ICAM-1 upregulation is not required for retinoic acid-induced human eosinophil survival

Active metabolites of vitamin A, retinoic acids (RAs), are known to play critical roles in mucosal immune responses and dramatically inhibit human eosinophil apoptosis, but the detailed mechanisms have not been elucidated. We previously screened for ICAM-1 (CD54) upregulation in RA-stimulated human eosinophils by gene microarray analysis. As ICAM-1 induction and activation were observed to have a role in maintenance of eosinophil survival, we tested the hypothesis that RAs prolong eosinophil survival through ICAM-1 outside-in signaling. Blood-derived isolated eosinophils cultured with 9-cis RA and all-trans RA showed significant upregulation of ICAM-1 mRNA and cell surface expression. TTNPB, a retinoic acid receptor agonist, also induced ICAM-1 expression, while HX630, a retinoid X receptor agonist, did not. Furthermore, an RAR antagonist, HX531, completely inhibited the effect of RAs. Upregulated ICAM-1 was associated with altered kinetics of Akt, ERK, and p38 MAP kinase phosphorylation through ICAM-1 cross-linking, but an ICAM-1-blocking antibody did not affect RA-mediated cell survival. These findings indicate that RAs induce functional ICAM-1 expression through RARs, but the induced ICAM-1 does not contribute to prolongation of eosinophil survival.