Retinoic acid inhibits basal and interferon-gamma-induced expression of intercellular adhesion molecule 1 in monocytic cells.

Retinoic acid (RA) and 1,25-(OH)2-vitamin D3 (1,25-D3) induced U937 cell maturation into distinct monocytic phenotypes, as demonstrated by up-regulation of CD23 by RA and CD14 by 1,25-D3. Differentiation by RA but not by 1,25-D3 was associated with reduction of basal and complete suppression of interferon-gamma (IFN-gamma)-stimulated intercellular adhesion molecule 1 (ICAM-1) expression. Induction of cyclooxygenase activity by RA and attenuation of basal ICAM-1 expression exhibited similar kinetics. Treatment with indomethacin prevented and prostaglandin E2 (PGE2), dibutyryl-cAMP, or forskolin mimicked reduction of basal ICAM-1 expression by RA, indicating that this effect of RA is mediated by PGE2 synthesis and subsequent cAMP elevation. In contrast, suppression of IFN-gamma-induced ICAM-1 expression by RA was only partly reversible by indomethacin, suggesting that inhibition of IFN-gamma stimulation was not completely due to cyclooxygenase induction. RA did not always counter-act IFN-gamma, as it cooperated with IFN-gamma in down-regulating very late activation antigen 4. Specific polymerase chain reaction and Northern blotting of ICAM-1 mRNA revealed that RA suppressed ICAM-1 induction by IFN-gamma at the transcriptional level. RA also blocked ICAM-1 induction by IFN-gamma in isolated human blood monocytes. In conclusion, inhibition of basal and stimulated ICAM-1 expression in monocytic cells may provide a mechanism for beneficial anti-inflammatory effects of retinoids.