Deletion of β‐adrenergic receptor 1, 2, or both leads to different bone phenotypes and response to mechanical stimulation

As they age, mice deficient for the β2‐adrenergic receptor ( Adrb2 −/− ) maintain greater trabecular bone microarchitecture, as a result of lower bone resorption and increased bone formation. The role of β1‐adrenergic receptor signaling and its interaction with β2‐adrenergic receptor on bone mass regulation, however, remains poorly understood. We first investigated the skeletal response to mechanical stimulation in mice deficient for β1‐adrenergic receptors and/or β2‐adrenergic receptors. Upon axial compression loading of the tibia, bone density, cancellous and cortical microarchitecture, as well as histomorphometric bone forming indices, were increased in both Adrb2 −/− and wild‐type (WT) mice, but not in Adrb1 −/− nor in Adrb1b2 −/− mice. Moreover, in the unstimulated femur and vertebra, bone mass and microarchitecture were increased in Adrb2 −/− mice, whereas in Adrb1 −/− and Adrb1b2 −/− double knockout mice, femur bone mineral density (BMD), cancellous bone volume/total volume (BV/TV), cortical size, and cortical thickness were lower compared to WT. Bone histomorphometry and biochemical markers showed markedly decreased bone formation in Adrb1b2 −/− mice during growth, which paralleled a significant decline in circulating insulin‐like growth factor 1 (IGF‐1) and IGF–binding protein 3 (IGF‐BP3). Finally, administration of the β‐adrenergic agonist isoproterenol increased bone resorption and receptor activator of NF‐κB ligand (RANKL) and decreased bone mass and microarchitecture in WT but not in Adrb1b2 −/− mice. Altogether, these results demonstrate that β1‐ and β2‐adrenergic signaling exert opposite effects on bone, with β1 exerting a predominant anabolic stimulus in response to mechanical stimulation and during growth, whereas β2‐adrenergic receptor signaling mainly regulates bone resorption during aging. © 2012 American Society for Bone and Mineral Research.