Functional α1-and β2-Adrenergic Receptors in Human Osteoblasts

Central (hypothalamic) control of bone mass is proposed to be mediated through beta 2-adrenergic receptors (beta 2-ARs). While investigations in mouse bone cells suggest that epinephrine enhances both RANKL and OPG mRNA via both beta-ARs and alpha-ARs, whether alpha-ARs are expressed in human bone cells is controversial. The current study investigated the expression of alpha 1-AR and beta 2-AR mRNA and protein and the functional role of adrenergic stimulation in human osteoblasts (HOBs). Expression of alpha 1B- and beta 2-ARs was examined by RT-PCR, immunofluorescence microscopy and Western blot (for alpha 1B-ARs). Proliferation in HOBs was assessed by 3 H-thymidine incorporation and expression of RANKL and OPG was determined by quantitative RT-PCR. RNA message for alpha 1B- and beta 2-ARs was expressed in HOBs and MG63 human osteosarcoma cells. alpha 1B- and beta 2-AR immunofluorescent localization in HOBs was shown for the first time by deconvolution microscopy. alpha 1B-AR protein was identified in HOBs by Western blot. Both alpha 1-agonists and propranolol (beta-blocker) increased HOB replication but fenoterol, a beta 2-agonist, inhibited it. Fenoterol nearly doubled RANKL mRNA and this was inhibited by propranolol. The alpha 1-agonist cirazoline increased OPG mRNA and this increase was abolished by siRNA knockdown of alpha 1B-ARs in HOBs. These data indicate that both alpha 1-ARs and beta 2-ARs are present and functional in HOBs. In addition to beta 2-ARs, alpha 1-ARs in human bone cells may play a role in modulation of bone turnover by the sympathetic nervous system. J. Cell. Physiol. 220: 267-275, 2009. (C) 2009 Wiley-Liss, Inc.