Background: The high rate of the recurrence and metastasis of osteosarcoma (OS) is the major cause of its poor prognosis. There is a strong correlation between tumor-associated neutrophils (TANs) and tumor progression, progression, and metastasis. This study aimed to identify potential markers that could predict OS metastasis based on analysis of TANs in the tissues of OS patients.

Methods: A single-cell sequencing dataset (GSE152048), containing seven primary OS lesions, two recurrent OS lesions, and two lung metastatic OS lesions was used for TANs subset identification using the R software (version 4.1.0, R Project for Statistical Computing, Vienna, Austria; https://www.r-project.org/). Immune cell infiltration and immune score were analyzed using CIBERSORT algorithm and ESTIMATE database, respectively. The differentially expressed genes (DEGs) of TANs were used for weighted gene co-expression network analysis (WGCNA) to screen key genes associated with OS metastasis. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) functional enrichment analysis were used to analyze the functions and signaling pathways involved in key genes. The mRNA levels of protein phosphatase 2 regulatory subunit B'gamma (PPP2R5C) were validated in human osteosarcoma cell lines U2-OS and MG63, human normal cervical endometrial cell line HUCEC, and human foreskin fibroblast (HFF-1) cell line by real-time qPCR (RT-qPCR). PPP2R5C-siRNA991 was transfected into U2-OS and MG63 for 48 h, then the expression levels of PPP2R5C, AKT serine/threonine kinase (AKT), and phospho-AKT (p-AKT) were determined by RT-qPCR and Western blotting. Cell proliferation, migration, and apoptosis were measured by cell counting kit-8 (CCK-8), Transwell, and flow cytometry, respectively.

Results: We identified TANs subsets in primary, metastatic, and recurrent OS. Immune infiltration analysis showed that TANs were expressed in OS. Compared with non-metastatic OS, metastatic OS had lower stromal score, immune score, ESTIMATE score, and higher tumor purity. WGCNA classified DEGs into five clusters, according to their function and identified PPP2R5C, protein phosphatase 2 regulatory subunit B'epsilon (PPP2R5E), tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein gamma (YWHAG) and CREB binding protein (CREBBP), as potential markers that may affect TANs-induced OS metastasis via hypoxia inducible factor 1 (HIF-1), phosphatidylinositol 3-kinases (PI3K)-AKT and Janus kinase (JAK)-signal transducers and activators of transcription (STAT) signaling pathways. In vitro experiments demonstrated that the mRNA and protein expressions of PPP2R5C, PPP2R5E, YWHAG, and CREBBP were highly expressed in U2-OS and MG63 cells (p < 0.01). Furthermore, PPP2R5C reduced proliferation and migration (p < 0.01) and increased apoptosis and p-AKT protein levels in U2-OS and MG6 cells (p < 0.01).

Conclusions: PPP2R5C affects OS metastasis via PI3K/AKT pathway, which may be a potential marker for OS metastasis and recurrence.