Reprogramming cancer cells to pluripotency

The epigenetic marks displayed by a cancer cell originate from two separate processes: The most prominent epigenetic signatures are associated with the cell of origin, i.e., the lineage and cell type identity imposed during development. The second set comprises those aberrant cancer-specific epigenetic marks that appear during tumor initiation or subsequent malignant progression. These are generally thought to associate with tumor-promoting pathways. As biochemical pathways regulating epigenetic mechanisms are potentially “druggable” and reversible, there is considerable interest in defining their roles in tumor genesis and growth, as they may represent therapeutic targets for treatment of human neoplasias.¹ Dawson MA, Kouzarides T. Cancer epigenetics: from mechanism to therapy. Cell 2012; 150:12 - 27; http://dx.doi.org/10.1016/j.cell.2012.06.013; PMID: 22770212 [Crossref], [PubMed], [Web of Science ®] , [Google Scholar] However, despite the potential importance of epigenetic modifications in human cancer, it has been difficult to determine when, where and how epigenetic disruptions occur, and if they have important functional roles in sustaining the malignant state.     

Differences among brain tumor stem cell types and fetal neural stem cells in focal regions of histone modifications and DNA methylation,broad regions of modifications,and bivalent promoters

Background

Aberrational epigenetic marks are believed to play a major role in establishing the abnormal features of cancer cells. Rational use and development of drugs aimed at epigenetic processes requires an understanding of the range, extent, and roles of epigenetic reprogramming in cancer cells. Using ChIP-chip and MeDIP-chip approaches, we localized well-established and prevalent epigenetic marks (H3K27me3, H3K4me3, H3K9me3, DNA methylation) on a genome scale in several lines of putative glioma stem cells (brain tumor stem cells, BTSCs) and, for comparison, normal human fetal neural stem cells (fNSCs).

Results

We determined a substantial “core” set of promoters possessing each mark in every surveyed BTSC cell type, which largely overlapped the corresponding fNSC sets. However, there was substantial diversity among cell types in mark localization. We observed large differences among cell types in total number of H3K9me3+ positive promoters and peaks and in broad modifications (defined as >50 kb peak length) for H3K27me3 and, to a lesser extent, H3K9me3. We verified that a change in a broad modification affected gene expression of CACNG7. We detected large numbers of bivalent promoters, but most bivalent promoters did not display direct overlap of contrasting epigenetic marks, but rather occupied nearby regions of the proximal promoter. There were significant differences in the sets of promoters bearing bivalent marks in the different cell types and few consistent differences between fNSCs and BTSCs.

Conclusions

Overall, our “core set” data establishes sets of potential therapeutic targets, but the diversity in sets of sites and broad modifications among cell types underscores the need to carefully consider BTSC subtype variation in epigenetic therapy. Our results point toward substantial differences among cell types in the activity of the production/maintenance systems for H3K9me3 and for broad regions of modification (H3K27me3 or H3K9me3). Finally, the unexpected diversity in bivalent promoter sets among these multipotent cells indicates that bivalent promoters may play complex roles in the overall biology of these cells. These results provide key information for forming the basis for future rational drug therapy aimed at epigenetic processes in these cells.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-724) contains supplementary material, which is available to authorized users.     

Milking the stroma in triple-negative breast cancer

Comment on: Witkiewicz AK, et al. Cell Cycle 2012; 1108–1117Investment in the post-genomic molecular dissection of breast cancer has resulted in an emphasis on prognostic and predictive markers, signatures derived to stratify the disease and the drive to generate targeted therapies. However, there remain significant challenges to individualize therapeutic targeting and improve the prognosis for the thousands of women who die each year from the heterogeneous range of breast cancers. This is particularly true for poor prognosis “triple-negative” breast cancers (TNBC), most prevalent in young and African American women, lacking the established therapeutic targets of estrogen receptor, progesterone receptor or HER2.Research has largely focused on the epithelial component of breast cancer rather than the tumor microenvironment, now recognized as a key hallmark of cancer.¹ In vitro, animal models and observations on clinical material² are now moving to consider physiological mechanisms by which stromal cells may influence breast epithelial and carcinoma cells.Witkiewicz et al.³ build on published evidence from the Lisanti group that cancer cells secrete hydrogen peroxide, initiating oxidative stress and aerobic glycolysis in tumor stroma, with L-lactate secretion from cancer-associated fibroblasts fueling oxidative mitochondrial metabolism in epithelial cancer cells: the “reverse Warburg effect.”They demonstrate stromal monocarboxylate transporter 4 (MCT4), detected by immunohistochemistry, as a functional marker of stromal hypoxia, oxidative stress, aerobic glycolysis and L-lactate efflux. High stromal MCT4 expression (but, critically, not epithelial MCT4) was associated with poor prognosis in TNBC patients. Combined high stromal MCT4 and loss of stromal caveolin-1 identify particularly poor prognostic TNBC.Thus, development of cancer may not lie solely in genetic or epigenetic epithelial changes, but with acquired functional changes in the stromal infrastructure of the breast. This supports the concept of epithelial malignant changes consequent with ecological and evolutionary opportunity.⁴ The “parasitic” character of tumor cells feeding off stromal cells highlights the need to seriously consider both ecological and biophysical concepts.⁵ We need to think beyond “intraspecific” competition among clonal subpopulations in the tumor and to consider tumor and stromal cells as distinct populations in a cancer ecosystem, with a range of “interspecific” competitive, exploitative and opportunistic interactions. Furthermore, the reverse Warburg effect relies on the inefficient diffusion of nutrients from stromal cells to tumor cells in a complex three-dimensional space. The extracellular space is brought to the foreground, and physical properties of molecular transport in this space may have as much impact on tumor growth as intricate cellular processes. The importance of the spatial arena is also apparent when contrasting the reverse Warburg effect with angiogenesis. In the former, tumor cells are exploiting their local environment, which will presumably be of limited yield, whereas angiogenesis taps the nutrients of the entire organism—­an effectively infinite reservoir for a growing tumor. In the reverse Warburg effect, a balance of ecological and biophysical factors underpins the sustainability of this mode of cancer nutrition. A two-compartment model coupling oxidative epithelial cells with glycolytic fibroblasts reflects increased expression of hypoxia-associated genes as a component part of prognostic stromal signatures.⁶ Further evidence of stromal/epithelial interaction comes from evidence that the effects of radiation on normal breast epithelium in vivo is at least partially dependent on the stromal context.⁷Manipulation of the tumor microenvironment to promote an anticancer phenotype challenges the cancer treatment paradigm. The long-established antidiabetes biguanide drugs offer a low-toxicity opportunity to disrupt the reverse Warburg effect. Metformin may target the cancer mitochondria³ and phenformin induce stromal sclerosis, at least in a breast cancer xenograft model,⁸ in addition to in vivo AMPK pathway and insulin-mediated systemic effects of metformin in women with breast cancer.⁹ The reverse Warburg effect challenges our therapeutic focus on breast cancer epithelium. Stromal MCT4 expression with caveolin-1 loss identifies poor prognostic TNBC patients and emphasizes the roles of the tumor microenvironment and ecological interactions between distinct populations of cells. The challenges now revolve around therapeutic manipulation of the stroma/epithelial interaction and the extracellular space, and testing these concepts in pre-invasive and metastatic settings where stromal changes may provide tissue niches of evolutionary opportunity for malignant cells.     

Reviewing once more the c-myc and Ras collaboration: Converging at the cyclin D1-CDK4 complex and challenging basic concepts of cancer biology

The c-myc is a proto-oncogene that manifests aberrant expression at high frequencies in most types of human cancer. C-myc gene amplifications are often observed in various cancers as well. Ample studies have also proved that c-myc has a potent oncogenicity, which can be further enhanced by collaborations with other oncogenes such as Bcl-2 and activated Ras. Studies on the collaborations of c-myc with Ras or other genes in oncogenicity have established several basic concepts and have disclosed their underlying mechanisms of tumor biology, including “immortalization” and “transformation”. In many cases, these collaborations may converge at the cyclin D1-CDK4 complex. In the meantime, however, many results from studies on the c-myc, Ras and cyclin D1-CDK4 also challenge these basic concepts of tumor biology and suggest to us that the immortalized status of cells should be emphasized. Stricter criteria and definitions for a malignantly transformed status and a benign status of cells in culture also need to be established to facilitate our study of the mechanisms for tumor formation and to better link up in vitro data with animal results and eventually with human cancer pathology.Key words: c-Myc, Cyclin D1, transformation, immortalization, oncogeneC-myc is the first proto-oncogene discovered and is known to participate in many cellular functions,¹ including maintenance of stem cell properties.² Most types of human cancer manifest aberrant expression of c-myc at high frequencies, and gene amplification occurs in many cases of various cancers as well. Ample studies have demonstrated that c-myc has a potent oncogenicity, which can be further enhanced by collaborations with other oncogenes such as a Ras mutant or with many extracellular growth stimuli that activate Ras, such as epidermal growth factor (EGF) or transforming growth factor α (TGFα). Studies on the collaborations of c-myc with Ras and other genes have provided us with mechanistic details behind several basic concepts of cancer biology, including the “two-hit principle”,³ “immortalization” and “transformation”. In the meantime, however, many results from these studies also challenge these basic concepts and thus confuse us. We now discuss the data on the collaborations of c-myc with Ras and other genes and present a perspective that these collaborations may converge at the cyclin D1-CDK4 complex. We also appeal to emphasize the importance of an immortalized status of cells and to establish stricter criteria to better define a transformed and benign statuses, so as to better connect in vitro results with animal data and with human cancer pathology.     

Semaphorin Signals on the Road to Cancer Invasion and Metastasis

Semaphorins are a large family of secreted and membrane-bound molecules initially implicated in the development of the nervous system and in axon guidance. More recently, they have been found to regulate cell adhesion and cell motility, angiogenesis, immune function and tumor progression. Notably, Semaphorins have been implicated with opposite functions in cancer: either as putative tumor suppressors and anti-angiogenic factors, or as mediating tumor angiogenesis, invasion and metastasis. Interestingly, Semaphorins may display divergent activities in different cell types. These multifaceted functions may be explained by the involvement of different kinds of semaphorin receptor complexes, and by the consequent activation of multiple signaling pathways, in different cells or different functional stages. Semaphorin signaling is largely mediated by the Plexins. However, semaphorin receptor complexes may also include Neuropilins and tyrosine kinases implicated in cancer. In this review, we will focus on major open questions concerning the potential role of Semaphorin signals in cancer.Key words: semaphorin, plexin, neuropilin, migration, tumor, metastasis, signalingOver twenty different Semaphorin genes are known in vertebrates. They were initially discovered as repelling cues for axons, in the wiring of the neural system. However, they are currently considered versatile signals regulating cell migration, angiogenesis, tissue morphogenesis, immune function and cancer.^1–2 Semaphorins have been implicated with opposite functions in tumor progression (summarized in Fig. 1). For example, Semaphorins 3B and 3F are putative tumor suppressors, while the expression of Semaphorin 3C, 3E and 5C has been associated with tumor invasion and metastasis. Interestingly, certain Semaphorins display divergent activities in different cell types. These varied functions of Semaphorins are likely to be explained by the involvement of different receptor complexes and multiple signaling pathways.Open in a separate windowFigure 1Semaphorin signals on the road to cancer invasion and metastasis. Semaphorins play a regulatory role on the main elements driving cancer progression. They can be seen as “stop” or “go” signals for tumor cells, as well as for stromal cells in the tumor microenvironment. The scheme features some examples of the semaphorin signals implicated so far. More information on the implicated receptors and functional activities of the different semaphorins are summarized in 相似文献   

An Integrated Approach to Uncover Driver Genes in Breast Cancer Methylation Genomes

Background

Cancer cells typically exhibit large-scale aberrant methylation of gene promoters. Some of the genes with promoter methylation alterations play “driver” roles in tumorigenesis, whereas others are only “passengers”.

Results

Based on the assumption that promoter methylation alteration of a driver gene may lead to expression alternation of a set of genes associated with cancer pathways, we developed a computational framework for integrating promoter methylation and gene expression data to identify driver methylation aberrations of cancer. Applying this approach to breast cancer data, we identified many novel cancer driver genes and found that some of the identified driver genes were subtype-specific for basal-like, luminal-A and HER2+ subtypes of breast cancer.

Conclusion

The proposed framework proved effective in identifying cancer driver genes from genome-wide gene methylation and expression data of cancer. These results may provide new molecular targets for potential targeted and selective epigenetic therapy.     

Metformin is synthetically lethal with glucose withdrawal in cancer cells

Comment on: Menendez JA, et al. Cell Cycle 2012; 11: 2782-92.In a recent issue of Cell Cycle, Menendez and colleagues proposed a novel concept, that metformin is synthetically lethal with glucose withdrawal in cancer cells.¹ Historically, synthetic lethality has focused on how tumor cells are responsive to certain agents that only harbor specific constitutive epigenetic or genetic lesions.² More recent data from several groups have uncovered that altered tumor microenvironment could be used to confer synthetic lethality to specific drugs, defined as “contextual synthetic lethality,” that is microenvironment-mediated. For example, hypoxia-induced HR (homologous repair) defect has been shown to be synthetically lethal to PARP inhibition, while PARP inhibition, per se, did not alter HR inhibition or function, thus providing a prime example of “contextual synthetic lethality.”³ In this report, Menendez et al. have elegantly connected the glucose-deprived tumor microenvironment in primary tumors as a synthetic lethal partner to metformin. Metformin is a FDA-approved drug to treat diabetic patients that is gaining momentum as a repurposing drug for cancer treatment.⁴ Using several different breast cancer cells with and without oncogenic activation, the authors have shown that the glucose-rich conditions of the in vitro experiments dictates the use of very high concentrations of metformin, which are not applicable to glucose-starved in vivo conditions. While other reports have alluded to the effect of glucose withdrawal in killing genetically compromised cells to therapeutic effect of metformin in vitro,⁵ Menendez et al have provided a logical explanation for the use of very high concentrations of metformin to achieve anticancer effects in vitro in the high glucose-rich environment used in these experiments, which are clinically not applicable in vivo in patients.Based on these findings, it can be envisaged that in the tumor microenvironment, where the cancer cells are under extreme nutritional and hypoxic stress (a niche for cancer stem cells), metformin treatment could favor synthetic lethality and hence effectively can attenuate tumor growth. The tumor microenvironment thus enables the bioenergetic switch in favor of glycolysis and dependence on glucose and glutamine as a rapid source of nutrition. While the authors’ data clearly depicts how metformin eliminates the tolerance of the breast cancer cells to fluctuations in glucose concentrations, it is important to understand how the availability of other dominant sources of energy, such as glutamine, might participate in this scenario. It is plausible that subtype of breast cancers, i.e., basal vs luminal, might depend on different energy sources, albeit to a different extent.⁶ This is important, because tumor cells often acquire metabolic adaptability toward available preferred energy source to adapt well to nutritional stress via autophagy and altered metabolism.⁷ Along these lines, the authors rationalize the therapeutic targeting of the cancer stem cells by metformin through its synthetic lethal activity to the hyperglycotic phenotype often seen in CSC to sustain their stemness.⁸ Further characterization of how metformin treatment alters the metabolic nodes in cancer stem cells and/or p53-null cells would explain the underpinning mechanisms for increased susceptibility of these indolent and aggressive cancer cells toward metformin.It is well documented that metformin, by inhibiting complex I of respiratory chain in mitochondria (ETCI), induces a decrease in the ATP levels, and that glucose depletion also decreases ATP levels, albeit to varying levels. Therefore, it is possible that simultaneous targeting of both pathways (glycolytic pathway and OXPHOS) caused ATP depletion below a critical threshold, resulting in cell death. This concept is supported by the elegant study⁹ highlighting the effectiveness of combination of glycolysis inhibition by 2-DG and metformin in several preclinical models exhibiting anti-tumor effects, including MB-MDA231 used in this study.Since recent studies indicate that inhibiting glucose uptake with small-molecule inhibitors led to a decline in cylcin E2 and p-RB levels,¹⁰ it is a possibility that cell cycle inhibitor levels are also regulated under glucose withdrawal conditions, sensitizing cells to cytotoxic effects of metformin in breast cancer cells.Considering data from several studies, a view that metformin treatment has pleotropic effects on several signaling pathways under glucose-free conditions seems a practical possibility. Overall, this work offers several new insights into glucose-dependent mechanisms underpinning the mode of action of metformin as a viable therapeutic strategy.     

Epigenetic therapy of cancer stem and progenitor cells by targeting DNA methylation machineries

Recent advances in stem cell biology have shed light on how normal stem and progenitor cells can evolve to acquire malignant characteristics during tumorigenesis. The cancer counterparts of normal stem and progenitor cells might be occurred through alterations of stem cell fates including an increase in self-renewal capability and a decrease in differentiation and/or apoptosis. This oncogenic evolution of cancer stem and progenitor cells, which often associates with aggressive phenotypes of the tumorigenic cells, is controlled in part by dysregulated epigenetic mechanisms including aberrant DNA methylation leading to abnormal epigenetic memory. Epigenetic therapy by targeting DNA methyltransferases (DNMT) 1, DNMT3A and DNMT3B via 5-Azacytidine (Aza) and 5-Aza-2’-deoxycytidine (Aza-dC) has proved to be successful toward treatment of hematologic neoplasms especially for patients with myelodysplastic syndrome. In this review, I summarize the current knowledge of mechanisms underlying the inhibition of DNA methylation by Aza and Aza-dC, and of their apoptotic- and differentiation-inducing effects on cancer stem and progenitor cells in leukemia, medulloblastoma, glioblastoma, neuroblastoma, prostate cancer, pancreatic cancer and testicular germ cell tumors. Since cancer stem and progenitor cells are implicated in cancer aggressiveness such as tumor formation, progression, metastasis and recurrence, I propose that effective therapeutic strategies might be achieved through eradication of cancer stem and progenitor cells by targeting the DNA methylation machineries to interfere their “malignant memory”.     

Developmental signaling pathways in cancer stem cells of solid tumors

Background

The intricate regulation of several signaling pathways is essential for embryonic development and adult tissue homeostasis. Cancers commonly display aberrant activity within these pathways. A population of cells identified in several cancers, termed cancer stem cells (CSCs) show similar properties to normal stem cells and evidence suggests that altered developmental signaling pathways play an important role in maintaining CSCs and thereby the tumor itself.

Scope of review

This review will focus on the roles of the Notch, Wnt and Hedgehog pathways in the brain, breast and colon cancers. We describe the roles these pathways play in normal tissue homeostasis through the regulation of stem cell fate in these three tissues, and the experimental evidence indicating that the role of these pathways in cancers of these is directly linked to CSCs.

Major conclusions

A large body of evidence is accumulating to indicate that the deregulation of Notch, Wnt and Hedgehog pathways play important roles in both normal and cancer stem cells. We are only beginning to understand how these pathways interact, how they are coordinated during normal development and adult tissue homeostasis, and how they are deregulated during cancer. However, it is becoming increasingly clear that if we are to target CSCs therapeutically, it will likely be necessary to develop combination therapies.

General significance

If CSCs are the driving force behind tumor maintenance and growth then understanding the molecular mechanisms regulating CSCs is essential. Such knowledge will contribute to better targeted therapies that could significantly enhance cancer treatments and patient survival. This article is part of a Special Issue entitled Biochemistry of Stem Cells.