All-trans-retinoic acid (ATRA) induces bone resorption, but the molecular mechanisms are unknown. We have studied the effect of ATRA on osteoprotegerin (OPG) and receptor activator of NF-kappaB ligand (RANKL) expression in human MG-63 osteosarcoma cells and primary osteoblast-like cultures. ATRA dose-dependently down-regulated protein levels of OPG in MG-63 cells, with a maximum (-56%) observed at a dose of 10(-6) M. This effect was confirmed with quantitative real-time PCR, where OPG mRNA was decreased after 4 h (-68%) in primary cultures and after 8 h (-87%) in MG-63 cells. The reduction in OPG expression was inhibited by a retinoic acid receptor (RAR)-antagonist and was mimicked by a RARbeta, gamma-agonist, indicating that the ATRA effect is mediated by these receptors. In primary cultures we found a threefold induction of RANKL mRNA expression. Thus, the RANKL/OPG ratio was markedly increased, suggesting a potential mechanism of ATRA-induced bone resorption. (C) 2004 Elsevier Inc. All rights reserved.