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.     

Mathematical model for the cancer stem cell hypothesis

Recent research on the origin of brain cancer has implicated a subpopulation of self-renewing brain cancer stem cells for malignant tumour growth. Various genes that regulate self-renewal in normal stem cells are also found in cancer stem cells. This implies that cancers can occur because of mutations in normal stem cells and early progenitor cells. A predictive mathematical model based on the cell compartment method is presented here to pose and validate non-intuitive scenarios proposed through the neural cancer stem cell hypothesis. The growths of abnormal (stem and early progenitor) cells from their normal counterparts are ascribed with separate mutation probabilities. Stem cell mutations are found to be more significant for the development of cancer than a similar mutation in the early progenitor cells. The model also predicts that, as previously hypothesized, repeated insult to mature cells increases the formation of abnormal progeny, and hence the risk of cancer.     

Modulation of tumorigenesis and oestrogen receptor‐α expression by cell culture conditions in a stem cell‐derived breast epithelial cell line

Background information. The common phenotypes of cancer and stem cells suggest that cancers arise from stem cells. Oestrogen is one of the few most important determinants of breast cancer, as shown by several lines of convincing evidence. We have previously reported a human breast epithelial cell type (Type 1 HBEC) with stem cell characteristics and ERα (oestrogen receptor α) expression. A tumorigenic cell line, M13SV1R2, was developed from this cell type after SV40 (simian virus 40) large T‐antigen transfection and X‐ray irradiation. The cell line, however, was not responsive to oestrogen for cell growth or tumour development. In the present study, we tested the hypothesis that deprivation of growth factors and hormones may change the tumorigenicity and oestrogen response of this cell line. Results. The M13SV1R2 cells lost their tumorigenicity after culturing in a growth factor/hormone‐deprived medium for >10 passages (referred to as R2d cells) concomitant with the expression of two tumour suppressor genes, namely those coding for maspin and α6 integrin. However, these cells acquired oestrogen responsiveness in cell growth and tumour development. By immunocytochemistry, Western blotting and flow cytometry analysis, oestrogen treatment of R2d cells was found to induce many important effects related to breast carcinogenesis, namely: (i) the emergence of a subpopulation of cells expressing CD44^+/high/CD24^?/low breast tumour stem cell markers; (ii) the induction of EMT (epithelial‐to‐mesenchymal transition); (iii) the acquisition of metastatic ability; and (iv) the expression of COX‐2 (cyclo‐oxygenase‐2) through a CD44‐mediated mechanism. Conclusion. An oestrogen‐responsive cell line with ERα and CD44⁺/CD24^?/low expression can be derived from breast epithelial stem cells. The tumorigenicity and oestrogen response of these cells could depend on the cell culture conditions. The findings of this study have implications in regard to the origins of (1) ERα‐positive breast cancers, (2) CD44⁺/CD24^?/low breast tumour stem cells and (3) the metastatic ability of breast cancer.     

Experimental chemotherapy and concepts related to the cell cycle

Scheduling of chemotherapy is limited by damage to normal tissues, and tolerated schedules are dependent on normal tissue recovery. Most anticancer drugs are more toxic to proliferating cells and the fall and recovery of granulocyte counts after chemotherapy may be explained by the effect of drugs on rapidly proliferating precursor cells in the bone marrow. It is argued that serious toxicity due to myelosuppression most often occurs because of damage to proliferating precursors that may be recognized in bone marrow rather than to stem cells. In contrast, therapy that is aimed at producing cure or long-term remission of tumours must be directed at killing tumour stem cells. The evidence that tumours contain a limited population of cells which can repopulate the tumour after treatment (and are therefore tumour stem cells) is reviewed critically. While there is quite strong evidence for a limited population of target cells, evidence from studies on metastases suggests that the tumour cells which may express this stem cell property may change with time. The stem cell concept has major implications for predictive assays. Although colony-forming assays appear to have a sound biological background for predicting tumour response, technical problems prevent them from being used routinely in patient management. Cells in tumours are known to be heterogeneous and at least three types of heterogeneity may influence tumour response to drug treatment: the development of subclones with differing properties including drug resistance; variation in cellular properties due to differentiation during clonal expansion; and variation in properties due to nutritional status and micro-anatomy. Heterogeneity in drug distribution within solid tumours may occur because of limited drug penetration from blood vessels, and nutrient-deprived cells in solid tumours may be expected to escape the toxicity of some anticancer drugs as well as being resistant to radiation because of hypoxia. This may occur both because nutrient-deprived cells have a low rate of cell proliferation, and also because of poor drug penetration to them. There is a need for improved understanding of the mechanisms that lead to cell death in tumours. If these mechanisms were understood, it might be possible to simulate them by therapeutic manoeuvres. Recent research from our laboratory suggests that the combination of low extracellular pH and hypoxia may be very toxic to cells in nutrient-deprived regions. Drugs which limit the cell's ability to survive in regions of acid pH may provide strategy for therapy of nutrient-deprived cells.     

Pathobiology of cancer metastasis: a short account

ABSTRACT: Cancer-initiating cells display aberrant functional and phenotypic characteristics of normal stem cells from which they evolved by accumulation of multiple cytogenetic and/or epigenetic alterations. Signal transduction pathways which are essential for normal stem cell function are abnormally expressed by cancer cells, with a cancer cell phenotype playing an essential role in cancerization and metastasis.Local tumour progression, metastasis and metastatic tumour growth are mediated by direct cell-to-cell and paracrine reciprocal interactions between cancer cells and various stromal cells including fibroblasts, macrophages, bone marrow derived stem cells and progenitor cells. These interactions mediate breakdown of basement membrane barriers and angiogenesis both locally at the invasive front of the primary tumour and at the distant metastatic site; attract primary tumour cells to the candidate metastatic site; and promote proliferation, survival and growth of primary tumour cells and of metastatic cells at their distant site.It is the purpose of this article to highlight the analogies between some of the genetic programs of normal stem cells, and of cancer cells participating in the process of metastasis.     

Immunoediting: evidence of the multifaceted role of the immune system in self-metastatic tumor growth

ABSTRACT: BACKGROUND: The role of the immune system in tumor progression has been subject to discussion for many decades. Numerous studies suggest that a low immune response might be beneficial, if not necessary, for tumor growth, and only a strong immune response can counter tumor growth and thus inhibit progression. METHODS: We implement a cellular automaton model previously described that captures the dynamical interactions between the cancer stem and non-stem cell populations of a tumor through a process of self-metastasis. By overlaying on this model the diffusion of immune reactants into the tumor from a peripheral source to target cells, we simulate the process of immune-system-induced cell kill on tumor progression. RESULTS: A low cytotoxic immune reaction continuously kills cancer cells and, although at a low rate, thereby causes the liberation of space-constrained cancer stem cells to drive self-metastatic progression and continued tumor growth. With increasing immune system strength, however, tumor growth peaks, and then eventually falls below the intrinsic tumor sizes observed without an immune response. With this increasing immune response the number and proportion of cancer stem cells monotonically increases, implicating an additional unexpected consequence, that of cancer stem cell selection, to the immune response. CONCLUSIONS: Cancer stem cells and immune cytotoxicity alone are sufficient to explain the three-step "immunoediting" concept - the modulation of tumor growth through inhibition, selection and promotion.     

Inducing stem cell differentiation using low intensity laser irradiation: a possible novel therapeutic intervention

Developments in adult stem cell (ASC) potentiation have contributed to excitement in the field of stem cell-based therapy. The use of ASCs not only increases therapeutic treatment possibilities but successful use of multipotent cells for gene therapy has been demonstrated in animal models [1]. Concurrent ability of stem cells (SCs) to either contribute to disease development, as identified in cancer stem cells (CSCs), or to replace diseased tissue by induced differentiation using selected growth factors, has highlighted the intricate molecular and cellular mechanisms. Adipose derived stem cells (ADSCs) are capable of self-renewal and respond well to induced differentiation [2]. Auto-immunity and transplant rejection may become minor limitations when selective induction of immunological nonresponsiveness to specific antigens or tissues become possible using autologous cell sources [3]. CSCs initiate tumorogenesis, can generate differentiated daughter cells or undergo self-renewal while thought to instigate tumour regeneration post-treatment. Therapy targeting CSCs has failed to provide feasible alternatives to conventional cancer treatment. Low intensity laser irradiation (LILI), induce a biostimulatory response in several tissue types in addition to a dose-response effect to the detriment of cellular degeneration. Potential of LILI to induce CSC differentiation and subsequent cytotoxic therapy to prevent tumour regeneration is explored in this mini-review.     

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.     

A conformation hypothesis for the suppressor and promoter functions of p53 in cell growth control and in cancer.

Cancer is a genetic disease caused by defective control of cell proliferation. As cancer cells divide, the genetic defect is inherited by each daughter cell, leading to tumour development with possible progression to malignancy. The identification of those genes linked with cancer is essential for our understanding of the regulation of cell proliferation and for the therapeutic management of cancer cell growth. Recent studies have revealed that p53 is the most commonly affected gene in human cancer. It is a single copy gene and functions in the regulation of cell proliferation. Mutation of p53 is linked with tumour development, and this may involve abnormal functioning of mutant p53 protein. A mutant allele of p53 is functionally temperature-sensitive and can promote or suppress cell proliferation. The tertiary structure of the mutant protein is also sensitive to temperature and adopts promoter and suppressor forms of p53. A conformation model for the functioning of p53 proposes that wild-type p53 is induced to change from suppressor to promoter form during the cell growth response. This model predicts that any mutation that deregulates the normal control of p53 conformation may lead to cancer.     

Immunotherapy of prostate cancer: should we be targeting stem cells and EMT?

Cancer stem cells have been implicated in a number of solid malignancies including prostate cancer. In the case of localised prostate cancer, patients are often treated with surgery (radical prostatectomy) and/or radiotherapy. However, disease recurrence is an issue in about 30% of patients, who will then go on to receive hormone ablation therapy. Hormone ablation therapy is often palliative in a vast proportion of individuals, and for hormone-refractory patients, there are several immunotherapies targeting a number of prostate tumour antigens which are currently in development. However, clinical responses in this setting are inconsistent, and it is believed that the failure to achieve full and permanent tumour eradication is due to a small, resistant population of cells known as ‘cancer stem cells’ (CSCs). The stochastic and clonal evolution models are among several models used to describe cancer development. The general consensus is that cancer may arise in any cell as a result of genetic mutations in oncogenes and tumour suppressor genes, which consequently result in uncontrolled cell growth. The cancer stem cell theory, however, challenges previous opinion and proposes that like normal tissues, tumours are hierarchical and only the rare subpopulation of cells at the top of the hierarchy possess the biological properties required to initiate tumourigenesis. Furthermore, where most cancer models infer that every cell within a tumour is equally malignant, i.e. equally capable of reconstituting new tumours, the cancer stem cell theory suggests that only the rare cancer stem cell component possess tumour-initiating capabilities. Hence, according to this model, cancer stem cells are implicated in both tumour initiation and progression. In recent years, the role of epithelial–mesenchymal transition (EMT) in the advancement of prostate cancer has become apparent. Therefore, CSCs and EMT are both likely to play critical roles in prostate cancer tumourigenesis. This review summarises the current immunotherapeutic strategies targeting prostate tumour antigens taking into account the need to consider treatments that target cancer stem cells and cells involved in epithelial–mesenchymal transition.     

In silico identification of thiostrepton as an inhibitor of cancer stem cell growth and an enhancer for chemotherapy in non–small‐cell lung cancer

Cancer stem cells (CSCs) play an important role in cancer treatment resistance and disease progression. Identifying an effective anti‐CSC agent may lead to improved disease control. We used CSC‐associated gene signatures to identify drug candidates that may inhibit CSC growth by reversing the CSC gene signature. Thiostrepton, a natural cyclic oligopeptide antibiotic, was the top‐ranked candidate. In non–small‐cell lung cancer (NSCLC) cells, thiostrepton inhibited CSC growth in vitro and reduced protein expression of cancer stemness markers, including CD133, Nanog and Oct4A. In addition, metastasis‐associated Src tyrosine kinase signalling, cell migration and epithelial‐to‐mesenchymal transition (EMT) were all inhibited by thiostrepton. Mechanistically, thiostrepton treatment led to elevated levels of tumour suppressor miR‐98. Thiostrepton combined with gemcitabine synergistically suppressed NSCLC cell growth and induced apoptosis. The inhibition of NSCLC tumours and CSC growth by thiostrepton was also demonstrated in vivo. Our findings indicate that thiostrepton, an established drug identified in silico, is an inhibitor of CSC growth and a potential enhancer of chemotherapy in NSCLC.     

Influence of stem-cell cycle time on accelerated re-population during radiotherapy in head and neck cancer

Objectives

Tumour re‐population during radiotherapy was identified as an important reason for treatment failure in head and neck cancers. The process of re‐population is suggested to be caused by various mechanisms, one of the most plausible one being accelerated division of stem‐cells (i.e. drastic shortening of cell cycle duration). However, the literature lacks quantitative data regarding the length of tumour stem‐cell cycle time during irradiation.

Materials and methods

The presented work suggests that if accelerated stem‐cell division is indeed a key mechanism behind tumour re‐population, the stem‐cell cycle time can drop below 10 h during radiotherapy. To illustrate the possible implications, the mechanism of accelerated division was implemented into a Monte Carlo model of tumour growth and response to radiotherapy. Tumour response to radiotherapy was simulated with different stem‐cell cycle times (between 2 and 10 h) after the initiation of radiotherapy.

Results

It was found that very short stem‐cell cycle times lead to tumour re‐population during treatment, which cannot be overcome by radiation‐induced cell kill. Increasing the number of radiation dose fractions per week might be effective, but only for longer cell cycle times.

Conclusion

It is of crucial importance to quantitatively assess the mechanisms responsible for tumour re‐population, given that conventional treatment regimens are not efficient in delivering lethal doses to advanced head and neck tumours.     

MicroRNAs: key players of taxane resistance and their therapeutic potential in human cancers

The successful long‐term use of taxane for cancer therapy is often prevented by the development of drug resistance in clinic. Thus, exploring the mechanisms involved is a first step towards rational strategies to overcome taxane resistance. Taxane resistance‐related microRNA (miRNAs) are under investigation and miRNAs could induce the taxane resistance of tumour cells by regulating cell cycle distribution, survival and/or apoptosis pathways, drug transports, epithelial–mesenchymal transition and cancer stem cell. This article summarizes current research involving miRNAs as regulators of key target genes for tanxanxe chemoresistance and discusses the complex regulatory networks of miRNAs. Also, the authors will envisage future developments towards the potential use of targeting miRNAs as a novel strategy for improving response of tumour patients to taxane. miRNAs play critical roles in taxane chemoresistance and the miRNA‐based therapies will be helpful for overcoming drug resistance and developing more effective personalized anti‐cancer treatment strategies. Further research studies should be performed to promote therapeutic–clinical use of taxane resistance‐related miRNAs in cancer patients, especially in those patients with taxane‐resistant cancers.     

Statins,stem cells,and cancer

The statins (3‐hydroxy‐3‐methylglutaryl coenzyme A reductase inhibitors) were proven to be effective antilipid agents against cardiovascular disease. Recent reports demonstrate an anticancer effect induced by the statins through inhibition of cell proliferation, induction of apoptosis, or inhibition of angiogenesis. These effects are due to suppression of the mevalonate pathway leading to depletion of various downstream products that play an essential role in cell cycle progression, cell signaling, and membrane integrity. Recent evidence suggests a shared genomic fingerprint between embryonic stem cells, cancer cells, and cancer stem cells. Activation targets of NANOG, OCT4, SOX2, and c‐MYC are more frequently overexpressed in certain tumors. In the absence of bona fide cancer stem cell lines, human embryonic stem cells, which have similar properties to cancer and cancer stem cells, have been an excellent model throwing light on the anticancer affects of various putative anticancer agents. It was shown that key cellular functions in karyotypically abnormal colorectal and ovarian cancer cells and human embryonic stem cells are inhibited by the statins and this is mediated via a suppression of this stemness pathway. The strategy for treatment of cancers may thus be the targeting of a putative cancer stem cell within the tumor with specific agents such as the statins with or without chemotherapy. The statins may thus play a dual prophylactic role as a lipid‐lowering drug for the prevention of heart disease and as an anticancer agent to prevent certain cancers. This review examines the relationship between the statins, stem cells, and certain cancers. J. Cell. Biochem. 106: 975–983, 2009. © 2009 Wiley‐Liss, Inc.     

Tamoxifen treatment failure in cancer and the nonlinear dynamics of TGFbeta

The process of cancer invasion involves a complex interplay between cell-cell and cell-medium adhesion, proteolytic enzyme secretion, cell birth and death processes, random and directed motility, and immune response, as well as many other factors. The growth factor TGF beta is known to have a complex effect on this process. It inhibits mitosis and promotes apoptosis in a concentration-dependent manner in vitro, and it is for this reason that its secretion is thought to be helpful in inhibiting tumour growth. However, recent in vitro and in vivo results have shown a significant effect of this growth factor in promoting the sensitivity of malignantly transformed cells to gradients of extracellular matrix proteins--an effect which tends to increase invasiveness. The drug tamoxifen has been demonstrated to be therapeutically effective in the treatment of patients with breast cancer; however, it is known also that many patients become resistant to the effect of this drug after a few years, and the reasons for this remain controversial. In this work we take our established model of cancer invasion (J. Theor. Biol. 216(1) (2002) 85), and extend it to include the effect of TGF beta. In so doing we demonstrate that a tamoxifen-stimulated upregulation of the secretion of TGF beta may give rise to a tumour which has a smaller number of cells but which has a greater invasiveness, greater metastatic potential, and a tumour histology which is known to correlate with a poorer prognosis. These data suggest that tamoxifen-stimulated secretion of TGF beta might explain treatment failure in some patients.