Microenvironment-derived APP promotes osteosarcoma malignant phenotypes and induces macrophage M2 polarization via CD74.

The tumor microenvironment plays a critical role in osteosarcoma (OS) progression. However, the mechanisms underlying intercellular communication remain incompletely understood. This study aimed to investigate the role of the APP-CD74 signaling axis in OS and its impact on tumor progression and immune remodeling. Integrated bioinformatics analyses, including bulk RNA-seq, single-cell RNA sequencing, and CellChat, were performed to identify key signaling pathways and cellular sources. In vitro experiments, including wound healing, Transwell invasion, colony formation, ELISA, and Western blot assays, were conducted to assess functional and mechanistic effects. Conditioned medium models and APP neutralization or CD74 knockdown were used to evaluate pathway dependency. A nude mouse xenograft model and immunohistochemistry were employed for in vivo validation. Bioinformatics analyses identified immune-related hub genes and revealed that APP is primarily derived from macrophages and endothelial cells, while CD74 is highly expressed in OS cells. Functional assays indicated that exogenous and microenvironment-derived APP enhanced OS cell migration, invasion, and clonogenicity in a CD74-dependent manner. Mechanistically, APP was associated with the activation of AKT/ERK/NF-κB signaling pathways via CD74. Furthermore, APP-CD74 signaling enhanced the secretion of immunoregulatory cytokines (IL-6, CCL2, TGF-β), driving macrophage polarization toward an M2-like phenotype. In vivo, APP accelerated tumor growth and increased tumor-associated macrophage infiltration, whereas CD74 knockdown markedly attenuated these effects. Collectively, the macrophage/endothelial-derived APP-CD74 axis promotes OS progression by activating tumor-associated signaling and remodeling the immune microenvironment. The APP-CD74 axis therefore warrants further investigation as a candidate therapeutic target.