Retinoic Acid Induces Aggregation of the Acute Promyelocytic Leukemia Cell Line NB-4 by Utilization of LFA-1 and ICAM-2

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Abstract
All-trans retinoic acid (tRA) is a potent differentiation agent that is effective therapy for acute promyelocytic leukemia (APL). However, 5% to 25% of patients develop retinoic acid syndrome, a potentially life-threatening complication in which the pathogenesis relates to adhesive alterations of APL cells. Therefore, we investigated the relationship between tRA-induced differentiation and the adhesive properties of APL cells. After confirming differentiation-related morphological changes of NB-4 cells in response to tRA, we showed that homotypic aggregation of NB-4 cells grown in tRA for 72 hours is dose-dependent with a median effective dose of approximately 50 nmol/L. Maximal aggregation occurred at mean and peak therapeutic serum concentrations (100 and 1,000 nmol/L, respectively). Aggregation also increased with the length of tRA exposure over 168 hours. Aggregation was inhibited by neutralizing antibodies against LFA-1 and ICAM-2. Notably, antibodies directed against VLA-4, other β2 integrins (Mac-1 and p150), or other potential LFA-1 counterstructures that were expressed on the cell surface (ICAM-1 and ICAM-3) did not block aggregation. Aggregation occurred with similar kinetics regardless of the presence of phorbol ester or the “activating” monoclonal antibody (MoAb) KIM 185, suggesting that the avidity of LFA-1 is not modulated on NB-4 cells in a manner similar to other leukocytes. Consistent with the prompt clinical effectiveness of methyl prednisolone sodium succinate (MPSS) in retinoic acid syndrome, MPSS rapidly inhibited homotypic aggregation in a dose-dependent manner. Thus, tRA alters the adhesive properties of APL cells by inducing the expression of high-avidity β2 integrins, aggregation is inhibited by LFA-1 and ICAM-2 MoAb, and tRA effects are rapidly reversible by MPSS. Taken together, our findings provide a clinically relevant system for study of LFA-1/ICAM-2 interaction and suggest a mechanism in part for retinoic acid syndrome and the effectiveness of MPSS in ameliorating retinoic acid syndrome.