RESUMO
Tumor necrosis factor (TNF)-α is a key cytokine regulator of bone and mediates inflammatory bone loss. The molecular signaling that regulates bone loss downstream of TNF-α is poorly defined. Here, we demonstrate that inactivating the pro-osteoblastogenic ERK-activated ribosomal S6 kinase RSK2 leads to a drastically accelerated and amplified systemic bone loss in mice ectopically expressing TNF-α [human TNF transgenic (hTNFtg) mice]. The phenotype is associated with a decrease in bone formation because of fewer osteoblasts as well as a drastically increased bone destruction by osteoclasts. The molecular basis of this phenotype is a cell autonomous increased sensitivity of osteoblasts and osteocytes to TNF-induced apoptosis combined with an enhancement of their osteoclast supportive activity. Thus, RSK2 exerts a strong negative regulatory loop on TNF-induced bone loss.
Assuntos
Reabsorção Óssea/metabolismo , Proteínas Quinases S6 Ribossômicas 90-kDa/metabolismo , Fator de Necrose Tumoral alfa/antagonistas & inibidores , Animais , Apoptose/genética , Reabsorção Óssea/genética , Reabsorção Óssea/patologia , Osso e Ossos/metabolismo , Osso e Ossos/patologia , Expressão Gênica , Humanos , Masculino , Camundongos , Camundongos Transgênicos , Osteoblastos/metabolismo , Osteoblastos/patologia , Osteoclastos/metabolismo , Osteoclastos/patologia , Proteínas Quinases S6 Ribossômicas 90-kDa/genética , Transdução de Sinais , Fator de Necrose Tumoral alfa/genética , Fator de Necrose Tumoral alfa/metabolismoRESUMO
A shift from osteoblastogenesis to adipogenesis is one of the underlying mechanisms of decreased bone mass and increased fat during aging. We now uncover a new role for the transcription factor Fra-1 in suppressing adipogenesis. Indeed, Fra1 (Fosl1) transgenic (Fra1tg) mice, which developed progressive osteosclerosis as a result of accelerated osteoblast differentiation, also developed a severe general lipodystrophy. The residual fat of these mice appeared immature and expressed lower levels of adipogenic markers, including the fatty acid transporter Cd36 and the CCAAT/enhancer binding protein Cebpa. Consequently accumulation of triglycerides and free fatty acids were detected in the serum of fasting Fra1tg mice. Fra-1 acts cell autonomously because the adipogenic differentiation of Fra1 transgenic primary osteoblasts was drastically reduced, and overexpression of Fra-1 in an adipogenic cell line blocked their differentiation into adipocytes. Strikingly, Cebpa was downregulated in the Fra-1-overexpressing cells and Fra-1 could bind to the Cebpa promoter and directly suppress its activity. Thus, our data add to the known common systemic control of fat and bone mass, a new cell-autonomous level of control of cell fate decision by which the osteogenic transcription factor Fra-1 opposes adipocyte differentiation by inhibiting C/EBPα.