Exosomal S100A9 Promotes Lung Metastasis of Adenoid Cystic Carcinoma via Activating Cancer-Associated Fibroblasts

Salivary adenoid cystic carcinoma (SACC) exhibits a high incidence of lung metastasis, which primarily contributes to patient mortality. The tumor microenvironment (TME) plays a critical role in facilitating tumor progression, yet its specific contribution to ACC metastasis remains unclear. In this study, we used single-cell transcriptomic analysis of primary ACC tumors and adjacent normal salivary gland to reveal strong intercellular communication between epithelial cells and cancer-associated fibroblasts (CAFs). We demonstrate that S100A9, a calcium-binding protein of the S100 family, is upregulated in a subset of ACC cells and their derived exosomes. Exosomes enriched with S100A9 reprogram normal fibroblasts into activated CAFs with elevated fibroblast activation protein (FAP) and α-smooth muscle actin (α-SMA) expression, enhanced migration, and increased contractility. RNA sequencing of fibroblasts treated with S100A9-enriched exosomes revealed activation of IL-17, TNF, and NF-κB signaling pathways, which are known to drive inflammation, extracellular matrix remodeling, and tumor-stroma interactions. Furthermore, activated CAFs promote epithelial-mesenchymal transition in ACC cells and facilitate lung metastasis through IL-17 signaling. These findings demonstrate tumor-derived exosomal S100A9 as a key mediator of intercellular communication between ACC cells and fibroblasts, identifying S100A9 and S100A9-enriched exosomes as potential therapeutic targets for modulating ACC lung metastasis.