Metastasis is the leading cause of death in breast cancer patients due to the lack of effective therapies. Elevated levels of paxillin expression have been observed in various cancer types, with tyrosine phosphorylation shown to play a critical role in driving cancer cell migration. However, the specific impact of the distinct tyrosine phosphorylation events of paxillin in the progression of breast cancer remains to be fully elucidated. Here, we found that paxillin overexpression in breast cancer tissue is associated with a patient's poor prognosis. Paxillin knockdown inhibited the migration and invasion of breast cancer cells. Furthermore, the phosphorylation of paxillin tyrosine residue 31 (Tyr31) was significantly increased upon the TGF-β1-induced migration and invasion of breast cancer cells. Inhibiting Fyn activity or silencing Fyn decreases paxillin Tyr31 phosphorylation. The wild-type and constitutively active Fyn directly phosphorylate paxillin Tyr31 in an in vitro system, indicating that Fyn directly phosphorylates paxillin Tyr31. Additionally, the non-phosphorylatable mutant of paxillin at Tyr31 reduces actin stress fiber formation, migration, and invasion of breast cancer cells. Taken together, our results provide direct evidence that Fyn-mediated paxillin Tyr31 phosphorylation is required for breast cancer migration and invasion, suggesting that targeting paxillin Tyr31 phosphorylation could be a potential therapeutic strategy for mitigating breast cancer metastasis.
