within the cell, (exemplified by the cell nuclei), in which p53, PUMA, NOXA, and caspase-2 gets activated on DNA damage and transmit pro-apoptotic signals to mitochondria.
      In the extrinsic caspase activation pathway, TNF superfamily ligands bind to DRs, causing oligomerization of DRs and recruitment of adaptor proteins via a death domain. In turn, adaptor proteins bind pro-caspase-8 via a death effector domain (DED) allowing pro-caspase-8 to be activated, an event that is critically dependent on the adaptor recruitment domain in the pro-caspase-8.
      • Pop C
      • Timmer J
      • Sperandio S
      • Salvesen GS
      The apoptosome activates caspase-9 by dimerization.
      Active caspase-8 then either activates pro-caspase-3 directly or amplifies the signal through Bid-cleavage (Figure 1).
      • Garrido C
      • Galluzzi L
      • Brunet M
      • Puig PE
      • Didelot C
      • Kroemer G
      Mechanisms of cytochrome c release from mitochondria.
      In the intrinsic pathway, apoptotic signals trigger increased mitochondrial outer membrane permeability (MOMP), followed by selective release of apoptogenic proteins from the mitochondrial inter membrane space to the cytosol (e.g., cytochrome c, Smac/DIABLO, and HtrA2/omi), all which promote caspase activation (Fig. 1).
      • Garrido C
      • Galluzzi L
      • Brunet M
      • Puig PE
      • Didelot C
      • Kroemer G
      Mechanisms of cytochrome c release from mitochondria.
      Cytosolic cytochrome c forms a complex with apoptosis protease-activating factor 1 (Apaf-1) and dATP (i.e., the apoptosome), in which the dimerization of pro-caspase-9 occurs, allowing its activation into caspase-9. This is followed by pro-caspase-3 activation.
      • Pop C
      • Timmer J
      • Sperandio S
      • Salvesen GS
      The apoptosome activates caspase-9 by dimerization.
      For these to efficiently result in apoptotic propagation, the concomitant alleviation of the caspase-blocking effect of inhibitor of apoptosis proteins (IAPs) is required. Hence, the release of Smac/DIABLO and HtrA2/omi, both which block IAPs and both which are released as a consequence of increased MOMP, leads to increased caspase-3 activity.
      • Garrido C
      • Galluzzi L
      • Brunet M
      • Puig PE
      • Didelot C
      • Kroemer G
      Mechanisms of cytochrome c release from mitochondria.
      In part, MOMP is controlled by Bcl-2 family proteins, and the anti-apoptotic members Bcl-xL and Bcl-2 both inhibit MOMP. Accordingly, pro-apoptotic members such as Bak or Bax, both which are activated by some BH3-only proteins (Bid, Bim, Bad, PUMA, and NOXA), can promote MOMP.
      • Puthalakath H
      • Strasser A
      Keeping killers on a tight leash: transcriptional and post-translational control of the pro-apoptotic activity of BH3-only proteins.
      An important regulator of mitochondria-mediated signaling is the tumor suppressor p53. Thus, p53 may induce expression of pro-apoptotic proteins (e.g., Bax, PUMA, Apaf-1) and/or repress anti-apoptotic proteins, including Bcl-2, in response to DNA damage.
      • Vousden KH
      • Lu X
      Live or let die: the cell's response to p53.
      In addition, p53 can re-localize to cytosol and in the same way as BH3-only proteins trigger Bak and/or Bax activation.
      • Leu JI
      • Dumont P
      • Hafey M
      • Murphy ME
      • George DL
      Mitochondrial p53 activates Bak and causes disruption of a Bak-Mcl1 complex.
      • Chipuk JE
      • Kuwana T
      • Bouchier-Hayes L
      • et al.
      Direct activation of Bax by p53 mediates mitochondrial membrane permeabilization and apoptosis.

      CASPASE ACTIVITY ADJUSTERS

      Caspase activity can be restrained by inhibitor of apoptosis proteins (IAPs), by heat shock proteins (HSPs), or by changes in protein kinase signaling. Briefly, IAPs (cIAP-1,-2, XIAP, and survivin) cause a structural block within the substrate-binding pocket of caspases, which impede substrate binding and target the bound caspases for proteosomal degradation.
      • Schimmer AD
      Inhibitor of apoptosis proteins: translating basic knowledge into clinical practice.
      HSPs (HSP90, HSP70, HSP60, and HSP27) can block caspase activity through sequestration of cytochrome c, inhibition of Bid redistribution, or Akt dephosphorylation or by blocking Apaf-1–mediated pro-caspase-9 or -3 activation.
      • Parcellier A
      • Gurbuxani S
      • Schmitt E
      • Solary E
      • Garrido C
      Heat shock proteins, cellular chaperones that modulate mitochondrial cell death pathways.
      The phosphatidylinositol 3-kinase (PI3-K)/Akt-dependent pathway and the Ras-activated mitogen-activated protein kinase (MAPK) pathway both influence apoptotic propensity.
      • Davis RJ
      Signal transduction by the JNK group of MAP kinases.
      Although Akt and MAPK/ERK mainly are activated by growth factors and inhibit pro-apoptotic signaling, the MAPKs p38 and JNK can also be activated in response to cellular stress (e.g., DNA damaging treatments) and be either pro- or anti-apoptotic depending on stimuli, duration, and cell type.
      • Davis RJ
      Signal transduction by the JNK group of MAP kinases.
      Both Akt and ERK may inhibit Bad or caspase-9 or antagonize Bim.
      • Puthalakath H
      • Strasser A
      Keeping killers on a tight leash: transcriptional and post-translational control of the pro-apoptotic activity of BH3-only proteins.
      • Ye K
      PIKE GTPase-mediated nuclear signalings promote cell survival.
      • Greer EL
      • Brunet A
      FOXO transcription factors at the interface between longevity and tumor suppression.
      JNK is known to regulate cytochrome c release in part by promoting Bax /Bak activation or complex formation or by inhibiting Bcl-2/Bcl-xL.
      • Tournier C
      • Hess P
      • Yang DD
      • et al.
      Requirement of JNK for stress-induced activation of the cytochrome c-mediated death pathway.
      • Lei K
      • Davis RJ
      JNK phosphorylation of Bim-related members of the Bcl2 family induces Bax-dependent apoptosis.
      • Ihrlund LS
      • Hernlund E
      • Viktorsson K
      • et al.
      Two distinct steps of Bak regulation during apoptotic stress signaling: different roles of MEKK1 and JNK1.
      • Yamamoto K
      • Ichijo H
      • Korsmeyer SJ
      BCL-2 is phosphorylated and inactivated by an ASK1/Jun N-terminal protein kinase pathway normally activated at G(2)/M.
      • Donovan N
      • Becker EB
      • Konishi Y
      • Bonni A
      JNK phosphorylation and activation of BAD couples the stress-activated signaling pathway to the cell death machinery.
      • Kharbanda S
      • Saxena S
      • Yoshida K
      • et al.
      Translocation of SAPK/JNK to mitochondria and interaction with Bcl-x(L) in response to DNA damage.
      JNK also promotes expression of Bak, Bax, and Bim.
      • Papadakis ES
      • Finegan KG
      • Wang X
      • et al.
      The regulation of Bax by c-Jun N-terminal protein kinase (JNK) is a prerequisite to the mitochondrial-induced apoptotic pathway.
      In a similar way, p38 controls apoptotic signaling by antagonizing or promoting the Bcl-2 family proteins.
      • Cai B
      • Chang S
      • Becker EB
      • Bonni A
      • Xia Z
      p38 MAP kinase mediates apoptosis through phosphorylation of Bimel at Ser65.
      • Grethe S
      • Coltella N
      • Di Renzo MF
      • Porn-Ares MI
      p38 MAPK downregulates phosphorylation of Bad in doxorubicin-induced endothelial apoptosis.
      • Choi SY
      • Kim MJ
      • Kang CM
      • et al.
      Activation of Bak and Bax through c-abl-protein kinase Cdelta-p38 MAPK signaling in response to ionizing radiation in human non-small cell lung cancer cells.

      ALTERATIONS IN APOPTOTIC SIGNALING PATHWAYS IN LUNG CANCER

      Loss of pro-caspase-8, FasL, or DRs expression (i.e., FasR or TRAIL-receptor 1) is all reported in SCLC.
      • Joseph B
      • Ekedahl J
      • Sirzen F
      • Lewensohn R
      • Zhivotovsky B
      Differences in expression of pro-caspases in small cell and non-small cell lung carcinoma.
      • Hopkins-Donaldson S
      • Ziegler A
      • Kurtz S
      • et al.
      Silencing of death receptor and caspase-8 expression in small cell lung carcinoma cell lines and tumors by DNA methylation.
      Moreover, increased expression of c-FLIP, a non-cleavable homologue to caspase-8, was observed in SCLC.
      • Shivapurkar N
      • Reddy J
      • Matta H
      • et al.
      Loss of expression of death-inducing signaling complex (DISC) components in lung cancer cell lines and the influence of MYC amplification.
      In a patient with NSCLC, material consisting of approximately 100 specimens, somatic mutations of TRAIL receptor 2 were found in approximately 10% of the patients.
      • Lee SH
      • Shin MS
      • Kim HS
      • et al.
      Alterations of the DR5/TRAIL receptor 2 gene in non-small cell lung cancers.
      The TRAIL receptor 2 mutations reported were located in the death domain, a region required for appropriate apoptotic signaling.
      • Lee SH
      • Shin MS
      • Kim HS
      • et al.
      Alterations of the DR5/TRAIL receptor 2 gene in non-small cell lung cancers.
      However, if the mutations in TRAIL receptor 2 influenced the patient response to chemo- or radiotherapy or overall survival remains to be examined. Decreased expression of Apaf-1 was reported in NSCLC tumors compared with normal lung, whereas pro-caspase-9 and -3 were up-regulated.
      • Krepela E
      • Prochazka J
      • Fiala P
      • Zatloukal P
      • Selinger P
      Expression of apoptosome pathway-related transcripts in non-small cell lung cancer.
      Enhanced expression of pro-caspase-3 was associated with poor prognosis in resected NSCLC,
      • Takata T
      • Tanaka F
      • Yamada T
      • et al.
      Clinical significance of caspase-3 expression in pathologic-stage I, nonsmall-cell lung cancer.
      whereas increased caspase-3 expression and/or activity was associated with increased survival in another study.
      • Volm M
      • Mattern J
      • Koomagi R
      Inverse correlation between apoptotic (Fas ligand, caspase-3) and angiogenic factors (VEGF, microvessel density) in squamous cell lung carcinomas.
      The localization of Apaf-1 to nucleus was also reported to predict survival in patients with early-stage NSCLC.
      • Besse B
      • Cande C
      • Spano JP
      • et al.
      Nuclear localization of apoptosis protease activating factor-1 predicts survival after tumor resection in early-stage non-small cell lung cancer.
      With respect to Bcl-2 family proteins, we reported that radioresistant NSCLC cells display no or little Bak or Bax activation compared with radiosensitive NSCLCs or SCLCs.
      • Viktorsson K
      • Ekedahl J
      • Lindebro MC
      • et al.
      Defective stress kinase and Bak activation in response to ionizing radiation but not cisplatin in a non-small cell lung carcinoma cell line.
      The prognostic value of Bcl-2 for survival and/or chemo- or radiotherapy responses among patients with lung cancer was recently reviewed.
      • Zhu CQ
      • Shih W
      • Ling CH
      • Tsao MS
      Immunohistochemical markers of prognosis in non-small cell lung cancer: a review and proposal for a multiphase approach to marker evaluation.
      Increased Bcl-2 expression was found in certain NSCLC subtypes and was proposed to have slightly good prognostic value.
      • Zhu CQ
      • Shih W
      • Ling CH
      • Tsao MS
      Immunohistochemical markers of prognosis in non-small cell lung cancer: a review and proposal for a multiphase approach to marker evaluation.
      In the same report, Bax was found not to have any prognostic value, even for chemotherapy responses.
      • Zhu CQ
      • Shih W
      • Ling CH
      • Tsao MS
      Immunohistochemical markers of prognosis in non-small cell lung cancer: a review and proposal for a multiphase approach to marker evaluation.
      The expression and prognostic value of IAPs and HSPs in NSCLC has also been reviewed.
      • Fennell DA
      Caspase regulation in non-small cell lung cancer and its potential for therapeutic exploitation.
      Briefly, c-IAP-1, XIAP, and survivin were reported to be differentially expressed in a panel of SCLC and NSCLC cell lines in a non-tumor type-dependent manner.
      • Ekedahl J
      • Joseph B
      • Grigoriev MY
      • et al.
      Expression of inhibitor of apoptosis proteins in small- and non-small-cell lung carcinoma cells.
      In contrast, c-IAP-2 was expressed at a significantly higher level in NSCLC lines compared with SCLC lines.
      • Ekedahl J
      • Joseph B
      • Grigoriev MY
      • et al.
      Expression of inhibitor of apoptosis proteins in small- and non-small-cell lung carcinoma cells.
      However, in a clinical LC material, c-IAP-1, -2, and XIAP were reported not to correlate to clinically related prognostic factors (e.g., tumor size, stage, histology, and grade) or to tumor chemotherapeutic response.
      • Ferreira CG
      • van der Valk P
      • Span SW
      • et al.
      Assessment of IAP (inhibitor of apoptosis) proteins as predictors of response to chemotherapy in advanced non-small-cell lung cancer patients.
      In contrast, a high survivin expression was reported to correlate to poor prognosis and local control after radiotherapy.
      • Choi N
      • Baumann M
      • Flentjie M
      • et al.
      Predictive factors in radiotherapy for non-small cell lung cancer: present status.
      Analysis of HSP72 and HSP27 in human NSCLC and SCLC cell lines did not reveal any correlation to radiosensitivity in the cell line panel.
      • Ekedahl J
      • Joseph B
      • Marchetti P
      • et al.
      Heat shock protein 72 does not modulate ionizing radiation-induced apoptosis in U1810 non-small cell lung carcinoma cells.
      When examining HSP27 and HSP70 in NSCLC clinical specimens, expression was found in 60% of the cases, and HSP70 expression was correlated to histopathological differentiation, clinical stages, smoking history, or lymph node metastasis.
      • Huang Q
      • Zu Y
      • Fu X
      • Wu T
      Expression of heat shock protein 70 and 27 in non-small cell lung cancer and its clinical significance.
      It is well established that lung cancer, especially NSCLC, is driven by increased growth factor signaling. Thus IGF-1R, EGF-R (erbB1), or K-Ras are all often over-expressed or constitutively active in NSCLC and/or SCLC and may cause increased anti-apoptotic signaling.
      • Reeve JG
      • Kirby LB
      • Brinkman A
      • Hughes SA
      • Schwander J
      • Bleehen NM
      Insulin-like growth-factor-binding protein gene expression and protein production by human tumour cell lines.
      • Arteaga CL
      EGF receptor mutations in lung cancer: from humans to mice and maybe back to humans.
      Moreover, we have also shown that deficiency in activation of MAPKs such as JNK and/or p38 may also contribute to impaired radiation-induced apoptotic responses.
      • Viktorsson K
      • Ekedahl J
      • Lindebro MC
      • et al.
      Defective stress kinase and Bak activation in response to ionizing radiation but not cisplatin in a non-small cell lung carcinoma cell line.
      Impeded JNK activity may result from increased expression of the phosphatase MKP1/CL100 within NSCLC cells.
      • Chattopadhyay S
      • Machado-Pinilla R
      • Manguan-Garcia C
      • et al.
      MKP1/CL100 controls tumor growth and sensitivity to cisplatin in non-small-cell lung cancer.

      APOPTOSIS-BASED THERAPIES

      Several concepts of increasing apoptotic signaling as a way to improve chemo- and radiotherapy responses have been introduced and are in preclinical development to allow clinical use or have entered into clinical trials.
      • Reed JC
      Drug insight: cancer therapy strategies based on restoration of endogenous cell death mechanisms.
      • Green DR
      • Kroemer G
      Pharmacological manipulation of cell death: clinical applications in sight?.
      One way is to reactivate death receptor signaling; this strategy has been tested for therapeutic purposes.
      • Kelley SK
      • Ashkenazi A
      Targeting death receptors in cancer with Apo2L/TRAIL.
      • Cretney E
      • Takeda K
      • Smyth MJ
      Cancer: novel therapeutic strategies that exploit the TNF-related apoptosis-inducing ligand (TRAIL)/TRAIL receptor pathway.
      TNFα was the first choice, and although it was capable of causing tumor cell kill, adverse toxic side effects abolished clinical applicability.
      • Kelley SK
      • Ashkenazi A
      Targeting death receptors in cancer with Apo2L/TRAIL.
      Instead, the TNFα-related apoptosis, inducing ligand TRAIL, which bind to TRAIL-R1 (DR4) and TRAIL-R2 (DR5/Killer), has shown promising results in the recombinant form alone or together with chemo- or radiotherapy.
      • Kelley SK
      • Ashkenazi A
      Targeting death receptors in cancer with Apo2L/TRAIL.
      Another approach that has been examined in several preclinical studies is to use agonistic TRAIL receptor monoclonal antibodies (mAbs).
      • Cretney E
      • Takeda K
      • Smyth MJ
      Cancer: novel therapeutic strategies that exploit the TNF-related apoptosis-inducing ligand (TRAIL)/TRAIL receptor pathway.
      Currently, phase I and II clinical trials with such mAbs directed against either DR4 (HGS-ETR1) or DR5 (HGS-ETR2) are ongoing for patients with NSCLC.
      • Cretney E
      • Takeda K
      • Smyth MJ
      Cancer: novel therapeutic strategies that exploit the TNF-related apoptosis-inducing ligand (TRAIL)/TRAIL receptor pathway.
      Using a tumor-specific gene delivery of the TRAIL gene to NSCLC cells in a xenograft mouse model, Chang et al.
      • Zhang X
      • Cheung RM
      • Komaki R
      • Fang B
      • Chang JY
      Radiotherapy sensitization by tumor-specific TRAIL gene targeting improves survival of mice bearing human non-small cell lung cancer.
      recently showed a radiosensitizing effect involving induction of apoptotic signaling, inhibition of tumor growth, and prolonged survival of the tumor-bearing mice.
      In preclinical lung cancer models, alleviation of IAP function using either antisense or siRNA or peptides mimicking the endogenous IAP inhibitor Smac has been tested either alone or in combination with chemo- or radiotherapy and have, to some extent, been promising.
      • Schimmer AD
      Inhibitor of apoptosis proteins: translating basic knowledge into clinical practice.
      • Yonesaka K
      • Tamura K
      • Kurata T
      • et al.
      Small interfering RNA targeting survivin sensitizes lung cancer cell with mutant p53 to adriamycin.
      • Yang L
      • Mashima T
      • Sato S
      • et al.
      Predominant suppression of apoptosome by inhibitor of apoptosis protein in non-small cell lung cancer H460 cells: therapeutic effect of a novel polyarginine-conjugated Smac peptide.
      • Bartling B
      • Lewensohn R
      • Zhivotovsky B
      Endogenously released Smac is insufficient to mediate cell death of human lung carcinoma in response to etoposide.
      Strategies in which Bcl-2/Bcl-xL expression is inhibited or BH3-mimetics applied have also been introduced. Thus, antisense against Bcl-2 (Oblimersen) has been tested in NSCLC and other tumor types and has reached phase III trials.
      • Herbst RS
      • Frankel SR
      Oblimersen sodium (Genasense bcl-2 antisense oligonucleotide): a rational therapeutic to enhance apoptosis in therapy of lung cancer.
      • Green DR
      • Kroemer G
      Pharmacological manipulation of cell death: clinical applications in sight?.
      Mutations in the p53 gene, which impede its function as a transcriptional regulator of apoptosis, are common in both NSCLC and SCLC (50% and 70%, respectively).
      • Tammemagi MC
      • McLaughlin JR
      • Bull SB
      Meta-analyses of p53 tumor suppressor gene alterations and clinicopathological features in resected lung cancers.
      Hence, one alternative to revert chemo- or radiotherapy resistance in lung cancer may therefore be to restore p53 function. This has been clinically tested in NSCLC by using wildtype-p53 gene transfer either alone or in combination with chemotherapy.
      • Lebedeva IV
      • Su ZZ
      • Sarkar D
      • Fisher PB
      Restoring apoptosis as a strategy for cancer gene therapy: focus on p53 and mda-7.
      Preclinical development of peptides or small molecules that can reactivate mutant p53 is also ongoing.
      • Green DR
      • Kroemer G
      Pharmacological manipulation of cell death: clinical applications in sight?.
      However, the clinical usefulness of such an approach awaits further studies.

      ACKNOWLEDGMENTS

      This study was supported by grants from the Swedish Cancer Society (to RL), the Stockholm Cancer Society (to RL and KV), and the Funds of the Karolinska Institutet.

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