Emerging role of exosome signalling in maintaining cancer stem cell dynamic equilibrium

Cancer stem cells (CSCs) are a small subset of heterogeneous cells existed in tumour tissues or cancer cell lines with self‐renewal and differentiation potentials. CSCs were considered to be responsible for the failure of conventional therapy and tumour recurrence. However, CSCs are not a static cell population, CSCs and non‐CSCs are maintained in dynamic interconversion state by their self‐differentiation and dedifferentiation. Therefore, targeting CSCs for cancer therapy is still not enough,exploring the mechanism of dynamic interconversion between CSCs and non‐CSCs and blocking the interconversion seems to be imperative. Exosomes are 30‐100 nm size in diameter extracellular vesicles (EVs) secreted by multiple living cells into the extracellular space. They contain cell‐state‐specific bioactive materials, including DNA, mRNA, ncRNA, proteins, lipids, etc. with their specific surface markers, such as, CD63, CD81, Alix, Tsg101, etc. Exosomes have been considered as information carriers in cell communication between cancer cells and non‐cancer cells, which affect gene expressions and cellular signalling pathways of recipient cells by delivering their contents. Now that exosomes acted as information carriers, whether they played role in maintaining dynamic equilibrium state between CSCs and non‐CSCs and their mechanism of activity are unknown. This review summarized the current research advance of exosomes’ role in maintaining CSC dynamic interconversion state and their possible mechanism of action, which will provide a better understanding the contribution of exosomes to dedifferentiation and stemness acquisition of non‐CSCs, and highlight that exosomes might be taken as the attractive target approaches for cancer therapeutics.     

Therapy targets in glioblastoma and cancer stem cells: lessons from haematopoietic neoplasms

Despite intense efforts to identify cancer‐initiating cells in malignant brain tumours, markers linked to the function of these cells have only very recently begun to be uncovered. The notion of cancer stem cell gained prominence, several molecules and signalling pathways becoming relevant for diagnosis and treatment. Whether a substantial fraction or only a tiny minority of cells in a tumor can initiate and perpetuate cancer, is still debated. The paradigm of cancer‐initiating stem cells has initially been developed with respect to blood cancers where chronic conditions such as myeloproliferative neoplasms are due to mutations acquired in a haematopoietic stem cell (HSC), which maintains the normal hierarchy to neoplastic haematopoiesis. In contrast, acute leukaemia transformation of such blood neoplasms appears to derive not only from HSCs but also from committed progenitors that cannot differentiate. This review will focus on putative novel therapy targets represented by markers described to define cancer stem/initiating cells in malignant gliomas, which have been called ‘leukaemia of the brain’, given their rapid migration and evolution. Parallels are drawn with other cancers, especially haematopoietic, given the similar rampant proliferation and treatment resistance of glioblastoma multiforme and secondary acute leukaemias. Genes associated with the malignant conditions and especially expressed in glioma cancer stem cells are intensively searched. Although many such molecules might only coincidentally be expressed in cancer‐initiating cells, some may function in the oncogenic process, and those would be the prime candidates for diagnostic and targeted therapy. For the latter, combination therapies are likely to be envisaged, given the robust and plastic signalling networks supporting malignant proliferation.     

MicroRNA‐622 is a novel mediator of tumorigenicity in melanoma by targeting Kirsten rat sarcoma

The network of molecular players is similar when comparing neural crest‐derived, actively migrating melanoblasts to melanoma cells. However, melanoblasts are sensitive to differentiation‐initiating signals at their target site (epidermis), while melanoma cells maintain migratory and undifferentiated features. We aimed at identifying downregulated genes in melanoma that are particularly upregulated in melanoblasts. Loss of such genes could contribute to stabilization of a dedifferentiated, malignant phenotype in melanoma. We determined that microRNA‐622 (miR‐622) expression was strongly downregulated in melanoma cells and tissues compared to melanocytes and melanoblast‐related cells. miR‐622 expression correlated with survival of patients with melanoma. miR‐622 re‐expression inhibited clonogenicity, proliferation, and migration in melanoma. Inhibition of miR‐622 in melanocytes induced enhanced migration. Kirsten rat sarcoma (KRAS) was identified as a major functional target of miR‐622 in melanoma. We conclude that miR‐622 is a novel tumor suppressor in melanoma and identify the miR‐622‐KRAS axis as potential therapeutic target.     

Role of VEGFR‐1 in melanoma acquired resistance to the BRAF inhibitor vemurafenib

The vascular endothelial growth factor receptor‐1 (VEGFR‐1) is a tyrosine kinase receptor frequently expressed in melanoma. Its activation by VEGF‐A or placental growth factor (PlGF) promotes tumour cell survival, migration and invasiveness. Moreover, VEGFR‐1 stimulation contributes to pathological angiogenesis and induces recruitment of tumour‐associated macrophages. Since melanoma acquired resistance to BRAF inhibitors (BRAFi) has been associated with activation of pro‐angiogenic pathways, we have investigated VEGFR‐1 involvement in vemurafenib resistance. Results indicate that human melanoma cells rendered resistant to vemurafenib secrete greater amounts of VEGF‐A and express higher VEGFR‐1 levels compared with their BRAFi‐sensitive counterparts. Transient VEGFR‐1 silencing in susceptible melanoma cells delays resistance development, whereas in resistant cells it increases sensitivity to the BRAFi. Consistently, enforced VEGFR‐1 expression, by stable gene transfection in receptor‐negative melanoma cells, markedly reduces sensitivity to vemurafenib. Moreover, melanoma cells expressing VEGFR‐1 are more invasive than VEGFR‐1 deficient cells and receptor blockade by a specific monoclonal antibody (D16F7 mAb) reduces extracellular matrix invasion triggered by VEGF‐A and PlGF. These data suggest that VEGFR‐1 up‐regulation might contribute to melanoma progression and spreading after acquisition of a drug‐resistant phenotype. Thus, VEGFR‐1 inhibition with D16F7 mAb might be a suitable adjunct therapy for VEGFR‐1 positive tumours with acquired resistance to vemurafenib.     

A Novel Regulatory Mechanism for Differentiation of Mesenchymal Stem Cell: Redox State of DJ‐1 Matters

Reactive oxygen species (ROS) are multifunctional gas transmitters with diverse biological actions (adverse vs beneficial) dependent on their level. The differentiation of vascular stem cells into smooth muscle cells (SMCs) might be involved in the pathogenesis of cardiovascular disorders including hypertension and atherosclerosis. Therefore, controlling the differentiation of vascular stem cells is a potential strategy for the treatment of vascular diseases. Nonetheless, it remains to be revealed whether ROS could mediate the differentiation of mesenchymal stem cells (MSCs) into SMCs. In addition, there are no redox (reduction–oxidation)‐sensitive molecules identified, which are responsible for the ROS‐induced differentiation of MSCs. In article number 1700208, Baek et al. [Proteomics 2017, 17, Issue 21] found that ROS mediate the differentiation of MSCs into SMCs through the modification of redox states of a multifunctional ROS‐responsive protein, DJ‐1, revealing a novel regulatory mechanism for differentiation of MSCs into SMCs and shedding light into the future development of stem‐cell‐targeted pharmacotherapy.     

Mouse 3T3 fibroblasts under the influence of fibroblasts isolated from stroma of human basal cell carcinoma acquire properties of multipotent stem cells

Background information. Multipotent mesenchymal stem cells can participate in the formation of a microenvironment stimulating the aggressive behaviour of cancer cells. Moreover, cells exhibiting pluripotent ESC (embryonic stem cell) markers (Nanog and Oct4) have been observed in many tumours. Here, we investigate the role of cancer‐associated fibroblasts in the formation of stem cell supporting properties of tumour stroma. We test the influence of fibroblasts isolated from basal cell carcinoma on mouse 3T3 fibroblasts, focusing on the expression of stem cell markers and plasticity in vitro by means of microarrays, qRT‐PCR (quantitative real‐time PCR) and immunohistochemistry. Results. We demonstrate the biological activity of the cancer stromal fibroblasts by influencing the 3T3 fibroblasts to express markers such as Oct4, Nanog and Sox2 and to show differentiation potential similar to mesenchymal stem cells. The role of growth factors such as IGF2 (insulin‐like growth factor 2), FGF7 (fibroblast growth factor 7), LEP (leptin), NGF (nerve growth factor) and TGFβ (transforming growth factor β), produced by the stromal fibroblasts, is established to participate in their bioactivity. Uninduced 3T3 do not express the stem cell markers and show minimal differentiation potential. Conclusions. Our observations indicate the pro‐stem cell activity of cancer‐associated fibroblasts and underline the role of epithelial—mesenchymal interaction in tumour biology.     

Using embryonic stem cells to form a biological pacemaker via tissue engineering technology

Biological pacemakers can be achieved by various gene‐based and cell‐based approaches. Embryonic stem cells (ESCs)‐derived pacemaker cells might be the most promising way to form biological pacemakers, but there are challenges as to how to control the differentiation of ESCs and to overcome the neoplasia, proarrhythmia, or immunogenicity resulting from the use of ESCs. As a potential approach to solve these difficult problems, tissue‐engineering techniques may provide a precise control on the different cell components of multicellular aggregates and the forming of a construct with‐defined architectures and functional properties. The combined interactions between ESC‐derived pacemaker cells, supporting cells, and matrices may completely reproduce pacemaker properties and result in a steady functional unit to induce rhythmic electrical and contractile activities. As ESCs have a high capability for self‐renewal, proliferation, and potential differentiation, we hypothesize that ESCs can be used as a source of pacemaker cells for tissue‐engineering applications and the ambitious goal of biological cardiac pacemakers may ultimately be achieved with ESCs via tissue‐engineering technology.     

Induced pluripotent stem cells and senescence: learning the biology to improve the technology

The discovery that adult somatic cells can be reprogrammed into pluripotent cells by expressing a combination of factors associated with pluripotency holds immense promise for a wide range of biotechnological and therapeutic applications. However, some hurdles—such as improving the low reprogramming efficiencies and ensuring the pluripotent potential, genomic integrity and safety of the resulting cells—must be overcome before induced pluripotent stem cells (iPSCs) can be used for clinical purposes. Several groups have recently shown that key tumour suppressors—such as members of the p53 and p16^INK4a/retinoblastoma networks—control the efficiency of iPSC generation by activating cell‐intrinsic programmes such as senescence. Here, we discuss the implications of these discoveries for improving the safety and efficiency of iPSC generation, and for increasing our understanding of different aspects of basic biology—such as the control of pluripotency or the mechanisms involved in the generation of cancer stem cells.     

Modifying the tumour microenvironment and reverting tumour cells: New strategies for treating malignant tumours

The tumour microenvironment (TME) plays a pivotal role in tumour fate determination. The TME acts together with the genetic material of tumour cells to determine their initiation, metastasis and drug resistance. Stromal cells in the TME promote the growth and metastasis of tumour cells by secreting soluble molecules or exosomes. The abnormal microenvironment reduces immune surveillance and tumour killing. The TME causes low anti‐tumour drug penetration and reactivity and high drug resistance. Tumour angiogenesis and microenvironmental hypoxia limit the drug concentration within the TME and enhance the stemness of tumour cells. Therefore, modifying the TME to effectively attack tumour cells could represent a comprehensive and effective anti‐tumour strategy. Normal cells, such as stem cells and immune cells, can penetrate and disrupt the abnormal TME. Reconstruction of the TME with healthy cells is an exciting new direction for tumour treatment. We will elaborate on the mechanism of the TME to support tumours and the current cell therapies for targeting tumours and the TME—such as immune cell therapies, haematopoietic stem cell (HSC) transplantation therapies, mesenchymal stem cell (MSC) transfer and embryonic stem cell‐based microenvironment therapies—to provide novel ideas for producing breakthroughs in tumour therapy strategies.     

Cancer stem cells and human malignant melanoma

Cancer stem cells (CSC) have been identified in hematological malignancies and several solid cancers. Similar to physiological stem cells, CSC are capable of self-renewal and differentiation and have the potential for indefinite proliferation, a function through which they may cause tumor growth. Although conventional anti-cancer treatments might eradicate most malignant cells in a tumor, they are potentially ineffective against chemoresistant CSC, which may ultimately be responsible for recurrence and progression. Human malignant melanoma is a highly aggressive and drug-resistant cancer. Detection of tumor heterogeneity, undifferentiated molecular signatures, and increased tumorigenicity of melanoma subsets with embryonic-like differentiation plasticity strongly suggest the presence and involvement of malignant melanoma stem cells (MMSC) in the initiation and propagation of this malignancy. Here, we review these findings in the context of functional properties ascribed to melanocyte stem cells and CSC in other cancers. We discuss the association of deregulated signaling pathways, genomic instability, and vasculogenic mimicry phenomena observed in melanoma subpopulations in light of the CSC concept. We propose that a subset of MMSC may be responsible for melanoma therapy-resistance, tumor invasiveness, and neoplastic progression and that targeted abrogation of a MMSC compartment could therefore ultimately lead to stable remissions and perhaps cures of metastatic melanoma.     

Using ABCG2-molecule-expressing side population cells to identify cancer stem-like cells in a human ovarian cell line

CSCs (cancer stem cells) are a small subset of cells within a tumour that possesses the characteristics of stem cells and are considered to be responsible for resistance to chemoradiation. Identification of CSCs through stem cell characteristics might have relevant clinical implications. In this study, SP (side population ) cells were sorted from a human ovarian cancer cell line by FACS to determine whether cancer stem cell-like SP cells were present. A very small fraction of SP cells (2.6%) was detected in A2780 cells. SP cells possessed the following characteristics: highly proliferative activity, marked ability for self-renewal in soft agar and culture medium, high expression of ABCG2, drug resistance to vinblastine in vitro, and strong tumourigenic potential in Balb/c nude mice. It is concluded that there exists in the A2780 cell line a small number of SP cells with high expression of ABCG2. The cells have the characteristics of cancer stem-like cells, and identification and cloning of such human SP cells can help in improving therapeutic approaches to ovarian cancer in patients.     

Protective roles of melatonin in central nervous system diseases by regulation of neural stem cells

Neural stem cells (NSCs) are immature precursors of the central nervous system (CNS), with self‐renewal and multipotential differentiation abilities. Their proliferation and differentiation are dynamically regulated by hormonal and local factors. Alteration in neurogenesis is associated with many neurological disorders. Increasing evidence suggests that modulation of NSCs can be a promising therapeutic approach for neural injury and neurodegenerative disorders. Melatonin, a pineal gland‐derived hormone, regulates the neuroimmuno‐endocrine axis and is functionally important to the circadian rhythm, tumour suppression and immunity. In the CNS, melatonin exerts neuroprotective effects in many diseases, such as Parkinson's disease, Alzheimer's disease and ischaemic brain injury. Emerging evidence suggests that it might also mediate such protective action by influencing proliferation and differentiation of NSCs. In this article, we review the current literature concerned with effects of melatonin on NSCs in different physiological and pathological conditions.     

Prospects for pluripotent stem cell‐derived cardiomyocytes in cardiac cell therapy and as disease models

The derivation of embryonic stem cells (hESC) from human embryos a decade ago started a new era in perspectives for cell therapy as well as understanding human development and disease. More recently, reprogramming of somatic cells to an embryonic stem cell‐like state (induced pluripotent stem cells, iPS) presented a new milestone in this area, making it possible to derive all cells types from any patients bearing specific genetic mutations. With the development of efficient differentiation protocols we are now able to use the derivatives of pluripotent stem cells to study mechanisms of disease and as human models for drug and toxicology testing. In addition derivatives of pluripotent stem cells are now close to be used in clinical practice although for the heart, specific additional challenges have been identified that preclude short‐term application in cell therapy. Here we review techniques presently used to induce differentiation of pluripotent stem cells into cardiomyocytes and the potential these cells have as disease models and for therapy. J. Cell. Biochem. 107: 592–599, 2009. © 2009 Wiley‐Liss, Inc.     

B‐cell acute lymphoblastic leukaemia: towards understanding its cellular origin

B‐cell acute lymphoblastic leukaemia (B‐ALL) is a clonal malignant disease originated in a single cell and characterized by the accumulation of blast cells that are phenotypically reminiscent of normal stages of B‐cell differentiation. B‐ALL origin has been a subject of continuing discussion, given the fact that human disease is diagnosed at late stages and cannot be monitored during its natural evolution from its cell of origin, although most B‐ALLs probably start off with chromosomal changes in haematopoietic stem cells. However, the cells responsible for maintaining the disease appear to differ between the different types of B‐ALLs and this remains an intriguing and exciting topic of research, since these cells have been posited to be responsible for resistance to conventional therapies, recurrence and dissemination. During the last years this problem has been addressed primarily by transplantation of purified subpopulations of human B‐ALL cells into immunodeficient mice. The results from these different reconstitution experiments and their interpretations are compared in this review in the context of normal B‐cell developmental plasticity. While the results from different research groups might appear mutually exclusive, we discuss how they could be reconciled with the biology of normal B‐cells and propose research avenues for addressing these issues in the future.     

A new gamboge derivative Compound 2 inhibits cancer stem‐like cells via suppressing EGFR tyrosine phosphorylation in head and neck squamous cell carcinoma

Cancer stem‐like cells represent a population of tumour‐initiating cells that lead to the relapse and metastasis of cancer. Conventional anti‐cancer therapeutic drugs are usually ineffective in eliminating the cancer stem‐like cells. Therefore, new drugs or therapeutic methods effectively targeting cancer stem‐like cells are in urgent need to successfully cure cancer. Gamboge is a natural anti‐cancer medicine whose pharmacological effects are different from those of conventional chemotherapeutical drugs and they can kill some kinds of cancer cells selectively. In this study, we identified a new gamboge derivative, Compound 2 (C2), which presents eminent suppression effects on cancer cells. Interestingly, when compared with cisplatin (CDDP), C2 effectively suppresses the growth of both cancer stem‐like cells and non‐cancer stem‐like cells derived from head and neck squamous cell carcinoma (HNSCC), inhibiting the formation of tumour spheres and colony in vitro, resulting in the loss of expression of multiple cancer stem cell (CSC)‐related molecules in HNSCC. Treating with C2 effectively inhibited the growth of HNSCC in BALB/C nude mice. Further investigation found that C2 notably inhibits the activation of epithelial growth factor receptor and the phosphorylation of its downstream protein kinase homo sapiens v‐akt murine thymoma viral oncogene homolog (AKT) in HNSCC, resulting in down‐regulation of multiple CSC‐related molecules in HNSCC. Our study has demonstrated that C2 effectively inhibits the stem‐like property of cancer stem‐like cells in HNSCC and may be a hopeful targeting drug in cancer therapy.     

Cancer heterogeneity—a multifaceted view

Cancers of various organs have been categorized into distinct subtypes after increasingly sophisticated taxonomies. Additionally, within a seemingly homogeneous subclass, individual cancers contain diverse tumour cell populations that vary in important cancer‐specific traits such as clonogenicity and invasive potential. Differences that exist between and within a given tumour type have hampered significantly both the proper selection of patients that might benefit from therapy, as well as the development of new targeted agents. In this review, we discuss the differences associated with organ‐specific cancer subtypes and the factors that contribute to intra‐tumour heterogeneity. It is of utmost importance to understand the biological causes that distinguish tumours as well as distinct tumour cell populations within malignancies, as these will ultimately point the way to more rational anti‐cancer treatments.EMBO reports advance online publication 12 July 2013; doi:10.1038/embor.2013.92     

Ablation of sensory nerves favours melanoma progression

The tumour mass is composed not only of heterogeneous neoplastic cells, but also a variety of other components that may affect cancer cells behaviour. The lack of detailed knowledge about all the constituents of the tumour microenvironment restricts the design of effective treatments. Nerves have been reported to contribute to the growth and maintenance of numerous tissues. The effects of sensory innervations on tumour growth remain unclear. Here, by using state‐of‐the‐art techniques, including Cre/loxP technologies, confocal microscopy, in vivo‐tracing and chemical denervation, we revealed the presence of sensory nerves infiltrating within the melanoma microenvironment, and affecting cancer progression. Strikingly, melanoma growth in vivo was accelerated following genetic ablation or chemical denervation of sensory nerves. In humans, a retrospective analysis of melanoma patients revealed that increased expression of genes related to sensory nerves in tumours was associated with better clinical outcomes. These findings suggest that sensory innervations counteract melanoma progression. The emerging knowledge from this research provides a novel target in the tumour microenvironment for therapeutic benefit in cancer patients.