Tumor Necrosis Factor alpha up-regulates the Expression of beta2 Adrenergic Receptor via NF-kappaB-dependent Pathway in Osteoblasts

Tumor necrosis factor alpha (TNFalpha) is a multifunctional inflammatory cytokine that regulates various cellular and biological processes. Increased levels of TNFalpha have been implicated in a number of human diseases including diabetes and arthritis. Sympathetic nervous system stimulation via the beta2-adrenergic receptor (beta2AR) in osteoblasts suppresses osteogenic activity. We previously reported that TNFalpha up-regulates beta2AR expression in murine osteoblastic cells and that this modulation is associated with TNFalpha inhibition of osteoblast differentiation. In our present study, we explored whether TNFalpha induces beta2AR expression in human osteoblasts and then identified the downstream signaling pathway. Our results indicated that beta2AR expression was increased in Saos-2 and C2C12 cells by TNFalpha treatment, and that this increase was blocked by the inhibition of NF-kappaB activation. Chromatin immunoprecipitation and luciferase reporter assay results indicated that NF-kappaB directly binds to its cognate elements on the beta2AR promoter and thereby stimulates beta2AR expression. These findings suggest that the activation of TNFalpha signaling in osteoblastic cells leads to an upregulation of beta2AR and also that TNFalpha induces beta2AR expression in an NF-kappaB-dependent manner.

TNFalpha Increases the Expression of beta2 Adrenergic Receptors in Osteoblasts

Tumor necrosis factor alpha (TNFalpha) is a multifunctional cytokine that is elevated in inflammatory diseases such as atherosclerosis, diabetes and rheumatoid arthritis. Recent evidence has suggested that beta2 adrenergic receptor (beta2AR) activation in osteoblasts suppresses osteogenic activity. In the present study, we explored whether TNFalpha modulates betaAR expression in osteoblastic cells and whether this regulation is associated with the inhibition of osteoblast differentiation by TNFalpha. In the experiments, we used C2C12 cells, MC3T3-E1 cells and primary cultured mouse bone marrow stromal cells. Among the three subtypes of betaAR, beta2 and beta3AR were found in our analysis to be upregulated by TNFalpha. Moreover, isoproterenol-induced cAMP production was observed to be significantly enhanced in TNFalpha-primed C2C12 cells, indicating that TNFalpha enhances beta2AR signaling in osteoblasts. TNFalpha was further found in C2C12 cells to suppress bone morphogenetic protein 2-induced alkaline phosphatase (ALP) activity and the expression of osteogenic marker genes including Runx2, ALP and osteocalcin. Propranolol, a beta2AR antagonist, attenuated this TNFalpha suppression of osteogenic differentiation. TNFalpha increased the expression of receptor activator of NF-kappaB ligand (RANKL), an essential osteoclastogenic factor, in C2C12 cells which was again blocked by propranolol. In summary, our data show that TNFalpha increases beta2AR expression in osteoblasts and that a blockade of beta2AR attenuates the suppression of osteogenic differentiation and stimulation of RANKL expression by TNFalpha. These findings imply that a crosstalk between TNFalpha and beta2AR signaling pathways might occur in osteoblasts to modulate their function.