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Apoptosis induction and tumor cell repopulation: The yin and yang of radiotherapy

The induction of tumor cell death is a central goal of radiotherapy. Surprisingly, a recent study sheds new light on this process, and the results presented by Huang et al. strongly question the benefit of radiation-induced apoptosis for the outcome of cancer radiotherapy.

In their study, Huang and coworkers describe that induction of apoptosis by radiotherapy stimulates rapid tumor cell repopulation - a process crucially dependent on caspase 3 activity [1]. These findings are of immense relevance for the clinical use of radiotherapy, particularly when combined with targeted agents aiming at radiosensibilization and enhanced apoptosis induction [2–8]. In contrast to the results presented by Huang et al., previous work has convincingly demonstrated that the increased induction of apoptotic cell death, for example by the combination of agonistic TRAIL antibodies with radiation, results in a pronounced benefit for long term tumor control in a colorectal xenograft model [9–11]. Parallel in vitro studies have revealed that enhanced caspase-mediated apoptosis is the underlying mechanism for the improved eradication of clonogenic tumor cells [10, 12]. Thus, the effect described by Huang and coworkers should be considered as a repopulation mechanism, which is of importance under specific, currently unknown circumstances. In this regard, it can be speculated that the balance between the apoptotic net cell kill, and the PGE2-driven tumor cell survival and repopulation accounts for the reported discrepancies.

Nevertheless, the study by Huang and coworkers is highly interesting, in particular because of the elegant experiments, with which the signaling cascade of apoptosis-induced tumor cell repopulation was unraveled. The downstream mechanisms identified by Huang et al. involve the caspase 3-dependent cleavage and activation of iPLA2 and the subsequent production of PGE2. During radiation-induced cell death in vitro PGE2 was shown to be released by tumor cells as well as by fibroblasts. In vivo (in experimental mouse models), both tumor and tumor stroma cells reportedly contributed to rapid tumor cell repopulation by few residual tumor cells in response to radiotherapy.

We would like to point out that the tumor stroma contains a highly interesting cell population, which might contribute to or even dominate the tumor-growth-stimulating PGE2 production: macrophages that govern the elimination of apoptosing cells and instigate tissue healing by producing a clearance-related cytokine milieu, including PGE2 [13–15]. Of note, Huang and coworkers observed that more macrophages were present in irradiated (apoptotic) tumors than in non-irradiated ones. These phagocytes have presumably been recruited by apoptotic cell-derived find-me signals, such as nucleotides and lysophosphatidylcholine, which - akin to PGE2 - are released in a caspase 3- or caspase 3- plus iPLA2-dependent manner, respectively [16–19]. Caspase 3 apparently is a key player in this context. So it should be taken into consideration that caspase 3 controls more processes than the release of PGE2 or phagocyte-recruiting attraction signals by apoptotic cells. Caspase 3 also shifts the balance between apoptosis, necrosis and autophagy as described by Huang et al., and orchestrates the central features of apoptosis, which have profound impact on macrophage activation and cytokine production after the engulfment of apoptotic cells. As such, externalization of phosphatidylserine, bleb formation and internucleosomal DNA fragmentation, known to be crucial for the subsequent anti-inflammatory cytokine production by macrophages [20–22], have been reported to depend on caspase 3 activity during apoptosis [23–27]. Thus, caspase 3-positive apoptosing cells recruit more macrophages, are more efficiently phagocytosed, and induce a stronger anti-inflammatory, wound-healing and growth promoting phagocyte response, including PGE2 production, than their caspase 3-negative counterparts. This might contribute or translate to the clinical observation by Huang et al. that elevated expression levels of activated caspase-3 were associated with a poor outcome in two patient cohorts with head and neck carcinoma or with advanced stage breast carcinoma.

Overall, Huang and coworkers suggest a scenario, in which the caspase 3-driven iPLA2-dependent PGE2 production by irradiated tumor and tumor stroma cells plays a pivotal role for radiation-induced tumor cell repopulation and for poor therapeutic outcome. We would like to add the clearance of apoptosing cells by macrophages, and the subsequently produced clearance-related anti-inflammatory milieu, including PGE2, to this model (Figure 1). Intriguingly, in their final step both processes rely on cyclooxygenase activity, thus re-opening the therapeutic perspective of cautious cyclooxygenase inhibition as an adjuvant to radiotherapy [28–30]. Up to now several clinical trials have documented that a safe combination of cyclooxygenase inhibitors (celecoxib) and radiotherapy is feasible, yet most of the trials were not adequately powered to detect meaningful differences in tumor control [31, 32]. Thus, further clinical trials, in particular phase III studies, are required to shed light onto this issue. In the same line, it should be addressed, whether caspase inhibition (upstream of PGE2 production) in combination with radiotherapy displays a benefit for the overall therapeutic outcome - provided that tumor cell systems utilizing the caspase 3-dependent, PGE2-driven tumor cell repopulation can reliably be identified.

Figure 1
figure 1

Radiotherapy-induced apoptosis leads to caspase 3-dependent tumor cell repopulation.

References

  1. Huang Q, Li F, Liu X, Li W, Shi W, Liu FF, O'Sullivan B, He Z, Peng Y, Tan AC, et al: Caspase 3-mediated stimulation of tumor cell repopulation during cancer radiotherapy. Nat Med. 2011, 17 (7): 860-866. 10.1038/nm.2385.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  2. Connell PP, Weichselbaum RR: A downside to apoptosis in cancer therapy?. Nat Med. 2011, 17 (7): 780-782. 10.1038/nm0711-780.

    Article  CAS  PubMed  Google Scholar 

  3. Itani W, Geara F, Haykal J, Haddadin M, Gali-Muhtasib H: Radiosensitization by 2-benzoyl-3-phenyl-6,7-dichloroquinoxaline 1,4-dioxide under oxia and hypoxia in human colon cancer cells. Radiat Oncol. 2007, 2: 1-10.1186/1748-717X-2-1.

    Article  PubMed Central  PubMed  Google Scholar 

  4. Liao X, Che X, Zhao W, Zhang D, Long H, Chaudhary P, Li H: Effects of propranolol in combination with radiation on apoptosis and survival of gastric cancer cells in vitro. Radiat Oncol. 2010, 5: 98-10.1186/1748-717X-5-98.

    Article  PubMed Central  PubMed  Google Scholar 

  5. Rubel A, Handrick R, Lindner LH, Steiger M, Eibl H, Budach W, Belka C, Jendrossek V: The membrane targeted apoptosis modulators erucylphosphocholine and erucylphosphohomocholine increase the radiation response of human glioblastoma cell lines in vitro. Radiat Oncol. 2006, 1: 6-10.1186/1748-717X-1-6.

    Article  PubMed Central  PubMed  Google Scholar 

  6. Rudner J, Ruiner CE, Handrick R, Eibl HJ, Belka C, Jendrossek V: The Akt-inhibitor Erufosine induces apoptotic cell death in prostate cancer cells and increases the short term effects of ionizing radiation. Radiat Oncol. 2010, 5: 108-10.1186/1748-717X-5-108.

    Article  PubMed Central  PubMed  Google Scholar 

  7. Welsh JW, Mahadevan D, Ellsworth R, Cooke L, Bearss D, Stea B: The c-Met receptor tyrosine kinase inhibitor MP470 radiosensitizes glioblastoma cells. Radiat Oncol. 2009, 4: 69-10.1186/1748-717X-4-69.

    Article  PubMed Central  PubMed  Google Scholar 

  8. Zerp SF, Stoter R, Kuipers G, Yang D, Lippman ME, van Blitterswijk WJ, Bartelink H, Rooswinkel R, Lafleur V, Verheij M: AT-101, a small molecule inhibitor of anti-apoptotic Bcl-2 family members, activates the SAPK/JNK pathway and enhances radiation-induced apoptosis. Radiat Oncol. 2009, 4: 47-10.1186/1748-717X-4-47.

    Article  PubMed Central  PubMed  Google Scholar 

  9. Marini P, Budach W, Niyazi M, Junginger D, Stickl S, Jendrossek V, Belka C: Combination of the pro-apoptotic TRAIL-receptor antibody mapatumumab with ionizing radiation strongly increases long-term tumor control under ambient and hypoxic conditions. Int J Radiat Oncol Biol Phys. 2009, 75 (1): 198-202. 10.1016/j.ijrobp.2009.04.038.

    Article  CAS  PubMed  Google Scholar 

  10. Marini P, Denzinger S, Schiller D, Kauder S, Welz S, Humphreys R, Daniel PT, Jendrossek V, Budach W, Belka C: Combined treatment of colorectal tumours with agonistic TRAIL receptor antibodies HGS-ETR1 and HGS-ETR2 and radiotherapy: enhanced effects in vitro and dose-dependent growth delay in vivo. Oncogene. 2006, 25 (37): 5145-5154.

    CAS  PubMed  Google Scholar 

  11. Marini P, Junginger D, Stickl S, Budach W, Niyazi M, Belka C: Combined treatment with lexatumumab and irradiation leads to strongly increased long term tumour control under normoxic and hypoxic conditions. Radiat Oncol. 2009, 4: 49-10.1186/1748-717X-4-49.

    Article  PubMed Central  PubMed  Google Scholar 

  12. Marini P, Jendrossek V, Durand E, Gruber C, Budach W, Belka C: Molecular requirements for the combined effects of TRAIL and ionising radiation. Radiother Oncol. 2003, 68 (2): 189-198. 10.1016/S0167-8140(03)00186-5.

    Article  CAS  PubMed  Google Scholar 

  13. Fadok VA, Bratton DL, Konowal A, Freed PW, Westcott JY, Henson PM: Macrophages that have ingested apoptotic cells in vitro inhibit proinflammatory cytokine production through autocrine/paracrine mechanisms involving TGF-beta, PGE2, and PAF. J Clin Invest. 1998, 101 (4): 890-898. 10.1172/JCI1112.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  14. Voll RE, Herrmann M, Roth EA, Stach C, Kalden JR, Girkontaite I: Immunosuppressive effects of apoptotic cells. Nature. 1997, 390 (6658): 350-351. 10.1038/37022.

    Article  CAS  PubMed  Google Scholar 

  15. Mosser DM, Edwards JP: Exploring the full spectrum of macrophage activation. Nat Rev Immunol. 2008, 8 (12): 958-969. 10.1038/nri2448.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  16. Elliott MR, Chekeni FB, Trampont PC, Lazarowski ER, Kadl A, Walk SF, Park D, Woodson RI, Ostankovich M, Sharma P, et al: Nucleotides released by apoptotic cells act as a find-me signal to promote phagocytic clearance. Nature. 2009, 461 (7261): 282-286. 10.1038/nature08296.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  17. Lauber K, Bohn E, Krober SM, Xiao YJ, Blumenthal SG, Lindemann RK, Marini P, Wiedig C, Zobywalski A, Baksh S, et al: Apoptotic cells induce migration of phagocytes via caspase-3-mediated release of a lipid attraction signal. Cell. 2003, 113 (6): 717-730. 10.1016/S0092-8674(03)00422-7.

    Article  CAS  PubMed  Google Scholar 

  18. Mueller RB, Sheriff A, Gaipl US, Wesselborg S, Lauber K: Attraction of phagocytes by apoptotic cells is mediated by lysophosphatidylcholine. Autoimmunity. 2007, 40 (4): 342-344. 10.1080/08916930701356911.

    Article  CAS  PubMed  Google Scholar 

  19. Peter C, Waibel M, Radu CG, Yang LV, Witte ON, Schulze-Osthoff K, Wesselborg S, Lauber K: Migration to apoptotic "find-me" signals is mediated via the phagocyte receptor G2A. J Biol Chem. 2008, 283 (9): 5296-5305.

    Article  CAS  PubMed  Google Scholar 

  20. Huynh ML, Fadok VA, Henson PM: Phosphatidylserine-dependent ingestion of apoptotic cells promotes TGF-beta1 secretion and the resolution of inflammation. J Clin Invest. 2002, 109 (1): 41-50.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  21. Stadler K, Frey B, Munoz LE, Finzel S, Rech J, Fietkau R, Herrmann M, Hueber A, Gaipl US: Photopheresis with UV-A light and 8-methoxypsoralen leads to cell death and to release of blebs with anti-inflammatory phenotype in activated and non-activated lymphocytes. Biochem Biophys Res Commun. 2009, 386 (1): 71-76. 10.1016/j.bbrc.2009.05.130.

    Article  CAS  PubMed  Google Scholar 

  22. Nagata S: DNA degradation in development and programmed cell death. Annu Rev Immunol. 2005, 23: 853-875. 10.1146/annurev.immunol.23.021704.115811.

    Article  CAS  PubMed  Google Scholar 

  23. Coleman ML, Sahai EA, Yeo M, Bosch M, Dewar A, Olson MF: Membrane blebbing during apoptosis results from caspase-mediated activation of ROCK I. Nat Cell Biol. 2001, 3 (4): 339-345. 10.1038/35070009.

    Article  CAS  PubMed  Google Scholar 

  24. Sebbagh M, Renvoize C, Hamelin J, Riche N, Bertoglio J, Breard J: Caspase-3-mediated cleavage of ROCK I induces MLC phosphorylation and apoptotic membrane blebbing. Nat Cell Biol. 2001, 3 (4): 346-352. 10.1038/35070019.

    Article  CAS  PubMed  Google Scholar 

  25. Enari M, Sakahira H, Yokoyama H, Okawa K, Iwamatsu A, Nagata S: A caspase-activated DNase that degrades DNA during apoptosis, and its inhibitor ICAD. Nature. 1998, 391 (6662): 43-50. 10.1038/34112.

    Article  CAS  PubMed  Google Scholar 

  26. Sakahira H, Enari M, Nagata S: Cleavage of CAD inhibitor in CAD activation and DNA degradation during apoptosis. Nature. 1998, 391 (6662): 96-99. 10.1038/34214.

    Article  CAS  PubMed  Google Scholar 

  27. Janicke RU, Sprengart ML, Wati MR, Porter AG: Caspase-3 is required for DNA fragmentation and morphological changes associated with apoptosis. J Biol Chem. 1998, 273 (16): 9357-9360. 10.1074/jbc.273.16.9357.

    Article  CAS  PubMed  Google Scholar 

  28. Mazhar D, Ang R, Waxman J: COX inhibitors and breast cancer. Br J Cancer. 2006, 94 (3): 346-350. 10.1038/sj.bjc.6602942.

    Article  PubMed Central  CAS  PubMed  Google Scholar 

  29. Solomon SD, McMurray JJ, Pfeffer MA, Wittes J, Fowler R, Finn P, Anderson WF, Zauber A, Hawk E, Bertagnolli M: Cardiovascular risk associated with celecoxib in a clinical trial for colorectal adenoma prevention. N Engl J Med. 2005, 352 (11): 1071-1080. 10.1056/NEJMoa050405.

    Article  CAS  PubMed  Google Scholar 

  30. Jendrossek V: Targeting apoptosis pathways by Celecoxib in cancer. Cancer Lett. 2011,

    Google Scholar 

  31. Gore E, Bae K, Langer C, Extermann M, Movsas B, Okunieff P, Videtic G, Choy H: Phase I/II trial of a COX-2 inhibitor with limited field radiation for intermediate prognosis patients who have locally advanced non-small-cell lung cancer: radiation therapy oncology group 0213. Clin Lung Cancer. 2011, 12 (2): 125-130. 10.1016/j.cllc.2011.03.007.

    Article  CAS  PubMed  Google Scholar 

  32. De Ruysscher D, Bussink J, Rodrigus P, Kessels A, Dirx M, Houben R, Wanders R: Concurrent celecoxib versus placebo in patients with stage II-III non-small cell lung cancer: a randomised phase II trial. Radiother Oncol. 2007, 84 (1): 23-25. 10.1016/j.radonc.2007.05.008.

    Article  CAS  PubMed  Google Scholar 

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Lauber, K., Munoz, L.E., Berens, C. et al. Apoptosis induction and tumor cell repopulation: The yin and yang of radiotherapy. Radiat Oncol 6, 176 (2011). https://doi.org/10.1186/1748-717X-6-176

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