Annexin A1 (AnxA1) originating from mature neutrophils and their microparticles (MPs) plays an important anti-inflammatory role during the resolution phase of inflammation. However, the role of AnxA1 during the process of granulocytic differentiation is still unknown. All-trans retinoic acid (ATRA) can induce acute promyelocytic leukemic (APL) cells to differentiate along the granulocytic lineage and has been used successfully in treating APL patients. In this study, we investigated whether or not AnxA1 contributed to the anti-inflammatory properties of ATRA-treated APL (NB4; ATRA-NB) cells using the transmigratory and adhesive assays. We found that ATRA was able to enhance the surface expression of AnxA1 and its receptor (FPR2/ALX) and the release of AnxA1-containing MPs from ATRA-NB4 cells, while the expression of annexin V was not elevated on the latter cells. Further studies demonstrated that exogenous AnxA1 could inhibit ATRA-NB4 cells in their transmigratory activity and adhesion to endothelial cells. In addition, the transmigratory activity of ATRA-NB4 cells can be significantly enhanced by pretreatment with a FPR2/ALX neutralizing antibody, suggesting that endogenous AnxA1 may contribute to the anti-migratory effects. Finally, ATRA-NB4-derived MPs could also inhibit recipient cells in their transmigratory and adhesive activities and these anti-inflammatory effects could be inhibited by pretreatment of MPs with a specific anti-AnxA1 antibody. Flowcytometry studies further demonstrated that FITC-labeled AnxA1 could be transported from MPs to the membrane of recipient ATRA-NB4 cells. We conclude that biologically active AnxA1 may play a role in the anti-inflammatory properties of ATRA-treated APL cells during the process of granulocytic differentiation.
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