Retinoids Stimulate Periosteal Bone Resorption by Enhancing the Protein RANKL, a Response Inhibited by Monomeric Glucocorticoid Receptor

Increased vitamin A (retinol) intake has been suggested to increase bone fragility. In the present study, we investigated effects of retinoids on bone resorption in cultured neonatal mouse calvarial bones and their interaction with glucocorticoids (GC). All-trans-retinoic acid (ATRA), retinol, retinalaldehyde, and 9-cis-retinoic acid stimulated release of Ca-45 from calvarial bones. The resorptive effect of ATRA was characterized by mRNA expression of genes associated with osteoclast differentiation, enhanced osteoclast number, and bone matrix degradation. In addition, the RANKL/OPG ratio was increased by ATRA, release of Ca-45 stimulated by ATRA was blocked by exogenous OPG, and mRNA expression of genes associated with bone formation was decreased by ATRA. All retinoid acid receptors (RAR alpha/beta/gamma) were expressed in calvarial bones. Agonists with affinity to all receptor subtypes or specifically to RAR alpha enhanced the release of Ca-45 and mRNA expression of Rankl, whereas agonists with affinity to RAR beta/gamma or RAR gamma had no effects. Stimulation of Rankl mRNA by ATRA was competitively inhibited by the RAR alpha antagonist GR110. Exposure of calvarial bones to GC inhibited the stimulatory effects of ATRA on 45Ca release and Rankl mRNA and protein expression. This inhibitory effect was reversed by the glucocorticoid receptor (GR) antagonist RU 486. Increased Rankl mRNA stimulated by ATRA was also blocked by GC in calvarial bones from mice with a GR mutation that blocks dimerization (GR(dim) mice). The data suggest that ATRA enhances periosteal bone resorption by increasing the RANKL/OPG ratio via RAR alpha receptors, a response that can be inhibited by monomeric GR.