Recent advances in cancer stem cells

The theory of cancer stem cells states that a subset of cancer cells within a tumor has the ability to self-renew and differentiate. Only those cells within a tumor that have these two properties are called cancer stem cells. This concept was first demonstrated in the study of leukemia where only cells with specific surface antigen profiles were able to cause leukemia when engrafted into immunodeficient mice. In recent years solid tumors were studied utilizing similar techniques in mice. Human tumors where evidence of cancer stem cells has been published include tumors of the breast, brain, pancreas, head and neck, and colon. If this difference in tumorigenicity of cancer cells also occurs in patients, then the ability to enrich for cancer stem cells lays an important groundwork for future studies where mechanisms involved in cancer stem cells can now be investigated.     

Perspectives on brain tumor formation involving macrophages, glia, and neural stem cells

The incidence of brain tumors is rising in children and the elderly, but little is known about the mechanisms underlying brain tumor initiation and progression. In the 1940s, Zimmerman and coworkers exploited the tumor-promoting potential of polycyclic hydrocarbons to produce brain tumor models in adult mice that simulated the neuropathology of human brain tumors. Based on these early findings and on recent neurobiological studies of stem cells, I propose that crystalline carcinogenic pellets surgically implanted in the central nervous system establish over time a microenvironment that fosters proliferation and genetic damage in neural stem cells and their progenitors. Moreover, activated glia (microglia and astrocytes) and recruited macrophages mediate these processes. Gradually local tissue fields, which normally restrict stem cell proliferation, become disorganized, leading to further stem cell proliferation, genetic damage, and eventual neoplasia. Depending on age, location, and the state of glial/macrophage activation, the resulting brain tumor may resemble transformed neural progenitors aborted in more or less differentiated states. This hypothesis integrates the general mechanisms by which neural stem cells, glia, and macrophages orchestrate the initiation and progression of brain cancer. Also discussed are implications of these concepts for the diagnosis and therapy of human brain tumors.     

Metastatic cancer stem cells: A new target for anti-cancer therapy?

Over the past few years, supporting evidence for the cancer stem cell hypothesis has been provided for an increasing number of tumor entities. According to this hypothesis, only a small population of undifferentiated cells with stem cell characteristics has the ability to form tumors through asymmetric division and subsequent differentiation of the progeny into the heterogeneous cell types that comprise a tumor. Recently, we were able to show that cancer stem cells are not only responsible for tumorigenesis, but that they contain a subpopulation characterized by CXCR4 expression which is exclusively capable of disseminating and subsequently providing the substrate for tumor metastasis. Of note, these recent advances in our understanding of cancer stem cell biology raise more questions than they answer. Some of these arising questions regarding the targeted elimination of these cancer stem cells will be addressed in this perspective.     

Systems biology and cancer stem cells

The identification, purification, and characterization of cancer stem cells holds tremendous promise for improving the treatment of cancer. Mounting evidence is demonstrating that only certain tumor cells (i.e. the cancer stem cells) can give rise to tumors when injected and that these purified cell populations generate heterogeneous tumors. While the cell of origin is still not determined definitively, specific molecular markers for populations containing these cancer stem cells have been found for leukemia, brain cancer, and breast cancer, among others. Systems approaches, particularly molecular profiling, have proven to be of great utility for cancer diagnosis and characterization. These approaches also hold significant promise for identifying distinctive properties of the cancer stem cells, and progress is already being made.     

Overexpression of Calcium-Permeable Glutamate Receptors in Glioblastoma Derived Brain Tumor Initiating Cells

Glioblastoma multiforme is the most malignant type of primary brain tumor with a poor prognosis. These tumors consist of a heterogeneous population of malignant cells, including well-differentiated tumor cells and less differentiated cells with stem cell properties. These cancer stem cells, known as brain tumor initiating cells, likely contribute to glioma recurrence, as they are highly invasive, mobile, resistant to radiation and chemotherapy, and have the capacity to self-renew. Glioblastoma tumor cells release excitotoxic levels of glutamate, which may be a key process in the death of peritumoral neurons, formation of necrosis, local inflammation, and glioma-related seizures. Moreover, elevated glutamate levels in the tumor may act in paracrine and autocrine manner to activate glutamate receptors on glioblastoma tumor cells, resulting in proliferation and invasion. Using a previously described culturing condition that selectively promotes the growth of brain tumor initiating cells, which express the stem cell markers nestin and SOX-2, we characterize the expression of α-amino-3-hydroxy-5-methyl-4-isozolepropionic acid (AMPA)-type glutamate receptor subunits in brain tumor initiating cells derived from glioblastomas. Here we show for the first time that glioblastoma brain tumor initiating cells express high concentrations of functional calcium-permeable AMPA receptors, compared to the differentiated tumor cultures consisting of non-stem cells. Up-regulated calcium-permeable AMPA receptor expression was confirmed by immunoblotting, immunocytochemistry, and intracellular calcium imaging in response to specific agonists. Our findings raise the possibility that glutamate secretion in the GBM tumor microenvironment may stimulate brain tumor derived cancer stem cells.     

Identification of single chain antibodies to breast cancer stem cells using phage display

Recent evidence suggests that most malignancies are driven by “cancer stem cells” sharing the signature characteristics of adult stem cells: the ability to self renew and to differentiate. Furthermore these cells are thought to be quiescent, infrequently dividing cells with a natural resistance to chemotherapeutic agents. These studies theorize that therapies, which effectively treat the majority of tumor cells but ‘miss’ the stem cell population, will fail, while therapies directed at stern cells can potentially eradicate tumors. In breast cancer, researchers have isolated ‘breast cancer stem cells’ capable of recreating the tumor in vivo and in vitro. Generated new tumors contained both additional numbers of cancer stem cells and diverse mixed populations of cells present in the initial tumor, supporting the intriguing self‐renewal and differentiation characteristics. In the present study, an antibody phage library has been used to search for phage displayed‐single chain antibodies (scFv) with selective affinity to specific targets on breast cancer stem cells. We demonstrate evidence of two clones binding specifically to a cancer stem cell population isolated from the SUMl59 breast cancer cell line. These clones had selective affinity for cancer stem cells and they were able to select cancer stem cells among a large population of non‐stem cancer cells in paraffin‐embedded sections. The applicability of these clones to paraffin sections and frozen tissue specimens made them good candidates to be used as diagnostic and prognostic markers in breast cancer patient samples taking into consideration the cancer stern cell concept in tumor biology. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

Epidermal growth factor plays a crucial role in mitogenic regulation of human brain tumor stem cells

A cancer stem cell population in malignant brain tumors takes an essential part in brain tumor initiation, growth, and recurrence. Growth factors, such as epidermal growth factor, fibroblast growth factor-2, vascular endothelial growth factor, platelet-derived growth factor, and hepatocyte growth factor, are shown to support the proliferation of neural stem cells and also may play key roles in gliomagenesis. However, the responsible growth factor(s), which controls maintenance of brain tumor stem cells, is not yet uncovered. We have established three cancer stem cell lines from human gliomas. These cells were immunoreactive with the neuronal progenitor markers, nestin and CD133, and established tumors that closely resembled the features of original tumor upon transplantation into mouse brain. Three cell lines retained their self-renewal ability and proliferation only in the presence of epidermal growth factor (>2.5 ng/ml). In sharp contrast, other growth factors, including fibroblast growth factor-2, failed to support maintenance of these cells. The tyrosine kinase inhibitors of epidermal growth factor signaling (AG1478 and gefitinib) suppressed the proliferation and self-renewal of these cells. Gefitinib inhibited phosphorylation of epidermal growth factor receptor as well as Akt kinase and extracellular signal-regulated kinase 1/2. Flow cytometric analysis revealed that epidermal growth factor concentration-dependently increased the population of CD133-positive cells. Gefitinib significantly reduced CD133-positive fractions and also induced their apoptosis. These results indicate that maintenance of human brain tumor stem cells absolutely requires epidermal growth factor and that tyrosine kinase inhibitors of epidermal growth factor signaling potentially inhibit proliferation and induce apoptosis of these cells.     

Bladder cancer stem cells

Stem cells are undifferentiated cells that renew themselves while simultaneously producing differentiated tissue- or organspecific cells through asymmetric cell division. The appreciation of the importance of stem cells in normal tissue biology has prompted the idea that cancers may also develop from a progenitor pool (the "cancer stem cell (CSC) hypothesis"), and this idea is gaining increasing acceptance among scientists. CSCs are sub-populations of cancer cells responsible for tumor initiation, differentiation, recurrence, metastasis, and drug resistance. First identified in the hematopoietic system, CSCs have also been discovered in solid tumors of the breast, colon, pancreas, and brain. Recently, the tissue-specific stem cells of the normal urothelium have been proposed to reside in the basal layer, and investigators have isolated phenotypically similar populations of cells from urothelial cancer cell lines and primary tumors. Herein, we review the CSC hypothesis and apply it to explain the development of the two different types of bladder cancer: noninvasive ("superficial") carcinoma and invasive carcinoma. We also examine potential approaches to identify CSCs in bladder cancer as well as therapeutic applications of these findings. While exciting, the verification of the existence of CSCs in bladder cancer raises several new questions. Herein, we identify and answer some of these questions to help readers better understand bladder cancer development and identify reasonable therapeutic strategy for targeting stem cells.     

脑肿瘤干细胞

脑肿瘤尤其是恶性脑胶质瘤,由于生长及复发快,预后极差,所以找到胶质瘤复发的根源,提高胶质瘤病人的存活率,已成为国内外的肿瘤生物学工作者和临床医学工作者亟待解决的难题。近年来肿瘤干细胞概念的提出及脑肿瘤干细胞的分离及鉴定,为脑肿瘤的研究提供了新的切入点,同时可成为肿瘤治疗新的靶标,为根治脑肿瘤带来了光明的前景。简要综述了脑肿瘤干细胞无限增殖、自我更新、多分化潜能的生物学特性,脑肿瘤干细胞的起源以及与脑肿瘤相关机制方面的研究进展,从而为今后脑肿瘤早期诊断、治疗以及以此为靶标的药物开发提供新的思路和方向。     

Distinct populations of cancer stem cells determine tumor growth and metastatic activity in human pancreatic cancer

Pancreatic adenocarcinoma is currently the fourth leading cause for cancer-related mortality. Stem cells have been implicated in pancreatic tumor growth, but the specific role of these cancer stem cells in tumor biology, including metastasis, is still uncertain. We found that human pancreatic cancer tissue contains cancer stem cells defined by CD133 expression that are exclusively tumorigenic and highly resistant to standard chemotherapy. In the invasive front of pancreatic tumors, a distinct subpopulation of CD133(+) CXCR4(+) cancer stem cells was identified that determines the metastatic phenotype of the individual tumor. Depletion of the cancer stem cell pool for these migrating cancer stem cells virtually abrogated the metastatic phenotype of pancreatic tumors without affecting their tumorigenic potential. In conclusion, we demonstrate that a subpopulation of migrating CD133(+) CXCR4(+) cancer stem cells is essential for tumor metastasis. Strategies aimed at modulating the SDF-1/CXCR4 axis may have important clinical applications to inhibit metastasis of cancer stem cells.     

Cancer stem cells in glioblastoma — molecular signaling and therapeutic targeting

Glioblastomas (GBMs) are highly lethal primary brain tumors. Despite current therapeutic advances in other solid cancers, the treatment of these malignant gliomas remains essentially palliative. GBMs are extremely resistant to conventional radiation and chemotherapies. We and others have demonstrated that a highly tumorigenic subpopulation of cancer cells called GBM stem cells (GSCs) promotes therapeutic resistance. We also found that GSCs stimulate tumor angiogenesis by expressing elevated levels of VEGF and contribute to tumor growth, which has been translated into a useful therapeutic strategy in the treatment of recurrent or progressive GBMs. Furthermore, stem cell-like cancer cells (cancer stem cells) have been shown to promote metastasis. Although GBMs rarely metastasize beyond the central nervous system, these highly infiltrative cancers often invade into normal brain tissues preventing surgical resection, and GSCs display an aggressive invasive phenotype. These studies suggest that targeting GSCs may effectively reduce tumor recurrence and significantly improve GBM treatment. Recent studies indicate that cancer stem cells share core signaling pathways with normal somatic or embryonic stem cells, but also display critical distinctions that provide important clues into useful therapeutic targets. In this review, we summarize the current understanding and advances in glioma stem cell research, and discuss potential targeting strategies for future development of anti-GSC therapies.     

TGF-beta in neural stem cells and in tumors of the central nervous system

Mechanisms that regulate neural stem cell activity in the adult brain are tightly coordinated. They provide new neurons and glia in regions associated with high cellular and functional plasticity, after injury, or during neurodegeneration. Because of the proliferative and plastic potential of neural stem cells, they are currently thought to escape their physiological control mechanisms and transform to cancer stem cells. Signals provided by proteins of the transforming growth factor (TGF)-beta family might represent a system by which neural stem cells are controlled under physiological conditions but released from this control after transformation to cancer stem cells. TGF-beta is a multifunctional cytokine involved in various physiological and patho-physiological processes of the brain. It is induced in the adult brain after injury or hypoxia and during neurodegeneration when it modulates and dampens inflammatory responses. After injury, although TGF-beta is neuroprotective, it may limit the self-repair of the brain by inhibiting neural stem cell proliferation. Similar to its effect on neural stem cells, TGF-beta reveals anti-proliferative control on most cell types; however, paradoxically, many brain tumors escape from TGF-beta control. Moreover, brain tumors develop mechanisms that change the anti-proliferative influence of TGF-beta into oncogenic cues, mainly by orchestrating a multitude of TGF-beta-mediated effects upon matrix, migration and invasion, angiogenesis, and, most importantly, immune escape mechanisms. Thus, TGF-beta is involved in tumor progression. This review focuses on TGF-beta and its role in the regulation and control of neural and of brain-cancer stem cells. This work was supported by the German Federal Ministry of Education and Research (BMBF no. 01GA0510 and no. 0312134) and by the Bavarian State Ministry of Sciences, Research and the Arts, "Forneurocell grant".     

The therapeutic implications of plasticity of the cancer stem cell phenotype

The cancer stem cell hypothesis suggests that tumors contain a small population of cancer cells that have the ability to undergo symmetric self-renewing cell division. In tumors that follow this model, cancer stem cells produce various kinds of specified precursors that divide a limited number of times before terminally differentiating or undergoing apoptosis. As cells within the tumor mature, they become progressively more restricted in the cell types to which they can give rise. However, in some tumor types, the presence of certain extra- or intracellular signals can induce committed cancer progenitors to revert to a multipotential cancer stem cell state. In this paper, we design a novel mathematical model to investigate the dynamics of tumor progression in such situations, and study the implications of a reversible cancer stem cell phenotype for therapeutic interventions. We find that higher levels of dedifferentiation substantially reduce the effectiveness of therapy directed at cancer stem cells by leading to higher rates of resistance. We conclude that plasticity of the cancer stem cell phenotype is an important determinant of the prognosis of tumors. This model represents the first mathematical investigation of this tumor trait and contributes to a quantitative understanding of cancer.     

Gene Expression Analysis of PTEN Positive Glioblastoma Stem Cells Identifies DUB3 and Wee1 Modulation in a Cell Differentiation Model

The term astrocytoma defines a quite heterogeneous group of neoplastic diseases that collectively represent the most frequent brain tumors in humans. Among them, glioblastoma multiforme represents the most malignant form and its associated prognosis is one of the poorest among tumors of the central nervous system. It has been demonstrated that a small population of tumor cells, isolated from the brain neoplastic tissue, can reproduce the parental tumor when transplanted in immunodeficient mouse. These tumor initiating cells are supposed to be involved in cancer development and progression and possess stem cell-like features; like their normal counterpart, these cells remain quiescent until they are committed to differentiation. Many studies have shown that the role of the tumor suppressor protein PTEN in cell cycle progression is fundamental for tumor dynamics: in low grade gliomas, PTEN contributes to maintain cells in G1 while the loss of its activity is frequently observed in high grade gliomas. The mechanisms underlying the above described PTEN activity have been studied in many tumors, but those involved in the maintenance of tumor initiating cells quiescence remain to be investigated in more detail. The aim of the present study is to shed light on the role of PTEN pathway on cell cycle regulation in Glioblastoma stem cells, through a cell differentiation model. Our results suggest the existence of a molecular mechanism, that involves DUB3 and WEE1 gene products in the regulation of Cdc25a, as functional effector of the PTEN/Akt pathway.     

microRNA-34a is tumor suppressive in brain tumors and glioma stem cells

We recently found that microRNA-34a (miR-34a) is downregulated in human glioma tumors as compared to normal brain, and that miR-34a levels in mutant-p53 gliomas were lower than in wildtype-p53 tumors. We showed that miR-34a expression in glioma and medulloblastoma cells inhibits cell proliferation, G1/S cell cycle progression, cell survival, cell migration and cell invasion, but that miR-34a expression in human astrocytes does not affect cell survival and cell cycle. We uncovered the oncogenes c-Met, Notch-1 and Notch-2 as direct targets of miR-34a that are inhibited by miR-34a transfection. We found that c-Met levels in human glioma specimens inversely correlate with miR-34a levels. We showed that c-Met and Notch partially mediate the inhibitory effects of miR-34a on cell proliferation and cell death. We also found that mir-34a expression inhibits in vivo glioma xenograft growth. We concluded that miR-34a is a potential tumor suppressor in brain tumors that acts by targeting multiple oncogenes. In this extra view, we briefly review and discuss the implications of these findings and present new data on the effects of miR-34a in glioma stem cells. The new data show that miR-34a expression inhibits various malignancy endpoints in glioma stem cells. Importantly, they also show for the first time that miR-34a expression induces glioma stem cell differentiation. Altogether, the data suggest that miR-34a is a tumor suppressor and a potential potent therapeutic agent that acts by targeting multiple oncogenic pathways in brain tumors and by inducing the differentiation of cancer stem cells.     

Role of platelets and breast cancer stem cells in metastasis

The high mortality rate of breast cancer is mainly caused by the metastatic ability of cancer cells, resistance to chemotherapy and radiotherapy, and tumor regression capacity. In recent years, it has been shown that the presence of breast cancer stem cells is closely associated with the migration and metastatic ability of cancer cells, as well as with their resistance to chemotherapy and radiotherapy. The tumor microenvironment is one of the main molecular factors involved in cancer and metastatic processes development, in this sense it is interesting to study the role of platelets, one of the main communicator cells in the human body which are activated by the signals they receive from the microenvironment and can generate more than one response. Platelets can ingest and release RNA, proteins, cytokines and growth factors. After the platelets interact with the tumor microenvironment, they are called "tumor-educated platelets." Tumor-educated platelets transport material from the tumor microenvironment to sites adjacent to the tumor, thus helping to create microenvironments conducive for the development of primary and metastatic tumors. It has been observed that the clone capable of carrying out the metastatic process is a cancer cell with stem cell characteristics. Cancer stem cells go through a series of processes, including epithelial-mesenchymal transition, intravasation into blood vessels, movement through blood vessels, extravasation at the site of the establishment of a metastatic focus, and site colonization. Tumor-educated platelets support all these processes.