Functional α1‐ and β2‐adrenergic receptors in human osteoblasts

Abstract

Central (hypothalamic) control of bone mass is proposed to be mediated through beta2-adrenergic receptors (beta2-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 alpha1-AR and beta2-AR mRNA and protein and the functional role of adrenergic stimulation in human osteoblasts (HOBs). Expression of alpha1B- and beta2-ARs was examined by RT-PCR, immunofluorescence microscopy and Western blot (for alpha1B-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 alpha1B- and beta2-ARs was expressed in HOBs and MG63 human osteosarcoma cells. alpha1B- and beta2-AR immunofluorescent localization in HOBs was shown for the first time by deconvolution microscopy. alpha1B-AR protein was identified in HOBs by Western blot. Both alpha1-agonists and propranolol (beta-blocker) increased HOB replication but fenoterol, a beta2-agonist, inhibited it. Fenoterol nearly doubled RANKL mRNA and this was inhibited by propranolol. The alpha1-agonist cirazoline increased OPG mRNA and this increase was abolished by siRNA knockdown of alpha1B-ARs in HOBs. These data indicate that both alpha1-ARs and beta2-ARs are present and functional in HOBs. In addition to beta2-ARs, alpha1-ARs in human bone cells may play a role in modulation of bone turnover by the sympathetic nervous system.

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

“…Altogether, our data indicate that b1-adrenergic signaling plays an independent and predominant role in the regulation of bone mass acquisition during growth and in response to loading, while b2-adrenergic signaling seems to regulates bone remodeling, the latter through direct effects on OPG/RANKL expression in osteoblasts. (15,31) Because adrenergic receptors are present not only in bone cells, but also in different cells types from the bone marrow (32) and in a variety of tissues, it was important to determine whether the low bone mass of Adrb1b2 À/À mice resulted from the alteration of local and/or systemic factors. Among the molecules regulating peak bone mass acquisition and longitudinal growth, IGF-1 plays a central role.…”

Effect of mesenchymal stem cells on hypoxia-induced desensitization of β2-adrenergic receptors in rat osteosarcoma cells

“…Cells (5x10 6 ) of COS1NR in 100 µl of phosphate-buffered saline (PBS) were inoculated into the subcutaneous tissue in the posteriors of F344 rats (groups 1 and 2). Each group consisted of 4 rats and the growth rate of the tumor was evaluated every week.…”

“…cells (5x10 6 ) of COS1NR in 100 µl of PBS were inoculated into the subcutaneous tissue in the posteriors of F344 rats. Subsequently, the same number of MSCs in 100 µl of PBS were directly injected into the circulation through the tail vein twice at weeks 1 and 3 (groups 1 and 2).…”

“…β2AR signaling induces c-fos gene expression in OS cell lines (5), modulates bone turn-over in osteoblasts (6) and affects the osteogenesis of MSCs via the cyclic AMP (cAMP)/PKA pathway (7). In addition, β2AR activation leads to the upregulation of hypoxia-inducible factor-1α (HIF-1α) via Akt and ERK1/2 signaling (8), that may be a mechanism by which β2AR signaling accelerates tumor growth in several types of cancer (9)(10)(11)(12)(13).…”

Genes and Molecular Pathways of the Osteogenic Process

“…Conversely, estrogens increase OPG production in osteoblasts, hence increase bone formation and reduce resorption (Zallone, 2006). More recently a role in the control of bone remodeling has been emerging for leptins, serotonin and insulin, probably acting through the RANKL/OPG system (Elefteriou, 2005;Huang et al, 2009). …”