Interactions between cancer stem cells and their niche govern metastatic colonization

Cancer stem cell-related gene periostin: a novel prognostic marker for breast cancer

We investigated the expression status of periostin in breast cancer stem cells and its clinical implications in order to lay a foundation for managing breast cancer. CD44+/CD24−/line- tumor cells (CSC) from clinical specimens were sorted using flow cytometry. Periostin expression status was detected in CSC cells and 1,086 breast cancer specimens by Western blot and immunohistochemistry staining, with the CSC ratio determined by immunofluorescence double staining. The relationship between the periostin protein and clinico-pathological parameters and prognosis was subsequently determined. As a result, CSC cells are more likely to generate new tumors in mice and cell microspheres that are deficient in NOD/SCID compared to the control group. Periostin protein was expressed higher in CSC cells compared to the control cells and was found to be related to CSC chemotherapy resistance. Moreover, periostin expression was found to be related to the CSC ratio in 1,086 breast cancer specimens (P = 0.001). In total, 334 (30.76%) of the 1,086 breast cases showed high periostin expression. After universal and Spearman regression correlation analysis, periostin was observed to be related to histological grade, CSC ratio, lymph node metastasis, tumor size, and triple-negative breast cancer (all P<0.05). Furthermore, periostin was shown to attain a significantly more distant bone metastasis and worse disease-specific survival than those with none or low-expressed periostin protein (P = 0.001). In the Cox regression test, periostin protein was detected as an independent prognostic factor (P = 0.001). In conclusion, periostin was found to be related to the CSC and an independent prognostic factor for breast cancer. It is also perhaps a potential target to breast cancer.

Activating mutations in β-catenin in colon cancer cells alter their interaction with macrophages; the role of snail

BACKGROUND

Tumor cells become addicted to both activated oncogenes and to proliferative and pro-survival signals provided by the abnormal tumor microenvironment. Although numerous soluble factors have been identified that shape the crosstalk between tumor cells and stroma, it has not been established how oncogenic mutations in the tumor cells alter their interaction with normal cells in the tumor microenvironment.

PRINCIPAL FINDINGS

We showed that the isogenic HCT116 and Hke-3 cells, which differ only by the presence of the mutant kRas allele, both stimulate macrophages to produce IL1β. In turn, macrophages enhanced Wnt signaling, proliferation and survival in both HCT116 and Hke-3 cells, demonstrating that signaling by oncogenic kRas in tumor cells does not impact their interaction with macrophages. HCT116 cells are heterozygous for β-catenin (HCT116(WT/MT)), harboring one wild type (WT) and one mutant (MT) allele, but isogenic lines that carry only the WT (HCT116(WT)) or MT β-catenin allele (HCT116(MT)) have been generated. We showed that macrophages promoted Wnt signaling in cells that carry the MT β-catenin allele, but not in HCT116(WT) cells. Consistent with this observation, macrophages and IL1β failed to stabilize Snail in HCT116(WT) cells, and to protect these cells from TRAIL-induced apoptosis. Finally, we demonstrated that HCT116 cells expressing dominant negative TCF4 (dnTCF4) or HCT116 cells with silenced Snail failed to stimulate IL1β production in macrophages, demonstrating that tumor cells activate macrophages via a Wnt-dependent factor.

SIGNIFICANCE

Our data demonstrate that oncogenic β-catenin mutations in tumor cells, and subsequent activation of Wnt signaling, not only trigger cell-intrinsic alterations, but also have a significant impact on the crosstalk of tumor cells with the tumor associated macrophages.

Fibronectin and stem cell differentiation - lessons from chondrogenesis

The extracellular matrix (ECM) is an intricate network of proteins that surrounds cells and has a central role in establishing an environment that is conducive to tissue-specific cell functions. In the case of stem cells, this environment is the stem cell niche, where ECM signals participate in cell fate decisions. In this Commentary, we describe how changes in ECM composition and mechanical properties can affect cell shape and stem cell differentiation. Using chondrogenic differentiation as a model, we examine the changes in the ECM that occur before and during mesenchymal stem cell differentiation. In particular, we focus on the main ECM protein fibronectin, its temporal expression pattern during chondrogenic differentiation, its potential effects on functions of differentiating chondrocytes, and how its interactions with other ECM components might affect cartilage development. Finally, we discuss data that support the possibility that the fibronectin matrix has an instructive role in directing cells through the condensation, proliferation and/or differentiation stages of cartilage formation.

The pro-metastatic role of bone marrow-derived cells: a focus on MSCs and regulatory T cells

Several bone marrow-derived cells have been shown to promote tumour growth and progression. These cells can home to the primary tumour and become active components of the tumour microenvironment. Recent studies have also identified bone marrow-derived cells—such as mesenchymal stem cells and regulatory T cells—as contributors to cancer metastasis. The innate versatility of these cells provides diverse functional aid to promote malignancy, ranging from structural support to signal-mediated suppression of the host immune response. Here, we review the role of mesenchymal stem cells and regulatory T cells in cancer metastasis. A better understanding of the bipolar nature of these bone marrow-derived cells in physiological and malignant contexts could pave the way for new therapeutics against metastatic disease.

Regulation of beta catenin signaling and parathyroid hormone anabolic effects in bone by the matricellular protein periostin

Periostin (Postn) is a matricellular protein preferentially expressed by osteocytes and periosteal osteoblasts in response to mechanical stimulation and parathyroid hormone (PTH). Whether and how periostin expression influences bone anabolism, however, remains unknown. We investigated the skeletal response of adult Postn(-/-) and Postn(+/+) mice to intermittent PTH. Compared with Postn(+/+), Postn(-/-) mice had a lower bone mass, cortical bone volume, and strength response to PTH. PTH-stimulated bone-forming indices were all significantly lower in Postn(-/-) mice, particularly at the periosteum. Furthermore, in vivo stimulation of Wnt-β-catenin signaling by PTH, as evaluated in TOPGAL reporter mice, was inhibited in the absence of periostin (TOPGAL;Postn(-/-) mice). PTH stimulated periostin and inhibited MEF2C and sclerostin (Sost) expression in bone and osteoblasts in vitro. Recombinant periostin also suppressed Sost expression, which was mediated through the integrin αVβ3 receptor, whereas periostin-blocking antibody prevented inhibition of MEF2C and Sost by PTH. In turn, administration of a Sost-blocking antiboby partially restored the PTH-mediated increase in bone mass in Postn(-/-) mice. In addition, primary osteoblasts from Postn(-/-) mice showed a lower proliferation, mineralization, and migration, both spontaneously and in response to PTH. Osteoblastic gene expression levels confirmed a defect of Postn(-/-) osteoblast differentiation with and without PTH, as well as an increased osteoblast apoptosis in the absence of periostin. These data elucidate the complex role of periostin on bone anabolism, through the regulation of Sost, Wnt-β-catenin signaling, and osteoblast differentiation.

Regulation of beta catenin signaling and parathyroid hormone anabolic effects in bone by the matricellular protein periostin

Periostin (Postn) is a matricellular protein preferentially expressed by osteocytes and periosteal osteoblasts in response to mechanical stimulation and parathyroid hormone (PTH). Whether and how periostin expression influences bone anabolism, however, remains unknown. We investigated the skeletal response of adult Postn −/− and Postn +/+ mice to intermittent PTH. Compared with Postn +/+ , Postn −/− mice had a lower bone mass, cortical bone volume, and strength response to PTH. PTH-stimulated bone-forming indices were all significantly lower in Postn −/− mice, particularly at the periosteum. Furthermore, in vivo stimulation of Wnt-β-catenin signaling by PTH, as evaluated in TOPGAL reporter mice, was inhibited in the absence of periostin ( TOPGAL;Postn −/− mice). PTH stimulated periostin and inhibited MEF2C and sclerostin (Sost) expression in bone and osteoblasts in vitro. Recombinant periostin also suppressed Sost expression, which was mediated through the integrin αVβ3 receptor, whereas periostin-blocking antibody prevented inhibition of MEF2C and Sost by PTH. In turn, administration of a Sost-blocking antiboby partially restored the PTH-mediated increase in bone mass in Postn −/− mice. In addition, primary osteoblasts from Postn −/− mice showed a lower proliferation, mineralization, and migration, both spontaneously and in response to PTH. Osteoblastic gene expression levels confirmed a defect of Postn −/− osteoblast differentiation with and without PTH, as well as an increased osteoblast apoptosis in the absence of periostin. These data elucidate the complex role of periostin on bone anabolism, through the regulation of Sost, Wnt-β-catenin signaling, and osteoblast differentiation.

Lysophosphatidic acid-induced ADAM12 expression mediates human adipose tissue-derived mesenchymal stem cell-stimulated tumor growth

Lysophosphatidic acid (LPA) is involved in mesenchymal stem cell-stimulated tumor growth in vivo. However, the molecular mechanism by which mesenchymal stem cells promote tumorigenesis remains elusive. In the present study, we demonstrate that conditioned medium from A549 human lung adenocarcinoma cells (A549 CM) induced the expression of ADAM12, a disintegrin and metalloproteases family member, in human adipose tissue-derived mesenchymal stem cells (hASCs). A549 CM-stimulated ADAM12 expression was abrogated by pretreatment of hASCs with the LPA receptor 1 inhibitor Ki16425 or by small interfering RNA-mediated silencing of LPA receptor 1, suggesting a key role for the LPA-LPA receptor 1 signaling axis in A549 CM-stimulated ADAM12 expression. Silencing of ADAM12 expression using small interfering RNA or short hairpin RNA abrogated LPA-induced expression of both α-smooth muscle actin, a marker of carcinoma-associated fibroblasts, and ADAM12 in hASCs. Using a xenograft transplantation model of A549 cells, we demonstrated that silencing of ADAM12 inhibited the hASC-stimulated in vivo growth of A549 xenograft tumors and the differentiation of transplanted hASCs to α-smooth muscle actin-positive carcinoma-associated fibroblasts. LPA-conditioned medium from hASCs induced the adhesion of A549 cells and silencing of ADAM12 inhibited LPA-induced expression of extracellular matrix proteins, periostin and βig-h3, in hASCs and LPA-conditioned medium-stimulated adhesion of A549 cells. These results suggest a pivotal role for LPA-stimulated ADAM12 expression in tumor growth and the differentiation of hASCs to carcinoma-associated fibroblasts expressing α-smooth muscle actin, periostin, and βig-h3.

Modeling the connection between primary and metastatic tumors

We put forward a model for cancer metastasis as a migration phenomenon between tumor cell populations coexisting and evolving in two different habitats. One of them is a primary tumor and the other one is a secondary or metastatic tumor. The evolution of the different cell phenotype populations in each habitat is described by means of a simple quasispecies model allowing for a cascade of mutations between the different phenotypes in each habitat. The cell migration event is supposed to be unidirectional and take place continuously in time. The possible clinical outcomes of the model depending on the parameter space are analyzed and the effect of the resection of the primary tumor is studied.

Molecular biology of chronic myeloid leukemia

Detailed information on the crystal structure of the pharmacologically targeted domains of the BCR-ABL molecule and on its intracellular signaling, which are potentially involved in growth, anti-apoptosis, metabolism and stemness, has made the study of chronic myeloid leukemia the most successful field in tumor biology. However, we now face the issue of drug resistance due to deregulation in the quality control of both DNA and protein. BCR-ABL is basically a misfolded protein with intrinsically disordered regions, which not only produces endoplasmic reticulum stress followed by unfolded protein response in some settings, but also conformational plasticity that may affect the structure of the whole molecule. The intercellular signaling derived from the leukemic cell microenvironment may influence the intracellular responses that take place in a manner both dependent on and independent of BCR-ABL tyrosine kinase activity.

Periostin as a biomarker of the amniotic membrane

Tracing the precise developmental origin of amnion and amnion-derived stem cells is still challenging and depends chiefly on analyzing powerful genetic model amniotes like mouse. Profound understanding of the fundamental differences in amnion development in both the disc-shaped primate and human embryo and the cup-shaped mouse embryo is pivotal in particular when sampling amniotic membrane from nonprimate species for isolating candidate amniotic stem cells. The availability of molecular marker genes that are specifically expressed in the amniotic membrane and not in other extraembryonic membranes would be instrumental to validate unequivocally the starting material under investigation. So far such amniotic markers have not been reported. We postulated that bone morphogenetic protein (BMP) target genes are putative amniotic membrane markers mainly because deficiency in one of several components of the BMP signaling cascade in mice has been documented to result in defective development of the early amnion. Comparative gene expression analysis of acknowledged target genes for BMP in different extraembryonic tissues, combined with in situ hybridization, identified Periostin (Postn) mRNA enrichment in amnion throughout gestation. In addition, we identify and propose a combination of markers as transcriptional signature for the different extraembryonic tissues in mouse.

The malignant social network: cell-cell adhesion and communication in cancer stem cells

Tumors contain a vastly complicated cellular network that relies on local communication to execute malignant programs. The molecular cues that are involved in cell-cell adhesion orchestrate large-scale tumor behaviors such as proliferation and invasion. We have recently begun to appreciate that many tumors contain a high degree of cellular heterogeneity and are organized in a cellular hierarchy, with a cancer stem cell (CSC) population identified at the apex in multiple cancer types. CSCs reside in unique microenvironments or niches that are responsible for directing their behavior through cellular interactions between CSCs and stromal cells, generating a malignant social network. Identifying cell-cell adhesion mechanisms in this network has implications for the basic understanding of tumorigenesis and the development of more effective therapies. In this review, we will discuss our current understanding of cell-cell adhesion mechanisms used by CSCs and how these local interactions have global consequences for tumor biology.

Tropism between hepatic and pulmonary metastases in colorectal cancers

In metastatic colorectal cancers, tumor cells are disseminated prior to surgical resection of the primary tumor but remain dormant until proper colonization mechanisms are activated. To identify the colonization mechanisms of the metastatic tumors, we conducted a pairwise comparison between primary colorectal cancers and metastatic tumors (n=12 pairs), including six hepatic pairs and six pulmonary pairs. The mRNA levels of 224 genes previously reported to be associated with metastasis, cytokines and angiogenesis were quantitatively determined by PCR arrays. Among them, 27 genes were duplicated or triplicated to show consistent expression. Unsupervised hierarchical clustering of the Ct values of metastasis-related genes revealed that liver metastases were indistinguishable from primary colorectal cancers (n=5/6), whereas lung metastases were highly diversified from one another and from the primary tumors (n=6/6). Cytokines and receptor gene expression array data also confirmed the divergence of pulmonary metastases from primary colorectal cancers (n=6/6). Heat map analyses of ΔCt values of the metastasis-related genes identified a 17-gene tropism signature that was sufficient not only to distinguish liver and the lung metastases, but also reconstituted the clustering of primary tumors with the hepatic metastases (n=17/18). In this pilot experiment, pulmonary metastases were significantly diverged from hepatic metastases that were indistinguishable from primary colorectal cancers. Further genomic and clinical studies are in progress to evaluate the potential of the tropism signature as a therapeutic target to inhibit the colonization of metastatic colorectal cancers.

ß3 integrin modulates transforming growth factor beta induced (TGFBI) function and paclitaxel response in ovarian cancer cells

Background

The extracellular matrix (ECM) has a key role in facilitating the progression of ovarian cancer and we have shown recently that the secreted ECM protein TGFBI modulates the response of ovarian cancer to paclitaxel-induced cell death.

Results

We have determined TGFBI signaling from the extracellular environment is preferential for the cell surface αvß3 integrin heterodimer, in contrast to periostin, a TGFBI paralogue, which signals primarily via a ß1 integrin-mediated pathway. We demonstrate that suppression of ß1 integrin expression, in ß3 integrin-expressing ovarian cancer cells, increases adhesion to rTGFBI. In addition, Syndecan-1 and −4 expression is dispensable for adhesion to rTGFBI and loss of Syndecan-1 cooperates with the loss of ß1 integrin to further enhance adhesion to rTGFBI. The RGD motif present in the carboxy-terminus of TGFBI is necessary, but not sufficient, for SKOV3 cell adhesion and is dispensable for adhesion of ovarian cancer cells lacking ß3 integrin expression. In contrast to TGFBI, the carboxy-terminus of periostin, lacking a RGD motif, is unable to support adhesion of ovarian cancer cells. Suppression of ß3 integrin in SKOV3 cells increases resistance to paclitaxel-induced cell death while suppression of ß1 integrin has no effect. Furthermore, suppression of TGFBI expression stimulates a paclitaxel resistant phenotype while suppression of fibronectin expression, which primarily signals through a ß1 integrin-mediated pathway, increases paclitaxel sensitivity.

Conclusions

Therefore, different ECM components use distinct signaling mechanisms in ovarian cancer cells and in particular, TGFBI preferentially interacts through a ß3 integrin receptor mediated mechanism to regulate the response of cells to paclitaxel-induced cell death.

Multifaceted tumor stromal fibroblasts

Tumors are highly complex tissues composed of neoplastic cells and different kinds of stromal cells. Tumor stromal cells, especially fibroblasts, play important roles during the multistep development of tumors. In this review, the two-faced characteristics of tumor stromal fibroblasts are discussed in the light of our current knowledge. For one thing, fibroblasts act as an "inflammation regulator" by secretion of cytokines and regulation of tumor immunity; for another, they act as a "damage healer" for cure of wounds by remodeling extracellular matrix or taking a part in the "foreign body reaction". Since the properties of fibroblasts are complicated, both aspects of fibroblasts for tumor development should be considered carefully in clinical studies to target cancer-associated fibroblasts.

OrganDots--an organotypic 3D tissue culture platform for drug development

INTRODUCTION

There is an urgent need for preclinical testing systems that more accurately reflect responses in human target organs. The use of ex vivo tissues taken out of the human body and kept alive for sufficient time to perform testing has until recently been limited by tissue availability and by the length of time tissues can be kept alive outside the body, however, recent advances in tissue handling and tissue culture techniques have now made it possible to envisage using such tissues for drug discovery on a scale that is of value for the evaluation of compounds prior to testing in humans.

AREAS COVERED

The article presents a method for generating 3D microtissues at the air-liquid interface 'OrganDots' which are formed by reaggregating primary tissues or stem cell-based material which may be useful in drug discovery and development. The article compares this method with other methods for obtaining ex vivo tissues and looks at their uses as surrogates to testing compounds in humans.

EXPERT OPINION

Reconstituting tissues in vitro has now reached a point where they can be used to profile the activity of compounds prior to in vivo testing. The ability to reconstitute tissues from primary material and the ability to synthesize new tissues in vitro from stem cells may lead to new testing systems that better reflect human pathophysiology and may allow individual differences to be expressed in vitro. These new drug testing systems should lead to more predictable in vitro drug testing systems in the near future.

The epithelial-mesenchymal transition under control: global programs to regulate epithelial plasticity

The epithelial to mesenchymal transition or EMT has become one of the most exciting fields in cancer research. Nevertheless, its relevance in tumor biology and the metastatic process still faces some controversy. Clarification may arise when considering the EMT as a reversible and often incomplete process, essentially a manifestation of strong epithelial plasticity. Transient cellular states are generated to fulfill specific requirements in each and all the steps of the metastatic process, from primary tumor cell detachment to dissemination and colonization. Opposing multiple cellular programs that promote or prevent EMT, thereby destabilizing or reinforcing epithelial integrity, play a central role in the inherent cellular dynamics of cancer progression. These cell biology programs not only drive cells towards the epithelial or the mesenchymal state but also impinge into multiple cellular and global responses including proliferation, stemness, chemo and immunotherapy resistance, inflammation and immunity, all relevant for the development of the metastatic disease.

To differentiate or not--routes towards metastasis

Why are many metastases differentiated? Invading and disseminating carcinoma cells can undergo an epithelial-mesenchymal transition (EMT), which is associated with a gain of stem cell-like behaviour. Therefore, EMT has been linked to the cancer stem cell concept. However, it is a matter of debate how subsequent mesenchymal-epithelial transition (MET) fits into the metastatic process and whether a MET is essential. In this Opinion article, I propose two principle types of metastatic progression: phenotypic plasticity involving transient EMT-MET processes and intrinsic genetic alterations keeping cells in an EMT and stemness state. This simplified classification integrates clinically relevant aspects of dormancy, metastatic tropism and therapy resistance, and implies perspectives on treatment strategies against metastasis.

Emerging roles of the tumor-associated stroma in promoting tumor metastasis

The stroma in human carcinomas consists of extracellular matrix and various types of non-carcinoma cells, mainly leukocytes, endothelial cells, fibroblasts, myofibroblasts and bone marrow-derived progenitors. The tumor-associated stroma actively supports tumor growth by stimulating neo-angiogenesis, as well as proliferation and invasion of apposed carcinoma cells. It has long been accepted that alterations within carcinoma cells mediate metastasis in a cell-autonomous fashion. Recent studies have, however, suggested an additional notion that cancer cells instigate local and systemic changes in the tumor microenvironment and contribute to niche formation for metastasis. Research, aiming to establish the roles of the tumor-associated stroma in facilitating the spread of carcinoma cells into distant organs, has provided an abundance of data and greater knowledge of the biology of metastatic carcinoma cells and associated stromal cells. This has stimulated further advances in the development of novel therapeutic approaches targeting tumor metastasis.

The bed and the bugs: interactions between the tumor microenvironment and cancer stem cells

Tumors have been increasingly recognized as organs with a complexity that approaches, and may even exceed, that of healthy tissues. When viewed from this perspective, the biology of a tumor can be understood only by studying tumor cell heterogeneity and the microenvironment that is constructed during the course of tumorigenesis and malignant progression. Recent work has revealed the existence of cancer stem cells, the "bugs", with the capacity for self-renewal and tumor propagation. In addition, it is now recognized that the tumor microenvironment, the "bed", plays a critical role in supporting cancer stem cells and also may promote neoplasia and malignant progression. The interdependence of the cell-intrinsic features of cancer, including the cancer stem cell "bugs" and the tumor microenvironment "bed", is only beginning to be understood. In this review, we highlight the rapidly evolving concepts about the interactions between tumor stem cells and their microenvironment, the insights gained from studying their normal tissue counterparts, and the questions and controversies surrounding this area of research, with an emphasis on breast and lung cancer. Finally, we address evidence supporting the notion that eliminating the bed as well as the bugs should lead to more effective and personalized cancer treatments that improve patient outcome.

New insights into the mechanisms of organ-specific breast cancer metastasis

Despite the substantial advances obtained in the treatment of localized malignancies, metastatic disease still lacks effective treatment and remains the primary cause of cancer mortality, including in breast cancer. Thus, in order to improve the survival of cancer patients it is necessary to effectively improve prevention or treatment of metastasis. To achieve this goal, complementary strategies can be envisaged: the first one is the eradication of established metastases by adding novel modalities to current treatments, such as immunotherapy or targeted therapies. A second one is to prevent tumor cell dissemination to secondary organs by targeting specific steps governing the metastatic cascade and organ-specific tropism. A third one is to block the colonization of secondary organs and subsequent cancer cell growth by impinging on the ability of disseminated cancer cells to adapt to the novel microenvironment. To obtain optimal results it might be necessary to combine these strategies. The development of therapeutic approaches aimed at preventing dissemination and organ colonization requires a deeper understanding of the specific genetic events occurring in cancer cells and of the host responses that co-operate to promote metastasis formation. Recent developments in the field disclosed novel mechanisms of metastasis. In particular the crosstalk between disseminated cancer cells and the host microenvironment is emerging as a critical determinant of metastasis. The identification of tissue-specific signals involved in metastatic progression will open the way to new therapeutic strategies. Here, we will review recent progress in the field, with particular emphasis on the mechanisms of organ specific dissemination and colonization of breast cancer.

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.

On the epigenetic origin of cancer stem cells

Epigenetic mechanisms are the key component of the dynamic transcriptional programming that occurs along the process of differentiation from normal stem cells to more specialized cells. In the development of cancer and according to the cancer stem cell model, aberrant epigenetic changes may ensure the property of cancer cells to switch cancer stem cell markers on and off in order to generate a heterogeneous population of cells. The tumour will then be composed of tumourigenic (cancer stem cells) and non-tumourigenic (the side population that constitutes the bulk of the tumour) cells. Characterizing epigenetic landscapes may thus help discriminate aberrant marks (good candidates for tumour detection) from cancer stem cell specific profiles. In this review, we will give some insights about what epigenetics can teach us about the origin of cancer stem cells. We will also discuss how identification of epigenetic reprogramming may help designing new drugs that will specifically target cancer stem cells.

Accessories to the crime: functions of cells recruited to the tumor microenvironment

Mutationally corrupted cancer (stem) cells are the driving force of tumor development and progression. Yet, these transformed cells cannot do it alone. Assemblages of ostensibly normal tissue and bone marrow-derived (stromal) cells are recruited to constitute tumorigenic microenvironments. Most of the hallmarks of cancer are enabled and sustained to varying degrees through contributions from repertoires of stromal cell types and distinctive subcell types. Their contributory functions to hallmark capabilities are increasingly well understood, as are the reciprocal communications with neoplastic cancer cells that mediate their recruitment, activation, programming, and persistence. This enhanced understanding presents interesting new targets for anticancer therapy.

Concepts of metastasis in flux: the stromal progression model

The ability of tumor cells to leave a primary tumor, to disseminate through the body, and to ultimately seed new secondary tumors is universally agreed to be the basis for metastasis formation. An accurate description of the cellular and molecular mechanisms that underlie this multistep process would greatly facilitate the rational development of therapies that effectively allow metastatic disease to be controlled and treated. A number of disparate and sometimes conflicting hypotheses and models have been suggested to explain various aspects of the process, and no single concept explains the mechanism of metastasis in its entirety or encompasses all observations and experimental findings. The exciting progress made in metastasis research in recent years has refined existing ideas, as well as giving rise to new ones. In this review we survey some of the main theories that currently exist in the field, and show that significant convergence is emerging, allowing a synthesis of several models to give a more comprehensive overview of the process of metastasis. As a result we postulate a stromal progression model of metastasis. In this model, progressive modification of the tumor microenvironment is equally as important as genetic and epigenetic changes in tumor cells during primary tumor progression. Mutual regulatory interactions between stroma and tumor cells modify the stemness of the cells that drive tumor growth, in a manner that involves epithelial-mesenchymal and mesenchymal-epithelial-like transitions. Similar interactions need to be recapitulated at secondary sites for metastases to grow. Early disseminating tumor cells can progress at the secondary site in parallel to the primary tumor, both in terms of genetic changes, as well as progressive development of a metastatic stroma. Although this model brings together many ideas in the field, there remain nevertheless a number of major open questions, underscoring the need for further research to fully understand metastasis, and thereby identify new and effective ways of treating metastatic disease.

Host-tumor interactions in nasopharyngeal carcinomas

Like other human solid tumors, nasopharyngeal carcinoma (NPC) is a tissue and a systemic disease as much as a cell disease. Tumor cell population in NPC is highly heterogeneous. Heavy infiltration by non-malignant leucocytes results at least in part from the production of abundant inflammatory cytokines by the malignant epithelial cells. There is indirect evidence that interactions between stromal and malignant cells contribute to tumor development. Peripheral blood samples collected from NPC patients contain multiple products derived from the tumor, including cytokines, non-cytokine tumor proteins, tumor exosomes and viral nucleic acids. These products represent a potential source of biomarkers for assessment of tumor aggressiveness, indirect exploration of cellular interactions and monitoring of tumor response to therapeutic agents. Most NPC patients are immunocompetent with evidence of active humoral and cellular immune responses against EBV-antigens at the systemic level. Tumor development is facilitated by local immunosuppressive factors which are not fully understood. Local accumulation of regulatory T-cells is probably one important factor. At least two NPC tumor products are suspected to contribute to their expansion, the cytokine CCL20 and the tumor exosomes carrying galectin 9. In the future, new therapeutic modalities will probably aim at breaking immune tolerance or at blocking cellular interactions critical for tumor growth.

Role of the wnt pathway in thyroid cancer

Aberrant activation of Wnt signaling is involved in the development of several epithelial tumors. Wnt signaling includes two major types of pathways: (i) the canonical or Wnt/β-catenin pathway; and (ii) the non-canonical pathways, which do not involve β-catenin stabilization. Among these pathways, the Wnt/β-catenin pathway has received most attention during the past years for its critical role in cancer. A number of publications emphasize the role of the Wnt/β-catenin pathway in thyroid cancer. This pathway plays a crucial role in development and epithelial renewal, and components such as β-catenin and Axin are often mutated in thyroid cancer. Although it is accepted that altered Wnt signaling is a late event in thyroid cell transformation that affects anaplastic thyroid tumors, recent data suggest that it is also altered in papillary thyroid carcinoma (PTC) with RET/PTC mutations. Therefore, the purpose of this review is to summarize the main relevant data of Wnt signaling in thyroid cancer, with special emphasis on the Wnt/β-catenin pathway.