Radiation resistance of cancer stem cells: the 4 R's of radiobiology revisited

NGF/γ-IFN inhibits androgen-independent prostate cancer and reverses androgen receptor function through downregulation of FGFR2 and decrease in cancer stem cells

Androgen-independent prostate cancer (AIPC) is difficult to treat. Present study is to explore the inhibitory effect of a cytokine environment on AIPC and its mechanism. We utilized nerve growth factor (NGF)/γ-interferon (γ-IFN) to change the cytokine environment. Animal models and 2 androgen receptor (AR)-negative prostate cancer cell lines were used to evaluate the effect of NGF/γ-IFN. Flow cytometry, immunocytochemistry, western blotting, Tunel assay, colony formation efficiency, gene microarray, and in vivo bioluminescence were used to discern the mechanisms within NGF/γ-IFN that effect the environment. In vitro, NGF/γ-IFN effectively inhibited the proliferation of AIPC cell lines and promoted the apoptosis of the cancer cells. In vivo, NGF/γ-IFN suppressed the growth and metastasis of a tumor mass that arose from the AIPC cell line. After NGF/γ-IFN treatment, the AR-negative cell lines re-expressed AR and were then able to respond to the androgen. Contrary to expectations, the proliferation of cells was inhibited after dihydrotestosterone was added, and the results indicated that NGF/γ-IFN decreased the proportion of cancer stem cells. NGF/γ-IFN worked mainly through the downregulation of fibroblast growth factor receptor 2.

Multispecies model of cell lineages and feedback control in solid tumors

We develop a multispecies continuum model to simulate the spatiotemporal dynamics of cell lineages in solid tumors. The model accounts for protein signaling factors produced by cells in lineages, and nutrients supplied by the microenvironment. Together, these regulate the rates of proliferation, self-renewal and differentiation of cells within the lineages, and control cell population sizes and distributions. Terminally differentiated cells release proteins (e.g., from the TGFβ superfamily) that feedback upon less differentiated cells in the lineage both to promote differentiation and decrease rates of proliferation (and self-renewal). Stem cells release a short-range factor that promotes self-renewal (e.g., representative of Wnt signaling factors), as well as a long-range inhibitor of this factor (e.g., representative of Wnt inhibitors such as Dkk and SFRPs). We find that the progression of the tumors and their response to treatment is controlled by the spatiotemporal dynamics of the signaling processes. The model predicts the development of spatiotemporal heterogeneous distributions of the feedback factors (Wnt, Dkk and TGFβ) and tumor cell populations with clusters of stem cells appearing at the tumor boundary, consistent with recent experiments. The nonlinear coupling between the heterogeneous expressions of growth factors and the heterogeneous distributions of cell populations at different lineage stages tends to create asymmetry in tumor shape that may sufficiently alter otherwise homeostatic feedback so as to favor escape from growth control. This occurs in a setting of invasive fingering, and enhanced aggressiveness after standard therapeutic interventions. We find, however, that combination therapy involving differentiation promoters and radiotherapy is very effective in eradicating such a tumor.

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.

Targeting a cornerstone of radiation resistance: cancer stem cell

In radiation oncology, cancer stem cells (CSCs) have become an important research field. In fact, it appears that most cancer types contain populations of cells that exhibit stem-cell properties. CSCs have the ability to renew indefinitely, which can drive tumor development and metastatic invasion. As those cells are classically resistant to conventional chemotherapy and to radiation therapy, they may contribute to treatment failure and relapse. Over past decades, preclinical research has highlighted that variations in the CSCs content within tumor could affect their radiocurability by interfering with mechanisms of DNA repair, redistribution in the cell cycle, tumor cells repopulation, and hypoxia. It is now possible to isolate particular cells expressing specific surface markers and thus better investigating CSCs pathways. Numerous inhibitory agents targeting these specific signaling pathways, such as Notch and Wnt/B-catenin, are currently evaluated in early clinical trials. By targeting CSCs, tumor radioresistance could be potentially overcome to improve outcome for patients with solid malignancies. Radiation therapy using ion particles (proton and carbon) may be also more effective than classic photon on CSCs. This review presents the major pathophysiological mechanisms involved in CSCs radioresistance and recent developments for targeted strategies.

SPECT/CT imaging of hNIS-expression after intravenous delivery of an oncolytic adenovirus and 131I

Oncolytic adenoviruses can be engineered for better tumor selectivity, gene delivery and be armed for imaging and concentrating radionuclides into tumors for synergistic oncolysis. We constructed Ad5/3-hTERT-hNIS where replication is controlled by hTERT-promoter. Ad5/3-hTERT-hNIS expresses hNIS for imaging of transgene expression and for treatment of infected tumors by radioiodine. Ad5/3-hTERT-hNIS efficiently killed prostate cancer cells and induced iodine uptake in vitro and in vivo after intratumoral virus administration. Survival of mice treated with intravenous Ad5/3-hTERT-hNIS significantly prolonged survival over mock or radioiodine only but the combination of virus with radioiodine was not more effective than virus alone. Temporal and spatial changes in hNIS-expression during therapy were detected with SPECT, demonstrating feasibility of evaluation of the combination therapy with hNIS-expressing adenoviruses and radioiodide.

Concise review: stem cell effects in radiation risk

Stem cells of normal mammalian tissues are defined as nonspecialized cells that have two critical properties: (a) the ability to renew themselves through cell division and (b) the potency to differentiate into other cell types. Therefore, they play a crucial role in development and in tissue homeostasis during adult life. Being long-lived, they can be the targets of environmental carcinogens leading to the accumulation of consecutive genetic changes. Hence, the genome of stem cells must be exceptionally well protected, and several protective mechanisms have evolved to ensure the genetic integrity of the stem cell compartment in any given tissue. Ionizing radiation exposure can disrupt tissue homeostasis both through the induction of cell killing/depletion of radiosensitive stem cells, leading to loss of tissue functionality and by genotoxic damage, increasing overall risk of cancer. We will review the current knowledge about radiation effects in adult stem cells of specific normal tissues, including skin, breast, and brain, examine parallels, as well as differences with cancer stem cells, and discuss the relevance of stem cell effects to radiation risk and radiotherapy.

CD24(-/low) stem-like breast cancer marker defines the radiation-resistant cells involved in memorization and transmission of radiation-induced genomic instability

A growing body of evidence attributes properties of chemo- and/or radiation-resistance to cancer stem cells (CSCs). Moreover, non-targeted delayed effects such as genomic instability, transmitted through many generations, can be observed in the progeny of surviving irradiated cells. As a consequence, we propose that radiation-resistance properties associated to CSCs could confer a key role to this subpopulation in the transmission of genomic instability. To test this hypothesis, we searched the CSC markers associated to radiation-resistance in breast cancer cell lines and studied the role of the resistant cells in the transmission of genomic instability. First, we show that irradiation induces a 2-4 weeks period of intense cell death leading to the emergence of chromosomal unstable cells during more than 35 population doublings. Then, among seven breast CSC markers, we identify CD24(-/low) labelling as a marker of radiation-resistance. We demonstrate that CD24(+) progeny of irradiated cells exclusively descends from CD24(-/low) cells. Finally, we show that delayed chromosomal instability is only expressed by CD24(+) cells, but is transmitted by stable surviving CD24(-/low) cells. So, for the first time a CSC marker, CD24, is associated with the transmission of genomic instability. This work may assign a new deleterious role to breast CSCs in aggressive recurrence after radiotherapy, as the transmitted genomic instability potentially leads tumour cells to acquire more aggressive characteristics.

Cancer stem cells, tumor dormancy, and metastasis

Tumor cells can persist undetectably for an extended period of time in primary tumors and in disseminated cancer cells. Very little is known about why and how these tumors persist for extended periods of time and then evolve to malignancy. The discovery of cancer stem cells (CSCs) in human tumors challenges our current understanding of tumor recurrence, drug resistance, and metastasis, and opens up new research directions on how cancer cells are capable of switching from dormancy to malignancy. Although overlapping molecules and pathways have been reported to regulate the stem-like phenotype of CSCs and metastasis, accumulated evidence has suggested additional clonal diversity within the stem-like cancer cell subpopulation. This review will describe the current hypothesis linking CSCs and metastasis and summarize mechanisms important for metastatic CSCs to re-initiate tumors in the secondary sites. A better understanding of CSCs' contribution to clinical tumor dormancy and metastasis will provide new therapeutic revenues to eradicate metastatic tumors and significantly reduce the mortality of cancer patients.

Impact of Autophagy on Chemotherapy and Radiotherapy Mediated Tumor Cytotoxicity: "To Live or not to Live"

Autophagy, a highly regulated cell “self-eating” pathway, is controlled by the action of over 34 autophagy-related proteins (collectively termed Atgs). Although they are fundamentally different processes, autophagy and apoptosis (type I programmed cell death), under certain circumstances, can be regulated by common signaling mediators. Current cancer therapies including chemotherapy and ionizing radiation are known to induce autophagy within tumor cells. However, autophagy plays a dual role of either pro-cell survival or pro-cell death in response to these cancer treatments, depending on the cellular context and the nature of the treatment. We review the current basic and translational cancer research literature on how autophagy impacts tumor cell survival (“to live”) and death (“not to live”) following treatment as well as the role of chemical mediators of autophagy.

Isolation and applications of prostate side population cells based on dye cycle violet efflux

This unit describes methods for digestion of human prostate clinical specimens and dye cycle violet (DCV) staining for identification, isolation, and quantitation of radiolabeled dihydrotestosterone (DHT) retention of side population cells. The principle of the side population assay is based on differential efflux of DCV, a cell-membrane-permeable fluorescent dye, by cells with high ATP-binding cassette (ABC) transporter activity. Cells with high ABC transporter activity that efflux DCV and fall in the lower left quadrant of a flow cytograph are designated as "side population" cells. This unit emphasizes tissue digestion, DCV staining, flow settings for sorting side population cells, and quantitation of radiolabeled DHT retention.

Associating GWAS Information with the Notch Signaling Pathway Using Transcription Profiling

Genome-wide association studies (GWAS) have identified SNPs associated with breast cancer. However, they offer limited insights about the biological mechanisms by which SNPs confer risk. We investigated the association of GWAS information with a major oncogenic pathway in breast cancer, the Notch signaling pathway. We first identified 385 SNPs and 150 genes associated with risk for breast cancer by mining data from 41 GWAS. We then investigated their expression, along with 32 genes involved in the Notch signaling pathway using two publicly available gene expression data sets from the Caucasian (42 cases and 143 controls) and Asian (43 cases and 43 controls) populations. Pathway prediction and network modeling confirmed that Notch receptors and genes involved in the Notch signaling pathway interact with genes containing SNPs associated with risk for breast cancer. Additionally, we identified other SNP-associated biological pathways relevant to breast cancer, including the P53, apoptosis and MAP kinase pathways.

Targeting autophagy during cancer therapy to improve clinical outcomes

Autophagy is a catabolic process that turns over long-lived proteins and organelles and contributes to cell and organism survival in times of stress. Current cancer therapies including chemotherapy and radiation are known to induce autophagy within tumor cells. This is therefore an attractive process to target during cancer therapy as there are safe, clinically available drugs known to both inhibit and stimulate autophagy. However, there are conflicting positive and negative effects of autophagy and no current consensus on how to manipulate autophagy to improve clinical outcomes. Careful and rigorous evaluation of autophagy with a focus on how to translate laboratory findings into relevant clinical therapies remains an important aspect of improving clinical outcomes in patients with malignant disease.

Are cancer stem cells radioresistant?

Based on findings that cancer cell clonogens exhibit stem cell features, it has been suggested that cancer stem-like cells are relatively radioresistant owing to different intrinsic and extrinsic factors, including quiescence, activated radiation response mechanisms (e.g., enhanced DNA repair, upregulated cell cycle control mechanisms and increased free-radical scavengers) and a surrounding microenvironment that enhances cell survival mechanisms (e.g., hypoxia and interaction with stromal elements). However, these radiosensitivity features are probably dynamic in nature and come into play at different times during the course of chemo/radiotherapy. Therefore, different molecularly targeted radiosensitization strategies may be needed at different stages of therapy. This article describes potential sensitization approaches based on the dynamics and changing properties of cancer stem-like cells during therapy.

Non-invasive in vivo imaging of tumor-associated CD133/prominin

Background

Cancer stem cells are thought to play a pivotal role in tumor maintenance, metastasis, tumor therapy resistance and relapse. Hence, the development of methods for non-invasive in vivo detection of cancer stem cells is of great importance.

Methodology/Principal Findings

Here, we describe successful in vivo detection of CD133/prominin, a cancer stem cell surface marker for a variety of tumor entities. The CD133-specific monoclonal antibody AC133.1 was used for quantitative fluorescence-based optical imaging of mouse xenograft models based on isogenic pairs of CD133 positive and negative cell lines. A first set consisted of wild-type U251 glioblastoma cells, which do not express CD133, and lentivirally transduced CD133-overexpressing U251 cells. A second set made use of HCT116 colon carcinoma cells, which uniformly express CD133 at levels comparable to primary glioblastoma stem cells, and a CD133-negative HCT116 derivative. Not surprisingly, visualization and quantification of CD133 in overexpressing U251 xenografts was successful; more importantly, however, significant differences were also found in matched HCT116 xenograft pairs, despite the lower CD133 expression levels. The binding of i.v.-injected AC133.1 antibodies to CD133 positive, but not negative, tumor cells isolated from xenografts was confirmed by flow cytometry.

Conclusions/Significance

Taken together, our results show that non-invasive antibody-based in vivo imaging of tumor-associated CD133 is feasible and that CD133 antibody-based tumor targeting is efficient. This should facilitate developing clinically applicable cancer stem cell imaging methods and CD133 antibody-based therapeutics.

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

Despite continuous improvements in cancer management, locoregional recurrence or metastatic spread still occurs in a high proportion of patients after radiotherapy or combined treatments. One underlying reason might be a low efficacy of current treatments on eradication of cancer stem cells (CSCs). It has been recognised for a long time, that only the small subpopulation of CSCs can cause recurrences and that all CSCs need to be killed for permanent tumour cure. However, only recently novel technologies have allowed to enrich CSCs and to investigate their biology. An emerging experimental and clinical database provides first hints that cell populations accumulated by putative stem cell markers or tumours that highly express such markers may be more radioresistant than their marker-negative counterparts. Other data support a higher tolerance of CSCs to hypoxia and preferential location in specific microenvironmental niches. However, conflicting data, methodological problems of the assays and a generally small database on only few tumour types necessitate further large and well-designed prospective experimental and clinical investigations that specifically address this question to corroborate this hypothesis. If such investigations confirm biological differences between CSCs and non-CSCs, this would imply that novel treatment strategies need to be tested specifically for their effect on CSCs. Another implication is that also biomarkers for prediction of local tumour control after radiotherapy or combined treatments need to reflect the behaviour of CSCs and not of the bulk of all cancer cells. This review discusses the importance of CSCs for treatment failure and challenges occurring from the CSC concept for cancer diagnosis, treatment and prediction of outcome. It is concluded that CSC-based endpoints and biomarkers are eventually expected to considerably improve tumour cure rates in the clinics through individualised tailoring of treatment.