Cancer stem cells in sarcomas: Getting to the stemness core

Glioblastoma stem cell metabolism and immunity

Despite enormous efforts being invested in the development of novel therapies for brain malignancies, there remains a dire need for effective treatments, particularly for pediatric glioblastomas. Their poor prognosis has been attributed to the fact that conventional therapies target tumoral cells, but not glioblastoma stem cells (GSCs). GSCs are characterized by self-renewal, tumorigenicity, poor differentiation, and resistance to therapy. These characteristics represent the fundamental tools needed to recapitulate the tumor and result in a relapse. The mechanisms by which GSCs alter metabolic cues and escape elimination by immune cells are discussed in this article, along with potential strategies to harness effector immune cells against GSCs. As cellular immunotherapy is making significant advances in a variety of cancers, leveraging this underexplored reservoir may result in significant improvements in the treatment options for brain malignancies.

Genetic and Epigenetic Characterization of Sarcoma Stem Cells Across Subtypes Identifies EZH2 as a Therapeutic Target

High-grade complex karyotype soft tissue sarcomas (STS) are a heterogeneous and aggressive set of cancers that share a common treatment strategy. Disease progression and failure to respond to anthracycline based chemotherapy, standard first-line treatment, is associated with poor patient outcomes. To address this, we investigated the contribution of STS cancer stem cells (STS-CSCs) to doxorubicin resistance. We identified a positive correlation between CSC abundance and doxorubicin IC 50 in resistant cell lines. We investigated if a common genetic signature across STS-CSCs could be targeted. Utilizing patient derived samples from five sarcoma subtypes we identified Enhancer of Zeste homolog 2 (EZH2), a member of the polycomb repressive complex 2 (PRC2) responsible for H3K27 methylation as being enriched in the CSC population. EZH2 activity and a shared epigenetic profile was observed across subtypes. Targeting of EZH2 using Tazemetostat, an FDA approved inhibitor specifically ablated the STS-CSC population. Treatment of doxorubicin resistant cell lines with tazemetostat resulted in a decrease in the STS-CSC population. Further, co-treatment was not only synergistic in the parent cell lines, but restored chemosensitivity in doxorubicin resistant lines. These data confirm the presence of shared genetic programs across distinct subtypes of CSC-STS that can be therapeutically targeted.

Advancing biomarker development for diagnostics and therapeutics using solid tumour cancer stem cell models

The cancer stem cell model hopes to explain solid tumour carcinogenesis, tumour progression and treatment failure in cancers. However, the cancer stem cell model has led to minimal clinical translation to cancer stem cell biomarkers and targeted therapies in solid tumours. Many reasons underlie the challenges, one being the imperfect understanding of the cancer stem cell model. This review hopes to spur further research into clinically translatable cancer stem cell biomarkers through first defining cancer stem cells and their associated models. With a better understanding of these models there would be a development of more accurate biomarkers. Making the clinical translation of biomarkers into diagnostic tools and therapeutic agents more feasible.

Noncanonical roles of p53 in cancer stemness and their implications in sarcomas

Impairment of the prominent tumor suppressor p53, well known for its canonical role as the "guardian of the genome", is found in almost half of human cancers. More recently, p53 has been suggested to be a crucial regulator of stemness, orchestrating the differentiation of embryonal and adult stem cells, suppressing reprogramming into induced pluripotent stem cells, or inhibiting cancer stemness (i.e., cancer stem cells, CSCs), which underlies the development of therapy-resistant tumors. This review addresses these noncanonical roles of p53 and their implications in sarcoma initiation and progression. Indeed, dysregulation of p53 family proteins is a common event in sarcomas and is associated with poor survival. Additionally, emerging studies have demonstrated that loss of wild-type p53 activity hinders the terminal differentiation of mesenchymal stem cells and leads to the development of aggressive sarcomas. This review summarizes recent findings on the roles of aberrant p53 in sarcoma development and stemness and further describes therapeutic approaches to restore normal p53 activity as a promising anti-CSC strategy to treat refractory sarcomas.

Tumour Microenvironment Stress Promotes the Development of Drug Resistance

Multi-drug resistance (MDR) is a leading cause of cancer-related death, and it continues to be a major barrier to cancer treatment. The tumour microenvironment (TME) has proven to play an essential role in not only cancer progression and metastasis, but also the development of resistance to chemotherapy. Despite the significant advances in the efficacy of anti-cancer therapies, the development of drug resistance remains a major impediment to therapeutic success. This review highlights the interplay between various factors within the TME that collectively initiate or propagate MDR. The key TME-mediated mechanisms of MDR regulation that will be discussed herein include (1) altered metabolic processing and the reactive oxygen species (ROS)-hypoxia inducible factor (HIF) axis; (2) changes in stromal cells; (3) increased cancer cell survival via autophagy and failure of apoptosis; (4) altered drug delivery, uptake, or efflux and (5) the induction of a cancer stem cell (CSC) phenotype. The review also discusses thought-provoking ideas that may assist in overcoming the TME-induced MDR. We conclude that stressors from the TME and exposure to chemotherapeutic agents are strongly linked to the development of MDR in cancer cells. Therefore, there remains a vast area for potential research to further elicit the interplay between factors existing both within and outside the TME. Elucidating the mechanisms within this network is essential for developing new therapeutic strategies that are less prone to failure due to the development of resistance in cancer cells.

Sarcoma treatment in the era of molecular medicine

Sarcomas are heterogeneous and clinically challenging soft tissue and bone cancers. Although constituting only 1% of all human malignancies, sarcomas represent the second most common type of solid tumors in children and adolescents and comprise an important group of secondary malignancies. More than 100 histological subtypes have been characterized to date, and many more are being discovered due to molecular profiling. Owing to their mostly aggressive biological behavior, relative rarity, and occurrence at virtually every anatomical site, many sarcoma subtypes are in particular difficult-to-treat categories. Current multimodal treatment concepts combine surgery, polychemotherapy (with/without local hyperthermia), irradiation, immunotherapy, and/or targeted therapeutics. Recent scientific advancements have enabled a more precise molecular characterization of sarcoma subtypes and revealed novel therapeutic targets and prognostic/predictive biomarkers. This review aims at providing a comprehensive overview of the latest advances in the molecular biology of sarcomas and their effects on clinical oncology; it is meant for a broad readership ranging from novices to experts in the field of sarcoma.

SOX2 Expression in Canine Neoplasia

SOX2 is a major transcriptional regulator of stem cell pluripotency and self-renewability. Its expression in cancer stem cells from several different tumor types in humans and rodent models directly implicates SOX2 in tumorigenicity, metastasis, drug resistance, recurrence, and poor survival. Our objective was to investigate the expression of SOX2 in canine neoplasia. Immunohistochemistry for SOX2 was performed in sets of 10 archived formalin-fixed paraffin-embedded tissues from 45 distinct canine neoplasms. Normal expression of SOX2 was evaluated in a canine tissue microarray. Strong and diffuse SOX2 intranuclear immunolabeling was consistently found in the majority of ectodermal (13/15) and endodermal tumors (5/7). Negative, variable, or inconsistent SOX2 intranuclear immunolabeling was detected in the majority of mesodermal tumors (10/16) and in tumors with dual or uncertain origin (5/7). Although further studies are necessary to understand mechanistically how SOX2 contributes to the biology of each tumor type, this study demonstrates the expression of SOX2 in a wide variety of canine cancers. In the future, screening methods based on cellular plasticity and pluripotency biomarkers may provide avenues for the rational design of therapeutic strategies that target vulnerable signals upstream or downstream of SOX2 in different cancers, and possibly offer novel clinical applications for SOX2 as a prognostic indicator.

Cancer Stem Cells in Soft-Tissue Sarcomas

Soft tissue sarcomas (STS) are a rare group of mesenchymal solid tumors with heterogeneous genetic profiles and clinical features. Systemic chemotherapy is the backbone treatment for advanced STS; however, STS frequently acquire resistance to standard therapies, which highlights the need to improve treatments and identify novel therapeutic targets. Increases in the knowledge of the molecular pathways that drive sarcomas have brought to light different molecular alterations that cause tumor initiation and progression. These findings have triggered a breakthrough of targeted therapies that are being assessed in clinical trials. Cancer stem cells (CSCs) exhibit mesenchymal stem cell (MSC) features and represent a subpopulation of tumor cells that play an important role in tumor progression, chemotherapy resistance, recurrence and metastasis. In fact, CSCs phenotypes have been identified in sarcomas, allied to drug resistance and tumorigenesis. Herein, we will review the published evidence of CSCs in STS, discussing the molecular characteristic of CSCs, the commonly used isolation techniques and the new possibilities of targeting CSCs as a way to improve STS treatment and consequently patient outcome.

SOX2 Expression and Transcriptional Activity Identifies a Subpopulation of Cancer Stem Cells in Sarcoma with Prognostic Implications

Stemness in sarcomas is coordinated by the expression of pluripotency factors, like SOX2, in cancer stem cells (CSC). The role of SOX2 in tumor initiation and progression has been well characterized in osteosarcoma. However, the pro-tumorigenic features of SOX2 have been scarcely investigated in other sarcoma subtypes. Here, we show that SOX2 depletion dramatically reduced the ability of undifferentiated pleomorphic sarcoma (UPS) cells to form tumorspheres and to initiate tumor growth. Conversely, SOX2 overexpression resulted in increased in vivo tumorigenicity. Moreover, using a reporter system (SORE6) which allows to monitor viable cells expressing SOX2 and/or OCT4, we found that SORE6+ cells were significantly more tumorigenic than the SORE6- subpopulation. In agreement with this findings, SOX2 expression in sarcoma patients was associated to tumor grade, differentiation, invasive potential and lower patient survival. Finally, we studied the effect of a panel of anti-tumor drugs on the SORE6+ cells of the UPS model and patient-derived chondrosarcoma lines. We found that the mithramycin analogue EC-8042 was the most efficient in reducing SORE6+ cells in vitro and in vivo. Overall, this study demonstrates that SOX2 is a pro-tumorigenic factor with prognostic potential in sarcoma. Moreover, SORE6 transcriptional activity is a bona fide CSC marker in sarcoma and constitutes an excellent biomarker for evaluating the efficacy of anti-tumor treatments on CSC subpopulations.

Serial Xenotransplantation in NSG Mice Promotes a Hybrid Epithelial/Mesenchymal Gene Expression Signature and Stemness in Rhabdomyosarcoma Cells

Serial xenotransplantation of sorted cancer cells in immunodeficient mice remains the most complex test of cancer stem cell (CSC) phenotype. However, we have demonstrated in various sarcomas that putative CSC surface markers fail to identify CSCs, thereby impeding the isolation of CSCs for subsequent analyses. Here, we utilized serial xenotransplantation of unsorted rhabdomyosarcoma cells in NOD/SCID gamma (NSG) mice as a proof-of-principle platform to investigate the molecular signature of CSCs. Indeed, serial xenotransplantation steadily enriched for rhabdomyosarcoma stem-like cells characterized by enhanced aldehyde dehydrogenase activity and increased colony and sphere formation capacity in vitro. Although the expression of core pluripotency factors (SOX2, OCT4, NANOG) and common CSC markers (CD133, ABCG2, nestin) was maintained over the passages in mice, gene expression profiling revealed gradual changes in several stemness regulators and genes linked with undifferentiated myogenic precursors, e.g., SOX4, PAX3, MIR145, and CDH15. Moreover, we identified the induction of a hybrid epithelial/mesenchymal gene expression signature that was associated with the increase in CSC number. In total, 60 genes related to epithelial or mesenchymal traits were significantly altered upon serial xenotransplantation. In silico survival analysis based on the identified potential stemness-associated genes demonstrated that serial xenotransplantation of unsorted rhabdomyosarcoma cells in NSG mice might be a useful tool for the unbiased enrichment of CSCs and the identification of novel CSC-specific targets. Using this approach, we provide evidence for a recently proposed link between the hybrid epithelial/mesenchymal phenotype and cancer stemness.

Adjuvant trans-arterial chemoembolization after hepatectomy significantly improves the prognosis of low-risk patients with R0-stage hepatocellular carcinoma

Background: Transcatheter arterial chemoembolization (TACE) is one of the local therapies most commonly used to treat intermediate-stage or advanced-stage hepatocellular carcinoma (HCC). However, the clinical benefits of PA-TACE (postoperative adjuvant TACE) for improving prognosis (progress-free survival [PFS] or overall survival [OS]) of low-risk HCC patients with R0-stage HCC after hepatectomy were not very clear.

Methods: From January 2005 to December 2012, 180 patients who underwent hepatectomy for HCC treatment were enrolled in this study, and the follow-up of these patients was ended in December 2017. Among these patients, 102 patients were performed PA-TACE 1 month later after R0 hepatectomy and 78 patients without adjuvant TACE after R0 hepatectomy. Survival analysis was calculated using the Kaplan–Meier statistical method. Differences between survival curves of different groups were tested using the univariate log-rank test. Multivariate Cox model was used to search for independent prognostic factors for progression or death and to acquire the adjusted HR.

Results: PA-TACE significantly improved the survival of HCC patients received surgical resection. The PFS (progress-free survival) of PA-TACE group (median PFS 52.0 months; 95% CI: 14.0–90.0) was significantly longer than the control group (median PFS 11.1 months; 95% CI: [7.9–14.3]; log-rank P<0.001); and the OS (in PA-TACE group (median OS 90.7 months; 95% CI: 84.4–97.0 months) was also much longer than that of control group (median OS 54.4 months; 95% CI: 38.2–70.6 months; log-rank p<0.001). Moreover, the benefits of PA-TACE are greater for low-risk patients than high-risk patients.

Conclusion: In patients with HCC, PA-TACE can significantly prolong progression-free survival and long-term OS. For low-risk patients, the benefits might be greater.