Tumor initiation in human malignant melanoma and potential cancer therapies

One Stone, Two Birds: N6-Methyladenosine RNA Modification in Leukemia Stem Cells and the Tumor Immune Microenvironment in Acute Myeloid Leukemia

Acute myeloid leukemia is the most common acute leukemia in adults, with accumulation of abundant blasts and impairment of hematogenic function. Despite great advances in diagnosis and therapy, the overall survival of patients with acute myeloid leukemia remains poor. Leukemia stem cells are the root cause of relapse and chemoresistance in acute myeloid leukemia. The tumor immune microenvironment is another trigger to induce recurrence and drug resistance. Understanding the underlying factors influencing leukemia stem cells and the tumor immune microenvironment is an urgent and unmet need. Intriguingly, N6-methyladenosine, the most widespread internal mRNA modification in eukaryotes, is found to regulate both leukemia stem cells and the tumor immune microenvironment. Methyltransferases and demethylases cooperatively make N6-methyladenosine modification reversible and dynamic. Increasing evidence demonstrates that N6-methyladenosine modification extensively participates in tumorigenesis and progression in various cancers, including acute myeloid leukemia. In this review, we summarize the current progress in studies on the functions of N6-methyladenosine modification in acute myeloid leukemia, especially in leukemia stem cells and the tumor immune microenvironment. We generalize the landscape of N6-methyladenosine modification in self-renewal of leukemia stem cells and immune microenvironment regulation, as well as in the initiation, growth, proliferation, differentiation, and apoptosis of leukemia cells. In addition, we further explore the clinical application of N6-methyladenosine modification in diagnosis, prognostic stratification, and effect evaluation. Considering the roles of N6-methyladenosine modification in leukemia stem cells and the tumor immune microenvironment, we propose targeting N6-methyladenosine regulators as one stone to kill two birds for acute myeloid leukemia treatment.

miRNA-200b Signature in the Prevention of Skin Cancer Stem Cells by Polyphenol-enriched Blueberry Preparation

Exposure of the skin to solar UV radiation leads to inflammation, DNA damage, and dysregulation of cellular signaling pathways, which may cause skin cancer. Photochemoprevention with natural products is an effective strategy for the control of cutaneous neoplasia. Polyphenols have been proven to help prevent skin cancer and to inhibit the growth of cancer stem cells (CSCs) through epigenetic mechanisms, including modulation of microRNAs expression. Thus, the current study aimed to assess the effect of polyphenol enriched blueberry preparation (PEBP) or non-fermented blueberry juice (NBJ) on expression of miRNAs and target proteins associated with different clinicopathological characteristics of skin cancer such as stemness, motility, and invasiveness. We observed that PEBP significantly inhibited the proliferation of skin CSCs derived from different melanoma cell lines, HS 294T and B16F10. Moreover, PEBP was able to reduce the formation of melanophores. We also showed that the expression of the CD133⁺ stem cell marker in B16F10 and HS294T cell lines was significantly decreased after treating the cells with PEBP in comparison to the NBJ and control groups. Importantly, tumor suppressors' miR-200s, involved in the regulation of the epithelial-to-mesenchymal transition and metastasis, were strikingly upregulated. In addition, we have shown that a protein target of the tumor suppressor miR200b, ZEB1, was also significantly modulated. Thus, the results demonstrates that PEBP possesses potent anticancer and anti-metastatic potentials and may represent a novel chemopreventative agent against skin cancer.

Immune evasion by cancer stem cells

Tumor immunity represents a new avenue for cancer therapy. Immune checkpoint inhibitors have successfully improved outcomes in several tumor types. In addition, currently, immune cell-based therapy is also attracting significant attention. However, the clinical efficacy of these treatments requires further improvement. The mechanisms through which cancer cells escape the immune response must be identified and clarified. Cancer stem cells (CSCs) play a central role in multiple aspects of malignant tumors. CSCs can initiate tumors in partially immunocompromised mice, whereas non-CSCs fail to form tumors, suggesting that tumor initiation is a definitive function of CSCs. However, the fact that non-CSCs also initiate tumors in more highly immunocompromised mice suggests that the immune evasion property may be a more fundamental feature of CSCs rather than a tumor-initiating property. In this review, we summarize studies that have elucidated how CSCs evade tumor immunity and create an immunosuppressive milieu with a focus on CSC-specific characteristics and functions. These profound mechanisms provide important clues for the development of novel tumor immunotherapies.

Cancer stem cells and macrophages: molecular connections and future perspectives against cancer

Cancer stem cells (CSCs) have been considered the key drivers of cancer initiation and progression due to their unlimited self-renewal capacity and their ability to induce tumor formation. Macrophages, particularly tumor-associated macrophages (TAMs), establish a tumor microenvironment to protect and induce CSCs development and dissemination. Many studies in the past decade have been performed to understand the molecular mediators of CSCs and TAMs, and several studies have elucidated the complex crosstalk that occurs between these two cell types. The aim of this review is to define the complex crosstalk between these two cell types and to highlight potential future anti-cancer strategies.

Role of Cancer Stem Cells in Cholangiocarcinoma and Therapeutic Implications

Cholangiocarcinoma (CCA) is the second most common type of liver cancer, and is highly aggressive with very poor prognosis. CCA is classified into intrahepatic cholangiocarcinoma (iCCA) and extra-hepatic cholangiocarcinoma (eCCA), which is further stratified into perihilar (pCCA) and distal (dCCA). Cancer stem cells (CSCs) are a subpopulation of cancer cells capable of tumor initiation and malignant growth, and are also responsible for chemoresistance. Thus, CSCs play an important role in CCA carcinogenesis. Surface markers such as CD133, CD24, CD44, EpCAM, Sox2, CD49f, and CD117 are important for identifying and isolating CCA CSCs. CSCs are present in the tumor microenvironment (TME), termed ‘CSC niche’, where cellular components and soluble factors interact to promote tumor initiation. Epithelial-to-mesenchymal transition (EMT) is another important mechanism underlying carcinogenesis, involved in the invasiveness, metastasis and chemoresistance of cancer. It has been demonstrated that EMT plays a critical role in generating CSCs. Therapies targeting the surface markers and signaling pathways of CCA CSCs, proteins involved in TME, and immune checkpoint proteins are currently under investigation. Therefore, this review focuses on recent studies on the roles of CSCs in CCA; the possible therapeutic strategies targeting CSCs of CCA are also discussed.

Tumor Microenvironment Versus Cancer Stem Cells in Cholangiocarcinoma: Synergistic Effects?

Cholangiocarcinoma (CCAs) may be defined as tumors that derived from the biliary tree with the differentiation in the biliary epithelial cells. This tumor is malignant, extremely aggressive with a poor prognosis. It can be treated surgically and its pathogenesis is poorly understood. The tumor microenvironment (TME) is a very important factor in the regulation of tumor angiogenesis, invasion, and metastasis. Besides cancer stem cells (CSCs) can modulate tumor growth, stroma formation, and migratory capability. The initial stage of tumorigenesis is characterized by genetic mutations and epigenetic alterations due to intrinsic factors which lead to the generation of oncogenes thus inducing tumorigenesis. CSCs may result from precancerous stem cells, cell de-differentiation, normal stem cells, or an epithelial-mesenchymal transition (EMT). CSCs have been found in the cancer niche, and EMT may occur early within the tumor microenvironment. Previous studies have demonstrated evidence of cholangiocarcinoma stem cells (CD133, CD24, EpCAM, CD44, and others) and the presence of these markers has been associated with malignant potential. The interaction between TME and cholangiocarcinoma stem cells via signaling mediators may create an environment that accommodates tumor growth, yielding resistance to cytotoxic insults (chemotherarapeutic). While progress has been made in the understanding of the mechanisms, the interactions in the tumorigenic process still remain a major challenge. Our review, addresses recent concepts of TME-CSCs interaction and will emphasize the importance of early detection with the use of novel diagnostic mechanisms such as CCA-CSC biomarkers and the importance of tumor stroma to define new treatments. J. Cell. Physiol. 231: 768-776, 2016. © 2015 Wiley Periodicals, Inc.

Expression of Multidrug Resistance Transporter ABCB5 in a Murine Model of Human Conjunctival Melanoma

Conjunctival melanoma (CM) is a rare ocular malignancy with a high tendency to reoccur locally and with a high risk of metastatic disease. Metastases are often unresponsive to conventional treatment. Recently, an animal model was set up using human CM cells. Orthotopic xenografts from human CM were created by subconjunctival injection of three different CM cell lines into NOD.Cg-Prkdc^scid IL2rg^tm1Wjl /SzJ (NSG) mice. Subconjunctival injection of cultured CM cells led to excellent subconjunctival growth, but no metastases were found. When single-cell suspensions were obtained from the subconjunctival xenografts and passaged in vivo, all mice developed metastases. As recent findings indicate that cancer stem cells are linked to tumor recurrences, we used this new murine model to determine the expression of the stem cell marker ABCB5 during tumor progression. Expression of the ABCB5 protein was determined in three cell lines and during different stages of tumor development as observed in our model. All three cell lines contained a subpopulation of cells positive for ABCB5. During tumor development, expression of ABCB5 increased during phases of tumor expansion. Furthermore, expression of ABCB5 was increased in metastases. Using this model for CM, we were able to initiate metastatic spread and determine the expression of the stem cell marker ABCB5 during different stages of tumor development, identifying ABCB5 as a potential novel therapeutic target. This study illustrates the potential of our newly established murine model.

Cancer stem cells as a target population for drug discovery

Cancer stem cells (CSCs) have been identified in a growing list of malignancies and are believed to be responsible for cancer initiation, metastasis and relapse following certain therapies, even though they may only represent a small fraction of the cells in a given cancer. Like somatic stem cells and embryonic stem cells, CSCs are capable of self-renewal and differentiation into more mature, less tumorigenic cells that make up the bulk populations of cancer cells. Elimination of CSCs promises intriguing therapeutic potential and this concept has been adopted in preclinical drug discovery programs. Herein we will discuss the progress of these efforts, general considerations in practice, major challenges and possible solutions.

Mek inhibition results in marked antitumor activity against metastatic melanoma patient-derived melanospheres and in melanosphere-generated xenografts

One of the key oncogenic pathways involved in melanoma aggressiveness, development and progression is the RAS/BRAF/MEK pathway, whose alterations are found in most patients. These molecular anomalies are promising targets for more effective anti-cancer therapies. Some Mek inhibitors showed promising antitumor activity, although schedules and doses associated with low systemic toxicity need to be defined. In addition, it is now accepted that cancers can arise from and be maintained by the cancer stem cells (CSC) or tumor-initiating cells (TIC), commonly expanded in vitro as tumorspheres from several solid tumors, including melanoma (melanospheres). Here, we investigated the potential targeting of MEK pathway by exploiting highly reliable in vitro and in vivo pre-clinical models of melanomas based on melanospheres, as melanoma initiating cells (MIC) surrogates. MEK inhibition, through PD0325901, provided a successful strategy to affect survival of mutated-BRAF melanospheres and growth of wild type-BRAF melanospheres. A marked citotoxicity was observed in differentated melanoma cells regardless BRAF mutational status. PD0325901 treatment, dramatically inhibited growth of melanosphere-generated xenografts and determined impaired tumor vascularization of both mutated- and wild type-BRAF tumors, in the absence of mice toxicity. These results suggest that MEK inhibition might represent a valid treatment option for patients with both mutated- or wild type-BRAF melanomas, affecting tumor growth through multiple targets.

Markers of circulating tumour cells in the peripheral blood of patients with melanoma correlate with disease recurrence and progression

BACKGROUND

Multimarker quantitative real-time polymerase chain reaction (qRT-PCR) represents an effective method for detecting circulating tumour cells in the peripheral blood of patients with melanoma.

OBJECTIVES

To investigate whether the phenotype of circulating melanoma cells represents a useful indicator of disease stage, recurrence and treatment efficacy.

METHODS

  Peripheral blood was collected from 230 patients with melanoma and 152 healthy controls over a period of 3years and 9months. Clinical data and blood samples were collected from patients with primary melanoma (early stages, 0-II, n=154) and metastatic melanoma (late stages, III-IV, n=76). Each specimen was examined by qRT-PCR analysis for the expression of five markers: MLANA, ABCB5, TGFβ2, PAX3d and MCAM.

RESULTS

 In total, 212 of the patients with melanoma (92%) expressed markers in their peripheral blood. Two markers, MLANA and ABCB5, had the greatest prognostic value, and were identified as statistically significant among patients who experienced disease recurrence within our study period, being expressed in 45% (MLANA) and 49% (ABCB5) of patients with recurrence (P=0·001 and P=0·031, respectively). For patients administered nonsurgical treatments, MCAM expression correlated with poor treatment outcome.

CONCLUSIONS

Circulating tumour cells were detectable at all stages of disease and long after surgical treatment, even when patients were considered disease free. Specifically, expression of ABCB5 and MLANA had significant prognostic value in inferring disease recurrence, while MCAM expression was associated with poor patient outcome after treatment, confirming multimarker qRT-PCR as a potential technique for monitoring disease status.

Identifying mRNA, microRNA and protein profiles of melanoma exosomes

BACKGROUND

Exosomes are small membranous vesicles secreted into body fluids by multiple cell types, including tumor cells, and in various disease conditions. Tumor exosomes contain intact and functional mRNAs, small RNAs (including miRNAs), and proteins that can alter the cellular environment to favor tumor growth. Molecular profiling may increase our understanding of the role of exosomes in melanoma progression and may lead to discovery of useful biomarkers.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study, we used mRNA array profiling to identify thousands of exosomal mRNAs associated with melanoma progression and metastasis. Similarly, miRNA array profiling identified specific miRNAs, such as hsa-miR-31, -185, and -34b, involved in melanoma invasion. We also used proteomic analysis and discovered differentially expressed melanoma exosomal proteins, including HAPLN1, GRP78, syntenin-1, annexin A1, and annexin A2. Importantly, normal melanocytes acquired invasion ability through molecules transported in melanoma cell-derived exosomes.

CONCLUSIONS/SIGNIFICANCE

Our results indicate that melanoma-derived exosomes have unique gene expression signatures, miRNA and proteomics profiles compared to exosomes from normal melanocytes. To the best of our knowledge, this is the first in-depth screening of the whole transcriptome/miRNome/proteome expression in melanoma exosomes. These results provide a starting point for future more in-depth studies of tumor-derived melanoma exosomes, which will aid our understanding of melanoma biogenesis and new drug-targets that may be translated into clinical applications, or as non-invasive biomarkers for melanoma.

Targeting apoptosis pathways in cancer stem cells

There is a significant void in cancer biology with regard to the elucidation of the mechanisms that underlie tumor formation and progression. Recently, the existence of a hierarchy within cancer cell populations has been demonstrated experimentally for several tumor types. The identification of a tumor cell subset that is capable of self-renewal and, concurrently, generation into more differentiated progeny has engendered new perspectives toward selective targeting of tumors. Although the identification of the so-called "cancer stem cells" (CSCs) is a leap in the study of cancer ontogenesis, therapeutic targeting of such cells is plagued by significant difficulties. CSCs are able to evade the control mechanisms that regulate cell survival and proliferation. Apoptosis is one of the most critical and well-studied mechanisms, governing tissue development and homeostasis through a complex network of molecules that mediate death and survival signals. Escape from such a finely tuned death program is a prerequisite for any tumor-initiating cell. Thus, many compounds have been developed to target cancer cells and induce apoptosis directly or indirectly. Several TRAIL receptor agonists are in Phase I or II trials, and IAP inhibitors are undergoing clinical examination to exploit their ability to enhance ionizing radiation- and chemotherapy-induced apoptosis. Further, the EGF-R/Akt pro-survival signaling axis is one of the most frequently explored sources of targets for indirect apoptosis induction, as evidenced by the significant amount of molecules designed to target this pathway and have been approved by the FDA or are under clinical evaluation. Despite the promise of these magic bullets, the absence of reliable clinical models has considerably diminished the therapeutic potential of targeted therapies considerably. A more systematic molecular characterization of the tumor-initiating cell population in many tumors will allow us to refine the stimuli that force CSCs to die, thus accelerating the development of more effective treatment for cancer.

Isolation of tumorigenic circulating melanoma cells

Circulating tumor cells (CTC) have been identified in several human malignancies, including malignant melanoma. However, whether melanoma CTC are tumorigenic and cause metastatic progression is currently unknown. Here, we isolate for the first time viable tumorigenic melanoma CTC and demonstrate that this cell population is capable of metastasis formation in human-to-mouse xenotransplantation experiments. The presence of CTC among peripheral blood mononuclear cells (PBMC) of murine recipients of subcutaneous (s.c.) human melanoma xenografts could be detected based on mRNA expression for human GAPDH and/or ATP-binding cassette subfamily B member 5 (ABCB5), a marker of malignant melanoma-initiating cells previously shown to be associated with metastatic disease progression in human patients. ABCB5 expression could also be detected in PBMC preparations from human stage IV melanoma patients but not healthy controls. The detection of melanoma CTC in human-to-mouse s.c. tumor xenotransplantation models correlated significantly with pulmonary metastasis formation. Moreover, prospectively isolated CTC from murine recipients of s.c. melanoma xenografts were capable of primary tumor initiation and caused metastasis formation upon xenotransplantation to secondary murine NOD-scid IL2Rγ(null) recipients. Our results provide initial evidence that melanoma CTC are tumorigenic and demonstrate that CTC are capable of causing metastatic tumor progression. These findings suggest a need for CTC eradication to inhibit metastatic progression and provide a rationale for assessment of therapeutic responses of this tumorigenic cell population to promising emerging melanoma treatment modalities.