Pharmacological inhibition of PPARγ increases osteoblastogenesis and bone mass in male C57BL/6 mice

Norisoboldine, a Natural Alkaloid from Lindera aggregata (Sims) Kosterm, Promotes Osteogenic Differentiation via S6K1 Signaling Pathway and Prevents Bone Loss in OVX Mice

SCOPE

Norisoboldine (NOR) is a major isoquinoline alkaloid component in the traditional Chinese herbal plant Lindera aggregata (Sims) Kosterm, with previously reported anti-osteoclast differentiation and antiarthritis properties. However, the roles of NOR on osteoblasts, bone marrow mesenchymal stem cells (BMSCs), and osteoporosis in vivo have never been well established.

METHODS AND RESULTS

This study investigates the ability of NOR to improve bone formation in vitro and in vivo. Osteoblasts and BMSCs are used to study the effect of NOR on osteogenic and adipogenic differentiation. It finds that NOR promotes osteogenic differentiation of osteoblasts and BMSCs, while inhibiting adipogenic differentiation of BMSCs by reducing the relative expression of peroxisome proliferator-activated receptor γ (Ppar-γ) and adiponectin, C1Q and collagen domain containing (Adipoq). Mechanistic studies show that NOR increases osteoblast differentiation through the mechanistic target of rapamycin kinase (mTOR)/ribosomal protein S6 kinase; polypeptide 1 (S6K1) pathway, and treatment with an mTOR inhibitor rapamycin blocked the NOR-induced increase in mineral accumulation. Finally, the study evaluates the therapeutic potential of NOR in a mouse model of ovariectomy (OVX)-induced bone loss. NOR prevents bone loss in both trabecular and cortical bone by increasing osteoblast number and phospho-S6K1 (p-S6K1) expression in osteoblasts.

CONCLUSION

NOR effects in enhancing osteoblast-induced bone formation via S6K1 pathway, suggesting the potential of NOR in osteoporosis treatment by increasing bone formation.

Exploring the Interplay between Bone Marrow Stem Cells and Obesity

Obesity, a complex disorder with rising global prevalence, is a chronic, inflammatory, and multifactorial disease and it is characterized by excessive adipose tissue accumulation and associated comorbidities. Adipose tissue (AT) is an extremely diverse organ. The composition, structure, and functionality of AT are significantly influenced by characteristics specific to everyone, in addition to the variability connected to various tissue types and its location-related heterogeneity. Recent investigation has shed light on the intricate relationship between bone marrow stem cells and obesity, revealing potential mechanisms that contribute to the development and consequences of this condition. Mesenchymal stem cells within the bone marrow, known for their multipotent differentiation capabilities, play a pivotal role in adipogenesis, the process of fat cell formation. In the context of obesity, alterations in the bone marrow microenvironment may influence the differentiation of mesenchymal stem cells towards adipocytes, impacting overall fat storage and metabolic balance. Moreover, bone marrow's role as a crucial component of the immune system adds another layer of complexity to the obesity-bone marrow interplay. This narrative review summarizes the current research findings on the connection between bone marrow stem cells and obesity, highlighting the multifaceted roles of bone marrow in adipogenesis and inflammation.

The effects of PPARγ inhibitor on bones and bone marrow fat in aged glucocorticoid-treated female rats

Progressive bone marrow (BM) fat accumulation is a common bone loss characteristic in older populations and glucocorticoid (GC)-induced skeletal destruction that is inversely associated with bone synthesis and directly associated with increased peroxisomal proliferator-activated receptor gamma (PPARγ) expression. PPARγ inhibition is an efficient therapeutic strategy for aged- and GC-related skeletal disorders. This study aimed to evaluate the effect of PPARγ inhibition on aged GC-treated female rats. It was hypothesised that bisphenol A diglycidyl ether (BADGE) could inhibit marrow adiposity and improve osteogenesis by inhibiting PPARγ, thereby preventing GC-induced osteoporosis (GIO). Female Sprague-Dawley rats (n = 32, age = 18 months) were randomly allocated to one of the following groups: (1) control, (2) BADGE (30 mg/kg/day, intraperitoneal), (3) methylprednisolone (MP; 30 mg/kg/day, subcutaneous), and (4) MP + BADGE. After eight weeks of treatment, bone density (BD) and trabecular bone microarchitectures were quantified by micro-computed tomography (CT), and BM adipocytes were quantified by histopathology. Additionally, mRNA and protein expression of adipogenic and osteogenic markers were quantified by reverse transcription-quantitative polymerase chain reaction. Furthermore, serum bone turnover biomarker levels were quantified by enzyme-linked immunosorbent assay. MP treatment led to marrow adipogenesis and bone deterioration. However, rats treated with MP + BADGE showed lower marrow adipogenesis, as indicated by smaller marrow adipocyte diameter, decreased density and area percentages, reduced expression of marrow adipogenic genes and proteins, improved BD and trabecular microarchitectures, increased expression of osteogenic genes and proteins, and higher levels of serum bone formation markers. These results were consistent with the differences observed between control and BADGE mono-treated rats. In conclusion, BADGE treatment attenuates BM adiposity and improves bone formation in aged GC-treated female rats by inhibiting PPARγ. Therefore, PPARγ might be a potential target for treating GIO in older populations.

Nanoparticles of naturally occurring PPAR-γ inhibitor betulinic acid ameliorates bone marrow adiposity and pathological bone loss in ovariectomized rats via Wnt/β-catenin pathway

AIMS

Postmenopausal osteoporosis is one of the world's biggest yet unnoticed health issues. After ovariectomy, declined estrogen level significantly contributes to the elevation of bone marrow adiposity and bone loss leading to osteoporosis. Therapeutics to prevent osteoporosis addressing various aspects are now in short supply. In this study we made an approach to synthesize nanoparticles of naturally occurring PPAR-γ inhibitor, betulinic acid (BA/NPs) and tested the same in altered bone metabolisms developed after ovariectomy.

MAIN METHODS

The osteogenic efficacy of BA/NPs was established in human and rat primary osteoblast cells using qRT-PCR and immunoblot analysis. Furthermore, lineage allocations of multipotent bone marrow stromal cells were evaluated. Various aspects of altered bone metabolism after ovariectomy such as bone marrow adiposity and pathological bone loss were evaluated using μCT and histological assessments.

KEY FINDINGS

BA/NPs exert potential osteogenic efficacy by modulating RUNX2 and BMP2. Mechanistically BA/NPs regulate osteoblastogenesis through Wnt/β-catenin signaling. Further, BA/NPs showed the potential to inhibit the differentiation of multipotent BMSCs towards adipogenesis while favouring the osteogenic lineage. In the in vivo study, increased bone marrow adiposity was reduced in ovariectomized rats after BA/NPs treatment as assessed by histology and μCT analysis. Loss of bone mineral density as a hallmark of pathological bone loss was also abrogated by BA/NPs.

SIGNIFICANCE

Our findings imply that BA/NPs could be used further as a viable drug lead to counteract various pathophysiological challenges after menopause.

Evaluation of the expression pattern and diagnostic value of PPARγ in malignant and benign primary bone tumors

PURPOSE

The quantifiable description of PPARγ expression pattern beside mechanistic in-vitro evidence will provide insights into the involvement of this mediator in tumor pathogenesis. This study is focused on illuminating the PPARγ gene and protein expression pattern, its association with tumor deterioration and its diagnostic value in different types of primary bone tumors.

METHODS

The expression pattern of PPARγ was investigated in the 180 bone tissues including 90 bone tumor tissues and 90 non-cancerous bone tissues. The local PPARγ expression level was assessed using real-time qRT-PCR and the PPARγ protein expression pattern was measured using immunohistochemistry. The correlation of PPARγ expression level with patients' clinic-pathological features, also the value of the variables in predicting PPARγ expression level in tumors and the value of PPARγ to discriminate tumor subtypes were assessed.

RESULTS

The mean PPARγ mRNA expression was significantly higher in bone tumors compared to healthy bone tissues, also the malignant tumors including osteosarcoma and Ewing sarcoma had the elevated level of PPARγ mRNA compared to GCT tumors. Consistently, the protein expression of PPARγ in the tumor site was significantly higher in the bone tumors and malignant tumors compared to non-cancerous and benign tumors, respectively. The PPARγ protein could predict malignant tumor features including tumor grade, metastasis and recurrence significantly. Moreover, PPARγ could potentially discriminate the patients from the controls also malignant tumors from benign tumors with significant sensitivity and specificity.

CONCLUSIONS

PPARγ might be involved in primary bone tumor pathogenesis and determining its molecular mechanism regarding bone cancer pathogenesis is of grave importance.

Genetic and pharmacologic suppression of PPARγ enhances NELL-1-stimulated bone regeneration

Recent investigations into mechanisms behind the development of osteoporosis suggest that suppressing PPARγ-mediated adipogenesis can improve bone formation and bone mineral density. In this study, we investigated a co-treatment strategy to enhance bone formation by combining NELL-1, an osteogenic molecule that has been extensively studied for its potential use as a therapeutic for osteoporosis, with two methods of PPARγ suppression. First, we suppressed PPARγ genetically using lentiviral PPARγ-shRNA in immunocompromised mice for a proof of concept. Second, we used a PPARγ antagonist to suppress PPARγ pharmacologically in immunocompetent senile osteopenic mice for clinical transability. We found that the co-treatment strategy significantly increased bone formation, increased the proliferation stage cell population, decreased late apoptosis of primary mouse BMSCs, and increased osteogenic marker mRNA levels in comparison to the single agent treatment groups. The addition of PPARγ suppression to NELL-1 therapy enhanced NELL-1's effects on bone formation by upregulating anabolic processes without altering NELL-1's inhibitory effects on osteoclastic and adipogenic activities. Our findings suggest that combining PPARγ suppression with therapeutic NELL-1 may be a viable method that can be further developed as a novel strategy to reverse bone loss and decrease marrow adiposity in age-related osteoporosis.

PTHG2 Reduces Bone Loss in Ovariectomized Mice by Directing Bone Marrow Mesenchymal Stem Cell Fate

Teriparatide, also known as 1-34 parathyroid hormone (PTH (1-34)), is commonly used for the treatment of osteoporosis in postmenopausal women. But its therapeutic application is restricted by poor metabolic stability, low bioavailability, and rapid clearance. Herein, PTHG2, a glycosylated teriparatide derivative, is designed and synthesized to improve PTH stability and exert more potent antiosteoporosis effect. Surface plasmon resonance (SPR) analysis shows that PTHG2 combines to PTH 1 receptor. Additional acetylglucosamine covalent bonding in the first serine at the N terminal of PTH (1-34) improves stability and increases protein hydrolysis resistance. Intermittent administration of PTHG2 preserves bone quality in ovariectomy- (OVX-) induced osteoporosis mice model, along with increased osteoblastic differentiation and bone formation, and reduced marrow adipogenesis. In vitro, PTHG2 inhibits adipogenic differentiation and promotes osteoblastic differentiation of bone marrow mesenchymal stem cells (BMSCs). For molecular mechanism, PTHG2 directs BMSCs fate through stimulating the cAMP-PKA signaling pathway. Blocking PKA abrogates the pro-osteogenic effect of PTHG2. In conclusion, our study reveals that PTHG2 can accelerate osteogenic differentiation of BMSCs and inhibit adipogenic differentiation of BMSCs and show a better protective effect than PTH (1-34) in the treatment of osteoporosis.

Effects of Total Flavonoids of Epimedium on Bone Marrow Adipose Tissue in Ovariectomized Rats

Bone marrow adipose tissue has brown fat characteristics. Several studies have demonstrated that total flavonoids of Epimedium (TFE) could prevent bone loss and reduce the white adiposity in bone marrow induced by ovariectomy (OVX) in rats. However, the effects of TFE on marrow brown fat in OVX rats remain unclear. In this word, we addressed this question expected to provide a new target for preventing and treating osteoporosis. Thirty-six 3-month-old female Sprague-Dawley rats were equally divided into Sham controls, OVX controls, and OVX treated with TFE. Chemical shift coding magnetic resonance was performed to detect marrow fat fraction at the left femur at baseline, 6 and 12 weeks post-OVX. Bone mineral density at the lumbar spine and femur was measured by dual-energy x-ray absorptiometry. Serum bone biomarkers by ELISA, trabecular bone microarchitecture at the proximal tibia by micro-CT, quantitative parameters of marrow adipocyte by hematoxylin, and eosin staining were evaluated. The marrow adipocyte gene and protein expressions profile were determined by real-time quantitative PCR and immunostaining in whole tibiae. We found that TFE treatment could decrease bone turnover rate and improved bone mineral density and trabecular microarchitecture in OVX rats. OVX resulted in marrow adipogenesis as evidenced by increased marrow fat fraction, larger marrow adipocyte size, increased adipocyte number and percentage of adipocyte area, marrow white adipocyte gene, and protein expression, including PPARγ2 and FABP4. These pathological changes induced by estrogen deficiency were restored by TFE treatment. TFE also increased brown adipocyte expressions of the transcription factor Ucp1 and Prdm16 in whole tibiae. There was no detectible protein expression of brown adipocyte markers in the proximal tibia. Taken together, TFE regulation of bone marrow adiposity in OVX rats is mediated, at least in part, via maintaining the reciprocity of white and brown adipose tissue.

Bone marrow adiposity during pathologic bone loss: molecular mechanisms underlying the cellular events

Bone marrow (BM) is a heterogeneous niche where bone marrow stromal cells (BMSCs), osteoblasts, osteoclasts, adipocytes, hematopoietic cells, and immune cells coexist. The cellular composition of BM changes with various pathophysiological states. A reduction in osteoblast number and a concomitant increase in adipocyte number in aging and pathological conditions put bone marrow adipose tissue (BMAT) into spotlight. Accumulating evidence strongly supports that an overwhelming production of BMAT is a major contributor to bone loss disorders. Therefore, BMAT-targeted therapy can be an efficient and feasible intervention for osteoporosis. However, compared to blocking bone-destroying molecules produced by BMAT, suppressing BMAT formation is theoretically a more effective and fundamental approach in treating osteoporotic bone diseases. Thus, a deep insight into the molecular basis underlying increased BM adiposity during pathologic bone loss is critical to formulate strategies for therapeutically manipulating BMAT. In this review, we comprehensively summarize the molecular mechanisms involved in adipocyte differentiation of BMSCs as well as the interaction between bone marrow adipocytes and osteoclasts. More importantly, we further discuss the potential clinical implications of therapeutically targeting the upstream of BMAT formation in bone loss diseases.

Deletion of PPARγ in Mesenchymal Lineage Cells Protects Against Aging-Induced Cortical Bone Loss in Mice

Bone loss in aging is linked with chronic low-grade inflammation and the accumulation of marrowfat in animals and humans. Peroxisome proliferator-activated receptor gamma (PPARγ), an adipogenic regulator, plays key roles in these biological processes. However, studies of the roles of PPARγ in age-related bone loss and inflammation are lacking. We hypothesized that deletion of PPARγ in bone marrow mesenchymal lineage cells would reduce bone loss with aging, potentially through a reduction in fat-generated inflammatory responses and an increase in osteoblastic activity. In the present study, we show that mice deficient of PPARγ in Dermo1-expressing mesenchymal lineage cells (Dermo1-Cre:PPARγ fl/fl) have reduced fat mass and increased cortical bone thickness but that deficiency of PPARγ had limited effect on protection of trabecular bone with aging as demonstrated by dual-energy X-ray absorptiometry, µCT, and histomorphometric analyses. Conditional knockout of PPARγ reduced serum concentrations of adipokines, including adiponectin, resistin, and leptin, and reduced marrow stromal cell expression levels of inflammation-related genes. Inflammation genes involved in the interferon signaling pathway were reduced the most. These results demonstrate that disruption of the master adipogenic regulator, PPARγ, has a certain protective effect on aging-induced bone loss, suggesting that regulation of adipose function and modulation of interferon signaling are among the key mechanisms by which PPARγ regulates bone homeostasis during aging process.

Erucic Acid-Rich Yellow Mustard Oil Improves Insulin Resistance in KK-Ay Mice

Obesity is a major risk factor for some metabolic disorders including type 2 diabetes. Enhancement of peroxisome proliferator-activated receptor (PPAR) γ, a master regulator of adipocyte differentiation, is known to increase insulin-sensitive small adipocytes. In contrast, decreased PPARγ activity is also reported to improve insulin resistance. We have previously identified erucic acid as a novel natural component suppressing PPARγ transcriptional activity. In this study, we investigated the effect of erucic acid-rich yellow mustard oil (YMO) on obese/diabetic KK-A^y mice. An in vitro luciferase reporter assay and mesenchymal stem cell (MSC) differentiation assay revealed that 25 µg/mL YMO significantly inhibited PPARγ transcriptional activity and differentiation of MSCs into adipocytes but promoted their differentiation into osteoblasts. In KK-A^y mice, dietary intake of 7.0% (w/w) YMO significantly decreased the surrogate indexes for insulin resistance and the infiltration of macrophages into adipose tissue. Furthermore, 7.0% YMO increased bone mineral density. These results suggest that YMO can ameliorate obesity-induced metabolic disorders.

Picolinic Acid, a Catabolite of Tryptophan, Has an Anabolic Effect on Bone In Vivo

Fractures attributable to osteoporosis have a severe impact on our older population. Reports of side effects with commonly prescribed osteoporosis drugs have led to the investigation of new and safer treatments with novel mechanisms of action. Picolinic acid (PIC), a catabolite of tryptophan, induces in vitro osteogenic differentiation of mesenchymal stem cells. Here we demonstrate that PIC has an anabolic effect on bone in vivo by increasing bone formation, bone mass, and bone strength in normal and ovariectomized C57BL/6 mice. Activation of the osteogenic pathways triggered this osteoanabolic response without any cross-related effects on mineral absorption or calciotropic hormones. Because PIC was also well tolerated and absorbed with no side effects, it is an ideal potential candidate for the treatment of osteoporosis. © 2020 American Society for Bone and Mineral Research.

Identification of key genes of human bone marrow stromal cells adipogenesis at an early stage

Background

Bone marrow adipocyte (BMA), closely associated with bone degeneration, shares common progenitors with osteoblastic lineage. However, the intrinsic mechanism of cells fate commitment between BMA and osteogenic lineage remains unclear.

Methods

Gene Expression Omnibus (GEO) dataset GSE107789 publicly available was downloaded and analyzed. Differentially expressed genes (DEGs) were analyzed using GEO2R. Functional and pathway enrichment analyses of Gene Ontology and Kyoto Encyclopedia of Genes and Genomes were conducted by The Database for Annotation, Visualization and Integrated Discovery and Gene set enrichment analysis software. Protein-protein interactions (PPI) network was obtained using STRING database, visualized and clustered by Cytoscape software. Transcriptional levels of key genes were verified by real-time quantitative PCR in vitro in Bone marrow stromal cells (BMSCs) undergoing adipogenic differentiation at day 7 and in vivo in ovariectomized mice model.

Results

A total of 2,869 DEGs, including 1,357 up-regulated and 1,512 down-regulated ones, were screened out from transcriptional profile of human BMSCs undergoing adipogenic induction at day 7 vs. day 0. Functional and pathway enrichment analysis, combined with modules analysis of PPI network, highlighted ACSL1, sphingosine 1-phosphate receptors 3 (S1PR3), ZBTB16 and glypican 3 as key genes up-regulated at the early stage of BMSCs adipogenic differentiation. Furthermore, up-regulated mRNA expression levels of ACSL1, S1PR3 and ZBTB16 were confirmed both in vitro and in vivo.

Conclusion

ACSL1, S1PR3 and ZBTB16 may play crucial roles in early regulation of BMSCs adipogenic differentiation.

The relationship between bone marrow adipose tissue and bone metabolism in postmenopausal osteoporosis

Postmenopausal osteoporosis (PMOP) is a prevalent skeletal disorder associated with menopause-related estrogen withdrawal. PMOP is characterized by low bone mass, deterioration of the skeletal microarchitecture, and subsequent increased susceptibility to fragility fractures, thus contributing to disability and mortality. Accumulating evidence indicates that abnormal expansion of marrow adipose tissue (MAT) plays a crucial role in the onset and progression of PMOP, in part because both bone marrow adipocytes and osteoblasts share a common ancestor lineage. The cohabitation of MAT adipocytes, mesenchymal stromal cells, hematopoietic cells, osteoblasts and osteoclasts in the bone marrow creates a microenvironment that permits adipocytes to act directly on other cell types in the marrow. Furthermore, MAT, which is recognized as an endocrine organ, regulates bone remodeling through the secretion of adipokines and cytokines. Although an enhanced MAT volume is linked to low bone mass and fractures in PMOP, the detailed interactions between MAT and bone metabolism remain largely unknown. In this review, we examine the possible mechanisms of MAT expansion under estrogen withdrawal and further summarize emerging findings regarding the pathological roles of MAT in bone remodeling. We also discuss the current therapies targeting MAT in osteoporosis. A comprehensive understanding of the relationship between MAT expansion and bone metabolism in estrogen deficiency conditions will provide new insights into potential therapeutic targets for PMOP.

Therapeutic potentials and modulatory mechanisms of fatty acids in bone

Bone metabolism is a lifelong process that includes bone formation and resorption. Osteoblasts and osteoclasts are the predominant cell types associated with bone metabolism, which is facilitated by other cells such as bone marrow mesenchymal stem cells (BMMSCs), osteocytes and chondrocytes. As an important component in our daily diet, fatty acids are mainly categorized as long-chain fatty acids including polyunsaturated fatty acids (LCPUFAs), monounsaturated fatty acids (LCMUFAs), saturated fatty acids (LCSFAs), medium-/short-chain fatty acids (MCFAs/SCFAs) as well as their metabolites. Fatty acids are closely associated with bone metabolism and associated bone disorders. In this review, we summarized the important roles and potential therapeutic implications of fatty acids in multiple bone disorders, reviewed the diverse range of critical effects displayed by fatty acids on bone metabolism, and elucidated their modulatory roles and mechanisms on specific bone cell types. The evidence supporting close implications of fatty acids in bone metabolism and disorders suggests fatty acids as potential therapeutic and nutritional agents for the treatment and prevention of metabolic bone diseases.

Erucic acid derived from rosemary regulates differentiation of mesenchymal stem cells into osteoblasts/adipocytes via suppression of peroxisome proliferator-activated receptor γ transcriptional activity

Osteoporosis is associated with increase in fat tissue in bone marrow in humans. Mesenchymal stem cells in bone marrow are induced to differentiate into osteoblasts rather than adipocytes by the stimulation of peroxisome proliferator-activated receptor (PPAR) γ antagonists. PPARγ antagonists are expected to be useful to prevent osteoporosis by regulating the lineages of mesenchymal stem cells in bone marrow, as well as the prevention of obesity. In this study, we explored natural components suppressing PPARγ transcriptional activity in rosemary. Separation of active fraction of rosemary extract by repeated high performance liquid chromatograph and PPARγ luciferase reporter assay identified erucic acid, one of the monounsaturated fatty acids, as an active component. Twenty-five-micrometer erucic acid significantly decreased PPARγ luciferase activity and enhanced the differentiation of mouse-delivered C3H10T1/2 cells into osteoblasts rather than adipocytes. Furthermore, 25-μM erucic acid significantly decreased the expression of adipocyte marker genes, while accelerating osteoblast marker genes. In conclusion, erucic acid is a novel natural component derived from rosemary regulating mesenchymal stem cell differentiation via suppression of PPARγ transcriptional activity.

Reporting Guidelines, Review of Methodological Standards, and Challenges Toward Harmonization in Bone Marrow Adiposity Research. Report of the Methodologies Working Group of the International Bone Marrow Adiposity Society

The interest in bone marrow adiposity (BMA) has increased over the last decade due to its association with, and potential role, in a range of diseases (osteoporosis, diabetes, anorexia, cancer) as well as treatments (corticosteroid, radiation, chemotherapy, thiazolidinediones). However, to advance the field of BMA research, standardization of methods is desirable to increase comparability of study outcomes and foster collaboration. Therefore, at the 2017 annual BMA meeting, the International Bone Marrow Adiposity Society (BMAS) founded a working group to evaluate methodologies in BMA research. All BMAS members could volunteer to participate. The working group members, who are all active preclinical or clinical BMA researchers, searched the literature for articles investigating BMA and discussed the results during personal and telephone conferences. According to the consensus opinion, both based on the review of the literature and on expert opinion, we describe existing methodologies and discuss the challenges and future directions for (1) histomorphometry of bone marrow adipocytes, (2) ex vivo BMA imaging, (3) in vivo BMA imaging, (4) cell isolation, culture, differentiation and in vitro modulation of primary bone marrow adipocytes and bone marrow stromal cell precursors, (5) lineage tracing and in vivo BMA modulation, and (6) BMA biobanking. We identify as accepted standards in BMA research: manual histomorphometry and osmium tetroxide 3D contrast-enhanced μCT for ex vivo quantification, specific MRI sequences (WFI and H-MRS) for in vivo studies, and RT-qPCR with a minimal four gene panel or lipid-based assays for in vitro quantification of bone marrow adipogenesis. Emerging techniques are described which may soon come to complement or substitute these gold standards. Known confounding factors and minimal reporting standards are presented, and their use is encouraged to facilitate comparison across studies. In conclusion, specific BMA methodologies have been developed. However, important challenges remain. In particular, we advocate for the harmonization of methodologies, the precise reporting of known confounding factors, and the identification of methods to modulate BMA independently from other tissues. Wider use of existing animal models with impaired BMA production (e.g., Pfrt^-/-, Kit^W/W-v) and development of specific BMA deletion models would be highly desirable for this purpose.

Endogenous Glucocorticoid Signaling in the Regulation of Bone and Marrow Adiposity: Lessons from Metabolism and Cross Talk in Other Tissues

PURPOSE OF REVIEW

The development of adiposity in the bone marrow, known as marrow adipose tissue (MAT), is often associated with musculoskeletal frailty. Glucocorticoids, which are a key component of the biological response to stress, affect both bone and MAT. These molecules signal through receptors such as the glucocorticoid receptor (GR), but the role of the GR in regulation of MAT is not yet clear from previous studies. The purpose of this review is to establish and determine the role of GR-mediated signaling in marrow adiposity by comparing and contrasting what is known against other energy-storing tissues like adipose tissue, liver, and muscle, to provide better insight into the regulation of MAT during times of metabolic stress (e.g., dietary challenges, aging).

RECENT FINDINGS

GR-mediated glucocorticoid signaling is critical for proper storage and utilization of lipids in cells such as adipocytes and hepatocytes and proteolysis in muscle, impacting whole-body composition, energy utilization, and homeostasis through a complex network of tissue cross talk between these systems. Loss of GR signaling in bone promotes increased MAT and decreased bone mass. GR-mediated signaling in the liver, adipose tissue, and muscle is critical for whole-body energy and metabolic homeostasis, and both similarities and differences in GR-mediated GC signaling in MAT as compared with these tissues are readily apparent. It is clear that GC-induced pathways work together through these tissues to affect systemic biology, and understanding the role of bone in these patterns of tissue cross talk may lead to a better understanding of MAT-bone biology that improves treatment strategies for frailty-associated diseases.

The Regulation of Marrow Fat by Vitamin D: Molecular Mechanisms and Clinical Implications

PURPOSE OF REVIEW

To review the available literature regarding a possible relationship between vitamin D and bone marrow adipose tissue (BMAT), and to identify future avenues of research that warrant attention.

RECENT FINDINGS

Results from in vivo animal and human studies all support the hypothesis that vitamin D can suppress BMAT expansion. This is achieved by antagonizing adipogenesis in bone marrow stromal cells, through inhibition of PPARγ2 activity and stimulation of pro-osteogenic Wnt signalling. However, our understanding of the functions of BMAT is still evolving, and studies on the role of vitamin D in modulating BMAT function are lacking. In addition, many diseases and chronic conditions are associated with low vitamin D status and low bone mineral density (BMD), but BMAT expansion has not been studied in these patient populations. Vitamin D suppresses BMAT expansion, but its role in modulating BMAT function is poorly understood.

Inhibition of PPARγ by bisphenol A diglycidyl ether ameliorates dexamethasone-induced osteoporosis in a mouse model

Objectives

Bisphenol A diglycidyl ether (BADGE) is an antagonist for PPARγ that reduces bone marrow adiposity and increases bone formation in some animal models of osteoporosis and osteonecrosis. However, the effect of BADGE treatment on glucocorticoid-induced osteoporosis is unknown. This study investigated the preventive effects of BADGE on steroid-induced osteoporosis in mice.

Methods

Thirty-six female C57BL/6J mice were randomly divided into normal (phosphate-buffered saline), model (50 mg/kg dexamethasone sodium phosphate [Dex]), and BADGE (30 mg/kg of BADGE, combined with Dex) groups. All groups received intraperitoneal injections of their treatments, daily for 4 weeks. Protein and mRNA expression levels of gene markers were measured. Micro-computed tomography was used to measure physical parameters of femurs. Bone histomorphology was analyzed by hematoxylin and eosin staining. ELISA was used to measure serum osteocalcin and C-terminal telopeptide of type I collagen (CTX-1).

Results

Glucocorticoid treatment enlarged the marrow fat, concomitant with bone deterioration; BADGE treatment reversed steroid-induced marrow adiposity. Compared with the model group, BADGE treatment improved bone quality and increased bone volume, while increasing osteogenic markers and reducing adipogenic markers at both mRNA and protein levels; moreover, it reduced serum CTX-1 and increased serum osteocalcin.

Conclusion

BADGE treatment ameliorates glucocorticoid-induced osteoporosis by inhibiting PPARγ.

Characterizing how probiotic Lactobacillus reuteri 6475 and lactobacillic acid mediate suppression of osteoclast differentiation

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Osteoporosis is a disease that impacts over 200 million people worldwide.

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The probiotic bacterium Lactobacillus reuteri (L. reuteri) has been shown to prevent bone loss during estrogen deficiency.

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Lactobacillic acid is important for L. reuteri-induced suppression of in vitro osteoclastogenesis.

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Osteoclastogenesis was inhibited by L. reuteri and lactobacillic acid via GPR120 signaling.

Osteoporosis is a disease that impacts over 200 million people worldwide. Taking into consideration the side effects stemming from medications used to treat this illness, investigators have increased their efforts to develop novel therapeutics for osteoporosis. In a previous study, we demonstrated that ovariectomy-induced bone loss in mice was prevented by treatment with the probiotic bacterium Lactobacillus reuteri 6475 (L. reuteri), an effect that correlated with reduced osteoclastogenesis in the bone marrow of L. reuteri treated mice. We also demonstrated that L. reuteri directly inhibited osteoclastogenesis in vitro. To better understand how L. reuteri impacts osteoclast formation, we used additional in vitro analyses to identify that conditioned supernatant from L. reuteri inhibited osteoclastogenesis at the intermediate stage of fused polykaryons. To elucidate the effect of L. reuteri treatment on host cell physiology, we performed RNAseq at multiple time points during in vitro osteoclastogenesis and established that L. reuteri downregulated several KEGG pathways including osteoclast differentiation as well as TNF-α, NF-κB, and MAP kinase signaling. These results were consistent with Western Blot data demonstrating that NF-κB and p38 activation were decreased by L. reuteri treatment. We further identified that lactobacillic acid (LA), a cyclopropane fatty acid produced by L. reuteri, contributed significantly to the suppression of osteoclastogenesis. Additionally, we demonstrated that L. reuteri is signaling through the long chain fatty acid receptor, GPR120, to impact osteoclastogenesis. Overall, these studies provide both bacterial and host mechanisms by which L. reuteri impacts osteoclastogenesis and suggest that long chain fatty acid receptors could be targets for preventing osteoclastogenesis.

HU-671, a Novel Oleoyl Serine Derivative, Exhibits Enhanced Efficacy in Reversing Ovariectomy-Induced Osteoporosis and Bone Marrow Adiposity

Oleoyl serine (OS), an endogenous fatty acyl amide (FAA) found in bone, has been shown to have an anti-osteoporotic effect. OS, being an amide, can be hydrolyzed in the body by amidases. Hindering its amide bond by introducing adjacent substituents has been demonstrated as a successful method for prolonging its skeletal activity. Here, we tested the therapeutic efficacy of two methylated OS derivatives, oleoyl α-methyl serine (HU-671) and 2-methyl-oleoyl serine (HU-681), in an ovariectomized mouse model for osteoporosis by utilizing combined micro-computed tomography, histomorphometry, and cell culture analyses. Our findings indicate that daily treatment for 6 weeks with OS or HU-671 completely rescues bone loss, whereas HU-681 has only a partial effect. The increased bone density was primarily due to enhanced trabecular thickness and number. Moreover, the most effective dose of HU-671 was 0.5 mg/kg/day, an order of magnitude lower than with OS. The reversal of bone loss resulted from increased bone formation and decreased bone resorption, as well as reversal of bone marrow adiposity. These results were further confirmed by determining the serum levels of osteocalcin and type 1 collagen C-terminal crosslinks, as well as demonstrating the enhanced antiadipogenic effect of HU-671. Taken together, these data suggest that methylation interferes with OS’s metabolism, thus enhancing its effects by extending its availability to its target cells.

Regulatory mechanisms and clinical manifestations of musculoskeletal aging

Aging is the strongest risk factor for degenerative bone and joint diseases. Clinical therapies for age-related musculoskeletal disorders face significant challenges as their pathogenic mechanisms remain largely unclear. This review article focuses on the recent advances in the understanding of regulatory mechanisms of musculoskeletal aging and their clinical relevance. We begin with the prevalence and socioeconomic impacts of major age-related musculoskeletal disorders such as sarcopenia, osteoporosis, osteoarthritis, and degenerative tendinopathy. The current understanding of responsible biological mechanisms involved in general aging is then summarized. Proposed molecular, cellular, and biomechanical mechanisms relevant to the clinical manifestations of aging in the musculoskeletal system are discussed in detail, with a focus on the disorders affecting muscle, bone, articular cartilage, and tendon. Although musculoskeletal aging processes share many common pathways with the aging of other body systems, unique molecular and cellular mechanisms may be involved in the aging processes of musculoskeletal tissues. Advancements in the understanding of regulatory mechanisms of musculoskeletal aging may promote the development of novel treatments for age-related musculoskeletal disorders. Finally, future research directions for major musculoskeletal tissues including functional interaction between the tissues and their clinical relevance to age-related musculoskeletal disorders are highlighted in the Future Prospects section. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1475-1488, 2019.

Treatment with an inhibitor of fatty acid synthase attenuates bone loss in ovariectomized mice

Bone and fat cells have an antagonistic relationship. Adipocytes exert a toxic effect on bone cells in vitro through the secretion of fatty acids, which are synthesized by fatty acid synthase (FAS). Inhibition of FAS in vitro rescues osteoblasts from fat-induced toxicity and cell death. In this study, we hypothesized that FAS inhibition would mitigate the loss of bone mass in ovariectomized (OVX) mice. We treated OVX C57BL/6 mice with cerulenin (a known inhibitor of FAS) for 6 weeks and compared their bone phenotype with vehicle-treated controls. Cerulenin-treated mice exhibited a significant decrease in body weight, triglycerides, leptin, and marrow and subcutaneous fat without changes in serum glucose or calciotropic hormones. These effects were associated with attenuation of bone loss and normalization of the bone phenotype in the cerulenin-treated OVX group compared to the vehicle-treated OVX group. Our results demonstrate that inhibition of FAS enhances bone formation, induces uncoupling between osteoblasts and osteoclasts, and favors mineralization, thus providing evidence that inhibition of FAS could constitute a new anabolic therapy for osteoporosis.

PGlyRP3 concerts with PPARγ to attenuate DSS-induced colitis in mice

Nutrients may modulate immunity through their transcription factors that act on both metabolic and immunity genes. It has been shown that the transcription factor of lipid ligands PPARγ physically binds the gene promoter of the peptidoglycan recognition protein (PGlyRP3), which showed anti-inflammatory action in vitro. It is hypothesized in the present work that olive oil feeding protects against toxicity of DSS-induced colitis via activation of the lipid transcription factor PPARγ that stimulates the anti-inflammatory PGlyRP3. Results: PGlyRP3 is expressed in mouse colon and up-regulated by olive oil feeding. Olive oil reduced mortality and severity scores of DSS-induced colitis and down-regulated the proinflammatory IL-1b, IL-6 and TNFα genes. This protective effect was accompanied by up-regulation of both PPARγ and PGlyRP3. Inhibition of PPARγ by its antagonist BADGE down-regulated PGlyRP3 and abolished the anti-inflammatory effect of olive oil feeding in this DSS-induced colitis model, reflecting the pivotal role of PPARγ binding nutrition and inflammation. Activation of PGlyRP3 by its ligand peptidoglycan was not responsible for the inflammation caused by peptidoglycan, since neutralization of TLR2 attenuated this inflammatory response without affecting the peptidoglycan-induced PGlyRP3 level. Olive oil activated the IκBα and inhibited NF-κB and cox-2 gene expressions, and p65 nuclear translocation in DSS-colitis mice, reflecting the involvement of the inhibition of NF-κB signaling pathway in the anti-inflammatory olive oil - PPARγ - PGlyRP3 access. This pathway was reactivated by the PPARγ antagonist BADGE. Conclusions: Olive oil regulates by the same transcription factor (PPARγ) both lipid metabolic and immune gene (PGlyRP3) expressions, exerting the anti-inflammatory effect, and protecting against DSS-induced colitis in mice.

Epigallocatechin‑3‑gallate protects against secondary osteoporosis in a mouse model via the Wnt/β‑catenin signaling pathway

Epigallocatechin‑3‑gallate (EGCG) is a polyphenolic compound extracted and isolated from green tea, which has a variety of important biological activities in vitro and in vivo, including anti‑tumor, anti‑oxidation, anti‑inflammation and lowering blood pressure. The aim of the present study was to investigate the protective effect of EGCG against secondary osteoporosis in a mouse model via the Wnt/β‑catenin signaling pathway. Reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR) and western blotting were used to analyze runt‑related transcription factor 2 and osterix mRNA expression, and the protein expression of cyclin D1, Wnt and β‑catenin, and suppressed peroxisome proliferator‑activated receptor γ protein expression. The protective effect of EGCG against secondary osteoporosis was examined and its potential mechanism was analyzed. Treatment with EGCG significantly decreased serum calcium, urinary calcium, body weight and body fat, and increased leptin levels in mice with secondary osteoporosis. In addition, EGCG treatment significantly inhibited the structure score of articular cartilage and cancellous bone in proximal tibia metaphysis in mice with secondary osteoporosis. Treatment also significantly decreased alkaline phosphatase activity, runt‑related transcription factor 2 and osterix mRNA expression. EGCG also significantly induced the protein expression of cyclin D1, Wnt and β‑catenin, and suppressed peroxisome proliferator‑activated receptor γ protein expression in mice with secondary osteoporosis. Taken together, these results suggest that EGCG may be a possible new drug in clinical settings.

Effect of Leptin on Marrow Adiposity in Ovariectomized Rabbits Assessed by Proton Magnetic Resonance Spectroscopy

OBJECTIVE

Leptin acts to influence bone metabolism through indirect hypothalamic relay and direct peripheral pathways. Leptin enhances the differentiation of mesenchymal stem cells to the osteoblast rather than the adipocyte lineage, but the in vivo impacts of leptin on ovariectomy (OVX)-induced marrow adiposity are poorly understood. In this work, we aimed to address this question.

METHODS

Forty-five female New Zealand rabbits were divided into sham + vehicle, OVX + vehicle, and OVX + leptin for 5 months. Magnetic resonance spectroscopy and dual-energy x-ray absorptiometry were performed to longitudinally evaluate marrow fat fraction and bone density at 0, 2.5, and 5 months, respectively. At the end of experiment, quantitative parameters of marrow adipocytes were assessed by histopathology.

RESULTS

Estrogen-deficient rabbits markedly exhibited expansion of marrow fat in a time-dependent manner, with a variation of marrow fat fraction (+19.7%) at 2.5 months relative to baseline conditions, and it was maintained until 5 months (+49.2%; all P < 0.001), which was accompanied by diminished bone density. Adipocyte diameter, density, and adipocytes area percentage in the OVX controls was increased by 50.7%, 76.3%, and 135.5%, respectively, relative to the sham controls (all P < 0.001). These OVX-induced marrow adiposity and bone loss were partly restored by leptin treatment. Treatment with leptin prevented OVX-induced increases in bone turnover in rabbits.

CONCLUSIONS

Early leptin administration inhibits the adipogenic effect of estrogen deficiency in terms of reverting marrow fat expansion seen in OVX rabbits. Magnetic resonance spectroscopy may be a useful tool for longitudinal and interventional assessments in osteoporosis.

Marrow Adipose Tissue: Its Origin, Function, and Regulation in Bone Remodeling and Regeneration

Marrow adipose tissue (MAT) is a unique fat depot in the bone marrow and exhibits close relationship with hematopoiesis and bone homeostasis. MAT is distinct from peripheral adipose tissue in respect of its heterogeneous origin, site-specific distribution, and complex and perplexing function. Though MAT is indicated to function in hematopoiesis, skeletal remodeling, and energy metabolism, its explicit characterization still requires further research. In this review, we highlight recent advancement made in MAT regarding the origin and distribution of MAT, the local interaction with bone homeostasis and hematopoietic niche, the systemic endocrine regulation of metabolism, and MAT-based strategies to enhance bone formation.

The effect of vitamin D and zoledronic acid in bone marrow adiposity in kidney transplant patients: A post hoc analysis

PURPOSE

Osteoblasts and adipocytes are derived from mesenchymal stem cells. An imbalance in the differentiation of these lineages could affect the preservation of bone integrity. Several studies have suggested the importance of this imbalance in the pathogenesis of osteoporosis after kidney transplant (KT), but the role of bone marrow adiposity in this process is not well known, and if the treatment with the anti-absorptive (zoledronic acid-ZA) drugs could attenuate bone loss. Thus, our objective was compare bone marrow adiposity, osteoblasts and osteocytes before and after KT, verify an association between bone remodeling process (Turnover, Volume, and Mineralization-TMV classification), the osteocyte sclerostin expression to evaluate if there is a role of Wnt pathway, as well as the effect of ZA on these cells.

METHODS

We studied 29 new living-donor KT patients. One group received ZA at the time of KT plus cholecalciferol for twelve months, and the other group received only cholecalciferol. Bone biopsies were performed at baseline and after 12 months of treatment. Histomorphometric evaluation was performed in bone and bone marrow adipocytes. Sclerostin (Scl) expression in osteocytes was evaluated by immunohistochemistry.

RESULTS

Some bone marrow adiposity parameters were increased before KT. After KT, some of them remained increased and they worsened with the use of ZA. In the baseline, lower bone Volume and Turnover, were associated with increased bone marrow adiposity parameters (some of them). After KT, both groups showed the same associations. Osteocyte Scl expression after KT decreased with the use of ZA. We observed also an inverse association between bone adiposity parameters and lower osteocyte sclerostin expression 12 months after KT.

CONCLUSION

In conclusion, the present study suggests that KT fails to normalize bone marrow adiposity, and it even gets worse with the use of ZA. Moreover, bone marrow adiposity is inversely associated with bone Volume and Turnover, which seems to be accentuated by the antiresorptive therapy.

BADGE, a synthetic antagonist for PPARγ, prevents steroid-related osteonecrosis in a rabbit model

Background

It was indicated that inhibition of PPARγ probably represents a novel therapy for steroid-related osteonecrosis. In this study, we investigated the preventive effects of PPARγ inhibition on steroid-related osteonecrosis in a rabbit model.

Methods

Rabbits were randomly divided into three groups (normal group, model group and BADGE group). Osteonecrosis was induced in rabbits in the model group and the BADGE group. The BADGE group also received bisphenol a diglycidyl ether(BADGE), a PPARγ antagonist, for 6 weeks.

Results

Histopathological results indicated that rabbits treated with BADGE exhibited significantly reduced osteonecrotic changes, incidence of osteonecrosis and bone marrow adiposity. Furthermore, BADGE-treated rabbits exhibited reduced intraosseous pressure and increased femoral blood perfusion. Micro-computed tomography and bone histomorphometry indicated that the BADGE group exhibited significantly improved bone quality and mineral appositional rate compared with the model group. Furthermore, the BADGE group showed a significant increase in circulating levels of the bone formation marker osteocalcin and reduced levels of the bone resorption marker TRACP. Overall, BADGE-treated rabbits exhibited reduced marrow adiposity concomitant with improved bone formation.

Conclusions

In conclusion, these observations demonstrated that pharmacological inhibition of PPARγ might represent an effective therapy for steroid-related osteonecrosis in the near future.

Marrow Fat-a New Target to Treat Bone Diseases?

PURPOSE OF REVIEW

The goal of this review is to summarize recent findings on marrow adipose tissue (MAT) function and to discuss the possibility of targeting MAT for therapeutic purposes.

RECENT FINDINGS

MAT is characterized with high heterogeneity which may suggest both that marrow adipocytes originate from multiple different progenitors and/or their phenotype is determined by skeletal location and environmental cues. Close relationship to osteoblasts and heterogeneity suggests that MAT consists of cells representing spectrum of phenotypes ranging from lipid-filled adipocytes to pre-osteoblasts. We propose a term of adiposteoblast for describing phenotypic spectrum of MAT. Manipulating with MAT activity in diseases where impairment in energy metabolism correlates with bone functional deficit, such as aging and diabetes, may be beneficial for both. Paracrine activities of MAT might be considered for treatment of bone diseases. MAT has unrecognized potential, either beneficial or detrimental, to regulate bone homeostasis in physiological and pathological conditions. More research is required to harness this potential for therapeutic purposes.

Curcumin Attenuates Pulmonary Inflammation in Lipopolysaccharide Induced Acute Lung Injury in Neonatal Rat Model by Activating Peroxisome Proliferator-Activated Receptor γ (PPARγ) Pathway

Background

This study aimed to investigate the therapeutic effect of curcumin in lipopolysaccharide (LPS) induced neonatal acute lung injury (ALI) and the possibly associated molecular mechanisms.

Material/Methods

ALI neonatal animal model was established by using LPS. Curcumin and/or peroxisome proliferator-activated receptor γ (PPARγ) inhibitor BADGE (bisphenol A diglycidyl ether) were administrated to animals. Lung edema was evaluated by PaO₂ and lung wet/dry weight ratio (W/D) measurements. EMSA was used to determine the PPARγ activity. Levels of high-mobility group box 1 (HMGB1), secretory receptor for advanced glycation end products (RAGE), tumor necrosis factor α (TNFα), interleukin 6 (IL6), and transforming growth factor β1 (TGFβ1) in bronchoalveolar lavage fluid (BALF) were examined by ELISA. Western blotting was used to evaluate the expression levels of HMGB1, RAGE, heme oxygenase 1 (HO1), TNFα, IL6, and TGFβ1 in lung tissue.

Results

Curcumin administration significantly improved lung function by increasing PaO₂ and decreasing W/D in neonatal ALI rats. Curcumin treatment upregulated the PPARγ activity and expression level of HO1 which were suppressed in lung tissue of neonatal ALI rats. Elevated levels of HMGB1, RAGE, TNFα, IL6, and TGFβ1 in both lung tissue and BALF from neonatal ALI rats were decreased dramatically by curcumin treatment. PPARγ inhibitor BADGE administration impaired curcumin’s alleviation on lung edema, inhibitory effects on inflammatory cytokine expression and recovery of PPARγ/HO1 signaling activation.

Conclusions

Curcumin alleviated lung edema in LPS-induced ALI by inhibiting inflammation which was induced by PPARγ/HO1 regulated-HMGB1/RAGE pro-inflammatory pathway.

Longitudinal assessment of marrow fat content using three-point Dixon technique in osteoporotic rabbits

OBJECTIVE

In this longitudinal pilot study, we aimed to investigate the intra-, interobserver, and scan-rescan reproducibility of marrow fat fraction (FF) measurements using three-point Dixon imaging in osteoporotic rabbits: comparison with histopathology.

METHODS

Twenty female rabbits were randomly assigned to sham-operation and ovariectomy in combination with daily methylprednisolone hemisuccinate groups (n = 10 per group). Marrow FF by three-point Dixon technique and bone density by dual-energy x-ray absorptiometry were assessed at baseline, 6 and 12 weeks after operation. Intra-, inter-reader, and scan-rescan reliability of FF measurements were evaluated using intraclass correlation coefficient (ICC) and Bland-Altman 95% limit of agreement. Histomorphometry was performed to quantify marrow adipocyte parameters.

RESULTS

Intra- and inter-reader reproducibility of FF measurements was "substantial" (ICC = 0.984 and 0.978, respectively). Although the ICC for scan-rescan reliability was excellent (ICC = 0.962), increased measurement variability was observed using Bland-Altman plot. Relative to the sham-operated rabbits, the adipocytes mean diameter, density, and percent adipocytes area in the osteoporotic rabbits increased by 23.4%, 68.9%, and 117.0%, respectively. Marrow FF was positively correlated with the quantitative parameters of adipocytes, particularly with percent adipocyte area, but inversely associated with bone density. At the relatively early stage, the percentage of bone loss was similar to that of elevated fatty marrow in the osteoporotic rabbits; at the later stage, the change for the latter outweighed that of the former.

CONCLUSIONS

Results of three-point Dixon technique demonstrated a very reproducible manner within and between observers and acceptable scan-rescan performance in the assessment of marrow fat in rabbits.

A Review on the Effects of Bisphenol A and Its Derivatives on Skeletal Health

Bisphenol A (BPA) is an endocrine disruptor which can bind to the oestrogen receptor. It also possesses oestrogenic, antiandrogenic, inflammatory and oxidative properties. Since bone responds to changes in sex hormones, inflammatory and oxidative status, BPA exposure could influence bone health in humans. This review aimed to summarize the current evidence on the relationship between BPA and bone health derived from cellular, animal and human studies. Exposure to BPA (0.5-12.5 µM) decreased the proliferation of osteoblast and osteoclast precursor cells and induce their apoptosis. Bisphenol AF (10 nM) enhanced transforming growth factor beta signalling but bisphenol S (10 nM) inhibited Wnt signalling involved in osteoblast differentiation in vitro. In animals, BPA and its derivatives demonstrated distinct effects in different models. In prenatal/postnatal exposure, BPA increased femoral bone mineral content in male rats (at 25 ug/kg/day) but decreased femoral mechanical strength in female mice (at 10 µg/kg/day). In oestrogen deficiency models, BPA improved bone mineral density and microstructures in aromatase knockout mice (at very high dose, 0.1% or 1.0% w/w diet) but decreased trabecular density in ovariectomized rats (at 37 or 370 ug/kg/day). In contrast, bisphenol A diglycidyl ether (30 mg/kg/day i.p.) improved bone health in normal male and female rodents and decreased trabecular separation in ovariectomized rodents. Two cross-sectional studies have been performed to examine the relationship between BPA level and bone mineral density in humans but they yielded negligible association. As a conclusion, BPA and its derivatives could influence bone health and a possible gender effect was observed in animal studies. However, its effects in humans await verification from more comprehensive longitudinal studies in the future.

Inhibition of Adipogenesis Is Involved in the Protective Effects of 1,25-Dihydroxy Vitamin D3 on the Radiation-Injured Bone Marrow Microenvironment in Mice

To explore the protective effects of 1,25-dihydroxy vitamin D3 (1,25-(OH)₂D₃) on the bone marrow microenvironment in mice after irradiation and the underlying molecular mechanisms, a total of 150 7-wk-old male BALB/c mice were randomly divided into a normal group, an irradiation (IR) group and an irradiation+1,25-(OH)₂D₃ (IR+VD3) group. The mice in the IR+VD3 group were treated with 6.0 Gy ⁶⁰Coγ rays, and 1,25-(OH)₂D₃ (dissolved in DMSO, 2.5 μg/kg) was administered once per day from 2 d before to 8 d after irradiation. Mice in the IR group were treated with the same dose of γ rays and an equal volume of DMSO. Subsequently, the body weights and the numbers of peripheral white blood cells (WBCs) were measured. Histological analysis of femur bone marrow was conducted to determine the proportion of adipose area as well. Finally, the expression of peroxisome proliferator-activated receptor-gamma (PPARγ) in bone marrow was detected by immunohistochemistry. After irradiation, the percentage of adipose area in the bone marrow was significantly increased, and the WBC number and body weight were markedly reduced. Compared with irradiation alone, the co-administration of 1,25-(OH)₂D₃ with irradiation markedly attenuated radiation-induced adipogenesis in bone marrow, resulted in fewer bone marrow stromal cells expressing PPARγ and enhanced the recovery of body weight and WBCs. These results indicate that 1,25-(OH)₂D₃ could accelerate the recovery of body weight and WBCs in irradiated mice and protect the bone marrow by inhibiting radiation-induced adipogenesis via the down-regulation of PPARγ expression.

NLRP3 inflammasome activation in mesenchymal stem cells inhibits osteogenic differentiation and enhances adipogenic differentiation

Osteoporosis is one of the most common skeletal disease featured by osteopenia and adipose accumulation in bone tissue. NLRP3 inflammasome activation is an essential player in aging-related chronic diseases like osteoporosis, particularly due to the causal caspase-1 activation and its correlation to adipose accumulation in bone tissue. Moreover, the expression of anti-aging/senescence SIRT1 was reported to decline along with aging. As the major cellular contributor of bone formation, mesenchymal stem cells (MSCs) are multipotent stem cells processing mutually exclusive differentiatability toward osteocytes or adipocytes. Therefore, we hypothesized that NLRP3 inflammasome activation promotes adipogenesis and repress osteogenesis in MSCs via inhibiting SIRT1 expression. We activated NLRP3 inflammasome in human MSCs via lipopolysaccharide and palmitic acid (LPS/PA) treatment for self-renewal maintenance, adipogenic differentiation or osteogenic differentiation. LPS/PA treatment significantly increased NLRP3 expression, decreased SIRT1 expression and promoted caspase-1 activity in MSCs. LPS/PA treatment also boosted adipogenesis of MSCs and suppressed osteogenesis. Moreover, inhibition of caspase-1 activity repressed adipogenic differentiation and partially improved osteogenic differentiation of MSCs with LPS/PA treatment. Our study demonstrated the pivotal roles of NLRP3 inflammasome and downstream mediator caspase-1 for the progress of osteo-differentiation MSCs, and offered novel therapeutic target of treatment for osteoporosis.

Differential effects of bisphenol A diglicydyl ether on bone quality and marrow adiposity in ovary-intact and ovariectomized rats

Bisphenol A diglycidyl ether (BADGE), a PPARγ2 antagonist, has been shown to inhibit marrow adipogenesis and promote bone formation in intact animals. We investigated the impact of BADGE on a new and more clinically relevant physiological model, the ovariectomized (OVX) rat model. Forty female Wistar rats were divided into four treatment groups for 12 wk (n = 10/group): sham+vehicle, sham+BADGE, OVX+vehicle, and OVX+BADGE. Postmortem analyses included MRI, micro-CT, serological test, histomorphometry, biomechanical tests, RT-PCR, and Western blot. Overall, OVX induced a sequential marrow fat expansion accompanied by bone deterioration. Compared with OVX controls, BADGE reduced fat fraction of the distal femur by 36.3%, adipocyte density by 33.0%, adipocyte size by 28.6%, adipocyte volume percentage by 57.8%, and adipogenic markers PPARγ2 and C/EBPα by ∼50% in OVX rats. Similar results were observed in sham rats vs. vehicle. BADGE could promote bone quality in sham rats; however, BADGE did not significantly improve trabecular microarchitecture, biomechanical strength, and dynamic histomorphometric parameters except for trabecular separation in OVX rats. We concluded that early BADGE treatment at a dose of 30 mg/kg attenuates marrow adiposity in ovary-intact and OVX rats and stimulates bone formation in ovary-intact rats but does not significantly rescue bone quality in OVX rats.

Panax notoginseng saponins mitigate ovariectomy-induced bone loss and inhibit marrow adiposity in rats

OBJECTIVE

Previous data have suggested that Panax notoginseng saponins (PNS) can prevent estrogen deficiency-induced bone loss by dual action: stimulation of new bone formation and inhibition of bone resorption. Marrow adipogenesis has been identified as a negative indicator of skeletal strength and integrity. This study assessed the effects of early PNS supplementation on bone microarchitecture preservation and marrow fat content in an ovariectomized rat model.

METHODS

Forty adult female Sprague-Dawley rats were randomly assigned to four equal groups for 12 weeks of treatment: (1) sham operation (SHAM) + vehicle; (2) ovariectomy (OVX) + vehicle; (3) OVX + 17β-estradiol (25 μg/kg); (4) OVX + PNS (300 mg/kg/d, PO). Marrow fat content of the femur was determined, using fat/water magnetic resonance imaging (MRI), at baseline and 6 and 12 weeks after operation. At the end of the experiment, bone turnover, trabecular microarchitecture, and marrow adipocytes were assessed by serum biomarkers, micro-computed tomography (micro-CT), and histopathology, respectively. The effects of PNS on adipocytic differentiation were reflected by expression levels of the adipogenic genes PPARγ2 and C/EBPα, as determined by reverse transcription-polymerase chain reaction.

RESULTS

Ovariectomized rats experienced remarkable increases in marrow fat content across time points, which were accompanied by elevated rate of bone turnover, global volumetric bone density, and trabecular microarchitecture deterioration. These OVX-induced pathological changes are reversible in that most of them could be mostly corrected upon 17β-estradiol treatment. PNS treatment significantly reduced marrow adipogenesis (adipocyte density, -27.2%; size, -22.7%; adipocyte volume-to-tissue volume ratio, -53.3%; all P < 0.01) and adipocyte marker gene expression, and prevented bone mass loss and microarchitecture deterioration. Moreover, PNS enhanced osteoblast activity but suppressed osteoclast turnover, as evidenced by decreased levels of serum C-terminal telopeptides of type I collagen and elevated levels of alkaline phosphatase.

CONCLUSIONS

PNS mitigates estrogen deficiency-induced deterioration of trabecular microarchitecture and suppresses marrow adipogenesis.

P-glycoprotein overexpression in bone marrow-derived multipotent stromal cells decreases the risk of steroid-induced osteonecrosis in the femoral head

P‐glycoprotein (P‐gp) plays a role in steroid‐induced osteonecrosis of the femoral head (ONFH), but the underlying mechanism remains unknown. We hypothesized that P‐gp overexpression can prevent ONFH by regulating bone marrow–derived multipotent stromal cell (BMSC) adipogenesis and osteogenesis. BMSCs from Sprague–Dawley rats were transfected with green fluorescent protein (GFP) or the multidrug resistance gene 1 (MDR1) encoding GFP and P‐gp. Dexamethasone was used to induce BMSC differentiation. Adipogenesis was determined by measuring peroxisome proliferator‐activated receptor (PPAR‐γ) expression and the triglyceride level. Osteogenesis was determined by measuring runt‐related transcription factor 2 (Runx2) expression and alkaline phosphatase activity. For in vivo experiments, rats were injected with saline, BMSCs expressing GFP (GFP‐BMSCs) or BMSCs expressing GFP‐P‐gp (MDR1‐GFP‐BMSCs). After dexamethasone induction, adipogenesis was determined by measuring PPAR‐γ expression and fatty marrow, whereas osteogenesis was detected by measuring Runx2 expression, trabecular parameters and the mineral apposition rate, followed by evaluation of the incidence of ONFH. Overexpression of P‐gp in BMSCs resulted in markedly decreased expression of adipogenic markers and increased expression of osteogenic markers. Compared with rats injected with saline, rats injected with GFP‐BMSCs showed reduced ONFH, and the injected GFP‐positive BMSCs attached to trabecular surfaces and exhibited an osteoblast‐like morphology. Compared with the rats injected with BMSCs expressing GFP alone, rats injected with BMSCs overexpressing GFP and P‐gp showed lower adipocytic variables, higher osteogenic variables and lower incidence of ONFH. Overexpression of P‐gp inhibited BMSC adipogenesis and promoted osteogenesis, which reduced the incidence of steroid‐induced ONFH.

MR spectroscopy for assessing the effects of oxytocin on marrow adipogenesis induced by glucocorticoid in rabbits

BACKGROUND

Previous studies suggest that oxytocin (OT) negatively modulates adipogenesis while promoting osteogenesis in vitro. Because of its effects on marrow stromal cells, OT might have potential utility in therapy for glucocorticoid-induced osteoporosis (GIO).

PURPOSE

To explore the effects of OT on marrow adipogenesis in a rabbit model of GIO.

MATERIAL AND METHODS

Thirty-six-month-old female New Zealand rabbits were randomly assigned to the control, GIO, and GIO + OT groups. Magnetic resonance (MR) spectroscopy and multi-detector computed tomography (MDCT) were performed to detect marrow fat content (MFC) and bone mineral density (BMD) at baseline, and 1, 2, and 3 months. After 3 months of treatment, marrow adipocytes were quantitatively evaluated by histopathology.

RESULTS

In the GIO group, MFC substantially increased from 34.1% to 43.2% at month 1, and it was maintained until month 3 (by 59.2%, all P <0.01). MFC values in the GIO group were significantly different from the control and OT-treated groups over time. Early OT treatment reversed marrow adiposity to levels of the controls. BMD values were significantly lower in the GIO group at months 2 and 3 compared to the controls; however, partial recovery of vertebral BMD (87.1% of baseline) and femoral BMD (89.3% of baseline) in the OT-treated group were observed. The mean diameter and density of adipocyte and percentage of adipocyte area increased by 30.0%, 70.1%, and 88.9%, respectively (all P <0.05) in the GIO group, but remained unchanged in the OT-treated group.

CONCLUSION

Early OT treatment was sufficient to eliminate glucocorticoid-induced marrow adiposity.

Orexin Plays a Role in Growth Impediment Induced by Obstructive Sleep Breathing in Rats

STUDY OBJECTIVES

The mechanisms linking sleep disordered breathing with impairment of sleep and bone metabolism/architecture are poorly understood. Here, we explored the role of the neuropeptide orexin, a respiratory homeostasis modulator, in growth retardation induced in an upper airway obstructed (AO) rat model.

METHODS

The tracheae of 22-day-old rats were narrowed; AO and sham-control animals were monitored for 5 to 7 w. Growth parameters, food intake, sleep/wake activity, and serum hormones were measured. After euthanasia, growth plate (GP) histology, morphometry, orexin receptors (OXR), and related mediators were analyzed. The effect of dual orexin receptor antagonist (almorexant 300 mg/kg) on sleep and GP histology were also investigated.

RESULTS

The AO group slept 32% less; the time spent in slow wave and paradoxical sleep during light period and slow wave activity was reduced. The AO group gained 46% less body weight compared to the control group, despite elevated food intake; plasma ghrelin increased by 275% and leptin level decreased by 44%. The impediment of bone elongation and bone mass was followed by a 200% increase in OX1R and 38% reduction of local GP ghrelin proteins and growth hormone secretagogue receptor 1a. Sry-related transcription factor nine (Sox9), a molecule mediating cartilage ossification, was downregulated and the level of transcription factor peroxisome proliferator-activated receptor gamma was upregulated, explaining the bone architecture abnormalities. Administration of almorexant restored sleep and improved GP width in AO animals.

CONCLUSIONS

In AO animals, enhanced expression of orexin and OX1R plays a role in respiratory induced sleep and growth abnormalities.

Eldecalcitol prevents endothelial dysfunction in postmenopausal osteoporosis model rats

Postmenopausal women have high incidence of cardiovascular events as estrogen deficiency can cause endothelial dysfunction. Vitamin D is reported to be beneficial on endothelial function, but it remains controversial whether vitamin D is effective for endothelial dysfunction under the treatment for osteoporosis in postmenopausal women. The aim of this study was to evaluate the endothelial protective effect of eldecalcitol (ELD) in ovariectomized (OVX) rats. ELD (20  ng/kg) was orally administrated five times a week for 4 weeks from 1 day after surgery. After that, flow-mediated dilation (FMD) as an indicator of endothelial function was measured by high-resolution ultrasound in the femoral artery of living rats. ELD ameliorated the reduction of FMD in OVX rats. ELD inhibited the increase in NOX4, nitrotyrosine, and p65 and the decrease in dimer/monomer ratio of nitric oxide synthase in OVX rat femoral arteries. ELD also prevented the decrease in peroxisome proliferator-activated receptor gamma (PPARγ) in femoral arteries and cultured endothelial cells. Although PPARγ is known to inhibit osteoblastogenesis, ELD understandably increased bone mineral density of OVX rats without increase in PPARγ in bone marrow. These results suggest that ELD prevented the deterioration of endothelial function under condition of preventing bone loss in OVX rats. This endothelial protective effect of ELD might be exerted through improvement of endothelial nitric oxide synthase uncoupling, which is mediated by an antioxidative effect through normalization of vascular PPARγ/NF-κB signaling.

Adipose tissue and bone: role of PPARγ in adipogenesis and osteogenesis

Peroxisome proliferator-activated receptor-γ (PPARγ) is a critical factor for the reciprocal regulation of adipogenesis and osteogenesis. Because of their insulin-sensitizing effect, PPARγ agonists, the thiazolidinediones (TZDs), have been widely used for the treatment of type 2 diabetes mellitus; however, the use of TZDs has also been revealed to cause bone loss and bone fractures. The nodal point of regulation of skeletal metabolism by PPARγ activation may reside in its role in cell fate determination of mesenchymal stem cells toward adipogenesis at the expense of osteogenesis. In addition, accumulating evidence demonstrates that PPARγ possesses a circadian expression profile and plays an important role in the skeletal and adipose metabolism regulated by the circadian clock network. Recently, we have shown that nocturnin, a circadian-regulated gene, enhances PPARγ activity, resulting in the suppression of osteogenesis and enhancement of adipogenesis, thus providing additional evidence of the link between circadian networks and PPARγ. In this review, we will focus on the emerging concept of PPARγ as a regulator for skeletal metabolism and summarize recent findings about one of the mechanisms on how PPARγ is connected to the circadian-regulatory system, which involves nocturnin.

Microbiota from Obese Mice Regulate Hematopoietic Stem Cell Differentiation by Altering the Bone Niche

The effect of metabolic stress on the bone marrow microenvironment is poorly defined. We show that high-fat diet (HFD) decreased long-term Lin(-)Sca-1(+)c-Kit(+) (LSK) stem cells and shifted lymphoid to myeloid cell differentiation. Bone marrow niche function was impaired after HFD as shown by poor reconstitution of hematopoietic stem cells. HFD led to robust activation of PPARγ2, which impaired osteoblastogenesis while enhancing bone marrow adipogenesis. At the same time, expression of genes such as Jag-1, SDF-1, and IL-7 forming the bone marrow niche was highly suppressed after HFD. Moreover, structural changes of microbiota were associated to HFD-induced bone marrow changes. Antibiotic treatment partially rescued HFD-mediated effects on the bone marrow niche, while transplantation of stools from HFD mice could transfer the effect to normal mice. These findings show that metabolic stress affects the bone marrow niche by alterations of gut microbiota and osteoblast-adipocyte homeostasis.

Effect of zoledronic acid on vertebral marrow adiposity in postmenopausal osteoporosis assessed by MR spectroscopy

OBJECTIVE

Zoledronic acid (ZOL) has a suppressive effect on marrow adiposity in ovariectomized rats. Currently, however, data on the effect of ZOL on marrow fat in humans are unavailable. The purpose of this work was to determine the in vivo effect of ZOL on bone remodeling and marrow adipogenesis in postmenopausal osteoporosis.

MATERIALS AND METHODS

In a 12-month, randomized, double-blind, placebo-controlled trial, we studied 100 postmenopausal women with osteoporosis who were randomly given either a single dose of intravenous infusion of ZOL (5 mg) or placebo. All subjects received adequate dietary calcium and vitamin D3. Main outcome measures included bone mineral density by dual-energy X-ray absorptiometry, vertebral marrow fat content by proton MR spectroscopy, serum markers of bone turnover by biochemical analysis.

RESULTS

Ninety percent of the participants completed the 12-month follow-up. With respect to baselines, marrow fat content reduced by 8.1% in the ZOL-treated women and increased by 3.0% in the controls (all p < 0.05). In addition, there were significant increases of bone mineral density by 2.8, 2.0, and 1.7% in the lumbar spine, femoral neck, and total hip, respectively, in the ZOL group compared with the placebo group. Serum levels of bone resorption marker CTX and bone formation marker BALP decreased by 33 and 18% in postmenopausal women receiving ZOL.

CONCLUSIONS

In postmenopausal women with osteoporosis, a single dose of ZOL therapy significantly reduced marrow adiposity. MR spectroscopy of vertebral marrow fat may therefore serve as a novel tool for BMD-independent efficacy assessment.

FFA-ROS-P53-mediated mitochondrial apoptosis contributes to reduction of osteoblastogenesis and bone mass in type 2 diabetes mellitus

This study evaluated the association between free fatty acid (FFA), ROS generation, mitochondrial dysfunction and bone mineral density (BMD) in type 2 diabetic patients and investigated the molecular mechanism. db/db and high fat (HF)-fed mice were treated by Etomoxir, an inhibitor of CPT1, MitoQ, and PFT-α, an inhibitor of P53. Bone metabolic factors were assessed and BMSCs were isolated and induced to osteogenic differentiation. FFA, lipid peroxidation and mtDNA copy number were correlated with BMD in T2DM patients. Etomoxir, MitoQ and PFT-α significantly inhibited the decrease of BMD and bone breaking strength in db/db and HF-fed mice and suppressed the reduction of BMSCs-differentiated osteoblasts. Etomoxir and MitoQ, but not PFT-α, inhibited the increase of mitochondrial ROS generation in db/db and HF-fed mice and osteoblasts. In addition, Etomoxir, MitoQ and PFT-α significantly inhibited mitochondrial dysfunction in osteoblasts. Moreover, mitochondrial apoptosis was activated in osteoblasts derived from db/db and HF-fed mice, which was inhibited by Etomoxir, MitoQ and PFT-α. Furthermore, mitochondrial accumulation of P53 recruited Bax and initiated molecular events of apoptotic events. These results demonstrated that fatty acid oxidation resulted in ROS generation, activating P53/Bax-mediated mitochondrial apoptosis, leading to reduction of osteogenic differentiation and bone loss in T2DM.

Icariin prevents ovariectomy-induced bone loss and lowers marrow adipogenesis

OBJECTIVE

Icariin prevents bone loss by stimulating new bone formation and by inhibiting bone resorption. However, less is known about how icariin affects marrow adiposity. This lack of information is a vital problem, as the degree of marrow adipogenesis may be an alternative indicator of the severity of osteoporosis in relation to the degree of osteogenesis and osteoblastogenesis. To explore this question, we tested the effects of icariin on bone mineral density (BMD) and marrow fat content in a rat model of postmenopausal osteoporosis.

METHODS

Thirty-six 3-month-old female Sprague-Dawley rats were randomly assigned to one of the following treatment groups: sham operation, ovariectomized controls, and ovariectomized rats treated orally with either 17β-estradiol or icariin for 12 weeks. BMD and marrow fat fraction were dynamically measured on weeks 0, 6, and 12. After 12 weeks of treatment, serum 17β-estradiol and bone biomarker levels were measured, and marrow adipocytes were quantitatively evaluated by histopathology.

RESULTS

Ovariectomized controls experienced a marked increase in fat fraction over time, with increases of 40% between weeks 0 and 6 and 69.4% between weeks 6 and 12 (P < 0.001). Marrow adiposity in ovariectomized controls was dramatically higher than that in sham rats on week 6; however, a reduction in BMD was detected in ovariectomized rats on week 12 (P < 0.001). Ovariectomized rats had levels of serum alkaline phosphatase and serum C-terminal telopeptide of type I collagen that were 49.4% and 67.2% higher, respectively, than those of sham rats (P < 0.001). The density, size, and volume of marrow adipocytes in ovariectomized controls were 57.3%, 29.5%, and 163% higher, respectively, than those in sham rats. Early icariin treatment decreased bone biomarker levels, inhibited bone degeneration, and restored marrow fat infiltration and adipocyte parameters to the levels observed in sham rats. Overall, the osteoprotective effect of icariin was comparable with that of 17β-estradiol; however, icariin did not produce uterine estrogenicity.

CONCLUSIONS

Early icariin treatment restores marrow adiposity in the estrogen-deficient rat model.

Perspectives on osteoporosis therapies

Osteoporosis is a skeletal disease which predisposes to fragility fractures with high morbidity and economic impact, and, therefore, the goal of any osteoporosis treatment is to reduce the fracture risk. In the various forms of osteoporosis an imbalance between bone resorption and apposition is present, that generally leads to a reduction of bone mineral density and bone quality, and finally to the increased fracture risk. Nowadays, several drugs are available with a demonstrated anti-fracturative effect obtained by inhibiting bone resorption or stimulating bone formation. However, their use is not free from limitations and side effects. Importantly, to date, the available antiresorptive drugs have also an inhibiting, though to a lesser extent, effect on bone apposition and, similarly, the anabolic drugs lead to an increase also of bone resorption. Advances in our knowledge about bone biology, with molecular insights into mechanisms underlying osteoblast, osteoclast, and osteocyte activity, have led to the recognition of new potential targets and consequently to the formulation of new therapeutic agents to treat osteoporosis. New potential developments among the antiresorptive drugs include cathepsin K inhibitors and among the osteoanabolic drugs those activating the Wnt signaling pathway, such as the monoclonal antibodies against sclerostin. The novelty of these compounds is that their mechanism of action gives the exciting possibility to uncouple bone resorption and bone formation, and data available so far appear to be promising. Finally, several new therapeutic targets are under investigation in preclinical studies which could open further approaches to treat osteoporosis in the future.