Sequencing of breast cancer stem cell populations indicates a dynamic conversion between differentiation states in vivo

Cancer stem cells: Masters of all traits

Cancer is a heterogeneous disease, which contributes to its rapid progression and therapeutic failure. Besides interpatient tumor heterogeneity, tumors within a single patient can present with a heterogeneous mix of genetically and phenotypically distinct subclones. These unique subclones can significantly impact the traits of cancer. With the plasticity that intratumoral heterogeneity provides, cancers can easily adapt to changes in their microenvironment and therapeutic exposure. Indeed, tumor cells dynamically shift between a more differentiated, rapidly proliferating state with limited tumorigenic potential and a cancer stem cell (CSC)-like state that resembles undifferentiated cellular precursors and is associated with high tumorigenicity. In this context, CSCs are functionally located at the apex of the tumor hierarchy, contributing to the initiation, maintenance, and progression of tumors, as they also represent the subpopulation of tumor cells most resistant to conventional anti-cancer therapies. Although the CSC model is well established, it is constantly evolving and being reshaped by advancing knowledge on the roles of CSCs in different cancer types. Here, we review the current evidence of how CSCs play a pivotal role in providing the many traits of aggressive tumors while simultaneously evading immunosurveillance and anti-cancer therapy in several cancer types. We discuss the key traits and characteristics of CSCs to provide updated insights into CSC biology and highlight its implications for therapeutic development and improved treatment of aggressive cancers.

Forward-reverse mutation cycles in cancer cell lines under chemical treatments

Spontaneous forward-reverse mutations were reported by us earlier in clinical samples from various types of cancers and in HeLa cells under normal culture conditions. To investigate the effects of chemical stimulations on such mutation cycles, the present study examined single nucleotide variations (SNVs) and copy number variations (CNVs) in HeLa and A549 cells exposed to wogonin-containing or acidic medium. In wogonin, both cell lines showed a mutation cycle during days 16-18. In acidic medium, both cell lines displayed multiple mutation cycles of different magnitudes. Genomic feature colocalization analysis suggests that CNVs tend to occur in expanded and unstable regions, and near promoters, histones, and non-coding transcription sites. Moreover, phenotypic variations in cell morphology occurred during the forward-reverse mutation cycles under both types of chemical treatments. In conclusion, chemical stresses imposed by wogonin or acidity promoted cyclic forward-reverse mutations in both HeLa and A549 cells to different extents.

Interconversion of Cancer Cells and Induced Pluripotent Stem Cells

Cancer cells, especially cancer stem cells (CSCs), share many molecular features with induced pluripotent stem cells (iPSCs) that enable the derivation of induced pluripotent cancer cells by reprogramming malignant cells. Conversely, normal iPSCs can be converted into cancer stem-like cells with the help of tumor microenvironment components and genetic manipulation. These CSC models can be utilized in oncogenic initiation and progression studies, understanding drug resistance, and developing novel therapeutic strategies. This review summarizes the role of pluripotency factors in the stemness, tumorigenicity, and therapeutic resistance of cancer cells. Different methods to obtain iPSC-derived CSC models are described with an emphasis on exposure-based approaches. Culture in cancer cell-conditioned media or cocultures with cancer cells can convert normal iPSCs into cancer stem-like cells, aiding the examination of processes of oncogenesis. We further explored the potential of reprogramming cancer cells into cancer-iPSCs for mechanistic studies and cancer dependencies. The contributions of genetic, epigenetic, and tumor microenvironment factors can be evaluated using these models. Overall, integrating iPSC technology into cancer stem cell research holds significant promise for advancing our knowledge of cancer biology and accelerating the development of innovative and tailored therapeutic interventions.

Metabolic dependencies of metastasis-initiating cells in female breast cancer

Understanding the mechanisms that enable cancer cells to metastasize is essential in preventing cancer progression. Here we examine the metabolic adaptations of metastasis-initiating cells (MICs) in female breast cancer and how those shape their metastatic phenotype. We find that endogenous MICs depend on the oxidative tricarboxylic acid cycle and fatty acid usage. Sorting tumor cells based upon solely mitochondrial membrane potential or lipid storage is sufficient at identifying MICs. We further identify that mitochondrially-generated citrate is exported to the cytoplasm to yield acetyl-CoA, and this is crucial to maintaining heightened levels of H3K27ac in MICs. Blocking acetyl-CoA generating pathways or H3K27ac-specific epigenetic writers and readers reduces expression of epithelial-to-mesenchymal related genes, MIC frequency, and metastatic potential. Exogenous supplementation of a short chain carboxylic acid, acetate, increases MIC frequency and metastasis. In patient cohorts, we observe that higher expression of oxidative phosphorylation related genes is associated with reduced distant relapse-free survival. These data demonstrate that MICs specifically and precisely alter their metabolism to efficiently colonize distant organs.

Tumor suppressor DEAR1 regulates mammary epithelial cell fate and predicts early onset and metastasis in triple negative breast cancer

Triple negative breast cancer (TNBC) is a disease of poor prognosis, with the majority classified as the basal-like subtype associated with epithelial-mesenchymal transition and metastasis. Because basal breast cancers originate from proliferative luminal progenitor-like cells upon dysregulation of proper luminal differentiation, genes regulating luminal-basal transition are critical to elucidate novel therapeutic targets to improve TNBC outcomes. Herein we demonstrate that the tumor suppressor DEAR1/TRIM62 is a critical regulator of luminal cell fate. DEAR1 loss in human mammary epithelial cells results in significantly enhanced mammosphere formation that is accelerated in the presence of TGF-β/SMAD3 signaling. Mammospheres formed following DEAR1 loss are enriched for ALDH1A1 and CK5 expression, EpCAM^-/CD49f⁺ and CD44^high/24^low basal-like epithelial cells, indicating that DEAR1 regulates stem/progenitor cell properties and luminal-basal progenitor transition. We show that DEAR1 maintains luminal differentiation as a novel ubiquitin ligase for SNAI2/SLUG, a master regulator driving stemness and generation of basal-like progenitor populations. We also identify a significant inverse correlation between DEAR1 and SNAI2 expression in a 103 TNBC case cohort and show that low DEAR1 expression significantly correlates with young age of onset and shorter time to metastasis, suggesting DEAR1 could serve as a biomarker to stratify early onset TNBCs for targeted stem cell therapies.

Differentiated glioma cell-derived Fibromodulin activates Integrin-dependent Notch signaling in endothelial cells to promote tumor angiogenesis and growth

Cancer stem cells (CSCs) alone can initiate and maintain tumors, but the function of non-cancer stem cells (non-CSCs) that form the tumor bulk remains poorly understood. Proteomic analysis showed a higher abundance of the extracellular matrix small leucine-rich proteoglycan fibromodulin (FMOD) in the conditioned medium of differentiated glioma cells (DGCs), the equivalent of glioma non-CSCs, compared to that of glioma stem-like cells (GSCs). DGCs silenced for FMOD fail to cooperate with co-implanted GSCs to promote tumor growth. FMOD downregulation neither affects GSC growth and differentiation nor DGC growth and reprogramming in vitro. DGC-secreted FMOD promotes angiogenesis by activating integrin-dependent Notch signaling in endothelial cells. Furthermore, conditional silencing of FMOD in newly generated DGCs in vivo inhibits the growth of GSC-initiated tumors due to poorly developed vasculature and increases mouse survival. Collectively, these findings demonstrate that DGC-secreted FMOD promotes glioma tumor angiogenesis and growth through paracrine signaling in endothelial cells and identifies a DGC-produced protein as a potential therapeutic target in glioma.

Bladder cancer cells shift rapidly and spontaneously to cisplatin-resistant oxidative phosphorylation that is trackable in real time

Genetic mutations have long been recognized as drivers of cancer drug resistance, but recent work has defined additional non-genetic mechanisms of plasticity, wherein cancer cells assume a drug resistant phenotype marked by altered epigenetic and transcriptional states. Currently, little is known about the real-time, dynamic nature of this phenotypic shift. Using a bladder cancer model of nongenetic plasticity, we discovered that rapid transition to drug resistance entails upregulation of mitochondrial gene expression and a corresponding metabolic shift towards the tricarboxylic acid cycle and oxidative phosphorylation. Based on this distinction, we were able to track cancer cell metabolic profiles in real time using fluorescence lifetime microscopy (FLIM). We observed single cells transitioning spontaneously to an oxidative phosphorylation state over hours to days, a trend that intensified with exposure to cisplatin chemotherapy. Conversely, pharmacological inhibition of oxidative phosphorylation significantly reversed the FLIM metabolic signature and reduced cisplatin resistance. These rapid, spontaneous metabolic shifts offer a new means of tracking nongenetic cancer plasticity and forestalling the emergence of drug resistance.

Breast cancer as an example of tumour heterogeneity and tumour cell plasticity during malignant progression

Heterogeneity within a tumour increases its ability to adapt to constantly changing constraints, but adversely affects a patient's prognosis, therapy response and clinical outcome. Intratumoural heterogeneity results from a combination of extrinsic factors from the tumour microenvironment and intrinsic parameters from the cancer cells themselves, including their genetic, epigenetic and transcriptomic traits, their ability to proliferate, migrate and invade, and their stemness and plasticity attributes. Cell plasticity constitutes the ability of cancer cells to rapidly reprogramme their gene expression repertoire, to change their behaviour and identities, and to adapt to microenvironmental cues. These features also directly contribute to tumour heterogeneity and are critical for malignant tumour progression. In this article, we use breast cancer as an example of the origins of tumour heterogeneity (in particular, the mutational spectrum and clonal evolution of progressing tumours) and of tumour cell plasticity (in particular, that shown by tumour cells undergoing epithelial-to-mesenchymal transition), as well as considering interclonal cooperativity and cell plasticity as sources of cancer cell heterogeneity. We review current knowledge on the functional contribution of cell plasticity and tumour heterogeneity to malignant tumour progression, metastasis formation and therapy resistance.

Lung Cancer Stem Cells-Origin, Diagnostic Techniques and Perspective for Therapies

Lung cancer remains one of the most aggressive solid tumors with an overall poor prognosis. Molecular studies carried out on lung tumors during treatment have shown the phenomenon of clonal evolution, thereby promoting the occurrence of a temporal heterogeneity of the tumor. Therefore, the biology of lung cancer is interesting. Cancer stem cells (CSCs) are involved in tumor initiation and metastasis. Aging is still the most important risk factor for lung cancer development. Spontaneously occurring mutations accumulate in normal stem cells or/and progenitor cells by human life resulting in the formation of CSCs. Deepening knowledge of these complex processes and improving early recognition and markers of predictive value are of utmost importance. In this paper, we discuss the CSC hypothesis with an emphasis on age-related changes that initiate carcinogenesis. We analyze the current literature in the field, describe our own experience in CSC investigation and discuss the technical challenges with special emphasis on liquid biopsy.

Phenotype-based single cell sequencing identifies diverse genetic subclones in CD133 positive cancer stem cells

Tumor heterogeneity is one of the ongoing huddles in the field of colon cancer therapy. It is evident that there are countless clones which exhibit different phenotypes and therefore, single cell analysis is inevitable. Cancer stem cells (CSCs) are rare cell population within tumor which is known to function in cancer metastasis and recurrence. Although there have been trials to prove intra-tumoral heterogeneity using single cell sequencing, that of CSCs has not been clearly elucidated. Here, we articulate the presence of heterogeneous subclones within CD133 positive cancer stem cells through single cell sequencing. As a proof of principle, we performed phenotype-based high-throughput laser isolation and single cell sequencing (PHLI-seq) of CD133 positive cells in a frozen tumor tissue obtained from a patient with colorectal cancer. The result proved that CD133 positive cells were shown to be heterogeneous both in copy number and mutational profiles. Single cancer stem cell specific mutations such as RNF144A, PAK2, PARP4, ADAM21, HYDIN, KRT38 and CELSR1 could be also detected in liver metastatic tumor of the same patient. Collectively, these data suggest that single cell analysis used to spot subclones with genetic variation within rare population, will lead to new strategies to tackle colon cancer metastasis.

Advances in histone deacetylase inhibitors in targeting glioblastoma stem cells

Glioblastoma multiforme (GBM) is a lethal grade IV glioma (WHO classification) and widely prevalent primary brain tumor in adults. GBM tumors harbor cellular heterogeneity with the presence of a small subpopulation of tumor cells, described as GBM cancer stem cells (CSCs) that pose resistance to standard anticancer regimens and eventually mediate aggressive relapse or intractable progressive GBM. Existing conventional anticancer therapies for GBM do not target GBM stem cells and are mostly palliative; therefore, exploration of new strategies to target stem cells of GBM has to be prioritized for the development of effective GBM therapy. Recent developments in the understanding of GBM pathophysiology demonstrated dysregulation of epigenetic mechanisms along with the genetic changes in GBM CSCs. Altered expression/activity of key epigenetic regulators, especially histone deacetylases (HDACs) in GBM stem cells has been associated with poor prognosis; inhibiting the activity of HDACs using histone deacetylase inhibitors (HDACi) has been promising as mono-therapeutic in targeting GBM and in sensitizing GBM stem cells to an existing anticancer regimen. Here, we review the development of pan/selective HDACi as potential anticancer agents in targeting the stem cells of glioblastoma as a mono or combination therapy.

New Insights Into the Role of Phenotypic Plasticity and EMT in Driving Cancer Progression

Tumor cells demonstrate substantial plasticity in their genotypic and phenotypic characteristics. Epithelial-mesenchymal plasticity (EMP) can be characterized into dynamic intermediate states and can be orchestrated by many factors, either intercellularly via epigenetic reprograming, or extracellularly via growth factors, inflammation and/or hypoxia generated by the tumor stromal microenvironment. EMP has the capability to alter phenotype and produce heterogeneity, and thus by changing the whole cancer landscape can attenuate oncogenic signaling networks, invoke anti-apoptotic features, defend against chemotherapeutics and reprogram angiogenic and immune recognition functions. We discuss here the role of phenotypic plasticity in tumor initiation, progression and metastasis and provide an update of the modalities utilized for the molecular characterization of the EMT states and attributes of cellular behavior, including cellular metabolism, in the context of EMP. We also summarize recent findings in dynamic EMP studies that provide new insights into the phenotypic plasticity of EMP flux in cancer and propose therapeutic strategies to impede the metastatic outgrowth of phenotypically heterogeneous tumors.

What is the potential use of platelet-rich-plasma (PRP) in cancer treatment? A mini review

Platelet-rich-plasma (PRP) is an autologous human platelet concentrate extracted from plasma. PRP has been investigated in order to be used in many fields, with emphasis on the musculoskeletal field applied to sports injuries, as well as on other medical fields such as cardiac surgery, gynecology, pediatric surgery, urology, ophthalmology and plastic surgery. Cancer treatment is another important field where PRP should be investigated; thus, it is important validating PRP preparation protocols to be used in clinical research. Many protocols should be revised since, overall, most studies do not provide necessary information to allow them to be multiplied or replicated. The current review focuses on several topics about cancer, mainly on innovative studies about PRP use as a feasible therapeutic alternative to treat bladder cancer - a field where it could play a key role. Relevant aspects such as platelets' contribution to immune regulation and the supportive role they play in innate and adaptive immune functions are also addressed. Another important topic reviewed in the current study refers to inflammatory process regulation associated with cancer and thrombosis sites, which indicated that tumor-induced platelet activation could be used as an important therapeutic target in the future. New aspects concerning nitric oxide's ability to restrain platelet adhesion and aggregation in order to slow metastasis progress in cancer patients provide an important advantage in cancer treatment. Finally, the current review has pointed out perspectives and the main concerns about, and possibilities of, PRP use in cancer treatment.

Epigenetic plasticity potentiates a rapid cyclical shift to and from an aggressive cancer phenotype

Highly tumorigenic, drug-resistant cancer stem-like cells drive cancer progression. These aggressive cells can arise repeatedly from bulk tumor cells independently of mutational events, suggesting an epigenetic mechanism. To test this possibility, we studied bladder cancer cells as they cyclically shifted to and from a cancer stem-like phenotype, and we discovered that these two states exhibit distinct DNA methylation and chromatin accessibility. Most differential chromatin accessibility was independent of methylation and affected the expression of driver genes such as E2F3, a cell cycle regulator associated with aggressive bladder cancer. Cancer stem-like cells exhibited increased E2F3 promoter accessibility and increased E2F3 expression that drove cell migration, invasiveness and drug resistance. Epigenetic interference using a DNA methylation inhibitor blocked the transition to a cancer stem-like state and reduced E2F3 expression. Our findings indicate that epigenetic plasticity plays a key role in the transition to and from an aggressive, drug-resistant phenotype.

Comparison of the Genomic Profile of Cancer Stem Cells and Their Non-Stem Counterpart: The Case of Ovarian Cancer

The classical cancer stem cell (CSC) model places CSCs at the apex of a hierarchical scale, suggesting different genetic alterations in non-CSCs compared to CSCs, since an ill-defined number of cell generations and time intervals separate CSCs from the more differentiated cancer cells that form the bulk of the tumor. Another model, however, poses that CSCs should be considered a functional state of tumor cells, hence sharing the same genetic alterations. Here, we review the existing literature on the genetic landscape of CSCs in various tumor types and as a case study investigate the genomic complexity of DNA obtained from matched CSCs and non-CSCs from five ovarian cancer patients, using a genome-wide single-nucleotide polymorphism (SNP) microarray.

Hypoxia-responsive nanoparticle based drug delivery systems in cancer therapy: An up-to-date review

Hypoxia is a salient feature observed in most solid malignancies that holds a pivotal role in angiogenesis, metastasis and resistance to conventional cancer therapeutic approaches, and thus enables cancer progression. However, the typical characteristics of hypoxic cells such as low oxygen levels and highly bio-reductive environment can offer stimuli-responsive drug release to aid in tumor-specific chemo, radio, photodyanamic and sonodynamic therapies. This approach based on targeting the poorly oxygenated tumor habitats offers the prospective to overcome the difficulties that arises due to heterogenic nature of tumor and could be possibly used in the design of diagnostic as well as therapeutic nanocarriers for targeting various types of solid cancers. Consequently, hypoxia triggered nanoparticle based drug delivery systems is a rapidly progressing research area in developing effective strategies to combat drug-resistance in solid tumors. The present review presents the recent advances in the development of hypoxia-responsive nanovehicles for drug delivery to heterogeneous tumors. The initial sections of the article provides insights into the development of hypoxia in growing cancer and its role in disease progression. The current limitations and the future prospective of hypoxia-stimulated nanomachines for cancer treatment are also discussed.

Interrogation of Phenotypic Plasticity between Epithelial and Mesenchymal States in Breast Cancer

Dynamic interconversions between transitional epithelial and mesenchymal states underpin the epithelial mesenchymal plasticity (EMP) seen in some carcinoma cell systems. We have delineated epithelial and mesenchymal subpopulations existing within the PMC42-LA breast cancer cell line by their EpCAM expression. These purified but phenotypically plastic states, EpCAMHigh (epithelial) and EpCAMLow (mesenchymal), have the ability to regain the phenotypic equilibrium of the parental population (i.e., 80% epithelial and 20% mesenchymal) over time, although the rate of reversion in the mesenchymal direction (epithelial-mesenchymal transition; EMT) is higher than that in the epithelial direction (mesenchymal-epithelial transition; MET). Single-cell clonal propagation was implemented to delineate the molecular and cellular features of this intrinsic heterogeneity with respect to EMP flux. The dynamics of the phenotypic proportions of epithelial and mesenchymal states in single-cell generated clones revealed clonal diversity and intrinsic plasticity. Single cell-derived clonal progenies displayed differences in their functional attributes of proliferation, stemness marker (CD44/CD24), migration, invasion and chemo-sensitivity. Interrogation of genomic copy number variations (CNV) with whole exome sequencing (WES) in the context of chromosome count from metaphase spread indicated that chromosomal instability was not influential in driving intrinsic phenotypic plasticity. Overall, these findings reveal the stochastic nature of both the epithelial and mesenchymal subpopulations, and the single cell-derived clones for differential functional attributes.

Modeling differentiation-state transitions linked to therapeutic escape in triple-negative breast cancer

Drug resistance in breast cancer cell populations has been shown to arise through phenotypic transition of cancer cells to a drug-tolerant state, for example through epithelial-to-mesenchymal transition or transition to a cancer stem cell state. However, many breast tumors are a heterogeneous mixture of cell types with numerous epigenetic states in addition to stem-like and mesenchymal phenotypes, and the dynamic behavior of this heterogeneous mixture in response to drug treatment is not well-understood. Recently, we showed that plasticity between differentiation states, as identified with intracellular markers such as cytokeratins, is linked to resistance to specific targeted therapeutics. Understanding the dynamics of differentiation-state transitions in this context could facilitate the development of more effective treatments for cancers that exhibit phenotypic heterogeneity and plasticity. In this work, we develop computational models of a drug-treated, phenotypically heterogeneous triple-negative breast cancer (TNBC) cell line to elucidate the feasibility of differentiation-state transition as a mechanism for therapeutic escape in this tumor subtype. Specifically, we use modeling to predict the changes in differentiation-state transitions that underlie specific therapy-induced changes in differentiation-state marker expression that we recently observed in the HCC1143 cell line. We report several statistically significant therapy-induced changes in transition rates between basal, luminal, mesenchymal, and non-basal/non-luminal/non-mesenchymal differentiation states in HCC1143 cell populations. Moreover, we validate model predictions on cell division and cell death empirically, and we test our models on an independent data set. Overall, we demonstrate that changes in differentiation-state transition rates induced by targeted therapy can provoke distinct differentiation-state aggregations of drug-resistant cells, which may be fundamental to the design of improved therapeutic regimens for cancers with phenotypic heterogeneity.

Systemic Metabolism, Its Regulators, and Cancer: Past Mistakes and Future Potential

There has been a resurgence of interest in cancer metabolism; primarily in the resetting of metabolism within malignant cells. Metabolism within cells has always been a tightly regulated process; initially in protozoans due to metabolic enzymes, and the intracellular signaling pathways that regulate these, being directly sensitive to the availability of nutrients. With the evolution of metazoans many of these controls had been overlaid by extra-cellular regulators that ensured coordinated regulation of metabolism within the community of cells that comprised the organism. Central to these systemic regulators is the insulin/insulin-like growth factor (IGF) system that throughout evolution has integrated the control of tissue growth with metabolic status. Oncological interest in the main systemic metabolic regulators greatly subsided when pharmaceutical strategies designed to treat cancers failed in the clinic. During the same period, however the explosion of new information from genetics has revealed the complexity and heterogeneity of advanced cancers and helped explain the problems of managing cancer when it reaches such a stage. Evidence has also accumulated implying that the setting of the internal environment determines whether cancers progress to advanced disease and metabolic status is clearly an important component of this local ecology. We are in the midst of an epidemic of metabolic disorders and there is considerable research into strategies for controlling metabolism. Integrating these new streams of information suggests new possibilities for cancer prevention; both primary and secondary.

Dynamism, Sensitivity, and Consequences of Mesenchymal and Stem-Like Phenotype of Cancer Cells

There are remarkable similarities in the description of cancer stem cells (CSCs) and cancer cells with mesenchymal phenotype. Both cell types are highly tumorigenic, resistant against common anticancer treatment, and thought to cause metastatic growth. Moreover, cancer cells are able to switch between CSC and non-CSC phenotypes and vice versa, to ensure the necessary balance within the tumor. Likewise, cancer cells can switch between epithelial and mesenchymal phenotypes via well-described transition (EMT/MET) that is thought to be crucial for tumor propagation. In this review, we discuss whether, and to which extend, the CSCs and mesenchymal cancer cells are overlapping phenomena in terms of mechanisms, origin, and implication for cancer treatment. As well, we describe the dynamism of both phenotypes and involvement of the tumor microenvironment in CSC reversion and in EMT.

Differentiation-state plasticity is a targetable resistance mechanism in basal-like breast cancer

Intratumoral heterogeneity in cancers arises from genomic instability and epigenomic plasticity and is associated with resistance to cytotoxic and targeted therapies. We show here that cell-state heterogeneity, defined by differentiation-state marker expression, is high in triple-negative and basal-like breast cancer subtypes, and that drug tolerant persister (DTP) cell populations with altered marker expression emerge during treatment with a wide range of pathway-targeted therapeutic compounds. We show that MEK and PI3K/mTOR inhibitor-driven DTP states arise through distinct cell-state transitions rather than by Darwinian selection of preexisting subpopulations, and that these transitions involve dynamic remodeling of open chromatin architecture. Increased activity of many chromatin modifier enzymes, including BRD4, is observed in DTP cells. Co-treatment with the PI3K/mTOR inhibitor BEZ235 and the BET inhibitor JQ1 prevents changes to the open chromatin architecture, inhibits the acquisition of a DTP state, and results in robust cell death in vitro and xenograft regression in vivo.

Resistance to therapy can be driven by intratumoral heterogeneity. Here, the authors show that drug tolerant persistent cell populations emerge during treatment, and these emergent populations arise through epigenetically mediated cell state transitions rather than sub population selection.

Targeting cancer stem cells and their niche: perspectives for future therapeutic targets and strategies

A small subpopulation of cells within the bulk of tumors share features with somatic stem cells, in that, they are capable of self-renewal, they differentiate, and are highly resistant to conventional therapy. These cells have been referred to as cancer stem cells (CSCs). Recent reports support the central importance of a cancer stem cell-like niche that appears to help foster the generation and maintenance of CSCs. In response to signals provided by this microenvironment, CSCs express the tumorigenic characteristics that can drive tumor metastasis by the induction of epithelial-mesenchymal-transition (EMT) that in turn fosters the migration and recolonization of the cells as secondary tumors within metastatic niches. We summarize here recent advances in cancer stem cell research including the characterization of their genetic and epigenetic features, metabolic specialities, and crosstalk with aging-associated processes. Potential strategies for targeting CSCs, and their niche, by regulating CSCs plasticity, or therapeutic sensitivity is discussed. Finally, it is hoped that new strategies and related therapeutic approaches as outlined here may help prevent the formation of the metastatic niche, as well as counter tumor progression and metastatic growth.

PDGF-AB rich-trombocyte lysate supplementation from breast cancer patients increased the proliferation of breast cancer stem cells

Background: Thrombocytosis in breast cancer (BC) patient was thought to play a role in the invasiveness of breast cancer stem cells (BCSCs). Modification of tumor microenvironment was proposed to increase the efficacy of anticancer therapy. This study was aimed to analyze the effect of platelet lysate (PL) as well as its PDGF-AB content as a tumor microenvironment on (CD24-/CD44+) BCSC proliferation.Methods: This was an experimental study that treated culture of BCSCs with PL from breast cancer (BC) patients or healthy donors. Venous blood from all subjects were subjected to prior hematology test and then processed to obtain platelet rich plasma  (PRP). Platelet counts in PRP were determined. PRP was processed to obtain PL. PDGF-AB contents in PL were measured. PL at concentrations of 0.01% (v/v) was supplemented into DMEM-F12 medium and used for culturing BCSCs (CD24-/CD44+ cells). After 48 hours, total cell count, population doubling time (PDT), and cell viability were calculated and their correlation with platelet count and PDGF-AB levels were analyzed.Results: BC patients (n=5) had higher platelet counts and PDGF-AB levels in PL compared to healthy donors (n=15), (p=0.02). PL from BC patients could stimulate the proliferation of BCSCs higher than healthy donors (p<0.001) and showed lower PDT value (p=0.001). Cell proliferation and PDT showed strong correlation with PDGF-AB level. This observation suggests that PDGF-AB has a role on BCSCs proliferation. PL showed no effect on BCSCs viability.Conclusion: Breast cancer patient platelet lysate stimulated BCSC proliferation.

Comparison of EpCAMhighCD44+ cancer stem cells with EpCAMhighCD44- tumor cells in colon cancer by single-cell sequencing

Cancer stem cells (CSCs) are considered to be responsible for tumorigenesis and cancer relapse. EpCAM^highCD44⁺ tumor cells are putative colorectal CSCs that express high levels of stem cell genes, while the EpCAM^highCD44^- population mostly contains differentiated tumor cells (DTCs). This study aims to determine whether single CSC (EpCAM^highCD44⁺) and DTC (EpCAM^highCD44^-) can be distinguished in terms of somatic copy number alterations (SCNAs). We applied fluorescence-activated cell sorting to isolate the CD45^-EpCAM^highCD44⁺ and CD45^-EpCAM^highCD44^- populations from two primary colon tumors, on which low-coverage single-cell whole-genome sequencing (WGS) was then performed ∼0.1x depth. We compared the SCNAs of the CSCs and DTCs at single-cell resolution. In total, 47 qualified single cells of the two populations underwent WGS. The single-cell SCNA profiles showed that there were obvious SCNAs in both the CSCs and DTCs of each patient, and each patient had a specific copy number alteration pattern. Hierarchical clustering and correlation analysis both showed that the SCNA profiles of CSCs and DTCs from the same patient had similar SCNA pattern, while there were regional differences in the CSCs and DTCs in certain patient. SCNAs of CSCs in the same patient were highly reproducible. Our data suggest that major SCNAs occurred at an early stage and were inherited steadily. The similarity of ubiquitous SCNAs between the CSCs and DTCs might have arisen from lineage differentiation. CSCs from the same patient had reproducible SCNA profiles, indicating that gain or loss in certain chromosome is required for colon cancer development.

Phenotypic Plasticity, Bet-Hedging, and Androgen Independence in Prostate Cancer: Role of Non-Genetic Heterogeneity

It is well known that genetic mutations can drive drug resistance and lead to tumor relapse. Here, we focus on alternate mechanisms-those without mutations, such as phenotypic plasticity and stochastic cell-to-cell variability that can also evade drug attacks by giving rise to drug-tolerant persisters. The phenomenon of persistence has been well-studied in bacteria and has also recently garnered attention in cancer. We draw a parallel between bacterial persistence and resistance against androgen deprivation therapy in prostate cancer (PCa), the primary standard care for metastatic disease. We illustrate how phenotypic plasticity and consequent mutation-independent or non-genetic heterogeneity possibly driven by protein conformational dynamics can stochastically give rise to androgen independence in PCa, and suggest that dynamic phenotypic plasticity should be considered in devising therapeutic dosing strategies designed to treat and manage PCa.

The viable circulating tumor cells with cancer stem cells feature, where is the way out?

With cancer stem cells (CSCs) became the research hotspot, emerging studies attempt to reveal the functions of these special subsets in tumorigenesis. Although various approaches have been used in CSCs researches, only a few could really reflect or simulate the microenvironment in vivo. At present, CSCs theories are still difficult to apply for clinical remedy because CSCs subpopulations are always hard to identify and trace. Thus an ideal approach for clinicians and researchers is urgently needed. Circulating tumor cells (CTCs), as the method of noninvasive-liquid biopsy, could be detected in the peripheral blood (PB) from many tumors and even could be treated as procurators for CSCs deeper researches from patient-derived sample. However, CTCs, as a diagnostic marker, also raise much controversy over theirs clinical value. Mechanisms causing CTCs to shed from the tumor have not been fully characterized, thus it is unclear whether CTCs represent the entire makeup of cancer cells in the tumor or only a subset. The heterogeneity of CTCs also caused different clinical outcomes. To overcome these unsolved problems, recently, CTC researches are not just depend on enumerations, whereas those CTC subsets that could expand in vitro may play a pivotal role in the metastatic cascade. Here, we retrospect the CTC developmental history and discourse upon the enrichment of viable CTCs in functional assays, probe the further avenue at the crossroad.

Tumor acidosis enhances cytotoxic effects and autophagy inhibition by salinomycin on cancer cell lines and cancer stem cells

Sustained autophagy contributes to the metabolic adaptation of cancer cells to hypoxic and acidic microenvironments. Since cells in such environments are resistant to conventional cytotoxic drugs, inhibition of autophagy represents a promising therapeutic strategy in clinical oncology. We previously reported that the efficacy of hydroxychloroquine (HCQ), an autophagy inhibitor under clinical investigation is strongly impaired in acidic tumor environments, due to poor uptake of the drug, a phenomenon widely associated with drug resistance towards many weak bases. In this study we identified salinomycin (SAL) as a potent inhibitor of autophagy and cytotoxic agent effective on several cancer cell lines under conditions of transient and chronic acidosis. Since SAL has been reported to specifically target cancer-stem cells (CSC), we used an established model of breast CSC and CSC derived from breast cancer patients to examine whether this specificity may be associated with autophagy inhibition. We indeed found that CSC-like cells are more sensitive to autophagy inhibition compared to cells not expressing CSC markers. We also report that the ability of SAL to inhibit mammosphere formation from CSC-like cells was dramatically enhanced in acidic conditions. We propose that the development and use of clinically suitable SAL derivatives may result in improved autophagy inhibition in cancer cells and CSC in the acidic tumor microenvironment and lead to clinical benefits.

Cancer cells exhibit clonal diversity in phenotypic plasticity

Phenotypic heterogeneity in cancers is associated with invasive progression and drug resistance. This heterogeneity arises in part from the ability of cancer cells to switch between phenotypic states, but the dynamics of this cellular plasticity remain poorly understood. Here we apply DNA barcodes to quantify and track phenotypic plasticity across hundreds of clones in a population of cancer cells exhibiting epithelial or mesenchymal differentiation phenotypes. We find that the epithelial-to-mesenchymal cell ratio is highly variable across the different clones in cancer cell populations, but remains stable for many generations within the progeny of any single clone—with a heritability of 0.89. To estimate the effects of combination therapies on phenotypically heterogeneous tumours, we generated quantitative simulations incorporating empirical data from our barcoding experiments. These analyses indicated that combination therapies which alternate between epithelial- and mesenchymal-specific treatments eventually select for clones with increased phenotypic plasticity. However, this selection could be minimized by increasing the frequency of alternation between treatments, identifying designs that may minimize selection for increased phenotypic plasticity. These findings establish new insights into phenotypic plasticity in cancer, and suggest design principles for optimizing the effectiveness of combination therapies for phenotypically heterogeneous tumours.

MicroRNA-17 is downregulated in esophageal adenocarcinoma cancer stem-like cells and promotes a radioresistant phenotype

Resistance to neoadjuvant chemoradiation therapy (CRT) remains a critical barrier to the effective treatment of esophageal adenocarcinoma (EAC). Cancer stem-like cells (CSCs) are a distinct subpopulation of cells implicated in the resistance of tumors to anti-cancer therapy. However, their role in the resistance of EAC to CRT is largely unknown. In this study, using a novel in vitro isogenic model of radioresistant EAC, we demonstrate that radioresistant EAC cells have enhanced tumorigenicity in vivo, increased expression of CSC-associated markers and enhanced holoclone forming ability. Further investigation identified a subpopulation of cells that are characterised by high aldehyde dehydrogenase (ALDH) activity, enhanced radioresistance and decreased expression of miR-17-5p. In vitro, miR-17-5p was demonstrated to significantly sensitise radioresistant cells to X-ray radiation and promoted the repression of genes with miR-17-5p binding sites, such as C6orf120. In vivo, miR-17-5p was significantly decreased, whilst C6orf120 was significantly increased, in pre-treatment EAC tumour samples from patients who demonstrated a poor response to neoadjuvant CRT. This study sheds novel insights into the role of CSCs in the resistance of EAC to CRT and highlights miR-17-5p as a potential biomarker of CRT sensitivity and novel therapeutic target in treatment resistant EAC.

Isolation and characterization of adult mammary stem cells from breast cancer-adjacent tissues

Normal adult mammary stem cells (AMSCs) are promising sources for breast reconstruction, particularly following the resection of breast tumors. However, carcinogenic events can potentially convert normal AMSCs to cancer stem cells, posing a safety concern for the use of AMSCs for clinical tissue regeneration. In the present study, AMSCs and autologous primary breast cancer cells were isolated and compared for their ability to differentiate, their gene expression profile, and their potential to form tumors in vivo. AMSCs were isolated from normal tissue surrounding primary breast tumors by immunomagnetic sorting. The pluripotency of these cells was investigated by differentiation analysis, and gene expression profiles were compared with microarrays. Differentially expressed candidate genes were confirmed by reverse transcription-polymerase chain reaction and western blot analyses. The in vivo tumorigenicity of these cells, compared with low-malignancy MCF-7 cells, was also investigated by xenograft tumor formation analysis. The results revealed that AMSCs isolated from normal tissues surrounding primary breast tumors were positive for the stem cell markers epithelial-specific antigen and keratin-19. When stimulated with basic fibroblast growth factor, a differentiation agent, these AMSCs formed lobuloalveolar structures with myoepithelia that were positive for common acute lymphoblastic leukemia antigen. The gene expression profiles revealed that, compared with cancer cells, AMSCs expressed low levels of oncogenes, including MYC, RAS and ErbB receptor tyrosine kinase 2, and high levels of tumor suppressor genes, including RB transcriptional corepressor 1, phosphatase and tensin homolog, and cyclin-dependent kinase inhibitor 2A. When injected into nude non-obese diabetic/severe combined immunodeficiency-type mice, the AMSCs did not form tumors, and regular mammary ductal structures were generated. The AMSCs isolated from normal tissue adjacent to primary breast tumors had the normal phenotype of mammary stem cells, and therefore may be promising candidates for mammary reconstruction subsequent to breast tumor resection.

3D modeling of cancer stem cell niche

Cancer stem cells reside in a distinct microenvironment called niche. The reciprocal interactions between cancer stem cells and niche contribute to the maintenance and enrichment of cancer stem cells. In order to simulate the interactions between cancer stem cells and niche, three-dimensional models have been developed. These in vitro culture systems recapitulate the spatial dimension, cellular heterogeneity, and the molecular networks of the tumor microenvironment and show great promise in elucidating the pathophysiology of cancer stem cells and designing more clinically relavant treatment modalites.

Estrogen Receptor β as a Therapeutic Target in Breast Cancer Stem Cells

Background

Breast cancer cells with tumor-initiating capabilities (BSCs) are considered to maintain tumor growth and govern metastasis. Hence, targeting BSCs will be crucial to achieve successful treatment of breast cancer.

Methods

We characterized mammospheres derived from more than 40 cancer patients and two breast cancer cell lines for the expression of estrogen receptors (ERs) and stem cell markers. Mammosphere formation and proliferation assays were performed on cells from 19 cancer patients and five healthy individuals after incubation with ER-subtype selective ligands. Transcriptional analysis was performed to identify pathways activated in ERβ-stimulated mammospheres and verified using in vitro experiments. Xenograft models (n = 4 or 5 per group) were used to study the role of ERs during tumorigenesis.

Results

We identified an absence of ERα but upregulation of ERβ in BSCs associated with phenotypic stem cell markers and responsible for the proliferative role of estrogens. Knockdown of ERβ caused a reduction of mammosphere formation in cell lines and in patient-derived cancer cells (40.7%, 26.8%, and 39.1%, respectively). Gene set enrichment analysis identified glycolysis-related pathways (false discovery rate < 0.001) upregulated in ERβ-activated mammospheres. We observed that tamoxifen or fulvestrant alone was insufficient to block proliferation of patient-derived BSCs while this could be accomplished by a selective inhibitor of ERβ (PHTPP; 53.7% in luminal and 45.5% in triple-negative breast cancers). Furthermore, PHTPP reduced tumor initiation in two patient-derived xenografts (75.9% and 59.1% reduction in tumor volume, respectively) and potentiated tamoxifen-mediated inhibition of tumor growth in MCF7 xenografts.

Conclusion

We identify ERβ as a mediator of estrogen action in BSCs and a novel target for endocrine therapy.

Breast cancer stem cell: the roles and therapeutic implications

Breast cancers have been increasingly recognized as malignancies displaying frequent inter- and intra-tumor heterogeneity. This heterogeneity is represented by diverse subtypes and complexity within tumors, and impinges on response to therapy, metastasis, and prognosis. Cancer stem cells (CSCs), a subpopulation of cancer cells endowed with self-renewal and differentiation capacity, have been suggested to contribute to tumor heterogeneity. The CSC concept posits a hierarchical organization of tumors, at the apex of which are stem cells that drive tumor initiation, progression, and recurrence. In breast cancer, CSCs have been proposed to contribute to malignant progression, suggesting that targeting breast cancer stem cells (BCSCs) may improve treatment efficacy. Currently, several markers have been reported to identify BCSCs. However, there is objective variability with respect to the frequency and phenotype of BCSCs among different breast cancer cell lines and patients, and the regulatory mechanisms of BCSCs remain unclear. In this review, we summarize current literature about the diversity of BCSC markers, the roles of BCSCs in tumor development, and the regulatory mechanisms of BCSCs. We also highlight the most recent advances in BCSC targeting therapies and the challenges in translating the knowledge into clinical practice.

The role of autophagy in the cross-talk between epithelial-mesenchymal transitioned tumor cells and cancer stem-like cells

Epithelial-mesenchymal transition (EMT) and cancer stem-like cells (CSC) are becoming highly relevant targets in anticancer drug discovery. A large body of evidence suggests that epithelial-mesenchymal transitioned tumor cells (EMT tumor cells) and CSCs have similar functions. There is also an overlap regarding the stimuli that can induce the generation of EMT tumor cells and CSCs. Moreover, direct evidence has been brought that EMT can give rise to CSCs. It is unclear however, whether EMT tumor cells should be considered CSCs or if they have to undergo further changes. In this article we summarize available evidence suggesting that, indeed, additional programs must be engaged and we propose that macroautophagy (hereafter, autophagy) represents a key trait distinguishing CSCs from EMT tumor cells. Thus, CSCs have often been reported to be in an autophagic state and blockade of autophagy inhibits CSCs. On the other hand, there is ample evidence showing that EMT and autophagy are distinct events. CSCs, however, represent, by themselves, a heterogeneous population. Thus, CSCs have been distinguished in predominantly non-cycling and cycling CSCs, the latter representing CSCs that self-renew and replenish the pool of differentiated tumor cells. We now suggest that the non-cycling CSC subpopulation is in an autophagic state. We propose also two models to explain the relationship between EMT tumor cells and these two major CSC subpopulations: a branching model in which EMT tumor cells can give rise to cycling or non-cycling CSCs, respectively, and a hierarchical model in which EMT tumor cells are first induced to become autophagic CSCs and, subsequently, cycling CSCs. Finally, we address the therapeutic consequences of these insights.

Single Amino Acid Variant Profiles of Subpopulations in the MCF-7 Breast Cancer Cell Line

Cancers are initiated and developed from a small population of stem-like cells termed cancer stem cells (CSCs). There is heterogeneity among this CSC population that leads to multiple subpopulations with their own distinct biological features and protein expression. The protein expression and function may be impacted by amino acid variants that can occur largely due to single nucleotide changes. We have thus performed proteomic analysis of breast CSC subpopulations by mass spectrometry to study the presence of single amino acid variants (SAAVs) and their relation to breast cancer. We have used CSC markers to isolate pure breast CSC subpopulation fractions (ALDH+ and CD44+/CD24- cell populations) and the mature luminal cells (CD49f-EpCAM+) from the MCF-7 breast cancer cell line. By searching the Swiss-CanSAAVs database, 374 unique SAAVs were identified in total, where 27 are cancer-related SAAVs. 135 unique SAAVs were found in the CSC population compared with the mature luminal cells. The distribution of SAAVs detected in MCF-7 cells was compared with those predicted from the Swiss-CanSAAVs database, where we found distinct differences in the numbers of SAAVs detected relative to that expected from the Swiss-CanSAAVs database for several of the amino acids.

Bulk pancreatic cancer cells can convert into cancer stem cells(CSCs) in vitro and 2 compounds can target these CSCs

Increasing evidence has confirmed the existence of cancer stem cells (CSCs) in both hematological malignancies and solid tumors. However, the origin of CSCs is still uncertain, and few agents have been capable of eliminating CSCs till now. The aim of this study was to investigate whether bulk pancreatic cancer cells could convert into CSCs under certain conditions and explore whether metformin and curcumin can kill pancreatic CSCs. Aspc1, Bxpc3 and Panc1 pancreatic cancer cells were cultured in stem cell culture medium (serum-free Dulbecco's modified Eagle medium/Nutrient Mixture F-12 containing basic fibroblast growth factor, epidermal growth factor, B27 and insulin) for 5 days and it was found that all the pancreatic cancer cells aggregated into spheres and expressed pancreatic cancer stem cell surface markers. Then characteristics of Panc1 sphere cells were analyzed and cytotoxicity assays were performed. The results show that Panc1 sphere cells exhibited CSC characteristics and were more resistant to conventional chemotherapy and more sensitive to metformin and curcumin than their parent cells. These findings suggested that bulk pancreatic cancer cells could acquire CSC characteristics under certain conditions, which may support the "yin-yang" model of CSCs (interconversion between bulk cancer cells and CSCs). These results also showed that metformin and curcumin could be candidate drugs for targeting pancreatic CSCs.

Microenvironment-Responsive Three-Pronged Approach Breaking Traditional Chemotherapy to Target Cancer Stem Cells for Synergistic Inoperable Large Tumor Therapy

Three-pronged nanoparticles (NPs) that can efficiently prohibit the proliferation of large tumor are developed for inoperable large tumor therapy. The NPs achieve spatially and temporally controlled release of drugs in target sites. The NPs induce the apoptosis of differentiated cancer cells, cancer stem cells, and vascular niches simultaneously. Importantly, the three-pronged NPs inhibit the growth of large tumors without recurrence.

Deciphering biological characteristics of tumorigenic subpopulations in human colorectal cancer reveals cellular plasticity

BACKGROUND

It is supposed that human colorectal cancer consists of a phenotypically distinct population of tumorigenic cancer cells known as cancer stem cells (CSCs) which play a pivotal role in cancer progression, maintenance, metastasis, and the relapse. The aim of this effort was to investigate and compare biological characterizations of CD133⁺ with CD133^- cell subsets isolated from both primary and metastatic human colorectal tumors.

MATERIALS AND METHODS

Using our optimized protocols, unfixed colorectal tumors were enzymatically and mechanically dissociated into single cells followed by evaluation of postdigestion viability. The obtained single cell suspensions were then subjected to cell sorting using magnetic beads according to CD133 marker. The resultant CD133⁺ and CD133^- cell subsets were cultured in specific cell culture medium followed by aldehyde dehydrogenases (ALDH) activity assessment and flow cytometric analyses.

RESULTS

The results demonstrate that CD133⁺ cells have smaller size and lower complexity of intracellular structure, sphere formation ability, and ALDH enzyme activity while CD133^- cells isolated from primary colon cancer samples were not able to form a sphere and did not show ALDH enzyme activity. Intriguingly, CD133^- cells isolated from metastatic colorectal cancer specimen were able to form a sphere and shown ALDH enzyme activity. The present study indicates that our results are in agreement with SC theory and possibility of the existence of cellular plasticity among cancer subpopulations should be portrayed.

CONCLUSION

We also conclude that this cellular plasticity is greatly affected by tumor microenvironment cues and the role of CSCs niche in cancer therapeutic strategies should be precisely considered.

Mathematical Modeling of the Role of Survivin on Dedifferentiation and Radioresistance in Cancer

We use a mathematical model to investigate cancer resistance to radiation, based on dedifferentiation of non-stem cancer cells into cancer stem cells. Experimental studies by Iwasa 2008, using human non-small cell lung cancer (NSCLC) cell lines in mice, have implicated the inhibitor of apoptosis protein survivin in cancer resistance to radiation. A marked increase in radio-sensitivity was observed, after inhibiting survivin expression with a specific survivin inhibitor YM155 (sepantronium bromide). It was suggested that these observations are due to survivin-dependent dedifferentiation of non-stem cancer cells into cancer stem cells. Here, we confirm this hypothesis with a mathematical model, which we fit to Iwasa's data on NSCLC in mice. We investigate the timing of combination therapies of YM155 administration and radiation. We find an interesting dichotomy. Sometimes it is best to hit a cancer with a large radiation dose right at the beginning of the YM155 treatment, while in other cases, it appears advantageous to wait a few days until most cancer cells are sensitized and then radiate. The optimal strategy depends on the nature of the cancer and the dose of radiation administered.

Breast Tumor Heterogeneity: Source of Fitness, Hurdle for Therapy

Tumor heterogeneity impinges on prognosis, response to therapy, and metastasis. As such, heterogeneity is one of the most important and clinically relevant areas of cancer research. Breast cancer displays frequent intra- and inter-tumor heterogeneity as the result of genetic and non-genetic alterations that often enhance the vigor of cancer cells. In-depth characterization and understanding of the origin of this phenotypic and molecular diversity is paramount to improving diagnosis, the definition of prognostic and predictive biomarkers, and the design of therapeutic strategies. Here, we summarize current knowledge about sources of breast cancer heterogeneity, its consequences, and possible counter-measures. We discuss especially the impact on tumor heterogeneity of the differentiation state of the cell-of-origin, cancer cell plasticity, the microenvironment, and genetic evolution. Factors that enhance cancer cell vigor are clearly detrimental for patients.

Hypoxia-responsive drug–drug conjugated nanoparticles for breast cancer synergistic therapy

(A) Chemical structure of IR–AZO–CA4 molecule and preparation of IR–AZO–CA4/CP NPs. (B) Schematic illustration of the hypoxia-responsive drug–drug conjugated NPs in hypoxic cells and normoxic cells.

Properties and feasibility of using cancer stem cells in clinical cancer treatment

Cancer treatment failure, drug resistance, or metastatic recurrence are thought to be caused mainly by the existence of a very small number of cancer stem cells (CSCs). The characteristics of this subgroup of cells include self-renewal, tumorigenesis, multiple differentiation and high invasiveness, metastasis, and drug resistance potential. Many studies have demonstrated that CSCs play important roles in tumor growth, spread and metastatic relapse after treatment, and are closely related to the prognosis of patients. From a therapeutic viewpoint, deep insights into the CSCs biology, development of specific therapeutic strategies for targeting CSCs, and characterization of their microenvironment could be an ideal way to combat cancer.

Developmental Insights into Breast Cancer Intratumoral Heterogeneity

Breast cancer is no longer considered a single disease, but instead is made up of multiple subtypes with genetically and most likely epigenetically heterogeneous tumors composed of numerous clones. Both the hierarchical cancer stem cell and clonal evolution models have been invoked to help explain this intratumoral heterogeneity. Several recent studies have helped define the functional interactions among the different cellular subpopulations necessary for the evolution of this complex ecosystem. These interactions involve paracrine interactions that include locally acting Wnt family members, reminiscent of the signaling pathways important for normal mammary gland development and stem cell self-renewal. In this review, we discuss the interactions among various cell populations in both normal and tumor tissues. A better understanding of these interactions, especially in the metastatic setting, will be important for the development of improved combinatorial therapies designed to prevent relapse and to ultimately decrease mortality.

Cancer Stem Cells: Plasticity Works against Therapy

Great successes in identification and deciphering of mechanisms of the adult stem cells regulation have given rise to the idea that stem cells can also function in tumors as central elements of their development, starting from the initial stage and continuing until metastasis. Such cells were called cancer stem cells (CSCs). Over the course of intense discussion, the CSCs hypothesis gradually began to be perceived as an obvious fact. Recently, the existence of CSCs has been indeed confirmed in a number of works. However, when are CSCs universal prerequisites of tumors and to what extent their role is essential for tumor evolution remains an issue far from resolved. Likewise, the problem of potential use of CSCs as therapeutic targets remains unsolved. The present review attempts to analyze the issue of cancer stem cells and the potential of targeting them in tumor therapy.

HPV Infection of the Head and Neck Region and Its Stem Cells

The human papillomavirus (HPV) is an etiologic agent associated with the development of head and neck squamous carcinoma (HNSCC)-in particular, oropharyngeal squamous cell carcinoma. The HPV-positive HNSCC is characterized by genetic alterations, clinical progression, and therapeutic response, which are distinct from HPV-negative head and neck cancers, suggesting that virus-associated tumors constitute a unique entity among head and neck cancers. Malignant stem cells, or cancer stem cells, are a subpopulation of tumor cells that self-renew, initiate new tumors upon transplantation, and are resistant to therapy, and their discovery has revealed novel effects of oncovirus infection in cancer. In this review, we provide a virus-centric view and novel insights into HPV-positive head and neck pathogenesis. We discuss the influence of cancer stem cells, HPV oncoproteins, altered molecular pathways, and mutations in cancer initiation and cancer progression. We compiled a catalogue of the mutations associated with HPV-positive HNSCC, which may be a useful resource for genomic-based studies aiming to develop personalized therapies. We also explain recent changes in mass vaccination campaigns against HPV and the potential long-term impact of vaccinations on the prevention and treatment of HPV-positive head and neck cancers.

mTOR inhibitors counteract tamoxifen-induced activation of breast cancer stem cells

Breast cancer cells with stem cell characteristics (CSC) are a distinct cell population with phenotypic similarities to mammary stem cells. CSCs are important drivers of tumorigenesis and the metastatic process. Tamoxifen is the most widely used hormonal therapy for estrogen receptor (ER) positive cancers. In our study, tamoxifen was effective in reducing proliferation of ER + adherent cancer cells, but not their CSC population. We isolated, expanded and incubated CSC from seven breast cancers with or without tamoxifen. By genome-wide transcriptional analysis we identified tamoxifen-induced transcriptional pathways associated with ribosomal biogenesis and mRNA translation, both regulated by the mTOR-pathway. We observed induction of the key mTOR downstream targets S6K1, S6RP and 4E-BP1 in-patient derived CSCs by tamoxifen on protein level. Using the mTOR inhibitors rapamycin, everolimus and PF-04691502 (a dual PI3K/mTOR inhibitor) and in combination with tamoxifen, significant reduction in mammosphere formation was observed. Hence, we suggest that the CSC population play a significant role during endocrine resistance through activity of the mTOR pathway. In addition, tamoxifen further stimulates the mTOR-pathway but can be antagonized using mTOR-inhibitors.

Glioblastoma stem cells (GSCs) epigenetic plasticity and interconversion between differentiated non-GSCs and GSCs

Cancer stem cells (CSCs) or cancer initiating cells (CICs) maintain self-renewal and multilineage differentiation properties of various tumors, as well as the cellular heterogeneity consisting of several subpopulations within tumors. CSCs display the malignant phenotype, self-renewal ability, altered genomic stability, specific epigenetic signature, and most of the time can be phenotyped by cell surface markers (e.g., CD133, CD24, and CD44). Numerous studies support the concept that non-stem cancer cells (non-CSCs) are sensitive to cancer therapy while CSCs are relatively resistant to treatment. In glioblastoma stem cells (GSCs), there is clonal heterogeneity at the genetic level with distinct tumorigenic potential, and defined GSC marker expression resulting from clonal evolution which is likely to influence disease progression and response to treatment. Another level of complexity in glioblastoma multiforme (GBM) tumors is the dynamic equilibrium between GSCs and differentiated non-GSCs, and the potential for non-GSCs to revert (dedifferentiate) to GSCs due to epigenetic alteration which confers phenotypic plasticity to the tumor cell population. Moreover, exposure of the differentiated GBM cells to therapeutic doses of temozolomide (TMZ) or ionizing radiation (IR) increases the GSC pool both in vitro and in vivo. This review describes various subtypes of GBM, discusses the evolution of CSC models and epigenetic plasticity, as well as interconversion between GSCs and differentiated non-GSCs, and offers strategies to potentially eliminate GSCs.

Implications of stemness-related signaling pathways in breast cancer response to therapy

There is accumulating evidence that breast cancer may arise from a small subpopulation of transformed mammary stem/progenitor cells, termed breast cancer-initiating cells (BCICs), responsible for initiation and maintenance of cancer. BCICs have been identified in clinical specimens based on CD44(+)/CD24(-/low) membrane expression and/or enzymatic activity of aldehyde dehydrogenase 1 (ALDH1+), or isolated and in vitro propagated as non-adherent spheres. This cell population has been demonstrated to be able to recreate, when injected in mice even at very low concentrations, the same histopathological features of the tumor they were derived from and to escape from current therapeutic strategies. Alterations in genes involved in stemness-related pathways, such as Wnt, Notch, and Sonic Hedgehog, have been proven to play a role in breast cancer progression. Targeting these key elements represents an attractive option, with a solid rationale, although possible concerns may derive from the poor knowledge of tolerance and efficacy of inhibiting these mechanisms without inducing severe side effects. In addition, efforts to develop alternative BCIC-targeted therapies against stemness markers (CD44 and ALDH1) and molecules involved in regulating EMT- and HER2-related pathways, or able to reverse the multi-drug resistance phenotype, or to induce differentiation and to control cell survival pathways are currently ongoing and encouraging results from pre-clinical studies have already been obtained using in vitro and in vivo models.