Caffeine displays dual effect on RANKL-induced osteoclast differentiation and bone resorption activity via adenosine...

The effects of caffeine on bone metabolism and orthodontic tooth movement are largely inconsistence. This study investigated the dual effect of caffeine on receptor activator of nuclear factor κΒ ligand (RANKL)-induced osteoclastogenesis in RAW 264.7 cells and the role of adenosine receptors in these effects.

RAW 264.7 cells were treated with RANKL with or without 1-300 μM of caffeine. Cell viability and the numbers of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells were evaluated. The effect of caffeine on osteoclast activity was validated by bone resorption assays to measure the release of a fluorescent probe, resorption pit areas, and visualization by scanning electron microscope (SEM). The role of adenosine receptor (AR) subtypes in caffeine-, inducing or -inhibiting osteoclast differentiation were analyzed using AR antagonists.

Caffeine at concentrations 1-100 μM did not affect cell viability. The number of TRAP positive cells, which represent osteoclast differentiation, increased with RANKL treatment and further increased with low concentrations of caffeine (1-10 µM). However, higher concentrations (30-300 µM) inhibited RANKL-induced TRAP formation. Similarly, the release of fluorescent probes and the formation of resorption pit areas, which represented osteoclast activity, increased at low concentrations and were reduced at high concentrations of caffeine. SEM micrographs demonstrated a pattern similar to the pit assay. In addition, the A_2AR antagonist tended to increase osteoclast differentiation, while the A₁R antagonist inhibited the caffeine effect.

Caffeine has a dual effect depending on the concentration. At low concentrations, caffeine enhanced osteoclast differentiation and activity, while higher concentrations diminished osteoclastogenesis. The A₁R receptor may be responsible for caffeine-induced osteoclast differentiation.