β2-Adrenergic receptor regulates osteoblast differentiation and migration and C-terminal β-catenin phosphorylation.

The sympathetic nervous system modulates bone mass in part by inhibiting bone formation through the osteoblastic β2-adrenergic receptor (β2AR), but understanding of the bone anabolic processes hindered by β2AR signaling is incomplete. Canonical Wnt signaling is anabolic in bone, promoting osteoblastic bone formation when the multifunctional β-catenin protein is unphosphorylated at N-terminal residues. In the present study, osteoblastic consequences of Wnt-independent β-catenin C-terminal phosphorylation downstream of β2AR activation were investigated. Treatment with the β-adrenergic agonist isoproterenol (ISO) decreased mineralized nodule formation in differentiating cultures of MC3T3 pre-osteoblasts and BMSCs from WT mice, but not BMSCs from β2AR deficient mice. After treatment with ISO or the β2AR agonist salbutamol there was increased phosphorylation of β-catenin at serine residues 552 and 675, whereas phosphorylation at canonical Wnt-dependent N-terminal sites was unaffected. These C-terminal serine phosphorylation events were inhibited by pre-treatment with β-adrenergic antagonist propranolol or PKA inhibitor H89, indicating that β-catenin phosphorylation downstream of β2AR occurs via G-protein coupled signaling. These phosphorylation events were also dependent on PAK4 kinase activation. Also, co-immunoprecipitation studies suggested enhanced interaction of β-catenin protein phosphorylated at Ser552 with cadherin 11. Finally, β-adrenergic stimulation increased osteoblast attachment and reduced migration, and transfection studies indicated a requirement for serine phosphorylated β-catenin in these effects. These data suggest that increased interaction between cadherin and the C-terminal serine-phosphorylated β-catenin species may contribute to β2AR delay of osteoblast migration. Thus, β-catenin is a novel downstream effector of β2AR signaling and may link β2AR activity to changes in osteoblast cell structure and function.