Background: Apolipoprotein O (APOO) influences the tumor microenvironment and cancer progression, but its role in macrophage regulation in breast cancer (BC) remains unclear. This study aimed to investigate APOO's effect on macrophage polarization and its molecular mechanisms in BC.

Methods: Gene expression data obtained from single-cell RNA sequencing (RNA-seq) and The Cancer Genome Atlas Breast Invasive Carcinoma (TCGA-BRCA) were analyzed to identify key BC-associated genes. Clinical relevance was validated with TCGA patient data. APOO was overexpressed in tumor cells and co-cultured with macrophages. M2 polarization was assessed by cluster of differentiation 206 (CD206) flow cytometry and quantitative real-time PCR (qRT-PCR), and the role of aarF domain containing kinase 2 (ADCK2) was evaluated by co-culture with ADCK2 knockdown macrophages.

Results: Our analysis identified anti-müllerian hormone (AMH), APOO, neurexophilin 4 (NXPH4), and vascular endothelial growth factor (VEGF) as key differentially expressed genes in BC. Among these, APOO was notably upregulated in tumor tissues as well as correlated significantly with poorer clinical outcomes (p < 0.05). Immune infiltration analysis indicated that higher APOO expression was linked to enhanced macrophage infiltration, with a notable increase in M2-like macrophages. Functional assays demonstrated that APOO promotes M2 macrophage polarization via the ADCK2 signaling pathway (p < 0.05).

Conclusions: These findings suggest that APOO may serve as both a potential biomarker and a therapeutic target in BC treatment strategies.