Arch. Pharm. Res. (2015) 38:408–413 DOI 10.1007/s12272-015-0563-1
REVIEW
Radiation treatment and cancer stem cells Yongjoon Suh • Su-Jae Lee
Received: 7 November 2014 / Accepted: 12 January 2015 / Published online: 27 January 2015 Ó The Pharmaceutical Society of Korea 2015
Abstract Radiotherapy is a standard treatment for many cancers and is frequently used as primary or adjuvant therapy, often in combination with surgery or chemotherapy or both. However, locoregional recurrence or metastatic spread still occurs in a high proportion of patients after radiotherapy. In this regard, emerging evidences suggest that sublethal radiation paradoxically promotes expansion of cancer stem cell population that is highly tumorigenic and is reminiscent of non-neoplasm stem cells. In this review, we discussed recent findings that demonstrate the increase in cancer stem cells after irradiation, and the possible cellular mechanisms with a perspective of tumor microenvironment. A further understating on the mechanistic mechanisms underlying radiation-enhanced malignant phenotypes might increase the efficacy of radiotherapy for cancer treatment. Keywords Ionizing radiation Cancer stem cells Tumor microenvironment
Introduction Radiotherapy is widely used as a standard treatment for many types of cancer including lung cancer, breast cancer and glioma. Ionizing radiation (IR) has been used clinically for cancer treatment since the exposure to high does of gamma IR causes severe DNA damages and thus can induces apoptosis of cancer cells. For this reason, radiotherapy was
Y. Suh S.-J. Lee (&) Laboratory of Molecular Biochemistry, Department of Life Science, Hanyang University, 17 Haengdang-Dong, Seongdong-Ku, Seoul 133-791, Korea e-mail: [email protected]
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also called as radiosurgery that uses gamma IR like a surgery knife. With an advent of modern technology such as stereotactic radiotherapy, the accuracy of radiotherapy was greatly enhanced and its usage is becoming indispensable for cancer treatment. However, in the other side, emerging clinical evidences suggest that sublethal doses of IR paradoxically promotes the malignant phenotypes in many cancers, causing relapse after treatment (Dirks 2008; Squatrito et al. 2010). In particular, the harmful effects of radiotherapy are frequently observed after exposure to doses of fractionated radiation that are not sufficient to induce cell death in cancers. Although the mechanisms underlying this effect are not well understood, it reflects the selective expansion of intrinsically IR-resistant cancer cells or acquisition of radio-resistance after exposure to sublethal IR. In this regard, several studies reported that sublethal radiation could promote expansion of cancer stem cell population that is a subpopulation of cancer and is resistant to conventional cancer therapies including radiotherapy. Since these cancer stem cells are highly tumorigenic and reminiscent of non-neoplasm stem cells, tumors could be repopulated by the small subset of cancer stem cells, even though the bulk of tumor is eradicated by radiation. For this reason, it seems reasonable to suggest that a cancer cure could be achieved only if this population is eliminated. Although the cancer stem cell model has been in debate (Hill and Perris 2007; Kelly et al. 2007; Tomasson 2009) and does not fit perfectly to the behavior of all cancer, it explains better the behavior of tumor recurrence after radiation treatment than a stochastic cancer model, in which all cancer cells have the same tumorigenic potential (Kummermehr 2001; Vescovi et al. 2006). Here, we review the findings that demonstrate the clinically harmful effect of radiation, focusing on expansion of cancer stem cells
Radiation treatment and cancer stem cells
and the possible mechanisms underlying radiationenhanced expansion of cancer stem cells in terms of tumor microenvironment.
Radiation-enhanced cancer stem cells Cancer stem cells are defined functionally as two properties, long term self-renewal and tumorigenic potential that contribute to tumor cellular heterogeneity. Although radiotherapy is purposed to eradicate cancer cells, many studies suggested that exposure to sublethal radiation increases cancer stem cells. Bao et al. has investigated the effect of radiation on the glioblastoma cell lines and surgical glioblastoma samples (Bao et al. 2006). When they treated mice bearing glioma cells D456MG xenograft tumors with radiation, they observed an increased percentage of CD133? cells that is well known cancer stem cell marker (Singh et al. 2004). For ex vivo radiation, cells isolated from D54MG xenografts were treated with 5 Gy, resulting in a similar increase in CD133? cells. Similar results were obtained with cells isolated from three human glioblastoma surgical samples radiated ex vivo with 2 Gy. The percentage of CD133-expressing cells as analyzed by FACS also correlated with the rate of tumor formation when implanted in mice. Likewise, Kim et al. reported that treatment of glioblastoma cells with radiation stimulates expansion of glioma stem cells through nitric oxide (NO) production mediated by inducible NO synthase (iNOS) (Kim et al. 2013a). They demonstrated that treatment with either single radiation (6 Gy) or fractionated radiation (2 Gy x3) resulted in the increase of CD133? Nestin? cell population along with expression of SOX2, b-catenin and Notch2. However, inhibition of iNOS attenuated the radiationenhanced increase of glioma stem cells. Gomez-Casal et al. also found that irradiation of non-small cell lung cancer (NSCLC) cells caused an increase in cancer stem cell population (Gomez-Casal et al. 2013). They treated A549 and H460 NSCLC cells with IR (5 Gy) and found that the irradiated and survived cancer cells displayed more sphere forming ability and expressed higher levels of cancer stem cell markers (CD24 and CD44), nuclear b-catenin and epithelial-mesenchymal cell transition (EMT) markers (Snail1, Vimentin and N-cadherin) than non-irradiated lung tumor sphere cells. By these findings that IR causes an increase in cancer stem cell population, we could ask a question on how sublethal radiation could increase cancer stem cell population. First, we could think that possibly, cancer stem cells are more resistant to radiation than the rest of cancer cells and could have more chance to be survived than noncancer stem cells by radiation. Thus, as in the Darwinian
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natural selection, cancer stem cells might be selected by radiation and be evaluated as a higher percentage than the pre-radiation status (Fig. 1). Indeed, many evidences are reported that cancer stem cells are more resistant to radiation than rest of cancer cells (Pajonk et al. 2010; Kim et al. 2011; Bao et al. 2006; Rycaj and Tang 2014). In this regard, Bao et al. suggested that the resistance of cancer stem cells is caused by their preferential activation of the DNA damage response (Bao et al. 2006). They showed that CD133? glioblastoma cells preferentially activate the signaling components that are involved in DNA damage checkpoint and thereby repair IR-induced DNA damage more efficiently than CD133- glioblastoma cells. However, previously, Kim et al. observed an increase of cancer stem cell population to a greater extent by fractionated radiation than a single high dose of radiation (Kim et al. 2013a). Since a higher single dose of radiation is predicted to be more powerful for selection of cancer stem cells than fractionated radiation, it is hard to explain this phenomena with the selection hypothesis. Second, exposure to sublethal radiation might stimulate proliferation (self-renewal) of cancer stem cells (Fig. 1). In line with this hypothesis, Gao et al. reported that irradiation modulates differentially glioma stem cell division kinetics (Gao et al. 2013). By simulation of cell growth kinetics with experimental data, they suggested that irradiation could cause a shift from asymmetric to symmetric division or a fast cycle of glioma stem cells. In third, by irradiation, some cancer cells might be adapted to radiation and acquire radio-resistance in association with self-renewal and tumorigenic capacity (Fig. 1). This hypothesis is supported by previous studies showing that somatic cells can be reprogrammed by Yamanaka transcription factors (Lorenzo et al. 2012; Takahashi and Yamanaka 2006). Importantly, these findings challenged the concept that differentiation of stem cells or progenitors into functional somatic cells is an irreversible process. Although
Fig. 1 Schematic models depicting cellular mechanisms underlying radiation-enhanced cancer stem cell population
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this de-differentiation occurred artificially in vitro and has not yet been observed in living organisms, cancer cells appear to undergo reprogramming, exploiting the stemness machinery for their malignant progression. In line with this hypothesis, many oncogenes, including Notch, Nanog, bcatenin and c-Myc, are also expressed in normal stem cells (Ben-Porath et al. 2008; Taylor et al. 2005). Moreover, many normal neural stem cell markers are also markers of cancer stem cells (Singh et al. 2004; Muto et al. 2012). In association with this hypothesis, Kim et al. found that nonstem cancer cells can be converted to cancer stem cells through the IL-6-JAK1-STAT3 signaling pathway (Kim et al. 2013b), implying that de-differentiation occurs in cancers. Yang et al. also suggested that there is dynamic equilibrium between cancer stem cells and non-stem cancer cells in breast cancer cell populations (Yang et al. 2012). Collectively, many emerging evidences suggest that radiotherapy could increase cancer stem cell population; however, its cellular mechanisms remain still obscure. Since targeting cancer stem cells is closely associated with clinical outcome, unraveling the cellular mechanism underlying radiation-enhanced cancer stem cells will merit further study.
Comparison of cancer stem cells with non-neoplastic stem cells Since highly tumorigenic subpopulation of cancer cells shares features with non-neoplastic stem cells, the special subpopulation was named as ‘‘cancer stem cells’’ or ‘‘tumor initiating cells’’ and so on. Gene expression profiles of cancer stem cells are similar with those of non-malignant normal stem cells (Ben-Porath et al. 2008; Taylor et al. 2005). Disruption of several stem cell-specific pathways disturbs proliferation and tumorigenesis of cancer stem cells (Bar et al. 2007; Clement et al. 2007; Fan et al. 2006). In addition, as neural stem cells (NSCs) does, glioma stem cells were also found in the perivascular regions suggesting that the maintenance of glioma stem cells might require similar specialized niche and similar regulatory mechanisms with NSCs. However, in contrary to NSCs that reside in a specialized niche such as subventricular zone and hippocampus dentate gyrus (Ihrie and Alvarez-Buylla 2008), glioma stem cells are able to infiltrate to other sites of brain parenchyma, where maintaining their tumorigenic potential in a new microenvironment, forming another brain tumor (Zhang et al. 2013; Nduom et al. 2012). Thus, despite the similarity of cancer stem cells with normal stem cells, we suppose that cancer stem cells might have their own distinct regulatory mechanisms different from non-neoplastic stem cells. In this regard, Eyler et al. reported that glioma stem cells depend on
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iNOS activity for growth and tumorigenicity, distinguishing them from non-glioma stem cells and normal neural progenitors (Eyler et al. 2011). They observed that NO is significantly elevated in CD133? glioma stem cells than CD133- non-glioma stem cells. However, importantly, NO levels and iNOS expression were not higher in CD133? NSCs than counterpart control neural progenitors. In parallel, genetic and pharmacologic blockade of iNOS inhibited only glioma stem cell growth but not normal NSCs. In agreement with this report, Kim et al. found that irradiation increases the expression levels of iNOS among NOS isoforms in glioma cells, thereby promotes glioma stem cell proliferation (Kim et al. 2013a). Previously, Yoon et al. also suggested that JNK signaling has a critical role in the maintenance of glioma stem cells. Although JNK signaling regulates a variety of cellular events such as proliferation and differentiation, its major physiological function is to induce apoptosis (Davis 2000). Indeed, JNK signaling-induced apoptosis was previously reported in rat NSCs (Kanzawa et al. 2006). Thus, in normal brain, activation of JNK signaling appears to induce apoptosis of NSCs. In contrast, JNK has been reported to be constitutively activated in malignant glioma (Tsuiki et al. 2003; Li et al. 2008; Cui et al. 2006). Thus, in contrast to normal NSCs, JNK signaling does not induce cellular apoptosis in glioma stem cells; rather has a critical role in the maintenance of stemness in malignant glioma cells. Because cancer stem cells have notable phenotypic and mechanistic similarities to normal stem cells, it raised the question of whether cancer therapy can be developed that eliminate cancer stem cells without eliminating normal stem cells. A further study on mechanistic differences between normal stem cells and cancer stem cells might be necessary to design new therapies that deplete cancer stem cells without damaging normal stem cells.
Radiation-altered tumor microenvironment and cancer stem cells Tumors are more than insular masses of proliferating cancer cells, in which multiple distinct types including recruited normal cells participate in heterotypic interaction one another. When viewed from this perspective, the behavior of cancer cells should be understood by studying the individual specialized cell types within the tumor microenvironment (Fig. 2). Indeed, many studies reported that the recruited normal cells also contribute to malignant cancer progression as active participants rather than passive bystanders in tumor microenvironment (Lu et al. 2014; Dirat et al. 2011; Su et al. 2014). In response of irradiation, many inflammatory cytokines or soluble factors are known to be secreted in tumor
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Fig. 2 Radiation-enhanced cancer stem cells in tumor microenvironment. Note that irradiation reduces tumor burden; however, it also promotes an increase in the secretion of soluble factors and reinforces autocrine or paracrine signaling, resulting in the increase of cancer stem cells in tumor microenvironment, and eventually tumor relapse
microenvironment (Siva et al. 2014; Muller and Meineke 2007; Derradji et al. 2008). Possibly, these radiation-induced secretory factors could contribute to the increase in cancer stem cell population (Fig. 2). In this regard, Kim et al. found that irradiation causes secretion of NO in glioblastoma microenvironment that can promote glioma stem cell population via autocrine signaling (Kim et al. 2013a). As a fail safe mechanism by following irradiation, cells can undergo senescence alternatively instead of apoptosis. Notably, radiation-induced senescent cells secrete many soluble factors that could alter the behavior of neighboring cells. Although the functional role of senescence-associated secretome remains unclear, these inflammatory cytokines and chemokines produced by senescent cells could provide a cancer-promoting microenvironment for their neighboring tumor and normal cells (Campisi and D’adda Di Fagagna 2007). Gomez-Casal et al. also reported that radiation survived NSCLC cells display cancer stem cells and EMT phenotypes (Gomez-Casal et al. 2013). Importantly, they found that platelet derived growth factor receptor (PDGFR)-beta was not detectable in NSCLC cells; however, it was highly expressed in radiation survived cells, suggesting that radiation could alter PDGFR signaling in tumor microenvironment and is involved in radiation-enhanced cancer stem cells. Intriguingly, cells that have not been directly exposed to IR have been found to behave as if they have been exposed to radiation and this phenomena was termed as ‘‘radiationinduced bystander effects’’ (Mothersill and Seymour 2004). In line with this concept, Lorimore et al. showed that radiation-caused chromosomal instability could be induced also in non-irradiated cells (Lorimore et al. 1998). They found that the genetic instability of non-irradiated cells is caused by signals from the irradiated cells. These studies implicate the complexity of radiation effect and also suggest that radiation-altered tumor microenvironment should be considered to minimize the harmful effect of radiotherapy for cancer. Meanwhile, Elvington et al. showed that modulation of tumor microenvironment can enhance the efficiency of
radiotherapy for cancer treatment (Elvington et al. 2014). Although complement is known as a proinflammatory effector mechanism of antitumor immunity, it is also important for effective clearance of apoptotic cells, which can be an anti-inflammatory and tolerogenic process. In their study, complement inhibition was found to promote inflammation within the tumor, induce an early influx of neutrophils, and promote a systemic antitumor immune response. Thus, they found that complement inhibition markedly improves therapeutic outcome of radiotherapy, an effect linked to early increases in apoptotic cell numbers and increased inflammation. Their study suggested that the harmful effect of radiotherapy could be mitigated by modulation of tumor microenvironment. In this review, we discussed the published reports providing evidences that sublethal radiation enhances an increase in cancer stem cell population. In parallel, we also proposed the possible mechanisms underlying radiationenhanced expansion of cancer stem cells based on the previous studies. Over the decade, the importance of tumor microenvironment has been recognized in the maintenance of cancer stem cells. In another decade, we anticipate that further study on the mechanistic mechanisms underlying radiation-enhanced expansion of cancer stem cells with a perspective of tumor microenvironment might increase the efficacy of radiotherapy for cancer treatment. Acknowledgments This work was supported by the National Research Foundation (NRF) and Ministry of Science, ICT & Future Planning, Korean government, through its National Nuclear Technology Program (2012M2A2A7035878).
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REVIEW
Radiation treatment and cancer stem cells Yongjoon Suh • Su-Jae Lee
Received: 7 November 2014 / Accepted: 12 January 2015 / Published online: 27 January 2015 Ó The Pharmaceutical Society of Korea 2015
Abstract Radiotherapy is a standard treatment for many cancers and is frequently used as primary or adjuvant therapy, often in combination with surgery or chemotherapy or both. However, locoregional recurrence or metastatic spread still occurs in a high proportion of patients after radiotherapy. In this regard, emerging evidences suggest that sublethal radiation paradoxically promotes expansion of cancer stem cell population that is highly tumorigenic and is reminiscent of non-neoplasm stem cells. In this review, we discussed recent findings that demonstrate the increase in cancer stem cells after irradiation, and the possible cellular mechanisms with a perspective of tumor microenvironment. A further understating on the mechanistic mechanisms underlying radiation-enhanced malignant phenotypes might increase the efficacy of radiotherapy for cancer treatment. Keywords Ionizing radiation Cancer stem cells Tumor microenvironment
Introduction Radiotherapy is widely used as a standard treatment for many types of cancer including lung cancer, breast cancer and glioma. Ionizing radiation (IR) has been used clinically for cancer treatment since the exposure to high does of gamma IR causes severe DNA damages and thus can induces apoptosis of cancer cells. For this reason, radiotherapy was
Y. Suh S.-J. Lee (&) Laboratory of Molecular Biochemistry, Department of Life Science, Hanyang University, 17 Haengdang-Dong, Seongdong-Ku, Seoul 133-791, Korea e-mail: [email protected]
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also called as radiosurgery that uses gamma IR like a surgery knife. With an advent of modern technology such as stereotactic radiotherapy, the accuracy of radiotherapy was greatly enhanced and its usage is becoming indispensable for cancer treatment. However, in the other side, emerging clinical evidences suggest that sublethal doses of IR paradoxically promotes the malignant phenotypes in many cancers, causing relapse after treatment (Dirks 2008; Squatrito et al. 2010). In particular, the harmful effects of radiotherapy are frequently observed after exposure to doses of fractionated radiation that are not sufficient to induce cell death in cancers. Although the mechanisms underlying this effect are not well understood, it reflects the selective expansion of intrinsically IR-resistant cancer cells or acquisition of radio-resistance after exposure to sublethal IR. In this regard, several studies reported that sublethal radiation could promote expansion of cancer stem cell population that is a subpopulation of cancer and is resistant to conventional cancer therapies including radiotherapy. Since these cancer stem cells are highly tumorigenic and reminiscent of non-neoplasm stem cells, tumors could be repopulated by the small subset of cancer stem cells, even though the bulk of tumor is eradicated by radiation. For this reason, it seems reasonable to suggest that a cancer cure could be achieved only if this population is eliminated. Although the cancer stem cell model has been in debate (Hill and Perris 2007; Kelly et al. 2007; Tomasson 2009) and does not fit perfectly to the behavior of all cancer, it explains better the behavior of tumor recurrence after radiation treatment than a stochastic cancer model, in which all cancer cells have the same tumorigenic potential (Kummermehr 2001; Vescovi et al. 2006). Here, we review the findings that demonstrate the clinically harmful effect of radiation, focusing on expansion of cancer stem cells
Radiation treatment and cancer stem cells
and the possible mechanisms underlying radiationenhanced expansion of cancer stem cells in terms of tumor microenvironment.
Radiation-enhanced cancer stem cells Cancer stem cells are defined functionally as two properties, long term self-renewal and tumorigenic potential that contribute to tumor cellular heterogeneity. Although radiotherapy is purposed to eradicate cancer cells, many studies suggested that exposure to sublethal radiation increases cancer stem cells. Bao et al. has investigated the effect of radiation on the glioblastoma cell lines and surgical glioblastoma samples (Bao et al. 2006). When they treated mice bearing glioma cells D456MG xenograft tumors with radiation, they observed an increased percentage of CD133? cells that is well known cancer stem cell marker (Singh et al. 2004). For ex vivo radiation, cells isolated from D54MG xenografts were treated with 5 Gy, resulting in a similar increase in CD133? cells. Similar results were obtained with cells isolated from three human glioblastoma surgical samples radiated ex vivo with 2 Gy. The percentage of CD133-expressing cells as analyzed by FACS also correlated with the rate of tumor formation when implanted in mice. Likewise, Kim et al. reported that treatment of glioblastoma cells with radiation stimulates expansion of glioma stem cells through nitric oxide (NO) production mediated by inducible NO synthase (iNOS) (Kim et al. 2013a). They demonstrated that treatment with either single radiation (6 Gy) or fractionated radiation (2 Gy x3) resulted in the increase of CD133? Nestin? cell population along with expression of SOX2, b-catenin and Notch2. However, inhibition of iNOS attenuated the radiationenhanced increase of glioma stem cells. Gomez-Casal et al. also found that irradiation of non-small cell lung cancer (NSCLC) cells caused an increase in cancer stem cell population (Gomez-Casal et al. 2013). They treated A549 and H460 NSCLC cells with IR (5 Gy) and found that the irradiated and survived cancer cells displayed more sphere forming ability and expressed higher levels of cancer stem cell markers (CD24 and CD44), nuclear b-catenin and epithelial-mesenchymal cell transition (EMT) markers (Snail1, Vimentin and N-cadherin) than non-irradiated lung tumor sphere cells. By these findings that IR causes an increase in cancer stem cell population, we could ask a question on how sublethal radiation could increase cancer stem cell population. First, we could think that possibly, cancer stem cells are more resistant to radiation than the rest of cancer cells and could have more chance to be survived than noncancer stem cells by radiation. Thus, as in the Darwinian
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natural selection, cancer stem cells might be selected by radiation and be evaluated as a higher percentage than the pre-radiation status (Fig. 1). Indeed, many evidences are reported that cancer stem cells are more resistant to radiation than rest of cancer cells (Pajonk et al. 2010; Kim et al. 2011; Bao et al. 2006; Rycaj and Tang 2014). In this regard, Bao et al. suggested that the resistance of cancer stem cells is caused by their preferential activation of the DNA damage response (Bao et al. 2006). They showed that CD133? glioblastoma cells preferentially activate the signaling components that are involved in DNA damage checkpoint and thereby repair IR-induced DNA damage more efficiently than CD133- glioblastoma cells. However, previously, Kim et al. observed an increase of cancer stem cell population to a greater extent by fractionated radiation than a single high dose of radiation (Kim et al. 2013a). Since a higher single dose of radiation is predicted to be more powerful for selection of cancer stem cells than fractionated radiation, it is hard to explain this phenomena with the selection hypothesis. Second, exposure to sublethal radiation might stimulate proliferation (self-renewal) of cancer stem cells (Fig. 1). In line with this hypothesis, Gao et al. reported that irradiation modulates differentially glioma stem cell division kinetics (Gao et al. 2013). By simulation of cell growth kinetics with experimental data, they suggested that irradiation could cause a shift from asymmetric to symmetric division or a fast cycle of glioma stem cells. In third, by irradiation, some cancer cells might be adapted to radiation and acquire radio-resistance in association with self-renewal and tumorigenic capacity (Fig. 1). This hypothesis is supported by previous studies showing that somatic cells can be reprogrammed by Yamanaka transcription factors (Lorenzo et al. 2012; Takahashi and Yamanaka 2006). Importantly, these findings challenged the concept that differentiation of stem cells or progenitors into functional somatic cells is an irreversible process. Although
Fig. 1 Schematic models depicting cellular mechanisms underlying radiation-enhanced cancer stem cell population
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this de-differentiation occurred artificially in vitro and has not yet been observed in living organisms, cancer cells appear to undergo reprogramming, exploiting the stemness machinery for their malignant progression. In line with this hypothesis, many oncogenes, including Notch, Nanog, bcatenin and c-Myc, are also expressed in normal stem cells (Ben-Porath et al. 2008; Taylor et al. 2005). Moreover, many normal neural stem cell markers are also markers of cancer stem cells (Singh et al. 2004; Muto et al. 2012). In association with this hypothesis, Kim et al. found that nonstem cancer cells can be converted to cancer stem cells through the IL-6-JAK1-STAT3 signaling pathway (Kim et al. 2013b), implying that de-differentiation occurs in cancers. Yang et al. also suggested that there is dynamic equilibrium between cancer stem cells and non-stem cancer cells in breast cancer cell populations (Yang et al. 2012). Collectively, many emerging evidences suggest that radiotherapy could increase cancer stem cell population; however, its cellular mechanisms remain still obscure. Since targeting cancer stem cells is closely associated with clinical outcome, unraveling the cellular mechanism underlying radiation-enhanced cancer stem cells will merit further study.
Comparison of cancer stem cells with non-neoplastic stem cells Since highly tumorigenic subpopulation of cancer cells shares features with non-neoplastic stem cells, the special subpopulation was named as ‘‘cancer stem cells’’ or ‘‘tumor initiating cells’’ and so on. Gene expression profiles of cancer stem cells are similar with those of non-malignant normal stem cells (Ben-Porath et al. 2008; Taylor et al. 2005). Disruption of several stem cell-specific pathways disturbs proliferation and tumorigenesis of cancer stem cells (Bar et al. 2007; Clement et al. 2007; Fan et al. 2006). In addition, as neural stem cells (NSCs) does, glioma stem cells were also found in the perivascular regions suggesting that the maintenance of glioma stem cells might require similar specialized niche and similar regulatory mechanisms with NSCs. However, in contrary to NSCs that reside in a specialized niche such as subventricular zone and hippocampus dentate gyrus (Ihrie and Alvarez-Buylla 2008), glioma stem cells are able to infiltrate to other sites of brain parenchyma, where maintaining their tumorigenic potential in a new microenvironment, forming another brain tumor (Zhang et al. 2013; Nduom et al. 2012). Thus, despite the similarity of cancer stem cells with normal stem cells, we suppose that cancer stem cells might have their own distinct regulatory mechanisms different from non-neoplastic stem cells. In this regard, Eyler et al. reported that glioma stem cells depend on
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iNOS activity for growth and tumorigenicity, distinguishing them from non-glioma stem cells and normal neural progenitors (Eyler et al. 2011). They observed that NO is significantly elevated in CD133? glioma stem cells than CD133- non-glioma stem cells. However, importantly, NO levels and iNOS expression were not higher in CD133? NSCs than counterpart control neural progenitors. In parallel, genetic and pharmacologic blockade of iNOS inhibited only glioma stem cell growth but not normal NSCs. In agreement with this report, Kim et al. found that irradiation increases the expression levels of iNOS among NOS isoforms in glioma cells, thereby promotes glioma stem cell proliferation (Kim et al. 2013a). Previously, Yoon et al. also suggested that JNK signaling has a critical role in the maintenance of glioma stem cells. Although JNK signaling regulates a variety of cellular events such as proliferation and differentiation, its major physiological function is to induce apoptosis (Davis 2000). Indeed, JNK signaling-induced apoptosis was previously reported in rat NSCs (Kanzawa et al. 2006). Thus, in normal brain, activation of JNK signaling appears to induce apoptosis of NSCs. In contrast, JNK has been reported to be constitutively activated in malignant glioma (Tsuiki et al. 2003; Li et al. 2008; Cui et al. 2006). Thus, in contrast to normal NSCs, JNK signaling does not induce cellular apoptosis in glioma stem cells; rather has a critical role in the maintenance of stemness in malignant glioma cells. Because cancer stem cells have notable phenotypic and mechanistic similarities to normal stem cells, it raised the question of whether cancer therapy can be developed that eliminate cancer stem cells without eliminating normal stem cells. A further study on mechanistic differences between normal stem cells and cancer stem cells might be necessary to design new therapies that deplete cancer stem cells without damaging normal stem cells.
Radiation-altered tumor microenvironment and cancer stem cells Tumors are more than insular masses of proliferating cancer cells, in which multiple distinct types including recruited normal cells participate in heterotypic interaction one another. When viewed from this perspective, the behavior of cancer cells should be understood by studying the individual specialized cell types within the tumor microenvironment (Fig. 2). Indeed, many studies reported that the recruited normal cells also contribute to malignant cancer progression as active participants rather than passive bystanders in tumor microenvironment (Lu et al. 2014; Dirat et al. 2011; Su et al. 2014). In response of irradiation, many inflammatory cytokines or soluble factors are known to be secreted in tumor
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Fig. 2 Radiation-enhanced cancer stem cells in tumor microenvironment. Note that irradiation reduces tumor burden; however, it also promotes an increase in the secretion of soluble factors and reinforces autocrine or paracrine signaling, resulting in the increase of cancer stem cells in tumor microenvironment, and eventually tumor relapse
microenvironment (Siva et al. 2014; Muller and Meineke 2007; Derradji et al. 2008). Possibly, these radiation-induced secretory factors could contribute to the increase in cancer stem cell population (Fig. 2). In this regard, Kim et al. found that irradiation causes secretion of NO in glioblastoma microenvironment that can promote glioma stem cell population via autocrine signaling (Kim et al. 2013a). As a fail safe mechanism by following irradiation, cells can undergo senescence alternatively instead of apoptosis. Notably, radiation-induced senescent cells secrete many soluble factors that could alter the behavior of neighboring cells. Although the functional role of senescence-associated secretome remains unclear, these inflammatory cytokines and chemokines produced by senescent cells could provide a cancer-promoting microenvironment for their neighboring tumor and normal cells (Campisi and D’adda Di Fagagna 2007). Gomez-Casal et al. also reported that radiation survived NSCLC cells display cancer stem cells and EMT phenotypes (Gomez-Casal et al. 2013). Importantly, they found that platelet derived growth factor receptor (PDGFR)-beta was not detectable in NSCLC cells; however, it was highly expressed in radiation survived cells, suggesting that radiation could alter PDGFR signaling in tumor microenvironment and is involved in radiation-enhanced cancer stem cells. Intriguingly, cells that have not been directly exposed to IR have been found to behave as if they have been exposed to radiation and this phenomena was termed as ‘‘radiationinduced bystander effects’’ (Mothersill and Seymour 2004). In line with this concept, Lorimore et al. showed that radiation-caused chromosomal instability could be induced also in non-irradiated cells (Lorimore et al. 1998). They found that the genetic instability of non-irradiated cells is caused by signals from the irradiated cells. These studies implicate the complexity of radiation effect and also suggest that radiation-altered tumor microenvironment should be considered to minimize the harmful effect of radiotherapy for cancer. Meanwhile, Elvington et al. showed that modulation of tumor microenvironment can enhance the efficiency of
radiotherapy for cancer treatment (Elvington et al. 2014). Although complement is known as a proinflammatory effector mechanism of antitumor immunity, it is also important for effective clearance of apoptotic cells, which can be an anti-inflammatory and tolerogenic process. In their study, complement inhibition was found to promote inflammation within the tumor, induce an early influx of neutrophils, and promote a systemic antitumor immune response. Thus, they found that complement inhibition markedly improves therapeutic outcome of radiotherapy, an effect linked to early increases in apoptotic cell numbers and increased inflammation. Their study suggested that the harmful effect of radiotherapy could be mitigated by modulation of tumor microenvironment. In this review, we discussed the published reports providing evidences that sublethal radiation enhances an increase in cancer stem cell population. In parallel, we also proposed the possible mechanisms underlying radiationenhanced expansion of cancer stem cells based on the previous studies. Over the decade, the importance of tumor microenvironment has been recognized in the maintenance of cancer stem cells. In another decade, we anticipate that further study on the mechanistic mechanisms underlying radiation-enhanced expansion of cancer stem cells with a perspective of tumor microenvironment might increase the efficacy of radiotherapy for cancer treatment. Acknowledgments This work was supported by the National Research Foundation (NRF) and Ministry of Science, ICT & Future Planning, Korean government, through its National Nuclear Technology Program (2012M2A2A7035878).
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