SNX9 Orchestrates Lung Metastasis via EGFR-ERK Signaling and Actin Cytoskeleton Remodeling in Breast Cancer

Objectives: Sorting nexin 9 (SNX9) participates in endocytic trafficking and has been connected to several malignancies, but its involvement in breast cancer (BC) remains incompletely resolved. This work was designed to examine whether SNX9 supports BC progression and investigate signaling and cytoskeletal processes associated with its activity. Methods: The clinical relevance of SNX9 was assessed using bioinformatics analysis of publicly available cancer databases. Lentiviral vectors were used to establish BC cell models with stable SNX9 overexpression or knockdown. Both cellular (proliferation and motility) and murine (tumor growth and metastatic colonization) experiments were implemented to functionally characterize the SNX9-mediated phenotypes. The underlying mechanisms were investigated via western blotting, immunofluorescence, co-immunoprecipitation, and pathway-focused analyses. Results: Across the analyzed datasets, greater SNX9 abundance was linked to worse overall survival outcomes in BC patients. Functionally, SNX9 upregulation conferred increased proliferative, migratory, and invasive capacities in vitro and contributed to both primary tumor enlargement and distant metastatic spread in vivo, whereas SNX9 depletion produced the reciprocal phenotypes. SNX9 silencing also increased the G2/M cell fraction and disrupted actin cytoskeletal organization mechanistically linked to reduced Ras-related C3 botulinum toxin substrate 1 (Rac1)/cell division cycle 42 homolog (Cdc42) activation and the subsequent impairment of lamellipodial and filopodial protrusion. Additionally, co-immunoprecipitation substantiated the physical coupling between SNX9 and the scaffold protein tyrosine kinase substrate with five SH3 domains (TKS5), which correlated with the invasive behavior of BC cells. Conclusion: Collectively, our findings establish SNX9 as a critical oncoprotein that drives BC progression by coordinating proliferative epidermal growth factor receptor (EGFR)/extracellular signal-regulated kinase 1/2 (ERK1/2) signaling and cytoskeletal dynamics through interactions with TKS5 and Rac1/Cdc42. Clinically, SNX9 qualifies as a promising prognostic classifier and a rational target for therapeutic intervention.