Reduced expression of interleukin-11 in bone marrow stromal cells of senescence-accelerated mice (SAMP6): relationship to osteopenia with enhanced adipogenesis

Low-intensity pulsed ultrasound regulates bone marrow mesenchymal stromal cells differentiation and inhibits bone loss by activating the IL-11-Wnt/β-catenin signaling pathway

BACKGROUND

Osteoporosis (OP) is a common metabolic bone disease. Low-intensity pulsed ultrasound (LIPUS) can effectively promote bone formation and fracture healing. The Wnt/β-catenin signaling pathway is crucial for regulating bone homeostasis and bone diseases, and its downregulation is one of the main mechanisms of osteoporosis pathogenesis. Interleukin-11 (IL-11), which is regulated by mechanical stress, is a key factor in bone remodeling. Here, we investigated the optimal intervention parameters for LIPUS, the relationships among LIPUS, IL-11, and the Wnt/β-catenin signaling pathway, and the effects of LIPUS on bone loss and potential molecular mechanisms in ovariectomized (OVX) mice.

METHODS

Bone marrow mesenchymal stromal cells (BMSCs) were subjected to LIPUS intervention for 0, 10, or 20 min to determine the optimal intervention time. The mediating role of IL-11 in LIPUS intervention was explored through IL-11 knockdown and overexpression. Finally, animal experiments were conducted to investigate the in vivo therapeutic effects of LIPUS.

RESULTS

The optimal intervention time for LIPUS was 20 min. LIPUS promoted IL-11 expression and upregulated the Wnt/β-catenin signaling pathway, thereby promoting osteogenic differentiation and inhibiting adipogenic differentiation of BMSCs. IL-11 mediates the regulation of the Wnt/β-catenin signaling pathway by LIPUS. Additionally, LIPUS effectively improved the bone microstructure in ovariectomized mice, inhibited bone loss, promoted IL-11 expression in bone tissue, and activated the Wnt/β-catenin signaling pathway in the femur.

CONCLUSION

Low-intensity pulsed ultrasound can regulate BMSCs differentiation and inhibit bone loss by promoting IL-11 expression and activating the Wnt/β-catenin signaling pathway.

Roles of IL-11 in the regulation of bone metabolism

Bone metabolism is the basis for maintaining the normal physiological state of bone, and imbalance of bone metabolism can lead to a series of metabolic bone diseases. As a member of the IL-6 family, IL-11 acts primarily through the classical signaling pathway IL-11/Receptors, IL-11 (IL-11R)/Glycoprotein 130 (gp130). The regulatory role of IL-11 in bone metabolism has been found earlier, but mainly focuses on the effects on osteogenesis and osteoclasis. In recent years, more studies have focused on IL-11's roles and related mechanisms in different bone metabolism activities. IL-11 regulates osteoblasts, osteoclasts, BM stromal cells, adipose tissue-derived mesenchymal stem cells, and chondrocytes. It's involved in bone homeostasis, including osteogenesis, osteolysis, bone marrow (BM) hematopoiesis, BM adipogenesis, and bone metastasis. This review exams IL-11's role in pathology and bone tissue, the cytokines and pathways that regulate IL-11 expression, and the feedback regulations of these pathways.

The Emerging Role of Interleukin-(IL)-11/IL-11R in Bone Metabolism and Homeostasis: From Cytokine to Osteokine

Interleukin-(IL)-11 is a cytokine involved in hematopoiesis, cancer metastasis, and inflammation. IL-11 belongs to the IL-6 cytokine family, binding to the complex of receptors glycoprotein gp130 and the ligand-specific-receptor subunits (IL-11Rα or their soluble counterpart sIL-11R). IL-11/IL-11R signaling enhances osteoblast differentiation and bone formation and mitigates osteoclast-induced bone resorption and cancer bone metastasis. Recent studies have shown that systemic and osteoblast/osteocyte-specific IL-11 deficiency leads to reduced bone mass and formation, but also adiposity, glucose intolerance, and insulin resistance. In humans, mutations of IL-11 and the receptor IL-11RA genes are associated with height reduction, osteoarthritis, and craniosynostosis. In this review, we describe the emerging role of IL-11/IL-11R signaling in bone metabolism by targeting osteoblasts, osteoclasts, osteocytes, and bone mineralization. Furthermore, IL-11 promotes osteogenesis and suppresses adipogenesis, thereby influencing the fate of osteoblast/adipocyte differentiation derived from pluripotent mesenchymal stem cells. We have newly identified IL-11 as a bone-derived cytokine that regulates bone metabolism and the link between bone and other organs. Thus, IL-11 is vital in bone homeostasis and could be considered a potential therapeutic strategy.

Osteoblast/osteocyte-derived interleukin-11 regulates osteogenesis and systemic adipogenesis

Exercise results in mechanical loading of the bone and stimulates energy expenditure in the adipose tissue. It is therefore likely that the bone secretes factors to communicate with adipose tissue in response to mechanical loading. Interleukin (IL)-11 is known to be expressed in the bone, it is upregulated by mechanical loading, enhances osteogenesis and suppresses adipogenesis. Here, we show that systemic IL-11 deletion (IL-11^-/-) results in reduced bone mass, suppressed bone formation response to mechanical loading, enhanced expression of Wnt inhibitors, and suppressed Wnt signaling. At the same time, the enhancement of bone resorption by mechanical unloading was unaffected. Unexpectedly, IL-11^-/- mice have increased systemic adiposity and glucose intolerance. Osteoblast/osteocyte-specific IL-11 deletion in osteocalcin-Cre;IL-11^fl/fl mice have reduced serum IL-11 levels, blunted bone formation under mechanical loading, and increased systemic adiposity similar to IL-11^-/- mice. Adipocyte-specific IL-11 deletion in adiponectin-Cre;IL-11^fl/fl did not exhibit any abnormalities. We demonstrate that osteoblast/osteocyte-derived IL-11 controls both osteogenesis and systemic adiposity in response to mechanical loading, an important insight for our understanding of osteoporosis and metabolic syndromes.

Hallmarks of aging and immunosenescence: Connecting the dots

Aging is a natural physiological process that features various and variable challenges, associated with loss of homeostasis within the organism, often leading to negative consequences for health. Cellular senescence occurs when cells exhaust the capacity to renew themselves and their tissue environment as the cell cycle comes to a halt. This process is influenced by genetics, metabolism and extrinsic factors. Immunosenescence, the aging of the immune system, is a result of the aging process, but can also in turn act as a secondary inducer of senescence within other tissues. This review aims to summarize the current state of knowledge regarding hallmarks of aging in relation to immunosenescence, with a focus on aging-related imbalances in the medullary environment, as well as the components of the innate and adaptive immune responses. Aging within the immune system alters its functionality, and has consequences for the person's ability to fight infections, as well as for susceptibility to chronic diseases such as cancer and cardiovascular disease. The senescence-associated secretory phenotype is described, as well as the involvement of this phenomenon in the paracrine induction of senescence in otherwise healthy cells. Inflammaging is discussed in detail, along with the comorbidities associated with this process. A knowledge of these processes is required in order to consider possible targets for the application of senotherapeutic agents - interventions with the potential to modulate the senescence process, thus prolonging the healthy lifespan of the immune system and minimizing the secondary effects of immunosenescence.

Effects and Mechanisms of Five Psoralea Prenylflavonoids on Aging-Related Diseases

During the aging process, senescent cells gradually accumulate in the organs; they secrete proinflammatory cytokines and other factors, collectively known as the senescence-associated secretory phenotype (SASP). SASP secretions contribute to “inflammaging,” which is a state of chronic, systemic, sterility, low-grade inflammatory microenvironment and a key risk factor in the development of aging-related diseases. Fructus psoraleae is a traditional Chinese medical herb best known for delaying aging and treating osteoporosis. Prenylflavonoids from fructus psoraleae are the main bioactive compounds responsible for its pharmacological applications, such as beaching, bavachinin, bavachalcone, isobavachalcone, and neobavaisoflavone. In previous decades, there have been some promising studies on the pharmacology of fructus psoraleae. Here, we focus on the anti-inflammatory and antiaging diseases of five psoralea prenylflavonoids, such as cardiovascular protection, diabetes and obesity intervention, neuroprotection, and osteoporosis, and discuss the mechanism of these active ingredients for better understanding the material basis and drug application of fructus psoraleae in Chinese medicine.

Dry-Fermented Soybean Food (Cheonggukjang) Ameliorates Senile Osteoporosis in the Senescence-Accelerated Mouse Prone 6 Model

Senile osteoporosis increases the risk of skeletal fractures with age. Cheonggukjang (CGJ), a traditional Korean dry fermented soybean product, has numerous therapeutic effects; however, its effects on bone mineral density (BMD) and bone metabolism in senile osteoporosis are unclear. In this study, we treated the senescence-accelerated mouse prone 6 (SAMP6) model of senile osteoporosis with CGJ to determine its potential for ameliorating and preventing osteoporosis progression. High-performance liquid chromatography analysis for isoflavone profiles revealed that short-term fermentation significantly increased the isoflavone aglycone content in soybeans. Thereafter, we fed 6-week-old SAMP6 mice with experimental diets containing 5% or 10% CGJ for 15 weeks. Microcomputed tomography revealed that CGJ supplementation effectively increased the BMD and relative bone length. In vitro, CGJ increased the osteopontin reactivity and upregulated the expression of Alp, Col1a1, Fak, Bmp2/4, Smad1/5/8, and Runx2 in osteoblasts, and decreased Cathepsin K reactivity and downregulated Rankl and Nfatc1 expression in osteoclasts. In addition, CGJ increased the osteoprotegerin/Rankl ratio. Collectively, these results demonstrate that CGJ can ameliorate the detrimental effects of senile osteoporosis by improving osteogenesis and decreasing osteoclast activity.

MECHANISMS IN ENDOCRINOLOGY: Bone marrow adiposity and bone, a bad romance?

Bone marrow adipocytes (BMA-) constitute an original and heterogeneous fat depot whose development appears interlinked with bone status throughout life. The gradual replacement of the haematopoietic tissue by BMA arises in a well-ordered way during childhood and adolescence concomitantly to bone growth and continues at a slower rate throughout the adult life. Importantly, BM adiposity quantity is found well associated with bone mineral density (BMD) loss at different skeletal sites in primary osteoporosis such as in ageing or menopause but also in secondary osteoporosis consecutive to anorexia nervosa. Since BMA and osteoblasts originate from a common mesenchymal stem cell, adipogenesis is considered as a competitive process that disrupts osteoblastogenesis. Besides, most factors secreted by bone and bone marrow cells (ligands and antagonists of the WNT/β-catenin pathway, BMP and others) reciprocally regulate the two processes. Hormones such as oestrogens, glucocorticoids, parathyroid and growth hormones that control bone remodelling also modulate the differentiation and the activity of BMA. Actually, BMA could also contribute to bone loss through the release of paracrine factors altering osteoblast and/or osteoclast formation and function. Based on clinical and fundamental studies, this review aims at presenting and discussing these current arguments that support but also challenge the involvement of BMA in the bone mass integrity.

The Spectrum of Fundamental Basic Science Discoveries Contributing to Organismal Aging

Aging research has undergone unprecedented advances at an accelerating rate in recent years, leading to excitement in the field as well as opportunities for imagination and innovation. Novel insights indicate that, rather than resulting from a preprogrammed series of events, the aging process is predominantly driven by fundamental non-adaptive mechanisms that are interconnected, linked, and overlap. To varying degrees, these mechanisms also manifest with aging in bone where they cause skeletal fragility. Because these mechanisms of aging can be manipulated, it might be possible to slow, delay, or alleviate multiple age-related diseases and their complications by targeting conserved genetic signaling pathways, controlled functional networks, and basic biochemical processes. Indeed, findings in various mammalian species suggest that targeting fundamental aging mechanisms (eg, via either loss-of-function or gain-of-function mutations or administration of pharmacological therapies) can extend healthspan; ie, the healthy period of life free of chronic diseases. In this review, we summarize the evidence supporting the role of the spectrum of fundamental basic science discoveries contributing to organismal aging, with emphasis on mammalian studies and in particular aging mechanisms in bone that drive skeletal fragility. These mechanisms or aging hallmarks include: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. Because these mechanisms are linked, interventions that ameliorate one hallmark can in theory ameliorate others. In the field of bone and mineral research, current challenges include defining the relative contributions of each aging hallmark to the natural skeletal aging process, better understanding the complex interconnections among the hallmarks, and identifying the most effective therapeutic strategies to safely target multiple hallmarks. Based on their interconnections, it may be feasible to simultaneously interfere with several fundamental aging mechanisms to alleviate a wide spectrum of age-related chronic diseases, including osteoporosis. © 2018 American Society for Bone and Mineral Research.

Glucagon-like peptide-1(GLP-1) receptor agonists: potential to reduce fracture risk in diabetic patients?

This review summarizes current knowledge about glucagon-like peptide 1 receptor agonists (GLP-1 RA) and their effects on bone metabolism and fracture risk. Recent in vivo and in vitro experiments indicated that GLP-1 RA could improve bone metabolism. GLP-1 could affect the fat-bone axis by promoting osteogenic differentiation and inhibiting adipogenic differentiation of bone mesenchymal precursor cells (BMSCs), which express the GLP-1 receptor. GLP-1 RA may also influence the balance between osteoclasts and osteoblasts, thus leading to more bone formation and less bone resorption. Wnt/β-catenin signalling is involved in this process. Mature osteocytes, which also express the GLP-1 receptor, produce sclerostin which inhibits Wnt/β-catenin signalling by binding to low density lipoprotein receptor-related protein (LRP) 5 and preventing the binding of Wnt. GLP-1 RA also decreases the expression of sclerostin (SOST) and circulating levels of SOST. In addition, GLP-1 receptors are expressed in thyroid C cells, where GLP-1 induces calcitonin release and thus indirectly inhibits bone resorption. Furthermore, GLP-1 RA influences the osteoprotegerin(OPG)/receptor activator of nuclear factor-κB ligand (RANKL)/receptor activator of nuclear factor-κB (RANK) system by increasing OPG gene expression, and thus reverses the decreased bone mass in rats models. However, a recent meta-analysis and a cohort study did not show a significant relationship between GLP-RA use and fracture risk. Future clinical trials will be necessary to investigate thoroughly the relationship between GLP-1 RA use and fracture risk in diabetic patients.

Vitamin K2 Prevents Glucocorticoid-induced Osteonecrosis of the Femoral Head in Rats

Glucocorticoid medication is one of the most common causes of atraumatic osteonecrosis of the femoral head (ONFH), and vitamin K₂ (VK₂) has been shown to play an important and beneficial role in bone metabolism. In this study, we hypothesized that VK₂ could decrease the incidence of glucocorticoid-induced ONFH in a rat model. Using in vitro studies, we investigated how bone marrow-derived stem cells in the presence of methylprednisolone proliferate and differentiate, specifically examining osteogenic-related proteins, including Runx2, alkaline phosphatase and osteocalcin. Using in vivo studies, we established glucocorticoid-induced ONFH in rats and investigated the preventive effect of VK₂. We employed micro-CT scanning, angiography of the femoral head, and histological and immunohistochemical analyses, which demonstrated that VK₂ yielded beneficial effects for subchondral bone trabecula. In conclusion, VK₂ is an effective antagonist for glucocorticoid on osteogenic progenitors. The underlying mechanisms include acceleration of BMSC propagation and promotion of bone formation-associated protein expression, which combine and contribute to the prevention of glucocorticoid-induced ONFH in rats.

The interrelationship between bone and fat: from cellular see-saw to endocrine reciprocity

The number of mature osteoblasts and marrow adipocytes in bone is influenced by the differentiation of the common mesenchymal progenitor cell towards one phenotype and away from the other. Consequently, factors which promote adipogenesis not only lead to fatty marrow but also inhibit osteoblastogenesis, resulting in decreased osteoblast numbers, diminished bone formation and, potentially, inadequate bone mass and osteoporosis. In addition to osteoblast and bone adipocyte numbers being influenced by this skewing of progenitor cell differentiation towards one phenotype, mature osteoblasts and adipocytes secrete factors which may evoke changes in the cell fate and function of each other. This review examines the endogenous factors, such as PPAR-γ2, Wnt, IGF-1, GH, FGF-2, oestrogen, the GP130 signalling cytokines, vitamin D and glucocorticoids, which regulate the selection between osteoblastogenesis and adipogenesis and the interrelationship between fat and bone. The role of adipokines on bone, such as adiponectin and leptin, as well as adipose-derived oestrogen, is reviewed and the role of bone as an energy regulating endocrine organ is discussed.

Parathyroid hormone (1-34) counteracts the suppression of interleukin-11 expression by glucocorticoid in murine osteoblasts: a possible mechanism for stimulating osteoblast differentiation against glucocorticoid excess

Glucocorticoid (GC) excess causes a rapid loss of bone with a reduction in bone formation. Intermittent PTH (1-34) administration stimulates bone formation and counteracts the inhibition of bone formation by GC excess. We have previously demonstrated that mechanical strain enhances interleukin (IL)-11 gene transcription by a rapid induction of ΔFosB expression and protein kinase C (PKC)-δ-mediated phosphorylation of phosphorylated mothers against decapentaplegic (Smad)-1. Because IL-11 suppresses the expression of dickkopf-1 and -2 and stimulates Wnt signaling, IL-11 appears to mediate at least a part of the effect of mechanical strain on osteoblast differentiation and bone formation. The present study was undertaken to examine the effect of PTH(1-34) and GCs on IL-11 expression in murine primary osteoblasts (mPOBs). PTH(1-34) treatment of mPOBs enhanced IL-11 expression in a time- and dose-dependent manner. PTH(1-34) also stimulated ΔFosB expression and Smad1 phosphorylation, which cooperatively stimulated IL-11 gene transcription. PTH(1-34)-induced Smad1 phosphorylation was mediated via PKCδ and was abrogated in mPOBs from PKCδ knockout mice. Dexamethasone suppressed IL-11 gene transcription enhanced by PTH(1-34) without affecting ΔFosB expression or Smad1 phosphorylation, and dexamethasone-GC receptor complex was bound to JunD, which forms heterodimers with ΔFosB. High doses of PTH(1-34) counteracted the effect of dexamethasone on apoptosis of mPOBs, which was blunted by neutralizing anti-IL-11 antibody or IL-11 small interfering RNA. These results demonstrate that PTH(1-34) and GCs interact to regulate IL-11 expression in parallel with osteoblast differentiation and apoptosis and suggest that PTH(1-34) and dexamethasone may regulate osteoblast differentiation and apoptosis via their effect on IL-11 expression.

Neopterin, inflammation-associated product, prolongs erythropoiesis suppression in aged SAMP1 mice due to senescent stromal-cell impairment

Anemia induced by inflammation is well known to be more serious in the elderly than in non-elderly adults; however, the reason why this is so remains unclear. Neopterin produced by monocytes during inflammation promotes myelopoiesis but suppresses B-lymphopoiesis and erythropoiesis, by activating stromal cells in mice. Here, age-related changes in the erythropoietic response to neopterin were determined using senescence accelerated mice (SAMP1) with senescence stromal-cell impairment. Intravenous injection of neopterin into young mice (8-12 weeks old) resulted in a decrease in erythroid progenitor cell number in the bone marrow (BM), concomitant with an increase in myeloid progenitor cell number over one week. Intravenous injection of neopterin into aged mice (30-36 weeks old) resulted in a prolonged decrease in erythroid progenitor cell number in the BM over three weeks and a limited increase in myeloid progenitor cell number over one day. Neopterin treatment induced a decrease in serum erythropoietin concentrations in young mice but not in aged mice. The gene expression of tumor necrosis factor α (TNF-α), a negative regulator of erythropoiesis, was up-regulated in the BM of both young and aged mice, and the degree of TNF-α up-regulation was the same in both groups. The gene expression of interleukin (IL)-11, a positive regulator of erythropoiesis, was also up-regulated over one day in both young and aged mice. However, IL-11 gene expression remained up-regulated thereafter in young mice, whereas it was rapidly down-regulated in aged mice. These data suggest that prolonged suppression of erythropoiesis in aged mice may be due to a decrease in the production of positive regulators rather than to an increase in the production of negative regulators. Our combined data suggest that age-related impairment of stromal cells induces serious anemia in the elderly during inflammation.

Dysregulated in vitro hematopoiesis, radiosensitivity, proliferation, and osteoblastogenesis with marrow from SAMP6 mice

The senescence accelerated-prone mouse variant 6 (SAMP6) shows normal growth followed by rapid aging, development of osteopenia, and shortened lifespan, compared with control R1 mice. Because oxidative stress is a fundamental mechanism of tissue aging, we tested whether cellular parameters that are associated with oxidative stress are impaired with marrow from SAMP6 mice. We compared in vitro hematopoiesis, irradiation sensitivity, proliferative potential, and osteoblastogenesis with marrow cells from SAMP6 and R1 mice. Marrow cells from SAMP6 mice showed shortened in vitro hematopoiesis; their stromal cells showed greater radiation sensitivity and decreased proliferation. Consistent with those properties, there was constitutive upregulation of transforming growth factor-β(1), an inhibitor of hematopoiesis, and of cell cycle inhibitory genes, p16(INK4A) and p19(ARF). Paradoxically, there was constitutive expression of osteoblast genes in stromal cells from SAMP6 mice, but in vitro matrix mineralization was impaired. These studies and data included in other reports indicate that impaired proliferation of osteoblast progenitors in SAMP6 marrow may be a major factor contributing to accelerated loss of bone mass. In sum, marrow from SAMP6 mice had diminished capacity for long-term hematopoiesis, increased radiosensitivity, and reduced proliferative capacity.

Suppression of zinc finger protein 467 alleviates osteoporosis through promoting differentiation of adipose derived stem cells to osteoblasts

Osteoblast and adipocyte are derived from common mesenchymal progenitor cells. The bone loss of osteoporosis is associated with altered progenitor differentiation from an osteoblastic to an adipocytic lineage. In this study, a comparative analysis of gene expression profiling using cDNA microarray and realtime-PCR indicated that Zinc finger protein 467 (Zfp467) involved in adipocyte and osteoblast differentiation of cultured adipose derived stem cells (ADSCs). Our results showed that RNA interference for Zfp467 in ADSCs inhibited adipocyte formation and stimulated osteoblast commitment. The mRNA levels of osteogenic and adipogenic markers in ADSCs were regulated by si-Zfp467. Zfp467 RNAi in ADSCs could restore bone function and structure in an ovariectomized (OVX)-induced osteoporotic mouse model. Thus Zfp467 play an important role in ADSCs differentiation to adipocyte and osteoblast. This has relevance to therapeutic interventions in osteoporosis, including si-Zfp467-based therapies currently available, and may be of relevance for the use of adipose-derived stem cells for tissue engineering.

Regulation of osteoblast differentiation by interleukin-11 via AP-1 and Smad signaling

Mechanical stress and parathyroid hormone (PTH) are major stimulators, and aging and glucocorticoids excess are important suppressors of osteoblast differentiation. Mechanical stress and PTH stimulate interleukin (IL)-11 expression in cells of osteoblast lineage by enhancing transcription of IL-11 gene via an increase in intracellular Ca²⁺. The elevated Ca²⁺ activates extracellular signal-regulated kinase (ERK) to enhance phosphorylation of cyclic AMP response element-binding protein (CREB), which binds to the fosB gene promoter and enhances ΔFosB expression. ΔFosB dimerizes with JunD on the IL-11 gene promoter to enhance its transcription. Both mechanical stress and PTH also stimulate phosphorylation of Smad1 via an activation of protein kinase Cδ (PKCδ). Phosphorylated Smad1 binds to the IL-11 gene promoter and forms complex with ΔFosB/JunD to further enhance IL-11 gene transcription. The increased IL-11 then suppresses expression of Wnt inhibitors, including Dickkopf 1 (Dkk1) and 2, and enhances Wnt signaling to stimulate osteoblast differentiation and inhibit adipocyte differentiation. The suppression of osteoblast differentiation by aging involves a decrease in IL-11 gene transcription by a reduction in JunD binding to the activator protein (AP)-1 site of the IL-11 gene promoter. Glucocorticoids inhibit transcriptional activation of IL-11 gene by an interaction of glucocorticoid-glucocorticoid receptor (GR) complex with ΔFosB/JunD heterodimer. Thus, factors that enhance osteoblast differentiation stimulate, and those which suppress osteoblast differentiation inhibit IL-11 gene transcription, and IL-11 enhances Wnt signaling by suppressing expression of its inhibitors. These observations are consistent with the notion that IL-11 mediates stimulatory and inhibitory signals of osteoblast differentiation by affecting Wnt signaling.

Zinc finger protein 467 is a novel regulator of osteoblast and adipocyte commitment

Osteoblasts and adipocytes are derived from common mesenchymal progenitor cells. The bone loss of osteoporosis is associated with altered progenitor differentiation from an osteoblastic to an adipocytic lineage. cDNA microarrays and quantitative real-time PCR (Q-PCR) were carried out in a differentiating mouse stromal osteoblastic cell line, Kusa 4b10, to identify gene targets of factors that stimulate osteoblast differentiation including parathyroid hormone (PTH) and gp130-binding cytokines, oncostatin M (OSM) and cardiotrophin-1 (CT-1). Zinc finger protein 467 (Zfp467) was rapidly down-regulated by PTH, OSM, and CT-1. Retroviral overexpression and RNA interference for Zfp467 in mouse stromal cells showed that this factor stimulated adipocyte formation and inhibited osteoblast commitment compared with controls. Regulation of adipocyte markers, including peroxisome proliferator-activated receptor (PPAR) γ, C/EBPα, adiponectin, and resistin, and late osteoblast/osteocyte markers (osteocalcin and sclerostin) by Zfp467 was confirmed by Q-PCR. Intra-tibial injection of calvarial cells transduced with retroviral Zfp467 doubled the number of marrow adipocytes in C57Bl/6 mice compared with vector control-transduced cells, providing in vivo confirmation of a pro-adipogenic role of Zfp467. Furthermore, Zfp467 transactivated a PPAR-response element reporter construct and recruited a histone deacetylase complex. Thus Zfp467 is a novel co-factor that promotes adipocyte differentiation and suppresses osteoblast differentiation. This has relevance to therapeutic interventions in osteoporosis, including PTH-based therapies currently available, and may be of relevance for the use of adipose-derived stem cells for tissue engineering.

Mechanical stress activates Smad pathway through PKCδ to enhance interleukin-11 gene transcription in osteoblasts

BACKGROUND

Mechanical stress rapidly induces ΔFosB expression in osteoblasts, which binds to interleukin (IL)-11 gene promoter to enhance IL-11 expression, and IL-11 enhances osteoblast differentiation. Because bone morphogenetic proteins (BMPs) also stimulate IL-11 expression in osteoblasts, there is a possibility that BMP-Smad signaling is involved in the enhancement of osteoblast differentiation by mechanical stress. The present study was undertaken to clarify whether mechanical stress affects BMP-Smad signaling, and if so, to elucidate the role of Smad signaling in mechanical stress-induced enhancement of IL-11 gene transcription.

METHODOLOGY/PRINCIPAL FINDINGS

Mechanical loading by fluid shear stress (FSS) induced phosphorylation of BMP-specific receptor-regulated Smads (BR-Smads), Smad1/5, in murine primary osteoblasts (mPOBs). FSS rapidly phosphorylated Y311 of protein kinase C (PKC)δ, and phosphorylated PKCδ interacted with BR-Smads to phosphorylate BR-Smads. Transfection of PKCδ siRNA or Y311F mutant PKCδ abrogated BR-Smads phosphorylation and suppressed IL-11 gene transcription enhanced by FSS. Activated BR-Smads bound to the Smad-binding element (SBE) of IL-11 gene promoter and formed complex with ΔFosB/JunD heterodimer via binding to the C-terminal region of JunD. Site-directed mutagenesis in the SBE and the AP-1 site revealed that both SBE and AP-1 sites were required for full activation of IL-11 gene promoter by FSS.

CONCLUSIONS/SIGNIFICANCE

These results demonstrate that PKCδ-BR-Smads pathway plays an important role in the intracellular signaling in response to mechanical stress, and that a cross-talk between PKCδ-BR-Smads and ΔFosB/JunD pathways synergistically stimulates IL-11 gene transcription in response to mechanical stress.

A subset of osteoblasts expressing high endogenous levels of PPARgamma switches fate to adipocytes in the rat calvaria cell culture model

BACKGROUND

Understanding fate choice and fate switching between the osteoblast lineage (ObL) and adipocyte lineage (AdL) is important to understand both the developmental inter-relationships between osteoblasts and adipocytes and the impact of changes in fate allocation between the two lineages in normal aging and certain diseases. The goal of this study was to determine when during lineage progression ObL cells are susceptible to an AdL fate switch by activation of endogenous peroxisome proliferator-activated receptor (PPAR)gamma.

METHODOLOGY/PRINCIPAL FINDINGS

Multiple rat calvaria cells within the ObL developmental hierarchy were isolated by either fractionation on the basis of expression of alkaline phosphatase or retrospective identification of single cell-derived colonies, and treated with BRL-49653 (BRL), a synthetic ligand for PPARgamma. About 30% of the total single cell-derived colonies expressed adipogenic potential (defined cytochemically) when BRL was present. Profiling of ObL and AdL markers by qRT-PCR on amplified cRNA from over 160 colonies revealed that BRL-dependent adipogenic potential correlated with endogenous PPARgamma mRNA levels. Unexpectedly, a significant subset of relatively mature ObL cells exhibited osteo-adipogenic bipotentiality. Western blotting and immunocytochemistry confirmed that ObL cells co-expressed multiple mesenchymal lineage determinants (runt-related transcription factor 2 (Runx2), PPARgamma, Sox9 and MyoD which localized in the cytoplasm initially, and only Runx2 translocated to the nucleus during ObL progression. Notably, however, some cells exhibited both PPARgamma and Runx2 nuclear labeling with concomitant upregulation of expression of their target genes with BRL treatment.

CONCLUSIONS/SIGNIFICANCE

We conclude that not only immature but a subset of relatively mature ObL cells characterized by relatively high levels of endogenous PPARgamma expression can be switched to the AdL. The fact that some ObL cells maintain capacity for adipogenic fate selection even at relatively mature developmental stages implies an unexpected plasticity with important implications in normal and pathological bone development.

Effect of intermittent treatment with human Parathyroid Hormone 1-34 in SAMP6 senescence-accelerated mice

We examined trabecular and cortical bone in the senescence-accelerated mouse prone 6 (SAMP6) murine model of senile osteoporosis after treatment with human PTH 1-34. Sixteen-week-old female SAMP6 mice were assigned to control and PTH groups. PTH (20 microg/kg) was administered sc 3 times a week for 12 weeks. The control mouse strain, senescence-accelerated mouse resistant 1 (SAMR1), was used for comparison. The femoral metaphysis and diaphysis were used to measure bone mineral density (BMD), analyze the trabecular and the cortical structure by micro-computed tomography, and for conducting the bone strength test. PTH significantly attenuated the loss of BMD, improved the trabecular bone microstructure, and increased the bone strength in the femoral metaphysis. We did not find any differences in the bone strength of the femoral diaphysis after PTH treatment, although the cortical bone volume and cortical thickness were improved. Although the cortical thickness increased, the cortical bone density decreased, likely because of the increase of cortical porosity in the distal metaphysis after administration of PTH.

Bone acquisition in healthy young females is reciprocally related to marrow adiposity

CONTEXT

Considerable evidence indicates that osteoblasts and adipocytes share a common progenitor cell in the bone marrow that is capable of mutually exclusive differentiation into the cell lineages responsible for bone and fat formation.

OBJECTIVE

The purpose of this study was to examine the relation between bone acquisition and changes in marrow adiposity.

DESIGN

This was a longitudinal study. OUTCOME MEASURES AND SUBJECTS: Computed tomography measurements of femoral cortical bone area (CBA), cross-sectional area (CSA), and marrow density, and dual-energy x-ray absorptiometry (DXA) measurements of total body fat and lean mass (LM) were obtained in 39 healthy females (15-20 yr of age) at baseline and 18-24 months later.

RESULTS

Marrow adiposity was inversely related to CBA at baseline and follow-up (r = 0.39 and 0.33; P = 0.015 and 0.039, respectively) but was not associated to CSA (r = 0.19 and 0.17; P = 0.24 and 0.32, respectively). The association between marrow fat and CBA persisted, even after controlling for body mass and DXA values of LM and femoral CSA. Gains in CBA during the course of the study were related to decreases in marrow fat (r = 0.41; P = 0.009), a relation that persisted, even after accounting for changes in bone size. Marrow fat was not associated to anthropometric measures or DXA values of body fat and LM (all r's between -0.15 and 0.19; P > 0.05).

CONCLUSIONS

Bone acquisition in the appendicular skeleton of healthy young females is inversely related to changes in marrow adiposity. These results provide support for the growing body of evidence indicating an inversely coupled relationship between osteogenesis and adipogenesis in the skeleton.

Influence of defective bone marrow osteogenesis on fracture repair in an experimental model of senile osteoporosis

Bone marrow osteogenesis in senile osteoporotic bone is impaired and, as such, may have significant implications on the successful outcome of fracture repair. Here we utilize a well-established murine model of senile osteoporosis, the P6 strain of senescence-accelerated mice (SAMP6), to investigate fracture healing in aged osteoporotic bone. A femoral osteotomy was created in SAMP6 and in non-osteoporotic age-matched control R1 senescence-resistant mice (SAMR1). The course of fracture healing was evaluated over a period of 42 days using quantitative microCT and histological analysis. The differentiation capabilities of bone mesenchymal progenitor cells derived from SAMP6 and SAMR1 mice was examined, and their osteogenic potential determined. Although preliminary in vitro analysis confirmed that bone marrow-derived stem cells (BMSC) isolated from SAMP6 mice had a reduced osteogenic capacity, no significant deficit in fracture repair as determined by quantitative microCT could be detected. This was supported by histology assessment, where complete bridging of the fracture gap was evident by day 28 and was fully healed day 42 in both SAMP6 and SAMR1 mice. Further in vitro studies revealed that periosteal-derived progenitor cells (PDPC) isolated from SAMP6 mice had an osteogenic potential comparable to that observed in SAMR1 mice. In conclusion, fracture healing in SAMP6 mice is not detrimentally affected by impairment of BMSC osteogenesis, suggesting that bone marrow-mediated repair processes are dispensable for normal bone healing in this senile osteoporotic fracture model. Furthermore, the influence of PDPC in the repair process may partly explain the absence of any detectable deficits in fracture repair in SAMP6 mice.

Mechanical stress induces Interleukin-11 expression to stimulate osteoblast differentiation

Molecular mechanism of mechanical stress-induced bone formation remains unclear. We demonstrate that mechanical unloading suppresses and reloading enhances Interleukin (IL)-11 gene expression in the hindlimb of mice in vivo. Mechanical stress to osteoblasts by fluid shear stress (FSS) in vitro rapidly and transiently enhances fosB gene transcription, stimulates binding of DeltaFosB/JunD complex to activator protein (AP)-1 site of the IL-11 gene promoter, and enhances IL-11 gene transcription. Anti-IL-11 antibody blocks mechanical stress-induced enhancement of osteoblastogenesis and suppression of adipogenesis, suggesting the requirement of IL-11 for the stimulation of osteoblast differentiation by mechanical stress. Down-regulation of DeltaFosB/JunD by small interfering RNA (siRNA) suppresses and overexpression of DeltaFosB/JunD enhances IL-11 gene promoter activity. Consistent with our previous observations that up-regulation of DeltaFosB depends upon activation of cyclic AMP response element-binding protein (CREB) via Ca(2+)-dependent activation of extracellular signal-regulated kinase (ERK) to phosphorylate CREB, mechanical stress-induced activation of IL-11 gene transcription is dependent upon Ca(2+)-ERK pathway. Present results also demonstrated that FSS to osteoblasts enhances canonical Wnt signaling in vitro, and that mechanical unloading induces and reloading suppresses the expression of a canonical Wnt signal inhibitor, dickkopf2 (Dkk2), in vivo. In addition, IL-11 siRNA enhances Dkk2 expression suppressed by FSS, and osteoblasts from IL-11 transgenic mice show reduced Dkk2 mRNA expression than those from wild-type mice. These observations are consistent with the notion that mechanical stress stimulates IL-11 gene transcription via an enhanced DeltaFosB/JunD binding to the IL-11 gene promoter, and that increased IL-11 enhances canonical Wnt signal at least in part via a reduction in Dkk2 expression to stimulate osteoblast differentiation.

Osteoclastogenesis by bone marrow-derived macrophages is enhanced in obese mice

Obesity induces a low-grade systemic chronic inflammatory condition for which macrophages are responsible. We hypothesized that obesity affects osteoclastogenesis by acting on bone marrow-derived macrophages (BMM). Male mice were fed a high-fat diet (45% of energy) or a standard diet (10% of energy) for 13 wk. We found that the density of the femurs of obese mice was significantly lower than that of the femurs of lean mice. Osteoclastogenesis was enhanced in the BMM from obese mice. Lower levels of interleukin (IL)-10 were generated by the BMM from obese mice than by those from lean mice upon stimulation of receptor activator of nuclear factor-kappaB ligand. Neutralization of IL-10 in the BMM from obese mice was not as effective in increasing osteoclast (OC) formation as that in those from lean mice. Exogenous IL-10 inhibited OC formation more strongly in the BMM from obese mice than those from lean mice. The elevated level of OC formation in the BMM from obese mice may thus be due to in part to the lower level of IL-10, a negative regulator of osteoclastogenesis. Our results suggest that obesity is associated with bone loss via enhanced osteoclastogenesis due to reduced IL-10 production by the BMM from obese mice.

Reciprocal relation between marrow adiposity and the amount of bone in the axial and appendicular skeleton of young adults

BACKGROUND

Studies in the elderly suggest a reciprocal relation between increased marrow adiposity and bone loss, supporting basic research data indicating that osteoblasts and adipocytes share a common progenitor cell. However, whether this relation represents a preferential differentiation of stromal cells from osteoblasts to adipocytes or whether a passive accumulation of fat as bone is lost and marrow space increases with aging is unknown. To address this question and avoid the confounding effect of bone loss, we examined teenagers and young adults.

METHODS

Using computed tomography, we obtained measurements of bone density and cross-sectional area of the lumbar vertebral bodies and cortical bone area, cross-sectional area, marrow canal area, and fat density in the marrow of the femurs in 255 sexually mature subjects (126 females, 129 males; 15-24.9 yr of age). Additionally, values for total body fat were obtained with dual-energy x-ray absorptiometry.

RESULTS

Regardless of gender, reciprocal relations were found between fat density and measures of vertebral bone density and femoral cortical bone area (r = 0.19-0.39; all P values < or = .03). In contrast, there was no relation between marrow canal area and cortical bone area in the femurs, neither between fat density and the cross-sectional dimensions of the bones. We also found no relation between anthropometric or dual-energy x-ray absorptiometry fat values and measures for marrow fat density.

CONCLUSIONS

Our results indicate an inverse relation between bone marrow adiposity and the amount of bone in the axial and appendicular skeleton and support the notion of a common progenitor cell capable of mutually exclusive differentiation into the cell lineages responsible for bone and fat formation.

Mechanical stimulation of bone formation is normal in the SAMP6 mouse

With aging, the skeleton may have diminished responsiveness to mechanical stimulation. The senescence-accelerated mouse SAMP6 has many features of senile osteoporosis and is thus a useful model to examine how the osteoporotic skeleton responds to mechanical loading. We performed in vivo tibial bending on 4-month-old SAMP6 (osteoporotic) and SAMR1 (control) mice. Loading was applied daily (60 cycles/day, 5 days/week) for 2 weeks at peak force levels that produced estimated endocortical strains of 1,000 and 2,000 microepsilon In a separate group of mice, sham bending was applied. Comparisons were made between right (loaded) and left (nonloaded) tibiae. Tibial bone marrow cells were cultured under osteogenic conditions and stained for alkaline phosphatase (ALP) and alizarin red (ALIZ) at 14 and 28 days, respectively. Tibiae were then embedded in plastic and sectioned, and endocortical bone formation was assessed based on calcein labels. Tibial bending did not alter the osteogenic potential of the marrow as there were no significant differences in ALP or ALIZ staining between loaded and nonloaded bones. Tibial bending activated the formation of endocortical bone in both SAMP6 and SAMR1 mice, whereas sham bending did not elicit an endocortical response. Both groups of mice exhibited bending strain-dependent increases in bone formation rate. We found little evidence of diminished responsiveness to loading in the SAMP6 skeleton. In conclusion, the ability of the SAMP6 mouse to respond normally to an anabolic mechanical stimulus distinguishes it from chronologically aged animals. This finding highlights a limitation of the SAMP6 mouse as a model of senile osteoporosis.

Characterization of mandibular bones in senile osteoporotic mice

At present, little is known about the age-related changes in jaw bones. The aim of this study was to characterize the mandibles of 6 month-old senile osteoporotic mice, SAMP6, and compare with those of age-matched controls, SAMR1. In comparison to SAMR1, SAMP6 showed thinner cortical bone, lower bone volume, and poorly organized collagen matrix. The collagen fibril diameter in SAMP6 was significantly smaller than that of SAMR1. In SAMP6 both collagen content and cross-links were lower than those of SAMR1, but the ratio of the major mature cross-link (pyridinoline) to its precursor reducible cross-link (dehydrodihydroxylysinonorleucine/its ketoamine) was higher in comparison to SAMR1. In addition, the extent of lysine hydroxylation of collagen was higher in SAMP6 than that of SAMR1. These results indicate that not only the quantity of collagen but also its quality are altered in SAMP6 and may result in the age-associated osteoporotic defects of mandibles.

Effects of soluble silicon compound and deep-sea water on biochemical and mechanical properties of bone and the related gene expression in mice

Silicon has been known as an essential element for bone formation. The silicon contents of sea water increase with increasing of depth: 1.8 ppm Si in deep-sea water (DW) at 612 m in depth versus 0.06 ppm in surface sea water (SW). The effects of soluble silicon (Si) and DW from which NaCl was eliminated were studied in comparison with tap water (TW) and SW in cell cultures and in animal experiments using the control strain of senescence accelerated mouse, SAMR1. Si at 10 ppm as sodium metasilicate or 10% DW in the alpha-MEM medium stimulated cellular viability, marker enzymes of osteoblast and osteoclast cell lines, and the (45)CaCl(2) uptake in those cells in comparison with the medium control. After weanling SAMR1 were maintained for 6 months on a diet containing 200 ppm Si and 39% of DW and SW, DW and Si improved bone biochemical indices such as femoral weight, mineral and collagen content, and marker enzymes of bone formation and resorption as well as mechanical properties as compared to TW. In the femoral bone marrow of SAMR1, the mRNA expression of bone morphogenetic protein-2 (BMP-2), interleukin-11 (IL-11), and runt-related transcription factor 2 (Runx 2), which stimulate osteoblast development as well as type I procollagen (COL1A1) mRNA, were significantly increased in both DW and Si groups. The expressions of both osteoprotegerin (OPG) and receptor activator of NF-kappaB ligand (RANKL) were also elevated, resulting in distinct increases of the OPG/RANKL ratio in both DW and Si groups. The results indicated that a soluble silicate and deep-sea water as its natural material stimulated cell growth in both osteoblasts and osteoclasts in cell culture and promoted bone metabolic turnover in favor of bone formation through stimulation of the related mRNA expression in animal experiments.

Longitudinal analysis of mesenchymal progenitors and bone quality in the stem cell antigen-1-null osteoporotic mouse

We performed a longitudinal analysis of bone quality in Sca-1-null mice. A tight temporal, site-specific association between Sca-1-deficient BMD deficiency and reduced mesenchymal progenitor frequency was observed. Defects in trabecular microarchitecture and mineralization were, at least partially, responsible for the age-related reduction in toughness of Sca-1(-/-) bones.

INTRODUCTION

We previously showed that stem cell antigen 1 (Sca-1)-null mice undergo normal bone development but exhibit significantly decreased bone mass characteristic of age-dependent osteoporosis. The objective of this study was to characterize the initiation and progression of the Sca-1 mutant skeletal phenotype at the cellular, structural, material, and mechanical levels.

MATERIALS AND METHODS

Sca-1-null and control mice were analyzed at 3, 5, 7, and 9 mo of age. In vitro osteoclastogenesis of bone marrow and spleen-derived progenitor populations was assessed. Bone marrow-derived mesenchymal progenitor frequency, along with osteogenic and adipogenic differentiation potential, was analyzed in vitro. Static histomorphometry of the sixth lumbar vertebrae was performed. Whole body, femoral, and vertebral BMD were assessed using DXA. Lumbar vertebrae were analyzed using microCT, back-scattered electron imaging, and compression tests. Three-point bending and femoral neck fracture tests were performed on excised femurs.

RESULTS

Sca-1-null mice displayed an age-dependent, cell-autonomous osteoclast deficiency in vitro. From 7 mo of age onward, reduced Sca-1-null femoral BMD was observed alongside reduced mesenchymal progenitor frequency, and decreased in vitro osteogenic and adipogenic differentiation potential. Sca-1-deficient mice exhibited reduced whole body BMD compared with controls at all time-points analyzed. Although no differences in spinal BMD were observed, Sca-1(-/-) vertebrae exhibited decreased bone formation, with a maximal difference at 7 mo of age, inferior trabecular microarchitecture, and a greater degree of mineralization. At all sites tested, Sca-1-null bones exhibited reduced energy to failure from 5 mo onward.

CONCLUSIONS

We showed a tight association within Sca-1-null mice between the initiation of stem cell defects and the exacerbation of deficiencies in bone quality at two sites clinically relevant to developing osteoporotic fractures. Sca-1-deficient mice, therefore, provide a novel and useful murine model of age-related osteoporosis, which with additional study, should further our understanding of the mechanisms underlying this increasingly prevalent disease.

Serum from postmenopausal women directs differentiation of human clonal osteoprogenitor cells from an osteoblastic toward an adipocytic phenotype

A consistent observation in osteoporosis is bone volume reduction accompanied by increased marrow adipose tissue. No single cause linking the two phenomena has yet been identified. In a human progenitor cell clone (hOP 7) derived from bone marrow, however, we have demonstrated that rabbit serum can direct differentiation away from an osteoblast lineage to one of adipocytes. We now report on whether human serum has a similar effect. Serum was collected from 10 pre- and 10 postmenopausal women and from the 10 postmenopausal women before and following 6-week hormone replacement therapy (HRT). hOP 7 cells were cultured with the various sera, and after 7-14 days adipocytogenesis was determined by oil red O staining and lipoprotein lipase (LPL) and glycerol 3-phosphate dehydrogenase (G3PDH) expression. Incubation with 10% premenopausal serum led to labeling of 10.9% of cells (P < 0.05) with oil red O, whereas application of 10% postmenopausal serum led to a much larger effect, 43.5% labeling (P < 0.001 with respect to premenopausal serum). Oil red O positivity was accompanied by loss of type I collagen expression and increased LPL and G3PDH expression. HRT did not reverse the adipocytogenic effect of postmenopausal serum. In conclusion, serum from postmenopausal women contains factors that steer hOP 7 bone progenitor cells toward an adipocytic phenotype, irrespective of HRT. The study suggests a role for serum factors in the development of fatty marrow in postmenopausal osteoporosis.

The function of connexin 43 on the differentiation of rat bone marrow cells in culture

Connexin (Cx) 43-mediated gap-junctional intercellular communication (GJC) mainly regulates the osteoblastic differentiation, but much of the function of Cx43 on the differentiation of bone marrow cells is unclear. This study is aimed to clarify relationship between the differentiation of rat bone marrow cells and the function of Cx43. Bone marrow cells derived from four-week-old Wistar strain rats were grown in the presence and absence of 18-alpha-glycyrrhetinic acid (AGA, 100 muM) to inhibit Cx43-mediated GJC. Expression of Cx43 gene and protein, and the level of intracellular cyclic adenosine monophosphate (cAMP) were determined as the assessment of the function in Cx43-mediated GJC, and alkaline phosphatase (ALP) activity and mineralization were measured as the assessment of osteoblastic differentiation. The Cx43 gene expression was first observed at 2 days, but under the condition in which rat bone marrow cells were treated with AGA, there was no significant effect on the Cx43 gene expression. By administrating AGA to rat bone marrow cells, all parameters of maturation but the Cx43 gene expression significantly decreased. The results of this experiment suggest that Cx43-mediated GJC plays a critical role in rat bone marrow cells, progress toward maturation.

Heme oxygenase-1 and interleukin-11 are overexpressed in stress-induced premature senescence of human WI-38 fibroblasts induced by tert-butylhydroperoxide and ethanol

Acute repeated exposures to subcytotoxic concentrations of tert-butylhydroperoxide and ethanol trigger premature senescence of human diploid fibroblasts. In the present work we found an increased mRNA and protein level of interleukin-11 and heme oxygenase-1 in premature senescence of WI-38 human diploid foetal lung fibroblasts induced by both tert-butylhydroperoxide and ethanol. We tested whether interleukin-11 and heme oxygenase-1 could protect against tert-butylhydroperoxide- or ethanol-induced premature senescence when stable overexpression was established using a retroviral vector-based transduction. No protective effect was found against the decrease of the proliferative potential, the increase of the proportion of senescence-associated ss-galactosidase positive cells and the increase of the mRNA levels of six senescence-associated genes.

A critical damping approach for assessing the role of marrow fat on the mechanical strength of trabecular bone

Several clinical findings revealed that post-menopausal osteoporosis and age-related osteopenia are accompanied by trabecular bone marrow fat (BMF) increase. To help understand this phenomenon, a vibrating string model is proposed, based on the hypothesis that, when bone marrow properties change, the trabecular bone structure remodels itself to preserve its critical damping state. It is found that an inverse relationship holds between trabecular average length and marrow damping coefficient. Such a result leads us to hypothesize the following bone-weakening mechanism. Since fat-rich bone marrow is a worse damper, a BMF increment causes an increase of trabecular average length, which is accomplished by the absorption of horizontal trabeculae (structurally less important than vertical trabeculae). The resulting bone patterns are in excellent agreement with clinical observations of osteoporotic bone. A definitive confirmation of the proposed mechanism will support a therapeutical approach to widespread osteopenic diseases aimed at avoiding, or limiting, BMF increase.

Molecular backgrounds of age-related osteoporosis from mouse genetics approaches

Backgrounds underlying age-related bone loss can be classified into two categories: systemic abnormality and osteoblast dysfunction. The former includes insufficiency of vitamin D or estrogen, causing a negative balance of calcium metabolism. We propose the contribution of an aging-suppressing gene, klotho, as a novel systemic factor, as a mouse deficient in the klotho gene exhibits multiple aging phenotypes including osteopenia with a low bone turnover. As a factor intrinsic to osteoblasts, we investigated the role of PPARgamma, a key regulator of adipocyte differentiation, based on the facts that osteoblasts and adipocytes share a common progenitor. Heterozygous PPARgamma-deficient mice exhibited high bone mass by stimulating osteoblastogenesis from bone marrow progenitors, and this effect became prominent with aging, indicating involvement of PPARgamma-dependent bone formation in the pathophysiology of age-related bone loss. The local environment of osteoblasts is mainly controlled by cytokines/growth factors, among which insulin-like growth factor-I (IGF-I) is the most possible candidate whose production and activity are decreased with aging. Bone phenotypes of deficient mice of insulin receptor substrates (IRS-1 and IRS-2), essential molecules for intracellular signaling of IGF-I, revealed that IRS-1 is essential to maintain bone turnover by up-regulating anabolic and catabolic functions of osteoblasts, while IRS-2 is needed to keep the predominance of the anabolic function over the catabolic function. A next task ahead of us will be to elucidate the network system of these factors underlying age-related osteoporosis.

Treatment of senile osteoporosis in SAMP6 mice by intra-bone marrow injection of allogeneic bone marrow cells

A substrain of the senescence-accelerated mouse, SAMP6 (senescence-accelerated mouse prone 6), spontaneously develops osteoporosis early in life. Therefore, this strain is a useful animal model for developing new strategies for the treatment of osteoporosis in humans. We succeeded in treating osteoporosis in SAMP6 mice after the onset of this disease, using a newly developed method of bone marrow transplantation (BMT): Allogeneic bone marrow cells obtained from normal mouse strains were directly injected into the bone marrow cavity of irradiated SAMP6 mice (intra-bone marrow BMT [IBM-BMT]). After the treatment with IBM-BMT, hematolymphoid cells were completely reconstituted by donor-derived cells, and bone marrow stromal cells were also found to be of donor origin. The treated SAMP6 mice showed histologically-normal trabecular bone. In addition, bone mineral density and urinary deoxypiridinoline, a hallmark of bone destruction, were normalized. When the message levels of cytokines (tumor necrosis factor alpha, interleukin-6 [IL-6], IL-11, and receptor activator of nuclear factor-kappa B ligand [RANKL]) were examined, IL-11, RANKL (from bone marrow stromal cells), and IL-6 (from osteoclasts), which regulate bone remodeling, were restored to levels similar to those in normal B6 mice. These findings indicate that not only the hemopoietic system but also the bone marrow microenvironment were normalized after IBM-BMT, resulting in an amelioration of the imbalance between bone absorption and formation.

1,25(OH)2D3 acts as a bone-forming agent in the hormone-independent senescence-accelerated mouse (SAM-P/6)

Recent studies suggest that vitamin D signaling regulates bone formation. However, the overall effect of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on bone turnover in vivo is still unclear. In this study, our aim was to examine the effect of 1,25(OH)2D3 on bone turnover in SAM-P/6, a hormone-independent mouse model of senile osteoporosis characterized by a decrease in bone formation. Male and female 4-mo-old SAM-P/6 mice were treated with 1,25(OH)2D3 (18 pmol/24 h) or vehicle for a period of 6 wk, and a group of age- and sex-matched nonosteoporotic animals was used as control. Bone mineral density (BMD) at the lumbar spine increased rapidly by >30 +/- 5% (P < 0.001) in 1,25(OH)2D3-treated SAM-P/6 animals, whereas BMD decreased significantly by 18 +/- 2% (P < 0.01) in vehicle-treated SAM-P/6 animals and remained stable in control animals during the same period. Static and dynamic bone histomorphometry indicated that 1,25(OH)2D3 significantly increased bone volume and other parameters of bone quality as well as subperiosteal bone formation rate compared with vehicle-treated SAM-P/6 mice. However, no effect on trabecular bone formation was observed. This was accompanied by a marked decrease in the number of osteoclasts and eroded surfaces. A significant increase in circulating bone formation markers and a decrease in bone resorption markers was also observed. Finally, bone marrow cells, obtained from 1,25(OH)2D3-treated animals and cultured in the absence of 1,25(OH)2D3, differentiated more intensely into osteoblasts compared with those derived from vehicle-treated mice cultured in the same conditions. Taken together, these findings demonstrate that 1,25(OH)2D3 acts simultaneously on bone formation and resorption to prevent the development of senile osteoporosis.

Impaired marrow osteogenesis is associated with reduced endocortical bone formation but does not impair periosteal bone formation in long bones of SAMP6 mice

We used the SAMP6 osteoporotic mouse to examine the link between marrow osteogenic potential and in vivo cortical bone formation. SAMP6 marrow supported less in vitro osteogenesis than marrow from SAMR1 controls; SAMP6 mice had a corresponding deficit in endocortical mineralizing surface. This marrow/endocortical defect did not affect the periosteum, where SAMP6 mice had normal to enhanced bone formation.

INTRODUCTION

With aging, there may be a reduction in the number or proliferative capacity of bone marrow osteoprogenitors that may contribute to age-related decreases in bone formation. To examine the link between the ability of the marrow to support osteogenesis and age-related changes in bone formation, we measured in vitro and in vivo indices of osteogenesis in a model of osteoporosis, the senescence-accelerated mouse SAMP6.

MATERIALS AND METHODS

Femora and tibias from SAMP6 and SAMR1 (control) mice were harvested at 2, 4, 6, and 12 months of age (168 bones total). Bone marrow cells were cultured under osteogenic conditions and stained for alkaline phosphatase (ALP) and alizarin red. Dynamic indices of bone formation were assessed histologically from calcein labels.

RESULTS

ALP+ and alizarin red-positive areas were significantly less in cultures from SAMP6 bones versus SAMR1 (p < 0.05), indicating less osteogenic potential. For example, SAMP6 tibial cultures had 21% less ALP+ area and 36% less alizarin red-positive area than SAMR1. Marrow from tibias had 2-fold greater osteogenesis than femoral marrow (p < 0.001). SAMP6 mice had a deficit in endocortical mineralizing surface across all age groups (p < 0.05), but no deficit in mineral apposition rate. Last, despite the marrow and endocortical deficits, SAMP6 mice had normal or slightly increased periosteal bone formation, consistent with their larger bone size.

CONCLUSION

SAMP6 bone marrow supports less in vitro osteogenesis than SAMR1, consistent with a lower concentration of marrow osteoprogenitors in SAMP6. SAMP6 mice have less endocortical mineralizing surface than SAMR1 at all ages but no detectable deficit in mineral apposition rate, which suggests a reduction in osteoblast number but normal function. Periosteal bone formation is unimpaired in SAMP6 mice, indicating that the marrow/endocortical defect does not affect the periosteal surface.

Morphological study of the parathyroid gland and thyroid C cell in senescence-accelerated mouse (SAMP6), a murine model for senile osteoporosis

SAMP6, a substrain of senescence-accelerated mouse, was developed as an animal model for senile osteoporosis. We investigated the morphology of the parathyroid gland and thyroid C cell, together with the serum parathyroid hormone (PTH) and calcitonin (CT) in SAMP6 and age-matched normal mice SAMR1. We did not find any significant differences between SAMR1 and SAMP6 at 1 month of age with regard to the serum PTH level and the morphology of the parathyroid glands. As compared with SAMR1, the serum PTH level was significantly higher in SAMP6 at 2, 5 and 12 months of age. In the parathyroid chief cells of SAMP6 at 2, 5 and 12 months of age, the Golgi complexes and the cisternae of the granular endoplasmic reticulum were well developed. Numerous secretory granules were located near the plasma membranes and mitoses were sometimes observed. There was no marked difference between SAMR1 and SAMP6 regarding the morphology of the thyroid C cells and the serum CT level. These findings suggest that the secretory activity of the parathyroid gland is stimulated in SAMP6 at 2, 5 and 12 months of age. The parathyroid follicle was sometimes found in SAMP6, and the significance of this structure was also discussed.

Morphological and mechanical properties of caudal vertebrae in the SAMP6 mouse model of senile osteoporosis

The senescence-accelerated mouse, strain P6 (SAMP6), is a model of senile osteoporosis with relatively low bone mineral density (BMD), low rates of bone formation and reduced long-bone bending strength. Seeking to extend previous descriptions of the SAMP6 skeletal phenotype, we assessed the morphological and mechanical properties of vertebrae from SAM mice at 4 and 12 months of age. We hypothesized that, relative to SAMR1 controls, vertebrae from SAMP6 mice have: (1) less trabecular bone, (2) increased endosteal and periosteal bone size and (3) decreased whole-bone strength. Caudal vertebrae from adult female mice (SAMR1 and SAMP6; 4 and 12 months; n = 10-11 per group) were evaluated by micro-computed tomography and mechanical compression testing. SAMP6 vertebrae had 33% less trabecular bone volume in cephalad and caudal end regions than SAMR1 (P < 0.001) due to significant reductions in both trabecular number (P = 0.002) and thickness (P < 0.001). In contrast to previous findings in SAMP6 long bones, SAMP6 vertebrae showed no evidence of increased overall bone dimensions compared to SAMR1, and in fact had smaller total bone area (TA) and endosteal area (EA) at 12 months of age. Whereas SAMR1 vertebrae showed signs of age-related expansion from 4 to 12 months, SAMP6 vertebral dimensions did not change with age. Mechanical properties of SAMP6 vertebrae were not significantly different than SAMR1 vertebrae (stiffness, yield force, ultimate force, displacement at ultimate force, energy to ultimate force; P > 0.05), another finding in contrast to previous results in SAMP6 long bones. In summary, reduced vertebral trabecular bone volume is another feature of the SAMP6 mice with relevance to senile osteoporosis. However, age-related bone expansion and reduced whole-bone strength were not evident in SAMP6 vertebrae, indicating that while the SAMP6 mouse has many features relevant to senile osteoporosis in humans, not all features are observed or detectable at all skeletal sites.

PPARgamma insufficiency enhances osteogenesis through osteoblast formation from bone marrow progenitors

Based on the fact that aging is associated with a reciprocal decrease of osteogenesis and an increase of adipogenesis in bone marrow and that osteoblasts and adipocytes share a common progenitor, this study investigated the role of PPARgamma, a key regulator of adipocyte differentiation, in bone metabolism. Homozygous PPARgamma-deficient ES cells failed to differentiate into adipocytes, but spontaneously differentiated into osteoblasts, and these were restored by reintroduction of the PPARgamma gene. Heterozygous PPARgamma-deficient mice exhibited high bone mass with increased osteoblastogenesis, but normal osteoblast and osteoclast functions, and this effect was not mediated by insulin or leptin. The osteogenic effect of PPARgamma haploinsufficiency became prominent with aging but was not changed upon ovariectomy. The PPARgamma haploinsufficiency was confirmed to enhance osteoblastogenesis in the bone marrow cell culture but did not affect the cultures of differentiated osteoblasts or osteoclast-lineage cells. This study demonstrates a PPARgamma-dependent regulation of bone metabolism in vivo, in that PPARgamma insufficiency increases bone mass by stimulating osteoblastogenesis from bone marrow progenitors.

Decreased AP-1 activity and interleukin-11 expression by bone marrow stromal cells may be associated with impaired bone formation in aged mice

Expression of an osteogenic cytokine, IL-11, is decreased in SAMP6. We show here that IL-11 transcription largely depends on AP-1 transcription factors, activities of which are decreased in SAMP6 as well as aged ICR mice. Therefore, diminished AP-1 activity and the resultant decline in IL-11 expression may play a role in impaired bone formation in the aged.

INTRODUCTION

Evidence suggests that impaired osteoblastogenesis contributes to aging-associated osteopenia. The P6 strain of senescence-accelerated mice (SAM) is an animal model of senile osteoporosis, which exhibits low bone mass caused by impaired bone formation. Bone marrow stromal cells from SAMP6 show decreased osteoblastogenesis and increased adipogenesis. We previously demonstrated that these abnormalities of SAMP6 stromal cells may be attributed to decreased expression of interleukin (IL)-11.

METHODS

In this study, we attempted to determine the molecular mechanism of decreased IL-11 expression by SAMP6 stromal cells by cloning and analyzing the mouse IL-11 gene promoter.

RESULTS AND CONCLUSIONS

We found that two tandem activating protein-1 (AP-1) sites that reside immediately upstream of TATA box play critical roles in IL-11 gene transcription. Gel shift analysis showed that binding activity to the IL-11 AP-1 sites was reduced in SAMP6 stromal cell nuclear extracts. Among multiple components of AP-1 transcription factors, Jun D binding was particularly decreased. Furthermore, decreased Jun D binding and IL-11 expression by stromal cells was also observed in aged mice of the ICR strain. Therefore, decreased AP-1 activity and a resultant decline in IL-11 expression by bone marrow stromal cells may play a role in impaired bone formation in the aged.

The role of stem cells in aging

The objectives of this review were first to critically review what is known about the effects of aging on stem cells in general, and hematopoietic stem cells in particular. Secondly, evidence is marshalled in support of the hypothesis that aging stem cells play a critical role in determining the effects of aging on organ function, and ultimately on the lifespan of a mammal. Aging has both quantitative and qualitative effects on stem cells. On balance, the qualitative changes are the more important since they affect the self-renewal potential, developmental potential, and interactions with extrinsic signals, including those from stroma. Although hematopoiesis is generally maintained at normal and life-supporting levels during normal aging, diminished function is acutely apparent when old stem cells are subjected to stress. There is ample evidence of diminished self-renewal capacity, restriction of the breadth of developmental potency, and decreased numbers of progeny of old stem cells subjected to hematopoietic demands. The prediction is made that whatever plasticity in developmental potential possessed by a young stem cell is lost during aging. Those parts of the world enjoying an ever-increasing standard of living are also inhabited by an increasingly elderly population. The effects of age on many physiological functions are not well studied or appreciated. A public health challenge to provide increased quality of life for this growing segment of the population requires more attention to the variable of age in experimental studies. Stem cell populations are likely to be a fruitful subject for studies of this type.

Impaired osteoclast formation in bone marrow cultures of Fgf2 null mice in response to parathyroid hormone

Fibroblast growth factor (FGF)-2 and parathyroid hormone (PTH) are potent inducers of osteoclast (OCL) formation, and PTH increases FGF-2 mRNA and protein expression in osteoblasts. To elucidate the role of endogenous FGF-2 in PTH responses, we examined PTH-induced OCL formation in bone marrow cultures from wild type and mice with a disruption of the Fgf2 gene. FGF-2-induced OCL formation was similar in marrow culture from both genotypes. In contrast, PTH-stimulated OCL formation in bone marrow cultures or co-cultures of osteoblast-spleen cells from Fgf2-/mice was significantly impaired. PTH increased RANKL mRNA expression in osteoblasts cultures from both genotypes. After 6 days of treatment, osteoprotegerin protein in cell supernatants was 40-fold higher in vehicle-treated and 30-fold higher in PTH-treated co-cultures of osteoblast and spleen cells from Fgf2-/mice compared with Fgf2+/+ mice. However, a neutralizing antibody to osteoprotegerin did not rescue reduced OCL formation in response to PTH. Injection of PTH caused hypercalcemia in Fgf2+/+ but not Fgf2-/mice. We conclude that PTH stimulates OCL formation and bone resorption in mice in part by endogenous FGF-2 synthesis by osteoblasts. Because RANKL- and interleukin-11-induced OCL formation was also reduced in bone marrow cultures from Fgf2-/mice, we further conclude that endogenous FGF-2 is necessary for maximal OCL formation by multiple bone resorbing factors.