GPR176 represses mitophagy to promote the progression of osteosarcoma by facilitating mTORC1 activity via PI3K-AKT pathway.

BACKGROUND: Osteosarcoma (OS) is a common primary malignant tumor of the bone. It is reported that abnormal GPR176 expression contributes to the occurrence and subsequent progression of tumors. However, the role of GPR176 in OS has not been elucidated. Thus, the aim of this study was to evaluate the role of GPR176 in the progression of OS. METHODS: The expression level and prognosis of GPR176 were explored based on Gene Expression Omnibus (GEO), TARGET and Genotype-Tissue Expression (GTEx) databases, as well as the involved pathways. Effects of GPR176 on the proliferation, migration, invasion, and apoptosis of U2OS cells, as well as in a mouse tumor xenograft model. Multiple parameters were employed to explore the role of GPR176 in mitophagy, including mitochondrial membrane potential (MMP), reactive oxygen species (ROS), and mitophagy-related proteins. The protein levels of downstream substrates of mTORC1 were analyzed by Western blot. RESULTS: The expression level of GPR176 was obviously elevated in OS, and increased GPR176 expression associated with poor prognosis in patients with OS. Gene Set Enrichment Analysis (GSEA) showed that GPR176 is mainly involved in the oxidative phosphorylation, retinol metabolism, and ErbB signaling pathways. GPR176 knockdown suppressed the proliferation, migration, and invasion of U2OS cells and enhanced their apoptosis. GPR176 downregulation also induced mitophagy in U2OS cells, as evidenced by an increase in ROS levels; a decrease in MMP, adenosine triphosphate (ATP), and mitochondrial DNA (mtDNA); and concomitant changes in mitophagy-related proteins. GPR176 knockdown suppresses mTORC1 activity in U2OS cells. Moreover, GPR176 knockdown represses the growth of tumor xenografts in vivo while promoting mitophagy. The levels of phosphorylated-mechanistic target of rapamycin complex 1 (p-mTORC1)/mTORC1, p-v-akt murine thymoma viral oncogene homolog 1 (AKT)/AKT, and p-phosphatidylinositol-3 kinase (PI3K)/PI3K were significantly downregulated following GPR176 knockdown. CONCLUSIONS: GPR176 is upregulated in OS and is associated with a poor prognosis. GPR176 suppresses mitophagy to promote OS progression by facilitating mTORC1 activity via the PI3K-AKT pathway.