Secretion of annexin V from cultured cells requires a signal peptide

Annexin V is an intracellular protein that lacks a hydrophobic signal peptide. However, there are several studies reporting the extracellular presence of annexin V. In this study, we designed transgenes of annexin V with or without an attached secretory signal peptide and investigated the secretion of the transgene products in COS-7 cells. The signal peptide, targeted annexin V to the endoplasmic reticulum (ER), the Golgi and culture media of transfected cells. In contrast, without the signal peptide, annexin V was present only in the cytoplasm and was not detected in the medium. To confirm our results we also evaluated the presence of extracellular annexin V in two cultured cell lines: BeWo, a choriocarcinoma cell model of placental trophoblasts, and human umbilical vein endothelial cells (HUVEC). Our results showed that annexin V was immunolocalized on the surfaces of both cells but could not be detected in the culture medium of either cell type. Our results suggest that the secretion of annexin V required the recombinant addition of a hydrophobic signal peptide and that the limited quantities of endogenous cell surface annexin V on BeWo and HUVEC cells is most likely derived from adjacent damaged cells.