R-spondin 2-LGR4 system regulates growth, migration and invasion, epithelial-mesenchymal transition and stem-like properties of tongue squamous cell carcinoma via Wnt/β-catenin signaling

BACKGROUND: R-spondins (Rspo) and leucine-rich repeat-containing G-protein-coupled receptors (LGR) play important roles in development, stem cells survival, and tumorigenicity by activating Wnt signaling pathway. Whether R-spondins-LGR signaling affects the progression of squamous cell carcinoma (SCC) remain unknown. This study aims to uncover the role of R-spodin2/LGR4 in tongue SCC (TSCC). METHODS: The expression of Rspo2 in TSCC specimens and its correlation with TSCC clinical outcome were evaluated. Levels of Rspo2 or LGR4 were altered by pharmacological and genetic approaches, and the effects on TSCC progression were assessed. FINDINGS: Aberrantly high levels of Rspo2 were detected in TSCC specimens. Its levels were closely related with lymph node metastasis, clinical stage and survival rate in patients with tongue SCC. Exogenous Rspo2 or overexpression of Rspo2 promoted growth, migration and invasion, epithelial-mesenchymal transition (EMT) and stem-like properties in SCC both in vivo and in vitro. Silence of Rspo2 abolished these phenotypes. LGR4 was functionally upregulated by Rspo2 in TSCC. Overexpression of Rspo2 increased, whereas Rspo2 silencing decreased the expression of LGR4, leading to subsequent phosphorylation of LRP6 and nuclear translocation of β-catenin in TSCC cell lines. This nuclear translocation of β-catenin was associated with a significant alteration in TCF-1, a downstream nuclear transcription factor of β-catenin, as well as its target genes: CD44, CyclinD1 and c-Myc. INTERPRETATION: Rspo2-LGR4 system regulates growth, migration and invasion, EMT and stem-like properties of TSCC via Wnt/β-catenin signaling pathway.