Annexin 1 Binds to U937 Monocytic Cells and Inhibits Their Adhesion to Microvascular Endothelium: Involvement of the α4β1 Integrin

Annexin 1 (ANX1), a calcium-binding protein, participates in the regulation of early inflammatory responses. Whereas some of its effects depend on intracellular interactions, a growing number of observations indicate that ANX1 may also act via autocrine/paracrine functions following externalization to the outer side of the plasma membrane. We studied the effects of ANX1 on leukocyte adhesion to endothelial cells using as a model system the monocytic cell line U937 and human bone marrow microvascular endothelial cells. Exogenous rANX1, as well as endogenous ANX1 externalized by U937 differentiated in vitro, inhibited monocyte firm adhesion to vascular endothelium. Both binding of ANX1 to U937 cells and ANX1-mediated inhibition of cell adhesion involved the short N-terminal domain of the ANX1 molecule. Under experimental conditions in which ANX1 inhibited U937 adhesion to human bone marrow microvascular endothelial cells, this protein specifically colocalized with the alpha 4 integrin, and a direct interaction between ANX1 and the alpha 4 integrin could be documented by immunoprecipitation experiments. Moreover, ANX1 competed with the endothelial integrin counterreceptor, VCAM-1, for binding to alpha 4 integrin. These results indicate that ANX1 plays an important physiological role in modulating monocyte firm adhesion to the endothelium.