Tumor‐Associated Neutrophils Regulate Breast Cancer Progression Through the <scp>AQP9</scp>/<scp>STAT3</scp> Signaling Pathway

Tumor-associated neutrophils (TANs) contribute to breast cancer (BC) progression, and aquaporin 9 (AQP9) plays a critical role in tumor development. However, the interactions between TANs and AQP9 in BC are poorly understood. Bioinformatics analyses and clinical samples revealed a positive correlation between neutrophil infiltration and AQP9 expression in BC. Treating BC cells with TAN-conditioned media significantly elevated AQP9 expression compared with neutrophil-conditioned and control media treatments. Immunohistochemical analysis revealed higher AQP9 protein expression in BC tissues than in adjacent normal tissues, and AQP9 expression was negatively correlated with recurrence-free survival and overall survival in patients with BC. Functional studies demonstrated that AQP9 promoted BC cell proliferation but did not affect migration or invasion. AQP9 knockdown markedly inhibited the ability of TANs to enhance BC cell proliferation, migration, and invasion. Intravenous and intratumoral injection of TANs in mice increased tumor growth rate, weight, and volume compared with controls; moreover, histological examination revealed lung metastasis in two mice and bone involvement in one mouse out of six in the TAN treatment group. AQP9 knockdown significantly reduced the tumor growth rate. In BC cells, TAN treatment elevated STAT3 phosphorylation, and this effect was amplified by AQP9 overexpression. In conclusion, TANs promote BC progression by enhancing STAT3 phosphorylation through AQP9 upregulation. AQP9 is crucial for TAN-mediated BC progression and is a potential target for immunotherapy in patients with BC.