Osteoclasts are acid-secreting polykaryons that have high energy demands and contain abundant mitochondria. How mitochondrial biogenesis is integrated with osteoclast differentiation is unknown. We found that the transcription of Ppargc1b, which encodes peroxisome proliferator-activated receptor-gamma coactivator 1beta (PGC-1beta), was induced during osteoclast differentiation by cAMP response element-binding protein (CREB) as a result of reactive oxygen species. Knockdown of Ppargc1b in vitro inhibited osteoclast differentiation and mitochondria biogenesis, whereas deletion of the Ppargc1b gene in mice resulted in increased bone mass due to impaired osteoclast function. We also observed defects in PGC-1beta-deficient osteoblasts. Owing to the heightened iron demand in osteoclast development, transferrin receptor 1 (TfR1) expression was induced post-transcriptionally via iron regulatory protein 2. TfR1-mediated iron uptake promoted osteoclast differentiation and bone-resorbing activity, associated with the induction of mitochondrial respiration, production of reactive oxygen species and accelerated Ppargc1b transcription. Iron chelation inhibited osteoclastic bone resorption and protected against bone loss following estrogen deficiency resulting from ovariectomy. These data establish mitochondrial biogenesis orchestrated by PGC-1beta, coupled with iron uptake through TfR1 and iron supply to mitochondrial respiratory proteins, as a fundamental pathway linked to osteoclast activation and bone metabolism.

Bone remodeling is characterized by the sequential, local tethering of osteoclasts and osteoblasts and is key to the maintenance of bone integrity. While bone matrix-mobilized growth factors, such as TGF-β, are proposed to regulate remodeling, no in vivo evidence exists that an osteoclast-produced molecule serves as a coupling factor for bone resorption to formation. We found that CTHRC1, a protein secreted by mature boneresorbing osteoclasts, targets stromal cells to stimulate osteogenesis. Cthrc1 expression was robustly induced when mature osteoclasts were placed on dentin or hydroxyapatite, and also by increasing extracellular calcium. Cthrc1 expression in bone increased in a high-turnover state (such as that induced by RANKL injections in vivo), but decreased in conditions associated with suppressed bone turnover (such as with aging and after alendronate treatment). Targeted deletion of Cthrc1 in mice eliminated Cthrc1 expression in bone, whereas its deficiency in osteoblasts did not exert any significant effect. Osteoclast-specific deletion of Cthrc1 resulted in osteopenia due to reduced bone formation and impaired the coupling process after resorption induced by RANKL injections, impairing bone mass recovery. These data demonstrate that CTHRC1 is an osteoclastsecreted coupling factor that regulates bone remodeling. IntroductionBone is constantly remodeled through the removal of old bone (resorption) and the replacement of new bone (formation) by hematopoietic-derived osteoclasts and mesenchymal-derived osteoblasts, respectively, to meet structural and metabolic demands (1, 2). It has long been believed that a preceding resorption phase is a prerequisite for the initiation of subsequent bone formation (3), which is also supported by recent clinical observations that treatment of osteoporotic patients with the potent antiresorptive drug alendronate blunts the anabolic action of parathyroid hormone (PTH) (4). Although the coupling of bone formation to resorption has been long recognized, the mechanism and the factors mediating this fundamental process in skeletal homeostasis are not fully understood (5).Recently, it was suggested that active TGF-β1 released from bone matrix during bone resorption is coupled to bone formation by inducing migration of marrow stromal cells to resorption sites (6). Bidirectional signaling between EPHRINB2 on osteoclasts and the receptor EPHB4 on osteoblasts has also been proposed to link bone resorption and formation through direct cell-cell contact (7). In addition, mature osteoclasts produce and secrete factors, such as WNT10B, BMP6, and the lipid mediator sphingosine-1-phosphate (S1P), that have been shown to stimulate osteoblast recruitment and survival (8-10). However, the in vivo function of these factors in the coupling process remains to be elucidated.We reasoned that a factor mediating the coupling reaction linking bone resorption to formation would have to meet the following criteria: (a) it should be produced and secreted/presented by osteoclasts; (b) it should exert a...

Hypertension and osteoporosis are two major age-related disorders; however, the underlying molecular mechanism for this comorbidity is not known. The renin-angiotensin system (RAS) plays a central role in the control of blood pressure and has been an important target of antihypertensive drugs. Using a chimeric RAS model of transgenic THM (Tsukuba hypertensive mouse) expressing both the human renin and human angiotensinogen genes, we showed in this study that activation of RAS induces high turnover osteoporosis with accelerated bone resorption. Transgenic mice that express only the human renin gene were normotensive and yet exhibited a low bone mass, suggesting that osteoporosis occurs independently of the development of hypertension per se. Ex vivo cultures showed that angiotensin II (AngII) acted on osteoblasts and not directly on osteoclast precursor cells and increased osteoclastogenesis-supporting cytokines, RANKL and vascular endothelial growth factor (VEGF), thereby stimulating the formation of osteoclasts. Knockdown of AT2 receptor inhibited the AngII activity, whereas silencing of the AT1 receptor paradoxically enhanced it, suggesting a functional interaction between the two AngII receptors on the osteoblastic cell surface. Finally, treatment of THM mice with an ACE inhibitor, enalapril, improved osteoporosis and hypertension, whereas treatment with losartan, an angiotensin receptor blockers specific for AT1, resulted in exacerbation of the low bone mass phenotype. Thus, blocking the synthesis of AngII may be an effective treatment of osteoporosis and hypertension, especially for those afflicted with both conditions.

Background:The osteoclastogenic cytokine RANKL is expressed in various cell types, including osteoblasts, osteocytes, and lymphocytes. Results: RANKL expression is induced during adipogenesis through the action of C/EBP␤ and/or C/EBP␦, and RANKL-positive preadipocytes increase in aging marrow along with down-regulation of osteoprotegerin. Conclusion: Adipogenesis is linked to osteoclastogenesis through RANKL expression. Significance: Increased marrow preadipocytes with aging may contribute to osteoporosis.

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