Approaches to Targeting Cancer Stem Cells in Solid Tumors

The Role of GREMLIN1, a Bone Morphogenetic Protein Antagonist, in Cancer Stem Cell Regulation

Cancer remains a leading cause of death globally, characterized by uncontrolled cell proliferation, tumor growth and metastasis. Bone morphogenetic proteins (BMPs) and their growth differentiation factor (GDF) relatives are crucial regulators of developmental processes such as limb, kidney and lung formation, cell fate determination, cell proliferation, and apoptosis. Cancer stem cells (CSCs) are a subpopulation of self-renewing cells within tumors that possess stemness properties and a tumor cell-forming capability. The presence of CSCs in a tumor is linked to growth, metastasis, treatment resistance and cancer recurrence. The tumor microenvironment in which CSCs exist also plays a critical role in the onset, progression and treatment resistance in many cancers. Growth factors such as BMPs and GDFs counterbalance transforming growth factor-beta (TGF-β) in the maintenance of CSC pluripotency and cancer cell differentiation. BMP signaling typically functions in a tumor suppressor role in various cancers by inducing CSC differentiation and suppressing stemness characteristics. This differentiation process is vital, as it curtails the self-renewal capacity that characterizes CSCs, thereby limiting their ability to sustain tumor growth. The interplay between BMPs and their secreted antagonists, such as GREM1, Noggin and Chordin, adds another layer of complexity to CSC regulation. Human cancers such as gastric, colorectal, glioblastoma, and breast cancer are characterized by GREMLIN1 (GREM1) overexpression, leading to inhibition of BMP signaling, facilitating the maintenance of pluripotency in CSCs, thus promoting tumorigenesis. GREM1 overexpression may also contribute to CSC immune evasion, further exacerbating patient prognoses. In addition to BMP inhibition, GREM1 has been implicated as a target of fibroblast growth factor (FGF) → Sonic hedgehog (Shh) signaling, as well as the Wnt/Frizzled pathway, both of which may contribute to the maintenance of CSC stemness. The complex role of BMPs and their antagonists in regulating CSC behavior underscores the importance of a balanced BMP signaling pathway. This article will summarize current knowledge of BMP and GREM1 regulation of CSC function, as well as conflicting data on the exact role of GREM1 in modulating CSC biology, tumor formation and cancer. Targeting this pathway by inhibiting GREM1 using neutralizing antibodies or small molecules may hold early-stage promise for novel therapeutic strategies aimed at reducing CSC burden in cancers and improving patient outcomes.

RANKL regulates differentially breast cancer stem cell properties through its RANK and LGR4 receptors

BACKGROUND

Breast cancer stem cells (BCSC) are a subpopulation responsible for cancer resistance and relapse. The receptor activator of nuclear factor kappa-Β ligand (RANKL) is a cytokine capable of activating RANK and LGR4 receptors. RANKL/RANK signaling maintains the self-renewal of BCSCs, however, the effect of RANKL via LGR4 remains unclear. Evidence from osteoclasts suggests that RANKL/LGR4 axis disrupts RANK signaling, leading to opposing cellular responses. Anti-RANKL inhibitors are potential agents for eradicating CSCs, but their effect on RANKL/LGR4 signal has not been demonstrated.

OBJECTIVE

This project aimed to elucidate the role of RANKL in regulating stemness depending on the expression of its receptors.

METHODS

We use in vitro and in vivo approaches to evaluate the effects of RANKL inhibition in stemness in low or high-LGR4 expressing cells. Furthermore, we analyze the effects of RANKL stimulation on the stemness of LGR4 or RANK overexpressing cells. Additionally, we evaluated the impact of RANKL/LGR4 signaling in the activity of Wnt/β-catenin and NF-κB signaling pathways.

RESULTS

Our findings indicated that elevated RANKL expression is related to a favorable prognosis in patients with high LGR4 levels. Furthermore, RANKL inhibition decreased BCSC properties in LGR4-low cell lines, while it promoted migration in LGR4-high cells. Additionally, the RANKL/RANK axis activated NF-κB signaling and enhanced BCSCs in RANK-overexpressing cells. In contrast, in LGR4-overexpressing cells, RANKL failed to activate NF-κB but instead inhibited the Wnt/β-catenin pathway, leading to a reduction in BCSCs.

CONCLUSION

Our findings suggest that RANKL exerts different responses according to the expression of its receptors.

Cancer-associated Fibroblasts Communicate with Breast Tumor Cells Through Extracellular Vesicles in Tumor Development

Breast cancer is the leading cause of cancer death among women worldwide. In solid tumors, the microenvironment plays a critical role in tumor development, and it has been described a communication between the different cell types that conform the stroma, including fibroblasts, pericytes, adipocytes, immune cells and cancer-associated fibroblasts. Intercellular communication is bidirectional, complex, multifactorial and is mediated by the secretion of molecules and extracellular vesicles. The extracellular vesicles are vesicles limited by two membranes that are secreted by normal and cancer cells into the extracellular space. Extracellular vesicle cargo is complex and includes proteins, miRNAs, DNA and lipids, and their composition is specific to their parent cells. Extracellular vesicles are taken up for neighboring or distant cells. Particularly, extracellular vesicles from breast cancer cells are taken up for fibroblasts and it induces the activation of fibroblasts into cancer-associated fibroblasts. Interestingly, cancer associated fibroblasts release extracellular vesicles that are taken up for breast cancer cells and promote migration, invasion, proliferation, epithelial-mesenchymal transition, changes in metabolism, chemoresistance, evasion of immune system and remodeling of extracellular matrix. In addition, the enrichment of specific cargos in extracellular vesicles of breast cancer patients has been suggested to be used as biomarkers of the disease. Here we review the current literature about the intercommunication between tumor cells and cancer associated fibroblasts through extracellular vesicles in breast cancer.

Combining radiation to EGFR and Bcl-2 blockade: a new approach to target cancer stem cells in head and neck squamous cell carcinoma

PURPOSE

The clinical outcome of head and neck squamous cell carcinoma (HNSCC) remains poor, partly due to the presence of resistant cancer stem cells (CSCs) which are responsible of recurrences. CSCs have low EGFR expression and, conversely, overexpress the anti-apoptotic Bcl-2 protein, which is involved in resistance to apoptosis and the invasion/migration capacities of tumour cells.

METHODS

The combination therapy of ABT-199, a Bcl-2 inhibitor, cetuximab an EGFR inhibitor, and radiation using an HNSCC model (SQ20B cell line) and its corresponding CSC subpopulation were evaluated in vitro (2D/3D cell proliferation; invasion/migration and apoptosis using videomicroscopy) and in vivo.

RESULTS

Cetuximab strongly inhibited 2D and 3D cell proliferation, as well as invasion/migration, only in non-CSC-SQ20B cells, whereas ABT-199 selectively inhibited these mechanisms in SQ20B/CSCs. The combination of irradiation + cetuximab + ABT-199 increased the inhibition of the 2D and 3D cell proliferation, invasion/migration, and resistance to apoptosis in both cell sub-populations. In addition, in a nude mouse model with heterotopic tumour xenograft, a treatment combining cetuximab + ABT-199 with fractional irradiation strongly delayed the tumour growth and increased in vivo lifespan without side effects.

CONCLUSION

Based on the present results, this triple combination therapy may represent a new opportunity for testing in clinical trials, particularly in locally advanced HNSCC.

Role of cancer stem cells in the development of giant cell tumor of bone

The primary bone tumor is usually observed in adolescence age group which has been shown to be part of nearly 20% of the sarcomas known today. Giant cell tumor of bone (GCTB) can be benign as well as malignant tumor which exhibits localized dynamism and is usually associated with the end point of a long bone. Giant cell tumor (GCT) involves mononuclear stromal cells which proliferate at a high rate, multinucleated giant cells and stromal cells are equally present in this type of tumor. Cancer stem cells (CSCs) have been confirmed to play a potential role in the development of GCT. Cancer stem cell-based microRNAs have been shown to contribute to a greater extent in giant cell tumor of bone. CSCs and microRNAs present in the tumors specifically are a great concern today which need in-depth knowledge as well as advanced techniques to treat the bone cancer effectively. In this review, we attempted to summarize the role played by cancer stem cells involving certain important molecules/factors such as; Mesenchymal Stem Cells (MSCs), miRNAs and signaling mechanism such as; mTOR/PI3K-AKT, towards the formation of giant cell tumor of bone, in order to get an insight regarding various effective strategies and research advancements to obtain adequate knowledge related to CSCs which may help to focus on highly effective treatment procedures for bone tumors.