New Insights about the Wnt/β-Catenin Signaling Pathway in Primary Bone Tumors and Their Microenvironment: A Promising Target to Develop Therapeutic Strategies?

Abstract

Osteosarcoma and Ewing sarcoma are the most common malignant primary bone tumors mainly occurring in children, adolescents and young adults. Current standard therapy includes multidrug chemotherapy and/or radiation specifically for Ewing sarcoma, associated with tumor resection. However, patient survival has not evolved for the past decade and remains closely related to the response of tumor cells to chemotherapy, reaching around 75% at 5 years for patients with localized forms of osteosarcoma or Ewing sarcoma but less than 30% in metastatic diseases and patients resistant to initial chemotherapy. Despite Ewing sarcoma being characterized by specific EWSR1-ETS gene fusions resulting in oncogenic transcription factors, currently, no targeted therapy could be implemented. It seems even more difficult to develop a targeted therapeutic strategy in osteosarcoma which is characterized by high complexity and heterogeneity in genomic alterations. Nevertheless, the common point between these different bone tumors is their ability to deregulate bone homeostasis and remodeling and divert them to their benefit. Therefore, targeting different actors of the bone tumor microenvironment has been hypothesized to develop new therapeutic strategies. In this context, it is well known that the Wnt/β-catenin signaling pathway plays a key role in cancer development, including osteosarcoma and Ewing sarcoma as well as in bone remodeling. Moreover, recent studies highlight the implication of the Wnt/β-catenin pathway in angiogenesis and immuno-surveillance, two key mechanisms involved in metastatic dissemination. This review focuses on the role played by this signaling pathway in the development of primary bone tumors and the modulation of their specific microenvironment.

Keywords: Wnt/β-catenin; bone sarcoma; bone tumor microenvironment.

Figure 1

The canonical Wnt/β-catenin signaling pathway. Left panel: in the absence of Wnt ligand, β-catenin is sequestered by a protein complex composed of dishevelled (Dvl), adenomatous polyposis coli (APC), Axin1/2, Wilms tumor gene on X chromosome protein (WTX) and two kinases responsible for the phosphorylation of β-catenin, CK1α (casein kinase 1 alpha) and GSK3β (glycogen synthase kinase 3 beta). Then, YAP/TAZ (yes-associated protein/transcriptional co-Activator with a PDZ-binding domain) proteins recruit β-TrCP (beta-transducin-repeat-containing protein), a ubiquitin ligase responsible for the ubiquitination of β-catenin and its degradation by the proteasome pathway. In the nucleus, the transcriptional proteins of the TCF/LEF family (T-cell factor/lymphoid enhancer-binding factor) interact with the transcriptional repressors groucho/TLE (transducin-like cnhancer of split), recruiting histone deacetylases (HDACs) responsible for repressing transcription. Right panel: Binding of the Wnt ligands to the frizzled (Fzd) receptor and low-density-lipoprotein-related protein 5/6 (LRP5/6) co-receptor complex induces the recruitment of the scaffold protein Dvl to Fzd and leads to LRP5/6 phosphorylation (P) by CK1α and GSK3β kinases. The β-catenin destruction complex is then trapped to the membrane through Axin/Fzd interaction, leading to its inactivation. In parallel, Axin proteins are degraded following poly-ADP-ribosylation by tankyrases (TNKS). Newly synthesized β-catenin accumulates in the cytoplasm and translocates into the nucleus where it interacts with the transcription factors of the TCF/LEF family and with histones modifying co-activators p300 or CREB binding protein (CBP), B cell CLL/lymphoma 9 (BCL-9), brahma-related gene 1 (BRG1), and pygopus. These transcription complexes activate the transcription of target genes such as cMYC, AXIN2, BIRC5 or CCND1.

Figure 2

The crucial role of Wnt/β-catenin signaling pathway in multiple steps of bone sarcoma progression and metastatic dissemination. (a) The canonical Wnt/β-catenin signaling pathway is able to enhance bone sarcoma cells proliferation, to induce an epithelial-mesenchymal transition (EMT)-like through secretion of fibulin-3 and to promote the acquisition of stem cells properties of bone sarcoma cancer stem cells (CSCs). (b) The Wnt/β-catenin pathway participates to the hijacking of the bone microenvironment by the bone sarcoma cells, leading to the establishment of a vicious cycle between bone remodeling and tumor cells proliferation associated with the release of pro-tumoral factors including Wnt ligands from the bone matrix. (c) The Wnt/β-catenin pathway promotes the modulation of the extracellular matrix (ECM), increasing secretion of extracellular matrix components such as tenascin C, fibronectin 1 or collagens and stimulates the ECM degradation by upregulation of proteolytic enzymes such as MMPs. (d) The Wnt/β-catenin pathway induces an over-expression of vascular endothelial growth factor (VEGF), the most important pro-angiogenic factor and is also able to modulate endothelial cell migration, leading to an increase in tumor-associated angiogenesis. (e) The Wnt/β-catenin signaling pathway participates to the establishment of an immune tolerance in the TME, enhancing pro-tumoral M2 macrophages polarization and inhibiting cytotoxic T cell infiltration and functions and inducing resistance to anti-PD1 or anti-PDL-1 therapy. The activation of the Wnt/β-catenin signaling pathway in dendritic cells leads to up-regulation of interleukin-10 (IL-10) and indoleamine 2,3-dioxygenase 1 (IDO) secretion leading to an inhibition of tumor-infiltrating lymphocytes (TILs) cytotoxic properties. By targeting both osteosarcoma and Ewing sarcoma cells and the bone TME, the Wnt/β-catenin signaling pathway participates to the disease progression and the establishment of lung metastases.