Annexin A6 Modulates the Secretion of Pro-Inflammatory Cytokines and Exosomes via Interaction with SNAP23 in Triple-Negative Breast Cancer Cells.

Annexin A6 (AnxA6) is a predominantly intracellular calcium-dependent membrane-binding multifunctional protein that is also detected extracellularly and in small extracellular vesicles (exosomes). We previously demonstrated that lapatinib resistance in triple-negative breast cancer (TNBC) cells is associated with AnxA6 upregulation and accumulation of cholesterol in late endosomes. Here, we investigated the fate of AnxA6 and cholesterol in lapatinib-resistant (LAP-R) cells and whether extracellular AnxA6 influences TNBC cell survival. We demonstrate that reduced expression of AnxA6 in LAP-R cells decreased the secretion of MCP-1/CCL2, CCL8/IL-8, DKK1, TSP-1, and OPN by antibody arrays. The secretion of exosomes was also markedly reduced in AnxA6-depleted LAP-R cells, while AnxA6 upregulation stimulated the release of MCP-1 and exosomes. Compared to the respective controls, exosome-associated AnxA6, Rab7, and cholesterol levels were increased in exosomes isolated from AnxA6-expressing LAP-R cells. Mechanistically, we demonstrated by co-immunoprecipitation, GST pulldown, and proximity ligation assays that AnxA6 interacts with SNAP23, a component of the membrane fusion machinery. Finally, blocking extracellular AnxA6 with neutralizing antibodies reduced the viability of AnxA6-low TNBC cells but had little effect on AnxA6-high cells. These findings suggest that extracellular AnxA6 is critical for the survival of highly proliferative AnxA6-low basal-like breast cancer cells and that AnxA6 influences TNBC progression by facilitating the secretion of pro-inflammatory cytokines and cholesterol-enriched exosomes.