Selective β2-adrenoreceptor signaling regulates osteoclastogenesis via modulating RANKL production and neuropeptides expression in osteocytic MLO-Y4 cells.

The β2-adrenergic receptor (β2-AR) signaling on bone cells is the major contributor in the effect of the sympathetic nervous system on bone turnover. However, it remains unclear whether receptor activator of nuclear factor κ-Β ligand (RANKL) modulation and neuropeptides expression in osteocytes are responsible for the mechanism. This study used β2-AR stimulation to investigate cell cycle and proliferation, the gene and protein expression of RANKL, and osteoprotegerin (OPG), as well as neuropeptides regulation in osteocytic MLO-Y4 cells. Clenbuterol (CLE; a β2-AR agonist) slightly promoted the growth of MLO-Y4 cells in a concentration-dependent effect but had no effect on the proliferation index. And the concentration of 10-8  M showed a significant increase in the S-phase fraction on day 3 in comparison with the control. Additionally, CLE-promoted osteoclast formation and bone resorption in osteocytic MLO-Y4 cell-RAW264.7 cell cocultures. RANKL expression level and the ratio of RANKL to OPG in MLO-Y4 cells were enhanced in CLE treatment but were rescued by blocking β2-AR signaling. However, neuropeptide Y and α-calcitonin gene-related peptide, two neurogenic markers, were inhibited in CLE treatment of MLO-Y4 cells, which was reversed by a β2-AR blocker. The results indicate that osteocytic β2-AR plays an important role in the regulation of RANKL/OPG and neuropeptides expression, and β2-AR signaling in osteocytes can be used as a new valuable target for osteoclast-related pathologic disease.