Endogenous TNFalpha lowers maximum peak bone mass and inhibits osteoblastic Smad activation through NF-kappaB.
Endogenous TNFalpha prevents the attainment of maximum achievable peak bone mass in vivo. In vitro, TNFalpha suppresses BMP-2- and TGFbeta-mediated Smad activation through induction of NF-kappaB. Consistently, pharmacological suppression of NF-kappaB augments osteoblast differentiation and mineralization in vitro.


BACKGROUND
Osteoporosis is a major health threat. Traditional therapeutic strategies have centered on anti-catabolic drugs that block bone resorption. Recently focus has shifted to anabolic agents that actively rebuild lost bone mass. Future strategies may involve elevating peak bone mass to delay osteoporosis development. Recent in vitro studies show that TNFalpha represses osteoblast differentiation and mineralization; however, the mechanisms are poorly understood and the impact of basal TNFalpha concentrations on the acquisition of peak bone mass in vivo is unknown.


METHODS
We examined peak BMD, bone volume, and bone turnover makers in mice deficient in TNFalpha or its receptors. We further examined the effect of TNFalpha on Smad-induced signaling by TGFbeta and BMP-2 in vitro using a Smad responsive reporter. The effect of TNFalpha-induced NF-kappaB signaling on Smad signaling and on in vitro osteoblast mineralization was examined using specific NF-kappaB inhibitors and activators, and effects of TNFalpha-induced NF-kappaB signaling on BMP-2-induced Runx2 mRNA were examined using RT-PCR.


RESULTS
Mice null for TNFalpha or its p55 receptor had significantly increased peak bone mass, resulting exclusively from elevated bone formation. In vitro, TNFalpha potently suppressed Smad signaling induced by TGFbeta and BMP-2, downregulated BMP-2-mediated Runx2 expression, and inhibited mineralization of osteoblasts. These effects were mimicked by overexpression of NF-kappaB and prevented by pharmacological NF-kappaB suppression.


CONCLUSIONS
Our data suggest that TNFalpha and NF-kappaB antagonists may represent novel anabolic agents for the maximization of peak basal bone mass and/or the amelioration of pathological bone loss.
Li
Yan
Y
Division of Endocrinology and Metabolism and Lipids, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
Li
Aimin
A
Strait
Karen
K
Zhang
Hongying
H
Nanes
Mark S
MS
Weitzmann
M Neale
MN
D000818:Animals; D015519:Bone Density / Q000235:genetics; D055396:Bone Morphogenetic Protein 2; D019485:Bone Morphogenetic Proteins / Q000378:metabolism; D002454:Cell Differentiation / Q000235:genetics; D002460:Cell Line; D015536:Down-Regulation / Q000235:genetics; D051379:Mice; D018345:Mice, Knockout; D016328:NF-kappa B / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D010006:Osteoblasts / Q000166:cytology / Q000378:metabolism*; D010012:Osteogenesis / Q000235:genetics; D010024:Osteoporosis / Q000188:drug therapy / Q000235:genetics / Q000378:metabolism; D047888:Receptors, Tumor Necrosis Factor, Type I / Q000172:deficiency / Q000378:metabolism; D015398:Signal Transduction / Q000235:genetics; D051785:Smad Proteins / Q000378:metabolism*; D016212:Transforming Growth Factor beta / Q000378:metabolism; D014409:Tumor Necrosis Factor-alpha / Q000037:antagonists & inhibitors / Q000172:deficiency / Q000378:metabolism*
C525719:Bmp2 protein, mouse; D055396:Bone Morphogenetic Protein 2; D019485:Bone Morphogenetic Proteins; D016328:NF-kappa B; D047888:Receptors, Tumor Necrosis Factor, Type I; D051785:Smad Proteins; D016212:Transforming Growth Factor beta; D014409:Tumor Necrosis Factor-alpha