Stem cell self-renewal in intestinal crypt

As a rapidly cycling tissue capable of fast repair and regeneration, the intestinal epithelium has emerged as a favored model system to explore the principles of adult stem cell biology. However, until recently, the identity and characteristics of the stem cell population in both the small intestine and colon has remained the subject of debate. Recent studies based on targeted lineage tracing strategies, combined with the development of an organotypic culture system, have identified the crypt base columnar cell as the intestinal stem cell, and have unveiled the strategy by which the balance between proliferation and differentiation is maintained. These results show that intestinal stem cells operate in a dynamic environment in which frequent and stochastic stem cell loss is compensated by the proliferation of neighboring stem cells. We review the basis of these experimental findings and the insights they offer into the mechanisms of homeostatic stem cell regulation.     

Regulation of self-renewal in normal and cancer stem cells

Mutations can confer a selective advantage on specific cells, enabling them to go through the multistep process that leads to malignant transformation. The cancer stem cell hypothesis postulates that only a small pool of low-cycling stem-like cells is necessary and sufficient to originate and develop the disease. Normal and cancer stem cells share important functional similarities such as 'self-renewal' and differentiation potential. However, normal and cancer stem cells have different biological behaviours, mainly because of a profound deregulation of self-renewal capability in cancer stem cells. Differences in mode of division, cell-cycle properties, replicative potential and handling of DNA damage, in addition to the activation/inactivation of cancer-specific molecular pathways confer on cancer stem cells a malignant phenotype. In the last decade, much effort has been devoted to unravel the complex dynamics underlying cancer stem cell-specific characteristics. However, further studies are required to identify cancer stem cell-specific markers and targets that can help to confirm the cancer stem cell hypothesis and develop novel cancer stem cell-based therapeutic approaches.     

DNA damage causes TP53-dependent coupling of self-renewal and senescence pathways in embryonal carcinoma cells

Comment on: Wu R, et al. Clin Cancer Res 2011; 17:7359–72     

Stem cell function, self-renewal, and behavioral heterogeneity of cells from the adult muscle satellite cell niche

Satellite cells are situated beneath the basal lamina that surrounds each myofiber and function as myogenic precursors for muscle growth and repair. The source of satellite cell renewal is controversial and has been suggested to be a separate circulating or interstitial stem cell population. Here, we transplant single intact myofibers into radiation-ablated muscles and demonstrate that satellite cells are self-sufficient as a source of regeneration. As few as seven satellite cells associated with one transplanted myofiber can generate over 100 new myofibers containing thousands of myonuclei. Moreover, the transplanted satellite cells vigorously self-renew, expanding in number and repopulating the host muscle with new satellite cells. Following experimental injury, these cells proliferate extensively and regenerate large compact clusters of myofibers. Thus, within a normally stable tissue, the satellite cell exhibits archetypal stem cell properties and is competent to form the basal origin of adult muscle regeneration.     

Stem cells and brain cancer

An increasing body of research is showing that cancers might contain their own stem cells. In fact, cancer cells, like stem cells, can proliferate indefinitely through a deregulated cellular self-renewal capacity. This raises the possibility that some features of tumor cells may be due to cancer stem cells. Stem cell-like cancer cells were isolated from several solid tumors. Now, evidence has shown that brain cancers, such as glioblastomas, medulloblastomas and astrocytomas, also contain cells that may be multipotent neural stem cell-like cells. In this review, we discuss the results of these studies, along with the molecular pathways that could be involved in cancer stem cell physiopathology.     

Stem cell self-renewal: centrosomes on the move

Three recent studies show that centrosome asymmetry correlates with self-renewal of Drosophila neural and germline stem cells and that equalizing centrosomes disrupts asymmetric cell division.     

Targeting proapoptotic protein BAD inhibits survival and self-renewal of cancer stem cells

Emerging evidence suggests that the resistance of cancer stem cells (CSC) to many conventional therapies is one of the major limiting factors of cancer therapy efficacy. Identification of mechanisms responsible for survival and self-renewal of CSC will help design new therapeutic strategies that target and eliminate both differentiated cancer cells and CSC. Here we demonstrated the potential role of proapoptotic protein BAD in the biology of CSC in melanoma, prostate and breast cancers. We enriched CD44⁺/CD24⁻ cells (CSC) by tumorosphere formation and purified this population by FACS. Both spheres and CSC exhibited increased potential for proliferation, migration, invasion, sphere formation, anchorage-independent growth, as well as upregulation of several stem cell-associated markers. We showed that the phosphorylation of BAD is essential for the survival of CSC. Conversely, ectopic expression of a phosphorylation-deficient mutant BAD induced apoptosis in CSC. This effect was enhanced by treatment with a BH3-mimetic, ABT-737. Both pharmacological agents that inhibit survival kinases and growth factors that are involved in drug resistance delivered their respective cytotoxic and protective effects by modulating the BAD phosphorylation in CSC. Furthermore, the frequency and self-renewal capacity of CSC was significantly reduced by knocking down the BAD expression. Consistent with our in vitro results, significant phosphorylation of BAD was found in CD44⁺ CSC of 83% breast tumor specimens. In addition, we also identified a positive correlation between BAD expression and disease stage in prostate cancer, suggesting a role of BAD in tumor advancement. Our studies unveil the role of BAD in the survival and self-renewal of CSC and propose BAD not only as an attractive target for cancer therapy but also as a marker of tumor progression.Although tumors initially respond positively to anti-cancer agents, several cancers, despite the best care and significant improvements in treatment, recur and progress to advanced stages of the disease. The mechanisms underlying this recurrence and metastasis are not clearly understood. Over the past decade, substantial evidence supported the cancer stem cell (CSC) hypothesis as a viable explanation for the initiation, progression and recurrence of cancer. According to this hypothesis, each tumor harbors a small subpopulation of specialized cells among cellular heterogeneity, known as CSC. These cells exhibit self-renewal property that drives tumorigenesis and plasticity to differentiate into multiple cell types contributing to tumor cellular heterogeneity. Support for this hypothesis came from the studies by Lapidot et al. who identified tumor-initiating cells in acute myeloid leukemia.^{1, 2} Subsequently, CSCs have been identified in several cancers.^{3, 4, 5, 6, 7, 8, 9, 10}Accumulating evidence suggests that current cancer therapies can only shrink tumors as they target and kill the differentiated cancer (DC) cells, but are unable to target the rare CSC population.^{11, 12} Thus, despite a wealth of information on DC cells, the active survival and self-renewal pathways in CSCs have not been characterized thoroughly. An understanding of the molecular mechanisms involved in the survival, self-renewal and resistance of CSCs to current therapeutic regimens is of immense clinical interest. This information will help in developing novel strategies for more effective treatments for cancer.Most anti-cancer drugs exert their effects through triggering the apoptotic pathways. However, malignant cancer cells can escape apoptosis by altering the expression level of proapoptotic and antiapoptotic BCL-2 family members. Considering the potential role of BCL-2 family members in tumorigenesis and cancer cell survival, their role in CSC biology has been increasingly studied.^{13, 14} BAD (BCL2-antagonist of cell death) is a member of the BH3-only BCL-2 family protein that when dephosphorylated promotes apoptosis by heterodimerizing with the antiapoptotic proteins BCL-XL and BCL-2.¹⁵ The cytotoxic effects of BAD are controlled by mechanisms that regulate its phosphorylation on at least two distinct serine residues, S112 and S136.^{16, 17, 18} Previously, we showed that phosphorylation at either site is sufficient to protect prostate cancer cells from apoptosis.^{19, 20, 21} We also showed that BAD promotes prostate tumor growth in mouse models.²² Clinically, while BAD expression was associated with relapse in tamoxifen-treated breast cancer patients,^{23, 24} phospho-BAD expression was associated with cisplatin resistance and poor overall survival in ovarian cancer.²⁵Our previous findings along with other reports showing the role of BAD in the apoptosis modulation and growth of DC cells^{19, 22, 26} prompted us to explore the potential role of BAD in the biology of CSCs. We started our investigation by assessing the role of BAD in survival and self-renewal of CSCs. As we observed a significant role for BAD in CSC''s biology, we extended our work to assess the BAD phosphorylation in CSCs of breast cancer patient tumors and for a potential correlation between BAD expression and disease progression in prostate cancer.     

Stem cells in normal breast development and breast cancer

Abstract.  The main focus of this review is the role of mammary stem cells in normal breast development and carcinogenesis. We have developed a new in vitro culture system that permits, for the first time, the propagation of mammary stem and progenitor cells in an undifferentiated state, which should facilitate the elucidation of pathways that regulate normal mammary stem-cell self-renewal and differentiation. Furthermore, we propose a model in which transformation of stem cells, or early progenitor cells, results in carcinogenesis. A key event in this process is the deregulation of normal self-renewal in these cells. Transformed mammary stem or progenitor cells undergo aberrant differentiation processes that result in generation of the phenotypic heterogeneity found in human and rodent breast cancers. This phenotypic diversity is driven by a small subset of mammary tumour stem cells. We will discuss the important implications of this mammary tumour stem-cell model.     

Telomeres,senescence, and hematopoietic stem cells

The replicative lifespan of normal somatic cells is restricted by the erosion of telomeres, which are protective caps at the ends of linear chromosomes. The loss of telomeres induces antiproliferative signals that eventually lead to cellular senescence. The enzyme complex telomerase can maintain telomeres, but its expression is confined to highly proliferative cells such as stem cells and tumor cells. The immense regenerative capacity of the hematopoietic system is provided by a distinct type of adult stem cell: hematopoietic stem cells (HSCs). Although blood cells have to be produced continuously throughout life, the HSC pool seems not to be spared by aging processes. Indeed, limited expression of telomerase is not sufficient to prevent telomere shortening in these cells, which is thought ultimately to limit their proliferative capacity. In this review, we discuss the relevance of telomere maintenance for the hematopoietic stem cell compartment and consider potential functions of telomerase in this context. We also present possible clinical applications of telomere manipulation in HSCs and new insights affecting the aging of the hematopoietic stem cell pool and replicative exhaustion. This work was supported by European Community Grant LSHC-CT-2004-502943 (MOL CANCER MED).     

Recent advances in cellular senescence, cancer and aging

How much do we know about the biology of aging from cell culture studies? Most normal somatic cells have a finite potential to divide due to a process termed cellular or replicative senescence. A growing body of evidence suggests that senescence evolved to protect higher eukaryotes, particularly mammals, from developing cancer. We now know that telomere shortening, due to the biochemistry of DNA replication, induces replicative senescence in human cells. However, in rodent cells, replicative senescence occurs despite very long telomeres. Recent findings suggest that replicative senescence is just the tip of the iceberg of a more general process termed cellular senescence. It appears that cellular senescence is a response to potentially oncogenic insults, including oxidative damage. In young organisms, growth arrest by cell senescence suppresses tumor development, but later in life, due to the accumulation of senescent cells which secret factors that can disrupt tissues during aging, cellular senescence promotes tumorigenesis. Therefore, antagonistic pleiotropy may explain in part, if not in whole, the apparently paradoxical effects of cellular senescence, though this still remains an open question.     

Autophagy,senescence and tumor dormancy in cancer therapy

The relationships between autophagy and apoptosis have been examined quite extensively and have often been shown to be reciprocally regulated responses to stresses such as exposure of the tumor cells to chemotherapeutic drugs and radiation. However, there is now evidence that autophagy may also play a role in tumor dormancy. Given that tumor dormancy and disease recurrence are poorly understood phenomenon which are nevertheless critical elements of patient morbidity and mortality, this commentary develops the postulate that autophagy and senescence may be related responses that influence the capacity of the tumor cell to maintain a prolonged state of growth arrest that can be succeeded by tumor regrowth and disease recurrence.     

Stem cell self-renewal: The role of asymmetric division

Asymmetric division occurs widely in different groups of organisms from single-celled to insects, mammals, and plants. The operation of asymmetrical division may differ widely in different organisms. In multicellular organisms, asymmetrical division is one of the essential features of stem cell biology. The data obtained assume one of the main biological functions of asymmetrical division to be maintenance of cell viability, beginning with stem cells. Cells continuously accumulate toxic inclusions, which are formed by damaged proteins which cannot be degraded by proteasomes. As a result of asymmetric division, these inclusions segregate into one of the daughter cells providing the ability of long-lived proliferation to another cell.     

Pancreatic stellate cells form a niche for cancer stem cells and promote their self-renewal and invasiveness

Chronic pancreatitis and pancreatic ductal adenocarcinoma (PDAC) are characterized by extensive fibrosis. Importantly, in PDAC, this results in poor vascularization and impaired drug delivery to the cancer cells. Therefore, the combined targeting of pancreatic tumor stroma and chemotherapy should enhance response rates, but the negative outcome of a recent phase III clinical trial for the combination of chemotherapy and hedgehog pathway inhibition suggests that other means also need to be considered. Emerging data indicate that elimination of cancer stem cells as the root of the cancer is of pivotal importance for efficient treatment of pancreatic cancer. Recently, we demonstrated in a highly relevant preclinical mouse model for primary pancreatic cancers that the combination of cancer stem cell-targeting strategies in combination with a stroma-targeting agent, such as a hedgehog pathway inhibitor and chemotherapy, results in significantly enhanced long-term and progression-free survival. In the present study, we demonstrate mechanistically that Nodal-expressing pancreatic stellate cells are an important component of the tumor stroma for creating a paracrine niche for pancreatic cancer stem cells. Secretion of the embryonic morphogens Nodal/Activin by pancreatic stellate cells promoted in vitro sphere formation and invasiveness of pancreatic cancer stem cells in an Alk4-dependent manner. These data imply that the pancreatic cancer stem cell phenotype is promoted by paracrine Nodal/Activin signaling at the tumor-stroma interface. Therefore, targeting the tumor microenvironment is not only able to improve drug delivery but, even more importantly, destroys the cancer stem cell niche and, therefore, should be an integral part of cancer stem cell-based treatment strategies.     

Stem cells and cancer; the polycomb connection

Proteins from the Polycomb group (PcG) are epigenetic chromatin modifiers involved in cancer development and also in the maintenance of embryonic and adult stem cells. The therapeutic potential of stem cells and the growing conviction that tumors contain stem cells highlights the importance of understanding the extrinsic and intrinsic circuitry controlling stem cell fate and their connections to cancer.