Hypoxia regulates ANXA1 expression to support prostate cancer cell invasion and aggressiveness

Annexin A1 (ANXA1) is a Ca²⁺-binding protein overexpressed in the invasive stages of prostate cancer (PCa) development; however, its role in this tumor metastatization is largely unknown. Moreover, hypoxic conditions in solid tumors have been related to poor prognosis in PCa patients. We have previously demonstrated that ANXA1 is implicated in the acquisition of chemo-resistant features in DU145 PCa cells conferring them a mesenchymal/metastatic phenotype. In this study, we have investigated the mechanisms by which ANXA1 regulates metastatic behavior in LNCaP, DU145 and PC3 cells exposed to hypoxia. ANXA1 was differentially expressed by PCa cell lines in normoxia whereas hypoxic stimuli resulted in a significant increase of protein expression. Additionally, in low oxygen conditions ANXA1 was extensively secreted out-side the cells where its binding to formyl peptide receptors (FPRs) induced cell invasion. Loss and gain of function experiments performed by using the RNA interfering siANXA1 and an ANXA1 over-expressing plasmid (MF-ANXA1), also confirmed the leading role of the protein in modulating LNCaP, DU145 and PC3 cell invasiveness. Finally, ANXA1 played a crucial role in the regulation of cytoskeletal dynamics underlying metastatization process, such as the loss of adhesion molecules and the occurrence of the epithelial to mesenchymal transition (EMT). ANXA1 expression increased inversely to epithelial markers such as E-cadherin and cytokeratins 8 and 18 (CKs) and proportionally to mesenchymal ones such as vimentin, ezrin and moesin. Our results indicated that ANXA1 may be a key mediator of hypoxia-related metastasis-associated processes in PCa.