Cancer stem cells at the crossroads of current cancer therapy failures--radiation oncology perspective

Inflammation and cancer stem cells

Cancer stem cells are becoming recognised as being responsible for metastasis and treatment resistance. The complex cellular and molecular network that regulates cancer stem cells and the role that inflammation plays in cancer progression are slowly being elucidated. Cytokines, secreted by tumour associated immune cells, activate the necessary pathways required by cancer stem cells to facilitate cancer stem cells progressing through the epithelial-mesenchymal transition and migrating to distant sites. Once in situ, these cancer stem cells can secrete their own attractants, thus providing an environment whereby these cells can continue to propagate the tumour in a secondary niche.

Cytokine-induced killer (CIK) cells bound with anti-CD3/anti-CD133 bispecific antibodies target CD133(high) cancer stem cells in vitro and in vivo

CD133 is a common marker of cancer stem cells (CSCs). We generated an anti-CD3/anti-CD133 bispecific antibody (BsAb) and bound it to the cytokine-induced killer (CIK) cells as effector cells (BsAb-CIK) to target CD133(high) CSCs. The killing of CD133(high) pancreatic (SW1990) and hepatic (Hep3B) cancer cells by the BsAb-CIK cells was significantly (p<0.05) higher than the killing by the parental CIK or by CIK cells bound with anti-CD3 (CD3-CIK) without CD133 targeting. In nude mice, the BsAb-CIK cells inhibited CD133(high) tumor growth significantly (p<0.05) more than that by CIK or CD3-CIK cells, or by the BsAb alone. BsAb-CIK cells co-cultured with CD133(high) cells produced significantly (p<0.05) higher amount of IFN-γ. Treatment with the BsAb-CIK cells significantly downregulated the expression of S100P and IL-18bp, but upregulated STAT1. The findings may help with the development of novel immunotherapies for patients with cancer containing CD133(high) CSCs by selectively targeting this cell population.

Clinical perspectives of cancer stem cell research in radiation oncology

Radiotherapy has a proven potential to eradicate cancer stem cells which is reflected by its curative potential in many cancer types. Considerable progress has been made in identification and biological characterisation of cancer stem cells during the past years. Recent biological findings indicate significant inter- and intratumoural and functional heterogeneity of cancer stem cells and lead to more complex models which have potential implications for radiobiology and radiotherapy. Clinical evidence is emerging that biomarkers of cancer stem cells may be prognostic for the outcome of radiotherapy in some tumour entities. Perspectives of cancer stem cell based research for radiotherapy reviewed here include their radioresistance compared to the mass of non-cancer stem cells which form the bulk of all tumour cells, implications for image- and non-image based predictive bio-assays of the outcome of radiotherapy and a combination of novel systemic treatments with radiotherapy.

Chemotherapy sensitizes colon cancer initiating cells to Vγ9Vδ2 T cell-mediated cytotoxicity

Colon cancer comprises a small population of cancer initiating stem cells (CIC) that is responsible for tumor maintenance and resistance to anti-cancer therapies, possibly allowing for tumor recapitulation once treatment stops. Combinations of immune-based therapies with chemotherapy and other anti-tumor agents may be of significant clinical benefit in the treatment of colon cancer. However, cellular immune-based therapies have not been experimented yet in the population of colon CICs. Here, we demonstrate that treatment with low concentrations of commonly used chemotherapeutic agents, 5-fluorouracyl and doxorubicin, sensitize colon CICs to Vγ9Vδ2 T cell cytotoxicity. Vγ9Vδ2 T cell cytotoxicity was largely mediated by TRAIL interaction with DR5, following NKG2D-dependent recognition of colon CIC targets. We conclude that in vivo activation of Vγ9Vδ2 T cells or adoptive administration of ex-vivo expanded Vγ9Vδ2 T cells at suitable intervals after chemotherapy may substantially increase anti-tumor activities and represent a novel strategy for colon cancer immunotherapy.

Dual-energy micro-computed tomography imaging of radiation-induced vascular changes in primary mouse sarcomas

PURPOSE

To evaluate the effects of radiation therapy on primary tumor vasculature using dual-energy (DE) micro-computed tomography (micro-CT).

METHODS AND MATERIALS

Primary sarcomas were generated with mutant Kras and p53. Unirradiated tumors were compared with tumors irradiated with 20 Gy. A liposomal-iodinated contrast agent was administered 1 day after treatment, and mice were imaged immediately after injection (day 1) and 3 days later (day 4) with DE micro-CT. CT-derived tumor sizes were used to assess tumor growth. After DE decomposition, iodine maps were used to assess tumor fractional blood volume (FBV) at day 1 and tumor vascular permeability at day 4. For comparison, tumor vascularity and vascular permeability were also evaluated histologically by use of CD31 immunofluorescence and fluorescently-labeled dextrans.

RESULTS

Radiation treatment significantly decreased tumor growth from day 1 to day 4 (P<.05). There was a positive correlation between CT measurement of tumor FBV on day 1 and extravasated iodine on day 4 with microvascular density (MVD) on day 4 (R(2)=0.53) and dextran accumulation (R(2)=0.63) on day 4, respectively. Despite no change in MVD measured by histology, tumor FBV significantly increased after irradiation as measured by DE micro-CT (0.070 vs 0.091, P<.05). Both dextran and liposomal-iodine accumulation in tumors increased significantly after irradiation, with dextran fractional area increasing 5.2-fold and liposomal-iodine concentration increasing 4.0-fold.

CONCLUSIONS

DE micro-CT is an effective tool for noninvasive assessment of vascular changes in primary tumors. Tumor blood volume and vascular permeability increased after a single therapeutic dose of radiation treatment.

Preclinical molecular imaging using PET and MRI

Molecular imaging fundamentally changes the way we look at cancer. Imaging paradigms are now shifting away from classical morphological measures towards the assessment of functional, metabolic, cellular, and molecular information in vivo. Interdisciplinary driven developments of imaging methodology and probe molecules utilizing animal models of human cancers have enhanced our ability to non-invasively characterize neoplastic tissue and follow anti-cancer treatments. Preclinical molecular imaging offers a whole palette of excellent methodology to choose from. We will focus on positron emission tomography (PET) and magnetic resonance imaging (MRI) techniques, since they provide excellent and complementary molecular imaging capabilities and bear high potential for clinical translation. Prerequisites and consequences of using animal models as surrogates of human cancers in preclinical molecular imaging are outlined. We present physical principles, values and limitations of PET and MRI as molecular imaging modalities and comment on their high potential to non-invasively assess information on hypoxia, angiogenesis, apoptosis, gene expression, metabolism, and cell trafficking in preclinical cancer research.

Targeting radioresistant osteosarcoma cells with parthenolide

Osteosarcoma is a devastating tumor of bone, primarily affecting adolescents. Osteosarcoma tumors are notoriously radioresistant. Radioresistant cancers, including osteosarcoma, typically exhibit a considerable potential for relapse and development of metastases following treatment. Relapse and metastatic potential can, in part, be due to a specific radioresistant subpopulation of cells with stem-like characteristics, cancer stem cells, which maintain the capacity to regenerate entire tumors. In the current study, we have investigated whether in vitro treatments with parthenolide, a naturally occurring small molecule that interferes with NF-κB signaling and has various other effects, will re-sensitize cancer stem cells and the entire cell population to radiotherapy in osteosarcoma. Our results indicate that parthenolide and ionizing radiation synergistically induce cell death in LM7 osteosarcoma cells. Importantly, the combination treatment results in a significant reduction in the viability of both the overall population of osteosarcoma cells and the cancer stem cell subpopulation. This effect is dependent on the ability of parthenolide to induce oxidative stress. Therefore, as a supplement to current multimodal therapy, parthenolide may sensitize osteosarcoma tumors to radiation and greatly reduce the prevalence of relapse and metastatic progression.

Sphere-forming tumor cells possess stem-like properties in human fibrosarcoma primary tumors and cell lines

Fibrosarcoma is a malignant soft tissue tumor of mesenchymal origin. Despite advances in medical and surgical treatment, patient survival rates have remained poor. According to the cancer stem cell hypothesis, tumors are comprised of heterogeneous cell populations that have different roles in tumor formation and growth. Cancer stem cells are a small cell subpopulation that exhibits stem-like properties to gain aggressiveness and recurrence. These cells have been identified in a variety of cancerous tumors, but not in human fibrosarcoma. In this study, we observed that HT1080 cells and primary fibrosarcoma cells formed spheres and showed higher self-renewal capacity, invasiveness and drug resistance compared with their adherent counterparts. Moreover, we demonstrated that the cells showed higher expression of the embryonic stem cell-related genes Nanog, Oct3/4, Sox2, Sox10 and their encoding proteins, as well as greater tumorigenic capacity in nude mice. In conclusion, our data suggest the presence of a stem-like cell population in human fibrosarcoma tumors, which provides more evidence for the cancer stem cell hypothesis and assistance in designing new therapeutic strategies against human fibrosarcoma.

Cancer stem cell targeting: the next generation of cancer therapy and molecular imaging

Cancer stem cells (CSCs) have the capacity to generate the heterogeneous lineages of all cancer cells comprising a tumor and these populations of cells are likely to be more relevant in determining prognosis. However, these cells do not operate in isolation, but instead rely upon signals co-opted from their microenvironment, making the targeting and imaging of CSCs within a cancer mass a daunting task. A better understanding of the molecular cell biology underlying CSC pathology will facilitate the development of new therapeutic targets and novel strategies for the successful eradication of cancer. In addition, the continued investigation of sensitive molecular-imaging modalities will enable more accurate staging, treatment planning and the ability to monitor the effectiveness of CSC-targeted therapies in vivo. In this review, we explore the possibilities and limitations of CSC-directed therapies and molecular imaging modalities.

The role of epigenetic regulation in stem cell and cancer biology

Normal development and homeostasis requires a carefully coordinated gene expression program. Appropriate transcriptional regulation is maintained, in part, through epigenetic modifications of both DNA and histones. It is now apparent that the epigenetic landscape is complex and carefully controlled to both silence and activate gene transcription and that these states remain exquisitely poised for reversal. The deregulation of epigenetics in cancer is common and results in both the activation of oncogenes and the silencing of tumor suppressors. A tremendous amount of research corroborates the existence in many tumor types of a cancer stem cell that is both the origin and cell type responsible for resistance of tumors to current therapies. As our understanding of cancer stem cell biology continues, it is apparent that these cells are also under the influence of epigenetic regulation. We will discuss the cancer stem cell hypothesis and the role of epigenetics in both normal and cancer stem cell biology.

Cancer stem cells: in the line of fire

Most tumours appear to contain a sub-population(s) of self-renewing and expanding stem cells known as cancer stem cells (CSCs). The CSC model proposes that CSCs are at the apex of a hierarchically organized cell population, somewhat akin to normal tissue organization. Selection pressures may also facilitate the stochastic clonal expansion of sub-sets of cancer cells that may co-exist with CSCs and their progeny, moreover the trait of stemness may be more fluid than hitherto expected, and cells may switch between the stem and non-stem cell state. A large body of evidence points to the fact that CSCs are particularly resistant to radiotherapy and chemotherapy. In this review we discuss the basis of such resistance that highlights the roles of ABC transporters, aldehyde dehydrogenase (ALDH) activity, intracellular signalling pathways, the DNA damage response, hypoxia and proliferative quiescence as being significant determinants. In the light of such observations, we outline strategies for the successful eradication of CSCs, including targeting the self-renewal controlling pathways (Wnt, Notch and Hedgehog), ALDH activity and ABC transporters, blocking epithelial mesenchymal transition (EMT), differentiation therapy and niche targeting.

Individualization of cancer treatment from radiotherapy perspective

Radiotherapy is today used in about 50% of all cancer patients, often in multidisciplinary approaches. With major advance in radiotherapy techniques, increasing knowledge on tumor genetics and biology and the continuous introduction of specifically targeted drugs into combined radio-oncological treatment schedules, individualization of radiotherapy is of high priority to further improve treatment outcomes, i.e. to increase long-term tumor cure and/or to reduce chronic treatment toxicity. This review gives an overview on the importance of predictive biomarkers for the field of radiation oncology. The current status of knowledge on potential biomarkers of tumor hypoxia, tumor cell metabolism, DNA repair, cancer stem cells and biomarkers for combining radiotherapy with inhibition of the epidermal growth factor receptor using monoclonal antibodies is described.

Immunotherapy of prostate cancer: should we be targeting stem cells and EMT?

Cancer stem cells have been implicated in a number of solid malignancies including prostate cancer. In the case of localised prostate cancer, patients are often treated with surgery (radical prostatectomy) and/or radiotherapy. However, disease recurrence is an issue in about 30% of patients, who will then go on to receive hormone ablation therapy. Hormone ablation therapy is often palliative in a vast proportion of individuals, and for hormone-refractory patients, there are several immunotherapies targeting a number of prostate tumour antigens which are currently in development. However, clinical responses in this setting are inconsistent, and it is believed that the failure to achieve full and permanent tumour eradication is due to a small, resistant population of cells known as 'cancer stem cells' (CSCs). The stochastic and clonal evolution models are among several models used to describe cancer development. The general consensus is that cancer may arise in any cell as a result of genetic mutations in oncogenes and tumour suppressor genes, which consequently result in uncontrolled cell growth. The cancer stem cell theory, however, challenges previous opinion and proposes that like normal tissues, tumours are hierarchical and only the rare subpopulation of cells at the top of the hierarchy possess the biological properties required to initiate tumourigenesis. Furthermore, where most cancer models infer that every cell within a tumour is equally malignant, i.e. equally capable of reconstituting new tumours, the cancer stem cell theory suggests that only the rare cancer stem cell component possess tumour-initiating capabilities. Hence, according to this model, cancer stem cells are implicated in both tumour initiation and progression. In recent years, the role of epithelial--mesenchymal transition (EMT) in the advancement of prostate cancer has become apparent. Therefore, CSCs and EMT are both likely to play critical roles in prostate cancer tumourigenesis. This review summarises the current immunotherapeutic strategies targeting prostate tumour antigens taking into account the need to consider treatments that target cancer stem cells and cells involved in epithelial--mesenchymal transition.

Colon Cancer Stem Cells: Bench-to-Bedside-New Therapeutical Approaches in Clinical Oncology for Disease Breakdown

It is widely accepted by the scientific community that cancer, including colon cancer, is a “stem cell disease”. Until a few years ago, common opinion was that all neoplastic cells within a tumor contained tumorigenic growth capacity, but recent evidences hint to the possibility that such a feature is confined to a small subset of cancer-initiating cells, also called cancer stem cells (CSCs). Thus, malignant tumors are organized in a hierarchical fashion in which CSCs give rise to more differentiated tumor cells. CSCs possess high levels of ATP-binding cassette (ABC) transporters and anti-apoptotic molecules, active DNA-repair, slow replication capacities and they produce growth factors that confer refractoriness to antineoplastic treatments. The inefficacy of conventional therapies towards the stem cell population might explain cancer chemoresistance and the high frequency of relapse shown by the majority of tumors. Nowadays, in fact all the therapies available are not sufficient to cure patients with advanced forms of colon cancer since they target differentiated cancer cells which constitute most of the tumor mass and spare CSCs. Since CSCs are the entities responsible for the development of the tumor and represent the only cell population able to sustain tumor growth and progression, these cells represent the elective target for innovative therapies.

Targeting the osteosarcoma cancer stem cell

Osteosarcoma is the most common type of solid bone cancer and the second leading cause of cancer-related death in pediatric patients. Many patients are not cured by the current osteosarcoma therapy consisting of combination chemotherapy along with surgery and thus new treatments are urgently needed. In the last decade, cancer stem cells have been identified in many tumors such as leukemia, brain, breast, head and neck, colon, skin, pancreatic, and prostate cancers and these cells are proposed to play major roles in drug resistance, tumor recurrence, and metastasis. Recent studies have shown evidence that osteosarcoma also possesses cancer stem cells. This review summarizes the current knowledge about the osteosarcoma cancer stem cell including the methods used for its isolation, its properties, and its potential as a new target for osteosarcoma treatment.

Cancer stem cells: problems for therapy?

Many, if not all, tumours contain a sub-population of self-renewing and expanding stem cells known as cancer stem cells (CSCs). The symmetric division of CSCs is one mechanism enabling expansion in their numbers as tumours grow, while epithelial-mesenchymal transition (EMT) is an increasingly recognized mechanism to generate further CSCs endowed with a more invasive and metastatic phenotype. Putative CSCs are prospectively isolated using methods based on either a surface marker or an intracellular enzyme activity and then assessed by a 'sphere-forming' assay in non-adherent culture and/or by their ability to initiate new tumour growth when xenotransplanted into immunocompromised mice-hence, these cells are often referred to as tumour-propagating cells (TPCs). Cell sub-populations enriched for tumour-initiating ability have also been found in murine tumours, countering the argument that xenografting human cells merely select human cells with an ability to grow in mice. Cancer progression can be viewed as an evolutionary process that generates new/multiple clones with a fresh identity; this may be a major obstacle to successful cancer stem cell eradication if treatment targets only a single type of stem cell. In this review, we first briefly discuss evidence that cancer can originate from normal stem cells or closely related descendants. We then outline the attributes of CSCs and review studies in which they have been identified in various cancers. Finally, we discuss the implications of these findings for successful cancer therapies, concentrating on the self-renewal pathways (Wnt, Notch, and Hedgehog), aldehyde dehydrogenase activity, EMT, miRNAs, and other epigenetic modifiers as potential targets for therapeutic manipulation.

Impact of stromal sensitivity on radiation response of tumors implanted in SCID hosts revisited

Severe combined immunodeficient (SCID) mice carry a germ-line mutation in DNA-PK, associated with deficiency in recognition and repair DNA double-strand breaks. Thus, SCID cells and tissues display increased sensitivity to radiation-induced postmitotic (clonogenic) cell death. Nonetheless, the single-radiation doses required for 50% permanent local control (TCD(50)) of tumors implanted in SCID mice are not significantly different from the TCD(50) values of the same tumors in wild-type hosts. Whereas the tumor stroma is derived from the host, the observation that tumors implanted in SCID mice do not exhibit hypersensitivity to radiation might imply that stromal endothelial elements do not contribute substantially to tumor cure by ionizing radiation. Here, we challenge this notion, testing the hypothesis that ASMase-mediated endothelial apoptosis, which results from plasma membrane alterations, not DNA damage, is a crucial element in the cure of tumors in SCID mice by single-dose radiotherapy (SDRT). We show that the endothelium in MCA/129 fibrosarcomas and B16 melanomas exhibits a wild-type apoptotic phenotype in SCID hosts, abrogated in tumors in SCID(asmase-/-) littermates, which also acquire resistance to SDRT. Conversion into a radioresistant tumor phenotype when implanted in SCID(asmase-/-) hosts provides compelling evidence that cell membrane ASMase-mediated microvascular dysfunction, rather than DNA damage-mediated endothelial clonogenic lethality, plays a mandatory role in the complex pathophysiologic mechanism of tumor cure by SDRT, and provides an explanation for the wild-type SDRT responses reported in tumors implanted in SCID mice.

Profiling protein expression and interactions: proximity ligation as a tool for personalized medicine

The ability to detect very low levels of expressed proteins has enormous potential for early diagnostics and intervention at curable stages of disease. An extended range of targets such as interacting or post-translationally modified proteins can further improve the potential for diagnostics and patient stratification, and for monitoring response to treatment. These are critical building blocks for personalized treatment strategies to manage disease. The past few decades have seen a remarkably improved understanding of the molecular basis of disease in general, and of tumour formation and progression in particular. This accumulated knowledge creates opportunities to develop drugs that specifically target molecules or molecular complexes critical for survival and expansion of tumour cells. However, tumours are highly variable between patients, necessitating the development of diagnostic tools to individualize treatment through parallel analysis of sets of biomarkers. The proximity ligation assay (PLA) can address many of the requirements for advanced molecular analysis. The method builds on the principle that recognition of target proteins by two, three or more antibodies can bring in proximity DNA strands attached to the antibodies. The DNA strands can then participate in ligation reactions, giving rise to molecules that are amplified for highly sensitive detection. PLA is particularly well suited for sensitive, specific and multiplexed analysis of protein expression, post-translational modifications and protein-protein interactions. The analysis of this extended range of biomarkers will prove critical for the development and implementation of personalized medicine.

Tumor senescence and radioresistant tumor-initiating cells (TICs): let sleeping dogs lie!

Preclinical data from cell lines and experimental tumors support the concept that breast cancer-derived tumor-initiating cells (TICs) are relatively resistant to ionizing radiation and chemotherapy. This could be a major determinant of tumor recurrence following treatment. Increased clonogenic survival is observed in CD24^-/low/CD44⁺ TICs derived from mammosphere cultures and is associated with (a) reduced production of reactive oxygen species, (b) attenuated activation of γH2AX and CHK2-p53 DNA damage signaling pathways, (c) reduced propensity for ionizing radiation-induced apoptosis, and (d) altered DNA double-strand or DNA single-strand break repair. However, recent data have shed further light on TIC radioresistance as irradiated TICs are resistant to tumor cell senescence following DNA damage. Taken together, the cumulative data support a model in which DNA damage signaling and repair pathways are altered in TICs and lead to an altered mode of cell death with unique consequences for long-term clonogen survival. The study of TIC senescence lays the foundation for future experiments in isogenic models designed to directly test the capacity for senescence and local control (that is, not solely local regression) and spontaneous metastases following treatment in vivo. The study also supports the targeting of tumor cell senescence pathways to increase TIC clonogen kill if the targeting also maintains the therapeutic ratio.